Topmost Atomic Layer Research Articles

Direct Observation of the Surface Segregation of Cu in Pd by Time-Resolved Positron-Annihilation-Induced Auger Electron Spectroscopy

Density functional theory calculations predict the surface segregation of Cu in the second atomic layer of Pd which has not been unambiguously confirmed by experiment so far. We report measurements on Pd surfaces covered with three and six monolayers of Cu using element selective positron-annihilation-induced Auger electron spectroscopy (PAES) which is sensitive to the topmost atomic layer. Moreover, time-resolved PAES, which was applied for the first time, enables the investigation of the dynamics of surface atoms and hence the observation of the segregation process. The time constant for segregation was experimentally determined to τ=1.38(0.21) h, and the final segregated configuration was found to be consistent with calculations. Time-dependent PAES is demonstrated to be a novel element selective technique applicable for the investigation of, e.g., heterogeneous catalysis, corrosion, or surface alloying.

Pinning of Adsorbed Cobalt Atoms by Defects Embedded in the Copper (111) Surface

Using a low-temperature scanning tunneling microscope (STM), we observe that Co adatoms are unusually strongly bound to a particular type of pinning centers on the Cu(111) surface. Using density-functional-theory calculations, the pinning centers are identified as Ag substitutional atoms embedded in the topmost atomic layer of the surface. These impurities are hardly detectable in the STM images as they have low topographic height and produce no standing-wave patterns. They do not affect the exchange coupling of the Co adsorbate with the substrate electrons; thus, the Kondo resonances measured on pinned and free Co adatoms show no detectable differences. Whereas free Co adatoms undergo significant surface diffusion already above 8 K, Ag-stabilized Co adatoms remain pinned up to 12.7 K.

Environmentally friendly plasma-based surface engineering technologies

Weakly ionised reactive plasmas are characterised by a very low degree of ionization, which rarely exceeds the order of 10−4, and by a very high degree of dissociation, which often reaches values above 50%. Thus the most numerous plasma species are free neutral atoms originated from the dissociation of the source gas molecules. Neutral atoms are chemically very reactive species, which makes such plasma suitable for material processing. At the same time the neutral atoms have a low kinetic energy and therefore they cannot penetrate into the bulk material, so their effect is restricted to the topmost atomic layers of the material surface hence weakly ionised, reactive plasmas are suitable for surface engineering. Here we present examples of weakly ionised plasma applications as environmentally friendly alternatives to processes that otherwise utilise aggressive chemicals and produce toxic waste.

Imaging Top and Bottom

Chemistry![Figure][1] CREDIT: HAN ET AL., ACS NANO 3 , 10.1021/NN901030X (2009) Scanning tunneling microscopy of adsorbed molecules on surfaces usually interrogates the topography of the topmost layer of atoms. For small molecules, this information is often sufficient to answer many structural questions, but for long-chain molecules, the bonding of the molecule to the surface can be obscured, especially at high surface coverages. Han et al. imaged both the topmost layer and the bonding interface of alkane thiols containing up to 10 carbon atoms adsorbed on gold. The application of an ac modulation of the tunneling gap allowed the buried methyl groups at the gold/sulfur interface to be imaged in a derivative mode. In the high coverage regime, the coexisting domains of different packing arrangements varied in their gold/sulfur bonding motifs in a manner consistent with previous x-ray diffraction studies. This technique will also be useful in studying other processes where changes in molecular tilt impact function. ACS Nano 3 , 10.1021/nn901030x (2009). [1]: pending:yes

Theoretical Insights into the Hydrated (10.4) Calcite Surface: Structure, Energetics, and Bonding Relationships

