Semi-transparent graphite films growth on Ni and their double-sided polymer-free transfer

Vertical heterostructure of graphite-MoS2 for gas sensing

2D materials, given their form-factor, high surface-to-volume ratio, and chemical functionality have immense use in sensor design. Engineering 2D heterostructures can result in robust combinations of desirable properties but sensor design methodologies require careful considerations about material properties and orientation to maximize sensor response. This study introduces a sensor approach that combines the excellent electrical transport and transduction properties of graphite film with chemical reactivity derived from the edge sites of semiconducting molybdenum disulfide (MoS2) through a two-step chemical vapour deposition method. The resulting vertical heterostructure shows potential for high-performance hybrid chemiresistors for gas sensing. This architecture offers active sensing edge sites across the MoS2 flakes. We detail the growth of vertically oriented MoS2 over a nanoscale graphite film (NGF) cross-section, enhancing the adsorption of analytes such as NO2, NH3, and water vapor. Raman spectroscopy, density functional theory calculations and scanning probe methods elucidate the influence of chemical doping by distinguishing the role of MoS2 edge sites relative to the basal plane. High-resolution imaging techniques confirm the controlled growth of highly crystalline hybrid structures. The MoS2/NGF hybrid structure exhibits exceptional chemiresistive responses at both room and elevated temperatures compared to bare graphitic layers. Quantitative analysis reveals that the sensitivity of this hybrid sensor surpasses other 2D material hybrids, particularly in parts per billion concentrations.

Graphene nanowalls formation investigated by Electron Energy Loss Spectroscopy

The properties of layered materials are significantly dependent on their lattice orientations. Thus, the growth of graphene nanowalls (GNWs) on Cu through PECVD has been increasingly studied, yet the underlying mechanisms remain unclear. In this study, we examined the GNWs/Cu interface and investigated the evolution of their microstructure using advanced Scanning transmission electron microscopy and Electron Energy Loss Spectroscopy (STEM-EELS). GNWs interface and initial root layers of comprise graphitic carbon with horizontal basal graphene (BG) planes that conform well to the catalyst surface. In the vertical section, the walls show a mix of graphitic and turbostratic carbon, while the latter becomes more noticeable close to the top edges of the GMWs film. Importantly, we identified growth process began with catalysis at Cu interface forming BG, followed by defect induction and bending at 'coalescence points' of neighboring BG, which act as nucleation sites for vertical growth. We reported that although classical thermal CVD mechanism initially dominates, growth of graphene later deviates a few nanometers from the interface to form GNWs. Nascent walls are no longer subjected to the catalytic action of Cu, and their development is dominated by the stitching of charged carbon species originating in the plasma with basal plane edges.

Continuous epitaxy of single-crystal graphite films by isothermal carbon diffusion through nickel

Multilayer van der Waals (vdW) film materials have attracted extensive interest from the perspective of both fundamental research^1-3 and technology^4-7. However, the synthesis of large, thick, single-crystal vdW materials remains a great challenge because the lack of out-of-plane chemical bonds weakens the epitaxial relationship between neighbouring layers^8-31. Here we report the continuous epitaxial growth of single-crystal graphite films with thickness up to 100,000 layers on high-index, single-crystal nickel (Ni) foils. Our epitaxial graphite films demonstrate high single crystallinity, including an ultra-flat surface, centimetre-size single-crystal domains and a perfect AB-stacking structure. The exfoliated graphene shows excellent physical properties, such as a high thermal conductivity of ~2,880 W m^-1 K^-1, intrinsic Young's modulus of ~1.0 TPa and low doping density of ~2.2 × 10¹⁰ cm^-2. The growth of each single-crystal graphene layer is realized by step edge-guided epitaxy on a high-index Ni surface, and continuous growth is enabled by the isothermal dissolution-diffusion-precipitation of carbon atoms driven by a chemical potential gradient between the two Ni surfaces. The isothermal growth enables the layers to grow at optimal conditions, without stacking disorders or stress gradients in the final graphite. Our findings provide a facile and scalable avenue for the synthesis of high-quality, thick vdW films for various applications.

