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The main objective of the ACS-PRF DNI grant was to explore one-dimensional quantum confinement effects in tailoring the thermoelectric properties of semiconductor nanowires, in particular low bandgap III-V semiconductors such as InAs, InSb. Over the past one and half years, the PI’s group has obtained significant progress on the synthesis and characterization of InAs nanowires with small diameter (and therefore strong quantum confinement effect). The research efforts have resulted five publications, among which three are work done by the PI’s group alone and two are collaborative work. The main progress and findings are summarized below.

To take advantage of the one-dimensional quantum confinement effect in controlling the thermopower and conductance of nanowires, the diameter of nanowire should be small enough and at the same time carriers in nanowire maintain good mobility. We performed electrical transport measurements of  InAs nanowires with small (20nm) diameter. Through temperature and magnetic field dependent conductance study, we showed that one-dimensional weak localization dominates the electron transport in nanowires with medium (a few hundred cm square per volt second) mobilities. This finding suggests that electrons have long quantum coherence length (longer than nanowire diameter) in InAs nanowires and interference between electron diffusion paths makes electron wavefunction being localized in a one-dimensional fashion. Furthermore, a perpendicular magnetic field is shown to be more effective than parallel magnetic field in suppressing the weak localization, again due to the small cross-section and wire geometry. These findings are published as two papers in leading journals of nanoscience and Physics: one in Nano Letters and one in Physical Review B. We have also started characterizing thermopower of InAs nanowires. Preliminary measurements showed oscillations in thermopower when electrons are added into or removed from nanowires by a gate voltage. This result is encouraging since it may reflect the corresponding thermopower control by filling or depletion of one-dimensional subbands.

Another interesting property we uncovered in the study of InAs nanowire is the existence of surface state and the use of surface state conduction for enhanced nanowire sensor response. Bulk InAs is known to have electron accumulation at surface due to surface states. With large surface-to-volume ratio, the surface electron layer plays a dominant role in the electrical properties of nanowire and may be exploited for sensing molecules adsorbed on surface. Indeed, we found that gas molecule adsorption onto InAs nanowire surface strongly affects its carrier density and mobility, making this material ideal for sensing applications (result published in Nano Letters). With the support of this PRF-DNI grant, the PI was also able to collaborate with colleagues and obtained two interesting publications in ACS Nano and Nano Letters on the selective growth of semiconducting carbon nanotubes and tuning nanowire biosensor sensitivity through electrolyte gating.

This PRF-DNI grant has had very positive impact on the PI’s career. As a junior faculty member, the PI is able to support postdoc and graduate/undergraduate students, obtain data, publications and establish collaborations with colleagues from other disciplines. This grant is also indispensable for the training of students, postdoc as it provides financial support for the stipends and materials and supplies used in experiments.