Surface Core-level Shifts Research Articles

Initial oxidation of low index Mg surfaces investigated by SCLS and DFT

Magnesium (Mg) is a metal having a high structural efficiency and a very high chemical reactivity at the same time. Its potential areas of applications include the automotive industry, biomedicine, and energy storage and generation. Further developments in these very diverse application areas require a more detailed investigation of the behavior of Mg surfaces under oxidative conditions. The basic mechanisms of magnesium reactivity are not yet fully understood and therefore need to be further investigated by a combination of theoretical and experimental studies on representative surfaces with different crystallographic orientations.For this, we measured in situ high-resolution Mg 2p core level spectra at critical low-index Mg(0001), Mg(101¯0), and Mg(112¯0) surfaces to obtain surface core level shifts (SCLSs) at the early stages of Mg oxidation. We also used density functional theory (DFT) to obtain theoretical estimates of the SCLSs for respective surfaces and associated possible reconstructions to guide the analysis of the experimental spectra and to rule out un-reconstructed Mg(112¯0). DFT simulations were also used to reveal the energies of O atom adsorption, and the formation of evolving oxide structures in the Mg surfaces.

Band energy diagrams of n-GaInP/n-AlInP(100) surfaces and heterointerfaces studied by X-ray photoelectron spectroscopy

Lattice matched n-type AlInP(100) charge selective contacts are commonly grown on n-p GaInP(100) top absorbers in high-efficiency III–V multijunction solar or photoelectrochemical cells. The cell performance can be greatly limited by the electron selectivity and valance band offset at this heterointerface. Understanding of the atomic and electronic properties of the GaInP/AlInP heterointerface is crucial for the reduction of photocurrent losses in III–V multijunction devices. In our paper, we investigated chemical composition and electronic properties of n-GaInP/n-AlInP heterostructures by X-ray photoelectron spectroscopy (XPS). To mimic an in-situ interface experiment with in-situ stepwise deposition of the contact material, 1 nm –50 nm thick n-AlInP(100) epitaxial layers were grown on n-GaInP(100) buffer layer on n-GaAs(100) substrates by metal organic vapor phase epitaxy. We observed (2 × 2)/c(4 × 2) low-energy electron diffraction patterns with characteristic diffuse streaks along the [011¯] direction due to PP dimers on both AlInP(100) and GaInP(100) as-prepared surfaces. Atomic composition analysis confirmed P-rich termination on both surfaces. Angle-resolved XPS measurements revealed a surface core level shift of 0.9 eV in P 2p peaks and the absence of interface core level shifts. We assigned the surface chemical shift in the P 2p spectrum to PP bonds on a surface. We found an upward surface band bending on the (2 × 2)/c(4 × 2) surfaces most probably caused by localized mid-gap electronic states. Pinning of the Fermi level by localized electronic states remained in n-GaInP/n-AlInP heterostructures. A valence band offset of 0.2 eV was derived by XPS and band alignment diagram models for the n-n junctions were suggested.

Modification of the Ge(0 0 1) subsurface electronic structure after adsorption of Sn

In this work, we investigate how the electronic structure of the Ge(001) surface is modified by the adsorption of Sn atoms. We extend a previously established growth model of the Sn layer formation on Ge(001) with a detailed analysis of surface core-level shifts, observing a prevalence of symmetric Sn ad-dimers at a Sn coverage above onemonolayer. The valence band structure of Ge(001) reveals the appearance of a non-dispersive electronic state after the adsorption of Sn. We correlate the presence of this state to the interaction of electronic states from a Sn ad-dimer configuration with the surface resonances of the Ge up-dimer. Post-deposition annealing leads to full incorporation of Sn and, consequently, to the disappearance of valence band state attributable to Sn ad-atoms. Notably, the adsorption and/or incorporation of Sn removes a Ge(001) surface state above the valence band maximum. The Fermi-level remains pinned close to the valence band maximum, indicating the initial stages of a Schottky barrier formation. Overall, these results provide new fundamental insights into the electronic structure of Sn on Ge(001), crucial for the development of SnGe electronics devices, and more generally of use for understanding the controlled alloying of isoelectronic layered materials.

First-Principles Estimation of Core Level Shifts for Hf, Ta, W, and Re.

