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Size-controllable growth of ZnO nanorods on Si substrate
Accepted Manuscript Size-controllable growth of ZnO nanorods on Si substrate Zhentao Yu, Hui Li, Yining Qiu, Xu Yang, Wu Zhang, Ning Xu, Jian Sun, Jiada Wu
PII:
S0749-6036(16)31371-4
DOI:
10.1016/j.spmi.2016.12.005
Reference:
YSPMI 4708
To appear in:
Superlattices and Microstructures
Received Date: 2 November 2016 Accepted Date: 5 December 2016
Please cite this article as: Z. Yu, H. Li, Y. Qiu, X. Yang, W. Zhang, N. Xu, J. Sun, J. Wu, Sizecontrollable growth of ZnO nanorods on Si substrate, Superlattices and Microstructures (2017), doi: 10.1016/j.spmi.2016.12.005. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Submitted to Superlattices and Microstructures, original
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Size-controllable growth of ZnO nanorods on Si substrate
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Zhentao Yua, Hui Lia, Yining Qiua, Xu Yanga, Wu Zhanga, Ning Xua,b, Jian Suna,b,c,*,
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Jiada Wua,b,* Department of Optical Science and Engineering, Fudan University, Shanghai 200433, China
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Shanghai Engineering Research Center of Ultra-Precision Optical Manufacturing, Fudan University, Shanghai 200433, China
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Key Lab for Micro and Nano Photonic Structures, Ministry of Education, Fudan
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University, Shanghai 200433, China
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Abstract
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Here we report a simple two-step chemical-solution-based method to grow highly
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oriented and size-controllable ZnO nanorods on ZnO-seeded Si substrate. The
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morphology of the grown ZnO nanorods was examined by field emission scanning
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electron microscopy. The structure was characterized by X-ray diffraction and Raman
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scattering spectrum. Photoluminescence spectra were measured at room temperature
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and low temperatures to evaluate the photoluminescence properties of the ZnO
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nanorods. The grown ZnO nanorods are structured with hexagonal wurtzite. The
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diameter and length of ZnO nanorods can be controlled by varying the crystal quality
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of the underlying ZnO seed layers. The crystal quality of the seed layers gets
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improved as the deposition time and annealing temperature for ZnO seed layers are
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increased. The effects of annealing on the ZnO nanorods were also studied.
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Keywords: ZnO, nanorod, seed layer, hydrothermal reaction, size-controllable growth
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*Corresponding author. E-mail addresses: [email protected] (J. Sun),
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[email protected] (J.D. Wu).
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1. Introduction With a direct and wide band gap (Eg = 3.37 eV), a large excitation binding energy
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(60 meV) at room temperature, and excellent properties such as high piezoelectricity,
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pyroelectricity, conductivity, transparency, thermal and chemical stability[1, 2], ZnO
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is an outstanding representative of II-VI group compound semiconductor materials
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having potential applications in a variety of fields. Compared with bulk ZnO, ZnO
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nanostructures, especially one-dimensional ZnO nanostructures (nanorods, nanotubes,
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nanowires, nanotapes, etc.) having a high specific surface area, usually show unique
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and excellent properties and behave better than bulk ZnO when being used in
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electrical, optical and magnetic fields, such as short-wavelength optoelectronic
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devices, solar energy conversion, transparent conducting coating materials, and
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sensors etc [3-9]. In addition, the high specific surface area of ZnO nanostructures
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provides an advantage to efficiently decorate the surface of ZnO nanostructures for
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the modification of properties. Therefore, the directional growth of high quality
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one-dimensional ZnO nanostructures on solid substrates and size controlling have
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become very promising and exciting.
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There are many methods for the preparation of one-dimensional ZnO nanorods.
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Generally speaking, they can be divided into gas phase techniques and liquid phase
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methods. Compared with the gas phase methods which are usually performed at high
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temperatures, the preparation of ZnO nanorods using liquid phase methods has great
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advantage and superiority because of its low energy consumption, large scale
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production and low temperature fabrication. In this paper, we use a two-step,
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wet-chemical process to grow ZnO nanorods on Si substrates. This process includes
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the deposition of a nanocrystalline ZnO thin film on Si substrate as a seed layer and
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the growth of ZnO nanorods on the ZnO seed layer by a low-temperature
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chemical-solution-based method. Highly oriented ZnO nanorods are grown by the
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thermal decomposition of Zn-formamide complexes in hydrothermal reaction [10-12].
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In this process, the crystalline grains in the seed layer work as nucleation centers for
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the subsequent growth of ZnO nanorods, and the length and diameter of ZnO
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nanorods are dependent on the crystal quality of the ZnO seed layer and hence are
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controllable by varying the crystal quality of ZnO seed layers. In addition, the growth
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of ZnO nanorods is performed at a temperature much lower than that for
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hydrothermal growth of ZnO nanorods using zinc salts [13, 14].
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2. Methods
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2.1 Deposition of ZnO seed layers
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N-type Si(100) wafers were used as substrates after being carefully cleaned
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sequentially with acetone, ethanol and de-ionized water in an ultrasound bath. A thin
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nanocrystalline ZnO layer was first deposited on Si substrate by electron cyclotron
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resonance plasma assisted pulsed laser deposition (ECR-PLD) [15]. Then some
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samples were annealed at 400 °C or 600 °C in air for 30 minutes. In the subsequent
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growth process of ZnO nanorods, the annealed nanocrystalline ZnO layer mainly has
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two functions: (a) working as a buffer layer to solve the lattice mismatch between
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ZnO nanorods and Si substrate, and (b) serving as a seed layer to assist the nucleation
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of ZnO and the growth of ZnO nanorods oriented along the direction vertical to Si
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substrate.
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2.2 Growth of ZnO nanorods
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The ZnO nanorods were grown on the ZnO seed layer with a low-temperature
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formamide-solution-based method. Analytical-grade formamide and metal Zn foils
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with a purity of 99.99% were used as raw materials of the hydrothermal reaction.
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Using pure metal Zn as zinc source can avoid introducing impurities which may be
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incorporated when using other zinc salts in usual wet-chemical reactions. Typically,
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one ZnO-seeded Si substrate and one Zn foil were immersed in 3 mL of 5% (volume
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ratio, v/v) formamide aqueous solution in a 10 mL glass ware. The glass ware was put
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in a thermostatic water bath pot for ZnO nanorods to be grown for 24 hours at a
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constant temperature of 65 °C which is much lower than those for hydrothermal
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growth of ZnO nanorods using zinc salts. The sample was then taken out to be
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ultrasonically cleaned in de-ionized water and dried in air. Some of the grown ZnO
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nanorods were annealed in air at 600 °C for one hour.
