Fabrication and Transport Properties of Te-Doped Bi Nanowire Arrays
- PDF / 80,568 Bytes
- 6 Pages / 612 x 792 pts (letter) Page_size
- 67 Downloads / 239 Views
FABRICATION AND CHARACTERIZATION The fabrication technique of the Te-doped Bi nanowire arrays is similar to that used in synthesizing pure Bi nanowires [6], except that pure Bi metal is replaced by the Tedoped Bi alloy in the pressure injection process. The host material for the doped Bi nanowires is a porous anodic alumina template which has self-assembled hexagonallypacked cylindrical channels. The wide bandgap of the anodic alumina provides excellent confinement of the electronic wavefunctions within the nanowires. The porous anodic alumina was prepared by a two-step anodization method [7] to control the pore size distribution and the regularity of the pore packing order. The Al substrate with a thickness of 0.25 mm was polished mechanically and electrochemically before the anodization process. It was first anodized in 4 wt% H2C2O4 at 45 V for 90 minutes at 19°C, and then dipped in an etch solution of H3PO4 and CrO3 to dissolve the anodic alumina layer before the second anodization. The resulting Al subtrate was reanodized for 250 minutes under conditions identical to the first anodization. The SEM image of the top surface of the anodic alumina template is shown in Figure 1(a). The asprepared anodic alumina has an average pore diameter of 40 nm with a channel length of 70 µm. It was found that the pore size and the template thickness were very reproducible for samples prepared by this two-step anodization method. The Te-doped Bi alloy was prepared by mixing the desired amounts of high-purity Bi (99.9999%) and Te (99.999%) in a quartz tube, which was then evacuated and sealed. The mixture was melted, and maintained at 600°C for 24 hours. The as-prepared Bi-Te alloy was then placed in the high-pressure reactor along with the porous alumina template. In the pressure injection process, Cu atoms were introduced into the metal melt to facilitate the Bi alloy filling by reducing the surface tension of Bi with respect to the template [8]. After the pressure injection process, the Al substrate and barrier layer underlying the nanowire arrays were etched away by 0.2 M HgCl2 and 5 wt% H3PO4, respectively, so that the nanowires were exposed on both sides of the template and electrical contacts could be made. The nanowire array was annealed in vacuum for 8 hours to release the pressure within the composite system before the transport measurements [8]. Figure 1(b) shows the SEM image of the bottom surface of the anodic
Figure 1. SEM images of (a) the top surface of the anodic alumina template after two-step anodization; and (b) the bottom surface of the anodic alumina template after pressure injection of molten Te-doped Bi. The average pore diameters of both templates are 40 nm.
alumina template after removal of the barrier layer. The bright spots indicate the channels that are filled with metal, and the dark spots represent unfilled channels. Figure 1(b) shows that most of the channels were successfully filled all the way to the bottom side of the template. X-ray diffraction pattern of the nanowire composite indicates that
Data Loading...
