The basis of organic spintronics： Fabrication of organic spin valvesl

Organic spintronics focuses on utilizing the spin degree of freedom in organic materials because of the long spin relaxation time. The vertical organic spin valve （OSV） is a typical sample structure used to study the spin transport phe- nomena. However, the fabrication of high quality OSVs is difficult, which results in controversial experiment results and hence hinders the development of organic spintronics. In this work, we describe our recent study on the fabrication of typical vertical organic spin valves, Lao.67Sro.33MnO3 （LSMO）/Alq3/Co. The LSMO bottom electrodes are annealed to obtain an atomically smooth surface and improved magnetic properties. The top Co electrodes are deposited by an indirect deposition method to reduce the interfusion between Co and Alq3. The controlled fabrication process provides much better performance and sample yield of OSVs.     

The effects of electric and magnetic fields on the current spin polarization and magnetoresistance in a ferromagnetic/organic semiconductor/ferromagnetic（FM/OSC/FM） system

From experimental results of spin polarized injection and transport in organic semiconductors(OSCs),we theoretically study the current spin polarization and magnetoresistance under an electric and a magnetic field in a ferromagnetic/organic semiconductor/ferromagnetic(FM/OSC/FM) sandwich structure according to the spin drift-diffusion theory and Ohm’s law.From the calculations,it is found that the interfacial current spin polarization is enhanced by several orders of magnitude through tuning the magnetic and electric fields by taking into account the specific characteristics of OSC.Furthermore,the effects of the electric and magnetic fields on the magnetoresistance are also discussed in the sandwich structure.     

PEDOT:PSS Schottky contacts on annealed ZnO films

Polycrystalline ZnO and ITO films on SiO₂ substrates are prepared by radio frequency (RF) reactive magnetron sputtering. Schottky contacts are fabricated on ZnO films by spin coating with a high conducting polymer, poly(3, 4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS) as the metal electrodes. The current-voltage measurements for samples on unannealed ZnO films exhibit rectifying behaviours with a barrier height of 0.72 eV (n=1.93). The current for the sample is improved by two orders of magnitude at 1 V after annealing ZnO film at 850 ℃, whose barrier height is 0.75 eV with an ideality factor of 1.12. X-ray diffraction, atomic force microscopy and scanning electron microscopy are used to study the properties of the PEDOT:PSS/ZnO/ITO/SiO₂. The results are useful for applications such as metal-semiconductor field-effect transistors and UV photodetectors.     

MAGNETIC PROPERTIES OF Pr2Co17-x Mx ( M＝Ga,Si) COMPOUNDS

The influence of Ga or Si substitution for Co on the structural and magnetic properties of Pr₂Co₁₇ compounds is investigated. All samples studied here are single phase and have the rhombohedral Th₂Zn₁₇-type structure. The unit-cell volume is found to increase linearly by the substitution of Ga for Co,but reduce by the substitution of Si for Co in Pr₂Co₁₇ compounds. In Pr₂Co_17-x M_x, the Curie temperature decreases monoto nically with increasing at an approximate rate of 153K per Ga atom and 175K per Si atom. The saturation magnetic moment of Pr₂Co_17-x M_x ( M=Ga,Si) decreases with increasing x. The rates of the decrease are larger than that expected as a simple dilution. For Pr₂Co_17-xSi_x ,the spin reorientation transition is observed above room temperature. The spin-reorientation temperature Tsrfirst decreases with increasing Si content and then increases at higher x values (x>2). The spin reorientat ion behavior is interpreted by the competition between the Pr and Co sublattice anisotropies. The easy magnetization direction in Pr₂Co_17-xGa_x compounds is perpendicular to the c-axis, and no spin reorientation transition is observed.     

Recent advances in spin transport in organic semiconductors

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.     