A simple first-principles approach is used to estimate the core level shifts observed in X-ray photoelectron spectroscopy for the 4f electrons of Hf, Ta, W, and Re; these elements were selected because their 4f levels are relatively close to the Fermi energy. The approach is first tested by modeling the surface core level shifts of low-index surfaces of the four elemental metals, followed by its application to the well-studied material TaSe2 in the commensurate charge density wave (CDW) phase, where agreement with experimental data is found to be good, showing that this approach can yield insights into modifications of the CDW. Finally, unterminated surface core level shifts in the hypothetical MXene Ta3C2 are modeled, and the potential of XPS for the investigation of the surface termination of MXenes is demonstrated.

On the signatures of oxygen vacancies in O1s core level shifts

Density functional theory calculations are used to investigate O1s surface core level shifts for MgO(100), ZnO(101¯0), In2O3(111) and CeO2(111). Shifts are calculated for the pristine surfaces together with surfaces containing oxygen vacancies and dissociated H2. Pristine surfaces show small negative shifts with respect to the bulk components and vacancies are found to have a minor effect on the O1s binding energies of neighboring oxygen atoms. OH-groups formed by H2 dissociation yield binding energies shifted to higher energies as compared to the oxygen atoms in the bulk. The results stress the difficulties in assigning core-level shifts and suggest that assignments of shifts in O1s binding energies to neighboring oxygen vacancies for the explored oxides should be reconsidered.

Revisiting surface core-level shifts for ionic compounds

The shifts of core-level binding energies can provide powerful information about the electronic structure of a material. Understanding the physical origin of these shifts for catalytically relevant oxides may provide important insight into their properties. This requires reliable theoretical methods which are able to relate the binding energy shifts to the electronic structure. In order to establish such a methodology, the CaO(100) surface to bulk core-level binding energy shifts have been studied with Hartree-Fock and density-functional theory methods using both cluster and periodic slab models. The shifts obtained from the different theoretical methods are compared with each other and with data from synchrotron-based x-ray photoelectron spectroscopy (XPS) measurements. With a common approximation for the slab model treatment of XPS, the predicted binding energy shifts are seriously in error. The origin of the error is identified as arising from a flawed treatment of the surface atom binding energies, and a method for correcting the failure is presented.

First-principles characterization of Mg low-index surfaces: Structure, reconstructions, and surface core-level shifts

In this paper, first-principles calculations provide structural characterization of three low-index Mg surfaces - Mg(0001), Mg(1010), and Mg(1120) - and their respective surface core-level shifts (SCLSs). Inspired by the close similarities between Be and Mg surfaces, we also explore the reconstruction of Mg(1120). Through the calculation of surface energies and the use of the angular-component decomposed density of states, we show that reconstructions are likely to occur at the Mg(1120) surface, similarly to what was found earlier for Be(1120). Indeed, the surface energy of some of the explored reconstructions is slightly lower than that of the unreconstructed surface. In addition, because of lattice symmetry, the morphology of the unreconstructed surface (1120) results in a steplike zig-zag chain packing, with topmost chains supporting a resonant, quasi-one-dimensional (1D), partially filled electronic state. As the presence of partially filled quasi-1D bands is a necessary condition for Peierls-like dimerization, we verify that the undimerized surface chain remains stable with respect to it. Some of the reconstructions, namely, the 2×1 and 3×1 added row reconstructions, induce a stronger relaxation of the topmost chains, increasing the coupling with lower layers and thus significantly damping the quasi-1D character of this state. The original approach followed offers a common and general framework to identify quasi-1D bands - even in the case of resonant electronic surface states - and to meaningfully compare calculated and measured SCLSs even in the presence of multicomponent peak contributions. (Less)

Microscopic Views of Atomic and Molecular Oxygen Bonding with epi Ge(001)-2 × 1 Studied by High-Resolution Synchrotron Radiation Photoemission.

In this paper, we investigate the embryonic stage of oxidation of an epi Ge(001)-2 × 1 by atomic oxygen and molecular O2 via synchrotron radiation photoemission. The topmost buckled surface with the up- and down-dimer atoms, and the first subsurface layer behaves distinctly from the bulk by exhibiting surface core-level shifts in the Ge 3d core-level spectrum. The O2 molecules become dissociated upon reaching the epi Ge(001)-2 × 1 surface. One of the O atoms removes the up-dimer atom and the other bonds with the underneath Ge atom in the subsurface layer. Atomic oxygen preferentially adsorbed on the epi Ge(001)-2 ×1 in between the up-dimer atoms and the underneath subsurface atoms, without affecting the down-dimer atoms. The electronic environment of the O-affiliated Ge up-dimer atoms becomes similar to that of the down-dimer atoms. They both exhibit an enrichment in charge, where the subsurface of the Ge layer is maintained in a charge-deficient state. The dipole moment that was originally generated in the buckled reconstruction no longer exists, thereby resulting in a decrease in the ionization potential. The down-dimer Ge atoms and the back-bonded subsurface atoms remain inert to atomic O and molecular O2, which might account for the low reliability in the Ge-related metal-oxide-semiconductor (MOS) devices.

