Overlayer Phase Research Articles

Structural investigation of flat overlayer and surface alloy of Sn on Mo(110)

Atomic structures of Sn on Mo(110) are investigated using low-energy electron diffraction (LEED) and density functional theory (DFT) calculation. Sn atoms occupy substitutional sites at elevated temperatures in (3 × 1) structure, corresponding to a coverage of 0.33 ML. The substitutional model agrees with previous LEED, workfunction, and STM results. For (1 × 3) structure at 0.67 ML, Sn atoms adsorb in the nearest three-fold hollow sites. With increasing Sn coverage, the (1 × 4) structure at 0.75 ML, Sn atoms start to fill four-fold hollow sites. On both overlayer phases, Sn structures are flat without significant buckling. Compared with a two-dimensional tin, stanene, which is a stacking of α-Sn(111), Sn overlayer on Mo(110) is not related to α-Sn(111) but to β-Sn(100), which is normal metallic

Thermodynamic Driving Forces for Substrate Atom Extraction by Adsorption of Strong Electron Acceptor Molecules.

A quantitative structural investigation is reported, aimed at resolving the issue of whether substrate adatoms are incorporated into the monolayers formed by strong molecular electron acceptors deposited onto metallic electrodes. A combination of normal-incidence X-ray standing waves, low-energy electron diffraction, scanning tunnelling microscopy, and X-ray photoelectron spectroscopy measurements demonstrate that the systems TCNQ and F4TCNQ on Ag(100) lie at the boundary between these two possibilities and thus represent ideal model systems with which to study this effect. A room-temperature commensurate phase of adsorbed TCNQ is found not to involve Ag adatoms, but to adopt an inverted bowl configuration, long predicted but not previously identified experimentally. By contrast, a similar phase of adsorbed F4TCNQ does lead to Ag adatom incorporation in the overlayer, the cyano end groups of the molecule being twisted relative to the planar quinoid ring. Density functional theory (DFT) calculations show that this behavior is consistent with the adsorption energetics. Annealing of the commensurate TCNQ overlayer phase leads to an incommensurate phase that does appear to incorporate Ag adatoms. Our results indicate that the inclusion (or exclusion) of metal atoms into the organic monolayers is the result of both thermodynamic and kinetic factors.

Связь критических явлений в механизме Ленгмюра–Хиншельвуда с типом фазовой диаграммы адсорбционного слоя: квадратная решетка, аддитивные смеси

Связь критических явлений в механизме Ленгмюра–Хиншельвуда с типом фазовой диаграммы адсорбционного слоя: квадратная решетка, аддитивные смеси

Energy level offsets and space charge layer formation at electrode-electrolyte interfaces: X-ray photoelectron spectroscopy analysis of Li-ion model electrodes

Energy level offsets and related space charge layer formation are crucial for the kinetic properties of functional electric interfaces. In this contribution, we give a concise overview of our results on model Li-ion cathode interfaces with different overlayer phases obtained by surface science methodology using X-ray photoelectron spectroscopy. In particular, we present electronic energy level alignment, space charge layer formation, and Li1s core level binding energy differences at thin film LiCoO2 surfaces covered by thin layers of solid state electrolyte, other ionic compounds and organic solvents. The data suggest a correlation between the binding energy of the Li1s emission and the extent of the space charge layer formation, which can be rationalized using the concept of ionic energy levels. The data also indicate that interface dipole potentials might be relevant for the formation of some of the investigated interfaces, and suggest that large electronic bulk energy level offsets are insufficient indicators for interface stability, as is usually assumed for electrode-electrolyte interfaces.

