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Optimizing the Field Emission Properties of ZnO Nanowire Arrays by Precisely Tuning the Population Density and Application in Large-Area Gated Field Emitter Arrays Yufeng Li, Zhipeng Zhang, Guofu Zhang, Long Zhao, Shaozhi Deng, Ningsheng Xu, and Jun Chen ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b13994 • Publication Date (Web): 09 Jan 2017 Downloaded from http://pubs.acs.org on January 11, 2017

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ACS Applied Materials & Interfaces

Optimizing the Field Emission Properties of ZnO Nanowire Arrays by Precisely Tuning the Population Density and Application in Large-Area Gated Field Emitter Arrays Yufeng Li, Zhipeng Zhang, Guofu Zhang, Long Zhao, Shaozhi Deng, Ningsheng Xu, Jun Chen* State Key Laboratory of Optoelectronic Materials and Technologies, Guangdong Province Key Laboratory of Display Material and Technology, School of Electronics and Information Technology, Sun Yat-sen University, Guangzhou 510275, People’s Republic of China

KEYWORDS: zinc oxide nanowires, thermal oxidation, screening effect, electrical conductivity, field emitter arrays

ABSTRACT: Zinc oxide (ZnO) nanowires are prepared for application in large area gated field emitter arrays (FEAs). By oxidizing Al-coated Zn films, the population density of the ZnO nanowires was tuned precisely by varying the thickness of the Al film. The nanowire density decreased linearly as the thickness of the Al film increased. Optimal field emission properties with a turn-on field of 6.21 V µm−1 and current fluctuations less than 1% are obtained. This can be explained by the minimized screening effect and good electrical conductivity of the back-

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contact layer. The mechanism responsible for the linear variation in the nanowire density is investigated in detail. Addressable FEAs using the optimal ZnO nanowire cathodes were fabricated and applied in a display device. Good gate-controlled characteristics and the display of video images are realized. The results indicate that ZnO nanowires could be applied in large area FEAs.

1. INTRODUCTION Large area gate-controlled field emitter arrays can emit electrons from specific area or pixel. These addressable field emitter arrays have important applications in flat panel X-ray source,1 flat panel X-ray detector,2 etc. For example, addressable flat panel X-ray source can not only reduce the volume of current X-ray inspection machine, but also dramatically reduces the dose of X-ray inspection and thus decrease the cancer risk. Therefore, development of large area addressable field emitter arrays is of great significance. Zinc oxide (ZnO) nanowires have been intensively studied as potential application as field emitters.3,4 ZnO nanowires can be synthesized with various methods including thermal evaporation/condensation,5 chemical vapor deposition,6 hydrothermal solution,7 and thermal oxidation.4 In the thermal oxidation method, ZnO nanowires grow directly from the Zn layer, without a catalyst and under mild temperatures. For application in gated FEAs, the ZnO nanowires

have

to

integrate

with

micro-structures.

Therefore,

compatibility

with

microfabrication processes is essential. In this aspect, a thermal oxidation method is advantageous because ZnO can easily be patterned using standard photolithography methods.


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Furthermore, the processes are compatible with glass substrates and are scalable to large areas, which have great application potential in large area FEAs. ZnO nanowires as field emitters should have a low turn-on field, high field emission current density, and good stability at high currents.1,3,4,8,9 Much effort has been devoted to improving the field emission properties of ZnO nanowire field emitters. Doping and coating have been adopted by some researchers to improve the field emission properties of ZnO nanowires. Z. S. Zhang et al.8 reported that Al-doped ZnO nanowires have enhanced field emission properties because of their increased carrier concentration, which resulted in an enhanced electrical conductivity. L. Liao et al.9 improved the field emission properties by coating ZnO nanowires with amorphous carbon nitride, which has a low work function. However, those doping or coating approaches are difficult to be compatible with the fabrication processes used for gated FEAs. Firstly, for the mentioned coating approach, because the coating was carried out after the ZnO nanowire growth. The coating will cover the whole structure and may affect the gated structure. For example, the insulation between the gate and cathode may be affected. On the other hand, in order to avoid these effects, additional photolithography step is needed for localized coating, which will complicate the fabrication approach. Secondly, the reported doping approach needs high temperature. The high temperature may limit the substrate we can use. In our study, we used sodalime glass for the purpose of large area field emitters application, which has a soften point of around 550 ℃. Furthermore, high temperature will affect the properties of gated structure. For example, the metal electrode is liable to be oxidized under high temperature. Also, the insulation layer between the gate and cathode will degrade during high temperature processes.


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The density of nanowires is the key parameter that affects the field emission properties of nanowire arrays because of screening effects. An optimal population density of ZnO nanowires could significantly diminish the screen effect and retain a sufficient amount of effectively emissive nanowires. Some researchers have controlled the density of ZnO nanowires/nanorods by patterned-selective growth,10 controlling the synthesis temperature,11 or the density of the catalyst.12 J. Liu et al.13 tuned the density of ZnO nanowires over a wide range by changing the thickness of the seed layer in the solution-phase growth process. Early attempts have been made to control the density of nanowires prepared by thermal oxidation. R. Mema et al.14 reported that in-plane tensile stresses could increase the CuO nanowire growth density during the oxidation of copper. For ZnO nanowire arrays prepared by thermal oxidation, tuning the density of the nanowires still remains an issue. The thermal oxidation method does not use a catalyst, so it is not possible to tune the density by adjusting the catalyst density. Varying the temperature is one approach. However, the temperature will affect the morphology of the nanowires and make precisely controlling the field emission (FE) properties difficult. Here, a facile method that is compatible with the fabrication processes for gated FEAs is proposed to tune the population density of ZnO nanowires during thermal oxidation. The density of ZnO nanowires was optimized by using Al-coated Zn films as the starting material and enhanced field emission properties were achieved. Moreover, the mechanism responsible for the variation of the nanowire density was studied, giving evidence of the growth mechanism of ZnO nanowire prepared by thermal oxidation.4 2. EXPERIMENTAL METHODS


