Zong-ang Li

Rectifying Properties of Oligo(Phenylene Ethynylene) Heterometallic Molecular Junctions: Molecular Length and Side Group Effects

The rectifying properties of α,ω-dithiol terminated oligo(phenylene ethynylene) molecules sandwiched between heterometallic electrodes, including the molecular length and side group effects, are theoretically investigated using the fully self-consistent nonequilibrium Greens function method combined with density functional theory. The results show nonlinear variation with changes in molecule length: when the molecule becomes longer, the current decreases at first and then increases while the rectification shifts in the opposite direction. This stems from the change in molecular eigenstates and the coupling between the molecule and electrodes caused by different molecular lengths. The rectifying behavior of heterometallic molecular junctions can be attributed to the asymmetric molecule-electrode contacts, which lead to asymmetric electronic tunneling spectra, molecular eigenvalues, molecular orbitals, and potential drop at reversed equivalent bias voltages. Our results provide a fundamental understanding of the rectification of heterometallic molecular junction, and a prediction of rectifiers with different rectification properties from those in the experiment, using electrodes with reduced sizes.

A method to study electronic transport properties of molecular junction: one-dimension transmission combined with three-dimension correction approximation (OTCTCA).

Based on the ab initio calculation, a method of one-dimension transmission combined with three-dimension correction approximation (OTCTCA) is developed to investigate electron-transport properties of molecular junctions. The method considers that the functional molecule provides a spatial distribution of effective potential field for the electronic transport. The electrons are injected from one electrode by bias voltage, then transmit through the potential field around the functional molecule, at last are poured into the other electrode with a specific transmission probability which is calculated from one-dimension Schrödinger equation combined with three-dimension correction. The electron-transport properties of alkane diamines and 4, 4′-bipyridine molecular junctions are studied by applying OTCTCA method. The numerical results show that the conductance obviously exponentially decays with the increase of molecular length. When stretching molecular junctions, steps with a certain width are presented in conductance traces. Especially, in stretching process of 4, 4′-bipyridine molecular junction, if the terminal N atom is broken from flat part of electrode tip and exactly there is a surface Au atom on the tip nearby the N atom, the molecule generally turns to absorb on the surface Au atom, which further results in another lower conductance step in the traces as the experimental probing.

Effect of Gate Electric Field on Single Organic Molecular Devices

Based on the first‐principles computational method and elastic scattering Greens function theory, we have investigated the effect of gate electric field on electronic transport properties of a series of single organic molecular junctions theoretically. The numerical results show that the molecular junctions that have redox centers and relatively large dipole moments parallel gate direction can respond to the gate electric field remarkably. The current‐voltage properties of 2,5‐dimethyl‐thiophene‐dithiol present N‐channel‐metal‐oxide‐semiconductor‐like characteristics. Its distinct current‐voltage properties can be understood from the evolution of eigenvalues, coupling energies, and atomic charges with gate electric field.

Gas-sensor property of single-molecule device: F2 adsorbing effect*

The single thiolated arylethynylene molecule with 9,10-dihydroanthracene core (denoted as TADHA) possesses pronounced negative differential conductance (NDC) behavior at lower bias regime. The adsorption effects of F2 molecule on the current and NDC behavior of TADHA molecular junctions are studied by applying non-equilibrium Greens formalism combined with density functional theory. The numerical results show that the F2 molecule adsorbed on the benzene ring of TADHA molecule near the electrode can dramatically suppresses the current of TADHA molecular junction. When the F2 molecule adsorbed on the conjugated segment of 9,10-dihydroanthracene core of TADHA molecule, an obviously asymmetric effect on the current curves induces the molecular system showing apparent rectifier behavior. However, the current especially the NDC behavior have been significantly enlarged when F2 addition reacted with triple bond of TADHA molecule.

