Non-epitaxial single-crystal 2D material growth by geometric confinement.

Two-dimensional (2D) materials and their heterostructures show a promising path for next-generation electronics1-3. Nevertheless, 2D-based electronics have not been commercialized, owing mainly to three critical challenges: i) precise kinetic control of layer-by-layer 2D material growth, ii) maintaining a single domain during the growth, and iii) wafer-scale controllability of layer numbers and crystallinity. Here we introduce a deterministic, confined-growth technique that can tackle these three issues simultaneously, thus obtaining wafer-scale single-domain 2D monolayer arrays and their heterostructures on arbitrary substrates. We geometrically confine the growth of the first set of nuclei by defining a selective growth area via patterning SiO2 masks on two-inch substrates. Owing to substantial reduction of the growth duration at the micrometre-scale SiO2 trenches, we obtain wafer-scale single-domain monolayer WSe2 arrays on the arbitrary substrates by filling the trenches via short growth of the first set of nuclei, before the second set of nuclei is introduced, thus without requiring epitaxial seeding. Further growth of transition metal dichalcogenides with the same principle yields the formation of single-domain MoS2/WSe2 heterostructures. Our achievement will lay a strong foundation for 2D materials to fit into industrial settings.

Higher superconducting transition temperature by breaking the universal pressure relation.

By investigating the bulk superconducting state via dc magnetization measurements, we have discovered a common resurgence of the superconducting transition temperatures (Tcs) of the monolayer Bi2Sr2CuO6+δ (Bi2201) and bilayer Bi2Sr2CaCu2O8+δ (Bi2212) to beyond the maximum Tcs (Tc-maxs) predicted by the universal relation between Tc and doping (p) or pressure (P) at higher pressures. The Tc of underdoped Bi2201 initially increases from 9.6 K at ambient to a peak at 23 K at 26 GPa and then drops as expected from the universal Tc-P relation. However, at pressures above 40 GPa, Tc rises rapidly without any sign of saturation up to 30 K at 51 GPa. Similarly, the Tc for the slightly overdoped Bi2212 increases after passing a broad valley between 20 and 36 GPa and reaches 90 K without any sign of saturation at 56 GPa. We have, therefore, attributed this Tc resurgence to a possible pressure-induced electronic transition in the cuprate compounds due to a charge transfer between the Cu 3[Formula: see text] and the O 2p bands projected from a hybrid bonding state, leading to an increase of the density of states at the Fermi level, in agreement with our density functional theory calculations. Similar Tc-P behavior has also been reported in the trilayer Br2Sr2Ca2Cu3O10+δ (Bi2223). These observations suggest that higher Tcs than those previously reported for the layered cuprate high-temperature superconductors can be achieved by breaking away from the universal Tc-P relation through the application of higher pressures.

Why In2O3 Can Make 0.7 nm Atomic Layer Thin Transistors.

In this work, we demonstrate enhancement-mode field-effect transistors by an atomic-layer-deposited (ALD) amorphous In2O3 channel with thickness down to 0.7 nm. Thickness is found to be critical on the materials and electron transport of In2O3. Controllable thickness of In2O3 at atomic scale enables the design of sufficient 2D carrier density in the In2O3 channel integrated with the conventional dielectric. The threshold voltage and channel carrier density are found to be considerably tuned by channel thickness. Such a phenomenon is understood by the trap neutral level (TNL) model, where the Fermi-level tends to align deeply inside the conduction band of In2O3 and can be modulated to the bandgap in atomic layer thin In2O3 due to the quantum confinement effect, which is confirmed by density function theory (DFT) calculation. The demonstration of enhancement-mode amorphous In2O3 transistors suggests In2O3 is a competitive channel material for back-end-of-line (BEOL) compatible transistors and monolithic 3D integration applications.

Intrinsic enhancement of the rate capability and suppression of the phase transition via p-type doping in Fe-Mn based P2-type cathodes used for sodium ion batteries.

