Shape-controlled synthesis of single-crystalline Fe₂O ₃ hollow nanocrystals and their tunable optical properties

Single-crystalline α-Fe₂O₃ hollow nanocrystals from nanotube to nanoring have been successfully synthesized by a facile hydrothermal method. Size and shape control of the hollow nanocrystals is achieved by simple adjustments of reactants' concentration and molar ratio without any surfactant assistance. The steady absorption spectra indicate a size-dependent blue shift of these hollow nanocrystals. The femtosecond optically heterodyne optical Kerr effect measurements show that the sample that has the smallest diameter possesses the largest third-order nonlinear optical susceptibility. This is ascribed to the remarkably enhanced local electric field in the small particle in accord with finite-difference time-domain simulations. These results reveal that both size and shape of these hollow nanocrystals have significant contributions to their optical response.