Paul Ahern - Overview of Micro & Nano Transducers

Overview of Micro- and Nano-Transducers
(December 2013)

Paul Ahern, 1School of Electronic Engineering, Dublin City University.
Reliability & Eco-Environmental Engineering, Bell Labs / Alcatel Lucent.
paul.ahern@alcatel-lucent.com

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Abstract— The aim of this paper is to present a review of
current transducer technology, fabrication methods and
materials pertinent to the nanotechnology and MEMS era. We
begin with an introduction to the concept of a transducer and the
historical context, and then review some specific application
classes of transducers where nanotechnology has already, or has
the possibility in the future, to have an impact on the transducer
device market. This review highlights the advantages of these
MEMS approaches to promote new transducer types, especially
those related to nanotechnology, and possible future research
directions are discussed.
Keywords — Accelerometer, gyroscope, inertial sensor,
resonator,
microfabrication,
MEMS,
silicon
sensors,
piezoelectricity, thin films, ultrasonic actuators.
I.

INTRODUCTION

A mounting cause for concern in the growing
microelectronics market is the slow pace of availability of
power efficient, high sensitivity and commercially viable
transducers. Thanks to generations of familiarity with silicon
VLSI components attempts are on-going in many other fields
to apply “Moore’s Law” type scaling improvements to
transducer technology to innovate and refine new
multidisciplinary transducer products. These efforts typically
have a low initial success rate as the numbers of transducers
with adequate performance at a suitably low price point do not
exist.
Novel transducers which make use of nano-sized devices
and materials have the potential to allow the ultra-sensitive
measurement of previously undetectable phenomena all the
way down to the molecular scale. Scaling transducers down to
the length scale of nanotechnology affords us the opportunity
to utilise their special properties for a wide variety of
measurements. Potential applications of such a shrink could
be utilised in diverse fields such as nanoelectronics,
biotechnology and medicine.
To date, there are only a handful of commercially available
transducer types which utilise micromechanical principles and
fabrication techniques, however the potential for micro and
eventually nano-sized transducers to have a disruptive effect
on the world of sensing and control is huge. In this review
paper we will undertake a whistle-stop tour of the subject, and
discuss the principles of fabrication, nanomaterials properties
and applications for a select sample of micro- and nanotransducer systems.

II. OVERVIEW OF TRANSDUCER OPERATION & FABRICATION
A transducer is at its most basic is simply a mechanical
device which reads an input signal and outputs an electrical or
optically detectable response1. The buttons of a computer
keyboard , the quartz crystal of a watch, and the light sensor of
a burglar alarm are all examples of a cheap, uncomplicated
input transducers2 with the light emitting diode (LED)
probably the currently most well-known example of a lowcost output transducer.
Many would consider the origin of the burgeoning field of
micro-transducer as being closely related to the publication of
a series of important research papers by a group of scientists
starting in the mid-1950’s. Charles Smith, then of Bell Labs
while on sabbatical from Case Institute of Technology,
published his study3 of uniaxial strain on the resistivity of
silicon and germanium devices in 1954 which opened the
possibility of having such a microscopic phenomenon as his
“electron transfer mechanism” giving direct usable
experimental data.
Paul and Pearson4, of Harvard and Bell Labs respectively,
continued this direction with their 1955 publication
concerning single high purity silicon crystals and their
behaviour of lowering their energy gap when in a high
pressure environment.
Feynman’s now infamous 1959
presentation at CalTech to the American Physical Society,
entitled “There’s Plenty of Room at the Bottom”, with
hindsight, is credited as spurring more interest into the nascent
concept of manipulating single atoms to build structures and
devices.
In 1961, Pfann and Thurston5 again of Bell Labs probed the
viability of a transducer which used the piezoresistive
response of diffusion-doped silicon and germanium to
evaluate longitudinal and transverse stresses. At Honeywell,
Tufte, Chapman and Long6 were the first in 1962 to create a
truly thin silicon diaphragm for pressure sensing by using
early semiconductor processing methods such as plasma
etching and selective oxidationa.

a
For a full and entertaining history of the explosion in research and new
companies in the field from the mid-sixties onwards, a good starting point is
Marc J. Madou’s “Fundamentals of Microfabrication & Nanotechnology” by
CRC press.

