Multiple Families of Magic-Sized ZnSe Quantum Dots via Noninjection

Nov 29, 2010 - Key Laboratory for the Chemistry and Molecular Engineering of Medicinal Resources (Ministry of Education),. School of Chemistry and ...
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J. Phys. Chem. C 2010, 114, 21921–21927

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Multiple Families of Magic-Sized ZnSe Quantum Dots via Noninjection One-Pot and Hot-Injection Synthesis Lai-Jun Zhang,†,‡,§ Xing-Can Shen,*,† Hong Liang,*,† and Jia-Ting Yao† Key Laboratory for the Chemistry and Molecular Engineering of Medicinal Resources (Ministry of Education), School of Chemistry and Chemical Engineering, Guangxi Normal UniVersity, Guilin 541004, People’s Republic of China, School of Chemistry and Chemical Engineering, Nanjing UniVersity, Nanjing 210093, People’s Republic of China, and School of Chemistry and Chemical Engineering, Shangrao Normal UniVersity, Shangrao 334001, People’s Republic of China ReceiVed: May 14, 2010; ReVised Manuscript ReceiVed: September 27, 2010

Four families of magic-sized ZnSe quantum dots (ZnSe MSQDs) with the first exciton absorption peaks at 280 nm (F280), 291 nm (F291), about 319 nm (F319), and 347 nm (F347) were synthesized using fatty amine-H2Se complex as selenium precursor via noninjection one-pot or hot-injection approaches. The formation, growth, and transformation of these families of ZnSe MSQDs were monitored mainly by ultraviolet-visible absorption spectroscopy. Relatively pure F291 and F347 were obtained via the combination of hot-injection technology with purification treatment and were further characterized using mass spectroscopy, infrared spectroscopy, fluorescence spectroscopy, transmission electron microscopy, and X-ray powder diffraction. F291 and F347 are speculated to be [(Zn7Se7)(OLA)2(OA)] (OLA ) oleylamine, OA ) octylamine) and [(Zn16Se15)(OLA)7], respectively, by analyzing the corresponding mass and the infrared and absorption spectra; both emit narrow exciton fluorescence with relatively small stokes shift of 9 and 2 nm, respectively. Ultrafine ZnSe nanorods with a diameter of about 1.7 nm formed via oriented attachment of F347 were observed using transmission electron microscopy. Fatty amine-H2Se complex has been demonstrated to be a simple, phosphine-free, low-cost, and effective selenium precursor for the synthesis of ZnSe MSQDs or regular quantum dots (RQDs). 1. Introduction Many researchers are focusing on the synthesis of highly monodisperse quantum dots (QDs) which may be very useful building blocks for functional materials in many scientific and technological fields.1 Magic-sized quantum dots (MSQDs) with well-defined size, structure, and precise atomic composition are of particular importance because of the reproducible physical, chemical, and optical properties as well as the corresponding applications.2 Recently, several research groups have successfully synthesized CdS,3 CdSe,4 CdTe,4a,5 and Cd3P26 MSQDs using hot-injection and noninjection one-pot wet-chemistry colloid synthesis approaches. Zinc selenide (ZnSe) is an important II-VI semiconductor with a wide direct band gap of 2.7 eV because of excellent absorption and photoluminescence properties,7 cadmium-free toxicity,8 and potential applications in such fields as solar cells,8 infrared detectors,9 blue-green lasers,10 light-emitting diodes,11 fluorescence probes,12 and photocatalysis.13 ZnSe MSQDs such as (ZnSe)13, (ZnSe)19, (ZnSe)33, and (ZnSe)34 have also been generated by laser ablation and have been studied by time-offlight mass spectrometry,14 but unlike CdSe MSQDs,4 ZnSe MSQDs were obtained rarely by wet-chemistry colloid approach, which may be partly explained by the following facts. In aqueous solution, the solubility product of ZnSe is 2 orders higher than that of CdSe, which means that for Se2-, the reactivity of Zn2+ relative to Cd2+ is significantly weaker. It is * To whom correspondence should be addressed. E-mail: hliang@ mailbox.gxnu.edu.cn (H. L.); [email protected] (X.-C. S.). † Guangxi Normal University. ‡ Nanjing University. § Shangrao Normal University.

