Giant Modulation of Refractive Index from Picoscale Atomic Displacements.

It is shown that structural disorder-in the form of anisotropic, picoscale atomic displacements-modulates the refractive index tensor and results in the giant optical anisotropy observed in BaTiS3, a quasi-1D hexagonal chalcogenide. Single-crystal X-ray diffraction studies reveal the presence of antipolar displacements of Ti atoms within adjacent TiS6 chains along the c-axis, and threefold degenerate Ti displacements in the a-b plane. 47/49Ti solid-state NMR provides additional evidence for those Ti displacements in the form of a three-horned NMR lineshape resulting from a low symmetry local environment around Ti atoms. Scanning transmission electron microscopy is used to directly observe the globally disordered Ti a-b plane displacements and find them to be ordered locally over a few unit cells. First-principles calculations show that the Ti a-b plane displacements selectively reduce the refractive index along the ab-plane, while having minimal impact on the refractive index along the chain direction, thus resulting in a giant enhancement in the optical anisotropy. By showing a strong connection between structural disorder with picoscale displacements and the optical response in BaTiS3, this study opens a pathway for designing optical materials with high refractive index and functionalities such as large optical anisotropy and nonlinearity.

The Hydrogen Evolution Activity of BaZrS3, BaTiS3, and BaVS3 Chalcogenide Perovskites.

Chalcogenide perovskites are a class of materials with electronic and optoelectronic properties desirable for solar cells, infrared optics, and computing. The oxide counterparts of these chalcogenides have been studied extensively for their electrocatalytic and photoelectrochemical properties. As chalcogenide perovskites are more covalent, conductive, and stable, we hypothesize that they are more viable as electrocatalysts than oxide perovskites. The goal of this synthetic, experimental, and computational study is to examine the hydrogen evolution reaction (HER) activity of three Barium-based chalcogenides in perovskite and related structures: BaZrS3, BaTiS3, and BaVS3. Potential energy surfaces for hydrogen adsorption on surfaces of these materials are calculated using density functional theory and the computational hydrogen electrode model is used to contrast overpotentials with experiment. Although both experiments and computations agree that BaVS3 is the most active of the three materials, high overpotentials of these materials make them less viable than platinum for HER. Our work establishes a framework for future studies in the chemical and electrochemical properties of chalcogenide perovskites.

A Hybrid Pulsed Laser Deposition Approach to Grow Thin Films of Chalcogenides.

Vapor-pressure mismatched materials such as transition metal chalcogenides have emerged as electronic, photonic, and quantum materials with scientific and technological importance. However, epitaxial growth of vapor-pressure mismatched materials are challenging due to differences in the reactivity, sticking coefficient, and surface adatom mobility of the mismatched species constituting the material, especially sulfur containing compounds. Here, a novel approach is reported to grow chalcogenides-hybrid pulsed laser deposition-wherein an organosulfur precursor is used as a sulfur source in conjunction with pulsed laser deposition to regulate the stoichiometry of the deposited films. Epitaxial or textured thin films of sulfides with variety of structure and chemistry such as alkaline metal chalcogenides, main group chalcogenides, transition metal chalcogenides, and chalcogenide perovskites are demonstrated, and structural characterization reveal improvement in thin film crystallinity, and surface and interface roughness compared to the state-of-the-art. The growth method can be broadened to other vapor-pressure mismatched chalcogenides such as selenides and tellurides. This work opens up opportunities for broader epitaxial growth of chalcogenides, especially sulfide-based thin film technological applications.

Colossal Optical Anisotropy from Atomic-Scale Modulations.

Materials with large birefringence (Δn, where n is the refractive index) are sought after for polarization control (e.g., in wave plates, polarizing beam splitters, etc.), nonlinear optics, micromanipulation, and as a platform for unconventional light-matter coupling, such as hyperbolic phonon polaritons. Layered 2D materials can feature some of the largest optical anisotropy; however, their use in most optical systems is limited because their optical axis is out of the plane of the layers and the layers are weakly attached. This work demonstrates that a bulk crystal with subtle periodic modulations in its structure-Sr9/8 TiS3 -is transparent and positive-uniaxial, with extraordinary index ne = 4.5 and ordinary index no = 2.4 in the mid- to far-infrared. The excess Sr, compared to stoichiometric SrTiS3 , results in the formation of TiS6 trigonal-prismatic units that break the chains of face-sharing TiS6 octahedra in SrTiS3 into periodic blocks of five TiS6 octahedral units. The additional electrons introduced by the excess Sr form highly oriented electron clouds, which selectively boost the extraordinary index ne and result in record birefringence (Δn > 2.1 with low loss). The connection between subtle structural modulations and large changes in refractive index suggests new categories of anisotropic materials and also tunable optical materials with large refractive-index modulation.

Charge Density Wave Order and Electronic Phase Transitions in a Dilute d-Band Semiconductor.

