Optical Materials 36 (2014) 1534–1541

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Optical Materials

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Synthesis and optical properties of core/shell ternary/ternaryCdZnSe/ZnSeS quantum dots

http://dx.doi.org/10.1016/j.optmat.2014.04.0200925-3467/� 2014 Elsevier B.V. All rights reserved.

⇑ Corresponding author at: Institute of Materials Science (IMS), VietnamAcademy of Science and Technology (VAST), 18 Hoang Quoc Viet, Cau Giay, Hanoi,Viet Nam. Tel.: +84 90 41 20 471; fax: +84 43 73 45 895.

E-mail addresses: phtnga@ims.vast.ac.vn, ngalamvn@gmail.com (P. Thu Nga).

Nguyen Hai Yen a, Willy Daney de Marcillac b,c, Clotilde Lethiec b,c, Phan Ngoc Hong a,b,c,Catherine Schwob b,c, Agnès Maître b,c, Nguyen Quang Liem a, Le Van Vu d, Paul Bénalloul b,c,Laurent Coolen b,c, Pham Thu Nga a,e,⇑a Institute of Materials Science (IMS), Vietnam Academy of Science and Technology (VAST), 18 Hoang Quoc Viet, Cau Giay, Hanoi, Viet Namb Sorbonne Universités, UPMC Univ Paris 06, UMR 7588, Institut de NanoSciences de Paris (INSP), Paris F-75005, Francec CNRS, UMR 7588, Institut de NanoSciences de Paris (INSP), Paris F-75005, Franced Center for Materials Science, University of Natural Science, VNUH, 334 Nguyen Trai, Hanoi, Viet Name Duy Tân University, Danang, Viet Nam

a r t i c l e i n f o

Article history:Received 25 December 2013Received in revised form 5 March 2014Accepted 16 April 2014Available online 10 May 2014

Keywords:Ternary QDsAlloyed QDsCdZnSe/ZnSeS core/shell QDsSingle-photon emitter

a b s t r a c t

In this paper we report on the synthesis of ternary/ternary alloyed CdZnSe/ZnSeS core/shell quantum dots(QDs) by embryonic nuclei-induced alloying process. We synthesized CdZnSe core QDs emitting in thespectral range of 530–607 nm with various Cd/Zn ratios, depending on the core synthesis temperature.By shelling ZnSeS on the CdZnSe core QDs, the average luminescence quantum yield is increased by a typ-ical factor of 2 up to 17, which we attribute to the reduction of number of non-emitting QDs. The single-photon emitter micro-photoluminescence study showed that the CdZnSe/ZnSeS core/shell QDs are goodsingle-photon emitters and their blinking properties were improved compared to the CdZnSe core QDs.Quantum yields up to 25% were measured for the core/shell samples, demonstrating the potential forhigh-quality ternary/ternary QDs fabrication.

� 2014 Elsevier B.V. All rights reserved.

1. Introduction

Semiconductor QDs have demonstrated their nice photolumi-nescence (PL) properties, such as tuning of the emission throughthe QD diameter, with applications for multiplexed biolabeling[1,2], solid-state lighting and display [3], sensors for agriculture[1,4–6] and single-photon emission [7–9]. Among various QDs,CdSe QDs have been studied most extensively [10–12]. By shellingCdSe core QDs with ZnS [13,14] or CdS [2,15], non-radiative decaychannels related to surface states could be passivated, conse-quently increasing the core/shell QDs luminescence quantum yield(LQY), which reaches almost 100% in some cases [16]. However,though CdSe/ZnS QDs emit strong luminescence, it also exhibitrandom ‘‘blinking’’ between emitting ‘‘on’’ states and non-emitting‘‘off’’ states [17]. To eliminate the blinking in a core QDs, a thicksuitable shell could be applied. For CdSe QDs, the thick CdS shellwas used to give a quasi-suppression of the blinking [2,18–20].

Many applications of QDs such as solid-state lighting or biolog-ical labeling require tuning the emission over the whole visiblespectrum. This is theoretically possible with the well-controlledCdSe QDs size; however for emission in the blue–green spectralregion it requires very small CdSe QDs, which are difficult to besynthesized with well-passivated surface and homogeneous size,resulting in low LQY and broad emission [21,22]. Alternatively,considering alloyed QDs, e.g., various ternary CdZnSe [21,25–31],CdSeTe [24] and CdZnS [23,32] which have been synthesized, theviolet–green region is accessible because the emission wavelengthdepends on their compositions. The tunable emissions fromthe mentioned QDs were reported in the spectral ranges of360–500 nm [27], 390–580 nm [26], 415–460 nm [32], 535–620 nm [21], respectively. Controlling the synthesis of ternaryQDs is however a delicate question. Different methods have beenpresented for the synthesis of alloyed CdZnSe QDs, and in particu-lar Zhong et al. distinguished two procedures [25,26]: for the firstone called ‘‘embryonic nuclei-induced alloying process’’ [25], bin-ary seeds are formed by injection of the first cationic precursor(either Cd or Zn), then the ternary QDs are grown by increase ofthe temperature and injection of the second cationic precursor.For the second procedure called ‘‘cationic exchange process’’ [26],

H.Y. Nguyen et al. / Optical Materials 36 (2014) 1534–1541 1535

binary QDs, for instance ZnSe, are first synthesized, then the sec-ond cationic precursor Cd2+ is injected and mixed with ZnSe so thatCdZnSe QDs are obtained. The obtained crystalline structures werefound to be hexagonal for the former synthesis [25] and zinc-blende for the latter [26].

