SCIENCE CHINA Physics, Mechanics & Astronomy. Recent advances in spin transport in organic semiconductors

Transcription

1 SCIENCE CHINA Physics, Mechanics & Astronomy Review January 2013 Vol.56 No.1: Progress of Projects Supported by NSFC Spintronics doi: /s Recent advances in spin transport in organic semiconductors JIANG ShengWei, YUE FengJuan, WANG Shen * & WU Di * National Laboratory of Solid State Microstructures and Department of Physics, Nanjing University, Nanjing , China Received October 15, 2012; accepted November 29, 2012; published online December 26, 2012 The spin relaxation time is long in organic semiconductors because of the weak spin-orbit and hyperfine interactions, leading to intensive study on spin transport in organic semiconductors. The rapid progress towards utilizing spin degree of freedom in organic electronic devices is occurring. While the spin injection, transport and detection in organic semiconductors are demonstrated, the fundamental physics of these phenomena remains unclear. This paper highlights recent progress that has been made, focusing primarily on present experimental work. organic spintronics, spin injection, spin transport, spin relaxation PACS number(s): Mk, Dc, Le, d Citation: Jiang S W, Yue F J, Wang S, et al. Recent advances in spin transport in organic semiconductors. Sci China-Phys Mech Astron, 2013, 56: , doi: /s *Corresponding author ( WANG Shen, break100@sina.com; WU Di, dwu@nju.edu.cn) In the past several decades organic electronics have aroused considerable interest because of unique advantages, such as low cost fabrication, lightweight, mechanical flexibility and relatively availability of chemical engineering of molecular properties. Remarkable progress has been made in electronics and optics on the basis of inorganic counterparts [1,2]. Several devices, such as organic light emitting diodes (OLEDs), organic thin-film transistor (OTFT) and organic photovoltaic cell, have been demonstrated based on organic semiconductor materials. OLEDs are already in commercial production as high efficient, bright and colorful displays in mobile phones and digital cameras. However, the spin degree of freedom is ignored in these organic devices. Organic semiconductors are suitable for spin transport because of the extremely long spin relaxation time, which stems from the weak spin-orbit coupling and hyperfine interaction. The spin transport phenomena are usually studied in an organic spin valve (OSV) device geometry, in which two ferromagnetic (FM) electrodes are separated by a thin organic layer. The resistance of the OSVs depends on the relative orientation of the magnetizations of the two FM electrodes, which is the same as metallic giant magnetoresistance (GMR) devices and magnetic tunneling junctions. The initial experiment was carried out in a lateral hybrid organic device La 0.7 Sr 0.3 MnO 3 (LSMO)/Sexithiophene (T 6 )/ LSMO in 2002 [3]. However, they did not measure the resistance with scanning magnetic field to show clear GMR loop, for example, the different resistance between two FM electrodes in parallel and antiparallel configurations since the coercivity of two LSMO electrodes was almost the same. Xiong et al. [4] demonstrated as high as 40% GMR effects at low temperature in LSMO/8-hydroxy-quinoline aluminium (Alq 3 )/Co vertical OSVs, shown in Figure 1. Triggered by this report, later, there has been increased research in this field. We present an overview of the experiments in the field of spin transport in organic semiconductors. In sect. 1, we briefly discuss the approaches of fabrication OSVs. Experiments on the interface of FM electrode/organic materials and spin injection into organic materials are discussed in sect. 2. In sect. 3 spin relaxation mechanisms are discussed. Science China Press and Springer-Verlag Berlin Heidelberg 2012 phys.scichina.com

