Interaction of magnetic transition metal dimers with spin-polarized hydrogenated graphene.

The coadsorption of hydrogen and transition metal dimers Fe2, Co2, Ni2, and FeCo on graphene is investigated using density functional theory calculations. Our work is motivated by observations that the magnetic moments of these transition metal dimers are large and that hydrogen adsorption partitions the graphene lattice into magnetic subdomains. Thus, we expect the magnetic dimers to interact strongly with the lattice. Our results show that the majority-spin direction of the lattice electronic states depends upon the dimer identity, the lattice spin polarization being in the same direction as the dimer spin polarization for Fe2 and FeCo, but opposite for Co2 and Ni2. We can understand this by examining the electronic density of states of the dimer and the lattice. We also show that coadsorption significantly increases the adsorption energies of both dimer and hydrogen leading to a more strongly-adsorbed dimer, while the bond length and magnetic moment of the upper dimer atom, the latter important for potential magnetic storage applications, are negligibly changed. Our work shows that the coadsorbed hydrogen and metal dimer interact over a long-range, this interaction being mediated by the hydrogen-induced spin-polarization of the graphene lattice. We obtain general insight into how the elemental identity of these magnetic dimers determines the spin-polarized states on the hydrogenated graphene lattice. These results could be important for potential applications of magnetic properties of decorated graphene lattices.

Anomalous Ferromagnetism of quasiparticle doped holes in cuprate heterostructures revealed using resonant soft X-ray magnetic scattering.

We report strong ferromagnetism of quasiparticle doped holes both within the ab-plane and along the c-axis of Cu-O planes in low-dimensional Au/d-La1.8Ba0.2CuO4/LaAlO3(001) heterostructures (d = 4, 8 and 12 unit-cells) using resonant soft X-ray and magnetic scattering together with X-ray magnetic circular dichroism. Interestingly, ferromagnetism is stronger at a hole doped peak and at an upper Hubbard band of O with spin-polarization degree as high as 40%, revealing strong ferromagnetism of Mottness. For in-ab-plane spin-polarizations, the spin of doped holes in O2p-Cu3d-O2p is a triplet state yielding strong ferromagnetism. For out-of-ab-plane spin-polarization, while the spins of doped holes in both O2p-O2p and Cu3d-Cu3d are triplet states, the spin of doped holes in Cu3d-O2p is a singlet state yielding ferrimagnetism. A ferromagnetic-(002) Bragg-peak of the doped holes is observed and enhanced as a function of d revealing strong ferromagnetism coupling between Cu-O layers along the c-axis.

Kinetics of Ge diffusion, desorption and pit formation dynamics during annealing of Si(0.8)Ge(0.2)/Si(001) virtual substrates.

Thermal stability of Si(0.8)Ge(0.2)/Si(001) virtual substrates (VS) is studied as a function of annealing temperature and time by X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM). Two regimes describing different Ge behavior are observed when the Si(0.8)Ge(0.2) VS is annealed. Heating the substrate from room temperature to 500 degrees C results in some degree of Ge segregation. The surface morphology however remains relatively smooth and there is no formation of 3D islands on the surface. Above 500 degrees C, Ge is preferentially lost from the surface and microscopic pits with edges aligned along 110 azimuth are formed. As temperature increases, Ge% decreases and the size of pits also increases. The decrease in Ge% and the formation of holes at the surface are attributed to Ge desorption from the surface. A kinetic model involving diffusion and desorption processes is proposed to describe the Ge behavior and pits formation in this regime.

Revisiting the vibrational spectra of silicon hydrides on Si(100)-(2x1) surface: What is on the surface when disilane dissociates?

Even though the decomposition of disilane on silicon surfaces has been extensively studied, the molecular mechanism for its decomposition has not been fully resolved. The general view motivated partly by spectroscopic data is that decomposition occurs through silicon-silicon bond dissociation although there is evidence from kinetics that silicon-hydrogen bond dissociation is important, and perhaps even dominant. Thus, we reexamine the assignment of the experimental vibrational peaks observed in disilane and silane adsorption in order to assess the evidence for the silicon hydride species that are formed during decomposition. We calculate the vibrational density of states for a number of silicon hydride species on the Si(100)-(2x1) surface using Car-Parrinello molecular dynamics. We obtain the calculated vibrational frequency in the adiabatic limit by extrapolating to zero orbital mass, calibrating our method using the well-established monohydride peak. The calculated vibrational frequencies of the monohydride are in good agreement experimental data. Our results show that the spectroscopic data for silicon hydrides does not preclude the occurrence of Si(2)H(5) on the surface thus providing evidence for silicon-hydrogen bond dissociation during disilane adsorption. Specifically, we find that an experimentally observed vibrational peak at 2150 cm(-1) that has generally been attributed to the trihydride SiH(3) is more likely to be due to Si(2)H(5). Our results also clear up the assignment of two peaks for monohydride species adsorbed at the edge of a growing terrace, and a peak for the dihydride species adsorbed in the interdimer configuration.

