Influence of the donor unit on the rectification ratio in tunnel junctions based on donor-acceptor SAMs using PTM units as acceptors (original) (raw)
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Journal of the American Chemical Society, 2017
This Communication describes the mechanism of charge transport across self-assembled monolayers (SAMs) of two donor-acceptor systems consisting of a polychlorotriphenylmethyl (PTM) electron-acceptor moiety linked to an electron-donor ferrocene (Fc) unit supported by ultraflat template-stripped Au and contacted by a eutectic alloy of gallium and indium top contacts. The electronic and supramolecular structures of these SAMs were well characterized. The PTM unit can be switched between the nonradical and radical forms, which influences the rectification behavior of the junction. Junctions with nonradical units rectify currents via the highest occupied molecular orbital (HOMO) with a rectification ratio R = 99, but junctions with radical units have a new accessible state, a single-unoccupied molecular orbital (SUMO), which turns rectification off and drops R to 6.
Charge Transport through Self-Assembled Monolayers of Compounds of Interest in Molecular Electronics
Journal of the American Chemical Society, 2002
The electrical properties of self-assembled monolayers (SAMs) on metal surfaces have been explored for a series of molecules to address the relation between the behavior of a molecule and its structure. We probed interfacial electron transfer processes, particularly those involving unoccupied states, of SAMs of thiolates or arylates on Au by using shear force-based scanning probe microscopy (SPM) combined with current-voltage (i-V) and current-distance (i-d) measurements. The i-V curves of hexadecanethiol in the low bias regime were symmetric around 0 V and the current increased exponentially with V at high bias voltage. Different than hexadecanethiol, reversible peak-shaped i-V characteristics were obtained for most of the nitro-based oligo(phenylene ethynylene) SAMs studied here, indicating that part of the conduction mechanism of these junctions involved resonance tunneling. These reversible peaked i-V curves, often described as a negative differential resistance (NDR) effect of the junction, can be used to define a threshold tip bias, VTH, for resonant conduction. We also found that for all of the SAMs studied here, the current decreased with increasing distance, d, between tip and substrate. The attenuation factor of hexadecanethiol was high, ranging from 1.3 to 1.4 Å-1 , and was nearly independent of the tip bias. The-values for nitro-based molecules were low and depended strongly on the tip bias, ranging from 0.15 Å-1 for tetranitro oligo(phenylene ethynylene) thiol, VII, to 0.50 Å-1 for dinitro oligo(phenylene) thiol, VI, at a-3.0 V tip bias. Both the VTH and values of these nitro-based SAMs were also strongly dependent on the structures of the molecules, e.g. the number of electroactive substituent groups on the central benzene, the molecular wire backbone, the anchoring linkage, and the headgroup. We also observed charge storage on nitro-based molecules. For a SAM of the dintro compound, V, ∼25% of charge collected in the negative scan is stored in the molecules and can be collected at positive voltages. A possible mechanism involving lateral electron hopping is proposed to explain this phenomenon.
Charge Transport and Rectification in Arrays of SAM-Based Tunneling Junctions
Nano Letters, 2010
This paper describes a method of fabrication that generates small arrays of tunneling junctions based on self-assembled monolayers (SAMs); these junctions have liquid-metal top-electrodes stabilized in microchannels and ultraflat (template-stripped) bottom-electrodes. The yield of junctions generated using this method is high (70-90%). The junctions examined incorporated SAMs of alkanethiolates having ferrocene termini (11-(ferrocenyl)-1-undecanethiol, SC 11 Fc); these junctions rectify currents with large rectification ratios (R), the majority of which fall within the range of 90-180. These values are larger than expected (theory predicts R e 20) and are larger than previous experimental measurements. SAMs of n-alkanethiolates without the Fc groups (SC n-1 CH 3 , with n ) 12, 14, 16, or 18) do not rectify (R ranged from 1.0 to 5.0). These arrays enable the measurement of the electrical characteristics of the junctions as a function of chemical structure, voltage, and temperature over the range of 110-293 K, with statistically large numbers of data (N ) 300-800). The mechanism of rectification with Fc-terminated SAMs seems to be charge transport processes that change with the polarity of bias: from tunneling (at one bias) to hopping combined with tunneling (at the opposite bias).
The Journal of Physical Chemistry B, 2004
The supramolecular association of tetrathiafulvalene (TTF) donors and bipyridinium acceptors is employed routinely to direct the formation of host/guest complexes and interlocked molecules in bulk solution. We have reproduced these donor/acceptor interactions at electrode/solution interfaces and demonstrated their pronounced influence on heterogeneous electron transfer. Specifically, we have synthesized a TTF with an oligomethylene arm terminated by a thiol group and assembled monolayers of this compound on gold. We have observed that the cyclic voltammogram of the immobilized TTF donors varies significantly upon addition of benzyl viologen, tetracyanoquinodimethane (TCNQ), or tetracyanoethylene (TCNE) acceptors to the electrolyte solution. Indeed, the supramolecular association of the complementary donors and acceptors results in a pronounced current decrease for the TTF redox waves. Consistently, the electrochemical response of the acceptors changes dramatically in the presence of TTF donors on the electrode surface. Instead, hexadecanethiolate monolayers, lacking the TTF donors at the termini of the oligomethylene chains, have a marginal influence on the voltammograms of the acceptors. Impedance measurements indicate that the charge-transfer resistance (R CT) for the reduction of the acceptors increases from less than 0.3 kΩ, at bare gold, to 324, 24, and 43 kΩ for benzyl viologen, TCNQ, and TCNE, respectively, at TTF-coated electrodes. By contrast, the electrode coating has a negligible influence on the cyclic voltammogram and impedance response of ferrocene, which cannot sustain donor/acceptor interactions with the immobilized TTFs. Thus, our results demonstrate that the interfacial complexation of complementary donors and acceptors has a dramatic effect on the heterogeneous electron transfer to and from the associated components.
