NiAl Surface Research Articles

Adsorption-induced constraint on delocalization of electron states in an Au chain on NiAl(110)

We have carried out a density functional study of the localized constraint on the delocalized, unoccupied resonance states in a mono atomic Au chain on a NiAl(110) surface by adsorption of a CO molecule on one of the adatoms in the chain. This constraint was observed recently by scanning tunnelling microscopy and spectroscopy. The repulsive interaction of the occupied 5${\sigma}$ molecular state with the unoccupied, resonance state in a Au adatom in the chain is found to break the degeneracy of the Au adatom-induced resonance states and to disrupt their delocalization in the chain.

THE OXIDATION OF NIAL: What Can We Learn from Ab Initio Calculations?

▪ Abstract We review here the theory of the early stages of oxidation of the (110) surface of Ni1−x Alx, based on ab initio calculations using a plane-wave pseudopotential method. The clean surface and several oxidized surfaces have been investigated, with oxygen coverages up to 2ML of oxygen (1ML = 3 O atoms per 2 surface Al atoms). The theory to date is a description in terms of equilibrium thermodynamics, with a comparison of the free energies of several surfaces of different composition, implemented at the atomic scale. Three environmental parameters are singled out as control variables in this treatment, namely the alloy composition x (assumed to be near 0.5), the temperature T and the partial pressure of oxygen pO2. With certain reasonable approximations an analytic formula for the surface energy σ is derived in terms of these variables and some constants that are calculated ab initio together with others that are derived from experimental thermodynamic tables. At oxygen pressures just above the threshold for bulk oxidation of NiAl, the calculations explain the observed formation of a thin film of alumina in place of NiAl surface layers, with the consequent dissolution of Ni into the bulk. Ab initio calculations illustrate how the energetics of supplying Al to the surface depends on bulk stoichiometry, which alters the relative stability of different surface oxidation states so as to favour oxidation more if the alloy is Al-rich than if it is Ni-rich.

First principles theory of artificial metal chains on NiAl(110) surface

Using first-principle calculations we have studied the stable phases of metal-atom chains on the (110) surface of NiAl. Our investigation is mainly focused on the technologically relevant case of Au chains, but the results will be analyzed in the broader framework of a family of metallic systems. We demonstrate that the nature of the adatom (Au, Mn, Ni, Cu, Al, H, C, Na, K, and Ca) is responsible for different levels of interaction with the substrate and gives rise to a variety of electronic behaviors. With some transition metals (such as Au, Mn, Ni, and Cu) the NiAl surface acts as a simple structural template for the formation of the artificial one-dimensional system and does not affect the electronic properties of the chains. With other atomic species (H, C, Na, K, and Ca) we observe substantially different couplings and stronger interactions. We demonstrate that the different electronic properties of the various adatoms are responsible for different couplings with the substrate and compare our findings with the existing experimental results. Finally, in the case of Au chains we have investigated the role of adatom-adatom interactions in the formation of such one-dimensional structures.

Tunneling rates in electron transport through double-barrier molecular junctions in a scanning tunneling microscope

The scanning tunneling microscope enables atomic-scale measurements of electron transport through individual molecules. Copper phthalocyanine and magnesium porphine molecules adsorbed on a thin oxide film grown on the NiAl(110) surface were probed. The single-molecule junctions contained two tunneling barriers, vacuum gap, and oxide film. Differential conductance spectroscopy shows that electron transport occurs via vibronic states of the molecules. The intensity of spectral peaks corresponding to the individual vibronic states depends on the relative electron tunneling rates through the two barriers of the junction, as found by varying the vacuum gap tunneling rate by changing the height of the scanning tunneling microscope tip above the molecule. A simple, sequential tunneling model explains the observed trends.

Structure of the Ultrathin Aluminum Oxide Film on NiAl(110)

The well-ordered aluminum oxide film formed by oxidation of the NiAl(110) surface is the most intensely studied metal surface oxide, but its structure was previously unknown. We determined the structure by extensive ab initio modeling and scanning tunneling microscopy experiments. Because the topmost aluminum atoms are pyramidally and tetrahedrally coordinated, the surface is different from all Al2O3 bulk phases. The film is a wide-gap insulator, although the overall stoichiometry of the film is not Al2O3 but Al10O13. We propose that the same building blocks can be found on the surfaces of bulk oxides, such as the reduced corundum (0001) surface.

