Person: Kempa, Thomas Jan
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Kempa
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Thomas Jan
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Kempa, Thomas Jan
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Publication Tuning Light Absorption in Core/Shell Silicon Nanowire Photovoltaic Devices through Morphological Design(American Chemical Society, 2012) Kim, Sun-Kyung; Day, Robert; Cahoon, James F.; Kempa, Thomas Jan; Song, Kyung-Deok; Park, Hong-Gyu; Lieber, CharlesSubwavelength 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.Publication Nanowire Architectures for Next-Generation Solar Cells and Photonic Devices(2013-02-15) Kempa, Thomas Jan; Lieber, Charles M.; Gordon, Roy; Cohen, AdamThis thesis presents the design and synthesis of nanowires (NW) with targeted and tunable optical properties. Moreover, we show how single and assembled NW devices can enable new photovoltaic (PV) and photonic platforms. Beginning with an investigation of axially modulated p-i-n junction NWs, we established several fundamental parameters dictating solar cell performance at the nanoscale and demonstratred the first series integration of multiple solar cells on a single NW. Thereafter, implementation of the first silicon NW photovoltaic device with radially modulated p-n junctions showed that power conversion efficiencies of 3-4% are attainable from a nanoscale architecture, exceeding efficiencies for many organic and hybrid organic-inorganic solar cells. Despite these achievements, the poor electrical characteristics and insufficient control over absorption properties characterizing the aforementioned devices would limit the promise of silicon NWs for next generation solar cells. We overcome these limitations with a class of polymorphic core/multi-shell silicon NWs with highly-crystalline hexagonally-faceted shells and embedded coaxial p/i/n junctions. NW PV devices 200-300 nm in diameter exhibit open-circuit voltages of 0.5 V and fill-factors of 73% under one-sun solar illumination. Single-NW wavelength-dependent photocurrent measurements agree quantitatively with FDTD simulations. Synthetic manipulation of NW size and morphology drives tuning of optical resonances such that optimized structures can yield current densities double those for films of comparable thickness. Further optimized NW devices achieve current densities of 17 mA/cm2 and power conversion efficiencies of 6%. We also present steps toward rational assembly of larger-scale NW PV arrays. Parallel integration of NWs preserves PV metrics while assembly of vertically-stacked NWs yields current densities of \(25 mA/cm^2\) and projected efficiencies of ~15% for \(1 \mu m\) thick assemblies. Finally, we present the first ever NW material possessing 3 degrees of structural freedom, thus expanding the NW "structome." Such NWs were achieved through the first demonstration of facet selective growth of silicon and germanium in the gas phase. Photonic devices based on this new material present intriguing optical properties, including selective attenuation, enhancement, and wavelength tunability of resonant cavity modes.Publication Design of Nanowire Optical Cavities as Efficient Photon Absorbers(American Chemical Society (ACS), 2014) Kim, Sun-Kyung; Song, Kyung-Deok; Kempa, Thomas Jan; Day, Robert; Lieber, Charles; Park, Hong-GyuPublication Facet-Selective Growth on Nanowires Yields Multi-Component Nanostructures and Photonic Devices(American Chemical Society, 2013) Kempa, Thomas Jan; Kim, Sun-Kyung; Day, Robert; Park, Hong-Gyu; Nocera, Daniel; Lieber, CharlesEnhanced 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.Publication Micelle-Encapsulated Quantum Dot-Porphyrin Assemblies asin VivoTwo-Photon Oxygen Sensors(American Chemical Society (ACS), 2015) Lemon, Christopher M.; Karnas, Elizabeth; Han, Xiaoxing; Bruns, Oliver T.; Kempa, Thomas Jan; Fukumura, Dai; Bawendi, Moungi G.; Jain, Rakesh; Duda, Dan; Nocera, DanielMicelles have been employed to encapsulate the supramolecular assembly of quantum dots with palladium(II) porphyrins for the quantification of O2 levels in aqueous media and in vivo. Förster resonance energy transfer from the quantum dot (QD) to the palladium porphyrin provides a means for signal transduction under both one- and two-photon excitation. The palladium porphyrins are sensitive to O2 concentrations in the range of 0–160 Torr. The micelle-encapsulated QD-porphyrin assemblies have been employed for in vivo multiphoton imaging and lifetime-based oxygen measurements in mice with chronic dorsal skinfold chambers or cranial windows. Our results establish the utility of the QD-micelle approach for in vivo biological sensing applications.Publication Semiconductor nanowire solar cells: synthetic advances and tunable properties(Walter de Gruyter GmbH, 2014) Kempa, Thomas Jan; Lieber, CharlesThe solar power received by Earth far exceeds global power demands. Despite this, infrastructure shortages and high capital costs prevent the wide-scale adoption of photovoltaics to displace conventional energy technologies relying on carbon-based fuels. In response, new concepts and materials have been explored to develop next-generation solar cells capable of operating more efficiently and cheaply. Over the past decade, single semiconductor nanowire (NW) and NW array devices have emerged as promising platforms with which to examine new concepts. Small distances in NWs allow for efficient charge separation while tunable photonic modes permit light absorption properties distinct from bulk materials. Furthermore, the synthesis and fabrication of NW devices presents new opportunities such as with incorporation of complex heterostructures or use of cheaper substrates. Here, we present a critical discussion of the benefits and remaining challenges related to utilization of NWs for solar energy conversion and emphasize the synthetic advances leading towards significant improvement in the electrical and optical performance of NW devices. We conclude by articulating the unique capabilities of solar cells assembled from multiple, distinct NWs.Publication Semiconductor Nanowires: A Platform for Exploring Limits and Concepts for Nano-Enabled Solar Cells(Royal Society of Chemistry, 2013) Kempa, Thomas Jan; Day, Robert; Kim, Sun-Kyung; Park, Hong-Gyu; Lieber, CharlesOver 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.