Center for Bioelectronics and Biosensors, the Biodesign Institute, Department of Electrical Engineering, Arizona State University, Tempe, Arizona 85287, USA.
Acc Chem Res. 2009 Mar 17;42(3):429-38. doi: 10.1021/ar800199a.
Electron movement within and between molecules--that is, electron transfer--is important in many chemical, electrochemical, and biological processes. Recent advances, particularly in scanning electrochemical microscopy (SECM), scanning-tunneling microscopy (STM), and atomic force microscopy (AFM), permit the study of electron movement within single molecules. In this Account, we describe electron transport at the single-molecule level. We begin by examining the distinction between electron transport (from semiconductor physics) and electron transfer (a more general term referring to electron movement between donor and acceptor). The relation between these phenomena allows us to apply our understanding of single-molecule electron transport between electrodes to a broad range of other electron transfer processes. Electron transport is most efficient when the electron transmission probability via a molecule reaches 100%; the corresponding conductance is then 2e(2)/h (e is the charge of the electron and h is the Planck constant). This ideal conduction has been observed in a single metal atom and a string of metal atoms connected between two electrodes. However, the conductance of a molecule connected to two electrodes is often orders of magnitude less than the ideal and strongly depends on both the intrinsic properties of the molecule and its local environment. Molecular length, means of coupling to the electrodes, the presence of conjugated double bonds, and the inclusion of possible redox centers (for example, ferrocene) within the molecular wire have a pronounced effect on the conductance. This complex behavior is responsible for diverse chemical and biological phenomena and is potentially useful for device applications. Polycyclic aromatic hydrocarbons (PAHs) afford unique insight into electron transport in single molecules. The simplest one, benzene, has a conductance much less than 2e(2)/h due to its large LUMO-HOMO gap. At the other end of the spectrum, graphene sheets and carbon nanotubes--consisting of infinite numbers of aromatic rings--have small or even zero energy gaps between the conduction and valence bands. Between these two limits are intermediate molecules with rich properties, such as perylene derivatives made of seven aromatic rings; the properties of these types of molecules have yet to be fully explored. Studying PAHs is important not only in answering fundamental questions about electron transport but also in the ongoing quest for molecular-scale electronic devices. This line of research also helps bridge the gap between electron transfer phenomena in small redox molecules and electron transport properties in nanostructures.
电子在分子内和分子间的运动——即电子转移——在许多化学、电化学和生物过程中都很重要。最近的进展，特别是在扫描电化学显微镜（SECM）、扫描隧道显微镜（STM）和原子力显微镜（AFM）方面，使得对单个分子内的电子运动进行研究成为可能。在本综述中，我们描述了单分子水平的电子输运。我们首先考察了电子输运（来自半导体物理）和电子转移（更一般的术语，指的是供体和受体之间的电子移动）之间的区别。这些现象之间的关系使我们能够将我们对电极之间单分子电子输运的理解应用于更广泛的其他电子转移过程。当电子通过分子的传输概率达到 100%时，电子输运效率最高；相应的电导则为 2e(2)/h（e 是电子的电荷，h 是普朗克常数）。在单个金属原子和连接在两个电极之间的金属原子链中已经观察到了这种理想的传导。然而，连接到两个电极的分子的电导通常要小几个数量级，并且强烈依赖于分子的固有性质及其局部环境。分子长度、与电极的连接方式、共轭双键的存在以及分子导线上可能的氧化还原中心（例如，二茂铁）的包含，对电导有显著影响。这种复杂的行为是导致各种化学和生物现象的原因，并且可能对器件应用有用。多环芳烃（PAHs）为单分子中的电子输运提供了独特的见解。最简单的苯，由于其较大的 LUMO-HOMO 能隙，其电导远小于 2e(2)/h。在光谱的另一端，由无限数量的芳香环组成的石墨烯片和碳纳米管具有导带和价带之间很小甚至为零的能隙。在这两个极限之间是具有丰富性质的中间分子，例如由七个芳香环组成的苝衍生物；这些类型的分子的性质尚未得到充分探索。研究 PAHs 不仅对于回答关于电子输运的基本问题很重要，而且对于正在进行的分子尺度电子器件的探索也很重要。这条研究线还有助于弥合小分子氧化还原分子中的电子转移现象与纳米结构中的电子输运性质之间的差距。
