Interaction and diffusion of potassium onCr2O3(0001)/

Abstract

The interaction of potassium atoms on top of Cr 2 O 3 ͑0001͒/Cr͑110͒ has been studied using work-function ͑⌬⌽͒, temperature programmed desorption ͑TPD͒, and optical second-harmonic generation ͑SHG͒ measurements. Potassium grows via the completion of a first layer, followed by a second layer in the form of two-dimensional ͑2D͒ islands, and at higher coverage 3D clusters are formed. This growth model is supported by and consistent with the results obtained from all three methods. Work-function data suggest that annealing at temperatures above 350 K results in the formation of a surface potassium oxide compound, provided the potassium coverage is higher than 0.5 monolayers ͑ML͒. Diffusion of alkali-metal atoms on an oxide surface is reported here over distances of several micrometers. This was measured using optical SH diffraction from coverage gratings that were generated by laser-induced thermal desorption. The activation energy for surface diffusion of potassium on Cr 2 O 3 ͑0001͒/Cr͑110͒ has been determined to be 11Ϯ0.5 kcal/mol with a preexponential factor D 0 ϭ10 5 cm 2 /sec in the coverage range of 1.5-2.5 ML, dropping to 9 kcal/mol and D 0 ϭ3 ϫ10 3 cm 2 /sec at a coverage of 3.0 ML. These results are consistent with the diffusion of atoms in the third layer, on top of two-dimensional potassium islands in the second layer, the activation energy represent the barrier for descending from the 2D islands.

The influence of potassium on the growth of ultra-thin films of para-hexaphenyl on muscovite mica(001)

“…The various base pressures were adjusted by intentionally leaking air into the baked UHV chamber. Only in the 10 −9 mbar pressure range and below, pure potassium desorption (from the metallic multilayer) can be observed, which is expected to take place at about 350 K. [24][25][26] However, at and above 2 × 10 −8 mbar no potassium desorbs in the temperature range where desorption of pure potassium is expected. There exist only small desorption peaks (note the logarithmic y-scale) at higher temperature, which are most probably due to cracking products of more stable potassium containing compounds.…”

“…But there is general agreement in the literature that potassium tends to form islands on non-metallic substrates. 26,32 With respect to the of potassium monolayer on mica one has to emphasize that the monolayer is so strongly bound that it cannot desorb (up to 1000 K), unlike for potassium desorption from metal surfaces, e.g., from nickel 24 or silver, 25 where monolayer desorption over a broad temperature range between 400 K and 1000 K is observed.…”

The Cooperative Molecular Field Effect

“…The experimental results indeed show that the electronic structure of adsorbed atoms differs radically from that of the corresponding isolated atoms. [13] If a layer of atoms is adsorbed, this can modify the work function of the surface, according to the dipole of the layer. The effective dipole of the layer may well differ from what can be extrapolated from isolated atom results, because of dipole±dipole interactions between the adsorbed atoms (how close can the dipoles approach each other, before dipole±dipole repulsion stops it?).…”

Adsorption of metal adatoms on FeO(111) and MgO(111) monolayers: Effects of charge state of adsorbate on rumpling of supported oxide film

“…23 The occurrence of positive charging has also been predicted for alkali metals on oxide thin films. 8,24,25 While the metal/oxide interface determines the electronic characteristics ͑e.g., the work function͒ of the support and, thus, its capacity to exchange electrons with the adsorbates, atomic relaxation of the oxide film may significantly contribute to stabilize charged adsorbates ͑polaroniclike effect͒. Due to their structural flexibility, this effect becomes particularly important on ultrathin films of insulating materials.…”