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Femtosecond-laser Microstructuring of Silicon:
Dopants and Defects
A thesis presented
by
Michael Anthony Sheehy
to
The Department of Chemistry and Chemical Biology
in partial fulfillment of the requirements
for the degree of
Doctor of Philosophy
in the subject of
Chemistry
Harvard University
Cambridge, Massachusetts
September 2004
c©2004 by Michael Anthony Sheehy
All rights reserved.
iii
Femtosecond-laser Microstructuring of Silicon: Dopants and Defects
Cynthia Friend Michael A. Sheehy
Abstract
This dissertation deals with the incorporation of elements into silicon using a fem-
tosecond laser in order to understand the source for below-band gap absorptance. Previous
experimental results indicate that irradiation of silicon with a femtosecond laser in the
presence of sulfur hexafluoride (SF6) leads to unique optical properties. The absorptance
for above-band gap radiation is increased to 95%; the more interesting result is that the
below-band gap absorptance goes from nearly 0% to 90%. In the first set of experiments
performed, we irradiated silicon in the presence of H2S, SiH4, and H2. The absorptance for
samples prepared in H2S is identical to that of samples prepared in SF6; the other samples
have a trailing edge of absorptance for energies below the band gap. This result indicated
that sulfur played a crucial role in the below-band gap absorptance.
The next set of experiments involved incorporating selenium and tellurium from
a powder source to investigate possible dependence of the optical properties on the size of
the dopant (selenium and tellurium have the same valence, but are larger in atomic size
than sulfur). Incorporation of these two elements also leads to near-unity absorptance for
below-band gap radiation. A comparison of the composition and the optical properties
before and after annealing showed that the source for below-band gap absorptance is likely
due to both the incorporated chalcogen and defects.
The final set of experiments deals with the incorporation of elements from other
families. These studies bolster the results of the previous research and provide further
iv
details on the interaction of the dopant with the laser-modified surface. We speculate on
some requirements the dopants must satisfy (i.e. atomic size and valence configuration)
and propose further research that can be done in this area.
These experiments provide significant insight into the optical absorption mecha-
nism and show that this material has great potential for devices that operate in the infrared
portion of the spectrum, such as infrared photodiodes and solar cells.
Table of Contents
Abstract iii
Table of Contents v
List of Figures vii
List of Tables ix
Acknowledgements x
Citations to Published Work xii
1 Introduction 1
2 Experimental Method 4
3 Background Information 8
4 The role of the background gas in the morphology and optical propertiesof laser-microstructured silicon 114.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114.2 Experimental . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 134.3 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 154.4 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
5 Chalcogen doping of silicon with a femtosecond laser above the ablationthreshold 265.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 265.2 Experimental . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 285.3 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 295.4 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
v
Table of Contents vi
6 Incorporating dopants from other families 406.1 Families III-V . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 416.2 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41
6.2.1 Valence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 426.2.2 Atomic size . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 436.2.3 Annealing and defects . . . . . . . . . . . . . . . . . . . . . . . . . . 44
6.3 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45
7 Future Work 487.1 Detailed annealing studies of sulfur samples . . . . . . . . . . . . . . . . . . 487.2 Further annealing of selenium and tellurium samples . . . . . . . . . . . . . 497.3 Optoelectronic applications . . . . . . . . . . . . . . . . . . . . . . . . . . . 507.4 Further materials characterization . . . . . . . . . . . . . . . . . . . . . . . 517.5 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52
A Rearrangement as a probe for radical formation: bromomethylcyclopropaneon oxygen-covered Mo(110) 53A.1 Experimental . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55A.2 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57
A.2.1 Temperature programmed reaction spectrometry . . . . . . . . . . . 57A.2.2 Fourier transform infrared vibrational spectroscopy . . . . . . . . . . 64A.2.3 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72A.2.4 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78
B The reaction of CH3NO2 on O-covered Mo(110): the effect of oxygen onproduct distribution 80
References 90
List of Figures
2.1 Schematic of the microstructuring setup. . . . . . . . . . . . . . . . . . . . . 52.2 Schematic of the spectrometer setup. . . . . . . . . . . . . . . . . . . . . . . 52.3 An example of a reflectance trace. . . . . . . . . . . . . . . . . . . . . . . . 62.4 An example of a transmittance trace. . . . . . . . . . . . . . . . . . . . . . . 72.5 An example of an absorptance trace. . . . . . . . . . . . . . . . . . . . . . . 7
3.1 Morphology of microstructures prepared in different gases. . . . . . . . . . . 93.2 Absorptance of samples prepared in different gases. . . . . . . . . . . . . . . 10
4.1 Morphology of samples prepared in various gases. . . . . . . . . . . . . . . . 164.2 Absorptance of samples prepared in various gases. . . . . . . . . . . . . . . 174.3 Morphology of samples prepared in dilute sulfur environments. . . . . . . . 184.4 Absorptance of samples prepared in dilute sulfur ambients. . . . . . . . . . 194.5 Morphology of structures prepared in H2S before and after annealing. . . . 204.6 Absorptance of samples prepared in H2S and annealed to various temperatures. 21
5.1 Scanning electron micrographs of the chalcogen samples. . . . . . . . . . . . 295.2 Scanning electron micrographs of the chalcogen samples after annealing. . . 305.3 Optical properties of chalcogen-doped microstructures. . . . . . . . . . . . . 315.4 Optical properties of chalcogen-doped microstructures after annealing at 775
K. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 325.5 Rutherford backscattering spectra for a sulfur-doped sample, before and after
annealing. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 325.6 Rutherford backscattering spectra for a selenium-doped sample, before and
after annealing. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 335.7 Rutherford backscattering spectra for a tellurium-doped sample, before and
after annealing. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
6.1 Absorptance of samples prepared using Family V dopants. . . . . . . . . . . 426.2 Absorptance of samples prepared using Family IV dopants. . . . . . . . . . 436.3 Absorptance of samples prepared using Family III dopants. . . . . . . . . . 446.4 Absorptance spectra after annealing P and Te samples. . . . . . . . . . . . 45
vii
List of Figures viii
7.1 Optical properties of annealed microstructures in different sulfur environments. 49
A.1 The radical rearrangement of the methylcyclopropyl to the 3-butenyl radical 55A.2 Temperature programmed reaction data following adsorption of multilayers
of bromomethylcyclopropane on oxygen-covered Mo(110) (θ0 = 0.67 ML). . 58A.3 Temperature programmed reaction after adsorption of 4-bromo-1-butene on
oxygen-covered Mo(110). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63A.4 Temperature programmed reaction data obtained after adsorption of multi-
layers of 3-buten-1-ol on oxygen-covered Mo(110). . . . . . . . . . . . . . . 65A.5 Infrared absorption spectra obtained after heating bromomethylcyclopropane,
4-bromo-1-butene, and 3-buten-1-ol to 450 K. . . . . . . . . . . . . . . . . . 66A.6 Difference spectra for infrared reflection absorption data obtained after heat-
ing the surface to 450 K. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69A.7 Infrared reflection absorption spectra after transient heating of bromomethyl-
cyclopropane adsorbed on oxygen-covered Mo(110) to various temperatures. 71A.8 Infrared spectra on 16O- and 18O-covered surfaces heated to various temper-
atures. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72A.9 Proposed reaction scheme for bromomethylcyclopropane on oxygen-covered
Mo(110). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73
B.1 Temperature programmed reaction for CH3NO2 on Mo(110)–(1×6)-O and0.4 ML O on Mo(110). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85
B.2 Reflectance absorption infrared spectroscopy data for CH3NO2 on Mo(110)–(1×6)-O at different temperatures. . . . . . . . . . . . . . . . . . . . . . . . 87
List of Tables
4.1 Infrared absorptance metric and sulfur content . . . . . . . . . . . . . . . . 174.2 Infrared absorptance and sulfur content of annealed H2S samples. . . . . . . 20
5.1 Physical data of chalcogens in silicion . . . . . . . . . . . . . . . . . . . . . 275.2 Chalcogen content of microstructured samples before and after annealing . 31
A.1 Mass fragmentation patterns of authentic samples . . . . . . . . . . . . . . 59A.2 Mass fragmentation patterns of products . . . . . . . . . . . . . . . . . . . . 60A.3 Infrared vibrational assignments for bromomethylcyclopropane, 4-bromo-1-
butene, and 3-buten-1-ol on oxygen covered Mo(110) . . . . . . . . . . . . . 67
B.1 Assignments for vibrational bands of molecular CH3NO2 . . . . . . . . . . . 86B.2 Assignments for vibrational bands of CH3NO2 adsorbed at 100 K and then
annealed to 500 K. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88
ix
Acknowledgements
Three weeks have passed since my defense, and I still struggle to find the words to
acknowledge everyone who has helped me in my time at Harvard. I have been fortunate to
be part of two research groups composed of outstanding graduate students and postdoctoral
fellows. I have found that I am part of many groups, all of which have helped me through
one situation or another. So, I would like to acknowledge the Mazur group, the Friend
group, the Thursday night crowd, the epidemic crew, the pool team, and the Five Families.
Without these people, graduate school would have been a lesser experience.
However, I feel I would be remiss if I did not specifically thank Jim Carey, Chris
Roeser, Brian Tull and Rafael Gattass. Jim and I worked together for a significant portion
of my graduate career and he has been a source of knowledge and a good friend. Chris
and I must have logged about 1000 hours of pool together and I would not ask for any of
that time back. Brian joined the project a couple years ago and has shared many moments
of the co-advisor relationship, good and bad, and I appreciate all the time we have spent
together. Rafa has simply been one of the kindest people I have had the good fortune to
know. I thank all of these people for making graduate school a fun place to work.
I would also like to thank my family for being a source of support and help. They
x
Acknowledgements xi
have been there, one and all, for the phone calls when I complained about the defense date
being pushed back and back again, when I got a job offer, when I changed projects, and
whenever else I needed them. They always listened and helped. For that, I am grateful.
Michael Sheehy
Cambridge, Massachusetts
September, 2004
Acknowledgements of Financial Support
This thesis is based on work supported by an NSF GK-12 fellowship and DOE
MRSEC.
Citations to Published Work
Parts of this dissertation cover research reported in the following articles:
[1] M. Sheehy, L. Winston, J. Carey, C. Friend, and E. Mazur Chem. of Materials, 2004.submitted.
[2] M. Sheehy, C. Friend, and E. Mazur Mat. Sci. and Eng. B., 2004. to be submitted.
[3] C. Crouch, J. Carey, M. Shen, E. Mazur, and F. Genin Appl. Phys. A., 2004. accepted.
[4] C. Wu, C. Crouch, L. Zhao, J. Carey, R. Younkin, J. Levinson, E. Mazur, R. Farrel,P. Gothoskar, and A. Karger Appl. Phys. Lett., vol. 78, p. 1850, 2001.
[5] T.-H. Her, R. Finlay, C. Wu, S. Deliwala, and E. Mazur Appl. Phys. Lett., vol. 73,p. 1673, 1998.
[6] R. Younkin, J. Carey, E. Mazur, J. Levinson, and C. Friend J. Appl. Phys., vol. 93,p. 2626, 2003.
[7] J. Levinson, I. Kretzschmar, M. Sheehy, L. Deiner, and C. Friend Surf. Sci, vol. 479,p. 273, 2001.
[8] L. Deiner, A. Chan, M. Sheehy, and C. Friend Surf. Sci. Lett., vol. 555, p. L127, 2004.
xii
For my parents, who taught me how to think,
and let me figure out what to think.
Great is the art of beginning, but greater is the art of ending.
Henry Wadsworth Longfellow
Chapter 1
Introduction
The study of laser material interactions is rich and varied, with a vast scope
that touches upon many fields of scientific endeavor, including biomedical applications,
ultrafast chemical reaction dynamics, communication networks and fundamental processes
in physics. Femtosecond lasers, while only recently developed, already possess a great range
of applications, with some of the richest being in materials science.
One of the most technologically important materials is silicon and its interaction
with a femtosecond pulse was proposed by this group to study laser-assisted reactive ion
etching. As serendipidity would have it, a wealth of interesting experiments was provoked
by this one experiment. The most interesting characteristic of the resultant material is the
near-unity absorptance for both above-band gap radiation and, more surprisingly, below-
band gap radiation. The resulting surface is covered with conical microstructures that are
quasi-periodic across the surface.
This thesis is an extension of previous work performed on the interaction of a
1
Chapter 1: Introduction 2
femtosecond laser with a silicon substrate and focuses on the chemistry and materials science
behind the absorptance for near-infrared radiation.
Organization of the Dissertation
Chapter 2 provides schematics of the experimental setup. Specifically, schematics
of the laser setup, the spectrometer setup, as well as some sample spectra taken using our
spectrometer.
Chapter 3 provides a brief summary of the experimental results acquired prior to
the beginning of the research described in latter chapters. This chapter is brief and serves
as an opportunity for the reader to become familiar with the relevant background.
Chapter 4 presents a study that contains an analysis of the impact of various
different properties of the background gas on the morphology of the substrate and the
absorptance of visible and near-infrared radiation. Experimental results from this chapter
show that absorptance of near-infrared radiation is not observed in samples created in a
background of H2, SiH4 and Ar, but it is observed when the background gas contains sulfur.
Chapter 5 is an extension of the previous chapter to study three chalcogens, sul-
fur, selenium, and tellurium. Experimental results show that incorporation of any of the
chalcogens leads to the near-unity absorptance of near-infrared radiation. Differences in
absorptance of near-infrared radiation are measured after annealing, lending insight into
the mechanism of absorption of photons of this energy.
Chapter 6 details a study where pentavalent (phosphorous and antimony), tetrava-
lent (carbon, silicon, and germanium), and trivalent (gallium) elemental dopants are used.
Near-unity absorptance of near-infrared radiation is also observed for some of these dopants,
Chapter 1: Introduction 3
indicating that the dopant need not be hexavalent (a chalcogen) to create a surface with
novel optoelectronic properties. Annealing data provides further information on the mech-
anism by which the microstructured surface absorbs infrared radiation.
Chapter 7 discusses some further research that could be done, questions that
remain unanswered, and a brief mention of some device results obtained by work done by
a colleague in parallel with this thesis.
Several chapters in this thesis are organized as papers that have or will be sub-
mitted to journals for publication and there is some repetition in the text, particularly the
experimental sections. However, after reading the various sections, the reader should have
a greater understanding of the material and how the morphology and optical properties of
this material evolve.
In Appendix A and B are two papers that have been published by Levinson et al.
and Deiner et al.. These results are based on research performed during the earlier stages
of the author’s career in which the focus was on studying model catalytic systems. The first
is a paper on the reaction of bromomethylcyclopropane on oxidized Mo(110). The second
is a study on the reaction of nitromethane on oxidized Mo(110).
Chapter 2
Experimental Method
The specifics of the experiments are described in detail in each individual chapter.
However, as the reader may be unfamiliar with the experiments described hereafter, a brief
summary of the laser setup and the spectrometer used are included below.
A schematic of the processing setup used in all experiments is shown in Figure 2.1.
A 1-kHz train of 100-fs, 800-nm laser pulses is focused to a spot 150 µm in diameter. The
sample is loaded via a quick access port onto a sample arm. The sample arm is connected
to a translation stage that can be directly controlled by a computer. The chamber can be
evacuated and then filled with any gas (or combination of gases).
