controllable hydrothermal synthesis of zno nanowires arrays on al-doped zno

Applied

Surface

Science

257 (2011) 10134–

10140
Contents

lists

available

at

ScienceDirect
Applied

Surface

Science
jou

rn

al

h

om

epa

g

e:

www.elsevier.com/locate/apsusc
Controllable

hydrothermal

synthesis

of

ZnO

nanowires

arrays

on

Al-doped

ZnO
seed

layer

and

patterning

of

ZnO

nanowires

arrays

via

surface

modification

of
substrate
Jin

Zhang
a
,

Wenxiu

Que
a,∗
,

Qiaoying

Jia
a
,

Xiangdong

Ye
b
,

Yucheng

Ding
b
a
Electronic

Materials

Research

Laboratory,

School

of

Electronic

and

Information

Engineering,

Xi’an

Jiaotong

University,

Xi’an

710049,

Shaanxi,

People’s

Republic

of

China

b
State

Key

Laboratory

of

Manufacturing

Systems

Engineering,

Xi’an

Jiaotong

University,

Xi’an

710049,

Shaanxi,

People’s

Republic

of

China
a

r

t

i

c

l

e

i

n

f

o
Article

history:
Received

11

December

2010
Received

in

revised

form

7

June

2011
Accepted

10

June

2011
Available online 7 July 2011
Keywords:
Zinc

oxide

Nanowires
Seed

layer
Fluorination
Photoluminescence
a

b

s

t

r

a

c

t
ZnO

nanowire

(NW)

arrays

are

assembled

on

the

Al-doped

ZnO

(AZO)

seed

layer

by

a

hydrothermal
process.

Effects

of

the

temperature

and

growth

time

of

the

hydrothermal

process

on

morphological

and
photoluminescence

properties

of

the

as-assembled

ZnO

NW

arrays

are

characterized

and

studied.

Results
indicate

that

the

length

and

diameter

of

the

ZnO

NWs

increase

with

a

lengthening

of

the

growth

time
at

80
◦
C

and

the

hydrothermal

temperature

has

a

significant

effect

on

the

growth

rate

and

the

photo-
luminescence

properties

of

the

ZnO

NW

arrays.

The

patterned

AZO

seed

layer

is

fabricated

on

a

silicon
substrate

by

combining

a

sol–gel

process

with

an

electron-beam

lithography

process,

as

well

as

a

surface

fluorination

technique,

and

then

the

ZnO

NW

arrays

are

selectively

grown

on

those

patterned

regions
of

the

AZO

seed

layer

by

the

hydrothermal

process.

Room-temperature

photoluminescence

spectra

of
the

patterned

ZnO

NW

arrays

shows

that

only

a

strong

UV

emission

at

about

380

nm

is

observed,

which
implies

that

few

crystal

defects

exist

inside

the

as-grown

ZnO

NW

arrays.
© 2011 Elsevier B.V. All rights reserved.
1.

Introduction
ZnO

is

a

semiconductor

with

exceptional

electronic

and

pho-
tonic

properties

as

well

as

great

thermal

stability

and

oxidation
resistance.

Recent

developments

and

capacity

to

synthesize

ZnO
nanostructures

with

different

shapes

[1–3]

have

led

to

novel
and

enhanced

properties

as

compared

to

its

bulk

form,

and

thus
enabling

it

to

have

many

attractive

applications.

For

example,

it
can

be

used

as

a

potential

material

for

nanodevice

assembly

and
applications

in

blue-UV

light

emitters

[4]

and

photodetectors

[5],
field

emission

devices

[6],

and

dye-sensitized

solar

cells

[7],

etc.
Indeed,

well-aligned

ZnO

nanowire

(NW)

arrays

have

been

formed

on

GaN,

AlN,

Al
1−x
Ga
x
N,

6H–SiC,

and

ZnO

buffer

layers

[8–10],

but
the

optical

properties

of

the

NWs

grown

on

buffer

layers

have

been
scarcely

investigated,

especially

with

respect

to

impurity

and

defect
distribution,

which

can

hinder

the

applications

of

the

NW

arrays.
In

recent

years,

a

number

of

the

ZnO

thin

films

doped

with

various
metallic

ions

have

been

extensively

studied

for

the

manipulation
of

their

optical

and

electrical

properties,

the

Al-doped

ZnO

(AZO)
thin

films

are

attractive

due

to

their

good

conductivity,

high

trans-
∗
Corresponding

author.