2 Jiang S W, et al. Sci China-Phys Mech Astron January (2013) Vol. 56 No Figure 1 Schematic diagram of LSMO/Alq 3 /Co OSV devices and magnetoresistance loop measured at 11 K. Adapted with permission (ref. [4]). Copyright 2004, NPG. In sect. 4 tunneling magnetoresistance (TMR) and GMR effects are discussed. We finally give a short summary in sect Fabrication of vertical organic spin-valve devices In the OSVs reported in 2004 [4], LSMO is used as bottom electrode because of its nearly full spin polarization, stableness in ambient air. Interestingly, the LSMO electrode can be clean and reused to fabricate OSVs without obvious degrading. Organic layer Alq 3 is deposited onto LSMO at room temperature. The top electrode Co is thermally evaporated to directly deposit onto Alq 3 layer. Unfortunately, the Co can easily penetrate into Alq 3 layer up to about 100 nm to form so-called ill-defined layer, leading to poor Co/ Alq 3 interfaces. And this ill-defined layer makes the sample yield low and reproducibility poor [5]. The ill-define layer is a key obstacle to obtain high quality OSV samples. So far there are three approaches to overcome the top electrode penetration effects to reduce the thickness of illdefine layer. The first approach is called buffer layer assist growth (BLAG) developed by Sun et al. [6], shown in Figure 2(a). The LSMO/Alq 3 bilayer is cooled down to low temperature to absorb a layer of Xe gas. The Co atoms form clusters on top of Xe layer. The clusters have a much lower diffusion rate into the organic spacer layer than individual atoms because of the larger size. The OSVs are greatly improved with a remarkable negative magnetoresistance (MR) of ~300% comparing with 12% MR in OSVs fabricated by regular deposition method. The second approach [7,8] is to deposit a layer of tunnel barrier of Al 2 O 3 or LiF on top of organic layer before depositing top FM electrodes, shown in Figure 2(b). The tunnel barrier serves as a protection layer during the top FM electrodes deposition. A room temperature MR of ~0.15% is demonstrated in the LSMO/Alq 3 / Al 2 O 3 /Co OSVs using this method. Since this method is very simple, it has been adopted by other groups. The third approach is called indirect deposition developed by Wang et al. [9], shown in Figure 3. In this approach, the evaporated Figure 2 (a) Schematic diagrams of a BLAG spin valve and a conventional OSV adapted with permission (ref. [6]). Copyright 2010, APS. (b) Schematic diagram of a LSMO/Alq 3 /Al 2 O 3 (or LiF)/Co OSV. FM metallic atoms are scattered by Ar atoms, which are intentionally introduced into vacuum chamber before deposition, to reduce their high kinetic energy. To let the Co atoms be scattered several times, the samples face away from the evaporation source. Therefore, the Co atoms are softly deposited onto the organic layer to significantly reduce the ill-defined layer thickness. The improvement interface leads to a room temperature of 0.07% in LSMO/ Alq 3 /Co OSVs. The deposition rate of this method is more than one order of magnitude lower than direct deposition method, but the sample yield of observing MR effect is nearly 100%. 2 Spin injection at FM/organic interfaces The large MR effects are frequently reported [10 15] in organic spintronics devices, however, one important fundamental question, spin-injection into organic materials, is not fully answered and remains. Cinchetti et al. [16] used two-photon photoemission spectroscopy to measure the spin injection efficiency at FM/ organic interfaces. A pulsed laser illuminates a few monolayer of CuPc on a Co film to excite electrons from Co. The energy of excited electrons is between the Co Fermi and the vacuum level of the hetero-junctions. Since the laser penetration depth is much larger than the inelastic mean free path of the electrons, only those excited electrons that cross

