Eli Sutter

Epitaxial graphene on ruthenium

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.

Why is anatase a better photocatalyst than rutile? - Model studies on epitaxial TiO2 films

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.

Enhanced Electrocatalytic Performance of Processed, Ultrathin, Supported Pd–Pt Core–Shell Nanowire Catalysts for the Oxygen Reduction Reaction

We report on the synthesis, characterization, and electrochemical performance of novel, ultrathin Pt monolayer shell-Pd nanowire core catalysts. Initially, ultrathin Pd nanowires with diameters of 2.0 ± 0.5 nm were generated, and a method has been developed to achieve highly uniform distributions of these catalysts onto the Vulcan XC-72 carbon support. As-prepared wires are activated by the use of two distinctive treatment protocols followed by selective CO adsorption in order to selectively remove undesirable organic residues. Subsequently, the desired nanowire core-Pt monolayer shell motif was reliably achieved by Cu underpotential deposition followed by galvanic displacement of the Cu adatoms. The surface area and mass activity of the acid and ozone-treated nanowires were assessed, and the ozone-treated nanowires were found to maintain outstanding area and mass specific activities of 0.77 mA/cm(2) and 1.83 A/mg(Pt), respectively, which were significantly enhanced as compared with conventional commercial Pt nanoparticles, core-shell nanoparticles, and acid-treated nanowires. The ozone-treated nanowires also maintained excellent electrochemical durability under accelerated half-cell testing, and it was found that the area-specific activity increased by ~1.5 fold after a simulated catalyst lifetime.

Size-Dependent Enhancement of Electrocatalytic Performance in Relatively Defect-Free, Processed Ultrathin Platinum Nanowires

We report on the synthesis, characterization, and electrocatalytic performance of ultrathin Pt nanowires with a diameter of less than 2 nm. An acid-wash protocol was employed in order to yield highly exfoliated, crystalline nanowires with a diameter of 1.3 +/- 0.4 nm. The electrocatalytic activity of these nanowires toward the oxygen reduction reaction was studied in relation to the activity of both supported and unsupported Pt nanoparticles as well as with previously synthesized Pt nanotubes. Our ultrathin, acid-treated, unsupported nanowires displayed an electrochemical surface area activity of 1.45 mA/cm(2), which was nearly 4 times greater than that of analogous, unsupported platinum nanotubes and 7 times greater than that of commercial supported platinum nanoparticles.

Chemistry under Cover: Tuning Metal―Graphene Interaction by Reactive Intercalation

Intercalation of metal atoms is an established route for tuning the coupling of graphene to a substrate. The extension to reactive species such as oxygen would set the stage for a wide spectrum of interfacial chemistry. Here we demonstrate the controlled modification of a macroscopic graphene-metal interface by oxygen intercalation. The selective oxidation of a ruthenium surface beneath graphene lifts the strong metal-carbon coupling and restores the characteristic Dirac cones of isolated monolayer graphene. Our experiments establish the competition between low-temperature oxygen intercalation and graphene etching at higher temperatures and suggest that small molecules can populate the space between graphene and metals, with the adsorbate-metal interaction being modified significantly by the presence of graphene. These findings open up new avenues for the processing of graphene for device applications and for performing chemical reactions in the confined space between a metal surface and a graphene sheet.

Interface Formation in Monolayer Graphene-Boron Nitride Heterostructures

The ability to control the formation of interfaces between different materials has become one of the foundations of modern materials science. With the advent of two-dimensional (2D) crystals, low-dimensional equivalents of conventional interfaces can be envisioned: line boundaries separating different materials integrated in a single 2D sheet. Graphene and hexagonal boron nitride offer an attractive system from which to build such 2D heterostructures. They are isostructural, nearly lattice-matched, and isoelectronic, yet their different band structures promise interesting functional properties arising from their integration. Here, we use a combination of in situ microscopy techniques to study the growth and interface formation of monolayer graphene-boron nitride heterostructures on ruthenium. In a sequential chemical vapor deposition process, boron nitride grows preferentially at the edges of existing monolayer graphene domains, which can be exploited for synthesizing continuous 2D membranes of graphene embedded in boron nitride. High-temperature growth leads to intermixing near the interface, similar to interfacial alloying in conventional heterostructures. Using real-time microscopy, we identify processes that eliminate this intermixing and thus pave the way to graphene-boron nitride heterostructures with atomically sharp interfaces.

