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On-chip surface modified nanostructured ZnO as functional pH sensors

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Zinc oxide (ZnO) nanostructures are promising candidates as electronic components for biological and chemical applications. In this study, ZnO ultra-fine nanowire (NW) and nanoflake (NF) hybrid structures have been prepared by Au-assisted chemical vapor deposition (CVD) under ambient pressure. Their surface morphology, lattice structures, and crystal orientation were investigated by scanning electron microscopy (SEM), x-ray diffraction (XRD), and transmission electron microscopy (TEM). Two types of ZnO nanostructures were successfully integrated as gate electrodes in extended-gate field-effect transistors (EGFETs). Due to the amphoteric properties of ZnO, such devices function as pH sensors. We found that the ultra-fine NWs, which were more than 50 μm in length and less than 100 nm in diameter, performed better in the pH sensing process than NW–NF hybrid structures because of their higher surface-to-volume ratio, considering the Nernst equation and the Gouy–Chapman–Stern model. Furthermore, the surface coating of (3-Aminopropyl)triethoxysilane (APTES) protects ZnO nanostructures in both acidic and alkaline environments, thus enhancing the device stability and extending its pH sensing dynamic range.
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On-chip surface modified nanostructured ZnO as functional pH sensors
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2015 Nanotechnology 26 355202
(http://iopscience.iop.org/0957-4484/26/35/355202)
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On-chip surface modied nanostructured
ZnO as functional pH sensors
Qing Zhang, Wenpeng Liu, Chongling Sun, Hao Zhang, Wei Pang,
Daihua Zhang and Xuexin Duan
State Key Laboratory of Precision Measuring Technology and Instruments, College of Precision Instrument
and Opto-electronics Engineering, Tianjin University, Tianjin 300072, Peoples Republic of China
E-mail: dhzhang@tju.edu.cn and xduan@tju.edu.cn
Received 4 March 2015, revised 18 May 2015
Accepted for publication 21 May 2015
Published 12 August 2015
Abstract
Zinc oxide (ZnO) nanostructures are promising candidates as electronic components for
biological and chemical applications. In this study, ZnO ultra-ne nanowire (NW) and nanoake
(NF) hybrid structures have been prepared by Au-assisted chemical vapor deposition (CVD)
under ambient pressure. Their surface morphology, lattice structures, and crystal orientation were
investigated by scanning electron microscopy (SEM), x-ray diffraction (XRD), and transmission
electron microscopy (TEM). Two types of ZnO nanostructures were successfully integrated as
gate electrodes in extended-gate eld-effect transistors (EGFETs). Due to the amphoteric
properties of ZnO, such devices function as pH sensors. We found that the ultra-ne NWs, which
were more than 50 μm in length and less than 100 nm in diameter, performed better in the
pH sensing process than NWNF hybrid structures because of their higher surface-to-volume
ratio, considering the Nernst equation and the GouyChapmanStern model. Furthermore, the
surface coating of (3-Aminopropyl)triethoxysilane (APTES) protects ZnO nanostructures in both
acidic and alkaline environments, thus enhancing the device stability and extending its
pH sensing dynamic range.
Keywords: ZnO nanostructures, pH sensor, eld-effect-transistors
(Some gures may appear in colour only in the online journal)
1. Introduction
In recent years, construction of functional bio-sensing devices
using nanomaterials has triggered tremendous interest in the
eld of nanotechnology [17]. The high surface-to-volume
ratio (S/V ratio) of the nanostructures has the unique advan-
tages of absorbing a lot more biomolecules (e.g., biomarkers
[8], enzymes [9], and bio-receptors [10]) at the sensor surface,
thus increases the sensor sensitivity [810]. Nanostructure-
based biosensors promise to revolutionize the eld of bio-
analytical research by offering ultrasensitive, rapid, and label-
free detection of the target biomolecules [1113]. ZnO is one
of the most promising materials to conduct as a nano-bio-
sensor. It has a wide, direct bandgap and large exciton
binding energy [14,15]. Besides, the diverse and abundant
ZnO nanostructures, such as nanowires (NWs) [1618],
nanoakes (NFs) [19], nanobelts [20,21], nanobows [22],
and nanohelices [23], open novel designs and applications of
nano-devices.
