本文介紹了通過半徑選擇技術中的水性膠體沉淀技術路線來合成CdTe和CdSe半導體納米晶體，CdTe / CdSe納米粒子的特點主要是通過紫外-可見分光光度法(紫外可見)進行分類。x射線衍射(XRD)也適用于對合成樣品進行分析。因其size-selective降水技術的影響納米顆粒被壓縮了。實驗結果表明,size-selective降水技術能夠凈化不同顆粒大小的CdTe和CdSe半導體納米晶體。從最基本的觀點來看，膠體半導體納米晶體量子點(SNCs)或(PQs)非常有趣，他們之間存在一個中間行為水晶和孤立的原子[1,2]。當半導體納米晶體的物理尺寸小于電子和孔之間的平均距離——激子波爾半徑(aB)時,激子不再自由移動,波的物理限制被影響。這種效應被稱為量子限制,因為它暴露了激子的量子本質晶體的大小。
 
Abstract
 
This paper presented a synthesis of CdTe and CdSe semiconductor nanocrystals by size-selective precipitation technique via the water-based colloidal route. CdTe/CdSe nanoparticles were characterized mainly by ultraviolet-visible spectrophotometry (UV-Vis). X-ray diffraction (XRD) was also applied to analyze the synthesized samples. And the influence of the size-selective precipitation technique on the nanoparticles was investigated by UV-Vis. The experimental results demonstrated that the size-selective precipitation are able to isolate CdTe/CdSe nanoparticles of different sizes and purify the nanoparticles as well.
 
Keywords: quantum dots; nanocrystal; CdTe/CdSe; size-selective precipitation; semiconductors.
 
INTRODUCTION
 
Colloidal semiconductor nanocrystalline (SNCs) or quantum dots (PQs) are extremely interesting from the fundamental point of view, and they present an intermediate behavior between crystals and isolated atoms [1,2]. When the physical size of the nanocrystalline semiconductor is less than the Bohr radius (aB) of the exciton, which is the average distance between the electron and hole, the exciton is no longer free to move, and feel the effects of the physical limits because of its wave nature [3]. This effect is called quantum confinement because it exposes the quantum nature of the exciton to the size of the crystal. Thus, pairs of electrons and holes confined in three dimensions lead to an increase in energy between the valence band and conduction band, according to the decrease of its size. Consequently, both optical absorption and emission tend to move to the blue region of the electromagnetic spectrum as their sizes are becoming smaller and smaller [3-6], leading to a rainbow of color emission in the region from ultraviolet to infrared..
 
Because these optical properties strongly dependent on the size, the techniques used to characterize PQs are the ultraviolet-visible spectrophotometry (UV-Vis) [7] and photoluminescence (PL) [8,9]. The main band in the UV-Vis spectra is due to the fundamental electronic transition, and the bandwidth is dependent on the size distribution of nanoparticles (NPs) [7,10]. Peng and his colleagues correlate, by polynomial relations, the wavelength of maximum absorption band (λmax) of the fundamental SNCs with their sizes, which were previously determined by transmission electron microscopy.. This work is still the best reference on the relationship between effect size and UV-Vis spectroscopy of SNCs, since it allows to determine the size of the nanoparticles from the sample corresponding λmax. In addition, this method has enabled the determination of the extinction coefficient and concentration of the nanocrystals by the Lambert-Beer’s law [7].#p#分頁標題#e#
 
Recently, Mulvaney and colaboradores proposed a reassessment of the size dependence of the absorption properties of CdSe quantum dots [11]. In this work, transmission electron microscopy provided appropriate measures to nanoparticles with sizes larger, with an error of only 0.17 nm for CdSe. However, for smaller particles (<2.5 nm) to determine the sizes became more difficult, leading to larger errors in measurements. For this reason, the authors introduced an atomistic model semiempirical pseudopotential proposed by Franceschetti et al., which provided a good agreement between the theoretical and experimental results for nanoparticles of smaller sizes [12].
 
The great interest in SNCs is due to their interesting optical and electronic properties, good chemical stability, high values of photoluminescence quantum yields (FF) and narrow excitonic emission and intensa [13-15]. These properties have enabled the application of these materials in various areas of technological interest as devices optoeletrônicos [16-18] and PV [19,20] amplifiers for telecommunications media [21.22], biological markers in the area médica23-28 and the detection of drugs in diagnostics médicos [29-31].
 
Although this technique is widely used to reduce dispersion of of SNCs, however, there are no studies that characterize the fractions in detail and verify the potential of this technique in the separation of sizes. Thus, this paper presented a synthesis of CdTe and CdSe semiconductor nanocrystals by size-selective precipitation technique via water-based colloidal route.
 
