Picometer-Precision Atomic Position Tracking through Electron Microscopy

The modern aberration-corrected scanning transmission electron microscopes (AC-STEM) have successfully achieved direct visualization of atomic columns ...

The Pennsylvania State University

Phase-pure samples of a metastable allotrope of silicon, Si-III or BC8, were synthesized by direct elemental transformation at 14 GPa and ∼900 K and also at significantly reduced pressure in the Na-Si system at 9.5 GPa by quenching from high temperatures ∼1000 K. Pure sintered polycrystalline ingots with dimensions ranging from 0.5 to 2 mm can be easily recovered at ambient conditions. The chemical route also allowed us to decrease the synthetic pressures to as low as 7 GPa, while pressures required for direct phase transition in elemental silicon are significantly higher. In situ control of the synthetic protocol, using synchrotron radiation, allowed us to observe the underlying mechanism of chemical interactions and phase transformations in the Na-Si system. Detailed characterization of Si-III using X-ray diffraction, Raman spectroscopy, (29)Si NMR spectroscopy, and transmission electron microscopy are discussed. These large-volume syntheses at significantly reduced pressures extend the range of possible future bulk characterization methods and applications.

Synthesis of Bulk BC8 Silicon Allotrope by Direct Transformation and Reduced-Pressure Chemical Pathways.

Whether an alloy is random or ordered can have profound effects on its properties. The close chemical similarity of W and Mo in the two-dimensional semiconductors MoS and WS has led to the expectation that WMoS is a random alloy. Here we report that triangular monolayer flakes of WMoS produced by sulfurization of MoO/WO are not only nonrandom, but also anisotropic: W and Mo form atomically thin chains oriented parallel to the edges of the triangle, especially around x ∼ 0.5, as resolved by aberration-corrected transmission electron microscopy. First-principles calculations reveal that the binding energies of striped and random alloys are nearly identical but that phase segregation at the growth edge favors one metal over another depending on the local sulfur availability, independent of the composition deeper inside the monolayer. Thus, atomically thin striping is kinetically driven and controlled by fluctuations that couple the local chemical potentials of metals and chalcogenide. Considering the nearly identical electronic properties but very different atomic masses of Mo and W, the resulting striped alloy is electronically isotropic, but vibrationally anisotropic. Phonon anomalies associated with the stripe ordering are predicted, as is an anisotropic thermal conductivity. More generally, fluctuation-driven striping provides a mechanism to produce in-plane subnanometer superlattices within two-dimensional crystals, with broad implications for controlling the electronic, optical, and structural properties of these systems.

Spontaneous Formation of Atomically Thin Stripes in Transition Metal Dichalcogenide Monolayers.

Two-dimensional materials offer a remarkably rich materials platform to study the origin of different material behaviors at the atomic level, and doping provides a key means of tailoring such materials' functional properties. The local atomic structure around such dopants can be critically important in determining the material's behavior as it could modulate scattering, catalytic activity, electronic and magnetic properties, and so forth. Here, using aberration-corrected scanning transmission electron microscopy (STEM) with sub-Ångstrom resolution in conjunction with density functional theory calculations, we demonstrate a strong coupling between Mo dopants and two types of defects in WS monolayers: sulfur monovacancies and grain boundaries. Although Mo does occupy a transition metal lattice site, it is not an ideal substitutional dopant: ∼80% of the S vacancies identified by STEM colocalize with Mo dopants, an affinity that appears to be enhanced by symmetry breaking of a partially occupied midgap defect state. Although a Mo dopant by itself does not considerably distort the WS lattice, it induces substantial lattice deformation by apparently facilitating the charging of a sulfur monovacancy paired with it, which is consistent with the results of first-principles calculations. This coupling of foreign substitutional dopants with vacancies could potentially be exploited to control the distribution and location of chalcogenide vacancies within transition metal dichalcogenides (TMD), by segregating vacancies into regions of high Mo concentration that are purposely placed away from active regions of TMD-based devices.

