Exploring PtAg onto silanized biogenic silica as an electrocatalyst for H evolution: A combined experimental and theoretical investigation.

The task of creating a remarkably stable and effective electrochemical catalyst for efficient hydrogen evolution is arduous, primarily due to the multitude of factors that need to be taken into account for the industrial utilization of Pt. In this work, hybrid formation through in-situ reduction of Pt onto biogenic porous silica (Pt-SiO) is tested for its use as an efficient catalyst for hydrogen production. Exceptionally high electrocatalytic activity and excellent reusability of catalysts up to 200 cycles have been demonstrated.

Resonant Raman Scattering Study of Strain and Defects in Chemical Vapor Deposition Grown MoS Monolayers.

The development of bottom-up synthesis routes for semiconducting transition metal dichalcogenides (TMDs) and the assessment of their defects are of paramount importance to achieve their applications. TMD monolayers grown by chemical vapor deposition (CVD) can be subjected to significant strain and, here, Raman and photoluminescence spectroscopies are combined to characterize strain in over one hundred MoS monolayer samples grown by CVD. The frequency changes of phonons as a function of strain are analyzed, and used to extract the Grüneisen parameter of both zone-center and edge phonons.

Effects of post-transfer annealing and substrate interactions on the photoluminescence of 2D/3D monolayer WS/Ge heterostructures.

The ultraflat and dangling bond-free features of two-dimensional (2D) transition metal dichalcogenides (TMDs) endow them with great potential to be integrated with arbitrary three-dimensional (3D) substrates, forming mixed-dimensional 2D/3D heterostructures. As examples, 2D/3D heterostructures based on monolayer TMDs (, WS) and bulk germanium (Ge) have become emerging candidates for optoelectronic applications, such as ultrasensitive photodetectors that are capable of detecting broadband light from the mid-infrared (IR) to visible range. Currently, the study of WS/Ge(100) heterostructures is in its infancy and it remains largely unexplored how sample preparation conditions and different substrates affect their photoluminescence (PL) and other optoelectronic properties.

Spatial Control of Substitutional Dopants in Hexagonal Monolayer WS : The Effect of Edge Termination.

The ability to control the density and spatial distribution of substitutional dopants in semiconductors is crucial for achieving desired physicochemical properties. Substitutional doping with adjustable doping levels has been previously demonstrated in 2D transition metal dichalcogenides (TMDs); however, the spatial control of dopant distribution remains an open field. In this work, edge termination is demonstrated as an important characteristic of 2D TMD monocrystals that affects the distribution of substitutional dopants.

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

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.

Superconductivity enhancement in phase-engineered molybdenum carbide/disulfide vertical heterostructures.

Stacking layers of atomically thin transition-metal carbides and two-dimensional (2D) semiconducting transition-metal dichalcogenides, could lead to nontrivial superconductivity and other unprecedented phenomena yet to be studied. In this work, superconducting α-phase thin molybdenum carbide flakes were first synthesized, and a subsequent sulfurization treatment induced the formation of vertical heterolayer systems consisting of different phases of molybdenum carbide-ranging from α to γ' and γ phases-in conjunction with molybdenum sulfide layers. These transition-metal carbide/disulfide heterostructures exhibited critical superconducting temperatures as high as 6 K, higher than that of the starting single-phased α-MoC (4 K).

Selective Synthesis of BiTe/WS Heterostructures with Strong Interlayer Coupling.

The vertical integration of atomically thin-layered materials to create van der Waals heterostructures (vdWHs) has been proposed as a method to design nanostructures with emergent properties. In this work, epitaxial BiTe/WS vdWHs are synthesized via a two-step vapor deposition process. It is calculated that the vdWH has an indirect band gap with a valence band edge that bridges the vdW gap, resulting in a quenched photoluminescence (PL) from the WS monolayer, reduced intensity of its resonance Raman vibrational peaks, improved vertical charge transport, and a decrease in the intensity of second harmonic generation (SHG).

Universal Substitutional Doping of Transition Metal Dichalcogenides by Liquid-Phase Precursor-Assisted Synthesis.

Doping lies at the heart of modern semiconductor technologies. Therefore, for two-dimensional (2D) semiconducting transition metal dichalcogenides (TMDs), the significance of controlled doping is no exception. Recent studies have indicated that, by substitutionally doping 2D TMDs with a judicious selection of dopants, their electrical, optical, magnetic, and catalytic properties can be effectively tuned, endowing them with great potential for various practical applications.