Experiment
 
Synthesis of CdTe quantum dots
 
The synthesis of CdTe PQs was held in colloidal aqueous medium in a two-step process. The first was to prepare a solution naht, a precursor of tellurium, and the second step in the addition of freshly prepared solution of tellurium precursor to a solution of cadmium, followed by continuous stirring and heating at 100 °C. To prepare the precursor solution was used tellurium tellurium powder; NaBH4 as reducing agent and pure water as a solvent. The reaction took place under argon atmosphere at room temperature. To prepare the precursor solution of cadmium, we used Cd (ClO4) 2.6H2O, thioglycolic acid (TGA) as binder and surface of the NPs of pure water as solvent. The pH of the dispersion was reached at 11.2 using a solution of NaOH 1.0 mol L-1, and the reaction was kept under argon atmosphere and stirring for 30 min. We used the molar ratio of Cd2 +-NT2: 1.00:0.50:2.40 TGA, respectively.
 
Were held three syntheses of CdTe, which were halted in reaction times of 6, 24 and 124 h, resulting in final samples luminescence (when irradiated with Xe light wavelength of 365 nm), green, yellow and red respectively. The monitoring of the synthesis was carried out by removing aliquots of the reaction medium at regular time intervals. The UV-Vis spectra of the samples were recorded using a spectrophotometer Varian Cary 50 Diode Array with quartz cells of 1 cm optical path.#p#分頁標題#e#
 
Synthesis of CdSe quantum dots
 
The synthesis of CdSe PQs was similar to that of CdTe and the main difference between them is in the preparation of the precursor solution of selenium. For CdSe, the first stage of synthesis also involves the preparation of the precursor solution of selenium NAHS, but for a different method. In this case, the solution is prepared from NaHSO drip of a solution of H2SO4 (10% v/v) in a certain amount of aluminum selenide (Al2Se3) under argon flow, which favored the formation of gas H2Se . This gas was bubbled into 100 mL of NaOH 0.05 mol L-1, leading to formation of NAHS. In the second step of the synthesis, part of the solution of NaHSO was transferred to the solution containing CdCl2 and thioglycolic acid (TGA). The pH was adjusted to 11.2 using a solution of NaOH 1.0 mol L-1. The system was subjected to prolonged heating and stirring, under an atmosphere of argon. We used the molar ratio Cd 2 +: Se2-: 1.00:0.50:2.40 TGA, respectively. The monitoring of the synthesis was also performed similarly to the synthesis of CdTe.
 
Selective precipitation of quantum dot’s size
 
At the end of the heating and agitation, the technique of size selective precipitation was applied to the NPs containing suns. First, the sun NPs containing the crude was concentrated to a quarter of its volume in a rotaevaporador coupled to a vacuum pump. Then certain amount of acetone was added, drop by drop, the dispersion of NPs to observe a turbidity. The suspension was then subjected to 15 min of agitation, followed by centrifugation. This process was repeated with supernatants obtained subsequently. At each step were recorded electronic absorption spectra of the supernatant and the precipitate, after redispersion in water.
 
RESULTS AND DISCUSSIONS
 
Synthesis and selective precipitation of CdTe nanocrystals
 
The preparation of the CdTe PQs was carried out in two experimental stages, which can be represented by Equations 1 and 2:
 
4NaBH4 + 2Te + 7H2O ~ 2NaHTe + Na2HTe + Na2B4O7 + 14H2(1) NaHTe + CdCl2 ~ CdTe + Na+ + 2Cl- + H+ (2)
 
Thus, each of the three syntheses of CdTe resulted in products with different emission colors, due to the different reaction times were submitted to (6, 24 and 124 h). Due to quantum confinement effect of these different emission colors correspond to NPs with sizes diferentes [5,32]. The set of optical absorption spectra of UV-Vis of the temporal evolution of the synthesis of CdTe with large green, yellow and red are found in Figure 1.
 
Figure 1. UV-visible absorption spectra of solutions containing CdTe nanocrystals.
 