Temperature-dependent structure, elasticity, and entropic stability of Bi phases on Cu{111}

We have used low energy electron microscopy (LEEM) to characterize the structure and stability of Bi phases on Cu{111}. As a function of temperature we find that the Cu{111}($\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3}$)R${30}^{\ensuremath{\circ}}$-Bi surface alloy phase gradually dealloys and is fully depleted from Bi at a temperature of 803 K. The dealloying leads to a defect induced change of its elastic properties. The Bi surface alloy phase coexists with a Bi overlayer phase that exhibits a sharp decrease in density in a narrow temperature interval just below the temperature where the surface alloy phase has fully dealloyed. LEEM is used to directly evaluate the structure as well as the entropic contributions that determine the stability of each of the phases.

Phase Transition Driven Discontinuity in Thermodynamic Size Selection

We show how an order-disorder phase transition in a two-dimensional system can discontinuously alter the shape and size of stress-stabilized self-assembled nanostructures. Low energy electron microscopy was used to study the dealloying of the Cu(111)-sqrt[3]×sqrt[3]-R30°-Bi surface alloy. The gradual expulsion of embedded bismuth from the alloy with increasing temperature induces a hard-hexagon-type order-disorder transition in the surface alloy. Our low energy electron microscopy results demonstrate how the loss of long-range order induces enormous changes in the domain patterns that the alloy forms with a Bi overlayer phase. We propose that the occurrence of phase transitions in one of the two surface phases that constitute a self-assembled domain pattern, provides a general, largely unexplored, mechanism that can be used to influence the morphological details of two-dimensional nanostructures.

Computational Studies of Experimentally Observed Structures of Sulfur on Metal Surfaces

First-principles electronic structure calculations were carried out to examine the experimentally observed structures of sulfur on close packed surfaces of a number of important metals - Ag(111), Cu(111), Ni(111), Pt(111), Rh(111), Re(0001) and Ru(0001). At low coverages ({le} 1/3 ML), the prediction is consistent with the typical pattern of preferred sulfur occupancy of threefold hollow sites, notably the fcc site on the (111) surfaces and the hcp site on the (0001) surfaces. Theoretical confirmation for the existence of pure sulfur overlayer phases on Pt(111), Rh(111), Re(0001) and Ru(0001) at higher coverages (> 1/3 ML) was provided. For the ({radical}7 x {radical}7) phase seen on Ag(111), the most preferred structure identified for adsorbed S trimer consists of an S atom on the top site bonded to two S atoms situated on the nearest neighbor off-bridge site positions. Among the different densely packed mixed sulfur-metal overlayer models suggested for the ({radical}7 x {radical}7) phase on Cu(111), the structure which consists of metal and S atoms in a hexagonal-like arrangement on the top substrate was found to be the most energetically favorable. For the (5{radical}3 x 2) phase on Ni(111), the calculations confirm the existence of clock-reconstructed top layer metal atoms onto whichmore » sulfur atoms are adsorbed.« less

Optical reflectance anisotropy of the Si/Cu(110) surface alloy

Measurements of the optical reflectance anisotropy (RA) of the Si/Cu(110)-c(2 × 2)surface alloy are reported. Significant changes in the RA response of Cu(110) are observed uponthe formation of the surface alloy, and with the growth of one-dimensional (1D) anisotropic Sichains on top of the surface alloy. The transitions between the surface states near the Fermi level(EF) at the symmetry point on the clean Cu(110) surface are no longer observed in RA spectra of 0.3 MLSi coverage. Peaks in RA spectra arising from transitions between surface-modified bands nearEF atthe L point are found to be sensitive to the formation of the surface alloy. The RA response of thec(2 × 2) surface alloy from 3.0 to 5.5 eV is simulated using a simple three-phase derivativemodel. The addition of an overlayer phase to this model makes it possible tosimulate higher coverage Si/Cu RA profiles where 1D Si chains cover the surfacealloy. The success of the models, in which discrete phases contribute to the RAresponse, supports the view that the Si chains grow on top of the intact c(2 × 2) alloy. Depositing between 1.2 and 1.8 ML Si results in no change to the RA spectroscopysignal, indicating that the signal remains sensitive to the covered alloy interface.

