Fabrication, structural characterization and ...

Home

Search

Collections

Journals

About

Contact us

My IOPscience

Fabrication, structural characterization and photoluminescence of Q-1D semiconductor ZnS hierarchical nanostructures

This content has been downloaded from IOPscience. Please scroll down to see the full text. 2006 Nanotechnology 17 2695 (http://iopscience.iop.org/0957-4484/17/10/042) View the table of contents for this issue, or go to the journal homepage for more

Download details: IP Address: 222.178.10.249 This content was downloaded on 08/10/2013 at 20:22

Please note that terms and conditions apply.

INSTITUTE OF PHYSICS PUBLISHING

NANOTECHNOLOGY

Nanotechnology 17 (2006) 2695–2700

doi:10.1088/0957-4484/17/10/042

Fabrication, structural characterization and photoluminescence of Q-1D semiconductor ZnS hierarchical nanostructures Jun Zhang1 , Yongdong Yang, Feihong Jiang, Jianping Li, Baolong Xu, Xichang Wang and Shumei Wang Research Center of Optoelectronic Functional Materials, Department of Physics, Yantai University, Yantai 264005, People’s Republic of China E-mail: [email protected]

Received 9 January 2006, in final form 6 April 2006 Published 8 May 2006 Online at stacks.iop.org/Nano/17/2695 Abstract Quasi-one-dimensional semiconductor ZnS hierarchical nanostructures have been fabricated by thermal evaporation of a mixture of ZnS nanopowders and Sn powders. Sn nanoparticles are located at or close to the tips of the nanowires (or nanoneedles) and served as the catalyst for quasi-one-dimensional ZnS nanostructure growth by a vapour–liquid–solid mechanism. The morphology and microstructure of the ZnS hierarchical nanostructures were measured by scanning electron microscopy and high-resolution transmission electron microscopy. The results show that a large number of ZnS nanoneedles were formed on the outer shells of a long and straight ZnS axial nanowire. The ZnS axial nanowires grow along the [001] direction, and ZnS nanoneedles are aligned over the surface of the ZnS nanowire in the radial direction. The room temperature photoluminescence spectrum exhibits a UV weak emission centred at 337 nm and one blue emission centred at 436 nm from the as-synthesized single-crystalline semiconductor ZnS hierarchical nanostructures.

1. Introduction Research in quasi-one-dimensional (Q-1D) nanostructures (nanotubes, nanowires and nanoribbons) has attracted a great deal of attention because they are abundant and of potential application in semiconductor and electronic technologies, especially for nanoelectronics and optoelectronics [1–10]. In the past few years, much effort has been devoted to develop the controlled growth of various Q-1D semiconductor nanocrystals via laser ablation-catalytic growth [11, 12], oxide-assisted growth [13], template-inducted growth [14], and solution– liquid–solid growth [15]. Q-1D semiconductor hierarchical nanostructures are the focus of considerable research interest. Current efforts are focused on developing synthetic methods to prepare and introduce function controllably at branch points 1 Author to whom any correspondence should be addressed.

0957-4484/06/102695+06$30.00

in both two- and three-dimensional structures. Most recently, several authors have prepared the hierarchical nanostructures of silica and semiconducting oxide [16–18]. For example, Ye and his co-workers reported a silicon nanowire hierarchical structure standing on silica microwires [16]. Wang’s research group [17] and Ren’s research group [18] reported ZnO hierarchical nanostructures, respectively. They have demonstrated that various semiconducting oxides, such as ZnO, In2 O3 , or SiOx hierarchical nanostructures, can be fabricated by a simple evaporation and condensation process, and they proposed that these novel structures could be applied in field emission, photovoltaics, supercapacitors, and so on. As an important II–VI semiconductor, zinc sulfide has received great attention in recent years for its potential application in optoelectronics [19–21]. This wide bandgap (3.66 eV at 300 K) semiconductor [22] has a high refractive index [21] and a high transmittance in the visible