clear that in the organic phase reaction, the reactivity of zinc precursor (Zn-precursor) is also weaker than that of cadmium precursor analogue. Uniform ZnSe QDs can be synthesized using an active organometallic compound (diethylzinc) as Znprecursor,15 however, diethylzinc is not only highly poisonous and expensive but also is flammable and therefore needs to be dealt with under severe oxygen-free and anhydrous conditions, which is unfavorable for the application in the industrial-scale production of ZnSe MSQDs and regular quantum dots (RQDs).16 Therefore, the use of the relatively stable compounds such as zinc carboxylates as Zn-precursor is often a better choice in the synthesis of ZnSe MSQDs and RQDs.16a,17 Thus, a facile, safe, and suitable selenium precursor (Seprecursor) becomes very important particularly for the synthesis of ZnSe MSQDs which often requires very slow growth rate.3-5 The Se-precursors used were often Se-TOP (trioctylphosphine selenide) or Se-ODE (Se dispersed in 1-octadecene) in the traditional synthesis of ZnSe QDs.17,18 However, a very high injection temperature (often higher than 240 °C) is necessary for the nucleation of ZnSe QDs when either Se-TOP or SeODE is used as the Se-precursor.17-19 Once the nucleation takes place at such a high temperature, the formed nuclei will inevitably grow rapidly into ZnSe RQDs. Therefore, it is difficult to give ZnSe MSQDs with very small size when using traditional Se-TOP or Se-ODE. A facile and available strategy is to appropriately enhance the reactivity of Se-precursor. Panda et al. successfully synthesized ZnSe MSQDs, ultrafine nanorods, and nanowires formed from MSQDs via oriented attachment using selenourea as the active Se-precursor.20 However, selenourea is not only sensitive to air, light, and water but also is very expensive, which makes

10.1021/jp1044282  2010 American Chemical Society Published on Web 11/29/2010

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it unsuitable for wide application in large-scale synthesis of ZnSe and other metal selenides MSQDs and RQDs. It is necessary to hunt for cheaper, easy to preserve, green raw materials to synthesize ZnSe MSQDs and RQDs. In this paper, we report several ZnSe MSQDs with welldefined optical characteristics synthesized using fatty amine-H2Se complex as Se-precursor with noninjection onepot or hot-injection technique. These ZnSe MSQDs were identified by ultraviolet-visible (UV-vis) absorption spectroscopy and electrospray ionization mass spectrometry (ESI-MS) and were further studied using infrared (IR) spectroscopy, photoluminescence (PL) spectroscopy, transmission electron microscopy (TEM), and X-ray powder diffraction (XRD). 2. Experimental Section 2.1. Chemicals. All reagents were available commercially and were used without any purification. Selenium powder (100 mesh, 99.5%) and 1-octadecene (ODE, technical grade 90%) were purchased from Alfa Aesar. Zinc acetate dihydrate (Zn(OAc)2 · 2H2O, AR), octylamine (OA, 99%), and sodium borohydride (NaBH4, AR) were purchased from Beijing chemical plant (China). n-Nonanoic acid (98%) and oleylamine (OLA, 80-90%; primary amine, 96%) were purchased from Aladdin Reagent Co., Ltd. (China). Ethanol, acetone, and cyclohexane were of analytical grade and were supplied by Shantou Xilong Chemical Co. Ltd. (China). 2.2. Synthesis of Zn-Precursor. Forty millimoles of Zn(OAc)2 · 2H2O was first dissolved in 300 mL of ethanol under stirring followed by addition of 120 mmol of n-nonanoic acid. This mixture was refluxed at 70 °C for 2 h. Then, the resulting white precipitate was centrifuged, was washed with doubledistilled water and ethanol, and finally was dried at 30 °C in vacuum. The Zn-precursor was defined as zinc nonanoate using IR spectrum (Figure S1), thermogravimetric (TG) curve (Figure S2), and elemental analysis in the Supporting Information. The reaction could be expressed as follows: Zn(OAc)2 · 2H2O + CH3(CH2)7COOH f Zn(OOC(CH2)7CH3)2. 2.3. Synthesis of Se-Precursor. Se-precursor solution was prepared freshly prior to use. In a 50 mL three-neck flask, 10 mL of distilled water was deoxygenated via argon gas flow at 50 °C for 10 min followed by addition of 1.5 mmol of NaBH4. Then, 0.5 mmol of selenium powder was added, and a colorless and transparent NaHSe solution formed after about 20 min. H2Se gas was produced by dropping 2 M H2SO4 solution into NaHSe solution after being cooled to room temperature and was introduced into the mixture of octylamine and oleylamine using argon gas as the carrier gas. After 3 h, the colorless and clear fatty amine mixture turned into a red-black solution, which was used as Se-precursor solution. The color change may be because fatty amine reacted with acidic gas (H2Se) to produce fatty amine-H2Se complex.21 The actual Se content in Se-precursor solution was about 0.4 mmol, which was calculated by weighing before and after absorbing H2Se gas. In the present study, a mixture of octylamine and oleylamine is selected as absorbing agent to synthesize Se-precursor. In fact, any ingredient, either octylamine or oleylamine, can also be used as absorbing agent. It can be found that octylamine has higher ability to absorb H2Se gas than oleylamine, however, its relatively low boiling point is not conducive to the synthesis of ZnSe QDs at the relatively high temperature. Thus, a mixture of octylamine and oleylamine is selected to simultaneously achieve high absorbing efficiency and boiling point in the following synthesis unless stated otherwise.

