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Volume 2 Number 7 21 February 2014 Pages 1167–1348
Journal of
Materials Chemistry C
Materials for optical and electronic devices
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Themed issue: Flexible Electronics
ISSN 2050-7526
PAPER
Ming-Qiang Zhu, Guozhen Shen et al.
High performance rigid and ﬂexible visible-light photodetectors based on
aligned X(In, Ga)P nanowire arrays
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Materials Chemistry C
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PAPER
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Cite this: J. Mater. Chem. C, 2014, 2,
1270
High performance rigid and ﬂexible visible-light
photodetectors based on aligned X(In, Ga)P
nanowire arrays†
Gui Chen,ab Bo Liang,ab Zhe Liu,ab Gang Yu,a Xuming Xie,ab Tao Luo,ab Zhong Xie,a
Di Chen,a Ming-Qiang Zhu*a and Guozhen Shen*b
InP and GaP nanowires (NWs) were synthesized via a simple thermal evaporation method for applications as
high performance visible-light photodetectors. Individual InP NW ﬁeld-eﬀect transistors (FETs) were
fabricated to study their electronic transport and photoresponse characteristics, which exhibited typical
n-type transistor characteristics with an eﬃcient electron mobility of 1.21 cm2 V1 s1, a fast response
time (0.1 s) and good sensitivity with a spectral responsivity of 779.14 A W1 and a high quantum
eﬃciency of 1.53 105% to visible light irradiation. Using the contact printing process, large scale
aligned InP NW arrays were assembled on both rigid SiO2/Si and ﬂexible PET substrates. Both rigid and
ﬂexible InP NW array based photodetectors demonstrated excellent photoresponse performance,
especially a faster response, for example, from 0.1 s to 80 ms. In addition, the ﬂexible InP NW array
Received 2nd August 2013
Accepted 29th August 2013
based photodetectors exhibited good ﬂexibility, good folding endurance and electrical stability. Using
similar processes, aligned GaP NW array based photodetectors were also fabricated on SiO2/Si and PET
substrates, which also exhibited fast, reversible, and stable photoresponse properties. These merits
DOI: 10.1039/c3tc31507j
demonstrate that the as-prepared InP and GaP NWs are good candidates with substantial potential for
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future electronic and optoelectronic nanodevice applications.
Introduction
One-dimensional (1D) semiconductor nanostructures have
attracted extensive interest due to their unique electronic,
optical, thermal and mechanical characteristics, which are
excellent building blocks for next-generation integrated nanodevices.1–8 Many studies have been carried out to fabricate
various types of nanodevices based on 1D nanostructures such
as solar cells, lithium-ion batteries, supercapacitors, gas
sensors, photodetectors, eld-eﬀect transistors (FETs), lightemitting diodes (LEDs) and so on.9–25 Among these nanodevices, photodetectors have gained particular attention
because of their wide applications in light-wave communication, imaging techniques and integrated circuits.26 Moreover,
compared with conventional photodetectors based on thinlms or bulk materials, photodetectors based on 1D
a
Wuhan National Laboratory for Optoelectronics and School of Optical and Electronic
Information, Huazhong University of Science and Technology, Wuhan 430074, China.
E-mail: [email protected]
b
State Key Laboratory for Superlattices and Microstructures, Institute of
Semiconductors, Chinese Academy of Sciences, Beijing 100083, China. E-mail:
[email protected]
† Electronic supplementary
10.1039/c3tc31507j
information
1270 | J. Mater. Chem. C, 2014, 2, 1270–1277
(ESI)
available.
See
DOI:
nanostructures show much better performance because of their
large surface-to-volume ratios and rational designed
surfaces.26–28
Semiconducting III–V compounds are of scientic and
technological importance because of their high electronic
mobility and eﬃcient luminescence properties. For example,
Indium phosphide (InP) has a direct band gap of 1.34 eV at
room temperature and the band gap of Gallium Phosphide
(GaP) is 2.26 eV. They have demonstrated great potential in
photonic and photoelectronic applications.29–32 To date, many
reports could be found for the synthesis of InP or GaP 1D
nanostructures with diﬀerent morphologies, such as nanowires, nanotubes, unconventional zigzag nanowires, nanosprings, etc.33–41 Optical properties and electronic transport
properties of the produced InP or Gap 1D nanostructures were
also studied. However, little work has been done to investigate
the photoresponse properties of 1D InP or GaP nanostructures,
regardless of their principally high potential for optical switches
and optical photodetectors.
