Journal of Nanoparticle Research

, Volume 11, Issue 4, pp 1017–1021

Photoluminescence properties of Eu3+-doped Cd1−xZnxS quantum dots

Authors

  • Karamjit Singh
    • Department of Applied SciencesChitkara Institute of Engineering & Technology
  • Sunil Kumar
    • Department of PhysicsMaharishi Markandeshwar University
  • N. K. Verma
    • School of Physics and Materials ScienceThapar University
    • Department of PhysicsPunjabi University
Research Paper

DOI: 10.1007/s11051-009-9586-1

Cite this article as:
Singh, K., Kumar, S., Verma, N.K. et al. J Nanopart Res (2009) 11: 1017. doi:10.1007/s11051-009-9586-1
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Abstract

Eu3+-doped Cd1−xZnxS (0 ≤ x ≤ 0.5) quantum dots (QDs) have been synthesized using wet chemical precipitation method. X-ray diffraction and transmission electron microscope have been used for the crystallographic and morphological characterization of synthesized nanomaterials. In order to understand the spectral characteristics of doped QDs, N2-laser induced time resolved spectra have been recorded. Excited state lifetime values for dichromatic emission (red and violet) attributed to 5D0 → 7FJ (J = 1, 2) transitions of Eu3+ and host lattice transitions have been calculated from the recorded luminescence decay curves. Decay time dependence on the dopant concentration (0.01–10 at. wt% of Cd2+) has been studied in detail.

Keywords

Quantum dotsTime resolved spectraDecay timeSynthesisSemiconductors

Introduction

In recent years, optical properties of doped semiconductor nanocrystals have attracted great attention as electronic structure and electromagnetic fields drastically modified due to confinement effects. Quantum size confinement affects not only the excitonic emission in the host, but also the luminescence from the dopants. Significant attention has been paid to transition/rare earth ions doped semiconductor nanocrystals to find out the potential applications in photonics and biophotonics (Bharagava et al. 1994; Reisfeld et al. 2000; Bol et al. 2002; Erwin et al. 2005). Recent studies have revealed that rare earth doped luminescent II–VI materials are promising candidates for applications in optical memories and color thin film electroluminescence devices (Okamoto et al. 1988; Jayaraj and Vallabhan 1991). In rare earth ions 4f electrons participate in luminescence. These are hardly influenced by their ligands due to the presence of 5s and 5p electrons surrounding them. Therefore, crystal field effects observable in 3d transition metal ions are not feasible in the case of rare earth ions. However, rare earth ion doped phosphors have emission in the visible range.

Higher quantum efficiency and lifetime shortening of intrinsic and extrinsic semiconductor quantum structures due to quantum confinement effects motivated to synthesize and characterize rare earth doped II–VI semiconductor nanocrystals. In bulk semiconductors, due to extreme dislocation of the electron or hole, the electron-hole exchange interaction term is very small, while in molecular size nanoparticles, due to confinement, the exchange term should be very large. Therefore, there may be a large enhancement of the oscillator strength from bulk to nanostructure materials, which enhances radiative recombination rate and causes life-time shortening. So it seems possible to design and fabricate more sensitive sensors or more efficient devices. Rare earth doped semiconductor quantum dots (QDs) are good candidates for fast and efficient phosphors and high density optical data storage applications. But there are only few reports on rare earth doped II–VI semiconductor QDs.

Doping of rare earth ions in ZnS nanoparticles has been discussed by Bhargava (1996). He pointed out that rare earth ion doped ZnS nanoparticles can be useful in producing efficient phosphor materials with a gamut of colors. ZnS host is able to produce red, green, and blue luminescence due to Tm3+, Tb2+, and Eu3+ dopants. Moreover, slow trapping sites or non-radiative recombination sites can be removed in nanoparticles by appropriate surface passivation. Chen et al. (2000) also investigated Eu2+-doped ZnS nanoparticles of 3 nm size. Qu et al. (2002) synthesized structurally and optically stable nanoparticles of Eu3+-doped ZnS with average size of about 3–5 nm by the chemical precipitation method adding α-methacrylic acid as the stabilizer. Papakonstantinou et al. (1998) synthesized Eu3+-doped ZnS nanoparticles embedded in a polymer matrix. It has been observed that introduction of Eu3+ greatly enhanced the host related photoluminescence (PL) by exciting with a 315 nm wavelength. Chowdhury and Patra (2006) studied role of Eu3+ concentration and surface coating on photophysical properties of CdS: Eu3+ nanocrystals. They reported that site symmetry of ions plays a very important role in the modifications of radiative and nonradiative relaxation mechanisms. Julian et al. (2006) reported one-pot sol–gel synthesis and optical characterization of Eu3+-doped CdS nanocrystals in SiO2 matrices.

