nanowires
The IR absorption characteristic of amorphous SiO2 has been studied before and three
major absorption peaks centered at 460, 810 and 1070 cm-1 have been confirmed by many researchers. These three absorption peaks reflect the rocking of an oxygen atom about an axis through the two silicon atoms, the symmetrical stretching of 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, respectively [5.17,5.18]. FTIR measurements were done on the silicon oxide nanowires at room temperature in the range of 500-4000 cm-1. Figure 5.6 shows the transmittance and absorbance of the nanostructures, with major absorption peak at 558 and 1075 cm-1, which can be attributed to Si-O-Si and Si-O stretching vibrations respectively. The other major absorption expected at 810 cm-1 was not observed this might be directly related to the purity of the nanostructures. The most intense peak at 1075 cm-1 is characterized by Si-O asymmetric stretching mode, whereby the adjacent O atoms execute the asymmetric motion in phase [5.19, 5.20].
155 Figure 5.6: FTIR spectra of SiOx nanowires.
156
5.4 Conclusions
A 60 nm thin film of gold was deposited on silicon substrates by sputtering to act as a catalyst and assist in the growth of FeSi nanowires; this was done to observe the VLS growth mechanism. There nanowires that were produced from the reaction were not FeSi nanowires; instead amorphous SiOx nanowires resulted in the final reaction
products. XRD and TEM results showed that the nanowires were amorphous and there was no FeSi present in the nanowires. Raman and FTIR also revealed signatures for amorphous silicon in the nanostructures. The absence of Fe and Au particles in the nanostructures provided proof that Fe and Au did not play any catalytic role in the growth of the SiOx nanowires. No catalyst tips were found on the tips of the nanowires,
which should be a typical scenario if the nanowires followed the VLS growth mechanism. The growth mechanism for these nanowires was explained by the formation of SiOx clusters which acted as nucleation centers for the growth of the
nanowires.
157
5.5 References
[5.1] J. Lee, C. Choi, T.Seong, Current Applied Physics, 2011, 11, 199-202.
[5.2] Z.Q Liu, W.Y Zhou, L.F Sun, D.S Tang, X.P Zou, Y.B Li, C.Y Wang, G. Wang, S.S Xie, Chemical Physical Letters, 2001, 341, 523-528.
[5.3] D.P Yu, Y.J Xing, Q.L Hang, H.F Yan, J. Xu, Z.H Xi, S.Q Feng, Physica E, 2001, 9, 305-309.
[5.4] C.H Liang, L.D Zhang, G.W Meng, Y.W Wang, Z.Q Chu, Journal of Non-Crystalline Solids, 2000, 277, 63-67.
[5.5] N.Wang, Y.Cai, R.Q Zhang, Materials Science and Engineering R, 2008, 60, 1-51. [5.6] Y. Liang, B. Xue, Y. Yumeng, N. Eryong, L. Donglai, S. Congli, F. Huanhuan, X. Jingjing, C.Yu, J.Yong, J. Zhifeng, S. Xiaosong, Applied Surface Science, 2011, 258, 1470-1473.
[5.7] K.A Dick, progress in Crystal Growth and Characterization of Materials, 2008, 54, 138-173.
[5.8] X.Chen, R.S Ruoff, Nano: Brief Reports and Reviews, 2007, 2, 91-95.
[5.9] R.S Wagner, W.C Ellis, Transactions Of The Metallic Society of AIME, 1965, 233, 1053-1063.
[5.10] X.Cao, Y.Kolyptin, R.Prozorov, G.Kataby, A.Gedanken, Journal of Material Chemistry, 1997, 7(12), , 2447-2451.
[5.11] M.Mebarki, A.Layadi, A.Guittoum, A.Benabbas, B.Ghebouli, M.Saad, N.Menni, Applied Surface Science, 2011, 257, 7025-7029.
[5.12] Y.Chen, C.Somsen, S.Milenkovic, A.W Hassel, Journal of Materials Chemistry, 2009, 1, , 1-5.
[5.13] S.Liang, X.Fang, T. Xia, Y.Qing, Z.Guo. Journal of Physical Chemistry C, 2010,114, 16187-16190.
