We discuss the fabrication and operation of GaN nanogap vacuum nanoelectronic diodes that operate in air and exhibit ultra-low turn-on voltage, high field emission current, excellent on-off ratio, and promising reliability and radiation hardness. We present experimental and modeling results on the characteristics of these devices at various nanogap sizes, operating pressures, and radiation environments. Preliminary results on the fabrication and characteristics of lateral GaN nano vacuum transistors will also be presented. These results provide key new insights into the behavior and potential of this new class of devices and point to future challenges and opportunities. Sandia National Laboratories is managed and operated by NTESS under DOE NNSA contract DE-NA0003525.

Previously we introduced a novel fin shape light-emitting diode (LED) architecture based on sub-micron n-ZnO/p-GaN heterojunctions that at high current densities demonstrated a droop-free behavior. Efficiency droop, which is the decline in internal quantum efficiency with increasing current density, is one of the significant challenges facing wide bandgap LEDs. In the present work, we discuss performance of fins of wide band gap materials such as AlGaN that are formed on GaN. Discussion also includes electroluminescence and different types of efficiencies of these LEDs for generation of UV wavelengths as well as the significance of the fin shape in improving their performance.

The need for new electronic devices and applications is driving the use of transition metal dichalcogenides (TMDCs) as suitable materials to integrate the next generation of novel applications. Thickness control of semiconductors is the great importance to attain specific electrochemical properties. In this work we investigate the thickness fluctuation of 2D MoSe2 structures synthesized by pulsed laser ablation in liquids (PLAL) by changing the choice of liquid phase. For the characterization we used atomic absorption spectrometer, high resolution electron microscopy, Raman spectroscopy and photoluminescence spectroscopy. Thinner MoSe2 nanosheets resulted from PLAL synthesis in reline compared with those produced in water.

Low-dimensional materials allow us to concentrate light down to atomic dimensions while offering improved strategies for controlling the flow of optical energy at such a scale, opening a fascinating range of applications. However, injection and extraction of externally propagating light into nanosystems remain a challenge because of the large size mismatch of ultraconfined polaritons with respect to the optical wavelength. This in/out-coupling problem in nanophotonics has been addressed following several strategies with mixed degrees of success, including free- and tunneling-electron injection, as well as the mediation of optimized couplers, which we discuss here along with recent results and appealing prospects.

Although silver-based telescope mirrors excel over other materials such as gold and aluminum in the visible-infrared spectral range, they require robust protective coatings to overcome their inherent low durability. Our research shows that a single-layer of aluminum oxide (AlOx) deposited through thermal atomic layer deposition (ALD) using trimethylaluminum (TMA) and water (H2O) at low temperatures (~60°C) serves as an acceptable protection coating without adversely impacting the optical performance of the mirrors. While the use of TMA and H2O as precursors in thermal ALD offers AlOx that performs decently in the field, it degrades quickly in environmental tests under high-humidity at high-temperature conditions, suggesting that there is room to improve. In this paper, two approaches by which ALD processes of AlOx protection coatings can be improved are investigated: exploring another precursor for oxygen and implementing a pre-deposition conditioning. The study is carried out by introducing two new processes – the use of Pure Ozone (PO) and Ozone-Ethylene Radical (OER) in comparison to thermal ALD with TMA and H2O. Our study shows that samples prepared by PO have the initial spectral reflectance lower than that of those prepared by the thermal ALD; however, reflectance of the PO samples remains nearly constant 1.6 times longer in the environmental test, suggesting promising characteristics of AlOx prepared with PO.

Using thermal atomic layer deposition (ALD), with trimethylaluminum (TMA) and water (H2O), it was found that coatings of aluminum oxide (AlOx) deposited on silver-based telescope mirrors operate as viable means to both protect the mirrors from corrosion and reap the benefits of the mirrors while only minimally impacting the mirror’s performance. However, as effective as AlOx coatings are in high temperature/high humidity (HTHH) testing, the mirrors tend to catastrophically fail after an extended period of time during the HTHH test. In order to further improve long-term resilience, the use of Pure Ozone (PO) as an oxygen precursor in replacing H2O is proposed and investigated. While the initial reflectance of the samples prepared with PO is slightly lower than those prepared with H2O, they exhibit better resilience during the HTHH test. In this paper, the study on progressive structural changes that occur in silver-based telescope mirrors protected by AlOx is described over several different stages during cyclic HTHH tests (i.e., temperature is cycled between two, high and low, temperatures). Two types of samples are prepared using either H2O or PO as an oxidation precursor and compared. The study reveals that the initial drop in spectral reflectance of the PO samples during the first HTHH cycle is associated with the surface reconfiguring itself in the presence of the extra energy which effectively tempers the surface to gain higher stability.

Despite the pandemic, recent two years showed significant progress in application of Tip Enhanced Raman Spectroscopy (TERS) and tip enhanced Photoluminescence (TEPL) to nanoscale spectroscopic characterization of lateral and vertical heterostructures of transition metal dichalcogenides (TMDCs). In this talk I'll discuss how TERS imaging revealed existence of the alloyed transition area at the junction of MoS2-WS2 heteromonolayers, it's width varying greatly along the junction line from over 500nm to less then 25nm ( pixel limited resolution in those experiments). Next, I'll demonstrate how ultra low frequency TERS imaging that can directly probe interlayer phonons in twisted vertical heterobilayers of WS2 and WSe2 can sense the layer decoupling in the nanobubbles formed within these heterostructures. In addition, I'll demonstrate that the Stokes/ anti-Stokes ratio of the first and second order modes in TERS spectra of WSe2 and WS2-WSe2 heterostructures can be dramatically different even when those bands are separated by less than 10 cm-1 in spectral space. Possible physical mechanisms of such dramatic difference will be discussed. Finally, I'll demonstrate a crucial importance of varied excitation wavelengths for TERS characterization of TMDCs and their vertical heterostructures. In case of the CVD-grown WS2-WSe2 vertical heterostructures, with 785 nm excitation the characteristic Raman bands of both constituents can be clearly seen, while TERS spectra of the same crystal obtained with the same TERS probe but with 638nm excitation showed greatly suppressed intensity of the WSe2 bands. In addition, I'll show that the TERS spectra of the monolayer WS2 collected with 671 nm excitation, contain a resonant band at 324 cm-1, which is completely absent in TERS spectra of the same crystal collected with 638 or 785 nm excitation, which assumes the presence of the resonance at this wavelength.

