Reverse micelle-derived Cu-doped Zn1−xCdxS quantum dots and their core/shell structure

doi:10.1016/j.jcis.2009.09.039

Journal of Colloid and Interface Science

Volume 341, Issue 1, 1 January 2010, Pages 59-63

https://doi.org/10.1016/j.jcis.2009.09.039 Get rights and content

Abstract

Reverse micelle chemistry-derived Cu-doped Zn₁₋_xCd_xS quantum dots (QDs) with the composition (x) of 0, 0.5, 1 are reported. The Cu emission was found to be dependent on the host composition of QDs. While a dim green/orange emission was observed from ZnS:Cu QDs, a relatively strong red emission could be obtained from CdS:Cu and Zn_0.5Cd_0.5S:Cu QDs. Luminescent properties of undoped QDs versus Cu-doped ones and quantum yields of alloyed ZnCdS versus CdS QDs are compared and discussed. To enhance Cu-related red emission of CdS:Cu and Zn_0.5Cd_0.5S:Cu core QDs, core/shell structured QDs with a wider band gap of ZnS shell are also demonstrated.

Graphical abstract

A 0.45 nm thick ZnS shell was overcoated on Zn_0.5Cd_0.5S:Cu core QDs, resulting in 9.4% of quantum yield from core/shell QDs.

Introduction

During the past decade, II–VI semiconducting nanocrystals (quantum dots, QDs) have been vigorously investigated as efficient luminescent materials, which have been proved to be promising in a number of application areas including biological markers [1], [2], [3], light-emitting diodes [4], [5], [6], solar cells [7], [8], and color-converted solid-state lighting devices [9], [10], [11].

Unlike undoped luminescent QDs, the luminescent properties of QDs doped with luminescent activators are typically governed by the activator-related quantum states situated in the semiconductor host band gap. Transition metal or rare earth ions could be doped into II–VI semiconductor nanocrystal hosts such as ZnS, CdS, ZnSe, and CdSe, however, the doping efficiency into a specific host is a quite different, strongly depending on the similarities of chemical properties (e.g., valence state, ionic radius) between host/dopant [12] and synthesis conditions. Compared to rare earth ions, doping by transition metal ions such as Mn²⁺[13], [14], [15], Cu²⁺[16], [17], [18], and Co²⁺[19] has been demonstrated to be more successful in ZnS, CdS, and ZnSe nanocrystal hosts.

Among them, Mn²⁺ ion (⁴T₁–⁶A₁ transition) is known to be a representative efficient dopant in the above host. Depending on the synthetic chemistry, surface passivation, and QD host composition, high quantum yields (QYs) up to 30–70% have been reported from Mn-doped QDs, indicating Mn²⁺ ion can serve as an efficient emitter in such QD hosts. On the other hand, Cu²⁺ ion is not an efficient emitter in II–VI semiconductor hosts. Most studies on Cu-doped QDs have focused on Cu-related emissions and their possible origins (which are still controversial). Even though the QY of 2–4% was reported by Meijerink et al. [20] from their ZnSe:Cu QDs of high temperature (310 °C) synthesis, the QYs in most Cu-doped QD-related studies are rarely found in the literature, presumably due to their poor efficiencies. This inefficient Cu emission in such QD hosts might be involved with the difficulty of Cu doping [19], [21] and/or the low quenching temperature (∼130 K) of Cu emission [20].

In order to produce QDs with a higher luminescent efficiency the core/shell structure is desirable. Highly enhanced QY and photostability have been well achieved in doped [15], [22] as well as undoped core/shell QDs [23], [24]. Based on the appropriate selection of a shell material that forms a type I electronic structure with a core material and has a similar lattice parameter to that of core, the effective surface passivation can be realized through epitaxial overcoating of shell layer on the core surface, leading to highly efficient, photostable QDs [15], [23], [24].

