Azimuth (United States)

We demonstrate the facile exfoliation of transition metal dichalcogenides (TMDCs) directly in neat acrylate monomers resulting in readily processable stable colloidal dispersions─bypassing conventional methodologies that require process-limiting solvents and nanomaterial ligand functionalization. As such, we likewise demonstrate the controlled ease of fabricating polymer/TMDC nanocomposite thin films via free radical and anionic polymerization with minimal to no post-exfoliation processing. Our polymer/TMDC nanocomposite films offer tailored effective optical properties (i.e., effective refractive index, neff, and effective extinction coefficient, keff) simply by adjusting the exfoliated TMDC colloidal dispersion concentration prior to thin film fabrication. Our nanocomposite films yield appealingly large changes in effective optical constants of up to ∼Δ2.2 for neff and ∼Δ1.8 for keff (as compared to neat polymer), which contrast is necessary to develop application-relevant optical coating systems. The direct exfoliation of TMDCs in neat monomers and the ease of thin film nanocomposite processing present alternative approaches for high-performance optical coating development with utility in sensing, imaging, and energy harvesting.

Tin hypothiodiphosphate (Sn2P2S6) is a ferroelectric semiconductor with electro-optic and photorefractive relevance. The lattice contains Sn2+ ions and (P2S6)4− anionic groups. In the present work, an Sn2P2S6 crystal is doped with Te during growth. Our results show that the Te ions occupy Sn sites and S sites, where they serve as electron and hole traps, respectively. Electron paramagnetic resonance (EPR) spectra from five trapped holes and one trapped electron are observed at low temperatures after exposure to 633 nm light. Spin-Hamiltonian parameters are extracted from the angular dependence of each spectrum. Holes are localized on (P2TeS5)4− units and form (P2TeS5)3− anions, with the five defects corresponding to Te on different S sites within the anion. These defects have large g shifts caused by the tellurium spin–orbit coupling parameter. Four of the five also have an isotropic hyperfine interaction with one Sn ion, which suggests that they have an adjacent Sn vacancy (leaving just one close Sn neighbor). Three trapped-hole spectra are photoinduced at 20 K and two appear after removing the light, warming to 120 K, and then returning to 20 K. Electrons are trapped in pairs at Te4+ ions occupying Sn2+ sites when the crystal is exposed to 633 nm light while at 20 K. The Te2+ ions that are formed have no observable EPR spectrum. Warming to 100 K releases one electron from Te2+ ions that have an adjacent Sn vacancy, and an EPR spectrum from Te3+ ions (5s25p) is seen when the crystal is returned to lower temperatures.

The synthesis of deuterated, sulfurated, proton-free, glassy polymers offers a route to optical polymers for infrared (IR) optics, specifically for midwave IR (MWIR) photonic devices. Deuterated polymers have been utilized to enhance neutron cross-sectional contrast with proteo polymers for morphological neutron scattering measurements but have found limited utility for other applications. We report the synthesis of perdeuterated d14-(1,3-diisopropenylbenzene) with over 99% levels of deuteration and the preparation of proton-free, perdeuterated poly(sulfur-random-d14-(1,3-diisopropenylbenzene)) (poly(S-r-d14-DIB)) via inverse vulcanization with elemental sulfur. Detailed structural analysis and quantum computational calculations of these reactions demonstrate significant kinetic isotope effects, which alter mechanistic pathways to form different copolymer microstructures for deutero vs proteo poly(S-r-DIB). This design also allows for molecular engineering of MWIR transparency by shifting C-H bond vibrations around 3.3 μm/3000 cm-1 observed in proteo poly(S-r-DIB) to 4.2 μm/2200 cm-1. Furthermore, the fabrication of thin-film MWIR optical gratings made from molding of deuterated-sulfurated, proton-free poly(S-r-d14-DIB) is demonstrated; operation of these gratings at 3.39 μm is achieved successfully, while the proteo poly(S-r-DIB) gratings are opaque at these wavelengths, highlighting the promise of MWIR sensors and compact spectrometers from these materials.

