Electronic Transport in Molecular Junction Based on C20 Cages

Choosing closed-ended armchair （5, 5） single-wall carbon nanotubes （CCNTs） as electrodes, we investigate the electron transport properties across an all-carbon molecular junction consisting of C20 molecules suspended between two semi-infinite carbon nanotubes. It is shown that the conductances are quite sensitive to the number of C20 molecules between electrodes for both configuration CF1 and double-bonded models： the conductances of C20 dimers are markedly smaller than those of monomers. The physics is that incident electrons easily pass the C20 molecules and are predominantly scattered at the C20-C20 junctions. Moreover, we study the doping effect of such molecular junction by doping nitrogen atoms substitutionally. The bonding property of the molecular junction with configuration CF1 has been analysed by calculating the Mulliken atomic charges. Our results have revealed that the C atoms in N-doped junctions are more ionic than those in pure-carbon ones, leading to the fact that N-doped junctions have relatively large conductance.     

Hydration effect on the electronic transport properties of oligomeric phenylene ethynylene molecular junctions

A first-principles computational method based on the hybrid density functional theory is developed to simulate the electronic transport properties of oligomeric phenylene ethynylene molecular junctions with H₂O molecules accumulated in the vicinity as recently reported by Na {\it et al.} [\wx{Nanotechnology}{18} 424001 (2007)]. The numerical results show that the hydrogen bonds between the oxygen atoms of the oligomeric phenylene ethynylene molecule and H₂O molecules result in the localisation of the molecular orbitals and lead to the lower transition peaks. The H₂O molecular chains accumulated in the vicinity of the molecular junction can not only change the electronic structure of the molecular junctions, but also open additional electronic transport pathways. The obvious influence of H₂O molecules on the electronic structure of the molecular junction and its electronic transport properties is thus demonstrated.     

Au-Si_(60)-Au分子结电子输运性质的理论计算

运用密度泛函理论对Si60团簇的结构进行几何优化,得到基态结构是一个直径为1.131 nm,平均键长为0.239 nm,分子最低未占据轨道与最高占据轨道能量差即能隙值为0.72 eV,具有C1点群的空心笼状结构.然后把它与两半无限的Au(100)-4×4电极相连构成Au-Si60-Au三明治结构分子结点,运用密度泛函理论结合非平衡格林函数的方法对其电子输运性质进行了第一性原理计算.当两电极的距离为1.74 nm时,分子结点的平衡电导为1.93G0(G0=2e2/h),然后在-2.0—2.0 V的电压范围内,计算了不同电压下的电导与电流,得到其I-V曲线成近线性关系,从分子前线轨道与透射谱分析了Si60分子的电子输运特性,讨论了电荷转移量与电导之间的关系.     

Controlling spin off state by gas molecules adsorption on metal-phthalocyanine molecular junctions and its possibility of gas sensor

Employing Green's function (GF) technique in combination with spin-polarized density functional theory (DFT), we study the electronic structure and magnetic properties of metal phthalocyanine (MPc) (M?=?Mn, Fe, Co, Ni, Cu, Zn) with or without four different gas molecules (NO, CO, O₂ and NO₂) adsorbing on the M atom of MPc molecule. The corresponding stable adsorption structural configurations and transport properties of MPc molecular junctions are also investigated. Our results indicate that the magnetic moment of MPc for M?=?Mn, Fe and Co can be modified by the specific gas molecule adsorption, which is mainly ascribed to competitive relation of HOMO-LUMO Gap and Hund's rules. However, for M?=?Ni, Cu and Zn, it is difficult to detect gas molecule because the interaction of M atom and these gases is most of weak van der Waals interaction. Remarkably, the spin of MPc molecule can be switched to a magnetic off-state by specific gas absorption, giving rise to a potential application on controllable spintronic devices. In addition, CO, NO, O₂ and NO₂ gas molecules can be detected selectively by measuring spin filter efficiency of these MPc molecular junctions. On the basis of our results, MPc (M?=?Mn, Fe, Co) molecular junctions can be considered as a promising nanosensor device to detect individual gas molecules.     