Zhang et al. 2.4. Synthesis of ZnSe MSQDs. Noninjection One-Pot Synthesis. A total of 228 mg of Zn-precursor (0.6 mmol), 2 mL ODE, and 1 mL of OLA were loaded into a 50 mL threeneck flask at room temperature under argon gas flow. Seprecursor solution was then added into a three-neck flask when Zn-precursor was dissolved to form a clear solution under continuous stirring. After 5 min, the reaction mixture was heated to 230 °C at a rate of 5 °C/min. A 0.1 mL aliquot of reaction solution was extracted from the reaction mixture every 10 °C and was directly dispersed into 0.5 mL of cyclohexane for recording UV-vis absorption spectroscopy. Hot-Injection Synthesis and Purification Treatment. In a 50 mL three-neck flask, the mixture of Zn-precursor (0.6 mmol), 2 mL ODE, and 1 mL of OLA were heated at 110 °C, 180 °C, and 220 or 230 °C in argon atmosphere, respectively. Then, freshly prepared Se-precursor solution was injected rapidly into the hot reaction mixture under vigorous stirring. Finally, 0.1 mL aliquot of reaction solution was extracted at different time intervals, was injected into 0.5 mL of acetone to flocculate ZnSe MSQDs and RQDs, and was centrifugated at the rate of 4000 r/min for 10 min. After decanting the upper clear liquid, a small amount of cyclohexane was added to dissolve the precipitation, and the above purification procedure was repeated. 2.5. Characterization. Purified ZnSe MSQDs and RQDs were dispersed into cyclohexane for UV-vis absorption spectroscopy, PL spectroscopy, and TEM measurements. The absorption spectra were collected using Varian cary-100 spectrophotometer in the wavelength range of 250-500 nm. PL spectra were collected using Shimadzu RF-5301 PC spectrofluorophotometer using excitation wavelengths of 260 and 300 nm for these ZnSe MSQDs samples synthesized at 110 and 180 °C, respectively. Time-resolved fluorescence decay curves were measured on Horiba JobinYvon FL3-TCSPC system. TEM images were obtained on Philips EM-400ST TEM and Hitachi H-9000 TEM operated at an accelerating voltage of 100 kV. ESI-MS for the purified MSQDs dispersed in chloroform were performed with Bruker HCT plus ion trap mass spectrometer. Purified ZnSe QDs samples dried in vacuum were used for collecting XRD data using Rigaku D/max 2500v/pc X-ray diffractometer equipped with a Cu KR radiation source (40 kV, 200 mA). 3. Results and Discussion 3.1. Noninjection One-Pot Synthesis of ZnSe MSQDs. Figure 1 displays UV-vis absorption spectra of a serials of aliquots taken from the noninjection one-pot synthetic reaction in the temperature range from 20 to 230 °C. As shown, there are two obvious absorption peaks at 279 nm (P2) and at 290 nm (P3) for these samples obtained at 20-160 °C, while no significant absorption feature is observed for the corresponding Zn-precursor solution. In this temperature range, the positions of these two peaks do not shift. It is believed that in the synthesis of semiconductor QDs, ultrastable MSQDs are formed if the absorption spectrum exhibits sharp peaks without the shift for a long reaction time or within a very wide temperature range.3-5 To describe conveniently, we usually name these ZnSe MSQDs on the basis of their first exciton absorption peak positions. Clearly, the absorption peak at 290 nm represents such a family of ZnSe MSQDs, which is defined as Family 290 (F290). For these reported CdS, CdSe, and CdTe MSQDs, two or more exciton absorption peaks were often observed, and the intensity ratio between these absorption peaks remained unchanged.3-5 However, in the present noninjection one-pot reaction, the absorption intensity of these two peaks and their

Multiple Families of Magic-Sized ZnSe Quantum Dots

Figure 1. UV-vis absorption spectra of ZnSe QDs synthesized via noninjection one-pot approach in the temperature range from 20 to 230 °C.