As can be seen in Figure 1, the methodology for synthesis of CdTe used PQs showed good reproducibility, since the meteors observed were consistent with reaction times and confirmed by the regions in which the maximum of the absorption bands were located in the respective spectra. It can also be observed in all spectra a shift of the absorption bands toward longer wavelengths during the course of synthesis, which is due to increase in size of NPs as synthesis proceeds. Moreover, it was observed in the first aliquots taken from the summaries over the presence of bands resolved. This can be explained by the fact that at the beginning of synthesis, the nucleation process is more effective, resulting in smaller and NPs dispersion tamanhos [33].However, as the reaction time increases, the growth path Ostwald rippening occurs, increasing the dispersion of sizes of the NPs. As a result, the bands become wider and less intense in the UV-Vis spectra.#p#分頁標題#e#
 
To study the technique of selective precipitation of sizes, we used the final solutions of the three previously reported syntheses. Figure 2 shows the UV-Vis spectra obtained for the fractions of precipitates and supernatants obtained in the synthesis of NPs with green and yellow luminescence end, after application of the technique of post-preparation in each. The corresponding spectra obtained in the case of NPs are red luminescence in Figure 3.
 
According to Figure 2, we can see that the technique of size selective precipitation was effective in the separation of CdTe NPs of different sizes in the three syntheses carried out so that the greatest gifts in NPs solutions were found in the first precipitate whereas in others, the trend was to separate the NPs with sizes smaller and smaller. This phenomenon is interesting, since the technique of selective precipitation of sizes separates the NPs in the opposite direction to the synthesis, allowing to obtain NPs of smaller sizes in the last fractions of precipitates.
 
Figure 2. UV-visible absorption spectra of solutions containing precipitates and supernatants obtained during the selective precipitation.
 
In the case of NPs green luminescence (Fig. 2A), it was possible to obtain two fractions of precipitates containing NPs of different sizes. In the first cast, the wavelength of maximum absorption was 453 nm. In the second precipitate, the maximum absorption was 423 nm, which is very close to the wavelength of maximum absorption of the first supernatant, which was at 420 nm. It also is notable in UV-Vis spectrum of the second precipitate the absorption band became narrower, indicating that the separation of sizes is more noticeable for smaller particles.
 
For NPs with yellow luminescence (Figure 2B), the separation of sizes is also evident, since the wavelength of maximum absorption of dispersions of precipitates are located at 504, 468 and 438 nm for the precipitate 1, 2 and 3 , respectively. In this case, the separation of NPs by size was more evident in the spectra of the precipitates that the supernatants.
 
For NPs red luminescence (Figure 3), were two fractions of precipitates, whose absorption bands were observed at 580 and 533 nm for the first and second precipitate, respectively. In this case, it was observed that the technique also allowed the separation by size of NPs, however, this separation was less pronounced than in the two previous situations. The reason for this is that due to the high polydispersity of NPs red luminescence, the absorption bands had lower intensities. The polydispersity of the medium was enhanced by the extensive reflux time that the system was submitted (124 h of reaction - five times larger than the reaction time to obtain NPs with yellow luminescence). Thus, in the case of NPs red luminescence, they are much larger and also presented with wide distribution of sizes. This is easily seen by the broad absorption bands for both supernatants, and for the precipitates.#p#分頁標題#e#
 
Figure 3. UV-visible absorption spectra of solutions containing precipitates and supernatants of CdTe with Luminescence in the Red.
 
Although the sizes of the separation of NPs has not been very evident by the analysis of optical absorption spectra of NPs red luminescence, the efficiency of the technique was still very evident considering the luminescence of the NPs present in precipitates and supernatants obtained during the process (images not shown). In this case, the first precipitate showed intense red luminescence, while meteors were observed in orange and yellow precipitates in 2 and 3, respectively, when the NPs were excited in xenon light, confirming that there was indeed separation of sizes by applying the technique but the samples still show high dispersion. This difference in color between the emission fractions of precipitates and supernatants was also notable in other syntheses analyzed. For NPs with green luminescence, the first precipitate showed very intense green luminescence, whereas in the second that was slightly greenish. For NPs with yellow luminescence, the latter precipitate presented Lumines ¬ cence pale yellow, intermediate, and the intense yellow-green observed to precipitate the first and third, respectively.
 
To complement the characterization of the samples, the crystalline structure of CdTe nanoparticles contained in one of the precipitates obtained by applying the technique of post-preparation, was determined by X-ray diffraction of the precipitate obtained in a three syntheses which was conducted to study the selective precipitation of sizes described above. Figure 4 shows the diffraction patterns obtained for the first precipitates of the samples with green and yellow luminescence, and the second sample precipitated red luminescence.
 
Figure 4. X-ray diffraction (XRD) spectra of CdTe samples isolated by post-preparation.
 
