3-1. Introduction
One-dimensional (1D) semiconductor nanostructures attract renewed interest because of their excellent optoelectronic properties and promising applications [3.1-3.2]. Among the various applications, field emission displays had attracted a lot of interest for their high aspect ratio, proper number densities, and high emission stability [3.3]. Several groups have been reported on field emission from nanorods / nanowires such as carbon nanotubes (CNTs) [3.4], diamond cones [3.5], Ni31Si12 nanowires [3.6] and ZnO nanorods [3.7-3.8] etc. Among 1D nanostructure, the ZnO nanorods are considered to be one of the most promising cold cathode materials due to their large exciton binding energy, strong radiation-oxidation resistance, and high thermal stability [3.9-3.10]
. Generally, the field emission property depends on the material work function, tip morphology and number density of nanorod emitters. But, still it’s a challenge to develop the nanorod emitters having good properties and high reliability. Therefore, improvement of the field emission performances is an important issue for their application in field emission displays.
It is well known that control the morphologies of the nanorods to improve their field emission properties because the vertically aligned ZnO nanorods have relatively large diameter and hexagonal structure at top end. Plasma treatment is an easy and fast process to control morphology of the nanorods. We recently reported the preparation of ZnO nanotip structures from the as-grown ZnO nanorods by using the
combination of chemical etching and Ar plasma treatment [3.11]. However, there is no literature that has ever been reported on the effect of oxygen plasma treatment on field emission properties of ZnO nanorods. In this chapter, we synthesized the ZnO nanorods by using solution method and employed oxygen plasma etching method to form nanotip on the as-grown nanorods. Effects of etching time on morphology of nanostructures, electrical and optical characteristics of ZnO nanorods are also investigated.
3-2. Experimental method
P-type Si (100) substrate was cleaned by a standard Radio Corporation of America (RCA) method. A thin film of zinc acetate was spin coated on the substrate for 10 times with a solution containing 5 mM zinc dehydrate (C4H6O4Zn.2H2O, 98 % purity) in ethanol. After deposition, the film was annealed at 350 ℃ for 30 min to produce ZnO buffer layer.Then, the aqueous solution synthesis of ZnO nanorod array was carried out at 90 ℃ in a sealed kettle placed in a quartz beaker. The ZnO coated substrates were immersed in a precursor solution for 3 hour. The solution containing of the 0.05 M zinc nitrate hexahydrate (Zn(NO3)2‧6H2O, 99.9% purity) and the 0.05 M methenamine (C6H12N4, 99.9% purity). After the reaction, the substrates were removed from the solution, rinsed with deionized water, and dried in the air. For making the nanorods with tip morphologies, nanorods were bound on a sputtering target by carbon tape and exposed to oxygen plasma for 0, 30, 60 and 120 s, respectively. For the plasma treatment, process pressure and rf-power were maintained at 5x10-2 Torr and 30 W, respectively.
The morphology, size distribution and crystal structure of all the nanorods were investigated by a field-emission scanning electron microscope (FE-SEM, Hitachi S-4700I), a transmission electron microscope (TEM, JEOL 2100F), and a X-ray
diffractor (XRD, Bede D1). The chemical composition was estimated by a energy dispersive X-ray spectrometer (EDS, Oxford ISIS300). The field-emission current-voltage (I-V) curves of all nanorod emitters were measured at a pressure of 2×10-6 Torr kept by a turbo molecular pump. A copper tip was employed to act as an anode with a tip area of 7.09×10-3 cm2 and p-type Si covered with ZnO emitters as an cathode with an area of 1cm2. We used micrometer (accuracy of ±1 μm) to adjust the distance between a copper anode and nanorod emitters. The distances of the as-grown and other nanorod emitters with the anode are 100 and 150 μm, respectively. The field emission properties are affected by the anode area and anode-cathode distance based on Filips model [3.19]. The turn-on and threshold fields are defined at current densities of 1 μA/cm2 and 1 mA/cm2, respectively. The dependence of the field emission current on the anode-cathode voltage was recorded using programmable Keithley 237 picoammeter measurement system. The stability measurements were also carried out for the ZnO nanorod emitters at temperature of 25, 50 and 100 ℃, respectively. The photoluminescence (PL) spectra were obtained using a He-Cd laser (325nm) as excitation source at room temperature.
3-3. Results and discussion
3-3-1 Morphology, crystal structure, composition and optical properties of ZnO nanorod array
Figures 3.1(a)-(d) show the FE-SEM morphologies of the as-grown and oxygen plasma treated (etching times of 30, 60 and 120 s, respectively) ZnO nanorod emitters.
