All cuprous sulfide nanostructures show apparent six-fold
rotational symmetry along the z-axis, a feature commonly reported
in the monoclinic crystal phase of low-chalcocite.13 Twinning
defects are the most commonly reported cause for this form of
symmetry in the crystals;14 however, other explanations are
possible. These include strain in the crystal lattice to accommodate
the 120 angle required of a hexagonal structure. Another possibility
stems from the high Cu ion mobility of chalcocite enabling
rapid phase transformations between the high and low temperature
stable forms of Cu2S.15 For the Cu2S system, three distinct
crystal phases are possible including a low-temperature stable
phase (monoclinic) stable below 104 C, a high-temperature
stable phase (hexagonal), and a higher-T phase (tetragonal). Such
ease of phase transition, due to the high cation mobility, may allow
the Cu2S hexagonal nanoprisms to form at 240 C in the high-T
stable hexagonal crystal phase, but upon cooling to room
temperature, experience rearrangements of the copper cation
lattice to the low-T stable monoclinic crystal phase. Thus, the
nanostructure could retain the bulk symmetry elements of
a hexagonal nanoprism, while undergoing a rearrangement of the
Cu cations. For both high-chalcocite (hexagonal) and low-chalcocite
(monoclinic), the hexagonal close-packed sulfur anion
lattice remains mainly unchanged between the two phases.13,15