From Figure 4 shows that the XRD patterns of samples are similar to each other and similar to the XRD patterns reported in the literature for CdTe obtained via synthesis coloidal [5,32,34,35].According to the Join Committee on Power Diffraction Standards (JCPDS) [36], peaks located at 23.7, 39.0 and 46.6 degrees (2) correspond to planes (111), (220) and (311), respectively, the cubic structure of zinc blend CdTe . Nevertheless, it was observed that the peaks showed a small shift to higher angles, which can be attributed to the formation of NPs of the type core / shell CdTe / CdS due to prolonged reflux in the dispersion is subjected in the presence of thiols ( in this case refers to the TGA) [35].The samples obtained with longer reflux showed peaks better defined and more intense due to its higher degree of crystallinity, as expected.
 
Moreover, one can observe the presence of thin and intense peaks at 31.8 and 45.6 degrees (2D) in samples with green and yellow luminescence, which are assigned to planes (111) and (200) NaCl, respectivamente [36]. This salt is formed as a byproduct of the synthesis reaction of the NPs, as shown in Equation 2.The presence of NaCl crystals was not observed for the sample with red luminescence, since this sample was used for the second precipitate obtained in the post-preparation technique used, indicating that the selective precipitation of sizes also functions as a form of purification of the fractions of NPs reaction byproducts.#p#分頁標題#e#
 
Figure 5. UV-visible absorption spectra of solutions during the synthesis course of CdSe. The time interval was 1 h.
 
Synthesis and selective precipitation of CdSe nanocrystals
 
To make the study of selective precipitation of CdSe of sizes in PQs, we applied the technique of post-preparation in a single synthesis for 13 h. The optical absorption spectra of the samples collected during the synthesis are shown in Figure 5. In the spectra of Figure 5 observe the appearance of the fundamental band at 375 nm from the second sample. This band remains in all spectra of other samples, but suffers no change of position even with the prolonged reaction time. Failure to shift this absorption band along the synthesis can be an indication that the rate of growth of CdSe NPs is very slow, requiring more time for NPs to expand their size and consequently resulted in the displacement of the bands for longer wavelengths, or that the NPs do not grow after the nucleation step. To check these hypotheses we applied the technique of selective precipitation of sizes, which surprizing allowed the separation of seven fractions of precipitates and supernatants, containing CdSe NPs with different sizes. The optical absorption spectra for redisperse any possible precipitates in water are shown in Figure 6. The spectra of the supernatants obtained in this preparation are shown in Figure 7.
 
Figure 6. UV-visible absorption spectra of precipitates during the synthesis course of CdSe.
 
Figure 7. UV-visible absorption spectra of supernatants during the synthesis course of CdSe.
 
It was observed in the spectra of the fractions of CdSe precipitates and supernatants from a shift of absorption maxima to lower wavelengths as the process of post-preparation was applied. Thus, the spectra of the precipitates show a shift of the band of 478 nm, precipitated in 1 to 366 nm, clearly more settled and also narrower in the precipitate 7. With respect to the supernatants, we observed two absorption maxima in the first supernatant at 367 and 397 nm, which become more pronounced and narrower from the supernatant 3. The fact that there is a narrowing and an improvement in the resolution of the absorption bands as the technique was applied supports the idea that the separation of sizes by measurements of optical absorption is most visible when it comes to smaller NPs, which was also observed for CdTe. Thus, the application of the technique, and allows efficient separation of different sizes in fractions of CdSe nanocrystals, also highlighted the polydispersity of NPs in the middle, which was not evident in the spectra of the samples collected during the synthesis.
 
Therefore, in the case of CdSe, the technique of selective precipitation of sizes has shown that the reaction had a broad distribution of sizes and not a slow growth kinetics, as apparently observed in the temporal evolution of synthesis, which explains the similarity between the absorption spectra obtained for samples collected during the synthesis. Thus, the post-preparation technique proved to be suitable for the separation of NPs of different sizes even in a very polydisperse NPs present in the smaller sizes, as in the case of CdSe, and not so great as in the third synthesis of CdTe NPs.#p#分頁標題#e#
 
CONCLUSIONS
 
The size-selective precipitation technique can effectively separate CdTe and CdSe nanoparticles according to their sizes. In our experiment, it was observed that the separation according to size was evident for CdTe in the dispersions containing nanoparticles of smaller sizes, because it is polydisperse a little way. For nanoparticle with different size in our experiment, this separation was significant and more evident by the difference in luminance of the color displayed by the fractions of precipitates and supernatants. In the case of CdSe, the technique can enable the separation of seven fractions of precipitates and supernatants containing nanoparticles with different sizes and also allow the better understanding of the CdSe synthesis. In summary, this paper presented a synthesis of CdTe and CdSe semiconductor nanocrystals by size-selective precipitation technique via the water-based colloidal route. The experimental results demonstrated that the size-selective precipitation are able to isolate CdTe/CdSe nanoparticles of different sizes and purify the nanoparticles as well.
 
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