Photoluminescence of organo-polysilane

Photoluminescence of organo-polysilane

Solid State Communications, Vol. 57, No. 8, pp. 635-637, 1986. Printed in Great Britain. 0038-1098/86 $3.00 + .00 Pergamon Press Ltd. PHOTOLUMINESCE...

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Solid State Communications, Vol. 57, No. 8, pp. 635-637, 1986. Printed in Great Britain.

0038-1098/86 $3.00 + .00 Pergamon Press Ltd.

PHOTOLUMINESCENCE OF ORGANO-POLYSILANE T. Kagawa, M. Fujino, K. Takeda and N. Matsumoto Research Laboratory, NTT, Musashino-shi, Tokyo 180, Japan

(Received 18 October 1985 by I¢. Sasaki) Photoluminescence of organo-polysilane is investigated. A comparison is made between the spectra of two kinds of organo-polysilane: methylpropyl-polysilane and methyl-phenyl-polysilane. A sharp peaked luminescence spectrum in the ultraviolet region is observed in both materials. An additional broad visible luminescence exists in methyl-phenyl-polysilane. These features are discussed using the band calculation results. The halfwidth of ultraviolet luminescence exhibits strong temperature dependence in spite of a slight Stokes shift.

1. INTRODUCTION ORGANO-POLYSILANE, which is an Si skeleton polymer having organic side chain, has been recently investigated as a semiconductor material. Impurity doping effect [1], photoabsorption spectrum [1, 2], and photoconductivity [3] have been reported. It has proved to be a new type of solvable and formative semiconductor. In this paper, photoluminescence (PL) of organopolysilane (SiR1R2)n is reported for the first time, where R1 and R2 are organic side chains. In order to clarify the side chain contribution to the PL property, two kinds of organo-polysilane are investigated: methylpropyl-potysilane (MPrPSi) and methyl-phenyl-polysilane (MPhPSi). The former contains only o electrons and the latter has 7r electrons in its side chain. The PL spectrum is discussed using band structure calculation. Additionally, the temperature dependence of the PL spectrum is also measured in order to study the effect of the electron phonon interaction. 2. EXPERIMENT Methyl-propyl-polysilane (MPrPSi) and methylphenyl-polysilane (MPhPSi) were respectively prepared from the polymerization of methyl-propyl-dichlorosilane and methyl-phenyl-dichlorosilane using sodium metal in boiling toluene. Their molecular structure are shown in Fig. 1, and their molecular weights were 10 6 and 7 x 104, respectively. Films of approximately 1/~m in thickness were formed on SiO2 substrate by the spin coating of solved molecules. X-ray diffraction shows that samples contain micro-crystals. Both MPrPSi and MPhPSi have strong light absorption peaks at 3.6 eV which is commonly observed in organopolysilane. A He-Cd laser (3.81 e V ) w a s thus

used for excitation with an excitation intensity of about 10 mW cm -2. Figure 2(a) and (b) show the PL spectra at 77 K. MPrPSi has a sharp peak at 3.59 eV while MPhPSi shows one at 3.55 eV in the ultraviolet region. In addition, MPhPSi has a broad peak at the low energy side, which is absent in MPrPSi. This difference in the PL spectra can be readily understood by the band structure calculation to be discussed in the next section. These spectra showed no variation when samples were excited by an N2 laser (3.68 eV). These observed spectra are therefore not due to Raman scattering but to PL. Figure 3 compares the detailed PL spectrum of MPrPSi in the ultraviolet region at several temperatures. At 5.8 K, the spectrum is resolved into three peaks at 3.59, 3.52 and 3.44eV. As these peaks are separated from each other by about 73meV, those at 3.52 and 3.44 eV are considered to be phonon side bands of the main peak at 3.59 eV. The lineshape of the main peak is Gaussian in the high energy side and Lorentzian in the low energy side. Figure 4 shows the temperature dependence of the halfwidth of the main peak. When the temperature is raised, the peak is broadened and the separation of the three peaks becomes obscure. In MPrPSi, the full width at half maximum (FWHM) is constant (45 meV) below 75 K and linearly increases with temperature above 75 K (AE = 7.5 kT). And the high energy side width at half maximum (HWHM) also increases with temperature above 75K. Peak energy was almost temperature independent in the 5.8-170 K range. In MPhPSi, on the other hand, the PL peak at 3.55 eV is broader than that of MPrPSi, and the FWHM is constant (90meV). MPhPSi is rapidly degraded by He-Cd laser illumination in the temperature region

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636

PHOTOLUMINESCENCE OF ORGANO-POLYSILANE

I

F

s~--- tC3HTJ n

c.,

Vol. 57, No. 8

1

T~'~n

(a)

(b)

Fig. 1. Molecular structure of (a)methyl-propyl-polysilane (MPrPSi) and (b) methyl-phenyl-polysilane OvlPhPSi).

i

90

I

i

180

270

360

POLARIZER ANGLE (DEG.) MPrPSi 77K

MPhPSi

Fig. 5. The polarizer angle dependence of transmitted excitation laser beam and luminescence. The polarizer was placed between the sample and monochromator. MPrPSi

2'.0

i

MPhPSi

i

2.5 3.0 ENERGY (eV)

2.0

3.5

2.5 3.0 ENERGY (eV)

3.5 ~

(b)

(o)

Fig. 2. PL spectra of (a) MPrPSi and (b) MPhPSi.

C.B, -10 -2 J _-2-Lz_-~-L-_---_-- - 3 - - - - _ - ~ - : - : ~

q0 tad

-15 X

-15 r

k

k

(o)

(b)

Fig. 6. The calculated band structures of (a) MPrPSi and (b) MPhPSi.

z l-

;.~' i~ '31E

'

ENERGY (eV)

Fig. 3. PL spectrum of MPrPSi at several temperatures.