As show in Figure 3.1 (a), the as-grown ZnO nanorods arrays present typical hexagonal shape structure with an average diameter of 230 nm, an average length of 1.4 μm and relatively high cover density. The morphology of ZnO nanorod emitters
reveals dramatic change due to the oxygen plasma treatment. While oxygen plasma treatment for 30 s, we find that the top of the nanorods becomes tower-shaped morphology. By increasing the treatment time to 60 s, the tip structure of the ZnO nanorod emitters is formed. The formation of acute nanotips morphology is due to the oxygen ion bombardment at the edge of the nanorods leading to isotropic etching [3.11]. Figure 3.1 (d) shows the morphology of the nanorods plasma treated for 120 s, indicating a destruction due to over etched and a decrease of aspect (c/a) ratio of the nanorods, which will degrade the performance of the emitters [3.12].
Figure 3.2 (a) indicates that X-ray diffraction patterns of the as-grown and oxygen plasma etched ZnO nanorod array exhibit single phase with hexagonal wurtzite structure, (002) preferred orientation and the lattice constant of ZnO nanorods are a=b=~3.256 Å , c=~5.204 Å (space group P63mc; JCPDS card NO.
36-1451). No characteristic diffraction peaks from impurities were detected. From Figure 3.2 (a), it shows that the intensity of the (002) diffraction peak increases with an increase of the oxygen plasma bombardment time up to 120 s. It indicates that the oxygen ions bombardment not only helps to form tip structure but also oxidizes the oxygen vacancies at the ZnO nanorods. The crystalline characteristics, morphology and composition of the ZnO nanorod with 60 sec oxygen plasma treatment studied using plane view TEM observations. The bright field image, selected area diffraction (SAD) pattern and high resolution images of the nanorod are shown in Figure 3.2 (b).
Figure 3.2 (b) shows that the nanorod has a small tip, which is consistent with the result of SEM observation and the tip angle approximately 100º can be obtained. The SAED pattern (insets in Figure 3.2 (b)) shows the preferred [0001] growth of the ZnO nanorods. The high resolution images shown in insets of figure 3(e) reveal the tip and side of nanorods have lattice planes with interplaner spaces of 2.54 Å and 2.58 Å , respectively, indicating the ZnO nanorods are wurtzite structure with [0001] direction.
It is also observed that the tip area is smooth surface and no crack, indicating that the oxygen plasma treatment is a fast and powerful nanoscale surface modification method. The EDS spectrum shown in Figure 3.2 (c) indicates that the constituent elements of the nanorod are only composed of Zn and O having atomic ratio of Zn/O is 48.7:51.3, which is close to stoichiometric ratio. No evidence of other impurities was found in the EDS spectrum. The Cu and C signals originated from the TEM grid.
Photoluminescence (PL) spectrum is a useful technology for characterizing the optical properties of nanostructures. The room-temperature PL spectra of the as-grown and oxygen plasma etched ZnO nanorod emitters are shown in Figure 3.3.
The strong UV emissions for those nanorods occur at about 378 nm, which comes from the recombination of exciton. The broad emission band located at about 550 nm, which is the green emission of the visible spectrum. These peaks occur from the oxygen vacancies of the nanorods [3.13-3.14]
. It is known that oxygen vacancies are the common defect in n-type ZnO, which are relative to visible emission. From the Figures 3.3, indicate the intensity of visible emission decreases, due to the decrease in oxygen vacancy concentration by oxygen plasma treatment. Inset of Figure 3.3 shows green emission peak intensity decreases, indicating clearly the decreased oxygen vacancy concentration and consequently, the enhanced intensity of UV emission during oxygen plasma treatment occurred. Previously, the reduced oxygen vacancy concentration of ZnO nanorods by annealing at various temperatures in an oxygen atmosphere was reported [3.15]. Thus, the reduced green emission is expected to occur by annealing at oxygen atmosphere. In contrast with our case, we were used a simple oxygen plasma treatment, which employed oxygen ion implantation into ZnO nanorods and make reparation the oxygen vacancy of ZnO nanorods and exhibit much better UV emission than the as-grown nanorods.
3-3-2 Field emission properties
Figures 3.4 (a) show the J-E curves of the as-grown ZnO and oxygen plasma etched ZnO nanorod emitters. The field emission current–voltage characteristics are analyzed by using the Fowler-Nordheim (F-N) equation [3.16-3.17]
: (b)).The field emission properties of the nanorods with various Ga/Zn molar ratios in the solutions and different oxygen plasma treatment times are listed in Tables 3.1.
