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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 127 records · Page 7
All sourcesNASA NTRSDOE OSTI
DOE OSTI2020

Geometrically Tunable Beamed Light Emission from a Quantum‐Dot Ensemble Near a Gradient Metasurface

Abstract Optical metasurfaces have been widely investigated in recent years as a means to tailor the wavefronts of externally incident light for passive device applications. At the same time, their use in active optoelectronic devices such as light emitters is far less established. This work explores their ability to control the radiation properties of a nearby continuous ensemble of randomly oriented incoherent dipole sources via near‐field interactions. Specifically, a film of colloidal quantum dots is deposited on a plasmonic metasurface consisting of a 1D array of metallic nanoantennas on a metal film. The array is designed to introduce a linear phase profile upon reflection, and a bi‐periodic nanoparticle arrangement is introduced to ensure adequate sampling of the desired phase gradient. Highly directional radiation patterns are correspondingly obtained from the quantum dots at an enhanced emission rate. The underlying radiation mechanism involves the near‐field excitation of surface plasmon polaritons at the metal film, and their selective diffractive scattering by the metasurface into well‐collimated beams along predetermined geometrically tunable directions. These results underscore the distinctive ability of metasurfaces to control radiation properties directly at the source level, which is technologically significant for the continued miniaturization and large‐scale integration of optoelectronic devices.

Wang, Xiaowei↗
DOE OSTI2017

Los Alamos Studies Perovskites for Efficient Optoelectronics

In the eternal search for next generation high-efficiency solar cells and LEDs, scientists at Los Alamos National Laboratory and their partners are gaining an extra degree of freedom in designing and fabricating efficient optoelectronic devices based on 2D layered hybrid perovskites. Industrial applications could include low cost solar cells, LEDs, laser diodes, detectors, and other nano-optoelectronic devices. The 2D, near-single-crystalline “Ruddlesden-Popper” thin films have an out-of-plane orientation so that uninhibited charge transport occurs through the perovskite layers in planar devices. The new research finds the existence of “layer-edge-states” at the edges of the perovskite layers which are key to both high efficiency of solar cells (greater than 12 percent) and high fluorescence efficiency (a few tens of percent) for LEDs. The spontaneous conversion of excitons (bound electron-hole pairs) to free carriers via these layer-edge states appears to be the key to the improvement of the photovoltaic and light-emitting thin film layered materials.

36 MATERIALS SCIENCE↗
DOE OSTI2026

High-performance nanodevices based on WGe 2⁢ N 4 monolayer

Two-dimensional (2D) 𝑀⁢𝐴 2 ⁢𝑍 4 -family monolayers (MLs) have emerged as promising semiconductors due to their element tunability and rich electronic and optoelectronic properties. In this work, using first-principles calculations, we investigate the electronic, mechanical, transport, and optoelectronic properties of WGe 2⁢ N 4 ML with a small indirect bandgap. Various nanodevices based on WGe 2 ⁢N 4 ML are studied, including pn-junction diodes, pin-junction field-effect transistors (FETs), and phototransistors. The present results reveal that the WGe 2⁢ N 4 ML exhibits high rigidity, thermal stability, and remarkable light absorption. These nanodevices demonstrate excellent performance: (1) the pn-junction diode shows a high rectification ratio and a near-Shockley-limit ideality factor, (2) the pin-junction FET exhibits significant gate voltage modulation capability with subthreshold swing as low as 71 mV/dec (close to the theoretical limit calculated based on the Boltzmann distribution), and (3) the phototransistor displays strong optoelectronic responses in the visible and ultraviolet regions. Furthermore, these findings establish WGe 2 ⁢N 4 ML as a versatile platform for developing high-performance, multifunctional nanoelectronic and optoelectronic devices, significantly expanding the application potential of 𝑀⁢𝐴 2⁢ 𝑍 4 -family materials.

