Publications

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Electrical power generation from a moderate-temperature thermal source by means of direct conversion of infrared radiation is important and highly desirable for energy harvesting from waste heat and micropower applications. Here, we demonstrate direct rectified power generation from an unbiased large-area nanoantenna-coupled tunnel diode rectifier called a rectenna. Using a vacuum radiometric measurement technique with irradiation from a temperature-stabilized thermal source, a generated power density of 8 nW/cm2 is observed at a source temperature of 450 °C for the unbiased rectenna across an optimized load resistance. The optimized load resistance for the peak power generation for each temperature coincides with the tunnel diode resistance at zero bias and corresponds to the impedance matching condition for a rectifying antenna. Current-voltage measurements of a thermally illuminated large-area rectenna show current zero crossing shifts into the second quadrant indicating rectification. Photon-assisted tunneling in the unbiased rectenna is modeled as the mechanism for the large short-circuit photocurrents observed where the photon energy serves as an effective bias across the tunnel junction. The measured current and voltage across the load resistor as a function of the thermal source temperature represents direct current electrical power generation.

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We experimentally demonstrated an actively tunable optical filter that controls the amplitude of reflected long-wave-infrared light in two separate spectral regions concurrently. Our device exploits the dependence of the excitation energy of plasmons in a continuous and unpatterned sheet of graphene on the Fermi-level, which can be controlled via conventional electrostatic gating. The filter enables simultaneous modification of two distinct spectral bands whose positions are dictated by the device geometry and graphene plasmon dispersion. Within these bands, the reflected amplitude can be varied by over 15% and resonance positions can be shifted by over 90 cm-1. Electromagnetic simulations verify that tuning arises through coupling of incident light to graphene plasmons by a grating structure. Importantly, the tunable range is determined by a combination of graphene properties, device structure, and the surrounding dielectrics, which dictate the plasmon dispersion. Thus, the underlying design shown here isapplicable across a broad range of infrared frequencies.

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We have developed an ambient temperature, SiO₂/Si wafer - scale process for Josephson junctions based on Nb electrodes and Ta x N barriers with tunable electronic properties. The films are fabricated by magnetron sputtering. The electronic properties of the Ta_xN barriers are controlled by adjusting the nitrogen flow during sputtering. This technology offers a scalable alternative to the more traditional junctions based on AlO_x barriers for low - power, high - performance computing.

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Properties of NbN and TaxN thin films grown at ambient temperatures on SiO2/Si substrates by reactive-pulsed laser deposition and reactive magnetron sputtering (MS) as a function of N2 gas flow were investigated. Both techniques produced films with smooth surfaces, where the surface roughness did not depend on the N2 gas flow during growth. High crystalline quality, (111) oriented NbN films with Tc up to 11 K were produced by both techniques for N contents near 50%. The low temperature transport properties of the TaxN films depended upon both the N2 partial pressure used during growth and the film thickness. The root mean square surface roughness of TaxN films grown by MS increased as the film thickness decreased down to 10 nm.

We have examined graphene absorption in a range of graphene-based infrared devices that combine either monolayer or bilayer graphene with three different gate dielectrics. Electromagnetic simulations show that the optical absorption in graphene in these devices, an important factor in a functional graphene-based detector, is strongly dielectricdependent. These simulations reveal that plasmonic excitation in graphene can significantly influence the percentage of light absorbed in the entire device, as well as the graphene layer itself, with graphene absorption exceeding 25% in regions where plasmonic excitation occurs. Notably, the dielectric environment of graphene has a dramatic influence on the strength and wavelength range over which the plasmons can be excited, making dielectric choice paramount to final detector tunability and sensitivity.

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We have developed and characterized novel in-situ corrosion sensors to monitor and quantify the corrosive potential and history of localized environments. Embedded corrosion sensors can provide information to aid health assessments of internal electrical components including connectors, microelectronics, wires, and other susceptible parts. When combined with other data (e.g. temperature and humidity), theory, and computational simulation, the reliability of monitored systems can be predicted with higher fidelity.

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Two-dimensional materials were explored through collaboration with Steve Howell and Catalyn Spataru, led by James Bartz during FY15 and FY16 at Sandia National Laboratories. Because of their two-dimensional nature, these materials may offer properties exceeding those of bulk materials. This work involved Density Functional Theory simulations and optical methods, instrumentation development, materials growth and materials characterization. Through simulation the wide variety of two dimensional materials was down-selected for fabrication and testing. Out of the two dimensional semiconductors studied, black phosphorus bilayers showed the strongest spectral absorption tuning with applied electric field. Laser scanning confocal microscopy, spectroscopy and atomic force microscopy allowed for identification of micron scale samples. A technique involving conductive tip atomic force microscopy and back-side illumination was developed simple assembly and characterization of material spectral response.

