多相复合体系的电磁输运
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摘要
近年来,多相复合体系是材料科学和凝聚态物理领域备受关注的研究对象。多相复合材料具有非均质的结构特征,构成多相复合材料的各单相(可以是不同组分或者是不同性质的各部分),按一定的统计规律分布在材料中,单相浓度、结构或者分布的改变会导致多相复合材料宏观物性的相应改变。表征多相复合体系的重要参量是多相复合体系的微结构,它主要包括材料的几何结构(描述单相的形状、尺寸等)、拓扑结构(描述单相之间的连接、相互作用等)和表面结构(描述材料表面的粗糙程度),它们的性质是和我们观察和测量的尺度密切相关的。对多相复合体系的研究由来已久,但是随着材料科学尤其是纳米科学的飞速发展,目前人们已着眼于人工设计、制备和组装各种介观多相复合材料,从而获得所需的各种独特性质。常见的材料已经涉及人们日常生活的方方面面,包括半导体、金属、超导体、铁磁体、氧化物陶瓷、高聚物等等,材料的结构包括微米或纳米尺度的多晶、颗粒、薄膜、纤维及其它一些非均质结构。对多相复合体系的研究也突破了之前的关于输运过程的若干观念和规律,使得人们必须重新研究这一体系的物理过程。
现代的透射电子显微镜(TEM)、扫描电子显微镜(STM)等技术已可揭示在介观乃至微观尺度上的材料的非均质结构,但是多相复合材料中的各单相的微结构特征以及定量解决微结构与材料宏观物性的关联问题尚未得到透彻的理解,需要更深入的讨论。理论和计算方法的不断进步可以帮助理解和预言多相复合体系的有效宏观物性,如体系的有效电导、有效介电常数、有效弹性模量等。迄今最主要的理论研究方法可分为两类:连续模型和离散模型。连续模型通过求解经典的连续输运方程,可以成功的解释许多非均质结构的有效宏观物性和输运性质。但是对于强无序的多相复合体系,各单相的性质差异很大,或者某个单相已经形成了较大团聚簇时,多数的连续模型都会失效。离散模型,由于其本质是建立在用晶格描述材料的微结构,因此可以在介观尺度上正确预言材料的性质,成为连续模型的有益补充。虽然对离散模型的研究已经获得一些成熟的理论,但是离散模型并未很好的运用到多相复合材料的具体输运性质的研究中去。
半金属氧化物的多晶、冷压缩粉末以及与其它材料的复合物等纳米颗粒体系是目前被广泛研究的一类多相复合体系。体系中存在纳米尺寸的半金属颗粒(晶粒)相和绝缘性的颗粒边界(晶界)相。颗粒边界相,包括各种自然和人工晶界、粉末颗粒表面、复合材料中的颗粒界面等多种情况。对该多相复合体系的磁输运性质的研究均发现了颗粒边界磁电阻效应(即由于大量颗粒边界的存在,导致低磁场下产生显著的电阻下降)。在半金属氧化物颗粒体系中,颗粒边界相的作用至关重要,颗粒边界磁电阻效应与穿越颗粒边界相的自旋输运过程密切相关。颗粒边界本身的性质,如局域态的存在、颗粒边界的磁、电性质等会导致不同的输运机制(直接隧穿、共振隧穿等)。半金属颗粒体系中存在的半金属颗粒相和颗粒边界相,在一定的外界条件(温度、磁场、电场和应力等)下,还会形成不同的拓扑结构,即不同的输运网络,它们都会导致复杂多变的颗粒边界磁电阻行为。对半金属氧化物多相复合体系的磁输运网络效应的研究,不仅在理论上具有重要的学术价值,同时还在技术和商业层面上有着广阔的应用前景,是材料科学和凝聚态物理研究领域中的前沿课题。
导电聚合物也是被广泛研究的一类多相复合体系,是近年来备受关注的新一代功能聚合物材料。对材料的微观结构分析表明,在高导电聚合物中同时存在结晶度高,链排列有序,电性能好的纳米尺度的“金属”相和链排列无序,电性能较差多孔相,整个体系呈现介观非均质的结构特征,可以看成是链连接的颗粒网络体系。聚合物的纳米颗粒体系中,金属颗粒之间的耦合不同于普通的颗粒混合体系是直接的点接触,而是通过穿越金属颗粒的聚合物长链来实现的,因此具有许多特殊的电输运现象,如它的金属绝缘转变不存在严格意义上的临界点,它的临界行为可以在相当大的温度范围内观察得到,并且会随外压、磁场等因素发生变化;体系的有效介电常数,有效电导也表现出许多异于普通金属的特征。但是目前已建立的模型尚无法对导电聚合物多相复合体系中链形成的金属相和多孔相,及其形成的拓扑结构对整个体系的电输运的过程的影响作出全面的描述。
近年来实验和理论研究表明,作为高迁移率有机薄膜晶体管有源层的半导体聚合物材料,如聚噻吩层状多晶材料中,聚合物长链的排列对迁移率有很大的影响。可以通过表面处理和自组织的方法改善半导体聚合物材料中链间的无序状态,从而提高了载流子的迁移率;在自组织的多晶聚噻吩体系中,颗粒边界的电子结构决定了体系迁移率的大小。迄今获得的半导聚合物的高迁移率是和聚合物的链结构的有序度,特别是颗粒边界处的性质密切相关的。但是目前已建立的模型尚无法对导电聚合物特殊的链形成的颗粒边界,及其对整个体系的电输运的过程的影响作出全面的描述。
本文对多相复合体系的输运性质的研究主要集中在:
1.半金属氧化物颗粒体系的磁输运
我们研究了锰氧化物多晶以及CrO_2冷压缩粉末颗粒体系的磁输运行为。我们发现对于多晶和粉末样品类的分散型多相复合体系,已有的电阻并联和串联模型的计算结果与实验测量的数据仍有较大的差异。我们考察其微结构特征,发现其中的半金属颗粒相被绝缘的颗粒边界相包裹,并完全分隔开,因此颗粒间输运占主导地位。我们着重考虑其介观非均质的特征,基于多相复合体系的离散模型,我们分析了电阻并联和串联模型的不足,首次提出和建立了电导存在一定分布的无规电阻网络模型,研究了体系的磁电阻效应随网络维度和电导无序分布程度的变化。
对半金属氧化物多晶或粉末样品,我们仅考虑其主导输运机制-直接隧穿机制,分别建立电阻网络模型和串、并联电阻模型,通过三者间的相互比较以及与实验结果的对比,说明基于连续模型的串、并联电阻模型均存在不足之处。我们首次通过计算直接证实了,对于高自旋极化的半金属氧化物颗粒体系,磁输运的网络(拓扑)效应是不容忽视的;随着电阻网络的维度(晶格配位数)增加,磁电阻减小,且该效应随自旋极化率增大而更明显;隧穿电导的无序分布影响磁电阻值,无序度大的颗粒体系将具有较大的磁电阻效应。这些结果有效的解释了相关的实验测量数据。
显然,与串、并联电阻模型相比,无规电阻网络模型更适合用于描述多相复合体系的微结构特征以及微结构和磁输运性质的关联。
2.高导电聚合物的电输运
