Solution‐Processed Reduced Graphene Oxide Films as Electronic Contacts for Molecular Monolayer Junctions

Abstract

In monolayer-based molecular electronics, functionalized molecules are utilized as active components; in electronic devices, these functionalities are accessed through stable electrical contacts. In such molecular junctions, a metallic contact can be used to complete a molecular electronic circuit. For practical applications, however, the formation of a soft contact is highly desired to avoid the possibility of a metal filamentary short circuit or molecular damage caused by the penetration of metal atoms into molecular monolayers during direct metal deposition. Therefore, contact fabrication techniques such as indirect metal evaporation, surfacediffusion-mediated deposition, and soft organic interlayer coating have been developed for molecular electronic devices. One successful technique used for the formation of stable molecular junctions involves the use of a conducting layer of an organic material such as single-walled nanotubes (SWNTs), a conducting polymer (e.g., poly(3,4-ethylenedioxythiophene):poly(4-styrenesulphonic acid), PEDOT:PSS), or multilayered graphene as a top electrode or a bridging interfacial layer between the active molecules and a top electrode. These layers can provide proper contact resistance to facilitate true molecular effects in monolayer-based devices composed of alkanethiols/alkanedithiols, photoisomers, p-conjugated organic molecules, or metal complexes. Nonetheless, although molecule-dependent electronic transport has been thoroughly investigated, previous interlayer junctions have not allowed for a clear elucidation of the intrinsic properties of functionalized molecules such as memory components in molecular monolayer devices. A reliable device system with a highly sensitive interlayer with molecular functionality is required for the further development of these molecular devices. To develop a highly sensitive interlayer with a high device yield, it is crucial to create an interlayer that is stable, both chemically and electrically, with a soft contact that is able to communicate between the functional molecular monolayer and a metal electrode. To optimize the sensitivity of the interlayer, its thickness should be easily controllable by varying the number of layers. For a stable junction, the interlayer should protect the molecular monolayers from the penetration of hot metal nanoparticles during the vapor deposition of a metallic top electrode because most functional molecular monolayers, which are only a few nanometres thick (< 2–3 nm), are vulnerable to this contamination. Chemically exfoliated graphene oxide (GO) consists of atomically thin sheets of oxidized graphite that are dispersible in various solvents and can be used to produce dispersible reduced graphene oxide (rGO) by chemical reduction. The conductivity of rGO is comparable to that of SWNT and is increased by thermal treatment, similar to graphite. Graphene and rGO are composed of sp carbon networks and can act as electrodes for electronic devices. One notable advantage of the use of rGO for organic electronics is its solution processability compared with that of graphene. For example, an rGO film electrode can be easily prepared by spin-coating an rGO solution onto a substrate or by spincoating with GO followed by vapor reduction. Additionally, the strong p–p interactions between rGO nanosheets result in a graphite interlayer distance of approximately 0.34 nm, leading to a high conductivity that is comparable to that of highly oriented pyrolytic graphite (HOPG). Electrical conduction in rGO thin films is treated as that through a semimetal such as graphite, and the contact resistance in rGO devices is negligible. Because rGO has high chemical stability, mechanical strength, and a work function comparable to that of gold, rGO has been considered to be a promising candidate for interfacial electronic contacts in monolayer molecular electronics. Herein, we report the development of a solution-processed electronic contact between rGO thin films and molecular components in monolayer-based devices. The rGO contact allows for stable monolayer junctions of alkanethiol monolayers (e.g., molecular resistors) and redox-active metal complex monolayers (e.g., molecular memories) that prevent the formation of metallic short circuits and exhibit excellent junction preservation. The effects of monolayer thickness and molecular functionality were examined in novel molecular junctions using rGO interlayers. Through the semimetallic rGO interlayer contact, the current hysteresis loops and threshold conductance [*] Dr. S. Seo, M. Min, Dr. J. Lee, Dr. H. Lee National Creative Research Initiative, Center for Smart Molecular Memory, Department of Chemistry, Samsung-SKKU Graphene Center, Sungkyunkwan University 300 Cheoncheon-dong, Jangan-gu, Suwon, Gyeonggi-do 440-746 (Korea) E-mail: hyoyoung@skku.edu