ZnS Nanoparticle-Assisted Synthesis and Optical Properties of ZnS

ABSTRACT: Tower-like ZnS nanostructures are facilely synthesized via a ZnS nanoparticle-assisted vapor deposition route. Scanning electron microscopy ...
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CRYSTAL GROWTH & DESIGN

ZnS Nanoparticle-Assisted Synthesis and Optical Properties of ZnS Nanotowers

2007 VOL. 7, NO. 8 1459-1462

Yugang Zhang,* Fang Lu, Zhenyang Wang, Huixin Wang, Mingguang Kong, Xiaoguang Zhu, and Lide Zhang* Key Laboratory of Materials Physics, Anhui Key Lab of Nanomaterials and Nanotechnology Institute of Solid State Physics, Chinese Academy of Sciences, Hefei 230031, People’s Republic of China ReceiVed January 7, 2007; ReVised Manuscript ReceiVed May 30, 2007

ABSTRACT: Tower-like ZnS nanostructures are facilely synthesized via a ZnS nanoparticle-assisted vapor deposition route. Scanning electron microscopy and transmission microscopy observations show that the tower-like structures are likely to be made up in a layer-by-layer fashion and consist of quasi-hexagonal plates. The growth of nanotowers is initiated by the preferred nucleation on the ZnS nanoparticles precoated on Si substrate, followed by alternate two-dimensional growth of hexagonal plates/layers inside nanotowers and one-dimensional stacking in the direction perpendicular to the plates/layers via vapor-solid growth processes. Such nanotowers, due to the existence of many interfacial defects, display distinct photoluminescence and Raman spectra from nanoplates prepared without the assistance of ZnS nanoparticles. The photoluminescence spectra of the nanotowers show two strong emission bands located at 394 and 592 nm. In Raman spectra, two anomalous modes at 233 and 488 cm-1 are observed. 1. Introduction With the ongoing miniaturization of microelectronics, lowdimensional nanoscale building blocks, such as nanoparticles, nanotubes, nanowires, nanobelts, and nanosheets, have attracted extensive interest because of their remarkable physical and chemical properties and the potential to revolutionize broad areas of nanotechnology.1-3As an important semiconductor with a direct wide band gap of 3.91 eV, ZnS has attracted considerable research interest due to its extensive usage in thin-film electroluminescent devices, infrared windows, flat panel displays, sensors, and lasers.4 So far, extensive efforts have been made on the synthesis of low dimensional ZnS nanostructures,5 including nanoparticles,6 nanowires,7 nanobelts,8-10 nanocables,11 and nanotubes.12 In comparison to the large amount of work on low dimensional nanostructures, there are relatively few reports on complex nanostructures, although these structures are highly desirable for many applications.13-16 In this work, single crystalline tower-like complex ZnS structures are formed via a ZnS nanoparticles-assisted vapor deposition route. We demonstrate that the growth of nanotowers is initiated by the preferred nucleation on the ZnS nanoparticles precoated on Si substrate, followed by alternate two-dimensional (2D) growth of hexagonal plates/layers inside nanotowers and one-dimensional (1D) stacking in the direction perpendicular to the plates/layers via vapor-solid (VS) growth processes. Such ZnS nanotowers display new photoluminescence emission bands and anomalous Raman scattering modes compared with nanoplates prepared without the assistance of ZnS nanoparticles. 2. Experimental Section The ZnS nanotowers were synthesized through an improved vapor deposition route in a horizontal tube furnace. ZnS powder (1 g, 99.999%) loaded in an alumina boat was placed in the central region of a ceramic tube, and Si wafer, which was spin-coated with a thin wurtzite ZnS nanoparticle film, was used as the substrate. The ZnS nanoparticle was synthesized by the method reported by Zhao.6 The tube was purged by high-purity argon for 3 h to eliminate oxygen in the tube, and then the argon was switched off. The system was then * To whom correspondence should be addressed. E-mail: ygzhang@ issp.ac.cn (Y.Z.); [email protected] (L.Z.).

