Quantum-well resonances and image states in the Ar/Cu(1 0 0) system

A theoretical study of the effect of an atomically thin rare gas layer on the dynamics of excited electronic states at metal surfaces is presented for the case of a few mono-layers of Ar on a Cu(1 0 0) surface. We develop a 3D-microscopic model with predictive capabilities of the interaction of an electron with an Ar layer physisorbed on a metal surface. It takes into account the 3D structure of the Ar layer as well as its dielectric character. The dynamics of the excited electron on the surface is treated within a wave-packet propagation approach. The calculations show that two different types of excited states are present at the Ar/Cu(1 0 0) surface. (i) Image states that are repelled into vacuum as compared to their position on clean Cu(1 0 0) surfaces, leading to a decrease of their binding energies and to an increase of their lifetimes. (ii) Quantum-well resonances, corresponding to quasi-stationary states localised inside the Ar layer; they are associated with the quantisation of the conduction band in the finite size Ar layer. The present results on image states nicely agree with very recent time-resolved two-photon-photo-emission experiments by Berthold, Feulner and Höfer.     

Effect of various adsorbates in dephasing and population decay of the Cu(1 0 0) image potential states

A theoretical study of the electron dynamics in image potential states on Cu(1 0 0) surfaces with different types of adsorbates is presented. Scattering of the image state electron by an adsorbate induces inter-band and intra-band transitions leading respectively to the population decay and to the dephasing of the image state. We compare results obtained with low coverage (typically 1 adsorbate atom per 1000 surface atoms) Cs, Ar, and a model electronegative adsorbates. As follows from our results, Cs adsorbates lead to both appreciable dephasing and decay, while electronegative adsorbates mostly affect the dephasing rate. The effect of low coverage Ar adsorbates is small, consistent with their neutrality.     

Densities of states of low-index surfaces of tantalum by photofield emission

The total energy distributions (TED) of the true photofield emission current from clean (1 0 0), (1 1 0) and (1 1 1) facets of Ta field emitters have been measured using a new method to remove the thermocurrent associated with laser heating. Each TED exhibits one or more prominent peaks that are interpreted by comparing them with ab initio calculations of the TED of the emission current based on density functional theory. The generally good agreement with experiment indicates that the same method can be used for accurate calculations of the densities of states of low-index surfaces of Ta. Each of the experimental TEDs shows, in addition to the prominent peaks, a set of weaker peaks that are not predicted by the calculation and whose spacing depends on the sharpness of the field emitter. These weaker peaks are interpreted as arising from size-effect resonances in the microcrystal at the apex of the field emitter.     

Surface wave scattering by a nano-object situated on a surface

The scattering of the surface electromagnetic waves by a nano-defect (object) on a surface was calculated. The scattered field has been considered as a field caused by the current generated by the self-consistent local field inside the defect. In turn, the self-consistent local field has been determined as a result of solution of the integral Lippmann-Schwinger equation. The effective susceptibility of the object has been calculated using a self-consistent procedure. The corrections of self-energy part due to direct and indirect electromagnetic interactions, as well as due to interaction with surface wave field are taken into account. The self-energy part is calculated analytically within the framework of the near-field approximation. The scattering indicatrisses in reciprocal space have been computed for different shapes of the scatterer. Strong dependence of the scattered field on geometry of the scatterer has been found and explained.     

Systematic studies on surface modifications by ARUPS on Shockley-type surface states

The quasi two-dimensional surface state on noble metal (1 1 1)-surfaces can be used as a sensitive probe for different surface modifications, adsorption processes, and interactions between adsorbate and substrate. Already one monolayer of physisorbed Xe on Au(1 1 1) is responsible for a characteristic shift of the Shockley state towards the Fermi level and the surface state experiences an increase in spin-orbit splitting of up to 35%. In contrast to the physisorption process of rare gases, a sub-monolayer coverage of an alkali metal, e.g., Na on Au(1 1 1), has the opposite effect on the Shockley state, i.e. an increase in binding energy, until it reaches the bottom of the L-gap and vanishes into the bulk states. Additionally, we studied the intermetallic system Ag/Au(1 1 1) which differs substantially from the other systems because of the similarity in the electronic structure between substrate and overlayer.     

