Session H24: Molecular Electronics and Quantum Dots

Thermopower and Electrical Conductance Measurements of Single Molecule Junctions

The thermopower and electrical conductance of metal-molecule-metal junctions is studied by trapping single molecules between two gold electrodes with either a temperature differential (thermopower) or voltage differential (electrical conductance) applied across the electrodes. The voltage differential generated due to a temperature differential across a single molecule of Benzenedithiol, Dibenzenedithiol and Tribenzenedithiol trapped between Au electrodes is measured. The sign of the measured thermopower is used to show unambiguously that electrical conduction in these single molecule junctions is p-type (hole). The electrical current in a metal-molecule-metal junction due to a voltage differential of $\sim $100 mV is measured. The effect of molecular structure on electrical conductance is studied by 1) systematically varying the length of aliphatic molecules and aromatic molecules 2) changing the end groups binding to the electrodes 3) by adding substituents to the molecules. It is seen that the electrical resistance of aliphatic and aromatic molecules increases exponentially with length, while there was little effect of end groups and substituents for the molecules that we studied. Further, aromatic molecules are found to be much less resistive than aliphatic molecules of similar length.   

Quantum Dots Tailored with Conjugated Polymer

Placing conjugated polymers (CPs) in direct contact with a quantum dot (QD) (i.e., preparing QD-CP nanocomposites) carries advantage over cases where QD aggregation dominates. Such QD-CP nanocomposites possess a well-defined interface that significantly promotes the charge or energy transfer between these two components. However, very few studies have centered on such direct integration and QD-CP nanocomposites confined in nanoscopic geometries have never been explored. Here we demonstrate an approach to graft vinyl functionalized poly(3-hexylthiophene) (P3HT) onto aryl-bromide functionalized CdSe QD surfaces. The photophysical properties of nanocomposites in nanoscopic confined geometries are studied.   

Plasmonically Enhanced Second-Harmonic Generation from Metallic/Organic Hybrid Self-Assembled Films

We have fabricated a new class of second order nonlinear optical materials by combining ionic self-assembled multilayer (ISAM) films with silver nanoparticle arrays in a non-centrosymmetric geometry. These hybrid films exhibit second-harmonic generation (SHG) efficiencies as much as 1600 times larger than unmodified, conventional ISAM films, which makes a three bilayer hybrid film perform at the same level as a micron thick, 700-1000 bilayer film. This was accomplished by using nanosphere lithography to deposit silver nanoparticles on the ISAM film, tuning the geometry of the particles to make their plasmonic resonances overlap the frequency of optical excitation. Even though the enhancement is already large, we suggest that further refinements of the techniques are expected to lead to additional enhancements of similar or larger magnitude.   

Reversible Photomechanical Switching of Individual Engineered Molecules at a Surface

We have spatially resolved reversible light-induced mechanical switching in a single organic molecule bound to a metal surface. Scanning tunneling microscopy (STM) was used to image the features of an individual azobenzene molecule on a gold surface before and after reversibly cycling its mechanical structure between \textit{trans} and \textit{cis} states via photo-actuation (i.e., photoisomerization). Azobenzene molecules were engineered to increase their surface photomechanical activity by attaching varying numbers of \textit{tert}-butyl (TB) ligands (``legs'') to the azobenzene phenyl rings. We find that azobenzene molecules lacking TB legs or having only two legs do not switch on a gold substrate under UV irradiation, while molecules synthesized with four TB legs can be photoswitched on gold. STM images of the functionalized molecules show that increasing the number of TB legs ``lifts'' the azobenzene molecules from the substrate, thereby increasing their photomechanical activity. The reversibility of the photoreaction, along with comparison of experimental data to \textit{ab initio} simulation of isomerized azobenzene, confirms the photo-induced \textit{trans-cis} conversion of single molecules.   

Magnetic Field Effect on Hybrid Exciton in a Quantum Dot Coated by an Organic Shell

We investigate the effect of magnetic field perturbations on the hybrid exciton in a semiconductor quantum dot coated by an organic material. The spatial confinement effect of electron and holes of the heterostructures have been considered together with the quantum confined Zeeman effect and the magnetic confinement. Upon the application of magnetic field the coupling term between the two kinds of excitons increases. An important result is the possibility of tuning the Wannier-Frenkel exciton resonance by applied magnetic fields.   

