Publications

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Based on the concept of band bending at metal/semiconductor interfaces as an energy filter for electrons, we present a theory for the enhancement of the thermoelectric properties of semiconductor materials with metallic nanoinclusions. We show that the Seebeck coefficient can be significantly increased due to a strongly energy-dependent electronic scattering time. By including phonon scattering, we find that the enhancement of ZT due to electron scattering is important for high doping, while at low doping it is primarily due to a decrease in the phonon thermal conductivity. © 2008 The American Physical Society.

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We present a simple top down approach based on nanoimprint lithography to create dense arrays of silicon nanowires over large areas. Metallic contacts to the nanowires and a bottom gate allow the operation of the array as a field-effect transistor with very large on/off ratios. When exposed to ammonia gas or cyclohexane solutions containing nitrobenzene or phenol, the threshold voltage of the field-effect transistor is shifted, a signature of charge transfer between the analytes and the nanowires. The threshold voltage shift is proportional to the Hammett parameter and the concentration of the nitrobenzene and phenol analytes. For the liquid analytes considered, we find binding energies of 400 meV, indicating strong physisorption. Such values of the binding energies are ideal for stable and reusable sensors.

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At reduced dimensionality, Coulomb interactions play a crucial role in determining device properties. While such interactions within the same carbon nanotube have been shown to have unexpected properties, device integration and multi-nanotube devices require the consideration of inter-nanotube interactions. We present calculations of the characteristics of planar carbon nanotube transistors including interactions between semiconducting nanotubes and between semiconducting and metallic nanotubes. The results indicate that inter-tube interactions affect both the channel behaviour and the contacts. For long channel devices, a separation of the order of the gate oxide thickness is necessary to eliminate inter-nanotube effects. Because of an exponential dependence of this length scale on the dielectric constant, very high device densities are possible by using high-κ dielectrics and embedded contacts. © 2006 IOP Publishing Ltd.

Electrical contacts to semiconductors play a key role in electronics. For nanoscale electronic devices, particularly those employing novel low-dimensionality materials, contacts are expected to play an even more important role. Here we show that for quasi-one-dimensional structures such as nanotubes and nanowires, side contact with the metal only leads to weak band re-alignment, in contrast to bulk metal-semiconductor contacts. Schottky barriers are much reduced compared with the bulk limit, and should facilitate the formation of good contacts. However, the conventional strategy of heavily doping the semiconductor to obtain ohmic contacts breaks down as the nanowire diameter is reduced. The issue of Fermi level pinning is also discussed, and it is demonstrated that the unique density of states of quasi-one-dimensional structures make them less sensitive to this effect. Our results agree with recent experimental work, and should apply to a broad range of quasi-one-dimensional materials.

In this letter, we examine the effects of discrete mobile dislocations on spinodal decomposition kinetics in lattice mismatched binary alloys. By employing a novel continuum model, we demonstrate that the effects of dislocation mobility on domain coarsening kinetics can be expressed in a unified manner through a scaling function, describing a crossover from t12 to t13 behavior. © 2005 American Institute of Physics.

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DNA-wrapped carbon nanotubes (DNA-CNT) have generated attention due the ability to disperse cleanly into solution, and by the possibility of sorting nanotubes according to size and conductivity. In order to learn more about the effects of DNA on the electrical transport characteristics of single wall carbon nanotubes, we fabricate and test a series of devices consisting of DNA-wrapped CNTs placed across gold, palladium, and palladium oxide electrodes. In addition, we look at how DNA functionalized CNTs react to presence of hydrogen, which has previously been shown to affect the conductivity of CNTs when in contact with palladium.

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We present nanometer-scale resolution, ballistic electron emission microscopy (BEEM) studies of Au/octanedithiol/n-GaAs (001) diodes. The presence of the molecule dramatically increases the BEEM threshold voltage and displays an unusual transport signature as compared to reference Au/GaAs diodes. Furthermore, BEEM images indicate laterally inhomogeneous interfacial structure. We present calculations that address the role of the molecular layer at the interface. Our results indicate that spatially resolved measurements add new insight to studies using conventional spatial-averaging techniques.

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We present theoretical performance estimates for nanotube optoelectronic devices under bias. Current-voltage characteristics of illuminated nanotube p-n junctions are calculated using a self-consistent nonequilibrium Green's function approach. Energy conversion rates reaching tens of percent are predicted for incident photon energies near the band gap energy. In addition, the energy conversion rate increases as the diameter of the nanotube is reduced, even though the quantum efficiency shows little dependence on nanotube radius. These results indicate that the quantum efficiency is not a limiting factor for use of nanotubes in optoelectronics.

The length scale of stress domain patterns formed at solid surfaces is usually calculated using isotropic elasticity theory. Because this length depends exponentially on elastic constants; deviations between isotropic and anisotropic elasticity can lead to large errors. Another inaccuracy of isotropic elasticity theory is that it neglects the dependence of elastic relaxations on stripe orientation. To remove these inaccuracies; we calculate the energy of striped domain patterns using anisotropic elasticity theory for an extensive set of surfaces encountered in experimental studies of self-assembly. We present experimental and theoretical evidence that elastic anisotropy is large enough to determine the stripe orientation when Pb is deposited on Cu(111). Our analytical and numerical results should be useful for analysis of a broad range of experimental systems.

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Pb deposition on Cu(111) causes the surface to self-assemble into periodically arranged domains of a Pb-rich phase and a Pb-poor phase. Using low-energy electron microscopy (LEEM) we provide evidence that the observed temperature-dependent periodicity of these self-assembled domain patterns is the result of changing domain-boundary free energy. We determine the free energy of boundaries at different temperatures from a capillary wave analysis of the thermal fluctuations of the boundaries and find that it varies from 22 meV/nm at 600 K to 8 meV/nm at 650 K. Combining this result with previous measurements of the surface stress difference between the two phases we find that the theory of surface-stress-induced domain formation can quantitatively account for the observed periodicities.

In this article we report on the electrical characteristics of single wall carbon nanotubes (SWCNTs) wrapped with single-stranded deoxyribonucleic acid (ssDNA). We fabricate these devices using a solution-based method whereby SWCNTs are dispersed in aqueous solution using 20-mer ssDNA, and are placed across pairs of Au electrodes using alternating current dielectrophoresis (ACDEP). In addition to current voltage characteristics, we evaluate our devices using scanning electron microscopy and atomic force microscopy. We find that ACDEP with ssDNA based suspensions results in individual SWCNTs bridging metal electrodes, free of carbon debris, while similar devices prepared using the Triton X-100 surfactant yield nanotube bundles, and frequently have carbon debris attached to the nanotubes. Furthermore, the presence of ssDNA around the nanotubes does not appear to appreciably affect the overall electrical characteristics of the devices. In addition to comparing the properties of several devices prepared on nominally clean Au electrodes, we also investigate the effects of self-assembled monolayers of C{sub 14}H{sub 29}-SH alkyl thiol and benzyl mercaptan on the adhesion and electrical transport across the metal/SWCNT/metal devices.