Recent Development of Halide Perovskite Materials and Devices for Ionizing Radiation Detection.

Ionizing radiation such as X-rays and γ-rays has been extensively studied and used in various fields such as medical imaging, radiographic nondestructive testing, nuclear defense, homeland security, and scientific research. Therefore, the detection of such high-energy radiation with high-sensitivity and low-cost-based materials and devices is highly important and desirable. Halide perovskites have emerged as promising candidates for radiation detection due to the large light absorption coefficient, large resistivity, low leakage current, high mobility, and simplicity in synthesis and processing as compared with commercial silicon (Si) and amorphous selenium (a-Se). In this review, we provide an extensive overview of current progress in terms of materials development and corresponding device architectures for radiation detection. We discuss the properties of a plethora of reported compounds involving organic-inorganic hybrid, all-inorganic, all-organic perovskite and antiperovskite structures, as well as the continuous breakthroughs in device architectures, performance, and environmental stability. We focus on the critical advancements of the field in the past few years and we provide valuable insight for the development of next-generation materials and devices for radiation detection and imaging applications.

Developing a Chemical and Structural Understanding of the Surface Oxide in a Niobium Superconducting Qubit.

Superconducting thin films of niobium have been extensively employed in transmon qubit architectures. Although these architectures have demonstrated improvements in recent years, further improvements in performance through materials engineering will aid in large-scale deployment. Here, we use information retrieved from secondary ion mass spectrometry and electron microscopy to conduct a detailed assessment of the surface oxide that forms in ambient conditions for transmon test qubit devices patterned from a niobium film. We observe that this oxide exhibits a varying stoichiometry with NbO and NbO2 found closer to the niobium film/oxide interface and Nb2O5 found closer to the surface. In terms of structural analysis, we find that the Nb2O5 region is semicrystalline in nature and exhibits randomly oriented grains on the order of 1-3 nm corresponding to monoclinic N-Nb2O5 that are dispersed throughout an amorphous matrix. Using fluctuation electron microscopy, we are able to map the relative crystallinity in the Nb2O5 region with nanometer spatial resolution. Through this correlative method, we observe that the highly disordered regions are more likely to contain oxygen vacancies and exhibit weaker bonds between the niobium and oxygen atoms. Based on these findings, we expect that oxygen vacancies likely serve as a decoherence mechanism in quantum systems.

Direct Patterning of Optoelectronic Nanostructures Using Encapsulated Layered Transition Metal Dichalcogenides.

Direct top-down nanopatterning of semiconductors is a powerful tool for engineering properties of optoelectronic devices. Translating this approach to two-dimensional semiconductors such as monolayer transition metal dichalcogenides (TMDs) is challenging because of both the small scales required for confinement and the degradation of electronic and optical properties caused by high-energy and high-dose electron radiation used for high-resolution top-down direct electron beam patterning. We show that encapsulating a TMD monolayer with hexagonal boron nitride preserves the narrow exciton linewidths and emission intensity typical in such heterostructures after electron beam lithography, allowing direct patterning of functional optical monolayer nanostructures on scales of a few tens of nanometers. We leverage this fabrication method to study size-dependent effects on nanodot arrays of MoS2 and MoSe2 as well as laterally confined electrical transport devices, demonstrating the potential of top-down lithography for nanoscale TMD optoelectronics.

Tuning of Optical Phonons in α-MoO3-VO2 Multilayers.

Merging the properties of VO2 and van der Waals (vdW) materials has given rise to novel tunable photonic devices. Despite recent studies on the effect of the phase change of VO2 on tuning near-field optical response of phonon polaritons in the infrared range, active tuning of optical phonons (OPhs) using far-field techniques has been scarce. Here, we investigate the tunability of OPhs of α-MoO3 in a multilayer structure with VO2. Our experiments show the frequency and intensity tuning of 2 cm-1 and 11% for OPhs in the [100] direction and 2 cm-1 and 28% for OPhs in the [010] crystal direction of α-MoO3. Using the effective medium theory and dielectric models of each layer, we verify these findings with simulations. We then use loss tangent analysis and remove the effect of the substrate to understand the origin of these spectral characteristics. We expect that these findings will assist in intelligently designing tunable photonic devices for infrared applications, such as tunable camouflage and radiative cooling devices.