Paul Ahern “Overview of Micro- and Nano-Transducers”

These devices can be broadly categorised as passive or
active; passive types operate due to the generation of some
shift in material properties such as capacitance or resistance
with an external power source required to give rise to the final
measurable output. Active transducers on the other hand
usually transforms the input signal directly into electrical
energy, such as the way a piezoceramic element materials
translates energy from mechanical to an electrical form.
Additional signal processing can be done either on-board
the transducer or elsewhere, but still the principle is using a
basic mechanical element such any kind of micro-sized beam
or diaphragm.
Typical transducers are made possible by the harnessing of
certain intrinsic solid state materials properties, and common
transducer devices such as thermometers, pressure and
moisture sensors all have their basis of operation in the
manifestation of a suitable material band gap, with their low
cost fabrication being based on very well understood silicon
planar technology7.
The differing current approaches used for MEMS-based
transducers are to either attempt to utilise nanotechnology to
fashion both the transducer and the required amplification
component8, or else as in the silicon case to attempt the
approach whereby an integrated circuit wafer as a 2D substrate
is used and we include the nano-sized transducer parts as
separate elements situated as closely as possible to the
patterned transistors amplifiers9. If the mechanical transducer
and the integrated circuit components can be incorporated
together in such a “hybrid” arrangement, then there is the
opportunity for mass production to give rise to inexpensive,
high quality transducers.
However there are benefits to taking a different, more
disruptive approach and pursuing miniaturisation using
nanotechnology principles to drive miniaturisation down
closer to the theoretical limits, with improved performance
and lower power consumption side benefits to the realm of
new measurements concepts for Nano-transducers that can
only be realised at the nanoscale.

SELECTED NOTABLE APPLICATIONS

III. NANO-RESONATORS
Resonators are devices which utilise an element, such as a
beam or a diaphragm, to detect or generate vibrations of a
certain frequency. The ability to discriminate different
frequencies is critical to their operation and usefulness.
Traditional resonators have been with us since the midsixties when Belyaev10 and co-workers showed their prototype
which was a simple diaphragm mounted on two supporting
brackets.
In the silicon age, micromachining and IC
fabrication techniques have been used successfully to design
new types of smaller resonator which contain a hollow cavity
and can operate much more efficiently in vacuum
environments. A typical representation of this type of device
is shown in Figure 1 opposite. The resonator device can then

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be affixed onto a printed circuit board which houses the
control electronics using industry standard adhesives.

Figure 1 – Schematic of the cross-section of a typical
silicon micro-machined diaphragm resonator. The PZT
layer is clamped between a bottom electrode of Pt/Ta
and a top electrode of Cr/Ag. (Image – Deshpande &
Saggere11, 2007)
The inclusion of a transparent cantilever allows an optical
detection scheme to be used if an electronic scheme is not
suitable for the application , which means the system is now a
optical
resonator
described
as
a
“microoptoelectromechanical” device, or MOEMS.
The further addition of lenses, prisms, mirrors and
refractive media can be used with such systems to build small,
fast devices for photonic waveguides, routing and modulation.
An early commercially available example of such a device is a
handheld barcode scanner which most readers would be
familiar with, however integration with other types of free
space optical components has given rise to interesting and
more complex devices.
One example of such a device is Tien12 and co-workers
surface laser for micro positioning which is sown in Figure 2
overleaf. Built upon a silicon substrate, it makes use of
freestanding MEMS components such as folding micro
mirrors, micro lenses and a resonant transducer operating at a
natural frequency of 29.2 kHz.


Figure 2 – Tien & co-workers design of a laser resonant
scanner built on a silicon substrate which makes use of
freestanding MEMS components.
In the future it is very likely that optical MEMS will
consume more and more of the market in this space, and that
increasingly expertise from the nanotechnology and
semiconductor fabrication arenas will be brought to bear on
making these devices which can directly interact with the
photonic packets being distributed in modern optical
networks.
Newly simplified spin on glass (SOG) processes such as
those described by Cheng13 and co-workers which is an
enabler for tighter integration of integrated photonics and
MOEMS devices which critically require smooth surfaces
with uniform nanometre scaled roughness values.
One compound which is attractive for this purpose is
biphenol A ether glycidyl due to its high optical transparency
above the 400nm wavelength window coupled with its very
low propagation loss profile, which makes it valuable for
fabricating optical components such as resonators and MachZender diodes.
Plasma treatments such as those detailed by Zebda14 and coworkers are critical in delivering interfacial surface
modification methods which allow these materials to grip to
plasma treated surfaces effectively to allow additive spin-on
processing. And pave the development for ultra-integrated
2.5D photonics structures. The problematic high thermal
budget associated with these processes can also be decreased,
when piezoceramic materials are present in the device, by
instead using one of the newer CVD-related deposition
techniques such as plasma enhanced chemical vapour
deposition (PECVD) or conformal atomic layer deposition
(ALD) methods.
IV. INERTIAL SENSORS
Once the exclusive preserve of aircraft, space rockets, and
cruise missiles, the last decade has seen MEMS-based inertial
sensors such as gyroscopes and accelerometer make rapid
inroads into first the luxury segment, and now the non-luxury

