Spectroscopic Characterization of Interlayer Contaminants within van der Waals Heterostructures

Van der Waals heterostructures (vdWHs) leverage the characteristics of disparate 2D material building blocks to create a myriad of structures with unique and desirable properties. A common fabrication strategy relies on polymeric stamps to assemble layers of 2D materials into vertical stacks. However, the properties of structures produced in this way frequently are degraded by contaminants, typically of unknown composition, trapped between the constituent layers. This contamination impedes the study and application of intrinsic heterostructures requiring pristine interfaces. Here, we use a photothermal induced resonance technique to obtain nanoscale infrared spectra and maps of the contamination with ~20 nm spatial resolution. Heterostructures comprised of WS₂, WSe₂, and hBN layers were found to contain significant amounts of polydimethylsiloxane and polycarbonate, corresponding to the stamp materials used in their construction. Additionally, we validate spectroscopically a previously reported “nano-squeegee” technique as an effective means of locally removing contaminants. These insights into the chemical makeup and sources corrupting vdWHs provide guidance for devising mitigation strategies and enhance capabilities for producing materials with precisely engineered properties.