Nanowires bismuth

We use the same approach to classify the different nanostructures for Titania. The term one-dimensional (ID) nanostructures indicate nanocrystals in which elongation only in one direction is above this threshold (about 10 nm). This class of ID nanostructures comprises different types of nano-ordered materials, such as nanorods, -wires, -coils, -fibers, -pillars (or -columns) and -tubes. We prefer to use the term quasi one-dimensional nanostructures, because often the dimensions are larger than the indicated threshold, although elongation along one main axis still exists. When the diameter of the nanorod, nanowire or nanotube becomes smaller, there is often a significant change in the properties with respect to crystalline solids or even two-dimensional systems. A bismuth nanowire is an excellent example, which transforms into a semiconductor, as the wire diameter becomes smaller.145... 

Nanowire systems have attracted a great deal of attention recently due to their technological potential They are of fundamental interest because they exhibit unique quantum confinement effects. In this article, advances in the fabrication of nanowires via template-assisted and laser-assisted approaches are reviewed. The structure and characteristics of different nanowire systems are discussed. To understand and predict the unusual properties of nanowires, we have developed a generalized theoretical model for the band structure of these onedimensional systems. A unique semimetal-semiconductor transition that occurs in bismuth nanowires is described. Transport measurements on bismuth and antimony nanowires illustrate that these novel materials are very different from their bulk counterparts. A transport... 

Figure 6(a) shows a SEM image of bismuth nanowires embedded in an anodic alumina template after pressure injection. The pore diameter of the template was about 42 nm, and the maximum pressure and temperature applied for the injection process were about 310 bar and 325°C, respectively. A small amount of copper was introduced into hquid bismuth to enhance its injection the copper atoms should be segregated from bismuth during solidification because they have zero solubihty in solid bismuth. Upon solidification, the copper flakes were brought to the top surface of Bi due to their... 

Fig. 6. (a) SEM image of the bottom surface of an anodic alumina template filled with bismuth. The pore diameter is 42 nm. (b) TEM micrograph of the cross section of a 65-nm bismuth nanowire array (Zhang et al., 1999). 

Fig. 7. (a) A HRTEM image of a 40-nm freestanding bismuth nanowire, showing lattice fringes. The amorphous surface layer is bismuth oxide formed upon air exposure of bismuth nanowire, (b) SAED pattern of a single Bi nanowire (Zhang et at, 1999). 

Figure 8 shows X-ray diffraction (XRD) patterns of bismuth nanowire arrays (Lin et al., 2000b). It illustrates that the crystal structure of bismuth nanowires is the same as that of bulk bismuth and that no copper phases were present. The nanowires have a preferred wire orientation dependent on their diameters. The major orientations of the 95-nm and 40-nm bismuth nanowire arrays were normal to the (202) and (012) lattice planes, respectively, indicating that most (> 80%) of the nanowires were oriented along the [1011] and [0112] directions for <7W > 60 nm and <7W < 50 nm, respectively (Zhang et al., 1999 Lin et al., 2000b). The existence of more than one dominant orientation in the 52-nm Bi nanowires (Fig. 8(b)) was... 

The smallest diameter attained for bismuth nanowires by the pressure injection method was about 13 nm, using a pressure of approximately 0.3 kbar (Zhang et al., 1999). Finer nanowires might be fabricated by increasing injection pressures (Huber et al., 2000), but it remains to be seen if the anodic alumina templates would retain their structural integrity under those high pressures. 

In a nanowire system, the quantized subband energy enm and the transport effective mass mzz along the wire axis are the two most important parameters and determine almost all the electronic properties. Due to the anisotropic carriers and the special geometric configuration (circular wire cross section and high aspect ratio of length to diameter), several approximations were used in earlier calculations to derive e m and mzz in bismuth nanowires. In the... 

