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The rapid development of micro-nano electronic systems has greatly increased the urgent demand for device miniaturization and integration. Molecular electronics seeks to assemble devices at the scale of individual molecules in a "bottom-up" manner and expects to assemble logic circuits or even molecular computers from a single molecule or several molecules. In recent years, with the development of micro & nano-fabrication technology, molecular electronics has made great progress. The core technology in this field is the manufacture and related applications based on the electronic model of "electrode-molecule-electrode junction". However, there are still many challenges in precision machining and fine control at the atomic scale in molecular electronics. Recently, Prof. Junyang Liu, Dr. Yi Zhao, Prof. Wenjing Hong and Prof. Zhongqun Tian et al. from the College of Chemistry and Chemical Engineering of Xiamen University published a review article entitled "The fabrication, characterization and functionalization in molecular electronics" in the International Journal of Extreme Manufacturing. In this paper, the fabrication and regulation at the atomic scale in molecular electronics are systematically summarized from three aspects: fabrication, characterization and functionalization. Figure 1 gives an overview of fabrication, characterization and functionalization in molecular electronics.
Figure 1. Overview of fabrication, characterization and functionalization in molecular electronics
Molecular electronics, Molecular junction, Molecular electronic device, Fabrication and functionalization
● Molecular electronics is a science that studies the quantum behavior of electrons tunneling through molecules, in which the electric property of a single-molecule is an intrinsic fingerprint property strongly correlated to its structure, offering opportunities to develop new operating strategies of electronic devices for future possible solution to extend and even surpass beyond the Moore’s Law.
● The electrode-molecule-electrode junction (molecular junction) is the basic unit and model of molecular electronics, which can be achieved by precise fabrication and regulation methods.
● The basic aspects of fabrication, characterization and functionalization in molecular electronics are described.
With the approach of sub-5 nm nodes in the complementary metal-oxide-semiconductor (CMOS) chip manufacturing, the size and machining accuracy of CMOS-based devices gradually enter molecular and even atomic scales (left in Figure 2). However, to keep pushing the limit into this scale, the quantum tunneling effect especially that between source and drain electrodes will ultimately become obvious and nonnegligible. Therefore, to further extend Moore’s Law and even surpass beyond it, new operating principles of electronic devices based on quantum tunneling effect need to be considered. At this point, molecular electronics and the corresponding developed research tools may offer future opportunities, demonstrating a new operating strategy for ‘Beyond CMOS’ (right in Figure 2).
Figure 2. Molecular electronics, a potential candidate for ‘Beyond CMOS’.
Molecular electronics, that is, the science which investigates the electronics at the molecular level. At least one dimension of the study system is nanoscale, or even a single molecule. Electrode-molecule-electrode junction (referred to as molecular junction) is the basic model of molecular electronics, as shown in Figure 3. For researches on molecular electronics, how to realize the connection between single molecule and external measurement circuit to form stable molecular junction, how to accurately acquire and adjust the electrical signals of molecular junctions, and how to realize the preparation of corresponding high-performance devices are the key scientific problems to be solved. This review is based on these issues.
Figure 3. Schematic of electrode - molecule - electrode (molecular junction).
3. Recent Advances
This review mainly revolves around the basic model molecular junction of molecular electronics, as shown in Figure 4. The section 1 briefly introduces the research background of molecular electronics. The section 2 briefly describes the theoretical background of the charge transport mechanism in molecular electronics. In section 3, we comprehensively summarize the fabrication methods of molecular junctions. It is divided into three categories, including static junctions, dynamic junctions and large-scale array junctions. The section 4 discusses the characterization and regulation of molecular electronics at the atomic scale. The section 5 is the functionalization of molecular junctions, that is, various molecular devices including molecular rectifiers, optical switches, transistors, etc. Finally, we briefly summarize some challenges and opportunities in this field.
Figure 4. Three essential aspects of molecular electronics: fabrication, characterization and functionalization.
