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Optical Field Response from
S tructured Nano- and Mesocomposites
A thesis submitted to the Faculty of Graduate Shidies and Resenrch in parfitai fuifiUment of the requirernents for the degree of
Master of Science
Department of Chemistry
McGU University
Montréal, Québec, Canada
@ March 1998
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Abstract
Molecular self-assembly (MSA) was used to fabricate multilayered composites on
silicon. silicon monoxide and silica substrates. 3-rnercaptopropyltrimethoxysilane
(MPTMS) was grafted ont0 the substrates, followed by chernisorbed silver colloidal
nanoparticles. 1.4-benzenedimethanethiol (BDMT) was used to chemisorb gold to silver
coiioidal nanoparticles forming an MPTMS-silver-methyl-BDMT-gold heterostmcture. A
self-poling stilbazole dye - (E)-[4-NN-(octadecyImethy1mino)styryl]pyridine (ODMASP) - was integrated with a mercaptoacetic acid (MAA) spacer into an MPTMS-
silver-MAA-ODMASP heterostmcture by MSA of ODMASP C i8 dkyl tails. ODMASP
on fractal silver aggregates enhanced the 2nd-order nonlinear susceptibility irnplying that
silver colloid intensified the electrical field to produce a i2' of 1.48 x c'/J' (4.00 x
IO'* esu). We used x-ray photoelectron spectroscopy (XPS), transmission electron
microscopy (TEM), ultraviolet-visible (UV-VIS) spectroscopy and second harmonic
generation (SHG) for characterization. We describeci the spthesis of a tolane dye - N-4- [ ( 4 ' - n i t r o d i p h e n y l e t h y n y l ) p h e n y l ] - b i s - 2 ' (NDPEPD) - by palladium-
cataiyzed cross-coupling. Corona poled NDPEPD-doped poly(styrene) films produced a
i2' of 1.5 x 1 o - ~ c~/J' (0.4 x 1 O* esu).
Résumé
La méthode de l'auto-assemblage moléculaire a été utilisée pour la fabrication de
composites en multicouches sur du silicium, du monoxyde de silicium, ainsi que sur des
substrats de silice. Le composé 3-mercaptopropyltriméthoxysilane (MPTMS) a été greffé
sur ces substrats, suivi de la chémisorption de monoparticules de colloïdes d'argent. Le
composé 1,4- benzénediméthanethiol (BDMT) a été utilisé pour réaliser la chémisorption
d'or sur des nanoparticules colloïdales d'argent, formant ainsi une hétérostructure de
composition MPTMS-argent-méthyle-BMDT-or. Par ailleurs, un colorant pouvant
aligner son moment dipolaire avec un champ dlecuique, le stilbazole - (a-[4-(NJV-
octadecylméthylamino)styryl]pyridine (ODMASP) - a été inséré à I'aide d'un maillon
d'acide mercaptoacétique (AMA) à l'intérieur de l'hétérostructure MPTMS-argent-
AMA-ODMASP par auto-assemblage moléculaire des chaînes alkyles Cis du ODMASP.
La présence d'ODMASP sur des surfaces fractales d'argent a pour effet d'augmenter la
valeur de i2' (susceptibilité non-linéaire de second ordre), impliquant ainsi que les
colloïdes d'argent intensifient le champ électrique produisant un f2' de 1.48 x 10'" c~/J', soit 4.00 x 10" esu. Les techniques de spectroscopie par photoélectrons-X (XPS), de
microscopie à transmission électronique (TEM), de spectroscopie ultraviolet-visible (UV-
VIS) et de génération harmonique de second ordre (SHG) ont éti5 utilisées pour la
caractérisation des materiaux. La synthèse d'un colorant tolane - N4[(4'-
nitrodiphényléthynylephényle]-bis-2"-dieoe (NDPEPD) - par reaction de
couplage catalysée au palladium a été décrite. Les films à base de poly(styrène) dopes au
NDPEPD donnent un 22ZIZ' de 1.5 x 1 0 - ~ c~/J', soit 0.4 x lu8 esu.
I want to express my gratitude to.. . - Dr. Mark Andrews for his enthusiasm, direction and support during my research - Dr. M. A. (Amir) Fardad for his friendship, encouragement and advice - Dr. Norbert Schühler for his fnendship and support in XPS analyses - Dn. Galina Milova, Ignacio Vargas-Baca, Andrew Brown and Xin Min Du for
their friendship, technical support and assistance - Dr. Georges Veilleux (INRS-UQAM) for XPS data and scheduling - Dr. Hojatollah Vali (Department of Earth and Planetary Sciences - McGill), Mn. Jeannie Mui and Matilda Cheung (Department of Anatomy - McGill) for TEM analyses and photo development
- Dn. Mark Kuzyk and Dennis Garvey (Department of Physics - Washington State University) for technical advice
- Dr. Fassil Ghebremichael (US Air Force Academy) for technical advice - Dr. Ashok Kakkar for his advice and support conceming SHG analyses - Drs. George Wong and Jianyao Wu (Materials Research Center, Department of
Physics and Astronomy - Northwestem University) for SHG analyses - Ms. Renée Charron for rnanaging my academic accounu - Emmanuelle for the abstract translation, Lu, Benji, Kevin, Tony for organic
syntheses; Jean and Wenbo for colloid syntheses, Nadim for MS analyses, Ed (Department of Chernical Engineering - McGill) for elemental analyses
- Kalai, James, Max and al1 lab colleagues (Photonic and Polyrneric Materials Research Group - McGill) and Sara, Peyman, Zakaria (Photonics Research Group - École Polytechnique de I'Univenite de Montréal) for their friendship and support
- the following people for friendship, advice and support.. . - William and Vivian, Hany and E.J., Jeff, Chris and Cindy, Amber, Jill, Cathy, Thomas,
Redina, Sam, Kelly and Sonia, Jay, George, James - Maria, Melanie, Gabriel and Linda, Tony, Grant and Francine, Paul, Tem, Myriam, Glen, Sharon, Eric, Nathalie, Tara, Kohei, Frank, Dam, Sherril, Jean-Claude
- Tina, Donna, Giovanna, Elana, Marianne, Eleanor, Paule - GCrard, Christophe, Bianca, Tobi, Kevin, Annie, Maria, Sung Dae, Jung, Josh,
Mariah, lennifer, Caroline, Soraya, Katie, Beth, Annette, Paul, Brian G., Man, David, Martin, Vickie, Karen, Jenny, Elsah, Hannah, Louis, Albert, Brian H
- Rania, Ngiap Kie, Shane, loy, Shanti, Christine, Kui, Carl, Matthew, Neil, Luc, Leujun, Maria, Hongwei, Daniel, Stephanie, Heather, Clare, Lana, Xiao Ming
- al1 the other professors, staff members and studenu at McGill who assisted me
Most importantly, 1 want to thank God, my farnily and all my fnends at home, especially Mom and Dad, Jan, Jeff and JO-Dee for al1 their love and support, encouragement and advice during my studies in Montréal.
Table of Contents
Abstract R6sumé Ackno w ledgrnents Table of Contents List of Tables List of Figures List of Abbreviations
Chapter 1
General Introduction
1.1.1 Overview 1.1.2 Molecular Self-Assembly (MSA) 1.1.3 Nonlinear Optical Organic Dyes 1.1.4 Optical Field Enhancement 1.1.5 Research Applications
Chapter 2
Introduction
2.1.1 Multilayered Composite Materials
2.2.1 Silver Colloid Hydrosol 2.2.2 Gold Colloid Hydrosol 2.2.3 MPTMS Organic Graft 2.2.4 Silver Nanoparticles 2.2.5 Methylation of MPTMS 2.2.6 MAA and BDMT Organic Spacen 2.2.7 Gold Nanoparticles 2.2.8 ODMASP Organic Dye 2.2.9 ODMASP in Poly(styrene) 2.2.10 Characterization
1 *.
11 .*. 111
iv vi
vii
Resuits and Discussion
2.3.1 X-ray Photoelectron Spectroscopy (XPS) 2.3.2 Transmission Electron Microscopy 2.3.1 Ultraviolet-Visible (W-VIS) Spectroscopy 2.3.2 Second Harmonic Generation (SHG)
Conclusions
Chapter 3
Introduction
3.1.1 NLOD Alternative - NDPEPD 3.1.2 NDPEPD Tolane Dye S ynthesis
3.2.1 PD and IPTE - Aryl Iodide Synthesis 3.2.2 NEDPA - Aryl Alkyne Spthesis 3.2.3 PD and IPTE with NEDPA - Palladium-catal yzed Cross-coupling 3.2.4 NDPEPD - Direct Thiolation 3.2.5 NDPEPD in Poly(styrene) 3.2.6 Chamcterization
Results and Discussion
3.3.1 Synthesis - NDPEPD 3.3.2 Cornparison - NDPEPD versus ODMASP
Conclusions
A. 1 Units Conversion A.2 XP Spcu?il Data
Table 1. I : Donor-acceptor systems
Table 1.2: Linear-nonlinear optical applications
Table 1.3: Poynting vector showing metai nanoparticle in electromagnetic field
Table 1.4: Cornparison of different scales of matter
Table 2.1 : MPTMS-silver-meth y 1-BDMT-gold heterostmcture on silicon wafer
Table 2.2: Elernental binding enrgy - MPTMS-silver-methyl-BDMT-gold heterostructure
on silicon wafer
Table 2.3a: MPTMS-methyl-silver-BDMT-gold heterostructure on silica glas slide
Table 2.3b: MPTMS-silver-MAA-ODMASP heterostructure on silica glas slide
Table 2.4: i2' - MPTMS-silver-MAA-ODMASP, MPTMS-silver and PS-DMASP
heterostructures
Table 3.1 : MAA and ODMASP - solvent, pH and temperature
Table 3.2: An, and P - tolane derivatives
Table 3.3: A.- and - stil bazole derivatives
Table 3.4: i2' - NDPEPD and DMASP in poly(styrene)
Table A- 1 : Unit Conversions
Table A-2: SHG measurements for MPTMS-silver-MAA-ODMASP, MPTMS-silver and
PS -DMAS P heterostructures
List of Figures
Figure 1.1 : Electrical field poling in doped NLOD polymers
Figure 1.2: Second harmonic generation (SHG)
Figure 1.3: Metal nanoparticle in electrical field (at Frohlich frequency)
Figure 2.1 : Schematic - MPTMS-silver-methyl-BDMT-gold heterostnicture on silicon
wafer and silicon monoxide grid
Figure 2.2: Schematic - MPTMS-silver-MAA-ODMASP composite on silica glass slide
Figure 2.3: Glass reactor vesse1 with silica glass slides in PTFE support rack
Figure 2.4: Fiowsheet - MPTMS-silver-methyl-BDMT-gold on silicon wafer and silicon
monoxide grids
Figure 2Sa: TE micrograph - MPTMS-silver heterostmcture on silicon monoxide grid
(82500 X magnification)
Figure 2.5b: TE micrograph - MPTMS-silver-methyl-BDMT-gold heierostructure on
silicon monoxide grid (82500 X magnification)
Figure 2.5~: TE micrograph - MPIhlS-silver-methyLBDMT-goid heterostmcture on
silicon monoxide grid (165000 X magnification)
Figure 2.6a: Histogram - silver nanoparticle size distribution
Figure 2.6b: Histogram - gold nanoparticle size distribution
Figure 2.7a: Logarithmic plot - silver nanoparticle h t a l dimension
Figure 2.7b: Logarithrnic plot - gold nanoparticle fractal dimension
Figure 2.8: Flowsheet - MPTMS-silver-MAA-ODMASP heterostnicture on silica glas
slide
Figure 2.9a: UV-VIS spectrurn - gold + BDMT (in solution)
Figure 2.9b: UV-VIS spectrurn - MPTMS-silverlgold-BDMT on silica glass slide
Figure 2.9~: W-VIS spectnim - MPTMS-silver-BDMT on silica glass slide
Figure 2.9d: UV-VIS spectrurn - MPTMS-silver-methyl-BDMT-gold on silica g l a s slide
Figure 2.10: ODMASP charge transfer between PR* States
Figure 2.1 1 : ODMASP chernisorbed onto MAA (acid-base)
Figure 2.12: W-VIS spectra - MPTMS-silver-MAA-ODMASP composite
Figure 2.13: ODMASP resonance (fundamental and second barmonic frequency)
vii
Figure 3.1 : Schematic - NDPEPD synthesis
Figure 3.2: Schematic - NDPEPD direct thiolation
Figwe 3.3: M>PEPD ground and excited (R-n*) states
Figure 3.4: ODMASP ground and excited (n-n*) states
Figure 3.5: NDPEPD and ODMASP orbital configurations
Figure 3.6: W-VIS spectnim - NDPEPD tolane in methanol
Figure A- l(contro1): XP spectrum - MPTMS + gold (Control)
Figure A-la: XP spectrum - MPTMS (exposed to air) (Sample A)
Figure A-lb: XP spectrum - MPTMS (unexposed to air) (Sample B)
Figure A-lc: XP spectrum - MPTMS-methyl (Sample C)
Figure A-Id: XP spectrum - MPTMS-methyl-silver (Sarnple D)
Figure A-le: XP spectnim - MPTMS-silver (Saniple E)
Figure A- I F: XP spectmm - MPTMS-silver-meth yl-gold (Sarnple F)
Figure A- 1 g: XP spectrum - MPTMS-silver-me th y l-BDMT (Sampie G)
Figure A-1 h: XP spectrum - MPTMS-silver-methyl-BDMT-gold (Sample H)
Figure A-Z(contro1): XP (S 2p) spectnim - MPTMS + gold (Control)
Figure A-2a: XP (S 2p) spectrum - MPTMS (exposed to air) (Sample A)
Figure A-2b: XP (S 2p) spectmm - MfTMS (unexposed to air) (Sample B)
Figure A-2c: XP (S 2p) spectnirn - MPTMS-mer h y1 (Sample C)
Figure A-2d: XP (S 2p) spectmm - MP'I'MS-merh yl-silver (Sample D)
Figure A-2e: XP (S 2p) spectrum - MPTMS-silvcr (Sample E)
Figure A-2f: XP (S 2p) spectrum - MPTMS-silver-methyl-gold (Sarnple F)
Figure A-2g: XP (S 2p) spectnrm - MPTMS-silver-methylBDMT (Sample G)
Figure A-2h: XP (S 2p) spectmm - MPTMS-silvcr-methyl-BDMT-gold (Sample H)
Figure A-3h: XP (Ag 3d) spectrum - MFTMS-si l ver-methyl-BDMT-gold (Sarnple H)
Figure A-4f: XP (Au 4f) spectrum - MPTMS-si1 ver-methyl-gold (Sample F)
Figure A-4h: XP (Au 4f) spectrum - MP'ïMS -s i : v er-methyl-BDMT-gold (Sarnple H)
List of Abbreviations
BDMT - t ,&benzenedimethanethiol BDT - I ,4-benzenedithiol
DMAP - 4iiimethylaminopyridine
DMASP - (E)-[4-(Nfl-dimeth ylamino)] styrylp yridine
'H NMR spectroscopy - proton nuclear magnetic resonance spectroscopy
PD - 4-iodophenyl-bis-2'-diethylethanoatylamine
PTE - 4-iodophenyl-bis-2'-(tert-butyldimethylsilylo~y)e~yl~ne LR - Lawesson's Reagent
MAA - mercaptoacetic acid
MPTMS - 3-mercaptopropyltrimethoxysilane MS - mass spectrometry
MSA - molecular self-assembly
NEDPA - 4-nitro-4'-ethynyldiphenylacetylene NDPEPD - N4[(J'-ni~odiphenylethynyI)phenyI]-bi~-Z'-diethanethioIarnine NDPEPPD - N4[(4'-nitrodiphenylethynyl)phenyl)phenyl]-bi~-2'-die~yle~moatyl~ne
NDPEPPTE - N - 4 - [ ( 4 ' - n i t r o d i p h e n y l e t h y n y 1 ) p h e n y l ) p ~ y l -
sily1oxy)ethylarnine
NLOD - nonlinear optical dye
OCB - optical chernical bench
ODM ASP - (E)-[4-(N&-octadecylmethylamino)s~l]pyridine PDEA - N-phenyldiethanolamine
PMT - photomultiplier tube
PS - poly(styrene)
F E E - poly(tetrafluoroethy1ene) gITPP - palladium tetra(tripheny1phosphine)
PDCDPP - palladium dichlomdiphenylphosphine
SHG - second hatmonic generation
TBA- AF - tetrabuty lammonium chlorideff luoride
TBDMS - tert-butyldimethylsilyl
TEM - transmission efectron microscopy
TMAF - tetramethylammonium fluoride
UV-VIS spectroscopy - ultraviolet-visible spectroscopy
XPS - x-ray photoelectron spectroscopy
" To solemnize this &y the glorious Sun
Stqs ni his course, and p l 9 the a l c h i s r ,
Turning, with qdendour of his precious eye,
nie meagre cloddy earth to glittertng gold. "
King Philip of France - King John, Act III, Scene 1.
William Shakespeare ( 1 564- 16 16)
Chapter 1
General Introduction
Photonic materials are central to the research desdbed in this thesis. Photonics requires
opticai input/output processes using photons instead of electrons.' In photonic materials
chemistry, we endeavor to undentand submicron level events that describe the nature of
material optical properties. Among these properties is optical nonlinearity, which is of
scientific and technoiogical interest? We investigate optical nonlinearity in some detail
later (see Section 1.1.2).
