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Self-assembled highly symmetrical ZnS nanostructures and their cathodoluminescence
Journal of Luminescence 131 (2011) 1095–1099
Contents lists available at ScienceDirect
Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
Self-assembled highly symmetrical ZnS nanostructures and their cathodoluminescence Baoyu Liu a,n, Long Hu b, Chengchun Tang b, Lu Liu a, Shaohua Li a, Jigong Qi a, Yuhan Liu a a b
College of Petroleum Engineering, Liaoning Shihua University, Fushun 113001, China School of Material Science and Engineering, Hebei University of Technology, Tianjin 300130, China
a r t i c l e i n f o
abstract
Article history: Received 12 October 2010 Received in revised form 25 January 2011 Accepted 3 February 2011 Available online 19 February 2011
Here we report the synthesis and characterization of self-assembled highly symmetrical, i.e., two-fold, three-fold, four-fold and multi-fold, ZnS nanostructures through a simple thermal evaporation process. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) analyses indicated that the ZnS nanostructures are composed of faceted crystalline ZnS nanorods with a diameter in the range of 200–600 nm and length up to 2 mm. In addition, all the branched ZnS nanorods have preferential orientation along the [0 0 2] direction of a wurtzite-type hexagonal structure. The cathodoluminescence measurement demonstrated that the ZnS nanostructures have a strong and uniform band-gap emission centered at 337 nm, indicating their good crystallinity and excellent optical property. & 2011 Elsevier B.V. All rights reserved.
Keywords: ZnS Nanostructures Synthesis Self-assemble Cathodoluminescence
1. Introduction Due to their unique and interesting geometries, semiconductor nanomaterials with peculiar morphologies and unconventional structures have attracted extensive research interest in the past years [1,2]. Compared with the formation of conventional nanostructures, the nucleation of three-dimensional (3D) complex nanostructures and their subsequent growth may rely on a selfassembly process [3,4]. In addition, some unexpected phenomena and interesting properties may also be found in these complex nanostructures and thus some specific applications based on these unconventional nanostructures in the field of optics, magnetism and electronics can be explored [5]. As an important group of II–VI semiconductors, ZnS possesses a direct wide-band-gap of 3.77 eV (hexagonal phase) and has been extensively used in electroluminescent devices, flat panel displays, infrared windows, lasers and sensors, and photocatalysis [6,7]. Recently, much effort has been paid to the synthesis of various ZnS nanostructures and their application exploration in diverse fields. For instance, low-dimensional ZnS nanostructures such as nanowires, nanobelts and nanotubes fabricated from either physical evaporation or chemical routines have been widely reported and their applications as ultraviolet photodetectors and luminescent phosphors have been realized [7–9]. Unlike
n
Corresponding author. Tel.: +86 413 6865005; fax: + 86 413 6860766. E-mail address: [email protected] (B. Liu).
0022-2313/$ - see front matter & 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2011.02.011
the normal synthesis of low-dimensional ZnS nanostructures, unconventional 3D ZnS nanostructures with peculiar morphologies and shapes can often be formed through a self-assembly process that possesses unexpected properties [3,5]. In this paper, we report the synthesis and characterization of highly symmetrical ZnS nanostructures through a self-assembly process. The crystallinity and microstructure of the as-synthesized ZnS nanostructures were studied using the high-resolution transmission electron microscopy (TEM) and the selected area electron diffraction (SAED) techniques. The optical properties of ZnS nanostructures were also investigated by cathodoluminescence measurement and wavelength-resolved techniques. Finally, the possible optical emission mechanism of the highly symmetrical ZnS nanostructures was discussed based on the microstructural and compositional analyses.
2. Experimental ZnS nanostructures with high structural symmetry were directly synthesized in a horizontal electrical resistance furnace through a feasible thermal evaporation process, as reported elsewhere [5]. The typical procedure for ZnS nanostructure growth can be simply described as follows: high-purity ZnS powders of micro-size (Sinopharm Chemical Reagent Co., Ltd.; purity 99.99%) loaded into an Al2O3 boat were placed at the center of a quartztube (140 cm in length and 40 mm in outer diameter) and then a piece of Si substrate (15 15 mm) was put downstream for the
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deposition of ZnS nanostructures, with a distance of 10 cm from the center. The quartz-tube sealed with silica gel plugs and connected with flowing Ar gas was finally moved into the furnace for thermal evaporation reaction. The reaction chamber was quickly heated to 1150 1C in 30 min under Ar flow (200 mL/min). After reacting at this temperature for 30 min, the reaction chamber was naturally cooled to room temperature under the protection of Ar gas. The ZnS nanostructures grown on Si substrate were collected for subsequent measurement and analysis. The morphological characterization and structure analysis were performed using a scanning electron microscope (SEM, JEOL, JSM-5600LV) and a 200 kV highresolution field-emission transmission electron microscope (TEM, FEI, Tecnai-G220). The composition analysis was carried out by an energy dispersive X-ray spectrometer (EDS) attached to the TEM. Optical property and spatially resolved cathodoluminescence (CL) images of ZnS nanostructures were obtained from a CL measuring system attached to an ultra-high-vacuum SEM (UHV-SEM). For typical CL measurement, the accelerating voltage and beam current were fixed at 5 kV and 1000 pA, respectively, and the spectral resolution was determined by grating and the slit width was typically 0.2 nm. More details about CL measurement can be found elsewhere [10].