Shaping the Atomic‐Scale Geometries of Electrodes to Control Optical and Electrical Performance of Molecular Devices

A straightforward method to generate both atomic-scale sharp and atomic-scale planar electrodes is reported. The atomic-scale sharp electrodes are generated by precisely stretching a suspended nanowire, while the atomic-scale planar electrodes are obtained via mechanically controllable interelectrodes compression followed by a thermal-driven atom migration process. Notably, the gap size between the electrodes can be precisely controlled at subangstrom accuracy with this method. These two types of electrodes are subsequently employed to investigate the properties of single molecular junctions. It is found, for the first time, that the conductance of the amine-linked molecular junctions can be enhanced ≈50% as the atomic-scale sharp electrodes are used. However, the atomic-scale planar electrodes show great advantages to enhance the sensitivity of Raman scattering upon the variation of nanogap size. The underlying mechanisms for these two interesting observations are clarified with the help of density functional theory calculation and finite-element method simulation. These findings not only provide a strategy to control the electron transport through the molecule junction, but also pave a way to modulate the optical response as well as to improve the stability of single molecular devices via the rational design of electrodes geometries.

Designing molecular rectifiers and spin valves using metallocene-functionalized undecanethiolates: one transition metal atom matters

By using the first-principles method, here we have theoretically investigated the effects of the head group on the rectifying and spin filtering properties of metallocenyl-functionalized undecanethiolate molecular junctions. It is found that the rectifying performance as well as the rectification direction of the molecular junctions can be largely modulated by choosing different metallocenyl head groups, i.e., chromocene (CrCp2), manganocene (MnCp2), ferrocene (FeCp2), cobaltocene (CoCp2), and nickelocene (NiCp2). More interestingly, large or even perfect spin filtering efficiency can be obtained for molecular junctions embedded with a magnetic metallocenyl head group (CrCp2, MnCp2, CoCp2, or NiCp2). Further analysis reveals that all of the frontier molecular orbitals around the Fermi energy are localized on the metallocenyl head group, which results in their monotonic evolutions under positive and negative bias voltage due to the electrostatic effect of external bias voltage. This contributes to the rectification observed for the molecular junctions. Meanwhile, alignments of the frontier molecular orbitals with respect to the Fermi energy and their spin properties can be dramatically changed by the metallocenyl head group, which essentially leads to the inversion of rectification direction and the remarkable spin filtering effect. Our result provides a feasible way to optimize the rectifying performance of alkanethiolate based molecular diodes, and it also suggests a good platform to obtain a high or even perfect spin filtering efficiency that has a wide use in the field of molecular spintronics.

Effect of H 2 O Adsorption on Negative Differential Conductance Behavior of Single Junction

Large negative differential conductance (NDC) at lower bias regime is a very desirable functional property for single molecular device. Due to the non-conjugated segment separating two conjugated branches, the single thiolated arylethynylene molecule with 9,10-dihydroanthracene core (denoted as TADHA) presents excellent NDC behavior in lower bias regime. Based on the ab initio calculation and non-equilibrium Green’s function formalism, the NDC behavior of TADHA molecular device and the H2O-molecule-adsorption effects are studied systematically. The numerical results show that the NDC behavior of TADHA molecular junction originates from the Stark effect of the applied bias which splits the degeneration of the highest occupied molecular orbital (HOMO) and HOMO-1. The H2O molecule adsorbed on the terminal sulphur atom strongly suppresses the conductance of TADHA molecular device and destroys the NDC behavior in the lower bias regime. Single or separated H2O molecules adsorbed on the backbone of TADHA molecule can depress the energy levels of molecular orbitals, but have little effects on the NDC behavior of the TADHA molecular junction. Aggregate of several H2O molecules adsorbed on one branch of TADHA molecule can dramatically enhance the conductance and NDC behavior of the molecular junction, and result in rectifier behavior.

Ambient molecule effects on the electronic transport of pyrene-1,8-dithiol molecular junction

Due to the small size, single-molecule device may be sensitive to the ambient molecules. Thus, it is significant for fabricating single-molecule sensors to understand the influence of ambient molecule on molecular device. Based on the ab initio calculations combined with non-equilibrium Greens function method, the adsorption effects of H2O, CO2 and NO2 molecule on the pyrene-1,8-dithiol molecular junctions are studied systematically. The numerical results show that, the influence of H2O or CO2 molecule on the pyrene-1,8-dithiol molecular junction is very slight when they are adsorbed on the pyrene-1,8-dithiol molecules, which attributes to the closed-shell ground states of these two molecules. Different from H2O and CO2 molecule, being a radical, NO2 molecule shows obvious influence on the electronic transport of pyrene1,8-dithiol molecular junctions. The system with NO2 adsorbate is more conductive in the positive and lower negative bias regime than those of the other two molecular systems, which is due to the evident coupling between the states of NO2 molecule and pyrene-1,8-dithiol molecule.