Herein, we report the synthesis of high-quality InP and GaP
NWs via a simple thermal evaporation method. Electrical
transport and photoresponse properties of the as-synthesized
InP NWs were studied by fabricating individual NW devices
on a SiO2/Si substrate. Using a simple contact printing process,
aligned InP and GaP NW arrays were easily obtained to fabricate
both rigid and exible photodetectors with a fast response to
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visible light irradiation. Especially, the as-fabricated exible
devices displayed extreme exibility, good folding endurance
and electrical stability.
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Experimental section
High-quality n-type X(In, Ga)P NWs were synthesized in a
horizontal furnace via a simple thermal evaporation method. In
a typical process, commercially available InP (or GaP) powder
was put in a quartz boat placed in a furnace. Silicon wafers
coated with gold nanoparticles (20 2.0 nm) were used as
substrates to collect the products in the downstream. The
reaction system was rst pumped to eliminate the remaining
air, and then high-purity Ar gas was introduced through the
system at a rate of 100 sccm. The furnace was rapidly heated to
950 C within 25 min and kept at that temperature for 2 h. Aer
reaction, the furnace was cooled to room temperature and darkyellow wool-like products were found deposited on the Si
substrate. The as-prepared product was characterized using an
X-ray diﬀractometer (XRD, X’pert Pro, PANalytical B.V., Netherlands), a scanning electron microscope (SEM, Hitachi S4800)
and a transmission electron microscope (TEM, JEOL JEM3000F) equipped with an energy-dispersive X-ray spectrometer
(EDS). The photoluminescence (PL) spectrum was collected at
room temperature with an HORIBA Jobin Yvon LabRAM Spectrometer HR 800 UV with a He–Cd laser line at 514 nm as the
excitation source.
To study the electrical transport and photoresponse properties of the NWs, single-NW eld-eﬀect transistors (FETs) and
individual nanowire photodetectors were fabricated via a
traditional photolithography process. The as-obtained NWs
were rst dispersed in alcohol and then deposited onto a
degenerately doped silicon wafer covered with a 500 nm SiO2
layer. Using the contact printing process,42 large scale aligned
InP or GaP NW arrays were assembled on both the rigid SiO2/Si
substrate and the exible PET substrate. Aer the wafer was
dried in air, UV lithography, thermal evaporation and li-oﬀ
processes were performed to pattern the Cr/Au drain and
source electrodes (10 nm/100 nm) on both ends of the NWs. The
electrical transport measurements of single nanowire devices
were conducted by the four-probe station with a semiconductor
characterization system (Keithley 4200-SCS). The incident
power of the light was measured using an Ophir NOVA power
meter. Monochromatic light from a source composed of a
tungsten lamp (250 W) and a monochromator (WDG15-Z) was
focused and guided onto the semiconductor NW. All measurements were performed in air and at room temperature.
Fig. 1 (a and b) SEM images of the as-synthesized InP NWs. (c) XRD
pattern of the InP NWs. (d) Room-temperature PL spectrum of InP
NWs.
In2O3 or other crystalline phases was found, revealing the
formation of a pure InP product. Fig. 1d depicts the roomtemperature PL spectrum of the as-synthesized product. From
the spectrum, it can be clearly seen that a strong emission peak
is located at 809 nm (about 1.53 eV). Compared with the bulk
InP PL (about 925 nm, 1.34 eV), it exhibits a distinct blue shi,
which can be explained by the defects in the NWs, according to
previously reported results.31
To further evaluate the crystal quality and elemental
composition of the as-synthesized NWs, the TEM, HRTEM
images and the EDS spectrum of InP NWs are recorded and the
corresponding results are shown in Fig. 2. TEM images of the
nanowire shown in Fig. 2a and b indicate that the NWs have
well-dened 1D geometry with smooth surfaces and uniform
diameter along the whole wire length. Fig. 2c shows the top
portion of several NWs, which clearly shows Au particles at the
Results and discussion
Fig. 1a and b show the SEM images of the as-synthesized InP
NWs at diﬀerent magnications, indicating the formation of
NWs with a very high yield. Typical NWs have a uniform
geometry with diameters in the range of 50–100 nm and the
lengths of several tens of micrometres. Fig. 1c depicts the XRD
pattern of the as-prepared product. All the diﬀraction peaks can
be assigned to cubic InP (JCPDS, no. 32-0452). No peak from
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(a)–(c) TEM images, (d) EDX spectrum and (e) HRTEM image of
the InP NWs.
