Electron field emission enhancement of hybrid Cu/CuO nanowires fabricated by rapid thermal reduction of CuO nanowires

Abstract

CuO nanowires (NWs) were grown via the thermal oxidation of Cu film deposited on a CuO/glass template. The reduction of CuO NWs was conducted at 250, 260, 270, and 280 °C in H₂ atmosphere. It was found that the surfaces of CuO NWs partially transformed into Cu via rapid thermal annealing (RTA) under H₂ atmosphere, forming hybrid Cu/CuO NWs. The field-enhancement factors of pure CuO NWs and hybrid Cu/CuO NWs prepared at 250, 260, and 270 °C were 2624, 5702, 7897, and 79 580, respectively. The results also show that the hybrid Cu/CuO NWs efficiently reduce the turn-on field from 6.4 to 0.9 V μm⁻¹.

---

Introduction

One-dimensional (1-D) semiconductor nanowires (NWs) have potential applications in electronics, optoelectronics, biosciences, and energy sciences.^1,2 Recently, 1-D semiconductor NWs have attracted much interest as a potential material for cold-cathode field emitters. Due to their large length-to-diameter ratio, considerable attention has been focused on the use of 1-D nanostructures for flat panel displays. Besides a large field enhancement factor, NWs have other advantages for field emitter applications, including high emission current density, low electric field and easy fabrication. Field emitters based on 1-D materials, such as ZnO, TiO₂, Ga₂O₃, In₂O₃, and CuO NWs have been demonstrated.^3–7 CuO, which has a narrow bandgap of 1.2 eV, is a natural p-type material with a monoclinic structure, which makes it useful in many applications such as high-temperature superconductors, catalysis, and gas sensors. There are various methods for synthesizing CuO nanostructures, such as microwave-assisted hydrothermal methods and catalysis methods.^8–10 Xu et al. proposed a simple method for growing CuO NWs via the direct heating of Cu foil in air.¹¹ It is also worthwhile to grow CuO NWs on other substrates such as Si or glass substrates to allow CuO NW integration into microelectromechanical systems.^12,13

The advantage of cold-cathode field emitters over conventionally adopted thermionic emitters is their reduced electric field. To lower the turn-on field of CuO NWs, Zhu et al. treated CuO NWs using O₂ and CF₄ plasma.¹⁴ The decrease of the turn-on field depends on the plasma treatment time. Tsai et al. grew CuO NWs doped with Zn, which obviously decreased the turn-on field.¹⁵ Hydrogen annealing is also a very effective method for improving the performance of 1-D semiconductor NWs for electronics and optoelectronics.¹⁶ The present study proposes an easier method for fabricating CuO field emitters that efficiently reduces the turn-on field. Large-area CuO NWs were grown on a glass substrate and then subjected to rapid thermal treatment under H₂ ambiance. The reduction under various temperatures and electron field emission characteristics are also discussed.

Experimental

Prior to CuO NW growth, Corning 1737 glass substrates were wet cleaned in acetone and deionized water. A 100 nm-thick CuO layer was deposited on a glass substrate to serve as the adhesion layer and then a 1μm-thick Cu film was deposited via DC sputtering. The samples were then annealed in air at 450 °C for 5 hours to grow CuO NWs. The reduction of CuO NWs was conducted via RTA under H₂ ambiance. Firstly, the samples were placed in the RTA chamber, which was vacuumed to 3 × 10⁻² Torr. The samples were then heated to various temperatures (i.e., 250, 260, 270 and 280 °C) at a heating rate of 10 °C s⁻¹. Once the temperature was stable, 3% H₂ (H₂ : N₂ = 3 : 97) was introduced into the chamber to treat the samples at 1 atm pressure for 10 min. The crystallographic properties and surface morphology of the as-synthesized and thermally reduced CuO NWs were then examined by X-ray diffraction (XRD, MAC MXP18), field emission scanning electron microscopy (FE-SEM, JEOL JSM-7001F), and transmission electron microscopy (TEM, JEOL JEM-2100F). To measure the field emission properties, the samples were placed in a vacuum chamber, which was pumped down to 4 × 10⁻⁶ Torr. The CuO NWs served as the cathode. A tungsten probe with a 1 mm-diameter plate was used as the anode. The gap between the anode and the NWs was carefully controlled using a micrometer (Mitutoyo). A Keithley 237 high-voltage source was then used to provide the sweeping voltage with a step of 10 V and to measure the emission current. The relationship between the emission current and the applied voltage was recorded automatically.

