34 research outputs found
Surface energy and stability of stress-driven discommensurate surface structures
A method is presented to obtain {\it ab initio} upper and lower bounds to
surface energies of stress-driven discommensurate surface structures, possibly
non-periodic or exhibiting very large unit cells. The instability of the
stressed, commensurate parent of the discommensurate structure sets an upper
bound to its surface energy; a lower bound is defined by the surface energy of
an ideally commensurate but laterally strained hypothetical surface system. The
surface energies of the phases of the Si(111):Ga and Ge(111):Ga systems and the
energies of the discommensurations are determined within eV.Comment: 4 pages RevTeX. 2 Figures not included. Ask for a hard copy (through
regular mail) to [email protected]
Probing the Surface Polarization of Ferroelectric Thin Films by X-ray Standing Waves
Understanding the mechanisms underlying a stable polarization at the surface
of ferroelectric thin films is of particular importance both from a fundamental
point of view and to achieve control of the surface polarization itself. In
this study, it is demonstrated that the X-ray standing wave technique allows
the polarization near the surface of a ferroelectric thin film to be probed
directly. The X-ray standing wave technique is employed to determine, with
picometer accuracy, Ti and Ba atomic positions near the surface of three
differently strained thin films grown on scandate
substrates, with a film as bottom electrode. This technique
gives direct access to atomic positions, and thus to the local ferroelectric
polarization, within the first 3 unit cells below the surface. By employing
X-ray photoelectron spectroscopy, a detailed overview of the oxygen-containing
species adsorbed on the surface, upon exposure to ambient conditions, is
obtained. The combination of structural and spectroscopic information allows us
to conclude on the most plausible mechanisms that stabilize the surface
polarization in the three samples under study. The different amplitude and
orientation of the local ferroelectric polarizations are associated with
surface charges attributed to the type, amount and spatial distribution of the
oxygen-containing adsorbates
Graphene on Ir(111): Physisorption with chemical modulation
The nonlocal van der Waals density functional (vdW-DF) approach is applied to calculate the binding of graphene to Ir(111). The precise agreement of the calculated mean height h = 3.41 Å ; of the C atoms with their mean height h = (3.38 ± 0.04) Å ; as measured by the X-ray standing wave (XSW) technique provides a benchmark for the applicability of the non-local functional. We find bonding of graphene to Ir(111) to be due to the van der Waals interaction with an antibonding average contribution from chemical interaction. Despite its globally repulsive character, in certain areas of the large graphene moiré unit cell charge accumulation between Ir substrate and graphene C atoms is observed, signaling a weak covalent bond formation
Quantitative structure of an acetate dye molecule analogue at the TiO2- acetic acid interface
The positions of atoms in and around acetate molecules at the rutile TiO2(110) interface with 0.1 M acetic acid have been determined with a precision of ±0.05 Å. Acetate is used as a surrogate for the carboxylate groups typically employed to anchor monocarboxylate dye molecules to TiO2 in dye-sensitised solar cells (DSSC). Structural analysis reveals small domains of ordered (2 x 1) acetate molecules, with substrate atoms closer to their bulk terminated positions compared to the clean UHV surface. Acetate is found in a bidentate bridge position, binding through both oxygen atoms to two five-fold titanium atoms such that the molecular plane is along the [001] azimuth. Density functional theory calculations provide adsorption geometries in excellent agreement with experiment. The availability of these structural data will improve the accuracy of charge transport models for DSSC
Underpotential deposition of Cu on Au(111) from neutral chloride containing electrolyte
Support by the Leverhulme Trust (RGP-2013-177) and EPSRC via a doctoral training grant (H.A.) is gratefully acknowledged.The structure of a chloride terminated copper monolayer electrodeposited onto Au(111) from a CuSO4/KCl electrolyte was investigated ex situ by three complementary experimental techniques (scanning tunneling microscopy (STM), photoelectron spectroscopy (PES), X-ray standing wave (XSW) excitation) and density functional theory (DFT) calculations. STM at atomic resolution reveals a stable, highly ordered layer which exhibits a Moire structure and is described by a (5×5) unit cell. The XSW/PES data yield a well-defined position of the Cu layer and the value of 2.16 A above the topmost Au layer suggests that the atoms are adsorbed in threefold hollow sites. The chloride exhibits some distribution around a distance of 3.77 Å in agreement with the observed Moire pattern due to a higher order commensurate lattice. This structure, a high order commensurate Cl overlayer on top of a commensurate (1×1) Cu layer with Cu at threefold hollow sites, is corroborated by the DFT calculations.PostprintPeer reviewe