Recently, ZnO nanostructures have been applied for
pH sensing applications due to their unique amphoteric
properties (reacting with both acidic and alkaline solutions)
[24]. Many biological processes release protons during bio-
chemical reactions [25], and various bioelectronics have been
developed based on pH sensors [26], such as glucose [27],
cholesterol [28], and intracellular sensing devices [25]. Ion-
sensitive eld-effect transistors (ISFETs) are the most popular
miniature pH sensors [26]. The mechanism of pH sensing
with an ISFET is based on the change of the surface prop-
erties of the gate dielectric, which responds to different
pH values. The amphoteric nature of the hydroxyl groups at
the solution-dielectric interface allows the surface charge state
to change as it protonates and deprotonates. The most
important component of an ISFET sensor is the pH-sensitive
Nanotechnology
Nanotechnology 26 (2015) 355202 (9pp) doi:10.1088/0957-4484/26/35/355202
0957-4484/15/355202+09$33.00 © 2015 IOP Publishing Ltd Printed in the UK1
membrane, which can respond to the H
+
ions in solutions
[29]. Previous studies have demonstrated the possibilities of
integrating ZnO as pH-sensitive membranes in ISFET.
However, most studies focused on the ZnO nanostructures
prepared by the liquid-phase deposition, which suffers non-
uniformity and relatively large feature size with uncontrol-
lable defects [30]. Therefore, when integrated into functional
devices, they have poor measurement repeatability. Besides,
due to their amphoteric properties, ZnO nanostructures tend to
dissolve in strong acidic or alkaline conditions, which limited
their pH sensing stability. Great efforts have been made to
modify the ZnO nanostructures with different doping mate-
rials (e.g., Si [31], Al [32], and Ta [33]), which stabilized the
ZnO nanostructures and extended their sensing dynamic
range. However, these methods require expensive and com-
plicated procedures, and potentially they would introduce
impurities.
In this work, we fabricated ZnO nanostructures in gas
phase with Au-assisted chemical vapor deposition (CVD),
which has the advantage of fabricating defect-free nanos-
tructures with ultra-high S/V ratio. Two types of nanos-
tructures with different S/V ratios were prepared. Their
pH sensing properties have been compared before and after
the integration of nanostructures with functional eld effect
transistor (FET) devices. In addition, in order to stabilize the
ZnO nanostructures in ionic solutions, a simple amino-sila-
nization step was introduced to modify the ZnO surface, and
therefore it extended their pH sensing range and further
increased their pH sensitivity.
2. Materials and methods
2.1. Materials
ZnO powder and graphite powder were purchased from
Beijing Gold Crown for the New Material Technology Co.,
Ltd (3-Aminopropyl) triethoxysilane (APTES) was purchased
from the Aladdin Industrial Corporation. NaH
2
PO
4
and
C
6
H
8
O
7
were purchased from Tianjin Heowns Biochem LLC.
All materials were used as received without additional
treatment.
2.2. Growth of ZnO Nanostructures by CVD
The ZnO nanostructures were prepared with the following
four steps. Firstly, a 300 nm SiO
2
lm was coated on a Si
substrate by chemical deposition (Applied Materials, Preci-
sion 5000) process to isolate Si from the catalyst metal. Then,
a thin Cr/Au layer was sputtered on the substrate by electron
beam evaporation (CHA-600), where the 5 nm Cr and
25 nm Au lm acted as conducting layer and catalyst,
respectively. Afterwards, NWs and NWNF hybrid nanos-
tructures were grown on the substrate by a vapor liquid solid
(VLS) process. Finally, the prepared substrates were synthe-
sized on a Si/SiO
2
substrate in a tube furnace.
