Optical Materials 36 (2014) 813–819

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Optical Materials

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Effect of amine addition on the synthesis of CdSe nanocrystals in liquidparaffin via one-pot method

0925-3467/$ - see front matter � 2013 Elsevier B.V. All rights reserved.http://dx.doi.org/10.1016/j.optmat.2013.12.006

⇑ Corresponding author. Address: Institute of Materials Science and Engineering,Ocean University of China, Songling Road 238, Qingdao 266100, Shandong Province,PR China. Tel.: +86 532 66781690; fax: +86 532 66781320.

E-mail address: jttian@ouc.edu.cn (J. Tian).

Jinqian Jia, Jintao Tian ⇑, Weiguo Tian, Wen Mi, Xiaoyun Liu, Jinhui Dai, Xin WangInstitute of Materials Science and Engineering, Ocean University of China, Songling Road 238, Qingdao 266100, PR China

a r t i c l e i n f o

Article history:Received 19 August 2013Received in revised form 9 November 2013Accepted 3 December 2013Available online 28 December 2013

Keywords:CdSe nanoparticlesAmineLiquid paraffinOne-pot method

a b s t r a c t

The effect of n-octylamine (OA) and octadecylamine (ODA) addition on the synthesis of CdSe nanocrystalsin liquid paraffin via one-pot method is investigated via the measurements of their ultraviolet–visibleabsorption and fluorescence emission spectra. Our results showed that the in situ added amines can acti-vate the formation reaction of Cd precursor and, as a result, substantially decrease the initial reactiontemperature and accelerate the particle growth. By adding OA at high temperature of 200 �C, remarkableimprovement on particle quality is achieved, giving relatively narrow size distribution of 33.1 nm andhigh photoluminescence quantum yield (PLQY) of 81.9% for the CdSe nanoparticles. OA addition at lowtemperature shows also good quality improvement for the nanoparticles. With regard to the primaryamine of ODA, it may be inappropriate for quality improvement of the CdSe nanoparticles from liquidparaffin via one-pot method.

� 2013 Elsevier B.V. All rights reserved.

1. Introduction

In the past few decades, there are lots of researches about thesynthesis of cadmium selenide (CdSe) nanocrystals [1–4].Whereas, how to synthesize high quality CdSe nanocrystals withnarrow size distribution and high luminescence efficiency are stillthe key topic in the field of material chemistry. In general, the par-ticle size and size distribution of CdSe nanoparticles are kinetic-dependent and can be managed by controlling the reactiontemperature and time. The luminescence efficiency, however, issurface covering-sensitive. Due to the large surface-to-volume ratioof nanocrystals, the most common reason for poor luminescenceefficiency is nonradiative recombination of light-generated chargeat surface traps [5,6]. Elimination of these traps can be achieved bygrowing an inorganic passivation shell of CdS, ZnSe and/or ZnS onthe bare CdSe core [7–9]. On the other hand, proper chemical mod-ification of the nanocrystal surface by using effective capping agentis another simple and efficient way for removing these traps [10].Amine, especially primary amine, can provide much better passiv-ation and therefore significantly enhance the photoluminescencequantum yield (PLQY) [11,12]. In addition, narrow size distributionof nanocrystals can also be achieved from amine addition [13].

Up to now, there have already large amounts of researchesrelated to amine addition in the literature [14–16]. However, the

involved methods for nanocrystal synthesis in these works aremainly injection-based. By contrast, one-pot synthesis is a simpleand efficient noninjection approach featuring easy handling withhigh synthetic reproducibility and large-scale production capabil-ity. Compared to those toxic, unstable, and expensive solvents suchas trioctylphosphine oxide and 1-octadecene previously used[1,17], liquid paraffin is less toxic, more stable at high temperature,rather cheap, and hence as a solvent greatly reduces raw materialcost and simplifies experimental process to some extent [18–20].In our previous study, CdSe nanocrystals have been successfullysynthesized in liquid paraffin via one-pot method [21]. Cadmiumstearate was chosen not only to form homogeneous cadmiumsource solution but also the involved long chain stearic acid toact as a sole capping ligand. From the viewpoint of both strongfluorescence emission and saving in raw materials, a reaction tem-perature of 200 �C and a molar ratio of Cd to Se of 4/1 were pro-posed. In this study, we would like to forward this research bydirectly adding another coordinating ligand of amine into the reac-tion solution to improve the quality of the CdSe nanoparticles. Ourpurpose is to clarify the effects of amine addition on the growthkinetics of the CdSe nanoparticles and the quality improvementsof both size distribution and PLQY.

