Polycrystalline Few-Layer Graphene as a Durable Anticorrosion Film for Copper.

Corrosion of metals in atmospheric environments is a worldwide problem in industry and daily life. Traditional anticorrosion methods including sacrificial anodes or protective coatings have performance limitations. Here, we report atomically thin, polycrystalline few-layer graphene (FLG) grown by chemical vapor deposition as a long-term protective coating film for copper (Cu). A six-year old, FLG-protected Cu is visually shiny and detailed material characterizations capture no sign of oxidation. The success of the durable anticorrosion film depends on the misalignment of grain boundaries between adjacent graphene layers. Theoretical calculations further found that corrosive molecules always encounter extremely high energy barrier when diffusing through the FLG layers. Therefore, the FLG is able to prevent the corrosive molecules from reaching the underlying Cu surface. This work highlights the interesting structures of polycrystalline FLG and sheds insight into the atomically thin coatings for various applications.

The unusual mechanism of partial Fermi level pinning at metal-MoS2 interfaces.

Density functional theory calculations are performed to unravel the nature of the contact between metal electrodes and monolayer MoS2. Schottky barriers are shown to be present for a variety of metals with the work functions spanning over 4.2-6.1 eV. Except for the p-type Schottky contact with platinum, the Fermi levels in all of the studied metal-MoS2 complexes are situated above the midgap of MoS2. The mechanism of the Fermi level pinning at metal-MoS2 contact is shown to be unique for metal-2D-semiconductor interfaces, remarkably different from the well-known Bardeen pinning effect, metal-induced gap states, and defect/disorder induced gap states, which are applicable to traditional metal-semiconductor junctions. At metal-MoS2 interfaces, the Fermi level is partially pinned as a result of two interface behaviors: first by a metal work function modification by interface dipole formation due to the charge redistribution, and second by the production of gap states mainly of Mo d-orbitals character by the weakened intralayer S-Mo bonding due to the interface metal-S interaction. This finding would provide guidance to develop approaches to form Ohmic contact to MoS2.

Revealing the improved stability of amorphous boron-nitride upon carbon doping.

We report on a large improvement of the thermal stability and mechanical properties of amorphous boron-nitride upon carbon doping. By generating versatile force fields using first-principles and machine learning simulations, we investigate the structural properties of amorphous boron-nitride with varying contents of carbon (from a few percent to 40 at%). We found that for 20 at% of carbon, the sp3/sp2 ratio reaches a maximum with a negligible graphitisation effect, resulting in an improvement of the thermal stability by up to 20% while the bulk Young's modulus increases by about 30%. These results provide a guide to experimentalists and engineers to further tailor the growth conditions of BN-based compounds as non-conductive diffusion barriers and ultralow dielectric coefficient materials for a number of applications including interconnect technology.

Large-area graphene single crystals grown by low-pressure chemical vapor deposition of methane on copper.

Graphene single crystals with dimensions of up to 0.5 mm on a side were grown by low-pressure chemical vapor deposition in copper-foil enclosures using methane as a precursor. Low-energy electron microscopy analysis showed that the large graphene domains had a single crystallographic orientation, with an occasional domain having two orientations. Raman spectroscopy revealed the graphene single crystals to be uniform monolayers with a low D-band intensity. The electron mobility of graphene films extracted from field-effect transistor measurements was found to be higher than 4000 cm(2) V(-1) s(-1) at room temperature.

Graphene films with large domain size by a two-step chemical vapor deposition process.

