Quantum dots (QDs) have been widely studied in both basic sciences and applications for many years.1,2 In comparison with organic dyes, QDs have many advantages, including tunable sizedependent emission, high photoluminescence (PL) quantum yields, broad excitation spectra and narrow emission bandwiths.3 The semiconductor QDs have been intensively investigated in versatile applications, including thin-film light emitting devices (LEDs),4 low-threshold lasers,5 optical amplifier media for telecommunication networks6,7 and biological labels.8-10