Tel.:

+86

29

82668679;

fax:

+86

29

82668794.
E-mail

address:



(W.

Que).
parency

and

relatively

low

cost

[11,12].

In

view

of

this,

the

use
of

a

lattice-matched

and

conducting

buffer

layer

may

circumvent
the

problem

and

lead

to

potential

integration

with

silicon

micro-
electronics

[13–15].

Therefore,

the

luminescent

and

electron

field
emission

properties

of

the

ZnO

NW

arrays

grown

on

the

AZO

seed
layers

are

also

reported

by

many

research

groups

[16,17].

Further-
more,

in

order

to

achieve

an

immense

potential

of

the

ZnO

NW
arrays,

it

is

important

and

necessary

to

have

a

good

control

for

the
spatial

arrangements

and

properties

of

the

ZnO

NW

arrays

[16].
In

this

paper,

the

ZnO

NW

arrays

were

grown

on

AZO

seed
layer,

which

was

deposited

by

a

sol–gel

process,

by

a

hydrother-
mal

method,

and

effects

of

the

temperature

and

growth

time

of
the

hydrothermal

process

on

the

morphological

and

photolumines-
cence

properties

of

the

as-grown

ZnO

NW

arrays

were

also

studied
and

discussed.

In

addition,

what

we

believe

to

be

the

first

report
on

the

fabrication

of

the

patterned

AZO

seed

layer

on

the

silicon
substrate

by

combining

a

sol–gel

process

with

an

electron-beam
lithography

process,

as

well

as

a

surface

fluorination

technique,
which

can

eliminate

the

effect

of

the

electron-beam

resist

on

the
boundary

of

the

patterned

AZO

seed

layer,

and

thus

the

ZnO

NW
arrays

could

be

successfully

grown

on

the

patterned

regions

of

the
AZO

seed

layer

by

employing

the

hydrothermal

process.

Further-
more,

the

photoluminescene

properties

of

the

selectively

grown
ZnO

NW

arrays

were

also

characterized

and

investigated.
0169-4332/$

–

see

front

matter ©

2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.apsusc.2011.06.163
J.

Zhang

et

al.

/

Applied

Surface

Science

257 (2011) 10134–

10140 10135
Fig.

1.

Schematic

representation

of

the

patterning

process

of

the

ZnO

NW

arrays

on

silicon

substrate.
2.

Experimental
2.1.

Preparation

of

the

AZO

seed

layer
In

order

to

compare

the

properties

of

the

ZnO

NW

arrays

grown
on

different

seed

layers,

the

ZnO

and

AZO

seed

layers

were

pre-
pared

by

the

sol–gel

technique.

Here,

the

Al-doped

concentration
of

the

AZO

seed

layer

was

1.0

at.%

with

respect

to

Zn

due

to

its
relatively

outstanding

performance

than

other

doping

concentra-
tions

as

shown

in

our

previous

report

[18].

The

ZnO

and

AZO
sols

were

prepared

as

follows.

In

briefly,

Zn(CH
3
COO)
2
·2H
2
O

was

first

dissolved

in

a

2-methoxyethanol

monoethanolamine

(MEA)-
deionized

water

solution

at

room

temperature.

The

molar

ratio

of

the

MEA

and

deionized

water

to

zinc

acetate

was

fixed

at

1
and

0.5,

respectively,

and

the

concentration

of

the

zinc

acetate
was

0.75

mol/L.

For

the

AZO

sol,

an

appropriate

amount

of

alu-
minum

doping

was

obtained

by

adding

AlCl
3
·6H
2
O

to

above

the

as-prepared

precursor

solution.

Then,

the

final

solution

was

stirred
at

60
◦
C

for

30

min

until

to

yield

a

clear

and

homogeneous

solution.
The

ZnO

and

AZO

seed

layers

were

deposited

onto

a

quartz

glass
substrate

by

a

multi-spin-coating

process

for

20

s

at

3000

rpm.

It

should

be

mentioned

that

after

spin-coating

one

layer,

the

coated
sample

should

be

preheated

in

the

air

at

200
◦
C

for

10

min

and
thus

the

one-layer

thin

film

with

about

50

nm

thick

can

be

eas-
ily

obtained.