© 2006 IOP Publishing Ltd Printed in the UK

2695

J Zhang et al

range [23, 24]. It is used as a key material for ultraviolet light-emitting diodes and injection lasers [25], as phosphors in cathode-ray tube and flat-panel displays [26], for thin film electroluminescence, and for IR windows [27]. ZnS is also a promising triboluminescence material [28]. Several Q-1D ZnS nanostructures, including nanowires, nanobelts/nanoribbons, nanosaws, and nanocables, have been reported [29], and their photoluminescence (PL) property has also been reported [30]. In this paper, we report the synthesizing Q-1D ZnS hierarchical nanostructures. Using the mixture of metal tin powders and ZnS nanopowders as the source material, ZnS hierarchical nanostructures have been synthesized. A large number of ZnS nanoneedles were formed on the outer shells of a long and straight ZnS nanowire. These nanoneedles are aligned over the surface of the ZnS nanowire in the radial direction. The tip of every ZnS nanowire and nanoneedle has a large head, which is identified as a Sn metal particle that served as the catalyst for the growth. The nanostructure has multioutlets and are ideal objects for the fabrication of nanoscale functional devices. It is anticipated that these novel structures will have some unique applications in nanophotonics.

(a)

20 µm

(b)

2. Experimental section The experimental set-up used for the synthesis consists of a horizontal tube furnace, an alumina tube, a gas supply and control system. ZnS hierarchical nanostructures were fabricated through thermal evaporation via the vapour–liquid– solid (VLS) mechanism. A mixture of commercial metal tin powders and high-purity ZnS nanopowders was put in an alumina boat, then the boat was inserted into an alumina tube. The alumina tube was placed inside a horizontal electronic resistance furnace with the centre of the boat positioned at the centre of the furnace. A silica plate as substrate was typically placed at 5–10 cm from the centre of the boat and the substrate was placed downstream of the gas flow. The temperature of the furnace was rapidly increased to 950 ◦ C from room temperature and kept at 950 ◦ C for 2 h under a constant flow gas of argon. Argon was introduced into the alumina tube through a mass-flow controller at rates of 50 standard cubic centimetres per minute (sccm). After the furnace was slowly cooled down to room temperature, the Ar flow was turned off. A layer of wool-like products was formed on the walls of the boat and the surface of the substrate. The as-prepared products were characterized and analysed by scanning electron microscopy (SEM) (JEOL JSM5610LV), high-resolution transmission electron microscopy (HRTEM) (JEOL 2010, 200 kV), and energy-dispersive xray spectrometer (EDS) (JEOL JED-2200) attached to the SEM instrument. The photoluminescence (PL) measurements (Hitachi, 850 spectrophotometer) were performed using an excitation wavelength of 254 nm and a filter wavelength of 310 nm with a Xe lamp at room temperature. The specimens for HRTEM were prepared by putting the as-prepared products in ethanol and immersing them in an ultrasonic bath for 5 min, then dropping a few drops of resulting suspension containing the synthesized materials onto a Cu grid coated with a holey carbon film. 2696

1 µm

(c) Branch wire Axial wire

1 µm

Figure 1. (a) A typical SEM image of the as-synthesized products. The morphology is similar to that of a bristled caterpillar, so we call it a bristled nanocaterpillar. (b) A magnified SEM image of the ZnS hierarchical nanostructures. (c) A SEM image of an individual ZnS hierarchical nanostructure.

3. Results and discussion SEM observation shown in figure 1(a) reveals that the products consist of a large number of wirelike nanostructures with typical lengths in the range of several tens of micrometres. The magnified SEM image of the as-prepared products is given in figure 1(b). A long and straight axial nanowire has a large head and a large number of nanoneedles were grown on the surface of the axial nanowire. Every nanoneedle also has a head. It can be clearly seen that the nanoneedles are aligned over the surface of the long and straight nanowire in the radial direction. The morphology shown here is similar to that of a bristled caterpillar, so we call it a bristled nanocaterpillar. A typical


* (112)

Intensity (a.u.)

(002)

(101)

(100)

(110)

(a)

* (103)

1 µm

30

40 50 2θ (degree)

Zn

(201)

(200)

(102)

20

*

**

60

70

Figure 2. XRD pattern of the ZnS hierarchical nanostructures. The unindexed starred peaks may be due to Sn nano- (or micro-) particles attached at the end of the ZnS nanowires.

