Mapping the first electronic resonances of a Cu phthalocyanine STM tunnel junction

Using a low temperature, ultrahigh vacuum scanning tunneling microscope (STM), dI/dV differential conductance maps were recorded at the tunneling resonance energies for a single Cu phthalocyanine molecule adsorbed on an Au(111) surface. We demonstrated that, contrary to the common assumption, such maps are not representative of the molecular orbital spatial expansion, but rather result from their complex superposition captured by the STM tip apex with a superposition weight which generally does not correspond to the native weight used in the standard Slater determinant basis set. Changes in the molecule conformation on the Au(111) surface further obscure the identification between dI/dV conductance maps and the native molecular orbital electronic probability distribution in space.     

Low-bias negative differential conductance controlled by electrode separation

The electronic transport properties of a single thiolated arylethynylene molecule with 9,10-dihydroanthracene core,denoted as TADHA, is studied by using non-equilibrium Green's function formalism combined with ab initio calculations.The numerical results show that the TADHA molecule exhibits excellent negative differential conductance(NDC) behavior at lower bias regime as probed experimentally. The NDC behavior of TADHA molecule originates from the Stark effect of the applied bias voltage, by which the highest occupied molecular orbital(HOMO) and the HOMO-1 are pulled apart and become localized. The NDC behavior of TADHA molecular system is tunable by changing the electrode distance.Shortening the electrode separation can enhance the NDC effect which is attributed to the possible increase of coupling between the two branches of TADHA molecule.     

Kondo effect of an adsorbed cobalt phthalocyanine (CoPc) molecule: the role of quantum interference

It has been shown that by distorting a CoPc molecule adsorbed on a Au(111) surface a Kondo effect is induced with a temperature higher than 200 K. We examine a model in which an atom with strong Coulomb repulsion (Co) is surrounded by four atoms on a square (molecule lobes), with two atoms above and below it representing the apex of the STM tip and an atom on the gold surface (all with a single atomic orbital). The Hamiltonian is solved exactly for the isolated cluster, and, after connecting the leads, the conductance is calculated by standard techniques. Quantum interference prevents the existence of the Kondo effect when the orbitals on the square do not interact (undistorted molecule); the Kondo resonance shows up after switching on that interaction. The weight of the Kondo resonance is controlled by the interplay of couplings to the STM tip and the gold surface and between the molecule lobes.     

Effect of gold electrode crystallographic orientations on charge transport through aromatic molecular junctions

ABSTRACT

We examined the electrical conduction through single-molecular junctions comprising of anthracenedithiol molecule coupled to two gold electrodes having ?1,0,1?, ?1,1,0? and ?1,1,1? crystallographic orientations. Owing to this jellium model, we evaluated the values of current and conductance using non-equilibrium Green's functions combined with extended Huckel theory. This data was further interpreted in terms of transmission spectra, density of states and their molecular orbital analysis for zero bias. We evinced the oscillating conductance in all three cases, due to the oscillation of orbital energy relative to Fermi level. Our detailed analysis suggested that electrode orientation can tune the molecule–electrode coupling and hence conduction. Anthracene molecular junction with ?1,1,0? orientation displayed favourable conduction, when compared to the other two orientations, thus can provide us an insight while designing futuristic molecular electronic devices.     

六元杂环分子电学特性的理论研究

在第一性原理基础上，利用弹性散射格林函数方法，研究了六元杂环分子结2,5-哒嗪二硫酚 、2,5-吡嗪二硫酚和2,5-嘧啶二硫酚的电子输运特性，分析了终端原子的选取对杂环分子吡 啶电学特性的影响. 利用分子前线轨道理论和微扰方法定量地确定了分子与金属的相互作用 能参数. 计算结果表明，2,5-哒嗪二硫酚具有较好的电学特性，而2,5-嘧啶二硫酚在外加电 压较低时电导值比较小. 对于吡啶分子，选取硒原子作为终端原子时，其导电特性优于分别 以氧原子和硫原子作为终端原子的情况. 关键词： 六元杂环分子 伏安特性 电子输运 分子电子学     

First-principles study of the switching characteristics of the 15,16-dinitrile DDP/CPD-based optical molecular switch with carbon nanotube electrodes

We have studied the switching characteristics of an optical molecular switch based on the 15,16-dinitrile dihydropyrene/cyclophanediene (DDP/CPD) molecule with two single-walled carbon nanotube (SWCNT) electrodes using first-principles transport calculations. It is shown that the DDP shows an overall higher conductance than the CPD at low bias which is independently of the SWCNT chirality. Meantime, the conductance of the molecular switch can be tuned by the chirality of the SWCNT.     

