Single crystalline materials have excellent quality as well as extreme performance capabilities, which has triggered a series of engineering applications in turbine blades in aircraft and silicon wafers in electronics. The nature as well as size of defects in these single crystalline materials therefore become of great interest when it comes to the direct measure of intrinsic material attributes and the design of superior material performance and attributes.
2D materials, such as graphene, for instance, are unique in the sense that they have zero bulk and normally represent the theoretical minimum material thickness. Defects in these materials often constitute holes or pores in an atomically thin membrane. These holes and pores in atomically thin membranes provide a possibility of creating a new form of membrane with transformative enhancements in selectivity and permeance. On the other hand, such defects ought to be minimized so that these materials can be used as membrane barriers in electronics as well as packaging.
The realization of these improvements will thus depend on developing an in-depth understanding of the nature as well as size of defects in atomically thin membranes. Atomically thin graphene is a perfect model system for probing defects in a material. Unfortunately, the fabrication and isolation of large area graphene continues to be a challenge.
Researchers led by Professor Rohit Karnik at and from Massachusetts Institute of Technology, United States, probed nanoscale mass transport across technologically relevant large area, (approximately 0.2 cm2), atomically thin single crystalline graphene synthesized form silicon carbide. This was perhaps the first instance of probing mass transport across a macroscopic thin 2D single crystal. Their work is published in peer-reviewed journal, Advanced Materials.
Lead author Piran R. Kidambi (currently an assistant professor at Vanderbilt University) and colleagues chose 10% porosity polycarbonate track etched membranes with about 200 nm cylindrical pores as supports for single crystalline graphene. A defined geometry minimized cross talk between pores and allowed for better interpretation of mass transport across single crystalline graphene. The authors then applied a layer-resolved graphene transfer process to exfoliate graphene from the silicon carbide substrate where a nickel stressor was previously deposited. Then an adhesive tape frame was used to offer mechanical stability in the process of exfoliation of the nickel single crystalline graphene stack form the silicon carbide substrate.
From the experiment, the authors demonstrated that transport behavior was consistent with the presence of sub-nanometer to nanometer size defects in silicon carbide-derived single crystalline graphene, considering that the graphene transfer process encompassing thermal evaporation of nickel was less likely to initiate defects.
Getting large area single crystals of 2-Dimensional materials have been the focus for electronic applications. The authors however observed from their results that for barrier applications, handling intrinsic defects within the crystal was equally important. In this study, the authors pursued the hypothesis whether single crystalline graphene possesses transport pathways. They found clear evidence of size-selective transport that could enable the realization of atomically thin membranes for small molecule separation or in desalting applications.
The outcomes of their study provide a fill for critical technological void in the field by offering a platform to analyze barrier attributes of large areas of 2D single crystals and establish the intrinsic quality of atomically thin components at the sub-nanometer to nanometer length scale over large areas to help in their development.
Piran R. Kidambi, Michael S. H. Boutilier, Luda Wang, Doojoon Jang, Jeehwan Kim, and Rohit Karnik. Selective Nanoscale Mass Transport across Atomically Thin Single Crystalline Graphene Membrane. Advanced Materials 2017, 29, 1605896.Go To Advanced Materials