金属有机配合物M-TCNQ微纳米结构的制备及电学性能研究

【作者】 叶春暖；

【导师】 陈国荣；

【作者基本信息】 复旦大学 ， 物理电子学， 2005， 博士

【摘要】 纳米材料在纳米电子学、光子学、磁器件等方面有着广阔的应用空间。自从Iijima发现纳米碳管以来，各种新颖的一维纳米材料如纳米线、纳米棒、纳米带和纳米同轴电缆等相继被发现，其中纳米线和纳米管是纳米尺度材料科学的前沿，它们最有意义的应用是在纳米电子学领域。纳米线既可以用作功能单元，也可以作为导电沟道，是研究电输运的良好模型体系；对纳米线电学性质的研究是实现纳米电子器件的基础。所以纳米电子学最近受到了全球范围的关注和快速发展。 目前，碳纳米管在场发射显示方面的研究获得了极大的进展，但走向实用化还是遭遇了难以克服的困境，主要由于半导体和金属性的碳纳米管共存且技术上很难实现分离。另外，碳纳米管制备时的温度较高，对设备要求也就较高，故而成本也难以降低。作为一种有机半导体，电荷转移型金属有机配合物M-TCNQ块体和薄膜材料具有电双稳特性，有的还具有准一维导电性，在光电开关、存储器件方面具有广泛的应用前景，被认为是理想的分子电子学材料。但是目前M-TCNQ的一维结构研究甚少，它的一维纳米结构可能具有独特的光、电特性。研究其一维生长的过程及工艺以及它们的潜在应用，例如：场发射，如何利用单根M-TCNQ纳米线组成纳电子器件等，对于了解M-TCNQ晶体的生长特性和在纳电子领域的应用具有重要科学意义。有关有机材料的功函数测试工作报道较少，研究M-TCNQ纳米线的场发射性能，有必要了解其功函数大小。 M-TCNQ具有柱状的堆积结构，一定条件下可以获得其一维生长。M-TCNQ薄膜在强电场作用下的纳秒内能够从高阻态转变为低阻态，这一特性可能使得M-TCNQ一维纳米结构具有优异的场发射性能。理论上，取向排列好、离散、有序分布的纳米线＼管阵列或均匀分布的薄膜都将具有良好的场发射性能。所以本论文围绕M-TCNQ一维纳米结构的制备和电学性能研究，重点研究了在M-TCNQ纳米线的蒸气输运反应法的制备工艺、生长机理及其场发射性能。 分别利用传统的溶液反应法、真空条件下的蒸气输运反应法和直接气相反应法制备M-TCNQ一维微纳米结构，通过SEM、XRD、XPS等测试手段，对它们的形貌、结构和成分进行了表征。重点研究了蒸气输运反应法制备M-TCNQ纳米线的工艺及生长机理。金属薄膜的厚度及均匀性、反应温度和时间、真空度、基片种类等对纳米线的形貌、晶体结构有直接影响。通过对反应时间的控制，观察到了处于不同生长阶段的Ag-TCNQ纳米线；结合银膜在不同条件下热处理引起的形貌变化，提出了取向纳米线和纳米带等形貌按VS机制的生长过程。 在上述M-TCNQ纳米结构制备研究的基础上，测试了Ag-TCNQ微纳米线及纳米线阵列的电学性质。首先利用电子束刻蚀技术和光刻技术分别制作由纳米线和微米线组成的两端器件，利用函数发生器、示波器和探针系统测试其I-V特性，发现了单根微纳米具有较好的开关特性和负电阻特性，有希望用于超高密度存储以至于纳米级的场效应晶体管；观察到了纳米线阵列也具有电双稳性质。然后将纳米线阵列制作成过电压保护器原型，结点数大大增加有可能提高安全系数，通过电容充放电测试了器件原型的转变时间。 测试了Ag-TCNQ纳米线样品的功函数。UPS谱测试得到的平均功函数为5.82eV；STM谱粗略测出局域功函数小于5eV。然后，利用自建的二极系统从实验上研究了M-TCNQ纳米线阵列的场致发射性能影响因素，总的说来制备M-TCNQ时的初始金属薄膜厚度、反应温度和时间以及测试时的极间距等对场发射性能均有影响。一般来说，金属膜厚度在10-20nm时的M-TCNQ纳米线具有较低的开启电压，约在2.0V／μm左右。 为了进一步改善场发射性能，在纳米线表面蒸镀低功函数的LiF膜，或者选用金属片作为纳米线生长的衬底进行了研究。表面镀LiF的Ag-TCNQ纳米线在5V／μm的场强下达到0.275mA／cm~2的发射电流，稳定性得到了较大的提高。在金属基片上制备的Ag-TCNQ纳米线在电场为2.0V／μm时的电流密度为0.5mA／cm~2，Cu-TCNQ纳米线在电场为0.7V／μm时的电流密度为0.6mA／cm~2，比Si基片上的样品有了较大的性能改善。 利用单根纳米线的电学性质和场发射样品的XRD谱解释了场发射中的电流“阶跃”现象。结合纳米线的导电性、纳米线与基片的接触界面以及纳米线尖端形貌分析了场电子的发射过程。利用经典F-N理论对场发射数据进行分析，估算出Ag-TCNQ的功函数约为1.29eV，Cu-TCNQ功函数为0.50eV。从实用角度看，需要进一步提高场发射电流和改善场发射稳定性。 希望以上对电荷转移型金属有机配合物M-TCNQ微纳米结构的制备及一些应用基础方面的研究对今后M-TCNQ的电子器件研制起到一些参考作用。更多还原