Finally,

the

sample

with

three

layers

was

post-heated

at

a

temperature

of

500
◦
C

for

1

h

in

the

air

[19].
2.2.

Hydrothermal

synthesis

of

the

ZnO

NWs

on

the

ZnO

and

AZO
seed

layers
Vertically

aligned

ZnO

NW

arrays

were

grown

in

a

Teflon-lined
stainless

steel

autoclave

by

immersing

the

substrates

deposited
with

the

ZnO

or

AZO

seed

layers

into

the

mixed

aqueous

solu-
tion,

which

includes

Zn(NO
3
)
2
(0.04

mol/L)

and

NaOH

(0.8

mol/L),
at

80–180
◦
C

for

1–3

h.

The

obtained

samples

were

then

washed

by
the

deionized

water

and

dried

in

the

air

at

80
◦
C

[20].
2.3.

Fabrication

of

the

patterned

ZnO

NW

arrays

on

silicon
substrate
Fig.

1

shows

the

fabrication

process

of

the

patterned

ZnO

NW
arrays

on

the

silicon

substrate.

Here,

the

electron-beam

resist
(ZEP520A

from

Zeon

Corp.)

was

first

spin-coated

on

the

silicon

sub-
strate

and

followed

the

coated

sample

was

put

in

an

oven

to

prebake
at

180
◦
C

for

30

min.

Then,

the

prebaked

sample

was

exposed

for
patterning

at

30

kV

under

a

high-resolution

electron-beam

lithog-
raphy

system

(CABL-9000C

Crestec

Corp.).

Subsequently,

the

silicon
substrate

with

the

patterned

resist

was

immerged

and

rinsed

in
a

ZMD-B

(Methyl

isobutyl

ketone

89%

and

Isopropyl

alcohol

11%)
solution

for

1

min

to

remove

the

exposed

EB-resist.

To

make

low
surface

energy

coatings

on

the

substrate,

the

silicon

substrate

with
patterned

resist

was

immerged

into

the

solution,

which

consists
of

2.0

vol.%

(Heptadecafluoro-1,1,2,2-tetradecyl)

trimethoxysilane
(SC-1060F,

from

Sicong

New

Materials

Corp.),

0.5

vol.%

acetic

acid
and

97.5

vol.%

isopropyl

alcohol,

for

2

h

and

then

picked

out.

Fol-
lowed

that

the

immerged

sample

was

heated

at

150
◦
C

in

an

oven
for

1

h

and

cooled

down

to

room

temperature,

the

residual

resist
was

then

removed

from

the

silicon

substrate

by

rinsing

it

with
chlorobenzene,

thus,

the

template

was

obtained.

Finally,

the

AZO
sol

was

spin-coated

on

the

patterned

silicon

substrate

and

the

ZnO
NW

arrays

were

selectively

grown

on

the

patterned

regions

of

the
AZO

seed

layer

by

the

hydrothermal

process.
The

structural

properties

of

the

ZnO

NW

arrays

were

char-
acterized

by

using

a

D/max-2400

X-ray

diffraction

spectrometer
(Rigaku)

with

Cu

K␣

radiation

and

operated

at

40

kV

and

100

mA
from

20

to

70
◦
in

2Â,

and

the

scanning

speed

was

15
◦
min
−1
at

a

step
of

0.02
◦
.

The

morphological

properties

of

the

ZnO

NW

arrays

were
observed

by

a

JEOL

JSM-7000F

field-emission

scanning

electron
10136 J.

Zhang

et

al.

/

Applied

Surface

Science

257 (2011) 10134–

10140
Fig.

2.

(a)

SEM

image

of

the

ZnO

seed

layer,

(b)

SEM

image

of

the

AZO

seed

layer.
microscopy

(FE-SEM).

The

UV–vis

absorption

spectra

and

transmit-
tance

spectra

of

the

ZnO

NW

array

films

were

characterized

by

a
JASCO

V-570

UV/VIS/NIR

spectrometer

and

the

photoluminescence
spectra

of

the

ZnO

NW

arrays

were

measured

at

room

temperature
by

a

FLUOROMAX-4

spectrometer.
3.