Controlling Ta phase in Ta/TaN bilayer by surface pre-treatment on TaN

Beyond 0.13 μm feature size, a bilayer of Ta/TaN is commonly used as a metal barrier and adhesive layer for copper multilevel interconnections. However, for the Ta films, cubic α-phase is preferred due to the lower resistivity, whereas high-resistive tetragonal β-phase should be avoided but easily formed by sputtering. Here, we demonstrate that the Ta overlayer phase can be effectively controlled by resputtering TaN surface using Ar prior to Ta growth. Upon surface treatment for enough time, α-phase forms instead of β-Ta. The intriguing phase change is attributed to the decrease of TaN lattice constant toward the surface, which renders a better lattice-matched template for α-Ta. From Auger depth profile, resputtering causes nitrogen deficiency in the surface layer of TaN to become TaN 0.1, leading to about 1.9% lattice mismatch. The mechanism for the phase change of the Ta overlayer was further verified by examining phase evolution of Ta deposited on various materials with different structure and lattice constants including W, TaN x ( x<1) and Ti.

Photoemission study of the anisotropic phase formation of Pb on Si(557)

Pb nanowires formed on the stepped surface of Si(557) were recently reported by Tegenkamp et al. to exhibit an intriguing insulator-to-metal transition upon cooling with the metallic character persistent to $4\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ [Phys. Rev. Lett. 95, 176804 (2005)]. High-resolution photoemission spectroscopy and low-energy-electron diffraction (LEED) are applied to systematically investigate the phase formation of Pb on Si(557) and to disclose its overall structural and electronic properties. Through LEED, we find two anisotropic phases around one monolayer coverage in addition to the well-known two-dimensional Pb overlayer phases observed on a flat Si(111) surface. In particular, the anisotropic phase denoted as $\ensuremath{\alpha}\ifmmode\times\else\texttimes\fi{}2$ with a $4\frac{2}{3}\ifmmode\times\else\texttimes\fi{}2$ periodicity ($\ifmmode\times\else\texttimes\fi{}2$ along step edges) has a good long-range order and is thought to be due to an ordered nanowire array. In $\mathrm{Si}\phantom{\rule{0.2em}{0ex}}2p$ spectra, an extremely narrow line shape and distinct surface components are identified for the $\ensuremath{\alpha}\ifmmode\times\else\texttimes\fi{}2$ phase supporting its good structural order. The $\mathrm{Pb}\phantom{\rule{0.2em}{0ex}}5d$ core-level spectra indicate that the $\ensuremath{\alpha}\ifmmode\times\else\texttimes\fi{}2$ phase consists of a single-dense Pb layer with two distinct Pb sites. The metallic nature of this phase is indicated by the asymmetric line shapes of $\mathrm{Pb}\phantom{\rule{0.2em}{0ex}}5d$ and $\mathrm{Si}\phantom{\rule{0.2em}{0ex}}2p$ and the finite density of states at Fermi level in valence-band photoemission.

Mechanisms of Metal Ion Sorption on Calcite: Composition Mapping by Lateral Force Microscopy

We show that lateral force microscopy (also known as frictional force microscopy) can differentiate between substrate and overlayer phases during an inorganic surface reaction. A calcite substrate is imaged in situ, while immersed in aqueous solutions of pH ∼ 6−9 containing metal ions (Cd2+, Sr2+, and La3+) at concentrations of 10-5 to 10-3 M. Cd2+ and Sr2+ passivate surface steps, initiating overgrowth only in solutions already supersaturated relative to their respective carbonates. In contrast, La3+ initiates overgrowth even in undersaturated conditions and carries the reaction to completion by scavenging carbonate anions directly from the dissolving calcite surface. Monomolecular surface steps play a central role, serving as both dissolution sites for the substrate and nucleation sites for the overgrowth.