Cu-doped QDs have been usually prepared by surface capping agent-assisted colloidal chemistries [16], [18], [27], [28], which often make the room temperature-preparation of core/shell QDs difficult because the capping molecules pre-existing on core QD surface hinder the subsequent formation of shell layer. Here, the first synthesis of doped Zn₁₋_xCd_xS:Cu QDs via a reverse micelle approach is reported. Variation of Cu-related emission with QD host composition, i.e., ZnS, CdS, and their alloy, are described. To the best of our knowledge, core/shell structured Cu-doped QDs with ZnS shell overcoating are for the first time synthesized through room temperature synthesis and their enhanced QY is reported.

Section snippets

Materials

Cadmium acetate dihydrate (Cd(CH₃COO)₂·2H₂O), copper (II) acetate monohydrate (Cu(CH₃COO)₂·H₂O), sodium sulfide (Na₂S), and zinc acetate (Zn(CH₃COO)₂) were used as precursors. Dioctyl sulfosuccinate, sodium salt (AOT), heptane, and deionized water were chosen to form a reverse micelle system. n-dodecanethiol (CH₃(CH₂)₁₁SH) was used to organically cap the surface of QDs.

Synthesis of Zn₁₋_xCd_xS:Cu QDs and their core/shell structure

In general, Zn₁₋_xCd_xS:Cu and its core/shell structured QDs were synthesized based on our previously reported reverse micelle

Results and discussion

The absorption and PL emission spectra of 0.5 mol% Cu-doped ZnS QDs are shown in Fig. 1. Significantly blue-shifted absorption peak (294 nm, 4.22 eV) of ZnS:Cu QDs versus bulk ZnS (∼3.6 eV) is a direct result of the quantum confinement effect. Their size can be approximated from the effective mass approximation, resulting in a diameter of ∼3.4 nm. Various emission bands from ultraviolet to infrared region have been reported from bulk and nano-sized ZnS:Cu [16], [25] and their origins are still under

Summary

Zn₁₋_xCd_xS:Cu QDs (x = 0, 0.5, 1) with a diameter of ∼3.6 nm were synthesized by a reverse micelle approach and their Cu-related emission properties with host composition were compared. First, ZnS:Cu QDs showed inefficient Cu-related green and orange emission bands. The low efficiency of Cu emission in ZnS QD host is postulated to be due to a large difference in solubility product constants between ZnS and CuS phase. The actual composition of Zn_0.5Cd_0.5S:Cu QDs was indirectly determined to be Zn_0.08

Acknowledgments

This work was financially supported by the Korea Energy Management Corporation (Energy Technology R&D, 2007-E-CM11-P-07). This work was partly supported by the IT R&D program of MKE/IITA (2009-F-020-01, Development of red nitride phosphor and self-assembly phosphorescent layer packaging technology for high rendition LED illumination).

References (28)

H. Yang et al.
Adv. Funct. Mater.
(2004)
S.F. Wuister et al.
J. Lumin.
(2003)
P. Yang et al.
Inorg. Chem. Commun.
(2001)
K. Manzoor et al.
Mater. Chem. Phys.
(2003)
A.A. Bol et al.
J. Lumin.
(2002)
W.Q. Peng et al.
Opt. Mater.
(2006)
B. Dubertret et al.
Science
(2002)
I.L. Medintz et al.
Nat. Mater.
(2005)
W.J. Parak et al.
Nanotechnology
(2005)
S. Coe et al.
Nature
(2002)

Q. Sun et al.

Nat. Photonics

(2007)

H. Yang et al.

J. Phys. Chem. B

(2003)

P.V. Kamat

J. Phys. Chem. C

(2008)

R.G. Xie et al.

Adv. Mater.

