Field emission from zinc oxide nanostructures and its degradation

Vacuum 83 (2009) 265–272

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Field emission from zinc oxide nanostructures and its degradation Jing Xiao, Gengmin Zhang*, Xin Bai, Ligang Yu, Xingyu Zhao, Dengzhu Guo Key Laboratory for the Physics and Chemistry of Nanodevices, Department of Electronics, Peking University, Beijing 100871, China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 30 September 2007 Received in revised form 11 March 2008 Accepted 8 July 2008

Arrays of zinc oxide (ZnO) nanowires and nanobelts were synthesized by the thermal evaporation of mixed powders of ZnO and graphite. Neither catalyst nor vacuum environment was involved in the fabrication. For comparison, the ZnO nanowires were grown on a pre-deposited transitional ZnO film on a brass substrate and the ZnO nanobelts were grown directly on a Si substrate. Their field emission properties were systematically measured. Current density of 10 mA/cm2 was achieved at the fields of 5.7 and 6.2 V/mm from the nanowires and nanobelts, respectively. Also, the emission sites were found to distribute uniformly on the whole cathode. In the preliminary test on the stability, the ZnO nanobelts, which were sharp at the tip but wide at the root, exhibited better robustness than the ZnO nanowires. The post-test scanning electron microscopy (SEM) observation showed that the degradation of their field emission capability resulted from the breaking of the nanowires, which was tentatively attributed to the resistive heating during the field emission. In contrast, the shedding of the ZnO from the substrate was not so serious as imagined. Ó 2008 Elsevier Ltd. All rights reserved.

Keywords: ZnO nanostructure Field emission Degradation

1. Introduction Cold electron sources based on field emission have such advantages over thermionic cathodes as low energy consumption, instant turn-on capability, and small energy spread [1]. As summarized by Xu and Huq, they have already found applications or demonstrated application perspectives in field emission displays (FEDs), lighting elements, electron microcolumns in lithography, mass spectrometer for space exploration, and radio frequency devices, etc. [2]. So far, the most mature cold electron sources are still Spindt-type field emitter arrays (Spindt FEAs) [3]. Meanwhile, one-dimensional nanometer-scale materials have also appeared to be quite competitive. The field emission devices that center on these nanomaterials can often deliver electrons at low fields. Also, their fabrication is relatively easy and inexpensive compared with that of the Spindt FEAs. In this context, field emission has been obtained from different nanostructures of ZnO and the results are quite encouraging [4–8]. ZnO is a wide band gap semiconductor (WBGS). Its band bending, which usually favors field emission by lowering surface barrier and bringing more electrons to the bottom of conduction band, can be quite dramatic under high field [9,10]. Recently, it was found that the field emission performance of ZnO nanorods was fully recoverable even after the exposure to O2, N2, Ar, and air in a severe vacuum condition of w2  104 Torr, indicating very good chemical stability of ZnO nanostructures [11].

* Corresponding author. Tel.: þ86 10 62751773; fax: þ86 10 62762999. E-mail addresses: [email protected], [email protected] (G. Zhang). 0042-207X/$ – see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2008.07.005

As reported in this paper, we have systematically studied the field emission properties of ZnO nanostructures, including the dependence of the emission current density on the apparent field between the anode and the cathode (J–E* behavior), the distribution of the emission sites and the variance of the emission current with time. Especially, for the future practical applications, we have paid particular attention to the mechanism behind the degradation of the field emission. Some preliminary results have been obtained and the corresponding analysis is also given.