intensity ratio (I290/I280) first increase and then decrease gradually, which indicates the formation of two or more ultrastable ZnSe MSQDs after mixing Zn-precursor and Se-precursor solutions.3,4d This means that in addition to F290, there is at least one other family of ZnSe MSQDs with a characteristic absorption peak near 280 nm in the reaction mixture. Interestingly, when the reaction temperature is higher than 160 °C, the absorption peak at 290 nm becomes weaker and disappears quickly with increasing reaction temperature accompanied by the emergence of three new sharp absorption peaks including 307 nm (P4), 322 nm (P5), and 344 nm (P6). It can be concluded that F290 gradually grows up into larger ZnSe MSQDs, and thus the amount of F290 decreases significantly.4d-f The formation of F344 is evidenced by the fixed first absorption peak at 344 nm. The F344 often has the first and second exciton absorption peaks at 344 and 328 nm, respectively, as observed by Panda et al.20 With the increase in reaction temperature, the intensity of the absorption peak near 322 nm first increases and then becomes weaker while the intensity ratio of peaks at 344 and near 322 (I344/I322) increases continuously. These results indicate that the appearance of the peak near 322 nm is due to the absorption overlap of two families of ZnSe MSQDs including the second exciton absorption peak (328 nm) of F344 and the first exciton absorption peak of another family of MSQDs which should be F319 as judged by the literature.22 The absorption peak at 307 nm is probably the second exciton absorption peak of F319. The further growth of F319 and F344 will occur when the reaction temperature is raised. As expected, broad and

J. Phys. Chem. C, Vol. 114, No. 50, 2010 21923 featureless absorbance begins to appear in the spectral range of 300-400 nm at 220 °C and further intensifies at 230 °C. This may be because F319 and F344 grow further to generate ZnSe RQDs or nanorods at the temperature higher than 220 °C (Figure 2d and Figure S3 of the Supporting Information). At the wavelengths shorter than 290 nm, the absorption peak near 280 nm (P2) becomes more obvious, and there appears another distinct absorption peak at 269 nm (P1). This indicates that smaller ZnSe MSQDs with the first absorption peak near 280 nm, namely, F280, exist in the reaction solution. The peak at 269 nm should be the second exciton absorption peak of F280. Thus, from the above analysis, we believe that the absorption peak at 279 nm in the temperature range of 20-160 °C should be the overlap of the first exciton peak of F280 and the second exciton peak of F290. As described above, four families of ZnSe MSQDs (F280, F290, F319, and F344) are successfully obtained via noninjection one-pot synthesis. The above results show that the formation, growth, and transformation of these ZnSe MSQDs are driven by thermodynamic equilibrium and are adjustable by reaction temperature.3,4e In addition, F280 and F290 can be obtained at room temperature indicating that the Se-precursor used here has much higher reactivity than the traditional SeTOP or Se-ODE precursor and provides an excellent candidate for the synthesis of ZnSe and other metal selenide MSQDs and RQDs at relatively mild conditions. 3.2. Hot-Injection Synthesis and Characterization of ZnSe MSQDs. 3.2.1. Effect of Injection/Growth Temperature. In noninjection one-pot synthesis, it has been shown that the formation, growth, transformation, and amount of these four ZnSe MSQDs are relevant to reaction temperature, which is revealed by UV-vis absorption spectrum. However, the obtained samples are often the mixture of several families of ZnSe MSQDs, which is unfavorable for further studying their corresponding structures and properties. Therefore, hot-injection technology and purification treatment were performed to obtain more pure ZnSe MSQDs or RQDs because both hot-injection technology and purification treatment enhance monodispersity.1b,d In hot-injection synthesis, four injection/growth temperatures including 110, 180, 220, and 230 °C were selected to synthesize different families of ZnSe MSQDs or RQDs according to the law obtained from the above noninjection one-pot synthesis. Figure 2 shows the evolution of UV-vis absorption spectra of ZnSe MSQDs or RQDs synthesized at the injection/growth temperatures of 110, 180, 220, and 230 °C. As shown in Figure 2a, the samples synthesized at 110 °C have two clear, sharp absorption peaks at 280 and 291 nm which do not shift during the reaction time over 120 min indicating the formation of ultrastable ZnSe MSQDs (F291). As compared with the noninjection one-pot synthesis, the first exciton absorption peak positions of the resulting ZnSe MSQDs red-shift by 1 nm in hot-injection synthesis, and the absorption intensity ratio (I291/ I280) becomes larger. It is believed that F290 obtained from noninjection one-pot synthesis and F291 obtained from hotinjection synthesis should be the same ZnSe MSQDs; the 1 nm red-shift of first exciton absorption peak is a consequence of both purification treatment and hot-injection technology. The resulting ZnSe sample (F291) will have higher purity after F280 is removed through purification treatment in a hot-injection synthesis. Moreover, as can be also seen from Figure 2a, the intensities of two characteristic absorption peaks (280 and 291 nm) of F291 reach the maximum at about 30 min indicating the maximal amount of F291 at this time. Hence, the amount of F291 decreases because longer reaction time enables F291

