JSSSJournal of Sensors and Sensor SystemsJSSSJ. Sens. Sens. Syst.2194-878XCopernicus GmbHGöttingen, Germany10.5194/jsss-4-103-2015Silicon micro-levers and a multilayer graphene membrane studied via
laser photoacoustic detectionZelingerZ.zelinger@jh-inst.cas.czJandaP.SuchánekJ.DostálM.KubátP.NevrlýV.BitalaP.CivišS.J. Heyrovský Institute of Physical Chemistry AS CR,
Prague, Czech RepublicFaculty of Safety Engineering, VŠB – Technical
University of Ostrava, Ostrava, Czech RepublicZ. Zelinger (zelinger@jh-inst.cas.cz)5March20154110310924July201426January20151February2015This work is licensed under a Creative Commons Attribution 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by/3.0/This article is available from https://jsss.copernicus.org/articles/4/103/2015/jsss-4-103-2015.htmlThe full text article is available as a PDF file from https://jsss.copernicus.org/articles/4/103/2015/jsss-4-103-2015.pdf
Laser photoacoustic spectroscopy (PAS) is a method that utilizes the sensing
of the pressure waves that emerge upon the absorption of radiation by
absorbing species. The use of the conventional electret microphone as a
pressure sensor has already reached its limit, and a new type of microphone
– an optical microphone – has been suggested to increase the sensitivity of
this method. The movement of a micro-lever or a membrane is sensed via a
reflected beam of light, which falls onto a position-sensing detector. The
use of one micro-lever as a pressure sensor in the form of a silicon
cantilever has already enhanced the sensitivity of laser PAS.
Herein, we test two types of home-made sensing elements – four coupled
silicon micro-levers and a multilayer graphene membrane – which have the
potential to enhance this sensitivity further. Graphene sheets possess
outstanding electromechanical properties and demonstrate impressive
sensitivity as mass detectors. Their mechanical properties make them suitable
for use as micro-/nano-levers or membranes, which could function as extremely
sensitive pressure sensors.
Graphene sheets were prepared from multilayer graphene through the
micromechanical cleavage of basal plane highly ordered pyrolytic graphite.
Multilayer graphene sheets (thickness ∼102 nm) were then mounted on
an additional glass window in a cuvette for PAS. The movements of the sheets
induced by acoustic waves were measured using an He–Ne laser beam reflected
from the sheets onto a quadrant detector. A discretely tunable CO2 laser
was used as the source of radiation energy for the laser PAS experiments.
Sensitivity testing of the investigated sensing elements was performed with
the aid of concentration standards and a mixing arrangement in a flow regime.
The combination of sensitive microphones and micromechanical/nanomechanical
elements with laser techniques offers a method for the study and development
of new, reliable and highly sensitive chemical sensing systems. To our
knowledge, we have produced the first demonstration of the feasibility of
using four coupled silicon micro-levers and graphene membranes in an optical
microphone for PAS. Although the sensitivity thus far remains inferior to
that of the commercial electret microphone (with an S/N ratio that is 5
times lower), further improvement is expected to be achieved by adjusting the
micro-levers and membrane elements, the photoacoustic system and the position
detector.
Introduction
The objective of this paper lies in the context of new sensing
technologies based on micromechanical sensing elements, including functional
materials for gas sensing. These elements could be employed in the laser
photoacoustic spectroscopy (PAS) method as part
of a sensitive optical microphone. This technique offers several advantages
compared with conventional spectroscopy; some of its major benefits include
the fact that the detected signal is directly proportional to the laser
intensity, the elimination of false absorption resulting from scattered light
and the lack of a need for photodetectors, which perform poorly in the
mid-infrared region.
PAS is a spectroscopic method that differs from other absorption
spectroscopic techniques in the manner in which the absorbed radiation is
detected. The absorbed light is converted into heat, which leads to gas
expansion. If the excitation light is modulated or chopped, then the
resulting pressure waves can be sensed by a microphone. Although conventional
condenser microphones have reached their limits of sensitivity, the
development of new pressure sensors offers an opportunity to increase the
sensitivity of this technique.
