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Superlattice (SL) phase change materials have shown promise to reduce the switching current and resistance drift of phase change memory (PCM). However, the effects of internal SL interfaces and intermixing on PCM performance remain unexplored, although these are essential to understand and ensure reliable memory operation. Here, using nanometer-thin layers of Ge

We investigated phase transitions of Co(OH)2 nanosheets by in situ scanning transmission electron microscopy heating experiments. The layered Co(OH)2 transformed to Co nanoparticles via CoO phases; spinel Co3O4 was formed with temperature reduction.

In this work, we designed a novel CuO/Al2CuO4 catalyst by a phase and interphase engineering approach, which enables the electrochemical conversion of carbon dioxide to ethylene with ultrahigh activity and selectivity.

Preparation of edge-rich two-dimensional (2D) transition metal dichalocogenides (TMDs) has been actively investigated with the aim to improve their electrical and catalytic properties. Here, we elucidate the role of potassium ions in oxidation of TMDs and suggest a consequent novel anisotropic etching mechanism driven by self-running oxide droplets. We discover that potassium-mediated oxidation of MoS2 leads to the formation of K-intercalated hexagonal-phase molybdenum oxides (h-KxMoO3), whereas orthorhombic-phase oxides are formed in the absence of potassium ions. Metastable h-KxMoO3 appears to have decomposed into oxide droplets at higher temperature. Self-running of the oxide droplets leads to layer-by-layer anisotropic etching of MoS2 along the armchair direction. The motion of the droplets appears to be triggered by the surface energy instability between the oxide droplets and the underlying MoS2 layer. This study opens new possibilities to design and manufacture novel edge-rich 2D TMDs that do not follow the equilibrium Wulff shape by modulating their oxidation with the assistance of alkali metals and also offers fundamental insights into the interactions between nanodroplets and 2D materials toward edge engineering.

Atomic-scale information is essential for understanding and designing unique structures and properties of two-dimensional (2D) materials. Recent developments in in situ transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM) enable research to provide abundant insights into the growth of nanomaterials. In this study, 2D MoS2 is synthesized on a suspended graphene substrate inside a TEM column through thermolysis of the ammonium tetrathiomolybdate (NH4)2MoS4 precursor at 500 °C. To avoid misinterpretation of the in situ STEM images, a deep-learning framework, DeepSTEM, is developed. The DeepSTEM framework successfully reconstructs an object function in atomic-resolution STEM imaging for accurate determination of the atomic structure and dynamic analysis. In situ STEM imaging with DeepSTEM enables observation of the edge configuration, formation, and reknitting progress of MoS2 clusters with the formation of a mirror twin boundary. The synthesized MoS2/graphene heterostructure shows various twist angles, as revealed by atomic-resolution TEM. This deep-learning framework-assisted in situ STEM imaging provides atomic information for in-depth studies on the growth and structure of 2D materials and shows the potential use of deep-learning techniques in 2D material research.

Conventional two-dimensional materials either have natural layered structures or are produced, with large surface areas, via physical or chemical synthesis. However, to form a two-dimensional material from a non-layered material, a method different from the existing ones is required. In this study, a surfactant-assisted method was utilized to synthesize Zn(OH)

Significant efforts in the catalytic applications of transition metal dichalcogenides (TMDs) have been made, primarily focused on the design and synthesis of their vertically aligned structures, including expanded interlayer spacing, to pursue the maximized exposure of active edge sites. However, vertically aligned TMD heterostructures are rare. In this work, we demonstrate vertical MoS2/WS2 heterostructures on reduced graphene oxide sheets (MoS2/WS2/rGO) by a one-pot synthesis, which show a high catalytic activity for the hydrogen evolution reaction (HER). The vertically aligned heterostructures were characterized by Raman spectroscopy, transmission electron microscopy, and energy dispersion spectroscopy. Compared with MoS2/rGO (with an onset potential of −125 mV and a Tafel slope of 81 mV dec−1), MoS2/WS2/rGO exhibited a much enhanced electrochemical HER performance with an onset potential of −113 mV and a Tafel slope of 44 mV dec−1. The electrochemical impedance results suggest that the enhanced catalytic activity of MoS2/WS2/rGO can be attributed to fast electron transfer in the TMD heterostructure. This work suggests great potential for TMD-based (photo)electrocatalysts through modification of their morphology and interlayer spacing.

