有机半导体薄膜的自旋弛豫机制研究

发布时间：2018-06-21 10:51

本文选题：有机半导体 + 自旋阀　；参考：《南京大学》2015年博士论文

【摘要】：自旋电子学主要研究如何利用电子的自旋自由度作为信息的载体,从上世纪80年代末到现在,在基础科研领域和工业应用方面都获得了巨大的成功和发展。有机自旋电子学是自旋电子学在新材料方向上发展起来的一个分支,是有机电子学(有机发光二极管OLED、有机太阳能电池等)和自旋电子学(巨磁电阻GMR、隧穿磁电阻TMR等)这两大研究领域交叉结合的地方,这也是它从一出生就吸引广泛关注的重要原因。有机材料相比于传统的无机材料,它有一些显著的特点和优势：首先,有机分子间通过范德瓦尔斯力发生相互作用(van der Waals interaction),这种作用和无机材料中的离子键、金属键等比较起来,相对较弱,因此,电子波函数主要定域在单个分子上,分子之间的波函数交叠较少,另外,有机电子学中广泛使用的有机薄膜通常情况下是非晶的,属于无序系统,这时候载流子的输运机制不再是无机材料中的能带图像,电子或者空穴像准自由粒子一样传输,而是变为跃迁(hopping)输运,载流子的迁移率比无机材料要低几个数量级,比如P型硅的迁移率一般可以达到450 cm2V-1s-1,而有机材料中比较常用的Alq3分子,其迁移率只在10-5 cm2V-1s-1量级；其次,有机材料主要由碳、氢、氧之类的轻元素构成,自旋轨道耦合作用较弱,作为有机材料骨架元素的C,核自旋为0,超精细相互作用很弱,所以有机材料的自旋弛豫时间较长,十分适合自旋极化的输运；最后,有机材料的种类十分丰富,各种官能团,可以组合出各式各样的有机分子,另外,有机薄膜的制备相对于无机材料来说比较简单,旋涂(spin-coating)、低温热蒸发等方法都可以制备出高质量的有机薄膜,同时,有机材料是柔性的,可以弯曲和扭转,比如曲面OLED显示器。有机自旋电子学利用有机材料的特性,把自旋自由度引入到有机体系中,结合成熟的有机电子学、自旋电子学,具有十分巨大的潜在应用价值。在本论文中,我们首先介绍了传统无机自旋电子学及其自旋弛豫理论,包括Elliott-Yafet机制、D'yakonov-Perel'机制、Bir-Aronov-Pikus机制以及超精细相互作用。接着,我们详细回顾了有机自旋电子学这十来年的发展,并介绍了目前有机自旋阀中的自旋弛豫机制研究进展,包括超精细相互作用和自旋轨道耦合作用,以及该方向目前还未解决的问题。最后,探讨了一下有机自旋电子学领域存在的一些基础性的开放问题,比如磁电阻温度关系、磁电阻偏压关系、传导电子自旋轨道耦合自旋弛豫机制的实验验证、自旋输运的调控等,我博士期间的研究课题主要集中在这其中的一些问题上面。我们主要研究了自旋阀中有机半导体薄膜对自旋弛豫的影响及调控。首先进行了底电极LSMO制备过程的优化,利用脉冲激光沉积系统PLD制备LSMO单晶薄膜,通过在流动氧气氛围下高温退火处理,LSMO薄膜的居里温度从320 K提高到了360 K,同时表面形貌达到了原子级别的平整度。在此基础上,我们又系统的研究了以刚制备的LSMO,退火优化的LSMO和LCMO为底电极的Alq3有机自旋阀的磁电阻效应。利用优化后的LSMO为底电极制备的有机自旋阀,相比于以刚制备的LSMO为底电极的有机自旋阀,磁电阻的大小得到了明显的提高,同时获得了高达2.2%的室温磁电阻效应。尽管这三种底电极的居里温度相差很大,分别为250 K的LCMO、320 K刚制备的LSMO、360 K退火优化的LSMO,但是在100 K之下,三种自旋阀器件的归一化磁电阻温度依赖关系几乎是一样的,结合它们在100 K以下相似的器件电阻温度依赖关系,我们认为：有机材料Alq3对输运自旋的弛豫作用导致了100 K以下三种自旋阀器件具有相同的磁电阻温度依赖关系。通常认为有机半导体的自旋轨道耦合(SOC)的强度以及对自旋弛豫作用的大小可以通过在有机分子中引入重金属离子得以加强,为了研究重金属离子引入的自旋轨道耦合对有机薄膜的自旋弛豫作用的影响,我们系统的测量了自旋在Ir(ppy)3和Alq3这两种有机材料中的磁输运现象。这两种分子具有类似的化学结构,主要区别在于：Ir(ppy)3含有重金属离子Ir3+, Alq3含有轻金属离子Al3+,预期Ir(PPY)3薄膜的自旋弛豫作用远强于Alq3薄膜的自旋弛豫作用。和预想的一样,光致发光谱的测量结果表明Ir(PPY)3分子SOC的强度远大于Alq3分子SOC的强度。但是,通过测量有机自旋阀的磁电阻大小随有机中间层厚度的关系,推导出Ir(PPY)3分子的自旋扩散长度比Alq3分子的长。考虑到Ir(PPY)3分子的载流子迁移率比Alq3的低,那么,Ir(ppy)3分子的自旋弛豫时间比Alq3的自旋弛豫时间长得多,这意味着Ir(ppy)3分子SOC的强度比Alq3分子SOC的强度弱。光致发光谱和磁输运测量看似矛盾的结果可以通过SOC的强度与有机分子能级状态相关加以解释：在有机分子配合物中,有机配位场通常是非常强的,Ir(ppy)3分子是一种5d过渡金属八面体配合物,八面体配位场导致Ir3+离子的5d轨道能级劈裂,形成一个能量较高的二重简并eg轨道,和一个能量较低的三重简并t2g轨道。重金属有机配合物在发光和输运时处于完全不同的状态,在光发射过程中,光子来源于激子的跃迁,Ir(ppy)3分子中的激子来源于金属到配体电荷转移态(metal-to-ligand-charge-transfer (MLCT)),电子定域在有机配体上,空穴来源于Ir4+离子的5d轨道。因此,Ir离子参与了光发射过程,同时,激子使Ir3+离子变成了MLCT态的Ir4+离子,Ir4+离子具有5个5d电子,填充在t2g轨道上,没有填满,此时自旋和轨道角动量不为零,从而,重金属离子的引入导致自旋轨道耦合作用的加强,对于光发射过程确实有较大的影响。但在输运时,Ir3+离子具有6个5d电子,填满t2g轨道,重金属离子处于零轨道角动量和零自旋角动量的基态,对通过π轨道输运的自旋的自旋轨道耦合作用几乎为零,弛豫作用较小。最后,我们研究了有机薄膜中的缺陷态对有机自旋阀器件电输运和磁输运的影响。我们通过往有机中间层Alq3主体分子中掺杂ZnPc有机分子的方式引入缺陷态,由于有机材料带隙一般比较大,热激发产生的载流子数量相对较少,缺陷的引入会对有机器件的ⅣV曲线带来比较大的影响,我们首先对电输运进行了研究,由于自旋输运和电荷输运是密切相关的,进一步又对自旋相关的效应进行了研究。通过掺杂引入缺陷态的方式,成功实现了有机自旋阀器件的记忆效应,通过外加偏压,能够使器件的电阻在高低阻态间变换。载流子在每个分子上停留的时间t=1/ωij∝ exp [(εj-εi)/kBT],通过比较载流子经过中间有机层所用的时间,我们认为：载流子经过缺陷trap的电阻远大于trap被电荷填满后,由于同种电荷的排斥作用,导致载流子绕着trap走的电阻,所以trap空的时候对应高阻态,trap被电荷填满的时候对应于低阻态。器件处于高电阻态时,磁电阻较小,低电阻态时,磁电阻较大,对此实验现象的定性解释如下：高阻态,载流子经过缺陷trap,会在缺陷分子上停留很长的时间,在超精细相互作用等效磁场下进动的时间变长,自旋弛豫的效果变强,从而导致磁电阻较小；低阻态时,载流子不经过缺陷trap,在分子上停留时间较短,自旋弛豫作用较弱,从而导致磁电阻效应较大。通过外加偏压的方法,实现了有机自旋阀器件高、低阻态的转变,进而实现了有机薄膜自旋弛豫作用大小的调控。
