  Sensing lower molecular weight in a diluted solution using a label-free
biosensor is challenging and requires a miniaturized plasmonic structure, e.g.,
a vertical Au nanorod (AuNR) array based metamaterials. The sensitivity of a
sensor mainly depends on transducer properties and hence for instance, the AuNR
array geometry requires optimization. Physical vapour deposition methods (e.g.,
sputtering and e-beam evaporation) require a vacuum environment to deposit Au,
which is costly, time-consuming, and thickness-limited. On the other hand,
chemical deposition, i.e., electroplating deposit higher thickness in less time
and at lower cost, becomes an alternative method for Au deposition. In this
work, we present a detailed optimization for electroplating based fabrication
of these metamaterials. We find that slightly acidic (6.0 < pH < 7.0) gold
sulfite solution supports immersion deposition, which should be minimized to
avoid uncontrolled Au deposition. Immersion deposition leads to plate-like (for
smaller radius AuNR) or capped-like, i.e., mushroom (for higher radius AuNR)
structure formation. The electroplating time and DC supply are the tuning
parameters that decide the geometry of the vertically aligned AuNR array in
area-dependent electroplating deposition. This work will have implications for
developing plasmonic metamaterial based sensors.