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The growth process of the ZnO nanorods is based on the homogeneous 3
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natural oxidation of metal Zn by naturally dissolved oxygen in water is limited by the
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existence of a naturally formed thin passive ZnO layer, and hence the rate of natural
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oxidation for metal Zn is very slow in water. This oxidation process can be
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remarkably accelerated by the addition of formamide [10, 16, 17]. In the presence of
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formamide, Zn-formamide complexes are formed by spontaneous oxidation reaction
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of metal Zn and oxygen at the surface of Zn foil contacting the solution
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(1)
facilitating the releasing of Zn ions into the solution by decomposition of Zn-formamide complexes [Zn(formamide)n]2+ → Zn2+ + n⋅formamide .
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Zn + n⋅formamide + H2O + 1/2 O2 → [Zn(formamide)n]2+ + 2OH− ,
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(2)
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Zn ions are therefore continuously supplied to support the formation of ZnO in the
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vicinity of the substrate and the self-assembled growth of one dimensional ZnO
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nanostructures at the surface of the substrate [18] Zn2+ + 2OH− → Zn(OH) ,
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Zn(OH) → ZnO + H2O .
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(3) (4)
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2.3 Sample characterization
The morphology of the samples was characterized by field emission scanning
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electron microscopy (FE-SEM) with a Hitachi S-4800 microscope. The crystal
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structure of the samples were characterized by X-ray diffraction technology using
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Cu Kα radiation with a Rigaku D/max-γB X-ray diffractometer. Raman scattering
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spectroscopy was used to characterize the lattice vibration modes of ZnO nanorods
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with a Jobin-Yvon LabRAM HR 800 UV spectroscope using 632.8-nm laser light to
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excite the samples. The photoluminescence (PL) was measured to evaluate the
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luminescence properties of ZnO nanorods at room temperature and low temperatures
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with 325-nm He−Cd laser light excitation.
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3. Results and discussion
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3.1 Morphology and structure of ZnO seed layer with varied deposition time 4
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°C-annealed ZnO seed layers deposited for 10, 20 and 30 minutes, respectively. The
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FE-SEM images indicate that the seed layers are composed of densely connected
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grains with the grain boundaries being clearly revealed. It seems that the mean size of
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the particles on the seed layer increases with the increase of the deposition time.
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Longer deposition time provided more ZnO precursors, resulting in thicker seed
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layers and larger grains.
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Figure 1 Top-view FE-SEM images of ZnO seed layers deposited by ECR-PLD method
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for (A) 10, (B) 20 and (C) 30 minutes and subsequently annealed at 600 °C in air.
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Figure 2 shows the XRD patterns of the ZnO seed layers deposited for 10, 20 and
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30 minutes followed by annealing at 600 °C in air. Besides the strong diffraction from
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the Si substrate, one diffraction peak at about 2θ ~ 34 ° appears in each pattern. This 5
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deposited ZnO layers are structured with hexagonal wurtzite and are grown highly
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oriented with their c-axes perpendicular to the substrates. As shown in Figure 2, the
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diffraction intensity increases and the full width at half-maximum (FWHM) decreases
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as the deposition time increases, revealing that the crystal structure improves and the
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crystal grains grow larger. On the lattice-mismatched Si, the initially deposited ZnO
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serves as the buffer layer for later deposited ZnO. Therefore, as the ZnO film grows
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thicker, the crystal structure gets better. According to the Scherrer formula D =
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Kλ/Bcosθ [19], where λ is the wavelength of the incident X-ray, B the diffraction peak
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width (FWHM) in radian, θ the Bragg angle, K a numerical constant set as 0.9, the
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mean size D of the crystal grains were calculated to be 30 31 32 nm for the ZnO seed
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layers deposited for 10, 20 and 30 minutes, respectively.
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Figure 2 XRD patterns of ZnO seed layers deposited by ECR-PLD method for 10 (a), 20
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(b) and 30 (c) minutes and subsequently annealed at 600 °C in air.
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3.2 Effect of seed layer deposition duration on ZnO nanorods
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We find that the size of ZnO nanorods is dependent on the crystal quality of ZnO
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in the seed layer. Annealed ZnO films with different deposition time (10, 20 and 30
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minutes) were used as seed layers for the growth of ZnO nanorods under the same
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hydrothermal reaction conditions. As shown by the FE-SEM images in Figure 3,
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uniform and dense nanorods have been grown on the ZnO-seeded substrates. The 6
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direction nearly vertical to the substrate. It can be seen that hexagonal ZnO nanorods
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with a mean diameter of 150 nm and length of 1.2 µm have been grown on the seed
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layer deposited for 10 minutes. On the ZnO seed layers deposited for 20 and 30
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minutes, the grown ZnO nanorods have larger mean diameters of 175 and 219 nm,
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respectively. And the lengths of the nanorods are also increased to 1.4 and 2.2 µm,
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respectively, as clearly shown in Figure 3 and its insets. As described above, the
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crystallinity of the ZnO seed layer improves as the deposition time for ZnO seed layer
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increases. This suggests that the size of the grown ZnO nanorods is dependent on the
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crystallinity of the ZnO seed layers.
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Figure 3 Top-view and cross-sectional (inset) FE-SEM images of ZnO nanorods grown on
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ZnO seed layers deposited for (A) 10, (B) 20 and (C) 30 minutes.
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10-, 20- and 30-minute deposited ZnO seed layers. The strong and narrow diffraction
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peak indexed to the (002) diffraction of hexagonal wurtzite ZnO at about 2θ ~ 34°
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reveals that the ZnO nanorods have been grown at high (002) preference with their
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c-axes perpendicular to the substrate. The FWHM of the (002) peak of the ZnO
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nanorods is about 0.1°, smaller than that of the seed layer (~ 0.3°). This indicates that
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the size of ZnO grains in the grown nanorods is larger than that in the seed layers. It is
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noted that with increasing seed layer deposition time, the FWHM of the (002) peak
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gets smaller, i.e. the grain size of the nanorods becomes larger. From the XRD data
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and using the Scherrer formula, the ZnO grain size was calculated to be 64 69 and 83
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nm, respectively, for the ZnO nanorods grown on the 10-, 20- and 30-minute
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deposited ZnO seed layers. It is well known that the nanocrystalline ZnO films serve
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as buffer layers to solve the problem of lattice mismatch between ZnO and the
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underlying Si(100) substrate. Besides that, the ZnO nanocrystalline grains in the seed
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layers act as the crystal nuclei for the hydrothermal growth of ZnO nanorods. Longer
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deposition time results in better crystallinity and larger crystalline grains, which are
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conducive for ZnO nanorods to be grown larger and longer. Thus we can control to
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some degrees the length and diameter of the ZnO nanorods by changing the
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deposition time of the seed layers.
