Growth and Characterization of Vertically Aligned Nonpolar [11̅00

Article pubs.acs.org/crystal

Growth and Characterization of Vertically Aligned Nonpolar [11̅00] Orientation ZnO Nanostructures on (100) γ‑LiAlO2 Substrate Chenlong Chen, Yan-Ting Lan, Mitch M.C. Chou,* Da-Ren Hang, Tao Yan, He Feng, Chun-Yu Lee, Shih-Yu Chang, and Chu-An Li NSC Taiwan Consortium of Emergent Crystalline Matreials, Department of Materials and Optoelectronic Science, National Sun Yat-Sen University, Kaohsiung 80424, Taiwan, Republic of China ABSTRACT: In this work, we demonstrate the capability of controlling the crystallographic orientation of ZnO nanostructure arrays grown along nonpolar [110̅ 0] orientation. The Au catalyst was used to initiate and guide the growth of ZnO nanostructures on (100) γ-LiAlO2 substrate. The morphology, structure, and optical properties of the as-synthesized ZnO nanostructures were characterized by field emission scanning electron microscopy, X-ray diffraction, field emission transmission electron microscope, monochromatic cathodoluminescence image, and photoluminescence spectroscopy. The ZnO nanostructure arrays exhibited special shape and were aligned vertically on the (100) γ-LiAlO2 substrate. A cooperative of Au-catalyzed VLS vertebral column growth along [11̅00] orientation, zinc-self-catalyzed VLS growth along [0001], and ZnO VS lateral expansion growth process was proposed for the special ZnO nanostructures.

1. INTRODUCTION Wurtzite structured zinc oxide (ZnO) is a very interesting semiconductor material with a wide direct band gap of 3.37 eV and a large exciton binding energy of 60 meV at room temperature. Meanwhile, it is a piezoelectric, pyroelectric, biocompatible material with high thermal and mechanical stability. These unique properties make ZnO a potential material for applications in electronic, optoelectronics, acoustics, and sensing.1 Nanoscale materials are expected to exhibit novel mechanical, chemical, electrical, and optical properties due to the surface and quantum confinement effects. A rich variety of ZnO nanostructures, such as nanowires,2 nanorods,3 nanobelts,4 nanocombs,5 nanorings,6 nanotube,7 nanohelixes,8 cages,9 and nanoflower,10 have attracted considerable research interest due to their unique opto-electrical properties and potential use as building blocks for future nanophotonics and nanoelectronics devices. Among those numerous ZnO nanostructures, vertically aligned ZnO nanostructure have gained special concern for its promising applications.11−14 ZnO has three fast growth directions: one c axis [0001] polar direction and the other two [11̅00] and [112̅0] nonpolar directions, which are perpendicular to the c axis.15,16 Being able to control the crystallographic orientation of ZnO nanostructure is strongly desirable for tuning the anisotropic optical and electrical properties.17,18 Precise control over sizes and sites growth of polar ZnO [0001] vertical nanorod arrays have already been demonstrated.19,20 However, there exists a spontaneous and strain-induced piezoelectric polarization in the [0001] direction of wurtzite ZnO.21 The strong internal polarization field causes spatial separation of © 2012 American Chemical Society

electrons and holes in multiple quantum well structures and leads to a reduced optical emission efficiency, as well as an undesirable red shift in the emission spectra, which is called quantum confined Stark effect (QCSE).22,23 The absence of polarization induced electrostatic field in the growth direction for nonpolar structure eliminates the deteriorated QCSE and significantly improved the performance of optoelectronic devices.23,24 Until now, no reports have shown the ability to grow ZnO nanomaterials along the nonpolar direction. The proper way to guide the ZnO crystallographic growth direction along nonpolar [11̅00] is to select a suitable single crystal substrate whose lattice matches well with (11̅00) m plane ZnO. Tetragonal γ-LiAlO2 crystal belongs to the space group P41212 in which each lithium and aluminum atom is coordinated at the center of tetrahedron, with four oxygen atoms.25 The (100) plane of γ-LiAlO2 crystal has similar atomic arrangement with the prismatic (11̅00) plane of wurtzite structure ZnO. Figure 1a exhibits rectangular nucleation sites for the rectangular plane (m plane) of ZnO on γ-LiAlO2(100) with anisotropic mismatches of only [0001]ZnO//[010]γ‑LiAlO2, cZnO(5.206 Å) ≅ aγ‑LiAlO2(5.169 Å) with 0.7%, and [112̅0]ZnO// [001]γ‑LiAlO2, 2aZnO(3.249 Å) ≅ cγ‑LiAlO2(6.268 Å) with 3.7%. Figure 1b shows the scheme of the crystallographic cells of ZnO and indicates that the ZnO [0001] axis lies in the growth plane and that the ZnO nanomaterials would grow along the nonpolar [110̅ 0] axis. Previously, we reported the growth and Received: September 24, 2012 Revised: October 31, 2012 Published: November 7, 2012 6208