2. Experimental procedure

Cadmium stearate (Cd(SA)2, 99.5%), selenium powder (Se,99.99%), n-octylamine (CH3(CH2)7NH2, OA), octadecylamine(CH3(CH2)17NH2, ODA), and rhodamine 6G are purchased fromAldrich Chemical Inc. Chemical grade liquid paraffin (boiling point

814 J. Jia et al. / Optical Materials 36 (2014) 813–819

higher than 300 �C) and other analytical grade chemicals of tolu-ene, acetone, trichloromethane, n-hexane, and methanol are fromBeijing Chemical Reagent Ltd., China. All the above raw materialsare used as received without any further purification. Details forthe synthesis of the CdSe nanoparticles in liquid paraffin via one-pot method can be found in our previous study [21], so it is onlybriefly described here. For a typical synthesis where ODA isin situ added, 0.2 mmol Cd(SA)2, 0.05 mmol Se, 0.2 (or 1.7) mmolODA, and 25 mL liquid paraffin are added into a 100 mL flask anddegassed with nitrogen for 30 min. The mixture is then heated to100 �C and maintained with continuous stirring for 30 min innitrogen to get a transparent solution. The solution is then heatedto 200 �C with a temperature increase rate of 0.46 �C/s and kept fora period of time. After that, 0.8 mL colloid solution is taken fromthe flask with a syringe and immediately transferred into 4 mLtoluene to prevent the nanocrystals from further growth. For thesynthesis where ODA is added at high temperature, the CdSe nano-particles contained solution is firstly prepared at 200 �C within 1 hfrom the above process. Then, a transparent solution containing1.7 mmol ODA and 6 mL liquid paraffin achieved at 80 �C is swiftlyinjected into the reaction flask and the reaction temperature ismaintained at 160 �C or 200 �C for a period of time up to 2 h. Forthe CdSe nanocrystals with OA addition, the synthetic process issimilar. Besides those added in situ and at high temperature of200 �C, amine addition at low temperature is also tried to get CdSenanoparticles. The nanoparticles are firstly synthesized at 200 �Cfor 1 h with a solution composition of 0.2 mmol Cd(SA)2,0.05 mmol Se, and 25 mL liquid paraffin in a 100 mL flask. Thesolution is then quickly cooled down to room temperature by add-ing 5 mL liquid paraffin. A reaction mixture of 8 mL CdSe solution,7 mL liquid paraffin, and 0.453 mmol OA (or ODA) is heated to a gi-ven temperature and the timer starts for annealing. In this case theconcentration of OA (or ODA) in the mixture is 30 mM and themolar ratio of OA (or ODA) to Cd is 8.5/1. The ultraviolet–visible(UV–Vis) absorption spectrum for the colloid sample is recordedwith a U-3010 Hitachi spectrophotometer. The fluorescencespectrum is obtained using a Fluorolog 3-P spectrofluorometer(Jobin Yvon). The slit width is 2 nm. The photoluminescence quan-tum yield (PLQY) of the nanocrystals is measured by comparing theintegrated photoluminescence (PL) intensity of the nanocrystalswith that of rhodamine 6G (PLQY = 95%) [22,23]. The X-ray diffrac-tion (XRD) patterns of the nanocrystals are obtained on a GermanBruker D8 Advance X-ray diffractometer. The Fourier transforminfrared (FT-IR) spectra are from a Nicolet iN10 IR microscope(Thermo Fisher). The transmission electron microscopy (TEM)images are obtained from a JEM-2100 electron microscope (JEOL).Details for the preparation of the CdSe powder sample for XRD andFT-IR measurements and the colloid sample for TEM observationcan be found in our previous study [21].

