Electron-energy-loss spectroscopy of C60 monolayer films on active and inactive surfaces

The characteristics of interaction between C₆₀ molecules and Si(1 1 1)-7×7, Ag/Si(1 1 1)-√3×√3 R30° and layered material MoS₂ surfaces have been investigated using electron-energy-loss spectroscopy (EELS). The EEL spectrum of C₆₀/Si(1 1 1)-7×7 shows a new peak at loss energy of 2.7 eV. This indicates the existence of charge transfer from the substrate to C₆₀ molecules. The EEL spectrum of a C₆₀ monolayer film grown on a cleaved surface of MoS₂ is almost the same as that of bulk C₆₀. The EEL spectrum of a C₆₀ monolayer film on an Ag/Si(1 1 1) surface is quite different from that on a clean Si(1 1 1)-7×7 surface, although the films on those substrates have the same epitaxial arrangement. Furthermore, intensities of energy-loss peaks of C₆₀/Ag/Si(1 1 1) are slightly smaller than those of C₆₀/MoS₂ in spite of having the same loss-energy. This suggests that the interaction between C₆₀ molecules and the Ag/Si(1 1 1) surface is stronger than that between C₆₀ molecules and the MoS₂ surface.     

Fluorine etching on the Si(111)-7×7 surfaces using fluorinated fullerene

Fluorine etching on the Si(1 1 1)-7×7 surfaces using fluorinated fullerene molecules as a fluorine source has been investigated. At room temperature, adsorbed fluorinated fullerene molecules reacted with the Si(1 1 1)-7×7 surface to create a localized distribution of fluorine on the surface. Nanoscale etch pits were created by annealing at 300 °C, due to the adsorption of the fluorine localized around the C₆₀F_x molecules. Annealing at 400 °C resulted in the delocalized fluorine distribution on the surface and healing of the etch pits, due to the enhancement of the diffusion of both the fluorine and silicon atoms. Subsequent annealing at 500 °C led to desorption of SiF₂ reactants formed on the surface. The fluorine diffusion process was found to be an elemental process in the etching because the diffusion of adsorbed fluorines is a key for the formation of the SiF₂ species and their subsequent desorption.     

UV–Vis and NMR spectroscopic investigations on effective and selective non-covalent interactions between fullerenes and calix[6]arene

Ground state non-covalent interactions between a macro cyclic calixarene receptor, namely, 4-tert-butylcalix[6]arene (1), and fullerenes (C₆₀ and C₇₀) were studied in toluene medium by absorption spectrophotometric method. Absorption band due to the charge transfer (CT) transition have been observed in the visible region between fullerenes and 1. Utilizing the CT absorption bands, various important physicochemical parameters like oscillator strength, resonance energy, transition dipole strength of the fullerene-1 complexes and ionization potential of 1 is determined in present investigations. From Jobs method of continuous variation, it is observed that both C₆₀ and C₇₀ form stable 1:1 complexes with 1. The most fascinating feature of the present study is that 1 binds selectively C₇₀ compared to C₆₀ as obtained from binding constant (K) data of C₆₀–1 (K_C60–1) and C₇₀–1 (K_C70–1) complexes, i.e., K_C60–1 = 32,400 dm³·mol^? 1 and K_C70–1 = 110,000 dm³.mol^? 1 and selectivity (K_C70–1/K_C60–1) = 3.4. ¹H NMR analysis provides very good support in favor of strong binding between C₇₀ and 1.     

Electronic charge transfer between Au nano-particles and TiO2-terminated SrTiO3(0 0 1) substrate

The interaction between Au nano-particles and oxide supports is recently discussed in terms of the catalytic activities. This paper reports the electronic charge transfer between Au nano-particles and TiO₂-terminated SrTiO₃(0 0 1) substrate, which is compared with that for stoichiometric(S)-, pseudo-stoichiometric(S¹)- and reduced(R)-TiO₂(1 1 0) supports. We observed the photoelectron spectra of Au 4f, O 2s, Ti 3p, and Sr 4p lines and also measured the work functions for Au/oxides supports using synchrotron-radiation light. As the results, all the O 2s, Ti 3p, and Sr 4p lines for Au/SrTiO₃(0 0 1) show lower binding energy shifts in a quite same manner and abrupt increase in the work function is seen in an initial stage. This clearly evidences an electronic charge transfer from the substrate to Au probably due to a much larger work function of Au than SrTiO₃(0 0 1), which leads to an upward band bending (0.3 eV) just like a Schottky contact. Electronic charge transfers also take place at Au/S- and Au/S¹-TiO₂(1 1 0) and Au/R-TiO₂(1 1 0) interfaces, where electrons are transferred from Au to S- and S¹-TiO₂ and from R-TiO₂ to Au, as predicted by ab initio calculations.     