2-dimensional systems↗
DOE OSTI2015

Insight into the epitaxial growth of high optical quality GaAs{sub 1–x}Bi{sub x}

The ternary alloy GaAs{sub 1–x}Bi{sub x} is a potentially important material for infrared light emitting devices, but its use has been limited by poor optical quality. We report on the synthesis of GaAs{sub 1–x}Bi{sub x} epi-layers that exhibit narrow, band edge photoluminescence similar to other ternary GaAs based alloys, e.g., In{sub y}Ga{sub 1–y}As. The measured spectral linewidths are as low as 14 meV and 37 meV at low temperature (6 K) and room temperature, respectively, and are less than half of previously reported values. The improved optical quality is attributed to the use of incident UV irradiation of the epitaxial surface and the presence of a partial surface coverage of bismuth in a surfactant layer during epitaxy. Comparisons of samples grown under illuminated and dark conditions provide insight into possible surface processes that may be altered by the incident UV light. The improved optical quality now opens up possibilities for the practical use of GaAs{sub 1–x}Bi{sub x} in optoelectronic devices.

36 MATERIALS SCIENCE↗
DOE OSTI2013

Solar cells

Organic photosensitive optoelectronic devices are disclosed. The devises are thin-film crystalline organic optoelectronic devices capable of generating a voltage when exposed to light, and prepared by a method including the steps of: depositing a first organic layer over a first electrode; depositing a second organic layer over the first organic layer; depositing a confining layer over the second organic layer to form a stack; annealing the stack; and finally depositing a second electrode over the second organic layer.

14 SOLAR ENERGY↗
NASA NTRS1989

Monolithically integrated two-dimensional arrays of optoelectronic threshold devices for neural network applications

A monolithic 10 x 10 two-dimensional array of 'optical neuron' optoelectronic threshold elements for neural network applications has been designed, fabricated, and tested. Overall array dimensions are 5 x 5 mm, while the individual neurons, composed of an LED that is driven by a double-heterojunction bipolar transistor, are 250 x 250 microns. The overall integrated structure exhibited semiconductor-controlled rectifier characteristics, with a breakover voltage of 75 V and a reverse-breakdown voltage of 60 V; this is attributable to the parasitic p-n-p transistor which exists as a result of the sharing of the same n-AlGaAs collector between the transistors and the LED.

Kim, J. H.↗
DOE OSTI2020

Influence of Hydrogen and Oxygen on the Structure and Properties of Sputtered Magnesium Zirconium Oxynitride Thin Films

Nitride materials with mixed ionic and covalent bonding character and resulting good charge transport properties are attractive for optoelectronic devices. Recently, Mg-based ternary nitride materials were found to have large dielectric constants and high absorption coefficients with bandgaps appropriate for photovoltaic applications. However, their degenerate carrier concentrations still hinder their possible applications in solar cells and related optoelectronic devices. Therefore, further understanding and engineering of the parameters controlling the material properties of these ternary nitrides is highly desirable. In this paper we report that the structural, optical and electrical properties of magnesium zirconium oxynitride (MZNO) thin films synthesized by combinatorial sputtering with a wide range of cation compositions can be affected by incorporation of oxygen and hydrogen. Excess oxygen improved the crystallinity of MZNO thin films whereas hydrogen attracted oxygen and formed Mg-rich oxide layers at the grain boundaries which in turn reduced the conductivity. On the other hand, optical properties are more sensitive to the composition – both cation and anion ratios – rather than the presence of hydrogen. Compared to cation-stoichiometric MZNO (10 19 –10 20 cm -3 ), substantial reduction of carrier concentration down to ~10 14 cm -3 was achieved under Mg-rich conditions by supplying hydrogen during growth. Photoluminescence measurements showed that the films prepared with hydrogen were optoelectronically active. Overall, this study demonstrates that the material properties of MZN thin films can be significantly influenced by incorporation of oxygen and hydrogen.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2020

Photoconductive Switch with High Sub-Bandgap Responsivity in Nitrogen-Doped Diamond

To evaluate nitrogen-doped diamond as a candidate for Photoconductive Semiconductor Switches (PCSS) triggered in the sub-bandgap visible range, we have fabricated and tested diamond PCSS from a set of diamond grades of varying nitrogen concentration. The nitrogen-doped diamond PCSS have higher responsivity than other diamond optoelectronic devices in the visible range. We present a novel demonstration of sub-bandgap triggered diamond PCSS, which when scaled to higher voltage, can compete with and even surpass performance of existing ultraviolet-triggered diamond PCSS. This work suggests that nitrogen-doped diamond is a promising candidate for high-power, fast optoelectronic devices.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Hybrid planar-graded heterojunction for organic photovoltaics

Disclosed herein are organic photosensitive optoelectronic devices comprising at least one hybrid planar-graded heterojunction. In particular, organic photosensitive optoelectronic devices are disclosed having two electrodes (110), (150) in superposed relation, a graded heterojunction layer (130) located between the two electrodes, and at least one photoactive layer (120), (140) adjacent to and interfacing with the graded heterojunction layer.

