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Electron ptychography has recently attracted considerable interest for high resolution phase-sensitive imaging. However, to date studies have been mainly limited to radiation resistant samples as the electron dose required to record a ptychographic dataset is too high for use with beam-sensitive materials. Here we report defocused electron ptychography using a fast, direct-counting detector to reconstruct the transmission function, which is in turn related to the electrostatic potential of a two-dimensional material at atomic resolution under various low dose conditions.

In close quarters: Nicotinamide adenine dinucleotide phosphate (NADPH) and NADP+ are cycled rapidly between ferredoxin–NADP+ reductase (FNR) and a dehydrogenase enzyme (E2) within the 5–100 nm pores of an indium tin oxide electrode. The electrode mimics the nanoconfinement strategies of natural systems, such as the chloroplast stroma or mitochondria in living cells.In living cells, redox chains rely on nanoconfinement using tiny enclosures, such as the mitochondrial matrix or chloroplast stroma, to concentrate enzymes and limit distances that nicotinamide cofactors and other metabolites must diffuse. In a chemical analogue exploiting this principle, nicotinamide adenine dinucleotide phosphate (NADPH) and NADP+ are cycled rapidly between ferredoxin–NADP+ reductase and a second enzyme—the pairs being juxtaposed within the 5–100 nm scale pores of an indium tin oxide electrode. The resulting electrode material, denoted (FNR+E2)@ITO/support, can drive and exploit a potentially large number of enzyme-catalysed reactions.

Monolayer MoS2is a wide-bandgap semiconductor suitable for use in high-temperature electronics. It is therefore important to understand its thermal stability. We report the results of a study on thermal degradation of MoS2monolayers supported on SrTiO3substrates in ultrahigh vacuum (UHV). Our studies were carried out on the (111), (110), and (001) terminations of SrTiO3substrates, but MoS2was found to degrade on all of these surfaces in a similar way. By scanning tunneling microscopy, we show that MoS2monolayer crystals maintain their structure up to 700 °C under UHV, at which point triangular etch trenches appear along the ⟨21̅1̅0⟩ lattice directions (i.e., sulfur-terminated edge directions) of the MoS2crystals. The trenches are due to the preferential loss of sulfur, allowing molybdenum to be oxidized by oxygen originating from the SrTiO3substrate. The intensity of the A-exciton photoluminescence (PL) peak and the E2g1and A1gRaman signals reduced significantly following treatment at this temperature. The crystals continue to degrade at higher annealing temperatures in UHV until they transform into MoOx(x= 2–3) particles at 900 °C, and the optical properties characteristic of MoS2are lost entirely in PL and Raman spectra. The initial sulfur loss and the formation of MoOxare confirmed by X-ray photoelectron spectroscopy. The macroscopic triangular shapes of the MoS2crystals are retained until the residual particles evaporate at above 1000 °C. The optical properties of the 700 and 800 °C UHV-annealed samples can be partially recovered upon sulfur annealing. This work establishes a pathway of the thermal degradation of SrTiO3-supported monolayer MoS2in vacuum from smooth MoS2crystals to crystals with sulfur vacancies (etch trenches), followed by MoO2and finally MoO3particles. We also demonstrate how sulfur annealing can be used to heal the defects.

A hybrid catalyst, Pt nanocrystals deposited on the surface of MoS2 vertically standing nanoplatelets, is synthesized via chemical vapor deposition and subsequent thermal annealing (TA) of Pt precursor. The hybrid material shows promising results as an electrocatalyst for the hydrogen evolution reaction (HER). By varying Pt synthesis conditions—precursor loading and TA temperature—the deposition sites, size, and morphology of the Pt nanostructure can be controlled. The size effect of a Pt nanoparticle on catalytic activity and sintering resistance is discussed. Results show that higher Pt loading yields better HER performance despite smaller specific surface area; higher TA temperature delivers larger average particle size of Pt crystals and lower HER activity. Larger average size leads to fast sintering and thus poor durability of the catalyst. On the basis of the correlation between HER performance and growth behaviors of Pt on MoS2 surfaces, an optimization route for a highly active and stable cocatalyst can be established. The optimized Pt-MoS2 catalyst (400 °C, 11 wt %) reported in this study possesses superior overpotential of 9 mV (close to zero), Tafel slope of 44 mV/dec, and moderate exchange current density of 373 μA/cm2; it exhibits activity degradation of 140 mV @ 20 mA/cm2 after 10 000 cycles. The Tafel slope indicates the combination of Volmer–Heyrovsky steps as HER mechanism in this particular hybrid catalyst system. The outstanding HER activity attributes to highly dispersed Pt nanoparticles grown on MoS2 basal surfaces, large MoS2 edge density, and Pt–S bonding effect induced activity improvement of MoS2 as well as 3D porous network assisted superaerophobic surface.

