ZnO Nanofi lms

ZnO Hollow-Sphere Nanofi lm-Based High-Performance and Low-Cost Photodetector

Min Chen , Linfeng Hu , Jiaxi Xu , Meiyong Liao , Limin Wu , * and Xiaosheng Fang *

Monodisperse hollow spheres have attracted considerable

interest in the past decades due to their well-defi ned mor-

phology, uniform size, low density, high surface area, and

potential applications in catalysis, photonic crystals, chromato-

graphy, protection of biologically active agents, fi llers (or

pigments, coatings), waste removal, and large bimolecular

release systems. [ 1 ] In recent years, some novel nanodevices

with unique properties have even been realized using semi-

conducting hollow spheres as the building blocks. [ 2–4 ] For

example, it has been found that the gas sensors fabricated

from a thin fi lm of WO 3 hollow spheres exhibits high sen-

sitivity to organic gases in an intermediate temperature

range; [ 2 ] tin-encapsulated hollow carbon spheres can effec-

tively accommodate the strain of volume change during Li +

insertion/extraction process and improve the performance

and durability of lithium batteries; [ 3 ] and dye-sensitized solar

cell using electrodes consisting of nanoembossed TiO 2 hollow

spheres exhibit outstanding light-harvesting effi ciency. [ 4 ]

However, to the best of our knowledge, there are still no

reports on the photodetectors constructed using semicon-

ducting hollow spheres as the building blocks, although

photodetectors show wide applications as binary switches in

imaging techniques and light-wave communications, as well

as in future memory storage and optoelectronic circuits. [ 5 ]

Our group has extensively reported the facile syntheses

of a variety of inorganic hollow-spheres from the corre-

sponding core/shell precursors. [ 6–10 ] Most recently, a novel

‘oil–water interface self-assembly’ has been reported as a

low-cost and universal strategy for the assembly of low-

dimensional nanostructures. [ 11 a–d] The nanostructures can be

well organized at an oil-water interface to form a high-quality

monolayer fi lm in a macroscopic scale due to the decrease

of interfacial energy. [ 11 e] This inspires us to self-assemble a

high-quality fi lm made of semiconductor hollow spheres by

this low-cost method, which should be a promising candidate

for the key sensing elements in optoelectronic devices due to

its high-area coverage ratio and the large surface-to-volume

ratio.

© 2011 Wiley-VCH Verlag Gmbsmall 2011, 7, No. 17, 2449–2453

DOI: 10.1002/smll.201100694

Dr. M. Chen , Dr. L. F. Hu , J. X. Xu , Dr. M. Y. Liao , Prof. L. M. Wu , Prof. X. S. Fang Department of Materials Science and Advanced Materials LaboratoryFudan UniversityShanghai 200433, PR China E-mail: lmw@fudan.edu.cn; xshfang@fudan.edu.cn

In this communication, the fi rst hollow-sphere nanofi lm-

based photodetector using ZnO hollow spheres as the

building blocks is presented by an ‘oil–water’ interfacial self-

assembly strategy. This is because ZnO is a very important

semiconductor with a wide room-temperature bandgap of

3.37 eV [ 12 ] and has been widely used as one of the most impor-

tant materials in the optoelectronic devices. [ 13 ] Well-defi ned

polystyrene (PS)/ZnO core/shell nanospheres were prepared

and then self-assembled at a hexane–water interface to form

a precursor fi lm. Annealing this precursor fi lm under optimal

conditions, a ZnO hollow-sphere nanofi lm with a densely

packed network structure was obtained. Finally, a UV photo-

detector was successfully constructed from the as-transformed

ZnO hollow-sphere nanofi lm (as illustrated in Figure 1 ). This

hollow-sphere nanofi lm-based photodetector displayed high

sensitivity, excellent stability, and fast response times, justi-

fying the effective utilization of the semiconducting hollow

spheres as the building blocks of UV photodetectors.