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Patterned ZnO nanowire arrays were prepared by thermally oxidizing Al-coated Zn films. Both the Zn and Al films were deposited by electron beam evaporation. First, 3.5-inch soda-lime glass substrates were ultrasonically cleaned in acetone, ethanol, and de-ionized water in sequence for 30 min each. Subsequently, a 500-nm-thick ITO layer was deposited by magnetron sputtering (direct current method, 4 A) as the electrode. After ultrasonically cleaning, following the process mentioned above, photoresist was spin coated on the substrates and they were exposed to ultraviolet light in a mask aligner machine. Rectangular pixel masks, 25×60 µm in size were used to grow the patterned ZnO nanowire arrays. Then, a Zn film (evaporative material, Zn granule, 20 mesh, purity 99.8%, from Alfa Aesar Corp.) with a thickness of 1 µm was deposited by electron beam evaporation. The deposition rate was 6 Å s−1 and the background pressure was below 1×10−3 Pa. After exposing the samples with Zn films to an atmosphere, an Al film (purity 99.5 %) was deposited by electron beam evaporation. The deposition rate was 3 Å s−1 and the background pressure was above 3×10−3 Pa. The thicknesses of the Al films varied from 30 to 210 nm to study its effect on optimizing the density of the ZnO nanowires. After deposition of both films, patterns on the Al-coated Zn thin films were formed by a lift-off method, in which the whole substrate was immersed in acetone to remove the photoresist. Then, the samples were dried by blowing them with N2. Finally, the patterned Al-coated Zn thin films were oxidized in a horizontal quartz tube furnace in air. The temperature was raised from 20 to 500 °C at the rate of 2.5 °C min−1 and held at 500 °C for 2 h. After thermal oxidation, the samples were cooled to room temperature naturally. In addition, a series of experiments were designed with different holding times at 500 °C to study the mechanisms responsible for the linear variation in the nanowire density.


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The surface and cross-sectional morphologies and crystal structures of the prepared nanowires were characterized by SEM (Zeiss SUPRA-55) and transmission electron microscopy (FEI Titan G2 60-300). The effects of UV emission and structural defects from the back-contact layer, oxidized from the Al-coated Zn films were investigated by room-temperature photoluminescence spectroscopy (PL, EDINBURGH FLSP920). The electrical conductivities of the back-contact layers were measured with a semiconductor analyzer (Keithley 4200SCS). Field emission measurements were investigated in a vacuum chamber with a base pressure of 2.5×10−5 Pa. A diode set-up was used for the field emission measurements and the distance between the anode and cathode was 500 µm. 3. RESULTS AND DISCUSSION 3.1. Morphology of the nanowires prepared from Al-coated Zn films. To tune the density of ZnO nanowires, Al films with various thickness of 30, 60, 90, 105, 120, 150, 180, and 210 nm were deposited on Zn films by electron beam evaporation (see figure S1 in supporting information). The Al film is not continuous and only nano-particles were deposited on the surface of the Zn micro-grains when the thickness of the Al film was 160 nm), the Al layer tended to cover all of the micro-grains and a continuous thick Al film formed. Both the gaps and edges of the grains were buried under the relatively thick Al film. Figure 1a shows the morphology of a ZnO nanostructure grown from a pure Zn film. Both nanowires and nanosheets existed in the product. However, most of them formed the nanosheet.


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The height of the ZnO nanosheet was ~2 µm. The average tip diameter was about 31 nm with the standard deviation of 5.3 nm and the diameter of the base was about ~250 nm (see inset of figure 1(a)). Those nanostructures grew randomly on the rough ZnO film. The density of ZnO nanostructures was estimated to be 5.5×108 cm−2. Some of the nanosheets separated into two parts, the upper part indicated that the sheets may have been formed by compactly aligned nanowires. Figure 1(b) ~ (j) shows the morphologies of ZnO nanowires prepared from Al-coated Zn films. The nanowires prepared from Al-coated Zn films had remarkable features. They were straight needles with sharp tips and narrow bases. Regardless of the Al film thickness, the ZnO nanowires all had lengths of ~2 µm. The nanowires prepared from Al-coated Zn films had tip diameters of ~21 nm with the standard deviation of 3.1 nm and base diameters of ~75 nm (see insets in figure 1(b) ~ (j)). For the ZnO nanowires grown from Zn films coated with different Al film thicknesses, the greatest difference was that the density of nanowires decreased from 1.2×109 cm−2 to 1.2×108 cm−2 as the Al thickness increased. The variation is less than 10% in one sample indicating that ZnO nanowires uniformly grow on the substrate. The variation in the population density with the Al thickness is shown in figure 2. The population density of ZnO nanowires decreased linearly as the thickness of the Al film increased from 30 to 210 nm. 3.2. Crystal structures and elemental compositions of ZnO nanowires prepared from Alcoated Zn films.