The fundamental properties of graphene are making it an attractive material for a wide variety of applications. Various techniques have been developed to produce graphene and recently we discovered the synthesis of large area graphene by chemical vapor deposition (CVD) of methane on Cu foils. We also showed that graphene growth on Cu is a surface-mediated process and the films were polycrystalline with domains having an area of tens of square micrometers. In this paper, we report on the effect of growth parameters such as temperature, and methane flow rate and partial pressure on the growth rate, domain size, and surface coverage of graphene as determined by Raman spectroscopy, and transmission and scanning electron microscopy. On the basis of the results, we developed a two-step CVD process to synthesize graphene films with domains having an area of hundreds of square micrometers. Scanning electron microscopy and Raman spectroscopy clearly show an increase in domain size by changing the growth parameters. Transmission electron microscopy further shows that the domains are crystallographically rotated with respect to each other with a range of angles from about 13 to nearly 30°. Electrical transport measurements performed on back-gated FETs show that overall films with larger domains tend to have higher carrier mobility up to about 16,000 cm(2) V(-1) s(-1) at room temperature.

Evolution of graphene growth on Ni and Cu by carbon isotope labeling.

Large-area graphene growth is required for the development and production of electronic devices. Recently, chemical vapor deposition (CVD) of hydrocarbons has shown some promise in growing large-area graphene or few-layer graphene films on metal substrates such as Ni and Cu. It has been proposed that CVD growth of graphene on Ni occurs by a C segregation or precipitation process whereas graphene on Cu grows by a surface adsorption process. Here we used carbon isotope labeling in conjunction with Raman spectroscopic mapping to track carbon during the growth process. The data clearly show that at high temperatures sequentially introduced isotopic carbon diffuses into the Ni first, mixes, and then segregates and precipitates at the surface of Ni forming graphene and/or graphite with a uniform mixture of (12)C and (13)C as determined by the peak position of the Raman G-band peak. On the other hand, graphene growth on Cu is clearly by surface adsorption where the spatial distribution of (12)C and (13)C follows the precursor time sequence and the linear growth rate ranges from about 1 to as high as 6 mum/min depending upon Cu grain orientation. This data is critical in guiding the graphene growth process as we try to achieve the highest quality graphene for electronic devices.

Transfer of large-area graphene films for high-performance transparent conductive electrodes.

Graphene, a two-dimensional monolayer of sp(2)-bonded carbon atoms, has been attracting great interest due to its unique transport properties. One of the promising applications of graphene is as a transparent conductive electrode owing to its high optical transmittance and conductivity. In this paper, we report on an improved transfer process of large-area graphene grown on Cu foils by chemical vapor deposition. The transferred graphene films have high electrical conductivity and high optical transmittance that make them suitable for transparent conductive electrode applications. The improved transfer processes will also be of great value for the fabrication of electronic devices such as field effect transistor and bilayer pseudospin field effect transistor devices.

Atomic Layer Deposition of Layered Boron Nitride for Large-Area 2D Electronics.

Hexagonal boron nitride (h-BN) has been considered a promising dielectric for two-dimensional (2D) material-based electronics due to its atomically smooth and charge-free interface with an in-plane lattice constant similar to that of graphene. Here, we report atomic layer deposition of boron nitride (ALD-BN) using BCl3 and NH3 precursors directly on thermal SiO2 substrates at a relatively low temperature of 600 °C. The films were characterized by X-ray photoelectron spectroscopy, atomic force microscopy, and transmission electron microscopy wherein the uniform, atomically smooth, and nanocrystalline layered-BN thin film growth is observed. The growth rate is ∼0.042 nm/cycle at 600 °C, a temperature significantly lower than that of h-BN grown by chemical vapor deposition. The dielectric properties of the ALD-BN measured from Metal Oxide Semiconductor Capacitors are comparable with that of SiO2. Moreover, the ALD-BN exhibits a 2-fold increase in carrier mobility of graphene field effect transistors (G-FETs/ALD-BN/SiO2) due to the lower surface charge density and inert surface of ALD-BN in comparison to that of G-FETs fabricated on bare SiO2. Therefore, this work suggests that the transfer-free deposition of ALD-BN on SiO2 may be a promising candidate as a substrate for high performance graphene devices.

Methods to Characterize Effective Thermal Conductivity, Diffusivity and Thermal Response in Different Classes of Composite Phase Change Materials.

