Birch-Type Hydrogenation of Few-Layer Graphenes: Products and Mechanistic Implications

Tafel-Kinetics-Controlled High-Speed Switching in a Electrochemical Graphene Field-Effect Transistor

Graphene field-effect transistors (FETs) have attracted tremendous attention owing to the single-atomic-layer thickness and high electron mobility for potential applications in next-generation electronics. With regards to switching methodology, the electric-field-induced metal-insulator transition offers a new strategy to produce a large on/off current ratio through reversible electrochemical hydrogenation of the graphene channels. Therefore, the performance of such electrochemical graphene FETs greatly relies on the kinetics of hydrogenation reaction. Here, we show that the switching time can be systemically controlled by the applied gate voltages and geometries of graphene channels. The turn-on and turn-off time display an exponential dependence on the gate voltages, manifesting the dominated Tafel-form kinetics of hydrogenation reaction in a two-dimensional limit. Moreover, the turn-off time is inversely proportional to the channel width but independent of the length, while the turn-on time relies on both the width and length, as well as the off-state gate voltage and duration. Our work improves the response time to the magnitude of tens of microseconds and advances the application of graphene-based electronic devices.

Nanostructure engineering of two-dimensional diamonds toward high thermal conductivity and approaching zero Poisson's ratio

Two-dimensional diamond, also called diamane, has attracted great research attention for its novel physical properties and potential applications in nanoelectronics, ultrasensitive resonators and thermal management. Compared with the hexagonal diamane, the physical properties of the rectangular diamane are less explored. In this work, using first-principles calculations, we conducted a comprehensive study on the electronic, phononic, thermal and mechanical properties of three types of rectangular diamanes. We found that rectangular diamanes possess a high Debye temperature (722-788 K) and a strong in-plane Young's modulus (405.9-575.9 N m^-1). We further show close to zero Poisson's ratio in the rectangular Pmma diamane. Moreover, based on the phonon Boltzmann transport equation, high room temperature lattice thermal conductivity (910-1807 W m^-1 K^-1) and strong configuration and orientation dependence are demonstrated. Phonon group velocity, relaxation time and characteristic square velocity are explored and it is demonstrated that phonon harmonic behavior is responsible for the remarkable configuration dependent thermal conductivity in rectangular diamanes. The present work underscores the use of nanostructure engineering to manipulate thermal conductivity of 2D diamond, which provides opportunities for developing effective thermal channeling devices.

Progress in Diamanes and Diamanoids Nanosystems for Emerging Technologies

New materials are the backbone of their technology-driven modern civilization and at present carbon nanostructures are the leading candidates that have attracted huge research activities. Diamanes and diamanoids are the new nanoallotropes of sp3 hybridized carbon which can be fabricated by proper functionalization, substitution, and via Birch reduction under controlled pressure using graphitic system as a precursor. These nanoallotropes exhibit outstanding electrical, thermal, optical, vibrational, and mechanical properties, which can be an asset for new technologies, especially for quantum devices, photonics, and space technologies. Moreover, the features like wide bandgap, tunable thermal conductivity, excellent thermal insulation, etc. make diamanes and diamanoids ideal candidates for nano-electrical devices, nano-resonators, optical waveguides, and the next generation thermal management systems. In this review, diamanes and diamanoids are discussed in detail in terms of its historical prospect, method of synthesis, structural features, broad properties, and cutting-edge applications. Additionally, the prospects of diamanes and diamanoids for new applications are carefully discussed. This review aims to provide a critical update with important ideas for a new generation of quantum devices based on diamanes and diamanoids which are going to be an important topic in the future of carbon nanotechnology.

