Array of ZnO nanoparticle-sensitized ZnO nanorods for UVphotodetection

Zahra Alaie • Shahram Mohammad Nejad •

Mohammad Hasan Yousefi

Received: 9 August 2013 / Accepted: 29 November 2013 / Published online: 12 December 2013

� Springer Science+Business Media New York 2013

Abstract Vertically aligned ZnO nanorod (NR) arrays

have been successfully synthesized on ITO-glass substrate

by hydrothermal growth. Chemical bath deposition method

was used to deposit ZnO nanoparticles (NPs) onto the ZnO

NRs. These structures were applied in fabricating ZnO NPs

sensitized ultraviolet (UV) photodetectors (PDs). Incorpo-

ration of ZnO NPs onto ZnO NRs results in distinct

improvement of optical properties of ZnO NRs, i.e., sig-

nificant enhancement of emission as well as effective

suppression of defects emission in ZnO. Furthermore, there

is a noticeable blue-shift in absorption spectra compared

with that of ZnO NRs structure. I–V characteristics show

that the sensitized structure improved photocurrent almost

twice that of unsensitized ZnO NRs. Consequently, these

findings may open new opportunities for the integration of

different ZnO nanostructures for application in UV region

particularly fabrication of UV PDs.

1 Introduction

Zinc oxide (ZnO) is a wide direct band gap n-type semi-

conductor (3.37 eV) with a large exciton binding energy

(60 meV) that makes it suitable for UV photodetection.

One dimensional (1D) ZnO nanostructures have been

shown the improved optical and electrical properties due to

the carriers being confined in a certain direction [1, 2].

These nanostructures are suitable for application of light

emitting diodes, solar cells, PDs and so on.

Surface modification of ZnO with narrow bandgap

materials such as ZnSe, CdS, ZnS has been recognized as

an efficient technique to improve the electro-optical prop-

erties of ZnO [3]. Recent efforts highlight the usefulness of

QD sensitized semiconductors in improving the photon-to-

photocurrent conversion efficiency of the photoelectro-

chemical cell [4].

The quantum confinement effect of nanocrystals (NCs)

makes it possible to generate multiple electron–hole pairs

per photon through impact ionization effect [4]. This work

uses sensitized semiconductor layer to improve amount of

absorption in UV region that is benefit to use in UV PDs.

In this study, we used wet chemical approach for fab-

rication of high density ZnO NP/ZnO NR arrays on ITO-

glass substrates.

2 Experimental

The fabrication procedure of ZnO NRs consists of two steps:

preparation of seed-layer and growth of NR arrays. In the

first step, solution of seed layer was prepared by dissolving

0.4 M zinc acetate dehydrate (Zn (CH3COO)2�2H2O) and

0.4 M ethanolamine (NH2CH2CH2OH) in 2-methoxyetha-

nol (CH3OCH2CH2OH). The resulting mixture was stirred

using a magnetic stirrer at 60 �C for 30 min to get a clear and

stable solution. The prepared solution was spin coated on the

ITO-glass substrates at 3,000 rpm for 1 min. Then, the

samples were dried and annealed in air at 450 �C for 1 h.

In the second step, the substrates were placed in a heated

solution (0.005 M) of zinc nitrate and (0.1 M) NaOH. This

solution held at 70� C for 5 h. Finally, the samples were

Z. Alaie (&) � S. Mohammad Nejad

Nanoptronics Research Center, Iran University of Science and

Technology, Tehran, Iran

e-mail: alaie.zahra@yahoo.com; Alaie_zahra@iust.ac.ir

M. H. Yousefi

Nanolab, Malke Ashtar University of Technology, Esfahan, Iran

123

J Mater Sci: Mater Electron (2014) 25:852–856

DOI 10.1007/s10854-013-1656-6

removed from the solution and immediately rinsed with

deionized water to remove residuals from the surface.

For deposition of ZnO NPs on the surface of ZnO NRs,

ZnO NPs must be uniformly formed on the surface of ZnO

NRs. In this work, the ZnO NCs deposited on ZnO NRs by

chemical bath deposition and it seems to satisfy this

requirement [5]. The colloidal solution of ZnO NPs was

fabricated by wet chemical method.

