Joseph A. Sulpizio

Transport measurements across a tunable potential barrier in graphene.

The peculiar nature of electron scattering in graphene is among many exciting theoretical predictions for the physical properties of this material. To investigate electron scattering properties in a graphene plane, we have created a gate-tunable potential barrier within a single-layer graphene sheet. We report measurements of electrical transport across this structure as the tunable barrier potential is swept through a range of heights. When the barrier is sufficiently strong to form a bipolar junction (n-p-n or p-n-p) within the graphene sheet, the resistance across the barrier sharply increases. We compare these results to predictions for both diffusive and ballistic transport, as the barrier rises on a length scale comparable to the mean free path. Finally, we show how a magnetic field modifies transport across the barrier.

Observation of a one-dimensional spin–orbit gap in a quantum wire

The ability to produce spin-polarized currents in a quantum wire is crucial for spin-based electronics. Fortunately, the spin–orbit interaction can be exploited to deliver pure spin currents, without charge currents, that travel in one direction only.

Molecular Junctions of Self-Assembled Monolayers with Conducting Polymer Contacts

We present a method to fabricate individually addressable junctions of self-assembled monolayers (SAMs) that builds on previous studies which have shown that soft conductive polymer top contacts virtually eliminate shorts through the SAMs. We demonstrate devices with nanoscale lateral dimensions, representing an order of magnitude reduction in device area, with high yield and relatively low device-to-device variation, improving several features of previous soft contact devices. The devices are formed in pores in an inorganic dielectric layer with features defined by e-beam lithography and dry etching. We replace the aqueous PEDOT:PSS conductive polymer used in prior devices with Aedotron P, a low-viscosity, amphiphilic polymer, allowing incorporation of self-assembled monolayers with either hydrophobic or hydrophilic termination with the same junction geometry and materials. We demonstrate the adaptability of this new design by presenting transport measurements on SAMs composed of alkanethiols with methyl, thiol, carboxyl, and azide terminations. We establish that the observed room-temperature tunnel barrier is primarily a function of monolayer thickness, independent of the terminal groups hydrophilicity. Finally, we investigate the temperature dependence of transport and show that the low-temperature behavior is based on the energy distribution of sites from which carriers can tunnel between the polymer and gold contacts, as described by a model of variable-range hopping transport in a disordered conductor.

An integrated capacitance bridge for high-resolution, wide temperature range quantum capacitance measurements.

We have developed a highly sensitive integrated capacitance bridge for quantum capacitance measurements. Our bridge, based on a GaAs HEMT amplifier, delivers attofarad (aF) resolution using a small AC excitation at or below k(B)T over a broad temperature range (4-300 K). We have achieved a resolution at room temperature of 60 aF/√Hz for a 10  mV ac excitation at 17.5 kHz, with an improved resolution at cryogenic temperatures, for the same excitation amplitude. We demonstrate the utility of our bridge for measuring the quantum capacitance of nanostructures by measuring the capacitance of top-gated graphene devices and cleanly resolving the density of states.

Erratum: “An integrated capacitance bridge for high-resolution, wide temperature range quantum capacitance measurements” [Rev. Sci. Instrum. 82, 053904 (2011)]

Arash Hazeghi, Joseph A. Sulpizio, Georgi Diankov, David Goldhaber-Gordon, and H. S. Philip Wong Citation: Rev. Sci. Instrum. 82, 129901 (2011); doi: 10.1063/1.3665097 View online: http://dx.doi.org/10.1063/1.3665097 View Table of Contents: http://rsi.aip.org/resource/1/RSINAK/v82/i12 Published by the AIP Publishing LLC.