We examine the atomic structure of chemical vapour deposition grown multilayer WS2pyramids using aberration corrected annular dark field scanning transmission electron microscopy coupled with anin situ heating holder. The stacking orders and specific types of defects after partial degradation by S and W atomic loss at high temperature are resolved layer-by-layer. Our study of an individual WS2pyramid with at least six layers, reveals a mixed 2H and 3R polytype stacking. Etching occurred both top and bottom of the WS2pyramid, which aids in determining the exact vertical layer stacking configurations in the thicker regions. We provide an extensive catalogue of the contrast profiles associated with defects in WS2as a function of layer number and stacking type, as imaged using ADF-STEM. These results provide extensive details about the identification of a wide range of defects in S2layers, and the unique ADF-STEM contrast patterns that arise from complex multilayer stacking.

Monolayer transition metal dichalcogenides (TMDs) have demonstrated great potential in next-generation electronics due to their unique optical and electronic properties. However, it remains challen...

We investigate the 2D-layer-dependent electronic and optoelectronic properties of lateral Au/graphene/WS2 photodetecting diodes. All 2D materials used [graphene and WS2 domains [monolayer (1L) and ...

Sustainability has become a critical concern in the semiconductor industry as hazardous wastes released during the manufacturing process of semiconductor devices have an adverse impact on human beings and the environment. The use of hazardous solvents in existing fabrication processes also restricts the use of polymer substrates because of their low chemical resistance to such solvents. Here, we demonstrate an environmentally friendly mechanical, bilayer lithography that uses just water for development and lift-off. We show that we are able to create arbitrary patterns achieving resolution down to 310 nm. We then demonstrate the use of this technique to create functional devices by fabricating a MoS2 photodetector on a polyethylene terephthalate (PET) substrate with measured response times down to 42 ms.

Precise controlled filling of point vacancies in hBN with carbon atoms is demonstrated using a focused electron beam method, which guides mobile C atoms into the desired defect site. Optimization of the technique enables the insertion of a single C atom into a selected monovacancy, and preferential defect filling with sub-2 nm accuracy. Increasing the C insertion process leads to thicker 3D C nanodots seeded at the hBN point vacancy site. Other light elements are also observed to bind to hBN vacancies, including O, opening up a wide range of complex defect structures that include B, C, N, and O atoms. The ability to selectively fill point vacancies in hBN with C atoms provides a pathway for creating non-hydrogenated covalently bonded C molecules embedded in the insulating hBN.

Various 2D materials can be assembled into vertical heterostructure stacks that emit strong electroluminescence. However, to date, most work is done using mechanical exfoliated materials, with little insights gained into the operation limits and failure mechanisms due to the limited number of devices produced and the device-to-device variances. However, when using chemical vapor deposition (CVD) grown 2D crystals, it is possible to construct dozens of devices to generate statistics and ensemble insights, providing a viable way toward scalable industrialization of 2D optoelectronics. In particular, the operation lifetime/duration of electroluminescence and subsequent failure mechanisms are poorly understood. Here, we demonstrate that all-2D vertical layered heterostructure (VLH) devices made using CVD-grown materials (Gr:h-BN:WS

Two-dimensional (2D) materials and van der Waals heterostructures with atomic-scale thickness provide enormous potential for advanced science and technology. However, insufficient knowledge of compatible synthesis impedes wafer-scale production. PdSe2 and Pd2Se3 are two of the noble transition-metal chalcogenides with excellent physical properties that have recently emerged as promising materials for electronics, optoelectronics, catalyst, and sensors. This research presents a feasible approach to synthesize PdSe2 and Pd2Se3 with inherently asymmetric structure on honeycomb lattice 2D monolayer substrates of graphene and MoS2. We directly deposit a molecular transition-metal precursor complex on the surface of the 2D substrates, followed by low-temperature selenization by chemical vapor flow. Parameter control leads to tuning of the material from monolayer nanocrystals with Pd2Se3 phase, to continuous few-layer PdSe2 films. Annular dark-field scanning transmission electron microscopy (ADF-STEM) reveals the structure, phase variations, and heteroepitaxy at the atomic level. PdSe2 with unconventional interlayer stacking shifts appeared as the kinetic product, whereas the bilayer PdSe2 and monolayer Pd2Se3 are the thermodynamic product. The epitaxial alignment of interlayer rotation and translation between the PdSe2 and underlying 2D substrate was also revealed by ADF-STEM. These results offer both nanoscale and atomic-level insights into direct growth of van der Waals heterostructures, as well as an innovative method for 2D synthesis by predetermined nucleation.

Two-dimensional (2D) materials and van der Waals heterostructures with atomic-scale thickness provide enormous potential for advanced science and technology. However, insufficient knowledge of compatible synthesis impedes wafer-scale production. PdSe

Magnetic resonance imaging (MRI) and computed tomography (CT) imaging with X-rays are crucial diagnostic techniques in medicine, especially in oncology for evaluating the response to treatment. Body movement causes image blurring and synchronized gating to the respiratory and cardiac cycles is required. Degradation of MRI and CT imaging by the presence of metal in electronic respiratory sensors has limited their use, with a preference for pressure balloons for detecting respiration, but these are cumbersome and insensitive. Here, graphene's role is studied as an electromagnetically transparent electrode in a piezoelectric graphene respiratory sensor (GRS) device designed specifically for dual gated MRI and CT imaging of small animals. The GRS is integrated into a 3D-printed cradle with all-carbon-based device life support (heating pad) and monitoring of small animals (electrocardiogram), enabling both heartbeat and respiration detection, significant improvements to throughput and reproducibility, and reduced animal suffering. This shows graphene's potential for a wide range of electromagnetic transparent electronics for medical imaging and diagnostics, beyond conventional metal electrodes.

