Structural defects in poly (methylphenylsilylene)

Volume 198, number 1,2

CHEMICAL PHYSICS LETTERS

2 October 1992

Structural defects in poly (methylphenylsilylene) M i c h i y a Fujiki NTT Basie Research Laboratories, Musashino, Tokyo 180, Japan

Received 1 April 1992; in final form 10 July 1992

The Si-based structural defects in poly(methylphenylsilylene) backbones were detected as two new 295i NMR signals and as two new Si-Si vibrational IR bands. The defects, consisting of an organosilyne unit and about three methylphenylsilylenemonomer units near the branch, were presumed to be in a fairly lengthened Si-Si bonding state. The empirical linear relationship between the relative intensity of broad photoluminescence and the defect density predicts the disappearance of the broad luminescence for a defect density of less than 1%.

Since the first reports o f soluble polysilylenes, which are silicon catenated g-bonded high polymers bearing organic substituents (for a recent review, see ref. [ 1]), considerable attention has been focused on these materials because of their unusual photophysical properties - intense nearoUV absorption, highly efficient photoluminescence, charge-carrier transport, non-linear optical susceptibility, spectral hole burning and their potential applications for photoresists and precursors o f SiC. Although most polysilylenes have a drift mobility o f about 10 -4 cm 2 V-~ s-~ at r o o m temperature, they have deep trap centers with an associated activation energy o f 0.10.3 eV from time-of-flight and thermally stimulated current measurements [ 2 - 4 ]. Poly(methylphenylsilylene) ( P M P S ) , which is a typical soluble polysilylene, exhibits an unusual dual photoluminescence below 77 K; a sharp (S) emission at 3.5 eV and a broad structureless (B) emission at 2.7 eV [ 5 - 7 ] . The S emission in PMPS is due to the lowest ~(Sic, Sio-*) state. Although Ito et al. recently concluded that the B emission in PMPS comes from the Si~phenyln* charge-transfer state [6], the origin of B emission is still controversial. This is because a number of polysilylenes, including some "dialkyl-substituted" polysilylenes, also reveal this type of broad emission in the visible region [ 1 ]. Correspondence to: M. Fujiki, NTT Basic Research Laboratories, Musashino, Tokyo 180, Japan.

The present study was initiated by the assumption that B emission in PMPS is associated with Si-based defects in the Si framework that trap holes. We report here that, in various PMPS samples prepared by different techniques, all photoluminescence, UV absorption, and IR absorption spectra are significantly influenced by the densities o f Si-based defects. The Si-based defects were presumed to be due to lengthened Si-Si bonding. Additionally, we suggest that the intensity of B emission will essentially disappear at a defect density in a PMPS chain o f less than 1%. The characteristics of the six PMPS samples used in this work are listed in table 1. All polymers were prepared by a Wurtz-type coupling reaction in dry toluene in an Ar atmosphere using sodium and methylphenyldichlorosilane (MPS) either with or without organotrichlorosilane [8]. After 1 h at 110°C, the reaction mixture was cooled to r o o m temperature and passed through a 0.45 lam PTFE filter under Ar gas pressure. Then the filtrate was poured into iso-propanol and the resulting precipitate was collected by centrifugation and dried at 60 ° C in a vacuum overnight. The yield o f all polymers ranged from 29% to 33%. Since scarcely any insoluble polymeric product was formed, undesirable crosslinking and gelation was minimal. Although we originally proposed to introduce branching Si units, regarded as a model of structural defects, by copolymerizing an organotrichlorosilane with the MPS monomer, a small number o f these defects were de-

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177

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2 October 1992

Table 1 Characteristics of PMPS samples Sample NO.

Mw ")

Si-based defect density per total Si units b)

Branched Si units per total Si units c~

Monomer feed d)

Sodium (addition order) e~

1

307000

0.025

0.01 f~

MPS= 1.00

dispersion (reverse)

2

712000 (84%) 5700 (16%) 9700

0.040

0.02 f~

MPS= 1.00

dispersion (reverse)

0.076

0.03 f~

MPS = 1.00

lump (normal)

3 4

12100

0.160

0.08

MPS/PS = 0.97/0.03

dispersion (normal)

5

10800

0.130

0.04

MPS/BS = 0.97/0.03

dispersion (normal)

6

7100

0.205

0.13

MPS/PS =0.92/0.08

dispersion (normal)

