Effect of Cd_0.5Zn_0.5S shells on temperature-dependent luminescence kinetics of CdSe quantum dots

Abstract

CdSe/Cd_0.5Zn_0.5S core/shell quantum dots (QDs) with high photoluminescence (PL) efficiency up to 85% were fabricated in organic solutions at high temperature via an anisotropic shell growth on CdSe nanorods. The core/shell QDs with PL peak wavelengths from green to red were obtained by controlling the size of the cores and the thickness of the array Cd_0.5Zn_0.5S shells. Both the cores and the core/shell QDs revealed narrow size distributions which resulted in narrow PL spectra. Green-emitting CdSe cores with a Se-rich surface revealed a long average lifetime of ∼44 ns. After being coated with Cd_0.5Zn_0.5S shells, the average lifetime of QDs decreased drastically up to ∼23 ns. The average decay time of the core/shell QDs depended on their shell thickness. The temperature-dependent PL in a temperature range of 293 to 393 K was investigated for CdSe cores and highly luminescent CdSe/Cd_0.5Zn_0.5S core/shell QDs. Luminescent quenching occurred with increasing temperature for the cores even though the cores exhibited high crystallinity. In contrast, with increasing temperature, the emission PL peak wavelength of the core/shell QDs shifts towards lower energies, the PL bandwidth increases a little and the PL efficiencies decrease slightly. The red-shifted degree of the PL spectra with temperature is small (less than 10 nm).

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Introduction

Quantum dots (QDs) with size ranges of less than 10 nm are of great interest due to their tunable optical properties such as broad absorption spectra, narrow and symmetric emission spectra, high fluorescence quantum yield, and photostability, as well as particle size-dependent photoluminescence (PL).¹ The properties of semiconductor QDs depend strongly on their composition, structure, surface chemistry, and capping strategies, especially for their PL efficiencies and PL peak wavelengths because of the tunable band gap of the QDs with compositions, structures, and sizes.² For example, the effective band gaps of type-I core/shell QDs are mostly governed by the size and composition of the core materials because carriers dwell mostly in the cores while those of type-II QDs have the conduction and valence band levels of the cores that are offset from those in the shells; hence carriers reside on opposite sides of the core/shell boundary.³ These special band gap natures make the QDs reversible PL spectral switching. In the case of well-synthesized QDs, which have less structure and surface defects as well as a highly PL efficiency, many interesting behaviors such as temperature-dependent emission color changes can be observed. This promotes the study and application of the QDs. Therefore, researchers focused on the preparation and property study of highly luminescent QDs. For example, type-II CdTe/CdSe core/shell QDs revealed a high and temperature-dependent PL.⁴ Despite these progresses, the temperature dependence of the properties of the QDs is still controversial and not fully understood.

It is well known, the band gap of bulk semiconductors depends strongly on temperature. In the case of nanoparticles, this dependence takes into account among the surface defects, the change in lattice parameter, and the temperature dependence of the electron–lattice interaction because of the particle sizes. The PL properties of QDs, such as spectral width and Stokes shifts, are depended strongly on the band gap and surface state. The study of PL spectra and PL-decay profiles is important and necessary to understand the temperature-dependent feature because the operating temperature is either elevated or reduced in many of the potential applications of semiconductor QDs. The defect-related PL spectrum with a large Stokes shift of the QDs is dominant, and the band-edge PL band is negligibly weak. The defect-related PL resulted in a difficulty to understand the temperature-dependent feature. It is necessary to prepare QDs with high PL efficiency to understand the mechanism of temperature-dependent PL. Recent development in the synthesis of colloidal QDs can allow many sophisticated nanostructures like core/shell heterostructures.³ For example, CdSe/ZnS and CdSe/ZnCdS core/shell QDs can exhibit bright and more stable PL compared with bare CdSe cores. Such bright PL is ascribed to a shell passivation which decreased the surface defects. The study of synthesis kinetics of the QDs makes a possibility to discuss the temperature-dependent PL of the QDs.

