Comprehensive Route to the Formation of Alloy ... - ACS Publications

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Comprehensive Route to the Formation of Alloy Interface in Core/ Shell Colloidal Quantum Dots Nathan Grumbach,† Richard K. Capek,† Evgeny Tilchin,† Anna Rubin-Brusilovski,† Junfeng Yang,#,⊥ Yair Ein-Eli,#,§ and Efrat Lifshitz*,†,‡ †

Schulich Faculty of Chemistry, ‡Solid State Institute and the Russell Berrie Nanotechnology Institute, and #Department of Materials Science and Engineering, Technion, Haifa 3200003, Israel § Key Laboratory of Materials Physics, Institute of Solid State Physics, Chinese Academy of Sciences, Hefei, People’s Republic of China ⊥ The Nancy and Stephen Grand Technion Energy Program, Technion-Israel Institute of Technology, Haifa 3200003, Israel S Supporting Information *

ABSTRACT: The electronic properties of colloidal quantum dots (CQDs) have shown intriguing potential in recent years for implementation in various optoelectronic applications. However, their chemical and photochemical stabilities, mainly derived from surface properties, have remained a major concern. This paper reports a new strategic route for the synthesis of surface-treated CQDs, the CdSe/CdS core/shell heterostructures, based on low-temperature coating of a shell constituent, followed by a programmed annealing process. A comprehensive follow-up of the stability and the optical properties through the various synthesis stages is reported, suggesting that the low-temperature coating is responsible for the formation of a sharp interface between the core and the shell, whereas a postcoating annealing process leads to the generation of a thin alloy interfacial layer. At the end of the process, the CdSe/CdS CQDs show a significant improvement of the photoluminescence quantum yield, as well as an exceptional photostability. Consequently, the work reported here provides a convenient generic route to the formation of core/shell CQDs to be employed as a procedure for achieving various other heterostructures.

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INTRODUCTION Colloidal quantum dots (CQDs) are of considerable interest due to their size-dependent optical and electronic properties, which allow implementation in photovoltaic cells,1 lightemitting diodes,2 photocatalysis,3 bioassays,4 and electronics.5 However, the photostability and emission efficiency of CQDs show a strong dependence on surface quality,6 so that a physical and electronic decoupling of the excitonic wave function from the environment is absolutely necessary. This can be achieved by overcoating the CQDs with an inorganic broad bandgap material, forming core/shell CQDs.7−12 These CQDs show a significant improvement of the photoluminescence quantum yield (PL QY), as well as a noteworthy increase in the photostability.13 CdSe CQDs are probably the most extensively investigated CQDs, due to the simplicity of their preparation using hot-injection processes,14 enabling accurate control of the particles’ size, shape, and size distribution.15,16 CdSe/CdS CQDs appear to possess the best luminescence properties, because the lattice mismatch between CdSe and CdS is low (3.9%).8 In 1997, Peng et al. reported a PL QY close to unity for CdSe/CdS CQDs using highly reactive precursors at low temperatures.17 Later, Li et al.7developed the “successive ion layer adsorption and reaction” (SILAR) coating method, employing less hazardous precursors. Here, high temperatures were mandatory to avoid co-nucleation of CdS CQDs from the © XXXX American Chemical Society

shell precursors, and the PL QY values reached were far lower than those obtained previously. Following the SILAR method a high coating temperature was adopted in many other cases.18−20 In 2011, Nan et al. synthesized CdSe/CdS CQDs with PL QY values close to unity and sharp emission line widths using a “thermal cycling” procedure.21 In 2013, Chen et al.22 reported a route to a so-called “new generation” of highquality CdSe/CdS CQDs, showing high PL QY and photostability; however, the route could not be generalized for the preparation of CdSe/CdS/ZnS CQDs.23 Other approaches suggested the design of alloy composition at the core−shell interface, to reduce a crystallographic mismatch and consequent defect sites.24 Such an alloyed interface also induced changes in carriers’ distribution with the existence of a gradual interface potential,25 which led to a suppression of an Auger process in nanostructures.26−28 The aim of this work was to generate high-quality core/shell CdSe/CdS CQDs with low surface and interface defects, focusing on the temperature program during coating. The thermal stability of core CdSe CQDs was first investigated while some experimental conditions were varied. The results Received: March 31, 2015 Revised: May 13, 2015

