We have studied the adsorption of the polyaromatic molecule 1,4"-paratriphenyldimethylacetone, which we have nicknamed Trima. The originality of this linear molecule is that it was designed and synthesized to have two functionalities. First, chemisorb itself to the surface by its two ends rather like a bridge. Second, the central part of the molecule could then be rotated by injecting electrons with the tip of the scanning tunneling microscope (STM). The length of the molecule corresponds exactly to the spacing between five dimers in a row on the Si(100)-2 x 1 surface. We found that the molecule adsorbs as expected on the clean silicon surface by using complementary STM and synchrotron radiation studies. Manipulation of individual molecules with the STM tip showed selective internal modifications that were highly voltage dependent. These manipulations were found to be compatible with an electronic excitation of the pi-pi* transition of the molecule.

The topographic and spectroscopic capabilities of the scanning tunnelling microscope (STM) have been used to explore the conductivity of hydrogenated diamond C(100)-(2×1) surfaces. It has been shown that the surface conductivity is determined by the interplay between various factors: the adsorption of atmospheric species on the surface, the doping concentration of the sample, the presence of sub-surface species and the presence of the top layer of hydrogen.

The electronic or quantum control of individual molecules with the scanning tunnelling microscope offers exciting perspectives on operating molecular nanomachines. This implies the use of semiconductor surfaces rather than metallic surfaces which would rapidly quench the electronic excitations. We review recent results illustrating the state of the art and the main problems which need to be solved: the choice, design and properties of functionalized organic molecules on semiconductor surfaces; the control of the inelastic electronic channels through a single molecule; and the search for well-controlled atomic-scale wide-band-gap semiconductor surfaces.

We have shown that the room-temperature adsorption of the biphenyl molecule on the Si(100) surface gives rise in majority to a bistable molecular configuration. The switching of the bistable molecule is activated at room temperature by thermal activation. By using a combination of room-temperature and low-temperature (30 K) scanning tunneling microscope (STM) topography, room-temperature STM manipulation, and near edge x-ray absorption fine structure spectroscopy, the nature of the bistable molecule, its adsorption geometry, and its interaction with the surface could be identified.

Molecular oxygen adsorption on the Si(100) 2x1 surface has been studied at low and room temperatures by combining ultraviolet photoemission spectroscopy and photon stimulated desorption (PSD) of ions near the Si (2p) excitation edge. PSD studies as a function of oxygen exposure and photon energy have revealed an oxygen precursor on the Si(100) 2x1 surface. This precursor has been observed only at low temperature (50 K) and very low oxygen exposures (<0.06 L). It consists in a molecular or atomic species bonded to a Si surface atom in a top position.

Molecular oxygen has been found to be easily adsorbed on the partially hydrogenated diamond C(100)-(2×1):H surfaces, whereas the clean and fully hydrogenated C(100) surfaces are completely inert to molecular oxygen. The partially hydrogenated diamond C(100)-(2×1) surfaces have been prepared by (i) in situ hydrogen photodesorption from the fully hydrogenated surface and (ii) in situ hydrogen adsorption on the clean surface. The surface reactivity has been monitored through the changes of the valence band photoemission spectra upon molecular oxygen exposure. These results suggest that oxygen adsorption occurs on the isolated carbon dangling bonds produced, on partially hydrogenated surfaces, from the breaking of the π-bonding of paired dangling bonds.

The scanning tunnelling microscope (STM) has been used to study the adsorption at room (300 K) and low (30 K) temperature of oxygen on the Si(1 1 1)-7×7 surface. Subsequently, STM manipulation has been used to find out which of the observed sites can be modified (displacement, transformation, desorption). It was found that the adsorption is quite different at 30 K compared to 300 K and that the manipulation at room temperature is essential in the identification of the sites. These results are in general agreement with recent conclusions from theoretical work [Phys. Rev. Lett. 84 (2000) 1724] and synchrotron radiation experiments [Phys. Rev. B 65 (2002) 035315].

Ion photodesorption is a powerful tool to investigate (i) how molecules adsorb on surfaces, (ii) the electronic relaxation of the excited states of the adsorbed species and (iii) their intramolecular dynamics. The temperature dependence of all these processes is of crucial interest, especially on semiconductor surfaces, to understand the role of phonons, charge carrier densities, thermal expansion and thermal activation. We have studied at 300 and 30 K the ion photodesorption processes following the Si(2p) core level excitation from O2 on Si(1 1 1)7×7. Detailed information have been obtained by analyzing the O+ ion kinetic energy distributions at both temperatures. The results suggest that, at low temperature, the lifetimes of the excited electronic states producing the ion desorption are significantly increased due to the reduced number of charge carriers. This offers interesting perspectives to control the dynamics of molecular systems on surfaces.

