Crystalline boron oxide nanowires on silicon substrate

Transcription

1 Physica E 27 (2005) Crystalline boron oxide nanowires on silicon substrate Qing Yang a, Jian Sha b, Lei Wang a, Yu Zou a, Junjie Niu a, Can Cui a, Deren Yang a, a State Key Laboratory of Silicon Materials, Zhejiang University, 20 Yu Gu Road, Hangzhou , People s Republic of China b Department of Physics, Zhejiang University, 20 Yu Gu Road, Hangzhou , People s Republic of China Received 26 November 2004; accepted 20 December 2004 Abstract Crystalline boron oxide nanowires have been synthesized on silicon substrates by chemical vapor deposition (CVD) process without the use of catalysts or templates. It is pointed out that the boron oxide nanowires are cubic and single crystalline, and the diameter of the nanowires is in the range of nm: Some of the nanowires branched, and the diameters of the branches and stems of the branched boron oxide nanowires are in the range of and nm; respectively. The crystallinity, morphology, and structure features of the as-prepared boron oxide nanowires were investigated by field emission scanning electron microscopy, X-ray diffraction, transmission electron microscopy, and selected area electron diffraction. Furthermore, Raman spectrum and Fourier transform infrared spectroscopy of the nanowires were also investigated. r 2005 Elsevier B.V. All rights reserved. PACS: Je; w; Gh Keywords: B 2 O 3 ; Nanowires; CVD 1. Introduction The trivalent elemental boron, characterized by its short covalent radius, sp 2 hybrid valence orbital, three-center deficiency bonds, high melting point of 2300 C, hardness that is similar to that of diamond, etc. has been intensively focused on [1]. Corresponding author. Tel.: ; fax: address: mseyang@zju.edu.cn (D. Yang). The curious discovery of new superconducting material MgB 2 above 40 K has accelerated boron and boron-related compounds research [2 4]. Boron oxide is one of the additives widely used to modify textural and acid-based properties of metal oxides such as Al 2 O 3 ; TiO 2 ; and MgO [5], and metal borate is an excellent antiwear and reducing friction additive [6]. One-dimensional (1D) materials, having the potential to go far beyond the limits of top-down manufacturing, are important for future nanodevices and fundamental /$ - see front matter r 2005 Elsevier B.V. All rights reserved. doi: /j.physe

