Para‐sexiphenyl is found to assemble into azimuthally oriented crystalline nanoneedles on a TiO2 substrate (see Figure). The needles grow after prolonged exposure until they completely tile the ...surface with a film of uniform thickness. This novel morphology combined with the unique orientation of the molecules (all parallel to each other and the substrate and all with their molecular planes tilted with respect to the substrate) appears useful for both light‐emitting diodes and field‐effect transistor applications.
Growth of sexithiophene films on both ordered and disordered TiO2(110) surfaces has been investigated by angle-resolved ultraviolet photoemission spectroscopy, atomic force microscopy, and X-ray ...diffraction including grazing-incidence characterization. The order (or disorder) of the TiO2(110)-1 x 1 surface has been observed to profoundly influence the electronic, morphological, and structural properties of the 6T films: the band alignment, which determines the injection efficiency of contacts, has been considerably modified by 0.6 eV, and a morphology with either needle-like or dendritic-like islands has been obtained. The changes in the 6T film properties are associated with the orientational modifications of sexithiophene molecules within the films, either flat-lying or upright standing, 6T(010) or 6T(100) crystallites, respectively. The growth of different crystallite orientations is argued to be controlled by the kinetics mediated by the (dis)-order of the TiO2(110) surface rather than exclusively by chemical interaction between the molecule and the substrate.
The interaction of O2 with Pd(111) in the temperature range from 300K to 1000K was studied by molecular beam adsorption, thermal desorption (TDS), low energy electron diffraction (LEED), ...high-resolution X-ray photoelectron spectroscopy (XPS) and high-resolution electron energy loss spectroscopy (HREELS). Using a capillary array doser and high effective oxygen pressures, evidence was found for the formation of a densely packed chemisorbed oxygen adlayer saturating at ΘO close to 1 and separately for subsurface migration of oxygen at elevated temperatures, but not at room temperature and below. Up to completion of a p(2×2) oxygen adlayer at 0.25ML surface coverage, the dissociative sticking probability of oxygen into the chemisorbed state is high and masks the much slower diffusion into the bulk. Beyond 0.25ML surface coverage, the adsorption rate into the chemisorbed state becomes small and the influence of bulk migration detectable. Exposure of the sample to high oxygen dosages at 1000K fills up the subsurface reservoir and subsequent sticking measurements are no longer influenced by oxygen loss to the bulk. The subsurface oxygen could be distinguished in both XPS and off-specular HREELS. These latter techniques revealed that considerable concentrations of oxygen in the near-surface region can build up, even at lower temperatures (523K) and oxygen exposures (40L). In contrast to chemisorbed oxygen atoms on Pd(111), the subsurface species cannot be removed by reaction with CO.
Surface pre-treatment of graphitic electrode materials for lithium ion cells has recently been shown to significantly reduce the irreversible consumption of material and charge due to the formation ...of the so-called solid electrolyte interphase (SEI) during battery charging. In this paper, we compare graphite powders and carbon fibres as model materials for X-ray photoemission spectroscopy (XPS) studies of the effects of surface pre-treatments. For carbon fibres, the surface carbon percentage was found to vary from 70–95% depending on the surface treatment, with corresponding changes in the relative proportion of graphitic compared to CO bonds, as determined from C 1s curve fits. In contrast, results from the graphite powders show very little change in surface chemical composition and an essentially constant C 1s lineshape dominated by graphitic carbon. SEM data show the carbon fibre cross-section to be composed of a radial array of layered graphite, leaving a surface consisting largely of prismatic planes, while the graphite powder consists of graphite platelets with the surface area predominantly of basal planes. We conclude that the chemical modification occurs at the prismatic planes, and that the powders are unsuitable as models for XPS studies of electrode surface modification, while the fibres are very well suited.
Low-dimensional oxide nanostructures supported on well-defined metal surfaces raise scientific interest both on a fundamental level and for potential technological applications. These systems may be ...regarded as artificially created hybrid materials with novel emergent properties, supporting new concepts of geometrical structure, electronics and magnetism, complex phase diagrams and particular chemical reactivity. The coupling of the oxide nano-phase to the metal support surface by electronic and elastic interactions together with the low dimensionality aspects are at the origin of the particular behaviour of these hybrid structures. By way of prototypical examples, this Prospective article highlights some of the novel properties of nano-oxides and, as a side aspect, comments on the aesthetics of their structural motifs.
Procedures for the vacuum deposition of thin histidine films on polycrystalline Au(111) and their characterization with high-resolution synchrotron-radiation-based photoelectron spectroscopy are ...reported. The chemical form of histidine (anionic vs zwitterionic) and the nature of its interactions with the substrate (strong ionic−covalent vs weak van der Waals bonding) in mono- and multilayer films are analyzed. It is shown that water adsorption on a pre-prepared histidine film at 100 K results in protonation of histidine molecules and partial formation of hydroxyl anions. These chemical effects are carefully differentiated from spectral changes associated with radiation damage of the histidine films.