Transport Behaviour of La0.8Sr0.2AlO3 Thin Film on Oxygen Deficient SrTiO3 Substrate

La_0.8Sr_0.2AlO₃ (LSAO) thin films are grown on SrTiO₃ (STO) and MgO substrates by laser molecular beam epitaxy. The LSAO thin film on oxygen deficient STO substrate exhibits metallic behaviour over the temperature range of 80--340K. The optical transmittance spectrum indicates that theLSAO thin films on MgO substrate are insulating at room temperature. The transport properties of LSAO thin films on STO substrates deposited in different oxygen pressure are compared. Our results indicate that oxygen vacancies in STO substrates should be mainly responsible for the transport behaviour of LSAO thin films.     

Performance Improvement of Bulk Heterojunction Organic Photovoltaic Cellby Addition of a Hole Transport Material

A novel photovoltaic cell with an active layer of poly（phenyleneethynylene） （PPE）/C60/N,N＇-diphenyl-N,N＇-di-（m-tolyl）-p-benzidine （TPD） is designed. In the active layer, PPE is the major component; C60 and TPD are the minor ones. Compared with a control BHJ device based on PPE/C60, the short circuit current density Jsc is increased by 1 order of magnitude, and the whole device performance is increased greatly, however the open circuit voltage Voc is largely decreased. The possible mechanism of the improved performance may be as follows： In the PPE/C60/TPD device, PPE, C60, and TPD serve as the energy harvesting material, the electron transport material, and the hole transport material, respectively. As the TPD and C60 are spatially separated by PPE, the charge recombination is effectively retarded.     

CO_2膨胀液体的分子动力学模拟

采用分子动力学方法,模拟了CO2膨胀甲醇体系、CO2膨胀乙醇体系的热力学性质和输运性质,以及对氯硝基苯在CO2膨胀甲醇体系、苯甲腈在CO2膨胀乙醇体系中的扩散性质。CO2膨胀甲醇体系的密度模拟值略高于实验值,而CO2膨胀乙醇体系的密度模拟值与实验值非常接近。模拟结果表明:CO2使甲醇和乙醇溶液的膨胀非常明显,当CO2的摩尔分数达到0.5时,溶液膨胀约100%;得到了CO2、甲醇、乙醇、对氯硝基苯以及苯甲腈的扩散系数,其中对氯硝基苯和苯甲腈在两种膨胀液体中的扩散系数与实验结果接近;通过扩散系数关联了两种膨胀液体的粘度,计算结果与修正的Wilke-Chang方程得到的体系粘度规律一致。     

First-principles calculation on the conductance of ruthenium-quasi cumulene-ruthenium molecular junctions

The conductance of a family of ruthenium-quasi cumulene-ruthenium molecular junctions including different numbers of carbon atoms, both in even numbers and odd numbers, are investigated using a fully self-consistent ab initio approach which combines the non-equilibrium Green’s function formalism with density functional theory. Our calculations demonstrate that although the overall transport properties of the Ru-quasi cumulene-Ru junctions with an even number of carbon atoms are different from those of the junctions with an odd number of carbon atoms, the difference between the corresponding current-voltage (I–V) characteristics of these molecular junctions declines to lesser than 16% when the voltage goes up. In each group, the molecular junctions give a large transmission around the Fermi level since the Ru-C π bonds can extend the π conjugation of the carbon chains into the Ru electrodes, and their I–V characteristics are almost linear and independent of the chain length, illustrating potential applications as conducting molecular wires in future molecular electronic devices and circuits.      

SELFCONSISTENT SEMICLASSICAL CALCULATION WITH EXTENDED SKYRME FORCES AND THE PROPERTIES OF ISOVECTOR GIANT RESONANCES

The properties of isovector giant resonances are discussed with the help of the nuclear ground state density profiles obtained from self consiotent semi-classical calculation.Both the component with ΔT₃=0 and the components with ΔT₃=±1 are considered.The isospin properties of the giant resonances are thereby studied.The results are in good agreement with HF-RPA calculation and experimental evidences.     