In all experiments involving a spectrometer, an integrating sphere was used to
account for both specular and diffuse reflection. Reflectance and transmittance values were
taken in 1-nm increments from 0.25 µm to 2.5 µm (these values represent the maximum
range of the spectrometer and integrating sphere).
In Figures 2.3–2.5 are actual spectra taken using our spectrometer. After taking
4
Chapter 2: Experimental Method 5
quick-access
door
quartz window
to roughing
pump
to gas-handling
manifold
computer-controlled
axeshand-controlled
axis
pressure gauges
mounting magnet
spotsize CCD
surface imaging CCD
white-light fiber lamp
800-nm,
100-fs laser pulses
focusing lens
Figure 2.1: A schematic representation of the setup we use in the microstructuring of silicon.
white
light
gratin
g
sam
ple
TiO
2
photodiode output
white
light
gratin
g
sam
ple
photodiode output
Figure 2.2: A schematic representation of the spectrometer used to measure the total ab-sorptance. Note that we account for both diffuse and specular reflection.
Chapter 2: Experimental Method 6
b
wavelength (µm)
reflecta
nce
0 1 2 3
1.0
0.8
0.6
0.4
0.2
0
a
Figure 2.3: An example of a reflectance trace as obtained from our spectrometer. The tracesare from a) an unstructured crystalline silicon wafer and b) from a wafer structured in SF6.
both reflectance (R) and transmittance (T) data, the two values are subtracted from 1 to
obtain the absorptance A (A = 1 − R − T ). Note that crystalline silicon has a feature at
1.1 µm in all spectra. This feature corresponds to the band gap of the material. Also, note
that the spectra for the structured samples have very low transmittance and reflectance as
well as near-unity absorptance for below-band gap radiation. This result is the motivation
behind the experiments described hereafter.
Chapter 2: Experimental Method 7
b
wavelength (µm)
tra
nsmitta
nce
0 1 2 3
1.0
0.8
0.6
0.4
0.2
0
a
Figure 2.4: An example of a transmittance trace as obtained from our spectrometer. Thetraces are from a) an unstructured crystalline silicon wafer and b) from a wafer structuredin SF6.
b
wavelength (µm)
absorp
tance
0 1 2 3
1.0
0.8
0.6
0.4
0.2
0
a
Figure 2.5: An example of an absorptance trace as obtained from our spectrometer. Thetraces are from a) an unstructured crystalline silicon wafer and b) from a wafer structuredin SF6.
Chapter 3
Background Information
The experiments and results described in the remaining chapters were inspired
by an experiment carried out six years ago. This preliminary result was a study of the
morphology of the silicon surface after femtosecond-laser irradiation. As shown in Figure
3.1, the morphology of the substrate after irradiation depends on the background gas.
Microstructures formed in SF6 are approximately 10 µm tall and have some nanomaterial
on the sidewalls of the microstrucutres. The radius of curvature at the tip is on the order
of several hundred nanometers. The microstructures formed in Cl2 are also sharp like the
samples from SF6, but they lack the nanomaterial. The microstructures formed in nitrogen
and air are larger and blunter than those formed in the halogen-containing gases. Structures
on surfaces prepared in both air and nitrogen have a large amount of nanomaterial on the
sidewalls.
These results stimulated our investigation of the role of the background gas in
etching the surface. Some of the questions regarding morphology are discussed in Chapter
8
Chapter 3: Background Information 9
a)
10 mµ
b)
c) d)
Figure 3.1: Scanning electron micrographs of surfaces created in a) SF6 b) Cl2 c) N2 andd) air.
4. For more details on the formation mechanism, dependence on the laser parameters, and
various other factors, the reader is referred to [1].
After analyzing the morphology, the optical properties were the next parameter
investigated. After irradiation, the surface goes from being highly reflective to grey or black,
depending on the background gas used and its pressure. Figure 3.2 shows that there is an
enhanced absorptance for above-band gap radiation for samples prepared in all gases. At
1.1 µm, however, differences in the optical properties exist. The absorptance for samples
prepared in air, nitrogen and chlorine all decrease monotonically for wavelengths longer than
1.1 µm. Samples prepared in SF6, however, have near-unity absorptance at wavelengths as
long as 2.5 µm.
Chapter 3: Background Information 10
c b
a
wavelength (µm)
absorp
tance
0 1 2 3
1.0
0.8
0.6
0.4
0.2
0
ed
Figure 3.2: Absorptance of samples created in a) SF6 b) Cl2 c) N2 and d) air. The tracefor crystalline silicon (e) is included for reference.
This particular result encouraged us to investigate the role of sulfur in the novel
optical properties of the material.
Chapter 4
The role of the background gas in
the morphology and optical
properties of laser-microstructured
silicon
4.1 Introduction
In the past several years, the use of lasers to incorporate impurities into silicon has
produced a wealth of possibilities for new optoelectronic device applications. Of particular
interest is the possibility of extending the response of silicon to wavelengths beyond the band
edge, with potential applications ranging from silicon-based infrared detectors to improved
solar cells.[2, 3, 4]
11
Chapter 4: The role of the background gas in the morphology and optical properties oflaser-microstructured silicon 12
Her et al. first reported the formation of sharp conical microstructures by fem-
tosecond laser irradiation of silicon in the presence of 500 Torr of SF6.[5] The conical mi-
crostructures are on the order of 10 µm tall, have a diameter at the tip of less than 1 µm,
and are quasi-periodic across the surface. The physical and chemical mechanisms for creat-
ing this microstructured surface are complex and include: laser ablation and melting of the
silicon substrate; substrate etching by reactive ions and fragments created in the intense
fields of the laser [6]; and redeposition of material from the ablation plume.
The optical properties of the microstructured silicon are of interest because of the
absorptance changes over a broad range of wavelengths. For large areas of microstructured
silicon, Wu et al. measured high absorptance at wavelengths from 0.25 µm to 2.5 µm for
samples created in SF6.[7] In the region from 0.25 µm to 1.1 µm, samples absorb 95%
of incident radiation; at 1.1 µm, corresponding to the band gap of crystalline silicon, the
absorptance decreases somewhat, but remains at 90% up to wavelengths as long as 2.5 µm.
The optical properties of microstructured silicon depend strongly on the gaseous
species present during irradiation. Younkin et al. found that samples created in N2, Cl2, and
air show enhanced optical absorptance for above-band gap radiation, but have absorptance
which decreases monotonically from the band edge (1.1 µm) to 2.5 µm.[8] To determine the
source of the differences in absorptance for infrared radiation between samples created in SF6
and samples from other background gases, Rutherford backscattering data measurements
were taken. They found 1% of sulfur in samples that had 90% absorptance for infrared
radiation. Based on these Rutherford backscattering measurements and because sulfur in
low concentrations is known to create discrete or localized energy states in the band gap
Chapter 4: The role of the background gas in the morphology and optical properties oflaser-microstructured silicon 13
of silicon, they concluded that the near-unity optical absorptance in the infrared is due to
the presence of high concentrations of sulfur impurities.[9] Experimental results show that
impurity concentrations on the order of 1016 cm−3 are sufficient to create an entire band of
states in the gap.[10]
Younkin et al. also examined the effect of the background gas on the morphology
of the resulting microstructures.[8] Microstructures formed in SF6 and Cl2 are sharper and
twice as dense as those formed in N2 and air. This difference is attributed to the ability of
halogens to create volatile compounds of silicon, which nitrogen and oxygen do not.
Crouch et al. analyzed the crystallinity of the samples using transmission electron
microscopy.[11] They found that the outer layer of the microstructures, a layer that is several
hundred nanometers thick, consists of silicon nanocrystallites embedded in an amorphous
and polycrystalline silicon network.
In order to gain a deeper understanding of the factors affecting the optical prop-
erties and morphology of these samples, we compare the morphologies and the optical
properties of the structured surfaces created in H2S, SF6, SiH4, H2, and a mixture of Ar
and SF6. The work presented here provides new insight into the chemistry that leads to
the formation of sharp microstructures as well as some of the critical factors for attaining
near-unity absorptance over a broad range of wavelengths.
4.2 Experimental
For all experiments described in this paper, we used high resistivity (ρ = 800–1200
Ω·cm), n-doped, Si(111) wafers cut into 10 × 10 mm2 pieces. The samples were loaded into a
Chapter 4: The role of the background gas in the morphology and optical properties oflaser-microstructured silicon 14
stainless steel vacuum chamber and evacuated to about 50 mTorr using a corrosion-resistant
mechanical pump. Subsequently, the system was backfilled to a pressure of 500 Torr with
a specific gas or mixture. For the pure gases, we backfilled the chamber to 500 Torr via a
gas-handling manifold. In experiments using a mixture of Ar and SF6, the desired quantity
of SF6 was first added to the chamber. The manifold was then evacuated and backfilled
with argon to bring the total pressure to 500 Torr.
All microstructuring was done using a 1-kHz train of 100-fs, 800-nm laser pulses
with a fluence of 10 kJ/m2 focused to a spot of 150 µm in diameter. The sample was
translated horizontally at 250 µm/s and stepped vertically 75 µm at the end of each row to
create near uniform exposure to the laser over large areas of silicon. With the parameters
listed above, approximately 600 pulses irradiate each spot on the surface.
We annealed microstructures formed in the presence of H2S in a separate stainless
steel chamber with a base pressure of 2×10−6 Torr in order to monitor changes in the
morphologies and the optical properties of samples annealed at different temperatures. The
samples were clamped to a tantalum foil and a thermocouple was spot welded to one of
these clamps, ensuring good mechanical contact between the thermocouple and the sample
surface. The sample was radiatively heated to the desired temperature by a tungsten
filament. The sample was annealed for 30 minutes at the desired temperature and the
background pressure never exceeded 5×10−4 Torr.
To measure the optical properties of the samples, we measured the infrared ab-
sorptance with an UV-VIS spectrophotometer equipped with an integrating sphere detector.
The reflectance (R) and transmittance (T) were measured for wavelengths in the range of
Chapter 4: The role of the background gas in the morphology and optical properties oflaser-microstructured silicon 15
0.25–2.5 µm, in 1-nm increments. The absorptance (A = 1−R−T ) was then plotted versus
the wavelength.
We used Rutherford backscattering measurements to study the chemical compo-
sition of the microstructured samples. We first dipped the samples for 10 minutes in a
10% HF solution to remove the native oxide, then rinsed the sample and placed it in the
backscattering chamber. The backscattering measurements were taken with 2.0-MeV al-
pha particles and an annular solid-state detector. We determined the composition of the
samples by fitting the data to simulated spectra.[10]
4.3 Results
Figure 4.1 shows scanning electron microscope images of surfaces prepared in H2S,
SF6, H2, and SiH4. The surfaces prepared in SF6 and H2S have nearly identical microstruc-
tures, are approximately 10 µm tall and 5 µm × 3 µm wide at the base. The radius of
curvature at the tip is slightly less than 1 µm and the average tip-to-tip spacing is about 4
µm. The sides of the microstructures are covered with nanometer-scale dendritically shaped
material. Figure 4.1 also shows that the microstructures formed in the presence of H2 and
SiH4 have a blunter shape than those formed in H2S and SF6 and are 10–12 µm in height
and 4 µm × 10 µm wide at the base. The microstructures have a much greater area at
the tip (4 µm × 8 µm) and a tip-to-tip spacing of 6 µm. The number and density of the
dendritic nanoparticles are lower than those formed in the presence of H2S and SF6.
Samples irradiated in the presence of H2S and SF6 are black to the eye, while those
irradiated in the presence of gases that do not contain sulfur are dull grey. Figure 4.2 shows
Chapter 4: The role of the background gas in the morphology and optical properties oflaser-microstructured silicon 16
a b
c d
5 µm
Figure 4.1: Scanning electron micrographs of surfaces created in a) H2S b) SF6 c) H2 andd) SiH4.
the optical absorptance for the silicon samples prepared in H2, SiH4, SF6, and H2S, along
with the absorptance for an unstructured silicon substrate. The microstructured surfaces
absorptance at wavelengths from 0.25 µm to 1.1 µm is 50% higher than that of crystalline
silicon. Samples irradiated in the presence of H2S and SF6 absorb 95% for above-band gap
radiation; those in H2 and SiH4 absorb 90% for above-band gap radiation.
Figure 4.2 also shows the below-band gap absorptance from 1.1 µm to 2.5 µm.
Samples irradiated in the presence of H2S and SF6 have flat, featureless 90% absorptance
for incident radiation in this wavelength range. Surfaces irradiated in the presence of H2
and SiH4 have an absorptance that falls monotonically for wavelengths longer than 1.1 µm.
Table 4.1 provides a metric of the absorptance at 2 µm for each of the samples listed above
that displays the changes described qualitatively above.
Chapter 4: The role of the background gas in the morphology and optical properties oflaser-microstructured silicon 17
wavelength (µm)
absorp
tance (%)
0 1 2 3
100
80
60
40
20
0
a
b
c
e
d
Figure 4.2: Absorptance spectra for a) H2S b) SF6 c) SiH4, and d) H2. The trace forcrystalline silicon (e) is included for reference.
Gas used Absorptance at 2 µm Sulfur content (±0.2%)
H2S 90% 1%
SF6 90% 1%
Ar + 1% SF6 90% 0.6%
Ar + 0.1% SF6 60% 0.2%
SiH4 33% none detected
Ar 15% none detected
H2 8% none detected
Table 4.1: Infrared absorptance at 2 µm and sulfur content, as measured by Rutherfordbackscattering, for various microstructured samples.
Chapter 4: The role of the background gas in the morphology and optical properties oflaser-microstructured silicon 18
a b
5 µm
Figure 4.3: Scanning electron micrographs of structures created in the presence of a) 1%partial pressure of SF6 and b) 0.1% partial pressure of SF6.
Figure 4.3 shows how the morphology of the microstructures depends on the partial
pressure of the sulfur containing gas. Microstructures formed in the presence of low partial
pressures of sulfur more closely resemble microstructures created in H2 and SiH4 than
those created in H2S and SF6. Individual microstructures from a 0.1% SF6 sample (partial
pressure of 500 mTorr) are 8 µm tall and 13 µm × 7 µm at the base. The structures taper
to 8 µm × 3 µm at the tip and the average tip-to-tip spacing is on the order of 8 µm. Those
created in 1% SF6 (partial pressure of 5 Torr) are 9 µm tall, 8 µm × 4.5 µm at the base and
taper to 4 µm × 2 µm at the tip (Figure 4.3). The tip-to-tip spacing is approximately 6 µm
for these structures, making them slightly higher in areal density than those from 0.1% SF6.
Microstructures formed in either 0.1% SF6 or 1% SF6 also have very little nanomaterial on
the sides, similar to the microstructures formed in H2 and SiH4.