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automobile supply chain. Anti-lock braking and skid control
systems in the average family hatchback owe thanks to the
advent of low cost, mass produced silicon based MEMS
sensors. On luxury models, additional features such as
adaptive active suspension systems, autosensing intelligent
cruise control, and advanced airbag protection systems are all
controlled by inertial transducers.
While historically inertial systems such as gyroscopes and
accelerometers have been composed of mainly mechanical
sensors, in recent decades there was an impetus to move the
foundation of such devices in to the MEMS and
nanotechnology space. The main drivers for this change have
been the marked performance and reliability improvements
possible, easier integration directly with the on-board digital
electronics as well as the lower costs attainable with
semiconductor manufacturing methods which allow many
thousands of devices to be created on a single silicon wafer
substrate, such as shown in Figure 3 below.

Figure 3 – Scanning Electron Microscope image of an early
silicon MEMS comb drive tuning fork gyroscope. The comb
structures allow detection of resonance along the axis
normal to the vibration plane. (Image – Barbour15 and coworkers/Draper Laboratories)
MEMS gyroscopes are an offshoot of the resonator type
device, and similarly to resonators are usually fabricated from
a monolithic substrate of silicon (or sometimes quartz). These
devices operate based on the physical principle of translating
an external angular change to an internal torsional force which
is detectable around the sensor’s axis, called a Coriolis force16.
For a silicon-based sensor, the change is directly measured
as a capacitance shift, whereas for quartz materials the change
is manifested as a piezoelectric perturbance. The electrical
output is then demodulated by the digital electronics,
amplified and fed to the digital output. Gyroscopic sensor
devices require the presence of a vacuum inside an
hermetically sealed device package, usually an Al2O3 ceramic
casing, in order to lower losses and obtain high Q resonance
conditions, and are intended to maintain
stable and


uncontaminated surroundings for the device for a typical 20year design lifetime17.
Semiconductor manufacturers are adept at squeezing every
last but of performance out of silicon devices, and inertial
sensors are no exception. Supplier like Samsung use their IC
experience to tune the manufacturing process to drop the noise
floor of the inertial sensor to its minimum using some novel
tweaks, such as employing differential drive voltage setups,
application of an effective interlayer DC bias after fabrication
and utilising high-aspect-ratio polysilicon structures in the
device front-end to maximise the desired frequency
response18.
Recent work by Zandi19 and co-workers has postulated how
microphotonics may be able to advance accelerometer devices
by using the familiar principles of a Fabry-Perot cavity. Their
scheme involved the attachment of a suspended movable mass
to one Distributed Bragg reflector (DBR) mirror – when there
is an acceleration, the movable mirror’s position changes and
thus the cavity width changes which can be seen as a
modification of the resonance conditions. This approach
allows an easily-integrated solution, shown in Figure 4 below,
which is independent of EMI and consumes very low power.

Figure 4 – Can microphotonic devices extend the
operational envelope for accelerometers? Zandi and coworkers silicon based Fabry-Perot accelerometer allows
integration of all components on a single substrate.