3. Results and discussion

3.1. In situ amine addition

The primary amine of ODA or OA is firstly in situ added and theresultant UV–Vis spectra for the colloid CdSe samples are shown inFig. 1a–e. All the colloid samples for Fig. 1 are prepared under thesame experimental conditions except for the added amine and itsamount. The reaction solution is heated from 100 �C to 200 �C witha temperature increase rate of 0.46 �C/s and then the timer starts.All the spectra are recorded without any post size-selective precip-itation. Fig. 1a shows that an incubation time of 1 min is neededwhen amine is absent, suggesting an initial reaction temperaturehigher than 200 �C. When ODA or OA is incorporated, the first exci-ton absorption peaks have been evidently observed from Fig. 1b–e

without any incubation time. This demonstrates that with aminethe CdSe nanoparticles can be obtained below 200 �C. Thus, theadded amine can decrease the initial reaction temperature. Inorder to confirm this result, UV–Vis measurements are performedon the colloid samples obtained at temperature below 200 �C. Byapplying Peng’s equation [24], the size of the CdSe nanoparticlesobtained both below and at 200 �C is calculated and shown inFig. 2a and b. Fig. 2a clearly shows that by applying ODA or OAin large amount the CdSe nanoparticles can be obtained at temper-ature as low as 180 �C. It should be pointed out that in these casesthe temperature increase is quite quick. When it is raised slowly,the nanoparticles can be obtained even at lower temperature.

Besides the initial reaction temperature, the added amine canalso greatly affect the growth kinetics of the CdSe nanoparticles,as shown in Fig. 2b. The particle growth is strongly acceleratedand the final size varies a lot with amine addition. In particular,amine addition in large amount leads to fast particle growth andsmall final size. In order to well understand the above growth fea-ture, it is helpful to numerically fit the experimental points inFig. 2b by applying the proposed model in our previous study[21]. Fig. 2c shows significant decrease of the growth rate againstreaction time for all the amine added samples. In particular, thesamples with large amount of amine have small growth rate inFig. 2c and large initial diameter in Fig. 2b. Here the initial diame-ter means the diameter at the moment the reaction solutionreaches the intended temperature of 200 �C and the timer beginsto start. One might have doubt that these results contradict withthe above description where amine addition in large amount favorsfast particle growth and small final size. In fact, by taking intoaccount those happened in the solution before temperature reach-ing 200 �C, these results do not contradict but coincide well witheach other. As demonstrated previously, the added amine can def-initely decrease the initial reaction temperature. This means thatby adding amine the reaction for the CdSe nanoparticles formationhas been occurred for a period time before starting the timer (themoment the temperature reaching 200 �C) and the length of thisreaction time varies with the amine amount. Large amount amineaddition reduces the initial reaction temperature a lot (Fig. 2a). Insuch a case the nanoparticles are formed at lower temperaturewith longer reaction time. As a result, the nanoparticles grow uplargely and the growth rate decreases a lot, leading to small growthrate and large initial diameter at the moment of the timer starting.

The chain length of amine is supposed to mainly affect diffusionof the involved molecules in the solution. The chain length of OA isshorter than that of ODA. Thus, OA has small steric hindrance andeasy diffusion in the solution and therefore activate the monomerformation reaction more. However, such positive effect, asexpected, might be rather less than the amount of the addedamine, especially within a short reaction time of 10 min as shownin Fig. 2b and 2c. The reason is that with a short reaction time theformed CdSe monomers are quite enough for nucleation andgrowth and long distance diffusion is not crucial. In this case diffu-sion resistance caused by chain length does not play an importantrole for the CdSe nanoparticles formation. As the nanoparticlescontinuously grow up, the monomers nearby are seriouslyexhausted and long distance diffusion is quite crucial. The additionof amine with long alkyl chain goes against particle growth due tothe large diffusion resistance and therefore low growth rate andsmall final particle size.

