Threshold ionization energy of C60 in the solid state

Chemical Physics 162 (1992) 433-438

North-Holland

Threshold ionization energy of CeO in the solid state

Naoki Sato Department of Chemutry. College ofArts and Sciences, Unwerslty of Tokyo, Komaba, Meguro, Tokyo 153, Japan

Yahachi Saito Department ofElectrlca1 Engrneermg, Faculty ofEngmeenng, MI’~ Umverslty, Tsu 514, Japan

and

Hisanori Shinohara Department of Chemlstryfor Materials, Faculty ofEngrneermg, Mi’e Unwersrty, Tsu 514, Japan

Received 23 December 199 1

The threshold ionization energy of C& vacuum-deposited thin films was determined to be 6.17 + 0.07 eV by ultraviolet pho-

toemisslon spectroscopy with low photon-energy excitations and retardmg-field analysis of photoemitted electrons. This method

also enabled to determine the work function to be 4.85 f. 0.05 eV. The obtained values can be compared with the values reported

so far and are discussed in terms of energetic relaxation of a CSO molecule via condensation to the sohd state.

1. Introduction

Since the recent success of macroscopic prepara- tion of CeO (buckminsterfullerene) [ 1,2 1, a number of spectroscopic studies have been carried out on this novel form of carbon cluster. Furthermore, its solid state properties have attracted much attention, particularly after superconductivity of a potassium doped CbO thin film was recently reported [ 3 1. Accordingly, both theoretical [ 4,6] and experimental [7-l 1 ] studies on the electronic structure of solid CeO are of

great importance and further study in great detail is necessary.

The highest occupied electronic states of solid Cd,, have been observed mainly by ultraviolet photoemission spectroscopy (UPS) using deflector analyzers for electron energy analysis and helium resonance lines (hv=21.2 and 40.8 eV) [7] and synchrotron radiation (40<hv<200 eV) [ 81 as light source. These high excitation energies and the deflector energy analyzer are not always useful to determine absolute values of the electron energy levels, which are definitely indispensable information on the elec-

tronic structure, because of the more severe electron scattering caused by the higher energy electrons and the possible ambiguity in energy calibration, respectively.

Concerning this, Lichtenberger et al. [ 71 reported that the first vertical ionization energies of Ceo in the gaseous and solid states are almost the same (7.6 eV). This result appears to be rather perplexing when one considers the electronic stabilization of a photoion- ized molecule, resulting from electrostatic polarization induced on the surrounding molecules in the solid [ 12 1. Such electrostatic stabilization energy, i.e. polarization energy, works in common organic molecular solids [ 13 1, so that their first ionization energies are usually smaller than those of their constituent molecules in the gas phase.

In the present work, we have measured ultraviolet photoemission spectra of vacuum-deposited thin films of CbO using a combination of a low-energy excitation light source and a spherical retarding-field electron energy analyzer, which has much higher ef- ficiency in detecting very low-energy electrons than deflector analyzers widely used for photoemission

0301-0104/92/$ 05.00 0 1992 Elsevier Science Publishers B.V. All nghts reserved

434 N. Sato et al. /Threshold ronrzatlon energy of solrd C,,

measurements. This combination enabled US to per- ments was an improved version of that reported in

form a much more accurate determination of the detail previously [20]. The light source, a water-

highest occupied energy levels, particularly the cooled-capillary hydrogen discharge tube attached to

threshold ionization energy, of solid CbO than those a 0.5 m Seya-Namioka monochromator, was used in

reported so far. Further, the obtained values lead to the photon energy range from 7 to 12 eV. The elec-

an estimate of the polarization energy of Go, and, tron energy analysis was done with a spherical retard-

comparing with the gas phase ionization energy in- ing-field analyzer, whose inside was coated with gold,

dicated by Lichtenberger et al. [ 71, we discuss the with an ac modulation method. This enabled energy

electronic polarizability of CeO in comparison with values to be determined with an experimental error

those of conventional organic solids. of GO.15 eV.