2 320 Q. Yang et al. / Physica E 27 (2005) research [7,8]. Till now, boron and boride compound nanostructures such as amorphous [9,10] and crystalline boron nanowires [11 13], crystalline boron nanobelts [14], MgB 2 nanowires [15,16], metal borate nanowires and nanotubes [17,18], and amorphous boron oxide nanowires with a diameter of nm on Mg surface [19] have been synthesized. To achieve the functionality of these quantum wires as building blocks in potential nanoscale electronic and mechanical devices, several objectives are important. One objective is to fabricate nanowires on silicon substrates and integrate them to useful devices. Forming nanowires by chemical vapor deposition (CVD) is especially attractive because of the compatibility of CVD with conventional semiconductor device fabrication and also the ease of scaling from research to production-size systems. Silicon nanotubes [20] and nanowires [21], silica nanowires [22], boron nanowires [12], and MgB 2 nanowires [16] etc. have been synthesized by our group using CVD as the fabrication method and nano-channel Al 2 O 3 or silicon as the substrates. Here, we report the preparation of crystalline boron oxide nanowires on silicon substrates using a simple chemical vapor deposition (CVD) method which is different from the amorphous boron oxide nanowires synthesized through infrared irradiation reported by Ma et al. [19], the assynthesized nanowires are single crystalline. For consideration of the compatibility with the integrated circuits, single crystal silicon substrates were used in our experiments. The optical properties of these nanowires have also been characterized by Raman scattering spectroscopy and Fourier transform infrared spectroscopy. 2. Experimental details The experimental apparatus consists of a horizontal tube furnace (75 cm in length), a reacting chamber made of quartz tube (f 5:5 150 cm), a rotary pump system, and a gas supply and control system. The ultimate vacuum for this configuration is 70 Pa: Several silicon substrates were placed downstream one by one from the center of the furnace to the end of the furnace, for collecting growth products. The furnace chamber was first pumped down to 100 Pa and heated. As the temperature reached 1000 C, the gas mixture of argon, hydrogen and diborane with a flow ratio of 50:20:1 was allowed into the chamber. The pressure and temperature in the chamber were kept at 2000 Pa and 1000 C, respectively. After 5 h deposition the chamber was cooled to room temperature with the furnace. Boron oxide nanowires were collected on the downstream place of 30 cm from the center of the furnace at the temperature of about 400 C. Field emission scanning electron microscopy (FESEM, Sirion), energy-dispersive X-ray spectroscopy (EDX) attached to the FESEM, transmission electron microscopy (TEM, JEM200CX, 200 kv), selected-area electron diffraction (SAED), and X-ray diffraction (XRD, D/maxrA, with CuKa radiation) were carried out for the morphological and structural observation. Raman scattering spectroscopy (Nicolet, Almega) and Fourier transform infrared (FTIR, Bruker, IFS 66v/S) were performed for the vibrational investigation. All the measurements were carried out at room temperature. 3. Results and discussion Fig. 1 shows the XRD pattern of the assynthesized nanowires. All the three diffraction peaks can be perfectly indexed to the cubic crystalline B 2 O 3 ða ¼ 10:05 AÞ; ( and the diffraction data are in agreement with JCPDS card of , not only in the peaks positions, but also in their relative intensity. The two peaks in the spectrum can be readily indexed as (3 1 0) and (4 2 0) crystal planes of the cubic structure B 2 O 3 ; respectively. The morphology of the B 2 O 3 nanowires was observed via SEM and TEM, as shown in Figs. 2 and 3. Fig. 2 displays the SEM image of the B 2 O 3 nanowires. A large quantity of curved and very long nanowires could be seen. The TEM image of the nanowires is depicted in Fig. 3(a); it can be seen that the diameter of the nanowires is in the range of nm: SAED pattern of the nanowires shows that the nanowires are single

3 Q. Yang et al. / Physica E 27 (2005) B 2 O CPS θ Fig. 1. XRD patterns of the boron oxide nanowires synthesized by CVD process. All the three diffraction peaks can be perfectly indexed to the cubic crystalline B 2 O 3 ða ¼ 10:05 ( AÞ; while the diffraction data are in agreement with JCPDS card of Fig. 3. TEM images of the B 2 O 3 nanowires. (a) TEM image of curved long boron oxide nanowires. (b) A branched nanowire. Fig. 2. SEM image of the boron oxide nanowires on silicon substrates. crystalline B 2 O 3 with the lattice constant of a ¼ 10:05 ( A; consistent with the XRD result shown in Fig. 1. Some branched boron oxide nanowires could also be found in the TEM characterization (Fig. 3(b)), which shows that the nanowires nucleate and grow on the sidewall of the backbone nanowire and form multiple nanojunctions. The diameters of the branches and stems are in the range of and nm; respectively. The SAED pattern taken from the branched nanowire shown in the inset of Fig. 3(b) illustrates that the branched nanowire is also a single crystal cubic lattice structure of boron oxide with the lattice constant of a ¼ 10:05 A: ( The ripple-like contrast observed in the TEM images is due to the strain resulting from the bending of the nanowires [23]. Fig. 4 shows the Raman spectra of the assynthesized B 2 O 3 nanowires (curve a) and bulk B 2 O 3 (curve b). There are two peaks in the spectrum of the B 2 O 3 nanowires. The dominant line at 878 cm 1 is generally attributed to a breathing mode of the three oxygen atoms within a boroxol ring [24]. Its broadened width and asymmetric shape compared with the Raman spectrum of bulk boron oxide could be due to the strain of the defects existing on the nanowires. The relatively weaker line at 500 cm 1 in the Raman spectrum of the boron oxide nanowires is attributed to the Si B stretching mode [25].

