Effect of Cd 0.5 Zn 0.5 S shells on...

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Effect of Cd0.5Zn

aSchool of Material Science and Engineering

China. E-mail: mse_yangp@ujn.edu.cnbDepartment of Materials Science and Engi

Hsinchu 300, Taiwan

Cite this: RSC Adv., 2014, 4, 43800

Received 21st July 2014Accepted 2nd September 2014

DOI: 10.1039/c4ra07399a

www.rsc.org/advances

43800 | RSC Adv., 2014, 4, 43800–438

0.5S shells on temperature-dependent luminescence kinetics of CdSe quantumdots

Ping Yang,*a Hsueh-Shih Chen,b Sha Zhang,a Jie Zhao,a Yingying Du,a Yanping Miao,a

Haiyan Hea and Yunshi Liua

CdSe/Cd0.5Zn0.5S 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 Cd0.5Zn0.5S 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 Cd0.5Zn0.5S

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/Cd0.5Zn0.5S 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).

Introduction

Quantum dots (QDs) with size ranges of less than 10 nm are ofgreat interest due to their tunable optical properties such asbroad absorption spectra, narrow and symmetric emissionspectra, high uorescence quantum yield, and photostability, aswell as particle size-dependent photoluminescence (PL).1 Theproperties of semiconductor QDs depend strongly on theircomposition, structure, surface chemistry, and capping strate-gies, especially for their PL efficiencies and PL peak wavelengthsbecause of the tunable band gap of the QDs with compositions,structures, and sizes.2 For example, the effective band gaps oftype-I core/shell QDs are mostly governed by the size andcomposition of the core materials because carriers dwell mostlyin the cores while those of type-II QDs have the conduction andvalence band levels of the cores that are offset from those in theshells; hence carriers reside on opposite sides of the core/shellboundary.3 These special band gap natures make the QDsreversible PL spectral switching. In the case of well-synthesizedQDs, which have less structure and surface defects as well as ahighly PL efficiency, many interesting behaviors such as

, University of Jinan, Jinan, 250022, P. R.

neering, National Tsing Hua University,

05

temperature-dependent emission color changes can beobserved. This promotes the study and application of the QDs.Therefore, researchers focused on the preparation and propertystudy of highly luminescent QDs. For example, type-II CdTe/CdSe core/shell QDs revealed a high and temperature-depen-dent PL.4 Despite these progresses, the temperature depen-dence of the properties of the QDs is still controversial and notfully understood.

It is well known, the band gap of bulk semiconductorsdepends 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 depen-dence of the electron–lattice interaction because of the particlesizes. The PL properties of QDs, such as spectral width andStokes shis, are depended strongly on the band gap andsurface state. The study of PL spectra and PL-decay proles isimportant and necessary to understand the temperature-dependent feature because the operating temperature is eitherelevated or reduced in many of the potential applications ofsemiconductor QDs. The defect-related PL spectrum with alarge Stokes shi of the QDs is dominant, and the band-edge PLband is negligibly weak. The defect-related PL resulted in adifficulty to understand the temperature-dependent feature. Itis necessary to prepare QDs with high PL efficiency to under-stand the mechanism of temperature-dependent PL. Recentdevelopment in the synthesis of colloidal QDs can allow many

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sophisticated nanostructures like core/shell heterostructures.3

For example, CdSe/ZnS and CdSe/ZnCdS core/shell QDs canexhibit bright and more stable PL compared with bare CdSecores. Such bright PL is ascribed to a shell passivation whichdecreased the surface defects. The study of synthesis kinetics ofthe QDs makes a possibility to discuss the temperature-depen-dent PL of the QDs.

The study of synthesis kinetics promotes to understand thePLmechanism of QDs. The temperature dependent PL of QDs isascribed that the variation of band gap can provide insight intothe exciton relaxation process and exciton–phonon interac-tions.1 However, the study on the temperature dependence ofsemiconductor nanocrystals have been rare and mostly limitedto CdSe, CdSe/ZnS and ZnSe.5–9 It is important and necessary tounderstand this feature, due to the potential applications ofsemiconductor QDs, the operating temperature is eitherelevated or reduced.10 For instance, Li et al. reported on the useof single QD as local temperature markers in the temperaturerange in the temperature range 25–50 �C.11 The temperaturedependence of PL is normally investigated at low temperature(e.g. less than room temperature). This may limit the applica-tion of QDs.