Monolayer 2H-phase MoS

Solar-driven water splitting is highly desirable for hydrogen fuel production, particularly if water oxidation is effectively sustained in a complete cycle and/or by means of stable and efficient photocatalysts of main group elements, for example, carbon and nitrogen. Despite extensive success on H2 production on polymer photocatalysts, polymers have met with very limited success for the rate-determining step of the water splitting-water oxidation reaction due to the extremely slow "four-hole" chemistry. Here, the synthesized metal-free oxygenated covalent triazine (OCT) is remarkably active for oxygen production in a wide operation window from UV to visible and even to NIR (up to 800 nm), neatly matching the solar spectrum with an unprecedented external quantum efficiency (even 1% at 600 nm) apart from excellent activity for H2 production under full arc irradiation, a big step moving toward full solar spectrum water splitting. Experimental results and DFT calculations show that the oxygen incorporation not only narrows the band gap but also causes appropriate band-edge shifts. In the end, a controlled small amount of oxygen in the ionothermal reaction is found to be a promising and facile way of achieving such oxygen incorporation. This discovery is a significant step toward both scientific understanding and practical development of metal-free photocatalysts for cost-effective water oxidation and hydrogen generation over a large spectral window.

The self-assembly of bowlic supramolecules on graphene surface is studied with single molecular sensitivity. This is achieved by incorporating a heavy metal tag in the form of a single W atom into the tip of the molecular structure, which enables the direct imaging of molecular distribution using annular dark-field scanning transmission electron microscopy (ADF-STEM) along with graphene as an electron transparent support. The bowlic molecules have nonplanar geometry, and their orientations with respect to their graphene substrate and with each other result in various packing configurations. Statistical data on intermolecular distances is obtained from numerous measurements of the bright contrast from the single metal atom tags. The analysis shows that the bowlic molecules lie sideways on the graphene surface with favorable head-to-tail stacking, rather than sitting vertically with the bowl facing toward the graphene surface. In thicker film regions, nanoscale lamellar fringes are observed, demonstrating that large-scale aligned packing extends into 3D. Image simulations and various molecular packing schemes are discussed to help interpret the ADF-STEM images and the possible range of molecular interactions occurring. These results aid the understanding of nonplanar supramolecular assemblies on van der Waals surfaces for potential applications in molecular recognition by porous films.

Two-dimensional (2D) tin disulfide (SnS2) crystals have been arousing immense attention for flexible electronics and integrated circuits in next generation because of their earth-abundant and nontoxic elemental components. Producing high quality crystal with controlled morphology, however, remains challenging due to the lack of understanding for its growth mechanism. Here, we demonstrate the direct chemical vapor deposition (CVD) growth and morphology control of 2D SnS2 on CVD-grown WS2 layers. In addition to the formation of type II van der Waals (vdW) vertical heterostructures with enhanced charge separation, the morphology of SnS2 is found to be highly dependent on the underlying substrate surface, where lateral growth could be stabilized with epitaxially aligned crystals on the defect-free surface whereas cluster growth appears on the defect-rich surface. This is attributed to the lower energy barrier of migration for adsorbed active species on the defect-free surface, resulting in facilitated surface diffusion compared to the defect-rich surface. Similar results also occur when switching the growth substrates to other 2D transition metal dichalcogenides such as MoS2 layers, showing the importance of defect-free 2D substrates on the SnS2 growth which is crucial for the applications in next-generation nanoelectronics such as photodetectors or light-emitting diodes.

UV-sensitive lateral all-two-dimensional (2D) photodetecting devices are produced by growing the large band gap layered GaS between graphene electrode pairs directly using chemical vapor deposition methods. The use of prepatterned graphene electrode pairs on the Si wafer enables more than 200 devices to be fabricated simultaneously. We show that the surface chemistry of the substrate during GaS leads to selective growth in graphene gaps, forming the lateral heterostructures, rather than on the surface of graphene. The graphene/GaS/graphene lateral photodetecting devices are demonstrated to be sensitive to UV light only, with no measurable response to visible light. Furthermore, we demonstrate UV-band discrimination in photosensing, with measured photocurrents only produced for middle-UV and not for near-UV wavelength regions. The detection limit could reach down to 2.61 μW/cm2 with a photoresponsivity as high as 11.7 A/W and a photo gain of 53.7 under 270 nm excitation. Gate-dependent modulation of the photocurrent is also demonstrated. The photodetectors exhibit long-term stability and reproducible ON-OFF switching behavior, with a response time lower than 60 ms. These results provide insights into how ultrathin UV sensing devices can be created using only 2D materials by exploiting large band gap 2D semiconductors such as GaS.

