UC Berkeley

In recent years semiconductor nanowires and nanotubes have garnered increased attention for their unique properties. With their nanoscale dimensions comes high surface area and quantum confinement, promising enhancements in a wide range of applications. 1-dimensional nanostructures are especially attractive for energy conversion applications where photons, phonons, and electrons come into play. Since the bohr exciton radius and phonon and electron mean free paths are on the same length scales as nanowire diameters, optical, thermal, and electrical properties can be tuned by simple nanowire size adjustments. In addition, the high surface area inherent to nanowires and nanotubes lends them towards efficient charge separation and superior catalytic performance.

In thermoelectric power generation, the nanoscale wire diameter can effectively scatter phonons, promoting reductions in thermal conductivity and enhancements in the thermoelectric figure of merit. To that end, single-crystalline arrays of PbS, PbSe, and PbTe nanowires have been synthesized by a chemical vapor transport approach. The electrical and thermal transport properties of the nanowires were characterized to investigate their potential as thermoelectric materials. Compared to bulk, the lead chalcogenide nanowires exhibit reduced thermal conductivity below 100 K by up to 3 orders of magnitude, suggesting that they may be promising thermoelectric materials. Smaller diameters and increased surface roughness are expected to give additional enhancements.

The solution-phase synthesis of PbSe nanowires via oriented attachment of nanoparticles enables facile surface engineering and diameter control. Branched PbSe nanowires synthesized by this approach showed near degenerately doped charge carrier concentrations. Compared to the bulk, the PbSe nanowires exhibited a similar Seebeck coefficient and a significant reduction in thermal conductivity in the temperature range 20 K to 300 K. Thermal annealing of the PbSe nanowires allowed their thermoelectric properties to be controllably tuned by increasing their carrier concentration or hole mobility. After optimal annealing, single PbSe nanowires exhibited a thermoelectric figure of merit (ZT) of 0.12 at 300 K. In addition, using a field-effect gated device, the Seebeck coefficient of single PbSe nanowires could be tuned from 64 to 193 μV*K^-1. This direct electrical field control of the electrical conductivity and Seebeck coefficient suggests a powerful strategy for optimizing ZT in thermoelectric devices and these results represent the first demonstration of field-effect modulation of the thermoelectric figure of merit in a single semiconductor nanowire. This novel strategy for thermoelectric property modulation could prove especially important in optimizing the thermoelectric properties of semiconductors where reproducible doping is difficult to achieve.

Recent theoretical work has shown large enhancements in ZT for single-crystal nanowires containing nanoscale interfaces along their lengths. M₂O₃(ZnO)_n (M = In, Ga, Fe) superlattice nanowires were synthesized via a novel solid-state diffusion approach to investigate this possible enhancement. Using atomic resolution Z-contrast STEM imaging a detailed structural analysis was performed on In_2-xGa_xO₃(ZnO)_n nanowires, leading to the discovery that octahedral inclusions within the superlattice structure are likely generated through a defect-assisted process. Single-nanowire thermal and electrical measurements onIn_2-xGa_xO₃(ZnO)_n reveal a simultaneous improvement in all contributing factors to the thermoelectric figure of merit, giving an order of magnitude enhancement over similar bulk materials at room temperature. This is the first report of enhancement of all three thermoelectric parameters (Seebeck coefficient, electrical conductivity, and thermal resistivity) for a nanowire system.

Photoelectrochemical water splitting is another exciting renewable energy application that can benefit from the high surface area of nanomaterials. Recently, (Ga_1-xZn_x)(N_1-xO_x) has gained widespread attention as a high efficiency material for visible-light-driven H₂ and O₂ generation. To improve the crystallinity of the material and reduce charge recombination (Ga_1-xZn_x)(N_1-xO_x) nanotubes were synthesized by epitaxial coating of GaN onto ZnO nanowires. The nanotubes were single-crystalline, solid solutions of GaN and ZnO with ZnO compositions up to 10% and bandgaps as low as 2.6 eV. Incorporation of In into these nanotubes pushed the absorption even further into the visible. After decoration with Rh_2-yCr_yO₃ nanoparticle cocatalysts, (Ga_1-xZn_x)(N_1-xO_x) nanotubes spontaneously generated H₂ in aqueous solutions under illumination. The photoanodic properties of these nanotubes are under investigation.

The significant reductions in thermal conductivity achieved using lead chalcogenide and In_2-xGa_xO₃(ZnO)_n nanowires highlight their use in thermoelectric power generation. The promise of 1-dimensional materials for energy conversion is further evident in the superior crystalline quality and high surface areas of the (Ga_1-xZn_x)(N_1-xO_x) nanotubes. As research continues along these direction we move ever closer toward implementation of nanowires and nanotubes for clean, renewable, and more efficient energy use.