Effect of Aspect Ratio on Field Emission Properties of ZnO Nanorod Arrays
© to the authors 2008
Received: 27 June 2008
Accepted: 1 August 2008
Published: 16 August 2008
ZnO nanorod arrays are prepared on a silicon wafer through a multi-step hydrothermal process. The aspect ratios and densities of the ZnO nanorod arrays are controlled by adjusting the reaction times and concentrations of solution. The investigation of field emission properties of ZnO nanorod arrays revealed a strong dependency on the aspect ratio and their density. The aspect ratio and spacing of ZnO nanorod arrays are 39 and 167 nm (sample C), respectively, to exhibit the best field emission properties. The turn-on field and threshold field of the nanorod arrays are 3.83 V/μm and 5.65 V/μm, respectively. Importantly, the sample C shows a highest enhancement of factorβ, which is 2612. The result shows that an optimum density and aspect ratio of ZnO nanorod arrays have high efficiency of field emission.
ZnO is an important functional II–VI semiconductor compounds and have been extensively investigated for their excellent optoelectronic, electronic, photochemical properties [1, 2]. Various methods including chemical, electro-chemical, and physical deposition techniques have been employed to synthesize 1D ZnO nanostructures [3–5]. On the other hand, the wet-chemical methods [6–8] have been used for producing varied ZnO one-dimensional (1D) nanostructures, such as nanotube , nanopencil , nanoneedle , and nanoscrew . Recent experiments have shown that the ZnO 1D nanostructures have excellent field emission properties, far better than other semiconductors [10, 11, 13, 14]. An important advantage of using aligned nanorods, nanowires, nanobelts, and nanotubes for field emission is their high aspect ratio. The field enhancement factor β is a key parameter which is determined by turn-on field, threshold field, and work function. Also, the value of β relates to the structure, shape, size, alignment, crystalline, aspect ratio, etc. . Many effects, such as morphological effects , surface states , and densities of nanorods  have been studied. Zhao et al.  investigated the morphological effects on the field emission of ZnO nanorod arrays. The result showed that the ZnO nanoneedle arrays exhibit excellent properties due to their small emitter radius and high nanorod density remarkably reduces the local field at the emitters owing to the screening effect. Wang et al.  investigated the density effects on the field emission of ZnO nanorods and pointed out that the mezzo density of ZnO nanorods had the best field emission properties. But there is no report about the aspect ratio effects on the field emission properties of ZnO nanorod arrays. In this work, ZnO nanrod arrays with different aspect ratios and densities are synthesized by controlling the reaction times and concentrations of solution. The field emission properties of ZnO nanorod arrays with different aspect ratios and densities have been investigated for showing the enhancement factor β was enhanced with increasing the aspect ratio of ZnO nanorods and the enhancement factor β was decreased with reducing the density of nanorods. An optimum density and aspect ratio of ZnO nanorod arrays (sample C) have high efficiency of field emission. A model has been used for the explanation of the results.
The ZnO nanorod arrays were prepared on a silicon wafer (4 × 5 cm2) through a multi-step hydrothermal process . Firstly ZnO nanocrystals colloid (4 × 10−3 M) is spin-coated 15 times on the silicon wafer at the speed of 3000 r/s to form a thick film of ZnO nanocrystals and ZnO nanocrystlas film annealed at 400 °C for 2 h under atmosphere. Then the silicon wafer is immersed into the aqueous solution (250 mL) of 0.04 M zinc nitrate hexahydrate/hexamethylenetetramine at 75 °C. After keeping it for 10 h in this solution, the surface of silicon wafer is coated for forming a layer of white film, which is washed by deionized water three times, and dried in air at room temperature. A piece of the silicon wafer (1 × 4 cm2) is cut as sample A. The remnant wafer (4 × 4 cm2) is reinserted into the aqueous solution (250 mL) of 0.04 M zinc nitrate hexahydrate/hexamethylenetetramine at 75 °C for 10 h and the sample B is obtained by cutting a piece from the above mentioned silicon wafer (1 × 4 cm2). The nanorods are able to form bundles if the sample B is immersed into 0.04 M aqueous solution of zinc nitrate hexahydrate/hexamethylenetetramine at 75 °C for 10 h. The concentration of reaction solution is reduced to 0.03 M and a remnant wafer (3 × 4 cm2) is kept into the aqueous solution (250 mL) of 0.03 M of zinc nitrate hexahydrate/hexamethylenetetramine at 75 °C for 10 h and the sample C (1 × 4 cm2) is obtained from the above method. Samples D and E are obtained by repeating the reaction process several times.
The arrays of ZnO nanorods are characterized and analyzed by field emission scanning electron microscopy (FESEM) and X-ray diffraction (XRD). The SEM images are obtained with a JEOL JSM 6700F field emission scanning electron microscope. The XRD patterns are recorded with a Japan Rigaku D/max-2500 rotation anode X-ray diffractometer equipped with graphite-monochromatized Cu Kα radiation (λ = 1.54178 Å), employing a scanning rate of 0.05°s−1in the 2θ range from 20° to 60°.
The field emission properties of ZnO nanorod arrays are measured using a two-parallel-plate configuration in a homemade vacuum chamber at a base pressure of ~1.0 × 10−6 Pa at room temperature. The sample is attached to one of the stainless-steel plates which is cathode with the other plate as anode. The distance between the electrodes is 300 μm. A direct current voltage sweeping from 0 to 5000 V was applied to the sample at a step of 50 V. The emission current is monitored using a Keithley 6485 picoammeter.
Results and Discussion
The morphological characteristic and field emission property of ZnO nanorod arrays. (r: the average radius;L: the length of nanorod;β: the field enhance factor;s: the spacing of nanorods)
Turn-on field (V/μm)
Threshold field (V/μm)
90 ± 2
2.5 ± 0.05
195 ± 10
125 ± 5
4.2 ± 0.05
183 ± 10
136 ± 5
5.3 ± 0.05
167 ± 10
150 ± 5
6.3 ± 0.1
143 ± 10
168 ± 5
7.4 ± 0.1
126 ± 10
The arrays of ZnO nanorod with different aspect ratios and densities are constructed using a multi-step hydrothermal process by controlling the reaction times and concentrations. The field emission properties of ZnO nanorod arrays are investigated. The results show that the aspect ratio and the density of nanorod arrays play key roles in the field emission. The sample C exhibits the best field emission properties in these samples. The field enhancement factorβ enhances with increasing the aspect ratio of the nanorod. For the small interspacing (s), the screening effect may become the domain factor which will decrease the field enhancement factorβ. When the interspacing (s) is larger than 167 nm, the enhancement factorβ increases with aspect ratio, linearly, while the screening effect can be negligible. But when thes is smaller than 167 nm, the screening effect becomes the domain factor. There exists a balance point (aspect ratio: 38.9, interspacing: 178 nm), in which the optimization field emission can be obtained.
This work was supported by the National Nature Science Foundation of China (20531060, 20473102, and 20571078) and the National Basic Research 973 Program of China (Grant No. 2006CB932100 and 2005CB623602).
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