Two-Step Growth of Uniform Monolayer MoS2 Nanosheets by Metal-Organic Chemical Vapor Deposition.

To achieve large area growth of transition metal dichalcogenides of uniform monolayer thickness, we demonstrate metal-organic chemical vapor deposition (MOCVD) growth under low pressure followed by a high-temperature sulfurization process under atmospheric pressure (AP). Following sulfurization, the MOCVD-grown continuous MoS2 film transforms into compact triangular crystals of uniform monolayer thickness as confirmed from the sharp distinct photoluminescence peak at 1.8 eV. Raman and X-ray photoelectron spectroscopies confirm that the structural disorders and chalcogen vacancies inherent to the as-grown MOCVD film are substantially healed and carbon/oxygen contaminations are heavily suppressed. The as-grown MOCVD film has a Mo/S ratio of 1:1.6 and an average defect length of ∼1.56 nm, which improve to 1:1.97 and ∼21 nm, respectively, upon sulfurization. The effect of temperature and duration of the sulfurization process on the morphology and stoichiometry of the grown film is investigated in detail. Compared to the APCVD growth, this two-step growth process shows more homogenous distribution of the triangular monolayer MoS2 domains across the entire substrate, while demonstrating comparable electrical performance.

A Library of Atomically Thin 2D Materials Featuring the Conductive-Point Resistive Switching Phenomenon.

Non-volatile resistive switching (NVRS) is a widely available effect in transitional metal oxides, colloquially known as memristors, and of broad interest for memory technology and neuromorphic computing. Until recently, NVRS was not known in other transitional metal dichalcogenides (TMDs), an important material class owing to their atomic thinness enabling the ultimate dimensional scaling. Here, various monolayer or few-layer 2D materials are presented in the conventional vertical structure that exhibit NVRS, including TMDs (MX2 , M = transitional metal, e.g., Mo, W, Re, Sn, or Pt; X = chalcogen, e.g., S, Se, or Te), TMD heterostructure (WS2 /MoS2 ), and an atomically thin insulator (h-BN). These results indicate the universality of the phenomenon in 2D non-conductive materials, and feature low switching voltage, large ON/OFF ratio, and forming-free characteristic. A dissociation-diffusion-adsorption model is proposed, attributing the enhanced conductance to metal atoms/ions adsorption into intrinsic vacancies, a conductive-point mechanism supported by first-principle calculations and scanning tunneling microscopy characterizations. The results motivate further research in the understanding and applications of defects in 2D materials.

Thinnest Nonvolatile Memory Based on Monolayer h-BN.

2D materials have attracted much interest over the past decade in nanoelectronics. However, it was believed that the atomically thin layered materials are not able to show memristive effect in vertically stacked structure, until the recent discovery of monolayer transition metal dichalcogenide (TMD) atomristors, overcoming the scaling limit to sub-nanometer. Herein, the nonvolatile resistance switching (NVRS) phenomenon in monolayer hexagonal boron nitride (h-BN), a typical 2D insulator, is reported. The h-BN atomristors are studied using different electrodes and structures, featuring forming-free switching in both unipolar and bipolar operations, with large on/off ratio (up to 107 ). Moreover, fast switching speed (<15 ns) is demonstrated via pulse operation. Compared with monolayer TMDs, the one-atom-thin h-BN sheet reduces the vertical scaling to ≈0.33 nm, representing a record thickness for memory materials. Simulation results based on ab-initio method reveal that substitution of metal ions into h-BN vacancies during electrical switching is a likely mechanism. The existence of NVRS in monolayer h-BN indicates fruitful interactions between defects, metal ions and interfaces, and can advance emerging applications on ultrathin flexible memory, printed electronics, neuromorphic computing, and radio frequency switches.

Growth of lateral graphene/h-BN heterostructure on copper foils by chemical vapor deposition.

The synthesis of lateral heterostructures assembled by atomically-thin materials with distinct intrinsic properties is important for future heterojunction-embedded two-dimensional (2D) devices. Here we report an etching-assisted chemical vapor deposition method to synthesize large-area continuous lateral graphene/hexagonal boron nitride (Gr/h-BN) heterostructures on carbon-containing copper foils. The h-BN film is first synthesized on the copper foil, followed by hydrogen etching, and then epitaxial graphene domains are grown to form continuous lateral heterostructures. Analyses, including Raman spectroscopy, atomic force microscopy, scanning electron microscopy, x-ray photoelectron spectroscopy, and ultraviolet-visible absorption spectroscopy, are used to characterize the coexistence of both materials and the highly continuous nature of this lateral heterostructure. This facile and scalable synthesizing method enables the potential usage of Gr/h-BN heterostructure in both fundamental studies and related 2D devices.

