- Email: [email protected]
Photoluminescence from silicon-chain cluster in poly(dimethylsilane) evaporated film
& *H __ __
applii surface science
EB ELXZVIER
Applied Surface Science I 13/l I4 (1997) 472-475
Photoluminescence from silicon-chain cluster in polyidimethylsilanej evaporated film *, Takeshi
Reiji Hattori Department
of Electrical Engineering.
Faculty
Sugano, Junji Shirafuji
of Engineering, Osaka lJnir,ersity, 2-l Yamada-Okn, Suitu, Osaka 565. Jupun
Abstract Photoluminescence (PL) and excitation (EX) spectra in epitaxially grown poly(dimethylsilane) (PDMS) films prepared by evaporation have been measured. The epitaxial films are prepared onto mechanically oriented poly(tetrafluoroethylene) (PTFE) layer or onto cleaved surface of alkali halide crystals. The EX peak is located in the lower photon energy tail of the broad absorption CABS) band; this is explained in terms of the occurrence of energy transfer from short delocalized regions to the longest delocalized regions in silicon main chain and the extremely poor transfer probability. The epitaxially grown films show higher photoluminescence intensity possibly because the existence of an increased number of delocalized regions. Keyword.~: Polysilane;
Poly(dimethylsilane);
Epitaxial growth; AFM; Photoluminescence
1. Introduction Polysilane
group
molecules
are composed
of sp”
substituents bounded to each Si atoms, and possesses interesting characteristics as one-dimensional like semiconductors [l], for example, wide optical gap materials, large exciton binding energy, photoluminescence with a high quantum yield [2,3] and photoconductivity with a high hole mobility [4,5]. Such characteristics are referred to the delocalized (Tbonded Si chain. Photoluminescent measurements of polysilanes have been carried out mainly in solution although the application, such as an electroluminescent (EL) diode, is realized in the state of solid films. In hybridized
Si principal
chain
and two organic
_ Corresponding author. Tel.: +8l-6-8797699; 8750506; e-mail: [email protected]. Ol69-4332/97/$17.00 Copyright PII SO 169.4332(96)00808-2
fax:
+ 81-6.
solution the molecules are isolated and there is little interaction between molecules. However, in solid films the interaction becomes very significant and the photoluminescent properties are considered to be different from that in solution. Therefore the optical properties and molecule structure in solid state must be revealed. Poly(dimethylsilane) (PDMS) employed in this work has the smallest side group among polysilane groups and all-rvans rigid conformation at room temperature. Moreover, the crystal and electronic structures have also been known [6]. PDMS has been considered to be the most suitable polysilane material for investigating one dimensionality of Si chain. Investigations on oriented polymer are one of the most effective way to explore novel functions of dimensionality and to improve their electrical and optical properties of the polymer materials. We have succeeded in obtaining aligned PDMS films by evap-
6 1997 Elsevier Science B.V. All rights reserved.
R. Hattori
et cd. /Applied
Sutface Science I13 / I14
oration on mechanically oriented poly(tetrafluoroethylene) (PTFE) substrates. The Si chain were aligned epitaxially to the direction of PTFE molecules and parallel to the substrate [7]. In addition, we attempted in this paper to align PDMS molecules normal to the substrate using cleaved surface of alkali halide. In this paper, the orientation of evaporated PDMS films was confirmed by high resolution atomic force microscopy (AFM) observation and discuss the mechanism of photoluminescence from these films.