The crystal structures of the prepared nanowires were investigated by

transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), and selective-area electron diffraction (SAED). Elemental analysis was also carried


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out using electron dispersive spectroscopy (EDS) integrated into the TEM instrument. Figure 3(a) shows a low magnification TEM image of an intact nanowire with a length of 1.3 µm, a tip diameter of 21 nm, and a base diameter of 53 nm. The sample was prepared from a Zn film coated with a 90-nm-thick Al film. Figure 3(b) shows a HRTEM lattice image of the near-top section of another nanowire. This indicates that the nanowires were structurally uniform without any obvious stacking faults. The spacing of 0.16 nm between adjacent lattice fringes matches the (11-20) orientation, which indicates that the growth direction of the nanowires could be indexed to be 11-20 (i.e., the α axes), one of the fast growth directions.15 The indexed zone axes (11-20), (10-10), and (10-11) are labeled on the corresponding SAED pattern. There were two sets of diffraction spots, indicating that the nanowires had a bi-crystalline structure. The adjacent grain boundaries could be narrowed and even crowned together in the volume as it expanded after melting, offering a driving force for the development of twin boundaries4. The EDS analysis is shown in the left inset of figure 3(a). Besides Zn and O in the nanowires, C and Cu were also present, which came from the supporting membrane and mesh in the TEM experiments. No Al was detected in the nanowires. Because of the low temperatures used during the growth process, Al from the starting material could not be doped into the ZnO nanowires. The above results demonstrate that the prepared ZnO nanowires had a wurtzite structure (JCPDS# 75-1533). 3.3. Growth behavior of ZnO nanowires grown from Al-coated Zn films. To study the growth behavior of ZnO nanowires prepared by oxidizing Al-coated Zn films, a sample of a Zn film coated with 90 nm Al was divided into several parts. Each part was thermally oxidized at 500 °C for different holding times, varying from 1, 10, 30, and 120 min. Their morphologies, observed by SEM, are shown in figure 4(b1) ~ (e1) and their cross-sectional SEM images are shown in figure 4(b2) ~ (e2). The Al thin film coating consisted of nano-particles distributed on


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the surfaces of the Zn grains, as shown in figure 4(a1) and its inset. The Al coating on each grain was not continuous. Figure 4(a2) and its inset show that there were fewer Al nano-particles at the edges of most of the micro-grains. The gaps between adjacent micro-grains were not filled with the Al nano-particles. These features of the Al-coated Zn films are the prerequisite to grow ZnO nanowires only at the local grain boundaries. Figure 4(b1) and (b2) shows that when the temperature increased to 500 °C, nanowire embryo grew at the grain boundaries, because of the edge-enhanced oxidation effects during the early stages of thermal oxidation. The ZnO nanowire growth and crystal orientation were based on the seed particles. Figure 4(c1) and (c2) corresponds to holding times in the first 10 min. Over this period, the early formed embryo grew into nanowires and more new embryo formed and grew. Additionally, submicron-sized mounds (labeled in the inset of figure 4(c1)) appeared around the edges of the grains. As the thermal oxidation time was prolonged, the lengths and diameters of the nanowires increased, as shown in figure 4(d1) and (d2). Meanwhile, the submicron-sized mounds became larger with time and piled up together around the grain boundaries for long thermal oxidation times. As the submicron-sized mounds expanded by a certain extent, some nanowires were buried either in the mounds or in the gaps between adjacent mounds. As the mounds continued to grow and occupy the edges of the grains, there were no spaces around the edges for new embryo/nanowires to grow. Thus, the density of nanowires could be maintained at a certain level. When the holding time reached 120 min or more, the length of the nanowires continued to increase, but the growth speed become slower, as shown in figure 4(e1) and (e2). Furthermore, the diameter and density of the nanowires do not increase any more. After growth, most of the nanowires had a length of 1–3 µm, a tip diameter of 10–50 nm, and a base diameter of 50–100 nm.


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3.4. Mechanisms of the growth and density control of ZnO nanowires. Based on the above experimental results, the mechanisms responsible for the growth and density control of ZnO nanowires were proposed. A schematic of the growth process is shown in figure 5. The initial Zn film consisted of hexagonal zinc grains (figure 5(a)). After coating the Al films by electron-beam evaporation, the Al thin films contained nano particles distributed on the surfaces of the Zn grains (figure 5(b)). The Al coating and its thickness were crucial factors to precisely tune the population density of the ZnO nanowires. When the temperature increased, Al was first oxidized to form amorphous Al2O3 because Al has a higher chemical reactivity than Zn and the Al particles had a large surface area to react with O2. After the temperature rose to around 200 °C, the Zn at the grain boundaries was oxidized because fewer Al2O3 particles covered the boundaries. As the Zn was oxidized, stress accumulated at the grain boundaries, which resulted from the lattice mismatch between Zn and ZnO.16 Grain boundaries were the original sites for nanowire growth as grain boundaries possess high stress because of the high oxidation speed at the zinc grain edges. As the temperature reached ~500 °C, nanowire embryo grew at the grain boundaries, caused by the edge-enhance oxidation effect (see figure 5(c)). For the samples with an Al coating, it was difficult for the Zn ions to diffuse outward to react with O ions because of the Al2O3 particle films that formed. The diffusion of metal cations can be described by the Vagner-oxidation theory:17

jm2 + = − jvm = Dvm

Cv'' m − Cv' m x

,

(1)


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where

jm2 +

is the outward diffusion flux of a metal cation,

jv m

is the inward diffusion flux of a

D cation vacancy, x is the thickness of the oxidized film, vm is the diffusion coefficient of a

cation vacancy,

Cv' m

is the vacancy concentration at a metal-oxide interface, and

vacancy concentration at an oxide-air interface.

Dvm (Cv'' m − Cv' m )

Cv'' m

is the

is constant when a

thermodynamic equilibrium is reached at the interface. Therefore, it is clear that the outward diffusion flux of the Zn cations became lower when the thickness of the Al2O3 particle-like film increased. The formation rate of nanowire embryo also became slower for thicker Al films. The density of nanowire embryo decreased as the thickness of the Al film increased from 60 nm to 120 nm at the beginning of the thermal oxidation process (holding time = 1 min) was demonstrated (see figure S2 in supporting information). Therefore, the thickness of the Al film had a crucial effect on the density of nanowire embryo. After the temperature rose above the melting point of Zn (around 500 °C), Zn micro-grains melted to a liquid state, which was active for diffusion and oxidation. As the heating procedure continued, more stress accumulated at the edges and boundaries of the Zn grains. The large amount of stress needed to be released, which was the driving force for the outward diffusion of Zn cations. Zn cations transported through the twin boundaries to the tips of the ZnO nanowires and then reacted with O anions. Figure 5(d) corresponds to this period, during which the fastest growth rate of nanowires was observed. Not only did the length and diameter of the nanowires increase, but submicron-sized mounds also formed around edges of the grains. The submicronsized mounds were formed by Zn cations diffusing outward and reacting with oxygen. The growth of submicron-sized mounds not only released the compressive stress, but also limited the growth of more seed particles.