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Figure 2. UV-vis absorption spectra of ZnSe QDs synthesized at the injection/growth temperatures of (a) 110 °C, (b) 180 °C, (c) 220 °C, and (d) 230 °C.

to further grow up into bigger ZnSe MSQDs. Much of the literature has shown that, although MSQDs are very stable structures, excess long reaction time and higher temperature will make MSQDs continue to grow and thus will generate larger MSQDs, even RQDs.3,4d-f At the injection/growth temperature of 180 °C, purified samples also have two very sharp peaks located at 347 and 328 nm (Figure 2b). These two absorption peaks should be the first and second exciton absorption peaks of F347. The first exciton absorption peak red-shifts about 3 nm as compared with the above noninjection one-pot synthesis and previous literature,20 which is also due to more pure F347 obtained by using hotinjection technology as well as purification treatment. In addition, broad absorption peaks near 290 nm for these samples with relatively short reaction times such as 4 and 5 min are distinguished, which indicates that a small amount of F291 coexists with F347. It is also clear that in hot-injection synthesis, F319 reduces significantly and F347 increases relatively in the amount as compared with noninjection one-pot synthesis. As can be seen, the sharpest and strongest absorption peak of F347 is at 60 min or so, which shows that the purest F347 and most F347 may be obtained at about 60 min. Subsequently, the absorbance tail increases possibly because of the increase in the degree of oriented attachment of ZnSe MSQDs. These results demonstrate that relatively pure F347 can be obtained by tuning reaction time. When the injection/growth temperature further increases to 220 °C (Figure 2c), absorption spectra become significantly different from those synthesized at 110 and 180 °C (Figure 2a, 2b). Although the sharp absorption peaks at 344 and 328 nm also exist at the period shorter than 20 min, the intensity of absorption at the wavelength longer than 350 nm is greatly enhanced, and a shoulder peak appears near 365 nm. Within 120 min, absorption shoulder peaks of ZnSe RQDs red-shift from 365 to 395 nm. These results shows that the larger sized ZnSe RQDs rather than F347 have become the main species at 220 °C; the remaining F347 will grow up into ZnSe RQDs,

and the particle size of ZnSe RQDs gradually increases with increasing reaction time. When the injection/growth temperature becomes higher, such as 230 °C (Figure 2d), the sharp absorption peak of F347 does not occur indicating that nucleation and growth processes become faster so as to form ZnSe RQDs in 4 min. To our knowledge, ZnSe RQDs were usually obtained above 240 °C using the traditional Se-TOP or Se-ODE precursor.17,18 The fact that ZnSe RQDs can form at 220-230 °C in the present system shows again that the used Se-precursor (fatty amine-H2Se complex) has higher reactivity than Se-TOP and Se-ODE precursors, which facilitates the nucleation and growth of ZnSe QDs. On the basis of these UV-vis absorption spectra analysis, it is concluded that ZnSe MSQDs could be synthesized at the relatively low temperature, which grow up into larger MSQDs or RQDs with increasing reaction temperature indicating that the formation, growth, and transformation of ZnSe MSQDs are driven by thermodynamic equilibrium.3,4e The structure and property of these purified ZnSe MSQDs were further studied. 3.2.2. ESI-MS Spectral Analysis. According to IR spectroscopy analysis (Figure S1 of the Supporting Information), it can be inferred that both F291 and F347 are modified by amine group from OLA and/or OA. As an effective and powerful means to determine the structure of MSQDs,2b,14b ESI-MS spectra of both the purified F291 and F347 synthesized at 110 and 180 °C for 30 min were collected, and the corresponding positive ion mode ESI-MS spectra are shown in Figure 3. Consistent with a previous report,14b positive ion ESI-MS spectra have a higher ratio of signal-to-noise than negative ion ESI-MS spectra. Therefore, the positive ion ESI-MS spectra of F291 and F347 are selectively analyzed, and several molecular fragments can be clearly identified by strong signals. As shown in Figure 3a, in the m/z range from 800 to 5000, the strongest signal is at m/z ) 1674. It can be speculated that m/z ) 1674 corresponds to F291 MSQDs with chemical formula [(Zn7Se7)(OLA)2(OA)] by combination of IR spectroscopy analysis (Figure S1 of the Supporting Information). Stable 7-nuclear ZnSe clusters have

Multiple Families of Magic-Sized ZnSe Quantum Dots

Figure 3. Positive ion mode ESI-MS spectra of the purified ZnSe samples synthesized for 30 min at (a) 110 °C and (b) 180 °C.

