Coadsorption of CO and O on Ir(1 0 0): First principles calculations

Density functional theory (DFT) calculations with generalized gradient approximation (GGA) are performed to investigate the structural, electronic and energetic properties of Ir(1 0 0)-(mCO+nO), m=1, 2; n=1, 2 and 3, coadsorption systems. The stability of coadsorbed oxygen is not affected by the increase of CO coverage from 0.25 ML to 0.50 ML. However, the stability of CO increases by increasing O coverage from 0.25 ML to 0.50 ML and 0.75 ML as a result of the rumpling of the topmost Ir layer.     

Structural stability and electronic structure of iron adsorption on the GaN(0 0 0 1) surface

In this work, we have investigated by means of first-principles spin-polarized calculations, the electronic and magnetic properties of iron (Fe) adsorption and diffusion on the GaN(0 0 0 1) surface using density functional theory (DFT) within a plane-wave pseudopotential scheme. In the surface adsorption study, results show that the most stable positions of a Fe adatom on GaN(0 0 0 1) surface are the H3 sites and T4 sites, for low and high Fe coverage respectively. We found that the Fe-H3 2 × 2 surface reconstruction exhibits a half-metallic behavior with a spin band gap and stable ferromagnetism ordering, which is a desirable property for high-efficiency magnetoelectronic devices. In addition, confirming previous experimental results, we found that the iron monolayers present a ferromagnetic order and a large thermal stability. This is interesting from a theoretical point of view and for its technological applications.     

Metastable magnetic configurations of (V, Cr, Mn and Fe) on Ir(0 0 1) surfaces

We present a theoretical study of the electronic and magnetic structure of the 3d-transition metals (M = V, Cr, Mn and Fe) in several overlayer systems. The electronic as well as magnetic structures are investigated for pseudomorphic overlayers (M/Ir(0 0 1)), ordered alloyed overlayers of the type M_0.5Ir_0.5/Ir(0 0 1) and ordered binary surface alloys of V, Cr, Mn and Fe transition metals on Ir(0 0 1) substrates. The calculations are performed with a self-consistent tight-binding method using the unrestricted Hartree-Fock approximation within the Hubbard model. We obtained metastable c(2 × 2) configurations for V, Cr and Mn and a p(1 × 1) configuration for Fe pseudomorphic overlayers. However, ferrimagnetic configuration has been obtained for the ordered surface alloys M_0.5Ir_0.5 and the binary alloyed overlayers on Ir(0 0 1) surfaces.     

Adsorption and decomposition of NO on O-covered planar and faceted Ir(2 1 0)

We report on the adsorption and decomposition of NO on O-covered planar Ir(2 1 0) and nanofaceted Ir(2 1 0) with variable facet sizes investigated using temperature programmed desorption (TPD), high-resolution electron energy loss spectroscopy (HREELS), and density functional theory (DFT). When pre-covered with up to 0.5 ML O, both planar and faceted Ir(2 1 0) exhibit unexpectedly high reactivity for NO decomposition. Upon increasing the oxygen coverage to 0.7 ML O, planar Ir(2 1 0) has little activity while faceted Ir(2 1 0) still remains active toward NO decomposition, although NO decomposition is completely inhibited when both surfaces are pre-covered by 1 ML O. NO molecularly adsorbs on O-covered Ir at 300 K. At low NO and oxygen coverage, NO adsorbs on the atop sites of planar Ir(2 1 0) while on the bridge and atop sites of faceted Ir(2 1 0) composed of (1 1 0) and {3 1 1} faces. No evidence for size effects in the decomposition of NO on O-covered faceted Ir(2 1 0) is observed for average facet size in the range 5-14 nm. Our findings should be of importance for development of Ir-based catalysts for NO decomposition under oxygen-rich conditions.     

Hydrogen adsorption and diffusion on Pd(1 1 1)

The adsorption, diffusion and ordering of hydrogen on Pd(1 1 1) was studied by scanning tunneling microscopy in the temperature range of 37-90 K. At low coverage isolated hydrogen atoms were observed. They formed √3×√3-1H islands as the coverage increased. Above 1/3 monolayer (ML) coverage areas of a new phase with √3×√3-2H structure were formed, with both structures coexisting between 1/3 and 2/3 ML. Finally a 1 × 1 structure was formed after high exposures of hydrogen above 50 K, with a coverage close to 1 ML. Atomically resolved images reveal that H binds to fcc hollow sites.     

