In-Situ Study of the Adsorption-Desorption Kinetics of N₂O, CO and CH₄ on Graphene with MIR and gFET DC Measurements

This thesis utilizes the time-resolved FT-IR absorption-reflection spectroscopy and the transfer characteristics of a graphene field effect transistor (gFET) device to characterize the adsorption and desorption kinetics of the environmentally harmful gases N_2O, CO and CH_4 on graphene. The chosen methods, in particular the IR based optical approach, are shown to replenish the information gaps existing in the widely utilized mass spectroscopy (MS) and the electrical gas sensing techniques. Notably, while the MS-based approaches are the standart for desorption studies[84, 83], they are not directly applicable to studies of adsorption. On the other hand, electrical sensing can probe both adsorption and desorption[94, 107] but has the drawback of not being able to detect the non-polar, non-charge transferring molecules such as CH_4. The time-resolved optical measurements on graphene used in this work can probe both the adsorption and desorption of polar as well as non-polar species with the same setup. The back-gated gFET’s used in this thesis have been produced via the CVD-growth of graphene on Cu substrates and subsequent transfer to SiO_2/Si substrates. The gFETs, installed in a custom-made vacuum chamber with electrical feedthroughs and gas-flow lines, thermocouples and a ceramic heating element, enable in-situ optical and electrical experiments within a Bruker Vertex 80V FT-IR spectrometer. A temperature range of 145 - 300 °C has been chosen for the temperature programmed adsorption/desorption (TPA/TPD) experiments to ensure complete desorption at the upper limit and steady-state adsorption at the lower limit. As importantly, the choice of this temperature range rules out H_2O as a competing, accidental adsorbate. The obtained time-resolved FT-IR signal, Dirac potential (V_D) and drain-source current (I_ds) of the gFET have been normalized to determine the change of the adsorbate concentration with time, providing the chemical kinetics of graphene/X (X=N_2O, CO and CH_4). The measurements show clear evidence for 1st order reactions with rate constants k = 7 x 10-3 s^-1. The derived adsorption/desorption activation energies, E_a, for of N_2O, CO and CH_4 on graphene are in excellent agreement with the available experimental or theoretical literature values. The E_a values for adsorption and desorption are in the range of 10-40 kJ/Mol (-19.6 (N_2O), -12.1 (CO), -9.5 (CH_4) for E_a_ads and 42.2 (N_2O), 18.7 (CO), 16.3 (CH_4) for E_a_des), indicative of weak intermolecular forces between graphene and adsorbates. According to the observed reaction rates the desorption step is significantly slower (by > 1 order of magnitude) than the adsorption step even at the elevated temperature of the experiments. Comparison of the optical and electrical studies yields close agreement between the FT-IR reflection-absorption signal and the results of the gFET Dirac potential measurements. Both methods deliver reliable kinetic data. The time-resolved FT-IR approach is, however, more versatile since it allows, e.g., investigations of non-polar adsorbates like CH_4. The time-dependent Dirac potential measurements on the other hand provide valuable information about the correlation of the charge transfer/carrier concentration in graphene and the kinetics of the adsorbate adsorption/desorption. The reaction rates, r = - dθ/dt, determined from the FT-IR and Dirac potential (V_D) studies, are identical within the experimental error thereby confirming a direct correlation between the adsorption/desorption of adsorbates and the carrier concentration in graphene. The comparison of the FT-IR and VD data with those of the gFET drain–source current (I_ds) measurements is, however, less satisfying. As Ids (or the conductance of the gFET) depends on two quantities, the carrier concentration n and the carrier mobility µ, both of them should be precisely known for the analysis of the TPD experiment with Ids as indicator. The change of n with desorption has been followed by the Dirac potential shift (∆VD) and µFET is derived from the linear part of an Ids-Vg plot, i.e., by the derivate dIds/dVg. At the start of the TPD experiments with N_2O and CO, I_ds is only weakly declining, less than expected from the clear reduction of n (as indicated by DeltaV_D) and an almost constant µ^FET. Accordingly, the Ids is not representative of the actual adsorbate concentration at this first stage of desorption. However, Ids follows the behavior seen in FT-IR and V_D in the main desorption region (200-250 °C) where the desorption rates are high due to the increased temperature. I_ds strongly declines in this region because of the reduced carrier concentration and a reduced mobility. The mobility here deviates by < 20 % from the value of pristine graphene. Evidently, µ^FET gets dependent on the adsorbate and charge carrier concentration with a non-linear correlation between n and µ. The significant but not very large change of µ^FET suggests that I_ds is still mainly governed by the charge transfer/loss of charge carriers in graphene. n and µFET further steadily decline with increasing adsorbate desorption. Close to the end of desorption, n gets < 10^11 cm^-2 according to the observed Dirac potential shifts. The carrier concentration induced by the adsorbates and the intrinsic carrier concentration of graphene at its neutrality point V_D = 0 V (at gate voltage of V_g = 0 V) are then of comparable magnitude. I_ds is non-linearly related to V_g at these conditions allowing no clear-cut evaluation of mobility. According to theoretical work, the behavior of mobility near the neutrality point of graphene essentially depends on the presence of long range Coulomb impurities (in this case adsorbates) and short range graphene defects. Competition between these two types of defects first leads to an enhancement and then a decline of mobility, or a peak in the carrier dependence of mobility. The I_ds data of this work present a first experimental proof of this modelling. At low adsorbate and low carrier concentrations, at the end of desorption at high temperature, I_ds exhibits a small but reproducible peak with approximately 5 percent change in intensity which is only explainable by an increase in the mobility. The reduction/loss of charged impurity scattering by the adsorbates is held responsible for this behavior. To shortly summarize, the in-situ TPA/TPD studies of graphene/X (X=N_2O, CO, CH_4) of this thesis provide new insight into the linkage between charged adsorbates and graphene properties during the progression of the adsorption/desorption process. Charge transfer to/from adsorbates and the corresponding changes of the graphene e^-/h^+ concentrations are the main quantities determining the time-dependend changes, i.e., the chemical kinet-ics of the graphene/adsorbate system. However, significant deviations from the expected electrical behaviour occur at low adsorbate concentrations. Especially the graphene carrier mobility is no more constant but declines with decreasing adsorbate/carrier concentration. As shown by the TPD experiments with I_ds, the electronic properties of graphene depend on the ratio of intrinsic charge carriers and charged impurities (adsorbates and other defects) close to the end of desorption. Concerning applications, these findings suggest that the commonly utilized conductance-based graphene gas sensors need to be re-evaluated near their detection limits.