烷烃分子的自旋输运和有机自旋阀器件的制备
摘要
1988年人们在Fe/Cr多层膜中发现了巨磁电阻效应,之后人们又在铁磁/绝缘体/铁磁的磁隧道结中发现了隧穿磁电阻效应。并且通过利用巨磁电阻效应和隧穿磁电阻效应,人们推出了如高灵敏度的磁盘读出头和磁性随机存储器等创新性的微电子产品,从而开拓了自旋电子学这一全新的学科。1992年,人们先后在Co/Cu和Co/Ag等复合颗粒膜体系中发现同样存在磁电阻效应,从而拓宽了磁电阻材料的研究范围。2004年,人们又在铁磁/有机半导体/铁磁的自旋阀器件中,发现了高达40%的低温磁电阻效应。铁磁/有机这种新的复合材料体系所具有的独特优点,引起了人们极大的兴趣。这是因为有机材料主要是由轻元素(C,H,N,O)组成,其自旋-轨道耦合作用很弱,并且超精细相互作用也很弱。这种弱的相互作用非常有利于自旋在有机材料中输运。自旋在有机材料中输运时,受到的自旋相关散射非常小,从而使得自旋的弛豫时间很长,因而可能具有更大的磁电阻效应和更强的自旋相关效应。另外有机材料具有制备简单、经济,易与大规模平面工艺相结合,可选材料丰富等优点,这使得有机自旋电子学具有非常大的应用潜力。
我们主要从两个方面对有机材料中的自旋输运性质进行了研究。一是铁磁/单分子复合纳米颗粒的制备及其自旋输运性质的研究。在铁磁/有机复合纳米颗粒体系中,至今人们只探索了极少数的材料体系,如在Fe3O4/聚苯乙烯或Fe3O4/聚碳酸脂等复合纳米颗粒中14T的磁场可获得约40%的低温磁电阻效应；在FeCo/十六烷基胺纳米颗粒中观察到了库仑阻塞效应等,这些研究都不能很好地控制有机材料的厚度,影响了进一步深入的研究。而我们成功的在Fe3O4纳米颗粒外利用自组装的方法包裹单分子层有机分子,并系统研究了自旋在有机单分子中输运时随有机分子长度的变化。二是对铁磁/有机半导体/铁磁三层膜结构的自旋阀器件的研究。我们设计了一种可靠的工艺,用来制备高质量,可重复的有机自旋阀器件。在制备有机自旋阀器件的过程中,我们改进了在有机半导体薄膜上生长铁磁金属上电极Co的方法。通过在真空腔中通入惰性气体的方法,使蒸发出来的金属Co原子与惰性气体原子多次碰撞交换能量,有效的降低了金属原子Co的能量,从而极大的抑制了生长过程中Co原子对有机半导体薄膜的穿透,得到可靠稳定的有机自旋阀器件。
本文工作主要包括以下几个部分：
一、Fe3O4/油酸单分子层复合纳米颗粒体系的磁电阻增强效应
通过自组装的方法成功制备了单分子层油酸分子包裹的Fe3O4纳米颗粒,发现油酸分子的包裹增强了体系中的磁电阻效应。傅里叶红外透射谱的测量表明,Fe3O4纳米颗粒表面成功包裹了单分子层的油酸分子,并且油酸分子全部以化学成键的方式与Fe3O4纳米颗粒结合,没有以物理吸附的方式与Fe3O4结合的油酸分子。被油酸完全包裹的Fe3O4纳米颗粒样品,在5.8 KOe的磁场下,室温具有7.3%,115 K具有17.5%的磁电阻。其磁电阻值是没有包裹油酸分子的Fe3O4纳米颗粒的2.4倍。这种磁电阻增强效应很好的证明了,自旋在有机分子内输运时受到的自旋相关散射非常小。
二、自旋在直链饱和烷烃分子中的输运性质研究
有机分子材料由于其自旋轨道耦合作用和超精细作用很弱,使得自旋在有机分子中输运时具有很长的自旋弛豫时间,在自旋电子学可能的应用方面具有极大的潜力。然而与相对成熟的无机自旋电子学相比,有机自旋电子学或者分子自旋电子学的研究还处于初始阶段。我们利用自组装的方法,成功制备了单分子层直链饱和烷烃基分子包裹的Fe3O4纳米颗粒,通过系统变化有机分子的长度,系统的研究自旋在碳链分子中的输运性质。样品的电阻值随有机分子长度的增加表现为指数增大,这种变化关系说明载流子在有机分子中是隧穿输运。室温下发现Fe3O4/有机分子具有极大的隧穿磁电阻,磁电阻值高达～21%。而随着分子长度的增加系统的磁电阻却并没有变化。表明自旋通过烷烃分子输运时基本没有受到散射。
三、气体散射方法制备有机自旋阀器件
我们设计了一种可靠的工艺,用来制备高质量,可重复的有机半导体自旋阀器件。常规的制备有机半导体自旋阀器件的方法,在生长铁磁上电极的过程中,由于铁磁金属的熔点很高,从而使得蒸发出来的铁磁金属原子具有很高的动能,在沉积到有机半导体薄膜上时,很容易破坏有机半导体薄膜,并且进入到有机半导体薄膜的内部,造成器件的短路等不良效应,所以很难得到高质量,可重复的有机半导体自旋阀器件。我们针对生长铁磁上电极的方法进行了改进,主要是通过在真空腔中通入惰性气体,蒸发出的Co原子经过被气体原子多次散射后降低动能,从而让Co轻柔的沉积在有机薄膜表面,降低Co/有机界面的互扩散而获得高质量的有机自旋阀器件。与传统的生长方法相比较,这种气体散射的方法极大的抑制了生长金属过程中钴原子对有机半导体的穿透。通过在单晶硅基底和有机半导体基底上,对比生长的Co薄膜的磁矩测量,证明了气体散射的方式可以极大的抑制Co原子的穿透效应。并且在使用气体散射的方法制备的自旋阀器件中观测到了0.07%的室温磁电阻效应。
