Ph. Dumas

Morphology of hydrogen-terminated Si(111) and Si(100) surfaces upon etching in HF and buffered-HF solutions

Abstract High resolution electron energy loss spectroscopy (EELS) and infrared absorption spectroscopy (IRAS) are used to characterize Si(111) and Si(100) surfaces after chemical etching in solutions of HF and buffered HF. Electron energy loss spectra confirm that the HF-etched silicon surfaces are completely terminated with hydrogen, and display essentially no loss features attributable to atomic or molecular impurities. IRAS and specular beam EELS measurements show that Si(111) surfaces become flatter as the pH of the etching solutions increases from 1 (concentrated HF) to 7.8 (ammonium fluoride solution). For high pH, the (111) surfaces are atomically flat and ideally H-terminated. In contrast, the Si(100) surfaces remain rough for all pH. The surface morphology, however, depends critically on the pH and varies most for pH between 5 and 7. The EELS spectra are dominated by impact scattering and the loss features show no measurable dispersion except for the atomically smooth, ideally H-terminated Si(111) surface.

Adsorption and reactivity of NO on Cu(111): a synchrotron infrared reflection absorption spectroscopic study

Infrared reflection absorption spectroscopy of NO adsorbed on Cu(111) has been performed in the frequency range 200–2500 cm−1. At low temperatures (T < 120 K), two different states were identified with increasing coverage: adsorption followed by dissociation. At coverages up to one monolayer, NO molecules adsorb on identical sites, which are suggested to be of threefold symmetry, with an upright geometry. Two ordered overlayers are formed in turn: p(3 × 3) and (√7 × √7)R19.1°. The infrared spectra show two absorption bands: the internal stretching mode of NO, which shifts upwards in frequency with increasing coverage, and a low frequency anti-absorption band. The anti-absorption band is assigned, based on its isotopic frequency dependence, to the hindered rotation of the NO molecules. After completion of the monolayer, the NO molecules react and adsorbed N2O molecules are found on the surface bound through the oxygen atom (CuON2 stretching mode at 352 cm−1) with their molecular axis (ONN) parallel to the surface plane. Desorption of the adsorbed N2O occurs at 120 K. No dimers, which are the reaction intermediates in the formation of N2O from NO, have been clearly identified.

Influence of silicon oxide on the morphology of HF-etched Si(111) surfaces: Thermal versus chemical oxide

Infrared reflection‐absorption measurements of the Si‐H stretching vibrations of HF‐etched Si(111) surfaces show that the structure of the H‐passivated surfaces depends strongly on the nature of the initial silicon‐oxide layer. For similar etching conditions, thermal oxides lead to much flatter surfaces than chemical oxides. A new processing sequence involving the removal of thermal oxide by buffered HF (pH=5), followed by etching in a 40% ammonium‐fluoride solution, produces a remarkably homogeneous H/Si(111)‐(1×1) surface, characterized by a 0.05 cm−1 broad Si‐H stretch‐mode.

Vibrational dynamics of the ideally H-terminated Si(111) surface

Picosecond sum-frequency generation measurements of the Si-H stretching vibration of an unreconstructed, ideally H-terminated Si(111) surface show that its lifetime is 0.8 ± 0.1 ns. High resolution infrared reflection-absorption measurements reveal a marked temperature dependence of the linewidth. frequency and intensity of the SiH stretching vibration. The width and frequency variations are completely accounted for by a weak coupling of this mode to a Si surface phonon centered at 210 ± 25 cm1. The loss in intensity, observed as the temperature is increased above 300 K, gives evidence for a strong coupling between the SiH stretching and bending modes.