It has been proposed that a cantilever-type pressure sensor be used in PAS in
place of microphones to achieve optimal sensitivity (Kauppinen et al., 2004;
Wilcken and Kauppinen, 2003; Kuusela and Kauppinen, 2007; Koskinen et al.,
2008, 2006). The primary benefits of a cantilever are the very low string
constant and the extremely wide dynamical range that can be achieved in the
cantilever movement. The string constant can be 2 or 3 orders of magnitude
smaller than that of the membrane of a condenser microphone, and the movement
of the cantilever can span tens of micrometres without suffering any non-linear or restricting effects. A
non-contact (deflection, interferometric) measurement of cantilever movement is required to avoid any damping caused by the probe and to maintain the wide
dynamic range (Li et al., 2012).
Micromechanical sensors represent a new branch of chemical sensing that
utilizes a microfabricated spring (cantilever), as originally applied in
atomic force microscopes (AFMs), for the recognition of interfacial mass- and
charge-transfer processes with very high sensitivity. A detection system
based on cantilever micromechanical behaviour already exists and is identical
to that employed in AFMs (Jalili and Laxminarayana, 2004). This system has
allowed the detection of analytes in concentrations down to the picomolar and
sub-picomolar levels to become feasible in a variety of processes, including
deposition/dissolution, adsorption/desorption (Ji et al., 2001), solution pH
changes and surface-confined charge-transfer reactions (Tabard-Cossa et al.,
2005). For detection, both bending- and frequency-readout micro-lever sensors
can be employed (Battiston et al., 2001). Further development will be focused
on attempts to construct cantilever arrays (Lang et al., 2005) modified by
various receptors to increase the selectivity of the sensor response (Grogan
et al., 2002) and to gain the ability to perform multicomponent analysis in a
single step. The major advantages of microcantilever array sensors are their
micrometre-scale size, high sensitivity and short response time (Berger et
al., 1997).
A good approximation to the ideal cantilever – i.e. a one-dimensional single
nanocrystal – can be found in multi-/single-layer graphite crystals – i.e.
multi-/single-layer graphene (MLG, SLG) (Novoselov et al., 2004). Graphene
sheets possess outstanding electromechanical properties (Geim and Novoselov,
2007; Lee et al., 2008; Castro Neto et al., 2009) and demonstrate impressive
sensitivity as mass detectors (Chen et al., 2009; Avdoshenko et al., 2012).
Because of its highly uniform (if defect-free) structure, relatively high chemical
stability, and outstanding mechanical properties such as a high Young's
modulus and a low specific weight, which allow it to reach a high
differential mass ratio and hence a high sensor sensitivity, graphene
represents almost the ideal material for nanomechanical sensors.
Nanomechanical vibrations have been investigated for potential application in
nanothermometers (Rahmat et al., 2010).
In previous studies (to improve the physical modelling of urban air
pollution), we have developed and employed laser photoacoustic spectrometry
(Zelinger et al., 2004, 2006, 2009). The objective of this paper is to
present the design and fabrication of several home-made sensing elements of
the cantilever-and-membrane type and to test and characterize their
mechanical properties with the aid of the developed PAS method. Transducers
composed of various materials (silicon, carbon) were employed in the design
of new gas-sensing elements.
The purpose of experiments that involve microphones, graphene-based membranes
and silicon cantilevers is to examine the differences among various sensors
to optimize the range of their utilization. Unlike microphones, cantilevers
allow for detection in both frequency and resonance modes in addition to the
deflection mode. In the case of a multilayer graphene membrane detector, we
have now proven the feasibility of its use. This detector represents a fusion
of a microphone membrane and a cantilever, the deflection of which is probed
by a reflected laser beam. Its prospective advantage lies in the possibility
of tuning its sensitivity by decreasing its stiffness. This can be
accomplished by decreasing the number of graphene layers, which will also
cause a shift in the resonant frequency. Most importantly, however, our
approach includes an examination of a multicantilever/multimode set-up, which
offers variability in both the mechanical properties and detection modes of
the cantilevers and thus the choice of the detector with the most suitable
response for a given application. There is also the possibility of receiving
signals from a multidetector system and thus of improving the
signal-to-noise
ratio and minimizing detector failure. Both features are important for
prospective utilization in detector units and for increasing automatic
sensor-network coverage.
