ZHANG ET AL.
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CXXXX American Chemical Society
Piezo-phototronic Effect Enhanced
Visible and Ultraviolet Photodetection
Using a ZnO?CdS Core?Shell
Fang Zhang,†,‡Yong Ding,†Yan Zhang,†Xiaoling Zhang,‡and Zhong Lin Wang†,§,*
†School of Material Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332-0245, United States,‡Key Laboratory of Cluster Science of
Ministry of Education, School of Chemistry, Beijing Institute of Technology, Beijing 100081,China, and§Beijing Institute of Nanoenergy and Nanosystems, Chinese
Academy of Sciences, Beijing, China
large surface-to-volume ratio and direct
pathway for charge transport.1?3A variety
belts have been developed, such as group
II?VI compounds ZnO (<3.37 eV, 370 nm)
and ZnS (<3.77 eV, 335 nm) as the typical
active materials for UV light and CdS (<2.40
eV,516 nm) and CdSe(<1.74 eV,712 nm) for
visible light.4?7However, pure semiconduc-
tor NWs cannot absorb wavelengths below
their band gap, which limits their wide-
spectral sensitivity in standard devices, such
as for visible/ultraviolet (UV) light imaging,
memory storage or switches. Herein we re-
port the performance of a single ZnO?CdS
core?shell micro/nanowire based photo-
detector, which shows an excellent sensi-
tivity and response from 372 to 548 nm.
This is one of the most effective hetero-
structure-based multiband photodetectors
for simultaneously detecting visible and UV
Moreover, the performance of such a
photon sensor has been largely enhanced
with the use of piezo-phototronic effect.
The wurtzite structured ZnO NW exhibits
highpolarization performance. Apiezoelec-
tric potential (piezopotential) is created in
the nanowire by applying a stress.9Recently,
by the coupling of piezoelectric, optical, and
semiconducting properties, the performance
of photodetector based on a single ZnO wire
has been improved, which is referred to as
nanostructures are considered as
the most sensitive and fast respon-
at NW?electrode contacts by using strain
work, based on the unique crystalline struc-
ture of the ZnO?CdS core?shell wire, the
performance of the photodetector can be
further enhanced by strain induced piezo-
of the Schottky barrier heights (SBHs) at the
*Address correspondence to
Received for review August 7, 2012
and accepted September 28, 2012.
The piezo-phototronic effect is about the use of the piezoelectric potential created inside some
on the photon sensitivity for a ZnO?CdS core?shell micro/nanowire based visible and UV
sensor. CdS nanowire arrays were grown on the surface of a ZnO micro/nanowire to form a
ZnO?CdScore?shellnanostructure by afacile hydrothermal method.Withthe twoendsofa
is sensitive simultaneously to both green light (548 nm) and UV light (372 nm). Furthermore,
the performance of the photon sensor is much enhanced by the strain-induced piezopotential
in the ZnO core through modulation of the Schottky barrier heights at the source and drain
contacts. This work demonstrates a new application of the piezotronic effect in photon
KEYWORDS: ZnO?CdS core?shell micro/nanowire.Schottky contact.
ZHANG ET AL.
’ NO. XX
tion nanomaterials by introducing a piezo-phototronic
RESULTS AND DISCUSSION
ZnO micro/nanowires were grown by thermal eva-
is a low-magnification transmission electron micro-
scopy (TEM) image, showing the details of a single ZnO
NW. A high-resolution transmission electron micro-
scopy (HRTEM) image and the corresponding select
area electron diffraction (SAED) pattern taken from the
wire is structurally uniform and single crystalline with
length direction along the c-axis. Figure 1b displays a
dark-field (DF) TEM image of a single ZnO?CdS NW;
the inset is the magnified DF image of the CdS NW
array with diameters of ∼100 nm and lengths of
∼500 nm, and the line-like contrasts are ascribed to
stacking faults normal to the length axis. To further
verify the crystalline structure of the CdS NWs, a high-
resolution image and SAED pattern were taken at the
marked tipregion ofaCdS NW, indicating that the CdS
NW grows along the c-axis and the lattice fringes can
be assigned to (0001). Figure 1c is a low-magnification
TEM image of CdS NW array grown on the ZnO NW
after hydrothermal reaction, showing the entire sur-
face of the ZnO NW is uniformly covered by CdS NW
array. A three-dimensional schematic configuration of
a single ZnO?CdS wire in Figure 1d exhibits the
oriented growth of CdS NW array toward outside on
firm the core?shell heterostructure, a more detailed
investigation was conducted using field-emission trans-
mission electron microscopy (FE-TEM). Figure 1e is a
bright-field (BF) and scanning transmission electron mi-
croscopy annular dark field (ADF) image of an individual
profile across the entire NW along the red line is pre-
sented in Figure 1f, which clearly demonstrates that Cd
and S are distributed at the shell while the Zn located
at the core.