Ruthenium oxide and strontium ruthenate electrodes for ferroelectric thin-films capacitors

Metallic ruthenium and ruthenium oxides, such as SrRuO₃ and RuO₂, are potential electrode materials for ferroelectric capacitors. The electrical properties (e.g. leakage currents) of such thin film devices are dependent on the electronic properties of the electrode/ferroelectric junctions and therefore also on the electrode work functions. During growth and processing of film-electrode layer structures the formation of sub-oxides within the electrode is possible, with their work functions being unknown. In order to obtain information for predicting device properties, we have systematically analysed the valence bands and work functions of RuO_x and SrRuO_y thin films with different oxidation states by using photoelectron spectroscopy techniques. The results suggest that Ru⁰ and Ru⁴⁺ ions are present in co-existence at the surfaces of oxygen-deficient polycrystalline films (inhomogeneous oxidation). For both oxygen-deficient materials the work function coincides with that of metallic ruthenium (4.6ǂ.1 eV). Only for fully oxidised ruthenium oxide and strontium ruthenate films (no Ru⁰ present at the surface) is the work function increased to 5.0 or 4.9 eV, respectively. As an example of importance for new dynamic random access memory applications, the junctions of Ba_1-xSr_xTiO₃ with SrRuO_y and RuO_x are discussed.     

Elastic and inelastic electron transport in metal–molecule(s)–metal junctions

An overview of studies on elastic and inelastic electron transport properties of molecular junction devices is presented. The development of the experimental fabrication and characterization of molecular junctions as well as the corresponding theoretical modeling is briefly summarized. The functions of molecular devices are generally governed by the intrinsic structure–property relationships, and strongly affected by various environment factors including temperature, solvent and intermolecular interactions. Those detailed structural and environmental information could be probed by a powerful tool of inelastic electron tunneling spectroscopy, for which the theoretical modeling becomes particularly important. With many successful examples, it is demonstrated that the combination of theoretical simulations and experimental measurements can help not only to understand the electron–phonon interaction, but more importantly also to accurately determine the real configurations of molecules inside the junctions.     

Spin-polarized transport properties of a pyridinium-based molecular spintronics device

By applying a first-principles approach based on non-equilibrium Green's functions combined with density functional theory, the transport properties of a pyridinium-based “radical-π-radical” molecular spintronics device are investigated. The obvious negative differential resistance (NDR) and spin current polarization (SCP) effect, and abnormal magnetoresistance (MR) are obtained. Orbital reconstruction is responsible for novel transport properties such as that the MR increases with bias and then decreases and that the NDR being present for both parallel and antiparallel magnetization configurations, which may have future applications in the field of molecular spintronics.     

Role of inter-tube coupling and quantum interference on electrical transport in carbon nanotube junctions

Due to excellent transport properties, Carbon nanotubes (CNTs) show a lot of promise in sensor and interconnect technology. However, recent studies indicate that the conductance in CNT/CNT junctions are strongly affected by the morphology and orientation between the tubes. For proper utilization of such junctions in the development of CNT based technology, it is essential to study the electronic properties of such junctions. This work presents a theoretical study of the electrical transport properties of metallic Carbon nanotube homo-junctions. The study focuses on discerning the role of inter-tube interactions, quantum interference and scattering on the transport properties on junctions between identical tubes. The electronic structure and transport calculations are conducted with an Extended Hückel Theory-Non Equilibrium Green's Function based model. The calculations indicate conductance to be varying with a changing crossing angle, with maximum conductance corresponding to lattice registry, i.e. parallel configuration between the two tubes. Further calculations for such parallel configurations indicate onset of short and long range oscillations in conductance with respect to changing overlap length. These oscillations are attributed to inter-tube coupling effects owing to changing π orbital overlap, carrier scattering and quantum interference of the incident, transmitted and reflected waves at the inter-tube junction.     

Temperature-Dependent Transport Properties in Oxide p-n Junction above Room Temperature

Oxide p-n junctions ofp-SrIn_0.1Ti_0.9O₃/n-SrNb_0.01Ti_0.99O₃ (SITO/SNTO) are fabricated by laser molecular beam epitaxy. The current-voltage characteristics of the SITO/SNTO p-n junction are investigated mainly in the temperature range of 300--400K. The SITO/SNTO junction exhibited good rectifying behaviour over the wholetemperature range. Our results indicate a possibility of application of oxide p-n junction in higher temperatures in future electronic devices.     