Tin Disulfide—An Emerging Layered Metal Dichalcogenide Semiconductor: Materials Properties and Device Characteristics

Layered metal dichalcogenides have attracted significant interest as a family of single- and few-layer materials that show new physics and are of interest for device applications. Here, we report a comprehensive characterization of the properties of tin disulfide (SnS2), an emerging semiconducting metal dichalcogenide, down to the monolayer limit. Using flakes exfoliated from layered bulk crystals, we establish the characteristics of single- and few-layer SnS2 in optical and atomic force microscopy, Raman spectroscopy and transmission electron microscopy. Band structure measurements in conjunction with ab initio calculations and photoluminescence spectroscopy show that SnS2 is an indirect bandgap semiconductor over the entire thickness range from bulk to single-layer. Field effect transport in SnS2 supported by SiO2/Si suggests predominant scattering by centers at the support interface. Ultrathin transistors show on-off current ratios >10(6), as well as carrier mobilities up to 230 cm(2)/(V s), minimal hysteresis, and near-ideal subthreshold swing for devices screened by a high-k (deionized water) top gate. SnS2 transistors are efficient photodetectors but, similar to other metal dichalcogenides, show a relatively slow response to pulsed irradiation, likely due to adsorbate-induced long-lived extrinsic trap states.

Electronic Structure of Few-Layer Epitaxial Graphene on Ru(0001)

The electronic structure of epitaxial monolayer, bilayer, and trilayer graphene on Ru(0001) was determined by selected-area angle-resolved photoelectron spectroscopy (micro-ARPES). Micro-ARPES band maps provide evidence for a strong electronic coupling between monolayer graphene and the adjacent metal, which causes the complete disruption of the graphene pi-bands near the Fermi energy. However, the perturbation by the metal decreases rapidly with the addition of further graphene sheets, and already an epitaxial graphene bilayer on Ru recovers the characteristic Dirac cones of isolated monolayer graphene. A graphene trilayer on Ru behaves like free-standing bilayer graphene. Density-functional theory based calculations show that this decoupling is due to the efficient passivation of metal d-states by the interfacial graphene layer.

Chemical Vapor Deposition and Etching of High-Quality Monolayer Hexagonal Boron Nitride Films

The growth of large-area hexagonal boron nitride (h-BN) monolayers on catalytic metal substrates is a topic of scientific and technological interest. We have used real-time microscopy during the growth process to study h-BN chemical vapor deposition (CVD) from borazine on Ru(0001) single crystals and thin films. At low borazine pressures, individual h-BN domains nucleate sparsely, grow to macroscopic dimensions, and coalescence to form a closed monolayer film. A quantitative analysis shows borazine adsorption and dissociation predominantly on Ru, with the h-BN covered areas being at least 100 times less reactive. We establish strong effects of hydrogen added to the CVD precursor gas in controlling the in-plane expansion and morphology of the growing h-BN domains. High-temperature exposure of h-BN/Ru to pure hydrogen causes the controlled edge detachment of B and N and can be used as a clean etching process for h-BN on metals.

Interface and Wetting Layer Effect on the Catalyst-Free Nucleation and Growth of GaN Nanowires†

To avoid catalyst-induced contaminations that might alter the electronic properties of the material, catalyst-free growth is preferable. However, the nucleation and growth mechanisms of GaN wires in the catalyst-free procedure are still under debate. Two mechanisms are usually invoked for the nucleation and the growth of NWs. One is based on the Ga-droplet formation followed by the well-established vapor–liquid–solid mechanism, [13] and the other is based on small GaN clusters as nucleation seeds and a vapor–solid growth process. [12] In the present work the formation of crystalline GaN nanoclusters as possible NW precursors in catalyst-free plasma-assisted MBE (PAMBE) growth is studied by high-resolution transmission electron microscopy (HRTEM) imaging. Details of the interface between the GaN layer and the substrate are investigated and discussed in connection with the mechanism of catalyst-free NW growth. GaN NWs were grown at 7808C by PAMBE without the use of catalysts, on clean Si(111) and oxidized Si(100) substrates, according to the procedure described elsewhere. [6] The GaN NWs were investigated by HRTEM using a JEOL 3000F