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Materials Letters 59 (2

Efficient field emission from tetrapod-like zinc oxide nanoneedles

Ke Yua,T, Yongsheng Zhangb, Rongli Xua, Shixi Ouyangc, Dongmei Lia, Laiqiang Luob,

Ziqiang Zhub, Jin Maa, Shijie Xiea, Shenghao Hana, Haoran Gengd

aDepartment of Microelectronics, School of Physics and Microelectronics, Shandong University, No. 5, Hongjialou,

Jinan, Shandong, 250100, People’s Republic of ChinabDepartment of Electronic Engineering, East China Normal University, Shanghai, 200062, People’s Republic of China

cState Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, Wuhan 430070,

P. R. China, and China Building Materials Academy, Beijing 100024, People’s Republic of ChinadSchool of Mater. Sci. and Eng. of Jinan University, Jinan, Shandong, 250022, People’s Republic of China

Received 24 November 2004; received in revised form 4 February 2005; accepted 10 February 2005

Available online 17 March 2005

Abstract

Tetrapod-like zinc oxide (ZnO) nanoneedles were fabricated using a simple and economical method of rapid heating high purity zinc

powders at 900 8C. No catalyst and vacuum were employed in the experiment. Field-emission measurements showed that the turn-on field of

the synthesized tetrapod-like ZnO nanoneedles was as low as 1.8 V/Am at the emission current density of 1.0 AA/cm2 and the emission

current density reached 1.0 mA/cm2 under an applied field of about 3.9 V/Am. The low turn-on field, high emission current density, and good

electron emission stability make the ZnO nanoneedles one of the promising candidates for field-emission displays.

D 2005 Elsevier B.V. All rights reserved.

PACS: 81.05.Dz; 81.10.Jt; 85.45.Db

Keywords: Zinc oxide nanoneedles; Thermal evaporation; Field emission

1. Introduction

Comprehensive research on carbon nanotubes (CNTs)

and their challenging applications has stimulated related

studies on other nanomaterials. Mechanical, physical, and

chemical properties, characteristic of a downsized geometry,

were displayed by certain nanostructures. Particularly, small

radii of curvature peculiar to low-dimensional nanostruc-

tures made some of them highly promising for electron field

emitter applications. CNTs have been widely investigated

for field emission (FE) for years, and there has been a large

number of laboratory work reported [1,2]. However, besides

the geometric factors, thermal stability and ambient insensi-

tivity are equally important to the operation of field emitters

[3]. As an oxide, zinc oxide (ZnO) exhibits high melting

0167-577X/$ - see front matter D 2005 Elsevier B.V. All rights reserved.

doi:10.1016/j.matlet.2005.02.001

T Corresponding author. Tel.: +86 531 642 1483; fax: +86 531 836 3848.

E-mail address: yk5188@263.net (K. Yu).

point (1975 8C) and large exciton binding energy (~60

meV) [4,5], which makes it have more thermal stability and

ambient insensitivity compared with CNTs. In addition,

ZnO is more resistant to radiation and oxidation, which is

essential to the application of FE. Therefore, ZnO-based

nanostructures are an appropriate alternative to CNT for

field-emission devices. The field-emission properties for

both ZnO nanowire arrays [6–8] and single ZnO nanotips

[9] have been studied, and this pioneering work demon-

strates that the FE properties of ZnO nanostructures are

fairly comparable to those of CNTs. However, the ZnO

nanorods reported in the literature [6–9] have relative large

diameter and a flat top end as well, which results in a small

aspect ratio. Such morphology may be a limitation to further

improvement of the FE properties [10]. Thus, to get better

ZnO-based field emitters, it is important to fabricate ZnO

nanostructures with sharp emission ends. In this paper, we

developed a catalyst-free method for mass production of

tetrapod-like ZnO nanoneedles. And a number of ZnO

005) 1866–1870

30 40 50 60 70

(201

)(112

)(2

00)

(103

)

(110

)

(102

)

(101

)

(002

)

(100

)

Inte

nsi

ty (

arb

. un

it)

2 Theta (degree)

Fig. 1. A typical XRD pattern of the ZnO nanoneedles synthesized by

thermal evaporation of metal zinc.

(a)

K. Yu et al. / Materials Letters 59 (2005) 1866–1870 1867

nanoparticles existed on the surface of some nanoneedles.

The field emission measurements confirmed that the

prepared ZnO nanoneedle film had a relatively low turn-

on field and uniform electron field emission with high

emission spot density.

(b)

Fig. 2. (a) A low-magnification SEM image of the synthesized ZnO

nanoneedles; (b) A SEM image of an individual tetrapod-like ZnO

nanoneedle. The inset highlights the one leg of the nanoneedle.

