發(fā)布時間：2018-06-21 10:51

  本文選題：有機半導(dǎo)體 + 自旋閥��； 參考：《南京大學(xué)》2015年博士論文

【摘要】：自旋電子學(xué)主要研究如何利用電子的自旋自由度作為信息的載體,從上世紀(jì)80年代末到現(xiàn)在,在基礎(chǔ)科研領(lǐng)域和工業(yè)應(yīng)用方面都獲得了巨大的成功和發(fā)展。有機自旋電子學(xué)是自旋電子學(xué)在新材料方向上發(fā)展起來的一個分支,是有機電子學(xué)(有機發(fā)光二極管OLED、有機太陽能電池等)和自旋電子學(xué)(巨磁電阻GMR、隧穿磁電阻TMR等)這兩大研究領(lǐng)域交叉結(jié)合的地方,這也是它從一出生就吸引廣泛關(guān)注的重要原因。有機材料相比于傳統(tǒng)的無機材料,它有一些顯著的特點和優(yōu)勢：首先,有機分子間通過范德瓦爾斯力發(fā)生相互作用(van der Waals interaction),這種作用和無機材料中的離子鍵、金屬鍵等比較起來,相對較弱,因此,電子波函數(shù)主要定域在單個分子上,分子之間的波函數(shù)交疊較少,另外,有機電子學(xué)中廣泛使用的有機薄膜通常情況下是非晶的,屬于無序系統(tǒng),這時候載流子的輸運機制不再是無機材料中的能帶圖像,電子或者空穴像準(zhǔn)自由粒子一樣傳輸,而是變?yōu)檐S遷(hopping)輸運,載流子的遷移率比無機材料要低幾個數(shù)量級,比如P型硅的遷移率一般可以達到450 cm2V-1s-1,而有機材料中比較常用的Alq3分子,其遷移率只在10-5 cm2V-1s-1量級；其次,有機材料主要由碳、氫、氧之類的輕元素構(gòu)成,自旋軌道耦合作用較弱,作為有機材料骨架元素的C,核自旋為0,超精細相互作用很弱,所以有機材料的自旋弛豫時間較長,十分適合自旋極化的輸運；最后,有機材料的種類十分豐富,各種官能團,可以組合出各式各樣的有機分子,另外,有機薄膜的制備相對于無機材料來說比較簡單,旋涂(spin-coating)、低溫?zé)嵴舭l(fā)等方法都可以制備出高質(zhì)量的有機薄膜,同時,有機材料是柔性的,可以彎曲和扭轉(zhuǎn),比如曲面OLED顯示器。有機自旋電子學(xué)利用有機材料的特性,把自旋自由度引入到有機體系中,結(jié)合成熟的有機電子學(xué)、自旋電子學(xué),具有十分巨大的潛在應(yīng)用價值。在本論文中,我們首先介紹了傳統(tǒng)無機自旋電子學(xué)及其自旋弛豫理論,包括Elliott-Yafet機制、D'yakonov-Perel'機制、Bir-Aronov-Pikus機制以及超精細相互作用。接著,我們詳細回顧了有機自旋電子學(xué)這十來年的發(fā)展,并介紹了目前有機自旋閥中的自旋弛豫機制研究進展,包括超精細相互作用和自旋軌道耦合作用,以及該方向目前還未解決的問題。最后,探討了一下有機自旋電子學(xué)領(lǐng)域存在的一些基礎(chǔ)性的開放問題,比如磁電阻溫度關(guān)系、磁電阻偏壓關(guān)系、傳導(dǎo)電子自旋軌道耦合自旋弛豫機制的實驗驗證、自旋輸運的調(diào)控等,我博士期間的研究課題主要集中在這其中的一些問題上面。我們主要研究了自旋閥中有機半導(dǎo)體薄膜對自旋弛豫的影響及調(diào)控。首先進行了底電極LSMO制備過程的優(yōu)化,利用脈沖激光沉積系統(tǒng)PLD制備LSMO單晶薄膜,通過在流動氧氣氛圍下高溫退火處理,LSMO薄膜的居里溫度從320 K提高到了360 K,同時表面形貌達到了原子級別的平整度。在此基礎(chǔ)上,我們又系統(tǒng)的研究了以剛制備的LSMO,退火優(yōu)化的LSMO和LCMO為底電極的Alq3有機自旋閥的磁電阻效應(yīng)。利用優(yōu)化后的LSMO為底電極制備的有機自旋閥,相比于以剛制備的LSMO為底電極的有機自旋閥,磁電阻的大小得到了明顯的提高,同時獲得了高達2.2%的室溫磁電阻效應(yīng)。盡管這三種底電極的居里溫度相差很大,分別為250 K的LCMO、320 K剛制備的LSMO、360 K退火優(yōu)化的LSMO,但是在100 K之下,三種自旋閥器件的歸一化磁電阻溫度依賴關(guān)系幾乎是一樣的,結(jié)合它們在100 K以下相似的器件電阻溫度依賴關(guān)系,我們認為：有機材料Alq3對輸運自旋的弛豫作用導(dǎo)致了100 K以下三種自旋閥器件具有相同的磁電阻溫度依賴關(guān)系。