ZnO Nanofilms
ZnO Hollow-Sphere Nanofilm-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-defined morphology, uniform size, low density, high surface area, and
potential applications in catalysis, photonic crystals, chromatography, protection of biologically active agents, fillers (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 semiconducting hollow spheres as the building blocks.[2–4] For
example, it has been found that the gas sensors fabricated
from a thin film of WO3 hollow spheres exhibits high sensitivity to organic gases in an intermediate temperature
range;[2] tin-encapsulated hollow carbon spheres can effectively 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 TiO2 hollow
spheres exhibit outstanding light-harvesting efficiency.[4]
However, to the best of our knowledge, there are still no
reports on the photodetectors constructed using semiconducting 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 corresponding 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 lowdimensional nanostructures.[11a–d] The nanostructures can be
well organized at an oil-water interface to form a high-quality
monolayer film in a macroscopic scale due to the decrease
of interfacial energy.[11e] This inspires us to self-assemble a
high-quality film 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.
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 Laboratory
Fudan University
Shanghai 200433, PR China
E-mail: [email protected]; [email protected]
DOI: 10.1002/smll.201100694
small 2011, 7, No. 17, 2449–2453
In this communication, the first hollow-sphere nanofilmbased photodetector using ZnO hollow spheres as the
building blocks is presented by an ‘oil–water’ interfacial selfassembly 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 important materials in the optoelectronic devices.[13] Well-defined
polystyrene (PS)/ZnO core/shell nanospheres were prepared
and then self-assembled at a hexane–water interface to form
a precursor film. Annealing this precursor film under optimal
conditions, a ZnO hollow-sphere nanofilm with a densely
packed network structure was obtained. Finally, a UV photodetector was successfully constructed from the as-transformed
ZnO hollow-sphere nanofilm (as illustrated in Figure 1). This
hollow-sphere nanofilm-based photodetector displayed high
sensitivity, excellent stability, and fast response times, justifying the effective utilization of the semiconducting hollow
spheres as the building blocks of UV photodetectors.
The detailed procedure for the ‘water–oil’ interfacial selfassembly of monodisperse PS/ZnO core/shell nanospheres
into a monolayer film can be seen in the Supporting Information (SI), Figure S1. The as-assembled film at the interface can be transferred onto various solid substrates, such as
quartz and silicon substrates. Due to its thickness of just a
few nanometers, the film deposited on the quartz substrate
exhibits a high transparency (as shown in Figure 1a). It is
noteworthy that the film assembled at the hexane–water
interface can also be easily transferred on a plastic substrate
with excellent flexibility (Figure 1b), offering the possibility
to fabricate flexible nanodevices by this simple strategy.
The precursor core/shell film deposited on a SiO2/Si substrate was then annealed at 600 °C for 3 h in air to produce a
ZnO hollow-sphere film.[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 film and the as-transformed
ZnO hollow-sphere film can be indexed to a hexagonal wurtzite ZnO phase (JCPDS 36-1451). Figure 2a,b show the
typical transmission electron microscopy (TEM) and highresolution TEM (HRTEM) images for the as-transformed
product, respectively, confirming 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
© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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M. Chen et al.
Figure 1. Photographs of the PS/ZnO nanosphere film assembled at a hexane–water interface, and the films mounted on a) quartz and
b) flexible plastic substrates. Schematic illustration of the fabrication procedures for ZnO hollow-sphere nanofilm photodetector: c) deposition of
the as-assembled PS/ZnO precursor film on a silicon substrate with a 200 nm SiO2 top layer, d) thermal transformation from a PS/ZnO precursor
nanofilm into a ZnO hollow-sphere nanofilm, and e) a complete ZnO hollow-sphere nanofilm photodetector.