Ternary transition metal dichalcogenide alloys with spatially graded bandgaps are an emerging class of two-dimensional materials with unique features, which opens up new potential for device applications. Here, visible–near-infrared and self-powered phototransistors based on spatially bandgap-graded MoS2(1−x)Se2x alloys, synthesized by a simple and controllable chemical solution deposition method, are reported. The graded bandgaps, arising from the spatial grading of Se composition and thickness within a single domain, are tuned from 1.83 to 1.73 eV, leading to the formation of a homojunction with a built-in electric field. Consequently, a strong and sensitive gate-modulated photovoltaic effect is demonstrated, enabling the homojunction phototransistors at zero bias to deliver a photoresponsivity of 311 mA W−1, a specific detectivity up to ~ 1011 Jones, and an on/off ratio up to ~ 104. Remarkably, when illuminated by the lights ranging from 405 to 808 nm, the biased devices yield a champion photoresponsivity of 191.5 A W−1, a specific detectivity up to ~ 1012 Jones, a photoconductive gain of 106–107, and a photoresponsive time in the order of ~ 50 ms. These results provide a simple and competitive solution to the bandgap engineering of two-dimensional materials for device applications without the need for p–n junctions.

The nature of the atomic configuration and the bonding within epitaxial Pt-graphene films is investigated. Graphene-templated monolayer/few-multilayers of Pt, synthesized as contiguous 2D films by room temperature electrochemical methods, is shown to exhibit a stable {100} structure in the 1–2 layer range. The fundamental question being investigated is whether surface Pt atoms rendered in these 2D architectures are as stable as those of their bulk Pt counterparts. Unsurprisingly, a single layer Pt on the graphene (Pt_1ML/GR) shows much larger Pt dissociation energy (−7.51 eV) than does an isolated Pt atom on graphene. However, the dissociation energy from Pt_1ML/GR is similar to that of bulk Pt(100), −7.77 eV, while in bi-layer Pt on the graphene (Pt_2ML/GR), this energy changes to −8.63 eV, surpassing its bulk counterpart. At Pt_2ML/GR, the dissociation energy also slightly surpasses that of bulk Pt(111). Bulk-like stability of atomically thin Pt–graphene results from a combination of interplanar PtC covalent bonding and inter/intraplanar metallic bonding. This unprecedented stability is also accompanied by a metal-like presence of electronic states at the Fermi level. Such atomically thin metal-graphene architectures can be a new stable platform for synthesizing 2D metallic films with various applications in catalysis, sensing, and electronics.

Two-dimensional (2D) transition metal dichalcogenides (TMDs), a burgeoning family of layered materials, have been offering researchers a fertile ground for harvesting novel properties and versatile applications. Lattice imperfections that are always present in crystalline TMDs can significantly influence their physical–chemical properties. Here, we cover how transmission electron microscopy (TEM) can be used to provide atomic-resolution studies of defect structures in monolayer/bilayer TMDs. Both phase contrast TEM imaging and scanning TEM are discussed. A brief overview is included on the pristine crystalline structures and properties of 2D materials, as well as the development of TEM. The defects summarized in this review are mainly categorized into intrinsic and extrinsic ones, based on whether foreign atoms are incorporated into the pure TMD system. We will discuss various atomic configurations of different defects, their dynamics under beam irradiation, and the transformation pathways for reconstructions.

Recent studies have revealed that van der Waals (vdW) heteroepitaxial growth of 2D materials on crystalline substrates, such as hexagonal boron nitride (hBN), leads to the formation of self-aligned grains, which results in defect-free stitching between the grains. However, how the weak vdW interaction causes a strong limitation on the crystal orientation of grains is still not understood yet. In this work, we have focused on investigating the microscopic mechanism of the self-alignment of MoS2 grains in vdW epitaxial growth on hBN. Using the density functional theory and the Lennard-Jones potential, we found that the interlayer energy between MoS2 and hBN strongly depends on the size and crystal orientation of MoS2. We also found that, when the size of MoS2 is several tens of nanometers, the rotational energy barrier can exceed ∼1 eV, which should suppress rotation to align the crystal orientation of MoS2 even at the growth temperature.

Monolayer molybdenum disulfide (MoS2) crystals grown on amorphous substrates such as SiO2 are randomly oriented. However, when MoS2 is grown on crystalline substrates, the crystal shapes and orientations are also influenced by their epitaxial interaction with the substrate. In this paper, we present the results from chemical vapor deposition growth of MoS2 on three different terminations of single crystal strontium titanate (SrTiO3) substrates. On SrTiO3(111), the monolayer MoS2 crystals form equilateral triangles with two main orientations, in which they align their ⟨21̅1̅0⟩-type directions (i.e., the sulfur-terminated edge directions) with the ⟨11̅0⟩-type directions on SrTiO3. This arrangement allows near-perfect coincidence epitaxy between seven MoS2 unit cells and four SrTiO3 unit cells. On SrTiO3(110), the MoS2 crystals tend to align their edges with both the ⟨11̅0⟩ and ⟨11̅2̅⟩ directions on SrTiO3 because these both provide favorable coincidence lattice registry. This distorts the crystal shapes and introduces an additional strain detectable by photoluminescence. When triangular MoS2 crystals are grown on SrTiO3(001), they again show a preference to align their edges with the ⟨11̅0⟩ directions on SrTiO3. Our observations can be explained if the interfacial van der Waals (vdW) bonding between MoS2 monolayers and SrTiO3 is greatest when maximum commensuration between the lattices is achieved. Therefore, a key finding of this paper is that the vdW interaction between MoS2 and SrTiO3 substrates determines the supported crystal shapes and orientations by epitaxial relations. Controlled crystal orientations make the growth of large sheets of MoS2 possible when there are multiple nucleation sites. This minimizes the number of grain boundaries and optimizes the electronic properties of the material, e.g., charge mobility, which is crucial for the application of monolayer MoS2 in next-generation nanoelectronic devices.

