Oriented Attachment Growth of Quantum-Sized CdS Nanorods by

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Oriented Attachment Growth of Quantum-Sized CdS Nanorods by Direct Thermolysis of Single-Source Precursor Zhiguo Li,† Jiehe Sui,*,‡ Xiaoli Li,§ and Wei Cai*,‡ †

School of Material Science and Engineering, Northeast Forestry University, Harbin, Heilongjiang 150001, P. R. China Department of Chemistry, School of Science Northeast Forestry University, Harbin, Heilongjiang 150001, P. R. China ‡ School of Material Science and Engineering, Harbin Institute of Technology, Harbin, Heilongjiang 150001, P. R. China §

bS Supporting Information ABSTRACT: Quantum-sized CdS nanorods were synthesized by direct thermal decomposition of a single-source precursor in a monosurfactant system. The CdS nanorods were uniform, had high crystallinity, and exhibited strong quantum confinement effect. The nanorod growth was controlled by an oriented attachment mechanism, and the morphology was determined by the competition between dipole attraction and steric repulsion of nanodots. Increasing precursor concentration and prolonging reaction time were favorable for the formation of CdS nanorods.

’ INTRODUCTION The fabrication of one-dimensional semiconductor nanorods or nanowires has attracted intense interest in the past several years because of their unusual optical and electric properties.1-8 Scientists are interested in the quantum size effect of the one-dimensional nanomaterials9,10 and their promising applications in light-emitting diodes,11-13 solar cells,14,15 biological labeling,16,17 catalysis,18 and so forth. The key issue for the research and manipulation of semiconductor nanorods is the controllable synthesis of nanorods with diameters smaller than the corresponding bulk exciton Bohr radius. It has been reported that semiconductor nanorods in the quantum confinement regime have been synthesized through the surfactantcontrolled rod-growth approach.19-27 The anisotropic growth of the nanorods occurs primarily because the different crystal faces of a growing nanocrystal have various surface energy, which enable different binding strengths with surfactant. Following adsorption of the surfactant, the crystal “faces” with lower binding energy will grow more rapidly than those with higher binding energy.28 However, such a method for nanorod formation simultaneously needs the following conditions: two or more surfactants that adsorb differently, high monomer concentrations, and high growth rates. Moreover, uniformity of the nanorods along the long axis has been a problem, and branching has often occurred.20,28-30 Oriented attachment growth as another promising growth approach for nanorods/wires within the quantum confinement r 2011 American Chemical Society

regime refers to the phenomenon that generates nanorod/wires by attaching existing dot-shaped nanocrystals along a given crystal orientation. Since the mechanism was proposed,31 it has successfully accounted for the formation of several systems, such as CdTe32,33 and PbSe nanowires34 and ZnO35 and ZnSe nanorods.36,37 In 2002, Tang et al.32 reported the growth of CdTe nanowires by spontaneous organization of single CdTe nanoparticles upon controllable removal of the protective shell of organic stabilizer. Since the nanorods were formed by attachment of the preproduced nanoparticles at room temperature, it took a long time (several days) and it was difficult to control the uniformity of the products, especially nanowire length. As a modification of the classical colloidal chemistry method for the formation of quantum dots, Peng et al.38 synthesized CdSe nanowires through oriented attachment at relatively low temperature (100-180 °C) in a single type or two different types of amines, which required injection of the Cd and Se precursors into the solution. The diameters of CdSe nanowires produced were within in the strong quantum confinement size regime. Jung et al.39 produced quantumsized cubic ZnS nanorods in hexadecylamine, which was also based on precursor injection technique (diethyl zinc was rapidly Received: October 31, 2010 Revised: January 1, 2011 Published: February 01, 2011 2258

