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Colloids and Surfaces A: Physicochem. Eng. Aspects 353 (2010) 232–237

Contents lists available at ScienceDirect

Colloids and Surfaces A: Physicochemical andEngineering Aspects

journa l homepage: www.e lsev ier .com/ locate /co lsur fa

hosphine-free synthesis of high quality zinc-blende CdSe nanocrystals in air

an Wanga,b,1, Xiudong Suna,∗, Wenjing Liuc,2, Xin Yub,1

Department of Applied Physics, Harbin Institute of Technology, Harbin 150001, ChinaDepartment of Physics, Harbin Medical University, Harbin 150081, ChinaDepartment of Astronautic Science and Mechanics, Harbin Institute of Technology, Harbin 150001, China

r t i c l e i n f o

rticle history:eceived 25 August 2009eceived in revised form 2 November 2009ccepted 7 November 2009vailable online 13 November 2009

a b s t r a c t

Based on a phosphine-free synthesis route, a safe, common, low-cost, non-coordinating solvent (liquidparaffin) was used as matrix for the synthesis of CdSe nanocrystals. High quality zinc-blende (cubic) CdSenanocrystals have been synthesized by using stearic acid as a ligand under the air condition. Adding n-hexadecylamine into reaction solution, highly homogeneous and luminescent CdSe nanocrystals in theless common zinc-blende crystal structure have been obtained. The reproducibility of the synthesis of

eywords:dSehosphine-freetearic acidexadecylamine

CdSe NCs by this route was discussed. Without any size sorting, the size distribution of the nanocrystalscan be readily controlled and the higher photoluminescence quantum efficiency of the nanocrystals wasup to 20–30%.

© 2009 Elsevier B.V. All rights reserved.

n airinc-blende crystal structure

. Introduction

Colloidal semiconductor nanocrystals (NCs, quantum dots) havettracted a great deal of interest in a variety of applications suchs light-emitting diodes [1], lasers [2], and biological label [3,4]ue to their size-tunable photoemission characteristics. CdSe is onef the most popular NCs showing almost full range visible lightmission. Many works on the synthesis of semiconductor NCs haveeen reported, including organometallic precursor route developedy Murray et al. [5]. However, some key chemicals used in thisraditional route are extremely toxic, expensive, explosive, andyrophoric, resulting in the poor reproducibility of NCs prepara-ion. This motivates us to develop alternative synthetic methodsor this well-studied model system. Recently, the synthesis of CdSen the coordinating solvent tri-n-octylphosphine oxide (TOPO) haseen extended to the less hazardous, more inert solvent, such as-octadecene [6–9]. Particularly, a safe, common, low-cost, non-

oordinating solvent (liquid paraffin) has been used as matrix forhe synthesis of CdSe NCs [10–16]. Dushkin’s group [11,12,14,15]eported the synthesis of high quality CdSe NCs in liquid paraf-n using stearic acid (SA) and trioctylphosphine (TOP) as solvents

∗ Corresponding author. Tel.: +86 451 86414129.E-mail addresses: lwang322@yahoo.com.cn (L. Wang), xdsun@hit.edu.cn

X. Sun), lwjhit@hit.edu.cn (W. Liu), hrl12319@163.com (X. Yu).1 Tel.: +86 451 86674918.2 Tel.: +86 451 86269699.

927-7757/$ – see front matter © 2009 Elsevier B.V. All rights reserved.oi:10.1016/j.colsurfa.2009.11.018

under N2. Yordanov et al. [16] proved various organic solvents (liq-uid paraffin, stearic acid, hexadecylamine) react with selenium byliberating H2Se which reacts further with the cadmium metal saltto form the corresponding nanosized CdSe.

