Axis dependent conduction polarity in the air-stable semiconductor, PdSe2.

Axis-dependent conduction polarity (ADCP) is a unique electronic phenomena in which the charge polarity of carrier conduction can differ from p-type to n-type depending on the direction of travel through the crystal. Most materials that exhibit ADCP are metals, and very few semiconducting materials exhibit this effect. Here, we establish that PdSe2, a ∼0.5 eV band gap semiconductor that is air- and water-stable, exhibits ADCP, through the growth and characterization of the transport properties of crystals with extrinsic p- and n-type doping levels of Ir and Sb, respectively, in the 1016-1018 cm-3 range. Electron doped PdSe2 exhibits p-type conduction in the cross-plane direction and n-type conduction along the in-plane directions above an onset temperature of 100-200 K that varies with doping level. Lightly p-doped samples show p-type thermopower in all directions at low temperatures, but above ∼360 K the in-plane thermopower turns negative. Density functional theory calculations indicate that the origin of ADCP arises from the complementary effective mass anisotropies in the valence and conduction bands in this material, which facilitate hole transport in the cross-plane direction, and electron transport along the in-plane directions. ADCP occurs at temperatures with sufficient thermal population of both carrier types to overcome the extrinsic doping levels to exploit the effective mass anisotropy. In total, the development of this stable semiconductor in which thermally or optically excited holes and electrons inherently migrate along different directions opens up numerous potential applications in a multitude of technologies.

Al Coordination and Ga Interstitial Stability in a ß-(Al0.2Ga0.8)2O3 Thin Film.

Alloying Al2O3 with Ga2O3 to form ß-(AlxGa1-x)2O3 opens the door to a large number of new possibilities for the fabrication of devices with tunable properties in many high-performance applications such as optoelectronics, sensing systems, and high-power electronics. Often, the properties of these devices are impacted by defects induced during the growth process. In this work, we uncover the crystal structure of a ß-(Al0.2Ga0.8)2O3/ß-Ga2O3 interface grown by molecular beam epitaxy. In particular, we determine Al coordination and the stability of Al and Ga interstitials and their effect on the electronic structure of the material by means of scanning transmission electron microscopy combined with density functional theory. Al atoms can substitutionally occupy both octahedral and tetrahedral sites. The atomic structure of the ß-(Al0.2Ga0.8)2O3/ß-Ga2O3 interface additionally shows Al and Ga interstitials located between neighboring tetrahedrally coordinated cation sites, whose stability will depend on the number of surrounding Al atoms. The presence of Al atoms near interstitials leads to structural distortions in the lattice and creates interstitial-divacancy complexes that will eventually form deep-level states below the conduction band (Ec) at Ec -1.25 eV, Ec -1.68 eV, Ec -1.78 eV, Ec -1.83 eV, and Ec -1.86 eV for a Ga interstitial surrounded by zero, one, two, three, and four Al atoms, respectively. These findings bring new insight toward the fabrication of tunable ß-(AlxGa1-x)2O3 heterostructure-based devices with controlled electronic properties.

CrxPt1-xTe2 (x ≤ 0.45): A Family of Air-Stable and Exfoliatable van der Waals Ferromagnets.

The development of thermally robust, air-stable, exfoliatable two-dimensional van der Waals ferromagnetic materials with high transition temperatures is of great importance. Here, we establish a family of magnetic alloys, CrxPt1-xTe2 (x ≤ 0.45), that combines the stability of the late transition metal dichalcogenide PtTe2 with magnetism from Cr. These materials are easily grown in crystal form from the melt, are stable in ambient conditions, and have among the highest concentrations of magnetic element substitution in transition metal dichalcogenide alloys. The highest Cr-substituted material, Cr0.45Pt0.55Te2, exhibits ferromagnetic behavior below 220 K, and the easy axis is along the c-axis of the material, as determined using a combination of neutron diffraction and magnetic susceptibility measurements. These materials are metallic, with appreciable magnetoresistance below the Curie temperature. Single-crystal and powder diffraction measurements indicate Cr readily alloys onto the Pt site and does not sit in the van der Waals space, allowing these materials to be readily exfoliated to the few-layer regime. In summary, this air-stable, exfoliatable, high transition temperature ferromagnet shows great potential as building block for future 2D devices.