Mechanistic Investigation of Molybdenum Disulfide Defect Photoluminescence Quenching by Adsorbed Metallophthalocyanines.

Lattice defects play an important role in determining the optical and electrical properties of monolayer semiconductors such as MoS2. Although the structures of various defects in monolayer MoS2 are well studied, little is known about the nature of the fluorescent defect species and their interaction with molecular adsorbates. In this study, the quenching of the low-temperature defect photoluminescence (PL) in MoS2 is investigated following the deposition of metallophthalocyanines (MPcs). The quenching is found to significantly depend on the identity of the phthalocyanine metal, with the quenching efficiency decreasing in the order CoPc > CuPc > ZnPc, and almost no quenching by metal-free H2Pc is observed. Time-correlated single photon counting (TCSPC) measurements corroborate the observed trend, indicating a decrease in the defect PL lifetime upon MPc adsorption, and the gate voltage-dependent PL reveals the suppression of the defect emission even at large Fermi level shifts. Density functional theory modeling argues that the MPc complexes stabilize dark negatively charged defects over luminescent neutral defects through an electrostatic local gating effect. These results demonstrate the control of defect-based excited-state decay pathways via molecular electronic structure tuning, which has broad implications for the design of mixed-dimensional optoelectronic devices.

Spatial Mapping of Electrostatic Fields in 2D Heterostructures.

In situ electron microscopy is an effective tool for understanding the mechanisms driving novel phenomena in 2D structures. However, due to practical challenges, it is difficult to address these technologically relevant 2D heterostructures with electron microscopy. Here, we use the differential phase contrast (DPC) imaging technique to build a methodology for probing local electrostatic fields during electrical operation with nanoscale spatial resolution in such materials. We find that, by combining a traditional DPC setup with a high-pass filter, we can largely eliminate electric fluctuations emanating from short-range atomic potentials. Using a method based on this filtering algorithm, a priori electric field expectations can be directly compared with experimentally derived values to readily identify inhomogeneities and potentially problematic regions. We use this platform to analyze the electric field and charge density distribution across layers of hBN and MoS2.

Valley-selective optical Stark effect of exciton-polaritons in a monolayer semiconductor.

Selective breaking of degenerate energy levels is a well-known tool for coherent manipulation of spin states. Though most simply achieved with magnetic fields, polarization-sensitive optical methods provide high-speed alternatives. Exploiting the optical selection rules of transition metal dichalcogenide monolayers, the optical Stark effect allows for ultrafast manipulation of valley-coherent excitons. Compared to excitons in these materials, microcavity exciton-polaritons offer a promising alternative for valley manipulation, with longer lifetimes, enhanced valley coherence, and operation across wider temperature ranges. Here, we show valley-selective control of polariton energies in WS2 using the optical Stark effect, extending coherent valley manipulation to the hybrid light-matter regime. Ultrafast pump-probe measurements reveal polariton spectra with strong polarization contrast originating from valley-selective energy shifts. This demonstration of valley degeneracy breaking at picosecond timescales establishes a method for coherent control of valley phenomena in exciton-polaritons.

Correction: Topology of transition metal dichalcogenides: the case of the core-shell architecture.

Correction for 'Topology of transition metal dichalcogenides: the case of the core-shell architecture' by Jennifer G. DiStefano et al., Nanoscale, 2020, 12, 23897-23919, DOI: 10.1039/D0NR06660E.

Making the Most of your Electrons: Challenges and Opportunities in Characterizing Hybrid Interfaces with STEM.

