In the last two decades surface science techniques have decisively contributed to our present knowledge of alkanethiol self-assembled monolayers (SAMs) on solid surfaces. These organic layers have been a challenge for surface scientists, in particular because of the soft nature of the organic material (which can be easily damaged by irradiation), the large number of atoms present in the molecules, and the complex physical chemistry involved in the self-assembly process. This challenge has been motivated by the appealing technological applications of SAMs that cover many fields of the emerging area of nanotechnology. Sulfur (S) is closely related to alkanethiols and can be used to understand basic aspects of the surface structure of SAMs. In this review we focus on the atomic/molecular structures of S-containing SAMs on Au(111). Particular emphasis is given to the substrate, adsorption sites, chemical state of the S–metal bond and also to the experimental and theoretical tools used to study these structures at the atomic or molecular levels.

The oxidation of the Rh(111) surface at oxygen pressures from 10(-10) mbar to 0.5 bar and temperatures between 300 and 900 K has been studied on the atomic scale using a multimethod approach of experimental and theoretical techniques. Oxidation starts at the steps, resulting in a trilayer O-Rh-O surface oxide which, although not thermodynamically stable, prevents further oxidation at intermediate pressures. A thick corundum like Rh2O3 bulk oxide is formed only at significantly higher pressures and temperatures.

The interaction of water with TiO is crucial to many of its practical applications, including photocatalytic water splitting. Following the first demonstration of this phenomenon 40 years ago there have been numerous studies of the rutile single-crystal TiO(110) interface with water. This has provided an atomic-level understanding of the water-TiO interaction. However, nearly all of the previous studies of water/TiO interfaces involve water in the vapour phase. Here, we explore the interfacial structure between liquid water and a rutile TiO(110) surface pre-characterized at the atomic level. Scanning tunnelling microscopy and surface X-ray diffraction are used to determine the structure, which is comprised of an ordered array of hydroxyl molecules with molecular water in the second layer. Static and dynamic density functional theory calculations suggest that a possible mechanism for formation of the hydroxyl overlayer involves the mixed adsorption of O and HO on a partially defected surface. The quantitative structural properties derived here provide a basis with which to explore the atomistic properties and hence mechanisms involved in TiO photocatalysis.

Surface x-ray diffraction has been employed to elucidate the surface structure of the (011)-(2 x 1) termination of rutile TiO2. The data are inconsistent with previously proposed structures. Instead, an entirely unanticipated geometry emerges from the structure determination, which is terminated by zigzag rows of twofold coordinated oxygen atoms asymmetrically bonded to fivefold titanium atoms. The energetic stability of this structure is demonstrated by ab initio total energy calculations.

The physics of transition metal oxide heterostructures is a subject of great interest due to the observations of new interface electronic and magnetic states. [ 1 ] The two-dimensional electron gas (2DEG) discovered at the interface between insulating LaAlO 3 (LAO) thin fi lms and bulk SrTiO 3 (STO) crystals stands as a model and has boosted the great expectations placed in oxide electronics. [ 2 , 3 ] Recently, it has been found that also the (001) SrTiO 3 surfaces, obtained by cleaving SrTiO 3 in ultra-high vacuum (UHV) [ 4 , 5 ] or by annealing in reducing conditions, [ 6 ] exhibit two-dimensional electronic states, independent of the bulk doping. From these studies it emerges that, despite possible different doping mechanisms, a similar band splitting characterizes the electronic properties of vacuum/STO and LAO/STO [ 7 ] interfaces.As matter of fact, while electron doping is believed to occur by creation of oxygen vacancies in STO surfaces, [ 5 , 6 , 8 ] in the case of LAO/STO interfaces the primary origin of the charge carriers is attributed to an electrostatic instability of the system. In particular, a scenario of electronic reconstruction has been proposed, [ 9 , 10 ] where a Zener breakdown takes place above a LAO thickness of 3 unit cells (uc), as experimentally observed. [ 11 , 12 ] According to this picture, once the electron transfer occurs, carriers spontaneously occupy bands composed by Ti-3d orbitals, starting with d xy -symmetry bands located within the fi rst adjacent SrTiO 3 layers. [ 13 ] Moreover, according to DFT calculation on LAO/STO bilayers, [ 14 ] the distribution of electrons accumulated within the fi rst STO layers gives rise to an electric fi eld, which in turn induces a rumpling of the SrO and TiO 2 interface atomic planes within the same region hosting the 2DEG.Thus, a polar instability scenario predicts for LAO/STO heterostructures structural and electronic modifi cations of the fi rst SrTiO 3 unit cells occurring simultaneously with the appearance of a 2D-metal. However, not all the experimental results are fully consistent with this picture. For example, second harmonic generation experiments suggested that polar distortions in STO appear below the LAO threshold thickness of 4-uc, i.e., before the realization of 2D electron system. [ 15 ] More recently, a rumpling of the STO interface layers was observed also in p-type LAO/STO insulating interfaces, as shown by grazing incidence X-ray diffraction (GIXD). [ 16 ] To clarify these issues, we have used a combination of advanced X-ray synchrotron-based spectroscopic and structural measurements to study the evolution of the electronic properties and ionic positions at LAO/STO interfaces as a function of the LAO thickness ( Figure 1 ). We fi nd that an orbital reconstruction occurs also in insulating 2-uc LAO/STO, before the appearance of the metallic state. Partial or complete inversion of the 3d-t 2g energy levels in insulating samples is correlated to partial or complete coverage of the STO surface by an ordered polar layer....

The structure and stability of alkanethiols self-assembled on Au(111) have been studied as a function of the molecular chain length by means of atomic force microscopy (AFM) and grazing incidence X-ray diffraction (GIXD). Below saturation, phases consisting of molecules with different tilt angles and periodicities are formed. Differences in the mechanical stability of these phases are revealed by AFM experiments and discussed in terms of the competition between intermolecular and molecule-substrate interactions as a function of chain length. For long molecules, intermolecular interactions play a dominant role which stabilizes the formation of closed packed 30 degrees tilted ( radical 3x radical 3)R30 degrees structures. For short molecules, the van der Waals interaction with the gold substrate favors the formation of a 50 degrees tilted phase in which the molecules are arranged in a rectangular configuration.

The c(4 x 2) structure of C16H33SH alkanethiol monolayers self-assembled on Au(111) has been studied by grazing incidence X-ray diffraction. This structure coexists on the surface with the (radical3x radical3)R30 degrees phase. The structural refinement of the c(4 x 2) phase has been accomplished by omitting the fractional order reflections common to both structures. The surface unit cell consists of four symmetry-independent molecules with atomic displacements related by couples, such that only two nonequivalent chains are present in the surface cell. The stability between neighbor chains is due to van der Waals interactions. The substrate plays an important and non-negligible role in the c(4 x 2) reconstruction. The lateral and normal substrate relaxations to the surface plane are small, and gold atom displacements are lower than 0.25 angstroms but contribute very strongly to the fractional order intensities. The molecular chains form a close packed structure tilted by 37 degrees from the surface normal with no indications of dimer formation between closest S atoms.

Using a combined experimental and theoretical approach, we show that the initial oxidation of a Rh͑553͒ surface, a surface vicinal to ͑111͒, undergoes step bunching when exposed to oxygen, forming lower-index facets. At a pressure of about 10 −6 mbar and a temperature of 380°C this leads to ͑331͒ facets with onedimensional oxide chains along the steps, coexisting with ͑111͒ facets. Further increase of the pressure and temperature results in ͑111͒ facets only, covered by an O-Rh-O surface oxide. Our density functional theory calculations provide an atomistic understanding of the observed behavior.