In this study, we present improved power characteristics and suppressed phase transition by incorporating elemental doping into a P2-type cathode of sodium ion batteries. A Cu-doped Fe-Mn based P2-type Na0.67Cu0.125Fe0.375Mn0.5O2 cathode was designed based on the calculations of the electronic structure and then examined experimentally. Using first principles, we introduced instrinsic p-type conductivity by elemental doping with Cu. Introduction of Cu generated electron holes above the Fermi level in the electronic structure, which is typical of p-type semiconductors. Charge analyses suggested that the hole generation was driven primarily by the greater reduced characteristics of Cu as compared with those of Fe and Mn. In addition, introduction of Cu retaining high reduced property also suppressed phase transition from the P2 to Z phase by Fe migration to empty Na layers mainly. Electrochemical experiments revealed improved power characteristics upon the introduction of p-type conductivity. This could be attributed to the increase in the electronic conductivity by hole generation in the valence band. This study suggests that the introduction of p-type conductivity could be a rational tactic for the development of promising cathode materials for high performance sodium ion batteries.

Core-shell PdAu nanocluster catalysts to suppress sulfur poisoning.

Reducing sulfur poisoning is significant for maintaining the catalytic efficiency and durability of heterogeneous catalysts. We screened PdAu nanoclusters with specific Pd : Au ratios based on Monte Carlo simulations and then carried out density functional calculations to reveal how to reduce sulfur poisoning via alloying. Among various nanoclusters, the core-shell structure Pd13Au42 (Pd@Au) exhibits a low adsorption energy of SO2 (-0.67 eV), comparable with O2 (-0.45 eV) and lower than CO (-1.25 eV), thus avoiding sulfur poisoning during the CO catalytic oxidation. Fundamentally, the weak adsorption of SO2 originates from the negative d-band center of the shell and delocalized charge distribution near the Fermi level, due to the appropriate charge transfer from the core to shell. Core-shell nanoclusters with a different core (Ni, Cu, Ag, Pt) and a Pd@Au slab model were further constructed to validate and extend the results. These findings provide insights into designing core-shell catalysts to suppress sulfur poisoning while optimizing catalytic behaviors.

First-Principles Study on Electron-Induced Excitations of Atomic Layer Deposition Precursors: Inelastic Electron Wave Packet Scattering with Cobalt Tricarbonyl Nitrosyl Co(CO)3NO Using Time-Dependent Density Functional Theory.

A quantitative study on inelastic electron scattering with a molecule is of significant importance for understanding the essential mechanisms of electron-induced gas-phase and surface chemical reactions in their excited electronic states. A key issue to be addressed is the quantitatively detailed inelastic electron collision processes with a realistic molecular target, associated with electron excitation that leads to potential ionization and dissociation reactions of the molecule. Using the real-time time-dependent density functional theory (TDDFT) modeling, we present quantitative findings on the energy transfers and internal excitations for the low energy (up to 270 eV) electron wave packet impact with the molecular target cobalt tricarbonyl nitrosyl (CTN, Co(CO)3NO) that is used as a precursor in electron-enhanced atomic layer deposition (EE-ALD) growth of Co films. Our modeling shows the quantitative dependence of the wave packet sizes, target molecule orientations, and impact parameters on the energy transfer in this inelastic electron scattering process. It is found that the wave packet sizes have little effect on the overall profile of the internal multiple excited states, whereas different target orientations can cause significantly different internal excited states. To evaluate the quantitative prediction capability, the inelastic scattering cross-section of a hydrogen atom is calculated and compared with the experimental data, leading to a constant scaling factor over the whole energy range. The present study demonstrates the remarkable potential of TDDFT for simulating the inelastic electron scattering process, which provides critical information for future exploration of electronic excitations in a wide range of electron-induced chemical reactions in current technological applications.

Real-time ab initio simulation of inelastic electron scattering using the exact, density functional, and alternative approaches.

