Article pubs.acs.org/crystal
Tip Shaping for ZnO Nanorods via Hydrothermal Growth of ZnO Nanostructures in a Stirred Aqueous Solution Chao-Yin Kuo,† Rong-Ming Ko,‡ Yung-Chun Tu,† Yan-Ru Lin,*,§ Tseng-Hsing Lin,† and Shui-Jinn Wang*,†,‡ †
Institute of Microelectronics, Department of Electrical Engineering, National Cheng Kung University, Tainan 701, Taiwan, R.O.C. Advanced Optoelectronic Technology Center, National Cheng Kung University, Tainan 701, Taiwan, R.O.C. § Department of Materials Engineering/Center for Thin Film Technologies and Applications, Ming Chi University of Technology, Taishan, Taipei 24301, Taiwan, R.O.C. ‡
S Supporting Information *
ABSTRACT: To enhance the field emission (FE) properties of emitters based on ZnO nanostructures, the growth of ZnO nanostructures on ZnO nanorods (NRs) (1−1.2 μm in length and ∼200 nm in diameter) in a disturbed hydrothermal growth (HTG) solution is demonstrated. Experimental results reveal that the degree of disturbance of the aqueous solution determines both the shape and location of the synthesized ZnO nanostructures. For stirring speeds of 300 and 600 rpm (rpm), NR-like ZnO nanostructures with a reduced uniform diameter (70−120 nm) and a tapered shape but a rough surface are grown on the basal plane of ZnO NRs, respectively. For stirring speeds of 900 and 1150 rpm, ZnO needles (40−70 nm and 15−20 nm in diameter, respectively) were synthesized along the {101̅0} planes of the ZnO NRs with coherent c-planes. FE characteristics of ZnO-NRs emitters with and without the second stage growth of ZnO nanostructures are reported and compared. Possible growth mechanisms which govern the physical characteristics of the ZnO nanostructures synthesized in the HTG process are proposed and discussed.
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°C in the HTG process, the morphology of ZnO evolved from nanorods (NRs) to nanopencils.16 The present study reports a simple two-step HTG method for growing and then decorating ZnO NRs on the vertical sidewalls of an aluminum-doped zinc-oxide (AZO) layer and modifying (or decorating) their tops for better field emission (FE) performance. In the first step of the HTG process, ZnO NRs are synthesized in a still aqueous solution of zinc nitrate. In the second step of the HTG process, ZnO nanostructures are grown on the ZnO NRs in an aqueous solution disturbed by a magnetic stirrer. Material analysis was conducted to clarify the effect of the stirring disturbance of the mixed solution during the second stage growth on the shape, size, and location of ZnO nanostructures. Scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (HRTEM) lattice images of the ZnO nanostructures grown in the second step of the HTG process are presented and analyzed. The impact of stirring speed for the aqueous solution on the size and shape of the synthesized ZnO nanostructures and the location of growth on ZnO NRs is investigated. Possible growth mechanisms of ZnO nanostructures in the stirred solution are
INTRODUCTION Zinc oxide (ZnO) is an important low-cost II−VI semiconductor material due to its catalytic, electrical, optoelectronic, and photoelectrochemical properties. In recent years, onedimensional (1D) ZnO nanostructures have attracted considerable attention because of their potential applications in lightemission nanodevices,1−3 solid-state sensors,4−7 and field emitters.8−14 The properties of ZnO nanostructures strongly depend on their dimensions and morphology. Controlling the dimensions and morphology of the nanostructures is thus crucial for basic fundamental studies and for applications such as building blocks of electronic and optoelectronic nanodevices. Numerous techniques have been employed for the synthesis of 1D ZnO nanostructures. Among them, the hydrothermal growth (HTG) method is attractive because it is a lowtemperature and low-cost process that is especially suitable for large-area production. To improve the performance of ZnOnanostructure-based devices, a number of research groups have attempted to control the morphology of 1D ZnO nanostructures using HTG processes. Vayssieres demonstrated that when the concentration of the precursors is decreased by an order of magnitude in the HTG process, the diameter of the ZnO rods can decrease by about an order of magnitude, suitable for nano-, meso-, and microscales.15 Ahsanulhaq et al. found that when the reaction temperature was increased from 70 to 110 © 2012 American Chemical Society
Received: October 4, 2011 Revised: May 19, 2012 Published: June 15, 2012 3849
dx.doi.org/10.1021/cg2013182 | Cryst. Growth Des. 2012, 12, 3849−3855
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Figure 1. Schematic diagram of the two-step HTG process flow and the typical devcie structure of prepared samples at the end of each step.