3. Results and discussions Fig. 1 shows representative morphology of the as-synthesized ZnS nanostructures. All these ZnS nanostructures grown from a self-assembly process are highly symmetrical in structure. They can be either two-fold (Fig. 1a), three-fold (Fig. 1b), four-fold (Fig. 1d) or multi-fold (Fig. 1e). Occasionally, some ZnS nanostructures with low structural symmetry can also be found, as presented in Fig. 1f. In addition, highly symmetrical ZnS nanostructure with a small rod stretching from the nucleation site of the three-fold crystal to form a tetrapod-like morphology can also be observed (Fig. 1c). Typically, the individual branch nanorods in the ZnS nanostructures exhibit excellent uniformity in diameter, morphology and length (Fig. 1a–f). The diameter of the branch ZnS nanorods is in the range of 200–600 nm that is mainly controlled by the initial nucleation size on the Si substrate. In addition, the structural symmetry of ZnS nanostructure was
also related with the initial nucleation size. Basically, two-fold or three-fold (highly structural symmetry) ZnS nanostructures can be obtained if the initial nucleation size is small, whereas a large nucleation size will lead to a ZnS nanostructure with relatively lower structural symmetry as shown in Fig. 1e and f, respectively. The length of the ZnS nanorods can be up to a maximum of 2mm after a 30 min growth. Compared with the other ZnS nanostructures guided by the metal catalyst [9], the growth rate of assynthesized ZnS nanostructures was quite lower due to the absence of a catalyst, which can effectively promote the growth of nanostructure at a high temperature. Different from the catalytic growth, no metal catalyst particle can be found on the flat or sharp tip of ZnS nanostructure and the flatness of the tip was obviously dependent on the diameter of the branch ZnS nanorods, as shown in Fig. 1a, d, e and f. From the SEM observations, one can also notice that the sidewall of each ZnS nanorod is faceted and forms a typical prism-like morphology, indicating that the ZnS nanorods were grown along the axial direction ([0 0 2]) in a wurtzite-type hexagonal structure. It is believed that the preferential growth along the axial direction plays a key role in the formation of such highly structurally symmetrical nanostructure as reported in a number of similar nanostructures [3] through a self-generated process. In order to systemically study the microstructure, crystallography and composition of the as-synthesized ZnS nanostructures, a high-resolution transmission electron microscope (HRTEM) equipped with an energy dispersive X-ray spectrometer (EDS) was employed and the detailed structural and compositional analyses were carried out on tens of ZnS nanostructures. As an example, Fig. 2a shows the low-magnification TEM image of a branched ZnS nanorod separated from the ZnS nanostructure by scraping the samples from the substrate surface. The ZnS nanorod shows a bulletin-like morphology and possesses a diameter of 200 nm that is consistent with the SEM images. In order to investigate the microstructure and crystallography and deduce the growth behavior of the ZnS nanostructure, three representative areas, namely, tip, shoulder and base, marked by the Arabic numbers 1, 2 and 3, respectively, were selected for HRTEM analysis. Fig. 2b was the HRTEM image of the nanorod tip (3 area) and it can be seen that all the atoms are regularly arranged following a given atom stacking sequence. By measuring
Fig. 1. (a–f) Representative SEM images of as-grown ZnS nanostructures with diverse morphologies and structural symmetries.
B. Liu et al. / Journal of Luminescence 131 (2011) 1095–1099
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Fig. 2. (a) Low-magnification TEM image of a branched ZnS nanorod scraped from the substrate; (b–d) high-resolution TEM images of the selected areas shown in (a); inset is the corresponding ED pattern recorded from the ZnS nanorod; (e) a representative EDS spectrum collected from the ZnS nanostructure.
