The Case for a Defect Genome Initiative.

The Materials Genome Initiative (MGI) has streamlined the materials discovery effort by leveraging generic traits of materials, with focus largely on perfect solids. Defects such as impurities and perturbations, however, drive many attractive functional properties of materials. The rich tapestry of charge, spin, and bonding states hosted by defects are not accessible to elements and perfect crystals, and defects can thus be viewed as another class of "elements" that lie beyond the periodic table. Accordingly, a Defect Genome Initiative (DGI) to accelerate functional defect discovery for energy, quantum information, and other applications is proposed. First, major advances made under the MGI are highlighted, followed by a delineation of pathways for accelerating the discovery and design of functional defects under the DGI. Near-term goals for the DGI are suggested. The construction of open defect platforms and design of data-driven functional defects, along with approaches for fabrication and characterization of defects, are discussed. The associated challenges and opportunities are considered and recent advances towards controlled introduction of functional defects at the atomic scale are reviewed. It is hoped this perspective will spur a community-wide interest in undertaking a DGI effort in recognition of the importance of defects in enabling unique functionalities in materials.

Recent Advances in 2D Material Theory, Synthesis, Properties, and Applications.

Two-dimensional (2D) material research is rapidly evolving to broaden the spectrum of emergent 2D systems. Here, we review recent advances in the theory, synthesis, characterization, device, and quantum physics of 2D materials and their heterostructures. First, we shed insight into modeling of defects and intercalants, focusing on their formation pathways and strategic functionalities. We also review machine learning for synthesis and sensing applications of 2D materials. In addition, we highlight important development in the synthesis, processing, and characterization of various 2D materials (e.g., MXnenes, magnetic compounds, epitaxial layers, low-symmetry crystals, etc.) and discuss oxidation and strain gradient engineering in 2D materials. Next, we discuss the optical and phonon properties of 2D materials controlled by material inhomogeneity and give examples of multidimensional imaging and biosensing equipped with machine learning analysis based on 2D platforms. We then provide updates on mix-dimensional heterostructures using 2D building blocks for next-generation logic/memory devices and the quantum anomalous Hall devices of high-quality magnetic topological insulators, followed by advances in small twist-angle homojunctions and their exciting quantum transport. Finally, we provide the perspectives and future work on several topics mentioned in this review.

Quantum Materials Manufacturing.

The quantum age is just around the corner. As quantum systems become more stable, robust, and mainstream, tackling the challenge of high-throughput manufacturing will require further developments in materials synthesis, characterization, assembly, and diagnostics. As the building blocks of future technologies scale down to atomic and molecular scales, a paradigm shift in manufacturing will begin to take shape. Inspired by a quantum manufacturing world that elevates the Materials Genome Initiative to the next level, a "human-in-the-loop" framework for high-throughput manufacturing, which addresses key opportunities and challenges to be overcome, is outlined.

Plasmon-Induced Enhanced Light Emission and Ultrafast Carrier Dynamics in a Tunable Molybdenum Disulfide-Gallium Nitride Heterostructure.

The effect of localized plasmon on the photoemission and absorption in hybrid molybdenum disulfide-Gallium nitride (MoS2-GaN) heterostructure has been studied. Localized plasmon induced by platinum nanoparticles was resonantly coupled to the bandedge states of GaN to enhance the UV emission from the hybrid semiconductor system. The presence of the platinum nanoparticles also increases the effective absorption and the transient gain of the excitonic absorption in MoS2. Localized plasmons were also resonantly coupled to the defect states of GaN and the exciton states using gold nanoparticles. The transfer of hot carriers from Au plasmons to the conduction band of MoS2 and the trapping of excited carriers in MoS2 within GaN defects results in transient plasmon-induced transparency at ~1.28 ps. Selective optical excitation of the specific resonances in the presence of the localized plasmons can be used to tune the absorption or emission properties of this layered 2D-3D semiconductor material system.

Wafer-Scale Lateral Self-Assembly of Mosaic Ti3C2Tx MXene Monolayer Films.

Bottom-up assembly of two-dimensional (2D) materials into macroscale morphologies with emergent properties requires control of the material surroundings, so that energetically favorable conditions direct the assembly process. MXenes, a class of recently developed 2D materials, have found new applications in areas such as electrochemical energy storage, nanoscale electronics, sensors, and biosensors. In this paper, we present a lateral self-assembly method for wafer-scale deposition of a mosaic-type 2D MXene flake monolayer that spontaneously orders at the interface between two immiscible solvents. ReaxFF molecular dynamics simulations elucidate the interactions of a MXene flake with the solvents and its stability at the liquid/liquid interface, the prerequisite for MXene flakes self-assembly at the interface. Moreover, facile transfer of this monolayer onto a flat substrate (Si, glass) results in high-coverage monolayer films with uniform thickness and homogeneous optical properties. Multiscale characterization of the resulting films reveals the mosaic structure and sheds light on the electronic properties of the films, which exhibit good electrical conductivity over cm-scale areas.

