Exploring the Potential Use of Metal-Supported Endofullerenes and Exofullerenes as Multistate Switches for Molecular Electronics

硅是制造晶体管和开关等当前电子元件的基础材料。随着电子集成的溶液处理技术(例如纳米光刻)的进步，每年都伴随着单个组件的尺寸减小。摩尔定律预测，在小型化过程中，电子元件很快就会达到比例极限。因此，研究硅基组件的替代品对于保持我们设计性能更好、成本效益更高、提供更大灵活性和降低能源消耗的电子设备的能力至关重要。分子电子学领域的目标是通过利用单个分子来执行单个组件的功能来促进电子设备的小型化[1]。分子开关被定义为在两种或两种以上状态下表现出稳定性的分子(例如，涉及电导、构象等的“开”和“关”)。一旦施加受控的外部微扰，无论是电的还是其他的，都会经历可逆的变化，从而使分子发生变化。以前的工作已经显示了具有多达四个和六个不同状态的多态分子开关[2，3]。我们在圣安德鲁斯的研究小组最近报道了一种使用面体内富勒烯Li@C60的多态单分子开关，它显示了14个可以统计访问的分子状态[4]。这是利用低温扫描隧道显微镜和光谱学实现的。在本论文中，我们打算进一步探索金属支载的富勒烯作为分子电子学的多态开关的应用。在我们想要解决的主题中：(1)Li@C60多重开关中的确切激发机制尚不清楚。我们认为它依赖于通过富勒烯笼子的超原子分子轨道的共振隧穿作为Li活化的一种手段[4]。我们将探索证明这一点或不证明这一点的策略。(2)其他内富勒烯物种是否表现出多态转换？(3)外富勒烯吸附在合适的载体上的研究迄今很少。我们将用扫描隧道显微镜研究由我们的有机化学合作者合成的几个精选体系。(4)光子(紫外光和近可见光)能否作为激发源在Li@C60中诱导Li跃迁？我们将把我们的扫描隧道显微镜装置与紫外光光源耦合起来，以研究外延富勒烯在新型纳米光电子器件中的潜在应用。本论文是多学科合作的一部分，涉及开发基于面内富勒烯的多态分子开关的理论和实验研究。我们的合作者是：(1)Eleanor Campbell教授(爱丁堡)，负责气相光谱测量；(2)Andreas Stasch博士(圣安德鲁斯)，负责部分外富勒烯的有机合成；(3)艾米·邱博士(A*STAR新加坡)，负责基于密度泛函的高水平计算。[1]Aviram，A.；Ratner，M.A.化学物理通讯1974，29,277-283.[2]Auwaarter，W.；Seufert，K.；Bischoff，F.；Ecija，D.；Vijayaraghavan，S.；Joshi S.；Klaypenberger，F.；Samudrala，N.；Barth，J.V.自然纳米技术2012，7，41-46.[3]Huang，T.；赵，J.；冯，M.；Popov，A.A.；Yang，S.；Dunsch，L.；Petek，H.Nano Letters 2011，11,5327-5332.[4]Chandler，H.J.；Stefan ou，M.；Campbell，E.E.B.；Schaub，R.Natural Communications 2019，10,2283.

Silicon is the base material for current electronic components such as transistors and switches. With progress in solution processing techniques for electronics integration (e.g. nanolithography), every year is accompanied with a reduction in size of individual components. Moore's law predicts that the electronic components will soon reach scaling limits in the miniaturization process. Hence, research into alternatives to silicon-based components is vital to sustain our ability to devise better performing and cost-effective electronic devices that afford greater flexibility and reduced energy consumption.The field of molecular electronics aims at advancing the miniaturization of electronic devices, by exploiting single molecules to perform the function of individual components [1]. A molecular switch is defined as a molecule that displays stability in two or more states (e.g. "on" and "off" involving conductance, conformation etc.) and upon application of a controlled external perturbation, electric or otherwise, undergoes a reversible change such that the molecule is altered. Previous work has shown multi-state molecular switches with up to four and six distinct states [2,3]. Our research group in St Andrews has recently reported on a multi-state single molecule switch using the endohedral fullerene Li@C60 that displays 14 molecular states which can be statistically accessed [4]. This was achieved using low temperature scanning tunnelling microscopy and spectroscopy. In this thesis, we propose to further explore the use of metal-supported fullerenes as multistate switches for molecular electronics. Amongst the topics we want to address: (1) The exact excitation mechanism operating in Li@C60 multi-switching is unknown. We believe that it relies on resonant tunnelling via the superatom molecular orbitals of the fullerene cage as a means of Li activation [4]. We will explore strategies to prove this or otherwise. (2) Do other endofullerene species display multi-state switching? (3) Exofullerenes adsorbed on adequate supports have been poorly investigated to date. We will study, by means of STM, a few selected systems synthesized by our organic chemistry collaborators. (4) Can photons (UV and near-VIS) be used as excitation source to induce Li switching in Li@C60? We will couple our STM setup with a UV source to investigate the potential use of exofullerenes towards novel nano-opto-electronic devices.The present thesis work is part of a multidisciplinary collaboration involving theoretical and experimental studies related to the development of multistate molecular switches based on endohedral fullerenes. Our collaborators are: (1) Prof. Eleanor Campbell (Edinburgh) for gas-phase spectroscopy measurements, (2) Dr Andreas Stasch (St Andrews) for the organic synthesis of selected exofullerenes, and (3) Dr Amy Khoo (A*STAR Singapore) for high-level DFT-based calculations. [1] Aviram, A.; Ratner, M. A. Chemical Physics Letters 1974, 29, 277-283.[2] Auwaarter, W.; Seufert, K.; Bischoff, F.; Ecija, D.; Vijayaraghavan, S.; Joshi, S.; Klappenberger, F.; Samudrala, N.; Barth, J. V. Nature Nanotechnology 2012, 7, 41-46.[3] Huang, T.; Zhao, J.; Feng, M.; Popov, A.A.; Yang, S.; Dunsch, L.; Petek, H. Nano Letters 2011, 11, 5327-5332.[4] Chandler, H.J.; Stefanou, M.; Campbell, E.E.B.; Schaub, R. Nature Communications 2019, 10, 2283.