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Figure 1. (a) Ball-and-stick model of m plane ZnO (11̅00) nucleation sites either on cations (upper rectangle) or anions (lower rectangle) of γLiAlO2(100). (b) Illustration of ZnO nanostructure grown along nonpolar [110̅ 0] direction and has its polar [0001] direction parallel to the growth plane.

Figure 2. Setup of chemical vapor deposition for the growth of nonpolar ZnO nanostructures on Au-coated (100) γ-LiAlO2 substrate.

characterization of nonpolar m plane ZnO epitaxial films on γLiAlO2 (100) substrate.26 We also used the intensively studied and well understood Au catalyst via a vapor−liquid−solid (VLS)27 process to initiate and guide the growth of LiAl5O8 nanorods on γ-LiAlO2 (100) substrate.28 In this report, a chemical vapor deposition (CVD) approach was presented to grow nonpolar ZnO nanomaterials with [11̅00] orientation on a lattice-matched γ-LiAlO2 (100) substrate. The Au-coated γ-LiAlO2 (100) substrate was placed directly on top of the source materials. The structural relationship between the ZnO rectangular nonpolar m plane and the γ-LiAlO2(100) substrate leads to the controlled vertical aligned growth of ZnO nanostructures array along nonpolar [11̅00] direction.

growth of ZnO nanomaterials, the substrates were cleaned sequentially in ultrasonic bath of acetone and ethyl alcohol and dried in nitrogen flow. The source materials, a mixture of graphite powder and ZnO powder with equal weight (0.5 g each), were introduced into an alumina boat. A 10 nm thin layer of Au catalyst was deposited on the (100) γ-LiAlO2 substrate by a Pelco SC-6 sputter coater before been placed on top of the source materials. Then, the alumina boat was placed in the center of a horizontal tube furnace, evacuated to a base pressure of 8 mtorr (1 Torr ≈ 133 Pa) and introduced into a constant Ar flow (50 sccm Ar; sccm stands for standard cubic centimeters per minute). The furnace was then heated up to 930 °C from room temperature with a heating rate of 40 °C/min. The Ar flow was changed to a constant premixed gas flow (49 sccm Ar and 1 sccm O2) and maintained this temperature for 5 min with a pressure of 10 Torr. It was followed by cooling down naturally under flowing gas. Characterization of the nonpolar [11̅00] orientation ZnO nanostructures: The surface morphology of the ZnO nanostructure was studied by a field emission scanning electron microscope (SEM, JEOL JSM-6700F). The orientation and crystal structure of assynthesized ZnO nanostructures were characterized by SIEMENS D5000 and Bruker D8 X-ray diffractometers, using Cu Kα (λ = 0.15406 nm) radiation at 40 kV and 30 mA. More microstructures and orientation characterizations of the ZnO nanostructures were carried out using a field emission transmission electron microscope (FE-TEM, Tecnai F20 G2) operated at 200 kV. The TEM samples were prepared

2. EXPERIMENTAL DETAILS Fabrication of the nonpolar [11̅00] orientation ZnO nanostructures: High-quality 2 in. γ-LiAlO2 single crystals were grown using the Czochralski pulling technique in our laboratory, and polished (100) γLiAlO2 substrates with root-mean-square roughness of 0.32−0.48 nm were used in the experiments.29 A CVD approach was presented to grow nonpolar ZnO nanomaterials with [11̅00] orientation (m-plane) on γ-LiAlO2 single crystal substrate, as shown in Figure 2. Before the 6209