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Figure 4 XRD patterns of ZnO nanorods grown on ZnO seed layers deposited for 10 (a),
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20 (b) and 30 (c) minutes.
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Figure 5 illustrates the PL spectra of the grown ZnO nanorods taken at room
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temperature. The ZnO nanorods were grown on the ZnO seed layers deposited for
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different durations. These nanorods exhibit some difference in luminescence
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properties. Under 325-nm light excitation, the ZnO nanorods emit an intense
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ultraviolet (UV) luminescence around 380 nm corresponding to the near-band-edge
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(NBE) emission of ZnO [20-22]. The defect-related visible luminescence can hardly
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be detected, revealing that the concentrations of defects in the ZnO nanorods are very
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low. However, the UV NBE emission is different in emission intensity and width.
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With prolonging the time for ZnO seed layer deposition, i.e. improving the crystal
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quality of ZnO seed layer, the UV ZnO NBE emission gets stronger and narrower. As
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compared to the NBE of the ZnO nanorods grown on the 10-minute deposited ZnO
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seed layer, that of the ZnO nanorods grown on the 30-minute deposited ZnO seed
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layer is nearly tripled in intensity. The intensity increase in the NBE emission can be
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attributed to the better crystallinity of the ZnO nanorods, consistent with the above
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XRD characterization which reveals that the grain size of the nanorods is increased
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with the increase of the time for ZnO seed layer deposition.
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Figure 5 Room-temperature PL spectra of ZnO nanorods grown on ZnO seed layers
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deposited for 10 (a), 20 (b) and 30 (c) minutes.
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3.3 Effect of seed layer annealing temperature on ZnO nanorods 9
ACCEPTED MANUSCRIPT As the size of ZnO nanorods is affected by the crystal quality of ZnO in the seed
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layer, it can be envisioned that the size of ZnO nanorods is not only dependent on the
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deposition time of ZnO see layers, but also on the annealing conditions of the seed
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layers. Here we demonstrate the effect of the seed layer annealing temperature on the
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ZnO nanorods. The 30-minute deposited ZnO films with different annealing
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temperatures (as-deposited, 400 and 600 °C) were used as seed layers for the growth
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of ZnO nanorods under the same hydrothermal reaction conditions. As shown by the
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FE-SEM images in Figure 6, the size and shape of the ZnO nanorods are strongly
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affected by the temperature at which the seed layer was annealed. The ZnO nanorods
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grown on the as-deposited seed layer have a shape of irregular hexagonal prisms with
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a smaller mean diameter of 105 nm and rougher surface than those grown on the
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annealed seed layers. And as the annealing temperature increases, the diameter of the
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nanorods gets larger.
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Figure 6 Top-view and cross-sectional (inset) FE-SEM images of ZnO nanorods grown on
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(A) as-deposited, (B) 400 °C-annealed and (C) 600 °C-annealed ZnO seed layers. The
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seed layers were deposited for 30 minutes.
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To explain the effects of the annealing temperature of the ZnO seed layers on the
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grown ZnO nanorods, the surface morphology and crystal structure of as-deposited
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and annealed ZnO films used as seed layers for ZnO nanorod growth were
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characterized. FE-SEM images in Figure 7 show the surface morphology of the
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30-minute deposited ZnO films before and after annealing in air for 30 minutes. It can
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be seen that for the as-deposited film, the grains seem imperfectly grown with vague
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boundaries, revealing poor crystal structure. After annealing, the films show
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well-grown grains and clear boundaries, an evidence of improved crystal structure of
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the annealed films. And the grains grow lager and the boundaries appear clearer as the
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temperature results in better crystal structure.
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Figure 7 Top-view FE-SEM images of ZnO films deposited by ECR-PLD for 30 minutes,
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(A) as-deposited, (B) annealed at 400 °C and (C) annealed at 600 °C.
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Figure 8 shows the XRD patterns of the as-deposited, 400 °C-annealed and 600
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°C-annealed ZnO films deposited for 30 minutes. We can see that the deposited ZnO
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films show wurtzite structure with c-oriented preference whether annealed or not.
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However, the annealed ZnO films have higher diffraction intensity and narrower
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diffraction width than the as-deposited one, and the diffraction gets stronger with
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smaller width as the annealing temperature increases, consistent with result obtained
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by FE-SEM examination. Combination of the crystal structure characterization for the
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as-deposited and different-temperature-annealed ZnO films with the size observation 12
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ZnO films confirms further that the size of the grown ZnO nanorods is strongly
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dependent on the crystal quality of the ZnO seed layer on which the ZnO nanorods are
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grown. The size dependence of the grown ZnO nanorods on the deposition time and
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annealing temperature of the seed layer suggests us an approach to control the size of
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ZnO nanorods by varying the crystal quality of the seed layer.
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Figure 8 XRD patterns of ZnO films deposited by ECR-PLD for 30 minutes, as-deposited
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(a), annealed at 400 °C (b) and annealed at 600 °C (c).
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3.4 The influence of post-grown annealing on the structure of nanorods Post-grown annealing in air has little influence on the morphology of the grown
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ZnO nanorods. However, the structure of the grown ZnO nanorods can be improved
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by annealing. Figure 9(A) shows the XRD patterns of as-grown and annealed ZnO
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nanorods grown on the 10-minute deposited ZnO seed layer. Only the (002)
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diffraction of hexagonal wurtizite ZnO can be observed in addition to a strong
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diffraction peak of Si substrate. After annealing, the intensity of the (002) diffraction
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peak is increased significantly, indicating the improvement in the crystallinity of the
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ZnO nanorods. As for the annealed nanorods, the (002) diffraction angle is 2θ002 =
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34.54° with an FWHM of 0.14°, according to Bragg formula [19], the lattice constant
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was calculated to be c = 0.519 nm. This value is well consistent with that from JCPDS
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data (JCPDS 36-1451).
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Figure 9 (A) XRD patterns of as-grown and annealed ZnO nanorods on 10-minute
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deposited ZnO seed layer. (B) Raman spectra of as-grown and annealed ZnO nanorods
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on 10-minute deposited ZnO seed layer. Raman spectrum recorded for Si (100) substrate
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is also shown for comparison.
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Raman backscattering measurement of the samples confirms that the ZnO
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nanorods had hexagonal wurtzite structure and the crystallinity of the ZnO nanorods
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is improved by annealing. For hexagonal wurtzite structure, the phonon modes
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belonging to the E2, E1, and A1 symmetries are Raman active. The E2 symmetry has
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low-frequency branch E2 (low) and high-frequency branch E2 (high), while the E1 and
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A1 symmetries can be divided into longitudinal optical (LO) and transverse optical 14
ACCEPTED MANUSCRIPT (TO) components. Therefore, wurtzite ZnO has six Raman active phonon vibration
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modes: E2 (low), E2 (high), A1 (TO), A1 (LO), E1 (TO) and E1 (LO) [23]. Figure 9(B)
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shows the Raman spectra of the as-grown and annealed ZnO nanorods grown on the
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10-minute deposited ZnO seed layer when being excited by the 632.8-nm laser beam.