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DOI: 10.1021/acs.jpcc.5b03086 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

stirring. The reaction was terminated by the injection of 10 mL of ODE. As prepared core CdSe CQDs were precipitated twice with a 2-propanol/ethanol mixture (1:1−1:2), separated by centrifugation, and redissolved in hexane. Synthesis of Core CdSe CQDs and Core−Shell CdSe/ CdS CQDs. The 0.1 M Cd precursor was prepared by dissolving 0.1 mmol of cadmium acetate and 0.2 mmol of HDA in 0.942 mL of ODE at 100 °C inside a nitrogen-filled glovebox. The 0.1 M S precursor was prepared by dissolving 0.1 mmol of sulfur in 1 mL of ODE at 160 °C. The coating procedure adopted has been reported for coating ZnS on small zinc blend CdSe CQDs.26 Initially, 7.5 mL of ODE was degassed under vacuum for 1 h at 100 °C. The ODE was cooled to 65 °C, and a solution of 3.7 × 10−4 mmol CdSe CQDs in hexane was injected (CdSe CQD concentration after evacuation of the hexane in ODE = 5 × 10−5 mol/L). Then, the Cd precursor solution was added at 65 °C. After degassing for 10 min under vacuum at 65 °C, the S precursor was added. The amounts of the added precursors have been calculated to reach the desired shell thickness, in general, three monolayers (MLs)27 (where one monolayer corresponds to an increase of 0.4 nm of the CQD radius). The reaction mixture was quickly heated to 100 °C and allowed to remain for 4 h at this temperature. In the case of postcoating annealing, the reaction mixture was heated in steps of 10 °C. The reaction mixture was maintained at each temperature for 30 mn. Aliquots were taken at the various temperatures and after varied time spans. The aliquots were mixed each with 1 mL of toluene and 100 μL of oleic acid under ambient conditions. Then the nanoparticles were precipitated with a mixture of 2-propanol and ethanol (1:1−1:2), separated by centrifugation, and redissolved in toluene. The precipitation process was repeated two times. Physical Measurements. UV−vis absorption spectra were recorded using a JASCO V-570 UV−vis−NIR spectrometer. Luminescence measurements were performed with a Fluorolog JY Horiba fluorescence spectrometer (range = 200−850 nm) using a slit size corresponding to a spectral resolution of 1 nm in excitation and 2 nm in emission. Photoluminescence quantum yield efficiency (PL QYE) was determined relative to Rhodamine 6G dissolved in ethanol (PL QYE of the Rhodamine 6G in ethanol = 95%).28 Transmission electron microscopy (TEM) pictures and energy dispersive X-ray (EDX) spectra were recorded with a JEOL 2200 FS electron microscope. Nanoparticle size determination was performed by direct analysis of TEM images. X-ray powder diffraction (XRD) patterns were obtained using a Philips PW1830 X-ray diffractometer. XRD samples of colloidal quantum dots were prepared by evaporating the solvent and resuspending the particles in an 80:20 hexane/ heptane mixture and subsequent drop casting on a 1 × 1 cm glass slide. The samples were measured with a Bruker D8 Discover. Raman spectra at room temperature were performed using a micro-Raman spectrometer (Horiba Jobin Yvon XploRa), with a spatial resolution of 1 μm × 1 μm. Laser excitation was provided by the 532 nm of a Nd:YAG laser. The samples were prepared by evaporating the solvent and resuspending the particles in an 80:20 hexane/heptane mixture and subsequent drop casting on a 1 × 1 cm glass slide. The PL decay curves of CdSe/CdS CQDs were recorded by exciting the samples with a Nd:YAG laser, Continum Minilite II (Eexc = 1.17 eV). The measurements used a laser flux of 90%) was purchased from Fluka. Toluene, hexane, and isopropanol were purchased from BioLab, and ethanol (96%) was purchased from Gadot. All solvents used were of technical grade. The spectroscopic grade ethanol used for PL QY measurements was purchased from Gadot. Rhodamine 6G (99%) used as reference dye for PL QYE measurements was purchased from Aldrich. All chemicals were used without further purification. All syntheses were performed under nitrogen atmosphere using the standard Schlenk technique. Synthesis of Core CdSe CQDs. CdSe CQDs with a diameter from 2.2 to 4.5 nm were prepared using cadmium oleate and trioctylphosphine selenide (TOPSe) as Cd and Se precursors with the addition of primary amines and by varying the experimental conditions. The initial reaction mixture always contained ODE, CdO, OA, and HDA or ODA as primary amines. TOPSe is injected at high temperature, and the reaction is stopped after a given reaction time. Table 1 Table 1. Conditions for the Synthesis of the Five CdSe CQDs with Different Diameters amount of CdO (mmol) core CdSe 2.2 nm core CdSe 2.6 nm core CdSe 2.9 nm core CdSe 3.4 nm core CdSe 4.4 nm

