Transport Properties Of Molecular Devices Research Articles

Exploring the Odd–Even Effect, Current Stabilization, and Negative Differential Resistance in Carbon-Chain-Based Molecular Devices

The transport properties of molecular devices based on carbon chains are systematically investigated using a combination of non-equilibrium Green’s function (NEGF) and density functional theory (DFT) first-principle methods. In single-carbon-chain molecular devices, a distinct even–odd behavior of the current emerges, primarily influenced by the density of states (DOS) within the chain channel. Additionally, linear, monotonic currents exhibit Ohmic contact characteristics. In ladder-shaped carbon-chain molecular devices, a notable current stabilization behavior is observed, suggesting their potential utility as current stabilizers within circuits. We provide a comprehensive analysis of the transport properties of molecular devices featuring ladder-shaped carbon chains connecting benzene-ring molecules. The occurrence of negative differential resistance (NDR) in the low-bias voltage region is noted, with the possibility of manipulation by adjusting the position of the benzene-ring molecule. These findings offer a novel perspective on the potential applications of atom chains.

Remarkable modulation of the spin transport properties of transition metal dibenzotetraaza[14]annulenes by adsorption of oxygen

Here, the spin transport properties of molecular devices comprised of a transition metal-dibenzotetraaza[14]annulene complex (TM-DBTAA, TM=Co, Cu, Ni) connected to two single-walled carbon nanotube (SWCNT) electrodes have been theoretically investigated by using the first-principles method. In addition, the effect of O2 adsorption on the TM atom on the spin transport properties of the investigated devices is inspected. The calculations reveal that Co-DBTAA behaves as a spin filter with SFE close to 100%. Upon O2 adsorption, the spin polarization of the current for Co-DBTAA is changed from spin-down to spin-up with the magnitude of SFE nearly unchanged. More interestingly, for the spin non-polarized and nonconducting Ni-DBTAA, the adsorption of an O2 molecule induces a spin-up polarized current with remarkable SFE larger than 90%. This work is helpful for future designing of molecular spin filters as well as gas sensors.

Effect of crystallographic orientations on transport properties of methylthiol-terminated permethyloligosilane molecular junction

The understanding of the influence of electrode characteristics on charge transport is essential in the field of molecular electronics. In this work, we investigate the electronic transport properties of molecular junctions comprising methylthiol-terminated permethyloligosilanes and face-centered crystal Au/Ag electrodes with crystallographic orientations of (111) and (100), based on the ab initio quantum transport simulations. The calculations reveal that the molecular junction conductance is dominated by the electronic coupling between two interfacial metal–S bonding states, which can be tuned by varying the molecular length, metal material of the electrodes, and crystallographic orientation. As the permethyloligosilane backbone elongates, although the σ conjugation increases, the decreasing of coupling induced by the increasing number of central Si atoms reduces the junction conductance. The molecular junction conductance of methylthiol-terminated permethyloligosilanes with Au electrodes is higher than that with Ag electrodes with a crystallographic orientation of (111). However, the conductance trend is reversed when the electrode crystallographic orientation varies from (111) to (100), which can be ascribed to the reversal of interfacial coupling between two metal–S interfacial states. These findings are conducive to elucidating the mechanism of molecular junctions and improving the transport properties of molecular devices by adjusting the electrode characteristics.

Electron transport properties of molecular devices based on silicon-1, 6-hexanedithiol-silicon

To reveal how the electronic transport properties of the silicon-sulfur molecular junction depend on its microscopic structure, we studied the configuration and electronic transport properties of a silicon-1, 6-hexanedithiol-silicon molecular device, based on density functional theory (DFT) combined with the non-equilibrium Green’s function (NEGF) method. Eight groups of molecular junctions were modeled, each consisting of a 1, 6-hexanedithiol molecule spanning two silicon tips. Following optimisation of the molecular structure, first of the 1, 6-hexanedithiol by itself and then of the junction as a whole, the zero-bias conductance and electronic transmission spectrum were calculated. The results show that silicon-sulfur molecular junctions have characteristics that differ significantly from those of gold-sulfur molecular junctions. The structural details of the silicon-sulfur molecular junction are shown to have a crucial influence on its electronic transport properties such as zero-bias piezoelectric conductance and transmission spectra.