2. Experimental 2.1. Sample fabrication Two categories of ZnO nanostructures were synthesized via thermal evaporation. Neither metallic catalysts nor a vacuum environment was required in the fabrication. Our previous work has shown that the drop of the field emission current from the ZnO nanowires was related to the decrease of the emission sites [12]. Thus, in the hope of improving the adherence of the nanowires to the substrate, this time some of the ZnO nanostructures were grown on a brass substrate coated with a layer of pre-deposited ZnO thin film. Before the growth of the ZnO nanostructures, a layer of ZnO film was electroplated on the brass substrate. First, a piece of commercial H62 brass (5  5 cm) was washed successively with acetone, deionized water and ethanol. Then, it was electrically polished and dipped into an electrolytic cell for Zn

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electrodeposition. The electrolyte was the solution of 370 g ZnSO4$7H2O, 75 g CoSO4$7H2O and 45 g Na2SO4 in 600 ml deionized water [13]. Here the CoSO4$7H2O and Na2SO4 were used to disperse the Zn ions and improve the conductivity of the solution, respectively. During the electroplating process, a Zn rod was used as the cathode and the polished brass was used as the anode. A constant voltage source was employed to provide a steady 5 V DC voltage between the anode and cathode. The electroplating lasted 10 min. In this process, the output current of the voltage source was between 2.0 and 3.0 A and the electrolyte was stirred magnetically all the time. As a result, a layer of Zn in grayish white was deposited on the surface of the brass substrate. Then the Zn-covered brass was cut into small pieces, which were all 0.5  0.5 cm2 in area, and submitted to a two-hour annealing at 200  C in air. The annealing oxidized the Zn layers and the surface of the small pieces turned black. The ZnO nanowires were fabricated using a similar process as before [10,14]. The synthesis process was carried out in a quartz tube in a horizontal furnace. The tube was 28 mm in inner diameter, 3 mm in thickness and 1.2 m in length. A quartz boat that contained the source materials, i.e., powders of ZnO (99.0% in purity) and graphite mixed in a molar ratio of 1:2, was pushed to the middle of the tube. Meanwhile, several ZnO-covered brass substrates were placed in sequence in another quartz boat near the downstream outlet of the tube. The temperature would not be uniform along the tube during heating. It would be highest in the middle and about 500  C lower around the substrate. Before the heating began, argon was introduced into the tube with a flow of 40 cm3/min for approximately 40 min. Then the furnace was switched on and the argon flow was lowered to 20 cm3/min. It took about 18 min for the temperature in the middle of the tube to reach 960  C and that around the substrate to 440  C accordingly. Then the relay started to work and the tube temperature was kept unchanged for 10 min. During this process, the source materials reacted with each other and the evaporated products were transported to the substrate, where the synthesis of ZnO nanostructures occurred. Finally, the furnace was switched off and the sample was allowed to cool down to room temperature in the tube with the gas still flowing. As will be shown in the following section, the above condition was found to yield non-aligned ZnO nanowires. Since the brass substrates were at different positions in the tube and thus experienced different temperatures during the fabrication, the ZnO nanowires on them somehow differed in concrete configuration. The samples obtained in this manner are hereafter referred to as the A-type samples. It is worth noting that the substrate temperature during the fabrication of the A-type samples, 440  C, was lower, though only narrowly, than the softening point of the glass used in FEDs, which is in the range of 500–600  C [15]. This relatively low fabrication temperature would be an advantage when the current technique is incorporated in the production process of FEDs. The mechanism of the decrease in field emission sites could be more complex than was maintained in our previous paper [12]. For a comparison with the results obtained from the A-type samples, ZnO nanostructures were also grown directly on Si substrate [16]. Zn powders, 4–10 mm in diameter, were dispersed in ethanol ultrasonically for 30 min and the resultant suspension was immediately dropped onto some 0.5  0.5 cm2 Si substrates, which had been obtained by cutting a 125 mm-diametered Si wafer and cleaned beforehand in an ultrasonic bath using acetone, deionized water and ethanol successively. After the ethanol evaporated in air, the substrates coated with the Zn particles were put into a quartz boat and placed in the middle of the quartz tube in the furnace. The two terminals of the quartz tube were open, thus the substrates were exposed to air during the growth process. Then the furnace was switched on and the temperature in the middle of the tube was elevated with a rate of 20  C/min until it reached 600  C. The