concn TOPSe (mol/L)

injection temp (°C)

reaction time

amine used

0.4

2

245

5s

HDA

0.4

2

245

30 s

HDA

0.2

1

280

2 min

HDA

0.2

0.25

280

16 min

ODA

0.1

0.125

280

32 min

ODA

summarizes the various experimental conditions (the initial amount of CdO, the concentration of the TOPSe solution, the temperature of injection, the reaction time, and the type of the amine used) leading to the different core diameters. A mixture of CdO and OA in a molar ratio of 1:4 and 7.5 mL of ODE was put in a 25 mL three-neck flask. The reaction mixture was degassed for 1 h at 100 °C under vacuum. Under nitrogen, the temperature was then raised to 300 °C until the solution turned clear, indicating the formation of cadmium oleate. Then the solution was cooled, and the amine was added in a molar ratio of 1:8 (Cd/amine). Afterward, the solution was heated to the desired temperature, and 2 mL of TOPSe (different concentrations in TOP) were injected under vigorous B


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Figure 1. (A) Absorption spectra of a solution containing CdSe CQDs (d = 2.6 nm) and the cadmium precursor only, at various temperatures. The absorption spectra have been normalized to have the same peak intensity. (B−D) Evolution with temperature of E1s−1s and the corresponding FWHM when heating a solution containing CdSe CQDs and the Cd precursor solution only, varying the concentration of the Cd precursor (for a core concentration of 5 × 10−5 M) (B), the core CdSe CQDs concentration (when the concentration of the Cd precursor in the solution is suitable for the addition of 3 MLs of shell) (C), and the CdSe CQDs size (D). (The solid lines have been added to guide the reader.)

intact until nearly 120 °C, beyond which an energy red-shift is observed. Figure 1B displays a plot of E1s‑1s (top) and the corresponding full width at half-maximum (FWHM, bottom) during the heating of a 5.0 × 10−5 M solution of the CQDs shown in Figure 1A, with and without the addition of Cd precursors at various concentrations (see legend in the top panel). It is seen that the pristine CQD solution undergoes a rapid energy red-shift and excitonic band broadening below 100 °C; however, in the presence of excess Cd precursors, spectral stability is maintained at higher temperatures. The changes above 100 °C are related to initiation of an Ostwald ripening process, which is slowed by an excess of Cd2+ ions in the solution. Figure 1C shows plots of E1s−1s and FWHM of the CQDs discussed in Figure 1A versus temperature, for solutions of core CdSe CQDs with core concentrations as indicated in the legend and with excess Cd precursor concentration equivalent to a shell coating of three MLs, where one monolayer corresponds to the (111) lattice-plane spacing. The plots reveal that the stability is preserved up to higher temperatures in solutions with the highest core concentration. Figure 1D represents plots of E1s−1s and FWHM of CQD

mJ/cm2, corresponding to a photon fluence of jp ≈ 1011 photons/cm2 per pulse. The number of photogenerated photons is estimated to be 10−4 ≪ 1, ensuring the generation of single excitons. The tr-PL curves were monitored by a photon multiplier tube, Hamamatsu NIR-PMT H10330-75. The PL decay curves were fitted to either a single or biexponential function, whereas the extracted decay time was corrected with respect to the quantum yield, supplying the radiative lifetime values.