Numerical analysis of the mechanism of carrier transport in organic light—emitting devces

The mechanism of carrier transport in organic light-emitting devices is numerically studied,on the basis of trappedcharge-limited conduction with an exponential trap distribution.The spatial distributions of the electrical potential,field and carrier density in the organic layer are calculated and analysed.Most carriers are distributed near the two electrodes,only a few of them are distributed over the remaining part of the orgaic layer,The carriers are accumulated near the electrodes,and the remaining region is almost exhausted of carriers.When the characteristic energy of trap distribution is greater than 0.3eV.it leads to a reduction of current density.In order to improve the device efficiency,organic materials with minor traps and low characteristic energy should be chosen.The diffusion current is the dominant component near the injection electrode.whereas the drift current dominates the remaining region of the organic layer.     

Performance of Organic Field Effect Transistors with Self-Improved Cu/Organic Interfaces

We fabricate pentacene-based organic field effect transistors （OFETs） with Cu as source and drain （S-D） electrodes. The fabricated devices stored for ten hours under ambient atmospheric conditions exhibit superior performance compared with the as-prepared devices. The field-effect mobility increases from 0. 012 to 0.03 cm^2 V^-1 s^-1, and the threshold voltage downshifts from -14 to -9 V. The on/off current ratios are close to the order of 10^4. The improved performance of the stored devices is attributed to the formation of thin Cu oxide at the Cu electrodes/organic interfaces. These results suggest a simple and available way to optimize device properties and to reduce fabrication cost for OFETs.     

Perfect Spin-Filtering in 4H-TAHDI-Based Molecular Devices： the Effect of N-Substitution

Based on the non-equilibrium Green＇s function formalism and spin-polarized density functional theory calcula- tions, we investigate the spin transport properties of HDI and terahydrotetraazahexaeene diimide （4H-TAHDI） with two ferromagnetic zigzag-edge graphene nanoribbon electrodes. Compared with ttDI, four carbon atoms in the hexacene part of 4H-TAHDI are substituted by nitrogen atoms. The results show that the nitrogen substitu- tion can improve significantly the spin-filtering performance and 4H-TAHDI can be used as a perfect spin filter. Our study indicates that suitable chemical substitution is a possible way to realize high-efficiency spin filters.     

Significant performance enhancement in AlGaN/GaN high electron mobility transistor by high-κ organic dielectric

The electrical properties of AlGaN/GaN high electron mobility transistor (HEMT) with and without high-κ organic dielectrics are investigated. The maximum drain current I_{D max} and the maximum transconductance g_{m max} of the organic dielectric/AlGaN/GaN structure can be enhanced by 74.5%, and 73.7% compared with those of the bare AlGaN/GaN HEMT, respectively. Both the threshold voltage V_T and g_{m max} of the dielectric/AlGaN/GaN HEMT are strongly dielectric-constant-dependent. Our results suggest that it is promising to significantly improve the performance of the AlGaN/GaN HEMT by introducing the high-κ organic dielectric.     

Three-dimensional vertical ZnO transistors with suspended top electrodes fabricated by focused ion beam technology

Three-dimensional(3D)vertical architecture transistors represent an important technological pursuit,which have distinct advantages in device integration density,operation speed,and power consumption.However,the fabrication processes of such 3D devices are complex,especially in the interconnection of electrodes.In this paper,we present a novel method which combines suspended electrodes and focused ion beam(FIB)technology to greatly simplify the electrodes interconnection in 3D devices.Based on this method,we fabricate 3D vertical core-double shell structure transistors with ZnO channel and Al₂O₃ gate-oxide both grown by atomic layer deposition.Suspended top electrodes of vertical architecture could be directly connected to planar electrodes by FIB deposited Pt nanowires,which avoid cumbersome steps in the traditional 3D structure fabrication technology.Both single pillar and arrays devices show well behaved transfer characteristics with an Ion/Ioff current ratio greater than 106 and a low threshold voltage around 0 V.The ON-current of the 2×2 pillars vertical channel transistor was 1.2μA at the gate voltage of 3 V and drain voltage of 2 V,which can be also improved by increasing the number of pillars.Our method for fabricating vertical architecture transistors can be promising for device applications with high integration density and low power consumption.     

