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Ambient Vacuum Frequency-modulated Kelvin probe force microscopy Scanning probe microscopy – transport in variable environments Raman characterisation Introduction Methods Acknowledgments Conclusions Ambient Vacuum Nitrogen 20 % R.H. 40 % R.H. 60 % R.H. Ambient Mono-layer 2LG island Work function maps of the mono-layer graphene transferred to SiO 2 (with 2LG islands) in controlled environmental conditions. • Significant decrease of WF and hole concentration in vacuum → desorption of water and other p-dopants present on the graphene surface. • Gradual increase of WF and hole concentration → water is a strong p dopant. • Saturation of WF and hole concentration → water saturates the graphene surface and is not the only dopant in ambient air. • Bi-layer graphene behaves as two decoupled layers, with only the top layer affected by humidity (response similar to mono-layer). • Weak interaction between mono-layer and twisted 2LG results in different response compared to de-coupled bi- layers. We acknowledge the support of EC grants Graphene Flagship and the IRD Graphene Project. The work was carried out as part of an Engineering Doctorate Program in Micro- and NanoMaterials and Technologies, financially supported by the EPSRC, the University of Surrey and the National Physical Laboratory. = 532 Mono-layer graphene: Narrow and symmetrical 2D peak → substrate is covered with mono-layer graphene. 2LG islands: Narrow and symmetrical, but shifted 2D peak → twisted (not AB stacked) 2LG graphene. • The two-dimensional nature of graphene makes it sensitive to environmental doping. • With water vapour being a significant component of the ambient air, graphene-based devices designed to operate in ambient air (e.g. sensors) will be significantly affected, as their electronic properties can change with humidity. • For the successful commercialisation of graphene-based devices, the complete understanding of the water-graphene interactions is necessary. • We employ simultaneously local Kelvin probe force microscopy (KPFM) and global transport measurements in the van der Pauw geometry to observe the effects of water on the work function (WF) and carrier concentration of chemical vapour deposition (CVD) grown graphene of different thicknesses transferred on SiO 2 . • We probe the local electronic properties of mono- and bi-layer graphene in a series of measurements in controlled environments, starting from ambient (~30 % R.H.), to vacuum, pure nitrogen and 20-60% relative humidity (RH). • Raman spectroscopy and mapping is used to assess the graphene thickness and structure. • KPFM showed significant decrease of WF in vacuum which is associated with desorption of water and other p-dopants present in ambient air. This was also observed as a decrease in hole concentration (transport measurements). • In vacuum, both the hole concentration and WF are higher for the mono-layer compared to bi-layer graphene on SiO 2 → screening of substrate charges. • Introduction of nitrogen and water vapour mixture with a gradual change of humidity from 20 to 60% RH results in an increase in the hole concentration and WF, however the value does not reach ambient levels → water vapour is not the only p-dopant in the ambient air that affects graphene. • Graphene surface saturates with water at humidity levels higher than ~40% R. H. This can be seen from the saturation in hole concentration and WF. • Upon exposure to ambient air, the carrier concentration is not fully restored → long restoring time is needed. • Raman spectroscopy/mapping indicated that the mono-layer sample is covered with 1LG film, decorated with twisted 2LG nucleation islands. The bi-layer samples are covered with decoupled (not AB stacked) 2LG films. N 2 20% R.H. 40% R.H. 60% R.H. Ambient Vacuum N 2 20% R.H. 40% R.H. 60% R.H. Bi-layer eV eV eV eV Work function maps of the bi-layer graphene transferred to SiO 2 in controlled environmental conditions. Ambient Vacuum Nitrogen 20% R.H. 40% R.H. 60% R.H. Ambient 4.3 4.4 4.5 4.6 4.7 4.8 4.9 5.0 5.1 5.2 5.3 Work function (eV) Environmental condition Mono-layer WF 2LG islands WF Bi-layer WF 2 4 6 8 10 12 14 16 18 20 Mono-layer hole concentration Bi-layer hole concentration Hole concentration (10 12 cm -2 ) G-Peak intensity 680 3000 1200 1400 1600 2600 2700 2800 Raman shift (cm -1 ) Mono-layer 2LG island 1 2LG island 2 2LG island D G D’ 2D 2D-Peak intensity 680 1450 G-Peak shift 1570 cm -1 1610 cm -1 Tip calibration: Φ Tip ≈Φ HOPG +eU CPD (Φ HOPG =4.48 eV) Mono-layer 2LG islands Substrate Effect of humidity on electronic properties of CVD graphene C. Melios 1,2 , V. Panchal 1 , C.E. Giusca 1 , A. Centeno 3 , A. Zurutuza 3 , S.R.P. Silva 2 and O. Kazakova 1 1 National Physical Laboratory, Hampton Road, Teddington, TW11 0LW, UK 2 Advanced Technology Institute, University of Surrey, Guildford, Surrey, GU2 7XH, UK 3 Graphenea SA, 20018 Donostia-San Sebastián, Spain Email: [email protected]
