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Applied Surface Science (v.256, #15)

Editorial Board (pp. ii).

Preface by Leszek Jurczyszyn Guest Editor; Robert Kucharczyk Guest Editor (pp. 4763-4763).

Adsorption of 2-chlorophenol on Cu₂O(111)–Cu_CUS: A first-principles density functional study by Mohammednoor Altarawneh; Marian W. Radny; Phillip V. Smith; John C. Mackie; Eric M. Kennedy; Bogdan Z. Dlugogorski; Aloysius Soon; Catherine Stampfl (pp. 4764-4770).

First-principles density functional theory and a periodic-slab model have been utilized to investigate the adsorption of a 2-chlorophenol molecule on a CuO(111) surface with a vacant Cu surface site, namely Cu₂O(111)–Cu_CUS. Several vertical and flat orientations have been studied. All of these molecular configurations interact very weakly with the Cu₂O(111)–Cu_CUS surface, an observation which also holds for clean copper surfaces and the Cu₂O(110):CuO surface. Hydroxyl-bond dissociation assisted by the surface was found to be endoergic by 0.42–1.72eV, depending predominantly on the position of the isolated H on the surface. In addition, the corresponding adsorbed 2-chlorophenoxy moiety was found to be more stable than a vacuum 2-chlorophenoxy radical by about 0.76eV. Despite these predicted endoergicities, however, we would predict the formation of 2-chlorophenoxy radicals from gaseous 2-chlorophenol over the copper (I) oxide Cu₂O(111)–Cu_CUS surface to be a feasible and important process in the formation of PCDD/Fs in the post-flame region where gas-phase routes are negligible.

Keywords: 2-Chlorophenol; PCDD/F; Polychlorodibenzo-; p; -dioxins; Polychlorodibenzofurans; CuO; DFT calculations

Defects of SiC nanowires studied by STM and STS by A. Busiakiewicz; A. Huczko; T. Dudziak; M. Puchalski; W. Kozłowski; M. Cichomski; S. Cudziło; Z. Klusek; W. Olejniczak (pp. 4771-4776).

For the first time the scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS) are employed to investigate the morphology and the surface electronic structure of the defective silicon carbide nanowires (SiCNWs). The SiCNWs produced via combustion synthesis route are studied. The STS measurements are performed in the current imaging tunneling spectroscopy mode (CITS) that allows us to determine the correlation between STM topography and the local density of electronic states (LDOS) around the bend of an isolated SiCNW. The measurements reveal fluctuations of LDOS in the vicinity of the defect. The local graphitisation and the inhomogeneous concentration of doping impurities (e.g. nitrogen, oxygen) are considered to explain these fluctuations of metallic-like LDOS in the vicinity of the SiCNW's deformation.

Keywords: Silicon carbide; Nanowire; Defect; STM; STS

Nanoscale patterning of functional perovskite-type complex oxides by pulsed laser deposition through a nanostencil by Cristian-Victor Cojocaru; Riad Nechache; Catalin Harnagea; Alain Pignolet; Federico Rosei (pp. 4777-4783).

We present studies on parallel nanoscale patterning of piezoelectrics/ferroelectrics via deposition through a nanostencil. Unlike other processes reported for oxide nanostructuring, we selectively deposit the material, directly, by interposing a nanosieve between the substrate and the deposition source. We show that this selective deposition can be realized even with materials as complex as perovskite oxides, both at room temperature and at high temperature. We elaborate on and analyze the performance of the nanostenciling approach for the growth of barium titanate BaTiO₃ on strontium titanate SrTiO₃(100). The patterned structures of ferroelectric materials are characterized by X-ray diffraction and imaged locally by scanning probe microscopy in piezoresponse mode to individually probe their functionality.

Keywords: Parallel patterning; Nanostencil lithography; Pulsed laser deposition (PLD); Perovskite complex oxides; Ferroelectric nanostructures; Barium titanate

Low-coverage alkali metal adsorption on the Ge(001)-p(1×2) surface by Barbara Czech; Paweł Mikołajczyk; Barbara Stankiewicz (pp. 4784-4788).

Alkali metals adsorbed on Ge(001)-p(1×2) exhibit a variety of atomic arrangements depending on the coverage. We present theoretical investigation of a quasi-isolated alkali atom adsorption on the Ge(001)-p(1×2) surface, with and without a missing-dimer defect. Stable adsorption sites are found and the corresponding adsorption energies are compared. For completeness, STM images are simulated for the energetically most favourable atomic configurations.