temperature was held constant for 6 h. Finally the furnace was switched off and the system cooled down naturally to room temperature. After this reaction, the Si substrates were found to be uniformly covered with a layer of white products. The samples fabricated under this condition are referred to as the B-type samples in the paper. 2.2. Sample characterization The morphology of the as-grown samples was observed with two scanning electron microscopes (SEMs, Amray 1910FE and FEI XL30 SFEG), respectively. The crystal structure of the products was characterized by X-ray diffraction (XRD, Rigaku DMAX-2400 diffractometer). A transmission electron microscope (TEM, Hitachi H-9000 NAR) was used to obtain more details of the morphology and the crystal structure of the samples. For the preparation of the TEM specimens, the as-grown products were first ultrasonically dispersed in ethanol for 15 min, and then several drops of the ethanol were applied onto a TEM grid. 2.3. Field emission measurement The field emission measurements were carried out in a lab-built ultrahigh vacuum system with a base pressure of 107 Pa. The sample was fixed on a Cu holder and used as the cathode. Two kinds of anodes were, respectively, used in the measurement. One was a glass screen coated with tin oxide (SnO2). The transparent anode technique allowed us to observe the two-dimensional distribution of the emission sites on the cathode. The other was a stainless steel anode. It could bear long-term bombardment of relatively high current density, thus it was used in the measurement of the J–E* behavior and the stability test. The cathode surface was separated from the transparent anode by 0.8 mm. The spacing between the stainless steel anode and the cathode could be adjusted with a micromanipulator outside the measurement chamber in a range of 0–1.5 mm. The J–E* curve and the dependence of the emission current on the time under a constant voltage (I–t curve) could be recorded automatically with a self-developed signal acquisition system. 3. Results and discussion 3.1. Characterization results The results of the SEM observation of the products are given in Fig. 1. The SEM images of two A-type examples are shown in Fig. 1(a)–(c) and those of one B-type sample are shown in Fig. 1(d) and (e). In all the samples, the substrates were found to be covered by a layer of islands whose diameter was of the order of 1–10 mm. In Fig. 1(a) and (b), each island was a cluster of random-oriented curly nanowires that entangled together into a web structure. The diameter and length of these nanowires were of the order of 10– 102 nm and 10–102 mm, respectively. Thus, their aspect ratio was very high. As will be shown later, this high aspect ratio led to a sufficiently large field enhancement and is considered as one of the key origins of the efficient field emission. In Fig. 1(c), the nanowires were also random-oriented, but they were straight and grew along the radial direction of the islands. The nanowires were with uniform diameters and lengths, which were of the order of 10 nm and 10 mm, respectively. Thus, they also had a high aspect ratio. In Fig. 1(d) and (e), nanobelts grew from the islands. These nanobelts, 1–10 mm in length, were wide at the root and became sharp at the tip. This sharp tip is also considered as conducive to the formation of high field.

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Fig. 1. SEM images of non-aligned nanowires and nanobelts. (a) and (b) An A-type sample; (c) another A-type sample; (d) and (e) a B-type sample (the scale bars are 10, 1, 10, 20 and 5 mm, respectively).

For the structural analysis of the above nanowires and nanobelts, the samples were submitted to the XRD and TEM analysis, and the results are given in Figs. 2 and 3, respectively. The XRD peaks acquired from an A-type sample are assigned to either the Cu–Zn alloy (JCPDS Card No. 25-1228 Cu5Zn8) or the hexagon-structured ZnO with lattice constants a ¼ 3.250 Å and c ¼ 5.206 Å (JCPDS Card No. 80-0075 ZnO). Since the former is attributable to the brass substrate, the nanowires are believed to be wurtzite-type ZnO. In the XRD of a B-type sample, with the exception of the strong Si peak due to the substrate, all the peaks arose from the ZnO crystal. The size of the nanostructures was estimated based on the Scherrer formula,