Figure 4.4 shows the absorptance for the microstructured surfaces created in mix-
tures of argon and sulfur and that of a sample created in pure argon. There is an increase
in both above-band and below-band gap absorptance for the samples created in low partial
Chapter 4: The role of the background gas in the morphology and optical properties oflaser-microstructured silicon 19
0 1 2 3
100
80
60
40
20
0
a
b
c
wavelength (µm)
absorp
tance (%)
Figure 4.4: Absorptance spectra for a) 1% SF6 b) 0.1% SF6 and c) Ar.
pressures of sulfur as compared to the absorptance for samples created in H2 and SiH4; the
absorptance for the argon sample more closely resembles that of H2 and SiH4 in that the
absorptance for below-band gap radiation decreases monotonically for wavelengths longer
than 1.1 µm. Both samples created in sulfur mixtures absorb 95% for above-band gap ra-
diation. The absorptance for below-band gap radiation can be seen qualitatively in Figure
4.4 and quantitative values for the absorptance at 2 µm are provided in Table 4.1.
Scanning electron microscope images taken before and after annealing at 900 K
(Figure 4.5) show that the morphology does not change within the resolution of the electron
microscope. Figure 4.6 shows the optical properties of samples annealed at temperatures
ranging from 475 K to 900 K. All annealed samples have an absorptance for 95% of above-
band gap radiation. The absorptance for below-band gap radiation decreases as the anneal
Chapter 4: The role of the background gas in the morphology and optical properties oflaser-microstructured silicon 20
a b
5 µm
Figure 4.5: Scanning electron micrographs of structures created in a) H2S and annealed tob) 900 K.
Anneal temperature Absorptance at 2 µm Sulfur content (±0.2%)
475 K 85% 1%
700 K 80% 1%
800 K 75% 1%
900 K 70% 1%
Table 4.2: Infrared absorptance at 2 µm and sulfur content of annealed H2S samples, asmeasured by Rutherford backscattering.
temperature increases, but the absorptance remains flat and featureless. The sample an-
nealed at 475 K absorbs 85% for below-band gap radiation (as compared to 90% absorptance
for unannealed samples). Those annealed to 700 K, 800 K and 900 K absorb 80%, 75% and
70% for below-band gap radiation, respectively, and the results are summarized in Table
4.2
Tables 4.1 and 4.2 contain compositional information obtained from Rutherford
backscattering. Structures created in H2S have a sulfur content of approximately 1% and
there is no measurable change in the sulfur content upon annealing at 900 K. The sample
Chapter 4: The role of the background gas in the morphology and optical properties oflaser-microstructured silicon 21
0 1 2 3
100
90
80
70
60
50
wavelength (µm)
absorp
tance (%)
b
a
c
d
Figure 4.6: Absorptance spectra for H2S samples annealed to a) 475 K, b) 700 K, c) 800K, and d) 900 K.
created in 1% SF6 contains 0.6% sulfur, while the 0.1% SF6 sample has a sulfur concentration
of 0.2%. We detected no sulfur in samples formed in SiH4, H2, or argon.
4.4 Discussion
Our experimental results in Figure 4.1 and Figure 4.3 demonstrate that the pres-
ence of sulfur in the background gas is crucial to create sharp, triangular microstructures.
The fact that the morphology is nearly identical for structures created in H2S and SF6
indicates that the presence of H and F, common etchants of silicon with differing etch rates,
are not as important as the presence of sulfur. In addition, there is a notable reduction
in tip area for the individual microstructures as the amount of sulfur containing species
present in the background increases. This can also be seen as a general sharpening of the
Chapter 4: The role of the background gas in the morphology and optical properties oflaser-microstructured silicon 22
microstructures or as an increase in the aspect ratio.
The morphology of the microstructured surfaces and the incorporation of sulfur
into the surface layer are responsible for the increased absorptance at wavelengths from
0.25 µm to 1.1 µm. Assuming the same absorption coefficient as for crystalline silicon,
two reflections on the sidewalls of the microstructured silicon increase the absorptance
from approximately 70% to about 90%. These reflections can account for the increased
absorptance seen in all microstructured samples, but fails to do so for the 5% increase seen
in all samples created in an ambient containing sulfur relative to that of samples created in
an ambient without sulfur. The incorporation of sulfur into this outer layer must therefore
create a material with an absorption coefficient that is greater than that of crystalline
silicon.
Because silicon has such a small absorption coefficient in the near infrared, mul-
tiple reflections on the sidewalls of the microstructures cannot explain the absorptance for
below-band gap radiation that we observe in any of our samples. Damage and disorder
introduced to the lattice by femtosecond-laser irradiation and incorporation of elements
of the background gas create a tail of states below the band gap by changing the bond
lengths, bond angles, and/or coordination of the crystalline silicon; the extent of damage
and disorder determines the width of the tail and the number of states available at each
wavelength.[12] These changes to the lattice explain the below-band gap absorptance we
measure in H2, SiH4, Ar, and 0.1% SF6, as well as a portion of the absorptance observed
in the samples created in a sulfur containing ambient.
The flat, featureless 90% absorptance for below-band gap radiation seen for sam-
Chapter 4: The role of the background gas in the morphology and optical properties oflaser-microstructured silicon 23
ples created in H2S, SF6 and a mixture of 1% SF6 and Ar, however, cannot be explained by
changes in morphology nor lattice damage alone. The microstructured surfaces created in
the presence of H2S and 1% SF6 have different morphologies, yet the infrared absorptance
traces are very similar. Also, the damage done to the lattice is similar for all samples be-
cause the laser fluence is the same in all experiments. The creation of high absorptance for
below-band gap radiation must therefore be a result of a parameter other than morphology
or lattice damage.
Rutherford backscattering measurements indicate that the concentration of sulfur
in our samples is greater than 0.6% (atomic concentration of about 1020 cm−3) for all sam-
ples that possess a 90% absorptance for below-band gap radiation. The solid solubility of
sulfur in silicon is less than 1015 cm−3 and a concentration incorporated in the microstruc-
tured samples is more than sufficient to create a broad band of absorption energies. In a
previous study of the electronic states that sulfur creates in silicon, 8 discrete donor states
in the gap of silicon are created from an atomic concentration of about 1016 cm−3, and
the deepest of these levels resides 614 meV below the conduction band edge [13]. If sulfur
creates an entire band of states from 614 meV below the conduction band edge to the con-
duction band, the gap between the top of the valence band and this new band would be
456 meV, corresponding to a wavelength of 2.7 µm.
The possibility also exists that the absorptance in the near-infrared region of the
spectrum may be due to sulfur bonding with silicon in a highly disordered network. Excess
sulfur atoms could stabilize the lattice in a non-equilibrium geometry and create a material
with a greater absorption coefficient at energies below the band gap of crystalline silicon.
Chapter 4: The role of the background gas in the morphology and optical properties oflaser-microstructured silicon 24
A study on the depth distribution of ion-implanted sulfur into silicon concludes that sulfur
clusters around defect sites induced by the ion beam, especially in the defect-rich transi-
tion layer between the implantation-damaged layer and the unperturbed crystalline silicon
layer.[14] Given the similarity in atomic concentrations of sulfur as well as the thickness of
the disordered layer between the implanted samples and our microstructured samples, we
expect that sulfur coordinates around any area with a high density of defects and stabilizes
the network. This distribution of atoms has a new electronic configuration that may cause
a broad band of absorption energies in the near infrared.
The absorptance measurements shown in Figure 4.6 combined with data from
Table 4.1 provide further evidence that the cause for near-unity absorptance for below-band
gap radiation is the combination of sulfur with a disordered silicon network. Upon annealing
to 900 K, the absorptance for below-band gap radiation drops to 70% , but the concentration
of sulfur is unchanged. There are several potential explanations for the observed drop in
absorptance for below-band gap radiation. One possibility is that sulfur diffuses out of an
active site, which is a location in the network where sulfur is coordinated in such a way as to
create below-band absorption. The diffusion coefficient for sulfur is relatively low and the
anneal time is short, indicating that diffusion out of an active site is unlikely.[15] Another
possibility is precipitation of the sulfur atoms into an inactive cluster. Such precipitation
has been observed for arsenic implanted in silicon.[16] Precipitation of sulfur into inactive
clusters requires diffusion of silicon out of an area with a high concentration of sulfur or
diffusion of sulfur into this type of area. While we cannot rule out the possibility that sulfur
forms precipitates, the likelihood of diffusion of either silicon or sulfur on this timescale
Chapter 4: The role of the background gas in the morphology and optical properties oflaser-microstructured silicon 25
and these temperatures is still unlikely. A third possibility we consider is a relaxation
of the sulfur-silicon network that makes the electronic structure begin to revert to that of
crystalline silicon. Various experimental data show that thermally induced relaxations (e.g.,
healing of defect sites not coordinated with silicon) in amorphous silicon occur over a wide
range of temperatures and depend on the distribution of bond lengths and bond angles in
the amorphous network and the dopant concentrations.[17, 18, 19] In particular, changes in
ion-implanted silicon material occur at temperatures as low as 400 K, which is well below any
of the anneal temperatures we used.[19] Because there is no change in sulfur concentration
on annealing and the annealing temperatures are relatively low, thermal relaxation is likely
the dominant mechanism for the reduction in near-infrared absorptance after annealing.
In conclusion, we show that incorporation of sulfur (on the order of 1%) into a
disordered silicon network created by laser irradiation leads to near-unity absorptance for
both above- and below-band gap radiation. The presence of sulfur is also an important
factor in developing sharp microstructures. The absorption mechanism for below-band gap
radiation may be either due to the creation of an impurity band resulting from the high
concentration of sulfur or due to the incorporation of sulfur into the disordered network
in such a way as to create a material with a new electronic structure. The decrease in
absorptance for below-band gap radiation observed upon annealing appears to be primarily
due to thermal relaxation of the disordered silicon network.
Chapter 5
Chalcogen doping of silicon with a
femtosecond laser above the
ablation threshold
5.1 Introduction
Chalcogen-doping of silicon is an active area of interest in materials science because
of potential applications in devices such as infrared detectors and thermal imaging devices.[2,
3, 4] Amorphous chalcogen silicon alloys show promise as stable photovoltaic materials.[20,
21, 22] Numerous studies on the diffusion of chalcogens in silicon and their interaction
with a silicon lattice have been carried out to understand the properties of chalcogen-doped
silicon.[23, 24, 14, 25, 26, 27]
We previously reported on the morphology and optical properties following femtosecond-
26
Chapter 5: Chalcogen doping of silicon with a femtosecond laser above the ablationthreshold 27
Element Solubility in c-Si Si-X bond strength Estimated diffusion length
atoms cm−3 a kJ mol−1 nm
S 1.76×1014 623 21
Se 2.90×1015 548 2
Te 3.50×1016 452 0
a Quoted solubilities are given at 1325 K, the only common temperature available in the
literature for these three elements.
Table 5.1: Physical data of chalcogens in silicon.
laser irradiation of silicon in the presence of several gases.[28] After irradiation, the surface
is covered with conical microstructures that are 10–12 µm in height. In addition to mor-
phological changes, the optical properties of the microstructured silicon differ from that
of crystalline silicon. Specifically, when sulfur is present in the background gas, there is
near-unity absorptance for radiation with energy below the band gap of crystalline silicon.
We concluded that incorporation of sulfur into an outer, disordered layer of these samples
is a critical factor in determining the optical properties.[11, 28]
In order to elucidate the role of the dopant in creating the near-unity absorptance
for below-band gap radiation, we present a set of experiments involving incorporation of
chalcogens — sulfur, selenium and tellurium. These elements are all in Group VI and
have the same valence electron configuration, but different atomic sizes and solubilities in
crystalline silicon (given in Table 5.1).
Chapter 5: Chalcogen doping of silicon with a femtosecond laser above the ablationthreshold 28
5.2 Experimental
For all experiments, we used a high resistivity (ρ = 8–12 Ω·m), n-doped Si(111)
substrate wafer that is 10 × 10 mm2. Approximately 2 mg of the desired dopant, in powder
form, was placed on the silicon wafer and manually dispersed across the surface using 0.5
mL of either toluene (for sulfur and selenium) or mineral oil (for tellurium). The solvent
served as a means of adhering the powder to the silicon surface and largely evaporated prior
to irradiation. We then placed the sample in a stainless steel chamber and evacuated the
chamber to less than 6.7 Pa using a corrosion-resistant mechanical pump. The chamber was
then filled with 6.7 × 104 Pa of N2. We irradiated the samples with a 1-kHz train of 100-fs,
800-nm laser pulses with a fluence of 10 kJ/m2 focused to a spot 150 µm in diameter. The
sample is raster-scanned at 250 µm/s and stepped vertically 50 µm at the end of each row
to create uniform exposure to the laser over large areas of silicon. After microstructuring,
samples are placed in an ultrasonic bath of methanol for 30 minutes to remove any residue.
To evaluate the optical properties of the samples, we measured the infrared ab-
sorptance with an UV-VIS-NIR spectrophotometer equipped with an integrating sphere
detector. The reflectance (R) and transmittance (T) were measured for wavelengths in the
range of 1.0–2.5 µm, in 1-nm increments to obtain the absorptance (A = 1 − R − T ) as a
function of wavelength.
To measure the composition of the material after irradiation, we use Rutherford
backscattering spectrometry. Before these measurements, we dip the samples for 10 minutes
in a 10% HF solution to remove any oxide layer. The backscattering measurements are
taken with 2.0-MeV alpha particles and an annular solid-state detector. We fit our data to
Chapter 5: Chalcogen doping of silicon with a femtosecond laser above the ablationthreshold 29
simulated spectra to determine the composition of the samples.[10]
To analyze changes in morphology, composition, and absorptance that occur with
heating, we annealed the samples in a vacuum oven at 775 K for 30 minutes. The base
pressure never exceeded 4.0 × 10−4 Pa.
5.3 Results
Figure 5.1 shows scanning electron microscope images of surfaces prepared using
sulfur, selenium and tellurium. The height of the structures is between 9 and 14 µm;
the width on the long axis varies from 6 µm to 9 µm; and the width on the short axis
ranges from 2 µm to 3 µm. The sulfur and tellurium microstructures have some nanoscale
structure on the walls of the microstrucutures; the selenium microstructures are smooth in
comparison. Figure 5.2 shows that there is no change in morphology, within the resolution
of the microscope, after annealing.
5 µm
a b c
Figure 5.1: Scanning electron micrographs of surfaces created in a) S, b) Se, and c) Te.
Figure 5.3 compares the absorptance for samples created in sulfur, selenium and
tellurium to that of crystalline silicon. All samples have 90% absorptance at wavelengths
from 1.2 µm to 2.5 µm; the absorptance for crystalline silicon at these wavelengths is 15%.
Chapter 5: Chalcogen doping of silicon with a femtosecond laser above the ablationthreshold 30
5 µm
a b c
Figure 5.2: Scanning electron micrographs of surfaces created in a) S, b) Se, and c) Te andannealed to 775 K.
Figure 5.4 shows that annealing for 30 minutes at 775 K in vacuum reduces the absorptance
at wavelengths between 1.2 µm to 2.5 µm by an amount that depends on the chalcogen
dopant. The absorptance for below-band gap radiation of the sample prepared in sulfur
decreases from 90% to 48%; the optical properties of the selenium sample are less affected
by annealing and the absorptance drops to 80%; annealing the tellurium sample causes very
little change in the optical properties and the absorptance drops only 1% to 89%.