Before the VLS process, the four- inch Si/SiO
2
/Cr/Au
wafer was cut into 0.8 cm ×0.8cm squares. For the CVD
process, source materials consisted of ZnO powder (0.8 g,
purity 99.99%) and graphite powder (0.8 g, purity 99.99%)
(w:w = 1:1). The source material was put in a tube furnace
under O
2
/Ar atmosphere. The furnace was heated up from
room temperature (20 °C) to 820 °C at the rate of
10 °C min
1
, and then from 820950 °C at the rate of
5 °C min
1
. The furnace was kept at 950 °C for growing
nanostructures. In order to grow NWs, the substrates were
mounted vertically in the chamber at the downstream of the
source materials, and to grow NWNF hybrid structures, the
substrates were put horizontally. The distance between the
substrates and the source materials was 1 2 mm. The
growing time was around 30 min. After these steps, the whole
system was naturally cooled down to room temperature.
2.3. Surface functionalization of ZnO
Before functionalization, ZnO NWs and NWNF hybrid
structures were treated with oxygen plasma for 30 s to remove
all the impurities and organic contaminants. This step was
followed by an overnight APTES vapor deposition under
reduced pressure in a desiccator to generate an amino layer on
the ZnO surface.
2.4. Preparation of testing pH solution
The pH test solution was prepared by dissolving NaH
2
PO
4
and C
6
H
8
O
7
in mill-Q water and adjusting to the desired
pH with a strong acid or alkali.
2.5. Characterization and measurement
The top-view and tilted-view microstructures of ZnO NWs
and NWNFs were characterized by scanning electron
microscopy (SEM, FEI Inspect F50). X-ray diffraction (XRD)
was carried out by D/MAX-2500 (Rigaku, Japan). Trans-
mission electron microscopy (TEM, FEI Tecnai G2 F20) was
used to characterize the lattice structure of ZnO NWs.
The pH sensing experiments were measured using an
extended-gate FET (EGFET) setup. A front-end sensing chip
and back-end transistors were integrated on a single printed
circuit board, accompanying signal amplication, and a data
acquisition interface for personal computers. The front-end
sensing chip was a micro-fabricated Au electrode with a
typical width of 20 μm. On top of the Au electrode, different
ZnO nanostructures were incorporated. A mixing cell (solu-
tion chamber) was then created on top of the sensing electrode
by epoxying thin-walled, 8 mm diameter PTFE tubing to the
chip surface and by inserting thinner tubing (0.5 mm) to serve
as the uid supply and return. The solution input tube was
placed directly over the central region of the sensor. This
system enabled continual mixing (equivalent to pipetting up-
and-down) throughout the course of the sensing measure-
ments. A miniature Ag/AgCl reference electrode (Harvard
Apparatus) was inserted into the solution chamber as a
solution gate during all tests. The I
d
V
g
characteristic of off-
chip EGFET at a constant V
ds
(100 mV) could be tested
thoroughly off-line before performing any measurements,
serving as a look-up table for converting the measured drain
2
Nanotechnology 26 (2015) 355202 Q Zhang et al
60000
50000
40000
30000
20000
10000
0
60000
80000
40000
20000
0
30 32 34 36 38 40
020406080100
2 Theta (Degree) 2 Theta (Degree)
Intensity
(arb. unit)
Intensity
(arb. unit)
Zn0 (002)
Zn0 (100)
Zn0 (101)
Zn0 (002)
Si (100)
Au
Figure 1. Characterizations of ZnO nanostructures. SEM images of (a) top-view of NWNFs, (b) tilted-view of NWNFs, (c) tilted-view of
NWs, and (d) zoom-in of (c). TEM images of (e) a typical view of a single NW with 10 μm in length and a selected area electron diffraction
(SAED) image (inset) give the [0001] growth direction, respectively. (f) Typical TEM image with HRTEM picture (top inset) and other
SAED picture (lower inset). XRD patterns of ZnO NWs with (g) wide range from 2080°; (h) narrow range from 3040°.
3
Nanotechnology 26 (2015) 355202 Q Zhang et al
current I
ds
back into the interface potential. IVand DC time
measurements were carried out by a customized system using
a National Instruments data acquisition card and two Keithley
2400 source measure units. All measurements were per-
formed at room temperature.
