Controllable oxidation of h-BN monolayer on Ir(111) studied by core-level spectroscopies

The effect of atomic oxygen adsorption on the structure and electronic properties of monolayer hexagonal boron nitride (h-BN) grown on Ir(111) has been studied using near edge X-ray absorption fine structure spectroscopy (NEXAFS), photoelectron spectroscopy (PES), and low-energy electron diffraction (LEED). It has been shown that the oxidation of the h-BN monolayer occurs through a gradual substitution of N by O in the h-BN lattice. This process leads to the formation of defect sites corresponding to three different types of the B atom environment (BN_3 ? xO_x with x = 1,2,3). The oxidation of the h-BN monolayer is very different from the case of graphene on Ir(111), where adsorption of atomic oxygen results mainly in the formation of epoxy groups [J. Phys. Chem. C. 115, 9568 (2011)]. A post-annealing of the h-BN monolayer after oxygen exposure results in further destruction of the B–N bonds and formation of a B₂O₃-like structure.     

Formation of a monolayer <Emphasis Type="Italic">h</Emphasis>-BN nanomesh on Rh (111) studied using <Emphasis Type="Italic">in-situ</Emphasis> STM

As a member of the 2D family of materials, h-BN is an intrinsic insulator and could be employed as a dielectric or insulating inter-layer in ultra-thin devices. Monolayer h-BN can be synthesized on Rh (111) surfaces using borazine as a precursor. Using in-situ variable-temperature scanning tunneling microscopy (STM), we directly observed the formation of h in real-time. By analyzing the deposition under variable substrate temperatures and the filling rate of the h-BN overlayer vacant hollows during growth, we studied the growth kinetics of how the borazine molecules construct the h-BN overlayer grown on the Rh surface.     

Single-layer model of the hexagonal boron nitride nanomesh on the Rh(111) surface

An alternative model of the hexagonal boron nitride (h-BN) on nanomesh on the Rh(111) surface is presented. It explains the observed ultraviolet photoelectron spectroscopy spectra and reproduces experimental STM images introducing, instead of two, only one strongly corrugated layer of h-BN covering the whole Rh surface. In order to optimize the geometry of the BN layer we calculate the forces by density functional theory and analyze the interactions in the system. The final geometry is a result of a competition between BN-metal attraction or repulsion and elastic properties of the isolated h-BN layer. The calculated bonding energy is around 0.33 eV per BN molecule with a corrugation close to 0.55 A.     

钴原子及其团簇在Rh(111)和Pd(111)表面的扫描隧道显微学研究

利用扫描隧道显微镜和扫描隧道谱(STM/STS)及单原子操纵,系统研究了单个钴原子(Co) 及其团簇在Rh (111)和Pd (111)两种表面的吸附和自旋电子输运性质. 发现单个Co原子在Rh (111)上有两种不同的稳定吸附位,分别对应于hcp和fcc空位, 他们的高度明显不同,在针尖的操纵下单个Co原子可以在两种吸附位之间相互转化. 在这两种吸附位的单个Co原子的STS谱的费米面附近都存在很显著的峰形结构, 经分析认为Rh (111)表面单个Co原子处于混价区,因此这一峰结构是d轨道共振 和近藤共振共同作用的结果.对于Rh (111)表面上的Co原子二聚体和三聚体, 其费米面附近没有观测到显著的峰,这可能是由于原子间磁交换相互作用 和原子间轨道杂化引起的体系态密度改变所共同导致.与Rh (111)表面不同, 在Pd (111)表面吸附的单个Co原子则表现出均一的高度.并且对于Pd (111)表面所有 单个Co原子及其二聚体和三聚体,在其STS谱的费米面附近均未探测到显著的电子结构, 表明Co原子吸附于Pd (111)表面具有与Rh (111)表面上不同的原子-衬底相互作用与自旋电子输运性质.     

Ti/CeOx(111) interfaces studied by XPS and STM

Low coverage of Ti was deposited on the well-ordered CeO_x(111) (1.5 < x < 2) thin films grown on Ru(0001) by physical vapor deposition at room temperature. The structure and interaction of Ti/ceria interfaces were investigated with X-ray photoelectron spectroscopy (XPS), low energy electron diffraction (LEED) and scanning tunneling microscopy (STM) techniques under ultrahigh vacuum conditions. XPS data indicate that the deposition of Ti on both oxidized and reduced ceria surfaces causes the partial reduction of Ce from + 4 to + 3 state. Ti is formally in the + 4 state. STM data show the formation of small atomic-like titania features at 300 K, which coalesce to form chain structures upon heating. It is demonstrated in the study that the deposition of Ti can form mixed metal oxides at the interface and modify both electronic and structural properties of the ceria support. The structural study of Ti/ceria interfaces can be a key for understanding the higher catalytic activity of the Ti–CeO_x mixed oxide catalysts as compared with the individual pure oxides.     

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.     

Some coadsorption effects on Rh(111)

ESD energy analysis is used to study the reaction products produced during coadsorption of CO-O₂ and CH₄-O₂ on Rh(111). Residence of CO₂ on the surface is confirmed by detection of a CO₂⁺ ionic component with energy of 1.8 eV. Adsorption and dissociation of CH₄ on oxygen- covered Rh(111) is inferred as a result of a low energy component present in the energy spectra of desorbed O⁺.     

Studies of alkali adsorption on Rh(111) using optical second-harmonic generation

Dramatic changes were observed in the optical second-harmonic generation from the Rh(111) crystal surface as the surface was covered with alkali atoms. For low alkali coverage, these changes are explained qualitatively in terms of optical transitions between alkali atomic-derived electronic states and, for high coverage, in terms of the plasmon resonance of the alkali layer. The results establish the surface specificity of second-harmonic generation to the topmost 1 or 2 atomic layers of alkali metals.     