Fig. 2
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tips of the NWs, revealing that the NWs were grown via a
conventional vapor–liquid–solid (VLS) mechanism. The EDS
spectrum shown in Fig. 2d reveals that the NW is composed of
In and P with a composition of ca. 1 : 1, close to the stoichiometry of InP. In the spectrum, signals from Cu and O elements
are from the TEM grid and the surface oxide layer of the NWs
(see ESI, Fig. S1†), respectively, further indicating the formation
of a pure InP product. The HRTEM image shown in Fig. 2e is
used to study the detailed structure of an individual InP nanowire. We can see from the image that the NW exhibited good
structural crystallinity. A periodically twinned zincblende (ZB)
crystal structure with the [111] grown direction was observed,
which is consistent with the literature value.34 Moreover, the
HRTEM image shown in the inset of Fig. 2e reveals two sets of
adjacent lattice fringes with interplanar spacings of 0.34 and
0.29 nm, respectively, corresponding to the (111) and (200)
planes of the zincblende (ZB) InP phase, which is in good
agreement with the XRD result.
In order to study the electronic transport properties of the
InP NWs, eld-eﬀect transistors (FETs) based on single InP
nanowires were fabricated on a SiO2/Si substrate, and their
corresponding results are shown in Fig. 3. Fig. 3a depicts the
drain current (Ids) versus source–drain (Vds) curves measured at
various gate voltages (Vgs). The single InP NW FET exhibits
obvious gate dependence with good conductivity. With a
Paper
positively increased gate voltage (0–20 V), the conductance of
the device gradually increases, revealing typical n-type semiconducting behavior, which is in agreement with the result
observed by Johnny C. Ho et al.34 Meanwhile, clear contact
resistance can be found at the low Vds regime, which may be due
to the energy barrier between electrodes and NWs induced by
the thick surface oxide layer of the InP NWs (see ESI, Fig. S1†).
The Ids–Vgs curve was also investigated for the same device and
the result is shown in Fig. 3b. From the curve, it can be seen that
for an identical Vds, Ids increases as Vgs increases, which is
consistent with an n-type transistor.
In addition, the threshold voltage (Vth) is determined to be
about 2.7 V by extrapolating the linear region of the Ids–Vgs curve
in Fig. 3b. The small threshold voltage is important to decrease
the power consumption. The transconductance (gm) of the
device can be derived from the linear region of Vgs shown in
Fig. 3b via the following equation:
gm ¼ dIds/dVgs
The value is calculated to be around 33 nS. The mobility (m)
of the single InP NW device was calculated by applying the
following equation:24
m ¼ gmL2/(VdsCi)
where L (4 mm) is the NW channel length and Ci is the NW
capacitance, which can be calculated from the equation:
Ci ¼ 2p303sL/ln(2h/r)
Fig. 3 Transistor characteristics of a back-gate single InP nanowire
ﬁeld-eﬀect transistor (FET) on a SiO2/Si substrate. (a) Ids–Vds curves at
various Vgs. (b) Ids–Vgs curve measured at Vds ¼ 10 V. The inset is
the SEM image of a nanowire transistor with a channel length of about
5 mm.
1272 | J. Mater. Chem. C, 2014, 2, 1270–1277
where h (500 nm) is the thickness of the dielectric SiO2 layer, r
(136.5 nm) is the NW radius, 3s is the relative dielectric constant
of SiO2 (3s ¼ 3.9) and 30 is the vacuum dielectric constant (30 ¼
8.85 1012 F m1). From the data shown in Fig. 3d, the
capacitance Ci and eﬀective mobility m can be estimated to be
4.36 1016 F and 1.21 cm2 V1 s1, respectively. The eﬀective
mobilities of our devices range from 1–10 cm2 V1 s1 according
to our experiments (Fig. S2†).
To study the photoresponse behavior of the InP nanowires,
single NW photodetectors were fabricated by using standard
lithography on a rigid SiO2/Si substrate. Monochromatic light
illumination was vertically on the device with two adjacent
(Cr/Au) electrodes and the corresponding photoresponse properties were determined (as shown in Fig. 4). A schematic illustration of the single NW photodetector is depicted in Fig. 4a.