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Langmuir injected into sulfur solution). The nanorods were attached from the nanoparticles produced at high temperature (300 °C), involving aging in oleylamine at low temperature (60 °C). As the above methods mentioned, it is necessary to prepare pre-existing32,33,35 or preproduced quantum dots38,39 in the first step used as “seeds” to form quantum wires. This made the synthesis complicated and difficult to control. Recently, Thoma et al.28 synthesized short CdSe nanorods with about 12 nm length using a single-source precursor at low reaction temperature (100 °C), while the CdSe nanorods produced were not “perfect” with “necks” contained in the individual nanorods, and the process took a relatively long time (72 h). Therefore, it is essential to further explore new methods based on the oriented attachment mechanism to prepare controllable quantum-sized nanorods. In this paper, we report the one-step synthesis of quantumsized CdS nanorods by direct thermal decomposition of a singlesource precursor, (Me4N)4[Cd10S4(SPh)16], in a single surfactant. The obtained CdS nanorods were uniform and had high crystallinity, and exhibited a strong quantum confinement effect. The effect of various reaction conditions on the nanorod growth was investigated, and the corresponding formation mechanism was discussed.

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Figure 1. TEM image of CdS nanorods synthesized in HDA.

’ EXPERIMENTAL SECTION Materials. Methanol and toluene used in the present study were of analytical grade. Hexadecylamine (HDA, 90%) was purchased from Aldrich Chemical and used without further purification. Cd(NO3)2 3 4H2O, triethylamine, acetonitral, sulfur, and tetramethylammonium chloride were purchased from Aldrich Chemical. (Me4N)4[S4Cd10(SPh)16] (SPh is phenyl thiolate) was prepared by the reported method.40 Synthesis of CdS Nanorods. The synthesis of CdS nanorods from cluster precursors was carried out in a modified manner as reported previously for the synthesis of CdS nanoparticles from (Me4N)4[Cd10S4(SPh)16].41 In a typical experiment, about 25 g of HDA was degassed under vacuum at 120 °C. While stirring, 1.2 g of (Me4N)4[Cd10S4(SPh)16] was added into HDA solution at 80 °C under N2 condition. The temperature of the reaction mixture was raised to 220 °C at a rate of 1.0 °C/min. After 5 h at 220 °C, the solution was cooled naturally to about 60 °C. Then, an excess of methanol (50 mL) was added, and the flocculant precipitate formed. The solid was separated by centrifugation (5000 rpm) and redispersed in toluene. The above procedure of centrifugation and isolation was then repeated several times for purification of the CdS nanorods. After purification, the dried samples are about 0.8 g (25 g/L). Characterization. Transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) were performed on a Tecnai S-Twin electron microscope operating at 200 kV. Samples for TEM and HRTEM were prepared by spreading a drop of dilute dispersion containing as-prepared products on amorphous carbon-coated copper grids and then drying at room temperature. X-ray powder diffraction (XRD) was recorded with a Rigaku D/max-γB diffractometer equipped with a rotating anode and a Cu KR source (λ = 0.154 056 nm). Samples for XRD measurements were solid powder. UV-vis absorption spectra were recorded at room temperature on a UV-2550 spectrophotometer. Samples were dispersed in toluene and placed in a 1 cm quartz cell. The photoluminescence (PL) spectrum was recorded at room temperature on an FP-6500 spectrophotometer. The excitation wavelength was set at 410 nm.

’ RESULTS AND DISCUSSION Synthesis of Quantum-Sized CdS Nanorods. The CdS nanorods were prepared on the large scale by direct thermal decomposition of the single-source precursor in HDA. Figure 1 shows the TEM

Figure 2. X-ray diffraction pattern of the CdS nanorods.

