High frequency FTIR absorption of sio2 si nanowires

Đây là một bài báo khoa học về dây nano silic trong lĩnh vực nghiên cứu công nghệ nano dành cho những người nghiên cứu sâu về vật lý và khoa học vật liệu.Tài liệu có thể dùng tham khảo cho sinh viên các nghành vật lý và công nghệ có đam mê về khoa học

High-frequency FTIR absorption of SiO 2 /Si nanowires Quanli Hu * , Hiroshi Suzuki, Hong Gao, Hiroshi Araki, Wen Yang, Tetsuji Noda National Institute for Materials Science, 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan Received 3 June 2003; in final form 29 July 2003 Published online: 26 August 2003 Abstract An IR absorption measurement of SiO 2 /Si nanowires made by thermal evaporation was conducted. In comparison with SiO 2 nanoparticles, enhancement absorption of SiO 2 /Si nanowires around 1130 cm À1 was observed. This en- hancement was considered to result from: (1) the interface effect of the open structure of chainlike SiO 2 /Si nanowires; (2) the vibration of an interstitial oxygen atom in a silicon single-crystalline core of nanowire; and. The longitudinal optical (LO) modes of Si–O–Si stretching in an amorphous SiO 2 outer shell of SiO 2 /Si nanowires were also discussed. Ó 2003 Elsevier B.V. All rights reserved. 1. Introduction IR absorption spectroscopy is useful for un- derstanding the structural and compositional properties of many kinds of oxides. The IR ab- sorption characteristic of amorphous SiO 2 has also been studied for many years [1]. Three major ab- sorption bands centered at 460, 810, and 1070 cm À1 have been confirmed by many researchers [2]. These three absorption peaks reflect the rocking of an oxygen atom about an axis through the two silicon, the symmetrical stretching of an oxygen atom along a line bisecting the axis through the two silicon atoms and asymmetrical stretching of an oxygen atom along a line parallel to the axis through the two silicon atoms, respectivel y. In addition, an increase in the structural disorder could enhance the relative intensity of the ab- sorption band at a higher-frequency side [3]. Among these absorption bands, the intensity of 1070 cm À1 is much stronger than the features at its high-frequency side even when the structural dis- order of amorphous SiO 2 has been increased by different ways [4]. On the other hand, silicon nanowires have been grown using the VLS mechanism [5,6], STM [7], and laser ablation or thermal evaporation [8–10]. Lee et al. reported that the thermal evaporation method is useful for the large-scale synthesis of Si nanowires, which can be explained by a new oxide- assisted mechanism that involves the use of an oxide to promote nanowire growth. Moreover, the double-layer structure of silicon nanowires is ob- served. The TEM images of these nanowires indicate that each nanowire consists of an inner single-crystalline core and an outer layer of SiO 2 Chemical Physics Letters 378 (2003) 299–304 www.elsevier.com/locate/cplett * Corresponding author. Fax: +81298592701. E-mail address: HU.Quanli@nims.go.jp (Q. Hu). 0009-2614/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2003.07.015 (or SiO) amorphous layer. The nanostructural effect on the IR absorption properties may be investigated by studying the vibration modes of Si–O on the surface of the chain-like amorphous SiO 2 of silicon nanowires and in the interface be- tween silicon crystal and amorphous SiO 2 . How- ever, the detailed structural features of the surface of silicon nanowires with an amorphous SiO 2 outer layer have not been clearly investigated be- cause the fabrication of silicon nanowires with different nanostructures is very difficult. The objectives of this work, after getting a mount of nanowires by thermal physical evapo- ration, are to study the IR absorption character- istics of SiO 2 /Si nanowires and the nanostructural effect on IR absorption characteristics at a higher frequency. 2. Experimental Si powder (99.99 wt% purity, 300 mesh) was used as raw material for the growth of silicon nanowires. After being ground in a mortar, the Si powder was pre-sintered at 1150 °C for 2 h in a vacuum of 10 À5 Pa. The growth of silicon nano- wires was conducted by a modified thermal evap- oration process in a three-st age horizontal furnace with three independent heating controllers. Semi- cut quartz tubes containing Si wafers were placed along the downstream region in an alumina tube to act as the substrate for the grown SiNWs products. Pre-sintered silicon powder was placed in a Al 2 O 3 crucible and evaporated under 1350 °C for 2–3 h. The pressure in the alumina tube was maintained at normal values by flowing Ar gas at a rate of 20 sccm. The temperature distribution along the alumina tube in the furnace was con- trolled by temperature-setting values at three points. Silicon nanowire products on a silicon wafer and a quartz tube were examined by field-emission scanning electron microscopy (FE-SEM, JEOL- 6700F) and energy dispersion (EDS) attached to FE-SEM. Transmission electron microscopy (TEM, JEOL-2010) was utilized to characterize the detailed microstruc ture features of silicon nanowires. The IR transmission measurements were conducted in a JEOL IR spectrometer model JIR-7000 with a Fourier transform infrared spec- trometer (FTIR). The sponge-like silicon nano- wires were mixed with high-purity KBr to make a measurement pellet. The spectral resolution in the experiments was 0.5 cm À1 in the range of 400–4000 cm À1 . 