High-brightness and Monodisperse Quaternary CuInZnS@ZnS Quantum Dots with Tunable and Long-lived Emission

doi:10.15541/jim20240426

Abstract

Abstract:

As an essential candidate for environment-friendly luminescent quantum dots (QDs), CuInS-based QDs have attracted more attention in recent years. However, several drawbacks still hamper their industrial applications, such as lower photoluminescence quantum yield (PLQY), complex synthetic pathways, uncontrollable emission spectra, and insufficient photostability. In this study, CuInZnS@ZnS core/shell QDs was prepared via a one-pot/three-step synthetic scheme with accurate and tunable control of PL spectra. Then their ensemble spectroscopic properties during nucleation formation, alloying, and ZnS shell growth processes were systematically investigated. PL peaks of these QDs can be precisely manipulated from 530 to 850 nm by controlling the stoichiometric ratio of Cu/In, Zn²⁺ doping and ZnS shell growth. In particular, CuInZnS@ZnS QDs possess a significantly long emission lifetime (up to 750 ns), high PLQY (up to 85%), and excellent crystallinity. Their spectroscopic evolution is well validated by Cu-deficient related intragap emission model. By controlling the stoichiometric ratio of Cu/In, two distinct Cu-deficient related emission pathways are established based on the differing oxidation states of Cu defects. Therefore, this work provides deeper insights for fabricating high luminescent ternary or quaternary-alloyed QDs.

Key words: quantum dot, CuInS, alloying, core/shell, ensemble spectroscopic, Cu-deficient related emission

CLC Number:

TQ174

CHEN Zi, ZHANG Aidi, GONG Ke, LIU Haihua, YU Gang, SHAN Qingsong, LIU Yong, ZENG Haibo. High-brightness and Monodisperse Quaternary CuInZnS@ZnS Quantum Dots with Tunable and Long-lived Emission[J]. Journal of Inorganic Materials, 2025, 40(4): 433-339.

Figures/Tables 21

Fig. 1 Optical and morphology characterization of the CuInS-based QDs (a) Synthetic route of the CuInS-based QDs; (b) Photograph of the CuInZnS@ZnS QDs with shell growth of 20 h (Cu : In : Zn is 1 : 2 : 3) under 365 nm showing brilliant luminescence; (c) Transient PL decays of the CuInZnS@ZnS QDs with different shell growth time (Cu : In : Zn is 1 : 2 : 3); (d) TEM images of the CuInZnS/ZnS QDs (Cu : In : Zn is 1 : 2 : 3) with shell growth of 20 h. Colorful figures are available on website

Fig. 2 Schematic depictions of relaxation processes in stoichiometric CuInS QDs (a), Cu-deficient CuInS QDs (b), and Cu-deficient CuInZnS@ZnS core/shell QDs (c) (a) Photon absorption is mainly due to the VB to CB transition. For the CuInS QDs with Cu/In ratio close to stoichiometric, the PL emission was due to radiative recombination of the CB electron with the hole existing in the intragap Cu+ state. (b) For the Cu-deficient CuInS QDs, there are three main processes. Process ①: a hole existing in the ground state forms Cu2+ defect, and it can directly recombine with the CB electron. Process ②: the recombination process was slow with the lifetime lasting hundreds of nanoseconds. To dominate the PL emission, another Cu vacancy trap (noted as VCu) quickly captured the photogenerated hole from the VB state, and formed a charge-compensated pair with the Cu2+ defect. Process ③: the trapped hole at VCu center radiatively recombined with the electron and finished the whole recombination process. (c) For the Cu-deficient core/shell QDs, the diffusion of Zn2+ ions occupied and decreased the VCu intragap states, and the thick ZnS shell eliminated the electron trap bands associated with the CB. The recombination of hot electron at CB edge and hole located at the intragap state (Cu+) dominated the PL decay process.

Fig. S1 Synthetic equipment of the CuInS-based QDs

Fig. S2 Digital photographs of CuInS QDs in a typical nucleation growth process under daylight lamp and UV lamp (365 nm) DDT was chosen as the sulfur source, surface ligand, and solvent. The reaction temperature was 200 ℃

Fig. S3 Digital photographs of CuInS QDs in a typical nucleation growth process under daylight lamp and UV lamp (320 nm)

Fig. S4 Temporal evolution of PL emission spectra of CuInS QDs synthesized with different molar stoichiometric ratios of Cu : In precursors (a-d) stand for 1 : 1, 1 : 2, 1 : 4, and 1 : 6. DDT was chosen as the sulfur source. The aliquots of QDs samples for the PL intensity test were fixed. PL spectra were recorded with excitation at 450 nm