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Compared with the Raman spectrum of the Si substrate, it can be seen that two
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obvious Raman signal peaks located at 99 and 438 cm−1 appear in the Raman
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spectrum taken from the as-grown ZnO nanorods. They correspond to the nonpolar
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optical phonon E2 (low) and E2 (high) models, respectively [23], confirming that the
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grown ZnO nanorods have hexagonal wurtzite structure. After annealing treatment,
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the sample exhibits enhanced intensities of the E2 (low) and E2 (high) signals. Besides
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the above two Raman signals, we can also observe weak peaks at around 330, 376 and
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580 cm−1. The signal at 330 cm−1 is identified as the second-order Raman scattering of
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the E2 mode of hexagonal ZnO [24], and the signal at 376 cm−1 is considered to be the
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A1 (TO) mode, while the last one is probably consisted of the A1 (LO) and E1 (LO)
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modes [23]. They all belong to the Raman scattering from wurtzite ZnO. The
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enhancement of the E2 (low) and E2 (high) signals and the emergence of the additional
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weak Raman signals demonstrate the improvement in the crystalline quality of ZnO
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nanorods after annealing.
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The luminescence properties can also reveal the effect of annealing on the
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structure. Hence PL spectra were measured to investigate the luminescence properties
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of the ZnO nanorods. Room temperature PL spectra of the as-grown and annealed
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ZnO nanorods are shown in Figure 10. As described above, the as-grown ZnO
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nanorods have the capability of emitting strong UV luminescence attributed to the
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NBE emission of ZnO. A detailed PL measurement reveals that the UV NBE
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emission peaks at 382 nm with an FWHM of 30 nm. Besides this NBE emission of
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ZnO, very weak and broad visible luminescence with the center at 520 nm is also
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observed as shown in the inset. It is usually believed that the green luminescence is
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associated with the defect levels in the band gap of ZnO introduced by defects such as
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oxygen vacancies and Zn interstice [25]. The intensity of the visible luminescence is
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very low compared with that of the NBE emission, indicating that the ZnO nanorods
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have good quality with low defect concentration. For the annealed ZnO nanorods, the UV luminescence peak blue-shifts to 378 nm
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with an FWHM of 13.5 nm. The narrowing in the width of UV emission peak
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indicates the improvement in the crystallinity of ZnO nanorods after annealing. In
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addition, the green luminescence becomes even weaker and could hardly be observed
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as shown in the inset of Figure 10, indicating that the defect concentration is almost
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negligible in the annealed ZnO nanorods. The PL measurement gives another
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evidence for the improvement in the quality of the ZnO nanorods due to post-grown
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annealing.
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Figure 10 Room temperature PL spectra of as-grown and annealed ZnO nanorods.
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At low temperatures, due to the suppression of non-radiative recombination and
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the freezing of phonons, the NBE emission increase significantly together with a
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blue-shift of the emission peak and a narrowing of the emission width. Figure 11(A)
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displays the PL spectra of the annealed ZnO nanorods taken at 10 K and 300 K. The
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inset in Figure 11(A) shows the high resolved NBE emission and its Gaussian
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deconvolution showing the emission components associated with various excitons
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involved in radiative transitions. The strongest emission peak at 3.38 eV is attributed
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to the neutral donor bound excitonic emission (D0X) [21], while the peak at the
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low-energy side of D0X is the neutral acceptor bound excitonic emission (A0X),
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indicating that the NBE emission results mainly from the bound exciton emission at
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associated with free excitons (FX). Besides the emission peaks associated with bound
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and free excitons, another emission peak can be resolved at 3.34 eV, which is
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considered as the radiative recombination processes caused by donor-to-acceptor
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(DAP) pair transitions [26].
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Figure 11(A) PL spectra of annealed ZnO nanorods recorded at 300 and 10 K. The inset
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shows the details of the PL recorded at 10 K. (B) Temperature-dependent PL spectra of
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annealed ZnO nanorods.
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Figure 11(B) shows the temperature-dependent PL spectra of the annealed ZnO 17
ACCEPTED MANUSCRIPT nanorods taken at temperatures from 10 K to 300 K. The wavelengths of the PL
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components including D0X, A0X, FX and DAP have an obvious red shift as the
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temperature increases, caused by the decrease of the energy band gap as the
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temperature increases [27]. Meanwhile, the PL intensity decreases rapidly, especially
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for the emissions associated with bound excitons, which decrease much faster than
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that associated with free excitons. Such an evolution of the PL spectrum can be
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explained as that the thermal energy is absorbed by the bound excitons which
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decompose to free ones with the increase of temperature. When the temperature is
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lower than 200 K, the NBE emission is dominated by the bound exciton-related
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emission, whereas at temperatures higher than 200 K, the free exciton emission
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dominates the NBE emission. As the temperature continues to rise, the bound exciton
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emission can hardly be observed, suggesting the complete decomposition of the
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bound excitons to free ones.
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4. Conclusions
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We have demonstrated the size-controllable growth of highly oriented ZnO
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nanorods on nanocrystalline ZnO-seeded Si substrates by hydrothermal method using
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formamide and Zn as the raw materials. The ZnO seed layers were deposited by
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plasma assisted pulsed laser deposition, and the crystal quality of ZnO in seed layers
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is dependent on deposition time and annealing temperature. The as-grown nanorods
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are hexagonal wurtzite in crystal structure with c-axis orientation. The diameter and
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length can be controlled by varying the crystal quality of ZnO in seed layers, with
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better crystal quality of ZnO seed layers resulting in larger and longer ZnO nanorods
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as well as better crystal structure. Post-grown annealing in air has little influence on
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the morphology of the grown ZnO nanorods, but improves the ZnO crystallinity.
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Under the 325-nm light excitation at room temperature, the ZnO nanorods emit a
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strong UV luminescence dominated by NBE emission of ZnO associated with
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radiative recombination of free excitons with almost the absence of visible
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defect-related emission. The photoluminescence increases as the temperature
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decreases and evolves to be dominated by the bound excitonic emission at neutral
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donors.
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Acknowledgement This work is supported by the National Natural Science Foundation of China
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(11275051) and
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(15ZR1403300).
the
Municipal
Natural
Science Foundation
of Shanghai
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ACCEPTED MANUSCRIPT Size-controllable growth of oriented ZnO nanorods on ZnO-seeded Si was demonstrated. ZnO nanorods were grown by hydrothermal method at a temperature as low as 65 °C.