Regarding the oxygen plasma treatment, the ZnO nanorod with 60 s treatment shows the best field emission properties. The turn-on field, threshold field, and field enhancement factor are found to be 2.42 V/μm, 3.61 V/μm, and 2268 respectively.
Based on Filips model, the β is approximately equal to r sd
1 , where s is dependent on screen effect, d the distance between anode and cathode and r the radius of the emitters. In our experiment, the different etched time emitters are considered with the same nanorods number density of 18.5 /μm2 from FE-SEM images and the same distance between tips and anode plate. Clearly, the nanorods with sharp tips have high β values. In our case, the optimum oxygen plasma etched time for the nanorods is 60 s.
These nanorod emitters show lower turn-on field, uniform morphology distribution and high crystallinity. On the other hand, the ZnO nanorods with 120 s oxygen plasma treatment show decrease in aspect ratio (c/a) and destruction of nanorods from FE-SEM observation, which would degenerate the field emission properties.
Figures 3.5 (a) and (b) depict the stability characteristics of 104 s at 25 ℃ for the
tip structured ZnO nanorod emitters and show the variations of turn-on field, lifetime and operation cycles for field emission device applications.
Figure 3.6 (a) depicts the J-E curves at temperatures of 25 ℃, 50 ℃ and 100 ℃ for tip structured ZnO nanorod emitters. It is indicated that the nanorod emitters can be successively and stably operated between the 25 and 100 ℃. During the cycling test for 1000 s, the turn-on fields are 2.33±0.25 V/μm, 2.41±0.33 V/μm and 2.34±0.16 V/μm; the threshold fields are 3.86±0.27 V/μm, 3.60±0.47 V/μm and 3.77±0.18 V/μm;
the field enhancement factor are 2239.0±115.9, 2289.5±141.1 and 2216.6±188.4 at the 25 ℃, 50 ℃ and 100 ℃, respectively, which can be calculated according to the J-E curves and they are shown in Figures 8(b)-(c). According to crystal defect theory, the probability () of oxygen ions to overcome the potential barrier and create probability of defect formation is increased. However, in the present case, the nanorod emitters still exhibit the stable and reversible field emission properties at temperatures 25-100 ℃. Thus, oxygen plasma etching process has the advantage of obtaining more
stable field emission characteristic up to 100 ℃. This phenomenon may be attributed to decreasing the concentration of oxygen vacancies of the nanorods and obtaining a better crystallinity after the treatment.
3-4. Conclusions
In summary, we successfully enhance the performance of ZnO employing oxygen plasma treatment. The synthesized process is simple, low cost and enable to large scale production. The (002) orientation and tip-structured morphology of the nanorod emitters were proved by XRD pattern, FE-SEM and TEM observations. The PL spectrum reveals that the green emission peak that occurred form oxygen vacancies after oxygen plasma treatment is lowered and obviously this phenomenon is due to reduced the concentration of oxygen vacancies of the nanorod emitters. The tip structured nanorod emitters can be formed after oxygen plasma treatment. Thus, through oxygen plasma treatment, the tip structured ZnO nanorod emitters have enhanced performance including lower turn-on and threshold fields, higher field enhancement factor, good stability characteristics over 104 s at room temperature. In addition, the tip structured ZnO nanorod emitters can successfully and stably operate up to 100 ℃ without notable degradation of emission properties. Therefore, these tip structured ZnO nanorod emitters could be used in electron field emission and light emitting applications in the future.
Figure 3.1 Typical FE-SEM images of ZnO nanorod with various oxygen
plasma treatment times (a) 0 s (as-grown), (b) 30 s, (c) 60 s and (g) 120 s.
respectively.
Figure 3.2 (a) XRD analysis of ZnO nanorod emitters with the various oxygen
plasma eching times. (b) TEM bright field image, corresponding SAED pattern, HR-TEM image and (c) EDS analysis of as-grown ZnO nanorod after 60 s oxygen plasma etching.
Figure 3.3 Room temperature PL spectra of ZnO nanorod emitters with the
various oxygen plasma etching times (inset show magnify of green emission areas).
Figure 3.4 (a) J-E curves and (b) F-N plots of ZnO nanorod emitters with
various oxygen plasma treatment times.
Figure 3.5 Stability at 25 ℃ of ZnO nanorodwith oxygen plasma etching for
60 s: (a) Turn-on and threshold fields, (b) Field emission enhanced factor, (c) 1st, 100th, 200th and 400th cycle respective J-E curves and (d) FE-SEM images of tip structured ZnO nanorod emitters for stability tests.
Figure 3.6 Stability at various temperatures of tip structure ZnO nanorod
emitters: (a) J-E curves, (b) Turn-on and threshold fields and (c) Field emission enhancement factor.