Large positive in-plane magnetoresistance induced by localized states at nanodomain boundaries in graphene

Graphene supports long spin lifetimes and long diffusion lengths at room temperature, making it highly promising for spintronics. However, making graphene magnetic remains a principal challenge despite the many proposed solutions. Among these, graphene with zig-zag edges and ripples are the most promising candidates, as zig-zag edges are predicted to host spin-polarized electronic states, and spin-orbit coupling can be induced by ripples. Here we investigate the magnetoresistance of graphene grown on technologically relevant SiC/Si(001) wafers, where inherent nanodomain boundaries sandwich zig-zag structures between adjacent ripples of large curvature. Localized states at the nanodomain boundaries result in an unprecedented positive in-plane magnetoresistance with a strong temperature dependence. Our work may offer a tantalizing way to add the spin degree of freedom to graphene

Application of single crystalline tungsten for fabrication of high resolution STM probes with controlled structure

The possibility to fabricate scanning tunneling microscopy (STM) probes with controlled electronic structure using single crystalline tungsten tips is discussed. High resolution power of oriented single crystalline probes is demonstrated in atomic and subatomic resolution STM studies of silicon, gallium telluride and graphite surfaces. The possibility of controllable selection of the tungsten tip atom electron orbitals responsible for the surface imaging in STM experiments is demonstrate

Fe Nanoclusters on the Ge(001) Surface Studied by Scanning Tunnelling Microscopy, Density Functional Theory Calculations and X-Ray Magnetic Circular Dichroism

The growth of Fe nanoclusters on the Ge(001) surface has been studied using low-temperature scanning tunneling microscopy (STM) and density functional theory (DFT) calculations. STM results indicate that Fe nucleates on the Ge(001) surface, forming well-ordered nanoclusters of uniform size. Depending on the preparation conditions, two types of nanoclusters were observed having either four or sixteen Fe atoms within a nanocluster. The results were confirmed by DFT calculations. Annealing the nanoclusters at 420 K leads to the formation of nanorow structures, due to cluster mobility at such temperature. The Fe nanoclusters and nanorow structures formed on the Ge(001) surface show a superparamagnetic behaviour as measured by X-ray magnetic circular dichroism

Selecting the tip electron orbital for scanning tunneling microscopy imaging with sub-?ngstrom lateral resolution

We report on scanning tunneling microscopy (STM) studies performed with single crystalline W[001] tips on a graphite(0001) surface. Results of distance-dependent STM experiments with sub-?ngstrom lateral resolution and density functional theory electronic structure calculations show how to controllably select one of the tip electron orbitals for high-resolution STM imaging. This is confirmed by experimental images reproducing the shape of the 5dxz,yz and 5dx2 ? y2 tungsten atomic orbitals. The presented data demonstrate that the application of oriented single crystalline probes can provide further control of spatial resolution and expand the capabilities of STM

Finite element method simulations of heat flow in fixed bed solar water splitting redox reactors

An improved design for radiation absorption and heat flow into materials with low thermal conductivity is demonstrated. The design was developed for application in fixed bed two step solar water splitting redox reactors. The fixed bed was assumed to be made from porous ceramic. The low thermal conductivity of the porous ceramic redox material is compensated for by changing the profile of the fixed bed. The profiling used was wedges cut into the material which allows concentrated solar radiation to be incident on a larger area of redox material than for a flat monolith design. The design is demonstrated to efficiently transfer heat to the bulk and greatly reduce re-radiation. For a wedge 9 cm in depth and 1.6 cm wide at the opening, heated with 500 kWm 2 incident radiation for 300 s, approximately double the amount of radiation is absorbed. The effects of thermal conductivity, emissivity and scaling on the efficiency of the design were investigated. The radiation absorption performance improved when scaled up. The improvement of the design over a flat plain bed is greater for lower emissivity. The improvement provided by the wedge design was found to decrease for increasing thermal conductivity, and eventually for high conductivity values it reduced performance. Using this method a larger amount of material with low thermal conductivity can be heated with the same power input and reduced radiation losses. The heat flow simulations were then coupled to an Arrhenius rate equation to investigate the possible improvement to the reaction efficiency and yield offered by the wedge design. Over a time of 300 s the efficiency and yield were seen to be approximately a factor of 4 times higher in the wedge case. Finally a concentrated solar cavity reactor based on the design is proposed

Growth and ordering of Ni(II) diphenylporphyrin monolayers on Ag(111) and Ag/Si(111) studied by STM and LEED

The room temperature self-assembly and ordering of (5,15-diphenylporphyrinato)nickel(II) (NiDPP) on the Ag(111) and Ag/Si(111)-(?3 ? ?3)R30? surfaces have been investigated using scanning tunnelling microscopy and low-energy electron diffraction. The self-assembled structures and lattice parameters of the NiDPP monolayer are shown to be extremely dependent on the reactivity of the substrate, and probable molecular binding sites are proposed. The NiDPP overlayer on Ag(111) grows from the substrate step edges, which results in a single-domain structure. This close-packed structure has an oblique unit cell and consists of molecular rows. The molecules in adjacent rows are rotated by approximately 17? with respect to each other. In turn, the NiDPP molecules form three equivalent domains on the Ag/Si(111)-(?3 ? ?3)R30? surface, which follow the three-fold symmetry of the substrate. The molecules adopt one of three equivalent orientations on the surface, acting as nucleation sites for these domains, due to the stronger molecule?substrate interaction compared to the case of the Ag(111). The results are explained in terms of the substrate reactivity and the lattice mismatch between the substrate and the molecular overlayer

Rotational transitions in a C60 monolayer on the WO2/W(110) surface

Variable-temperature scanning tunneling microscopy (STM) is shown to be an effective technique to study two-dimensional phase transitions. Observations show that a monolayer of C60 deposited on an ultrathin WO2 layer grown on the W(110) surface undergoes a structural phase transition at 259 K, similar in temperature to that of bulk C60. In turn, a kinetic transition has been observed at 220 K, which is significantly higher than that of the bulk C60 crystal (90 K). This difference is attributed to interactions between the molecular overlayer and the substrate, as well as correlation effects within the C60 film. Different types of molecular nanomotion, such as rotation, spinning, and switching between different orientations, have been observed. STM measurements are supported by density functional theory calculations, which provide confirmation of different orientations of C60 on the WO2 thin film

Self-assembly of Fe nanocluster arrays on templated surfaces

The growth of Fe nanoclusters on the Ge(001) and MoO2/Mo(110) surfaces has been studied using low-temperature scanning tunneling microscopy (STM) and X-ray magnetic circular dichroism (XMCD). STM results indicate that at low coverage Fe atoms self-assemble on both surfaces into well-separated nanoclusters, which nucleate at equivalent surface sites. Their size, shape, and the observed spatial separation are dictated by the substrate and depend on preparation conditions. Annealing the Fe nanoclusters on Ge(001) at 420 K leads to the formation of linear nanocluster arrays, which follow the Ge dimer rows of the substrate, due to cluster mobility at such temperature. In turn, linear Fe nanocluster arrays are formed on the MoO2/Mo(110) surface at room temperature at a surface coverage greater than 0.5 monolayer. This is due to the more pronounced row pattern of the MoO2/Mo(110) surface compared to Ge(001). These nanocluster arrays follow the direction of the oxide rows of the strained MoO2/Mo(110) surface. The Fe nanoclusters formed on both surfaces show a superparamagnetic behavior as measured by XMCD. (C) 2012 American Institute of Physics. [doi:10.1063/1.3676207