micromachines
Article
A Highly Sensitive Humidity Sensor Based on
Ultrahigh-Frequency Microelectromechanical
Resonator Coated with Nano-Assembled
Polyelectrolyte Thin Films
Wenpeng Liu, Hemi Qu *, Jizhou Hu, Wei Pang, Hao Zhang and Xuexin Duan *
State Key Laboratory of Precision Measuring Technology & Instruments, Tianjin University,
Tianjin 300072, China; [email protected] (W.L.); [email protected] (J.H.);
[email protected] (W.P.); [email protected] (H.Z.)
* Correspondence: [email protected] (H.Q.); [email protected] (X.D.); Tel.: +86-22-2740-1002 (H.Q. & X.D.)
Academic Editor: Stefano Mariani
Received: 10 March 2017; Accepted: 27 March 2017; Published: 5 April 2017
Abstract: We developed a highly sensitive humidity sensor based on the combination of
ultrahigh-frequency film bulk acoustic resonator (FBAR) and nano-assembled polyelectrolyte (PET)
thin films. The water molecule absorption efficiency was optimized by forming loosely-packed PET
nanostructures. Then, the humidity sensing characteristics were analyzed in terms of sensitivity,
linearity, reversibility, stability and detection limit. As a result, PET-coated FBAR exhibits excellent
humidity sensitivity of 2202.20 Hz/ppm, which is five orders of magnitude higher than quartz crystal
microbalance (QCM). Additionally, temperature dependence was investigated with the result that
PET-coated FBAR possessed a higher sensitivity at low temperature. Furthermore, we realized the
selective detection of water vapor from volatile organic compounds (VOCs) with respect to the
polarity property. Owing to the high sensitivity, miniaturized size and ultrahigh operating frequency,
PET-coated FBAR is uniquely favorable as a wireless humidity sensor node to integrate into wireless
sensor networks (WSNs).
Keywords: humidity sensor; film bulk acoustic resonator (FBAR); ultrahigh frequency;
microelectromechanical resonator; nano-assembled polyelectrolyte (PET) thin films; wireless sensor
networks (WSNs)
1. Introduction
Monitoring of environmental humidity is crucial in fields ranging from meteorology, agriculture
and industry to medicine/healthcare development and food science [1–3]. Based on this need, many
types of humidity sensors based on different transducing techniques have been developed. Among
them, capacitive and resistive humidity sensors have been dominating the markets over the years [4,5].
Other approaches, including surface acoustic waves (SAW) [6], quartz crystal microbalance (QCM) [7],
field effect transistor (FET) [8], and fluorescent emission [9], have also been adopted to monitor
humidity with success. Recently, the trend towards the implementation of wireless sensor networks
(WSNs) [10], in which the spatially-distributed miniaturized sensors monitor physical or environmental
conditions through wireless communication functionalities, requires sensors be small and consume
less energy. As one type of microelectromechanical resonators, film bulk acoustic resonator (FBAR)
has emerged to meet the trend [11,12]. FBAR usually operates at two orders of magnitude higher
frequency than QCM and is cable of detecting small mass changes at even pictogram order. Hence, it
has been extensively employed for fundamental science and practical engineering [13,14]. It features
a greatly reduced dimension of the sensing element, which is beneficial to integrate into miniaturized
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platform and obtain lab-on-a-chip. Also, the compatibility with conventional complementary metal
oxide semiconductor (CMOS) technology promotes the mass production of FBAR with low cost. All
these advantages make FBAR a good candidate as humidity sensor to integrate into WSNs in the
development of internet of things. Nevertheless, the FBAR-based humidity sensor reported in the
literature suffers from the poor sensing characteristics with respect to the sensitivity, detection limit
and stability [11,12,15].
As a crucial component of surface-based sensors, the sensitive layer functionalized on the sensing
area enhances the sensitivity by providing more absorption sites for analytes. Nowadays, materials
with nanostructures such as nanofiber [16], nanowire [17], nanopore [18], and nanotube [19], have
been adopted as humidity-sensitive layers to improve the humidity sensing characteristics. Owing
to the large surface area, these nanostructured materials provide tremendous absorption sites for
water molecules to enhance the surface activity. Among them, nanostructured polyelectrolytes are
of particular interest due to the flexibility, low cost and long-term stability. Molecular surface
self-assembly is a unique approach for the formation of nanostructured polyelectrolyte (PET) thin
films with controlled thickness at nanometer range [20,21]. It is based on alternate exposure of
charged substrate to PET solutions with oppositely charged polycations and polyanions, which
enable electrostatically driven self-assembly of these PETs [22]. In a wide variety of research fields,
molecular surface self-assembly is one of the most prominent methods to build ultrathin PET films,
with many distinct advantages over other methods, including simplicity, controllable thickness and no
requirements of complex equipment.
In this work, we are motivated to develop a highly sensitive humidity sensor using
ultrahigh-frequency FBAR coated with nano-assembled PET thin films. The water vapor absorption
behavior of different morphology of PET thin films was discussed. Then, the sensing characteristics
including linearity, sensitivity, stability, reversibility, selectivity and detection limit were evaluated.
Temperature dependence for humidity measurement was investigated in order to ensure the
performance within a wide range of applications. We also analyzed the selective absorption of water
molecules from volatile organic compounds (VOCs). Owing to the high sensitivity, miniaturized size
and ultrahigh operating frequency, FBAR coated with nano-assembled PET thin films is a potential
candidate as a wireless humidity sensor to realize the integration with WSNs.
2. Materials and Methods
2.1. Materials
Poly(sodium 4-styrenesulfonate) (PSS, Mw = 70,000), poly(diallyldimethytlammonium
choride) (PDDA, Mw < 10,000), poly(acrylic acid) (PAA, Mw = 160,000) and poly(allylamine
hydrochloride) (PAH, Mw = 58,000), were purchased from Sigma Aldrich (St. Louis, MO, USA).
(3-Aminopropyl)triethoxysilane (APTES) were purchased from Aladdin Industrial Corporation
(Shanghai, China). All chemicals were used without further purification. N-propyl alcohol (NPA) and
n-hexane were purchased from Real & Lead Chemical Co. Ltd. (Tianjin, China). PSS/PDDA solutions
were prepared by dissolving them separately into ultrapure water, and ultrasonicated for 15 min.
2.2. Experimental Setup
Figure 1 depicts the schematic diagram of the experimental setup. A customized gas delivery
system was designed to deliver water vapor at a constant temperature of 28 ◦ C. In the system, a channel
(Ch1) was used to bubble out the water vapor through liquid phase by nitrogen gas (99.999%), while
another channel (Ch2) filled with nitrogen gas was employed as a dilution line to adjust the relative
concentration of final mixed gas by changing the ratio of two channel flow rates via flow controllers.
Before each experiment, a commercial humidity sensor was introduced to calibrate the volume
concentration of water vapor inside the detecting chamber. FBAR coated with nano-assembled PET
thin films was mounted onto an evaluation board, and then connected to a computer-controlled
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vector
Santa
Rosa,
CA,
USA)
to simultaneously
record
the
vector network
networkanalyzer
analyzer(VNA,
(VNA,B5071C,
B5071C,Keysight,
Keysight,
Santa
Rosa,
CA,
USA)
to simultaneously
record
frequency
shift.
the frequency shift.
showing
thethe
experimental
setup
for humidity
sensing
using polyelectrolyte
(PET)Figure 1.
