C. Didiot

Nanopatterning the electronic properties of gold surfaces with self-organized superlattices of metallic nanostructures

The self-organized growth of nanostructures on surfaces could offer many advantages in the development of new catalysts, electronic devices and magnetic data-storage media. The local density of electronic states on the surface at the relevant energy scale strongly influences chemical reactivity, as does the shape of the nanoparticles. The electronic properties of surfaces also influence the growth and decay of nanostructures such as dimers, chains and superlattices of atoms or noble metal islands. Controlling these properties on length scales shorter than the diffusion lengths of the electrons and spins (some tens of nanometres for metals) is a major goal in electronics and spintronics. However, to date, there have been few studies of the electronic properties of self-organized nanostructures. Here we report the self-organized growth of macroscopic superlattices of Ag or Cu nanostructures on Au vicinal surfaces, and demonstrate that the electronic properties of these systems depend on the balance between the confinement and the perturbation of the surface states caused by the steps and the nanostructures superlattice. We also show that the local density of states can be modified in a controlled way by adjusting simple parameters such as the type of metal deposited and the degree of coverage.

Observation of a Mott Insulating Ground State for Sn/Ge(111) at Low Temperature

We report an investigation on the properties of 0.33 ML of Sn on Ge(111) at temperatures down to 5 K. Low-energy electron diffraction and scanning tunneling microscopy show that the (3x3) phase formed at approximately 200 K, reverts to a new ((square root 3)x(square root 3))R30 degrees phase below 30 K. The vertical distortion characteristic of the (3x3) phase is lost across the phase transition, which is fully reversible. Angle-resolved photoemission experiments show that, concomitantly with the structural phase transition, a metal-insulator phase transition takes place. The ((square root 3)x(square root 3))R30 degrees ground state is interpreted as the formation of a Mott insulator for a narrow half-filled band in a two-dimensional triangular lattice.

ARPES and STS investigation of Shockley states in thin metallic films and periodic nanostructures

Due to their extreme surface sensitivity, the Shockley states of (111) noble metal surfaces can be used to study the modifications of atomic and electronic properties of epitaxial ultra thin films and self-organized nanostructures. In metallic interfaces, the different parameters of the Shockley surface state bands (energy, effective mass and eventually spin?orbit splitting) have been shown to be strongly thickness dependent. It was also possible by scanning tunneling spectroscopy to evidence a spectroscopic signature of buried interfaces. Moreover, superperiodic surface structures like the reconstruction on Au(111) vicinal surfaces or self-organized nanodots, lead to spectacular spectroscopic effects. In the vicinal Au(23?23?21) surface, the opening of tiny energy gaps associated with the reconstruction potential of such surfaces has been evidenced. Peculiar growth on these Au vicinal surfaces allows us to obtain high quality self-assembled metallic nanostructures which exhibit homogeneous electronic properties on a large spatial scale resulting from a coherent scattering of the Shockley states.

Symmetry breaking and gap opening in two-dimensional hexagonal lattices

The inhibition in wave propagation at band gap energies plays a central role in many areas of technology such as electronics (electron gaps), nanophotonics (light gaps) and phononics (acoustic gaps), among others. Here we demonstrate that metal surfaces featuring free-electron- like bands may become semiconducting by periodic nanostructuration. We combine scanning tunneling spectroscopy and angle-resolved photoemisssion to accurately determine the energy-dependent local density of states and band structure of the Ag/Cu(111) noble metal interface patterned with an array of triangular dislocations, demonstrating the existence of a 25meV band gap that extends over the entire surface Brillouin zone. We prove that this gap is a general consequence of symmetry reduction in close-packed metallic overlayers; in particular, we show that the gap opening is due to the symmetry lowering of the wave vector group at the K point from C3v to C3.