Chapter 1
CHAPTER – 1
Introduction
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Chapter 1
Sr.
No.
CONTENTS
Page
No.
1.1 Preamble
03
1.2 Sensor: Definition and Classification
04
1.3 Introduction to Volatile Organic Compounds (VOCs)
06
1.4 Semiconductor Gas/Vapour Sensors
1.4.1 Thin Film Sensors: Advantages and Drawbacks
1.4.2 Sensing Mechanism
1.4.3 Choice of Semiconducting material
1.4.4 Performance Parameters
(a) Improvement of the Performance
(b) Solution to the Problem of Selectivity and Cross
Sensitivity: Array of Sensors and Electronic Nose
07
1.5 Importance of Indium Tin Oxide (ITO) Material
024
1.6 Reports on ITO based gas/vapour sensors
024
1.7 Aim of the Present Work
025
References
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1.1 Preamble
The understanding, imitation and extension of human sensing abilities have
been one of the prime pre-occupations of scientists and technocrats in the
field of electronics and instrumentation. This leads to a better, efficient
dealing of life with an enhanced standard of living; and, surely, towards a
more accurate understanding of the properties of Nature; - both of which are
the objectives laid for the applied scientific research.
Development of accurate, precise, sensitive, selective, stable sensor devices
with fast response and capability of interfacing with instrumentation system,
and which is reliable and viable, having low production cost for all different
variables in Nature is indeed a challenge for the scientific community.
After the successful breakthrough by audio and video sensing systems, and
thereby, the development of the modern communication system imitating
and extending our ‘ears’ ad ‘eyes’, the next task the scientists are looking
for is the development of electronic device to replicate the human ‘nose’,
or the olfactory system. Days are not far off when once such ‘e-nose’
devices are developed successfully, the complete ‘odour communication
system’, with both the transmitter and the receiver sections developed,
would facilitate the “smell” to be detected, transmitted, reproduced and
communicated. The odour sensor, in the form of “a Sensor detecting
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Volatile Organic Compounds”, would also facilitate in detecting (and
thereby controlling) the indoor-outdoor pollution, presence of explosives,
deterioration of food, and help medical diagnostics with non-invasive
detection. Semiconductor Vapour Sensor is a Chemical Sensor, which
fulfils the requirement of providing a reliable electronic nose system
when fabricated and connected as an Array driving a suitable
discriminative and/or quantifying pattern recognition system, using
statistical or artificial intelligence approaches.
1.2
Sensor: Definition and Classification
Sensor is an input interfacing device which detects qualitatively and/or
quantitatively the presence of a particular parameter under test. It is an input
transducer to any instrumentation system, which may or may not possess the
transduction function but basically it senses or detects or perceives the
change in the input variables, which can be physical, chemical or biological in
nature. A sensor along with a transducer is connected to the input processing
unit of any instrumentation or communication system or control unit.
Table 1.1 shows the classifications of sensors based upon different criteria.
Research and Development in Chemical sensors, which detect qualitatively
and quantitatively the chemical variables have been in great demand today,
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Table 1.1 Classifications of Sensors
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as, compared to the sensors detecting physical properties of matter, chemical
sensors, are less studied and hence lagging behind as far as the performance
criteria are concerned.
1.3
Introduction to Volatile Organic Compounds (VOCs)
Volatile organic compounds (VOCs) mean any compounds of carbon,
excluding carbon monoxide, carbon dioxide, carbonic acid, metallic
carbides or carbonates, and ammonium carbonate, which participate in
atmospheric photochemical reactions [1].
Volatile organic compounds (VOCs) are emitted as gases from certain solids
or liquids. VOCs include a variety of chemicals, some of which may have
short- and long-term adverse health effects. Concentrations of many VOCs
are consistently higher indoors (up to ten times higher) than outdoors
[2]. Formaldehyde, one of the best known VOCs, is one of the indoor air
pollutants that need to be measured due to its harmful effects.