Keywords: Germanium; Alkali metals; Adsorption; Surface relaxation and reconstruction; DFT calculations; STM simulation

Investigation of InSb(110) and InSb(111) surfaces by means of target current (VLEED) spectroscopy and LEED by A. Dittmar-Wituski; A. Grudziński; Ł. Lademann; A. Mikołajczyk; Ł. Andraszyk; M. Roszak (pp. 4789-4795).

In this paper, we present experimental results of target current (TCS) and LEED investigation of well-oriented InSb(110) and InSb(111)-A and InSb(111)-B crystals. TCS results are interpreted in terms of very low electron diffraction (VLEED) and empty band signatures. To examine to which extent the TCS spectra reflect the bulk or surface electronic properties of InSb(110), thin layers of indium have been evaporated and the corresponding spectrum changes have been investigated.

Keywords: InSb; Target current spectroscopy (TCS); VLEED

Order–disorder transition for corrugated Au layers by Keisuke Fukutani; N. Lozova; S.M. Zuber; P.A. Dowben; P. Galiy; Yaroslav B. Losovyj (pp. 4796-4800).

Atomic-scale structure of the growth of a gold film on (112) plane of Mo single crystal was investigated by means of low energy electron diffraction (LEED) and scanning tunneling microscopy (STM) up to two monolayers (ML) of gold coverage. Both LEED and STM results establish that Au grows on Mo(112) in a layer-by-layer mode, for at least the first two monolayers. A number of ordered structures are formed and both the first and second layers adopt the Mo(112) 1×1 surface structure upon completion. For some gold layers on Mo(112), notably the 1.66 monolayer 3×1 and 1.75 monolayer 4×1 gold overlayers, we find evidence of a phase transition associated with increasing disorder in gold layers with structural corrugation and anisotropic band structure. The signature of this phase transition, at temperatures in the range of 400–500K, is a sharp decrease in the overlayer effective Debye temperature.

Keywords: PACS; 63.20.kr; 64.90.+b; 68.35.Ja; 72.10.FkGold overlayers; Debye temperature; Electron–phonon coupling; Phase transitions

The growth and stability of Fe layers on the Mo(111) surface by Sz. Klein; S. Stepanovskyi; J. Śliwiński (pp. 4801-4805).

The initial growth and the stability of Fe layers on the Mo(111) surface was studied with Auger electron spectroscopy, low energy electron diffraction, scanning tunneling microscopy and thermal desorption spectroscopy. At room temperature at least the first two monolayers grow layer-by-layer. The first layer is stable up to about 1200K. Excess Fe starts to agglomerate at about 400K and forms with increasing temperature thick flat-top islands which start to sublime at a somewhat below 1200K. A strong decrease of the adsorption energy with coverage was found in the first monolayer. No {211} or {110} micro-faceting could be seen at any coverage upon annealing.

Keywords: Faceting; TDS; AES; STM; Molybdenum

Theoretical study of CO and Pb adsorption on the Ni(111) and Ni₃Al(111) surfaces by K. Kośmider (pp. 4806-4812).

Adsorption of CO molecules and Pb atoms on the Ni(111) and Ni₃Al(111) substrates is studied theoretically within an ab initio density-functional-theory approach. Stable adsorption sites and the corresponding adsorption energies are first determined for stoichiometric surfaces. The three-fold hollow sites (fcc for Pb and hcp for CO) are found most favourable on both substrates. Next, the effect of surface alloying by a substitution of selected topmost substrate atoms by Pb or Ni atoms on the adsorption characteristics is investigated. When the surface Al atoms of the Ni₃Al(111) substrate are replaced by Ni atoms, the Pb and CO adsorption energies approach those for a pure Ni(111) substrate. The Pb alloying has a more substantial effect. On the Ni₃Al(111) substrate, it reduces considerably adsorption energy of CO. On the Ni(111) substrate, CO binding strengthens slightly upon the formation of the Ni(111)p(2×2)-Pb surface alloy, whereas it weakens drastically when the Ni(111)(3×3)R30°-Pb surface alloy is formed.