Defect Coupling and Sub-Angstrom Structural Distortions in WMoS Monolayers.

Large-area (∼cm) films of vertical heterostructures formed by alternating graphene and transition-metal dichalcogenide (TMD) alloys are obtained by wet chemical routes followed by a thermal treatment at low temperature. In particular, we synthesized stacked graphene and WMoS alloy phases that were used as hydrogen evolution catalysts. We observed a Tafel slope of 38.7 mV dec and 96 mV onset potential (at current density of 10 mA cm) when the heterostructure alloy was annealed at 300 °C. These results indicate that heterostructures formed by graphene and WMoS alloys are far more efficient than WS and MoS by at least a factor of 2, and they are superior compared to other reported TMD systems. This strategy offers a cheap and low temperature synthesis alternative able to replace Pt in the hydrogen evolution reaction (HER). Furthermore, the catalytic activity of the alloy is stable over time, i.e., the catalytic activity does not experience a significant change even after 1000 cycles. Using density functional theory calculations, we found that this enhanced hydrogen evolution in the WMoS alloys is mainly due to the lower energy barrier created by a favorable overlap of the d-orbitals from the transition metals and the s-orbitals of H; with the lowest energy barrier occurring for the WMoS alloy. Thus, it is now possible to further improve the performance of the "inert" TMD basal plane via metal alloying, in addition to the previously reported strategies such as creation of point defects, vacancies and edges. The synthesis of graphene/WMoS produced at relatively low temperatures is scalable and could be used as an effective low cost Pt-free catalyst.

Low-temperature Synthesis of Heterostructures of Transition Metal Dichalcogenide Alloys (WMoS) and Graphene with Superior Catalytic Performance for Hydrogen Evolution.

Polymer dielectrics are the preferred materials of choice for power electronics and pulsed power applications. However, their relatively low operating temperatures significantly limit their uses in harsh-environment energy storage devices, e.g., automobile and aerospace power systems. Herein, hexagonal boron nitride (h-BN) films are prepared from chemical vapor deposition (CVD) and readily transferred onto polyetherimide (PEI) films. Greatly improved performance in terms of discharged energy density and charge-discharge efficiency is achieved in the PEI sandwiched with CVD-grown h-BN films at elevated temperatures when compared to neat PEI films and other high-temperature polymer and nanocomposite dielectrics. Notably, the h-BN-coated PEI films are capable of operating with >90% charge-discharge efficiencies and delivering high energy densities, i.e., 1.2 J cm , even at a temperature close to the glass transition temperature of polymer (i.e., 217 °C) where pristine PEI almost fails. Outstanding cyclability and dielectric stability over a straight 55 000 charge-discharge cycles are demonstrated in the h-BN-coated PEI at high temperatures. The work demonstrates a general and scalable pathway to enable the high-temperature capacitive energy applications of a wide range of engineering polymers and also offers an efficient method for the synthesis and transfer of 2D nanomaterials at the scale demanded for applications.

High-Performance Polymers Sandwiched with Chemical Vapor Deposited Hexagonal Boron Nitrides as Scalable High-Temperature Dielectric Materials.

The effective use of the van der Waals 2D materials relies on their successful transfer from the growth substrate onto other target substrates in the form of large films or flakes. In particular, it is important to transfer such atomically thin samples to various substrates with minimal sample damage and exposure to etchants and chemicals to realize their applications. Here we develop a universal transfer method that not only is free of reactive etchants, but also can maintain the film morphology intact with no tears and cracks. We show a variety of different 2D crystals and thin films with various sizes and thicknesses transferred from different substrates, i.e. metal-organic chemical vapor deposition-grown WSe coalesced thin films on sapphire and mechanically exfoliated 2D crystalline flakes on titanium nitride. Further examination by transmission electron microscopy indicates successful transfer of all the samples. This study presents a universal and etchant-free transfer method that can be used to transfer a variety of 2D crystals and other nanostructures from/to various substrates.

Etchant-free transfer of 2D nanostructures.