3. Results and discussions
3.1. Characterization of ZnO nanostructures
In this work, ZnO NWs and NWNFs were successfully
prepared through carefully controlling the CVD process
conditions. As shown in SEM images (gures 1(a)(d)), the
NWNF hybrid structures are around 500 nm in width, while
the NWs are less than 100 nm; both structures extended more
than 50 μm in length. Thus, the nanostructures feature an
ultra-high S/V ratio. As shown in gures 1(a) and (b), the
NWNF hybrid structures are uniformly distributed
throughout the whole substrate, and the NFs are grown on the
top of NWs. As for the NW structures (gures 1(c) and (d)),
the NWs are arranged vertically on the substrate with a great
uniformity in diameter and length. Compared to the NWs, the
NWNF hybrid structures are more diverse in position,
dimension, and shape.
TEM is employed to investigate the crystal structures of
ZnO nanostructures. Figures 1(e) and (f) show the TEM
images of a typical single ZnO NW with a diameter of 80 nm.
The inset of gure 1(e) is the electron diffraction pattern of a
selected area, which demonstrates that the NW grows along
[0001], and the volume is free from dislocations. Inset of
gure 1(f) presents a high-resolution TEM (HRTEM) image,
which shows a lattice distance of c= 5.2 Å. This conrms the
high crystallization quality of ZnO NWs with a clean and
structurally perfect surface. The surface of the NW is atom-
ically sharp and has no sheathed amorphous phase.
Figures 1(g) and (h) show the XRD patterns of ZnO NWs
and NWNF hybrid structures. The XRD images clearly
show the diffraction peaks of ZnO (002) and Si (100) in
gure 1(g). Here, the ZnO (002) diffraction peak is from ZnO
nanostructures, and the Si (100) diffraction peak is from the
Si substrate. Figure 1(h) shows the XRD result from 30° to
40°, which conrms the hexagonal wurtzite phase of the ZnO
nanostructures. The ZnO nanostructures are highly crystal-
line, grown by the CVD process at high temperature (950 °C).
The ZnO diffraction peaks are clearly detected, and the peaks
intensity agrees with SEM and TEM results.
3.2. Device integration
An EGFET setup is used to evaluate the pH sensing proper-
ties of the ZnO nanostructures (gure 2). An EGFET is
developed from an ISFET and has the advantage of main-
taining higher device stability by connecting an auxiliary
sensing electrode to the gate terminal of a conventional
metallic oxide semiconductor eld effect transistor (MOSFET),
thus separating the FET device from the chemical or biological
environment and preventing potential damage of the FET [6
8]. Such a system also facilitates the integrating of ZnO
nanostructures into the FET measurement. Au is used as the
catalyst for ZnO growth in the CVD process, and, in principle,
such an Au layer can be used as the sensing electrode of
EGFET. Thus ZnO nanostructures and EGFETs can share the
same Au layer. However, the Au layer will melt below 900°C
and is not able to survive the high temperature during ZnO
growth (typically during 900950 °C). To integrate the
nanostructures into the EGFET, a metal layer (Cr) is deposited
between the Au and the substrate before growing ZnO. This Cr
layer can survive the high temperature process and strengthen
the adhesion between ZnO nanostructures and the substrate.
The high conductivity of Cr effectively allows for the electrical
changes of ZnO during pH sensing.
3.3. pH sensing results
Figure 3shows the pH sensing results of bare ZnO nanos-
tructures. Figures 3(a) and (b) are typical static measurements
of the I
ds
V
g
curve from the ZnO nanostructures, which are
connected to the gate terminal of the EGFET. A clear dif-
ference can be observed after exposure to different
pH solutions, which indicates successful sensing. We calcu-
lated the transconductance (g
m
)bylineartting of the IV
curve and found that the g
m
change is very small (<4.74%),
and the change of the I
ds
V
g
induced by different pH solutions
can be described as a parallel (threshold voltage) shift of the
I
ds
V
g
. It is also noticed that by changing the same pH, the
threshold voltage shift of the ZnO NW modied electrode
Figure 2. Schematic diagram of (a) ZnO nanostructure-based
EGFET pH sensor, and (b) APTES-modied ZnO nanostructures.