Figures 5.5-5.7 show the Rutherford backscattering spectra for samples prepared
using the 3 chalcogens, before and after annealing. The results of the simulations are
summarized in Table 5.2. The samples made with sulfur powder contain about 1% sulfur in
a layer that is 200 nm thick, before and after annealing. The analogous selenium samples
both contain approximately 0.7% selenium. Prior to annealing, the tellurium spectrum is
best simulated with two layers. The outermost layer is approximately 20 nm thick and
contains 7% tellurium; the next layer is 200 nm thick and contains 1.5% tellurium. After
annealing the spectrum can be simulated with only one layer that is 200 nm thick and
contains 1.3% tellurium.
Chapter 5: Chalcogen doping of silicon with a femtosecond laser above the ablationthreshold 31
a,b,c
wavelength (µm)
absorp
tance
0 1 2 3
1.0
0.8
0.6
0.4
0.2
0
d
Figure 5.3: Absorptance of samples prepared in a) S, b) Se, and c) Te. The trace forcrystalline silicon (d) is included for reference. Note: samples irradiated with only mineraloil or toluene on the surface have a trailing edge of absorptance after the band edge ofsilicon, similar to the SiH4 trace.
Sample Layer Thickness, nm Chalcogen content
sulfur 1 200 1%
sulfur, annealed 1 200 1%
selenium 1 200 0.7%
selenium, annealed 1 200 0.7%
tellurium 1 20 7%
2 200 1.5%
tellurium, annealed 1 200 1.3%
Table 5.2: Results of simulations to fit the Rutherford backscattering spectra indicating thecontent of chalcogen-doped silicon.
Chapter 5: Chalcogen doping of silicon with a femtosecond laser above the ablationthreshold 32
c
b
a
wavelength (µm)
absorp
tan
ce
0 1 2 3
1.0
0.8
0.6
0.4
0.2
0
d
Figure 5.4: Absorptance of samples prepared in a) S, b) Se, and c) Te after annealing at775 K. The trace for crystalline silicon (d) is included for reference.
energy (MeV)
yie
ld (
co
un
ts)
1 1.2 1.4 1.6 1.8 2
1600
1200
800
400
0
Figure 5.5: Rutherford backscattering spectra for a sulfur-doped sample. The solid line isbefore annealing and the dashed spectrum is taken after annealing at 775 K.
Chapter 5: Chalcogen doping of silicon with a femtosecond laser above the ablationthreshold 33
energy (MeV)
yie
ld (
co
un
ts)
1 1.2 1.4 1.6 1.8 2
1600
1200
800
400
0
Figure 5.6: Rutherford backscattering spectra for a selenium-doped sample. The solid lineis before annealing and the dashed spectrum is taken after annealing at 775 K.
energy (MeV)
yie
ld (
co
un
ts)
1 1.2 1.4 1.6 1.8 2
1600
1200
800
400
0
Figure 5.7: Rutherford backscattering spectra for a tellurium-doped sample. The solid lineis before annealing and the dashed spectrum is taken after annealing at 775 K.
Chapter 5: Chalcogen doping of silicon with a femtosecond laser above the ablationthreshold 34
5.4 Discussion
Prior to a discussion of the interaction of the chalcogens with the silicon lattice in
this material, however, a brief summary of the laser-material interaction and the composi-
tion of our material are given. During irradiation of silicon with a femtosecond laser above
the ablation threshold, a volume of silicon is ablated and a thin layer of silicon is melted.
The molten silicon then begins to resolidify and the velocity of the resolidifcation front can
be as high as 10 m/s.[29, 30] The dopant from the surface powder is incorporated into the
molten silicon layer and becomes trapped in the solid by this high velocity resolidification
front. This trapping, known as solute trapping, creates concentrations of dopants that can
be far in excess of the solid solubility for any of these elements in crystalline silicon.[31]
Another result of the rapid resolidification after femtosecond irradiation is a high density of
point defects (vacancies and interstitials).[30] For the laser pulses used in these experiments,
transmission electron microscopy indicates that the region affected by the laser is several
hundred nanometers thick.[11] This surface layer contains nanocrystalline grains of silicon
(10–50 nm in diameter) interspersed throughout a disordered network and contains about
1% of the chalcogen used.
In previous work, we concluded that incorporating sulfur from a gaseous back-
ground source into this disordered silicon network is the source of the below-band gap
absorptance.[28] In this work we also observe near-unity absorptance for microstructured
samples created in sulfur, selenium, and tellurium from a powder dopant source. With
the results described above, we can now provide further detail on how the absorptance for
below-band gap radiation is created. Specifically, we propose 3 different chemical envi-
Chapter 5: Chalcogen doping of silicon with a femtosecond laser above the ablationthreshold 35
ronments the chalcogen may occupy in the disordered network and assess the viability of
each.
The first possibility we consider is that the chalcogen is incorporated into an amor-
phous network. Several studies of amorphous alloys composed of silicon and a chalcogen,
a-Siy :Xz (X = S, Se, or Te ), show that the band gap of the material changes as a function
of the mole fraction z.[20, 21, 22] In the above studies for z less than 0.15, however, none of
the measured band gaps are less than 1.25 eV, which corresponds to a band gap at a wave-
length of 994 nm. The band gaps of these materials are all larger than that of crystalline
silicon. Even though transmission electron microscopy indicates that the microstructured
samples have some amorphous character and 1% chalcogen concentration, we do not believe
this chemical environment is responsible for the absorptance for below-band gap radiation.
Another possibility is that the chalcogen occupies substitutional sites in nanocrys-
talline grains with a low concentration of defects. Previous studies have shown that doping
silicon with a chalcogen using thermal diffusion, which inherently restricts the concentra-
tion to the solubilitiy limit, creates discrete states in the band gap.[13, 32] The number and
energy of these states vary with the chalcogen used. However, they all create deep levels
in the band gap of silicon, where the deepest-lying state is 614 meV below the conduction
band edge for sulfur, 593 meV for selenium, and 411 meV for tellurium.[13, 32] With a
concentration of the chalcogen that is several orders of magnitude higher than the solid
solubility, the chalcogen may create one or more impurity bands that spread around any or
all of the discrete states seen in doping crystalline silicon.[9] Based on our data, it is not
possible to determine whether an impurity band of this nature overlaps with the conduction
Chapter 5: Chalcogen doping of silicon with a femtosecond laser above the ablationthreshold 36
band, with the valence band, or lies in between the two.
The annealing data provide some evidence that substitutional chalcogens in the
nanocrystals are not likely to be the cause of the below-band gap absorptance. During
the annealing, the chalcogen diffuses out of the nanocrystals (because the concentration
is in excess of the solid solubility) to a grain boundary or to the amorphous material
between the crystalline phases. The change in concentration in a nanocrystalline grain can
be estimated from the diffusion length of the different chalcogens. From the bulk diffusion
studies, we estimate that more than 80% of the sulfur and one quarter of the selenium
escape the nanocrystals; the tellurium remains effectively trapped under these conditions.
In a spherical nanocrystal (d = 50 nm) with a uniform distribution of the chalcogen, a
diffusion length of 21 nm (in the case of sulfur) would reduce the concentration in the
nanocrystal by 80%; there would be a larger reduction in any smaller nanocrystals. For a
selenium sample, the same assumptions would reduce the concentration of selenium in a
50-nm diameter nanocrystal by 10%. A larger decrease, up to 50% for a 10-nm diameter
nanocrystal, would occur in smaller nanocrystals. If the chalcogen concentration in the
nanocrystals were proportional to the below-band gap absorptance, we would expect to see
a larger drop in the absorptance for both the sulfur and selenium samples after annealing.
Although we cannot refute this mechanism entirely, we expect that it is a more complicated
organization of the silicon and the chalcogen that gives rise to the absorptance for below-
band gap radiation.
A third possibility that might contribute to absorptance for below-band gap radi-
ation is a high concentration of defects created during laser irradiation that is stabilized by
Chapter 5: Chalcogen doping of silicon with a femtosecond laser above the ablationthreshold 37
chalcogen incorporation. Defects in a lattice affect the local structure in many ways, includ-
ing changes in coordination, bond length, and/or bond angle of neighboring silicon atoms.
Any of these changes to the local atomic arrangement leads to modification of the elec-
tronic structure and therefore the optical properties; the extent of the changes determines
the resulting optical properties of the material. There are numerous examples of defects
(i.e. point defects, divacancies, and more extended defect clusters) in silicon that create
electronic states in the band gap of silicon.[33, 34] Irradiating silicon with a femtosecond
laser and incorporating a high concentration of the chalcogen may also create defect states
that are not seen in the aforementioned studies, which primarily use ion-beam implantation.
The chalcogen may stabilize a defect-rich network in several ways, for example by coordi-
nating with dangling bonds, by bonding with more than four silicon atoms, or by bonding
with fewer than four silicon atoms. Our data does not allow for the determination of the
precise nature of the defect(s) responsible, but given the extent and number of possible
changes created by the defects and chalcogen incorporation, it is possible that a new band
or several bands of absorption energies in the band gap of crystalline silicon is created.
The changes in absorptance seen after annealing can be accounted for by one
or more changes in the defect-chalcogen network. Regardless of the exact nature of the
chalcogen-defect system that leads to absorptance for below-band gap radiation, the changes
in absorptance we observe must be due to diffusion of silicon, the chalcogen, or a defect.
The mobile species could be silicon moving through the disordered network. We assume,
however, that the diffusivity of silicon is similar in all three samples because the material is
primarily composed of silicon. Furthermore, if we use gas phase Si-X (X = S, Se, or Te) bond
Chapter 5: Chalcogen doping of silicon with a femtosecond laser above the ablationthreshold 38
strength as a qualitative estimate of the silicon-chalcogen bond strength in this disordered
network, we would expect to see the greatest drop in absorptance in the tellurium sample
because the silicon-tellurium has the weakest bond strength. If the chalcogen is the mobile
species, then we expect the smallest change for tellurium. The diffusion length of tellurium
is essentially zero at 775 K, and the diffusion length in this highly defective network is likely
smaller than that in crystalline silicon because of the number of traps. The last possibility is
that it is the defect that is diffusing, possibly in concert with the chalcogen. If the chalcogen
and the defect diffuse as a pair, we would also expect the chalcogen to dictate the diffusion
rate and therefore observe the smallest change in the tellurium sample. Approximating the
mobility of a defect alone in this network is not possible and we are unable to estimate the
strength of the interaction between the different chalcogens and the defect(s). The results
of the annealing experiments show that the first mechanism is not consistent and both the
second and third are plausible.
In conclusion, we have shown that incorporation of sulfur, selenium, and tellurium
into the microstructured silicon material leads to near-unity absorptance for below-band gap
radiation. After annealing, sulfur shows a significant drop in absorptance for below-band gap
radiation, selenium shows a moderate reduction, and tellurium shows essentially no change.
Based on these data, we attribute the near-unity absorptance to a high concentration of
trapped chalcogen dopants that coordinate with a highly defective silicon network in such
way as to modify the electronic structure of the outer layer and thereby create these novel
optical properties. Annealing the samples results in relaxations of this network toward
a more thermodynamically stable configuration, which is either due to diffusion of the
Chapter 5: Chalcogen doping of silicon with a femtosecond laser above the ablationthreshold 39
chalcogen, the defects, or a combination of the two.
Chapter 6
Incorporating dopants from other
families
After a detailed study of the interaction of the chalcogens with the disordered
network (Chapter 5), we turned our attention to other families of the periodic table. We
wanted to discover whether the optical properties created by chalcogen incorporation were
unique to Family VI. Therefore, we investigated doping with Family V (phosphorous and
antimony), Family IV (carbon, silicon, and germanium), and Family III (gallium and in-
dium). We show the results of these experiments and include a discussion of the results.
Preparation of samples discussed in the following sections is the same as described
in Sections 4.2 and 5.2.
40
Chapter 6: Incorporating dopants from other families 41
6.1 Families III-V
Figure 6.1 shows that incorporating either phosphorous or antimony leads to near-
unity absorptance for below-band gap radiation. This result implies that there are dopants
other than those from Family VI that can lead to the novel optical properties. As Figure
6.2 shows, Family IV does not create this near-unity absorptance. The silicon doping is
taken from irradiation in a background of silane; carbon doping is done by irrradiating in
CF4; germanium doping was done with germanium powder. In Figure 6.2, the absorptance
for all samples begins to drop for wavelengths longer than 1.1 µm. Finally, in Figure 6.3
we see that incorporation of gallium and indium also possess high absorptance for below-
band gap radiation. The absorptance, however, is not as high as samples prepared in sulfur
ambients. These results are of particular interest because of the potential applications in
optoelectronic devices. However, we will discuss these possibilities in Chapter 7 and focus
our discussion here on the material properties.
6.2 Discussion
This section is broken into 3 sections in order to address 3 separate aspects. The
first is the valence effect of the dopant on creating the near-unity absorptance. The second
section deals with the atomic size effect that plays a role in creating the novel optical
properties. The final section deals with a brief annealing study that has been performed
and the implications of those results on our overall understanding of the material.
Chapter 6: Incorporating dopants from other families 42
a,b
wavelength (µm)
absorp
tance
0 1 2 3
1.0
0.8
0.6
0.4
0.2
0
c
Figure 6.1: Absorptance of samples prepared in a) phosphorous and b) antimony (the noisiertrace is that of the antimony-doped sample). The trace for crystalline silicon (c) is includedfor reference.
6.2.1 Valence
Throughout the course of this research project, including results done prior to this
thesis, there have been a variety of dopants used. The range has spanned from Family
III to Family VIII. After these experiments, we now know that doping with elements from
Families III, V, and VI all lead to enhanced absorptance (Note: we here avoid using the
term near-unity because of the fact that Family III absorbs approximately 80% at below-
band gap wavelengths.); incorporation of elements from Families IV, VII, and VIII do not.
In order to obtain this below-band gap absorptance, the dopant must be able to coordinate
with the lattice in a specific way. Interestingly, dopants from Family III, which are subvalent
of silicon, also lead to significantly enhanced absorptance at below-band gap wavelengths.
Chapter 6: Incorporating dopants from other families 43
wavelength (µm)
absorp
tance
0 1 2 3
1.0
0.8
0.6
0.4
0.2
0
d
b
c
a
Figure 6.2: Absorptance of samples prepared in a) Ge, b) SiH4, and c) CF4. The trace forcrystalline silicon (d) is included for reference.
The lower absorptance seen for Family III dopants as compared to Families V and VI may
be due to this valence difference.
6.2.2 Atomic size
In addition to the valence dependence discussed in 6.2.1, there is also a size de-
pendence. Irradiation of silicon in the presence of nitrogen does not lead to the near-unity
absorptance, but irradiation in the presence of a phosphorous powder does. These elements
have the same valence configuration, but different atomic sizes (the neutral atomic radius
for nitrogen is 65 pm and that for phosphorous is 100 pm). Also, irradiating silicon in
air (where the concentration of oxygen is approximately 20%) does not lead to near-unity
absorptance for below-band gap radiation; incorporating any of the three elements directly
Chapter 6: Incorporating dopants from other families 44
wavelength (µm)
absorp
tance
0 1 2 3
1.0
0.8
0.6
0.4
0.2
0
c
ba
Figure 6.3: Absorptance of samples prepared in a) gallium and b) indium (the noisierspectrum is that of the indium-doped sample). The trace for crystalline silicon (c) is includedfor reference.
below oxygen (i.e. sulfur, selenium or tellurium) does. The neutral atomic radius for tel-
lurium is 140 pm. Along with the valence requirements, incorporating an element with
an atomic radius between 100 pm and 140 pm (up to 155 pm if one considers indium)
leads to near-unity absorptance. Larger atomic radii may also work, though we have no
experimental results to test the extent of this atomic size effect.