In Vivo Partial Restoration of Neural Activity across Severed Mouse Spinal Cord Bridged with Ultralong Carbon Nanotubes.

Electrically bridging severed nerves in vivo has transformative healthcare implications, but current materials are inadequate. Carbon nanotubes (CNTs) are promising, with low impedance, high charge injection capacity, high flexibility, are chemically inert, and can electrically couple to neurons. Ultralong CNTs are unexplored for neural applications. Using only ultralong CNTs in saline, without neuroregeneration or rehabilitation, we partially restored neural activity across a severed mouse spinal cord, recovering 23.8% of the intact amplitude, while preserving signal shape. Neural signals are preferentially facilitated over artifact signals by a factor of ×5.2, suggesting ultralong CNTs are a promising material for neural-scaffolding and neural-electronics applications.

Nonlinear Dark-Field Imaging of One-Dimensional Defects in Monolayer Dichalcogenides.

One-dimensional defects in two-dimensional (2D) materials can be particularly damaging because they directly impede the transport of charge, spin, or heat and can introduce a metallic character into otherwise semiconducting systems. Current characterization techniques suffer from low throughput and a destructive nature or limitations in their unambiguous sensitivity at the nanoscale. Here we demonstrate that dark-field second harmonic generation (SHG) microscopy can rapidly, efficiently, and nondestructively probe grain boundaries and edges in monolayer dichalcogenides (i.e., MoSe2, MoS2, and WS2). Dark-field SHG efficiently separates the spatial components of the emitted light and exploits interference effects from crystal domains of different orientations to localize grain boundaries and edges as very bright 1D patterns through a Cerenkov-type SHG emission. The frequency dependence of this emission in MoSe2 monolayers is explained in terms of plasmon-enhanced SHG related to the defect's metallic character. This new technique for nanometer-scale imaging of the grain structure, domain orientation and localized 1D plasmons in 2D different semiconductors, thus enables more rapid progress toward both applications and fundamental materials discoveries.

Evidence of a purely electronic two-dimensional lattice at the interface of TMD/Bi2Se3 heterostructures.

When 2D materials are vertically stacked, new physics emerges from interlayer orbital interactions and charge transfer modulated by the additional periodicity of interlayer atomic registry (moiré superlattice). Surprisingly, relatively little is known regarding the real-space distribution of the transferred charges within this framework. Here we provide the first experimental indications of a real-space, non-atomic lattice formed by interlayer coupling induced charge redistribution in vertically stacked Bi2Se3/transition metal dichalcogenide (TMD) 2D heterostructures. Robust enough to scatter 200 keV electron beams, this non-atomic lattice generates selected area diffraction patterns that correspond excellently with simulated patterns from moiré superlattices of the parent crystals suggesting their location at sites of high interlayer atomic registry. Density functional theory (DFT) predicts concentrated charge pools reside in the interlayer region, located at sites of high nearest-neighbor atomic registry, suggesting the non-atomic lattices are standalone, reside in the interlayer region, and are purely electronic.

Oxygen-Induced In Situ Manipulation of the Interlayer Coupling and Exciton Recombination in Bi2Se3/MoS2 2D Heterostructures.

Two-dimensional (2D) heterostructures are more than a sum of the parent 2D materials, but are also a product of the interlayer coupling, which can induce new properties. In this paper, we present a method to tune the interlayer coupling in Bi2Se3/MoS2 2D heterostructures by regulating the oxygen presence in the atmosphere, while applying laser or thermal energy. Our data suggest that the interlayer coupling is tuned through the diffusive intercalation and deintercalation of oxygen molecules. When one layer of Bi2Se3 is grown on monolayer MoS2, an influential interlayer coupling is formed, which quenches the signature photoluminescence (PL) peaks. However, thermally treating in the presence of oxygen disrupts the interlayer coupling, facilitating the emergence of the MoS2 PL peak. Our density functional theory calculations predict that intercalated oxygen increases the interlayer separation ∼17%, disrupting the interlayer coupling and inducing the layers to behave more electronically independent. The interlayer coupling can then be restored by thermally treating in N2 or Ar, where the peaks will requench. Hence, this is an interesting oxygen-induced switching between "non-radiative" and "radiative" exciton recombination. This switching can also be accomplished locally, controllably, and reversibly using a low-power focused laser, while changing the environment from pure N2 to air. This allows for the interlayer coupling to be precisely manipulated with submicron spatial resolution, facilitating site-programmable 2D light-emitting pixels whose emission intensity could be precisely varied by a factor exceeding 200×. Our results show that these atomically thin 2D heterostructures may be excellent candidates for oxygen sensing.

