SYNTHESIS AND OPTICAL PROPERTIES OF

NANO: Brief Reports and Reviews Vol. 8, No. 6 (2013) 1350067 (5 pages) © World Scienti¯c Publishing Company DOI: 10.1142/S1793292013500677

SYNTHESIS AND OPTICAL PROPERTIES OF CRYSTALLINE Si1x Gex Oy NANORODS C. W. ZHOU*,§, K. F. CAI*,‡, D. H. YU† and Y. DU* *Functional Materials Research Laboratory Tongji University, 1239 Siping Road, Shanghai 200092, P. R. China †

Bragg Institute, Australian Nuclear Science and Technology Organization New Illawarra Road, Lucas Heights, NSW 2234, Australia

NANO 2013.08. Downloaded from www.worldscientific.com by TONGJI UNIVERSITY on 12/29/13. For personal use only.

‡[email protected]

Received 3 March 2013 Accepted 16 July 2013 Published 14 August 2013

Crystalline Si1x Gex Oy nanorods were successfully synthesized by a chemical vapor deposition method using germanium as a starting material on Au-coated Si (111) substrate at 1000 C in a °owing high-purity Ar atmosphere. Most of the nanorods have smooth surfaces, with diameters ranging from 50 nm to 165 nm and lengths longer than several microns. Ultraviolet-visible absorption spectrum of the nanorods shows a maximum absorption wavelength at 203 nm. Photoluminescence spectrum of the nanorods exhibits an ultraviolet emission peak at 336 nm. The growth of the nanorods follows a vaporliquidsolid mechanism. Keywords: Oxides; nanostructures; vapor deposition; electron microscopy; optical properties.

1. Introduction One-dimensional (1D) nanostructures such as nanorods,1 nanowires2 and nanotubes,3 have attracted great attention recently because of their novel physical and chemical properties. These unique properties make 1D nanostructures as essential components in future functional devices. Oxides, as the basis of functional materials, have diverse nanostructures and endless new phenomena and applications.4 Silicon-based 1D oxide nanostructures are of great interest to both science and technology. Because of their strong photoluminescence at room temperature and high refractive index, silicon oxide 1D nanostructures are

potential materials applied in advanced optical devices.57 Si1x Gex Oy is isostructural to SiOx . Optical devices with 1D Si1x Gex Oy nanostructures will be able to work with a higher-frequency source.4 Moreover, the optical properties may be improved by Ge doping because GeO2 is more refractive than SiO2 6 and Ge has signi¯cantly higher hole and electron mobilities than Si. Furthermore, the exciton Bohr radius of Ge (24.3 nm) is much larger than that of Si (4.9 nm), which means that quantum con¯nement e®ect in Ge can be possibly achieved with larger-sized nanostructures.8 Until now, there are only few reports on the growth of Si1x Gex Oy 1D nanostructures. For

§

Present address: Shanghai Ship Building Technology Research Institute, No. 851, Zhong Shan Nan Er Rd, Shanghai, 200032, P. R. China. 1350067-1


C. W. Zhou et al.

example, He et al.4 fabricated amorphous Si1x Gex Oy nanowires on Au-coated silicon substrate by annealing Si0:8 Ge0:2 alloys in a quartz tube furnace at 9001200 C in N2. Ko et al.6 prepared amorphous Si1x Gex Oy nanowires on Si substrate via carbothermal reduction of GeO2/CuO powders at 1100 C in Ar, where the growth of the nanowires was catalyzed by CuSiGe. Huang et al.8 synthesized amorphous SiGeOx nanotubes on Au-coated Si substrate by heating GeI4 and Ge powders, respectively, in a conventional three-zone furnace system in Ar. In this work, we synthesized crystalline Si1x Gex Oy nanorods on Au-coated Si substrate by heating Ge powders in a simple quartz tube furnace at 1000 C in Ar. The growth mechanism of the nanorods was proposed. The ultraviolet-visible (UV-Vis) spectra and photoluminescence (PL) properties of the nanorods were studied.

heated to 1000 C and held at 1000 C for 60 min, the furnace was turned o® and cooled down to room temperature. One layer of gray product was observed on the substrate. The product grown on the substrate was examined by X-ray di®raction (XRD, Bruker D8 Advance, with CuK radiation, ¼ 1:5406 A), ¯eld emission scanning electron microscopy (FESEM, Hitachi, S4800) and transmission electron microscopy (TEM, JEOL, JEM-2100F) equipped with electron energy dispersive X-ray spectroscopy (EDX, EDAX). The sample for TEM examination was ultrasonically dispersed in absolute ethanol for 10 min before being dropped on a carbon-coated copper grid. The UV-Vis spectra of the sample were measured using an Agilent 8453 UV-Vis spectrometer at room temperature. The PL spectra of the sample were measured using a PK LS-55 °uorescence spectrometer with a Xe lamp at room temperature. The excitation wavelength was 220 nm.

2. Experimental Details One-side-polished Si (111) wafer (n-type, electrical resistivity 25 30 ohm cm) was used as substrate. The Si substrate about 5 mm 10 mm in size was cleaned with a conventional method9 and dried with Ar gas. A 5-nm-thick Au layer was thermally deposited on each substrate using an electron beam evaporation system (EMS550 X). For the growth of nanostructures, 3 g of Ge powder (99.999%) was loaded at the closed end of a small quartz tube while the substrate was placed at the open end of the quartz tube as shown in Fig. 1. The small quartz tube was transferred into a quartz tube furnace. Before heating, the system was evacuated by a mechanical pump, °ushed ¯ve times with highpurity Ar gas, and then the Ar gas was kept °owing into the system at a rate of 30 mL/min. After being

Fig. 1. Schematic diagram of the main part of the quartz tube system.