Keywords: Ni; Ni; ₃; Al; Pb; CO; Adsorption; Surface alloying; DFT calculations

Array of double Au–Ag chains on the Si(557) surface by M. Krawiec; M. Jałochowski (pp. 4813-4817).

Using scanning tunneling microscopy we study the topographic properties of Ag structure on the Au induced, highly ordered Si(557) surface. Topography measurements show that a small amount of Ag (0.25ML) deposited on that surface leads to considerable modifications of the one-dimensional structure induced by Au atoms. In particular, we observe two different chains on each terrace, which are identified as Si adatoms and Ag chain structures. The STM topography of those chains strongly depends on the bias voltage, indicating an important role of electronic effects in this system.

Keywords: Silicon; High index surfaces; Surface structure; Atomic chain; STM

Coadsorption of samarium with oxygen on the molybdenum (211) surface by M. Kuchowicz (pp. 4818-4821).

Samarium atoms exist in two different electronic configurations, namely, divalent (Sm²⁺) and trivalent (Sm³⁺). Up to now, those two electronic configurations have been connected with the existence of Sm atoms in different valence states. Recent theoretical calculations performed by Yakovkin show that both electronic configurations have similar LDOS around the Fermi level, and as a result, should give a similar photoelectron emission in the valence band region, which was identified earlier as the emission from the Sm²⁺ state. The Sm³⁺ signal in photoelectron emission could originate from contaminations of Sm by other elements, e.g. oxygen. To check the influence of O on the electronic structure of Sm, the XPS experiments of coadsorption of O and Sm have been performed.

Keywords: Samarium; Molybdenum; Oxygen; Coadsorption; Electronic structure; Mixed valence; XPS

Fluorine, chlorine and iodine adsorption on the Ge(001) surface: Comparative study for the coverage of 0.75 and 1 monolayer by P. Mikołajczyk; B. Stankiewicz (pp. 4822-4828).

Theoretical study of the adsorption of 0.75 and 1 monolayer of F, Cl, and I on the Ge(001)-p(1×2) surface is presented. Various stable atomic configurations are considered, their surface electronic properties are discussed by means of the density-of-states analysis, and the corresponding STM images are simulated. While the results obtained for Cl and I are similar to those known for Br, the F-covered surfaces exhibit noticeably different features.

Keywords: Germanium; Fluorine; Chlorine; Iodine; Adsorption; Surface relaxation and reconstruction; Density functional calculations; STM images

Energy level alignment of catechol molecular orbitals on ZnO(112¯0) and TiO₂(110) surfaces by Sylvie Rangan; Jean-Patrick Theisen; Eric Bersch; R.A. Bartynski (pp. 4829-4833).

The occupied and unoccupied electronic structure of catechol adsorbed onto two single crystal surfaces, rutile TiO₂(110) and wurtzite ZnO(112¯0), have been investigated using UV-photoemission and inverse photoemission spectroscopies (UPS and IPS) in an ultra-high vacuum environment. To aid in assignment of the spectral features, model metal-bound catechol structures were calculated using a DFT approach. From these measurements, the energy alignment of the catechol-related states with respect to the substrates band edges is directly determined and is in good agreement with a direct injection process of the photoexcited electron into the substrate conduction band, resulting in the characteristic absorption properties of adsorbed catechol.

Keywords: Photoemission; Inverse photoemission; Energy alignment; Photoexcitation; Direct injection

Adsorption of Nd on the Mo(110) surface by Marcin Wiejak; Maciej Jankowski; Ivan Yakovkin; Jan Kołaczkiewicz (pp. 4834-4838).

The adsorption of Nd on the Mo(110) surface has been studied by low energy electron diffraction and Auger electron spectroscopy. It has been found that at low coverages Nd adatoms form a rich amount of dilute ( n×2) commensurate structures, which can be explained as forming zigzag chains oriented along the [110] direction. Monte Carlo simulations indicate that the formation of the zigzag chain structures is initiated by the indirect lateral interaction between Nd adatoms.

Keywords: Low energy electron diffraction; Nd structures; Low-index transition metal surfaces; Monte Carlo simulations

Spectroscopic ellipsometry study of the dielectric response of Au–In and Ag–Sn thin-film couples by A.A. Wronkowska; A. Wronkowski; K. Kukliński; M. Senski; Ł. Skowroński (pp. 4839-4844).