(2005)

Cited by (25)

II-VI core/shell quantum dots and doping with transition metal ions as a means of tuning the magnetoelectronic properties of CdS/ZnS core/shell QDs: A DFT study
2022, Journal of Molecular Graphics and Modelling
This paper examines the alterations in the properties of II-VI Quantum Dots (QDs) when these are coated with a shell made of another material of the same family and investigates the structural, electronic and magnetic properties of doped CdS/ZnS core/shell QDs. The core/shell QDs have been constructed by building the shell over the bare core QD and it is found that this construction of a shell over the bare QD can bring about dramatic changes in its optical properties. On changing the shell by varying either the cation or the anion, substantial variations are brought about in the band gap and electrophilicity. The trend of Fermi energies is more negative for core/shell QDs than for the QDs without a shell, and the value is almost the same for core/shell QDs with the same core. Swapping of the core and the shell materials brings greater stability in the case of shells of the wider band gap materials. Binding energy data demonstrates that the CdS/ZnS, CdSe/ZnSe, CdSe/CdS core/shell systems are more stable than ZnS/CdS, ZnSe/CdSe, CdS/CdSe core/shell systems, respectively. An augmentation in the properties is found on doping the QD with transition metal ions. The binding energies are found to be functions of the kind of dopant as well as the spin multiplicity and account for the stability of one spin state over the other at a specific site of the QD. The most fascinating property that plays a decisive role in the extant work is the introduction of magnetism in core/shell QDs as a result of the entry of unpaired electrons within the CdS/ZnS QDs on doping with transition metal ions. The deviation of the observed magnetic moments from the expected values increases as the dopant is varied from Mn²⁺ to Fe²⁺ to Co²⁺ to Ni²⁺ to Cu²⁺. Hirshfeld charge analysis shows that the doped ion accepts negative charge from the sulfide ions in the core, with the smallest charge transfer seen in the case of Hg²⁺ ions. As we move from Mn²⁺ to Hg²⁺, the trend followed for the Hirshfeld charges indicates that the overall charge on the core is lower and that on the shell is higher for all the doped cases in comparison to the undoped CdS/ZnS core/shell QD. The band gap values reveal that the Fe²⁺ doped CdS/ZnS core/shell structures have the smallest band gaps. Hence, we expect that this paper will help researchers to develop a strategy to produce QDs of the anticipated properties for various applications, and transition metal ions can be successfully employed for modification of various magnetoelectronic properties of the host semiconductor for future applications in nanotechnology.
Photoluminescence properties of Cu-doped CdTeSe alloyed quantum dots versus laser excitation power and temperature
2020, Journal of Luminescence
Citation Excerpt :
Meanwhile, CdSexTe1-x based solar cells have highly power-conversion efficiencies with high stability [6,7]. It has also been found that nanostructured ternary alloy semiconductors are also suitable hosts to dope transition-metal (TM) ions [12,13], in which TM ions are usually Mn [12,14], Cu [15,16], Co [17], and Ni [18]. The doping remarkably improves the application and operation ranges of ternary semiconductors for optoelectronic and/or spintronic devices.
Ternary alloy semiconductors have been proven to be suitable host lattices, which could be doped with transition metals (TM) to control the bandgap energy (E_g) and optoelectronic properties. Through many works performed on TM-doped ternary alloys, their photoluminescence (PL) properties versus the laser excitation power (P_ex) and temperature (T) have not been investigated in detail. This work presents a careful study on the effect of P_ex and T on the PL properties of zinc-blend Cd_1-xCu_xTe_0.5Se_0.5 alloyed quantum dots (QDs) with x = 0, 0.005 and 0.05, which were synthesized by a wet chemical method. At T = 300 K and P_ex = 0.1 mW, we observed the PL spectra of x = 0 and 0.005 exhibiting two emissions peaked at about 644 and 820 nm associated with near-band-edge excitons (E_high) and lattice defects and Cu²⁺ ions (E_low), respectively. Meanwhile, the PL spectrum of x = 0.05 shows only one emission (E_low) associated with Cu²⁺ ions. Interestingly, at 300 K, as increasing P_ex from 0.1 to 100 mW, we observed the redshift of E_high and the blueshift of E_low. If fixing P_ex at 0.1 mW, the study of PL spectra in the range of T = 15–300 K indicated that the emission-peak positions of E_high(T) and E_low(T) could be described by Varshni's empirical law while their intensity could be described by the Arrhenius law. Physical mechanisms related to these phenomena were also discussed.
