Field emission characteristics of zinc oxide nanowires synthesized by vapor-solid process

N A N O E X P R E S S

Open Access

Field emission characteristics of zinc oxide

nanowires synthesized by vapor-solid process

Shou-Yi Kuo

and Hsin-I Lin

Abstract

Vertically aligned ZnO nanowire (NW) arrays have been synthesized on silicon substrates by chemical vapor deposition. The growth of ZnO NWs may be dominated by vapor-solid nucleation mechanism. Morphological, structural, optical, and field emission characteristics can be modified by varying the growth time. For growth time that reaches 120 min, the length and diameter of ZnO NWs are 1.5μm and 350 nm, respectively, and they also show preferential growth orientation along the c-axis. Room-temperature photoluminescence spectra exhibit a sharp UV emission and broad green emission, and the enhanced UV-to-green emission ratio with increasing growth time might originate from the reduced concentration of surface defects. Furthermore, strong alignment and uniform distribution of ZnO NWs can also effectively enhance the antireflection to reach the average reflectance of 5.7% in the visible region. The field emission measurement indicated that the growth time plays an important role in density- and morphology-controlled ZnO NWs, and thus, ZnO NWs are expected to be used in versatile optoelectronic devices.

Keywords: ZnO nanowires; 1D nanostructures; Field emission; Near band edge Background

Recently, semiconductor one-dimensional (1D) nanostruc-tures have been attracting much attention in fundamental research and in potential applications for nanodevices. There are numerous studies on 1D nanostructures of Si, Ge, and III-V and also on oxide systems such as tin oxide (SnO2), silicon oxide (SiO2), indium tin oxide (ITO), zinc oxide (ZnO), and aluminum oxide (Al2O3). Among them, ZnO has been expected to be one of the most important optoelectronic materials with piezoelectricity, biocompati-bility, wide bandgap (approximately 3.37 eV), and large ex-citon binding energy (approximately 60 meV) at room temperature [1,2]. Due to their exceptional physical and chemical properties, 1D ZnO nanostructures, such as nano-rods, nanowires (NWs), nanotubes, and nanoneedles, are very attractive as well. Arrays of vertically aligned ZnO nanostructures are considered to be a promising candidate for applications in blue UV light emitters, field emission devices, high-efficiency photonic devices, photovoltaic de-vices, and biosensors [3-10]. So far, various kinds of

high-quality and well-aligned 1D ZnO nanostructures have been realized using phase transport, metal-organic vapor-phase epitaxy, pulsed laser deposition, and wet chemistry methods [11-15]. Vapor–liquid-solid (VLS) and vapor-solid (VS) processes have been employed by many researchers for the growth of 1D ZnO nanostructures because of its simple procedure and relatively low cost. The VLS model has been used to explain the growth mechanism at high temperatures, while the VS model dominates at low tem-peratures. From the viewpoint of applications, a high-temperature process might damage or deteriorate optoelec-tronic devices. A low-temperature VS process would be more suitable for the integration of 1D ZnO-based devices. Besides, the important characteristic of the field emission of ZnO NWs is rarely investigated, which could be a candi-date for field electron emitters due to their high aspect ratios, negative electron affinity, and mechanical and chem-ical stability. In this paper, we report a simple synthesis of ZnO NWs on a silicon substrate using the VS process at a relatively low growth temperature (550°C).

Methods

ZnO NWs were synthesized in a horizontal tube furnace system equipped with a 90-cm-long quartz tube,

three-* Correspondence:[email protected]

1_{Department of Electronic Engineering, Chang Gung University, Taoyuan,}

Taiwan

2_{Advanced Optoelectronic Technology Center, National Cheng-Kung}

University, Tainan 701, Taiwan

© 2014 Kuo and Lin; licensee Springer. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly credited.

zone heating system, gas inlet, and pump out. A 1 × 1 cm-sized, n-type Si(100) has been used as the deposited substrate. Before being loaded, the silicon substrate was etched using hydrofluoric acid and cleaned ultrasonically with ethanol and deionized water. After finishing sub-strate pretreatment, the silicon subsub-strates were coated with 8-nm-thick Au films as buffer layer by a DC sput-ter. An alumina boat loaded with zinc powder (100 mesh, 99.99%) was placed at the center of the quartz tube, and the silicon substrates were placed a few centi-meters downstream from the source. After loading, the quartz tube was heated up to 550°C under a constant high-purity Ar gas (150 sccm, 99.99%). The temperature was held at the peak temperature for 60, 90, 120 min, re-spectively. After evaporation, the system was naturally cooled down to room temperature under flowing argon gas.