Self-assembly of C60 fullerenes on quasi-one-dimensional Si(111)4 × 1-In surface

Using scanning tunneling microscopy observations, self-assembly of C₆₀ fullerenes in the course of room-temperature adsorption onto Si(111)4 × 1-In reconstruction and after subsequent annealing at temperatures ranging from 150 to 450 °C has been studied. Adsorbed C₆₀ fullerenes have been found to occupy off-centered positions on In-atom rows forming linear chains with a maximal length of eight C₆₀ molecules. Intermolecular spacing within the regular chains equals three lattice constants of Si(111) surface. Two energetically different adsorption states of C₆₀ have been detected, one of which is occupied preferentially at room temperature, while occupation of the second (more tight) state dominates at temperature above ~ 150 °C. In the first state, C₆₀ fullerene resides plausibly in a continuous rotation, while in the second state a C₆₀ molecule is fixed tightly in a single orientation with a C₆₀ hexagon pointing upward. Transition of C₆₀ fullerenes to the more stable state is accompanied by expelling In atoms from the Si(111)4 × 1-In reconstruction.     

Radiation-stimulated modification of C60 films on Si-oxide surfaces

The electronic structure of C₆₀/SiO_x/Si(1 0 0) interface was studied by photoemission (valence-bands, C 1s, and Si 2p core levels) and near-edge X-ray absorption fine structure (C 1s threshold) spectroscopies. It was concluded that the SiO_x/Si surface is non-reactive with respect to the interaction with C₆₀. The exposure of the C₆₀/SiO_x/Si system under non-monochromatic synchrotron radiation causes modification of the electronic structure of this system. It is explained by polymerization of the C₆₀ molecules and arising a strong ionic-like interaction of the polymerized C₆₀ with the SiO_x surface. Annealing of this system up to temperatures of 550–625 °C leads to complete desorption of the C₆₀ molecules from the non-irradiated sample areas while the modified by radiation fullerenes remain attached to the substrate.     

Infrared study of triruthenium dodecacarbonyl interactions with gold

We present here a study of the interaction of triruthenium dodecacarbonyl Ru₃(CO)₁₂ with gold surfaces using time-evolved and temperature-programmed infrared reflection absorption spectroscopy (IRAS) and STM. Ru₃(CO)₁₂ exhibits drastically different adsorption/desorption behavior on high-index surfaces of gold in comparison to the smooth Au(111) surface. On the smooth Au(111) surface, the adsorption of Ru₃(CO)₁₂ at 200 K is observed to be molecular and reversible with the molecule's Ru₃-plane oriented essentially perpendicular to the surface in the first and second layer. In the multilayer (> 3 ML), the molecule is oriented parallel (or moderately inclined) to the surface. On high-index gold surfaces, prepared by partial annealing of rough gold films, the molecules dissociate. Vibrational spectra reveal dissociation of carbonyl to Ru and CO at elevated temperature (> 250 K) with the formation of CO covered Ru-islands and the subsequent desorption of CO from Ru-islands. Increasing amounts of CO observed with increasing surface roughness demonstrate that the rate of Ru₃(CO)₁₂ dissociation is related directly to the surface roughness of the gold surface. STM images reveal at low coverage the formation of 2-D islands of carbonyl fragments with lateral sizes of 1 to 1.5 nm and at higher coverage the formation of larger 3-D islands of 1 to 3 layers and lateral sizes above 10 nm.     

Adsorption and diffusion of an Au atom and dimer on a θ-Al2O3 (001) surface

With density-functional calculations we have investigated adsorption and diffusion of an Au atom and an Au₂ dimer on a θ-Al₂O₃(001) surface. The surface structure of θ-Al₂O₃(001) has an armchair-like configuration containing flat and trench areas and the Au_n (n = 1 or 2) cluster prefers to adsorb on the flat area. A single Au atom adsorbs on an O–Al bridge site with adsorption energy 0.35 eV, whereas an Au₂ dimer bonds to the oxide with adsorption energy 0.78 eV, with one Au coordinated singly to a surface O. Formation of Au₂ from Au₁ is favored, with a negligible energy barrier. The calculated energy barriers for diffusion indicate that an Au atom diffuses more rapidly than an Au₂ dimer but both prefer to diffuse anisotropically, along the flat area of the θ-Al₂O₃(001) surface.     