Inspired by the unique architectures composed of hard and soft materials in natural and biological systems, synthetic hybrid structures and associated soft-hard interfaces have recently evoked significant interest. Soft matter is typically dominated by fluctuations even at room temperature, while hard matter (which often serves as the substrate or anchor for the soft component) is governed by rigid mechanical behavior. This dichotomy offers considerable opportunities to leverage the disparate properties offered by these components across a wide spectrum spanning from basic science to engineering insights with significant technological overtones. Such hybrid structures, which include polymer nanocomposites, DNA functionalized nanoparticle superlattices and metal organic frameworks to name a few, have delivered promising insights into the areas of catalysis, environmental remediation, optoelectronics, medicine, and beyond. The interfacial structure between these hard and soft phases exists across a variety of length scales and often strongly influence the functionality of hybrid systems. While scanning/transmission electron microscopy (S/TEM) has proven to be a valuable tool for acquiring intricate molecular and nanoscale details of these interfaces, the unusual nature of hybrid composites presents a suite of challenges that make assessing or establishing the classical structure-property relationships especially difficult. These include challenges associated with preparing electron-transparent samples and obtaining sufficient contrast to resolve the interface between dissimilar materials given the dose sensitivity of soft materials. We discuss each of these challenges and supplement a review of recent developments in the field with additional experimental investigations and simulations to present solutions for attaining a nano or atomic-level understanding of these interfaces. These solutions present a host of opportunities for investigating and understanding the role interfaces play in this unique class of functional materials.

Topology of transition metal dichalcogenides: the case of the core-shell architecture.

Non-planar architectures of the traditionally flat 2D materials are emerging as an intriguing paradigm to realize nascent properties within the family of transition metal dichalcogenides (TMDs). These non-planar forms encompass a diversity of curvatures, morphologies, and overall 3D architectures that exhibit unusual characteristics across the hierarchy of length-scales. Topology offers an integrated and unified approach to describe, harness, and eventually tailor non-planar architectures through both local and higher order geometry. Topological design of layered materials intrinsically invokes elements highly relevant to property manipulation in TMDs, such as the origin of strain and its accommodation by defects and interfaces, which have broad implications for improved material design. In this review, we discuss the importance and impact of geometry on the structure and properties of TMDs. We present a generalized geometric framework to classify and relate the diversity of possible non-planar TMD forms. We then examine the nature of curvature in the emerging core-shell architecture, which has attracted high interest due to its versatility and design potential. We consider the local structure of curved TMDs, including defect formation, strain, and crystal growth dynamics, and factors affecting the morphology of core-shell structures, such as synthesis conditions and substrate morphology. We conclude by discussing unique aspects of TMD architectures that can be leveraged to engineer targeted, exotic properties and detail how advanced characterization tools enable detection of these features. Varying the topology of nanomaterials has long served as a potent methodology to engineer unusual and exotic properties, and the time is ripe to apply topological design principles to TMDs to drive future nanotechnology innovation.

Lithography-free IR polarization converters via orthogonal in-plane phonons in α-MoO3 flakes.

Exploiting polaritons in natural vdW materials has been successful in achieving extreme light confinement and low-loss optical devices and enabling simplified device integration. Recently, α-MoO3 has been reported as a semiconducting biaxial vdW material capable of sustaining naturally orthogonal in-plane phonon polariton modes in IR. In this study, we investigate the polarization-dependent optical characteristics of cavities formed using α-MoO3 to extend the degrees of freedom in the design of IR photonic components exploiting the in-plane anisotropy of this material. Polarization-dependent absorption over 80% in a multilayer Fabry-Perot structure with α-MoO3 is reported without the need for nanoscale fabrication on the α-MoO3. We observe coupling between the α-MoO3 optical phonons and the Fabry-Perot cavity resonances. Using cross-polarized reflectance spectroscopy we show that the strong birefringence results in 15% of the total power converted into the orthogonal polarization with respect to incident wave. These findings can open new avenues in the quest for polarization filters and low-loss, integrated planar IR photonics and in dictating polarization control.

Direct Visualization of Electric-Field-Induced Structural Dynamics in Monolayer Transition Metal Dichalcogenides.

Layered transition metal dichalcogenides offer many attractive features for next-generation low-dimensional device geometries. Due to the practical and fabrication challenges related to in situ methods, the atomistic dynamics that give rise to realizable macroscopic device properties are often unclear. In this study, in situ transmission electron microscopy techniques are utilized in order to understand the structural dynamics at play, especially at interfaces and defects, in the prototypical film of monolayer MoS2 under electrical bias. Through our sample fabrication process, we clearly identify the presence of mass transport in the presence of a lateral electric field. In particular, we observe that the voids present at grain boundaries combine to induce structural deformation. The electric field mediates a net vacancy flux from the grain boundary interior to the exposed surface edge sites that leaves molybdenum clusters in its wake. Following the initial biasing cycles, however, the mass flow is largely diminished and the resultant structure remains stable over repeated biasing. We believe insights from this work can help explain observations of nonuniform heating and preferential oxidation at grain boundary sites in these materials.

