Piezo-phototronic Effect Enhanced Visible and Ultraviolet Photodetection Using a ZnO-CdS Core-Shell Micro/nanowire

Article (PDF Available)inACS Nano 6(10):9229-36 · September 2012with41 Reads
DOI: 10.1021/nn3035765 · Source: PubMed
Abstract
The piezo-phototronic effect is about the use of the piezoelectric potential created inside some materials for enhancing the charge carrier generation or separation at the metal-semiconductor contact or pn junction. In this paper, we demonstrate the impact of the piezo-phototronic effect 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-CdS core-shell nanostructure by a facile hydrothermal method. With the two ends of a ZnO-CdS wire bonded on a polymer substrate, a flexible photodetector was fabricated, which 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 detectors.
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CXXXX American Chemical Society
Piezo-phototronic Eect Enhanced
Visible and Ultraviolet Photodetection
Using a ZnOCdS CoreShell
Micro/nanowire
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
One-dimensional semiconductor
nanostructures are considered as
the most sensitive and fast respon-
sive materials for photon sensors due to their
large surface-to-volume ratio and direct
pathway for charge transport.
13
Avariety
of semiconductor nanowires (NWs) and nano-
belts have been developed, such as group
IIVI 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.
47
However, 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 ZnOCdS
coreshell micro/nanowire based photo-
detector, which shows an excellent sensi-
tivity and response from 372 to 548 nm.
This is one of the most eective hetero-
structure-based multiband photodetectors
for simultaneously detecting visible and UV
light illumination.
8
Moreover, the performance of such a
photon sensor has been largely enhanced
with the use of piezo-phototronic eect.
The wurtzite structured ZnO NW exhibits
high polarization performance. A piezoelec-
tric potential (piezopotential) is created in
the nanowire by applying a stress.
9
Recently,
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
the piezo-phototronic eect.
10,11
The physical
mechanism of this eect is to tune the charge
transport/separation/recombination process
at NWelectrode contacts by using strain
induced piezoelectric eld in the wire.
12
In our
work, based on the unique crystalline struc-
ture of the ZnOCdS coreshell wire, the
performance of the photodetector can be
further enhanced by strain induced piezo-
potential in the ZnO core through modulation
of the Schottky barrier heights (SBHs) at the
source and drain contacts. This result provides
* Address corresponde nce to
zlwang@gatech.edu.
Received for review August 7, 2012
and accepted September 28, 2012.
Published online
10.1021/nn3035765
ABSTRACT
The piezo-phototronic eect is about the use of the piezoelectric potential created inside some
materialsfor enhancing the charge carriergeneration or separation at the metalsemiconductor
contact or pn junction. In this paper, we demonstrate the impact of the piezo-phototronic eect
on the photon sensitivity for a ZnOCdS coreshell micro/nanowire based visible and UV
sensor. CdS nanowire arrays were grown on the surface of a ZnO micro/nanowire to form a
ZnOCdS coreshell nanostructure by a facile hydrothermal method. With the two ends of a
ZnOCdS wire bonded on a polymer substrate, a exible photodetector was fabricated, which
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 eect in photon
detectors.
KEYWORDS: ZnOCdS coreshell micro/nanowire .Schottky contact .
piezopotential .photodetector .piezo-phototronic eect
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a new method to optimize the performance of multifunc-
tion nanomaterials by introducing a piezo-phototronic
eect.
RESULTS AND DISCUSSION
ZnO micro/nanowires were grown by thermal eva-
poration described in the previous literature.
13
Figure 1a
is a low-magnication 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 diraction (SAED) pattern taken from the
edge of ZnO NW are presented, indicating that the ZnO
wire is structurally uniform and single crystalline with
length direction along the c-axis. Figure 1b displays a
dark-eld (DF) TEM image of a single ZnOCdS NW;
the inset is the magnied 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 tip region of a CdS 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-magnication
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 conguration of
a single ZnOCdS wire in Figure 1d exhibits the
oriented growth of CdS NW array toward outside on
the surface of ZnO wire at the normal direction. To con-
rm the coreshell heterostructure, a more detailed
investigation was conducted using eld-emission trans-
mission electron microscopy (FE-TEM). Figure 1e is a
bright-eld (BF) and scanning transmission electron mi-
croscopy annular dark eld (ADF) image of an individual
ZnOCdS NW. An energy dispersive X-ray (EDX) line scan
prole 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 ZnOCdS micro/nanowire-based
photodetector was then constructed by a standard
Figure 1. (a) Low-magnication TEM image of a ZnO nanowire, HRTEM image, and SAED pattern taken from the marked edge.