Chemically Resolved Interface Structure of Epitaxial Graphene on SiC(0001)
Atomic-layer 2D crystals have unique properties that can be significantly modified through interaction with an underlying support. For epitaxial graphene on SiC(0001), the interface strongly influences the electronic properties of the overlaying graphene. We demonstrate a novel combination of x-ray scattering and spectroscopy for studying the complexities of such a buried interface structure. This approach employs x-ray standing wave-excited photoelectron spectroscopy in conjunction with x-ray reflectivity to produce a highly resolved chemically sensitive atomic profile for the terminal substrate bilayers, interface, and graphene layers along the SiC[0001] direction. DOI: 10.1103/PhysRevLett.111.215501 PACS numbers: 61.48.Gh, 61.05.cm, 68.49.Uv, 79.60.Ài Epitaxial graphene (EG) grown on the Si-terminated face of silicon carbide [SiC Early studies revealed that EG/SiC(0001) possesses a complex 6 p 3 Â 6 p 3R30 (6R3) reconstructed interfacial layer [10], referred to herein as the interfacial, or EG 0 , layer. This layer has significant influence on the growth, morphology, and electronic behavior of the overlaying graphene Because of the importance of the interfacial layer to the behavior of EG/SiC(0001), there have been numerous efforts to characterize its structure, including low-energy electron diffraction In this Letter we detail the structure of the interface by employing a suite of x-ray characterization techniques, including depth-sensitive XPS, x-ray standing waveenhanced XPS (XSW-XPS), and x-ray reflectivity (XRR). These tools, when employed collectively, provide the chemically specific structural information necessary to clarify previously unknown details of the EG/SiC(0001) interface. This approach ultimately enables the construction of a chemically resolved interfacial map with sub-Å resolution along the SiC[0001] direction. The XSW technique affords conventional photoelectron spectroscopy with high spatial resolution due to the influence of the XSW [here produced by the SiC(0006) Bragg reflection] on the photoabsorption process. A depiction of this phenomenon is shown i
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Hard X -Ray Photoelectron Spectroscopy (HAXPES) characterisation of electrochemical passivation oxide layers on Al-Cr-Fe Complex Metallic Alloys (CMA).
A Hard X-ray Photoelectron Spectroscopy (HAXPES) characterisation of the passivation layers formed by electrochemical polarisation of Al–Cr–Fe complex metallic alloys is presented. By employing X-ray excitation energies from 2.3 to 10.0 keV, the depth distributions of Al- and Cr-oxide and hydroxide species in the (Al,Cr)-containing passive layers could be determined. Simultaneous analyses of the shallow Al 2s and deep Al 1s core level lines (respectively, more bulk- and surface-sensitive) provided complementary information to effectively determine the depth-resolved contributions of hydroxide and oxide species within the passivation layer. A Cr threshold concentration of 18 (at.%) was found for effective passivation at pH 1
Photoelectron spectroscopy of transition metal oxide interfaces
In the present paper we review applications of the photoelectron spectroscopy (PES) technique to the investigation of transition metal oxide (TMO) interfaces. We summarize very briefly some of the principle, specific characteristics of TMOs. Because of the buried nature of the interfaces, the photoelectrons must penetrate certain thicknesses of material, which is easier with higher kinetic energies in the keV range. Thus, we also briefly summarize some of the hallmarks of hard X-ray photoelectron spectroscopy (HAXPES), before presenting four explicit samples of the analysis of TMO interfaces: The LaAlO3/SrTiO3 (0 0 1) interface, which had attracted attention because of the discovery of a sheet of high mobility electrons below the thin layer of LaAlO3; superlattices of the two insulators CaCuO2 and SrTiO3, which can be prepared to become superconducting at about 40 K; epitaxial films of the 90 K superconductor GdBa2Cu3O7−δ on NdGaO3; and finally the analysis of the nucleation of the 90 K superconductor YBa2Cu3O7−δ on SrTiO3 (0 0 1). The latter two cases of the investigation of the superconducting films are examples of photoelectron spectroscopy by X-ray standing wave (XSW) excitation. Because of this, the bare essential features of the XSW technique are also briefly reviewed