2. Preparation
of PDMS films
Two types of epitaxially grown PDMS films were prepared by evaporating PDMS powder. It has already been known that only the direction of Si chains in evaporated PDMS films on amorphous substrates can be controlled by the deposition rate [8,9]. At low deposition rates Si chains tend to align normal to the substrate surface with little coherence between Si chains. On the other hand, at high deposition rates, Si chains have a tendency to lie parallel to the substrate surface. However, by employing oriented polymer layer or cleaved crystalline substrates crystalline PDMS films with c-axis ether normal or parallel to the substrate surface could be epitaxially grown. In order to prepare the films with main chains lying parallel to the substrate surface, PDMS was deposited onto a highly oriented layer of PTFE at a high deposition rate. The highly oriented PTFE layer was prepared by mechanical deposition method [IO]. It was confirmed by X-ray diffraction and polarized optical absorption measurements that the Si chains lay along the direction of aligned PTFE molecule orientation and that the (1 IO) crystalline plane was parallel to the PTFE layer [9]. On the other hand, the films with main chains normal to the substrate surface were prepared by depositing onto a cleaved surface of alkali halide crystals (KBr. NaCl, KC11 at a low deposition rate. The crystallographic structure of PDMS has assigned in two different manners [6,1 I]. The results of AFM observation was discussed on the basis of the monoclinic unit cell with CI= 7.45, b = 7.24, c = 3.89 A and y = 67.1” for PDMS crystalline structure
[61.
f 1997)
473
472-475
3. AFM observation High resolution AFM images of PDMS films and substrate surfaces taken in the contact mode are shown in Fig. I(a-dl. The AFM observation of PDMS films was carried out in methyl alcohol because of fragility of PDMS. The surface of highly oriented PTFE layer has a wavy structure due to alignment of PTFE molecules with 7.5 A spacing (Fig. l(a)) which a 11ows the epitaxial growth [7]. Fig. l(b) shows the aligned PDMS molecules with the spacing of about 8 A in parallel to PDMS molecules. This value of the period is in agreement with the distance of PTFE molecules in Fig. l(a) and is also consistent with the result of XRD measurement that the (1 IO) plane is parallel to the substrate surface [9]. Fig. l(c) and Fig. l(d) show the atomic image of KBr cleaved surface and the PDMS surface image,
(d) Fig. I High resolution AFM images. (a) Surface of mechanically oriented PTFE layer. Twist patterns represent the helix structure of PTFE moleculea. (b) Surface of evaporated PDMS on PTFE layer. Period is about 8 a which agrees with 8. I i of (7 10) plane separation of PDMS crystal. Cc) (100) cleaved surface of KBr. Bright spots represent the positions of K or Br atoms. Cd) Surface of evaporated PDMS film on KBr cleaved surface. Inner surface of PDMS film in contact with KBr was observed. Structure of bright spots are coincide well with that of (001) plane of PDMS crystal.
474
respectively. the spacing crystal. This of PDMS on
R. Hattori
et al./Applied
Surface Science 113/
114 (1997) 472-475
The lattice constant of KBr is close to between the (010) planes of PDMS may be the reason for epitaxial growth KBr cleaved surface.
4. Optical properties In order to examine the effect of epitaxial growth on optical properties of PDMS, PL, EX and ABS spectra of two types (c-axis I or II to the substrate surface) of epitaxial PDMS films were measured and compared to those of PDMS powder. ABS spectra of the epitaxial films show a characteristic peak at 4.1 eV (300 nm) and broad absorption tail in the low energy region (Fig. 2(b) and Fig. 2c)), while the powder material shows a broad peak at 3.6 eV (344 nm) (Fig. 2(a)). The photoluminescence and absorption in polysilanes are associated with u - u * band gap transitions in silicon backbone. The broad absorption seems to be discussed in relation to the distribution of electron delocalized length in silicon backbone assuming that the ABS photon energy is determined only by quantum effect in the silicon-chain [ 121. The absorption peak energy of 4.1 eV corresponds to the number of silicon atoms in delocalized region (cluster size) of 10. However, the EX peak of the evaporated films
3
Fig. 2. Room
4 PHOTON ENERGY (eV)
temperature PL, EX and ABS spectra of PDMS powder (a), PDMS film evaporated on PTFE layer (b) and on KBr cleaved surface (cl.
