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Graphene is a promising material for sensing applications because of its large specific surface area and low noise. In many applications, graphene will inevitably be in contact with oxygen since it is the second most abundant gas in the atmosphere. Therefore, it is of interest to understand how this gas affects the sensor properties. In this work, the effect of oxygen on the low-frequency noise of suspended graphene is demonstrated. Devices with suspended graphene nanoribbons with a width (W) and length (L) of 200 nm were fabricated. The resistance as a function of time was measured in a vacuum and pure oxygen atmosphere through an ac lock-in method. After signal processing with wavelet denoising and analysis, it is demonstrated that oxygen causes random telegraphy signal (RTS) in the millisecond scale, with an average dwell time of 2.9 milliseconds in the high-resistance state, and 2 milliseconds in the low-resistance state. It is also shown that this RTS occurs only at some periods, which indicates that, upon adsorption, the molecules take some time until they find the most energetically favorable adsorption state. Also, a slow-down in the RTS time constants is observed, which infers that less active sites are available as time goes on because of oxygen adsorption. Therefore, it is very important to consider these effects to guarantee high sensitivity and high durability for graphene-based sensors that will be exposed to oxygen during their lifetime.

Valley Hall effect has been observed in asymmetric single-layer and bilayer graphene systems. In single-layer graphene systems, asymmetry is introduced by aligning graphene with hexagonal boron nitride (hBN) with a near-zero twist angle, breaking the sub-lattice symmetry. Although a similar approach has been used in bilayer graphene to break the layer symmetry and thereby observe the valley Hall effect, the bilayer graphene was sandwiched with hBN on both sides in those studies. This study looks at a much simpler, non-encapsulated structure where hBN is present only at the top of graphene. The crystallographic axes of both hBN and bilayer graphene are aligned. A clear signature of the valley Hall effect through non-local resistance measurement ($R_{\rm{NL}}$) was observed. The observed non-local resistance could be manipulated by applying a displacement field across the heterostructure. Furthermore, the electronic band structure and Berry curvature calculations validate the experimental observations.

Molecular doping of graphene is intriguing since the electronic and vibrational properties of graphene are not only determined by the interfacial charge transfer process, but they can also be influenced by adsorbate–graphene hybridization, adsorbate–adsorbate interactions, as well as Fermi-level pinning of the molecule. In this work, we investigate the change in vibrational properties of graphene induced by an organic electron acceptor molecule in the presence of intervening spacers of ultra-thin molybdenum disulphide (MoS2) of variable thickness, down to a single layer. Mechanically exfoliated single-layer graphene and few-layer MoS2 crystals are combined to form heterostructures, which are then subjected to molecular doping by means of an organic hole-dopant, 7,7,8,8-tetracyanoquinodimethane (TCNQ). Variations in Raman G-peak parameters are discussed in terms of molecular doping of graphen e, when the dopant:graphene separation is controllably increased. We show that dopant-separation dependent Fermi level variations of graphene cannot be completely accounted within a simple charge transfer model. The additional mechanisms relevant to the complex nature of molecular doping are discussed.