Creation of stable molecular junctions with a custom-designed scanning tunneling microscope (original) (raw)
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Optical and transport studies of single molecule tunnel junctions based on self-assembled monolayers
Solid State Communications, 2005
We have fabricated a variety of novel molecular tunnel junctions based on selfassembled-monolayers (SAM) of two-component solid-state mixtures of molecular wires (1,4 methane benzene-dithiol; Me-BDT with two thiol anchoring groups), and molecular insulator spacers (1-pentanethiol; PT with one thiol anchoring group) at different concentration ratios, r of wires/spacers, which were sandwiched between two metallic electrodes such as gold and cobalt. FTIR spectroscopy and surface titration were used, respectively to verify the formation of covalent bonds with the electrodes, and obtain the number of active molecular wires in the device. The electrical transport properties of the SAM devices were studied as a function of (i) r-value, (ii) temperatures, and (iii) different electrodes, via the conductance and differential conductance spectra. The measurements were used to analyze the Me-BDT density of states near the electrode Fermi level, and the properties of the interface barriers. We measured the Me-BDT single molecule resistance at low bias and gold electrodes to be 6x10 9 Ohm. We also determine the energy difference, ∆ between the Me-BDT HOMO level and the gold Fermi level to be about 1.8 eV. In addition we also found that the temperature dependence of the SAM devices with r < 10 -4 is much weaker than that of the pure PT device (or r = 0), showing a small interface barrier.
Journal of the American Chemical Society, 2010
The charge transport characteristics of a family of long conjugated molecular wires have been studied using the scanning tunneling microscope break junction technique. The family consists of four wires ranging from 3.1 to 9.4 nm in length. The two shortest wires show highly length dependent and temperature invariant conductance behavior, whereas the longer two wires show weakly length dependent and temperature variant behavior. This trend is consistent with a model whereby conduction occurs by two different mechanisms in the family of wires: by a coherent tunneling mechanism in the shorter two and by an incoherent charge hopping process in the longer wires. The temperature dependence of the two conduction mechanisms gives rise to a phenomenon whereby at elevated temperatures longer molecules that conduct via charge hopping can yield a higher conductance than shorter wires that conduct via tunneling. The evolution of molecular junctions as the tip retracts has been studied and explained in context of the characteristics of individual transient current decay curves.
Physical chemistry chemical physics : PCCP, 2016
An advanced understanding of the molecule-electrode contact interfaces of single-molecule junctions is a necessity for real world application of future single-molecule devices. This study aims to elucidate the change in the contact tunnelling barrier induced by junction extension and how this change affects the resulting junction conductance. The contact barrier of Au-octanedithiol/octanediamine-Au junctions was studied under triangle (TRI) mechanical modulations using the modified scanning tunneling microscopy (STM) break junction technique. The experimental results reveal that as the junction separation extends, the contact barrier of octanedithiol follows a unique trend, a linear increase followed by a plateau in barrier height, which is in contrast to that of octanediamine, a nearly rectangle barrier. We propose a modified contact barrier model for the unique barrier shape of octanedithiol, based on which the calculation agrees well with the experimental data. This study shows u...
Nano Letters, 2008
We report on single molecule electron transport measurements of two oligophenylenevinylene (OPV3) derivatives placed in a nanogap between gold (Au) or lead (Pb) electrodes in a field effect transistor device. Both derivatives contain thiol end groups that allow chemical binding to the electrodes. One derivative has additional methylene groups separating the thiols from the delocalized π-electron system. The insertion of methylene groups changes the open state conductance by 3−4 orders of magnitude and changes the transport mechanism from a coherent regime with finite zero-bias conductance to sequential tunneling and Coulomb blockade behavior.