Cd1−xZnxS as a direct band gap semiconductor has attracted renewed interest for applications in solar cells, photoconductive devices, and display devices. Moreover, rare earth doped Cd1−xZnxS QDs will be very good candidates for display and sensor applications. To the best of our knowledge there is no report on the synthesis and PL studies of Eu3+-doped Cd1−xZnxS QDs. This article reports first time ‘laser induced time resolved laser spectroscopy’ of rare earth doped quaternary semiconductor QDs. Time resolved laser spectroscopic measurements and hence, calculated decay time values are very beneficial to make phosphor calibration curves for future optoelectronic industrial applications. High peak power pulsed laser excitation is capable of exciting the short lived shallow trapping states of QDs, which seems impossible with conventional lamps.

Experimental

Eu3+-doped Cd1−xZnxS (0 ≤ x ≤ 0.5) QDs have been synthesized using well known bottom-up synthesis technique wet chemical precipitation method. Analytical reagent grade chemicals: cadmium acetate [(CH3COO)2Cd · 2H2O], zinc acetate (C4H6O4Zn · 2H2O), europium acetate [Eu(CH3CO2)3 · H2O], sodium sulphide (Na2SxH2O), and polyvinyl pyrrolidone (PVP) [(C6H9NO)n] have been used without further purification. About 0.5 M solution of cadmium acetate, zinc acetate, europium acetate, and sodium sulphide were prepared in separate beakers. Then solutions of cadmium, zinc, and europium precursors were mixed in the stoichiometric proportion under vigorous stirring, 4 mL of 2% PVP aqueous solution was added to total 50 mL volume of reaction mixture, before drop wise addition of aqueous sodium sulphide. PVP will act as the capping agent to avoid the agglomeration of QDs. The resulting precipitates were centrifuged and dried in vacuum oven for 10–12 h continuously.

X-ray diffraction patterns of the synthesized samples have been recorded using a Panalytical’s X’Pert Pro Powder X-ray diffractometer with Cu Kα radiation (λ = 1.541 Å) in the 2θ range 20–70o. From the line broadening of the XRD diffractogram average crystallite size has been calculated using Scherrer formula (Cullity 1978). Transmission electron microscope (TEM) images have been recorded using JEOL JEM 2000 Ex. Type TEM for average particle size determination.

Time resolved luminescence spectra have been recorded using high peak power (10 kW), pulsed N2-laser excitation. Nanophosphors pasted on the perspex sample holder are placed at 45o to the laser beam. The phosphorescence from the phosphor at an angle of 90o to the laser beam is collected by a fast photomultiplier tube (RCA 8053 PMT) through assembly of monochromator (wavelength selective element) and glass slab (UV radiation filter). Decay signals were recorded in the digital storage oscilloscope coupled with PC. Excited state lifetime values have been calculated from recorded multi-exponential decay curves. Details of the experimental set-up have been already reported by Bhatti et al. (2004, 2005).

Results and discussion

Broad XRD patterns have been recorded for all the samples; one such X-ray diffractogram is shown in Fig. 1. This reveals that the synthesized nanomaterials exhibit a zinc-blende crystal structure. The three diffraction peaks correspond to (111), (220), and (311) planes of the cubic crystalline CdZnS. XRD analysis show no characteristics peaks of impurity phases. XRD peak broadening confirm the nanosize formation. Average crystallite size calculated from the recorded XRD patterns is ~4 nm.
https://static-content.springer.com/image/art%3A10.1007%2Fs11051-009-9586-1/MediaObjects/11051_2009_9586_Fig1_HTML.gif
Fig. 1

XRD pattern of Eu3+ (10 at.wt%)-doped Cd0.5Zn0.5S QDs

Figure 2 shows the transmission electron micrograph of the Eu3+-doped nanocrystalline Cd0.5Zn0.5S. Average particle size calculated from the TEM is ~4 nm, which is similar to the average crystallite size calculated from XRD. So all the particles are single nanocrystals having size comparable or less than the Bohr exciton radius (Bohr exciton size varies with value of x in Cd1−xZnxS).
https://static-content.springer.com/image/art%3A10.1007%2Fs11051-009-9586-1/MediaObjects/11051_2009_9586_Fig2_HTML.jpg
Fig. 2

