Adolf Winkler

Characterization of Step-Edge Barriers in Organic Thin-Film Growth

Detailed understanding of growth mechanisms in organic thin-film deposition is crucial for tailoring growth morphologies, which in turn determine the physical properties of the resulting films. For growth of the rodlike molecule para-sexiphenyl, the evolution of terraced mounds is observed by atomic force microscopy. Using methods established in inorganic epitaxy, we demonstrate the existence of an additional barrier (0.67 electron volt) for step-edge crossing—the Ehrlich-Schwoebel barrier. This result was confirmed by transition state theory, which revealed a bending of the molecule at the step edge. A gradual reduction of this barrier in the first layers led to an almost layer-by-layer growth during early deposition stage. The reported phenomena are a direct consequence of the complexity of the molecular building blocks versus atomic systems.

Nanotexture Switching of Single-Layer Hexagonal Boron Nitride on Rhodium by Intercalation of Hydrogen Atoms†

The interaction of atomic hydrogen with a single layer of hexagonal boron nitride on rhodium leads to a removal of the h-BN surface corrugation. The process is reversible as the hydrogen may be expelled by annealing to about 500 K whereupon the texture on the nanometer scale is restored. This effect is traced back to hydrogen intercalation. It is expected to have implications for applications, like the storage of hydrogen, the peeling of sp2-hybridized layers from solid substrates or the control of the wetting angle, to name a few.Playing nano-tectonics: The interaction of atomic hydrogen with a single layer of hexagonal boron nitride on rhodium leads to the removal of the h-BN surface corrugation (see picture; blue region: corrugated, orange region: flat). This change of surface texture arises from the intercalation of hydrogen atoms between the h-BN skin and the metal, and can be restored by annealing to about 600 K to expel the hydrogen atoms.

Characterization of alkanethiol self-assembled monolayers on gold by thermal desorption spectroscopy.

SAM formation of undecanethiol (UDT) and mercaptoundecanoic acid (11-MUA) on Au(111) and on gold foils, using wet chemical preparation methods as well as physical vapor deposition (PVD) in UHV, has been studied by means of thermal desorption spectroscopy (TDS), low energy electron diffraction (LEED), and Auger electron spectroscopy (AES). The main aim of this study was to explore the possible application of TDS to characterize the quality of a SAM and to determine its thermal stability. The influence of various parameters, like substrate pretreatment, film formation method, and type of the functional end group, has been studied in detail. Three different temperature regimes can be identified in TDS, which yields specific information about the organic layer: Desorption of disulfides around 400 K can be shown to result from standing molecules in a well-defined SAM. Desorption of intact molecules and of molecules with split-off sulfur is observed around 500 K, resulting from lying molecules. Finally, desorption of an appreciable amount of gold-containing molecules is observed around 700 K. This is more pronounced for 11-MUA than for UDT, and in addition more pronounced for solution-based SAMs than for PVD prepared SAMs. These results emphasize the important role of gold adatoms in SAM formation, as recently discussed in the literature.

Epitaxial growth of quaterphenyl thin films on gold(111)

The crystal structure and molecular arrangement of para-quaterphenyl (4P) grown on single crystalline Au(111) was investigated over a wide thickness range. The molecular arrangement in the first monolayer, as investigated with low energy electron diffraction, shows a highly regular structure. This wetting layer is defined by adsorbate–substrate interactions and forms a prestage for the epitaxial growth of 4P single crystalline islands, as observed in x-ray diffraction. Two similar orientations of the 4P bulk phase are observed, with the (211) and (311) planes parallel to the Au(111) surface. The alignment of the molecules was kept unchanged from the first monolayers up to a film thickness of 200 nm.

A Study on the Formation and Thermal Stability of 11-MUA SAMs on Au(111)/Mica and on Polycrystalline Gold Foils

In this article we present a comprehensive study of 11-mercaptoundecanoic acid self-assembled monolayer (SAM) formation on gold surfaces. The SAMs were prepared in ethanolic solution, utilizing two different substrates: Au(111)/mica and polycrystalline gold foils. Several experimental methods (X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy, and atomic force microscopy) reveal a well-defined SAM. The main focus of this work, however, was to test the stability of these SAMs by thermal desorption spectroscopy. The spectra show different desorption peaks indicating different adsorption states and/or decomposition products on the surface. The assumed monolayer peak, which can be attributed to desorption of the intact molecule, is detected at 550 K. Further desorption peaks can be found, which result, e.g., from cracking of the S-C bond on the surface, depending on the substrate quality and on the residence time under ambient conditions.

Adsorption, initial growth and desorption kinetics of p-quaterphenyl on polycrystalline gold surfaces

Thermal desorption spectroscopy (TDS) and X-ray photoelectron spectroscopy (XPS) have been applied to investigate the kinetics of adsorption, layer growth and desorption of p-quaterphenyl (P4P) on polycrystalline gold surfaces. Two different desorption peaks resulting from a monolayer (wetting layer) and a multilayer can be observed. The multilayer predominantly grows in form of a continuous film at 93 K, whereas at room temperature needle-like island growth is observed. A rearrangement (island formation) of the continuous multilayer takes place during heating prior to desorption. The influence of carbon on the adsorption/desorption kinetics of the monolayer has been studied in detail. On the clean surface some amount of adsorbed P4P dissociates and hydrogen is released at about 650 and 820 K, respectively. No dissociation of P4P takes place on the carbon-covered surface. The intact P4P molecules of the monolayer desorb in form of two broad peaks around 420 and 600 K, the multilayer desorbs with zero order above 350 K. Quantitative measurements with a quartz microbalance yield a mean thickness of 0.27 nm for the monolayer, suggesting that the P4P molecules are lying flat on the surface, for both the clean and the carbon-covered surface.

