In what could prove to be a significant advance in fabricating new technologies, scientists discovered a new self-assembly mechanism that surprisingly drives negatively charged molecules to clump together to form islands when graphene is supported by an electrical insulator. Under these conditions, different charge interactions are not diminished, as they are when graphene is supported by a metallic substrate. At low concentrations, individual adsorbed molecules repel each other, but with increasing concentration, the molecules form two-dimensional islands. It was determined by theory that the flow of extra electrons into the islands from graphene keeps the molecules together. The electronic driving forces and stabilization energies are sufficient to overcome the repulsion between the negative charges.
This self-assembly mechanism can be used to tune the electronic properties of graphene layers in devices and control how electrons flow through the graphene. This mechanism permits atomic-scale patterning of electronic properties, which cannot be achieved with conventional lithographic techniques currently being used in the semiconductor industry.
Silicon has been successful because it is an electronically tunable semiconductor material that can be used in electronic devices. Graphene has distinct advantages over silicon for many applications due to its higher electron mobility and a very stable crystal structure, but it can be difficult to precisely tune. One way to tune the electronic properties of graphene is to adsorb molecules onto its surface. For example, negatively charged molecules on a graphene surface pull electrons from the graphene layer, changing its electronic properties. However, efforts to controllably assemble such negatively charged molecules have been limited because negatively charged species repel each other. Now scientists led by the University of California-Berkeley and Lawrence Berkeley National Laboratory have discovered that this repulsion can be overcome and two-dimensional islands can be controllably formed by negatively charged molecules on graphene supported by an insulator. Through microscopy and theoretical modeling, they determined that the underlying insulator was key to altering the nature of the interactions between the negatively charged molecules and graphene. These molecules are known to extract electrons from its substrate. At low surface concentrations, the negatively charged molecules separately accept electrons from the underlying graphene and repel each other, as expected because like charges repel each other. Remarkably and counterintuitively, at higher concentrations, these charged molecules clump together to form ordered islands. This usual behavior is explained by theory as the donation of extra electrons to the islands of molecules by the graphene when compared to the donation to a single molecule. This extra charge makes it energetically more favorable to form islands. Surprisingly, this behavior observed on graphene substrate supported by an insulator does not occur when graphene is supported by a metal. This molecular self-assembly provides a possible alternative to patterning graphene using conventional lithographic techniques. Atomic-scale tuning of the properties of graphene layers could enable the fabrication of new devices based on graphene that cannot be made using silicon.
Michael F. Crommie
University of California-Berkeley
Lawrence Berkeley National Laboratory
This work was supported by the DOE Office of Science (Office of Basic Energy Sciences) (scanning transmission microscopy imaging, sample synthesis, theory), and the Molecular Foundry (graphene growth and characterization) and National Energy Research Scientific Computing Center (computational resources), DOE Office of Science User Facilities. This work was also supported by the National Science Foundation (device characterization), Department of Defense (graduate fellowship), Swiss National Science Foundation (postdoctoral fellowship), National Research Foundation (National University of Singapore), Austrian Science Fund (fellowship at University of California-Berkeley), MEXT Japan Elemental Strategy Initiative (synthesis of BN crystals), and Japan Society for the Promotion of Science (characterization of BN crystals and development of high pressure BN synthesis instrumentation).
H. Z. Tsai, A. A. Omrani, S. Coh, H. Oh, S. Wickenburg, Y. W. Son, D. Wong, A. Riss, H. S. Jung, G. D. Nguyen, G. F. Rodgers, A. S. Aikawa, T. Taniguchi, K. Watanabe, A. Zettl, S. G. Louie, J. Lu, M. L. Cohen, and M. F. Crommie, "Molecular self-assembly in a poorly screened environment: F4TCNQ on graphene/BN." ACS Nano 9, 12168 (2015). [DOI: 10.1021/acsnano.5b05322]