SEM image (figure 1(c)) of an individual ZnS hierarchical nanostructure provided further detail of morphology. The image showed that the three nanoneedles were grown on the surface of a long and uniform axial nanowire. XRD pattern of the products shown in figure 2 reveals that the whole spectrum can be indexed in peak positions to wurtzite phase ZnS, in good agreement with the reported values of the bulk hexagonal ZnS phase. The lattice constants of the crystalline phase are a0 = 0.383 nm and b0 = 0.626 nm (JCPDS 10-434) [31]. The unindexed starred peaks may be due to Sn nano- (or micro-) particles attached at the end of the ZnS nanowires. The compositional homogeneity of the ZnS hierarchical nanostructures was further confirmed using EDS. An individual central axial wire was shown in figure 3(a). EDS spectra reveal the presence of Zn, S, and Sn. The Zn, S, and Sn elemental mapping of the axial wire demonstrates the composite nanowire structure. EDS measurements made on the stem indicate that the stem is composed of Zn and S, and the tip is composed only of Sn. In addition, the elemental mappings from a nanoneedle shown in figure 3(b) also further reveal that the tip is composed only of Sn; the stem is composed mainly of Zn and S. The molecular ratio of Zn/S of the nanowire calculated from the EDS quantitative analysis data is close to that of a bulk ZnS crystal. Therefore, EDS microanalysis shows that the products are large quantities of ZnS hierarchical nanostructures. Detailed structural information of the ZnS nanowires was obtained from the high-resolution TEM (HRTEM). A lowmagnification TEM image of a segment of a ZnS nanowire and its tip is shown in figure 4(a). It can be seen that the linear segment was crystalline with a uniform diameter of 50 nm. The tip of the nanowire has a large head, which is identified as a Sn metal particle that served as the catalyst for the growth. Selected area electron-diffraction (SAED) patterns (inset in figure 4(a)) indicate that the ZnS nanowire has hexagonal symmetry. The HRTEM image is shown in figure 4(b); the clear lattice fringes indicated a single-crystal structure of the nanowire. The spacing between the lattice planes perpendicular to the nanowire axis is about 0.315 nm, which agrees well with the (002) spacing of wurtzite ZnS. In this

S

Sn

(b)

500 nm

S

Zn

Sn

Figure 3. (a) SEM image of a central axial wire. The long and straight central axial wire has a large head at the tip. The elemental mappings of the axial wire demonstrate the composite wire structure. The results indicate that the stem is composed of Zn and S, and the tip is composed only of Sn. (b) SEM image of a nanoneedle. The needle has a large head at the tip. The elemental mappings of the nanoneedle indicate that the needle is composed of Zn and S, and the tip is composed only of Sn. (This figure is in colour only in the electronic version)

image, the adjacent lattice plane (arrow-heads) corresponds to the distance between two (002) crystal planes, indicating 001 as the growth direction for the ZnS nanowire. A TEM image of a segment of a branch ZnS nanostructure and its tip (inset) is shown in figure 4(c). The image shows a junction consisting of an axial nanowire and a branch of a nanoneedle. An HRTEM image for the ZnS branch structure is shown in figure 4(d). The growth direction for the ZnS branch structure is [0110]. These data from SEM, TEM, and HRTEM altogether reveal that the growth of the ZnS hierarchical nanostructures may be dominated by the VLS process [32]. In our experiment, the Sn droplet is located at the growth front of the wire and acts as the catalytic active site. The solidified spherical droplets at the tips of the nanowires are commonly considered to be the evidence for the operation of the VLS mechanism. From the experimental results demonstrated above, the Sn particles are liquid droplets at the growth temperature due to their low 2697

J Zhang et al

(b)

(a)

Nanowire axis

Sn

0.315 nm ZnS [001] 2 nm 100 nm

(d)

(c)

Branch axis

Branch wire [0110]

Axial wire

100 nm

Figure 4. (a) A TEM image of a central axial wire. The nanowire has a uniform diameter of 50 nm. The tip of the nanowire has a large head, which is identified as a Sn metal particle that served as the catalyst for the growth of the VLS mechanism. SAED patterns (inset) indicate that the axial wire has hexagonal structure. (b) HRTEM image of the crystalline ZnS nanowire. The (002) planes (spacing 0.315 nm) are perpendicular to the growth direction. (c) A TEM image of a junction consisting of an axial nanowire and a branch of a nanoneedle. The tip of the needle has a large head (inset). (d) HRTEM image of the branch ZnS nanoneedle. The growth direction is [0110].

