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The preparation and cathodoluminescence of ZnS nanowires grown by chemical vapor deposition
Applied Surface Science 261 (2012) 665–670
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Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
The preparation and cathodoluminescence of ZnS nanowires grown by chemical vapor deposition Meng-Wen Huang a , Yin-Wei Cheng b , Ko-Ying Pan c , Chen-Chuan Chang c , F.S. Shieu a , Han C. Shih a,b,c,∗ a b c
Department of Materials Science and Engineering, National Chung Hsing University, Taichung 402, Taiwan, ROC Institute of Materials Science and Nanotechnology, Chinese Culture University, Taipei 111, Taiwan, ROC Department of Materials Science and Engineering, National Tsing Hua University, Hsinchu 300, Taiwan, ROC
a r t i c l e
i n f o
Article history: Received 3 March 2012 Received in revised form 19 August 2012 Accepted 22 August 2012 Available online 28 August 2012 Keywords: Zinc sulfide Nanowires X-ray photoelectron spectrometer Cathodoluminescence
a b s t r a c t Single crystal ZnS nanowires were successfully synthesized in large quantities on Si (1 0 0) substrates by simple thermal chemical vapor deposition without using any catalyst. The morphology, composition, and crystal structure were characterized by field emission scanning electron microscopy (FESEM), X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), energy-dispersive X-ray spectroscopy (EDX), X-ray photoelectron spectroscopy (XPS), and cathodoluminescence (CL) spectroscopy. SEM observations show that the nanowires have diameters about 20–50 nm and lengths up to several tens of micrometers. XRD and TEM results confirmed that the nanowires exhibited both wurtzite and zinc blende structures with growth directions aligned along [0 0 0 2] and [1 1 1], respectively. The CL spectrum revealed emission bands in the UV and blue regions. The blue emissions at 449 and ∼581 nm were attributed to surface states and impurity-related defects of the nanowires, respectively. The perfect crystal structure of the nanowires indicates their potential applications in nanotechnology and in the fabrication of nanodevices. Crown Copyright © 2012 Published by Elsevier B.V. All rights reserved.
1. Introduction Since the discovery of carbon nanotubes [1], study of nanomaterials and their applications has been on a rise because these materials often demonstrate very different properties. Onedimensional semiconducting nanomaterials have been successfully synthesized and have attracted much attention and gained importance due to their special properties. Furthermore, nanomaterials with high-aspect-ratio structures and large surface areas offer exciting research possibilities to study and tap their novel physical or chemical properties, including their unique optical, magnetic and electronic characteristics [2–5]. As a result, the synthesis and characterization of one-dimensional metal oxide nanostructures have attracted considerable attention from the research community. Among the metal oxide materials, zinc sulfide (ZnS) is an important II B–VI group semiconductor with a wide direct band gap (3.7 eV at 300 K) and high refractive index. The luminescence properties of ZnS nanowires are eliciting great interest because of their potential applications in flat-panel displays [6], sensors, and lasers [7,8]. Recently, many novel methods of fabricating metal oxide nanomaterials have been reported. One-dimensional
ZnS nanostructures such as nanoparticles [9,10], nanorods [11,12], nanowires [13,14], nanobelts [15,16] and nanotubes [17] have been produced by different synthesis techniques including thermal evaporation [18], liquid crystal template method [19], and laser ablation [20]. In this study, we report a simple and low cost process to prepare high-purity ZnS nanowires with a high yield on silicon (1 0 0) substrates by chemical vapor deposition without the use of any catalysts. The nanowires were formed by the vapor–solid (VS) mechanism. This method can be used to continuously synthesize and produce single crystalline ZnS nanowires at a large scale. Furthermore, a series of experiments indicated that wurtzite (hexagonal) and sphalerite (face-centered cubic) phases coexisted in the as-synthesized product. Characterization by cathodoluminescence spectroscopy showed that these nanowires absorbed strongly in the near-UV to green region of the electromagnetic spectrum.
2. Experimental 2.1. Preparation of ZnS nanowires
∗ Corresponding author at: Institute of Materials Science and Nanotechnology, Chinese Culture University, Taipei 111, Taiwan, ROC. Fax: +886 2 28618701. E-mail address: [email protected] (H.C. Shih).