image of the nanorods. It can be seen that the products are uniform and straight nanorods with an aspect ratio of about 10:1. The average diameter of the nanorods is 4 nm, which is in the quantum confinement regime. The synthesized nanorods could be well-dispersed in organic solvents such as toluene (as shown in Figure 1 inset). From the HRTEM image (Figure 1 inset) of individual CdS nanorods, it can be observed that the space between adjacent planes is 0.336 nm, which is consistent with the interplanar distance of the (002) planes of hexagonal CdS, indicating that the nanorods are elongated along the c-axis. Figure 2 shows the XRD pattern of the CdS nanorods. The CdS diffraction pattern shows obvious broadened peaks compared to those of the bulk CdS crystals. The XRD pattern is consistent with predominantly the hexagonal phase. The (110), (103), and (112) planes of wurtzite CdS are clearly distinguishable in the pattern (JCPDS file no. 41-1049). From the XRD pattern, a stronger and narrower (002) peak is observed in contrast to other peaks, and it is also stronger than that of nanoparticles with 4 nm diameter (as shown in Supporting Information Figure S1)). This further confirms that the nanorods are elongated along the c-axis. Figure 3 shows the room-temperature UV-vis absorption (a) and PL (b) spectra of the as-synthesized CdS nanorods. From the absorption spectrum (Figure 3a), it can be seen that the excitonic 2259

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Figure 3. UV-vis absorption (a) and PL (b) spectra of CdS nanorods.

absorption peak is well-defined with a maximum value at 412 nm. It is well-known that the excitonic absorption peak is associated with the lowest optical transition. Therefore, it provides a simple way to determine the nanocrystal band gap. The first absorption peak of the as-prepared CdS nanorods is 412 nm, corresponding to a band gap of 3.01 eV. Compared with the value of bulk CdS (2.42 eV),42 the band edge of CdS nanorods is blue-shifted, which is indicative of quantum confinement of CdS nanorods. Figure 3b shows the PL spectrum of the CdS nanorods with the emission maximum at 590 nm. The PL spectrum has a large red shift compared to the absorption spectrum, which is indicative of the large Stokes shift of the nanorods. The red emission is assigned to the trap emission, which is observed in poorly passivated and/or defect-containing materials. In the as-synthesized products, the lack of stoichiometry (the ratio of Cd to S is 1.2:1) determined by XPS (Supporting Information Figure S2) can cause surface defects on the CdS nanorods. Therefore, the trap emission observed from the CdS nanorods is ascribed to the surface defects. In the previous work,41 we synthesized CdS quantum dots with low precursor concentration (1.0 g (Me4N)4[Cd10S4(SPh)16] in 55 g HDA) by a similar procedure. The hexagonal CdS quantum dots with average of 4.3 nm diameter were produced at 220 °C for 3 h. At that precursor concentration, the products are still quantum dots even though the reaction time was prolonged to 7 h (Supporting Information Figure S3). While increasing the precursor concentration, the products are nanorods as shown in Figure 1. These results demonstrate that the morphology of the CdS products can be controlled by adjusting the reaction conditions. Thus, it is necessary to investigate the influence of precursor concentration and reaction time on the CdS growth in the following sections. Effect of Precursor Concentration on CdS Nanorod Growth. In order to investigate the effect of precursor concentration on the growth of CdS nanorods, the precursor concentration was as follows: 1.0 g precursor/60 g HDA, 1.0 g precursor/30 g HDA, 1.0 g precursor/20 g HDA, and 2.0 g precursor/20 g HDA, respectively. All the samples for measurements are used as prepared, without any further purification. Figure 4 shows the TEM images of the CdS samples obtained at different precursor concentrations. It can be seen that the obtained products are quantum dots at low precursor concentration (1.0 g precursor/60 g HDA, as shown in Figure 4a). The average particle size of the quantum dots is about 4.0 nm. After increasing the precursor concentration to 1.0 g precursor/30 g HDA, the products mainly contained nanodots with a small quantity of nanorods (about 30%). It can be observed from Figure 4b that,

Figure 4. TEM images of CdS samples prepared at different precursor concentrations: (a) the CdS samples are nanodots at 1.0 g precursor/ 60 gHDA; (b) the CdS samples contain nanodots and nanorods at 1.0 g precursor/30 g HDA; (c) the CdS samples contain nanodots (I), “oligomers” (II), and nanorods (III) at 1.0 g precursor/20 gHDA; (d) the CdS sample are uniform nanorods at 2.0 g precursor/20 g HDA uniform nanorods.