15 GEOTHERMAL ENERGY↗
DOE OSTI2016

Coexistence of negative photoconductivity and hysteresis in semiconducting graphene

Solution-processed graphene quantum dots (GQDs) possess a moderate bandgap, which make them a promising candidate for optoelectronics devices. However, negative photoconductivity (NPC) and hysteresis that happen in the photoelectric conversion process could be harmful to performance of the GQDs-based devices. So far, their origins and relations have remained elusive. Here, we investigate experimentally the origins of the NPC and hysteresis in GQDs. By comparing the hysteresis and photoconductance of GQDs under different relative humidity conditions, we are able to demonstrate that NPC and hysteresis coexist in GQDs and both are attributed to the carrier trapping effect of surface adsorbed moisture. We also demonstrate that GQDs could exhibit positive photoconductivity with three-order-of-magnitude reduction of hysteresis after a drying process and a subsequent encapsulation. Considering the pervasive moisture adsorption, our results may pave the way for a commercialization of semiconducting graphene-based and diverse solution-based optoelectronic devices.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2015

Silicon-nitride photonic circuits interfaced with monolayer MoS 2

Herein we report integration of monolayer molybdenum disulphide with silicon nitride microresonators assembled by visco-elastic layer transfer techniques. Evanescent coupling from the resonator mode to the monolayer is confirmed through measurements of cavity transmission. The absorption of the monolayer semiconductor flakes in this geometry is determined to be 850 dB/cm, which is larger than that of graphene and black phosphorus with the same thickness. This technique can be applied to diverse monolayer semiconductors for assembling hybrid optoelectronic devices such as photodetectors and modulators operating over a wide spectrum range.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Low Contact Barrier in 2H/1T' MoTe 2 In-Plane Heterostructure Synthesized by Chemical Vapor Deposition

Metal–semiconductor contact has been an important topic in the semiconductor industry because it influences device performance remarkably. Conventional metals have served as the major contact material in electronic and optoelectronic devices, but such a selection becomes increasingly inadequate for emerging novel materials such as two-dimensional (2D) materials. Deposited metals on semiconducting 2D channels usually form large resistance contacts due to the high Schottky barrier. A few approaches have been reported to reduce the contact resistance but they are not suitable for large-scale application or they cannot create a clean and sharp interface. Here, a chemical vapor deposition (CVD) technique is introduced to produce large-area semiconducting 2D material (2H MoTe 2 ) planarly contacted by its metallic phase (1T' MoTe 2 ). We demonstrate the phase-controllable synthesis and systematic characterization of large-area MoTe 2 films, including pure 2H phase or 1T' phase, and 2H/1T' in-plane heterostructure. Theoretical simulation reflects a lower Schottky barrier in 2H/1T' junction than in Ti/2H contact, which is confirmed by electrical measurement. This one-step CVD method to synthesize large-area, seamless-bonding 2D lateral metal–semiconductor junction can improve the performance of 2D electronic and optoelectronic devices, paving the way for large-scale 2D integrated circuits.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Review of using gallium nitride for ionizing radiation detection

With the largest band gap energy of all commercial semiconductors, GaN has found wide application in the making of optoelectronic devices. It has also been used for photodetection such as solar blind imaging as well as ultraviolet and even X-ray detection. Unsurprisingly, the appreciable advantages of GaN over Si, amorphous silicon (a-Si:H), SiC, amorphous SiC (a-SiC), and GaAs, particularly for its radiation hardness, have drawn prompt attention from the physics, astronomy, and nuclear science and engineering communities alike, where semiconductors have traditionally been used for nuclear particle detection. Several investigations have established the usefulness of GaN for alpha detection, suggesting that when properly doped or coated with neutron sensitive materials, GaN could be turned into a neutron detection device. Work in this area is still early in its development, but GaN-based devices have already been shown to detect alpha particles, ultraviolet light, X-rays, electrons, and neutrons. Furthermore, the nuclear reaction presented by {sup 14}N(n,p){sup 14}C and various other threshold reactions indicates that GaN is intrinsically sensitive to neutrons. This review summarizes the state-of-the-art development of GaN detectors for detecting directly and indirectly ionizing radiation. Particular emphasis is given to GaN's radiation hardness under high-radiation fields.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2021

