Electronic structures of polysilanes and related compounds

Journal of Electron Spectroscopy

Electronic

structures

of polysilanes

and Related Phenomena

78 (19%) 403406

and related compounds

K. Seki,a H. Ishii,a*b A. Yuyama, a M. Watanabe,c K. Fukui,d E. Ishiguro,e J. Yamazaki,b S. Hasegawa,b K. Horiuchi,f T. Ohta,g H. Isaka,h M. Fujino,h M. Fujikip K. FurukawaP and N. Matsumotoh aDepartment of Chemistry, Faculty of Science, Nagoya University, Furocho, Chikusaku, Nagoya 464-01, Japan bJnstitute for Molecular Science, Myodaiji, Okazaki 444, Japan CResearch Institute for Scientific Measurements, Tohoku University, Katahira, Aobaku, Sendai 980, Japan dDepartment of Electronic Engineering, Faculty of Engineering, Fukui University, Bunkyo, Fukui 910, Japan eCollege of Education, University of the Ryukyus, Senbaru- 1, Nishihara-cho, Okinawa 903-O1, Japan fDepartment of Materials Science, Faculty of Science, Hiroshima University, Kagamiyama, Higashi-hiroshima 724, Japan gDepartment of Chemistry, Faculty of Science, The University of Tokyo, Hongo, Bunkyoku Tokyo 113, Japan hN’I’T Basic Research Laboratories, Wakamiya, Morinosato, Atsugi 243-01, Japan UPS and Si K- and LED-edge NEXAFS spectra of various polysilanes, poly(dimethylsiloxane), and octakis(fbutyl)octa-silacubane (OTBOSC) were measured for elucidating their occupied and vacant electronic structures, in comparison with the results for polysilazane. The results revealed that the electronic structures are sensitive for the main chain structure, pendant, and the dimensionality of the Si-containing backbone. 1. INTRODUCTION Polysilanes (SiRR’)n are Si analogues of polyethylene (CH2)n. Usually the side groups R and R are organic groups rather than H, for the protection from hydrolysis. These polymers have attracted attention due to their unique optical and electronic properties [ 1,2]. The valence electrons are delocalized over the Si chain, and the long SiSi bond length results in a smaller HOMO-LUMO gap than in (CH2)n As a result, the fundamental absorption is in the near-W region even without unsaturated bond, and the absorption often leads to the cleavage of SiSi bonds. Further, the smaller electronegativity of Si than that of C, enables hole doping. These properties make polysilanes attractive for possible applications as photoresists, semiconductors, and initiators of radical reactions [ 1,2]. Polysilanes are also interesting from basic viewpoints. It can be regarded as a l-dimensional analogue of Si crystal, with a significant difference that Si is not luminescent, while polysilanes are [3]. There have been extensive studies comparing wide variety of Si family including other compounds [3]. As is evident from these facts, the elucidation of the electronic structure of these systems forms the 0368-2048/%/$15.00 0 1996 Elsevier Science B.V. All rights reserved PI1 SO368 - 2048 (%) 02760-o

basis for understandiig their properties. For such studies, ultraviolet photoelectron spectroscopy (UPS) and near edge X-ray absorption fine structure (NEXAFS) spectroscopy are powerful tools. There has been some UPS studies of polysilanes [4-61, but they were either of low spectral quality or limited to the top part of the valence levels. The study by polarized NEXAFS was reported for oriented poly(dihexylsilane) at the Si K edge [7]. In this work, we report (i) the UPS study of several polysilanes, (ii) NEXAFS spectra of polysilanes and related compounds, and (iii) the UPS study of a Si cluster compound octakis(t-butyl)octasilacubane (OTBOSC)@] with eight Si atoms at the comers of a cube. The results are discussed with stress on the comparison of the electronic structure of these systems i.e. how the chemical structure affects the electronic structure of S&derived systems. Some part of the present results are already reported in preliminary publications [9, lo]. 2. EXPERIMENTAL The samples of polydimethylsilane (SiMe2)n (Me = CH3) and polydimethylsiloxane [Si(CH3)2_ O]n were supplied from Shinnissokako and Chisso,

404

respectively. Poly(methylpropylsilane) [SiMeP& (PI= n-C3H7), poly(methylphenylsilane)[SiMePh]n (ph = CgHg), poly(diphenylsilane) (SiPh2)n, and OTBOSC were synthesizedat NTT. The UPS spectra were measured at the beamline 8B2 of UVSOR facility at IMS with a plane grating monochromator [l 11. The samples were prepared as cast films from solution and transferred to the UPS system in Ar atmosphere for preventing oxidation. For OTBOSC, the sample was prepared as in-situ evaporated film of ca. 20 nm thickness. The Si K edge NEXAFS spectra were measured at the beamline 11B of Photon Factory at National Institute for High Energy Physics (KEK-PF) using a double crystal monochromator [ 121. The spectra of Si LRa edge were measured at beamline 8B 1 of WSOR with a 2.2 m Rowland-circle grazing incidence monochromator [ 131 and at beamline 11A of KEK-PF with a Grasshopper monochromator. All the spectra were measured as total electrons either as the drain current or using a channeltron. The samples were prepared as solution-cast films, except for (S&k&n pmpared as evaporated films. 3. RESULTS

AND DISCUSSION

In Fig. 1 we show the UPS spectra of (SiMePr), and (SiMePh)n as representatives of polysilanes with aliphatic and aromatic pendants. In these spectra, the peaks at high binding energy region (18-25 eV) and medium energy region (10 -

