采用新型二维晶体构筑非对称单分子结及其电子传输机理的研究

Incorporation of single molecules into electronic devices holds great promise for decreasing the size and increasing the packing-density of emerging nano-electronic systems. Electronic components at the molecular level as well as taking advantage of their small size, could utilize their electronic properties and structural diversity to increase device density and enhance device functionality beyond present limits. Single molecule electronics has greatly advanced in recent years and it is now possible to reliably measure the electrical properties of single molecules in junctions. The most widely used tools for reliably measuring single molecule conductance are the scanning tunneling microscope (STM) and the conducting atomic force microscope (c-AFM). In the STM-based break-junction (STM-BJ) technique single molecules are captured between a gold STM tip and a gold substrate, such that a gold-molecule-gold junction is formed. The STM measures the current which flows through the junction in the single molecule analogue of an ammeter. To date most single molecule electronics measurements have focused on the use of gold contacts. However, it is now important that single molecule electronics research addresses the challenge of other contact materials which will have impact in the eventual implementation of molecular electronics. In this respect gold has a number of drawbacks; including its non-compatibility with complementary metal-oxide-semiconductor (CMOS) technologies, its mobility and also expense. Replacing gold electrodes in the proposed study is aimed towards providing cost effective and industrially scalable materials for integrated single molecule electronics beyond the current limits of miniaturization. .The aim of this project is to develop the use of two-dimensional (2D) materials for single molecule electronic devices, including molybdenum disulfide (MoS2) or tungsten disulfide (WS2), molybdenum diselenide (MoSe2) or tungsten diselenide (WSe2). Novel 2D materials have already been foreseen as an important future technology and work on these materials has only just begun. When one of the dimensions is extremely reduced, the 2D nanomaterials exhibit unique properties, such as a transition from indirect to direct semiconductor properties. Inspired by the structural analogue of graphene, 2D layer materials with nature semiconducting properties are ideal candidates to replace gold in future nanoelectronics. There are many challenges to be addressed in this project, which include the development of substrates and STM methods for single molecule measurements suited to these substrates, the evaluation of appropriate terminal chemical group for binding to these substrates, achieving reproducible contacts and stable junctions and measuring temperature-dependent conductance and electrochemical properties for these new device configurations. In the longer term this project will contribute new ideas towards the next generation of nanostructured devices.

作为电子器件的组装材料，单分子的应用给新兴的分子电子学领域带来了希望。将单分子与两端电极相连形成单分子结，电子在分子中的输运可通过扫描隧道显微镜（STM）来测定。迄今为止，大部分单分子电子元件的测定都采用了金触头（即金STM针尖和金基底），只有少量的研究工作涉及如氧化铟锡或者非金（Ag和Pt）电极。金有着许多弱点，它与互补的金属氧化物半导体技术非兼容性，它的移动性和昂贵价格。若能用其他材料取代金，解决其他触头材料的难题，这将对分子电子学的最终成功产生深远的影响。本研究提出以新型二维材料（MoS2, MoSe2, WS2, WSe2 等）作为一端电极，构筑可靠和可复制的分子结。二维材料具有类似石墨烯的优异特性，当一维度极小时，二维半导体会直接转变成导体。研究内容还包括揭示分子长度、锚定基团对于单分子电荷传输的影响。并以此为基础，开展以温度、气氛、电化学和PH值触发的单分子开关器件的研究。

本项目利用先进的扫描隧道显微镜技术研发了以新型二维材料作为一端电极的各种分子结。建立了诸如分子长度、锚定基团、电极材料等重要参数，对分子结电学性能的影响，并通过电化学环境获得纳米尺度上新型分子结的有效控制。具体研究内容如下：.1..项目掌握了高质量石墨烯、MoS2基底材料的制备工艺，建立了工艺参数优化与石墨烯导电薄膜性能的关系，论证了石墨烯可以用来取代金属作为电极材料，从而构建出了相应的金/分子/石墨烯非对称性分子结并测量了单分子的电导，实验的结果也通过理论计算得到了进一步的证实。.2..通过项目执行，我们发现使用石墨烯基底形成的非对称性分子结，整个体系衰减系数非常小。长链化合物仍然可以保持较好的电导率，有望大大降低单分子电子元件的损耗。.3..通过项目执行开发了一套无需人工值守，自动筛选电导台阶曲线的数据分析算法，极大的减少了分子电子学中数据分析的时间，避免了人工筛选带来的误差。.4..研究团队首次报道了一种完全不含金属的分子结，采用碳纤维作为顶部电极，石墨烯作为底部电极，构建了两端都是碳原子电极的碳钎维/烷烃分子/石墨烯单分子结。使用碳纤维代替了金电极后，其分子节与金-金电极体系相比依然有着类似的性能，展现了碳纤维作为分子器件电极材料的潜力。.5..通过以苯环分子为主链的共轭分子作为连接分子，讨论了金电极和石墨烯电极对分子电导的影响，以及锚定基团对分子电导的影响。进一步掌握了金/石墨烯体系下的共轭分子的电子转移特性，对构建共轭分子为基础的分子器件具有积极的意义。