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When the holding time for thermal oxidation was 120 min or more, the length of the nanowires continued to increase, but with a slower speed (see figure 5(e)). There were no nanowires growing on the surfaces of the grains because of the compact Al2O3 particle-like films and the lower localized stress (shown in the magnified inset of figure 5(e)). The above growth mechanism explains the experimental results, where the population density of ZnO nanowires decreased linearly as the Al film thickness increased. The density of nanowires could be controlled accurately by controlling the thickness of the Al coating film, which controls the outward diffusion of Zn cations. 3.5. Influence of the Al film on the growth. To further understand how the Al coating film influenced the growth of nanowires, a sample with a Zn film coated with 120-nm-thick Al film was prepared on a glass substrate. Then, the film was peeled off the substrate and underwent thermal oxidation. Both sides of the film were oxidized directly (see figure S3 in supporting information). It is found that randomly dense submicron-sized mounds were formed on the pure Zn film layer, while the hexagonal surface features of the grains were maintained after thermal oxidation and submicron-sized mounds sequentially formed at the edges of grains from Alcoated Zn film layer. Therefore, the Al coating was found to be the main reason for the morphology difference between the two sides of the layers. Without the Al coating, the oxidization process of the Zn film was fast because the Zn film directly faced the oxygen atmosphere. Zn micro-grains melted into a liquid state when the temperature rose above the melting point of Zn during thermal oxidation. Both resulted in a mass of submicron-sized mounds forming quickly and randomly. Moreover, the formed oxidized layer was compact. But for the Al coated Zn film layer, the compact Al2O3 particle-like film with a high melting point coated the surfaces of the grains, which was similar to a shapely mound to protect the hexagonal


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grain features. Submicron-sized mounds only appeared around the edges of the micro-grains sequentially. Some nanowires were buried in the submicron-sized mounds or in the gaps between the adjacent submicron-sized mounds, which enhanced the mechanical strength of the nanowires. As the Al thickness increased, the size of the mounds and their number decreased simultaneously, affecting the outward diffusion of Zn cations. In addition, EDS spectroscopy was used to analyze the semi-quantitative elements in the oxidized films (see figure S4 in supporting information). The cross-sectional morphologies of oxidized pure Zn film and Al-coated Zn film layer were measured (see figure S3 in supporting information). As for the pure Zn film, the interlayer between the nanowires and substrate became entirely hollow after thermal oxidation. From the above discussion, a mass of submicron-sized mounds formed quickly and randomly on this sample. Therefore, the process dramatically consumed the Zn source, which is the reason for the formation of the hollow region. On the contrary, for the sample prepared from a Zn film coated with a 120-nm-thick Al film, there were no hollow regions in the interlayer after thermal oxidation for 180 min. From the above discussion, not only did the nanowires grow longer, but submicron-sized mounds also piled up around the grain boundaries during the later stages of thermal oxidation. Most importantly, no submicron-sized mounds existed on the surfaces of the grains. Fewer submicron-sized mounds required a small amount of the Zn source from the starting film. As a result, there were no hollow regions formed in the interlayer and the shape of the ZnO micro-grains remained the same. Surplus Zn source was available for overlong nanowire growth by long thermal oxidation times. These differences in the morphologies of the interlayers may have led to the difference in their conductivity and thus they affected the FE properties. This will be discussed later.


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For the samples prepared from Zn films coated with Al, tiny ZnO nanowire embryo only grew at the grain boundaries at the beginning of the thermal oxidation process. Thinner Al2O3 particlelike films covered the edges of grains, which meant that there were finite sites where the Zn cations could diffuse outward. Thus, there was a narrow path to release the large stress, which led to a large rate and high intension during the oxidized process. Therefore, tiny ZnO nanoembryo could grow to needle-like nanowires with sharp tips, narrow bases, and straight trunk features because of the rapid and intense oxidation process. Conversely, for the samples prepared on Zn films without an Al film, the nanosheets had a bamboo-leaf-like morphology with flexible tips and wide bases, attributed to the wide path to release the stress. By thermally oxidizing the pure Zn films without an Al coating, it was difficult to ensure the growth of nanowires/nanosheets in every experiment. Occasionally, nanowalls grew, depending on the repeatability of the ambient conditions in furnace. Nanoscale material growth is influenced by the ambient humidity, barometric pressure, and sample position.18 For the Alcoated Zn films, the particle state only gave a dot of space around the grain boundaries to release the stress accumulated during thermal oxidation. The effect from the limited growth space was deeper than the effect from the ambient conditions. Therefore, nano-embryo were located in the dot of space where the nanowires grew. It was difficult to grow nanowalls or nanobarriers, as they need either line space or face space. 3.6. UV emission, structural defects, and electrical conductivity of the back-contact layer oxidized from Al-coated Zn films. Although the Al component in the starting Al-coated Zn films did not contribute to the growth of ZnO nanowires, it is believed to have become part of the oxidized Al-coated Zn films (named the back-contact layer) in the region where ZnO nanowires grew after thermal oxidation. During field emission, the electron is transported from


