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Powering and controlling the operation of a single molecule adsorbed on a surface can be achieved by using the tip of a scanning tunnelling microscope (STM) as an atomic-size source of electrons. We review the various electronic excitation processes induced by the electrons from the STM tip which are able to activate the functions of a molecular nanomachine. In particular, we review recent results illustrating the electronic control of molecular dynamics at the level of a single molecule.

Ion photostimulated desorption (PSD) is a specific surface sensitive process. This paper reviews ion PSD following core level excitation of molecules adsorbed on the Si(111) 7 × 7 and Si(100) 2 × 1 surfaces. Several aspects of ion PSD will be discussed; (i) the use of ion PSD as a tool for investigating the adsorption configurations of O2 on Si(111) 7 × 7, (ii) the relevance of ion PSD for probing the physisorption–chemisorption transition of benzene molecules on Si(111), (iii) the comparison between ion PSD of methanol adsorbed on Si(111) and photofragmentation of the methanol molecule in the gas phase, (iv) the comparison between ion PSD of formic acid adsorbed on the Si(111) and Si(100) surfaces and (v) the chemical selectivity of ion PSD following core level excitation of NO dissociatively adsorbed on Si(111). Finally, the interplay between ion dynamics and electronic relaxation following core level excitation and Auger processes will be discussed in the case of O+ photodesorption from O2 adsorbed on Si(111) 7 × 7, as investigated via isotope and temperature effects on both the intensities and kinetic energies of the desorbed ions.

In this work we present scanning tunnelling microscopy (STM) imaging and spectroscopy of a highly p-doped wide bandgap semiconducting 4H-SiC(0001) surface. Whereas n- and p-doped 6H-SiC or n-doped 4H-SiC surfaces can be relatively easily imaged with the STM, the p-doped 4H-SiC cannot be imaged due to the absence of any surface conductivity. This is very surprising given the presence of a p-doped, degenerate epitaxial layer. The behaviour can be explained by the formation of a Schottky barrier either between the tip and the surface or between the surface and the sample holder, depending on the polarity of the applied voltage. We found that prolonged and repeated exposures of the SiC surface to a Si atomic flux followed by thermal annealing are required before the surface conductivity is sufficient to allow STM images to be recorded. The result is the deposition of overlayers of Si, with structures similar to Si(111) 7 × 7, Si(113) 3 × 2, and Si(110) 16 × 2 rather than the expected stable SiC(0001) 3 × 3 reconstruction. We have further demonstrated the ability of scanning tunnelling spectroscopy to distinguish between the Si and the SiC phases based on the difference in their bandgaps.

International audience; The fabrication of thin films of colloidal semiconductor nanocrystals is attracting much attention due to their exceptional optoelectronic properties. This requires the development of new methods for depositing nanocrystals under well-controlled conditions. Here, we report the use of the pulse injection method to deposit CdSe nanocrystals under ultrahigh vacuum (UHV) on clean and well-ordered surfaces. The deposition of nanocrystals has been tested by x-ray photoelectron spectroscopy (XPS) and near edge x-ray absorption fine structure spectroscopy. Special attention has been paid to the preparation of very pure solutions of CdSe nanocrystals using cadmium stearate, trioctylphosphine oxide (TOPO) and the TOP/Se adduct for the nanocrystals synthesis followed by dissolution in pentane. It has been found that CdSe nanocrystals adsorb with similar sticking coefficients on graphite, hydrogenated silicon (100) and hydrogenated diamond (100) surfaces. Furthermore, the XPS analysis has revealed that the surface of theCdSe nanocrystal isCd rich,which has important consequences for the optical and chemical properties. This ability to deposit semiconductor nanocrystals under UHV conditions on clean and well-ordered surfaces opens up new perspectives for studying in a reliable manner all their chemical, electronic and optical properties.

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 photofragmentation study of O2 adsorbed on Si(111)7 × 7 shows that two markedly different regimes can be selected by varying the sample temperature and the O2 coverage. At room temperature the photofragmentation is essentially dominated by direct processes whereas at low temperature (30 K) the photofragmentation evolves from indirect to direct processes when increasing the O2 coverage from 0.04 to 0.2 L. These effects are tentatively assigned to the temperature and coverage dependence of the adsorption sites of O2 on Si(111)7 × 7.