2. Experimental

The tetrapod-like ZnO nanoneedles were fabricated by

direct thermal evaporation of high purity zinc powders

(purity: 99.999%) under a flow of high-purity Ar as carrier

gas and air as reaction gas ambient. Neither metal catalyst

nor graphite/carbon additive was employed, which is

different from conventional thermal evaporation. A fused-

quartz tube was inserted into a horizontal tube furnace and

heated to 900 8C. High purity Zn powders (3.0 g) were

placed on a quartz plate, and then it was pushed to the center

of the quartz tube. After heating for about 3 min under a

constant flow of Ar (120 sccm), a cluster was observed on

the top of the zinc powders and while fog appeared in the

tube. A large amount of white products were found on the

surface of the plate after the quartz plate was pulled out of

the tube furnace slowly. Some products were ultrasonic

dispersed in ethanol for the morphology characterization by

scanning electron microscopy (SEM) [JEOL-JSM-6700 F].

The crystal structures of the synthesized products were

investigated by X-ray diffractometer (XRD)[D/max 2550 V,

Cu Ka radiation] and transmission electron microscopy

(TEM) [Philips Tecnai 20 U-TWIN].

A synthesized film with the thickness of about 20 Amwas formed on the nickel substrate by the electrophoretic

deposition (EPD) technique, which has been reported in our

previous paper [11]. Field emission measurements were

performed with diode structure in a vacuum chamber under

a pressure of 5�10-5 Pa. The synthesized film deposited on

the nickel substrate (as a cathode) was separated from a

phosphor/ ITO/glass anode by two Teflon spacers. The

measured area was 9�9 mm2. Through a window of the

vacuum chamber, the distribution of the field emission sites

on fluorescent anode was recorded with a camera. Mean-

while, the emission current versus voltage curve was

measured with standard electronic instruments after the bias

voltage sweeps were conducted several times for the emitter

to reach a stable emission for each given applied field.

3. Results and discussion

Fig. 1 displays a powder XRD pattern of the synthesized

product. The stronger diffraction peaks can be indexed as

those from the known wurtzite-structured ZnO with lattice

constants of a =0.324 nm and c =0.521 nm. No diffraction

peaks corresponding to Zn or other impurities are detected.

Fig. 2(a) shows a SEM image of the synthesized product.

Many needle-shaped ZnO nanostructures were observed.

K. Yu et al. / Materials Letters 59 (2005) 1866–18701868

The length of the needles is in the range of 5–10 Am. The

SEM image of a single tetrapod-like ZnO nanoneedle is

shown in Fig. 2(b). The crystal consists of four needle-

shaped tetrahedrally arranged legs connected at the center,

forming a tetrapod structure. One leg of the single ZnO

crystal is highlighted in the inset. Its hexagonal end planes

with a diameter of about 50 nm can be clearly seen, which

indicates that it grows along the b0001N direction. Fig. 3(a)

shows a low-magnification TEM image of the tetrapod-like

ZnO nanoneedle. The diameters of the nanoneedles gradu-

ally decrease from the root to form a sharp tip. The

magnified image of several nanoneedles is shown in the

inset of Fig. 3(a). The apex angle of the tip is estimated as

small as 88, exhibiting excellent configuration of the as-

synthesized nanoneedles for field emission applications.

The structural details of the tetrapod-like ZnO nanoneedles

were analyzed using HRTEM. The HRTEM image shown in

Fig. 3(b) reveals the atomic structure of a leg of the

tetrapod-like ZnO nanoneedles, while the inset shows a

(c)

(a)

Fig. 3. (a) A low-magnification TEM image of the tetrapod-like ZnO nanoneedle

image of a single nanoneedle. The growth direction is along b0001N direction. Th

image of a number of ZnO single crystal particles on the surface of some nanon

selected area electron diffraction pattern. The continuous

lattice fringes indicate that the nanoneedle is of single

crystal nature, and the growth direction of every leg is along

b0001N c axis, which is in agreement with the result drawn

from the inset in Fig. 2(b). More interestingly, The HRTEM

image in Fig. 3(c) reveal a number of ZnO single crystal

particles with average size of 4 nm on the surface of some

nanoneedles. These nanoparticles with sharp geometry

morphology as emitters have a high field enhancement

factor, which could make it easy to emit electron.

The growth mechanism of the tetrapod-like ZnO nano-

structures in our experiment is different from the conven-

tional vapor–liquid–solid (VLS) growth mechanism because

no metal catalyst was employed. Since Zn evaporation is a

violent breakout process at 900 8C, the amount of oxygen is

limited and some suboxides (ZnOx x b1) should form. Zn

and Zn suboxides (melting point about 419 8C) should be in

vapor phases in the beginning of our experiment [12]. At

relatively low temperature, Zn and ZnOx vapors should

(b)

s. The inset is a magnified image of several nanoneedles. (b) The HRTEM

e inset shows the selected area electron diffraction pattern. (c) The HRTEM

eedles.

0 1 3 4 6 82 5 7500

750

1000

1250

1500

Time (hour)

(a)

(b)

2.0 2.5 3.0 3.5 4.0 4.5 5.0

0

1x103

2x103

3x103

300 µm 200 µm 100 µm

Field strength (V/µm)

0.2 0.3 0.4 0.5-2

0

2

4

Ln

(J/E

2 )(µ

Aµ

m2 V

-2)

1/E(µmV -1)

µµµ

µµ

µ

Cu

rren

t d

ensi

ty (

µA /c

m2 )

µ

µ

Cu

rren

t d

ensi

ty (

ÍA/c

m2 )

Fig. 4. (a) Emission current density versus electric field (J–E) plots of ZnO

nanoneedle films at different anode-sample separations and their corre-

sponding F–N plots (inset). (b) A curve of emission current density versus

time recorded at an applied electric field of 4.0 V/Am.