For synthesis of ZnO NCs, 50 ml aqueous solution of

(0.05 M) poly ethylene glycol was dissolved to a vessel of

50 ml (0.1 M) ZnCl2 solution. The ZnO NR layers were

immersed into the solution of ZnCl2 and poly ethylene

glycol. Then, an aqueous solution of 50 ml (0.2 M) KOH

was slowly injected into the vessel. After the end of

reaction, the samples were washed by deionized water

three times and dried in air. The presence of the ZnO NPs

on the surface of ZnO NR array was confirmed from the

TEM results. TEM image of sensitized layer and ZnO NPs

were shown in Fig. 1. Clearly, the surface of the ZnO NR is

uniformly coated with a number of ZnO NPs, as shown in

Fig. 1a. After sensitization by NPs, the surface of ZnO NR

becomes rougher. By detaching the NPs from the surface of

NRs, it was also possible to evaluate sizes of ZnO NPs.

TEM analysis was carried out to confirm the average size

of ZnO NPs and the results are shown in Table 1. The

average size of ZnO NPs was about 18.7 nm.

3 Results and discussion

3.1 XRD

The crystal phase and orientation of ZnO NRs/ZnO NCs

were analyzed by X-ray diffraction (Fig. 2). It is note-

worthy to mention that only single crystalline phase of ZnO

was formed. All diffraction peaks were identified as

belonging to the hexagonal ZnO structure and no charac-

teristic peak of any other impurities is observed in the

spectrum. A high single crystalline of ZnO NRs can pro-

vide direct electrical pathways to collect excited electrons.

It is suitable for photodetection devices.

Figure 2 compares the XRD spectra recorded from the

ZnO NR films before and after ZnO NPs deposition. Fig-

ure 2a shows the typical XRD pattern of as-prepared ZnO,

which can be indexed to a hexagonal structure of ZnO

(JCPDS: 75-1526) with lattice constants a = 3.22 A and

c = 5.2 A. The predominant (002) peak suggest that the

ZnO NRs grew with their c-axis orientation normal to the

substrate surface. Considering the growth direction of the

ZnO NRs, the XRD result is fairly consistent with the

excellent vertical alignment of ZnO NR arrays on the

substrate observed in the SEM image.

Figure 2c shows the X-ray diffraction pattern of ZnO

NPs. The X-ray diffraction pattern exhibits prominent

broad peaks at planes of the hexagonal phase of ZnO.

Therefore, ZnO NPs and ZnO NRs have the same crystal

phase. The lattice parameters have been calculated and are

found to be a = 3.24 A and c = 5.18 A.

Salt

ZnO NPs

(a) (b)Fig. 1 TEM images of

(a) sensitized ZnO NR layer

with ZnO NPs and (b) ZnO NPs

Table 1 Statistical analysis of ZnO NPs in TEM images

Figure Area (nm2) Perimeter (nm) Radius (nm)

Spherical 1,087.809 116.918 18.60807

Spherical 543.9045 82.67349 13.1579

Spherical 783.2225 99.20819 15.78947

Spherical 1,153.078 120.3744 19.15818

Spherical 1,414.152 133.307 21.21647

Spherical 1,392.396 132.2776 21.05263

Spherical 1,414.152 133.307 21.21647

Spherical 1,261.859 125.9245 20.04151

Spherical 1,936.3 155.988 24.82627

Spherical 870.2473 104.5746 16.64357

Spherical 979.0282 110.9181 17.65317

Spherical 739.7102 96.41303 15.34461

Spherical 348.0989 66.13879 10.52632

Spherical 804.9787 100.5766 16.00727

Spherical 1,327.127 129.1401 20.55329

Spherical 2,110.35 162.8479 25.91805

Spherical 1,327.127 129.1401 20.55329

Average 1,146.679 117.631 18.72156

J Mater Sci: Mater Electron (2014) 25:852–856 853

123

They are found to be in close agreement with JCPDS

(79-0205) data. The grain size has been calculated using

Scherre’s equation [6]:

L ¼ K kb cosh

where k is the radiation wavelength, k = 0.90, and h is the

Bragg angle. The average grain size of ZnO NCs estimated

from the prominent peaks of XRD pattern is about 42 nm.

Figure 2b shows the XRD spectrum of ZnO NRs/ZnO

NPs where the peaks corresponding to reflections from the

lattice planes of the ZnO NRs and NCs have been

observed. After deposition of ZnO NPs, the diffraction

peaks are observed broaden compared to ZnO NRs. The

broadness of the diffraction peaks due to ZnO NPs indi-

cates that the size of ZnO NP crystallites is very small, as

calculated by the Debye–Scherrer formula.