Results

and

discussion
SEM

images

and

XRD

patterns

of

both

the

ZnO

and

AZO

seed
layer,

which

are

deposited

on

the

quartz

glass

substrate,

are

showed
in

Figs.

2

and

3,

respectively.

It

can

be

observed

from

Fig.

2

that
the

AZO

seed

layer

has

a

smaller

grain

size

as

compared

to

that
of

the

ZnO

seed

layer,

and

the

(0

0

2)

diffraction

peak

of

the

AZO
seed

layer

in

intensity

is

higher

than

that

of

the

ZnO

seed

layer
as

seen

in

Fig.

3.

It

is

indicated

that

the

AZO

seed

layer

has

a

better
crystalline

orientation

(0

0

2)

than

that

of

the

ZnO

seed

layer,

which
coincides

with

those

reported

in

our

previous

work

[18].

Fig.

4(a)
shows

the

transmittance

spectra

of

the

ZnO

and

AZO

seed

layers
as

well

as

the

corresponding

ZnO

NW

arrays

grown

on

these

seed
layers.

Results

indicate

that

all

the

ZnO

and

AZO

thin

films

exhibit

a
transmittance

of

higher

than

85%

in

the

visible

region.

However,

it
is

worthy

to

note

that

the

transmittance

of

the

AZO

thin

film

layer
is

obviously

higher

(90%)

than

that

of

the

ZnO

thin

film

layer,

which
is

probably

related

to

the

optimized

crystalline

orientation

of

the
(0

0

2)

and

the

seed

layer

with

smaller

grain

size.

Furthermore,

the
transmittance

of

the

ZnO

NW

arrays,

which

are

grown

on

the

ZnO
20 30 40 50 60 70
Intensity (a.u.)
2 Th

eta / de

gree
ZnO
AZO
(002)
Fig.

3.

XRD

patterns

of

the

ZnO

and

AZO

seed

layers.
or

AZO

seed

layer

at

80
◦
C

for

1

h,

is

still

above

40%

in

the

visible
region.

In

addition,

the

transmittance

of

the

ZnO

NW

arrays

grown
on

the

AZO

seed

layer

is

lower

than

that

grown

on

ZnO

seed

layer
owing

to

its

high

light

scattering

and

decrease

light

transmittance
[18].

Fig.

4(b)

shows

the

absorption

spectra

of

the

obtained

samples.
It

is

found

that

the

absorption

peak

of

the

AZO

thin

film

layer

has

a
blue

shift

as

compared

to

that

of

the

ZnO

thin

film

layer

due

to

the
Burstein–Moss

effect

[21,22].
300 400 500 600 700 800
30
40
50
60
70
80
90
100
(a) (b)
Trans (%)
Wavelength

(nm)

1
2
3
4
5
325 350 375 400 425
0.0
0.1
0.2
0.3
0.4
0.5
0.6
ZnO
AZO
Abs (a.u.)
Wavelength /

nm
Fig.

4.

(a)

Transmittance

spectra

of

the

quartz

glass

(curve

1),

AZO

thin

film

(curve

2),

ZnO

thin

film

(curve

3),

ZnO

NW

arrays

grown

on

the

ZnO

seed

layer

at

80
◦
C

for

1

h

(curve

4)

and

ZnO

NW

arrays

grown

on

the

AZO

seed

layer

at

80
◦
C

for

1

h

(curve

5).

(b)

Absorption

spectra

of

the

ZnO

and

AZO

thin

films.
J.

Zhang

et

al.

/

Applied

Surface

Science

257 (2011) 10134–

10140 10137
Fig.

5.

SEM

images

of

the

ZnO

NWs

grown

on

the

ZnO

and

AZO

seed

layers:

(a),

(b)

and

(c)

are

SEM

images

of

the

ZnO

NWs

grown

on

the

ZnO

seed

layer

at

80
◦
C

for

1

h,
2

h

and

3

h,

respectively,

(d),

(e)

and

(f)

are

SEM

images

of

the

ZnO

NWs

grown

on

the

AZO

seed

layer

at

80
◦
C

for

1

h,

2

h

and

3

h,

respectively,

(g)

and

(h)

are

SEM

images

of
the

ZnO

NWs

grown

on

the

ZnO

seed

layer

for

1

h

at

130

and

180
◦

C,

respectively,

(i)

and

(j)

are

SEM

images

of

the

ZnO

NWs

grown

on

the

AZO

seed

layer

for

1

h

at

130

and
180
◦
C,

respectively.
Fig.