(b) Dark-eld TEM image of a ZnOCdS nanowire, HRTEM image, and SAED pattern taken from the marked tip of a CdS
nanowire. (c) Low-magnication TEM image of a ZnOCdS wire. (d) Three-dimensional structure model of a ZnOCdS
coreshell wire, showing the structure relationship between ZnO wire core and CdS nanowire array shell. (e) STEM image of a
ZnOCdS nanowire. (f) The corresponding EDS elemental prole of Cd, S, and Zn along the red line in panel e.
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procedure (see the Experimental Section). The sche-
matic structure and optical image of the photodetector
are given in Supporting Information, Figure S1a,b
showing the measurement setup. First, the performance
of a single ZnOCdS wire photodetector (device #1)
under green light illumination (λ= 548 nm) is summar-
ized. Figure 2a shows some typical IVcharacteristics of
the ZnOCdS wire in the dark and under illumination
at dierent light intensities from 1.79 10
7
W/cm
2
to
1.43 10
3
W/cm
2
. Signicantly, the absolute current
increased from 0.6 nA (dark current) to 10.2 nA (4.48
10
5
W/cm
2
) and further to 37.6 nA (1.43 10
3
W/cm
2
)
at an applied voltage of 2 V. Figure 2b illustrates the
measured current response of a single ZnOCdS wire
based photodetector under 548 nm light illumination at a
bias of 2.0 V. With the light irradiation oand 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-
ponse speed faster than 0.5 s. The excellent photocurrent
reproducibility and stability indicate that the developed
ZnOCdS wire is a great candidate for applications in
visible light detection.
Additionally, the intensity dependences of photo-
currents (I
light
I
dark
) are plotted in Figure 2c. As we can
see, the photocurrent increased with the optical power
and showed no saturation at high power levels, oer-
ing a large dynamic range from 10
7
to 10
3
W/cm
2
.
The sensitivity dened as (I
light
I
dark
)/I
dark
was found
to be 6 10
3
% (1.43 10
3
W/cm
2
), which is about 10
3
times higher than CdS nanoribbon (550 nm, <5%) and
6 times higher than the aligned networks of CdS NWs
on SiO
2
substrates.
14,15
Although the reason for this
remarkable enhancement is not quite clear, we pro-
pose that it may be understood from the nanostructure
of the CdS shell and band energy alignment of the
ZnOCdS wire. Generally, one-dimensional nanostruc-
ture arrays have the advantage of low reectance
due to light scattering and trapping, endowing them
with superior optical absorption compared with a one-
dimensional nanostructure.
16,17
Therefore, the CdS NW
array grown on a ZnO wire is more ecient in generating
a higher photocurrent, and thus increases the sensitivity
of the photodetector. On the other hand, a type-II hetero-
structure with a staggered alignment at the heterjunction
in the ZnOCdS wire facilitates the spatial separation of
the photon-generated carriers and can decrease the
recombination of the electronhole pairs, therefore sig-
nicantly increasing the photocurrent and sensitivity of
the device.
18,19
Such an enhancement eect 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.
21
The total responsivity of the photodetector
Ris dened as
R¼Iph
Pill
¼ηextq
hνΓG(1)
Pill ¼Iilldl (2)
Figure 2. (a) Typical IVcharacteristics of single ZnOCdS wire-based device, excited by green light centered at 548 nm. (b)
Repeatable response of a single ZnOCdS wire-based device, excited by green light centered at 548 nm (1.43 mW/cm
2
) (c)
Absolute photocurrent of a single ZnOCdS wire-based device measured as a function of the excitation intensity. (d) The
derived photon responsivity relative to excitation intensity on the ZnOCdS wire.
