Michael J. Gordon

Importance of diffuse scattering phenomena in moth-eye arrays for broadband infrared applications

Moth-eye (ME) arrays with varying aspect ratios and profile heights were fabricated in Si using a general colloidal lithography and reactive ion etching technique. Antireflective (AR) properties of the arrays were rigorously assessed from the near to far infrared (λ=2-50 μm) using transmission and reflection measurements via dispersive and Fourier transform infrared spectroscopy and modeled using an effective medium approximation (EMA). Infrared transmission of low aspect ratio structures (~2) matched the EMA model, indicating that the most important factor for AR at higher wavelengths is structure height. High aspect ratio structures (>6) were highly transmissive (>90% of theoretical maximum) over a large bandwidth in the mid-infrared (20-50 μm). Specular reflectance, total transmission, and diffuse reflectance (DR) measurements indicate that ME structures do not reach the theoretical maximum at near-infrared wavelengths due to DR and forward scattering phenomena. Ultimately, correlating optical performance with feature geometry (pitch, profile, height, etc.) over multiple length scales allows intelligent design of ME structures for broadband applications.

Simple colloidal lithography method to fabricate large-area moth-eye antireflective structures on Si, Ge, and GaAs for IR applications

A two-step colloidal lithography process (Langmuir–Blodgett dip coating + reactive ion etching) was developed to fabricate single and double-sided moth-eye structures in Si, Ge, and GaAs for antireflection applications in the IR. Large increases in transmittance were obtained in all three material platforms (up to 97% single-side and 91% absolute transmittance) over the λ = 4−20+ μm region. Effective medium theory and the transfer matrix method were used to predict IR optical response of moth-eye substrates as well as investigate the effect of protuberance shape on antireflectance behavior. Overall, it is demonstrated that colloidal lithography and etching provide an easy and generic way to synthesize moth-eyes in different IR material platforms.

Bio-inspired, sub-wavelength surface structures for ultra-broadband, omni-directional anti-reflection in the mid and far IR

Quasi-ordered moth-eye arrays were fabricated in Si using a colloidal lithography method to achieve highly efficient, omni-directional transmission of mid and far infrared (IR) radiation. The effect of structure height and aspect ratio on transmittance and scattering was explored experimentally and modeled quantitatively using effective medium theory. The highest aspect ratio structures (AR = 9.4) achieved peak transmittance of 98%, with >85% transmission for λ = 7-30 μm. A detailed photon balance was constructed by measuring transmission, forward scattering, specular reflection and diffuse reflection to quantify optical losses due to near-field effects. In addition, angle-dependent transmission measurements showed that moth-eye structures provide superior anti-reflective properties compared to unstructured interfaces over a wide angular range (0-60° incidence). The colloidal lithography method presented here is scalable and substrate-independent, providing a general approach to realize moth-eye structures and anti-reflection in many IR-compatible material systems.

Catalytic molten metals for the direct conversion of methane to hydrogen and separable carbon

Hydrogen from methane in molten metal The hydrogen used in making ammonia and other industrial reactions is produced mainly through steam reformation of methane over nickel catalysts. This high-temperature process also releases carbon dioxide, a greenhouse gas. Upham et al. used nickel dissolved in molten bismuth to pyrolyze methane to release hydrogen and form carbon, which floats to the surface of the melt, where it can be removed. Carbon formation on steam-reforming catalysts is usually a deactivating side reaction, but in the new process, the carbon can be stored or incorporated into composite materials. Science, this issue p. 917 Molten metal alloys catalyze methane pyrolysis to form hydrogen and removable surface carbon. Metals that are active catalysts for methane (Ni, Pt, Pd), when dissolved in inactive low–melting temperature metals (In, Ga, Sn, Pb), produce stable molten metal alloy catalysts for pyrolysis of methane into hydrogen and carbon. All solid catalysts previously used for this reaction have been deactivated by carbon deposition. In the molten alloy system, the insoluble carbon floats to the surface where it can be skimmed off. A 27% Ni–73% Bi alloy achieved 95% methane conversion at 1065°C in a 1.1-meter bubble column and produced pure hydrogen without CO2 or other by-products. Calculations show that the active metals in the molten alloys are atomically dispersed and negatively charged. There is a correlation between the amount of charge on the atoms and their catalytic activity.

Fabrication and optical behavior of graded-index, moth-eye antireflective structures in CdTe

A simple and scalable method, based on dip-coat colloidal lithography, mask reduction, and plasma-based pattern transfer, is presented to create graded-index, moth eye-inspired antireflective features on II–VI semiconductors. Hexagonal arrays of isolated conical frusta with tunable geometry (top diameter = 200–1300 nm, pitch = 310–2530 nm, and height = 790–7100 nm) were realized by isotropic etching of various size silica colloid masks before pattern transfer into the underlying substrate. Substantial increases in single-side direct and total infrared (IR) transmission across the 4–20 μm range (9%–15% for CdTe thin films and 18% for bulk CdTe) were achieved, in excellent agreement with transfer matrix calculations and finite difference time domain optical simulations. The fabrication method presented can be used to enhance efficiency in multiple IR application areas including photovoltaics, optical system components, detectors, and focal plane array imagers.

Enhanced light extraction from free-standing InGaN/GaN light emitters using bio-inspired backside surface structuring

Light extraction from InGaN/GaN-based multiple-quantum-well (MQW) light emitters is enhanced using a simple, scalable, and reproducible method to create hexagonally close-packed conical nano- and micro-scale features on the backside outcoupling surface. Colloidal lithography via Langmuir-Blodgett dip-coating using silica masks (d = 170-2530 nm) and Cl2/N2-based plasma etching produced features with aspect ratios of 3:1 on devices grown on semipolar GaN substrates. InGaN/GaN MQW structures were optically pumped at 266 nm and light extraction enhancement was quantified using angle-resolved photoluminescence. A 4.8-fold overall enhancement in light extraction (9-fold at normal incidence) relative to a flat outcoupling surface was achieved using a feature pitch of 2530 nm. This performance is on par with current photoelectrochemical (PEC) nitrogen-face roughening methods, which positions the technique as a strong alternative for backside structuring of c-plane devices. Also, because colloidal lithography functions independently of GaN crystal orientation, it is applicable to semipolar and nonpolar GaN devices, for which PEC roughening is ineffective.

Testing Predictions from Density Functional Theory at Finite Temperatures:

We perform a critical assessment of the accuracy of DFT-based methods in predicting stable phases within the Co-Pt binary alloy. Statistical mechanical analysis applied to zero kelvin DFT predictions yields finite-temperature results that can be directly compared with experimental measurements. The predicted temperature-composition phase diagram is qualitatively incompatible with experimental observations, indicating that the predicted stability of long-period superstructures as ground states in the Co-Pt binary is incorrect. We also show that recently suggested methods to better align DFT and experiment via the hybrid functional HSE06 are unable to resolve the discrepancies in this system. Our results indicate a need for better verification of DFT based phase stability predictions, and highlight fundamental flaws in the ability of DFT to treat late 3

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-Like Ground States in Co-Pt

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Microplasmas for direct, substrate-independent deposition of nanostructured metal oxides

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