Spatial Mapping of Hot-Spots at Lateral Heterogeneities in Monolayer Transition Metal Dichalcogenides.

Lateral heterogeneities in atomically thin 2D materials such as in-plane heterojunctions and grain boundaries (GBs) provide an extrinsic knob for manipulating the properties of nano- and optoelectronic devices and harvesting novel functionalities. However, these heterogeneities have the potential to adversely affect the performance and reliability of the 2D devices through the formation of nanoscopic hot-spots. In this report, scanning thermal microscopy (SThM) is utilized to map the spatial distribution of the temperature rise within monolayer transition metal dichalcogenide (TMD) devices upon dissipating a high electrical power through a lateral interface. The results directly demonstrate that lateral heterojunctions between MoS2 and WS2 do not largely impact the distribution of heat dissipation, while GBs of MoS2 appreciably localize heating in the device. High-resolution scanning transmission electron microscopy reveals that the atomic structure is nearly flawless around heterojunctions but can be quite defective near GBs. The results suggest that the interfacial atomic structure plays a crucial role in enabling uniform charge transport without inducing localized heating. Establishing such structure-property-processing correlation provides a better understanding of lateral heterogeneities in 2D TMD systems which is crucial in the design of future all-2D electronic circuitry with enhanced functionalities, lifetime, and performance.

Morphological Engineering of Winged Au@MoS2 Heterostructures for Electrocatalytic Hydrogen Evolution.

Molybdenum disulfide (MoS2) has been recognized as a promising cost-effective catalyst for water-splitting hydrogen production. However, the desired performance of MoS2 is often limited by insufficient edge-terminated active sites, poor electrical conductivity, and inefficient contact to the supporting substrate. To address these limitations, we developed a unique nanoarchitecture (namely, winged Au@MoS2 heterostructures enabled by our discovery of the "seeding effect" of Au nanoparticles for the chemical vapor deposition synthesis of vertically aligned few-layer MoS2 wings). The winged Au@MoS2 heterostructures provide an abundance of edge-terminated active sites and are found to exhibit dramatically improved electrocatalytic activity for the hydrogen evolution reaction. Theoretical simulations conducted for this unique heterostructure reveal that the hydrogen evolution is dominated by the proton adsorption step, which can be significantly promoted by introducing sufficient edge active sites. Our study introduces a new morphological engineering strategy to make the pristine MoS2 layered structures highly competitive earth-abundant catalysts for efficient hydrogen production.

Site-Specific Positioning and Patterning of MoS2 Monolayers: The Role of Au Seeding.

Monolayers of transition metal dichalcogenides (TMDs) are attractive for various modern semiconductor devices. However, the limited control over the location, yield, and size distribution of the products using current synthesis methods has severely limited their large-scale applicability. Herein, we identify the ability to use metal ( e. g., Au) nanoparticles to seed the growth of MoS2 monolayers and thereby provide a means to achieve programmable and controllable synthesis. In this study, prepatterned Au seeds are used as heterogeneous nucleation sites to induce the formation of desired geometries of MoS2 monolayers via chemical vapor deposition. Our experimental and theoretical results shed light on the growth mechanism driving the formation of MoS2 monolayers at these sites, revealing that the seeding effect originates from the favorable formation energy of MoS2 on the Au surface. A field-effect transistor with a predesigned channel geometry exhibits electronic performance that compares nicely with previously reported MoS2 monolayer devices. We believe this study contributes fundamental insights into controlled synthesis of TMD monolayers, making integration of these materials into emerging electronic devices more attainable.

Intrinsic Transport in 2D Heterostructures Mediated through h-BN Tunneling Contacts.