Surface-state conduction through dangling-bond states

The surface-state conduction through dangling-bond states on the (1 1 1) ideal surfaces of group-IV semiconductors is studied theoretically. Localization strength of wave functions is an important factor determining the surface-state conduction. Approximate expressions for the decay constants of the dangling-bond states are presented using a tight-binding model. The origin of the difference in the decay constants of diamond, Si, and Ge is clarified in terms of linear combination of the states at the top of valence bands and the bottom of conduction bands. The ballistic conductance from a probe to the surfaces is calculated using the Landauer formalism. The surface-state conductance is much larger than the bulk-state one, which is due to small band dispersions of the dangling-bond states.     

The surface science of xerography

Over the past four decades xerography, the dry ink marking process developed by the photocopy industry, has grown from nothing into a $170 billion industry worldwide. This amazing commercial success is due to the fact that during this period, xerographic technology experienced constant and often-dramatic improvement created by sustained industrywide research and development. Indeed, the development of the xerographic copying and printing industry is one of the great applied surface science successes of all time. In this article we outline the story of the advances in xerographic technology during the past four decades, describe the profound dependence on these advances of the control of surface and interface properties of increasingly sophisticated multi-component materials systems, and indicate the potential impact on the industry of the continuing development of the surface and interface science of the multi-component materials packages used in xerographic technology.     

Degeneracy pressure of electrons: a new effect for Pb islands grown on Si(1 1 1) substrate

We have found that the degeneracy pressure of electrons (DPE) inside Pb islands grown on a silicon substrate plays a crucial role in stabilizing the islands. In most cases, at a metal-semiconductor interface charge spilling takes place due to the difference of Fermi energies between the two materials, which makes DPE decrease along with the energy of the system. Based on this new effect, calculations of energy as a function of height are carried out for Pb islands grown on Si(1 1 1)-() and -(7 × 7) phases, which have most stable heights of 5 and 7 monolayers (ML), respectively. Our results explain why these most stable heights are observed. Using this new effect supplemented with experimental data, all the preferred heights of the Pb islands on Si(1 1 1)-(7 × 7) can be explained too.     

Anisotropy of electron structure at InAs(1 1 1) surfaces by laser pump-and-probe photoemission spectroscopy

The electronic structure and the electron dynamics of the clean InAs(1 1 1)A 2 × 2 and the InAs(1 1 1)B 1 × 1 surfaces have been studied by laser pump-and-probe photoemission spectroscopy. Normally unpopulated electron states above the valence band maximum (VBM) are filled on the InAs(1 1 1)A surface due to the conduction band pinning above the Fermi level (E_F). Accompanied by the downward band banding alignment, a charge accumulation layer is confined to the surface region creating a two dimensional electron gas (2DEG). The decay of the photoexcited carriers above the conduction band minimum (CBM) is originated by bulk states affected by the presence of the surface. No occupied states were found on the InAs(1 1 1)B 1 × 1 surface. This fact is suggested to be due to the surface stabilisation by the charge removal from the surface into the bulk. The weak photoemission intensity above the VBM on the (1 1 1)B surface is attributed to electron states trapped by surface defects. The fast decay of the photoexcited electron states on the (1 1 1)A and the (1 1 1)B surfaces was found to be τ_1 1 1 A ? 5 ps and τ_1 1 1 B ?  4 ps, respectively. We suggest the diffusion of the hot electrons into the bulk is the decay mechanism.     

Channel length effect in a MOSFET structure by scanning capacitance microscopy

Depth dependent carrier density and trapped charges in a metal-oxide-semiconductor field effect transistor (MOSFET) like structure have been studied using scanning capacitance microscopy (SCM). For a MOSFET structure, since minority carrier can be provided by the source and drain diffusions, its response time is shorter than that of metal-oxide-semiconductor (MOS) system. So the high frequency C-V relation is slightly different from that of MOS capacitor and shows the characteristics dependent on the channel length. Bias dependent SCM images which represent the depth dependent carrier density and detrapping time constant of trapped charges in the oxide layer were observed to see the channel effect in a MOSFET structure.     