Here, we developed Deng’s [10] routes by using stearic acidinstead of oleic acid as ligand and presented another much cheaperand greener non-TOP-based route for the synthesis of high qualitysize-controlled CdSe dots in the zinc-blende (ZB) lattice structure.Different from Yordanov’s report [16], our synthesis of CdSe NCswas performed by injection of Cd-stearic acid (Cd-SA) solution intoSe solution at the temperature above Se melting point rather thanthe injection of Se solution into Cd-SA solution. This is because Secannot be dissolved directly in liquid paraffin at room tempera-ture without TOP or tributyl phosphine (TBP). In our paper, weperformed the experiments in air and compared these data withthose obtained under N2. Without any size sorting, the quality ofCdSe dots formed by this scheme is comparable to the CdSe NCs pre-pared by TOP-based synthesis route. The obtained NCs are all in ZBphase, while in the TOP-based route the synthesized CdSe NCs are inthe common wurtzite (W, hexagonal) phase. The scheme requiresvery simple equipments and condition. So it is reproducible andcan be readily scaled up for industrial production.

Taking into account the growing demand on high luminescent

semiconductor NCs, we improved our synthesis by introducingan additional coordinating component n-hexadecylamine (HDA)to the reaction solution. Although HDA is long known to con-siderably improve quality of colloidal CdSe [17–19], the effect ofhexadecylamine in paraffin was not studied plainly in the previous

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apers. Focusing of the size distribution was observed during parti-le growth. The surface of as-prepared CdSe NCs can be passivatedithout isolating the NCs from the crude solution either by growing

n inorganic shell or by surface modification with alkylamines andhe highest photoluminescence quantum efficiency of the CdSe NCsas up to 20–30% at room temperature. In addition, we find that

he existing of HDA accelerates the nucleation processes, slowesown the growth rates, and therefore smaller CdSe NCs can bebtained. The high quality of these CdSe NCs has been confirmedy absorption and photoluminescence (PL) spectroscopy, powder-ray diffraction (P-XRD) and transmission electron microscopy

TEM).

. Experimental

.1. Chemicals

CdO (99.5%), Se powder (99.9%), stearic acid (analytical grade),araffin liquid (chemical grade), anhydrous ethanol (analyticalrade), chloroform (analytical grade), Rhodamine B (with a pho-oluminescence quantum yield of 97% in anhydrous ethanol) werebtained from Sinopharm Chemical Reagent Co., Ltd, China. Hex-decylamine (90%) was obtained from Aldrich. All chemicals andolvents were used without further purification.

.2. Synthesis

A mixture of 10 mmol of CdO, 40 mmol of SA, and 10 mL ofaraffin liquid was loaded into a three-neck flask and heated. Atbout 160 ◦C, the reddish CdO powder was dissolved gradually.ept heating at the temperature until the solution became clear andenerated a light yellowish solution of cadmium stearate (solution). In another three-neck flask, 1 mmol of Se powder in 50 mL ofaraffin liquid was carefully heated with rapid stirring (solution B).t about 200 ◦C, the solution became clear and then turned wineed. Then about 5 mL of solution A (containing about 2 mmol ofd precursor) was swiftly injected into solution B at 240 ◦C. Theemperature dropped to 230 ◦C immediately and remained at thisalue during the NCs growth. The whole process was carried outnder air or N2. For comparison, CdSe NCs were prepared by the

ynthesis adding 20 mmol of HDA to the solution A. To monitorhe reaction during the growth process, aliquots of the mixtureere removed from the reaction flask at regular time intervals. To

solate the CdSe NCs from the organic subproducts and unreactedaterials, the hot reaction mixture should be fast cooled down in

ig. 1. Temporal evolution of the UV–vis absorption spectra and the PL spectra of the Cdnder N2 with HDA, and (d) under air with HDA.

chem. Eng. Aspects 353 (2010) 232–237 233

cold chloroform. The supernatant was separated from the precipi-tation by centrifugation. Hot anhydrous ethanol was added to thesupernatant and then centrifuged. The CdSe NCs were precipitatedout and dissolved in chloroform for characterization (samples). Thesamples were diluted with chloroform directly for characterizationwithout any size sorting and UV–vis and PL spectra were recorded.Finally the rest reaction mixture was cooled down to room tem-perature by adding double volume anhydrous ethanol. Removingthe solidified supernatant followed by centrifugation resulted in areddish precipitate. The precipitate was further washed with hotanhydrous ethanol several times, and dried in the air for XRD char-acterization.