3 144 Jiang S W, et al. Sci China-Phys Mech Astron January (2013) Vol. 56 No. 1 the CuPc/Co interface and reach the CuPc surface region can absorb the second photons to have sufficient energy to emit to vacuum, as shown in Figure 4(a). The energy and the spin component of photo-emitted electrons are analyzed to obtain information of the spin injection from Co into CuPc. By varying the CuPc thickness, they conclude that spin injection efficiency from Co to CuPc is as high as 85% at room temperature, shown in Figure 4(b). However, the spin injection efficiency measured by this technique is hot electron injection. The injection efficiency must be different from the MR measurement, in which the electrons at Fermi energy dominate the transport. Schulz et al. [8] systematically studied OSVs with and without a thin interfacial layer of polar material, LiF, between FM electrode and organic spacer. In the OSVs of NiFe/Alq 3 /FeCo, the devices show a negative MR (Figure 5(b)), which indicates that the resistance of the magnetization of two FM electrodes in parallel configuration is larger than that in antiparallel configuration. This result is in strong contrast with the OSVs of NiFe/LiF/Alq 3 /FeCo, in which the sign of MR reverses to positive (Figure 5(a)). These phenomena are attributed to the down shift of Alq 3 highest occupied molecular orbital (HOMO) with respect to the NiFe Fermi energy by the dipole moment of LiF (Figures 5(c) and (d)). Since the carriers are extracted from (injected into) HOMO level, the spin extraction (injection) probability depends on the FM electrodes spin- majority and minority density of state (DOS) at HOMO level. The minority DOS of NiFe is higher than the majority DOS at Alq 3 HOMO energy level without LiF, leading to more minority spin transport in Alq 3. However, because of the shift of HOMO level by LiF, the corresponding minority DOS of NiFe at HOMO level is lower than the majority DOS at HOMO energy level with LiF. This results in more majority spins extracted from (injected into) Alq 3 layer and hence the sign of MR reversal. Using the direct spectroscopic technique low-energy muon spin rotation (LE- SR) spectrum [17], the spin polarization of carriers in Alq 3 controlled by LiF interfacial layer is directly observed. These results indicate that the FM/organic interfaces play an important role in spin injection and tuning the interface can control the sign and magnitude of MR. Since the FM/organic interfaces are crucial for spin injection, the interaction between organic materials and FM Figure 3 Schematic diagram of the indirect deposition method and magnetoresistance of LSMO/Alq 3 /Co OSVs and hysteresis loops of Co and LSMO measured at 300 K (ref. [9]). Figure 4 (a) Schematic diagram of the two-photon photoemission experiments. (b) Spin injection efficiency as a function of CuPc thickness. Adapted with permission (ref. [16]). Copyright 2009, NPG.

4 Jiang S W, et al. Sci China-Phys Mech Astron January (2013) Vol. 56 No Figure 5 MR for OSVs with (a) and without (b) LiF. (c) The energy level alignment in the OSVs with (c) and without (d) LiF. Adapted with permission (ref. [8]). Copyright 2011, NPG. electrodes is widely studied. For example, Zhan et al. investigate the interface of Alq 3 /LSMO [18] and Alq 3 /Co [19], respectively, by means of photoelectron spectroscopy. The interface dipoles are both observed at the Alq 3 /LSMO and Alq 3 /Co interface. At the Alq 3 /LSMO interface, a strong interface dipole shifts down the energy diagram of the Alq 3 ~0.9 ev with respect to the vacuum level. At the Alq 3 /Co interface, a strong interface dipole that reduces the effective work function of cobalt by about 1.5 ev is observed. The results help to understanding the electronic structure of the FM electrode/organic material interface and represent a significant step toward the definition of the interface parameters for the efficient spin injection into organic materials. Later, by low-temperature spin-polarized scanning tunneling microscopy, Brede and Atodiresei et al. [20,21] investigated the spin- and energy-dependent tunneling through a single organic molecule adsorbed on FM films. A complex energy dependent magnetic structure created at the organic molecule-surface interface resembles the p z -d exchange type mechanism. The strong interactions between the FM surface and molecules may yield to an inversion of the injected spin polarization. This needs further transport measurements to