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The diameter and length of ZnO nanorods depend on the crystal quality of seed layer. The size of nanorods can be controlled by changing the deposition time of seed layer.
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The size can also be controlled by varying the annealing temperature of seed layer.
PII:
S0749-6036(16)31371-4
DOI:
10.1016/j.spmi.2016.12.005
Reference:
YSPMI 4708
To appear in:
Superlattices and Microstructures
Received Date: 2 November 2016 Accepted Date: 5 December 2016
Please cite this article as: Z. Yu, H. Li, Y. Qiu, X. Yang, W. Zhang, N. Xu, J. Sun, J. Wu, Sizecontrollable growth of ZnO nanorods on Si substrate, Superlattices and Microstructures (2017), doi: 10.1016/j.spmi.2016.12.005. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Submitted to Superlattices and Microstructures, original
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Size-controllable growth of ZnO nanorods on Si substrate
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Zhentao Yua, Hui Lia, Yining Qiua, Xu Yanga, Wu Zhanga, Ning Xua,b, Jian Suna,b,c,*,
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Jiada Wua,b,* Department of Optical Science and Engineering, Fudan University, Shanghai 200433, China
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Shanghai Engineering Research Center of Ultra-Precision Optical Manufacturing, Fudan University, Shanghai 200433, China
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c
Key Lab for Micro and Nano Photonic Structures, Ministry of Education, Fudan
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University, Shanghai 200433, China
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Abstract
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Here we report a simple two-step chemical-solution-based method to grow highly
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oriented and size-controllable ZnO nanorods on ZnO-seeded Si substrate. The
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morphology of the grown ZnO nanorods was examined by field emission scanning
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electron microscopy. The structure was characterized by X-ray diffraction and Raman
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scattering spectrum. Photoluminescence spectra were measured at room temperature
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and low temperatures to evaluate the photoluminescence properties of the ZnO
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nanorods. The grown ZnO nanorods are structured with hexagonal wurtzite. The
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diameter and length of ZnO nanorods can be controlled by varying the crystal quality
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of the underlying ZnO seed layers. The crystal quality of the seed layers gets
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improved as the deposition time and annealing temperature for ZnO seed layers are
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increased. The effects of annealing on the ZnO nanorods were also studied.
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Keywords: ZnO, nanorod, seed layer, hydrothermal reaction, size-controllable growth
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*Corresponding author. E-mail addresses: [email protected] (J. Sun),
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[email protected] (J.D. Wu).
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1. Introduction With a direct and wide band gap (Eg = 3.37 eV), a large excitation binding energy
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(60 meV) at room temperature, and excellent properties such as high piezoelectricity,
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pyroelectricity, conductivity, transparency, thermal and chemical stability[1, 2], ZnO
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is an outstanding representative of II-VI group compound semiconductor materials
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having potential applications in a variety of fields. Compared with bulk ZnO, ZnO
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nanostructures, especially one-dimensional ZnO nanostructures (nanorods, nanotubes,
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nanowires, nanotapes, etc.) having a high specific surface area, usually show unique
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and excellent properties and behave better than bulk ZnO when being used in
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electrical, optical and magnetic fields, such as short-wavelength optoelectronic
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devices, solar energy conversion, transparent conducting coating materials, and
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sensors etc [3-9]. In addition, the high specific surface area of ZnO nanostructures
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provides an advantage to efficiently decorate the surface of ZnO nanostructures for
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the modification of properties. Therefore, the directional growth of high quality
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one-dimensional ZnO nanostructures on solid substrates and size controlling have
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become very promising and exciting.
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There are many methods for the preparation of one-dimensional ZnO nanorods.
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Generally speaking, they can be divided into gas phase techniques and liquid phase
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methods. Compared with the gas phase methods which are usually performed at high
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temperatures, the preparation of ZnO nanorods using liquid phase methods has great
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advantage and superiority because of its low energy consumption, large scale
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production and low temperature fabrication. In this paper, we use a two-step,
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wet-chemical process to grow ZnO nanorods on Si substrates. This process includes
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the deposition of a nanocrystalline ZnO thin film on Si substrate as a seed layer and
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the growth of ZnO nanorods on the ZnO seed layer by a low-temperature
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chemical-solution-based method. Highly oriented ZnO nanorods are grown by the
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thermal decomposition of Zn-formamide complexes in hydrothermal reaction [10-12].
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In this process, the crystalline grains in the seed layer work as nucleation centers for
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the subsequent growth of ZnO nanorods, and the length and diameter of ZnO
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nanorods are dependent on the crystal quality of the ZnO seed layer and hence are
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controllable by varying the crystal quality of ZnO seed layers. In addition, the growth
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of ZnO nanorods is performed at a temperature much lower than that for
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hydrothermal growth of ZnO nanorods using zinc salts [13, 14].
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2. Methods
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2.1 Deposition of ZnO seed layers
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N-type Si(100) wafers were used as substrates after being carefully cleaned
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sequentially with acetone, ethanol and de-ionized water in an ultrasound bath. A thin
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nanocrystalline ZnO layer was first deposited on Si substrate by electron cyclotron
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resonance plasma assisted pulsed laser deposition (ECR-PLD) [15]. Then some
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samples were annealed at 400 °C or 600 °C in air for 30 minutes. In the subsequent
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growth process of ZnO nanorods, the annealed nanocrystalline ZnO layer mainly has
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two functions: (a) working as a buffer layer to solve the lattice mismatch between
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ZnO nanorods and Si substrate, and (b) serving as a seed layer to assist the nucleation
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of ZnO and the growth of ZnO nanorods oriented along the direction vertical to Si
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substrate.
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2.2 Growth of ZnO nanorods
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The ZnO nanorods were grown on the ZnO seed layer with a low-temperature
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formamide-solution-based method. Analytical-grade formamide and metal Zn foils
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with a purity of 99.99% were used as raw materials of the hydrothermal reaction.
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Using pure metal Zn as zinc source can avoid introducing impurities which may be
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incorporated when using other zinc salts in usual wet-chemical reactions. Typically,
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one ZnO-seeded Si substrate and one Zn foil were immersed in 3 mL of 5% (volume
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ratio, v/v) formamide aqueous solution in a 10 mL glass ware. The glass ware was put
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in a thermostatic water bath pot for ZnO nanorods to be grown for 24 hours at a
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constant temperature of 65 °C which is much lower than those for hydrothermal
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growth of ZnO nanorods using zinc salts. The sample was then taken out to be
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ultrasonically cleaned in de-ionized water and dried in air. Some of the grown ZnO
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nanorods were annealed in air at 600 °C for one hour.