30

I

I

20

10

Evac=

BindingEnergyI eV

Figure 1 UPS spectra of (SiMePr),, Ph]nathv=4OeV

and [SiMe-

15 eV) show good correspondence with the C2s and C2p parts of the UPS spectra of propane [14] and benzene [ 141, respectively, indicating the dominant contribution from the pendants, This dominance is reasonable when we consider that 4 or 7 C atoms per Si atom are contained in the repeating unit.. Band calculations [ 161 indicate that the highest valence states of polysilanes are formed by the Si main chain. To confIrm this, the UPS spectrum of p=nmthyhetmsilane S&Mel0 [17] corresponds well with that of (SiMePr)n (Fig. 1) in this region., The spectrum of (SiMePh)n differs slightly fkom that of (SiMePr)n in this region. This may be due to the mixing of one of the doubly degenerate 1c HOMOs of benzene with the delocalizd G orbital in the main chain [5]. This will raise the HOMO mainly formed by the Si cr-orbitals. Actually, the ionization threshold energy of (SiMePh)n (5.3 eV) [5] is smaller than that of (SiMePr)n (5.65eV) [18]. By comparing these spectra with that of (SiMe2, 0)n [ 151, we note that the latter lacks the low binding energy part below 10 eV. This should be due to the break of a-conjugation among Si atoms by the insertion of 0 atoms. Reflecting this, the threshold ionization energy of (SiMe2O)n is as high as 8.3 eV [14]. The 3dimensional analogues Si and Si02 have threshold energies of 5.35 eV [19] and 10.6 eV [20], respectively. The lowering from polysilane to Si should be due to the 3dimensional Coordination, while the threshold of SiO2 is even larger than that of (SiMe2O)n due to the increased coordination of electronegative 0 atoms. In Figures 2 and 3, we show the Si K-edge and Lm_edge NEKAFS spectra Of several pOlysilatES in comparison with the spectra of polysilazane (SiH2-NH), [21,22] and [SiMe2-O]n. The small spin+rbit splitting of LR,III edge (0.6 eV) does not allow us to separate the Ln and L~R contributions. The L-edge spectra of all@ polysilanes [(SiMe2)n and (SiePr)n] resemble each other. The reported K-edge spectra of poly(dihexylsilane) (Si(n+jH13)2)n E7]also commponds well with that of These findings indicate the similarity (SiMe2)n among the vacant states of these polymers. On the other hand the spectra of polysilanes with aromatic pendants [(SiMePh)n and (Si@-EtPh)2)tJ (Et = C2H5) resemble each other, but differ from the spectra of alkyl-substituted ones; In the Kedgespectm,alowenergypeakappears,andthe Ledge spectra show broad features instead of sharp ones in the spectra of alkyl polysilanes.

405

/Q\

-siT

Qi

n

t+

1635

1840

1850

1860

Photon Energy I eV

I:

95

100

105

110

115

Q

n

120

Photon Energy I eV

Figure 2. Si K-edge NEXAFS spectra of (SiMe2),, (SiMePr)n, (SiMePh)n, (Sib-Et-Ph)2)n, (SiH2NHlll[2 119~dC3~9-ohz. This finding is difficult to explain if we assume that the excitation is confined in the Si main chain. Instead, the present results suggest that there is a rather significant mixing of the unoccupied levels of the pendants with those of the Si main chain. More detailed analysis of these results requires the character of vacant states. In K-edgespectraof Si, electron is excited into vacant orbitals of 3p character, while in the L-edge spectra into orbit& of 3s and 3d characters. For a polysilaue chain in planar-zigzag conformation, band calculation [16] indicates that the lowest vacant spa* band consists of the contribution from Si3s and SDpy orbitals (y is vertical to the chain axis and in the plane formed by the Si atoms), and the orbital of the pendant . When the character of this orbital is changed, contribution from both Si3s and 3py orbitals will be affected. This offers a possible explanation of the observed difference between the NEXAFS of polysilanes with alkyl and aromatic pendants, although the energy of this band was calculated to be similar between (SiMePr)n and (SiMePh)n [16]. At both K- and L-edges, the spectra of (SiH2NH), and (S&&q-0), show shift of the spectral features to the high energy side. As shown in the inset of Fig. 3 as functions of the averaged electro-

Fig. 3. Si LE,m edge NEXAFS spectra of (SiMeZ)n, (SiMeR)n, (SiMePh)n, (SiH2-NH), [22], and (SiMe2Qn. The inset shows the shift of absorption edges and Sip binding energy by XPS as functions of averaged electronegativity of 4 atoms coordinating Si (see text). negativity of 4 coordinating atoms x tkom that of crystalline Si, these shifts are almost parallel with the Sip binding energy, indicating that the main origin of the shift in the absorption is the shift of the Sils and 2p levels by the bonding with incmasmgly elecuonegative elements of N and 0. Finally, we examine the UPS spectrum of OTBOSC in Fig. 4. It is different from that of (SiMePr)n also shown in Fig. 4 in that distinct peaks are observed at the topmost valence region. This should be due to the regular arrangement of eight Si atoms in OTBOSC with high symmetry, leading to the large degree of degeneracy. Similar situation is observed in the UPS spectmm of C60 [24]. The quasi-3dimensional delocalization results in the extension of a-conjugated levels extending to about 5 eV below the vacuum level. This corresponds to the low valence excitation energy of 1.9 eV in the visible region compared to those of polysilanes in the W region. Detailed analysis of this UPS specuum by MO calculations is in progress.

406

10

&act0

BindingEnergy I eV Figure 4. UPS spectrum of OTBOSC at hv = 40 eV compared with that of (SiMePr), . ACKNOWLEDGMENTS

This work was carried out under the approval of the Program Advisory Committee of Photon Factory (Approval No. 91-0251, and a Joint Studies Program of Institute for Molecular Science (No. 6H2 171. It was supported in part by the Grants-inAids from the Ministry of Education, Science and Culture of Japan (Nos.07NFO303 and 072132161. REFERENCES

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