Point-defect properties of, and sputtering events in, the {001} surfaces of Ni3Al. II. Sputtering events at and near surfaces

Atomic recoil events at and near {001} surfaces of Ni3Al due to elastic collisions between electrons and atoms have been simulated by molecular dynamics to obtain the sputtering threshold energy as a function of atomic species, recoil direction and atomic layer of the primary recoil atom. The minimum sputtering energy occurs for adatoms and is 3.5 and 4.5 eV for Al and Ni adatoms on the Ni–Al surface (denoted ‘M’), respectively, and 4.5 eV for both species on the pure Ni surface (denoted ‘N’). For atoms within the surface plane, the minimum sputtering energy is 6.0 eV for Al and Ni atoms in the M plane and for Ni atoms in the N surface. The sputtering threshold energy increases with increasing angle, θ, between the recoil direction and surface normal, and is almost independent of azimuthal angle, ϕ, if θ<60°; it varies strongly with ϕ when θ>60°, with a maximum at ϕ = 45° due to ⟨{110}⟩ close-packed atomic chains in the surface. The sputtering threshold energy increases significantly for subsurface recoils, except for those that generate efficient energy transfer to a surface atom by a replacement collision sequence. The implications of the results for the prediction of the mass loss due to sputtering during microanalysis in a FEG STEM are discussed.

Vibrational spectroscopy of individual doping centers in a monolayer organic crystal

A scanning tunneling microscope (STM) is used to study individual Ag doping centers in a monolayer of C60 molecules supported on a thin Al2O3 film grown on the NiAl(110) surface. Vibronic states of the doping centers are observed with differential conductance (dIdV) spectroscopy. The double-barrier nature of the junction results in bipolar transport: same states participate in charge transport at both bias voltage polarities. Identification of the dIdV features corresponding to bipolar conduction enables a new mode of vibrational spectroscopy with STM.

A work function study of ultra-thin alumina formation on NiAl(1 1 0) surface

We have investigated the oxidation of NiAl(1 1 0) surface at 1020 and 670 K using ultra-violet photoelectron spectroscopy, Kelvin probe, X-ray photoelectron spectroscopy and low-energy electron diffraction. The work function change during oxidation was monitored in situ as a function of oxygen exposure. It was observed that the work function decreased by 0.6 eV after 7.9 Å of well-ordered Al 2O 3 formation on NiAl(1 1 0) at 1020 K. The formation of the interfacial dipole layer was the main factor that determined the work function and XPS binding energy shifts of Al 2O 3 energy levels. The work function decreased by 0.8 eV after 5.1 Å of amorphous Al 2O 3 formation at 670 K. The oxide layer structure was one of the key factors that determined the work function of the Al 2O 3/NiAl(1 1 0) system.

Self-assembly and dynamics of oxide nanorods on NiAl(110)

We observe the spontaneous formation of parallel oxide rods upon exposing a clean NiAl(110) surface to oxygen at elevated temperatures (850--1350 K). By following the self-assembly of individual nanorods in real time with low-energy electron microscopy (LEEM), we are able to investigate the processes by which the rods lengthen along their axes and thicken normal to the surface of the substrate. At a fixed temperature and ${\mathrm{O}}_{2}$ pressure, the rods lengthen along their axes at a constant rate. The exponential temperature dependence of this rate yields an activation energy for growth of $1.2\ifmmode\pm\else\textpm\fi{}0.1\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$. The rod growth rates do not change as their ends pass in close proximity $(&lt;40\phantom{\rule{0.3em}{0ex}}\mathrm{nm})$ to each other, which suggests that they do not compete for diffusing flux in order to elongate. Both LEEM and scanning tunneling microscopy (STM) studies show that the rods can grow vertically in layer-by-layer fashion. The heights of the rods are extremely bias dependent in STM images, but occur in integer multiples of approximately 2-\AA{}-thick oxygen-cation layers. As the rods elongate from one substrate terrace to the next, we commonly see sharp changes in their rates of elongation that result from their tendency to gain (lose) atomic layers as they descend (climb) substrate steps. Diffraction analysis and dark-field imaging with LEEM indicate that the rods are crystalline, with a lattice constant that is well matched to that of the substrate along their length. We discuss the factors that lead to the formation of these highly anisotropic structures.