我们主要研究了高导电聚苯胺和聚吡咯的电子动态。考察该多相复合体系的微结构,我们发现在高导电聚合物中同时存在结晶度高,链排列有序,电性能好的“金属”微晶相和链排列无序,电性能较差多孔相;金属相始终被多孔相分隔开;金属颗粒没有固定的形状并具有量子尺寸;金属相之间的耦合是通过穿越金属区域的聚合物长链来实现的。计算表明,颗粒间的转移积分要比链间积分大两个数量级左右,而颗粒间的相互作用其本质是电子通过链内共价键的过程。因此,我们发展了以前的理论工作,首次基于无规电阻网络模型,首次提出了链连接的颗粒体系模型,并考虑了电子的相干和非相干的输运过程,以及逾渗效应,很好的解释了高导电聚合物在金属绝缘转变过程中的频率响应的特点。
在我们的电阻网络模型中,我们在格点上定义不同浓度的金属和绝缘两种性质的颗粒,格点之间的电阻代表了不同相之间的链耦合。这种链耦合随探测频率和温度的改变,会引起宏观电导及介电常数的变化,从而导致高导电聚合物会在临界耦合附近发生金属绝缘相变。我们的计算结果表明,高导电聚合物是处于金属绝缘相变边缘的特殊体系,它的低能结构主要取决相干和非相干两种输运机制之间的竞争,从而导致介电常数由正到负的变化以及电导的非单调变化,我们计算结果很好的解释了实验结果。
3.半导聚合物的电输运
我们主要研究了层状多晶的聚噻吩薄膜材料中的电输运过程。在聚噻吩的多晶样品中,链排列有序的层状结晶区,电子的迁移率很高,但是在链排列无序的晶界(颗粒边界)处,电子的迁移率小几个数量级,因此在颗粒边界处的电子的行为决定了整个体系的迁移率大小。薄膜晶体管迁移率提高的主导因素是提高聚合物链结构的尤其是晶界处的有序化。
我们构造了微晶取向不同导致的几种聚合物晶界结构,研究无序的颗粒边界对电输运性质的影响。我们首次用第一性原理的方法研究了聚噻吩的晶界处的电子结构,计算结果表明,聚噻吩层状多晶颗粒体系的电输运是各向异性的,无序度不同的晶界结构,会导致薄膜晶体管沟道电导相差几个数量级,直接证实了颗粒边界对半导聚合物电输运的作用至关重要。
综合以上的讨论,对于多相复合体系,无论是半金属氧化物颗粒体系还是导电/半导聚合物体系,都具介观非均质的结构特点,体系中存在着性质差异较大的两相。其中每相的形状、尺寸等(对应多相复合体系的几何结构),以及不同相之间的分布、连接、相互作用等(对应多相复合体系的拓扑结构)共同决定了多相复合体系的输运性质。我们基于离散模型建立的输运无规电阻网络模型可以再现复杂的微结构对输运过程的影响,从而方便的分析与输运相关的其它行为,如体系的有效电导、介电常数等。
In recent years, the multiphase composite systems have attracted much attention in material science and condensed physics. The multiphase composites are characteristic of the heterogeneous structure and all the phases, which may have different elements or physical properties from each other, are distributed with certain statistical law. The changes of the concentration, structure and distribution of the single phases will all lead to the changes of the macroscopic properties of the system accordingly. The most important parameter to characterize such multiphase composite systems is the microscopic morphology, which consists of materials' geometry, mainly referring to the sizes and shapes of the phases, and materials' topology, describing the how the phases connect to one another, and materials' surface structure, reflecting the roughness features. Of course all them are closely related to the length scale we choose to observe and measure the materials. There has been a long history since we studied such systems and with the rapid progress of the material science and nano-technology, people are now focusing on the design, preparation, and assembly of various mesco-composites to achieve particular functions. The man-made composites have been utilized in all aspects of our daily life, ranging from semiconductors, metals, superconductors, magnets and oxide-ceramics to polymers. They may exist as micro- or nano- polycrystal, grains, thin film, fiber and other heterogeneous structures. The studies of the multiphase composites have broken some of the old concepts and rules, making people to re-investigate the physical process governing such systems.