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The growth process of the ZnO nanorods is based on the homogeneous 3
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natural oxidation of metal Zn by naturally dissolved oxygen in water is limited by the
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existence of a naturally formed thin passive ZnO layer, and hence the rate of natural
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oxidation for metal Zn is very slow in water. This oxidation process can be
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remarkably accelerated by the addition of formamide [10, 16, 17]. In the presence of
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formamide, Zn-formamide complexes are formed by spontaneous oxidation reaction
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of metal Zn and oxygen at the surface of Zn foil contacting the solution
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(1)
facilitating the releasing of Zn ions into the solution by decomposition of Zn-formamide complexes [Zn(formamide)n]2+ → Zn2+ + n⋅formamide .
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Zn + n⋅formamide + H2O + 1/2 O2 → [Zn(formamide)n]2+ + 2OH− ,
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(2)
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Zn ions are therefore continuously supplied to support the formation of ZnO in the
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vicinity of the substrate and the self-assembled growth of one dimensional ZnO
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nanostructures at the surface of the substrate [18] Zn2+ + 2OH− → Zn(OH) ,
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Zn(OH) → ZnO + H2O .
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(3) (4)
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2.3 Sample characterization
The morphology of the samples was characterized by field emission scanning
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electron microscopy (FE-SEM) with a Hitachi S-4800 microscope. The crystal
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structure of the samples were characterized by X-ray diffraction technology using
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Cu Kα radiation with a Rigaku D/max-γB X-ray diffractometer. Raman scattering
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spectroscopy was used to characterize the lattice vibration modes of ZnO nanorods
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with a Jobin-Yvon LabRAM HR 800 UV spectroscope using 632.8-nm laser light to
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excite the samples. The photoluminescence (PL) was measured to evaluate the
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luminescence properties of ZnO nanorods at room temperature and low temperatures
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with 325-nm He−Cd laser light excitation.
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3. Results and discussion
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3.1 Morphology and structure of ZnO seed layer with varied deposition time 4
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°C-annealed ZnO seed layers deposited for 10, 20 and 30 minutes, respectively. The
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FE-SEM images indicate that the seed layers are composed of densely connected
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grains with the grain boundaries being clearly revealed. It seems that the mean size of
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the particles on the seed layer increases with the increase of the deposition time.
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Longer deposition time provided more ZnO precursors, resulting in thicker seed
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layers and larger grains.
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Figure 1 Top-view FE-SEM images of ZnO seed layers deposited by ECR-PLD method
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for (A) 10, (B) 20 and (C) 30 minutes and subsequently annealed at 600 °C in air.
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Figure 2 shows the XRD patterns of the ZnO seed layers deposited for 10, 20 and
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30 minutes followed by annealing at 600 °C in air. Besides the strong diffraction from
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the Si substrate, one diffraction peak at about 2θ ~ 34 ° appears in each pattern. This 5
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deposited ZnO layers are structured with hexagonal wurtzite and are grown highly
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oriented with their c-axes perpendicular to the substrates. As shown in Figure 2, the
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diffraction intensity increases and the full width at half-maximum (FWHM) decreases
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as the deposition time increases, revealing that the crystal structure improves and the
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crystal grains grow larger. On the lattice-mismatched Si, the initially deposited ZnO
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serves as the buffer layer for later deposited ZnO. Therefore, as the ZnO film grows
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thicker, the crystal structure gets better. According to the Scherrer formula D =
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Kλ/Bcosθ [19], where λ is the wavelength of the incident X-ray, B the diffraction peak
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width (FWHM) in radian, θ the Bragg angle, K a numerical constant set as 0.9, the
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mean size D of the crystal grains were calculated to be 30 31 32 nm for the ZnO seed
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layers deposited for 10, 20 and 30 minutes, respectively.
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Figure 2 XRD patterns of ZnO seed layers deposited by ECR-PLD method for 10 (a), 20
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(b) and 30 (c) minutes and subsequently annealed at 600 °C in air.
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3.2 Effect of seed layer deposition duration on ZnO nanorods
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We find that the size of ZnO nanorods is dependent on the crystal quality of ZnO
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in the seed layer. Annealed ZnO films with different deposition time (10, 20 and 30
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minutes) were used as seed layers for the growth of ZnO nanorods under the same
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hydrothermal reaction conditions. As shown by the FE-SEM images in Figure 3,
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uniform and dense nanorods have been grown on the ZnO-seeded substrates. The 6
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direction nearly vertical to the substrate. It can be seen that hexagonal ZnO nanorods
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with a mean diameter of 150 nm and length of 1.2 µm have been grown on the seed
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layer deposited for 10 minutes. On the ZnO seed layers deposited for 20 and 30
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minutes, the grown ZnO nanorods have larger mean diameters of 175 and 219 nm,
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respectively. And the lengths of the nanorods are also increased to 1.4 and 2.2 µm,
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respectively, as clearly shown in Figure 3 and its insets. As described above, the
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crystallinity of the ZnO seed layer improves as the deposition time for ZnO seed layer
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increases. This suggests that the size of the grown ZnO nanorods is dependent on the
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crystallinity of the ZnO seed layers.
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Figure 3 Top-view and cross-sectional (inset) FE-SEM images of ZnO nanorods grown on
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ZnO seed layers deposited for (A) 10, (B) 20 and (C) 30 minutes.
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10-, 20- and 30-minute deposited ZnO seed layers. The strong and narrow diffraction
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peak indexed to the (002) diffraction of hexagonal wurtzite ZnO at about 2θ ~ 34°
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reveals that the ZnO nanorods have been grown at high (002) preference with their
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c-axes perpendicular to the substrate. The FWHM of the (002) peak of the ZnO
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nanorods is about 0.1°, smaller than that of the seed layer (~ 0.3°). This indicates that
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the size of ZnO grains in the grown nanorods is larger than that in the seed layers. It is
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noted that with increasing seed layer deposition time, the FWHM of the (002) peak
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gets smaller, i.e. the grain size of the nanorods becomes larger. From the XRD data
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and using the Scherrer formula, the ZnO grain size was calculated to be 64 69 and 83
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nm, respectively, for the ZnO nanorods grown on the 10-, 20- and 30-minute
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deposited ZnO seed layers. It is well known that the nanocrystalline ZnO films serve
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as buffer layers to solve the problem of lattice mismatch between ZnO and the
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underlying Si(100) substrate. Besides that, the ZnO nanocrystalline grains in the seed
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layers act as the crystal nuclei for the hydrothermal growth of ZnO nanorods. Longer
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deposition time results in better crystallinity and larger crystalline grains, which are
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conducive for ZnO nanorods to be grown larger and longer. Thus we can control to
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some degrees the length and diameter of the ZnO nanorods by changing the
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deposition time of the seed layers.