A new discipline, called spintronics has emerged since the discoveries of giant magnetoresistance and tunnelling magnetoresistance in multilayer structure which revolutionized applications such as magnetic recording and read head in hard disk. Consequently the effect in Co/Cu and Co/Ag granular systems which observed in 1992 widely developed the area of fundamental research. In 2004, Xiong et al. demonstrated a large magnetoresistance (MR) over 40% in ferromagnetic/organic semiconductor/ferromagnetic sandwich structure. Organic spintronics attract much attention due to the weak spin orbit coupling and hyperfine interactions base on the light element (such as C, H, N, and O) which are the main composition of organic materials. The extremely weak spin orbit coupling and hyperfine interactions courses a very long spin relaxation time hence a great potential of stronger magnetoresistance effect or spin relate effect. Beside the low-temperature processing, the tunable electronic properties, and the flexible in mechanics, it makes organic materials great candidate for spintronics applications.
This thesis concerns on two aspects. One is the spin transport properties in self-assembled monolayer of alkane molecules on Fe3O4 nanoparticles systems. A few efforts are carried out on the ferromagnetic/organic nanoparticle systems. Such as a low temperature magnetoresistance about 40% is observed under 14 T magnetic field in Fe3O4/Polystyrene system. And in FeCo/Hexadecylamine system, a Coulomb blockade effect is observed. However the distance between the nanoparticles is difficult to be controlled by those approaches. We successfully fabricate the self-assembled monolayers of alkane molecules on Fe3O4 nanoparticles with monolayer thickness on the particle surface, and with this approach we carefully study the length dependent spin transport properties. On the other we developed a reliable technique for fabricating high quality, reproducible organic spin-valve (OSV) devices using indirect deposition (ID) method, which relies on the evaporated metallic atoms scattering with inset gas to reduce energy, to deposit the ferromagnetic electrode Co on top of organic semiconductors. This method significantly suppresses the penetration of Co atoms into organic semiconductor layer during deposition process.
