Yu-Ming Lin

Nanowires and nanotubes

Nanowires and nanotubes are now at the forefront of materials science at the nanoscale. This article starts with introductory comments about nanowires and nanotubes and then addresses in more detail the special structure and properties of bismuth nanowires and carbon nanotubes, which are considered as prototype examples of nanowires and nanotubes. Both nano-materials are important for the new nanoscience concepts that they introduce and for their promise for practical applications. Both provide a system that is simple enough so that detailed calculations of their properties can be carried out, and predictions about their physical behavior can be made. The occurrence and control of unusual and unique properties of specific nanostructures are the drivers for the exploitation of nanoscience in nanotechnology applications.

Transport properties of Bi nanowire arrays

To explain various temperature-dependent resistivity measurements [R(T)] on bismuth (Bi) nanowires as a function of wire diameter down to 7 nm, a semiclassical transport model is developed, which explicitly considers anisotropic and nonparabolic carriers in cylindrical wires, and the relative importance of various scattering processes. R(T) of 40 nm Bi nanowires with various Te dopant concentrations is measured and interpreted within this theoretical framework.

Anomalously high thermoelectric figure of merit in Bi1−xSbx nanowires by carrier pocket alignment

Electronic transport calculations were carried out for Bi1−xSbx nanowires (0⩽x⩽0.30) of diameters 10u200anm⩽dW⩽100u200anm at 77 K. A band structure phase diagram was generated, showing the dependence of the relative band edge positions on diameter and composition. Calculations of the thermoelectric figure-of-merit (ZT) predict that the performance of Bi1−xSbx nanowires is superior to that of Bi nanowires and to that of the bulk alloy. An exceptionally high value of ZT for p-type nanowires at 77 K was found for dW∼40u200anm and x∼0.13, which is explained by the coalescence in energy of up to ten valence subband edges to maximize the density-of-states at the Fermi energy.

Semimetal–semiconductor transition in Bi1−xSbx alloy nanowires and their thermoelectric properties

The resistivity of Bi1−xSbx nanowire arrays exhibits complex variations as a function of Sb content x and temperature T due to the unique semimetal-to-semiconductor (SM–SC) transition experienced by the nanowires. Seebeck coefficient measurements show enhanced thermopower due to Sb alloying and the reduction in wire diameter. The theoretical model not only explains these transport measurements, but also suggests a useful technique to experimentally determine (i) whether the wire is semimetallic or semiconducting, (ii) the carrier concentration, and (iii) the conditions for the SM–SC transition.

Making electrical contacts to nanowires with a thick oxide coating

Techniques are presented for making ohmic contacts to nanowires with a thick oxide coating. Although experiments were carried out on Bi nanowires, the techniques described in this paper are generally applicable to other nanowire systems. Metal electrodes are patterned to individual Bi nanowires using, electron beam lithography. Imaging the chemical reaction on the atomic scale with in situ high-resolution transmission electron microscopy shows that annealing in H-2 or NH3 can reduce the nanowires oxide coating completely. The high temperatures required for this annealing, however, are not compatible with the lithographic techniques. Low-resistance ohmic contacts to individual bismuth nanowires are achieved using a focused ion beam (FIB) to first sputter away the oxide layer and then deposit Pt contacts. By combining electron beam lithography and FIB techniques, ohmic contacts stable from 2 to 400 K are successfully made to the nanowires. A method for preventing the burnout of nanowires from electrostatic discharge is also developed.

Chapter 1 – Quantum Wells and Quantum Wires for Potential Thermoelectric Applications

In this chapter, the predictions of an enhancement in the thermoelectric figure of merit of low-dimensional material systems relative to their corresponding bulk counterparts, as well as the present state of experimental confirmation of these predictions, are reviewed. Progress with specific quantum well, quantum wire, and quantum dot materials systems, such as the lead salts, Si-Ge, and bismuth is discussed. To date most of the effort has gone into proof-of-principle studies, though actual demonstration of the highest thermoelectric figure of merit ( Z 3D T ) of any material to date has been seen in the low-dimensional system PbSe 0.98 Te 0.02 -PbTe, where the enhancement is attributed to quantum dot formation associated with the interface between the PbTe and PbSe 0.98 Te 0.02 . In this chapter, particular attention is given to a discussion of the structure and properties of bismuth quantum wires, which are still at an early state of research. Bismuth nanowires, however, offer significant promise for practical applications, because they can be self-assembled and are predicted to have desirable thermoelectric properties when they have wire diameters in the 5- to 10-nm range. Though temperature-dependent resistance measurements have been carried out for Bi nanowires in this diameter range, reliable thermoelectric measurements have not yet been reported.

Intersubband transitions in bismuth nanowires

Optical absorption associated with the one-dimensional joint density of states of an intersubband transition in bismuth nanowires is reported. The previously observed strong absorption in bismuth nanowires at ∼1000 cm−1 is here shown to depend on the wire diameter and on the polarization of the incident light. The absorption line shape, the decreasing frequency with increasing wire diameter, and the polarization dependence of the reflectivity, all indicate that this resonance is due to an intersubband absorption resulting from quantum-confinement effects.

Transport properties of Bi1−xSbx alloy nanowires synthesized by pressure injection

Various transport measurements are performed to assess the alloying and size effects in sub-100 nm Bi1−xSbx (0⩽x⩽0.15) nanowires. Temperature-dependent resistance measurements exhibit non-monotonic trends as x increases, and a theoretical model is presented to explain the features which are related to the unusual band structure of Bi1−xSbx systems. Magnetoresistance measurements of these Bi1−xSbx nanowires show interesting size-dependent behaviors similar to those in Bi nanowires.

Plasmon excitation modes in nanowire arrays

Electron energy loss spectrometry and energy-filtered transmission electron microscopy reveal characteristic plasmon excitations in both isolated Bi nanowires and an array of Bi nanowires within an Al2O3 matrix. As the average nanowire diameter decreases from 90 to 35 nm, both the volume plasmon energy and peak width increase. In addition, a lower-energy excitation is present in a very localized region at the Bi–Al2O3 interface. These results are discussed in the context of quantum confinement and the influence of interfaces on the electronic properties of nanocomposite materials.

Thermoelectric investigation of bismuth nanowires

An enhanced thermoelectric figure of merit, ZT, has been predicted for bismuth in the low-dimensional form of Bi nanowires. To obtain ZT experimentally, both the Seebeck coefficient, S, as well as the electrical resistivity, /spl rho/, must be determined, in addition to the thermal conductivity, not discussed in this work. A technique for measuring the electrical resistivity of individual Bi nanowires by a 4-point method was developed and carried out using electron-beam lithography techniques. A pattern of four electrodes was affixed on top of single Bi nanowires, and measurements of current versus voltage were made. Measurements of the Seebeck coefficient of arrays of Bi nanowires within an alumina template were also made. We report details of the experimental procedures as well as some preliminary results from measurements of the temperature dependence of both S and /spl rho/ for Bi nanowire arrays within an anodic alumina template.