extremely sensitive NO 2 detection. The gas exposed uppermost layer of the multi layer device is screened from the SiC by the intermediate layers leading to a p-type nature with a higher concentration of charge carriers and therefore, a lower gas response. The single layer graphene device is thought to undergo an n-p transition upon exposure to increasing concentrations of NO 2 indicated by a change in response direction. This transition is likely to be due to the transfer of electrons to NO 2 making holes the majority carriers.

We compare the three most commonly used scanning probe techniques to obtain a reliable value of the work function in graphene domains of different thickness. The surface potential (SP) of graphene is directly measured in Hall bar geometry via a combination of electrical functional microscopy and spectroscopy techniques, which enables calibrated work function measurements of graphene domains in ambient conditions with values Φ1LG ~4.55 ± 0.02 eV and Φ2LG ~ 4.44 ± 0.02 eV for single- and bi-layer, respectively. We demonstrate that frequency-modulated Kelvin probe force microscopy (FM-KPFM) provides more accurate measurement of the SP than amplitude-modulated (AM)-KPFM. The discrepancy between experimental results obtained by different techniques is discussed. In addition, we use FM-KPFM for contactless measurements of the specific components of the device resistance. We show a strong non-Ohmic behavior of the electrode-graphene contact resistance and extract the graphene channel resistivity.

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The efficiency of flexible photovoltaic and organic light emitting devices is heavily dependent on the availability of flexible and transparent conductors with at least a similar workfunction to that of Indium Tin Oxide. Here we present the first study of the work function of large area (up to 9 cm2) FeCl3 intercalated graphene grown by chemical vapour deposition on Nickel, and demonstrate values as large as 5.1 eV. Upon intercalation, a charge density per graphene layer of 5 ⋅ 1013 ± 5 ⋅ 1012 cm−2 is attained, making this material an attractive platform for the study of plasmonic excitations in the infrared wavelength spectrum of interest to the telecommunication industry. Finally, we demonstrate the potential of this material for flexible electronics in a transparent circuit on a polyethylene naphthalate substrate.

A pivotal issue for the fabrication of electronic devices on epitaxial graphene on SiC is controlling the number of layers and reducing localized thickness inhomogeneities. Of equal importance is to understand what governs the unintentional doping of the graphene from the substrate. The influence of substrate surface topography on these two issues was studied by work function measurements and local surface potential mapping. The carrier concentration and the uniformity of epitaxial graphene samples grown under identical conditions and on substrates of nominally identical orientation were both found to depend strongly on the terrace width of the SiC substrate after growth.

Using environmental scanning Kelvin probe microscopy, we show that the position of the Fermi level of single layer graphene is more sensitive to chemical gating than that of double layer graphene. We calculate that the difference in sensitivity to chemical gating is not entirely due to the difference in band structure of 1 and 2 layer graphene. The findings are important for gas sensing where the sensitivity of the electronic properties to gas adsorption is monitored and suggest that single layer graphene could make a more sensitive gas sensor than double layer graphene. We propose that the difference in surface potential between adsorbate-free single and double layer graphene, measured using scanning kelvin probe microscopy, can be used as a noninvasive method of estimating substrate-induced doping in epitaxial graphene.

We study an epitaxial graphene monolayer with bilayer inclusions via magnetotransport measurements and scanning gate microscopy at low temperatures. We find that bilayer inclusions can be metallic or insulating depending on the initial and gated carrier density. The metallic bilayers act as equipotential shorts for edge currents, while closely spaced insulating bilayers guide the flow of electrons in the monolayer constriction, which was locally gated using a scanning gate probe.

We present a new instrument, "Boreas", a cryogen-free methane (CH 4 ) preconcentration system coupled to a dual-laser spectrometer for making simultaneous measurements of δ 13 C(CH 4 ) and δ 2 H(CH 4 ) in ambient air. Excluding isotope ratio scale uncertainty, we estimate a typical standard measurement uncertainty for an ambient air sample of 0.07‰ for δ 13 C(CH 4 ) and 0.9‰ for δ 2 H(CH 4 ), which are the lowest reported for a laser spectroscopy-based system and comparable to isotope ratio mass spectrometry. We trap CH 4 (∼1.9 μmol mol −1 ) from ∼5 L of air onto the front end of a packed column, subsequently separating CH 4 from interferences using a controlled temperature ramp with nitrogen (N 2 ) as the carrier gas, before eluting CH 4 at ∼550 μmol mol −1 . This processed sample is then delivered to an infrared laser spectrometer for measuring the amount fractions of 12 CH 4 , 13 CH 4 , and 12 CH 3 D isotopologues. We calibrate the instrument using a set of gravimetrically prepared amount fraction primary reference materials directly into the laser spectrometer that span a range of 500−626 μmol mol −1 (CH 4 in N 2 ) made from a single pure CH 4 source that has been isotopically characterized for δ 13 C(CH 4 ) by IRMS. Under the principle of identical treatment, a compressed ambient air sample is used as a working standard and measured between air samples, from which a final calibrated isotope ratio is calculated. Finally, we make automated measurements of both δ 13 C(CH 4 ) and δ 2 H(CH 4 ) in over 200 ambient air samples and demonstrate the application of Boreas for deployment to atmospheric monitoring sites.