Julio A. Martinez

Bioelectronic silicon nanowire devices using functional membrane proteins

Modern means of communication rely on electric fields and currents to carry the flow of information. In contrast, biological systems follow a different paradigm that uses ion gradients and currents, flows of small molecules, and membrane electric potentials. Living organisms use a sophisticated arsenal of membrane receptors, channels, and pumps to control signal transduction to a degree that is unmatched by manmade devices. Electronic circuits that use such biological components could achieve drastically increased functionality; however, this approach requires nearly seamless integration of biological and manmade structures. We present a versatile hybrid platform for such integration that uses shielded nanowires (NWs) that are coated with a continuous lipid bilayer. We show that when shielded silicon NW transistors incorporate transmembrane peptide pores gramicidin A and alamethicin in the lipid bilayer they can achieve ionic to electronic signal transduction by using voltage-gated or chemically gated ion transport through the membrane pores.

Carbon nanotube transistor controlled by a biological ion pump gate.

We report a hybrid bionanoelectronic transistor that has a local ATP-powered protein gate. ATP-dependent activity of a membrane ion pump, Na(+)/K(+)-ATPase, embedded in a lipid membrane covering the carbon nanotube, modulates the transistor output current by up to 40%. The ion pump gates the device by shifting the pH of the water layer between the lipid bilayer and nanotube surface. This transistor is a versatile bionanoelectronic platform that can incorporate other membrane proteins.

Enhanced thermoelectric figure of merit in SiGe alloy nanowires by boundary and hole-phonon scattering

We report the thermoelectric characteristics of individual p-type SiGe alloy nanowires for diameters of 100 to 300 nm and temperatures between 40 to 300 K. A technique that allows for electrical and thermal characterization on the same nanowire was developed in this work. Experimental data provide evidence of the scattering of low-frequency phonons by the boundary of the nanowires. The thermal conductivities for SiGe alloy nanowires with different free carrier concentrations reveal that the long free path phonons are also scattered by hole-phonon interactions. Combined boundary and hole-phonon scattering mechanisms with alloy scattering resulted in thermal conductivities as low as 1.1 W/m-K at 300 K, which is one of the lowest measured for SiGe alloys and is comparable to that of bulk silica. The enhanced thermal properties observed in this work yielded ZT close to 0.18 at 300 K—more than a factor of 2 higher than the bulk SiGe alloy.

Highly efficient biocompatible single silicon nanowire electrodes with functional biological pore channels.

Nanoscale electrodes based on one-dimensional inorganic conductors could possess significant advantages for electrochemical measurements over their macroscopic counterparts in a variety of electrochemical applications. We show that the efficiency of the electrodes constructed of individual highly doped silicon nanowires greatly exceeds the efficiency of flat Si electrodes. Modification of the surfaces of the nanowire electrodes with phospholipid bilayers produces an efficient biocompatible barrier to transport of the solution redox species to the nanoelectrode surface. Incorporating functional alpha-hemolysin protein pores in the lipid bilayer results in a partial recovery of the Faradic current due to the specific transport through the protein pore. These assemblies represent a robust and versatile platform for building a new generation of highly specific biosensors and nano/bioelectronic devices.

Understanding ultrafast carrier dynamics in single quasi-one-dimensional Si nanowires

We use femtosecond optical pump-probe spectroscopy to study ultrafast carrier dynamics in single quasi-one-dimensional silicon nanowires. By isolating individual nanowires, we avoid complications resulting from the broad size and alignment distribution in nanowire ensembles, allowing us to directly probe ultrafast carrier dynamics. Spatially-resolved experiments demonstrate the influence of surface-mediated mechanisms on carrier dynamics in a single NW, while polarization-resolved experiments reveal a clear anisotropy in carrier lifetimes measured parallel and perpendicular to the long axis of the NW, due to density-dependent Auger recombination. These results suggest the possibility of tailoring carrier relaxation in a single nanowire for a desired application.

Distributed feedback gallium nitride nanowire lasers

Achieving single-mode laser operation in nanowire lasers remains a challenge due to a lack of mode selection approaches. We have implemented single-mode lasing using distributed feedback by externally coupling gallium nitride nanowires to a dielectric grating to achieve mode-control. The effective periodicity of the grating experienced by the nanowire was altered using nanomanipulation to change the angular alignment between the nanowire and the grating. The effective periodicity controls the spectral location of the distributed feedback stop-band. Single-mode emission was achieved at an alignment, where the designed periodicity of the grating was experienced by the nanowire.

Contribution of radial dopant concentration to the thermoelectric properties of core-shell nanowires

We report the thermoelectric characteristics of core-shell p-type germanium nanowires (GeNWs) (lightly doped core, heavily doped shell). Overall, the thermoelectric characteristics are dominated by the heavily doped shell. Experimental data indicate that surface states produce dopant deactivation when the heavily doped shell is removed. Under this situation, the thermoelectric figure of merit is degraded. Etching the heavily doped shell resulted in a rough germanium nanowire with a thermal conductivity close to 1.1 W/m-K at 300 K, which is one of the smallest k measured for nanowires and comparable to the thermal conductivity of bulk SiO2.

Space-and-time-resolved spectroscopy of single GaN nanowires

Spatially-resolved ultrafast transient absorption measurements on a single GaN nanowire give insight into carrier relaxation dynamics as a function of the laser polarization and position on the nanowire on a femtosecond timescale.

Simultaneous Thermoelectric and Optoelectronic Characterization of Individual Nanowires.

Semiconducting nanowires have been explored for a number of applications in optoelectronics such as photodetectors and solar cells. Currently, there is ample interest in identifying the mechanisms that lead to photoresponse in nanowires in order to improve and optimize performance. However, distinguishing among the different mechanisms, including photovoltaic, photothermoelectric, photoemission, bolometric, and photoconductive, is often difficult using purely optoelectronic measurements. In this work, we present an approach for performing combined and simultaneous thermoelectric and optoelectronic measurements on the same individual nanowire. We apply the approach to GaN/AlGaN core/shell and GaN/AlGaN/GaN core/shell/shell nanowires and demonstrate the photothermoelectric nature of the photocurrent observed at the electrical contacts at zero bias, for above- and below-bandgap illumination. Furthermore, the approach allows for the experimental determination of the temperature rise due to laser illumination, which is often obtained indirectly through modeling. We also show that under bias, both above- and below-bandgap illumination leads to a photoresponse in the channel with signatures of persistent photoconductivity due to photogating. Finally, we reveal the concomitant presence of photothermoelectric and photogating phenomena at the contacts in scanning photocurrent microscopy under bias by using their different temporal response. Our approach is applicable to a broad range of nanomaterials to elucidate their fundamental optoelectronic and thermoelectric properties.

Polarization anisotropy of transient carrier dynamics in single Si nanowires

We present the first ultrafast time-resolved, polarization-dependent experiments on both single- and ensemble-silicon nanowires using non-degenerate spectroscopy. Anisotropy was observed for polarizations perpendicular and parallel to the nanowire.