Effects of polarons and bipolarons on spin polarized transport in an organic device

A polaron–polaron interaction model is suggested to study the spin injection and transport in an organic semiconductor (OSC) device. The evolutions of spin polarons and spinless bipolarons are calculated from the drift-diffusion equations, in which both the polaron–bipolaron transition and the spin-flipping of a spin polaron are included. Then the spin polarized current is obtained. It is found that the polarons are responsible for the spin polarized transport in an OSC. Different from the case in a normal inorganic semiconductor, spinless bipolarons will affect the spin polarization of the OSC device. Finally, effects of the spin-flip time and the mobility of the carriers on the spin polarization in an organic device are discussed.     

Charge current polarization and magnetoresistance in ferromagnetic/organic semiconductor/ferromagnetic devices

Spin-polarized injection and transport in ferromagnetic/organic semiconductor/ferromagnetic devices are studied theoretically. Based on the spin diffusion theory and Ohm’s law, we obtain the charge current polarization and the magnetoresistance, which takes into account the special carriers in organic semiconductors. From the calculation, it is found that the charge current polarization decreases exponentially from the ferromagnetic layer into the organic layer and polarons are effective spin carriers in organic semiconductors for polarized charge current. To get an apparent magnetoresistance in an organic device, it is better to adopt a spin-dependent interface, and the thickness of the organic interlayer is much smaller than the spin diffusion length. Spin polarons are effective carriers for gaining remarkable magnetoresistance in ferromagnetic/organic semiconductor/ferromagnetic devices.     

Encoding Information on the Excited State of a Molecular Spin Chain

The quantum states of nano-objects can drive electrical transport properties across lateral and local-probe junctions. This raises the prospect, in a solid-state device, of electrically encoding information at the quantum level using spin-flip excitations between electron spins. However, this electronic state has no defined magnetic orientation and is short-lived. Using a novel vertical nanojunction process, these limitations are overcome and this steady-state capability is experimentally demonstrated in solid-state spintronic devices. The excited quantum state of a spin chain formed by Co phthalocyanine molecules coupled to a ferromagnetic electrode constitutes a distinct magnetic unit endowed with a coercive field. This generates a specific steady-state magnetoresistance trace that is tied to the spin-flip conductance channel, and is opposite in sign to the ground state magnetoresistance term, as expected from spin excitation transition rules. The experimental 5.9 meV thermal energy barrier between the ground and excited spin states is confirmed by density functional theory, in line with macrospin phenomenological modeling of magnetotransport results. This low-voltage control over a spin chain's quantum state and spintronic contribution lay a path for transmitting spin wave-encoded information across molecular layers in devices. It should also stimulate quantum prospects for the antiferromagnetic spintronics and oxides electronics communities.     

Trap effect of triplet excitons on magnetoresistance in organic devices

In an organic bipolar device, injected electrons and holes can form spin singlet and triplet excitons, which are manipulated by an applied magnetic field. We suppose that the localized intra-molecule triplet exciton has a blocking effect on charge carrier transport by assuming that the intra-molecule triplet exciton can increase the on-site binding and make the electron states more localized. By considering the magnetic field-dependent transition between singlet and triplet excitons, from the master equation based on the hopping mechanism, we calculate the magnetoresistance (MR) in organic devices and compare the results with some experimental data. Our research reveals the importance of hyperfine interaction in organic magnetoresistance (OMAR). Especially, our investigation indicates that a bipolar organic device should have a larger MR value than a unipolar one due to the trap effect of triplet excitons on hopping electrons or holes, which is confirmed by some experimental observations.     

Hybrid Interface States and Spin Polarization at Ferromagnetic Metal–Organic Heterojunctions: Interface Engineering for Efficient Spin Injection in Organic Spintronics

Ferromagnetic metal–organic semiconductor (FM‐OSC) hybrid interfaces have been shown to play an important role for spin injection in organic spintronics. Here, 11,11,12,12‐tetracyanonaptho‐2,6‐quinodimethane (TNAP) is introduced as an interfacial layer in Co‐OSCs heterojunctions with an aim to tune the spin injection. The Co/TNAP interface is investigated by use of X‐ray and ultraviolet photoelectron spectroscopy (XPS/UPS), near edge X‐ray absorption fine structure (NEXAFS) and X‐ray magnetic circular dichroism (XMCD). Hybrid interface states (HIS) are observed at Co/TNAP interfaces, resulting from chemical interactions between Co and TNAP. The energy level alignment at the Co/TNAP/OSCs interface is also obtained, and a reduction of the hole injection barrier is demonstrated. XMCD results confirm sizeable spin polarization at the Co/TNAP hybrid interface.     