Lucky Number 13: A 13-Layer Polytype of the Alkyne Hydrogenation Catalyst CaGaGe.

Polytypism, the ability of materials to form crystal structures with different stacking sequences, occasionally causes materials with the same stoichiometry and similar local structures to have profoundly different properties. Herein, we discover a metastable 13-layer trigonal (13T) polytype of CaGaGe, a layered intermetallic phase comprised of [GaGe]2- honeycombs separated by Ca2+. 13T-CaGaGe is synthesized from arc-melting the elements, and its structure is elucidated via neutron powder diffraction. Air-stable 13T-CaGaGe has one misaligned [GaGe]2- layer for every 13 and transforms into the more stable 4-layer hexagonal (4H) CaGaGe polytype after annealing at 500 °C. Transition-metal-free 13T-CaGaGe shows remarkable activity in the catalytic hydrogenation of phenylacetylene to styrene and ethylbenzene, much higher than the 4H polytype. This work identifies the first 13-layer polytype for any crystal structure and further establishes the influence of polytypism on catalysis.

Recent Advances in Corrosion Science Applicable To Disposal of High-Level Nuclear Waste.

High-level radioactive waste is accumulating at temporary storage locations around the world and will eventually be placed in deep geological repositories. The waste forms and containers will be constructed from glass, crystalline ceramic, and metallic materials, which will eventually come into contact with water, considering that the period of performance required to allow sufficient decay of dangerous radionuclides is on the order of 105-106 years. Corrosion of the containers and waste forms in the aqueous repository environment is therefore a concern. This Review describes the recent advances of the field of materials corrosion that are relevant to fundamental materials science issues associated with the long-term performance assessment and the design of materials with improved performance, where performance is defined as resistance to aqueous corrosion. Glass, crystalline ceramics, and metals are discussed separately, and the near-field interactions of these different material classes are also briefly addressed. Finally, recommendations for future directions of study are provided.

Electrochemical metrics for corrosion resistant alloys.

Corrosion is an electrochemical phenomenon. It can occur via different modes of attack, each having its own mechanisms, and therefore there are multiple metrics for evaluating corrosion resistance. In corrosion resistant alloys (CRAs), the rate of localized corrosion can exceed that of uniform corrosion by orders of magnitude. Therefore, instead of uniform corrosion rate, more complex electrochemical parameters are required to capture the salient features of corrosion phenomena. Here, we collect a database with an emphasis on metrics related to localized corrosion. The six sections of the database include data on various metal alloys with measurements of (1) pitting potential, Epit, (2) repassivation potential, Erp, (3) crevice corrosion potential, Ecrev, (4) pitting temperature, Tpit, (5) crevice corrosion temperature, Tcrev, and (6) corrosion potential, Ecorr, corrosion current density, icorr, passivation current density, ipass, and corrosion rate. The experimental data were collected from 85 publications and include Al- and Fe-based alloys, high entropy alloys (HEAs), and a Ni-Cr-Mo ternary system. This dataset could be used in the design of highly corrosion resistant alloys.

Layer- and gate-tunable spin-orbit coupling in a high-mobility few-layer semiconductor.

Spin-orbit coupling (SOC) is a relativistic effect, where an electron moving in an electric field experiences an effective magnetic field in its rest frame. In crystals without inversion symmetry, it lifts the spin degeneracy and leads to many magnetic, spintronic, and topological phenomena and applications. In bulk materials, SOC strength is a constant. Here, we demonstrate SOC and intrinsic spin splitting in atomically thin InSe, which can be modified over a broad range. From quantum oscillations, we establish that the SOC parameter α is thickness dependent; it can be continuously modulated by an out-of-plane electric field, achieving intrinsic spin splitting tunable between 0 and 20 meV. Unexpectedly, α could be enhanced by an order of magnitude in some devices, suggesting that SOC can be further manipulated. Our work highlights the extraordinary tunability of SOC in 2D materials, which can be harnessed for in operando spintronic and topological devices and applications.