2.3. Characterization

Ultraviolet and visible absorption (UV–vis) spectra wererecorded with a UV-2550 (SHIMADZU, Japan) spectrophotome-ter at room temperature. Photoluminescence (PL) spectra weremeasured with a RF-5301PC (SHIMADZU, Japan) fluorescence spec-trometer. The diameters of CdSe NCs were calculated from theexcitonic absorption peak using a simple effective mass approxima-tion (EMA) model proposed by Brus [20]. The photoluminescencequantum yield (PL QY) was obtained by comparison with a stan-dard (Rhodamine B in ethanol) and using data derived from theluminescence and the absorption spectra [21].

X-ray powder diffraction (XRD) spectra were collected on aJapan SHIMADZU X-ray diffractometer using Cu target with a scan-ning rate of 8.0000◦/min. The accelerating voltage was set at 40 kV.Scatter and divergence slits of 1.0◦ and a 0.3-mm collection slit wereused. Purified powder CdSe NCs were crushed into fine powder forthe XRD analysis.

A Tecnai F20(FEI) microscope operating at 160 kV was usedfor transmission electron microscopy (TEM). High-resolution TEM(HRTEM) was performed at 200 kV. Samples for TEM and HRTEMwere prepared by putting a drop of n-hexane diluted solution ofCdSe NCs onto an amorphous carbon substrate supported on acopper grid and then allowing the solvent to evaporate at roomtemperature.

3. Results and discussion

Fig. 1 compares the UV–vis absorption spectra and the PL spectrafor a series of samples prepared at the growth temperature of 230 ◦Cunder N2 without HDA (a), under air without HDA (b), under N2with HDA (c), and under air with HDA (d).

Se NCs growth at 230 ◦C: (a) under N2 without HDA, (b) under air without HDA, (c)

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34 L. Wang et al. / Colloids and Surfaces A: P

In Fig. 1a, the small CdSe NCs (2.5 nm in diameter) with anbsorption peak at 506 nm were formed in 1 s and next devel-ped to bigger NCs (3.2 nm in diameter) with an absorption peak at61 nm in 10 s, then the CdSe NCs grew hardly and the absorbanceeak reached 569 nm (3.4 nm in diameter) by the end of 30 min,

ndicating the nucleation and growth process were less than 10 s.hen the reaction was performed in open air, the CdSe NCs with

n absorption peak at 499 nm (2.4 nm) were obtained until 30 sFig. 1b), and developed to bigger NCs (3.0 nm in diameter) with anbsorption peak at 546 nm in 60 s, then the CdSe NCs grew graduallynd at last the bigger NCs with 4.7 nm in diameter (the absorbanceeak of the NCs reached 616 nm) were obtained in 60 min, ando obvious “defocusing” of the size distribution was observed. Inontrast, very fast “focusing” of the size distribution was observeduring the particle growth in the presence of HDA and no “defo-using” appeared after 30 min at the growth temperature of 230 ◦CFig. 1c and d). In Fig. 1c, the smaller CdSe NCs with an absorptioneak at 493 nm (2.3 nm in diameter) were formed in 3 s and thenhe NCs grew gradually to form NCs with an absorption peak at45 nm (3.0 nm in diameter) in 30 min. In Fig. 1d, when the processas performed under air in the presence of HDA, the smaller CdSeCs with an absorption peak at 472 nm (2.1 nm in diameter) were

ormed in 3 s, and developed to bigger NCs (2.5 nm in diameter)ith an absorption peak at 511 nm in 10 s, then the CdSe NCs grew

lowly to form NCs with an absorption peak at 529 nm (2.7 nm iniameter) by the end of 30 min, and no “defocusing” of the sizeistribution was observed.

The absorption peaks of Fig. 1a and b (in the absence of HDA)re broad and the absorption peaks of Fig. 1c and d (in the pres-nce of HDA) are very narrow, especially the absorption spectrumf Fig. 1c possesses several resolved electronic transitions, indi-ating the advancement of crystallization. Accordingly, the fullidths at half-maximum (FWHM) of the band edge PL spectra

re in the range of 26–32 nm (Fig. 1c), 28–43 nm (Fig. 1d) respec-ively. According to the relation between the FWHM and the realanocrystal size, reported by Dushkin et al. [22], the small FWHM ofhe CdSe NCs reveals that the sizes of the CdSe NCs could be close to