The study of synthesis kinetics promotes to understand the PL mechanism of QDs. The temperature dependent PL of QDs is ascribed that the variation of band gap can provide insight into the exciton relaxation process and exciton–phonon interactions.¹ However, the study on the temperature dependence of semiconductor nanocrystals have been rare and mostly limited to CdSe, CdSe/ZnS and ZnSe.^5–9 It is important and necessary to understand this feature, due to the potential applications of semiconductor QDs, the operating temperature is either elevated or reduced.¹⁰ For instance, Li et al. reported on the use of single QD as local temperature markers in the temperature range in the temperature range 25–50 °C.¹¹ The temperature dependence of PL is normally investigated at low temperature (e.g. less than room temperature). This may limit the application of QDs.

In this paper, we prepared CdSe cores with high crystallinity via an organic synthesis. Green-emitting CdSe cores with rice-like morphology revealed PL efficiency of 10% while that of red one (a PL peak of 624 nm) with a rod shape is 1%. The cores were coated with a Cd_0.5Zn_0.5S shell to get a high PL efficiency up to 85%. Both of the cores and core/shell QDs revealed narrow size distribution. The decay time of the cores and core/shell QDs was investigated. The core/shell QDs revealed a short average life time compared with the cores. The temperature dependence of PL was investigated from 293 to 393 K. The PL spectra of the core/shell QDs were red-shifted with increasing temperature. However, the PL of the cores was quenched with increasing temperature. Because the core/shell QDs exhibited narrow PL spectra and still retained relative high PL efficiencies with increasing temperature, they will be utilizable for further applications.

Experimental

Cadmium oxide (99.99%), cadmium acetate dihydrate (Cd(Ac)₂·2H₂O, 98%), zinc acetate (Zn(Ac)₂, 99.99%), selenium (99.5%, 100 mesh), sulfur (99.98%, powder), octadecylphosphonic acid (ODPA, 97%), trioctylphosphine (TOP, 90%), oleic acid (OA, 90%), and trioctylamine (TOA) were purchased from Sigma Aldrich. All chemicals were used directly without any further purification except for TOP. The pure water was obtained from a Milli-Q synthesis system (18 Ω cm).

The synthesis of CdSe cores were completed in N₂ atmosphere by modifying a published method.¹² Typically, CdO of 0.54 mmol was added in a three-neck round-bottom flask with 180 mg of ODPA, and 5 mL of TOA with N₂ flowing and stirring at 300 °C until the CdO completely dissolved. The TOPSe solution with Se powder of 1 mmol and 1.5 mL of TOP was then injected into the precursor solution of cadmium with vigorous stirring. The solution was then kept at 300 °C for 2 min, followed by cooling down to room temperature. After that, 15 mL of hexane and 50 mL of ethanol were added to precipitate samples. The resulting sample was centrifuged, washed with copious ethanol, and re-dispersed in 15 mL of toluene. Finally, samples were precipitated with ethanol, and re-dispersed in 15 mL of toluene for subsequent shell coating.

To coat with a Cd_xZn_1−xS shell on CdSe cores, typically, a Cd and Zn precursor with Cd(Ac)₂·2H₂O of 0.05 mmol, Zn(Ac)₂ of 0.05 mmol, 2 mL of OA, and 5 mL of TOA were prepared in a three-neck round-bottom flask with N₂ flow and stirring at 300 °C. The toluene solution of CdSe cores was injected in the precursor solution with vigorous stirring, followed by the injection of the TOPS solution with S powder of 0.20 mmol and 0.5 mL of TOP. The mixture was kept at 300 °C with stirring for further certain time, followed by cooling down to room temperature. The resulting samples were precipitated, washed with ethanol, and re-dispersed in 15 mL of toluene.

