ABSTRACT
Gold nanoparticle-porphyrin assemblies were formed by binding functionalized porphyrins to gold nanoparticles (Au-NPs). Spectroscopic properties of hybrids and binding strength of porphyrins to Au-NPs were observed based on number and type of linker moieties using fluorescence spectroscopy. Binding appears to be dependent on number rather than type of linker moieties present on the porphyrin molecules, as tetraaminophenyl porphyrin shows the highest binding among the molecules we studied and causes agglomeration of nanoparticles due to presence of four linker groups. The inner filter effects of Au-NPs are considerably high due to their high extinction coefficient and cause large errors in the evaluation of quenching efficiencies. We have described a very simple method to calculate the inner filter effects of Au-NPs by first loading them with porphyrins and then replacing them with nonfluorescent ligands. The difference in the fluorescence of unbound porphyrins in the presence and absence of Au-NPs describes their inner filter effects.
ABSTRACT
Metal/organic interfaces critically determine the characteristics of molecular electronic devices, because they influence the arrangement of the orbital levels that participate in charge transport. Studies on self-assembled monolayers show molecule-dependent energy-level shifts as well as transport-gap renormalization, two effects that suggest that electric-field polarization in the metal substrate induced by the formation of image charges plays a key role in the alignment of the molecular energy levels with respect to the metal's Fermi energy. Here, we provide direct experimental evidence for an electrode-induced gap renormalization in single-molecule junctions. We study charge transport through single porphyrin-type molecules using electrically gateable break junctions. In this set-up, the position of the occupied and unoccupied molecular energy levels can be followed in situ under simultaneous mechanical control. When increasing the electrode separation by just a few ångströms, we observe a substantial increase in the transport gap and level shifts as high as several hundreds of meV. Analysis of this large and tunable gap renormalization based on atomic charges obtained from density functional theory confirms and clarifies the dominant role of image-charge effects in single-molecule junctions.
ABSTRACT
We have investigated charge transport in ZnTPPdT-Pyr (TPPdT: 5,15-di(p-thiolphenyl)-10,20-di(p-tolyl)porphyrin) molecular junctions using the lithographic mechanically controllable break-junction (MCBJ) technique at room temperature and cryogenic temperature (6 K). We combined low-bias statistical measurements with spectroscopy of the molecular levels in the form of I(V) characteristics. This combination allows us to characterize the transport in a molecular junction in detail. This complex molecule can form different junction configurations, having an observable effect on the trace histograms and the current-voltage (I(V)) measurements. Both methods show that multiple, stable single-molecule junction configurations can be obtained by modulating the interelectrode distance. In addition we demonstrate that different ZnTPPdT-Pyr junction configurations can lead to completely different spectroscopic features with the same conductance values. We show that statistical low-bias conductance measurements should be interpreted with care, and that the combination with I(V) spectroscopy represents an essential tool for a more detailed characterization of the charge transport in a single molecule.
ABSTRACT
We present the formation of single-molecule devices based on nanometre-spaced platinum electrodes. The electrodes are fabricated using a self-breaking electromigration method which yields nanogaps with long-term stability at room temperature [Prins et al., APL, 2009, 94, 123108.]. The stability at room temperature allows for detailed comparison of the device electrical properties before and after deposition of the molecules. In this way, conductance as a result of direct tunneling between the electrodes can be distinguished from conductance through the molecule. After molecule deposition, some devices display transport in the strong coupling regime while others are in the weak-coupling Coulomb blockade regime. Gated transport is observed in the latter case.
