Anomalous scaling behaviour of cobalt cluster size distributions on graphite,epitaxial graphene and carbon-rich (6√3 × 6√3)R30°

The influence of the underlying interface on adsorption of cobalt (Co) is investigated by comparing the nucleation and growth of Co at room temperature on three carbon (C) surfaces, i.e. highly oriented pyrolytic graphite (HOPG), epitaxial graphene/SiC(0001) (hereafter abbreviated as EG) and precursor of EG i.e. C-rich (6√3 × 6√3)R30°/SiC(0001) (hereafter abbreviated as 6√3). On all three surfaces, Co adopts Volmer–Weber growth mode via formation of three-dimensional dome-shaped nanoclusters. Co clusters formed on 6√3 surface are smaller but denser than Co/HOPG or Co/EG. Scaling analysis reveals a critical nucleus size, i* = 1 (atom) and the smallest stable cluster (i* + 1) would be a dimer. Co/HOPG and Co/EG have the same order of magnitude for their cluster densities and sizes. Scaling analyses however show that the i* for Co/EG (i* = 3) is larger than Co/HOPG (i* = 0) and in this respect the smallest stable cluster would be tetramer and monomer respectively. This difference is attributed to the influence of an interface situated between graphene and SiC bulk. It appears that EG is more inert than HOPG towards the adsorption of Co and may act as a better substrate to host Co clusters.     

Hydrogen bonds induced supramolecular self-assembly of azobenzene derivatives on the highly oriented pyrolytic graphite surface

The self-assembly of azobenzene derivatives (C_nAzCOOH) with various lengths of peripheral alkyl chains (with carbon number of n = 8, 10, 12, 14, 16) were observed by scanning tunneling microscopy on highly oriented pyrolytic graphite (HOPG) surface. The effect of van der Waals interactions and the intermolecular hydrogen bonding on the two-dimensional self-assembly was systematically studied. No alkyl-chain length effect was observed according to the STM images. All kinds of C_nAzCOOH adopting the same pattern self-assembled on the HOPG surface, suggesting the formation of the two-dimensional structures was dominated by the hydrogen bonding of the functional groups. It could be found that two C_nAzCOOH molecules formed a hydrogen-bonded dimer with “head-to-head” fashion as expected; however, the dimers organized themselves in the form of relative complex lamellae. Three dimers as a group arranged side by side and formed a well-defined stripe with periodic dislocations due to the registry mechanism of the alkyl chain with the underlying HOPG surface. The hydrogen bonds between the adjacent dimers in one lamella were formed and dominated the self-assembled pattern.     

Theoretical understanding of adlayer structure,thermal stability and electronic property of graphene molecules

The geometry of hexafluorotribenzo[a,g,m]coronene with n-carbon alkyl chains [FTBC-Cn (n = 4, 6, 8, 12)] and their supramolecule self-assembly on a highly oriented pyrolytic graphite (HOPG) surface has been optimized by molecular dynamics simulations using COMPASS force field at 0 K, 298 K, 333 K and 353 K. Electronic properties and intermolecular interactions in graphene supramolecule assembly have been studied by the first principle methods based on the density functional theory (DFT). It is indicated that the thermal stability and electronic properties of graphene molecules can be tunable by attaching alkyl chains to a triangular graphene sheet, and changing the length of the alkyl chain, and self-assembling on a certain substrate. The main results are as follows. The geometry and energy gap of the FTBC-Cn single molecule and their supramolecule self-assembly on HOPG are both stable with the changes of the temperature from 0 K to 353 K and the number of carbon atoms on the alkyl chain. The simulation results of geometry, energy gap as well as STM images of graphene supramolecule assembly are in good agreement with the corresponding experimental results in room temperature. Furthermore, the electronic properties of graphene supramolecule assembly at the temperatures of 0 K, 333 K and 353 K are also predicted. When a triangular graphene molecule attached with six alkyl chains, the energy gaps are increased and stabilized at the temperature from 0 K to 353 K. After FTBC-Cn molecule self-assembly on a HOPG substrate, the energy gap is reduced but still stable.     

H2 splitting on Pt,Ru and Rh nanoparticles supported on sputtered HOPG

The equilibrium hydrogen exchange rate between adsorbed and gas phase hydrogen at 1 bar is measured for Pt, Ru and Rh nanoparticles supported on a sputtered HOPG substrate. The particles are prepared by Electron Beam Physical Vapor Deposition and the diameter of the particles varies between 2 and 5 nm. The rate of hydrogen exchange is measured in the temperature range 40–200 °C at 1 bar, by utilization of the H–D exchange reaction. We find that the rate of hydrogen exchange increases with the particle diameter for all the metals, and that the rate for Ru and Rh is higher than for Pt. In the case of Pt, the equilibrium dissociative sticking probability, S, is found to be nearly independent of particle diameter. For Ru and Rh, S is found to depend strongly on particle diameter, with the larger particles being more active. The apparent energy of desorption at equilibrium, E_app, shows a dramatic increase with decreasing particle diameter for diameters below 5 nm for Ru and Rh, whereas E_app is only weakly dependent on particle diameter for Pt. We suggest that the strong variation in the apparent desorption energy with particle diameter for Ru and Rh is due to the formation of compressed hydrogen adlayers on the terraces of the larger particles. Experiments are also carried out in the presence of 10 ppm CO. Pt is found to be very sensitive to CO poisoning and the H–D exchange rate drops below the detection limit when CO is added to the gas mixture. In the case of Ru and Rh nanoparticles, CO decreases the splitting rate significantly, also at 200 °C. The variation of the sensitivity to CO poisoning with particle diameter for Ru and Rh is found to be weak.     

Pt-chain induced formation of Ge nanowires on the Ge(001) surface

A new reconstructed Pt/Ge(001)–4 × 2 surface structure of 0.25 ML Pt deposition is suggested based on density functional theory. The Ge dimers form nanowire arrays on a Pt-chain modified Ge(001) surface in which the chain is located between the two quasi-dimer rows and below the Ge nanowire. The simulated scanning tunneling microscope (STM) images of the surface are in excellent agreement with the previously observed STM features and sample bias dependence. It is the nanowire Ge dimers and not the Pt atoms that contribute to the STM images for occupied states at high sample biases, contrary to what has always been assumed in experiments. The surface bands of the Pt chain and quasi-dimer rows exhibit quasi-one-dimensional metallic behavior in the direction of the nanowire. When changing from the 4 × 2 to the 4 × 4 structure, there are likely pseudogaps opened at the new surface Brillouin zone boundary, which simultaneously reduce the metallicity. This may be related to the Peierls instability. The interaction between the Pt chain and the quasi-dimer row, as well as the inter-quasi-dimer row interaction, is of essential importance for stabilization.     

Dependence of mark size on STM pulse voltage in heat-assisted magnetic probe recording on a cobalt–nickel–platinum thin film

This paper presents a method of heat-assisted magnetic probe recording on perpendicular medium. Electrical current from scanning tunneling microscope (STM) is employed as the major heating source. Recording medium is a strongly-coupled CoNi/Pt multilayered film. Pulses with amplitude of 3–7 V and duration of 200 ns were applied between the STM tip and the recording medium. Experiments show that magnetic marks with an average size of 184 nm were formed for voltages above 4 V. A model is built to simulate the formation of magnetic domains in the processing of probe-based magnetic recording. Simulation results agree with experiments well.     

Atomic surface structures on multi-layered cuprate superconductor Ba2Ca4Cu5O10(O1−xFx)2 observed by STM

We investigate the scanning tunneling microscopy (STM) on the multi-layered cuprate superconductor Ba₂Ca₄Cu₅O₁₀(O_1?xF_x)₂ (F0245, T_c = 79 K, x = 0.72).The STM images show clear atomic lattice structures and large random spot structures. Among the regular square-lattice atomic corrugation with the period of the lattice constant a ～ 0.38 nm, another kind of atomic spots arranged into the fourfold cross shaped clusters is clearly observed along the diagonal direction with the period of 0.26 nm. These clusters are being distributed inhomogeneously, which are due to the charge imbalance associated with the apical O/F rate. The apical O and F sites are also identified from the positions of such clusters in the STM topographic images.     

