Direct chemisorption site selectivity for molecular halogens on the Si(111)-(7 x 7) surface

The Many "Facets" of Halide Ions in the Chemistry of Colloidal Inorganic Nanocrystals

Over the years, scientists have identified various synthetic "handles" while developing wet chemical protocols for achieving a high level of shape and compositional complexity in colloidal nanomaterials. Halide ions have emerged as one such handle which serve as important surface active species that regulate nanocrystal (NC) growth and concomitant physicochemical properties. Halide ions affect the NC growth kinetics through several means, including selective binding on crystal facets, complexation with the precursors, and oxidative etching. On the other hand, their presence on the surfaces of semiconducting NCs stimulates interesting changes in the intrinsic electronic structure and interparticle communication in the NC solids eventually assembled from them. Then again, halide ions also induce optoelectronic tunability in NCs where they form part of the core, through sheer composition variation. In this review, we describe these roles of halide ions in the growth of nanostructures and the physical changes introduced by them and thereafter demonstrate the commonality of these effects across different classes of nanomaterials.

Dissipation of the excess energy of the adsorbate-thermalization via electron transfer

A new scenario for the thermalization process of adsorbates at solid surfaces is proposed. The scenario is based on the existence of an electric dipole layer in which the electron wavefunctions extend over the positive ions, creating a strong local electric field which drags the electrons into the solid interior and repels the positive ions. During adsorption the electrons tunnel into the solid interior, conveying the excess energy. The positive ions are retarded by the field, losing the excess kinetic energy, and are located smoothly into the adsorption sites. In such a scheme, the excess energy is not dissipated locally, avoiding melting or the creation of defects which is in accordance with experiments. The scenario is supported by ab initio calculation results, including density function theory of the slabs representing the AlN surface and the Schrodinger equation for the time evolution of hydrogen-like atoms at the solid surface.

Characterization of initial halogen adsorption on Si(111) surface by scanning tunnelling microscopy: correlation with optical measurements

Initial adsorption processes of halogen atoms on a Si(111)-(7 × 7) surface were studied by means of scanning tunnelling microscopy (STM). The adsorption sites of halogen atoms were clarified directly with STM, and the results were compared with the partial coverage at each site, estimated previously from surface differential reflectance and thermal desorption spectroscopic analyses. The microscopic geometry of the atomic structure showed a good correspondence with the optical measurements, especially in terms of the density of the reacted sites. Bromine atoms were predominantly adsorbed near already adsorbed bromine, while chlorine atoms were almost randomly adsorbed. Polybromide formation occurred at coverage levels above 0.1 ML. Bromine atoms break the back-bonds of Si adatoms at lower levels of coverage than do chlorine atoms. The reason for the difference in adsorption behaviour between chlorine and bromine is discussed.

Two-electron dissociation of single molecules by atomic manipulation at room temperature

Using the tip of a scanning tunnelling microscope (STM) to mechanically manipulate individual atoms and molecules on a surface is now a well established procedure. Similarly, selective vibrational excitation of adsorbed molecules with an STM tip to induce motion or dissociation has been widely demonstrated. Such experiments are usually performed on weakly bound atoms that need to be stabilized by operating at cryogenic temperatures. Analogous experiments at room temperature are more difficult, because they require relatively strongly bound species that are not perturbed by random thermal fluctuations. But manipulation can still be achieved through electronic excitation of the atom or molecule by the electron current tunnelling between STM tip and surface at relatively high bias voltages, typically 1-5 V. Here we use this approach to selectively dissociate chlorine atoms from individual oriented chlorobenzene molecules adsorbed on a Si(111)-7 x 7 surface. We map out the final destination of the chlorine daughter atoms, finding that their radial and angular distributions depend on the tunnelling current and hence excitation rate. In our system, one tunnelling electron has nominally sufficient energy to induce dissociation, yet the process requires two electrons. We explain these observations by a two-electron mechanism that couples vibrational excitation and dissociative electron attachment steps.

Decoration of surfaces with size-selected clusters and molecular manipulation at room temperature: precision and uncertainty in organizing atoms

The deposition onto surfaces of clusters of atoms, prepared and size-selected in the gas phase, is, like atomic or molecular manipulation with the scanning tunnelling microscope, an appealing (but parallel) route to the creation of nanoscale surface features. Both of these seemingly orthogonal approaches allow, in principle, a selected number of atoms to be organized, and both are strongly affected by the lateral thermal diffusion of the constituent atoms, molecules or clusters over the surface. In this sense, the room-temperature (as opposed to cryogenic-temperature) regime can be regarded as a hostile environment for organizing atoms. In this paper we review recent achievements in size-selected cluster deposition and molecular manipulation at room temperature and thus address the fundamental question: with what precision can we organize atoms at room temperature?