Theory of electrical rectification in a molecular monolayer (original) (raw)
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Applied Physics A, 2016
In this research work, we compare the rectification trends of two symmetrical and one asymmetrical molecular junction formed with gold and silver electrodes bridging benzenedithiol molecule. The origin of rectification is attributed to both molecular bias drop and asymmetric molecule-electrode coupling. The electronic transport properties are computed by using semi-empirical extended Huckel method combined with non-equilibrium Green's function framework. The results are fully rationalized by analysing the distribution of molecular orbitals with changing bias voltage, available density of states and area of transmission spectra spanned within bias window, transmission eigenstates and transmission pathways. We deduce through this work that the molecular rectification is not only the property of asymmetric molecule-metal coupling, but molecular bias also plays vital role in stemming asymmetric I-V characteristics. Our results suggest how to realize molecular rectification by using different electrode materials which act as Schottky barriers in molecular junctions that emulate p-n junction diode in semiconductor electronics.
Rectification and stability of a single molecular diode with controlled orientation
Nature Chemistry, 2009
In the molecular electronics field it is highly desirable to engineer the structure of molecules to achieve specific functions. In particular, diode (or rectification) behaviour in single molecules is an attractive device function. Here we study charge transport through symmetric tetraphenyl and non-symmetric diblock dipyrimidinyldiphenyl molecules covalently bound to two electrodes. The orientation of the diblock is controlled through a selective deprotection strategy, and a method in which the electrode-electrode distance is modulated unambiguously determines the current-voltage characteristics of the single-molecule device. The diblock molecule exhibits pronounced rectification behaviour compared with its homologous symmetric block, with current flowing from the dipyrimidinyl to the diphenyl moieties. This behaviour is interpreted in terms of localization of the wave function of the hole ground state at one end of the diblock under the applied field. At large forward current, the molecular diode becomes unstable and quantum point contacts between the electrodes form.
The Journal of Physical Chemistry B, 2001
Unimolecular rectification was detected between oxide-free Au electrodes for a Langmuir-Blodgett (LB) monolayer of the zwitterionic D + -π-Amolecule hexadecylquinolinium tricyanoquinodimethanide, C 16 H 33 Q-3CNQ. The top gold pad was deposited by a process that cools the metal vapor before deposition. The maximum rectification ratio is 27.5 at 2.2 V (the average rectification ratio is 7.55). The currents are as large as 9.04 × 10 4 electrons molecule -1 s -1 . The result reinforces previous work with oxide-bearing Al electrodes, but the currents with Au electrodes are larger by 3-5 orders of magnitude. Rectification was also seen for a nine-monolayer Z-type LB film between similar Au electrodes but not for a monolayer of arachidic acid. The direction of enhanced electron current is from the negatively charged dicyanomethylene end of C 16 H 33 Q-3CNQ to the quinolinium ring, as predicted by the Aviram-Ratner analysis. However, there is a good fit to the behavior expected for quantum conduction dominated by a single molecular level. The best-fit energy for this level is 1.31 ( 0.25 eV above the Fermi level of the Au electrode.
Journal of the American Chemical Society, 2016
Two layers of molecular oligomers were deposited on flat carbon electrode surfaces by electrochemical reduction of diazonium reagents, then a top contact applied to complete a solid-state molecular junction containing a molecular bilayer. The structures and energy levels of the molecular layers included donor molecules with relatively high energy occupied orbitals and acceptors with low energy unoccupied orbitals. When the energy levels of the two molecular layers were similar, the device had electronic characteristics similar to a thick layer of a single molecule, but if the energy levels differed, the current voltage behavior exhibited pronounced rectification. Higher current was observed when the acceptor molecule was biased negatively in eight different bilayer combinations, and the direction of rectification was reversed if the molecular layers were also reversed. Rectification persisted at very low temperature (7 K), and was activationless between 7 and 100 K. The results are ...
Current rectification in molecular junctions produced by local potential fields
Physical Review B, 2010
The transport properties of a octane-dithiol (ODT) molecule coupled to Au(001) leads are analyzed using density functional theory and non-equilibrium Green functions. It is shown that a symmetric molecule can turn into a diode under influence of a local electric field created by an external charged probe. The origin of the asymmetry of the current-voltage (I − V ) dependence is traced back to the appearance of a probe induced quasi-local state in the pseudogap of the ODT molecule. The induced state affects electron transport, provided it is close to the Fermi level of the leads. An asymmetric placement of the charged probe along the alkane chain makes the induced quasi-local state in the energy gap very sensitive to the bias voltage and results in rectification of the current. The results based on DFT are supported by independent calculations using a simple one-particle model Hamiltonian.
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.
Physical Review B, 2006
We report on the development of an accurate first-principles computational scheme for the charge transport characteristics of molecular monolayer junctions and its application to hexanedithiolate ͑C6DT͒ devices. Starting from the Gaussian basis set density-functional calculations of a junction model in the slab geometry and corresponding two bulk electrodes, we obtain the transmission function using the matrix Green's function method and analyze the nature of transmission channels via atomic projected density of states. Within the developed formalism, by treating isolated molecules with the supercell approach, we can investigate the current-voltage characteristics of single and parallel molecular wires in a consistent manner. For the case of single C6DT molecules stretched between Au͑111͒ electrodes, we obtain reasonable quantitative agreement of computed conductance with a recent scanning tunneling microscope experiment result. Comparing the charge transport properties of C6DT single molecules and their monolayer counterparts in the stretched and tilted geometries, we find that the effect of intermolecular coupling and molecule tilting on the charge transport characteristics is negligible in these devices. We contrast this behavior to that of the-conjugated biphenyldithiolate devices we have previously considered and discuss the relative importance of molecular cores and molecule-electrode contacts for the charge transport in those devices.
Molecular Rectification in a Metal−Insulator−Metal Junction Based on Self-Assembled Monolayers
Journal of the American Chemical Society, 2002
An electrical junction formed by mechanical contact between two self-assembled monolayers (SAMs)sa SAM formed from an dialkyl disulfide with a covalently linked tetracyanoquinodimethane group that is supported by silver (or gold) and a SAM formed from an alkanethiolate SAM that is supported by mercurysrectifies current. The precursor to the SAM on silver (or gold) was bis(20-(2-((2,5-cyclohexadiene-1,4-diylidene)dimalonitrile))decyl)) disulfide and that for the SAM on mercury was HS(CH2)n-1CH3 (n ) 14, 16, 18). The electrical properties of the junctions were characterized by current-voltage measurements. The ratio of the conductivity of the junction in the forward bias (Hg cathodic) to that in the reverse bias (Hg anodic), at a potential of 1 V, was 9 ( 2 when the SAM on mercury was derived from HS(CH2)15CH3. The ratio of the conductivity in the forward bias to that in the reverse bias increased with decreasing chain length of the alkanethiol used to form the SAM on mercury. These results demonstrate that a single redox center asymmetrically placed in a metal-insulator-metal junction can cause the rectification of current and indicate that a fixed dipole in the insulating region of a metal-insulator-metal junction is not required for rectification.