"~ Weight-averaged molecular weight determined by gel permeation chromatography based on calibration with polystyrene standards. b~ Determined from integrated intensity between two extra 295i signals and total 29Sisignals by 29SiNMR (79.46 or 59.59 MHz, gated decoupling without NOE, CDC13). c~ Determined from integrated intensity between methyl and phenyl protons by IH NMR (299.95 MHz, CDC13). d~Nominal molar ratio. MPS, methylphenyldichlorosilane; PS, phenyltrichlorosilane; BS, n-butyltrichlorosilane. ~ "Normal" means that monomers are added to the mixture of sodium and toluene, while "reverse" means that sodium is added to the mixture of monomers and toluene. r) Phenyl protons are slightly in excess. tected even in P M P S h o m o p o l y m e r s . All P M P S samples c o n t a i n e d a negligible a m o u n t o f inorganic impurities, such as N a and Fe (0.001-0.004 wt%). The oxygen content due to S i - O - S i linkages were less than 0.1 wt% in all p o l y m e r samples according to elemental analysis and the present P M P S samples are analytically pure. Fig. l a shows solution 29Si N M R spectra ( b y gated decoupling t e c h n i q u e without N O E ) o f three P M P S samples containing varying Si defect densities (Nos. l, 4, and 6). The 29Si N M R spectra show two b r o a d signals a r o u n d - 35 and - 4 5 ppm, in the vicinity o f the three sharp m a i n signals located at - 3 9 . 3 , - 3 9 . 9 a n d - 4 1 . 3 p p m due to stereochemical t r i a d configurations [9 ]. Because the intensity o f these N M R signals a r o u n d - 35 and - 45 p p m increases with an increase in the n o m i n a l m o l a r ratio between M P S and the organotrichlorosilane in the copolymerization process (see fig. 1 and table I ), they are logically related to Si-based structural defects d e r i v e d from the i n t r o d u c t i o n o f Si branches. The space-filling m o d e l for a P M P S backbone with organosilyne branching suggests that the inherent large steric interaction among these substituents near the branching will result in b o n d r e h y b r i d i z a t i o n 178

which could change the Si-Si b o n d length a n d / o r S i Si b o n d angle. It has been reported that a 15% elongation o f the S i - S i b o n d in peralkylated disilane causes an unusual down-field shift of 49 p p m relative to unstrained peralkylated disilane in the 295i N M R spectrum [ 10 ]. F r o m fig. 1a, the relative ratio o f integrated intensities between the 295i signal at - 3 5 p p m and that at - 4 5 p p m is evaluated to be about 3. Hence, we consider the cause o f the two extra b r o a d 29Si signals, based on the simple assumption that a 1.5% elongation o f S i - S i b o n d (about 0.035/~) contributes to a down-field effect o f 5 ppm. A strained Si branch itself, which consists o f three such lengthened S i - S i bonds, will be deshielded by 15 p p m relative to an unstrained Si branch. Since the 29Si signal o f an unstrained organosilicon connected with three silicon units is n o r m a l l y observed around - 6 0 p p m [11 ], the 29Si signal o f the strained Si branch is expected to a p p e a r near - 4 5 p p m . Similarly, each o f three strained methylphenylsilylene units adjacent to the strained organosilyne, which is c o m p o s e d o f a strained S i - S i b o n d a n d an unstrained S i - S i bond, will a p p e a r near - 3 5 p p m relative to an unstrained methylphenylsilylene catenate. Thus, the two additional 295i signals around

Volume 198,number 1,2

CHEMICALPHYSICSLETTERS

(a)

2 October 1992

(b)

1

I

I

-20

-30

I -40

I

I -50

-60

chemical shift vs Me4Si/ppm

600

I

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1

500

400

wavenumber /

c m "1

I 300

Fig. 1. (a) The 295iNMR spectra (79.46 MHz, gated decouplingwithout NOE, CDC13,tetramethylsilane as reference, 45 ° pulse angle, 0.5 s relaxation time) of PMPS with varyingdefect densities. (b) The IR spectra (thin film on KBr cast from toluene solution) spectra of PMPS with varyingdefect densities. Sample Nos. l, 4 and 6 are listed in table 1. Arrowsindicate peaks derived from Si-based defects. - 3 5 and - 4 5 ppm could be identified tentatively as three strained methylphenylsilylene units and a strained organosilyne itself, respectively. Fig. lb shows the IR spectra of three PMPS samples with varying Si defect densities (Nos. 1, 4, and 6). The shoulders in the IR spectra at 485 and 440 c m - t can be seen and are associated with an intense peak at 461 cm -~ due to the Si-Si asymmetric stretching mode of the Si backbone [ 12 ]. It is proposed that the existence of Si-based structural defects causes shifts relative to the normal Si-Si backbone vibrations. Hence, these shoulders reflect the difference of the Si-Si bonds in the vicinity of a Si branch, and their intensities increase with the increase of the nominal molar ratio of organotrichlorosilane incorporated in the polymer. Other chemical structures should be considered for the defects. Although the 295i signal of poly(methylphenylsiloxane) is normally observed at - 3 2 to