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where Ris the responsivity, I
ph
is the photocurrent, P
ill
is
the illumination power on the photodetector, η
ext
is
the external quantum eciency (EQE), qis the elec-
tronic charge, his Planck's constant, νis the frequency
of the light, Γ
G
is the internal gain, I
ill
is the excitation
power, dis the diameter of the ZnOCdS wire, and lis
the spacing between two electrodes. Remarkably, the
calculated responsivity Rof the present device is
approximately 11 A W
1
at an intensity of 1.79
10
7
W/cm
2
of green light illumination (548 nm). This
corresponds to an EQE of 2.49 10
3
% if the internal
gain Γ
G
is assumed to be 1. These values indicate, as for
visible/UV light sensor, our device based on single
ZnOCdS wire exhibits ultrahigh responsivity Rand
EQE as well as wide spectral sensitivity (11 A W
1
,2.49
10
3
%, 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).
8
Besides, the decrease of
the responsivity with high light intensities could be
ascribed to the hole-trapping saturation and the
Schottky barrier being transparent at high 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
ZnOCdS 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
Rshows 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 eect 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), electronhole 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 signicantly optimized
compared to that of pure CdS or ZnO nanostructures
by combining the high UV and visible light sensitivity,
justifying the eective utilization of the present ZnO
CdS wire as the UV/visible light photodetector.
As described in our previous work, the internal
piezoelectric eld formed inside ZnO can tune the
charge transport/separation process at the contact
and thus optimize the photoresponse of a single ZnO
wire.
10
To investigate the eects of the piezopotential
on the performance of our photodetector (device #2),
IVcharacteristics of a single ZnOCdS wire under a
variety of compressive and tensile strain were mea-
sured under illumination of green light (548 nm, 1.43
10
3
W/cm
2
), as shown in Figure 3a. The asymmetric
IVcurves show excellent rectication behavior under
all straining conditions and indicate two unequal back-
to-back Schottky barriers at contacts, with Schottky
barrier height (SBH) at the drain side (φ
d
) being much
higher than that at the source side (φ
s
). In positive bias
range, the photocurrent response of the device in-
creased and decreased under compressive and tensile
strain, respectively. The IVcurves could be recovered
as the strains were released, and the extensive study
indicates that the IVbehavior 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 metalsemiconductormetal (MSM) struc-
ture. To understand the changes of IVcurves with
strains in Figure 3a, we consider the current transport
with illumination to be described by a relationship
based on the thermionic emissiondiusion theory for
V.3kT/q 77 mV,
2224
thus the changes of SBH Δφ
s
(at 2.0 V, black curve) and Δφ
d
(at 2.0 V, red curve)
with strain are calculated by
25,26
ΔφBkT ln[I(εzz)=I(0)] (3)
where φ
B
is the SBH, kis the Boltzmann constant, I(ε
zz
)
and I(0) are the current measured through the
ZnOCdS wire at a xed bias V with and without being
Figure 3. (a) Typical IVcharacteristics of a single ZnOCdS wire-based device under dierent compressive and tensile
strains, excited by green light centered at 548 nm (1.43 mW/cm
2
). (b) The derived change in SBH as a function of stain using
the thermionic emissiondiusion model. Black curve and red curve are the SBH change for source contact and drain contact
at a sourcedrain bias of V= 2 and 2 V, respectively.
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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 eect.
23,25
The
band structure change (e.g., the piezoresistive eect
and parasitic capacitance eect) is a nonpolar eect on
the electric transport; while the piezotronic eect is a
polarized eect using the piezopotential as a gate
voltage to tune charge carrier transport at the semi-
conductormetal contact.
27,28
If we ignore the piezo-
resistive eect and use the experimental observed IV
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
at dierent slopes, indicating an asymmetric change of
SBHs at the source and drain contacts, which is the
piezotronic eect.
To illustrate the piezopotential eect on the perfor-
mance of the photodetector upon visible/UV light,
respectively, we measured compressive strain-dependent
IVcharacteristics of a single ZnOCdS wire (device
#3) under illumination of green light (548 nm, 1.43
10
3
W/cm
2
) and UV light (372 nm, 6.36 10
5
W/cm
2
),
asshowninFigure4.Theabsolutecurrentofthe
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 Δφ
d
(red curve) and Δφ
s
(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 eect,
23,27
which agrees well with the results
in Figure 3b. Figure 4d shows the responsivity of the
photodetector under dierent compressive strains
upon green light (green curve) and UV light (red curve)
illumination, respectively. It is noticed that the respon-
sivity of the present ZnOCdS wire-based photode-
tector is signicantly enhanced by more than 10 times
compared to that of the unstrained wire, indicating the
piezo-phototronic eect on the ZnOCdS wire as
visible/UV photodetector.