Adsorption/desorption of H2 and CO on Zn-modified Pd(111)

The adsorption and thermal desorption of H(2) and CO on clean and Zn covered Pd(111) surfaces were studied using temperature programmed desorption (TPD), low energy electron diffraction, and Auger electron spectroscopy. The obtained H(2) and CO-TPD results reveal that thick Zn layers (approximately 10 ML) prepared at low temperature (150 K) block the adsorption of H(2) and CO. However, the ZnPd surface alloy which is formed at temperatures above 300 K shows a different behavior. The amount of hydrogen adsorbed on surface sites is reduced by about 1/2 on the ZnPd surface alloy whereupon the diffusion of hydrogen into the subsurface region is not influenced. The initial sticking coefficient decreases from 0.5 on the clean surface to 0.14 on the ZnPd alloy. The TPD spectra for CO on the ZnPd surface alloy show that the heat of adsorption is shifted to much lower values than on clean Pd, yielding a desorption energy of 71+/-2 kJ mol(-1) at low CO coverages. The saturation coverage equals 0.5 ML which means that each Pd atom of the ZnPd surface alloy is occupied by one CO admolecule. Interestingly, however, the initial sticking coefficient for CO on the ZnPd surface alloy is still unity, as on the clean Pd surface.

Angular and energy distributions of D2 molecules desorbing from sulfur and oxygen modified V(111) surfaces

The energy and angular distribution of deuterium molecules desorbing from a vanadium (111) surface modified either by oxygen or by sulfur has been studied, using time-of-flight spectroscopy. It has been shown that the desorption flux contains two contributions, a thermal and a hyperthermal contribution. The mean translational energy of the hyperthermal part can be described by 〈E〉=8.3⋅kTs and 5.8⋅kTs for the sulfur and oxygen covered V(111) surface, respectively. Interestingly, the mean translational energy of the hyperthermal contribution is independent of the desorption angle. The angular distribution of the hyperthermal desorption flux is forward focused and can be described by cos3.3 θ and cos4.3 θ functions for the sulfur and oxygen modified surface, respectively. From the angular flux distribution and the angle independent mean translational energy of the hyperthermal contribution one can conclude that normal energy scaling does not exist for this adsorption/desorption channel. This is mainly due to...

Initial Steps of Rubicene Film Growth on Silicon Dioxide.

The film growth of the conjugated organic molecule rubicene on silicon dioxide was studied in detail. Since no structural data of the condensed material were available, we first produced high quality single crystals from solution and determined the crystal structure. This high purity material was used to prepare ultrathin films under ultrahigh vacuum conditions, by physical vapor deposition. Thermal desorption spectroscopy (TDS) was applied to delineate the adsorption and desorption kinetics. It could be shown that the initial sticking coefficient is only 0.2 ± 0.05, but the sticking coefficient increases with increasing coverage. TDS further revealed that first a closed, weakly bound bilayer develops (wetting layer), which dewets after further deposition of rubicene, leading to an island-like layer. These islands are crystalline and exhibit the same structure as the solution grown crystals. The orientation of the crystallites is with the (001) plane parallel to the substrate. A dewetting of the closed bilayer was also observed when the film was exposed to air. Furthermore, Ostwald ripening of the island-like film takes place under ambient conditions, leading to films composed of few, large crystallites. From TDS, we determined the heat of evaporation from the multilayer islands to be 1.47 eV, whereas the desorption energy from the first layer is only 1.25 eV.

Film growth, adsorption and desorption kinetics of indigo on SiO2

Organic dyes have recently been discovered as promising semiconducting materials, attributable to the formation of hydrogen bonds. In this work, the adsorption and desorption behavior, as well as thin film growth was studied in detail for indigo molecules on silicon dioxide with different substrate treatments. The material was evaporated onto the substrate by means of physical vapor deposition under ultra-high vacuum conditions and was subsequently studied by Thermal Desorption Spectroscopy (TDS), Auger Electron Spectroscopy, X-Ray Diffraction, and Atomic Force Microscopy. TDS revealed initially adsorbed molecules to be strongly bonded on a sputter cleaned surface. After further deposition a formation of dimers is suggested, which de-stabilizes the bonding mechanism to the substrate and leads to a weakly bonded adsorbate. The dimers are highly mobile on the surface until they get incorporated into energetically favourable three-dimensional islands in a dewetting process. The stronger bonding of molecules within those islands could be shown by a higher desorption temperature. On a carbon contaminated surface no strongly bonded molecules appeared initially, weakly bonded monomers rather rearrange into islands at a surface coverage that is equivalent to one third of a monolayer of flat-lying molecules. The sticking coefficient was found to be unity on both substrates. The desorption energies from carbon covered silicon dioxide calculated to 1.67 ± 0.05 eV for multilayer desorption from the islands and 0.84 ± 0.05 eV for monolayer desorption. Corresponding values for desorption from a sputter cleaned surface are 1.53 ± 0.05 eV for multilayer and 0.83 ± 0.05 eV for monolayer desorption.