Defects related emission and nanosecond optical power limiting in CuS quantum dots

We report optical and nonlinear optical properties of CuS quantum dots and nanoparticles prepared through a nontoxic, green, one-pot synthesis method. The presence of surface states and defects in the quantum dots are evident from the luminescent behavior and enhanced nonlinear optical properties measured using the open aperture Z-scan, employing 5 ns laser pulses at 532 nm. The quantum dots exhibit large effective third order nonlinear optical coefficients with a relatively lower optical limiting threshold of 2.3 J cm⁻², and the optical nonlinearity arises largely from absorption saturation and excited state absorption. Results suggest that these materials are potential candidates for designing efficient optical limiters with applications in laser safety devices.     

Oxidation of ultra-thin zinc films on Rh(100) surface

The oxidation of submonolayer zinc films on Rh(100) surface by O₂ gas has been studied using low-energy electron diffraction (LEED), Auger electron spectroscopy (AES), and scanning tunneling microscopy (STM). With a zinc coverage of 0.8 ML, an atomically flat ultra-thin zinc oxide film formed at an oxygen partial pressure of 2 × 10^? 8 mbar and a temperature of 150 °C. The zinc oxide film showed a c(16 × 2) LEED pattern. The high resolution STM image of the zinc oxide film showed single dotted spots and double dotted spots arranged linearly and periodically along the [01¯1] direction. We propose an atomic arrangement model of the film accounting for the LEED pattern, the STM image, and the atomic arrangement of the bulk ZnO(0001) surface.     

Dynamic instabilities during the continuous electro-oxidation of CO on poly- and single crystalline Pt electrodes

Dynamic instabilities during bulk CO electro-oxidation on poly- and single crystalline rotating Pt electrodes in different electrolytes were investigated experimentally. In sulphuric and perchloric electrolytic media, only bistability is observed. The dependence of the width of the bistable regime on some parameters is discussed. The addition of small amounts of chloride ions induces current oscillations under potentiostatic conditions on polycrystalline Pt, Pt(1 1 0) and Pt(1 0 0) electrodes. Existence range, shape and mean frequency of the mainly irregular kinetic oscillations vary significantly with the crystallographic structure of the electrode surface.     

The effects of annealing and growth temperature on the morphologies of Bi nanostructures on HOPG

We report on the growth of ultra-thin bismuth (Bi) films on the basal plane of highly ordered pyrolitic graphite (HOPG) substrates; we investigate the morphologies of films grown at room temperature and then annealed at high temperature, and the morphologies of Bi structures grown at high temperature. Films grown at room temperature nucleate flat islands on the HOPG terraces, and 1D nanorods from HOPG step edges, the islands and rods both have heights in the range 1–3 nm. During annealing, the flat islands break up into groups of aligned, ～ 2.5 nm tall rods. For films grown at high temperature, terrace nucleation is almost nonexistent, and 1D structures grow from step edges, with heights up to 30 nm. Finally, we observe rods with distinctively different morphologies corresponding to (110) and (111) orientations, and infer a surface energy driven Bi(110) to Bi(111) orientation transition. We speculate that the dominant mechanism for the reorientation is coalescence.     

Determining the work function of a carbon-cone cold-field emitter by in situ electron holography

Cold-field emission properties of carbon cone nanotips (CCnTs) have been studied in situ in the transmission electron microscope (TEM). The current as a function of voltage, i(V), was measured and analyzed using the Fowler–Nordheim (F–N) equation. Off-axis electron holography was employed to map the electric field around the tip at the nanometer scale, and combined with finite element modeling, a quantitative value of the electric field has been obtained. For a tip-anode separation distance of 680 nm (measured with TEM) and a field emission onset voltage of 80 V, the local electric field was 2.55 V/nm. With this knowledge together with recorded i(V) curves, a work function of 4.8 ± 0.3 eV for the CCnT was extracted using the F–N equation.     