3 PHOTON ENERG; (eV)
Fig. 3. Room temperature PL, EX spectra (a) and ABS spectra (b) of evaporated films on KBr, KCI, NaCl cleaved surfaces and quartz plate. These films were deposited in the same run. (b) ABS spectra.
appears at the low energy tail of the broad ABS band. This means that the energy transfer occurs from the short delocalized region to the longer ones and that the photoluminescence takes place dominantly from the longest delocalized regions in silicon chains. The probability of energy transfer should be very little in evaporated films because the EX spectrum shape has no relation to ABS one. The PL spectra of the evaporated films are poorly symmetrical to EX spectra, because the PL spectra are narrower than EL spectra. This fact also suggests the energy transfer between PDMS chains. Fig. 3(a and b) show the PL, EX and ABS spectra at 300 K for the evaporated films simultaneously prepared on alkali halide cleaved surfaces and on fused quartz plate. The absorption intensity around 3.6 eV where the EX spectra have a peak, and PL intensity are both in order of KBr, KCl, NaCl. This order corresponds to that of laitice constants. The lattice constant of KBr (6.6 A) is closer to the distance between (010) planes (6.7 A) than other alkali halides. This suggests that the PL intensity can be enhanced by increasing number of long delocalized regions by careful epitaxial growth. The results in Fig. 3 suggest that the number of effectively long delocalized regions in the films may rely on how much the epitaxial growth conditions are satisfied. The effective length of delocalized regions does not necessarily correspond to Si chain length, because the PDMS molecules arriving at the substrates in evaporation have the same length for each films
R. Hattori et al. /Applied
Surface Science I13/
which were prepared simultaneously in the same run. Therefore it is reasonable that the length of Si chain is long enough and the delocalized region is limited by randomness or imperfection of conformation of Si chain.
114 (19971472-475
475
powder samples. This work was partially supported by 1996 Grant-in-Aid for Encouragement of Young Scientist (A) (No. 08750365) form the Ministry of Education, Science and Culture of Japan.
References 5. Summary The PL spectrum from evaporated PDMS oriented films along with the EX and ABS spectra were measured. The large difference between the positions of ABS and EX peaks is explained in terms of
energy transfer process from short delocalized regions to the longest delocalized region and the extremely poor transfer probability. In order to increase the number of the longest delocalized regions or to increase the PL intensity, epitaxial growth of PDMS onto highly oriented and lattice-matched substrates is favorable.
Acknowledgements
The authors are much indebted to T. Fujiki, S. Kawasaki and R. Nishida of Advanced Technology Center, Osaka Gas Co., Ltd. for supplying PDMS
111R.D. Miller 121Y. Majima,
and J. Michl, Chem. Rev. 89 (1989) 1359. K. Kawata, Y. Nakano and S. Hayase, J. Polym. Sci. Polym. Chem. (1995). 20 (1987) [31 L.A. Harrah and J.M. Zeigler, Macromolecules 601. 141 Y. Majima, H. Nishizawa, T. Hiraoka, Y. Nakano and S. Hayase, Jpn. J. Appl. Phys. 34 (1995) 3820. [51 R.G. Kepler, J.M. Zeigler, L.A. Harrah and S.R. Kurtz, Phys. Rev. B 35 (1987) 2818. 161S. Furukawa and K. Takeuchi, Solid State Commun. 87 (1993) 931. [71 R. Hattori, Y. Aoki, T. Sugano, J. Shirafuji and T. Fujiki, to be submitted.
Bl K. Takeuchi,
M. Mizoguti, M. Kira, M. Shimana, S. Furukawa and M. Tamura, J. Phys., Condens. Matter 6 (1994) 10705. [91 R. Hattori, J. Shirafuji, Y. Aoki, T. Fujiki. S. Kawakasi and R. Nishida, J. Non-Crys. Solids, to be published. [lOI J.C. Wittmann and P. Smith, Nature 352 (1991) 414. [Ill A.J. Lovinger, F.C. Schilling, F.J. Padden, Jr. and F.A. Bovey, Macromolecules 24 (1991) 132. [I21 Y. Kanemitsu, K. Suzuki, Y. Nakayoshi and Y. Matsumoto, Phys. Rev. B 46 (1992) 3916.
applii surface science
EB ELXZVIER
Applied Surface Science I 13/l I4 (1997) 472-475
Photoluminescence from silicon-chain cluster in polyidimethylsilanej evaporated film *, Takeshi
Reiji Hattori Department
of Electrical Engineering.