6.2.3 Annealing and defects
One final result that merits discussion is a change in absorptance seen after an-
nealing samples prepared in phosporous and tellurium. The anneal conditions are the same
as discussed in 5.3. As discussed in 5.4, we determined that the mobile species after anneal-
ing is either the chalcogen or other defects in the network, perhaps coordinated with the
Chapter 6: Incorporating dopants from other families 45
wavelength (µm)
absorp
tance
0 1 2 3
1.0
0.8
0.6
0.4
0.2
0
c
a
b
Figure 6.4: Absorptance spectra for a) Te and b) P taken after annealing at 775 K. Thetrace for crystalline silicon (c) is included for reference.
chalcogen. We thought it more likely that it was a defect other than the chalcogen. This
conclusion gains further proof based on the phosphorous annealing results show in Figure
6.4. Phosphorous has a smaller diffusion coefficient in crystalline silicon than tellurium, yet
there is a much larger drop in the absorptance. This new data implies that the drop in
absorptance upon annealing is not solely due to diffusion of the dopant.
6.3 Conclusions
This information in turn implies that the optically active component is a defect
other than the dopant itself or perhaps a defect coordinated with this dopant. Because
there is such a broad range of absorption energies, there must also be a broad range of
environments which can absorb these energies. With femtosecond-laser irradiation of a
Chapter 6: Incorporating dopants from other families 46
material, a large number of defects can be incorporated. Concurrent with the damage
induced by the laser is the incorporation of a high percentage of these dopant elements.
These dopants could then coordinate with the silicon network in such a way as to stabilize
defects that are otherwise unstable.
Furthermore, we can begin to narrow our scope of defects that are responsible. For
example, we know from previous results that irradiating silicon in vacuum does not lead to
near-unity absorptance for below-band gap radiation. The concentration of silicon vacancies
and interstitials after irradiating would likely be the same. However, because we do not
observe the near-unity absorptance below the band gap, there must be another contributing
factor. We also know that only incorporation of specific elements creates this near-unity
absorptance. As discussed in 6.2.1 and 6.2.2, there appear to be a subset of parameters
that the dopant must satisfy in order to create the below-band gap absorptance.
Through the use of these new dopants from Families III, IV and V, we have gained
further insight into the mechanism by which this material absorbs below-band gap radia-
tion. It appears as though there is a very specific, local environment which is created by
femtosecond-laser irradiation. This environment depends also on the incorporated dopant to
stabilize this network in this unique arrangement. As the material is annealed, the absorp-
tance drops. The extent of the drop is dependent on the dopant used. Based on comparisons
of the diffusion and compositional analysis, we believe these changes are predominantly due
to relaxations in the disordered layer. There are a number of possible changes, including
dangling bond removal, relaxations from a strained network to a more crystalline network,
etc. Because there is a flat and featureless absorptance over a broad range of energies, it
Chapter 6: Incorporating dopants from other families 47
is likely that there is more than one type of thermal event which leads to these changes in
absorptance during annealing.
Chapter 7
Future Work
While we have learned a great deal about the material and its formation through
the experiments described in the preceding chapters, there remains a wealth of research to
be performed. We provide some of the possibilities here.
7.1 Detailed annealing studies of sulfur samples
After annealing the samples prepared in H2S at different temperatures, it is pos-
sible to approximate an activation energy using an Arrhenius plot. However, depending
on the method of sulfur incorporation (i.e sulfur powder, SF6, and H2S), the absorptance
changes to differing extents. This difference is shown in Figure 7.1. We do not yet under-
stand the source of these differences, but they may emanate from the differing mobility of
optically active defects in the disordered layer. For example, there are numerous examples
in the literature that discuss the effect of hydrogen in silicon. In our case, hydrogen may
coordinate with dangling bonds or other types of defects in the disordered layer and prevent
48
Chapter 7: Future Work 49
wavelength (µm)
absorp
tance
0 1 2 3
1.0
0.8
0.6
0.4
0.2
0
a
e
b
c
d
Figure 7.1: Absorptance measurements taken for samples prepared in b) H2S, c) sulfurpowder, and d) SF6 and then annealed at 775 K. A trace before annealing (a) and one ofcrystalline silicon (e) are included for reference.
relaxation that occurs more readily in the case of the sulfur powder or SF6
7.2 Further annealing of selenium and tellurium samples
In Chapter 5, we discussed the composition and optical properties of samples pre-
pared in selenium and tellurium, before and after annealing. Only one anneal temperature
was discussed because there was a significant amount of insight gained from these studies
alone. In the future, further work with different temperatures and analysis of the material
will be useful. Of particular interest would be the change in composition as the material
is annealed at higher temperatures. While we have some information in this area from
working with SF6 samples, more can be gained using selenium or tellurium. The peak for
Chapter 7: Future Work 50
sulfur in Rutherford backscattering is so close to that of silicon that it is not possible to see
if there is any clustering of the elements at the sample surface, at the interface between the
disordered layer and the unperturbed substrate, etc. Because selenium and tellurium are
both heavier than silicon, the distribution of the dopant within the disordered layer can be
measured. In our initial studies, we did not see any change in the distribution. Anneals at
higher temperatures or for longer periods of time may produce interesting changes.
7.3 Optoelectronic applications
This material has recently been shown to be a surprisingly good photodiode as well
as showing a photovoltaic response. Specifically, the responsivity at below-band gap wave-
lengths (out to 1.6 µm) is only one order of magnitude below that of more exotic materials
such as InGaAs. With further engineering and research, this gap could be narrowed. Due
to the significant difference in cost between these materials, it is not necessary to surpass
these commercial photodiodes.
One of the more exciting possibilities, though, is the use of dopants other than
sulfur in the development of these photodiodes. In the manufacture of our photodiodes, the
anneal temperature is critical in creating a rectifying junction. The tradeoff with annealing
is that the absorptance for below-band gap radiation drops with increasing anneal tem-
perature. Using selenium rather than sulfur may then lead to a more robust photodiode.
Annealing selenium samples at 775 K has a smaller effect on the absorptance for below-band
gap radiation than the corresponding effect for sulfur samples. It may be possible to anneal
selenium to a higher temperature than sulfur without seeing the drop in responsivity at
Chapter 7: Future Work 51
below-band gap wavelengths.
However, it is possible that the removal of these optically active defects is the
effect that creates the rectifying junction. Regardless, there are numerous possibilities for
improving the optoelectronic properties of this material.
7.4 Further materials characterization
Though we have incorporated a fair number of elemental dopants, there exist a
great deal more that could be incorporated. Interesting results may be generated by incor-
porating metals, for example. Metallic dopants may reduce the resistivity of the material,
though the optical absorptance may not be the same. There are two primary goals to
further characterization of this material. The first is that more work must be done on un-
derstanding the dopant’s role in order to improve the device characterization. The second
goal is simply a scientific curiosity that needs to be addressed.
In terms of the device applications, there are several experiments of particular
interest. Given that indium and gallium both possess high absorptance for below-band gap
radiation, it will be interesting to investigate the photodiode response after using a p-type
dopant. The substrate doping can also be varied to determine the role of the dopant in
creating this unique material. For further information on the photodiode characterization
that has already been done, the reader is referred to [1].
Chapter 7: Future Work 52
7.5 Conclusion
As can be seen in the above paragraphs, there are a wide variety of experiments
that could be performed and indeed some of them are already underway. The parameter
space is vast, but the reader should now have an improved understanding of the unique
material that is created. A project that began as laser-assisted reactive ion etching has
blossomed into an area that has provided experimental and scientific questions that include
biology, chemistry, physics and materials science. The future in this area is quite promising
and we look forward to seeing the developments as they become available.
Appendix A
Rearrangement as a probe for
radical formation:
bromomethylcyclopropane on
oxygen-covered Mo(110)
The study of radical reactions on transition metal surfaces is of great interest
because of its importance in hydrocarbon processing; e.g., the synthesis of alternative fuels
and the production of chemical building blocks. Accordingly, there have been extensive
studies of alkyl sources on metal surfaces as a means of studying radicals. Among the most
widely studied precursors of radical reactions on surfaces are alkyl halides.
Alkyl halides have been studied extensively on a variety of transition metal sur-
faces. The reactivity of both linear an cyclic alkyl halides have been studied on Cu
53
Appendix A: Rearrangement as a probe for radical formation: bromomethylcyclopropaneon oxygen-covered Mo(110) 54
[35, 20, 36, 37, 38, 39], Ag [40], Ni [41], Rh [42, 43, 44, 45], and Pt [46]. In all of these cases,
there is the presumption that the carbon-halide bond breaks to yield a transient radical
species.
We have likewise investigated the reactions of radical species with oxygen on
Mo(110). In earlier work, we investigated the direct addition of methyl radicals to oxy-
gen on Mo(110) [47]. In these studies, we demonstrated that methyl radicals readily add to
surface oxygen, forming adsorbed methoxy at 100 K. An interesting aspect of this system is
that the reaction can be reversed thermally. Methyl radicals are evolved into the gas phase
via homolytic C–O bond cleavage on O-covered Mo(110) [47, 48, 49, 50, 51]. In all cases,
C–O bond cleavage occurs above 400 K. For alkoxides other than methoxide, the transient
radical formed from C–O bond cleavage undergoes rapid β-hydrogen elimination, yielding
the corresponding alkene. For example, ethoxide, formed from ethanol, yields ethene at 460
K [48].
Notably, we have previously shown that the methylcyclopropyl radical, formed
from C–O bond dissociation of methylcyclopropoxide, rearranges to a butenyl radical that
is trapped on the surface [52]. The use of radical rearrangement as a gauge for determining
radical lifetimes and relative reaction time scales (i.e., as a free radical clock) [53, 54] is a
well-established technique in both gas- and solution-phase chemistry [55, 56]. In particular,
the radical rearrangement of the methylcyclopropyl to the 3-butenyl radical (Figure A.1)
is a well-calibrated example of a fast-reacting radical clock [57]. The ring-opened radical,
formed from methycycloproxide, adds to both oxygen and open Mo centers. The ring-
opened 3-buten-1-oxide is identified using infrared spectroscopy. Thus our earlier studies
Appendix A: Rearrangement as a probe for radical formation: bromomethylcyclopropaneon oxygen-covered Mo(110) 55
established that the transient methylcyclopropyl radical has a sufficiently long lifetime to
rearrange prior to addition to oxygen. Therefore, this is an excellent system to probe for
formation of the transient radical in the case of the corresponding Br-compound.
Figure A.1: The radical rearrangement of the methylcyclopropyl to the 3-butenyl radical
In this study, we extend our earlier investigations of radical reactions on oxygen-
covered Mo(110) by investigating bromomethylcyclopropane as a potential source of the
methylcyclopropyl radical. Surprisingly, no rearrangement is detected upon C–Br bond
dissociation. Instead, the methylcyclopropoxide species is formed. These results indicate
that oxygen displaces the Br, i.e. that a radical is not formed. These results are discussed
in the context of using alkyl halides as sources of radicals in surface reactions.
A.1 Experimental
All experiments were performed in two ultrahigh vacuum chambers described pre-
viously with bases pressures of ≤ 2 × 10−10 Torr [58, 59]. Both chambers were equipped
with a a UTI quadrupole mass spectrometer, low energy electron diffraction (LEED) op-
tics, and an Auger spectrometer with a cylindrical mirror analyzer. The infrared spectra
Appendix A: Rearrangement as a probe for radical formation: bromomethylcyclopropaneon oxygen-covered Mo(110) 56
were collected using a single beam, clean air purged Fourier transform infrared spectrom-
eter (Nicolet, Series 800) and averaged over 500 scans using an MCT detector at 4 cm−1
resolution; the scan time was approximately 3 min. Sample spectra were compared to a
background spectrum taken immediately after the sample scans by flashing the crystal to
760 K. The background scan was initiated after the crystal had returned to a baseline
temperature of approximately 130 K.
The Mo(110) crystal (Metal Crystals) could be cooled to 100 K, heated to 900 K
radiatively, or heated to 2300 K via electron impact bombardment. Prior to each experi-
ment, the Mo(110) surface was cleaned by oxidation at 1200 K in 1×10−9 of O2 for 5 min.
The crystal temperature was allowed to return to approximately 100 K and subsequently
flashed to 2300 K to remove residual oxygen. No surface carbon or oxygen was detected
in the Auger electron spectra of the surface after this treatment. A sharp (1×1) low en-
ergy diffraction pattern was also observed. The oxygen-covered surface (θ0=2/3 ML) was
prepared by saturating the surface at 100 K in 1×10−9 Torr of O2 for 1 min followed by
flashing transiently to 500 K.
Bromomethylcyclopropane, 4-bromo-1-butene, and 3-buten-1-ol were purchased
from Aldrich. Several freeze-pump-thaw cycles were performed to increase the purity of
these samples, and each material was characterized by comparison with its tabulated gas-
phase mass spectra [60]. After dosing a particular species onto the oxygen-covered crystal
at 100 K, the crystal was positioned approximately 2 mm from the aperture (3 mm in
diameter) of the mass spectrometer shield during the collection of temperature programmed
reaction data. The crystal was biased at –70 V during temperature programmed reaction
Appendix A: Rearrangement as a probe for radical formation: bromomethylcyclopropaneon oxygen-covered Mo(110) 57
to minimize reactions induced by the electrons generated by the mass spectrometer. The
mass spectrometer was computer interfaced, and the data were collected with a program
capable of collecting up to 16 separate ion intensity profiles during a single experiment. The
heating rate was constant at 10 ± 2 K/s between 100 and 750 K.
A.2 Results
A.2.1 Temperature programmed reaction spectrometry
Competition between desorption and reaction to produce open-chain hydrocar-
bons is observed during temperature programmed reaction of bromomethylcyclopropane on
oxygen-covered Mo(110) (θ0 = 2/3 ML). Three hydrocarbon products — 1,3-butadiene,
1-butene, and ethene — are formed between 400 and 600 K (Figure A.2). 1-Butene is
formed in two poorly resolved peaks at 450 and 525 K, denoted as α and β. 1,3-Butadiene
forms coincidentally with the β-butene peak at 525 K. Ethene is formed in an asymmetric
peak with a maximum at approximately 560 K. The asymmetry of this peak is mainly due
to overlap from 1-butene and 1,3-butadiene fragmentation, indicated by the shaded area
(Figure A.2). The ethene yield peaks as the production of C4 species diminishes. All prod-
ucts are identified by quantitative comparison of mass spectral data with the fragmentation
patters measured for the most intense masses of authentic samples (Tables A.1 and A.2).
Water and H2 formation accompany the hydrocarbon production; note that hydro-
gen is lost during butadiene formation and that ethene formation must yield surface-bound
hydrocarbon fragments that ultimately dehydrogenate. Water is produced in an asymmet-
ric peak at ∼560 K. A minor amount of gaseous H2 is also detected between 400 and 600 K.