The growth of the ZnS or ZnSe shell on the alloyed QDs wasreported, resulting in the core/shell ternary/binary structures[29,32]. For CdZnSe/ZnSe QDs, a suppression of blinking of theemission at 600–650 nm was demonstrated that is attributed toa gradient of the composition of alloy [31]. CdZnSe QDs have alsobeen used for the fabrication of light-emitting diode (LED) withthe spectral properties better than those of CdSe/ZnS QDs [33].To improve the spectral characteristics and optical properties, bin-ary/ternary core/shell structure like CdSe/CdZnS QDs or core/mul-tishell one like CdSe/(CdS/ZnCdS/ZnS) QDs have also beenperformed [34,35]. However, to our knowledge, there has beenno report on the synthesis of ternary/ternary core/shell QDs. Thisis possibly due to the particular difficulty in controlling the synthe-sis conditions of ternary/ternary QDs. One can expect the improve-ment of the optical properties from ternary/ternary core/shell QDsbecause of reduction of the lattice mismatch between the core andthe shell that are resulted from the smooth change in the compo-sition at the interface.

In this paper, we report on the synthesis of ternary/ternaryCdZnSe/ZnSeS core/shell QDs. The size and alloy composition, aswell as the photophysical properties of the QDs obtained at differ-ent synthesis temperatures are discussed. As the synthesis temper-ature increased from 285 �C to 310 �C, CdZnSe QDs obtained couldemit strong photoluminescence (PL) ranging from 530 nm to607 nm. We then have studied the ZnSeS shell growth with differ-ent Se/S precursor ratios. After shelling CdZnSe core QDs withZnSeS, increases of the LQY by 2–16 times and values of LQY upto 25% have been determined for the CdZnSe/ZnSeS core/shellQDs. The PL spectra and PL decay-times from the core/shell QDswere measured and analyzed. Finally, single-photon emittermicroscopy was performed showing much suppression of the PLblinking in the CdZnSe/ZnSeS core/shell QDs.

2. Experimental

2.1. Chemicals

We used the following reagents (from Aldrich): cadmium ace-tate (Cd(Ac)2, 99.9%), zinc acetate (Zn(Ac)2, 99.9%), selenium pow-der (Se, 99.99%), hexamethyl disilthiane (TMS)2S,trioctylphosphine oxide (TOPO, 99%), trioctylphosphine (TOP,90%) and hexadecylamine (HDA, 99%). All chemicals were usedwithout further purification.

2.2. Synthesis of the ternary alloyed CdZnSe core QDs

For the synthesis of the ternary core QDs we used the embry-onic nuclei-induced alloying procedure described in [25], with ini-tial growth of ZnSe seeds and subsequent growth of the CdZnSeQDs. Our choice of the precursors was modified with respect toRef. [25], so that the reaction conditions were adjusted accordingly.All synthetic routes were carried out using standard airless proce-dures by filling the reaction flask by ultra-pure nitrogen gas flow.

In order to fabricate the cadmium stock solution, we dissolved0.025 g of cadmium acetate in 0.54 ml TOP at 80 �C in nitrogengas. Similarly, we obtained the zinc stock solution by dissolving0.0875 g of zinc acetate in 1 ml TOP at 140 �C in nitrogen gas atmo-sphere, and we obtained the TOP–Se precursor by dissolving0.135 g of Se in 1.665 ml of TOP at 120 �C in nitrogen gas. Themolar ratios of precursors were thus the same for all samples

and equal to 0.2/0.8 for the Cd/Zn ratio and 1/3.3 for the(Cd + Zn)/Se ratio.

Briefly, 3.325 g of TOPO and 1.6625 g of HDA were poured into athree-neck reaction flask. Nitrogen gas was used to remove watervapor and oxygen out of the reaction flask at room temperaturefor 30 min, then at 120 �C for one hour. We first injected theTOP–Se precursor into the flask under vigorous stirring and heatingat temperatures up to 100 �C in nitrogen gas atmosphere. Kept stir-ring and heating the reactor up to 190 �C, and at that moment zincstock precursor solution was injected into the reaction flask inorder to form the ZnSe nanocrystallite seeds. Then temperaturewas increased up to 280 �C, at that time we injected the cadmiumstock solution into the reactor. As the temperature of the liquid inthe reaction flask dropped to �260 �C, the nucleation of the alloyedCdZnSe QDs started shaping quickly. The alloyed CdZnSe QDs weregrown for typically 20 min. at different temperatures of 285, 300and 310 �C for samples A–C, respectively.