Molecular mechanisms for disilane chemisorption on Si(100)-(2 x 1).

The dissociative chemisorption of disilane is an important elementary process in the growth of silicon films. Although factors governing the rate of film growth such as surface temperature and disilane flux have been extensively studied experimentally by a large number of groups, the molecular mechanism for disilane adsorption is not well established. In particular, although it is generally held that chemisorption occurs via silicon-silicon bond dissociation, there have been a number of suggestions that silicon-hydrogen bond dissociation also occurs. We consider this issue in detail hereby examining a number of different paths that disilane can take to chemisorb. In addition to silicon-silicon bond dissociation paths, we examine three different mechanisms for silicon-hydrogen bond dissociation, for each path considering both adsorption at interdimer and intradimer sites. The calculated barriers are critically compared to experimental data. We conclude that silicon-hydrogen bond dissociation is likely, finding two zero barrier paths for chemisorption at interdimer sites, and a precursor-mediated path with a low barrier. We also find two precursor states, and show that each can lead to chemisorption via either silicon-silicon or silicon-hydrogen bond dissociation. Finally, we calculated the barriers for reaction of coadsorbed disilyl and hydrogen to form gas phase silane. Our calculations are performed using density-functional theory within a planewave ultrasoft pseudopotential methodology. We traced the reaction paths with the nudged-elastic band technique.

Disilane chemisorption on Si(x)Ge(1-x)(100)-(2 x 1): molecular mechanisms and implications for film growth rates.

At low temperatures, hydrogen desorption is known to be the rate-limiting process in silicon germanium film growth via chemical vapor deposition. Since surface germanium lowers the hydrogen desorption barrier, Si(x)Ge((1-x)) film growth rate increases with the surface germanium fraction. At high temperatures, however, the molecular mechanisms determining the epitaxial growth rate are not well established despite much experimental work. We investigate these mechanisms in the context of disilane adsorption because disilane is an important precursor used in film growth. In particular, we want to understand the molecular steps that lead, in the high temperature regime, to a decrease in growth rate as the surface germanium increases. In addition, there is a need to consider the issue of whether disilane adsorbs via silicon-silicon bond dissociation or via silicon-hydrogen bond dissociation. It is usually assumed that disilane adsorption occurs via silicon-silicon bond dissociation, but in recent work we provided theoretical evidence that silicon-hydrogen bond dissociation is more important. In order to address these issues, we calculate the chemisorption barriers for disilane on silicon germanium using first-principles density functional theory methods. We use the calculated barriers to estimate film growth rates that are then critically compared to the experimental data. This enables us to establish a connection between the dependence of the film growth rate on the surface germanium content and the kinetics of the initial adsorption step. We show that the generally accepted mechanism where disilane chemisorbs via silicon-silicon bond dissociation is not consistent with the data for film growth kinetics. Silicon-hydrogen bond dissociation paths have to be included in order to give good agreement with the experimental data for high temperature film growth rate.

Sputter roughening of inhomogeneous surfaces: impurity pinning and nanostructure shape selection.

A model is proposed for sputter roughening of inhomogeneous systems with slowly sputtered impurity particles randomly distributed in the bulk. Surface inhomogeneity, which develops as a result of coupling between the time evolution of the local surface impurity concentration and the local surface shape, is tuned by changing the dependence of the sputtering probability upon impurity concentration. In 1+1 dimensions, we find long-time scaling exponents that are consistent with Kardar-Parisi-Zhang (KPZ) values. However, for a range of surface inhomogeneity, impurity pinning results in a persistent growth regime where the surface roughens rapidly. We correlate this rapid roughening to fluctuations of the impurity concentration at the surface. Roughening in this regime leads to the formation of cones whose shape is determined by material property and sputtering flux, suggesting a unique method of nanostructure fabrication. In 2+1 dimensions, a similar variation of the roughening behavior with surface inhomogeneity is observed. For small surface inhomogeneity, there is an initial exponential roughening followed by power-law roughening with an effective growth exponent much smaller than KPZ. For larger surface inhomogeneity two power-law roughening regimes are observed, with an initial rapid roughening that crosses over to slower roughening; the effective exponent in each of these regimes increases with surface inhomogeneity. The surface morphology observed in the simulations is considerably noisier than experimental data for InP and GaSb. Our model shows noisy nonlinear pattern formation in contrast to the marked long-range hexagonal ordering seen in experiments. However, the scaling behavior is robust enough that roughening kinetics similar to that observed experimentally can be obtained depending upon the values of inhomogeneity and the strength of the nonlinear term in the model.