Figure 2. SEM and TEM images of prepared samples (a) after the first step of the HTG process and (b) after the second step (300 rpm) of the HTG process. (c) TEM image of the ZnO NR shown in part b. The inset shows a wider view of part c.
also proposed. The experimental findings and growth mechanism presented and proposed in this work might shed light on the physical insights that govern the growth of nanostructure for the decoration of parent ZnO NRs. In addition, the effectiveness in improving the FE characteristics of emitters based on the tip-decorated ZnO NRs proposed in this work has also been confirmed experimentally, which might
open low voltage operable FE devices for sensors and vacuum electronics applications.
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EXPERIMENT
The process flow for the fabrication of lateral type ZnO NRs emitters and the typical device structure of the prepared samples at the end of each step are schematically illustrated in Figure 1. It comprises three 3850
dx.doi.org/10.1021/cg2013182 | Cryst. Growth Des. 2012, 12, 3849−3855
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Figure 3. SEM images of samples prepared with various stirring speeds in the second step of the HTG process: (a) 600, (b) 900, and (c) 1150 rpm.
Figure 4. Schematic diagram illustrating the setup used for FE current measurement (a) and experimental FE characteristics of electron emitters based on lateral ZnO NRs prepared by a conventional one-step HTG process (b) and the proposed two-step HTG process with different stirring speeds (c). The as-deposited AZO layer had a doping concentration of 4.1 × 1020 cm−3 and a resistivity of 1.1 × 10−3 Ω·cm according to Hall measurements and four-point probe resistivity measurements, respectively. After a photolithographic process, samples with a twoelectrode configuration,17 i.e., a pair of vertical AZO sidewalls with a spacing (LM) of 6−10 μm and a width (W) of 500 μm, were obtained. This finishes the two-electrode structure preparation step. Note that the two-electrode structure allows a selective growth of ZnO NRs on the side walls of the AZO seed layer openings. In addition, the anode− cathode (or the tip-to-tip) distance L can be well controlled in the
main steps, including the lateral type two-electrode structure preparation, the first-step HTG process for the lateral growth of ZnO NRs, and the second-step HTG process for the tip shaping of ZnO NRs obtained from the first-step HTG process. In the experiments, Si wafers with a 300-nm-thick SiO2 top layer were used as substrates. At first, a bilayer structure comprising an AZO (200 nm) seeding layer and a Pt (300 nm) nonseeding layer was deposited via sputtering and e-beam deposition in sequence onto the SiO2 layer. Note that the Pt layer was employed to serve as a contact pad and to suppress the growth of ZnO NRs on the top surface of the AZO layer. 3851
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along the prismatic planes of the ZnO NRs obtained in the first step of the HTG process. To examine the effectiveness of the two-step HTG process in improving the FE characteristics of lateral ZnO NRs, the FE characteristics of electron emitters based on lateral ZnO NRs prepared by the proposed two-step HTG approach were measured in a vacuum chamber with a base pressure of 5 × 10−6 Torr at room temperature. Note that the sample was first heated for 15 min to degas and a Keithley 237 source-measure unit was used for measuring the current−voltage (I−V) characteristics. A bias voltage with a sweep step of 0.4 V was applied between the anode and the cathode. The prepared samples were measured one after the other under exactly the same conditions. Figure 4 shows the typical FE current-field intensity (I−E) characteristics of the electron emitters. The inset shows the corresponding Fowler−Nordheim (FN) plot. Note that the FN plots show a linear relationship, implying that the quantum tunneling effect is the main mechanism for the FE. Though it is still a challenging issue in reliably comparing the field emission characteristics of different emitters which have a very short interelectrode distance L (