A single-crystal diamond substrate (SCDS) with a (100) surface orientation was submerged in liquid gallium containing a small amount of dissolved silicon, and exposed to a mixture of methane and hydrogen at 1 atm and 900 ºC. New growth diamonds were found that are single crystal square pyramids with (111) facets and that are homoepitaxial to the substrate, as proven by scanning and transmission electron microscopy, and small angle X-ray scattering and diffraction. Raman spectroscopy with 13C-labeling prove that the methane as well as the SCDS are the carbon source for the newly grown diamond. This approach opens up new ways for growing diamond in liquid metal systems.

We map the lattice orientation of graphene grains (domains) larger than a centimeter by measuring the orientation of cracks in a WS2 film grown on graphene that had been previously grown by chemica...

We thank UNIST Central Research Facilities (UCRF) and Y. K. Kim for the cross-sectional high-resolution TEM images. This work was supported by Samsung Electronics (Samsung-SKKU Graphene/2D Center), the research fund (NRF-2017R1E1A1A01074493 and NRF-2019R1A4A1027934), the IBS (IBS-R-019-D1) and a grant (CASE-2013M3A6A5073173) from the Centre for Advanced Soft Electronics under the Global Frontier Research Program via the National Research Foundation of the Ministry of Science and ICT, South Korea. The NEXAFS experiments performed at the 4D, 6D and 10A2 beamlines of the Pohang Accelerator Laboratory (PAL) were supported in part by the Ministry of Science and ICT, POSTECH and UNIST. M.C. acknowledges support from Leverhulme Trust Research Grant RPG-2019-227. S.R., A.A. and M.C. acknowledge the European Union Horizon 2020 research and innovation programme for grant number 785219 and 881603 (Graphene Flagship). A.A. is supported by Project MECHANIC (PCI2018-093120) funded by Ministerio de Ciencia, Innovación y Universidades. The Catalan Institute of Nanoscience and Nanotechnology is funded by the CERCA Programme/Generalitat de Catalunya and supported by the Severo Ochoa Centres of Excellence programme, funded by the Spanish Research Agency (grant number SEV-2017-0706).

The electronic, catalytic, and optical properties of transition metal dichalcogenides (TMDs) are significantly affected by oxidation, and using oxidation to tune the properties of TMDs has been actively explored. In particular, because transition metal oxides (TMOs) are promising hole injection layers, a TMD-TMO heterostructure can be potentially applied as a p-type semiconductor. However, the oxidation of TMDs has not been clearly elucidated because of the structural instability and the extremely small quantity of oxides formed. Here, we reveal the phases and morphologies of oxides formed on two-dimensional molybdenum disulfide (MoS2) using transmission electron microscopy analysis. We find that MoS2 starts to oxidize around 400 °C to form orthorhombic-phase molybdenum trioxide (α-MoO3) nanosheets. The α-MoO3 nanosheets so formed are stacked layer-by-layer on the underlying MoS2 via van der Waals interaction and the nanosheets are aligned epitaxially with six possible orientations. Furthermore, the band gap of MoS2 is increased from 1.27 to 3.0 eV through oxidation. Our study can be extended to most TMDs to form TMO-TMD heterostructures, which are potentially interesting as p-type transistors, gas sensors, or photocatalysts.

A key challenge in the development of two-dimensional (2D) devices is the fabrication of metal–semiconductor junctions with minimal contact resistance and depinned energy levels. An ideal solution for practical applications is to make contacts between 2D van der Waals semiconductors and 2D van der Waals metals. Here we report the wafer-scale production of patterned layers of metallic transition metal ditellurides on different substrates. Our tungsten ditelluride and molybdenum ditelluride layers, which are grown using a tellurization process applied to a precursor transition metal layer, have an electronic performance comparable to that of mechanically exfoliated flakes and can be combined with the 2D semiconductor molybdenum disulfide. The resulting metal–semiconductor junctions are free from significant disorder effects and Fermi-level pinning, and are used to create monolayer molybdenum disulfide field-effect transistors. The Schottky barrier heights of the devices also largely follow the trend of the Schottky–Mott limit. Two-dimensional metallic WTe2 and MoTe2 layers can be combined with a semiconducting MoS2 monolayer to create metal–semiconductor junctions that are free from substantial disorder effects and Fermi-level pinning.

