  GISAXS has been used to study morphology change of $\alpha$-Fe$_2$O$_3$
nanocubes after annealing processes. A submonolayer of the nanocubes was
deposited on a Si(100) substrate. While an annealing at 400 $^\circ$C in a
vacuum does not change a GISAXS pattern from as-prepared nanocubes
submonolayer, annealing in the air at the same temperature altered the GISAXS
pattern significantly. SEM image showed that the air-annealed nanocubes were
coated with thin layers which were identified as amorphous carbon layers based
on Raman measurements. GISAXS simulations from morphologies of nanocube with 38
nm side-length and core-shell (nanocube-core and 7 nm thick carbon-shell)
nicely reproduced measured patterns from the vacuum annealed and the
air-annealed nanocubes, respectively. The current study provides new approach
for in-situ characterization of carbon deposition on a uniform shape
nanoparticle through monitoring of deposited carbon thickness.