Faculty
Sugano, Junji Shirafuji
of Engineering, Osaka lJnir,ersity, 2-l Yamada-Okn, Suitu, Osaka 565. Jupun
Abstract Photoluminescence (PL) and excitation (EX) spectra in epitaxially grown poly(dimethylsilane) (PDMS) films prepared by evaporation have been measured. The epitaxial films are prepared onto mechanically oriented poly(tetrafluoroethylene) (PTFE) layer or onto cleaved surface of alkali halide crystals. The EX peak is located in the lower photon energy tail of the broad absorption CABS) band; this is explained in terms of the occurrence of energy transfer from short delocalized regions to the longest delocalized regions in silicon main chain and the extremely poor transfer probability. The epitaxially grown films show higher photoluminescence intensity possibly because the existence of an increased number of delocalized regions. Keyword.~: Polysilane;
Poly(dimethylsilane);
Epitaxial growth; AFM; Photoluminescence
1. Introduction Polysilane
group
molecules
are composed
of sp”
substituents bounded to each Si atoms, and possesses interesting characteristics as one-dimensional like semiconductors [l], for example, wide optical gap materials, large exciton binding energy, photoluminescence with a high quantum yield [2,3] and photoconductivity with a high hole mobility [4,5]. Such characteristics are referred to the delocalized (Tbonded Si chain. Photoluminescent measurements of polysilanes have been carried out mainly in solution although the application, such as an electroluminescent (EL) diode, is realized in the state of solid films. In hybridized
Si principal
chain
and two organic
_ Corresponding author. Tel.: +8l-6-8797699; 8750506; e-mail: [email protected]. Ol69-4332/97/$17.00 Copyright PII SO 169.4332(96)00808-2
fax:
+ 81-6.
solution the molecules are isolated and there is little interaction between molecules. However, in solid films the interaction becomes very significant and the photoluminescent properties are considered to be different from that in solution. Therefore the optical properties and molecule structure in solid state must be revealed. Poly(dimethylsilane) (PDMS) employed in this work has the smallest side group among polysilane groups and all-rvans rigid conformation at room temperature. Moreover, the crystal and electronic structures have also been known [6]. PDMS has been considered to be the most suitable polysilane material for investigating one dimensionality of Si chain. Investigations on oriented polymer are one of the most effective way to explore novel functions of dimensionality and to improve their electrical and optical properties of the polymer materials. We have succeeded in obtaining aligned PDMS films by evap-
6 1997 Elsevier Science B.V. All rights reserved.
R. Hattori
et cd. /Applied
Sutface Science I13 / I14
oration on mechanically oriented poly(tetrafluoroethylene) (PTFE) substrates. The Si chain were aligned epitaxially to the direction of PTFE molecules and parallel to the substrate [7]. In addition, we attempted in this paper to align PDMS molecules normal to the substrate using cleaved surface of alkali halide. In this paper, the orientation of evaporated PDMS films was confirmed by high resolution atomic force microscopy (AFM) observation and discuss the mechanism of photoluminescence from these films.
