Binding sites for cluster atoms: Ir on Ir(111)

Clusters of indium atoms on iridium (111) have been examined with the field ion microscope. From observations on highly symmetrical trimers, tetramers, and heptamers it appears that in larger clusters the atoms prefer to sit in bulk (fcc) sites, even though single adatoms favor surface (hcp) sites. These larger clusters thus serve to continue the normal fcc structure.     

Stacking-fault nucleation on Ir(111)

Variable temperature scanning tunneling microscopy experiments reveal that in Ir(111) homoepitaxy islands nucleate and grow both in the regular fcc stacking and in the faulted hcp stacking. Analysis of this effect in dependence on deposition temperature leads to an atomistic model of stacking-fault formation: The large, metastable stacking-fault islands grow by sufficiently fast addition of adatoms to small mobile adatom clusters which occupy in thermal equilibrium the hcp sites with a significant probability. Using parameters derived independently by field ion microscopy, the model accurately describes the results for Ir(111) and is expected to be valid also for other surfaces.     

Ar adsorptions on Al (111) and Ir (111) surfaces: a first-principles study

We investigate the adsorptions of Ar on Al (111) and Ir (111) surfaces at the four high symmetry sites, i.e., top, bridge, fcc- and hcp-hollow sites at the coverage of 0.25 monolayer (ML) using the density functional theory within the generalized gradient approximation of Perdew, Burke and Ernzerhof functions. The geometric structures, the binding energies, the electronic properties of argon atoms adsorbed on Al (111) and Ir (111) surfaces, the difference in electron density between on the Al (111) surface and on the Ir (111) surface and the total density of states are calculated. Our studies indicate that the most stable adsorption site of Ar on the Al (111) surface is found to be the fcc-hollow site for the (2 × 2) structure. The corresponding binding energy of an argon atom at this site is 0.538 eV/Ar atom at a coverage of 0.25 ML. For the Ar adsorption on Ir (111) surface at the same coverage, the most favourable site is the hcp-hollow site, with a corresponding binding energy of 0.493 eV. The total density of states (TDOS) is analysed for Ar adsorption on Al (111) surface and it is concluded that the adsorption behaviour is dominated by the interaction between 3s, 3p orbits of Ar atom and the 3p orbit of the base Al metal and the formation of sp hybrid orbital. For Ar adsorption on Ir (111) surface, the conclusion is that the main interaction in the process of Ar adsorption on Ir (111) surface comes from the 3s and 3p orbits of argon atom and 5d orbit of Ir atom.     

The adsorption of CO on Ir(111) investigated with FT-IRAS

The adsorption of CO on Ir(111) has been investigated with Fourier transform infrared reflection-absorption spectroscopy, temperature programmed desorption, and low-energy electron diffraction. At sample temperatures between 90 and 350 K, only a single absorption band, above 2000 cm⁻¹, has been observed at all CO coverages. For fractional coverages above approximately 0.2, the bandwidth becomes as narrow as 5.5 cm⁻¹. The linewidth is attributed mainly to inhomogeneous broadening at low CO coverages and to the creation of electron-hole pairs at higher CO coverages. The coverage-dependent frequency shift of the IR band can be described quantitatively using an improved dipolar coupling model. The contribution of the dipole shift and the chemical shift to the total frequency shift were separated using isotopic mixtures of CO. The chemical shift is positive with a constant value of approximately 12 cm⁻¹ for all coverages, whereas the dipole shift increases with coverage up to a value of 36 cm⁻¹ at a coverage of 0.5 ML.     

Chemisorptive bonding and reaction of nitric oxide on Ir(100) and Ir(111) surfaces

Chemisorption of nitric oxide on single crystal Ir(111) and Ir(100)?(5 × 1) has been studied by UV-photoelectron spectroscopy, thermal desorption and low energy electron diffraction. At 300 K, partially dissociative adsorption is observed on both surfaces, confirming the borderline location of Ir in the Periodic Table with respect to molecular versus dissociative adsorption. Three different molecular chemisorption phases are distinguished in the UPS spectra through distinctly different 1π-level energies. A skewed orientation associated with a possible rehybridization and bending of the nitrosyl-metal bound for chemisorption on the Ir(111) surface is inferred both from a splitting of the 1π level and from observation of relative intensity variations in photoemission using a polarized photon source.     