the distance between the parallel neighboring atomic planes, an interplanar distance of 0.626 nm can be confirmed that corresponds to the d-spacing value of (0 0 2) plane in a wurtzite-type ZnS crystal. From the HRTEM image shown in Fig. 2b, one can also notice that the edge of the flat growth front is parallel to the (0 0 2) plane of hexagonal ZnS crystal, suggesting that the ZnS nanorod was in fact grown along the preferential [0 0 2] direction, which can also be confirmed from the electron diffraction (ED) pattern shown in the inset of Fig. 2. The atomic lattice image of the shoulder region (area 2 in Fig. 2a) was presented in Fig. 2c and the single crystalline characteristic can be verified again. The distances of 0.62 and 0.38 nm match well with the standard lattice d-spacings of (0 0 2) and (1 0 0) planes of a bulky ZnS crystal. As marked in Fig. 2c, the sidewall facet of the ZnS nanorod can be confirmed as (1 0 0) plane, whereas the tilted one corresponds to the (1 0 1) plane. The angle between the tilted plane and radial plane was measured to be 621, which is in good agreement with the theoretical value between (1 0 1) and (0 0 2) planes. Fig. 2d shows the HRTEM image of the base region of the ZnS nanorod (area 3 in Fig. 2a) and one can find that all the atoms exhibit the same atomic stacking sequence as the above two areas. Based on the detailed microstructure analysis, it can be concluded that the as-grown ZnS nanostructures were single crystal and no structural defect such as twin or stacking fault was found [5]. In addition, the crystalline characteristic of the ZnS nanostructures could be further demonstrated by the regularly arranged ED spots. Though the individual ZnS nanorods are of single crystal, it should be mentioned that they may be grown from the initial nucleation site in the form of twin, as observed in tetrapod-branched ZnS nanorod architectures [11]. Fig. 2e shows
a representative EDS spectrum from tens of investigated samples. Only Zn and S peaks could be detected (Cu signal comes from the TEM grids) within the resolution limit of EDS measurement and their atomic ratio approaches Zn:S¼1:1. In addition, no oxygen signal was detected during the EDS analysis, suggesting that the high purity of as-synthesized ZnS nanostructures is in good agreement with the absence of any amorphous oxide layers, as occasionally found in most nanostructures [12]. Different from the catalytic growth of one-dimensional ZnS nanowires, in which the well-known vapor–liquid–solid (V–L–S) mechanism is involved [13], formation of the highly symmetrical ZnS nanostructures may follow a vapor–solid (V–S) process. From SEM and TEM observations, it can be seen that the growth of the ZnS nanostructures is consistent with their crystallographic characteristics. Basically, the formation of ZnS nanostructure with high structural symmetry can be explained as follows: first, the Zn droplets decomposed from the ZnS precursors deposit on the SiO2 surface at low temperature zone and combine with S vapor to form some ZnS nuclei. Due to polarity, these ZnS nuclei prefer to evolve into some nanoscaled, faceted ZnS crystals and combine together in the form of twin boundaries via a self-organization process, as proposed in the formation of dart-shaped tricrystal (DST) and some tetrapod nanostructures [2,3]. Second, the nucleated ZnS crystals will gradually transform to some faceted ZnS nanorods under the continuous supply of Zn droplets and S vapors. When the reaction is stopped, the growth of these ZnS nanorods will terminate. From TEM analysis, it can be seen that all the ZnS nanorods have a preferential orientation along [0 0 2] direction and this may be related to the lower formation energy of this plane. It is believed that the symmetry of ZnS nanostructures
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is directly dependent on the initial numbers of the twin boundaries formed on the ZnS nuclei. In addition, it should be mentioned that the oxide layer (SiO2) may also play a key role in the self-organization of these highly symmetrical ZnS nanostructures, as demonstrated in the formation of ZnS hepta- and tetra-pods, 3D hierarchical ZnO nanostructures, and ZnS DST [2,3,14]. ZnS is a key wide-band-gap semiconductor with a direct bandgap energy of 3.77 eV (for hexagonal wurtzite phase) that corresponds to a wavelength of 329 nm. Thus, it can be used to fabricate some highly sensitive ultraviolet optoelectronic devices such as ultraviolet photodetector and H2 sensor [7,15]. However, most of the ZnS crystals, either bulk or nanostructures, exhibit strong green emissions rather than the intrinsic band-gap ultraviolet emission due to the existence of large amount of structural defects such as twin or stacking fault [5]. In addition, the impurity contamination in ZnS lattice such as O element can also lead to the shift of emission wavelength towards the visible range. Especially, the band-gap emission is rather challenging in nanostructured ZnS materials due to its easy oxidation or contamination. Fig. 3 shows a typical CL spectrum recorded