Investigation of factors that influence the quality of van der Waals heterostructures made by direct pick-up

Two dimensional material van der Waals heterostructures, such as twisted bilayer graphene, can be assembled by the prevailing pick-up procedure.\footnote{K. Kim et al. \textbf{Nano Lett.} 16.3:1998 (2016)} \footnote{Y. Cao et al. \textbf{Phys. Rev. Lett.} 117.11:116804(2016)} However, the size and the quality of the devices are limited by defects such as ``Bubbles'' in the samples. We investigate sources that can impair the quality of van der Waals heterostructures using optical, atomic force microscope, and scanning electron microscope imaging. Choices of polymer layers for picking up, schemes of stacking heterostructures, and influences of pre- and post- furnace annealing for stack fabrication are studied. In conjunction with transport measurements, we refine the procedure for making high-quality twisted bilayer graphene and other heterostructure devices.