although the length of the obtained nanorods is not uniform, the width of nanorods is identical to the diameter of the nanodots (about 4.0 nm). As the precursor concentration is further increased to 1.0 g precursor/20 g HDA, quantum dots (I), “oligomers” (II) (known as “string-of-pearls” in the literature32,38), and nanorods (III) are formed as shown in Figure 4c. It is interesting to note that all three morphologies are in the diameter of 4.0 nm. At high concentration (2.0 g precursor/20 g HDA), uniform and straight nanorods are produced and the diameters are also about 4.0 nm, as observed from TEM images (Figure 4d). These results demonstrate that precursor concentration significantly influences the morphology of the CdS products. Increasing precursor concentration is favorable for the formation of CdS nanorods. On the basis of the above results, it can be reasonably deduced that the nanorods are aggregated from nanodots during the synthesis process, which we call the oriented attachment. This mechanism involves two stages including formation of nanodots and aggregation of a string of nanodots. The appearance of “oligomers”32,34,35 as shown in Figure 4c has been considered key evidence of the oriented attachment mechanism. Thus, during the process of thermal decomposition of the precursor, the nanodots were first produced, and then, the nanodots attached to each other to form nanorods. It can be proven from Figure 4b,c that the diameters of the nanodots, “oligomers”, and nanorods are almost identical at the same reaction conditions. It has been proven that dipolar attraction between semiconductor nanodots could direct their oriented attachment into a linear chain.32,34,39 The inherent anisotropy of crystal structure or crystal surface reactivity was identified in previous studies32,33 as the driving force for the one-dimensional growth. Giersig and co-workers have shown that the well-defined facets of Ag nanoparticles may have different polarizability and reactivity, which leads to the oriented formation of Ag nanowires.43 In the case of quasi-spherical crystalline CdS nanodots, the lack of central 2260

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Langmuir symmetry due to the distribution of polar facets may result in dipolar attraction along the polar faces of nanodots. For the CdS nanocrystals (wurtzite structure), the (002) plane is polarized, which leads to dipolar attraction of CdS nanoparticles along the c-axis. It is well-known that the dipole-dipole attraction is favored for the oriented attachment of nanodots, but there is another steric or electrostatic repulsion to prevent nanodot aggregation. The steric repulsion originates from the stabilizing agent molecules on the nanodot surface (such as HDA), which results in the repulsion of nanodots from each other. On the nanodot surfaces, the stabilizing agent molecules maintain a dynamic equilibrium during the absorption and detachment process at a certain temperature. At high reaction temperature, the absorption and detachment rate of stabilizing agent molecules on the nanodot surface is accelerated. The acceleration of the absorption and detachment rate of molecules reduces the mutual electrostatic repulsion of nanoparticles. When the attractive force between nanodots dominates over the steric repulsion of molecules, the oriented attachment of nanodots occurs during the synthesis process. Thus, the product is determined by the repulsion and attractive forces between nanoparticles. At low precursor concentration, the dipolar attraction between nanodots is weaker, and the steric repulsion is predominant. At this condition, nanodots are obtained as shown in Figure 4a. For all the above synthesis, the experimental condition is the same with the exception of precursor concentrations. It has been demonstrated that the nanodots produced under the same reaction temperature and time have the same size, indicating that the concentration of nanodots in the solution is increased by increasing precursor concentration. It can be assumed that the increase of nanodot concentration in solution should strengthen the attractive forces between nanodots. Therefore, the attractive force is increased gradually by increasing the precursor concentration, consequently leading to the formation of oligomers and nanorods (Figure 4b,c) based on oriented attachment. When the dipolar attraction becomes predominant by increasing the precursor concentration to 2.0 g precursor/20 g HDA, uniform and straight nanorods are produced (Figure 4d). The very specific ordering of nanodots into linear chains is mainly attributed to the directional dipolar attractions originating from the opposite polarity of the crystal planes. Thus, it can be reasonably concluded that the competition between dipolar attraction and steric repulsion influences the growth of the nanocrystals. This phenomenon can be further proven by the following low-temperature reaction. In comparison with the hightemperature reaction (220 °C) under the same precursor concentration, pearl-necklace-shaped nanorods were obtained at 150 °C as shown in Figure 5. Under low temperature, the absorption and detachment rate of HDA molecules on the nanodot surface is decreased, which leads to an increase in steric repulsion. Therefore, well-defined smooth nanorods could not be formed in the low-temperature condition. These observations suggest that the nanorods are assembled from individual nanodots via oriented attachment mechanism, rather than the multiple surfactant-mediated routes. It is conceivable that the impurities in the HDA may act as a second surfactant, yet this is far below the weight fractions typically associated with surfactant-mediated anisotropic growth. One might also suggest that the reaction byproduct (such as thiophenol) might act as a second surfactant. Although it has recently been shown that thiophenol can coadsorb with HDA, it does not play a role as a second surfactant in a face-selective