Subsequently, a pair of Cr/Au electrodes was deposited on
the as-transformed ZnO hollow-sphere nanofilm on a SiO2/Si
substrate using an Au microwire as the mask, and the morphology of the resulting ZnO hollow-sphere nanofilm device
is shown in Figure 3a. Figure 3b 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 μA was detected at an
applied voltage of 5.0 V (about 53 times enhancement compared 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
be low. Figure 3c 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 efficiency 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.[15a] Neverthelesss, the present device shows high
signal-to-noise ratio with a high photocurrent of few μA and a low dark current of
the order of nA, which is strongly desirable for its practical application. The high
signal-to-noise ratio of the present device
Figure 2. Typical a) TEM and b) HRTEM images of the ZnO hollow spheres, and c) corresponding also indicates a high sensitivity, which may
SAED pattern taken from a single ZnO hollow sphere. d,e) SEM images of the ZnO hollow- be attributed to a high light-absorption
efficiency of our hollow spheres because
sphere nanofilm deposited on a SiO2/Si substrate.
a single ZnO hollow sphere (Figure 2c) shows six diffuse diffraction 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 structure ensures the porosity of the hollow spheres, which can
further be confirmed by the nitrogen adsorption/desorption
measurements shown in SI, Figure S3. The Brunauer–
Emmett–Teller (BET) specific surface area and the average
pore size of the as-transformed ZnO hollow spheres are
around 9.77 m2 g−1 and 70.6 nm, respectively, suggesting
a large surface-to-volume ratio of the hollow spheres.
Figure 2d,e show the scanning electron microscopy (SEM)
images of the as-transformed film. 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 film should have a high surface area and therefore be suitable for both UV and gas detectors.
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ZnO Hollow-Sphere Nanofilm-Based Photodetector
trum measured under alternating current mode is further shown in Figure
3e, which is relatively lower than that
2
obtained by a direct current mode rea1
sonably. It can be clearly seen that the
0
photocurrent increases by about three
-1
350 nm
orders of magnitude when the device
400 nm
500 nm
is illuminated by a light of an energy
-2
600 nm
dark
above the threshold excitation energy
-3
of ZnO (≈3.37 eV, 370 nm). The high
-5 -4 -3 -2 -1 0 1 2 3 4 5
spectral selectivity of the light waveVoltage (V)
length less than 370 nm suggests that
(c)
(d)
this device is indeed ‘visible-blind’
15
1E-23
and highly UV-sensitive. Also investi10
1E-24
gated were the responses of the device
5
under different working atmospheres,
1E-25
0
as shown in Figure 3f. The photocurrent
1E-26
-5
of the device measured in vacuum
conditions of 1 Pa is about 4.4 times
-10
1E-27
higher than that in ambient conditions,
-15
-5 -4 -3 -2 -1 0 1 2 3 4 5
200
300
400
500
600
demonstrating that the photocurrent
Wavelength (nm)
Voltage (V)
can be enhanced by decreasing the gas
pressure of the environment. The result
(e)
(f)
1
15
confirms the existence of the oxygen
10
chemisorption/desorption on the ZnO
0.1
5
hollow-sphere surface.
0.01
The photoconductive mechanism
0
in the ZnO hollow spheres includes
1E-3
-5
the generation of free carriers and the
1Pa
-10
1E-4
air
electrical transport through the inter-15
face between two neighboring spheres
1E-5
and the metal/ZnO interface. The high
200
300
400
500
600
-5 -4 -3 -2 -1 0 1 2 3 4 5
Wavelength (nm)
Voltage (V)
background electron concentration in
ZnO always provides the Ohmic or
(g) 3.0
(h)
injection-type electric contact, which
2.5
contributes to the high photoresponsivity with a quantum efficiency much
2.0
larger than 1. The role of metal/semi1.5
conductor interface has been discussed
previously,[16] which is not special for
1.0
the ZnO hollow spheres. Here, the
0.5
focus is on the photogeneration of free
carriers for the ZnO hollow spheres
0
100
200
300
400
500
and the electric transport between two
Time (S)
neighboring spheres. It is generally
Figure 3. a) A typical SEM image of the ZnO hollow-sphere nanofilm photodetector. b) I–V
accepted that the absorption/desporcharacteristics of the ZnO photodetector illuminated with light of lights of 350 nm, 400 nm, tion of oxygen molecules governs the
500 nm, 600 nm, and under dark conditions. c) The responsivity versus applied-voltage
generation of free carriers for ZnO:
characteristic under illumination of 350 nm light. d) A spectral photoresponse of the device
i) the adsorbed oxygen molecules onto
measured at a bias of 5.0 V at diffraction wavelengths ranging from 210 to 630 nm. e) A quantified
responsivity of the photodetector at diffraction wavelengths ranging from 210 to 630 nm by an the hollow-sphere surfaces capture free
alternating current mode. f) I–V characteristics of the device when illuminated with a light of electrons from the n-type ZnO [O2(g) +
350 nm measured in air and in vacuum (1 Pa). g) Response time of the photodetector measured e− → O2−(ad)], creating a depletion
in air at a bias of 5.0 V. h) A transient response generated by illuminating the ZnO film device layer near the surface. This reduces
with a 350 nm light pulse chopped at a frequency of 100 Hz. The light power intensity was kept at
the electrical conductivity; ii) Under
1. 32 mW cm−2 for all measurements.
