Publications

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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.