1. Cartoon
Cartoon
showing
experimental
setup
for humidity
sensing
using polyelectrolyte
(PET)-coated
filmacoustic
bulk acoustic
resonator
(FBAR).
coated film bulk
resonator
(FBAR).
2.3. Device Fabrication
and
bottom
electrode.
It
FBAR comprises
comprises aa piezoelectric
piezoelectriclayer
layersandwiched
sandwichedby
bythe
thetop
topelectrode
electrode
and
bottom
electrode.
is
fabricated
through
standard
semiconductor
manufacturing
technology,
as
shown
in
Figure
2a–d.
It is fabricated through standard semiconductor manufacturing technology, as shown in Figure 2a–d.
Briefly, an
an air
air cavity
cavity was
was first
first created
created on
on the
the silicon
silicon substrate
substrate by reactive ion etching, and filled with
Briefly,
phosphosilicate glass
glass (PSG)
(PSG) as sacrificial
sacrificial layer
layer by chemical vapor deposition.
deposition. A 600-nm molybdenum
phosphosilicate
(Mo) and 800-nm high-quality c-axis oriented aluminium nitride (AlN) were sequentially
sequentially deposited
by radio frequency (RF) sputter and patterned to create the bottom
bottom electrode
electrode and
and piezoelectric
piezoelectric layer.
layer.
Another 600 nm Mo and 400 nm AlN were sequentially deposited and patterned as the top electrode
Another
and passivation
passivation layer.
layer. A
A 10
10 nm/100
nm/100 nm Cr/Au
Cr/Aucomposite
composite film
film was
was then
then deposited
deposited by physical vapor
deposition and
and patterned
patterned to form
form the
the pads.
pads. Finally, PSG
PSG was
was etched
etched by
by diluted
diluted hydrofluoric
hydrofluoric acid
acid to
to
deposition
completely
release
the
device.
completely release the device.
2.4. Molecular Surface Self-Assembly of Polyelectrolytes (PETs)
(PETs)
As shown in Figure 2e–g, FBAR was first exposed to the air plasma for 5 min to form hydroxyl
groups on a passivation
passivation layer [23], followed
followed by amino-silanization
amino-silanization with APTES in a chemical gas
deposition system (Yield Engineering Systems, Inc., Livermore, CA, USA). Then we adopted both
dipping-assisted and spinning-assisted
methods to realize the molecular surface self-assembly of
spinning-assisted methods
PETs in
in aa layer-by-layer
layer-by-layer fashion.
fashion. For
For the
the dipping-assisted
dipping-assisted method,
method, NH
NH22-functionalized FBAR was
immerged into PSS solution for 2 min and rinsed by ultrapure
ultrapure water
water for
for 30
30 s.
s. The PSS-coated FBAR
was then
dipped
into
PDDA
solution
with
the
same
procedure,
resulting
then dipped into PDDA solution with the same procedure, resulting in
in the
the first
firstPSS/PDDA
PSS/PDDA
bilayer.
films
withwith
various
thickness
were accomplished
by tuningbythetuning
number
of PSS/PDDA
bilayer. PET
PETthin
thin
films
various
thickness
were accomplished
the
number of
bilayers.
For
multiple
assembly,
a homemade
fully automatic
dipping robot
was
used
to
PSS/PDDA
bilayers.
Forbilayers
multiple
bilayers assembly,
a homemade
fully automatic
dipping
robot
was
maintain
the uniformity
of the nano-assembled
PET thinPET
films.
Forfilms.
the spinning-assisted
method,
used to maintain
the uniformity
of the nano-assembled
thin
For the spinning-assisted
NH
FBAR was
fixed
onto
a spin-coating
machine,
the spinning
speed speed
was set
at
method,
NH2-functionalized
FBAR
was
fixed
onto a spin-coating
machine,
the spinning
was
2 -functionalized
2000
solutions
were alternately
pumped
onto FBAR
syringes
by 200 µL/s
forμL/s
5 s,
set atrpm.
2000PSS/PDDA
rpm. PSS/PDDA
solutions
were alternately
pumped
onto via
FBAR
via syringes
by 200
for 5 s, followed by spin-drying for 15 s. An ellipsometer was used to record the thickness of nanoassembled PET thin films.
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followed by spin-drying for 15 s. An ellipsometer was used to record the thickness of nano-assembled
PET
thin films.
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PDDA
(a)
PSS
NH3+ NH 3+ NH3+ NH 3+
(b)
(g)
PSS
NH3+ NH3+ NH 3+ NH3+
(c)
(f)
NH 3+ NH3+ NH3+ NH3+
(d)
(e)
Figure
process
of of
FBAR
and
molecular
surface
self-assembly
of
Figure2.2.Schematic
Schematicillustrating
illustratingthe
thefabrication
fabrication
process
FBAR
and
molecular
surface
self-assembly
PETs.
(a) Etching
of airofcavity
and deposition
of phosphosilicate
glass (PSG);
deposition
of bottom
of PETs.
(a) Etching
air cavity
and deposition
of phosphosilicate
glass (b)
(PSG);
(b) deposition
of
electrode
and piezoelectric
layer; (c) deposition
top electrode
and electrode
passivation
layer;
(d) deposition
bottom electrode
and piezoelectric
layer; (c) of
deposition
of top
and
passivation
layer;
of
pads andofrelease
of PSG;
(e) amino-silanization
of FBAR; (f) nano-assembly
of poly(sodium
(d)Au
deposition
Au pads
and release
of PSG; (e) amino-silanization
of FBAR; (f) nano-assembly
of
4-styrenesulfonate)
(PSS);
(g)
nano-assembly
of
poly(diallyldimethytlammonium
choride)
(PDDA).
poly(sodium 4-styrenesulfonate) (PSS); (g) nano-assembly of poly(diallyldimethytlammonium
choride) (PDDA).
3. Results and Discussion
3. Results and Discussion
3.1. Characterization of FBAR
3.1. Characterization
FBAR of FBAR is compatible with standard wafer-scale semiconductor
The fabricationofprocess
technology,
hence FBAR
possesses
competitive
advantages
for miniaturization
and mass
production
The fabrication
process
of FBAR
is compatible
with
standard wafer-scale
semiconductor
with
low
cost,
which
is
particularly
important
for
integration
as
wireless
sensor
node
into
WSNs.
technology, hence FBAR possesses competitive advantages for miniaturization and mass production
Figure
3a shows
the comparison
of the
size between
FBAR and
QCM, indicating
significantly
smaller
with low
cost, which
is particularly
important
for integration
as wireless
sensor node
into WSNs.
Figure
footprints
of
FBAR.
The
top-left
inset
of
Figure
3a
shows
the
top-view
scanning
electron
microscopy
3a shows the comparison of the size between FBAR and QCM, indicating significantly smaller
(SEM)
image
FBARThe
with
the pentagonal
sensing
area ofthe
20,000
µm2 . This
pentagonal
area
footprints
of of
FBAR.
top-left
inset of Figure
3a shows
top-view
scanning
electronsensing
microscopy
2. This pentagonal
is(SEM)
defined
by the
top electrode
an AlNsensing
passivation
layer protects
top
image
of FBAR
with the with
pentagonal
area oflayer.