By combining high-resolution X-ray photoelectron and electron energy loss spectroscopies a comprehensive analysis of the adsorption of CO on Pd(1
1
1) at 300 K has been performed. The characteristic ...fingerprints for various CO–Pd(1
1
1) bonding configurations have been identified from the decomposition analysis of the adsorbate C
1s and the substrate Pd
3d
5/2 core-level photoemission spectra obtained after CO adsorption at 120 K. The c(4×2) structure at 0.5 monolayer (ML) and the (2×2)-3CO structure at 0.75 ML formed at low temperature have been used for calibration purposes. The core-level results are consistent with CO adsorbing in a mixture of fcc and hcp threefold hollow sites in the c(4×2) structure and of hollow and on-top sites in the (2×2) structure, as reported in the literature. For CO adsorption at 300 K, a different site occupation is evidenced by the presence of two components in the C
1s and Pd
3d
5/2 core-level and C–O stretching vibration lineshapes. At coverages up to 0.1 ML only fcc threefold hollow sites in a
(
3
×
3
)R30°
structure are occupied. The population of these sites continues with increasing the CO coverage up to 0.32 ML, but in addition adsorption of CO in bridge sites takes place. The latter sites become predominant at the 300 K saturation coverage of 0.5 ML, and are considered to form domain boundaries in a poorly ordered
(
3
×
3
)R30°
phase with split low energy electron diffraction reflexes. Between 0.32 and 0.5 ML the domain boundary regions grow at the expense of the
(
3
×
3
)R30°
domains.
Cerium oxide is an important catalytic material known for its ability to store and release oxygen, and as such, it has been used in a range of applications, both as an active catalyst and as a ...catalyst support. Using scanning tunneling microscopy and Auger electron spectroscopy, we investigated oxygen interactions with CeO x nanoclusters on a complete graphene monolayer-covered Ru(0001) surface at elevated temperatures (600–725 K). Under oxidizing conditions (P O2 = 1 × 10–7 Torr), oxygen intercalation under the graphene layer is observed. Time dependent studies demonstrate that the intercalation proceeds via spillover of oxygen from CeO x nanoclusters through the graphene (Gr) layer onto the Ru(0001) substrate and extends until the Gr layer is completely intercalated. Atomically resolved images further show that oxygen forms a p(2 × 1) structure underneath the Gr monolayer. Temperature dependent studies yield an apparent kinetic barrier for the intercalation of 1.21 eV. This value correlates well with the theoretically determined value for the reduction of small CeO2 clusters reported previously. At higher temperatures, the intercalation is followed by a slower etching of the intercalated graphene (apparent barrier of 1.60 eV). Vacuum annealing of the intercalated Gr leads to the formation of carbon monoxide, causing etching of the graphene film, demonstrating that the spillover of oxygen is not reversible. In agreement with previous studies, no intercalation is observed on a complete graphene monolayer without CeO x clusters, even in the presence of a large number of point defects. These studies demonstrate that the easily reducible CeO x clusters act as intercalation gateways capable of efficiently delivering oxygen underneath the graphene layer.
We report on an interface-stabilized strained c(4
×
2) phase formed by cobalt oxide on Pd(1
0
0). The structural details and electronic properties of this oxide monolayer are elucidated by ...combination of scanning tunneling microscopy data, high resolution electron energy loss spectroscopy measurements and density functional theory. The c(4
×
2) periodicity is shown to arise from a rhombic array of Co vacancies, which form in a pseudomorphic CoO(1
0
0) monolayer to partially compensate for the compressive strain associated with the large lattice mismatch (∼9.5%) between cobalt monoxide and the substrate. Deviation from the perfect 1:1 stoichiometry thus appears to offer a common and stable mechanism for strain release in Pd(1
0
0) supported monolayers of transition metal rocksalt monoxides of the first transition series, as very similar metal-deficient c(4
×
2) structures have been previously found for nickel and manganese oxides on the same substrate.
Two-dimensional (2D) WO3 nanosheets exhibit a range of novel properties and functionalities that render them attractive for advanced nanotechnologies. However, at the ultimate 2D limit of ...single-layer thickness, the structural properties of WO3 are unclear. Here, we fabricated, using molecular beam epitaxy techniques, a crystalline 2D WO3 overlayer on a Ag(100) surface and unveiled its geometric, electronic, and vibrational structure via a combination of state-of-the-art experimental (microscopic and spectroscopic) and computational techniques. The 2D WO3 phase forms a bilayer with a staggered arrangement of WO6 octahedra, linked together by corner- and edge-sharing, which is significantly different from the cubic and monoclinic WO3 bulk structures, but resembles a bilayer of the α-MoO3 layered bulk lattice. Such a 2D WO3 bilayer on Ag(100) is a robust nonpolar structure, which is incommensurate in various rotational orientations, weakly coupled to the metal substrate, and, according to the density functional theory calculations, should survive as a stable freestanding layer, that is, as a nanosheet.
PDF