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ZnO layer to ZnO nanowires directly and then tunnelling to the vacuum. We think the thin layer of aluminium oxide will not directly affect the field emission. The field emission behaviours are mainly influenced by morphologies，density and conductivities of ZnO nanowires and ZnO layer, while the dielectric inhomogeneity of aluminium oxide can be neglected for field emission behaviours of ZnO nanowires. However, the aluminium oxide layer together with Al layer would hinder the transportation of oxygen to the Zn layer during the thermal process, causing the formation of defects such as oxygen vacancies in ZnO layer. And thus, then conductivity of ZnO layer can be changed. To characterize the back-contact layer, the room-temperature photoluminescence (PL) spectra (figure 6(a)) and electrical conductivity, i.e., I–V curves (figure 6(b)), from the samples were investigated in detail. The samples prepared from Al-coated Zn films varied with Al thicknesses of 0 nm, 30 nm, 60 nm, 90 nm, 120 nm, 150 nm, and 180 nm. The information from the measurements mostly came from the back-contact layer. Little came from the ZnO nanowires because the influence of the ZnO nanowires was limited to the nanoscale. The characteristics of the PL spectra could be distinguished into two parts: an intrinsic ultraviolet (UV) emission peak for ZnO around 380 nm and a broad green emission band around 525 nm. It has been reported that the UV emission band arises from the near band-edge transitions of ZnO with wide band gaps19 and the broad green emission band in the visible region is from structural defects (such as singly ionized oxygen vacancies).20 Table 1 shows the peaks for samples with different Al film thicknesses, which resulted in a small shift. In particular, the green emission rose as the Al thickness increased from 30 to 120 nm and fell as the Al thickness decreased from 120 to 180 nm. This indicates that the concentration of oxygen vacancies or other


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structural defects were generated as the Al thickness increased in the thin scale, and lowered as the Al thickness increased in the relatively thick scale. It is known that Al acts as Al3+ and competes with the Zn ions to consume the residual O ions in the ZnO matrix, which results in a decrease in the oxygen concentration in ZnO.21, 22 Thus, it is believed that Al consumes the residual O more successfully for Al thicknesses greater than 120 nm. Figure 6(b) shows that the electrical conductivity of the back-contact layer varied for different Al thicknesses with an applied bias around ±2 V. Another electrode made of indium tin oxide (ITO) was used, with a film resistance around 80 Ω. Compared with the samples prepared from Al-coated Zn films, those prepared from Zn films without Al had high resistances, resulting from the hollow region in the back-contact layer after thermal oxidation. For samples with different Al thicknesses, the conductivities of the back-contact layer varied with the concentration of oxygen vacancies (PL spectra) as the Al thickness increased. The samples with Al thicknesses between 90 and 120 nm possessed the best electrical conductivities because the higher concentration of oxygen vacancies likely created many electron carriers.23, 24 3.7. Enhanced field emission properties for ZnO nanowires prepared from Al-coated Zn films. Here, the field emission properties of ZnO nanowires prepared from Al-coated Zn films are investigated in detail. Here, we use the simplified F-N formula to estimate the relative field enhancement factor differences from ZnO nanowires when the Zn films are coated with difference thickness of Al film. The Fowler-Nordheim equation can be described as:25

J=A

β 2E2 φ 3/ 2 exp( − B ) φ βE ,

(2)


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where J is the emission current density (µA cm−2), E is the electrical field (V µm−1), φ is the work function of the emitter, and A and B are the F-N constants with values of 1.54×10−6 (A eV V−2) and 6.53×109 (V m−1 eV−3/2 ), respectively. The work function of ZnO was assumed to be 5.28 eV.26 Therefore, the field enhancement factor (β) could be calculated from the slope of the ln(J/E2) versus 1/E plot according to the F-N plot. The effective field emission area was 3.672 cm2 in the 3.5 inch samples and the turn-on field was defined at a current density of 10 µA cm−2. It should be stated that the simplified F-N formula is only valid for a basic approximation in relative field enhancement factor, and actual emission parameters need to be extracted from accurate field emission theory. 27,28 Figure 7 shows the field emission J-E plots with the corresponding F-N plots (see inset) for samples of ZnO nanowires prepared from Al-coated Zn films. Figure 7(b) ~ (e) summarizes the density, turn-on field, field emission current at 4150 V, and the field enhancement factor versus the Al thickness. For the samples grown from Zn films without an Al coating, the turn-on field at a current density of 10 µA cm−2 was about 7.8 V µm−1 and the field emission current was 80 µA at 4150 V. First, compared with the ZnO nanowires prepared from pure Zn films, the ZnO nanowires prepared from Al-coated Zn films had lower turn-on fields and higher field emission currents. The enhanced field emission may have been caused by the good conductivity of backcontact layer and the optimized morphology of the samples grown from Al-coated Zn films. This phenomenon is attributed to the remarkable features, the sharp tip, narrow base, needle-like, perfectly straight ZnO nanowires prepared by the Al-coated Zn films. Conversely, the ZnO nanowires grown from Zn films without an Al coating were flexible and wide, leading to a relatively low aspect ratio. The work function of the nanowires was defined to be the same value when calculating b. The b of the samples prepared from Zn films without an Al coating were


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larger than most of the samples prepared from Al-coated Zn films, which is in contrast with the J-E and F-N plots. This was caused by the difference in the work function between the former and latter samples. Secondly, for the ZnO nanowires prepared from Al-coated Zn films, the turn-on field initially decreased (the emission current initially increased) as the Al coating thickness increased from 30 to 120 nm and then increased (the emission current decreased) when the Al thickness increased from 135 to 180 nm. An optimal turn-on field and emission current were obtained from the samples grown from 120-nm-thick Al-coated Zn films with a turn-on field of 6.21 V µm−1 and a current of 2.15 mA at 4150 V. The β changed with the emission current. We believe that a screening effect was the main reason for the observed results. For the samples with a high density, the screening effect may have been lower than the actual field at the tips of the nanowires. While, for the samples with a low density, the number of emitters was low. Therefore, an optimal density was expected. In our study, the value was ~6×108 cm−2 for the samples prepared from Zn films coated with a 120-nm-thick Al layer. The screening effects in the samples prepared under these conditions were effectively diminished. We have compared the field emission properties of our ZnO nanowires with some results reported in the literature (Table 2). It is worth noting that the turn-on field of our ZnO nanowires is higher than those reported in the literature.3,8,11,29,30 We attribute the relatively high turn-on field to following reasons. One reason is that the field enhancement factor of our nanowire is low, which mainly due to the relatively small height of the nanowire. The second reason is that the measured area of our ZnO nanowires was much larger than those reported in the literature. Usually it is difficult to achieve a uniform emission over large area. Therefore, the local current