5

shows

the

SEM

images

of

the

ZnO

NW

arrays

grown

on
the

ZnO

and

AZO

seed

layer

at

80–180
◦
C

for

1–3

h,

respectively.
The

insets

as

shown

in

Fig.

5

are

the

cross-section

of

the

ZnO

NWs
arrays.

Fig.

6

shows

that

the

TEM

images

and

the

selected

area
electron

diffraction

(SAED)

pattern

of

the

ZnO

NWs

which

grown
on

the

AZO

seed

layer

at

80
◦
C

for

1

h.

Fig.

6(a)

is

a

typical

low-
magnification

image

of

the

synthesized

ZnO

NW.

The

diameter

of
the

tip

is

slightly

smaller

than

the

bottom.

The

atomic

arrangements
10138 J.

Zhang

et

al.

/

Applied

Surface

Science

257 (2011) 10134–

10140
Fig.

6.

TEM

images

of

the

ZnO

NWs

grown

on

the

AZO

seed

layer

(a)

low-magnification

image,

(b)

high-magnification

image,

(c)

corresponding

selected

area

electron
diffraction

pattern

(SAED).
of

the

ZnO

NW

are

seen

in

Fig.

6(b).

It

clearly

shows

the

ZnO

(0

0

2)
fringes

perpendicular

to

the

wire

axis

are

on

average

separated

by
0.26

nm,

indicating

the

crystalline

ZnO

NWs

growth

along

the

ZnO
(0

0

2)

direction.

Also,

the

diffraction

pattern

confirms

that

the

ZnO
NWs

have

a

single

crystalline

growth

along

ZnO

(0

0

2)

as

shown
in

Fig.

6(c).

In

addition,

the

similar

results

are

also

observed

for

the
ZnO

NWs

grown

on

the

ZnO

seed

layer

at

80
◦
C

for

1

h.

The

values
of

the

length

and

diameter

of

the

ZnO

NWs,

which

is

obtained

from
Fig.

5

are

summarized

in

Fig.

7.

Corresponding

to

the

different

seed
layers

(ZnO,

AZO),

the

ZnO

NWs

grown

on

the

ZnO

seed

layer

are
labeled

as

ZnO-NWZ,

and

the

ZnO

NWs

grown

on

the

AZO

seed
layer

are

labeled

as

ZnO-NWA.

It

can

be

seen

from

Fig.

5

that

all

the
ZnO

NW

arrays

obtained

under

different

hydrothermal

conditions
are

vertically

aligned

on

the

seed

layers.

For

the

hydrothermal

tem-
perature

at

80
◦
C

and

the

growth

time

between

1

and

3

h,

the

length
and

diameter

of

the

ZnO-NWZ

and

the

ZnO-NWA

increase

with

a
lengthening

of

the

growth

time

as

shown

in

Fig.

7.

However,

it

is
also

interesting

to

note

for

the

same

growth

time

that

the

length

of
the

ZnO-NWA

is

much

longer

that

of

the

ZnO-NWZ

and

the

diam-

eter

of

the

ZnO-NWA

is

much

smaller

than

that

of

the

ZnO-NWZ.
These

results

are

probably

related

to

the

crystal

grain

size

of

the
seed

layer,

that

is

to

say,

the

bigger

the

crystal

grain

size

is,

the
shorter

and

wider

the

grown

ZnO

NW

is

as

shown

in

Fig.

5(a)–(f).
In

other

words,

the

aspect

ratio

of

the

ZnO-NWA

is

higher

than

that
of

the

ZnO-NWZ.

Moreover,

the

distance

among

the

ZnO-NWA

is
bigger

than

that

among

the

ZnO-NWZ.

That

is

because

the

distance
among

the

ZnO

NWs

is

also

dependent

on

the

crystal

interspaces
and

the

crystal

grain

size

of

the

seed

layer.

As

can

be

seen

in

Fig.

2,
the

quantity

of

the

crystal

grains

within

the

unit

area

of

the

AZO

seed

layer

is

more

than

that

of

the

ZnO

seed

layer,

which

leads

to

a
larger

number

of

the

ZnO

NWs

can

be

grown

on

the

AZO

seed

layer
as

shown

in

Fig.