Growth, structural evolution and electronic properties of ultrathin films of Sn on W(110)

Growth and structure of Sn on W(110) from submonolayer to thick film (up to 8 monolayers) coverages at room temperature (RT) and as-well-as for higher temperatures (HT) (up to 673 K) have been studied using low energy electron diffraction (LEED), x-ray photoemission spectroscopy (XPS) and angle-resolved photoemission spectroscopy (ARPES) techniques. Surface structural evolution was studied where (3 × 1), (1 × 3) and (1 × 4) LEED patterns were observed for RT whereas (3 × 1) and (1 × 4) LEED patterns were observed for growth at 673 K due to the energetically available adsorption sites for Sn atoms on W(110) surface. The absence of alloy formation between Sn and W was confirmed by the XPS data where surface core level shift of Sn 3d was observed whereas no shift was observed for W 4f levels. Detailed electronic structures were studied along Γ¯ - H¯ and Γ¯ - N¯ directions for Sn deposited on W(110) with the help of ARPES. ARPES data for RT and HT case is mostly similar. A pair of splitted bands was observed due to the deposition of Sn near the surface projected band gap.

Surface structure analysis of Eu Zintl template on Ge(001)

The surface reconstructions and surface core level shifts of 0.25 monolayer (ML), 0.5 ML and 1.0 ML of europium (Eu) on the Ge(001) surface are investigated experimentally and theoretically, using molecular beam epitaxy (MBE), reflection high-energy electron diffraction (RHEED), x-ray photoelectron spectroscopy (XPS), and density functional theory (DFT). Starting with a clean Ge(001) surface we find that the surface reconstruction changes from the initial (2 × 1) to a (3 × 2), (8 × 1) and a final (3 × 1) pattern with increasing Eu coverage of 0.25 ML, 0.5 ML and 1.0 ML, respectively. XPS scans demonstrate that the Ge 3d core level shifts to higher binding energies with increasing Eu coverage. We find that Eu is in the 2+ oxidation state, transferring its charge to the Ge surface, resulting in the disappearance of the asymmetry of the dimer tilt at a coverage of 0.5 ML of Eu. From density functional calculations we find that the Ge dimers flatten out at 0.5 ML Eu coverage and partially break apart at 1.0 ML Eu.

Native and graphene-coated flat and stepped surfaces of TiC

Titanium carbide attracts growing interest as a substrate for graphene growth and as a component of the composite carbon materials for supercapacitors, an electrode material for metal-air batteries. For all these applications, the surface chemistry of titanium carbide is highly relevant and being, however, insufficiently explored especially at atomic level is a subject of our studies. Applying X-ray photoelectron spectroscopy (XPS) to clean (111) and (755) surfaces of TiC, we were able to obtain the detailed spectroscopic pattern containing information on the plasmon structure, shake up satellite, the peak asymmetry and, finally, surface core level shift (SCLS) in C 1s spectra. The latter is essential for further precise studies of chemical reactions. Later on, we studied interface between TiC (111) and (755) and graphene and found the SCLS variation due to strong chemical interaction between graphene and substrate. This interaction is also reflected in the peculiar band structure of graphene probed by angle-resolved photoelectron spectroscopy (ARPES). Based on LEED data the structure is close to (7√3 × 7√3)R30°, with graphene being slightly corrugated. We found that similarly to the graphene on metals, the chemical interaction between graphene and TiC can be weakened by means of intercalation of oxygen atoms underneath graphene.

