The rise of two-dimensional (2D) materials research took place following the isolation of graphene in 2004. These new 2D materials include transition metal dichalcogenides, mono-elemental 2D sheets, and several carbide-and nitride-based materials. The number of publications related to these emerging materials has been drastically increasing over the last five years. Thus, through this comprehensive review, we aim to discuss the most recent groundbreaking discoveries as well as emerging opportunities and remaining challenges. This review starts out by delving into the improved methods of producing these new 2D materials via controlled exfoliation, metal organic chemical vapor deposition, and wet chemical means. We look into recent studies of doping as well as the optical properties of 2D materials and their heterostructures. Recent advances towards applications of these materials in 2D electronics are also reviewed, and include the tunnel MOSFET and ways to reduce the contact resistance for fabricating highquality devices. Finally, several unique and innovative applications recently explored are discussed as well as perspectives of this exciting and fast moving field.

Abstract:Recent advances in scanning transmission electron and scanning probe microscopies have opened exciting opportunities in probing the materials structural parameters and various functional properties in real space with angstrom-level precision. This progress has been accompanied by an exponential increase in the size and quality of datasets produced by microscopic and spectroscopic experimental techniques. These developments necessitate adequate methods for extracting relevant physical and chemical information from the large datasets, for which a priori information on the structures of various atomic configurations and lattice defects is limited or absent. Here we demonstrate an application of deep neural networks to extract information from atomically resolved images including location of the atomic species and type of defects.We develop a "weakly-supervised" approach that uses information on the coordinates of all atomic species in the image, extracted via a deep neural network, to identify a rich variety of defects that are not part of an initial training set. We further apply our approach to interpret complex atomic and defect transformation, including switching between different coordination of silicon dopants in graphene as a function of time, formation of peculiar silicon dimer with mixed 3-fold and 4-fold coordination, and the motion of molecular "rotor". This deep learning based approach resembles logic of a human operator, but can be scaled leading to significant shift in the way of extracting and analyzing information from raw experimental data. Keywords:STEM, neural networks, weakly-supervised learning, graphene, TMDC. 3In the last decade, the proliferation of electron microscopy and scanning probe microscopy techniques have generated massive amounts of data on local chemical structure and atomic transformation. [1][2][3] Since the advent of aberration corrected Scanning Transmission Electron Microscopy (STEM), atomically resolved images of multiple materials classes ranging from multiferroics, semiconductors, and superconductors have become common. [4][5][6][7][8] The further impetus to this field was given by the development of atomically resolved dynamic studies, when the dynamic changes in matter on the atomic level are visualized. These traditionally include the thermal and chemical processes enabled by advanced thermal and environmental holders. 9,10 More recently, progressively more attention is being attracted to the dynamic processes induced by the electron beam irradiation, 11-14 especially promising in the context of e-beam atomic fabrication. [15][16][17] Similar advances are achieved in the field of atomically resolved scanning tunneling (STM) and atomic force microscopy (AFM). The recent famous examples include direct imaging of chemical bonds in molecules, 18 visualizing atomic collapse in artificial nuclei on graphene, 19 and inferring mechanisms behind fundamental physical phenomena, such as high-T c superconductivity, from single atom defect induced scattering patterns. 20 I...

Compared with their bulk counterparts, atomically thin two-dimensional (2D) crystals exhibit new physical properties, and have the potential to enable next-generation electronic and optoelectronic devices. However, controlled synthesis of large uniform monolayer and multi-layer 2D crystals is still challenging. Here, we report the controlled synthesis of 2D GaSe crystals on SiO2/Si substrates using a vapor phase deposition method. For the first time, uniform, large (up to ~60 μm in lateral size), single-crystalline, triangular monolayer GaSe crystals were obtained and their structure and orientation were characterized from atomic scale to micrometer scale. The size, density, shape, thickness, and uniformity of the 2D GaSe crystals were shown to be controllable by growth duration, growth region, growth temperature, and argon carrier gas flow rate. The theoretical modeling of the electronic structure and Raman spectroscopy demonstrate a direct-to-indirect bandgap transition and progressive confinement-induced bandgap shifts for 2D GaSe crystals. The 2D GaSe crystals show p-type semiconductor characteristics and high photoresponsivity (~1.7 A/W under white light illumination) comparable to exfoliated GaSe nanosheets. These 2D GaSe crystals are potentially useful for next-generation electronic and optoelectronic devices such as photodetectors and field-effect transistors.