Effect of graphene flake size on functionalisation: quantifying reaction extent and imaging locus with single Pt atom tags

Here, the locus of functionalisation on graphene-related materials and the progress of the reaction is shown to depend strongly on the starting feedstock. Five characteristically different graphite sources were exfoliated and functionalized using a non-destructive chemical reduction method. These archetypical examples were compared via a model reaction, grafting dodecyl addends, evaluated with TGA-MS, XPS and Raman data. A general increase in grafting ratio (ranging from 1.1 wt% up to 25 wt%) and an improvement in grafting stoichiometry (C/R) were observed as flake radius decreased. Raman spectrum imaging of the functionalised natural flake graphite identified that grafting is directed towards flake edges. This behaviour was further corroborated, at atomistic resolution, by functionalising the graphene layers with bipyridine groups able to complex single platinum atoms. The distribution of these groups was then directly imaged using aberration-corrected HAADF-STEM. Platinum atoms were found to be homogeneously distributed across smaller graphenes; in contrast, a more heterogeneous distribution, with a predominance of edge grafting was observed for larger graphites. These observations show that grafting is directed towards flake edges, but not necessary at edge sites; the mechanism is attributed to the relative inaccessibility of the inner basal plane to reactive moieties, resulting in kinetically driven grafting nearer flake edges. This phenomenology may be relevant to a wide range of reactions on graphenes and other 2d materials.

Surface Functionalization of Graphene-Based Materials: Biological Behavior, Toxicology, and Safe-By-Design Aspects

The increasing exploitation of graphene-based materials (GBMs) is driven by their unique properties and structures, which ignite the imagination of scientists and engineers. At the same time, the very properties that make them so useful for applications lead to growing concerns regarding their potential impacts on human health and the environment. Since GBMs are inert to reaction, various attempts of surface functionalization are made to make them reactive. Herein, surface functionalization of GBMs, including those intentionally designed for specific applications, as well as those unintentionally acquired (e.g., protein corona formation) from the environment and biota, are reviewed through the lenses of nanotoxicity and design of safe materials (safe-by-design). Uptake and toxicity of functionalized GBMs and the underlying mechanisms are discussed and linked with the surface functionalization. Computational tools that can predict the interaction of GBMs behavior with their toxicity are discussed. A concise framing of current knowledge and key features of GBMs to be controlled for safe and sustainable applications are provided for the community.

Substrate Engineering for CVD Growth of Single Crystal Graphene

Single crystal graphene (SCG) has attracted enormous attention for its unique potential for next-generation high-performance optoelectronics. In the absence of grain boundaries, the exceptional intrinsic properties of graphene are preserved by SCG. Currently, chemical vapor deposition (CVD) has been recognized as an effective method for the large-scale synthesis of graphene films. However, polycrystalline films are usually obtained and the present grain boundaries compromise the carrier mobility, thermal conductivity, optical properties, and mechanical properties. The scalable and controllable synthesis of SCG is challenging. Recently, much attention has been attracted by the engineering of large-size single-crystal substrates for the epitaxial CVD growth of large-area and high-quality SCG films. In this article, a comprehensive and comparative review is provided on the selection and preparation of various single-crystal substrates for CVD growth of SCG under different conditions. The growth mechanisms, current challenges, and future development and perspectives are discussed.

Complex three-dimensional graphene structures driven by surface functionalization

The origami technique can provide inspiration for fabrication of novel three-dimensional (3D) structures with unique material properties from two-dimensional sheets. In particular, transformation of graphene sheets into complex 3D graphene structures is promising for functional nano-devices. However, practical realization of such structures is a great challenge. Here, we introduce a self-folding approach inspired by the origami technique to form complex 3D structures from graphene sheets using surface functionalization. A broad set of examples (Miura-ori, water-bomb, helix, flapping bird, dachshund dog, and saddle structure) is achieved via molecular dynamics simulations and density functional theory calculations. To illustrate the potential of the origami approach, we show that the graphene Miura-ori structure combines super-compliance, super-flexibility (both in tension and compression), and negative Poisson's ratio behavior.