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Figure 4 XRD patterns of ZnO nanorods grown on ZnO seed layers deposited for 10 (a),
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20 (b) and 30 (c) minutes.
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Figure 5 illustrates the PL spectra of the grown ZnO nanorods taken at room
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temperature. The ZnO nanorods were grown on the ZnO seed layers deposited for
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different durations. These nanorods exhibit some difference in luminescence
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properties. Under 325-nm light excitation, the ZnO nanorods emit an intense
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ultraviolet (UV) luminescence around 380 nm corresponding to the near-band-edge
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(NBE) emission of ZnO [20-22]. The defect-related visible luminescence can hardly
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be detected, revealing that the concentrations of defects in the ZnO nanorods are very
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low. However, the UV NBE emission is different in emission intensity and width.
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With prolonging the time for ZnO seed layer deposition, i.e. improving the crystal
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quality of ZnO seed layer, the UV ZnO NBE emission gets stronger and narrower. As
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compared to the NBE of the ZnO nanorods grown on the 10-minute deposited ZnO
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seed layer, that of the ZnO nanorods grown on the 30-minute deposited ZnO seed
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layer is nearly tripled in intensity. The intensity increase in the NBE emission can be
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attributed to the better crystallinity of the ZnO nanorods, consistent with the above
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XRD characterization which reveals that the grain size of the nanorods is increased
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with the increase of the time for ZnO seed layer deposition.
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Figure 5 Room-temperature PL spectra of ZnO nanorods grown on ZnO seed layers
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deposited for 10 (a), 20 (b) and 30 (c) minutes.
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3.3 Effect of seed layer annealing temperature on ZnO nanorods 9
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layer, it can be envisioned that the size of ZnO nanorods is not only dependent on the
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deposition time of ZnO see layers, but also on the annealing conditions of the seed
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layers. Here we demonstrate the effect of the seed layer annealing temperature on the
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ZnO nanorods. The 30-minute deposited ZnO films with different annealing
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temperatures (as-deposited, 400 and 600 °C) were used as seed layers for the growth
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of ZnO nanorods under the same hydrothermal reaction conditions. As shown by the
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FE-SEM images in Figure 6, the size and shape of the ZnO nanorods are strongly
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affected by the temperature at which the seed layer was annealed. The ZnO nanorods
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grown on the as-deposited seed layer have a shape of irregular hexagonal prisms with
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a smaller mean diameter of 105 nm and rougher surface than those grown on the
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annealed seed layers. And as the annealing temperature increases, the diameter of the
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nanorods gets larger.
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Figure 6 Top-view and cross-sectional (inset) FE-SEM images of ZnO nanorods grown on
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(A) as-deposited, (B) 400 °C-annealed and (C) 600 °C-annealed ZnO seed layers. The
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seed layers were deposited for 30 minutes.
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To explain the effects of the annealing temperature of the ZnO seed layers on the
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grown ZnO nanorods, the surface morphology and crystal structure of as-deposited
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and annealed ZnO films used as seed layers for ZnO nanorod growth were
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characterized. FE-SEM images in Figure 7 show the surface morphology of the
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30-minute deposited ZnO films before and after annealing in air for 30 minutes. It can
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be seen that for the as-deposited film, the grains seem imperfectly grown with vague
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boundaries, revealing poor crystal structure. After annealing, the films show
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well-grown grains and clear boundaries, an evidence of improved crystal structure of
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the annealed films. And the grains grow lager and the boundaries appear clearer as the
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Figure 7 Top-view FE-SEM images of ZnO films deposited by ECR-PLD for 30 minutes,
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(A) as-deposited, (B) annealed at 400 °C and (C) annealed at 600 °C.
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Figure 8 shows the XRD patterns of the as-deposited, 400 °C-annealed and 600
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°C-annealed ZnO films deposited for 30 minutes. We can see that the deposited ZnO
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films show wurtzite structure with c-oriented preference whether annealed or not.
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However, the annealed ZnO films have higher diffraction intensity and narrower
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diffraction width than the as-deposited one, and the diffraction gets stronger with
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smaller width as the annealing temperature increases, consistent with result obtained
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by FE-SEM examination. Combination of the crystal structure characterization for the
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as-deposited and different-temperature-annealed ZnO films with the size observation 12
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ZnO films confirms further that the size of the grown ZnO nanorods is strongly
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dependent on the crystal quality of the ZnO seed layer on which the ZnO nanorods are
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grown. The size dependence of the grown ZnO nanorods on the deposition time and
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annealing temperature of the seed layer suggests us an approach to control the size of
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ZnO nanorods by varying the crystal quality of the seed layer.
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Figure 8 XRD patterns of ZnO films deposited by ECR-PLD for 30 minutes, as-deposited
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(a), annealed at 400 °C (b) and annealed at 600 °C (c).
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3.4 The influence of post-grown annealing on the structure of nanorods Post-grown annealing in air has little influence on the morphology of the grown
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ZnO nanorods. However, the structure of the grown ZnO nanorods can be improved
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by annealing. Figure 9(A) shows the XRD patterns of as-grown and annealed ZnO
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nanorods grown on the 10-minute deposited ZnO seed layer. Only the (002)
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diffraction of hexagonal wurtizite ZnO can be observed in addition to a strong
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diffraction peak of Si substrate. After annealing, the intensity of the (002) diffraction
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peak is increased significantly, indicating the improvement in the crystallinity of the
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ZnO nanorods. As for the annealed nanorods, the (002) diffraction angle is 2θ002 =
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34.54° with an FWHM of 0.14°, according to Bragg formula [19], the lattice constant
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was calculated to be c = 0.519 nm. This value is well consistent with that from JCPDS
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data (JCPDS 36-1451).
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Figure 9 (A) XRD patterns of as-grown and annealed ZnO nanorods on 10-minute
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deposited ZnO seed layer. (B) Raman spectra of as-grown and annealed ZnO nanorods
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on 10-minute deposited ZnO seed layer. Raman spectrum recorded for Si (100) substrate
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is also shown for comparison.
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Raman backscattering measurement of the samples confirms that the ZnO
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nanorods had hexagonal wurtzite structure and the crystallinity of the ZnO nanorods
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is improved by annealing. For hexagonal wurtzite structure, the phonon modes
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belonging to the E2, E1, and A1 symmetries are Raman active. The E2 symmetry has
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low-frequency branch E2 (low) and high-frequency branch E2 (high), while the E1 and
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A1 symmetries can be divided into longitudinal optical (LO) and transverse optical 14
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modes: E2 (low), E2 (high), A1 (TO), A1 (LO), E1 (TO) and E1 (LO) [23]. Figure 9(B)
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shows the Raman spectra of the as-grown and annealed ZnO nanorods grown on the
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10-minute deposited ZnO seed layer when being excited by the 632.8-nm laser beam.