Sn

ZnS [0001]

(a)

Sn

(b)

(c)

(d)

Figure 5. Schematic diagram showing the two-stage growth of ZnS junctions.

melting point (232 ◦ C), and serve as the sites for adsorption of ZnS vapour. The central axial wire grows quickly along [001] with the Sn particle at the tip; smaller droplets of Sn particles can be formed on the axial nanowire surface, 2698

which leads to the growth of ZnS hierarchical structure. All of the grown branches are likely to be confined in the surface because the entire piece is a single-crystalline structure. When the tip of the branch grows bigger and bigger, more ZnS vapour will be adsorbed by the catalytic active site, and the diameter of the branch will be increased. While the lower part of the branch shows the extreme sharp morphology, its tip exhibits a large head. Therefore, based on the growth models of the Sn-catalysed self-assembled nanowire–nanoribbon junction arrays of ZnO [17] and the Sncatalysed thermal-evaporation synthesis of tetrapod-branched ZnSe nanorod architectures [33], the growth of the ZnS hierarchical nanostructure presented in the present study can be separated into two stages. The first stage is a fast growth of the ZnS axial nanowire along [0001] with Sn as the catalyst (figure 5(a)). The growth rate is so high that a slow increase in the size of the Sn droplet has little influence on the diameter of the ZnS axial nanowire; thus, the ZnS axial nanowire has a fairly uniform shape along the growth direction. The second stage of the growth is the nucleation and epitaxial growth of the nanoneedles due to the arrival of the tiny Sn droplets onto the ZnS nanowire surface (figure 5(b)). This stage is much slower than the first stage. Since Sn is in liquid state at the growth temperature, it tends to adsorb the newly arriving Sn species


exhibit the emission property at 436 nm. However, the origin of the luminescence of ZnS hierarchical nanostructures is a rather complicated process. Therefore, further detailed work is needed to clarify the underlying mechanism for the PL spectrum of the ZnS hierarchical nanostructures.

PL Intensity (a.u.)

436 nm

4. Conclusions

337 nm

300

400

500

600

700

Wavelength (nm) Figure 6. Room-temperature PL spectrum of as-synthesized ZnS hierarchical nanostructures.

and grows into a larger size particle (figure 5(c)). Therefore, the diameter of the ZnS nanoneedle increases as the size of the Sn particle at the tip becomes larger, resulting in the formation of a needle-like structure (figure 5(d)). The direct evidence comes from the SEM images (in figures 1(b), (c) and 3(b)) and TEM image (in figure 4(c)) observations. The room-temperature PL spectrum of the as-synthesized ZnS hierarchical nanostructures was measured with excitation wavelength of 254 nm. As shown in figure 6, the spectrum exhibits one weak emission peak at 337 nm and one strong emission peak at 436 nm from the as-synthesized product. Previous works on wurtzite ZnS nanowires have found the peaks of ZnS in the range 330–340 nm [34]. The emission peak of ZnS corresponds to the bandgap emission. Because it is a bandgap semiconductor (3.66 eV) at room temperature, ZnS is used as a key material for UV light-emitting diodes. However, direct bandgap luminescence of pure ZnS nanocrystals has rarely been reported. Lee’s group [29e] reported that the ZnS nanowires exhibit a strong and stable bandgap emission centred at 3.66 eV, which is blueshifted by 47 nm relative to the emission from the bulk ZnS. The luminescence mechanism might be attributed to the polytype modulated nanostructure sizing at the several angstrom scale in the as-synthesized ZnS nanowires. In our experiment, the weak UV peak at 337 nm also corresponds to the bandgap emission of ZnS; the mechanism could be due to the quantum size effect. Recently, the PL properties of nanocrystalline doped ZnS have been extensively studied. Nanocrystalline ZnS doped with Cu2+ , Mn2+ , or the rare earth ions (Re2+ ) can exhibit emission from the blue to the green [34]. These emission properties can be due to transitions from the conduction band of ZnS to the different levels of excited Cu2+ , Mn2+ , or Re2+ in the ZnS bandgap [29e]. In our experiment, is metal Sn exerting some influence on PL spectra or not? Because metal tin powders and high-purity ZnS nanopowders are the source material, metal Sn not only serves as a catalyst, but also simultaneously supplies doped ions. Based on the previous reports [34, 29e] and our experimental result, it was reasonable to explain that the strong emission peak at 436 nm might be attributed to the transitions from the conduction band of ZnS to the different levels of excited Sn2+ in the ZnS bandgap. That is to say, nanocrystalline ZnS doped with Sn2+ could

