V.K. Medvedev

Lithium field desorption microscope : a new tool for surface investigations

Abstract A new method of imaging a surface with field-desorbed metal (Li + ) ions is reported. The long-lasting supply of the imaging substance (Li) is achieved by surface diffusion of Li from a multilayer of Li deposited on the shank of the tip. The surfaces of [110]-oriented W and [111]-oriented Rh tips were imaged in continuous, pulsed and mixed continuous-pulsed field modes. Different types of images, formed by ions field-desorbed from di- or triatomic Li layers on the surface, were observed. The mechanisms of image formation in the lithium field desorption microscope (FDM) are discussed on the basis of known properties of field desorption and surface diffusion of Li in comparison with experimental data. The perspectives and possible applications of Li FD for surface investigations are outlined.

Field desorption and field evaporation of metals: In memoriam Professor J.H. Block

Abstract In this review we present a detailed study, both experimental and theoretical, of the field desorption and field evaporation of alkali- and transition metals looking in particular at the site specificity and the coverage dependence. A novel experimental approach based on the retarding potential analysis of metal ions emitted in a continuous field desorption mode is used. With this approach, absolute values of the field ion appearance energy have been measured and binding energies have been obtained for atoms extracted from selected surface sites under high field conditions. We discuss results of the mass-to-charge resolved retarding potential analysis of lithium ions, desorbed from W(111), and of rhodium ions evaporated from Rh(100) and Rh(111). Appearance energies of Li+ and Rh2+ were derived from the ion retardation curves, and activation energy data were evaluated from desorption rate measurements. Applying a thermionic cycle, the binding energies of Li adatoms on W(111) as well as of Rh at Rh(100) and Rh(111) step sites are obtained. The cluster embedded in jellium model, based on density functional theory, is used to interpret the experimental data. Local field enhancements, binding and activation energies are calculated for Li field desorption and Rh field evaporation as a function of field strength and surface geometry.

Investigation of adsorption and coadsorption of O2 and CO on Rh by O2+ field ion and Li+ field desorption microscopies

Abstract A recently developed new method, the lithium field desorption microscopy as well as the O2+ field ion microscopy were applied for studying the clean, the carbon monoxide- and the oxygen-covered surfaces of [100]- and [111]-oriented Rh tips. The catalytic CO-oxidation reaction on the Rh surface was studied in situ using Li+ and O2+ field ion modes of imaging. The peculiarities of imaging of different surfaces in the Li FDM are discussed in terms of field-stimulated surface diffusion of Li, of the localization of the Li field desorption process and the lateral interaction between Li adatoms. A roughening of the oxygen-covered Rh surface is observed and explained in terms of an oxygen-caused reconstruction of the Rh surface. A substantial influence of the electron density modification near the surface, caused by the presence of Li, on the CO-oxidation reaction on Rh is detected. The promoting role of the enhanced electron density in the catalytic oxidation reaction is discussed on the basis of present experimental data and known alkali-gas coadsorption properties.

Interface control in the chemical vapor deposition of titanium dioxide on silicon(100)

Abstract A novel deposition sequence has been developed for the growth of carbon-free, amorphous titanium dioxide thin films on p-type Si(100) using titanium tetrakis–isopropoxide (TTIP) as the metalorganic precursor. The deposition process occurs in an ultra-high vacuum chamber and consists of three stages. Initially, a continuous layer of titanium was deposited on silicon at a substrate temperature of 650 K using a Ti sublimator. The substrate temperature was then lowered to 300 K and the titanium layer was oxidized using a gas phase oxygen source to form a TiOx buffer layer. Subsequently, a TiO2 thin film was deposited by MOCVD using TTIP at low temperature (below 650 K). Auger electron spectroscopy (AES) performed at each stage of growth showed no detectable carbon contamination. Additional AES measurements suggest that at the process temperature, titanium grows on silicon according to the Stranski–Krastanov mode, which permits the uniform growth of the titanium layer. Finally, X-ray diffraction (XRD) studies show that the thermal decomposition of TTIP produces amorphous TiO2 at 650 K. This unique process allows for the production of a high-quality amorphous titanium dioxide thin film without interfacial carbon contamination.

Investigations of the CO-oxidation on Pt with lithium field desorption microscopy

Abstract The clean, carbon monoxide- and oxygen-covered surfaces of a [110]-oriented Pt tip were studied in the lithium field desorption microscope (Li FDM). Surface diffusion of Li on clean, carbon monoxide- and oxygen-covered Pt surfaces was investigated with field electron microscopy. A correlation between the peculiarities of the field-stimulated surface diffusion of Li on surfaces of bcc and fcc crystals, and the different mechanism of Li FDM image formation of W and Pt are observed and discussed in terms of sub- and multilayer Li surface diffusion. The catalytical CO-oxidation reaction on Pt was visualized in Li FDM. The influence of the Li on the CO-oxidation process was observed and discussed on the basis of present experimental data and known alkali-gas coadsorption properties.