Carbon doping of WS monolayers: Bandgap reduction and p-type doping transport.

Chemical doping constitutes an effective route to alter the electronic, chemical, and optical properties of two-dimensional transition metal dichalcogenides (2D-TMDs). We used a plasma-assisted method to introduce carbon-hydrogen (CH) units into WS monolayers. We found CH-groups to be the most stable dopant to introduce carbon into WS, which led to a reduction of the optical bandgap from 1.

Light-Emitting Transition Metal Dichalcogenide Monolayers under Cellular Digestion.

2D materials cover a wide spectrum of electronic properties. Their applications are extended from electronic, optical, and chemical to biological. In terms of biomedical uses of 2D materials, the interactions between living cells and 2D materials are of paramount importance.

Optical identification of sulfur vacancies: Bound excitons at the edges of monolayer tungsten disulfide.

Defects play a significant role in tailoring the optical properties of two-dimensional materials. Optical signatures of defect-bound excitons are important tools to probe defective regions and thus interrogate the optical quality of as-grown semiconducting monolayer materials. We have performed a systematic study of defect-bound excitons using photoluminescence (PL) spectroscopy combined with atomically resolved scanning electron microscopy and first-principles calculations.

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

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.

Photoluminescence Segmentation within Individual Hexagonal Monolayer Tungsten Disulfide Domains Grown by Chemical Vapor Deposition.

We show that hexagonal domains of monolayer tungsten disulfide (WS) grown by chemical vapor deposition (CVD) with powder precursors can have discrete segmentation in their photoluminescence (PL) emission intensity, forming symmetric patterns with alternating bright and dark regions. Two-dimensional maps of the PL reveal significant reduction within the segments associated with the longest sides of the hexagonal domains. Analysis of the PL spectra shows differences in the exciton to trion ratio, indicating variations in the exciton recombination dynamics.

Defect Coupling and Sub-Angstrom Structural Distortions in WMoS 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.

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

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.

Ultrasensitive molecular sensor using N-doped graphene through enhanced Raman scattering.

As a novel and efficient surface analysis technique, graphene-enhanced Raman scattering (GERS) has attracted increasing research attention in recent years. In particular, chemically doped graphene exhibits improved GERS effects when compared with pristine graphene for certain dyes, and it can be used to efficiently detect trace amounts of molecules. However, the GERS mechanism remains an open question.

Excitonic Effects in Tungsten Disulfide Monolayers on Two-Layer Graphene.

Light emission in atomically thin heterostructures is known to depend on the type of materials and the number and stacking sequence of the constituent layers. Here we show that the thickness of a two-dimensional substrate can be crucial in modulating the light emission. We study the layer-dependent charge transfer in vertical heterostructures built from monolayer tungsten disulfide (WS2) on one- and two-layer epitaxial graphene, unravelling the effect that the interlayer electronic coupling has on the excitonic properties of such heterostructures.

Electric-Field-Assisted Directed Assembly of Transition Metal Dichalcogenide Monolayer Sheets.

Directed assembly of two-dimensional (2D) layered materials, such as transition metal dichalcogenides, holds great promise for large-scale electronic and optoelectronic applications. Here, we demonstrate controlled placement of solution-suspended monolayer tungsten disulfide (WS2) sheets on a substrate using electric-field-assisted assembly. Micrometer-sized triangular WS2 monolayers are selectively positioned on a lithographically defined interdigitated guiding electrode structure using the dielectrophoretic force induced on the sheets in a nonuniform field.

Ultrasensitive gas detection of large-area boron-doped graphene.

Heteroatom doping is an efficient way to modify the chemical and electronic properties of graphene. In particular, boron doping is expected to induce a p-type (boron)-conducting behavior to pristine (nondoped) graphene, which could lead to diverse applications. However, the experimental progress on atomic scale visualization and sensing properties of large-area boron-doped graphene (BG) sheets is still very scarce.

Tellurium-Assisted Low-Temperature Synthesis of MoS2 and WS2 Monolayers.

Chemical vapor deposition (CVD) is a scalable method able to synthesize MoS2 and WS2 monolayers. In this work, we reduced the synthesis temperature by 200 °C only by introducing tellurium (Te) into the CVD process. The as-synthesized MoS2 and WS2 monolayers show high phase purity and crystallinity.