Self-induced polarization rotation of laser beam in fullerene (C70) solutions

Nonlinear self-rotation of elliptically polarized laser pulses (λ = 532 nm, τ_FWHM ~ 12 ns) in toluene, benzene and binary mixture (toluene + ethanol) solutions of fullerene C₇₀ has been investigated experimentally. Absolute values and signs of the nonlinear refractive indices (n₂) and nonlinear optical susceptibilities χ⁽³⁾(ω, ? ω, ω) of C₇₀ solutions in toluene and benzene at different values of polarization ellipse (θ = 0.2 ÷ 0.8) have been determined. High-resolution transmission electron microscope studies of C₇₀ solutions showed that in toluene + ethanol mixtures ball-shaped C₇₀ clusters are formed with particle sizes in the range ~ 100 ÷ 500 nm. It has been demonstrated, that the clusters sizes depend on the C₇₀ concentration and volume fraction of ethanol in toluene. Correlation between the processes of C₇₀ clusters formation in solutions and the values of polarization self-rotation angle of transmitted laser beam has been demonstrated. Physical mechanisms of laser induced optical activity in fullerene solutions have been discussed.     

New evaluation of fullerene molecules on Si(111)-(7 × 7) reconstructed structure using non-contact scanning non-linear dielectric microscopy

Fullerene (C₆₀) molecules on an Si(111)-(7 × 7) surface have been investigated using non-contact scanning non-linear dielectric microscopy (NC-SNDM) under an ultra-high vacuum. The topography, the interface between the C₆₀ molecule and Si adatoms, and the internal structure of the C₆₀ molecules were successfully investigated. For ～ 0 ML and ～ 0.4 ML coverage, both phase reversal sites and sites without phase reversal could be observed in the first order phase (θ1) image. On the other hand, for 1 ML coverage, phase reversal could not be identified. These results indicate that charge transfer only occurred from Si adatoms to C₆₀ molecules at three-fold symmetric sites on the Si(111)-(7 × 7) surface, and the electric dipole moment is reflected in the electronic state of the C₆₀ molecules. The internal structure of C₆₀ molecules was clearly observed in topography by the second order amplitude (A₂) feedback signal for 1 ML coverage, reflecting the LDOS originating from the t_1u orbital.     

Preparation and characterization of Fe3O4(1 1 1) nanoparticles and thin films on Au(1 1 1)

Fe₃O₄ nanoparticles and thin films were prepared on the Au(1 1 1) surface and characterized using X-ray photoelectron spectroscopy (XPS) and scanning tunneling microscopy (STM). Fe₃O₄ was formed by annealing α-Fe₂O₃(0 0 0 1) structures on Au(1 1 1) at 750 K in ultrahigh vacuum (UHV) for 60 min. Transformation of the α-Fe₂O₃(0 0 0 1) structures into Fe₃O₄ nanoparticles and thin films was supported by XPS. STM images show that during the growth procedure used, Fe₃O₄ initially appears as nanoparticles at low coverages, and forms thin films at ～2 monolayer equivalents (MLE) of iron. Two types of ordered superstructures were observed on the Fe₃O₄ particles with periodicities of ～50 and ～42 Å, respectively. As the Fe₃O₄ particles form more continuous films, the ～50 Å feature was the predominant superstructure observed. The Fe₃O₄ structures at all coverages show a hexagonal unit cell with a ～3 Å periodicity in the atomically resolved STM images.     

Electrical properties of a nanoparticle-networked film

We have developed a new technique for the preparation of conducting nanoparticle films, which consisted of gold nanoparticles networked with binding molecules such as butanethiol, pentanehtiol, hexanethiol, and heptanethiol, on a polystyrene substrate through a one-step straightforward procedure. The film conductivity became significantly higher with a decrease in the alkylchain length, depending on the number of carbon atoms (n) of the binder molecule between adjacent Au nanoparticles, in which it was changed from an insulator (n > 10) to conductor (n = 4). The film resistivity (2.1 μΩ cm) prepared using butanethiol corresponds to that of planar gold (1.3 μΩ cm).     

