  One-atom thick crystalline layers and their vertical heterostructures carry
the promise of designer electronic materials that are unattainable by standard
growth techniques. In order to realize their potential it is necessary to
isolate them from environmental disturbances in particular those introduced by
the substrate. But finding and characterizing suitable substrates, and
minimizing the random potential fluctuations they introduce, has been a
persistent challenge in this emerging field. Here we show that Landau-level
(LL) spectroscopy is exquisitely sensitive to potential fluctuations on both
local and global length scales. Harnessing this technique we demonstrate that
the insertion of an intermediate graphene layer provides superior screening of
substrate induced disturbances, more than doubling the electronic mean free
path. Furthermore, we find that the proximity of hBN acts as a nano-scale
vacuum cleaner, dramatically suppressing the global potential fluctuations.
This makes it possible to fabricate high quality devices on standard SiO2
substrates.