The coadsorption of hydrogen and potassium on Ir(111)

The adsorption of gas-phase atomic hydrogen on potassium-precovered Ir(111) surfaces was investigated. Even very low coverages of potassium adatoms strongly inhibit the dissociative adsorption of molecular hydrogen. However, using gas-phase atomic hydrogen allows us to overcome the activation barrier for dissociative hydrogen adsorption. In addition, abstraction of hydrogen adatoms by impinging atomic hydrogen occurs. The probabilities and cross-sections for both reactions and the maximum number of hydrogen adsorption sites are derived and compared to data obtained on other surfaces. Furthermore, a kinetic isotope effect in the desorption of hydrogen and deuterium was observed. Implications of these results with respect to the potassium-hydrogen interaction are discussed.     

Controllable p-doping of graphene on Ir(111) by chlorination with FeCl3

The in situ chlorination of graphene on Ir(111) has been achieved by depositing FeCl(3) followed by its thermal decomposition on the surface into FeCl(2) and Cl. This process is accompanied by an intercalation of Cl under graphene and formation of an epitaxial FeCl(2) film on top, which can be removed upon further annealing. A pronounced hole doping of graphene has been observed as a consequence of the annealing-assisted intercalation of Cl. This effect has been studied by a combination of core-level and angle-resolved photoelectron spectroscopies (CL PES and ARPES, respectively), near-edge x-ray absorption fine structure (NEXAFS) spectroscopy and low-energy electron diffraction (LEED). The ease of preparation, the remarkable reproducibility of the doping level and the reversibility of the doping upon annealing are the key factors making chlorination with FeCl(3) a promising route for tuning the electronic properties in graphene.     

采用(meta)-GGA泛函对Ag(111)、Rh(111)和Ir(111)上乙烯吸附的理论研究

本文针对在多种催化反应的重要中间体乙烯,使用(meta)-GGA等级的包含PBE,BEEF-vdW,SCAN以及SCAN+rVV10在内的多种交换关联泛函,系统研究了在过渡金属表面(Ag,Rh和Ir)上乙烯吸附势能面对泛函的依赖关系. 研究发现,对于乙烯在贵金属Ag(111)上的吸附,除了PBE外,BEEF-vdW,SCAN以及SCAN+rVV10均能预测出物理吸附态的存在. 对于乙烯在Rh(111)面的吸附,SCAN和SCAN+rVV10预测在化学吸附位之前存在有物理吸附前驱态,而基于PBE和BEEF-vdW的势能面并没有发现前驱态的存在. 而对于乙烯在Ir(111)上的吸附,BEEF-vdW也能微弱地预测出化学吸附前驱态的存在. 研究结果表明,无论在哪一种金属表面上,四种泛函中SCAN+rVV10给出的吸附能最强,其次是SCAN,最后是PBE或者BEEF-vdW.     

Scalable synthesis of graphene on single crystal Ir(111) films

We have investigated single crystal Ir(111) films grown heteroepitaxially on Si(111) wafers with yttria-stabilized zirconia (YSZ) buffer layers as possible substrates for an up-scalable synthesis of graphene. Graphene was grown by chemical vapor deposition (CVD) of ethylene. As surface analytical techniques we have used scanning tunneling microscopy (STM), low-energy electron diffraction, scanning electron microscopy, and atomic force microscopy. The mosaic spread of the metal films was below 0.2° similar to or even below that of standard Ir bulk single crystals, and the films were basically twin-free. The film surfaces could be improved by annealing so that they attained the perfection of bulk single crystals. Depending on the CVD conditions a lattice-aligned graphene layer or a film consisting of different rotational domains were obtained. STM data of the non-rotated phase and of the phases rotated by 14° and 19° were acquired. The quality of the graphene was comparable to graphene grown on bulk Ir(111) single crystals.     

