Aryl functionalization as a route to band gap engineering in single layer graphene devices

Chemical functionalization is a promising route to band gap engineering of graphene. We chemically grafted nitrophenyl groups onto exfoliated single-layer graphene sheets in the form of substrate-supported or free-standing films. Our transport measurements demonstrate that nonsuspended functionalized graphene behaves as a granular metal, with variable range hopping transport and a mobility gap ～0.1 eV at low temperature. For suspended graphene that allows functionalization on both surfaces, we demonstrate tuning of its electronic properties from a granular metal to a semiconductor in which transport occurs via thermal activation over a transport gap ～80 meV from 4 to 300 K. This noninvasive and scalable functionalization technique paves the way for CMOS-compatible band gap engineering of graphene electronic devices.     

Heat conduction across monolayer and few-layer graphenes

We report the thermal conductance G of Au/Ti/graphene/SiO(2) interfaces (graphene layers 1 ≤ n ≤ 10) typical of graphene transistor contacts. We find G ≈ 25 MW m(-2) K(-1) at room temperature, four times smaller than the thermal conductance of a Au/Ti/SiO(2) interface, even when n = 1. We attribute this reduction to the thermal resistance of Au/Ti/graphene and graphene/SiO(2) interfaces acting in series. The temperature dependence of G from 50 ≤ T ≤ 500 K also indicates that heat is predominantly carried by phonons through these interfaces. Our findings suggest that metal contacts can limit not only electrical transport but also thermal dissipation from submicrometer graphene devices.     

Thermal stability of graphene and nanotube covalent functionalization

We study kinetic factors governing the diffusion and desorption of covalently grafted phenyl and dichlorocarbene radicals on graphene and carbon nanotubes. Our ab initio calculations of reaction rates show that isolated phenyls can easily desorb and diffuse at room temperature. On the contrary, paired phenyls are expected to remain grafted to the surface up to a few hundred degrees Celsius. In the case of dichlorocarbene, no clustering is observed; at room temperature, the isolated radicals remain covalently attached to small-diameter nanotubes but desorb easily from graphene. Our results on the thermal behavior of side moieties on graphitic surfaces could be used to optimize the tradeoff between reactivity and conductance of nanotubes in the process of covalent functionalization.     

Graphene-based materials: synthesis, characterization, properties, and applications

Graphene, a two-dimensional, single-layer sheet of sp(2) hybridized carbon atoms, has attracted tremendous attention and research interest, owing to its exceptional physical properties, such as high electronic conductivity, good thermal stability, and excellent mechanical strength. Other forms of graphene-related materials, including graphene oxide, reduced graphene oxide, and exfoliated graphite, have been reliably produced in large scale. The promising properties together with the ease of processibility and functionalization make graphene-based materials ideal candidates for incorporation into a variety of functional materials. Importantly, graphene and its derivatives have been explored in a wide range of applications, such as electronic and photonic devices, clean energy, and sensors. In this review, after a general introduction to graphene and its derivatives, the synthesis, characterization, properties, and applications of graphene-based materials are discussed.     

First-principles study of heat transport properties of graphene nanoribbons

We use density-functional theory and the nonequilibrium Green's function method as well as phonon dispersion calculations to study the thermal conductance of graphene nanoribbons with armchair and zigzag edges, with and without hydrogen passivation. We find that low-frequency phonon bands of the zigzag ribbons are more dispersive than those of the armchair ribbons and that this difference accounts for the anisotropy in the thermal conductance of graphene nanoribbons. Comparing our results with data on large-area graphene, edge effects are shown to contribute to thermal conductance, enhance the anisotropy in thermal conductance of graphene nanoribbons, and increase thermal conductance per unit width. The edges with and without hydrogen passivation modify the atomic structure and ultimately influence the phonon thermal transport differently for the two ribbon types.     

Alignment of colloidal graphene quantum dots on polar surfaces

Controlling the orientation of nanostructures with anisotropic shapes is essential for taking advantage of their anisotropic electrical, optical, and transport properties in electro-optical devices. For large-area alignment of nanocrystals, so far orientations are mostly induced and controlled by external physical parameters, such as applied fields or changes in concentration. Herein we report on assemblies of colloidal graphene quantum dots, a new type of disk-shaped nanostructures, on polar surfaces and the control of their orientations. We show that the orientations of the graphene quantum dots can be determined, either in- or out-of-plane with the substrate, by chemical functionalization that introduces orientation-dependent interactions between the quantum dots and the surfaces.     