通常認為有機半導(dǎo)體的自旋軌道耦合(SOC)的強度以及對自旋弛豫作用的大小可以通過在有機分子中引入重金屬離子得以加強,為了研究重金屬離子引入的自旋軌道耦合對有機薄膜的自旋弛豫作用的影響,我們系統(tǒng)的測量了自旋在Ir(ppy)3和Alq3這兩種有機材料中的磁輸運現(xiàn)象。這兩種分子具有類似的化學(xué)結(jié)構(gòu),主要區(qū)別在于：Ir(ppy)3含有重金屬離子Ir3+, Alq3含有輕金屬離子Al3+,預(yù)期Ir(PPY)3薄膜的自旋弛豫作用遠強于Alq3薄膜的自旋弛豫作用。和預(yù)想的一樣,光致發(fā)光譜的測量結(jié)果表明Ir(PPY)3分子SOC的強度遠大于Alq3分子SOC的強度。但是,通過測量有機自旋閥的磁電阻大小隨有機中間層厚度的關(guān)系,推導(dǎo)出Ir(PPY)3分子的自旋擴散長度比Alq3分子的長�？紤]到Ir(PPY)3分子的載流子遷移率比Alq3的低,那么,Ir(ppy)3分子的自旋弛豫時間比Alq3的自旋弛豫時間長得多,這意味著Ir(ppy)3分子SOC的強度比Alq3分子SOC的強度弱。光致發(fā)光譜和磁輸運測量看似矛盾的結(jié)果可以通過SOC的強度與有機分子能級狀態(tài)相關(guān)加以解釋：在有機分子配合物中,有機配位場通常是非常強的,Ir(ppy)3分子是一種5d過渡金屬八面體配合物,八面體配位場導(dǎo)致Ir3+離子的5d軌道能級劈裂,形成一個能量較高的二重簡并eg軌道,和一個能量較低的三重簡并t2g軌道。重金屬有機配合物在發(fā)光和輸運時處于完全不同的狀態(tài),在光發(fā)射過程中,光子來源于激子的躍遷,Ir(ppy)3分子中的激子來源于金屬到配體電荷轉(zhuǎn)移態(tài)(metal-to-ligand-charge-transfer (MLCT)),電子定域在有機配體上,空穴來源于Ir4+離子的5d軌道。因此,Ir離子參與了光發(fā)射過程,同時,激子使Ir3+離子變成了MLCT態(tài)的Ir4+離子,Ir4+離子具有5個5d電子,填充在t2g軌道上,沒有填滿,此時自旋和軌道角動量不為零,從而,重金屬離子的引入導(dǎo)致自旋軌道耦合作用的加強,對于光發(fā)射過程確實有較大的影響。但在輸運時,Ir3+離子具有6個5d電子,填滿t2g軌道,重金屬離子處于零軌道角動量和零自旋角動量的基態(tài),對通過π軌道輸運的自旋的自旋軌道耦合作用幾乎為零,弛豫作用較小。最后,我們研究了有機薄膜中的缺陷態(tài)對有機自旋閥器件電輸運和磁輸運的影響。我們通過往有機中間層Alq3主體分子中摻雜ZnPc有機分子的方式引入缺陷態(tài),由于有機材料帶隙一般比較大,熱激發(fā)產(chǎn)生的載流子數(shù)量相對較少,缺陷的引入會對有機器件的ⅣV曲線帶來比較大的影響,我們首先對電輸運進行了研究,由于自旋輸運和電荷輸運是密切相關(guān)的,進一步又對自旋相關(guān)的效應(yīng)進行了研究。通過摻雜引入缺陷態(tài)的方式,成功實現(xiàn)了有機自旋閥器件的記憶效應(yīng),通過外加偏壓,能夠使器件的電阻在高低阻態(tài)間變換。載流子在每個分子上停留的時間t=1/ωij∝ exp [(εj-εi)/kBT],通過比較載流子經(jīng)過中間有機層所用的時間,我們認為：載流子經(jīng)過缺陷trap的電阻遠大于trap被電荷填滿后,由于同種電荷的排斥作用,導(dǎo)致載流子繞著trap走的電阻,所以trap空的時候?qū)��?yīng)高阻態(tài),trap被電荷填滿的時候?qū)��?yīng)于低阻態(tài)。器件處于高電阻態(tài)時,磁電阻較小,低電阻態(tài)時,磁電阻較大,對此實驗現(xiàn)象的定性解釋如下：高阻態(tài),載流子經(jīng)過缺陷trap,會在缺陷分子上停留很長的時間,在超精細相互作用等效磁場下進動的時間變長,自旋弛豫的效果變強,從而導(dǎo)致磁電阻較��；低阻態(tài)時,載流子不經(jīng)過缺陷trap,在分子上停留時間較短,自旋弛豫作用較弱,從而導(dǎo)致磁電阻效應(yīng)較大。通過外加偏壓的方法,實現(xiàn)了有機自旋閥器件高、低阻態(tài)的轉(zhuǎn)變,進而實現(xiàn)了有機薄膜自旋弛豫作用大小的調(diào)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.
【學(xué)位授予單位】：南京大學(xué)
【學(xué)位級別】：博士
【學(xué)位授予年份】：2015
【分類號】：TB383.2

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