Effect of spin excited states on electron transport through an organic ferromagnetic device

Spin excited states in an organic ferromagnet are proposed and investigated on the basis of an extended SSH + Heisenberg (SSH = Su-Schrieffer-Heeger) model. It is found that a spin excited state will form a local distortion of the spin density wave (SDW) of π-electrons while the lattice configuration of main chain has no obvious change. Then the spin-polarized transport properties through an organic ferromagnetic device are investigated with the Landauer-Büttiker formula and Green’s function method. It is obtained that the current will be spin polarized due to the existence of SDW in the ferromagnetic molecule. Both the total current and the spin-polarized current will be modulated when the SDW is excited. The total current through the device is suppressed by the spin excitation of side radicals, through which a conductance switch function may be realized. Compared with ground state, the spin polarization has no obvious change in a low spin excited state and the device still has spin-filter function. Finally, spin excitations induced by temperature is studied and we find that an organic ferromagnetic device can hold a high spin polarization when temperature is not too high.     

Spontaneous spin polarization in organic thiophene oligomers

We studied the spin polarization phenomenon of injected charges in organic thiophene oligomer by using extended Su–Schrieffer–Heeger (SSH) model including electron–electron interaction, spin–orbit coupling as well as spin-flip effect. Our simulation shows that a charged carrier is spontaneously spin polarized, which has a lower energy than the non-polarized one. This polarization is related with the amount of injected charges and the polymerization of the molecule.     

Designing cross-linked carbon nanotubes as perfect spin filter and spin valve

By applying nonequilibrium Green’s functions in combination with density-function theory, the spin-dependent transport properties of cross-linked carbon nanotube spintronic devices are investigated. Our calculations show that the perfect spin filtering effect with the almost 100% spin polarization, and the magnetoresistance effect with a magnetoresistance ratio larger than 10⁴${10}^4$% can be observed in the device. The occurrence of the perfect spin-filtering and magnetoresistance effects in the cross-linked carbon nanotube spintronic device provides the possibility for further improving the integration level of carbon nanotube networks. Moreover, the mechanisms for these interesting phenomena are suggested.     

Large Annular Dipoles Bounded between Single-Atom Co and Co Cluster for Clarifying Electromagnetic Wave Absorbing Mechanism

It is very challenging to demonstrate the intrinsic feature and absorption mechanism for electromagnetic (EM) wave absorber since dipole polarization loss is always discussed together with magnetic loss, conductive loss, defects/interfacial polarization, and so on. To address this issue, here, a kind of atomic composites is reported, including single-atom Co and Co cluster with controllable atom dipole to tune the polarization and establish the link between dipole polarization and the EM wave absorption. Using a chemical synthesis route, the atomic composites are fabricated, including Co single-atom (SA) sites and cluster (Cs) on nitrogen-doped graphitic carbon (Co_1+Cs/NGC). Due to the special design, the effect of magnetic loss, conductive loss, and interfacial polarization on EM wave dissipation can be ignored so that it can only highlight dielectric loss caused by dipole polarization. And, by controlling the Co atoms concentration, it can tune the valence state of Co atoms between 0 to +2 to control dipole polarization and relaxation. As a result, the Co_1+Cs/NGC-2 with Co concentration of 6.0 wt% exhibits optimized dipole moments and thus excellent absorption performance (the reflection loss exceeds −54.3 dB, and the effective absorption bandwidth with RL ≤−10 dB reaches 7.0 GHz at 2.0 mm) due to the effective dipole polarization caused by the large annular dipole bounded between Co SA sites and Co Cs. This study proposes a simplified model to clarify EM wave absorption mechanism from atom view.     

Tuning of a Vertical Spin Valve with a Monolayer of Single Molecule Magnets

The synthesis and the chemisorption from solution of a terbium bis‐phthalocyaninato complex suitable for the functionalization of lanthanum strontium manganite (LSMO) are reported. Two phosphonate groups are introduced in the double decker structure in order to allow the grafting to the ferromagnetic substrate actively used as injection electrode in organic spin valve devices. The covalent bonding of functionalized terbium bis‐phthalocyaninato system on LSMO surface preserves its molecular properties at the nanoscale. X‐ray photoelectron spectroscopy confirms the integrity of the molecules on the LSMO surface and a small magnetic hysteresis reminiscent of the typical single molecule magnet behavior of this system is detected on surface by X‐ray magnetic circular dichroism experiments. The effect of the hybrid magnetic electrode on spin polarized injection is investigated in vertical organic spin valve devices and compared to the behavior of similar spin valves embedding a single diamagnetic layer of alkyl phosphonate molecules analogously chemisorbed on LSMO. Magnetoresistance experiments have evidenced significant alterations of the magneto‐transport by the terbium bis‐phthalocyaninato complex characterized by two distinct temperature regimes, below and above 50 K, respectively.     