STM observation of the origin of electrochemical promotion on metal catalyst-electrodes interfaced with YSZ and β″-Al2O3

Scanning tunneling microscopy (STM) was used to investigate the surfaces of Pt(111) single crystals interfaced with YSZ and β″-Al₂O₃ at atmospheric pressure. In both cases the STM imaged the reversible electrochemically controlled dosing (backspillover) of O²⁻ species and of Na⁺ species on Pt(111) surface respectively, which both form a (12 × 12) hexagonal structure on the Pt(111) surface. On the mechanistic side, the STM has confirmed the backspillover mechanism of electrochemical promotion and metal support interactions.     

Scanning tunneling microscopy on iron-chalcogenide superconductor Fe(Se, Te) single crystal

We have investigated the iron-chalcogenide superconductor Fe(Se, Te) using a low-temperature scanning tunneling microscopy/spectroscopy (STM/STS) technique. STM topography at 4.9 K shows clear regular square arrangements of spots with the lattice spacing ～0.37 nm, from which what we observe are attributed to Se or Te atomic plane. In the topography, brighter and darker atomic spots are randomly distributed, which are most probably due to Te and Se atoms, respectively. For the FeTe compound, the topography exhibits clusters of the bright spots probably arising from separated iron atoms distributing over several Te lattice sites. The STS measurements clarify the existence of the large-size gap with 2Δ = 0.4–0.6 eV.     

Optical detection of organic hydrides with platinum-loaded tungsten trioxide

Tungsten trioxide powder with loading 0.1 wt% platinum (Pt/WO₃) was prepared for optical detection of organic hydrides such as cyclohexane, decalin by impregnation with PtCl₆^2? and subsequent calcination in air at 500 °C. The scanning electron microscopic observation of Pt/WO₃ shows that the Pt particles with mean diameters of 80–100 nm were on the surface of the WO₃ powder. The Pt/WO₃ showed coloration for 13% cyclohexane at higher 100 °C and for 1.3% cyclohexane at 200 °C. The in-situ XRD results of the Pt/WO₃ in coloring/bleaching change indicate that the coloring of Pt/WO₃ was caused by transformation of WO₃ to tungsten bronze. The analysis of reacted gas demonstrates that Pt on WO₃ produces only hydrogen and benzene through dehydrogenation of cyclohexane over 100 °C. It was founded that the Pt/WO₃ has potential of optical detection of organic hydrides by heating at higher 100 °C.     

Coarsening phenomena of metal nanoparticles and the influence of the support pre-treatment: Pt/TiO2(110)

One of the technologically most important requirements for the application of oxide-supported metal nanoparticles (NPs) in the fields of molecular electronics, plasmonics, and catalysis is the achievement of thermally stable systems. For this purpose, a thorough understanding of the different pathways underlying thermally-driven coarsening phenomena, and the effect of the nanoparticle synthesis method, support morphology, and degree of support reduction on NP sintering is needed. In this study, the sintering of supported metal NPs has been monitored via scanning tunneling microscopy combined with simulations following the Ostwald ripening and diffusion-coalescence models. Modifications were introduced to the diffusion-coalescence model to incorporate the correct temperature dependence and energetics. Such methods were applied to describe coarsening phenomena of physical-vapor deposited (PVD) and micellar Pt NPs supported on TiO₂(110). The TiO₂(110) substrates were exposed to different pre-treatments, leading to reduced, oxidized and polymer-modified TiO₂ surfaces. Such pre-treatments were found to affect the coarsening behavior of the NPs.No coarsening was observed for the micellar Pt NPs, maintaining their as-prepared size of ~ 3 nm after annealing in UHV at 1060 °C. Regardless of the initial substrate pre-treatment, the average size of the PVD-grown NPs was found to increase after identical thermal cycles, namely, from 0.5 ± 0.2 nm to 1.0 ± 0.3 nm for pristine TiO₂, and from 0.8 ± 0.3 nm to 1.3 ± 0.6 nm for polymer-coated TiO₂ after identical thermal treatments. Although no direct real-time in situ microscopic evidence is available to determine the dominant coarsening mechanism of the PVD NPs unequivocally, our simulations following the diffusion-coalescence coarsening route were in significantly better agreement with the experimental data as compared to those based on the Ostwald-ripening model. The enhanced thermal stability of the micellar NPs as compared to the PVD clusters might be related to their initial larger NP size, narrower size distribution, and larger interparticle distances.     

