Au3-Decorated graphene as a sensing platform for O2 adsorption and desorption kinetics.

The adsorption and desorption kinetics of molecules is of significant fundamental and applied interest. In this paper, we present a new method to quantify the energy barriers for the adsorption and desorption of gas molecules on few-atom clusters, by exploiting reaction induced changes of the doping level of a graphene substrate. The method is illustrated for oxygen adsorption on Au3 clusters. The gold clusters were deposited on a graphene field effect transistor and exposed to O2. From the change in graphene's electronic properties during adsorption, the energy barrier for the adsorption of O2 on Au3 is estimated to be 0.45 eV. Electric current pulses increase the temperature of the graphene strip in a controlled way and provide the required thermal energy for oxygen desorption. The oxygen binding energy on Au3/graphene is found to be 1.03 eV and the activation entropy is 1.4 meV K-1. The experimental values are compared and interpreted on the basis of density functional theory calculations of the adsorption barrier, the binding energy and the activation entropy. The large value of the activation entropy is explained by the hindering effect that the adsorbed O2 has on the fluxional motion of the Au3 cluster.

Interaction of graphene with Aunclusters: a first-principles study.

The interaction between Aun(n= 1-6) clusters and graphene is studied using first-principles simulations, based on density functional theory. The computed binding energy between Aunand graphene depends on the number of atoms in the cluster and lies between -0.6 eV and -1.7 eV, suggesting (weak) chemisorption of the clusters on graphene, rather than physisorption. Overall, the electronic properties, spin-orbit interaction and spin texture, as well as the transport properties of graphene strongly depend on the precise size of the Aunclusters. Doping of graphene is predicted for clusters with an odd number of Au atoms, due to overlap between Ausand carbonpzstates close to the Fermi level. On the other hand, there is no charge transfer between even size Au clusters and graphene, but a gap is formed at the Dirac cone, due to the breaking of the pseudo spin inversion symmetry of graphene's lattice. The adsorbed Aunclusters induce spin-orbit interactions as well as spin and pseudo spin interactions in graphene, as indicated by the splitting of the electronic band structure. A hedgehog spin texture is also predicted for adsorbed clusters with an even number of Au atoms. Ballistic transport simulations are performed to study the influence of the adsorbed clusters on graphene's electronic transport properties. The influence of the cluster on the electron transmission across the structure depends on the mixing of the valence orbitals in the transport energy window. In the specific case of the Au3/graphene system, the adsorbed clusters reduce the transmission and the conductance of graphene. The Au3clusters act as 'scattering centers' for charge carriers, in agreement with recent experimental studies.

Restoring self-limited growth of single-layer graphene on copper foil via backside coating.

The growth of single-layer graphene (SLG) by chemical vapor deposition (CVD) on copper surfaces is very popular because of the self-limiting effect that, in principle, prevents the growth of few-layer graphene (FLG). However, the reproducibility of the CVD growth of homogeneous SLG remains a major challenge, especially if one wants to avoid heavy surface treatments, monocrystalline substrates and expensive equipment to control the atmosphere inside the growth system. We demonstrate here that backside tungsten coating of copper foils allows for the exclusive growth of SLG with full coverage by atmospheric pressure CVD implemented in a vacuum-free furnace. We show that the absence of FLG patches is related to the suppression of carbon diffusion through copper. In the perspective of large-scale production of graphene, this approach constitutes a significant improvement to the traditional CVD growth process since (1) a tight control of the hydrocarbon flow is no longer required to avoid FLG formation and, consequently, (2) the growth duration necessary to reach full coverage can be drastically shortened.

Nano-SQUIDs with controllable weak links created via current-induced atom migration.

As the most sensitive magnetic field sensor, the superconducting quantum interference device (SQUID) became an essential component in many applications due to its unmatched performance. Through recently achieved miniaturization, using state-of-the-art fabrication methods, this fascinating device extended its functionality and became an important tool in nanomaterial characterization. Here, we present an accessible and yet powerful technique of targeted atom displacement in order to reduce the size of the weak links of a DC nano-SQUID beyond the limits of conventional lithography. The controllability of our protocol allows us to characterize in situ the full superconducting response after each electromigration step. From this in-depth analysis, we reveal an asymmetric evolution of the weak links at cryogenic temperatures. A comparison with time resolved scanning electron microscopy images of the atom migration process at room temperature confirms the peculiar asymmetric evolution of the parallel constrictions. Moreover, we observe that when electromigration has sufficiently reduced the junction's cross section, superconducting phase coherence is attained in the dissipative state, where magnetic flux readout from voltage becomes possible.