Appendix A: Rearrangement as a probe for radical formation: bromomethylcyclopropaneon oxygen-covered Mo(110) 58
Figure A.2: Temperature programmed reaction data following adsorption of multilayersof bromomethylcyclopropane on oxygen-covered Mo(110) (θ0 = 0.67 ML). The ions shownare characteristic of the products indicated: 1,3-butadiene (m/z=54), 1-butene (m/z=41),ethene (m/z=28), water (m/z=18), and dihydrogen (m/z=2). All data are uncorrectedfor fragmentation and in some cases contain contributions from other products. Carbonmonoxide (m/z=28) is produced at high temperature (inset). The signal at 41 amu isrepresentative of 1-butene, as it is the most abundant fragment ion and has no contributionsfrom 1,3-butadiene formation (Table A.1). All spectra are taken with a heating rate of 10± 2 K/s. The shaded area displays the contribution of 1-butene (37% of amu 41) and1,3-butadiene fragmentation (111% of amu 54) to the m/z=28 peak. The low-temperaturefeatures are all fragments of desorbing bromomethylcyclopropane.
Appendix A: Rearrangement as a probe for radical formation: bromomethylcyclopropaneon oxygen-covered Mo(110) 59
Species m/z
26 27 28 29 39 41 53 54 55 56
Bromomethycyclopropane 22 57 32 24 45 8 21 22 100 56
1,3-Butadiene 27 58 78 1 100 48 70 19
1-Butene 14 37 37 14 54 100 7 4 21 37
Ethene 67 62 100 2
Table A.1: Mass fragmentation patterns of authentic samples
Some of the H2 arises from adsorption of hydrogen from the background. In independent ex-
periments, hydrogen adsorbed from the background also desorbs as H2 between 400 and 550
K with an intensity approximately one-third that measured for bromomethylcycloproprane.
Hence, all proceses involving C–H bond cleavage have to occur below 600 K.
CO is formed, via coupling of adsorbed carbon and oxygen, starting around 850
K and peaking at 950 K (Figure A.2 inset). The carbon can be deposited via complete
decomposition of the bromomethylcyclopropane and/or in conjunction with ethene elimi-
nation. The ratio of ethene (560 K) to CO (950 K) is 1:12 after correction for contributions
from 1-butene and 1,3-butadiene fragmentation, indicating that there is adsorbed carbon
arising from non-selective decomposition of adsorbed species in addition to carbon left be-
hind in the evolution of ethene from the C4 intermediates. Calculation of the ionization
efficiencies, ε, based on the number of electrons in the molecule [61] leads to a very similar
value, ε∼1 for CO and C2H4. C4H6 has a somewhat higher ionization efficiency of ε∼1.7.
Basic sensitivity factors (SB) calculated using ionization efficiencies and mass spectrometer
Appendix A: Rearrangement as a probe for radical formation: bromomethylcyclopropaneon oxygen-covered Mo(110) 60
Species m/z26 27 28 29 39 41 53 54 55 56
Intensities from multilayerpeak for bromomethylcyclo-propane
21 55 39 26 45 11 18 20 100 25
All products from reactionpeaks for bromomethylcyclo-propane, 400–600 K
80 107 132 8 100 42 34 49 28 17
Residual correcting for 1-butene
95 118 150 3 100 41 62 25
Residual after correcting for1-butene and 1,3-butadieneleaving ethene
94 84 100
All products from reactionpeak for 4-bromo-1-butene
68 96 115 8 100 27 37 52 24 10
Residual correcting for 1-butene
75 101 123 5 100 41 60 22
Residual after correcting for1-butene and 1,3-butadieneleaving ethene
107 95 100
All products from reactionpeak for 3-buten-1-ol
62 92 111 13 100 40 40 62 28 12
Residual correcting for 1-butene
72 99 122 10 100 48 77 24
Residual after correcting for1-butene and 1,3-butadieneleaving ethene
101 92 100
a 1-Butene was identified on the basis of peaks at 41 and 56 amu, the most intense ionand parent, respectively. The other hydrocarbons were identified by subtracting the con-tribution of 1-butene to other masses in the product spectrum. The residual 54 amu signalis attributed to 1,3-butadiene. After accounting for fragmentation of both 1-butene and1,3-butadiene, significant signals at 26, 27, and 28 amu remain, which are attributed toethene.
Table A.2: Mass fragmentation patterns of productsa
Appendix A: Rearrangement as a probe for radical formation: bromomethylcyclopropaneon oxygen-covered Mo(110) 61
transmission values for the respective fragment ions lead to SB=1.1, 1.2, and 1.3 × 10−3 for
CO, C2H4, and C4H6, respectively. The similarity of the values for CO and C2H4 should
allow for similar detection efficiencies for these ions. Thus, if the C2 fragment left behind
during ethene desorption was the only source for evolution of CO, a CO:C2H4 ratio of 2:1
would be expected, instead of the measured ration of 12:1. The peak area ratio of the 54
amu at 525 K to the 28 amu peak at 950 K is 1:3. The formation of CO coincides with
desorption of Br at 860 K peaking at 1000 K (data not shown).
A detailed analysis of the mass spectral data indicates that no other C-containing
products evolve. Specifically, there is no detectable production of C3-hydrocarbons, cyclic
species, or oxygenates. The fact that there is no residual intensity in the range of 36–44 amu
after accounting for the three primary reaction products rules out the possibility that C3-
species or oxygenates are formed. The absence of intensity at 31 and 32 amu further confirms
that no oxygen-containing species are evolved, other than water, because these fragments
are characteristic of oxygenates. Further, the fact that the only change in the spectrum for
reaction on 18O-labeled Mo(110) is a shift of 2 amu for the water peak indicates that no
surface oxygen is incorporated in any product other than water. The possible formation of
cyclic compounds (e.g, cyclopropane, methylcyclopropane, cyclobutane, and cyclobutene)
is ruled out based on key differences between their relative mass fragment intensities and
those of the desorbed products. Similarly, formation of methane (16 amu) and acetylene
(26 amu) is ruled out. Finally, no other species with more than four carbons are formed
based on a comprehensive search of masses in the 2–140 amu range, i.e. no masses above
57 amu are detected.
Appendix A: Rearrangement as a probe for radical formation: bromomethylcyclopropaneon oxygen-covered Mo(110) 62
The formation of open-chain hydrocarbon products during temperature-programmed
reaction of bromomethylcyclopropane demonstrates that the C3-ring opens at some point
along the reaction path. Hence, the reactions of the linear analogues, 4-bromo-1-butene and
3-buten-1-ol, were also studied as a means of accessing the same intermediate(s) as those
formed from bromomethylcylopropane.
The temperature-programmed reaction spectrum of 4-bromo-1-butene (Figure A.3)
is similar to that of bromomethylcyclopropane, which suggests that both react via the same
intermediate. Again, 1,3-butadiene, 1-butene, ethene, water, and H2 are produced in the
range of 400–600 K. Interestingly, α-butene production is enhanced relative to the reac-
tion of bromomethylcyclopropane. The relative yield of products is similar for 4-bromo-
1-butene and bromomethylcyclopropane; however, the absolute amount of product forma-
tion is smaller for 4-bromo-1-butene. For example, the amount of CO formed via atom
re-combination is ∼70% that formed from bromomethylcyclopropane. The ratio of CO
production to ethene formation is 11:1, which is similar to the bromomethylcyclopropane.
The butadiene:CO ratio is 1:12, which is smaller than the ratio of 1:3 for bromomethylcy-
clopropane. Quantitative analysis of the mass spectra was again employed to identify the
products using the same approach as described for bromomethylcyclopropane (Table A.2).
Reaction of 3-buten-1-ol on oxygen-covered Mo(110) also leads to simialr products
(Figure A.4) as in the bromomethylcyclopropane and 4-bromo-1-butene systems. However,
there are also some distinct differences. For instance, the α-butene peak is missing, so
that butene is formed in a single, symmetric peak at 550 K. The butadiene peak is also
narrower; however, the peak temperature is nearly the same as for the Br-compounds. Table
Appendix A: Rearrangement as a probe for radical formation: bromomethylcyclopropaneon oxygen-covered Mo(110) 63
Figure A.3: Temperature programmed reaction data obtained after adsorption of 4-bromo-1-butene on oxygen-covered Mo(110) showing formation of 1,3-butadiene (m/z=54), 1-butene (m/z=41), ethene (m/z=28), water (m/z=18), and dihydrogen (m/z=2). Data areuncorrected for fragmentation. The features at ∼220 K are due to desorption of molecular4-bromo-1-butene. The shaded area displays the contribution of 1-butene (37% of amu 41)and 1,3-butadiene fragmentation (111% of amu 54) to the m/z=28 peak. All spectra aretaken with a heating rate of 10 ± 2 K/s.
Appendix A: Rearrangement as a probe for radical formation: bromomethylcyclopropaneon oxygen-covered Mo(110) 64
A.2 summarizes the results of the quantitative analysis of the mass spectra performed to
identify the products. The similar formation temperatures and product mass distributions
measured for the alcohol and the Br-compounds suggest that the products formed above
500 K evolve from a common intermediate, most likely a surface-bound butenoxy species.
Water also forms at 560 K in the reactions of 3-buten-1-ol. The oxygen in the
water originates mainly from the surface-bound oxygen, since most water formed from
reaction on 18O-covered Mo(110) is H218O. There is also some production of H2
16O at the
same temperature which arises from reaction with oxygen deposited when the hydrocarbon
products are evolved. Carbon monoxide is again formed at high temperature (data not
shown).
The selectivity for hydrocarbon production from 3-buten-1-ol is greater than for
the Br-compounds, based on the smaller CO:butadiene ratio of 6:1 and the almost negligible
amount of C2H4 (CO:C2H4=53:1) estimated after subtraction of the 1-butene and 1,3-
butadiene contributions. Furthermore, the absolute amount of C4H6 formed compared to
bromomethylcyclopropane is 328% indicating a much higher selectivity for C4-hydrocarbon
formation for the alcohol.
A.2.2 Fourier transform infrared vibrational spectroscopy
Infrared absorption studies provide evidence that the ring in bromomethylcyclo-
propane opens in the range of 400–450 K and that butenoxy is formed from all three reac-
tants studied. Our evidence for ring opening is that the infrared spectra obtained for all
three reactants — bromomethylcyclopropane, 4-bromo-1-butene, and 3-buten-1-ol — after
heating to 450 K are very similar (Figure A.5). Most notably, there are peaks at 1645 cm−1
Appendix A: Rearrangement as a probe for radical formation: bromomethylcyclopropaneon oxygen-covered Mo(110) 65
Figure A.4: Temperature programmed reaction data obtained after adsorption of mul-tilayers of 3-buten-1-ol on oxygen-covered Mo(110) showing formation of 1,3-butadiene(m/z=54), 1-butene (m/z=41), ethene (m/z=28), water (m/z=18), and dihydrogen(m/z=2). All spectra are taken with a heating rate of 10 ± 2 K/s and are uncorrectedfor fragmentation. The shaded area displays the contribution of 1-butene (37% of amu 41)and 1,3-butadiene fragmentation (111%) of amu 54) to the m/z=28 trace.
Appendix A: Rearrangement as a probe for radical formation: bromomethylcyclopropaneon oxygen-covered Mo(110) 66
Figure A.5: Infrared absorption spectra obtained after heating bromomethylcyclopropane,4-bromo-1-butene, and 3-buten-1-ol to 450 K. All reactants were adsorbed onto oxygen-covered Mo(110) at ∼100 K and transiently heated to 450 K. These data are referred tothe corresponding surface heated transiently to 760 K.
and near 3090 cm−1 that are assigned to the ν(C=C) and the alkene ν(=C–H) modes [62],
respectively, in all three cases. These results demonstrated unequivocally that the ring in
bromomethylcyclopropane opens below 450 K (Table A.3).
There are also important differences in the infrared spectrra obtained after heating
the Br-compounds and the alcohol to 450 K (Figure A.5). Most notably, there is a peak
at 1242 cm−1 in the spectra of the Br-compounds that is absent in the spectrum of the
alcohol. We assign this feature to a δ(Mo-CH2) bending-mode of an alkyl species bound to
Appendix A: Rearrangement as a probe for radical formation: bromomethylcyclopropaneon oxygen-covered Mo(110) 67
Infraredvibrationalassign-ments
c-(C3H5)CH2O-bromomethyl-cyclopropane
C4H7O-bromomethyl-cyclopropane
C4H7-bromomethyl-cyclopropane
Intermediatesfrom 4-bromo-1-butenea,b
Intermediatefrom 3-buten-1-olb
ν(C–O) 880 895 890–930 888–940 942ν(C–H) 2957, 3014 2960 2962 2916, 2934ν(C–H)c 3090 3091 3088ν(C–C–O) 1029 1041d 1040 1038 1037ν(C=C) 1646 1646 1645Ring modes 1394,1433δ(Mo–CH2) 1243 1242
a Mixture of linear alkyl and alkoxy species; see text.b Infrared data at 450 K.c Alkene.d Mixture of both C–C and C–C–O stretches; see text.
Table A.3: Infrared vibrational assignments for bromomethylcyclopropane, 4-bromo-1-butene, and 3-buten-1-ol on oxygen covered Mo(110).
the Mo(110), based on analogy with surface-bound alkyls, such as methylene [37, 63, 64].
The presence of this peak indicates that there is competition between addition to open
metal centers and oxygen following C–Br bond dissociation and ring opening.
The other major difference in the spectra obtained after heating the three reactants
to 450 K is in the region below 1000 cm−1. The intermediate formed from the alcohol is
presumed to be the butenoxy species, based on analogy with other alcohols studied on
oxygen-covered Mo(110) [47, 48, 49, 50, 51]. The most prominent features in the spectrum
for the alkoxy are centered at 942 and 1037 cm−1 (Figure A.5). The most prominent feature
in the spectra obtained after heating the two Br-compounds to 450 K is a peak at ∼1040
cm−1, which is similar to the alkoxy. However, there is not a well-defined peak near 940
cm−1 for bromomethylcyclopropane, and the peak near 940 cm−1 for 4-bromo-1-butene is
Appendix A: Rearrangement as a probe for radical formation: bromomethylcyclopropaneon oxygen-covered Mo(110) 68
also not as pronounced as in the infrared spectrum of the alcohol. The latter differences
are attributed to additional peaks near 900 cm−1 due to the Mo-alkyl.
The contributions of the additional species to the infrared spectra of the Br-
compounds at 450 K are highlighted in the differences between spectra for the Br-compounds
and 3-buten-1-ol (Figure A.6). By subtracting infrared absorption traces of the linear alkoxy
species after scaling the spectra so that the peaks in the 930–940 cm−1 region are equal in
intensity, the remaining features must be attributed to a surface species other than 3-buten-
1-oxy. The difference spectra obtained between the alcohol and the two Br-compounds are
virtually identical, indicating that the remaining intermediate is similar in the two cases.
Infrared peaks in the 1245 cm−1 region have been assigned as δ(Mo-CH2) modes [63, 65].
The residual peaks in the vicinity of 891 and 1040 cm−1 are attributed to ν(C–C) modes.
Thus, we assign the residual peaks to a straight-chain alklyl species; however, based on our
results we cannot rule out the formation on other surface species, such as an oxametallacycle
[66].