The work presented in this dissertation consists of the following parts:
Ⅰ. Enhanced magnetoresistance in self-assembled monolayer of oleic acid molecules on Fe3O4 nanoparticles
Spin transport through molecules is investigated using self-assembled monolayers of oleic acid molecules on half metallic Fe3O4 nanoparticles. Fourier transform infrared spectroscopy measurements indicate that one(?)monolayer molecules chemically bond to the Fe3O4 nanoparticles and the physically absorbed molecules do not exist in the samples. The MR of cold-pressed, molecule fully covered nanoparticles is up to 7.3% at room temperature and 17.5% at 115 K under a field of 5.8 kOe. The MR ratio is more than two times larger than that of pure Fe3O4 nanoparticles. This enhanced MR is likely arising from weak spin scattering while carriers hop through the oleic acid molecules.
Ⅱ. Investigation on spin transport properties of alkane molecules
Organic molecular materials are known to have long spin relaxation time owing to the weak spin-orbit and hyperfine interactions; therefore offer unique advantages for potential spintronic applications. Compared with the mature inorganic spintronics which has already found applications in computer memories and read heads, however, organic or molecular spintronics is still at its infancy. For example, robust room-temperature spin-conserving building blocks have yet to be demonstrated with molecular junctions. In this work, we successfully fabricate molecular junction spin valves comprised of superparamagnetic Fe3O4 nanoparticles self-assembled with alkane molecules of different lengths d as the spacer. Although the resistance spans～two decades as the molecular length d varies from 0.7 to 2.5 nm, remarkably, a very large room-temperature tunneling magnetoresistance ratio of～21% stays approximately constant. This molecular length independent spin valve magnetoresistance, originated from the even weaker spin dependent scattering in alkane molecules.
Ⅲ. Fabricating organic spin-valve devices using indirect deposition
We report a reliable technique for fabricating high quality, reproducible organic spin valve devices. We use indirect deposition method, which relies on the evaporated metallic atoms scattering with inset gas to reduce energy, to deposit the ferromagnetic electrode Co on top of organic layer Alq3. This method significantly suppresses the penetration of Co atoms into Alq3 layer during deposition process, in comparison with devices fabricated by conventional direct deposition method. The improved Alq3/Co interface is further confirmed by comparing the magnetic moment of depositing Co onto Alq3 and Si substrates by indirect and direct deposition method. And a spin-valve effect with magnetoresistance of 0.07% at room temperature is demonstrated.