from an individual ZnS nanostructure under an applied accelerating voltage of 5 KV and a beam current of 1000 pA. A sharp emission peak centered at 337 nm and a broad emission range from 400–800 nm can be observed. Obviously, the 337 nm peak originates from the band-gap emission, whereas the broad emission peak in the visible range may result from the defect-related emission. The 337 nm band-gap emission further demonstrated good crystallinity and high purity of the as-synthesized ZnS nanostructures, as verified by HRTEM and EDS analyses (Fig. 2), and was consistent with the previous results [2,7]. For the visible emission, generally it is believed that some catalyst-introduced deep levels or Zn vacancy-related structural defect in the ZnS lattice host may induce such emission. In addition, some structural defects such as point defects, microtwins and dopants (Mn2 + ) incorporated into the ZnS lattice were also reported [16],
and various defect-related emissions in the range of 400–800 nm have been observed in diverse ZnS structures [9,17,18]. In this work, no microtwin or stacking fault was found in ZnS nanostructures during HRTEM observation and ED analysis so the effect of these structural defects on the visible emission can be basically excluded. The main reason responsible for this defect-related emission can be considered as a result from some elemental sulfur species, as verified by X-ray photoelectron spectroscopy (XPS) and photoluminescence measurement [19]. In addition, though the impurity contamination coming from both catalyst and foreign dopants (Si or O) was not detected within the resolution limit of EDS measurement, some Si or O impurity with a much lower content may still accumulate in the ZnS lattice, and thus generate some deep impurity levels between the conduction and valence bands of ZnS crystal. Therefore, the broad emission peak in the visible range may be caused by a combined effect coming from Zn/S vacancies and Si/O impurities. In order to further study the origin of the two band emissions, wavelength-resolved CL properties of the as-synthesized ZnS nanostructures were investigated. From the CL spectrum, it can be known that the ZnS nanostructures exhibited a sharp band-gap emission centered at 337 nm and a broad visible emission with maximum peak intensity at 463 nm. Therefore, the 337 and 463 nm wavelengths were used to examine the emission origin of the two bands, respectively. Fig. 4a shows a typical SEM image of a ZnS nanostructure and its corresponding CL images, taken at 337 and 463 nm. It can be seen that the 337 nm emission was quite uniform, whereas the 463 nm emission is uneven. In addition, the 337 nm CL image exhibited a rather brighter emission, whereas the 463 nm emission is a little weak, indicating the excellent band-gap emission of as-synthesized ZnS nanostructures. With strong ultraviolet emission, it is believed that our ZnS nanostructures hold promising application in the field of nanooptoelectronic devices such as ultraviolet photodetectors.
4. Conclusion
Intensity (a. u.)
337 nm
200
400
600
800
1000
Wavelength (nm) Fig. 3. CL spectrum collected from a four-fold symmetrical ZnS nanostructure.
In summary, complex ZnS nanostructures with two-fold, three-fold, four-fold or multi-fold structural symmetry have been synthesized through a simple thermal evaporation process. All the ZnS nanostructures are wurtzite-type hexagonal phase and each branched nanorod in the ZnS nanostructure is a single crystalline, possessing a diameter in the range of 200–600 nm and a length up to 2 mm. In addition, all the branched ZnS nanorods exhibit a preferential orientation along [0 0 2] direction. CL measurements showed that the ZnS nanostructures have strong band-gap emission with a centered wavelength of 337 nm and a broad emission in the visible range. It is believed that the ZnS nanostructures with highly structural symmetry and strong ultraviolet emission may find some potential application in the field of optical and optoelectronic devices.
Fig. 4. (a) SEM image of an individual ZnS nanostructure; (b, c) CL images taken at 337 and 463 nm, respectively. The scale bar is 500 nm.
B. Liu et al. / Journal of Luminescence 131 (2011) 1095–1099
References [1] L. Rashmi, B. Bednarz, G. Hackens, V. Farhi, I. Bayot, Huynen, Solid State Commun. 134 (2005) 217. [2] X. Fan, X.M. Meng, X.H. Zhang, W.S. Shi, W.J. Zhang, A. Zapien, C.S. Lee, S.T. Lee, Angew. Chem. Int. Ed. 45 (2006) 2568. [3] G.Z. Shen, Y. Bando, J.Q. Hu, D. Golberg, Appl. Phys. Lett. 90 (2007) 123101. [4] L.S. Zhong, J.S. Hu, H.P. Liang, A.M. Cao, W.G. Song, L.J. Wan, Adv. Mater. 18 (2006) 2426. [5] B.D. Liu, Y. Bando, Z.E. Wang, C.Y. Li, M. Gao, M. Mitome, X. Jiang, D. Golberg, Cryst. Growth Des. 10 (2010) 4143. [6] A.J. Mieszawska, R. Jalilian, G.U. Sumanasekera, F.P. Zamborini, Small 3 (2007) 722. [7] X.S. Fang, Y. Bando, M. Liao, U.K. Gautam, C. Zhi, B. Dierre, B.D. Liu, T.Y. Zhai, T. Sekiguchi, Y. Koide, D. Golberg, Adv. Mater. 21 (2009) 2034. [8] T.Y. Zhai, Z.J. Gu, Y. Ma, W.S. Yang, L.Y. Zhao, J.N. Yao, Mater. Chem. Phys. 100 (2006) 281.