Modern technologies like tunneling electron microscope (TEM), scanning tunneling microscope (STM) have shown up the composites' heterogeneous structure at mesoscopic or even microscopic length scale. The microscopic morphology and its relation to the macroscopic physical features, however, have not been understood thoroughly and need our further discussion. Development of the theory and calculation method may help to better understand and predict the effective physical properties, like the effective conductivity, effective dielectric constant and effective module of such systems. The most successful approaches ever used are the continuum models and the discrete models. Based on solving the classical continuum equations of transport, the continuum models can predict the effective features and transport properties of the composite materials. Rarely, however, can such continuum models provide accurate predictions of the macroscopic properties of strongly disordered multiphase materials. In particular, when the contract between phases is large or the phases form large clusters, most continuum models break down. At the same time, due to their very nature, the discrete models, which are based on a lattice representation of a material's morphology, have the ability for providing accurate predictions for the effective properties of such heterogeneous systems. In this sense, the discrete models are good complementary to the continuum models. Though theories have been established about such discrete models, they have not been well applied to the studies of the transport characters in multiphase composite systems.
Half-metallic oxides granular systems like polycrystalline samples, cold pressed powers and other composites are one of the most widely studied multiphase composite systems. The nano-size half metallic grain phase always coexists with the grain boundarie (GB) regions. The "grain boundaries" here include not only natural but also some artificial GB junctions, grain surfaces in pressed powders and various other composites. Grain-boundary magnetoresistance effect is found due to the existence of lots of GBs, which play a key role in the magneto-transport process in such systems, because the magneteresistance is closely related to the spin-polarized process in the boundaries. Actually, the properties of GBs, such as the density of local spins inside, the electronic and magnetic features of GBs will result in different transport mechanisms like direct tunneling, resonant tunneling and etc.. When applied magnetic or electrical field or under the press, the half-metallic and GBs phases will form some kinds of topology structures, i.e. different transport network, and result in complex grain-boundary magnetoresistance behaviors accordingly. Therefore, half-metallic multiphase composites have fundamental meanings and extensive technological applications.