The detailed procedure for the ‘water–oil’ interfacial self-

assembly of monodisperse PS/ZnO core/shell nanospheres

into a monolayer fi lm can be seen in the Supporting Infor-

mation (SI), Figure S1. The as-assembled fi lm at the inter-

face can be transferred onto various solid substrates, such as

quartz and silicon substrates. Due to its thickness of just a

few nanometers, the fi lm deposited on the quartz substrate

exhibits a high transparency (as shown in Figure 1 a). It is

noteworthy that the fi lm assembled at the hexane–water

interface can also be easily transferred on a plastic substrate

with excellent fl exibility (Figure 1 b), offering the possibility

to fabricate fl exible nanodevices by this simple strategy.

The precursor core/shell fi lm deposited on a SiO 2 /Si sub-

strate was then annealed at 600 ° C for 3 h in air to produce a

ZnO hollow-sphere fi lm. [ 14 ] As shown in SI, Figure S2, all the

diffraction peaks in the X-ray diffraction (XRD) patterns of

the PS/ZnO core/shell precursor fi lm and the as-transformed

ZnO hollow-sphere fi lm can be indexed to a hexagonal wur-

tzite ZnO phase (JCPDS 36-1451). Figure 2 a,b show the

typical transmission electron microscopy (TEM) and high-

resolution TEM (HRTEM) images for the as-transformed

product, respectively, confi rming the hollow nature of the

product. It is evident that the spherical morphology was well

maintained during the annealing, and the shell of each hollow

sphere is composed of numorous ZnO nanocrystals with size

of ≈ 10–20 nm. The observed d spacings correspond well with

the (100) and (101) planes, respectively. Individual hollow

spheres are, on the other hand, polycrystalline in nature. A

selected area electron diffraction (SAED) pattern taken from

2449H & Co. KGaA, Weinheim wileyonlinelibrary.com

M. Chen et al.

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communications

Figure 1 . Photographs of the PS/ZnO nanosphere fi lm assembled at a hexane–water interface, and the fi lms mounted on a) quartz and b) fl exible plastic substrates. Schematic illustration of the fabrication procedures for ZnO hollow-sphere nanofi lm photodetector: c) deposition of the as-assembled PS/ZnO precursor fi lm on a silicon substrate with a 200 nm SiO 2 top layer, d) thermal transformation from a PS/ZnO precursor nanofi lm into a ZnO hollow-sphere nanofi lm, and e) a complete ZnO hollow-sphere nanofi lm photodetector.

a single ZnO hollow sphere (Figure 2 c) shows six diffuse dif-

fraction rings, which can be indexed as the (100), (101), (102),

(110), (103), and (200) planes of wurtzite ZnO, starting from

inner to outer ring, respectively. Such a polycrystalline struc-

ture ensures the porosity of the hollow spheres, which can

further be confi rmed by the nitrogen adsorption/desorption

measurements shown in SI, Figure S3. The Brunauer–

Emmett–Teller (BET) specifi c surface area and the average

pore size of the as-transformed ZnO hollow spheres are

around 9.77 m 2 g − 1 and 70.6 nm, respectively, suggesting

a large surface-to-volume ratio of the hollow spheres.

Figure 2 d,e show the scanning electron microscopy (SEM)

images of the as-transformed fi lm. The substrate is densely

covered by a large number of ZnO hollow spheres with

average diameter of about 260 nm. Since some spaces

between the interconnected hollow spheres are still observed,

the annealed fi lm should have a high surface area and there-

fore be suitable for both UV and gas detectors.

0 www.small-journal.com © 2011 Wiley-VCH Verlag Gm

Figure 2 . Typical a) TEM and b) HRTEM images of the ZnO hollow spheres, anSAED pattern taken from a single ZnO hollow sphere. d,e) SEM images sphere nanofi lm deposited on a SiO 2 /Si substrate.

Subsequently, a pair of Cr/Au electrodes was deposited on

the as-transformed ZnO hollow-sphere nanofi lm on a SiO 2 /Si

substrate using an Au microwire as the mask, and the mor-

phology of the resulting ZnO hollow-sphere nanofi lm device

is shown in Figure 3 a. Figure 3 b shows the I–V curves of the

device illuminated with radiation of different wavelengths

and under dark conditions, respectively. It can be seen that

the photoresponsivity just shows very slight changes when the

wavelength of the light sources are 600 nm (1.68 mW cm − 2 ),

500 nm (2.81 mW cm − 2 ) and 400 nm (2.02 mW cm − 2 ). When the

device was illuminated by a 350 nm UV light at 1.32 mW cm − 2 ,

a drastic increase of current up to 2.6 μ A was detected at an

applied voltage of 5.0 V (about 53 times enhancement com-

pared with a dark current of 50 nA). The nonlinear behavior

of the photocurrent curve is attributed to nonOhmic contact

between the ZnO hollow-sphere and the Cr/Au electrodes.

To fabricate a high-performance photodetector, the detector

responsivity needs to be high while the dark current needs to

bH & Co. KGaA, Weinhe

d c) corresponding of the ZnO hollow-

be low. Figure 3 c displays the responsivity

versus applied-voltage characteristic under

illumination of 350 nm light. The spectral

response at 350 nm is about 13.5 A W − 1

at a 5 V bias, corresponding to an external

quantum effi ciency of 4783%. The present

performance is comparable or superior

to a near-UV-light photodetector based

on ZnO and ZnS nanostructures. [ 15 ]

For example, a device based on hybrid

polymer/zinc oxide nanorods prepared

by low-temperature solution processes

exhibited a response of 0.18 A W − 1 at

300 nm by applying a bias of −2 V. [ 15 a] Nev-

erthelesss, the present device shows high

signal-to-noise ratio with a high photocur-

rent of few μ A and a low dark current of

the order of nA, which is strongly desir-

able for its practical application. The high

signal-to-noise ratio of the present device

also indicates a high sensitivity, which may

be attributed to a high light-absorption

effi ciency of our hollow spheres because

im small 2011, 7, No. 17, 2449–2453

ZnO Hollow-Sphere Nanofilm-Based Photodetector

Figure 3 . a) A typical SEM image of the ZnO hollow-sphere nanofi lm photodetector. b) I–V characteristics of the ZnO photodetector illuminated with light of lights of 350 nm, 400 nm, 500 nm, 600 nm, and under dark conditions. c) The responsivity versus applied-voltage characteristic under illumination of 350 nm light. d) A spectral photoresponse of the device measured at a bias of 5.0 V at diffraction wavelengths ranging from 210 to 630 nm. e) A quantifi ed responsivity of the photodetector at diffraction wavelengths ranging from 210 to 630 nm by an alternating current mode. f) I–V characteristics of the device when illuminated with a light of 350 nm measured in air and in vacuum (1 Pa). g) Response time of the photodetector measured in air at a bias of 5.0 V. h) A transient response generated by illuminating the ZnO fi lm device with a 350 nm light pulse chopped at a frequency of 100 Hz. The light power intensity was kept at 1. 32 mW cm − 2 for all measurements.

-5 -4 -3 -2 -1 0 1 2 3 4 5-3

-2

-1

0

1

2

3

Cur

rent

(µ A

)

Voltage (V)

350 nm 400 nm 500 nm 600 nm dark

-5 -4 -3 -2 -1 0 1 2 3 4 5-15

-10

-5

0

5

10

15

Voltage (V)

Res

pons

ivity

(A

/W)

200 300 400 500 600

1E-27

1E-26

1E-25

1E-24

1E-23

Res

pons

ivity

(a.

u.)

Wavelength (nm)

200 300 400 500 600

1E-5

1E-4

1E-3

0.01

0.1

1

Wavelength (nm)

Res

pons

ivity

(A

/W)

-5 -4 -3 -2 -1 0 1 2 3 4 5

-15

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-5

0

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10

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1Pa air

Cur

rent

(µA

)

Voltage (V)

0 100 200 300 400 500

0.5

1.0

1.5

2.0

2.5

3.0

Time (S)

Cur

rent

(µA

)(a) (b)

(c) (d)

(e) (f)

(g) (h)

of their large active surface. Figure 3 d depicts the photon-re-

sponse spectra of the device as a function of the incident light

wavelength at a bias of 5.0 V. A quantifi ed responsivity spec-

© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheimsmall 2011, 7, No. 17, 2449–2453

trum measured under alternating cur-

rent mode is further shown in Figure

3 e, which is relatively lower than that

obtained by a direct current mode rea-

sonably. It can be clearly seen that the

photocurrent increases by about three

orders of magnitude when the device

is illuminated by a light of an energy

above the threshold excitation energy

of ZnO ( ≈ 3.37 eV, 370 nm). The high

spectral selectivity of the light wave-

length less than 370 nm suggests that

this device is indeed ‘visible-blind’

and highly UV-sensitive. Also investi-

gated were the responses of the device

under different working atmospheres,

as shown in Figure 3 f. The photocurrent

of the device measured in vacuum

conditions of 1 Pa is about 4.4 times

higher than that in ambient conditions,

demonstrating that the photocurrent

can be enhanced by decreasing the gas

pressure of the environment. The result

confi rms the existence of the oxygen

chemisorption/desorption on the ZnO

hollow-sphere surface.