The evolution of the infrared spectra as the bromomethylcyclopropane-covered
surface is transiently heated provides more inofrmation on the temperature required for
C–Br bond cleavage and ring opening for bromomethylcyclopropane (Figure A.6). The
spectrum at 100 K is representative of multilayers of the intact molecule, which is con-
sistent with temperature programmed reaction. Comparison of the multilayer spectrum
with the vapor-phase IR [67] shows good agreement. Thus, we assign the modes in the
1225 cm−1 region to C–C ring modes of intact bromomethylcyclopropane and the 1428
cm−1 peak to the CH2 scissors mode [68]. The CH(Br) in-plane bend of bromomethylcyclo-
Appendix A: Rearrangement as a probe for radical formation: bromomethylcyclopropaneon oxygen-covered Mo(110) 69
Figure A.6: Difference spectra for infrared reflection absorption data obtained after heatingbromomethylcyclopropane and 3-buten-1-ol (solid curve) and 4-bromo-1-butene (dashedcurve) on oxygen-covered Mo(110) to 450 K.
propane has been assigned as 1269 cm−1, in analogy to studies on Cu [39]. By 350 K, intact
bromomethylcyclopropane is no longer present on the surface as indicated by the disap-
pearance of the associated peaks. Ring modes at 1394 and 1433 cm−1 (associated with an
adsorbed cyclic species) and a δ(Mo–CH2) mode at 1242 cm−1 (associated with an adsorbed
alkyl species) are present in the spectrum at 350 K. These modes start to appear around
220 K (data not shown), which we interpret as the point at which the C–Br bond breaks
and both metal- and oxygen-substituted alkyl and alkoxy intermediates are formed, respec-
tively; similar observations are made for 4-bromo-1-butene. Most notably no ring opening
Appendix A: Rearrangement as a probe for radical formation: bromomethylcyclopropaneon oxygen-covered Mo(110) 70
occurs at 350 K, because no ν(C=C) mode at 1646 cm−1 is detected. Thus, we assert that
the bromine in bromomethylcyclopropane is substituted by either oxygen or a Mo atom at
around 220 K to form methylcyclopropyl–O and methylcyclopropyl–Mo. Notably, there is a
strong correspondence between the infrared spectrum for methylcylopropyl–O formed from
hydroxymethylcyclopropane [52] and the spectrum for bromomethylcyclopropane heated to
350 K on oxygen-covered Mo(110).
Further heating to 450 K induces ring opening, signified by the disappearance of
the modes associated with the cyclopropyl ring at 1394 and 1433 cm−1 (Figure A.7) and
the appearance of the 1646 cm−1 peak. The intensity at 1242 cm−1 increases and the
mode at 1041 cm−1 gains intensity while the 1029 cm−1 peak loses intensity after heating
bromomethylcyclopropane to 450 K.
Isotopic shifts due to 18O substitution are expected to be small due to intramolec-
ular coupling of vibrations near 1000 cm−1 for alcohols with two or more carbons [69].
Nevertheless, changes below 950 cm−1 are observed that are consistent with formation of
both alkoxide and alkyl species from reaction of the Br-compounds on the O-covered sur-
face. Differences between the spectra obtained after heating the Br-compounds to 350 and
450 K on 16O- and the 18O-covered surfaces show that there are subtle shifts in the low-
frequency region (Figure A.8). There is no difference between 16O- and 18O-labeled spectra
for modes above 1050 cm−1. Difference spectra highlight changes upon labeling (Figure
A.8(c) and (f)). Peaks that are sensitive to 18O-labeling should appear as differential peaks
with positive and negative areas of equal amplitude in the difference spectra. It should
be noted that at least part of the difference may be due to intensity variation. Since the
Appendix A: Rearrangement as a probe for radical formation: bromomethylcyclopropaneon oxygen-covered Mo(110) 71
Figure A.7: Infrared reflection absorption spectra after transient heating of bromomethyl-cyclopropane adsorbed on oxygen-covered Mo(110) to various temperatures.
positive peak at ∼880 cm−1 in Figure A.8(c) lacks an equal negative counterpart, this peak
is assigned primarily to C–C stretching modes. However, the difference in peak widths in
the vicinity of 880 cm−1 in Figure A.8(a) and (b) suggests that this peak has some C–O
stretch character and that the corresponding peak for the 18O trace may coalesce with the
C–C component of the peak. The small positive peak at ∼1040 cm−1 in Figure A.8(c) may
indicate some C–O character as well. Similarly, the difference spectrum in Figure A.8(f)
compares the surface intermediates formed at 450 K for the 16O- and 18O-covered surfaces.
A differential but asymmetric peak is observed at ∼890 cm−1, again suggesting partial C–C
and C–O character in the broad peak centered at 892 cm−1 (Figure A.8(d)). A double peak
Appendix A: Rearrangement as a probe for radical formation: bromomethylcyclopropaneon oxygen-covered Mo(110) 72
Figure A.8: Infrared spectra for bromomethylcyclopropane on a) 16O- and on b) 18O-labeledsurfaces heated transiently to 350 K; c) the difference between the spectra in a) and b);bromomethylcyclopropane on d) 16O- and e) 18O-labeled surfaces heated transiently to 450K; and f) the difference between the spectra in d) and e). Shaded regions in a) and c)represent the peak intensity attributed to C–18O stretching (see text).
now appears more clearly at 1040 cm−1, indicating C–O character in this region.
A.2.3 Discussion
Our studies clearly demonstrate that the C3-ring of bromomethylcyclopropane
does not open upon C–Br bond scission. This indicates that bromine is replaced by oxygen
to form adsorbed methylcylopropoxide (Figure A.9), on a time scale that is more rapid
than rearrangement. This result suggests the possibility that the reaction proceeds via a
concerted mechanism rather than formation of a radical upon C–Br bond scission.
The proposed formation of methycyclopropoxide from replacement of Br by O is
supported by the fact that there is strong evidence for C–Br bond scission, commencing
at ∼220 K and that the intermediates remaining on the surface have an intact cyclopropyl
ring. Our data are consistent with formation of both alkyl and alkoxy intermediates as the
C–Br bond breaks. There is no evidence for ring opening up to 400 K (data not shown).
Appendix A: Rearrangement as a probe for radical formation: bromomethylcyclopropaneon oxygen-covered Mo(110) 73
Figure A.9: Proposed reaction scheme for bromomethylcyclopropane on oxygen-coveredMo(110).
Appendix A: Rearrangement as a probe for radical formation: bromomethylcyclopropaneon oxygen-covered Mo(110) 74
Specifically, the ring modes at 1394 and 1433 cm−1 persist and there is no visible ν(C=C)
mode at 1645 cm−1 (Figure A.7).
Infrared spectra provide evidence that methylcyclopropoxide and Mo-methylcyclo-
propyl species are present on the surface after heating bromomethycyclopropane to 350 K.
There is a strong correspondence between the spectra obtained for methylcylopropoxide
formed from hydroxymethylcyclopropane [52] and the intermediate formed from heating
bromomethycyclopropane to 350 K. Furthermore, a detailed analysis of the infrared spectra
obtained on 16O- and 18O-covered surfaces provides evidence for C–O bond formation.
There is also clear evidence for formation of a metal-alklyl species as C–Br bond
cleavage is induced by heating bromomethylcyclopropane to 350 K. The presence of the
alkyl species is indicated by the δ(Mo–CH2) mode at 1242 cm−1 and the development of
peaks involving C–C bonds below 900 cm−1.
The fact that there is no detectable ring opening upon C–Br bond scission sug-
gests that either this reaction step does not proceed via a radical mechanism or that the
radical is so short-lived that it does not rearrange. In other words, we propose that the
methylcyclopropoxde results from substitution of bromine by oxygen in a concerted process.
If C–Br bond dissociation yielded a methylcyclopropyl radical, rearrangement should have
been observed at temperatures as low as 220 K, the temperature where the onset of C–Br
bond cleavage occurs (data not shown). The timescale for radical addition to oxygen should
be the same independent of the source of the radical. We have already established that the
methylcyclopropyl radical formed from C–O bond scission in methylcyclopropoxide rear-
ranges prior to addition to surface oxygen to form the butenoxy species [52]. As discussed
Appendix A: Rearrangement as a probe for radical formation: bromomethylcyclopropaneon oxygen-covered Mo(110) 75
in earlier work, the observation of rearrangement places a lower bound on the time scale
for addition of the radical to oxygen of ∼1 ns [52]. We note that the temperature for C–Br
bond dissociation is lower than for C–O bond breaking — 300 vs. 400 K (data not shown).
The rate of rearrangement of the radical will depend on temperature. The rate constant
at 300 K is estimated to be 8.76 × 107 s−1, i.e. a characteristic reaction time of 11 ns,
based on an expression for the T-dependence of the rate derived by Halgren et al. [57]. We
propose that rearrangement should still occur at this temperature if a radical is formed,
and that the absence of ring opening is strong evidence for displacement of Br by oxygen,
as opposed to formation of a transient radical species. We are currently investigating sub-
stituted hydroxymethylcyclopropanes that have lower rate constants in order to determine
the time scale necessary for addition to oxygen as well as the thermal and electron induced
ring opening of bromomethylcyclopropane on oxygen-covered Mo(110).
The fact that C–Br bond scission does not generate a radical has important im-
plications regarding the use of alkyl halides to model the reactions of alkyl radicals. Alkyl
halides have been used extensively to model the reactions of radical species on metal sur-
faces. Our work shows that there are at least some instances where C–X bond cleavage
does not generate a radical. Therefore, it is important to establish whether a transient
radical is, indeed, formed on other surfaces if such studies are to be used as models for
radical reaction. In our case, it is possible that the surface oxygen affects the mechanism
for C–Br bond dissociation. The oxygen creates a partial negative charge at the surface
that may affect the interaction of the Br-compound with the surface. In addition, oxygen
occupies adsorption sites that would otherwise be available for bromine. These points are
Appendix A: Rearrangement as a probe for radical formation: bromomethylcyclopropaneon oxygen-covered Mo(110) 76
begin addressed in independent studies.
Once the methylcyclopopropoxide intermediate is formed from bromomethylcy-
clopropane, it behaves identically to hydroxymethylcyclopropane. Specifically, heating to
∼400 K induces homolytic C–O bond breaking, which generates a cyclopropylmethyl rad-
ical. The radical quickly rearranges to a 3-butenyl radical and is trapped on the surface
to yield ring-opened alkyl and alkoxy moieties (Figure A.9). Heating the surface to higher
temperatures then leads to selective product formation and to non-selective decomposition
on the surface.
The elimination of a radical via homolytic C–O bond cleavage in methylcyclo-
propoxide is consistent with previous studies of alcohols on oxygen-covered Mo(110) [47,
48, 49, 50, 51]. Generally, C–O bond dissociation of alkoxides occurs above 500 K on
oxygen-covered Mo(110). Direct evidence for formation of a radical is observed in the case
of methoxy-metal radicals are evolved into the gas phase at ∼550 K [70, 71]. Alkoxides
with longer chains react via a radial mechanism to form a combination of alkane and alkene
products in conjunction with water in the range of 500–600 K. In these case the tempera-
ture required for C–O bond dissociation correlates with the homolytic C–O bond strength,
providing evidence for a radical intermediate [50, 70]. Furthermore, isotopic labeling studies
showed that the predominant products, alkenes, are formed via dissociation of the C–O bond
followed by dehydrogenation at the 2-carbon [50]. It is well known that dehydrogenation
of radical species occurs via elimination of hydrogen at the carbon adjacent to the radical
site, the 2-carbon. In contrast, it is well established that dehydrogenation prior to C–O
bond cleavage occurs preferentially at the C–H bonds adjacent to the oxygen because they
Appendix A: Rearrangement as a probe for radical formation: bromomethylcyclopropaneon oxygen-covered Mo(110) 77
are the weakest and most subject to attack. Dehydrogenation of primary and secondary
alkoxides yields aldehydes and ketones, respectively, on Rh(111) [72] or Pd(111) [73], for
example.
The observation of ring opening in the reactions of methylcyclopropoxide is in it-
self strong evidence for a radical mechanism, as discussed in previous work [52]. A surface-
mediated ring opening mechanism can be ruled out based on recent experiments performed
in our laboratory using 1-cyclopropylethanol [52]. Notably, the ethylcyclopropyl radical
cannot rearrange homogeneously like the methycyclopropyl radical. Hence, the reactions
of 1-cyclopropylethanol are a good test for a possible surface-mediated ring opening; how-
ever, no ring opening is observed. Only C5 species are evolved (all below 350 K); none
of these species are straight-chain products and none contain oxygen. The absence of a
surface-mediated mechanism implies that the transient methylcylopropyl radical rearranges
independently of the surface — i.e., it shows ‘gas-phase’ behavior in the vicinity of the
surface.
Upon elimination and subsequent rearrangement of the methycyclopropyl radi-
cal to the butenyl radical, hydrocarbons are produced. 1,3-Butadiene is formed via β-H
elimination from the butenyl radical, whereas 1-butene is formed via hydrogenation. The
hydrogen eliminated from the radical reacts with surface oxygen to form water. These re-
actions are very similar to those previous observed for other alkoxides, e.g. ethoxide [48],
on oxygen-covered Mo(110).
Differences in the temperature programmed reaction spectra for the three reactants
when correlated with infrared data also show that the metal-bound alkyl contributes to the
Appendix A: Rearrangement as a probe for radical formation: bromomethylcyclopropaneon oxygen-covered Mo(110) 78
reaction products. Specifically, our data indicate that the metal-bound alkyl react to form
C4-hydrocarbons at lower temperature than the alkoxide and that they are the primary
source of ethene. Specifically, the product peaks are sharp for the alcohol and are broad
for the Br-compounds. Notably, the lower temperature α-butene formation peak is also
absent in the temperature programmed reaction spectrum of the 3-buten-1ol. In addition,
the 1242 cm−1 peak attributed to a metal-bound alkyl is absent in the infrared spectrum of
the 3-buten-1-ol but present for both Br-compounds after heating to 350 K. The correlation
between the β-butene formation and the δ(Mo-CH2) peak is evidence that the β-butene
evolved at low temperature arises form the alkyl. The fact that the amount of ethene
produced in the reactions of 3-buten-1-ol is negligible also leads to the conclusion that the
ethene produced from the reactions of the Br-compounds arises mainly from decomposition
of the metal-bound alkyl. This is consistent with other studies of alkoxides on O-covered
Mo(110) since there are no examples of gaseous hydrocarbon elimination via C–C bond
dissociation in any alkoxide studied to date.
A.2.4 Conclusion
No rearrangement occurs subsequent to C–Br bond scission for bromomethylcyclo-
propane on oxygen-covered Mo(110). Instead, the methylcyclopropyl group is transferred to
oxygen. The absence of rearrangement indicates that a radical intermediate is not formed,
since the methycyclopropyl radical is known to rearrange prior to addition to oxygen on this
surface. As a result, methylcyclopropoxide and metal-bound methylcyclopropyl are formed
after heating to 350 K. Subsequently, rearrangement occurs upon dissociation of the C–
O bond of methylcyclopropoxide commencing at ∼400 K. The transient methylcylopropyl
Appendix A: Rearrangement as a probe for radical formation: bromomethylcyclopropaneon oxygen-covered Mo(110) 79
radical rearranges and the ring-opened butenyl species is trapped on the surface. Addition
to oxygen yields 3-buten-1-oxy and addition to the metal affords the butenyl-Mo moiety.