Figure 4. Representative TEM image of ZnSe samples (F347) synthesized at 180 °C. (a) 30 min (HRTEM), (b) 30 min, (c) 120 min, and (d) selected area in c.

been theoretically calculated by Deglmann et al.23 Similarly, the signals in Figure 3b at m/z ) 1654, 3501, and 4103 may be assigned to [(Zn8Se6)(OLA)2(OA)], [(ZnSe)15(OLA)5], and [(Zn16Se15)(OLA)7], respectively. From these results, it can be also speculated that the chemical composition of F347 MSQDs should be [(Zn16Se15)(OLA)7]. As the reaction temperature increases, about two atomic layers for the 7-nuclear ZnSe MSQDs obtained at 110 °C grow up into three atomic layers for 16-nuclear ZnSe MSQDs at 180 °C. According to the lattice constant (0.5667 nm) of zinc-blende ZnSe (PCPDS # 37-1463), it can be calculated that F347 has a particle size of about 1.7 nm, which can be further confirmed by TEM measurement. 3.2.3. TEM Analysis. The morphology and size of ZnSe samples (F347) synthesized at 180 °C for 30 and 120 min were observed using TEM. Figure 4a and b shows the representative TEM images of F347 taken at 30 min. In the higher-magnification TEM image (Figure 4a), many nearly spherical MSQDs marked by red circles are observed to be about 2 nm, which is very close to the above calculated size. Additionally, some particles appear to be 3-4 nm in diameter because of the aggregation, coalescence, and overlap of two or more ZnSe MSQDs.4d Besides nearly spherical MSQDs (Figure 4a), a very small amount of nanorods with about 2 nm diameter can also be found in the lower-magnification TEM image (Figure 4b). Such ZnSe nanorods increase significantly for ZnSe sample

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Figure 5. XRD patterns of ZnSe MSQDs (F347) synthesized at 180 °C for (a) 30 min and (b) 120 min as well as (c) ZnSe RQDs synthesized at 230 °C for 120 min.

obtained at 120 min (Figure 4c), which have the same diameter as the nanorods at 30 min. One can see that these nanorods are directly stringed with MSQDs, and some MSQDs fuse completely to form nanorods (Figure 4d). These results indicate that ZnSe nanorods are formed possibly via an oriented attachment growth mechanism, that is, F347 formed at the first stage orientedly grow up into nanorods of uniform diameter with increasing reaction time. The existence of these nanorods leads to an absorption tail in the spectrum range between 350 and 400 nm. The longer the reaction time, the larger the amount of ZnSe nanorods, and the more significant the absorption tail. For ZnSe samples (F291) synthesized at 110 °C, it is very difficult to measure accurately using TEM because of the too small particle size. 3.2.4. XRD Analysis. Figure 5 shows XRD patterns of ZnSe MSQDs (F347) synthesized at 180 °C for 30 min and for 120 min and ZnSe RQDs synthesized at 230 °C for 120 min. Bulk ZnSe standard diffraction cards of cubic (PCPDS #37-1463) and hexagonal (PCPDS #15-0105) phases are also shown as references. As compared with their diffraction peaks in the range of 20-80°, it can be found that ZnSe MSQDs samples synthesized at 180 °C for both 30 and 120 min exhibit hexagonal wurtzite structure; on the contrary, diffraction peak positions of ZnSe RQDs synthesized at 230 °C match well with those of bulk ZnSe with cubic zinc-blende structure. Thus, inconsistent with the fact that wurtzite structure is the thermodynamically stable structure of bulk ZnSe, ZnSe samples in the present system show an anisotropic wurtzite structure at the lower reaction temperature and a zinc-blende structure at higher temperature, which can be explained by fatty amine templateassisted oriented attachment growth mechanism.20,24 Fatty amine molecules bind to ZnSe MSQDs on the surface via the formation of coordination bond between nitrogen atom of fatty amine and Zn2+ thereby inhibiting the growth of Zn2+-containing crystal plane and leading to a different growth rate at different crystal faces.20,24 Thus, a longer reaction time will produce more ZnSe nanorods, which is identical with the above TEM observation. For ZnSe RQDs synthesized at 230 °C, with larger diameter than MSQDs, the amount of fatty amine molecules relative to Zn atoms on ZnSe QDs surface decreases greatly; therefore, the template-assisted ability of fatty amine significantly decreases leading to the isotropic cubic zinc-blende structure. 3.2.5. Optical Property of F347 and F291. Figure 6a shows the representative UV-vis absorption and PL spectra of F347 synthesized at 180 °C. As mentioned above and shown also in Figure 6a, F347 has two sharp absorption peaks located at 347 and 328 nm, respectively, with the energy gap of 176 meV between these two peaks. The peak at 347 nm is the h transition absorption, first exciton peak caused by 1Se-1S3/2