3.2 UV–vis

The room temperature UV–vis absorption spectra of ZnO

NPs, ZnO NRs and double layer ZnO NPs/ZnO NRs were

measured. Figure 3a shows the absorbance spectra of the

ZnO NR thin films grown before and after of ZnO NPs

deposition. The absorbance spectra of the ZnO NPs have

been shown at Fig. 3b. The absorption edges of the ZnO

NR films before and after of ZnO NPs deposition occur at

about 410 and 380 nm, respectively. It can be seen that the

absorption edge moves towards a lower-wavelength with

deposition of ZnO NPs. They contain more active sites on

surface and larger band gap thus leading improvement of

the UV absorption of the sensitized layer.

20 40 60 80

112

201

004103 200110102

101100

112

201

004

Inte

nsi

ty(a

.u.)

2θ

(a) ZnO NRs (b) ZnO NRs+ZnO NPs (c) ZnO NPs

100

002

101110 103 200102

100101

102 110 103 200

112

201

004

(a)

(b)

(c)

Fig. 2 X-ray diffraction spectrum of (a) ZnO NRs, (b) ZnO NRs/

ZnO NPs and (c) ZnO NPs. Coincidence with Lines of hexagonal

structure are shown

300 350 400 450 500 550 600 650

Inte

nsi

ty (

a.u

)

Wavelength (nm)

NPs Absorption

350 400 450 500 550 600 650 700 750 8000

2

4

6

8

10

12

Ab

sorp

tio

n (

a.u

)

Wavelength (nm)

350 400 450 500 550 600 650 700 750 8000

2

4

6

8

10

12

Tra

nsm

issi

on

(a.

u)

Wavelength (nm)

Before NP Deposition After NP Deposition

Before NP Deposition After NP Deposition

Before NP Deposition After NP Deposition

(a) (b)

(c) (d)

Fig. 3 (a) UV–vis spectra of

thin films, (b) UV–vis spectra of

ZnO NPs solution,

(c) transmission spectra and

(d) the (a hm)2 versus hm curve

of thin films

854 J Mater Sci: Mater Electron (2014) 25:852–856

123

The band gap enlargement of ZnO NPs is in agreement

with the theoretical calculation based on the effective mass

model. The relationship between band gap and size of ZnO

NPs can be obtained using effective mass model [7]. Using

this model for spherical particles, the band gap Eg (eV) can

be approximately written as:

Eg ¼ Eg bulkð Þ þ �h2p2

2er2

1

me

þ 1

mh

� �� 1:8e2

4p�r�0r

where Eg(bulk) is the bulk energy gap, r is the particle radius,

me is the effective mass of the electrons, mh is the effective

mass of the holes, er is the relative permittivity, e0 is the

permittivity of free space, �h is Planck’s constant divided by

2p, and e is the charge of the electron. Band gap of ZnO NPs

can be calculated from the effective mass model with Eg

(bulk) = 3.37 eV, me = 0.24 m0, mh = 0.45 m0, and

er = 3.7, where m0 is the free electron mass. Energy band

gap for average radius of ZnO NPs (which obtained from

TEM image) are estimated about 3.377 eV. Enlargement

effects are predominant when the ZnO NP size is less than

exciton Bohr radius. It was reported that PL data and

absorption data of ZnO NPs show the same tendency, i.e.

they predict the size-dependent energy gap [8]. However, the

PL data of NPs is not accurately same tendency since it

represents the emission from a relaxed state and the exciton

binding energy [8]. It also reported a size-dependent Stokes

shift of the PL maximum relative to the absorption onset.

This Stokes shift has been observed in other II–VI and III–V

NPs and extrapolated exponentially or hyperbolically [9]. It

was previously reported as either size-dependent electron–

phonon processes or size dependent spin–orbit exchange

interaction may contribute to the Stokes shift. However, the

origin of the size-dependent Stokes shift in semiconductor

NP systems remains unclear at present [9]. In this paper,

energy band gap of ZnO NPs/ZnO NRs, have been shown the

same tendency, i.e. enlargement of band gap after deposition

of ZnO NPs on ZnO NRs that you can see from Fig. 3.

By using the transmission spectra (Fig. 3c), the optical

band gap (Eg) of thin film can be estimated by using the

conventional Tauc equation[10]:

ahm ¼ Aðhm� EgÞn=2

Here a (a = -ln T) is the absorption coefficient, hm is the

photon energy, A is a constant. As a direct band gap

semiconductor, the optical band gap (Eg) of ZnO thin film

can be estimated by the zero crossing of the rising edge of

the (a hm)2 vs hm curve, as shown in Fig. 3d.