Tunable photoluminescent Cu-doped CdS/ZnSe type-II core/shell quantum dots
2019, Journal of Luminescence
We used wet chemical methods to fabricate CdSm/ZnSen type-II core/shell (C/S) quantum dots (QDs) doped with different Cu contents (x), where m is the index related to the size (d) of the CdS core and n is the number of monolayers of the ZnSe shell. X-ray diffraction analyses indicated that all QDs crystallized in the zinc blende structure. The formation of type-II heterostructures was confirmed by Raman scattering spectroscopy. Studying the photoluminescence (PL) properties, we have found PL spectra of QDs showing two emissions. The first one is associated with the near band-edge excitonic transition of type-II C/S structures while the other is mainly associated with the Cu²⁺ impurity. We have also found that the Cu²⁺ level in C/S QDs is stable versus varying n and occupies above the valence bands of CdS and ZnSe. Particularly, by changing the values of d (or m), n, and x in the core and shell parts, we would tune effectively both emission intensity and peak position. While the exciton emission could be tuned from 495 to about 615 nm, the Cu-related emission could be from 700 to about 945 nm. Also, the lifetime related to the Cu emission of the doped C/S QDs also enhances remarkably. We believe that these Cu-doped CdS/ZnSe type-II C/S QDs are potential for near-infrared applications.
Multi-phase structures of boron-doped copper tin sulfide nanoparticles synthesized by chemical bath deposition for optoelectronic devices
2018, Journal of Physics and Chemistry of Solids
Citation Excerpt :
Furthermore, the addition of a dopant to the semiconductor may also improve the performance of the material. Generally, doping with metals such as Ga [23], Zn [24], Mn [25], Cu [26], or Ag [27] increases the number of n-type carriers in a p-type semiconductor. Previous studies showed that doping with metals generated thermal instability and induced electron-hole pair recombination, which is detrimental for solar cell applications [28].
We investigated the influence of boron doping on the structural, optical, and electrical properties of copper tin sulfide (CTS) nanoparticles coated on a WO₃ surface and synthesized using chemical bath deposition. Boron doping at concentrations of 0.5, 1.0, 1.5, and 2.0 wt% was investigated. The X-ray diffraction pattern of CTS showed the presence of monoclinic Cu₂Sn₃S₇, cubic Cu₂SnS_3, and orthorhombic Cu₄SnS₄. Boron doping influenced the preferred orientation of the nanoparticles for all phase structures and produced a lattice strain effect and changes in the dislocation density. Increasing the concentration of boron in CTS from 0.5 wt% to 2.0 wt% reduced the band gap for all phases of CTS from 1.46 to 1.29 eV and reduced the optical transmittance. Optical constants, such as the refractive index, extinction coefficient, and dissipation factor, were also obtained for B-doped CTS. The dispersion behavior of the refractive index was investigated in terms of a single oscillator model and the physical parameters were determined. Fourier transform infrared spectroscopy confirmed the successful synthesis of CTS nanoparticles. Cyclic voltammetry indicated that optimum boron doping (<1.5 wt% for all phases) resulted in desirable p-n junction behavior for optoelectronic applications.
Biomolecule-assisted, cost-effective synthesis of a Zn<inf>0.9</inf>Cd<inf>0.1</inf>S solid solution for efficient photocatalytic hydrogen production under visible light
2018, Cuihua Xuebao/Chinese Journal of Catalysis
Citation Excerpt :