- 3 4 ppm [ 13], the 295i signal at - 3 5 ppm cannot be assigned to a Si-O-Si linkage in a PMPS chain based on the low-oxygen analysis. The 13C N M R spectra (by DEPT) for all PMPS samples except No. 5 provide no indication of other possible defect structures, such as Si-(CH2),-Si bonds, because these spectra show only CH3 and CH (=phenyl) carbon peaks, while the PMPS No. 5 sample showed methylene signals due to the butyl substituent. Additionally, a Si-branched phenyl substituent in a PMPS chain may be formed in the vigorous Wurtz reaction. However, this possibility can be excluded since overtone bands characteristic of di-substituted benzene rings (ortho, meta and para positions) were clearly not detected between 1700 and 1950 c m - t [14]. As seen in table 1, a small amount of the defect signals was detected by 29Si NMR, ~H NMR and IR spectroscopies even in PMPS homopolymers (Nos. 179

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CHEMICAL PHYSICS LETTERS

1, 2, and 3). These indicate the formation of Si branches, which can probably be assigned to phenylsilyne moieties, during the polymerization. These may result from the presence of a small quantity of phenyltrichlosilane in the starting monomer, as suggested by a recent report [ 15]. However, it should be noted that, from the electron-impact ionization mass spectrum of MPS monomer, the Si-CH3 bond dissociates ten to twenty times more easily than the Si-C1 and Si-phenyl bonds. Therefore, as an alternative explanation of branching in homopolymer, we point out the possibility that PhSiC12 radical species incorporate into a polymer backbone throughout a radical-like chain propagation process in the Wurtztype reaction. Fig. 2 illustrates the absorption, photoluminescence, and excitation spectra o f a PMPS (No. 3) thin film at 4 K. All PMPS thin films have two intense absorption bands near 3.7 and 4.6 eV, respectively, due to Sio-Sig* and ph~-phx* transitions [ 16 ]. As reported previously [5-7 ], all PMPS samples show two emission bands: S emission at 3.5 eV with a full width at half maximum (fwhm) of 0.15 eV and B

5 o eO3 Q ¢-

0 09

E

i Q_

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g

(d)~

(1)

0

6.0

5.0

4.0

Lt

I

3.0

2.0

1.0

photon energy / eV

Fig. 2. The optical spectra of PMPS thin film (sample No. 3) at 4 K. (a) Absorption spectrum (b) photoluminescence spectrum excited at 4.77 eV (c) excitation spectrum monitored at 3.44 eV, and (d) excitation spectrum monitored at 2.76 eV. Arrows suggest the positions of band tails.

180

2 October 1992

emission at 2.7 eV with a fwhm of 0.6 eV. The excitation spectrum of B emission, however, differs from the absorption spectrum and exhibits a much broader band around 3.9 eV, with a tail ranging from 3.1 to 3.5 eV. The excitation spectrum of S emission, on the other hand, is almost identical to the absorption spectrum. This indicates that at least two energetically different states exist in a PMPS chain and that B emission originates from the tail of the broader band around 3.9 eV. The existence of the tail is further supported by the fact that B emission is still observed with excitation ranging from 3.1 to 3.5 eV and the very weak absorption tail observed in this energy region. Soluble Si-branched PMPS incorporating strained local structures may be regarded as a suitable model compound of insoluble hydrogenated amorphous silicon (a-Si:H) which contains many metastable SiSi bonding states. By analogy with the physics of aSi: H [ 17,18 ], the spectral tail near 3.1-3.5 eV that is observed in either the excitation or absorption spectrum of PMPS could be evidence of localized electronic states known as the band tails of both con. duction and valence bands. The band tails are derived from the lengthened Si-Si bonds. The lengthened Si-Si bond corresponds to the so-called "weak bond" in a-Si:H. Since the paramagnetic shielding term in a 29Si N M R chemical shift is inversely proportional to the energy gap between the ground state and the low-lying excited state [ 19 ], the hypothesis that the 29Si N M R peak at - 35 p p m is due to lengthened Si-Si bonds is consistent with the idea that a band tail state exists in a PMPS chain. This might be related to the observation that the solid-state z9si N M R spectra (by CP/MAS technique) of the present PMPS samples show a single but strongly asymmetric 29Si signal near - 4 2 p p m which tails downfield. The existence of the band tails is thought to lead both to the definitive trap centers of carriers and to the recombination sites of photoexcited energy. Recently, Wilson and Weidman observed a broad photoluminescence around 2.6-2.9 eV exhibiting a nonexponential decay for both a series of hexylsilyne-branched poly (hexylmethylsilylene)s and poly(hexylsilyne) [15]. They reported that the broad photoluminescence behavior is closely associated with "self-trapped" relaxation based on a "bandtaillike" model for amorphous semiconductors. They