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[9] C.J. Barrelet, Y. Wu, D.C. Bell, C.M. Lieber, J. Am. Chem. Soc. 125 (2003) 11498. [10] J.Q. Hu, Y. Bando, D. Golberg, Small 1 (2005) 95. [11] S.C. Lyu, Y. Zhang, H. Ruh, H.J. Lee, C.J. Lee, Chem. Phys. Lett. 367 (2003) 717. [12] R.S. Wagner, J. Appl. Phys. 38 (1967) 1554. [13] J.Y. Lao, J.G. Wen, Z.F. Ren, Nano Lett. 3 (2003) 235. [14] Z.G. Chen, J. Zou, G. Liu, H.F. Lu, F. Li, G.Q. Lu, H.M. Cheng, Nanotechnology 19 (2008) 055710. [15] S. Yanagida, H. Kawakami, Y. Midori, H. Kizumoto, C.J. Pac, Y. Wada, Bull. Chem. Soc. Jpn. 68 (1995) 1811. [16] Z.G. Chen, J. Zou, D.W. Wang, L.C. Yin, G. Liu, Q. Liu, C.H. Sun, X. Yao, F. Li, X.L. Yuan, T. Sekiguchi, G.Q. Lu, H.M. Cheng, Adv. Func. Mater. 19 (2009) 484. [17] C.H. Liang, Y. Shimizu, T. Sasaki, H. Umehara, N. Koshizaki, J. Phys. Chem. B 108 (2004) 9728. [18] C.H. Ye, X.S. Fang, G.H. Li, L.D. Zhang, Appl. Phys. Lett. 85 (2004) 3035. [19] B. Dierre, X.L. Yuan, T. Sekiguchi, Sci. Technol. Adv. Mater. 11 (2010) 043001.
Contents lists available at ScienceDirect
Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
Self-assembled highly symmetrical ZnS nanostructures and their cathodoluminescence Baoyu Liu a,n, Long Hu b, Chengchun Tang b, Lu Liu a, Shaohua Li a, Jigong Qi a, Yuhan Liu a a b
College of Petroleum Engineering, Liaoning Shihua University, Fushun 113001, China School of Material Science and Engineering, Hebei University of Technology, Tianjin 300130, China
a r t i c l e i n f o
abstract
Article history: Received 12 October 2010 Received in revised form 25 January 2011 Accepted 3 February 2011 Available online 19 February 2011
Here we report the synthesis and characterization of self-assembled highly symmetrical, i.e., two-fold, three-fold, four-fold and multi-fold, ZnS nanostructures through a simple thermal evaporation process. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) analyses indicated that the ZnS nanostructures are composed of faceted crystalline ZnS nanorods with a diameter in the range of 200–600 nm and length up to 2 mm. In addition, all the branched ZnS nanorods have preferential orientation along the [0 0 2] direction of a wurtzite-type hexagonal structure. The cathodoluminescence measurement demonstrated that the ZnS nanostructures have a strong and uniform band-gap emission centered at 337 nm, indicating their good crystallinity and excellent optical property. & 2011 Elsevier B.V. All rights reserved.
Keywords: ZnS Nanostructures Synthesis Self-assemble Cathodoluminescence
1. Introduction Due to their unique and interesting geometries, semiconductor nanomaterials with peculiar morphologies and unconventional structures have attracted extensive research interest in the past years [1,2]. Compared with the formation of conventional nanostructures, the nucleation of three-dimensional (3D) complex nanostructures and their subsequent growth may rely on a selfassembly process [3,4]. In addition, some unexpected phenomena and interesting properties may also be found in these complex nanostructures and thus some specific applications based on these unconventional nanostructures in the field of optics, magnetism and electronics can be explored [5]. As an important group of II–VI semiconductors, ZnS possesses a direct wide-band-gap of 3.77 eV (hexagonal phase) and has been extensively used in electroluminescent devices, flat panel displays, infrared windows, lasers and sensors, and photocatalysis [6,7]. Recently, much effort has been paid to the synthesis of various ZnS nanostructures and their application exploration in diverse fields. For instance, low-dimensional ZnS nanostructures such as nanowires, nanobelts and nanotubes fabricated from either physical evaporation or chemical routines have been widely reported and their applications as ultraviolet photodetectors and luminescent phosphors have been realized [7–9]. Unlike
n
Corresponding author. Tel.: +86 413 6865005; fax: + 86 413 6860766. E-mail address: [email protected] (B. Liu).
0022-2313/$ - see front matter & 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2011.02.011
the normal synthesis of low-dimensional ZnS nanostructures, unconventional 3D ZnS nanostructures with peculiar morphologies and shapes can often be formed through a self-assembly process that possesses unexpected properties [3,5]. In this paper, we report the synthesis and characterization of highly symmetrical ZnS nanostructures through a self-assembly process. The crystallinity and microstructure of the as-synthesized ZnS nanostructures were studied using the high-resolution transmission electron microscopy (TEM) and the selected area electron diffraction (SAED) techniques. The optical properties of ZnS nanostructures were also investigated by cathodoluminescence measurement and wavelength-resolved techniques. Finally, the possible optical emission mechanism of the highly symmetrical ZnS nanostructures was discussed based on the microstructural and compositional analyses.