Conductance switching of a phthalocyanine molecule on an insulating surface

We study the electron transport through the double-barrier junction consisted of the phthalocyanine molecule adsorbed on a NaCl bilayer on a metal substrate and the STM tip from first principles. The hydrogen tautomerization reaction happened in the molecule changes the spatial extensions of the molecular π orbitals under the tip, leading to junction conductance switching. Shifting the molecule to locate on different ions also varies the conductance. The transport channels of the tautomers on different adsorbed sites are identified.     

Scanned probe imaging of nanoscale conducting channels in Pt/alkanoic acid monolayer/Ti devices

The mechanisms responsible for switching in metal/molecule/metal systems are subjects of intense research. We report here a scanned probe technique that maps the conductance of a planar molecular junction (Pt/stearic acid monolayer/Ti) under mechanical perturbation by using an atomic force microscope (AFM) to apply a localized force to a molecular junction while measuring the junction conductance. Such mechano-conductance maps reveal that transport through the molecular device is dominated by nanoscale conducting channels, which emerged or disappeared when the junction is switched into higher or lower conductance states. A quantitative model that combines quantum tunneling with the growth of nano-asperities effectively describes the experimental conductance data across a wide range of device conductance. PACS 68.37.Ps; 73.23.-b; 73.63.-b; 81.07.Pr; 85.65.+h     

Investigating molecular charge transfer complexes with a low temperature scanning tunneling microscope

Electron donor-acceptor molecular charge transfer complexes (CTCs) formed by alpha-sexithiophene (6T) and tetrafluoro-tetracyano-quinodimethane (F4TCNQ) on a Au(111) surface are investigated by scanning tunneling microscopy, spectroscopy, and spectroscopic imaging at 6 K. New hybrid molecular orbitals are formed in the CTCs, and the highest occupied molecular orbital of the CTC is mainly located on the electron accepting F4TCNQ while the lowest unoccupied molecular orbital is predominantly positioned on the electron donating 6T. We observed the conductance switching of F4TCNQ inside CTCs, which may find potential applications in novel molecular device operations.     

Retrieving angular distributions of high-order harmonic generation from a single molecule

We present a novel method to retrieve angular distributions of high-order harmonic generation from a single molecule. This technique uses an iterative procedure based only on experimental results of time and angle-dependent harmonic signals, and no actual shape of molecular orbital is assumed. The molecular axis distribution in a target gas can simultaneously be deduced in this procedure. The angle-dependent signal retrieved for a single N2 and O2 molecule is demonstrated to reflect the highest occupied molecular orbital, excluding the ambiguity due to the imperfect alignment.     

Cross polarization effect of donor–acceptor group on a potential single‐molecule transistor

Theoretical investigation of the polarization effect on a potential single‐molecule transistor has been studied with density functional theory. 4,4′‐(2‐Amino‐5‐nitro‐1,4‐phenylene)bis(ethyne‐2,1‐diyl) dibenzenethiol (AN‐OPE), containing a donor and an acceptor (D–A) crossed to its oligo(p‐phenylene‐ethynylene) (OPE) backbone, was used as a prototype for this study. Simulation results indicate that AN‐OPE has a higher on/off ratio on conductance than OPE because of the larger polarization along the D–A direction. This high on/off ratio was proved by the 20 times variation in molecular charge, 15 times variation in bond lengths, 49 times variation in polarizability, 9 times variation in the rotation angles, and 13 times variation in the highest occupied molecular orbital–lowest unoccupied molecular orbital gaps under the same gate using B3LYP/6‐31G (d, p). And results imply that conjugated molecules with a cross D–A structure could be a good direction for constructing a better single‐molecule field‐effect transistor. Copyright © 2014 John Wiley & Sons, Ltd.     