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 aects 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 ZnOCdS
heterostructure.
29
When the ZnOCdS 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
Figure 4. (a,b) Typical IVcharacteristics of a single ZnOCdS wire-based device under dierent compressive strains, excited
by green light centered at 548 nm and UV light centered at 372 nm. (c) The derived change in SBH as a function of compressive
strains using the thermionic emissiondiusion model. Black curve and red curve are the SBH change for source contact and
drain contact at a sourcedrain bias of V= 2 and 2 V, respectively. (d) The change of responsivity under compressive strains,
excited by green light centered at 548 nm and UV light centered at 372 nm; R
0
is set as responsivity under zero strain.
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moving into the ZnO, the high carrier mobility in the
high-crystalline ZnO core makes it an eective channel
for conducting electrons, while the holes are trans-
ported through CdS. The separation of the 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 ZnOCdS wire device under illumination.
Since the c-axis (polarization direction) of the CdS
NW array is approximately normal to direction of strain,
only a band structure change (e.g., piezoresistance) can
be produced in the CdS shell along the length of the
ZnOCdS wire, which produces an equal eect to the
SBHs at the source and drain contacts regardless of the
polarity of the voltage. While, as the c-axis ([0001]) of
ZnO NW is positioned in alignment with strain direc-
tion from the source to drain side, a piezopotential
drop of approximately V
þ
V
=P
X
Lcan be induced
along the length of the ZnO core (where P
X
is axial
polarization and Lis the length of the ZnO core). Thus,
the piezopotential at the source and drain sides can be
qualitatively described as V
þ
and 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 Δφ
pz
, the drain has a negative piezo-
potential (V
p
) leading to a increased SBH (φ
d
þΔφ
pz
),
and the source has a positive piezopotential (V
p
þ
)
leading to a decreased SBH (φ
s
Δφ
pz
). 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
when a positive bias voltage Vis applied at the drain side,
the voltage drop occurs mainly at the reversely biased
Schottky barrier φ
s
at the source side. Consequently, the
piezopotential induced decrease of SBH (φ
s
Δφ
pz
)at
the source side allows an signicant increase of photo-
current and thus the photon responsivity.
Once the photodetector is under the illumination of
UV light, electronhole 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 eect.
CONCLUSIONS
In summary, we rst fabricated a novel ZnOCdS
micro/nanowire with the CdS NW array as the shell and
a ZnO micro/nanowire as the core. Based on a single
ZnOCdS 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, electronhole pair separation, and transfer in the
ZnOCdS heterostructure under light illumination of green light centered at 548 nm and UV light centered at 372 nm. (b) The
proposed sandwich model of the device, that is, two back-to-back Schottky diodes connected to a ZnO core and CdS shell,
respectively, and simulation of the piezopotential distribution in the ZnO core under compressive strain. (c) Schematic energy
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 eect of switching the piezoelectric potential on the SBH.
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light (372548 nm). The photocurrent and sensitivity
of the ZnOCdS wire photodetector is 10
3
times high-
er than that of CdS nanoribbon, and the responsivity
(11 A W
1
, 548 nm, at 2 V bias) is nearly 100 times
higher than that of ZnSe-nanobelt-based blue/UV-light
sensor (0.12 A W
1
, 400 nm, at 30 V bias). Moreover, the
performance of the ZnOCdS 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 eect when
thedeviceissubjectedtoa0.31% compressive
strain. This investigation extends the application
of the piezo-phototronic eect in a wide spectrum
photon detector.
EXPERIMENTAL SECTION
ZnOCdS 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.
13
Then, the CdS NW array was grown
on ZnO micro/nanowire by hydrothermal method. Briey,
1 mmol Cadmium nitrate (Cd(NO
3
)
2
34H
2
O) and 3 mmol
Thiourea (CH
4
N
2
S) 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 Teon-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 nal ZnOCdS wire products
were characterized by TEM [JEOL 100CX at 100 kV, JEOL
4000EX at 400 kV, TF30 at 300 kV].