Results and discussion

According to previous research, CuO NWs are difficult to reduce to Cu at below 200 °C,¹⁷ and thus the treatment temperature in this experiment was higher than 250 °C. Fig. 1a and b show the cross-sectional and top-view SEM images, respectively, of the as-synthesized CuO NWs with a 1 μm-thick Cu film. High density NWs were uniformly distributed on the surface. The average length and diameter of the CuO NWs were around 6 μm and 60 nm, respectively. Fig. 1c–f show the SEM images of the CuO NWs reduced via RTA at 250, 260, 270, and 280 °C for 10 min, respectively. Compared with the as-synthesized CuO NWs, the length of NWs decreased with annealed temperature in the range of 250 to 270 °C. Fig. 2 shows top-view SEM images of H₂-treated CuO NWs at 250–280 °C. From Fig. 1b and 2, it can be observed that the density of CuO NWs decreased with increasing temperature, and then the CuO NWs disappeared after treatment at 280 °C.

Fig. 1 (a) Cross-sectional and (b) top-view SEM images of as-synthesized CuO NWs. Cross-sectional SEM images of reduced CuO NWs treated at (c) 250, (d) 260, (e) 270, and (f) 280 °C for 10 min, and at 270 °C for (g) 20 and (h) 30 min.

Fig. 2 Top-view SEM images of H₂-treated CuO NWs at various temperatures.

Fig. 3a shows XRD patterns of the CuO NWs before and after H₂ treatment at various temperatures for 10 min. For the as-synthesized CuO NWs (without H₂ treatment), the XRD peaks located at 2θ = 32.5°, 35.6°, 38.8°, 48.7°, 53.5°, 58.3°, 61.6°, 66.2°, 68.1°, 72.4°, and 75.3° are indexed to (110), (1̄11), (111), (2̄02), (020), (202), (1̄13), (3̄11), (220), (311), and (2̄22) of monoclinic CuO, respectively. No peaks related to Cu were found, which means that the Cu layer had completely oxidized into CuO NWs. The as-synthesized samples were then treated with H₂ gas via RTA at 250, 260, 270, and 280 °C, respectively. For a treatment temperature of 260 °C, the Cu-related peaks of (111), (200) and (220) were observed. The intensity of the Cu peaks increased with increasing treatment temperature. After annealing at 280 °C, the peaks of the CuO phases almost disappeared, and thus the Cu phase dominated the whole sample. It should be noted that in addition to the CuO-related peaks, two new peaks corresponding to Cu₂O (111) and Cu (111) appeared at 250 °C. However, at this stage, the intensity of the Cu₂O peak was much higher than that of the Cu peak. With increasing temperature, the intensities of Cu₂O and Cu peaks both increased. Nevertheless, the increasing intensity of Cu-related peaks was much more obvious than that of the Cu₂O peak in the range of 250 to 280 °C. This indicates that Cu₂O formed at the initial stage of the reduction process. As the temperature was increased, Cu₂O seeds were then continuously reduced to Cu. The reduction occurred as follows:

2CuO + H₂ → Cu₂O + H₂O (1)

Cu₂O + H₂ → 2Cu + H₂O (2)

Fig. 3 (a) XRD patterns of as-synthesized CuO NWs and CuO NWs reduced at various temperatures. (b) XRD patterns of CuO NWs treated at 270 °C for various times.

Park et al. reported that for the oxidization of Cu to CuO NWs, Cu₂O seeds were firstly formed to relieve the initial compressive stress in Cu film and then transform into the CuO phase.¹² These results imply the reactive paths for the oxidation of Cu–Cu₂O–CuO and the reduction of CuO–Cu₂O–Cu are the same. Fig. 3b shows the reductive results for various treatment times. At 270 °C, the CuO-related peaks disappeared and transformed into Cu₂O and Cu peaks after 30 min treatment, and thus all CuO NWs disappeared on the sample surface (Fig. 1g and h).

To further understand the relationship between the NW structure and the field emission properties, the structural characteristics were examined using high-resolution SEM and TEM. Fig. 4 shows the surface morphology of the CuO NWs after H₂ treatment for 10 min. The NW surface was still very smooth after treatment at 250 °C (Fig. 4a). However, nanoparticles formed on the NW surface at 260 °C (Fig. 4b). When the temperature was further increased to 270 °C (Fig. 4c and d), the size and density of these nanoparticles became higher than those of the sample treated at 260 °C. Fig. 5 shows the TEM images of the CuO NWs after H₂ treatment at various temperatures. Similar to the results of SEM images, nanoparticles formed on the NW surface while the temperature was higher than 260 °C. For the treatment temperature of 270 °C, the average diameter of these nanoparticles was around 20 nm. To identify its composition, energy-dispersive X-ray spectroscopy (EDS) spectra of the primary NW (point A) and the nanoparticle (point B) were analysed (Fig. 5c, e and f). The Cu : O ratio of point A was around 54 : 46, which meant its intrinsic CuO domination. However, the concentration of Cu at point B was much higher than that of oxygen, which indicates its partial reduction by H₂ (Cu : O = 80 : 20). Fig. 5d shows the interfacial high-resolution TEM image on the place C of Fig. 5c. The interplanar spacing of CuO lattice fringe patterns was calculated to be 0.251 nm. This value corresponded well with the spacing calculated for (1̄11) crystallographic planes for monoclinic CuO (cell constants a = 0.469 nm, b = 0.342 nm, c = 0.513 nm, β = 99.54°). The right side of Fig. 5d shows the reductive results of Cu(111) with interplanar spacing of 0.205 nm. Fig. 5g schematically depicts the reduction mechanism of the CuO NWs.