A multistep diffusion-mediated process was developed to control the nucleation density, size, and lateral growth rate of WSe domains on c-plane sapphire for the epitaxial growth of large area monolayer films by gas source chemical vapor deposition (CVD). The process consists of an initial nucleation step followed by an annealing period in HSe to promote surface diffusion of tungsten-containing species to form oriented WSe islands with uniform size and controlled density. The growth conditions were then adjusted to suppress further nucleation and laterally grow the WSe islands to form a fully coalesced monolayer film in less than 1 h. Postgrowth structural characterization demonstrates that the WSe monolayers are single crystal and epitaxially oriented with respect to the sapphire and contain antiphase grain boundaries due to coalescence of 0° and 60° oriented WSe domains. The process also provides fundamental insights into the two-dimensional (2D) growth mechanism. For example, the evolution of domain size and cluster density with annealing time follows a 2D ripening process, enabling an estimate of the tungsten-species surface diffusivity. The lateral growth rate of domains was found to be relatively independent of substrate temperature over the range of 700-900 °C suggesting a mass transport limited process, however, the domain shape (triangular versus truncated triangular) varied with temperature over this same range due to local variations in the Se/W adatom ratio. The results provide an important step toward atomic level control of the epitaxial growth of WSe monolayers in a scalable process that is suitable for large area device fabrication.

Diffusion-Controlled Epitaxy of Large Area Coalesced WSe Monolayers on Sapphire.

Expanding the application space of transparent electrodes toward the ultraviolet range has been found challenging when utilizing the conventional approach to degenerately dope semiconductors with band gaps larger than ZnO or InO. Here, it is shown that the correlated metal SrNbO with  < 1 is ideally suited as a UV-transparent electrode material, enabling UV light-emitting diodes for a wide range of applications from water disinfection to polymer curing. It is demonstrated that SrNbO thin films can be grown by radio frequency (RF) sputtering and that they remain in the perovskite phase despite a sizeable Sr deficiency. The electrical and optical properties are characterized and compared to those of commonly used indium tin oxide (ITO) and Sn-doped GaO transparent conductor standards. SrNbO films were found to have sheet resistances as low as 30 Ω sq with optical transmission at a wavelength of 280 nm up to 86%, marking a two-order-of-magnitude increase over the performance of traditional UV-transparent conductors. The compatibility of SrNbO with a physical vapor deposition technique that is widely employed in the transparent conductor coating industry along with the robustness of the highly electrically conducting and optically transparent perovskite phase makes it an ideal transparent electrode for applications in the UV spectrum.

Sputtered SrNbO as a UV-Transparent Conducting Film.

Metalattices are crystalline arrays of uniform particles in which the period of the crystal is close to some characteristic physical length scale of the material. Here, we explore the synthesis and properties of a germanium metalattice in which the ∼70 nm periodicity of a silica colloidal crystal template is close to the ∼24 nm Bohr exciton radius of the nanocrystalline Ge replica. The problem of Ge surface oxidation can be significant when exploring quantum confinement effects or designing electronically coupled nanostructures because of the high surface area to volume ratio at the nanoscale. To eliminate surface oxidation, we developed a core-shell synthesis in which the Ge metalattice is protected by an oxide-free Si interfacial layer, and we explore its properties by transmission electron microscopy (TEM), Raman spectroscopy, and electron energy loss spectroscopy (EELS). The interstices of a colloidal crystal film grown from 69 nm diameter spherical silica particles were filled with polycrystalline Ge by high-pressure confined chemical vapor deposition (HPcCVD) from GeH. After the SiO template was etched away with aqueous HF, the Ge replica was uniformly coated with an amorphous Si shell by HPcCVD as confirmed by TEM-EDS (energy-dispersive X-ray spectroscopy) and Raman spectroscopy. Formation of the shell prevents oxidation of the Ge core within the detection limit of XPS. The electronic properties of the core-shell structure were studied by accessing the Ge 3d edge onset using STEM-EELS. A blue shift in the edge onset with decreasing size of Ge sites in the metalattices suggests quantum confinement of the Ge core. The degree of quantum confinement of the Ge core depends on the void sizes in the template, which is tunable by using silica particles of varying size. The edge onset also shows a shift to higher energy near the shell in comparison with the Ge core. This shift along with the observation of Ge-Si vibrational modes in the Raman spectrum indicate interdiffusion of Ge and Si. Both the size of the voids in the template and core-shell interdiffusion of Si and Ge can in principle be tuned to modify the electronic properties of the Ge metalattice.