STM manipulation of a subphthalocyanine double-wheel molecule on Au(111)

A new class of double-wheel molecules is manipulated on a Au(111) surface by the tip of a scanning tunneling microscope (STM) at low temperature. The double-wheel molecule consists of two subphthalocyanine wheels connected by a central rotation carbon axis. Each of the subphthalocyanine wheels has a nitrogen tag to monitor its intramolecular rolling during an STM manipulation sequence. The position of the tag can be followed by STM, allowing us to distinguish between the different lateral movements of the molecule on the surface when manipulated by the STM tip.     

The chemisorption of Co on Rh(111)

The adsorption of CO on Rh(111) has been studied by thermal desorption mass spectrometry and low-energy electron diffraction (LEED). At temperatures below 180 K, CO adsorbs via a mobile precursor mechanism with sticking coefficient near unity. The activation energy for first-order CO desorption is 31.6 kcal/mole (ν_d = 10^13.6s^?1) in the limit of zero coverage.As CO coverage increases, a (√3 ×√3)R30^u overlayer is produced and then destroyed with subsequent formation of an overlayer yielding a (2 × 2) LEED pattern in the full coverage limit. These LEED observations allow the absolute assignment of the full CO coverage as 0.75 CO molecules per surface Rh atom. The limiting LEED behavior suggests that at full CO coverage two CO binding states are present together.     

Self-limited growth of a thin oxide layer on Rh(111)

The oxidation of the Rh(111) surface at oxygen pressures from 10(-10) mbar to 0.5 bar and temperatures between 300 and 900 K has been studied on the atomic scale using a multimethod approach of experimental and theoretical techniques. Oxidation starts at the steps, resulting in a trilayer O-Rh-O surface oxide which, although not thermodynamically stable, prevents further oxidation at intermediate pressures. A thick corundum like Rh2O3 bulk oxide is formed only at significantly higher pressures and temperatures.     

STM study of Au(111) growth on mica

Gold films were thermally evaporated in vacuum on heated cleaved mica substrates. The substrate temperature was immediately decreased after finishing the growth. Samples prepared at various temperatures from 580 K to 890 K and evaporation rates smaller than 1 nm/s were studied by a scanning tunneling microscope (STM) under ambient conditions. Structure and defects of Au(111) surfaces were investigated and discussed with respect to growth conditions.     

Hydrogen-adsorption induced atomic rearrangement of a Pb monolayer on Si(111)

We have observed interesting H-atom adsorption induced atomic rearrangements of a Pb monolayer on the Si(111) with a scanning tunneling microscope. A hexagonal ringlike pattern is formed around the point defect. The interactions among nearby H-adsorbed defects can even produce interferencelike superstructures. Phase boundaries are found to either enhance or suppress the formation of the interference pattern. These phenomena are produced by an intricate interplay between electronic and atomic interactions as perturbed by the adsorbed hydrogen atoms.     

Deposition of monolayer and bulk lead on Ag(111) studied in vacuum and in an electrochemical cell

The early stages of Pb condensation on Ag(111) have been studied in ultrahigh vacuum and in an electrochemical cell. The vacuum deposited films were investigated by AES, LEED and work function measurements, and the electrolytically deposited films in situ by cyclic voltammetry, potential step experiments and reflectance spectroscopy. Despite the very different environments a similar growth behaviour is observed. In both cases deposition starts with the formation of a uniform monolayer on Ag(111), which occurs in three steps: random adsorption at very low coverages followed by a √3 × √3 superstructure and finally, a hexagonal close packed layer. From an analysis of the AES data for the vacuum deposited film a particular case of Stranski-Krastanov type is found for the subsequent growth. The formation of ordered submonolayer structures which is clearly detectable by LEED, is inferred for the electrolytically formed layer since a similar coverage dependence in the reflectance is observed. From the underpotential shift of the electrolytically formed monolayer an excess binding energy of about 0.3 eV is deduced for the Pb adatoms on Ag(111).     

Intercalation of silver atoms under a graphite monolayer on Ni(111)

The process of silver intercalation under a graphite monolayer (GM) grown on the (111) nickel single-crystal face, GM/Ni(111), is studied. The experiments were conducted in ultrahigh vacuum. The systems were formed in situ in a vacuum chamber under direct monitoring of each stage in the formation of the systems by angle-resolved UV photoelectron spectroscopy and LEED. The possibility of silver intercalation in the GM/Ni(111) system was studied in the course of deposition of various amounts of the metal on the given subject with subsequent heat treatment. It was established that the process occurs optimally under cyclic alternation of the operations of adsorbate (Ag) deposition on the GM/Ni(111) surface and subsequent annealing of the system. In the intermediate stages of GM/Ag/Ni(111) formation, the GM on Ni(111) was found to exist in two phases. Ag intercalation under a graphite monolayer on Ni(111) at room temperature was verified.     

Scanning tunneling microscopy study of a Ag monolayer on Cu(111)

The structure of submonolayer Ag deposited on Cu(111) has been examined with scanning tunneling microscopy. The long diffusion length of Ag leads to the formation of large (111)-like islands nucleating from the “downhill” side of Cu step edges. The growth fronts of the Ag islands are mostly straight and lie along the 〈1&#x0304;10〉 directions. The Ag overlayer is apparently slightly compressed relative to bulk Ag, and forms a mixture of superstructures rotated slightly relative to the substrate.