20,000The
μmpassivation
sensing area
electrode
from
vapor
and
thus
improves
device
stability.
Additionally,
selective
amino-silanization
is defined by the top electrode with an AlN passivation layer. The passivation layer protects top
on
the passivation
layer
ensures
the selective
assembly
PET thin films
on sensing
area without
electrode
from vapor
and
thus improves
device
stability.ofAdditionally,
selective
amino-silanization
contaminating
Au
pads.
on the passivation layer ensures the selective assembly of PET thin films on sensing area without
Figure 3b presents
contaminating
Au pads.the magnitude of impedance over frequency, as well as the quality factor
whichFigure
is supposed
to be the
an indicator
of of
energy
losses over
in thefrequency,
system [24].
The impedance
at series
3b presents
magnitude
impedance
as well
as the quality
factor
9 Hz) and parallel resonant frequency ( f = 1.48 × 109 Hz) is
resonant
frequency
(
f
=
1.44
×
10
s
p
which is supposed to be an indicator of energy losses in the system [24]. The impedance at series
0.838
Ω and
2435 Ω (respectively.
factorresonant
is as high
as 2427 when
operating
at ultrahigh
resonant
frequency
= 1.44 × 109The
Hz)quality
and parallel
frequency
( = 1.48
× 109 Hz)
is 0.838 Ω
9
frequency
of
1.44
×
10
Hz,
indicating
high
acoustic
energy
density
and
well-trapped
waves
within
and 2435 Ω respectively. The quality factor is as high as 2427 when operating at ultrahigh frequency
the
piezoelectric
layer.
In
addition,
according
to
Sauerbrey’s
equation
[25],
the
frequency
shift
of
9
of 1.44 × 10 Hz, indicating high acoustic energy density and well-trapped waves within the
piezoelectric layer. In addition, according to Sauerbrey’s equation [25], the frequency shift of masssensitive FBAR for a certain mass load increases proportional to the resonance frequency squared;
hence, ultrahigh-frequency FBAR shows markedly enhanced sensitivity for humidity sensing.
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mass-sensitive FBAR for a certain mass load increases proportional to the resonance frequency squared;
hence, ultrahigh-frequency FBAR shows markedly enhanced sensitivity for humidity sensing.
Figure 3. FBAR characterization. (a) A comparison of the size between FBAR and quartz crystal
microbalance (QCM). The top-left inset shows the top-view scanning electron microscopy (SEM) image
of FBAR; (b) magnitude of impedance and Q value over frequency.
3.2. Morphology Optimization of Nano-Assembled PET Thin Films
Nano-assembled PET thin films provide abundant absorption sites for water molecules by
enlarging the surface area, leading to an enhanced sensitivity. Therefore, efforts are generally put
into the optimization of the nanostructure of PET thin films to obtain excellent sensing performance.
Here, we prepared the nano-assembled PET thin films with different morphology and investigated
the morphology dependence of humidity sensing performance. The morphology can be tuned by
choosing assembly methods and varying PET concentration. Thus, three types of PET thin films were
assembled on FBAR surface at three conditions, which are summarized in Table 1.
Table 1. Preparation of polyelectrolyte (PET) thin films with different morphology 1 .
1
Sample
Assembly Methods
PET Concentration
1
2
3
Dipping-assisted
Dipping-assisted
Spinning-assisted
0.2 mg/mL
20 mg/mL
20 mg/mL
FBAR frequency shift was recorded to monitor the total mass of PETs up to the same amount for each sample.
We first analyzed the concentration dependence of PET thin films’ morphology and their water
molecule absorption efficiency. Figure 4a shows the responses of FBARs for humidity measurement.
Obviously, responses of PET thin films assembled in concentrated solutions (20 mg/mL) are higher
than in dilute solutions (0.2 mg/mL). Such discrepancy can be explained by the different conformation
of PET chains in different concentration. During the assembly process, PET chains interdigitate
and form complexes with oppositely charged ones which have been formerly absorbed onto the
surface, resulting in a certain internal roughness of PET thin films [26]. In concentrated solutions, the
conformation of PET chains is less extended than in dilute solutions [27], leading to a higher internal
roughness and loosely-packed nanostructure, as shown in Figure 5a.
The Brunauer–Emmett–Teller (BET) equation is introduced to characterize the nanostructure of
PET thin films with respect to the specific surface area (SBET ) [18,28]:
1
ν
p0
p
−1
=
c−1
p
1
×
+
νm c
p0
νm c
(1)
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SBET =
νm Ns
Va
(2)
where p represents the partial vapor pressure and p0 represents the saturated vapor pressure, ν is
the absorbed mass of vapor, νm is the monolayer absorbed mass of vapor, c is the BET constant, N
Micromachines 2017, 8, 116
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is Avogadro’s number, s is the adsorption cross section of the absorbed species, V is the molar
volume
the vapor,
vapor,and
and aisisthe
themass
mass
sensitive
layer.
a consequence,
thethin
PET
thin
volume of
of the
of of
thethe
sensitive
layer.
As aAs
consequence,
the PET
films
2
films
fabricated
20 mg/mL
solutions
possess
a larger
of 518.54
m /g
fabricated
in 20 in
mg/mL
solutions
possess
a larger
specificspecific
surfacesurface
area ofarea
518.54
m2/g than
in than
0.2
2 /g, as illustrated in Figure 4b
2
in
0.2
mg/mL
solutions
whose
specific
surface
area
is
338.26
m
mg/mL solutions whose specific surface area is 338.26 m /g, as illustrated in Figure 4b and
and
Supplementary
Figure
S1. Therefore,
the loosely-packed
nanostructure
withspecific
larger surface
specific
Supplementary
Figure
S1. Therefore,
the loosely-packed
nanostructure
with larger
surface
area absorbs
more molecules
water molecules
and results
in higher
responses.
5b,c present
area absorbs
more water
and results
in higher
responses.
FigureFigure
5b,c present
the
the
morphology
of
PET
thin
films
nano-assembled
in
concentrated
and
dilute
solutions
respectively.
morphology of PET thin films nano-assembled in concentrated and dilute solutions respectively. PET
PET
fabricated
in 20
mg/mL
solutions
uniform
thaninin0.2
0.2mg/mL
mg/mLsolutions.
solutions.
thin thin
filmsfilms
fabricated
in 20
mg/mL
solutions
areare
lessless
uniform
than
Quantitatively,
bilayers fabricated
fabricated in
in 20
20 mg/mL
mg/mL
Quantitatively, the
the root-mean-square
root-mean-square roughness
roughness of PSS/PDDA
PSS/PDDA bilayers
solutions
0.03 nm
nm in
in 0.2
0.2 mg/mL
mg/mLsolutions.
solutions.This
This
solutionsisis1.09
1.09nm
nm±± 0.06 nm, while it decreases to 0.95 nm ±
± 0.03
isisin
inconsistence
consistence with
with the
the BET
BET analysis.
analysis.
