Aluminum thin films are often utilized for UV applications where other metals like gold and silver are ineffective, but the efficiency of Al in structures like mirror coatings and bandpass filters can be limited by the presence of native oxide. Previous work has investigated the development of thermal atomic layer etching (ALE) methods to remove this native oxide prior to subsequent encapsulation with fluoride dielectric materials deposited by atomic layer deposition (ALD). This ALE process has involved cyclic exposure to trimethylaluminum and HF in the presence of alkali materials which has been shown to enhance of the oxide removal efficacy at a given substrate temperature. In this current work we investigate ALE co-reaction with cesium fluoride which is shown to enable aluminum oxide etching at temperatures as low as 125 °C and reduce the amount of etch damage experienced by the Al layer. We report on the fabrication of broadband protected Al mirror coatings using this ALE/ALD approach and summarize ongoing and future NASA astrophysics missions that will benefit from this development.

Silver (Ag) and aluminum (Al) are common metal-bases for broadband telescope mirrors, but suffer from low durability. Ag, while highly reflective in the visible range, is readily corroded necessitating a protective coating like aluminum oxide (AlOx) that impairs the blue-UV response. Similarly, Al performs well from 90-2000 nm, but requires a protective coating like aluminum fluoride (AlFx), as the metal-base is readily oxidized. Optimizing the metal/dielectric interfaces of AlOx/Ag and AlFx/Al, then are necessary to create durable high-performing mirrors. Current manufacturing methods employ physical vapor deposition (PVD) and atomic layer deposition (ALD) independently; introducing failure points at the metal/dielectric interface. The study examines using sputtering atomic layer augmented deposition (SALAD) as a solution by seamlessly integrating the capabilities of PVD and ALD without breaking vacuum. Further, a focus is placed on multi-physics modeling, environmental testing, characterization, and assessment of degradation mechanisms.

Traditional atomic layer deposition (ALD) systems, designed for small ~200 mm substrates, limit ALD's application in astronomical instrumentation. Our previous work introduced a meter-scale ALD (MSALD) system, accommodating larger ~900 mm substrates. Utilizing the MSALD system, we prepared aluminum oxide (AlOx) to protect silver-based telescope mirrors, demonstrating scalable ALD processes with optimized parameters. This opened avenues for diverse materials like aluminum in telescope mirror applications. Our current investigation explores the MSALD systems’ potential to create aluminum fluoride (AlFx) protection coatings for aluminum-based telescope mirrors operated in the far UV spectral range. Unlike traditional AlOx, ALD of AlFx is uncommon, posing new challenges for achieving a uniform 2 nm coating due to the oxidation of Al surfaces. Our study, along with MSALD system modifications, yields crucial insights into scaling ALD for AlFx coatings, offering unique solutions to enhance Al-mirrors' performance and durability.

Silver (Ag) excels as a metal-base for astronomical mirrors for its high reflectivity across the visible to infrared spectral range. However, Ag degrades quickly in an observatory environment, necessitating a protective coating like aluminum oxide (AlOx). Our study compares using water (H2O) and high-purity ozone (PO) as oxygen precursors for AlOx a protective coating on Ag using low-temperature atomic layer deposition (ALD). At ~80% purity, PO allows for higher quality films compared to that of H2O, while offering a reduced deposition time. After enduring high humidity high temperature (HTHH) testing, H2O samples showed a substantial reduction in reflectivity (~30%), while PO samples boasted a minimal reflectivity reduction (~12%). Ellipsometry revealed a 74 nm phase shift, compared to a 6 nm shift for H2O and PO respectively; indicating improved structural integrity. AFM and EDS analysis revealed H2O samples underwent erratic structural changes compromising integrity, while PO samples showed minimal structural change.