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Compared with the Raman spectrum of the Si substrate, it can be seen that two
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obvious Raman signal peaks located at 99 and 438 cm−1 appear in the Raman
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spectrum taken from the as-grown ZnO nanorods. They correspond to the nonpolar
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optical phonon E2 (low) and E2 (high) models, respectively [23], confirming that the
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grown ZnO nanorods have hexagonal wurtzite structure. After annealing treatment,
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the sample exhibits enhanced intensities of the E2 (low) and E2 (high) signals. Besides
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the above two Raman signals, we can also observe weak peaks at around 330, 376 and
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580 cm−1. The signal at 330 cm−1 is identified as the second-order Raman scattering of
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the E2 mode of hexagonal ZnO [24], and the signal at 376 cm−1 is considered to be the
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A1 (TO) mode, while the last one is probably consisted of the A1 (LO) and E1 (LO)
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modes [23]. They all belong to the Raman scattering from wurtzite ZnO. The
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enhancement of the E2 (low) and E2 (high) signals and the emergence of the additional
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weak Raman signals demonstrate the improvement in the crystalline quality of ZnO
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nanorods after annealing.
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The luminescence properties can also reveal the effect of annealing on the
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structure. Hence PL spectra were measured to investigate the luminescence properties
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of the ZnO nanorods. Room temperature PL spectra of the as-grown and annealed
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ZnO nanorods are shown in Figure 10. As described above, the as-grown ZnO
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nanorods have the capability of emitting strong UV luminescence attributed to the
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NBE emission of ZnO. A detailed PL measurement reveals that the UV NBE
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emission peaks at 382 nm with an FWHM of 30 nm. Besides this NBE emission of
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ZnO, very weak and broad visible luminescence with the center at 520 nm is also
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observed as shown in the inset. It is usually believed that the green luminescence is
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associated with the defect levels in the band gap of ZnO introduced by defects such as
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oxygen vacancies and Zn interstice [25]. The intensity of the visible luminescence is
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very low compared with that of the NBE emission, indicating that the ZnO nanorods
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have good quality with low defect concentration. For the annealed ZnO nanorods, the UV luminescence peak blue-shifts to 378 nm
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with an FWHM of 13.5 nm. The narrowing in the width of UV emission peak
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indicates the improvement in the crystallinity of ZnO nanorods after annealing. In
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addition, the green luminescence becomes even weaker and could hardly be observed
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as shown in the inset of Figure 10, indicating that the defect concentration is almost
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negligible in the annealed ZnO nanorods. The PL measurement gives another
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evidence for the improvement in the quality of the ZnO nanorods due to post-grown
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annealing.
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Figure 10 Room temperature PL spectra of as-grown and annealed ZnO nanorods.
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At low temperatures, due to the suppression of non-radiative recombination and
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the freezing of phonons, the NBE emission increase significantly together with a
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blue-shift of the emission peak and a narrowing of the emission width. Figure 11(A)
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displays the PL spectra of the annealed ZnO nanorods taken at 10 K and 300 K. The
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inset in Figure 11(A) shows the high resolved NBE emission and its Gaussian
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deconvolution showing the emission components associated with various excitons
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involved in radiative transitions. The strongest emission peak at 3.38 eV is attributed
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to the neutral donor bound excitonic emission (D0X) [21], while the peak at the
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low-energy side of D0X is the neutral acceptor bound excitonic emission (A0X),
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indicating that the NBE emission results mainly from the bound exciton emission at
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associated with free excitons (FX). Besides the emission peaks associated with bound
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and free excitons, another emission peak can be resolved at 3.34 eV, which is
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considered as the radiative recombination processes caused by donor-to-acceptor
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(DAP) pair transitions [26].
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Figure 11(A) PL spectra of annealed ZnO nanorods recorded at 300 and 10 K. The inset
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shows the details of the PL recorded at 10 K. (B) Temperature-dependent PL spectra of
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annealed ZnO nanorods.
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Figure 11(B) shows the temperature-dependent PL spectra of the annealed ZnO 17
ACCEPTED MANUSCRIPT nanorods taken at temperatures from 10 K to 300 K. The wavelengths of the PL
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components including D0X, A0X, FX and DAP have an obvious red shift as the
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temperature increases, caused by the decrease of the energy band gap as the
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temperature increases [27]. Meanwhile, the PL intensity decreases rapidly, especially
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for the emissions associated with bound excitons, which decrease much faster than
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that associated with free excitons. Such an evolution of the PL spectrum can be
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explained as that the thermal energy is absorbed by the bound excitons which
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decompose to free ones with the increase of temperature. When the temperature is
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lower than 200 K, the NBE emission is dominated by the bound exciton-related
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emission, whereas at temperatures higher than 200 K, the free exciton emission
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dominates the NBE emission. As the temperature continues to rise, the bound exciton
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emission can hardly be observed, suggesting the complete decomposition of the
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bound excitons to free ones.
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4. Conclusions
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We have demonstrated the size-controllable growth of highly oriented ZnO
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nanorods on nanocrystalline ZnO-seeded Si substrates by hydrothermal method using
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formamide and Zn as the raw materials. The ZnO seed layers were deposited by
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plasma assisted pulsed laser deposition, and the crystal quality of ZnO in seed layers
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is dependent on deposition time and annealing temperature. The as-grown nanorods
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are hexagonal wurtzite in crystal structure with c-axis orientation. The diameter and
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length can be controlled by varying the crystal quality of ZnO in seed layers, with
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better crystal quality of ZnO seed layers resulting in larger and longer ZnO nanorods
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as well as better crystal structure. Post-grown annealing in air has little influence on
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the morphology of the grown ZnO nanorods, but improves the ZnO crystallinity.
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Under the 325-nm light excitation at room temperature, the ZnO nanorods emit a
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strong UV luminescence dominated by NBE emission of ZnO associated with
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radiative recombination of free excitons with almost the absence of visible
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defect-related emission. The photoluminescence increases as the temperature
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decreases and evolves to be dominated by the bound excitonic emission at neutral
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donors.
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Acknowledgement This work is supported by the National Natural Science Foundation of China
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(11275051) and
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(15ZR1403300).
the
Municipal
Natural
Science Foundation
of Shanghai
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ACCEPTED MANUSCRIPT Size-controllable growth of oriented ZnO nanorods on ZnO-seeded Si was demonstrated. ZnO nanorods were grown by hydrothermal method at a temperature as low as 65 °C.
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The diameter and length of ZnO nanorods depend on the crystal quality of seed layer. The size of nanorods can be controlled by changing the deposition time of seed layer.
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The size can also be controlled by varying the annealing temperature of seed layer.