In this paper, we prepared CdSe cores with high crystallinityvia an organic synthesis. Green-emitting CdSe cores with rice-like morphology revealed PL efficiency of 10% while that of redone (a PL peak of 624 nm) with a rod shape is 1%. The coreswere coated with a Cd0.5Zn0.5S shell to get a high PL efficiencyup to 85%. Both of the cores and core/shell QDs revealed narrowsize distribution. The decay time of the cores and core/shell QDswas investigated. The core/shell QDs revealed a short averagelife time compared with the cores. The temperature dependenceof PL was investigated from 293 to 393 K. The PL spectra of thecore/shell QDs were red-shied with increasing temperature.However, the PL of the cores was quenched with increasingtemperature. Because the core/shell QDs exhibited narrow PLspectra and still retained relative high PL efficiencies withincreasing temperature, they will be utilizable for furtherapplications.

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), octadecylphos-phonic acid (ODPA, 97%), trioctylphosphine (TOP, 90%), oleicacid (OA, 90%), and trioctylamine (TOA) were purchased fromSigma Aldrich. All chemicals were used directly without anyfurther purication except for TOP. The pure water wasobtained from a Milli-Q synthesis system (18 U cm).

The synthesis of CdSe cores were completed in N2 atmo-sphere by modifying a published method.12 Typically, CdO of0.54 mmol was added in a three-neck round-bottom ask with180 mg of ODPA, and 5 mL of TOA with N2 owing and stirringat 300 �C until the CdO completely dissolved. The TOPSe solu-tion with Se powder of 1 mmol and 1.5 mL of TOP was theninjected into the precursor solution of cadmium with vigorousstirring. The solution was then kept at 300 �C for 2 min,

This journal is © The Royal Society of Chemistry 2014

followed by cooling down to room temperature. Aer that, 15mL of hexane and 50 mL of ethanol were added to precipitatesamples. The resulting sample was centrifuged, washed withcopious ethanol, and re-dispersed in 15 mL of toluene. Finally,samples were precipitated with ethanol, and re-dispersed in 15mL of toluene for subsequent shell coating.

To coat with a CdxZn1�xS shell on CdSe cores, typically, a Cdand Zn precursor with Cd(Ac)2$2H2O of 0.05 mmol, Zn(Ac)2 of0.05 mmol, 2 mL of OA, and 5 mL of TOA were prepared in athree-neck round-bottomask withN2 ow and stirring at 300 �C.The toluene solution of CdSe cores was injected in the precursorsolution with vigorous stirring, followed by the injection of theTOPS 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 certaintime, followed by cooling down to room temperature. Theresulting samples were precipitated, washed with ethanol, andre-dispersed in 15 mL of toluene.

Elemental analysis of QD samples was performed using aninductively coupled plasma atomic emission spectrometer(IR1S Advantage, Nippon Jarrell-Ash Co. Ltd.). The molar ratioof Zn/Cd in the shell was estimated through minus the Cdamount of the cores used during preparation of the shell. Theconcentration of QDs was estimated by their adsorption at rstabsorption peak. Transmission electron microscopy (TEM)observations were carried out using JEM 2100 (JEOL Ltd.) andH-1000 (Hitachi) electron microscopes. The absorption and PLspectra of samples were recorded using Hitachi F-4600 and U-4100 spectrometers, respectively. Both excitation and emissionslits used for the measurement of PL spectra are 5 nm. PLlifetime measurements were carried out using the time-corre-lated single-photon-counting spectrouorometer system (lex ¼370 nm, Fluorocube-01, JY-IBH, Horiba). The PL efficiency ofsamples was estimated in comparison with a standard rhoda-mine 6G solution (PL efficiency h0 of 95%) under the similaroptical path length and optical density conditions.13,14

Results and discussion

Fig. 1 shows the absorption and PL spectra of CdSe cores andCdSe/Cd0.5Zn0.5S core/shell QDs. Two kinds of CdSe cores(samples 1 and 5) were created through adjusting the injectedrates of TOPSe solutions. Except for the rst adsorption peak,absorption shoulders or maxima in the UV region andunstructured absorption features at longer wavelengths around440 to 500 nm were observed in the absorption spectra of CdSecores (samples 1 and 5). Such multiple absorption features mayrelate the absorption coefficient depended on the excitingphoton polarization at a given photon energy and also reect anarrow size dispersion of CdSe QDs.15,16 The PL peak wavelengthof samples 1 and 5 are 557 and 624 nm, respectively. Because ofa Se-rich surface, their PL efficiency is low (10% for sample 1and 1% for sample 5). Table 1 illustrates the properties of thesecores and core/shell QDs. Aer being coated with a Cd0.5Zn0.5Sshell, signicant red-shi in both the absorption and PL spectra(a maximum PL peak wavelength of 652 nm for sample 6) of thecore/shell QDs and increased PL efficiencies up to 85% (sample2) were observed. This phenomenon is ascribed to the surface