Analog neuromorphic computing hardware, despite their application-specific energy efficiency, are not easily reconfigured for generality, thus impeding them from competing with entrenched general purpose digital processors. Here we address this fundamental limitation by enabling a single neuromorphic component to be functionally reconfigured to express neuronal, synaptic, interconnect and switching behaviors. Via precise voltage-controlled on-chip injection of oxygen vacancy defects into VO2, a Mott insulator, we nucleate and stabilize phase coexistence across various oxides of vanadium, much like spinodal decomposition of a water-oil mixture, and tune the overall phase transition properties. Such phase coexistence is not possible in purely electronic components, and has not been achieved electrochemically under normal chip operating conditions. Using electrochemically controlled phase coexistence, we demonstrate both functionally tunable computing, such as reconfigurable logic gates, and also unusually stable information retention, with 1% loss over 14 years in ambient conditions. On-chip defect-tuned phase coexistence paves the path for functionally dense and dynamically reconfig

We present the fabrication and operation of GaN vacuum electron nanodiodes operating by field emission and in air. The devices exhibit low turn-on voltage, high field emission current, and excellent radiation hardness. Experimental and modeling results on the characteristics of these devices at various nanogap sizes, operating pressures, temperature, and radiation environments are discussed. Preliminary results on the fabrication and characteristics of lateral GaN nano vacuum transistors will also be shown. Sandia National Laboratories is managed and operated by NTESS under DOE NNSA contract DE-NA0003525.

We present results on III-V semiconductor nanowires and metal halide perovskite nanowires. Focus is on optical and electrical design and characterization, especially of nanowire arrays for optoelectronic applications. Emphasis is given to the strong impact of material and geometry choice on device performance. We give examples of how electromagnetic optics modelling and drift-diffusion electrical modelling can be used to aid in both device design and analysis of characterization results.

We discuss the fabrication and characterization of flexible piezoelectric motion sensors (PMSs) with single-electrode (SE) configuration. As a response medium of the devices, InN nanowires (NWs), formed on Si(111) substrates by a plasma-assisted molecular beam epitaxy, were used. For comparison, the PMSs with two electrodes (TEs) were prepared. The performances of the PMSs were analyzed depending on device parameters such as bending frequency, operation time, relative humidity, and bending cycle. For example, the output voltage of the SE-PMS with the InN NWs aligned along the bending direction was measured to be 10.65 V, which is higher than that (7.1 V) of the TE device. To evaluate the possibility for real-life applications, the flexible PMSs were attached to a human finger and their output signals were measured depending on the movement angle.

The continuous research on electronics, biocompatible materials and nanomaterials has led to the design of a new generation of wearable devices that can be employed in direct contact with the body of the user, which is attractive for real-time, non-invasive health monitoring. For the satisfaction of such requirements, hydrogel-based conductive devices are often proposed as promising candidates for these applications, thanks to their softness, flexibility, and biocompatibility. Here we report the synthesis of conductive hybrid hydrogels containing two-dimensional (2D) MoS2. The nanoflakes are integrated in the polymeric matrix creating an anisotropic structure, which helps to generate mismatch stress for a strain sensing under a certain stimulus, thus allowing the gel to give an electrical response to pressure.

Colloidal quantum dots (QDs) integrated Micro-LEDs are key technologies for next-generation displays such as AR/VR. However, fundamental scientific issues such as the low internal quantum efficiency (IQE) of Micro-LEDs and low quantum dots light conversion efficiency (LCE) remain to be resolved for the commercialization of this technology. In this talk, the impact of Micro-LED Epi and chip designs as well as GaN free-standing substrates on the improvement of IQE of Micro-LEDs will be reported. Mechanisms of proposed 0D–2D hybrid optical scatterers for improving the LCE of QDs will also be discussed.

Micro light-emitting diodes (μ-LEDs) coupled with color conversion phosphors are among the most promising technologies for future display and artificial light sources. Here, we demonstrate down-converting μ-LED phosphors based on CsPbBr3 perovskite nanocrystals grown directly within perfectly sealed mesoporous silica nanospheres (NSs). Through a selective sintering technology, we can synthesize CsPbBr3/SiO2 NSs with emission efficiency >87% at high temperatures (>500oC) without causing interparticle cross-linking or aggregation. The resulting CsPbBr3-SiO2 NSs with uniform size, good solution dispersion, ultra-stable, and high brightness meet the technical requirements of photolithographic inks and can achieve highly uniform μ-LED color conversion patterns with pixels smaller than 20 μm.

