  Electromagnetic interference (EMI) shielding coating materials with
thicknesses in the microscale are required in many sectors, including
communications, medical, aerospace and electronics, to isolate the
electromagnetic radiation emitted from electronic equipment. We report a spray,
layer-by-layer (LbL) coating approach to fabricate micron thick, highly-ordered
and electrically-conductive coatings with exceptional EMI shielding
effectiveness (EMI SE >4830 dB/mm), through the alternating self-assembly of
negatively-charged reduced graphene oxide (RGO) and a positively-charged
polyelectrolyte (PEI). The microstructure and resulting electrical properties
of the (PEI/RGO)n LbL structures are studied as function of increasing mass of
graphene deposited per cycle (keeping the PEI content constant), number of
deposited layers (n), flake diameter and type of RGO. A strong effect of the
lateral flake dimensions on the electrical properties is observed, which also
influences the EMI SE. A maximum EMI SE of 29 dB is obtained for a 6 um thick
(PEI/RGO)10 coating with 19 vol.% loading of reduced
electrochemically-exfoliated graphene oxide flakes with diameters ~3um. This SE
performance exceeds those previously reported for thicker graphene papers and
bulk graphene/polymer composite films with higher RGO or graphene nanoplatelets
contents, which represents an important step towards the fabrication of thin
and light-weight high-performance EMI shielding structures.