A structural study of the interaction of methanethiol with Pt [formula omitted] using X-ray standing waves

In combination with surface characterisation by synchrotron radiation X-ray photoelectron spectroscopy, the normal incidence X-ray standing wave (NIXSW) technique has been applied to a determination of the structure of surface phases formed on Pt(1 1 1) by reaction with methanethiol. On surfaces heated to ⩾ ≈500 K, producing only coadsorbed atomic S and C, the S atoms are found to occupy fcc hollow sites (directly above Pt atoms in the third layer) in a geometry essentially identical to that of simple ordered S overlayer phases on Pt(1 1 1) with a S–Pt layer spacing of 1.67 Å, but with possible fractional co-occupation of a complex S phase. On a surface annealed to ≈223 K only a surface methanethiolate (CH 3S–) species is believed to be present, the favoured model involves a tilted off-atop bonding such that the S atoms are located offset from the fcc hollow sites, with frustrated rotational vibrations of large amplitude, although an alternative model based on co-occupation of atop and fcc hollow sites is also consistent with the NIXSW data.

Adsorption of O 2 and NO on Pd nanocrystals supported on Al 2O 3/NiAl( [formula omitted]): overlayer and edge structures

Adsorbate-induced overlayers on nanosized metal clusters have been studied with atomic resolution for the first time. Using scanning tunneling microscopy on supported palladium nanocrystals it was found that oxygen forms a p(2×2) overlayer phase and nitric oxide a c(4×2) overlayer phase. Adsorption of oxygen has a prominent effect on the cluster edges. On the basis of density functional theory calculations the effect is assigned to an oxygen-induced edge reconstruction.

Potential-dependence of CO adlayer structures on Pt(111) electrodes in acid solution: Evidence for a site selective charge transfer

The oxidation of carbon monoxide on a Pt(111) electrode surface is studied under transient and stationary reaction conditions in a 0.1 M HClO4 aqueous solution as a function of the applied electrode potential. The optical response is investigated by optical second harmonic generation (SHG) at 280 nm and infrared absorption spectroscopy (IRAS) starting at low potentials where the electrode is not reactive [e.g., 0.1 V vs the reversible hydrogen electrode (RHE)] towards more positive potentials where CO oxidation is initiated. For transient reaction conditions, in the absence of CO dissolved in solution, the oxidation starts at about 0.5 V vs. RHE and consists of a fast oxidation of about 10% of the adlayer and of a second reaction with slower kinetics involving the removal of the complete CO layer (overlayer stripping). Under steady-state conditions in CO-saturated solution the CO adlayer is stable up to 0.9 V vs RHE. At 0.63 V an overlayer phase transition is indicated by a 20% increase of the isotropic component of the second harmonic (SH) intensity (pp-polarization). The same potential region for this phase transition in the adlayer structure is derived from IRAS spectra after correcting for the effect of the thin layer electrolyte in IRAS measurements. The disappearance of hollow sites, the appearance of bridge sites, as well as an increased occupation of on-top sites at 0.63 V is interpreted as corresponding to the adlayer phase transition. The observations are consistent with a transition from the c(2×2) to the (√19×√19) adlayer structure of CO. The high sensitivity of SHG with regard to structural phase transitions of the CO adlayer is explained by distinct charge transfer contributions to the second-order surface susceptibility at different coordination sites.

Photoemission study of K on graphite

The physical and electronic structure of the dispersed and $(2\ifmmode\times\else\texttimes\fi{}2)$ phases of K/graphite have been characterized by valence and core-level photoemission. Charge transfer from K to graphite is found to occur at all coverages, and includes transfer of charge to the second graphite layer. A rigid band description is reasonably successful in describing important aspects of the data, and our results are consistent with a shift of approximately $0.4 \mathrm{eV}$ in the surface graphite layer for both phases. The C $1s$ line shape and binding-energy shift as a function of charge transfer can be understood qualitatively by taking into account rigid band effects and the effects of a core hole on the density of states. For the $(2\ifmmode\times\else\texttimes\fi{}2)$ phase the metallic overlayer contributes extrinsic satellites to the C $1s$ line shape. The K $3p$ spectrum is strongly affected by the overlayer phase, and in addition indicates very little variation in the substrate charge distribution as a function of coverage in the dispersed phase. The lack of an interface K $3p$ binding-energy shift for a K bilayer or multilayer is ascribed to a weak K-graphite bond for metallic overlayers. The results have implications for the interpretation of photoelectron spectra of alkali graphite intercalation compounds (GIC's).