First stages of oxide growth on Al(1 1 0) and core-level shifts from density functional theory calculations

The formation of islands of O-atoms is the dominant mode of growth of the oxide in the first stages of oxidation of Al(1 1 1). It is however unknown if a similar mechanism exists for other low index surfaces of Al. We performed a density functional theory (DFT) and ab initio molecular dynamics investigation of the first stages of the oxidation of Al(1 1 0) using two distinct models: a homogeneous surface disposition of O-atoms; and a model where the O-atoms are close-spaced forming clusters or islands. We investigated the surface reactions with oxygen up to a coverage of 2 ML and found that for both models the adsorption energy per dissociating O2(g) becomes more negative with increasing coverage. Our results show that for coverages up to 1.25 ML the oxide forms clusters or islands while for coverages higher than 1.5 ML the oxide covers the surface homogeneously. This is because the O-atoms bind preferably to neighboring sites even at the minimum coverage. With increasing coverage, the clusters of O start to form stripes along the [1 1¯ 0] direction. The work function (ϕ) of the surface decreases when going from bare Al(1 1 0) to up to 1 ML coverage of O-atoms, but for coverages of 1.25 ML and higher, ϕ increases. The Al 2p surface core level shifts (SCLS) shift towards higher binding energies with increasing surface coverage of O-atoms and start to approach the values of Al 2p in Al2O3 already at a coverage of 2ML. A relation between the SCLS and the coordination number of Al to O-atoms was made. The Al 2p SCLS increases with increasing coordination to O-atoms, for single, twofold and three-fourfold coordinated cations. For the O-atoms that terminate the surface at the short-bridge sites, the SCLS of O 1s, is largely affected by the proximity to other O-atoms. These results demonstrate that the cooperative effects between surface bound O-atoms have important roles in the mechanism of growth of the oxide at Al(1 1 0), and similarly to what happens for Al(1 1 1), the initial oxidation of Al(1 1 0) proceeds via the formation of islands of O-atoms.

A Chemical View on X-ray Photoelectron Spectroscopy: the ESCA Molecule and Surface-to-Bulk XPS Shifts.

In this paper we remind the reader of a simple, intuitive picture of chemical shifts in X-ray photoelectron spectroscopy (XPS) as the difference in chemical bonding between the probed atom and its neighbor to the right in the periodic table, the so called Z+1 approximation. We use the classical ESCA molecule, ethyl trifluoroacetate, and 4d-transition metals to explicitly demonstrate agreement between core-level shifts computed as differences between final core-hole states and the approach where each core-ionized atom is replaced by a Z+1 atom. In this final state, or total energy picture, the XPS shift arises due to the more or less unfavorable chemical bonding of the effective nitrogen in the carbon geometry for the ESCA molecule. Surface core level shifts in metals are determined by whether the Z+1 atom as an alloy segregates to the surface or is more soluble in the bulk. As further illustration of this more chemical picture, we compare the geometry of C 1s and O 1s core-ionized CO with that of, respectively, NO+ and CF+ . The scope is not to propose a new method to compute XPS shifts but rather to stress the validity of this simple interpretation.

Interaction of Sulfur Dioxide and Near-Ambient Pressures of Water Vapor with Cuprous Oxide Surfaces

The interaction of water vapor and sulfur dioxide (SO2) with single crystal cuprous oxide (Cu2O) surfaces of (100) and (111) termination was studied by photoelectron spectroscopy (PES) and density functional theory (DFT). Exposure to near-ambient pressures of water vapor, at 5 × 10–3 %RH and 293 K, hydroxylates both Cu2O surfaces with OH coverage up to 0.38 copper monolayers (ML) for (100) and 0.25 ML for (111). O 1s surface core level shifts indicate that the hydroxylation lifts the (3,0;1,1) reconstruction of the clean (100) surface. On both clean Cu2O terminations, SO2 adsorbs to unsaturated surface oxygen atoms to form SO3 species with coverage, after a saturating SO2 dose, corresponding to 0.20 ML on the Cu2O(100) surface and 0.09 ML for the Cu2O(111) surface. Our combined DFT and PES results suggest that the SO2 to SO3 transformation is largely facilitated by unsaturated copper atoms at the Cu2O(111) surface. SO3-terminated surfaces exposed to low doses of water vapor (≤100 langmuirs) in ultrahigh v...

Atomic configuration of hydrogenated and clean tantalum(111) surfaces: Bond relaxation, energy entrapment and electron polarization

By studying the tantalum (Ta)(111) surface with X-ray photoemission spectroscopy and density functional theory, we determined binding energy values for the clean Ta(111) (+3.068 eV) and hydrogenated Ta(111) (+3.421 eV) surfaces with an isolated atom level of 18.977 eV. Using the bond–band barrier and zone-selective electron spectroscopy correlation, we investigated the mechanism of hydrogenation adsorption on the Ta(111) surface. We found the local densities of states of the first layer of Ta atoms in the reconstructed structure, which formed on the adsorbent hydrogen of the surface chemical bond contracts and dipole polarization. Moreover, we showed that on the Ta(111) surface, the hydrogen-induced surface core level shifts are dominated by quantum entrapment and are proportional to the calculated hybridized orbitals of the valence band. The latter is therefore correlated to the local surface chemical reactivity and is useful for other adsorbate systems on transition metals.

