绝缘衬底上有机分子电子学器件的构建—分子结构对电学特性的影响

With the development of information technology, the scale of electronic devices is becoming smaller and smaller. The conventional Si-based transistor is approaching its physical limit. More and more work is being undertaken on the development of new materials and devices. Organic molecules, due to their small size and diverse properties, attract great attention. Mechanically controllable break junction (MCBJ) is one of the most commonly used techniques that are able to measure electronic properties of a single molecular device. However, only one molecule device could be studied by using MCBJ at a time, which limited its applicability in the integration of multiple molecular devices. Scanning tunneling microscopy (STM) could be used to characterize molecular device on a substrate with very high spatial resolution, whereas only vertical properties of a single molecule could be measured. Another problem about STM measurement is that we have to use a conductive substrate for obtaining a tunneling current.. Electrical properties of molecular devices are undoubtedly determined by their chemical structures. However, the detailed molecular structure of an organic molecule, in a sub-molecular level, could not be observed experimentally until the chemical bonds of a pentacene molecule were visualized recently by using non-contact atomic force microscopy (NC-AFM). In this proposal, we will utilize ultra high vacuum NC-AFM/STM to study the effects of molecular chemical structure on its electrical properties. We will prepare some organic molecules with special functions on an insulating NaCl/Cu(111) substrate. Then we study their chemical structure and electronic structure in detail by using NC-AFM and STM. Next we construct electrical connection between molecule and conductive wire and measure the electrical properties. Molecular structure will be modified by using precise atom manipulation technique with NC-AFM and its influence on electrical properties will be studied. This work will deepen the understanding of relationship between chemical structures and their electrical properties of organic molecules, and will be helpful for people to design molecular electronic devices in future.

信息技术的发展要求电子学器件的尺寸越来越小。传统的Si基晶体管的尺寸正在接近它的物理极限。有机功能分子由于本身的小尺寸和丰富的功能，使其在构建未来电子学器件方面吸引了越来越多的注意。有机分子自身的化学结构无疑将决定分子功能器件的性能。然而，以前由于人们无法在实验上直接观测单个分子的化学结构，使人们对分子结构-物性关系的认识很难深入下去。最近几年非接触式原子力显微镜的发展，尤其在对分子化学结构进行高分辨率成像方面，取得了长足的进步。这为人们进行这一方面的研究提供了机遇。本项目将利用非接触式原子力显微镜对具有一定电子学特性的功能分子进行化学结构的详细表征。在亚分子层面上对其结构进行修饰。探索在绝缘衬底上制备分子器件，并测量其在水平面内电学特性的方法。全面了解分子的内部化学结构以及外部环境对其电子学性能的影响。为未来按照人类意愿设计分子电子学器件提供依据。

由于本身的小尺寸和丰富的特性，有机分子在构建纳米功能器件方面具有极大的潜力。分子电子学中的一个关键部件为分子开关。一种实现该功能的有效途径为构建具有固定转轴的可转动的分子器件。因此研究有机单分子转动行为的调控方式对于分子电子学中关键器件的设计具有重要意义。有机分子不仅可用于构建分子电子学器件，也可用于构建分子机械器件。而转动的分子可作为分子转子，用于组建更为复杂的分子机器。目前，人们已经实现了对分子转子开关状态的控制，然而分子转子在各转动构型的停留时间分布的精确控制仍然是有挑战性的问题。另一方面，尽管分子转子的转动激发方式有多种形式，如电、光和热等，其转动行为总是受转动势能面控制。因此，调节势能面对未来分子转子的可控运动极为重要。然而目前尚缺少对其转动势能面的有效调制方法。本项目在Au(111)表面构建了 (t-Bu)4-NiPc单分子转子，利用低温扫描隧道显微镜对其结构进行了表征，并系统研究了其在液氮温度下的转动行为。通过扫描隧道显微系统测量时间分辨的隧穿电流谱(I-t)，我们发现分子在两个相邻转动构型上的停留时间分布可由针尖-分子间相互作用可控调制。分析表明针尖下构型的势能随针尖-分子间距离减小而降低。此外，改变样品偏压也可有效调节转动势能。增加样品负偏压导致针尖下构型的势能增大。分子在两个吸附构型间的能量差随针尖高度和样品偏压变化率分别为 ∼0.3 meV/pm 和 ∼18 meV/V。另一方面，转动势垒也可得到有效调节。进一步分析表明其势能调控机制为范德华相互作用和分子偶极与电场间静电相互作用。本项目研究结果为探索外力调控固体表面分子转子的转动势能面提供了新途径，也为分子电子学中的单分子开关的设计提供了新思路。