5.

Furthermore,

it

is

also

observed

for

the

same
growth

time

(1

h)

but

different

hydrothermal

temperatures

that

the
length

of

the

ZnO-NWZ

increases

and

the

diameter

of

the

ZnO-NWZ
enlarge

extremely

with

the

increase

of

the

hydrothermal

tempera-
ture

as

compared

to

that

of

the

ZnO-NWA.

A

reasonable

explanation
is

that

the

small

crystal

grain

of

the

AZO

seed

layer

restricts

the
cross-growth

of

the

ZnO

NWs.

The

length

of

the

ZnO-NWA

also
increases

with

the

raising

of

the

hydrothermal

temperature

from
80
◦
C

to

130
◦
C,

but

when

the

hydrothermal

temperature

is

further
up

to

180
◦
C,

the

length

of

the

ZnO-NWA

is

shorter

than

that

of

the
ZnO-NWA

grown

at

130
◦
C.

A

possible

explanation

is

as

follows.

As
reported

in

Refs.

[23,24],

the

hydrothermal

synthesis

of

the

ZnO
NWs

is

a

dynamic

balance

process

as

follows.
[Zn(OH)
n
]
n−2−
→

ZnO

+

H
2
O

+

[OH]
−
,

(n

=

2,

4)

(1)

ZnO

+

2[OH]
−
→

[ZnO
2
]
2−
+

H
2
O

(2)
Thus,

the

[Zn(OH)
n
]
n−2−
groups

dehydrate

at

the

surface

of

the
ZnO

seed

layer

to

form

the

ZnO

molecules,

H
2
O

molecules

and
[OH]
−
,

and

the

formed

[OH]
−
dissolves

the

ZnO

molecules

to

form
[ZnO
2
]
2−

groups

at

the

same

time.

While

the

supersaturation

of
the

[Zn(OH)
n
]
n−2−
groups

is

high

enough,

the

growth

rate

of

the
ZnO

NWs

will

be

much

higher

than

the

dissolution

rate.

While

the
hydrothermal

temperature

increased

to

130
◦
C,

the

supersatura-
tion

and

the

molecule

energy

of

the

[Zn(OH)
n
]
n−2−
groups

achieve
the

best

values,

which

leads

to

a

fast

growth

rate

of

the

ZnO
NWs.

While

the

hydrothermal

temperature

further

increases

up
1

2

3

4

5
1
2

3
(a)

(b)
Length of th

e Zn

O NWs / µm


Growth time / hour
80
100
120
140
160
180
Temperature of chemical bath /
o
C
0 50 100 150 200 250 300 350
1
2
3
Diameter of th

e Zn

O NWs / n

m
Growth time / hour
80
100
120
140
160
180
Temperature of chemical bath /
o
C
Fig.

7.

Effect

of

the

hydrothermal

growth

time

and

temperature

on

the

length

and

diameter

of

the

ZnO

NWs:

(a)

a

relationship

between

the

length

of

the

ZnO

NWs

and

the
hydrothermal

growth

time

and

temperature;

(b)

a

relationship

between

the

diameter

of

the

ZnO

NWs

and

the

hydrothermal

growth

time

and

temperature.
J.

Zhang

et

al.

/

Applied

Surface

Science

257 (2011) 10134–

10140 10139
to

180
◦
C,

the

growth

rate

of

the

ZnO

NWs

decreases

due

to

the
decrease

of

the

supersaturation

of

the

[Zn(OH)
n
]
n−2−
groups.

It
can

be

concluded

based

on

above

results

and

discussion

that

the
growth

rate

of

the

ZnO

NWs

is

determined

by

the

supersaturation
of

the

[Zn(OH)
n
]
n−2−
groups

and

the

concentration

of

the

[OH]
−
in
the

solution

and

the

fastest

growth

rate

of

the

ZnO-NWA

can

be
obtained

at

130
◦
C.

Due

to

the

effect

of

the

Al-doping

on

the

seed
layer,

the

ZnO-NWA

has

a

higher

aspect

ratio

than

that

of

the

ZnO-
NWZ,

which

is

more

suitable

for

those

potential

applications

in

the
dye-sensitized

solar

cells,

luminescent

and

electron

field

emission
devices.
Fig.