3. Results and discussion After the fabrication of silicon nanowires by the thermal evaporation method, products with dif- ferent sizes of nanowires and surface states could be obtained in different temperature ranges because the size of silicon nanowires can be controlled by the variation of ambient temperature and pressure [9]. Three products were selected from different temperature ranges: (1) sample A: nanoparticles of silicon oxides (SiO 2 ) with 40–60 nm average parti- cle size taken from the temperature region of 1000 K; (2) sample B: thick silicon nanowires (di- ameter: 800–1000 nm; length: 100–400 lm) taken from the temperature region of 1173 K; and (3) sample C: thin silicon nanowires (diameter: 50–150 nm; length: 40–100 lm) taken from the tempera- ture region of 1373 K. Fig. 1 shows FE-SEM im- ages of the three samples described above with different nanostructures. In addition, the SAED (selected-area electron diffraction) analysis indi- cated that sample A is almos t amorphous SiO 2 ;on the other hand, the single nanowire in samples B and C has a double-layer structure, which has a crystalline silicon core and an amorphous SiO 2 outer shell (20–100 nm thickness). In Fig. 2, the absorption spectra of sample A taken from the temperature region of 1000 K are presented. Here, only the position of the absorp- tion peak is investigated. It shows the well-known transversal optica l (TO) resonances, the Si–O–Si rocking vibrational mode (468 cm À1 ), the O–Si–O bending mode (808 cm À1 ), and the Si–O asym- metric stretchi ng mode (1082 cm À1 ). Furthermore, the strongest absorption peak locates at the high- er-frequency side around 1082 cm À1 . The absorp- tion spectra of samples B and C taken from the temperature regions of 1173 and 1300 K are also shown in Fig. 2. Compared with that of sample A, 300 Q. Hu et al. / Chemical Physics Letters 378 (2003) 299–304 the absorption bands of samples B and C centered at 1000–1300 cm À1 showed some interesting vari- ations. Namely, the strongest absorption peak lo- cates at 1130 cm À1 , a phenomenon that has never been observed before from either bulk or film samples, and there is a shoulder of absorption spectra at the higher-frequency side around 1170– 1200 cm À1 . In order to understand clearly the in- fluence of Si–O bond stretching on the shape and intensity of the spectra of samples B and C at a higher frequency, a detailed analysis of IR spectra in the range of 1000–1300 cm À1 is given in Fig. 3. This analysis shows that IR absorption spectra in the range of 1000–1300 cm À1 can be deconvoluted in four absorption bands with a symmetrical Gaussian shape centered at about 1070, 1130, 1160, and 1200 cm À1 . The relative ratio among the areas of the four absorption bands above, which reflect the relative distribution for each stretching mode, is shown in Table 1. Table 1 indicates that the TO asymmetric stretching modes of the Si–O bond dominate in the range of 1000–1300 cm À1 of sample A. Two other weak modes, longitudinal optical (LO) asymmetric stretching modes at 1160 cm À1 and asymmetric stretching (TO) modes at 1200 cm À1 , also contribute to the absorption spectra around 1000–1300 cm À1 . If we only consider the effect of nanoscale size, we should find similar enhancement on higher frequency absorption of IR spectrum in nanopar- ticle and nanowire. Because either nanoparticles or nanowires have a strong surface tension to cause the distortion and the shortening of Si–O bond length. And this will produce more intensity at higher frequency. However if we further consider the structural characteristics of nanoparticles and nanowires, the differences on IR spectrum are very obvious. First of all, the difference comes from effect of crystalline field of silicon single crystal core. In SiO 2 /Si nanowires, the crystalline field of silicon core could bring some influence on the Si–O vibration of outer SiO 2 layer or interface. This kind of influence may increase the energy gap be- tween excited state and ground state for Si–O vi- bration absorption to cause the increasing of higher frequency absorption in nanowires. Oppo- sitely, the crystalline field effect does not exist in nanoparticles of SiO 2 because it has no silicon single crystal core. Second, the difference comes from the interface structure of the SiO 2 /Si nano - wires. In SiO 2 /Si nanowires, the interface between silicon core and SiO 2 outer shell has a large ratio in the structure of nanowires body. And there are a lot of point defects such as vacancy and broken bonds of Si–O on these