The Chemical Design Principles for Axis-Dependent Conduction Polarity.

The recent discovery that specific materials can simultaneously exhibit n-type conduction and p-type conduction along different directions of the single crystal has the potential to impact a broad range of electronic and energy-harvesting technologies. Here, we establish the chemical design principles for creating materials with this behavior. First, we define the single-carrier and multicarrier mechanisms for axis-dependent conduction polarity and their identifying band structure fingerprints. We show using first-principles predictions that the AMX (A = Ca, Sr, Ba; M = Cu, Ag, Au; X = P, As, Sb) compounds consisting of MX honeycomb layers separated by A cations can exhibit p-type conduction in-plane and n-type conduction cross-plane via either mechanism depending on the doping level. We build up the band structure of BaCuAs using a molecular orbital approach to illustrate the structural origins of the two different mechanisms for axis-dependent conduction polarity. In total, this work shows this phenomenon can be quite prevalent in layered materials and reveals how to identify prospective materials.

Direct imaging of the nitrogen-rich edge in monolayer hexagonal boron nitride and its band structure tuning.

Identification of edge atoms and tracking the edge structure evolution of two-dimensional (2D) crystals at the scale of individual atoms is critical for understanding the edge-dominated properties and behavioral responses to external field stimuli. Here, direct imaging of the edge configuration of monolayer hexagonal boron nitride (h-BN) is demonstrated at the atomic scale, by using aberration-corrected transmission electron microscopy. Tracking of the edge atoms revealed that a nitrogen-terminated zigzag arrangement dominates along the edge, naturally leading to nitrogen rich (N-rich) characteristics in this area, while the stoichiometric interior of the h-BN monolayer is maintained. Both top-down fabrication and bottom-up growth were proposed to obtain novel h-BN flakes with an N-rich ratio larger than 1% when the size is reduced to the threshold of 25 nm. Furthermore, density functional theory calculations revealed that a new bandgap of ∼3 eV is created by the N-rich characteristics, and h-BN transforms into an n-type semiconductor by self-doping. The results call for the development of ultra-small h-BN islands to be used in intriguing 2D electronic devices with a photoresponse function to visible light.

The Fermi surface geometrical origin of axis-dependent conduction polarity in layered materials.

Electronic materials generally exhibit a single isotropic majority carrier type, electrons or holes. Some superlattice1,2 and hexagonal3-5 materials exhibit opposite conduction polarities along in-plane and cross-plane directions due to multiple electron and hole bands. Here, we uncover a material genus with this behaviour that originates from the Fermi surface geometry of a single band. NaSn2As2, a layered metal, has such a Fermi surface. It displays in-plane electron and cross-plane hole conduction in thermopower and exactly the opposite polarity in the Hall effect. The small Nernst coefficient and magnetoresistance preclude multi-band transport. We label this direction-dependent carrier polarity in single-band systems 'goniopolarity'. We expect to find goniopolarity and the Fermi surface geometry that produces it in many metals and semiconductors whose electronic structure is at the boundary between two and three dimensions. Goniopolarity may enable future explorations of complex transport phenomena that lead to unprecedented device concepts.

Raman Spectroscopy, Photocatalytic Degradation, and Stabilization of Atomically Thin Chromium Tri-iodide.

As a 2D ferromagnetic semiconductor with magnetic ordering, atomically thin chromium tri-iodide is the latest addition to the family of two-dimensional (2D) materials. However, realistic exploration of CrI3-based devices and heterostructures is challenging due to its extreme instability under ambient conditions. Here, we present Raman characterization of CrI3 and demonstrate that the main degradation pathway of CrI3 is the photocatalytic substitution of iodine by water. While simple encapsulation by Al2O3, PMMA, and hexagonal BN (hBN) only leads to modest reduction in degradation rate, minimizing light exposure markedly improves stability, and CrI3 sheets sandwiched between hBN layers are air-stable for >10 days. By monitoring the transfer characteristics of the CrI3/graphene heterostructure over the course of degradation, we show that the aquachromium solution hole-dopes graphene.

Three-dimensional imaging of individual point defects using selective detection angles in annular dark field scanning transmission electron microscopy.