(c) The test circuit picture.
4
Nanotechnology 26 (2015) 355202 Q Zhang et al
(gure 3(b)) is larger than that of the NWNF-modied elec-
trode (gure 3(a)) (average 34.74 mV pH
1
for NWNFs,
while 36.65 mV pH
1
for NWs).
More reliable pH determination is achieved by real-time
measurements from the same device. Figure 3(c) shows real-
time FET current (I
ds
) responses of ZnO NWs by exposure to
different pH solutions, with V
g
xed at 0.7 V and V
ds
xed at
0.1 V. This sensor immersed for 300s in each buffer solution.
The I
ds
decreased each time when the pH value increased
from 49 and increased as the pH value decreased from 94.
This measurement exhibits excellent real-time response with
multiple scans, which proves the reliability of ZnO NWs as
pH sensors. The potentials of the sensor in gure 3(c) exhibit
almost complete overlapping with less than 0.05 mV differ-
ences, indicating excellent recovery.
Since the solution pH change will induce the surface
potential change of ZnO, we converted the device current
responses to surface potential changes by dividing the
FET transconductance. The surface potential changes of
ZnO NWs and NWNFs in different pH solutions are com-
pared in gure 3(d). The ZnO NWNF sensor is found to
have weak pH sensitivity at 34.74 mV pH
1
, compared with
36.65 mV pH
1
for the ZnO NW sensor.
The ZnO NWs perform better than ZnO NWNF hybrid
structures for pH sensing (125% times higher in signal
intensity). This is due to the fact that NW structures have a
higher S/V ratio than NWNF hybrids, which indicates higher
surface-site densities. The pH response of ZnO nanostructures
is due to the ZnO surface, which can protonate or deproto-
nate, based on the electrolyte solution pH. The surface-site
densities of the ZnO are critical in their pH sensing ability,
according to the Nernst equation and the GouyChapman
Stern model [26]. As shown in formulas [1] and [2] below,
the H
+
absorption percentage of ZnO NWs is larger than that
of NWsNFs when they function as pH sensors.
In acid buffer: ZnO H ZnOH (1)
ss
() ()
+=
++
In basic buffer: ZnO 2H O Zn(OH) H (2)
ss
() 2 3( )
+= +
+
Besides the morphology effects, the chemical surface
functionalization of ZnO nanostructures has also been
investigated to improve their pH sensing performance. In this
work, APTES is used to modify the surface of the ZnO
nanostructures. The results are shown in gure 4. Figures 4(a)
and (b) show the comparison of the static sensor responses for
the APTES-modied ZnO nanostructure sensors, in which the
V
ds
is xed at 0.1 V. The threshold voltage is observed to shift
from lower to higher potential as the pH increases, and the
voltage shift of the NWs is higher than the NWNF hybrids,
which is similar to non-functionalized ZnO. The results in
Figure 3. Sensing response of V
g
versus I
ds
under different buffer solutions of (a) ZnO NWNF hybrid structure and (b) ZnO NW-based
EGFET sensor. (c) Real-time I
ds
responses to buffer solutions of a ZnO NW-based sensor in a pH 4-9-4 loop cycle at V
g
of 0.7 V. (d) The
variation of the reference electrode potential (V
g
) as a function of pH value for ZnO NWs and ZnO NWNF hybrid-based sensors.
5
Nanotechnology 26 (2015) 355202 Q Zhang et al
gure 4(c) show that this sensor is stable during the whole
test. The overlapping difference is less than 0.01 mV, indi-
cating good reproducibility. The APTES- modied sensor
maintains good repeatability under this pH sensing procedure;
this is because APTES can protect ZnO nanostructures in both
acid and alkaline solutions. The real-time response of the
APTES-modied ZnO NW sensor has also been investigated
(gure 4(d)). This sensor has been immersed for 300 s in each
pH buffer solution. The I
ds
differs with the pH value change,
Figure 4. After APTES modication, the sensing responses of V
g
versus I
ds
under different buffer solutions of (a) ZnO NWNF hybrid
structures and (b) a ZnO NW-based EGFET sensor. (c) Repeatability and stability measurement of an APTES-modied ZnO NW-based
sensor under a pH value 4-6-9 and repeated four times. (d) Real-time I
ds
response to buffer solutions of an APTES-modied ZnO NW-based
sensor in a pH 2-9-2 loop cycle at V
g
of 0.7 V.
