Techniques and influencing factors for single molecule electronic conductance measurements
Article
Cheng, P, Li, Y, Chang, S. (2020). Techniques and influencing factors for single molecule electronic conductance measurements
. 36(11), 1-21. 10.3866/PKU.WHXB201909043
Cheng, P, Li, Y, Chang, S. (2020). Techniques and influencing factors for single molecule electronic conductance measurements
. 36(11), 1-21. 10.3866/PKU.WHXB201909043
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 singlemolecule electronic conductance measurements. In this review, we first provide a detailed introduction and classification of the current wellaccepted 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 singlemolecule 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.
