  Large area van der Waals (vdW) thin films are assembled materials consisting
of a network of randomly stacked nanosheets. The multi-scale structure and the
two-dimensional nature of the building block mean that interfaces naturally
play a crucial role in the charge transport of such thin films. While single or
few stacked nanosheets (i.e. vdW heterostructures) have been the subject of
intensive works, little is known about how charges travel through multilayered,
more disordered networks. Here we report a comprehensive study of a
prototypical system given by networks of randomly stacked reduced graphene
oxide 2D nanosheets, whose chemical and geometrical properties can be
controlled independently, permitting to explore percolated networks ranging
from a single nanosheet to some billions with room temperature resistivity
spanning from 10-5 to 10-1 ohm m. We systematically observe a clear transition
between two different regimes at a critical temperature T*: Efros-Shklovskii
variable range hopping (ESVRH) below T* and power law (PL) behavior above.
Firstly, we demonstrate that the two regimes are strongly correlated with each
other, both depending on the charge localization length xi, calculated by
ES-VRH model, which corresponds to the characteristic size of overlapping sp2
domains belonging to different nanosheets. Thus, we propose a microscopic model
describing the charge transport as a geometrical phase transition, given by the
metal-insulator transition associated with the percolation of quasi-1D
nanofillers with length xi, showing that the charge transport behavior of the
networks is valid for all geometries and defects of the nanosheets, ultimately
suggesting a generalized description on vdW and disordered thin films.