Thermal transport across twin grain boundaries in polycrystalline graphene from nonequilibrium molecular dynamics simulations

We have studied the thermal conductance of tilt grain boundaries in graphene using nonequilibrium molecular dynamics simulations. When a constant heat flux is allowed to flow, we observe sharp jumps in temperature at the boundaries, characteristic of interfaces between materials of differing thermal properties. On the basis of the magnitude of these jumps, we have computed the boundary conductance of twin grain boundaries as a function of their misorientation angles. We find the boundary conductance to be in the range 1.5 × 10(10) to 4.5 × 10(10) W/(m(2) K), which is significantly higher than that of any other thermoelectric interfaces reported in the literature. Using the computed values of boundary conductances, we have identified a critical grain size of 0.1 μm below which the contribution of the tilt boundaries to the conductivity becomes comparable to that of the contribution from the grains themselves. Experiments to test the predictions of our simulations are proposed.     

Graphene - a promising material for organic photovoltaic cells

As a promising two-dimensional nanomaterial with outstanding electronic, optical, thermal, and mechanical properties, graphene has been proposed for many applications. In this Progress Report we summarize and discuss comprehensively the advances made so far for applications of graphene in organic photovoltaic (OPV) cells, including that for transparent electrodes, active layers and interfaces layer in OPV. It is concluded that graphene may very likely play a major role in new developments/improvements in OPVs. The future studies for this area are proposed to focus on the following: i) improving the conductivity without comprising the transparency as a transparent electrode material; ii) controlling the sheet sizes, band structure and surface morphology for use as a electron acceptor material, and iii) controlling and improving the functionalization and compatibility with other materials as interface layer material.     

Transfer-free electrical insulation of epitaxial graphene from its metal substrate

High-quality, large-area epitaxial graphene can be grown on metal surfaces, but its transport properties cannot be exploited because the electrical conduction is dominated by the substrate. Here we insulate epitaxial graphene on Ru(0001) by a stepwise intercalation of silicon and oxygen, and the eventual formation of a SiO(2) layer between the graphene and the metal. We follow the reaction steps by X-ray photoemission spectroscopy and demonstrate the electrical insulation using a nanoscale multipoint probe technique.     

Structure and electronic transport in graphene wrinkles

Wrinkling is a ubiquitous phenomenon in two-dimensional membranes. In particular, in the large-scale growth of graphene on metallic substrates, high densities of wrinkles are commonly observed. Despite their prevalence and potential impact on large-scale graphene electronics, relatively little is known about their structural morphology and electronic properties. Surveying the graphene landscape using atomic force microscopy, we found that wrinkles reach a certain maximum height before folding over. Calculations of the energetics explain the morphological transition and indicate that the tall ripples are collapsed into narrow standing wrinkles by van der Waals forces, analogous to large-diameter nanotubes. Quantum transport calculations show that conductance through these "collapsed wrinkle" structures is limited mainly by a density-of-states bottleneck and by interlayer tunneling across the collapsed bilayer region. Also through systematic measurements across large numbers of devices with wide "folded wrinkles", we find a distinct anisotropy in their electrical resistivity, consistent with our transport simulations. These results highlight the coupling between morphology and electronic properties, which has important practical implications for large-scale high-speed graphene electronics.     

Thermal conduction across the one-dimensional interface between a MoS<Subscript>2</Subscript> monolayer and metal electrode

The thermal conductance across the one-dimensional (1D) interface between a MoS2 monolayer and Au electrode (edge-contact) has been investigated using molecular dynamics simulations.Although the thermal conductivity of monolayer MoS2 is 2-3 orders of magnitude lower than that of graphene,the covalent bonds formed at the interface enable interfacial thermal conductance (ITC) that is comparable to that of a graphene-metal interface.Each covalent bond at the interface serves as an independent channel for thermal conduction,allowing ITC to be tuned linearly by changing the interfacial bond density (controlling S vacandes).In addition,different Au surfaces form different bonding configurations,causing large ITC variations.Interestingly,the S vacancies in the central region of MoS2 only slightly affect the ITC,which can be explained by a mismatch of the phonon vibration spectra.Further,at room temperature,ITC is primarily dominated by phonon transport,and electron-phonon coupling plays a negligible role.These results not only shed light on the phonon transport mechanisms across 1D metal-MoS2 interfaces,but also provide guidelines for the design and optimization of such interfaces for thermal management in MoS2-based electronicdevices.     