To investigate inelastic electron scattering, which is ubiquitous in various fields of study, we carry out ab initio study of the real-time dynamics of a one-dimensional electron wave packet scattered by a hydrogen atom using different methods: the exact solution, the solution provided by time-dependent density functional theory (TDDFT), and the solutions given by alternative approaches. This research not only sheds light on inelastic scattering processes but also verifies the capability of TDDFT in describing inelastic electron scattering. We revisit the adiabatic local-density approximation (ALDA) in describing the excitation of the target during the scattering process along with a self-interaction correction and spin-polarized calculations. Our results reveal that the ALDA severely underestimates the energy transferred in the regime of low incident energy particularly for a spin-singlet system. After demonstrating alternative approaches, we propose a hybrid ab initio method to deal with the kinetic correlation alongside TDDFT. This hybrid method would facilitate first-principles studies of systems in which the correlation of a few electrons among many others is of interest.

Molecularly Thin Electrolyte for All Solid-State Nonvolatile Two-Dimensional Crystal Memory.

A molecularly thin electrolyte is developed to demonstrate a nonvolatile, solid-state, one-transistor (1T) memory based on an electric-double-layer (EDL) gated WSe2 field-effect transistor (FET). The custom-designed monolayer electrolyte consists of cobalt crown ether phthalocyanine and lithium ions, which are positioned by field-effect at either the surface of the WSe2 channel or an h-BN capping layer to achieve "1" or "0", respectively. Bistability in the monolayer electrolyte memory is significantly improved by the h-BN cap with density functional theory (DFT) calculations showing enhanced trapping of Li+ near h-BN due to a ∼1.34 eV increase in the absolute value of the adsorption energy compared to vacuum. The threshold voltage shift between the two states corresponds to a change in charge density of ∼2.5 × 1012 cm-2, and an On/Off ratio exceeding 104 at a back gate voltage of 0 V. The On/Off ratio remains stable after 1000 cycles and the retention time for each state exceeds 6 h (max measured). When the write time approaches 1 ms, the On/Off ratio remains >102, showing that the monolayer electrolyte-gated FET can respond on time scales similar to existing flash memory. The data suggest that faster switching times and lower switching voltages could be feasible by top gating.

Reversible Anionic Redox Activities in Conventional LiNi1/3 Co1/3 Mn1/3 O2 Cathodes.

Redox reactions of oxygen have been considered critical in controlling the electrochemical properties of lithium-excessive layered-oxide electrodes. However, conventional electrode materials without overlithiation remain the most practical. Typically, cationic redox reactions are believed to dominate the electrochemical processes in conventional electrodes. Herein, we show unambiguous evidence of reversible anionic redox reactions in LiNi1/3 Co1/3 Mn1/3 O2 . The typical involvement of oxygen through hybridization with transition metals is discussed, as well as the intrinsic oxygen redox process at high potentials, which is 75 % reversible during initial cycling and 63 % retained after 10 cycles. Our results clarify the reaction mechanism at high potentials in conventional layered electrodes involving both cationic and anionic reactions and indicate the potential of utilizing reversible oxygen redox reactions in conventional layered oxides for high-capacity lithium-ion batteries.

Pocketlike Active Site of Rh1/MoS2 Single-Atom Catalyst for Selective Crotonaldehyde Hydrogenation.

Selective hydrogenation of unsaturated aldehydes to unsaturated alcohols is a valuable but challenging task for synthesizing fine chemicals. We report that single Rh atoms anchored to the edges of 2D MoS2 sheets can efficiently convert crotonaldehyde to crotyl alcohol with 100% selectivity via a steric confinement effect of pocketlike active sites. Characterization results suggest that the synthesized Rh1/MoS2 single-atom catalysts (SACs) possess a unique geometric and electronic configuration, which confines the adsorption mode of the reactant molecule by a steric effect. The DFT calculations suggest that the MoS2 sheets terminate with oxidized Mo edges and the Rh1 stably anchors at the Mo cation vacancy site, which can facilely dissociate H2 to H atoms. The dissociated H atoms spill over to react with the edge O atoms to form OH species and create an HO-Mo-Rh1-Mo-OH configuration, resembling a pocketlike active site, which confines the adsorption mode of the crotonaldehyde due to steric effects. Such specific adsorption configuration yields 100% selectivity. The strategy of constructing pocketlike active centers with single metal atoms and 2D nanosheets opens new approaches to designing highly selective SACs for specific classes of catalytic transformations.