Oxide-Free Three-Dimensional Germanium/Silicon Core-Shell Metalattice Made by High-Pressure Confined Chemical Vapor Deposition.

Dilute magnetic semiconductors (DMS), achieved through substitutional doping of spin-polarized transition metals into semiconducting systems, enable experimental modulation of spin dynamics in ways that hold great promise for novel magneto-electric or magneto-optical devices, especially for two-dimensional (2D) systems such as transition metal dichalcogenides that accentuate interactions and activate valley degrees of freedom. Practical applications of 2D magnetism will likely require room-temperature operation, air stability, and (for magnetic semiconductors) the ability to achieve optimal doping levels without dopant aggregation. Here, room-temperature ferromagnetic order obtained in semiconducting vanadium-doped tungsten disulfide monolayers produced by a reliable single-step film sulfidation method across an exceptionally wide range of vanadium concentrations, up to 12 at% with minimal dopant aggregation, is described. These monolayers develop p-type transport as a function of vanadium incorporation and rapidly reach ambipolarity. Ferromagnetism peaks at an intermediate vanadium concentration of ~2 at% and decreases for higher concentrations, which is consistent with quenching due to orbital hybridization at closer vanadium-vanadium spacings, as supported by transmission electron microscopy, magnetometry, and first-principles calculations. Room-temperature 2D-DMS provide a new component to expand the functional scope of van der Waals heterostructures and bring semiconducting magnetic 2D heterostructures into the realm of practical application.

Monolayer Vanadium-Doped Tungsten Disulfide: A Room-Temperature Dilute Magnetic Semiconductor.

Realization of wafer-scale single-crystal films of transition metal dichalcogenides (TMDs) such as WS requires epitaxial growth and coalescence of oriented domains to form a continuous monolayer. The domains must be oriented in the same crystallographic direction on the substrate to inhibit the formation of inversion domain boundaries (IDBs), which are a common feature of layered chalcogenides. Here we demonstrate fully coalesced unidirectional WS monolayers on 2 in. diameter -plane sapphire by metalorganic chemical vapor deposition using a multistep growth process to achieve epitaxial WS monolayers with low in-plane rotational twist (0.09°). Transmission electron microscopy analysis reveals that the WS monolayers are largely free of IDBs but instead have translational boundaries that arise when WS domains with slightly offset lattices merge together. By regulating the monolayer growth rate, the density of translational boundaries and bilayer coverage were significantly reduced. The unidirectional orientation of domains is attributed to the presence of steps on the sapphire surface coupled with growth conditions that promote surface diffusion, lateral domain growth, and coalescence while preserving the aligned domain structure. The transferred WS monolayers show neutral and charged exciton emission at 80 K with negligible defect-related luminescence. Back-gated WS field effect transistors exhibited an / of ∼10 and mobility of 16 cm/(V s). The results demonstrate the potential of achieving wafer-scale TMD monolayers free of inversion domains with properties approaching those of exfoliated flakes.

Nasim Alem

Department of Materials Science & Engineering

The Pennsylvania State University

Picometer-Precision Atomic Position Tracking through Electron Microscopy

Nasim Alem

.css-o250i3{font-size:18px;font-family:Open Sans,sans-serif;line-height:21.6px;}Department of Materials Science & Engineering

The Pennsylvania State University

Picometer-Precision Atomic Position Tracking through Electron Microscopy

Department of Materials Science & Engineering