Growth of Cr on Ir(1 1 1) studied by scanning tunneling microscopy

We have studied the room-temperature growth of Cr on Ir(1 1 1) by scanning tunneling microscopy. Even in the low-coverage regime, up to a total coverage of 2 monolayers (ML), Cr does not grow in the layer-by-layer mode. Instead, we observe islands with local coverages Θ between 1 ML and 5 ML. While the 1st layer growth is pseudomorphic, sporadic defect lines are observed in the 2nd layer. For Θ ? 3 ML periodic one-dimensional dislocation lines appear indicating the onset of strain relief. Scanning tunneling spectroscopy reveals that islands with Θ = 1 ML exist in two modifications. Though their tunneling spectra are qualitatively rather similar, direct comparison shows that the main peak is shifted by about 15 mV, resulting in peak positions of −0.255 V and −0.270 V. We interpret these two modifications as regular fcc Cr and Cr which exhibits a faulted hcp stacking on Ir(1 1 1), respectively. The assignment of fcc to areas directly attached to substrate steps together with the evolution of the ratio of the different ML-areas with coverage leads to the conclusion that hcp is the more favorable stacking.     

Coverage-dependent magnetization of 3d transition-metal adatoms on Co(0 0 1) in the submonolayer regime

The electronic structure of 3d transition-metal atoms on face-centered cubic Co(0 0 1) substrate is determined within ab initio density functional calculations in the gradient corrected approach. Calculations are performed for ordered surface configuration with coverage equal to 0.25, 0.5, 0.75 and 1 ML. For Ni and Fe a ferromagnetic coupling with the Co atoms is always obtained independently of the concentration. Moreover the values of the magnetic moments remain similar. For Mn a ferromagnetic coupling is obtained for low-coverage whereas an in-plane antiferromagnetic coupling is found for a complete Mn overlayer on Co(0 0 1). Also, for Sc, Ti, V and Cr a drastic modification of the magnetic map is observed when we go from low-coverage to the monolayer. Cr (Mn) adatoms present antiferromagnetic (ferromagnetic) coupling with Co(0 0 1) for x = 0.25 whereas an in-plane antiferrimagnetic coupling is obtained for x = 1.00.     

Methanol adsorption on silicon (0 0 1)

Using a first-principles pseudopotential technique, we have investigated the adsorption of CH₃OH on the Si(0 0 1) surface. We have found that, in agreement with the overall experimental picture, the most probable chemisorption path for methanol adsorption on silicon (0 0 1) is as follows: the gas phase CH₃OH adsorbs molecularly to the electrophilic surface Si atom via the oxygen atom and then dissociates into Si-OCH₃ and H, bonded to the electrophilic and nucleophilic surface silicon dimer atoms, respectively. Other possible adsorption models and dissociation paths are also discussed. Our calculations also suggest that the most probable methanol coverage is 0.5 ML, i.e., one molecule per Si-Si dimer, in agreement with experimental evidences. The surface atomic and electronic structures are discussed and compared to available theoretical and experimental data. In addition, we propose that a comparison of our theoretical STM images and calculated vibrational modes for the adsorbed systems with detailed experimental investigations could possibly confirm the presented adsorption picture.     

Effect of hydrogenation on the electronic structure of the P/Si(0 0 1)-(1 × 2) surface

Ab initio calculations, based on pseudopotentials and density functional theory (DFT), have been performed to investigate the effect of hydrogenation on the electronic properties of P/Si(0 0 1)-(1 × 2) surface. In parallel with this, the electronic band structure of the hydrogenated and non-hydrogenated P/Si(0 0 1)-(1 × 2) surface have been calculated for half- and full-monolayer P. For the mixed Si-P dimer structure, we have identified two occupied and one unoccupied surface state, which correspond to 0.5 ML coverage of P. When this surface is terminated with H, we see that two occupied states completely disappeared and that one unoccupied state is shifted towards the conduction band. A similar calculations for the 1 ML coverage of P have been also carried out. It is seen that the unoccupied state C₁ appeared in the P/Si(0 0 1)-(1 × 2) surface is passivated when this surface is terminated with the H atoms. To explain the nature of the surface states, we have also plotted the total and partial charge densities at the point of the Surface Brillouin Zone (SBZ).     

