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Excimer materials have been widely studied and have generated significant interest for their applications in many optoelectronic devices. However, a thorough investigation of the entire process involved in excimer formation, enhancement, and annihilation in solid materials is still lacking. Here we designed a crystal based on an anthracene derivative with dissociative, molecular-ordered dimer assembly, and studied the formation and evolution of an anthracene excimer as a function of pressure. During the initial stage of pressurization, the fluorescence intensity arising from anthracene monomers gradually decreases. With continued compression, the two anthracene units become increasingly closer allowing strong intermolecular π–π interactions to develop that lead to excimer formation at 3.5 GPa, accompanied with a phase transition. The fluorescence intensity then keeps increasing with pressure and reaches its maximum at 5.6 GPa due to the strengthening of the excimer and the increased structural defects. Meanwhile the fluorescence color shows a continuous redshift, which initially results from conformation planarization and then excimer evolution. After releasing pressure back to ambient conditions, the structural changes in the sample are reversible, while the fluorescence signal preserves some high-pressure features due to the partial retention of the π–π interactions between the anthracene dimers. This study reveals the evolution of an excimer and its intrinsic photophysical properties, and provides guidance for future research on pressure-sensitive fluorescent devices.

We report the topochemical reduction of epitaxial thin films of the cubic perovskite BaZrO3. Reduction with calcium hydride yields n-type conductivity in the films, despite the wide band gap and low electron affinity of the parent material. X-ray diffraction studies show concurrent loss of out-of-plane texture with stronger reducing conditions. Temperature-dependent transport studies on reduced films show insulating behavior (decreasing resistivity with increasing temperature) with a combination of thermally activated and variable-range hopping transport mechanisms. Time-dependent conductivity studies show that the films are stable over short periods, with chemical changes over the course of weeks leading to an increase in electrical resistance. Neutron reflectivity and secondary ion mass spectrometry indicate that the source of the carriers is most likely hydrogen incorporated from the reducing agent occupying oxygen vacancies and/or interstitial sites. Our studies introduce topochemical reduction as a viable pathway to electron-dope and meta-stabilize low electron affinity and work function materials.

We use time-resolved photoluminescence to study excited-state carrier mobility and recombination rates in BaZrS3 and Ba3Zr2S7, and we refine a semiconductor physics model to the data. Both materials have favorable properties for optoelectronics.

We combine generalized spectroscopic ellipsometry and polarization-resolved infrared micro-spectroscopy to characterize the birefringence and dichroism of several engineered anisotropic A1+xBX3 materials across the infrared wavelength range.

Crystalline solids exhibiting glass-like thermal conductivity have attracted substantial attention both for fundamental interest and applications such as thermoelectrics. In most crystals, the competition of phonon scattering by anharmonic interactions and crystalline imperfections leads to a non-monotonic trend of thermal conductivity with temperature. Defect-free crystals that exhibit the glassy trend of low thermal conductivity with a monotonic increase with temperature are desirable because they are intrinsically thermally insulating while retaining useful properties of perfect crystals. However, this behavior is rare, and its microscopic origin remains unclear. Here, we report the observation of ultralow and glass-like thermal conductivity in a hexagonal perovskite chalcogenide single crystal, BaTiS3, despite its highly symmetric and simple primitive cell. Elastic and inelastic scattering measurements reveal the quantum mechanical origin of this unusual trend. A two-level atomic tunneling system exists in a shallow double-well potential of the Ti atom and is of sufficiently high frequency to scatter heat-carrying phonons up to room temperature. While atomic tunneling has been invoked to explain the low-temperature thermal conductivity of solids for decades, our study establishes the presence of sub-THz frequency tunneling systems even in high-quality, electrically insulating single crystals, leading to anomalous transport properties well above cryogenic temperatures., Defect-free crystals showing the glassy trend of low thermal conductivity with a monotonic increase with temperature are desirable but rare. Here, the authors observe atomic tunneling associated with low and glass-like thermal conductivity in a hexagonal perovskite chalcogenide single-crystal BaTiS3.

