Cong Rimin, Luo Yunjun , Li Guoping, Wang Xiaoqing
(School of Material Science and Engineering, Beijing Institute of Technology, Beijing 100081, China)

Abstract At present, Poly (amido amine) (PAMAM) dendrimers were used as templates for in-situ synthesis of nano-particles because of their well-defined shape, inner nano-holes and controllable end-groups on the surface. In this paper, well-dispersed CdS quantum dots (Q-CdS) were prepared through the protection of methyl ester-terminated generation 3.5 PAMAM dendrimers (G3.5) using H₂S and Na₂S respectively; the differences between the two methods were discussed; the adjustment of the photoluminescence (PL) color of Q-CdS was realized; and the effects of excess Zn²⁺ and Cd²⁺ ions on the optical properties of Q-CdS was studied in detail. It was found that the contrasts in size, size distribution and the room temperature photoluminescence quantum yield (RT-PQY) of Q-CdS prepared using the two methods were caused by the differences in the nucleation process, position and surface defects of Q-CdS. The photoluminescence (PL) color of Q-CdS was tunable by controlling the particle size, which was changed by varying the Cd²⁺/G3.5 molar ratio. The RT-PQY of Q-CdS after adding excess Cd²⁺ and Zn²⁺ ions were improved nearly 9 and 10 times respectively; and their PL lifetimes increased about 5 and 8 times respectively. These results were attributed to the decrease of surface defects and the formation of the Schottky-like barrier.
Keywords Q-CdS; PAMAM dendrimers; Optical property; Room temperature PL quantum yield; PL lifetime

1. INTROUDUCTION
Owing to their tunable photoluminescence (PL) color, longer PL lifetime, higher RT-PQY and optical stability than organic dyes, Ⅱ-Ⅵ semiconductor quantum dots are promising to find many novel applications, such as fluorescence probes, photo catalysts, and photoelectric converters ^[1-2]. Before realizing their widespread applications, the following factors are necessary: high RT-PQY, tunable size, narrow size-distribution, controllable solubility both in organic and inorganic solvents et al. But most of nano-particles prepared using the common methods can not meet these needs simultaneously. Therefore, a lot of researchers have been engaged in searching for new approaches to solve the problems. The development of dendrimer provides a new method for preparing nano-materials. The nano holes inside the dendrimer can act as reaction vessels and prevent the nano-particles from aggregation. Since the abundant end groups are easy to be modified, the solubility of semiconductor nano-particles both in organic and inorganic solvent will be controllable. Some nano-particles such as Ag, Cu, Pt and Pd clusters ^[3-8] were synthesized by using PAMAM dendrimers as templates, which were called PAMAM dendrimer nano-composites (PAMAM DNC). It has been found that PAMAM dendrimers were excellent stabilizers in preparing compound semiconductor nano-particles. Sooklal ^[9] prepared CdS quantum dots (Q-CdS) using carboxyl and amine terminated PAMAM dendrimers as stabilizing agents, and the average diameter of Q-CdS was about 3nm. But the Q-CdS/PAMAM DNC aggregated gradually in solution and formed large aggregates with the size over 100nm. Lemon ^[10] prepared Q-CdS using hydroxyl-terminated PAMAM dendrimers as templates and no aggregation of Q-CdS formed. He found that the size of Q-CdS grew with the increase of generation of PAMAM dendrimers. Also, Wu ^[11] synthesized Q-CdS that located on the surface of the amine-terminated PAMAM dendrimers of generation 8.0. But the PL lifetime of the Q-CdS was less than 2 ns, and it was affected slightly by excess metal ions. In this paper, Q-CdS was prepared by a novel method: Cd²⁺ ions coordinated with the inner groups of G3.5 PAMAM dendrimer fully, and then Q-CdS was prepared by adding S^2- ions. We found that the methyl ester-terminated PAMAM dendrimer of generation 3.5 was another excellent inner template, the size-distribution of Q-CdS was very narrow, and the size grew with the Cd²⁺/PAMAM molar ratio. Differing from the results of Wu, excess Cd²⁺ and Zn²⁺ metal ions increased the PL intensity and PL lifetime greatly.
    In this paper, Q-CdS was prepared by adding two sulfide sources: Na₂S and H₂S. The differences between the two methods were discussed carefully. Furthermore, the effects of Cd²⁺/PAMAM molar ratio on the size and PL color of Q-CdS, effects of excess Cd²⁺ and Zn²⁺ ions on the optical properties were discussed in detail. The morphology of Q-CdS was measured by HRTEM, and their optical properties were characterized by UV-vis spectra, PL spectra, and PL decay curves.