2. Experimental 3. Results and discussion

The CsO samples have been prepared following the method described by Kratschmer et al. [ 11. A graphite rod was evaporated by resistive heating under a partial helium pressure of ( 1.3-2.7) X lo4 Pa. The carbon soot-like deposit was scrapped from the inside of the vacuum chamber. Soluble products were extracted from the deposit by the Soxhlet method with boiling benzene. The extraction was judged to be complete (8 h) when dark red-brown material ceases to leach from the Soxhlet thimble. The so obtained liquid was known to contain &, and other fullerenes in addition to CbO [ 14-l 7 1. The concentrated extract was subjected to column chromatography using neu- tral alumina (ICN BIOMEDICALS Akt. I) with hexanes as eluent [ 18 1. The elution with hexanes then produced with colored bands: the first (purple-ma- genta) consisting of pure (99%) Go. The sample pu- rity (with respect to CT0 and larger fullerenes) was checked using high-pressure liquid chromatography. Evaporation of the eluent was performed at 250°C. The CeO powder obtained was further purified by vacuum sublimation at 430°C under a vacuum of < low5 Pa.

Fig. 1 shows ultraviolet photoemission spectra of a vapor-deposited CeO film, observed with four different excitation source energies, as energy distribution curves (EDCs) of photoemitted electrons plotted versus the retardation voltage, V,. The stopping voltage, V,, which corresponds to the onset for photoemission of electrons with the highest kinetic energy, in- creases with the source energy. On the other hand, the saturation voltage, V,, which is determined using the vacuum level in the sample solid relative to that

11 27

Thin solid films of C&, were prepared by vacuum deposition for in situ measurement of the threshold ionization energy. The C& films were vacuum-deposited onto copper substrates at room temperature, using a simple gun for vapor deposition of the organic thin films [ 191, under a vacuum of 10eJ Pa. Their thicknesses were about 20 nm, measured on a quartz oscillator monitor.

0 1 2 3 L 5 v, / v

The threshold ionization energy of CGO was determined for these films by ultraviolet photoemission spectroscopy. The apparatus used for these measure-

Fig. 1. PhotoemIssion spectra of a vapor-deposited CeO film as

observed versus the retardation voltage with a spherical retard-

mg-field analyzer.

N. Sato et al. /Threshold lomzatlon energy ofsolld C,, 435

in the electron energy analyzer, i.e. vacuum-deposited polycrystalline gold, does not change, being I’, = 0.00 V for these spectra. Besides, the work function of the gold coated inside of the analyzer has beforehand been determined to be 4.85 + 0.05 eV from the photoelectron spectra of a copper substrate only and gold evaporated onto them. The above result on V, therefore shows that the work function, @, of the CeO thin film is 4.85 eV, when its Fermi level is assumed to coincide with the Fermi level in the gold analyzer through the copper substrate and electric wires. This value is larger than those of two prototyp- ical states of carbon in the solid state, i.e. diamond and graphite: the work functions of diamond and graphite have been reported to be 4.5 [ 211 and 4.7 eV [ 22 1, respectively.

Re-plotting the observed photoemission spectra as depicted in fig. 1 by shifting each spectrum along the abscissa over the difference of the excitation source energy and a particular photon energy, one can obtain fig. 2. The abscissa of this figure, the ionization energy in the solid state, Z,, is found by determining the threshold ionization energy, Zih, using the relation:

Z;h=hv-e(V,-V,). (1)

The Zq value of solid C6,, has practically been determined to be 6.17 f 0.07 eV from averaging the values obtained from the spectra for different excitation source energies. In fig. 2 thus obtained, two spectral

1 Fig. 2. Photoemission spectra of a vapor-deposlted CGO film plot-

ted versus the ionization energy in the solid state. The ionization

threshold and positions of the spectral features are indicated by

arrows and vertical bars, respectively.

features can be distinguished for the spectra above hu = 9.25 eV, while the second lowest band is not perfectly separated due to the contribution of secondary scattered electrons. The maxima of the features are denoted by vertical bars in fig. 2. To determine the position of the second feature, we have referred to difference spectra found by subtracting the assumed contribution of the secondary scattered electrons from the original spectra for hv= 9.25 and 9.84 eV.