Conducting polymers system is another widely studied multiphase composites, and is a new kind of functional materials as well. Microstructure studies point out that there are nanosized crystalline regions where the polymer chains are regularly and densely packed and therefore can be treat as metallic grains. Outside these regions the order in chain arrangement is poor and the chains form amorphous media. The whole system has the feature of heterogeneous disorder, and the two phases couple into a chain-linked granular network. The coupling between the metallic grains is provided by single chains, which may extend into several regions, therefore the electrical connection is totally different from that of dot connection in conventional granular systems. Some novel transport phenomenon are find such as it has no well defined critical point for its disorder induced metal-insulator transition, which has been observed over a relatively wide temperature range and can be tuned by applying external pressure and/or magnetic fields, the same to its effective conductivity and dielectric coefficient. But to date no models can give a comprehensive description about the conducting polymer multiphase composite systems.
Theoretical and experimental researches have proved that high performance of semi-conducting polymer thin film transistors with high mobility resulted from the increase of the improvement of polymer structures. The most studied poly(3-alkylthiophene)s can achieve great increase in mobility by self-organize to polycrystalline lamella structures after some deposit surface treatment process, which will lead to good polymer chain packing. And the boundaries between the crystalline parts determine the charge transport and the high mobility achieved so far all related to the properties of these boundaries. But theoretical explanations are still lack.
The purpose of our work is focused on the charge- and magneto-transport in the two multiphase composite systems. The main results are listed as follows:
1. The magneto-transport in half-metallic granular systems
We study the magnet-transport process in half-metallic granular systems, including manganite polycrystalline system and CrO_2 cold pressed powders. We found that the existing resistor series connection model and parallel connection model can't well describe the experimental results. Investigating the detailed micro-structure in such dispersed systems, we find that most of the half-metallic are packed and separated by the grain boundaries. The transport across the grain boundaries dominates. Considering the mesoscopic heterogeneous characters, we establish a random resistor network (RRN) model with the resistors are distributed, which reflect the disorder of the system and may overcome the disadvantages of the previous series and parallel connection model. We can conveniently use RRN model to discuss the effect of the network dimensionality and the resistors random distribution on the magneto-transport process since it is based on the discrete model.
For granular half-metallic oxides like some polycrystalline samples and pressed powders, we only consider the direct tunneling mechanism since it dominates in such systems. We build the random resistor network (RRN) model and compare it with the series connection model and parallel connection model. The latter two are based on the continuum model therefore are not sufficient to describe the transport process. Our calculations prove that for the half-metallic granular systems with high spin polarization, the transport network effects can't be ignored. With the increase of the dimension of the network, the magnetoresistance value decreases and such effect is more prominent for higher spin polarization materials. The random distribution of the tunneling resistance will influence the magnetotransport in such a way that the more disordered system has the larger magnetoresistance value, and all these well agree with the related experimental data.
We conclude that in half-metallic multiphase composites, the RRN model is a much more better candidate to describe the micro-structure and its relation with the magnetotransport properties.
2. The charge—transport of highly conducting polymer systems
We study the carrier dynamics in conducting polymers such as polypyrrole and polyaniline. We observe that the systems are composite medium with metallic regions randomly distributed in the amorphous parts. Within the metallic regions, the polymer chains are regularly and densely packed, outside which the poorly arranged chains form amorphous host. The grains have quantum size their shapes can't be described by a single geometric form. The energy scale for interchain charge transfer is about two orders of magnitude smaller than that for intergrain charge transfer. The intergrain transport is essentially an intrachain process that proceeds along the covalently bound chains. We further the previous work, and based on the random resistor network (RRN) model we propose for the first time a chain linked granular system. Including the coherent and incoherent transport mechanism in our RRN model, we reproduce the dielectric response through an metal-insulator transition in conducting polymers.
In our RRN model, we place metallic and insulating dots on the lattice sites. The resistors inserted between them represent different couplings, which may change with exploring frequency and temperature. The effective conductivity and dielectric can therefore be calculated within the framework of RRN model at different external conditions. Our calculation show that the highly conducting polymers are a particular composite near metal to insulator transition and the low energy carrier dynamics is determined by the competition between the coherent and incoherent channels, which result in the unusual frequency dependent conductivity and multiple zero crossing of dielectric function of highly conducting polymers. Our calculations are in good agreement with the experimental results.
3. The charge—transport of semi-conducting polymer systems
In this section we study the charge transport in semi-conducting polymers of poly-(3hexylthiophene), (P3HT) with polycrystalline lamella structures, where in the crystalline parts, the chain packing densely and orderly, while the parts between them form the disordered regions. The mobility in the disordered boundaries is several orders less than that of the crystalline parts, therefore determines the effective mobility of the composite system. The improvement in the structures of these boundaries is most dominant factor.