We report a reversible three-state and dual color photoluminescence (PL) intensity modulation of quantum dots (QDs) by electrochemically applying voltages on the Prussian blue (PB) substrate. PB acts as the electro-switchable materials because the applied voltage controls the oxidation state of iron ions in PB. Depending on the oxidation states of iron ions and their redox potential, the charge transfer from QDs to PB can be allowed or blocked, acting as a main mechanism of PL intensity modulation of QDs in multistate. Engineering heterostructures of QDs give rise to additional controllability of PL intensity modulation. The CdS shell on top of CdSe core QDs acts as a hole blocking layer, whereas the ZnSe shell acts as an electron blocking layer. With the combination of the applied voltage and its core/shell heterostructure, we could selectively quench or recover PL intensity of two different QDs which gives dual color tunability.

In the past years, we have been working with "self-assembly" --- Self-assembly of atoms, molecules and nanomaterials on two-dimensional (2D) surface. For 2D self-assembly, asymmetric interface is required such as air-liquid, gas-solid and liquid-solid interfaces, even for the case of nanomaterials. Self-assembly of metal nanoparticles revealed great potential for both fundamental and application via coupling of localized plasmons. Regularly aligned small sized metal nanoparticles (< 10 nm) with nanogap can excite collective mode on their surface. It results in extremely high refractive index and extinction coefficient, regarded as metamaterials (metasurfaces), and exhibits quite unique optical properties. One example is "Electromagnetically induced transparency (EIT)" induced by multilayered metal nanoparticles on mirror and brought up vivid metallic full-colors as a result of LSPR band splitting. In this talk, 2D self-assembly of luminescent QDs such as CdSe/ZnS and perovskite nanocrystals are also introduced with their unique optical properties.

Metal halide perovskite solar cells have achieved efficiencies exceeding 26%, at par with crystalline Silicon. However, concerns of long-term stability and open questions about upscaled manufacturing persist. I will show how atomic layer deposition (ALD) can unlock further progress towards increased efficiency and long-term stability. Permeation barriers prepared by ALD as integral part of the device architecture suppress thermally driven decomposition of the perovskite and inhibit detrimental diffusion of halide species [1]. At the same time, ALD enables novel processing options for the preparation of semitransparent cells [2] and ultra-thin loss-less interconnects for tandem architectures [3] with the prospects to reach efficiency levels beyond 30% [4]. As ALD is originally a vacuum-based batch-processing technique, I will address the prospects of upscaling ALD for high-throughput manufacturing by the introduction of spatial ALD (S-ALD). [1] K. O. Brinkmann et al., Nat. Comms. 2017, 8, 13938. [2] T. Gahlmann et al., Adv. Energy Mater. 10, 1903. [3] K. O. Brinkmann et al., Nature 604, 280 (2022). [4] K.O. Brinkmann et al. Nat. Rev. Mater. DOI: 10.1038/s41578-023-00642-1.

This study proposes a novel approach to streamline the manual identification and classification of 2D materials on-chip devices, crucial for rapid prototyping. Leveraging high-resolution imaging and smart stitching techniques, our method achieves a comprehensive representation of the material landscape. Advanced image processing algorithms, including mask-RCNN segmentation, extract key material attributes such as surface area and morphology. A tailored U-Net model is trained for precise material identification, encompassing parameters like composition and thickness. Performance evaluation involves state-of-the-art model architectures and hyperparameter optimization. By automating the material identification process and integrating with a sophisticated transfer system, manual intervention is minimized, expediting prototyping workflows. This framework not only enhances efficiency but also aligns with contemporary trends in materials science and machine learning research, fostering advancements in rapid prototyping capabilities.

In the course of our study, we characterized MoSSe and MoSeS Janus materials derived from MoS2 and MoSe2 correspondingly. Preliminary Raman characterization of the as-synthesized crystal performed in a single point with the single excitation wavelength (532nm) showed weak Raman peaks of the precursors, what was misinterpreted as incomplete conversion. SPM and TERS imaging revealed that the precursor monolayers featured multiple nanoscale multi-layer islands. These islands have been identified in the Janus crystals transferred to Au or Ag via the SPM imaging and their composition was confirmed by TERS imaging. The morphology of the Janus crystals derived from MoS2 and MoSe2 was also fundamentally different. In the course of conversion of MoSe2 to MoSeS accumulated tensile strain led to physical breakage of the crystals. TERS showed that the gaps between the domains in MoSeS monolayers seen in SPM images were physical cracks. Conversely, compressive strain appearing in MoSSe results in the formation of wrinkles that after the transfer to Au or Ag look like cracks in MoSeS, but in reality there is no physical breakage in these crystals.

