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Issue Date: 16-Mar-2017
Authors: Piazza, Aurora
Title: Thermally induced effects in controlled atmosphere on Graphene and Molybdenum disulfide
Abstract: A wide interest has grown in recent years for the 2D materials. The graphene (Gr) is the 2D carbon material with a zero energy band gap, that turned out to be relevant due to its electrical, transport and optical properties and is considered the pioneer of emerging 2D solids. A key requirement for some applicative tasks is constituted by the need to open an energy gap in the Gr electronic levels distribution to induce, for example, a higher current modulation (i.e., on/off current ratio) in Gr-based field effect transistors. Another target is the doping to induce a shift of the Fermi level for the use of graphene in electronics devices and modulate the charge carriers. In this context, controllable and stable p- or n-type doping represent prominent goals to tailor Gr sheet resistance for specific electronics and optoelectronics applications, such as ultra-thin exible circuits and interconnects transparent conductive electrodes for the next generation solar cells and/or touch screens. It is known that oxygen is a p-type dopant for graphene, active also by thermal treatments. The mechanisms responsible for p-type doping of substrate supported monolayer graphene (Gr) by thermal treatments in oxygen ambient have been investigated by micro-Raman spectroscopy, atomic force microscopy (AFM), considering commonly employed dielectric substrates, such as SiO2 and Al2O3 thin films grown on Si. While a high p-type doping (~10^13 cm^-2) is observed for Gr on SiO2, no significant doping is found for Gr samples on the Al2O3 substrate, suggesting a key role of the Gr/SiO2 interface states in the trapping of oxygen responsible for the Gr p-type doping. Furthermore, we investigated the doping stability of Gr on SiO2 during subsequent thermal treatments in nitrogen (N2), carbon dioxide (CO2), water (H2O) or in vacuum controlled atmospheres. These processes induce only minor effects on the doping of Gr but for H2O and principally affect its defectiveness, suggesting that the literature reported air influence on the doping depends on water present in the atmosphere. Alongside the Gr, the MoS2 transition metal dichalcogenides is one of the stable 2D materials of interest. The potentiality to produce Van der Waals heterostructures combining Gr and MoS2, in particular, is pushing their study. Indeed, the non-null band gap, good chemical sensitivity and photo response of MoS2 pave the way for applications in optoelectronics, sensing and photovoltaic devices, for example. In this context, the possibility to tune the properties of MoS2 by opportune treatments is of interest, also to evaluate the thermal effects in the case of heterostructures. The MoS2 treatment in O2 has evidenced a progressive erosion of the flakes without relevant spectral changes in their central zone, during in-situ measurements, whereas the formation of MoO3 on the flakes edges is observed indicative of the oxygen activated transformation.
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