Theoretical study of photoisomerization of azobenzene derivatives on Au(111)

Azobenzene and its various substituted derivatives are organic molecules that can be made to photoisomerize reversibly in solution between the \textit{cis} and \textit{trans} isomers. Scanning tunneling microscopy (STM) experiments have recently shown that photoisomerization is also possible in vacuum on a Au(111) surface. We use \textit{ab initio} pseudopotential density-functional theory to confirm and analyze the experimental results by simulating STM images of the isomers, and we also study how the molecules adsorb on the surface and why some azobenzene derivatives can photoisomerize on the surface while others cannot.   

Reliable and Versatile Molecular Electrodes

Further advancements of molecular electronics will require a reliable and easily scalable electrode fabrication scheme with dimensional control to molecular lengths. We have produced versatile molecular junction (MJ) with high yield ( 90{\%}) long device life ($>$1year) using simple photolithography and thin film methods. The critical electrode dimension is readily set to the length of a molecule by the thickness of an insulator film at a pattern edge. A variety of MJs were prepared by attaching paramagnetic molecular clusters to span the exposed edge of metal-insulator-metal tunnel junctions. Magnetic (Co, NiFe and Ni) and nonmagnetic (Cu, Pd, Ta and Au) metal electrodes and Al$_{2}$O$_{3}$ insulator were utilized. After molecule attachment $\sim $5000{\%} increase in current over bare tunnel junction current was observed. Control experiments including the use of neat solvents, using junction widths longer than molecules, use of insulating molecules, and the reversible binding of molecule to top electrode confirm the successful fabrication of molecular electrodes. MJs were photoactive producing $\sim $60mV photo voltage with white light irradiation. Large magneto-resistance effects were seen with magnetic electrodes.   

Manipulation of Kondo Effect via Two-Dimensional Molecular Self-Assembly

We report manipulation of a Kondo resonance originated from the spin-electron interactions between a two dimensional molecular assembly of TBrPP-Co molecules and a Cu(111) surface at 4.6 K using a low temperature scanning tunneling microscope. By manipulating nearest-neighbor molecules with a scanning tunneling microscope tip we are able to tune the spin-electron coupling of the center molecule inside a small hexagonal molecular assembly in a controlled step-by-step manner. The Kondo temperature increases from 105 to 170 K with a decreasing the number of nearest neighbor molecules from six to zero. This Kondo temperature variation is originated from the scattering of surface electrons by the molecules located at the edges of the molecular layer, which reduces spinelectron coupling strength for the molecules inside the layer. Investigations on different molecular arrangements indicate that the observed Kondo resonance is independent on the molecular lattice. This work is financially supported by US-DOE grant DE-FG02-02ER46012.   

Spatial correlation of photoisomerization of functionalized azobenzene molecules on a surface

Photoactive azobenzene molecules have great potential for nanoscale opto-mechanical applications. We report a scanning tunneling microscopy (STM) study of the time-dependence of photo-switching tetra-\emph{tert}-butyl-azobenzene (TTB-AB) molecules on Au(111). ``Switched'' molecule concentrations were measured as a function of exposure time to various incident light wavelengths until stationary concentrations were reached. We examined the spatial correlations of the photo-switching rates. Scanning tunneling spectroscopy was used to reveal the possible dependence of switching dynamics on the electronic structure of the islands. Implications for organic photoactive devices will be discussed.   

Electron transport through the building blocks of proteins

We investigate two-terminal charge transport through single oligopeptide molecules, thiol-bonded to gold leads. Applying \emph{ab initio} and semi-empirical techniques, we calculate equilibrium and non-equilibrium results in the Landauer formalism. The conductance and current thus obtained are consistent with the recent experimental results of X. Y. Xiao, B. Q. Xu, and N. J. Tao (\emph{J. Am.\ Chem.\ Soc.} {\bf 126}, 5370; \emph{Angew.\ Chem.\ Int. \ Ed.} {\bf 43}, 6148). This theory furthermore provides a straightforward explanation of the striking current rectification seen in those experiments.