In single-molecule transistors, we observe inelastic cotunneling features that correspond energetically to vibrational excitations of the molecule, as determined by Raman and infrared spectroscopy. ...This is a form of inelastic electron tunneling spectroscopy of single molecules, with the transistor geometry allowing in situ tuning of the electronic states via a gate electrode. The vibrational features shift and change shape as the electronic levels are tuned near resonance, indicating significant modification of the vibrational states. When the molecule contains an unpaired electron, we also observe vibrational satellite features around the Kondo resonance.
We have studied charge injection across the metal/organic semiconductor interface in bottom-contact poly(3-hexylthiophene) (P3HT) field-effect transistors, with Au source and drain electrodes ...modified by self-assembled monolayers (SAMs) prior to active polymer deposition. By using the SAM to engineer the effective Au work function, we markedly affect the charge injection process. We systematically examine the contact resistivity and intrinsic channel mobility and show that chemically increasing the injecting electrode work function significantly improves hole injection relative to untreated Au electrodes.
We report Kondo resonances in the conduction of single-molecule transistors based on transition metal coordination complexes. We find Kondo temperatures in excess of 50 K, comparable to those in ...purely metallic systems. The observed gate dependence of the Kondo temperature is inconsistent with observations in semiconductor quantum dots and a simple single-dot-level model. We discuss possible explanations of this effect, in light of electronic structure calculations.
We report electronic transport measurements of single-molecule transistor devices incorporating bipyridyl-dinitro oligophenylene-ethynylene dithiol (BPDN-DT), a molecule known to exhibit conductance ...switching in other measurement configurations. We observe hysteretic conductance switching in 8% of devices with measurable currents and find that dependence of the switching properties on gate voltage is rare when compared to other single-molecule transistor devices. This suggests that polaron formation is unlikely to be responsible for switching in these devices. We discuss this and alternative switching mechanisms.
Six customized phenylene-ethynylene-based oligomers have been studied for their electronic properties using scanning tunneling microscopy to test hypothesized mechanisms of stochastic conductance ...switching. Previously suggested mechanisms include functional group reduction, functional group rotation, backbone ring rotation, neighboring molecule interactions, bond fluctuations, and hybridization changes. Here, we test these hypotheses experimentally by varying the molecular designs of the switches; the ability of the molecules to switch via each hypothetical mechanism is selectively engineered into or out of each molecule. We conclude that hybridization changes at the molecule−surface interface are responsible for the switching we observe.
Thiolate self-assembly on gold has proven to be a valuable technique for assembling monolayers on a wide variety of substrates. However, the oxidative instability of the thiols, especially aromatic ...thiols and α,ω-dithiols, presents several difficulties. Shown here is that thiocyanates, easily synthesized stable thiol derivatives, can be directly assembled on gold surfaces with no auxiliary reagents required. Assembly is complete in 24 h and leaves a similar gold thiolate structure as seen in typical thiol self-assembled monolayers.
Thiocyanate assembly is shown to be an effective method for assembling thiolate structures on platinum, silver, and gold. The assemblies were studied by infrared reflection spectroscopy and X-ray ...photoelectron spectroscopy (XPS). Two cyanide species were identified on the surfaces: the first corresponding to adsorbed cyanide and the second to a form commonly seen as an intermediate during cyanide etching of metals. The presence of the second species supports the theory that cyanide is leaving the surface as M(CN) x , resulting in a thiolate monolayer. Comparison of thiocyanate assemblies on evaporated gold and silver to those on template-stripped gold demonstrates the integral role of surface morphology in the expulsion of (CN)ads from the surface of the metals.
A series of transition metal coordination complexes designed to assemble on gold surfaces was synthesized, their electronic structure and transitions analyzed, and their magnetic properties studied. ...By taking advantage of recently developed thiocyanate assembly protocols, these molecules were then assembled onto a gold surface, without the need for an inert atmosphere, to give a loosely packed monolayer. The assembled molecules exhibit properties similar to that of the bulk molecules, indicating little change in molecular structure outside of chemisorption.
In semiconductor quantum dots, the Kondo temperature has been observed to depend exponentially on the gate voltage. This dependence arises because in these structures the gate capacitively shifts the ...energy of the singly occupied Kondo-active level relative to the chemical potential of the conduction electrons in the source and drain. In single-molecule transistors incorporating transition metal complexes, we find that the expected gate dependence in the Kondo regime is not observed. While the data show that the gate does shift electronic levels, the Kondo temperature found from both the differential conductance temperature and bias voltage dependence is approximately independent of gate voltage. We discuss possible explanations for this surprising observation, including the possible effect of molecular vibrational modes.
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In order to investigate one of the proposed molecular electronics switching mechanisms, we synthesized several molecules whose cores are unable to undergo conformational rotation. Preparation ...of these molecules, all of which are terminated with the thioacetyl moiety, takes advantage of an improved synthesis of 2,7-dibromophenanthrene, use of the reduced form of phenanthrenequinone as a protecting group, and various Sonogashira and halogen to Li to S conversions.
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