In this thesis, we explore methods to chemisorb colloidal metal nanoparticles onto
various silicon-based surfaces by covalent bonding to organic 3-mercaptopropyl-
trimethoxysilane (MPTMS), grafted ont0 the glass. We select metal particles with
diameters on the order of 4.5- 16.25 nm to intensify laser optical field interactions,
panicularly at the metd plasma frequency. We investigate the possibility of enhancing
the optical nonlinearity of nonlinear optical dyes (NLODs) chemisorbed ont0
nanoparticle surfaces.
This thesis establishes protocols to chemisorb metal nanoparticles to opticd waveguide
surfaces. These surfaces include silicon wafer, silicon monoxide coated TEM grids and
silica glas slides.
Accordingly, the objectives of this thesis are as follows:
1) To assemble nano- and mesoscale multilayered composites composed of silver and
gold nanoparticles. organic spacers and NLODs (ODMASP stilbazole),
2) To demonstrate that gold nanoparticles can be chemisorbed onto silver. selectively.
3) To illustrate 22' enhancement of an NLOD (ODMASP stilbazole) fkom silver
nanoparticles on an optical cheMcal bench (OCB),
4) To synthesize a novel NLOD (MIPEPD tolane) that chemisorbs onto silver or gold
nanoparticle surfaces.
In Section 1.1.2, we provide a definition of molecular self-assembly with examples.
Section 1.1.3 follows with a brief review of the linear and nonlinear optical response of
organic NLODs. We then describe in Section 1.1.4 the enhancement of the local
electromagnetic field sunounding an isolated metal particle where we allude to
ODMASP local field enhancement h m silver surface plasmons. Finally, Section 1.1.5
diagrams the modified OCB waveguide required to perform our f 2 ) nonlinear optical
measurements and outlines the applications of our research.
in Chapter 2, we outline 2 main types of multilayered composites with organothiol
spacers 1,4-benzenedimethanethiol (BDMT) and mercaptoacetic acid ( M M ) .
1) MPTMS-silver-methyl-BDMT-gold heterostmcture on silicon wafer, silicon
monoxide @ds and silica glass slides.
2) MPTMS-silver-MAA-ODMASP heterostnicture on silica glas slides.
We anaiyze the rnultilayer growth sequence of silver-BDMT-gold by XPS and TEM and
compare W-VIS and ft'" nonlinear optical responses fiom SHG in the MPTMS-silver-
MAA-ODMASP heterostrucnire. In Chapter 3, we describe the synthesis of an M D D
(NDPEPD tolane) that chemisorbs ont0 silver without the use of an MAA spacer. We
compare i2) of both doped ODMASP and doped NDPEPD in poly(styrene) or
PS-ODMASP and PS-NDPEPD composites.
1.1.2 Molecular Self-Assembly (MSA)
Ordered organic thin films show considerable scientific and technological promise.
Several examples include thin-film optics, sensors and tramducers, photoresists and
surface coatings (for lubrication, adhesion and ~ e t t i n ~ ) . ~ These applications require
ordered patteming of molecular layers on the surface. Ordered monolayen can arise by a
process termed molecular self-assembly (MSA), which may involve the adsorption of
alkyl acids, alkylthiols or alkylchlorosilanes on noble rnetals such as silver, gold and
oxides such as silica4 In Our research, we use ordered monolayen to constmct intricate
multilayered heterostructuns. MSA exploits the chemical interactions between the
molecular adsorbate and the substrate so that spontaneous assembly occun. Chernical
kinetics and themodynarnics control the rate and stability of an assembled structure.
Ordering in monolayers increases with dkyl length. Hydrophobie alkyl tails of at Ieast
C12 assemble easily in a tram configuration.' Adsorption cnergy increases linearly with
alkyl Iength as described b y
where -A& is the adsorption free energy, - d e h is the adsorption free energy per
methylene group, Nc is the number of carbons in the alkyl and W is the energy per
methylene group.6 The typical adsorption energy for alkylthiols is approximately 750
1mo1-' per methylene group. We focus on chemical adsorption or chernisorption of
organothiols ont0 metal surfaces using MSA. Assembly of organothiol on silver and gold
metai d a c e s has k e n previously reviewed?
In out research, we exploit MSA to self-pole ODMASP and fonn an ordered structure.
We use MSA to promote the self-poling of ODMASP required for second hannonic
generation (SHG). MSA requins no electrical poling to dign ODMASP dipoles since we
exploit the favorable themodynamics associated with the ordering of hydrophobie Ci8
alkyl tails. We pursue the origin of SHG in the following section.
1.1.3 Nonünear Optical Organic Dyes
Nonbear Optics
Nonlinear optical processes depend on microscopic (molecular) and mucroscopic (buik)
polarizability of matter.
(i) Microscopic PolPrizPbüity
At the microscopic level. the poiarizabiiity, pi, of a medium cm be represented by a
Taylor series expansion?
where a,, fijk and are the linear polarizability, fint and second hyperpolarizability
tenson, respectively (in the molecular frame of reference, i-j-k). These tensors relate the
induced polarization, pi, to the electrical field components, Ei, E,, Ek and El.
(ü) Macroscopic Polarizabüity
At the macroscopic level of bulk matter, the polarizability of a medium P I , is similarly
exprcssed as a Taylor series expansion?
nd rd where if)!,, i2)IJx and i3)[ja are nonlinear susceptibility tensors of 2 , 3 and 4' rank.
These tensors relate the induced polarization, PI, to the electrical field components Ei, E,,
Ek and El. E represents the electrical field of a plane of light that may be expressed as
E = Eocos(an), where o represents the electricai field frequency and t represents time?
The l t , 2nd- and 3dsrder nonlinear susceptibilities are related to Eo cos(a*), Eo cos(2air)
and Eo cos(3@), respective1 y.
Even-ordered ternis of the nonlinear susceptibility ~ 2 ' I J f i i 4 '~ ,~ , ...) are non-zero only
for anisotropic media (noncentrosymmetxic - with no inversion center). Nomally, we
nquire electrical field poling to achieve i2' for noncentrosymmetry. Otherwise,
centrosymmetric dye dipoles would cancel and, therefore, /3= O and i2' would not exist.
We use electrical poling by electrical point corona discharge to dign dipoles in the PS-
ODMASP and PS-NDPEPD composites while we simultaneously monitor XI2' using
SHG analYsis.'O
Heating is supplied until temperatures graduaily approach the polymer glass transition
temperature, Tg, or the polymer softening temperature (see Figure 1.1). Polymer softening
allows ODMASP or NDPEPD dipoles to align (by rotation and translation in the polymer
matnx) under an applied electrical field. Dipoles continue to align until the Tg is reached.
Afterwards, the temperature is gradually decreased to room temperature with the applied
field on. These dipoles now remain aligned or poled inside the cooled and hardened
pol ymer.
increase temperature
poling
T<T,
decrease temperature
Figure 1.1: Electrical field poling in doped NLOD polymers
As described in Section 3.2.6, we use corona discharge poling in our PS-ODMASP and
PS-NDPEPD composites to align dipoles. However, we may completely circumvent
elecûical poling by nsorting to MSA. We chwse to use self-poling ODMASP to align
ODMASP diples on a silver nanoparticle surface without the electrical poling
cequirement. By self-poling, we obtain noncentrosymmetry and thus, may measure f2' .
In this thesis we are interested in SHG processes. Optical SHG is a nonlinear optical
process that converts photons at frequency a, to produce one photon at ftequency 2a.
Figure 1.2 diagrams the frequency conversion.
Figure 1.2: Second harmonic generation (SHG)
201
dl
In particular, we are interested in SHG at the silver-ODMASP interface on our
multilayered silica glass slide composites.
nonlincar opticai medium
Orgaaic Dyes
For practical applications. organic NLODs should have environmental thermal stability
and no loss of polar orientational order. Moreover. NLODs must be processible to gain
acceptance in optical device applications. Organic ODMASP and NDPEPD are of
interest since they filfi11 these materials desiderata. In addition, improvements in
molecular design and synthesis inspires us to use ODMASP and NDPEPD.
In ODMASP and NDPEPD. nonlinear optical effects originate in donor-acceptor charge
transfer interactions." Donor functional groups donate electron density and acceptor
hinctional groups accept electron density to polarize the dipole asymmetrically
(polarization in opposite directions). A conjugated carbon structure mediates electron
charge m s f e r through pn-electron orbital delocalizations. Table 1.1 lis& typical donor-
acceptor s ystems.
Table 1.1
Donor-acceptor systems (reproduced from Nalwa et al. 12)
- -- - -
Donor px-structure ~cceptor
m2, mCH3, benzene NOz, NO, CN. OOH, N(w312, m, azobenzene COO-, CONH2, N2H3, Ft Cl, Br, 1, s tilbazole CONHR. CONR2, SH, SR. OR, CH3, tolane CHO, SSI, S02R, OH, NWCOCH3, biphen y1 so2c3F7, s-3.
OCH,. SCH3, benzy lidene COR, COCF3, CF3. oc6k c(cH3)3, polyene COCH3, H=C(CN)2,
COOCHs, O-, S' and C2(CN)3, SaNH2, aromatic N2'. W+, N(CH3);.
and aromatic
Many donor-acceptor systems exhibit optical nonlinearity that has potential in optical
device applications. Outiined in Table 1.2 are relevant examples.
Table 1.2
Linear-nonlinear optical applications (reproduced from Chemla and zysd3)
Order Macroscopic Polarizability Effects Application susce~tibilitv
1 %lfj a re flection optical fibers
2 0' f l second hannonic fnquenc y doublers generation
w+a,+2cu
frequency rnixing optical mixers ar *@ + 0 3
parme tric opticai pararnetric amplification oscillators a 3 + @I + cuz
Pockels effect electro-optical c l l + o + w modulators
Y 4-wave mixing Raman coherent spec troscop y
phase gratings reai time holography
Kerr effect ultra-high speed optical gates
optical bistability amplifiers, amplitude choppers, logic gates
1.1.4 Optical Field Enhancement
Experiments enhancing optical field interactions are well hown. These experiments have
used surface-enhanced Raman scattenng (SERS)'~ and surface-enhanced second
harmonic generation (SESHG)" with noble metal nanoparticles and films. It is
ceasonable to assume that optical fields derived from these metals may enhance the
2"-order susceptibility of ODMASP. Noble metals such as silver and gold may enhance
the optical field by exciting intense nonlinear sources near the metd surface. These
nonlinear sources are due to surface plasmons. In principle, surface plasmons are
responsible for a large electromagnetic field confined to the silver interface. A surface-
propagating mode radiates a nonlinear polarization field. This surface mode may be used
to increase the local fields near an organic dye such as ODMASP, and therefore, the i2' nonlinear optical susceptibility as well. We describe these surface plasmons and local
fields in the following section.
Local Field Factors
We may show that local field factors contribute to increase the i2' optical nonlinearity.
Initially, we cm associate local field factors and optical nonlinearity in the following
equations.16 We may relate the 1"-order nonlinear susceptibility, fi', to the microscopic
polarizability, a, through
2" = NaFIFI 141
where N is the NLOD number density and FI and FI are the local field factors. The local
field factors are tenson in the buik hune of reference. IJ-K. Similarly, we can relate the
2"d- and 3%rder nonlinear susceptibilities, i2', and i3), to the lst- and 2d-order
rnicroscopic hyperpolarizabilities, fl, and y, respectively, through
We note that the local electrical fields, FI, F,, FK and FL, differ from the applied field, E.
These local fields arise from 2 sources: the applied field and the field generated by the
dye that is polarized by the applied field. Therefore. the fields FI and E, are reiated to Lu
by
where LIJ is the 2nd-rank local field tensor, FI is the local field and EJ is the applied field.
From standard micro- and macroscopic nonlinear susceptibility equations, it can be
shown that
f $k = dVP',d) 181
where N is the dye nurnber density and ('2'qk is the znd-order local field susceptibility for
k2'. The tensor function <''k may be described according to
<"& = (Lt, j* ,Ljj;&*)}f2'i ,2.
where the exponential function i2' depends on the local field ten SOB Liie ,Lj* and Lu*. We
see therefore, that the bulk nonlinear susceptibility is a strong function of the local field
tenson. Thus, i2' increases by the 3d power. Therefore. small increases in ODMASP
local fields c m result in large increases in i2'. Thus, we may enhance the optical
nonlinearity of the silver-ODMASP composites through a local field effect.
Surface Plasmons
The way that surface plasmons at a metal surface can be used to enhance ODMASP local
fields is discussed in this section. Quantum mechanics describes surface plasmons as
quantized electmmagnetic plasma waves that arise from resonant couplings with the
conduction electron gas or plasma (quanta of free electrons) oscillation^.'^ Classical
mechanics describes a surface plasmon in terms of a simple hannonic oscillator
approximation.
With the Drude model. we may represent the optical behavior of a free electron metd by
where E is the dielecbic function with plasmon frequency, q,, and damping factor, y.
We defined the plasma frequency through a+2 = fVe2/ma, where N is the number density
of free electrons and m is the effective mass of an electron. Longitudinal electrical field
oscillations occur when d = q,2 and da) = O. These oscillations result from
electrornagnetic fields interacting on the surface of the metai. Figure 1.3 depicts the
Poynting vector that represents the magnitude and direction of the rate of transfer of
electrornagnetic energy around a metal pariicle.'8 At the surface plasmon energy, field
lines distort near the metal particle. The distorted field increases the electromagnetic field
emanating from the particle surface. ~ i e ' ~ first analyzed linear optical responses of metal
nanoparticles. Mie theory accounts for light absorption and scattering from particles
under applied fields. Typically, gold hydrosols show surface plasmon absorbance, where
gold nanoparticles (eu. 5.2-20 nm particle diameter) have a maximum absorbance (ca.
520 nm)?' In addition, silver hydrosols ai about 410 nm aiso show a surface plasmon
absorbance (ca. 8-10 nm particle diameter) have a maximum absorbance (CU. 410 nm).*'
These nanoparticles may be integrated with molecular NLODs to form assembled
multilayers. We describe the applications of multilayered composite films in the
following section.
Figure 13: Poynting vector showing a metai nanoparticle in an electrornajpetic field
1.1.5 Research Applications
Ordered multilayered composite films ranging in thickness from a few nanometers (a
monolayer) to several hundred nanometers. show considerable technological promise.
Electronic and optical devices presently incorporate structures that are in this range.
Composite film heterostnictures have been proposed to replace both passive and active
components traditionally fabncated with other materials. Some applications include
optical waveguides, sensors, detectors, displays and photoresists.? In order to fabricate
these devices, we need to draw upon 2 emerging field disciplines - materials chemistry
and photonics.
Nano- and M e s o d e
Our intenst is to develop composite heterostnictures from nano- and mesoscale matter.
Nanoscale structures or nanostructures occupy nanometer scale dimensions of 1 - 100 nm.
Similarly, mesostructures occupy nanometer scale dimensions of 10' nm (approximately
100-500 nm)? Figure 1.4 classifies these dimensions. Materials chernistry allows us to
manipulate and construct matter using the "tools" of MSA (see Section 1.1.1). Now, we
may see how MSA may be used to fabricate nano- and mesoscale matter. In the present
research, we use MSA to assemble nanoscale ODMASP and NDPEPD into multilayered
heterostructures. Colloidal nanoparticles occupy mesoscale dimensions and are
chemisorbed onto grafted MPTMS
) Nanoscale Mesoscale Microscale Macroscale I
1 - 1 0 nrn - 100 nm ci~un 1 p l 0 mm
Figure 1.4: Cornparison of different scaies of matter
OpticaI Chernid Benches
Chemical surface reactions cm be probed by combining techniques of integrated optics
with surface spectroscopy. To do so we use microscopic laboratories (optical chernical
benches or OCBs) where we combine the chemical and spectral domains by carrying out
research in both using an optical waveguide. In fact, the OCB is a modified waveguide,
whose surface acts as a "benchtop" to perform chemical reactions involving assembled
nano- and mesostructum. Figure 1.5 shows an OCB acting as an opticai waveguide with
n, representing the waveguide substrate and ni repnsenting the optical film composite.
Total internai reflection occurs inside the wavepide." At the film-substrate interface el > sin"(nrhz) and at the air-substrate interface 6 > sin-'(n3/n2).
Figure 1.5: Opticai Chemical Bench Waveguide
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Cbapter 2
Introduction
2.1.1 Muitiiayered Composite Materiils
In this chapter, we use some of the MSA pnnciples outlined in Chapter 1. We reiierate
Our objectives here. Firstly, we show how nano- and mesoscale multilayer hetero-
structures are assembled. We use silver and gold nanoparticles, organic spacen and
ODMASP stilbazole to compose these heterostnictures. Secondly, we illustrate how
silver nanoparticles can be selectively chemisorbed ont0 gold nanoparticle surfaces.
Thirdiy, we demonstrate how i21f, is enhanced from ODMASP on silver nanoparticles
using OCB substrates. 8
In this section, we diagram the multilayend construction of each composite. Figure 2.1
outlines the development of the MPTMS-silver-rnethyl-BDMT-goid heterostructure. We
show the reaction sequence in each stage. Figure 2.1 a shows the initial grafting of
MPThlS on the substrate surface. Figure 2.1 b shows the chernisorption of silver
nanoparticles onto the MPTMS tenninal sulfhydryl groups, followed by rnethylation (see
Figure 2. lc). Methylation covers unreacted terminai sulfhydryl. Figure 2. Id shows the
chemisorption of the BDMT spacer ont0 silver followed by gold nanoparticles in Figure
2. le. Finally, Figure 2.lf shows the complete heterostructure. We use XPS. TEM and
W-VIS spectroscopy to characterize the composite heterosuuctures on silicon wafer,
silicon monoxide TEM grids and silica glas slides.
