Bottom-up approach for the low-cost synthesis of graphene-alumina nanosheet interfaces using bimetallic alloys

Recent progress in the theoretical design of two-dimensional ferroelectric materials

Two-dimensional (2D) ferroelectrics (FEs), which maintain stable electric polarization in ultrathin films, are a promising class of materials for the development of various miniature functional devices. In recent years, several 2D FEs with unique properties have been successfully fabricated through experiments. They have been found to exhibit some unique properties either by themselves or when they are coupled with other functional materials (e.g., ferromagnetic materials, materials with 5d electrons, etc.). As a result, several new types of 2D FE functional devices have been developed, exhibiting excellent performance. As a type of newly discovered 2D functional material, the number of 2D FEs and the exploration of their properties are still limited and this calls for further theoretical predictions. This review summarizes recent progress in the theoretical predictions of 2D FE materials and provides strategies for the rational design of 2D FE materials. The aim of this review is to provide guidelines for the design of 2D FE materials and related functional devices.

Electronic and Magnetic Properties of the Graphene/Y/Co(0001) Interfaces: Insights from the Density Functional Theory Analysis

The effect of Y intercalation on the atomic, electronic, and magnetic properties of the graphene/Co(0001) interface is studied using state-of-the-art density functional theory calculations. Different structural models of the graphene/Y/Co(0001) interface are considered: (i) graphene/Y/Co(0001), (ii) graphene/1ML-YCo2/Co(0001), and (iii) graphene/bulk-like-YCo2(111). It is found that the interaction strength between graphene and the substrate is strongly affected by the presence of Y at the interface and the electronic structure of graphene (doping and the appearance of the energy gap) is defined by the Y concentration. For the Co-terminated interfaces between graphene and the metallic support in the considered systems, the electronic structure of graphene is strongly disturbed, leading to the absence of the linear dispersion for the graphene π band; in the case of the Y-terminated interfaces, a graphene layer is strongly n-doped, but the linear dispersion for this band is preserved. Our calculations show that the magnetic anisotropy for the magnetic atoms at the graphene/metal interface is strongly affected by the adsorption of a graphene layer, giving a possibility for one to engineer the magnetic properties of the graphene/ferromagnet systems.

Integration of graphene and two-dimensional ferroelectrics: properties and related functional devices

Ferroelectric (FE) thin films have been investigated for many years due to their broad applications in electronic devices. It was recently demonstrated that FE functionality persists in ultrathin films, possibly even in monolayers of two-dimensional (2D) FEs. However, the feasibility of 2D-based FE functional devices remains an open challenge. Here, we employ density-functional-theory calculations to propose and document the possible integration of graphene with 2D FE materials on metal substrates in the form of functional FE devices. We show that monolayers of proposed M₂O₃ (M = Al, Y) in the quintuple layer (QL) In₂Se₃ structure are stable 2D FE materials and that QL-M₂O₃ is a functional tunnel barrier in a graphene/QL-M₂O₃/Ru heterostructure. The QL-M₂O₃ barrier width can be modulated by its polarization direction, whereby the heterostructure can function as a prototype ferroelectric tunnel junction. Moreover, alternating the polarization of QL-M₂O₃ modulates the doping type of graphene, enabling the fabrication of graphene p-n junctions. By design, the proposed heterostructures can in principle be fabricated by intercalation, which is known to produce high-quality, large-scale 2D-based heterostructures.

Dirac Electron Behavior for Spin-Up Electrons in Strongly Interacting Graphene on Ferromagnetic Mn5Ge3

An elegant approach for the synthesis of graphene on the strong ferromagnetic (FM) material Mn₅Ge₃ is proposed via intercalation of Mn in the graphene-Ge(111) interface. According to the density functional theory calculations, graphene in this strongly interacting system demonstrates the large exchange splitting of the graphene-derived π band. In this case, only spin-up electrons in graphene preserve the Dirac-electron-like character in the vicinity of the Fermi level and the K point, whereas such behavior is not detected for the spin-down electrons. This unique feature of the studied gr-FM-Mn₅Ge₃ interface that can be prepared on the semiconducting Ge can lead to its application in spintronics.