In summary, with a mixture of metal Sn powder and ZnS nanopowder as the source material, ZnS hierarchical nanostructures have been synthesized. ZnS axial nanowires are straight and long; the tip of a nanowire has a large head. A large number of ZnS nanoneedles were aligned over the surface of the ZnS nanowire in the radial direction. Every ZnS nanoneedle also has a large head. HRTEM images show that the ZnS nanowires and ZnS nanoneedles are pure, structurally uniform, and single crystalline. Therefore, the method may suggest a way of making a heterojunction between nanowires for so-called bottom-up manufacturing techniques.

Acknowledgment This work was supported by the National Natural Science Foundation of China (grant No 60277023).

References [1] Duan X F, Huang Y, Cui Y, Wang J and Leiber C M 2001 Nature 409 66 [2] Cui Y and Leiber C M 2001 Science 291 851 [3] Huang M H, Mao S, Feick H, Yan H Q, Wu Y Y, Kind H, Weber E, Russo R and Yang P D 2001 Science 292 1897 [4] Biork M T, Ohlsson B J, Saaa T, Persson A I, Thelander C, Magusson M H, Deppert K, Wallenburg L R and Samuelson L 2002 Appl. Phys. Lett. 80 1058 [5] Xia Y, Yang P, Sun Y, Wu Y, Mayers B, Gates B, Yin Y, Kim F and Yan H 2003 Adv. Mater. 15 353 [6] Wang Z L, Kong X Y, Ding Y, Gao P, Hughes W L, Yang R and Zhang Y 2004 Adv. Funct. Mater. 14 943 [7] Pan Z W, Dai Z R and Wang Z L 2001 Science 291 1947 [8] Dai Z R, Pan Z W and Wang Z L 2003 Adv. Funct. Mater. 13 9 [9] Gao P X and Wang Z L 2005 Small 1 945 [10] Fustin C-A, Lohmeijer B G G, Duwez A-S, Jonas A M, Schubert U S and Gohy J-F 2005 Adv. Mater. 17 1162 [11a] Yang P D and Lieber C M 1996 Science 273 1896 [11b] Morales A M and Lieber C M 1998 Science 279 208 [11c] Zhang Y F, Tang Y H, Wang N, Yu D P, Lee C S, Bello I and Lee S T 1998 Appl. Phys. Lett. 72 1835 [11d] Huynh W U, Peng X G and Aliviasators A P 1998 J. Am. Chem. Soc. 120 5343 [11e] Wang W Z, Geng Y, Yan P, Liu F Y, Xie Y and Qian Y T 1999 J. Am. Chem. Soc. 121 4026 [12a] Duan X F and Lieber C M 2000 J. Am. Chem. Soc. 122 188 [12b] Yu D P, Sun X S, Lee C S, Bello I, Lee S T, Gu H D, Leung K M, Zhou G W, Dong Z F and Zhang Z 1998 Appl. Phys. Lett. 72 1966 [13a] Zhang R Q, Lifshitz Y and Lee S T 2003 Adv. Mater. 15 635 [13b] Lee S T, Zhang Y F, Wang N, Tang Y H, Bello I, Lee C S and Chung Y W 1999 J. Mater. Res. 14 4503 [14a] Han W, Fan S, Li Q and Hu Y 1997 Science 277 1287 [14b] Pan Z W et al 2000 Adv. Mater. 12 1186 [14c] Li Y, Meng G W and Zhang L D 2000 Appl. Phys. Lett. 76 2011 [15a] Jiang Y, Wu Y, Mo X, Yu W C, Xie Y and Qian Y T 2000 Inorg. Chem. 39 2964