We reveal a self-limiting mechanism during the formation of a specific type of circular nanopore in monolayer WS2 that limits its diameter to sub-nm. A single W atom vacancy (triangular nanopore) is transformed into the self-limiting nanopore (SLNP) through the atomic restructuring of S atoms around the area, reducing the number of dangling bonds at the nanopore edge by shifting them further in-plane with W-W bonding instead. Bond rotations in WS2 help accommodate the electron beam induced atomic loss and ensure the stability of the SLNP. The SLNP shows significant improvement in diameter stability during electron beam irradiation compared to other triangular nanopores in WS2 that typically continue to expand in diameter during atom loss. The atomic structure of these SLNPs is studied using aberration-corrected scanning transmission electron microscopy with an in situ heating holder, revealing that the SLNPs are mostly formed at a temperature of ∼500 °C, which is a balance between thermally activated S vacancy diffusion and sufficient S vacancy density to initiate local atomic reconstruction. At higher temperatures ( i. e., 1000 °C), S vacancies quickly migrate away into long line vacancies, resulting in low S vacancy density and rapidly expanding holes generated at the edges of the line vacancies. At room temperature, S vacancy migration is low and vacancy density is very high, which limits atomic reconstruction, and instead many small holes open up. These results provide insights into the factors that lead to uniform sized nanopores in the sub-nm range in transition-metal dichalcogenides.

Heterophase homojunction formation in atomically thin 2D layers is of great importance for next-generation nanoelectronics and optoelectronics applications. Technologically challenging, controllable transformation between the semiconducting and metallic phases of transition metal chalcogenides is of particular importance. Here, we demonstrate that controlled laser irradiation can be used to directly ablate PdSe

Mediator atoms catalyse bond formation and breaking in graphene and its defects., The structural transformations of graphene defects have been extensively researched through aberration-corrected transmission electron microscopy (AC-TEM) and theoretical calculations. For a long time, a core concept in understanding the structural evolution of graphene defects has been the Stone-Thrower-Wales (STW)–type bond rotation. In this study, we show that undercoordinated atoms induce bond formation and breaking, with much lower energy barriers than the STW-type bond rotation. We refer to them as mediator atoms due to their mediating role in the breaking and forming of bonds. Here, we report the direct observation of mediator atoms in graphene defect structures using AC-TEM and annular dark-field scanning TEM (ADF-STEM) and explain their catalytic role by tight-binding molecular dynamics (TBMD) simulations and image simulations based on density functional theory (DFT) calculations. The study of mediator atoms will pave a new way for understanding not only defect transformation but also the growth mechanisms in two-dimensional materials.

We show controlled production of atomically thin nanowells in bilayer WS

Nanoscale pore formation on chemical vapor deposition grown monolayer MoS2 is achieved using oxygen plasma etching through a nanoporous silicon mask, creating round pores of ∼70 nm in diameter. The microscale areas with high porosity were successfully patterned via the usage of silicon masks. Thermal annealing in air after the pore formation in the monolayers results in the gradual enlargement of the pores, providing an effective method of controlling edge-to-area ratio of MoS2 crystals. The photoluminescence of the nanoporous MoS2 exhibits rapid increase and blue-shift due to facile p-doping during the thermal annealing process compared to pristine MoS2. This method of fabricating porous transition metal dichalcogenide layers with controlled edge densities presents opportunities in various applications that require atomically thin nanomaterials with controlled pore density and edge sites, such as filtration, electrocatalysis, and sensing.

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.

We show how control over the chemical vapor deposition (CVD) reaction chemistry of molybdenum disulfide (MoS2) by hydrogen addition can enable the direct growth of centimeter-scale continuous films of vertically stacked MoS2 monolayer on graphene under atmospheric pressure conditions. Hydrogen addition enables longer CVD growth times at high temperature by reducing oxidation effects that would otherwise degrade the monolayer graphene. By careful control of nucleation density and growth time, high-quality monolayer MoS2 films could be formed on graphene, realizing all CVD-grown vertically stacked monolayer semimetal/semiconducting interfaces. Photoluminescence spectroscopy shows quenching of MoS2 by the underlying graphene, indicating a good interfacial charge transfer. We utilize the MoS2/graphene vertical stacks as photodetectors, with photoresponsivities reaching 2.4 A/W under 135 μW 532 nm illumination. This approach provides insights into the scalable manufacturing of high-quality two-dimensional elec...