Switching characteristics of anthraquinone molecular devices based on graphene electrodes

With the development of microelectronics and the miniaturization of electronic devices, the use of molecular materials to construct various components in electronic circuits has become a most likely development trend. Compared with silicon-based semiconductor components, molecular electronic device has the advantages of small size, high integration, low energy consumption and fast response. In recent years, more and more molecules have been used to design molecular devices such as molecular diodes, molecular switches, molecular field effect transistors and molecular memories. In this paper, sandwich structure devices based on graphene nanoribbon electrodes are constructed. The first-principles calculation method combining density functional theory and non-equilibrium Green’s function is adopted to design the molecular devices with functional characteristics. The effects of redox reactions on the electrical transport properties of molecular devices are systematically discussed. The main research contents of this paper are as follows. The switching characteristics of an anthraquinone molecular device based on graphene electrode are studied. The zigzag-edge nanoribbons and armchair-edge graphene nanoribbons are selected as electrodes. Considering the two isomers of anthraquinone (HQ) and anthraquinone (AQ) molecules in the redox reaction, the double electrode molecular junction is constructed. The effects of redox reaction and electrode structure on the switching characteristics of anthraquinone molecular devices are discussed. It is found that the current in the HQ configuration is significantly greater than that in the AQ configuration, regardless of the zigzag-edge graphene electrode or the armchair-edge graphene electrode. That is, under the redox reaction, the anthraquinone molecules show significant switching characteristics. The switching ratio of zigzag-edge graphene electrode is selected to reach a maximum of 3125, and that of armchair-edge graphene electrode is selected to maximum of 1538. In addition, when the armchair-edge graphene is used as an electrode in the HQ configuration, the negative differential resistance is obviously between 0.7 and 0.9 V.

Electronic transport induced by doping on the electrodes in molecular devices

The electronic transport properties of molecular device based on photochromic diarylethene with carbon nanotube electrode are investigated by density functional theory and non-equilibrium Green’s function. The devices with open and closed configurations show a switching effect. It is found that doping of different amounts of nitrogen atoms on left electrodes results in different electronic transport properties. In addition, we discuss the observed oscillation of current in the devices induced by doping using transmission eigenstates and transmission spectra of the device. The local density of states of the device is calculated to analyze the observed rectifying behavior. The results suggest that doping of nitrogen atoms on the left electrode can be considered as a factor to modulate the electronic transport properties of molecular device.

Optimizing the conductance switching performance in photoswitchable dimethyldihydropyrene/cyclophanediene single-molecule junctions

Designing molecular switches with high stability and performance is still a great challenge in the field of molecular electronics. For this aim, key factors influencing the charge transport properties of molecular devices require to be carefully addressed. Here, by using the nonequilibrium Green's function method in combination with the density functional theory, effect of bridging manner (i.e., linkage sites) for dimethyldihydropyrene (DHP)/cyclophanediene (CPD) photoswitchable molecule sandwiched between two Au(111) electrodes has been theoretically investigated. The computational results show that the fully conjugated DHP is more conductive than the less conjugated CPD, manifesting an evident switching effect. It is also found that the ON/OFF switching performance of DHP/CPD is dramatically modulated by the bridging manner. Further analysis attributes the switching feature to the different alignments of conducting HOMO with the Fermi energy for DHP and CPD. And it also reveals that local bond contributions to the electron transmission pathway are closely related to both the conjugation characteristics and bridging manner of the core molecule. This work suggests that the bridging manner of core molecule in single-molecule junction plays an essential role in determining the switching performance and should be carefully addressed in future design of molecular switches.

High-performance spin rectification in gallium nitride-based molecular junctions with asymmetric edge passivation

The spin transport properties of molecular devices constructed from zigzag gallium nitride nanoribbons (ZGaNNRs) are investigated by applying the non-equilibrium Green’s function formalism in combination with density functional theory. The computational results indicate that ZGaNNR systems show spin rectification with a high efficiency, approaching nearly 109, giant magnetoresistance with a ratio up to 108, perfect spin-filtering, and negative differential resistance effects. Importantly, our results reveal that intrinsic rectification can be observed regardless of their width. The microscopic origins of the rectification are revealed and discussed in terms of a spin-resolved transmission spectrum, the band structures of the ZGaNNRs, and the molecular projected self-consistent Hamiltonian. Our findings could be useful for designing GaN-based spintronic nanodevices.

Effect of Anchoring Groups on the Conduction Properties of Terphenyl Molecules Connected to Copper Leads

We report equilibrium and non-equilibrium conductance of terphenyl molecules with different anchoring groups including sulfur and nitrogen atom. The corresponding molecules are terphenyl-dithiols (TPDT) and diamino-terphenyl (DATP). The non-equilibrium Green's function (NEGF) technique has been implemented on the density functional tight-binding (DFTB) code to perform computations of the electronic transport properties of molecular devices. The NEGFs are used to compute the electronic density self-consistently with open boundary condition naturally encountered in transport problem which is imposed by the potentials at the contacts. As result, the value of the molecular conductance with amine groups is higher about ten times than the thiol anchored group.