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.~ SOk

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0

-

/ ~ NPrPSi

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,. . . . . .

,. . . . . . /'-f 100 200 TEMPERATURE(K) -

-

-

Fig. 4. Temperature dependence of half width of ultraviolet PL peak of MPrPSi and MPhPSi. FWHM and HWHM mean the full width at half maximum and high energy side width at half maximum, respectively.

higher than 77K and the PL spectrum significantly changes, i.e., the peak at 3.55 eV disappears and the lower energy luminescence shown in Fig. 2(b) increases. The linear polarized light of the H e - C d laser was utilized to selectively excite partial molecules in the film according to the selection rule of wavefunction symmetry. And the excited molecules will emit polarized light if the radiative recombination is intramolecular event. A polarizer was put between a sample and a monochromator. Figure 5 shows the polarizer angle dependence of detected luminescence at 77 K for MPrPSi. The dependence of excitation light is also shown for comparison. The excitation light intensity was monitored by a laser beam transmitted through the sample. Valleys of excitation light intensity are not exactly zero because laser beam is scattered in the sample. Although the ratio of peaks to valleys of about 2 was rather small due to the light scattering in the film, good correlation was found between the excitation light and luminescence. This feature did not vary with temperature. Similar data were also obtained for ultraviolet PL in MPhPSi. The observed polarized luminescence indicates that the photoexcitation and radiative recombination are both intramolecular events. 3. DISCUSSION Figure 6(a) and (b) show the calculated band structure of MPrPSi and MPhPSi. Calculation was

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PHOTOLUMINESCENCE OF ORGANO-POLYSILANE

performed using Slater Coster LCAO with Harrison method. A detailed calculation will be presented elsewhere [4]. The conduction and valence bands of MPrPSi consist of bonding p states and antibonding s states of Si-Si bonding, respectively. The calculated energy gap between them is 3.0 eV. The optical transition is thus considered to occur between these bands. On the other hand, one of the principal band structure features in MPhPSi is the occupied intruder bands originating from bonding state of phenyl. These bands are shown by the dotted lines in Fig. 6(b). The energies of these intruder bands sensitively vary with the rotation of phenyl around the Si-C bonding. The energy bands caused by phenyl are therefore considered to be widely distributed in the gap between the Si skeleton bonding and anti-bonding bands in such a spin coated film as used in this study. The wave functions of the intruder bands are located on the phenyl side chain and are spatially separated with Si skeleton. The optical transition probability between the intruder bands and the skeleton anti-bonding conduction band is thus considered to be small compared with transition between skeleton bonding and anti-bonding bands. Consequently, it is clear that the luminescence in the ultraviolet region is results from the transition between skeleton bonding and anti-bonding bands. Additionally, the peculiar lower energy broad luminescence observed in MPhPSi is caused by the transition between the Si-Si anti-bonding conduction band and the intruder bands. Let us now consider the temperature dependence of the half-width shown in Fig. 4. The temperature independent width below 75 K shows the broadening due to the intrinsic randomness. The strong temperature dependence of FWHM above 75 K of MPrPSi, on the other hand, cannot be explained by the simple thermal distribution of carriers in bands. (Thermal distribution causes the half width AE = In 2 kT = 0.7 kT.) Therefore, the electron phonon interaction should play some role [5] in the PL spectrum in the ultraviolet region. The undetectably small Stokes shift between the photoabsorption and the PL peaks indicates that the relaxation energy due to electron phonon interaction is very small. The electron phonon coupling constant can therefore be said to be rather small. This means that the FWHM exhibits strong temperature dependence in spite of the weak electron phonon coupling.

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These phenomena cannot be explained by so far developed theories [5] which are the perturbation valid when the band width and the relaxation energy due to the electron phonon coupling are sufficiently larger than the phonon energy. Therefore, the co-existence of the three effects, namely intrinsic randomness, electron phonon interaction, and extending effect of carriers due to the transfer integral, should be strictly taken into account to achieve a complete understanding the PL spectrum. 4. SUMMARY The photoluminescence of two organo-polysilanes was reported for the first time. Both of these organopolysilanes (MPrPSi and MPhPSi) exhibit a PL peak in the ultraviolet region, which is considered to be a phenomenon characteristic for organo-polysilane. MPrPSi has broad luminescence in the visible region. These features were discussed using band calculation results. The visible luminescence in MPhPSi was attributed to the intruder band due to the phenyl side chain. The half width of the ultraviolet PL peak exhibits a strong temperature dependence in spite of the small electron phonon coupling constant. Acknowledgements - The authors are grateful to Dr. K. Sugii for his fruitful discussions. They are also grateful to Mr. H. Ban for supply of methyl-phenyl-polysilane samples.

REFERENCES 1. 2. 3. 4. 5.

R. West, L.D. David, P.I. Djrovichi, K.L. Stearley, K.S.V. Srinivasan & H. Yu, J. Amer. Chem. Soc. 103, 7352 (1981). R.D. Miller, D. Hofer, J. Robolt & G.N. Fickes, J. Amer. Chem. Soc. 107, 2172 (1985). M. Fujino, T. Kagawa, S. Furukawa & N. Matsumoto, to be published. K. Takeda, H. Teramae & N. Matsumoto, to be published. Y. Toyozawa, Relaxation o f Elementary Excitations, (Edited by R. Kubo & E. Hanamura), Springer Series in Solid-State Science No. 18, Springer, Berlin, Heidelberg, New York (1980); Y. Toyozawa, Organic Molecular Aggregates, (Edited by P. Reineker, H. Haken & H.C. Wolf) (ibid. No. 49).