An individual ZnO?CdS micro/nanowire-based
photodetector was then constructed by a standard
(b) Dark-field TEM image of a ZnO?CdS nanowire, HRTEM image, and SAED pattern taken from the marked tip of a CdS
nanowire. (c) Low-magnification TEM image of a ZnO?CdS wire. (d) Three-dimensional structure model of a ZnO?CdS
ZnO?CdS nanowire. (f) The corresponding EDS elemental profile of Cd, S, and Zn along the red line in panel e.
ZHANG ET AL.
’ NO. XX
procedure (see the Experimental Section). The sche-
are given in Supporting Information, Figure S1a,b
showing the measurement setup. First, the performance
of a single ZnO?CdS wire photodetector (device #1)
under green light illumination (λ = 548 nm) is summar-
ized. Figure 2a shows some typical I?V characteristics of
the ZnO?CdS wire in the dark and under illumination
at different light intensities from 1.79 ? 10?7W/cm2to
1.43 ? 10?3W/cm2. Significantly, the absolute current
increased from 0.6 nA (dark current) to 10.2 nA (4.48 ?
at an applied voltage of 2 V. Figure 2b illustrates the
measured current response of a single ZnO?CdS wire
bias of 2.0 V. With the light irradiation off and on, the
current of the device exhibits two distinct states, includ-
ing a low-current state of 8.1 nA (which could reduce to
0.6 nA after 2 h) in the dark and a high-current state of
34.6 nA under light illumination. The current increases
very sharply from one state to another state, with a res-
reproducibility and stability indicate that the developed
ZnO?CdS wire is a great candidate for applications in
visible light detection.
Additionally, the intensity dependences of photo-
and showed no saturation at high power levels, offer-
ing a large dynamic range from 10?7to 10?3W/cm2.
The sensitivity defined as (Ilight? Idark)/Idarkwas found
times higher than CdS nanoribbon (550 nm, <5%) and
6 times higher than the aligned networks of CdS NWs
on SiO2substrates.14,15Although the reason for this
remarkable enhancement is not quite clear, we pro-
of the CdS shell and band energy alignment of the
ture arrays have the advantage of low reflectance
due to light scattering and trapping, endowing them
with superior optical absorption compared with a one-
a higher photocurrent, and thus increases the sensitivity
in the ZnO?CdS wire facilitates the spatial separation of
the photon-generated carriers and can decrease the
recombination of the electron?hole pairs, therefore sig-
nificantly increasing the photocurrent and sensitivity of
the device.18,19Such an enhancement effect has also
been observed in ZnO/ZnS biaxial nanobelts.20
Moreover, the responsivity (R) is also a critical
parameter to determine the capability of a photo-
detector.21The total responsivity of the photodetector
R is defined as
Pill ¼ Iilldl (2)
Figure 2. (a) Typical I?V characteristics of single ZnO?CdS wire-based device, excited by green light centered at 548 nm. (b)
Repeatable response of a single ZnO?CdS wire-based device, excited by green light centered at 548 nm (1.43 mW/cm2) (c)
Absolute photocurrent of a single ZnO?CdS wire-based device measured as a function of the excitation intensity. (d) The
derived photon responsivity relative to excitation intensity on the ZnO?CdS wire.
ZHANG ET AL.
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the illumination power on the photodetector, ηextis
the external quantum efficiency (EQE), q is the elec-
tronic charge, h is Planck's constant, ν is the frequency
of the light, ΓGis the internal gain, Iillis the excitation
power, d is the diameter of the ZnO?CdS wire, and l is
the spacing between two electrodes. Remarkably, the
calculated responsivity R of the present device is
approximately 11 A W?1at an intensity of 1.79 ?