2. Experimental ZnS nanostructures with high structural symmetry were directly synthesized in a horizontal electrical resistance furnace through a feasible thermal evaporation process, as reported elsewhere [5]. The typical procedure for ZnS nanostructure growth can be simply described as follows: high-purity ZnS powders of micro-size (Sinopharm Chemical Reagent Co., Ltd.; purity 99.99%) loaded into an Al2O3 boat were placed at the center of a quartztube (140 cm in length and 40 mm in outer diameter) and then a piece of Si substrate (15 15 mm) was put downstream for the
1096
B. Liu et al. / Journal of Luminescence 131 (2011) 1095–1099
deposition of ZnS nanostructures, with a distance of 10 cm from the center. The quartz-tube sealed with silica gel plugs and connected with flowing Ar gas was finally moved into the furnace for thermal evaporation reaction. The reaction chamber was quickly heated to 1150 1C in 30 min under Ar flow (200 mL/min). After reacting at this temperature for 30 min, the reaction chamber was naturally cooled to room temperature under the protection of Ar gas. The ZnS nanostructures grown on Si substrate were collected for subsequent measurement and analysis. The morphological characterization and structure analysis were performed using a scanning electron microscope (SEM, JEOL, JSM-5600LV) and a 200 kV highresolution field-emission transmission electron microscope (TEM, FEI, Tecnai-G220). The composition analysis was carried out by an energy dispersive X-ray spectrometer (EDS) attached to the TEM. Optical property and spatially resolved cathodoluminescence (CL) images of ZnS nanostructures were obtained from a CL measuring system attached to an ultra-high-vacuum SEM (UHV-SEM). For typical CL measurement, the accelerating voltage and beam current were fixed at 5 kV and 1000 pA, respectively, and the spectral resolution was determined by grating and the slit width was typically 0.2 nm. More details about CL measurement can be found elsewhere [10].
3. Results and discussions Fig. 1 shows representative morphology of the as-synthesized ZnS nanostructures. All these ZnS nanostructures grown from a self-assembly process are highly symmetrical in structure. They can be either two-fold (Fig. 1a), three-fold (Fig. 1b), four-fold (Fig. 1d) or multi-fold (Fig. 1e). Occasionally, some ZnS nanostructures with low structural symmetry can also be found, as presented in Fig. 1f. In addition, highly symmetrical ZnS nanostructure with a small rod stretching from the nucleation site of the three-fold crystal to form a tetrapod-like morphology can also be observed (Fig. 1c). Typically, the individual branch nanorods in the ZnS nanostructures exhibit excellent uniformity in diameter, morphology and length (Fig. 1a–f). The diameter of the branch ZnS nanorods is in the range of 200–600 nm that is mainly controlled by the initial nucleation size on the Si substrate. In addition, the structural symmetry of ZnS nanostructure was
also related with the initial nucleation size. Basically, two-fold or three-fold (highly structural symmetry) ZnS nanostructures can be obtained if the initial nucleation size is small, whereas a large nucleation size will lead to a ZnS nanostructure with relatively lower structural symmetry as shown in Fig. 1e and f, respectively. The length of the ZnS nanorods can be up to a maximum of 2mm after a 30 min growth. Compared with the other ZnS nanostructures guided by the metal catalyst [9], the growth rate of assynthesized ZnS nanostructures was quite lower due to the absence of a catalyst, which can effectively promote the growth of nanostructure at a high temperature. Different from the catalytic growth, no metal catalyst particle can be found on the flat or sharp tip of ZnS nanostructure and the flatness of the tip was obviously dependent on the diameter of the branch ZnS nanorods, as shown in Fig. 1a, d, e and f. From the SEM observations, one can also notice that the sidewall of each ZnS nanorod is faceted and forms a typical prism-like morphology, indicating that the ZnS nanorods were grown along the axial direction ([0 0 2]) in a wurtzite-type hexagonal structure. It is believed that the preferential growth along the axial direction plays a key role in the formation of such highly structurally symmetrical nanostructure as reported in a number of similar nanostructures [3] through a self-generated process. In order to systemically study the microstructure, crystallography and composition of the as-synthesized ZnS nanostructures, a high-resolution transmission electron microscope (HRTEM) equipped with an energy dispersive X-ray spectrometer (EDS) was employed and the detailed structural and compositional analyses were carried out on tens of ZnS nanostructures. As an example, Fig. 2a shows the low-magnification TEM image of a branched ZnS nanorod separated from the ZnS nanostructure by scraping the samples from the substrate surface. The ZnS nanorod shows a bulletin-like morphology and possesses a diameter of 200 nm that is consistent with the SEM images. In order to investigate the microstructure and crystallography and deduce the growth behavior of the ZnS nanostructure, three representative areas, namely, tip, shoulder and base, marked by the Arabic numbers 1, 2 and 3, respectively, were selected for HRTEM analysis. Fig. 2b was the HRTEM image of the nanorod tip (3 area) and it can be seen that all the atoms are regularly arranged following a given atom stacking sequence. By measuring
Fig. 1. (a–f) Representative SEM images of as-grown ZnS nanostructures with diverse morphologies and structural symmetries.