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density may be underestimated when we calculate the current density from I-V data obtained from a large area sample. The field emission current stabilities were measured for each sample (see figure S5 in supporting information). The highest stability was observed for samples prepared from Zn films coated with an Al thickness of 120 nm, the current fluctuation was less than 1% for the period of 180 min. The good current stability was mainly determined by a good electrical conductivity of the back-contact layers, which accounted for the good thermal conductance. Moreover, with the FE characteristic measurements, the field emission images that were recorded for the ZnO nanowires prepared from Zn films coated with different Al thickness are shown in figure 7(g) – (m). Figure f(a) displays a typical SEM image of a ZnO nanowire pattern. All of the field emission images have a scale of 3.5 inches with an area of 8 cm × 4.5 cm. Uniform emission was observed for all samples, except for the one prepared from a Zn film coated with a 180-nm-thick Al film. This was attributed to the uniform geometry of the ZnO nanowires. The most uniform emission with a large-density field emission site was observed for the samples prepared from Zn films coated with 120- and 135-nm-thick Al films (the population densities were ~6×108 cm−2 and ~5.6×108 cm−2). In summary, for the ZnO nanowires prepared from Zn films coated with a 120-nm-thick Al film, the optimal field emission properties occurred at a turn-on field of 6.21 V µm−1 and a FE current of 2.15 mA at 4150 V (i.e., 585.5 µA cm−2 at 8.3 V µm−1). For these samples, good stability at a current of 1000 µA and uniform emission images were achieved. The performance of this type of emitter was also verified in gate-structured EFAs.


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3.8. Application in large area gated FEAs using an optimal ZnO nanowire cathode. ZnO nanowires with an optimal density were applied in gate-structured FEAs.31 One pixel (figure 8(a)) of the gated FEAs consists of 30 single dot matrixes. Each part of the gated FEAs substrate is annotated in a schematic (figure 8(b)) and a cross-sectional view from a SEM image (figure 8(c)). Good compatibility was found for the ZnO nanowires grown in gate-structured FEAs. Cathode substrates and phosphor screens were sealed together with a low-melting glass frit at a distance of 500 µm. The samples were pumped to 1×10−5 Pa and baked at 250 °C for 10 hrs. The sealed samples (figure 8(d)) were tested as individual pixels by applying a scanning voltage. The measurements were carried out with an anode voltage of 5850 V, a gate voltage of 110 V, and a cathode voltage of 90 V. A full screen, cartoon of a running dog and Chinese characters could be well displayed, verifying the addressing capability of the gated FEAs (figure 8(e) ~ (j)). The encapsulated device works fine after 1 year demonstration operation, indicating its long lifetime. The characteristics of gated FEAs were measured in the fully-sealed device. All of the gate electrodes were linked together and a voltage was applied by a Keithley 6487. A high voltage was applied to the anode electrode and the cathode electrode was grounded. The currents in the three circuits were measured separately with an ammeter. Figure 9 shows the dependencies of the anode (Ianode) and gate currents (Igate) on the anode (Vanode) and gate voltages (Vgate). The difference in the value between Ianode and Igate was similar to the cathode current (Icathode). For a fixed Vanode, Ianode increased with increasing Vgate (figure 9 (a)). This is because the electric field between the cathode and gate reduced the turn-on electric field from the emitter, which led to more emission. In figure 9(b), Igate is the leakage current across the insulating layer between the gate and cathode. For a low Vanode, the electric field between the cathode and anode (ECA) was


20

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lower than the electric field established between the cathode and gate (ECG). An Igate with a positive value transported from the gate to the cathode and increased with increasing Vgate. However, the ECA was higher than the ECG for high Vanode values. An Igate with a negative value transported from the cathode to the gate. Moreover, Igate decreased with an increasing Vgate because the increasing ECG reduced the ECA. Overall, the leakage current was much lower than Ianode, which is beneficial to device performance. In the gated FEAs structure, the distance between the gate and cathode is less than 15 µm. However, the distance between the anode and cathode reaches 500 µm. The gate has to be much closer in proximity to the emitter than the anode, indicating that the ECG generated by Vgate has a greater effect on the emission current than the ECA of the Vanode at the emitter (ZnO nanowire cathode). The gain factor (α) describes how much of an effect Vgate has on the anode emission current over Vanode, which can be calculated using:32, 33

α =−

dVanode dVgate

I anode = const .

.

(3)

In figure 9(a), at a constant Ianode of 200 µA, there are two data groups, (Vanode=3834 V, Vgate=65 V) and (Vanode=3907 V, Vgate=55 V). Thus, the estimated α was 73/10=7.3. In addition, the transconductance (gm) can describe how much Vgate is required to reach the amount of Ianode, determined using:32, 33

gm =

dI anode dVgate

Vanode = const .

.