The synthesis was performed in a conventional horizontal quartz tube furnace. The single crystal ZnS nanowires were synthesized on silicon substrates by chemical vapor deposition. A pure
0169-4332/$ – see front matter. Crown Copyright © 2012 Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2012.08.079
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silicon (1 0 0) wafer was subjected to ultrasonic cleaning in ethanol for 30 min. Then, the wafer was treated with dilute HF (1 wt%) for 45 s and with deionized water and subsequently blow dried with nitrogen. High-purity Zn (0.1 g, Aldrich, −325 mesh, 99.99%), S (0.1 g, Aldrich, −325 mesh, 99.99%) and C (0.03 g, Aldrich, −325 mesh, 99.99%) powders, which acted as the precursor materials, were placed into an alumina boat and positioned at the constant temperature zone of the horizontal tube furnace. The cleaned silicon substrates were placed in the low temperature zone at about 10 cm downstream from the aluminum boat and flow of argon, which acted as the carrier gas. After being purged by high-purity argon (99.99%, 150 sccm) thrice, the quartz tube was pumped down to the required pressure of 5 × 10−5 Torr. The flow rates of Ar were controlled by a mass flow meter. During growth, the pressure inside the quartz tube was maintained at 1 × 10−1 Torr. The temperature of the furnace was raised to 1000 ◦ C at a ramping rate of 20 ◦ C/min with the Ar flow rate of 20 sccm. During synthesis the furnace temperature was maintained at 1000 ◦ C for 1 h, after which the furnace was allowed to cool naturally to room temperature. A white layer found to be deposited on the Si wafer was subjected to characterization. 2.2. Characterization of the product The surface morphology of the sample was characterized using a field emission scanning electron microscope (FESEM, JEOL JSM-6500F). The chemical composition was estimated by energy-dispersive spectroscopy (EDX) in the transmission electron microscope (TEM) and X-ray photoelectron emission spectroscopy (XPS, Perkin-Elmer, Model PHI1600). The crystal structure of the as-grown products was analyzed by X-ray diffraction (Shimadzu Lab XRD-6000). Cu K␣ radiation with a wavelength of 1.54056 A´˚ was used with the generator operated at 43 kV and 30 mA. Microstructures of the nanowires were obtained by TEM (JEOL 2010) operated at 200 kV. The high-resolution transmission electron microscopy (HRTEM) images and the selected area electron diffraction (SAED) patterns of the ZnS nanowires were also acquired with the same system. The optical properties of the as-prepared nanowires were measured by cathodoluminescence (CL) spectroscopy on a FESEM (JEOL-JSM-7001F) at room temperature. Luminescence generated by an electron beam (15 kV, 85 A) was focused using a parabolic mirror and lens onto the entrance slit of a monochromator with a 1200 lines/min grating system blazed at 300 nm.
Fig. 1. Typical FESEM images of ZnS nanowires synthesized via chemical vapor deposition. (a) Low-magnification image, and (b) high-magnification image.
wurtzite phase (hexagonal, lattice constants, a = b = 0.3822 nm, c = 0.6260 nm, Card ID 79-2204) were identified. Diffraction peaks belonging to Zn or any other phases were absent in the XRD patterns.
3. Results and discussion 3.1. Analysis of the morphology of ZnS nanowires Fig. 1 shows the representative SEM images at two different magnifications. The ZnS nanowires were randomly aligned and covered the entire substrate surface with reasonable uniformity. The nanowires exhibited a one-dimensional morphology with a high density over a significant field of view. The nanowires were uniform and were 20–50 nm in diameter and several tens of micrometers in length. 3.2. Analysis by X-ray diffraction Structural identification for the as-prepared ZnS nanowires was performed by XRD. Fig. 2 shows the XRD pattern of the as-synthesized product, which indicates that the nanowires were highly crystalline. The diffraction peaks were indexed by referring to the JCPDS database, and sphalerite phase (face center cubic, with lattice constants, a = b = c = 0.5345 nm, Card ID 80-0020) and
Fig. 2. The XRD pattern of ZnS nanowires revealing the wurtzite and sphalerite phase.
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Fig. 3. A low magnification TEM images of ZnS nanowires (a) wurtzite phase and (d) sphalerite phase. HRTEM images of an individual (b) wurtzite phase and (e) sphalerite phase of ZnS nanowire along with the SAD patterns in the corresponding inset. EDX spectrum taken from the ZnS nanowires (c) wurtzite phase and (f) sphalerite phase.