New high-k gate dielectrics are highly necessary in facilitating the continuous down-scaling of metal–oxide–semiconductor devices to the sub-10 nm range. This study presents ultrathin organic hydrocarbon (HC) films as a novel high-k gate insulator for metal–insulator–semiconductor (MIS) devices. During inductively-coupled plasma chemical vapor deposition with CH4 and H2 gases, the growth temperature greatly affects the structure of the carbon layers and consequently their dielectric characteristics. Specifically, sp2-rich dielectric HC layers are formed below 600 °C, whereas highly-ordered sp2-hybridized graphene is formed at 950 °C. The k value of the resulting HC films increases up to a maximum value of 90 at 350 °C. Moreover, the MIS devices exhibit excellent gate-insulating properties, including almost no hysteresis in the capacitance–voltage curve, low leakage current, and high dielectric strength, which surpass those of existing high-k gate oxides. These results reveal that the organic HC films are a promising next-generation high-k gate dielectric material for sub-10 nm node Si and organic semiconductor technologies.

Notwithstanding the numerous density functional studies on the chemically induced transformation of multilayer graphene into a diamond-like film carried out to date, a comprehensive convincing experimental proof of such a conversion is still lacking. We show that the fluorination of graphene sheets in Bernal (AB)-stacked bilayer graphene grown by chemical vapour deposition on a single-crystal CuNi(111) surface triggers the formation of interlayer carbon-carbon bonds, resulting in a fluorinated diamond monolayer ('F-diamane'). Induced by fluorine chemisorption, the phase transition from (AB)-stacked bilayer graphene to single-layer diamond was studied and verified by X-ray photoelectron, UV photoelectron, Raman, UV-Vis and electron energy loss spectroscopies, transmission electron microscopy and density functional theory calculations.

Defect engineering is one of the key technologies in materials science, enriching the modern semiconductor industry and providing good test-beds for solid-state physics. While homogenous doping prevails in conventional defect engineering, various artificial defect distributions have been predicted to induce desired physical properties in host materials, especially associated with symmetry breakings. Here, we show layer-by-layer defect-gradients in two-dimensional PtSe

ZnO, which can exist in various dimensions such as bulk, thin films, nanorods, and quantum dots, has interesting physical properties depending on its dimensional structures. When a typical bulk wurtzite ZnO structure is thinned to an atomic level, it is converted into a hexagonal ZnO layer such as layered graphene. In this study, we report the atomic arrangement and structural merging behavior of graphene-like ZnO nanosheets transferred onto a monolayer graphene using aberration-corrected TEM. In the region to which an electron beam is continuously irradiated, it is confirmed that there is a directional tendency, which is that small-patched ZnO flakes are not only merging but also forming atomic migration of Zn and O atoms. This study suggests atomic alignments and rearrangements of the graphene-like ZnO, which are not considered in the wurtzite ZnO structure. In addition, this study also presents a new perspective on the atomic behavior when a bulk crystal structure, which is not an original layered structure, is converted into an atomic-thick layered two-dimensional structure.

Wafer-scale growth of transition metal dichalcogenides with precise control over the number of layers, and hence the electronic state is an essential technology for expanding the practical application of 2D materials. Herein, a new growth method, phase-transition-induced growth (PTG), is proposed for the precisely controlled growth of molybdenum disulfide (MoS2 ) films consisting of one to eleven layers with spatial uniformity on a 2 in. wafer. In this method, an energetically unstable amorphous MoSx Oy (a-MoSx Oy ) phase is effectively converted to a thermodynamically stable crystalline MoS2 film. The number of MoS2 layers is readily controlled layer-by-layer by controlling the amount of Mo atoms in a-MoSx Oy , which is also applicable for the growth of heteroatom-inserted MoS2 . The electronic states of intrinsic and Nb-inserted MoS2 with one and four layers grown by PTGare are analyzed based on their work functions. The work function of monolayer MoS2 effectively increases with the substitution of Nb for Mo. As the number of layers increases to four, charge screening becomes weaker, dopant ionization becomes easier, and ultimately the work function increases further. Thus, better electronic state modulation is achieved in a thicker layer, and in this respect, PTG has the advantage of enabling precise control over the film thickness.