Figure 2.2 outlines the development of the MPTMS-silver-MAA-ODMASP hetero-
structure. Again, Figure 2.2a shows the grafting of MPTMS. Next. Figure 2.2b shows the
chemisorption of silver nanoparticles, foilowed by the chemisorption of the MAA spacer
ont0 silver (see Figure 2.2~). Figure 2.2d shows ionic chemisorption of ODMASP onto
MAA by acid-base reaction. Finaily, Figure 2.2e shows the complete heterostnicture.
We use UV-VIS spectroscopy and SHG to characterize the composite on silica glas and
FO-glas slides.
To constnict these composite heterostruchires, we initially "acid-etch" the substrate by
oxidation and hydrolysis of surface silica to give silanol at the air-silica interface.' Then
we derivatize the substrate by grafting an hdPTMS subsenictural layer. We graft MPTMS
onto the substrate surface by alcohol condensation reactions. These reactions involve
MPTMS methoxy groups reacting with silanol groups to form siloxane bridges (Si-O-Si)
and methanol by nucleophilic silanol addition ont0 MPTMS silicon. Hydrolysis involves
adventitious water on the substrate surface replacing the MPTMS methoxy groups with
hydroxyl groups and forming water. We exploit the property of interfacial surface
adhesion with MPTMS. Thus, an MPTMS grafi layer forms a substnicture in which we
may introduce other subsequent adlayen on top to fabricate the complete multilayered
heterostructure.
Glassware was cleaned in alcoholic sodium hydroxide (ethanol (95 % IV)-water-sodium
hydroxide, 10: 1 : 1 v/v/w) for 12 houn and then rinsed with hydrochloric acid soiution
(10 % IV). Cleaned glassware was then rinsed with distilled ~ i l l i ~ o r e @ water and dried in
an oven at 150" C for 30 minutes. Hydrosols were prepared in distilled water obtained
from the Milli-Q-Ultrapure ~ i l l i ~ o r e ~ ~ a t e r S ystem (22 ~ i l l i ~ a k @ filter).
Metal Colioid - Hydrosol Formation
2.2.1 Süver CoiIoid Hydrosol
Materiais
Silver nitrate (99.98 95) was acquired from Engelhard Industries of Canada Sodium
borohydride (98 8) was obtained from Aldrich Chernical.
Methoci
Silver colloid hydrosols wen prepared using the method of Creighton et al.' An ice bath
was prepared to hold a 500 mL Erlenmeyer reaction flask. ~ i l l i ~ o r e ~ water (1 L) was
collected in a IL Erlenmeyer flask. In a separate 50 mL stoppered Erlenmeyer flask,
silver nitrate (3.93 x 10 -~ mrnol, 6.66 mg) was weighed (with minimum exposure to
light). Sodium borohydnde (9.3 1 x lu2 rnrnol, 3 S O mg) was weighed in another 50 rnL
Erlenmeyer flask and transferred to the 500 mL, Erlenmeyer reaction flask with distilled
water (75 mL). The silver nitrate was dissolved in water (1 8 mL) and refrigerated for 15-
20 minutes. Afterwards, the 500 rnL Erlenmeyer reaction flask (containing the sodium
borohydride solution) was stirred. Silver nitrate solution (9 mL) was added into the
reaction flask at a rate of 1 drop per second using a 10 rnL pipette. Stimng was continued
for 45 minutes until a deep golden-yellow colored solution was fonned. The hydrosol
was covered and stored in the nfrigerator.
2.2.2 Gold Coiloid EiydtosoI
Materials
Hydrogen tetrachloroaurate hydrate (99.999 95) was obtained from Aldrich Chemical.
Sodium citrate (99.0 9) was acquired from A&C American Chernicais.
Method
Gold colloid hydrosols were prepared by an adaptation of the rnethod of Weia et al,) In a
250 mL Erlenmeyer flask, hydrogen temhloroaurate hydrate (0.100 m o l , 3.41 x
1 0-2 g) was measured and dissolved in water (100 m. ) . Sodium citrate ( 1 J O m o l ,
0.50 g) was dissolved in water (100 mL). In another 250 mL Erlenmeyer reaction flask
hydrogen tetrachloroaurate hydrate solution (5 mL) was added to water (90 mL). The
solution was heated to 90" C for 15 minutes with continuous stimng. Sodium citrate
solution (5 rnL. 0.5% w/w) was immediaiely added at a rate of 1 drop per second using a
5 mL pipette. Heating was continued at 90' C for 30 minutes. The resulting deep rosé-
pink colored hydrosol solution was gradually cooled in an ice bath, covered and stored in
the refrigerator.
Muitiiayered Composite - Heterostructure Fabrication
Multilayered composites were fabricated using the method of Andrews et d4 Al1 reactor
vessel glassware (Figure 2.3) was soaked in an alcoholic sodium hydroxide solution
(ethanol(95 % /v)-water-sodium hydroxide, 10: 1 : 1 v/v/w) for 12 hours and rinsed in
hydrochionc acid solution (10 96 vlv). Afterwards, the glassware was rinsed with distilled
~ i l l i ~ o r e @ water and dried in an oven at 150' C for 30 minutes. A Schlenk line apparatus
was used to perform d1 experiments involving the reactor vessel under a nitrogen
atmosphere.
MPTMS was initially grafted ont0 silicon wafer, silicon monoxide grids and silica glass
slides. MPTMS terminal sulthydryl groups were exposed on the surface.
Substrates
Silica glass slides (23 x 10 x 0.7 mm) and silicon wafen (<LOO> crystal plane) were
obtained from VWR Scientific Instruments and Si-Tcch Limited, respective1 y.
Transmission electron microscope (TEM) copper grids (300 mesh - surface-treated with
silicon monoxide to form coated silica) were received from Soquelec Limited. The TEM
grids are referred to as silicon monoxide grids.
Materiais
Sulfunc acid (36.5-38.0 46 IV) was purchased from J. T. Baker and hydrogen peroxide
(30 % IV) was obtained fiom BDH. 3-mercaptopropyltrimethoxysilane (MPTMS) was
acquired from United Chemicai Technologies and used without further purification.
Acetone (99.8 %), chloroform (99.5 1). toluene (99.8 %) and hexane (98.5 %) were
purchased from Caledon Laboratories. Toluene and hexane were further distilled fiom
calcium hydride and dried over sodium under a nitrogen atmosphere.
rerictcir vesse1 with silica glass slides in P T E support rack
Glass reactor vessel labels
1 A - silica glass slides
1 B - PTFE support rack
1 C - reactor vessel (bonom)
1 D - silicone rubber O-ring
1 E - reactor vesse1 (top)
F - side opening
G - PTFE key valve (side opening)
H - top opening (with silicon rubber O-ring) to condenser and Schlenk linc
Method
(a) Substrate Cleaasing
Silica glas slides were inserted vertically in a poly(tetrafiuoroethylene) (PTFE) support
rack (B). Aitematively, silicon <100> crystal plane wafer fragments (23 x 10 mm) cut
from full silicon wafers with a diarnond-tipped pen were inserted venically into the
support rack. Silicon monoxide grids were held in PTFE capsules and then placed into
the support rack.
The PTFE support rack was then placed into a glass reactor vessel (C) (Figure 2.3). A
pironha solution was prepared using concentrated sulfkic acid (H2SO4) and hydrogen
peroxide (H202) (1 : 1 V/V) and was added (50 mL) carefully into the glas reactor vessel
and heated for 1-2 hours (silicon monoxide grids required only 5 minutes of heating).
NOTE: Extreme caution is required (face shield and gloves). Afterwards, the solution
was carefully poured out while manipulating the support rack with PTFE tweezers. The
substrates were immersed in ~ i l l i ~ o r e ' water and the PTFE support rack was spun for 5
minutes to rinse the substrates. After rinsing, the water was then carefully poured out
while supporting the support rack with tweezers. The rinse procedure was repeated 3
times with ~il l i~ore@ water. Aftewards, acetone was adàed to the reactor vessel and the
support rack containing the samples was spun for an additional 5 minutes. Silicon
monoxide grids were f'urther rinsed wiih chloroform. The acetone was decanted while
retaining the rack with tweezers. Acetone was evaporated by spinning the support rack in
the reactor vessel under an air atmosphere for 10 minutes.
(b) Substrate Derivatization with MPTMS
The reactor vessel was equipped with a water condenser placed into the top opening (H)
of the reactor vessel as in Figure 2.3. The condenser was attached to a Schlenk vacuum
line apparatus. The vessel was clamped and sealed by a silicone rubber O-ring (D) so that
reactor vessel bottom (C) and top (E) were secured and air-tight. Then the vessel was
dned by vacuum and mild heat (with a heat gun) and filled with a nitrogen aimosphere.
Toluene (50 mL) was syringed under a nitrogen flow through the valve (G) into the
reactor vessel containing the silicon wafer, silicon monoxide gids or silica glass slides.
The syringe was inserted through a septum into the side opening (F) of the reactor vessel
(see Figure 2.3). MPTMS (1 mL) was syringed into the reactor vessel through the side
opening (F) as well (NOTE: Ptior to use, MPTMS was transferred into a dry 50 mL side-
amed flask and stored under nitrogen). After addition of MPTMS, the MPTMS-toluene
solution was heated to 40-50' C and the support rack was spun continuously for 20-24
houn under a nitrogen atmosphere. The MPTMS-toluene solution was then aliowed to
cool. The condenser was removed from the top opening (H) of the reactor vessel (while
under a nitrogen flow). A septum was installed to seal the opening. MPTMS-toluene was
cannulated out through the top opening (H) under a nitrogen flow. The substrates were
rinsed 3 times with toluene by syringe (to add toluene) and cannula (to remove toluene).
After the final rinse, the substrates were dried under vacuum and stored in the reactor
vessel under nitrogen.
Modifications of this procedure were required for silicon wafers and silicon monoxide
grids. Silicon wafers containing the MPTMS-toluene solution were heated for 12 houn
instead of 20-24 hours. Silicon monoxide gids were plred into PTFE capsules
(uncapped) and onented verticaily in the nactor vessel (C) on top of the support rack (B).
Hexane (25 rnL) was added instead of toluene. MPTMS (0.5 mL) was added and the
MPTMS-hexane solution was heated to 30-35" C for 12-16 hours.
2.24 Silver Nanoparticies
Silver nanoparticles were chernisorbed ont0 MPTMS terminal sulfhydryl groups.
Method
The silica glas slides in the PTFE support rack were deposited into silver hydrosol in
another nactor vessel containing silver coiloid hydrosol and soaked (without stirring) for
12 hours at room temperatun. The color of the solution changed h.om golden-yellow to
aubunisrange. Substrates were removed and then rinsed once with ~i l l i~ore@ water
from a Pasteur pipette. The support rack containing the substrates was stored under
nitrogen. The silicon wafers were treated in the same way as the silica glas slides. The
solution changed in color from golden-yeiiow to charcoal-gray. The silicon monoxide
grids were treated in the following way. The g d s were placed into individual PTFE
capsules (capped) containing silver colloid hydrosol instead of being placed into the
reactor vessel. The solution changed color from golden-yellow to auburn-orange. After
thorough rinsing with ~ i l l i ~ o r e " water, the grids were placed into the capsules
(uncapped) and seaied again in the reactor vesse1 under a nitrogen atmosphere as
previously descnbed.
2.2.5 Methylation of MITMS
Where required, MPTMS substrate grafts were methylated by reaction with
dimethy lsulfate.
Materiais
Toluene (99.8 %) and uiethylamine (99 + 9%) were obtained from Alcirich Chernical.
Toluene was further distilled from calcium hydride and cûied over sodium under a
nitrogen acmosphere. Dimethylsulfate (99.7 %) was obtained fiom Eastman Kodak.
Methoà
Warm toluene (35-40' C) was syringed through a septum into the reactor vessel top
opening (H) under a nitrogen flow. Dimethylsulfate (1 mL) and triethylamine (1 mL)
were also syinged. The PTFE support rack was spun in the resulting solution for 15
minutes at room tempemure. The toluenedimethylsuIfate-triethyIamine solution was
then cannulated out through a septum covering opening (H) of F i p 2.3 under a
nitrogen flow. Wafers nacted in this way were exposed to air and washed with
~ i l l i p r e @ water, followed by methanol.
2*2*6 MAA and BDMT Organic Spacers
MAA and BDMT organic spacers were chemisorbed ont0 silver nanoparticles. Spacers
were used to bridge silver to ODMASP (with MAA) and silver to gold (with BDMT).
Materials
MAA (97 %) and BDMT (98 %) were acquired from Aldrich Chernical and used as
received. Absolute ethanol(100 46 /v) and hexane (98.5 %) were purchased from
Commercial Alcohols and Caledon Laboratories, respectively. Hexane was distilled from
calcium hydnde and dried over sodium under a nitrogen atmosphere.
(a) Silver Nanoparticle Derivatization with MAA Spacer
Siiica Glass Slides
Glass slides were inserted vertically in the PTFE support rack and placed into the reactor
vessel. Absolute ethanol(50 rnL) was syringed through a septum into the top opening
(H). MAA (2.0 rnL) was added in the sarne way. The reactor vessel containing the MAA-
ethanol solution was then heated in an oil bath (30-35' C) for 12 hours under a nitrogen
atmosphere. The support rack was spun continuously to ensure proper rnixing.
Aftenuards, the MAA-ethano1 solution was cannulated out through a septum covering
opening (H) of the reactor vessel. Ethanol was then injected to rinse the slides. The rinse
was cannulated out of the vesse1 under a nitrogen flow. This process was repeated 3
times. The slides were dried under vacuum and stored in the reactor vessel under a
nitrogen atrnosphere.
(b) Siiver Nanoparticle Derivatization with BDMT Spacer
(i) Suicon Wafers
Wafers were inserted vertically in the PTFE support rack and placed into the reactor
vessel. Hexane (50 mL) was syringed through a septum into the top opening (H) of the
reactor vessel under a nitmgen flow. BDMT (5.87 x IO-' mol, 10 mg) was carehilly
added through the top opening (H) as well. The reactor vessel was heated to 30-35' C in
an oil bath for 12 hours. The support rack was spun continuously to ensure proper
stirring. The BDMT-hexane solution was cannulated under a nitrogen flow. Hexane was
then injected to rinse the wafers. The rime was cannulated out of the vessel under
nitrogen. This process was repeated 3 times. The wafers were dried under vacuum and
stored in the reactor vessel under a nitrogen atmosphere.
(ü) Süicon Moaoxide G tids
Grids were deposited into the PTFE capsules (uncapped). The capsules were inserted
vertically in the PTFE support rack (B) and placed into the reactor vessel (C). Hexane (25
mL) was syringed into the reactor vessel through a septum into the top opening (H).
BDMT (5.87 x lo-' mol, 10 mg) was added in the same way. The PTFE support rack was
spun for 12 hours at room temperature. Aftewards, the BMDT-hexane solution was
cannulated out of the vessel under a nitrogen flow. Hexane was injected to rinse the grids.
The inse was cannulated out of the vessel under nitrogen. This process was repeated 3
times. The grids were dried under vacuum and stored in the reactor vessel under a
nitrogen atmosphere.
2.2.7 Gold Nanoparticles
Gold nanoparticles were chemisorbed ont0 BDMT terminal sulfhydryl groups.
Method
Gold colloid hydrosol was added in the sarne way as silver (sce Section 2.2.4). Silica
glas slides, silicon wafers and silicon monoxide grids were placed into gold hydrosol for
12 hours. The color of the solution remained ros6-pink. The substrates were rinsed with
~ i l l i ~ o r e @ water, followed by methanol. Substrates were placed in the reactor vessel and
stored under a nitrogen atmosphere.
A control sarnple was made by adding BDMT to silica wafer under the same conditions
as the addition of BDMT spacer onto silver nanoparticles (see Section 2.2.6). The wafer
was rinsed with hexane to nmove any physisorbed BDMT. Gold colloid was added in
the sarne way as discussed previously, followed by a distilIed ~illipore@ water rinse.
2.2.8 ODMASP Orgsaic Dye
ODMASP was chemisorbed ont0 MAA by acid-base reaction.
Materiais
ODMASP was obtained from Professor G. D. Darling, McGill University. Montréal, PQ,
and used without further purification. Methanol(99.9 9%) and toluene (99.8 %) were
acquired from Caiedon Laboratories. Poly(styrene) (MW 45000) was purchased from
Aldrich Chernical.
Method
(a) Dip-coaüng
ODMASP (0.3 - 0.9 % fw) was dissolved in a methanol-toluene solution (50 ml.
1 :9 V/V) accordhg to the method of Darling et al? The silica glass slides in the PTFE
support rack were exposed to ODMASP-methanol-toluene solution for 12- 16 hours. Al1
the sarnples were rinsed with the methanol-toluene solution and then dned under a
nitrogen Row.
(b) Sph-coating
ODMASP (0.3 - 0.9 % Iw) was dissolved in methanol-toiuene (1 :9 v/v). filtering of the
solution was done through a 0.2 mm ~ c r o d i s c ~ CR PTFE filter (Gelman Sciences). A
Pasteur pipette was used to add ODMASP-methanol-toluene solution to cover each slide.
The solution was spin-coated ont0 the slides using a photo-resist spin-coater (Headway
Research) at 1500 and 2000 rpm for 30 seconds. Sampies were rinsed with the methanol-
toluene solution and dried in air.
L2.9 ODMASP in Poly(styrene)
Substrates
iTO-glas slides (25 x 37.5 x 1 mm - la0 Nsquare surface resistivity - coated on one
side) were obtained from Delta Technologies, Ltd. Slides were taped on the coated side to
allow for an exposed electncal contact ana to apply a voltage for corona discharge
poling.