Resonant Raman and Exciton Coupling in High-Quality Single Crystals of Atomically Thin Molybdenum Diselenide Grown by Vapor-Phase Chalcogenization.

We report a detailed investigation on Raman spectroscopy in vapor-phase chalcogenization grown, high-quality single-crystal atomically thin molybdenum diselenide samples. Measurements were performed in samples with four different incident laser excitation energies ranging from 1.95 eV ⩽ Eex ⩽ 2.71 eV, revealing rich spectral information in samples ranging from N = 1-4 layers and a thick, bulk sample. In addition to previously observed (and identified) peaks, we specifically investigate the origin of a peak near ω ≈ 250 cm-1. Our density functional theory and Bethe-Salpeter calculations suggest that this peak arises from a double-resonant Raman process involving the ZA acoustic phonon perpendicular to the layer. This mode appears prominently in freshly prepared samples and disappears in aged samples, thereby offering a method for ascertaining the high optoelectronic quality of freshly prepared 2D-MoSe2 crystals. We further present an in-depth investigation of the energy-dependent variation of the position of this and other peaks and provide evidence of C-exciton-phonon coupling in monolayer MoSe2. Finally, we show how the signature peak positions and intensities vary as a function of layer thickness in these samples.

Tunable and laser-reconfigurable 2D heterocrystals obtained by epitaxial stacking of crystallographically incommensurate Bi2Se3 and MoS2 atomic layers.

Vertical stacking is widely viewed as a promising approach for designing advanced functionalities using two-dimensional (2D) materials. Combining crystallographically commensurate materials in these 2D stacks has been shown to result in rich new electronic structure, magnetotransport, and optical properties. In this context, vertical stacks of crystallographically incommensurate 2D materials with well-defined crystallographic order are a counterintuitive concept and, hence, fundamentally intriguing. We show that crystallographically dissimilar and incommensurate atomically thin MoS2 and Bi2Se3 layers can form rotationally aligned stacks with long-range crystallographic order. Our first-principles theoretical modeling predicts heterocrystal electronic band structures, which are quite distinct from those of the parent crystals, characterized with an indirect bandgap. Experiments reveal striking optical changes when Bi2Se3 is stacked layer by layer on monolayer MoS2, including 100% photoluminescence (PL) suppression, tunable transmittance edge (1.1→0.75 eV), suppressed Raman, and wide-band evolution of spectral transmittance. Disrupting the interface using a focused laser results in a marked the reversal of PL, Raman, and transmittance, demonstrating for the first time that in situ manipulation of interfaces can enable "reconfigurable" 2D materials. We demonstrate submicrometer resolution, "laser-drawing" and "bit-writing," and novel laser-induced broadband light emission in these heterocrystal sheets.

Vapor-Phase-Gating-Induced Ultrasensitive Ion Detection in Graphene and Single-Walled Carbon Nanotube Networks.

Designing ultrasensitive detectors often requires complex architectures, high-voltage operations, and sophisticated low-noise measurements. In this work, it is shown that simple low-bias two-terminal DC-conductance values of graphene and single-walled carbon nanotubes are extremely sensitive to ionized gas molecules. Incident ions form an electrode-free, dielectric- or electrolyte-free, bias-free vapor-phase top-gate that can efficiently modulate carrier densities up to ≈0.6 × 1013 cm-2 . Surprisingly, the resulting current changes are several orders of magnitude larger than that expected from conventional electrostatic gating, suggesting the possible role of a current-gain inducing mechanism similar to those seen in photodetectors. These miniature detectors demonstrate charge-current amplification factor values exceeding 108 A C-1 in vacuum with undiminished responses in open air, and clearly distinguish between positive and negative ions sources. At extremely low rates of ion incidence, detector currents show stepwise changes with time, and calculations suggest that these stepwise changes can result from arrival of individual ions. These sensitive ion detectors are used to demonstrate a proof-of-concept low-cost, amplifier-free, light-emitting-diode-based low-power ion-indicator.

Transparent Electrophysiology Microelectrodes and Interconnects from Metal Nanomesh.

