used for bubble expansion, summarized in the plot (Fig. 4b). The
transferred SiNW-BBFs show a clear decrease in NW separation
and increase in density as the starting SiNW concentration
increases from 0.01 to 0.22 wt%. We find that the NWseparation
can be systematically varied over an order of magnitude from 50
to 3.0 mm as concentration increases from 0.01 to 0.22 wt%,
respectively; correspondingly NW density increases from 4.0
104 to 4.0 106 cm 2. Compared with submicron spacing achieved
by the LB technique, the NW spacing produced by the BBF
approach is relatively modest, but it is still useful for applications
such as nanoelectronic sensor arrays.39 The spacing (density)
versus wt% curves show some saturation at the higher SiNW
concentrations. This tendency towards saturation is believed to
be in part attributed to NW aggregation observed at higher
concentrations. Further optimizing of the surface chemistry for
the preparation of uniform higher wt% solutions would allow us
to test the separation limits achievable with this approach and
also to extend to different polymer systems besides epoxy, which
offers great flexibility and compatibility with polymer industry.