Nanowedge island formation on Mo(1 1 0)

The deposition of ultrathin Fe films on the Mo(1 1 0) surface at elevated temperatures results in the formation of distinctive nanowedge islands. The model of island formation presented in this work is based on both experiment and DFT calculations of Fe adatom hopping barriers. Also, a number of classical molecular dynamics simulations were carried out to illustrate fragments of the model. The islands are formed during a transition from a nanostripe morphology at around 2 ML coverage through a Bales-Zangwill type instability. Islands nucleate when the meandering step fronts are sufficiently roughened to produce a substantial overlap between adjacent steps. The islands propagate along the substrate [0 0 1] direction due to anisotropic diffusion/capture processes along the island edges. It was found that the substrate steps limit adatom diffusion and provide heterogeneous nucleation sites, resulting in a higher density of islands on a vicinal surface. As the islands can be several layers thick at their thinnest end, we propose that adatoms entering the islands undertake a so-called “vertical climb” along the sides of the island. This is facilitated by the presence of mismatch-induced dislocations that thread to the sides of the islands and produce local maxima of compressive strain. Dislocation lines also trigger initial nucleation on the surface with 2-3 ML Fe coverage. The sides of the nanowedge islands typically form along low-index crystallographic directions but can also form along dislocation lines or the substrate miscut direction.     

Adsorption site, core level shifts and charge transfer on the Pd(1 1 1)-I(√3 × √3) surface

We use core level photoelectron spectroscopy and density functional theory (DFT) to investigate the iodine-induced Pd(1 1 1)-I(√3 × √3) structure formed at 1/3 ML coverage. From the calculations we find that iodine adsorbs preferentially in the fcc hollow site. The calculated equilibrium distance is 2.06 Å and the adsorption energy is 68 kcal/mol, compared to 2.45 Å and 54 kcal/mol in the atop position. The adsorption energy difference between fcc and hcp hollows is 1.7 kcal/mol. Calculated Pd 3d surface core level shift on clean Pd(1 l 1) is 0.30 eV to lower binding energy, in excellent agreement with our experimental findings (0.28-0.29 eV). On the Pd(1 1 1)-I(√3 × √3) we find no Pd 3d surface core level shift, neither experimentally nor theoretically. Calculated charge transfer for the fcc site, determined from the Hirshfeld partitioning method, suggests that the iodine atom remains almost neutral upon adsorption.     

Growth mode and novel structure of ultra-thin KCl layers on the Si(1 0 0)-2 × 1 surface

This study investigates ultra-thin potassium chloride (KCl) films on the Si(1 0 0)-2 × 1 surfaces at near room temperature. The atomic structure and growth mode of this ionic solid film on the covalent bonded semiconductor surface is examined by synchrotron radiation core level photoemission, scanning tunneling microscopy and ab initio calculations. The Si 2p, K 3p and Cl 2p core level spectra together indicate that adsorbed KCl molecules at submonolayer coverage partially dissociate and that KCl overlayers above one monolayer (ML) have similar features in the valance band density of states as those of the bulk KCl crystal. STM results reveal a novel c(4 × 4) structure at 1 ML coverage. Ab initio calculations show that a model that comprises a periodic pyramidal geometry is consistent with experimental results.     

NMR of Xe on CO/Ir(1 1 1) and on multilayer Xe/Ir(1 1 1)

Nuclear magnetic resonance (NMR) is performed on monolayer (ML) amounts of adsorbed ¹²⁹Xe on a single crystal substrate. The inherently low sensitivity of NMR is overcome by using highly nuclear spin polarized ¹²⁹Xe that has been produced by optical pumping. A polarization of 0.8 is regularly achieved which is 10⁵ times the thermal (Boltzmann) polarization. The experiments are performed with a constant flux of xenon atoms impinging on the surface, typically 4 ML/s. The chemical shift (σ) of ¹²⁹Xe is highly sensitive to the Xe local environment. We measured profoundly different shifts for the Xe bulk, for the surface of the Xe bulk, and for Xe on CO/Ir(1 1 1). The growth of the bulk is seen in a phase transition like change of σ as a function of temperature at constant Xe flux. At temperatures where no bulk forms at a flux of 4 ML/s, the xenon exchange rate was measured by a spin inversion/recovery method. The exchange time of Xe is found to be 0.24 s at 63.4 K and 64.4 K and somewhat longer at 61.2 K. An analysis is given involving the desorption out of the second layer and fast mixing of first and second layer atoms at these temperatures.     

