单分子电导测量技术及其影响因素

doi:10.3866/PKU.WHXB201909043

摘要/Abstract

摘要：

分子电子学是纳米技术的一个重要应用领域，其最终目的是基于单个分子或分子阵列构建功能器件，实现与宏观器件相同的功能。为了实现这一目的，测量和控制单分子内部的电荷传递方式是非常有必要的。在单分子电学领域，存在着多种多样的单分子电导测量技术和环境影响因素。这篇综述对分子电子学发展至今所包含的单分子电导测量技术进行了分类总结，将所有技术归纳为固结法和裂结法两大类，并对每一类技术做了展开阐述。除此之外，本文还详细地介绍了当前学术界比较关心的内容：单分子电导影响因素。本文从内部因素(锚定基团、电极、目标分子)和外部因素(电压、温度、溶剂、pH值等)两个角度出发，对不同电导影响因素进行了比较全面的介绍。另外，本文也对近几年发展的调控分子电导的新型手段(通过能级调控以及光、热等刺激)进行了概述，并对利用这些手段实现的单分子尺度化学反应的相关研究进行了总结。最后，本文对这些测量技术以及电导调控手段在单分子电学领域内的潜在应用进行了总结和展望。

关键词: 单分子电导测量, 固结法, 裂结法, 锚定基团, 电导影响因素

Abstract:

Molecular electronics is an important field for the application of nanotechnologies with an ultimate goal of building functional devices using single molecules or molecular arrays to realize the same functionality as macroscopic devices. To attain this goal, reliable techniques for measuring and manipulating electron transfer processes through single molecules are essential. There are various techniques and many environmental factors influencing single-molecule electronic conductance measurements. In this review, we first provide a detailed introduction and classification of the current well-accepted techniques in this field for measuring single-molecule conductance. All available techniques are summarized into two categories: the fixed junction technique and break junction technique. The break junction technique involves repeatedly forming and breaking molecular junctions by mechanically controlling a pair of electrodes moving into and out of contact in the presence of target molecules. Single-molecule conductance can be determined from the conductance plateaus that appear in typical conductance decay traces when molecules bind two electrodes during their separation process. In contrast, the fixed junction technique is to fix the distance between a pair of electrodes and measure the conductance fluctuations when a single molecule binds the two electrodes stochastically. Both techniques comprise different application methods and have been employed preferentially by different groups. Specific features of both techniques and their intrinsic advantages are compared and summarized in Section 4.

Next, we systematically summarize the factors affecting the molecular conductance during the course of measurements, which are the focus of the current academic community in the field. As shown in the middle of the image above, the electrode, anchoring group, and target molecule are the three key elements in constructing a single-molecule junction. The contact geometry between the molecule and the electrode and the associated coupling strength can profoundly affect the conductance characteristics of single molecules. The properties of anchoring groups can determine whether a molecule is primarily transported by electrons or holes. A good selection of electrode materials can improve the yield of single-molecule junctions and facilitate strong electronic couplings with the target molecules. The conductance characteristics of saturated and conjugated molecules are quite different due to their diverse band gaps. Moreover, various substituents can be modified onto the backbone of the molecules, which can raise or lower the energy level of the frontier orbitals of molecules to different degrees, thereby affecting the conductance properties of target molecules. In addition to these factors for the key elements, the investigated molecules can be measured in a variety of environments, including organic solvents, high vacuum, aqueous solution, ionic liquids, or the atmosphere, and the work function of the metal contact may change in different environments. The change in work function can change the gap between the electrode Fermi energy and the frontier orbital of a target molecule, influencing the measured conductance. Additionally, both the electrical field orientation and the bias values applied have a significant effect on the molecular structures and thus their conductance characteristics. Temperature can also affect the charge transport when hopping dominates the transport mechanism. Meanwhile, pH affects the interactions between H⁺/OH^- and the anchoring groups, which indirectly induces a change in the tunneling barrier.

In this review, the influencing factors are comprehensively illustrated from two perspectives: internal factors (anchoring group, electrode, and target molecule) and external factors (voltage, temperature, solvent, pH value, and others). In addition, new approaches for modulating the molecular conductance (modulation of energy levels, light or heat stimulation, and others) that have been developed in recent years are reviewed, and investigations of chemical reactions at the single-molecule level using these methods are highlighted. Finally, the potential applications of these techniques and correlated modulating approaches are summarized and a prospective is provided for the field of single-molecule electronics.

Key words: Single molecule conductance measurement, Fixed junction, Break junction, Anchoring group, Influencing factor of conductance

程鹏坤, 李云川, 常帅. 单分子电导测量技术及其影响因素[J]. 物理化学学报, 2020, 36(11): 1909043.

Pengkun Cheng, Yunchuan Li, Shuai Chang. Techniques and Influencing Factors for Single Molecule Electronic Conductance Measurements[J]. Acta Physico-Chimica Sinica, 2020, 36(11): 1909043.

链接本文: https://www.whxb.pku.edu.cn/CN/10.3866/PKU.WHXB201909043

https://www.whxb.pku.edu.cn/CN/Y2020/V36/I11/1909043

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Techniques	Categories	Nanogap between the electrodes	Single molecule level?	Advantages	Disadvantages
Fixed junction	Electromigration	~1 nm ⁶	Possible (when the length of molecule is approximately equal to the size of nanogap)	High reproducibility ⁶, High stability, Belonging to nano-scale devices, Nanogap can be characterized with precision	Difficulty in repeatedly obtaining single molecule junctions, Needing nano fabrication, Low compatibility with different lengths of investigated molecules
	On-wire lithography	2 to several hundred nanometers ^{3, 4}	Possible (when the length of molecule is approximately equal to the size of nanogap)	High stability, Belonging to nano-scale devices, Nanogap can be characterized with precision	Difficulty in accurately controlling the gap distance, Difficulty in repeatedly obtaining single molecule junctions, Needing nano fabrication, Low compatibility with different lengths of investigated molecules
	STM-FJ	0–3 nm	Yes	The size of nanogap can be conveniently adjusted，High stability	For unknown molecules, a range of gap size needs to be characterized to determine the optimal gap conditions for single molecule conductance measurements
Break junction	STM-BJ	In the process of dynamic change	Yes	Fast creation of repeated molecular junctions	Needing repeatedly breaking metal contacts, thus limiting the metal selection, Difficulty in constructing molecular junctions with different metal electrode materials ⁵⁸
Break junction	MCBJ	In the process of dynamic change	Yes	Fast creation of repeated molecular junctions	Needing repeatedly breaking metal contacts, thus limiting the metal selection, Difficulty in constructing molecular junctions with different metal electrode materials ⁵⁸