Stable and Active Oxidation Catalysis by Cooperative Lattice Oxygen Redox on SmMn2O5 Mullite Surface.

The correlation between lattice oxygen (O) binding energy and O oxidation activity imposes a fundamental limit in developing oxide catalysts, simultaneously meeting the stringent thermal stability and catalytic activity standards for complete oxidation reactions under harsh conditions. Typically, strong O binding indicates a stable surface structure, but low O oxidation activity, and vice versa. Using nitric oxide (NO) catalytic oxidation as a model reaction, we demonstrate that this conflicting correlation can be avoided by cooperative lattice oxygen redox on SmMn2O5 mullite oxides, leading to stable and active oxide surface structures. The strongly bound neighboring lattice oxygen pair cooperates in NO oxidation to form bridging nitrate (NO3-) intermediates, which can facilely transform into monodentate NO3- by a concerted rotation with simultaneous O2 adsorption onto the resulting oxygen vacancy. Subsequently, monodentate NO3- species decompose to NO2 to restore one of the lattice oxygen atoms that act as a reversible redox center, and the vacancy can easily activate O2 to replenish the consumed one. This discovery not only provides insights into the cooperative reaction mechanism but also aids the design of oxidation catalysts with the strong O binding region, offering strong activation of O2, high O activity, and high thermal stability in harsh conditions.

Polarity governs atomic interaction through two-dimensional materials.

The transparency of two-dimensional (2D) materials to intermolecular interactions of crystalline materials has been an unresolved topic. Here we report that remote atomic interaction through 2D materials is governed by the binding nature, that is, the polarity of atomic bonds, both in the underlying substrates and in 2D material interlayers. Although the potential field from covalent-bonded materials is screened by a monolayer of graphene, that from ionic-bonded materials is strong enough to penetrate through a few layers of graphene. Such field penetration is substantially attenuated by 2D hexagonal boron nitride, which itself has polarization in its atomic bonds. Based on the control of transparency, modulated by the nature of materials as well as interlayer thickness, various types of single-crystalline materials across the periodic table can be epitaxially grown on 2D material-coated substrates. The epitaxial films can subsequently be released as free-standing membranes, which provides unique opportunities for the heterointegration of arbitrary single-crystalline thin films in functional applications.

Quantum Transport and Band Structure Evolution under High Magnetic Field in Few-Layer Tellurene.

Quantum Hall effect (QHE) is a macroscopic manifestation of quantized states that only occurs in confined two-dimensional electron gas (2DEG) systems. Experimentally, QHE is hosted in high-mobility 2DEG with large external magnetic field at low temperature. Two-dimensional van der Waals materials, such as graphene and black phosphorus, are considered interesting material systems to study quantum transport because they could unveil unique host material properties due to the easy accessibility of monolayer or few-layer thin films at the 2D quantum limit. For the first time, we report direct observation of QHE in a novel low-dimensional material system, tellurene. High-quality 2D tellurene thin films were acquired from recently reported hydrothermal method with high hole mobility of nearly 3000 cm2/(V s) at low temperatures, which allows the observation of well-developed Shubnikov-de Haas (SdH) oscillations and QHE. A four-fold degeneracy of Landau levels in SdH oscillations and QHE was revealed. Quantum oscillations were investigated under different gate biases, tilted magnetic fields, and various temperatures, and the results manifest the inherent information on the electronic structure of Te. Anomalies in both temperature-dependent oscillation amplitudes and transport characteristics were observed that are ascribed to the interplay between the Zeeman effect and spin-orbit coupling, as depicted by the density functional theory calculations.

Quantum-Confined Electronic States Arising from the Moiré Pattern of MoS2-WSe2 Heterobilayers.

