www.elsevier.com/locate/apsusc

Applied Surface Science 253 (2007) 8923–8927

Growth and field emission of tungsten oxide nanotip

arrays on ITO glass substrate

Kai Huang, Qingtao Pan, Feng Yang, Shibi Ni, Deyan He *

School of Physics Science and Technology, Lanzhou University, Lanzhou 730000, China

Received 8 April 2007; received in revised form 6 May 2007; accepted 6 May 2007

Available online 16 May 2007

Abstract

High-density and uniformly aligned tungsten oxide nanotip arrays have been deposited by a conventional thermal evaporation on ITO glass

substrates without any catalysts or additives. The temperature of substrate was 450–500 8C. It was shown that the tungsten oxide nanotips are

single-crystal grown along [0 1 0] direction. For commercial applications, field emission of the tungsten oxide nanotip arrays was characterized in a

poor vacuum at room temperature. The field emission behaviors are in agreement with Fowler–Nordheim theory. The turn-on field is 2.8 V mm�1

as d is 0.3 mm. The excellent field emission performances indicated that the tungsten oxide nanotip arrays grown by the present approach are a

good candidate for application in vacuum microelectronic devices.

# 2007 Elsevier B.V. All rights reserved.

PACS : 81.07.�b; 81.10.�h; 79.70.+q

Keywords: Tungsten oxide; Nanotip arrays; Field emission

1. Introduction

There has been increasing interest in development of field

emitters based on various nanomaterials for many applications,

such as flat panel displays and other vacuum microelectronic

devices [1–3]. Field emitters with nanostructures can increase

the aspect ratio and the field enhancement factor, leading to

lower the turn-on voltage for field emission [4–6]. For emitter

with a nanotip configuration, the tip cone angle u and the tip

radius Rtip are two important factors for its field emission

behaviors [7]. In order to obtain excellent field emission

performances, extensive efforts have been devoted to synthe-

size nanostructures with nanosize tips so far [7,8].

Tungsten oxide is an n-type semiconductor with a work

function in the range of 5.59–5.70 eV which makes it attractive

for the field emitter applications [9,10]. In recent years,

extensive and exhaustive efforts have been devoted to study the

synthesis of tungsten oxide nanowire arrays with tungsten

powders as the source during the thermal process [8,11,12], but

the temperature of substrate is higher than 1000 8C. Although

* Corresponding author. Fax: +86 931 8913554.

E-mail address: hedy@lzu.edu.cn (D.Y. He).

0169-4332/$ – see front matter # 2007 Elsevier B.V. All rights reserved.

doi:10.1016/j.apsusc.2007.05.006

tungsten oxide nanowires synthesized at low temperature have

been reported [13], they are not nanowire arrays. For practical

applications, the lower temperature of substrate to synthesize

tungsten oxide nanowire arrays is still a challenge.

In this paper, we have first successfully deposited one-

dimensional tungsten oxide nanotip arrays on ITO glass

substrates without any catalysts and additives by a conventional

thermal evaporation technique. The temperature of substrate

was 450–500 8C. It was shown that the tungsten oxide

nanowires are single-crystal grown along [0 1 0] direction.

Field emission of the tungsten oxide nanotip arrays was

characterized in a poor vacuum at room temperature. The

excellent performance of the field emission indicated that the

tungsten oxide nanotip arrays are a good candidate for

application in vacuum microelectronic devices.

2. Experimental

The tungsten oxide nanowires were grown on ITO glass

substrates by a conventional thermal evaporation using

tungsten trioxide powder (99.9%) as a source without any

catalysts and additives. The powder was placed at the center of

the alumina tube which was inserted in the horizontal tube

Fig. 2. XRD pattern of the tungsten oxide nanotip arrays on ITO glass substrate.

K. Huang et al. / Applied Surface Science 253 (2007) 8923–89278924

furnace. The ITO glass substrate (10 mm � 10 mm) was set at

the end of the alumina tube. After the tube was evacuated down

to a pressure of about 2 � 10�3 Torr, the temperature in the

center of the tube was elevated to 1100 8C at a rate of

20 8C min�1, and the temperature at the end of the tube was

controlled at about 450–500 8C. During the evaporation, the

total pressure of the chamber was maintained at 200 Torr by

introducing the gas flows of Ar and O2 at a total rate of

100 sccm. The evaporation time was 2 h. After the evaporation,

the furnace was naturally cooled down to room temperature.

