The (2√3×3)rect. phase of alkylthiolate self-assembled monolayers on Au(111): a symmetry-constrained structural solution

Low-energy electron-diffraction (LEED) patterns of the Au(111)(2√3×3)rect.-butylthiolate surface phase (a structure also seen in longer alkane chain thiolate self-assembled monolayers) show missing diffracted beams characteristic of glide symmetry, but do not show the larger set of missing beams found in surface x-ray diffraction (SXRD). The difference can be attributed to the greatly enhanced role of multiple scattering in LEED, but the combination of symmetry constraints placed on possible structural models by the observed SXRD and LEED beam extinctions greatly reduces the number of possible structural models. Only three such models are identified, one of which is clearly incompatible with other published experimental data. The relative merits of the remaining models, both involving Au adatom-thiolate moieties, are discussed in the light of the results of previous experimental studies

Photoelectron diffraction: from phenomenological demonstration to practical tool

The potential of photoelectron diffraction—exploiting the coherent interference of directly-emitted and elastically scattered components of the photoelectron wavefield emitted from a core level of a surface atom to obtain structural information—was first appreciated in the 1970s. The first demonstrations of the effect were published towards the end of that decade, but the method has now entered the mainstream armoury of surface structure determination. This short review has two objectives: First, to outline the way that the idea emerged and the way this evolved in my own collaboration with Neville Smith and his colleagues at Bell Labs in the early years: Second, to provide some insight into the current state-of-the art in application of (scanned-energy mode) photoelectron diffraction to address two key issue in quantitative surface structure determination, namely, complexity and precision. In this regard a particularly powerful aspect of photoelectron diffraction is its elemental and chemical-state specificity

The local adsorption site of methylthiolate on Au(1 1 1): Bridge or atop?

Measurements of the local adsorption geometry of the S head-group atom in the Au(1 1 1)(√3 × √3)R30°–CH3S surface have been made using normal incidence X-ray standing waves (NIXSW) and S 1s scanned-energy mode photoelectron diffraction on the same surface preparations. The results confirm that the local adsorption site is atop an Au atom in a bulk-continuation site with a S–Au bondlength of 2.42 ± 0.02 Å, and that there can be no significant fraction of coadsorbed bridging species as recently proposed in a combined molecular dynamics/experimental study by Mazzarello et al. [R. Mazzarello, A. Cossaro, A. Verdini, R. Rousseau, L. Casalis, M.F. Danisman, L. Floreano, S. Scandolo, A. Morgante, G. Scoles, Phys. Rev. Lett. 98 (2007) 016102]. The results do not, however, clearly distinguish the different local reconstruction (adatom) models proposed for this surface

Local methylthiolate adsorption geometry on Au(111) from photoemission core-level shifts

The local adsorption structure of methylthiolate in the ordered Au(111)-(√3×√3)R30° phase has been investigated using core-level-shift measurements of the surface and bulk components of the Au 4f7/2 photoelectron binding energy. The amplitude ratio of the core-level-shift components associated with surface Au atoms that are, and are not, bonded to the thiolate is found to be compatible only with the previously proposed Au-adatom-monothiolate moiety in which the thiolate is bonded atop Au adatoms in hollow sites, and not on an unreconstructed surface, or in Au-adatom-dithiolate species

A photoelectron diffraction investigation of vanadyl phthalocyanine on Au(1 1 1)

Scanned-energy mode photoelectron diffraction using the O 1s and V 2p emission perpendicular to the surface has been used to investigate the orientation and internal conformation of vanadyl phthalocyanine (VOPc) adsorbed on Au(1 1 1). The results confirm earlier indications from scanning tunnelling microscopy that the Vdouble bond; length as m-dashO vanadyl bond points out of, and not into, the surface. The Vdouble bond; length as m-dashO bondlength is 1.60 ± 0.04 Å, not significantly different from its value in bulk crystalline VOPc. However, the V atom in the adsorbed molecule is almost coplanar with the surrounding N atoms and is thus pulled down into the approximately planar region defined by the N and C atoms by 0.52 (+0.14/−0.10) Å, relative to its location in crystalline VOPc. This change must be attributed to the bonding interaction between the molecule and the underlying metal surface

The local adsorption structure of benzene on Si(001)-(2 × 1): a photoelectron diffraction investigation

Scanned-energy mode C 1s photoelectron diffraction has been used to investigate the local adsorption geometry of benzene on Si(001) at saturation coverage and room temperature. The results show that two different local bonding geometries coexist, namely the 'standard butterfly' (SB) and 'tilted bridge' (TB) forms, with a composition of 58 ± 29% of the SB species. Detailed structural parameter values are presented for both species including Si–C bond lengths. On the basis of published measurements of the rate of conversion of the SB to the TB form on this surface, we estimate that the timescale of our experiment is sufficient for achieving equilibrium, and in this case our results indicate that the difference in the Gibbs free energy of adsorption, ΔG(TB)−ΔG(SB), is in the range −0.023 to +0.049 eV. We suggest, however, that the relative concentration of the two species may also be influenced by a combination of steric effects influencing the kinetics, and a sensitivity of the adsorption energies of the adsorbed SB and TB forms to the nature of the surrounding benzene molecules

Spin resonance in the superconducting state of Li1−x_{1-x}Fex_{x}ODFe1−y_{1-y}Se observed by neutron spectroscopy

We have performed inelastic neutron scattering measurements on a powder sample of the superconductor lithium iron selenide hydroxide Li$_{1-x}$Fe$_{x}$ODFe$_{1-y}$Se ($x \simeq 0.16, y \simeq 0.02$, $T_{\rm c} = 41$\,K). The spectrum shows an enhanced intensity below $T_{\rm c}$ over an energy range $0.64\times2\Delta < E < 2\Delta$, where $\Delta$ is the superconducting gap, with maxima at the wave vectors $Q_1 \simeq 1.46$\,\AA$^{-1}$ and $Q_2 \simeq 1.97$\,\AA$^{-1}$. The behavior of this feature is consistent with the spin resonance mode found in other unconventional superconductors, and strongly resembles the spin resonance observed in the spectrum of the molecular-intercalated iron selenide, Li$_{0.6}$(ND$_{2}$)$_{0.2}$(ND$_{3}$)$_{0.8}$Fe$_{2}$Se$_{2}$. The signal can be described with a characteristic two-dimensional wave vector $(\pi, 0.67\pi)$ in the Brillouin zone of the iron square lattice, consistent with the nesting vector between electron Fermi sheets

Adsorption structure of glycine on TiO2(1 1 0): a photoelectron diffraction determination

High-resolution core-level photoemission and scanned-energy mode photoelectron diffraction (PhD) of the O 1s and N 1s states have been used to investigate the interaction of glycine with the rutile TiO2(1 1 0) surface. Whilst there is clear evidence for the presence of the zwitterion View the MathML sourceCH2COO− with multilayer deposition, at low coverage only the deprotonated glycinate species, NH2CH2COO is present. Multiple-scattering simulations of the O 1s PhD data show the glycinate is bonded to the surface through the two carboxylate O atoms which occupy near-atop sites above the five-fold-coordinated surface Ti atoms, with a Ti–O bondlength of 2.12 ± 0.06 Å. Atomic hydrogen arising from the deprotonation is coadsorbed to form hydroxyl species at the bridging oxygen sites with an associated Ti–O bondlength of 2.01 ± 0.03 Å. Absence of any significant PhD modulations of the N 1s emission is consistent with the amino N atom not being involved in the surface bonding, unlike the case of glycinate on Cu(1 1 0) and Cu(1 0 0)