Charge Transport Study of in-Situ Cesium Doped Monolayer Graphene
Authors: Aygar Ayse Melis , Szkopek Thomas 
Published: May 2020

Modifying graphene with surface adsorbates is a widely studied way of modifying graphene’s electronic properties. Different graphene/adatom systems have shown that it is possible to open a band gap in graphene1, engineer topological insulators2 and create magnetic moments3. The atomic thickness of graphene maximizes the effect of adsorbates, enabling chemical sensors that achieve single molecule sensitivity4. Alkali metal adatoms are a particularly efficient dopant for graphene due to their high electronegativity. Lithium/graphene systems have been studied for in the context of low-dimensional superconductivity5 and achieving ultrafast diffusion of lithium6 for battery applications.
However, doping graphene with alkali metal adatoms bears challenges. Experiments with alkali metals must be conducted in an ultra-high vacuum or inert gas environment due to the high reactivity of the alkali. Alkali metal adsorbed graphene is not air stable, which imposes limits on the available characterization methods. An impurity free graphene surface must be achieved prior to adsorption to achieve efficient adsorption and avoid unwanted parasitic reactions. This problem is particularly acute for graphene samples prepared by the use of polymer handles. Alkali metal atoms can also intercalate under the graphene layer depending on substrate and sample preparation conditions. Moreover, it is possible for adatoms to form clusters on graphene surface7which negatively impacts both doping uniformity and efficiency.
In this work, we report a new method of alkali doping of graphene to reach ultra-high doping (~1014cm-2) for charge transport studies beyond the limit of ionic liquid gating. We work with chemical vapor deposition grown graphene transferred onto quartz substrates. Quartz is the substrate of choice due to its chemical inertness and optical transparency, with the latter enabling non-invasive Raman spectroscopy through the substrate. Micron scale graphene devices were prepared using lithography-based methods and the samples were thermally annealed in a nitrogen glove box environment to desorb water prior to alkali doping. The graphene was subsequently exposed to cesium vapor at different temperatures using a flip-chip method that allows hermetic sealing of the air sensitive samples in an inert gas environment with a liquid cesium source of cesium vapour. We measured the in-situ variation of graphene resistivity as cesium atoms are adsorbed onto the graphene surface, and resistivity is modulated by charge transfer doping. Evidence of ultra-high doping is shown via Raman spectroscopy performed through the quartz substrate window, with G-peak shifts from 1589 cm-1 up to 1608 cm-1. Hall measurements in-situ further confirm the strong doping. We measured the electronic transport properties of cesium doped graphene at temperatures as low as 1.2 K and under magnetic fields up to 7 T. Weak-localization, magnetoresistance and Hall resistance are measured and analyzed.

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