Ultrasensitive Room-Temperature Piezoresistive Transduction in Graphene-Based Nanoelectromechanical Systems

The low mass and high quality factors that nanomechanical resonators exhibit lead to exceptional sensitivity in the frequency domain. This is especially appealing for the design of ultrasensitive force and mass sensors. The sensitivity of a nanomechanical mass and force sensor depends on its mass and quality factor; a low resonator mass and a higher quality factor reduce both the minimum resolvable mass and force. Graphene, a single atomic layer thick membrane is an ideal candidate for nanoelectromechanical resonators due to its extremely low mass and high stiffness. Here, we show that by employing the intrinsic piezoresistivity of graphene to transduce its motion in nanoelectromechanical systems, we approach a force resolution of 16.3 ± 0.8 aN/Hz^1/2 and a minimum detectable mass of 1.41 ± 0.02 zeptograms (10^–21 g) at ambient temperature. Quality factors of the driven response of the order of 10³ at pressures ∼10^–6 Torr on several devices are also observed. Moreover, we demonstrate this at ambient temperature on chemical-vapor-deposition-grown graphene to allow for scale-up, thus demonstrating its potential for applications requiring exquisite force and mass resolution such as mass spectroscopy and magnetic resonance force microscopy

Controlled Preferential Oxidation of Grain Boundaries in Monolayer Tungsten Disulfide for Direct Optical Imaging

Synthetic 2D crystal films grown by chemical vapor deposition are typically polycrystalline, and determining grain size within domains and continuous films is crucial for determining their structure. Here we show that grain boundaries in the 2D transition metal dichalcogenide WS₂, grown by CVD, can be preferentially oxidized by controlled heating in air. Under our developed conditions, preferential degradation at the grain boundaries causes an increase in their physical size due to oxidation. This increase in size enables their clear and rapid identification using a standard optical microscope. We demonstrate that similar treatments in an Ar environment do no show this effect, confirming that oxidation is the main role in the structural change. Statistical analysis of grain boundary (GB) angles shows dominant mirror formation. Electrical biasing across the GB is shown to lead to changes at the GB and their observation under an optical microscope. Our approach enables high-throughput screening of as-synthesized WS₂ domains and continuous films to determine their crystallinity and should enable improvements in future CVD growth of these materials

Electroluminescence Dynamics across Grain Boundary Regions of Monolayer Tungsten Disulfide

We study how grain boundaries (GB) in chemical vapor deposition (CVD) grown monolayer WS₂ influence the electroluminescence (EL) behavior in lateral source-drain devices under bias. Real time imaging of the WS₂ EL shows arcing between the electrodes when probing across a GB, which then localizes at the GB region as it erodes under high bias conditions. In contrast, single crystal WS₂ domains showed no signs of arcing or localized EL. Analysis of the eroded GB region shows the formation of micro- and nanoribbons across the monolayer WS₂ domains. Comparison of the EL spectrum with the photoluminescence spectrum from the monolayer WS₂ shows close agreement, indicating the EL emission comes from direct band gap excitonic recombination. These results provide important insights into EL devices that utilize CVD grown monolayer transition metal dichalcogenides when GBs are present in the active device region

Growth of Large Single-Crystalline Monolayer Hexagonal Boron Nitride by Oxide-Assisted Chemical Vapor Deposition

We show how an oxide passivating layer on the Cu surface before the growth of h-BN by chemical vapor deposition (CVD) can lead to increased domain sizes from 1 to 20 μm by reducing the nucleation density from 10⁶ to 10³ mm^–2. The h-BN domains within each Cu grain are well-oriented, indicating an epitaxial relationship between the h-BN crystals and the Cu growth substrates that leads to larger crystal domains within the film of ∼100 μm. Continuous films are grown and show a high degree of monolayer uniformity. This CVD approach removes the need for low pressures, electrochemical polishing, and expensive substrates for large-area continuous films of h-BN monolayers, which is beneficial for industrial applications that require scalable synthesis

Shape Evolution of Monolayer MoS₂ Crystals Grown by Chemical Vapor Deposition

Atmospheric-pressure chemical vapor deposition (CVD) is used to grow monolayer MoS₂ two-dimensional crystals at elevated temperatures on silicon substrates with a 300 nm oxide layer. Our CVD reaction is hydrogen free, with the sulfur precursor placed in a furnace separate from the MoO₃ precursor to individually control their heating profiles and provide greater flexibility in the growth recipe. We intentionally establish a sharp gradient of MoO₃ precursor concentration on the growth substrate to explore its sensitivity to the resultant MoS₂ domain growth within a relatively uniform temperature range. We find that the shape of MoS₂ domains is highly dependent upon the spatial location on the silicon substrate, with variation from triangular to hexagonal geometries. The shape change of domains is attributed to local changes in the Mo:S ratio of precursors (1:>2, 1:2, and 1:<2) and its influence on the kinetic growth dynamics of edges. These results improve our understanding of the factors that influence the growth of MoS₂ domains and their shape evolution