【Abstract】 One-dimensional semiconductor nanomaterials are now being studied for their novel physical properties. Among them, nanowires and nanotubes are at the frontier of materials science owing to their unique chemical, physical and mechanical properties, with the most interesting applications in nanoelectronics. At present, various one-dimensional nano-materials, for instance, carbon nanotubes, have been synthesized and their field emission properties were studied, but the commercial realization in field emission display are still full of challenges, because of the complicated synthesis technique and difficulty in purifying the mixture of the metallic and semiconductor carbon nanotubes.Metal-organic complexes of M-TCNQ have columnar stacking structure, which can be used to synthesize the nanowires. So the interest to study their electrical property, and further realize the novel nano-electronic devices was initiated. In this dissertation, the synthesis and field electron emission properties of M(Ag、Cu)-TCNQ nanowires were studied.Firstly, growth conditions of Ag、Cu-TCNQ micro/nanostructures by the vapor transport reaction were systematically investigated. It is exciting that the semiconducting nanowires of Cu-TCNQ phase I was obtained by this method which is hard to synthesize by the solution reaction method. It was also found that the thickness of M(Ag、 Cu) film, reaction temperature and time greatly influence the diameter, density and alignment of the resulting nanowires. The formation processes of Ag-TCNQ nanowires proceed through the steps of nucleation and directional growth, which are governed by the vapor-solid growth mechanism. Other morphologies such as belts and flowers were also obtained due to some uncontrollable variations in the growth conditions.The I-Vcurve for a two-terminal device with a single Ag-TCNQ nanowire shows that it has the electric bistable property, similar to the Ag-TCNQ films. A prototype of over-voltage protector made of the nanowires array has good performance with the response time shorter than 870 ns.The field-induced electron emission from the Ag、 Cu-TCNQ nanowires were measured, respectively. Results show that M-TCNQ nanowires grown with the 10-20 nm metal film have a low turn-on field of about 2.0 V/μm, and the nanowires on metal substrate has current density of about 0.5 mA/cm~2 at 2.0 V/μm for Ag-TCNQ, 0.6mA/cm以~2 at 0.7 V/μm for Cu-TCNQ, respectively, higher than those nanowires on Si substrate. The process of electron emission is similar to the two-process model. The stability in emission current can be improved by depositing a LiF layer on the surface of the M-TCNQ nanowires. Therefore, M-TCNQ nanowires are believed to be a candidate for displaying information.In summary, the synthesis of one-dimensional M-TCNQ nanowires and their potential application in field emission were investigated. These studies may be useful for the crystal growth, organic nano-devices such as electric switches and information storage, and potential application in cold cathode materials for information display.更多还原