Manganite/Alq3 interfaces investigated by impedance spectroscopy technique

With the general objective of studying interfaces between ferromagnetic materials and organic semiconductors, we report ac impedance investigations on La_0.7Sr_0.3MnO₃ (LSMO)/tris(8-hydroxyquinoline)aluminum (Alq3)/Al and Indium Tin Oxide (ITO)/Alq3/Al heterostructures, in the frequency range between 20 Hz and 1 MHz. The comparison of the equivalent circuits deduced to fit the experimental ac responses allows isolating a specific RC contribution which can be attributed to the LSMO/Alq3 interface region. Using the information obtained from our ac measurements, we propose a model which fits the temperature dependence of the magnetoresistance in spin valves combining LSMO electrodes and Alq3 layers.     

Preparation and assessment of reliable organic spin valves

The long spin relaxation time of organic semiconductors up to millisecond level make organic spintronics an emerging and promising field, which has attracted wide range of interests since the birth of the first spin valve. Organic spin valve with sandwich structure is the typical device for spintronic study, and it is generally the vertical structure. During the device fabrication process, the organic semiconductor spacer is easily to be penetrated by top ferromagnetic metal atoms with high energy, and thus causes the formation of ill-defined layer at the interface between the top electrode and organic spacer. The ill-dedfined layer commonly includes filaments or pinholes, which will seriously affect the spin transport performance and spin-related multifunctional exploration. Therefore, reliable organic spin valve charactered by filament- and pinhole-free at the interface is strongly required. So far, a series of reliable spin valve preparation methods have been developed. In this review, following a simply introduction of spin valve, the advances of reliable spin valve preparation methods have been systematically discussed. Subsequently, the assessment approaches of reliable spin valve are presented. Finally, the challenges and perspectives towards the preparation and assessment of reliable spin valve are outlined to clarify the future efforts needed in this area.     

Energy level alignment and interactive spin polarization at organic/ferromagnetic metal interfaces for organic spintronics

Energy level alignment and spin polarization at tetracyanoquinodimethane/Fe and acridine orange base/Fe interfaces are investigated by means of photoelectron spectroscopy and X-ray magnetic circular dichroism (XMCD), respectively, to explore their potential application in organic spintronics. Interface dipoles are observed at both hybrid interfaces, and the work function of Fe is increased by 0.7 eV for the tetracyanoquinodimethane (TCNQ) case, while it is decreased by 1.2 eV for the acridine orange base (AOB) case. According to XMCD results, TCNQ molecule has little influence on the spin polarization of Fe surface. In contrast, AOB molecule reduces the interfacial spin polarization of Fe significantly. Induced spin polarization of the two organic molecules at the interfaces is not observed. The results reveal the necessity of investigating the magnetic property changes of both the OSC and the FM during the process of energy level alignment engineering.     

Vertical Phase Separation in Small Molecule:Polymer Blend Organic Thin Film Transistors Can Be Dynamically Controlled

Blending of small‐molecule organic semiconductors (OSCs) with amorphous polymers is known to yield high performance organic thin film transistors (OTFTs). Vertical stratification of the OSC and polymer binder into well‐defined layers is crucial in such systems and their vertical order determines whether the coating is compatible with a top and/or a bottom gate OTFT configuration. Here, we investigate the formation of blends prepared via spin‐coating in conditions which yield bilayer and trilayer stratifications. We use a combination of in situ experimental and computational tools to study the competing effects of formulation thermodynamics and process kinetics in mediating the final vertical stratification. It is shown that trilayer stratification (OSC/polymer/OSC) is the thermodynamically favored configuration and that formation of the buried OSC layer can be kinetically inhibited in certain conditions of spin‐coating, resulting in a bilayer stack instead. The analysis reveals here that preferential loss of the OSC, combined with early aggregation of the polymer phase due to rapid drying, inhibit the formation of the buried OSC layer. The fluid dynamics and drying kinetics are then moderated during spin‐coating to promote trilayer stratification with a high quality buried OSC layer which yields unusually high mobility >2 cm² V^?1 s^?1 in the bottom‐gate top‐contact configuration.     

Interface‐Assisted Sign Inversion of Magnetoresistance in Spin Valves Based on Novel Lanthanide Quinoline Molecules

Molecules are proposed to be an efficient medium to host spin‐polarized carriers, due to their weak spin relaxation mechanisms. While relatively long spin lifetimes are measured in molecular devices, the most promising route toward device functionalization is to use the chemical versatility of molecules to achieve a deterministic control and manipulation of the electron spin. Here, by combining magnetotransport experiments with element‐specific X‐ray absorption spectroscopy, this study shows the ability of molecules to modify spin‐dependent properties at the interface level via metal–molecule hybridization pathways. In particular, it is described how the formation of hybrid states determines the spin polarization at the relevant spin valve interfaces, allowing the control of macroscopic device parameters such as the sign and magnitude of the magnetoresistance. These results consolidate the application of the spinterface concept in a fully functional device platform.     

