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

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.


Synthesis of Green-Emitting 'Giant' Quantum dots:
In synthesis of green-emitting core/ shell gradient alloy QDs, Single-step or one-pot synthesis is a typical synthetic procedure to obtain the colloidal quantum dots. In a three neck 50 ml round bottom flask 3.41 mmol of ZnO, 0.14 mmol of Cd(OAc) 2 were mixed gently with 7 ml of OA at 150 C with purging nitrogen gas in the RB flask. After reaching the temperature of the mixture at 150 C, 15 ml ODE was also injected. After adding the ODE, the temperature of the mixture was heated to 310 C.
At the same time, we also prepared the 4 vials of different anionic stock solutions. After reaching the temperature of RB flask at 310 C, a solution of 5 mmol of S and 5 mmol of Se in 5 ml of TOP swiftly injected into the RB flask at 10 min the aliquot was collected. After 10 minutes, the growth of CdSe/ZnS quantum dots, at the same temperature a stock solution of 1.6 mmol of S which was dissolved in 2.4 ml of ODE again injected for obtaining the uniquely large size CdSe/ZnS/ZnS QDs through the ZnS overcoating then hold for 12 min. In the same continuation, a zinc stock solution prepared with 2.86 mmol of Zn(OAc) 2 which was dissolved in the 4 ml of ODE and 1 ml OA injected in the RB flask at 310 C. After injection the temperature lower down and then the temperature was set at 270 C, a S stock solution Electronic Supplementary Material (ESI) for Nanoscale Advances. This journal is © The Royal Society of Chemistry 2021 which prepared by 9.65 mmol of S in 5 ml of TOP injected dropwise in the RB flask with the rate of 0.5ml/min to form a thick ZnS shell. This is called SILAR process in which the solution is injected in the portions with dropwise. 1,2 After completing the injection, the growth of quantum dots was continuing at 270 C for 20 min. The resulting CdSe/ZnS and CdSe/ZnS/ZnS colloidal quantum dots were precipitated in the ethanol and washed with solvent combination ethanol-hexane mixture in the ratio of 1:4 by centrifugation on 12000 rpm. Washed and purified quantum dots were dispersed in the hexane for further analysis and characterization.

UV-Vis absorption and PL measurements
CdSe/ZnS 'Giant' QDs and aliquots were dispersed in hexane with absorbance scanning mode UV-Visible spectra were collected in UV-Visible spectrophotometer UV-2600i Shimadzu. Photoluminescence or PL spectra of CdSe/ZnS 'Giant' QDs and aliquots were recorded with FlouroLog-3 (Horiba Jobin Yvon).

PL measurements and PLQY
Dye Cumarine 153 was used as a reference in ethanol (QY=0.546) for the measure of photoluminescence quantum yield (PLQY). For the measurements of PLQY, we have matched optical density 0.17 at 420 nm wavelength. The identical instrumental analysis were used to measure the areas under the fluorescence spectra curve.

Powder X-ray diffraction measurements
Green emitting 'Giant' QDs were dropped cast on the surface of neat and clean glass slides for powder X-ray diffraction which were measured with Empyrean PANalytical X-Ray Diffractometer with Cu-Kα X-radiation (λ = 1.5406 Å) at 40 kV and 30 mA power.

Transmission electron microscope (TEM) analysis
Transmission electron microscope (TEM) was used to characterization of size, distribution size, and structure of CdSe/ZnS QDs and the sample prepared by drop of an optimum solution of CdSe/ZnS QDs in hexane on copper (Cu) grid coated with carbon film using JEOL JEM-2100 High-Resolution Transmission Electron Microscope with 0.23 nm point resolution.

Time resolved photoluminescence (TRPL) analysis
Time resolved photoluminescence was measured by time-correlated single photon counting by TCSPC spectrometer (Horiba Jobin Yvon IBH) with laser diode at 372 nm excitation laser source and PL decay fitted and analysis by IBH DAS6 software.

Photostability measurements
Photostability was measured by a photoluminescence spectrometer (Horiba Jobin Yvon IBH) in the hexane. All four samples were kept for 48 hours under continuous UV irradiation with 365 nm of excitation at power density of 8 W/cm 2 then the spectra was recorded with photoluminescence (PL) intensity and time-resolved photoluminescence (TRPL) were taken after different time intervals at 0, 3, 6, 12, 24, and 48 hours.

Temperature dependent stability measurements
For the temperature dependent PL spectra, the samples were dispersed in toluene. Then temperature-dependent PL spectra of four QDs were recorded with Horiba Jobin Yvon IBH using with TC1 temperature controller Quantum northwest along with koolance with100-240 VAC and temperature from 10 °C to 90 °C.

Time resolved photoluminescence (TRPL) decay parameters analysis
The photoluminescence decay parameters of the as-synthesized and treated CdSe/ZnS 'giant' quantum dots and calculated radiative (k r ) and non-radiative (k nr ) rate constant. The lifetimes are calculated in nanosecond (ns).
PL lifetime decay parameters of sample-1 to sample-4. PL lifetime, K r, and K nr values were calculated as per below equations.