Experimental
A discretely tunable home-made CO2 laser emitting at rotation–vibration
transitions of CO2 in the bands Σu+(0001) –
Σg+(1000) and Σu+(0001) –
Σg+(0200) in a spectral range of 9–11 µm
was used as the source of radiation energy for laser photoacoustic spectroscopy (PAS). The
photoacoustic
(PA) cell was designed as a cylindrical cell of 300 mm
in length and 6 mm in diameter equipped with a microphone as well as either
a graphene membrane or a cantilever sensor, which could be exchanged with
each other or for a different membrane or cantilever. All sensor units were
placed in the middle of the longitudinal dimension of the cell. The total
internal volume of the PA cell was 19.5 cm3. To obtain the largest
possible PA signal, it is necessary that the interior space of the PA cell,
which is not exposed to laser radiation, is minimized. This means that in the
case of a cylindrical PA cell, the inner diameter of the PA cell must be
minimized with respect to the laser beam. The diameter of the laser beam that
was used was approximately 9–10 mm, and the laser beam was focused by a
lens with a 25 cm focus onto the centre of the PA cell; this set-up
motivated the geometry of the PA cell described above.
The sensitivity levels of the sensing elements were tested using
concentration standards based on the permeation method (Okeeffe and Ortman,
1966; Barratt, 1981). The concentration standards were prepared in the form
of closed tubes composed of permeable material (silicon, polyethylene,
Teflon), and these standards were placed in a thermostatted chamber at
35 ∘C for long-term weighing and storage. For measurements, each
concentration standard was placed in a chamber through which carrier air was
flowing. Using such a mixing arrangement, we were able to prepare a given
concentration level in a flow regime (flow rate 1 cm3 s-1) and
under atmospheric pressure, with methanol vapour as the testing gas. The
CO2 laser was tuned to the 9 P(34) CO2 laser line
(1033.488 cm-1), which corresponds to the fundamental CO stretching
band of methanol, where the maximal absorption cross section is 72×10-20 cm2 (Loper et al., 1980).
We studied several AFM-based silicon cantilevers and graphene sheets via
laser PAS. The membranes for the photoacoustic detector – graphene sheets –
were prepared from MLG through micromechanical cleavage of basal plane
Highly Ordered Pyrolytic Graphite (HOPG,
ZYH grade, Bruker, USA) (Novoselov et al., 2004). Graphene sheets in the form
of circular membranes (Fig. 1a) and AFM-based silicon cantilevers in a square
arrangement (Fig. 1b) were tested. The MLG sheets (thickness ≈102 nm) were mounted on an additional glass window in the cuvette for
PAS (Fig. 1a). The movements of the graphene sheets or the cantilever ends
that were induced by acoustic waves were measured by an He–Ne laser beam
that was reflected from the sheets or the ends onto a quadrant detector (a
red-enhanced quad-cell silicon photodiode, SD 085-23-21-021, Laser
Components). The signals from both the microphone and the quadrant detector
were processed by an oscilloscope (LeCroy 9361).
Two types of cantilevers and one type of membrane were used: commercial
(Bruker) silicon cantilevers of the OTESPA (metallized) type or the NP
(metallized) type, with an arm length of 200 µm and with resonant
frequencies of 300 kHz in ambient gas and 10 kHz in
liquid, and home-made multilayer graphene membranes. The primary benefits of
using layered graphene as a membrane lie in its high elasticity (YM ∼1 TPa, defect free), the possibility of modifying its stiffness by simply
removing graphene layers (down to the level of a single monolayer) and thus
increasing its bending sensitivity, and, finally, its reflectivity, which
allows the bending of the membrane to be probed using a reflected laser beam.