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Figure 7. UV-vis absorption spectra of F280 synthesized at 180 °C using the ODE/OA/OLA mixture as the adsorbing agent of H2Se gas over 120 min.

Figure 6. Representative UV-vis absorption and PL spectra of (a) F347 and (b) F291 synthesized at 180 and 110 °C, respectively. (C) Time-resolved decay curves of F347 and F291.

while the peak at 328 nm is the second exciton peak caused by 1Se-2Sh3/2 transition absorption.19a The dual-band emission, consisting of a narrow peak and a broad peak, is observed from the representative PL spectrum. The low-energy broader peak is mainly due to strong surface-state emission caused by defects and high surface to volume ratio,16a while the highenergy peak at 349 nm is very close to the first exciton absorption peak and therefore belongs to typical exciton PL peak. For F347, Stokes shift and full width at half-maximum (fwhm) are only, respectively, 2 and 8 nm; such a small Stokes shift and fwhm are typical for many other MSQDs but are not found for the previously reported ZnSe MSQDs.3-5,15,16 It is the first report on ZnSe MSQDs with such a small Stokes shift and a narrow exciton PL peak. Figure 6b shows the representative UV-vis absorption and PL spectra of F291 synthesized at 110 °C. F291 has two sharp absorption peaks located at 291 and 280 nm with an energy gap of 167 meV between these two peaks. The PL property of F291 reported for the first time shows that F291 gives exciton emission with the peak at 300 nm and very weak surface state emission; the corresponding Stokes shift and fwhm of exciton emission are, respectively, 9 and 16 nm. Additionally, the representative time-resolved decay curves of F347 and F291 are also shown in Figure 6c. Both decay curves can be well fitted by a biexponential function consisting of a fast relaxation component with a decay time constant of a few tens of picoseconds and a slow component with a decay time constant of a few nanoseconds. The calculated effective decay lifetimes of F347 and F291 are about 3.1 and 2.7 ns, respectively, which indicate that exciton radiative decay is the dominant recombination process.25 Thus, all the above

results show that the present synthesis system is very effective for the synthesis of high-quality ZnSe MSQDs such as F347 and F291. 3.2.6. Effect of Se-Precursor SolWent. As describe above, Se-precursor (fatty amine-H2Se complex) prepared using the mixture of oleylamine and octylamine as absorbing agent has very high reactivity and can be used to synthesize F347 at 180 °C. The case is not the same when 1 mL ODE is added into this absorbing agent. Figure 7 is UV-vis absorption spectra of ZnSe QDs samples synthesized using the mixture of ODE (1 mL), OA (1 mL), and OLA (1 mL) as absorbing agent of H2Se gas over 120 min. Although three sharp exciton absorption peaks at 258, 269, and 280 nm occur in the range of 250-300 nm, no sharp exciton absorption peak corresponding to F347 is found after injecting the Se-precursor solution containing ODE over 120 min in the range of 300-400 nm. This fact shows that this reaction at 180 °C does not produce F291, F319, and F347 but does produce smaller F280 upon addition of ODE into the absorbing agent. As can also be seen, the resulting F280 is very stable without visible growth within 30 min, which also indicates that relatively pure F280 can be obtained using the Se-precursor solution containing ODE. At 180 °C, in addition to the increase in the amount of F280, the longer reaction time does not generate larger ZnSe MSQDs but does generate ZnSe RQDs as indicated by the gradually enhanced absorption tail other than the sharp exciton peak at above 280 nm. This may be because of the formation of stable Se species in the Seprecursor solution containing ODE. During the absorbing process, H2Se gas reacts with ODE containing unsaturated double bonds allowing Se2- to bond to the double-bond site of ODE thus making Se-precursor more stable and difficult to react with Zn-precursor.26,27 The Se-precursor solution containing ODE obtained in the present reaction system is similar to Se-ODE precursor solution obtained via heating Se powder in ODE solvent at above 180 °C. No formation of F347, F319, or F291 upon addition of ODE into fatty amine absorbing agent indicates that the Se-ODE precursor solution is too stable to generate ZnSe MSQDs such as F347, F319, and F291 at 180 °C, which in turn illustrates that the fatty amine-H2Se complex obtained using OLA/OA mixture as the absorbing agent of H2Se gas is a highly reactive and effective Se-precursor for the synthesis of ZnSe MSQDs such as F347, F319, and F291.