verify. The efficiency of spin injection into inorganic semiconductors depends on the relative resistance ratio between FM electrodes and semiconductors, known as conductivity mismatch problem [22]. To enhance the spin injection efficiency, a tunnel or Schottky barrier is usually introduced at FM/semiconductor interface [23 26]. However, the validity of this problem for organic semiconductors is not known. Recently, Yue et al. [27] reported on magneto-transport measurements in OSVs of LSMO/interfacial layer/alq 3 /Co. The interfacial layer is a thin layer of copper phthalocyanine (CuPc), of which HOMO and lowest unoccupied molecular orbital (LUMO) level are in the gap of Alq 3 (Figure 6(a)). Therefore, with the interfacial layer of CuPc, the interfacial resistance between LSMO and Alq 3 is expected to be lower. Indeed, this is further verified by X-ray photoemission spectroscopy (XPS) and ultraviolet photoemission spectroscopy (UPS) [28]. The corresponding resistance of device with CuPc is more than one order of magnitude smaller than that of device without CuPc, reflecting that the reduction of the interface barrier height by interfacial CuPc layer. Meanwhile the MR ratio decreased down to only ~0.4% from ~6% for the device without CuPc interlayer at the same bias voltage. This MR ratio decrease reveals the lower spin injection efficiency. The findings indicate that the conductivity mismatch problem is still applicable to organic materials and the interfacial resistance has a critical impact on spin injection efficiency. 3 Spin relaxations in organic semiconductors The sources for spin relaxation are spin-orbit and hyperfine

5 146 Jiang S W, et al. Sci China-Phys Mech Astron January (2013) Vol. 56 No. 1 Figure 6 (a) Schematic energy level diagrams of OSVs of LSMO/CuPc/Alq 3 /Co (device A) and LSMO/Alq 3 /Co (device B). (b) MR response of devices with and without CuPc. Adapted with permission (ref.[ 27]). Copyright 2012, AIP. interactions. The dominate spin relaxation mechanisms in organic materials are still under debate. Both spin relaxation mechanisms have experiments and theoretical calculations to support. The molecules mainly compose of C and H atoms. The most abundant isotopic form of these atoms are 12 C (>98%) and 1 H (>99%). Since the nuclear spin of 12 C is zero, the spin relaxation mainly originates from H. To investigate the role of H in the spin relaxation, Nguyen et al. [29] replaced the hydrogen atoms ( 1 H, nuclear spin I=1/2) in the polymer poly(dioctyloxy)phenylenevinylene (H-DOOPPV) with deuterium atoms ( 2 H, I=1, D-DOOPPV). The hyperfine interaction in D-DOOPPV is much smaller than that in H- DOOPPV. Consequently, they observe that the MR increases more than one order of magnitude, shown in Figure 7. This result strongly suggests that hyperfine interaction significantly contributes to spin relaxation. Recently, using LE- SR technique, Drew et al. [17] directly measured the spin diffusion length in Alq 3. The LE-µSR spectra yield the probability distribution of the local magnetic field at the muon sites, which contains information of local magnetic field originated from the spin polarization current. By controlling a high electrical voltage applied on the sample, the stopping distribution of moun can be controlled and calculated. Therefore, the distribution of spin polarized carriers is directly measured. They obtain the spin diffusion length as a function of temperature, shown in Figure 8(a). Later, based on spin relaxation by spin-orbit interaction, Yu [30] simulated the spin diffusion length in Alq 3 as a function of temperature, which was in a good agreement with their results, strongly suggesting that the spin-orbit interaction contribute to spin relaxation as shown in Figure 8(b). There are two spin relaxation mechanisms: Elliott-Yafet (EY) and D yakonov-perel (DP), originated from spinorbit interaction [31]. The DP mechanism is directly related with the carrier mobility, but EY mechanism is related with the inversion of carrier mobility. Pramanik et al. [11] measured the MR of Co/Alq 3 /Ni nanowire. They obtained the spin diffusion length of ~4 nm at low temperature, calculated from the Julliere model according to the MR ratio. The up-limit of spin relaxation time is estimated to be as long as 1 s from the spin diffusion length based on the available carrier mobility of cm 2 V 1 s 1. In the nanowire OSVs, they believe that the charged surface likely causes additional Coulomb scattering, leading to the reduction of the carrier mobility. Comparing with the spin diffusion length of 45 nm in thin films of Alq 3 [4], the spin relaxation is attributed to the EY mechanism. 