2699

J Zhang et al

[15b] Jiang Y, Wu Y, Yuan S W, Xie B, Zhang S Y and Qian Y T 2001 J. Mater. Res. 10 2805 [16] Ye C H, Zhang L D and Fang X S 2004 Adv. Mater. 16 1019 [17] Gao P X and Wang Z L 2002 J. Phys. Chem. B 106 12653 [18a] Luo J Y, Wen J G and Ren Z F 2002 Nano Lett. 2 1287 [18b] Luo J Y, Huang J Y, Wang D Z and Ren Z F 2003 Nano Lett. 3 235 [19] Kishimoto S, Kato A, Naito A, Sakamoto Y and Iida S 2002 Phys. Status Solidi b 1 391 [20] Sun L, Liu C, Liao C and Yan C 1999 J. Mater. Chem. 9 1655 [21] Jiang X, Xie Y, Lu L, Zhu L, He W and Qian Y 2001 Chem. Mater. 13 1213 [22] Pawaskar N R, Sathaye S D, Mhadbhade M M and Patil K R 2002 Mater. Res. Bull. 37 1539 [23] Elidrissi B, Addou M, Regragui M, Bougrine A, Kachouane A and Bernecde J C 2001 Mater. Chem. Phys. 68 175 [24] Yamaga S, Yoshikawa A and Kasai H 1998 J. Cryst. Growth 86 252 [25] Yamamoto T, Kishimoto S and Iida S 2001 Physica B 308 916 [26] Bredol M and Merikhi J 1998 J. Mater. Sci. 33 471 [27] Calandra P, Goffredi M and Trurco Liberi V 1999 Colloids Surf. A 160 9 [28a] Walton A J 1997 Adv. Phys. 26 887 [28b] Xu C N, Watanabe T, Akiyama M and Zhang X G 1999 Appl. Phys. Lett. 74 1236 [29a] Jiang Y, Meng X M, Liu J, Xie Z Y, Lee C S and Lee S T 2003 Adv. Mater. 15 323 [29b] Ma C, Moore D, Li J and Wang Z L 2003 Adv. Mater. 15 228

2700

[29c] Moore D, Ronning C, Ma C and Wang Z L 2004 Chem. Phys. Lett. 385 8 [29d] Zhu Y C, Brando Y and Uemura Y 2003 Chem. Commun. 836 [29e] Jiang Y, Meng X M, Liu J, Hong Z R, Lee C S and Lee S T 2003 Adv. Mater. 15 1195 [29f] Ding Y, Wang X D and Wang Z L 2004 Chem. Phys. Lett. 398 32 [29g] Barrelet C J, Wu Y, Bell D C and Lieber C M 2003 J. Am. Chem. Soc. 125 11498 [29h] Moore D F, Ding Y and Wang Z L 2004 J. Am. Chem. Soc. 126 14372 [30a] Wang Z W, Daemen L L, Zhao Y, Zha C S, Downs R T, Wang X D, Wang Z L and Hemleys R J 2005 Nat. Mater. 4 922 [30b] Fang X S, Ye C H, Zhang L D, Wang Y H and Wu Y C 2005 Adv. Funct. Mater. 15 63 [30c] Yao W T, Yu S H, Pan L, Li J, Wu Q S, Zhang L and Jiang J 2005 Small 1 320 [30d] Fang X S, Ye C H, Peng X S, Wang Y H, Wu Y C and Zhang L D 2004 J. Cryst. Growth 263 263 [31] Powder Diffraction File. Inorganic Vol. No. 10-434 file. Published by the Joint Committee on Powder Diffraction Standards, 1601 Park Lane, Swarthmere, PA 19081, USA [32] Wagner R S and Ellis W C 1964 Appl. Phys. Lett. 4 89 [33] Hu J Q, Bando Y and Golberg D 2005 Small 1 95 [34a] Song W, Qian Y, Min J, Li D, Wang L, Shi W and Liu Y 2002 Solid State Commun. 121 475 [34b] Bol A A and Meijerink A 2001 J. Phys. Chem. B 105 10197 [34c] Bol A A and Meijerink A 2001 J. Phys. Chem. B 105 10203