Probing the mechanical properties of vertically-stacked ultrathin graphene/Al2O3 heterostructures

The superior intrinsic mechanical properties of graphene have been widely studied and utilized to enhance the mechanical properties of various composite materials. However, it is still unclear how heterostructures incorporating graphene behave, and to what extent graphene influences their mechanical response. In this work, a series of graphene/Al₂O₃ composite films were fabricated via atomic layer deposition of Al₂O₃ on graphene, and their mechanical behavior was studied using an experimental-computational approach. The inclusion of monolayer chemical vapor deposited graphene between ultrathin Al₂O₃ films (1.5-4.5 nm thickness) was found to enhance the overall stiffness by as much as 70% compared to a pure Al₂O₃ film of similar thickness (∼150 GPa to ∼250 GPa). Here, for the first time, the combination of graphene and Al₂O₃ in vertically-stacked heterostructures results in advanced hybrid films of unprecedented mechanical stiffness that also possess qualities desirable for graphene-based transistors and flexible electronics.

Nitrogen-doped graphene and graphene quantum dots: A review onsynthesis and applications in energy, sensors and environment

Doping of nitrogen is a promising strategy to modulate chemical, electronic, and structural functionalities of graphene (G)and graphene quantum dots (GQDs) for their outstanding properties in energy and environmental applications.This paper reviews various synthesis approaches of nitrogen-doped graphene (N-G) and nitrogen-doped graphene quantum dots (N-GQDs).;Thermal, ultrasonic, solvothermal, hydrothermal, and electron-beam methods have been applied to synthesize N-G and N-GQDs.These nitrogen-doped carbon materials are characterized to obtain their structural configurations in order to achieve better performance in their applications compared to only either graphene or graphene quantum dots.Both N-G and N-GQDs may be converted into functional materials by integrating with other compounds such as metal oxides/nitrides, polymers, and semiconductors.These functional materials demonstrate superior performance over N-G and N-GQDs materials.Examples of applications of N-G and N-GQDs include supercapacitors, batteries, sensors, fuel cells, solar cells, and photocatalyst.

Chemistry below graphene: decoupling epitaxial graphene from metals by potential-controlled electrochemical oxidation

While high-quality defect-free epitaxial graphene can be efficiently grown on metal substrates, strong interaction with the supporting metal quenches its outstanding properties. Thus, protocols to transfer graphene to insulating substrates are obligatory, and these often severely impair graphene properties by the introduction of structural or chemical defects. Here we describe a simple and easily scalable general methodology to structurally and electronically decouple epitaxial graphene from Pt(111) and Ir(111) metal surfaces. A multi-technique characterization combined with ab-initio calculations was employed to fully explain the different steps involved in the process. It was shown that, after a controlled electrochemical oxidation process, a single-atom thick metal-hydroxide layer intercalates below graphene, decoupling it from the metal substrate. This decoupling process occurs without disrupting the morphology and electronic properties of graphene. The results suggest that suitably optimized electrochemical treatments may provide effective alternatives to current transfer protocols for graphene and other 2D materials on diverse metal surfaces.

Self-Exfoliated Metal-Organic Nanosheets through Hydrolytic Unfolding of Metal-Organic Polyhedra

Few-layers thick metal-organic nanosheets have been synthesized using water-assisted solid-state transformation through a combined top-down and bottom-up approach. The metal-organic polyhedra (MOPs) convert into metal-organic frameworks (MOFs) which subsequently self-exfoliate into few-layered metal-organic nanosheets. These MOP crystals experience a hydrophobicity gradient with the inner surface during contact with water because of the existence of hydrophobic spikes on their outer surface. When the amount of water available for interaction is higher, the resultant layers are not stacked to form bulk materials; instead few-layered nanosheets with high uniformity were obtained in high yield. The phenomenon has resulted high yield production of uniformly distributed layered metal-organic nanosheets from three different MOPs, showing its general adaptability.

Surface chemistry and catalysis confined under two-dimensional materials

Interfaces between 2D material overlayers and solid surfaces provide confined spaces for chemical processes, which have stimulated new chemistry under a 2D cover.