Figure 5. The variation of gate potential (V
g
) of an EGFET as a
function of pH value change for both APTES-modied and non-
modied ZnO NWs and ZnO NWNF hybrid-based sensors.
Figure 6. Statistical analysis of pH sensing results of ZnO NWs and
NWNF hybrid structures with and without APTES coatings.
6
Nanotechnology 26 (2015) 355202 Q Zhang et al
which also indicates reliable performance in the pH sensing
procedure. The potentials of the sensors in gure 4(d) (and
also gure 3(c)) exhibit a pH response hysteresis [38], which
shows average differences of less than 0.1 mV. Considering
that the sensitivities of ZnO NWs and APTES-coated ZnO
NWs are 36.65 mV pH
1
and 43.22 mV pH
1
, respectively,
the sensitivity and hysteresis are acceptable for the
pH electrode.
Figure 5compares the pH sensitivities of the APTES-
modied and non-modied ZnO nanostructures. In both NWs
and NWNF sensors, the APTES-modied sensors have
higher pH sensitivity than the non-modied sensors
(approximately 185% times higher). This is due to the fact
that APTES can induce more surface sites on ZnO nanos-
tructures, which is critical in pH sensing, as discussed above.
Besides the improvement of pH sensitivity, APTES can
protect ZnO nanostructures in an extremely acidic environ-
ment. We are able to get a pH response at 2, which is not
possible for bare ZnO.
Another consideration is the stability of the APTES
coatings. Silane can be coated either in solution or in gas
phase. In the solution phase, silane molecules tend to react
with each other via cross-linking of the alkoxy units, resulting
in rough, defective, and unordered multilayers. Such multi-
layers of silanization are less stable in aqueous solutions due
to the aqueous hydrolysis of the alkoxysilanes, thus suffering
degradation over time. However, in gas-phase silanization of
small silane molecules (e.g., APTES), usually a monolayer is
formed, and such a coating has better stability in aqueous
solutions. In a recent published paper [39], it has been
demonstrated that APTES-coated ZnO nanocrystals show
rather high aqueous stability. Regarding our results, as is
shown in gure 4(d), the APTES-modied ZnO NWs have
been exposed in pH value 2-4-6-9 and repeated four times.
For each step, the APTES-modied ZnO NW-based sensor
would immerse into the pH solution for 300 s during the test
then be washed in a buffer solution for another 300 s. So the
total test for stability is around 2 h. During this period, we did
not observe degradation of the APTES coatings.
It is also noticed that the device integrated with NWs and
modied with APTES performs better as a pH sensor, which
is in agreement with the results from the non-modied
sensors. The NWs have a higher S/V ratio compared with
NWNF hybrid structures; thus the APTES coating will result
in higher NH
2
density as well. To get more statistical results,
we plotted the pH sensing results with ve different samples.
As shown in gure 6, the differences of ZnO NWs and NW-
NF hybrid structures with and without APTES are statistically
different.