Interest in low-dimensional magnetic systems surged with the discovery of magnetism in some quasi-2D materials. While ferro- and ferrimagnetic compounds gained attention, antiferromagnetic semiconductors like Metal-transition phospho-trichalcogenides (MPX3) remained less studied. MPX3s are quasi-2D van der Waals semiconductors with diverse antiferromagnetic spin configurations. In this talk, I will describe the properties of acoustic phonons in such materials probed by Brillouin-Mandelstam inelastic light scattering. Acoustic phonons carry heat and contribute to electron–phonon, and magnon–phonon scattering processes. We observed significant variations in acoustic phonon velocities among materials with similar structures. Correlations with available thermal transport data underscore the importance of our findings for understanding layered vdW semiconductors. Authors acknowledge the NSF DMR project No. 2205973 “Controlling Electron, Magnon, and Phonon States in Quasi‐2D Antiferromagnetic Semiconductors for Enabling Novel Device Functionalities” and NSF MRI project No. 2019056 “Development of a Cryogenic Integrated Micro-Raman-Brillouin-Mandelstam Spectrometer.”

Recent advancements have positioned van der Waals heterostructures as a platform for developing optical and optoelectronic devices with unprecedented properties. This talk focuses on our recent research investigating interaction among excitons in two-dimensional materials, exploring possible realization of macroscopic quantum coherent states and applications to nonlinear optics. I will describe the observation of the giant excitonic optical nonlinearity facilitated by exciton-hole interactions. I will also introduce our recent efforts to achieve tightly bound interlayer excitons with low disorder, which lays the groundwork for forming exciton condensates. Finally, we will discuss how these findings have substantial implications for advancing classical and quantum optical information processing and communication.

The bright emission from thick flakes makes gallium selenide a fantastic material for understanding the relationship between local strain and optical response. Here, we investigate complex strain distributions by transferring gallium selenide flakes onto nanostructures patterned in close proximity, enabling the study of a variety of strain distributions, such as uniaxial, biaxial, and triaxial strain within a single flake. Our findings reveal that finite strain distributions and resulting bandgap shifts occur in regions of gallium selenide suspended between closely-spaced nanostructures, in good agreement with strain distributions simulated using finite element analysis. This research paves the way for designer strain distributions and tailorable nanophotonic behavior in two-dimensional materials.

Beyond-silicon technology demands ultrahigh-performance field-effect transistors (FETs). Transition metal dichalcogenides (TMDs) provide an ideal material platform, but the device performances such as contact resistance, on/off ratio, and mobility are often limited by the presence of interfacial residues caused by transfer procedures. Here, we show an ideal residue-free transfer approach using polypropylene carbonate (PPC) with a negligible residue coverage of ~0.08% for monolayer MoS2 in the centimeter scale. By incorporating bismuth semimetal contact with atomically-clean monolayer MoS2-FET on h-BN substrate, we obtain an ultralow Ohmic contact resistance RC of ~78 Ω-µm, approaching the quantum limit, and a record-high on/off ratio of ~10^11 at 15 K. Such an ultraclean fabrication approach could be the ideal platform for high-performance electrical devices using large-area semiconducting TMDs.

A key signature of topological boundary modes, namely a non-local response, has remained elusive in topological superconductors(TSC). Here we focus on 1D higher-order TSC (HOTSC) chiral modes in FeTe0.55Se0.45, demonstrating they mediate electron co-tunneling (EC) over macroscopic distances, owing to the topological-protection long-term coherence. We found the hinge-mediated EC emerges in an anomalous large range, and produces a robust conductance plateau. Meanwhile, such plateau is robust against increasing temperature and magnetic field, and disappear without hinge contact or topological nontrivial phase of materials. Thus, our experiments reveal the first proof of robust, long-range, and non-local response from the 1D chiral hinge modes in \ch{FeTe_{0.55}Se_{0.45}}, providing a new methodology to explore TSC.