Epitaxial growth and phase transition in multilayers of the organic semiconductor PTCDA on InAs(0 0 1)

Abstract We study the initial stages of growth of the organic molecular semiconductor 3,4,9,10-perylenetetracar☐ylic dianhydride (PTCDA) on In-terminated InAs(0 0 1) using scanning tuneling microscopy (STM) and low-energy electron diffraction (LEED). The first 1–2 monolayers (ML) of PTCDA interact relatively strongly with the surface and form a two-dimensional (2D) overlayer with a periodicity similar to that of the original (4 × 2)/c(8 × 2) reconstruction. STM shows that molecular ordering is due to two preferential alignments of the molecules on the substrate which depend upon the molecular orbital symmetry and the atomic-scale substrate potential. A phase transition occurs for coverages greater than ∼ 2ML where bulk-like three-dimensional (3D) PTCDA clusters begin to form with a unique orientation with respect to the substrate. The film orientation results from the alignment of high-symmetry planes of the 3D PTCDA clusters to high-symmetry planes of the 2D PTCDA overlayer. This partially commensurate epitaxial growth supports previously formulated models of “point-on-line coincidence” and is apparently a general phenomenon of some weakly interacting molecular interfaces. The present example of PTCDA grown on InAs(0 0 1) is of interest due to the passivating nature of the 2D overlayer phase which forms a weakly interacting template for further bulk-like PTCDA growth in the Stranski-Krastanov mode.

Surface scattering of hyperthermal (10–50 eV) neutral C 60 molecules

We have studied the scattering of neutral C 60 molecules from surfaces at the impact energy range of 10–50 eV. The reactive interaction of hyperthermal C 60 with a clean polycrystalline nickel surface was found to be strongly dependent upon surface temperature and C 60 beam dose. Following adsorption and decomposition of C 60 on the substrate, two different types of carbon overlayer phases were identified. The high temperature (passivated) phase gave rise to direct inelastic scattering in the form of sharply-peaked angular and energy distributions. The scattering dynamics of C 60 molecular beams from this surface was probed by high resolution angular and energy distributions and vibrational energy measurements at various impact energies, surface temperatures and scattering and incidence angles. The scattered energy scales linearly with impact energy and kinetic energy losses vary with scattering angle from ∼85% to ∼40% (peak values). We have developed an improved method for measuring average vibrational energy of large polyatomic molecule or cluster and applied it to the surface scattered C 60. Vibrational excitation was found to be below 2% of impact energy, showing that C 60 is nearly non-deformable at these collision energies. This value is much lower than estimated before for higher energies C 60 + ion surface scattering. Analyzing the results we find nearly complete decoupling between normal and tangential energy losses. The agreement obtained between model prediction and experiment provides evidence in favor of spherical potential barrier geometry. The tangential losses are described in terms of various models of rotational excitation, and the involvement of translational slip during the scattering process is concluded. The C 60 surface interaction well depth was determined from the scattering results and compared with the measured desorption energy. Our detailed results describe the first real “bouncing” regime for C 60 scattering off surfaces, in agreement with recent molecular dynamics calculation.