onodisperse (narrow size distributions). This means HDA plays anmportant role to form the CdSe NCs with very narrow size distribu-ions. Adding HDA into reaction solution reduces the diffusion ratesf the monomer and therefore limits nanocrystal growth so that

he narrower size distributions can be achieved [23,24]. Comparedith the CdSe NCs obtained under N2 (Fig. 1a and c), the absorp-

ion peaks of the CdSe NCs formed in air (Fig. 1b and d) broaden aittle. No obvious “defocusing” of the size distribution is observedfter long-term heating in all of Fig. 1. Continuous “focusing” of

ig. 2. Diameter (a) and full-width-half-maximum (b) as functions of time for the CdSe Nnd the corresponding plots (solid plots) obtained at almost the same conditions as Fig. 1

chem. Eng. Aspects 353 (2010) 232–237

the size distribution occurs during the entire growth process, mak-ing postpreparative size selection unnecessary. This indicates thatusing SA as ligand in liquid paraffin for the synthesis of CdSe NCs(NCs) is more perfect than oleic acid and all UV–vis and PL spectraof the as-prepared CdSe NCs are comparable with the CdSe sam-ples obtained by the TOPO–TOP synthesis [5,6,20,25,26]. We alsofind the sharper absorption peaks can be obtained by adding HDAto the Cd precursor but not Se.

In order to study the reproducibility of the synthesis of CdSeNCs, another series experiments were carried out at almost thesame conditions as Fig. 1a, c and d. The corresponding data of theparticle diameter and full-width-half-maximum are presented inFig. 2. Because the data, represented in Fig. 1b, are not sufficient in300 s, we did not perform the experiment at the condition as Fig. 1b.Fig. 2 shows the plots of the particle diameter (a) and full-width-half-maximum (b) as functions of time for the CdSe NCs preparedas in Fig. 1a (open circles), Fig. 1c (open triangles), Fig. 1d (openstars) and the corresponding plots (solid plots) obtained at almostsame the conditions as Fig. 1. The data of the particles diametersin Fig. 2a are also well fitted by the expression derived by Dushkinet al. [22]. Fig. 2a shows there are two distinct kinetic regimes in30 min: during the first 30 s, the average size increases relativelyrapidly, and subsequently, the nanocrystals grow more slowly. Thenucleation process with pure stearate system occurs in a slowerrate (open circles, circles) and then the growth of NCs goes on,thereby bigger NCs are formed in comparison to those in the case ofstearate–HDA system (open triangles, triangles, open stars, stars).Similar phenomena have been described in Talapin’s report [19].We believe HDA can cause a faster nucleation rate for the forma-tion of the smaller nucleus and the precursors capped with HDA areless reactive than the ones without HDA so that the growth rates ofthe particles are slower. Nanocrystal particles grow inside a shellof coordinating ligand molecules (HDA), which hinders the attach-ing of monomers, reducing the growth rates of the particles. AlsoHDA influences the “focusing” of the size distribution. In Fig. 2b,the nanocrystal size distribution is “focused” (FWHM is small) in3 s in the absence of HDA (open circles, circles), and then the sizedistribution broadens (FWHM becomes greater). Peng et al. [24]pointed out at any given monomer concentration, there exists acritical size. At first, the NCs in solution are all slightly larger thanthe critical size, the smaller NCs grow faster than the larger ones,

and “focusing” of the size distribution occurs. When the monomerconcentration is depleted due to growth, the critical size becomeslarger than the average size present. Some smaller NCs are shrink-ing, while larger ones are still growing and “defocusing” occurs.Fig. 2b shows that the nanocrystal size distribution is “focused”

Cs prepared as in Fig. 1a (open circles), Fig. 1c (open triangles), Fig. 1d (open stars).