3. Results and Discussion Figure 2 shows the XRD patterns of the sample grown on a Si substrate and a pure Si substrate. The di®raction peak at 2 ¼ 28:60 is attributed to the Si (111) substrate (JCPDS card 26-1481). The one at 2 ¼ 26:53 corresponds to the (101) di®raction of the hexagonal GeO2 (JCPDS card 83-2480). FESEM image (see Fig. 3) shows that the sample contains two size distributions of nanostructures with diameters in the range of 50165 nm (thin)

Fig. 2. XRD patterns of the Si substrate and the sample with the Si substrate.

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Synthesis and Optical Properties of Crystalline Si1x Gex Oy Nanorods


Fig. 3.

FESEM image of the sample.

and 200550 nm (thick). The length of the nanostructures is about several microns. The amount of the thinner nanostructures is more than the thicker ones. As shown in Fig. 4(a), TEM image reveals both straight and curved nanorods. Black Au nanoparticles ( 60160 nm) were observed at one end of some nanorods, indicating that the nanorods were catalytically grown. As the size of the nanorods is determined by the size of Au nanoparticles, the

product consists of nanorods with di®erent sizes (see Fig. 3). An isolated nanorod [see Fig. 4(b)] was chosen for selected area electron di®raction (SAED). The SAED pattern [see Fig. 4(c)] con¯rms that the nanorod is in a crystalline form. As indicated in the schematic diagram of Fig. 4(d), the corresponding measured parameters are R1 ¼ R2 ¼ 2:829 1/nm, and ¼ 92:1 . With these values, the corresponding di®raction plane spacings are calculated: d1 ¼ d2 ¼ 0:3535 nm. However, according to the reported data of GeO2 (JCPDS card 83-2480), the interplanar distances of (101) and (011) planes are 0.3370 nm. Obviously, the lattice parameters from SAED analyses do not match well with those reported XRD data (JCPDS card 83-2480). The derivation may be due to the product being not pure GeO2 . Quantitative EDX analysis [see Fig. 4(e)] of the nanorods reveals that the atomic ratio of Si:Ge: O ¼ 0.04:0.96:1.52 and thus the chemical formula of the nanorods can be expressed as Six Ge1x O2y (0 < x < 1; 0 < y < 1). The C and Cu signals are from the TEM sample holder. It is interesting to note that the crystalline nanorods are transformed into amorphous state after having been exposed to a

Fig. 4. (a) TEM image of the sample; (b) TEM image of an isolated nanorod of the sample; (c) SAED image of the isolated nanorod in 4(b); (d) schematic diagram of general lattice parameters and (e) EDX spectrum recorded on the nanorods in 4(a). 1350067-3

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AuSi eutectic point ( 363 C), the Si contained Au ¯lm collapses to form AuSi droplets due to surface tension. As the temperature further increases, Ge vapor formed from the melted Ge powder and the residual oxygen are absorbed by the AuSi droplets to form AuSiGeO alloy droplets. When the alloy droplets become supersaturated, Six Ge1x O2y species start to nucleate, precipitate and grow. Eventually, the Six Ge1x O2y nanorods are formed.

4. Conclusions


Fig. 5. UV-visible absorption spectrum of the sample, the inset is the photoluminescence spectrum of the sample.

prolonged high-energy electron beam bombardment during the TEM analysis. This deformation may be induced by localized heating from electron beam, which has been shown to be capable of deforming nanostructures10 and inducing phase changes.11 Furthermore, the nanorods are likely to have a reduced melting point due to thermodynamic size e®ect.12 The lower melting point represents reduced bond strengths. This indicates that one can possibly control and manipulate the length and/or even the shape of a nanorod through the electron irradiation at de¯ned locations. Figure 5 shows the UV-visible absorption spectrum of the nanorods. It can be clearly seen from Fig. 5 that the maximum absorption wavelength (max ) of the nanorods is 203 nm. The PL spectrum of the nanorods is depicted in the inset of Fig. 5. It can be seen from the PL spectrum that the central wavelength of the luminescence is around 336 nm ultraviolet light. Signi¯cant blue shift of the central wavelength is observed as compared with the reported values of 390 nm and 415 nm from Refs. 4 and 6. The PL luminescence of the nanorods may be attributed to oxygen-de¯cient defects such as O3Si-GeO3 and O3Ge-GeO3 in the nanorods.4 On the basis of the above analysis results and previous reports,9,13 a vaporliquidsolid growth mechanism for the Si1x Gex Oy nanorods was proposed as follows: As the temperature increases, the Si atoms from the Si substrate gradually di®use into the Au ¯lm, and when the temperature is above the

The crystalline Six Ge1x O2y (0 < x < 1; 0 < y < 1) nanorods have been successfully synthesized. The nanorods were catalytically grown by Au particles. The UV-Vis absorption spectrum of the nanorods shows a maximum absorption wavelength at 203 nm. Room temperature PL spectrum of the nanorods exhibited an ultraviolet emission peak at 336 nm. The strong light emission from the nanorods is attributed to the oxygen de¯ciency. The UV emission from the nanorods may facilitate future applications in optical devices.

Acknowledgment This work was ¯nancially supported by the National Natural Science Foundation of China (50872095).

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