2. EXPERIMENTAL SECTION
2.1 Materials and Instruments
All chemicals used were of analytical grade. Cadmium acetate (Cd(CH₃COO)₂?/FONT>2H₂O), zinc acetate (Zn(CH₃COO)₂?/FONT>2H₂O), ferrous sulfide (FeS), sulfuric acid (H₂SO₄), sodium sulfide (NaS?9H₂O), sodium hydroxide (NaOH), acetic acid, methanol, ethylenediamine, methyl acrylate, quinine sulphate et al were purchased from Beijing Chemical Reagent Inc. Methanol, ethylenediamine and methyl acrylate were purified by distillation. H₂S gas was prepared by dripping H₂SO₄ of 1mol/L into the flask loaded with surplus FeS. PAMAM dendrimers of generation 3.5 were synthesized according to ref [12].
    High resolution transmission electron microscopy (HRTEM) image was performed on a JEM-2010 TEM at 200Kv, and the TEM image was performed on a Hitachi H-700 TEM. UV-Vis absorption spectra were measured at room temperature with an UV-2102-PC Spectrophotometer (Shanghai UNICO Instrument Inc, China). The PL spectra were recorded at room temperature with a Cary Eclipse fluorescence spectrophotometer (Varian, USA). The PL lifetime was measured with a FL900 spectrometer (Edinburgh Instruments Ltd. U.K.).
2.2 Synthesis of Q-CdS under the protection of G3.5 
Firstly, cadmium acetate methanol solution of 1.0×10^-2 mol/L was dripped into PAMAM dendrimer of G3.5 methanol solution of 5.0×10^-4 mol/L, then methanol was added till the concentration of G3.5 reached 1.0×10^-4 mol/L. The reaction solution was agitated rapidly at room temperature for 48h, after that, Cd²⁺ ions coordinated with the G3.5 completely ^[13]. Then a little excess H₂S or equivalent freshly prepared sodium sulfide solution (the molar ratio of methanol to water was 9:1) were added one-shot, and transparent Q-CdS/PAMAM DNC was synthesized. In this experiment, Q-CdS of different size was prepared at different Cd²⁺/G3.5molar ratio. These reactions were all carried out at the room temperature. The RT-PQY was measured by comparison to a quinine sulphate reference of the same absorbance (±1%) ^[14].

3. RESULTS AND DISCUSSTION
3.1 Effects of S^2- ion sources       
Well-dispersed Q-CdS were prepared using Na₂S and H₂S respectively at the same PAMAM concentration (1×10^-4mol/L) and Cd²⁺/PAMAM molar ratio (2.5:1). One could see from the HRTEM images of Q-CdS shown in Figure 1 that the average diameter of Q-CdS was 1.5nm (made by using Na₂S) and 3.8nm (made by using H₂S). As shown in Figure 1-c, the size distribution of the Q-CdS made using Na₂S was narrower than that of Q-CdS made using H₂S.

Figure 1 HRTEM image of Q-CdS made using Na₂S (a) and TEM image of Q-CdS made using H₂S (b); Histograms of Q-CdS size distribution when using Na₂S (black histogram) and H₂S (sparse bias histogram) as S^2- ion source (c).

    The absorption and PL spectra of the Q-CdS prepared using H₂S were at the low-energy direction and the spectra were broader than that of Q-CdS prepared using Na₂S (as shown in Figure 2), which indicated that the size of Q-CdS prepared using H₂S was larger and the size distribution was broader. The conclusions obtained from TEM and UV-Vis absorption spectra were consistent, so we characterized the size of Q-CdS using the absorption spectra during the following experiments.

Figure 2 Absorption spectra (a) and PL spectra (excitated at 348nm) (b) of Q-CdS prepared using two sulfide sources: (1) H₂S, (2) Na₂S.