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

Table 2 Composition of CdSe and CdSe/Cd0.5Zn0.5S QDs

Sample QDs Cd (mmol) Zn (mmol)

5 CdSe 1.13 � 10�3 N/A

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passivation of a Cd0.5Zn0.5S composite shell because the inter-mediate CdS layer could relieve the lattice mismatch betweenthe CdSe core and the ZnS shell. The thickness of theCd0.5Zn0.5S shell increased with prolonging reaction time,which resulted in a gradual red-shi of PL spectra (samples 2 to4). The full width at half maximum (FWHM) of PL spectra wasincreased slightly aer being coated with the Cd0.5Zn0.5S shell.The red-shi of PL peak wavelengths is ascribed to the forma-tion of type-II core/shell heterostructure. Because of the overlapof absorption of core/shell QDs and the PL peak of the core, theelectronic charge transfer between the core and shell occurred.Because of the difference of the growth kinetic of CdS and ZnS(CdS with small Ksp value compared with ZnS), a gradientcomposite shell with CdS-rich inside and ZnS-rich outside wascreated even though the precursors of Cd and Zn were mixeddirectly. Thus, the shell layers of CdS and ZnS might not beclearly separated. The gradient structure would be benecial toimprove stability by reducing lattice mismatch between CdSand ZnS layers and to decrease the toxicity of the QDs forapplications. Furthermore, inductively coupled plasma-atomic

Table 1 Properties of CdSe and CdSe/Cd0.5Zn0.5S QDs

Sample Composition Core used

1 CdSe N/A2 CdSe/Cd0.5Zn0.5S Sample 13 CdSe/Cd0.5Zn0.5S Sample 14 CdSe/Cd0.5Zn0.5S Sample 15 CdSe N/A6 CdSe/Cd0.5Zn0.5S Sample 5

43802 | RSC Adv., 2014, 4, 43800–43805

emission spectroscopy (ICP-AES) mass analysis revealed thatthe molar ratio of Cd/Zn in the shell is almost 1/1. Table 2illustrates the element analysis result of CdSe cores (sample 5)and CdSe/Cd0.5Zn0.5S core/shell QDs (sample 6). This conrmsthe shell composition of Cd0.5Zn0.5S.

Fig. 2 shows the TEM images of initial CdSe cores (samples 1and 5) and CdSe/Cd0.5Zn0.5S core/shell QDs (samples 2, 3, 4, and6). Sample 1 (CdSe QDs) revealed a rice-like morphology asshown in Fig. 2a while sample 5 exhibited a rod shape as shownin Fig. 2e. The well-developed lattice fringe of CdSe QDs wasobserved in the insets in Fig. 2a and e. The CdSe QDs have necrystallinity which indicates the QDs with less surface defects.However, the PL efficiency of the QDs is low as indicated inTable 1 (10% for sample 1 and 1% for sample 5). This isascribed a Se-rich surface which decreases the edge emission ofthe QDs. Aer being coated with a Cd0.5Zn0.5S shell, the lengthof the core/shell QDs barely increased in comparing with that ofthe cores. This is ascribed to an anisotropic growth during shellcoating. With increasing reux time, the thickness of theCd0.5Zn0.5S shell increased. Because of ligands adsorbed onpolar {001} facets of CdSe in a TOP–ODPA–TOA solution, CdSeQDs prepared at high temperature exhibits a hexagonal crystalstructure and prefers to grow along the {002} direction (lengthdirection). A CdS shell prefers to grow on the {00�1} facet(diameter direction) of CdSe. Therefore, Cd0.5Zn0.5S shell didnot deposit on the reactive {002} planes at the CdSe nanorodends.