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Fig. 5. The cathodoluminescence (CL) spectra of the as-synthesized ZnS nanowires, which exhibit maximum intensity centered at ∼449 nm.
3.3. Analysis by TEM and EDX study
Fig. 4. The XPS spectra of the as-synthesized ZnS nanowires: (a) survey scan, (b) Zn 2p, and (c) S 2p.
Detailed microstructures and morphologies of the nanowires were obtained by TEM and HRTEM. The chemical composition and stoichiometry of the ZnS nanowires were investigated by EDX. Fig. 3(a) shows the low-magnification bright field TEM image of the wurtzite phase ZnS nanowires. The TEM images reveal straight nanowires with a uniform diameter of ∼20 nm. Fig. 3(b) shows the HRTEM image of a single ZnS nanowire with wurtzite phase. The images show the crystal structures and growth directions of the individual nanowires. The lattice spacing of the crystallographic planes measured to be 0.628 nm (marked with arrowheads) corresponds to the distance between the (0 0 1) planes of the ZnS nanowires. [0 0 0 2] was found to be the primary growth direction of the wurtzite ZnS nanowires. The SAED pattern presented in the insert of Fig. 3(b) with the [1 0 0] zone axis shows that the nanowires were single-crystal structure. Fig. 3(c), which shows results of the EDX analysis of the nanowires, reveals their chemical purity. According to the EDX analysis results, the only elements that were found in the as-synthesized product were Zn and S, which confirms the chemical composition of the nanowires. Further, from the elemental analysis of individual nanowires by EDX, the atomic percentages of Zn and S were found to be 45.53 and 54.47%, respectively. These values were consistent with the elemental composition of ZnS within the experimental error and are in accordance with the results of obtained from the XRD pattern. Fig. 3(d) shows the low-magnification bright field TEM image of sphalerite phase ZnS nanowires with a diameter of ∼18 nm. Fig. 3(e) shows the corresponding HRTEM image of these ZnS nanowires. A typical SAED pattern acquired from these nanowires shown in the inset of Fig. 3(e) indicates the existence of twinning. Adjacent to the twin plane, two sphalerite structures of ZnS are present at an angle of 70.5◦ . The lattice spacing (marked with arrowheads) measured to be 0.532 nm corresponds to the distance between the (1 1 1) planes of the sphalerite phase of ZnS and the planes coincide with the growth direction. [1 1 1] was shown to be the primary growth direction of these ZnS nanowires. Twinning configurations are thought to be easily formed during the phase transformation for the wurtzite and sphalerite structure. However, surface energy plays an important role in determining the structural stability. At high temperatures, the high energy surface of wurtzite can be transferred to sphalerite, which induces the (0 0 1) plane of wurtzite to change to the (1 1 1) plane of sphalerite structure [21]. Fig. 3(f) shows the corresponding EDX spectra of the nanowires. The EDX analysis revealed the presence of only Zn and S, which confirms
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Fig. 6. Schematic illustration of the growth mechanism of the ZnS nanowires.
the elemental composition of the as-synthesized product and the product was characterized as ZnS nanowires. The percentages of Zn and S were estimated to be 58.31 and 41.69%, respectively, which is consistent with the results obtained from the XRD and TEM analyses. 3.4. Analysis of chemical bonding and composition To determine the chemical composition of the ZnS nanowires prepared by chemical vapor deposition, the nanowires were further subjected to analysis by XPS. The binding energy value at 284.5 eV has have been referenced to the C 1s orbital, which arises from the carbon contamination on the surface of the sample. Fig. 4(a) shows the results of the XPS analysis of the ZnS nanowires. From the survey spectrum, it can be concluded that the product is of high purity. The spectra revealed the presence of peaks corresponding to the binding energies of Zn 2p3/2 and Zn 2p1/2 orbitals located at 1027 eV and 1050 eV, respectively, as is shown in Fig. 4(b). The energy difference of 23 eV between these two peaks is in good agreement with the standard value of 22.97 eV [22]. The peaks corresponding to the binding energy of the S 2p3/2 orbital located at 165 eV shown in Fig. 4(c) illustrates the presence of S(VI). These results are consistent with the binding energy values reported by Wagner [23] and indicate the formation of stoichiometric ZnS. 3.5. Analysis by cathodoluminescence spectroscopy To the best of our knowledge, there have been few literature reports on the CL properties of ZnS nanomaterials. Fig. 5 shows strong and broad emissions in the room-temperature CL spectrum acquired from the as-synthesized ZnS nanowires. The emission peak was mainly located in the blue region with its maximum intensity centered at ∼449 nm. Characteristic peaks were also located at 357, 449 and 581 nm, respectively. The emission peak at 357 nm (3.47 eV) has been attributed to the direct band-to-band emission induced by quantum-confinement effects in the nanostructures [24]. However, the high-intensity blue-emission peak at 449 nm (2.76 eV) has previously been attributed to the combination of free charge carriers at defect sites, possibly at the surface of the ZnS nanowires [25]. In addition, the emission peaks at 581 nm (2.13 eV) can be attributed to the impurity-related defects in the nanowires induced by the twin sphalerite structure of ZnS.
