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10 to 80 pim per hour. The nanowires grew extending from the wall of the quartz tube towards the tip of the copper fingers. RESULTS A TEM image of the representative morphology of the nanowires is shown in Fig. 1. It can be seen that the sample of the Si nanowires, without any pretreatment, consists almost entirely of nanowires with diameters ranging from 3 to 43 nm and length up to a few hundreds of pm. The majority of the diameters falls between 11 nm and 24 nm with a distribution peak at -16 nm. Most of the wires are smoothly curved with some short straight sections; while some of them possess bends and kinks. A selected-area electron diffraction (SAED) pattern taken from this area is shown in the insert in Fig.1. The diffraction rings exhibit the feature of crystalline Si with the lattice index shown in the pattern. The chemical composition of the nanowires was only silicon and oxygen by in-situ investigating in the TEM with energy-dispersive x-ray spectroscopy (EDS) and ex-situ with Rutherford backscattering spectroscopy (RBS).

Fig. 1. A TEM image of the Si nanowires. The inset is a selected area electron diffraction pattern. Fig.2 shows a high resolution TEM image of a Si nanowire with a diamiter of 30.7 nm taken along the [110] direction. It can be seen that the nanowire consists of a crystalline Si core and a -5 nm thick disorder outer layer. The outer layer was found to be silicon oxide by EDS and XRD measurements (see Fig.3 also). The contrast of the Si core revealed a complicated feature in which many defects exist. Twins on the (1 T1T) plane appear in the form of narrow bands lying along 994

the [1 T2] direction, which is approximately parallel to the axis direction of the nanowire. There exists also some stacking faults in the form of equally spaced planar feature.

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5 nm. Fig. 2. A high-resolution TEM image of a Si nanowire taken along the [110] direction. Fig. 3 shows an XRD spectrum of Si nanowires. For comparison, we have also plotted the XRD spectrum of Si powder with an average grain size of 5 pim, and that of a Si nanowire sample after oxidization in air at -1000 °C. Notably, the broad peak at 0.411 rum can be attributed to the surface silicon oxide, since the Si nanowires were invariably oxidized upon exposure in air. Due to the very high surface-to-volume ratio of Si nanowires, the contribution of surface oxide layer to the XRD spectrum is thus clearly evident. The d-spacings of the Si nanowires as calculated from XRD spectrum are identical to those of bulk Si. However, the full width at half maximum (FWHM) of the XRD peaks of Si nanowires is about 3.6 times wider than that of the reference Si powder. This increased peak width can be due to the small diameter as well as the structural defects of Si nanowires (Fig. 1). The small-size effect on the XRD peak width may be deduced by the Scherrer equation, although its applicability to the one-dimensional Si nanowires is questionable. Using the width of the (111) XRD peak, we estimated the equivalent crystallite size of Si nanowires to be