Tunneling spectroscopy of lander molecules on coinage metal surfaces

Self-Assembly of Molecular Landers Equipped with Functional Moieties on the Surface: A Mini Review

The bottom-up fabrication of supramolecular and self-assembly on various substrates has become an extremely relevant goal to achieve prospects in the development of nanodevices for electronic circuitry or sensors. One of the branches of this field is the self-assembly of functional molecular components driven through non-covalent interactions on the surfaces, such as van der Waals (vdW) interactions, hydrogen bonding (HB), electrostatic interactions, etc., allowing the controlled design of nanostructures that can satisfy the requirements of nanoengineering concepts. In this context, non-covalent interactions present opportunities that have been previously explored in several molecular systems adsorbed on surfaces, primarily due to their highly directional nature which facilitates the formation of well-ordered structures. Herein, we review a series of research works by combining STM (scanning tunneling microscopy) with theoretical calculations, to reveal the processes used in the area of self-assembly driven by molecule Landers equipped with functional groups on the metallic surfaces. Combining these processes is necessary for researchers to advance the self-assembly of supramolecular architectures driven by multiple non-covalent interactions on solid surfaces.

Variation of Kondo temperature induced by molecule-substrate decoupling in film formation of bis(phthalocyaninato)terbium(III) molecules on Au(111)

We demonstrate that the lattice formation of an adsorbed molecule decouples the molecule-substrate interaction to change the Kondo resonance, which occurs due to interactions between conduction electrons and the molecule's unpaired spin. The double-decker bis(phthalocyaninato)terbium(III) complex, which is single-molecule magnet and forms a Kondo resonance on a Au(111) surface through an unpaired π-radical spin, is studied using scanning tunneling microscopy/spectroscopy (STM/STS). In the STS spectra, an unusual sharp, strong peak (peak A) is found only for the molecule in a film. The peak position of peak A (εA) cyclically shifts by several hundred millivolts as the STS tip position shifts along the outer circle of the molecule, reflecting the tilting of the upper phthalocyanine (Pc) ligand from the flat-lying lower Pc ligand. The Kondo resonance, which is detected as a sharp peak at the Fermi level, also shows cyclic variations of the peak width and intensity. As εA approaches EF, the Kondo temperature (TK) increases. We propose a model that peak A originates from the singly occupied molecular orbital state whose energy is shifted by an unscreened final state effect due to a decrease in the molecule-substrate chemisorptive interaction. We further examine this model using density functional theory calculations, confirming a decreased molecule-substrate interaction for molecules in the film compared to that of isolated molecules. Further calculations of a tilted upper Pc ligand configuration show a site-dependent, cyclic variation of the molecule-substrate interaction within a molecule.

The frontier orbitals of a push-pull azobenzene adsorbed on a metal surface in different bonding geometries investigated by scanning tunneling spectroscopy and spectroscopy mapping

The electronic structure of 4-anilino-4'-nitroazobenzene superstructures formed on Au(111) at 250 K is investigated by low temperature scanning tunneling microscopy, scanning tunneling spectroscopy, and dI/dV mapping at 5 K. Changes in the dI/dV maps of this push-pull molecule reflect the spatial distribution of the frontier orbitals on the molecular scale. Spectra of the trans- and the cis∗-isomer differ between themselves and in different parts of supramolecular assemblies. The relative importance of these differences is discussed.

Adsorption configuration effects on the surface diffusion of large organic molecules: the case of Violet Lander

Violet Lander (C(108)H(104)) is a large organic molecule that when deposited on Cu(110) surface exhibits lock-and-key like behavior [Otero et al., Nature Mater. 3, 779 (2004)]. In this work, we report a detailed fully atomistic molecular mechanics and molecular dynamics study of this phenomenon. Our results show that it has its physical basis on the interplay of the molecular hydrogens and the Cu(110) atomic spacing, which is a direct consequence of the matching between molecule and surface dimensions. This information could be used to find new molecules capable of displaying lock-and-key behavior with new potential applications in nanotechnology.

Supramolecular architectures on surfaces formed through hydrogen bonding optimized in three dimensions

Supramolecular self-assembly on surfaces, guided by hydrogen bonding interactions, has been widely studied, most often involving planar compounds confined directly onto surfaces in a planar two-dimensional (2-D) geometry and equipped with structurally rigid chemical functionalities to direct the self-assembly. In contrast, so-called molecular Landers are a class of compounds that exhibit a pronounced three-dimensional (3-D) structure once adsorbed on surfaces, arising from a molecular backboard equipped with bulky groups which act as spacer legs. Here we demonstrate the first examples of extended, hydrogen-bonded surface architectures formed from molecular Landers. Using high-resolution scanning tunnelling microscopy (STM) under well controlled ultrahigh vacuum conditions we characterize both one-dimensional (1-D) chains as well as five distinct long-range ordered 2-D supramolecular networks formed on a Au(111) surface from a specially designed Lander molecule equipped with dual diamino-triazine (DAT) functional moieties, enabling complementary NH...N hydrogen bonding. Most interestingly, comparison of experimental results to STM image calculations and molecular mechanics structural modeling demonstrates that the observed molecular Lander-DAT structures can be rationalized through characteristic intermolecular hydrogen bonding coupling motifs which would not have been possible in purely planar 2-D surface assembly because they involve pronounced 3-D optimization of the bonding configurations. The described 1-D and 2-D patterns of Lander-DAT molecules may potentially be used as extended molecular molds for the nucleation and growth of complex metallic nanostructures.

Large molecules on surfaces: deposition and intramolecular STM manipulation by directional forces

Intramolecular manipulation of single molecules on a surface with a scanning tunnelling microscope enables the controlled modification of their structure and, consequently, their physical and chemical properties. This review presents examples of intramolecular manipulation experiments with rather large molecules, driven by directional, i.e. chemical or electrostatic, forces between tip and molecule. It is shown how various regimes of forces can be explored and characterized with one and the same manipulation of a single molecule by changing the tip-surface distance. Furthermore, different deposition techniques under ultrahigh vacuum conditions are discussed because the increasing functionality of such molecules can lead to fragmentation during the heating step, making their clean deposition difficult.