A photon-stimulated desorption (PSD) process consists of the production of ions and neutrals upon irradiation with light of proper wavelength. Here, we present a short review on the PSD mechanisms and the parameters influencing it, such as the adsorbate–substrate coupling, the light polarization, the surface orientation, the temperature, the mass of the leaving fragments as well as the co-adsorption effects. Furthermore, as an example, we will present a PSD investigation of formic acid on Si(1 1 1)7×7 in the energy ranges including the C 1s and the O 1s thresholds. D+, C+, CD+, O+, CO+, DCO+ ions are produced in both energy ranges. The excitation in the C 1s energy range offers a less efficient substrate-mediated quenching with respect to the excitation in the O 1s one, with a consequent more structured photoexcitation spectrum. In the energy range including the O 1s the transitions to π*(CO) and σ*(C–D) levels produce selectively either D+ or CDO+. O+ is produced both by a transition to a π*(CO) level on through a shake-off process, and the remaining fragments are generally produced through secondary electrons.

The surface electronic structure of the hydrogenated diamond C(100)-(2 x 1):H surface was studied using scanning tunneling microscopy (STM), valence band and core-level photoemission, and near-edge x-ray-absorption fine structure. The hydrogenated diamond surface was prepared ex situ by hydrogen plasma treatment. The STM topographies of the hydrogenated diamond surface, recorded with an atomic resolution, indicated the atomically smooth diamond surface. The current-voltage (I-V) spectroscopy was used to study the transport of electrons injected from the STM tip into the diamond surface. It has been shown that the electron transport is determined not only by the surface states available but it depends crucially on the amount of subsurface hydrogen and the charge redistribution at the surface. Annealing of the diamond surface induces desorption of the subsurface hydrogen. That affects the electron transport as evidenced from the analysis of the I-V spectroscopy curves. A qualitative model for the electron transport, taking into account the specific electronic structure of the hydrogenated diamond surface, has been proposed.

Selective ion photodesorption from NO adsorbed on Si(111)7×7 has been investigated by core electron excitation. In a first step, the adsorption of nitric oxide on the clean Si(111)7×7 surface has been studied over the temperature range from 15 K to room temperature using ultraviolet photoemission spectroscopy, x-ray photoemission spectroscopy, and photon stimulated desorption of ions. At room temperature, NO was found to react with the surface silicon adatoms to form silicon oxynitride sites (SiNxOy). At low temperature and low coverage, in addition to a permanent dissociative adsorption, a transient molecular adsorption has been observed. At low temperature and high coverage, NO was found to be condensed. In a second step, ion photodesorption induced by core electron excitation of both the adsorbate and the substrate has been studied. Quite interestingly, selective ion photodesorption was observed by core electron excitation of both dissociatively adsorbed NO at room temperature and condensed NO molecules at low temperature. An “ultrafast dissociation” is proposed to explain the results of condensed NO molecules at low temperature.

We investigated the use of ion photodesorption as a tool to monitor the transition from the physisorbed to the chemisorbed state on a surface. The adsorption of benzene on Si(111) 7 × 7 in the temperature range 40–300 K is chosen as a prototype. The D+ ion photodesorption yield was monitored as a function of temperature at various benzene exposures. Comparative measurements of the C 1s photoelectron yield in the same temperature range enable the physisorbed to chemisorbed state transition to be distinguished from that of the multilayer to the chemisorbed state. We find the onset at 110 K in the first case, and at 130–140 K in the second case. These results demonstrate that ion photodesorption is a potentially interesting method to identify physisorption to chemisorption transitions of adsorbed molecules on surfaces.

The toluene adsorption on Si(111)7×7 was studied by valence band synchrotron-radiation photoemission at room temperature as a function of exposure and time. We found that the adsorption process occurs preferentially on rest atoms and that toluene adsorption is time dependent, since the adsorption features disappear and the rest atoms related feature re-appears as a function of time. The features time evolution is quadratic, hinting at a thermal desorption process. The high desorption rate allows us to rule out a dissociative adsorption involving the methyl group. Our conclusion is that toluene is molecularly π-bonded through the aromatic electrons on silicon rest atoms.

Resonant core level ionization by photon irradiation is chemically selective. When applied to produce chemical reactions such as bond breaking on surfaces, this selectivity can be used to pattern atomic-scale structures. This is illustrated in the case of the hydrogenated GaAs (110) surface terminated by both As and Ga atoms. It has been possible to desorb selectively hydrogen atoms from As or Ga atomic sites by tuning the Synchrotron Radiation photon energy to the As3d and Ga3p core level energy region respectively. The efficiency of the resonant core level surface bond breaking has been studied in the case Of O-2 on Si (111) 7 x 7. Isotope and temperature effects have shown that the surface bond breaking efficiency is greatly enhanced for low mass desorbing species and low temperature.