In this work, we systematically studied the rectifying properties of molecular junction based on asymmetric tunneling and hopping charge transport in a single electronic state model using Landauer formula and Marcus theory. We first analyzed the asymmetric I-V characteristics and revealed distinct physical origins of the rectification under the two types of transports. We found significant difference in I-V characteristics of the two and the hopping transport can afford a much higher rectification ratio than tunneling. Next, the effect of key physical parameters on rectification performance under tunneling and hopping, like asymmetric factor, energy barrier, temperature and molecule-electrode coupling et al, were extensively evaluated, which provided a theoretical baseline for molecular diode design and performance modulation. At last, we further analyzed representative experimental results using the two models. We successfully reproduced the experimental results by adjusting the mo...
ACS Applied Materials & Interfaces, 2020
Control over the energy level alignment in molecular junctions is notoriously difficult, making it challenging to control basic electronic functions such as the direction of rectification. Therefore, alternative approaches to control electronic functions in molecular junctions are needed. This paper describes switching of the direction of rectification by changing the bottom electrode material M = Ag, Au, or Pt in M−S(CH 2) 11 S−BTTF//EGaIn junctions based on self-assembled monolayers incorporating benzotetrathiafulvalene (BTTF) with EGaIn (eutectic alloy of Ga and In) as the top electrode. The stability of the junctions is determined by the choice of the bottom electrode, which, in turn, determines the maximum applied bias window, and the mechanism of rectification is dominated by the energy levels centered on the BTTF units. The energy level alignments of the three junctions are similar because of Fermi level pinning induced by charge transfer at the metal−thiolate interface and by a varying degree of additional charge transfer between BTTF and the metal. Density functional theory calculations show that the amount of electron transfer from M to the lowest unoccupied molecular orbital (LUMO) of BTTF follows the order Ag > Au > Pt. Junctions with Ag electrodes are the least stable and can only withstand an applied bias of ±1.0 V. As a result, no molecular orbitals can fall in the applied bias window, and the junctions do not rectify. The junction stability increases for M = Au, and the highest occupied molecular orbital (HOMO) dominates charge transport at a positive bias resulting in a positive rectification ratio of 83 at ±1.5 V. The junctions are very stable for M = Pt, but now the LUMO dominates charge transport at a negative bias resulting in a negative rectification ratio of 912 at ±2.5 V. Thus, the limitations of Fermi level pinning can be bypassed by a judicious choice of the bottom electrode material, making it possible to access selectively HOMO-or LUMO-based charge transport and, as shown here, associated reversal of rectification.
Tuning Charge Transport Properties of Asymmetric Molecular Junctions
The Journal of Physical Chemistry C
Charge transport characteristics of asymmetric molecules containing a 9,9'-spirobifluorene platform coupled covalently to a phenylene ethynylene linker capped with either a thiol or a nitrile end group are investigated by break junction techniques. It is shown that the platform provides very good electronic coupling with metallic leads and the differences in the charge transport depend solely on the type of the anchoring group at the opposite end of the molecule. The SH-terminated molecule has one order of magnitude higher conductance compared to CN-terminated one and the charge transport path depends on the end group utilized. By a combined experimental break junction techniques and theoretical DFT calculations, it was demonstrated that in molecules containing SH-terminated phenylene ethynylene wire attached to the 9,9'-spirobifluorene platform the charge is transported through fluorene unit and covalently coupled phenylene ethynylene linker. For CN-terminated molecules the charge is transported through the thiolate termini of the 9,9'-spirobifluorene tripod. These studies demonstrate the potential of spirobifluorene platform for the bottom-up approach to molecular architectures by its immobilization with all three thiol groups to one of the electrodes without compromising charge transport via the conjugated backbone.
Radical Enhanced Charge Transport in Single-Molecule Phenothiazine Electrical Junctions
Angewandte Chemie (International ed. in English), 2017
We studied the single-molecule conductance through an acid oxidant triggered phenothiazine (PTZ) based radical junction using the Mechanically Controllable Break Junction technique. The electrical conductance of the radical state was enhanced by up to 200 times compared to the neutral state, with high stability lasting for at least two months and high junction formation probability at room-temperature. Theoretical studies revealed that the conductance increase is due to a significant decrease of HOMO-LUMO gap and also the enhanced transmission close to the HOMO orbital when the radical forms. The large conductance enhancement induced by the formation of stable PTZ radical molecule will lead to promising applications in single-molecule electronics and spintronics.