heated to 1200 °C and maintained at this temperature for 30 min. During the peak heating temperature, argon was introduced into the reaction system as a carrier gas at 10 sccm. The desired samples were deposited onto the Si substrate, which was about 14 cm away from the source material, and the local temperature was about 800 °C. The assynthesized products were characterized using X-ray diffraction (XRD, Philips X’Pert, Cu KRline: 0.15419 nm), field emission scanning electron microscopy (FE-SEM, Sirion 200), and high-resolution transmission electron microscopy (HRTEM) (JEOL-2010). Photoluminescence (PL) and Raman spectra were recorded using LABRAMHR micro-Raman spectrometer (Jobin-Yvon) excited with the 325 nm He-Cd laser or 514.5-nm Ar+ laser.

3. Results and Discussion 3.1. Structural Characterization. Figure 1a shows a SEM image of the products synthesized on the ZnS nanoparticlescoated Si substrate. It can be seen that there are numerous towerlike nanowhiskers with sharp tips or flat tops over the entire surface of the substrate. Detailed observation (Figure 1b,c) shows that the nanowhiskers are of layer-by-layer tower-like structures. Such structure has a length of about 400-600 nm and a diameter of about 200-300 nm in the middle. The diameters gradually become smaller along the axial direction from the bottom to the top, leading to the formation of the tapered structures with sharp tips (Figure 1b) or flat tops (Figure 1c). Each layer of the nanotower has quasi-hexagonal shape and uniform thickness of about 30-40 nm. The crystalline phase of the as-synthesized products was identified by the XRD pattern shown in Figure 1d1. All diffraction peaks can be readily indexed to the wurtzite ZnS crystal structure, compared with standard diffraction of wurtzite ZnS powders (JCPDS Card 36-1450) shown at the bottom of Figure 1d. The microstructure of the as-synthesized ZnS nanotowers was studied by HRTEM and electron diffraction (ED). Figure 2a gives the TEM image of a single nanotower with a lateral dimension of 450-500 nm, of which the terrace-like side profile just reflects its layered structure. Figure 2b shows the HRTEM image taken from one layer (marked with a white square in Figure 2a) of the nanotower. The planar spacing is about 0.31 nm, which corresponds to the spacing for the (0002) planes of the ZnS wurtzite structure. The inset image in Figure 2b shows the corresponding selected area electron diffraction (SAED)

10.1021/cg0700162 CCC: $37.00 © 2007 American Chemical Society Published on Web 07/12/2007

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Figure 1. Low-magnification (a) and high-magnification (b, c) SEM images of the as-synthesized nanotowers. (d1-4) correspond to XRD patterns of nanotower, products only using nanoparticle film as precursor, nanoplates, and annealed nanotower, respectively. The bottom in (d) shows the standard diffraction of wurtzite ZnS powders.

Figure 2. (a) TEM image of a single ZnS nanotower. (b) HRTEM image of part from one layer in the nanotower (marked by a white square in a) and its corresponding selected area diffraction pattern (inset). (c) HRTEM image of the interlayer part of the nanotower (marked by a white rectangle in a).

Figure 3. (a, b) Schematic illustration for the growth mechanism of the ZnS nanotowers.

pattern, which can be indexed to that of the [011h0] zone axis of the hexagonal ZnS structure. Figure 2c shows the HRTEM image of the interlayer part (marked with a white rectangle in Figure 2a) of the nanotower. It can be seen that each layer of the nanotower is of a single crystalline ZnS wurtzite structure with (0002) planes parallel to each other. In addition, there are many defects at joints of the layers, and the lattice fringes are not well discernible. 3.2. Growth Mechanism. The growth of the ZnS nanotowers could be ascribed to the VS mechanism,17 ruling out the traditional vapor-liquid-solid mechanism because no alloy drops are observed on the top of the nanostructures in the SEM and TEM images. According to the above, it is proposed that

the growth of the nanotowers may proceed in two steps, namely, preferred nucleation and alternate 2D growth and 1D stacking, as illustrated in Figure 3. First, at the high-temperature zone, ZnS vapor is produced by evaporation of ZnS powder. In the course of transferring, the supersaturation degree of ZnS increases together with the decreasing of the temperature, which leads the ZnS molecules to prefer to condense on the precoated ZnS nanoparticles that could serve as nuclei. Subsequently, at the proper growth temperature zone, the ZnS nanoparticles would preferentially grow along two equal 〈101h0〉 directions into 2D hexagonal nanoplates. Under stable conditions, the central position of a formed hexagonal ZnS nanoplates may be a new favored