with oxygen to form carbon monoxide and carbon dioxide. Hence, ZnO structures were absent in our sample. Many growth mechanisms for ZnS nanomaterials have been proposed on the basis of their structures and experimental conditions used for their synthesis [11,23]. Among these, the vapor–solid (VS) mechanism has been most prevalently accepted. We suggest that the VS growth mechanism may also explain the production of ZnS nanowires under the experimental conditions used in the present study, because the synthesis did not involve any catalyst. The nucleation and growth of ZnS nanowires are schematically illustrated in Fig. 6. We believe that in addition to the temperature, the concentration of Zn and S vapors are also crucial for the formation ZnS nanostructures. When the precursors were heated to 1000 ◦ C, nucleation occurred spontaneously over a large area of the Si substrate. The concentrations of ZnS vapor surrounding the ZnS nuclei were sufficient to promote the growth of nanowires. The ZnS nanowires were eventually produced by the following reactions: Zn(s) + S(s) → ZnS(g)ZnS(g) → ZnS(s)
(1)
3C(s) + 2O2 (g) → 2CO(g) + CO2 (g)
(2)
4. Conclusions In summary, rapid large-scale production of ZnS nanowires on a Si substrate has been achieved by thermal evaporation of Zn, S, and C powders without the use of any catalyst. The nanowires were synthesized via the VS mechanism. The production process was simple, did not require a catalyst, and was efficient with a very high yield. The nanowires were 20–50 nm in diameter and up to several tens of nanometers in length. Analysis by XRD and TEM confirmed that single-crystalline wurtzite and sphalerite ZnS coexisted in the as-synthesized nanowires. The growth direction of the ZnS nanowires in wurtzite and sphalerite phases were along [0 0 0 2] and [1 1 1], respectively. At room temperature, the ZnS nanowires of both phases showed almost identical luminescence spectra in the blue region. The blue emission could be explained by the direct band-to-band emission, sulfur vacancies, self-activated sulfur defects on the surfaces, and interstitial lattice defects. As the ZnS nanowires absorbed strongly in the near-UV to blue regions, they may be appropriate to be used as UV/blue LED phosphor materials. Acknowledgment
3.6. Growth mechanism Our experiments suggest that carbon played a dominant role in the growth of ZnS nanowires. Carbon, a reducing agent, reacts
The authors would like to thank the National Science Council of the Republic of China (Taiwan), for financially supporting this research under Contract No. NSC-99-2922-I-007-357.