Room-Temperature Ferromagnetism of Co-Doped CeO2 Thin Films on Si（111） substrates

Using Co2O3 as the Co source, doped cerium oxide thin films with the composition of Ce0.97C00.03O2-δ （CCO） are deposited on Si（111） and glass substrates by pulse laser deposition technique. X-ray diffraction reveals that CCO films with （111） preferential orientation are grown on Si, while the fihn on glass is polycrystalline with nanocrystal. X-ray photoelectron spectroscopy shows that the （Jo displaces the （;e atom and exists in high spin state rather than low spin state, which contributes to the room-temperature ferromagnetism confirmed by vibration sample magnetometer. I~ilms on Si and glass are different in ferromagnetism, which is believed to be induced by different film microstructures. Based on these results, the possible ferromagnetism in this insulating film is discussed. Anyway, successful fabrication of CCO films with room-temperature ferromagnetism on Si substrates is of great importance in both technological and theoretical aspects.     

Interplay of Rashba effect and spin Hall effect in perpendicular Pt/Co/MgO magnetic multilayers

The interplay of the Rashba effect and the spin Hall effect originating from current induced spin–orbit coupling was investigated in the as-deposited and annealed Pt/Co/MgO stacks with perpendicular magnetic anisotropy. The above two effects were analyzed based on Hall measurements under external magnetic fields longitudinal and vertical to dc current,respectively. The coercive field as a function of dc current in vertical mode with only the Rashba effect involved decreases due to thermal annealing. Meanwhile, spin orbit torques calculated from Hall resistance with only the spin Hall effect involved in the longitudinal mode decrease in the annealed sample. The experimental results prove that the bottom Pt/Co interface rather than the Co/MgO top one plays a more critical role in both Rashba effect and spin Hall effect.     

A density-functional theory investigation on desorption of O2 on Sn（111） and its comparison with initial oxidation on the X（111） （X=Si,Ge, Sn,Pb） surfaces

<正>The first-principles calculations are performed to investigate the adsorption of O₂ molecules on an Sn（111） 2×2 surface.The chemisorbed adsorption precursor states for O₂ are identified to be along the parallel and vertical channels, and the surface reconstructions of Sn（lll） induced by oxygen adsorption are studied.Based on this,the adsorption behaviours of O₂ on X（111）（X=Si,Ge,Sn,Pb） surfaces are analysed,and the most stable adsorption channels of O₂ on X（111）（X=Si,Ge,Sn,Pb） are identified.The surface reconstructions and electron distributions along the most stable adsorption channels are discussed and compared.The results show that the O₂ adsorption ability declines gradually and the amount of charge transferred decreases with the enhancement of metallicity.     

Organic-ferromagnetic hetero-structures with spin transport properties and fundamental physical effects

Organic spintronics refers to control spin dependent transport through organic materials.In the last two decades,extraordinary development has been achieved for organic-spintronics.A series of theoretical and experimental studies have been done to reveal the mechanisms of spin dependent transport properties.The theoretical analysis is based on the non-equilibrium Green’s function formalism provides a mathematical framework for solving the transmission coefficients in the Landauer formula from atomistic first principles without any phenomenological parameters.In this article,we provide a brief theoretical review on organic spintronics devices and device physics therein.     

Effect of proportion on rectification in organic co-oligomer spin rectifiers

The rectification behaviours in organic magnetic/nonmagnetic co-oligomer spin rectifiers are investigated theoretically.It is found that both the charge current and the spin current through the device are rectified at the same time.By adjusting the proportion between the magnetic and nonmagnetic components,the threshold voltage and the rectification ratio of the rectifier are modulated.A large rectification ratio is obtained when the two components are equal in length.The intrinsic mechanism is analysed in terms of the asymmetric localization of molecular orbitals under biases.The effect of molecular length on the rectification is also discussed.These results will be helpful in the future design of organic spin diodes.     

