Gui-Chao Hu

Stretch or contraction induced inversion of rectification in diblock molecular junctions

Based on ab initio theory and nonequilibrium Greens function method, the effect of stretch or contraction on the rectification in diblock co-oligomer molecular diodes is investigated theoretically. Interestingly, an inversion of rectifying direction induced by stretching or contracting the molecular junctions, which is closely related to the number of the pyrimidinyl-phenyl units, is proposed. The analysis of the molecular projected self-consistent Hamiltonian and the evolution of the frontier molecular orbitals as well as transmission coefficients under external biases gives an inside view of the observed results. It reveals that the asymmetric molecular level shift and asymmetric evolution of orbital wave functions under biases are competitive mechanisms for rectification. The stretching or contracting induced inversion of the rectification is due to the conversion of the dominant mechanism. This work suggests a feasible technique to manipulate the rectification performance in molecular diodes by use of the mechanically controllable method.

Length-dependent inversion of rectification in diblock co-oligomer diodes

The rectifying direction of diblock co-oligomer molecular diodes is investigated theoretically by analyzing the asymmetric bias effects on the molecular orbitals. The results reveal two competitive mechanisms in determining the rectifying direction, asymmetric energy shift of eigenstates and asymmetric spatial localization of wave functions upon the reversal of bias voltage. It is demonstrated that the dominated mechanism may be converted between the two mechanisms by changing the molecular length, which induces an inversion of the rectification. This work indicates the relative orientation of the two moieties is not sufficient to decide the rectifying direction of co-oligomer diodes.

Multi-state magnetoresistance in ferromagnet/organic-ferromagnet/ferromagnet junctions

Spin-dependent transport through a ferromagnetic metal/organic-ferromagnet/ferromagnet metal junction is investigated theoretically. It is demonstrated that the current through the device strongly depends on the alignment of the magnetization orientations of the electrodes and interlayer. The spin-related electron tunnelling between the ferromagnetic electrodes suffers a further spin selection induced by the spin-polarized states of the central organic ferromagnet. This work indicates an intriguing prospect of organic ferromagnets in spintronic devices, such as four-state magnetoresistance manipulated by a magnetic field.

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.

Towards Rectifying Performance at the Molecular Scale

Molecular diode, proposed by Mark Ratner and Arieh Aviram in 1974, is the first single-molecule device investigated in molecular electronics. As a fundamental device in an electric circuit, molecular diode has attracted an enduring and extensive focus during the past decades. In this review, the theoretical and experimental progresses of both charge-based and spin-based molecular diodes are summarized. For the charge-based molecular diodes, the rectifying properties originated from asymmetric molecules including D–σ–A, D–π–A, D–A, and σ–π type compounds, asymmetric electrodes, asymmetric nanoribbons, and their combination are analyzed. Correspondingly, the rectification mechanisms are discussed in detail. Furthermore, a series of strategies for modulating rectification performance is figured out. Discussion on concept of molecular spin diode is also involved based on a magnetic co-oligomer. At the same time, the intrinsic mechanism as well as the modulation of the spin-current rectification performance is introduced. Finally, several crucial issues that need to be addressed in the future are given.

Spin-dependent transport and functional design in organic ferromagnetic devices

Organic ferromagnets are intriguing materials in that they combine ferromagnetic and organic properties. Although challenges in their synthesis still remain, the development of organic spintronics has triggered strong interest in high-performance organic ferromagnetic devices. This review first introduces our theory for spin-dependent electron transport through organic ferromagnetic devices, which combines an extended Su–Schrieffer–Heeger model with the Green’s function method. The effects of the intrinsic interactions in the organic ferromagnets, including strong electron–lattice interaction and spin–spin correlation between π-electrons and radicals, are highlighted. Several interesting functional designs of organic ferromagnetic devices are discussed, specifically the concepts of a spin filter, multi-state magnetoresistance, and spin-current rectification. The mechanism of each phenomenon is explained by transmission and orbital analysis. These works show that organic ferromagnets are promising components for spintronic devices that deserve to be designed and examined in future experiments.

Structural and electronic properties of SiC/AlN core/shell nanowires: a first-principles study

The structural stabilities and electronic properties of passivated and unpassivated SiC/AlN core/shell nanowires (CSNWs) along [0001] direction are investigated by using first-principles calculations with density functional theory. Our calculations demonstrate that thick AlN shell and small ratio of SiC core make the SiC/AlN CSNWs more stable. The band gaps decrease with the increasing of the CSNWs diameters. After passivation at the surface, type of SiC/AlN heterostructure changes and the mobility can be improved by increasing the CSNWs diameter and the SiC core ratio. These results provide an effective way to modulate the electronic properties of SiC/AlN structure, which is useful for fabrications and applications of CSNWs.

Density Functional Theory Calculations of Charge-Induced Spin Polarization in Pentacene

Based on density functional theory (DFT) calculations, we investigate the spin-related properties of spinless-hole injected organic molecule pentacene (Pc). DFT calculations reveal that there is spontaneous spin polarization in Pc when spinless-hole is injected. The charge-induced magnetic moment of Pc increases linearly with the increasing of the extra hole charge amount and its maximum can be up to 1 μB per injected spinless-hole per Pc molecule. The magnetic moment is expected due to the injected unpaired charge. The injected hole will preferably fill the spin-splitted carbon p z orbitals, which makes the Pc molecule spin polarize.

Adsorption properties of chloroform molecule on graphene: Experimental and first-principles calculations

Adsorption properties of chloroform molecule (CHCl3) on graphene surface are studied experimentally and theoretically. Based on the density functional theory (DFT) calculations, effects of different adsorption configurations and different adsorption distances on the system’s conductivity properties are discussed, and the comparisons with the experimental results are made. It is found that band gap appears when the adsorption distance is 1.0 A, which is about 0.32 eV near the Fermi level. However, the band gap is nearly zero when the adsorption distance is increased to 1.5 A, so the conductivity of the system will be increased with the increasing of the adsorption distances. The density of states, the adsorption energy and the effective masses are also calculated and the analyses are consistent with the experimental results. Our results reveal that graphene could be used to build sensors or as a catalyst for molecular adsorption.

Spin polarization properties at the spinterface of thiophene/Fe(100): First principles calculations

Based on density functional theory (DFT), the spin polarization properties of a thiophene molecule which is adsorbed at Fe (100) surface are discussed. A variety of horizontal and vertical adsorption configurations as well as their influences on the spin density distributions are studied in detail. The spin polarization comes from the p-d orbital coupling between the thiophene molecule and the electrode, which leads to the molecules’ energy level shifting and the density of states (DOS) broadening, so the two spin states near the Fermi level are exchange split. It is also found that the interfacial spin polarization is different under different contact configurations, and the biggest one will be obtained when the S atom is directly placed above the Fe atom at the horizontal direction. On the other hand, interface spin inversion can be obtained by adjusting the adsorption position, which will be helpful to build spin sensors.