D ¼

Kl Wcos q

where K is the Scherrer constant, D the grain size, W the full width at half maximum in radian, q the angle of diffraction and l the wave length of the X-ray [17]. For sample A, the diameter of the nanowire is estimated to be about 14 nm. This value is obviously smaller than the average diameter of the nanowires observed with the SEM and TEM. The discrepancy probably arose from the spurious broadening of the XRD peaks caused by the transitional ZnO film between the nanowires and the brass substrate. The ZnO film might have contained small-sized grains and thus led to XRD peaks with quite

large W. The peaks of the nanowires could have been well covered by those of the grains in the film. For sample B, the grain size along the [0001] direction was estimated to be about 58 nm. This value is in accordance with the SEM observation if taken as the thickness of the nanobelts. A TEM was further employed to investigate some individual nanowires from the samples. Bifurcation structure was often observed and one example is given in Fig. 3(a). Both the branches are uniform in diameter. The right branch was picked up for the high-resolution transmission electron microscopy (HRTEM) observation. As shown in Fig. 3(b), the spacing between two adjacent lattice planes is 0.52 nm, commensurate with that between the (0001) planes in ZnO. This result is in agreement with that of the XRD. Moreover, it has shown that the nanowires grew along the c-axis. 3.2. J–E* behaviors The field emission performance of the above samples was studied in the following three aspects: (1) the J–E* behavior, (2) the distribution of the emission sites over the cathode surface and (3) the temporal stability of the field emission. The most important parameters that reflect the emission capability of a cold electron source are the turn-on field and threshold field, which are defined as the fields that extract 10 mA/

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Fig. 2. The XRD analysis.

cm2 and 1 mA/cm2 current densities, respectively [4,18]. For the avoidance of ambiguity in concept, it has to be emphasized that the ‘‘field’’ in the above definition is not the actual local field around the ZnO nanowire, which is not directly measurable. Instead, the ‘‘field’’ in the above definition means the average field between the anode and the cathode. It is simply the quotient of the applied voltage to the cathode–anode separation, denoted by E* ¼ V/d. For the distinction with the actual local field E, this average field E* is termed as the ‘‘apparent field’’. The J–E* behaviors of an A-type sample and a B-type sample were acquired and are, respectively, shown in Fig. 4. Here both samples were 0.5  0.5 cm2 in area and measured with an anode– cathode separation of 0.5 mm. From Fig. 4, the turn-on fields of samples A and B are determined to be 5.7 and 6.2 V/mm, respectively. Also, their threshold fields are determined to be 8.6 and 10.4 V/mm, respectively. The Fowler–Nordheim theory is used for the formulation of the field emission from the ZnO nanostructure arrays [19,20]:

  J ¼ AE2 exp  BF3=2 =E where J is the field emission current density, F is the work function, E is the local electric field at the emission sites, as defined above, and A and B can be approximated by two constants. E cannot be measured directly and is usually related to the applied voltage V by setting E ¼ bV/d ¼ bE*, where b is the field enhancement factor. Theoretically, b is determined by the aspect ratio of the emitter [21,22]. The J–E* behavior of our sample followed the FN formula approximately, as is suggested by the FN plots in Fig. 4(b). Using the above formulae, b can be computed by the slope of the FN plots when the work function is known a priori. If the generally accepted value of the work function of bulk ZnO, 5.3 eV, is used [23], b is calculated to be w1 103 for both the two samples. In a pure geometric consideration, such a high enhancement factor does not seem to have been available from our ZnO nanostructures. As indicated by Fig. 1, the nanowires were not well aligned along the direction perpendicular to the substrate. This disagreement somehow indicates one of the drawbacks of the application of the FN theory in the field emission from a nanowire array. For comparison, some most representative or most recent works on the field emission from ZnO nanostructures are given in Table 1. It is reasonable to say that our result is quite competitive, especially after noticing that the turn-on field was defined at a lower current

Fig. 3. TEM observation of individual ZnO nanowires. (a) TEM image of two ZnO nanowire branches. (b) HRTEM image of the right branch.

