Terahertz Self-Mixing Reflection Interferometry and studies of adsorption/desorption at graphene surfaces
Optics, Quantum Optics and Physics of Atoms, Molecules and Plasmas
Physical Chemistry of Molecules, Liquids and Interfaces, Biophysical Chemistry
Final Report Abstract
The adsorption/desorption kinetics of N2O, CO and CH4 on graphene have been investigated by temperature programmed desorption with two different methods: time-resolved MIR reflectionabsorption spectroscopy and the electrical characterization of a graphene field effect transistor (gFET). To this end, a vacuum-tight measurement chamber has been constructed featuring a BaF2 IR- transparent window, gas flow lines, a 4-lane vacuum tight electrical feedthrough, 2 thermocouples and a heating element (up to 500 0C with 0.1 K increment). Graphene on SiO2/p-doped Si (or a gFET device) is placed on the heating element and serves as substrate to which the gas molecules N2O, CO and CH4 are adsorbed on/desorbed from. The custom-made measurement chamber is placed into a Bruker Vertex 80 V FTIR spectrometer. The set-up enabled precise optical and electrical Temperature Programmed Adsorption/Desorption studies. The experiments, carried out in-situ at elevated temperatures (145 < T < 300 0C) in a time range of 0 to 600 s, allow to determine the adsorption/desorption rate constants k and activation energies Ea of said gases on graphene, uninfluenced by H2O as omnipresent concurrent adsorbate. Both methods provide similar kinetic data for N2O and CO. In case of non-polar CH4 the gFET experiment, however, fails and reliable kinetic data is only gained by the optical approach. The desorption rate constants kdes of 108 to 109 molecules cm-2s-1 for the 3 gases are 1-2 orders of magnitude lower than that of adsorption, in line with the general observation of fast adsorption but slow desorption in graphene gas sensors. Adsorption activation energies -19.6(N2O), -12.1 (CO) and -9.5 (CH4) kJ/Mol and desorption activation energies of 42.2 (N2O), 27.2 (CO) and 16.3 (CH4) kJ/Mol, both derived from ln k vs T^-1 plots, suggest weak interaction between the adsorbates and graphene by intermolecular forces. Concerning electrical results, mainly the concentration of charge carriers changes with adsorption/desorption. The field effect mobility µ^e- of the gFET is unchanged (or changed only minorly) during desorption/reversal of charge transfer. The drain-source current Ids of the gFET shows a maximum in case of almost complete desorption and low graphene charge carrier concentrations as one approaches the graphene state specified by the Dirac potential VD of 0 V. It can be explained by the reduction/loss of scattering of the graphene conduction electrons in the graphene/adsorbate interface, thereby providing an experimental proof of the importance of charged impurity scattering suggested in the literature by theoretical work. Time-resolved MIR spectroscopy and in-situ electrical measurements on a graphene-FET substrate have been demonstrated for the first time as an alternative to semiconductor or mass spectrometerbased approaches of surface kinetics analysis. Adsorption/Desorption kinetics of N2O, CO and CH4 on graphene are experimentally investigated for the first time by in-situ optical and electrical temperature-programmed measurements. No experimental literature values exist for N2O on graphene for comparison with our work. The in-situ optical and electrical techniques may be extended to other environmentally hazardous or commercially relevant gases as well as other 2D materials in order to get a more comprehensive view on the kinetic properties of 2D material/gas interfaces. Concerning possible applications, we would like to mention highly selective optical gas sensors and lab-on-chip sensors based on graphene.
Publications
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Time-Resolved MIR Reflection–Absorption Spectroscopy of N2O, CO, and CH4 Adsorption on Graphene. The Journal of Physical Chemistry C, 127(10), 4998-5003.
Akça, Sefa; Lüders, Laura v.; Düsberg, Georg S.; Elsaesser, Wolfgang; Nicoloso, Norbert & Riedel, Ralf