Figure 15 shows the calculated DOS for electrons in a 40-nm bismuth nanowire compared to that of bulk bismuth. The DOS in nanowires is a superposition of one-dimensional transport channels, each located at a quantized subband energy snm. We note that the DOS in nanowires has sharp peaks at the subband edges, whereas that in a bulk material is a smooth monotonic function of energy. The enhanced DOS at the subband edges of nanowires has important implications for many applications, such as in optics (Black et al, 2000) and thermoelectrics (Hicks and Dresselhaus, 1993). 

Fig. 15. Calculated effective densities of states for 40-nm bismuth nanowires (solid curve) and bulk bismuth (dashed curve). The zero energy refers to the band edge of bulk bismuth. The nonparabolic effects of the electron carriers are considered in these calculations.

Fig. 17. Calculated total carrier density (electrons and holes) as a function of temperature for bulk 3D bismuth and bismuth nanowires of different diameters oriented along the [0112] direction.

The lower carrier density of the 80-nm nanowires compared to bulk bismuth is due to the smaller band overlap in the former. For the 40-nm bismuth nanowires, the carrier density has a temperature dependence similar to bulk bismuth at high temperatures, but it drops rapidly with decreasing temperature at low temperatures. Because the carrier density is highly dependent on wire diameter, the transport properties of bismuth nanowires are expected to be highly sensitive to wire diameter, as will be shown experimentally in the section temperature-dependent resistivity of nanowires. ... 

Fig. 18. SEM image of a 70-nm bismuth nanowire with four electrodes attached to the nanowire. The circle on the large left electrode is a reference point used to find the nanowire and to attach electrodes to it by a lithographic process (Cronin et al., 1999).

Figure 19(a) shows the temperature dependence of resistance R(T) for bismuth nanowire arrays (dw = 7 - 200 nm) synthesized by vapor deposition and measured by Heremans et al. (2000). Hong et al. (1999) reported similar resistance measurements on bismuth wires of larger diameters (200 nm to 2, uni) prepared by electrochemical deposition (Fig. 19(b)). These two studies... 

Fig. 19. (a) Measured temperature dependence of resistance for bismuth nanowire arrays of various wire diameters dw (Heremans et al, 2000). (b) R(T)/R(290 K) for bismuth wires of larger dw measured by Hong et al. (1999). (c) Calculated R(T)/R(300 K) of 36-nm and 70-nm bismuth nanowires (Lin et al, 2000b). The dashed curve refers to a 70-nm poly crystalline wire with increased boundary scattering. 

Fig. 20. (a) Measured R(T)/R(270 K) for 40-nm bismuth nanowires prepared with alloys of different Te doping levels, (b) The calculated temperature dependence of for 40-nm undoped and Te-doped bismuth nanowires of different Nd. The dashed and solid lines are fitting curves corresponding to undoped and Te-doped Bi nanowires, respectively. 

Black, M. R., Lin, Y.-M., Dresselhaus, M. S., Tachibama, M., Fang, S., Rabin, O., Ragot, F., Eklund, P. C., and Dunn, B., Measuring the dielectric properties of nanostructures using optical reflection and transmission bismuth nanowires in porous alumina. MRS Symp. Proc. 581, 623 (2000). 

Liu, K., Chien, C. L., Searson, P. C, and Kui, Y. Z., Structural and magneto-transport properties of electrodeposited bismuth nanowires. Appl. Phys. Lett. 73,1436 (1998a). 

Sun, X., Zhang, Z., and Dresselhaus, M. S., Theoretical modeling of thermoelectricity in bismuth nanowires. Appl. Phys. Lett. 74,4005 (1999b). 

Zhang, Z., Gekhtman, D., Dresselhaus, M. S., and Ying, J. Y., Processing and characterization of single-crystalline ultrafine bismuth nanowires. Chem. Mater. 11,1659 (1999). 

J-Bismuth molybdate, hydrodynamic cavitation, 33-34 Bismuth nanowires... 

Pressure injection bismuth nanowires, 175-177 experimental setup, 174 nanowire fabrication, 173-177 template requirements, 175 Washburn equation, 174-175 Pressure swing adsorption, adsorption, 80 Protein microtube-mediated synthesis, nanostructured materials, 15-16 Purification, olefin-diene, 117... 

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