Dynamical scaling of sputter-roughened surfaces in 2+1 dimensions.

The asymptotic scaling behavior of sputter-roughened surfaces is of great current interest. In particular, the disparately wide-ranging values of the growth exponent found experimentally, and whether sputter-roughening belongs to the Kardar-Parisi-Zhang universality class in 2+1 dimensions, are two interesting issues. We address these issues using simulations of an atomistic model. The asymptotic scaling appears to be Edwards-Wilkinson. Crossover behavior in the model leads to effective growth exponents that vary widely depending upon the regime of observation.

α-Fe2O3 nanotubes-reduced graphene oxide composites as synergistic electrochemical capacitor materials.

We present a facile approach for the fabrication of a nanocomposite comprising α-Fe(2)O(3) nanotubes (NTs) anchored on reduced graphene oxide (rGO) for electrochemical capacitors (ECs). The hollow tubular structure of the α-Fe(2)O(3) NTs presents a high surface area for reaction, while the incorporation of rGO provides an efficient two-dimensional conductive pathway to allow fast, reversible redox reaction. As a result, the nanocomposite materials exhibit a specific capacitance which is remarkably higher (~7 times) than α-Fe(2)O(3) NTs alone. In addition, the nanocomposites show excellent cycling life and large negative potential window. These findings suggest that such nanocomposites are a promising candidate as negative electrodes in asymmetrical capacitors with neutral electrolytes.

Configuration dependent critical nuclei in the self assembly of magic clusters.

Evidence for the formation of various 2-D structures possessing different numbers of Co-Si magic clusters (size approximately 10.0 +/- 0.5 A), configurations and lifetimes are studied in real time on a Si(111)-(7 x 7) surface at elevated temperature in the STM. Observations of individual cluster diffusion, attachment and detachment dynamics resolve unequivocally the question of self assembly over surface reconstruction. The smallest stable structure consisting of seven individual Co-Si magic clusters arranged in a hexagonal closed packed formation (i = 7) is found to retain sufficient cohesive energy to avoid dissociation. A configuration dependent critical 2-D nuclei (i* = 6) is determined to exist in facilitating the self assembly dynamics.

The dissociative adsorption of silane and disilane on Si(100)-(2 x 1).

We investigate the dissociative adsorption of silane and disilane on Si(100)-(2 x 1) using pseudopotential planewave density functional theory calculations. These are important steps in the growth of silicon films. Although silane has been studied computationally in some detail previously, we find physisorbed precursor states for the intradimer and interdimer channels. The silane energetics calculated here are in good agreement with experimental data and previous theoretical estimates and provide us with a useful reference point for our disilane calculations. Disilane has not been studied as intensively as silane. We investigate both silicon-silicon bond cleavage and silicon-hydrogen bond cleavage mechanisms, and for each we investigate intradimer, interdimer, and inter-row channels. As in the case of silane, we also find precursor states in the adsorption path in agreement with molecular beam experiments. The qualitative picture that emerges is that adsorption takes place through a weakly bound precursor state with a transition state to chemisorption that is low lying in energy relative to the gas phase. This is in good agreement with experimental data. However, the calculated energetics are only in fair agreement with experiments, with our transition state to chemisorption being about 0.02 eV above the gas phase while experimentally it is estimated to be approximately 0.28 eV below the gas phase. This suggests that accurate theoretical characterization of these weakly bound precursor states and the adsorption barriers requires further computational work.

Reassessment of the molecular mechanisms for H2 thermal desorption pathways from Si(1-x)Gex(001)-(2x1) surfaces.