Unfortunately, these conventional approaches usually involve the use of high temperatures above 160 °C and the presence of toxic chemicals including organic solvents, surfactants, and reducing agent to control the activity of the ions [14–17]. In particular, the use of expensive and highly poisonous sulfur agents including Na2S, thioactamide, thiourea, and dimethyl sulfoxide leads to the liberation of a large amount of environmentally toxic H2S, and thus, is not suitable for large-scale production [18–23]. Keeping the above points in mind, our aim was to develop a facile, green, template-free hydrothermal route under mild conditions for the synthesis of ZnxCd1-xS solid solutions by employing the nontoxic biomolecule L-cystine as the sulfur source.
A series of alloyed Zn-Cd-S solid solutions with a cubic zinc blende structure were fabricated hydrothermally with the assistance of L-cystine under mild conditions. The products were characterized by XRD, TEM, HRTEM, XPS, UV-vis, and BET techniques, and the photocatalytic performance for the reduction of water to H₂ on the solid solutions was evaluated in the presence of S²⁻/SO₃²⁻ as hole scavengers under visible light illumination. Among all the samples, the highest photocatalytic activity was achieved over Zn_0.9Cd_0.1S with a rate of 4.4 mmol h⁻¹ g⁻¹, even without a co-catalyst, which far exceeded that of CdS. Moreover, Zn_0.9Cd_0.1S displayed excellent anti-photocorrosion properties during the photoreduction of water into H₂. The enhancement in the photocatalytic performance was mainly attributed to the efficient charge transfer in the Zn_0.9Cd_0.1 alloyed structure and the high surface area. This work provides a simple, cost-effective and green technique, which can be generalized as a rational preparation route for the large-scale fabrication of metal sulfide photocatalysts.
Biomolecule-assisted hydrothermal synthesis of Zn<inf>x</inf>Cd<inf>1−x</inf>S nanocrystals and their outstanding photocatalytic performance for hydrogen production
2017, International Journal of Hydrogen Energy
Citation Excerpt :
An effective strategy to overcome these drawbacks is to fabricate ZnxCd1−xS solid solution photocatalysts based on CdS and wide bandgap semiconductor materials of ZnS (Eg = 3.6 eV) to facilitate the separation and transport of the photogenerated charge carriers [13]. Up to date, tremendous efforts have been devoted to the synthesis of ZnxCd1−xS solid solutions with a variety of sizes and morphologies to achieve high photocatalytic performance [14–22]. However, these techniques usually require high temperatures over 160 °C together with expensive and poisonous sulfur sources such as Na2S [14], sulfur powder [15], thioactamide [16], thiourea [17–19], dimethyl sulfoxide [20], thioglycolic acid [21] as well as thiols [22], resulting in the liberation of nauseous scent H2S.
To achieve scalable applications in solar hydrogen production, it is necessary to develop visible-light-responsive photocatalysts that are highly efficient, cost-effective, stable and environmentally-benign. Here narrow bandgap Zn–Cd–S solid solution photocatalysts (E_g = 2.11–2.53 eV) were prepared via a facile and green hydrothermal strategy under mild conditions. Amazingly, over the naked Zn_0.5Cd_0.5S photocatalyst, an extraordinarily high H₂ production activity in Na₂S–Na₂SO₃ aqueous solution is achieved up to 18.3 mmol h⁻¹ g⁻¹ with an apparent quantum efficiency of 73.8% per 50 mg under 420 nm light irradiation, which, to our knowledge, outperforms cocatalyst-free metal sulfide photocatalysts previously reported to date. Such super high performance arises from the enhanced visible-light-absorption capacity, suitable conduction and valence band potential together with the facilitated charge transport in Zn–Cd–S solid solutions. This work may open an avenue for the green preparation of inexpensive photocatalysts for solar H₂ production.

View all citing articles on Scopus

View full text

Reverse micelle-derived Cu-doped Zn1−xCdxS quantum dots and their core/shell structure

Abstract

Graphical abstract

Introduction

Section snippets

Materials

Synthesis of Zn1−xCdxS:Cu QDs and their core/shell structure

Results and discussion

Summary

Acknowledgments

Adv. Funct. Mater.

J. Lumin.

Inorg. Chem. Commun.

Mater. Chem. Phys.

J. Lumin.

Opt. Mater.

Science

Nat. Mater.

Nanotechnology

Nature

Nat. Photonics

J. Phys. Chem. B

J. Phys. Chem. C

Adv. Mater.

Reverse micelle-derived Cu-doped Zn₁₋_xCd_xS quantum dots and their core/shell structure

Synthesis of Zn₁₋_xCd_xS:Cu QDs and their core/shell structure