We here build several boundary structures resulting from the crystallites orientations and study their electronic structures using the first principle method. Our calculation results show that the charge transport in such system is mesco-anisotropic, and the conductance for different boundary structures ranges for sever orders in magnitude. Our calculations give a theoretical proof that it is necessary to consider heterogeneous structures and especially the electronic process in boundaries.
In summary, the multiphase composite systems, for both the half-metallic granular materials and conducting polymers, are characterized with the heterogeneous structures. There are always two or more phases, which have remarkable difference in physical features. Both the materials' geometry, mainly including the sizes and shapes of the phases, and materials' topology, describing the how the phases connect to one another, determine the transport process and therefore the macro-properties of such systems. Our RRN model, since based on the discrete model, can successfully use to describe the microstructures of the composites and reproduce many experimental results.
现代的透射电子显微镜(TEM)、扫描电子显微镜(STM)等技术已可揭示在介观乃至微观尺度上的材料的非均质结构,但是多相复合材料中的各单相的微结构特征以及定量解决微结构与材料宏观物性的关联问题尚未得到透彻的理解,需要更深入的讨论。理论和计算方法的不断进步可以帮助理解和预言多相复合体系的有效宏观物性,如体系的有效电导、有效介电常数、有效弹性模量等。迄今最主要的理论研究方法可分为两类:连续模型和离散模型。连续模型通过求解经典的连续输运方程,可以成功的解释许多非均质结构的有效宏观物性和输运性质。但是对于强无序的多相复合体系,各单相的性质差异很大,或者某个单相已经形成了较大团聚簇时,多数的连续模型都会失效。离散模型,由于其本质是建立在用晶格描述材料的微结构,因此可以在介观尺度上正确预言材料的性质,成为连续模型的有益补充。虽然对离散模型的研究已经获得一些成熟的理论,但是离散模型并未很好的运用到多相复合材料的具体输运性质的研究中去。
半金属氧化物的多晶、冷压缩粉末以及与其它材料的复合物等纳米颗粒体系是目前被广泛研究的一类多相复合体系。体系中存在纳米尺寸的半金属颗粒(晶粒)相和绝缘性的颗粒边界(晶界)相。颗粒边界相,包括各种自然和人工晶界、粉末颗粒表面、复合材料中的颗粒界面等多种情况。对该多相复合体系的磁输运性质的研究均发现了颗粒边界磁电阻效应(即由于大量颗粒边界的存在,导致低磁场下产生显著的电阻下降)。在半金属氧化物颗粒体系中,颗粒边界相的作用至关重要,颗粒边界磁电阻效应与穿越颗粒边界相的自旋输运过程密切相关。颗粒边界本身的性质,如局域态的存在、颗粒边界的磁、电性质等会导致不同的输运机制(直接隧穿、共振隧穿等)。半金属颗粒体系中存在的半金属颗粒相和颗粒边界相,在一定的外界条件(温度、磁场、电场和应力等)下,还会形成不同的拓扑结构,即不同的输运网络,它们都会导致复杂多变的颗粒边界磁电阻行为。对半金属氧化物多相复合体系的磁输运网络效应的研究,不仅在理论上具有重要的学术价值,同时还在技术和商业层面上有着广阔的应用前景,是材料科学和凝聚态物理研究领域中的前沿课题。
导电聚合物也是被广泛研究的一类多相复合体系,是近年来备受关注的新一代功能聚合物材料。对材料的微观结构分析表明,在高导电聚合物中同时存在结晶度高,链排列有序,电性能好的纳米尺度的“金属”相和链排列无序,电性能较差多孔相,整个体系呈现介观非均质的结构特征,可以看成是链连接的颗粒网络体系。聚合物的纳米颗粒体系中,金属颗粒之间的耦合不同于普通的颗粒混合体系是直接的点接触,而是通过穿越金属颗粒的聚合物长链来实现的,因此具有许多特殊的电输运现象,如它的金属绝缘转变不存在严格意义上的临界点,它的临界行为可以在相当大的温度范围内观察得到,并且会随外压、磁场等因素发生变化;体系的有效介电常数,有效电导也表现出许多异于普通金属的特征。但是目前已建立的模型尚无法对导电聚合物多相复合体系中链形成的金属相和多孔相,及其形成的拓扑结构对整个体系的电输运的过程的影响作出全面的描述。
近年来实验和理论研究表明,作为高迁移率有机薄膜晶体管有源层的半导体聚合物材料,如聚噻吩层状多晶材料中,聚合物长链的排列对迁移率有很大的影响。可以通过表面处理和自组织的方法改善半导体聚合物材料中链间的无序状态,从而提高了载流子的迁移率;在自组织的多晶聚噻吩体系中,颗粒边界的电子结构决定了体系迁移率的大小。迄今获得的半导聚合物的高迁移率是和聚合物的链结构的有序度,特别是颗粒边界处的性质密切相关的。但是目前已建立的模型尚无法对导电聚合物特殊的链形成的颗粒边界,及其对整个体系的电输运的过程的影响作出全面的描述。
本文对多相复合体系的输运性质的研究主要集中在:
1.半金属氧化物颗粒体系的磁输运
我们研究了锰氧化物多晶以及CrO_2冷压缩粉末颗粒体系的磁输运行为。我们发现对于多晶和粉末样品类的分散型多相复合体系,已有的电阻并联和串联模型的计算结果与实验测量的数据仍有较大的差异。我们考察其微结构特征,发现其中的半金属颗粒相被绝缘的颗粒边界相包裹,并完全分隔开,因此颗粒间输运占主导地位。我们着重考虑其介观非均质的特征,基于多相复合体系的离散模型,我们分析了电阻并联和串联模型的不足,首次提出和建立了电导存在一定分布的无规电阻网络模型,研究了体系的磁电阻效应随网络维度和电导无序分布程度的变化。
对半金属氧化物多晶或粉末样品,我们仅考虑其主导输运机制-直接隧穿机制,分别建立电阻网络模型和串、并联电阻模型,通过三者间的相互比较以及与实验结果的对比,说明基于连续模型的串、并联电阻模型均存在不足之处。我们首次通过计算直接证实了,对于高自旋极化的半金属氧化物颗粒体系,磁输运的网络(拓扑)效应是不容忽视的;随着电阻网络的维度(晶格配位数)增加,磁电阻减小,且该效应随自旋极化率增大而更明显;隧穿电导的无序分布影响磁电阻值,无序度大的颗粒体系将具有较大的磁电阻效应。这些结果有效的解释了相关的实验测量数据。
显然,与串、并联电阻模型相比,无规电阻网络模型更适合用于描述多相复合体系的微结构特征以及微结构和磁输运性质的关联。
2.高导电聚合物的电输运