Effect of electrode position and cross section size on transport properties of molecular devices

Many investigations indicate that molecular electronics opens up possibilities for continually miniaturizing the electronic devices beyond the limits of the standard silicon-based technologies. There have been significant experimental and theoretical efforts to build molecular junctions and to study their transport properties. The electron transport in molecular device shows clearly quantum effect, and the transport property for molecular device would be strongly affected by chemical and structural details, including the contact position and method between molecule and electrodes, the angle between two electrodes connecting to the molecule. Till now, the micro-fabrication technology still does not guarantee metal electrodes contacting the molecules surfaces ideally. During molecular device fabrication, any tiny variations for the contact configuration usually exist in the molecular device, which would change the device transport property. Hence, it is necessary to investigate the effects of electrode position and electrode cross section size on the transport property.We take Au-benzene-1, 4-dithiol (BDT)-Au (Au-BDT-Au) molecular junctions as example, and systematically calculate its transport properties with various contact positions, and several electrode cross section sizes. The contact face for Au electrode is set to be the (001) face. In the calculations, the density functional theory combined with the Keldysh non-equilibrium Green's function formalism is utilized. The local density approximation is selected as an exchange correlation potential, and atomic core is determined by the standard norm conserving nonlocal pseudo-potential.Our investigations show that the relative position between the electrodes plays a crucial role in the transport behavior of Au-BDT-Au device. When both electrodes are set to be at the counter-position, the preferable transport behavior could be found. The counter-position indicates that the two electrodes are on the same line, which is beneficial to the fabrication. As the angle, which is defined as the angle of electrode deviating from the axis, is larger than five degrees, the transport behavior deteriorates. Hence, the angle for the electrode deviating from its axis should be less than five degrees. To study the effect of electrode cross section size, we calculate the transport properties for three electrode cross sections, i.e. 3×4, 4×4 and 5×4 supercell. Our calculations indicate that when electrode cross section is less than 4×4, the transmission, near the Fermi level, is discontinuous, which would deteriorate the transport performance. Hence, the section size of electrode should not be less than 4×4. This research will provide a scientific index for the electrode position and its cross section size during the fabrication.

The electronic transport properties of B40 fullerenes with chalcogens as anchor atoms.

Fullerenes are the most popular molecules to use in applications related to molecular electronics because of their superconductive nature. These molecules show a diverse range of properties, including optical, electronic, and structural characteristics. In this work, we focused on the electronic transport properties of molecular devices consisting of the fullerene B40 or B40 with different anchor atoms between two gold electrodes in a two-probe configuration. The elements used as anchor atoms in the B40 molecules were oxygen, selenium, and sulfur, i.e., chalcogens. The current characteristics of these fullerene-based molecular devices were calculated and analyzed. The analysis highlighted the superior electrical conductivity of the pure B40 device compared to the devices based on its chalcogen-anchored variants. The conductivities of the molecular devices were ranked as follows: pure B40 > selenium-anchored > sulfur-anchored > oxygen-anchored B40. It was also noted that the devices based on B40 and its chalcogen-anchored variants gave nonzero conductance values at zero bias. These results pave the way for the application of these molecules in future nanodevices utilizing extremely small bias voltages.

Effect of length and negative differential resistance behavior in conjugated molecular wire tetrathiafulvalene devices

The electronic transport properties of molecular devices constructed by conjugated molecular wire tetrathiafulvalene (TTF) have been studied by applying nonequilibrium Green’s functions in combination with the density-functional theory. Two molecular junctions with different wire lengths have been considered. The results show that the current–voltage curves of TTF devices can be modulated by the length of the molecular wire and negative differential resistance behaviors are observed in these systems. The mechanisms have been proposed for the length effect and negative differential resistance behavior.

Designing multi-functional devices based on two benzene rings molecule modulated with Co and N atoms

We investigate the electronic transport properties in molecular devices consisting of two phenyl-rings connected by Co and N atoms by using nonequilibrium Green’s function method and density function theory. The molecule is coupled to two zigzag graphene nanoribbon electrodes. It is found that giant magnetoresistance effect exists in both the coplanar and the perpendicular conformations at low biases. And the total current decreases a lot, on order of 100, when the orientation of the phenyl ring changes from coplanar site to perpendicular site. This indicates a molecular switcher via conformational control. What’s more, perfect dual spin-filtering effect and rectifying behavior can be realized by modulating the external magnetic field. So a multi-functional device is achieved in our designed molecular junction. Detailed explanations via transmission spectra, distribution of molecular projected self-consistent Hamiltonian states are given to the above useful phenomenon.