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Figure 3. (a) Top view SEM image of ZnO nanostructures, (b) SEM image recorded by 45° tilt to the sample along the axis parallel to the wires, (c) SEM image recorded by 45° tilt to the sample along the axis perpendicular to the wires, (d) XRD pattern of the ZnO nanostructures grown on γLiAlO2(100) substrate.

vertically stacked along nonpolar [11̅00] direction. Figure 3c is the typical SEM image recorded by 45° tilt to the sample along the axis perpendicular to the wires. It clearly indicates that the nanowires, stemmed from the top of stacked bases, were cross acclivitous grown. The rectangular stacking morphologies of the bases were due to the thickness contrast. The cross acclivitous morphology, that is nanostructures 180° in-plane invert symmetrical distributed on the substrate, is induced by the equal epitaxial orientation probability of ZnO [0001] and ZnO [0001̅] along γ-LiAlO2 [001] (as can be seen in Figure 1). X-ray diffraction (XRD) was used to investigate the crystalline orientation and quality of ZnO nanostructure grown on γ-LiAlO2(100) substrate. As shown in Figure 3d, the two peaks are indexed as (11̅00) and (22̅00) m plane diffraction of the wurtzite ZnO, indicating that the nanostructures grow perfectly along the [11̅00] nonpolar direction. The in-plane epitaxial relationship between ZnO nanostructures and γ-LiAlO2 substrate was determined to be [0001]ZnO// [010]γ‑LiAlO2 and [112̅0]ZnO//[001]γ‑LiAlO2, by XRD φ-scan, as shown in Figure 4. The detailed microstructural characterizations of the ZnO nanostructures were carried out by using FE-TEM. Figure 5a shows a bright-field TEM image of three typical ZnO nanostructures labeled by A, B, and C. The image is taken with g = 11̅00 near the [112̅0] zone axis of nanostructure A.

using needlepoint scratched method assisted by an optical microscopy. Cathodoluminescence (CL) images were acquired by using a Gatan MonoCL3 spectrometer in a JEOL JSM 7000F SEM system. CL measurements were performed with 10 kV acceleration voltage of the electron beam energy at room temperature. Photoluminescence (PL) was excited by using a continuous wave (CW) He−Cd laser (325 nm). The power is 50 mW, and the spot size is 300 μm. The emission spectra were analyzed by a Jobin-Yvon Triax 550 monochromator with a 0.025 nm resolution and detected by a Hamamatsu photomultiplier tube and standard photon counting electronics. Low-temperature PL measurements were performed in the Cryo Model 22 closed cycle helium cryostat.

3. RESULTS AND DISCUSSION 3.1. Morphology and Structural Analysis of Vertically Aligned Nonpolar [11̅00] Orientation ZnO Nanostructures. Figure 3a shows a top view SEM image of ZnO nanostructures grown on Au-coated γ-LiAlO2 (100) substrate. Most of the ZnO nanostructures are constituted by a vertically aligned base and a lain nanowire. The lain nanowires stemmed from the top of the bases, grew orderly, and ended with a changed direction tip. Other bases have a vertical dagger tip on the top. To understand the ZnO nanostructures more detailed, SEM image recorded by 45° tilt to the sample along the axis parallel to the wires is shown in Figure 3b. It presents that the side of the base is constructed by several sized inequality hexagons, typical morphology of c plane wurtzite ZnO, 6210