2.3. Growth of the ternary alloyed ZnSeS shell on the CdZnSe core QDs

The shells were grown following a modified version of the suc-cessive ion layer adsorption and reaction (SILAR) procedure fromRefs. [34,36], originally described by Li, Peng and colleagues [15].The Zn precursor stock solution was prepared by mixing 0.165 gof zinc acetate and 1.88 ml of TOP, then heating to �140 �C innitrogen gas atmosphere until a clear solution was formed. Forthe TOP–Se, we dissolved 0.026 g Se in 0.33 ml TOP, at a tempera-ture of �120 �C in nitrogen gas (it is important that the solutionshould be clear after it is cooled down to room temperature) andmixed it with (TMS)2S. Different values of the Se/S molar ratio(noted x/1�x) were considered: x = 0.2, 0.4, 0.6 and 0.8. The molarratio of the precursors for growing the ZnSeS shell was always tobe Zn/(Se + S) = 1.37/1.

For shelling, 0.25 mM of the as-prepared CdZnSe ternary QDs inTOPO and HDA was filled into the reaction flask with nitrogen gasprotection. For one hour at 50 �C the evacuation of the air and there-fill of nitrogen gas were done several times to clean the atmo-sphere from the air. Then, temperature was raised to 240 �C forshelling with the precursor injection in five steps, each step con-sisting in consecutive addition of metal then chalcogenide addi-tion: for each step the zinc-stock solution was injected dropwisewith a rate of 1–2 drops per second (a drop is of 50 ll), under vig-orous stirring. Then, the mixture of (TMS)2S and TOP–Se was addedand the shelling temperature was kept at 240 �C for 15 min undervigorous stirring. The shell growth temperature was chosen suffi-ciently high for a good mixture of the ZnSeS alloy but 45–70 �Cbelow the core growth temperature in order to avoid any furthergrowth of the core QDs or diffusion between the core and shell.The volume of precursor solution injected at each step was calcu-lated in order to match the reaction stoichiometry to the area of QDsurface to be covered, as described in [37]. After the last step, theheater was removed and the mixture was cooled down to stopthe reaction. When the temperature reached 70 �C, the core/shellQDs were dispersed in organic solvent (toluene or choloroform).

2.4. Characterization of ternary CdZnSe/ZnSeS core/shell QDs

The size of the core QDs and the shell thickness were deter-mined by the transmission electron microscopy (TEM) with a JEOLJem 1010 microscope operating at 100 kV. The powder X-ray dif-fraction (XRD, Siemens D5005) was used to confirm the wurtzite(w) or zinc-blende (zb) crystalline structure. The XRD patternswere compared with the tabulated values of bulk CdSe (JCPDS cardNo. 19-191 (zb) and 8-459 (w)), ZnSe (JCPDS 37-1463 (zb) and 15-105 (w)) and ZnS (JCPDS 5-566 (zb) and 39-1363 (w)). The energy-

Fig. 1. TEM images of 5 nm (sample A), 4.2 nm (B) and 4.8 nm (C) CdZnSe core QDs.

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dispersive X-ray spectroscopy (EDS) was used to check the pres-ence of the Cd, Zn, Se and S elements in our ternary QD samples.

For optical characterizations, all the QD samples were diluted intoluene. In the PL measurements a pulsed nitrogen laser (337 nm,pulse width 0.6 ns, repetition rate of 10 Hz) was use as the excita-tion source. The PL from the samples was collected by an opticalfiber on the same side as the excitation light, then was analyzedby a Jobin–Yvon Spectrometer HR460 and detected by a multi-channel CCD detector (2000 pixel). The PL decays were analyzedwith a PM Hamamatsu R5600U and a Tektronix TDS 784A scopewith the time resolution of 1 ns. Finally, in order to measure theLQY, we measured for each sample the ratio of the integrated emis-sion (under Cd–He laser 442-nm excitation) to the 442-nm absorp-tion, and compared it with the ratio measured under the sameconditions for a rhodamine 6G sample. The concentration wasadjusted in order to keep the absorption at 442 nm in the range3–8% which was found adequate in our previous work [38].

For individual QD measurements, the QDs were spin-coated ona glass coverslip and then covered them by a 50-nm layer of PMMAto protect them from the oxidation. The sample was imaged with amicroscope and a x100 oil-immersion objective with 1.45 numer-ical aperture. On the one hand, for blinking studies (imaging tens ofQDs at the same time), a portion of the sample was illuminated at436 nm by a mercury-vapor lamp. The sample was imaged by aCCD camera, with a pixel size of 6.3 lm corresponding to a resolu-tion of 63 nm. The CCD rate was of 10 frames per second. On theother hand, for time-resolved studies, a single QD was excited bya 405-nm pulsed diode laser. Its emission was selected by a pin-hole located in the objective image plane, and detected by two ava-lanche photodiodes in Hanbury-Brown and Twiss configuration,connected to a Picoharp acquisition card. The overall setup pro-vided a 400-ps resolution.