Growth of Ce on Rh(1 1 1)

We have studied the growth of cerium films on Rh(1 1 1) using STM (scanning tunneling microscopy), LEED (low energy electron diffraction), XPS (X-ray photoelectron spectroscopy) and AES (Auger electron spectroscopy). Measurements of the Ce films after room temperature deposition showed that Ce is initially forming nanoclusters in the low coverage regime. These clusters consist of 12 Ce atoms and have the shape of pinwheels. At a coverage of 0.25 ML (monolayer, ML) an adatom layer with a (2 × 2) superstructure is observed. Above 0.4 ML, Rh is diffusing through pinholes into the film, forming an unstructured mixed layer. Annealing at 250 °C leads to the formation of ordered Ce-Rh compounds based on the bulk compound CeRh₃. At a coverage of 0.1 ML, small ordered (2 × 2) surface alloy domains are observed. The exchanged Rh atoms form additional alloy islands situated on the pure Rh(1 1 1) surface, showing the same (2 × 2) superstructure as the surface alloy. At a coverage of 0.25 ML, the surface is completely covered by the surface alloy and alloy islands. The (2 × 2) structure is equivalent to a (1 1 1)-plane of CeRh₃, contracted by 6%. Annealing a 1 ML thick Ce layer leads to a flat surface consisting of different rotational domains of CeRh₃(1 0 0). The Rh needed for alloy formation comes from 50 Å deep pits in the substrate. Finally we show that LEIS (low energy ion scattering) is not suitable for the characterization of Ce and CeRh films due to strong effects of neutralization.     

Oxygen-induced nano-faceting of Ir(2 1 0)

The adsorption of oxygen and the nanometer-scale faceting induced by oxygen have been studied on Ir(2 1 0). Oxygen is found to chemisorb dissociatively on Ir(2 1 0) at room temperature. The molecular desorption process is complex, as revealed by a detailed kinetic analysis of desorption spectra. Pyramid-shaped facets with {3 1 1} and (1 1 0) orientations are formed on the oxygen-covered Ir(2 1 0) surface when annealed to T?600 K. The surface remains faceted for substrate temperatures T<850 K. For T>850 K, the substrate structure reverts to the oxygen-covered (2 1 0) planar state and does so reversibly, provided that oxygen is not lost due to desorption or via chemical reactions upon which the planar (2 1 0) structure remains. A clean faceted surface was prepared through the use of low temperature surface cleaning methods: using CO oxidation, or reaction of H₂ to form H₂O, oxygen can be removed from the surface while preserving (“freezing”) the faceted structure. The resulting clean faceted surface remains stable for T<600 K. For temperatures above this value, the surface irreversibly relaxes to the planar state.     

Phase mixing and phase separation accompanying the catalytic oxidation of CO on Ir{1 0 0}