    Since the ligand field intensity of tertiary amine and amido groups inside the G3.5 PAMAM dendrimer was stronger than that of peripheral methyl esters, nearly all Cd²⁺ ions coordinated with the inner groups first ^[10] when the Cd²⁺/G3.5 molar ratio is less than 16 (the maximum coordination number of a G3.5 dendrimer molecule with Cd²⁺ ions in methanol ^[12]). Therefore, Cd²⁺ ions in-situ reacted with S^2- ions, and the Q-CdS was enclosed in the nano holes of G3.5 dendrimers. Since the aggregation of Q-CdS was prevented by the branched chains of G3.5, the Q-CdS colloid was still clear after a year. The size of Q-CdS prepared using both S^2- ion sources was less than the hydrodynamic diameter (about 4.5nm) of G3.5 PAMAM dendrimers.
    The differences between the size and size-distribution of Q-CdS prepared using the two methods were attributed to this reason: the nucleation process of Q-CdS in the two methods were different. When a H₂S molecule dissolved in the methanol solution, the S^2- ion combined with a Cd²⁺ ion to form a CdS molecule. While the other two H⁺ ions exchanged with the Cd²⁺ ions that coordinated with those tertiary amine groups inside the PAMAM dendrimer, leading to continued loss of Cd²⁺ into solution. The extricated Cd²⁺ combined with other S^2- ions both inside and outside the G3.5 dendrimer, causing the nucleation process occurred both inside and outside the dendrimer. The growth of CdS nuclei was not limited by the nano holes of dendrimers, but by the peripheral methyl esters of many G3.5, resulting in larger CdS particles and broader particle size distribution. However, when Na₂S was used as the sulfide source, the Na⁺ ion was less effective at displacing the Cd²⁺ coordinated with G3.5 dendrimers. The nucleation process then occurred inside the dendrimer, leading to more uniform nucleation, more nuclei, and smaller final particle size. Differences on the size and size distribution of Q-CdS were more obvious when the Cd²⁺/PAMAM molar ratio was larger.
    The PL spectra shown in Figure 2 were measured at the excitation wavelength of 348nm, and were band edge emitting. The PL peak position of Q-CdS was 491nm (H₂S as sulfide source) and 453nm (Na₂S as sulfide source) respectively, suggesting that Q-CdS synthesized by using H₂S was larger than that by using Na₂S. While the PL intensity of Q-CdS prepared by using H₂S was much lower. These mainly attributed to the larger particle size and broader particle size distribution of Q-CdS prepared using H₂S. In addition, the little excess H₂S formed sulfur-dangling bonds on the Q-CdS surface, resulting in the increase of nonradiative routes and the decrease of the PL efficiency. Consequently, the Q-CdS used in the following experiments were all prepared by using Na₂S. Moreover, the one-shot feeding of Na₂S induced very high nuclei concentration, which followed by nuclei growing inside G3.5 dendrimers with high Cd²⁺ and S^2- ions concentration ^[15]. This process resulted in narrower size distribution, narrower UV-vis absorption and PL spectra, and higher PL efficiency. Furthermore, the large stokes shift between UV-vis absorption spectra and PL spectra will make it easier to be resolved when using as PL probes. The optical properties of Q-CdS changed little after storage at room temperature for six months in dark, which suggested the optical properties of Q-CdS were very stable.
3.2 The tunable PL color of Q-CdS
In order to control the photoluminescence color, we synthesized Q-CdS of different size by changing the Cd²⁺/G3.5 molar ratio. The Cd²⁺/S^2- molar ratio was fixed as 1:1 during the preparation. As shown in Figure 3, the absorption spectra shifted to the red with the increase of Cd²⁺/G3.5 molar ratio, indicating the formation of larger Q-CdS. Correspondingly, the maximum PL wavelength shifted from 430nm to 474nm, i.e. the PL color changed from purple to blue. Furthermore, the PL efficiency increased with the raise of Cd²⁺/G3.5 molar ratio slowly, and reached the maximum when the molar ratio was 2.5. Then the PL efficiency decreased with the further increase of Cd²⁺/G3.5molar ratio，which was caused by the effects of the small size. On the other hand, the curvature of the Q-CdS increased with the decrease of diameter, which brought more dangling bonds and more surface defects, leading to lower PL efficiency. Under these two contrary effects, the PL efficiency of Q-CdS reached its maximum value when the Cd²⁺/G3.5 molar ratio was 2.5.

Figure 3 Absorption spectra (a) and PL spectra (b) of Q-CdS at different Cd²⁺/G3.5 molar ratio: (1) 0.5, (2) 1, (3) 2.5, (4) 5.

    Relationships between the size, PL color of Q-CdS and Cd²⁺/G3.5 molar ratio were given in Table 1. The size of Q-CdS was estimated by Brus model ^[16] according to the absorption edge values.