We acknowledge support from the National Science Foundation (NSF) under Grant No. 1751736, “CAREER: Fundamentals of Complex Chalcogenide Electronic Materials,” from the MIT Skoltech Program, and from “la Caixa” Foundation MISTI Global Seed Funds. Financial support from the Spanish Ministry of Economy, Competitiveness and Universities, through the “Severo Ochoa” Programme for Centres of Excellence in R&D (Grant No. SEV-2015-0496) and Projects No. MAT2015-73839-JIN (MINECO/FEDER, EU) and No. PID2019-107727RB-I00, and from Generalitat de Catalunya (Grant No. 2017 SGR 1377) is acknowledged. I.F. acknowledges Ramon y Cajal Contract No. RYC-2017- 22531. S.F. acknowledges support from the NSF Graduate Research Fellowship under Grant No. 1122374. The work at Caltech was supported by National Science Foundation Grant No. DMR-1606858. J.R., B.Z., and S.N. acknowledge support from Army Research Office under Award No. W911NF-19- 1-0137 and Air Force Office of Scientific Research under Award No. FA9550-16-1-0335. N.Z.K. and J.M.R. acknowledge support from the U.S. Department of Energy under Grant No. DE-SC0012375 and the DOD-HPCMP for computational resources. N.Z.K. thanks Dr. Michael Waters and Dr. Xuezeng Lu for helpful discussions. S.F. and R.J. acknowledge David Bono and Brian Neltner for helpful discussions and technical assistance.

Functional CsPbI3 perovskite phases are not stable at ambient conditions and spontaneously convert to a non-perovskite δ phase, limiting their applications as solar cell materials. We demonstrate the preservation of a black CsPbI3 perovskite structure to room temperature by subjecting the δ phase to pressures of 0.1 – 0.6 GPa followed by heating and rapid cooling. Synchrotron X-ray diffraction and Raman spectroscopy indicate that this perovskite phase is consistent with orthorhombic γ-CsPbI3. Once formed, γ-CsPbI3 could be then retained after releasing pressure to ambient conditions and shows substantial stability at 35% relative humidity. First-principles density functional theory calculations indicate that compression directs the out-of-phase and in-phase tilt between the [PbI6]4− octahedra which in turn tune the energy difference between δ- and γ-CsPbI3, leading to the preservation of γ-CsPbI3. Here, we present a high-pressure strategy for manipulating the (meta)stability of halide perovskites for the synthesis of desirable phases with enhanced materials functionality., Inorganic lead halide perovskites are structurally unstable, which prevents their application in solar cells. Here the authors synthesize, using high pressure and temperature, a perovskite CsPbI3 phase that is metastably preserved to ambient conditions through a structural deformation induced at high pressure.

Transition metal perovskite chalcogenides, a class of materials with rich tunability in functionalities, are gaining increased attention as candidate materials for renewable energy applications. Perovskite oxides are considered excellent n-type thermoelectric materials. Compared to oxide counterparts, we expect the chalcogenides to possess more favorable thermoelectric properties such as lower lattice thermal conductivity and smaller band gap, making them promising material candidates for high temperature thermoelectrics. Thus, it is necessary to study the thermal properties of these materials in detail, especially thermal stability, to evaluate their potential. In this work, we report the synthesis and thermal stability study of five compounds, \alpha-SrZrS$_3$, \beta-SrZrS$_3$, BaZrS$_3$, Ba$_2$ZrS$_4$, and Ba$_3$Zr$_2$S$_7$. These materials cover several structural types including distorted perovskite, needle-like, and Ruddlesden-Popper phases. Differential scanning calorimeter and thermo-gravimetric analysis measurements were performed up to 1200{\deg}C in air. Structural and chemical characterizations such as X-ray diffraction, Raman spectroscopy, and energy dispersive analytical X-ray spectroscopy were performed on all the samples before and after the heat treatment to understand the oxidation process. Our studies show that perovskite chalcogenides possess excellent thermal stability in air at least up to 600{\deg}C., Comment: 9 Figures

Transition metal perovskite chalcogenides (TMPCs) are explored as stable, environmentally friendly semiconductors for solar energy conversion. They can be viewed as the inorganic alternatives to hybrid halide perovskites, and chalcogenide counterparts of perovskite oxides with desirable optoelectronic properties in the visible and infrared part of the electromagnetic spectrum. Past theoretical studies have predicted large absorption coefficient, desirable defect characteristics, and bulk photovoltaic effect in TMPCs. Despite recent progresses in polycrystalline synthesis and measurements of their optical properties, it is necessary to grow these materials in high crystalline quality to develop a fundamental understanding of their optical properties and evaluate their suitability for photovoltaic application. Here, we report the growth of single crystals of a two-dimensional (2D) perovskite chalcogenide, Ba3Zr2S7, with a natural superlattice-like structure of alternating double-layer perovskite blocks and single-layer rock salt structure. The material demonstrated a bright photoluminescence peak at 1.28 eV with a large external luminescence efficiency of up to 0.15%. We performed time-resolved photoluminescence spectroscopy on these crystals and obtained an effective recombination time of ~65 ns. These results clearly show that 2D Ruddlesden-Popper phases of perovskite chalcogenides are promising materials to achieve single-junction solar cells., 4 Figures