© The Royal Society of Chemistry 2018. When secondary domains nucleate and grow on the surface of monolayer MoS2, they can extend across grain boundaries in the underlying monolayer MoS2 and form overlapping sections. We present an atomic level study of overlapping antiphase grain boundaries (GBs) in MoS2 monolayer-bilayers using aberration-corrected annular dark field scanning transmission electron microscopy. In particular we focus on the antiphase GB within a monolayer and track its propagation through an overlapping bilayer domain. We show that this leads to an atomically sharp interface between 2H and 3R interlayer stacking in the bilayer region. We have studied the micro-nanoscale "meandering" of the antiphase GB in MoS2, which shows a directional dependence on the density of 4 and 8 member ring defects, as well as sharp turning angles 90°-100° that are mediated by a special 8-member ring defect. Density functional theory has been used to explore the overlapping interlayer stacking around the antiphase GBs, confirming our experimental findings. These results show that overlapping secondary bilayer MoS2 domains cause atomic structure modification to underlying anti-phase GB sites to accommodate the van der Waals interactions.

Triangular nanovoids in 2D materials transition metal dichalcogenides have vertex points that cause stress concentration and lead to sharp crack propagation and failure. Here, the atomistic mechanics of back folding around triangular nanovoids in monolayer WS2 sheets is examined. Combining atomic-resolution images from annular dark-field scanning transmission electron microscopy with reactive molecular modelling, it is revealed that the folding edge formation has statistical preferences under geometric conditions based on the orientation mismatch. It is further investigated how loading directions and strong interlayer friction, interplay with WS2 lattice's crack preference, govern the deformation and fracture pattern around folding edges. These results provide fundamental insights into the combination of fracture and folding in flexible monolayer crystals and the resultant Moire lattices.

In order to understand the physical properties of materials it is necessary to determine the 3D positions of all atoms. There has been significant progress towards this goal using electron tomography. However, this method requires a relatively high electron dose and often extended acquisition times which precludes the study of structural dynamics such as defect formation and evolution. In this work we describe a method that enables the determination of 3D atomic positions with high precision from single high resolution electron microscopic images of graphene that show dynamic processes. We have applied this to the study of electron beam induced defect coalescence and to long range rippling in graphene. The latter strongly influences the mechanical and electronic properties of this material that are important for possible future applications.

The high-bias and breakdown behavior of suspended mono- and few-layer WS2 was explored by in situ aberration-corrected transmission electron microscopy. The suspended WS2 devices were found to undergo irreversible breakdown at sufficiently high biases due to vaporization of the WS2. Simultaneous to the removal of WS2 was the accompanying formation of few-layer graphene decorated with W and WS2 nanoparticles, with the carbon source attributed to organic residues present on the WS2 surface. The breakdown of few-layer WS2 resulted in the formation of faceted S-depleted WS2 tendrils along the vaporization boundary, which were found to exhibit lattice contraction indicative of S depletion, alongside pure W phases incorporated into the structure, with the interfaces imaged at atomic resolution. The combination of observing the graphitization of the amorphous carbon surface residue, W nanoparticles, and S-depleted WS2 phases following the high-bias WS2 disintegration all indicate a thermal Joule heating breakdown mechanism over an avalanche process, with WS2 destruction promoted by preferential S emission. The observation of graphene formation and the role the thin amorphous carbon layer has in the prebreakdown behavior of the device demonstrate the importance of employing encapsulated heterostructure device architectures that exclude residues.

Pt-nanocrystal:MoS2 hybrid materials have promising catalytic properties for hydrogen evolution, and understanding their detailed structures at the atomic scale is crucial to further development. Here, we use an in situ heating holder in an aberration-corrected transmission electron microscope to study the formation of Pt nanocrystals directly on the surface of monolayer MoS2 from a precursor on heating to 800 °C. Isolated single Pt atoms and small nanoclusters are observed after in situ heating, with two types of preferential alignment between the Pt nanocrystals and the underlying monolayer MoS2. Strain effects and thickness variations of the ultrasmall Pt nanocrystal supported on MoS2 are studied, revealing that single atomic planes are formed from a nonlayered face-centered cubic bulk Pt configuration with a lattice expansion of 7-10% compared to that of bulk Pt. The Pt nanocrystals are surrounded by an amorphous carbon layer and in some cases have etched the local surrounding MoS2 material after heating. Electron beam irradiation also initiates Pt nanocrystal etching of the local MoS2, and we study this process in real time at atomic resolution. These results show that the presence of carbon around the Pt nanocrystals does not affect their epitaxial relationship with the MoS2 lattice. Single Pt atoms within the carbon layer are also immobilized at high temperature. These results provide important insights into the formation of Pt:MoS2 hybrid materials.

The edges of 2D materials show novel electronic, magnetic, and optical properties, especially when reduced to nanoribbon widths. Therefore, methods to create atomically flat edges in 2D materials are essential for future exploitation. Atomically flat edges in 2D materials are found after brittle fracture or when electrically biasing, but a simple scalable approach for creating atomically flat periodic edges in monolayer 2D transition metal dichalcogenides has yet to be realized. Here, we show how heating monolayer MoS2 to 800 °C in vacuum produces atomically flat Mo terminated zigzag edges in nanoribbons. We study this at the atomic level using an ultrastable in situ heating holder in an aberration-corrected transmission electron microscope and discriminating Mo from S at the edge, revealing unique Mo terminations for all zigzag orientations that remain stable and atomically flat when cooling back to room temperature. Highly faceted MoS2 nanoribbon constrictions are produced with Mo rich edge structures that have theoretically predicted spin separated transport channels, which are promising for spin logic applications.

