Growth of diamond in liquid metal at 1 atm pressure.

Natural diamonds were (and are) formed (thousands of million years ago) in the upper mantle of Earth in metallic melts at temperatures of 900-1,400 °C and at pressures of 5-6 GPa (refs. 1,2). Diamond is thermodynamically stable under high-pressure and high-temperature conditions as per the phase diagram of carbon3. Scientists at General Electric invented and used a high-pressure and high-temperature apparatus in 1955 to synthesize diamonds by using molten iron sulfide at about 7 GPa and 1,600 °C (refs. 4-6). There is an existing model that diamond can be grown using liquid metals only at both high pressure and high temperature7. Here we describe the growth of diamond crystals and polycrystalline diamond films with no seed particles using liquid metal but at 1 atm pressure and at 1,025 °C, breaking this pattern. Diamond grew in the subsurface of liquid metal composed of gallium, iron, nickel and silicon, by catalytic activation of methane and diffusion of carbon atoms into and within the subsurface regions. We found that the supersaturation of carbon in the liquid metal subsurface leads to the nucleation and growth of diamonds, with Si playing an important part in stabilizing tetravalently bonded carbon clusters that play a part in nucleation. Growth of (metastable) diamond in liquid metal at moderate temperature and 1 atm pressure opens many possibilities for further basic science studies and for the scaling of this type of growth.

Spiral-Driven Vertical Conductivity in Nanocrystalline Graphene.

The structure of graphene grown in chemical vapor deposition (CVD) is sensitive to the growth condition, particularly the substrate. The conventional growth of high-quality graphene via the Cu-catalyzed cracking of hydrocarbon species has been extensively studied; however, the direct growth on noncatalytic substrates, for practical applications of graphene such as current Si technologies, remains unexplored. In this study, nanocrystalline graphene (nc-G) spirals are produced on noncatalytic substrates by inductively coupled plasma CVD. The enhanced out-of-plane electrical conductivity is achieved by a spiral-driven continuous current pathway from bottom to top layer. Furthermore, some neighboring nc-G spirals exhibit a homogeneous electrical conductance, which is not common for stacked graphene structure. Klein-edge structure developed at the edge of nc-Gs, which can easily form covalent bonding, is thought to be responsible for the uniform conductance of nc-G aggregates. These results have important implications for practical applications of graphene with vertical conductivity realized through spiral structure.

Phase-Engineered WS2 Monolayer Quantum Dots by Rhenium Doping.

Transition metal dichalcogenides (TMDs) occur in the thermodynamically stable trigonal prismatic (2H) phase or the metastable octahedral (1T) phase. Phase engineering of TMDs has proven to be a powerful tool for applications in energy storage devices as well as in electrocatalysis. However, the mechanism of the phase transition in TMDs and the synthesis of phase-controlled TMDs remain challenging. Here we report the synthesis of Re-doped WS2 monolayer quantum dots (MQDs) using a simple colloidal chemical process. We find that the incorporation of a small amount of electron-rich Re atoms in WS2 changes the metal-metal distance in the 2H phase initially, which introduces strain in the structure (strained 2H (S2H) phase). Increasing the concentration of Re atoms sequentially transforms the S2H phase into the 1T and 1T' phases to release the strain. In addition, we performed controlled experiments by doping MoS2 with Re to distinguish between Re and Mo atoms in scanning transmission electron microscopy images and quantified the concentration range of Re atoms in each phase of MoS2, indicating that phase engineering of WS2 or MoS2 is possible by doping with different amounts of Re atoms. We demonstrate that the 1T' WS2 MQDs with 49 at. % Re show superior catalytic performance (a low Tafel slope of 44 mV/dec, a low overpotential of 158 mV at a current density of 10 mA/cm2, and long-term durability up to 5000 cycles) for the hydrogen evolution reaction. Our findings provide understanding and control of the phase transitions in TMDs, which will allow for the efficient manufacturing and translation of phase-engineered TMDs.

Fabrication of p-type 2D single-crystalline transistor arrays with Fermi-level-tuned van der Waals semimetal electrodes.

High-performance p-type two-dimensional (2D) transistors are fundamental for 2D nanoelectronics. However, the lack of a reliable method for creating high-quality, large-scale p-type 2D semiconductors and a suitable metallization process represents important challenges that need to be addressed for future developments of the field. Here, we report the fabrication of scalable p-type 2D single-crystalline 2H-MoTe2 transistor arrays with Fermi-level-tuned 1T'-phase semimetal contact electrodes. By transforming polycrystalline 1T'-MoTe2 to 2H polymorph via abnormal grain growth, we fabricated 4-inch 2H-MoTe2 wafers with ultra-large single-crystalline domains and spatially-controlled single-crystalline arrays at a low temperature (~500 °C). Furthermore, we demonstrate on-chip transistors by lithographic patterning and layer-by-layer integration of 1T' semimetals and 2H semiconductors. Work function modulation of 1T'-MoTe2 electrodes was achieved by depositing 3D metal (Au) pads, resulting in minimal contact resistance (~0.7 kΩ·µm) and near-zero Schottky barrier height (~14 meV) of the junction interface, and leading to high on-state current (~7.8 µA/µm) and on/off current ratio (~105) in the 2H-MoTe2 transistors.

