Yan Pennec

Zwitterionic self-assembly of l-methionine nanogratings on the Ag(111) surface

The engineering of complex architectures from functional molecules on surfaces provides new pathways to control matter at the nanoscale. In this article, we present a combined study addressing the self-assembly of the amino acid l-methionine on Ag(111). Scanning tunneling microscopy data reveal spontaneous ordering in extended molecular chains oriented along high-symmetry substrate directions. At intermediate coverages, regular biomolecular gratings evolve whose periodicity can be tuned at the nanometer scale by varying the methionine surface concentration. Their characteristics and stability were confirmed by helium atomic scattering. X-ray photoemission spectroscopy and high-resolution scanning tunneling microscopy data reveal that the l-methionine chaining is mediated by zwitterionic coupling, accounting for both lateral links and molecular dimerization. This methionine molecular recognition scheme is reminiscent of sheet structures in amino acid crystals and was corroborated by molecular mechanics calculations. Our findings suggest that zwitterionic assembly of amino acids represents a general construction motif to achieve biomolecular nanoarchitectures on surfaces.

Supramolecular Gratings for Tuneable Confinement of Electrons on Metal Surfaces

The engineering of electron wave functions in reduced dimensions has allowed researchers to explore and visualize fundamental aspects of quantum mechanics1,2 and has also led to new ideas for advanced materials and devices3,4. The scanning tunnelling microscope, in particular, has been used to create two-dimensional structures such as quantum corrals by moving individual atoms on metal surfaces and then probing the quasi two-dimensional surface state electron gases confined therein5,6,7,8,9,10. However, this serial approach is time-consuming and not suited to producing ensembles of nanostructures for the control of electrons. Here we introduce a novel bottom-up method for the fabrication of nanoscale confinement structures on the Ag(111) surface. Scanning tunnelling spectroscopy data show that self-assembled molecular gratings act as one-dimensional resonators, and allow us to tune the characteristics of quantum-well states. We also demonstrate zero-dimensional confinement in quantum corrals down to 2xa0×xa05xa0nm in size by positioning single Fe atoms, which act as additional electron reflectors, in the molecular gratings.

Dimerization Boosts One-Dimensional Mobility of Conformationally Adapted Porphyrins on a Hexagonal Surface Atomic Lattice

We employed temperature-controlled fast-scanning tunneling microscopy to monitor the diffusion of tetrapyridylporphyrin molecules on the Cu(111) surface. The data reveal unidirectional thermal migration of conformationally adapted monomers in the 300-360 K temperature range. Surprisingly equally oriented molecules spontaneously form dimers that feature a drastically increased one-dimensional diffusivity. The analysis of the bonding and mobility characteristics indicates that this boost is driven by a collective transport mechanism of a metallosupramolecular complex.

Molecular nanoscience and engineering on surfaces

Molecular engineering of low-dimensional materials exploiting controlled self-assembly and positioning of individual atoms or molecules at surfaces opens up new pathways to control matter at the nanoscale. Our research thus focuses on the study of functional molecules and supramolecular architectures on metal substrates. As principal experimental tools we employ low-temperature scanning tunnelling microscopy and spectroscopy. Here we review recent studies in our lab at UBC: controlled manipulation of single CO molecules, self-assembled biomolecular nanogratings on Ag(111) and their use for electron confinement, as well as the organisation, conformation, metalation and electronic structure of adsorbed porphyrins.

Cleaving-temperature dependence of layered-oxide surfaces.

The surfaces generated by cleaving nonpolar, two-dimensional oxides are often considered to be perfect or ideal. However, single particle spectroscopies on Sr2RuO4, an archetypal nonpolar two-dimensional oxide, show significant cleavage temperature dependence. We demonstrate that this is not a consequence of the intrinsic characteristics of the surface: lattice parameters and symmetries, step heights, atom positions, or density of states. Instead, we find a marked increase in the density of defects at the mesoscopic scale with increased cleave temperature. The potential generality of these defects to oxide surfaces may have broad consequences to interfacial control and the interpretation of surface sensitive measurements.