(4)


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In figure 9(a), at a constant Vanode of 3850 V, there are two data groups, (Ianode=212.3 mA, Vgate=65 V), and (Ianode=156.6 mA, Vgate=55 V). Thus, the estimated gm was 55.7/10=5.57 mS. α and gm indicated that Vgate had a greater effect on the emission current than Vanode, and Ianode was strongly dependent on Vgate. 4. CONCLUSIONS By coating an Al film on the starting Zn film, enhanced field emission was observed from the ZnO nanowires prepared by thermal oxidation. The population density of the ZnO nanowires could be tuned by varying thickness of the Al film. ZnO nanowires with optimal emission performance were obtained. The enhancement in the field emission was attributed to the optimized morphology, conductivity, and population density. This emitter with the optimal field emission properties was successfully applied in gate-structured FEAs. Good gate-controlled characteristics and the display of video images were realized. The results indicated that ZnO nanowires could be used in large area FEAs.

FIGURE LEGENDS Figure 1. Top view SEM image of (a) ZnO nanowires without Al film coating, and nanowires prepared from Zn film coated with Al of different thickness: (b) Al-30 nm, (c) Al-60 nm, (d) Al90 nm, (e) Al-105 nm, (f) Al-120 nm, (g) Al-135 nm, (h) Al-150 nm, (i) Al-180 nm, (j) Al-210 nm. The inset of each sub-image is corresponding high resolution image. Figure 2. For nanowires prepared from Al coated Zn film, variation of population density with Al thickness from 30 nm to 210 nm and its linear fitting.


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Figure 3. (a) The bright field TEM image of ZnO nanowire prepared by Zn film coated with 90nm Al film. Inset (down right) shows the magnification of middle section. Inset (up left) is EDS spectrum of ZnO nanowire at circle region (The table gives the element distribution). (b) HRTEM lattice image of near-top section from another nanowire. Inset show the corresponding Selective area electron diffraction (SAED) pattern (down right). Figure 4. Top view SEM image of the sample prepared by Zn film coated with Al of 90 nm (a1) as-deposited film and thermal oxidation at 500 ℃ for different time: (b1)1 min, (d1)10 min, (c1)30 min, (e1)120min. Their corresponding cross-sectional view SEM image (a2) as-deposited film and thermal oxidation at 500 ℃ for different time: (b2)1 min, (c2)10 min, (d2)30 min, (e2)120min.The inset of each sub-image is corresponding high resolution image. Figure 5. Schematic of growth process for ZnO nanowires grown from Al coated Zn film. (a) Structure of as-deposited Zn film, (b) structure of Al coated Zn film, morphology from the (c) early, (d) medium, (e) later stage of thermal oxidation. Figure 6. (a) The room-temperature PL spectra of samples. (b) The electrical conductivity (I-V) curve of samples (inset shows the magnification of wireframe region). The measured target is back-contact layer oxidized from Zn film coated with Al of different thickness, on where grew ZnO nanowires. Figure 7. (a) Field emission J-E plot with the corresponding F-N plot (inset) obtained from 3.5 inch samples of ZnO nanowires prepared by Zn film coated with Al of different thickness; (b)~(e) the population density, turn-on field, field emission current at 4150 V, and field enhancement factor versus Al thickness. (f) Typical SEM image of ZnO nanowires pattern; (g)~(m) field emission image recorded from ZnO nanowires prepared by Zn film coated with Al of different thickness.


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Figure 8. Addressable field emission device with the anode-cathode separation of 500 µm. (a) Top view SEM image of structure of gated FEAs; (b) schematic of two pixels of gated FEAs structure; (c) cross-sectional view SEM image of structure of gated FEAs substrate; (d) The sealed gated FEAs with phosphor screen; (e) full screen, (f) cartoon of a running dog, (g)~(j) Chinese characters controlled by FEAs. Figure 9. Field emission characteristics of fully sealed gated FEAs: (a) anode current (Ianode) and (b) gate current (Igate) as a function of anode voltage (Vanode) and gate voltage (Vgate).

FIGURES


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Figure 1. Top view SEM image of (a) ZnO nanowires without Al film coating, and nanowires prepared from Zn film coated with Al of different thickness: (b) Al-30 nm, (c) Al-60 nm, (d) Al90 nm, (e) Al-105 nm, (f) Al-120 nm, (g) Al-135 nm, (h) Al-150 nm, (i) Al-180 nm, (j) Al-210 nm. The inset of each sub-image is corresponding high resolution image.


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Figure 2. For nanowires prepared from Al coated Zn film, variation of population density with Al thickness from 30 nm to 210 nm and its linear fitting.


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Figure 3. (a) The bright field TEM image of ZnO nanowire prepared by Zn film coated with 90nm Al film. Inset (down right) shows the magnification of middle section. Inset (up left) is EDS spectrum of ZnO nanowire at circle region (The table gives the element distribution). (b) HRTEM lattice image of near-top section from another nanowire. Inset show the corresponding Selective area electron diffraction (SAED) pattern (down right).


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Figure 4. Top view SEM image of the sample prepared by Zn film coated with Al of 90 nm (a1) as-deposited film and thermal oxidation at 500 ℃ for different time: (b1)1 min, (d1)10 min, (c1)30 min, (e1)120min. Their corresponding cross-sectional view SEM image (a2) as-deposited film and thermal oxidation at 500 ℃ for different time: (b2)1 min, (c2)10 min, (d2)30 min, (e2) 120min.The inset of each sub-image is corresponding high resolution image.


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Figure 5. Schematic of growth process for ZnO nanowires grown from Al coated Zn film. (a) Structure of as-deposited Zn film, (b) structure of Al coated Zn film, morphology from the (c) early, (d) medium, (e) later stage of thermal oxidation.


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Figure 6. (a) The room-temperature PL spectra of samples. (b) The electrical conductivity (I-V) curve of samples (inset shows the magnification of wireframe region). The measured target is back-contact layer oxidized from Zn film coated with Al of different thickness, on where grew ZnO nanowires.