Single-layer graphene (SLG) membranes have great promise as ultrahigh flux, high selectivity membranes for gas mixture separations due to their single atom thickness. It remains a central question whether SLG membranes of a requisite area can exist under an imposed pressure drop and temperatures needed for industrial gas separation. An additional challenge is the development of techniques to perforate or otherwise control the porosity in graphene membranes to impart molecularly sized pores, the size regime predicted to produce high gas separation factors. Herein, we report fabrication, pressure testing, temperature cycling, and gas permeance measurements through free-standing, low defect density SLG membranes. Our measurements demonstrate the remarkable chemical and mechanical stability of these 5 μm diameter suspended SLG membranes, which remain intact over weeks of testing at pressure differentials of >0.5 bar, repeated temperature cycling from 25 to 200 °C, and exposure to 15 mol % ozone for up to 3 mi...

Monolayer tungsten disulfide (WS 2 ) offers great prospects for use in optoelectronic devices due to its direct bandgap and high photoluminescence intensity. Here, we show how the controlled addition of hydrogen into the chemical vapor deposition growth of WS 2 can lead to the formation of centimeter scale continuous monolayer films at ambient pressure without the need for seed molecules, specially prepared substrates, or low pressure vacuum systems. Modifications of the reaction conditions, including growth time and hydrogen to argon ratio, allow for control over the domain size, film coverage, and film uniformity of the prepared WS 2 film. The combined control of hydrogen and a double-furnace system enables an increases in the growth rate of WS 2 , which results in fully merged films with cm 2 coverage and reduced multilayer content. Field effect transistors are fabricated to demonstrate that WS 2 has high quality for electronic applications. Our ambient pressure chemical vapor deposition reaction is simple and efficient, ideal for mass-production of large area monolayer WS 2 .

We have studied the atomic structure of small secondary domains that nucleate on monolayer MoS2 grown by chemical vapour deposition (CVD), which form the basis of bilayer MoS2. The small secondary bilayer domains have a faceted geometry with three-fold symmetry and adopt two distinct orientations with 60° rotation relative to an underlying monolayer MoS2 single crystal sheet. The two distinct orientations are associated with the 2H and 3R stacking configuration for bilayer MoS2. Atomic resolution images have been recorded using annular dark field scanning transmission electron microscopy (ADF-STEM) that show the edge termination, lattice orientation and stacking sequence of the bilayer domains relative to the underlying monolayer MoS2. These results provide important insights that bilayer MoS2 growth from 60° rotated small nuclei on the surface of monolayer MoS2 could lead to defective boundaries when merged to form larger continuous bilayer regions and that pure AA' or AB bilayer stacking may be challenging unless from a single seed.

Three-dimensionally structured graphitic materials are of interest for electrochemical applications as electrodes and catalyst supports. Many synthetic approaches to these materials require a preformed three-dimensional template and a carbon source, especially when highly crystalline materials are required. In this report, we utilise nickel acetate, which contains both a metal catalyst and a carbon source, as the sole component in the synthesis of both three-dimensional multi-layered carbon and graphene spherical structures (GS) and GS-carbon nanotube hybrid materials (GS-CNT), depending on the annealing procedure. By varying the synthesis conditions of these materials we are able to control the crystallinity of the structures. We show that the rapid introduction of the precursor into the hot zone is a key factor in the formation of the structures. This method demonstrates the utility of cheap metal salts as the sole component of the scalable synthesis of three-dimensional hybrid graphitic materials. We then utilise the novel materials as supports for platinum (Pt) nanocrystals produced using a thermal annealing approach and compare the effect of the carbon support on the active surface area and durability in cyclic voltammetry experiments.

We use annular dark-field scanning transmission electron microscopy (ADF-STEM) to study how solution-deposited molecules bind to the edges and surface regions around nanopores in MoS

Highlights Due to the Se composition and thickness gradient within single MoS2(1−x)Se2x domains, the bandgap of MoS2(1−x)Se2x is gradually tuned from 1.83 to 1.73 eV.The homojunction phototransistors at zero bias deliver a photoresponsivity of 311 mA W−1, a specific detectivity up to ~ 1011 Jones, and an on/off ratio up to ~ 104.The biased devices yield a champion photoresponsivity of 191.5 A W−1, a specific detectivity up to ~ 1012 Jones, and a photoconductive gain of 106–107. Electronic supplementary material The online version of this article (10.1007/s40820-019-0361-2) contains supplementary material, which is available to authorized users., Ternary transition metal dichalcogenide alloys with spatially graded bandgaps are an emerging class of two-dimensional materials with unique features, which opens up new potential for device applications. Here, visible–near-infrared and self-powered phototransistors based on spatially bandgap-graded MoS2(1−x)Se2x alloys, synthesized by a simple and controllable chemical solution deposition method, are reported. The graded bandgaps, arising from the spatial grading of Se composition and thickness within a single domain, are tuned from 1.83 to 1.73 eV, leading to the formation of a homojunction with a built-in electric field. Consequently, a strong and sensitive gate-modulated photovoltaic effect is demonstrated, enabling the homojunction phototransistors at zero bias to deliver a photoresponsivity of 311 mA W−1, a specific detectivity up to ~ 1011 Jones, and an on/off ratio up to ~ 104. Remarkably, when illuminated by the lights ranging from 405 to 808 nm, the biased devices yield a champion photoresponsivity of 191.5 A W−1, a specific detectivity up to ~ 1012 Jones, a photoconductive gain of 106–107, and a photoresponsive time in the order of ~ 50 ms. These results provide a simple and competitive solution to the bandgap engineering of two-dimensional materials for device applications without the need for p–n junctions. Electronic supplementary material The online version of this article (10.1007/s40820-019-0361-2) contains supplementary material, which is available to authorized users.