Study on the electronic transport properties of the C14 monocyclic ring: The first-principles calculations

The transport properties of C₁₄ monocyclic ring sandwiched between two Al(1 0 0) electrodes are investigated by first-principle calculations. The variation of the equilibrium conductance as the function of the separation distance between the molecule and the electrodes is studied. C₁₄ monocyclic ring shows metallic behavior according to the calculated equilibrium conductance. Electron transmission occurs through the lowest unoccupied molecular orbital (LUMO). With gate-voltage applied, it is found that the positive and negative gate-voltages can bring very different effect on the variation of equilibrium conductance. We also calculate the effects of adsorbing other atoms on the carbon ring such as oxygen and sulfur atoms. The results indicate that adsorption of this kind of electron-accepting impurity will decrease the conductance of the system.     

First-principles study on current through a single π conjugate molecule for analysis of carrier injection through an organic/metal interface

We have studied current flow through a single π conjugate molecule weakly adsorbed to, and sandwiched between, two electrodes, focusing specifically on carrier injection through an organic/metal interface. This is the first calculation to investigate the effects of the orientation of the molecule and the electrode material on current using a first-principles method: in the past, most calculations of current were based on the assumption of covalent bonding of a molecule to electrodes. We modeled two systems in which a naphthalene molecule is sandwiched between gold (Au) or aluminum (Al) electrodes. First, in both systems, the current through the molecule depends on the orientation of the molecule. This indicates that electrons mainly transfer through the π channel, which is the overlap between the molecular π orbital and the electrode orbital. Next, the current in the Au-naphthalene-Au system is higher than that in the Al-naphthalene-Al system. This shows that Au is more suitable as an electrode material than Al. Therefore, the orientation of the molecule at an organic/metal interface and the materials comprising the electrodes play a key role in carrier injection through the interface.     

Single C59N molecule as a molecular rectifier

We report a new kind of experimental realization of a molecular rectifier, which is based on a single azafullerene C59N molecule in a double-barrier tunnel junction via the single electron tunneling effect. An obvious rectifying effect is observed. The positive onset voltage is about 0.5-0.7 V, while the negative onset voltage is about 1.6-1.8 V. Theoretical analyses show that the half-occupied molecular orbital of the C59N molecule and the asymmetric shift of the molecular Fermi level when the molecule is charged are responsible for the molecular rectification.     

Current spin polarization of a platform molecule with compression effect

Using the first-principles method, the spin-dependent transport properties of a novel platform molecule containing a freestanding molecular wire is investigated by simulating the spin-polarized scanning tunneling microscope experiment with Ni tip and Au substrate electrodes. Transport calculations show that the total current increases as the tip gradually approaches to the substrate, which is consistent with the conductance obtained from previous experiment. More interestingly, the spin polarization (SP) of current modulated by compression effect has the completely opposite trend to the total current. Transmission analyses reveal that the reduction of SP of current with compression process originates from the promotion of spin-down electron channel, which is controlled by deforming the molecule wire. In addition, the density of states shows that the SP of current is directly affected by the organic-ferromagnetic spinterface. The weak orbital hybridization between the Ni tip and propynyl of molecule results in high interfacial SP, whereas the breaking of the C $\equiv$ C triple of propynyl in favor of the Ni-C-C bond induces the strong orbital hybridization and restrains the interfacial SP. This work proposes a new way to control and design the SP of current through organic-ferromagnetic spinterface using functional molecular platform.     

Gold–sulfur bond breaking in Zn(II) tetraphenylporphyrin molecular junctions

Quantized conductance measured from single tetraphenylporphyrin molecules with and without a central zinc(II) [Zn(II)] atom was measured using a molecular break junction (MBJ) method. From the conductance histograms we observed an additional 1.7 Å stretch for two-state conductance in a single Zn(II) tetraphenylporphyrin (ZnTPP) molecule as compared to single state conductance in a free tetraphenylporphyrin (TPP) molecule, i.e., no central Zn(II) atom. First-principles density functional calculations, using an electrode–molecule–electrode model, are completed to provide insight into the mechanisms attributed to bond stretching, and eventual bond breaking, to better understand the additional 1.7 Å of stretching observed with ZnTPP.     