Thermal transport in suspended and supported few-layer graphene

We report thermal conductivity (κ) measurements from 77 to 350 K on both suspended and supported few-layer graphene using a thermal-bridge configuration. The room temperature value of κ is comparable to that of bulk graphite for the largest flake, but reduces significantly for smaller flakes. The presence of a substrate lowers the value of κ, but the effect diminishes for the thermal transport in the top layers away from the substrate. For the suspended sample, the temperature dependence of κ follows a power law with an exponent of 1.4 ± 0.1, suggesting that the flexural phonon modes contribute significantly to the thermal transport of the suspended graphene. The measured values of κ are generally lower than those from theoretical studies. We attribute this deviation to the phonon-boundary scattering at the graphene-contact interfaces, which is shown to significantly reduce the apparent measured thermal conductance of graphene.     

Ethylene irradiation: a new route to grow graphene on low reactivity metals

A novel technique for growing graphene on relatively inert metals, consisting in the thermal decomposition of low energy ethylene ions irradiated on hot metal surfaces in ultrahigh vacuum, is reported. By this route, we have grown graphene monolayers on Cu(111) and, for the first time, on Au(111) surfaces. For both noble metal substrates, but particularly for Au(111), our scanning tunneling microscopy and spectroscopy measurements provide sound evidence of a very weak graphene-metal interaction.     

Direct nanoscale imaging of ballistic and diffusive thermal transport in graphene nanostructures

We report direct imaging of nanoscale thermal transport in single and few-layer graphene with approximately 50 nm lateral resolution using high vacuum scanning thermal microscopy. We observed increased heat transport in suspended graphene where heat is conducted by ballistic phonons, compared to adjacent areas of supported graphene, and observed decreasing thermal conductance of supported graphene with increased layer number. Our nanothermal images suggest a mean-free-path of thermal phonons in supported graphene below 100 nm.     

Direct CVD Growth of Graphene on Traditional Glass: Methods and Mechanisms

Chemical vapor deposition (CVD) on catalytic metal surfaces is considered to be the most effective way to obtain large‐area, high‐quality graphene films. For practical applications, a transfer process from metal catalysts to target substrates (e.g., poly(ethylene terephthalate) (PET), glass, and SiO₂/Si) is unavoidable and severely degrades the quality of graphene. In particular, the direct growth of graphene on glass can avoid the tedious transfer process and endow traditional glass with prominent electrical and thermal conductivities. Such a combination of graphene and glass creates a new type of glass, the so‐called “super graphene glass,” which has attracted great interest from the viewpoints of both fundamental research and daily‐life applications. In the last few years, great progress has been achieved in pursuit of this goal. Here, these growth methods as well as the specific growth mechanisms of graphene on glass surfaces are summarized. The typical techniques developed include direct thermal CVD growth, molten‐bed CVD growth, metal‐catalyst‐assisted growth, and plasma‐enhanced growth. Emphasis is placed on the strategy of growth corresponding to the different natures of glass substrates. A comprehensive understanding of graphene growth on nonmetal glass substrates and the latest status of “super graphene glass” production are provided.     

石墨烯增强铜基复合材料研究进展

铜(Cu)基复合材料具有优异的力学、热学、电学及耐磨和耐腐蚀等性能,广泛应用于各种工业技术领域。石墨烯(Graphene,Gr)具有二维平面结构和优异的综合性能,是金属基复合材料理想的增强相。石墨烯增强铜基复合材料拓展了铜及其合金的应用范围,适当的制备方法可以使其在保持优异导电导热性能的同时拥有更好的力学性能。石墨烯在铜基体中的存在形式主要以还原氧化石墨烯、石墨烯纳米片或与金属氧化物/碳化物纳米颗粒连接,旨在增强两者之间的界面结合。因此,石墨烯在铜基体中的结构完整性及存在形式直接影响了其性能的优劣。本文综述了Cu/Gr复合材料的制备及模拟方法、复合材料的性能评价及力学性能与功能特性的相互影响规律。指明Cu/Gr复合材料的发展关键在于：(1)分散性与界面结合;(2)三维石墨烯结构的构建;(3)界面结合对力学性能与功能特性的影响及两者间的相互协调。     