3. Results for the ternary CdZnSe core QDs

3.1. Structural characterization

We first characterize the CdZnSe core QDs, as obtained beforethe shell growth. Various core growth conditions (temperatureand reaction time) were used as summarized in Table 1. We alsoreport in Table 1 the core diameter, ranging from 4.2 nm to 5 nmas estimated from TEM images as shown in Fig. 1. For each sample,the measured core diameters typically displayed a ±0.5–1 nminhomogeneity. Let us point out that a significant portion of thismeasured inhomogeneity is introduced by the uncertainty of mea-suring the QD size on the TEM image, so that the actual QD size dis-tribution is probably of 5–10%.

We also performed energy dispersive X-ray spectrometry (EDS).The results, reported in Table 1, show a decrease of the Zn/Cd ratiofrom A to C. It also shows however some non-stoichiometric(Cd + Zn)/Se values. Similar results were reported in [35], showingthat the ligand cleaning procedure preliminary to EDS measure-ment can alter significantly the composition of the surface of theQDs: for CdSe QDs, Cd/Se ratios between 3 and 3.8 were measuredand attributed to a lack of Se atoms on the QD surface due to

Table 1Growth conditions (temperature and reaction time after TOP–Cd injection) and QD diameteof ternary CdZnSeQDs.

Sample name Reaction temperature (�C) Reaction time (min)

A 285 20B 300 20C 310 17

sample cleaning (typically two thirds of the QD atoms are on thesurface) [35]. Although the quantitative compositions obtainedfrom EDS may be affected by the cleaning procedure, EDS demon-strates the presence of the three elements Cd, Zn and Se in the QDs.

The crystalline structure of QDs alloy can be different from thebulk alloy and is difficult to be predicted. Bulk ZnSe can have azinc-blende (zb) phase or a metastable wurtzite (w) phase; onthe other hand bulk CdSe can have a wurtzite phase or a metasta-ble zinc-blende phase [34,39]. In the literature, both wurtzite [24]and zinc-blende [27] structures have been reported for CdZnSeQDs. For QDs grown on ZnSe seeds, a wurtzite structure wasreported in [26], although a zinc-blende structure has also beenfound for ZnSe QDs [25]. Experimentally, the analysis of the XRDdata is complicated because of the broadening of the diffractionpeaks due to the absence of long-range order.

Fig. 2 shows the XRD patterns of the three QD samples (synthe-sized with the conditions indicated in Table 1, namely A and B andC) and the tabulated diffraction lines of bulk CdSe and ZnSe. SimilarXRD patterns are obtained for the three samples synthesized attemperatures from 285 �C to 310 �C. We find experimental XRDpeaks between the tabulated peaks corresponding to the wurtzitephases of CdSe and ZnSe, which would be in agreement with awurtzite CdZnSe alloy (although, given the width of the experi-mental spectra, we cannot exclude a polytype of zinc-blende andwurtzite phases). The wurtzite structure would be consistent withthe structure reported in [26] with a ZnSe-seeded growth process.

r (from TEM images) and Cd/Zn and (Cd + Zn)/Se ratios from EDS data of three samples

QD diameter (nm) Cd/Zn amount (Cd + Zn)/Se amount

5.0 0.3/0.7 3.34.2 0.4/0.6 24.8 0.5/0.5 1.6

Fig. 2. Powder XRD patterns of CdZnSe ternary QD cores with different growthtemperature, from 285 �C (A) to 310 �C (C). Bulk diffraction peaks for zinc blende(zb) and wurtzite (w) ZnSe (top) and zb and w CdSe (bottom) are indexed foridentification purpose (for bulk CdSe [(JCPDS card 19-191 (zb) and 8–459 (w)] andbulk ZnSe [JCPDS card 37-1463 (zb) and 15–105 (w)]).

H.Y. Nguyen et al. / Optical Materials 36 (2014) 1534–1541 1537

3.2. Optical properties

Fig. 3(a) plots the absorption and PL spectra of the three sam-ples A–C of CdZnSe core QDs. The absorption spectra display a clearexcitonic peak showing high quality of the QDs and their corre-sponding energies are much higher than the bandgap energy ofbulk CdSe. The absorption and emission wavelengths and emission

Fig. 3. (a) Absorption (dotted lines) and photoluminescence (full lines) spectra ofthe three CdZnSe core samples: A–C (norm. units). (b) Photoluminescence decaycurves (in ln scale) of the samples A–C (full lines) and stretched-exponential fits(dotted lines).

line widths are presented in Table 2, showing a large dependenceon the QDs synthesis protocol. The QD emission wavelength rang-ing from 530 nm for sample A to 607 nm for sample C coulddepend on both the alloy bandgap and on the QD diameter (d).However, given the similar sizes of these samples, we expect thatmost contribution to the optical transition energy comes fromthe change in the QDs compositions (Zn/Cd ratio). One can see alsothe larger Stokes shifts in samples B and C as compared to that ofsample A that may originate from the localization of charge carri-ers in the alloy, as similarly observed in In(Zn)P QDs [40].