We propose a new scanning transmission electron microscopy (STEM) technique that can realize the three-dimensional (3D) characterization of vacancies, lighter and heavier dopants with high precision. Using multislice STEM imaging and diffraction simulations of ß-Ga2O3 and SrTiO3, we show that selecting a small range of low scattering angles can make the contrast of the defect-containing atomic columns substantially more depth-dependent. The origin of the depth-dependence is the de-channeling of electrons due to the existence of a point defect in the atomic column, which creates extra "ripples" at low scattering angles. The highest contrast of the point defect can be achieved when the de-channeling signal is captured using the 20-40mrad detection angle range. The effect of sample thickness, crystal orientation, local strain, probe convergence angle, and experimental uncertainty to the depth-dependent contrast of the point defect will also be discussed. The proposed technique therefore opens new possibilities for highly precise 3D structural characterization of individual point defects in functional materials.

NaSn2As2: An Exfoliatable Layered van der Waals Zintl Phase.

The discovery of new families of exfoliatable 2D crystals that have diverse sets of electronic, optical, and spin-orbit coupling properties enables the realization of unique physical phenomena in these few-atom-thick building blocks and in proximity to other materials. Herein, using NaSn2As2 as a model system, we demonstrate that layered Zintl phases having the stoichiometry ATt2Pn2 (A = group 1 or 2 element, Tt = group 14 tetrel element, and Pn = group 15 pnictogen element) and feature networks separated by van der Waals gaps can be readily exfoliated with both mechanical and liquid-phase methods. We identified the symmetries of the Raman-active modes of the bulk crystals via polarized Raman spectroscopy. The bulk and mechanically exfoliated NaSn2As2 samples are resistant toward oxidation, with only the top surface oxidizing in ambient conditions over a couple of days, while the liquid-exfoliated samples oxidize much more quickly in ambient conditions. Employing angle-resolved photoemission spectroscopy, density functional theory, and transport on bulk and exfoliated samples, we show that NaSn2As2 is a highly conducting 2D semimetal, with resistivities on the order of 10-6 Ω·m. Due to peculiarities in the band structure, the dominating p-type carriers at low temperature are nearly compensated by the opening of n-type conduction channels as temperature increases. This work further expands the family of exfoliatable 2D materials to layered van der Waals Zintl phases, opening up opportunities in electronics and spintronics.

Probing carbonate in bone forming minerals on the nanometre scale.

To devise new strategies to treat bone disease in an ageing society, a more detailed characterisation of the process by which bone mineralises is needed. In vitro studies have suggested that carbonated mineral might be a precursor for deposition of bone apatite. Increased carbonate content in bone may also have significant implications in altering the mechanical properties, for example in diseased bone. However, information about the chemistry and coordination environment of bone mineral, and their spatial distribution within healthy and diseased tissues, is lacking. Spatially resolved analytical transmission electron microscopy is the only method available to probe this information at the length scale of the collagen fibrils in bone. In this study, scanning transmission electron microscopy combined with electron energy-loss spectroscopy (STEM-EELS) was used to differentiate between calcium-containing biominerals (hydroxyapatite, carbonated hydroxyapatite, beta-tricalcium phosphate and calcite). A carbon K-edge peak at 290 eV is a direct marker of the presence of carbonate. We found that the oxygen K-edge structure changed most significantly between minerals allowing discrimination between calcium phosphates and calcium carbonates. The presence of carbonate in carbonated HA (CHA) was confirmed by the formation of peak at 533 eV in the oxygen K-edge. These observations were confirmed by simulations using density functional theory. Finally, we show that this method can be utilised to map carbonate from the crystallites in bone. We propose that our calibration library of EELS spectra could be extended to provide spatially resolved information about the coordination environment within bioceramic implants to stimulate the development of structural biomaterials.

Phonon-induced diamagnetic force and its effect on the lattice thermal conductivity.

