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The electronic properties of graphene are highly sensitive to atoms and molecules adsorbed on its surface and to changes in its environment, such as temperature and humidity. In this paper, we examine the effect of humidity on the local carrier concentration of different types of graphene prepared by mechanical exfoliation, chemical vapour deposition and epitaxial growth on SiC, using in-situ Raman spectroscopy. We present a systematic and comparative study of the changes in Raman response using a vector analysis method to produce spatial maps of doping variation as a function of humidity. We also quantify the humidity induced carrier concentration change for different types of graphene. This study illustrates the effects of humidity on the electronic properties of graphene and provides a simple, contactless and quantitative method to directly evaluate water-induced doping effects in graphene, that are crucial for tailored device performance.

Control of atomic-scale interfaces between materials with distinct electronic structures is crucial for the design and fabrication of most electronic devices. In the case of two-dimensional materials, disparate electronic structures can be realized even within a single uniform sheet, merely by locally applying different vertical gate voltages. Here, we utilize the inherently nano-structured single layer and bilayer graphene on silicon carbide to investigate lateral electronic structure variations in an adjacent single layer of tungsten disulfide (WS2). The electronic band alignments are mapped in energy and momentum space using angle-resolved photoemission with a spatial resolution on the order of 500 nm (nanoARPES). We find that the WS2 band offsets track the work function of the underlying single layer and bilayer graphene, and we relate such changes to observed lateral patterns of exciton and trion luminescence from WS2., Sharp atomic interfaces between materials dictate the interface’s electronic properties. The authors use angle-resolved photoemission spectroscopy with a spatial resolution of ~500 nm to investigate the nanoscale electronic band structure and band alignment in a lateral heterostructure composed of WS2 placed on alternating nano-stripes of monolayer and bilayer graphene.

The effects of humidity on the electronic properties of mono- and bi-layer graphene prepared by chemical vapour deposition and transferred on SiO 2 are investigated via simultaneous global transport and local work function measurements using the van der Pauw method and Kelvin probe force microscopy, respectively. It is found that mono-layer graphene on SiO 2 is extremely sensitive to water vapour, with water molecules acting as physisorbed loosely bound p-dopants. In the case of a bi-layer stack, produced by double transfer of two graphene layers, the layers are randomly oriented and decoupled with respect to each other. As a consequence, the bottom layer of the bi-layer stack is mostly affected by substrate charges, while the top graphene layer behaves as a decoupled layer, which is externally doped by the water vapour. Moreover, we provide evidence that ambient humidity is only partly responsible for the p-doping of graphene. These findings will assist in the development of reliable graphene-based electronics such as sensors working in ambient air.

The effects of humidity on the electronic properties of quasi-free standing one layer graphene (QFS 1LG) are investigated via simultaneous magneto-transport in the van der Pauw geometry and local work function measurements in a controlled environment. QFS 1LG on 4H-SiC(0001) is obtained by hydrogen intercalation of the interfacial layer. In this system, the carrier concentration experiences a two-fold increase in sensitivity to changes in relative humidity as compared to the as-grown epitaxial graphene. This enhanced sensitivity to water is attributed to the lowering of the hydrophobicity of QFS 1LG, which results from spontaneous polarization of 4H-SiC(0001) strongly influencing the graphene. Moreover, the superior carrier mobility of the QFS 1LG system is retained even at the highest humidity. The work function maps constructed from Kelvin probe force microscopy also revealed higher sensitivity to water for 1LG compared to 2LG in both QFS 1LG and as-grown systems. These results point to a new field of applications for QFS 1LG, i.e., as humidity sensors, and the corresponding need for metrology in calibration of graphene-based sensors and devices., 16 pages, 4 figures

We study local electronic properties of graphene oxide (GO) and reduced graphene oxide (RGO) on metallic (Pt) and insulating (Si 3 N 4 ) substrates in controlled humidity environment. We demonstrate that the supporting substrate plays a crucial role in interaction of these materials with water, with Pt making both GO and RGO insensitive to humidity variations and change in environment. On the other hand, in the case of Si 3 N 4 substrate a significant difference between GO and RGO with respect to humidity variations is demonstrated, indicating complete water coverage at ∼60% R.H for RGO and ∼30% R.H. for GO. Irrespective of the substrate, both GO and RGO demonstrate relative independence of their electronic properties on the material thickness, with similar trends observed for 1 and 2 layers when subject to humidity variations. This indicates a relatively minor role of material thickness in GO-based humidity sensors.

