Effect of the Molecule–Metal Interface on the Surface-Enhanced Raman Scattering of 1,4-Benzenedithiol

The influence of the number of molecule–metal interactions on the surface-enhanced Raman scattering (SERS) spectroscopy of 1,4-benzenedithiol (BDT) was investigated. For this purpose, a series of SERS-active samples were prepared featuring one or two molecule–metal interfaces. Molecules were adsorbed on the surface of a rough Au substrate, or sandwiched between Au nanoparticles (NPs) and a flat Au(111) substrate in a “sphere–plane” disposition. In the presence of the Au surface(s), vibrational energy and intensity of the SERS spectra differs significantly from the bulk. Molecule–metal charge transfer upon chemisorption weakens intramolecular bonds, resulting in the observed red shift of the breathing and CC stretching modes. This effect was found to be more pronounced for samples with multiple molecule–metal interfaces. In addition, the SERS spectra of BDT featured additional <i>b</i>₂ signals not present in the bulk spectra. Chemical enhancement of the <i>b</i>₂ modes takes place by means of photoinduced charge transfer from an occupied molecular orbital to an unoccupied metal orbital. Analysis of the normalized SERS intensity revealed a larger scattering enhancement for the samples with a sphere–plane disposition arising from the stronger electromagnetic enhancement effect via plasmonic localization of optical fields. Complementary studies on 4-aminobenzenethiol support these findings

Effect of Mechanical Strain on Electric Conductance of Molecular Junctions

Electromechanical properties of single molecular junctions are investigated using scanning tunneling microscopy based break junction method. Two types of molecular junctions consisting of π-conjugated backbones with and without coordinative bonding (i.e., Co­((4-aniline)-terpyridine)₂ complex and oligo­(phenylene-ethynylene) derivative) are prepared between two Au electrodes. Electronic transport measurements revealed molecular conductance of ca. 10^–4 <i>G</i>₀ (<i>G</i>₀ = 2<i>e</i>²/<i>h</i>) for both of the molecular junctions. Then we assessed the electronic transport properties of the two types of molecular junctions under mechanical strain in their compression–elongation cycle. We found significant asymmetric electromechanical response for all covalent systems of the oligo­(phenylene-ethynylene) derivative, while the Co complex with the coordinative bonding exhibits symmetric modulation of the electronic transport property in the compression–elongation cycle of the molecular junctions. The asymmetric and symmetric electromechanical behavior can be, respectively, ascribed to rigid covalent bonding in the π-conjugated backbone and flexible coordinative bonding at the metal center. This study demonstrates potential tunability of the molecular conductance under mechanical stimulus

Acetylene Used as a New Linker for Molecular Junctions in Phenylene–Ethynylene Oligomer Langmuir–Blodgett Films

Langmuir and Langmuir–Blodgett films have been fabricated from an acetylene-terminated phenylene–ethynylene oligomer, namely 4-((4-((4-ethynylphenyl)­ethynyl)­phenyl)­ethynyl)­benzoic acid (HOPEA). Characterization of the Langmuir film by surface pressure vs area per molecule isotherms and Brewster angle microscopy reveals the formation of a high quality monolayer at the air–water interface. One layer Langmuir–Blodgett (LB) films were readily fabricated by the transfer of HOPEA Langmuir films onto solid substrates by the withdrawal of the substrate. The deposition mode was Z-type. Quartz crystal microbalance (QCM) experiments confirm the formation of directionally oriented, monolayer LB films, in which the HOPEA molecules are linked to the gold substrate by attachment through the acid group. The morphology of these films was analyzed by atomic force microscopy (AFM), which revealed an optimum transference surface pressure of 18 mN m^–1 for the formation of homogeneous films. Cyclic voltammetry also showed a significant blockage of gold electrodes covered by HOPEA monolayers. Electrical properties of HOPEA monolayers sandwiched between a bottom gold electrode and a gold STM (scanning tunneling microscope) tip have been recorded, revealing that the acetylene group is an efficient linker for electron transport. In addition, the STM experiments indicate a nonresonant tunneling mechanism of charge transport through these metal–molecule–metal junctions

Die Beanspruchung des Bergmanns beim Herrichten und Ausrauben eines Strebes Schlussbericht

TIB Hannover: FR 3073+a / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman