Graphene has been used to explore the fascinating electronic properties of ideal two-dimensional carbon, and shows great promise for quantum device architectures. The primary method for isolating graphene, micromechanical cleavage of graphite, is difficult to scale up for applications. Epitaxial growth is an attractive alternative, but achieving large graphene domains with uniform thickness remains a challenge, and substrate bonding may strongly affect the electronic properties of epitaxial graphene layers. Here, we show that epitaxy on Ru(0001) produces arrays of macroscopic single-crystalline graphene domains in a controlled, layer-by-layer fashion. Whereas the first graphene layer indeed interacts strongly with the metal substrate, the second layer is almost completely detached, shows weak electronic coupling to the metal, and hence retains the inherent electronic structure of graphene. Our findings demonstrate a route towards rational graphene synthesis on transition-metal templates for applications in electronics, sensing or catalysis.

We demonstrated that platinum (Pt) oxygen-reduction fuel-cell electrocatalysts can be stabilized against dissolution under potential cycling regimes (a continuing problem in vehicle applications) by modifying Pt nanoparticles with gold (Au) clusters. This behavior was observed under the oxidizing conditions of the O2 reduction reaction and potential cycling between 0.6 and 1.1 volts in over 30,000 cycles. There were insignificant changes in the activity and surface area of Au-modified Pt over the course of cycling, in contrast to sizable losses observed with the pure Pt catalyst under the same conditions. In situ x-ray absorption near-edge spectroscopy and voltammetry data suggest that the Au clusters confer stability by raising the Pt oxidation potential.

The prototypical photocatalyst TiO2 exists in different polymorphs, the most common forms are the anatase- and rutile-crystal structures. Generally, anatase is more active than rutile, but no consensus exists to explain this difference. Here we demonstrate that it is the bulk transport of excitons to the surface that contributes to the difference. Utilizing high –quality epitaxial TiO2 films of the two polymorphs we evaluate the photocatalytic activity as a function of TiO2-film thickness. For anatase the activity increases for films up to ~5 nm thick, while rutile films reach their maximum activity for ~2.5 nm films already. This shows that charge carriers excited deeper in the bulk contribute to surface reactions in anatase than in rutile. Furthermore, we measure surface orientation dependent activity on rutile single crystals. The pronounced orientation-dependent activity can also be correlated to anisotropic bulk charge carrier mobility, suggesting general importance of bulk charge diffusion for explaining photocatalytic anisotropies.

In situ low-energy electron microscopy ͑LEEM͒ of graphene growth combined with measurements of the graphene structure and electronic band structure has been used to study graphene on Pt͑111͒. Growth by carbon segregation produces macroscopic monolayer graphene domains extending continuously across Pt͑111͒ substrate steps and bounded by strongly faceted edges. LEEM during cooling from the growth temperature shows the propagation of wrinkles in the graphene sheet, driven by thermal stress. The lattice mismatch between graphene and Pt͑111͒ is accommodated by moiré structures with a large number of different rotational variants, without a clear preference for a particular interface geometry. Fast and slow growing graphene domains exhibit moiré structures with small ͓e.g., ͑3 ϫ 3͒ G , ͑ ͱ 6 ϫ ͱ 6͒R2 G , and ͑2 ϫ 2͒R4 G ͔ and large unit cells ͓e.g., ͑ ͱ 44 ϫ ͱ 44͒R15 G , ͑ ͱ 52ϫ ͱ 52͒R14 G , and ͑8 ϫ 8͒ G ͔, respectively. A weak substrate coupling, suggested by the growth and structural properties of monolayer graphene on Pt͑111͒, is confirmed by maps of the band structure, which is close to that of isolated graphene aside from minimal hole doping due to charge transfer from the metal. Finally, the decoupled graphene monolayer on Pt͑111͒ appears impenetrable to carbon diffusion, which self-limits the graphene growth at monolayer thickness. Thicker graphene domains, which can form at boundaries between monolayer domains, have been used to characterize the properties of few-layer graphene on Pt͑111͒.

Black phosphorus (BP) has attracted significant interest as a monolayer or few-layer material with extraordinary electrical and optoelectronic properties. However, degradation in air and other environments is an unresolved issue that may limit future applications. In particular the role of different ambient species has remained controversial. Here, we report systematic experiments combined with ab-initio calculations that address the effects of oxygen and water in the degradation of BP. Our results show that BP rapidly degrades whenever oxygen is present, but is unaffected by deaerated (i.e., O 2 depleted) water. This behavior is rationalized by oxidation involving a facile dissociative chemisorption of O 2 , whereas H 2 O molecules are weakly physisorbed and do not dissociate on the BP surface. Oxidation (by O 2 ) turns the hydrophobic pristine BP surface progressively hydrophilic. Our results have implications on the development of encapsulation strategies for BP, and open new avenues for exploration of phenomena in aqueous solutions including solution-gating, electrochemistry, and solutionphase approaches for exfoliation, dispersion, and delivery of BP.