[5.14] Z.Q Liu, Z.W Pan, L.F Sun, D.S Tang, W.Y Zhou, G. Wang, L.X Qian, S.S Xie, Journal of Physics and Chemistry of Solids, 2000, 61, 1171-1175.
[5.15] J. Qu, Z. Zhao, X. Wang, J. Qiu, Y. Gogotsi, Materials Express, 2012, 2, 157-163.
158
[5.16] S.K Srivastava, P.K Singh, V.N Singh, K.N Sood, D. Haranath, V. Kumar, Physica E, 2009, 41, 1545-1549.
[5.17] T.Noda, H. Suzuki, H. Araki, W. Yang, Y. Shi, M. Tosa, Applied Surface Science, 2005, 241, 231-235.
[5.18] Y.W Wang, C.H Liang, G.W Meng, X.S Peng, L.D Zhang, Journal of Materials Chemistry, 2002, 12, 651-653.
[5.19] E.M Duraia, Z.A mansurov, S. Tokmolden, G.W Beall, Physica B, 2010, 405, 1176-1180.
[5.20] J. Niu, D. Yang, J. Sha, J. N Wang, M. Li, Materials Letters, 2007, 61, 894-896.
159
CHAPTER 6
______________________________
6. Summary
Synthesis of stoichiometric FeSi nanowires and nanofibers was done successfully by means of a chemical vapor deposition method at an optimized temperature of 1100oC. We have optimized the synthesis conditions, specifically temperature; experiments were done in the temperature range of 200oC to 1100oC. We found that at substrate temperatures lower than 1000oC, the nanostructures which results from those reactions have big diameters with values up to 600 nm and often contain chlorine impurities. When doing reactions at very low temperatures, in most instances, no nanowire growth was observed. However FeSi nanowires synthesized at temperatures above 1000oC had smaller diameters. The preferred growth method/mechanism for these nanowires was through the oxide assisted or self-catalytic growth method, which does not require a metal catalyst. The nanowire growth was simply aided by the native silicon oxide layer on the substrates. The nanowires have a crystalline core and an amorphous shell covering them. Characterization of the nanowires was done using the following techniques: XRD, SEM, TEM, FTIR, PL and Raman spectroscopy.
160
Photoluminescence studies were done on the nanostructures on the FeSi nanowires and fibers. There were observed features between the UV and the near infrared region (410 nm to 1772 nm), which could not be assigned to FeSi, with its band gap value of 0.1 eV, the material is expected to emit in the far infrared region and could not be observed due to instrument limitation. These results were attributed to intraband transitions and were explained by using band structure calculations using local density approximation (LDA) and generalized gradient approximation (GGA).
Attempts made to grow FeSi nanowires via the assistance of metal catalyst proved to be unsuccessful. Thin films of gold were deposited on silicon substrates and placed in a quartz tube furnace at a temperature of 1100oC. Instead of single crystalline FeSi nanowires with gold catalyst tips, as would be expected from a VLS reaction, amorphous SiOx nanowires were obtained. The growth mechanism was explained by
the formation of clusters which act as nucleation centers for the formation of the nanowires. The SiOx nanowires were characterized by XRD, SEM, TEM, FTIR and
Raman spectroscopy.
161
6.1 Future Plans
Figure 6.1 shows preliminary gas sensing results of a powder that was collected from the wall of the quartz tube. The powder was tested for hydrogen sensing at 300oC and showed good response for different concentrations. We would like to establish more reliable contact between the powder and electrodes because of noise in the results.
Figure 6.1: Hydrogen sensing at 300oC of a FexSixClxOx ceramic.
162
The other preliminary work involved dropping nanowires that were dispersed on ethanol on an interdigitated device (see Figure 6.2). The dropping method has its shortcomings as nanowires and other structures fall anywhere on the device. The future plan is to weld the nanowires using FIB-SEM in order to have more accurate electrical and gas sensing measurements.
Figure 6.2: Nanowire between two electrodes of the interdigitated device.
163
Appendix A-Publication
164
165
166
167
168
169