G. Comtet

Quantum Interference Channeling at Graphene Edges

Electron scattering at graphene edges is expected to make a crucial contribution to the electron transport in graphene nanodevices by producing quantum interferences. Atomic-scale scanning tunneling microscopy (STM) topographies of different edge structures of monolayer graphene show that the localization of the electronic density of states along the C-C bonds, a property unique to monolayer graphene, results in quantum interference patterns along the graphene carbon bond network, whose shapes depend only on the edge structure and not on the electron energy.

Imaging and spectroscopy of individual CdSe nanocrystals on atomically resolved surfaces

Imaging and spectroscopy of individual CdSe nanocrystals have been performed with the scanning tunneling microscope (STM) on atomically resolved hydrogenated Si(100) surfaces. The CdSe nanocrystals have been deposited under vacuum onto the surface by using the pulse valve method. Two different types of CdSe nanocrystals, capped either with trioctylphosphine oxide ligands or with cadmium stearate ligands, have been studied to optimize their anchoring to the surface. The I(V) spectroscopy shows a characteristic resonant excitation spectrum through the unoccupied levels of the nanocrystals with no significant charging effect. This suggests that the nanocrystals are weakly coupled to the surface, thus requiring a stronger coupling with the STM tip to achieve a measurable tunnel current. These results demonstrate the importance of depositing nanocrystals on clean and atomically well-defined surfaces for reliable measurement of their properties.

SiC(0001) 3 × 3 Heterochirality Revealed by Single-Molecule STM Imaging

We report a description of the SiC(0001) 3 x 3 silicon carbide reconstruction based on single-molecule scanning tunneling microscopy (STM) observations and density functional theory calculations. We show that the SiC(0001) 3 x 3 reconstruction can be described as contiguous domains of right and left chirality distributed at the nanoscale, which breaks the to date supposed translational invariance of the surface. While this surface heterochirality remains invisible in STM topographies of clean surfaces, individual metal-free phthalocyanine molecules chemisorbed on the surface act as molecular lenses to reveal the surface chirality in the STM topographies. This original method exemplifies the ability of STM to probe atomic-scale structures in detail and provides a more complete vision of a frequently studied SiC reconstruction.

Atomic-scale STM experiments on semiconductor surfaces: towards molecular nanomachines.

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.

Initial stage of NO adsorption on Si(100)-(2×1) studied by synchrotron radiation photoemission and photodesorption

The NO adsorption on the Si(100)-(2 x 1) surface was investigated by synchrotron radiation photoemission and photodesorption in the energy ranges including the valence band and the Si 2p, N 1s and O 1s core levels. The study was performed both ass function of NO exposure and as a function of temperature in the range 20-300 K. The photoemission experiments show clear evidence of a dissociative adsorption process both at room temperature as well as at temperatures as low as 20 K. Furthermore, the silicon surface states are involved in the adsorption process. The core level spectroscopy shows a complex adsorption pattern of the atomic species, which might involve a sub-surface migration of nitrogen atoms. The photodesorption yields only O+ in the Si 2p and O Is energy ranges. No nitrogen ion desorption is detected. In the Si 2p energy range the O+ photodesorption pattern follows the enhanced secondary electron yield when crossing the ionization threshold. In the O Is energy range the O+ photodesorption pattern is interpreted in terms of a partial sub-surface migration of oxygen atoms