The QD emission energy EQD has been described by ECdSe,QD =ECdSe,bulk + 1.83/d1.06 in Ref. [41] for a CdSe QD and EZnSe,QD =EZnSe,bulk + 2.08/d1.19 in Ref. [42] for a ZnSe QD. For bulk Cd1�xZnxSealloys, a quadratic relation can be used to describe the bulk alloybandgap ECdZnSe,bulk as a function of the Zn fraction x. These ele-ments suggest that the QD emission energy ECdZnSe,QD (in eV) canbe described by the following empirical relation:

ECdZnSe;QD ¼ ECdZnSe;bulk þ ð1� xÞ1:83=d1:06 þ x2:08=d1:19 ð1Þ

Kim et al. have used this relation with ECdZnSe,bulk = 1.74�(1�x) + 2.6�x�0.35x(1�x) to estimate x by using the emissionwavelength and the TEM value of d [21]. By using the samemethod, we find x of the order of 0.4, 0.1 and 0 for samples A–C,respectively. The latter value is surely false as we know from theEDS data that the QDs are CdZnSe alloys. There is thus probablya bias on the x value obtained from the empirical Eq. (1), and pos-sibly due to errors on the measured d or on the bulk bandgap value,as different values are reported in the literature (1.74–1.8 eV forCdSe [43]). However, we can make the qualitative estimation, fromEq. (1), that the amount of Zn is decreased in QDs synthesized athigher temperature, which would be in agreement with the com-position values obtained by EDS (Table 1), while addition of theCd precursor to ZnSe seeds induces more Cd as the temperatureis increased.

Fig. 3(b) shows the plots in ln scale of the PL decay curves fromthe three samples. The PL decays are non-exponential with a largecontribution of the fast decay component of sample A, which sug-gests an important contribution from non-radiative channels. Thedecay curves of samples B and C are closer to an exponential decayprocess. A reasonable fit is obtained with the stretched exponentialfunction: exp(-(t/t0)b), with, for the core samples A–C, the respec-tive characteristic times t0 = 6.3, 13 and 12.6 ns and exponentsb = 0.48, 0.64 and 0.64 (plotted as dotted lines on fig. 2(b)): similarparameters are found for the samples B and C, with shorter charac-teristic time for sample A and a lower b factor, indicating a strongerdeviation from exponential shape.

Finally, the measured LQY are reported in Table 3. The LQY islow for sample B (11%) and very low for sample A (2%), which con-firms the presence of stronger non-radiative decay channels forsample A. Both samples should benefit from addition of a cappingshell in order to improve their LQY.

As a short conclusion for CdZnSe core QDs, we have found that(i) a lower reaction temperature leads to a higher Zn concentrationindicated by a shorter emission wavelength and (ii) for the lowesttemperature (sample A) larger non-radiative decay channelsappear. The improvements of the synthesis parameters in order

Table 2Absorption peak, PL peak wavelength and PL line width in nm of the CdZnSe QDs.

Sample (nm) A B C

Absorption wavelength 526 554 588Emission wavelength 530 570 607Emission line width 34 35 43

Table 3Properties of the CdZnSe/ZnSeS QDs, obtained from TEM images and ensemble optical measurements, as a function of the CdZnSe core sample (A and B) and the Se/S shell nominalratio (1–4 corresponding respectively to a Se/S ratio of 0.2/0.8, 0.4/0.6, 0.6/0.4 and 0.8/0.2). The decay characteristics t0 (in ns) and b are obtained by fitting the decay curves with astretched exponential: exp(�(t/t0)b). The LQY (in%) was obtained by comparison with a rhodamine 6G Ref. [38].

Sample Se/S precursor ratio Diameter (nm) Absorption wavelength (nm) Emission wavelength (nm) Emission line width (nm) t0(ns)/b decay factors LQY (%)

A – 5.0 526 530 34 6.3/0.48 2A1 0.2/0.8 6.0 536 560 46 3.3/0.50 14A2 0.4/0.6 5.5 – 589 63 4.4/0.55 26A3 0.6/0.4 5.9 536 562 44 3.9/0.64 4A4 0.8/0.2 6.3 – 578 61 5.4/0.6 4B – 4.2 554 570 35 13/0.64 11B1 0.2/0.8 5.1 556 572 37 11/0.6 18B2 0.4/0.6 5.1 553 567 38 14/0.62 21B3 0.6/0.4 5.3 554 569 37 10/0.59 25B4 0.8/0.2 4.4 555 571 36 8/0.62 3

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to achieve both emission in the blue region and low non-radiativechannels are under progress.

In the following we describe the synthesis of the core/shell QDs,focusing on core samples A and B which showed the shortest emis-sion wavelengths and shelling with ZnSeS by different Se/S ratios.

