A two-dimensional molecular sieve has been realized. It consists of a host matrix of molecularly engineered building blocks self-assembled at the liquid-solid interface. The simultaneous size- and shape-dependent dynamics of different guest molecules is observed in situ, in real time with submolecular resolution using a scanning tunneling microscope both at the liquid-solid interface and under vacuum. The temperature-dependent dynamics reveals that the diffusion proceeds through thermally activated channeling between single-molecule surface cavities.

Plasmonic hotspots in single gold nanostars are located at the tips and can be excited selectively by laser light as evidenced by photoelectron emission microscopy. Selectivity is achieved through wavelength and polarization of the excitation light. Comparing photoelectron emission intensity and dark-field scattering spectra of the same individual nanostars reveals differences in terms of observable plasmon resonance wavelengths and field enhancements. Differences are explained with the underlying near- and far-field processes of the two techniques.

Short range surface plasmon resonators are investigated at the nanometer scale. Gold nanorods (30 nm in diameter) were microfabricated and probed by photoemission electron microscopy under direct laser light excitation. Resonances presenting various numbers of lobes occur for specific rod lengths. A simple analytical model shows that the successive resonant lengths differ by a multiple of one-half of the wavelength of the supported short-range surface plasmon polariton.

We report the observation by scanning tunneling microscopy of the emergence of chirality through the self-assembly into monolayers of nonchiral discotic liquid crystals, hexakis-2,3,6,7,10,11alkyloxytriphenylene, on the nonchiral surface of highly oriented pyrolitic graphite. A chiral ordering transition appears when increasing the triangular aspect ratio of the molecules, through the tuning of alkoxy-side-chain length. The mechanism of this symmetry breaking, which involves steric hindrance, superlattice formation, and conformational mobility, can be understood in the light of a simple frustrated triangular Ising net. [S0031-9007(98)05401-5] PACS numbers: 61.30.Gd, 83.70.Jr Microscopic order in molecular assemblies is known to play a central role in a lot of physical processes. Numerous properties of molecular materials are conditioned by the preservation of some symmetry requirements in their molecular arrangement. Besides its importance in life sciences, chiral asymmetry has considerable consequences on physical properties like optical rotation or optical evenharmonic generation. It is usually achieved by selecting molecules that are themselves chiral, but it can also occur through the chiral ordering of nonchiral molecules. Recently, such chiral phases composed of nonchiral rod-shaped molecules have been observed in Langmuir monolayers [1].New trends for molecularly designed materials and nanostructures raise the problem of assembling molecules into the required well ordered structure [2]. The selfassembly technique takes advantage of weak intermolecular interactions during thin-film growth in order to achieve a spontaneous organization [2,3]. This requires an accurate knowledge of these forces and of how to control them, either at a molecular level or through external parameters such as substrate-surface interactions. Liquid crystals have provided us with unique models to understand such weak interactions, both as concerns intermolecular forces and molecule-surface forces. Discotic liquid crystals [4] are disk-shaped molecules made of a rigid core surrounded by equatorial flexible chains. They form columnar mesophases where molecular disks stack into columns. They have been observed to form also self-assembled monolayers at liquid-solid interface [5,6].Scanning tunneling microscopy (STM) has become a major tool to study molecular structure, dynamic and electronic properties of such monolayers, formed on conducting substrates [7][8][9][10][11]. A unique advantage of this technique is the possibility to study local order without averaging over several domains.In this paper, we report the STM observation of the self-assembled monolayers formed by a series of discotic liquid-crystal molecules on a highly oriented pyrolitic graphite (HOPG) surface. We observe the emergence of a chiral order when increasing the triangular aspect ratio of the molecules, although neither the molecule nor the surface present any chirality. The mechanism of this symmetry breaking, which involves steric hindrance, superlattice form...