Elemental analysis of QD samples was performed using an inductively coupled plasma atomic emission spectrometer (IR1S Advantage, Nippon Jarrell-Ash Co. Ltd.). The molar ratio of Zn/Cd in the shell was estimated through minus the Cd amount of the cores used during preparation of the shell. The concentration of QDs was estimated by their adsorption at first absorption peak. Transmission electron microscopy (TEM) observations were carried out using JEM 2100 (JEOL Ltd.) and H-1000 (Hitachi) electron microscopes. The absorption and PL spectra of samples were recorded using Hitachi F-4600 and U-4100 spectrometers, respectively. Both excitation and emission slits used for the measurement of PL spectra are 5 nm. PL lifetime measurements were carried out using the time-correlated single-photon-counting spectrofluorometer system (λ_ex = 370 nm, Fluorocube-01, JY-IBH, Horiba). The PL efficiency of samples was estimated in comparison with a standard rhodamine 6G solution (PL efficiency η₀ of 95%) under the similar optical path length and optical density conditions.^13,14

Results and discussion

Fig. 1 shows the absorption and PL spectra of CdSe cores and CdSe/Cd_0.5Zn_0.5S core/shell QDs. Two kinds of CdSe cores (samples 1 and 5) were created through adjusting the injected rates of TOPSe solutions. Except for the first adsorption peak, absorption shoulders or maxima in the UV region and unstructured absorption features at longer wavelengths around 440 to 500 nm were observed in the absorption spectra of CdSe cores (samples 1 and 5). Such multiple absorption features may relate the absorption coefficient depended on the exciting photon polarization at a given photon energy and also reflect a narrow size dispersion of CdSe QDs.^15,16 The PL peak wavelength of samples 1 and 5 are 557 and 624 nm, respectively. Because of a Se-rich surface, their PL efficiency is low (10% for sample 1 and 1% for sample 5). Table 1 illustrates the properties of these cores and core/shell QDs. After being coated with a Cd_0.5Zn_0.5S shell, significant red-shift in both the absorption and PL spectra (a maximum PL peak wavelength of 652 nm for sample 6) of the core/shell QDs and increased PL efficiencies up to 85% (sample 2) were observed. This phenomenon is ascribed to the surface passivation of a Cd_0.5Zn_0.5S composite shell because the intermediate CdS layer could relieve the lattice mismatch between the CdSe core and the ZnS shell. The thickness of the Cd_0.5Zn_0.5S shell increased with prolonging reaction time, which resulted in a gradual red-shift of PL spectra (samples 2 to 4). The full width at half maximum (FWHM) of PL spectra was increased slightly after being coated with the Cd_0.5Zn_0.5S shell. The red-shift of PL peak wavelengths is ascribed to the formation of type-II core/shell heterostructure. Because of the overlap of absorption of core/shell QDs and the PL peak of the core, the electronic charge transfer between the core and shell occurred. Because of the difference of the growth kinetic of CdS and ZnS (CdS with small K_sp value compared with ZnS), a gradient composite shell with CdS-rich inside and ZnS-rich outside was created even though the precursors of Cd and Zn were mixed directly. Thus, the shell layers of CdS and ZnS might not be clearly separated. The gradient structure would be beneficial to improve stability by reducing lattice mismatch between CdS and ZnS layers and to decrease the toxicity of the QDs for applications. Furthermore, inductively coupled plasma-atomic emission spectroscopy (ICP-AES) mass analysis revealed that the molar ratio of Cd/Zn in the shell is almost 1/1. Table 2 illustrates the element analysis result of CdSe cores (sample 5) and CdSe/Cd_0.5Zn_0.5S core/shell QDs (sample 6). This confirms the shell composition of Cd_0.5Zn_0.5S.

Fig. 1 Absorption and PL spectra of QDs. (a) Samples 1 to 4. (b) Samples 5 and 6.