600
4.0x105
Purge
518.54
500
H2O
400
2
SBET (m /g)
Δf (Hz)
0.0
5
-4.0x10
373ppm
314.24
200
746ppm
-8.0x105
-1.2x106
338.26
300
Dip @ 20mg/ml
Dip @ 0.2mg/ml
Spin @ 20mg/ml
0
1
2
100
1119ppm
1492ppm
3
Time (h)
(a)
4
5
6
0
Dip@20mg/ml
[email protected]/ml
Spin@20mg/ml
(b)
Figure 4.4. The morphology
morphology dependence
Figure
dependence of
of humidity
humidity sensing
sensingvia
via PET-coated
PET-coatedFBAR.
FBAR.(a)
(a)Real-time
Real-time
responsesofofFBARs
FBARswith
withthe
the
volume
concentration
water
vapor
changing
from
to 1492
responses
volume
concentration
of of
water
vapor
changing
from
373373
ppmppm
to 1492
ppm.
ppm. Nano-assembled
PET
thinwere
films
were fabricated
via dipping-assisted
method
in 20 solutions
mg/mL
Nano-assembled
PET thin
films
fabricated
via dipping-assisted
method in
20 mg/mL
solutions
(black
curve), dipping-assisted
method
in 0.2 mg/mL
solutions
(red and
curve)
and spinning(black
curve),
dipping-assisted
method in
0.2 mg/mL
solutions
(red curve)
spinning-assisted
assisted
method
in
20
mg/mL
solutions
(blue
curve);
(b)
specific
surface
area
of
nano-assembled
PET
method in 20 mg/mL solutions (blue curve); (b) specific surface area of nano-assembled PET thin films.
thin films.
Similar results can be acquired from the comparative analysis between different assembly methods.
Similar results can be acquired from the comparative analysis between different assembly
As illustrated in Table 1, dipping-assisted and spinning-assisted methods were adopted to tune the
methods. As illustrated in Table 1, dipping-assisted and spinning-assisted methods were adopted to
morphology of PET thin films in 20 mg/mL solutions. Figure 4a,b present the responses of FBAR and
tune the morphology of PET thin films in 20 mg/mL solutions. Figure 4a,b present the responses of
BET fitting results for humidity measurement respectively. Responses of PET thin films assembled via
FBAR and BET fitting results for humidity measurement respectively. Responses of PET thin films
dipping-assisted method are higher than via spinning-assisted method, and the specific surface area of
assembled via dipping-assisted method are higher than via spinning-assisted method, and the
spinning-assisted PET thin films is 314.24 m2 /g, smaller than that of2 dipping-assisted ones. The small
specific surface area of spinning-assisted PET thin films is 314.24 m /g, smaller than that of dippingspecific
surface area of spinning-assisted PET thin films could be explained by the air shear force driven
assisted ones. The small specific surface area of spinning-assisted PET thin films could be explained
by
process
[29].byThe
shear force
extends
theair
conformation
of PET the
chains,
leading to
bythe
the spinning
air shear force
driven
theair
spinning
process
[29]. The
shear force extends
conformation
aoflow
internal
roughness
and
tightly-packed
nanostructure.
Figure
5d
presents
the
morphology
PET chains, leading to a low internal roughness and tightly-packed nanostructure. Figure 5dof
spinning-assisted
PET thinoffilms
assembled in PET
20 mg/mL
solutions.
Theinfilms
featuresolutions.
the smoothest
presents the morphology
spinning-assisted
thin films
assembled
20 mg/mL
The
surface
with
the
root-mean-square
roughness
of
0.90
nm
±
0.03
nm.
films feature the smoothest surface with the root-mean-square roughness of 0.90 nm ± 0.03 nm.
For
films were
were assembled
assembled in
in 20
20 mg/mL
mg/mL
For the
the following
following humidity
humidity measurements,
measurements, PSS/PDDA
PSS/PDDA films
solutions
via
the
dipping-assisted
method
in
order
to
obtain
a
large
specific
surface
area
and
improve
solutions via the dipping-assisted method in order to obtain a large specific surface area and improve
the
assembly of
of PSS/PDDA
PSS/PDDAbilayers
bilayerson
on
thesensitivity.
sensitivity.Since
SincePET
PET thin
thin films
films are
are not
not rigid
rigid films,
films, excessive
excessive assembly
FBAR
surface
leads
to
the
dissipation
of
acoustic
energy
from
the
piezoelectric
layer,
which
causes
FBAR surface leads to the dissipation of acoustic energy from the piezoelectric layer, which causes
the
the consequent
consequent decrease
decreaseof
ofsensitivity.
sensitivity.Additionally,
Additionally,the
theclassic
classic
theloss
lossof
of quality
quality factor,
factor, resulting
resulting in
in the
Sauerbrey equation is valid only when the maximum mass load of PET thin films does not exceed
two percent of the resonant frequency [23]. Therefore, we controlled the frequency shift resulting
from the PET assembly within 1.44 × 109 Hz × 2% = 2.88 × 107 Hz. Figure 6 displays the frequency shift
of FBAR and the thickness growth of PSS/PDDA films during the molecular surface assembly on
FBAR surface. The PSS/PDDA films grow in an approximately exponential behavior with controlled
Micromachines 2017, 8, 116
7 of 13
Sauerbrey equation is valid only when the maximum mass load of PET thin films does not exceed two
percent of the resonant frequency [23]. Therefore, we controlled the frequency shift resulting from
Micromachines
2017, 8, 116
of 13
the
PET assembly
within 1.44 × 109 Hz × 2% = 2.88 × 107 Hz. Figure 6 displays the frequency 7shift
of FBAR and the thickness growth of PSS/PDDA films during the molecular surface assembly on
thickness
at nanometer
assembly
cycles, the thickness
of PSS/PDDA
extends
to
Micromachines
2017,PSS/PDDA
8, 116range. After
7 of 13
FBAR
surface.
The
films 30
grow
in an approximately
exponential
behavior films
with controlled
7 Hz.
210
nm
and
the
resonant
frequency
of
FBAR
decreases
by
2.1
×
10
thickness at nanometer range. After 30 assembly cycles, the thickness of PSS/PDDA films extends to
thickness at nanometer range. After 30 assembly cycles, the thickness of PSS/PDDA films extends to
210 nm
and the resonant frequency of FBAR decreases by 2.1 × 107 Hz.
210 nm and the resonant frequency of FBAR decreases by 2.1 × 107 Hz.
Figure Comparison
5. Comparisonofofloosely-packed
loosely-packed and
tightly-packed
of PET
thin thin
films.films.
Figure
and tightly-packed
tightly-packednanostructure
nanostructure
of PET
PET
Figure 5.