Phonons are displacements of atoms around their rest positions in a crystalline solid. They carry sound and heat, but are not classically associated with magnetism. Here, we show that phonons are, in fact, sensitive to magnetic fields, even in diamagnetic materials. We do so by demonstrating experimentally that acoustic phonons in a diamagnetic semiconductor (InSb) scatter more strongly from one another when a magnetic field is applied. We attribute this observation to the magnetic-field sensitivity of the anharmonicity of the interatomic bonds that govern the probability of phonon-phonon interactions. The displacements of atoms locally affect the orbital motion of valence band electrons, which, in the presence of an external magnetic field, spatially modulates the orbital diamagnetism around the displaced atoms. The spatial gradient in magnetic moment results in an anharmonic magnetic force exerted on the displaced atom. The process is modelled by ab initio calculations that, without the use of a single adjustable parameter, reproduce the observed 12% decrease in the lattice thermal conductivity under a 7 T magnetic field at a temperature of 5.2 K.

Improving the stability and optical properties of germanane via one-step covalent methyl-termination.

Two-dimensional van der Waals materials have shown great promise for a variety of electronic, optoelectronic, sensing and energy conversion applications. Since almost every atom in these two-dimensional crystals is exposed to the surface, covalent surface termination could provide a powerful method for the controlled tuning of material properties. Here we demonstrate a facile, one-step metathesis approach that directly converts CaGe2 crystals into mm-sized crystals of methyl-terminated germanane (GeCH3). Replacing --H termination in GeH with --CH3 increases the band gap by ~0.1 eV to 1.7 eV, and produces band edge fluorescence with a quantum yield of ~0.2%, with little dependence on layer thickness. Furthermore, the thermal stability of GeCH3 has been increased to 250 °C compared with 75 °C for GeH. This one-step metathesis approach should be applicable for accessing new families of two-dimensional van der Waals lattices that feature precise organic terminations and with enhanced optoelectronic properties.

Independent ordering of two interpenetrating magnetic sublattices in the double perovskite Sr2CoOsO6.

The insulating, fully ordered, double perovskite Sr2CoOsO6 undergoes two magnetic phase transitions. The Os(VI) ions order antiferromagnetically with a propagation vector k = (1/2, 1/2, 0) below TN1 = 108 K, while the high-spin Co(II) ions order antiferromagnetically with a propagation vector k = (1/2, 0, 1/2) below TN2 = 70 K. Ordering of the Os(VI) spins is accompanied by a structural distortion from tetragonal I4/m symmetry to monoclinic I2/m symmetry, which reduces the frustration of the face centered cubic lattice of Os(VI) ions. Density functional theory calculations show that the long-range Os-O-Co-O-Os and Co-O-Os-O-Co superexchange interactions are considerably stronger than the shorter Os-O-Co interactions. The poor energetic overlap between the 3d orbitals of Co and the 5d orbitals of Os appears to be responsible for this unusual inversion in the strength of short and long-range superexchange interactions.

Atomic-scale engineering of the electrostatic landscape of semiconductor surfaces.

A low-temperature scanning tunneling microscope was used in conjunction with density functional theory calculations to determine the binding sites and charge states of adsorbed Ga and Mn atoms on GaAs(110). To quantify the adatom charge states (both +1e), the Coulomb interaction with an individual Mn acceptor is measured via tunneling spectroscopy and compared with theoretical predictions. Several methods for positioning these charged adatoms are demonstrated, allowing us to engineer the electrostatic landscape of the surface with atomic precision.

Progress, challenges, and opportunities in two-dimensional materials beyond graphene.

Graphene's success has shown that it is possible to create stable, single and few-atom-thick layers of van der Waals materials, and also that these materials can exhibit fascinating and technologically useful properties. Here we review the state-of-the-art of 2D materials beyond graphene. Initially, we will outline the different chemical classes of 2D materials and discuss the various strategies to prepare single-layer, few-layer, and multilayer assembly materials in solution, on substrates, and on the wafer scale. Additionally, we present an experimental guide for identifying and characterizing single-layer-thick materials, as well as outlining emerging techniques that yield both local and global information. We describe the differences that occur in the electronic structure between the bulk and the single layer and discuss various methods of tuning their electronic properties by manipulating the surface. Finally, we highlight the properties and advantages of single-, few-, and many-layer 2D materials in field-effect transistors, spin- and valley-tronics, thermoelectrics, and topological insulators, among many other applications.