Cadmium sulfide quantum dots stabilized by castor oil and ricinoleic acid

Castor oil and ricinoleic acid (an isolate of castor oil) are environmentally friendly bio-based organic surfactants that have been used as capping agents to prepare nearly spherical cadmium sulfide quantum dots (QDs) at 230, 250 and 280 °C. The prepared quantum dots were characterized by Ultra violet–visible (UV–vis), Photoluminescence (PL), Transmission Electron Microscopy (TEM), High Resolution Transmission Electron Microscopy (HRTEM) and X-ray diffraction (XRD) giving an overall CdS QDs average size of 5.14±0.39 nm. The broad XRD pattern and crystal lattice fringes in the HRTEM images showed a hexagonal phase composition of the CdS QDs. The calculated/estimated average size of the prepared castor oil capped CdS QDs for various techniques were 4.64 nm (TEM), 4.65 nm (EMA), 5.35 nm (UV–vis) and 6.46 nm (XRD). For ricinoleic acid capped CdS QDs, the average sizes were 5.56 nm (TEM), 4.78 nm (EMA), 5.52 nm (UV–vis) and 8.21 nm (XRD). Optical properties of CdS QDs showed a change of band gap energy from its bulk band gap of 2.42–2.82 eV due to quantum size confinement effect for temperature range of 230–280 °C. Similarly, a blue shift was observed in the photoluminescence spectra. Scanning electron microscope (SEM) observations show that the as-synthesized CdS QDs structures are spherical in shape. Fourier transform infra-red (FTIR) studies confirms the formation of castor oil and ricinoleic acid capped CdS QDs.     

Band-gap engineering of ZnSe quantum dots via a non-TOP green synthesis by use of organometallic selenium compound

Single-step green synthesis of ZnSe quantum dots (QDs) from 1,2,3-selenadiazole and zinc acetate resulted in formation of high-quality mono-disperse ZnSe with engineered band-gap. The present method is a non-TOP green route where oleic acid is used as a surfactant. The size quantization effect can be monitored by UV–visible spectroscopy which shows in the range 370–387 nm (3.20–3.35 eV), a blue shift of about 70–90 nm with respect bulk ZnSe. Photoluminescence measurement revealed band-gap emission at ∼390 nm (3.18 eV, Stokes shift of <10 nm with FWHM <30 nm). Broadened XRD pattern indicated formation of cubic ZnSe and the estimation of particle size from the line broadening at 〈1 1 1〉 matched well the TEM analysis.     

Simultaneous two color image capture for sub-diffraction localization fluorescence microscopy

A sub-diffraction limit fluorescence localization microscope was constructed using a standard cooled 1.4 mega-pixel fluorescence charge-coupled device (CCD) camera to simultaneously resolve closely adjacent paired quantum dots on a flat surface with emissions of 540 and 630 nm. The images of the overlapping Airy discs were analyzed to determine the center of the point spread function after noise reduction using Fourier transformation analysis. The Cartesian coordinates of the centers of the point spread functions were compared in serial images. Histograms constructed from serial images fit well to Gaussian functions for resolving two quantum dots separated by as little as 10 nm in the x–y coordinates. Statistical analysis of multiple pairs validated discrimination of inter-fluorophore distances that vary by 10 nm. The method is simple and developed for x–y resolution of dilute fluorophores on a flat surface, not serial z sectioning.     

Study of CO diffusion on stepped Pt(1 1 1) surface by scanning tunneling microscopy

We have used a time-dependent tunneling current mode based on scanning tunneling microscopy/spectroscopy (STM/STS) to study the tracer diffusion of CO molecules along steps and on terraces of Pt(1 1 1). The results show that the hopping rate of CO molecules along steps is about 10 times faster than that on terraces in the measured temperature range. The diffusion activation energies are 5.1 kcal/mol and 3.8 kcal/mol on terraces and along steps, respectively. The lower activation energy and faster hopping rate for CO molecules diffusing along steps provide evidence that steps provide fast diffusion channels for CO molecules on stepped Pt(1 1 1) surfaces.