Two-dimensional heavy fermions in the van der Waals metal CeSiI.

Heavy-fermion metals are prototype systems for observing emergent quantum phases driven by electronic interactions1-6. A long-standing aspiration is the dimensional reduction of these materials to exert control over their quantum phases7-11, which remains a significant challenge because traditional intermetallic heavy-fermion compounds have three-dimensional atomic and electronic structures. Here we report comprehensive thermodynamic and spectroscopic evidence of an antiferromagnetically ordered heavy-fermion ground state in CeSiI, an intermetallic comprising two-dimensional (2D) metallic sheets held together by weak interlayer van der Waals (vdW) interactions. Owing to its vdW nature, CeSiI has a quasi-2D electronic structure, and we can control its physical dimension through exfoliation. The emergence of coherent hybridization of f and conduction electrons at low temperature is supported by the temperature evolution of angle-resolved photoemission and scanning tunnelling spectra near the Fermi level and by heat capacity measurements. Electrical transport measurements on few-layer flakes reveal heavy-fermion behaviour and magnetic order down to the ultra-thin regime. Our work establishes CeSiI and related materials as a unique platform for studying dimensionally confined heavy fermions in bulk crystals and employing 2D device fabrication techniques and vdW heterostructures12 to manipulate the interplay between Kondo screening, magnetic order and proximity effects.

Amorphous silicon carbide high contrast gratings as highly efficient spectrally selective visible reflectors.

We report spectrally selective visible wavelength reflectors using hydrogenated amorphous silicon carbide (a-SiC:H) as a high index contrast material. Beyond 610nm and through the near infrared spectrum, a-SiC:H exhibits very low loss and exhibits an wavelength averaged index of refraction of n = 3.1. Here we design, fabricate, and characterize such visible reflectors using a hexagonal array of a-SiC:H nanopillars as wavelength-selective mirrors with a stop-band of approximately 40 nm full-width at half maximum. The fabricated high contrast grating exhibits reflectivity R >94% at a resonance wavelength of 642nm with a single layer of a-SiC:H nanopillars. The resonance wavelength is tunable by adjusting the geometrical parameters of the a-SiC:H nanopillar array, and we observe a stop-band spectral center shift from 635 nm up to 642 nm. High contrast gratings formed from a-SiC:H nanopillars are a promising platform for various visible wavelength nanophotonics applications.

Construction of a Semiconductor-Biological Interface for Solar Energy Conversion: p-Doped Silicon/Photosystem I/Zinc Oxide.

The interface between photoactive biological materials with two distinct semiconducting electrodes is challenging both to develop and analyze. Building off of our previous work using films of photosystem I (PSI) on p-doped silicon, we have deposited a crystalline zinc oxide (ZnO) anode using confined-plume chemical deposition (CPCD). We demonstrate the ability of CPCD to deposit crystalline ZnO without damage to the PSI biomaterial. Using electrochemical techniques, we were able to probe this complex semiconductor-biological interface. Finally, as a proof of concept, a solid-state photovoltaic device consisting of p-doped silicon, PSI, ZnO, and ITO was constructed and evaluated.

Unlocking Higher Power Efficiencies in Luminescent Solar Concentrators through Anisotropic Luminophore Emission.

The luminescent solar concentrator (LSC) offers a potential pathway for achieving low-cost, fixed-tilt light concentration. Despite decades of research, conversion efficiency for LSC modules has fallen far short of that achievable by geometric concentrators. However, recent advances in anisotropically emitting nanophotonic structures could enable a significant step forward in efficiency. Here, we employ Monte Carlo ray-trace modeling to evaluate the conversion efficiency for anisotropic luminophore emission as a function of photoluminescence quantum yield, waveguide concentration, and geometric gain. By spanning the full LSC parameter space, we define a roadmap toward high conversion efficiency. An analytical function is derived for the dark radiative current of an LSC to calculate the conversion efficiency from the ray-tracing results. We show that luminescent concentrator conversion efficiency can be increased from the current record value of 7.1-9.6% by incorporating anisotropy. We provide design parameters for optimized luminescent solar concentrators with practical geometrical gains of 10. Using luminophores with strongly anisotropic emission and high (99%) quantum yield, we conclude that conversion efficiencies beyond 28% are achievable. This analysis reveals that for high LSC performance, waveguide losses are as important as the luminophore quantum yield.

Silicon Heterojunction Microcells.

We report the design, fabrication, and characterization of silicon heterojunction microcells, a new type of photovoltaic cell that leverages high-efficiency bulk wafers in a microscale form factor, while also addressing the challenge of passivating microcell sidewalls to mitigate carrier recombination. We present synthesis methods exploiting either dry etching or laser cutting to realize microcells with native oxide-based edge passivation. Measured microcell performance for both fabrication processes is compared to that in simulations. We characterize the dependence of microcell open-circuit voltage (Voc) on the cell area-perimeter ratio and examine synthesis processes that affect edge passivation quality, such as sidewall damage removal, the passivation material, and the deposition technique. We report the highest Si microcell Voc to date (588 mV, for a 400 µm × 400 µm × 80 µm device), demonstrate Voc improvements with deposited edge passivation of up to 55 mV, and outline a pathway to achieve microcell efficiencies surpassing 15% for such device sizes.