Self-assembled structures of amino acids and unnatural amino acid on metal surfaces

Abstract

Two-dimensional Interlocking Chiral Honeycomb Structures of an Unnatural Amino Acid in a Single Domain

We have studied the two-dimensional self-assembled structures of a functionalized unnatural amino acid, $(S)-\beta-methyl$ $naphthalen-1- \gamma$-aminobutyric$ acid $(\gamma^2 -1-naphthylalanine)$, to investigate the formation of self-assembled nanostructures at 150 K by using scanning tunneling microscopy (STM), density functional theory (DFT) calculations, and high resolution photoemission spectroscopy (HRPES). Interestingly, STM analysis at a low coverage revealed interlocking chiral honeycomb structures that consist of both counter-clockwise and clockwise configurations in a single domain, which suggests that the intermolecular interac-tions between $\gamma^2$ -1-naphthylalanine molecules play a crucial role in the formation of those self-assembled structures in spite of the low coverage. As the amount of adsorbed $\gamma^2$ -1-naphthylalanine was increased, a well-ordered square closed packing structure was observed along with another structural domain in which higher molecules are trapped in the pores of the hexagonal molecular assembly. On the other hand, naphthalene on a Au(111) surface only forms a well-ordered hexagonal closed packing structure at high coverages. HRPES analysis indicates that there is no strong covalent bonding between $\gamma^2$ -1-naphthylalanine and the Au(111) surface, supporting that $\gamma^2$ -1-naphthylalanines have strong tendency to form the self-assembled structures mainly through intermolecular interactions. Thus, we conclude that modifying molecules to facilitate intermolecular interactions can potentiate the formation of well-ordered self-assembly nanostructures.

Comparison of the Coverage-dependent Adsorption Selectivity of Leucine and Tyrosine adsorbed on the Cu(110) Surface

The coverage-dependent adsorption selectivity of leucine and tyrosine molecules adsorbed on the Cu(110) surface was investigated and compared to obtain the bonding configurations and stable adsorption structures at two distinct coverages (low and high). These features were determined using density functional theory (DFT) calculations, high-resolution photoemission spectroscopy (HRPES), and reflection-absorption infrared spectroscopy (RAIRS). The DFT calculations indicates that an “O-H dissociation bonded structure” is the most stable structure of leucine on the Cu(110) surface, and an “O-H dissociated and N-dative bonded structure” is the most stable structure of tyrosine on the Cu(110) surface. In addition, according to HRPES and RAIRS results of at low coverage level (~0.20 ML), leucine shows “O-H dissociation bonded structure” on the Cu(110) surface whereas tyrosine shows “O-H dissociated and N-dative bonded structure” on the Cu(110) surface. In contrast, at higher coverage (~0.70 ML), we found the opposite results that the leucine on the Cu(110) surface shows “O-H dissociated and N-dative bonded structure” whereas tyrosine on the Cu(110) surface shows “O-H dissociation bonded structure”. These results were consistent with the results of the DFT calculations, which indicate that adsorption geometries yield the lowest adsorption energies at each coverage.

Nitrogen Doping via $NH_3$ Treatment as a Strategy for Increasing $Pd-TiO_2$ Surface Defect Sites Related to Catalytic Activity

This study examined the catalytic activities of three distinct nitrogen doped $Pd@TiO_2$ ($N-Pd@TiO_2$) nanoparticles, post-annealed $(at 700, 800, and 900^\circ C)$ after fabrication on silicon substrates. The catalytic activities were analysed by measuring the oxidation of benzenethiol, thioacetic acid, and thiobenzoic acid by using X-ray diffraction (XRD) patterns, scanning electron microscopy (SEM), scanning transmission x-ray microscopy (STXM), high-resolution photoemission spectroscopy (HRPES), and scanning photoelectron mi-croscopy (SPEM). Systematically, the study by HRPES and SPEM indicated that nitrogen was predominantly doped around PdO nanostructure. Particularly, we prove that photocatalytic activity in effective nitrogen doped surface area is far higher than in undoped area of $Pd@TiO_2$ nanoparticles. Besides to the band gap narrowing, nitrogen doping leads to generate $Ti^{3+}$ species or oxygen vacancies site of the surface, which leads to enhance the activity of surface photocatalysis.