B. Liu et al. / Journal of Luminescence 131 (2011) 1095–1099
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Fig. 2. (a) Low-magnification TEM image of a branched ZnS nanorod scraped from the substrate; (b–d) high-resolution TEM images of the selected areas shown in (a); inset is the corresponding ED pattern recorded from the ZnS nanorod; (e) a representative EDS spectrum collected from the ZnS nanostructure.
the distance between the parallel neighboring atomic planes, an interplanar distance of 0.626 nm can be confirmed that corresponds to the d-spacing value of (0 0 2) plane in a wurtzite-type ZnS crystal. From the HRTEM image shown in Fig. 2b, one can also notice that the edge of the flat growth front is parallel to the (0 0 2) plane of hexagonal ZnS crystal, suggesting that the ZnS nanorod was in fact grown along the preferential [0 0 2] direction, which can also be confirmed from the electron diffraction (ED) pattern shown in the inset of Fig. 2. The atomic lattice image of the shoulder region (area 2 in Fig. 2a) was presented in Fig. 2c and the single crystalline characteristic can be verified again. The distances of 0.62 and 0.38 nm match well with the standard lattice d-spacings of (0 0 2) and (1 0 0) planes of a bulky ZnS crystal. As marked in Fig. 2c, the sidewall facet of the ZnS nanorod can be confirmed as (1 0 0) plane, whereas the tilted one corresponds to the (1 0 1) plane. The angle between the tilted plane and radial plane was measured to be 621, which is in good agreement with the theoretical value between (1 0 1) and (0 0 2) planes. Fig. 2d shows the HRTEM image of the base region of the ZnS nanorod (area 3 in Fig. 2a) and one can find that all the atoms exhibit the same atomic stacking sequence as the above two areas. Based on the detailed microstructure analysis, it can be concluded that the as-grown ZnS nanostructures were single crystal and no structural defect such as twin or stacking fault was found [5]. In addition, the crystalline characteristic of the ZnS nanostructures could be further demonstrated by the regularly arranged ED spots. Though the individual ZnS nanorods are of single crystal, it should be mentioned that they may be grown from the initial nucleation site in the form of twin, as observed in tetrapod-branched ZnS nanorod architectures [11]. Fig. 2e shows
a representative EDS spectrum from tens of investigated samples. Only Zn and S peaks could be detected (Cu signal comes from the TEM grids) within the resolution limit of EDS measurement and their atomic ratio approaches Zn:S¼1:1. In addition, no oxygen signal was detected during the EDS analysis, suggesting that the high purity of as-synthesized ZnS nanostructures is in good agreement with the absence of any amorphous oxide layers, as occasionally found in most nanostructures [12]. Different from the catalytic growth of one-dimensional ZnS nanowires, in which the well-known vapor–liquid–solid (V–L–S) mechanism is involved [13], formation of the highly symmetrical ZnS nanostructures may follow a vapor–solid (V–S) process. From SEM and TEM observations, it can be seen that the growth of the ZnS nanostructures is consistent with their crystallographic characteristics. Basically, the formation of ZnS nanostructure with high structural symmetry can be explained as follows: first, the Zn droplets decomposed from the ZnS precursors deposit on the SiO2 surface at low temperature zone and combine with S vapor to form some ZnS nuclei. Due to polarity, these ZnS nuclei prefer to evolve into some nanoscaled, faceted ZnS crystals and combine together in the form of twin boundaries via a self-organization process, as proposed in the formation of dart-shaped tricrystal (DST) and some tetrapod nanostructures [2,3]. Second, the nucleated ZnS crystals will gradually transform to some faceted ZnS nanorods under the continuous supply of Zn droplets and S vapors. When the reaction is stopped, the growth of these ZnS nanorods will terminate. From TEM analysis, it can be seen that all the ZnS nanorods have a preferential orientation along [0 0 2] direction and this may be related to the lower formation energy of this plane. It is believed that the symmetry of ZnS nanostructures