磁性电极作为衬底的有机电致发光器件

有机电致发光器件(Organic Light-Emitting Device, OLED)已成为当今最热门的研究领域之一。以钛酸锶(100)作为基底, 采用RF磁控溅射镀膜系统制成磁性电极La1-xSrxMnO3(LSMO)薄膜, 为了增加钛酸锶基底LSMO薄膜的透光率, 对该基底进行了双面光学抛光。在此基础上, 以LSMO为衬底, 制作了结构为LSMO/NPB/Alq3/CsF/Mg:Ag的有机电致发光器件。器件大约在14 V时启亮, 在25 V时, 器件达到最大亮度。在磁场作用下, 研究了器件的亮度-电压-电流特性。在大约150 mT磁场下, 器件的发光亮度增大10%。研究结果表明: 由于经LSMO注入发光层内部的电子和空穴自旋方向被部分极化, 发光层单线态与三线态激子的形成比率增加。由于发光材料Alq3是单线态有机材料, 因而, 器件发光亮度增大。     

Spin Filtering through Single‐Wall Carbon Nanotubes Functionalized with Single‐Stranded DNA

High spin polarization materials or spin filters are key components in spintronics, a niche subfield of electronics where carrier spins play a functional role. Carrier transmission through these materials is “spin selective,” that is, these materials are able to discriminate between “up” and “down” spins. Common spin filters include transition metal ferromagnets and their alloys, with typical spin selectivity (or, polarization) of ≈50% or less. Here carrier transport is considered in an archetypical one‐dimensional molecular hybrid in which a single wall carbon nanotube (SWCNT) is wrapped around by single stranded deoxyribonucleic acid (ssDNA). By magnetoresistance measurements it is shown that this system can act as a spin filter with maximum spin polarization approaching ≈74% at low temperatures, significantly larger than transition metals under comparable conditions. Inversion asymmetric helicoidal potential of the charged ssDNA backbone induces a Rashba spin‐orbit interaction in the SWCNT channel and polarizes carrier spins. The results are consistent with recent theoretical work that predicted spin dependent conductance in ssDNA‐SWCNT hybrid. Ability to generate highly spin polarized carriers using molecular functionalization can lead to magnet‐less and contact‐less spintronic devices in the future. This can eliminate the conductivity mismatch problem and open new directions for research in organic spintronics.     

Modulating magnetic ordering of the zigzag-edge trigonal graphene by functionalizations

A pristine zigzag-edge trigonal graphene (ZTG) is a magnetic semiconductor, thus its spin polarization is extremely low. Here, we report the calculated results on enhancing the spin magnetism of a zigzag-edge trigonal graphene (ZTG) by functionalizations, including the heteroatom doping, edge modifications, and introducing topologic defects. It is found that the ZTG features a good tuning ability for functionalizations to improve its spin polarization. When one boron (B) atom is doped to replace one carbon atom in the B sublattice of graphene, a higher spin polarization can be achieved, and the edge modification by Cu, Co, O or B atom can modulate the magnetic ordering significantly due to the spin-polarized charge transfer between the ZTG and terminations, especially for O and Co terminations. And also, the introduced defect (a vacancy and a Stone–Wales defect) can obviously tune local magnetic structures owing to geometrically structural deformations (variations of bond length and bond angle). For these behaviors, in-depth analyses are performed. Our findings suggest that the desirable functionalized ZTG structures might promise importantly potential applications for developing nano-scale spintronics devices.     

Information Processing With Pure Spin Currents in Silicon: Spin Injection, Extraction, Manipulation, and Detection

We demonstrate that information can be transmitted and processed with pure spin currents in silicon. Fe/Al₂O₃ tunnel barrier contacts are used to produce significant electron spin polarization in the silicon, generating a spin current which flows outside of the charge current path. The spin orientation of this pure spin current is controlled in one of three ways: 1) by switching the magnetization of the Fe contact; 2) by changing the polarity of the bias on the Fe/Al₂O₃ "injector" contact, which enables the generation of either majority or minority spin populations in the Si, providing a way to electrically manipulate the injected spin orientation without changing the magnetization of the contact itself; and 3) by inducing spin precession through the application of a small perpendicular magnetic field. Spin polarization by electrical extraction is as effective as that achieved by the more common electrical spin injection. The output characteristics of a planar silicon three-terminal device are very similar to those of nonvolatile giant magnetoresistance metal spin-valve structures.