The structures of the samples were characterized by X-ray

diffraction (XRD) (Rigaku RINT2400 with Cu Ka radiation),

scanning electron microscopy (SEM) (Hitachi S800 FEG), and

high-resolution transmission electron microscopy (HRTEM)

(JEM 2100, 200KV).

3. Results and discussion

Fig. 1a is a low magnification SEM titled image of a typical

tungsten oxides nanowire arrays grown on ITO glass substrate.

It can be seen that uniformly aligned tungsten oxide nanowires

with an average height of�5 mm were vertically formed on the

substrate. Fig. 1b is an enlarged SEM image, showing that the

nanowires gradually shrink in diameter from 50–100 to

Fig. 1. (a) Low magnification SEM tilted image of a tungsten oxide nanotip

arrays grown on ITO glass substrate. (b) Enlarged SEM image.

5–10 nm. Such nanotips are well separated from each other.

The structure of the tungsten oxide nanotip arrays was also

characterized using XRD measurement. As shown in Fig. 2, the

diffraction peaks can be well indexed as the monoclinic cell of

W18O49 with cell constants of a = 18.28 A, b = 3.77 A,

c = 13.98 A and b = 115.208. The (0 1 0) diffraction peak is

the strongest one, suggesting that the [0 1 0] is the preferred

growth direction of the nanostructure. No peak from the ITO

substrate can be observed, probably because of the high-density

of the nanotips.

To further characterize the microstructures of the W18O49

nanotips, HRTEM images were taken for individual nanotip.

The low magnification TEM images shown in Fig. 3a and b

Fig. 3. (a and b) Low magnification TEM images of W18O49 nanotips. Scale bar

100 nm. (c) HRTEM image of the rectangle-enclosed area of (b), inset is a fast

Fourier transform pattern of the square-enclosed area of (c). Scale bar 10 nm.

(d) Enlarged HRTEM image. Scale bar 5 nm.

K. Huang et al. / Applied Surface Science 253 (2007) 8923–8927 8925

exhibit that the tungsten oxide nanotips have a segmental tip-

on-tip laddered structure. The fast Fourier transform (FFT)

pattern (inset of Fig. 3c) calculated for the square-enclosed

areas shown in Fig. 3c confirms the nanotip to be monoclinic

W18O49. Fig. 3d presents the enlarged HRTEM image of the

same nanotip. The image clearly reveals that the inter-plane

distance (d-spacing) is 3.78 A and the monoclinic W18O49

nanotip was grown along [0 1 0] direction. For monoclinic

W18O49, the lowest energy surfaces are likely to be the {0 1 0}-

type surfaces, and the nanowires are inclined to be bounded by

{0 1 0}-type facets. Thus, the preferred growth direction of the

tungsten oxide nanowires is [0 1 0].

To research the morphologies of tungsten oxide nanos-

tructures vary with the temperature of substrate, nanowires

were grown at several different temperatures by changing the

placement of substrates downstream in the furnace. The role of

temperature on the structure determined how much the vapor

concentration would want to condense [14]. At a relatively high

temperature region of 500–550 8C, the irregular and curved

nanowires with 100–200 nm diameter and more than 10 mm

length were formed (Fig. 4a). As the temperature decrease to

400–450 8C, only shorter nanotip arrays can be found in

Fig. 4b. The tungsten oxide vapor concentration and the

supersaturation level seem to be high at high temperature and

Fig. 4. Tungsten oxide nanowires grown at different temperature of the

substrate: (a) 500–550 8C; (b) 400–450 8C.

the favorable temperature zone is the important reasons to form

the best nanowire arrays.

No catalyst was used during the evaporation, and the vapor–

solid growth mode may be suitable for the present growth

process [15]. The evaporated tungsten trioxide powder directly

deposited on ITO glass substrate and grew into one-

dimensional nanostructure. Their alignment can be explained

in term of a competition process [16]. Initially, a layer of

nanoparticles was first formed on the surface of the substrate.