The fluorescence performance of the CdSe nanocrystals synthe-sized with in situ amine addition is examined using a spectrofluo-rometer and the results are shown in Fig. 3a and b. The CdSenanoparticles formed without amine addition shows a rather nar-row size distribution. The derived full width at half maximum(FWHM) of the PL peak at t = 10 min is as narrow as 26.7 nm. Smallamount of ODA or OA broadens the size distribution of the

Abs

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Fig. 1. UV–Vis spectra of the CdSe nanocrystals synthesized in liquid paraffin via one-pot method at 200 �C with in situ amine addition.

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Fig. 2. Calculated particle size (a and b) of the CdSe nanocrystals via Peng’s equation [24]. The derived growth rate of the CdSe nanocrystals is also shown in this figure (c).

J. Jia et al. / Optical Materials 36 (2014) 813–819 815

nanoparticles a lot, leading to an FWHM increase as large as 31% att = 10 min. Size focusing is observed at the initial growth stage of0–10 min. No visible defocusing phenomenon occurs in this case.

With large amount of amine addition, size focusing and defocusingare rather noticeable, though the FWHM increase at the turningpoint of t = 10 min is only 50% in comparison with that without

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60 No ODA (or OA) ODA= 8 mM ODA= 68 mM OA = 8 mM OA = 68 mM

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Y (

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(b)

Fig. 3. FWHM of the PL peak (a) and measured PLQY (b) against reaction time for the CdSe nanocrystals.

Table 1Summary of experimental data of particle size, FWHM of the PL peak, and PLQY obtained in this study. The bold values in this table denote nanocrystals with ralatively narrowFWHM and high PLQY.

Adding way Type Amount, mM Temperature, �C Time Size, nm FWHM, nm PLQY, %

No – – 200 1 h 2.75 27.6 28

In situ ODA 8 200 2 h 3.02 36.3 18.4ODA 68 200 2 h 2.63 44.1 41.6OA 8 200 2 h 3.20 36.3 24.8OA 68 200 5 min 2.73 40.5 32.7

High temperature OA 68 160 9.5 min 2.80 33.1 63.1OA 68 200 9 min 2.54 33.1 81.9ODA 68 160 14 min 2.52 32.0 14.9ODA 68 200 9.5 min 2.57 33.1 21.9

Low temperature OA 30 120 20 min 2.64 33.1 74.0OA 30 160 10 min 2.77 30.9 69.0OA 30 200 10 mn 2.39 38.7 90.3ODA 30 120 20 min 2.30 39.5 52.3ODA 30 160 1 min 2.00 39.4 36.2

816 J. Jia et al. / Optical Materials 36 (2014) 813–819

amine addition. High growth rate caused by large amount amine issupposed to be responsible for the initial notable focusing whilethe subsequent remarkable defocusing might be related to the Ost-wald ripening. The effect of amine addition on the PLQY is shownin Fig. 3b. Small amount of amine impairs the PL emission to someextent that is caused by a poorer surface coating from fastergrowth. In the case of amine addition in large amount, however,fluorescence enhancement is observed. The obtained experimentaldata on particle size, FWHM, and PLQY are summarized in Table 1.From the viewpoint of relatively narrow size distribution and highPLQY as well as short reaction time for energy saving, an in situaddition of OA with large amount is preferable for the qualityimprovement of the CdSe nanoparticles in liquid paraffin via one-pot method.

3.2. Reaction mechanism with amine addition

The above results demonstrate that the added amine cangreatly affect both the initial reaction temperature and the subse-quent growth kinetics of the CdSe nanoparticles. It is essential andmeaningful to clarify the mechanism during this process. Therehave already large amounts of researches about quality improve-ments of CdSe nanoparticles with amine addition in the literature[25–27]. Detail illustration on the mechanism of the added amine