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A common and non-trivial problem with generally all types
of accelerometer devices is the question of how to try and
limit cross-talk from other directions which may be mistaken
as input signals in the axis of interest. Maximisation of
sensitivity and simultaneous suppression of unwanted forces
and excitations is a large and on-going area for research.
These optical types of accelerometer however allow a very
low value of unwanted cross-axis sensitivity, with Zandi and
co-workers reporting that their initial prototype devices
achieved <0.5% on the orthogonal (in-plane) axis and <0.01%
for the z-axis (out-of-plane); to put this in perspective, this is
performance that could simply not be attained by traditional
mechanical acceleration sensors, no matter how much they are
shrunk in size.
For very high resolution sensing, another approach is to
harness quantum mechanical tunnelling effects in the device.
First envisaged at the Jet Propulsion Laboratory, in this case a
constant tunnelling current can be maintained between a fixed
tip and a moveable microstructural element which is within a
few Angstroms of separation, with a counter-electrode
employed to sense the displacement in a closed-loop system20.
Preservation of the closed-loop mode is vital, as the ratio of
tunnelling current: distance is exceptionally large and
critically limits the usable measurement range if employed in
the open loop arrangement, and the tunnelling barrier height
for the opposing electrodes can vary by one order of
magnitude in air, which hinders device sensitivity21.
An example of one such tunnelling device can be seen in
Figure 5 overleaf. Using tunnelling as a sensing mechanism, a
device can measure very small sized displacements with high
sensitivity in a small footprint, however they fall down in the
area of their low-frequency noise levels and their high voltage
requirements.


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frequency noise which has so far generally been noticeably
high in measurements made using these types of nanotubebased devices.
A proposed workaround for these limitations is the
proposed inclusion of AC bridging rectifier, such as the wellunderstood Wheatstone bridge, onto the transducer in order to
resolve the typically non-linear output of the transducer. Once
this response has been accounted for however, these devices
offer excellent sensitivity as shown in a study carried out by
Minot23 and co-workers where a device was successfully
fabricated with a measured sensitivity limit of 0.1nN/Hz 1/2 at
low frequencies.

Figure 5 – Yeh & Najafi’s design22 (top) and as-fabricated
electron micrograph (bottom) of their micro-machined
accelerometer which operates based on the principles of
tunnelling. The device boasts their high resolution with a
wide dynamic range contained within
a small footprint.
V. NANO-STRAIN GAUGES
In the physical world, gradual miniaturisation of macrosized transducers for sensing applications has a finite limit,
dictated by the device signal to noise (SNR) value which
becomes larger as the device geometry shrinks.
For
transducers such as strain gauges, SWCNTs have an
advantage due to their high strain sensitivity caused by
changes to the band gap energy due to distortion of the atomic
lattice.
This atomic-level effect is a clear example of the type of
quantum mechanical phenomena that govern the properties of
materials at the nanosystem level, and gives rise to a
detectable resistance change which encompasses up to two
orders of magnitude for even minor strain energies in the
region of 1-3%. A diagrammatical illustration of this
mechanism in terms of the band gap of the CNT can be seen in
Figure 6 opposite.
A significant impediment to the ready use of CNTs as a
strain sensitive material is their non-linear behaviour due to
temperature. The strain response has the effect of limiting the
strain sensitivity at very small magnitudes. Other issues are
surmounting contact resistance issues which have been shown
to limit SNR due to thermal noise, as well as controlling low

Figure 6 – Diagrammatical illustration of how pressure can
be measured in a nano-sized strain transducer. At (a) there
is no strain so the CNT acts as a p-type semiconductor. At
(b) transport in the CNT is interrupted by the creation of a
depletion region. At (c) effectively a p-n-p junction has
formed in the middle of the CNT and tunnelling can now
increase as the size of  decreases. (Image from Minot24 &
co-workers)
In terms of the fabrication method, we once again encounter
the question of a top-down vs. bottom-up approach when it
comes to the question of how to fabricate the CNT, whilst the
tried and trusted method of AFM manipulation has been used
to position the CNT beam on the silicon substrate. Large
metallic contacting pads for the device can be grown by
conventional Chemical Vapour Deposition (CVD) based
methods, with gold pads providing the best contact and lowest
resistance.
The SWCNT based transducer created by
Stampfer and co-workers can be seen in Figure 7 opposite.


resonance conditions,
simulations.

5

in

agreement

with

numerical

Figure 8 – Lee and co-workers experimental results
showing the Raman shift observed when a SWCNT is
subjected to increasing pressure from a probing AFM tip.
The G mode at 1590cm-1 shows marked changes as the
strain is applied.

