Heteroepitaxial Growth of Orientation-Ordered ZnS Nanowire Arrays

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J. Phys. Chem. C 2008, 112, 12299–12303

12299

Heteroepitaxial Growth of Orientation-Ordered ZnS Nanowire Arrays Guozhen Shen,*,†,‡ Yoshio Bando,†,‡,§ Dmitri Golberg,†,‡,§ and Chongwu Zhou† Department of Electrical Engineering, UniVersity of Southern California, Los Angeles, California 90089, Nanoscale Materials Center, National Institute for Materials Science, Namiki 1-1, Tsukuba, Ibaraki 305-0044, Japan, and World Premier International Center for Materials Nanoarchitectonics (MANA), National Institute for Materials Science (NIMS), Namiki 1-1, Tsukuba, Ibaraki 305-0044, Japan ReceiVed: May 6, 2008; ReVised Manuscript ReceiVed: June 10, 2008

Zinc sulfide (ZnS) is an important functional material for electronics and optoelectronics. In this paper, heteroepitaxial growth of ZnS nanowire arrays on Zn3P2 crystals, which are structural uniform with preferred (0001) growth directions, was fulfilled by using a simple thermal evaporation method. Studies found that hexagonal ZnS and tetragonal Zn3P2 showed a good orientation relationship of [010]Zn3P2//[12j10]ZnS and (101)Zn3P2//(0002)ZnS, which makes it possible to obtain well-aligned ZnS nanowire arrays epitaxially grown on Zn3P2 crystals. As-grown ZnS nanowire arrays show a strong broad green emission band centered at 554 nm and a weak emission band at about 802 nm. Field-emission studies found that they show good field emissions with a turn-on field of about 3.72 V/µm. 1. Introduction Zinc sulfide (ZnS) is an important functional material for electronics and optoelectronics. It has a wide band gap of 3.8 eV at room temperature. It is a key material for ultraviolet lightemitting diodes, injection lasers, cathode ray tubes, flat panel displays, and IR windows.1 Other important properties of ZnS include photoluminescence, electroluminescence, triboluminescence, and photocatalyst.2–4 The growth of ZnS bulk crystals and thin films has received attention for decades. Recently, onedimensional (1-D) nanostructures have attracted intense research interest because of their unique physical properties and potential applications as building blocks in nanoscale electronics and optoelectronics.5 Consequently, much effort has been devoted to the synthesis and properties studies of 1-D ZnS nanostructures including nanowires, nanobelts, and nanotubes, as well as their complex 3-D objects.3,4,6–8 Very recently, it was found that ZnS nanowires showed very good field emission properties although the work function for ZnS is relatively larger and bulk ZnS does not show significant field emissions.9 It is well-known that aligned nanostructures with a high packing density can significantly enhance the material field emission properties.10,11 Thus it is still of great interest to produce well-aligned ZnS nanowires to improve the field emission properties. The formation of well-ordered single-crystalline nanowires with controlled crystal orientation is needed for application to nanoscale devices and sensors.12 Especially, the formation of heteroepitaxial interfaces has proven to be useful in the development of numerous device concepts, as well as in the investigation of low-dimensional phenomena. Heteroepitaxial growth of nanowires on bulk substrates provides a very powerful approach to produce well-aligned crystal orientated nanowire arrays.13–15 As-grown vertically standing epitaxial nanowires as a device platform avoid pick-and-place approaches or nanomanipulations. In this paper, we reported the synthesis of orienta* Corresponding author. E-mail: [email protected]; [email protected]. † University of Southern California. ‡ Nanoscale Materials Center, National Institute for Materials Science. § World Premier International Center for Materials Nanoarchitectonics, National Institute for Materials Science.