Anomalous hexagonal superstructure of aluminum oxide layer grown on NiAl(110) surface

A modified method for the fabrication of a highly crystallized layer of aluminum oxide on a NiAl(110) surface is reported. The fabrication method involves the multistep selective oxidation of aluminum atoms on a NiAl(110) surface resulting from successive oxygen deposition and annealing. The surface morphology and local electronic structure of the novel aluminum oxide layer were investigated by high-resolution imaging using scanning tunneling microscopy (STM) and current imaging tunneling spectroscopy. In contrast to the standard fabrication method of aluminum oxide on a NiAl(110) surface, the proposed method produces an atomically flat surface exhibiting a hexagonal superstructure. The superstructure exhibits a slightly distorted hexagonal array of close-packed bright protrusions with a periodicity of 4.5 ± 0.2 nm. Atomically resolved STM imaging of the aluminum oxide layer reveals a hexagonal arrangement of dark contrast spots with a periodicity of 0.27 ± 0.02 nm. On the basis of the atomic structure of the fabricated layer, the formation of α-Al₂O₃(0001) on the NiAl(110) surface is suggested.

Switchable graphene-substrate coupling through formation/dissolution of an intercalated Ni-carbide layer

Control over the film-substrate interaction is key to the exploitation of graphene’s unique electronic properties. Typically, a buffer layer is irreversibly intercalated “from above” to ensure decoupling. For graphene/Ni(111) we instead tune the film interaction “from below”. By temperature controlling the formation/dissolution of a carbide layer under rotated graphene domains, we reversibly switch graphene’s electronic structure from semi-metallic to metallic. Our results are relevant for the design of controllable graphene/metal interfaces in functional devices.

Chemical gating of epitaxial graphene through ultrathin oxide layers

We achieved a controllable chemical gating of epitaxial graphene grown on metal substrates by exploiting the electrostatic polarization of ultrathin SiO2 layers synthesized below it. Intercalated oxygen diffusing through the SiO2 layer modifies the metal-oxide work function and hole dopes graphene. The graphene/oxide/metal heterostructure behaves as a gated plane capacitor with the in situ grown SiO2 layer acting as a homogeneous dielectric spacer, whose high capacity allows the Fermi level of graphene to be shifted by a few hundreds of meV when the oxygen coverage at the metal substrate is of the order of 0.5 monolayers. The hole doping can be finely tuned by controlling the amount of interfacial oxygen, as well as by adjusting the thickness of the oxide layer. After complete thermal desorption of oxygen the intrinsic doping of SiO2 supported graphene is evaluated in the absence of contaminants and adventitious adsorbates. The demonstration that the charge state of graphene can be changed by chemically modifying the buried oxide/metal interface hints at the possibility of tuning the level and sign of doping by the use of other intercalants capable of diffusing through the ultrathin porous dielectric and reach the interface with the metal.

CVD-Enabled Graphene Manufacture and Technology

Integrated manufacturing is arguably the most challenging task in the development of technology based on graphene and other 2D materials, particularly with regard to the industrial demand for “electronic-grade” large-area films. In order to control the structure and properties of these materials at the monolayer level, their nucleation, growth and interfacing needs to be understood to a level of unprecedented detail compared to existing thin film or bulk materials. Chemical vapor deposition (CVD) has emerged as the most versatile and promising technique to develop graphene and 2D material films into industrial device materials and this Perspective outlines recent progress, trends, and emerging CVD processing pathways. A key focus is the emerging understanding of the underlying growth mechanisms, in particular on the role of the required catalytic growth substrate, which brings together the latest progress in the fields of heterogeneous catalysis and classic crystal/thin-film growth.

Growth from behind: Intercalation-growth of two-dimensional FeO moiré structure underneath of metal-supported graphene

Growth of graphene by chemical vapor deposition on metal supports has become a promising approach for the large-scale synthesis of high quality graphene. Decoupling of the graphene from the metal has been achieved by either mechanical transfer or intercalation of elements/molecules in between the metal and graphene. Here we show that metal stabilized two-dimensional (2D)-oxide monolayers can be grown in between graphene and the metal substrate thus forming 2D-heterostructures that enable tuning of the materials properties of graphene. Specifically, we demonstrate the intercalation-growth of a 2D-FeO layer in between graphene and Pt(111), which can decouple the graphene from the metal substrate. It is known that the 2D-FeO/Pt(111) system exhibits a moiré-structure with locally strongly varying surface potential. This variation in the substrate surface potential modifies the interface charge doping to graphene locally, causing nanometer-scale variation in its work function and Fermi-level shifts relative to its Dirac point.