Giant magnetoresistance effect and spin filters in phthalocyanine-based molecular devices

The spin transport properties of molecular devices constructed by hydrogen–phthalocyanine and transition metal (TM)–phthalocyanine molecule with zigzag graphene nanoribbon electrodes are investigated by the Keldysh nonequilibrium Green’s function method in combination with the density functional theory. The results show that there exists giant magnetoresistance in both the hydrogen–phthalocyanine and the TM-phthalocyanine systems. The magnetoresistance ratio is much bigger than that found by Schmaus et al. [S. Schmaus, A. Bagrets, Y. Nahas, T. K. Yamada, A. Bork, M. Bowen, E. Beaurepaire, F. Evers, W. Wulfhekel, Nature Nanotechnology 6 (2011) 185–9] in single hydrogen–phthalocyanine-Co electrodes system. Moreover, it is found that the chromium–phthalocyanine molecular device is a good spin filtering device with nearly 100% spin filtering efficiency at a wide bias voltage region. The mechanisms are proposed for these phenomena.

Anchor position and donor/acceptor effects on transport properties in fused benzene-substituted oligothiophene molecular device

Utilizing non-equilibrium Green's functions in combination with density-functional theory, the transport properties of molecular devices based on fused benzene-substituted oligothiophenes were explored theoretically. The results show that the positions of anchor group and push–pull substituent groups can affect the conduction properties significantly, and the favorite current direction of the investigated devices can be tuned by applied bias. The first-principle calculations demonstrate that the forward current is favorable under low bias due to the asymmetric shift of transmission peaks of those that are derived from the resonant tunneling through HOMO state, while the asymmetric localization of HOMO states under high bias is beneficial to the reverse current.

The effect of asymmetrical electrode on the transport properties of molecular devices

By applying nonequilibrium Green's functions in combination with the density functional theory, we have investigated the electronic transport properties of molecular devices consisting of the carbon atomic chain coupling with symmetry and asymmetry Au electrodes. The asymmetry Au electrodes systems display good rectifying behavior. The main origin of this phenomenon is that a molecular core coupling with asymmetry electrodes can generate two asymmetrical Schottky barriers at both extended molecule regions. This rectification is also explained by the calculated transmission spectrum and the spatial distribution of the LUMO and HOMO states.

Effect of edge passivation on electronic and transport properties of carbon nanotube-based molecular devices

The electronic and transport properties of the molecular devices of finite-sized carbon nanotubes (CNTs) have been studied by combining the density-functional theory and Green's function method. Different edge-passivated types are considered for the edge carbon atoms at the two open ends. It is shown that the electronic and transport properties of the finite-size CNTs are sensitive to the passivation types of edge atoms. An asymmetry passivation method which can tune the spin density of states of CNTs has been proposed. The electron transmission can be manipulated by modifying the CNTs, such as edge passivation types and concentrations.

Effects of partial hydrogenation on electronic transport properties in C60 molecular devices

By using nonequilibrium Green's functions in combination with the density–function theory, we investigate electronic transport properties of molecular devices with pristine and partial hydrogenation. The calculated results show that the electronic transport properties of molecular devices can be modulated by partial hydrogenation. Interestingly, our results exhibit negative differential resistance behavior in the case of the imbalance H-adsorption in C60 molecular devices under low bias. However, negative differential resistance behavior cannot be observed in the case of the balance H-adsorption. A mechanism is proposed for the hydrogenation and negative differential resistance behavior.

Negative differential resistance behavior in doped C82 molecular devices

By using the first-principle calculations and nonequilibrium Green functions method, the electronic transport properties of molecular devices constructed by C82, C80BN and C80N2 were studied. The results show that the electronic transport properties of molecular devices are affected by doped atoms. Negative differential resistance (NDR) behavior can be observed in certain bias regions for C82 and C80BN molecular devices but cannot be observed for C80N2 molecular device. A mechanism for the negative differential resistance behavior was suggested.

Construction and Conductance Measurement of Single Molecule Junctions

Molecular electronics has become an important research field in the past decade, and molecular devices can be used as molecular wires, switches, rectifiers, and transistors etc. Construction of metal/molecule/metal (MMM) junctions is the most effective method for investigating the charge transport properties of molecular devices. However, the measurement of individual molecule junctions at the nanoscale is still very challenging because of many technical difficulties. This paper reviews the recent progress and the challenges in the measurement of single molecule conductance, and summarizes investigation of the charge transport mechanism.