The sensitivity to water vapour of one-, two-, and three-layer epitaxial graphene (1, 2, and 3LG) is examined in this study. It is unambiguously shown that graphene's response to water, as measured by changes in work function and carrier density, is dependent on its thickness, with 1LG being the most sensitive to water adsorption and environmental concentration changes. This is furthermore substantiated by surface adhesion measurements, which bring evidence that 1LG is less hydrophobic than 2LG. Yet, surprisingly, it is found that other contaminants commonly present in ambient air have a greater impact on graphene response than water vapor alone. This study indicates that graphene sensor design and calibration to minimize or discriminate the effect of the ambient, in which it is intended to operate, are necessary to insure the desired sensitivity and reliability of sensors. The present work will aid in developing models for realistic graphene sensors and establishing protocols for molecular sensor design and development.

The interaction of graphene oxide of varying reduction degrees with dielectric and metallic surfaces is probed in this study, in order to assess the influence that the supporting substrate has on the electronic properties of as-produced graphene oxide and its reduced form. Lateral inhomogeneities in the distribution of substrate trapped charged impurities are found to affect the electronic properties of reduced graphene oxide, giving rise to significant in-plane variations of the local electrostatic potential on reduced one-layer sheets supported on dielectric substrates. On the contrary, no such surface potential fluctuations are identified on as-produced graphene oxide sheets, or on graphene oxide layers deposited on a metallic substrate. Thicker, two-layer reduced graphene oxide sheets show effective screening of the electrostatic effects caused by charge impurities trapped in the substrate. The current study provides a useful account of the limitations that device performance could face when attempting to tune the electronic structure of graphene oxide via functionalization, highlighting the role of substrate-related disorder affecting the behaviour of nanodevices. The role of the substrate is particularly important for applications where electronic properties of graphene oxide are especially targeted, such as transparent conducting films, sensors and electrochemical devices.

The properties of epitaxial graphene on the C-face of SiC are investigated using comprehensive structural, chemical and electrical analyses. By matching similar nanoscale features on the surface potential and Raman spectroscopy maps, individual domains have been assigned to graphene patches of 1–5 monolayers thick, as well as bare SiC substrate. Furthermore, these studies revealed that the growth proceeds in an island-like fashion, consistent with the Volmer-Weber growth mode, illustrating also the presence of nucleation sites for graphene domain growth. Raman spectroscopy data shows evidence of large area crystallites (up to 620 nm) and high quality graphene on the C-face of SiC. A comprehensive chemical analysis of the sample has been provided by X-ray photoelectron spectroscopy investigations, further supporting surface potential mapping observations on the thickness of graphene layers. It is shown that for the growth conditions used in this study, 5 monolayer thick graphene does not form a continuous layer, so such thickness is not sufficient to completely cover the substrate.

Light emission in atomically thin heterostructures is known to depend on the type of materials and the number and stacking sequence of the constituent layers. Here we show that the thickness of a two-dimensional substrate can be crucial in modulating the light emission. We study the layer-dependent charge transfer in vertical heterostructures built from monolayer tungsten disulfide (WS2) on one- and two-layer epitaxial graphene, unravelling the effect that the interlayer electronic coupling has on the excitonic properties of such heterostructures. We bring evidence that the excitonic properties of WS2 can be effectively tuned by the number of supporting graphene layers. Integrating WS2 monolayers with two-layer graphene leads to a significant enhancement of the photoluminescence response, up to 1 order of magnitude higher compared to WS2 supported on one-layer graphene. Our findings highlight the importance of substrate engineering when constructing atomically thin-layered heterostructures.

The sensitivity of graphene to the surrounding environment is given by its {\pi} electrons, which are directly exposed to molecules in the ambient. The high sensitivity of graphene to the local environment has shown to be both advantageous but also problematic for graphene-based devices, such as transistors and sensors, where the graphene carrier concentration and mobility change due to ambient humidity variations. In this review, recent progress in understanding the effects of water on different types of graphene, grown epitaxially and quasi-free standing on SiC, by chemical vapour deposition on SiO2, as well as exfoliated flakes, are presented. It is demonstrated that water withdraws electrons from graphene, but the graphene-water interaction highly depends on the thickness, layer stacking, underlying substrate and substrate-induced doping. Moreover, we highlight the importance of clear and unambiguous description of the environmental conditions (i.e. relative humidity) whenever a routine characterisation for carrier concentration and mobility is reported (often presented as a simple figure-of-merit), as these electrical characteristics are highly dependent on the adsorbed molecules and the surrounding environment., Comment: 37 pages, 15 figures