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[13] X.M. Meng, J. Liu, Y. Jiang, W.W. Chen, C.S. Lee, I. Bello, S.T. Lee, Chemical Physics Letters 382 (2003) 434. [14] Y. Wang, L. Zhang, C. Liang, G. Wang, X. Peng, Chemical Physics Letters 357 (2002) 314. [15] Q. Li, C. Wang, Applied Physics Letters 83 (2003) 359. [16] Y. Jiang, X.M. Meng, J. Liu, Z.Y. Xie, C.S. Lee, S.T. Lee, Advanced Materials 15 (2003) 323. [17] X. Wang, P. Gao, J. Li, C.J. Summers, Z.L. Wang, Advanced Materials 14 (2002) 1732. [18] Y.C. Zhu, Y. Bando, D.F. Xue, Applied Physics Letters 82 (2003) 1769. [19] X. Jiang, Y. Xie, J. Lu, L. Zhu, W. He, Y. Qian, Chemistry of Materials 13 (2001) 1213. [20] X.F. Duan, C.M. Lieber, Advanced Materials 279 (2000) 208. [21] B. Gilber, B.H. Frazer, H. Zhang, F. Huang, J.F. Banfirld, D. Haskel, J.C. Lang, G. Srajer, G. De Stasio, Physical Review B 66 (2002) 245205. [22] J.F. Moulder, W.F. Stickle, P.E. Sobol, K.D. Bomben, Handbook of X-ray Photoelectron Spectroscopy, Perkin-Elmer Corp., Eden Prarie, MN, USA, 1992. [23] C.D. Wagner, A.V. Naumkin, A.K. Vass, J.W. Allison, C.J. Powell, J.R. Rumble Jr., NUST X-ray Photoelectron Spectroscopy Database, Version 3.4. [24] D. Denzler, M. Olschewski, K. Sattler, Journal of Applied Physics 84 (1998) 2841. [25] S. Kar, S. Chaudhuri, Journal of Physical Chemistry B 109 (2005) 3298.
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Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
The preparation and cathodoluminescence of ZnS nanowires grown by chemical vapor deposition Meng-Wen Huang a , Yin-Wei Cheng b , Ko-Ying Pan c , Chen-Chuan Chang c , F.S. Shieu a , Han C. Shih a,b,c,∗ a b c
Department of Materials Science and Engineering, National Chung Hsing University, Taichung 402, Taiwan, ROC Institute of Materials Science and Nanotechnology, Chinese Culture University, Taipei 111, Taiwan, ROC Department of Materials Science and Engineering, National Tsing Hua University, Hsinchu 300, Taiwan, ROC
a r t i c l e
i n f o
Article history: Received 3 March 2012 Received in revised form 19 August 2012 Accepted 22 August 2012 Available online 28 August 2012 Keywords: Zinc sulfide Nanowires X-ray photoelectron spectrometer Cathodoluminescence
a b s t r a c t Single crystal ZnS nanowires were successfully synthesized in large quantities on Si (1 0 0) substrates by simple thermal chemical vapor deposition without using any catalyst. The morphology, composition, and crystal structure were characterized by field emission scanning electron microscopy (FESEM), X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), energy-dispersive X-ray spectroscopy (EDX), X-ray photoelectron spectroscopy (XPS), and cathodoluminescence (CL) spectroscopy. SEM observations show that the nanowires have diameters about 20–50 nm and lengths up to several tens of micrometers. XRD and TEM results confirmed that the nanowires exhibited both wurtzite and zinc blende structures with growth directions aligned along [0 0 0 2] and [1 1 1], respectively. The CL spectrum revealed emission bands in the UV and blue regions. The blue emissions at 449 and ∼581 nm were attributed to surface states and impurity-related defects of the nanowires, respectively. The perfect crystal structure of the nanowires indicates their potential applications in nanotechnology and in the fabrication of nanodevices. Crown Copyright © 2012 Published by Elsevier B.V. All rights reserved.
1. Introduction Since the discovery of carbon nanotubes [1], study of nanomaterials and their applications has been on a rise because these materials often demonstrate very different properties. Onedimensional semiconducting nanomaterials have been successfully synthesized and have attracted much attention and gained importance due to their special properties. Furthermore, nanomaterials with high-aspect-ratio structures and large surface areas offer exciting research possibilities to study and tap their novel physical or chemical properties, including their unique optical, magnetic and electronic characteristics [2–5]. As a result, the synthesis and characterization of one-dimensional metal oxide nanostructures have attracted considerable attention from the research community. Among the metal oxide materials, zinc sulfide (ZnS) is an important II B–VI group semiconductor with a wide direct band gap (3.7 eV at 300 K) and high refractive index. The luminescence properties of ZnS nanowires are eliciting great interest because of their potential applications in flat-panel displays [6], sensors, and lasers [7,8]. Recently, many novel methods of fabricating metal oxide nanomaterials have been reported. One-dimensional
ZnS nanostructures such as nanoparticles [9,10], nanorods [11,12], nanowires [13,14], nanobelts [15,16] and nanotubes [17] have been produced by different synthesis techniques including thermal evaporation [18], liquid crystal template method [19], and laser ablation [20]. In this study, we report a simple and low cost process to prepare high-purity ZnS nanowires with a high yield on silicon (1 0 0) substrates by chemical vapor deposition without the use of any catalysts. The nanowires were formed by the vapor–solid (VS) mechanism. This method can be used to continuously synthesize and produce single crystalline ZnS nanowires at a large scale. Furthermore, a series of experiments indicated that wurtzite (hexagonal) and sphalerite (face-centered cubic) phases coexisted in the as-synthesized product. Characterization by cathodoluminescence spectroscopy showed that these nanowires absorbed strongly in the near-UV to green region of the electromagnetic spectrum.