Table 1 Properties of CdSe and CdSe/Cd_0.5Zn_0.5S QDs

Sample	Composition	Core used	FWHM (nm)	PL efficiency (%)	PL peak wavelength (nm)
1	CdSe	N/A	23.8	10	557
2	CdSe/Cd0.5Zn0.5S	Sample 1	25.4	85	590
3	CdSe/Cd0.5Zn0.5S	Sample 1	26.8	79	594
4	CdSe/Cd0.5Zn0.5S	Sample 1	27.2	76	597
5	CdSe	N/A	24.6	1	624
6	CdSe/Cd0.5Zn0.5S	Sample 5	28	61	652

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Table 2 Composition of CdSe and CdSe/Cd_0.5Zn_0.5S QDs

Sample	QDs	Cd (mmol)	Zn (mmol)
5	CdSe	1.13 × 10^−3	N/A
6	CdSe/Cd0.5Zn0.5S	2.23 × 10^−3	1.15 × 10^−3

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Fig. 2 shows the TEM images of initial CdSe cores (samples 1 and 5) and CdSe/Cd_0.5Zn_0.5S core/shell QDs (samples 2, 3, 4, and 6). Sample 1 (CdSe QDs) revealed a rice-like morphology as shown in Fig. 2a while sample 5 exhibited a rod shape as shown in Fig. 2e. The well-developed lattice fringe of CdSe QDs was observed in the insets in Fig. 2a and e. The CdSe QDs have fine crystallinity which indicates the QDs with less surface defects. However, the PL efficiency of the QDs is low as indicated in Table 1 (10% for sample 1 and 1% for sample 5). This is ascribed a Se-rich surface which decreases the edge emission of the QDs. After being coated with a Cd_0.5Zn_0.5S shell, the length of the core/shell QDs barely increased in comparing with that of the cores. This is ascribed to an anisotropic growth during shell coating. With increasing reflux time, the thickness of the Cd_0.5Zn_0.5S shell increased. Because of ligands adsorbed on polar {001} facets of CdSe in a TOP–ODPA–TOA solution, CdSe QDs prepared at high temperature exhibits a hexagonal crystal structure and prefers to grow along the {002} direction (length direction). A CdS shell prefers to grow on the {001̄} facet (diameter direction) of CdSe. Therefore, Cd_0.5Zn_0.5S shell did not deposit on the reactive {002} planes at the CdSe nanorod ends.

Fig. 2 TEM images of CdSe (samples 1 and 5) and CdSe/Cd_0.5Zn_0.5S core/shell QDs (samples 2, 3, 4, and 6). (a) Sample 1. (b) Sample 2. (c) Sample 3. (d) Sample 4. (e) Sample 5. (f) Sample 6. Well-developed lattice fringe was observed in insets in (a) and (e).

To further investigate the effect of Cd_0.5Zn_0.5S shell on the properties of CdSe cores, Fig. 3 shows the luminescence decay curves of samples 1 to 4 measured at the maximum PL peak, λ_ex = 370 nm. Reproduced curves for data shown in Table 3 are plotted as thin white lines in Fig. 3. A biexponential function as follows was used to fit the decay curves:

F(t) = A + B₁ exp(−t/τ₁) + B₂ exp(−t/τ₂)

where τ₁ and τ₂ represent the time constants, and B₁ and B₂ represent the amplitudes of the fast and slow components, respectively. The curves were well fitted and the corresponding fit parameters are listed in Table 3. Average lifetime τ_av is calculated using the formula as below.¹³

τ_av = (B₁τ₁² + B₂τ₂²)/(B₁τ₁ + B₂τ₂)

Fig. 3 PL decay curves (measured at maximum emission peak, λ_ex = 374 nm) of CdSe cores and CdSe/Cd_0.5Zn_0.5S core/shell QDs (samples 1 to 4). Reproduced curves for data shown in Table 3 are plotted as thin white lines.