The structure of as-grown samples was analyzed by X-ray diffraction (XRD; D5005, Siemens AG, Munich, Germany) using CuKα1 radiation. The morphology and microstruc-ture were investigated by scanning electron microscopy (SEM; S-4300, Hitachi, Tokyo, Japan). Photoluminescence (PL) measurement was performed at room temperature using λ = 325 nm of excitation of a He-Cd laser source (IK3401R-F, Kimmon Koha Co., Ltd., Tokyo, Japan). Field emission was measured at room temperature in a vacuum ambient of 3.5 × 105Torr. The distance between the anode and the tip of the ZnO NWs was 18μm, and the emission current was monitored with a Keithley 237 electrometer (Cleveland, OH, USA) and recorded at 1.0-s intervals by applying a sweep step of 10 V.

Results and discussion

XRD was used to acquire the crystallographic property of the ZnO NWs. Shown in Figure 1 are the XRD pat-terns of NWs grown at 550°C for 60, 90, and 120 min, respectively. Obviously, only the diffraction peak of ZnO (002) appears in the XRD profiles without the existence of secondary phases and clusters. This indicates that the ZnO NWs are preferentially oriented in the c-axis direc-tion. While increasing the growth duration from 60 to 120 min, the intensity of ZnO(002) diffraction plane in-creased as well. As expected, the highly enhanced (002) peak can be regarded as a result of enhanced NW growth in both vertical and lateral orientations.

Figure 2a,b,c,d,e,f shows the cross-sectional and plane-view FESEM images of the ZnO NWs for different growth durations. It is notable that both the average length and diameter of the NWs increase as the growth time is increased. In addition, the areal densities of ZnO NWs are 5.2 × 109, 2.9 × 109, and 1.8 × 109/cm2 with growth time of 60, 90, and 120 min, respectively. By varying the growth time from 60 to 120 min, the diame-ters of ZnO NWs increased from several tens to several

hundreds of nanometers, and the lengths increased from 200 nm to 1.5μm accordingly. It is also noteworthy that the ZnO NWs were almost aligned to the substrate sur-face. These observations are consistent with the XRD re-sults. In a typical metal-catalyzed VLS mechanism, nanosized metal clusters play a critical role in forming li-quid droplets that adsorb the gas-phase reactants where nanorod growth occurs. Hence, metallic nanoparticles with spherical shape are commonly found at the end of nanorods grown by the metal-catalyzed VLS method. Since no metallic particle was observed on the top of the ZnO NWs, we could rule out the possibility of a VLS-like mechanism and claim that the VS model dominates the nanowire growth.

Photoluminescence of the obtained ZnO NWs synthe-sized at different growth times was also investigated at room temperature, and the results are shown in Figure 3. The PL spectra consist of a sharp and strong UV emis-sion peak centered at about 380 nm and a weak green emission centered at about 500 nm. The UV emission is attributed to the near-band-edge (NBE) emission, and the green emission is related to the intrinsic defects in the ZnO samples. When the growth time increased, the intensity of NBE emission (INBE) also increased while the green emission (Igreen) decreased. Since ZnO NWs were fabricated under a fixed growth temperature, the im-provement of crystal quality might play a minor role. Thus, an increase in the NBE-to-green emission ratio with increasing growth time could result from the re-duced concentration of surface defects. Generally, the green emission is attributed to single ionized oxygen va-cancies (Vo) [16]. Recently, it has been recognized that the surface states that originated from large surface-to-volume ratios seriously influence the PL features in nanomaterials. As manifested in the SEM images, the average diameter becomes smaller with decreasing

20 30 40 50 60 70 80 90 min 60 min 120 min Intensity (a.u.) 2 (degrees) ZnO(002) NWs grown for

Figure 1 XRD patterns of ZnO NWs grown at 550°C for 60, 90, and 120 min, respectively.

growth time. Having a larger surface-to-volume ratio in nanostructures means a larger density of surface states. Therefore, higher surface states of ZnO NWs with a smaller diameter can be responsible for the origin of the enhanced green emission.

In order to investigate the influence of the ZnO NWs on light scattering, the spectral dependence of the total reflect-ance of nanowire arrays was analyzed. Figure 4 displays the reflectance spectra of ZnO NWs with different growth times of 60, 90, and 120 min. We can observe that the sili-con substrates covered by ZnO NWs have lower reflectance spectra in the range of 400 to 800 nm. This figure shows that the ZnO NWs with a growth time of 120 min have the lowest average reflectance of about 5.7% throughout the visible range (approximately 9.7% for 60 min and approxi-mately 7.6% for 90 min). That is simply because it has been realized that ZnO NWs with strong alignment, high aspect ratio, and uniform distribution can effectively enhance the antireflection coatings (ARCs) by trapping light and leading Figure 2 Cross-sectional and top-view FESEM images of ZnO NWs grown at different growth times. (a, d) NWs grown for 60 min, (b, c, e, f) from a Zn source at 550C for (a) 60 min (b) 90 min, and (c) 120 min of reaction times.