2. Experimental 2.1. Preparation of ZnS nanowires
∗ Corresponding author at: Institute of Materials Science and Nanotechnology, Chinese Culture University, Taipei 111, Taiwan, ROC. Fax: +886 2 28618701. E-mail address: [email protected] (H.C. Shih).
The synthesis was performed in a conventional horizontal quartz tube furnace. The single crystal ZnS nanowires were synthesized on silicon substrates by chemical vapor deposition. A pure
0169-4332/$ – see front matter. Crown Copyright © 2012 Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2012.08.079
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silicon (1 0 0) wafer was subjected to ultrasonic cleaning in ethanol for 30 min. Then, the wafer was treated with dilute HF (1 wt%) for 45 s and with deionized water and subsequently blow dried with nitrogen. High-purity Zn (0.1 g, Aldrich, −325 mesh, 99.99%), S (0.1 g, Aldrich, −325 mesh, 99.99%) and C (0.03 g, Aldrich, −325 mesh, 99.99%) powders, which acted as the precursor materials, were placed into an alumina boat and positioned at the constant temperature zone of the horizontal tube furnace. The cleaned silicon substrates were placed in the low temperature zone at about 10 cm downstream from the aluminum boat and flow of argon, which acted as the carrier gas. After being purged by high-purity argon (99.99%, 150 sccm) thrice, the quartz tube was pumped down to the required pressure of 5 × 10−5 Torr. The flow rates of Ar were controlled by a mass flow meter. During growth, the pressure inside the quartz tube was maintained at 1 × 10−1 Torr. The temperature of the furnace was raised to 1000 ◦ C at a ramping rate of 20 ◦ C/min with the Ar flow rate of 20 sccm. During synthesis the furnace temperature was maintained at 1000 ◦ C for 1 h, after which the furnace was allowed to cool naturally to room temperature. A white layer found to be deposited on the Si wafer was subjected to characterization. 2.2. Characterization of the product The surface morphology of the sample was characterized using a field emission scanning electron microscope (FESEM, JEOL JSM-6500F). The chemical composition was estimated by energy-dispersive spectroscopy (EDX) in the transmission electron microscope (TEM) and X-ray photoelectron emission spectroscopy (XPS, Perkin-Elmer, Model PHI1600). The crystal structure of the as-grown products was analyzed by X-ray diffraction (Shimadzu Lab XRD-6000). Cu K␣ radiation with a wavelength of 1.54056 A´˚ was used with the generator operated at 43 kV and 30 mA. Microstructures of the nanowires were obtained by TEM (JEOL 2010) operated at 200 kV. The high-resolution transmission electron microscopy (HRTEM) images and the selected area electron diffraction (SAED) patterns of the ZnS nanowires were also acquired with the same system. The optical properties of the as-prepared nanowires were measured by cathodoluminescence (CL) spectroscopy on a FESEM (JEOL-JSM-7001F) at room temperature. Luminescence generated by an electron beam (15 kV, 85 A) was focused using a parabolic mirror and lens onto the entrance slit of a monochromator with a 1200 lines/min grating system blazed at 300 nm.
Fig. 1. Typical FESEM images of ZnS nanowires synthesized via chemical vapor deposition. (a) Low-magnification image, and (b) high-magnification image.
wurtzite phase (hexagonal, lattice constants, a = b = 0.3822 nm, c = 0.6260 nm, Card ID 79-2204) were identified. Diffraction peaks belonging to Zn or any other phases were absent in the XRD patterns.