4. Results for the ternary/ternary CdZnSe/ZnSeS core/shell QDs

4.1. Structural characterization

Samples A and B were covered by a ZnSeS shell with, for eachsample, different concentrations of the Se and S precursors. Theobtained core/shell samples are labeled 1–4 corresponding to theSe/S ratio of 0.2/0.8, 0.4/0.6, 0.6/0.4 and 0.8/0.2 from the precur-sors, respectively.

The diameters of QDs including core and shell are extractedfrom the TEM images and shown in Table 3. The mean QD diameteris the same for all core/shell samples of the same series (the differ-ences between samples of a same series are not significant giventhe 0.5–1 nm distribution of measured size in a given sample):about 6.0 nm for the samples series A and 5.2 nm for the samplesseries B, corresponding to a 1-nm shell for both series. Sample B4is an exception as the shell thickness measured was only 0.2 nm(the shell thickness is then lower than the measurement uncer-tainty, but EDS and XRD pattern have confirmed the presence ofthe ZnSeS shell).

We have checked the structure of all samples prepared by usingXRD method. However, in Fig. 4 we represent only the XRD pat-

Fig. 4. Powder XRD patterns of CdZnSe ternary QD cores (sample B) and CdZnSe/ZnSeS core/shell (samples B1–B4). Bulk diffraction peaks for wurtzite ZnS (top),ZnSe (middle) and CdSe (bottom) are shown.

terns of sample B (the CdZnSe core QDs) and their core/shell struc-tures named as B1–B4. For a 4.2-nm core QDs with a 1-nm shell,we estimate by geometric considerations that about half of theatoms are located in the QD core and half in the QD shell. The threepeaks at 43�, 47� and 51� are characteristic to the wurtzite struc-ture (the zinc-blende structure creates only two peaks). For thecore, as discussed previously we find a polytype zinc-blende/wurtzite structure (two peaks and a smaller one). For the core/shellsamples, the structure is clearly wurtzite, indicating that the shellgrows in wurtzite phase. The peak positions for the core/shell QDsare shifted to higher angles as compared to the core samplebecause of the ZnSeS shell contribution with smaller lattice con-stant. In this cases, some compression might happen in the coreQDs. Although the samples B1 – B4 are synthesized with very dif-ferent Se/S precursor ratios, the XRD patterns indicate to be simi-lar, suggesting that Se and S concentrations in the shell did notclearly influence to the structure of the shell.

4.2. Optical properties of QDs ensemble

The absorption and PL spectra, and the PL decay curves of all A,B core QDs and their core/shell structures are plotted in Fig. 5. Themain spectral characteristics (peak positions and width, decay fac-tors t0 and b obtained by a stretched-exponential fit) are reportedin Table 3. For the series A, the PL spectra of the core/shell QDs arebroad and differently red-shifted as compared to the core QDs,namely sample A. This is possibly due to the less stable structureof the core QDs synthesized at 285 �C. We have observed similareffect with our CdZnSe/ZnS QDs (data not shown here), with a16-nm red-shift by addition of the 0.5-nm ZnS shell and a 20-nmred-shift by addition of the 0.8-nm or 1.4-nm ZnS shell, becauseof the tunnel penetration of the charges into the shell whichdecreases the confinement energy. The charges penetration insidethe shell is expected to be larger for the ZnSeS shell than for theZnS shell as ZnSeS has a smaller bandgap that explains the largerred-shift. Indeed the redshift for samples A1–A4 (ZnSeS shell)ranges from 30 to 59 nm, larger than the 20-nm red-shift for theZnS shell. Optimizing the shell composition is necessary to com-promise between the red-shift and the lattice mismatch. For thesamples with core B, the emission band from all samples is atthe same wavelength (within 2–3 nm) and the emission line widthis always 37 nm, indicating the stability of the core and goodreproducibility of the shell. The emission wavelength is exactlythe same as for the CdZnSe core, which is in contrast with sampleseries A. The presence of a significant ZnSeS shell in samples B1–B4has been demonstrated with EDS, TEM and XRD. One may suggestthat the alloy composition is not uniform inside the core B, with aZn-richer surface so that the electron–hole pair is better confinedand the shell has less influence. Alloy gradient effects have beenreported previously for CdZnSe QDs [24]. Another possibility

Fig. 5. (a) Absorption (dotted lines) and PL (full lines) spectra of two CdZnSesamples and eight CdZnSe/ZnSeS samples (norm. units). (b) PL decay curves of thesesamples (in ln scale; full lines) and fit by a stretched exponential (dotted lines).

Fig. 6. (a) PL dynamics of four typical A1 QDs; (b) distribution of the fraction of ‘‘on’’times for sample A1; (c) distribution of the durations of each ‘‘on’’ (blue) and ‘‘off’’(green) state duration. (For interpretation of the references to color in this figurelegend, the reader is referred to the web version of this article.)

H.Y. Nguyen et al. / Optical Materials 36 (2014) 1534–1541 1539

would be that the red-shift due to electron–hole penetration intothe shell is balanced by a blue-shift caused by a diffusion of S intothe CdZnSe core. Such a blue-shift upon shell addition has beenobserved for CdZnS/ZnS QDs [32]. Further characterization wouldbe necessary to discriminate between these interpretations.