Optical anisotropy is a fundamental building block for linear and nonlinear optical components such as polarizers, wave plates, and phase-matching elements1–4. In solid homogeneous materials, the strongest optical anisotropy is found in crystals such as calcite and rutile5,6. Attempts to enhance anisotropic light–matter interaction often rely on artificial anisotropic micro/nanostructures (form birefringence)7–11. Here, we demonstrate rationally designed, giant optical anisotropy in single crystals of barium titanium sulfide (BaTiS3). This material shows an unprecedented, broadband birefringence of up to 0.76 in the mid- to long-wave infrared, as well as a large dichroism window with absorption edges at 1.6 μm and 4.5 μm for light with polarization along two crystallographic axes on an easily accessible cleavage plane. The unusually large anisotropy is a result of the quasi-one-dimensional structure, combined with rational selection of the constituent ions to maximize the polarizability difference along different axes.

The growth of crystalline compound semiconductors on amorphous and non-epitaxial substrates is a fundamental challenge for state-of-the-art thin-film epitaxial growth techniques. Direct growth of materials on technologically relevant amorphous surfaces, such as nitrides or oxides results in nanocrystalline thin films or nanowire-type structures, preventing growth and integration of high-performance devices and circuits on these surfaces. Here, we show crystalline compound semiconductors grown directly on technologically relevant amorphous and non-epitaxial substrates in geometries compatible with standard microfabrication technology. Furthermore, by removing the traditional epitaxial constraint, we demonstrate an atomically sharp lateral heterojunction between indium phosphide and tin phosphide, two materials with vastly different crystal structures, a structure that cannot be grown with standard vapor-phase growth approaches. Critically, this approach enables the growth and manufacturing of crystalline materials without requiring a nearly lattice-matched substrate, potentially impacting a wide range of fields, including electronics, photonics, and energy devices.

We propose multilayers of perovskite and related ternary oxides with a general formula of ABO 3 as high reflectivity mirrors, especially for use in water window region (2.3–4.4 nm). The high reflectivity combinations of oxide multilayers were deduced using evolutionary search algorithms such as genetic algorithms, and the calculation speed was accelerated via parallel computing methods We also propose a figure of merit for X-ray reflectivity in periodic multilayers systems, which can simplify the future efforts on identifying material combinations, and the search through this multi-dimensional parameter space. The highest reflectivity value was found to be over 33% at 3.1 nm in the water window region. The effect of interface roughness was simulated and the decrease in reflectivity was found to be modest for practically achievable roughness values. This work establishes the foundation for future experimental and theoretical studies towards achieving high reflectivity x-ray mirrors of complex oxide multilayers.

Recent developments in nanophotonics have provided a clear roadmap for improving the efficiency of photonic devices through control over absorption and emission of devices. These advances could prove transformative for a wide variety of devices, such as photovoltaics, photoelectrochemical devices, photodetectors, and light-emitting diodes. However, it is often challenging to physically create the nanophotonic designs required to engineer the optical properties of devices. Here, we present a platform based on crystalline indium phosphide that enables thin-film nanophotonic structures with physical morphologies that are impossible to achieve through conventional state-of-the-art material growth techniques. Here, nanostructured InP thin films have been demonstrated on non-epitaxial alumina inverted nanocone (i-cone) substrates via a low-cost and scalable thin-film vapor-liquid-solid growth technique. In this process, indium films are first evaporated onto the i-cone structures in the desired morphology, followed by a high-temperature step that causes a phase transformation of the indium into indium phosphide, preserving the original morphology of the deposited indium. Through this approach, a wide variety of nanostructured film morphologies are accessible using only control over evaporation process variables. Critically, the as-grown nanotextured InP thin films demonstrate excellent optoelectronic properties, suggesting this platform is promising for future high-performance nanophotonic devices.

We investigate the variation of the band gap across the Ruddlesden-Popper (RP) series $({\mathrm{A}}_{n+1}{\mathrm{B}}_{n}{\mathrm{X}}_{3n+1})$ in model chalcogenide, oxide, and halide materials to understand the factors influencing the band gap evolution with $n$. In contrast to the oxides and halides, we find the band gaps of the chalcogenides evolve differently with the thickness of the perovskite blocks in these natural superlattices. We show that octahedral rotations (i.e., deviation of the B-X-B bond angles from ${180}^{\ensuremath{\circ}})$ and quantum confinement effects compete to decide the band gap evolution of RP phases. The insights gained here will allow us to rationally design layered perovskite phases for electronics and optoelectronics.