Materials
ODMASP was acquired from Professor G. D. Darling, McGill University, Montréal, PQ
and used without further purification. Polystynne (MW 45000) was obtained from
Aldrich Chernical. Methanol (99.9 %) and toluene (99.8 %) were obtained from Caledon
Laboratories and used without further purification.
Method
Poly(styrene) (PS) (2.68 x IO-' mol, 1.2 10 g) was measured into a 5 Dram glas via1 and
dissolved into a methanol-toluene solution (1:4 v/v). ODMASP (1 .O mol 96) was
dissolved into the PS-methanol-toluene solution and stimd with a magnetic stir bar.
Continuous stirring was maintained for 1 hour. The solution was filtered through a 0.2
mm ~crodisc~ CR PTFE filter (Gelman Sciences) into another 5 Dram glas vial. The
solution was dipensed onto an KO-glas slide (onto the ITO-coated side) using a Pasteur
pipette and spin-coated at 1500 and 2000 rpm for 30 seconds using a photo-resist spin-
coater (Headway Research) to produce films of thickness 1-3 p.
XP spectra were recorded using an ESCALAB 220i-XL (Fisons Instruments) spectro-
meter (Al K, radiation source). Each sarnple area (1 x 1 mm) was exposed to a 10 keV
electron source at 200 W power and 5 x lo0l3 atm (4 x 1 0 ~ ' ~ Torr) intemal chamber
pressure. Binding energies were refennced to the Au fin line at 84.0 eV. Data were
convertcd with ~c f i~ se@ V 1 . 7 ~ and d y z e d using or pro@ V2.01 software. Data were
acquired at the Institut National de la Recherche Scientifique-Université du Québec à
Montréal (Énergie et Matdriaux, INRS-UQAM), Varennes, PQ. TEM analyses were
performed using the Je01 JEM-2000FX and the Philips 400 EM electron microscopes at
33000 X magnification. Micrographs were developed using 2.5 X and 5 X magnification
from the negatives. Particle size diameten were measured using Sigma ~can@ Pro 4
software. UV-VIS measurements were taken using an HP8452 UV-Visible spectrophoto-
meter (mercury larnp) with a reference silica glas slide (0.7 mm thickness). Film
thicknesses for 2'' were measured using a Dektak (Sloan Technology) profilorneter from
the Photonics Research Group, École Polytechnique de Montréal, Montréal, PQ.
SHG measurements for analyzing the i2' nonlinear optical susceptibility were perfomed
using a Q-switched Quanta-Ray DCR neodyrnium-doped yttrium-aluminum gamet
(Nd:YAG) laser (output A = 1064 nm) with a 10 ns pulse width at 5 ml and a 10 Hz
repetition rate (set Figure 2.4). Sample and x-cut quartz crystal reference mounts (Oriel
Instruments) with rotation speeds of 2' per minute €rom O - 60' were used, in addition to
photomultiplier tubes (PMTs) (Hamatsu Company) and dual-channel boxcar integrators
(Stanford Company). The SHG signai was recorded from -50' to +50° for the quartz
reference and from O - 60" for the sample, and then amplified and averaged using the
boxcar integrators. In Figure 2.4, a minor kaction of the incident laser light transmits
through the 2 0 zcut quartz crystal reference to produce SHG. The PMT detects the SHG
signai and uses the signal as a reference channel for removing laser fluctuations in the
SHG signal from the sample. The major fraction of the incident laser light enters the
sample surface to produce the SHG signal. The fundamental harmonic bearn from the
laser passes through a polarizer and a half-wave plate to p-polmize (O0 with respect to the
plane of incidence) the fundamental harmonic. A cut-off fdter placed in front of the beam
path of the laser then filters the fundamental harmonic. The laser enters the sample and
the SHG signal from the sample suface results. The fundamental light reflected from the
surface propagates dong with the SHG to a thin 2-cut quartz crystal, frorn which more
SHG signai is produced. The phase relationship between these 2 sources of SHG is
detennined by the position of the z-cut quartz dong the beam path. By translating the z-
cut quartz paralle1 to the beam path while monitoring the SHG, an interference pattem
results, which describes the relative phase between the fundamental hannonic incident on
the sample surface and the SHG produced at the surface. The phase difference can be
extracteci €rom the interference pattern (Maker fnnges) by replacing the sample with
another zcut quartz of known orientation and then repeating the interference expriment.
Aftenvards. the SHG light enters through another polarizer set to p-polarize the second
harmonie. A filter separates the second harmonic fiom the fundamental harmonic signal.
The SHG signal enten a PMT and boxcar integrator monitors the output. Maximum i2' was determined using Microcalc Clrigin@ 3.73 software. Al1 measurements were
performed in the Materids Research Center and the Department of Physics and
Astronomy, Northwestem University, Evanston. IL.
Q-switched Nd:YAG laser ( A = 1064 nm) lem
boxcar integrator
and cornputer
I v
polarizer * . -
PMT monochromator quartz reference
4 * u
Pm
Figure 2.4: Second hamonic generation (SHG) apparatus for measuring i2'
Resuits and Discussion
2.3.1 X-ray Photoelectron Spectroscopy (XPS)
In this section, we locus on the microstructural analyses of the multilayered composites.
We use XPS to follow the layer-by-layer construction of the multilayered MPTMS-
silver-methyl-BDMT-gold heterostmcture. A flowsheet ouilines how the sarnples were
fabricated and characterized (see Figure 2.5). A control sample was used to determine
whether or not BDMT andor gold were deposited ont0 silicon. Figure 2.5, Tables 2.1 and
2.2 summarize the results.
Table 2.1
MPTMS-silver-methyl-BDMT-gold heterostmcture on silicon wafer
Samp ies Layers
Con trol
Sample A
Sarnple B
Sample C
Sample D
Sample E
Sample F
Sample G
Sample H
BDMT + gold
MPTMS (exposed to air)
MPTMS (unexposed to air)
MPTMS-methyl
MPTMS-meth yl-silver
MPTMS -s il ver
MPTMS-silver-meth yl-gold
MPTMS-silver-methyl-BDMT
MPTMS-silver-meth y 1-BDMT-gold
Figure 2.5
Flowsheet - MPTMS-silver-methyl-BDMT-gold heterosenicture on silicon wafer and silicon rnonoxide grids
Sarnple A s Silicon <IO> Wafers
Silicon Monoxide TEM Gnds
(exclusion of air)
Graft MPTMS
r
Addition of
Colloidd Silver
Sample F 9 Sample G 9
Addition of
Colloidal Silver
Addition 1 1
Colloidal Gofd
1 Addition 1
Addition El Addition
Colloidal m-
Addition
(exclusion of air)
Table 2.2
Elemental binding energy6 MPTMS-silver-methyl-BDMT-gold heterosenicture
on silicon wafer
Element Theoretical Sample Sample Sample Sample Sample Sample Sample Sample Values A B C D E F G H (eV) (eV) (eV1 (eV) (eV) (eV) (eV) (eV) (eV)
Appendix Figures A-1 to A-4 (inclusive) contain the XP spectra. Repnsentative features
indicate silicon, carbon, oxygen, sulfur, silver and goid. Sulfur 2pln and 2pm Iines
monitor sulfur-silver, sulfur-carbon and sulfur-gold interactions. Spectra were not
deconvoluted, thus binding energies are derived from peak maxima of spectra.
Control Experiments
These initial experiments eliminate the possibility of defects within the layers. We use the
following control samples to venfy that we were fabricating the correct multilayered
heterostmcture. Figures A- l(contro1) and A-2(control) show the XP spectra for these
samples. As described in the Section 2.2.7, silicon wafer fragments wen immersed in
BDMT-hexane solution, rinsed and scanned for the presence of sulfur by XPS. No
evidence of physi- or chemisorbed BDMT was found. Similady, silicon wafer fragments
that were exposed to gold hydrosol showed no evidence of physi- or chemisorbed gold.
From these results, we were able to continue with the layer-by-layer construction of the
heterostnicture.
Samples A and B: MPTMS (exposed and unexposed to air)
These experiments show the result of exposing the MPTMS graft to air. We wanted to
see if oxidation of MPTMS terminal sulfhydryl functional groups was a factor. We
prepared two samples - Sarnple A and Sarnple B. Sample A was exposed to air (see
Figures A- l a and A-2a and Sample B was not exposed. Sulfur S 2pin and S 2~~~ bands
(sulfur S 2p doublet band) illustrate that Samples A and B differ somewhat. Sarnple A
(exposed to air) shows an unresolved sulfur S 2p doublet band. while Sarnple B
(unexposed to air) shows a mon resolved sulfur S 2p doublet band. Sarnple B shows the
sulfur S 2p doublet band approaching a 1:2 ratio7 for S 2pinand S 2mn. The difference in
oxidized versus unoxidized sulfur may affect the multilayer uniformity. We explain this
difference in the following. In Sample A, less MPTMS terminal sulfhydryl groups are
available for silver nanoparticles to chemisorb onto since some sulfhdryl groups may
already be oxidized. Consequentiy, these oxidized sulfhydryl groups are not able to react
and chemisorb ont0 silver nanoparticles.
Sample C: MPTMS-mcthyl
These next experiments show the methylation of M P T M S terminal suifhydryl groups. We
wanted to sec if MPTMS cm k methylated. If so, we wanted to see the extent of
methylation. These experïments will be important in loter experiments when gold is
chemisorbed oato the silver surface. Figures A-lc and A-2c show spectra for methylated
MPMS. Methylation reduces the intensity of the sulfur S 2p doublet band of free
sulfhydryl (see Figure A-2c). A shifk to an increased oxidation state of the sulfur S 2p
doublet band results from a different chernical environment. We attribute the S 2p band
shift to rnethylation, but we still see the original S 2p band of umeacted terminal
sulfhydryl groups at 165.8 and 164.9 eV. Therefore, not dl the MPTMS sulfhydryl
groups have ken methylated. Other overlapping contributions from incomplete
rnethylation may be due to hydrogen bonding of H from sulfhydryl to dimethylsulfate to
form RS-H-O. Leavell et al.' detemined the extent of hydrogen bonding from sirnilar
chernical shifts. Therefore, we attribute these overlapping bands to hydrogen bonding
interactions.
SampIe D: MPTMS-methyl + siiver
Now that methylation of MPTMS was demonstrated, these next experiments show the
extent of methylation after exposure to silver nanoparticles. MPTMS sulfhydryl groups
are initially methylated and then silver nanoparticles are added. Figures A- ld and A-2d
show spectra for methylated MPTMS, followed by exposure to colloidal silver. We
conclude that not al1 the MPTMS sulfhydryl groups are methylated since the sulfhydryl
sulhr S 2p doublet band remains. Therefore, silver nanoparticles chemisorb ont0 the
remaining exposed MPTMS terminal sulfhydryl groups. We observe the presence of both
methylated and chemisorbed silver in Figure A-2d.
Sample E: MPTMS-silver
These expenments show the chemisorption of silver nanoparticles on M P I U S . Figures
A-le and A-2e show specva of MPTMS after exposure to colloidal silver. The sulfur
S 2p doublet band is unresolved (as in Sample A). Figure A-3e shows silver Ag 3d3R and
Ag 3dsn bands. These bands are more intense than those in Sample D, therefore,
indicating the chemisorption of more silver.
Sample F: MPTMS-dver-wthyl-gold
These experiments show that gold nanoparticles do not chemisorb onto the substrate,
methylated MPTMS, or to silver nanoparticles that have not k e n derivatized with
BDMT spacers. Figures A-If, A-2f and A-4f show spectra of MPTMS afkr exposure to
colloidal silver, methylation, and then colloidai gold. Gold does not chemisorb ont0
M P T M S sulfhydryl since methyl covers the available sulfur. Sulfur S 2p doublet bands
confirm complete methylation (see Fipn A-2f). Samples C and D are very sirnilar to
Sample F since methylation occurs in dl.
Sample G : MPTMS-silver-meth y 1-BDMT
These experiments show the chemisorption of the BDMT spaccr on silver nanoparticles.
Figures A- lg and A-2g show spectra for MPTMS after exposure to colloidal silver,
methylation and BDMT. Figure Adg shows sulfur S 2p doublet bands from exposed
BDMT terminal sulfhydryl. The MPTMS terminal sulfhydryl spectnim appears tc be
screened b y the silver nanoparticle and meth yl adlayea. Therefore, we do not see
MPTMS sulthydryl. Sample A and B are similar to Sample G since free exposed terminal
sulfhydryl is observed. Thenfore, BDMT chemisorbs successfully since BDMT contains
exposed sulfhydryl.
Sample H: Mm-silver-metbyCBDIMT-gold
These experiments show the chemisorption of gold nanoparticles ont0 BDMT. Figures
A- lh, A-2h. A-3h and A-4h show spectra for MPTMS after exposure to colloidal silver,
methylation, BDMT and colloidal gold. Gold covers BDMT well as indicated by the
reduction in intensity for the BDMT sulfhydryl compared to Sample G. A broadened line
at 174-166 eV may indicate varied oxidations States of the sulfur-gold interaction. The
gold Au 4fin band shifts indicating, perhaps, an incnase in oxidation state. Therefore,
this shift may provide additional confirmation that gold chernisorbs onto BDMT
sulfhydryl. h addition, the chemisorption of gold and BDMT dramatically damps and
reduces silver Ag 3du2 and Ag 3dsn band intensities (see Figure A-4h). Wagner and
~ i l o e n ~ amibuted damping effects to the metdlic scnening from gold.
Now, we have the complete MPTMS-silver-methyl-BDMT-gold heterosmicture. We
have clearly show that we can monitor the layer-by-layer sequence of the multilayered
composite using XPS. We now use transmission electron microscopy (TEM) to analyze
the surface in the next section.
In this section, we use TEM to probe the MPTMS-silver-methyl-BDMT-gold multilayer
and image the surface topography in detail. Figure 2.5 shows a flowsheet outlining the
fabrication of the multilayered composites on silicon monoxide treated copper TEM
grids. We investigate the MPTMS-silver-methyl-BDMT-gold heterostmcture (Sample H)
(see Table 2.1). Sample H shows grafted MPTMS, followed by exposure to colloidal
silver, methyiation of MPTMS sulfhydryl groups, and exposure to BDMT and colloidal
gold.
The purpose of these experiments was to see the METMS-silver-methyl-BDMT-gold
construction (see Figures 2.6a and 2.6b). We see a random distribution of both silver
(light-colored particles) and gold (darkcolored particles) nanoparticles. Silver particles
do not ovedap since they are bound to the surface by MPTMS only. These silver
nanoparticles are polydisperse with 4.50-9.00 nm diametea and possess average
diameters of eu. 6.57 f 1 .O2 nm. Figure 2.7a shows a histogram of the silver nanoparticle
size distribution. In Figure 2.6b, we see that the gold particles (larger dark-colored
particles) chernisorb exclusively ont0 BDMT and not onto other gold particles or the
silicon monoxide grid surface. Controls confirm that gold particles do not chernisorb
directly ont0 silver particles (smaller light-colored particles) without the BDMT spacer or
the silicon monoxide grid. Gold coverage is less than that of silver and may be due to low
coverage of BDMT spacer or diffennt adsorption kinetics for gold on BDMT versus
MPTMS. Solvent and temperature effects may influence the adsorption thermodynamics
since low reaction temperatures (30-35T) were used. Silver oxides may prevent
chernisorption of BDMT as well. Gold nanoparticles are polydisperse and confirrn the
observations by orem mus.'^ These polydisperse gold particles have 9.75- 16-75 nm
diameters and possess an average particle diameter of ca. 12.59 I 0.75 nm. Figure 2%
shows a nanoparticle size distribution that agrees with observations of Turkevich et al.
We observe that gold nanoparticle coverage is similar to that of silver (see Figure 2.6a).
The larger gold particles appear to cover silver only. We see variations in density of
nanoparticle distributions in certain regions in Figures 2.6a and 2.6b. These density
variations are due to the gold particle diameters being larger than silver. Not al1 the silver
particles are covered by gold. therefore, the particle density appears lower. If we assume
that at least one gold particle covers one silver particle, then the total area of particle
coverage would increase. As a result, mon of the grid surface would be occupied.
if we observe closely in Figure 2.6b, we notice interesting phenomena in the nanoparticle
distribution (see Figures 2.7a and 2%). In region A, we see separate silver particles
(smaller light-colored particles). Some gold particles (larger dark-colored particles) are
seen chemisorbed ont0 the silver. In region B. we see ciendritic formations of silver
particles. These formations are indicative of fractal behavior and we will explore this
detail in the next section. In ngion C, we see separate silver particles only. No separate
gold particles are observed. This observation clearly confirms that gold chemisorbs ont0
BDMT (which is chernisorbed ont0 silver). Gold does not chemisorb onto the silicon
monoxide grid itself since we do no< see separately scanered gold particles. Al1 gold
particles are added onto silver. We thus confm that gold does not chemisorb onto
MPTMS terminal sulfhydryl due to methylation. In ngion D, we notice dark linear bands
on several silver particles. These bands may imply silver particles eclipsed or twinned by
gold.'2 In region E, we observe aggregated gold particles. Several particles appear to be
hexagonally aggregated. We amibute this aggregation to hexagonal close packing of
silver. This packing distribution may be nlated to a parameter that quantifies how
distribution affects the propagation of light. We denote this parameter of dimension, d. h
hexagonally close packed silver, the dimension is trivial (d = 2). From the rnicrograph,
we observe an apparent random packing distribution, therefore, we assume that another
non-trivial dimension does exist. We will now explore this other dimension next.
Figure 2.7a: Histogram - silver nanoparticle size distribution
Figure 2.7b: Histogram - gold nanoparticle size distribution
Fractai Dimension
In Figure 2.6% we see a nanoparticle distribution showing apparent dilation symrnetry.
