J. Phys. Chem. C 2007, 111, 12673-12676
12673
Enhanced Field Emission Performance of ZnO Nanorods by Two Alternative Approaches Changhui Ye,* Yoshio Bando, Xiaosheng Fang, Guozhen Shen, and Dmitri Golberg Nanoscale Materials Center, National Institute for Materials Science, Namiki 1-1, Tsukuba 305-0044, Ibaraki, Japan ReceiVed: May 21, 2007; In Final Form: June 29, 2007
Standard ZnO nanorods with a diameter of ∼150 nm show poor field emission performance. In an attempt to dramatically improve the field emission properties, we developed two alternative methods: The first one is to decrease the tip radius of nanorods down to ∼5 nm. The second one is to decorate the nanorods by small metal particles. By either of these two approaches, we can obtain excellent field emission performances, typically turn-on field (the electric field at which the current density reaches 10 µA/cm2) as small as 2 V/µm and current density as large as 1 mA/cm2 can be readily achieved. The physical essence of the excellent performances is the geometrically enhanced local field near the tip of the nanorods and the lowered work function of ZnO nanorods by the donation of electrons from the nanoparticles.
1. Introduction ZnO, as an important wide band gap semiconductor, has attracted much attention owing to its wide applications, including UV lasers, optoelectronics, and photocatalysis.1 Especially, ZnO nanorods have been investigated as electron field emitters due to the high-temperature chemical stability.2-11 However, the field emission mechanism from ZnO nanorods has been insufficiently studied, and the effort in improving the field emission performances is still lacking. Therefore, it is essential to evaluate the field emission properties in detail for ZnO nanorods. According to the theory, the field emission is related to two essential parameters, namely, the field enhancement factor and the work function of the emitting material as indicated in formula 112-14
J)
(
)
Aβ2E2 Bφ3/2 exp φ βE
(1)
where J is the field emission current density in the unit of A/cm2, E is the applied field in the unit of V/µm, φ is the work function of the emitter, A and B are constants with values of 1.56 × 10-10 A‚V-2‚eV and 6.83 × 103 V‚eV-3/2‚µm-1, respectively, and β is the field enhancement factor. The field enhancement factor is defined as the ratio of the local field to the applied field and is determined by the shape and the morphology of the emitters and inverse proportionally related to the tip radius of the emitters according to Edgcombe and Valdre12 and Filip et al.13 for the cylinder shape as shown in formula 2 and Charbonnier et al.14 for the cone shape as shown in formula 3 (with minor modifications)
R is the radius of the bottom of the cone, and θ is the hemiangle of the cone. The work function is a material-related parameter and has no relation with the geometry of the emitter. The morphology effect on the field emission performances is extensively explored;15-17 however, there are still two obstacles to achieve ideal results. In one hand, producing a tip radius smaller than 5 nm is still a challenge; in the other hand, even though small tips could be produced, the field emission performances still have room to improve. Therefore, we have to explore other alternatives to further improve the performances. As mentioned in the previous section, the work function of the emitter is another key factor for the field emission property. This factor is a material-related one; however, that does not mean it is a fixed number. For example, the reported work function for carbon emitters ranges from 1 to 12 eV,12 and there is also evidence that the work function of the emitters can be varied in a large range by annealing in different atmospheres.18,19 Furthermore, improved field emission performances of ZnO nanorods can also be achieved by lowering the work function through doping or coupling with other materials.20-22 Accordingly, we adopted two alternative approaches to improve the field emission performances of the ZnO nanorods arrays. The first one is to decrease the tip radius of ZnO nanorods by controlling the growth processes, and the second one is to lower the work function of ZnO by modifying it with metal particles with a smaller work function. The size-dependent field emission performances of ZnO nanorods and the field emission performances of metal nanoparticle decorated ZnO nanomaterials is also discussed. 2. Experimental Section
d β=1+s r
(2)
1.7d β=s (rR tan θ)0.5
(3)
where s is the screening factor, d is the spacing from the tip of the emitter to the anode plate, r is the tip radius of the emitter, * Corresponding author. E-mail:
[email protected].
Synthesis of ZnO nanorod arrays by the solution method was carried out in a mixed solution of zinc nitrate and hexamethyltetramine (HMTA) at 90-98 °C, and the ZnO nanorod arrays grew on a Si substrate precoated with ZnO seeds prepared by decomposition of zinc acetate in ethanol solution.23 Although ZnO nanorod arrays were well-ordered, the diameters of the nanorods were generally large (∼80 nm). Alternatively, we prepared the ZnO seeds by decomposition of zinc acetate in ethanol/1-propanol mixed solution; then we could prepare cone-
10.1021/jp073928n CCC: $37.00 © 2007 American Chemical Society Published on Web 08/09/2007
12674 J. Phys. Chem. C, Vol. 111, No. 34, 2007
Ye et al.