Oxygen-rich Ti1−xO2 pillar growth at a gold nanoparticle–TiO2 contact by O2 exposure

In-situ gas-injection transmission electron microscopy revealed that a pillar grew at the edge of the interface of a gold nanoparticle and a TiO₂ substrate during exposure to O₂ gas at 100 Pa. The pillar was found to have a titanium-deficient chemical composition of Ti_1 ? xO₂ (x > 0) by electron energy loss spectroscopy (EELS). The spectra showed a chemical shift of oxygen and titanium ions to have ionic states of Ti³⁺ and O^y? (y < 3/2). The formation of the Ti_1 ? xO₂ at the contact edge of gold–Ti_1 ? xO₂ interface is discussed from the perspective of an O₂ affinity, which plays an important role in CO oxidation process of supported gold particle.     

Crystal chemistry and physical properties of the ternary compounds RE5B4C5 (RE=Ce,Pr, Nd)

The ternary rare-earth metal boride carbides RE₅B₄C₅ (RE=Ce, Pr, Nd) were prepared by arc melting the mixtures of the pure elements and subsequent annealing at 1270 K. Their crystal structures were determined from single crystal X-ray diffraction data. They crystallize in the Ce₅B₄C₅ type of structure (space group Pna2₁, Z=8). Two new compounds were found, Pr₅B₄C₅: a=24.592(2) Å, b=8.4563(5) Å, c=8.4918(5) Å, R₁=0.043 (wR₂=0.076) for 2871 reflections with I_o?2σ(I_o)); for Nd₅B₄C₅: a=24.301(1) Å, b=8.3126(5) Å, c=8.3545(4) Å, R₁=0.035 (wR₂=0.069) for 3707 reflections with I_o?2σ(I_o)). Its structural arrangement consists of a three-dimensional framework of rare-earth atoms resulting from the stacking of slightly corrugated two-dimensional square nets, leading to voids filled with B₄C₄, B₃C₃, BC₂ finite chains and isolated carbon atoms. Ce₅B₄C₅ is an antiferromagnet at 5 K followed by a metamagnetic transition in elevated external fields. Pr₅B₄C₅ and Nd₅B₄C₅ are ferromagnets below T_C=12 and 15 K, respectively.     

Growth of ultrathin layers of Au on LiNbO3(0 0 0 1) measured with atomic force microscopy

Atomic force microscopy (AFM) has been used to characterize the growth of Au deposited via evaporation onto the positive face of single crystalline, lithium niobate, LiNbO₃(0 0 0 1) surface. In order to study the mechanisms for the ordering and aggregation of a noble metal on this ferroelectric surface, topographic and phase contrast imaging of the fractional surface coverage of Au were performed. Atomically flat, uniformly poled LiNbO₃ surfaces were prepared via an ambient high temperature anneal and served as a support for the thin gold films. These gold atomic layers were grown using electron bombardment evaporation sources under ultra-high vacuum (UHV) conditions and subsequently characterized under both vacuum and ambient environments. Using AFM it was found that gold preferentially nucleates at the top of LiNbO₃ substrate step edges. With increased coverage, island formation proceeds due to local aggregation of adsorbed gold on each substrate terrace. Based on local imaging of the growth morphology, the data is discussed in terms of thin film growth mechanisms as well as the influence of native surface features such as defects and charge distribution. Understanding growth mechanisms for gold layers on ferroelectric surfaces allows for a fuller appreciation of how atomic deposition of metal atoms on patterned poled LiNbO₃ surfaces would occur as well as yielding greater insight on the atomic characteristics of metals on ferroelectric interfaces.     

Vibration spectroscopy of water on stepped gold surfaces

The vibration spectrum of H₂O (ice) adsorbed at low temperatures on Au(1 0 0), Au(1 11 1), and Au(1 1 5) is studied using electron energy loss spectroscopy. On the Au(1 0 0) surface, the spectra show the presence of the typical H-bonded network of water molecules for all coverages from the submonolayer into the multilayer range. The absence of a non-H-bonded OH-stretching mode is indicative for the “H-down bilayer”. On stepped surfaces, on the other hand, a considerable fraction of the H-atoms remains in the non-H-bonded state; surprisingly even in the multilayer range, and even after annealing. The fraction of non-H-bonded hydrogen atoms scales with the step density. Spectral features of water adsorbed at step-sites are isolated after annealing a surface exposed to small doses of H₂O. The results are discussed in the context of recent theoretical studies as well as in conceivable relation to the experimentally found reduction of the Helmholtz-capacitance on stepped Au(1 1 n) electrodes.     

