Chemical Identification of Interlayer Contaminants within van der Waals Heterostructures
Jeffrey J. Schwartz, ?Hsun-Jen Chuang, Matthew R. Rosenberger, Saujan V. Sivaram, Kathleen M. McCreary, B.T. Jonker, Andrea Centrone
Van der Waals heterostructures (vdWHs) leverage the characteristics of two-dimensional (2D) material building blocks to create a myriad of structures with unique and desirable properties. Several commonly employed fabrication strategies rely on polymeric stamps to assemble layers of 2D materials into vertical stacks. However, the properties of such heterostructures frequently are degraded by contaminants, typically of unknown composition, trapped between the constituent layers. Such contaminants therefore impede studies of the intrinsic properties of heterostructures and hinder their application. Here, we use the photothermal induced resonance (PTIR) technique to obtain infrared spectra and maps of the contaminants down to a few attomoles and with nanoscale resolution. Heterostructures comprised of WS2, WSe2, and hBN layers were found to contain significant amounts of polydimethylsiloxane (PDMS) and polycarbonate, corresponding to the stamp materials used in their construction. Additionally, we confirm that an atomic force microscope-based nanosqueegee technique is an effective method for locally removing contaminants by comparing spectra within as-fabricated and cleaned regions. Having identified the source of the contaminants, we demonstrate that cleaning PDMS stamps with isopropanol or toluene prior to vdWH fabrication reduces PDMS contamination within the structures. The general applicability of the PTIR technique for identifying the sources corrupting vdWHs provides valuable guidance for devising mitigation strategies (e.g., stamp cleaning or pre-/post-treatments) and enhances capabilities for producing materials with precisely engineered properties.