Synthesis and Optical Properties of ZnS Nanotowers

Figure 4. (a) SEM image of products only using nanoparticle film as the precursor and the hexagonal cross-section (inset). (b) Lowmagnification of nanoplates. (c) High-magnification of nanoplates and hexagonal cross-section (inset). (d1) TEM image of a single ZnS nanoplate, its corresponding selected area diffraction pattern (d2), and its HRTEM image (d3).

nucleation site to form the next nanoplate. In this way, the plates/ layers tend to grow into a hexagonal shape. Simultaneously, in the direction perpendicular to the plates/layers, the plates/layers prefer to stack along the 〈0001〉 zone axis of the wurtzite ZnS structure. Such a process is repeated again and again, leading to a hexagonal, layered morphology. Subsequently, ZnS vapor decreases gradually during growth, while the bottom plates/ layers have a longer growth time. As a result, tapered-structural nanotowers with gradually decreasing diameters from the bottom to the top come into being.18 To validate our proposition, we prepared samples following the same method mentioned in the experimental section while without ZnS powder. The products are confirmed as the wurtzite structure of ZnS by XRD pattern, as shown in Figure 1d2. According to the SEM observations (see Figure 4a), the original small nanoparticles on the Si substrate became larger and grew into hexagonal prism-like particles with diameters about 100150 nm, which is smaller than those of nanotowers. The inset

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in Figure 4a clearly shows that the prism has a regular hexagonal top surface, consistent with the symmetry characteristic of ZnS along the 〈0001〉 direction, indicating that this surface is plane {0001}. These hexagonal {0001} surfaces of the particles would be serve as favored nucleation sites for ZnS vapor, since ZnS is a kind of anisotropic crystal with polar plane {0001} that has higher surface energy than that of low-index nonpolar surfaces, such as {21h1h0} and {011h0}. Of course, when using ZnS powder as the source material, the sizes of particles become larger during the growth of nanotowers, which is consistent with the experimental results. To further study the growth habit of ZnS at the designed temperature zone, we prepared another sample following the same process except that there is no ZnS nanoparticle film on the Si substrate. Figure 4b,c shows the SEM images of the products, which show that platelike nanostructures with a thickness about 20 nm are formed on the substrate. The inset in Figure 4b shows that the nanoplates have a hexagonal cross-section. The products are also confirmed as a wurtzite structure of ZnS by XRD measurement (see Figure 1d3). Figure 4d1 shows the TEM image of a single fractured ZnS nanoplate, which was broken by ultrasonication. The ED pattern taken from the plate was shown in Figure 4d2, which can also be indexed to that of the [0001] zone axis of the ZnS wurtzite crystal. The HRTEM image (see Figure 4d3) shows that the planar spacing is about 0.33 nm, which corresponds to the spacing for the (101h0) planes of the ZnS wurtzite structure. The results confirm that ZnS crystal prefers to grow along 〈101h0〉 directions into 2D nanostructures at the designed temperature zone. 3.3. Photoluminescence. The PL properties of the assynthesized ZnS nanotowers were studied at room temperature using a PL spectrum excited with a 325-nm He-Cd laser. For comparison, the PL spectra of ZnS nanoplates and nanotowers annealed in Ar at 600 °C for 6 h were also measured, and all the results are shown in Figure 5a. The XRD pattern of the annealed nanotowers is given in Figure 1d4, which confirms that they are also wurtzite structure of ZnS. Both nanoplates and nanotowers exhibit two emission bands. A multipeak Gauss fit shows that the ZnS nanoplates have a strong blue emission band centered at about 423 nm (2.93 eV) and a weak and broad green emission band centered at about 510 nm (2.43 eV). The ZnS nanotowers show a strong yellow emission band centered at about 592 nm (2.09 eV) besides a weak violet emission band centered at about 394 nm (3.14 eV). While for the nanotowers annealed in Ar, the violet emission becomes stronger, whereas the yellow emission becomes very weaker. The violet, blue, and green emission has been attributed to the recombination of electrons from the energy level of sulfur vacancies (neutral donor) with the holes from the valence band,19 surface state,7,20,21 and some point defects, such as isolated Zn vacancies in the single negative charged state (neutral acceptor),22 respectively. Although nanotowers and nanoplates are both wurtzite ZnS structures, only the nanotowers exhibit the yellow emission; hence, this yellow emission could not be ascribed to impurities. It was well-known that there were large amounts of defects, such as dangling bonds, at the interfaces of nanomaterials, which may generate additional energy levels in the band gap.23 For the ZnS nanotowers, as observed from the HRTEM image (see Figure 2c), there are many structural defects at the joints of their layers. It is therefore reasonable to believe that additional defect energy levels in the band gap might be generated by these interfacial defects. The e-h recombination at these centers leads to the yellow emission in the ZnS nanotower. Our assignment is verified by the very weak yellow emission from the annealed

