Marek Szymonski

Energy distributions of atoms sputtered from alkali halides by 540 eV electrons

Abstract The emission of halogen and alkali atoms, occurring under bombardment of alkali halides with electrons has been investigated. The electron energy was 540 eV and the temperature of the target was varied between room temperature and 400°C. We measured the energy distribution of the emitted neutral particles with a time of flight method. It was found that either diffusing interstitial halogen atoms or moving holes dominate the sputtering process above 200°C. Below 150°C alkali halides with lattice parameter s/d ≥ 0.33 show emission of non-thermal halogen atoms. s is the interionic space between two halogen ions in a direction and d is the diameter of a halogen atom. In general the energy distribution of the alkali and halogen atoms is thermal above 200°C, but not Maxwellian.

An organometallic route to long helicenes

Along with the recent progress in the development of advanced synthetic methods, the chemical community has witnessed an increasing interest in promising carbon-rich materials. Among them, helicenes are unique 3D aromatic systems that are inherently chiral and attractive for asymmetric catalysis, chiral recognition and material science. However, there have been only limited attempts at synthesizing long helicenes, which represent challenging targets. Here, we report on an organometallic approach to the derivatives of undecacyclic helicene, which is based on intramolecular [2 + 2 + 2] cycloisomerization of aromatic hexaynes under metal catalysis closing 6 new cycles of a helicene backbone in a single operation. The preparation of nonracemic compounds relied on racemate resolution or diastereoselective synthesis supported by quantum chemical (density functional theory) calculations. The fully aromatic [11]helicene was studied in detail including the measurement and theoretical calculation of its racemization barrier and its organization on the InSb (001) surface by STM. This research provides a strategy for the synthesis of long helical aromatics that inherently comprise 2 possible channels for charge transport: Along a π-conjugated pathway and across an intramolecularly π-π stacked aromatic scaffold.

The sputtering processes during 6 kev Xe ion beam bombardment of halides

Abstract Mass selected energy distributions of sputtered atoms and molecules from AgBr, AgF, CdI2 and PbI2 have been measured using a time of flight method. The analysis of experimental data shows a complex character of the sputtering which can be explained by a simultaneous appearance of three different processes: collision cascade, thermal spike and thermal evaporation of decomposed target material. The analytical expressions for energy distributions given by the three sputtering mechanisms are in very good agreement with the experimental spectra. Physical parameters of the energy distributions: E b (surface binding energy), T sp (mean temperature of the spike) and T (macroscopic surface temperature) are obtained and their physical meaning is discussed. A comparison between existing sputtering theories and results of the investigations is presented.

On the model of the electron sputtering process of alkali halides

Abstract A model of electron induced sputtering of alkali halides is described. It is assumed that fast neutral particles are emitted when H-centres, taking part in the focused replacement sequence, cross the surface. When the focused replacement sequence is terminated at a certain distance from the surface, the H-centre may diffuse to the surface, giving rise to thermal particles. The model allows calculations of the sputtering yields for thermal, non-thermal and molecular emission. The results of the calculations are in good agreement with the experimental data obtained by different authors.

Sputtering of an AgAu alloy by bombardment with 6 keV Xe+ ions

The energy distributions of silver and gold atoms sputtered from an AgAu alloy by bombardment with a 6 keV Xe+ ion beam have been measured and compared with those from the pure elements. Total sputtering yields of the investigated samples have also been measured. The results show that in addition to collision cascades, spike effects contribute to the sputtering. The experimentally deduced binding energies of Ag and Au atoms in the AgAu alloy surface are lower than for pure elements which accounts for the higher sputtering yield of the AgAu alloy which the authors observed. The existing experimental data on composition changes of the AgAu surface during ion bombardment can be explained in the light of present measurements.

Electron sputtering of alkali halides a study of its dependence on the beam energy and target temperature

Abstract Experimental results are presented on the sputtering of alkali halides by electrons with varying electron energy and different target temperatures. The energy distributions of the emitted alkali and halogen atoms have been measured with a time of flight method. It was found that with increasing temperature the ratio of thermally emitted halogen atoms to non-thermally emitted halogen atoms increases exponentially. We also observed that this ratio is dependent on the electron energy. The experimental results can be understood if it is assumed that the thermal emission of halogen atoms is caused by Vk-centres, created by electron impact A model is described which explains the experimentally observed sputtering yields.

Atomic-resolution images of radiation damage in KBr

The first steps of electron irradiation induced modification of a KBr(1 0 0) surface have been studied by dynamic force microscopy with atomic resolution. Rectangular pits of monatomic depth with not more than one kink site per pit have been found. The atomic structure of KBr(1 0 0) is preserved at the bottom of the pits. Possible deexcitation and desorption mechanisms are discussed based on these results

Spikes in low energy sputtering of silver and gold

Abstract The energy distributions of atoms sputtered from Ag and Au targets by 6 keV Ar and Xe ion bombardment have been measured. It is shown that the spike mechanism, considered as a parallel process to the collision cascade, leads to a consistent explanation of the results.

Adsorption and Self-Assembly of Large Polycyclic Molecules on the Surfaces of TiO2 Single Crystals

Titanium dioxide is one of the most frequently studied metal oxides, and its (110) rutile surface serves as a prototypical model for the surface science of such materials. Recent studies have also shown that the (011) surface is relatively easy for preparation in ultra-high vacuum (UHV) and that both the (110) and (011) surfaces could be precisely characterized using scanning tunneling microscopy (STM). The supramolecular self-assembly of organic molecules on the surfaces of titanium dioxide plays an important role in nanofabrication, and it can control the formation and properties of nanostructures, leading to wide range of applications covering the fields of catalysis, coatings and fabrication of sensors and extends to the optoelectronic industry and medical usage. Although the majority of experiments and theoretical calculations are focused on the adsorption of relatively small organic species, in recent years, there has been increasing interest in the properties of larger molecules that have several aromatic rings in which functional units could also be observed. The purpose of this review is to summarize the achievements in the study of single polycyclic molecules and thin layers adsorbed onto the surfaces of single crystalline titanium dioxide over the past decade.

Supramolecular ordering of PTCDA molecules: the key role of dispersion forces in an unusual transition from physisorbed into chemisorbed state.

Adsorption and self-assembly of large π-conjugated 3,4,9,10-perylene tetracarboxylic dianhydride (PTCDA) molecules on rutile TiO(2)(110) surface have been investigated using a combination of high-resolution scanning tunneling microscopy (STM), low-energy electron diffraction, and density functional theory calculations with inclusion of Grimme treatment of the dispersion forces (DFT-D). Evolution of the STM images as a function of PTCDA coverage is caused by transition of the adsorption mode from physisorbed single adspecies and meandering stripes into spontaneously ordered chemisorbed molecular assemblies. This change in the adsorption fashion is accompanied by significant bending of the intrinsically flat, yet elastic, PTCDA molecule, which allows for strong electronic coupling of the dye adspecies with the TiO(2) substrate. Extensive DFT-D modeling has revealed that adsorption is controlled by interfacial and intermolecular dispersion forces playing a dominant role in the adsorption of single PTCDA species, their self-organization into the meandering stripes, and at the monolayer coverage acting collectively to surmount the chemisorption energy barrier associated with the molecule bending. Analysis of the resulting density of states has revealed that alignment of the energy levels and strong electronic coupling at the PTCDA/TiO(2) interface are beneficial for dye sensitization purposes.