  Short nanopores find extensive applications capitalizing on their high
throughput and detection resolution. Ionic behaviors through long nanopores are
mainly determined by charged inner-pore walls. When pore lengths decrease to
sub-200 nm, charged exterior surfaces provide considerable modulation to ion
current. We find that the charge status of inner-pore walls affects the
modulation of ion current from charged exterior surfaces. For 50-nm-long
nanopores with neutral inner-pore walls, charged exterior surfaces on the
voltage (surfaceV) and ground (surfaceG) sides enhance and inhibit ion
transport by forming ion enrichment and depletion zones inside nanopores,
respectively. For nanopores with both charged inner-pore and exterior surfaces,
continuous electric double layers enhance ion transport through nanopores
significantly. The charged surfaceV results in higher ion current by
simultaneously weakening ion depletion at pore entrances and enhancing the
intra-pore ion enrichment. The charged surfaceG expedites the exit of ions from
nanopores, resulting in a decrease in ion enrichment at pore exits. Through
adjustment in the width of charged-ring regions near pore boundaries, the
effective charged width of the charged exterior is explored at ~20nm. Our
results may provide a theoretical guide for further optimizing the performance
of nanopore-based applications, like seawater desalination, biosensing, and
osmotic energy conversion.