我们主要研究了高导电聚苯胺和聚吡咯的电子动态。考察该多相复合体系的微结构,我们发现在高导电聚合物中同时存在结晶度高,链排列有序,电性能好的“金属”微晶相和链排列无序,电性能较差多孔相;金属相始终被多孔相分隔开;金属颗粒没有固定的形状并具有量子尺寸;金属相之间的耦合是通过穿越金属区域的聚合物长链来实现的。计算表明,颗粒间的转移积分要比链间积分大两个数量级左右,而颗粒间的相互作用其本质是电子通过链内共价键的过程。因此,我们发展了以前的理论工作,首次基于无规电阻网络模型,首次提出了链连接的颗粒体系模型,并考虑了电子的相干和非相干的输运过程,以及逾渗效应,很好的解释了高导电聚合物在金属绝缘转变过程中的频率响应的特点。
在我们的电阻网络模型中,我们在格点上定义不同浓度的金属和绝缘两种性质的颗粒,格点之间的电阻代表了不同相之间的链耦合。这种链耦合随探测频率和温度的改变,会引起宏观电导及介电常数的变化,从而导致高导电聚合物会在临界耦合附近发生金属绝缘相变。我们的计算结果表明,高导电聚合物是处于金属绝缘相变边缘的特殊体系,它的低能结构主要取决相干和非相干两种输运机制之间的竞争,从而导致介电常数由正到负的变化以及电导的非单调变化,我们计算结果很好的解释了实验结果。
3.半导聚合物的电输运
我们主要研究了层状多晶的聚噻吩薄膜材料中的电输运过程。在聚噻吩的多晶样品中,链排列有序的层状结晶区,电子的迁移率很高,但是在链排列无序的晶界(颗粒边界)处,电子的迁移率小几个数量级,因此在颗粒边界处的电子的行为决定了整个体系的迁移率大小。薄膜晶体管迁移率提高的主导因素是提高聚合物链结构的尤其是晶界处的有序化。
我们构造了微晶取向不同导致的几种聚合物晶界结构,研究无序的颗粒边界对电输运性质的影响。我们首次用第一性原理的方法研究了聚噻吩的晶界处的电子结构,计算结果表明,聚噻吩层状多晶颗粒体系的电输运是各向异性的,无序度不同的晶界结构,会导致薄膜晶体管沟道电导相差几个数量级,直接证实了颗粒边界对半导聚合物电输运的作用至关重要。
综合以上的讨论,对于多相复合体系,无论是半金属氧化物颗粒体系还是导电/半导聚合物体系,都具介观非均质的结构特点,体系中存在着性质差异较大的两相。其中每相的形状、尺寸等(对应多相复合体系的几何结构),以及不同相之间的分布、连接、相互作用等(对应多相复合体系的拓扑结构)共同决定了多相复合体系的输运性质。我们基于离散模型建立的输运无规电阻网络模型可以再现复杂的微结构对输运过程的影响,从而方便的分析与输运相关的其它行为,如体系的有效电导、介电常数等。
In recent years, the multiphase composite systems have attracted much attention in material science and condensed physics. The multiphase composites are characteristic of the heterogeneous structure and all the phases, which may have different elements or physical properties from each other, are distributed with certain statistical law. The changes of the concentration, structure and distribution of the single phases will all lead to the changes of the macroscopic properties of the system accordingly. The most important parameter to characterize such multiphase composite systems is the microscopic morphology, which consists of materials' geometry, mainly referring to the sizes and shapes of the phases, and materials' topology, describing the how the phases connect to one another, and materials' surface structure, reflecting the roughness features. Of course all them are closely related to the length scale we choose to observe and measure the materials. There has been a long history since we studied such systems and with the rapid progress of the material science and nano-technology, people are now focusing on the design, preparation, and assembly of various mesco-composites to achieve particular functions. The man-made composites have been utilized in all aspects of our daily life, ranging from semiconductors, metals, superconductors, magnets and oxide-ceramics to polymers. They may exist as micro- or nano- polycrystal, grains, thin film, fiber and other heterogeneous structures. The studies of the multiphase composites have broken some of the old concepts and rules, making people to re-investigate the physical process governing such systems.
Modern technologies like tunneling electron microscope (TEM), scanning tunneling microscope (STM) have shown up the composites' heterogeneous structure at mesoscopic or even microscopic length scale. The microscopic morphology and its relation to the macroscopic physical features, however, have not been understood thoroughly and need our further discussion. Development of the theory and calculation method may help to better understand and predict the effective physical properties, like the effective conductivity, effective dielectric constant and effective module of such systems. The most successful approaches ever used are the continuum models and the discrete models. Based on solving the classical continuum equations