One of the aims of temperature-programmed desorption experiments is to facilitate identification of molecular pathways for desorption. The authors provide a rigorous assessment of the difficulty of doing this for H(2)/Si((1-x))Ge(x)(100)-(2x1). An extensive series of density functional calculations using both cluster and slab methods is performed. The resulting desorption barriers are used to compute thermal desorption spectra. A mean-field approximation is used to treat the populations of the various adsites present on the surface. The authors find a number of significant results. First, slab and cluster calculations do not appear to predict consistent differences in desorption barriers between intradimer and interdimer channels. Second, they find that a germanium atom affects the desorption barrier significantly only if it is present at the adsite. A germanium atom adjacent to an adsite or in the second layer influences the desorption barrier negligibly. Both cluster and slab calculations consistently predict a decrease of approximately 0.3-0.4 eV per germanium atom at the adsite. Third, current analysis of thermal desorption spectra in the literature, although yielding good fits to experimental data, is not rigorous. The authors' calculated spectra can be fitted rather well by assuming, as in current analysis of experimental data, three independent second-order channels, even though the underlying molecular pathways used to calculate the spectra are considerably different. Fourth, the authors' results highlight the importance of treating the rearrangement of hydrogen and germanium atoms at the surface during the thermal desorption process. This is generally not taken into account in kinetics modeling of desorption spectra.

Nucleation and growth of cobalt nanostructures on highly oriented pyrolytic graphite.

Cobalt in the form of three-dimensional (3D) hemispherical clusters (size approximately 10-30 nm) were observed to grow on pristine graphite surfaces via a Volmer-Weber growth mode. X-Ray photoelectron spectroscopy (XPS) reveals that these clusters are physisorbed on the surface. In the presence of minute surface contamination, the morphology of Co changes into a mixture of irregular and hemispherical three-dimensional islands. The formation of irregular islands appears to be mediated by the chemical interactions between Co and the surface contaminants as evidenced from analysis of the carbon pi-pi* transitions. Further analysis of size distribution of Co nanoclusters grown on pristine surfaces shows a critical nucleus size of i* = 1, i.e. a Co dimer forms the smallest stable cluster on a pristine graphite surface.

Evidence for hydrogen desorption through both interdimer and intradimer paths from Si(100)-(2 x 1).

Despite intensive work there are still controversial issues about desorption and adsorption of hydrogen on Si(100)-(2 x 1). In particular, the relative importance of the various interdimer- and intradimer-desorption paths is not clear. Nanosecond-pulse-laser desorption data have been used to argue that the 4H interdimer path is important, while data from thermal-desorption time-of-flight measurements suggest a large translationally hot contribution which cannot arise from the 4H interdimer path. The observation of a translationally hot desorption fraction at low to medium coverage can be accounted for by including the 2H interdimer path in quantum dynamical calculations. In this paper we investigate this issue further and present evidence that supports the inclusion of the intradimer path. Specifically, our results show that the intradimer and 3H interdimer paths provide the major contributions to the translationally hot fraction in the desorbate. Our conclusions are based on density-functional calculations of hydrogen translational excitation, mean-field analysis of thermal-desorption experiments over a range of ramp rate, and Monte Carlo simulations of nanosecond-pulse-laser experiments.

Hydrogen desorption kinetics from the Si(1-x)Gex(100)-(2x1) surface.

We study the influence of germanium atoms upon molecular hydrogen desorption energetics using density functional cluster calculations. A three-dimer cluster is used to model the Si((1-x))Ge(x)(100)-(2x1) surface. The relative stabilities of the various monohydride and clean surface configurations are computed. We also compute the energy barriers for desorption from silicon, germanium, and mixed dimers with various neighboring configurations of silicon and germanium atoms. Our results indicate that there are two desorption channels from mixed dimers, one with an energy barrier close to that for desorption from germanium dimers and one with an energy barrier close to that for desorption from silicon dimers. Coupled with the preferential formation of mixed dimers over silicon or germanium dimers on the surface, our results suggest that the low barrier mixed dimer channel plays an important role in hydrogen desorption from silicon-germanium surfaces. A simple kinetics model is used to show that reasonable thermal desorption spectra result from incorporating this channel into the mechanism for hydrogen desorption. Our results help to resolve the discrepancy between the surface germanium coverage found from thermal desorption spectra analysis, and the results of composition measurements using photoemission experiments. We also find from our cluster calculations that germanium dimers exert little influence upon the hydrogen desorption barriers of neighboring silicon or germanium dimers. However, a relatively larger effect upon the desorption barrier is observed in our calculations when germanium atoms are present in the second layer.