L. Wang et al. / Colloids and Surfaces A: Physico

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Fig. 4 shows the XRD patterns obtained for dots prepared underN2 without HDA (A, Fig. 1a), under air without HDA (B, Fig. 1b),under N2 with HDA (C, Fig. 1c), under air with HDA (D, Fig. 1d) at230 ◦C, and under air with HDA at 260 ◦C (E). As is often observed

ig. 3. Room-temperature absorption and emission spectra of CdSe NCs monitoreduring the growth at 230 ◦C (a) and 260 ◦C (b) under air in the presence of HDA.

lmost right after injection in the presence of HDA (open triangles,riangles, open stars, stars), and then the size distribution broad-ns (FWHM becomes greater), too. It may be because the initialucleation is homogeneous due to the fine dispersion of monomerapped by HDA. Fig. 2b also shows that the size distribution of theCs prepared in air (open stars, stars) is broader than that under N2

open triangles, triangles). This probably because the NCs in air areot as stable as the ones under N2 and therefore tend to aggregate,hich broaden the size distribution. Although the data of reproduc-

ive experiments (solid plots) are not very close to the origin oneshollow plots), probably due to slight variances of the experimentalperations, the trends of the plots are similar.

In our study, the growth kinetics of NCs also depends on tem-erature. Fig. 3 presents the room-temperature absorption andmission spectra of CdSe NCs monitored during the growth at30 ◦C (a) and 260 ◦C (b) under air in the presence of HDA. It isotable that it is safe to work directly at the temperature below00 ◦C as the boiling points of all the reagents used in our syn-hesis are all above 300 ◦C and stable in air. At lower temperatureFig. 3a), the nucleation rates are slower (the smaller CdSe NCsere formed until 3 s) and the growth rates are faster (absorptioneaks from 472 nm to 535 nm). In contrast, the particles are nucle-ted immediately after injection and the growth rates are slowabsorption peaks from 469 nm to 499 nm) at higher growth tem-erature (Fig. 3b). This means the initial nuclei sizes are smallernd the final particle sizes are also smaller at higher temperatures.ence, smaller NCs can be obtained at higher temperatures. It is

n agreement with Bullen’s report [6], indicating nucleation is sub-tantially faster as the temperature increasing. The sizes ranges ofdSe NCs at higher temperatures are narrower than those at loweremperatures. It means the growth rates of the particles reduce ashe temperature increasing. This is not in agreement with Bullen’seport [6]. In all cases of Fig. 3, sufficiently narrow particle size dis-ributions are maintained during the growth of the nanocrystalsntil 900 s. After that, the spectra start to change as in an Ostwaldipening, with a marked broadening. The broadening eventuallyevelops into distinct peaks (Fig. 3b). The appearing of “defocus-

ng” reveals the development of new well-defined species, ratherhan a general broadening due to classical crystal coarsening. This

ay be explained by the following scheme: at the end of the reac-ive growth regime, particle etching triggered by Oxygen (existingn air) and water (formed by reactant decomposition) [27] takeslace. This process leads to the fusion of the faceted crystals and aubsequent aggregation.

chem. Eng. Aspects 353 (2010) 232–237 235

The PL QY was obtained by using a conventional route, inte-grating the PL band of CdSe in chloroform and comparing theintensity to that of Rhodamine B in anhydrous ethanol. The photo-luminescence quantum efficiency of as-prepared CdSe NCs at roomtemperature shows no distinct difference between the CdSe NCsprepared in air and the one formed under N2. But obvious differ-ence was observed when HDA was added into the reaction solution.The room-temperature PL QY of as-prepared CdSe NCs in the pres-ence of HDA reaches 20–30% and has a tendency to decrease asthe sizes of the particles increasing, while it is in the range of5–10% in the absence of HDA. This enhancement of the band edgePL may be attributed to two reasons. The one is the bonding ofamines containing long alkyl chains to the nanocrystal surface pro-vides much better passivation of surface traps participating in thenon-radioactive recombination processes [21,28]. Another reasonprobably is liquid paraffin and hexadecylamine react with seleniumgenerating appreciable amount of H2Se [16] and therefore formingquite more corresponding CdSe, which enhances the photolumi-nescence quantum efficiency.

In general, lower growth temperature (230 ◦C) and in theabsence of HDA result in a broader sizes ranges (2.4–4.7 nm). In thepresence of HDA, the smaller nearly monodisperse CdSe NCs canbe formed and the synthesis of CdSe NCs is very reproducible. Onthe contrary, at a rather high growth temperature (260 ◦C) also withHDA, “defocusing” can appear and growth processes are somewhatdifficult to control.