The upper le inset in Fig. 4b shows the SEM image of the
device with a channel width of 5 mm between the two adjacent
electrodes. Fig. 4b shows the I–V plots of the device illuminated
with 633 nm light of diﬀerent intensities and in the dark. It can
be seen that the photocurrent increases as the light intensity
increases, consistent with the result that the charge carrier
photogeneration eﬃciency is proportional to the absorbed
photon ux. The corresponding dependence of a photodetector
on light intensity can be tted to a power law (Fig. 4c), I ¼ APb,43
where I is the photocurrent, A is a proportionality constant, P is
the light intensity, and b is an empirical value. From the curves,
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Fig. 4 (a) Schematic illustration of a single InP NW photodetector on a
SiO2/Si substrate. (b) Photocurrent versus voltage plots of the device in
the dark and under 633 nm light illumination at diﬀerent intensities.
The upper left inset: SEM image of the device. (c) Photocurrent versus
light intensity plot at a bias of 2 V. The corresponding function is I ¼
10.4P0.75. (d) Photocurrent versus time plots of the device obtained
using red light (633 nm) at an intensity of 3.15 mW cm2 at a bias of 2 V.
the photocurrent shows a strong dependence on light intensity
and depicts a relationship of I P0.75, indicating superior
photocurrent capability of the InP NWs. The photoresponse
switching behavior of the device was measured by periodically
switching the 633 nm light on and oﬀ at varying bias voltages, as
shown in Fig. 4d and Fig. S3.† At identical applied voltages, the
photocurrent increases sharply and reaches a steady state at the
“ON” state upon light illumination, and it decreases quickly at
the “OFF” state aer the light was turned oﬀ, revealing the
excellent stability and reproducibility of the single InP NW
photodetectors. Meanwhile, from the response curve, the
response and decay times can be estimated to be about 0.1 and
0.46 s, respectively.
The spectral responsivity (Rl) and external quantum eﬃciency (EQE) are two key parameters used to evaluate the suitability of NWs for photodetector applications.44 The spectral
responsivity (Rl) of the photodetector, dened as the photocurrent per unit power on the eﬀective area of the photodetector, is measured at a light intensity of 3.15 mW cm2 at 2 V
bias and can be calculated to be 779.14 A W1 by the
relationship:
Rl ¼
DI
PS
(1)
where DI(DI ¼ Iphoto Idark) is the diﬀerence between the
photocurrent and dark current, P is the light power irradiated
on the NWs, S is the eﬀective illuminated area on a single
nanowire. The external quantum eﬃciency (EQE) is dened as
the number of electrons detected per incident photon and is
expressed by the relationship:
EQE ¼
hc
Rl
el
(2)
where h is Planck’s constant, c is the velocity of light, e is the
electronic charge and l is the exciting wavelength. In our
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experiments, the single InP NW photodetector was irradiated by
red light (633 nm) at a light intensity of 3.15 mW cm2. The
EQE of the photodetector (1.53 105%) can be calculated at a
bias of 2 V.
To investigate the photocurrent properties of the aligned InP
NW arrays, photodetectors on SiO2/Si substrates were rst
fabricated. Fig. S4† shows the SEM image of the large scale
ordered NW arrays obtained on a rigid SiO2/Si substrate by the
contact printing process. Fig. 5a depicts the schematic illustration of the aligned InP NW array device. A red light with a
typical wavelength of 633 nm was used as an irradiation
source. The power of the light can be controlled from 0.96–
3.15 mW cm2. The distance between the device and the irradiation source is kept at about 10 cm. All the measurements
were performed at room temperature in air. Fig. 5b displays the
photocurrent versus time plots of the single InP NW photodetector on a rigid SiO2/Si substrate irradiated with the 633 nm
light at a bias of 2 V, 4 V and 8 V, respectively. The light intensity
was kept constant at 3.15 mW cm2. The irradiation source was
switched on and oﬀ periodically at 10 s intervals. From the
curve, it can be seen that the device exhibits excellent photoresponse properties. At an identical light intensity, the photocurrent increases with an increase in bias voltage. At a bias of
(a) Schematic illustration of the aligned InP NW array photodetector on a SiO2/Si substrate. (b) Photocurrent versus time plots of
the aligned InP NW array photodetector on a SiO2/Si substrate using
633 nm light at an intensity of 3.15 mW cm2 at a bias of 2 V, 4 V, and 8
V. (c) Current versus voltage plots of the device at diﬀerent intensities
at the same bias of 2 V. The upper left inset: SEM image of the device.