Evidences of a double resonant ionization mechanism in sputtering of metals

We present a theory of resonant charge exchanges, between sputtered atoms and metal surfaces, in which surface effects occur as quasi-molecular correlations in the diatomic systems formed, in the collision cascade, between secondary emitted atoms and their nearest-neighbor substrate atoms that have provided the last impulse for ejection. We set up a generalized Anderson-Newns Hamiltonian, from first principles, using a truncated and orthonormal set of states obtained from the valence orbitals of the diatomic system and from a continuous basis of jellium wave functions. We calculate the one-electron matrix elements appearing in the equations of motion for the annihilation operators of the truncated set in comparison with those resulting from the basic theory of resonant charge transfer. We determine the ionization probability of secondary emitted atoms versus their final emission velocities and we find it to be in good agreement with experimentally derived data on the Cu⁺/Cu-system. We support the hypothesis that the bare Anderson-Newns hopping mechanism needs to be completed with another charge transfer channel at the low energies of secondary ion emission.     

Electronic transport at semiconductor surfaces––from point-contact transistor to micro-four-point probes

The electrical properties of semiconductor surfaces have played a decisive role in one of the most important discoveries of the last century, transistors. In the 1940s, the concept of surface states––new electron energy levels characteristic of the surface atoms––was instrumental in the fabrication of the first point-contact transistors, and led to the successful fabrication of field-effect transistors. However, to this day, one property of semiconductor surface states remains poorly understood, both theoretically and experimentally. That is the conduction of electrons or holes directly through the surface states. Since these states are restricted to a region only a few atom layers thick at a crystal surface, any signal from them might be swamped by conduction through the underlying bulk semiconductor crystal, as well as greatly perturbed by steps and other defects at the surface. Yet recent results show that this type of conduction is measurable using new types of experimental probes, such as the multi-tip scanning tunnelling microscope and the micro-four-point probe. The resulting electronic transport properties are intriguing, and suggest that semiconductor surfaces should be considered in their own right as a new class of electronic nanomaterials because the surface states have their own characters different from the underlying bulk states. As microelectronic devices shrink even further, and surface-to-volume ratios increase, surfaces will play an increasingly important role. These new nanomaterials could be crucial in the design of electronic devices in the coming decades, and also could become a platform for studying the physics of a new family of low-dimensional electron systems on nanometre scales.     

Measurement of the surface charge density of CO-saturated Pt(1 1 1) electrodes as a function of potential: the potential of zero charge of Pt(1 1 1)

From measurements of the charge flowing upon immersion, at controlled potential, of a CO-covered Pt(1 1 1) electrode in a 0.1 M HClO₄ solution, the corresponding surface charge density vs. potential curve was obtained, and from this the potential of zero charge (pzc) of the CO-covered Pt(1 1 1) electrode. From these data it was estimated that the error incurred when the potential of zero total charge (pztc) of Pt(1 1 1) electrodes is determined by the CO-charge displacement method is of approximately 50 mV at pH 1 and of approximately 90 mV at pH 3. Furthermore, the experimentally determined pzc of the CO-covered Pt(1 1 1) electrode has allowed us to make an estimation of the potential of zero free charge (pzfc) of Pt(1 1 1) electrodes.     

Quantitative analysis of the surface reconstruction induced band-gap in the Shockley state on monolayer systems on noble metals

One monolayer of Ag deposited on Cu(1 1 1) shows two kinds of characteristic reconstruction, depending on the conditions of the preparation: the incommensurate moiré structure appears for one monolayer prepared at 200 K whereas a monolayer deposited at room temperature (or higher) exhibits a quasi-commensurate triangular structure. By high-resolution ARUPS measurements on the triangular structure we find an opening of a gap in the Shockley state band, which is a signature of the super-lattice. On the other hand, no gap opening is observed on the moiré structure. In addition, we show that the Shockley state plays an important role in the adsorption process of rare gas atoms on these surfaces. ARUPS experiments on adsorbed Xe on 0.6 ML Ag/Cu(1 1 1) show clearly that the Xe atoms favor the adsorption on the Ag islands, before the Cu terraces will be covered at higher Xe exposure.     