我们主要从两个方面对有机材料中的自旋输运性质进行了研究。一是铁磁/单分子复合纳米颗粒的制备及其自旋输运性质的研究。在铁磁/有机复合纳米颗粒体系中,至今人们只探索了极少数的材料体系,如在Fe3O4/聚苯乙烯或Fe3O4/聚碳酸脂等复合纳米颗粒中14T的磁场可获得约40%的低温磁电阻效应；在FeCo/十六烷基胺纳米颗粒中观察到了库仑阻塞效应等,这些研究都不能很好地控制有机材料的厚度,影响了进一步深入的研究。而我们成功的在Fe3O4纳米颗粒外利用自组装的方法包裹单分子层有机分子,并系统研究了自旋在有机单分子中输运时随有机分子长度的变化。二是对铁磁/有机半导体/铁磁三层膜结构的自旋阀器件的研究。我们设计了一种可靠的工艺,用来制备高质量,可重复的有机自旋阀器件。在制备有机自旋阀器件的过程中,我们改进了在有机半导体薄膜上生长铁磁金属上电极Co的方法。通过在真空腔中通入惰性气体的方法,使蒸发出来的金属Co原子与惰性气体原子多次碰撞交换能量,有效的降低了金属原子Co的能量,从而极大的抑制了生长过程中Co原子对有机半导体薄膜的穿透,得到可靠稳定的有机自旋阀器件。
本文工作主要包括以下几个部分：
一、Fe3O4/油酸单分子层复合纳米颗粒体系的磁电阻增强效应
通过自组装的方法成功制备了单分子层油酸分子包裹的Fe3O4纳米颗粒,发现油酸分子的包裹增强了体系中的磁电阻效应。傅里叶红外透射谱的测量表明,Fe3O4纳米颗粒表面成功包裹了单分子层的油酸分子,并且油酸分子全部以化学成键的方式与Fe3O4纳米颗粒结合,没有以物理吸附的方式与Fe3O4结合的油酸分子。被油酸完全包裹的Fe3O4纳米颗粒样品,在5.8 KOe的磁场下,室温具有7.3%,115 K具有17.5%的磁电阻。其磁电阻值是没有包裹油酸分子的Fe3O4纳米颗粒的2.4倍。这种磁电阻增强效应很好的证明了,自旋在有机分子内输运时受到的自旋相关散射非常小。
二、自旋在直链饱和烷烃分子中的输运性质研究
有机分子材料由于其自旋轨道耦合作用和超精细作用很弱,使得自旋在有机分子中输运时具有很长的自旋弛豫时间,在自旋电子学可能的应用方面具有极大的潜力。然而与相对成熟的无机自旋电子学相比,有机自旋电子学或者分子自旋电子学的研究还处于初始阶段。我们利用自组装的方法,成功制备了单分子层直链饱和烷烃基分子包裹的Fe3O4纳米颗粒,通过系统变化有机分子的长度,系统的研究自旋在碳链分子中的输运性质。样品的电阻值随有机分子长度的增加表现为指数增大,这种变化关系说明载流子在有机分子中是隧穿输运。室温下发现Fe3O4/有机分子具有极大的隧穿磁电阻,磁电阻值高达～21%。而随着分子长度的增加系统的磁电阻却并没有变化。表明自旋通过烷烃分子输运时基本没有受到散射。
三、气体散射方法制备有机自旋阀器件
我们设计了一种可靠的工艺,用来制备高质量,可重复的有机半导体自旋阀器件。常规的制备有机半导体自旋阀器件的方法,在生长铁磁上电极的过程中,由于铁磁金属的熔点很高,从而使得蒸发出来的铁磁金属原子具有很高的动能,在沉积到有机半导体薄膜上时,很容易破坏有机半导体薄膜,并且进入到有机半导体薄膜的内部,造成器件的短路等不良效应,所以很难得到高质量,可重复的有机半导体自旋阀器件。我们针对生长铁磁上电极的方法进行了改进,主要是通过在真空腔中通入惰性气体,蒸发出的Co原子经过被气体原子多次散射后降低动能,从而让Co轻柔的沉积在有机薄膜表面,降低Co/有机界面的互扩散而获得高质量的有机自旋阀器件。与传统的生长方法相比较,这种气体散射的方法极大的抑制了生长金属过程中钴原子对有机半导体的穿透。通过在单晶硅基底和有机半导体基底上,对比生长的Co薄膜的磁矩测量,证明了气体散射的方式可以极大的抑制Co原子的穿透效应。并且在使用气体散射的方法制备的自旋阀器件中观测到了0.07%的室温磁电阻效应。
A new discipline, called spintronics has emerged since the discoveries of giant magnetoresistance and tunnelling magnetoresistance in multilayer structure which revolutionized applications such as magnetic recording and read head in hard disk. Consequently the effect in Co/Cu and Co/Ag granular systems which observed in 1992 widely developed the area of fundamental research. In 2004, Xiong et al. demonstrated a large magnetoresistance (MR) over 40% in ferromagnetic/organic semiconductor/ferromagnetic sandwich structure. Organic spintronics attract much attention due to the weak spin orbit coupling and hyperfine interactions base on the light element (such as C, H, N, and O) which are the main composition of organic materials. The extremely weak spin orbit coupling and hyperfine interactions courses a very long spin relaxation time hence a great potential of stronger magnetoresistance effect or spin relate effect. Beside the low-temperature processing, the tunable electronic properties, and the flexible in mechanics, it makes organic materials great candidate for spintronics applications.