(d) Photocurrent versus light intensity plot at a bias of 2 V. The corresponding function is I ¼ 89.6P0.71. (e and f) Photocurrent rise and
decay of the device measured at a bias of 2 V and at a light intensity of
3.15 mW cm2.
Fig. 5
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2 V, the dark current is around 18.8 nA. However, under red
light irradiation the photocurrent could approach 208.3 nA with
an ON/OFF current ratio of around 11.1. At 4 V bias the dark
current is 24.1 nA, the photocurrent is 423.2 nA and the ON/OFF
ratio is around 17.6. At a higher bias of 8 V, the dark current is
29.8 nA, the photocurrent increases to 577.3 nA, and the
ON/OFF ratio is around 19.4. Photocurrent versus voltage plots
of the device at diﬀerent intensities are depicted in Fig. 5c. From
the curve, it can be seen that the photoconductance increased
obviously when the device was illuminated. The photocurrent
increases with an increase in light intensity as well.
Fig. 5d shows the photocurrent versus light intensity plot at a
bias of 8 V. The dependence relationship was tted with the
power law as I P0.71, revealing excellent photocapture in the
aligned InP NW arrays. The enlarged portions of a typical
ON/OFF cycle measured at a bias of 2 V at the light intensity of
3.15 mW cm2 are shown in Fig. 5e and f, respectively. The rise
time and decay time are 80 ms and 2.08 s, respectively. Current
versus voltage (I–V) curves of the single NW photodetectors in
the dark and under illumination with light of various wavelengths were also measured and shown in Fig. S5.† These
results demonstrate that the InP NWs have enormous potential
application as highly photosensitive detectors and eﬃcient
photoswitches.
To evaluate the potential of the aligned NW arrays for
application in exible electronics, the photodetectors based on
aligned InP NW arrays were fabricated on a exible PET
substrate to explore their electrical characteristics. To study the
photoresponse of the exible device, the same red light
(633 nm) was used as an irradiation source. The distance
between the exible device and the irradiation source is also
kept at about 10 cm. All the measurements were performed at
room temperature. Fig. 6a shows the photocurrent versus
(a) Current versus voltage plots of the aligned InP NW array
photodetector on a PET substrate at diﬀerent intensities. The upper
left inset: the digital image of the device. (b) Photocurrent versus light
intensity plot at a bias of 2 V. The corresponding function is I ¼
67.8P0.49. (c) Photocurrent versus time plots of the device under illumination with light of various wavelengths. The light intensity is kept
constant at 3.15 mW cm2. (d) Photocurrent rise of the device
measured at a bias of 2 V.
Fig. 6
1274 | J. Mater. Chem. C, 2014, 2, 1270–1277
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voltage plots of the exible device measured in the dark and
under illumination with red light of various intensities (from
0.32 to 3.15 mW cm2). The voltage was swept from 2 V to 2 V.
It can be seen clearly that the exible device based on aligned
InP NW arrays is also sensitive to the red light and has excellent
photoresponse characteristics. At identical voltages the photocurrent increases with an increase in light intensity. The upper
le inset in Fig. 6a shows a digital image of the exible
photodetector, demonstrating its excellent exibility. The light
intensity dependence of the photocurrent measured at a bias of
2 V is shown in Fig. 6b. From the curve, it can be seen that the
photocurrent increases almost linearly with the increased light
intensity, which is very similar to the device on a rigid substrate.
Moreover, by tting the measured data in the curve, the corresponding function is I P0.49, revealing that the photocurrent
exhibits good dependence on light intensity, which further
indicates excellent photocapture in the InP NWs. Fig. 6c shows
the photocurrent versus time plots of the aligned InP NW array
photodetector on the PET substrate irradiated by light of
diﬀerent wavelengths. The light intensity was kept constant at
3.15 mW cm2. The interval of 10 seconds was used to control
the ON and OFF states of the irradiation source. From the curve,
it can be seen that the aligned InP NW array exible device
exhibits excellent photoresponse properties with the current
ON/OFF ratios of 3.8, 5.3, 5.7 and 7.5 for diﬀerent-wavelength
light of 350, 520, 633 and 880 nm, respectively. At identical
light intensity and applied voltage, the photocurrent increases
with an increase in light wavelength (from 350 to 633 nm).