Flexible, Air-Stable, High-Performance Heaters Based on Nanoscale-Thick Graphite Films

Graphite sheets are known to exhibit remarkable performance in applications such as heating panels and critical elements of thermal management systems. Industrial-scale production of graphite films relies on high-temperature treatment of polymers or calendering of graphite flakes; however, these methods are limited to obtaining micrometer-scale thicknesses. Herein, we report the fabrication of a flexible and power-efficient cm²-scaled heater based on a polycrystalline nanoscale-thick graphite film (NGF, ∼100 nm thick) grown by chemical vapor deposition. The stability of these NGF heaters (operational in air over the range 30-300 °C) is demonstrated by a 12-day continuous heating test, at 215 °C. The NGF exhibits a fast switching response and attains a steady peak temperature of 300 °C at a driving bias of 7.8 V (power density of 1.1 W/cm²). This excellent heating performance is attributed to the structural characteristics of the NGF, which contains well-distributed wrinkles and micrometer-wide few-layer graphene domains (characterized using conductive imaging and finite element methods, respectively). The efficiency and flexibility of the NGF device are exemplified by externally heating a 2000 μm-thick Pyrex glass vial and bringing 5 mL of water to a temperature of 96 °C (at 2.4 W/cm²). Overall, the NGF could become an excellent active material for ultrathin, flexible, and sustainable heating panels that operate at low power.

Graphene and g-C3N4-Based Gas Sensors

The efficient monitoring of the environment is currently gaining a continuous growing interest in view of finding solutions for the global pollution issues and their associated climate change. In this sense, two-dimensional (2D) materials appear as one of highly attractive routes for the development of efficient sensing devices due, in particular, to the interesting blend of their superlative properties. For instance, graphene (Gr) and graphitic carbon nitride g-C3N4 (g-CN) have specifically attracted great attention in several domains of sensing applications owing to their excellent electronic and physical-chemical properties. Despite the high potential they offer in the development and fabrication of high-performance gas-sensing devices, an exhaustive comparison between Gr and g-CN is not well established yet regarding their electronic properties and their sensing performances such as sensitivity and selectivity. Hence, this work aims at providing a state-of-the-art overview of the latest experimental advances in the fabrication, characterization, development, and implementation of these 2D materials in gas-sensing applications. Then, the reported results are compared to our numerical simulations using density functional theory carried out on the interactions of Gr and g-CN with some selected hazardous gases’ molecules such as NO2, CO2, and HF. Our findings conform with the superior performances of the g-CN regarding HF detection, while both g-CN and Gr show comparable detection performances for the remaining considered gases. This allows suggesting an outlook regarding the future use of these 2D materials as high-performance gas sensors.

Fast, wafer-scale growth of a nanometer-thick graphite film on Ni foil and its structural analysis

The growth of graphite on polycrystalline Ni by chemical vapor deposition (CVD) and the microstructural relation of the graphitic films and the metallic substrate continues to puzzle the scientific community. Here, we report the wafer-scale growth of a nanometer-thick graphite film (∼100 nm, NGF) on Ni foil via a fast-thermal CVD approach (5 min growth). Moreover, we shed light on how localized thickness variations of the NGF relate to the Ni surface topography and grain characteristics. While on a macro-scale (mm²), the NGF film looks uniform-with a few hundred highly ordered graphene layers (d₀₀₀₂ = 0.335 nm), when studied at the micro- and nano-scales, few-layer graphene sections can be identified. These are present at a density of 0.1%-3% areas in 100 µ m², can be as thin as two layers, and follow an epitaxial relation with the {111} fcc-Ni planes. Throughout the 50 cm² NGF, the sharp graphite/substrate interfaces are either composed of a couple of NiC_x layers or a graphene layer. Moreover, the NGF was successfully transferred on SiO₂/Si substrate by a wet chemical etching method. The as-produced NGFs could complement or offer an alternative to the mm-thick films produced from natural graphite flakes or polymer sheets.