The distribution appears to be the same when viewed on a dilated scale. Such a
distribution is a fraftal. Using the method developed by Weitz et al? we exarnined the
MPTMS-silver-methyl-BDMT-gold sample for a non-trivial Fracta1 dimension. Figure
2.6b shows how the fractal dimension is calculated. We assume an average particle mass
(2.0 x IO-'' g for each silver particle and 3.7 x 10-17 g for each gold partick) and then
draw concentric circles of radius, R, around a point of mass. M, and count the number of
particles within each circle (inclusively). A fracta1 structure will scale according to
where d represents the Euclidean dimension of space. No scale invariance occurs when
d = dr , where dr represents the fractal dimension and quantitatively characterizes the
long-range dilation symrnetry. A fractal dimension with scale invariance occurs when
d > dl (where df is not an integer). We count the number of silver nanoparticles in each
concentric circle and determine the Euclidean dimension (see Figure 2.6b). We follow the
sarne method for counting the number of gold nanoparticles. The larger and darker gold
nanoparticles are counted. Bias in counting may be minimized by counting nanoparticles
in different circular regions and averaging the nanoparticle numbers that have the same
circular diameter, R. Figure 2.8a shows a plot of h ( M ) versus h ( R ) for silver
nanoparticles where the slope gives d!, the fractal dimension. Similady, Figure 2.8b
shows a plot determinhg the fracta1 dimension for gold nanoparticles.
WR)
Figure 2.8a: Logarithrnic plot - silver nanoparticle fractal dimension
Figure 2.8b: Logarïthmic plot - gold nanoparticle fractal dimension
Silver nanoparticles occupy a fracta1 dimension, df, of 1.45 i 0.08. Similarly, gold
nanopanicles appear to imprint, imbed into and replicate the silver fractd topography as
well. niese gold nanoparticles occupy a fractal dimension of 1.76 f 0.13. Andrews et
al.13 discovered similar fractal dimensions of metai nanoparticles embedded into
poly(pyrro1idone) on TEM grids. The measured fractal dimension agrees with the
diffision limited aggregation (DM) model on surfaces. This model involves the
diffision and adsorption of individual nanoparticles ont0 the outer vacant edges of an
existing aggregate. More specifically. the depositiondifision-aggregation (DDA) model
may better chvacterize nanoparticle monolayer for~nation.'~ This model diffen fiom the
DLA mode1 since it involves nanoparticle deposition onto an existing monolayer
aggregate and the difision of each nanoparticle towards a vacancy in the monolayer.
We now use W-VIS spectroscopy to analyze the various rnultilayered composites fiom
the measured absorbance spectra. Tables 2.3a and 2.3b define the compositions of the
heterostmctures that were studied on silica glas slides.
Table 2.3a
MPTMS-silver-methyl-BDMT-goId heterostnicture on silica glass slide
Sarnple Layer
Control Gold (in solution)
Gold-BDMT (in solution)
F MPTMS-silver/gold
MPTMS-silver/gold-BDMT
G MPTMS-silver
MPTMS-silver-BDMT
H MPTMS-silver
MPTMS-si1 ver-me th y 1-BDMT-gold
Table 23b
MPTMS-silver-MAA-ODMASP heterostnicture on silica glas slide
Sample Layer
Control MPTMS-silver-ODMASP
A MPTMS-silver
B MIPTMS -silver-MAA
C ODMASP
D MPTMS-silver-MAA-ODMSP
Figure 2.9
Flowsheet - MPTMS-silver-MAA-ODMASP on silica glass slide
Silica Glass Slides s Addition
Colloidal
of MAA
1
Addition of
ODMASP
Addition of
ODMASP
i) MPTMS-silver-methyi-BDMT-gold hefemstnicture
We now examine the W-VIS spectra of the following samples to determine the effect of
silver and gold with the BDMT spacer in the multilayered composite heterostnicture.
Control Smple: gold + BDMT (in solution)
These experiments show the result of chemisorption of BDMT on gold in gold hydrosol.
This LW-VIS control sarnple differs From the X P S control sample in that we perform this
control expriment in solution instead of on a silicon wafer. Figure 2.10a shows gold
hydrosol before and after exposure to BDMT. When BDMT chemisorbs ont0 gold
colloid, the absorption spectnim exhibits a shoulder centered near 650 nm.
Sample F: MPTMS-süver/gold-BDMT
These expenments show the result of chemisorption of BDMT on silver and gold
nanoparticles. This UV-VIS Sample F differs from the XPS Sample F in that we adsorb
colloidal silver first. then colloidal gold and BDMT. For the XPS Sample F, we
chemisorbed silver, methylated MPTMS, chemisorbed BDMT to silver and then
chemisorbed gold. Figure 2. lob shows spectra for equimolar colloidal silver and gold on
grafted MPTMS before and afier B D W chemisorption. Large broadening of the
absorption specuum arises from silver and gold surface plasmon band overlap. The silver
plasmon absorbance maxirnizes at 461 nm and 467 nm.
Sample G: MPTMS-silver-BDMT
These experiments show the chemisorption of BDMT on silver nanoparticles. This W-
VIS Sample G is identical to the XPS Sarnple G with the exception of MPTMS
methylation. Figure 2.10~ shows spectra for colloidal silver on grafted MPTMS before
and dter exposure to BDMT. The silver plasmon blue-shifts from 452 nm to 44L nm.
The absorption spectnim undergoes broadening from 300-375 nm and therefon, indicates
silver-BDMT interaction.
Sample H: MPTMS-suver-methyl-BDhlT-gold
These experiments show the chemisorption of gold nanoparticles on BDMT chemisorbed
ont0 colloidal silver. This W-VIS Sample H is identical to the XPS SampIe H. Figure
2.10d shows spectra for colloidal silver on grafted MPTMS before and after methylation
of MPTMS, followed by exposure to BDMT and colloidal gold. The silver plasmon blue-
shifts fiom 452- nm. We observe a visible color change from golden-yellow to rosé-
pink. This observation may indicate that the silver plasmon couples with gold. Conon et
confirmed that silver and gold coupling does indeed occur by blue-shifting in the
UV-VIS spectrum.
300 400 500 600 700 800 Wavekngth (nm)
Figure 2.10a: UV-VIS spectrum - gold-BDMT in aqueous solution
Wavekngth (nm)
Figure 2.lOb: UV-VIS specmm - WS-silverlgold-methyi on silica glass slide
300 460 500 600 700 Wavelength (nm)
Figure 2.10~: UV-VIS spectmrn - MPTMS-silver-methyl-BDMT on silica g l a s slide
Wavdmgth (nm)
Figure 2.lOd: W-VIS spectrum - MPTMS-silver-methyl-BDW-gold on silica giass slide
ü) MPTMS-silver-MAA-ODMASP heterostructure
We examine the W-VIS spectra of the foiiowing sarnples to determine the effect of
silver on ODMASP. ODMASP possesses polarizable pz-electron orbitals and electron
delocaiization occurs h m pxsrbital energy o ~ e r l a ~ . ' ~ Figure 2.1 1 shows the fomal
charge transfer process fiom the donor (dkylamino group) to the acceptor (pyridinium
group) to fonn the excited state.
t Excitai Statc
Figure 2.11: ODMASP charge transfer between rr-a* States
ODMASP probably forms an amphiphilic layer on the silver surface involving the Cis
hydrophobie alkyl tails and pyridinium hydophilic head (see Figure 2.12). The ODMASP
pyridinium nitrogen (lone electron pair) interacts by acid-base reaction with a proton
fiom MAA carboxylic acid. The MAA carboxylate anion delocalizes negative charge
whe~as the pyridinium nitrogen retains and localizes the positive charge. Acid-base ionic
chemisorption of ODMASP ont0 MAA positions ODMASP close to the silver surface.
Cross et al." c o d h e d that a Ci &y1 spacer positions the NLOD close enough to the
silver surface without disruption in charge transfer.
N""C'"Hn' I Hy drop ho bic La yer
// Hyperpolarizable Layer
Silver Nanopartic le Surface
Figure 2.12: ODMASP chernisorbed onto MAA (acid-base)
We conducted the following control experiments. Initially, we attempted to chemisorb
MAA ont0 the silica glass slides by alcohol condensation reactions. We also endeavond
to chemisorb ODMASP directly ont0 the glas slides by ionic and hydrogen bonding
interactions using the methanol-toluene (1 :9 vlv) solution for ionically chemisorbing
ODMASP ont0 MAA. As well. we attempted to chemisorb ODMASP ont0 silver by van
der Waals attraction with the methanol-toluene solution. From these control expenments
we conclude that both MAA and ODMASP do not chemisorb significantly ont0 acid-
etched, unfunctionalized silica glas slides and ODMASP alone does not chemisorb
significantiy ont0 the MPT'MS-silver heterostnxcture either.
400 500 600 700 000
Wavekngth (nm)
Figure 2.13: UV-VIS spectra - MPTMS-silver-MAA-ODMASP heterostnicture
on silica glass slides
These next experiments show the layer-by-layer construction of the MPTMS-silver-
MAA-ODMASP heterostnicture. Figure 2.13 shows spectra of colloidal silver on grafted
MPTMS (Sample A), followed by exposure to MAA (Sample B), and then exposun to
ODMASP (Sample D). We see in Sample A the signature of the surface plasmon band
maximum at 415 nm as confirmed by Bohren et a1.18 Subsequent adsorption of MAA in
Sample B suppresses the plasmon absorbance. Kreibig et al. observed that silvet-sulfur
interactions alter the fne electron density and darnp the plasmon absorbance. Sample C
shows free ODMASP with a maximum absorbance wavelength (A-) at 394 nm. We see
that this absorbance corresponds to the first electronic transition (So -+SI) and overlaps
with the silver plasmon absorbance (compare Samples A and C). Therefore. plasmon
enhancernents a cu r and promote the elecmnic excitation of ODMASP. Sample D shows
ODMASP reacting with MAA to form the MPTMS-silver-MAA-ODMASP
heterostructure. Decher et al?' obsmed that ionic acid-base chemisorption was indeed
an excellent way to construct multilayereà heterosmicnires. Spectral broadening implies
agpgation of ODMASP monomers as noted for stilbazolium dye salts by Xu et af?
Initialiy, we observe that the silver plasmon band absorbance maximum red-shifts from
344 nm to 445 nm. We may offer general explanations for the spectrum features. Firstly,
we rnay invoke a chemical effect. This effect occm when the ODMASP causes changes
in the ODMASP electronic stnicture. Secondly, we may invoke an electromagnetic
effect. This effect occurs when the plasma osciliations contribute to the molecular
eigenstates. In the fint case, protonation of ODMASP by MAA modifies the
eigenenergies and eigenhinctions of the adsorbate. In methanol, protonated ODMASP
shows a strong red-shifted absorption band at 470 nm and a weak absorption band at 320
nm. If these bands exist, then image-dipole interactions between ODMASP-H+ and the
silver surface would repnsent an additional perturbation on the ODMASP eigenstates.
Therefore, the absorption energy spectnim may simply represent protonated ODMASP
on MAA superimposed on the plasmon band of silver. In the second case, however, the
resonant response of the metai nanoparticles may actually by strongly coupled to the
absorption from the ODMASP layer. Measurernents from rhodamine B on colloidal silver
adsorbed ont0 glas reveal strong coupling between the adsorbate and silver? The
plasmon absorbance at 520 nm for uncoated silver nanoparticles splits with the bands
blue- and red-shified from the 520 nm plasmon band of the uncoated nanoparticles. The
strength of coupling depends on the degree of overlap of the absorption spectra of the dye
with the silver plasmon, the particle size and shape and the coating thickness.* From
Glass et al.18 and ~ a ~ e n , ~ ~ we suggest that the band in the plasma resonance of our coated
silver nanoparticles may be associated with the opticai resonance of the conduction
electrons through the dispersion in the reai part of the ODMASP-El? dielectric function
and through the absorption of ODMASP. In the next section, we suggest how this optical
nsonance effect can indeed enhance i2)
2.3.4 Second Harmonic Generation
We proceed to measure the SHG from the i2) nonlinear optical susceptibility. These
measurements were obtained from the multilayered composite containing both MPTMS-
silver and MPTMS-silver-MAA-ODMASP betetosüuctures on RO-glass slides. We
examine the obtained maximum as outlined in the experirnentai conditions (see
Section 2.2.10). In SHG experiments involving 22' measurements, both fundamental (a)
and second hamonic (20) waves propagate in the same direction. The wave vector
difference between the waves is described by
where k = n d c and k represents the wave vector, n is the refractive index, a, is the
fundamental frequency, Zn, is the second h m n i c frequency and c is the speed of light.
Since Ak t O, harmonic field components do not propagate in phase. Effective phase
matching occun when the second harmonic coincides with the fundamental wavelength.
Nonlinear optical susceptibility for SHG was measured by the Maker fringe techniquez
as described by Jerphagnon and ~ur t z? In this technique, the plane of the sample on
ï ï0-glas is rotated through an angle from O* to 60' to produce a fiinge pattern.
Birefringence phase matching was achieved through angle tuning, which uses the angle
dependence of the refractive index of the p-polarized wave relative to the optical wis. It
is usehil to define a nomalized f ' for p-polarized excitation as
where I, is the intensity of the excitation laser pulse and 12., is the intensity of the
comsponding SHG signal. A 1064 nm wavelength was chosen to have a Za, wavelength
overlap one of the observed electronic UV-VIS spectral bands. An alternative to the
birefringence phase matching is the conversion of the second harmonic wave to Cerenkov
radiation." On the basis of resonance enhancement, the SHG signal with 2n,
at 532 nm may become enhanced by the broad electronic transition band of ODMASP at
344 nm as well as from the overlapping silver plasmon band ai 445 nm (see Figure
2.13d). Thenfore, the nonlinear response may have contributions from both bands.
From these experiments, we show the 2'' response of the MPTMS-silver-MAA-
ODMASP heterostructure in Table 2.4.
Table 2.4 suggests that the iZCZ' from ODMASP may be enhanced by silver surface
plasmon excitations by the laser optical field and therefore, cocf~nas our previous
conjecture that a coupled enhancement may indeed occur.
Table 2.4
i2' for MPTMS-silver-MAA-ODMASP, MPTMS-silver and PS-DMASP heterostnictures
He terostruc ture f ' ( IO-* C ~ I J ~ , 10-* esu)
MPTMS-silver-MAA-ODMASP 1.48,4.00
The increase in i2' of ODMASP on silver suggests the influence of increased
hyperpolarizability, number density and local fields. We discuss the effects of these
factors show in our experiments later on.
Originally, we prepared and obtained samples of ODMASP for SHG experiments. The
fint sample consisted of ODMASP doped into poly(styrene) to form a PS-ODMASP
composite. The concentration was approximately 1 mol 46 ODMASP, which arnounts to
a number density of approximately 500 ODMASP monomers per nm2 (1 pm film
thickness). Due to thermal degradation of our sample, we used (a-[4-(N&-dimethyl-
arnino)]styrylpyridine (DMASP) from Marks et al? for cornparison. A sample that was
pied by coma discharge gave a P' of 0.29 IC 110'" ~ I J ~ (0.78 x 10' esu). ODMASP
chemisorbed ont0 MAA in the MPTMS-silver-MAA-ODMASP heterostnicture gave a
2'' of 1 A8 x luU C?/J~ (4.00 x lu8 esu). Both samples were exposed to 1064 nm light
from an Nd:YAG laser for SHG. These experiments were performed to show that the
silver nanoparticle optical fieids enhance i2' of the MPTMS-silver-MAA-ODMASP
heterostructure. This enhancement is not available in the PS-DMASP composite.
Orientational Order
We must therefore explain the differences in the measured i2' values in terms of (1)
differences in sample preparation relating to orientational order and (2) possible roles for
surface plasmon enhancements. Let us fmt consider the orientational order. Differences
in ;S2' rnay possibly be due to differences in polar orientational order. Corona poling
foms a nonequilibrium distribution of oriented monomeric DMASP. These dipoles will
spontaneously relax towards a random configuration (increased configurational entropy)
unless physically impeded by the host polymer matrix. Various techniques for impeding
the relaxation process may provided by lattice hardening for anchoring NLODs. DMASP
in poly(styrene) corresponds to the unanchored lattice hardening since the polyrner is not
cross-linked, and the DMASP is not covaiently chemisorbed to the polymer. Therefore.
DMASP polar orientation correlations c m decay immediately after the corona poling
field ceases at room temperature. Since the nonlinear susceptibility was measured
immediately after corona field poling, the measured 2'' most Iikely reflects the value that
would be obtained for the highest degree of order. This i2' assumes that the corona
poling process is efficient. We rneasured the growth of XI2' in situ and then maximized
the polar orientational order.
Surface Plasmon Enhancement
Let us now consider the surface plasmon enhancements. We consider the effects at the
metal interface. Optical SHG has been produced from roughened silver surfaces2' and
colloidal silver monolayers on Langmuir-Blodgett films. Typicall y, nonlinear effects in
the silver derive ffom electrîcal field gradients and surface effects to produce SESHG.
The second harrnonic field amplitude generated from an interfacial nonlinear polarization
on the silver surface is obtained by the use of an effective tensor. In actuality, the
measured X<Z' is temed the effective i2' and derives fiom the i2jM=, x"'La;fEl, f and X"'6m tensor components as described by ~revet~' in the fonn
where i2)sJa, i 2 j S , , f l I J P and i2)s,, are the surface 2"srder nonlinear susceptibility
tenson, E is the applied field and i3'=, i3L, XI3'= and i3'= are the 3*-order nonlinear
susceptibility tensors. Since the second harmonic field amplitude is proportional to the
tensor amplitude, we can expect a nonlinear intensity for the response to be proportional
to the quaciratic of the tensor amplitude as shown in Equation [12]. We use f ' to
represent i2)@, or in essence, ~ 2 J ~ c Z Z X , i2)&- and iZ)Cfim, throughout the thesis, when
refemng to our nonlinear optical susceptibility measunments. Usually, i2 '~ , dominates but in the case of surface-enhanced SHG (SESHG) the other tensor
components must be accounted for. We will explore SESHG regarding our experirnental
samples next.