Probing the Melting Transitions in Phase-Change Superlattices via Thin Film Nanocalorimetry.

Phase-change superlattices with nanometer thin sublayers are promising for low-power phase-change memory (PCM) on rigid and flexible platforms. However, the thermodynamics of the phase transition in such nanoscale superlattices remain unexplored, especially at ultrafast scanning rates, which is crucial for our fundamental understanding of superlattice-based PCM. Here, we probe the phase transition of Sb2Te3 (ST)/Ge2Sb2Te5 (GST) superlattices using nanocalorimetry with a monolayer sensitivity (∼1 Å) and a fast scanning rate (105 K/s). For a 2/1.8 nm/nm Sb2Te3/GST superlattice, we observe an endothermic melting transition with an ∼240 °C decrease in temperature and an ∼8-fold decrease in enthalpy compared to those for the melting of GST, providing key thermodynamic insights into the low-power switching of superlattice-based PCM. Nanocalorimetry measurements for Sb2Te3 alone demonstrate an intrinsic premelting similar to the unique phase transition of superlattices, thus revealing a critical role of the Sb2Te3 sublayer within our superlattices. These results advance our understanding of superlattices for energy-efficient data storage and computing.

Air-stable van der Waals PtTe2 conductors with high current-carrying capacity and strong spin-orbit interaction.

High-performance van der Waals (vdW) integrated electronics and spintronics require reliable current-carrying capacity. However, it is challenging to achieve high current density and air-stable performance using vdW metals owing to the fast electrical breakdown triggered by defects or oxidation. Here, we report that spin-orbit interacted synthetic PtTe2 layers exhibit significant electrical reliability and robustness in ambient air. The 4-nm-thick PtTe2 synthesized at a low temperature (∼400°C) shows intrinsic metallic transport behavior and a weak antilocalization effect attributed to the strong spin-orbit scattering. Remarkably, PtTe2 sustains a high current density approaching ≈31.5 MA cm-2, which is the highest value among electrical interconnect candidates under oxygen exposure. Electrical failure is caused by the Joule heating of PtTe2 rather than defect-induced electromigration, which was achievable by the native TeOx passivation. The high-quality growth of PtTe2 and the investigation of its transport behaviors lay out essential foundations for the development of emerging vdW spin-orbitronics.

Morphologically Controlled Synthesis of Reduced-Dimensional ZnO/Zn(OH)2 Nanosheets.

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)2 (a nonlayered transition metal oxide) nanosheets. This study described the synthesis of Zn(OH)2 nanosheets using an anionic sulfate layer and demonstrated a method of controlling the thickness and shape of the synthesized nanosheets by varying the surfactant concentration. Further, the characteristics of oxygen evolution reaction using ZnO/Zn(OH)2 nanosheets, obtained by annealing the synthesized sheets, as catalysts were studied.

Thermally driven phase transition of cobalt hydroxide sheets via cobalt oxides to cobalt nanoparticles.

Transition metal oxides, which include many stoichiometric variations, are formed into various crystal structures by the atomic arrangement of cations and anions according to stoichiometric composition and are used for a wide range of applications based on this. Among them, cobalt oxide, which has wide crystal structures depending on the lattice points of the anion and the valence of the Co cation, from its hydroxide formula, is attracting a lot of attention due to its interesting catalytic properties due to its crystal structure. In this study, using the synthesized Co(OH)2 nanosheets, the real-time behavior of the phase transition that occurs when continuous heat is applied to the sample has been systematically analyzed using an aberration-corrected scanning transmission electron microscope. The layered Co(OH)2 phase passes through hexagonal CoO and cubic CoO phases to finally become Co nanoparticles, but when the temperature is dropped in the hexagonal phase, spinel Co3O4 is formed. These results suggest that various phases included in transition metal oxides can be selectively implemented according to temperature range control.

Atomic transistors based on seamless lateral metal-semiconductor junctions with a sub-1-nm transfer length.

The edge-to-edge connected metal-semiconductor junction (MSJ) for two-dimensional (2D) transistors has the potential to reduce the contact length while improving the performance of the devices. However, typical 2D materials are thermally and chemically unstable, which impedes the reproducible achievement of high-quality edge contacts. Here we present a scalable synthetic strategy to fabricate low-resistance edge contacts to atomic transistors using a thermally stable 2D metal, PtTe2. The use of PtTe2 as an epitaxial template enables the lateral growth of monolayer MoS2 to achieve a PtTe2-MoS2 MSJ with the thinnest possible, seamless atomic interface. The synthesized lateral heterojunction enables the reduced dimensions of Schottky barriers and enhanced carrier injection compared to counterparts composed of a vertical 3D metal contact. Furthermore, facile position-selected growth of PtTe2-MoS2 MSJ arrays using conventional lithography can facilitate the design of device layouts with high processability, while providing low contact resistivity and ultrashort transfer length on wafer scales.