interfaces. Therefore, these could also bring the strong absorption intensity at higher frequency. Fig. 1. The FE-SEM images of samples A, B, and C. Q. Hu et al. / Chemical Physics Letters 378 (2003) 299–304 301 In another aspect, themselves of SiO 2 /Si nano- wires with micron-meter order length and nano- meter-order diameter bring the disorder in some extent in the measurement pellet. According to Gaskell [11], the intensity of the absorption band of amorphous SiO 2 at the high-energy side of the main stretching mode can be enhanced in samples with a large degree of structure disorder increased by different means, for instance, by ion bom- bardment. In the present work, the mixi ng of nanoscale structures of SiO 2 /Si nanowires obvi- ously results in a more open structure with free volume and surface to produce disordered effect, which cannot be achieved by the ordinary me- chanical-grounding method. Thi s is one of reasons for the enhanced intensity of the band around 1000–1300 cm À1 . Furthermore, the absorption band centered at 1130 cm À1 indicates that the existence of an ox- ygen atom dissolved in silicon nanowires cannot be ignored. It is known that the SiO 2 outer shell reacts with molten silicon, especially on the in- terface between SiO 2 outer layer and silicon core during the formation of silicon nanowires [12]. The oxygen in turn may dissolve in silicon single crystalline core to a certain extent. Some of them may form interstitial oxygen atoms. The intersti- tial oxygen atoms are assumed to be bound to two neighboring silicon atoms in regular lattice sites [13]. This means that two neighboring silicon atoms give up their covalent bond and engage with an interstitial oxygen atom instead, forming an isosceles triangle with Si–O–S i at the corners. In Fig. 4, the interface between amorphous SiO 2 and single crystalline silicon can be clearly ob- served. And the point defects and broken bonds to be found on the interface also provide an evidence for above discussion. Moreover, in samples B and C, the enhance- ment of the absorption band at 1160 cm À1 ,to which the LO modes of Si–O–Si stretching con- tribute, is observed. Usually, in an infinite bulk sample of amorphous SiO 2 , the LO modes can Fig. 2. The IR absorption spectra around 400–2000 cm À1 for the samples A, B, and C. Table 1 Relative distribution for each stretching mode from the fitting analysis 1070 1130 1160 1200 (cm À1 ) (cm À1 ) (cm À1 ) (cm À1 ) Sample A 0.91 0 0.04 0.05 Sample B 0.48 0.1 0.21 0.21 Sample C 0.52 0.06 0.08 0.34 302 Q. Hu et al. / Chemical Physics Letters 378 (2003) 299–304 only be observed by using polarized light with some angles of incidence because the electro- magnetic wave such as infrared wave cannot in- teract with longitudinal phonons. For example, Berreman carried out experiments in oblique in- cidence with p-polarized light (with the electric field vector parallel to the plane of incidence) [14]. But the excitation of longitudinal optical reso- nances is possible if the film is sufficiently thin compared to the incoming wavelength. In the present work, the absorption band at 1160 cm À1 was attributed to the fact that the thickness of amorphous SiO 2 (20–100 nm) was much smaller than the wavelength of the infrared optical reso- nances (2500–25 000 nm). In addition, the inter- face and boundary in SiO 2 /Si nanowires have very high ratio to produce many chance that may cause considerable absorption of LO modes, too. These also cause the strong signals in the region of the LO vibration modes. Finally, in sample A, most of the nanoparticles have no silicon single-crystalline core or chainlike structure; the influence from the interstitial oxygen vibration mode is either weak or zero. Therefore, the strongest absorption peak locates at 1082 cm À1 , which only comes from the influence of the TO resonances of the Si–O–Si asymmetric stretching modes. The shoulder of the absorption band around the 1170 cm À1 results from the co- existence of the LO and the TO resonances of Si–O–Si asymmetric stretching modes. Further work is underway to investigate the thickness variation of the amorphous SiO 2 outer- Fig. 4. The HRTEM image of one part of sample C. Fig. 3. The deconvolution in four absorption bands with symmetrical Gaussian shape centered at about 1070, 1130, 1160, and 1200 cm À1 for samples A, B, and C. Q. Hu et al. / Chemical Physics Letters 378 (2003) 299–304 303 shell dependence of IR absorption intensity and position. 4. Conclusion Enhancement of the relative intensity of IR absorption spectra around 1000–1300 cm À1 from an open structure with a free volume of SiO 2 /Si nanowires was observed. The interface effect and interstitial oxygen from SiO 2 /Si nanostructural characteristics were suggested to result in the en- hancement of above vibration modes. In addition, the stronger LO resonances at 1160 cm À1 was found, which depend on the nanoscale size effect of SiO 2 outer layer in SiO 2 /Si nanowires. 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