Graphene on diamond has been attracting considerable attention due to the unique and highly beneficial features of this heterostructure for a range of electronic applications. Here, ultra-high vacuum X-ray photoelectron spectroscopy is used for in-situ analysis of the temperature dependence of the Ni-assisted thermally-induced graphitization process of intrinsic nanocrystalline diamond thin films (65 nm thickness, 50-80 nm grain size) on silicon wafer substrates. Three major stages of diamond film transformation are determined from XPS during the thermal annealing in the temperature range from 300 °C to 800 °C. Heating from 300 °C causes removal of oxygen; formation of the disordered carbon phase is observed at 400 °C; the disordered carbon progressively transforms to graphitic phase whereas the diamond phase disappears from the surface from 500 °C. In the well controllable temperature regime between 600-700 °C, the nanocrystalline diamond thin film is mainly preserved, while graphitic layers form on the surface as the predominant carbon phase. Moreover, the graphitization is facilitated by a disordered carbon interlayer that inherently forms between diamond and graphitic layers by Ni catalyst. Thus, the process results in formation of a multilayer heterostructure on silicon substrate.

Molybdenum disulfide (MoS₂) and nanocrystalline diamond (NCD) have attracted considerable attention due to their unique electronic structure and extraordinary physical and chemical properties in many applications, including sensor devices in gas sensing applications. Combining MoS₂ and H-terminated NCD (H-NCD) in a heterostructure design can improve the sensing performance due to their mutual advantages. In this study, the synthesis of MoS₂ and H-NCD thin films using appropriate physical/chemical deposition methods and their analysis in terms of gas sensing properties in their individual and combined forms are demonstrated. The sensitivity and time domain characteristics of the sensors were investigated for three gases: oxidizing NO₂, reducing NH₃, and neutral synthetic air. It was observed that the MoS₂/H-NCD heterostructure-based gas sensor exhibits improved sensitivity to oxidizing NO₂ (0.157%·ppm^-1) and reducing NH₃ (0.188%·ppm^-1) gases compared to pure active materials (pure MoS₂ achieves responses of 0.018%·ppm^-1 for NO₂ and -0.0072%·ppm^-1 for NH₃, respectively, and almost no response for pure H-NCD at room temperature). Different gas interaction model pathways were developed to describe the current flow mechanism through the sensing area with/without the heterostructure. The gas interaction model independently considers the influence of each material (chemisorption for MoS₂ and surface doping mechanism for H-NCD) as well as the current flow mechanism through the formed P-N heterojunction.

Synthetic diamond films are routinely grown using chemical vapor deposition (CVD) techniques. Due to their extraordinary combination of intrinsic properties, they are used as the functional layers in various bio-optoelectronic devices. It is a challenge to grow the dimensional layers or porous structures that are required. This study reviews the fabrication of various porous diamond-based structures using linear antenna microwave plasma (LAMWP) chemical vapor deposition (CVD), a low-cost technology for growing diamond films over a large area (>1 m²) at low pressure (<100 Pa) and at low temperature (even at 350 °C). From a technological point of view, two different approaches, i.e., templated diamond growth using three different prestructured (macro-, micro-, and nanosized) porous substrates and direct bottom-up growth of ultra-nanoporous diamond (block-stone and dendritelike) films, are successfully employed to form diamond-based structures with controlled porosity and an enhanced surface area. As a bottom-up strategy, the LAMWP CVD system allows diamond growth at as high as 80% CO₂ in the CH₄/CO₂/H₂ gas mixture. In summary, the low-pressure and cold plasma conditions in the LAMWP system facilitate the growth on three-dimensionally prestructured substrates of various materials that naturally form porous self-standing diamond structures.