Dependence of h-BN Film Thickness as Grown on Nickel Single-Crystal Substrates of Different Orientations.

Chemical vapor deposition (CVD) of two-dimensional materials has been an active area of research in recent years because it is a scalable process for obtaining thin films that can be used to fabricate devices. The growth mechanism for hexagonal boron nitride (h-BN) on metal catalyst substrates has been described to be either surface energy-driven or diffusion-driven. In this work, h-BN is grown in a CVD system on Ni single-crystal substrates as a function of Ni crystallographic orientation to clarify the competing forces acting on the growth mechanism. We observed that the thickness of the h-BN film depends on the Ni substrate orientation, with the growth rate increasing from the (100) surface to the (111) surface and the highest on the (110) surface. We associate the observed results with surface reactivity and diffusivity differences for different Ni orientations. Boron and nitrogen diffuse and precipitate from the Ni bulk to form thin multilayer h-BN. Our results serve to clarify the h-BN CVD growth mechanism which has been previously ascribed to a surface energy-driven growth mechanism.

In Situ Observation of Initial Stage in Dielectric Growth and Deposition of Ultrahigh Nucleation Density Dielectric on Two-Dimensional Surfaces.

Several proposed beyond-CMOS devices based on two-dimensional (2D) heterostructures require the deposition of thin dielectrics between 2D layers. However, the direct deposition of dielectrics on 2D materials is challenging due to their inert surface chemistry. To deposit high-quality, thin dielectrics on 2D materials, a flat lying titanyl phthalocyanine (TiOPc) monolayer, deposited via the molecular beam epitaxy, was employed to create a seed layer for atomic layer deposition (ALD) on 2D materials, and the initial stage of growth was probed using in situ STM. ALD pulses of trimethyl aluminum (TMA) and H2O resulted in the uniform deposition of AlOx on the TiOPc/HOPG. The uniformity of the dielectric is consistent with DFT calculations showing multiple reaction sites are available on the TiOPc molecule for reaction with TMA. Capacitors prepared with 50 cycles of AlOx on TiOPc/graphene display a capacitance greater than 1000 nF/cm(2), and dual-gated devices have current densities of 10(-7)A/cm(2) with 40 cycles.

Simultaneous Electrochemical Reduction and Delamination of Graphene Oxide Films.

Here we report an electrochemical method to simultaneously reduce and delaminate graphene oxide (G-O) thin films deposited on metal (Al and Au) substrates. During the electrochemical reaction, interface charge transfer between the G-O thin film and the electrode surface was found to be important in eliminating oxygen-containing groups, yielding highly reduced graphene oxide (rG-O). In the meantime, hydrogen bubbles were electrochemically generated at the rG-O film/electrode interface, propagating the film delamination. Unlike other metal-based G-O reduction methods, the metal used here was either not etched at all (for Au) or etched a small amount (for Al), thus making it possible to reuse the substrate and lower production costs. The delaminated rG-O film exhibits a thickness-dependent degree of reduction: greater reduction is achieved in thinner films. The thin rG-O films having an optical transmittance of 90% (λ = 550 nm) had a sheet resistance of 6390 ± 447 Ω/□ (ohms per square). rG-O-based stretchable transparent conducting films were also demonstrated.

Thermal Oxidation of WSe2 Nanosheets Adhered on SiO2/Si Substrates.

Because of the drastically different intralayer versus interlayer bonding strengths, the mechanical, thermal, and electrical properties of two-dimensional (2D) materials are highly anisotropic between the in-plane and out-of-plane directions. The structural anisotropy may also play a role in chemical reactions, such as oxidation, reduction, and etching. Here, the composition, structure, and electrical properties of mechanically exfoliated WSe2 nanosheets on SiO2/Si substrates were studied as a function of the extent of thermal oxidation. A major component of the oxidation, as indicated from optical and Raman data, starts from the nanosheet edges and propagates laterally toward the center. Partial oxidation also occurs in certain areas at the surface of the flakes, which are shown to be highly conductive by microwave impedance microscopy. Using secondary ion mass spectroscopy, we also observed extensive oxidation at the WSe2-SiO2 interface. The combination of multiple microcopy methods can thus provide vital information on the spatial evolution of chemical reactions on 2D materials and the nanoscale electrical properties of the reaction products.