on the synthesis of nanoparticles, especially in liquid paraffin viaone-pot method, however, is quite few. As pointed out in our pre-vious study [21], the formation of CdSe nanoparticles in liquid par-affin via one-pot method consists of three stages: monomerformation, nucleation and growth. Cadmium stearate dissolvesinto liquid paraffin to form a Cd precursor. Se powder reacts withliquid paraffin to form a Se precursor of gaseous H2Se [28]. Theformed Cd and Se precursors react to achieve soluble CdSe mono-mers. The accumulation of the CdSe monomers leads to supersatu-ration until a ‘‘critical’’ concentration is reached and nucleationbegins. The nuclei then continuously grow up to nanocrystals[29]. With this process, it is supposed that the added amine canaffect both the formation of CdSe monomers and the subsequentexhaustion for nucleation and growth. Besides, in this study allthe involved solvent and ligands of liquid paraffin, cadmium stea-rate, and ODA or OA possess long alkyl chain molecular structures.It is clear that the added amine molecules have been involved intothe chemical reaction during the particle formation process other-wise no great acceleration would be caused by such long alkylchain molecules.

The added amine molecules are believed to activate the chem-ical reaction for Cd precursor formation [30], leading to a greatacceleration on particle growth. With the addition of ODA or OA,the long alkyl chain amine molecules are inclined to attach to

J. Jia et al. / Optical Materials 36 (2014) 813–819 817

Cd(SA)2 by coordinating nitrogen atom with Cd atom via alone pair of electrons and/or by forming hydrogen bond betweenH atom in –NH2 and O atom in –COO– [31]. Such attachmentwould lead to redistribution of the electron cloud between–COO– and Cd atom and virtually weaken their interaction. As aresult, Cd atom becomes easier to escape from –COO– bound toreact with Se precursor. The reaction for forming Cd precursor istherefore activated by amine. The added amine molecules mightalso be helpful for Se precursor formation of gaseous H2Se. It hasbeen proved that amine can contribute to the formation of H2S[32]. Since S and Se are in the same group, the promotion of aminemolecules on H2Se formation can be expected. Through the abovemechanisms, CdSe monomers can be formed at relatively low tem-perature with amine addition [30]. As large amount of amine isadded, much more CdSe monomers are formed, which leads tothe formation of large amount of nuclei and relatively low criticalsize. Meanwhile, nearly all the nuclei grow up with high growthrate and the final size is small.

Since both stearic acid molecules and amine are involved in thereaction solution, it is essential to further clarify the bonding nat-ure of the two ligands on the surface of the formed CdSe nanopar-ticles. The measured FT-IR spectra are shown in Fig. 4. As seen,both the CdSe nanoparticles with and without OA addition arealmost identical in absorption peak with the raw material ofCd(SA)2, suggesting similar bonding nature of –COO– to Cd atomin all these cases. Such bonding is so strong that the long alkyl stea-ric acid molecules cannot be washed off from the surface of thenanoparticles. No absorption peak is found to be responsible forOA, a fact of OA molecules having been completely washed offfrom the surface of the nanoparticles. The coordination of N atomwith Cd atom and/or the hydrogen bonding between H atomin –NH2 and O atom in –COO– are therefore weak. Similar phenom-enon has also been observed by several researches [31,33]. Thepeaks at 3320 cm�1 and 1640 cm�1 are from –NH2 group in ODA.Since the same washing process is applied to the ODA and OAadded colloid samples, a detectable residual of ODA sounds reason-able because ODA is more difficult to be washed off than OA due toits longer alkyl chain.

3.3. High temperature amine addition

The results in Fig. 3 show that the CdSe nanoparticles obtainedwithout amine addition have a rather small FWHM of 27.6 nm anda low PLQY of 28% (t = 1 h) while in the case of the in situ OA addi-tion in large amount the FWHM of the PL peak and PLQY are

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Fig. 4. FT-IR spectra measured on the powder of the CdSe nanocrystals with andwithout amine addition.

40.5 nm and 32.7% at t = 5 min (see data in Table 1). It is knownthat for high quality CdSe nanoparticles a narrow size distributionand a high PLQY are quite essential. Thus, one might image that theformation of the CdSe nanoparticles with high quality can beachieved by dividing the synthesis process into two steps, namelythe nanoparticles synthesis and subsequent surface modificationwith suitable ligand, so that both a narrow size distribution anda high PLQY can be obtained. With this image, the CdSe nanoparti-cles are firstly synthesized at 200 �C for 1 h in a simple solutionsystem, followed by surface passivation at 160 �C or 200 �C withlarge amount of ODA or OA.