3.4. Discussion of the role of APTES
The amino silane functionalization can protect ZnO in both
acid and alkaline solutions and enhance their chemical sta-
bility. More importantly, according to the Nernst equation
[3037], when all the surfaces of ZnO are protonated (in acid
buffer) or de-protonated (in alkaline buffer) under ideal
conditions, the surface sites can achieve the highest level; thus
we can get the highest surface potential changes. The sensi-
tivity of such a device is supposed to reach 59.1 mV pH
1
(Nernst potential). However, the protonation/deprotonation
process actually cannot reach such extreme conditions and
can only keep dynamic equilibrium according to H
+
ion
concentration. As a result, enhancing the S/V ratio and
increasing surface protonation/ deprotonation sites can
effectively push the pH response closer to the limit value of
the Nernst equation. The introduction of APTES functiona-
lization can ideally increase more sites for protonation/
deprotonation compared with the bare ZnO, as indicated in
gure 7(a). Therefore, it is likely that the pH response of such
a device is contributed not only by ZnO (equations (1), (2))
but also by APTES (equation (3)). Covalently linking APTES
to the ZnO surface results in a surface terminating in both
NH
2
and ZnO groups, which have different dissociation
constants, pKa. At low pH, the NH
2
group is protonated to
NH3
+(equation (3)), which is a dominant reaction compared
with ZnO protonation (equation (1)). This has been proven by
the extension of the pH sensing range (gure 5) and is less
damaging of ZnO after APTES coating. Thus the APTES
protonation functions as a positive gate and decreases the
conductance. At high pH, ZnOH is deprotonated to
Zn(OH)3(s)
(equation (2)), which correspondingly causes an
increase in conductance. The observed linear response can be
attributed to an approximately linear change in the total sur-
face charge density (versus pH) because of the combined acid
and alkaline behavior of both surface groups.
APTES in solution : NH H NH (3)
23
−+
++
Besides the binding sites, the conductance of the ZnO is
another critical factor for reaching the limit value of the
Nernst equation. For a typical n-type ZnO semiconducting
material, APTES modication can provide electronics so as to
reduce the bandgap (the distance of conduction band E
c
and
valence band E
v
) of ZnO, as gure 7(b) shows. Obtaining
additional electronics, the E
c
of ZnO is getting close to the
Fermi level (E
f
), making it much easier to translate the
pH signal from the solution to the sensing board. The
pH response can be delivered with less loss in the transfer
process through more conductive ZnO. As a result, such
combinations of these effects indicate that the APTES
Figure 7. (a) The schematic diagram shows ZnO surface sites to
protonate/deprotonate before (right) and after (left) APTES mod-
ication. (b) Energy-band diagrams of the interface of ZnO and
APTES, where APTES provides electronics to ZnO so as to decrease
the E
c
and make ZnO more conductive.
7
Nanotechnology 26 (2015) 355202 Q Zhang et al
functionalization leads to higher pH sensitivity, as shown in
gure 5.
4. Conclusion
ZnO nanostructures are known for their wide, direct bandgap
and large exciton binding energy. They are promising nano-
materials to conduct as nanoelectronics. ZnO nanostructures
grown from the gas phase (CVD process) usually have better
quality compared with the solution process; however, the high
temperature process hinders their device integration. In this
study, defect-free ZnO nanostructures grown from the CVD
process are successfully prepared and integrated into EGFET.
Due to the unique amphoteric properties of ZnO, such devices
function as pH sensors. Two types of ZnO nanostructures
(ultra-ne NWs and NWNF hybrid structures) are prepared,
and their surface morphology, lattice structures, crystal
orientations, and pH sensing properties are carefully com-
pared. Furthermore, ZnO nanostructures are coated with
APTES monolayers to enhance their stability and pH sensing
dynamic range. Among the four types of ZnO nanostructures,
pH sensors made from the APTES-modied ZnO NWs
exhibit the highest pH sensitivity and largest dynamic
pH sensing range, which are due to their high surface to
volume ratio and surface coating effects (increasing more
surface sites for protonation/deprotonation and higher con-
ductivity). Such surface modication methods are much
simpler compared with the previously reported doping
method and can be applied to other types of nanomaterial-
based devices.
Acknowledgments
The authors gratefully acknowledge nancial support from
Tianjin Applied Basic Research and Advanced Technology
(14JCYBJC41500) and the 111 Project (B07014).
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... The current saturates when the drain-source voltage reaches the difference between the gate-source voltage (V GS ), which is related to the voltage of the RE, and modified threshold voltage (V T ). For the linear region, the relationship is defined by the equation: (38) and for the saturation region, when V DS = V GS − V T , the relationship is defined by ...
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