This thesis concerns on two aspects. One is the spin transport properties in self-assembled monolayer of alkane molecules on Fe3O4 nanoparticles systems. A few efforts are carried out on the ferromagnetic/organic nanoparticle systems. Such as a low temperature magnetoresistance about 40% is observed under 14 T magnetic field in Fe3O4/Polystyrene system. And in FeCo/Hexadecylamine system, a Coulomb blockade effect is observed. However the distance between the nanoparticles is difficult to be controlled by those approaches. We successfully fabricate the self-assembled monolayers of alkane molecules on Fe3O4 nanoparticles with monolayer thickness on the particle surface, and with this approach we carefully study the length dependent spin transport properties. On the other we developed a reliable technique for fabricating high quality, reproducible organic spin-valve (OSV) devices using indirect deposition (ID) method, which relies on the evaporated metallic atoms scattering with inset gas to reduce energy, to deposit the ferromagnetic electrode Co on top of organic semiconductors. This method significantly suppresses the penetration of Co atoms into organic semiconductor layer during deposition process.
The work presented in this dissertation consists of the following parts:
Ⅰ. Enhanced magnetoresistance in self-assembled monolayer of oleic acid molecules on Fe3O4 nanoparticles
Spin transport through molecules is investigated using self-assembled monolayers of oleic acid molecules on half metallic Fe3O4 nanoparticles. Fourier transform infrared spectroscopy measurements indicate that one(?)monolayer molecules chemically bond to the Fe3O4 nanoparticles and the physically absorbed molecules do not exist in the samples. The MR of cold-pressed, molecule fully covered nanoparticles is up to 7.3% at room temperature and 17.5% at 115 K under a field of 5.8 kOe. The MR ratio is more than two times larger than that of pure Fe3O4 nanoparticles. This enhanced MR is likely arising from weak spin scattering while carriers hop through the oleic acid molecules.
Ⅱ. Investigation on spin transport properties of alkane molecules
Organic molecular materials are known to have long spin relaxation time owing to the weak spin-orbit and hyperfine interactions; therefore offer unique advantages for potential spintronic applications. Compared with the mature inorganic spintronics which has already found applications in computer memories and read heads, however, organic or molecular spintronics is still at its infancy. For example, robust room-temperature spin-conserving building blocks have yet to be demonstrated with molecular junctions. In this work, we successfully fabricate molecular junction spin valves comprised of superparamagnetic Fe3O4 nanoparticles self-assembled with alkane molecules of different lengths d as the spacer. Although the resistance spans～two decades as the molecular length d varies from 0.7 to 2.5 nm, remarkably, a very large room-temperature tunneling magnetoresistance ratio of～21% stays approximately constant. This molecular length independent spin valve magnetoresistance, originated from the even weaker spin dependent scattering in alkane molecules.
Ⅲ. Fabricating organic spin-valve devices using indirect deposition
We report a reliable technique for fabricating high quality, reproducible organic spin valve devices. We use indirect deposition method, which relies on the evaporated metallic atoms scattering with inset gas to reduce energy, to deposit the ferromagnetic electrode Co on top of organic layer Alq3. This method significantly suppresses the penetration of Co atoms into Alq3 layer during deposition process, in comparison with devices fabricated by conventional direct deposition method. The improved Alq3/Co interface is further confirmed by comparing the magnetic moment of depositing Co onto Alq3 and Si substrates by indirect and direct deposition method. And a spin-valve effect with magnetoresistance of 0.07% at room temperature is demonstrated.
引文
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