A novel and better approach at detecting non-uniformities in the optical properties of two-dimensional materials could potentially open the door to new uses for these materials.

«The Two-Dimensional Crystal Consortium (2DCC) is a world leader in 2D materials research and my lab often works with the 2DCC doing materials characterization for novel 2D materials,» said Slava V. Rotkin, Frontier Professor of Engineering Science and Mechanics with an appointment in the Materials Research Institute at Penn State. «There is a big challenge in these studies: Frequently, optical properties of 2D materials are not uniform in space. Furthermore, they may vary at a very small spatial scale, down to a single atom.»

Rotkin and other researchers were able to take one step toward a possible solution, which was outlined in ACS Nano. While Rotkin stresses they only gave a demonstration of the principle in the study, the solution they propose was used for van der Waals heterostructures which could enable sensors made with 2D materials, materials that are one to a few atoms thick.

Sensors can be developed that enable sensing of bio-, chemical and/or medical analytes of interest. Analytes are specific chemicals targeted for measurement or analysis. A good sensor detects these analytes with minimal sample preparation, in an abbreviated time frame, with low detection limits, and using samples containing substances other than the key analyte.

Identifying and understanding variability of properties in materials could be extremely important for applications of 2D materials as sensors. The sensor material typically can only interact with the analyte at the surface. Thus, the material’s surface is an active area, while material’s volume is not. The larger the ratio of surface to volume, the lower the fraction of material which cannot be used. Such atomically thin materials have the ultimate surface-to-volume ratio for sensor use and may possess surface non-uniformities at the nanometer scale. This includes atomic impurities, adsorbates, defects, wrinkles, ruptures, etc. Such features can modulate the optical properties.

«Despite this being critical for effectiveness in certain application of 2D materials, there is currently no truly effective approach to detect these variabilities,» Rotkin said. «Due to their being so tiny, they are undetectable by optical tools and non-optical tools cannot resolve optical contrast.»

The researchers conducted experiments using a heterostructure material made of graphene, the 2D material version of graphite, and the inorganic compound molybdenum disulfide. The molybdenum disulfide gives a photoluminescence signal that detects the amount of charge transfer between the graphene and the molybdenum disulfide layers. Therefore, it can detect changes due to the bio analyte, which in this case is the cancer treatment drug doxorubicin, that can affect the charge.

Story Source: Materials provided by Penn State. Original written by Jamie Oberdick. Note: Content may be edited for style and length.