Multiple Families of Magic-Sized ZnSe Quantum Dots 4. Conclusions By using fatty amine-H2Se complex as Se-precursor, four families of ZnSe MSQDs including F280, F291, F319, and F347 were successfully synthesized via noninjection one-pot or hotinjection approach. Of these families, relatively pure F347 and F291 were obtained through the combination of hot-injection technology and purification treatment, while relatively pure F280 could be obtained when using the mixture of fatty amine and ODE as the absorbing agent of H2Se gas. F347 and F291 are speculated to be [(Zn16Se15)(OLA)7] and [(Zn7Se7)(OLA)2(OA)], respectively, both of which can emit exciton PL with relatively small Stokes shift and narrow bandwidth. Ultrafine ZnSe nanorods with diameter of about 1.7 nm formed via oriented attachment of F347. Fatty amine-H2Se complex has been demonstrated to be a simple, phosphine-free, low-cost, and effective selenium precursor for the synthesis of ZnSe MSQDs and RQDs and can be potentially applied in synthesizing other selenide QDs. Acknowledgment. The authors thank the support of the National Natural Science Foundation of China (Nos. 20261001 and 20701010), Natural Science Foundation of Guangxi Province (Nos. 0728094 and 0339022), and the Department of Education of Jiangxi Province [GJJ (2007)348]. Supporting Information Available: IR spectroscopy, TG curve, and elemental analysis of Zn-precursor; TEM characterization. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) (a) Park, J.; Joo, J.; Kwon, S. G.; Jang, Y.; Hyeon, T. Angew. Chem., Int. Ed. 2007, 46, 4630–4660. (b) de Mello Donega´, C.; Liljeroth, P.; Vanmaekelbergh, D. Small 2005, 1, 1152–1162. (c) Cheng, Y.; Wang, Y.; Bao, F.; Chen, D. J. Phys. Chem. B 2006, 110, 9448–9451. (d) Zhang, L. J.; Shen, X. C.; Liang, H.; Guo, S. Y.; Liang, Z. H. J. Colloid Interface Sci. 2010, 342, 236–242. (2) (a) Park, Y.-S.; Dmytruk, A.; Dmitruk, I.; Kasuya, A.; Takeda, M.; Ohuchi, N.; Okamoto, Y.; Kaji, N.; Tokeshi, M.; Baba, Y. ACS Nano 2009, 4, 121–128. (b) Kasuya, A.; Sivamohan, R.; Barnakov, Y. A.; Dmitruk, I. M.; Nirasawa, T.; Romanyuk, V. R.; Kumar, V.; Mamykin, S. V.; Tohji, K.; Jeyadevan, B.; Shinoda, K.; Kudo, T.; Terasaki, O.; Liu, Z.; Belosludov, R. V.; Sundararajan, V.; Kawazoe, Y. Nat. Mater. 2004, 3, 99–102. (c) Seifert, G. Nat. Mater. 2004, 3, 77–78. (3) Li, M. J.; Ouyang, J. Y.; Ratcliffe, C. I.; Pietri, L.; Wu, X. H.; Leek, D. M.; Moudrakovski, I.; Lin, Q.; Yang, B.; Yu, K. ACS Nano 2009, 3, 3832–3838. (4) (a) Dai, Q. Q.; Li, D. M.; Chang, J. J.; Song, Y. L.; Kan, S. H.; Chen, H. Y.; Zou, B.; Xu, W. P.; Xu, S. P.; Liu, B. B.; Zou, G. T. Nanotechnology 2007, 18. (b) Kucur, E.; Ziegler, J.; Nann, T. Small 2008, 4, 883–887. (c) Kudera, S.; Zanella, M.; Giannini, C.; Rizzo, A.; Li, Y. Q.; Gigli, G.; Cingolani, R.; Ciccarella, G.; Spahl, W.; Parak, W. J.; Manna, L. AdV. Mater. 2007, 19, 548–552. (d) Ouyang, J.; Zaman, M. B.; Fu Jian, Y.; Johnston, D.; Li, G.; Xiaohua, W.; Leek, D.; Ratcliffe, C. I.; Ripmeester,

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