  Short nanopores have various applications in biosensing, desalination, and
energy conversion. Here, the modulation of charged exterior surfaces on ionic
transport is investigated through simulations with sub-200 nm long nanopores
under applied voltages. Detailed analysis of ionic current, electric field
strength, and fluid flow inside and outside nanopores reveals that charged
exterior surfaces can increase ionic conductance by increasing both the
concentration and migration speed of charge carriers. The electric double
layers near charged exterior surfaces provide an ion pool and an additional
passageway for counterions, which lead to enhanced exterior surface conductance
and ionic concentrations at pore entrances and inside the nanopore. We also
report that charges on the membrane surfaces increase electric field strengths
inside nanopores. The effective width of a ring with surface charges placed at
pore entrances (Lcs) is considered as well by studying the dependence of the
current on Lcs. We find a linear relationship between the effective Lcs and the
surface charge density and voltage, and an inverse relationship between the
geometrical pore length and salt concentration. Our results elucidate the
modulation mechanism of charged exterior surfaces on ionic transport through
short nanopores, which is important for the design and fabrication of porous
membranes.