4 322 Q. Yang et al. / Physica E 27 (2005) Raman Intensity (a) (b) Raman Shift (cm -1 ) Absorbance Wavenumbers (cm -1 ) Fig. 4. Raman spectra of the boron oxide nanowires (a) compared with that of bulk boron oxide (b). Fig. 5. FTIR spectrum of the boron oxide nanowires. Compared with the Si B stretching mode in bulk structure at 515 cm 1 ; the downshift of it might be caused by the tensile deformation between the Si substrate and the boron oxide nanowires. The existence of the Si B bond shows that the assynthesized boron oxide nanowires are chemically connected with silicon substrates. Fig. 5 shows the FTIR spectrum of the B 2 O 3 nanowires subtracting the spectrum of the silicon substrate. For eliminating the influence of the silicon substrate, a same silicon plate without the boron oxide nanowires was measured by means of FTIR at the same measurement conditions. It is well known that the band at around 3207:4cm 1 is related to O H stretching bond with the hydrogen being involved in a hydrogen bridge bond [26]. The small peak appears at 2260:4cm 1 ; it can be attributed to O Si H bond [27]. The three bands at , , and 1456:1cm 1 can be attributed to B O B stretching mode, B O H d bond, and B O terminal bond, respectively [28]. The other two bands at and 2508:9cm 1 are attributed to B H terminal bonds [29]. The existences of B H, B O H, and O H bonds show that the boron oxide nanowires are hydrogenated because of the gas of diborane and the hydrogen used for synthesis of the nanowires. Vapor liquid solid (VLS) mechanism [30], solution liquid solid (SLS) mechanism [31], vaporphase (VS) mechanism [32], and oxide-assisted growth mechanism [33] are the most successful mechanisms for explaining the growth of 1D nanostructures. In VLS and SLS mechanisms, the 1D growth of nanowires is mainly induced by liquid droplets at the tip of nanowires. It seems that the VLS and SLS model are not suited for our case because no metal catalysts were used in our experiments. In addition, no metal or alloy particles can be found at the tips of the boron oxide nanowires. We suggest that our use of the CVD method to grow crystalline boron oxide nanowires may involve a mechanism complex of oxide-assisted and VS mechanism. The growth process might be as follows: first, diborane decomposed and formed boron clusters that condensed on the substrates placed in the center of the furnace to form boron particles. Then the particles were oxidized by the residual oxygen and impure reaction gases in the quartz tube to form B 2 O 3 particles. The chemical reactions might be B 2 H 6! 2B þ 3H 2, (1) 4B þ 3O 2! 2B 2 O 3. (2) Boron oxide has a low melting point ( 450 C). When heated to 1000 C; the boron oxide is molten. The molten boron oxide produced B 2 O 3 vapor, and B 2 O 3 vapor was transported by the carrier gas to the low-temperature region and became super cold droplets. When the temperature is lower than the melting point of B 2 O 3 ; the super cold droplets will condense on the substrates