The tunable optoelectronic properties of stacked two-dimensional (2D) crystal monolayers are determined by their stacking orientation, order, and atomic registry. Atomic-resolution Z-contrast scanning transmission electron microscopy (AR-Z-STEM) and electron energy loss spectroscopy (EELS) can be used to determine the exact atomic registration between different layers, in few-layer 2D stacks; however, fast optical characterization techniques are essential for rapid development of the field. Here, using two- and three-layer MoSe2 and WSe2 crystals synthesized by chemical vapor deposition, we show that the generally unexplored low frequency (LF) Raman modes (<50 cm(-1)) that originate from interlayer vibrations can serve as fingerprints to characterize not only the number of layers, but also their stacking configurations. Ab initio calculations and group theory analysis corroborate the experimental assignments determined by AR-Z-STEM and show that the calculated LF mode fingerprints are related to the 2D crystal symmetries.

Synthesized two-dimensional GaSe/MoSe2 misfit heterostructures form p-n junctions with a gate-tunable photovoltaic response.

Artificial inorganic leafs are developed by organizing light harvesting, photoinduced charge separation, and catalysis modules (Pt/N‐TiO2) into leaf‐shaped hierarchical structures using natural leaves as biotemplates (see figure). The enhanced light‐harvesting and photocatalytic water‐splitting activities stem from the reproduction of the leafs' complex structures and self‐doping of nitrogen during synthesis. The research may represent an important first step toward the design of novel artificial solar‐energy transduction systems based on natural concepts, particularly on mimicking the structural design.

Phosphor-converted white light-emitting diodes for indoor illumination need to be warm-white (i.e., correlated color temperature ,4000 K) with good color rendition (i.e., color rendering index .80). However, no single-phosphor, single-emitting-center-converted white light-emitting diodes can simultaneously satisfy the color temperature and rendition requirements due to the lack of sufficient red spectral component in the phosphors' emission spectrum. Here, we report a new yellow Ba 0.93 Eu 0.07 Al 2 O 4 phosphor that has a new orthorhombic lattice structure and exhibits a broad yellow photoluminescence band with sufficient red spectral component. Warm-white emissions with correlated color temperature ,4000 K and color rendering index .80 were readily achieved when combining the Ba 0.93 Eu 0.07 Al 2 O 4 phosphor with a blue light-emitting diode (440-470 nm). This study demonstrates that warm-white light-emitting diodes with high color rendition (i.e., color rendering index .80) can be achieved based on single-phosphor, single-emitting-center conversion.

Unique twisted bilayers of MoSe2 with multiple stacking orientations and interlayer couplings in the narrow range of twist angles, 60 ± 3°, are revealed by low-frequency Raman spectroscopy and theoretical analysis. The slight deviation from 60° allows the concomitant presence of patches featuring all three high-symmetry stacking configurations (2H or AA', AB', and A'B) in one unique bilayer system. In this case, the periodic arrangement of the patches and their size strongly depend on the twist angle. Ab initio modeling predicts significant changes in frequencies and intensities of low-frequency modes versus stacking and twist angle. Experimentally, the variable stacking and coupling across the interface are revealed by the appearance of two breathing modes, corresponding to the mixture of the high-symmetry stacking configurations and unaligned regions of monolayers. Only one breathing mode is observed outside the narrow range of twist angles. This indicates a stacking transition to unaligned monolayers with mismatched atom registry without the in-plane restoring force required to generate a shear mode. The variable interlayer coupling and spacing in transition metal dichalcogenide bilayers revealed in this study may provide an interesting platform for optoelectronic applications of these materials.