Volume 198, number 1,2

CHEMICAL PHYSICS LETTERS

also suggested the existence of branching even in a poly(hexylmethylsilylene) homopolymer, which was formerly considered to be a completely linear chain, based on the observation of a weak broad emission with a nonexponential decay. Interestingly, this feature resembles the B-emission dynamics in "unbranched" PMPS [20]. Consequently, the similarity between the B-emission behavior for PMPS and the broad, photoluminescence for all allegedly "unbranched" polysilylenes, alkylsilyne-branched poly(dialkylsilylene), and network-type poly(alkylsilyne)s strongly suggests that the origin of these broad emissions is basically identical and that these emissions come from strained local structures incorporated in a Si skeleton. This idea closely relates to the previous argument that Sio electrons are not conjugated over a polymer backbone but are localized as segments [3]. In this regard, although Ito et al. originally assigned the B emission in PMPS to the Sio-phenyln* charge-transfer state based on the

band calculation of an ideal PMPS framework with a trans-planar syndiotactic sequence [ 6,16 ], this oversimplified picture may be unable to explain the electronic and optical properties of an actual PMPS sample in a noncrystalline state. This consideration may be supported by the following observation. We preliminary obtained solution 295i N M R spectra with a high signal-to-noise ratio (by gated decoupling without NOE, in CDC13) of several poly(methylpropylsilylene) (PMPrS) samples prepared by different techniques [21 ]. In addition to five well-resolved main signals of the polymer backbone centered at - 3 2 . 6 ppm, four broad unidentified 29Si signals for all PMPrS samples were detected weakly ranging from - 35 to - 4 3 ppm. These four signals are presumed to be a sort of Si-based defect incorporating in the polymer chain. From a comparison between two solid-state 295i NMR spectra (by MAS with and without CP, at room temperature), the four 295i signals existed in only

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60

80

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3.50 100

0

I

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20

40

60

80

0.0 100

(Si defect) "1

Fig. 3. (a) The relative ratio of broademission to the total emissions of PMPS thin film under4.77 eV excitation at 4 K. (b) The values of the lowest Sio-Si~ absorption maximum and its fwhm of PMPS (in tetrahydrofuranat room temperature) as a function of the reciprocal of the Si-based defect densities in PMPS backbone. 181

Volume 198, number 1,2

CHEMICAL PHYSICS LETTERS

noncrystalline phase. P M P r S is known to be one o f the dialkylated polysilylenes showing a b r o a d , but weak visible photoluminescence [ 1,15 ]. W h e n the P M P r S sample was p r e p a r e d in the presence o f a crownether as cocatalyst [ 22 ], both the four 295i signals and the b r o a d emission t e n d e d to increase markedly with each other. Recently, Ito et al. suggested that this b r o a d emission in P M P r S comes from an impurity i n t r o d u c e d during synthesis, since its excitation spectrum differs from the corresponding absorption spectrum [23 ]. Thus, the b r o a d emission in PMPrS probably relates to the four 29Si signals, namely, a sort o f structural defect in a p o l y m e r chain. Fig. 3a plots the relative ratio o f B emission to the total emission at 4.77 eV excitation (which is the c o m m o n b a n d in the excitation spectra o f both the S a n d B emissions) as a function o f the reciprocal o f the defect densities in the polymer chain (which reflects the average n u m b e r o f continuous unstrained Si m o n o m e r units i f defects are r a n d o m l y distributed). Fig. 3b shows both the energies o f the Sio-Sio-* absorption m a x i m u m a r o u n d 3.7 eV a n d their fwhm as a function o f the reciprocal o f the defect densities. The relationship between the ratio o f B emission relative to the total emission and the reciprocal o f the defect density is linear. This plot suggests that the Bemission b a n d will d i s a p p e a r when the defect density is less than about 1%. Also, the extrapolated value o f the lowest S i o - S i ~ absorption m a x i m u m and fwhm approaches 3.58 V (2m~,=346 n m ) a n d 0.20 eV, respectively, for defect densities below 1%. I f the structural defects are also trap centers, these results suggest the possibility o f further increasing the holedrift mobilities in P M P S films by further reducing their defect densities. In summary, the Si-based structural defects in a p o l y ( m e t h y l p h e n y l s i l y l e n e ) backbone were identified for the first time b y the observation o f two new 29Si N M R signals a n d two new S i - S i v i b r a t i o n a l IR bands. The defects, which consist o f an organosilyne unit a n d about three methylphenylsilylene units near the branch, were presumed to contain lengthened S i Si bonds. These S i - S i b o n d s are expected to act as the b a n d tails o f the valence and conduction bands. The empirical linear relationship between the relative intensity o f b r o a d photoluminescence a n d the defect density suggests that the b r o a d luminescence will d i s a p p e a r at a defect density o f less than 1%. 182