density in some of the cited works. Of course, owing to the disparity in the experimental conditions, such as the anode–cathode separation, the significance of this comparison should somehow be qualified. Actually, as shown in the next sections, the current work stresses more on the emission stability and degradation mechanism of the ZnO nanostructures. Within the authors’ knowledge, though similar work has been done on other field emitters, say carbon nanotube (CNT), it has still hardly been addressed in the study of ZnO. 3.3. Stability test Besides the J–E* behaviors, the stability of the field emission from our samples was also tested. The I–t curves were recorded under a constant voltage. For both samples the initial current density was set at 300 mA/cm2. For samples A and B, the operating voltages were 3.70 and 4.25 kV, respectively. Inevitably a small number of gas molecules would be ionized during the field emission measurement and the bombardment of the cathode by the resulting ions could constitute a threat to the nanostructures. A

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2.0

a

J (mA/cm2)

1.5

1.0

0.5 Sample A Sample B 0.0 0

2

4

6

8

10

12

E* (V/µm)

ln (J/E2)

-4

b

-6

-8

-10

0.08

Sample A Sample B 0.10

269

The test on an A-type sample lasted 72 h, as shown in Fig. 5(a). It was stopped when the current dropped to 25% the initial value. As shown in Fig. 5(b), while the emission sites distributed uniformly over the whole sample before the test, only a few of them survived the test. In the hope of finding some clues on the mechanism of the degradation of the field emission, the samples were observed again with the SEM after the stability test. Fig. 5(c)–(e) shows the morphology of the post-test A-type samples. Fig. 5(c) was obtained at the edge of a sample, in which one can clearly see that the ZnO film was still on the substrate after the long-term field emission measurement. The picture was intentionally taken at the edge, so that the ZnO film could be obviously distinguished from the underlying brass substrate. In other regions, no brass substrate was exposed. That is, the ZnO thin film was still quite intact after the stability test. Fig. 5(d) shows the debris of the broken nanowires. The above result of the SEM observation suggests that the degradation of the field emission from the A-type samples was mainly ascribable to the breaking of the ZnO nanowires. Relatively speaking, the shedding of the ZnO from the brass substrate was not very serious and thus played only an insignificant role. So far, many mechanisms have been put forward for the interpretation of the degradation and failure of the field emission [31–35]. As shown in Fig. 5(d), most of the ZnO nanowires were broken in the stability test. Here two factors are considered to have been possibly responsible for this breaking: resistive heating and mechanical failure [31]. In a simplified estimation, the stress across the nanowire section under the electric field E, as defined in the previous part, is the absolute value of the eigenvalues of the Maxwell stress tensor:

1 2

s ¼ 30 E 2 0.12

0.14

0.16

0.18

1/E* Fig. 4. J–E* behaviors of samples A and B. (a) The J–E* behaviors; (b) the corresponding FN plots.

high anode–cathode voltage meant a high ion kinetic energy. For the time being, the voltage still could not be further lowered owing to some technological restrictions in our experiment. Nonetheless, due to the maturity of the modern microelectronics technology, if the ZnO nanostructures are indeed applied as a practical field emitter, the actual anode–cathode separation or anode–grid separation can be much smaller than those in our current study. Therefore, the ions that bomb the cathode might have lower kinetic energy and therefore cause less damage. The distribution of the emission sites on the samples was also, respectively, observed using the transparent anode both before and after the stability tests.

where 30 is the dielectric constant [36]. Here the value of the electric field, E, is calculated using the apparent field in the stability test, 7.4 and 8.5 V/mm, and the field enhancement factor calculated above, w103. Also, two major approximations are invoked. First, the nanowire is assumed to be parallel to the electric field; second, the nanowire is assumed to be a perfect conductor and thus its inside is free from field. Under these approximations, s is estimated to be less than 1 GPa. Due to the fact that the tenability of the above two approximations cannot be guaranteed, this result is not very reliable. In fact, as argued in Section 3.2, the field enhancement factor was probably overestimated, thus the actual stress inside a ZnO nanowire could be obviously smaller than 1 GPa. Since the ultimate tensile strength (UTS) of ZnO is reported to be as high as 8–10 GPa [37], we tend to believe that the mechanical failure did not constitute the major reason for the nanowire breaking. Consequently, it is reasonable to consider that the resistive heating played the decisive role in breaking the ZnO nanowires [38]. From Fig. 5(d) one can see that most of the breaking events did not occur at the roots of the nanowires and the remnants of some