Revealing Strain-Induced Effects in Ultrathin Heterostructures at the Nanoscale

Two-dimensional materials are being increasingly studied, particularly for flexible and wearable technologies because of their inherent thickness and flexibility. Crucially, one aspect where our understanding is still limited is on the effect of mechanical strain, not on individual sheets of materials, but when stacked together as heterostructures in devices. In this paper, we demonstrate the use of Kelvin probe microscopy in capturing the influence of uniaxial tensile strain on the band-structures of graphene and WS₂ (mono- and multilayered) based heterostructures at high resolution. We report a major advance in strain characterization tools through enabling a single-shot capture of strain defined changes in a heterogeneous system at the nanoscale, overcoming the limitations (materials, resolution, and substrate effects) of existing techniques such as optical spectroscopy. Using this technique, we observe that the work-functions of graphene and WS₂ increase as a function of strain, which we attribute to the Fermi level lowering from increased p-doping. We also extract the nature of the interfacial heterojunctions and find that they get strongly modulated from strain. We observe that the strain-enhanced charge transfer with the substrate plays a dominant role, causing the heterostructures to behave differently from two-dimensional materials in their isolated forms

High-Performance All 2D-Layered Tin Disulfide: Graphene Photodetecting Transistors with Thickness-Controlled Interface Dynamics

Tin disulfide crystals with layered two-dimensional (2D) sheets are grown by chemical vapor deposition using a novel precursor approach and integrated into all 2D transistors with graphene (Gr) electrodes. The Gr:SnS₂:Gr transistors exhibit excellent photodetector response with high detectivity and photoresponsivity. We show that the response of the all 2D photodetectors depends upon charge trapping at the interface and the Schottky barrier modulation. The thickness-dependent SnS₂ measurements in devices reveal a transition from the interface-dominated response for thin crystals to bulklike response for the thicker SnS₂ crystals, showing the sensitivity of devices fabricated using layered materials on the number of layers. These results show that SnS₂ has photosensing performance when combined with Gr electrodes that is comparable to other 2D transition metal dichalcogenides of MoS₂ and WS₂

Large Dendritic Monolayer MoS₂ Grown by Atmospheric Pressure Chemical Vapor Deposition for Electrocatalysis

The edge sites of MoS₂ are catalytically active for the hydrogen evolution reaction (HER), and growing monolayer structures that are edge-rich is desirable. Here, we show the production of large-area highly branched MoS₂ dendrites on amorphous SiO₂/Si substrates using an atmospheric pressure chemical vapor deposition and explore their use in electrocatalysis. By tailoring the substrate construction, the monolayer MoS₂ evolves from triangular to dendritic morphology because of the change of growth conditions. The rough edges endow dendritic MoS₂ with a fractal dimension down to 1.54. The highly crystalline basal plane and the edge of the dendrites are visualized at atomic resolution using an annular dark field scanning transmission electron microscope. The monolayer dendrites exhibit strong photoluminescence, which is indicative of the direct band gap emission, which is preserved after being transferred. Post-transfer sulfur annealing restores the structural defects and decreases the n-type doping in MoS₂ monolayers. The annealed MoS₂ dendrites show good and highly durable HER performance on the glassy carbon with a large exchange current density of 32 μA cm_geo^–2, demonstrating its viability as an efficient HER catalyst

Doping Graphene Transistors Using Vertical Stacked Monolayer WS₂ Heterostructures Grown by Chemical Vapor Deposition

We study the interactions in graphene/WS₂ two-dimensional (2D) layered vertical heterostructures with variations in the areal coverage of graphene by the WS₂. All 2D materials were grown by chemical vapor deposition and transferred layer by layer. Photoluminescence (PL) spectroscopy of WS₂ on graphene showed PL quenching along with an increase in the ratio of exciton/trion emission, relative to WS₂ on SiO₂ surface, indicating a reduction in the n-type doping levels of WS₂ as well as reduced radiative recombination quantum yield. Electrical measurements of a total of 220 graphene field effect transistors with different WS₂ coverage showed double-Dirac points in the field effect measurements, where one is shifted closer toward the 0 V gate neutrality position due to the WS₂ coverage. Photoirradiation of the WS₂ on graphene region caused further Dirac point shifts, indicative of a reduction in the p-type doping levels of graphene, revealing that the photogenerated excitons in WS₂ are split across the heterostructure by electron transfer from WS₂ to graphene. Kelvin probe microscopy showed that regions of graphene covered with WS₂ had a smaller work function and supports the model of electron transfer from WS₂ to graphene. Our results demonstrate the formation of junctions within a graphene transistor through the spatial tuning of the work function of graphene using these 2D vertical heterostructures

Chemical Vapor Deposition Growth of Two-Dimensional Monolayer Gallium Sulfide Crystals Using Hydrogen Reduction of Ga₂S₃

Two-dimensional gallium sulfide (GaS) crystals are synthesized by a simple and efficient ambient pressure chemical vapor deposition (CVD) method using a single-source precursor of Ga₂S₃. The synthesized GaS structures involve triangular monolayer domains and multilayer flakes with thickness of 1 and 15 nm, respectively. Regions of continuous films of GaS are also achieved with about 0.7 cm² uniform coverage. This is achieved by using hydrogen carrier gas and the horizontally placed SiO₂/Si substrates. Electron microscopy and spectroscopic measurements are used to characteristic the CVD-grown materials. This provides important insights into novel approaches for enlarging the domain size of GaS crystals and understanding of the growth mechanism using this precursor system