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RESULTS AND DISCUSSION The stability of the preprepared core CdSe CQDs is required for an optimal shell coating and was examined after CQD redispersion in solution and heating and upon variation of some experimental conditions such as a change in concentration or the presence of excess precursors. Figure 1A demonstrates a representative set of absorption spectra of core CdSe CQDs with a diameter of 2.6 nm, heated to various temperatures as indicated in the legend. The absorption curves are characterized by a few excitonic band transitions; the energy of the lowest exciton is labeled hereafter as E1s−1s. The absorption curves stay C


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The Journal of Physical Chemistry C solutions (5.0 × 10−5 M) with variable core diameter (see legend) versus temperature, all examined in the presence of excess Cd precursor equivalent a shell growth of three MLs. The figure suggests comparable behavior regardless of core diameter, whereas the stability of the larger cores is maintained at higher temperatures. The coating program included three stages in the following order: injection of shell precursors into a solution containing dispersed clean cores; shell coating up to a restricted temperature; and annealing of the entire core/shell CQDs. The stages are referred to hereafter as injection, coating, and annealing. Because Figure 1 recommends performing the shell coating within a restricted temperature window with relatively high core CQD concentration, most of the reaction described here involved shell precursor injection at 65%, without broadening of the FWHM. Figure 4C shows plots similar to those given in Figure 4B; however, the coating process occurred at 130 °C. This process led to improvement of the PL QY, although it was accompanied by FWHM broadening. Comparison between panels B and C

Figure 2. TEM images of (A) core CdSe CQDs (d = 3.4 nm) and (B) core/shell CdSe/CdS CQDs after 4 h of coating.

indicated conditions (panel B). Additional TEM images related to aliquots extracted at interim stages during the coating process and TEM images with different magnifications are shown in the Supporting Information (SI; Figure S1). It is seen from Figure 2 and Figure S1 that the size of the CdSe/CdS CQDs increases gradually with extension of the coating duration, when the shape is nearly preserved and there is no indication of CdS conucleation. Furthermore, additional images displayed in Figure S1 reveal a high homogeneity and a small size distribution within each sample. Table 2 summarizes the gradual changes in diameter and sulfur mole fraction (determined by EDX and defined as Fs = [S]/([Se] + [S]) during shell coating at two different temperatures. The numbers indicated in the right column correspond to the

Table 2. Evolution of the Diameter and the S Fraction of CdSe/CdS CQDs during a Coating Process Restricted to 100 or 130 °C 100 °C coating temperature size (nm) 5.8 nm S fraction (%) 80%

130 °C coating temperature

core CdSe

l h coating

2 h coating

4 h coating

core CdSe

1 h coating

2 h coating

4 h coating

3.42 ± 0.09

5.05 ± 0.18 57.4 ± 9

5.42 ± 0.12 69.1 ± 1.2

5.82 ± 0.09 79.5 ± 3.2

3.38 ± 0.08

5.77 ± 0.05 81.8 ± 1.4

5.81 ± 0.07 79.6 ± 2.5

5.79 ± 0.06 80.4 ± 3.4

D


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Figure 3. (A) PL emission spectra (left side) and absorption spectra (right side) of CdSe cores (d = 3.4 nm) and corresponding CdSe/CdS core/ shell CQDs extracted at various stages during the coating process with three MLs of CdS at 100 °C. The absorption spectra have been normalized. The emission spectra are scaled relative to the absorbance at the excitation wavelength. (B−D) Evolution with temperature (light blue background) as well as versus reaction time after reaching the coating temperature (light red background) of E1s−1s (black squares), PL QY (blue triangles), and the corresponding FWHM (red circles) during the coating process when the coating temperature was limited to 100 °C (B), 130 °C (C), and 160 °C (D). (Data points are connected to guide the reader.)

Figure 5A exhibits Raman spectra in the range of 100−450 cm−1, performed on 3.4 nm core CdSe CQDs and on the corresponding CdSe/CdS CQDs with a shell thickness of three MLs, followed different stages of the coating/annealing processes (see legend). The Raman spectrum of core CQDs is composed by a single dominated band at 200 cm−1, related to the fundamental longitudinal optical (LO) phonon. The spectrum of the core/shell CQDs after coating shows two asymmetric bands,24,25,29 corresponding to the contribution of the core CdSe (near 200 cm−1) and the shell CdS (near 300 cm−1), whereas each band consists of two sub-bands (see Lorentzian function fits), related to the first-order LO phonons and to surface optical (SO) phonons. The Raman spectra of core/shell CQDs after annealing stages exhibit the appearance of a third sub-band overlapping the CdSe and the CdS spectral regime (see purple-colored Lorentzian functions), associated with LO mode of CdSexS1−x alloyed component,24,29 marked as CdSealloy and CdSalloy in the figure. This sub-band becomes more prominent when the annealing temperature is increased. Thus, Raman spectra suggest that the coating stage generates CdSe/CdS with a sharp interface, whereas the annealing stage induces interdiffusion and formation of an alloy interfacial layer. The frequencies of the various sub-bands resolved in Figure 5A are presented in table format in the accompanying SI (Table S1). Figure 5B represents XRD spectra of the samples described in Figure 5A and of a control sample, composed of homogeneously alloyed CdSe0.2S0.8 CQDs with composition and size identical to those of other samples shown in this figure (see legend). More details concerning the preparation of this