Figure 3 Photoluminescence spectra of ZnO NWs grown at different growth times. Near-band-edge emission and green emission are labeled.

to a broadband suppression in the reflection [17,18] Ac-cordingly, we expect that longer ZnO NWs have a much higher chance for the incident photons interacting with the NWs' surfaces, and therefore, the absorption cross section would be considerably larger than the short ones as we in-crease the growth time.

Figure 5 shows the field emission I-V plots for the ZnO nanowire with different growth times. Note that all samples show similar emission current–voltage (I-V) characteristics despite the different growth times. There are two different regions manifested in the I-V curve of all samples. In the low-voltage region, the emission current is low and seems to be independent of the ap-plied voltage. Once the voltage is increased further, the emitted current increases dramatically and the turn-on

voltages are 410, 440, and 550 V for growth times of 120, 90, and 60 min, respectively.

In order to analyze the emission behavior, the I-V characteristics of ZnO NWs are interpreted using the Fowler-Nordheim (FN) equation: J ¼ aV 2 β2 d2φexp bdφ3=2 βV   ð1Þ

where J is the current density, V is the applied voltage, β is the work function, d is the emitting distance, β is the field enhancement factor, and a and b are the constants. As shown in the inset of Figure 5, factor β in the FN equation represents the degree of field emission en-hancement. For a nanostructured emitter, the β value is related to its work function, morphology, crystallinity, conductivity, and density. By assuming 5.2 eV as the work function value for ZnO NWs, field enhancement factors were calculated to be 642, 492, 396 for growth times of 60, 90, and 120 min, respectively [19-21]. The field emission properties of ZnO NWs are expected to improve as their length is increased, which can be qualitatively explained by the increase in the field enhancement factor β for long NWs. However, our experimental results contradict the anticipation. The phenomenon can be ascribed to the compensation by the increase of their diameter. Based on our experi-mental results, the growth time plays an important role in density and morphology control of ZnO NWs and thus modifies the optoelectronic properties for versatile devices.

Figure 4 Reflectance spectra of ZnO nanowires grown for 60, 90, and 120 min, respectively.

0 100 200 300 400 500 600 700 0.000 0.002 0.004 0.006 0.008 0.010 0.012

Current

()

Applied voltage (V)

Figure 5 Field emission characteristics of ZnO NWs. They were grown for 60, 90, and 120 min, respectively. The inset shows Fowler-Nordheim plots of ln(I/V2

Conclusions

In summary, the vertical arrays of well-aligned c-axis orientation ZnO NWs have been synthesized on silicon substrate by VS growth mechanism at a relatively low growth temperature. By varying the growth time, we can adjust the areal density, length, and diameter of ZnO NWs and modify the structural and optoelectronic properties accordingly. PL spectra measured at room temperature exhibit a sharp UV peak and broad green band, corresponding to the NBE and defect-related emissions, respectively. When the growth time in-creased, the average diameter of NWs became larger and thus the surface-to-volume ratio became lower. Therefore, higher surface states of ZnO NWs with smaller diameters can be responsible for the origin of enhanced green emission. ZnO NWs with strong align-ment and uniform distribution can also minimize the re-flectance to 5.7% in the visible region. In addition, field emission features revealed that the growth time plays an important role in density- and morphology-controlled ZnO NWs. It is reasonable to expect that the ZnO NWs can be modified to meet the requirements for versatile optoelectronic devices.

Abbreviations

NBE:Near-band-edge; PL: Photoluminescence; SEM: Scanning electron microscopy; XRD: X-ray diffraction.

Competing interests

The authors declare that they have no competing interests. Authors_{’ contributions}

H-IL designed and carried out the experiment and statistical analysis and participated in drafting the manuscript. S-YK supervised the research and revised the manuscript. Both authors read and approved the final manuscript.

Acknowledgements

This work was supported by the Green Technology Research Center of Chang Gung University and the National Science Council (NSC) of Taiwan under contract numbers NSC100-2815-C-155-013-E, NSC100-2112-M-182-004, and NSC101-2112-M-182-003-MY3.