[Abstract]:Spintronics mainly studies how to use the spin freedom of electrons as the carrier of information. From the end of the 80s to the end of the last century, great success and development have been achieved in the field of basic scientific research and industrial applications. Sub studies (organic light-emitting diodes (OLED, organic solar cells, etc.) and spintronics (giant magnetoresistance GMR, tunneling magnetoresistance TMR, etc.) intersecting the two major research areas, which are also important reasons for attracting wide attention from birth. Organic materials have some significant characteristics and advantages compared to traditional inorganic materials. First of all, the organic molecules are interacted with the van der Waals interaction, which is relatively weak compared with the ionic bonds and metal bonds in the inorganic materials. Therefore, the electronic wave functions are mainly localized on a single molecule, and the wave function between molecules is less overlapping. In addition, it is widely used in organic electronics. The organic film used is usually amorphous and belongs to an unordered system. At this time the transport mechanism of the carrier is no longer a band image in the inorganic material. The electron or hole is transmitted like a quasi free particle, but is transformed into a transition (hopping) transport, and the transfer rate of the carrier is a few orders of magnitude lower than that of the inorganic material, such as the P type silicon. In general, the mobility can reach 450 cm2V-1s-1, while the mobility of Alq3 molecules in organic materials is only 10-5 cm2V-1s-1. Secondly, the organic materials are composed mainly of light elements such as carbon, hydrogen and oxygen. The spin orbit coupling is weak, as the C of the organic material skeleton element, the nuclear spin is 0, and the hyperfine interaction is weak. Therefore, the spin relaxation time of organic materials is long and is very suitable for spin polarization transport. Finally, the types of organic materials are very rich, various functional groups can combine various organic molecules. In addition, the preparation of organic thin films is relatively simple compared with inorganic materials, spin-coating, low temperature thermal evaporation and other methods. High quality organic thin films can be prepared. At the same time, organic materials are flexible and can be flexed and torsional, such as surface OLED displays. Organic spintronics, using the properties of organic materials, introduced spin freedom into the organic system, and combined with mature organic electronics and spintronics, it has a great potential application value. In this paper, we first introduce the traditional inorganic spintronics and their spin relaxation theories, including the Elliott-Yafet mechanism, the D'yakonov-Perel'mechanism, the Bir-Aronov-Pikus mechanism and the hyperfine interaction. Then, we reviewed the development of the organic spintronics in ten years, and introduced the current organic spin valves. The research progress of spin relaxation mechanism, including hyperfine interaction and spin orbit coupling, and the current unsolved problems in this direction. Finally, some basic open problems in the field of organic spintronics, such as magnetoresistance temperature relation, magnetoresistance bias relation, conduction electron spin orbit coupling, are discussed. The experimental verification of the spin relaxation mechanism, the regulation of spin transport, and so on, the research subject during my doctor's period mainly focused on some of these problems. We mainly studied the effect and regulation of the organic semiconductor thin film on the spin relaxation in the spin valve. First, the optimization of the preparation process of