27 research outputs found
The order-disorder character of the (3x3) to (sqrt3 x sqrt3)R30° phase transition of Sn on Ge(111)
Growing attention has been drawn in the past years to the \alpha-phase
(1/3 monolayer) of Sn on Ge(111), which undergoes a transition from the low
temperature (3x3) phase to the room temperature (\sqrt3 x \sqrt3)R30° one.
On the basis of scanning tunnelling microscopy experiments, this transition was
claimed to be the manifestation of a surface charge density wave (SCDW), i.e. a
periodic redistribution of charge, possibly accompanied by a periodic lattice
distortion, which determines a change of the surface symmetry.
Recent He diffraction studies of the (3x3) long range order have shown the
transition to be of the order-disorder type with a critical temperature Tc=220
K and belonging to the 3-state Potts' universality class. These findings
clearly exclude an SCDW driven mechanism at 220 K, but they cannot exclude the
occurence of a displacive transition at higher temperature. Here we present
photoelectron diffraction data taken at 300 K and photoemission data taken up
to 500 K (which is the maximum temperature where the (\sqrt3 x \sqrt3)R30°
is stable) . From our analysis it is shown that the atomic structure of the Sn
overlayer does not change throughout the transition up to 500 K. As a
consequence the displacive hypothesis must be discarded in favour of a genuine
order-disorder model.Comment: replaced Fig.
Adsorption of Cs on InAs(111) surfaces
Caesiated InAs(111)B (1 x 1) and InAs(111)A (2 x 2) surfaces have been studied by photoelectron spectroscopy. On the InAs(111)B a new (root 3 x root 3)R30 degrees reconstruction was observed. During Cs evaporation remarkably small changes are observed in the lone pair states, and no sign of an accumulation layer at the surface can be observed. Instead, the additional charge provided by Cs is rapidly transported towards the bulk. On the InAs(111)A cesium behaves as a typical electropositive alkali metal donator that enhances the already existing accumulation layer. (c) 2005 Elsevier B.V. All rights reserved
Study of spatial homogeneity and nitridation of an Al nanopattern template with spectroscopic photoemission and low energy electron microscopy
We report a study on the spatial homogeneity and nitridation of a nanopattern template using a spectroscopic photoemission and low energy electron microscopy. The template was composed of Al nanodots which were patterned into a SiO2/Si(1 1 1) surface using e-beam lithography and reactive ion etching. The template exhibited a global inhomogeneity in terms of the local topography, Al composition and structure of the individual nanopatterns. After nitridation, the individual nanopatterns were diminished, more corrugated and faceted. The nitridated nanopatterns were structurally ordered but differently orientated. The nitridation effectively removed the fluorine contaminants by decomposition of the fluorocarbon sidewalls, resulting in the AlN nanopatterns and partially nitridated Si substrate surface outside the nanopattern domains. (C) 2012 Elsevier B. V. All rights reserved
Photoemission and low energy electron microscopy study on the formation and nitridation of indium droplets on Si (111)7 x 7 surfaces
The formation and nitridation of indium(In) droplets on Si (1 1 1)7 x 7, with regard to In droplet epitaxy growth of InN nanostructures, were studied using a spectroscopic photoemission and low energy electron microscopy, for the In coverages from 0.07 to 2.3 monolayer (ML). The results reveal that the In adatoms formed well-ordered clusters while keeping the Si (1 1 1)7 x 7 surface periodicity at 0.07 ML and a single root 3 x root 3 phase at 0.3 ML around 440-470 degrees C. At 0.82 ML, owing to the presence of structurally defect areas beside the 7 x 7 domains, 3-D In droplets evolved concomitantly with the formation of 4 x 1-In cluster chains, accompanied by a transition in surface electric property from semiconducting to metallic. Further increasing the In to 2.3 ML led to a moderate increase in number density and an appreciable lateral growth of the droplets, as well as the multi-domain In phases. Upon nitridation with NH3 at similar to 480 degrees C, besides the nitridation of the In droplets, the N radicals also dissociated the In - Si bonds to form Si - N. This caused a partial disintegration of the ordered In phase and removal of the In adatoms between the In droplets
Changing adsorption mode of FePc on TiO2(110) by surface modification with bipyridine
Surface modification of reactive oxide substrates to obtain a less strongly interacting template for dye adsorption may be a way to enhance performance in dye-sensitized solar cells. In this work, we have investigated the electronic and structural properties of 4,4(')-bipyridine (bipy) as modifier adsorbed on the TiO2(110) surface. The modified surface is then coated with iron phthalocyanine (FePc) and the properties of this heterostructure are investigated with synchrotron based photoelectron spectroscopy, x-ray absorption spectroscopy, and scanning tunneling microscopy. We find that a saturated monolayer consisting of standing bipy molecules with one nitrogen atom pointing outward is formed on the oxide surface. FePc adsorb in molecular chains along the [001] direction on top of bipy and ordered in a tilted arrangement with adjacent molecules partially overlapping. Already from the first layer, the electronic properties of FePc resemble those of multilayer films. FePc alone is oxidized on the TiO2(110) surface, but preadsorbed bipy prevents this reaction. The energy level lineup at the interface is clarified. (C) 2008 American Institute of Physics
In-situ evaluation of dye adsorption on TiO2 using QCM
We measured the adsorption characteristics of two organic dyes; triphenylamine-cyanoacrylic acid (TPA-C) and phenoxazine (MP13), on TiO2, directly in a solution based on quartz crystal microbalance (QCM). Monitoring the adsorbed amount as a function of dye concentration and during rinsing allows determination of the equilibrium constant and distinction between chemisorbed and physisorbed dye. The measured equilibrium constants are 0.8 mM(-1) for TPA-C and 2.4 mM(-1) for MP13. X-ray photoelectron spectroscopy was used to compare dried chemisorbed layers of TPA-C prepared in solution with TPA-C layers prepared via vacuum sublimation; the two preparation methods render similar spectra except a small contribution of water residues (OH) on the solution prepared samples. Quantitative Nanomechanical Mapping Atomic Force Microscopy (QNM-AFM) shows that physisorbed TPA-C layers are easily removed by scanning the tip across the surface. Although not obvious in height images, adhesion images clearly demonstrate removal of the dye
Creation of a metallic channel at the Sn/InAs(111)B surface studied using synchrotron-radiation photoelectron spectroscopy
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