The decay curves of samples B1–B3 are similar to the decaycurve of the core sample B (with B3 slightly slower). The decay isslightly faster for B4, suggesting an increase in defect contributionsto the non-radiative decay, which may be explained by its thinnershell (0.2 nm instead of 1 nm for the other samples, as measuredby TEM). Values of the LQY are given in Table 3. For series B, thequantum yield is improved by a factor of 2 as compared to the core(with the exception of sample B4, which confirms that the qualityof the shell is poorer for this sample). Quantum yields up to 25% areobtained. On the other hand, the decay curves are not modified forB1–B3 as compared to the core, indicating that the total (radia-tive + non-radiative) decay rate is not modified, and suggestingthat both the radiative and non-radiative rates are not modified.The LQY being the ratio of the radiative to total decay rates, shouldthen not be modified either. However, it should be kept in mindthat the QDs ‘‘blink’’. The decay curves originate only from theemitting ‘‘on’’ QDs, so they show that the decay rates are not mod-ified for the emitting QDs. The quantum yield measurement, on theother hand, is an average over all (‘‘on’’ emitting and ‘‘off’’ non-emitting) QDs, as it is the ratio of emission to absorption and all(‘‘on’’ and ‘‘off’’) QDs absorb. The increase of the quantum yieldupon shell addition may thus be attributed to a reduced blinking(less ‘‘off’’ QDs), without significant modification of the radiativeand non-radiative decay rates of the ‘‘on’’ QDs.

For the samples with core A, all decays are about 1.5 times fas-ter than for the core sample. The LQY of the core/shell QDs isincreased by a factor 2.6–17 with respect to the core sample, with

a maximum value of 26%. This could be explained by an increase ofthe radiative rate; however an x1.5 increase of the total decay rateand a x17 quantum yield increase would correspond to a x25increase in radiative decay rate, which would be quite unrealistic.We will discuss this further in the next section and propose anexplanation by the decrease of the non-radiative rate.

4.3. Blinking properties from individual QDs

To further characterize the photophysical properties, the QDsfrom various CdZnSe core and CdZnSe/ZnSeS core/shell samplesof series A were spin-coated onto a glass substrate and imagedby a microscope onto a CCD camera. With a proper dilution, theQD were separated by a few microns and could be observed indi-vidually. Typical intensity fluctuations for 4 QDs of sample A1are plotted in Fig. 6, with a 100-ms time resolution. On thesecurves, the PL dynamics exhibit switching between a stable fluo-rescent ‘‘on’’ state and a non-fluorescent ‘‘off’’ state or the so-calledblinking. This blinking of the PL is well-known for CdSe/ZnS QDs[17] and has been attributed to the charge exchanges between

Fig. 7. (a) PL dynamics of a single QD (sample A1). (b) Decay curves (in ln scale)measured when the QD intensity was between 0.6 and 1.5 kcps (blue), 2.5 and4.5 kcps (green) and above 6.5 kcps (red). (c) Intensity autocorrelation functiong(2)(s), plotted as the histogram of the time delays between one photon onphotodiode 1 and one photon on photodiode 2. (For interpretation of the referencesto color in this figure legend, the reader is referred to the web version of thisarticle.)

1540 H.Y. Nguyen et al. / Optical Materials 36 (2014) 1534–1541

the QD core and trap states in its shell of environment. The clearswitching between two well-defined ‘‘on’’ and ‘‘off’’ states consti-tutes a reasonable photostability, as is usually observed for thehigh quality CdSe/ZnS QDs.

For each nanocrystal, we can define a threshold between ‘‘on’’and ‘‘off’’ states so that the duration of the successive ‘‘on’’ and‘‘off’’ states can be estimated. For a collection of about 50 QDs foreach sample, we determined the portion of the total acquisitiontime that was spent by each QD in the ‘‘on’’ state. Average portionsof time spent in the ‘‘on’’ emitting state for the three samples A, A1and A4 have been determined to be 25%, 35% and 40%, respectively,showing that the shell has reduced the QD blinking by increasingthe duration time in the ‘‘on’’ state. This increase alone cannotaccount for the x2–x10 increase in LQY obtained by ensembleluminescence measurements (Table 3), but it has been shown that,in a typical QD sample, a large portion of the QDs are ‘‘dead’’ [39]:they never emit light because their non-radiative channels arealways too high. We can thus assume that shell addition resultsin a reduction of the portion of ‘‘dead’’ QDs. If, for samples A, A1and A4 the LQY in solution is respectively 2%, 14% and 4% whilethe QDs spent resp. 25%, 35% and 40% of their time in the ‘‘on’’state, we deduce that roughly resp. 8%, 40% and 10% of QDs arenot ‘‘dead’’, showing that shell addition can increase the numberof QDs which are not ‘‘dead’’ up to 5 times.

