Marc Walker

Sulfur‐Functionalized Graphene Oxide by Epoxide Ring‐Opening

The treatment of graphene oxide (GO) with potassium thioacetate followed by an aqueous work-up yields a new material via the ring-opening of the epoxide groups. The new material is a thiol-functionalized GO (GO-SH) which is able to undergo further functionalization. Reaction with butyl bromide gives another new material, GO-SBu, which shows significantly enhanced thermal stability compared to both GO and GO-SH. The thiol-functionalized GO material showed a high affinity for gold, as demonstrated by the selective deposition of a high density of gold nanoparticles.

Weak mismatch epitaxy and structural feedback in graphene growth on copper foil

AbstractGraphene growth by low-pressure chemical vapor deposition on low cost copper foils shows great promise for large scale applications. It is known that the local crystallography of the foil influences the graphene growth rate. Here we find an epitaxial relationship between graphene and copper foil. Interfacial restructuring between graphene and copper drives the formation of (n10) facets on what is otherwise a mostly Cu(100) surface, and the facets in turn influence the graphene orientations from the onset of growth. Angle resolved photoemission shows that the electronic structure of the graphene is decoupled from the copper indicating a weak interaction between them. Despite this, two preferred orientations of graphene are found, ±8° from the Cu[010] direction, creating a non-uniform distribution of graphene grain boundary misorientation angles. Comparison with the model system of graphene growth on single crystal Cu(110) indicates that this orientational alignment is due to mismatch epitaxy. Despite the differences in symmetry the orientation of the graphene is defined by that of the copper. We expect these observations to not only have importance for controlling and understanding the growth process for graphene on copper, but also to have wider implications for the growth of two-dimensional materials on low cost metal substrates.

Clean wurtzite InN surfaces prepared with atomic hydrogen

Conventional methods of surface preparation for III–V semiconductors, such as thermal annealing and sputtering, are severely limited for InN, resulting in In-enrichment and the introduction of donorlike defects. This is explained in terms of the unusually low Γ-point conduction band minimum of InN with respect to its Fermi stabilization energy. Here, low energy atomic hydrogen irradiation is used to produce clean wurtzite InN surfaces without such detrimental effects. A combination of x-ray photoelectron spectroscopy (XPS) and high-resolution electron-energy-loss spectroscopy was used to confirm the removal of atmospheric contaminants. Low energy electron diffraction revealed a (1×1) surface reconstruction after cleaning. Finally, XPS revealed In∕N intensity ratios consistent with a predominantly In polarity InN film terminated by In-adlayers in analogy with c-plane GaN{0001}-(1×1) surfaces.

van der Waals epitaxy of monolayer hexagonal boron nitride on copper foil: growth, crystallography and electronic band structure

We investigate the growth of hexagonal boron nitride (h-BN) on copper foil by low pressure chemical vapour deposition (LP-CVD). At low pressure, h-BN growth proceeds through the nucleation and growth of triangular islands. Comparison between the orientation of the islands and the local crystallographic orientation of the polycrystalline copper foil reveals an epitaxial relation between the copper and h-BN, even on Cu(100) and Cu(110) regions whose symmetry is not matched to the h-BN. However, the growth rate is faster and the islands more uniformly oriented on Cu(111) grains. Angle resolved photoemission spectroscopy measurements reveal a well-defined band structure for the h-BN, consistent with a band gap of 6 eV, that is decoupled from the copper surface beneath. These results indicate that, despite a weak interaction between h-BN and copper, van der Waals epitaxy defines the long range ordering of h-BN even on polycrystalline copper foils and suggest that large area, single crystal, monolayer h-BN could be readily and cheaply produced.

Covalent modification of exfoliated fluorographite with nitrogen functionalities

Fluorographene, as a completely fluorinated counterpart of graphene, holds great promise for applications in high performance materials such as batteries, dielectrics or biosensors. However, its chemical inertness and low solubility hinders its use in practical devices. Here, water-soluble exfoliated fluorographite is obtained by covalent modification with amino groups. The simplicity of our method may be easily extended to study the effect that chemical modification has on the properties of single layer fluorographene, which have not yet been studied. Moreover, we have further employed the new nitrogen functionalities to selectively bind gold nanoparticles, which may enable novel or enhanced sensing, catalytic or electrochemical applications.