Vibronic Spectroscopy of Single C60 Molecules and Monolayers with the STM

A scanning tunneling microscope is used to study the differential conductance (dI/dV) of single C(60) molecules in isolation and in monolayers adsorbed on NiAl(110) and on an ultrathin alumina film grown on the NiAl(110) surface. On the oxide layer, the electronic bands in the dI/dV spectra display a series of equally spaced features, attributed to the vibronic states of the molecules, which are absent when the molecules are adsorbed on the metal. A comparison between the molecular spectra measured on the oxide film reveals the effect of adsorption temperature and geometry, as well as intermolecular interactions on the vibronic features.

Interaction of Water with Ordered θ-Al2O3 Ultrathin Films Grown on NiAl(100)

The structure of an ordered, ultrathin theta-Al(2)O(3) film grown on a NiAl(100) single-crystal surface was studied by Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS), and low-energy electron diffraction (LEED), and its interaction with water was investigated with temperature programmed desorption (TPD) and XPS. Our results indicate that H(2)O adsorption on the theta-Al(2)O(3)/NiAl(100) surface is predominantly molecular rather than dissociative. For theta(H)()2(O) < 1 ML (ML = monolayer), H(2)O molecules were found to populate Al(3+) cation sites to form isolated H(2)O species aligned in a row along the cation sites on the oxide surface with a repulsive interaction between them. For theta(H)()2(O) > 1 ML, three-dimensional ice multilayers were observed to form, which then desorb during TPD with approximate zero-order kinetics as expected. A small extent of H(2)O dissociation was observed to occur on the theta-Al(2)O(3)/NiAl(100) surface, which was attributed to the presence of a low concentration of oxygen atom vacancies. Titration of these defect sites with adsorbed H(2)O molecules revealed an estimated defect density of 0.05 ML for the theta-Al(2)O(3)/NiAl(100) system consistent with the ordered nature of the synthesized oxide film.

Tailoring electronic properties of atomic chains assembled by STM

Atomic chains were assembled from single Au and Pd atoms on a NiAl(110) surface, using the tip of a scanning tunneling microscope. The electronic properties of the chains were investigated by spatially resolved conductance spectroscopy and microscopy, revealing the development of a one-dimensional, free-electron-like band. Onset energy, effective electron mass, and spatial localization of the band were influenced by structural and chemical modifications of the chains. The experiments demonstrate the effects of interatomic spacing and elemental composition of the chains, as well as the roll of local impurities and adsorption on the properties of the one-dimensional electronic system.

Electronic properties of artificial Au chains with individual Pd impurities

Artificial Au atomic chains with individual Pd impurities were assembled from single metal atoms with a scanning tunneling microscope on a NiAl(110) surface. Scanning tunneling spectroscopy (STS) revealed an electronic resonance 2.15 eV above the Fermi energy localized within 4 A of single Pd atom impurities and two electronic resonances 2.25 eV and 2.95 eV above the Fermi energy localized within 8 A of Pd dimer impurities. The emergence of these localized resonances was studied by STS at each stage of the atom-by-atom assembly. Additionally, conductance images of the chains revealed delocalized electronic density oscillations in the pure Au segments of the chains.

In-Plane and Out-of-Plane Diffraction ofH2from Metal Surfaces

We have measured in-plane and out-of-plane diffraction of H2 and D2 molecular beams scattered by reactive Pd(111) and nonreactive NiAl(110) surfaces at 140-150 meV. A comparison with six-dimensional quantum dynamics and classical trajectory calculations shows for the first time that accurate diffraction patterns can be obtained from state-of-the-art potential energy surfaces based on density functional theory. Our measurements show that, at general incidence conditions, out-of-plane diffraction is much more important than was assumed in previous experiments.