3. Results and discussion 3.1. Analysis of the morphology of ZnS nanowires Fig. 1 shows the representative SEM images at two different magnifications. The ZnS nanowires were randomly aligned and covered the entire substrate surface with reasonable uniformity. The nanowires exhibited a one-dimensional morphology with a high density over a significant field of view. The nanowires were uniform and were 20–50 nm in diameter and several tens of micrometers in length. 3.2. Analysis by X-ray diffraction Structural identification for the as-prepared ZnS nanowires was performed by XRD. Fig. 2 shows the XRD pattern of the as-synthesized product, which indicates that the nanowires were highly crystalline. The diffraction peaks were indexed by referring to the JCPDS database, and sphalerite phase (face center cubic, with lattice constants, a = b = c = 0.5345 nm, Card ID 80-0020) and
Fig. 2. The XRD pattern of ZnS nanowires revealing the wurtzite and sphalerite phase.
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Fig. 3. A low magnification TEM images of ZnS nanowires (a) wurtzite phase and (d) sphalerite phase. HRTEM images of an individual (b) wurtzite phase and (e) sphalerite phase of ZnS nanowire along with the SAD patterns in the corresponding inset. EDX spectrum taken from the ZnS nanowires (c) wurtzite phase and (f) sphalerite phase.
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Fig. 5. The cathodoluminescence (CL) spectra of the as-synthesized ZnS nanowires, which exhibit maximum intensity centered at ∼449 nm.
3.3. Analysis by TEM and EDX study
Fig. 4. The XPS spectra of the as-synthesized ZnS nanowires: (a) survey scan, (b) Zn 2p, and (c) S 2p.
Detailed microstructures and morphologies of the nanowires were obtained by TEM and HRTEM. The chemical composition and stoichiometry of the ZnS nanowires were investigated by EDX. Fig. 3(a) shows the low-magnification bright field TEM image of the wurtzite phase ZnS nanowires. The TEM images reveal straight nanowires with a uniform diameter of ∼20 nm. Fig. 3(b) shows the HRTEM image of a single ZnS nanowire with wurtzite phase. The images show the crystal structures and growth directions of the individual nanowires. The lattice spacing of the crystallographic planes measured to be 0.628 nm (marked with arrowheads) corresponds to the distance between the (0 0 1) planes of the ZnS nanowires. [0 0 0 2] was found to be the primary growth direction of the wurtzite ZnS nanowires. The SAED pattern presented in the insert of Fig. 3(b) with the [1 0 0] zone axis shows that the nanowires were single-crystal structure. Fig. 3(c), which shows results of the EDX analysis of the nanowires, reveals their chemical purity. According to the EDX analysis results, the only elements that were found in the as-synthesized product were Zn and S, which confirms the chemical composition of the nanowires. Further, from the elemental analysis of individual nanowires by EDX, the atomic percentages of Zn and S were found to be 45.53 and 54.47%, respectively. These values were consistent with the elemental composition of ZnS within the experimental error and are in accordance with the results of obtained from the XRD pattern. Fig. 3(d) shows the low-magnification bright field TEM image of sphalerite phase ZnS nanowires with a diameter of ∼18 nm. Fig. 3(e) shows the corresponding HRTEM image of these ZnS nanowires. A typical SAED pattern acquired from these nanowires shown in the inset of Fig. 3(e) indicates the existence of twinning. Adjacent to the twin plane, two sphalerite structures of ZnS are present at an angle of 70.5◦ . The lattice spacing (marked with arrowheads) measured to be 0.532 nm corresponds to the distance between the (1 1 1) planes of the sphalerite phase of ZnS and the planes coincide with the growth direction. [1 1 1] was shown to be the primary growth direction of these ZnS nanowires. Twinning configurations are thought to be easily formed during the phase transformation for the wurtzite and sphalerite structure. However, surface energy plays an important role in determining the structural stability. At high temperatures, the high energy surface of wurtzite can be transferred to sphalerite, which induces the (0 0 1) plane of wurtzite to change to the (1 1 1) plane of sphalerite structure [21]. Fig. 3(f) shows the corresponding EDX spectra of the nanowires. The EDX analysis revealed the presence of only Zn and S, which confirms
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Fig. 6. Schematic illustration of the growth mechanism of the ZnS nanowires.