of transport, the continuum models can predict the effective features and transport properties of the composite materials. Rarely, however, can such continuum models provide accurate predictions of the macroscopic properties of strongly disordered multiphase materials. In particular, when the contract between phases is large or the phases form large clusters, most continuum models break down. At the same time, due to their very nature, the discrete models, which are based on a lattice representation of a material's morphology, have the ability for providing accurate predictions for the effective properties of such heterogeneous systems. In this sense, the discrete models are good complementary to the continuum models. Though theories have been established about such discrete models, they have not been well applied to the studies of the transport characters in multiphase composite systems.
Half-metallic oxides granular systems like polycrystalline samples, cold pressed powers and other composites are one of the most widely studied multiphase composite systems. The nano-size half metallic grain phase always coexists with the grain boundarie (GB) regions. The "grain boundaries" here include not only natural but also some artificial GB junctions, grain surfaces in pressed powders and various other composites. Grain-boundary magnetoresistance effect is found due to the existence of lots of GBs, which play a key role in the magneto-transport process in such systems, because the magneteresistance is closely related to the spin-polarized process in the boundaries. Actually, the properties of GBs, such as the density of local spins inside, the electronic and magnetic features of GBs will result in different transport mechanisms like direct tunneling, resonant tunneling and etc.. When applied magnetic or electrical field or under the press, the half-metallic and GBs phases will form some kinds of topology structures, i.e. different transport network, and result in complex grain-boundary magnetoresistance behaviors accordingly. Therefore, half-metallic multiphase composites have fundamental meanings and extensive technological applications.
Conducting polymers system is another widely studied multiphase composites, and is a new kind of functional materials as well. Microstructure studies point out that there are nanosized crystalline regions where the polymer chains are regularly and densely packed and therefore can be treat as metallic grains. Outside these regions the order in chain arrangement is poor and the chains form amorphous media. The whole system has the feature of heterogeneous disorder, and the two phases couple into a chain-linked granular network. The coupling between the metallic grains is provided by single chains, which may extend into several regions, therefore the electrical connection is totally different from that of dot connection in conventional granular systems. Some novel transport phenomenon are find such as it has no well defined critical point for its disorder induced metal-insulator transition, which has been observed over a relatively wide temperature range and can be tuned by applying external pressure and/or magnetic fields, the same to its effective conductivity and dielectric coefficient. But to date no models can give a comprehensive description about the conducting polymer multiphase composite systems.