Table 1 Some previous results on the field emission from ZnO nanostructures Sample type

Turn-on field (V/mm) (at 10 mA/cm2 if default)

Threshold field (V/mm) (at 1 mA/cm2 if default)

Field enhancement factor (b)

Reference

Nanowires Needle-like nanowires Nanopencils Nanocavity Nanonails Nanoscrews Nanosheets Sheet array Sample A Sample B

6.0 (at 0.1 mA/cm2) 11 (at 0.01 mA/cm2) 3.7 4.1 (distinguished from the background noise) 7.9 3.6 6.8 (at 1 mA/cm2) 6.13 (at 1 mA/cm2) 5.7 6.2

11 24 (at 0.1 mA/cm2) 4.6 (at 1.3 mA/cm2) 11.6 – 11.2 (at 1.2 mA/cm2) – – 8.6 10.4

847 372 2300 1035 – – 749 1  103 1  103 1  103

[4] [24] [25] [26] [27] [28] [29] [30] This work This work

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Current Density (mA/cm2)

270

350

a

300 250

b

200 150 100 50 0

0

12

24

36

48

60

72

Time (Hour)

c

d

e

f

Fig. 5. The test on the stability of field emission from an A-type sample. (a) The I–t curve; (b) the emission site distribution before (left) and after (right) the stability test; (c)–(e) the SEM images of the sample after the stability tests (the scale bars are 50, 2 and 200 mm, respectively). (f) An SEM image of the stainless steel anode after the stability test (the scale bar is 2 mm).

nanowires were still left on the sample. Since the heat losing was relatively easy at the root, the heating-induced breaking usually occurred at somewhere along the nanowire. The occurrence of this heating-related process has been confirmed by the SEM observation of the stainless steel anode. Fig. 5(f) shows that some nanowires could be found on the stainless steel anode. They deposited on the anode after leaving the cathode due to the breaking. Furthermore, as shown in Fig. 5(e), many pits emerged on the ZnO film after the stability test. They are believed to have arisen from the arcing between the anode and the cathode initiated by the field emission [39]. As summarized by Bonard and Klinke [31], the arcing events are usually caused by a high field emission current, anode outgassing, or local evaporation of cathode material that create a conduction channel between the electrodes, leading to a discharge that destroys the emitter [39,40]. During the field emission measurement, when the voltage was high, momentarily irregular flashing between the cathode and the anode could be

noticed. Also, the pressure of the vacuum chamber could rise temporarily from 107 to 105 Pa. As shown in Fig. 6(a), the stability test on a B-type sample lasted 9 days, much longer than that on the A-type sample, because the daily decrease in current was much less dramatic. The test was finally concluded when the current dropped intermittently to about half the initial value. Obviously, the B-type sample outperformed the A-type sample in term of stability of field emission. The comparison of the emission site distribution before and after the stability, which is given in Fig. 6(b), also shows that a large number of emission site were retained after the test. The SEM images of the post-test B-type samples are given in Fig. 6(c) and (d). On the one hand, Fig. 6(c) shows that the density of islands remained almost unchanged after the stability test, suggesting that large-scale dropping of the ZnO islands from the underlying substrate did not occur. On the other hand, Fig. 6(d) indicates that indeed most of the nanobelts on several islands were lost. Thus, the

J. Xiao et al. / Vacuum 83 (2009) 265–272

Current Density (µ µA/cm2)

a

271

c

350 300 250 200 150 100 50 0 0

24

48

72

96

120

144

168

192

216

d

Time (Hour)

b

Fig. 6. The test on the stability of field emission from a B-type sample. (a) The I–t curve; (b) the emission site distribution before (left) and after (right) the stability test. (c) and (d) The SEM images of the sample after the stability tests (the scale bars are 20 and 5 mm, respectively).