Figure 1. SEM images of ZnO nanorod arrays (a) and nanocones (b). The insets in (a) and (b) exhibit the enlarged view of the nanorods and the nanocones.
shaped ZnO nanorods with tip radius as small as ∼5 nm. The ZnO nanorods were decorated with Pt or Ag nanoparticles by reduction of Pt and Ag salts in H2O/ethanol solution at room temperature. Scanning electron microscopy (SEM, JSM-6700F) images were taken from the different samples deposited on a silicon wafer. Transmission electron microscopy and high-resolution transmission electron microscopy (HRTEM, JEM 3000F, 300 kV) images and electron dispersive spectroscopy (EDS) spectra were recorded for the different products. The samples were sonicated in ethanol for several minutes, and several drops of the solutions were dripped onto the carbon-coated copper grids. The field emission properties were measured in a vacuum chamber at a pressure lower than 10-7 torr. A cylindrical-shaped aluminum probe with tip area of 1 mm2 was used as the anode. 3. Results and Discussion Figure 1, parts a and b, reveals SEM images of the as-synthesized ZnO nanorod arrays and the nanocones, respectively. The nanorods are well-aligned and ∼80 nm in radius. The nanocones have tip radius as small as ∼5 nm and are not as well-aligned as the nanorods. The insets in Figure 1, parts a and b, show the high-magnification SEM images of the nanorods and nanocones, respectively. Considering that the nanorods have a radius as large as 80 nm and taking the screening factor s as unity, the field enhancement factor β is calculated as ∼1000 when d is 100 µm according to formula 2. The nanocones generally have semiangles smaller than 5°. Taking the typical value of r as 5 nm, R as 20 nm, d as 100 µm, θ as 5°, and s as 1, we can calculate the field enhancement factor β as ∼7 × 104 according to formula 3. However, as the screening factor s is much smaller than unity when the emitters are densely packed (small spacing in between), the experimentally observed β value should as well be much smaller than the calculated ideal value. Figure 2, parts a and b, shows TEM images of the ZnO nanorods decorated with Pt nanoparticles and ZnO nanocones, respectively. Figure 2, parts c and d, exhibits HRTEM images of these two samples, respectively. The surface of the nanocones is relatively smooth, whereas that of the nanorods decorated with Pt nanoparticles is rough. The size of the Pt particles is around 10 nm, and some of the particles are faceted. The EDS spectrum recorded on the Pt particles on the surface of ZnO nanorods as shown in Figure 2d is exhibited in Figure 2e, where signals of carbon and copper come from the carbon-coated copper grid, and the Pt signal reveals clearly that the surface particles are indeed composed of Pt element.
Figure 2. TEM images of ZnO nanocones with tip radius as small as ∼5 nm (a), and Pt nanoparticle decorated ZnO nanorods (b). HRTEM images of one ZnO nanocone (c) and Pt nanoparticle decorated ZnO nanorod (d), respectively. EDS spectrum of Pt particles on the surface of ZnO nanorods (e).
Figure 3a shows the J-E curves for the field emission of ZnO nanorods, ZnO nanocones, and the nanorods decorated with Pt and Ag nanoparticles, respectively, at the separation of 100 µm between the tip of the emitters and the anode. From this figure we can determine the turn-on field as ∼9.2, 3.7, 2.6, and 1.9 V/µm, respectively, for the four samples. The current densities of ZnO nanocones and ZnO nanorods decorated with Pt nanoparticles are larger than 1 mA/cm2. By plotting ln(J/E2) versus 1/E and fitting the data with Fowler-Nordheim relation (formula 3), we get linear curves as shown in Figure 3b. On the basis of these results, we can conclude that decreasing the
Field Emission Performance of ZnO Nanorods
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Figure 3. (a) J-E curves of the ZnO nanomaterials. The square symbols are for ZnO nanorods, the triangle symbols are for the ZnO nanocones, the star symbols are for the Pt nanoparticle decorated ZnO nanorods, and the dot symbols are for the Ag nanoparticle decorated ZnO nanorods. (b) The Fowler-Nordheim plots corresponding to the curves in (a).
Figure 4. Schematic illustrations of the energy levels of ZnO and Fermi levels of Pt and Ag. φ is the work function, Ef is the Fermi level, V is the vacuum level, CB and VB are the conduction and valence band edges of ZnO, respectively, Ef1 and Ef2 are the Fermi levels of the stoichiometric and the realistic ZnO, and φ1 and φ2 are the corresponding work functions of ZnO.