The phase change materials (PCMs) used in devices for thermal energy storage (TES) and management are often characterized by low thermal conductivity, a limit for their applicability. Composite PCMs (C-PCM), which combine active phase (proper PCM) with a passive phase with high conductivity and melting temperature have thus been proposed. The paper deals with the effect of length-scale on thermal characterization methods of C-PCM. The first part of the work includes a review of techniques proposed in the scientific literature. Up to now, special focus has been given to effective thermal conductivity and diffusivity at room or low temperature, at which both phases are solid. Conventional equipment has been used, neglecting length-scale effect in cases of coarse porous structures. An experimental set-up developed to characterize the thermal response of course porous C-PCMs also during active phase transition at high temperature is then presented. The setup, including high temperature-heat flux sensors and thermocouples to be located within samples, has been applied to evaluate the thermal response of some of the above C-PCMs. Experimental test results match Finite Elements (FE) simulations well, once a proper lattice model has been selected for the porous passive phase. FE simulations can then be used to estimate temperature difference between active and passive phase that prevents considering the C-PCM as a homogeneous material, to describe it by effective thermo-physical properties. In the engineering field, under these conditions, the design steps for TES systems design cannot be simplified by considering C-PCMs as homogeneous materials in FE codes.

Nonvolatile Memories Based on Graphene and Related 2D Materials.

The pervasiveness of information technologies is generating an impressive amount of data, which need to be accessed very quickly. Nonvolatile memories (NVMs) are making inroads into high-capacity storage to replace hard disk drives, fuelling the expansion of the global storage memory market. As silicon-based flash memories are approaching their fundamental limit, vertical stacking of multiple memory cell layers, innovative device concepts, and novel materials are being investigated. In this context, emerging 2D materials, such as graphene, transition metal dichalcogenides, and black phosphorous, offer a host of physical and chemical properties, which could both improve existing memory technologies and enable the next generation of low-cost, flexible, and wearable storage devices. Herein, an overview of graphene and related 2D materials (GRMs) in different types of NVM cells is provided, including resistive random-access, flash, magnetic and phase-change memories. The physical and chemical mechanisms underlying the switching of GRM-based memory devices studied in the last decade are discussed. Although at this stage most of the proof-of-concept devices investigated do not compete with state-of-the-art devices, a number of promising technological advancements have emerged. Here, the most relevant material properties and device structures are analyzed, emphasizing opportunities and challenges toward the realization of practical NVM devices.

Stress-Induced Crystallization of Thin Hf1- XZr XO2 Films: The Origin of Enhanced Energy Density with Minimized Energy Loss for Lead-Free Electrostatic Energy Storage Applications.

Increasing interest in the development of alternative energy storage technologies has led to efforts being taken to improve the energy density of dielectric capacitors with high power density. However, dielectric polymer materials still have low energy densities because of their low dielectric constant, whereas Pb-based materials are limited by environmental issues and regulations. Here, the energy storage behaviors of atomic layer-deposited Hf1- XZr XO2 ( X = 0-1) thin films (10 nm) and the phase transformation mechanism associated with an enhancement of their energy density are reported using unipolar pulse measurements. Based on electrical and material characterization, the energy density and energy efficiency are dependent on the Zr content, and stress-induced crystallization by the encapsulating Hf1- XZr XO2 films with TiN top electrodes prior to annealing can enhance the energy density (up to 47 J/cm3 at a small voltage value of 3.5 MV/cm) while minimizing energy loss even at low process temperatures (400 °C). This work will facilitate the realization of Hf1- XZr XO2-based capacitors for lead-free electrostatic energy storage applications.

Quantum engineering of transistors based on 2D materials heterostructures.