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Self-heating induced failure of graphene devices synthesized from both chemical vapor deposition (CVD) and epitaxial means is compared using a combination of infrared thermography and Raman imaging. Despite a larger thermal resistance, CVD devices dissipate >3x the amount of power before failure than their epitaxial counterparts. The discrepancy arises due to morphological irregularities implicit to the graphene synthesis method that induce localized heating. Morphology, rather than thermal resistance, therefore dictates power handling limits in graphene devices.

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Oxidation of exfoliated gallium selenide (GaSe) is investigated through Raman, photoluminescence, Auger, and X-ray photoelectron spectroscopies. Photoluminescence and Raman intensity reductions associated with spectral features of GaSe are shown to coincide with the emergence of signatures emanating from the by-products of the oxidation reaction, namely, Ga₂Se₃ and amorphous Se. Furthermore, photoinduced oxidation is initiated over a portion of a flake highlighting the potential for laser based patterning of two-dimensional heterostructures via selective oxidation.

We developed new detector technologies to identify the presence of radioactive materials for nuclear forensics applications. First, we investigated an optical radiation detection technique based on imaging nitrogen fluorescence excited by ionizing radiation. We demonstrated optical detection in air under indoor and outdoor conditions for alpha particles and gamma radiation at distances up to 75 meters. We also contributed to the development of next generation systems and concepts that could enable remote detection at distances greater than 1 km, and originated a concept that could enable daytime operation of the technique. A second area of research was the development of room-temperature graphene-based sensors for radiation detection and measurement. In this project, we observed tunable optical and charged particle detection, and developed improved devices. With further development, the advancements described in this report could enable new capabilities for nuclear forensics applications.

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Vertically aligned, untangled planarized arrays of multiwall carbon nanotubes (MWNTs) with Ohmic back contacts were grown in nanopore templates on arbitrary substrates. The templates were prepared by sputter depositing Nd-doped Al films onto W-coated substrates, followed by anodization to form an aluminum oxide nanopore array. The W underlayer helps eliminate the aluminum oxide barrier that typically occurs at the nanopore bottoms by instead forming a thin WO3 layer. The WO3 can be selectively etched to enable electrodeposition of Co catalysts with control over the Co site density. This led to control of the site density of MWNTs grown by thermal chemical vapor deposition, with W also serving as a back electrical contact. Ohmic contact to MWNTs was confirmed, even following ultrasonic cutting of the entire array to a uniform height.

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Nanoantennas are an enabling technology for visible to terahertz components and may be used with a variety of detector materials. We have integrated subwavelength patterned metal nanoantennas with various detector materials for infrared detection: midwave infrared indium gallium arsenide antimonide detectors, longwave infrared graphene detectors, and shortwave infrared germanium detectors. Nanoantennas offer a means to make infrared detectors much thinner, thus lowering the dark current and improving performance. The nanoantenna converts incoming plane waves to more tightly bound and concentrated surface waves. The active material only needs to extend as far as these bound fields. In the case of graphene detectors, which are only one or two atomic layers thick, such field concentration is a necessity for usable device performance, as single pass absorption is insufficient. The nanoantenna is thus the enabling component of these thin devices. However nanoantenna integration and fabrication vary considerably across these platforms as do the considerations taken into account during design. Here we discuss the motivation for these devices and show examples for the three material systems. Characterization results are included for the midwave infrared detector. © 2014 SPIE.

Nanoantennas are an enabling technology for visible to terahertz components and may be used with a variety of detector materials. We have integrated subwavelength patterned metal nanoantennas with various detector materials for infrared detection: midwave infrared indium gallium arsenide antimonide detectors, longwave infrared graphene detectors, and shortwave infrared germanium detectors. Nanoantennas offer a means to make infrared detectors much thinner, thus lowering the dark current and improving performance. The nanoantenna converts incoming plane waves to more tightly bound and concentrated surface waves. The active material only needs to extend as far as these bound fields. In the case of graphene detectors, which are only one or two atomic layers thick, such field concentration is a necessity for usable device performance, as single pass absorption is insufficient. The nanoantenna is thus the enabling component of these thin devices. However nanoantenna integration and fabrication vary considerably across these platforms as do the considerations taken into account during design. Here we discuss the motivation for these devices and show examples for the three material systems. Characterization results are included for the midwave infrared detector. © 2014 SPIE.