5 Q. Yang et al. / Physica E 27 (2005) placed at the low-temperature zone as the nuclei of the nanostructures and boron oxide nanowires formed finally. Some nanowires could nucleate on the defects of other nanowires and formed branched nanowires. It should be noted that boron oxide nanowires could not form when boron oxide powders were put in the center of the furnace and the furnace was heated to 1000 C under the flow of a gas mixture of Ar and H 2 : The reason might be that the volatilization of bulk B 2 O 3 is too fast at a high temperature and too much B 2 O 3 vapor formed at the same time. Therefore, just boron oxide particles, but no boron oxide nanowires, could form. However, in the CVD system, the quantity of diborane could be controlled, and the process of forming B 2 O 3 vapor is slow. B 2 O 3 nanowires could form through vapor solid mechanism. Additionally, when the temperature was not high enough (lower than 950 C) and the vacuum degree was high, boron nanowires formed instead of boron oxide nanowires because of the low oxidization of boron clusters [12]. 4. Conclusions In summary, crystalline boron oxide nanowires have been synthesized on silicon substrates by the CVD process without the use of catalysts or templates. The diameter of the nanowires is in the range of nm: Some of the nanowires are branched. Raman spectrum and FTIR spectroscopy have been checked on the nanowires. The broadened width and asymmetric shape of peaks in the Raman spectrum could be due to the strain of the defects existing on the nanowires. The FTIR spectrum of the B 2 O 3 nanowires shows that the nanowires are hydrogenated. The growth mechanism of the B 2 O 3 nanowires may involve a mechanism complex of oxide-assisted and VS mechanism. Acknowledgements This work is financially supported by the National Natural Science Foundation of China (Project nos and ), and the key project of the Education Department of China. And the authors would thank Mr. Youwen Wang for his great help in TEM and SEM measurement. References [1] W. Diets, H. Helmberger, Boron Semiconductor Devices, Plenum, New York, 1965, p [2] J. Nagamatsu, N. Nakagawa, T. Muranaka, Y. Zenitani, J. Akimitsu, Nature 410 (2001) 63. [3] J.S. Slusky, N. Rogado, K.A. Regan, M.A. Hayward, P. Khalifah, T. He, Nature 410 (2001) 343. [4] C.B. Eom, M.K. Lee, J.H. Chol, L.J. Belenky, Nature 411 (2001) 558. [5] M.A. Aramendia, V. Borau, C. Jimenez, J.M. Marinas, A. Porras, F.J. Urbano, J. Mater. Chem. 9 (1999) 819. [6] Z.S. Hu, R. Lai, F. Lou, L.G. Wang, Z.L. Chen, G.X. Chen, J.X. Dong, Wear 252 (2002) 370. [7] M.S. Gudiksen, L.J. Lauhon, J. Wang, D.C. Smith, C.M. Lieber, Nature 415 (2002) 617. [8] P. Calvert, Nature 399 (1999) 210. [9] L. Cao, Z. Zhang, L. Sun, C. Gao, M. He, Y. Wang, Y. Li, X. Zhang, G. Li, J. Zhang, W. Wang, Adv. Mater. 13 (2001) [10] L. Cao, K. Hahn, Y. Wang, C. Scheu, Z. Zhang, C. Gao, Y. Li, X. Zhang, L. Sun, W. Wang, Adv. Mater. 14 (2002) [11] C.J. Otten, O.R. Lourie, M.F. Yu, J.M. Cowley, J. Am. Chem. Soc. 124 (2002) [12] Q. Yang, J. Sha, J. Xu, Y. Ji, X. Ma, J. Niu, H. Hua, D. Yang, Chem. Phys. Lett. 379 (2003) 87. [13] Y. Wang, Appl. Phys. Lett. 82 (2003) 272. [14] Z. Wang, Y. Shimizu, T. Sasaki, K. Kawaguchi, K. Kimura, N. Koshizaki, Chem. Phys. Lett. 368 (2003) 663. [15] Y. Wu, B. Messer, P. Yang, Adv. Mater. 13 (2001) [16] Q. Yang, J. Sha, X. Ma, Y. Ji, D. Yang, Supercond. Sci. Technol. 17 (2004) L31. [17] R. Ma, Y. Bando, T. Sato, Appl. Phys. Lett. 81 (2002) [18] R. Ma, Y. Bando, D. Golberg, T. Sato, Angew. Chem. Int. Ed. 42 (2003) [19] R. Ma, Y. Bando, Chem. Phys. Lett. 374 (2003) 358. [20] J. Sha, J. Niu, X. Ma, J. Xu, X. Zhang, Q. Yang, D. Yang, Adv. Mater. 14 (2002) [21] J. Niu, J. Sha, X. Ma, J. Xu, D. Yang, Chem. Phys. Lett. 367 (2003) 528. [22] J. Niu, J. Shaa, N. Zhang, Y. Ji, X. Ma, D. Yang, Physica E 23 (2004) 1. [23] C. Ma, D. Moore, J. Li, Z. Wang, Adv. Mater. 15 (2003) 228. [24] A.H. Verhoef, H.W. Hartog, J. Non-Cryst. Solids 180 (1994) 102.

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