Electronic Structures of Halogenated Polysilanes
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Here, we investigate the electronic influences of side-chain halogenations theoretically and predict their electronic and optical properties. We use the first principles calculation based on the LDF method. For systematic understanding of X's influence, the transplaner (TP) zigzag Si backbone is limited for the present consideration. Bond lengths and angles are deduced from those of our previous PSi [3]. RESULT AND DISCUSSION a-n mixing by X replacement The characteristic feature of the PSi's electronic structure is the existence of the direct band-edge at point F (Fig. 1). The optical transition is allowed over this band gap. The highest occupied valence band (HOVB) state is formed by the ppa coupling between the skeleton Si's 3p atomic orbitals (AOs) (B2.), and the lowest unoccupied valence band LUCB state is formed by the spa coupling between the skeleton Si's 3s and 3p hybridized AOs [12]. Electrons and holes at these band-edge states are delocalized well along the Si main chain. A characteristic of the valence band is that it consists of the following three characteristic bands, sa-, quasi-4r, and pa bands.
8- A 6
10-
B3u
BBlU 2u
A'
BA'
3u JA
2
0) &_-" (D -4-
-6-
........ g
0 .7B HOVB 2g Ag) a 9Aý g quasi-nt
A"
quaso-c•
-
ULL a Pa
A A'
B2u B 3u
I
A'at
S
A'
s-10-
.
3.89eV
I-10
wave number X
IFwave number X
Figure 1: Calculated electronic structure of parent PSi. Energy is referred from HOVB state in electron-volts,
Figure 2: Calculated electronic structure of FPSi. Energy is referred from HOVB state in electron-volts.
Figure 2 shows the calculated electronic structure of fluoropolysilane (FPSi), in which one of the side chain H atoms in the unit cell is replaced by a F atom. While the 386
outline of PSi's electronic characteristics is maintained, novel levels appear both in the band gap and in the valence band. The strongly localized features of these levels are attributed to their little E - k dispersions, and the level in the band gap is not occupied by electrons (unoccupied 16calized level, ULL). The fluoronation also complicates the band structure near the valence band edge. These electronic characteristics can be expressed by considering the skeleton-side-chain interaction between the PSi's delocalized a electrons and the F's (X's) localized n electrons. First, we focus on the localized level appearing deep in the valence band. This level is the most energetically stable and shows no E - k dispersion. According to Mulliken charge analysis, the contribution of the F's 2s AO to this level amounts to 95%. The strong localized nature of this level also means it has little interaction with the PSi main chain. Above this localized level, the sa- bands and quasi-it bands are found, although their energy degeneracies at point X are broken by the symmetry lowering from D2h to C8 due to the side-chain fluoronation. Now, we discuss the upper edge of the valence band and the novel level ULL in the LUCB band gap. In the present TP zigzag skeleton form, the following three orbital compon
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