conclusion obtained from the A-type samples is reinforced by the comparison between the pre-test and the post-test B-type samples. For better understanding how the ZnO nanowires were actually broken, an in situ observation of the behavior of an individual ZnO nanowire during field emission, usually under an SEM, is necessary. At the moment the efforts are being made in this aspect. As a guidance to the practical applications, we could arrive at the conclusion that two factors are very crucial for the obtainment of stable ZnO field emission. Firstly, the nanostructure should be uniform over the whole cathode surface in geometric parameters, such as length and thickness. If the nanostructures are not uniform in these parameters, the field emission mainly comes from a few sites only and these heavily burdened sites are quite susceptible to the damage caused by resistive heating. Secondly, a reasonable geometric shape is also required. If field emission is available from some ZnO nanostructures with relatively large size, say w0.1–1 mm [41,42], good stability can be expected. There exists a contradiction between the demand for a high field enhancement and that for a good robustness. Generally speaking, a high aspect ratio is favorable to the former but unfavorable to the latter. In this regard, as indicated by the above results and discussion, the nanobelt structure of the B-type sample could be a good compromise. On the one hand, its sharp tip can result in high field enhancement; on the other hand, its wide root in connection with the underlying islands conduces to heat loss and thus is not so prone to be broken by the resistive heating.

4. Conclusion Arrays of non-aligned ZnO nanowires and nanobelts were fabricated via a simple method of thermal evaporation, in which neither foreign catalyst nor vacuum environment was required. The

ZnO nanowire arrays were grown on a layer of ZnO film that was electrochemically deposited beforehand. Field emission current densities of 10 mA/cm2 and 1 mA/cm2 were obtained under apparent fields 5.7 and 8.6 V/mm, respectively. The results of the stability test showed that the breaking of these nanowires with a high aspect ratio, which was ascribable to resistive heating, was the major reason why most of the initial emission sites eventually failed. Also, the ZnO nanobelts, which were sharp at tip and wide at root, were grown directly on Si substrates without using any transitional layer. Field emission current densities of 10 mA/cm2 and 1 mA/cm2 were obtained under apparent fields 6.2 and 10.4 V/mm, respectively. A large number of the nanobelts survived the stability test, indicating that the shedding of the ZnO from the substrate was not a very serious concern.

Acknowledgement This work was supported by the National Natural Science Foundation of China (Nos. 90606023 and 60771004) and the MOST of China (2006CB932402). The sample characterization was supported by the Instrumental Analysis Fund of Peking University.

References [1] Li XH, Yang CF, Bai GD, Zhang FQ, Liao FJ, Feng JJ, et al. Appl Surf Sci 2003;215:249. [2] Xu NS, Huq SE. Mater Sci Eng R 2005;48:47. [3] Spindt CA. J Appl Phys 1968;39:3504. [4] Lee CJ, Lee TJ, Lyu SC, Zhang Y, Ruh H, Lee HJ. Appl Phys Lett 2002;81:3648. [5] Shen XP, Yuan AH, Hu YM, Jiang Y, Xu Z, Hu Z. Nanotechnology 2005;16:2039. [6] Zhang YS, Yu K, Ouyang SX, Zhu ZQ. Mater Lett 2006;60:522. [7] Zhu YW, Zhang HZ, Sun XC, Feng SQ, Xu J, Zhao Q, et al. Appl Phys Lett 2003;83:144.