4 Tunneling MR using organic semiconductor as a tunneling barrier The organic semiconductor devices described above have relative thick organic layers, ~100 nm, through which the carriers hop. In fact, using ultrathin organic layer, the TMR can be realized. Santos et al. [12] observed a spin-polarized tunneling through an ultrathin layer of Alq 3 in And the TMR of 4.6% is obtained in a tunneling junction of

6 Jiang S W, et al. Sci China-Phys Mech Astron January (2013) Vol. 56 No Figure 7 (a) MR loop of LSMO/H-DOOPPV/Co OSVs and the corresponding polymer structure, which contains hydrogen atoms. (b) MR loop of LSMO/D-DOOPPV/Co OSVs and the corresponding polymer structure, which contains deuterium atoms. Adapted with permission (ref. [29]). Copyright 2010, NPG. Figure 8 (a) Spin diffusion length (red symbols) as a function of temperature, measured by LE-µSR technique, and MR as a function of temperature. Adapted with permission (ref. [17]). Copyright 2009, NPG. (b) The black curve is the calculated spin diffusion length as a function of temperature based on the spin relaxation by spin-orbit interaction. The purple symbols are the experimental data points of (a). Adapted with permission (ref. [30]). Copyright 2011, APS. Co/Al 2 O 3 /Alq 3 /NiFe at room temperature as shown in Figure 9. The tunneling characteristics, such as the currentvoltage behavior, temperature and bias dependence of the TMR, show the good quality of the organic tunnel barrier. The spin polarization of the tunneling current through the Alq 3 layer is directly measured by using superconducting Al as a spin detector. Later, the same group obtains the TMR of 6% at room temperature in a magnetic tunnel junction of Co/Al 2 O 3 /rubrene/fe/coo, in which the rubrene is an amorphous organic semiconductor [32]. In a series of devices with different thickness of rubrene and superconductor Al as one of electrode, a spin diffusion length of 13.3 nm is estimated at low temperature by spin polarized tunneling. Since the carrier mobility in crystalline rubrene is expected to be seven orders of magnitude higher, the spin diffusion length is predicted to reach even millimeters in single crystal rubrene. Besides the room temperature MR and a spinconserved tunneling even at room temperature, they report that an Al 2 O 3 seed layer can greatly modify the growth of organic layer [33]. These findings show that spin conserved transport through organic semiconductors is possible, which can lead to the development of spin-based molecular electronics. Li et al. [34] chose 3,4,9,10-perylene-teracarboxylic di-

7 148 Jiang S W, et al. Sci China-Phys Mech Astron January (2013) Vol. 56 No. 1 Figure 9 TMR of Co/Al 2 O 3 /Alq 3 /Py tunneling junctions, measured with 10 mv bias. The inset shows the temperature dependence of junction resistance and the chemical structure of Alq 3. Adapted with permission (ref. [12]). Copyright 2007, APS. anhydride (PTCDA) as tunneling barrier, which had been used in organic thin-film transistors and organic light-emitting diodes, to probe the transport behavior. Using a thin layer of PTCDA dusted with alumina at the organic/fm interfaces, a MR of 20% has been achieved at 20 K. The MR ratio decreases by a factor of 2 when the temperature is increased to 300 K. They found that the carriers coupled with the molecular vibration. They attributed the strong temperature dependence of MR to spin scattering in the high-temperature regime since spin scattering (or