10?7W/cm2of green light illumination (548 nm). This
corresponds to an EQE of ∼2.49 ? 103% if the internal
visible/UV light sensor, our device based on single
ZnO?CdS wire exhibits ultrahigh responsivity R and
EQE as well as wide spectral sensitivity (11 A W?1, 2.49 ?
103%, 548 nm, at 2 V bias), compared with individual
ZnSe-nanobelt-based blue/UV-light sensor (0.12 A W?1,
37.2%, 400 nm, at 30 V bias).8Besides, the decrease of
the responsivity with high light intensities could be
ascribed to the hole-trapping saturation and the
Schottkybarrier beingtransparentathigh light inten-
sity, as has also been observed from ZnO nanowire-
based UV light photodetectors.10
In addition to the good sensitivity to green light, the
ZnO?CdS wire-based photodetector also exhibits ex-
cellent response to UV light. Supporting Information,
Figure S2 displays the performance of the device at
wavelength of 372 nm. As expected, the responsivity
R shows a notable increase by nearly 1 order of
magnitude when the device is illuminated by 372 nm
UV light, which is evident from the comparison of
Figure 2d and Supporting Information, Figure S2d in
the scale of the vertical axis. This enhancement should
be ascribed to the coupling effect of the ZnO core and
CdS shell. Since the excitation energy of the UV light
(∼3.34 eV, 372 nm) is larger than the band gap of CdS
(∼2.4 eV) and ZnO (∼3.3 eV), electron?hole pairs can
be generated both in the core and shell parts of the
wire, which results in the increased amount of photo-
carries. Consequently, the performance of the ZnO?
CdS wire photodetector is significantly optimized
compared to that of pure CdS or ZnO nanostructures
by combining the high UV and visible light sensitivity,
justifying the effective utilization of the present ZnO?
CdS wire as the UV/visible light photodetector.
As described in our previous work, the internal
piezoelectric field formed inside ZnO can tune the
charge transport/separation process at the contact
and thus optimize the photoresponse of a single ZnO
wire.10To investigate the effects of the piezopotential
on the performance of our photodetector (device #2),
I?V characteristics of a single ZnO?CdS wire under a
variety of compressive and tensile strain were mea-
10?3W/cm2), as shown in Figure 3a. The asymmetric
I?V curvesshow excellentrectificationbehavior under
to-back Schottky barriers at contacts, with Schottky
barrier height (SBH) at the drain side (φd) being much
higher than that at the source side (φs). In positive bias
range, the photocurrent response of the device in-
creased and decreased under compressive and tensile
strain, respectively. The I?V curves could be recovered
as the strains were released, and the extensive study
indicates that the I?V behavior is introduced by strain
rather than poor or unstable contact.
As is well studied, the Schottky barrier at the inter-
face of a metal and semiconductor is an important
factor in determining electrical transport properties of
the metal?semiconductor?metal (M?S?M) struc-
ture. To understand the changes of I?V curves with
strains in Figure 3a, we consider the current transport
with illumination to be described by a relationship
V . 3kT/q ≈ 77 mV,22?24thus the changes of SBH Δφs
(at 2.0 V, black curve) and Δφd(at ?2.0 V, red curve)
with strain are calculated by25,26
ΔφB∼ ?kT ln[I(εzz)=I(0)]
where φBis the SBH, k is the Boltzmann constant, I(εzz)
and I(0) are the current measured through the
Figure 3. (a) Typical I?V characteristics of a single ZnO?CdS wire-based device under different compressive and tensile
strains, excited by green light centered at 548 nm (1.43 mW/cm2). (b) The derived change in SBH as a function of stain using
at a source?drain bias of V = 2 and ?2 V, respectively.
ZHANG ET AL.
