Direct chemical vapour deposited graphene synthesis on silicon oxide by controlled copper dewettting

In this paper we present a novel method for direct uniform graphene synthesis onto silicon oxide in a controlled manner. On a grooved silicon oxide wafer is copper deposited under a slight angle and subsequently the substrate is treated by a typical graphene synthesis process. During this process directional dewetting of the copper into the grooves was observed. A layer of graphene was directly deposited onto the silicon oxide while the copper was retracting from the surface. This method opens new possibilities for graphene devices, since the technique can be applied wafer-scale

Frequency dependent AC electroosmotic flow in nanochannels

We report frequency-dependent bidirectional AC electroosmotic flow (AC-EOF) in a nanochannel with double layer overlap. This work follows our report in μTas 2008 of unidirectional AC-EOF in nanochannels [1]. Observed is a bidirectional pumping behavior; simulations of the low frequency pumping confirm a direction opposite to that of AC-EOF in microchannels. By this frequency-dependent bidirectional pumping, nanochannel AC-EOF behaves in fundamentally different way than microchannel AC-EOF. Generally, the results are of importance for the understanding of ion and liquid transport in nanoconfinement

Oxidation kinetics of transition metals exposed to molecular and atomic oxygen

In this work we analyze the oxidation of 30 nm polycrystalline transition metals films (Hf, Ta, Mo and Ru) at low temperatures (298 K to 473 K) upon the exposure to two different species: molecular oxygen and atomic oxygen. Using in-situ spectroscopic ellipsometry and in-vacuum X-ray photoelectron spectroscopy, we verify the oxide growth kinetics and the final stoichiometry after each exposure condition for the four metals, and explore the particularities present in each oxide growth mechanics. The temperature-dependent analysis enabled to experimentally obtain the dissociation energy of molecular oxygen at polycrystalline O covered surfaces. By applying the principles of coupled currents and the Cabrera-Mott oxide growth mechanism, we extract values of the energy barrier for oxidation and the field formed in both oxygen molecule-metal and atom-metal interaction, exploring the differences between both exposure conditions. We demonstrate that in oxide growth at low temperatures two key points should be highlighted: (i) the strong dependence of surface potential on reactive oxygen coverage; (ii) the interrelation between exposure conditions and crystalline oxide formation. The obtained results and analysis contribute to the understanding of oxidation processes at low temperatures, advancing the knowledge required for the design and synthesis of thin metal and oxide films

Bifunctional catalytic effect of Mo2C/oxide interface on multi-layer graphene growth

The role of the Mo2C/oxide interface on multi-layer graphene (MLG) nucleation during a chemical vapor deposition (CVD) process is investigated. During the CVD process, MLG growth is only observed in the presence of a Mo2C/SiO2 interface, indicating that the chemical reactions occurring at this interface trigger the nucleation of MLG. The chemical reaction pathway is explained in four steps as (1) creation of H radicals, (2) reduction of the oxide surface, (3) formation of C–C bonds at O–H sites, and (4) expansion of graphitic domains on the Mo2C catalyst. Different Mo2C/oxide interfaces are investigated, with varying affinity for reduction in a hydrogen environment. The results demonstrate a catalyst/oxide bifunctionality on MLG nucleation, comprising of CH4 dehydrogenation by Mo2C and initial C–C bond formation at the oxide interface

Atomic H diffusion and C etching in multilayer graphene monitored using a y based optical sensor

In this work, the authors expose transferred multilayer graphene on a yttrium based hydrogen sensor. Using spectroscopic ellipsometry, they show that graphene, as well as amorphous carbon reference films, reduce diffusion of hydrogen to the underlying Y layer. Graphene and C are both etched due to exposure to atomic H, eventually leading to hydrogenation of the Y to YH2 and YH3. Multilayer graphene, even with defects originating from manufacturing and transfer, showed a higher resistance against atomic H etching compared to amorphous carbon films of a similar thickness

Geometrically programmable bidirectional pump using rotating magnetic microspheres

We report geometrically programmable bidirectional pumping in microchannels using magnetic microspheres which rotate around magnetic disks under influence of an external rotating magnetic field. Geometric programming of the pumping direction is obtained by locating the magnetic disks on the inside or the outside curve of a microchannel as shown in Figure 1. A second degree of freedom in pumping direction is offered by the rotation direction of the external field. Pumping rate is controlled by the rotational frequency