As one of the most fundamental physical phenomena, charge density wave (CDW) order predominantly occurs in metallic systems such as quasi-1D metals, doped cuprates, and transition metal dichalcogenides, where it is well understood in terms of Fermi surface nesting and electron-phonon coupling mechanisms. On the other hand, CDW phenomena in semiconducting systems, particularly at the low carrier concentration limit, are less common and feature intricate characteristics, which often necessitate the exploration of novel mechanisms, such as electron-hole coupling or Mott physics, to explain. In this study, an approach combining electrical transport, synchrotron X-ray diffraction, and density-functional theory calculations is used to investigate CDW order and a series of hysteretic phase transitions in a dilute d-band semiconductor, BaTiS3 . These experimental and theoretical findings suggest that the observed CDW order and phase transitions in BaTiS3 may be attributed to both electron-phonon coupling and non-negligible electron-electron interactions in the system. This work highlights BaTiS3 as a unique platform to explore CDW physics and novel electronic phases in the dilute filling limit and opens new opportunities for developing novel electronic devices.

Remote Oxygen Scavenging of the Interfacial Oxide Layer in Ferroelectric Hafnium-Zirconium Oxide-Based Metal-Oxide-Semiconductor Structures.

Discovery of ferroelectricity in HfO2 has sparked a lot of interest in its use in memory and logic due to its CMOS compatibility and scalability. Devices that use ferroelectric HfO2 are being investigated; for example, the ferroelectric field-effect transistor (FEFET) is one of the leading candidates for next generation memory technology, due to its area, energy efficiency and fast operation. In an FEFET, a ferroelectric layer is deposited on Si, with an SiO2 layer of ∼1 nm thickness inevitably forming at the interface. This interfacial layer (IL) increases the gate voltage required to switch the polarization and write into the memory device, thereby increasing the energy required to operate FEFETs, and makes the technology incompatible with logic circuits. In this work, it is shown that a Pt/Ti/thin TiN gate electrode in a ferroelectric Hf0.5Zr0.5O2 based metal-oxide-semiconductor (MOS) structure can remotely scavenge oxygen from the IL, thinning it down to ∼0.5 nm. This IL reduction significantly reduces the ferroelectric polarization switching voltage with a ∼2× concomitant increase in the remnant polarization and a ∼3× increase in the abruptness of polarization switching consistent with density functional theory (DFT) calculations modeling the role of the IL layer in the gate stack electrostatics. The large increase in remnant polarization and abruptness of polarization switching are consistent with the oxygen diffusion in the scavenging process reducing oxygen vacancies in the HZO layer, thereby depinning the polarization of some of the HZO grains.

Local Epitaxial Templating Effects in Ferroelectric and Antiferroelectric ZrO2.

Nanoscale polycrystalline thin-film heterostructures are central to microelectronics, for example, metals used as interconnects and high-K oxides used in dynamic random-access memories (DRAMs). The polycrystalline microstructure and overall functional response therein are often dominated by the underlying substrate or layer, which, however, is poorly understood due to the difficulty of characterizing microstructural correlations at a statistically meaningful scale. Here, an automated, high-throughput method, based on the nanobeam electron diffraction technique, is introduced to investigate orientational relations and correlations between crystallinity of materials in polycrystalline heterostructures over a length scale of microns, containing several hundred individual grains. This technique is employed to perform an atomic-scale investigation of the prevalent near-coincident site epitaxy in nanocrystalline ZrO2 heterostructures, the workhorse system in DRAM technology. The power of this analysis is demonstrated by answering a puzzling question: why does polycrystalline ZrO2 transform dramatically from being antiferroelectric on polycrystalline TiN/Si to ferroelectric on amorphous SiO2/Si?

Time-resolved photoluminescence studies of perovskite chalcogenides.

Chalcogenides in the perovskite and related crystal structures ("chalcogenide perovskites" for brevity) may be useful for future optoelectronic and energy-conversion technologies inasmuch as they have good excited-state, ambipolar transport properties. In recent years, several studies have suggested that semiconductors in the Ba-Zr-S system have slow non-radiative recombination rates. Here, we present a time-resolved photoluminescence (TRPL) study of excited-state carrier mobility and recombination rates in the perovskite-structured material BaZrS3, and the related Ruddlesden-Popper phase Ba3Zr2S7. We measure state-of-the-art single crystal samples, to identify properties free from the influence of secondary phases and random grain boundaries. We model and fit the data using a semiconductor physics simulation, to enable more direct determination of key material parameters than is possible with empirical data modeling. We find that both materials have Shockley-Read-Hall recombination lifetimes on the order of 50 ns and excited-state diffusion lengths on the order of 5 µm at room temperature, which bodes well for ambipolar device performance in optoelectronic technologies including thin-film solar cells.

Antiferroelectric negative capacitance from a structural phase transition in zirconia.