Control of Relative Tunneling Rates in Single Molecule Bipolar Electron Transport

The influence of relative electron tunneling rates on electron transport in a double-barrier single-molecule junction is studied. The junction is defined by positioning a scanning tunneling microscope tip above a copper phthalocyanine molecule adsorbed on a thin oxide film grown on the NiAl(110) surface. By tuning the tip-molecule separation, the ratio of tunneling rates through the two barriers, vacuum and oxide, is controlled. This results in dramatic changes in the relative intensities of individual conduction channels, associated with different vibronic states of the molecule.

Computational study of electron states in Au chains on NiAl(110)

We have carried out a density functional study of unoccupied, resonance states in a single Au atom, dimers, a trimer, and infinite Au chains on the NiAl(110) surface. Two inequivalent orientations of the ad-chains with substantially different interatomic distances were considered. From the study of the evolution of the electron states in an Au chain from being isolated to adsorbed, we find that the resonance states derive from the $6s$ states of the Au atoms, which hybridize strongly with the substrate states and develop a $p$-like polarization. The calculated resonance states and the local density of states (LDOS) images were analyzed in a simple tight-binding, resonance model. This model clarifies (1) the physics of direct and substrate-mediated adatom-adatom interactions and (2) the physics behind the enhancements of the LDOS at the ends of the adatom chains, and (3) the physical meaning of the ``particle-in-box'' model used in the analysis of observed resonance states. The calculated effective mass and band bottom energy are in good agreement with experimental data obtained from scanning tunnelling spectroscopy.

Mechanisms of Reversible Conformational Transitions in a Single Molecule

The reversible interconversion between two nonplanar conformations of single Zn(II) Etioporphyrin I molecules adsorbed on a NiAl(110) surface at 13 K was induced by a scanning tunneling microscope (STM). The threshold voltage for the conformational change at negative sample bias depends linearly on the tip-sample distance, suggesting an electrostatic force mechanism. The reverse conversion involves inelastic electron tunneling via a molecular electronic resonance at 1.25 eV. In contrast with the photon-induced conformational changes, an electrically induced mechanism is realized with the STM.

Formation of epitaxial Al 2O 3/NiAl(1 1 0) films: aluminium deposition

Structure of epitaxial Al 2O 3 layers formed on NiAl(1 1 0) substrates has been studied by means of reflection high-energy electron diffraction (RHEED). The elucidated structure was compared to the model suggested for 0.5 nm-thick Al 2O 3 layers [K. Müller, H. Lindner, D.M. Zehner, G. Ownby, Verh. Dtsch. Phys. Ges. 25 (1990) 1130; R.M. Jaeger, H. Kuhlenbeck, H.J. Freund, Surf. Sci. 259 (1991) 235]. The stepwise growth of Al 2O 3 film, involving deposition and subsequent oxidation of aluminium onto epitaxial 0.5 nm-thick Al 2O 3 layers, has been investigated. Aluminium was deposited at room temperature, whereas its oxidation took place during annealing at 1070 K. The Al 2O 3 thickness was monitored by means of Auger electron spectroscopy (AES). It was found that Al 2O 3 layer follows the structure of 0.5 nm thick Al 2O 3 film, although a tilting of Al 2O 3(1 1 1) surface plane with respect to NiAl(1 1 0) surface appeared after Al deposition.

Effects of surface cleaning on oxidation of NiAl(1 1 0)

The effects of Ar + ion sputtering on the surface of NiAl(1 1 0) and its oxidation behaviors were investigated using XPS and LEED. The surface roughness increased and the surface aluminium concentration decreased after several times of cleaning and oxidation of the NiAl(1 1 0) surface. This surface was shown to be less reactive under 1020 K oxidation but more reactive under 500 K than the first-cleaned NiAl(1 1 0) surface. After heating the samples with oxide over-layer to about 1320 K for about 15 min, the oxide over-layer was completely disappeared and a clean NiAl(1 1 0) surface was obtained. This result provided a new way to clean the NiAl(1 1 0) surface.

Single-atom spin-flip spectroscopy.

We demonstrate the ability to measure the energy required to flip the spin of single adsorbed atoms. A low-temperature, high-magnetic field scanning tunneling microscope was used to measure the spin excitation spectra of individual manganese atoms adsorbed on Al2O3 islands on a NiAl surface. We find pronounced variations of the spin-flip spectra for manganese atoms in different local environments.