5. Comparison of
loosely-packed and
nanostructure
of
(a) Schematic illustration of the loosely-packed and tightly-packed nanostructure of poly(sodium 4(a)
of the
theloosely-packed
loosely-packedand
andtightly-packed
tightly-packednanostructure
nanostructureofof
poly(sodium
(a) Schematic illustration of
poly(sodium
4styrenesulfonate)/poly(diallyldimethytlammonium choride) (PSS/PDDA) bilayers; (b) atomic-force
4-styrenesulfonate)/poly(diallyldimethytlammonium
choride)(PSS/PDDA)
(PSS/PDDA)bilayers;
bilayers; (b)
(b) atomic-force
styrenesulfonate)/poly(diallyldimethytlammonium
choride)
atomic-force
microscopy (AFM) height image showing the morphology of PSS/PDDA bilayers assembled via
microscopy
(AFM)
image showing
the
of
bilayers
assembled via
microscopy
(AFM) height
height
showing
the morphology
morphology
of PSS/PDDA
PSS/PDDA
bilayers
dipping-assisted
methodimage
in 20 mg/mL
solutions.
The root-mean-square
roughness
is 1.09assembled
nm ± 0.06 via
dipping-assisted
method
in
20
mg/mL
solutions.
The
root-mean-square
roughness
is
1.09
nm
±nm
0.06
nm;
dipping-assisted
method
in 20showing
mg/mLthe
solutions.
Theofroot-mean-square
± 0.06
nm; (c) AFM height
image
morphology
PSS/PDDA bilayersroughness
assembled is
via1.09
dipping(c)
AFM
height
image
showing
the
morphology
of
PSS/PDDA
bilayers
assembled
via
dipping-assisted
assisted
in 0.2 mg/mL
solutions.
The root-mean-square
roughness
0.95 nm ± 0.03
nm;
nm; (c)
AFMmethod
height image
showing
the morphology
of PSS/PDDA
bilayersis assembled
via dippingmethod
0.2 mg/mL
solutions.
The
root-mean-square
0.95
nmassembled
± 0.03
nm;
AFM
height
(d) in
AFM
height
image
showing
the
morphology
of roughness
PSS/PDDAisbilayers
via(d)
spinningassisted
method
in 0.2
mg/mL
solutions.
The root-mean-square
roughness
is 0.95
nm
± 0.03
nm;
image
showing
the
morphology
of
PSS/PDDA
bilayers
assembled
via
spinning-assisted
method
in
assisted
method
in
20
mg/mL
solutions.
The
root-mean-square
roughness
is
0.90
nm
±
0.03
nm.
The
(d) AFM height image showing the morphology of PSS/PDDA bilayers assembled via spinningroot-mean-square
roughness
was
obtained
from
three
different
scanning
areas
for
each
sample.
20
mg/mL
solutions.
The
root-mean-square
roughness
is
0.90
nm
±
0.03
nm.
The
root-mean-square
assisted method in 20 mg/mL solutions. The root-mean-square roughness is 0.90 nm ± 0.03 nm. The
roughness
was obtained
from was
threeobtained
differentfrom
scanning
for each
sample.
root-mean-square
roughness
threeareas
different
scanning
areas for each sample.
Figure 6. Frequency shift of FBAR and thickness growth of PSS/PDDA bilayers during the molecular
surface self-assembly on NH2-functionalized FBAR.
3.3. Humidity
Sensingshift
Characteristics
Figure
6. Frequency
Frequency
of FBAR
FBAR and
and thickness
thickness growth
growthof
ofPSS/PDDA
PSS/PDDA bilayers
bilayers during
during the
the molecular
molecular
Figure
6.
shift of
surface
self-assembly
on
NH
2
-functionalized
FBAR.
surface self-assembly on NH2 -functionalized FBAR.
3.3.1. Sensitivity and Linearity
In order
to measure
the humidity and investigate the sensitivity of FBAR, we prepared six
3.3. Humidity
Sensing
Characteristics
FBARs coated with different numbers of PSS/PDDA bilayers. Figure 7a shows the real-time responses
3.3.1. Sensitivity and Linearity
In order to measure the humidity and investigate the sensitivity of FBAR, we prepared six
Micromachines 2017, 8, 116
8 of 13
3.3. Humidity Sensing Characteristics
3.3.1. Sensitivity and Linearity
In order to measure the humidity and investigate the sensitivity of FBAR, we prepared six FBARs
8 of 13
coated with different numbers of PSS/PDDA bilayers. Figure 7a shows the real-time responses of
FBARs
as as
a function
ofofvolume
to 1866
1866 ppm.
ppm.
of
FBARs
a function
volumeconcentration
concentrationofofwater
watervapor
vaporranging
ranging from
from 373
373 ppm
ppm to
Obviously, FBARs
response time
time for
for humidity
humidity measurement.
measurement. With
With the
the growing
growing number
number
Obviously,
FBARs present
present aa fast
fast response
of
PSS/PDDA
bilayers
and
volume
concentration,
responses
of
FBARs
increase
gradually,
which
of PSS/PDDA bilayers and volume concentration, responses of FBARs increase gradually, which
indicates that
that more
more water
water molecules
molecules are
are absorbed
absorbed into
into the
the PET
PET thin
thin films.
Figure 7b
7b and
and Table
Table 22
indicates
films. Figure
present
the
calibration
curves
and
corresponding
sensitivity
(defined
as
the
calibration
curve
slope
present the calibration curves and corresponding sensitivity (defined as the calibration curve slope
with
the
unit
of
Hz/ppm),
as
well
as
the
linearity
(defined
as
R-Square)
respectively.
As
a
result,
the
with the unit of Hz/ppm), as well as the linearity (defined as R-Square) respectively. As a result, the
frequency shift
shift presents
presents an
an excellent
excellent linearity
linearity within
within the
the volume
volume concentration
concentration ranging
ranging from
from 373
373 ppm
ppm
frequency
to
1866
ppm.
The
sensitivity
increases
from
143.61
Hz/ppm
to
2202.20
Hz/ppm
with
the
growing
to 1866 ppm. The sensitivity increases from 143.61 Hz/ppm to 2202.20 Hz/ppm with the growing
thickness of
ofPET
PETthin
thinfilms,
films,which
which
five
orders
of magnitude
higher
a typical
[30,31].
thickness
is is
five
orders
of magnitude
higher
thanthan
a typical
QCMQCM
[30,31].
The
The markedly
enhanced
sensitivity
shows
a great
advantage
for humidity
measurement.
markedly
enhanced
sensitivity
shows
a great
advantage
for humidity
measurement.
Micromachines 2017, 8, 116
6x106
Purge
2.0x106
5x10
H2O
-2.0x106
373ppm
-4.0x10
5
15
25
10
20
30
4x106
-Δf (Hz)
Δf (Hz)
0.0
6
746ppm
6
1119ppm
10
20
30
5
15
25
-6.0x106
0
2
4
1492ppm
Time (h)
6
3x106
2x106
1x106
1866ppm
8
0
400
800
1200
1600
2000
Volume concentration (ppm)
(a)
(b)
Figure 7. Humidity sensing via PET-coated FBAR. (a) Real-time responses of FBARs with the volume
Figure 7. Humidity sensing via PET-coated FBAR. (a) Real-time responses of FBARs with the volume
concentration
of water
water vapor
vapor changing
changing from
from 373
373 ppm
ppm to
to 1866
1866 ppm.
ppm. The
The number
number of
of PSS/PDDA
PSS/PDDA bilayers
concentration of
bilayers
ranges
from
5
to
30;
(b)
frequency
shift
with
respect
to
the
various
volume
concentration
of water
ranges from 5 to 30; (b) frequency shift with respect to the various volume concentration of water
vapor
vapor
and number
of PSS/PDDA
bilayers.
and number
of PSS/PDDA
bilayers.