Transport in multiterminal graphene nanodevices

We study the transport properties of multiterminal graphene nanodevices using the Landauer-Buttiker approach and the tight binding model. We consider a four-terminal device made at the crossing of a zigzag and armchair nanoribbons and two types of T-junction devices. The transport properties of graphene multiterminal devices are highly sensitive to the details of the junction region. Thus the properties are drastically different from those on the armchair and zigzag counterparts. In the cross-junction device, we see a conductance dip in the armchair lead associated with a conductance peak in the zigzag lead. We find that this effect is enhanced in a T-junction device with one armchair sidearm.     

Ultra-High Interfacial Thermal Conductance via Double hBN Encapsulation for Efficient Thermal Management of 2D Electronics

Heat dissipation is a major limitation of high-performance electronics. This is especially important in emerging nanoelectronic devices consisting of ultra-thin layers, heterostructures, and interfaces, where enhancement in thermal transport is highly desired. Here, ultra-high interfacial thermal conductance in encapsulated van der Waals (vdW) heterostructures with single-layer transition metal dichalcogenides MX₂ (MoS₂, WSe₂, WS₂) sandwiched between two hexagonal boron nitride (hBN) layers is reported. Through Raman spectroscopic measurements of suspended and substrate-supported hBN/MX₂/hBN heterostructures with varying laser power and temperature, the out-of-plane interfacial thermal conductance in the vertical stack is calibrated. The measured interfacial thermal conductance between MX₂ and hBN reaches 74 ± 25 MW m⁻² K⁻¹, which is at least ten times higher than the interfacial thermal conductance of MX₂ in non-encapsulation structures. Molecular dynamics (MD) calculations verify and explain the experimental results, suggesting a full encapsulation by hBN layers is accounting for the high interfacial conductance. This ultra-high interfacial thermal conductance is attributed to the double heat transfer pathways and the clean and tight vdW interface between two crystalline 2D materials. The findings in this study reveal new thermal transport mechanisms in hBN/MX₂/hBN structures and shed light on building novel hBN-encapsulated nanoelectronic devices with enhanced thermal management.     

Growth of few layer graphene from commercial C2 hydrocarbons at low temperature and controlled surface functionalization

Graphene is mostly grown from methane on copper foils at a high temperature about 1000°C. In this research, a commercial ethylene-acetylene-ethane mixture was used as a clean precursor for graphene synthesis on nickel foils in a chemical vapor deposition reactor at 750°C. Furthermore, controlled functionalization of graphene sheets was achieved via hydrothermal oxidation at moderate pressure and temperature using nitric acid. Broadened 2D band and G band frequencies in Raman spectra indicated that pristine graphene (PG) was of high quality with low defects. X-ray diffraction results confirmed that PG has five layers. Transmission electron microscopy and N₂ adsorption-desorption analyses affirmed that the graphene is of a good quality, large surface area (562 m²/g) and small pore size. Fourier transform infrared spectroscopy confirmed functionalization process performance. Thermogravimetric analysis affirmed that the thermal stability of PG was drastically decreased after the functionalization process.     

Spectroscopy and microscopy of graphene on metals:

Graphene is an intriguing object of condensed mater physics. Discovery of its fascinating transport properties renewed the interest to the studies of graphene on metallic surfaces. That leads to the technological breakthrough showing that graphene on metals can be used as a protective layer and that its synthesis on metals is the most promising way to prepare huge graphene layers. However, all technology‐related and fundamental studies of the graphene‐metal interfaces require the understanding of the bonding mechanism at the interface and the consequent modifications of the electronic structure of graphene compared to the free‐standing case. Here we consider two representative examples of the strongly and weakly bonded graphene on metals demonstrating how surface science methods help to understand the origin of the bonding at the graphene‐metal interface. These methods help to trace all modifications in the electronic structure of graphene on the microscopic and macroscopic levels.