the bottom electrode LSMO was carried out, and the pulse laser deposition was used. LSMO single crystal film is prepared by system PLD. The Curie temperature of LSMO film is increased from 320 K to 360 K by annealing at high temperature in the atmosphere of mobile oxygen, and the surface morphology reaches the level of atomic level. On this basis, we systematically studied the Alq3 organic of the LSMO, the annealed LSMO and LCMO as the bottom electrode. The magnetoresistance effect of the spin valve. The organic spin valve prepared by the optimized LSMO is compared with the organic spin valve with the LSMO as the bottom electrode, and the magnetoresistance is up to 2.2% at the room temperature, although the Curie temperature of the three bottom electrodes is very different, respectively. For 250 K LCMO, 320 K just LSMO, 360 K annealing optimized LSMO, but under 100 K, the normalized magnetoresistance dependence of the three spin valve devices is almost the same, combined with their resistance temperature dependence of similar devices below 100 K, we believe that the relaxation effect of organic material Alq3 on the transport spin results in the effect of the relaxation effect of the organic material Alq3 on the transport spin Three kinds of spin valve devices below 100 K have the same magnetoresistance dependence. It is generally believed that the strength of the spin orbit coupling (SOC) and the size of the spin relaxation can be strengthened by introducing heavy metal ions into the organic molecules. In order to study the spin orbit coupling introduced by heavy metal ions, the pair is used to study the spin orbit coupling. The influence of the spin relaxation of the mechanical thin films, we systematically measured the magnetic transport phenomena in two kinds of organic materials, Ir (PPy) 3 and Alq3. These two molecules have similar chemical structures. The main difference is that Ir (PPy) 3 contains heavy metal ions Ir3+, Alq3 contains light metal ions Al3+, and the spin relaxation of Ir (PPY) 3 thin films is expected. Using the spin relaxation effect far stronger than the Alq3 film. As expected, the measurements of photoluminescence spectra show that the strength of the Ir (PPY) 3 molecule SOC is much greater than that of the Alq3 molecule SOC. However, the spin diffusion length of the Ir (PPY) 3 molecule is derived by measuring the relationship between the magnetoresistance of the organic spin valve and the thickness of the organic intermediate layer. Considering that the carrier mobility of the Ir (PPY) 3 molecule is lower than that of Alq3, then the spin relaxation time of the Ir (PPy) 3 molecule is much longer than the spin relaxation time of Alq3, which means that the intensity of SOC of the Ir (PPy) 3 molecule is weaker than the SOC of the Alq3 molecule. The seemingly contradictory results of the photoluminescence spectrum and magnetic transport measurement can be obtained through the intensity and the presence of SOC. The state dependence of the molecular energy level is explained: in organic molecular complexes, the organic coordination field is usually very strong. The Ir (PPy) 3 molecule is a 5D transition metal eight surface complex, and the eight surface coordination field causes the 5D orbital energy level splitting of the Ir3+ ion, forming a higher energy degenerate eg orbit and a lower energy three weight. Degenerate t2g orbit. Heavy metal organic complexes are in a completely different state of luminescence