Proper derivation of CH3NH3PbX3 (CH3NH3+ = methyl ammonium or MA+; X- = Cl-, Br-, I-) optical constants is a critical step toward the development of high-performance electronic and optoelectronic perovskite devices. To date, the optical dispersion regimes at, above, and below the band gap of these materials have been inconsistently characterized by omitting or under-approximating anomalous spectral features (from ultraviolet to infrared wavelengths). In this report, we present the rigorous optical dispersion data analysis of single crystal MAPbBr3 involving variable angle spectroscopic ellipsometry data appended with transmission intensity data. This approach yields a more robust derivation of MAPbBr3 optical constants (refractive index, n, and extinction coefficient, k) for both anomalous (absorptance) and normal (no absorptance) optical dispersion regimes. Using the derived optical constants for our MAPbBr3 single crystals, illustrative modeled solar cell device designs are presented in relation to non-realistic designs prepared using representative optical constants reported in the literature to date. In comparison, our derived optical dispersion data enables the modeled design of realistic planar perovskite solar cell (PSC) optical performance where the active layer (MAPbBr3) is optimized for maximum solar radiation absorption. We further demonstrate optimized modeled planar PSC designs with minimal parasitic optical absorptance in non-active PSC device layers resulting in improved performance at broad angles of incidence (approximately 0-70°). Our robust derivation of MAPbBr3 optical properties is expected to impact the optical dispersion data analysis of all perovskite analogs and expedite targeted development of, for example, solar cell, light-emitting diode, photo and X-ray/γ-ray detector, and laser system technologies.

Nonlinear optical effects in epsilon-near-zero materials have become an area of significant interest to the community within the last 10 years. From demonstrations of refractive index tuning, enhanced harmonic generation, and time varying interactions, a wide range of effects and nonlinear properties have been shown to be influenced by the epsilon-near-zero condition. In this talk, we will a higher level look at refractive index tuning in epsilon-near-zero materials (predominately free-carrier-based materials), highlighting why the epsilon-near-zero conditions provides increased index tuning as well as what features are intrinsic to the epsilon-near-zero condition and what features are dictated by external effects. Breaking down the interaction is intended to highlight what properties are difficult to control and what properties can be readily tuned to maximize effects. This is intended to highlight trade-offs in performance and implementation and facilitate discussion related to selecting the best materials, excitation conditions, and designs as epsilon-near-zero materials attempt to transition from laboratory curiosities to potential applications.

Smart electronic circuits that support neuromorphic computing on the hardware level necessitate materials with memristive, memcapacitive, and neuromorphic- like functional properties; in short, the electronic response must depend on the voltage history, thus enabling learning algorithms. Here we demonstrate volatile ferroelectric switching of Sn2P2S6 at room temperature and see that initial polarization orientation strongly determines the properties of polarization switching. In particular, polarization switching hysteresis is strongly imprinted by the original polarization state, shifting the regions of non-linearity toward zero-bias. As a corollary, polarization switching also enables effective capacitive switching, approaching the sought-after regime of memcapacitance. Landau–Ginzburg–Devonshire simulations demonstrate that one mechanism by which polarization can control the shape of the hysteresis loop is the existence of charged domain walls (DWs) decorating the periphery of the repolarization nucleus. These walls oppose the growth of the switched domain and favor back-switching, thus creating a scenario of controlled volatile ferroelectric switching. Although the measurements were carried out with single crystals, prospectively volatile polarization switching can be tuned by tailoring sample thickness, DW mobility and electric fields, paving way to non-linear dielectric properties for smart electronic circuits.

Nb-doped ZnO (NZO) thin films were prepared using reactive magnetron co-sputtering of Nb and Zn sources. Tailoring the infrared optical properties of NZO films can be achieved and applied for desired optical coating applications.

UO2's electrical properties and U's inherently high fission energy release make a UO2-based direct conversion neutron detector a tantalizing possibility, but the evolution of even rudimentary devices is inhibited by the absence of large, high quality single crystals of UO2. Hydrothermally grown UO2 is incorporated into a prototype device here, and laser light is used as a surrogate for neutron-induced fission events. Excitation sources with wavelengths of 325, 488, 532, and 633 nm are incident on the device for a variety of timescales to test the detection responses, resolution, and saturation behaviors. The non-amplified changes in the current are presented, including when the illumination's diameter, power, and location are altered. Photocurrent changes follow a positively correlated linear trend with power and device efficiency is inversely correlated with illumination wavelength. The impact of illumination time on the detection efficiency is discussed. Illumination event detection is independent of illumination surface area size and largely independent of the illumination location. The ramifications of these results with regard to the development of UO2 as a neutron detector are considered.