K. Yu et al. / Materials Letters 59 (2005) 1866–1870 1869

condense into liquid droplets. These droplets, as growing

nuclei, react with oxygen in tube furnace and grow up

gradually into one-dimensional ZnO nanostructures. Based

on the above explanation, it is assumed that the growth of

the products is a Zn self-catalytic VLS process. Why does

ZnO show a tetrapod-like morphology? The reason is not

very clear up to now. A possible growth mode based on a

(a) (b)

Fig. 5. The electron emission images of the ZnO nanoneedles on a fluorescent scree

(c) at an anode-sample separation of 300 Am.

multiple inversion-twin embryo is proposed by Iwanaga et

al. [13].

Fig. 4(a) shows three typical curves of an emission

current density as a function of an applied electric field

measured at different anode-sample separations of 100 Am,

200 Am, and 300 Am, respectively. The turn-on and

threshold field, defined as the electric field required to

produce a current density of 1 AA/cm2 and 1 mA/cm2,

respectively, were sensitive to the anode-sample separation,

A larger separation led to lower turn-on and lower threshold

fields. This might originate from the effect of field

distribution on active emitters at different separations. A

turn-on field of 1.8 V/Am and a threshold field of 3.9 V/Amwere obtained at a 300 Am spacing, whereas 2.2 V/Am and

4.6 V/Am were determined at a 100 Am. The turn-on field is

much lower than that of the aligned ZnO nanowires [7], and

comparable to the turn-on field (2.6 V/Am) of multi-wall

carbon nanotubes (MWNTs) [14]. Since the work function

of ZnO is very large (about 5.3 eV), the very low turn-on

field and threshold field of the synthesized products may be

attributed to high aspect ratio of the tetrapod-like ZnO

nanoneedles. The inset displays the corresponding Fowler–

Nordheim (FN) plots obtained at the different anode-sample

separations. These straight lines indicated that the emitting

electrons were mainly resulted from field emission [15,16].

The approximately parallel F–N plots also suggested that

the emission was independent of the anode-sample distance,

only a function of the applied voltage. According to FN

theory, field emission current density J can be expressed as

ln( J /E2=� (BU3 / 2 /b)V–1+ ln (A a b2 /U), herein E, the

applied field, b, field enhancement factor, U, the work

function of the emitter surface (eV), B and A, constants, and

a, the effective emission area. The slope the FN plots is

related to the work function of the field emitters, while the

line intercepts on the vertical axis are related to both the

work function and the actual emission area of the active

emitters. Assuming the work function of the emitter remains

constant during the emission process, the different intercepts

represent the change of effective emission area at various

anode–cathode distances. As the anode-sample distance

decreases, the electrical field on the cathode surface

becomes less uniform, which results in a smaller effective

(c)

n under different applied fields of 3.5 V/Am (a), 3.9 V/Am (b), and 4.5 V/Am

K. Yu et al. / Materials Letters 59 (2005) 1866–18701870

emission area or a smaller intercept. Emission stability of

the sample was tested at a constant electric field of 4.0 V/

Am, as shown in Fig. 4(b). No obvious degradation of

current density was observed. The emission current fluctua-

tion was with F3% during 8 h while the current density was

kept at 1000 AA/cm2, which exhibits a better emission

stability than CNTs. Such good emission stability makes the

ZnO nanoneedles highly valuable for practical application

as field emitters and an appropriate alternative to CNTs. Fig.

5 shows the electron emission images of the ZnO nano-

needles film on the fluorescent screen under different

applied field of 3.5 V/Am, 3.9 V/Am, and 4.5 V/Am at

anode-sample separations of 300 Am. At the applied field of

4.5 V/Am, the emission spot density (ESD), namely the

electron emission spot number per unit area, could be

estimated to be ~104 cm-2 from the magnified photograph.

The homogeneous emission and high ESD of the sample

may be attributed to a large number of ZnO nanoparticles as

emitters on the surface of the nanoneedles contributing to

emission.

4. Conclusions

Tetrapod-like ZnO nanoneedles were successfully syn-

thesized by direct thermal evaporation of high purity metal

zinc powders. The synthesized films exhibited very low

turn-on field of 1.8 V/Am at the current density of 1.0 AA/

cm2. The emission current density reached 1 mA/cm2 at a

bias electrical field of 3.9 V/Am. The low turn-on field and

threshold field is comparable to that for CNTs, which

indicates that low-dimensional ZnO is a promising material

for field emission displays and could be a substitute to CNTs

for field emission emitters.

Acknowledgments

This work was supported by Foundation of State Key

Laboratory of Advanced Technology for Materials Syn-

thesis and Processing (Wuhan University of Technology,

China, No. WUT2004 Z01), the NSF of China

(No.60476004), and GS of East China Normal University.

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