Thus, we have determined the energetic positions of the two lowest features in the ultraviolet photoemission spectra of solid Ceo to be Z: ~6.85 eV and Zf = 8.20 eV. These values can be compared with those reported so far: Zf = 7.6 eV and Zf = 8.95 eV [ 71, Z: =7.3 eV and Zs =8.6 eV (from fig. 2 in ref. [ 81) (both relative to the vacuum level), and Ek=2.3 eV and Ei=3.7 eV [23], where E,, is the binding energy relative to the Fermi level. It is ob- vious that the energy differences for the two features, i.e. AZ=Zz -Ii, are in good agreement with each other: 1.35eVinthiswork, 1.35eV [7], 1.3eV [8], and 1.4 eV [ 241. This shows that our spectra are es- sentially identical to the lower energy parts of the spectra already reported.

On the other hand, it is noted that the absolute energy values are different among the respective measurements: the Zl value scatters within 0.75 eV. Lich- tenberger et al. [7] have claimed that their “first vertical ionization energy”, which corresponds to Z: above, might have uncertainties though it is carefully determined. Weaver et al. have mentioned an “inde- pendent check” in energy determination [ 8 1, which was however obtained by X-ray photoelectron spectroscopic studies of again Ceo on InP( 1 lo), whose electronic structure is not perfectly clarified. Here again, we take into account that the values in these references were determined using both higher excitation source energies and deflector electron energy analyzers, which are not always useful to determine absolute energy values. By contrast, our measurement system with lower excitation sources as well as a spherical retarding-field analyzer is useful to determine such values in the region of the photoemission threshold.

The Z: value, thus determined as 6.85 eV, will solve the problem derived from the data by Lichtenberger et al. [ 71 whether the first vertical ionization energies of Go in the gaseous and solid states are almost

436 N. Sato et al. /Threshold ronxatlon energy of solid C,,

the same or not, as pointed out earlier. If we compare this value with the corresponding one in the gas phase, ZL =7.61 kO.02 eV, reported as a preliminary value [ 7 ] (besides, 7.68 eV has been obtained with a DV- Xa calculation [ 25 ] ), the intermolecular relaxation shift energy, R, or AE, [ 13,241, of CeO is determined to be 0.76 eV, which is evidently finite though even smaller than those of organic solids, typically 1.1 eV

[ 13,241. Concerning this, we will continue with a more de-

tailed discussion on the energetic relaxation of a CeO molecule in the solid state. In the case of most organic solids, the intermolecular relaxation shift is clearly determined from the comparison of photo-

electron spectra in the gaseous and solid state, since they correspond well with each other because of the persistence of the molecular (electronic) identity even in the solid. However, previous studies have shown that the polarization energy, which is charac- terized by

P, =z; -I:” , (2)

where Zi is the adiabatic ionization energy of a molecule in the gas phase, stands for a relaxation energy of a more bulk nature [ 13,26,27 1. We have therefore tried to estimate the P, value of CeO: P, < 1.4 eV, assuming that Zi < 7.6 eV from the preliminary ZL value of Lichtenberger et al. [ 7 1. Although this value is much smaller than those of common organic compounds, e.g. polycyclic aromatic hydrocarbons, typically 1.7 eV, it has been proved that the intermolecular relaxation effects involved in the photoionization process exist even in the case of CGO.