Roothaan-Hartree-Fock molecular orbital methods were applied to investigate the ground-state structural, energetic properties, and bonding relationships of the hydrated (10.4) calcite surface. The adsorption of water molecules was modeled at the 6-31G(d,p) level of theory using Ca(n)(CO(3))(n) slab cluster models (4 <or= n <or= 18) with a varying number of H(2)O monomers (2 <or= (H(2)O)(n) <or= 6) interacting with the surface. Modeling results add fresh insights into the detailed 3D structural registry of the first and second hydration layers and the reconstructed (10.4) calcite surface, complementary to the information acquired from earlier atomistic, density functional, X-ray scattering, and grazing incidence X-ray diffraction studies. Both the modeled energies and geometries agree best with results of earlier density functional calculations, supporting the associative character of adsorbed water molecules. Two adsorption configurations are postulated: (i) H(2)O molecules interacting with surface Ca through ionic bonding and by hydrogen bonding to a surface O with their dipole slightly oblique above the surface (1st hydration layer), and (ii) H(2)O molecules that hydrogen bond to surface O and to H(2)O molecules in the first hydration layer with their dipole nearly parallel to the surface (2nd hydration layer). These interactions are consistent with the "chemisorption" and "physisorption" of H(2)O on calcite surfaces, proposed on the basis of previous thermogravimetric and Fourier-transformed infrared studies. Most significant is the distortion of the surface Ca-O octahedra caused by the relaxation (and possibly rupture) of some Ca-O bonds upon hydration, weakening the topmost atomic layer. These findings are consistent with interpretations of X-ray photoelectron spectroscopy, density functional theory, and electrokinetic studies that suggest the preferential release of surface Ca atoms over surface CO(3) groups upon hydration of the cleavage surface. These insights will help to elucidate mechanisms of carbonate mineral dissolution, the rearrangement of surface layers, ion replacement, charge development, and solute transport through subsurface lattice layers.

Switching a ferroelectric film by asphyxiation

Scientists have found that the spontaneous ferroelectric polarization can be fully and reversibly flipped by varying partial oxygen pressure above the surface of an epitaxially compressed film. The inward polarized state is stabilized by ordered oxygen vacancies in the topmost atomic layer.

Characterization of Bimetallic Systems with UPS and PAX: Gold on Platinum and Palladium Surfaces

In the present study the thermal behavior of ultrathin Au films deposited on different noble metal single crystal surfaces was studied by means of ultraviolet photoelectron spectroscopy and photoemission of adsorbed xenon. The primary goal of this work was to demonstrate the power of photoemission of adsorbed xenon as an easy to handle and very surface sensitive technique for the characterization of multi-component systems. All investigated systems, Au–Pt(111), Au–Pd(111) and Au–Pd(110) show a particular dependence of their electronic and structural properties on the annealing temperature. Au multilayer films with a thickness of 3 monolayer prepared at 150 K undergo in all cases a smoothening by annealing up to room temperature. While on Pd this effect is mainly detectable on the topmost atomic layer the ordering process on Pt is also affecting the deeper layers. Further annealing of the Au–Pd system to 450 K and the Au–Pt system to 750 K, respectively, results in the onset of the alloying process at the interface between metal film and substrate. After annealing to 650 and 950 K, respectively, the alloying finally extends to the very surface. The complete loss of Au from the surface can be detected at 1050 K on Pd and at 1150 K on Pt. The orientation of the substrate has only a weak influence, showing up e.g. in additional electronic states of the not alloyed Au–Pd(111) and Au–Pd(111) systems. Adsorption of CO as a probe molecule illustrates the formation of different adsorption sites on the bimetallic systems during the Au diffusion process.

Atomic structure and electronic properties of Ni 3Al(0 0 1) surface

We present the results of experimental and theoretical study of the structural and electronic properties of the (0 0 1) surface of an ordered, paramagnetic Ni 3Al alloy. Experimental part of this work is based on STM measurements which enabled us to obtain, for the first time, images of the Ni 3Al(0 0 1) surface with atomic resolution. Topographies of these images indicate the presence of a superstructure with the square unit cell and the lattice parameter equal to the distance between nearest surface Al or Ni atoms. We have observed that this order of the Ni 3Al(0 0 1) surface has a long-range character. This is remarkably different from the Ni 3Al(1 1 1) surface, where a (2 × 2) order (indicated by previous STM study) appears only within small surface domains. Our theoretical study of Ni 3Al(0 0 1) is based on ab initio calculations performed in the framework of density-functional theory and the use of a plane wave basis set. The obtained results indicate that changes of the atomic positions caused by the relaxation process are small and practically limited to the three topmost atomic layers. However, in the relaxed structure, surface Al atoms have higher vertical positions than surface Ni atoms, in accordance with experimental data provided by LEED measurements. We have also analyzed the differences between results of structural calculations performed independently using LDA and GGA descriptions of the exchange-correlation effects, and found that the GGA results are in a considerably better agreement with LEED data than the corresponding LDA results or the results of earlier theoretical studies.