Fig. 4 Surface morphology of CuO NWs treated at (a) 250, (b) 260, and (c) 270 °C. (d) Enlarged image of CuO NWs treated at 270 °C.

Fig. 5 TEM images of CuO NW treated at (a) 250, (b) 260, and (c) 270 °C for 10 min, and (d) the high-resolution image of place C in (c). EDS analysis of (c) at (e) point A and (f) point B. (g) Schematic diagram of reduction process of CuO NWs.

Fig. 6a shows the field emission characteristics measured from these samples. For the as-synthesized CuO NWs, the field emission current increased slowly when the applied electric field was small. The turn-on field of the as-synthesized CuO NWs was 6.4 V μm⁻¹. At the turn-on field, the emission current was around 1.05 × 10⁻⁵ A cm⁻². When the applied bias was increased further, the measured emission current increased exponentially. After the samples were treated under H₂ ambiance, the turn-on field tended to lower values. The turn-on fields of the samples treated at 250, 260, and 270 °C were measured as 4.6, 3.3, and 0.9 V μm⁻¹, respectively. To further understand the turn-on field decrease of the H₂ treated NWs, the measured current density–electric field (J–E) curves were analyzed using the Fowler–Nordheim (F–N) equation J = (Aβ²E²/Φ)exp(−BΦ^3/2/βE), where J is the current density (A cm⁻²), E is the applied electric field (V μm⁻¹), β is the field enhancement factor, A = 1.56 × 10⁻⁶ (A eV V⁻²), B = 6.83 × 10⁷ (V cm⁻¹ eV^−3/2), and Φ is the work function of the emitting material.¹⁸ The work function of CuO is 5.2 eV.¹⁹ It should be noted that the NWs also included Cu₂O and Cu components after H₂ treatment, and thus the work function of these hybrid Cu/CuO NWs was not exactly 5.2 eV. However, to simplify the calculation of the enhancement factor, the work function of the H₂-treated NWs was assumed to be 5.2 eV. Fig. 6b replots ln(J/E²) as a function of 1/E. The field enhancement factor β of the as-synthesized CuO NWs was calculated to be 2624. After H₂ treatment, the β values were higher than that of the as-synthesized CuO NWs. Samples treated at 250, 260, and 270 °C had β values of around 5702, 7897, and 79 580, respectively. It has been reported that field emission depends strongly on the morphology, material quality and NW density. From Fig. 1, although the as-synthesized CuO NWs had the highest aspect ratio, they had the lowest β value. This implies that other factors affect the field emission properties more strongly than do the aspect ratio and density of CuO NWs.

Fig. 6 (a) Field emission characteristics of as-synthesized CuO NWs and hybrid Cu/CuO NWs treated at 250, 260, and 270 °C. (b) F–N plot of ln(J/E²) versus 1/E.

Fig. 7a schematically shows the band diagram of the as-synthesized CuO NWs. When a strong external field was applied between the NW tips and the vacuum, the conduction band was bended to Fermi energy. The energy band of NWs was distorted which resulted in a narrowing of the tunneling barrier. The accumulated electrons could thus tunnel through the barrier. Fig. 7b shows the band structure diagram of the enhanced field emission performance of H₂-treated CuO NWs.

Fig. 7 Schematic band diagram of (a) pure CuO NWs and (b) hybrid Cu/CuO NWs with strong external field. Inset shows schematic diagram of electron transfer and emission for hybrid Cu/CuO NWs.

After the reduction process, the CuO NWs were partially reduced to Cu, forming hybrid Cu/CuO NWs. During measurement, the electrons generated by the external field were transported from the CuO NWs to Cu. There are two possible mechanisms for the enhancement of the field emission performance (Fig. 7b). Firstly, the nanoparticles collect electrons from the hybrid NWs and transported them to the tip of the NWs. Electrons were then emitted by the external field. Secondly, nanoparticles form on the surface of CuO NWs, providing other paths for electron emission. The work functions of CuO and Cu are 5.2 eV and 4.65 eV, respectively.^19,20 A smaller work function results in a thinner barrier,^20,21 and thus the electrons concentrated at the nanoparticles will be emitted more easily than those of pure CuO NWs. A low turn-on field of 0.9 V μm⁻¹ can thus be achieved by the hybrid Cu/CuO NWs.

