After the synthesis of graphene, in the first year of this century, a wide research field on two-dimensional materials opens. 2D materials are characterized by an intrinsic high surface to volume ratio, due to their heights of few atoms, and, differently from graphene, which is a semimetal with zero or near zero bandgap, they usually have a semiconductive nature. These two characteristics make them promising candidate for a new generation of gas sensing devices. Graphene oxide, being an intermediate product of graphene fabrication, has been the first graphene-like material studied and used to detect target gases, followed by MoS₂, in the first years of 2010s. Along with MoS₂, which is now experiencing a new birth, after its use as a lubricant, other sulfides and selenides (like WS₂, WSe₂, MoSe₂, etc.) have been used for the fabrication of nanoelectronic devices and for gas sensing applications. All these materials show a bandgap, tunable with the number of layers. On the other hand, 2D materials constituted by one atomic species have been synthetized, like phosphorene (one layer of black phosphorous), germanene (one atom thick layer of germanium) and silicone (one atom thick layer of silicon). In this paper, a comprehensive review of 2D materials-based gas sensor is reported, mainly focused on the recent developments of graphene oxide, exfoliated MoS₂ and WS₂ and phosphorene, for gas detection applications. We will report on their use as sensitive materials for conductometric, capacitive and optical gas sensors, the state of the art and future perspectives.

In this paper we accurately revisit the mechanical exfoliation and layer number determination of MoS ₂ . By modelling the exfoliation itself as a random vertical (lateral) exfoliation (fragmentation) phenomenon, a rationale is given to optimise the number of iterations in the scotch-tape peeling technique and we rigorously demonstrate that since the second cleavage the occurrence of monolayers is the most likely. For the unambiguous layer number identification, experiments have been carried out with a unified complementary approach based on optical microscopy, atomic force microscopy, resonant and non resonant Raman spectroscopy, and photo-luminescence spectroscopy. The experimental analysis has been carried out on a statistically significant set of few-layer MoS ₂ flakes (from one to five layers). The work stresses the strong need of such complementary multi-technique approach to really unambiguously determine the layer number of flakes (that neither optical microscopy, nor AFM alone can give). Optical microscopy contrast analysis experiments (carried out on flakes deposited onto 270 nm SiO ₂ /Si(1 0 0) substrates) demonstrate that for few-layer MoS ₂ (from the mono to the epta-layer) the optical contrast is weakly depending on the radiation wavelength and is a non-monotonic function of the layer number. Thus, flakes from ten to twelve layers exhibit the same contrast of monolayers, as demonstrated by parallel AFM analysis. AFM clearly shows that the stacking between the silicon oxide substrate and the first MoS ₂ layer is strongly unpredictable and likely depending on the degree of surface contamination of the substrate. A parallel systematic resonant Raman study is performed ranging from one to five layers clearly exhibiting systematic layer dependent spectral features in the one-five layers range. For the first time a layer estimation number based on a quantitative analysis of resonant Raman and PL spectra is proposed.

The structural and electronic properties of MnxGe1−x alloys (x⩽0.15) fabricated by ion implantation are investigated by means of x-ray diffraction and synchrotron radiation photoemission spectroscopy. The diffraction patterns point to the presence of ferromagnetic Mn5Ge3 nanoparticles; however, valence band spectra, interpreted by means of accurate ab initio calculations including Hubbard-like correlations, show clear fingerprints of an effective substitutional Mn dilution in the Ge semiconducting host.