Processing and characterization of high-conductance bismuth wire array composites
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M.J. Graf Department of Physics, Boston College, Chestnut Hill, Massachusetts 02467
C.A. Foss, Jr. Department of Chemistry, Georgetown University, Washington, District of Columbia 20057-2222
P. Constant Laser Research Laboratory, Howard University, Washington, District of Columbia 20059 (Received 25 August 1999; accepted 1 June 2000)
We fabricated Bi nanowire array composites with wire diameters from 30 to 200 nm by high-pressure injection (HPI) of Bi melt into porous anodic alumina templates. The composites were dense, with Bi volume fraction in excess of 50%. The parallel Bi nanowires, whose length appeared to be limited only by the thickness of the host template (up to 55 m), terminated at both sides of the composite in the Bi bulk. The individual Bi nanowire crystal structure was rhombohedral, with the same lattice parameters as that of bulk Bi; the wires in the array were predominantly oriented with the trigonal axis along the wire length. Low contact resistance was achieved by bonding the composite to copper electrodes.
I. INTRODUCTION
Progress in the study of one-dimensional quantum wire systems has been slow due to the difficulty of fabricating such materials. Since Bi has the smallest electron effective mass among all known materials, quantum confinement effects in Bi are more manifest and can be observed in nanowires of larger diameter than those of any other nanowire material. One technique that is known to be applicable to the problem of fabricating Bi nanowires and that yields an array of ultrafine nanowires embedded in a porous dielectric template is the highpressure injection (HPI) of the conducting melt.1 This technique has been applied to fabricate a variety of metal and semiconductor nanowires arrays.2–4 The HPI technique was successfully applied by Gurtvitch5 to the synthesis of single nanowires of Bi in glass pipes. There are other template-based methods for making Bi nanowires. The Taylor technique of drawing glass capillaries filled with Bi has been applied by Glocker and Skove6 to making fine Bi microwires. Brandt et al.7 and Nikolaeva et al.8 have applied a similar technique to the fabrication of Bi nanowires ranging in size from 200 nm to several micrometers. Recently, the HPI technique has been employed to fabricate fine Bi nanowire arrays for thermoelectric applications.9–11 For such applications, it is important to minimize the template contribution to the thermal conductivity, and therefore, only samples with very high Bi content, such as wire arrays or networks, are of interest. Bulk Bi, a semimetal, and Bi1−xSbx, a semiconducting 1816
J. Mater. Res., Vol. 15, No. 8, Aug 2000
alloy, have the highest thermoelectric figure-of-merit Z at 100 K. Quantum size effects are predicted to result in an enhancement of Z for fine wires. Bogachek et al.12 studied theoretically the quantum transport in 3D nanowires. For small-diameter wires they predict conductance quantization that is manifested by the steplike behavior of the conductance and the appearance of thermopower peaks as a function
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