Elsevier

Applied Surface Science

Volume 456, 31 October 2018, Pages 876-881
Applied Surface Science

Full Length Article
A facile route for highly efficient color-tunable Cu-Ga-Se/ZnSe quantum dots

https://doi.org/10.1016/j.apsusc.2018.06.199Get rights and content

Highlights

  • Color-tunable Cu-Ga-Se/ZnSe quantum dots (QDs) were firstly obtained via a facile heating-up method.

  • The synthesis process is highly efficient and environmentally-friendly.

  • The tunable emission (485–630 nm) can be acquired by introducing sulfur ion and varying the Cu/Ga molar ratio.

  • The QDs are promising candidates for the exploration of novel and efficient LEDs.

Abstract

I-III-VI group “green” quantum dots (QDs) are receiving increasing attention in photoelectronic conversion applications. In this work, a facile one-pot route directly dissolving copper iodide, gallium acetylacetonate and selenium powder as precursors into a mixed solution of dodecanethiol, oleylamine (OLA) and octadecene was developed to synthesize Cu-Ga-Se/ZnSe (CGSe/ZnSe) core/shell QDs. The resulting CGSe/ZnSe QDs with a relatively high photoluminescence quantum yield (PL QY) of 77.73% can be obtained and the particle size control from 4.20 to 5.73 nm can be realized via the variation of OLA dosage. The tunable emission from 485 to 630 nm can be acquired by introducing sulfur ion into CGSe host materials (anionic alloying) and varying the Cu/Ga precursor molar ratio. Furthermore, the PL recombination process in CGSe/ZnSe QDs is discussed using the PL decay curve. The proposed one-pot synthesis route for highly efficient color-tunable CGSe/ZnSe QDs is facile, rapid and environmentally-friendly. The as-prepared QDs are promising candidates for the exploration of novel and efficient light-emitting diodes.

Introduction

Group I-III-VI semiconductor quantum dots (QDs), as cadmium- and lead-free visible emitters, have been emerging as a promising candidate for the fabrication of light-emitting diodes (LEDs) [1], [2], [3], [4], [5], [6]. A high degree of compositional flexibility of such I-III-VI QDs and the control of size or cationic off-stoichiometry, enable a fine tailoring of band gap and consequential photoluminescence (PL) in a wide range. The most intensively studied I–III–VI QDs are indium group, such as Cu-In-S (CIS) [2], [3], [7], [8], [9], the emission of which ranges typically from yellow to near-IR. The PL quantum yield (QY) is typically low in bare CIS QDs, but remarkably improves during the last few years up to 80% or higher, depending on shell material of a higher band gap passivating the core surface. Encouraged by the above promising PL results, successive synthetic efforts have been extended to other ternary Ag-In-S (AIS) [4], Cu-Ga-S (CGS) [5] QDs and further quaternary Zn-Cu-In-S (ZCIS) [10], Zn-Ag-In-S (ZAIS) [11] and Cu-In-Ga-S (CIGS) [12] ones. However, compared with other I-III-VI compounds, rare work has been reported about the luminescent property of Cu-Ga-Se (CGSe) QDs.

CuGaSe2 is receiving significant attention as thin film absorbing layers in photovoltaic cells because of their direct band gap character, high absorption coefficient, and facile band gap tunability [13], [14]. To the best of our knowledge, however, this material in the form of colloidal QDs reported in the previous work are almost of no or negligible fluorescence [15], [16] and study on the systematically controllable synthesis of color-tunable CGSe QDs is still lacking. The methods for the synthesis of CGSe QDs in only a few literatures were carried out mostly in hot injection and there still remain method-related obstacles in realizing high-performance light-emitting CGSe QDs, since the preparation of the Se precursor in the preparation of the metal selenide QDs generally requires heating of Se powders in octadecene (ODE) under a protective atmosphere for a long time (up to 30 min). On the contrary, one-pot synthesis of CGSe QDs is still in their infancy stage.

Herein, we employed a facile one-pot synthesis method to prepare the ternary CGSe QDs. The deposition of ZnSe shells on CGSe QDs gives rise to great enhancement of PL intensity and the PL QY is up to 77.73%. To the best of our knowledge, this is the first reported synthesis of CGSe/ZnSe QDs with high PL QY. TEM analysis reveals that the particle size can be controlled from 4.20 to 5.73 nm via the increase of the OLA dosage, which also results in the red-shift of both absorption band edges and PL peak positions. Color-tunable PL emissions from 500 to 630 nm can be accomplished by tailoring the Cu/Ga molar ratios. Besides, tunable-emission with the PL peak positions from 485 to 563 nm via introducing sulfur ion into CGSe host materials (anionic alloying) can also be acquired. Other synthesis conditions such as shell thickness, nucleation temperature and different zinc precursors were systematically optimized to investigate their influence on the ultraviolet and visible (UV–vis) absorption and PL spectra. In addition, the PL recombination process in CGSe/ZnSe QDs is discussed using the PL decay curve.

Section snippets

Materials

Copper iodide (CuI, 99.5%), gallium acetylacetonate [Ga(acac)3, 99.0%], selenium powder (Se, 99.99%), zinc iodide (ZnI2, 99.0%), zinc acetate [Zn(OAc)2, 98%], zinc stearate (ZnSt2, 13.5–15.5% Zn basis), dodecanethiol (DDT, 98.0%), oleylamine (OLA, 90%) and octadecene (ODE, 90%) were purchased from Sinopharm Chemical Reagent Co. Ltd. All chemicals were used without further purification.

Synthesis of CGSe/ZnSe core/shell QDs

To synthesize CGSe/ZnSe core/shell QDs, CuI (0.0625 mmol), Ga(acac)3 (0.25 mmol) and Se powder (1 mmol) were

Results and discussion

The synthetic procedures used for the CGSe/ZnSe QDs are schematically described in Fig. 1. Typically, CuI, Ga(acac)3 and Se powder were mixed with DDT, OLA and ODE in a three-necked flask. When degassed and backfilled with nitrogen gas, the mixture was heated to 120 °C. The metal ions, Cu+ and Ga3+, could react with DDT to form bridged complexes, (-Cu-SR-Ga-SR-)n, similar to Cu-In thiolate CuIn(SR)x [17]. Thermal decomposition of the metal-thiolate complexes was carried out at 240 °C to

Conclusions

In summary, highly efficient CGSe/ZnSe QDs with the PL QY up to 77.73% have been successfully prepared via a facile one-pot route directly dissolving CuI, Ga(acac)3 and Se powder as precursors into a mixed solution of DDT, OLA, and ODE. The shell thickness, nucleation temperature and different zinc precursors play an important role in the synthesis of the CGSe/ZnSe QDs. The action mechanism of the OLA is investigated by the comprehensive analysis of the PL, HRTEM, EDS and XRD. The particle size

Acknowledgements

This work was supported by the National Natural Science Foundation of China (NSFC, No. 61675049, NSFC, No. 61377046, and NSFC, No. 61177021).

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