Low-temperature growth of large-scale,single-crystalline graphene on Ir(111)

Iridium is a promising substrate for self-limiting growth of graphene. However, single-crystalline graphene can only be fabricated over 1120 K. The weak interaction between graphene and Ir makes it challenging to grow graphene with a single orientation at a relatively low temperature. Here, we report the growth of large-scale, single-crystalline graphene on Ir(111) substrate at a temperature as low as 800 K using an oxygen-etching assisted epitaxial growth method. We firstly grow polycrystalline graphene on Ir. The subsequent exposure of oxygen leads to etching of the misaligned domains.Additional growth cycle, in which the leftover aligned domain serves as a nucleation center, results in a large-scale and single-crystalline graphene layer on Ir(111). Low-energy electron diffraction, scanning tunneling microscopy, and Raman spectroscopy experiments confirm the successful growth of large-scale and single-crystalline graphene. In addition, the fabricated single-crystalline graphene is transferred onto a SiO_2/Si substrate. Transport measurements on the transferred graphene show a carrier mobility of about 3300 cm~2·V~(-1)·s~(-1). This work provides a way for the synthesis of large-scale,high-quality graphene on weak-coupled metal substrates.     

Step edge diffusion and the structure of nanometer-size Ir islands on the Ir(111) surface

We report an FIM study of the structure of nanometer-size Ir islands on the Ir(111) surface. In this experiment, the number of atoms in an island is carefully controlled by field evaporation and vapor deposition. When this number can be fitted to a hexagonal atomic arrangement, the stable structure is found to be a perfect hexagon. In other cases, an addition of one ledge atom can reverse the symmetry of a small island or change its shape. We also compare diffusion of adatoms on the Ir(311) and (331) surfaces to that of ledge-atoms along the A- and B-type steps of the (111) layer, and the relative binding energies of a ledge atom at these steps.     

NO在Ir(111)表面吸附与解离的第一性原理研究

本文系统研究了NO在Ir(111)表面的吸附,解离,以及可能的N_2生成机理.结果表明,顶位吸附的NO,其解离能垒较高(3.17 eV),不会发生解离,而三重Hcp和Fcc空位吸附的NO发生解离,能垒分别为1.23和1.28 eV.N_2是唯一的生成物,不会有副产物N_2O的产生.其最可能的反应路径为N和NO经过N_2O中间体而生成N_2,而不是直接N提取和N-N聚合产生N_2的机理.     

Adsorption of acetylene and ethylene on a clean Ir(111) surface

The adsorption of C₂H₂ and C₂H₄ on Ir(111) is studied within the temperature range 180–500 K by the HREELS and XPS methods. The absolute concentration of hydrocarbon coverage is estimated by XPS. Data are obtained on the kinetics of adsorption of the two gases at different temperatures. It is established by HREELS studies that at 180 K C₂H₄ forms ethylidyne (CCH₃ whereas C₂H₂ is adsorbed as CCH and ethylidyne species. At 300 K both kinds of species are found on the Ir(111) surface after C₂H₂ or C₂H₄ exposures. The ethylidyne decomposes completely to CCH at 500 K, which can be accompanied by polymerization of adsorbed hydrocarbon species.     

甲氧基在Ir(111)表面吸附的密度泛函理论研究

本文采用第一性原理和周期平板模型相结合的方法,对甲氧基在Ir(111)表面top,bridge,fcc和hcp位的吸附模型进行了构型优化、能量计算、Mulliken电荷布居分析以及差分电荷密度计算.结果表明,甲氧基通过氧原子与金属表面相互作用时,垂直吸附在fcc位是最有利的吸附构型,吸附能为2.26eV,此时电子从金属表面向甲氧基转移.吸附过程中C-O键振动频率发生红移,表明在该表面C-O键容易被活化.结合差分电荷密度分析表明,吸附时CH3O中氧的2p原子轨道和铱的dz2原子轨道相互作用形成σ键.     

甲氧基在Ir(111)表面吸附的密度泛函理论研究

本文采用第一性原理和周期平板模型相结合的方法,对甲氧基在Ir(111)表面top, bridge, fcc和hcp位的吸附模型进行了构型优化、能量计算、Mulliken电荷布居分析以及差分电荷密度计算。结果表明,甲氧基通过氧原子与金属表面相互作用时,垂直吸附在fcc位是最有利的吸附构型,吸附能为2.26 eV,此时电子从金属表面向甲氧基转移。吸附过程中C-O键振动频率发生红移,表明在该表面C-O键容易被活化。结合差分电荷密度分析表明,吸附时CH3O中氧的2p原子轨道和铱的dz2原子轨道相互作用形成σ键。