The optical band gap for the ZnO NR film grown by

hydrothermal method is estimated to be 3.02 eV. This low

band gap can be attributed to wide height of ZnO NRs and

therefore high thickness (1.4 lm) of thin film (that was

determined by SEM cross section image of thin film), low

strain, high lattice constant and low crystal quality [11, 12].

By the deposition of ZnO NPs on ZnO NRs, the optical

band gap shifts to 3.12 eV. It was found that there is a

broadening of the band gap about 100 meV between the

two samples. These results are consistent with those

observed for UV–Vis spectrum.

3.3 Photoluminescence (PL)

Figure 4 shows the room-temperature PL spectra of the as-

grown ZnO NR arrays and ZnO NPs/ZnO NRs. A sharp

near-band-edge emission at 381 nm and a broad very small

visible light emission have been observed. It is due to the

little concentration of oxygen vacancies on the surface of

synthesized material [13].

As you can see in Fig. 4, UV emission intensity for ZnO

NPs/ZnO NRs is relatively enhanced (higher three order)

compared to the ZnO NR arrays. Stronger UV emission of

ZnO NPs/ZnO NRs is due to lower defects and higher

surface-to-volume ratio compared to ZnO NR array.

3.4 SEM

The morphology of the ZnO NRs and ZnO NPs/ZnO NRs

double layer were checked by SEM images (Fig. 5).

SEM images reveal highly compact aligned ZnO NRs

that are single crystalline (Fig. 5a). It can be seen (Fig. 5b)

that ZnO NRs have hexagonal structure. The length of NR

360 400 440 480 520 560 600 640 680 720

30

60

90

120

150

180

210

240

Inte

nsi

ty (

a.u

.)

wavelength (nm)

Before NPs Deposition

360 400 440 480 520 560 600 640 680 720 760

0

100

200

300

400

500

600

700

800

Inte

nsi

ty (

a.u

)

Wavelength (nm)

After NPs Deposition

(a) (b)Fig. 4 PL spectra of (a) the

ZnO NR arrays and (b) the ZnO

NPs/ZnO NR thin film

J Mater Sci: Mater Electron (2014) 25:852–856 855

123

arrays was about 1.3 lm and their average diameter was

about 52.76 nm. The SEM image of double-layer is pre-

sented in Fig. 5c. Clearly, the surface of the ZnO NRs is

coated with ZnO NPs.

3.5 UV photodetection

By coating Au on ZnO NR arrays as cathode, metal–

semiconductor-metal UV PDs have been fabricated. Upon

illumination, conductivity increases (Fig. 6). In fact, the

photoexcited unpaired electrons increases the conductivity

[14]. Change of the photocurrent to the dark current is

maximally about 10 mA for bare ZnO NRs. By deposition

of ZnO NPs on ZnO NR array, change of the photocurrent

to the dark current becomes two orders of bare ZnO NRs,

i.e. maximally about 20 mA. The large part of the ZnO NR

surfaces is covered with NPs; therefore, a large numbers of

extra carriers were generated under UV light. The photo

generated electrons in NPs will be conducted to ZnO NRs

and result in enhancement of device detectivity.

4 Conclusion

In conclusion, we report a simple chemical approach, for

the near-room-temperature growth of the novel double-

layer ZnO NP/ZnO NR arrays. The sensitized structure has

been shown improved UV light absorption and emission

due to higher surface-to-volume ratio than that of ZnO NR

arrays. SEM image of double-layer ZnO NPs/ZnO NRs

shows that the surface of NRs fully covered by ZnO NPs.

Also, electrical properties of the sensitized structure

enhance about two orders. The improvement of optical and

electrical properties is of great significance for application

of this structure in UV photodetection.

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Fig. 5 SEM images of (a) ZnO NR array, (b) cross section of single ZnO NR and (c) ZnO NRs coated with ZnO NPs

2 4 6 8 10

10-2

10-1

Cu

rren

t (A

)

Voltage (v)

UV on for ZnO NPs/ZnO NRs thin film UV on for ZnO NRs thin film Dark

Fig. 6 Plot of I versus V of ZnO NR thin film before and after UV

illumination

856 J Mater Sci: Mater Electron (2014) 25:852–856

123