Person:

Day, Robert

Over the past decade extensive studies of single semiconductor nanowire and nanowire array photovoltaic devices have explored the potential of these materials as platforms for a new generation of efficient and cost-effective solar cells. This feature review discusses strategies for implementation of semiconductor nanowires in solar energy applications, including advances in complex nanowire synthesis and characterization, fundamental insights from characterization of devices, utilization and control of the unique optical properties of nanowires, and new strategies for assembly and scaling of nanowires into diverse arrays that serve as a new paradigm for advanced solar cells.

Enhanced synthetic control of the morphology, crystal structure, and composition of nanostructures can drive advances in nanoscale devices. Axial and radial semiconductor nanowires are examples of nanostructures with one and two structural degrees of freedom, respectively, and their synthetically tuned and modulated properties have led to advances in nanotransistor, nanophotonic, and thermoelectric devices. Similarly, developing methods that allow for synthetic control of greater than two degrees of freedom could enable new opportunities for functional nanostructures. Here we demonstrate the first regioselective nanowire shell synthesis in studies of Ge and Si growth on faceted Si nanowire surfaces. The selectively deposited Ge is crystalline, and its facet position can be synthetically controlled in situ. We use this synthesis to prepare electrically addressable nanocavities into which solution soluble species such as Au nanoparticles can be incorporated. The method furnishes multicomponent nanostructures with unique photonic properties and presents a more sophisticated nanodevice platform for future applications in catalysis and photodetection.

Subwavelength diameter semiconductor nanowires can support optical resonances with anomalously large absorption cross sections, and thus tailoring these resonances to specific frequencies could enable a number of nanophotonic applications. Here, we report the design and synthesis of core/shell p-type/intrinsic/n-type (p/i/n) Si nanowires (NWs) with different sizes and cross-sectional morphologies as well as measurement and simulation of photocurrent spectra from single-NW devices fabricated from these NW building blocks. Approximately hexagonal cross-section p/i/n coaxial NWs of various diameters (170–380 nm) were controllably synthesized by changing the Au catalyst diameter, which determines core diameter, as well as shell deposition time, which determines shell thickness. Measured polarization-resolved photocurrent spectra exhibit well-defined diameter-dependent peaks. The corresponding external quantum efficiency (EQE) spectra calculated from these data show good quantitative agreement with finite-difference time-domain (FDTD) simulations and allow assignment of the observed peaks to Fabry–Perot, whispering-gallery, and complex high-order resonant absorption modes. This comparison revealed a systematic red-shift of equivalent modes as a function of increasing NW diameter and a progressive increase in the number of resonances. In addition, tuning shell synthetic conditions to enable enhanced growth on select facets yielded NWs with approximately rectangular cross sections; analysis of transmission electron microscopy and scanning electron microscopy images demonstrate that growth of the n-type shell at (860^{\circ}C) in the presence of phosphine leads to enhanced relative Si growth rates on the four {113} facets. Notably, polarization-resolved photocurrent spectra demonstrate that at longer wavelengths the rectangular cross-section NWs have narrow and significantly larger amplitude peaks with respect to similar size hexagonal NWs. A rectangular NW with a diameter of 260 nm yields a dominant mode centered at 570 nm with near-unity EQE in the transverse-electric polarized spectrum. Quantitative comparisons with FDTD simulations demonstrate that these new peaks arise from cavity modes with high symmetry that conform to the cross-sectional morphology of the rectangular NW, resulting in low optical loss of the mode. The ability to modulate absorption with changes in nanoscale morphology by controlled synthesis represents a promising route for developing new photovoltaic and optoelectronic devices.

Nanowires hold significant promise for both fundamental studies and technological applications ranging from energy conversion to electronics to biological sensing. The detailed understanding of nanowire synthesis and the realization of new synthetic approaches have enabled precise control over their size, morphology, and composition, and, consequently, their material properties. While much of the work on synthesis in the literature relates to axial nanowire growth, where growth proceeds in the direction of its long axis, this thesis has focused on probing the unique opportunities of shell growth, where material deposits radially around a nanowire core. To this end, I will show, first, that faceted Si core/shell nanowires can be synthesized with embedded pn junctions and that these structures can function as efficient photovoltaic devices with enhanced light absorption properties distinct from bulk Si devices. Second, through choice of reactants and reaction conditions used for shell growth, we demonstrate fine control over the size and morphology of these nanowires, which, in turn, drastically enhances their light absorption at particular wavelengths. Finally, we report for the first time a growth phenomenon that is unique to one-dimensional materials and which combines the underlying physics of the Plateau-Rayleigh instability with crystal growth. By exploiting this phenomenon, which we term Plateau-Rayleigh crystal growth, we demonstrate the growth of periodic shells on one-dimensional substrates. Specifically, we show that for conditions near the Plateau-Rayleigh instability the deposition of Si onto uniform-diameter Si cores, Ge onto Ge cores, and Ge onto Si cores can generate diameter-modulated core/shell nanowires. Rational control of deposition conditions enabled tuning of distinct morphological features, including diameter-modulation periodicity, amplitude and cross-sectional anisotropy. More generally, Plateau-Rayleigh crystal growth highlights the opportunities in understanding the thermodynamics and kinetics unique to crystal growth on nanowires and other low dimensional systems.