tion-ordered ZnS nanowire arrays epitaxially grown on Zn3P2 crystals. As hexagonal ZnS and tetragonal Zn3P2 show a good orientation relationship of [010]Zn3P2//[12j10]ZnS and (101)Zn3P2// (0002)ZnS, well-aligned ZnS nanowire arrays were epitaxially grown on Zn3P2 crystals. As-grown ZnS nanowire arrays show good field emissions with a turn-on field of about 3.72 V/µm. 2. Experimental Section Orientation-ordered ZnS nanowire arrays were grown epitaxially on Zn3P2 crystals in a self-made high-frequency induction furnace.16 The furnace consists of a fused quartz tube and an induction-heated cylinder made of high-purity graphite coated with a carbon-fiber thermoinsulating layer. One inlet C pipe and one outlet C pipe are set on its top and base, respectively. In a typical process, a graphite crucible containing ZnS and Zn3P2 powders was placed in the center cylinder, where ZnS was located in the center while Zn3P2 was downstream 4 cm away from ZnS. After evacuation of the quartz tube to approximately 20 Pa, a flow of pure Ar was introduced into the furnace and maintained through the top inlet at a flow rate of 50 standard cubic centimeters per minute (sccm) at ambient pressure. The crucible was rapidly heated to 1250 °C in 10 min and kept at that temperature for 1 h before being cooled to room temperature. The products were collected for characterization by using field emission scanning electron microscopy (SEM, JEOL JSM6700), and high-resolution transmission electron microscopy (HRTEM, JEOL JEM-3000F) equipped with an energydispersive X-ray spectrometer (EDS). Photoluminescence (PL) was studied at room temperature with use of a continuous-wave He-Cd laser with 325 nm line. Field emission properties of the orientation-ordered ZnS nanowire arrays were studied by using a field emission system composed of a vacuum chamber of low pressure (∼1.1 × 10-5 Pa) and a rod-like copper probe with a cross-sectional area of 1 mm2. The copper probe acts as the anode while ZnS nanowire arrays act as the cathode. 3. Results and Discussion Typical SEM images of the obtained ZnS nanostructures were shown in Figure 1. Panels a and b of Figure 1 are the side view

10.1021/jp8039687 CCC: $40.75  2008 American Chemical Society Published on Web 07/19/2008

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Figure 1. (a, b) Side view, (c-e) tilt view, and (f) top view SEM images of the products, clearly showing the formation of well-aligned ZnS nanowires arrays.

SEM images of two as-synthesized ZnS nanowire arrays. They clearly show that the as-synthesized sample has a dominant morphology consisting of well-aligned arrays of ZnS nanowires. All of the nanowires are orientationally aligned with uniform diameters of several tens of nanometers and lengths of ca. 500 nm. Side view images (Figure 1c-e) and a top view image (Figure 1f) give further evidence of the formation of well-aligned ZnS nanowire arrays. In most cases, there exist two clear features: one is that the orientation-ordered ZnS nanowire arrays grew on top of a Zn3P2 crystal (see the following EDS data). The other one is that most of the ZnS nanowires have bigger diameters on the top then the bottom. One possible mechanism is that during vapor phase synthesis, ZnS vapors generated at high temperature are transferred to the low-temperature region and deposit on Zn3P2 crystals and grow into small ZnS nanowires. Since no catalyst was used, the formation of ZnS nanowires is mainly controlled by the well-known vapor-solid (VS) mechanism. The generated ZnS vapors will continuously absorb on the Zn3P2 crystals and push up the preformed small ZnS nanowires and increase their lengths. Usually, the top has a longer time to absorb the ZnS vapors than the bottom part, which results in the formation of the nanowire with decreased diameters from the top to the bottom.17 Detailed composition and microstructural analyses of the aligned nanowire arrays were carried out by using transmission electron microscopy (TEM) with an X-ray energy dispersive spectrometer (EDS). Figure 2a showed a TEM image of the ZnS nanowires grown on a Zn3P2 crystal, which have lengths of about 500 nm. EDS spectra were taken on the nanowires and the bottom crystal and the spectra were shown in Figure 2, panels b and c, respectively. Figure 2b of the nanowires showed the presence of Zn and S with an approximate atomic ratio of 1:1, confirming that the products are well-aligned ZnS nanowire arrays. As expected, the bottom crystal is Zn3P2, judging from Figure 2c. Figure 2d depicts a high-magnification TEM image of a ZnS nanowire array. It clearly shows that the ZnS nanowires are well-aligned with uniform diameters of ca. 40 nm. A highresolution TEM (HRTEM) image of the ZnS nanowires is shown in Figure 2e. The clearly resolved lattice perpendicular to the growth direction is 0.31 nm, corresponding to the (0001)