Table 1 Relationships between the size, PL color of Q-CdS and Cd²⁺/G3.5 molar ratio

3.3 Effects of excess Cd²⁺ and Zn²⁺ ions on the PL efficiency
The PL efficiency of semiconductor nano-particles was affected greatly by the surface conditions, and the surface conditions of colloidal nano-particles were influenced by the colloidal solution conditions firstly. Researchers found that excess metal ions influenced the optical properties of CdS apparently ^[17]. In this paper, the effect of excess Cd²⁺ and Zn²⁺ ions on the optical properties of Q-CdS was investigated carefully. Since the solubility constant of CdS was very low (3.6×10^-29 mol²/L²), we assumed that all Cd²⁺ ions had turned into CdS. For convenience, the excess metal ions were counted by the molar ratio of metal ions to CdS molecule. The changes of UV-vis absorption spectra and PL spectra of different samples were compared and discussed in detail. The samples added Cd²⁺ ions and Zn²⁺ ions were named sample 1 and sample 2 respectively. As shown in Figure 4, the UV-vis absorption spectra of sample 1 shifted to the red comparing with that of the sample without excess metal ions (blank sample), and the absorption edge wavelengths became shorter with the increase of excess Cd²⁺. While the UV-vis absorption spectra of sample 2 all shifted to the blue comparing with that of the blank sample. The PL spectra of samples before and after the addition of excess metal ions at the excitation wavelength of 348nm were shown in Figure 5. The PL intensities of both samples with excess metal ions increased dramatically. Sample 2 reached the largest PL-PQY (0.18) when the Zn²⁺/CdS molar ratio was 3, while the PL peak position unchanged. Sample 1 reached the largest PL-PQY (0.11) when the Cd²⁺/CdS molar ratio was 4, but the PL spectrum shifted to the red slightly. Relationships between PL intensities at the peak position and Cd²⁺/CdS molar ratios (solid line), Zn²⁺/CdS molar ratios (dotted line) were fitted with a Boltzmann function. Before the PL intensity attained its maximum value, the curves were fitted well. But the PL intensity of both samples decreased slowly with the further increase of excess metal ions.

Figure 4 (a) UV-Vis absorption spectra of sample 1 when Cd²⁺/CdS molar ratio was 0, 1, 2, 3, 4, and 5; (b) UV-vis absorption spectra of sample 2 when Zn²⁺/CdS molar ratio was 0, 1, 2, 3, 4, and 5.

    The principle of above phenomena was analyzed as follows. It was well-known that CdS colloidal particles were electronegative when S^2- ions were equivalent or excess during the preparation process. The dangling bonds of S^2- ions adsorbed on the surface of Q-CdS increased nonradiative recombination centers. Therefore, the PL intensity was low for Q-CdS prepared at the equivalent Cd²⁺/S^2- molar ratio. When Cd²⁺ ions were over-dose, excess Cd²⁺ ions reacted with the S^2- ions located on the surface of Q-CdS, increasing their size at some extent. This fact was verified by the small red shifts of the UV-vis absorption spectra and PL peak positions. Since the solubility constant of ZnS (1.2×10^-23 mol²/L²) is much larger than that of CdS, the excess Zn²⁺ ions can not replace Cd²⁺ ions to form a ZnS shell. Consequently, the UV-vis absorption spectra and PL spectra did not shift to the red. Moreover, the UV-vis adsorption spectra shifted to the blue with the increase of the excess Zn²⁺ions. It seemed as if the size of Q-CdS became smaller according to the Brus model, whereas the PL peak position was fixed, indicating that the size of Q-CdS was unaltered. This inconsistency will be explained in the next section.
    Moore ^[16] described this phenomenon using the model of surface Schottky-like barrier caused by surplus metal ions. In this paper, excess Cd²⁺ and Zn²⁺ ions compressed the electric double layers of the CdS colloidal particles to the area very near the surface, which reduced the surface defects such as the sulfur dangling bonds. At the same time, a Schottky-like barrier came into being and the charge carriers were confined within the Q-CdS ^[17], thus the nonradiative recombination of carriers was restricted, leading to the higher PL efficiency. Also the activation energy of electron transition was increased slightly owing to the limitation of the Schottky-like barrier, which resulted in the absorption spectra of Q-CdS shifting to the blue slightly with the increase of excess metal ions. Vazquez-Olmos ^[18] also observed this phenomenon during studying the activation of CdS nano-particles by Mn²⁺ and Cu²⁺ ions.

Figure 5 (a) PL spectra of Q-CdS before and after adding excess metal ions: (1) blank sample, (2) sample 2 (Zn²⁺/CdS molar ratio was 3) (3) sample 1 (Cd²⁺/CdS molar ratio was 4); (b) PL intensities of Q-CdS before and after adding Cd²⁺ ions (solid line) and Zn²⁺ ions (dotted line) at the PL peak position.