Conclusions

Hybrid Cu/CuO NWs were fabricated by partial reduction of pure CuO NWs via RTA in H₂ ambiance. The turn-on field and field-enhancement factor β of the as-synthesized CuO NWs were 6.4 V μm⁻¹ and 2624, respectively. Compared with pure CuO NWs, the turn-on field was reduced from 6.4 to 0.9 V μm⁻¹, and the field-enhancement factor β was enhanced from 2624 to 79 580 by rapid thermal reduction. The nanoparticles not only form electron transmission channels to the tip, but also provide emission sources for electrons. Such structures effectively enhance the field emission properties, giving them potential for field emission devices.

Acknowledgements

This work was supported by the Ministry of Science and Technology of Taiwan, under Contract no. MOST 103-2221-E-492-014-MY3.

Notes and references

1. M. S. Gudiksen, L. J. Lauhon, J. Wang, D. C. Smith and C. M. Lieber, Nature, 2002, 415, 617–620.
2. Z. L. Wang and J. H. Song, Science, 2006, 312, 242–246.
3. Q. X. Zhang, K. Yu, W. Bai, Q. Y. Wang, F. Xu, Z. Q. Zhu, N. Dai and Y. Sun, Mater. Lett., 2007, 61, 3890–3892.
4. G. Liu, F. Li, D. W. Wang, D. M. Tang, C. Liu, X. Ma, G. Q. Lu and H. M. Cheng, Nanotechnology, 2008, 19, 025606.
5. G. Sinha, A. Datta, S. K. Panda, P. G. Chavan, M. A. More, D. S. Joag and A. Patra, J. Phys. D: Appl. Phys., 2009, 42, 185409.
6. L. Q. Hu, Z. X. Lin, T. L. Guo, L. Yao, J. J. Wang, C. H. Yang, Y. A. Zhang and K. L. Zheng, Acta Phys. Sin., 2006, 55, 6136–6140.
7. W. Y. Sung, W. J. Kim, S. M. Lee, H. Y. Lee, Y. H. Kim, K. H. Park and S. Lee, Vacuum, 2007, 81, 851–856.
8. W. Z. Wang, Q. Zhou, X. M. Fei, Y. B. He, P. C. Zhang, G. L. Zhang, L. Peng and W. J. Xie, CrystEngComm, 2010, 12, 2232–2237.
9. C. M. Tsai, G. D. Chen, T. C. Tseng, C. Y. Lee, C. T. Huang, W. Y. Tsai, W. C. Yang, M. S. Yeh and T. R. Yew, Acta Mater., 2009, 57, 1570–1576.
10. L. Liqing, H. Kunquan, G. Xing and X. Mingxiang, Appl. Phys. A: Mater. Sci. Process., 2014, 115, 1147–1150.
11. C. H. Xu, C. H. Woo and S. Q. Shi, Chem. Phys. Lett., 2004, 399, 62–66.
12. Y. W. Park, N. J. Seong, H. J. Jung, A. Chanda and S. G. Yoon, J. Electrochem. Soc., 2010, 157, 119–124.
13. H. T. Hsueh, S. J. Chang, F. Y. Hung, W. Y. Weng, C. L. Hsu, T. J. Hsueh, S. S. Lin and B. T. Dai, J. Electrochem. Soc., 2011, 158, 106–109.
14. Y. W. Zhu, A. M. Moo, T. Yu, X. J. Xu, X. Y. Gao, Y. J. Liu, C. T. Lim, Z. X. Shen, C. K. Ong, A. T. S. Wee, J. T. L. Thong and C. H. Sow, Chem. Phys. Lett., 2006, 419, 458–463.
15. T. Y. Tsai, C. L. Hsu, S. J. Chang, S. I. Chen, H. T. Hsueh and T. J. Hsueh, IEEE Electron Device Lett., 2012, 33, 887–889.
16. X. H. Huang, Z. Y. Zhan, K. P. Pramoda, C. Zhang, L. X. Zheng and S. J. Chua, CrystEngComm, 2012, 14, 5163–5165.
17. Q. Yong, T. Staedler and J. Xin, Nanotechnology, 2007, 18, 4.
18. R. H. Fowler and L. Nordheim, Proc. R. Soc. London, Ser. A, 1928, 119, 173–181.
19. L. Liao, Z. Zhang, B. Yan, Z. Zheng, Q. L. Bao, T. Wu, C. M. Li, Z. X. Shen, J. X. Zhang, H. Gong, J. C. Li and T. Yu, Nanotechnology, 2009, 20, 085203.
20. B. S. Khan, T. Mehmood, A. Mukhtar and M. Tan, Mater. Lett., 2014, 137, 13–16.
21. J. Ding, M. Wang, X. Zhang and C. Ran, RSC Adv., 2013, 3, 14073–14079.

---