For sample A1, we also plot in Fig. 6(c) the distribution of the‘‘on’’ and ‘‘off’’ states duration (collected over 50 QDs). These 2plots, in log–log scale, agree reasonably with a power-law distribu-tion, and they can be fit by a t�1.5 function for the ‘‘on’’ times and at�1.4 function for the ‘‘off’’ times. These power-law distributions(Levy laws) with the 1.5 coefficient are in very good agreementwith previous results on the blinking statistics of CdSe/ZnS QDs[44]. The blinking properties of the CdZnSe/ZnSeS QDs are thusqualitatively of same nature as for the CdSe/ZnS nanocrystals. Var-ious mechanisms have been proposed to explain this power-lawdistribution, involving either a distribution of trapping states withdifferent trapping/detrapping rates [45] or the random-walk fluc-tuations of the emission wavelength [46].

4.4. Single-photon emitter time-resolved properties

The PL properties of sample A1 were characterized with better(400 ps) time resolution by exciting a single nanocrystal with apulsed 400-nm laser and detecting with a photon-counting ava-lanche photodiode. Fig. 7 plots the detected intensity for a typicalQD (in photon counts/s), with a 45-ms resolution. We find an ‘‘off’’state intensity of about 1000 counts/s, which corresponds to theelectronic and optical background. The ‘‘on’’ states typically rangebetween 4 and 14 kcps, with a large distribution of intensityvalues.

Fig. 7(b) shows the different intensity levels corresponding todifferent decay rates. We select the photons detected when theemission intensity was between 0.6 and 1.5 kcps (blue), 2.5 and4.5 kcps (green), and above 6.5 kcps (red) and we plot the threecorresponding decay curves in Fig. 7(b). The decay curve is almosta single exponential (with 18 ns decay time) for the highest emis-sion intensities, and faster and non-exponential (1/e decay in1.5 ns) for the lowest intensities. This indicates that the intensityfluctuations are caused by fluctuating non-radiative decay chan-nels, as was observed previously for CdSe/ZnS [47] and CdSe/CdS[48] QDs: the faster decay and lower intensities are correlatedbecause they correspond to higher non-radiative rates.

Finally we plot in Fig. 7(c) the histogram of the values of thedelay s between one photon detected on photodiode 1 and onephoton detected on photodiode 2. This curve constitutes the inten-sity autocorrelation function g(2)(s) and characterizes the lightemission quantum properties. We used here a pulsed laser with

a 400-ns period between the pulses, so that when a photon isdetected it is more likely that another photon is detected after adelay which is a multiple of 400 ns. This appears on the g(2)(s)curve as peaks for values of s multiples of 400 ns. However, ats = 0, there is no peak (or very weak peak due to background fluo-rescence from the substrate). This corresponds to the single-pho-ton emission property (antibunching): only one photon can beemitted after one laser pulse and never are two photons detectedat the same time. This property has been demonstrated for CdSe/ZnS QDs [7,49] and recently for CdZnSe/ZnSe QDs emitting at600 nm [31].

5. Conclusion

In summary, we have successfully synthesized high-qualityCdZnSe ternary core and ternary/ternary CdZnSe/ZnSeS core/shellQDs using embryonic nuclei-induced alloying process. Dependingon the Cd/Zn ratios and the synthesis temperature, CdZnSe coreQDs could strongly luminescence in the spectral range of 530–607 nm. This corresponds to a decrease of the Zn concentrationin the core QDs as the reaction temperature increased. By shelling

H.Y. Nguyen et al. / Optical Materials 36 (2014) 1534–1541 1541

ZnSeS on the CdZnSe core QDs, the average LQY is increased by atypical factor of 2 up to 17 and up to LQY values of 25%, showingthe potential for high-quality ternary/ternary QD synthesis. Weattribute this increase in LQY to the reductions of both the average‘‘off’’ time and the percentage of ‘‘dead’’ non-emitting QDs. Byusing single-photon emitter spectroscopy, we have demonstratedfor the CdZnSe/ZnSeS core/shell QDs a complete photon antibun-ching. We confirmed the reduction of the average ‘‘off’’ time byshelling the core QDs and showed that blinking property wasimproved comparing to that of the CdZnSe core QDs. We are cur-rently optimizing the synthesis parameters in order to take advan-tage of the core and shell concentrations to tune the emissionfurther to the blue region and to reduce blinking and non-radiativechannels.

Acknowledgments

This work was funded by Vietnam National Foundation for Sci-ence and Technology Development (NAFOSTED, Project 103.06-2011.03), the PICS cooperation project between CNRS and VAST(Project Number 5724) and by the Centre de Compétences C’Nano– Ile de France (NanoCrisPho and NanoPlasmAA projects) and theAgence Nationale de la Recherche (Delight project). The authorsthank the National Key Laboratory for Electronic Materials andDevices – IMS for the use of its facilities and Pr Le Van Vu for hisadvice on XRD analysis.

Appendix A. Supplementary data

Supplementary data associated with this article can be found, inthe online version, at http://dx.doi.org/10.1016/j.optmat.2014.04.020.

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