To further investigate the effect of Cd0.5Zn0.5S shell on theproperties of CdSe cores, Fig. 3 shows the luminescence decaycurves of samples 1 to 4 measured at the maximum PL peak, lex¼ 370 nm. Reproduced curves for data shown in Table 3 areplotted as thin white lines in Fig. 3. A biexponential function asfollows was used to t the decay curves:

F(t) ¼ A + B1 exp(�t/s1) + B2 exp(�t/s2)

where s1 and s2 represent the time constants, and B1 and B2represent the amplitudes of the fast and slow components,

FWHM (nm)PL efficiency(%)

PL peak wavelength(nm)

23.8 10 55725.4 85 59026.8 79 59427.2 76 59724.6 1 62428 61 652

6 CdSe/Cd0.5Zn0.5S 2.23 � 10�3 1.15 � 10�3

This journal is © The Royal Society of Chemistry 2014

Fig. 2 TEM images of CdSe (samples 1 and 5) and CdSe/Cd0.5Zn0.5Score/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-developedlattice fringe was observed in insets in (a) and (e).

Fig. 3 PL decay curves (measured at maximum emission peak, lex ¼374 nm) of CdSe cores and CdSe/Cd0.5Zn0.5S core/shell QDs (samples1 to 4). Reproduced curves for data shown in Table 3 are plotted as thinwhite lines.

Table 3 Components B1 and B2, time constants s1 and, s2, and averagelifetime sav of CdSe cores and CdSe/Zn0.5Cd0.5S QDs

Sample Composition B1 (%) s1/ns B2 (%) s2/ns sav/ns

1 CdSe 33 10.5 67 47.7 44.12 CdSe/Cd0.5Zn0.5S 19 10.6 81 29.9 29.93 CdSe/Cd0.5Zn0.5S 23 8.7 77 30.2 28.54 CdSe/Cd0.5Zn0.5S 27 3.9 73 24.4 23.2

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respectively. The curves were well tted and the correspondingt parameters are listed in Table 3. Average lifetime sav iscalculated using the formula as below.13

sav ¼ (B1s12 + B2s2

2)/(B1s1 + B2s2)

Average life time sav values are shown in Table 3. Because ofuctuation in the uorescence decay or the stochastic nature ofthe ground-state dipole moment,17 the PL decay plot of CdSecores is also tted well by biexponential. For CdSe cores, the fastdecay (s1) is 10.5 ns and the slow one (s2) is 47.7 ns, corre-sponding to the recombination of intrinsic excitons and theinterplay between excitons and surface traps, respectively.18 Theaverage uorescence lifetime sav of sample 1 is 44.4 ns. Thebiexponential dynamics indicates more than one radiativerecombination channel of excitons existed. Compared with theCdSe cores, the core/shell QDs (samples 2 to 4) have a smallvalue of s1 and s2 components, resulting in a decreased averagelifetime (sav). The average lifetime sav of samples 2 to 4 are 29.9,28.5, and 23.2 ns, respectively as illustrated in Table 3. Withincreasing the thickness of the shell, the average lifetimedecreased. This lifetime lengthening for a thin shell andshortening again for a thick shell might be related to the vari-ation of thickness and composition of the shell because theshell structure will inuence the exciton recombinationprocess.

To extract the radiative lifetime from the PL decay timeusually a suited kinetic model has to be applied providing areasonable tting function.19 The decay curves show best-ttingparameters by using biexponential tting functions. For CdSe/Cd0.5Zn0.5S core/shell QDs, the shell growth, which happens inan epitaxial manner, eliminates nonradiative traps at the core/shell interface, and the intrinsic excitons can be conned wellwithin QDs by a thick shell, so there is a lower possibility thatexcitons are trapped at surface states around QDs' surround-ings.18 Furthermore, the average uorescence lifetime of QDheterostructures increased when wide band gap CdS and ZnSshells over coated. This is ascribed to the position of the energylevels of semiconductor heterostructures having strong size-and composite-dependences, making the degree of carrierlocalization in core/shell QDs strongly sensitive to the dimen-sions of both the core and shell.13 For CdTe/CdS core/shell QDs,the lifetime increased aer a CdS shell coating.20 However, inour experiments, the core/shell QDs had a decrease in theiraverage uorescence lifetime compared with the cores. Both ofs1 and s2 components decreased for the core/shell QDscompared with the cores. Such effect with high PL efficiency

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Fig. 5 Temperature dependence of PL spectra of CdSe/Cd0.5Zn0.5Score/shell QDs at different temperatures. Dash line at 373 K and solidline at room temperature.