We report the topochemical reduction of epitaxial thin films of the cubic perovskite BaZrO

Anisotropic photonic materials with linear dichroism are crucial components in many sensing, imaging, and communication applications. Such materials play an important role as polarizers, filters, and waveplates in photonic devices and circuits. Conventional crystalline materials with optical anisotropy typically show unidirectional linear dichroism over a broad wavelength range. The linear dichroism conversion phenomenon has not been observed in crystalline materials. The investigation of the unique linear dichroism conversion phenomenon in quasi-1D hexagonal perovskite chalcogenide BaTiS3 is reported. This material shows a record level of optical anisotropy within the visible wavelength range. In contrast to conventional anisotropic optical materials, the linear dichroism polarity in BaTiS3 makes an orthogonal change at an optical wavelength corresponding to the photon energy of 1.78 eV. First-principles calculations reveal that this anomalous linear dichroism conversion behavior originates from the different selection rules of the parallel energy bands in the BaTiS3 material. Wavelength-dependent polarized Raman spectroscopy further confirms this phenomenon. Such a material, with linear dichroism conversion properties, could facilitate the sensing and control of the energy and polarization of light, and lead to novel photonic devices such as polarization-wavelength selective detectors and lasers for multispectral imaging, sensing, and optical communication applications.

Perovskite chalcogenides are gaining substantial interest as an emerging class of semiconductors for optoelectronic applications. High-quality samples are of vital importance to examine their inherent physical properties. We report the successful crystal growth of the model system, BaZrS3 and its Ruddlesden–Popper phase Ba3Zr2S7 by a flux method. X-ray diffraction analyses showed the space group of Pnma with lattice constants of a = 7.056(3) Å, b = 9.962(4) Å, and c = 6.996(3) Å for BaZrS3 and P42/mnm with a = 7.071(2) Å, b = 7.071(2) Å, and c = 25.418(5) Å for Ba3Zr2S7. Rocking curves with full width at half maximum of 0.011° for BaZrS3 and 0.027° for Ba3Zr2S7 were observed. Pole figure analysis, scanning transmission electron microscopy images, and electron diffraction patterns also establish the high quality of the grown crystals. The octahedral tilting in the corner-sharing octahedral network is analyzed by extracting the torsion angles., Air Force Office of Scientific Research (Award FA9550-16-1-0335), Army Research Office (Award W911NF-19-1- 0137)

Mid-wave infrared (IR) and long-wave IR spectral ranges are of growing interest in various applications such as thermal imaging, thermography-based remote sensing, and night vision. Materials widely used for IR photodetectors in this regime include cadmium mercury telluride alloys and nanostructures of compound semiconductor. The materials development for IR optics will drive down the cost of IR optical systems and enable larger scale deployment. Here, we report a mid-wave IR responsive material composed of earth abundant and non-toxic elements, Sr1+xTiS3. It has a highly anisotropic quasi-one-dimensional structure similar to hexagonal perovskites. We grew large, high quality single crystals and studied its anisotropic optical properties. We observed two distinct optical absorption edges at ~2.5 um and ~5 um, respectively, for linear polarizations along two principal axes. The material demonstrated strong and broadband linear dichroism spanning mid-wave IR and long-wave IR, with a dichroitic ratio of up to 22., 4 Figures

Black silicon (bSi) wafers with a high density of high-aspect ratio nanopillars have recently been suggested to have mechanical bactericidal activity. However, it remains unclear whether bSi with the nanopillars can kill only growing bacterial cells or also dormant spores that are harder to kill. We have reexamined the cidal activity of bSi on growing cells, dormant and germinated spores of B. subtilis, and dormant spores of several other Bacillus species by incubation on bSi wafers with and without nanopillars. We found that the bSi wafers with nanopillars were indeed very effective in rupturing and killing the growing bacterial cells, while wafers without nanopillars had no bactericidal effect. However, bSi wafers with or without nanopillars gave no killing or rupture of dormant spores of B. subtilis, Bacillus cereus or Bacillus megaterium, although germinated B. subtilis spores were rapidly killed. This work lays a foundation for novel bactericidal applications of bSi by elucidating the limits of mechanical bactericidal approaches.

The authors report the epitaxial growth and the electrical properties, especially the metal-to-insulator transition, of vanadium dioxide (VO2) thin films synthesized on [LaAlO3]0.3[Sr2AlTaO6]0.7 (LSAT) (111) (LSAT) substrates by pulsed laser deposition. X-ray diffraction studies show that the epitaxial relationship between the VO2 thin films and LSAT substrate is given as VO2(020)||LSAT(111) and V O 2 [ 001 ] | | LSAT [ 11 2 ¯ ]. The authors observed a sharp 4 orders of magnitude change in the longitudinal resistance for the VO2 thin films around the transition temperature. The authors also measured distinct Raman spectra below and above the transition point indicating a concomitant structural transition between the insulator and metallic phases, in agreement with past investigations.