양전자 소멸 Auger 전자 에너지 측정을 위한 Time of Flight의 분해도 향상에 관한 이론적 연구

저에너지(수 eV) 양전자 빔을 이용하여 도체나 반도체의 표면/계면의 물리화학적 특성 분석에 독특한 유용성이 보고 되고 있다. 기존의 표면 분석법에 비해 표면의 선택도가 향상되어 반도체 소자의 박막 두께가 얇아지는 최신기술에 적합한 분석법으로 주목을 받고 있다. 물질표면에 조사된 저에너지 양전자는 표면 근처의 image potential에 포획이 되어 표면에 있는 전자들과 쌍소멸하며 Auger 전자를 방출한다. 표면으로부터 방출된 Auger 전자의 에너지를 측정함으로 원자의 화학적 구별이 가능하므로 검출기의 에너지 분해도가 중요하다. 기존의 ExB 형태의 에너지 측정기는 분해도가 <TEX>$6{\sim}10\;eV$</TEX> 정도이고 특정한 에너지 영역만을 일정시간 스캔하여 스펙트럼을 측정하므로 측정시간이 길어진다는 단점이 있다. 반면에 Time-Of-Flight(TOF) 시스템은 방출되는 전자들의 에너지를 동시에 검출하므로 측정시간이 단축되어 측정 효율이 향상된다. 에너지 분해도를 높이기 위해서는 측정하고자 하는 전자의 진행거리를 길게 할수록 좋으나, 공간적 제약을 고려한 reflected TOF 시스템과 retarding tube을 이용한 linear TOF 시스템의 에너지 분해도를 이론적으로 시뮬레이션하였다. Since the presence of the chemical impurities and defect at surfaces and interfaces greatly influence the properties of various semiconductor devices, an unambiguous chemical characterization of the metal and semiconductor surfaces become more important in the view of the miniaturization of the devices toward nano scale. Among the various conventional surface characterization tools, Electron-induced Auger Electron Spectroscopy (EAES), X-ray Photoelectron Spectroscopy (XPS) and Secondary Electron Ion Mass Spectroscopy (SIMS) are being used for the identification of the surface chemical impurities. Recently, a novel surface characterizaion technique, Positron-annihilation induced Auger Electron Spectroscopy (PAES) is introduced to provide a unique method for the analysis of the elemental composition of the top-most atomic layer. In PAES, monoenergetic positron of a few eV are implanted to the surface under study and these positrons become thermalized near the surface. A fraction of the thermalized positron trapped at the surface state annihilate with the neighboring core-level electrons, creating core-hole excitations, which initiate the Auger process with the emission of Auger electrons almost simultaneously with the emission of annihilating gamma-rays. The energy of electrons is generally determined by employing ExB energy selector, which shows a poor resolution of <TEX>$6{\sim}10eV$</TEX>. In this paper, time-of-flight system is employed to measure the electrons energy with an enhanced energy resolution. The experimental result is compared with simulation results in the case of both linear (with retarding tube) and reflected TOF systems.

The effect of adsorbed sulfur on surface segregation in a polycrystalline Pd70Cu30 alloy

Surface segregation in alloys can be influenced by the presence of adsorbed species. In this work, the effect of adsorbed sulfur on surface segregation in a Pd70Cu30 alloy was studied for sulfur coverages from zero through saturation and for temperatures from 400 to 1000K. X-ray photoemission spectroscopy (XPS) was used to determine the alloy composition in the near-surface region (∼7 atomic layers) and low energy ion scattering spectroscopy (LEISS) was used to probe the composition of the topmost atomic layer of the alloy. Surface segregation was observed to depend on both the presence of adsorbed sulfur and heat-treatment history. The near-surface region of the clean alloy was enriched in Pd relative to the bulk, but the topmost atomic layer was enriched in Cu. Adsorbed sulfur caused a reversal of the Cu enrichment of the topmost surface, resulting in a top layer that contained only Pd and S atoms. Segregation reversal may be driven by the formation of thermodynamically favored Pd–S bonds at the terminating surface of the alloy.