Table 2. Sensitivity and linearity of FBARs coated with various number of PSS/PDDA bilayers with
Table 2. Sensitivity and linearity of FBARs coated with various number of PSS/PDDA bilayers with
the volume concentration of water vapor changing from 373 ppm to 1866 ppm.
the volume concentration of water vapor changing from 373 ppm to 1866 ppm.
Number of Bilayers
Number of
5 Bilayers
5
10
10
15
15
20
20
25
25
30
30
Sensitivity (Hz/ppm)
Sensitivity143.61
(Hz/ppm)
143.61
345.91
345.91
500.53
500.53
886.11
886.11
1317.20
1317.20
2202.20
2202.20
R-Square
R-Square
0.97625
0.97625
0.99695
0.99695
0.99576
0.99576
0.99892
0.99892
0.98583
0.98583
0.98989
0.98989
3.3.2. Reversibility
Reversibility
It is worth mentioning that the resonant frequency recovers to initial level after adjusting the
volume concentration
the
reversible
absorption
behavior
of
concentrationof
ofwater
watervapor
vaporback
backtoto0 0ppm,
ppm,indicating
indicating
the
reversible
absorption
behavior
the
nano-assembled
PET
thin
films.
of the
nano-assembled
PET
thin
films.The
Thereversible
reversiblesensing
sensingofofhumidity
humidity sensors
sensors allows
allows accurate
calibration prior
prior to
to humidity
humiditymeasurement.
measurement.Additionally,
Additionally,ititisisvery
veryattractive
attractive
from
economical
point
from
economical
point
of
of view by reusing the device. Therefore, FBAR coated with nano-assembled PET thin films
represents a promising candidate for the development of regenerative electronic humidity sensors.
Additionally, it is clear that responses take a longer time to recover to initial level after exposing
to higher volume concentration of water vapor. During desorption at low volume concentration, the
large interspaces in PET thin films make water molecules pass through the materials easily. This
contributes to the fast recovery of the humidity sensor. At high volume concentration, more water
Micromachines 2017, 8, 116
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1.2x10
4
1.0x10
4
8.0x10
3
6.0x10
3
4.0x10
3
2.0x10
3
0.0
FBAR
FBAR+[PSS/PDDA]30
Δf (Hz)
Δf (Hz)
view by reusing the device. Therefore, FBAR coated with nano-assembled PET thin films represents
a promising candidate for the development of regenerative electronic humidity sensors.
Additionally, it is clear that responses take a longer time to recover to initial level after exposing to
higher volume concentration of water vapor. During desorption at low volume concentration, the large
interspaces 2017,
in PET
thin films make water molecules pass through the materials easily. This contributes
Micromachines
8, 116
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to the fast recovery of the humidity sensor. At high volume concentration, more water molecules are
absorbed
into the
PET thindesorption
films and PET
thin films
are gelled.
are shortened,
are
shortened,
hindering
of water
molecules
fromThus,
PET the
thininterspaces
films. Therefore,
longer
hindering
desorption
offor
water
molecules
from PET
thin films.
longer level.
purging time is
purging
time
is required
recovering
the sensor
to initial
level atTherefore,
higher humidity
required for recovering the sensor to initial level at higher humidity level.
3.3.3. Detection Limit, Noise and Stability
3.3.3. Detection Limit, Noise and Stability
Considering that the noise level exhibited by PET-coated FBAR determines the theoretical
Considering
the noise
levelwe
exhibited
by the
PET-coated
determines
thecoating
theoretical
detection
limit for that
humidity
sensing,
measured
noise forFBAR
both FBAR
without
and
detection
limit
for
humidity
sensing,
we
measured
the
noise
for
both
FBAR
without
coating
FBAR coated with 30 PSS/PDDA bilayers at ambient conditions for 10 min, as shown in Figureand
8a.
FBAR
coated
30 PSS/PDDA
bilayers
at ambient
conditions
min,
asdeciles
shownof
infrequency
Figure 8a.
The
noise
levelwith
(defined
as the deviation
between
the first
deciles for
and10the
nine
The noise
level (defined
asisthe
deviation
between
deciles
the nine
deciles
of frequency
shift)
shift)
for uncoated
FBAR
2.32
× 103 Hz,
while the
the first
noise
level and
for FBAR
coated
with
30 PSS/PDDA
3 Hz, while the noise level for FBAR coated with 30 PSS/PDDA bilayers
for
uncoated
FBAR
is
2.32
×
10
3
bilayers is 3.20 × 10 Hz. The slight increase of noise level after PET assembly origins from the
is 3.20 × 103 Hz.ofThe
slight increase
ofwhich
noise level
after
PET assembly
from wave
the inhomogeneity
inhomogeneity
PSS/PDDA
bilayers
shows
negative
effect onorigins
the acoustic
propagation
of
PSS/PDDA
bilayers
which
shows
negative
effect
on
the
acoustic
wave
propagation
[32].
Meanwhile,
[32]. Meanwhile, assuming the sensitivity of 2202.20 Hz/ppm and the signal-to-noise ratio
of 3, the
assuming
the
sensitivity
of
2202.20
Hz/ppm
and
the
signal-to-noise
ratio
of
3,
the
theoretical
theoretical detection limit of FBAR coated with 30 PSS/PDDA bilayers for humidity is estimated
to
detection
limit
of
FBAR
coated
with
30
PSS/PDDA
bilayers
for
humidity
is
estimated
to
be
as
low
as
be as low as 43.64 ppm [33].
43.64We
ppm
[33].
also conducted the stability measurement for PET-coated FBAR. Figure 8b shows the
We also
conducted
stability
forbilayers
PET-coated
Figure
8b shows
the hours.
frequency
frequency
drift
of FBARthe
coated
withmeasurement
30 PSS/PDDA
afterFBAR.
exposure
in air
for seven
As
drift
of
FBAR
coated
with
30
PSS/PDDA
bilayers
after
exposure
in
air
for
seven
hours.
As
shown,
the
4
shown, the maximum frequency drift is limited to 3 × 10 Hz during testing, much less than the
maximum shift
frequency
drift
is limitedmeasurement.
to 3 × 104 Hz This
during
testing, the
much
lessstability
than theof
frequency
shift
frequency
for the
humidity
indicates
high
FBAR-based
for the humidity
humidity
sensor. measurement. This indicates the high stability of FBAR-based humidity sensor.
4x10
4
3x10
4
2x10
4
1x10
4
0
0
2
4
6
Time (min)
(a)
8
10
-1x10
4
-2x10
4
0
1
2
3
4
5
6
7
Time (h)
(b)
Figure 8. Noise and stability measurements of FBAR at ambient conditions. (a) Noise of both FBAR
Figure 8. Noise and stability measurements of FBAR at ambient conditions. (a) Noise of both FBAR
without coating and FBAR coated with 30 PSS/PDDA bilayers; (b) frequency drift of FBAR coated
without coating and FBAR coated with 30 PSS/PDDA bilayers; (b) frequency drift of FBAR coated
with 30 PSS/PDDA bilayers.
with 30 PSS/PDDA bilayers.