plane of wurtzite ZnS. The inset shows the selected-area electron diffraction (SAED) pattern taken from the nanowires, which is along the [100] zone axis of ZnS. The results suggest that the entire ZnS nanowires are single-crystalline ZnS with a wurtzite structure grown along the [0001] direction. To study the interface and orientation relationship between ZnS and Zn3P2, a HRTEM image taken from the interface domain is shown in Figure 2f. The resolved lattice fringes are 0.62 nm for the (0001) plane of wurtzite ZnS and 0.33 nm for the (202) plane of tetragonal Zn3P2. The interface between the ZnS and Zn3P2 fragment can be clearly seen from the image, where some defects can be found. This image clearly shows the crystal orientation configuration between ZnS and Zn3P2 as [010]Zn3P2// [12j10]ZnS and (101)Zn3P2//(0002)ZnS, which indicates that ZnS nanowires grow epitaxially on Zn3P2 crystals. We studied the ZnS products grown on Zn3P2 after 20 min. Figure 3 is a SEM image indicating the initial state of the ZnS products. It can be clearly seen that numerous small ZnS nanowires already grew on Zn3P2 crystals. With the increase of reaction time, aligned ZnS nanowires with uniform lengths were obtained as already discussed in Figure 1. Panels a and b of Figure 4 show the unit cells of hexagonal wurtzite ZnS and tetragonal Zn3P2, respectively. Structurally, hexagonal ZnS with a space group of P63mc has cell parameters of a ) 3.820 Å and c ) 6.257 Å, while tetragonal Zn3P2 with a space group of P42/nmc has cell parameters of a ) 8.097 Å and c ) 11.45 Å. As HRTEM results have already proved that ZnS and Zn3P2 have crystal orientation relationships as [010]Zn3P2//[12j 10]ZnS and (101)Zn3P2//(0002)ZnS, a cross-section structural model of the well-aligned ZnS nanowires grown on Zn3P2 projected along the [010]Zn3P2// [12j 10]ZnS layout were drawn here to study their epitaxial relationships and the model is shown in Figure 4c. It can be seen that during the initial growth, P atoms in Zn3P2 can be substituted by S atoms in ZnS on the interface due to the similar radius of S (0.26 Å) and P (0.31 Å). Calculations reveal that the lattice mismatches parallel and perpendicular to the [010]Zn3P2//[12j 10]ZnS layout are 5.84% and 5.5%, respectively. To match with each other, ZnS and Zn3P2 will

Orientation-Ordered ZnS Nanowire Arrays

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Figure 2. (a) TEM image of the synthesized ZnS nanowires array. EDS spectra taken from (b) ZnS nanowires and (c) bottom Zn3P2 crystals. (d) High-magnification TEM image showing the aligned ZnS nanowires. (e) HRTEM image of the ZnS nanowires. The inset shows the SAED pattern. (f) HRTEM image showing the interface between ZnS nanowires and the Zn3P2 crystal.

Figure 3. SEM image of aligned ZnS nanowires grown on Zn3P2 microcrystals for a short time.

expand and shrink within a several atom distance region in a horizontal plane and result in good heteroepitaxial growth. On the basis of the above crystallographic relationship between hexagonal ZnS and tetragonal Zn3P2, it is believed that aligned ZnS nanowires can be grown on Zn3P2 crystals with other shapes. Figure 5 depicts an example of this idea, which indicates a ZnS product grown on Zn3P2 plates. Aligned ZnS nanowires were found to grow vertically on both sides of the Zn3P2 plates. This can be easily explained from the abovediscussed crystallographic relationship. An enlarged SEM image of the product was shown in the inset in Figure 5. The arrow indicates the growth directions of ZnS nanowires, which are along the ((101) of Zn3P2.

Figure 4. Schematic of a unit cell of (a) hexagonal ZnS and (b) tetragonal Zn3P2. (c) Interface configuration between hexagonal ZnS and tetragonal Zn3P2, revealing a perfect epitaxial relationship.

Room temperature photoluminescence (PL) properties of the ZnS nanowire arrays were investigated by using a He-Cd laser

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Figure 5. Aligned ZnS nanowires grown on Zn3P2 plates.

Figure 6. Room temperature PL spectrum of the synthesized ZnS product.

line at 325 nm as the excitation source. Figure 6 depicts a PL spectrum taken from the product. It shows two emission bands. One is a strong broad green emission band centered at 554 nm, which may originate from some self-activated centers, cacancy states, element sulfur species on the ZnS nanowires surface, or interstitial states associated with the peculiar ZnS nanostructures or interfaces as previously reported on ZnS nanostructures.18 The other is a weak emission band at about 802 nm, which obviously originates from the free exciton emission near the band edge of tetragonal Zn3P2.19 Finally, field emission (FE) properties of the orientationordered ZnS nanowire arrays were studied. The whole field emission measurement system is composed of a vacuum chamber with a low pressure of ∼1.1 × 10-5 Pa and a rodlike copper probe with a cross-section area of 1 mm2, which plays the role of anode, with the orientation-ordered ZnS nanowire arrays acting as the cathode. A dc voltage increasing from 200 to 1100 V is applied to the ZnS nanostructures at different anode-cathode separations with an increasing voltage step of 20 V. Figure 7 shows the field emission current density, J, versus applied field, E, curves at anode-sample distances of 50 and 100 µm of the ZnS nanostructures, respectively. Here, we define the turn-on field (Eto) and the threshold field (Ethr) as the electronic fields required to produce a current density of 10 µA cm-2 and 10 mA cm-2, respectively. A turn-on field of about 3.72 V/µm is obtained for the ZnS nanowire arrays. The value is comparable to previous reports on 1-D ZnS nanostructures9 and it suggests that the present synthesized orientation-ordered ZnS nanowire arrays with low turn-on field can be used as valuable field emitters. The emission characteristics were also