The photoconductive mechanism

in the ZnO hollow spheres includes

the generation of free carriers and the

electrical transport through the inter-

face between two neighboring spheres

and the metal/ZnO interface. The high

background electron concentration in

ZnO always provides the Ohmic or

injection-type electric contact, which

contributes to the high photorespon-

sivity with a quantum effi ciency much

larger than 1. The role of metal/semi-

conductor interface has been discussed

previously, [ 16 ] which is not special for

the ZnO hollow spheres. Here, the

focus is on the photogeneration of free

carriers for the ZnO hollow spheres

and the electric transport between two

neighboring spheres. It is generally

accepted that the absorption/despor-

tion of oxygen molecules governs the

generation of free carriers for ZnO:

i) the adsorbed oxygen molecules onto

the hollow-sphere surfaces capture free

electrons from the n -type ZnO [O 2 (g) +

e− → O 2 − (ad) ], creating a depletion

layer near the surface. This reduces

the electrical conductivity; ii) Under

UV illumination, electron–hole pairs

are generated. The holes migrate to

the surface along the potential gradient and combine with

oxygen, inducing desorption of oxygen from the ZnO sur-

face [h + + O 2 − (ad) → O 2 (g) ] (as illustrated in Figure 4 a). This

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M. Chen et al.

2452

communications

Figure 4 . Schematic illustration of a) oxygen-adsorption process in the dark and oxygen-desorption process upon UV illumination of the ZnO hollow spheres, b) the SP–SP junction barrier for electron transfer in the hollow-sphere network, showing a decrease in SP–SP junction barrier height from the light-off state to light-on state.

hole-trapping process results in an increase in the free-carrier

concentration and a decrease in the width of the depletion

layer, leading to an apparent enhancement in photocurrent; [ 17 ]

iii) Under vacuum, oxygen desorption becomes more evi-

dent. Therefore, the concentration of free electrons is higher

in vacuum than in air, considering that oxygen acts as a trap

for electrons. This interprets the enhancement in the photo-

current under vacuum. On the other hand, due to the exist-

ence of physical boundaries between the hollow spheres, the

charge transfer among the hollow spheres is hopping-like,

which was evidenced by temperature-conductivity measure-

ments (not shown here). However, it was noticed that the

boundary barriers should be low enough for charge transfer

under UV illumination, since photocurrent gain (quantum

effi ciency is much large than 1) was observed in the current

study. [ 18 ]

The response speed is a key parameter which determines

the capability of a photodetector to follow a quickly varying

optical signal. The response time in Figure 3 g reveals that the

ZnO nanofi lm photodetector has a very fast response speed,

and excellent stability and repeatability. A 350 nm light pulse

chopped at a frequency of 100 Hz was employed to further

investigate the detailed photoresponse times of this device.

As shown in Figure 3 h, the rise time ( t r ) and decay time ( t d ),

respectively defi ned as the time taken for the current to

increase from 10% to 90% of the peak value or vice versa, [ 19 ]

www.small-journal.com © 2011 Wiley-VCH Verlag Gm

are both measured to be < 5 ms. In order to more accurately

estimate the response times, the collection step was further

decreased to 40 μ s under the resolution of the apparatus. It

shows that both the t r and t d are about 467 and 940 μ s, respec-

tively (SI, Figure S4). [ 20 ]

The present ZnO hollow-sphere nanofi lm device there-

fore has a much faster response time than the individual ZnO

nanostructure-based photodetectors reported previously (gen-

erally larger than 100 ms). [ 21 ] The reason might be ascribed

to the different conduction mechanisms between these two

kinds of devices. For the individual nanostructure-based

photodevice, the resistance is determined by the nanostructure

itself, thus the conductivity should be mainly governed

by the oxygen chemisorption/desorption as mentioned above.