We investigate the local surface potential and Raman characteristics of as-grown and ex-situ hydrogen intercalated quasi-free standing graphene on 4H-SiC(0001) grown by chemical vapor deposition. Upon intercalation, transport measurements reveal a change in the carrier type from n- to p-type, accompanied by a more than three-fold increase in carrier mobility, up to μh ≈ 4540 cm2 V−1 s−1. On a local scale, Kelvin probe force microscopy provides a complete and detailed map of the surface potential distribution of graphene domains of different thicknesses. Rearrangement of graphene layers upon intercalation to (n + 1)LG, where n is the number of graphene layers (LG) before intercalation, is demonstrated. This is accompanied by a significant increase in the work function of the graphene after the H2-intercalation, which confirms the change of majority carriers from electrons to holes. Raman spectroscopy and mapping corroborate surface potential studies.

We present experimental studies of both local and macroscopic electrical effects in uniform single- (1LG) and bi-layer graphene (2LG) devices as well as in devices with non-uniform graphene coverage, under ambient conditions. DC transport measurements on sub-micron scale Hall bar devices were used to show a linear rise in carrier density with increasing amounts of 2LG coverage. Electrical scanning gate microscopy was used to locally top gate uniform and non-uniform devices in order to observe the effect of local electrical gating. We experimentally show a significant level of electric field screening by 2LG. We demonstrate that SGM technique is an extremely useful research tool for studies of local screening effects, which provides a complementary view on phenomena that are usually considered only within a macroscopic experimental scheme.

We directly correlate the local (20-nm scale) and global electronic properties of a device containing mono-, bi- and tri-layer epitaxial graphene (EG) domains on 6H-SiC(0001) by simultaneously performing local surface potential measurements using Kelvin probe force microscopy and global transport measurements. Using well-controlled environmental conditions, where the starting state of the surface can be reproducibly defined, we investigate the doping effects of N2, O2, water vapour and NO2 at concentrations representative of the ambient air. We show that presence of O2, water vapour and NO2 leads to p-doping of all EG domains. However, the thicker layers of EG are significantly less affected by the atmospheric dopants. Furthermore, we demonstrate that the general consensus of O2 and water vapour present in ambient air providing majority of the p-doping to graphene is a common misconception. We experimentally show that even the combined effect of O2, water vapour, and NO2 at concentrations higher than typically present in the atmosphere does not fully replicate the state of the EG surface in ambient air. All doping effects can be reproducibly reversed by vacuum annealing. Thus, for EG gas sensors it is essential to consider naturally occurring environmental effects and properly separate them from those coming from targeted species., 4 Figures, 1 Table

cited By 9; International audience; Prior to the implementation of multi-walled carbon nanotubes in microelectronic devices, investigating their electronic structure down to the nanometer scale is necessary. In that prospect, we used scanning tunneling microscopy (STM) to study the detailed atomic scale structure of double-walled carbon nanotubes, each comprising two rolled monolayers of graphene. Atomically resolved STM images usually displayed a motif and periodicity similar to that found in graphite but, on selected regions, atomically resolved motifs with a clearly defined superstructure were observed. This phenomenon has been reported previously but without a suitable explanation. We discuss the origin of this behavior in terms of modified stacking sequences due to the mismatch in registry between the chiral angles of the inner and the outer shells, associated with the interaction between the two carbon monolayers. These phenomena must be taken into account for the realization of lateral interference devices based on carbon nanotubes or graphene layers.

We present a study of quasi-free-standing monolayer graphene obtained by intercalation of Au atoms at the interface between the carbon buffer layer (Bu-L) and the silicon-terminated face (0001) of 4H-silicon carbide. Au intercalation is achieved by deposition of an atomically thin Au layer on the Bu-L followed by annealing at 850 degrees C in an argon atmosphere. We explore the intercalation of Au and decoupling of the Bu-L into quasi-free-standing monolayer graphene by surface science characterization and electron transport in top-gated electronic devices. By gate-dependent magnetotransport we find that the Au-intercalated buffer layer displays all properties of monolayer graphene, namely gate-tunable ambipolar transport across the Dirac point, but we find no observable enhancement of spin-orbit effects in the graphene layer, despite its proximity to the intercalated Au layer. Funding Agencies|Swedish Foundation for Strategic Research (SSF)Swedish Foundation for Strategic Research [IS14-0053, GMT14-0077, RMA15-0024]; Korean-Swedish Basic Research Cooperative Program of the NRF, Korea [NRF-2017R1A2A1A18070721]; Knut and Alice Wallenberg FoundationKnut & Alice Wallenberg Foundation; Chalmers Area of Advance NANO; European Unions Horizon 2020 research and innovation programme Graphene Flagship [881603]