Nitrogen dioxide (NO₂) is a potential hazard to human health at low concentrations, below one part per million (ppm). NO₂ can be monitored using gas sensors based on multi-layered graphene operating at ambient temperature. However, reliable detection of concentrations on the order of parts per million and lower is hindered by partial recovery and lack of reproducibility of the sensors after exposure. We show how to overcome these longstanding problems using ultraviolet (UV) light. When exposed to NO₂, the sensor response is enhanced by 290 % − 550 % under a 275 nm wavelength light emitting diode irradiation. Furthermore, the sensor's initial state is completely restored after exposure to the target gas. UV irradiation at 68 W/m2 reduces the NO₂ detection limit to 30 parts per billion (ppb) at room temperature. We investigated sensor performance optimization for UV irradiation with different power densities and target gases, such as carbon oxide and ammonia. Improved sensitivity, recovery, and reproducibility of UV-assisted graphene-based gas sensors make them suitable for widespread environmental applications.

In this study, we investigate a Schottky junction based on solution-processed multilayered graphene (MLG). We present a rectifying device obtained with a straightforward approach, that is drop-casting a few microliters of MLG solution simultaneously onto Si, Si-SiO2 and Si-SiO2-Cr/Au surface. Monitoring the modulation of Schottky barrier height while operating in reverse bias, we study the behavior of such prepared MLG-Si/junction (MLG-Si/J) when exposed to oxidizing atmosphere, especially to nitrogen oxide (NO2). We finally compare the sensing behavior of MLG-Si/J at 1 ppm of NO2 with that of a chemiresistor-based on similarly prepared solution-processed MLG. Our study thus opens the path towards low-cost highly sensitive graphene-based heterojunctions advantageously fabricated without any complexity in the technological process.

Chemical vapour deposition (CVD) has emerged as the dominant technique to combine high quality with large scale production of graphene. The key challenge for CVD graphene remains the transfer of the film from the growth substrate to the target substrate while preserving the quality of the material. Avoiding the transfer process of single or multi-layered graphene (SLG-MLG) has recently garnered much more interest. Here we report an original method to obtain a 4-inch wafer fully covered by MLG without any transfer step from the growth substrate. We prove that the MLG is completely released on the oxidized silicon wafer. A hydrogen peroxide solution is used to etch the molybdenum layer, used as a catalyst for the MLG growth via CVD. X-ray photoelectron spectroscopy proves that the layer of Mo is etched away and no residues of Mo are trapped beneath MLG. Terahertz transmission near-field imaging as well as Raman spectroscopy and atomic force microscopy show the homogeneity of the MLG film on the entire wafer after the Mo layer etch. These results mark a significant step forward for numerous applications of SLG-MLG on wafer scale, ranging from micro/nano-fabrication to solar cells technology.

Humidity sensing is fundamental in some applications, as humidity can be a strong interferent in the detection of analytes under environmental conditions. Ideally, materials sensitive or insensitive towards humidity are strongly needed for the sensors used in the first or second case, respectively. We present here the sensing properties of multi-layered graphene (MLG) upon exposure to different levels of relative humidity. We synthesize MLG by chemical vapor deposition, as shown by Raman spectroscopy, Atomic Force Microscopy (AFM) and Scanning Electron Microscopy (SEM). Through an MLG-based resistor, we show that MLG is scarcely sensitive to humidity in the range 30%–70%, determining current variations in the range of 0.005%/%relative humidity (RH) well below the variation induced by other analytes. These findings, due to the morphological properties of MLG, suggest that defective MLG is the ideal sensing material to implement in gas sensors operating both at room temperature and humid conditions.

A method to grow multi layers graphene (MLG) just by thermal annealing in an inert atmosphere is reported. A molybdenum (Mo) catalyst layer is used in combination with a solid amorphous carbon (a-C) source on top or below the Mo layer. The formation of MLG directly on top of the catalyst substrate surface is confirmed by Raman spectroscopy, atomic force microscopy, cross-section transmission electron microscopy, electron energy loss spectroscopy and x-ray photoelectron spectroscopy. Growth of MLG on top of the Mo catalyst is demonstrated both with a-C below and above the Mo layer. The growth mechanism is attributed to the diffusion of a-C through the Mo layer and precipitation into the graphene at the surface, similar to the growth by chemical vapour deposition (CVD) on a Ni catalyst. The role of the inert Ar/H ₂ atmosphere, carbon thickness, catalyst thickness, anneal time and anneal temperature are reported. Fast growth of MLG (5 min) at 915 °C is demonstrated. The quality of MLG prepared by thermal annealing is at least as good as that of MLG synthesized by CVD. The relevant achievements presented in this study make the proposed technique a promising alternative to CVD based MLG.