Aluminium phthalocyanine chloride thin films for temperature sensing

This study presents the fabrication and temperature sensing properties of sensors based on aluminium phthalocyanine chloride(AlPcCl)thin films.To fabricate the sensors,50-nm-thick electrodes with 50-μm gaps between them are deposited on glass substrates.AlPcCl thin films with thickness of 50–100 nm are deposited in the gap between electrodes by thermal evaporation.The resistance of the sensors decreases with increasing thickness and the annealing at 100℃ results in an increase in the initial resistance of sensors up to 24%.The sensing mechanism is based on the change in resistance with temperature.For temperature varying from 25℃ to 80℃,the change in resistance is up to 60%.Simulation is carried out and results obtained coincide with experimental data with an error of±1%.     

Morphology evolution of SmCox permanent magnetic nanoparticles

Sm-Co nanoparticles (NPs) are promising candidates for preparing superstable magnets and exchange-coupled nanocomposite magnets with unprecedented magnetic properties.However,the morphology evolution of the NPs remains unclear.Here,single crystalline SmCox(x=4.07,4.79,6.94,and 8.61) NPs with dimensions below the critical size of a single magnetic domain were synthesized.These NPs consist of Sm₂Co₇,SmCo₅,and Sm2Co17phases with divergent typical morphologies.An evolution model for the different morphological characteristics was proposed based on phase-structure changes and surface-energy calculations using the density functional theory.The results show that these SmCo_4.79 NPs can be well aligned along the easy magnetization axis and exhibit an ultrahigh coercivity of 5.7 T,thus enabling to advance the control of NP morphology and to facilitate their use in superstable or nanocomposite magnets.     

Tactile and temperature sensors based on organic transistors:Towards e-skin fabrication

Tactile and temperature sensors are the key components for e-skin fabrication.Organic transistors,a kind of intrinsic logic devices with diverse internal configurations,offer a wide range of options for sensor design and have played a vital role in the fabrication of e-skin-oriented tactile and temperature sensors.This research field has attained tremendous advancements,both in terms of materials design and device architecture,thereby leading to excellent performance of resulting tactile/temperature sensors.Herein,a systematic review of organic transistor-based tactile and temperature sensors is presented to summarize the latest progress in these devices.Particularly,we focus on spotlighting various device structures,underlying mechanisms and their performance.Lastly,an outlook for the future development of these devices is briefly discussed.We anticipate that this review will provide a quick overview of such a rapidly emerging research direction and attract more dedicated efforts for the development of next-generation sensing devices towards e-skin fabrication.     

Study of depth-dependent tetragonal distortion of quaternary AIInGaN epilayer by Rutherford backscattering/channeling

<正>A 240-nm thick Al_0.4In_0.02Ga_0.58N layer is grown by metal organic chemical vapour deposition,with an over 1-μm thick GaN layer used as a buffer layer on a substrate of sapphire（0001）.Rutherford backscattering and channeling are used to characterize the microstructure of AlInGaN.The results show a good crystalline quality of AlInGaN(x_min= 1.5%) with GaN buffer layer.The channeling angular scan around an off-normal（1213） axis in the {1010} plane of the AlInGaN layer is used to determine tetragonal distortion e_T,which is caused by the elastic strain in the AlInGaN.The resulting AlInGaN is subjected to an elastic strain at interfacial layer,and the strain decreases gradually towards the near-surface layer.It is expected that an epitaxial AlInGaN thin film with a thickness of 850 nm will be fully relaxed （e_T=0）.     

Electric dipolar interaction assisted growth of single crystalline organic thin films

We report on a forest-like-to-desert-like pattern evolution in the growth of an organic thin film observed by using an atomic force microscope. We use a modified diffusion limited aggregation model to simulate the growth process and are able to reproduce the experimental patterns. The energy of electric dipole interaction is calculated and determined to be the driving force for the pattern formation and evolution. Based on these results, single crystalline films are obtained by enhancing the electric dipole interaction while limiting effects of other growth parameters.