Adverse Effect of PTFE Stir Bars on the Covalent Functionalization of Carbon and Boron Nitride Nanotubes Using Billups-Birch Reduction Conditions

The functionalization of nanomaterials has long been studied as a way to manipulate and tailor their properties to a desired application. Of the various methods available, the Billups-Birch reduction has become an important and widely used reaction for the functionalization of carbon nanotubes (CNTs) and, more recently, boron nitride nanotubes. However, an easily overlooked source of error when using highly reductive conditions is the utilization of poly(tetrafluoroethylene) (PTFE) stir bars. In this work, we studied the effects of using this kind of stir bar versus using a glass stir bar by measuring the resulting degree of functionalization with 1-bromododecane. Thermogravimetric analysis studies alone could deceive one into thinking that reactions stirred with PTFE stir bars are highly functionalized; however, the utilization of spectroscopic techniques, such as Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy, tells otherwise. Furthermore, in the case of CNTs, we determined that using Raman spectroscopy alone for analysis is not sufficient to demonstrate successful chemical modification.

Giant thermal conductivity in diamane and the influence of horizontal reflection symmetry on phonon scattering

Diamane, a chemically derived two-dimensional material, shows many superior physical and chemical properties similar to diamond thin films. Through the Peierls-Boltzmann transport equation, we reveal giant thermal conductivity in diamane with a stacking order of both AB and AA (respectively, abbreviated as D-AB and D-AA, hereafter) which are both comparable to that of diamond. Like in graphene, the phonon transport falls into the hydrodynamic regime even at room temperature, and the major contribution to the total thermal conductivity comes from the out-of-plane acoustic phonon modes (>40%). In addition, the thermal conductivity shows a dependence on the stacking order, namely, the thermal conductivity of D-AA, ∼2240 W m-1 K-1 at 300 K, is around 15% larger than that of D-AB, which is due to the strong restriction on the phonon scattering phase space induced by the horizontal reflection symmetry in D-AA. Such a kind of restriction, not limited to single atomic plane systems, is a general feature in two-dimensional materials with a horizontal reflection symmetry.

Sticking of atomic hydrogen on graphene

Recent years have witnessed an ever growing interest in the interactions between hydrogen atoms and a graphene sheet. Largely motivated by the possibility of modulating the electric, optical and magnetic properties of graphene, a huge number of studies have appeared recently that added to and enlarged earlier investigations on graphite and other carbon materials. In this review we give a glimpse of the many facets of this adsorption process, as they emerged from these studies. The focus is on those issues that have been addressed in detail, under carefully controlled conditions, with an emphasis on the interplay between the adatom structures, their formation dynamics and the electric, magnetic and chemical properties of the carbon sheet.

Protection from Below: Stabilizing Hydrogenated Graphene Using Graphene Underlayers

We show that dehydrogenation of hydrogenated graphene proceeds much more slowly for bilayer systems than for single layer systems. We observe that an underlayer of either pristine or hydrogenated graphene will protect an overlayer of hydrogenated graphene against a number of chemical oxidants, thermal dehydrogenation, and degradation in an ambient environment over extended periods of time. Chemical protection depends on the ease of oxidant intercalation, with good intercalants such as Br₂ demonstrating much higher reactivity than poor intercalants such as 1,2-dichloro-4,5-dicyanonbenzoquinone (DDQ). Additionally, the rate of dehydrogenation of hydrogenated graphene at 300 °C in H₂/Ar was reduced by a factor of roughly 10 in the presence of a protective underlayer of graphene or hydrogenated graphene. Finally, the slow dehydrogenation of hydrogenated graphene in air at room temperature, which is normally apparent after a week, could be completely eliminated in samples with protective underlayers over the course of 39 days. Such protection will be critical for ensuring the long-term stability of devices made from functionalized graphene.

Towards stoichiometric analogues of graphene: graphane, fluorographene, graphol, graphene acid and others

Stoichiometric derivatives of graphene, having well-defined chemical structure and well-defined chemical bonds, are of a great interest to the 2D materials research.