Study of CO surface diffusion on COW(111) by analysis of CO+ field ion rate fluctuations

Abstract An analysis of field ion rate fluctuations is proposed for studying the mechanism of the reactive imaging gas supply in a field ion microscope (FIM). In the experiment CO + ions emitted from a few selected surface sites of the apex plane of a [111]-oriented W tip and mass-to-charge resolved in a magnetic sector field are registered. The CO + ion rate fluctuations are analyzed by a method developed by Gomer for analyzing field electron current fluctuations. The CO gas supply toward the probed surface sites is found to have a diffusion character, confirming the role of the weakly bound CO layer in the localized field ionization of CO used as imaging gas in a FIM.

Temperature and field dependence of protonated water cluster emission from field adsorbed water layers on platinum

Water ion cluster formation in high electric fields was characterized on a Pt emitter tip over temperatures ranging from 170 to 300 K. Ion clusters emitted from the field-adsorbed water layer were mass resolved using a Wien filter. Two series of tests were conducted to characterize the ion cluster formation. In the first, tip temperature and water pressure were held constant while the applied field at the tip was ramped linearly with time. As the field was increased, water ion clusters H+(H2O)n were emitted, beginning with large n clusters (up to 7) and proceeding through each lower n cluster in turn. The ion emission onset field was found to depend on the value of n and is lower for larger n clusters. The onset of ionization for each cluster was observed to be relatively constant as a function of temperature, however, ion signal intensity was temperature dependent. For the second series of tests, tip temperature was cyclically ramped while the applied field was held constant. The cyclical ramping experiments provided Arrhenius graphs showing thermal deactivation energies for ion emission from the tip. The observed deactivation corresponds to thermal desorption of the nth solvating water molecule of the ion cluster, i.e. the energy associated with the solvation of the proton by the nth solvating water molecule. These energies were found to be 0.85, 0.76 and 0.55 eV for n=3, 4 and 5, respectively.

Non-linear dynamics of CO oxidation reaction on Pt and Rh as studied with the lithium field desorption microscope

Abstract The carbon monoxide- and the oxygen-covered surfaces of [110]-oriented Pt tips and of [100] and [111]-oriented Rh tips were studied in the lithium field desorption microscope (Li FDM). The catalytic CO-oxidation reaction on Rh and Pt surfaces was studied in situ using Li + and O + 2 field ion modes of imaging. The influence of the modification of electron density near the surface, caused by the presence of Li, on the CO-oxidation reaction on Pt and Rh has been observed. The promoting role of the enhanced electron density in the catalytic oxidation reaction is discussed on the basis of present experimental data and known alkali-gas coadsorption properties.

Appearance energy spectroscopy of CO+ field ions emitted from W(111)

Field ion appearance energy spectroscopy of CO + field ions emitted from a few surface sites of the W(111) plane was performed. Using a newly developed retarding potential technique, absolute values for the CO + appearance energies were derived from mass-to-charge resolved retarding potential analyses of field ions of the carbon monoxide supplied as an imaging gas in the field ion microscope. From analysis of the CO + field ion appearance energies, field ionization of CO on the W(111) surface was found to proceed by field desorption. Applying a thermionic cycle, field-dependent values of the binding energy for CO on W(111) at high coverages were obtained. A comparison of the results for CO/W(111) with data for the field-free binding energy of CO on rough W planes, measured independently, is made. The mechanism of field ion imaging of metal surfaces with CO + ions and the field dependent chemisorption of CO on different metal surfaces is discussed on the basis of experimental results and theoretical calculations.

New type of metal ion source: Surface diffusion Li+ ion source

A surface diffusion metal ion source, a new type of metal ion source, is explored. In this device a field desorption process is used to achieve an almost monoenergetic continuous flux of Li+ ions from a [111]‐oriented W field emitter. Earlier difficulties with the continuous supply of adatoms, required to produce measurable desorption rates, were overcome by making use of solid state surface diffusion from the Li multilayer reservoir at the shank of the field emitter. The high density of the ion beam (an ion current of 10−12 A was achieved from a W trimer), the extremely narrow energy distribution (full width at half‐maximum of 0.25 eV) and the stable geometric form of the emitter itself during the operation are advantages of the new ion source which may be important in different areas of nanotechnology.