The optical behaviors of the CdSe nanoparticles formed via theprocess described above are shown in Fig. 5. The derived particlesize in Fig. 5a clearly shows a size decrease followed by a substan-tial increase, leading to a turning point at a reaction time ofapproximately 10 min. The size decrease is believed to be due tothe small nanoparticles that are formed due to the addition of largeamount of amine. These small nanoparticles broaden the size dis-tribution with large FWHM, leading to a defocusing process inFig. 5b. Fig. 5c shows significant enhancement on PLQY for thenanoparticles with amine addition. OA shows great enhancementon PLQY while ODA does not work at all. This can be well under-stood by considering the difference of their molecular structures.OA molecules have shorter chain length and therefore easily dif-fuse onto the surface of the synthesized nanoparticles, leading tocreation of large capping densities [34,35]. By applying largeamount of OA, the PLQY at t = 9 min is successfully enhanced from28% to 63.1% (160 �C) and to 81.9% (200 �C) while the correspond-ing FWHM is only from 27.6 nm to 33.1 nm. Thus, the CdSe nano-particles with relatively narrow size distribution of 33.1 nm andhigh PLQY of 81.9% have been achieved in liquid paraffin via one-pot method by high temperature amine addition in this study.

3.4. Low temperature amine addition

The amine addition described above is conducted at hightemperature of 200 �C, and the CdSe nanoparticles with rela-tively narrow size distribution of FWHM of 33.1 nm and highPLQY of 81.9% have been achieved. It is meaningful to exploreif amine ligand can be added into the reaction solution at lowtemperature to get CdSe nanoparticles with high quality so thatconsumption of energy can be reduced. Details for amine addi-tion in this way have been described in the section of experi-mental procedure. The optical behaviors of the CdSenanoparticles obtained through this process are shown inFig. 6. The size of the nanoparticles without amine at t = 1 h is2.75 nm while with amine it is 2.76 nm in Fig. 6a (OA addition,120 �C and 160 �C, t = 0), suggesting very little growth of thenanoparticles during the period of time for temperatureincrease. A linear size decrease is observed for the colloid samplewith OA addition at 120 �C. The growth rate of the nanoparticlesin this case is believed to be quite mild. Small nanoparticles areformed with amine addition while those previously obtained athigh temperature of 200 �C grow very little, leading to a mono-tonic decreasing tendency. The size broadening and PLQYenhancement are also observed for this colloid sample inFig. 6b and c. The derived FWHM and PLQY for a reaction timeof 20 min is 33.1 nm and 74.0%. The CdSe nanoparticles can beformed at a temperature as low as 120 �C. In the case of the col-loid sample kept at 160 �C with OA addition, the size does notdecrease but keeps nearly constant. The growth rate of the nano-particles at this temperature is moderate. The favorite FWHMand PLQY is 30.9 nm and 69.0% with a reaction time of 10 min.For the colloid sample kept at 200 �C, the variations of the par-ticles size, FWHM, and PLQY are similar to those in Fig. 5. Thehighest PLQY of 90.3% is observed at t = 10 min in this case while

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Y (

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OA = 68 mM 160 °C ODA= 68 mM 160 °C OA = 68 mM 200 °C ODA= 68 mM 200 °C

(c)

Fig. 5. Optical behaviors of the CdSe nanocrystals synthesized in liquid paraffin via one-pot method with amine addition at high temperature of 200 �C. (a) Calculated particlesize, (b) derived FWHM of the PL peak and (c) measured PLQY.

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Fig. 6. Optical behaviors of the CdSe nanocrystals synthesized in liquid paraffin via one-pot method with amine addition at low temperature. (a) Calculated particle size, (b)derived FWHM of the PL peak and (c) measured PLQY.

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NCs formed

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(311)(220)(111)

(b)(a) Without amine With amine

Fig. 7. TEM images (a) and XRD patterns (b) of the CdSe nanocrystals synthesized in liquid paraffin via one-pot method. The nanocrystals are obtained at 200 �C for 2 hwithout amine addition, and at 200 �C for 1 h followed by OA addition (68 mM) at 200 �C for 9 min.