A previous study revealed that hole diffusion and oxygen

desorption are quite slow, leading to a slow response speed

of the individual nanostructure-based photodetector. [ 22 ] In

contrast, for the present device, the fi lm can be regarded as

a percolated network of polycrystalline hollow ZnO spheres,

whose boundary resistance is usually several orders of mag-

nitude larger than that of an individual nanostructure. [ 23 ] Fur-

thermore, the physical contact between the adjacent hollow

spheres inside the present fi lm device will scatter the carriers

and result in junction barriers. Therefore, the electron con-

duction of such a fi lm should be dominated by a combination

of both the grain-boundary barriers inside each ZnO hollow

sphere and the junction barriers between the ZnO hollow

spheres (denoted as ‘SP–SP’ junction barriers). The SP–SP

junctions can be analogous to two back-to-back Schottky bar-

riers. Upon illumination, the increased carrier density in ZnO

hollow spheres would narrow the barrier width or lower the

effective barrier height (as illustrated in Figure 4 b). Since the

narrowed barriers allow easier electron tunneling and trans-

portation, this process results in a signifi cant increase in the

conductivity of the hollow-sphere network. [ 24 ] It is generally

accepted that the light-induced barrier height modulation is

much faster than the oxygen-diffusion process. [ 22 ] Therefore,

the time response speed for our ZnO network device is much

faster than that of the individual nanostructure-based ZnO

devices. SI, Table S1 summaries a comparison of the photo-

conduction properties based on the present ZnO hollow-

sphere nanofi lm with other ZnO nanostructures, including

nanoparticles, nanowires and nanorods. [ 25–31 ] These key

parameters are comparable to or better than those of other

ZnO nanostructures with different shapes.

In summary, a high-quality ZnO hollow-sphere nanofi lm-

based photodetector has been successfully constructed for

the fi rst time by the ‘water–oil’ interfacial assembly of PS/

ZnO core/shell nanospheres and followed by an annealing

treatment. This nanofi lm photodetector showed high sensi-

tivity, good stability, and fast response times. It is quite prom-

ising for applications such as optical communications, fl ame

sensing, missile launch (Our present device has the potential

for detecting the UV radition and hands over coordinates of

the threatening missile) and so forth. This study is the fi rst

case of semiconducting hollow-sphere nanofi lm-based photo-

detector. The procedure can be easily extended to other semi-

conductor nanospheres, such as TiO 2 , ZnS, or CdS hollow

spheres, and these works are already underway.

bH & Co. KGaA, Weinheim small 2011, 7, No. 17, 2449–2453

ZnO Hollow-Sphere Nanofilm-Based Photodetector

Experimental Section

MonodispersePS/ZnO core/shell nanospheres were prepared by a template-assisted route similar to the procedures in our pre-vious study. [ 8 ] The PS/ZnO core/shell nanospheres were dispersed at a hexane–water interface to form a self-assembled densely packed fi lm. The fi lm was transferred onto a SiO 2 (200 nm)/Si sub-strate, and annealed at 600 ° C for 3 h in air to obtain a ZnO hollow-sphere nanofi lm. A UV photodetector was then fabricated from the as-transformed ZnO hollow-sphere nanofi lm (see Supporting Infor-mation for details). The current–voltage ( I–V ) characteristics of the ZnO nanofi lm photodetector were measured using an Advantest Picoammeter R8340A and a dc voltage source R6411. A spec-tral response for different wavelengths was recorded by using a xenon lamp (500 W). A transient response was recorded by using a 350 MHz Tektronix (TDS 500B) oscilloscope with a 50 V impedance by illuminating the ZnO hollow-sphere fi lm with a 350 nm light pulse chopped at a frequency of 100 Hz.

Supporting Information

Supporting Information is available from the Wiley Online Library or from the author.

Acknowledgements

This work was supported by the National Natural Science Founda-tion of China (Grant Nos. 21001028, 51002032 and 21074023), Science & Technology Foundation of Shanghai (0952nm01000, 10JC1401900) and the innovative team of Ministry of Education of China (IRT0911) and Shanghai Chenguang Foundation (11CG06).

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Received: April 12, 2011Published online: July 21, 2011

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