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is directly dependent on the initial numbers of the twin boundaries formed on the ZnS nuclei. In addition, it should be mentioned that the oxide layer (SiO2) may also play a key role in the self-organization of these highly symmetrical ZnS nanostructures, as demonstrated in the formation of ZnS hepta- and tetra-pods, 3D hierarchical ZnO nanostructures, and ZnS DST [2,3,14]. ZnS is a key wide-band-gap semiconductor with a direct bandgap energy of 3.77 eV (for hexagonal wurtzite phase) that corresponds to a wavelength of 329 nm. Thus, it can be used to fabricate some highly sensitive ultraviolet optoelectronic devices such as ultraviolet photodetector and H2 sensor [7,15]. However, most of the ZnS crystals, either bulk or nanostructures, exhibit strong green emissions rather than the intrinsic band-gap ultraviolet emission due to the existence of large amount of structural defects such as twin or stacking fault [5]. In addition, the impurity contamination in ZnS lattice such as O element can also lead to the shift of emission wavelength towards the visible range. Especially, the band-gap emission is rather challenging in nanostructured ZnS materials due to its easy oxidation or contamination. Fig. 3 shows a typical CL spectrum recorded from an individual ZnS nanostructure under an applied accelerating voltage of 5 KV and a beam current of 1000 pA. A sharp emission peak centered at 337 nm and a broad emission range from 400–800 nm can be observed. Obviously, the 337 nm peak originates from the band-gap emission, whereas the broad emission peak in the visible range may result from the defect-related emission. The 337 nm band-gap emission further demonstrated good crystallinity and high purity of the as-synthesized ZnS nanostructures, as verified by HRTEM and EDS analyses (Fig. 2), and was consistent with the previous results [2,7]. For the visible emission, generally it is believed that some catalyst-introduced deep levels or Zn vacancy-related structural defect in the ZnS lattice host may induce such emission. In addition, some structural defects such as point defects, microtwins and dopants (Mn2 + ) incorporated into the ZnS lattice were also reported [16],
and various defect-related emissions in the range of 400–800 nm have been observed in diverse ZnS structures [9,17,18]. In this work, no microtwin or stacking fault was found in ZnS nanostructures during HRTEM observation and ED analysis so the effect of these structural defects on the visible emission can be basically excluded. The main reason responsible for this defect-related emission can be considered as a result from some elemental sulfur species, as verified by X-ray photoelectron spectroscopy (XPS) and photoluminescence measurement [19]. In addition, though the impurity contamination coming from both catalyst and foreign dopants (Si or O) was not detected within the resolution limit of EDS measurement, some Si or O impurity with a much lower content may still accumulate in the ZnS lattice, and thus generate some deep impurity levels between the conduction and valence bands of ZnS crystal. Therefore, the broad emission peak in the visible range may be caused by a combined effect coming from Zn/S vacancies and Si/O impurities. In order to further study the origin of the two band emissions, wavelength-resolved CL properties of the as-synthesized ZnS nanostructures were investigated. From the CL spectrum, it can be known that the ZnS nanostructures exhibited a sharp band-gap emission centered at 337 nm and a broad visible emission with maximum peak intensity at 463 nm. Therefore, the 337 and 463 nm wavelengths were used to examine the emission origin of the two bands, respectively. Fig. 4a shows a typical SEM image of a ZnS nanostructure and its corresponding CL images, taken at 337 and 463 nm. It can be seen that the 337 nm emission was quite uniform, whereas the 463 nm emission is uneven. In addition, the 337 nm CL image exhibited a rather brighter emission, whereas the 463 nm emission is a little weak, indicating the excellent band-gap emission of as-synthesized ZnS nanostructures. With strong ultraviolet emission, it is believed that our ZnS nanostructures hold promising application in the field of nanooptoelectronic devices such as ultraviolet photodetectors.
4. Conclusion
Intensity (a. u.)
337 nm
200
400
600
800
1000
Wavelength (nm) Fig. 3. CL spectrum collected from a four-fold symmetrical ZnS nanostructure.
In summary, complex ZnS nanostructures with two-fold, three-fold, four-fold or multi-fold structural symmetry have been synthesized through a simple thermal evaporation process. All the ZnS nanostructures are wurtzite-type hexagonal phase and each branched nanorod in the ZnS nanostructure is a single crystalline, possessing a diameter in the range of 200–600 nm and a length up to 2 mm. In addition, all the branched ZnS nanorods exhibit a preferential orientation along [0 0 2] direction. CL measurements showed that the ZnS nanostructures have strong band-gap emission with a centered wavelength of 337 nm and a broad emission in the visible range. It is believed that the ZnS nanostructures with highly structural symmetry and strong ultraviolet emission may find some potential application in the field of optical and optoelectronic devices.
Fig. 4. (a) SEM image of an individual ZnS nanostructure; (b, c) CL images taken at 337 and 463 nm, respectively. The scale bar is 500 nm.
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