Ultra-high depth resolution SIMS for the interface analysis of complex low-dimensional structures

The flexibility of sputter sectioning, in combination with the principal advantages of the mass spectrometric techniques, such as large dynamic ranges both in the mass separation and detection systems, has enabled the dynamic secondary ion mass spectrometry (SIMS) to be an extremely sensitive technique for the analysis of solid surfaces and thin films. SIMS depth profiling is now possible at depth resolutions down to ∼2nm with quantification data obtainable from the topmost atomic layer onwards into the depth. With optimized experimental conditions, like extremely low primary beam current (down to ∼10nA) and low impact energy (below 1keV), appropriate impact angle and with effective profile reconstruction approaches, ultra-high resolution SIMS depth profiling has enabled interfacial composition analysis of ultra-thin films, Bragg mirrors, quantum wells, hetero-structures, etc. and interfacial alloys in metallic multilayer with high precision and repeatability. The paper addresses some of our recent studies on high depth resolution SIMS analysis of such low-dimensional structures and their interfaces.

A systematic density functional study of ordered sulfur overlayers on Cu(1 1 1) and Ag(1 1 1): Influence of the adsorbate coverage

The interaction of atomic S with the Cu(1 1 1) and Ag(1 1 1) surfaces has been studied using density functional calculations and periodic slab models. Relevant properties were obtained as a function of the S coverage reaching values as low as θ = 1/28 ML. Some properties such as perpendicular distance from the S atom to the surface, the adsorbate net charge or the vibrational frequency for the normal model corresponding to the perpendicular distance to the surface can be considered as converged a relatively high coverage ( θ = 1/4 ML). However, the adsorption energy and properties related to the electronic structure of the metal substrate exhibit small but noticeable variations depending on the coverage. A particularly interesting finding is that presence of a small amount of S on the metal surface provokes significant distortions of the topmost atomic layer and induces a significant variation of the surface work function. The implications of these findings for the reactivity of the S covered Cu(1 1 1) and Ag(1 1 1) metal surfaces are discussed.

Surface segregation in a polycrystalline Pd 70Cu 30 alloy hydrogen purification membrane

X-ray photoelectron spectroscopy (XPS) and low energy ion scattering spectroscopy (LEISS) have been used to study the effects of various surface preparations and thermal treatments on the composition of the near-surface region (∼7 atomic layers) and the topmost atomic layer of a polycrystalline Pd 70Cu 30 alloy. Palladium enrichment (relative to the bulk composition) is observed in the XPS-accessible near-surface region, but copper enrichment is observed in the topmost atomic layer. At temperatures above ∼800 K, where the bulk, the near-surface region and the topmost atomic layer are likely in thermodynamic equilibrium, segregation to the top layer can be described in terms of a simple thermodynamic model. Temperature programmed desorption (TPD) of H 2 and CO from the annealed surfaces illustrates the impact of segregation and atomic distribution in the top layer on surface chemical activity.

Identification of surface optical phonon in wurtzite InN epitaxial thin films by coherent phonon spectroscopy

Coherent phonon spectroscopy of wurtzite InN epitaxial thin films was carried out with time-resolved second-harmonic generation technique. In addition to the bulk A1(LO) phonon and A1(LO)–plasmon coupling modes, a phonon mode at 543cm−1 was identified and characterized. It is found that this phonon mode is localized in the topmost few atomic layers and sensitive to surface modification. This vibration mode is described as the opposite motion between the In and N atoms along the c axis. This phonon mode is assigned to the surface optical phonon of InN.