2. Preparation
of PDMS films
Two types of epitaxially grown PDMS films were prepared by evaporating PDMS powder. It has already been known that only the direction of Si chains in evaporated PDMS films on amorphous substrates can be controlled by the deposition rate [8,9]. At low deposition rates Si chains tend to align normal to the substrate surface with little coherence between Si chains. On the other hand, at high deposition rates, Si chains have a tendency to lie parallel to the substrate surface. However, by employing oriented polymer layer or cleaved crystalline substrates crystalline PDMS films with c-axis ether normal or parallel to the substrate surface could be epitaxially grown. In order to prepare the films with main chains lying parallel to the substrate surface, PDMS was deposited onto a highly oriented layer of PTFE at a high deposition rate. The highly oriented PTFE layer was prepared by mechanical deposition method [IO]. It was confirmed by X-ray diffraction and polarized optical absorption measurements that the Si chains lay along the direction of aligned PTFE molecule orientation and that the (1 IO) crystalline plane was parallel to the PTFE layer [9]. On the other hand, the films with main chains normal to the substrate surface were prepared by depositing onto a cleaved surface of alkali halide crystals (KBr. NaCl, KC11 at a low deposition rate. The crystallographic structure of PDMS has assigned in two different manners [6,1 I]. The results of AFM observation was discussed on the basis of the monoclinic unit cell with CI= 7.45, b = 7.24, c = 3.89 A and y = 67.1” for PDMS crystalline structure
[61.
f 1997)
473
472-475
3. AFM observation High resolution AFM images of PDMS films and substrate surfaces taken in the contact mode are shown in Fig. I(a-dl. The AFM observation of PDMS films was carried out in methyl alcohol because of fragility of PDMS. The surface of highly oriented PTFE layer has a wavy structure due to alignment of PTFE molecules with 7.5 A spacing (Fig. l(a)) which a 11ows the epitaxial growth [7]. Fig. l(b) shows the aligned PDMS molecules with the spacing of about 8 A in parallel to PDMS molecules. This value of the period is in agreement with the distance of PTFE molecules in Fig. l(a) and is also consistent with the result of XRD measurement that the (1 IO) plane is parallel to the substrate surface [9]. Fig. l(c) and Fig. l(d) show the atomic image of KBr cleaved surface and the PDMS surface image,
(d) Fig. I High resolution AFM images. (a) Surface of mechanically oriented PTFE layer. Twist patterns represent the helix structure of PTFE moleculea. (b) Surface of evaporated PDMS on PTFE layer. Period is about 8 a which agrees with 8. I i of (7 10) plane separation of PDMS crystal. Cc) (100) cleaved surface of KBr. Bright spots represent the positions of K or Br atoms. Cd) Surface of evaporated PDMS film on KBr cleaved surface. Inner surface of PDMS film in contact with KBr was observed. Structure of bright spots are coincide well with that of (001) plane of PDMS crystal.
474
respectively. the spacing crystal. This of PDMS on
R. Hattori
et al./Applied
Surface Science 113/
114 (1997) 472-475
The lattice constant of KBr is close to between the (010) planes of PDMS may be the reason for epitaxial growth KBr cleaved surface.
4. Optical properties In order to examine the effect of epitaxial growth on optical properties of PDMS, PL, EX and ABS spectra of two types (c-axis I or II to the substrate surface) of epitaxial PDMS films were measured and compared to those of PDMS powder. ABS spectra of the epitaxial films show a characteristic peak at 4.1 eV (300 nm) and broad absorption tail in the low energy region (Fig. 2(b) and Fig. 2c)), while the powder material shows a broad peak at 3.6 eV (344 nm) (Fig. 2(a)). The photoluminescence and absorption in polysilanes are associated with u - u * band gap transitions in silicon backbone. The broad absorption seems to be discussed in relation to the distribution of electron delocalized length in silicon backbone assuming that the ABS photon energy is determined only by quantum effect in the silicon-chain [ 121. The absorption peak energy of 4.1 eV corresponds to the number of silicon atoms in delocalized region (cluster size) of 10. However, the EX peak of the evaporated films
3
Fig. 2. Room
4 PHOTON ENERGY (eV)
temperature PL, EX and ABS spectra of PDMS powder (a), PDMS film evaporated on PTFE layer (b) and on KBr cleaved surface (cl.