Fig. S5 Temporal evolution of PL spectra of CuInS QDs, CuInZnS QDs, and CuInZnS@ZnS QDs with different growth time (a-d) stand for the stoichiometric ratio of Cu : In at 1 : 1, 1 : 2, 1 : 4, and 1 : 6. PL spectra were recorded with excitation at 450 nm

Fig. S6 Temporal evolution of UV-Vis absorption spectra of CuInS QDs, CuInZnS QDs, CuInZnS@ZnS QDs synthesized with different molar stoichiometric ratios of Cu : In precursors (a-d) stand for the stoichiometric ratio of Cu : In at 1 : 1, 1 : 2, 1 : 4, and 1 : 6. The absorption shoulder/onset is more blue-shifted with less Cu/In ratio

Fig. S7 Temporal evolution of PL central emission peaks for CuInZnS@ZnS QDs with different shell growth time The stoichiometric ratios of Cu : In are 1 : 1, 1 : 2, 1 : 4, and 1 : 6

Fig. S8 Temporal evolution of PLQY for CuInZnS@ZnS QDs with different shell growth time The stoichiometric ratios of Cu : In are 1 : 1, 1 : 2, 1 : 4, and 1 : 6

Fig. S9 Representative transient PL decays for CuInZnS QDs The stoichiometric ratios of Cu : In are 1 : 1, 1 : 2, 1 : 4, and 1 : 6

Fig. S10 Representative transient PL decays of CuInZnS@ZnS QDs with different ZnS shell growth time The stoichiometric ratio of Cu : In is 1 : 4

Fig. S11 TEM image of CuInS QDs (the stoichiometric ratio of Cu : In at 1 : 2) with reaction time of 30 min The red triangle frames indicate the shapes of the CuInS QDs

Fig. S12 TEM image of CuInZnS QDs (the stoichiometric ratio of Cu : In : Zn at 1 : 2 : 3) with Zn etching time of 90 min The red triangle frames indicate the shapes of the CuInZnS QDs

Fig. S13 TEM images of CuInZnS@ZnS QDs (the stoichioetric ratio of Cu : In : Zn at 1 : 2 : 3) with ZnS shell growth time of 5 h The insert showing their representative high- esolution TEM images

Fig. S14 Size distribution histograms for CuInZnS/ZnS QDs (Cu : In : Zn at 1 : 2 : 3) with shell growth of 20 h To build the histograms, over 100 particles were measured

Fig. S15 XRD patterns of CuInS QDs (Cu : In stoichiometric ratio at 1 : 2) and (Zn)CuInS/ZnS QDs (Cu : In : Zn stoichioetric ratio at 1 : 2 : 3)

Table S1 Relevant parameters for CuInS QDs synthesized with different molar stoichiometric ratios of Cu : In. λem at the PL central emission peak from the QDs solution when excited at 450 nm. The amounts of DDT (10 mL) and CuI (0.1 mmol) were held fixed

Cu : In precursor	λ_em/nm	PLQY/%	PL decay/ns
1 : 1	710	3.3	264 (65)
1 : 2	633	8.8	270 (71)
1 : 4	625	18.9	293 (86)
1 : 6	618	21.0	299 (95)

Table S2 Relevant parameters for CuInZnS alloyed QDs synthesized with differnt molar stoichiometric ratios of Cu : In. λem is the PL central emission peak from the QDs solution when excited at 450 nm. The amounts of DDT (10 mL) and CuI (0.1 mmol) were held fixed

Cu : In precursor	λ_em/nm	PLQY/%	PL decay/ns
1 : 1	675	14.7	277 (102)
1 : 2	596	16.8	284 (101)
1 : 4	590	24.8	284 (110)
1 : 6	581	36.3	282 (112)

Table S3 PL lifetime of CuInZnS@ZnS QDs with the stoichiometric ratio of Cu : In at 1 : 2 after excited at 450 nm.

Shell reaction time/h	PL decay/ns
3	588 (204)
5	647 (208)
8	714 (214)
10	724 (211)
12	729 (219)
15	751 (222)
20	755 (231)

Table S4 Relevant parameters for CuInZnS@ZnS QDs with the stoichiometric ratio of Cu : In at 1 : 4, synthesized with different ZnS shell growth time. λem is the PL central emission peak from the QDs solution when excited at 450 nm. The amounts of DDT (10 mL) and CuI (0.1 mmol) were held fixed.

Shell reaction time/h	PL emission peak/nm	PLQY/%	PL decay/ns
5	546	57	509 (171)
10	536	50	530 (177)
15	531	53	558 (174)
20	530	58	549 (165)

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