Next, random tungsten oxide nanowires start to grow, and the

nanowires have no orientation ordering. As a result, the growth

of the nanowires follows two major steps: first, the nanowires

grown parallel or nearly parallel to the substrate surface will

terminate at some length when they hit other nanowires.

Second, the nanowires growing along normal or nearly normal

directions continue to growth to form large length nanowires.

Thus, the nanowires tend to be aligned as the growth continues.

Filed emission measurements were carried out at room

temperature. Considering the practical application of the

emitters, a relative poor vacuum of�1.0 � 10�5 Torr was used

to test their ability to withstand poor vacuum conditions. Fig. 5a

shows the current density–electric field (J–E) characteristics of

tungsten oxide nanotip arrays measured at different values of d.

The turn-on field, which is defined as the field required at a

current density of 10 mA cm�2, is about 6.2, 4.1, and

2.8 V mm�1 at d = 0.1, 0.2, and 0.3 mm, respectively.

Compared with the reported results for three-dimensional

tungsten oxide nanowire networks [11], lower turn-on field and

higher current density of field emission were obtained for

tungsten oxide nanotips array. As shown in Fig. 3d, the tungsten

oxide nanotip grown in the present work has a needle shape,

which leads to a higher aspect ratio. The nanotips are well

separated from each other (Fig. 1b), which decreases the screen

effect and increases the field enhancement factor. And the fact

that, as the cathode, the nanostructures are vertically grown on

the substrates benefits for the field emission.

The dependence of the emission current density on the

electric field follows Fowler–Nordheim (F–N) equation [11]

J ¼�

E2b2

f

�exp

��Bf3=2

Eb

�; (1)

where B = 6.83 � 109 eV�3/2 V m�1, b is the field enhance-

ment factor, and f is the work function of emitter material.

Fig. 5b shows the corresponding F–N plots of Fig. 5a. All the F–

N plots at different values of d are close to straight lines and in

agreement with F–N theory, which indicate that the emission of

the nanotips array is a tunneling and cold electron emission

process [17]. The field enhancement factor was calculated from

the slope (�Bf3/2b�1) of the F–N plot (ln J/E2 versus E�1)

assuming f is 5.7 eV. By analyzing the data shown in Fig. 5b, b

is estimated to be 1095, 1532, and 2116 as d is 0.1, 0.2, and

0.3 mm, respectively, which is high enough for various field

emission applications.

b is an important parameter for describing field emission,

and discussing the relationship between b and d is very helpful

for practicality of the technology in producing field emission

K. Huang et al. / Applied Surface Science 253 (2007) 8923–89278926

displays [18,19]. In order to explain the relationship between b

and d, a two-region FE model has been proposed to discuss the

relationship. As shown in Fig. 5c, in region d, the field Ed is

almost uniform, and in field enhanced region h, the field Eh is

significantly enhanced at the tips of the nanowires. b is usually

defined as b = Eh/E0, while the mean field of the vacuum gap E0

is given by E0 = (Edd + Ehh)/(d + h) [18]. Considering d� h,

we can obtain

1

b¼ h

dþ 1

b0

; (2)

Fig. 5. (a) Field emission curves of tungsten oxide nanotip arrays measured

with different electrode spacing. (b) The F–N plots of the emission current. Inset

is the relationship between 1/b and 1/d. (c) Schematic diagram about the field

distribution in the vacuum gap and the two-region FE model.

where b0 = Eh/Ed is the absolute enhancement factor, which is

determined by the emitting surface and independent of h, d, and

applied voltage. Therefore, the relationship between 1/b and 1/

d should be a linear function. As shown in inset of Fig. 5b, the

experimental curve is almost fitted to be a straight line, and can

be described by Eq. (2). Increasing the vacuum gap will gain

higher b or higher local filed (Eh). The electron can be easier

emitted from the nanotips at the higher Eh, and thus the turn-on

filed (E0) is relative lower.

The remarkable performance reveals that the tungsten oxide

nanotip arrays can be served as a good candidate for

commercial application in poor vacuum microelectronic

devices, particularly flat panel displays.

