Transport Measurements of Individual Bi Nanowires
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close-packed array of empty channels, which extend continuously from one side of the template to the other. The channel diameter can be made between 7 and 200nm by varying the electrochemical growth parameters. The template is then filled with either molten Bi [1,2,3] or with Bi from the vapor phase [4]. From X-ray diffraction measurements, it was found that the deposited Bi crystallizes with a preferred orientation with respect to the wire axis. Doping of the Bi with Te has also been studied, and is discussed elsewhere [1]. For measurements on individual nanowires, the nanowires are removed by dissolving the template in acid to leave a solution of free standing nanowires. In addition to the large body of work describing the fabrication of Bi nanowires, extensive theoretical work has been done on this system in predicting its electronic properties [5]. One of the most interesting phenomena that occurs in low dimensional systems is the quantum confinement shifting of the energy of the band edges. This effect increases as both the wire diameter decreases, as shown in Fig. 1, and also as the temperature decreases [1]. The calculation of band shifts in this work assumes a circular wire cross-section, corresponding to the experimental situation, whereas previous calculations had assumed a square wire cross-section [5]. Here we choose the trigonal crystalline orientation for simplicity to demonstrate the concept, since the three L-point electron pockets are degenerate for this orientation. In bulk form, Bi is a semimetal, with a band overlap of 38meV. As the wire diameter gets smaller than a critical diameter dc, the conduction band no longer overlaps the valence band and the material becomes a semiconductor with a band gap. As one might expect, the properties of the material change dramatically below this dc. Since the Bi nanowire samples prepared in this work are not oriented along the trigonal axis, the semimetal to semiconductor transition occurs at a slightly different value of dc than shown in Fig. 1. 2-POINT RESISITANCE MEASURMENTS OF Bi NANOWIRE ARRAYS Measurements of arrays of Bi nanowires in the anodic alumina templates can be made by attaching leads to either side of the sample with silver paint or low temperature solder. Since we don't know how many wires are contributing to the conduction or the precise conduction length, it is not possible at present to deduce the resistivity of the sample from these measurements. However these samples can provide the temperature dependence of the resistivity. Shown in Fig. 1 is the resistance normalized to room temperature for samples of various wire diameters in comparison to bulk Bi. We find the temperature dependence of the resistance of the nanowires to be different from that of bulk Bi. The bulk Bi sample shows semimetallic behavior where the resistance increases with temperature. For larger diameter wires (~200nm), a peak in the resistance is observed below 100K. We understand this behavior on the basis of the decrease in the band overlap, as depicted in Fig. 1, and
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