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Figure 5. TEM image of pearl-necklace-shaped structure formed at 150 °C.

fashion.44 Such suppositions have been proven by Thoma28 et al., who performed a similar synthesis using a 90/10 wt % HDA/ TOPSe mixture and obtained pearl-necklace-shaped CdSe nanorods, which is similar to the results obtained in HDA. The above experiment demonstrates that the presence of impurity, even up to 10 wt %, could not alter the reaction product. On the basis of these results, it can be concluded that the growth of anisotropic nanostructures is dependent on the choice of precursor as well as the synthetic procedure. The surface chemical composition of CdS nanorods is studied by FTIR (Supporting Information Figure S4) and XPS (Figure S2). The results show that the surface of the nanorods is capped by the HDA molecule and no other surfactant molecule can be observed, which indicates that the absorption nature of HDA is favorable for the formation of anisotropic morphology under high precursor concentration. With these observations taken into account, HDA is used as solvent without any purification in the experiment. This provides a facile, low-cost, and high-yield method to synthesize CdS nanorods. Figure 6 shows the room-temperature UV-vis absorption of the obtained samples at different precursor concentrations. Welldefined absorption spectra can be seen with maximum peaks at 406 nm (Figure 6 a), 410 nm (Figure 6 b), 411 nm (Figure 6 c), and 425 nm (Figure 6 d), corresponding to band gaps of 3.05 eV, 3.02 eV, 3.02 eV, and 2.92 eV, respectively. It is clearly demonstrated that all four samples exhibit a strong quantum confinement effect. From the absorption spectra, second exciton bands with less-defined features can also be observed, which could be assigned to the higher spin-orbit component of the 1s-2s transition of CdS nanocrystals. The sharp first absorption peak and the observation of higher-energy second excitonic transitions in the absorption spectra are indicative of monodispersed nanocrystals.45 Effect of Reaction Time on the Growth of CdS Nanorods. As the dipolar attraction is a kind of long-distance interaction, the formation process of oriented CdS nanorods should take time to achieve. Since three different CdS nanostructures are formed for the sample at 1.0 g precursor/20 g HDA precursor concentration, it provides a suitable object to explore the effect of reaction time on the growth process. In order to study the effect of reaction time on the growth of nanocrystals, the reaction time was set to 0 h, 1 h, 3 h, 5 h, and 7 h at 220 °C. Figure 7 shows TEM images of CdS samples prepared at different reaction times. It can be seen that the ratio of nanorods increases with increasing reaction time. When the reaction is up to 7 h, uniform nanorods with 4.0 nm diameter are formed. The length of the nanorods is increased with reaction time, implying that the remaining nanodots are 2261