I will discuss how symmetry violations provide unique opportunities for nanophotonics, tailoring wave interactions in nanostructures for a wide range of functionalities. In particular, I will discuss broken geometrical symmetries for localized surface polaritons, the physics of moiré photonics, in-plane inversion symmetry breaking for valleytronics and nonradiative state control, time-reversal symmetry breaking for optical nonreciprocity, and parity-time symmetry breaking. Overall, this talk aims to present the role of low symmetry and broken symmetry in controlling nanoscale light and its widespread applications for optical technologies.

Solar-powered photochemical water splitting using suspensions of photocatalyst nanoparticles is an attractive route for economical production of green hydrogen. SrTiO3 based photocatalysts have been intensely investigated due to their stability and recently demonstrated near-100% external quantum yield at wavelengths below 400 nm. To extend the optical absorption into the visible range, SrTiO3 nanoparticles can be alloyed with various transition metals. Here we demonstrate that alloying SrTiO3 nanoparticles with ~1% Rh introduces mid-gap recombination centers that degrade the photocarrier lifetime from ~90 ps to ~1 ps, and lower the maximum achievable external quantum yield by an order of magnitude. By trapping the free conduction band electrons normally introduced by oxygen vacancies, Rh-induced traps change the charge transport mechanism from band to trap-mediated space-charge limited conduction, and drastically reduce the built-in electric fields needed for charge separation. Our results illustrate why and how the solar to hydrogen efficiency of Rh-doped SrTiO3 nanoparticles remains low despite extended optical absorption. Furthermore, the absence of built-in fields within Rh doped SrTiO3 nanoparticles suggests a new mechanism for photocatalytic reactions, where modest e-h separation can be achieved with a difference in mobility between electrons and holes.

Thermal camouflage enables the object to be invisible from infrared detection system, by assimilating their thermal radiation (TR) signal into surroundings. While extremely low TR is required for thermal camouflage of a high temperature object, proposed materials is still limited in engineering emissivity, not controlling surface temperature for TR suppression. Here, we propose a titanium carbide (MXene) based selective emitter (SE) for suppression of TR. With reducing thermal conductivity of MXene, designed SE exhibits low surface temperature even for a few micron thickness. Additionally, with interference effects in multilayer systems, designed SE show radiative cooling effects for additional TR reduction.

Enabled by designer local field patterns and electromagnetic modes, photonic metasurfaces have provided unprecedented control over light wave propagation. We summarize recent developments in light-matter interactions in the strong-field regime in semiconductor (Si, Ge, GaAs, GaP) metasurfaces. We will discuss high-efficiency optical harmonic generation and ultrafast all-optical switching, with an outlook toward novel optoelectronic devices and material platforms.

Optimizing the trade-off between high-speed and energy consumption in today's optoelectronic devices is becoming more complex. Achieving high-speed photonics with heterogeneous material integration requires millimeter-to-centimeter-scale footprints. Search for an electro-optic modulator with high speed, energy economy, and compactness continues. These results were achieved using a 2D material optical modulator integrated on a Silicon photonics platform. A vertical distributed-Bragg-reflector cavity boosts the electro-optic response while reducing the driving voltage by nearly 40 times while retaining modulation depth (5.2 dB/V). Low-power modulators provide high photonic chip density and performance (60 GHz), which is crucial for signal processing and analog and neuromorphic photonic computers. Next, I will share the implementation of the concept of the electrical Scale Length Theory asserts that for high-performance devices with a nominal factor, such as FETs, both the channel thickness and length scale with the nominal factor. When this concept is applied to photodetectors, the gain-bandwidth product (GBP), where is the distance between the electrodes and the route for collecting photo-generated carriers, is equal to the slot width. Here, we experimentally show that a waveguide-integrated, plasmonic MoTe2-based photodetector can efficiently detect light (0.8 A W-1) at 1310 nm at high speed (>30GHz) with 1 V bias voltage. Compact, efficient, and performance photodetectors and modulators are key for next-generation systems with applications in machine intelligence, network edge-processing, data-centers or cyber-security.

This conference presentation was prepared for the Low-Dimensional Materials and Devices 2022 conference at SPIE Optics + Photonics 2022.

This conference presentation was prepared for the Low-Dimensional Materials and Devices 2022 conference at SPIE Optics + Photonics 2022.

This conference presentation was prepared for the Low-Dimensional Materials and Devices 2022 conference at SPIE Optics + Photonics 2022.

This conference presentation was prepared for the Low-Dimensional Materials and Devices 2022 conference at SPIE Optics + Photonics 2022.

This conference presentation was prepared for the Low-Dimensional Materials and Devices 2022 conference at SPIE Optics + Photonics 2022.

This conference presentation was prepared for the Low-Dimensional Materials and Devices 2022 conference at SPIE Optics + Photonics 2022.