单硫醇分子结的几何结构和电输运性质:压力效应与末端基团效应

利用杂化密度泛函理论,研究了以甲基、醇基、羧基为末端基团的烷烃硫醇分子与金电极形成分子结的过程,得到了分子结的几何结构与外加压力的关系. 并在此基础上,利用弹性散射格林函数方法研究了烷烃硫醇分子的电输运性质. 研究结果表明,对于C₁₁S分子来说,当两电极距离大于2.1 nm时,该分子结断裂;对于C₁₁SOH和C₁₀SCOOH来说,相应的分子结断裂的电极距离基本相同(2.15 nm). 在相同的外加压力(4.0 nN)下,C_{11关键词： 压力 末端基团 烷烃硫醇分子 电输运性质}     

噻吩低聚物分子器件电输运特性的理论研究

利用从头计算方法和弹性散射格林函数理论,对不同噻吩低聚物分子的电输运性质进行理论研究．结果显示,由于分子几何结构对称性的不同使得末端基团跟金电极的连接方式不同,导致了分子与电极间耦合常数以及分子轨道的扩展性不同．出现了同系列的噻吩低聚物分子中较长的分子比较短的分子导电性更好的反常现象.     

Molecular rectification of thiol-linked Au|PTCDI-[CH2]n|Au junctions

The electronic transport properties of the PTCDI-[CH₂]_n(0≤n≤6) molecular junctions with different molecular lengths are theoretically investigated via the first-principles density functional theory (DFT) and non-equilibrium Green's function (NEGF) method. Our results show that the transport properties depend on molecular lengths. The equilibrium conductance of the probed systems decreases exponentially with the increasing number n of the CH₂ unit. With n≥1, the rectifying effect has been found. In the n=6 case, a significant rectification ratio of 72.6 is achieved at the bias of ±2.1 V in our probed voltage range. The rectification effect arises from asymmetric molecular structures. Our results suggest these molecules have great potential application in the molecular-scale device.     

Design and First-principles Study of a Fullerene Molecular Device

By using open-ended armchair （6, 6） single-wall carbon nanotubes as electrodes, we investigate the electron transport properties of an all-carbon molecular junction based on the C82 molecule. We find the most stable system among different isomers by performing structural optimization calculations of the Cs2 isomers and the C82 extended molecules. The calculated results show that the C82 -C2 （3） isomer and the C82 extended molecule with C82-C2 isomer are most stable. For the all-carbon hybrid system consisting of C82-C2 extended molecules, it is shown that the Landauer conductance can be tuned over several orders of magnitude both by changing the distance between two electrodes and by changing the orientation of the C82 molecule or rotating one of the tubes around the symmetry axis of the system at a fixed distance. Also, we find the most stable distance between two electrodes from the total energy curve. This fact could make this all-carbon molecular system a possible candidate for a nanoelectronic switch. Moreover, we interpret the conductance mechanism for such a molecular device.     

The spin-dependent electronic transport properties of M(dcdmp)2 (M = Cu,Au, Co,Ni) molecular devices based on zigzag graphene nanoribbon electrodes

The spin-dependent electronic transport properties of M(dcdmp)₂ (M = Cu, Au, Co, Ni; dcdmp = 2,3-dicyano-5,6-dimercaptopyrazyne) molecular devices based on zigzag graphene nanoribbon (ZGNR) electrodes were investigated by density functional theory combined nonequilibrium Green's function method (DFT-NEGF). Our results show that the spin-dependent transport properties of the M(dcdmp)₂ molecular devices can be controlled by the spin configurations of the ZGNR electrodes, and the central 3d-transition metal atom can introduce a larger magnetism than that of the nonferrous metal one. Moreover, the perfect spin filtering effect, negative differential resistance, rectifying effect and magnetic resistance phenomena can be observed in our proposed M(dcdmp)₂ molecular devices.     

Advanced simulation of conductance histograms validated through channel-sensitive experiments on indium nanojunctions

We demonstrate a self-contained methodology for predicting conductance histograms of atomic and molecular junctions. Fast classical molecular-dynamics simulations are combined with accurate density functional theory calculations predicting both quantum transport properties and molecular-dynamics force field parameters. The methodology is confronted with experiments on atomic-sized indium nanojunctions. Beside conductance histograms the distribution of individual channel transmission eigenvalues is also determined by fitting the superconducting subgap features in the I-V curves. The remarkable agreement in the evolution of the channel transmissions demonstrates that the simulated ruptures are able to reproduce a realistic statistical ensemble of contact configurations, whereas simulations on selected ideal geometries show strong deviations from the experimental observations.