8

shows

room

temperature

photoluminescence

(PL)

spectra
(excite

at

365

nm,

1

nm

slit

width)

of

the

ZnO

NW

arrays

grown

on
the

AZO

seed

layers

at

different

temperatures

of

80
◦

C,

130
◦
C

and
180
◦
C

for

1

h,

respectively.

The

inset

shows

the

partial

enlarged

PL

spectra

from

390

to

420

nm.

The

intensities

of

these

PL

spectra
are

also

normalized

by

the

thickness

of

the

ZnO

NW

array

films.

It
can

be

seen

that

with

the

increase

the

hydrothermal

temperature,
the

position

of

the

peaks

occurred

red-shift

and

the

intensity

of

the
peaks

decreases

gradually.

It

is

probably

related

to

an

increase

of
the

crystal

defects

in

the

ZnO

NW

arrays

due

to

higher

growth

tem-
perature

[25–27].

When

the

ZnO

NWs

grow

at

80
◦
C,

a

sharp

and
strong

UV

peak

at

380

nm,

which

can

be

assigned

to

the

intrinsic
excitation

of

ZnO,

dominates

the

PL

spectra

and

no

other

peaks

are
observed

in

the

spectrum

curve,

indicating

that

few

crystal

defects
exist

in

the

ZnO

NW

arrays

grown

on

the

AZO

seed

layer

at

80
◦
C
for

1

h.

It

should

be

mentioned

here

that

the

similar

results

are
also

observed

for

the

ZnO

NW

arrays

grown

on

the

AZO

seed

layer
at

80
◦
C

for

longer

growth

time.

However,

with

the

increase

the
hydrothermal

temperature

to

130
◦
C

or

above,

a

relative

weak

band
380 390 400 410 420 430 440 450
0.0
5.0x10
5
1.0x10
6
1.5x10

6
2.0x10
6
Normalized Intensity (a.u.)
Wavelength /

nm
ZnO

NWs

gro

wn at

80
o
C for

1h
ZnO

NWs

gro

wn at

130
o

C for

1h
ZnO

NWs

gro

wn at

180
o
C for

1h
390 400 410 420
Intensity (a.u.)
Wavelength

/ nm
Fig.

8.

Normalized

PL

spectra

of

the

ZnO

NW

arrays

grown

on

the

AZO

seed

layer
for

1

h

at

80,

130

and

180
◦
C,

respectively.
between

390

and

420

nm

can

be

clearly

observed

as

shown

in

the
inset

of

Fig.

7,

indicating

that

some

crystal

defects

start

to

occur

due
to

higher

growth

temperature,

which

may

lead

to

the

red-shift

of
the

PL

peaks

and

the

decrease

of

the

PL

peaks

in

intensity.

That

is

to
say,

with

the

increase

the

hydrothermal

temperature,

the

dissolu-
tion

rate

of

the

[OH]
−
will

be

intensified

on

the

surface

of

the

ZnO
NWs,

and

it

is

probably

to

lead

to

more

crystal

defects

in

the

ZnO
NWs

due

to

the

corrosiveness

of

the

aqueous

solution.
As

shown

in

Fig.

1,

the

silicon

substrate

is

first

patterned

with

EB
resist

and

EB

exposal,

then

the

low

surface

energy

coating

over

the
outside

of

the

patterned

region

is

derived

by

the

surface

fluorizated
process

on

the

silicon

substrate

by

using

fluoric

organic

solvents.
Fig.

9.

SEM

images

of

the

patterned

ZnO

NW

arrays

(a)

top-view

of

the

ZnO

NW

arrays,

(b)

line

width

of

1

␮m,

(c)

line

width

of

500

nm,

(d)

line

width

of

200

nm,

(e)

line

width

of

100

nm

and

(f)

line

width

of

50

nm,

respectively.
10140 J.

Zhang

et

al.

/

Applied

Surface

Science

257 (2011) 10134–

10140
380

390

400

410

420

430

440

450
0.0
2.0x10
5

4.0x10
5
6.0x10
5
8.0x10
5
1.0x10
6
1.2x10
6
Intensity (a.u.)
Wavelength / nm
390 40

041

042

0
Intensity (a.u.)
Wavelength

/ nm
Fig.

10.