suggests the optimal temperature program, retaining the coating up to 100 °C and applying a postcoating annealing up to a given temperature. Figure 4D represents the evolution when applying this most favorable temperature program for the coating of core CdSe CQDs with different diameters, as indicated in the legend. The plots expose the existence of an ideal annealing temperature for each core size. Table 3 lists the ultimate annealing temperature and corresponding best PL QY values achieved for different core CdSe diameters and shell CdS thicknesses. The coating and annealing of smaller core CdSe CQDs led to higher PL QY values up to 92%, comparable to experiments reported in other works.22 Performing the annealing at a temperature >160 °C results in a remarkable decrease in the PL QY and in a significant red-shift of E1s−1s, as described in the SI (Figure S3). The improvement of the PL QY values by controlling other experimental parameters such as the initial core concentration, the amount of the primary amine used for shell addition, or the amine chain length, is reported in the SI (Figure S4). Figure 4E represents PL decay curves of CdSe/CdS CQDs with core diameter of 4.4 nm and shell width up to three MLs that were subjected to different coating or annealing stages (see legend). Figure 4F exhibits plots of the extracted radiative lifetime versus time and temperature. The plots reveal extreme lifetime elongation (up to 20 ns) after coating, and a slight increase of the lifetime during the annealing phase; however, the lifetime decreases if the annealing is performed above 130 °C. E


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Figure 4. (A) PL emission spectra (left side) and absorption spectra (right side) of CdSe cores (d = 3.4 nm) and corresponding CdSe/CdS core/ shell CQDs extracted at various stages during the coating/annealing process, when the coating temperature was limited to 100 °C. The absorption spectra have been normalized. The emission spectra are scaled relative to the absorbance at the excitation wavelength. (B−C) Evolution with reaction time once reaching the coating temperature (light red background) and with annealing temperature (light green background) of E1s−1s (black squares), PL QY (blue triangles), and the corresponding FWHM (red circles) during the coating/annealing process when coating was retained at 100 °C (B) and 130 °C (C). (Data points are connected to guide the reader). (D) Evolution with temperature of the PL QY during the coating of different core CdSe CQDs with three ML of CdS (coating retained at 100 °C), followed by an annealing up to 160 °C. The various phases of the coating process are marked by different background colors: injection phase, light blue background; annealing phase, light green background. (The continuous lines have been added to guide the reader.) (E) Single-exciton PL decay dynamics of CdSe CQDs (d = 4.4 nm) and the corresponding CdSe/CdS CQDs during a coating/annealing process. The PL spectra have been normalized and are represented in a logarithmic scale. (F) Evolution of the radiative lifetime of the CQDs described in (E) during the coating/annealing process. The various phases of the coating process are marked by different background colors: injection phase, light blue background; coating phase, light red background; annealing phase, light green background. (Data points are connected to guide the reader.)

Table 3. Ideal Annealing Temperature and the Corresponding Optimal PL QY Value for Various Core CdSe Diameters and Shell CdS Widths 1 ML shell core core core core core

2.2 2.6 2.9 3.4 4.4

nm nm nm nm nm

130 °C, 67% 130 °C, 52% 150 °C, 27%

2 ML shell 140 140 140 140 140

°C, °C, °C, °C, °C,

3 ML shell

75% 59% 61% 43% 32%

140 140 150 130 130

°C, °C, °C, °C, °C,

92% 82% 68% 65% 51%

4 ML shell 130 130 120 120 120

°C, °C, °C, °C, °C,

92% 86% 70% 61% 45%

7 ML shell 120 120 120 120 120

°C, °C, °C, °C, °C,

85% 78% 53% 49% 44%

“smoothing” of the interface, as observed by Garcia-Santamaria et al.30 Furthermore, the appearance of a thin alloy interface also results in two competing effects.23,31 First, the potential step function at the valence band interface broadens to a potential gradient, further confining the holes to the lowest energy state. Second, the exponential decay of the electron wave functions further penetrates into the shell regime due to the relatively low potential barrier and the gradual interface, raising the delocalization of the electrons over the entire particle.32 Recent work led by Boldt et al.23 has shown that such hole wave function confinement and electron wave function delocalization converts the electronic structure to a quasi-type

