Francesca Matino

Spin-Crossover Complex on Au(111): Structural and Electronic Differences Between Mono- and Multilayers

Submono-, mono- and multilayers of the Fe(II) spin-crossover (SCO) complex [Fe(bpz)2 (phen)] (bpz=dihydrobis(pyrazolyl)borate, phen=1,10-phenanthroline) have beenprepared by vacuum deposition on Au(111) substrates and investigated with near edge X-ray absorption fine structure (NEXAFS) spectroscopy and scanning tunneling microscopy (STM). As evidenced by NEXAFS, molecules of the second layer exhibit a thermal spin crossover transition, although with a more gradual characteristics than in the bulk. For mono- and submonolayers of [Fe(bpz)2 (phen)] deposited on Au(111) substrates at room temperature both NEXAFS and STM indicate a dissociation of [Fe(bpz)2 (phen)] on Au(111) into four-coordinate complexes, [Fe(bpz)2 ], and phen molecules. Keeping the gold substrate at elevated temperatures ordered monolayers of intact molecules of [Fe(bpz)2 (phen)] are formed which can be spin-switched by electron-induced excited spin-state trapping (ELIESST).

Electronic decoupling of a cyclophane from a metal surface

Electronic self-decoupling of an organic chromophore from a metal substrate is achieved using a naphtalenediimide cyclophane to spatially separate one chromophore unit of the cyclophane from the substrate. Observations of vibronic excitations in scanning tunneling spectra demonstrate the success of this approach. These excitations contribute a significant part of the tunneling current and give rise to clear structure in scanning tunneling microscope images. We suggest that this approach may be extended to implement molecular functions at metal surfaces.

Rectification in Supramolecular Zinc Porphyrin/Fulleropyrrolidine Dyads Self‐Organized on Gold(111)

Self-assembled donor/acceptor dyads are of current interest as they are biomimetic to the natural photosynthetic conversion system. Herein, we present an ultrahigh-vacuum scanning tunneling microscopy and scanning tunneling spectroscopy (UHV-STM/STS) study of ex situ self-assembled supramolecular dyads consisting of fulleropyrrolidines (PyC(2)C(60)) axially ligated to zinc(II) tetraphenylporphyrin (ZnTPP), self organized on a 4-aminothiophenol (4-ATP) self-assembled monolayer on gold(111). These dyads show both bias-polarity-dependent apparent height in STM images and highly rectifying behavior in tunneling spectroscopy. First-principles density functional theory calculations clarify the conformational and electronic properties of the 4-ATP/ZnTPP/PyC(2)C(60) system. Interestingly, we find easier tunneling for electrons moving from the acceptor side of the dyads to the donor side, in the inverse-rectifying sense with respect to previously reported molecular rectifiers. Such behavior cannot be explained as an elastic resonant tunneling process, but it can by using a model based on the Aviram-Ratner mechanism.

Zinc Porphyrin‐Driven Assembly of Gold Nanofingers

Nanofingers of gold covered by porphyrins are prepared by a combination of atomic manipulation and surface self-organization. A submonolayer of zinc(II) 5,10,15,20-tetrakis(4-tert-butylphenyl)-porphyrin (ZnTBPP) axially ligated to a self-assembled monolayer of 4-aminothiophenol (4-ATP) on Au(111) is prepared and studied using a combination of ultrahigh vacuum techniques. Under the electric field produced by the STM tip, the relatively weakly bound Au surface atoms along the discommensuration lines become mobile due to the strong bond to 4-ATP, while the tendency of the porphyrins towards self-assembly result in a collective motion of gold clusters. The clusters diffuse onto the surface following well-defined pathways along the [112] direction and then reach the step edges where they assembled, thus forming nanofingers. First-principles density functional theory calculations demonstrate the reduction of the binding energies between the surface gold clusters and the substrate induced by adsorption of thiols. Scanning tunneling microscopy images show assemblies across three adjacent discommensuration lines of the Au(111)-(22 x square root 3) reconstruction, which collectively diffuse along these lines to form islands nucleated at step edges.

X-ray excited visible luminescence spectroscopy of organic materials using a portable optical spectrometer.

The use of a portable video telescope, mounted externally to a beamline endstation, to obtain synchrotron-radiation-excited visible luminescence, is described. Real-time video monitoring permits simple and quick alignment, and allows a visual record of the luminescence experiment. The telescope is fibre-optic-coupled to an optical spectrometer. Examples are given of X-ray excited optical spectroscopy from organic materials for light-emitting-diode applications.

Rectifying behaviour of self assembled porphyrin/fullerene dyads on Au(111)

Here we present an Ultra High Vacuum Scanning Tunnelling Microscopy (UHVSTM) and Scanning Tunnelling Spectroscopy (STS) study of self assembled donor-acceptor conjugate dyads, consisting of fulleropyrrolidines and metallo-porphyrins immobilized on gold. The coverage in the fulleropyrrolidine layers was optimized up to obtain isolated protrusions which we identify with isolated dyads since their lateral dimensions are consistent with the fullerene size. The STS study reveals a diode-like asymmetric behaviour of the dyads, different from the surrounding areas. We investigate also the influence of the tunneling conditions on the rectifying ratio which is found to be dependent on the initial set point conditions and to increase by increasing the tip-sample distance.