Compared with microphone membranes, cantilevers and graphene membranes
exhibit lower damping and lower stiffness, and, therefore, higher sensitivity
in general can be achieved. The graphene was of grade ZYH (Bruker, USA). The
thickness of the multilayer graphene (MLG) could be simply determined through
an AFM profile measurement. The thickness of the graphene that was used in
these experiments was typically in the range of 102 nm, although this
thickness could be further decreased.
Schematic illustrations of the construction of the sensing elements
and their implementations: (a) graphene-based sensing elements,
(b) AFM-cantilever-based sensing elements, and (c)
schematic illustration of the graphene membrane detector assembly on the
cuvette window sealed with a silicone O-ring, where the red lines represent
the laser beam and the arrow indicates the pressure wave.
The graphene membrane was attached to the glass window by an epoxy glue film
and was pressed by an O-ring over the circular opening through which the
pressure wave from the cuvette was transferred to the membrane. The membrane
deflection was measured using a laser beam reflected from the membrane onto
the quadrant detector. Details regarding the membrane mounting are provided
in Fig. 1c.
The micromechanical sensing elements were installed in the cuvette for PAS
together with a sensitive microphone (electret microphone EK 23024, Farnell)
for comparison. The experimental set-up is depicted in Fig. 2. Quantitative
measurements were performed using a phase-sensitive lock-in amplifier
(Stanford Research Systems SR530 lock-in amplifier). The CO2 laser beam
was modulated by a chopper, and the photoacoustic signals from the microphone
and the graphene were demodulated by the lock-in amplifier and subsequently
processed using a PC. The signals from the microphone and from the
cantilevers or graphene membrane were passed through preamplifiers to lock-in
amplifiers. The reference signal from the chopper, i.e. the frequency of
interruptions of the laser beam, served as a reference signal for the lock-in
amplifiers. The input signal amplitude for the lock-in amplifiers was 3 mV
for the microphone and 5 mV for the cantilever and graphene sensors.
Experimental set-up for the tests of the sensing elements.
Results and discussion
AFM-based silicon cantilevers and MLG sheets were tested as photoacoustic
detectors in these studies. Figure 1 shows a schematic diagram of the set-up
for the testing of the MLG-based and silicon–AFM–cantilever-based sensing
elements (henceforth referred to as graphene sensors and cantilever sensors,
respectively, for brevity). In the case of a classical capacitive microphone,
there is a space both in front of and behind the membrane that is connected
by a balancing channel to eliminate any damping of the membrane's movement
during its return. The same function is also performed in our sensing
elements. The free ends of the cantilevers in the cantilever sensors can move
freely in space. In the case of the graphene sensors, an additional glass
slide is used as a specialized holder for the MLG-sheet-based membrane
(Fig. 1a). The elimination of damping effects is proven by the experimental
data presented in Fig. 3a. Figure 3a presents the motion of the MLG sheet as
indicated by the He–Ne laser beam reflected onto the quadrant detector,
which translates the movement into a voltage amplitude proportional to the
pressure changes inside the chamber. The low-frequency (4 Hz) modulation of
the laser by the chopper causes symmetrical movements of the graphene
membrane.
Experimental signals recorded from the graphene-membrane-based
sensing elements at low frequency: (a) 4 Hz and (b)
23 Hz.
The experimental data presented in Fig. 3a demonstrate the detection of the
motion of the graphene sensor in the form of rising and falling exponentials.