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maintenance is explained by the reduction in the nonradiativedecay channel for CdSe/Cd0.5Zn0.5S QDs. Most possibly, this isascribed to the Se-rich surface of CdSe cores who revealed a longlifetime compared with in literature.21 Compared with sample 1(CdSe cores), sample 2 (CdSe/Cd0.5Zn0.5S QDs) exhibited similars1, decreased s2, decreased B1, and increased B2. The increasedB2 in the PL decay curves is ascribed to the radiative recombi-nation of carriers because it accounts for more and more of thetotal PL when the efficiency of QDs is increased.19,21,22 Thedecreased lifetime is ascribed to the CdS interlayer whichincreases the radiative decay channel. The same cation at thesurface a CdSe core and a CdS interlayer results a larger bandoffset in the conduction band. Therefore, the wave function ofthe electron can dislocate in the shell completely. An additionalZnS outside shell with a substantially wide band gap efficientlyconne both electrons and holes within the CdSe/Cd0.5Zn0.5SQDs and substantially enhancing the spatial indirect radiativerecombination.

The temperature-dependence of the band gap (Eg) of bulksemiconductors is well described by the Varshni relation, whichhas been shown to be also valid for semiconductor QDs,23

Eg ¼ E0 � aT2/(T + b) (1)

where a is the temperature coefficient, b is the approximateDebye temperature of the material, and E0 is the band gap at0 K. Similarly with bulk materials, the band gap change ofsemiconductor QDs as temperature increases results in thetemperature dependence of their optical properties. as a result,Fig. 4 shows the red shi in the PL peak wavelength astemperature increased for CdSe/Cd0.5Zn0.5S QDs (samples 2, 3,4, and 6). The degree of red shi in the PL peak wavelength ofthe QDs increased slightly with temperature. Fig. 5 shows the PLspectra of samples 4 and 6 at room temperature and 373 K.

In temperature dependent PL spectra of core/shell QDs, thePL peak shapes remain symmetric. The PL intensity of the QDsdecreased slightly. This is ascribed to the change of ligand onthe surface with increasing temperature. The PL peak wave-length of the QD was red-shied with increasing temperature.Typically, the radiative channels become more dominant and

Fig. 4 Red-shifted PL peak wavelengths of CdSe/Cd0.5Zn0.5S core/shell QDs with increasing temperature.

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the effective band gap increases as temperature decreases. Thisphenomenon may also be explained by an Arrhenius equation.In addition, we did not observed the red-shi of PL peakwavelength of CdSe cores. The PL of CdSe cores was quenchedwith heat-treatment. This is ascribed to the cores with highcrystallinity which normally expects good phonon propertywhich favors temperature quenching. The thermal quenchingof PL competes with radiative exciton recombination and can beexplained in terms of the decomposition of excitons. Non-radiative recombination generated by the decomposition ofexcitons. Therefore, the temperature quenching of the QDs areobserved, both in colloidal suspensions and in solvent-freesystems, such as the QDs in polymeric matrices.24 Similarphenomenon was observed in our previous paper.13 Greenemitting CdTe QDs with low crystallinity revealed PL quenchingbecause of the change of their surface state with increasingtemperature. In current experiment, the PL of CdSe coresquenched with increasing temperature because of their Se-richsurface which is the origin of decreasing edge emission.Furthermore, the core/shell QDs did not exhibit PL temperaturequenching because of their high PL efficiency.

In fact, the major nonradiative carrier relaxation channel insemiconductors is due to phonon quenching.25 The non-radiative rate becomes high when the phonon coupling isstrong, and the PL is more sensitive to temperature change.26

Therefore, the PL efficiency of QDs decreases normally withincreasing temperature. In our experiments, the PL intensity ofCdSe/Cd0.5Zn0.5S QDs decreased slightly with increasingtemperature.

Conclusions

CdSe cores with high crystallinity were created via an organicsynthesis. The core was coated with a gradient Cd0.5Zn0.5S shell.Compared with the core, the core/shell QDs revealed red-shiedPL spectra, drastically increased PL efficiencies up to 85%, and ashort lifetime. Both of the cores and the core/shell QDsexhibited narrow PL spectra which indicated a narrow sizedistribution. The temperature-dependent PL dynamics in CdSeQDs with a low PL efficiency and core/shell QDs with a high PLefficiency was further investigated from 293 to 393 K. The PL

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spectra of the core/shell QDs were red-shied with increasingtemperature. However, the PL of the cores was quenched withincreasing temperature. The PL intensity of the core/shell QDsdecreased slightly with increasing temperature. This is usefulfor further applications.

Acknowledgements

This work was supported in part by the program for TaishanScholars, the projects from National Natural Science Founda-tion of China (51202090) and Outstanding Young ScientistsFoundation Grant of Shandong Province (BS2012CL004 andBS2012CL006).

Notes and references

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