Crystalline materials with broken inversion symmetry can exhibit a spontaneous electric polarization, which originates from a microscopic electric dipole moment. Long-range polar or anti-polar order of such permanent dipoles gives rise to ferroelectricity or antiferroelectricity, respectively. However, the recently discovered antiferroelectrics of fluorite structure (HfO2 and ZrO2) are different: A non-polar phase transforms into a polar phase by spontaneous inversion symmetry breaking upon the application of an electric field. Here, we show that this structural transition in antiferroelectric ZrO2 gives rise to a negative capacitance, which is promising for overcoming the fundamental limits of energy efficiency in electronics. Our findings provide insight into the thermodynamically forbidden region of the antiferroelectric transition in ZrO2 and extend the concept of negative capacitance beyond ferroelectricity. This shows that negative capacitance is a more general phenomenon than previously thought and can be expected in a much broader range of materials exhibiting structural phase transitions.

Emergent interface vibrational structure of oxide superlattices.

As the length scales of materials decrease, the heterogeneities associated with interfaces become almost as important as the surrounding materials. This has led to extensive studies of emergent electronic and magnetic interface properties in superlattices1-9. However, the interfacial vibrations that affect the phonon-mediated properties, such as thermal conductivity10,11, are measured using macroscopic techniques that lack spatial resolution. Although it is accepted that intrinsic phonons change near boundaries12,13, the physical mechanisms and length scales through which interfacial effects influence materials remain unclear. Here we demonstrate the localized vibrational response of interfaces in strontium titanate-calcium titanate superlattices by combining advanced scanning transmission electron microscopy imaging and spectroscopy, density functional theory calculations and ultrafast optical spectroscopy. Structurally diffuse interfaces that bridge the bounding materials are observed and this local structure creates phonon modes that determine the global response of the superlattice once the spacing of the interfaces approaches the phonon spatial extent. Our results provide direct visualization of the progression of the local atomic structure and interface vibrations as they come to determine the vibrational response of an entire superlattice. Direct observation of such local atomic and vibrational phenomena demonstrates that their spatial extent needs to be quantified to understand macroscopic behaviour. Tailoring interfaces, and knowing their local vibrational response, provides a means of pursuing designer solids with emergent infrared and thermal responses.

Direct Observation and Control of Surface Termination in Perovskite Oxide Heterostructures.

Interfacial behavior of quantum materials leads to emergent phenomena such as quantum phase transitions and metastable functional phases. Probes for in situ and real time surface-sensitive characterization are critical for control during epitaxial synthesis of heterostructures. Termination switching in complex oxides has been studied using a variety of probes, often ex situ; however, direct in situ observation of this phenomena during growth is rare. To address this, we establish in situ and real time Auger electron spectroscopy for pulsed laser deposition with reflection high energy electron diffraction, providing structural and compositional surface information during film deposition. Using this capability, we show the direct observation and control of surface termination in heterostructures of SrTiO3 and SrRuO3. Density-functional-theory calculations capture the energetics and stability of the observed structures, elucidating their electronic behavior. This work demonstrates an exciting approach to monitor and control the composition of materials at the atomic scale for control over emergent phenomena and potential applications.

High frequency atomic tunneling yields ultralow and glass-like thermal conductivity in chalcogenide single crystals.

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.

Linear Dichroism Conversion in Quasi-1D Perovskite Chalcogenide.

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.

Electron Doping BaZrO3 via Topochemical Reduction.

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.

Confined Liquid-Phase Growth of Crystalline Compound Semiconductors on Any Substrate.

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.

Controlled Electrochemical Intercalation of Graphene/h-BN van der Waals Heterostructures.

Electrochemical intercalation is a powerful method for tuning the electronic properties of layered solids. In this work, we report an electrochemical strategy to controllably intercalate lithium ions into a series of van der Waals (vdW) heterostructures built by sandwiching graphene between hexagonal boron nitride (h-BN). We demonstrate that encapsulating graphene with h-BN eliminates parasitic surface side reactions while simultaneously creating a new heterointerface that permits intercalation between the atomically thin layers. To monitor the electrochemical process, we employ the Hall effect to precisely monitor the intercalation reaction. We also simultaneously probe the spectroscopic and electrical transport properties of the resulting intercalation compounds at different stages of intercalation. We achieve the highest carrier density >5 × 1013 cm2 with mobility >103 cm2/(V s) in the most heavily intercalated samples, where Shubnikov-de Haas quantum oscillations are observed at low temperatures. These results set the stage for further studies that employ intercalation in modifying properties of vdW heterostructures.

Analysis of killing of growing cells and dormant and germinated spores of Bacillus species by black silicon nanopillars.

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.

Scalable Indium Phosphide Thin-Film Nanophotonics Platform for Photovoltaic and Photoelectrochemical Devices.

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.