These observations are the result of the absorption of radiation, which heats
the gas in the chamber. By virtue of the fixed volume, the time-varying
thermal fluctuations generate pressure variations (sound waves), which are
detected by the graphene sensor. The following conditions correspond to the
typical case and are assumed to apply: the source is modulated such that the
time profile approximates a square wave of period T; the molecular
radiative and collisional relaxation times are small and can be neglected in
comparison with the period of the chopper modulation T; and the detector
chamber is a cylinder of length l and radius a, where l≫a. Under
these conditions, it can be theoretically derived that the time dependence of
the pressure response p(t) is given in the following forms:
p(t)=c1+c2(1-e-t/τ)0≤t≤T/2,p(t)=c2eT/2τe-t/τT/2≤t≤T,
where τ is the thermal relaxation time to the temperature of the
chamber walls and
c1=Δp(e-T/2τ-1)(e-T/2τ-eT/2τ),c2=Δp(1-e-T/2τ)(e-T/2τ-eT/2τ),
where Δp corresponds to the fully developed pressure amplitude (T→∞).
A gradual change from an exponential waveform to a sine waveform occurs as
the modulation frequency is increased, at a frequency of approximately 23 Hz
(Fig. 3b). Such waveforms are suitable for processing using a phase-sensitive
amplifier (lock-in amplifier), thus allowing us to compare the sensitivities
of the microphone and the sensing elements. The detection chamber was
equipped with both the microphone and the tested sensing element (graphene
sheet or silicon cantilevers). Both signals – from the microphone and from
the quadrant detector (representing the detection of the motion of the
sensing element: the graphene sensor or the cantilever sensor) – were
processed using lock-in amplifiers. As a testing gas, we used methanol
vapour. The detection chamber was connected to the flow system for precise
monitoring of the flow rate of the carrier air using mass flow meters and
controllers (FMA Series, OMEGA). A sampling chamber, into which the
concentration standards were inserted, was placed in the flow system just in
front of the detection chamber. These concentration standards provided a
calibration of the measurements based on the permeation method (Okeeffe and
Ortman, 1966; Stellmack and Street, 1983; Zelinger et al., 1988) and the
long-term weighing of the standards.
Equivalent measurements were performed for the cantilever sensors (Fig. 1b).
Silicon cantilevers offer the advantage of better optical reflection compared
with the diffuse reflection from the MLG-sheet-based sensing elements
(Fig. 1a). These experiments directly followed the research work previously
performed by Kaupinnen et al. (Kauppinen et al., 2004; Koskinen et al., 2006,
2008; Kuusela and Kauppinen, 2007).
Waveforms were simultaneously recorded to monitor the time dependence of the
intensity of the laser radiation, the time dependence of the response of the
microphone as processed by the lock-in amplifier, and the time dependence of
the response of the graphene sensor as processed by the lock-in amplifier;
the results are depicted in Fig. 4. The concentration of the methanol vapour
that was introduced into the detection chamber was ∼10 ppm. The
following steps were performed during each 500 s scan: the laser was
initially off (OFF) – the laser was turned on (ON) – the laser was turned
off (OFF) – the laser was turned on (ON) – the laser was turned off (OFF).
Both the microphone and the graphene sensor responded to the turning on and
off of the laser (Fig. 4). A rise and fall in the signals from both sensors
can be observed upon the powering on and off of the laser, respectively, in
Fig. 4; the magnitude of the difference in the signal is determined
predominantly by the concentration of the absorbing gas, i.e. 10 ppm of
methanol. The background signal level (the difference between the signals
recorded in the ON and OFF states of the laser at zero concentration of the
absorbing gas) is almost negligible; in the case of the microphone, it is
approximately twice as high as the noise level, and in the case of the
cantilever, it is approximately half of the noise level.