Mapping biocurrents at both microsecond and single-cell resolution requires the combination of optical imaging with innovative electrophysiological sensing techniques. Here, we present transparent electrophysiology electrodes and interconnects made of gold (Au) nanomesh on flexible substrates to achieve such measurements. Compared to previously demonstrated indium tin oxide (ITO) and graphene electrodes, the ones from Au nanomesh possess superior properties including low electrical impedance, high transparency, good cell viability, and superb flexibility. Specifically, we demonstrated a 15 nm thick Au nanomesh electrode with 8.14 Ω·cm2 normalized impedance, >65% average transmittance over a 300-1100 nm window, and stability up to 300 bending cycles. Systematic sheet resistance measurements, electrochemical impedance studies, optical characterization, mechanical bending tests, and cell studies highlight the capabilities of the Au nanomesh as a transparent electrophysiology electrode and interconnect material. Together, these results demonstrate applicability of using nanomesh under biological conditions and broad applications in biology and medicine.

Charge transfer in crystalline germanium/monolayer MoS2 heterostructures prepared by chemical vapor deposition.

Heterostructuring provides novel opportunities for exploring emergent phenomena and applications by developing designed properties beyond those of homogeneous materials. Advances in nanoscience enable the preparation of heterostructures formed incommensurate materials. Two-dimensional (2D) materials, such as graphene and transition metal dichalcogenides, are of particular interest due to their distinct physical characteristics. Recently, 2D/2D heterostructures have opened up new research areas. However, other heterostructures such as 2D/three-dimensional (3D) materials have not been thoroughly studied yet although the growth of 3D materials on 2D materials creating 2D/3D heterostructures with exceptional carrier transport properties has been reported. Here we report a novel heterostructure composed of Ge and monolayer MoS2, prepared by chemical vapor deposition. A single crystalline Ge (110) thin film was grown on monolayer MoS2. The electrical characteristics of Ge and MoS2 in the Ge/MoS2 heterostructure were remarkably different from those of isolated Ge and MoS2. The field-effect conductivity type of the monolayer MoS2 is converted from n-type to p-type by growth of the Ge thin film on top of it. Undoped Ge on MoS2 is highly conducting. The observations can be explained by charge transfer in the heterostructure as opposed to chemical doping via the incorporation of impurities, based on our first-principles calculations.

Protecting the properties of monolayer MoS2 on silicon based substrates with an atomically thin buffer.

Semiconducting 2D materials, like transition metal dichalcogenides (TMDs), have gained much attention for their potential in opto-electronic devices, valleytronic schemes, and semi-conducting to metallic phase engineering. However, like graphene and other atomically thin materials, they lose key properties when placed on a substrate like silicon, including quenching of photoluminescence, distorted crystalline structure, and rough surface morphology. The ability to protect these properties of monolayer TMDs, such as molybdenum disulfide (MoS2), on standard Si-based substrates, will enable their use in opto-electronic devices and scientific investigations. Here we show that an atomically thin buffer layer of hexagonal-boron nitride (hBN) protects the range of key opto-electronic, structural, and morphological properties of monolayer MoS2 on Si-based substrates. The hBN buffer restores sharp diffraction patterns, improves monolayer flatness by nearly two-orders of magnitude, and causes over an order of magnitude enhancement in photoluminescence, compared to bare Si and SiO2 substrates. Our demonstration provides a way of integrating MoS2 and other 2D monolayers onto standard Si-substrates, thus furthering their technological applications and scientific investigations.

Graphene-aluminum nitride NEMS resonant infrared detector.

The use of micro-/nanoelectromechanical resonators for the room temperature detection of electromagnetic radiation at infrared frequencies has recently been investigated, showing thermal detection capabilities that could potentially outperform conventional microbolometers. The scaling of the device thickness in the nanometer range and the achievement of high infrared absorption in such a subwavelength thickness, without sacrificing the electromechanical performance, are the two key challenges for the implementation of fast, high-resolution micro-/nanoelectromechanical resonant infrared detectors. In this paper, we show that by using a virtually massless, high-electrical-conductivity, and transparent graphene electrode, floating at the van der Waals separation of a few angstroms from a piezoelectric aluminum nitride nanoplate, it is possible to implement ultrathin (460 nm) piezoelectric nanomechanical resonant structures with improved electromechanical performance (>50% improved frequency×quality factor) and infrared detection capabilities (>100× improved infrared absorptance) compared with metal-electrode counterparts, despite their reduced volumes. The intrinsic infrared absorption capabilities of a submicron thin graphene-aluminum nitride plate backed with a metal electrode are investigated for the first time and exploited for the first experimental demonstration of a piezoelectric nanoelectromechanical resonant thermal detector with enhanced infrared absorptance in a reduced volume. Moreover, the combination of electromagnetic and piezoelectric resonances provided by the same graphene-aluminum nitride-metal stack allows the proposed device to selectively detect short-wavelength infrared radiation (by tailoring the thickness of aluminum nitride) with unprecedented electromechanical performance and thermal capabilities. These attributes potentially lead to the development of uncooled infrared detectors suitable for the implementation of high performance, miniaturized and power-efficient multispectral infrared imaging systems.