2 October 1992

The author is indebted to Dr. N o b u o M a t s u m o t o for his c o m m e n t s on the electronic structures of hydrogenated amorphous silicon and polysilylenes. This work was stimulated by unpublished photoluminescence data o f polysilylenes made by Hiroaki Isaka. The electron-impact ionization m a s s spectrum o f methylphenyldichlorosilane was measured by Hiroshi Kojima.

References [1]R.D. Miller and J. Michl, Chem. Rev. 89 (1989), and references therein. [2] R.G. Kepler, J.M. Zeigler,L.A. Harrah and S.R. Kurtz, Phys. Rev. B 35 (1987) 2818. [3] M. Abkowitz, F.E. Knier, H.-J. Yuh, R.J. Weagley and M. Stolka, Solid State Commun. 62 (1987) 547. [4] L.M. Samuel, P.N. Sanda and R.D. Miller, Chem. Phys. Letters 159 (1989) 227. [ 5 ] T. Kagawa, M. Fujino, K. Takeda and N. Matsumoto, Solid State Commun. 57 (1986) 635. [6] O. Ito, M. Terajima, T. Azumi, N. Matsumoto, K. Takeda and M. Fujino, Macromolecules 22 (1989) 1718. [ 7 ] L.A. Harrah and J.M. Zeigler, J. Polym. Sci. Polym. Letters 25 (1987) 205. [8] K. Matyjaszewski and H.K. Kim, Polym. Bull. 22 (1989) 253. [9]J. Maxka, F.K. Mitter, D.R. Powell and R. West, Organometallics 10 (1991) 660. [ 10] N. Wiberg, H. Schuster, A. Simon and K. Peters, Angew. Chem. Intern. Ed. Engl. 25 (1986) 79. [ll]P.A. Bianconi, F.C. Schilling and T.W. Weidman, Macromolecules 22 (1989) 1697. [ 12] H. Kuzmany, J.F. Rabolt, B.L. Farmer and R.D. Miller, J. Chem. Phys. 85 (1986) 7413. [ 13 ] G. Engelhardt, M. M~igiand E. Lippmaa, J. Organomet. Chem. 54 (1973) 115. [ 14] K. Nakanishi, P.H. Solomon and N. Furutachi, Sekigaisen Kyushu Supekutoru (Nankodo, Tokyo, 1960). [ 15] W.L. Wilson and T.W. Weidman, J. Phys. Chem. 95 ( 1991 ) 4568. [ 16 ] K. Takeda, H. Teramae and N. Matsumoto, J. Am. Chem. Soc. 108 (1986) 8186. [ 17 ] R. Car and M. Parrinello, Phys. Rev. Letters 60 ( 1988 ) 204. [ 18 ] N. Mott, Conduction in non-crystalline materials (Oxford Univ. Press, Oxford, 1987). [ 19 ] H. Marsmann, in: NMR 17: Oxygen-17 and Silicon-29, eds. P. Diehl, E. Fluck and R. Kosfeld (Springer, Berlin, 1981 ). [20] M. Terazima, O. Ito and T. Azumi, Chem. Phys. Letters 160 (1989) 319. [21 ] M. Fujiki, unpublished results. [22] M. Fujino and H. Isaka, J. Chem. Soc. Chem. Commun. (1989) 466. [23 ] O. Ito, M. Terazima and T. Azumi, J. Am. Chem. Soc. 112 (1991) 444.