inelastic transport) in organic systems is seen to be significantly dependent on temperature, thus affecting coherent spin transport. By studying of the inelastic tunneling spectrum, the carriers couple with the molecular vibration. TMR has also been observed in organic based spintronic devices with tetraphenyl porphyrin (TPP) as the spacer layer between LSMO and Co films [35]. In addition to the traditional method to fabricate organic tunnel junctions, Barraud et al. [36] used an atomic force microscope (AFM) nanolithography technique to fabricate less than 10 nm size magnetic tunnel junctions of LSMO/ Alq 3 /Co (Figure 10(a)). The magnetic tunnel junctions exhibited a MR response of up to 300% at low temperature (Figure 10(b)). Furthermore, a spin transport model that describes the role of interfacial spin-dependent metal/molecule hybridization on the effective polarization allowing enhancement and even sign reversal of injected spins is proposed. This result shows the FM/interface play an important role in spin injection and the magnitude of the MR ratio. It is apparent that the mechanisms of spin injection and tunneling through organic layers are different. Lin et al. [37] systematically studied the device of Co/AlOx/rubrene (5 50 nm)/fe and introduce a criterion for distinguishing between spin injection and tunneling. For rubrene thickness thinner than 15 nm, the devices exhibit tunneling transport behavior. It is identified by the transport phenomena decay exponentially with increasing rubrene thickness and is weak temperature dependence. In contrast, for rubrene thickness larger than 15 nm, the devices show injection behavior, characterized by strongly temperature dependent and highly nonlinear I-V traces. Interestingly, they only observed MR in tunneling region. However, Yoo et al. [38] observed MR in tunneling and spin injection region in devices of LSMO/ rubrene/fe. In their study, they found that the MR behavior as function of bias field and temperature are significant differences in both regions. In addition to the vertical magnetic tunnel junctions with organic thin films as tunnel barriers, the FM/molecule hybrid nanoparticles are used to form granular organic tunnel junctions [39], similar to granular giant magnetoresistive systems [40]. This approach is simple and can be used to study the spin transport at the molecular scale. For example, Wang et al. [41] studied the magneto-transport in the assemblies of superparamagnetic Fe 3 O 4 nanoparticles selfassembled with alkane molecules of different lengths d as the spacer. The resistivity,, can be described as an exponential function of the molecular length d, ~exp( d) and spans ~ two decades as the molecular length d varies from 0.7 to 2.5 nm, reflecting that the carriers tunnel through molecules and the resistance is dominated by the molecules. Remarkably, the TMR ratio remains approximately constant around ~21% at room temperature as shown in Figure 11, Figure 10 (a) Schematic drawing of the organic magnetic tunnel junctions with nanometer size. (b) MR of LSMO/Alq 3 /Co magnetic tunnel junctions measured at 2 K and 5 mv. The inset is the I-V curves for FM electrodes in parallel (I PA ) and antiparallel (I AP ) configurations. Adapted with permission (ref. [36]). Copyright 2010, NPG.

8 Jiang S W, et al. Sci China-Phys Mech Astron January (2013) Vol. 56 No Figure 11 (a) Schematic diagram of the molecule coated Fe 3 O 4 nanoparticle assembly. (b) MR measured at 300 K for bare 9 nm Fe 3 O 4 nanoparticles and 9 nm Fe 3 O 4 nanoparticles coated with different length molecules. Adapted with permission (ref. [41]). Copyright 2011, AIP. independent of the molecular length, suggesting ideal roomtemperature spin-conserving transport in alkane molecules. 