3.4. Temperature Dependence
3.4. Temperature Dependence
It is known that the sensitivity of sensors is influenced by the environmental temperature [34–
It is known that the sensitivity of sensors is influenced by the environmental temperature [34–36].
36]. In order to ensure that PET-coated FBAR is be able to perform humidity measurement within a
In order to ensure that PET-coated FBAR is be able to perform humidity measurement within a wide
wide range of wireless applications, we evaluate the sensor sensitivity at different temperature
range of wireless applications, we evaluate the sensor sensitivity at different temperature ranging
ranging◦ from 0 °C
to 35 °C, as shown in Figure 9. It is clear that responses of PET-coated FBAR vary
from 0 C to 35 ◦ C, as shown in Figure 9. It is clear that responses of PET-coated FBAR vary with the
with the environmental temperature. The sensitivity increases gradually at low temperature and the
environmental temperature. The sensitivity increases gradually at low temperature and the maximum
maximum sensitivity to humidity obtained is 1295.11 Hz/ppm at 0 °C, more than seven times higher
than at 35 °C. The enhancement of sensitivity is due to the high partial partition coefficient in
absorption equilibrium at low temperature [37,38]. In a specific water vapor concentration, the
frequency of FBAR is stable when the absorption and desorption of water molecules in PET thin films
reach a dynamic equilibrium. As the environmental temperature decreases, the water molecules
possess less kinetic energy. This hinders the escape of water molecules from PET thin films and
Micromachines 2017, 8, 116
10 of 13
sensitivity to humidity obtained is 1295.11 Hz/ppm at 0 ◦ C, more than seven times higher than at
35 ◦ C. The enhancement of sensitivity is due to the high partial partition coefficient in absorption
equilibrium at low temperature [37,38]. In a specific water vapor concentration, the frequency of FBAR
is stable when the absorption and desorption of water molecules in PET thin films reach a dynamic
equilibrium. As the environmental temperature decreases, the water molecules possess less kinetic
energy. This hinders the escape of water molecules from PET thin films and increases partial partition
coefficient,
well
as the amount of water molecules in PET thin films. Therefore, cooling device
Micromachines as
2017,
8, 116
10 of 13
is preferred to further enhance the sensor sensitivity for ultralow humidity measurement. Results
suggest
that normalization
withthat
respect
to the environmental
temperature
is needed for
PET-coated
measurement.
Results suggest
normalization
with respect
to the environmental
temperature
is
FBAR
forfor
practical
applications
inpractical
humidityapplications
measurement.
needed
PET-coated
FBAR for
in humidity measurement.
1.5x103
5°C
15°C
25°C
35°C
1.2x103
−Δf (Hz)
2.0x106
0°C
10°C
20°C
30°C
Sensitivity (Hz/ppm)
2.5x106
1.5x106
1.0x106
9.0x102
6.0x102
3.0x102
5
5.0x10
400
800
1200
1600
Volume concentration (ppm)
(a)
2000
0.0
0
5
10
15
20
25
30
35
Temperature (°C)
(b)
Figure
Figure 9.9. Temperature
Temperature dependence
dependence of
of PET-coated
PET-coated FBAR
FBAR for
for humidity
humidity sensing.
sensing. (a)
(a) Frequency
Frequency shift
shift
◦ C to 35 ◦ C and the volume
with
respect
to
both
the
environmental
temperature
ranging
from
0
with respect to both the environmental temperature ranging from 0 °C to 35 °C and the volume
concentration
concentrationof
ofwater
watervapor
vaporchanging
changingfrom
from373
373ppm
ppm to
to 1866
1866 ppm;
ppm; (b)
(b) sensitivity
sensitivity of
of PET-coated
PET-coated FBAR
FBAR
with the environmental temperature changing from 0 ◦ C to 35 ◦ C.
with the environmental temperature changing from 0 °C to 35 °C.
3.5.
3.5. Selectivity
Selectivity Analysis
Analysis
The
sensors shows
shows the
the capability
capability to
todiscriminate
discriminatethe
theanalytes
analytesofofinterest
interestfrom
froma
The selectivity
selectivity of
of sensors
aparticular
particularinterference.
interference.InInpractice,
practice,the
theselectivity
selectivityofof gas
gas sensors
sensors is
is usually
usually achieved
by
achieved by the
the
functionalization
of
sensing
area
with
sensitive
layer
[39].
In
our
experiments,
the
selectivity
of
functionalization of sensing area with sensitive layer [39]. In our experiments, the selectivity
of PETPET-coated
FBAR
analyzed
by introducing
VOCs
as interference.
coated FBAR
was was
analyzed
by introducing
VOCs
as interference.
After
molecular
surface
self-assembly
of
different
After molecular surface self-assembly of different numbers
numbers of
ofPSS/PDDA
PSS/PDDA bilayers
bilayers on
on FBAR
FBAR
surface,
we
compared
responses
for
the
measurements
of
water,
NPA
and
n-hexane
vapors.
Figure
10a
surface, we compared responses for the measurements of water, NPA and n-hexane vapors. Figure
plots
the three-dimensional
cube of
FBAR
responses
to different
vapors vapors
with respect
the relative
10a plots
the three-dimensional
cube
of FBAR
responses
to different
with to
respect
to the
concentration
(in
terms
of
p/p
)
and
the
number
of
PSS/PDDA
bilayers.
The
two-dimensional
diagram
0
relative concentration (in terms
of / ) and the number of PSS/PDDA bilayers. The twoof
the cube’s diagram
cross section
shows
thecross
relative
concentration
thickness
dependent or
property
of
dimensional
of the
cube’s
section
shows theorrelative
concentration
thickness
the
humidity
sensor.
As
shown
in
Figure
10b,
responses
to
water
vapor
are
higher
than
NPA
and
dependent property of the humidity sensor. As shown in Figure 10b, responses to water vapor are
n-hexane
vapors
a sequence
of water
> n-hexane.
is attributed
to This
the variations
of
higher than
NPAwith
and n-hexane
vapors
with>aNPA
sequence
of waterThis
> NPA
> n-hexane.
is attributed
vapor
polarity
[40].
Water property
molecules
are Water
highlymolecules
polarized are
andhighly
NPA molecules
to the molecule’s
variations of
vapor property
molecule’s
polarity
[40].
polarized
are
moderately
polarized,
while
n-hexane
molecules
show
no
polarity
property.
As
a
result,
polar
and NPA molecules are moderately polarized, while n-hexane molecules show no polarity property.
PSS/PDDA
films
absorb
more
water
molecules
and
less
NPA
molecules,
while
a
negligible
amount
As a result, polar PSS/PDDA films absorb more water molecules and less NPA molecules, while a
of
n-hexaneamount
molecules.
Moreover,molecules.
in contrastMoreover,
to water and
vapors,
the increment
the bilayer
negligible
of n-hexane
in NPA
contrast
to water
and NPAinvapors,
the
number
weakens
the
sensor
signal
during
N-hexane
vapor
measurement,
as
shown
in
Figure
10b.
increment in the bilayer number weakens the sensor signal during N-hexane vapor measurement, as
This
is due
to the fact
PSS/PDDA
act as PSS/PDDA
a passivation
layer
toas
prevent
the penetration
shown
in Figure
10b.that
Thispolar
is due
to the factfilms
that polar
films
act
a passivation
layer to
of
non-polar
n-hexane
molecules
onto
FBAR
surface.