Surface electronic structure of α-Mo2C(0 0 0 1)

The electronic structure of α-Mo₂C(0 0 0 1) has been investigated by angle-resolved photoemission spectroscopy utilizing synchrotron radiation. A sharp peak is observed at 3.3 eV in normal-emission spectra. Since the peak shows no dispersion as a function of photon energy and is sensitively attenuated by oxygen adsorption, the initial state of the peak is attributed to a surface state. Resonant photoemission study shows that the state includes substantial contribution of 4d orbitals of the Mo atoms in the second layer. The emissions with constant kinetic energies of 22 and 31 eV above the Fermi level (E_F) are found in normal-emission spectra, and these emissions are interpreted as originating from the Mo N₁N₂₃V and N₂₃VV Auger transitions, respectively.     

Chemical and electronic properties of sulfur-passivated InAs surfaces

Treatment with ammonium sulfide ((NH₄)₂S_x) solutions is used to produce model passivated InAs(0 0 1) surfaces with well-defined chemical and electronic properties. The passivation effectively removes oxides and contaminants, with minimal surface etching, and creates a covalently bonded sulfur layer with good short-term stability in ambient air and a variety of aqueous solutions, as characterized by X-ray photoelectron spectroscopy, atomic force microscopy, and Hall measurements. The sulfur passivation also preserves the surface charge accumulation layer, increasing the associated downward band bending.     

Surface core level shifts in photo-electron spectra from the Ca, Sr and Ba titanates

Photoelectron binding energy (BE), shifts observed in core level spectra of Ca, Sr and Ba present at the outer most surface with respect to the bulk in the respective single crystal titanates can be correlated to the inverse of their respective electro-negativities. Such trends can be explained within the context of the charge potential model. This model, which does not account for final state effects, indicates that (a) the direction of the BE shift results from chemical environment variations (an inter-atomic effect sometimes referred to in Madulung constant terms), and (b) BE shift variations result from electron density variations on the atom the photo-electron emanated from (an intra-atomic effect, which can also be relayed in ionic radii and electro-negativity terms).     

A DFT investigation of potential energy surface and vibrational properties of hydrogen adsorbed on the Rh(1 1 1) surface

Vibrational properties of hydrogen on the Rh(1 1 1) surface have been investigated theoretically. The potential energy surface for this system has been calculated within the density functional theory. The potential is found to be very anharmonic. The wave functions and their energies for the hydrogen motion on the potential energy surface (PES) have been calculated and assigned by using discrete variable representation. It was found that the vibrational wave function is localized at hollow site in the ground state for hydrogen on Rh(1 1 1). Higher excited states are of delocalized nature and mixed parallel and perpendicular character. Our results are in good agreement with the observed vibrational spectra of hydrogen on the Rh(1 1 1) surface.     

First-principles study on the electronic structures of iron phthalocyanine monolayer

The electronic band structure and magnetic properties of iron phthalocyanine (FePc) monolayer were investigated by using the first-principles all-electron full-potential linearized augmented plane wave energy band method. It is found that the ferromagnetic FePc monolayer is energetically more stable than the paramagnetic one. The exchange interaction, which splits the majority and minority bands, influences strongly on the electronic structure near the Fermi level (E_F). Magnetic moment of the central Fe atom is calculated to 1.95 μ_B. The range of the positive polarization of Fe site is larger in the out-of-plane than in the in-plane direction. The FePc ligand remains paramagnetic. The presence of states at E_F indicates the metallic character of FePc monolayer both for the paramagnetic and ferromagnetic states. However, the large density of states at E_F of the majority spins in the ferromagnetic state is expected to cause a phase transition to insulating antiferromagnetic state from the metallic ferromagnetic one.