Understanding the electronic transport of monolayer transition metal dichalcogenides (TMDs) and their heterostructures is complicated by the difficulty in achieving electrical contacts that do not perturb the material. Typically, metal deposition on monolayer TMDs leads to hybridization between the TMD and the metal, which produces Schottky barriers at the metal/semiconductor interface. In this work, we apply the recently reported hexagonal boron nitride (h-BN) tunnel contact scheme to probe the junction characteristics of a lateral TMD heterostructure grown via chemical vapor deposition. We first measure the electronic properties across the junction before elucidating optoelectronic generation mechanisms via scanning photocurrent microscopy. We find that the rectification ratio measured using the encapsulated, tunnel contact scheme is almost 2 orders of magnitude smaller than that observed via conventional metal contact geometry, which implies that the metal/semiconductor Schottky barriers play large roles in this aspect. Furthermore, we find that both the photovoltaic as well as hot carrier generation effects are dominant mechanisms driving photoresponse, depending on the external biasing conditions. This work is the first time that this encapsulation scheme has been applied to lateral heterostructures and serves as a reference for future electronic measurements on this material. It also simultaneously serves as a framework to more accurately assess the electronic transport characteristics of 2D heterostructures and better inform future device architectures.

Optically Active 1D MoS2 Nanobelts.

Transition metal dichalcogenides can be synthesized in a wide range of structures. 1D geometries, including nanotubes and nanowires, are especially intriguing due to enhanced light-matter interactions stemming from both the thickness and width possessing subwavelength dimensions. In this letter, we demonstrate the synthesis of 1D MoS2 nanobelts through chemical vapor deposition and examine the mechanism driving the formation of this material. We also report enhanced light scattering within these structures. Finally, we investigate the phototransistor behavior of MoS2 nanobelts and observed a photoresponsivity around 1.5 A/W, an order of magnitude greater than analogous multilayer 2D MoS2 sheets reported previously.

Superior Plasmonic Photodetectors Based on Au@MoS2 Core-Shell Heterostructures.

Integrating plasmonic materials into semiconductor media provides a promising approach for applications such as photosensing and solar energy conversion. The resulting structures introduce enhanced light-matter interactions, additional charge trap states, and efficient charge-transfer pathways for light-harvesting devices, especially when an intimate interface is built between the plasmonic nanostructure and semiconductor. Herein, we report the development of plasmonic photodetectors using Au@MoS2 heterostructures-an Au nanoparticle core that is encapsulated by a CVD-grown multilayer MoS2 shell, which perfectly realizes the intimate and direct interfacing of Au and MoS2. We explored their favorable applications in different types of photosensing devices. The first involves the development of a large-area interdigitated field-effect phototransistor, which shows a photoresponsivity ∼10 times higher than that of planar MoS2 transistors. The other type of device geometry is a Si-supported Au@MoS2 heterojunction gateless photodiode. We demonstrated its superior photoresponse and recovery ability, with a photoresponsivity as high as 22.3 A/W, which is beyond the most distinguished values of previously reported similar gateless photodetectors. The improvement of photosensing performance can be a combined result of multiple factors, including enhanced light absorption, creation of more trap states, and, possibly, the formation of interfacial charge-transfer transition, benefiting from the intimate connection of Au and MoS2.

Au@MoS2 Core-Shell Heterostructures with Strong Light-Matter Interactions.

There are emerging opportunities to harness diverse and complex geometric architectures based on nominal two-dimensional atomically layered structures. Herein we report synthesis and properties of a new core-shell heterostructure, termed Au@MoS2, where the Au nanoparticle is snugly and contiguously encapsulated by few shells of MoS2 atomic layers. The heterostructures were synthesized by direct growth of multilayer fullerene-like MoS2 shell on Au nanoparticle cores. The Au@MoS2 heterostructures exhibit interesting light-matter interactions due to the structural curvature of MoS2 shell and the plasmonic effect from the underlying Au nanoparticle core. We observed significantly enhanced Raman scattering and photoluminescence emission on these heterostructures. We attribute these enhancements to the surface plasmon-induced electric field, which simulations show to mainly localize within the MoS2 shell. We also found potential evidence for the charge transfer-induced doping effect on the MoS2 shell. The DFT calculations further reveal that the structural curvature of MoS2 shell results in a modification of its electronic structure, which may facilitate the charge transfer from MoS2 to Au. Such Au@MoS2 core-shell heterostructures have the potential for future optoelectronic devices, optical imaging, and other energy-environmental applications.