Direct Optical Determination of Interfacial Transport Barriers in Molecular Tunnel Junctions
Journal of the American Chemical Society, 2013
Molecular electronics seeks to build circuitry using organic components with at least one dimension in the nanoscale domain. Progress in the field has been inhibited by the difficulty in determining the energy levels of molecules after being perturbed by interactions with the conducting contacts. We measured the photocurrent spectra for large-area aliphatic and aromatic molecular tunnel junctions with partially transparent copper top contacts. Where no molecular absorption takes place, the photocurrent is dominated by internal photoemission, which exhibits energy thresholds corresponding to interfacial transport barriers, enabling their direct measurement in a functioning junction.
Electron transport properties of single molecular junctions under mechanical modulations
Journal of Physics: Condensed Matter, 2012
Electron transport behaviors of single molecular junctions are very sensitive to the atomic scale molecule-metal electrode contact interfaces, which have been difficult to control. We used a modified scanning probe microscope-break junction technique (SPM-BJT) to control the dynamics of the contacts and simultaneously monitor both the conductance and force. First, by fitting the measured data into a modified multiple tunneling barrier model, the static contact resistances, corresponding to the different contact conformations of single alkanedithiol and alkanediamine molecular junctions, were identified. Second, the changes of contact decay constant were measured under mechanical extensions of the molecular junctions, which helped to classify the different single molecular conductance sets into specific microscopic conformations of the molecule-electrode contacts. Third, by monitoring the changes of force and contact decay constant with the mechanical extensions, the changes of conductance were found to be caused by the changes of contact bond length and by the atomic reorganizations near the contact bond. This study provides a new insight into the understanding of the influences of contact conformations, especially the effect of changes of dynamic contact conformation on electron transport through single molecular junctions.
Experimental Observation of Real Time Molecular Dynamics Using Electromigrated Tunnel Junctions
The Journal of Physical Chemistry C, 2017
Single molecule tunnel junctions (SMTJs) can provide important physical insights into electronic and vibrational phenomena at the molecular scale. However, observations and analysis are typically confined to sufficiently low temperatures as to suppress molecular motion and the resulting stochastic fluctuations in the tunneling current. In this work, we introduce and experimentally validate a methodology whereby a slightly higher temperature (9 K) compared to a typical SMTJ study can be used to induce sparse fluctuations in the inelastic tunneling current and provide the fingerprints of dynamics between the conformational states of the molecule. Two examples of benzene dithiol and cysteine are studied in electromigratively formed W/Au nanowire SMTJs on SiO 2 at 9 K. The second-order transform of the tunneling current reveals the expected vibrational spectra. However, we show that temporal fluctuations can be analyzed using a hidden Markov Model to reveal dynamics assigned to millisecond rearrangements of the molecule, with apparent energy barriers ranging from 35 to 66 meV, consistent with theoretical predictions. The observed transitions are consistent with a model of lateral migration of the thiol-anchored molecules in an asymmetric junction. The use of temperature in SMTJs in this way can provide new insights into molecule dynamics in confined volumes and at electrode interfaces.
The Journal of Physical Chemistry C, 2009
This paper reports analysis of direct (low bias regime that corresponds to Simmons model) and field emission, Fowler-Nordheim (FN) tunneling, and a crossover between these regimes in molecular nanojunction. We have fabricated molecular devices based on a heterogenius mixture of molecular wires of 2-[4-(2mercaptoethyl)-phenyl]ethanethiol (Me-PET) as self-assembled monolayer (SAM) molecules incorporated into the matrix of molecular insulator spacers [penthane 1-thiol (PT)] at a concentration ratio of r ) 10 -6 wires/spacers. The monolayer is sandwiched between two gold (Au) electrodes. A temperature-depended conductivity in this structure was analyzed at both low and elevated biases using models of direct tunneling (Simmons model) and field emission (FN) regime. A crossover voltage, V trans , between these two regimes was determined at different temperatures. Comparison of temperature-dependent Simmons and FN barriers, (Φ B Sim (T) and Φ B FN (T)), and transition bias (V trans (T)) reveals an anomaly in position of V trans (T) with respect to Φ B Sim (T) and Φ B FN (T) at low temperatures (15-100 K). The change in slopes of Φ B Sim (T), Φ B FN (T)