producing stable bubbles. A similar method was used to prepare
homogeneous SiNW–epoxy solutions except that 5,6-epoxyhexyltriethoxysilane
was used to functionalize the surface of
the SiNWs (Fig. 2b). A homogeneous solution, which can be
aided by appropriate nanostructure surface functionalization,
is important for uniform distribution of nanomaterials in the
resulting bubble films.
The viscosity, which is determined by epoxy curing time, is
a key parameter that determines the best stage for blowing
bubbles. The desired viscosity range for blowing large bubbles is
15 to 25 Pa s (Fig. 2c). Bubbles break easily when blowing at higher
viscosity and only small bead-like bubbles could be blown at lower
viscosity. Addition ofTHFto the nanostructure–epoxy solution is
important for two reasons. First, the solvent helps disperse
nanomaterials, and second, it prolongs the cross-linking process.
The latter is important for achieving stable viscosity for bubble
expansion, and in the absence of THF we were unable to produce
bubbles by this overall approach.
Controlled bubble initiation, expansion, and transfer were
done using a 50 mm diameter circular die with a gas inlet at the
bottom and outlet at the top surface (Fig. 3a). The nanomaterial–
epoxy solution was deposited on the die surface and blown into
a bubble by flowing gas at a pressure of 150 to 200 kPa
(Fig. 3b–d). The upward bubble expansion was stabilized and
controlled by a motor-driven ring. Bubbles were expanded in an
elongated spherical shape (along the vertical axis), and sample
substrates, such as silicon wafers, were fixed close to the central