4. Conclusions

In conclusion, high-density, uniformly aligned tungsten

oxide nanotip arrays have been deposited on ITO glass

substrates by a conventional thermal evaporation technique

without any catalysts and additives. The temperature of

substrate was 450–500 8C. The nanowires are �5 mm in length

and 50–100 nm in diameter. Field emission behaviors of

tungsten oxide nanotip arrays were studied under a poor

vacuum at room temperature. The experimental data were well

in agreement with F–N theory. The excellent field emission

performances of the tungsten oxide nanotip arrays indicate that

they are a good candidate for application in vacuum

microelectronic devices.

Acknowledgements

The authors appreciate the financial support of the

Specialized Research Fund for the Doctoral Program of Higher

Education (No. 20040730029) and the Teaching and Research

Award Program for Outstanding Young Teachers in High

Education Institutions of MOE, China.

References

[1] K.B.K. Teo, E. Minoux, L. Hudanski, F. Peauger, J.-P. Schnell, L.

Gangloff, P. Legagneux, D. Dieumegard, G.A.J. Amaratunga, W.I. Milne,

Nature 437 (2005) 968.

[2] W.B. Choi, D.S. Chung, J.H. Kang, H.Y. Kim, Y.W. Jin, I.T. Han, Y.H. Lee,

J.E. Jung, N.S. Lee, G.S. Park, J.M. Kim, Appl. Phys. Lett. 75 (1999)

3129.

[3] R. Seelaboyina, J. Huang, W.B. Choi, Appl. Phys. Lett. 88 (2006)

194104.

[4] S.S. Fan, M.G. Chapline, N.R. Franklin, T.W. Tombler, A.M. Cassell, H.J.

Dai, Science 283 (1998) 512.

[5] V.N. Tondare, C. Balasubramanian, S.V. Shende, D.S. Joag, V.P. Godbole,

S.V. Bhorakar, M. Bhadbhade, Appl. Phys. Lett. 80 (2002) 4813.

[6] Z.L. Wang, R.P. Gao, W.A. de Heer, P. Poncharal, Appl. Phys. Lett. 80

(2002) 856.

[7] Y.H. Yang, B. Wang, N.S. Xu, G.W. Yang, Appl. Phys. Lett. 89 (2006)

043108.

[8] J. Zhou, L. Gong, S.Z. Deng, J. Chen, J.C. She, N.S. Xu, R. Yang, Z.L.

Wang, Appl. Phys. Lett. 87 (2005) 223108.

[9] G. Vida, V.K. Josepovits, M. Gyor, P. Deak, Microsc. Microanal. 9 (2003)

337.

[10] M. Gillet, R. Delamare, E. Gillet, Eur. Phys. J. D 34 (2005) 291.

K. Huang et al. / Applied Surface Science 253 (2007) 8923–8927 8927

[11] J. Zhou, Y. Ding, S.Z. Deng, L. Gong, N.S. Xu, Z.L. Wang, Adv. Mater. 17

(2005) 2107.

[12] L.F. Chi, N.S. Xu, S.Z. Deng, J. Chen, J.C. She, Nanotechnology 17 (2006)

5590.

[13] K.Q. Hong, M.H. Xie, H.S. Wu, Nanotechnology 17 (2006) 4830.

[14] S.H. Dalal1, D.L. Baptista, K.B.K. Teo, R.G. Lacerda, D.A. Jefferson, W.I.

Milne, Nanotechnology 17 (2007) 4811.

[15] W.Z. Pan, Z.R. Dai, Z.L. Wang, Science 291 (2001) 1947.

[16] J. Zhou, S.Z. Deng, L. Gong, Y. Ding, J. Chen, J.X. Huang, J. Chen, N.S.

Xu, Z.L. Wang, J. Phys. Chem. B 110 (2006) 10296.

[17] Q.H. Li, Q. Wan, Y.J. Chen, T.H. Wang, H.B. Jia, D.P. Yu, Appl. Phys.

Lett. 85 (2004) 636.

[18] D.Y. Zhong, G.Y. Zhang, S. Liu, T. Sakurai, E.G. Wang, Appl. Phys. Lett.

80 (2002) 506.

[19] X.Y. Xue, L.M. Li, H.C. Yu, Y.J. Chen, Y.G. Wang, T.H. Wang, Appl.

Phys. Lett. 89 (2006) 043118.