Optical properties and phase composition of In–Au and Sn–Ag ultra-thin films grown by sequential evaporating and co-depositing of metals in a vacuum were investigated combining X-ray diffraction and spectroscopic ellipsometry methods. The atomic concentration ratios of bilayer and co-deposited samples were the same, i.e. In(Sn):Au(Ag)=1:2. The XRD patterns indicated creation of AuIn, AuIn₂, Au₃In₂, Au₉In₄ and Ag₃Sn intermetallic compounds at room temperature. The effective complex dielectric functions of the composite layers,〈ε˜(E)〉=〈ε1(E)〉+i〈ε2(E)〉, were determined from ellipsometric quantities Ψ and Δ measured in a photon energy range of 0.6–6.5eV. The free-carrier parameters (unscreened plasma frequency and free-carrier damping) and optical resistivity were evaluated using a semiclassical Drude–Lorentz model of the effective dielectric function. There was noticed a distinct influence of phase composition and surface morphology on the optical constants and conductivity of the samples: ρ_op changed from approximately 15μΩcm to 37μΩcm for Ag–Sn structures, composed of β-Sn and Ag₃Sn phases, and from 21μΩcm to 83μΩcm for Au–In multiphase system. Lower resistivity demonstrated diffusive layers formed after deposition of an In(Sn) thin film on the noble metal underlayer.

Keywords: PACS; 66.30.Pa; 78.66.Bz; 78.20.Ci; 07.60.Fs; 78.70.CkInterdiffusion in nanoscale solids; Intermetallic compounds; Dielectric function; Optical resistivity; Spectroscopic ellipsometry; X-ray diffractometry

Valence of “divalent” rare earth metals by I.N. Yakovkin (pp. 4845-4849).

It is generally recognized that light rare earths change their valence from 2 to 3 when forming a bulk metal while remaining divalent at the surface. However, performed DFT calculations ultimately indicate that the higher-binding-energy peaks in photoemission spectra (like the −5.3eV peak for Sm), characteristic of the trivalent 4f n⁻¹5d¹ configuration, correspond not to the ground state, but to excited states induced by radiation. This means that the trivalent state is not inherent for the bulk of divalent rare earths, and therefore they do not become trivalent.

Keywords: Rare earth metals; Valence; Density of states; DFT calculations

Band alignments at SrZrO₃/Ge(001) interface: Thermal annealing effects by M. Yang; W.S. Deng; Q. Chen; Y.P. Feng; L.M. Wong; J.W. Chai; J.S. Pan; S.J. Wang; C.M. Ng (pp. 4850-4853).

High-κ dielectrics SrZrO₃ were prepared on Ge(001) substrate using pulse laser deposition, and band alignments and thermal annealing effects were studied with high resolution X-ray photoemission spectroscopy. Valence and conduction band offsets at this interface were measured to be 3.26eV and 1.77eV, respectively. Interfacial Ge oxide layers were found at the interface. After annealing at 600°C, the interfacial Ge oxide layers were eliminated, and the valence band offset increased to 3.50eV, but the amorphous SrZrO₃ became polycrystalline in the meantime.

Keywords: High-; κ; Ge-FETs; Gate dielectrics

Molecule–solid interaction: Electronic states of anthracene-9-carboxylic acid adsorbed on the surface of TiO₂ by Agata Zdyb; Stanisław Krawczyk (pp. 4854-4858).

Electronic excited states of athracene-9-carboxylic acid chemisorbed on the surface of TiO₂ nanoparticles were investigated by means of the electroabsorption (Stark effect) spectroscopy at low temperature. In addition to a fraction of molecules adsorbed with no significant spectral changes, two forms absorbing at lowered energy (25,600cm⁻¹ and 24,900cm⁻¹) were observed, that exhibit a large increase in the excited-minus-ground state dipole moments, 2.7D and 9.5D, respectively, as well as changed molecular polarizabilities. The dipole moments are interpreted as indicators of partial charge-transfer character of the excited state, corresponding to the optical electron transfer from the adsorbate onto TiO₂ concomitantly with the photon absorption. Consequences of these observations are shortly discussed in the context of the electron transfer process in dye-sensitized solar cells.

Keywords: Titanium dioxide; Charge-transfer; Dye-sensitized solar cell; Stark effect

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Applied Surface Science (v.256, #15)