In addition to the ON–OFF mode of the laser, we also tested a similar
ON–OFF mode for the concentration standard (Fig. 5). When we inserted a
concentration standard (∼2 ppm) into the sample chamber (concentration
ON), both the signal from the microphone and that from the graphene sensor
(as processed by the lock-in amplifier) increased; see Fig. 5a. The same
measurement was performed using the microphone and cantilever system; see
Fig. 5b. Then, we removed the concentration standard from the flow system
(concentration OFF) and monitored the resulting decrease in the signal
(Fig. 5c, d). The responses of the microphone and graphene sensor are shown
in Fig. 5c, and the responses of the microphone and cantilever system are
shown in Fig. 5d. The perturbations observed at the beginning of the increase
in the concentration signal (Fig. 5a, b) and at the beginning of the decrease
in the concentration signal (Fig. 5c, d) were caused by pressure disturbances
created in the sample chamber when the concentration standard was inserted
and removed.
Experimental data for comparison of the signals detected by the
microphone and by the graphene-based membranes at the same gas concentration
(methanol, ∼ 10 ppm), as processed by the lock-in amplifiers.
Experimental signals recorded from the lock-in amplifiers for a
generated concentration of methanol of ∼ 2 ppm: (a)
concentration ON for the microphone and graphene sensor, (b)
concentration ON for the microphone and cantilever system, (c)
concentration OFF for the microphone and graphene sensor, and (d)
concentration OFF for the microphone and cantilever system.
A comparison of the noise levels indicated by the experimental data presented
in Fig. 5 yields the corresponding signal-to-noise ratios (S/N). The
graphene sensors and the cantilever systems have different connections to the
PA cell. The connection is in the middle of the cell and thus exerts a strong
influence on the acoustic properties of the interior of the cell. In the
connection of the graphene sensors or the cantilever systems to the PA cell,
different regions are created that affect the acoustic signal gain, giving
rise to a difference in the resulting sensitivities of the two systems. In
the case of the microphone and the graphene sensor, the signal-to-noise ratio
for the microphone was S/N≈25 and that for the graphene sensor was
S/N≈5. In the case of the microphone and the cantilever system, the
signal-to-noise ratio for the microphone was S/N≈50 and that for the
cantilever system was S/N≈36. It is observed that the signal
detected by the graphene sensor suffered from a higher noise level than did the signal from the cantilever. Possible reasons for this finding could lie
in the adsorption properties of graphene with respect to those of methanol
vapour; this phenomenon will be the subject of further studies. The
cantilever system is not as strongly affected by the adsorption properties of
the gas, and, therefore, its sensitivity approaches that of the microphone.
Thus far, the sensitivity of these elements has been inferior to that of a
commercial microphone. We expect that further enhancement of the sensitivity
can be achieved by adjusting the elements in combination with the entire
photoacoustic system. To determine the optimal responses of the membranes,
graphene sheets of different thicknesses will be prepared, and the dimensions
of the cuvette (and thus the resonant frequency) will be varied. Reduction of
the diffusion of the reflected laser beam can be achieved by depositing a
thin layer of metal onto the membranes. Future work will focus on few-layer
graphene cantilevers/membranes and single-layer graphene, which will be
prepared using the chemical vapor deposition (CVD) technique and then transferred to supports, thus allowing for free-standing
mounting.
Conclusions
The first experimental results of analytical testing of
MLG-sheet-based sensing elements as well as results obtained for AFM-based
silicon cantilever systems are presented. A method for the comparison of the
responses of various sensing systems, including graphene sensors, cantilever
systems and microphones, has been developed. We obtained the first
quantitative data regarding the sensitivity of the tested graphene sensors
and cantilever systems by applying concentration standards to generate
concentrations of methanol in a well-defined way. The combination of
sensitive microphones and micromechanical elements with advanced laser
techniques offers possible opportunities for the development of new, reliable
and highly sensitive chemical sensing systems.
Acknowledgements
The authors are grateful for the financial support provided via project no.
LD14022 within the framework of COST action TD 1105 funded by the Ministry of
Education, Youth and Sports of the Czech Republic, via project no. 14-14696S
funded by the Grant Agency of the Czech Republic and via project no.
R200401401 in the framework of the regional cooperation of the Academy of
Sciences of the Czech Republic. Edited by:
A. L. Spetz Reviewed by: three anonymous referees
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