Surface electronic structure of epi germanium (001)-2 × 1

Photoemission from an epi Ge(001)-2 × 1 surface is presented using synchrotron radiation as a probe. The topmost surface atoms are buckled with the up-dimer and down-dimer atoms exhibiting surface core-level shifts (SCLSs) of −0.492 and −0.178 eV, respectively. The subsurface layer shows a +0.083 eV SCLS. The final-state effect suffices to explain the sign of the shift. The electron affinity and ionization potential for the epi Ge surface are 4.36 and 5.09 eV, respectively. An argument contrasting the current results with those of existing reports with non-epi surfaces is also given. Non-epi surfaces possess Ge surfaces with isolated single atoms or small droplets that affect Ge’s contact with dielectric layers and the electric performances of the Ge metal–oxide–semiconductor structure.

Zintl layer formation during perovskite atomic layer deposition on Ge (001).

Using in situ X-ray photoelectron spectroscopy, reflection high-energy electron diffraction, and density functional theory, we analyzed the surface core level shifts and surface structure during the initial growth of ABO3 perovskites on Ge (001) by atomic layer deposition, where A = Ba, Sr and B = Ti, Hf, Zr. We find that the initial dosing of the barium- or strontium-bis(triisopropylcyclopentadienyl) precursors on a clean Ge surface produces a surface phase that has the same chemical and structural properties as the 0.5-monolayer Ba Zintl layer formed when depositing Ba by molecular beam epitaxy. Similar binding energy shifts are found for Ba, Sr, and Ge when using either chemical or elemental metal sources. The observed germanium surface core level shifts are consistent with the flattening of the initially tilted Ge surface dimers using both molecular and atomic metal sources. Similar binding energy shifts and changes in dimer tilting with alkaline earth metal adsorption are found with density functional theory calculations. High angle angular dark field scanning transmission microscopy images of BaTiO3, SrZrO3, SrHfO3, and SrHf0.55Ti0.45O3 reveal the location of the Ba (or Sr) atomic columns between the Ge dimers. The results imply that the organic ligands dissociate from the precursor after precursor adsorption on the Ge surface, producing the same Zintl template critical for perovskite growth on Group IV semiconductors during molecular beam epitaxy.

Adsorption of hydrogen on stable and metastable Ir(100) surfaces

Using the combination of high resolution core level spectroscopy and density functional theory we present a detailed spectroscopic study for all clean and hydrogen covered phases of Ir(100). The results are complemented by an investigation of the hydrogen desorption process from various phases using temperature programmed desorption spectroscopy and scanning tunneling microscopy. In total, all experimentally determined core level shifts match very well with those predicted by density functional theory based on established structural models. In particular, we find for the (bridge site) adsorption on the unreconstructed 1×1 phase that the initial core level shift of surface Ir atoms is altered by +0.17eV for each Ir–H bond formed. In the submonolayer regime we find evidence for island formation at low temperatures. For the H-induced deconstructed 5×1-H phase we identify four different surface core level shifts with two of them being degenerate. Finally, for the reconstructed 5×1-hex phase also four surface components are identified, which undergo a rather rigid core level shift of +0.15eV upon hydrogen adsorption suggesting a similarly homogeneous charge transfer to all Ir surface atoms. Thermodesorption experiments for the 5×1-H phase reveal two different binding states for hydrogen independent of the total coverage. We conclude that the surface always separates into patches of fully covered deconstructed and uncovered reconstructed phases. We could also show by tunneling microscopy that with the desorption of the last hydrogen atom from the deconstructed unit cell the surface instantaneously reverts into the reconstructed state. Eventually, we could determine the saturation coverage upon molecular adsorption for all phases to be θmax1×1−H=1.0ML, θmax5×1−H=0.8ML, and θmax5×1−hex−H≥1.0ML.