The device was fabricated following the method described in
the literature.
10,11
In brief, two ends of a single ZnOCdS wire
were xed 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-
vice and make it optically transparent, exible, and robust under
repeated mechanical strains. (see Supporting Information, Fig-
ure S1). One end of the device was axed 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
device was monitored by a Nikon Eclipse Ti inverted microscope
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 lters and determined by a thermopile power
meter (Newport 818P-001-12). Meanwhile the IVcharacteris-
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
nancial interest.
Acknowledgment. Research was supported by Airforce,
MURI, U.S. Department of Energy, Oce 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,
Guang Zhu, Caofeng Pan, Simiao Niu and Lin Dong for technical
assistance.
Supporting Information Available: (1) Schem atic diagram
and optical microscopy image of a ZnOCdS wire-based
photodetector, schematic diagram of the measurement sys-
tem; (2) performance of single ZnOCdS wire-based photo-
detector under illumination of UV light centered at 372 nm.
This material is available free of charge via the Internet at
http://pubs.acs.org.
REFERENCES AND NOTES
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ARTICLE
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    Full-text · Article · Sep 2015
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  • [Show abstract] [Hide abstract] ABSTRACT: A facile and cost-effective fabrication approach of active strain sensor based on individual ZnO micro/nanowire was demonstrated. By connecting a ZnO micro/nanowire along polar growth direction with two Ag electrodes on flexible polystyrene (PS) substrate, the fabricated strain sensor was obtained as a typical M-S-M structure. The I-V characteristic of the device was highly sensitive to the strain caused by the obvious change of Schottky barrier height (SBH). Furthermore, both of the symmetric and asymmetric changes of the SBH at the source and drain were observed during device testing process. The respective contribution of piezoresistance effect and the piezoelectric effect to the change of SBHs were also systematically investigated.
    Article · Jan 2013
  • [Show abstract] [Hide abstract] ABSTRACT: We investigate the electronic band structures of Ge/Si core–shell nanowires (CSNWs) and devise a way to realize the electron quantum well at Ge core atoms with first-principles calculations. We reveal that the electronic band engineering by the quantum confinement and the lattice strain can induce the type-I/II band alignment transition, and the resulting type-I band alignment generates the electron quantum well in Ge/Si CSNWs. We also find that the type-I/II transition in Ge/Si CSNWs is highly related to the direct to indirect band gap transition through the analysis of charge density and band structures. In terms of the quantum confinement, for [100] and [111] directional Ge/Si CSNWs, the type-I/II transition can be obtained by decreasing the diameters, whereas a [110] directional CSNW preserves the type-II band alignment even at diameters as small as 1 nm. By applying a compressive strain on [110] CSNWs, the type-I band alignment can be formed. Our results suggest that Ge/Si CSNWs can have the type-I band alignment characteristics by the band structure engineering, which enables both n-type and p-type quantum-well transistors to be fabricated using Ge/Si CSNWs for high-speed logic applications.
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  • [Show abstract] [Hide abstract] ABSTRACT: Branched ZnO-CdS double-shell NW array on the surface of a carbon fiber (CF/ZnO-CdS) was successfully synthesized via a facile two-step hydrothermal method. Based on a single CF/ZnO-CdS wire on a polymer substrate, a flexible photodetector was fabricated, which exhibited ultrahigh photon responsivity under illuminations of blue light (1.11×105 A/W, 8.99×10-8 W/cm2, 480 nm), green light (3.83×104 A/W, 4.48×10-8 W/cm2, 548 nm) and UV light (1.94×105 A/W, 1.59×10-8 W/cm2, 372 nm) respectively. The responsivity of this broadband photon sensor was enhanced further by as high as 60% when the device was subjected to a -0.38% compressive strain. This is because that the strain induced a piezopotential in ZnO which tunes the barrier height at the ZnO-CdS heterojunction interface, leading to an optimized optoelectronic performance. This work demonstrates a promising application of piezo-phototronic effect in nano-heterojunction array based photon detectors.
    Article · Apr 2013
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    Full-text · Article · Apr 2013
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