272

J. Xiao et al. / Vacuum 83 (2009) 265–272

[8] Dong LF, Jiao J, Tuggle DW, Petty JM, Elliff SA, Coulter M. Appl Phys Lett 2003;82:1096. [9] Wang RZ, Wang B, Wang H, Zhou H, Huang AP, Zhu MK, et al. Appl Phys Lett 2002;81:2782. [10] Chen L, Zhang GM, Wang MS, Zhang QF. Chin Phys 2005;14:181. [11] Kim DH, Jang HS, Lee SY, Lee HR. Nanotechnology 2004;15:1433. [12] Wang WW, Zhang GM, Yu LG, Bai X, Zhang ZX, Zhao XY. Physica E 2007;36:86. [13] Cao Y, Ding GF, Yang CS. Electroplat Pollut Control 2003;23:9 (in Chinese). [14] Yao BD, Chan YF, Wang N. Appl Phys Lett 2002;81:757. [15] Talin AA, Dean KA, Jaskie JE. Solid State Electron 2001;45:963. [16] Wen XG, Fang YP, Pang Q, Yang CL, Wang JN, Ge WK, et al. J Phys Chem B 2005;109:15303. [17] Patterson AL. Phys Rev 1939;56:978. [18] Bonard JM, Maier F, Sto¨ckli T, Chaˆtelain A, Heer WA, Salvetat JP, et al. Ultramicroscopy 1998;73:7. [19] Fowler RH, Nordheim L. Proc R Soc London Ser A 1928;119:173. [20] Gomer R. Field emission and field ionization. Cambridge: Harvard University Press; 1961. p. 9. [21] Filip V, Nicolaescu D, Tanemura D, Okuyama F. Ultramicroscopy 2001;89:39. [22] Wang M, Shang XF, Li ZH, Wang XQ, Xu YB. Acta Phys Sin 2006;55:797 (in Chinese). [23] Minami T, Miyata T, Yamamoto T. Surf Coat Tech 1998;108:583. [24] Tseng YK, Huang CJ, Cheng HM, Lin IN, Liu KS, Chen IC. Adv Funct Mater 2003;13:811.

[25] Wang RC, Liu CP, Huang JL, Chen SJ, Tseng YK, Kung SC. Appl Phys Lett 2005;87:013110. [26] Zhao Q, Zhang HZ, Zhu YW, Feng SQ, Sun XC, Xu J, et al. Appl Phys Lett 2005;86:203115. [27] Shen GZ, Bando Y, Liu BD, Golberg D, Lee CJ. Adv Funct Mater 2006;16:410. [28] Liao L, Li JC, Liu DH, Liu C, Wang DF, Song WZ, et al. Appl Phys Lett 2005;86:083106. [29] Chin KC, Poh CK, Chong GL, Lin J, Sow CH, Wee ATS. Appl Phys A 2008; 90:623. [30] Wang C, Wang FF, Fu XQ, Wang TH. Chin Phys 2007;16:3545. [31] Bonard JM, Klinke C. Phys Rev B 2003;67:115406. [32] Kim DH, Kim TS, Ahn BK, Shin HY, Lee DG, Cho HK, et al. Mater Sci Forum 2005;475:1771. [33] Wang MS, Wang JY, Jin CH, Chen Q, Peng LM. Mater Sci Forum 2005;475: 4071. [34] Wang MS, Peng LM, Wang JY, Chen Q. J Phys Chem B 2005;109:110. [35] Saito Y, Seko K, Kinoshita J. Diamond Relat Mater 2005;14:1843. [36] Vanderlinde J. Classical electromagnetic theory. New York: Wiley; 1993. p. 84. [37] Kulkarni1 AJ, Zhou M, Ke FJ. Nanotechnology 2005;16:2749. [38] Dolan WW, Dyke WP, Trolan JK. Phys Rev 1953;91:1054. [39] Dyke WP, Trolan JK, Martin EE, Barbour JP. Phys Rev 1953;91:1043. [40] Sowers AT, Ward BL, English SL, Nemanich RJ. J Appl Phys 1999;86:3973. [41] Ni SL, Chang YQ, Long Y, Ye RC. Acta Phys Sin 2006;55:5409 (in Chinese). [42] Xuan K, Yan XH, Ding SL, Yang YR, Xiao Y, Guo ZH. Chin Phys 2006;15:460.