High stability of flexible perovskite transparent conductive oxide film via van der Waals heteroepitaxy

Transparent conducting oxides (TCOs), such as indium-tin oxide (ITO) and indium gallium zinc oxide (IGZO), are essential components in the design of flexible optoelectronic devices. However, it is still a challenge to prepare low-cost flexible transparent conductive film for achieving the flexible functionality together with high optical transparency and electrical conductivity. Herein, fluorphlogopite (mica) is selected as the bendable and transparent substrate for the fabrication of low-cost perovskite CaVO 3 TCO epitaxial film. Benefitting from thin mica substrate with high Young’s modulus of 200 GPa, flexible CaVO 3 TCO epitaxial film exhibits high mechanical stability in terms of electrical conductivity and optical transparency after bending up to 10 5 cycles. The change of resistance can reflect the bending curvature and strain state. Finally, our results provide a new pathway to design superior mechanical flexibility and stability CaVO 3 TCO films for application in flexible, light-weight, and portable smart optoelectronic devices.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Identification of an organic semiconductor superlattice structure of pentacene and perfluoro-pentacene through resonant and non-resonant X-ray scattering

Highly crystalline and stable molecular superlattices are grown with the smallest possible stacking period using monolayers (MLs) of the organic semiconductors pentacene (PEN) and perfluoro-pentacene (PFP). Superlattice reflections in X-ray reflectivity and their energy dependence in resonant soft X-ray reflectivity measurements show that PFP and PEN MLs indeed alternate even though the coherent ordering is lost after ~ 4 ML. The observed lattice spacing of 15.9 Å in the superlattice is larger than in pure PEN and PFP films, presumably because of more upright standing molecules and lack of interdigitation between the incommensurate crystalline PEN and PFP layers. The findings are important for the development of novel organic quantum optoelectronic devices.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Synthesis, microstructure, and electronic band structure properties of nanocrystalline neodymium-doped bismuth titanate ferroelectric films fabricated by the sol–gel method

Graphical abstract: X-ray diffraction indicates that the films are polycrystalline with the pure perovskite phase. Ten Raman active modes and one silicon substrate mode can be observed. The A{sub 1} {sub g}[Bi] at about 59 cm{sup −1} is unchanged whereas the B{sub 1} {sub g} and A{sub 1} {sub g}[Ti] phonon modes shift towards higher frequencies. Photoluminescence shows that the intensities of the two peaks increase with Nd concentration except the Bi{sub 3}NdTi{sub 3}O{sub 12} film, due to the smallest grain size and oxygen vacancy defects. Good optical functions of the BNT films are achieved due to the SE suggesting potential applications in ferroelectric-based optoelectronic devices. - Abstract: Bi{sub 4−x}Nd{sub x}Ti{sub 3}O{sub 12} (BNT) films with different Nd contents (from 0 to 1 with 0.25 intervals) are prepared by the sol–gel process. The Nd substitution effects on the preferred orientation, surface morphology, phonon modes, emission bands, andelectronic band structures of the BNT films are investigated by microscopy, Raman scattering, photoluminescence, and spectroscopic ellipsometry (SE) at room temperature. X-ray diffraction indicates that the films are polycrystalline with the pure perovskite phase. Ten Raman active modes and one silicon substrate mode can be observed. The A{sub 1} {sub g}[Bi] at about 59 cm{sup −1} is unchanged whereas the B{sub 1} {sub g} and A{sub 1} {sub g}[Ti] phonon modes shift towards higher frequencies. Photoluminescence shows that the intensities of the two peaks increase with Nd concentration except the Bi{sub 3}NdTi{sub 3}O{sub 12} film, due to the smallest grain size and oxygen vacancy defects. Good optical functions of the BNT films are achieved due to the SE suggesting potential applications in ferroelectric-based optoelectronic devices.

36 MATERIALS SCIENCE↗