Low-Voltage Voltammetric Electrowetting of Graphite Surfaces by Ion Intercalation/Deintercalation

We demonstrate low-voltage electrowetting at the surface of freshly cleaved highly oriented pyrolytic graphite (HOPG). Using cyclic voltammetry (CV), electrowetting of a droplet of a sodium perchlorate solution is observed at moderately positive potentials on high-quality (low step edge coverage) HOPG, leading to significant changes in the contact angle and relative contact diameter that are comparable to the results of the widely studied electrowetting on dielectric (EWOD) system, but over a much lower voltage range. The electrowetting behavior is found to be reasonably fast, reversible, and repeatable for at least 20 cyclic scans (maximum tested). In contrast to classical electrowetting, e.g., EWOD, the electrowetting of the droplet on HOPG occurs with the intercalation/deintercalation of anions between the graphene layers of graphite, driven by the applied potential, observed in the CV response, and detected by X-ray photoelectron spectroscopy. The electrowetting behavior is strongly influenced by those factors that affect the extent of the intercalation/deintercalation of ions on graphite, such as potential range scan rate, potential polarity, quality of the HOPG substrate (step edge density and step height), and type of anion in the solution. In addition to perchlorate, sulfate salts also promote electrowetting, but some other salts do not. Our findings suggest a new mechanism for electrowetting based on ion intercalation, and the results are important to fundamental electrochemistry as well as to diversifying the means by which electrowetting can be controlled and applied.

Antiferromagnetism at T > 500 K in the layered hexagonal ruthenate SrRu2O6

We report an experimental and computational study of the magnetic and electronic properties of the layered Ru(V) oxide SrRu2O6 (hexagonal, P3¯1m), which shows antiferromagnetic order with a Neel temperature of 563(2) K, among the highest for 4d oxides. Magnetic order occurs both within edge-shared octahedral sheets and between layers and is accompanied by anisotropic thermal expansivity that implies strong magnetoelastic coupling of Ru(V) centers. Electrical transport measurements using focused-ion-beam–induced deposited contacts on a micron-scale crystallite as a function of temperature show p-type semiconductivity. The calculated electronic structure using hybrid density functional theory successfully accounts for the experimentally observed magnetic and electronic structure, and Monte Carlo simulations reveal how strong intralayer as well as weaker interlayer interactions are a defining feature of the high-temperature magnetic order in the material.

Synthesis and controlled growth of osmium nanoparticles by electron irradiation.

We have synthesised osmium nanoparticles of defined size (1.5-50 nm) on a B- and S-doped turbostratic graphitic structure by electron-beam irradiation of an organometallic osmium complex encapsulated in self-spreading polymer micelles, and characterised them by transmission electron microscopy (TEM), high-resolution TEM (HRTEM), and atomic force microscopy (AFM) on the same grid. Oxidation of the osmium nanoparticles after exposure to air was detected by X-ray photoelectron spectroscopy (XPS).

Heteroepitaxial Growth of Ferromagnetic MnSb(0001) Films on Ge/ Si(111) Virtual Substrates

Molecular beam epitaxial growth of ferromagnetic MnSb(0001) has been achieved on high quality, fully relaxed Ge(111)/Si(111) virtual substrates grown by reduced pressure chemical vapor deposition. The epilayers were characterized using reflection high energy electron diffraction, synchrotron hard X-ray diffraction, X-ray photoemission spectroscopy, and magnetometry. The surface reconstructions, magnetic properties, crystalline quality, and strain relaxation behavior of the MnSb films are similar to those of MnSb grown on GaAs(111). In contrast to GaAs substrates, segregation of substrate atoms through the MnSb film does not occur, and alternative polymorphs of MnSb are absent.

Spectroscopic and nonlinear optical properties of the four positional isomers of 4α-(4-tert-butylphenoxy)phthalocyanine

The spectroscopic and nonlinear optical properties of the positional isomers of metal free 4α-(4-tert-butylphenoxy) phthalocyanine are presented. Second order nonlinear polarizability (β), imaginary hyperpolarizability (Im(γ)) and imaginary susceptibility (Im[χ(3)]) values were determined for the four positional isomers. The measured β values of the four isomers displayed the following trend, C4h (34.0 × 10−5 m MW−1) > D2h (28.8 × 10−5 m MW−1) > C2v (22.8 × 10−5 m MW−1) > Cs (13.7 × 10−5 m MW−1).