Potassium-induced reconstructive phase transitions on Au(100)

The adsorption of potassium on the quasi-hexagonally reconstructed (“5 × 20”) Au(100) surface has been studied in the temperature range between 130 K and 300 K employing LEED, AES, XPS, and work function (Δф) measurements. At room temperature, an intermediate work function maximum is correlated with a transition of the quasi-hexagonal substrate surface into a (1 × 2) phase caused by a missing-row reconstruction of the Au(100) surface. The lifting of the quasi-hexagonal Au reconstruction is also accompanied by a sudden decrease of the potassium 2p32 core level binding energy by ∼ 0.9 eV. Further adsorption of potassium leads to the formation of complex LEED patterns (among others a c(4 × 4) phase) until the monolayer saturation is attained around Θ ≈ 0.38. Lowering the surface temperature to 130 K does not stabilize the clean surface reconstruction sufficiently to allow formation of stable potassium overlayer phases on the (“5 × 20”)-reconstructed Au(100) surface; instead Δф and LEED measurements at 130 K indicate disorder and partial inhibition of the transition into the reconstructed (1 × 2) phase pointing to a thermally activated process. All in all, we confirm results of a recent STM/LEED/AES study on the Au(100)K system and provide further evidence for the idea whereafter elevated potassium coverages may favor interlayer mixing and/or growth of a potassium—gold alloy.

Charge transfer and structure in C60 adsorption on metal surfaces.

The charge state and structure of ${\mathrm{C}}_{60}$ monolayers deposited on Au(110), polycrystalline Ag, and Ni(110) have been investigated by electron-energy-loss spectroscopy and low-energy electron diffraction. The vibrational excitation spectra have been used to obtain a quantitative determination of the molecular charge state of the adsorbed ${\mathrm{C}}_{60}$ molecules. On both the Au(110) and Ag surfaces the charge transfer from the surface to the ${\mathrm{C}}_{60}$ is found to be (1\ifmmode\pm\else\textpm\fi{}1) electrons and that on Ni(110) is (2\ifmmode\pm\else\textpm\fi{}1) electrons. These estimates are supported by the metallic nature of the low-energy (0--7.5 eV) electronic excitations of the monolayers. For submonolayer coverages of ${\mathrm{C}}_{60}$, rectangular overlayer phases are observed on Ni(110) in contrast to the close-packed islands seen on the other metals. This difference in the low-coverage structure can be related to differing substrate-adsorbate interactions, directly reflected by the ${\mathrm{C}}_{60}$ charge state.

Phase diagram and overlayer structures for CO on Rh(110)

Low energy electron diffraction (LEED) patterns have been recorded for CO on Rh(110) and analyzed as a function of sample temperature or CO dose using a video-LEED system. The results provide a diagram of boundaries and existence regions for overlayer phases. The overlayer is a disordered lattice gas at absolute CO coverages (θ CO) below about 0.08 for all temperatures. A pattern characterized by splitting of c(2 × 2) spots appears most intense for θ CO ≈ 0.3–0.6 below 180 K. The “split” c(2 × 2) pattern converts to a pattern with diffuse, broad c(2 × 2) spots above 180 K. Overlapping diffraction features from “(3 × 2)”, “(4 × 2)”, and “(5 × 2)” symmetries are observed for θ CO above about 0.62 and below 240 K. The “(5 × 2)” predominates by θ CO ≈ 0.8. With further increase in θ CO, (2 × 1) LEED spots appear with the “(5 × 2)” features. A pure (2 × 1) pattern appears at monolayer θ CO. Overlayer structural models with CO molecules in short-bridge sites (off-top models) or tilted from on-top adsorption sites (tilt models) are proposed to account for features in the LEED patterns. Off-top models giving rise to pseudo-hexagonal ordering are favored for the “split” c(2 × 2) overlayer. The “c(2 × 2)” overlayer contains uniformly sized domains with ideal c(2 × 2) ordering and disordered domain walls. Increasing 3-, 4-, and 5-fold symmetry in off-top adlayers arises from uniform, commensurate compression along [110]. In tilt overlayer models by comparison, the “( n × 2)” structures contain CO molecules in rows with local (2 × 1) symmetry separated by 3, 4, or 5 unit cell spacings. A zig-zag tilt model for reconstruction in the (2 × 1) CO phase is proposed.