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Figure 5b is the enlarged bright-field TEM image taken with g = 110̅ 0 near the [112̅0] zone axis of C. Figure 5c is the select area diffraction pattern of C. ZnO nanostructure A displays left side straight and right side nonuniform morphology. The composition of the Au tip was confirmed by energy-dispersive X-ray spectroscopy. The obvious brightness contrast existing in the base region results from thickness difference of the ZnO nanostructure. The electron diffraction pattern of A shows that the entire ZnO nanostructure is a single crystal with vertical along [11̅00] and horizontal along [0001]. Two vertical lines near left side crossing the nanostructure can be clearly seen in A (indicated by two white arrows). The crystalline defects can be identified as stacking faults oriented parallel to the ZnO c plane. This suggests that the vertebral column of the ZnO nanostructures were columnar grown by Au catalyst guided via VLS mechanism. The lateral c axis of [11̅00] oriented vertebral column consists of positive (0001) Zn and negative (0001̅) O terminated polar surfaces. The O terminated (0001̅) polar surface is inert, and the Zn terminated (0001) polar surface is chemically active.5 The chemically active (0001) Zn terminated surface elongation growth is via zinc-self-catalyzed VLS.16 The ZnO vapor epitaxial deposits on the side surface and expansible grows into hexagonal shape analogy to vapor-solid (VS) growth of nanonail.30 The simultaneous occurring of Au-catalyzed, zinc-self-catalyzed, and VS growth might be due to the high concentration of Zn on the substrate. A vertical line near left side crossing the nanostructures can also be clearly seen in nanostructure B. Figure 5b clearly shows the growth orientation of B changes four times from [110̅ 0] direction to the tip of the ZnO nanostructure. An Au tip indicated by a dark arrow was also confirmed by energydispersive X-ray spectroscopy. The select area electron diffraction pattern of B shows that the entire ZnO nanostructure is a single crystal in despite of the change in growth direction. A stacking faults vertical line is also seen in nanostructure C (indicated by white arrow). The Au tip confirmed by energydispersive X-ray spectroscopy is indicated by a dark arrow. The select area electron diffraction pattern shows the base of ZnO nanostructure grown along [11̅00] and the nanowire run along [11̅01̅], [11̅02̅], etc. The nanostructural morphologies were determined by the in situ gas-phase supersaturation and the surface energy of the growing surface planes.31 It was observed that the ZnO

Figure 4. XRD φ-scan showing in-plane epitaxial relationship between ZnO nanostructures and γ-LiAlO2(100) substrate.

Figure 5. (a) Bright-field TEM image of the ZnO nanostructures with g = 110̅ 0 near the [112̅0] zone axis of A. (b) Bright-field TEM image of the ZnO nanostructures with g = 11̅00 near the [112̅0] zone axis of C. (c) Selected area electron diffraction pattern of C.

Figure 6. (a) Room temperature CL spectrum of ZnO nanostructures. (b) Monochromatic CL image at UV emission peak of the ZnO nanostructures excited by 10 kV electron acceleration voltage. 6211

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conditions.35,36 This intense free excitonic peak at room temperature strongly suggests that the nonpolar [101̅0] orientation ZnO nanomaterial has a high optical quality. Figure 7b shows the low temperature PL spectrum of ZnO nanostructures at 10 K. A strong and sharp donor-bound exciton emission (D0X) peaked at 3.364 eV with a narrow full width at half-maximum (fwhm) of 8 meV was observed in the band edge region. The sencondary luminescence at 3.318 eV can be assigned to basal plane stacking faults (BSF).37 3.3. Characterization of ZnO Nanostructures Grown at Various Growth Temperature and Cooling Atmosphere. The growth orientation of ZnO nanostructures on Aucoated γ-LiAlO2(100) substrate depends strongly on the growth temperature. Figure 8a is the SEM image of ZnO nanostructures grown at the temperature of 900 °C on Aucoated γ-LiAlO2(100) substrate. Most areas are occupied by tilted triangular shape ZnO nanobelts rather than vertically grown ZnO nanomaterials. The morphologies of ZnO nanostructures grown on Aucoated γ-LiAlO2(100) substrates also depend on the cooling atmosphere. Figure 8b is the SEM image of ZnO nanostructures grown at 930 °C and followed by cooling down naturally under no flowing gas. The tip is apparently different from that of the structure in Figure 3a. This might be due to the exhaustion of oxygen source before the temperature down to occurrence of further changed orientation. 3.4. Proposed Formation Mechanism of Vertically Aligned Nonpolar [11̅00] Orientation ZnO Nanostructures. Figure 9 schematically illustrates the growth mechanism of the as-synthesized ZnO nanostructures. During the heating process, the coated Au thin layer self-agglomerated to form small droplets. The Zn vapor coming from carbothermal reduction of ZnO powder reacted with the Au solvent on the substrate to form alloy droplets.34 Similar to the Au-catalyzed VLS growth of polar (0002) ZnO nanorods, in the presence of oxygen, the supersaturated Au catalyst droplets initiate and guide the growth of ZnO nanostructures, and the epitaxial orientation relationship between the ZnO rectangular prism nonpolar m plane and the γ-LiAlO2(100) substrate leads to the formation of rectangular ZnO nanorods instead of hexagonal ZnO underneath the Au catalyst droplets. The growth underneath the Au catalyst results in vertical growth of rectangular ZnO belts along nonpolar [11̅00] direction. The nature wurtize ZnO c planes consist of chemically inert O terminated (0001̅) surface and chemically active Zn terminated (0001) surface.5 The (0001) surface tends to have Zn clusters at the growth front and favors to initiate zinc-self-catalyzed VLS growth of ZnO, resulting in the asymmetric growth behavior.16 In this report, Au-coated γLiAlO2(100) substrate was placed on the top of ZnO powder and graphite source materials. Although growth by the Aucatalyzed VLS process dominates, the specific Zn-rich growth condition can still allow a certain degree of zinc-self-catalyzed VLS growth along [0001] accompanied by ZnO lateral expansion growth via VS mechanism. Consequently, the special vertically aligned nonpolar ZnO nanostructures were successfully obtained. If the Au-coated γ-LiAlO2(100) substrate was placed conventionally downstream of ZnO powder and graphite source materials, that is to say, Zn is not rich enough, it yields crystalline of LiAl5O8 nanorods rather than ZnO nanomaterial.28