Direct observation of individual nanometer-sized light-emitting structures on porous silicon surfaces

We present the first observations of nanometer resolution mapping of visible-light emission from 85% porous silicon. Using an STM tip as a local source of electrons to excite cathodoluminescence, simultaneous topographic and light intensity profiles are reported. The photon maps reveal contrast in emission intensity on a scale relevant to the proposed dimensions of the luminescence process (~ 4 nm). We suggest that excitation of individual quantum confinement structures has been observed. The technique overcomes the wide disparity in length scales of the photons ((600 ÷ 900) nm) and the characteristic size of the origins of the luminescence ((1 ÷ 5) nm) which have precluded previous experimental correlations of structural and luminescent behaviour.

Light-induced molecular motion of azobenzene-containing molecules: a random-walk model

Since it was first evidenced in 1995, light-induced mass motion in layers of azobenzene-containing molecules has led to diverging interpretations, and it remains partly unexplained. In this paper, we discuss a light-driven random-walk model where moving chromophores drag the molecule to which they are grafted. It consists in a diffusion motion of the azobenzene functions where each random step follows an isomerizing absorption. After a summary of the main characteristics of the motion, we present the hypotheses of the model and we show how it suits the experimental observations reported. In the frame of this model, where each azobenzene function is put in motion by light, we assess the distance over which an azobenzene-containing molecule can be dragged. We also estimate the energetic output of this dragging process. Finally, we discuss the microscopic origin of these molecular motors and we compare it to the model of thermal ratchets introduced by Feynman and extensively resorted to in Biology nowadays.

CO interaction with co-adsorbed C2H4 on Cu(111) as revealed by friction with the conduction electrons

Abstract Simultaneous measurements of the DC resistance and IR reflectivity changes of thin epitaxially grown copper (111) films have been performed during co-adsorption of CO with C 2 H 4 . The linear dependence of these two quantities, which has previously been observed for CO on Cu(111) films, also holds for this coadsorbed system. Importantly, we have shown that the linearity factor is independent of the adsorbate type, as predicted by the scattering model of the conduction electrons. Co-adsorption of CO with C 2 H 4 results in markedly more pronounced changes in IR reflectance and in resistance; these changes are higher than the sum of the individual changes of each species. The results are interpreted as a consequence of covalent through-metal bonding of the adsorbates via the lowest unoccupied molecular orbitals.

Effect of end-substitution of hexyl chains on the growth and electrical properties of quaterthiophene thin films

We present here a detailed study of the growth process of quaterthiophene (4T) based thin films, especially the influence of end-substitution of alkyl chains on the film formation. A radical change in the growth mechanism introduced by the side chains could be observed by atomic force microscopy (AFM) measurements. While the unsubstituted quaterthiophene shows a typical island growth, an almost perfect layer-to-layer growth was found for the α,ω-dihexylquaterthiophene (DH4T). The alteration in the growth mode leads to differences of one order in the grain size going from 4T to DH4T, which results finally in an enhancement of the mobility measured in the field-effect transistor (FET) of the same order. The results reveal a change of the growth process induced by molecular engineering leading to an improvement of the film morphology towards larger grain sizes and thus to an enhancement of the electronic properties of the organic semiconductor thin films.

Cooperative Segregation of Boron at Si(111)

Interface segregation of boron at Si(111) surfaces has been studied at the atomic scale using a scanning tunnelling microscope (STM). During the segregation process (produced by thermal annealing), strong cooperative effects take place which cannot be explained by a simple nearest-neighbour pair interaction model. Although average surface concentrations of boron in the investigated temperature range could be calculated in terms of a Fowlers model, the comparison between STM and simulated images suggests that one must introduce longer-range interactions to describe the atomic-scale configurations.

Characterization of the C60Ag(111) interface by sum-frequency generation

The C60Ag(111) interface is studied by infrared reflection absorption (IRAS) and infrared-visible sum-frequency generation (SFG) spectroscopies for infrared frequencies between 1300 and 1500 cm−1. Both IRAS and SFG spectroscopies reveal a vibrational mode located around 1445 cm−1. This mode is assigned to the Ag(2) “pentagonal pinch” mode of C60, which, although being Ramanactive and infrared-inactive in the free molecule, becomes infrared-active upon adsorption of the molecule.