Justin M. Foley

Analysis of the operation of thin nanowire photoelectrodes for solar energy conversion

The solar energy conversion properties of thin Si and GaP nanowire photoelectrodes in photoelectrochemical cells have been examined through sets of finite-element simulations. A discussion describing the motivation behind nanostructured, high aspect ratio semiconductor photoelectrode designs and a brief survey of current experimental results reported for nanostructured semiconductor photoelectrodes in photoelectrochemical cells are presented first. An analysis is then shown that outlines the primary recombination pathways governing the steady-state current-potential behaviors of thin, cylindrical nanowire photoelectrodes, with explicit expressions detailing the differences between planar and cylindrical photoelectrodes arising from the solution of carrier fluxes in planar and cylindrical geometries. Results from finite-element simulations used to model the key features of thin nanowire photoelectrodes under low-level injection conditions are shown that illustrate which recombination pathway(s) is operative under various experimental conditions. Specifically, the respective effects of non-uniform doping, tapering along the length, variation in charge carrier mobilities and lifetimes, changes in nanowire radius, and changes in the density of surface defects on the observable photocurrent-potential responses are reported. These cumulative results serve as guides for future experimental work aimed at improving the attainable solar energy conversion efficiencies of doped semiconductor nanowire photoelectrodes. Lastly, separate simulations that model lightly doped nanowire photoelectrodes under high-level injection conditions are discussed. These results suggest discrete, ohmic-selective contacts may afford a way to circumvent the stringent doping requirements discussed herein for thin nanowire photoelectrodes.

Benchtop Electrochemical Liquid–Liquid–Solid Growth of Nanostructured Crystalline Germanium

An electrochemical liquid-liquid-solid (ec-LLS) process that produces large amounts of crystalline semiconductors with tunable nanostructured shapes without any physical or chemical templating agent is presented. Electrodeposition of Ge from GeO(2)(aq) solutions followed by dissolution into a liquid Hg electrode, saturation of the liquid alloy, and precipitation can yield polycrystalline Ge(s) under ambient conditions. A unique advantage of ec-LLS is that it involves precipitation under electrochemical control, where the applied bias precisely defines the flux of Ge into the liquid electrode. Fidelity of the saturation and precipitation of Ge from liquid electrodes affords a variety of material morphologies, including dense films of oriented nanostructured filaments with large aspect ratios (>10(3)). Electrodeposition involving a liquid electrolyte, a liquid electrode, and a solid deposit under ambient conditions represents a conceptually unexplored direct wet-chemical route for the preparation of bulk quantities of crystalline group-IV semiconductors without the time- and energy-intensive processing steps required in traditional preparations of semiconductor materials.

Comparison of majority carrier charge transfer velocities at Si/polymer and Si/metal photovoltaic heterojunctions

Two sets of silicon (Si) heterojunctions with either Au or PEDOT:PSS contacts have been prepared to compare interfacial majority carrier charge transfer processes at Si/metal and Si/polymer heterojunctions. Current-voltage (J-V) responses at a range of temperatures, wavelength-dependent internal quantum yields, and steady-state J-V responses under illumination for these devices are reported. The cumulative data suggest that the velocity of majority carrier charge transfer, vn, is several orders of magnitude smaller at n-Si/PEDOT:PSS contacts than at n-Si/Au junctions, resulting in superior photoresponse characteristics for these inorganic/organic heterojunctions.

Narrowband mid-infrared transmission filtering of a single layer dielectric grating

We experimentally demonstrate a simple narrowband transmission filter using a silicon/air grating, exhibiting broadband high-reflectance between 8 and 14 μm and narrow transmission peaks at slightly off-normal incidence. We explain the response in terms of guided modes in a dielectric slab waveguide, with numerical modal analysis corroborating an intuitive slab waveguide picture and the experimental results. The filtered wavelength depends predictably on the grating period, making wavelength selection straightforward, while the length-scale independent operation principle shows promise for enabling similar filtering capabilities at higher frequencies.

Broadband long-wavelength infrared Si/SiO2 subwavelength grating reflector

A long-wavelength IR broadband reflector is demonstrated using a high-index-contrast subwavelength grating based on a Si/SiO2 system. The field response has been computationally and experimentally verified to exhibit broadband reflectance in the spectral range of 13-16 μm with Δλ/λ=18.5% for reflectance >70%. The gratings exhibit incident field polarization dependence with an average extinction ratio of 1:1.6.

Normal incidence narrowband transmission filtering capabilities using symmetry-protected modes of a subwavelength, dielectric grating.

We computationally study a normal incidence narrowband transmission filter based on a subwavelength dielectric grating that operates through Fano interference between supported guided leaky modes of the system. We characterize the filtering capabilities as the cross section of the grating is manipulated and suggest techniques for experimental demonstration. Using group theory, we study the plane wave coupling to the supported modes that leads to broadband reflectance and narrowband transmittance responses for rectangular, pentagonal, rhomboidal, and right trapezoidal cross-sectional geometries.

Investigating Student Motivation and Performance in Electrical Engineering and Its Subdisciplines

Factors influencing choice of major in electrical engineering and later curricular and professional choices are investigated. Studies include both quantitative and qualitative analyses via student transcripts, surveys, and focus groups. Student motivation for choosing an electrical engineering major and later subdiscipline in the field is interpreted through expectancy-value theory, where primary factors of strong perceived value of future professional opportunities and strong influence of course instructors are identified. Performances in select required electrical engineering courses appear to serve as predictors for student choice of subdiscipline emphasis. In contrast, participation in student professional activities does not show statistically significant correlations with subdiscipline. Curricular and professional choices appear to be explained by expectancy-value theory with inclusion of socializers. The findings suggest that early and integrative exposure of all electrical engineering technical areas, including high-quality teaching, may provide an optimal basis for students to make future decisions on academic path and participation in professional activities.

2017 IEEE Education Society Awards, 2017 Frontiers in Education Conference Awards, and Selected IEEE Awards

The IEEE Education Society, the IEEE Computer Society, and the American Society for Engineering Education Educational Research and Methods Division (ASEE ERM) sponsored the 46th Frontiers in Education (FIE) Conference in Erie, PA, USA, held October 12–15, 2016. James Sluss, President of the Society, Russell Meier, President of the ASEE ECE Division, and Elizabeth Eschenbach, Chair, FIE Steering Committee, presented awards sponsored by HP Enterprise, the FIE Conference, the Education Society of IEEE, and the ECE Division of ASEE to the 2016 recipients of these awards.

Suspended Si/air high contrast subwavelength gratings for long-wavelength infrared reflectors

We report broadband reflectance in the long-wavelength infrared (LWIR, 8-12 μm) utilizing suspended-Si, high-index-contrast subwavelength gratings (HCGs). Iterative design optimization using finite element analysis software has been performed accounting for silicon’s wavelength-dependent index of refraction and extinction coefficient. Grating arrays were fabricated using commercial silicon-on-insulator (SOI) substrates, photolithography and reactive ion etching; subsequent selective wet etching of SiO2 was used to provide suspended Si/air gratings. Fourier transform infrared (FTIR) spectroscopy demonstrates broadband, polarization–dependent reflectance between 8.5 and 12 μm, which agrees with the simulated response.

Erratum: Analysis of the operation of thin nanowire photoelectrodes for solar energy conversion (Energy & Environmental Science (2012) 5 (5203-5220) DOI:10.1039/C1EE02518J)

Erratum : Strong and binder free structured zeolite sorbents with very high CO2-over-N-2 selectivities and high capacities to adsorb CO2 rapidly (vol 5, pg 7664, 2012)