PL

spectra

of

the

patterned

ZnO

NW

arrays

grown

on

the

AZO

seed

layer

at
80
◦
C

for

1

h.
It

should

be

mentioned

here

that

this

kind

of

surface

fluorination
technique

is

a

universal

method

for

patterning

sol–gel

thin

films.
Thus,

when

the

AZO

sol

is

spin-coated

on

the

silicon

substrate,

the
AZO

sol

cannot

be

deposited

on

the

low

surface

energy

region

due
to

its

low

adhesion,

but

the

AZO

sol

can

be

firmly

deposited

on
those

patterned

regions.

In

addition

to,

this

process

can

eliminate
the

effect

of

the

resist

on

the

boundary

of

the

patterned

AZO

seed
layer.

In

order

to

achieve

a

good

photoluminescence

property,

the
patterned

ZnO

NW

arrays

are

only

grown

at

80
◦
C

for

1

h.

Images
of

the

patterned

ZnO

NW

arrays

are

shown

in

Fig.

9.

It

can

be

seen
that

the

ZnO

NW

arrays

can

be

selectively

and

sharply

grown

on
the

patterned

regions

of

the

AZO

seed

layer.

Fig.

9(a)

shows

the
patterned

ZnO

NW

arrays

at

a

large

feature

size

area.

Fig.

9(b)–(g)
show

the

as-grown

ZnO

NW

arrays

on

the

patterned

regions

with
a

width

of

1

␮m,

500

nm,

200

nm,

100

nm,

and

50

nm,

respectively.
It

can

be

clearly

observed

from

Fig.

9

that

vertically

aligned

ZnO
NW

arrays

can

be

easily

grown

on

the

patterned

regions

of

the
AZO

seed

layer.

The

as-grown

ZnO

NW

arrays

show

an

acicular
morphology,

and

the

average

length

and

diameter

of

the

ZnO

NWs
are

around

1

␮m

and

50

nm,

respectively.

Fig.

10

shows

the

room
temperature

PL

spectrum

(excite

at

365

nm,

1

nm

slit

width)

of

the
patterned

ZnO

NW

arrays

grown

on

the

AZO

seed

layers.

It

is

found
that

only

a

sharp

and

strong

UV

peak

at

380

nm

dominates

the

PL
spectrum.

The

inset

shows

the

partial

enlarged

PL

spectrum

from
390

to

420

nm,

and

no

other

peaks

are

observed

in

the

curve.

These
results

indicate

that

there

are

few

crystal

defects

to

exist

in

the
patterned

ZnO

NW

arrays

grown

on

the

sol–gel

derived

AZO

seed
layer

at

the

hydrothermal

temperature

of

80
◦
C

for

1

h.
4.

Conclusions
The

vertically

aligned

ZnO

NW

arrays

have

been

successfully
grown

on

the

AZO

seed

layer

by

the

hydrothermal

method.

Effects
of

the

hydrothermal

parameters

on

the

morphological

and

pho-
toluminescence

properties

of

the

ZnO

NW

arrays

have

been

also
studied.

Results

indicate

that

the

fastest

growth

rate

of

the

ZnO-
NWA

can

be

obtained

at

130
◦
C

and

the

ZnO-NWA

has

the

higher
aspect

ratio

than

that

of

the

ZnO-NWZ

due

to

the

effect

of

the

Al-
doping

on

the

seed

layer.

Furthermore,

the

patterned

ZnO-NWA
arrays

with

strong

PL

emission

and

few

crystal

defects

have

been
obtained

by

combining

the

sol–gel

process

with

the

electron-beam
lithography

process,

as

well

as

the

surface

fluorination

technique,
which

is

probably

suitable

for

the

applications

in

the

luminescent
and

electron

field

emission

devices.
Acknowledgments
This

work

was

supported

by

the

Ministry

of

Science

and

Tech-
nology

of

China

through

863-project

under

grant

2009AA03Z218,
the

Major

Program

of

the

National

Natural

Science

Foundation

of
China

under

grant

no.

90923012,

and

Xi’an

Applied

Materials

Inno-
vation

Fund

(XA-AM-200909).
Appendix

A.

Supplementary

data
Supplementary

data

associated

with

this

article

can

be

found,

in
the

online

version,

at

doi:10.1016/j.apsusc.2011.06.163.

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