eq. (3). For this purpose we have beforehand calculated a and p: the latter is directly obtained from the reported lattice constants, i.e. fee, a= 14.198 A and Z=4 [ 291. On the other hand, a can be calculated to be 7.82~ lo-39 Fm2 from a d-function potential

model [ 30,3 1 ] using the data on the reported molecular geometries: d( C-C) = 1.46 A and d( C=C) = 1.40 8, [ 32 1. Thus we obtain P, = 1.11 eV, putting these a and p values in eq. (3). This value, which is even smaller than the experimentally obtained value above, supports that the polarization energy of C,, is deti- nitely smaller than those of most organic solids. The

small polarization energy of Cho could be attributed to the small Cw value. the value is much smaller than

that of e.g. circumcoronene (Cs4H18): @=9.78x 1 O-39 Fm2, which is calculated with the same method

as applied above, using widely known data on the C- H and C-C bond lengths [ 33 1. (Circumcoronene is a supposed planar polycyclic aromatic hydrocarbon, whose molecular structure, having a high symmetry ( Dbh), is shown in fig. 3, and has a molecular weight comparable with Ceo.) By the way, the P, value of a single layer “molecule” of graphite, i.e. another phase of solid carbon, could be estimated to be 1.2 eV [ 13 ] from extrapolation of the Zg values of a number of large polycyclic aromatic hydrocarbons [ 341 and the Zih or $ value of graphite [ 201. It can be noted that the P, values are similarly small for both Ceo and graphite.

Further, an approximate expression for P, has been derived from an evaluation of the interaction energy between an ion and the induced dipoles on the surrounding molecules [ 12,13 1. It generally leads to values in good agreement with those observed experimentally [ 13,28 1. This relation, given in SI units, is:

Further, we will comment on the semiconductivity of solid Cbo. Theoretical calculations of the electronic structure show that the fee Ceo crystal is a semicon- ductive material with a direct energy gap ( 1.5 eV) at the Brillouin zone boundary [ 41. The optical absorp- tion edge therefore determines the observed energy gap, Eg, to be 1.95 eV [ 151. Comparing this value with the @ and I:” values obtained above, it is clear that the Fermi level is located more closely to the bot-

P, = 6.99 ( e/4mo)2@p4’3 , (3)

where to, au, and p are the permittivity of a vacuum, the mean molecular polarizability, and the molecular-packing density in the solid, respectively. This last term is defined as the number density of molecules in the unit crystal cell.

Here we try to calculate the P, value of Ceo using Fig. 3. Molecular structure of circumcoronene.

N. Sate et al. / Threshold lonlzation energy OJ Jolid C,, 437

tom of the conduction band than to the top of the valence band in the energy gap, as depicted in fig. 4. This simple energy scheme indicates that the CGO crystal in an n-type semiconductor. This is consistent with the remarkable enhancement of the electric con-

ductivities of CeO films upon potassium doping [ 3 1, which is considered to increase the charge carrier density by electron transfer from potassium atoms to C 60.

Finally, the bottom of the conduction band corresponds to the electron affinity level of the crystal, in accordance with the polarization model for the electronic structure of organic molecular crystals [ 12,35 1. The electron affinity in the solid, A,, is therefore calculated to be 4.22 eV from Zih -Er In the polarization model the following relation is also fundamental:

A,=A,+P_ , (4)

where A, and P_ are the electron affinity of a molecule in the gas phase and the polarization energy for an anion in the solid, respectively. The latter is often assumed to be similar in magnitude with P, in a simple polarization model of organic solids. Thus the A, value of a Ceo molecule is estimated to be 3.1 eV using eq. (4) on this assumption. This is slightly larger than the measured values 2.6-2.8 eV [36] and 2.650 2 0.050 eV [ 371 as well as a calculated value of 2.73 eV [25]. This difference may suggest that the assumption mentioned above is not valid and P_ is

Ftg. 4. Energy scheme of solid &,. VL: vacuum level, CB: bottom

of the conduction band, I&: the Fermi level, and VB: top of the valence band.

larger than P, by about 0.4 eV in the case of Go. Such discrepancies between P, and P_ values for particular compounds have been reported and/or eluci- dated earlier, e.g. P, > P_ for anthracene, and on the contrary, P, < P_ for tetracyanoquinodimethane (TCNQ) [ 381. A larger polarization energy for an anion could therefore support the more enhanced stability of C, states in solid.

Acknowledgement

One of the authors (NS) is grateful to the Nissha Aid for Academic Research for partial financial support to this work.

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Threshold ionization energy of C60 in the solid state