Received: 23 December 2013 Accepted: 14 January 2014 Published: 11 February 2014

References

1. Jiang CY, Sun XW, Lo GQ, Kwong DL, Wang JX: Improved dye-sensitized solar cells with a ZnO-nanoflower photoanode. Appl Phys Lett 2007, 90:263501.

2. Xu L, Shen H, Li X, Zhu R: Enhanced ultraviolet emission from ZnO thin film covered by TiO2 nanoparticles. Chin Opt Lett 2009, 7:953–955. 3. Huang MH, Mao S, Feick H, Yan H, Wu Y, Kind H, Weber E, Russo R, Yang P:

Room-temperature ultraviolet nanowire nanolasers. Science 1897, 2001:292.

4. Manoharan MP, Desai AV, Neely G, Haque MA: Synthesis and elastic characterization of zinc oxide nanowires. J Nanomater 2008, 2008:849745. 5. Ng HT, Han J, Yamada T, Nguyen P, Chen YP, Meyyappan M: Single crystal

nanowire vertical surround-gate field-effect transistor. Nano Lett 2004, 4:1247.

6. Heo YW, Tien LC, Kwon Y, Norton DP, Pearton SJ, Kang BS, Ren F: Depletion-mode ZnO nanowire field-effect transistor. Appl Phys Lett 2004, 85:2274.

7. Lee CH, Yoo J, Doh YJ, Yi GC: ZnO/Mg0.2Zn0.8O coaxial nanorod heterostructures for high-performance electronic nanodevice applications. Appl Phys Lett 2009, 94:043504.

8. Li QH, Liang YX, Wan Q, Wang TH: Oxygen sensing characteristics of individual ZnO nanowire transistors. Appl Phys Lett 2004, 85:6389. 9. Weissenberger D, Gerthsen D, Reiser A, Prinz GM, Feneberg M, Thonke K,

Zhou H, Sartor J, Fallert J, Klingshirn C, Kalt H: Influence of the measurement procedure on the field-effect dependent conductivity of ZnO nanorods. Appl Phys Lett 2009, 94:042107.

10. Wang XD, Song JH, Liu J, Wang ZL: Direct-Current nanogenerator driven by ultrasonic waves. Science 2007, 316:102.

11. Pan ZW, Dai ZR, Wang ZL: Nanobelts of semiconducting oxides. Science 1947, 2001:291.

12. Wu JJ, Liu SC: Low-temperature growth of well-aligned ZnO nanorods by chemical vapor deposition. Adv Mater 2002, 14:215.

13. Park WI, Kim DH, Jung SW, Yi GC: Metalorganic vapor-phase epitaxial growth of vertically well-aligned ZnO nanorods. Appl Phys Lett 2002, 80:4232.

14. Hartanto AB, Ning X, Nakata Y, Okada T: Growth mechanism of ZnO nanorods from nanoparticles formed in a laser ablation plume. Appl Phys A 2004, 78:299.

15. Vayssieres L, Keis K, Lindquist SE, Hagfeldt A: Purpose-built anisotropic metal oxide material: 3D highly oriented microrod array of ZnO. J Phys Chem B 2001, 105:3350.

16. Hu JW, Bando Y: Growth and optical properties of single-crystal tubular ZnO whiskers. Appl Phys Lett 2003, 82:1401.

17. Lee YJ, Ruby DS, Peters DW, McKenzie BB, Hsu JWP: ZnO nanostructures as efficient antireflection layers in solar cells. Nano Lett 2008, 8:1501–1505. 18. Lee C, Bae SY, Mobasser S, Manohara H: A novel silicon nanotips

antireflection surface for the micro sun sensor. Nano Lett 2005, 5:2438–2442.

19. Bai XD, Wang EG, Gao PX, Wang ZL: Measuring the work function at a nanobelt tip and at a nanoparticle surface. Nano Lett 2003, 3:1147. 20. Hsu CL, Su CW, Hsueh TJ: Enhanced field emission of Al-doped ZnO

nanowires grown on a flexible polyimide substrate with UV exposure. RSC Adv 2014, 4:2980–2983.

21. Mosquera E, Bernal J, Zarate RA, Mendoza F, Katiyar RS, Morell G: Growth and electron field-emission of single-crystalline ZnO nanowires. Mater Lett 2013, 93:326–329.

doi:10.1186/1556-276X-9-70

Cite this article as: Kuo and Lin: Field emission characteristics of zinc oxide nanowires synthesized by vapor-solid process. Nanoscale Research Letters 2014 9:70.

Submit your manuscript to a

journal and benefi t from:

7 Convenient online submission 7 Rigorous peer review

7 Immediate publication on acceptance 7 Open access: articles freely available online 7 High visibility within the fi eld

7 Retaining the copyright to your article