Correction: Thermally driven phase transition of cobalt hydroxide sheets via cobalt oxides to cobalt nanoparticles.

Correction for 'Thermally driven phase transition of cobalt hydroxide sheets via cobalt oxides to cobalt nanoparticles' by Aram Yoon et al., Nanoscale Horiz., 2022, https://doi.org/10.1039/d2nh00218c.

Unveiling the Effect of Superlattice Interfaces and Intermixing on Phase Change Memory Performance.

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 Ge2Sb2Te5 and Sb2Te3 in SL-PCM, we uncover that both switching current density (Jreset) and resistance drift coefficient (v) decrease as the SL period thickness is reduced (i.e., higher interface density); however, interface intermixing within the SL increases both. The signatures of distinct versus intermixed interfaces also show up in transmission electron microscopy, X-ray diffraction, and thermal conductivity measurements of our SL films. Combining the lessons learned, we simultaneously achieve low Jreset ≈ 3-4 MA/cm2 and ultralow v ≈ 0.002 in mushroom-cell SL-PCM with ∼110 nm bottom contact diameter, thus advancing SL-PCM technology for high-density storage and neuromorphic applications.

In-situ formation of co particles encapsulated by graphene layers.

The process of encapsulating cobalt nanoparticles using a graphene layer is mainly direct pyrolysis. The encapsulation structure of hybrids prepared in this way improves the catalyst stability, which greatly reduces the leaching of non-metals and prevents metal nanoparticles from growing beyond a certain size. In this study, cobalt particles surrounded by graphene layers were formed by increasing the temperature in a transmission electron microscope, and they were analyzed using scanning transmission electron microscopy (STEM). Synthesized cobalt hydroxide nanosheets were used to obtain cobalt particles using an in-situ heating holder inside a TEM column. The cobalt nanoparticles are surrounded by layers of graphene, and the number of layers increases as the temperature increases. The interlayer spacing of the graphene layers was also investigated using atomic imaging. The success achieved in the encapsulation of metallic nanoparticles in graphene layers paves the way for the design of highly active and reusable heterogeneous catalysts for more challenging molecules.

Defect-gradient-induced Rashba effect in van der Waals PtSe2 layers.

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 PtSe2 films developed by selective plasma treatments, which break spatial inversion symmetry and give rise to the Rashba effect. Scanning transmission electron microscopy analyses reveal that Se vacancies extend down to 7 nm from the surface and Se/Pt ratio exhibits linear variation along the layers. The Rashba effect induced by broken inversion symmetry is demonstrated through the observations of nonreciprocal transport behaviors and first-principles density functional theory calculations. Our methodology paves the way for functional defect engineering that entangles spin and momentum of itinerant electrons for emerging electronic applications.

Silica Particle-Mediated Growth of Single Crystal Graphene Ribbons on Cu(111) Foil.

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 Cu<101> direction 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 (C2 H4 ) and attachment of C atoms or species ("vapor solid" or VS growth) at the edges. It is found, based on the taper angle of the graphene ribbon, that the taper angle is determined by the growth temperature and the growth rates are independent of the particle size. The activation enthalpy (1.73 ± 0.03 eV) for longitudinal ribbon growth on Cu(111) from ethylene is lower than that for VS growth at the edges of the GRs (2.78 ± 0.15 eV) and for graphene island growth (2.85 ± 0.07 eV) that occurs concurrently.

Folding and Fracture of Single-Crystal Graphene Grown on a Cu(111) Foil.

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 the <110> and <211> 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.

Spiral Growth of Adlayer Graphene.

The morphology of as-grown graphene in chemical vapor deposition (CVD) experiments is sensitive to the reaction environment. Understanding the mechanism of formation of different graphene morphologies is essential to achieve controlled graphene CVD growth. Here the growth and formation mechanism of adlayer graphene spirals are reported. An adlayer graphene spiral is formed by fast propagation of the tips of spiral arms along the edge of the first graphene layer. The driving force to form spirals is the limited availability of carbon diffusing from the Cu surface through the edge of the first graphene layer. In addition, it is found that graphene onions are formed by overlapping graphene spirals with clockwise and anticlockwise arms. Based on these features, a kinetic Monte Carlo (kMC) method is demonstrated using which all the observed graphene spiral structures are successfully reproduced at the atomic level. This study thus unravels the hither-to unresolved mechanism of graphene onion growth and paves the way to the controllable growth of few-layer graphene by increasing the carbon supply at the edge of the first layer graphene.

Precise Layer Control and Electronic State Modulation of a Transition Metal Dichalcogenide via Phase-Transition-Induced Growth.

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.

Elucidation of Novel Potassium-Mediated Oxidation and Etching of Two-Dimensional Transition Metal Dichalcogenides.

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.

In Situ Scanning Transmission Electron Microscopy Study of MoS2 Formation on Graphene with a Deep-Learning Framework.

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.