However, under 880 nm light irradiation the photocurrent can
be reduced to 87.8 nA. The commonly observed phenomenon
can be originated from the enhanced absorption of high-energy
photons at or near to the surface region of semiconductor
materials.43
In addition, the exible device also exhibits good stability
and reproducibility, and is sensitive to ultraviolet light and
visible light. Compared with the aligned InP NW array device
(208.3 nA) on a rigid SiO2/Si substrate, the exible device
(107.4 nA) measured under identical conditions (light: 633 nm,
intensity: 3.15 mW cm2, bias voltage: 2 V) has much lower
photocurrent, which is usually attributed to the worse contact
between NW and the exible substrate compared with the rigid
SiO2/Si substrate according to previous reports.42 Fig. 6d shows
the rise time of the exible device measured using diﬀerentwavelength light at a bias of 2 V and the light intensity of
3.15 mW cm2. It can be seen that the rise time on 350, 520, 633
and 880 nm light is about 0.13 s, 0.15 s, 0.11 s and 0.21 s,
respectively. These results demonstrate that the exible aligned
InP NW array photodetectors have excellent stability and fast
response towards visible light, which may have application in
exible optoelectronic switches and photodetectors.
In order to accommodate to practical application of exible
electronic devices, the electrical properties of the exible device
aer bending should remain unchanged. We also measured
here the electrical properties of our exible photodetectors. The
exible device was xed on two X–Y mechanical stages. Each
end of the device was placed on one stage. By adjusting the
distance of the two adjacent stages, the bending curvature of the
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diﬀerent states (the upper insets in Fig. 7c). It demonstrates that
the photocurrent of the exible device was hardly inuenced by
external bending stress. These results indicate the extreme
exibility, good folding endurance and electrical stability of the
exible device.
Moreover, we also fabricated aligned GaP nanowire array
photodetectors on both rigid SiO2/Si substrate and exible PET
substrates by the same process. The characterization of asgrown GaP nanowires is shown in Fig. S6.† The photoconductivity of the aligned GaP nanowire arrays can be investigated.
The SEM image of the device is shown in the upper inset of
Fig. 8a. The Cr/Au (10 nm/100 nm) parallel electrodes with
8 mm separation were deposited on aligned nanowire arrays
printed on a SiO2/Si substrate. The uncovered part of the
nanowire arrays was exposed to light. Fig. 8a shows typical
current versus voltage plots of the device measured in the dark
and under 550 nm light illumination at diﬀerent intensities,
respectively. It can be seen clearly that at identical voltages, the
photocurrent of the device increases as the intensity increases,
which is in good agreement with the result of the InP nanowire
array based device observed in Fig. 5c. The photocurrent versus
light intensity can be measured at a bias voltage of 2 V under
Fig. 7 Current versus voltage plots of the ﬂexible aligned InP NW array
based photodetector under red light illumination. (a) Without bending
and (b) after 20, 40, 60, 80, 100, 120 and 140 cycles of bending. (c) I–T
curve of the ﬂexible device when bent with various curvatures under a
bias voltage of 2 V. The upper insets are the corresponding digital
images of the device in ﬁve bending states.
exible device was precisely controlled. The electrical stability
of the exible aligned InP NW array based photodetector was
examined at various bending curvatures. As shown in the upper
insets of Fig. 7c, ve diﬀerent bending states of the exible
device were studied and labelled as state I, II, III, IV, and V,
respectively. Bending of the device from states I to V followed by
releasing of it back to state I was considered as one cycle. Fig. 7b
shows the I–V curves of the device aer bending for various
cycles. From the curves, it can be seen that, compared with the
conductance of the exible device without bending (Fig. 7a), the
conductance endurance of the device remains almost constant
even aer 20, 40, 60, 80, 100, 120 and 140 cycles of bending,
revealing the good folding endurance of the exible aligned InP
NW array photodetector. The bending stress is also a key factor
aﬀecting the properties of the exible device. Fig. 7c shows the
I–T curve of the exible photodetector under illumination. It
can be seen that the photocurrent ow through the exible
device nearly kept unchanged at a xed voltage of 2 V at ve
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Fig. 8 (a) Current versus voltage plots of the aligned GaP NW array
photodetector on a SiO2/Si substrate at diﬀerent intensities. The upper
left inset: the SEM image of the device. (b) Photocurrent versus light
intensity plot at a bias of 2 V of the rigid device. The corresponding
function is I P0.33. (c) Photocurrent versus time plots of the rigid
device under illumination with light of various wavelengths. (d) Zoomin view of middle cycle at a bias of 2 V when illuminated with a 550 nm