5 Summary Organic spintronics is an interdisciplinary research field encompassing spintronics and organic electronics. The spin transport in organic semiconductors exhibits rich phenomena. In the beginning of this field, several experiments focus on the demonstration of spin injection or tunneling through organic materials. The organic materials are used from small molecules to polymers and the device structures are from vertical to in-plane organic spin valves or tunnel junctions. Interestingly, the Hanle effect [42], which is caused by spin procession in non-magnetic spacer during transport, observed in metallic and inorganic semiconductors is still absent in organic materials. This effect is believed to be a conclusive experiment to demonstrate the spin injection. Recently, the research moves to the understanding of the fundamental physics and possible application in utilizing spin degree of freedom in organic electronic devices. The OLEDs is recently demonstrated to control the light emission intensity by manipulating spin [43]. The spin injection and the relaxation mechanisms are still not well understood and under controversial. For example, although it is well accepted that the role of FM/organic interface is important in spin injection, how the interface influences the spin injection in OSVs is not well studied. The spin-orbit and hyperfine interactions are both supported by experiments and theoretical calculations to be the origins of the spin relation in organic semiconductors. Unlike inorganic semiconductors, the relative strong spin-orbit interactions can be used to manipulate spin. The very weak spin-orbit interactions in organic materials make it difficult to manipulate the spin, particularly by the optical means. This field is still in its infancy and it is attracted more attention because of the rich physics and potentially important applications. This work was supported by the National Natural Science Foundation of China (Grant Nos , and ), the National Basic Research Program of China (Grant Nos. 2010CB and 2013CB922103), the Priority Academic Program Development of Jiangsu Higher Education Institutions, and the Fundamental Research Funds for the Central Universities. 1 Friend R H, Gymer R W, Holmes A B, et al. Electroluminescence in conjugated polymers. Nature, 1999, 397: Voss D. Cheap and cheerful circuits. Nature, 2000, 407: Dediu V, Murgia M, Matacotta F C, et al. Room temperature spin polarized injection in organic semiconductor. Solid State Commun, 2002, 122: Xiong Z H, Wu D, Vardeny V, et al. Giant magnetoresistance in organic spin-valves. Nature, 2004, 427: Vinzelberg H, Schumann J, Elefant D, et al. Low temperature tunneling magnetoresistance on (La,Sr)MnO 3 /Co junctions with organic spacer layers. J Appl Phys, 2008, 103: Sun D L, Yin L F, Sun C J, et al. Giant magnetoresistance in organic spin valves. Phys Rev Lett, 2010, 104: Dediu V, Hueso L E, Bergenti I, et al. Room-temperature spintronic effects in Alq 3 -based hybrid devices. Phys Rev B, 2008, 78: Schulz L, Nuccio L, Willis M, et al. Engineering spin propagation across a hybrid organic/inorganic interface using a polar layer. Nat Mater, 2011, 10: Wang S, Shi Y J, Lin L, et al. Room-temperature spin valve effects in La 0.67 Sr 0.33 MnO 3 /Alq 3 /Co devices. Synth Met, 2011, 161: Pramanik S, Bandyopadhyay S, Garre K, et al. Normal and inverse spin-valve effect in organic semiconductor nanowires and the background monotonic magnetoresistance. Phys Rev B, 2006, 74: Pramanik S, Stefanita C G, Patibandla S, et al. Observation of extremely long spin relaxation times in an organic nanowire spin valve. Nat Nanotechnol, 2007, 2: Santos T S, Lee J S, Lekshmi I C, et al. Room-temperature tunnel magnetoresistance and spin-polarized tunneling through an organic semiconductor barrier. Phys Rev Lett, 2007, 98: Wang F J, Yang C G, Vardeny Z V. Spin response in organic spin valves based on La 2 3 Sr 1 3 MnO 3 electrodes. Phys Rev B, 2007, 75: Lin L, Pang Z Y, Wang F G, et al. Large room-temperature magnetoresistance and temperature-dependent magnetoresistance inversion in La 0.67 Sr 0.33 MnO 3 /Alq 3 -Co nanocomposites/co devices. Solid State Commun, 2011, 151: Morley N A, Rao A, Dhandapani D, et al. Room temperature organic spintronics. J Appl Phys, 2008, 103: 07F Cinchetti M, Heimer K, Wüstenberg J P, et al. Determination of spin injection and transport in a ferromagnet/organic semiconductor heterojunction by two-photon photoemission. Nat Mater, 2009, 8: Drew A J, Hoppler J, Schulz L, et al. Direct measurement of the electronic spin diffusion in a fully fuctional organic spin valve by low energy muon spin rotation. Nat Mater, 2009, 8:

9 150 Jiang S W, et al. Sci China-Phys Mech Astron January (2013) Vol. 56 No Zhan Y Q, Bergenti I, Hueso L E, et al. Alignment of energy levels at the Alq 3 La 0.7 Sr 0.3 MnO 3 interface for organic spintronic devices. Phys Rev B, 2007, 76: Zhan Y Q, Jong M P, Li F H, et al. Energy level alignment and chemical interaction at Alq 3 /Co interfaces for organic spintronic devices. Phys Rev B, 2008, 78: Brede J, Atodiresei N, Kuck S, et al. Spin- and energy-dependent tunneling through a single molecule with intramolecular spatial resolution. Phys Rev Lett, 2010, 105: Atodiresei N, Brede J, Lazic P, et al. Design of the local spin polarization at the organic-ferromagnetic interface. Phys Rev Lett, 2010, 105: Schmidt G, Ferrand D, Molenkamp L W, et al. Fundamental obstacle for electrical spin injection from a ferromagnetic metal into a diffusive semiconductor. Phys Rev B, 2000, 62: R4790 R Rashba E I. Theory of electrical spin injection: Tunnel contacts as a solution of the conductivity mismatch problem. Phys Rev B, 2000, 62: R16267 R Zhu H J, Ramsteiner M, Kostial H, et al. Room-temperature spin injection from Fe into GaAs. Phys Rev Lett, 2001, 87: Jonker B T, Kioseoglou G, Hanbicki A T, et al. Electrical spin-injection into silicon from a ferromagnetic metal/tunnel barrier contact. Nat Phys, 2007, 3: Han W, Pi K, McCreary K M, et al. Tunneling spin injection into single layer graphene. Phys Rev Lett, 2010, 105: Yue F J, Shi Y J, Chen B B, et al. Manipulating spin injection into organic materials through interface engineering. Appl Phys Lett, 2012, 101: Grobosch M, Dorr K, Gangineni R B, et al. Energy level alignment and injection barriers at spin injection contacts between La 0.7 Sr 0.3 Mn- O 3 and organic semiconductors. Appl Phys Lett, 2008, 92: Nguyen T D, Markosian G H, Wang F J, et al. Isotope effect in spin response of -conjugated polymer films and devices. Nat Mater, 2010, 9: Yu Z G. Spin-orbit coupling, spin relaxation, and spin diffusion in organic solids. Phys Rev Lett, 2011, 106: Zutic I, Fabian J, Sarma S D. Spintronics: Fundamentals and applications. Rev Mod Phys, 2004, 76: Shim J H, Raman K V, Park Y J, et al. Large spin diffusion length in an amorphous organic semiconductor. Phys Rev Lett, 2008, 100: Raman K V, Watson S M, Shim J H, at al. Effect of molecular ordering on spin and charge injection in rubrene. Phys Rev B, 2009, 80: Li K, Chang Y, Agilan S. Organic spin valves with inelastic tunneling characteristics. Phys Rev B, 2011, 83: Xu W, Szulczewsk G J, LeClair P, et al. Tunneling magnetoresistance observed in La 0.67 Sr 0.33 MnO 3 /organic molecule/co junctions. Appl Phys Lett, 2007, 90: Barraud C, Seneor P, Mattana R, et al. Unravelling the role of the interface for spin injection into organic semiconductors. Nat Phys, 2010, 6: Lin R, Wang F, Rybicki J, et al. Distinguishing between tunneling and injection regimes of ferromagnet/organic semiconductor/ferromagnet junctions. Phys Rev B, 2010, 81: Yoo J, Jang H W, Prigodin V N, et al. Giant magnetoresistance in ferromagnet/organic semiconductor/ferromagnet heterojunctions. Phys Rev B, 2009, 80: Wang S, Yue F J, Wu D, et al. Enhanced magnetoresistance in selfassembled monolayer of oleic acid molecules on Fe 3 O 4 nanoparticles. Appl Phys Lett, 2009, 94: Xiao J Q, Jiang J S, Chien C L. Giant magnetoresistance in nonmultilayer magnetic systems. Phys Rev Lett, 1992, 68: Wang S, Yue F J, Shi J, et al. Room-temperature spin-dependent tunneling through molecules. Appl Phys Lett, 2011, 98: Johnson M, Silsbee R H. Interfacial charge-spin coupling: Injection and detection of spin magnetization in metals. Phys Rev Lett, 1985, 55: Nguyen T D, Ehrenfreund E, Vardeny Z V. Spin-polarized lightemitting diode based on an organic bipolar spin valve. Science, 2012, 337: