To date graphene and graphene-derived materials have created an immense research interests due to its extraordinary physical, chemical, and physiochemical properties, which delineated graphene as an outstanding material for future electronics, optics, and energy-harvesting devices. Typically, graphene has high mobility and optical transparency along with excellent mechanical properties and chemical inertness. Single-layer graphene exhibits ultrahigh optical transmissivity (∼98%), which allows passing through wide range of light wave lengths, thus designated as an ever-reported material for an optically conducting window. Furthermore, graphene's optical, electrical, and electrocatalytic properties can be tuned by applying different chemical functionalization protocols, which make it one of the most suitable candidates for advanced applications in optoelectronic and energy-harvesting devices. This review is intended to summarize the most important experimental results from the recent publications concerning the fascinating properties of graphene electrodes and their applications in various types of solar cells. Furthermore, the state of the art of different graphene synthesis processes and functionalization for the applications in solar cells are also discussed in this review.

Oxide membranes are the foundation of several electrochemical devices and sensors, where functionality is related to selective transport of electrons and ions through a membrane or physical responses from an external perturbation. The ability to engineer power sources and sensors for the rapidly growing field of autonomous systems requires high power density and specific energy. Clamped free-standing nanoscale membranes provide an experimentally tunable platform to explore the limits of dimensionality reduction for such purposes. This review addresses the following: (i) advancing experimental methods to fabricate nanoscale oxide membranes that can sustain a chemical potential gradient, thermomechanically stable under large thermal cycles, and can be electrically interrogated with negligible parasitic loss; (ii) a representative example of high performance energy devices, solid oxide fuel cells, utilizing such membranes; and (iii) a brief discussion on emerging research directions broadly in the areas of condensed matter sciences and energy conversion and storage intersecting low-dimensional complex oxide materials.

Atomically thin films, such as graphene, graphene oxide, hexagonal-boron nitride (h-BN), and molybdenum disulfide (MoS2), have attracted intensive studies to explore their properties and potential applications as next generation materials due to their outstanding mechanical, electrical, thermal, and optical properties. The study of the mechanical behavior of this class of materials is in particular interesting as it not only physically determines the potential application fields where these materials can be utilized but also has revealed unique mechanical size effects and phenomena. Researchers have been studying the mechanical properties such as elastic modulus, strength, friction, and fracture behavior of atomically thin films for over a decade now. Here, we review recent results of the mechanical characterization and understanding of this class of materials.

Graphene's layered structure has opened new prospects for the exploration of properties of other monolayer-thick two-dimensional (2D) layered crystals. The emergence of these inorganic 2D atomic crystals beyond graphene promises a diverse spectrum of properties. For example, hexagonal-boron nitride (h-BN), a layered material closest in structure to graphene is an insulator, while niobium selenide (NbSe2), a transition metal dichalcogenide, is metallic, and monolayers of other transition metal dichalcogenides such as molybdenum disulfide (MoS2) and tungsten disulfide (WS2) are direct band gap semiconductors. The rich spectrum of properties exhibited by these 2D layered material systems can potentially be engineered on-demand and creates exciting prospects for using such systems in device applications ranging from electronics, photonics, energy harvesting, flexible electronics, transparent electrodes, and sensing. A review of the structure, properties, and the emerging device applications of these materials is presented in this paper. While the layered structure of these materials makes them amenable to mechanical exfoliation for quickly unveiling their novel properties and for fabricating proof-of-concept devices, an overview of the synthesis routes that can potentially enable scalable avenues for forming these 2D atomic crystals is also discussed.

Phonon thermal conduction was explored and discussed through a combined theoretical and simulation approach in this work. The thermal conductivity κ of polycrystalline graphene was calculated by molecular dynamics simulations based on a hexagonal patch model in close consistency with microstructural characterization in experiments. The effects of grain size, alignment, and temperature were identified with discussion on the microscopic phonon scattering mechanisms. The effective thermal conductivity was found to increase with the grain size and decrease with the mismatch angle and dislocation density at the grain boundaries (GBs). The ∼T−1 temperature dependence of κ is significantly weakened in the polycrystals. The effect of GBs in modifying thermal transport properties of graphene was characterized by their effective width and thermal conductivity as an individual phase, which was later included in a predictive effective medium model that showed degraded reduction in thermal conductivity for grains larger than a few micrometers.

Besides graphene and hexagonal boron nitride, transition metal dichalcogenides (TMDs) also exhibit a layered structure in which the layers weakly interact via van der Waals forces. Semiconducting TMDs in bulk are indirect band gap materials. However, an isolated sheet exhibits a direct gap. This particular behavior makes them very attractive in terms of optical properties. Moreover, NbS2 and NbSe2 in bulk and their monolayers are metallic. Density functional theory calculations were carried out to study different TMD bilayer systems. First, different bilayer geometries with different stackings were considered. It was found that the indirect and direct band gaps compete; however, the indirect band gap always dominates. Surprisingly, bilayer heterostructures of different TMDs have been found to possess direct band gaps. Finally, heterobilayers composed of one metallic monolayer and a semiconducting layer are predicted as novel metallic van der Waals solids that might find applications in new two-dimensional nanodevices.

Doping of a heteroatom such as nitrogen in carbon nanomaterials provides a means to tailor their electronic properties and chemical reactivities. In this article, we present simple methods to synthesize carbon quantum dots (CQDs) with high nitrogen doping content (18–22%), involving the reaction of glucose in the presence of urea under hydrothermal conditions or by microwave irradiation. The N-doped carbon quantum dots (N-CQDs) show high aqueous solubility and tunable photoluminescence (PL). Interaction of N-CQDs with exfoliated graphene or dimethylaniline quenches PL. Interaction of N-doped as well as undoped C-dots with electron-donating tetrathiafulvalene and electron-withdrawing tetracyanoethylene has been examined. The intense blue PL of CQDs has been exploited to produce white light by mixing the CQDs with yellow light emitting ZnO nanoparticles or graphene oxide. The N-doped CQDs exhibit superior photocatalytic activity compared to pristine CQDs.

Field emission (FE) measurements are reported from carbon nanotube (CNT) fibers and laser-patterned free standing films fabricated by direct online condensation from a floating catalyst chemical vapor deposition reactor. Fiber and film cathodes showed stable emission in the 1–2 mA current (I) range at maximum cathode temperatures less than 1000 °C; film cathodes show localized heating at the triangular tips and higher maximum temperatures than the fibers. Fowler–Nordheim (FN) analysis indicated a change in the morphology of the emitters with increasing external electrical field (Eext). Fiber cathode I–Eext data are interpreted as FN emission from the fiber tip which is eventually limited by space-charge effects. At higher Eext, FN emission from the fiber sidewall occurs. The single fiber cathode stopped emitting abruptly when field induced self-heating effects became significant. For CNT films, self-heating effects can destroy a portion of the film, but FE can still occur from other areas.

Commonly used techniques for cleaning copper substrates before graphene growth via chemical vapor deposition (CVD), such as rinsing with acetone, nitric, and acetic acid, and high temperature hydrogen annealing still leave residual adventitious carbon on the copper surface. This residual carbon promotes graphene nucleation and leads to higher nucleation density. We find that copper with an oxidized surface can act as a self-cleaning substrate for graphene growth by CVD. Under vacuum conditions, copper oxide thermally decomposes, releasing oxygen from the substrate surface. The released oxygen reacts with the carbon residues on the copper surface and forms volatile carbon monoxide and carbon dioxide, leaving a clean copper surface free of carbon for large-area graphene growth. Using oxidized electropolished copper foil leads to a reduction in graphene nucleation density by over a factor of 1000 when compared to using chemically cleaned oxygen free copper foil.

Two-dimensional crystals are an important class of materials for novel physics, chemistry, and engineering. Germanane (GeH), the germanium-based analogue of graphane (CH), is of particular interest due to its direct band gap and spin–orbit coupling. Here, we report the successful co-deposition growth of CaGe2 films on Ge(111) substrates by molecular beam epitaxy and their subsequent conversion to germanane by immersion in hydrochloric acid. We find that the growth of CaGe2 occurs within an adsorption-limited growth regime, which ensures stoichiometry of the film. We utilize in situ reflection high energy electron diffraction (RHEED) to explore the growth temperature window and find the best RHEED patterns at 750 °C. Finally, the CaGe2 films are immersed in hydrochloric acid to convert the films to germanane. Auger electron spectroscopy of the resulting film indicates the removal of Ca, and RHEED patterns indicate a single-crystal film with an in-plane orientation dictated by the underlying Ge(111) substrate. X-ray diffraction and Raman spectroscopy indicate that the resulting films are indeed germanane. Ex situ atomic force microscopy shows that the grain size of the germanane is on the order of a few micrometers, being primarily limited by terraces induced by the miscut of the Ge substrate. Thus, optimization of the substrate could lead to the long-term goal of large area germanane films.

Vertically aligned graphene was grown by plasma-enhanced chemical vapor deposition using methane feedstock. Optical emission spectroscopy (OES) was used to monitor the plasma species, and Raman spectroscopy was used for characterizing the properties of as-grown vertically aligned graphene. OES-derived information on plasma species, such as C, C2, CH, and H, are correlated with the properties of the vertically aligned graphene. Graphene grown at 250 W and 15 sccm exhibited the lowest amount of defects. Although OES peak intensities occurred at the highest power and lowest flow conditions, the OES peak ratios of plasma species had a greater dependence on flow rate and exhibited a saddle point in the atomic C/H ratio corresponding to optimal growth involving the lowest amount of overall defects. Plasma diagnostics provides a valuable approach to optimize growth characteristics and material properties.

Epitaxial graphene of uniform thickness prepared on SiC is of great interest for various applications. On the Si-face, large area uniformity has been achieved, and there is a general consensus about the graphene properties. A similar uniformity has yet not been demonstrated on the C-face where the graphene has been claimed to be fundamentally different. A rotational disorder between adjacent graphene layers has been reported and suggested to explain why multilayer C-face graphene show the π-band characteristic of monolayer graphene. Utilizing low energy electron microscopy, x-ray photoelectron electron microscopy, low energy electron diffraction, and photoelectron spectroscopy, we investigated the properties of C-face graphene prepared by sublimation growth. We observe the formation of micrometer-sized crystallographic grains of multilayer graphene and no rotational disorder between adjacent layers within a grain. Adjacent grains are in general found to have different azimuthal orientations. Effects on C-face graphene by hydrogen treatment and Na exposure were also investigated and are reported. Why multilayer C-face graphene exhibits single layer electronic properties is still a puzzle, however.

This article explores the growth of graphene under low-pressure Ar conditions. Carbon- and silicon-face 4H–SiC samples are subjected to epitaxial graphene growth at 1600 °C in vacuum, in 1 mbar argon, or in 10 mbar of argon. High-resolution x-ray scattering is used to characterize all graphene films. On the C-face, specular scans reveal a bimodal distribution of thicknesses that decrease with increasing Ar pressure. Thin and thick regions are approximately 15 and 46 monolayers in C-face graphene grown at high vacuum, 14 and 42 monolayers thick in graphene grown at 1 mbar, and 12 and 32 monolayers thick in graphene grown at 10 mbar. Azimuthal scans confirm in all cases that graphene layers are epitaxial and display expected crystallographic relationships with the underlying SiC substrate. In-plane azimuthal scans show the rotational disorder increases as pressure increases. Peaks in radial scans are asymmetric, suggesting the grain structure has a bimodal distribution of large and small domains. The sample displaying the lowest average Hall mobility (grown at 1 mbar) has the largest population of small crystallites (coherence length on the order of ∼30 nm). Variations in structure and mobility of C-face graphene are attributed to inadequate control of Si sublimation during growth.

Graphene, a single atomic sheet of sp2-bonded carbon atoms arranged in a honeycomb lattice, exhibits extraordinary electrical and mechanical properties, attracting much attention in both academia and industry. The preparation of high quality large-area graphene and the tuning of graphene electronic properties are important topics in this field. In this feature paper, we review our recent work on epitaxial graphene (EG) on SiC(0001). First, we introduce the bottom-up growth mechanism of the first few EG layers on SiC(0001), and the modification of graphene electronic properties by means of surface transfer doping with electron withdrawing materials (F4-TCNQ and MoO3). Next, we summarize the adsorption behaviors of organic (PTCDA, ClAlPc, and C60F48) and inorganic (bismuth) materials on EG/SiC(0001). Finally, as an example of tuning the electronic properties of graphene by reducing its dimensionality, we demonstrate the molecular self-assembly of atomically precise armchair graphene nanoribbons with varying widths and electronic structures.

We present a route for direct growth of boron nitride via a polyborazylene to h-BN conversion process. This two-step growth process ultimately leads to a >25x reduction in the root-mean-square surface roughness of h-BN films when compared to a high temperature growth on Al2O3(0001) and Si(111) substrates. Additionally, the stoichiometry is shown to be highly dependent on the initial polyborazylene deposition temperature. Importantly, chemical vapor deposition (CVD) graphene transferred to direct-grown boron nitride films on Al2O3 at 400 °C results in a >1.5x and >2.5x improvement in mobility compared to CVD graphene transferred to Al2O3 and SiO2 substrates, respectively, which is attributed to the combined reduction of remote charged impurity scattering and surface roughness scattering. Simulation of mobility versus carrier concentration confirms the importance of limiting the introduction of charged impurities in the h-BN film and highlights the importance of these results in producing optimized h-BN substrates for high performance graphene and TMD devices.

Dispersions containing 1 mg/mL of several carbon nanomaterials were used to deposit films containing 1 to 20 layers. The electrical properties of the composite films were characterized via impedance spectroscopy along two directions: in-plane on the film topmost surface and also through the thickness. It was found that carbon black nanoparticles never achieved full in-plane interconnection while the multiwalled carbon nanotube (MWNT) and single-walled carbon nanotube were already percolated at one layer. Graphite flakes showed a complete percolation curve that allowed its resistance to change by 6–7 orders of magnitude. The differences in the microstructure, electrical response, and thermal decomposition behavior of these carbon nanomaterial–paper substrate films were explained by detailed equivalent circuit analysis of the impedance spectra. Interpretation was supplemented by scanning electron microscopy images and thermal analysis via Differential Scanning Calorimetry/Thermogravimetric Analysis (DSC/TGA). Thru-plane electrical properties were for the most part similar, although only films with short MWNT showed a clearly infiltrated network structure.