Table 3 Components B₁ and B₂, time constants τ₁ and, τ₂, and average lifetime τ_av of CdSe cores and CdSe/Zn_0.5Cd_0.5S QDs

Sample	Composition	B1 (%)	τ1/ns	B2 (%)	τ2/ns	τav/ns
1	CdSe	33	10.5	67	47.7	44.1
2	CdSe/Cd0.5Zn0.5S	19	10.6	81	29.9	29.9
3	CdSe/Cd0.5Zn0.5S	23	8.7	77	30.2	28.5
4	CdSe/Cd0.5Zn0.5S	27	3.9	73	24.4	23.2

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Average life time τ_av values are shown in Table 3. Because of fluctuation in the fluorescence decay or the stochastic nature of the ground-state dipole moment,¹⁷ the PL decay plot of CdSe cores is also fitted well by biexponential. For CdSe cores, the fast decay (τ₁) is 10.5 ns and the slow one (τ₂) is 47.7 ns, corresponding to the recombination of intrinsic excitons and the interplay between excitons and surface traps, respectively.¹⁸ The average fluorescence lifetime τ_av of sample 1 is 44.4 ns. The biexponential dynamics indicates more than one radiative recombination channel of excitons existed. Compared with the CdSe cores, the core/shell QDs (samples 2 to 4) have a small value of τ₁ and τ₂ components, resulting in a decreased average lifetime (τ_av). The average lifetime τ_av of samples 2 to 4 are 29.9, 28.5, and 23.2 ns, respectively as illustrated in Table 3. With increasing the thickness of the shell, the average lifetime decreased. This lifetime lengthening for a thin shell and shortening again for a thick shell might be related to the variation of thickness and composition of the shell because the shell structure will influence the exciton recombination process.

To extract the radiative lifetime from the PL decay time usually a suited kinetic model has to be applied providing a reasonable fitting function.¹⁹ The decay curves show best-fitting parameters by using biexponential fitting functions. For CdSe/Cd_0.5Zn_0.5S core/shell QDs, the shell growth, which happens in an epitaxial manner, eliminates nonradiative traps at the core/shell interface, and the intrinsic excitons can be confined well within QDs by a thick shell, so there is a lower possibility that excitons are trapped at surface states around QDs' surroundings.¹⁸ Furthermore, the average fluorescence lifetime of QD heterostructures increased when wide band gap CdS and ZnS shells over coated. This is ascribed to the position of the energy levels of semiconductor heterostructures having strong size- and composite-dependences, making the degree of carrier localization in core/shell QDs strongly sensitive to the dimensions of both the core and shell.¹³ For CdTe/CdS core/shell QDs, the lifetime increased after a CdS shell coating.²⁰ However, in our experiments, the core/shell QDs had a decrease in their average fluorescence lifetime compared with the cores. Both of τ₁ and τ₂ components decreased for the core/shell QDs compared with the cores. Such effect with high PL efficiency maintenance is explained by the reduction in the nonradiative decay channel for CdSe/Cd_0.5Zn_0.5S QDs. Most possibly, this is ascribed to the Se-rich surface of CdSe cores who revealed a long lifetime compared with in literature.²¹ Compared with sample 1 (CdSe cores), sample 2 (CdSe/Cd_0.5Zn_0.5S QDs) exhibited similar τ₁, decreased τ₂, decreased B₁, and increased B₂. The increased B₂ in the PL decay curves is ascribed to the radiative recombination of carriers because it accounts for more and more of the total PL when the efficiency of QDs is increased.^19,21,22 The decreased lifetime is ascribed to the CdS interlayer which increases the radiative decay channel. The same cation at the surface a CdSe core and a CdS interlayer results a larger band offset in the conduction band. Therefore, the wave function of the electron can dislocate in the shell completely. An additional ZnS outside shell with a substantially wide band gap efficiently confine both electrons and holes within the CdSe/Cd_0.5Zn_0.5S QDs and substantially enhancing the spatial indirect radiative recombination.