The effective synthesis of two-dimensional (2D) heterostructures is essential for their use in electronic devices. In this study, by using atomic layer deposition (ALD), 2D transition metal dichalcogenide (TMD) heterostructures were grown by a halide precursor. This study shows the growth characteristics of the fluoride precursor compared to the chloride precursor used for the synthesis of the TMD on the graphene layer and the other TMD layer. Additionally, a carbonyl precursor was used for comparison with the halide precursor in terms of the thermal stability. From these experiments, the fluoride precursor was adequate for synthesizing on the graphene, however, was inappropriate for the TMD/TMD heterostructure because of its etching characteristic. Meanwhile, the chloride precursor was appropriate for the TMD/TMD heterostructure, even for a low binding energy with the substrate, but was inadequate in forming the TMD/graphene heterostructure, even if the ALD cycle increased. Through our experiments, we show, for the first time, that there exists a suitable halide precursor for a 2D layer for a substrate.

Among the different growth mechanisms for two-dimensional (2D) hexagonal boron nitride (h-BN) synthesized using chemical vapor deposition, spiraling growth of h-BN has not been reported. Here we report the formation of intertwined double-spiral few-layer h-BN that is driven by screw dislocations located at the antiphase boundaries of monolayer domains. The microstructure and stacking configurations were studied using a combination of dark-field and atomic resolution transmission electron microscopy. Distinct from other 2D materials with single-spiral structures, the double-spiral structure enables the intertwined h-BN layers to preserve the most stable AA′ stacking configuration. We also found that the occurrence of shear strains at the boundaries of merged spiral islands is dependent on the propagation directions of encountering screw dislocations and presented the strained features by density functional theory calculations and atomic image simulations. This study unveils the double-spiral growth of 2D h-BN multilayers and the creation of a shear strain band at the coalescence boundary of two h-BN spiral clusters. National Research Foundation (NRF) Accepted version This work was supported by IBS-R019-D1, the National Research Foundation (NRF) grant funded by the Korea government (MSIT) (2018R1A2A2A05019598), and the NRF-ANR Joint (NRF2016-NRF-ANR001).

The growth of high-quality graphene on copper substrates has been intensively investigated using chemical vapor deposition (CVD). It, however, has been considered that the growth mechanism is different when graphene is synthesized using a plasma CVD. In this study, we demonstrate a dual role of hydrogen for the graphene growth on copper using an inductively coupled plasma (ICP) CVD. Hydrogen activates surface-bound carbon for the growth of high-quality monolayer graphene. In contrast, the role of an etchant is to manipulate the distribution of the graphene grains, which significantly depends on the plasma power. Atomic-resolution transmission electron microscopy study enables the mapping of graphene grains, which uncovers the distribution of grains and the number of graphene layers depending on the plasma power. In addition, the variation of electronic properties of the synthesized graphene relies on the plasma power.

Intercalation in black phosphorus (BP) can induce and modulate a variety of the properties including superconductivity like other two-dimensional (2D) materials. In this perspective, spatially controlled intercalation has the possibility to incorporate different properties into a single crystal of BP. We demonstrate anisotropic angstrom-wide (∼4.3 A) Cu intercalation in BP, where Cu atoms are intercalated along a zigzag direction of BP because of its inherent anisotropy. With atomic structure, its microstructural effects, arising from the angstrom-wide Cu intercalation, were investigated and extended to relation with macrostructure. As the intercalation mechanism, it was revealed by in situ transmission electron microscopy and theoretical calculation that Cu atoms are intercalated through top-down direction of BP. The Cu intercalation anisotropically induces transition of angstrom-wide electronic channels from semiconductor to semimetal in BP. Our findings throw light on the fundamental relationship between microstructure changes and properties in intercalated BP, and tailoring anisotropic 2D materials at angstrom scale.