New perspectives in molecular electronics are opening up through controlled surface molecular synthesis. The first step of such a synthesis implies the adsorption of a polyfunctional molecule, which might use one functional group for the surface adsorption, and the other one(s) for further reactions. Here we present an adsorption study of Si(111)7×7 of geranyl-acetone (C13H22O, E-5,9 undecadien-one) characterized by a ketone and two unconjugated double bonds. The study has been performed by temperature and coverage dependent valence band photoemission and room temperature scanning tunneling microscopy. The use of these combined techniques allows us to infer that the interaction between the geranyl acetone and the silicon surface occurs selectively through the rest atom and the carbonyl group, most likely through the oxygen atom. The geranyl acetone does not undergo any fragmentation upon adsorption Si(111)7×7, as has been observed for smaller molecules on the same surface {acetaldehyde [Y. Bu, J. Breslin, M. C. Lin, J. Phys. Chem. B 101, 1872 (1997)] for instance}. The interaction of the chain with the surface is weak and is characterized in the STM images as a darkening of one adatom in positive bias, around the reacted rest atom.

Molecular oxygen adsorption on a Si(111) 7 X 7 surface is investigated by ion photodesorption and valence-band photoemission from very low O-2 exposures up to the saturation of the adatom dangling bonds. It was found that, at 30 K, the O-2 molecule is adsorbed dissociatively on a silicon adatom, with one oxygen atom on top of the silicon atom and one oxygen atom inserted into the silicon backbond. This single adsorption configuration is not stable at 300 K. At 300 K, the O-2 molecule dissociates on a silicon adatom, with two, oxygen atoms inserted into the two silicon backbonds. Double-ad sorption configurations, resulting from the successive adsorption of two O-2 molecules on the same silicon adatom, have been identified as SiO4 tetrahedral-type configurations coexisting at 300 K with double adsorption configurations having four subsurface O atoms. Finally, the diffusion of O-2 molecules on the surface at 30 K, and their subsequent reaction on single adsorption configurations, are proposed to explain the observed time dependence of the adsorption configuration populations.

The electronic structure of partially hydrogenated Si(100)-(2 X 1) surfaces, prepared by controlled thermal annealing and nonthermal photon stimulated desorption of fully hydrogenated Si(100) surfaces, has been investigated by using valence band photoemission. Thermal and nonthermal desorption are found to produce very specific electronic surface structures. This led us to the discovery of two specific surface states having binding energies of 1.0 and 0.7 eV associated with the isolated Si dimers and single Si dangling bonds, respectively.

For the first time hydrogen induced surface states were experimentally observed on the hydrogenated diamond C(1 0 0)-(2×1):H surface. The photoemission spectra of the clean, ex situ and in situ hydrogenated diamond (1 0 0) surface were recorded at grazing incident angles in order to improve the surface sensitivity. The careful analysis of the photoemission spectra, recorded at different photon energies, indicates the presence of the hydrogen induced states in the valence band. The photoemission spectra, measured at different incident angles, suggest the surface character of the hydrogen induced states. These surface states, located at 8.8, 11.3–11.5 and 16.9 eV, displayed a slight dispersion with the photon energy and were interpreted as C–H surface states of the C(1 0 0)-(2×1):H surface.

Surface electronic states of the partially hydrogenated diamond C(L00)-(2x1):H surface were studied by near-edge x-ray absorption fine structure and C 1s core level photoemission. Partially hydrogenated surfaces were prepared by synchrotron irradiation of the monohydride-terminated surface or by hydrogen adsorption on the clean surface. A new surface core-exciton state produced at a photon energy of 282.5 eV has been assigned to single dangling bonds of the partially hydrogenated surface. Monitoring this new feature has been found to be a powerful method to study hydrogen kinetics during (i) photon irradiation of a fully hydrogenated diamond surface, (ii) adsorption of atomic hydrogen on a clean diamond surface, and (iii) photon irradiation of a fully hydrogenated surface followed by thermal annealing. From the analysis of dangling-bond distribution, it follows that no preferential pairing of hydrogen on the C-C dimers occurs during hydrogen adsorption at room temperature. In contrast, thermal annealing induces pairing of the single dangling bonds into the pi -bonded configuration, the pairing process being accompanied by hydrogen desorption. This observation suggests that the activation barrier of hydrogen thermal diffusion is only slightly lower than that of thermal desorption.

Nanocrystalline carbon films possessing a prevailing diamond character are deposited by a direct current glow discharge chemical vapor deposition method using a 9:91 vol % methane to hydrogen gas mixture. In the present work the evolution and properties of nanodiamond films deposited by this method onto silicon substrates as a function of time were studied by various complementary techniques. Our analysis showed that prior to formation and growth of continuous films of a predominantly nanodiamond character, a graphitic phase is formed. After the nanodiamond phase is stabilized, near edge x-ray adsorption fine structure measurements proved the predominant diamond character of the film to be about 80%. By electron energy loss spectroscopy analysis the sp2-like character of the nanodiamond grain boundaries has been determined. The nanodiamond films were found to be thermally stable up to temperatures of ∼950 °C as established by vacuum heating. By scanning electron microscopy and atomic force microscopy the morphology of the films was examined showing that the formation of the nanodiamond phase is initially accompanied with an increase in surface roughness which decreases with film growth. By high-resolution transmission electron microscopy it was determined that the continous nanodiamond films are composed of diamond nanocrystallites, 3–5 nm in diameter.