nanobelt changed its directions between [0110̅ ] and [0113̅ ] during growth, which was suggested as a result of variation of growth temperature.32 During the cooling process in our experiment, the concentration of Zn source vapor, which generated from carbon thermal reduction of ZnO powder, decreased, while the flowing gas stayed constant. So it is probable that the decreasing temperature plays an important role in the change of growth direction for ZnO nanowire grown by Au-catalyzed VLS mechanism. When the temperature is below the eutectic temperature of the catalyst alloy or the reactant is no longer available, Au-catalyzed VLS growth terminates.33 3.2. Optical Properties of Vertically Aligned Nonpolar [11̅00] Orientation ZnO Nanostructures. CL spectrum was taken at room temperature and shown in Figure 6a. The accelerating voltage of the electron beam was 10 kV. The ZnO nanostructures emitted both ultraviolet (UV) light at a photon energy of 3.27 eV and broad green light peaking at 2.4 eV. Figure 6b shows the monochromatic CL image at UV emission peak of the ZnO nanostructures. Bright ZnO nanostructures are observed in the image, which indicates the nanostructures make major contributions to the UV luminescence. Room temperature PL spectrum of the ZnO nanostructures is shown in Figure 7a. Similar to the results of CL, a strong UV emission at 3.27 eV was detected, accompanied by a week, broader green emission with a peak at about 2.4 eV. The UV emission corresponds to the near band edge free excitonic emission.34 The deep-level green emission is due to oxygenvacancy (V O ), which is generated in Zn-rich growth

Figure 7. (a) Room temperature PL spectrum of the ZnO nanostructures. (b) PL spectrum of the ZnO nanostructures taken at a temperature of 10 K. 6212

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Figure 8. (a) SEM image of ZnO nanostructures grown on Au-coated γ-LiAlO2(100) substrate at the temperature of 900 °C, (b) SEM image of ZnO nanostructures grown on Au-coated γ-LiAlO2(100) substrate at 930 °C and followed by cooling down naturally under no flowing gas.

Figure 9. Schematic model of the growth mechanism of ZnO nanostructures.

National Sun Yat-Sen University. We acknowledge financial support from AIM for the Top University Plan of National Sun Yat-sen University, Taiwan.

4. CONCLUSIONS In summary, vertically well-aligned ZnO nanostructure arrays were epitaxially grown along nonpolar [11̅00] direction on Aucoated γ-LiAlO2(100) substrate. The growth mechanism of the nonpolar [11̅00] orientation ZnO nanostructure is suggested to be a cooperative of Au-catalyzed VLS vertebral column growth along [11̅00] orientation, zinc-self-catalyzed VLS growth along [0001], and lateral expansion via VS growth process. The realization of controlling ZnO nanostructure arrays crystalline along nonpolar direction could open a new door to understand the attractive properties of ZnO nanomaterials for developing new generation nanodevices with high performance.

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REFERENCES

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We would like to express our thanks to Professor Li-Wei Tu in Department of Physics, National Sun Yat-Sen University, for the help with CL measurements. This work is partly supported by the National Science Council of Taiwan (NSC 101-2119-M110-001) and Center for Nanoscience & Nanotechnology of 6213

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