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Figure 7. Field-emission properties of the synthesized well-aligned ZnS nanowire arrays measured at anode-sample distances of 50 and 100 µm, respectively.

analyzed by using Fowler-Nordheim (F-N) theory, which has been proven to be useful in describing FE from electron emittors. By plotting ln(1/V2) versus 1/V, a straight line was obtained, which is in good agreement with the field emission behavior and follows well with the F-N description. In summary, due to the good orientation relationship of [010]Zn3P2//[12j10]ZnS and (101)Zn3P2//(0002)ZnS, orientationordered ZnS nanowire arrays were epitaxially grown on Zn3P2 crystals via a simple thermal evaporation method. Assynthesized ZnS nanowires have diameters of several tens of nanometers and lengths of ca. 500 nm and are single crystals with preferred (0001) growth directions. They exhibit excellent field emission properties with a low turn-on field of 3.72 V/µm, which are expected to find many potential applications as field nanoemitters and in nanoscale electric and optoelectric devices. Acknowledgment. We thank the Center for Electron Microscope and Microanalysis (CEMMA) at University of Southern California for using their facilities. The authors acknowledge financial support partially from the National Science Foundation (CCF-0726815 and CCF-0702204). G.Z.S. thanks Dr. Zhi and Dr. Fang at the Nanoscale Materials Center, National Institute for Materials Science for their help on setting up the induction furnace. References and Notes (1) (a) Sun, L.; Liu, C.; Liao, C.; Yan, C. J. Mater. Chem. 1999, 9, 1655. (b) Jiang, X.; Xie, Y.; Lu, J.; Zhu, L.; He, W.; Qian, Y. T. Chem. Mater. 2001, 13, 1213. (2) (a) Prevenslik, T. V. J. Lumin. 2000, 87, 1210. (b) Bredol, M.; Merikhi, J. J. Mater. Sci. 1998, 33, 471. (c) Calandra, P.; Goffredi, M.; Liveri, V. T. Colloids Surf. A 1999, 160, 9. (3) (a) Shen, G. Z.; Bando, Y.; Golberg, D. Appl. Phys. Lett. 2006, 88, 123107. (b) Shen, G. Z.; Bando, Y.; Hu, J. Q.; Golberg, D. Appl. Phys. Lett. 2007, 90, 123101. (4) (a) Moore, D.; Ding, Y.; Wang, Z. L. Angew. Chem., Int. Ed. 2006, 45, 5150. (b) Moore, D. F.; Ding, Y.; Wang, Z. L. J. Am. Chem. Soc. 2004, 126, 14372. (5) (a) He, J. H.; Zhang, Y. Y.; Liu, J.; Moore, D.; Bao, G.; Wang, Z. L. J. Phys. Chem. C 2007, 111, 12152. (b) Barrelet, C. J.; Wu, Y.; Bell, D. C.; Lieber, C. M. J. Am. Chem. Soc. 2003, 125, 11498. (6) (a) Zhu, Y. C.; Bando, Y.; Xue, D. F.; Golberg, D. AdV. Mater. 2004, 16, 831. (b) Hu, J. Q.; Bando, Y.; Zhan, J. H.; Golberg, D. Angew. Chem., Int. Ed. 2004, 43, 4606. (c) Yin, L. W.; Bando, Y.; Zhan, J. H.; Li, M. S.; Golberg, D. AdV. Mater. 2005, 17, 1972. (7) (a) Kar, S.; Chaudhuri, S. J. Phys. Chem. B 2005, 109, 3298. Kar, S.; Biswas, S.; Chaudhuri, S. Nanotechnology 2005, 16, 3074. (b) Hao, Y. F.; Meng, G. W.; Wang, Z. L.; Ye, C. H.; Zhang, L. D. Nano Lett. 2006, 6, 1650. (8) (a) Jiang, Y.; Meng, X. M.; Liu, J.; Xie, Z. Y.; Lee, S. C.; Lee, S. T. AdV. Mater. 2003, 15, 323. (b) Fan, X.; Meng, X. M.; Zhang, X. H.;

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