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4. Conclusions In summary, single crystalline wurtzite ZnS nanotowers have been synthesized by a developed ZnS nanoparticle-assisted vapor deposition method. The preferred nucleation and subsequent alternate 2D growth and 1D stacking are responsible for the formation of the nanotowers. The anomalous characteristics that appeared in both the PL and Raman spectra are closely related to the interfacial defects in the nanotowers. Such ZnS nanotowers are promising to be used to design advanced semiconductor nanodevices such as optoelectronics and gas sensors. This nanoparticles-assisted technique will be constructive to better understand the behavior of ZnS nanostructures in a vapor-phase synthetic system. It represents an effective strategy to construct layered structures of some other materials with new properties. Acknowledgment. This work was financially supported by the National Major Project of Fundamental Research: Nanomaterials and Nanostructures (Grant No. 2005CB23603), the Special fund for President Scholarship, Chinese Academy of Science, and the National Natural Science Foundation of China (Grant No. 90406008). References

Figure 5. (a) Room-temperature PL spectra and (b) Raman spectra of the as-synthesized samples. (1-3) correspond to ZnS nanoplates, nanotowers, and annealed nanotowers, respectively.

ZnS nanotowers because of the decrease of interface defect density by annealing. 3.4. Raman Scattering. Raman scattering was performed at room temperature to investigate the crystal quality and vibration 4 properties of ZnS nanotowers. Wurtzite ZnS belongs to the C6V space group, with two formula units per primitive cell. At the Γ point of the Brillouin zone, the normal lattice vibration modes are predicted on the basis of group theory: Γopt ) A1(z) + 2B1 + E1(x, y) + 2E2.24 Among these, E1, E2, and A1 are the firstorder Raman active modes, and B1 is forbidden. Moreover, the E1 and A1 modes split into LO and TO components. Figure 5b2 gives the Raman spectrum of the ZnS nanotowers. The Raman spectra of nanoplates and annealed nanotowers are also given in Figure 5, panels b1 and b3, respectively. It can be seen that all the modes of ZnS nanoplates are attributed to the vibration of wurtzite ZnS crystal,25,26 and the identification of these modes is listed on the top left in Figure 5b. Compared with the modes of nanoplates, two additional peaks at 233 and 488 cm-1 are observed in the spectrum of ZnS nanotowers. These peaks have not been reported in the previous studies. As is known, when the lattice distortion or defects increase, the k ) 0 selection rule for the first-order Raman scattering is relaxed, and the phonon scattering will not be limited to the center of the Brillouin zone; hence, phonon dispersion near the Brillouin zone center should be considered as well. As a result, the symmetryforbidden modes will be observed.27,28 Therefore, the existence of large amounts of defects at the interfaces of nanotowers might activate the two abnormal vibration modes at 233 and 488 cm-1. After the nanotowers are annealed at annealed in Ar, the interfacial defect density decreases, leading to the decrease or even disappearance of the abnormal modes.

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