When the synthesis is performed in higher vapor pressure
solvents such as tributylphosphine or octadecene, the
morphology of the Cu2S nanoprisms remains unchanged;
however, overall yield is markedly lower. Since changing the
functional group or chain length of the solvent seems to have
little effect on the shape of the nanostructures, we find that the
primary role of the solvent is simply to provide a growth medium
for the Cu2S and prevent premature volatilization of the
precursor. Additionally, the use of a low vapor pressure solvent
and resulting slow evaporation rate allows for the neat
arrangement and packing of the Cu2S hexagonal disks. As can be
seen in Fig. 2A-B, the hexagonal disks preferentially arrange with
the hexagonal face touching the substrate surface. High packing
densities can be achieved through this synthesis over large
continuous areas of the substrate surface (Fig. 2B) with coverage
as large as the surface area (36 mm2) of the substrate used. Use of
slow evaporation of a solvent to aid packing of nanostructures
has been shown previously in the formation of aligned nanorod
arrays11 and the formation of nanoparticle superlattices.12 This
phenomenon of spontaneous orientation due to solvent drying is
an important step towards the incorporation of these nanostructures
to photovoltaic devices. The use of this synthetic
method could obviate secondary assembly steps in Cu2S