Atomically thin layers of B-N-C-O with tunable composition.

In recent times, atomically thin alloys of boron, nitrogen, and carbon have generated significant excitement as a composition-tunable two-dimensional (2D) material that demonstrates rich physics as well as application potentials. The possibility of tunably incorporating oxygen, a group VI element, into the honeycomb sp(2)-type 2D-BNC lattice is an intriguing idea from both fundamental and applied perspectives. We present the first report on an atomically thin quaternary alloy of boron, nitrogen, carbon, and oxygen (2D-BNCO). Our experiments suggest, and density functional theory (DFT) calculations corroborate, stable configurations of a honeycomb 2D-BNCO lattice. We observe micrometer-scale 2D-BNCO domains within a graphene-rich 2D-BNC matrix, and are able to control the area coverage and relative composition of these domains by varying the oxygen content in the growth setup. Macroscopic samples comprising 2D-BNCO domains in a graphene-rich 2D-BNC matrix show graphene-like gate-modulated electronic transport with mobility exceeding 500 cm(2) V(-1) s(-1), and Arrhenius-like activated temperature dependence. Spin-polarized DFT calculations for nanoscale 2D-BNCO patches predict magnetic ground states originating from the B atoms closest to the O atoms and sizable (0.6 eV < E g < 0.8 eV) band gaps in their density of states. These results suggest that 2D-BNCO with novel electronic and magnetic properties have great potential for nanoelectronics and spintronic applications in an atomically thin platform.

Chemical Vapor Deposition Synthesized Atomically Thin Molybdenum Disulfide with Optoelectronic-Grade Crystalline Quality.

The ability to synthesize high-quality samples over large areas and at low cost is one of the biggest challenges during the developmental stage of any novel material. While chemical vapor deposition (CVD) methods provide a promising low-cost route for CMOS compatible, large-scale growth of materials, it often falls short of the high-quality demands in nanoelectronics and optoelectronics. We present large-scale CVD synthesis of single- and few-layered MoS2 using direct vapor-phase sulfurization of MoO2, which enables us to obtain extremely high-quality single-crystal monolayer MoS2 samples with field-effect mobility exceeding 30 cm(2)/(V s) in monolayers. These samples can be readily synthesized on a variety of substrates, and demonstrate a high-degree of optoelectronic uniformity in Raman and photoluminescence mapping over entire crystals with areas exceeding hundreds of square micrometers. Because of their high crystalline quality, Raman spectroscopy on these samples reveal a range of multiphonon processes through peaks with equal or better clarity compared to past reports on mechanically exfoliated samples. This enables us to investigate the layer thickness and substrate dependence of the extremely weak phonon processes at 285 and 487 cm(-1) in 2D-MoS2. The ultrahigh, optoelectronic-grade crystalline quality of these samples could be further established through photocurrent spectroscopy, which clearly reveal excitonic states at room temperature, a feat that has been previously demonstrated only on samples which were fabricated by micro-mechanical exfoliation and then artificially suspended across trenches. Our method reflects a big step in the development of atomically thin, 2D-MoS2 for scalable, high-quality optoelectronics.

Ultrafast Intrinsic Photoresponse and Direct Evidence of Sub-gap States in Liquid Phase Exfoliated MoS2Thin Films.

2-Dimensional structures with swift optical response have several technological advantages, for example they could be used as components of ultrafast light modulators, photo-detectors, and optical switches. Here we report on the fast photo switching behavior of thin films of liquid phase exfoliated MoS2, when excited with a continuous laser of λ = 658 nm (E = 1.88 eV), over a broad range of laser power. Transient photo-conductivity measurements, using an optical pump and THz probe (OPTP), reveal that photo carrier decay follows a bi-exponential time dependence, with decay times of the order of picoseconds, indicating that the photo carrier recombination occurs via trap states. The nature of variation of photocurrent with temperature confirms that the trap states are continuously distributed within the mobility gap in these thin film of MoS2, and play a vital role in influencing the overall photo response. Our findings provide a fundamental understanding of the photo-physics associated with optically active 2D materials and are crucial for developing advanced optoelectronic devices.