Infared spectroscopy is used to identify these intermediates. The same species are formed
from the reaction of 4-bromo-1-butene. The 3-buten-1-oxy species is also formed from the
reaction of 3-buten-1-ol on oxygen-covered Mo(110). Upon further heating, 1,3-butadiene,
1-butene, ethene, water, and dihydrogen evolve between 450 and 600 K. Carbon monoxide
is also formed above 900 K, due to reaction of surface carbon and oxygen. The same prod-
ucts are formed from all three reactants — bromomethylcyclopropane, 4-bromo-1-butene
and 3-buten-1-ol. These studies raise the issue as to whether or not the reactions of alkyl
halides on metal surface can be used to model radical reactions.
Appendix B
The reaction of CH3NO2 onO-covered Mo(110): the effect ofoxygen on product distribution
The selective catalytic reduction of NOx by hydrocarbons is an attractive alterna-
tive to current technologies for combating automotive emissions. While it is well established
that methane, for example, increases NOx reduction over zeolites [74, 75], rare earth oxides
[76, 77], and metal-doped oxides [77, 78], debate continues regarding the mechanism by
which this occurs. It is possible that methyl radicals prevent catalyst poisoning by reacting
away oxygen deposited by N–O bond scission. It is also possible that methyl radicals attack
NOx directly, forming an intermediate with carbonnitrogen bonds, such as nitromethane
[75, 79, 80, 81]. Nitromethane could then decompose via a variety of routes [75, 82, 83] to
yield reduction products and oxygenates. Again, the formation and evolution of oxygenates
are a key part of the reduction process as removal of oxygen from the catalyst surface
prevents the blockage of active sites.
Molybdenum trioxide has been found to increase activity for NO reduction by CO
and H2 over transition metal based catalysts [84]. In addition, molybdenum trioxide (a
80
Appendix B: The reaction of CH3NO2 on O-covered Mo(110): the effect of oxygen onproduct distribution 81
-MoO3) is known to promote the partial oxidation of methane to formaldehyde [85, 86, 87,
88]. These studies have motivated our studies of species relevant to NOx reduction and
hydrocarbon oxidation over several oxidized Mo surfaces [89, 90, 91]. We are particularly
interested in the roles of oxygen in different coordination sites. Most recently, we have
investigated the reaction of nitromethane on a thin-film oxide of Mo(110) and found C–N
bond retention to be the predominant pathway [92]. In addition, some C–N bond rupture
to evolve methyl radical and a small amount of formaldehyde evolution was observed.
In the present work, we evaluate the reactivity of nitromethane on two oxygen-
covered Mo(110) surfaces (θ0=0.66 ML and θ0=0.40 ML) that do not contain dissolved
oxygen. High coordination sites are exclusively populated under these conditions. Only
twofold sites are occupied at 0.4 ML, whereas a mixture of twofold and quasi-threefold are
populated at 0.66 ML [64]. The reactivity of nitromethane on these surfaces is similar to
the thin film oxide. C–N bond retention predominates and some methyl radical is evolved.
All experiments were performed in a stainless steel ultrahigh vacuum chamber
described in detail elsewhere [92, 59]. Briefly, the chamber has a base pressure of 1×10−10
Torr and is equipped with a quadrupole mass spectrometer (UTI Model 100 C), an Auger
electron spectrometer, and low energy electron diffraction optics (LEED). In addition, it is
interfaced with a Fourier transform infrared spectrometer (Nicolet, Series 800).
The saturated oxygen overlayer (θ0=0.66 ML), Mo(110)–(1 × 6)-O, was prepared
by directed dosing of O2 such that the background pressure rose to 1×10−9 Torr for 1 min.
The crystal temperature during dosing was 100 K. Subsequent to evacuation, the sample was
heated to 500 K using the same temperature profile as in temperature programmed reaction
Appendix B: The reaction of CH3NO2 on O-covered Mo(110): the effect of oxygen onproduct distribution 82
(average dT/dt=10 K/s). This procedure saturates high coordination sites on the surface
but does not induce dissolution of oxygen into the bulk [64]. The overlayer containing 0.4
ML of oxygen was prepared by directed dosing of O2 such that the background pressure rose
by 1×10−10 Torr for 15 s with the crystal temperature maintained at 100 K during dosing.
Transient heating to 500 K followed dosing. The oxygen coverage of the resulting surface
was calibrated using Auger electron spectroscopy as compared to the reference spectrum of
0.66 ML O on Mo(110).
Nitromethane, CH3NO2 (Sigma Aldrich, 99%) was stored in a glass equilibration
flask and subject to several freeze-pump-thaw cycles prior to use. The crystal was biased
at –90 V during nitromethane dosing to prevent reactions induced by stray electrons from
the ion gauge [92].
Temperature programmed reaction up to 800 K was also performed with the crystal
biased at –90 V. Radiative heating (average dT/dt=10 K/s) was used to reach temperatures
up to 800 K and electron bombardment heating (average dT/dt=15 K/s) was used to reach
temperatures up to 1600 K. The filament was briefly flashed prior to each data collection
in order to minimize contribution from the filament. Between eight and sixteen masses
were monitored during each experiment and the temperature was measured with a W/Re
(5%/26%) thermocouple.
Infrared spectroscopy was performed at 4 cm−1 resolution at a crystal temperature
of 120 K. Five hundred scans were collected per acquisition and data was recorded using a
liquid nitrogen cooled MCT-A semiconductor photodiode detector.
The products observed in the reaction of nitromethane on Mo(110)–(1 × 6)-O
Appendix B: The reaction of CH3NO2 on O-covered Mo(110): the effect of oxygen onproduct distribution 83
(θ0=0.66 ML) are HCN, H2O, and ·CH3 (Figure B.1). In addition, molecular desorption is
observed at 150 K (multilayers) and 180 K (monolayer) (data not shown).
The temperatures for nitromethane desorption are similar to those reported for
other surfaces [92, 4, 93, 94]. A small amount of nonselective decomposition occurs, based
on the fact that CO and N2 are evolved above 800 K via atom recombination (data not
shown).
Hydrogen cyanide (m/z=27) and water (m/z=18) are the main products of CH3NO2
reaction on Mo(110)–(1 × 6)-O. They are identified based on an analysis using fragmenta-
tion patterns of authentic HCN and H2O samples [20]. The HCN and H2O traces have the
same shape and they both peak at 670 K. The amount of HCN and water observed for the
reaction of CH3NO2 on the O-covered surface is virtually the same as the amount observed
on the thin-film oxide [92].
Methyl radicals (m/z=15) are evolved in a broad peak between 500 and 800 K.
Methyl radical was identified based on a comparison of the observed m/z=15 : m/z=16 in-
tensity ratio of 1.0:0.8 to that of methane, 0.9:1.0 (as measured NIST). The ratio we observe
is similar to our previous studies of methyl radical from methoxy for which independent
infrared studies confirmed that methyl radical was formed [59]. Furthermore, reaction of
CD3NO2 yields some CD3H (m/z=19), but no CD4 (m/z=20), the expected methane iso-
tope. The amount of methyl radical evolution is 5% greater on O-covered Mo(110) than
it is on the thin-film oxide, for which the peak temperature for methyl radical evolution is
in the same range: ∼675 K. No other products were observed in temperature programmed
reaction up to 800 K during a comprehensive mass search from 2 to 100 amu.
Appendix B: The reaction of CH3NO2 on O-covered Mo(110): the effect of oxygen onproduct distribution 84
Hydrogen cyanide, water and methyl radical are produced during reaction of
CH3NO2 on the 0.40 ML oxygen overlayer (Figure B.1). The yields of HCN and H2O
are approximately 10% greater compared to Mo(110)–(1×6)-O (θ0=0.66 ML); whereas the
yield of methyl radical is 10% lower. In addition, the peak temperature for HCN and H2O
evolution is ∼20 K lower on the 0.40 ML and methyl radical peak temperature is ∼15
K lower. No formaldehyde is detected. The amount of adsorbed carbon and nitrogen is
greater on the surface containing 0.40 ML of oxygen. A sloping background makes precise
quantification impossible, but the increase is in the range of 50%. The greater yields of
nonselective decomposition product and CH bond scission product indicate that the total
amount of CH3NO2 that reacts is greater for the surface with lower oxygen coverage (θ0=0.4
ML) compared to either the (1 × 6)-O layer or the thin film oxide.
Reflection absorption infrared spectroscopy provides evidence for molecular ad-
sorption at 100 K on the saturated oxygen overlayer (Figure B.2, Table B.1). All peaks
observed in the 100 K spectrum correspond to peaks of molecular nitromethane [92]. The
shoulder at 1590 cm−1 may be due to a trace amount of nitrosomethane (CH3NO), as re-
ported for nitromethane on Pt(111) [94]. Significantly, there are no peaks visible in the
terminal oxygen region (Mo=O). The coverage of nitromethane is only slightly greater than
saturation so it is not likely that terminal oxygen is present but undetected due to the
presence of a multilayer.
Upon heating to 300 K, all N–O bonds dissociate and terminal oxygen, signified by
the ν(Mo=O) peak at 981 cm−1, is formed [92]. The assignment of this peak is confirmed
by isotopic shifts measured for reaction on Mo(110)–(1×6)-18O. A peak at 950 cm−1 was
Appendix B: The reaction of CH3NO2 on O-covered Mo(110): the effect of oxygen onproduct distribution 85
Figure B.1: Temperature programmed reaction data showing traces for HCN, H2O, and·CH3 evolution for CH3NO2 on Mo(110)–(1×6)-O (black line) and on 0.4 ML O on Mo(110)(grey line).
observed in addition to the 981 cm−1 peak on the 18O-labeled surface. In addition, the
peak did not shift in studies of CD3NO2 [92]. This indicates that the peak at 950 cm−1 is
not due to any carbon- or hydrogen-containing mode. Although studies of 13CH3NO2 were
not performed on Mo(110)–(1 × 6)-O, there was no shift in the analogous peak assigned
Appendix B: The reaction of CH3NO2 on O-covered Mo(110): the effect of oxygen onproduct distribution 86
Normal mode CH3NO2 on 0.66 ML O CH3NO2 on thin film oxideformed on Mo(110)a
ν(CH) 2960 2960νs(CH) 2925
νas(NO2) 1560 1558νs(NO2) 1375 1383δa(CH3) 1437 1436νs(CH3) 1421 1423r(CH3) 1097 1092
a Vibrational assignments taken from Reference [92]
Table B.1: Assignments for vibrational bands (energies given in cm−1) of molecularCH3NO2.
to terminal oxygen on the thin-film oxide for the 13C-isotope [92], consistent with our
assignment. Peaks at 1290, 1396, 2842, and 2904 cm−1 also appear after heating to 300 K.
All of these peaks are assigned to methylimido (CH3N) based on our previous work (Table
B.2) and on studies of CD3NO2 [92]. The methylimido and terminal oxygen peaks persist
to 500 K (Figure B.2); the terminal oxygen peak sharpens after heating such that two peaks
at 985 and 1010 cm−1 are resolved.
By 800 K, past the point of all low temperature product evolution, the only peaks
visible on the surface correspond to different types of terminal oxygen: 984, 997, and 1026
cm−1. The most intense terminal oxygen peak on O-covered Mo(110) is at 1026 cm−1 which
has been previously assigned to oxygen at step sites [95]. In the reaction of nitromethane
on the thin film oxide, most terminal oxygen resides at terrace sites [92, 95].
Carbon-nitrogen bond retention predominates in nitromethane reaction on all oxi-
dized Mo(110) surfaces studied. Furthermore, methylimido is identified as the intermediate
that yields HCN, H2O and methyl radical in all cases. The primary difference between the
Appendix B: The reaction of CH3NO2 on O-covered Mo(110): the effect of oxygen onproduct distribution 87
Figure B.2: Reflectance absorption infrared spectroscopy data for CH3NO2 on Mo(110)–(1×6)-O in the regions from a) 850–1700 cm−1, and b) 2700–3100 cm−1 (i) as dosed at 100K; followed by annealing to : (ii) 300 K, (iii) 500 K, and (iv) 800 K.
thin-film oxide and the oxygen-covered surfaces is that formaldehyde is only formed on the
thin-film oxide.
The coverage of chemisorbed oxygen affects reactivity and selectivity due to the
strong driving force for Mo bonding to O, C, and N. As the oxygen coverage decreases, the
propensity for O dissociation and C–H bond scission increases. Hence, the yields of HCN,
H2O, and nonselective decomposition products, C, N, and O, all increase. These results
indicate that both overall oxidation state and the presence of vacancies on the surface play
an important role in determining selectivity and reactivity.
Appendix B: The reaction of CH3NO2 on O-covered Mo(110): the effect of oxygen onproduct distribution 88
Normal mode CH3NO2 on Mo(110)– CD3NO2 on Mo(110)– CH3NO2 (CD3NO2) on(1×6)-O (1×6)-O thin film oxide formed
on Mo(110)ρ(CH3) 1072 1034 1072 (1034)ν(CN) 1290 1296 1288 (1296)
δs(CH3) 1396 1088 1396 (1088)νs(CH) 2906 2050 2906 (2050)
2δs(CH3) 2845 2842
a Vibrational assignments taken from Reference [92]
Table B.2: Assignments for vibrational bands (energies given in cm−1) of CH3NO2 adsorbedat 100 K and then annealed to 500 K.
Maintaining a high surface oxidation state is crucial to partial oxidation of hydro-
carbons [96]. This has been demonstrated for the partial oxidation of propene [96] and also
for the reaction of methoxy on two different types of thin film oxides [97, 98]. It is believed
that the higher the oxidation state of the surface the less likely it is that an oxygenate will
become irreversibly bound. The fact that O-covered Mo(110) is essentially zero-valent and,
therefore, binds oxygen very strongly is consistent with the proposal that high oxidation
states are important. In this work, low oxidation state favors C–N bond retention and
oxygen loss to the surface. Conversely, on the higher oxidation state thin film oxide, some
oxygenate is evolved.
Even though high oxidation state is important, population of oxygen in high co-
ordination sites on the surface also mitigates N–O dissociation and C–H activation. As the
coverage of oxygen on the O-covered surface decreases, the temperature of surface mediated
C–H bond scission decreases and the selectivity for C–H bond scission products increases.
Previous work involving alcohols on clean and O-covered Mo(110) confirms that oxygen
Appendix B: The reaction of CH3NO2 on O-covered Mo(110): the effect of oxygen onproduct distribution 89
moderates the high activity of Mo(110) [99, 100, 91]. This may be due to a site blocking
effect in which oxygen blocks sites that would ordinarily activate C–H bonds.
In conclusion, we have found that the reactivity of nitromethane on O-modified
Mo(110) depends on overall oxidation state and distribution of oxygen on the surface. To
confirm the connection between oxygenate evolution and overall surface oxidation states,
further studies are planned on systems with higher oxidation state. The presence of oxygen
on the surface of Mo(110) also moderates C–H bond activation and the propensity for N–O
dissociation.
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