Fabrication of Molecular Junction
The molecular junction is the fundamental model of molecular devices. Stable molecular junctions require the construction of reliable contacts between atomic-scale electrodes and target molecules: first, the electrodes need to be precisely controlled to fit the size of the molecules and even adjust their conformation; second, the molecule should be modified with anchoring groups at both ends for better linkage; both of these require some degree of atomic precision control. The author mainly summarizes them into three categories for discussion: one is a series of "static molecular junction" technologies prepared with fixed electrodes, which cannot directly adjust the distance; second is the "dynamic molecular junction" technology by multiple repeated construction of single-molecule junctions, which based on the repeated opening and closing of electrodes; The third is the "large-area array junction" method developed for integrated molecular junctions. Figure 5 gives the comparisons of different approaches of molecular junctions fabrication.
Figure 5. Comparison of different molecular junctions fabricated by different approaches.
Precise characterization and regulation of molecular junction
After the accurate fabrication of molecular junction, how to characterize and regulate the properties of the molecular junction become the next important issue. In this part, the author mainly concludes and discusses the following three aspects: Firstly, charge transport properties through molecular junction characterized by electrical measurement are the top priority; Secondly, the electrical responses by the outer stimulation such as optoelectronic signal or the spectroscopic information can be obtained under the photo-illumination; what’s more, the corresponding thermoelectric properties can be extracted when adding temperature control. Figure 6 shows some cases of electrical characterization, spectroscopy characterization and thermoelectric characterization of molecular junction introduced in this part.
Figure 6. Some examples of (a) electrical characterization, (b) spectroscopy characterization, (c) thermoelectric characterization of molecular junctions.
Functionalization of molecular junction
The ultimate goal of molecular electronics is to design electronic components from the atomic and molecular levels, which can realize the same functions as silicon-based electronic devices. The molecular junctions, are unable to provide a research foundation for the preparation of functional molecular electronic devices through accurately manufactured, precisely characterized and elegantly controlled. Furthermore, researchers can control the charge transport capacity of molecular junction by changing the temperature, magnetic field, light illumination, external electric field and electrochemical potential of the molecular junction, so as to realize the construction of molecular devices with specific functions. Figure 7 shows some examples of molecular junction applications in rectifiers, switches, transistors and spintronic devices.
Figure 7. Some examples of molecular junction applications in (a) rectifiers, (b) switches, (c) transistors, and (d) spintronic devices.
Although molecular electronics has made great progresses in recent years, there are still many challenges involved in precise machining and fine regulation at atomic scale. First, it is necessary to optimize and perfect the existed manufacturing characterization methods. By combining the most advanced micro- & nano-fabrication equipment, a molecular junction can be obtained with a processing accuracy close to a single atom and the increased manufacturing stability. The second is to make more complex configurations of molecular junctions to achieve multifunctional detection and regulation. In addition to its own development, molecular electronics can also be used as a research tool to be compatible and mutually promoted with the development of other disciplines. Thirdly, starting from the ultimate goal of molecular electronics, how to implement the prototypical molecular junctions into functional circuit components is the core problem in future exploration. How to form a stable and reliable molecular junction with an adjustable molecule-electrode interface is still the key challenge, whereas the molecular electronic device can really be moving forward into application after well solved. Therefore, the next crucial step for molecular electronics is to develop manufacturing technologies to realize the construction of integrated molecular electronic devices with high stability and reproducibility.
5. About the Authors
Dr. Junyang Liu is an associate professor and doctoral supervisor in the College of Chemistry and Chemical Engineering, Xiamen University, who is also a selectee of the Nanqiang Young Scholars Project B. He also joined Tan Kah Kee Innovation Laboratory (IKKEM), one of the first provincial laboratories in Fujian Province, as the assistant minister of condition guarantee department. He was supported by several projects from the National Natural Science Foundation of China (NSFC). His research mainly engages in the research of molecular electronics, using molecular engineering, micro and nano-fabrications technology, in-situ spectroscopy technology to construct single-molecule scale functional materials and devices and characterize their structure-function relationship, to explore the material and device basis of future information technology. He has published 16 papers in Chem, Angew. Chem. Int. Ed., Sci. Adv., Acc. Chem. Res., et al. as first author or corresponding author.