tip radius of ZnO nanorods and decoration with Pt and Ag nanoparticles indeed enhance the field emission performances. We can also extract the field enhancement factor as 120 and 2600, respectively, for ZnO nanorods and nanocones, which are smaller than the calculated field enhancement factors as expected. The screening factors can be estimated as 0.12 and 0.04, respectively, for these two samples. Apparently, if less densely packed ZnO nanocones (larger s value) of similar size can be grown, the field emission performances can be further improved accordingly. Why could the decoration of the surface of ZnO nanorods with Pt and Ag nanoparticles enhance the field emission performance? To answer this question, we further investigated the band structure and energy levels of ZnO, Pt, and Ag. From the schematic illustrations in Figure 4, the work function of n-type ZnO without heavy doping should lie within 4.2-5.8 eV, and the value for the intrinsic (undoped, n-type) ZnO is generally taken as 5.3 eV.24 It is clear that Ag with a work function of 4.3 eV can donate electrons to ZnO readily, whereas Pt with a work function of 5.6 eV can only donate electrons to ZnO slightly. The donation of electrons enhances the field emission performance of ZnO nanorods. From the perspective of the difference in work function, electron donation by Ag should be much easier than by Pt; however, from the experiments we find that the Pt is better in improving the field emission performances. The reason may lie in that Ag may be partially oxidized in room humidity, which decreases the electron-transfer rate. Further work is underway to elucidate the uncertainty. We have also investigated size effect in the field emission performance of ZnO. The preparation of ZnO nanorods with
Figure 5. Schematic diagram of size effect in field emission performance. NR 150 and NR 100 are ZnO nanorods with diameters of 150 and 100 nm, respectively. NT 50 is ZnO nanotubes with wall thickness of 50 nm. NC 8 is cone-shaped ZnO nanorods with tip diameter of 8 nm. QR 10 is ZnO quantum rods with diameter of 10 nm. Ag and Pt are deposited nanoparticles on size surfaces of ZnO nanomaterials. The dotted lines are guides for the eyes.
different diameters, quantum rods, and nanotubes is similar to the procedures described in the Experimental Section and will be reported in another paper. As seen in Figure 5, with reduction of the diameter of ZnO nanomaterials, the turn-on field also decreases, and the decoration by metal nanoparticles dramatically decreases the turn-on field. However, an interesting finding is worth noting. Pt nanoparticle decoration works better for ZnO nanomaterials with larger diameter, and with decrease of diameter of ZnO nanomaterials, the enhancement of field emission performance gets smaller. For ZnO nanocones, virtually, the decoration with Pt nanoparticles worsens the field performance. The results imply that there is a size range within which metal nanoparticle decoration works in enhancing the field emission performance of ZnO nanomaterials. We believe the competition between the bulk and surface effect is the underlying reason for this anti-intuitive phenomenon. 4. Conclusions In summary, we adopted two alternative approaches to enhance the field emission properties of ZnO nanomaterials (turn-on field as small as 2 V/µm and current density as large as 1 mA/cm2) by either down-tuning the tip radius of the nanorods or decorating the nanorods with small metal particles. It is noteworthy that for the validity of the concept of decorating with metal particles, the metal should be stable against oxidation and have a work function smaller than that of the emitting material. We can also conclude that the larger the work function of the emitting materials, the more effective the electron donation. In addition, there is a size range within which metal nanoparticle decoration works in enhancing the field emission
12676 J. Phys. Chem. C, Vol. 111, No. 34, 2007 performance of ZnO nanomaterials. The high field emission current density and the low turn-on field of ZnO nanocones and the metal particle decorated ZnO nanorods render them potential candidates in future applications in field emission display panels and light sources. Acknowledgment. This work was supported by the Japan Society for the Promotion of Science (JSPS) (Fellowship tenable at the National Institute for Materials Science, Tsukuba, Japan). C.Y. thanks JSPS for the fellowship. References and Notes (1) Wang, Z. L.; Kong, X. Y.; Ding, Y.; Gao, P.; Hughes, W. L.; Yang, R.; Zhang, Y. AdV. Funct. Mater. 2004, 14, 943. (2) Cao, B. Q.; Cai, W. P.; Duan, G. T.; Li, Y.; Zhao, Q.; Yu, D. P. Nanotechnology 2005, 16, 2567. (3) Lee, C. J.; Lee, T. J.; Lyu, S. C.; Zhang, Y.; Ruh, H.; Lee, H. J. Appl. Phys. Lett. 2002, 81, 3648. (4) Zhu, Y. W.; Zhang, H. Z.; Sun, X. C.; Feng, S. Q.; Xu, J.; Zhao, Q.; Xiang, B.; Wang, R. M.; Yu, D. P. Appl. Phys. Lett. 2003, 83, 144. (5) Yang, Y. H.; Wang, B.; Xu, N. S.; Yang, G. W. Appl. Phys. Lett. 2006, 89, 043108. (6) Banerjee, D.; Jo, S. H.; Ren, Z. F. AdV. Mater. 2004, 16, 2028. (7) Zhang, H.; Yang, D. R.; Ma, X. Y.; Que, D. L. J. Phys. Chem. B 2005, 109, 17055. (8) Wan, Q.; Yu, K.; Wang, T. H.; Lin, C. L. Appl. Phys. Lett. 2003, 83, 2253.
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