Graphene as a sensing platform for the adsorption and desorption kinetics of gas molecules on few-atom clusters

Few-atom clusters show great promise as catalysts for a number of industrially relevant chemical reactions due to their high reactivity and size-dependent physical properties. To fully exploit their catalytic properties, a detailed understanding of the adsorption and desorption reaction of gas molecules on the clusters is of paramount importance. In this regard, graphene offers interesting possibilities. Graphene, a two-dimensional material consisting of a hexagonal carbon lattice, has many remarkable properties, including a strong sensitivity to adsorbed particles.

In this thesis, size-selected cluster beams were deposited on the graphene transistors. Cluster beams were produced in the KU Leuven magnetron sputtering set-up. Graphene transistors were produced by the tools available in the KU Leuven NanoCentre. Electronic measurements give information on the doping of graphene by these clusters. When the cluster-decorated graphene transistors are exposed to gas molecules, an activation energy barrier for adsorption can be obtained by correlating the change in doping to the frequency of collision between the gas molecules and the clusters. It is shown that this activation energy barrier is dependent on both the cluster size and element, which shows that (a fraction of) the clusters remain (partly) isolated on the graphene surface.

Considerable attention is given to the controlled heating of the graphene transistor in order to induce desorption of gas molecules. Specifically, heating the graphene transistor through current annealing or through an external heater were investigated. It is shown that by controlled heating of the graphene transistor, gas desorption can be induced, of which the resulting change in graphene doping can be measured. It is shown that, to correctly understand the desorption behaviour of gas molecules from clusters, the dynamical configuration of clusters needs to be taken into account. This so-called cluster fluxionality significantly increases the entropy of the transition state during desorption.

To obtain insight in the mechanism that holds the clusters in place, the evolution of the electronic properties of cluster-decorated graphene transistors was investigated under elevated temperatures. It was found that there is a striking difference between few-atom Au and Ni clusters. While the Au cluster decorated sample’s properties were not noticeably affected by the elevated temperatures, the Ni cluster-decorated samples showed a return to the undoped state upon heating the sample by a few tens of degrees, which was attributed to coalescence of the Ni clusters.

Lastly, it was investigated whether the cluster mobility could be suppressed by introducing defects which could serve as anchor points for the clusters. To this end, the graphene transistors were bombarded with Ar+ ions. By use of Raman spectroscopy measurements, it was found that the defects are indeed created upon Ar+ bombardment. Upon deposition of Au3 clusters immediately after Ar+ bombardment, it could be seen that oxygen exposure induced n-doping, as opposed to p-doping for Au3 clusters on pristine graphene. This is consistent with density functional theory calculations and gives evidence that the Au3 clusters indeed attach to the defects.