EC-XPS Analysis of Oxygen Species Adsorbed on Pt-Nanoparticle Model Electrodes

The research and development of highly active cathode catalysts for the oxygen reduction reaction (ORR) is one of the most important subjects to achieve high efficiency in polymer electrolyte fuel cells (PEFCs). In order to understand the ORR mechanism and find clues to the design of high-performance cathode catalysts, it is essential to analyze surface the oxidation states of catalysts and the adsorbed oxygen species in the ORR. Recently, we have clarified the surface oxidation states of Pt polycrystalline and low-index single-crystal electrodes as a function of the electrode potential by using X-ray photoelectron spectroscopy combined with an electrochemical cell (EC–XPS).1 In practical PEFCs, however, Pt nanoparticles supported on carbon black have been employed as the anode and cathode catalysts. In the present research, we have prepared a model electrode of Pt nanoparticles supported on a carbon substrate and have examined the electronic structure of the Pt nanoparticles and the oxygen species adsorbed on the surface by using EC–XPS. Pt nanoparticles were prepared by a colloidal method,2 and dispersed on polished glassy carbon (GC). It was clarified by scanning electron microscopy (SEM) that Pt nanoparticles of ca. 3 nm diameter were well dispersed on the carbon substrate. We employed angle-resolved, grazing incidence X-ray photoelectron spectroscopy (AR-GIXPS), which is more surface-sensitive than conventional XPS, in order to analyze the small amount of Pt nanoparticles supported on the flat GC substrate. Figure 1 shows Pt 4f XP spectra of the Pt/GC model electrode after emersion from the 0.1 M HF solution at various electrode potentials. It was found that the peaks assigned to Pt 4f7/2 and 4f5/2 shifted to higher binding energies with increasing electrode potential. This peak shift can be ascribed to the surface core level shift induced by the surface oxidation of the Pt nanoparticles. At E > 1.1 V, peaks for bulk PtO, with binding energies of 73 and 77 eV, commenced to increase due to further oxidation of the Pt nanoparticles. Figure 2a shows O 1s XP spectra at the Pt/GC model electrode and a GC electrode (without any Pt) at 0.9 V, indicated by red and black lines, respectively. The O1s spectrum of the Pt/GC model electrode was found to include photoelectron signals attributed to oxygen species formed on the GC substrate, such as quinone and carboxyl groups. Then, we extracted the photoelectron signals originating from oxygen species adsorbed on the Pt nanoparticles by subtracting the O1s spectrum of the GC from that of the Pt/GC model electrode. Figure 2b shows the O1s difference spectrum for oxygen species adsorbed on the Pt nanoparticles. Thus, we succeeded in deconvoluting the difference spectrum into three components, H2Oad, OHad and Oad formed on the Pt nanoparticles. On the basis of the deconvoluted photoelectron intensities, it was found that H2Oad decreased with increasing electrode potential, while OHad increased. The Oad species was found to appear at E > 0.8 V. The onset potential of the OHad formation at the Pt/GC electrode was less positive than that for polycrystalline Pt, suggesting that the surface oxidation proceeded more easily at the Pt nanoparticle surfaces than at the polycrystalline surface. This work was supported by funds for the SPer-FC Project of NEDO, Japan References 1 M. Wakisaka, H. Suzuki, S. Mitsui, H. Uchida, and M. Watanabe, Langmuir, 25, 1897 (2009). 2 M. Watanabe, M. Uchida, and S. Motoo, J. Electroanal. Chem., 199, 311 (1986). Figure 1

Skin-depth lattice strain, core-level trap depression and valence charge polarization of Al surfaces

Clarifying the origin for surface core-level shift (SCLS) and gaining quantitative information regarding the coordination-resolved local strain, binding energy (BE) shift and cohesive energy change have been a challenge. Here, we show that a combination of the bond order–length–strength (BOLS) premise, X-ray photoelectron spectroscopy (XPS) and the ab initio density functional theory (DFT) calculations of aluminum (Al) [Formula: see text] energy shift of Al surfaces has enabled us to derive such information, namely, (i) the [Formula: see text] energy of an isolated Al atom [Formula: see text] and its bulk shift (0.499 eV); (ii) the skin lattice contracts by up to 12.5% and the BE density increases by 70%; and (iii) the cohesive energy drops up to 38%. It is affirmed that the shorter and stronger bonds between under-coordinated atoms provide a perturbation to the Hamiltonian and hence lead to the local strain, quantum entrapment and valence charge polarization. Findings should help in understanding the phenomena of surface pre-melting and skin-high elasticity, in general.