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Figure 6. UV-vis spectra of the CdS samples synthesized at different precursor concentrations: (a) 1.0 g precursor/60 g HDA; (b) 1.0 g precursor/ 30 g HDA; (c) 1.0 g precursor/20 g HDA; (d) 2.0 g precursor/20 g HAD.

attached to the nanorods. From TEM observation, it should be noted that the diameter of the nanodots and nanorods is about 4 nm with different reaction times (Figure 7a-d), which is within the typical strong quantum confinement size regime of the CdS nanocrystals. The UV-vis spectra of the samples with different reaction times are shown in Figure 8, demonstrating the obvious quantum confinement effect. It can also be seen that the absorption peaks are shifted from 410 nm to 416 nm when the reaction time is less than 3 h and varies slightly from 416 nm to 418 nm as the reaction time increases above 3 h. TEM results show that the products contain a large quantity of nanodots when the reaction time is less than 3 h. The diameter of CdS nanoparticles increases from 3.8 nm to 4.1 nm when the reaction time increases from 0 h to 3 h, while the nanoparticle size increases slightly with the prolongation of the reaction due to monomer consumption by the reaction.41 Therefore, it can be deduced that the red shift of the absorption peak of the samples is attributed to the variation of CdS nanoparticle size. The evolution of CdS nanocrystal growth with different times clearly demonstrates that the formation of CdS nanorods is an oriented attachment of nanodots. The above observations suggest that the formation process of CdS nanorods can be roughly divided into two stages: the nanodots are first produced, and then, the nanodots orient and attach to form nanorods. During the latter stage, the nanodots aggregate to form linear particle chains, in which the (002) planes of the CdS nanocrystals are almost perfectly aligned, as shown in the Figure 1 inset. On the basis of TEM images from intermediate stages (Figure 9), a closer inspection of selected parts of the nanoparticle aggregation shows that at some points there is adhesion of adjacent nanoparticles, finally leading to the well-defined nanorods. It has been shown that similar phenomena with both Ag43 and ZnO35 nanoparticles occurred when these were crystallographically well oriented. In the present study, HRTEM clearly shows that there is a random orientation of the crystallographic faces, as can be seen in Figure 9a. The (002) plane of the first particle and the corresponding plane in the adjacent particle are not aligned. However, fusion only takes place when the suitable facets are facing each other, as shown in Figure 9b. The particle chains further coalesce to form straight smooth nanorods, and the remaining

Figure 7. TEM images of CdS samples synthesized at different reaction times at 220 °C: (a) 0 h, (b) 1 h, (c) 3 h, (d) 5 h, (e) 7 h.

Figure 8. UV-vis spectra of the CdS samples synthesized at 220 °C reacting for different reaction times: (a) 1 h, (b) 3 h, (c) 5 h, (d) 7 h.

nanodots are attached to the nanorods as the reaction proceeds to lengthen the nanorods, as shown in Figure 9c. Mani E. and Rajdip B.47 have developed a model by a modified Brownian collision frequency to explain the formation of nanorods from a colloidal suspension of spherical nanodots. The results reveal that linear pearl-chain aggregates by the oriented attachment of nanodots during the early stages of synthesis, since it occurs on a 2262

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Figure 9. HRTEM images of particle chains by CdS nanodot oriented attachment via aggregation processes (a) and (b). HRTEM image of nanorods formed by fusion of several nanodots (c).