control sample are given in the SI (Figure S4). The XRD spectra show a pronounced shift toward larger 2θ values for samples that were annealed at the highest temperature. A more important shift was also found in the homogeneous alloy sample, further supporting the occurrence of alloying at the core/shell interface during the annealing process. It is commonly agreed that growth of a CdS shell on core CdSe leads to a compressive strain in the core and a tensile strain in the shell.24 An alloying interface reduces the lattice strain, which is beneficial for reaching high PL QY values, most likely by reducing the number of defects at the CdSe/CdS interface.13 The FWHM narrowing is also a consequence of the F


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which is tantamount to the preparation of the so-called “new generation” core/shell CQDs. These new generation CdSe/ CdS CQDs are characterized by (a) a weak red-shift of their bandgap energy during the coating process in comparison to traditional syntheses, (b) high PL QY values, (c) sharp spectral lines, and (d) high photostability. Finally, comprehensive methods developed in this work, for example, low-temperature coating/high-temperature annealing, should provide a route to more sophisticated and more stable core/shell systems, such as CdSe/CdS/ZnS CQDs.

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ASSOCIATED CONTENT

* Supporting Information S

Additional TEM images of core/shell CdSe/CdS CQDs during the coating process; discussion on the absorption spectra of CdSe cores and CdSe/CdS core/shell CQDs for energies >2.5 eV; evolution of the PL QY and E1S‑1S during the coating/ annealing process, when annealing is performed up to extremely high temperatures; dependence of the PL QY on other experimental parameters (primary amine used, its chain length, core concentration); details for the interpretation of the Raman spectra, including the values of the peak frequencies and the intensity ratio; details on the preparation and characterization of the homogeneously alloyed CdSe0.2S0.8 CQDs prepared for XRD measurements. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b03086.

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AUTHOR INFORMATION

Corresponding Author

*(E.F.) E-mail: [email protected]. Phone: +972 (0)4 829-3987. Mail: Schulich Faculty of Chemistry, Technion-Israel Institute of Technology, Haifa 3300002, Israel. Notes

The authors declare no competing financial interest.

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Figure 5. (A) Raman spectra of core CdSe CQDs (d = 3.4 nm), of core/shell CdSe/CdS CQDs after a coating with 3 ML of CdS (coating restricted to 100 °C), after an annealing to 130 and 160 °C. (B) XRD patterns of core/shell CdSe/CdS CQDs in the same conditions and XRD spectra of CdSe0.2S0.8 CQDs with a homogeneous alloyed composition.

ACKNOWLEDGMENTS We thank Roman Vaxenburg and Aron Safran for help and fruitful discussions. We acknowledge support of the Israel Council for High Education − Focal Area Technology (No. 872967), the Volkswagen Stiftun, (No .88116), the Niedersachsen-Deutsche TechnionGesellschaft E.V (No. ZN2916), and the Israeli Science Foundation (Project 985/11). N.G. thanks the Israel Ministry of Immigrant Absorption for the generous fellowship.

II structure, with a consequent red-shift of E1s−1s.33,34 This is further supported by an increase in the radiative lifetime for such particles. Applying these conclusions to the synthesized CdSe/CdS CQDs, it can be assumed that the observed red-shift as well as the increasing radiative lifetime after annealing is the result of the hole wave function confinement and the electronic wave function delocalization. Finally, recent studies26,27 exposed a considerably reduced Auger recombination rate and blinking behavior upon creation of alloying interface. In principle, this could also allow a substantial enhancement of the quantum efficiency.

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REFERENCES

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CONCLUSION To conclude, a low-temperature coating/high-temperature annealing process has been developed in CdSe/CdS CQD synthesis. Simple low-temperature coating approaches allow control in the formation of sharp CdSe/CdS interfaces. A controlled postcoating annealing at elevated temperatures forms a thin alloy interface, achieving exceptionally high-quality materials, as pronounced in their high PL QY and stability, G


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