A two-dimensional (2D) heterobilayer system consisting of MoS2 on WSe2, deposited on epitaxial graphene, is studied by scanning tunneling microscopy and spectroscopy at temperatures of 5 and 80 K. A moiré pattern is observed, arising from lattice mismatch of 3.7% between the MoS2 and WSe2. Significant energy shifts are observed in tunneling spectra observed at the maxima of the moiré corrugation, as compared with spectra obtained at corrugation minima, consistent with prior work. Furthermore, at the minima of the moiré corrugation, sharp peaks in the spectra at energies near the band edges are observed for spectra acquired at 5 K. The peaks correspond to discrete states that are confined within the moiré unit cells. Conductance mapping is employed to reveal the detailed structure of the wave functions of the states. For measurements at 80 K, the sharp peaks in the spectra are absent, and conductance maps of the band edges reveal little structure.

Atomic Insights into Phase Evolution in Ternary Transition-Metal Dichalcogenides Nanostructures.

Phase engineering through chemical modification can significantly alter the properties of transition-metal dichalcogenides, and allow the design of many novel electronic, photonic, and optoelectronics devices. The atomic-scale mechanism underlying such phase engineering is still intensively investigated but elusive. Here, advanced electron microscopy, combined with density functional theory calculations, is used to understand the phase evolution (hexagonal 2H→monoclinic T'→orthorhombic Td ) in chemical vapor deposition grown Mo1-x W x Te2 nanostructures. Atomic-resolution imaging and electron diffraction indicate that Mo1-x W x Te2 nanostructures have two phases: the pure monoclinic phase in low W-concentrated (0 < x ≤ 10 at.%) samples, and the dual phase of the monoclinic and orthorhombic in high W-concentrated (10 < x < 90 at.%) samples. Such phase coexistence exists with coherent interfaces, mediated by a newly uncovered orthorhombic phase Td '. Td ', preserves the centrosymmetry of T' and provides the possible phase transition path for T'→Td with low energy state. This work enriches the atomic-scale understanding of phase evolution and coexistence in multinary compounds, and paves the way for device applications of new transition-metal dichalcogenides phases and heterostructures.

Tuning electronic and magnetic properties of Mn-mullite oxide sub-nanoclusters via MnOn polyhedron units.

Ternary oxide nano-clusters compared to unary metallic and binary ones potentially exhibit more remarkable properties due to their higher stoichiometric flexibility in addition to cluster size variations. Herein, by combining with the structural searching scheme CALYPSO, we have built a series of Mn-mullite oxide clusters (SmxMnyOz)n {(xyz) = (125); (115); n = 1-4, 8} prior to investigation of their geometric and electronic structures via first-principles calculations. In small size regime (n < 4), (SmxMnyOz)n prefer nonstoichiometric (Sm1Mn1O5)n phases composed of nonmagnetic MnO4 tetrahedrons. When n ≧ 4, the clusters tend to develop as stoichiometric (Sm1Mn2O5)n species, including magnetic MnOn polyhedrons and Mn-Mn dimers, which contribute 3d-orbitals (dz2 and/or dx2-y2) around the Fermi levels. The different magnetic behaviors of nonstoichiometric and stoichiometric species originate from the distinct couplings of MnOn polyhedronal units, wherein Mn atoms experience different ligand fields and thus display different spin states. Such findings enable the tuning of electronic properties and potential applications in heterogeneous catalysis, electrochemical catalysis, and the related fields via engineering cluster size and stoichiometry.

Hexacyanometallates for sodium-ion batteries: insights into higher redox potentials using d electronic spin configurations.