3.4 Effects of excess metal ions on the PL lifetime
The PL lifetime relates intimately to the surface conditions of the semiconductor nano-particles, and is called a “structure-sensitive” parameter. Also it is expected that the semiconductor quantum dots possess longer PL lifetime when used as fluorescence probes. In this paper, excess Zn²⁺ ions and Cd²⁺ ions effectively reduced the surface defects, eliminated the nonradiative pathways, increased the PL RT-PQY, but also prolonged the PL lifetime of Q-CdS remarkably. PL decay curves shown in Figure 6 were fitted with the following bi-exponential function,, yielding the fitting parameters in Table 2. The PL lifetime of Q-CdS in our case was 12 times longer than that reported by Wu ^[11], which was about 2 ns. After adding excess Zn²⁺ and Cd²⁺ ions, both the fast decay part and the slow part of the PL decay increased remarkably, and besides, the ratio of the fast decay part increased dramatically. Such long PL lifetime will expand the usage of Q-CdS/PAMAM nano-composites.
    The photoluminescence of Q-CdS comprised mainly two emitting states with different energies. The fast decay corresponded to an emitting state with a short lifetime, while the slow decay came from an emitting state with a long lifetime. Since the lifetime was determined by both radiative decay and nonradiative decay, the prolonged lifetime of samples meant that the radiative decay dominated. These phenomena were attributed to the decrease of surface defects and the formation of the Schottky-like barrier. Furthermore, Zn²⁺ ions were more effective than Cd²⁺ ions on eliminating defects on the Q-CdS surface. Also, samples with higher RT-PQY possessed longer PL lifetime.

Figure 6 PL decay curves of Q-CdS before and after adding excess metal ions: (1) blank sample, (2) sample 1 (Cd²⁺/CdS molar ratio was 4), (3) sample 2 (Zn²⁺/CdS molar ratio was 3)

Table 2 Fitting parameters of the bi-exponential PL decay curves

4. CONCLUSIONS
We prepared Q-CdS by using Na₂S and H₂S respectively under the protection of G3.5 PAMAM dendrimers. The comparison of Q-CdS synthesized by both methods showed (i) H₂S disrupted the coordination bonds of Cd²⁺ with G3.5, therefore the dendrimer did not effectively restrict the growth of Q-CdS, (ii) Na₂S was a better S^2- ions source, since the Q-CdS prepared using Na₂S was enclosed inside the dendrimer, and the G3.5 PAMAM dendrimer played the role of inner templates. The PL color of Q-CdS was controlled by changing the Cd²⁺/G3.5 molar ratio at a fixed PAMAM dendrimer concentration. The PL intensity reached up to the maximum when the Cd²⁺/G3.5 molar ratio was 2.5. The effects of excess Zn²⁺ and Cd²⁺ ions on the optical properties of Q-CdS were studied and the results showed that the RT-PQY and PL lifetime increased greatly due to the decrease of the surface defects and the formation of the Schottky-like barrier.
    Since the abundant end-groups of PAMAM dendrimers are easy to be modified to control the solubility of Q-CdS in organic or inorganic solvent, the Q-CdS/PAMAM DNC are promising to be assembled into films or used as long-lifetime fluorescence probes.

Acknowledgments  Thanks for the sustentation of the Fundamental Research Foundation of Beijing Institute of Technology (No. BIT-UBF-200504E4203).

聚酰胺-胺树形分子模板法制备CdS量子点及其光学性能研究
丛日敏，罗运军，李国平，王晓青
（北京理工大学材料科学与工程学院 中国北京100081）
摘要 聚酰胺-胺（PAMAM）树形分子为单分散的球形结构，具有纳米级空腔和易于改性的密集的表面官能团，被广泛地用作制备纳米粒子的模板，并能促进这种纳米材料在自组装及检测材料方面的应用。本文以3.5代PAMAM树形分子（64个甲酯端基）为模板，分别以H₂S和Na₂S为硫源原位合成了分散良好的CdS量子点。分别研究了两种硫源对制备CdS量子点的影响、CdS量子点发光颜色的可调节性、以及过量的Zn²⁺ 和Cd²⁺ 离子对其光学性能的影响。实验发现用Na₂S制备的CdS量子点的尺寸更小，尺寸分布更窄，室温光致发光效率更高，这是由于CdS量子点的生长过程和位置不同，表面缺陷状态不同造成的。通过改变Cd²⁺离子与树形分子的比值可以改变CdS量子点的尺寸，从而调节其光致发光颜色。过量的Zn²⁺ 和Cd²⁺ 离子对CdS量子点的吸收和发光性能都有较大影响，分别能使其室温光致发光效率提高9倍和10倍；并使其发光寿命分别延长5倍和8倍。这是由于过量的Zn²⁺ 和Cd²⁺ 离子能够形成类Schottky 能垒，并能有效消除CdS量子点的表面缺陷。
关键词 CdS量子点，PAMAM树形分子，光学性能，室温光致发光效率，光致发光寿命