Quasibound electronic states of the CO2+ ion are investigated by means of photoion-photon-of-fluorescence coincidence (PIFCO) and ion kinetic energy spectroscopy (IKES) methods. Energies of the lowest four electronic states (3 Pi , 1 Sigma +), 3 Pi , 3 Sigma + and 1 Pi are accurately measured at 41.25+or-0.05, 41.3+or-0.1, 41.8+or-0.1 and 43.8+or-0.1 eV respectively. The latter two states dissociate in less than 1 mu s, while the 3 Pi and 1 Sigma 1 states can survive for a few mu s. Furthermore, the lack of detected fluorescence is discussed and an upper bound is derived for the fluorescence efficiency of CO2+.

The deposition of one monolayer of silicon on a Ag(lll) substrate induces the formation of silicene structures exhibiting different ordered phases, including a ()R30°, a (4×4) and a ()R13.9° superstructures. In tms paper we focus on the ()R30° phase. Using a combination of scanning tunneling microscopy and LEED observations, we show that this phase corresponds to a 11° rotation of the silicene sheet relative to the substrate orientation.

International audience; In this paper, the scattering of surface plasmon polaritons (SPPs) into photons at holes is investigated. A local, electrically excited source of SPPs using a scanning tunneling microscope (STM) produces an outgoing circular plasmon wave on a thick (200 nm) gold film on glass containing holes of 250, 500 and 1000 nm diameter. Fourier plane images of the photons from holescattered plasmons show that the larger the hole diameter, the more directional the scattered radiation. These results are confirmed by a model where the hole is considered as a distribution of horizontal dipoles whose relative amplitudes, directions, and phases depend linearly on the local SPP electric field. An SPPYoung's experiment is also performed, where the STMexcited SPP wave is incident on a pair of 1 m diameter holes in the thick gold film. The visibility of the resulting fringes in the Fourier plane is analyzed to show that the polarization of the electric field is maintained when SPPs scatter into photons. From this SPPYoung's experiment, an upper bound of 200 nm for the radius of this STMexcited source of surface plasmon polaritons is determined.

Inelastic electron tunnelling excitation of propagating surface plasmon polaritons (SPPs) on a thin gold film is demonstrated. This is done by combining a scanning tunnelling microscope (STM) with an inverted optical microscope. Analysis of the leakage radiation in both the image and Fourier planes unambiguously shows that the majority (up to 99.5%) of the detected photons originate from propagating SPPs with propagation lengths of the order of 10 µm. The remaining photon emission is localized under the STM tip and is attributed to a tip-gold film coupled plasmon resonance as evidenced by the bimodal spectral distribution and enhanced emission intensity observed using a silver STM tip for excitation.

We have used an atomic force microscope (AFM) to manipulate and study ligand-capped cadmium selenide nanorods deposited on highly oriented pyrolitic graphite (HOPG). The AFM tip was used to manipulate (i.e., translate and rotate) the nanorods by applying a force perpendicular to the nanorod axis. The manipulation result was shown to depend on the point of impact of the AFM tip with the nanorod and whether the nanorod had been manipulated previously. Forces applied parallel to the nanorod axis, however, did not give rise to manipulation. These results are interpreted by considering the atomic-scale interactions of the HOPG substrate with the organic ligands surrounding the nanorods. The vertical deflection of the cantilever was recorded during manipulation and was combined with a model in order to estimate the value of the horizontal force between the tip and nanorod during manipulation. This horizontal force is estimated to be on the order of a few tens of nN.

The ratio of double-to-single photoionisation cross sections for silver has been determined in the 28-80 eV energy range along with the Ag+ and Ag2+ ionic yield curves. In the 29-62 eV photon energy range direct double photoionisation corresponds to removing either a 4d and 5s electron together or two 4d electrons. The direct double photoionisation processes have a relatively high cross section since the ratio Ag2+/Ag+ is about 25% at 60 eV. Two sharp increases of Ag2+/Ag+ at 61.5 and 69 eV are considered to be due to Auger processes related to the opening of the 4p 2P3/2,1/2 ionisation channels.