Ultra-light Hierarchical Graphene Electrode for Binder-Free Supercapacitors and Lithium-Ion Battery Anodes.

A mild and environmental-friendly method is developed for fabricating a 3D interconnected graphene electrode with large-scale continuity. Such material has interlayer pores between reduced graphene oxide nanosheets and in-plane pores. Hence, a specific surface area up to 835 m(2) g(-1) and a high powder conductivity up to 400 S m(-1) are achieved. For electrochemical applications, the interlayer pores can serve as "ion-buffering reservoirs" while in-plane ones act as "channels" for shortening the mass cross-plane diffusion length, reducing the ion response time, and prevent the interlayer restacking. As binder-free supercapacitor electrode, it delivers a specific capacitance up to 169 F g(-1) with surface-normalized capacitance close to 21 µF cm(-2) (intrinsic capacitance) and power density up to 7.5 kW kg(-1), in 6 m KOH aqueous electrolyte. In the case of lithium-ion battery anode, it shows remarkable advantages in terms of the initiate reversible Coulombic efficiency (61.3%), high specific capacity (932 mAh g(-1) at 100 mA g(-1)), and robust long-term retention (93.5% after 600 cycles at 2000 mAh g(-1)).

Revealing the planar chemistry of two-dimensional heterostructures at the atomic level.

Two-dimensional (2D) atomic crystals and their heterostructures are an intense area of study owing to their unique properties that result from structural planar confinement. Intrinsically, the performance of a planar vertical device is linked to the quality of its 2D components and their interfaces, therefore requiring characterization tools that can reveal both its planar chemistry and morphology. Here, we propose a characterization methodology combining (micro-) Raman spectroscopy, atomic force microscopy and time-of-flight secondary ion mass spectrometry to provide structural information, morphology and planar chemical composition at virtually the atomic level, aimed specifically at studying 2D vertical heterostructures. As an example system, a graphene-on-h-BN heterostructure is analysed to reveal, with an unprecedented level of detail, the subtle chemistry and interactions within its layer structure that can be assigned to specific fabrication steps. Such detailed chemical information is of crucial importance for the complete integration of 2D heterostructures into functional devices.

Enhanced dielectric performance in polymer composite films with carbon nanotube-reduced graphene oxide hybrid filler.

The electrical conductivity and the specific surface area of conductive fillers in conductor-insulator composite films can drastically improve the dielectric performance of those films through changing their polarization density by interfacial polarization. We have made a polymer composite film with a hybrid conductive filler material made of carbon nanotubes grown onto reduced graphene oxide platelets (rG-O/CNT). We report the effect of the rG-O/CNT hybrid filler on the dielectric performance of the composite film. The composite film had a dielectric constant of 32 with a dielectric loss of 0.051 at 0.062 wt% rG-O/CNT filler and 100 Hz, while the neat polymer film gave a dielectric constant of 15 with a dielectric loss of 0.036. This is attributed to the increased electrical conductivity and specific surface area of the rG-O/CNT hybrid filler, which results in an increase in interfacial polarization density between the hybrid filler and the polymer.

Flexible and transparent dielectric film with a high dielectric constant using chemical vapor deposition-grown graphene interlayer.

We have devised a dielectric film with a chemical vapor deposited graphene interlayer and studied the effect of the graphene interlayer on the dielectric performance. The highly transparent and flexible film was a polymer/graphene/polymer 'sandwich-structure' fabricated by a one-step transfer method that had a dielectric constant of 51, with a dielectric loss of 0.05 at 1 kHz. The graphene interlayer in the film forms a space charge layer, i.e., an accumulation of polarized charge carriers near the graphene, resulting in an induced space charge polarization and enhanced dielectric constant. The characteristic of the space charge layer for the graphene dielectric film, the sheet resistance of the graphene interlayer, was adjusted through thermal annealing that caused partial oxidation. The dielectric film with higher sheet resistance due to the oxidized graphene interlayer had a significantly lower dielectric constant compared to that with the graphene with lower interlayer sheet resistance. Oxidizing the graphene interlayer yields a smaller and thinner space charge density in the dielectric film, ultimately leading to decreased capacitance. Considering the simplicity of the fabrication process and high dielectric performance, as well as the high transparency and flexibility, this film is promising for applications in plastic electronics.