3 PHOTON ENERG; (eV)
Fig. 3. Room temperature PL, EX spectra (a) and ABS spectra (b) of evaporated films on KBr, KCI, NaCl cleaved surfaces and quartz plate. These films were deposited in the same run. (b) ABS spectra.
appears at the low energy tail of the broad ABS band. This means that the energy transfer occurs from the short delocalized region to the longer ones and that the photoluminescence takes place dominantly from the longest delocalized regions in silicon chains. The probability of energy transfer should be very little in evaporated films because the EX spectrum shape has no relation to ABS one. The PL spectra of the evaporated films are poorly symmetrical to EX spectra, because the PL spectra are narrower than EL spectra. This fact also suggests the energy transfer between PDMS chains. Fig. 3(a and b) show the PL, EX and ABS spectra at 300 K for the evaporated films simultaneously prepared on alkali halide cleaved surfaces and on fused quartz plate. The absorption intensity around 3.6 eV where the EX spectra have a peak, and PL intensity are both in order of KBr, KCl, NaCl. This order corresponds to that of laitice constants. The lattice constant of KBr (6.6 A) is closer to the distance between (010) planes (6.7 A) than other alkali halides. This suggests that the PL intensity can be enhanced by increasing number of long delocalized regions by careful epitaxial growth. The results in Fig. 3 suggest that the number of effectively long delocalized regions in the films may rely on how much the epitaxial growth conditions are satisfied. The effective length of delocalized regions does not necessarily correspond to Si chain length, because the PDMS molecules arriving at the substrates in evaporation have the same length for each films
R. Hattori et al. /Applied
Surface Science I13/
which were prepared simultaneously in the same run. Therefore it is reasonable that the length of Si chain is long enough and the delocalized region is limited by randomness or imperfection of conformation of Si chain.
114 (19971472-475
475
powder samples. This work was partially supported by 1996 Grant-in-Aid for Encouragement of Young Scientist (A) (No. 08750365) form the Ministry of Education, Science and Culture of Japan.
References 5. Summary The PL spectrum from evaporated PDMS oriented films along with the EX and ABS spectra were measured. The large difference between the positions of ABS and EX peaks is explained in terms of
energy transfer process from short delocalized regions to the longest delocalized region and the extremely poor transfer probability. In order to increase the number of the longest delocalized regions or to increase the PL intensity, epitaxial growth of PDMS onto highly oriented and lattice-matched substrates is favorable.
Acknowledgements
The authors are much indebted to T. Fujiki, S. Kawasaki and R. Nishida of Advanced Technology Center, Osaka Gas Co., Ltd. for supplying PDMS
111R.D. Miller 121Y. Majima,
and J. Michl, Chem. Rev. 89 (1989) 1359. K. Kawata, Y. Nakano and S. Hayase, J. Polym. Sci. Polym. Chem. (1995). 20 (1987) [31 L.A. Harrah and J.M. Zeigler, Macromolecules 601. 141 Y. Majima, H. Nishizawa, T. Hiraoka, Y. Nakano and S. Hayase, Jpn. J. Appl. Phys. 34 (1995) 3820. [51 R.G. Kepler, J.M. Zeigler, L.A. Harrah and S.R. Kurtz, Phys. Rev. B 35 (1987) 2818. 161S. Furukawa and K. Takeuchi, Solid State Commun. 87 (1993) 931. [71 R. Hattori, Y. Aoki, T. Sugano, J. Shirafuji and T. Fujiki, to be submitted.
Bl K. Takeuchi,
M. Mizoguti, M. Kira, M. Shimana, S. Furukawa and M. Tamura, J. Phys., Condens. Matter 6 (1994) 10705. [91 R. Hattori, J. Shirafuji, Y. Aoki, T. Fujiki. S. Kawakasi and R. Nishida, J. Non-Crys. Solids, to be published. [lOI J.C. Wittmann and P. Smith, Nature 352 (1991) 414. [Ill A.J. Lovinger, F.C. Schilling, F.J. Padden, Jr. and F.A. Bovey, Macromolecules 24 (1991) 132. [I21 Y. Kanemitsu, K. Suzuki, Y. Nakayoshi and Y. Matsumoto, Phys. Rev. B 46 (1992) 3916.