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Figure 7. (a) Field emission J-E plot with the corresponding F-N plot (inset) obtained from 3.5 inch samples of ZnO nanowires prepared by Zn film coated with Al of different thickness; (b)~(e) the population density, turn-on field, field emission current at 4150 V, and field enhancement factor versus Al thickness. (f) Typical SEM image of ZnO nanowires pattern; (g)~(m) field emission image recorded from ZnO nanowires prepared by Zn film coated with Al of different thickness.


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Figure 8. Addressable field emission device with the anode-cathode separation of 500 µm. (a) Top view SEM image of structure of gated FEAs; (b) schematic of two pixels of gated FEAs structure; (c) cross-sectional view SEM image of structure of gated FEAs substrate; (d) The sealed gated FEAs with phosphor screen; (e) full screen, (f) cartoon of a running dog, (g)~(j) Chinese characters controlled by FEAs.


32

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Figure 9. Field emission characteristics of fully sealed gated FEAs: (a) anode current (Ianode) and (b) gate current (Igate) as a function of anode voltage (Vanode) and gate voltage (Vgate).

TABLE LEGEND Table 1. Corresponding to PL spectra, the UV emission peak of samples prepared by Zn film coated with Al of different thickness.


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TABLE Table 1. Corresponding to PL spectra, the UV emission peak of samples prepared by Zn film coated with Al of different thickness. Thickness nm

0

30

60

90

120

135

150

180

Peak nm

379

380

381

380

379

378

377

377

Table 2. Comparison of the field emisison results of typical ZnO nanostructures ZnO nanostructure morphology

Substr ate

Preparation method

ZnO/Si hierarchical nanotrees Nanotips

Si

Aqueous solution

Si

Injector-like nanostructure Nanowires

Si

Nanowires

Si

Nanowires

Carbon cloth Zinc foil ITO glass

Metalorganic chemical vapor deposition Vapour phase transport Thermal evaporation Aqueous solution Aqueous solution Aqueous solution Thermal oxidation

Nanospike Nanowires

Si

Preparat ion tempera ture 95 °C

Turn-on field (V/µm)

β

Meas ured area

2.18 (10 µA/cm2)

8100

10×10 (3) mm2

650 °C

1.1 (10 µA/cm2)

18334

unkno w

580 °C

1.85 (10 µA/cm2) 6.46 (1 mA/cm2) 7.1 (10 µA/cm2) 0.27 (10 µA/cm2) 2.11 (10 µA/cm2) 6.21 (10 µA/cm2)

4386

4 mm2 (11)

unko wn 862

19.6 mm2 0.79m m2 1.77 mm2 1.77 mm2 4.5×8 cm2

950~100 0 °C 80 °C 95 °C unknow 500 °C

4.6 × 104 5601 1064

Ref.

(8)

(12) (13) (29) (30) Our work

ASSOCIATED CONTENT


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Supporting Information. Morphologies of Zn films and Al-coated Zn films; The density of nanowire embryo; The growth behaviours of pure Zn film and Al-coated Zn film; Analysis of the high resolution SEM image from region D in figure S3(c); Field emission current stability. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *E-mail address: [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. J.C., S.D. and N.X. conceived and designed the experiments. Y.L., G.Z. and L.Z. performed all experiments. Y.L., Z.Z and J.C. wrote the paper. All authors discussed and analyzed the results. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT The authors gratefully acknowledge the financial support of the project from the National Key Research and Development Program of China (Grant No.2016YFA0202001), National Key Basic Research Program of China (Grant No. 2010CB327703, 2013CB933601), State Key


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Laboratory of Optoelectronic Materials and Technologies, the Fundamental Research Funds for the Central Universities, and Guangzhou Science Technology and Innovation Commission. REFERENCES (1) Chen, D. K.; Song, X. M.; Zhang, Z. P.; Li, Z. P.; She, J. C.; Deng, S. Z.; Xu, N. S.; Chen, J. Transmission Type Tlat-panel X-ray Tource Using ZnO Nanowire Field Emitters. Appl. Phys. Lett. 2015, 107, 243105. (2) Takiguchi, Y.; Nanba, M.; Honda, Y.; Watabe, T.; Egami, K. N.; Taniguchi, M.; Mimura, H. Lag Characteristics of Flat Image Sensor Consisting of Field Emitter Array and High-gain Avalanche Rushing Amorphous Photoconductor Target. Applied Physics Express, 2010, 3, 027001. (3) Lv, S.; Li, Z.; Chen, C.; Liao, J.; Wang, G.; Li, M.; Miao, W. Enhanced Field Emission Performance of Hierarchical ZnO/Si Nanotrees with Spatially Branched Heteroassemblies. Acs Appl. Mater. Inter. 2015, 7, 13564−13568. (4) Zhang, Z.; Song, X.; Chen, Y.; She, J.; Deng, S.; Xu, N.; Chen, J. Controllable Preparation of 1-D and Dendritic ZnO Nanowires and Their Large Area Field-emission Properties. J. Alloy. Compd. 2017, 690, 304–314. (5) Wang, X.; Chen, K.; Zhang, Y.; Wan, J.; Warren, O. L.; Oh, J.; Li, J.; Ma, E.; Shan, Z. Growth Conditions Control the Elastic and Electrical Properties of ZnO Nanowires. Nano Lett. 2015, 15, 7886-7892. (6) Stehr, J. E.; Chen, W. M.; Reddy, N. K.; Tu, C. W.; Buyanova, I. A. Efficient Nitrogen Incorporation in ZnO Nanowires. Sci. Rep. 2015, 5, 13406. (7) Nair, S. S. Directing the Growth of ZnO Nanostructures on Flexible Substrates Using Low Temperature Aqueous Synthesis. Rsc Adv. 2015, 5, 90881-90887.


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Optimizing the Field Emission Properties of ZnO ... - ACS Publications

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