It is believed that the capping ligand SA and HDA not only deter-mine the rates of nucleation and growth but also play major rolesin determining the sizes of the nucleus. Significantly, the repro-ducibility of luminescence quantum yield is not as better as theone of the nanocrystal sizes. In practice, the pure SA (without HDA)system is not recommended to synthesize small NCs because of thefast growth rates of the NCs (as shown in Fig. 1a and b), but the timedelay of initial nucleation of this system makes the initial injectiontake as long as tens of seconds and the larger CdSe NCs are obtained.

Fig. 4. XRD pattern of CdSe NCs prepared (A) under N2 without HDA (Fig. 1a), (B)under air without HDA (Fig. 1b), (C) under N2 with HDA (Fig. 1c), (D) under airwith HDA (Fig. 1d) at 230 ◦C, and (E) under air with HDA at 260 ◦C. The lower barsrepresent the position of the diffraction peaks for the bulk ZB lattice structure, andtheir lengths are proportional to their intensity.

236 L. Wang et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 353 (2010) 232–237

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or samples with narrow size distribution, the P-XRD patterns alsohow a narrow peak in the small-angle region. All particles arerystalline, and the positions of diffraction peaks at wide anglesatch those of the bulk phases of cubic CdSe (zinc-blende phase)odifications. The relative lower growth temperature may play

n important role in the form of the ZB phase [29]. In the presenttudy, the ZB phase is preferred for growth at the moderate tem-erature (220–260 ◦C), while the W phase is preferred for growth atigher temperatures (about 300 ◦C) [18,29,30]. In thermodynamics,B phase is the most stable form at lower temperature, while W isore stable at high temperature. SA acting capping agent maybe

s another reason for the form of the ZB phase. In some reports, atower temperatures (240–260 ◦C) and in a solvent containing HDA,ynthesis always produced CdSe NCs with the W structure [18,31].oreover, Mohamed et al thought the crucial parameter in con-

rolling the lattice structure was not the temperature but ratherhe type of ligand [32]. The role of the SA in our synthesis routes ofhe CdSe NCs is not only to dissolve CdO powder and form homoge-eous Cd solution but also help to form ZB structure. As expected,he width of the diffraction peaks is considerably broadened withhe particle size decreasing. By using the Scherrer formula, the

ean sizes of the NCs can be estimated from the peak width atalf-maximum. The mean crystals sizes (in Fig. 4 (from the bottomo the top)) are 3.3 nm, 4.8 nm, 3.0 nm, 2.9 nm, and 2.6 nm respec-ively. The calculated values (A, C and D) are in agreement with theize fitting from excitonic absorption peaks (shown in Fig. 2).

Fig. 5 shows the TEM (left) and HRTEM (right) pictures of thedSe NCs prepared in air in the presence of HDA (samples 30 s

rom Fig. 1d). Fig. 5 (left) shows the nanocrystals are dotlike with aigh uniformity in size. Also, they assemble in regular superlatticesFig. 5, right). The TEM and the HRTEM confirmed the size distri-ution of the CdSe NCs is nearly monodisperse with the averageize of 2.6 nm, which is consisted with the size calculated by thebsorption peak of the NCs at 514 nm.

. Conclusions

In summary, our results show that SA-paraffin liquid synthesisoute can be carried out open to air and yields of ZB structure. Fairlyarrow size distributions of the CdSe NCs are obtained in the pres-nce of effective capping agents HDA. The use of HDA generally can

roduce CdSe NCs with higher photoluminescence quantum effi-iency and smaller sizes. We believe that it is due to the fact that theA and paraffin liquid are very stable in the atmosphere, while TOPnd TBP are unstable in the atmosphere (thus the process must beerformed under nitrogen or in the glovebox). Choosing paraffin

[

in diameter) prepared under air in the presence of HDA.

liquid, SA and HDA as the reaction medium significantly simpli-fies the reaction for green and low-cost synthesis of the CdSe NCs.These features may significantly increase the possibility of usingCdSe nanocrystals for display and tagging applications. Moreoverthis scheme is reproducible and simple and thus can be readilyscaled up for industrial production.

Acknowledgment

The current investigations were financially supported by theTechnology Research Fund of Heilongjiang Provincial EducationDepartment (No. 11531169), China.

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