Figure 10. TEM image of the as-prepared CdS nanorods at 220 °C for 8 h by thermal decomposition of 1.2 g precursor in 20 g HDA.

time scale smaller than the coalescence time scale of nanodots present within an aggregation. The slower coalescence step leads to the transformation of the linear pearl-chain aggregate into a smooth nanorod over a longer time scale of many hours. According to the calculated results, during the CdS nanorod formation process, a mixture of nanodots and linear aggregations can be observed, because the time scale of oriented attachment of nanodots into linear chains is smaller than the coalescence time of nanodots. Thus, it can be observed that the nanodots and nanorods are coexist in the sample when the reaction time is less than 5 h (Figure 7a-d). With the reaction proceeding longer than 5 h, well-defined smooth nanorods are obtained (Figure 7e). The formation time for CdS nanorods at high temperature is less than that at room temperature (several days for CdTe nanowires)32 or at low temperature (100 °C, 72 h for CdSe nanowires).28 In conclusion, the nanorod formation process could be clarified as follows: Nanodots are produced first by thermal decomposition of the precursor in HDA. When the precursor concentration is low, the nanodots cannot aggregate with each other, and quantum dots are obtained. As the precursor concentration increases above 1.0 g precursor/30 g HDA, nanodots are oriented and attached to form a pearl-necklace structure, which further coalescences to form straight smooth nanorods. The dipolar attraction is the driving force for the formation of nanorods, while steric repulsion also influences the growth process. Thus, the final morphology of the products is determined by the competition between dipolar attraction and steric repulsion. The smooth and uniform quantum-sized CdS nanorods could be synthesized by increasing the reaction time within a certain precursor concentration range. Figure 10 shows the nanorods obtained by thermal decomposition of 1.2 g precursor in 20 g HDA at 220 °C for reaction time of 8 h. The nanorods are uniform and straight with an aspect ratio of 12:1.

’ CONCLUSION In summary, quantum-sized CdS nanorods were synthesized in one step by direct thermal decomposition of (Me4N)4[Cd10S4(SPh)16] in HDA. The CdS nanorod formation obeys an oriented attachment mechanism, and the formation process is influenced by the reaction conditions. The well-defined straight nanorods can be prepared by controlling the competition between dipolar attraction and steric repulsion of nanodots. By increasing the precursor concentration, the dipolar interaction between nanodots dominates over the steric repulsion to produce nanorods at high temperature (above 200 °C), and uniform nanorods were obtained when the reaction was carried out up to 7 h. The approach, without an injection procedure, allows largescale preparation of CdS nanorods with diameters in the quantum confinement range. These nanorods can form stable colloidal dispersions and provide for easy solution processing and device integration. ’ ASSOCIATED CONTENT

bS

Supporting Information. XRD patterns of CdS nanoparticles and nanorods, XPS and FTIR spectra of the nanorods, as well as TEM image of CdS quantum dots obtained with low precursor concentration at 7 h. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail addresses: [email protected]; [email protected].

’ ACKNOWLEDGMENT Financial support was given by the Excellent Youth Foundation of Heilongjiang Province of China (No. JC200715), the Fundamental Research Funds for the Central Universities (No. DL09BB44), and the National Natural Science Foundation of China (No. 31000270). ’ REFERENCES (1) Holmes, D.; Doty, R. C.; Johnston, K. P.; Korgel, B. A. Science 2000, 287, 1471. (2) Hu, J.; Li, L.-S.; Yang, W.; Manna, L.; Wang, L. W.; Alivisatos, A. P. Science 2001, 292, 2060. (3) Wang, J.; Gudiksen, M. S.; Duan, X.; Cui, Y.; Lieber, C. M. Science 2001, 293, 1455. (4) Hu, J.-T.; Odom, T. W.; Lieber, C. M. Acc. Chem. Res. 1999, 32, 435. (5) Hanrath, T.; Korgel, B. A. J. Am. Chem. Soc. 2002, 124, 1424. (6) Xia, Y.; Yang, P.; Sun, Y.; Wu, Y.; Mayers, B.; Gates, B.; Yin, Y.; Kim, F.; Yan, H. Adv. Mater. 2003, 15, 353. (7) Lee, S. M.; Cho, S. N.; Cheon, J. Adv. Mater. 2003, 15, 441. 2263

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dx.doi.org/10.1021/la1043552 |Langmuir 2011, 27, 2258–2264