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strained, respectively. It has been reported that the
SBH shifts under strain are attributed to a combination
of band structure and piezoelectric effect.23,25The
band structure change (e.g., the piezoresistive effect
the electric transport; while the piezotronic effect is a
polarized effect using the piezopotential as a “gate”
voltage to tune charge carrier transport at the semi-
conductor?metal contact.27,28If we ignore the piezo-
resistive effect and use the experimental observed I?V
as the starting curve to derive the change in SBH (see
eq 3), we get the results shown in Figure 3b, in which
both the SBHs at the source and drain contacts were
decreased step-by-step with application of a variable
strain from 0.22% tensile to ?0.31% compressing but
SBHs at the source and drain contacts, which is the
To illustrate the piezopotential effect on the perfor-
mance of the photodetector upon visible/UV light,
respectively, we measured compressive strain-dependent
I?V characteristics of a single ZnO?CdS wire (device
#3) under illumination of green light (548 nm, 1.43 ?
as shown in Figure 4. The absolute current of the
photodetector at a positive bias of 2.0 V increased
gradually from 40 to 632 nA under green light illumina-
tion, with application of a variable strain from 0% to
?0.31% (Figure 4a). Meanwhile, the current increased
from 12.9 to 144 nA under UV light illumination
(Figure 4b). Figure 4c shows the changes of SBH Δφd
(red curve) and Δφs(black curve) with strains at a bias
of 2 V. The result indicates that both the SBHs at the
source and drain contacts were decreased with in-
creased compressive strains. The asymmetric decreas-
ing tendency implies the participation of the piezo-
potential effect,23,27which agrees well with the results
in Figure 3b. Figure 4d shows the responsivity of the
photodetector under different compressive strains
illumination, respectively. It is noticed that the respon-
sivity of the present ZnO?CdS wire-based photode-
tector is significantly enhanced by more than 10 times
piezo-phototronic effect on the ZnO?CdS wire as
It is interesting to understand the piezopotential
enhanced performance of a strained photodetector
under illumination of visible light and UV light, respec-
tively. Here we use a schematic diagram (Figure 5) to
illustrate how piezoelectric polarization affects the
SBHs at the source and drain contacts and current
output of the device under illumination. Figure 5a
displays the type-II band alignment of ZnO?CdS
heterostructure.29When the ZnO?CdS wire is under
the illumination of green (548 nm) light, photogener-
ated electrons are injected from the conduction band
(CB) of the photoexcited CdS into the CB of ZnO,
leading to a high concentration of electrons in the CB
of ZnO. Meanwhile, with a large amount of electrons
excited by green light centered at 548 nm and UV light centered at 372 nm; R0is set as responsivity under zero strain.
ZHANG ET AL.
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moving into the ZnO, the high carrier mobility in the
for conducting electrons, while the holes are trans-
portedthroughCdS. The separation ofthe charge carrier
types minimizes their recombination rate, thus, increas-
ing the photocurrent. With considering the Schottky
contact characteristics of the device, the conductance is
mainly dominated by the local contacts.
Figure 5b shows the piezopotential distribution in a
strained ZnO?CdS wire device under illumination.
Since the c-axis (polarization direction) of the CdS
be produced in the CdS shell along the length of the
ZnO?CdS wire, which produces an equal effect to the
polarity of the voltage. While, as the c-axis () of
ZnO NW is positioned in alignment with strain direc-
tion from the source to drain side, a piezopotential
drop of approximately Vþ? V?= PXL can be induced
along the length of the ZnO core (where PXis axial
polarization and L is the length of the ZnO core). Thus,
the piezopotential atthe source anddrain sides canbe
qualitatively described as Vþand V?, which are of the
same magnitude but opposite in sign. As shown in
Figure 5c, with the piezopotential modulations to the
SBH denoted by Δφpz, the drain has a negative piezo-
potential (Vp?) leading to a increased SBH (φdþ Δφpz),
and the source has a positive piezopotential (Vpþ)
leading to a decreased SBH (φs ? Δφpz). Therefore,
based on the strain induced piezopotential, an asym-
metric change of SBHs at two contacts is introduced,
which is just what we observed experimentally in
Figures 3b and 4c. According to a previous report,30
the voltage drop occurs mainly at the reversely biased
Schottky barrier φsat the source side. Consequently, the
piezopotential induced decrease of SBH (φs? Δφpz) at
the source side allows an significant increase of photo-
current and thus the photon responsivity.
Once the photodetector is under the illumination of
UV light, electron?hole pairs can be generated in the
ZnO NW core and CdS NW array shell, and the perfor-
mance of the photodetector can be also enhanced by
the strain-induced piezoelectric effect.
