Luminescence mechanisms in 6H-SiC nanocrystals

Experimental conditions allowing consequent selection of a dominating photoluminescence mechanism in 6H-SiC nanocrystals at room temperature are reported. Electrostatic screening of surface states involved in radiative transitions can be efficiently achieved by polar ethanol molecules. This leads to a preponderant radiative channel between the electronic levels corresponding to the impurity atoms (N and Al). This radiative channel is deactivated by centrifugation-induced selection of the smallest colloidal 6H-SiC nanocrystals in which the probability to have both donor and acceptor atoms is negligible. Consequently, for these smallest 6H-SiC nanocrystals with switched off transitions between surface states and impurity levels, quantum-confinement effect can be clearly observed. The formation of energy subbands in the 6H-SiC nanocrystals is then evidenced from photoluminescence excitation and absorption measurements performed on the centrifuged colloidal nanosuspension. A most probable mean diameter of 1.9 nm for these particles is deduced from calculation of energy levels in the effective-mass approximation.