From our samples, we should see evidence of SHG as a result of surface enhancement of
colloidal silver in the MPTMS-silver heterosmcture. From our measurements, the
control sarnple of silver nanoparticles in the MITMS-silver heterostructure produced no
SHG after exposure to 1064 nm laser light. Thecefore, no direct contribution of 532 nm
light from the silver nanoparticle layer was observed using Our conditions of sarnple
preparation and measurement. We may explain this occumnce in terms of nanoparticle
distribution. Our MPTMS-silver heterosenictural samples on TEM grids showed fractal
distributions of silver nanoparticles. Fractal distributions have not yet been explorcd
extensively for SESHG. Depending on the extent of fractality, the SESHG signal may or
may not be detected. McGum et al." concluded chat multiple scattering by disordered
distributions such as fractals may decrease SESHG. As we explore further, the details for
this occurrence will become clear.
We see that SESHG in silver occm as laser light impinges on the surface. We may
describe the subsequent SESHG prwess by the following. ûn the süver surface, we may
use describe a free electron gas at the silver-air interfa~e?~ We assume that the electron
density is a hinction of oscillations at both the fundamental and second harmonies of the
incident electromagnetic wave. These harmonic oscillations superimpose upon each other
and occur at certain incident angles. Some of these angles correspond to surface silver
plasmon modes of the impinging laser. Unfortunately, fractd distributions rnay not
support these oscillations at al1 incident angles. For silicon monoxide TEM grids, a
fractai distribution was confirmed. We infer that these nanoparticle fractal distributions
rnay be similar for the ITO-glass samples that were measured for 22' during SHG
éxperirnents. We deduce that the varied surface topography of silver particies rnay inhibit
SESHG, especially since formation of silver oxides easily occurs. Ishida et al?'
concluded that the kinetics of MSA depend on the surface conditions of silver and gold.
Adsorption (both chemi- and physisorption) from various chemicai species do indeed
occur and rnay contaminate the surface. Somo jai et confimed that carbon
monoxide, oxygen, water and hydrocarbons chemisorb onto silver. In addition, Richter et
al? noted that water and hydrocarbons tend to physisorb ont0 silver. Thenfore,
adsorption of chernical species evidently does occur on the silver nanoparticle surface.
Therefore, the i2' response from SHG due to permeating SESHG electromagnetic waves
rnay not occur at d l . We have shown that the silver nanoparticles alone do not produce
any SHG signal. Similady, samples of residual ODMASP on underivatized silica glus
slides gave no SHG signal. We show that accidental adsorption of ODMASP in the
interstices between silver nanoparticles on the silica substrate cannot contribute to the
signal. Enhancements in i2' rnay be caused by resonance of surface plasmon modes at
the second hmonic wavelength. We descnbe this enhancement in the following. SHG
signal radiating from ODMASP pemeates into the silver particles to excite a plasma
response (see Section 1.1.4). We speculate that an excitational resonance occurs from a
coupled state between the plasma oscillations and the photons, therefore forming surface
plasmons. The plasmons focus and align electromagnetic fields of incident radiation into
the surrounding ODMASP layer. Pocluand et al? concluded that d2' becornes enhanced
on various silver surfaces. Consequently, the resulting surface plasmon resonance at the
silver-ODMASP interface enhances the XI2' response From SHG.
hfîuences on d2) i) Quaternization
Explanations for the differences in the i2' nsponse in our samples (see Table 2.4) may
be attributed to various factors including ODMASP molecular hyperpolarizability, self-
poling effkiency, local fields and number density. Park et al." surmised that an increase
in /la dye such as ODMASP may simply be accounted for by the effect of quaternization
of the pyridyl nitrogen. The increase in f l would therefore, increase the eficiency for
SHG. Other researchers do not account for an incnase in j from quatemization.
Therefore, whether or not quaternization is a factor in raising p remains questionable.
ü) Polar Order
Another explanation for the difference in the values of i2' obtained from the 2 systems
may be sought in the polar order of ODMASP on the silver nanoparticles. ODMASP self-
poles by van der Waals and electrostatic interactions. Cis alkyl tails promote hyàrophobic
van der Waals interaction among other alkyls. Porter et aL3* concluded from FT-IR and
electron diffraction experiments that stilbazole alkyl tails greater than Clo promote better
self-poling. Cross et al." concluded that Ci* self-poles the most efficiently using Ca, CIO
and C14 alkyl tails. In addition, Pincreases as alkyl carbon chah length increases from Ci
to Ci* (see Tables 3.2 and 3.3). This f l increase results from improved crystailization of
monomeric ODMASP. Ashwell et al?* confirmed that increases in Band i2' up to Czo
alkyl for stilbazoles in general such as ODMASP.
iii) Number Density
Local fields are associated with the dipole-induced-dipole interactions among monomeric
ODMASP for the MPTMS-silver-MAA-ODMASP heterostnicture. These interactions
affect the crystaiIization and thenfore, the ODMASP number density. High crystallinity
produces increased numbcr âensity and larger 8. Girling et al?' noted that increased
number density and fllead to an increase in the local fields. Steinhoff et al." observed
larger local fields in crystalline stilbazole venus monomeric stilbamle.
We speculate that the MPTMS-silver-MAA-ODMASP samples contain a low degree of
ODMASP crystallinity. Low crystailinity may be due to a number of reasons. Among
these reasons includes steric interactions as described by UlmanO4' A large free volume in
ODMASP occurs fkom the hydrocarbon Ci8 alkyl tail, which may lower the packing
efficiency leading to effective crystallization. Effectively, a lower y2' would result due to
a lower ODMASP number density. We calculated an estimated 1 .S ODMASP monomers
per nm2 on silver (assuming each ODMASP occupies a "foorprint" area of 54 nm2 on the
surface). Initidly, we assumed a hexagonal close-packing pattern. However, metal
particles aggregated into a fractal dimension as shown by the MFFïMS-silver-rnethyl-
BDMT-gold heterostmcture on TEM grids (see Figures 2.6a and 2.6b). We have yet to
ascertain if the MPTMS-silver-MAA-ODMASP heterostmcture on the silica glass OCB
slide shows fractal character as well. If we see a fracta1 dimension, then we may assume
that the number density of ODMASP on silver is much lower than previously estimated
since silver particles are pnsumably fractally (not hexagonally) distributed. By Our
estimation, there is a very low number density in the MPTMS-silver-MAA-ODMASP
heterostmcture compared to the PS-DMASP composite. We would othenvise assume that
the large DMASP number density in PS-DMASP would produce an exceptionally large
i2) - much 1-r than that for the MPTMS-silver-MAA-OMDAPS heterostmcture. This
consideration supports our conjecture that the fractal silver nanoparticle distribution
increases f ' . Andrews et al?' enhanced local fields of i3) in fracta1 silver nanoparticles
in poly(methy1methacrylate) composites. Since local fields may be enhanced up to 106
fiom fractal silver, we therefon assume that a sirnilar response in f2' of locai fields
occurs in the MPTMS-silver-MAA-ODMASP heterostructurc. From the data, we observe
a larger $' in MFTMS-silver-MAA-ODMASP even though the ODMASP number
density is much lower than expected. Therefore, we may conclude that local field
enhancement ofi2' occurs fiom the fractal silver nanopaiticles.
Conclusions
summery
New types of nano- and mesoscale multilayered composites were prepared. We
fabricated MPTMS-silver-methyl-BDMT-gold heterostructures and successfully used
XPS to follow the formation of each layer.
We concluded that BDMT and gold were not chernisorbed ont0 the silicon wafer. As
well, we monitored the sulf'ur S 2p doublet band to confirm the methylation and
chemisorption of colloidal silver, and the chemisorption of BDMT and colloidal gold.
Methylation was effective but did not envelop al1 the exposed M P T M S terminal
sulfhydryl groups. Monitoring of the silver Ag 3d doublet band showed that the band was
darnped after chemisorption of colloidal gold ont0 BDMT.
We confirmed by using TEM that colloidal gold nanoparticles could be chernisortted
selectively ont0 a colloidal silver nanoparticle sublayer. Gold chemisorbed exclusively on
top of silver by the BDMT spacer with no overlapping of pld-gold or silver-silver
nanoparticles. Colloidal silver and gold nanoparticles were measured to be 6.57 f 1 .O2
nm and 12.59 I0 .75 nm in diameter. respectively. We calculated the fractal dimension of
coiloidai silver to be 1.45 f 0.08. Following the addition of BDMT. colloidal gold
nanoparticles were imprinted and replicated into the fracta1 pattern of previously
deposited colloidal silver nanoparticles to occupy a fractal dimension of 1.76 f 0.13.
We also fabricated gold-BDMT and MPTMS-silverlgold-BDMT, MPTMS-silver-BDMT
and MPTMS-silver-methyl-BDMT-gold heterostructures on silica glass slides. Overall,
the W-VIS spectra revealed a silver surface plasmon absorbance blue-shifi due to a
coupled silver-gold charge transfer. We fabrîcated the MPTMS-silver-MAA-ODMASP
heterostmcture as well. We calculated the ODMASP number density in MPTMS-silver-
MAA-ODMASP (1.5 ODMASP per d) and in PS-DMASP (500 DMASP per nm2 in 1
pn thick film). Fuidly, we obtained a i2' of 1 A8 n 1 & J ~ (4.00 x 1 O-' esu) and 0.29
x IO-= Cfl2 (0.78 x 1 0 ~ esu), mspectively, for the 2 samples as well. The low number
density in the MPTMS-silver-MAA-ODMASP heterostructure points to a Low extent of
ODMASP crystallization (influencing P). The silver plasmon absorbance using W-VIS
spectroscopy revealed a nd-shift due to ODMASP. We postulated that resonant
excitations may be nsponsible for i2' enhancernents and proposed potential models.
Future Research
We suggest hirther improvements in design strategies for fabricating heterostnictures
incorporating siiver and gold nanoparticles and ODMASP. Proving enhancement of 2'' from silver rnay be made by the following:
1) Measure the 2*' of ODMASP or DMASP quatemized in poly(styrene),
2) Tune the laser wavelength into the piasmon band maximum to see if 2') increases as
the plasmon is excited,
3) Use a squaraine dye that does not absorb at 532 nm (second harmonie).
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Chapter 3
Introduction
3.1.1 NLOD Alternative - NDPEPD In this chapter, we design an NLOD to follow some of the MSA principles outlined in
Chapter 1. We reiterate Our objective here. We describe the synthesis of a novel NLOD
(NDPEPD tolane) that chemisorbs directly ont0 silver or gold nanoparticle surfaces
without the use of a spacer.
Tolanes are ngid linear structures that self-align very efficiently onto OCBS.' In general,
NLODs can be chemisorbed to fom heterostructural OCBs. in our structures, we initially
chose ODMAPS to use in the MPTMS-silver-MAA-ODMAPS. We capitalized on self-
poling since it is driven by the hydrophobic interactions associated with the hydrocarbon
alkyl tail. Self-poling circumvents electrical field poling to bnng about alignment for i2'. Several factors are important to ODMAPS chernisorption. orientation and optical
response. Solvent polarity and hydrogen bonding are important. In addition, polarity and
hydrogen bonding affect the excited state. For example. polar solvents stabilize the
excited state more than the ground state? As stated pnviously, Darling et al.' used a
methanol-toluene (1 :9 v/v) solvent mixture to chemisorb ODMAPS onto an MAA spacer.
Care must be taken to controi the pH of the medium since the pyridinium nitrogen may
be protonated by water (in methanol) prior to the acid-base interaction with ODMAPS-
MAA. Therefore, speculations arise in the nliability of ODMAPS chernisorption ont0 the
MAA spacer.
As an aitemative to ODMAPS, we proposed the use of N4[(4'-nitrodiphenylethyny1)-
phenyll-bis-2"-diethanethiolamine (NDPEPD), which could chemisorb directly onto the
siiver surface by the sulfhydryl group without the use of any additional spacer. We
surmise that this alternative circumvents the diff'icuity associated with spacer
incorporation into the heterostructue. Consequently, a more extensive chemisorption of
the NLOD ont0 the silver surface may be possible with NDPEPD. The availability of the
disulfhydryl groups allows for an increased degree of chemisorption ont0 the silver
surface. Contrarily, ODMAPS only has a monosulfhydryl group from the MAA spacer
for chernisorption. Therefore, NDPEPD has a larger probability of chemisorption ont0
the surface than ODMAPS. As well, NDPEPD can self-pole by MSA more eficientiy
than ODMAPS. ODMAPS bas Cia carbon alkyl tails that are dependent more extensive1 y
on kinetics and themodynamics to promote aiignment. NDPEPD pz-orbital interactions
occur between adjacent phenyl groups of monomeric NDPEPD. Such interactions may
lead to phenyl x-r stacking of NDPEPD? In contrast to ODMAPS, NDPEPD has no
stenc interactions from the Cl* carbon alkyl tails with which to contend. ODMAPS has a
hydrophobie Cla alkyl tail that cm sterically interfere with self-poling due to the large
imposed free volume. On the contrary, NDPEPD cm aggregate and pole due to linear
molecular rigidity. A detailed discussion comparing NDPEPD and ODMAPS. as well as
addressing the advantages of NDPEPD will be discussed later (Section 3.3.2). Now, we
will explore and propose the preparation of NDPEPD next.
3.13 NDPEPD Tolane Dye Synthesis
The preparation of N4-[(4-nitrodiphenyIethynyl)phenyl)phenyl]-bis-2"-die~mehiol-
amine (NDPEPD) was carried out according to Figure 3.1.
OH PDEA
PdlP(C6Hdh (2 mol 96) Cul (2 mol %) diùopmpylamine 8S0 C
Figure 3.1: Schematic - NDPEPD synthesis
N-phenyldiethanolamine (PDEA) was iodinated to form [ I l from the aryl amine
according to the procedure of ~rewster? The hydroxyl group in [ I l was protected by both
acetylation6 and silanation? Purified [3] was synthesized and palladium-catalyzed cross-
coupling of [2a] and [2b] each with [3] was perfomed to produce [4a] and [4b] as
described by Mardcr et d8 The hydroxyl group in [4] was deprotected by deacety1ation9
and desilanation.1° Thiolation was performed to convert hydroxyl to sulfhydryl according
to the procedure by ~ishio" involving Lawesson's nagent to form [6]. Conversion of
hydroxyl to suLfhydry1 fier crosscoupling instead of before was perfonned to prevent
contamination of the palladium catalyst.12 The triphenylphosphine ligands on the
palladium metal center easily absmcts sulfur from thiosulfates and thi~alk~ls . '~
Therefore, protection of hydroxyl by sulfhydryl is not feasible in these circumstances.
Sulfhydryl may be easily removed and chemisorbed onto the catalyst surface. Mfication
of [2] to [6] was accomplished by flash column chromatography with silica using hexane-
ethylacetate or methanol-dichloromethane as the eluting solvent mixture.
3.2.1 IPD and IPTE - Aryl Iodkie Spthesis
[l] 4-iodop hen y ldiethmolamine
Materiais
iV-phenyldiethanolamine (97%), sodium bicarbonate (99.7 + %), iodine (99.999 + %),
chloroform (99.8 56) and sodium hydroxide (97 96) were obtained from Aldrich
Chernical. Methanol(99.5 + %) was obtained from Caledon Laboratones. Al1 chernicals
were used without further purification.
Method
N-phenyldiethanolamine (PDEA) (120 rnrnol, 2 1.75 g) and sodium bicarbonate
( 1 80 mmol. 1 5.12 g) were measund into a 100 mL round bottom fias k and dissolved in
methanol(20 mL). The flask was cooled to 10-15' C in an ice bath and the cream-white
solution was stirred for 5 minutes. Distilled water was added and the solution was stirred
for an additional 20 minutes. Iodine (100 mmol, 25.38 g) was slowly added. The olive-
yellow solution was stirred for another 30 minutes. Cnide [1] was collected as a matte-
black viscous oil on a Buchner funnel under vacuum. Chloroform was added to dissolve
crude [l], which was extracted using a separatory funnel. Dilute sodium hydroxide
solution (10 % w/v) was added to separate [1] in the organic phase from ammonium salts
in the aqueous phase. The organic phase was isolated and the solvent was removed using
a rotating evaporator under vacuum. Charcoal-black crystals were produced in yield:
30.22 g (82 %). MS ml2 307 (m, 263 (MC - 1-C&N(CH2-H)), 18 1 (AC - C a -
N(CH2CH20H)2). 45 (M+ - CH2CH20H); MS mh calcd for Ci&Ii402NI: 307.00; Found:
306.71. 'H NMR (CDCi3: 6 3.54 (t, 4H. -NC&on arnino, J = 5.00 Hz), 3.81 (t, 4H,
- C w - on arnino, J = 5.20 Hz), 6.46 (d. 2H, -CEJOL on phenyl, J = 9.20 Hz), 7.45 (d,
W, Ica- on phenyl, J = 9.00 Hz) ppm. Anal. Calcd for C1&402M: C, 39.10; H, 4.59;
N, 4.56. Found: C, 41.27; H, 5.01; N, 4.84.