Figure 7 – High magnification SEM images showing topdown (a) and isometric (b) views of the nano strain gauge
fabricated by Stampfer25 and co-workers which uses a
SWCNT draped across CVD gold pads.
The conductance of the CNT beam strongly depends on the
chirality (armchair or zigzag) of the carbon atoms in the
wrapped sheet. Investigations by Maki 26 and co-workers of
photoluminescence (PL) spectrum measurements taken from
individual SWCNT demonstrated the presence of an energy
shift which is directly due to the band gap change caused by
the elastic strain in the system.
The band gap change is understood to be caused by the
displacement of elastic strain of the SWCNT under stretching,
and immediately before the ultimate failure stress is reached a
distinct emission intensity reduction event can be seen. It is
envisaged that these type of devices could be useful in the
future when applied to the potential application of nano-sized
tunable LEDs.
Lee27 and co-workers also looked at SWCNTs under strain,
but this time using in-situ Raman spectroscopy as reproduced
in Figure 8 opposite. They also recognised changes in the
intensity, specifically in the radial breathing mode frequency,
due to increases in strain which in turn leads to a change in

One somewhat counterintuitive finding was that increasing
the length of the nanotube did not give rise to an increase in
the axial bond length or a decrease in the resonant frequency.
When strains grew above a level of ∼2% some permanent
damage was seen to occur in the lattice structure of the CNT
which was distinct from other effects in the band structure
which were recoverable in materials very close in chirality to
the “armchair” geometries of (11,10) and (10,9).
It could be argued that previous work by Reich28 had
predicted this somewhat, as it found that there are differences
in chiral versus achiral tubes in terms of mixing of dominant
optical modes, as it was postulated that high-energy Eigen
modes can no longer be neatly categorised into purely
circumferential or axial modes which are used to respectively
describe the “armchair” and “zig-zag” nanotubes geometries,
as illustrated in Figure 9 on the following page in Wildoer and
co-workers seminal “Nature” article from 1998.


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Figure 9 – Illustration of how a planar sheet of grapheme
is folded into a nanotube by rolling the sheet along a
wrapping vector, C. (Image – Wildoer29 and co-workers,
Nature)
VI. PIEZOELECTIRC NANO-TRANSDUCERS
Piezoelectric films are a staple part of many different types
of transducer device. One specific role they play is in the field
of high frequency ultrasonic transducers for use in a wide
variety of applications. In the medical field, high frequency
ultrasound scanners are used routinely to give detailed
viewing of anatomical structures such as the skin, eyes and
circulatory system, with high spatial resolution traded off
against penetration depth.
Piezoceramic films such as PZT have shown promise as a
possible ultrasound transducer which could be fabricated in a
MEMS array to allow high frequency readout over a large
focal area with low losses30. These layers can be fabricated by
a variety of methods, from sintering to modified sol-gel
reactions to aerosol deposition31, however the effective
response of these very thin piezoelectric films can be
markedly different to the properties it exhibits in the bulk.
As can be seen in figure 10 opposite, subtle changes in the
deposition process can have a large effect on the surface
properties of the as-deposited film which in turn can limit the
feasibility of later nano-scale patterning and lithography.
Polymeric electrostrictives such as PVDF have also been
evaluated32 as potential transducers for ultrasound scanning
due to their low dielectric constant and the added benefits of
high flexibility and low acoustic impedance which is very
compatible with biological specimens – however its low
coupling factor at present does not make it a suitably attractive
choice as a transmitting material, unless modified as a copolymer with trifuoroethylene (TrFE) where it has found
success for uncomplicated annular array transducers 33.

Figure 10 – Electron micrographs from Zhu34 and coworkers showing the influence of sintering temperature and
separation method from the titanium substrate. Since a
variety of factors such as film density, crack volume,
porosity and grain size critically determine the transducer’s
operating frequency, processing conditions must be very
carefully controlled to give a film suitable for the desired
application.
Zinc Oxide (ZnO) is another material with many thin-film
transducer applications. Specifically within the realm of
biomedical imaging, a novel approach suggested by Feng35
and co-workers (among others) suggests that acoustic
focussing results can be achieved by fabricating dome-shaped
diaphragms on a sapphire or silicon substrate, with the
transducer’s effective focal region dictated by the diameter of
the domes in the array.
One successful method which has been described to create
an array of these dome-shaped Parylene diaphragms is by a
low temperature lost wax moulding processing route, where
toluene is used to remove the wax balls from the domes at
room temperature leaving an array of stress-free Parylene


domes as shown in Figure 11 below. In the final step, the
piezoelectric ZnO layer is then deposited on top of the
supporting Parylene domes.