In summary, we first fabricated a novel ZnO?CdS
a ZnO micro/nanowire as the core. Based on a single
ZnO?CdS wire, we developed a photodetector exhibit-
ing an excellent response on the wide-range visible/UV
Figure 5. (a) Schematic diagram exhibiting the energy band structure, electron?hole pair separation, and transfer in the
proposed sandwich model of the device, that is, two back-to-back Schottky diodes connected to a ZnO core and CdS shell,
band diagram illustrating the Schottky barriers at the source and drain contacts of an unstrained (dotted line) and
compressive strained (solid line) ZnO wire, which shows the effect of switching the piezoelectric potential on the SBH.
ZHANG ET AL.
’ NO. XX
light (372?548 nm). The photocurrent and sensitivity
of the ZnO?CdS wire photodetector is 103times high-
er than that of CdS nanoribbon, and the responsivity
(11 A W?1, 548 nm, at 2 V bias) is nearly 100 times
performance of the ZnO?CdS wire photodetector
upon illumination of visible and UV light can be both
further enhanced for more than 10 times with the
participation of the piezo-phototronic effect when
the device is subjected to a ?0.31% compressive
strain. This investigation extends the application
of the piezo-phototronic effect in a wide spectrum
ZnO?CdS micro/nanowires were synthesized via a two-step
process. First, ZnO micro/nanowires were synthesized by a
high-temperature thermal evaporation process as described
in our previous work.13Then, the CdS NW array was grown
on ZnO micro/nanowire by hydrothermal method. Briefly,
1 mmol Cadmium nitrate (Cd(NO3)234H2O) and 3 mmol
Thiourea (CH4N2S) were added to a given amount (80 mL)
of distilled water. ZnO micro/nanowires were subsequently
added to the result solution and the mixture was transferred
into a Teflon-lined stainless autoclave (25 mL capacity). The
autoclave was sealed and maintained at 200 ?C for 12 h. After
the system cooled down, the product was collected and
vacuum-dried. The detailed microscopic structures of the
precursor ZnO wires and the final ZnO?CdS wire products
were characterized by TEM [JEOL 100CX at 100 kV, JEOL
4000EX at 400 kV, TF30 at 300 kV].
the literature.10,11In brief, two ends of a single ZnO?CdS wire
were fixed on a PS substrate (typical length of ∼3 cm, width of
∼1 cm, and thickness of 0.5 mm) tightly by silver pastes which
serviced as the source and drain electrodes. Then a thin layer of
polydimethylsiloxane (PDMS) was utilized to package the de-
repeated mechanical strains. (see Supporting Information, Fig-
ure S1). One end of the device was affixed on a 3D stage with
movement resolution of 1 μm, which kept the device in focus
under a microscope during the measurement process. Another
3D stage connecting to the free end of the device was used to
produce a compressive or tensile strain on the substrate. The
system and excited by monochromatic UV light (centered at
372 nm) and green light (centered at 548 nm) with a Nikon
Intensilight C-HGFIE lamp as the source. The optical power
density illuminated on the device was varied by means of
neutral density filters and determined by a thermopile power
meter (Newport 818P-001-12). Meanwhile the I?V characteris-
tics of the device were recorded by Keithley 487 picoammeter/
voltage source in conjunction with a GPIB controller (National
Instruments GPIB-USB-HS, NI 488.2).
Conflict of Interest: The authors declare no competing
Acknowledgment. Research was supported by Airforce,
MURI, U.S. Department of Energy, Office of Basic Energy
Sciences (DE-FG02-07ER46394), NSF (CMMI 0403671), the
Knowledge Innovation Program of the Chinese Academy of
Sciences (KJCX2-YW-M13), National Nature Science Foundation
of China (No. 21275018) and the 111 Project in China (B07012).
Thanks are extended to Qing Yang, Ying Liu, Jianjun Chen,
Supporting Information Available: (1) Schematic diagram
and optical microscopy image of a ZnO?CdS wire-based
photodetector, schematic diagram of the measurement sys-
tem; (2) performance of single ZnO?CdS wire-based photo-
detector under illumination of UV light centered at 372 nm.
This material is available free of charge via the Internet at
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