Anisotropic Stark effect of carbon monoxide: emergent orbital cooperativity

ABSTRACT

Applying external electric fields to molecules gives rise to spectral shifting and splitting, a phenomenon known as the Stark effect. However, a fundamental question of how electronic structures of molecules are modified by electric fields is still not well understood. By applying electric fields to a carbon monoxide molecule, herein we have successfully addressed the fundamental question at orbital scales and discovered that the Stark effect exhibits anisotropic characters depending on the direction of the electric fields with respect to the molecular axis. Based on the fact that applying electric fields along the molecular axis always preserved the orthogonality between the sigma and pi electrons, we found that orbital resemblance-based cooperativity can only operate within either the sigma system in which sigma electrons somehow prefer to resemble each other or the pi electron system in which the 1π electrons experience polarization-based self-resemblance. However, switching the electric field vertical to the molecular axis breaks down the orthogonality between the sigma system and pi electron systems, opening up electronic channels that allow σ electron systems to resemble π electrons. Such orbital cooperativity represents a new physical effect beyond the conventional Stark effect. Moreover, we have found that applying electric fields to the molecule would modify its molecular orbital diagram, depending on the directions of the electric fields; the electric field along the carbon-to-oxygen direction basically retains the MO diagram of the free CO molecule, with noticeable intra-orbital electron redistributions, whereas the oxygen-to-carbon electric field does create new states of molecular orbital contributions.     

Quantum transport of the single metallocene molecule

The Quantum transport of three single metallocene molecule is investigated by performing theoretical calculations using the non-equilibrium Green's function method combined with density functional theory. We find that the three metallocen molecules structure become stretched along the transport direction, the distance between two C_p rings longer than the other theory and experiment results. The lager conductance is found in nickelocene molecule, the main transmission channel is the electron coupling between molecule and the electrodes is through the Ni d_xz and d_yz orbitals and the s, d_xz, d_yz of gold. This is also confirmed by the highest occupied molecular orbital resonance at Fermi level. In addition, negative differential resistance effect is found in the ferrocene, cobaltocene molecules, this is also closely related with the evolution of the transmission spectrum under applied bias.     

1,8-二巯基芘分子电学特性的理论研究

在第一性原理的基础上 ,对 1,8 二巯基芘分子的电学特性进行了理论研究 .采用了 3个Au原子构成的团簇来模拟Au表面 .首先利用密度泛函理论计算了 1,8 二巯基芘分子的电子结构及其和Au表面的相互作用 ,再利用前线轨道理论和微扰理论定量地确定了该分子和Au表面的相互作用能常数 .最后利用弹性散射格林函数法研究了该分子结的伏 安特性 .计算结果表明 ,分子中的硫原子和Au原子形成很强的共价键 .当外加偏压小于 1V时分子结存在电流禁区 ,随着偏压升高 ,分子结的电导出现平台结构 .分子结的电导特性和其电子结构密切相关 ,扩展分子轨道为电荷的迁移提供了通道 ,而局域轨道对电流贡献很小     

Switching behavior induced by different substituents of group in single molecular device

We investigate the electronic transport properties of photochromic azobenzene-based molecular devices with Au electrodes using non-equilibrium Green’s function and density functional theory. A reversible switching behavior between cis and trans isomerization is found in the device. In addition, the substituent of ?NH₂ on the right end hydrogen atom of azobenzene molecule reduces the switching ratio of current, consequently the disappearance of switching behavior, while the substituent of ?NO₂ improves the switching ratio of current. We discuss the different electronic transport induced by different substituents through the transmission spectra, localized density of states, molecular projected self-consistent Hamiltonian and transmission pathways. The observed polarization effect under bias is explained by the evolution of molecular projected self-consistent Hamiltonian of LUMO level. The results indicate that the electron-withdrawing group ?NO₂ substituting right terminal hydrogen of azobenzene molecule becomes a candidate for improving the performance of molecular device.