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VII. NANOSCALE POLYMER-BASED ACTUATORS
There are numerous benefits to creating artificial elements
which can actuate at the nanoscale. One approach has been to
utilise semiconducting polymeric materials to this end as they
possess large strain fields when correctly stimulated, and also
this allows the possibility of incorporating the actuating
mechanism directly with the control system in one monolithic
package.
While these Electro Active Polymer (EAP) materials such
as Trans-polyacetylene (PA), Poly(para-phenylene) (PPP) and
poly(pyrrole) (PPy) are somewhat conductive in their natural
state, success has been reported with doping methods which
allows them to display metal-like characteristics, with the
incorporating of the doping species38 into the polymer chain
giving rise to a net change in volume which increases the
actuating properties of the substrate. A summary of their
principles of operation can be seen in Figure 13 below.

Figure 11 – A novel fabrication method by Feng and coworkers for creating ZnO transducers on a silcon substrate
which are dome-shaped to take advantage of acoustic
focussing enhancement. (Image – Feng et al)
Piezoelectric ultrasonic transducers are an essential part of
the semiconductor manufacturing process; they are intrinsic to
the thermosonic wire bonding process which is central to the
packaging of silicon integrated circuits as they allow
transmission of acoustic energy from an electrical input to the
bonding interface, as shown diagrammatically in Figure 12
below. This has the effect of greatly improving the bond
quality of the solder interface36 and thus is an enabling
technology which allows faster, stronger and more repeatable
bonding technology with higher packaging pitches.
An experimental study by Wang37 and co-workers showed
that moving to smaller sizes of transducer design had the
advantage of allowing better frequency selection and
minimising detrimental vibration modes which can occur close
to the working frequency, due to a decrease in the vibration
coupling between the radial and longitudinal directions.

Figure 12 – Schematic diagram of a typical ultrasonic
transducer used in microelectronic bonding (Image from
Wang et al).

Figure 13 – Illustration of the mechanism of operation of a
flexible bilayer Electro Active Polymer (EAP) actuating
element. Oxidation and reduction mechanisms are
harnessed to cause motion as a result of expanding and
contracting the opposing polymeric layers. These
transitions are complemented by modification of the
polymer chain, driving the introduction and termination
of molecular entanglements. (Image: Ryhanen et al,2010.
“Nanotechnologies for Future Mobile Devices”,
Cambridge University Press)
When compared to familiar piezoelectric materials which
are typically used in this area, the polymer based solutions
have the major advantage of requiring an order of magnitude
less voltage with a much higher associated displacement
distance. The downside is that current designs will only
operate in the presence of a suitable electrolyte which provides
the ionic contact between the EAP layers, normally a lithium
salt, which currently limits the commercial applications of
such devices.
With that said, one material which has garnered significant
interest is free-standing films of poly-pyrrole (PPy) which are
fused with double sided adhesives to form a simple flexible


bilayer actuator39 with platinum wires used as the connecting
electrode. Providing that the poly-pyrrole film is uniform and
smooth enough, the actuators response can be modulated
solely using the magnitude of the applied charge, with no
current influence.
Heating of the Pt electrode lowers efficiency and response
in the system, so a further refinement of a trilayer design was
developed where two layers of EAP are employed and the Pt
electrode is no longer needed, with the encouraging result that
the ensuing actuator was able to move an object thousands of
times its own mass whilst also giving a feedback response
signal of the required power needed40.
Further research which removed the need for a liquid
electrolyte employed the use of a solid electrolytic layer such
as poly(epichlorohydrin-co-ethylene oxide) and showed some
promising results41. The angular velocity of the actuator in air
was comparable to that achieved in the electrolytic medium, as
was the electrical energy consumed by the motion with the
same broadly linear relationship between movement speed and
applied current noted.
Taking matters into the nanotechnology realm, SWCNTs
with their high Young’s modulus and high electric
conductivity have been touted as ideal replacement actuator
materials using the same vein of fabrication, lamination with
adhesives to give arrays of nanofibres. Challenges due to
separating the nanotubes were been surmounted by Fukushima
and co-workers42 successful preparation of “Bucky Gels” such
as those shown in Figure 14 below, using room temperature
ionic liquids that contain dispersed high-purity HiPco
SWCNTs and are built up using a rudimentary layer by layer
addition casting process, which has no stringent pre-requisites
save for a agate mortar and a hot plate which can be held at
80C.

Figure 14 – Scanning Electron Microscope image of
Fukushima & co-workers’ “Bucky Gel” polymeric nanoactuator. Layer (a) is the polymeric electrode composed of
dispersed SWCNTs, and (b) the ionic-liquid electrolyte
material.