Chemical vapour deposition of carbon-containing precursors on metal substrates is currently the most promising route for the scalable synthesis of large-area, high-quality graphene films1. However, there are usually some imperfections present in the resulting films: grain boundaries, regions with additional layers (adlayers), and wrinkles or folds, all of which can degrade the performance of graphene in various applications2–7. There have been numerous studies on ways to eliminate grain boundaries8,9 and adlayers10–12, but graphene folds have been less investigated. Here we explore the wrinkling/folding process for graphene films grown from an ethylene precursor on single-crystal Cu–Ni(111) foils. We identify a critical growth temperature (1,030 kelvin) above which folds will naturally form during the subsequent cooling process. Specifically, the compressive stress that builds up owing to thermal contraction during cooling is released by the abrupt onset of step bunching in the foil at about 1,030 kelvin, triggering the formation of graphene folds perpendicular to the step edge direction. By restricting the initial growth temperature to between 1,000 kelvin and 1,030 kelvin, we can produce large areas of single-crystal monolayer graphene films that are high-quality and fold-free. The resulting films show highly uniform transport properties: field-effect transistors prepared from these films exhibit average room-temperature carrier mobilities of around (7.0 ± 1.0) × 103 centimetres squared per volt per second for both holes and electrons. The process is also scalable, permitting simultaneous growth of graphene of the same quality on multiple foils stacked in parallel. After electrochemical transfer of the graphene films from the foils, the foils themselves can be reused essentially indefinitely for further graphene growth. Restricting the initial growth temperatures used for chemical vapour deposition of graphene on metal foils produces optimum conditions for growing large areas of fold-free, single-crystal graphene.

The authors report the growth of micrometer-long single-crystal graphene ribbons (GRs) (tapered when grown above 900 °C, but uniform width when grown in the range 850 °C to 900 °C) using silica particle seeds on single crystal Cu(111) foil. Tapered graphene ribbons grow strictly along the Cu101direction on Cu(111) and polycrystalline copper (Cu) foils. Silica particles on both Cu foils form (semi-)molten Cu-Si-O droplets at growth temperatures, then catalyze nucleation and drive the longitudinal growth of graphene ribbons. Longitudinal growth is likely by a vapor-liquid-solid (VLS) mechanism but edge growth (above 900 °C) is due to catalytic activation of ethylene (C

A single-crystal graphene film grown on a Cu(111) foil by chemical vapor deposition (CVD) has ribbon-like fold structures. These graphene folds are highly oriented and essentially parallel to each other. Cu surface steps underneath the graphene are along the110 and211 directions, leading to the formation of the arrays of folds. The folds in the single-layer graphene (SLG) are not continuous but break up into alternating patterns. A "joint" (an AB-stacked bilayer graphene) region connects two neighboring alternating regions, and the breaks are always along zigzag or armchair directions. Folds formed in bilayer or few-layer graphene are continuous with no breaks. Molecular dynamics simulations show that SLG suffers a significantly higher compressive stress compared to bilayer graphene when both are under the same compression, thus leading to the rupture of SLG in these fold regions. The fracture strength of a CVD-grown single-crystal SLG film is simulated to be about 70 GPa. This study greatly deepens the understanding of the mechanics of CVD-grown single-crystal graphene and such folds, and sheds light on the fabrication of various graphene origami/kirigami structures by substrate engineering. Such oriented folds can be used in a variety of further studies.

Graphene-based two-dimensional heterostructures are of substantial interest both for fundamental studies and their various potential applications. Particularly interesting are atomically thin semiconducting oxides on graphene, which uniquely combine a wide band gap and optical transparency. Here, we report the atomic-scale investigation of a novel self-formation of a ZnO monolayer from the Zn metal on a graphene oxide substrate. The spontaneous oxidation of the ultrathin Zn metal occurs by a reaction with oxygen supplied from the graphene oxide substrate, and graphene oxide is deoxygenated by a transfer of oxygen from O-containing functional groups to the zinc metal. The ZnO monolayer formed by this spontaneous redox reaction shows a graphene-like structure and a band gap of about 4 eV. This study demonstrates a unique and straightforward synthetic route to atomically thin two-dimensional heterostructures made from a two-dimensional metal oxide and graphene, formed by the spontaneous redox reaction of a very thin metal layer directly deposited on graphene oxide.