A chemical vapor deposition method is developed for thickness‐controlled (one to four layers), uniform, and continuous films of both defective gallium(II) sulfide (GaS): GaS0.87 and stoichiometric GaS. The unique degradation mechanism of GaS0.87 with X‐ray photoelectron spectroscopy and annular dark‐field scanning transmission electron microscopy is studied, and it is found that the poor stability and weak optical signal from GaS are strongly related to photo‐induced oxidation at defects. An enhanced stability of the stoichiometric GaS is demonstrated under laser and strong UV light, and by controlling defects in GaS, the photoresponse range can be changed from vis‐to‐UV to UV‐discriminating. The stoichiometric GaS is suitable for large‐scale, UV‐sensitive, high‐performance photodetector arrays for information encoding under large vis‐light noise, with short response time (

Platinum atomic layers grown on graphene were investigated by atomic resolution transmission electron microscopy (TEM). These TEM images reveal the epitaxial relationship between the atomically thin platinum layers and graphene, with two optimal epitaxies observed. The energetics of these epitaxies influences the grain structure of the platinum film, facilitating grain growth via in-plane rotation and assimilation of neighbor grains, rather than grain coarsening from the movement of grain boundaries. This growth process was enabled due to the availability of several possible low-energy intermediate states for the rotating grains, the Pt-Gr epitaxies, which are minima in surface energy, and coincident site lattice grain boundaries, which are minima in grain boundary energy. Density functional theory calculations reveal a complex interplay of considerations for minimizing the platinum grain energy, with free platinum edges also having an effect on the relative energetics. We thus find that the platinum atomic layer grains undergo significant reorientation to minimize interface energy (via epitaxy), grain boundary energy (via low-energy orientations), and free edge energy. These results will be important for the design of two-dimensional graphene-supported platinum catalysts and obtaining large-area uniform platinum atomic layer films and also provide fundamental experimental insight into the growth of heteroepitaxial thin films.

Chalcogenide glasses as nanoscale thin films have become leading candidates for several optical and photonic technologies, ranging from reflective displays and filters to photonic memories. Current material systems, however, show strong optical absorption which limits their performance efficiencies and complicates device level integration. Herein, we report sputter deposited thin films of GeSe3, which are low loss and in which the flexible nature of the atomic structure results in thermally activated tunability in the refractive index as well as in the film’s physical volume. Such changes, which occur beyond a threshold temperature are observed to be accumulative and directed towards a more equilibrium amorphous state of the film, instead of crystallization. Our results provide insights into a new type of configurability that is based on strong coupling in the material’s opto-structural properties. The low optical losses in this material system combined with the tunability in the optical properties in the visible and near-infrared have direct application in higher performing optical coatings and in corrective optics.

Molybdenum disulfide (MoS2 ) has been extensively utilized as an electrocatalyst for the hydrogen evolution reaction (HER) with edges as the primary active catalytic sites. Previous work on edge-rich MoS 2 nanoplatelet 3D porous films (3D-ER-MoS2 ) shows they are promising catalysts, yet have basal planes that are inactive. Here we demonstrate how hydrogen annealing and oxygen plasma etching of 3D-ER-MoS 2 generates defects and increases active site density within the basal plane, leading to dramatic improvements in HER catalytic activity. We also explore the critical processing parameters for electrocatalytic enhancement. Significantly enriched edge density was revealed for both routes. O2 plasma treatment was more effective in increasing the number of edges by creating micro-cracks and local surface damage on the basal planes as well as structuring saw-toothed edges; H2 etching mainly introduced irregular shaped basal surface nanopores and strips. By controlling processing parameters, optimum surface area/active sites density enhancement can be achieved, together with the robust 3D porous architecture and superaerophobic surface. The defect-rich MoS 2 catalysts exhibit excellent HER activity: 700 °C – H2 – MoS2 shows Tafel slope of 94 mV/dec and low onset overpotential of 193 mV; 15 min – O2 – MoS2 performs amongst the best with excellent exchange current density of 57 μA/cmgeo−2. Our defect engineered 3D-ER MoS2 exhibits jgeo (η = 0.5 mV) of 6-fold and 38-fold compared to monolayer MoS2 subject to similar process, for O2 plasma and H2 etching approaches respectively. Our study demonstrates an effective way to realize high performance Pt-free electrocatalysts for hydrogen generation by dual enhancement via edge enrichment and basal surface activation.