Paul Ahern - Overview of Micro & Nano Transducers

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 nano-

transducer 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 low-

cost 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.

Overview of Micro- and Nano-Transducers

(December 2013)

Paul Ahern,

1School of Electronic Engineering, Dublin City University.

2Reliability & Eco-Environmental Engineering, Bell Labs / Alcatel Lucent.

[email protected]

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

1

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 mid-

sixties 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

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 “micro-

optoelectromechanical” 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.


2

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 Mach-

Zender diodes.

Plasma treatments such as those detailed by Zebda14

and co-

workers 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

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 co-

workers/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


3

uncontaminated surroundings for the device for a typical 20-

year 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 co-

workers silicon based Fabry-Perot accelerometer allows

integration of all components on a single substrate.

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.


4

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 macro-

sized 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

frequency noise which has so far generally been noticeably

high in measurements made using these types of nanotube-

based devices.

A proposed workaround for these limitations is the

proposed inclusion of AC bridging rectifier, such as the well-

understood 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/Hz1/2

at

low frequencies.

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.


5

Figure 7 – High magnification SEM images showing top-

down (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 Maki26

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

resonance conditions, in agreement with numerical

simulations.

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.

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.


6

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 co-

polymer with trifuoroethylene (TrFE) where it has found

success for uncomplicated annular array transducers33

.

Figure 10 – Electron micrographs from Zhu34

and co-

workers 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


7

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.

Figure 11 – A novel fabrication method by Feng and co-

workers 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).

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 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


8

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

80C.

Figure 14 – Scanning Electron Microscope image of

Fukushima & co-workers’ “Bucky Gel” polymeric nano-

actuator. Layer (a) is the polymeric electrode composed of

dispersed SWCNTs, and (b) the ionic-liquid electrolyte

material.

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.


9

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. Najafi47

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.

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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