Structural and magnetic properties of thin epitaxial Fe films on (1 1 0) GaAs prepared by metalorganic chemical vapor deposition

Iron films have been grown on (1 1 0) GaAs substrates by atmospheric pressure metalorganic chemical vapor deposition at substrate temperatures (T_s) between 135°C and 400°C. X-ray diffraction (XRD) analysis showed that the Fe films grown at T_s between 200°C and 330°C were single crystals. Amorphous films were observed at T_s below 200°C and it was not possible to deposit films at T_s above 330°C. The full-width at half-maximum of the rocking curves showed that crystalline qualities were improved at T_s above 270°C. Single crystalline Fe films grown at different substrate temperature showed different structural behaviors in XRD measurements. Iron films grown at T_s between 200°C and 300°C showed bulk α-Fe like behavior regardless of film thickness (100–6400 Å). Meanwhile, Fe films grown at 330°C (144 and 300 Å) showed a biaxially compressed strain between substrate and epilayer, resulting in an expanded inter-planar spacing along the growth direction. Magnetization measurements showed that Fe films (>200 Å) grown at 280°C and 330°C were ferromagnetic with the in-plane easy axis along the [1 1 0] direction. For the thinner Fe films (⩽200 Å) regardless of growth temperature, square loops along the [1 0 0] easy axis were very weak and broad.     

On the encapsulation of azafullerenes inside the single-walled carbon nanotubes: Density-functional theory based treatments

We theoretically studied the encapsulation of azafullerene (C₅₉N) inside the single-walled carbon nanotubes (SWCNTs) from the first-principles. Adsorption energy is calculated, and the azafullerene affinities for the typical semiconducting and metallic nanotubes are investigated and compared with those of pure C₆₀ fullerene. It has been found that the azafullerene as well as the fullerene affinity for the semiconducting nanotubes is stronger than that for the metallic ones, and the energy values and binding distances are typical for the physisorption. Our first-principles results indicate that the interaction between SWCNTs and azafullerenes is comparable with the nanotubes-C₆₀ system. The charge analysis shows, however, that the charges have been transferred from the cage to the tube in the azafullerene peapods, while in the fullerene peapods the charges were found to be transferred from the tube to the fullerene nanocage. Furthermore, it was found that the interaction between the considered fullerenes and host nanotubes strongly depends on the tube diameters.     

Electrical and structural properties of stepped,partially reconstructed Au(11n) surfaces in HClO4 and H2SO4 electrolytes

We have studied the interface capacitance and the voltammograms of Au(11n) (n = 5, 7, 11, 17) and of Au(100) electrodes in 5 mM HClO₄ and 5 mM H₂SO₄ after immersion into the electrolyte solution at ?0.4 V versus a saturated calomel electrode. The minima of the capacitance curves measured in positive sweeps continuously shift towards positive potentials as function of 1/n. All voltammograms, even that of Au(1 1 5), display peaks that are characteristic for lifting of surface reconstructions, albeit at different potentials. Thus, all vicinal surfaces appear to have at least sections that allow reconstruction. This inference is consistent with STM-profiles of an Au(1 1 9) surface which displays a wide range of local inclination angles corresponding to local (11n)-orientations with 3.5 < n < ∞. A numerical analysis of the voltammograms shows the existence of three different ranges of transition potentials for the lifting of the reconstruction. The transition potentials are assigned to three different structures of the reconstructed phase as either observed by experiment or proposed by theory.     

Effects of Immersion Temperature on Self-Assembled Monolayers of Octanethiol on Au(111)

Self-assembled monolayers (SAMs) of 1-octanethiol (OT) on Au(111) surfaces, prepared at immersion temperatures between 300 K and 363 K in a sealed stainless steel chamber, were studied by scanning tunneling microscopy (STM). An oblique (√3 × √3)R30° OT-SAM structure was observed below 348 K, whereas a superstructure (3×√7)R11° covered the gold surfaces at 363 K. The highly resolved STM images permitted assignment of four symmetry-inequivalent OT molecules per 8.7 × 7.6 Å² unit cell at 363 K. Differences in the topographical heights of the molecules were attributed to the binding of OT sulfur head groups at different adsorption sites on Au(111). This structure was not observed in stirring reflux at a high temperature, which indicates a higher pressure (> 1 atm) in the chamber may be one of crucial factors for structural transition. As the immersion temperature increased, a lower density of vacancy islands and a higher fraction of island area were observed.