The temperature-dependence of the band gap (E_g) of bulk semiconductors is well described by the Varshni relation, which has been shown to be also valid for semiconductor QDs,²³

E_g = E₀ − αT²/(T + β) (1)

where α is the temperature coefficient, β is the approximate Debye temperature of the material, and E₀ is the band gap at 0 K. Similarly with bulk materials, the band gap change of semiconductor QDs as temperature increases results in the temperature dependence of their optical properties. as a result, Fig. 4 shows the red shift in the PL peak wavelength as temperature increased for CdSe/Cd_0.5Zn_0.5S QDs (samples 2, 3, 4, and 6). The degree of red shift in the PL peak wavelength of the QDs increased slightly with temperature. Fig. 5 shows the PL spectra of samples 4 and 6 at room temperature and 373 K.

Fig. 4 Red-shifted PL peak wavelengths of CdSe/Cd_0.5Zn_0.5S core/shell QDs with increasing temperature.

Fig. 5 Temperature dependence of PL spectra of CdSe/Cd_0.5Zn_0.5S core/shell QDs at different temperatures. Dash line at 373 K and solid line at room temperature.

In temperature dependent PL spectra of core/shell QDs, the PL peak shapes remain symmetric. The PL intensity of the QDs decreased slightly. This is ascribed to the change of ligand on the surface with increasing temperature. The PL peak wavelength of the QD was red-shifted with increasing temperature. Typically, the radiative channels become more dominant and the effective band gap increases as temperature decreases. This phenomenon may also be explained by an Arrhenius equation. In addition, we did not observed the red-shift of PL peak wavelength of CdSe cores. The PL of CdSe cores was quenched with heat-treatment. This is ascribed to the cores with high crystallinity which normally expects good phonon property which favors temperature quenching. The thermal quenching of PL competes with radiative exciton recombination and can be explained in terms of the decomposition of excitons. Nonradiative recombination generated by the decomposition of excitons. Therefore, the temperature quenching of the QDs are observed, both in colloidal suspensions and in solvent-free systems, such as the QDs in polymeric matrices.²⁴ Similar phenomenon was observed in our previous paper.¹³ Green emitting CdTe QDs with low crystallinity revealed PL quenching because of the change of their surface state with increasing temperature. In current experiment, the PL of CdSe cores quenched with increasing temperature because of their Se-rich surface which is the origin of decreasing edge emission. Furthermore, the core/shell QDs did not exhibit PL temperature quenching because of their high PL efficiency.

In fact, the major nonradiative carrier relaxation channel in semiconductors is due to phonon quenching.²⁵ The nonradiative rate becomes high when the phonon coupling is strong, and the PL is more sensitive to temperature change.²⁶ Therefore, the PL efficiency of QDs decreases normally with increasing temperature. In our experiments, the PL intensity of CdSe/Cd_0.5Zn_0.5S QDs decreased slightly with increasing temperature.

Conclusions

CdSe cores with high crystallinity were created via an organic synthesis. The core was coated with a gradient Cd_0.5Zn_0.5S shell. Compared with the core, the core/shell QDs revealed red-shifted PL spectra, drastically increased PL efficiencies up to 85%, and a short lifetime. Both of the cores and the core/shell QDs exhibited narrow PL spectra which indicated a narrow size distribution. The temperature-dependent PL dynamics in CdSe QDs with a low PL efficiency and core/shell QDs with a high PL efficiency was further investigated from 293 to 393 K. The PL spectra of the core/shell QDs were red-shifted with increasing temperature. However, the PL of the cores was quenched with increasing temperature. The PL intensity of the core/shell QDs decreased slightly with increasing temperature. This is useful for further applications.

Acknowledgements

This work was supported in part by the program for Taishan Scholars, the projects from National Natural Science Foundation of China (51202090) and Outstanding Young Scientists Foundation Grant of Shandong Province (BS2012CL004 and BS2012CL006).

Notes and references

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