818 J. Jia et al. / Optical Materials 36 (2014) 813–819

J. Jia et al. / Optical Materials 36 (2014) 813–819 819

the corresponding FWHM of 38.7 nm is moderate. Particle for-mation is also observed with ODA addition at low temperatureof 120 �C. However, the size distribution is larger and theenhancement on PLQY is quite limited. The colloid sample withODA addition at 160 �C shows very strange behaviors. The sizeof the CdSe nanoparticles decreased obviously within one minand then it disappears. The reason might be related to its poordispersion in the solution. Anyway, from these experimentaldata it is clear that ODA is not appropriate for quality improve-ment of the CdSe nanoparticles in this study.

3.5. Particle morphology and crystallization

The effect of amine addition on the particle morphology andcrystallization of the CdSe nanocrystals synthesized in liquid paraf-fin via one-pot method are characterized via TEM observation andXRD measurement. The results are shown in Fig. 7. As seen fromFig. 7a, round particles are observed both for those with and with-out amine addition. The XRD spectra in Fig. 7b are also comparablein each other. This suggests that amine addition does not give asubstantial effect on the particle morphology and crystallizationof the CdSe nanoparticles.

4. Conclusion

In this study primary amines of ODA and OA are added into thereaction solution for quality improvement of the CdSe nanoparti-cles from liquid paraffin via one-pot method. Our results demon-strate that the in situ addition of amine can decrease the initialreaction temperature to some extent. The particle growth isstrongly accelerated. The added amine molecules are believednot only to activate the formation reaction of Cd precursor but alsoto be probably helpful for Se precursor formation of gaseous H2Seand, as a result, the great acceleration of particle growth. In thiscase the enhancement on luminescence efficiency is quite limited.For OA or ODA addition at high temperature of 200 �C, better sur-face passivation that is caused by the addition of large amount ofamine are supposed to be responsible for the significant enhance-ment on PLQY of the CdSe nanoparticles. The CdSe nanoparticleswith relatively narrow size distribution of 33.1 nm and high PLQYof 81.9% are obtained by high temperature amine addition in thisstudy. With regard to the OA addition at low temperature, thehighest PLQY of 90.3% is observed with a reaction time of 10 minwhile the corresponding FWHM of 38.7 nm is moderate. The pri-mary amine of ODA might be inappropriate for quality improve-ment of the CdSe nanoparticles. Amine addition does not give asubstantial effect on the particle morphology and crystallizationof the CdSe nanoparticles.

Acknowledgement

This work is financially supported by the Natural Science Foun-dation of Shandong Province of China (ZR2013EMM017).

References

[1] C.B. Murray, D.J. Norris, M.G. Bawendi, J. Am. Chem. Soc. 115 (1993) 8706.[2] Z.A. Peng, X.G. Peng, J. Am. Chem. Soc. 123 (2001) 183.[3] Z.T. Deng, L. Cao, F.Q. Tang, B.S. Zou, J. Phys. Chem. B 109 (2005) 16671.[4] S. Sapra, A.L. Rogach, J. Feldmann, J. Mater. Chem. 16 (2006) 3391.[5] C. Landes, C. Burda, M. Braun, M.A. El-Sayed, J. Phys. Chem. B 105 (2001) 2981.[6] C.M. Donega, S.G. Hickey, S.F. Wuister, D. Vanmaekelbergh, A. Meijerink, J.

Phys. Chem. B 107 (2003) 489.[7] J.J. Li, Y.A. Wang, W.Z. Guo, J.C. Keay, T.D. Mishima, M.B. Johnson, X.G. Peng, J.