Direct delamination of graphene for high-performance plastic electronics.

Electrochemical delamination is developed to transfer graphene for plastic electronics. The use of a sacrificial support during transfer is eliminated by depositing the target polyimide substrate directly onto graphene. A continuous and residue-free graphene surface with less line disruptions (such as ripples and wrinkles) is obtained on the target polyimide substrate, and good mechanical durability as well as low sheet resistance is obtained. The properties are competitive with conventional transparent conducting films.

The role of surface oxygen in the growth of large single-crystal graphene on copper.

The growth of high-quality single crystals of graphene by chemical vapor deposition on copper (Cu) has not always achieved control over domain size and morphology, and the results vary from lab to lab under presumably similar growth conditions. We discovered that oxygen (O) on the Cu surface substantially decreased the graphene nucleation density by passivating Cu surface active sites. Control of surface O enabled repeatable growth of centimeter-scale single-crystal graphene domains. Oxygen also accelerated graphene domain growth and shifted the growth kinetics from edge-attachment-limited to diffusion-limited. Correspondingly, the compact graphene domain shapes became dendritic. The electrical quality of the graphene films was equivalent to that of mechanically exfoliated graphene, in spite of being grown in the presence of O.

Enhancement of the electrical properties of graphene grown by chemical vapor deposition via controlling the effects of polymer residue.

Residual polymer (here, poly(methyl methacrylate), PMMA) left on graphene from transfer from metals or device fabrication processes affects its electrical and thermal properties. We have found that the amount of polymer residue left after the transfer of chemical vapor deposited (CVD) graphene varies depending on the initial concentration of the polymer solution, and this residue influences the electrical performance of graphene field-effect transistors fabricated on SiO2/Si. A PMMA solution with lower concentration gave less residue after exposure to acetone, resulting in less p-type doping in graphene and higher charge carrier mobility. The electrical properties of the weakly p-doped graphene could be further enhanced by exposure to formamide with the Dirac point at nearly zero gate voltage and a more than 50% increase of the room-temperature charge carrier mobility in air. This can be attributed to electron donation to graphene by the -NH2 functional group in formamide that is absorbed in the polymer residue. This work provides a route to enhancing the electrical properties of CVD-grown graphene even when it has a thin polymer coating.

Millimeter-size single-crystal graphene by suppressing evaporative loss of Cu during low pressure chemical vapor deposition.

Millimeter-size single-crystal monolayer graphene is synthesized on polycrystalline Cu foil by a method that involves suppressing loss by evaporation of the Cu at high temperature under low pressure. This significantly diminishes the number of graphene domains, and large single crystal domains up to ∼2 mm in size are grown.

Reduced graphene oxide/copper nanowire hybrid films as high-performance transparent electrodes.

Hybrid films composed of reduced graphene oxide (RG-O) and Cu nanowires (NWs) were prepared. Compared to Cu NW films, the RG-O/Cu NW hybrid films have improved electrical conductivity, oxidation resistance, substrate adhesion, and stability in harsh environments. The RG-O/Cu NW films were used as transparent electrodes in Prussian blue (PB)-based electrochromic devices where they performed significantly better than pure Cu NW films.

Growth of adlayer graphene on Cu studied by carbon isotope labeling.

The growth of bilayer and multilayer graphene on copper foils was studied by isotopic labeling of the methane precursor. Isotope-labeled graphene films were characterized by micro-Raman mapping and time-of-flight secondary ion mass spectrometry. Our investigation shows that during growth at high temperature, the adlayers formed simultaneously and beneath the top, continuous layer of graphene and the Cu substrate. Additionally, the adlayers share the same nucleation center and all adlayers nucleating in one place have the same edge termination. These results suggest that adlayer growth proceeds by catalytic decomposition of methane (or CH(x), x < 4) trapped in a "nano-chemical vapor deposition" chamber between the first layer and the substrate. On the basis of these results, submillimeter bilayer graphene was synthesized by applying a much lower growth rate.