Determination of Compton profiles at solid surfaces from first-principles calculations

Projected momentum distributions of electrons, i.e. Compton profiles above the topmost atomic layer have recently become experimentally accessible by kinetic electron emission in grazing-incidence scattering of atoms at atomically flat single crystal metal surfaces. Sub-threshold emission by slow projectiles was shown to be sensitive to high-momentum components of the local Compton profile near the surface. We present a method to extract momentum distribution, Compton profiles, and Wigner and Husimi phase space distributions from ab-initio density-functional calculations of electronic structure. An application for such distributions to scattering experiments is discussed.

Grazing scattering of fast [formula omitted] ions from an Al(0 0 1) surface under axial surface channeling

10keV C60+ clusters are scattered from a clean and flat Al(001) surface under a grazing angle of incidence. The angular distributions of clusters are observed as function of the azimuthal angle of rotation of the target close to the 〈110〉 direction in the surface plane. For sufficiently small impact energies with respect to the motion along the surface normal, fractions of fragmented clusters are small, and projectiles are specularly reflected from the topmost layer of surface atoms. For a slight detuning of the azimuthal angle of incidence with respect to the 〈110〉 direction, scattered C60 clusters show a characteristic azimuthal deflection in the opposite direction to the sense of rotation of the target surface. The data are analyzed in terms of computer simulations based on classical trajectories calculated from an effective interaction potential for C60+ clusters in front of the surface.

Probing surface magnetism with ion beams

Ion beams can be used to probe magnetic properties of surfaces by a variety of different methods. Important features of these methods are related to trajectories of atomic projectiles scattered from the surface of a solid target and to the electronic interaction mechanisms in the surface region. Both items provide under specific conditions a high sensitivity for the detection of magnetic properties in the region at the topmost layer of surface atoms. This holds in particular for scattering under planar surface channeling conditions, where under grazing impact atoms or ions are reflected specularly from the surface without penetration into the subsurface region. Two different types of methods are employed based on the detection of the spin polarization of emitted or captured electrons and on spin blocking effects for capture into atomic terms. These techniques allow one to probe the long range and short range magnetic order in the surface region.

Long Range Substrate Mediated Mass Transport on Metal Surfaces Induced by Adatom Clusters

Mass transport at surfaces can proceed either (i) by hopping diffusion of atoms on top of the surface from one site to another or (ii) by propagation of small displacements from one atom to the next within the topmost atomic layer. In the latter case, a long range substrate mediated mass transport has been postulated but never observed explicitly. Experimental and theoretical evidence is shown here for the occurrence of such a mechanism on the reconstructed Au(111) surface, where the movement is shown to be well described by a soliton.

Correct application of Fresnel’s equations for intensity analysis of angle-resolved photoemission data

The electromagnetic field relevant for the excitation process in angle-resolved photoemission is studied. We show that Fresnel’s equations together with the known bulk dielectric constants can be used to calculate the complex vector potential at the metal surface. A model is developed which accounts correctly for the special experimental geometry with focused light. It is used to calculate the variation of photoemission intensity with changing light incidence angle and polarization. Experimental data for the photoemission intensity as a function of light incidence angle are presented for direct transitions out of bulk, surface and adsorbate states at a Cu(1 1 0) surface. The comparison to our model shows that the application of copper bulk optical constants is justified even when electronic states are localized to the topmost atomic layer.

Bi-induced (2 × 6), (2 × 8), and (2 × 4) reconstructions on the InAs(1 0 0) surface

Low-energy electron diffraction (LEED), scanning-tunneling microscopy (STM), and synchrotron-radiation photoemission results show that Bi induces (2 × 6), (2 × 8), and (2 × 4) reconstructions on the InAs(1 0 0) surface with decreasing Bi coverage. The α2-like structural model, established previously for the clean InAs(1 0 0)(2 × 4) surface, is proposed for Bi/InAs(1 0 0)(2 × 4), and two Bi 5d core-level components of this reconstruction are interpreted within the context of this model. For the Bi/InAs(1 0 0)(2 × 6) surface we propose a tentative model where two topmost atomic layers consist of Bi atoms. Some possible reasons why Bi forms chain-like (2 × 6) and (2 × 8) reconstructions, instead of the prototypical c(4 × 4) stabilized normally by As on III-As(1 0 0), are discussed.