Additionally,
PET-coated
FBAR
also
prevent the penetration of non-polar n-hexane molecules onto FBAR surface. Additionally,shows
PETnegligible
responses
to non-polar
carbon
dioxide to
(COnon-polar
a dominating
ambient
2 ), which is
coated FBAR
also shows
negligible
responses
carbon
dioxide gas
(COunder
2), which
is a
conditions
Figure
S2).
dominating(Supplementary
gas under ambient
conditions
(Supplementary Figure S2).
It is observed from Figure 10b that the detection limit of the sensor for humidity measurement
reaches 0.5% (16.66 ppm), which exceeds the theoretical detection limit (43.64 ppm) of FBAR coated
with 30 PSS/PDDA bilayers. This derives from the two-stage manner of FBAR-based humidity sensor:
the sensitivity of FBAR coated with 15 PSS/PDDA bilayers is 2068.60 Hz/ppm when the relative
concentration is less than 1.5% (55.98 ppm), while it decreases to 127.33 Hz/ppm when the relative
concentration ranges from 1.5% (55.98 ppm) to 32% (1194 ppm). This is in accord with the results
Micromachines 2017, 8, 116
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It is observed from Figure 10b that the detection limit of the sensor for humidity measurement
reaches 0.5% (16.66 ppm), which exceeds the theoretical detection limit (43.64 ppm) of FBAR coated
with 30 PSS/PDDA bilayers. This derives from the two-stage manner of FBAR-based humidity sensor:
the sensitivity of FBAR coated with 15 PSS/PDDA bilayers is 2068.60 Hz/ppm when the relative
concentration is less than 1.5% (55.98 ppm), while it decreases to 127.33 Hz/ppm when the relative
concentration ranges from 1.5% (55.98 ppm) to 32% (1194 ppm). This is in accord with the results
reported in the literature [30,31,41]. Further investigations are being undertaken with the purpose of
a full understanding
of the two-stage absorption behavior.
Micromachines 2017, 8, 116
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Figure 10. Frequency shift for the measurement of water, n-propyl alcohol (NPA) and n-hexane
Figure 10. Frequency shift for the measurement of water, n-propyl alcohol (NPA) and n-hexane vapors.
vapors. FBARs were coated with 5, 10 and 15 PSS/PDDA bilayers respectively. (a) Three-dimensional
FBARs cube
wereof
coated
5, 10 and
15 PSS/PDDA
(a) Three-dimensional
cube
FBARwith
responses
to water,
NPA and bilayers
n-hexane respectively.
vapors; (b) frequency
shift for the
of FBAR
responses to
NPAand
and
n-hexane
vapors;
(b) frequency shift for the measurements of
measurements
of water,
water, NPA
n-hexane
vapors
respectively.
water, NPA and n-hexane vapors respectively.
4. Conclusions
In summary, we proposed a highly sensitive humidity sensor based on the combination of
4. Conclusions
ultrahigh-frequency microelectromechanical resonator and nano-assembled PET thin films. We first
In optimized
summary,thewe
proposedofaPET
highly
sensitive
humidity
sensor
based
on theabsorption
combination of
morphology
thin films
in order
to improve
the water
molecule
ultrahigh-frequency
microelectromechanical
resonator and
nano-assembled
PET thin
films.aWe first
efficiency. Compared
with tightly-packed nanostructure,
loosely-packed
nanostructure
possesses
larger
specific
surface area,
provides
absorption
sites for water
molecules.
As a result,
optimized
the
morphology
of which
PET thin
filmsmore
in order
to improve
the water
molecule
absorption
the
FBAR-based
humidity
sensor
was
extremely
sensitive
with
good
linearity,
stability,
reversibility
efficiency. Compared with tightly-packed nanostructure, loosely-packed nanostructure possesses
and low detection limit. Since the partial partition coefficient of water vapor in absorption
a larger specific surface area, which provides more absorption sites for water molecules. As a result,
equilibrium increases when the environmental temperature decreases, PET-coated FBAR possesses a
the FBAR-based
humidity sensor was extremely sensitive with good linearity, stability, reversibility
higher sensitivity at low temperature. Furthermore, we analyzed the selective detection of water
and lowvapor
detection
limit.with
Since
the partial
partition
coefficient
oftowater
vapor
in absorption
equilibrium
from VOCs
respect
to the polarity
property.
Owing
the high
sensitivity,
miniaturized
increases
the environmental
temperature
decreases,
FBAR
possesses
sizewhen
and ultrahigh
operating frequency,
FBAR coated
with PETPET-coated
thin films is found
to be
a potentiala higher
candidate
as atemperature.
wireless humidity
sensor to realize
integration
with
WSNs. detection of water vapor
sensitivity
at low
Furthermore,
wethe
analyzed
the
selective
from VOCs with respect to the polarity property. Owing to the high sensitivity, miniaturized size and
Supplementary Materials: The following are available online at www.mdpi.com/link, Figure S1: Fitting results
ultrahigh
operating
frequency,
thin films
is of
found
a potential
candidate
as
, , to beand
using
the
of specific
surface
area using FBAR
the BETcoated
equation,with
TablePET
S1: Extracted
value
a wireless
sensor
realize the
integration
WSNs.
BEThumidity
equation, Figure
S2: to
Comparison
of frequency
shift with
between
CO2 and water vapor with respect to the
various volume concentration.
Supplementary
Materials: The following are available online at www.mdpi.com/2072-666X/8/4/116/s1,
Acknowledgments: The authors gratefully acknowledge financial support from the Natural Science Foundation
Figure S1:
Fitting
results of specific surface area using the BET equation, Table S1: Extracted value of A, B,
of China (NSFC No. 61674114), Tianjin Applied Basic Research and Advanced Technology (14JCYBJC41500),
νm and Sand
using
the BET equation, Figure S2: Comparison of frequency shift between CO2 and water vapor
BETthe
111 Project (B07014).
with respect to the various volume concentration.
Author Contributions: X.D. and H.Q. conceived and supervised the project. J.H. and W.L. designed and
performed the experiments. J.H. and W.L. analyzed the data. J.H., W.L., W.P. and H.Z. fabricated the device.
W.L. wrote the manuscript. X.D. and H.Q. commented on the manuscript. All authors discussed the results.
Conflicts of Interest: The authors declare no conflict of interest.
Micromachines 2017, 8, 116
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Acknowledgments: The authors gratefully acknowledge financial support from the Natural Science Foundation
of China (NSFC No. 61674114), Tianjin Applied Basic Research and Advanced Technology (14JCYBJC41500), and
the 111 Project (B07014).
Author Contributions: X.D. and H.Q. conceived and supervised the project. J.H. and W.L. designed and
performed the experiments. J.H. and W.L. analyzed the data. J.H., W.L., W.P. and H.Z. fabricated the device. W.L.
wrote the manuscript. X.D. and H.Q. commented on the manuscript. All authors discussed the results.
Conflicts of Interest: The authors declare no conflict of interest.
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