Quantum engineering entails atom-by-atom design and fabrication of electronic devices. This innovative technology that unifies materials science and device engineering has been fostered by the recent progress in the fabrication of vertical and lateral heterostructures of two-dimensional materials and by the assessment of the technology potential via computational nanotechnology. But how close are we to the possibility of the practical realization of next-generation atomically thin transistors? In this Perspective, we analyse the outlook and the challenges of quantum-engineered transistors using heterostructures of two-dimensional materials against the benchmark of silicon technology and its foreseeable evolution in terms of potential performance and manufacturability. Transistors based on lateral heterostructures emerge as the most promising option from a performance point of view, even if heterostructure formation and control are in the initial technology development stage.

Publisher Correction: Quantum engineering of transistors based on 2D materials heterostructures.

In the version of this Perspective originally published, in the email address for the author Giuseppe Iannaccone, the surname was incorrectly given as "innaconne"; this has now been corrected in all versions of the Perspective. Also, an error in the production process led to Figs. 1, 2 and 3 being of low resolution; these have now been replaced with higher-quality versions.

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.

Removal of a Frontal Sinus Osteoma and Reconstruction by a Custom-Made Implant with Neuronavigation Assistance.

The authors report the surgical treatment of an extensive right frontal sinus osteoma assisted by neuronavigation and reconstruction by a hydroxyapatite custom-made implant. The patient presents with ptosis, hypoglobus, and proptosis of the right eye, without any visual impairment. Computed tomographic (CT) scan showed a very large bony mass involving right frontal sinus and displacing the orbital roof. A stereolithographic model-guided planning was carried out to obtain a practical simulation of the surgical operation and it was submitted to a new CT scan to acquire the reference point to realize the neuronavigation assistance, and to achieve the template to realize the hydroxyapatite custom-made implant. Intraoperatively, with the help of neuronavigation assistance, osteotomies were performed by piezoelectric device. The reconstruction was made using a hydroxyapatite custom-made implant. The procedure was damage free, the bony mass was excised, and the orbital roof was repaired without any adverse effects. Postsurgical CT scan and scintigraphy showed a good reconstruction and a good-quality osteoblasts activity on the borders of the implant. Osteoma is a benign slow-growing bone tumor, usually involving the frontal sinus. Navigational assistance offers a very important help to perform safe osteotomies. Hydroxyapatite custom-made implant seems to be an excellent reconstructive method.

Oxygen-Promoted Chemical Vapor Deposition of Graphene on Copper: A Combined Modeling and Experimental Study.

Mass production of large, high-quality single-crystalline graphene is dependent on a complex coupling of factors including substrate material, temperature, pressure, gas flow, and the concentration of carbon and hydrogen species. Recent studies have shown that the oxidation of the substrate surface such as Cu before the introduction of the C precursor, methane, results in a significant increase in the growth rate of graphene while the number of nuclei on the surface of the Cu substrate decreases. We report on a phase-field model, where we include the effects of oxygen on the number of nuclei, the energetics at the growth front, and the graphene island morphology on Cu. Our calculations reproduce the experimental observations, thus validating the proposed model. Finally, and more importantly, we present growth rate from our model as a function of O concentration and precursor flux to guide the efficient growth of large single-crystal graphene of high quality.

Dislocation driven spiral and non-spiral growth in layered chalcogenides.

Two-dimensional materials have shown great promise for implementation in next-generation devices. However, controlling the film thickness during epitaxial growth remains elusive and must be fully understood before wide scale industrial application. Currently, uncontrolled multilayer growth is frequently observed, and not only does this growth mode contradict theoretical expectations, but it also breaks the inversion symmetry of the bulk crystal. In this work, a multiscale theoretical investigation aided by experimental evidence is carried out to identify the mechanism of such an unconventional, yet widely observed multilayer growth in the epitaxy of layered materials. This work reveals the subtle mechanistic similarities between multilayer concentric growth and spiral growth. Using the combination of experimental demonstration and simulations, this work presents an extended analysis of the driving forces behind this non-ideal growth mode, and the conditions that promote the formation of these defects. Our study shows that multilayer growth can be a result of both chalcogen deficiency and chalcogen excess: the former causes metal clustering as nucleation defects, and the latter generates in-domain step edges facilitating multilayer growth. Based on this fundamental understanding, our findings provide guidelines for the narrow window of growth conditions which enables large-area, layer-by-layer growth.