[fa] 4-iodophenyi-bk-2'dethyletb8tloaty1amine or IPD
Materials
Acetic anhydride (98 %), 4dimeihylaminopyridine (DMAP) (99 + %), and pyridine
(99 + %), ethanol(95 %), ethylacetate (99.5 + %), sodium sulfate (99 + 8) were obtained
from Aldrich Chernical and used without further purification. Toluene (99.8 %) was
obtained from Caledon Laboratories.
M e a d
Purificd [l] (1.00 mmol, 0.291 g), acetic anhydride (2.50 rnmol, 0.32 mg) and DMAP
(10 mg) were measured into a 100 mL round bonom flask and dissolved in pyridine
(50 mL). The solution was stirred for 1 hour at room temperature and later transferred to
a separatory hinnel. Ethanol(10 mL), ethylacetate (10 mL), water (10 mL) and potassium
carbonate (17.5 mmol, 2.02 g) were added and extracted in succession (in the aqueous
phase). The organic phase was isofated and the ethylacetate nmoved by rotating
evaporator under vacuum. Toluene was added to assist removal of pyridine and crude
[2a] was dried over sodium sulfate. Cnide [2a] was separated and purified by flash
column chromatography on silica (70-230 mesh) using hexane-ethylacetate (4: 1,3: 1 and
2: 1 V/V in sequence) as the eluting solvent. The procedure is described at the end of the
section. A peach-orange powder was obtained in yield: 0.25 g (65 %). MS rnh 39 1 (m, 26 1 (M+ - I-C&-N(CH2CH2-0)2), 84 (M+ - CH2CH2-O-COCH& MS m/z calcd for
CLaLB0W: 391.14; Found: 391.15. 'H NMR (CDC13): S 2.04 (s, 6H, -C& on acetyl),
3.59 (t, 4H, -NC&- on amino. 1 = 6.30 Hz), 4.2 1 (t, 4H, -C&û- on amino, J = 6.10 Hz),
6.56 (d, 2H, --CN- on phenyl, J = 9.00 Hz), 7.47 (d, 2H. 1CCI-J- on phenyl. J = 9.20
Hz) ppm. Anal. Calcd for Ci&804h?k C. 43.00; H, 5.41; N, 4.18. Found. C, 43.23; H.
5.56; N, 4.25.
Materials
Terr-butyldimethylsilyl (TBMDS) chloride (97 8). triethylamine (99 + %),
4-dimethylaminopyridine (DMAP) (99 + 96). and dichloromethane (99.6 %) were
obtained from Aldrich Chemical and used without hirther purification.
Methoà
Purified [l] (3.49 m o l , 1.02 g), TBDMS chloride (16.2 mrnol. 2.45 g), triethylarnine
(19.5 mmol, 1.97 g), DMAP (10 mg) and dichloromethane (25 mL) were combined in a
100 mL round bottom flask and stined for 12 hours. Chloroform, distilled water and
dilute hydrochloric acid were added to neuulilize excess triethylarnine. Cnide [2b] was
isolated by rernoval of triethylammonium salts in the aqueous phase by separatory Funnel
and the organic phase was removed by rotating evaporator under vacuum. Cnide [2b] was
separated and purified using flash colurnn chromatography on silica (70-230 mesh) using
methanol-dichloromethane ( 1 9 vlv) as the eluting solvent. The procedure is descnbed at
the end of the section. A cherry-nd powder was obtained in yield: 1.42 g (76 %). MS rn/z
535 (m, 409 (M+ - C&+QCH~CHI-O-S~(CH~)~C(CH~)~)~), 334 (M+ - N(CH2CH2-O-
Si(CH3)2C(CH3)3)2), 3 1 8 (M+ - C&-N(CH2CH20Si(CH3)2)). MS mlz calcd for
C&,&NISi2: 535.18; Found: 535.13. 'H NMR (CDCI,): 6 0.03 (s, 12H. -C& on
methyl), 0.89 (s, f8H, -C& on ter?-butyl), 3.45 (t, 4H. -NC&- on amino. J = 6.40 Hz),
3.70 (t, 4H, -C&O- on amino, J = 6.10 Hz), 6.45 (d, 2H. =aCN- on phen y 1.3 = 9.20
Hz), 7.40 (d, 2H, ICCH- on phenyl, J = 9.20 Hz) ppm. And. Calcd for C2H4202NISil: C,
49.33: H, 7.90; N, 2.62. Found: C, 51.78; H, 8.54; N, 2.71.
33.2 NEDPA - Aryl Ailcyne Synthesis
[3] 4-nitro4'=ethynyldiphenylacetylene or NEDPA
Materials
1-nitro4iodobenzene (99.8 %), 2-methyl-3-butyn-2-oI(99.5 4b). palladium dichiorodi-
(triphenylphosphine) (99.5 %), diisopropylamine (99 $6). sodium hydroxide (97 45). and
1,Lkliiodobenzene (99.5 %), toluene (99.8 96) were obtained from Aldnch Chemical.
Cuprous iodide (99.999 Q) was obtained from Fisher Scientific. Toluene and
diisopropylamine were further distilled from calcium hydride and dried over sodium
under a nitrogen atmosphere.
Meîhoà
Purified [3] was synthesized and obtained from Professor T. B. Manier, University of
Waterloo, Waterloo. ON. 1 -nitro4iodobenzene (1 .ûû mmol, 0.23 g), 2-rnethyi-3-butyn-
2-01 ( 1 .O rnrnol, 0.76 mg), palladium dichlorodi(tripheny1phosp hine) (PDCDTPP)
(2 mol %) and cuprous iodide (2 mol 96) were added to a triple-necked 50 mi, round
bottom flask under an argon atmosphere in a glove box. The top-middle and top-left flask
openings were attached to a condenser and a Schlenk line apparatus. respectively.
Diisopropylamine (20 mL) was added through a septum into the top-right opening by
syringe under a nitmgen flow. The solution was refluxed for 15 minutes and solvent was
later cannulated under a nitrogen flow. Sodium hydroxide (15.6 mmol, 0.50 g) was
dissolved into toluene (50 mL) and added into the top-right opening through a septum by
syringe under a nitrogen flow. The naction was refîuxed for an additional 15 minutes and
cmde ethynylbenzene (0.8 1 mmol, 0.1 1 g) was isolated. Solvent was cannulated under a
nitrogen flow. I ,4diiodobenzene (2.43 mrnol, 0.79 mg), PDCDTPP (2 mol a), cuprous
iodide (2 moi %) and diisopropylamine (20 mL) were added to the triple-necked 50 m .
round bottom flask under the same previous conditions. The solution was refluxed for 15
minutes to form 1 -iodo4(4'-nitrophenylethyny1)benzene. Al1 the previous procedures
were repeated to produce 131. Solvent was rernoved and crude [3] was extracted into hot
toluene and filtend through a pad (2 cm deep) of silica gel (70-230 mesh). A lemon-
yellow powder was obtained in yield: 0.18 g (71 96). MS mlz 247 (M+) 2 17,200. 189,
174, 153, 136, 105.89.77. MS rn/z calcd for Ci&02N: 247.06; Found: 246.80. 'H
NMR (CDC13): 6 3.21 (s, lH, =Cg on terminal ethpyl), 7.5 1 (s, 4H, = C C - and - CHCC= on phenyl near to terminal ethynyl), 7.66 (d, 2H, -ClJCC= on phenyl near to
center ethynyl, J = 8.85 Hz), 8.22 (d, 2H, -NCW- on phenyl near to nitro, J = 9.03 Hz)
ppm. Anal. Calcd for C1&O2N: C, 77.72; H. 3.67; NT 5.67. Found: C, 78.48; H, 3.70; N,
4.81.
33.3 IPD and IPTE with NEDPA - Palladium-catalyzeà Cross-coupiing
Materiais
Palladium tetra(tripheny1phosphine) (99.9 9) and diisopropylarnine (99 Sb) were
obtained from Aldrich Chernical. Cuprous iodide was obtained from Fisher Scientific
(99.999 %). Diisopropylamine was further distilled from calcium hydride and dned over
sodium under a nitrogen atmosphere.
Method
Coupling of [îa] with [3] to form [4a] was performed. Palladium tetra(inpheny1-
phosphine) (PT'ïPP) was measured (2 mol %) under an argon atmosphere in a @ove box
and deposited into a triple-necked 50 mL round bottom flask equipped with a condenser
(located on the top-middle opening) and sealed with a septum. [2a] (2.96 mmol, 1.59 g)
was added into a 25 mL side-amed flask. Diisopropylamine (5 mL) was added by
syringe under a nitrogen atmosphen and the solution was bubbled under a nitrogen flow
by syringe needle for 20 minutes. Cuprous iodide (2 mol 46) and 4nitro-4'-ethynyldi-
phenylacetylene (1.19 mmol. 0.50 g) were carefhlly added to the topright opening of the
flask while under a nitrogen flow. [2a] was canaulated and added into the flask at a rate
of 1 &op per second under a nitrogen flow. Diisopropylamine (15 rnL) was used to rime
the inside of the flask. The solution was refluxed for 10-15 minutes and the diisopropyl-
amine solvent was removed under vacuum. Cnide [4a] was separated and purified by
flash column chromatography with silica (70-230 mesh) using hexane-ethylacetate (4: 1 to
2: 1 vlv). The procedure is described at the end of the section. An orange-yellow powder
was produced in yield: 1 .O3 g (73 %). 'H NMR (CDC13): 6 3.66 (t. 4H. -NC&- on amino,
J = 6.10 Hz). 4.24 (t, 4H, - C w - on amino. J = 6.10 Hz), 6.72 (d. 2H, -CHCN- on
phenyl near to amino. J = 8.53 Hz), 7.39 (d, 2H, -CCCIJ- on phenyl near to amino.
J = 8.3 1 Hz), 7.50 (S. 4H, S C C E - and -=Ck on center phenyl), 7.65 (d, 2H, -ECCE
on phenyl near to nitro, J = 8.53 Hz), 8.22 (d, 2H, -NCCH- on phenyl near to nitro,
J = 8.3 1 Hz) ppm. Mass spectrometric and elemental analyses were not completed for
[4a] due tu distorted mass spectra resulting from product decomposition.
Materiais
Palladium tetra(tripheny1phosphine) (99.9 %) and diisopropylamine (99 %) were
obtained from Aldrich Chernical. Cuprous ioâide (99.999 %) was obtained from Fisher
Scientific. Diisopropylamine was further distilled hom calcium hydnde and dned over
sodium under a nitrogen atmosphere.
Method
Coupling of [2b] with [3] to form [4b] was followed from the previous procedure. Cnide
[4b] was separated and purified by flash colurnn chromatography with silica (70-230
mesh) using methanoi-dichloromethane (1 :9 v/v). The procedure is described at the end
of the section. An orange-nd powder was produced in yield: 1.32 g (68 46). MS rn/z 655
(m, 535 (MC - CC-CaH4-CC-C&N(CH2CH2-O-Si(CH3)2C(CH3)3)2)), 409 (W - C a - N(CH2CH2-O-Si(CH&C(CH3)3)2), 147 (M+ - 02N-Cd&-CC); MS mlz calcd for
C3&1500,@2Si2: 654.33; Found: 654.41. 'H NMR (mg): 6 0.03 (S. 4H. -CE& on
methyi), 0.86 (s, 18H, CH3 on rert-butyt), 3.47 (t, 4H, -NC& on amino, J = 0.04 Hz),
3.72 (t, 4H, -CHHO- on amino, J = 0.04 Hz), 6.47 (d, 2H, -CIJCN- on phenyl near to
arnino, J = 0.05 Hz), 7.3 1 (d, 2H, - C C - on phenyl near to amino, J = 0.05 Hz), 7.50 (s,
4H, <CC- and -Cm& on center phenyl), 7.69 (d, 2H, - C C - on phenyl near to
nitro, J = 0.05 Hz), 8.25 ( d 2H, - N C - on phenyl near to nitro, J = 0.06 Hz) ppm. Anal.
Calcd for C38H500&2Siz: C, 69.69; H, 7.69; N, 4.27. Found. C. 69.93; H, 7 3 5 ; N, 4.30.
3.2.4 NDPEPD - Direct Thiolation
Materiais
Methanol(99.9 %) was obtained frorn Caiedon Laboratones. Chloroform (99.8 96).
potassium carbonate (99.99 %), tetrabutylammoniurn chloride (TBAC) (99 + 96) and
sodium hydroxide (97 + %) were acquired frorn Aldrich Chemical md used without
further purification.
Method
Puiified [4a] (2.23 mmol, 1 .O3 g) was added into a 100 mL round bottom flask and
partially dissolved in methanol-chloroform-water ( 1 : 1 : 1 v/v/v). Potassium carbonate
(3.62 mmol, 0.5 g), TBAC (4.44 mmol, 1.23 g) and sodium hydroxide (2.08 mmol,
0.10 g) were added. Unfortunately, decomposition of [4a] did not permit further analysis.
Materials
Teinmethylammonium fluoride (TMAF) tetrahydrate (98 %) in acetonit.de (99 + %),
methanol(99.9 %), toluene (99.8 96) and hydrochloric acid (37 % wlv) were acquired
from Aldrich Chemical.
Method
Purified [4b] (2.12 mmol, 1.32 g) was added into a 100 mL round bottom flask and
dissolved in TMAF tetrahydrate. A methanol-toluene (5: 1 vlv) solution and h ydrochloric
acid (0.5 rnL) were added and the solution was heated to SOO C. Solvent was removed
using a rotating evaporator under vacuum. Cnide [Sb] was separated and purïfied by
silica gel flash column chromatography (70-230 mesh) using hexane-ethylacetate (4: 1 to
2:1 v h ) . The procedure is described at the end of the section. A cherry-red powder was
produced in yield: 0.92 g (82 %). MS m/z 426 (M+), 379 (M' - Cs&çC-C6&-cC-C6H4-
N(CH2CHtOH)2), 305 (W - CC-C&-CC-C&-N(CH2CH20H)z), 28 1 (M+ - C A - C C -
Ca)4-N(CHzCHzOH)2), 146 (M+ - 02N-Cd&-CC); MS mlz caicd for Cz&IuO&:
426.16; Found: 426.45. 'H NMR (CDC13): 6 2.10 (s, 2H, broad terminal OH), 3.5 1 (t, 1H,
-NC&- on arnino. J = 6.10 Hz), 3.76 (t, 4H, -C&O- on amino, J = 6.10 Hz), 6.69 (d, 2H,
-CEJCN- on phenyl near to arnino, J = 8.54 Hz), 7.23 (d, 2H. =CCC& on phenyl near to
arnino, J = 8.30 Hz). 7.39 (s, 4H, S C C H - and -CHCC= on center phenyl), 7.65 (d, W ,
-CHCCs near to nitro, J = 8.54 Hz), 8.21 (d. 2H, -NCC& near to nitro, J = 8.30 Hz)
ppm. Anal. Calcd for C26HZo4N2: C, 73.24; H. 5.20; N, 6.57. Found. C, 73.48; H. 5.60;
N, 6.8 1.
[a] N4-[(4'-nitrodiphenylethynyl)phenyl]-b&-2"die~me~iola~e or NDPEPD
Materiais
2.4-bis-(4-methoxypheny1)- 1,3,2,4-dithiadiphosphetane 2,4disulfide (Lawesson's
reagent) (97 %) in toluene was obtained fiom Aldrich Chemicai.
Met hod
[Sb] was converted to [6] by direct thiolation. Lawesson's reagent (5.86 mmol, 0.8 1 g)
and [Sb] (2.37 mmol, 0.18 g) were added under an argon atmosphere in a glove box and
deposited into a triple-necked 50 mL round bottom flask equipped with a condenser
(located on the top-middle opening). Toluene (20 mL) was added by syringe under a
nitrogen atmosphere and the solution was s h e d for 24 hours under a nitrogen
atmosphere at room temperature. Toluene was cannulated under a nitrogen flow. Cnide
[6] was separated and purified by silica gel flash column chromatography (70-230 mesh)
using toluene-methanol(2: 1 to 1: 1 vlv) to produce an orange-brown powder in yield:
0.73 g (70 96). MS m/z 458 (M?), 424 (M+ - HzS), 246 ((W - C&-N(CHZCH~SH)~); MS
m/z caicd for C&IUO~N~S~: 458.1 1; Found: 458.12. 'H NMR (CDC13: 6 1.33 (s, 2H,
broad terminal -Su, J = 6.15 Hz), 3.76 (t, 4H, -NC& on amino, J = 6.10 Hz), 4.06 (t,
4H, -Cas- on amino. J = 6.10 HZ), 6.69 (d, 2H, -CuCN- on phenyl near to amino,
J = 8.54 HZ), 7.42 (d, 2H. -CCCI-J- on phenyl near to amino. J = 8.30 Hz), 7.51 (s, 4H,
=CCCH- and -CHCG on center phenyl), 7.65 (d, 2H, -CHCG near to nitro, J = 8.54
Hz), 8.23 (d, 2H, -NCCH- near to nitro, J = 8.30 Hz) ppm. Anal. Calcd for
C ~ ~ H Z O ~ N ~ S ? : C, 68.1 1 ; H, 4.84; N, 6.1 1. Found. C, 68.65; H, 4.95; N, 6.16.
Süica-Gel Column Preparation for Flash ~hromatography'~
Materials
Hexane (98.5 %). ethylacetate (99.5 + %), silica gel (75-230 mesh), chloroform (99.8 %),
methanol(99.8 %), dichloromethane (99.6 %) were obtained from Aldrich Chernical and
used without further purification.