the elemental composition of the as-synthesized product and the product was characterized as ZnS nanowires. The percentages of Zn and S were estimated to be 58.31 and 41.69%, respectively, which is consistent with the results obtained from the XRD and TEM analyses. 3.4. Analysis of chemical bonding and composition To determine the chemical composition of the ZnS nanowires prepared by chemical vapor deposition, the nanowires were further subjected to analysis by XPS. The binding energy value at 284.5 eV has have been referenced to the C 1s orbital, which arises from the carbon contamination on the surface of the sample. Fig. 4(a) shows the results of the XPS analysis of the ZnS nanowires. From the survey spectrum, it can be concluded that the product is of high purity. The spectra revealed the presence of peaks corresponding to the binding energies of Zn 2p3/2 and Zn 2p1/2 orbitals located at 1027 eV and 1050 eV, respectively, as is shown in Fig. 4(b). The energy difference of 23 eV between these two peaks is in good agreement with the standard value of 22.97 eV [22]. The peaks corresponding to the binding energy of the S 2p3/2 orbital located at 165 eV shown in Fig. 4(c) illustrates the presence of S(VI). These results are consistent with the binding energy values reported by Wagner [23] and indicate the formation of stoichiometric ZnS. 3.5. Analysis by cathodoluminescence spectroscopy To the best of our knowledge, there have been few literature reports on the CL properties of ZnS nanomaterials. Fig. 5 shows strong and broad emissions in the room-temperature CL spectrum acquired from the as-synthesized ZnS nanowires. The emission peak was mainly located in the blue region with its maximum intensity centered at ∼449 nm. Characteristic peaks were also located at 357, 449 and 581 nm, respectively. The emission peak at 357 nm (3.47 eV) has been attributed to the direct band-to-band emission induced by quantum-confinement effects in the nanostructures [24]. However, the high-intensity blue-emission peak at 449 nm (2.76 eV) has previously been attributed to the combination of free charge carriers at defect sites, possibly at the surface of the ZnS nanowires [25]. In addition, the emission peaks at 581 nm (2.13 eV) can be attributed to the impurity-related defects in the nanowires induced by the twin sphalerite structure of ZnS.
with oxygen to form carbon monoxide and carbon dioxide. Hence, ZnO structures were absent in our sample. Many growth mechanisms for ZnS nanomaterials have been proposed on the basis of their structures and experimental conditions used for their synthesis [11,23]. Among these, the vapor–solid (VS) mechanism has been most prevalently accepted. We suggest that the VS growth mechanism may also explain the production of ZnS nanowires under the experimental conditions used in the present study, because the synthesis did not involve any catalyst. The nucleation and growth of ZnS nanowires are schematically illustrated in Fig. 6. We believe that in addition to the temperature, the concentration of Zn and S vapors are also crucial for the formation ZnS nanostructures. When the precursors were heated to 1000 ◦ C, nucleation occurred spontaneously over a large area of the Si substrate. The concentrations of ZnS vapor surrounding the ZnS nuclei were sufficient to promote the growth of nanowires. The ZnS nanowires were eventually produced by the following reactions: Zn(s) + S(s) → ZnS(g)ZnS(g) → ZnS(s)
(1)
3C(s) + 2O2 (g) → 2CO(g) + CO2 (g)
(2)
4. Conclusions In summary, rapid large-scale production of ZnS nanowires on a Si substrate has been achieved by thermal evaporation of Zn, S, and C powders without the use of any catalyst. The nanowires were synthesized via the VS mechanism. The production process was simple, did not require a catalyst, and was efficient with a very high yield. The nanowires were 20–50 nm in diameter and up to several tens of nanometers in length. Analysis by XRD and TEM confirmed that single-crystalline wurtzite and sphalerite ZnS coexisted in the as-synthesized nanowires. The growth direction of the ZnS nanowires in wurtzite and sphalerite phases were along [0 0 0 2] and [1 1 1], respectively. At room temperature, the ZnS nanowires of both phases showed almost identical luminescence spectra in the blue region. The blue emission could be explained by the direct band-to-band emission, sulfur vacancies, self-activated sulfur defects on the surfaces, and interstitial lattice defects. As the ZnS nanowires absorbed strongly in the near-UV to blue regions, they may be appropriate to be used as UV/blue LED phosphor materials. Acknowledgment
3.6. Growth mechanism Our experiments suggest that carbon played a dominant role in the growth of ZnS nanowires. Carbon, a reducing agent, reacts
The authors would like to thank the National Science Council of the Republic of China (Taiwan), for financially supporting this research under Contract No. NSC-99-2922-I-007-357.
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