light. (e) I–V plots of the aligned GaP NW array based ﬂexible device in
the dark and under 550 nm light illumination. The upper left inset: the
digital image of the device. (f) I–T plots of the ﬂexible device at a bias of
2 V. The light intensity is kept constant at 3.15 mW cm2.
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Journal of Materials Chemistry C
550 nm light illumination. The dependence relationship can be
tted with the power law as I P0.33, revealing excellent photocapture in the GaP NWs. Fig. 8c depicts the typical current
versus time (I–T) properties of the devices under diﬀerent
wavelength illumination of 325 nm, 450 nm, 490 nm, 550 nm
and 633 nm, respectively. The device is further proved to exhibit
excellent photoresponse properties. At an identical applied
voltage (2 V), the photocurrent markedly increases as light
wavelength is increased from 325 to 550 nm, and decreases
under 633 nm light illumination. At these wavelengths of the
light, the photoresponse exhibits an obvious increase when the
device is illuminated with light of energy above the threshold
excitation energy of 2.26 eV (549 nm). The photocurrent of
0.83 nA was recorded when the device was illuminated with
550 nm light at a light intensity of 3.15 mW cm2. The photocurrent under 520 nm light irradiation was more than 2 times
that under dark conditions. Furthermore, the analysis from an
enlarged view of the photoresponse process (Fig. 8d) indicates
that the device has a fast detection time. The rise time and decay
time of the device are about 60 ms and 2.1 s, respectively.
Flexible photodetectors based on aligned GaP NW arrays also
were fabricated on the PET substrate, and the device was
measured under the same conditions used in the rigid device.
Photoresponse properties of the exible device are shown in
Fig. 8e and f. From the curves, the exible devices also exhibit
excellent photoresponse performance. Under 550 nm light
illumination, the photocurrent dramatically increases
compared to the dark state. Furthermore, the device shows
outstanding mechanical exibility and electrical stability. All
the above results indicate that the device based on aligned GaP
NW arrays on both rigid SiO2/Si and exible PET substrates has
excellent stability, reproducibility, and a fast detection time,
which will exhibit good advantage for application in the next
generation high-speed and high-sensitivity large scale photodetectors and photoswitches.
Conclusions
High-quality X(In, Ga)P NWs were fabricated via a simple
thermal evaporation method. Individual InP NW devices on a
rigid SiO2/Si substrate were fabricated to study their electronic
transport and photoresponse characteristics. Single InP NW
photodetectors showed a fast response time (0.1 s) and good
sensitivity to visible light. Using the contact printing process,
photodetectors based on aligned InP and GaP NW arrays were
fabricated on a rigid SiO2/Si substrate and a PET substrate,
respectively. Both the rigid and the exible devices exhibited
excellent photoresponse performance to visible light, for
example, good wavelength selectivity, light intensity sensitivity
and a quite fast response speed of a couple of tenths of milliseconds. Flexible photodetectors based on aligned InP and GaP
NW arrays displayed extreme exibility, good folding endurance
and electrical stability. Our results reveal that the as-synthesized
X(In, Ga)P NWs have enormous potential application in highly
photosensitive detectors and eﬃcient photoswitches for nanoscale optoelectronics.
1276 | J. Mater. Chem. C, 2014, 2, 1270–1277
Paper
Acknowledgements
This work was supported by the National Natural Science
Foundation (61377033, 51002059, 20874025, 21174045), the
Program for National Basic Research Program of China (grant
no. 2013CB922104), the 973 Program of China (2011CB933300),
and the Program for New Century Excellent Talents of the
University in China (grant no. NCET-11-0179). Special thanks
are due to the Analytical and Testing Center of HUST and the
Center of Micro-Fabrication and Characterization (CMFC) of
WNLO for using their facilities.
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