UV illumination, electron–hole pairs
are generated. The holes migrate to
of their large active surface. Figure 3d depicts the photon-re- the surface along the potential gradient and combine with
sponse spectra of the device as a function of the incident light oxygen, inducing desorption of oxygen from the ZnO surwavelength at a bias of 5.0 V. A quantified responsivity spec- face [h+ + O2− (ad) → O2(g)] (as illustrated in Figure 4a). This
(a)
(b)
Responsivity (a.u.)
Current (µA)
Current (µA)
Responsivity (A/W)
Responsivity (A/W)
Current (µA)
3
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M. Chen et al.
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 evident. 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 photocurrent under vacuum. On the other hand, due to the existence of physical boundaries between the hollow spheres, the
charge transfer among the hollow spheres is hopping-like,
which was evidenced by temperature-conductivity measurements (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
efficiency 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 3g reveals that the
ZnO nanofilm 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 3h, the rise time (tr) and decay time (td),
respectively defined as the time taken for the current to
increase from 10% to 90% of the peak value or vice versa,[19]
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are both measured to be <5 ms. In order to more accurately
estimate the response times, the collection step was further
decreased to 40 μs under the resolution of the apparatus. It
shows that both the tr and td are about 467 and 940 μs, respectively (SI, Figure S4).[20]
The present ZnO hollow-sphere nanofilm device therefore has a much faster response time than the individual ZnO
nanostructure-based photodetectors reported previously (generally 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 film can be regarded as
a percolated network of polycrystalline hollow ZnO spheres,
whose boundary resistance is usually several orders of magnitude larger than that of an individual nanostructure.[23] Furthermore, the physical contact between the adjacent hollow
spheres inside the present film device will scatter the carriers
and result in junction barriers. Therefore, the electron conduction of such a film 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 barriers. 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 4b). Since the
narrowed barriers allow easier electron tunneling and transportation, this process results in a significant 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 photoconduction properties based on the present ZnO hollowsphere nanofilm 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 nanofilmbased photodetector has been successfully constructed for
the first time by the ‘water–oil’ interfacial assembly of PS/
ZnO core/shell nanospheres and followed by an annealing
treatment. This nanofilm photodetector showed high sensitivity, good stability, and fast response times. It is quite promising for applications such as optical communications, flame
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 first
case of semiconducting hollow-sphere nanofilm-based photodetector. The procedure can be easily extended to other semiconductor nanospheres, such as TiO2, ZnS, or CdS hollow
spheres, and these works are already underway.
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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 previous study.[8] The PS/ZnO core/shell nanospheres were dispersed
at a hexane–water interface to form a self-assembled densely
packed film. The film was transferred onto a SiO2 (200 nm)/Si substrate, and annealed at 600 °C for 3 h in air to obtain a ZnO hollowsphere nanofilm. A UV photodetector was then fabricated from the
as-transformed ZnO hollow-sphere nanofilm (see Supporting Information for details). The current–voltage (I–V) characteristics of the
ZnO nanofilm photodetector were measured using an Advantest
Picoammeter R8340A and a dc voltage source R6411. A spectral 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 film with a 350 nm light
pulse chopped at a frequency of 100 Hz.
[12]
[13]
[14]
[15]
[16]
Supporting Information
[17]
[18]
Supporting Information is available from the Wiley Online Library
or from the author.
[19]
[20]
Acknowledgements
This work was supported by the National Natural Science Foundation 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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© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Received: April 12, 2011
Published online: July 21, 2011
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