Theoretical and experimental researches have proved that high performance of semi-conducting polymer thin film transistors with high mobility resulted from the increase of the improvement of polymer structures. The most studied poly(3-alkylthiophene)s can achieve great increase in mobility by self-organize to polycrystalline lamella structures after some deposit surface treatment process, which will lead to good polymer chain packing. And the boundaries between the crystalline parts determine the charge transport and the high mobility achieved so far all related to the properties of these boundaries. But theoretical explanations are still lack.
The purpose of our work is focused on the charge- and magneto-transport in the two multiphase composite systems. The main results are listed as follows:
1. The magneto-transport in half-metallic granular systems
We study the magnet-transport process in half-metallic granular systems, including manganite polycrystalline system and CrO_2 cold pressed powders. We found that the existing resistor series connection model and parallel connection model can't well describe the experimental results. Investigating the detailed micro-structure in such dispersed systems, we find that most of the half-metallic are packed and separated by the grain boundaries. The transport across the grain boundaries dominates. Considering the mesoscopic heterogeneous characters, we establish a random resistor network (RRN) model with the resistors are distributed, which reflect the disorder of the system and may overcome the disadvantages of the previous series and parallel connection model. We can conveniently use RRN model to discuss the effect of the network dimensionality and the resistors random distribution on the magneto-transport process since it is based on the discrete model.
For granular half-metallic oxides like some polycrystalline samples and pressed powders, we only consider the direct tunneling mechanism since it dominates in such systems. We build the random resistor network (RRN) model and compare it with the series connection model and parallel connection model. The latter two are based on the continuum model therefore are not sufficient to describe the transport process. Our calculations prove that for the half-metallic granular systems with high spin polarization, the transport network effects can't be ignored. With the increase of the dimension of the network, the magnetoresistance value decreases and such effect is more prominent for higher spin polarization materials. The random distribution of the tunneling resistance will influence the magnetotransport in such a way that the more disordered system has the larger magnetoresistance value, and all these well agree with the related experimental data.
We conclude that in half-metallic multiphase composites, the RRN model is a much more better candidate to describe the micro-structure and its relation with the magnetotransport properties.
2. The charge—transport of highly conducting polymer systems
We study the carrier dynamics in conducting polymers such as polypyrrole and polyaniline. We observe that the systems are composite medium with metallic regions randomly distributed in the amorphous parts. Within the metallic regions, the polymer chains are regularly and densely packed, outside which the poorly arranged chains form amorphous host. The grains have quantum size their shapes can't be described by a single geometric form. The energy scale for interchain charge transfer is about two orders of magnitude smaller than that for intergrain charge transfer. The intergrain transport is essentially an intrachain process that proceeds along the covalently bound chains. We further the previous work, and based on the random resistor network (RRN) model we propose for the first time a chain linked granular system. Including the coherent and incoherent transport mechanism in our RRN model, we reproduce the dielectric response through an metal-insulator transition in conducting polymers.
In our RRN model, we place metallic and insulating dots on the lattice sites. The resistors inserted between them represent different couplings, which may change with exploring frequency and temperature. The effective conductivity and dielectric can therefore be calculated within the framework of RRN model at different external conditions. Our calculation show that the highly conducting polymers are a particular composite near metal to insulator transition and the low energy carrier dynamics is determined by the competition between the coherent and incoherent channels, which result in the unusual frequency dependent conductivity and multiple zero crossing of dielectric function of highly conducting polymers. Our calculations are in good agreement with the experimental results.
3. The charge—transport of semi-conducting polymer systems
In this section we study the charge transport in semi-conducting polymers of poly-(3hexylthiophene), (P3HT) with polycrystalline lamella structures, where in the crystalline parts, the chain packing densely and orderly, while the parts between them form the disordered regions. The mobility in the disordered boundaries is several orders less than that of the crystalline parts, therefore determines the effective mobility of the composite system. The improvement in the structures of these boundaries is most dominant factor.
We here build several boundary structures resulting from the crystallites orientations and study their electronic structures using the first principle method. Our calculation results show that the charge transport in such system is mesco-anisotropic, and the conductance for different boundary structures ranges for sever orders in magnitude. Our calculations give a theoretical proof that it is necessary to consider heterogeneous structures and especially the electronic process in boundaries.
In summary, the multiphase composite systems, for both the half-metallic granular materials and conducting polymers, are characterized with the heterogeneous structures. There are always two or more phases, which have remarkable difference in physical features. Both the materials' geometry, mainly including the sizes and shapes of the phases, and materials' topology, describing the how the phases connect to one another, determine the transport process and therefore the macro-properties of such systems. Our RRN model, since based on the discrete model, can successfully use to describe the microstructures of the composites and reproduce many experimental results.
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