TEM image of Eu3+ (10 at.wt%)-doped Cd0.5Zn0.5S QDs

N2-laser (λ = 337.1 nm) excitation causes dichromatic (violet & red) emission from Eu3+-doped Cd1−xZnxS QDs. Excitation cross-section of Eu3+ ions is negligible relative to Cd1−xZnxS excitation at 337.1 nm. The non-radiative energy transfer process is taking place from the excited state of host to the levels of Eu3+ ions. Violet emission (λ1 ≈ 430 nm) corresponds to host related emission, whereas red emission (λ2 ≈ 617 nm) attributed to 5D0 → 7F2 transition of Eu3+ ions. Orange colored very feeble emission corresponding to 5D0  → 7F1 transition has also been observed. But no time resolved spectra have been recorded for orange emission due to low intensity. Chowdhury and Patra (2006) have also reported the red (614 nm) and orange (590 nm) emission of Eu3+ ions. Multi-exponential decay curves have been recorded for all the samples. Figure 3 shows one such decay curve recorded for Eu3+-doped Cd1−xZnxS QDs. Multi-exponential nature of decay curves confirm the emission from multi-level trapping states. Verma et al. (2003) and Bhatti et al. (2004) have reported the detailed description about the peeling-off the multi-exponential decay curves into exponential components using peeling-off method of Bube and the calculation of excited state lifetime values.
https://static-content.springer.com/image/art%3A10.1007%2Fs11051-009-9586-1/MediaObjects/11051_2009_9586_Fig3_HTML.gif
Fig. 3

Decay curve for Eu3+-doped Cd1−xZnxS QDs measured at 617 nm

Table 1 shows decay time values for violet and red emission. Three values of decay time corresponding to each sample for each emission wavelength have been calculated due to emission from multilevel trapping states. Decay time values for violet emission varies from 0.05 to 2.85 μs, whereas excited state lifetime values for red emission varies from 0.06 to 2.72 ms. Recorded results show lifetime shortening for both the emissions with increasing concentration of Zn2+ in Cd1−xZnxS QDs. This is due band gap broadening with increasing concentration of zinc, which quenches the emission from deep traps, whereas the shallow trap state emission becomes dominant and on the other hand probability of energy transfer from host to Eu3+ increases with increasing concentration of zinc. Table 1 shows red colored decay becomes slow with increasing concentration of Eu3+ ions, which indicates that the Eu3+ ions at higher concentration are in lower site symmetry.
Table 1

Room temperature excited state lifetime values for Eu3+-doped Cd1−xZnxS QDs

Sr. No.

Sample

Decay time (μs) for 430 nm emission

Decay time (ms) for 617 nm emission

τ1

τ2

τ3

τ1

τ2

τ3

1.

CdS: Eu3+ (0.01%)

0.52

1.10

2.85

0.17

0.42

1.88

2.

CdS: Eu3+ (0.10%)

0.50

1.03

2.67

0.19

0.53

2.22

3.

CdS: Eu3+ (1.00%)

0.49

0.99

2.33

0.23

0.59

2.43

4.

CdS: Eu3+ (10.00%)

0.47

0.93

2.01

0.28

0.65

2.72

5.

Cd0.9Zn0.1S:Eu3+ (0.01%)

0.51

1.08

2.79

0.15

0.39

1.73

6.

Cd0.9Zn0.1S:Eu3+ (0.10%)

0.49

0.98

2.37

0.18

0.47

2.06

7.

Cd0.9Zn0.1S:Eu3+ (1.00%)

0.46

0.90

1.95

0.22

0.54

2.31

8.

Cd0.9Zn0.1S:Eu3+ (10.00%)

0.42

0.87

1.83

0.27

0.63

2.68

9.

Cd0.7Zn0.3S: Eu3+ (0.01%)

0.37

0.75

1.62

0.11

0.31

1.49

10.

Cd0.7Zn0.3S: Eu3+ (0.10%)

0.34

0.69

1.39

0.14

0.36

1.61

11.

Cd0.7Zn0.3S: Eu3+ (1.00%)

0.29

0.61

1.24

0.18

0.46

2.10

12.

Cd0.7Zn0.3S:Eu3+ (10.00%)

0.24

0.51

1.15

0.21

0.51

2.17

13.

Cd0.5Zn0.5S: Eu3+ (0.01%)

0.18

0.38

1.02

0.06

0.16

0.83

14.

Cd0.5Zn0.5S: Eu3+ (0.10%)

0.14

0.30

0.93

0.09

0.24

1.16

15.

Cd0.5Zn0.5S: Eu3+ (1.00%)

0.10

0.21

0.63

0.13

0.35

1.54

16.

Cd0.5Zn0.5S:Eu3+ (10.00%)

0.05

0.13

0.41

0.16

0.41

1.77

Conclusions

Eu3+-doped Cd1−xZnxS QDs have been synthesized at room temperature using a simple wet chemical precipitation method. Synthesized QDs of 4 nm size have zinc-blende crystal structure. Lifetimes shortening in the dichromatic emission of Eu3+-doped Cd1−xZnxS QDs have been reported with increasing concentration of Zn2+. But the rare earth related decay time becomes slow with increase in concentration of Eu3+ ions due to lower site symmetry.

Acknowledgements

Authors are grateful to RSIC, Punjab University, Chandigarh and especially Mr. Jagtar Singh for crystallographic and morphological studies.

Copyright information

© Springer Science+Business Media B.V. 2009