Direct Measurement of the Absolute Seebeck Coefficient for Pb and Cu at 300 K to 450 K

The utilization of thermal fluctuations or Johnson/Nyquist noise as a spectroscopic technique to experimentally measure transport properties is applied to Pb and Cu metal films. Through cross-correlation and autocorrelation functions obtained from power spectral density measurements, multiple transport coefficients are obtained through the Green-Kubo formalism. Supported rigorously by the underlying fluctuation-dissipation theory, this new experimental technique provides a direct measurement of absolute thermoelectric coefficients in addition to the electrical resistivity, electronic contribution to thermal conductivity, Lorentz number and various diffusion coefficients. This work reports the validation results of the experiment accomplished through the use of materials with thermoelectric properties widely accepted by the thermoelectric community, Pb and Cu. Further validation of the data was accomplished by comparing resistivity results to standard collinear four-probe resistivity measurements. Thermal fluctuation data for Pb at 300 K resulted in 5.9% and 2.02% agreement with the published Seebeck and four-probe resistivity data respectively. The Cu thermal fluctuation measurements at 300 K showed agreement within 3.76% and 6.14% for the published Seebeck and four-probe data respectively thus lending further credibility to the experimental method and underlying theory.

Nonmetallic Conduction Property of a DNA Templated Gold Nanowire

Nanowires based on DNA are exciting materials with several possible applications in nanoelectronics because of the self-assemble capability for the designed nanostructure. In this study, we have carried out electrical and thermal conduction measurements on a metallized single DNA molecule. The measured values of the electrical and thermal conductivity were about 1.42 × 101 S/cm and 149.8 W/mK at room temperature, respectively. The measured value of the Lorentz number was about 3.6 × 10−4, which is incompatible with that predicted by the Wiedemann-Franz law. The temperature dependent electrical conductivity shows that electron transport in metallized DNA occurs by the hopping process similar to that in nonmetallized DNA. Atomic force microscopy reveals nanoscale discontinuities in the gold layer around the DNA. While the gold layer assists the DNA in electron conduction, the overall conduction of the metallized DNA is dominated by the DNA rather than the coating. These results suggest that the DNA is potentially a better thermal conductor than the metal coating and that its effective conductivity may be large. This interesting physical property may make the DNA useful for bioapplications involving significant heat transfer.

Atomic Ordering, Electronic Structure, and Transport Properties of LAST-m Systems

In recent years, LAST-m (AgPbmSbTem+2) and related materials have emerged as potential high performance high temperature thermoelectrics. These compounds are obtained by starting from PbTe, and replacing pairs of Pb2+ ions by (Ag1+, Sb3+) pairs. One example is LAST-18. When optimally doped, this compound has thermoelectric figure of merit ZT=1.7 at 700K. This large ZT is most likely due to very low lattice thermal conductivity, caused by phonon scattering from nanostructures. These nanostructures involve clustering and ordering of Ag, Sb, and Pb ions. Possible origins of this atomic ordering and how the presence of nanostructures affects the electronic structure near the band gap region are discussed. The temperature (T) dependence of electrical conductivity σ (∼T2.2 in the range 300K <T< 900K) in n-type PbTe is analyzed in terms of the T-dependence of different physical quantities contributing to transport. We find that the dominant contribution comes from the explicit T-dependence of relaxation time rather than its energy dependence. The T-dependence of chemical potential is also significant in the concentration range of interest. Electronic thermal conductivity for constant field (κel,E) and for constant current (κel,J) are found to differ considerably at high temperatures and the Weidemann-Franz (WF) law κel,J = LoσT, where Lo =2x10−8WΩ/K is the Lorentz number, overstimates κel,J by nearly 60% at 800K for carrier concentration n=5x1019/cm3. As a result, one tends to underestimate the lattice contribution κlatt = κexp - κel,J. We give theoretical values of effective Lorentz number L = κel.J/σT for different n and T.