A kinetic Monte Carlo simulation method of van der Waals epitaxy for atomistic nucleation-growth processes of transition metal dichalcogenides.

Controlled growth of crystalline solids is critical for device applications, and atomistic modeling methods have been developed for bulk crystalline solids. Kinetic Monte Carlo (KMC) simulation method provides detailed atomic scale processes during a solid growth over realistic time scales, but its application to the growth modeling of van der Waals (vdW) heterostructures has not yet been developed. Specifically, the growth of single-layered transition metal dichalcogenides (TMDs) is currently facing tremendous challenges, and a detailed understanding based on KMC simulations would provide critical guidance to enable controlled growth of vdW heterostructures. In this work, a KMC simulation method is developed for the growth modeling on the vdW epitaxy of TMDs. The KMC method has introduced full material parameters for TMDs in bottom-up synthesis: metal and chalcogen adsorption/desorption/diffusion on substrate and grown TMD surface, TMD stacking sequence, chalcogen/metal ratio, flake edge diffusion and vacancy diffusion. The KMC processes result in multiple kinetic behaviors associated with various growth behaviors observed in experiments. Different phenomena observed during vdW epitaxy process are analysed in terms of complex competitions among multiple kinetic processes. The KMC method is used in the investigation and prediction of growth mechanisms, which provide qualitative suggestions to guide experimental study.

Sub-10 nm Tunable Hybrid Dielectric Engineering on MoS2 for Two-Dimensional Material-Based Devices.

The successful realization of high-performance 2D-materials-based nanoelectronics requires integration of high-quality dielectric films as a gate insulator. In this work, we explore the integration of organic and inorganic hybrid dielectrics on MoS2 and study the chemical and electrical properties of these hybrid films. Our atomic force microscopy, X-ray photoelectron spectroscopy (XPS), Raman, and photoluminescence results show that, aside from the excellent film uniformity and thickness scalability down to 2.5 nm, the molecular layer deposition of octenyltrichlorosilane (OTS) and Al2O3 hybrid films preserves the chemical and structural integrity of the MoS2 surface. The XPS band alignment analysis and electrical characterization reveal that through the inclusion of an organic layer in the dielectric film, the band gap and dielectric constant can be tuned from ∼7.00 to 6.09 eV and ∼9.0 to 4.5, respectively. Furthermore, the hybrid films show promising dielectric properties, including a high breakdown field of ∼7.8 MV/cm, a low leakage current density of ∼1 × 10-6 A/cm2 at 1 MV/cm, a small hysteresis of ∼50 mV, and a top-gate subthreshold voltage swing of ∼79 mV/dec. Our experimental findings provide a facile way of fabricating scalable hybrid gate dielectrics on transition metal dichalcogenides for 2D-material-based flexible electronics applications.

Synthesis of Graphene Films on Copper Foils by Chemical Vapor Deposition.

Over the past decade, graphene has advanced rapidly as one of the most promising materials changing human life. Development of production-worthy synthetic methodologies for the preparation of various types of graphene forms the basis for its investigation and applications. Graphene can be used in the forms of either microflake powders or large-area thin films. Graphene powders are prepared by the exfoliation of graphite or the reduction of graphene oxide, while graphene films are prepared predominantly by chemical vapor deposition (CVD) on a variety of substrates. Both metal and dielectric substrates have been explored; while dielectric substrates are preferred over any other substrate, much higher quality graphene large-area films have been grown on metal substrates such as Cu. The focus here is on the progress of graphene synthesis on Cu foils by CVD, including various CVD techniques, graphene growth mechanisms and kinetics, strategies for synthesizing large-area graphene single crystals, graphene transfer techniques, and, finally, challenges and prospects are discussed.