Method
Hexane-ethylacetate (4: 1 v/v) was added into a flask containing the dry silica gel to
produce a white slurry. The silica gel was transferred to a glass column (4 cm - inner
diameter) and filled to at least 30-50 cm in depth. Solvent levels were always maintained
above the silica gel level to provide efficient separation dunng elution. A sand layer
(0.5 cm) was placed on top of the silica gel after allowing the silica gel to settle. The
solvent level was lowered to just above the sand level layer. Chloroform was used to
dissolve the product and the solution was then carefully transferred into the colurnn by
Pasteur pipette. Hexane-ethylacetate was carefully added beginning with 4: 1 v/v portions
(NOTE: methanol-dichloromethane was used in some cases). Gradually. 3: 1 and 2: 1 v/v
portions were added as the separation proceeded. Fractions were collected in test tubes
and simultaneously anaiyzed with thin layer chromatography (70-230 mesh) using the
same eluting solvent. The product was isolated by cornparhg the starting material to the
gathered fractions. Fractions containing the product were collected and solvent was
removed by rotating evaporator under vacuum.
3.2.5 NDPEPD in Poly(styrene)
Su bs trates
ITO-glass slides (25 x 37.5 x 1 mm - 100 #square surface resisitivity - coated on one
side) were obtained from Delta Technologies, Ltd. Slides were taped on the coated side to
allow for an exposed electricai contact area to apply a voltage for corona discharge
poling.
Materials
NDPEPD was synthesized by us and used after final purification. Polystyrene (MW
45000) was obtained from Aldnch Chernical. Methanol(99.9 96) and toluene (99.8 %)
were obtained from Caledon Laboratones and used without hirther purification.
Method
Poly(styrene) (PS) (2.68 x IO-' mol, 1.210 g) was measured into a 5 D m glass viai and
dissolved into a methanol-toluene solution (1:4 v/v). NDPEPD (1 .O mol Z) was dissolved
into the PS-methanol-toluene solution and stirred with a magnetic stir bar. Continuous
stimng was rnaintained for 1 hour. The solution was filtered through a 0.2 mm ~crodisc@
CR PTFE filter (Gelman Sciences) into another 5 Dram glass vial. The solution was
dipensed ont0 an iT0-glass slide (onto the ITO-coated side) using a Pasteur pipette and
spin-coated at 1500 and 2000 rpm for 30 seconds using a photo-resist spin-coater
(Headway Research) to produce films of thickness 1-3 m.
Mass spectrometry was perfonned using an MS25RFA mass spectrometer (Kratos
Analytical Instrumentation) with a source temperature of 250°C and a 70 eV source
voltage. 'H NMR spectroscopy was recorded using the Gemini-200 NMR (Bruker
Instruments) at 200 MHz resonance frequency. 'H chenilcd shifis were nferenced to the
intemal standard tetramethylsilane (TMS) and d l spectra were recorded in deuterated
chloroform (CDC13). Elemental analyses were conducted using a 240 XA elementd
analyzer (CEC Instruments) in the Department of Chernical Engineering, McGill
University. Montréal, PQ. Film thickness was measured using a Dektak (Sloan
Technology) profilorneter provided by the Photonics Research Group, École
Polytechnique de Montr&d, Montréal, PQ. Electncal poling by electrical point corona
discharge poling at elevated temperatures was followed from Monazavi et al." and was
performed simu1taneously with SHG measurement for i21Z' nonlinear optical susceptibility
(see Figure 2.4). Samples were heated to 150' C over a 40 minute interval with an
electrical field of 4.0 kV (1.0 cm tip-to-piane gap) and cooled to room temperature over a
30 minute interval. SHG measurements for determining i2' was performed using a
Quanta-Ray DCR Q-switched neodymiurn-doped yttrium-aluminum gamet (Nd:YAG)
laser (output A = 1064 nm) with a 10 ns pulse width at 5 ml, a 10 Hz repetition rate and
equipped with optical lens components (Oriel Instruments). Sarnple and z-cut quartz
reference mounts (Oriel Instruments) with rotating speed 2' per minute from O - 60°.
photomultiplier tubes (PMTs) (Harnatsu Company) and duai-channel boxcar integrators
(Stanford Company) were used. The SHG signal was recorded from -50' to +50° for the
quartz reference and from O to 60" for the sample, and amplified using PMTs and
averaged using the boxcar integrators. Maximum 22' was calculated using Microcaic
Origina 3.73 software. Al1 rneasurements were performed in the Materials Research
Center and the Department of Physics and Astronorny, Northwestern University,
Evanston, IL.
Results and Discussion
3.3.1 Synthesis - NDPEPD
Iodination of N-phenyldiethanolamine (PDEA) was initially perfomed by dissolving
PDEA into methanol instead of water to improve solubility. Sodium bicarbonate extracts
hydrogen iodide to form sodium iodide and water (favorable equilibrium to prevent
hydrogen iodide formation - reduction). Further purification of [1] is necessary since
PDEA was not initial1 y purified (recrystailized).
The exposed hydroxyl groups in [ I l required protection. Acetic anhydride was used to
acetylate PD hydroxyl in the presence of pyridine base and catalytic DMAP? DMAP is
104 times more active as an acetylation cataiyst than pyridine alone.l6 Nucleophilic
addition of hydroxyl at carbonyl (acetic anhydride) gives the acetyl ester [2a]. Another
method for acetylation includes using acyl chloride in pyridine.'7 Similarly, TBDMS
chloride was used to silanate PD by reacting with catalytic DMAP in pyridine base.
Nucleophilic addition of hydroxyl to the silicon cation forms the silyl ether [2b]. TBDMS
is useful for a wide range of alcohols and is more stable to hydrolysis than trimethylsilyl
or dimethylisopropylsilyl ether. Other methods for silanation include using imida~ole~~
and dilithium s~lfide.'~
Palladium-cataiyzed cross-coupling of [2a] and [2b] each with [3] produce [4a] and [4b].
PTTPP catalyst initiates and propagates the cataiytic cycle.21 ~ d * reduces to pd0 by
oxidative coupling of [3]. P'iTPP oxidizes by addition of [2a] or [2b] but the presence of
thiol during coupling forms disulfide bridges by oxidative coupling. Consequenily, sulfur
contaminates PTTPP catalyst and depresses yields. Tram-metallation is performd by
reductive elimination of [4a] or [4b] and ETTPP is qenerated."
Deacetylation was used to deprotect acetylated hydroxyl (ester). Insolubility of [4a] in
methanolthloroforrn-water prevented deacetylation. Phase iraasfer TBAC improved
deacetylation at the aqueous-organic interface. Potassium carbonate was added initially to
deacetylate the [4a]? However, deacetylation was inhibited by insolubility of [4a].
Therefore, sodium hydroxide was added to aid in acetyl removal. Unfominately, [4a]
decornposed soon thereafter, and ammonium salts precipitated. Another method for
deacetylation involving potassium cyanide in ethanol for transesterification may have
been possible.u Nevertheless, rnilder conditions were necessary to remove acetyl. Thus,
we turned to TBDMS since deprotection procedures were extremely mild and limited the
risk for decomposition of product.
Desilanation ruas used to deprotect silanated hydroxyl (silyl ether). A common approach
used €or mild desilanation involves TBAF tetrahydrate in THF. Unfortunately, gelation
occurred due to steric restraints in [Sb]. Other less efficient methods for silyl removal
include boron trifluoride etherate in potassium," hydrofluoric acid in acet~nitrile.~ We
resorted to TMAF tetrahydrate since effective fluoridation was accomplished with
minimal side reactions? TBDMS was easily rernoved using fluotide ion by fluoride
nucleophilic addition on silicon.
Figure 3.2 shows the direct thiolation of hydroxyl in [5bl to form sulfhydryl in [6] with
Lawesson's reagent (LR). LR or (2.4-bis-(4-methoxypheny1)- l,3,2,4dithiadiphos-
phetane-2,4-disulfide) converts hydroxyl directly to sulfhydryl. The mechanisrn for the
direct thiolation postulates an O-alkylphosphonodithioic acid intermediate."
Nucleophilic addition of [Sb] on LR forms the intermediate. Subsequent P-SH
nucleophilic addition follows by cleavage to form [6]. We considered other methods for
thiolation. However, these methods involve multiple and complex synthetic steps.
1) acid hydrolysis of Bunte salts2' (S-alkyl thiosulfates to foxm thiols),
2) acid hydrolysis of aceta12' (to fom thioacetai and then thiol),
3) acid hydrolysis (peroxyformic acid) of hydroxypheny130 (to form thiols).
Chernical ionization (CI) methods wen used to analyze [Il to [4a] and [4b] and fast-atom
bombardment (FAB) methods were used to analyze [Sb] and [6]. 'H NMR spectra
assignments are given in tenns of chernical shifts. Most elemental analyses erron are
greater than the f 0.4 % acceptable range for carbon and hydrogen probably due to
impurities. These data include values for [l], [2b], [3] and [6].
Figure 3.2: Schematic - NDPEPD direct thiolation
3.3.2 Cornparison - NDPEPD versus ODMAPS
NDPEPD has 3 phenyl units and 2 ethynyl units (see Figure 3.2). Dialkylarnino is the
donor and nitro is the acceptor. ODMAPS has I phenyl, 1 pyridyl and 1 ethenyl unit (see
Figure 3.3). Dialkylarnino is the donor and pyridinium is the acceptor. Both NDPEPD
and ODMAPS have extended conjugation that increases linear and nonlinear
polarizability. Nonlinear poluizability increases by specific donor-acceptor
combinations. Marder er al?' descrîbed donor-acceptor influences on the Is'order
molecular hyperpolarizability, p.
Projecting pont0 the z-axis with the dipole moment, p, results in large nonlinear i2', as
shown by
i 2 j = (kem [ 151
where collinearity is important for and k. NDPEPD is molecularly linear and rigid
with ,u and pdirected dong the z-axis (set Figure 3.3). Therefore, the largest f l component (m ont0 pz allows for charge transfer to fom the excited state on the z-axis.
Figure 3.3: NDPEPD ground and excited (rr-x*) States
Optimal poling of the dipole moment, p, produces optimal 2''. Donor and acceptor
rernain conjugated by pz-electron orbital interactions even during unrestrîcted phenyl
rotation in tolanes?* Banoukas et al." obsewed that tolanes still exhibit peven after 90"
rotations about the z-axis plane. Stilbazoles do not exhibit #Mer z-ais plane rotations
since the px-electron conjugation is broken. Unrestricted phen y 1 rotation in tolanes
contrasts with the restncted phenyl rotation in stilbazoles.
ODMAPS is molecuIarly rigid with p and Pdirected along the z-axis as well (see Figure
3.4). A along allows for charge transfer to occur and form the excited state along the
Figure 3.4: ODMAPS ground and excited (x-x*) States
As mentioned previously, the structural conformation (E)- or trans- from ethenyl nstricts
phenyl rotation in ODMAPS. Stilbazoles have inherently larger f l than tolanes even with
identical donors and acceptors (see Tables 3.2 and 3.3). Both s tilbazole and stilbene have
large and i2', and these are 40-50 % larger than those of tolane. Cheng et al."
attnbuted lower to sp2-hybridized phenyl and sp-hybridized ethynyl interactions (see
Figure 3.5). Diminished electron transfer in ethynyl compared with ethenyl accounts for
large differences in rr-electron energy leveis.
NDPEPD carbon sp2- sp hybridized orbitals
ODMAPS carbon SC- sp2
hybridized orbitals
Figure 3.5: NDPEPD and ODMAPS orbital configurations
However, recent experiments indicated that eth yn y1 significantl y improves opticd
tran~~arenc~.~' In stilbazole derivatives, nonlinearity increases but optical transparency
decreases. A compromise between transparency and nonlinearity must be considered
when optimizing the optical performance. This compromise is known as the nonlinearîty-
transparency trade-off. Marder et aLM demonstrated improvements in both opticd
nonlinearity and transparency in tolanes by increasing the ir-electron conjugation length.
Tables 3.2 and 3.3 show the relationship between Band An, for tolanes and stilbazoles.
In Table 3.2, CN is an acceptor with increasing donor strength from CH30 to N(CH3)2.
With NO2 as an acceptor, the acceptor strength increases as shown Sy die increasing
and p. Donor strength increases as the carbon number of the alkylamino group is
lengthened. Increases in carbon number promotes alkyl tail self-poling and contributes to
p and i2'. In addition, the presence of sulfhydryl appears to increase A- and P substantiall y.
Table 3.1
A- and f l for tolane derivatives (adapted from Marder et
Tolane LU P (Donor : Accepter) (nm) (10 -49 C 3 m/J 3 2 , 10"~esu)
NOTE: All measurements were measured in CHC13. Electric field induced second harmonic (EFISH) measurernents for fl were obtained at 1064 nm.
Table 3.2 A- and f l for stilbazole derivatives
(adapted from Bubeck et al?')
S tilbazole LU f l (Donor : Accepter) (nm) (10 -49 C 3 m 3 /J 2 , 10;" esu)
N=R : N(C3&0H), 354 nm 6.61, 178
CH3-N'=R : N(C16HB)2 475 nm 1 1.2,304
Ch-W-R : N(CiaHnh 325 nm 9.28,250
H - f i R : NCH,(CliH3,) 394 nm -
NOTE: N=R represents pyridinium
In Figure 3.6, the A, for NDPEPD is 455 nm. NDPEPD may be more affected by the
overlapping silver surface plasmon absorbance at 415 nm and may provide a more
effective optical field enhancement than ODMAPS in the MPTMS-silver-MAA-
ODMAPS heterostnicture. Therefore, NDPEPD may overall be a better option than
ODMAPS for use in MPTMS-silver-NLOD heterostructures,
500 6b 700 Wavekngth (nm)
Figure 3.6: UV-VIS spectrum - NDPEPD tolane in methmol
We use NDPEPD and DMAPS (at the bottom of Tables 3.2 and 3.3). In general,
stilbazole has larger ,û than tolam, therefore al1 things king equal, DMAPS should
possess larger 2'' than NDPEPD. Moylan et al? also noted that tolanes (including
NDPEPD) possess lower f l than stilbazoles (including ODMAPS). In our i2' measurements, we simultaneousl y monitor ia responses as poling occurs until maximum
f ' . Table 3.4 compares i2' (1 -5 x 10-23 &J2 or 0.4 x 1 0 ~ CSU) for PS-NDPEPD to those
for other NLODs doped into poly(styrene).
Table 3.3
i2' for NDPEPD and DMAPS in poly(styrene)
NLOD doped into f ' 23 3 2 Poly(stpne) (10- C IJ , lu8 esu)
PS-NDPEPD 1.49,0.40
PS-Disperse Red 1" 4.46, 1.20
PS-DMAPS~~ 2.90,0.78
Differences between 2'' for PS-NDPEPD and PS-DMAPS result from lower ,ûof
NDPEPD. as discussed previously. Overall, the i2' for PS-NDPEPD is comparable io
many similar organic NLODs and may prove to be better in the nonlinearity-transparency
trade-off regime.2g
Conclusions
summnry
We successfully synthesized an alternative NLOD tolane (NDPEPD) that possesses
spacers that can be chemisorbed direcily ont0 silver or gold nanoparticle surfaces without
the requirement of an additional separate spacer. Palladium-catalyzed cross-coupling of
IPD and NEDPA provided access to design the linear NDPEPD. Direct thiolation with
Lawesson's reagent was used to convert hydroxyl directly to suuhydryl. Thiolation was
not performed prior to cross-coupling due to inevitable contamination of the palladium
catal yst. Desilanation of TBDMS with TMAF proved to be more effective than
deacetylation with potassium carbonate and sodium hydroxide in rernoving protecting
groups on [4b] and [4a], respectively.
23 3 2 i2' for PS-NDPEPD (1.5 x 1U C /J or 0.4 x 10" esu) compared favorably to those
PS-DMAPS (2.90 x 10'~ or 0.78 x 10-~ esu). In both cases, the AC2' of the organic
NDPEPD and DMAPS were comparable those of the organic NLODs listed. Potentiai for
effective silver surface plasmon optical field enhancements were evident with the use of
NDPEPD, which has A- at 455 nm. Therefore, NDPEPD demonstrates itself as a
cornpetitive NLOD alternative to potentially replace ODMAPS in the MPTMS-silver-
MAA-ODMAPS heterostructure.
Future Research
We suggest further exploration of design strategies for advancing NDPEPD tolane
synthesis. P'lWP cataiyst contamination may be eliminated by starting with extrernely
pure IPD and NEDPA. Improvernents in #3 while maintaining transparency may be
incorporated into NDPEPD for uses in optical device applications. NDPEPD may also be
anchored into polyrner to prevent relaxation decay after electrical field poling, thereby
alleviating i2' decay in PS-NDPEPD composites.
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Appendix
Units Conversion Table A-1 lis& cornmon units used in nonlinear optics for MKS, SI (système
international) and esu (electrostatic unit) systems. Units for i2' follows from the MKS
system, as used in the literature. Q defines the conversion factor, and N is the physical
quantity of interest.
Table A- 1
Units for polarizability, hyperpolarizability and nonlinear susceptibility
Ph ysicai MKS SI esu* NsiQ = Na" NMKSQ = N a Quantity A
Units described by the esu* system are often quoted as esu units.
Note:
C = coulomb
SC = statcoulomb (also represents esu units)
J =joule
m = meter
pm = picorneter
V = volt
Units Conversion
Table A- 1 lists common units used in nonlinear optics for MKS, SI (système
international) and esu (electrostatic unit) systerns. Units for 2" follows frorn the MKS
system, as used in the Iiteratun. Q defines the conversion factor. and N is the physical
quantity of interest.
Table A- 1
Units for polarizability, hyperpolarizability and nonlinear susceptibility
Uni& descnbed by the esu* system are often quoted as esu units.
Note:
C = coulomb
SC = statcoulomb (also rcpmnts esu units)
J = joule
m = meter
pm = picorneter
V = volt