Am. Chem. Soc. 125 (2003) 12567.[8] P. Reiss, J. Bleuse, A. Pron, Nano. Lett. 2 (2002) 781.[9] H.B. Shen, H.Z. Wang, Z.J. Tang, J.Z. Niu, S.Y. Lou, Z.L. Du, L.S. Li, CrystEngComm

11 (2009) 1733.[10] G. Kalyuzhny, R.W. Murray, J. Phys. Chem. B 109 (2005) 7012.[11] D.V. Talapin, A.L. Rogach, A. Kornowski, M. Haase, H. Weller, Nano. Lett. 1

(2001) 207.[12] L.H. Qu, X.G. Peng, J. Am. Chem. Soc. 124 (2002) 2049.[13] H.W. Yang, N.N. Fan, W.L. Luan, S. Tu, Nanoscale. Res. Lett. 4 (2009) 344.[14] R.F. Li, J. Lee, B.C. Yang, D.N. Horspool, M. Aindow, F. Papadimitrakopoulos, J.

Am. Chem. Soc. 127 (2005) 2524.[15] E.E. Foos, J. Wilkinson, A.J. Makinen, N.J. Watkins, Z.H. Kafafi, J.P. Long, Chem.

Mater. 18 (2006) 2886.[16] W. Kim, S.J. Lim, S. Jung, S.K. Shin, J. Phys. Chem. C 114 (2010) 1539.[17] J. Jasieniak, C. Bullen, J.V. Embden, P. Mulvaney, J. Phys. Chem. B 109 (2005)

20665.[18] B. Xing, W.W. Li, H.J. Dou, P.F. Zhang, K. Sun, J. Phys. Chem. C 112 (2008) 14318.[19] L. Wang, X.D. Sun, W.J. Liu, B.Y. Liu, Mater. Chem. Phys. 120 (2010) 54.[20] X.Y. Ye, Y. Fang, Y.S. Hu, T. Xia, W.D. Zhuang, Z. Long, Mater. Lett. 61 (2007)

5026.[21] J.Q. Jia, J.T. Tian, W. Mi, W.G. Tian, X.Y. Liu, J.H. Dai, X. Wang, J. Nanopart. Res.

15 (2013) 1724.[22] K.R. Choudhury, Y. Sahoo, T.Y. Ohulchanskyy, P.N. Prasad, Appl. Phys. Lett. 87

(2005) 073110.[23] S.W. Kim, J.P. Zimmer, S. Ohnishi, J.B. Tracy, J.V. Frangioni, M.G. Bawendi, J. Am.

Chem. Soc. 127 (2005) 10526.[24] W.W. Yu, L.H. Qu, W.Z. Guo, X.G. Peng, Chem. Mater. 15 (2003) 2854.[25] D.V. Talapin, A.L. Rogach, I. Mekis, S. Haubold, A. Kornowski, M. Haase, H.

Weller, Colloids Surf. A 202 (2002) 145.[26] S.N. Sharma, H. Sharma, G. Singh, S.M. Shivaprasad, Mater Chem Phys 11

(2008) 471.[27] T.C. Kippeny, M.J. Bowers II, A.D. Dukes III, J.R. McBride, R.L. Orndorff, M.D.

Garrett, S.J. Rosenthal, J. Chem. Phys. 128 (2008) 084713.[28] A.Y. Zhang, Q. Ma, M.K. Lu, G.J. Zhou, C.Z. Li, Z.G. Wang, J. Phys. Chem. C 113

(2009) 15492.[29] X.G. Peng, J. Wickham, A.P. Alivisatos, J. Am. Chem. Soc. 120 (1998) 5343.[30] N. Pradhan, D. Reifsnyder, R.G. Xie, J. Aldana, X.G. Peng, J. Am. Chem. Soc. 129

(2007) 9500.[31] O. Chen, Y. Yang, T. Wang, H.M. Wu, C.G. Niu, J.H. Yang, J.H. Cao, J. Am. Chem.

Soc. 133 (2011) 17504.[32] G.G. Yordanov, H. Yoshimura, C.D. Dushkin, Colloid. Polym. Sci. 286 (2008)

813.[33] Z. Li, X.G. Peng, J. Am. Chem. Soc. 133 (2011) 6578.[34] C. Bullen, P. Mulvaney, Langmuir 22 (2006) 3007.[35] J.K. Cooper, A.M. Franco, S. Gul, C. Corrado, J.Z. Zhang, Langmuir 27 (2011)

8486.