A fundamental understanding of anomalous redox mechanisms in hexacyanometallate compounds, compared with conventional NaMO2 systems (M: transition metals), is presented based on first-principles calculations and experimental validations. From theoretical calculations, we identified low-spin and high-spin states of Fe ions coordinated by the cyanide group (-CN) with the same oxidation state (Fe2+) in Na2Fe2(CN)6. Considering the site dependency of d electronic spin configurations based on the crystal field theory (CFT) of transition metals (TMs), we calculated the thermodynamic mixing energy using Na2Fe2(CN)6 and Na2Mn2(CN)6 for obtaining a thermodynamically stable phase of Na2FeMn(CN)6. The phase stabilities of Na2Fe2-xMnx(CN)6 among many atomic configurations and lattice parameters originating from octahedral structures (i.e., Fe(CN)6 and Mn(NC)6) are highly dependent on the electronic structures of TMs with spin states. From partial density of states (PDOS) and spatial electron distributions, it was observed that Fe2+ in the low-spin state (t) and Mn2+ in the high-spin states (t and e) in the stable phase lead to higher redox potentials (∼3.55 V vs. Na/Na+) with the removal of Na+ as compared to that of Na2Fe2(CN)6. In addition, lattice parameters from x = 0 to x = 1 in Na2Fe2-xMnx(CN)6 are increased due to the larger ionic radius of Mn2+ in the high-spin states. On the other hand, Fe2+ in the high-spin states (t and e) and Mn2+ in the low-spin state (t) in the most unstable phase of Na2FeMn(CN)6 would have lower redox potentials. Based on the fundamental correlation between redox potentials and CFT with spin configurations of TMs, we suggest a material design concept for intercalation compounds with higher energy densities for rechargeable battery systems.

First principles study of the Mn-doping effect on the physical and chemical properties of mullite-family Al2SiO5.

Transition metal (TM) modification is a common strategy for converting an earth-abundant mineral into a cost-effective catalyst for industrial applications. Among a variety of minerals, Al2SiO5, which has three phases, andalusite, sillimanite and kyanite, is emerging as a promising candidate for new catalyst development. In this paper, we use Mn to demonstrate the rationale of 3d TM doping at the Al sites in each of these three phases through first-principles calculations and the cluster expansion method. The results of cluster expansion show that Mn has a strong site preference for the six-coordinated Al octahedral chains in the andalusite and sillimanite phases, while distributing randomly in the kyanite phase. Moreover, Mn can only replace Al in sillimanite and kyanite in low concentrations; however, higher concentrations of Mn can replace Al in andalusite. We found that the concentration sensitivity is due to the Jahn-Teller distortion and 3d orbital splitting. This finding can also explain the low doping concentrations of other 3d TMs (Fe, Cr and V) in Al2SiO5 compounds. Based on the calculated Helmholtz free energy, we constructed a (MnxAl1-x)AlSiO5 temperature-composite phase diagram, which explains the physical mechanisms behind the results for 3d transition metal doping and phase transitions in Al2SiO5. This work could shed light on the related physics, chemistry, and geoscience of (MnxAl1-x)AlSiO5, and more importantly, a design rationale for the engineering of cheap catalysts.

Theoretical Demonstration of the Ionic Barristor.

In this Letter, we use first-principles simulations to demonstrate the absence of Fermi-level pinning when graphene is in contact with transition metal dichalcogenides (TMDs). We find that formation of either an ohmic or Schottky contact is possible. Then we show that, due to the shallow density of states around its Fermi level, the work function of graphene can be tuned by ion adsorption. Finally we combine work function tuning of graphene and an ideal contact between graphene and TMDs to propose an ionic barristor design that can tune the work function of graphene with a much wider margin than current barristor designs, achieving a dynamic switching among p-type ohmic contact, Schottky contact, and n-type ohmic contact in one device.

Covalent Nitrogen Doping and Compressive Strain in MoS2 by Remote N2 Plasma Exposure.

Controllable doping of two-dimensional materials is highly desired for ideal device performance in both hetero- and p-n homojunctions. Herein, we propose an effective strategy for doping of MoS2 with nitrogen through a remote N2 plasma surface treatment. By monitoring the surface chemistry of MoS2 upon N2 plasma exposure using in situ X-ray photoelectron spectroscopy, we identified the presence of covalently bonded nitrogen in MoS2, where substitution of the chalcogen sulfur by nitrogen is determined as the doping mechanism. Furthermore, the electrical characterization demonstrates that p-type doping of MoS2 is achieved by nitrogen doping, which is in agreement with theoretical predictions. Notably, we found that the presence of nitrogen can induce compressive strain in the MoS2 structure, which represents the first evidence of strain induced by substitutional doping in a transition metal dichalcogenide material. Finally, our first principle calculations support the experimental demonstration of such strain, and a correlation between nitrogen doping concentration and compressive strain in MoS2 is elucidated.