and transport. During light emission, photons are derived from exciton transitions, excitons in Ir (PPy) 3 molecules are derived from metal to ligand charge transfer states (metal-to-ligand-charge-transfer (MLCT)), and electron locals are on organic ligands, and holes originate from Ir4+ Therefore, the 5D ions of the ions are involved in the light emission process, and the exciton makes the Ir3+ ions become the Ir4+ ions of the MLCT state. The Ir4+ ions have 5 5D electrons, which are filled in the t2g orbit, and are not filled, and the spin and orbital angular momentum are not zero at this time, thus the introduction of the heavy metal ions leads to the strengthening of the spin orbit coupling effect. For the light, the coupling effect of the spin orbit is strengthened. The Ir3+ ion has 6 5D electrons, filled with t2g orbits, and the heavy metal ions are in the ground state of zero orbital angular momentum and zero spin angular momentum, and the spin orbit coupling effect is almost zero and the relaxation is smaller. Finally, we studied the organic film. The effect of the defect state on the electrical transport and magnetic transport of organic spin valve devices. We introduce the defective state by doping ZnPc organic molecules into the Alq3 main molecules in the organic intermediate layer, because the band gap of organic materials is generally large, the number of carriers produced by the thermal excitation is relatively small, and the introduction of the collapse will bring about the IV V curve of organic devices. In the larger influence, we first study the electrical transport. The spin transport is closely related to the charge transport, and the spin dependent effect is further studied. The memory effect of the organic spin valve device is successfully realized through the doping of the defective state, and the resistance of the device can be made by the external bias. The time that the carrier stays on each molecule t=1/ Omega ij exp [(epsilon j- I) /kBT], by comparing the time used by the carrier through the intermediate organic layer, we think that the resistance of the carrier passing through the defective trap is much greater than that of the trap being filled with the charge, which causes the carrier to walk around the trap. The resistance is corresponding to the high resistance state when the trap is empty. When the trap is filled with the charge, it corresponds to the low resistance state. When the device is in the high resistance state, the magnetoresistance is smaller and the low resistance state is larger. The qualitative explanation of the experimental phenomenon is as follows: high resistance state, the carrier passes through the trapping trap, and will stay for a long time on the defect molecule and exceed the defect molecule. The time of the precession in the equivalent magnetic field is longer and the effect of the spin relaxation is stronger, which leads to the smaller magnetoelectric resistance. In the low resistivity state, the carrier does not pass through the defect trap, the residence time is shorter in the molecule and the spin relaxation is weak, which leads to the larger magnetoresistance effect. The organic spin valve is realized by the applied bias method. The change of high and low resistivity state of the device can further control the spin relaxation effect of organic thin films.
【学位授予单位】：南京大学
【学位级别】：博士
【学位授予年份】：2015
【分类号】：TB383.2

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