8

The recorded displacement performance and operating
lifetimes of these actuators are best in class in terms of their in
air, low-voltage characteristics, and this is before they are
further fine-tuned by material optimisation by tweaking the
properties of both the underlying polymer network and the
ionic liquid electrolyte layer.
VIII. SMART BUILDING MATERIALS
A further interesting use of carbon nanotube-based
transducers is where cement-based sensors can be utilised for
monitoring the dynamic strain in building structures. The data
provided is useful as it contains all the dynamic character of
the input and thus provides a useful sensor for the emerging
market of "SHM" or “Structural Health Monitoring"
applications.
The use of nanomaterials as an additive to building
materials has shown a lot of promise in the last two decades
providing new materials which have strain sensitive properties
and can allow smart sensing of changes in mechanical
properties, even if typically this has been done only under
static loads. The smart sensing effect is due to the change in
the electrical signal generated by the increase in the concrete’s
mass resistivity during crack initiation and growth, and an
attendant decrease in the resistivity during subsequent crack
closure43.
Research by Materazzi44 and co-workers showed that the
frequency response of cement with added CNTs (Figure 15
below) showed an approximately linear relationship as the
load frequency increased, which suggested that this
mechanism could be used to measure dynamic strain
responses in buildings.

Figure 15 – SEM images from research by Materazzi and
co-workers of cement with added MWCNTs, demonstrating
the efficacy of mixing nan tubes’ in water (a) and in cement
paste(b) after curing.


Early work by Fu45 and co-workers had showed that
cyclic load and unloading causes damage to the cement
matrix which means that the short CNTs used have a great
probability of touching each another which decreases the
overall resistivity of the composite material. Further
investigation into this effect continues today, with the cause
of this enhanced dispersion phenomenon now understood to
be due to surface oxidation46, leading to the production of
new functional groups on the surface of the nanotubes.
The addition of carbon nanotubes serves to create ideal
smart materials from the point of view of self-monitoring,
but an issue is ensuring that there is adequate dispersion of
the fibres in the aggregate mixture. This can be helped
somewhat by ozone treatment of the fibres before they are
introduced into the mixture, a phenomenon which has
broader importance outside the sphere of smart construction
materials and applies to all nano transducers. Najafi 47 and
co-workers showed that exposing samples of MWCNTs to a
UV – ozone treatment for an exposure time of 60 minutes
increased their solubility in polar organic solvents by up to
320% compared to an untreated CNT.
Recent work by Chang & Liu48 investigated and
compared some suggested functionalisation mechanisms
and concluded that this “ozone-mediated” process is an
effective way to functionalize MWCNTs with a wide range
of polymer layers, including traditionally non-reactive
polymers which were considered to be not readily
amalgamated with MWCNTs. Figure 16 below shows
some high resolution TEM of functionalised MWCNTs, a
method which has applications outside just the smart
buildings research area.

Figure 16 – Chang & Lu’s high resolution transmission
electron microscopy (HR-TEM) analysis of ozone-mediated
MWCNTs which have been functionalised with polymer
layers.

9

IX. CONCLUSIONS & FUTURE DIRECTIONS
Throughout this review paper it has been illustrated that the
current nanotechnologic era affords us with some very
exciting opportunities to improve traditional transducer
designs and materials.
Completely new transducer schemes and principles of
operation and fabrication can be enabled by harnessing the
unique attributes of nanoscale systems, effects and materials.
This is especially true in the field of carbon nanotubes due to
their unique convergence of desirable material properties.
We can envision a future where new complex transducer
arrays could be merged at the near atomic level with
functionalised surfaces, metamaterials and advanced digital
signal electronics to give rise to faster, most sensitive devices
which we can utilise in ever more meaningful and adaptive
ways.
Indeed, it is around this very idea of boundless integration,
energy efficiency and ubiquity that we can sense a way to use
nanotechnology to drive new solutions in a variety of fields.
However, further advances in synthesis, fabrication and signal
processing will be needed in order to harness the potential
benefits for low cost commercial nano-transducers, as well as
research groups which straddle the different interdisciplinary
areas where MEMS-based transducers can have such a large
impact.
X. ACKNOWLEDGMENT
The author would like to thank Dr. Patrick McNally of the
School of Electronic Engineering in Dublin City University
for his proposal of this review topic, as well as his patience
and gusto in explaining many of the important theoretical
principles and concepts that underlie the field of micro- and
nano-transducers.


10

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