Electron transport through extended metal atom chains

在过去的40年里，电子工业的急剧扩张是建立在集成电路中硅基半导体元件尺寸逐渐缩小的基础上的。事实上，摩尔在他1965年被广泛引用的论文中预测了这一趋势，他预测这种电路的性能每12个月大约增加2倍（“摩尔定律”）。然而，半导体电路的小型化不能无限期地继续下去，我们正迅速接近量子效应将阻止利用现有技术进一步大幅提高计算机性能的阶段。因此，分子电子学领域在过去几年中迅速发展，该领域寻求识别和开发构成集成电路的晶体管的更小的分子类似物。简单地说，电子电路中任何元件的关键特性都是控制电流的能力。因此，大多数工作都集中在共轭碳基材料上，其中离域pi系统提供了一个明显的电子传递途径。然而，最近，两个实验小组（Peng，台北和Cotton，德克萨斯州）独立开发了一种基于有机材料绝缘护套包围的金属原子延伸链的替代策略。这些延伸金属原子链（EMACs）和宏观电线表面上的相似之处非常吸引人，但是，在更微妙的层面上，定义核心的金属-金属键的柔韧性为控制电子传递提供了一系列可能性。这个提议的最初灵感来自于我们对一个相当简单的EMAC的电子结构的探索，它包含一个只有三个钴原子的金属核心。这个物种非凡的结构化学一直是一个谜：分子显然可以存在至少两种不同的形式，其结构明显依赖于温度。利用密度泛函理论，我们证明了该系统的独特性质是通过分子内电子密度的双向再分配产生的：电子在一个方向上通过sigma框架运动，在另一个方向上通过delta轨道的流形运动，以响应温度的细微变化。这一发现促使我们提出一个非常简单的问题：如果这种电子的双向流动是对如此细微的环境变化的反应，那么如果系统被置于外加电压下，就像在集成电路中一样，会发生什么？最令人兴奋的可能性是，这种分子可能充当分子整流器，使电流在一个方向上（例如，通过sigma框架）比在另一个方向上更容易流动。结二极管的电流整流可能是导致20世纪50年代晶体管发展的最重要的发现，分子类似物的发现很可能对下一代计算机架构产生类似的影响。早在1974年，Aviram和Ratner就提出了分子整流器（基于共轭芳烃）的基本特征，但尚未考虑到金属链可能以类似的方式起作用，尽管是通过完全不同的机制。我们的目标是利用我们对隔离emac电子结构的详细了解作为平台，探索它们在外加电压下的行为。最终，我们希望利用我们从理论研究中获得的知识，为未来分子电子学元件的设计构建一个合理的框架。

The dramatic expansion of the electronics industry over the past 40 years has been based on the progressive reduction in size of the silicon-based semiconductor components of integrated circuits. Indeed, Moore anticipated this trend in his much-quoted 1965 paper, where he predicted an approximate 2-fold increase in the performance of such circuits every 12 months ('Moore's law'). The minituarisation of semi-conductor circuits cannot, however, continue indefinitely, and we are rapidly approaching the stage where quantum effects will prevent further dramatic improvements in computer performance using existing technology. As a result, the field of molecular electronics, which seeks to identify and develop much smaller molecular analogues of the transistors that make up integrated circuits, has expanded rapidly over the past few years.At a simplistic level, the key property of any component of an electronic circuit is its ability to control the flow of electric current. As a result, most work has focussed on conjugated carbon-based materials, where the delocalised pi system provides an obvious electron transport pathway. Very recently, however, an alternative strategy based on extended chains of metal atoms surrounded by an insulating sheath of organic material has been developed independently by two experimental groups (Peng, Taipei and Cotton, Texas). The superficial resemblance between these Extended Metal Atom Chains (EMACs) and macroscopic wires is very appealing, but, at a more subtle level, the inate flexibility of the metal-metal bonds that define the core offers a range of possibilities for controlling electron transport.The initial inspiration for this proposal came from our exploration of the electronic structure of a rather simple EMAC, containing a metal core of just three cobalt atoms. The remarkable structural chemistry of this species had been a long-standing mystery: the molecule could apparently exist in at least two distinct forms, whose structures were markedly dependent on temperature. Using density functional theory, we showed that the unique properties of this system arise through a bi-directional redistribution of electron density within the molecule: electrons move in one direction through the sigma framework and in the other through the manifold of delta orbitals in response to subtle changes in temperature. This discovery prompted us to ask a very simple question: if this bi-directional flow of electrons occurs in response to such subtle environmental changes , what might happen if the system is placed under an applied voltage, as it would be in an integrated circuit? The most exciting possibility is that the molecule might act as a molecular rectifier, allowing current to flow more easily in one direction (through the sigma framework, for example) than the other. Rectification of current in junction diodes was perhaps the single most important discovery that led to the development of the transistor in the 1950's, and the discovery of molecular analogues is likely to have a similar impact in the next generation computer architecture. Aviram and Ratner set out the basic features of a molecular rectifier (based on conjugated aromatics) as early as 1974, but the possibility that metal chains may act in a similar way, albeit through a completely different mechanism, has not yet been considered. We aim to use our detailed understanding of the electronic structure of the isolated EMACs as a platform to explore how they behave when placed under an applied voltage. Ultimately, we hope to use the knowledge gained from our theoretical study to construct a rational framework for the future design of components for molecular electronics.