Shell thickness dependent photostability studies of green-emitting "Giant" quantum dots

Abstract

Highly efficient green-emitting core/shell giant quantum dots have been synthesized through a facile "one-pot" gradient alloy approach. Furthermore, an additional ZnS shell was grown using the "Successive Ionic Layer Adsorption and Reaction" (SILAR) method. Due to the faster reactivity of Cd and Se compared to an analogue of Zn and S precursors it is presumed that CdSe nuclei are initially formed as the core and gradient alloy shells simultaneously encapsulate the core in an energy-gradient manner and eventually thick ZnS shells were formed. Using this gradient alloy approach, we have synthesized four different sized green-emitting giant core-shell quantum dots to study their shell thickness-dependent photostability under continuous UV irradiation, and temperature-dependent PL properties of nanocrystals. There was a minimum effect of the UV light exposure on the photostability beyond a certain thickness of the shell. The QDs with a diameter of ≥8.5 nm show substantial improvement in photostability compared to QDs with a diameter ≤ 7.12 nm when continuously irradiated under strong UV light (8 W cm^-2, 365 nm) for 48 h. The effect of temperature on the photoluminescence intensities was studied with respect to the shell thickness. There were no apparent changes in PL intensities observed for the QDs ≥ 8.5 nm, on the contrary, for example, QDs with <8.5 nm in diameter (for ∼7.12 nm) show a decrease in PL intensity at higher temperatures ∼ 90 °C. The synthesized green-emitting gradient alloy QDs with superior optical properties can be used for highly efficient green-emitters and are potentially applicable for the fabrication of green LEDs.

Fig. 1. Schematic illustration of the synthesis of green-emitting QDs using gradient chemical composition synthesis method.

Fig. 2. Absorbance (blackline) and PL intensity (redline) of the synthesized QDs dispersed in hexane (a) sample-1, (b) sample-2, (c) sample-3, and (d) sample-4. Inset shows the digital photographs of colloidal solution taken under UV light (λ_ex = 365 nm) irradiation.

Fig. 3. (a) PL spectra showing a blue shift from wavelength 547 to 512 nm. (b) Atomic percentage with increasing reaction time (c) change in the PLQY with respect to reaction time (d) time-resolved photoluminescence decay profiles of sample-1 to sample-4.

Fig. 4. TEM image of (a) sample-1 (b) sample-2 (c) sample-3 (d) sample-4. HR-TEM images of (e–h) sample-1 to sample-4 with a scale bar of 5 nm. Inset shows their d-spacing values (°A).

Fig. 5. XRD pattern of the synthesized QDs at various reaction times of sample-1 to sample-4 (bottom to top) respectively. Inset shows the digital images of thin films taken under UV light (λ_ex = 365 nm) irradiation.

Fig. 6. The PL decay curve of graded alloy green-emitting QDs after the irradiation time of 0 hours and 48 hours of (a) sample-1 (b) sample-2 (c) sample-3 and (d) sample-4. Inset shows the PL spectra of the samples after 0 hours (black line) and 48 hours (red line) (e) normalized PL intensity and (f) the lifetime in ns of sample-1 to sample-4 with increasing irradiation time.

Fig. 7. Temperature-dependent stability test of QDs at a different temperatures from 10 to 90 °C and the PL spectra of (a) sample-1 (b) sample-2 (c) sample-3 and (d) sample-4 respectively (e) normalized PL intensity of the four samples with temperature (°C).