The co-adsorption of CO and O on the unreconstructed (1 × 1) phase of Ir{1 0 0} was examined by low energy electron diffraction (LEED) and temperature programmed desorption (TPD). When CO is adsorbed at 188 K onto the Ir{1 0 0} surface precovered with 0.5 ML O, a mixed c(4 × 2)-(2O + CO) overlayer is formed. All CO is oxidised upon heating and desorbs as CO₂ in three distinct stages at 230 K, 330 K and 430 K in a 2:1:2 ratio. The excess oxygen left on the surface after all CO has reacted forms an overlayer with a LEED pattern with p(2 × 10) periodicity. This overlayer consists of stripes with a local p(2 × 1)-O arrangement of oxygen atoms separated by stripes of uncovered Ir. When CO is adsorbed at 300 K onto the surface precovered with 0.5 ML O an apparent (2 × 2) LEED pattern is observed. LEED IV analysis reveals that this pattern is a superposition of diffraction patterns from islands of c(2 × 2)-CO and p(2  × 1)-O structures on the surface. Heating this co-adsorbed overlayer leads to the desorption of CO₂ in two stages at 330 K and 430 K; the excess CO (0.1 ML) desorbs at 590 K.LEED IV structural analysis of the mixed c(4 × 2) O and CO overlayer shows that both the CO molecules and the O atoms occupy bridge sites. The O atoms show significant lateral displacements of 0.14 Å away from the CO molecules; the C-O bond is slightly expanded with respect to the gas phase (1.19 Å); the modifications of the Ir substrate with respect to the bulk-terminated surface are very small.     

Effect of hydrogenation on B/Si(0 0 1)-(1 × 2)

Ab initio calculations, based on pseudopotentials and density functional theory, have been performed to investigate the effect of hydrogenation on the atomic geometries and the energetics of substitutional boron on the generic Si(0 0 1)-(1 × 2) surface. For a single B atom substitution corresponding to 0.5 ML coverage, we have considered two different sites: (i) the mixed Si-B dimer structure and (ii) boron substituting for the second-layer Si to form Si-B back-bond structure, which is energetically more favorable than the mixed Si-B dimer by 0.1 eV/dimer. However, when both of these cases are passivated by hydrogen atoms, the situation is reversed and the Si-B back-bond case becomes 0.1 eV/dimer higher in energy than the mixed Si-B dimer case. For the B incorporation corresponding to 1 ML coverage, among the substitutional cases, 100% interdiffusion into the third layer of Si and 50% interdiffusion into the second layer of Si are energetically similar and more favorable than the other cases that are considered. However, when the surface is passivated with hydrogen, the B atoms energetically prefer to stay at the third layer of the Si substrate.     

On-surface and sub-surface oxygen on ideal and reconstructed Cu(1 0 0)

In order to understand the first steps of the Cu(1 0 0) oxidation we performed first principles calculations for on-surface and sub-surface oxygen on this surface. According to our calculations, the adsorption energies for all on-surface site oxygen atoms increase, whereas the energies of the sub-surface atoms decrease with the increasing oxygen coverage. At coverage 1 ML and higher on the reconstructed surface, structures including both on- and sub-surface atoms are energetically more favourable than structures consisting only of on-surface adsorbates. On the ideal (1 0 0) surface this change can be perceived at coverage 0.75 ML.     

Benzene adsorption and oxidation on Ir(1 1 1)

Adsorption, decomposition and oxidation of benzene on Ir(1 1 1) was studied by high resolution (synchrotron) XPS, temperature programmed desorption and low energy electron diffraction. Molecular adsorption of benzene on Ir(1 1 1) is observed between 170 K and 350 K. Above this temperature both desorption and decomposition of benzene take place. An ordered adsorbate structure was observed upon adsorption around 335 K. Decomposition involves C-C bond breaking as the formation of CH_ad is observed. The presence of a saturated O_ad layer (0.5 ML) weakens molecular benzene adsorption and suppresses decomposition.     

Spectroscopic interpretation for Na-induced phase transitions on Na/Si(1 1 1)3 × 1

Na adsorption at room temperature causes the Na/Si(1 1 1)3 × 1 surface with Na coverage of 1/3 monolayer (ML) to transit into the Na/Si(1 1 1)6 × 1 surface at 1/2 ML and sequentially into the Na/Si(1 1 1)3 × 1 surface at 2/3 ML. The phase transition was studied by Si 2p core-level photoemission spectroscopy. The detailed line shape analysis of the Si 2p core-level spectrum of the Na/Si(1 1 1)3 × 1 surface (2/3 ML) is presented and compared to the Na/Si(1 1 1)3 × 1 surface (1/3 ML) which is composed of Si honeycomb chain-channel structures. This suggests that as additional Na atoms form atomic chains resulting in the Na/Si(1 1 1)3 × 1 surface (2/3 ML), the inner atoms of the Si honeycomb chain-channel structure is buckled due to the additional Na atoms.