Semiconductor gas sensors can be used for the detection of volatile organic
compounds (VOCs) as, vapours in the gaseous form respond to the oxide
semiconductors.
VOCs are highly reactive and hence their oxidation
reduction process is very fast with oxide based semiconductor materials.
Gas sensing for detection and identification of odorants, vapours, or
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pollutant
is
of
significant importance for many industries and
organizations, which include food industries, military and humanitarian
organizations, land mines, petro‐chemicals and manufacturing companies,
airport security and custom inspections agencies [3].
Challenges for VOCs sensors
VOCs have very low boiling point so their evaporation rate process is fast.
VOCs are an often mixed with interfering gases or vapours so it is difficult
to have their selective detection. As the production continues to increase
and as at times they emit such low levels of gas that it has proved difficult
to implement technically and economically viable VOC sensors. Lin et al [4]
reported long term study on industrial VOCs emissions of six typical
industries including emission characteristics, environmental impact and
health risk assessment, and control challenge analysis with the purpose to
explore industry VOCs emissions and offer some original baselines for
national control and management of industry VOC emissions as shown in
Table 1.2.
1.4
Semiconductor Gas/Vapour sensors
Semiconductor gas/vapour sensors are devices which allow determination
of the ambient gas atmosphere by exploiting the change in their surface
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Table 1.2. Summary of VOCs emission from typical industries [4]
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conductance in presence of oxidizing or reducing gases. They are fabricated
from the Oxide Semiconductor materials such as Tin Oxide, Indium Oxide,
Zinc Oxide etc.
In their simplest form, they are conductometric devices. Due to their
chemical composition and properties, semiconducting oxide gas sensors
are well-suited for a wide range of applications and for the detection of
many reactive gases and vapours. Over the past few decades, researchers
and engineers have dedicated their efforts to develop both materials and
devices with the characteristics of high sensitivity, good selectivity, and
reliability to detect and analyze the gaseous ambience.Depending upon the
material used and the gases/vapours that need to be detected, typical
operating temperatures of the semiconductor device ranges between
room
temperature to higher Temperature [5]. Usually, these devices
follow empirical approach, and different materials have been tried to be
used for gas/vapour sensor application. Current research and development
efforts are focused on the synthesis of materials that have high relative
change in resistance and/or selectivity to vapours of interest,
improvements in sensor geometries and fabrication (especially in the area
of microstructure), and the applications of sensor analysis techniques for
solving complex, real time problems [6]. The oxide gas semiconductor can
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be distinguished by many parameters, the most common is given by, (i) the
classification by fabrication technology into ceramic, thick film and thin film
and (ii) by the device structure which makes disc or structured devices.
1.4.1 Thin Film Gas Sensors: Advantages and Drawbacks
Thin film semiconductor gas sensors are direct gas sensing device
which can be used for on-line and continuous monitoring of
gases/vapours.
Their advantages include: simplicity in design, low cost, low power
consumption, high selectivity, ease of miniaturization, large surfaceto-volume ratio, high sensitivity, fast response time, higher reliability,
safe evaporation, not requiring periodic calibration, reduced sensor
poisoning, improved Signal to Noise ratio, simultaneous detection of
multiple VOCs, broad dynamic range, ease of interface with
electronic system, possibility of developing array of sensors and
electronic nose.
The disadvantages of using thin film semiconductor sensors include
requiring special techniques, instruments for fabrication and trained
manpower to operate complex fabrication system, and the thin films
are not robust and need to be handled with care.
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1.4.2 Sensing Mechanism
The oxygen adsorbed in molecular form on the surface of the oxide
semiconductor can be ionised and get transformed into O2-, O-, O2depending upon the physic-sorption or chemi-sorption processes it
undergoes, and, the operating temperature of the device as shown
in Eqs. 1 to 4.[7-9]
O 2 ( gas )  O 2 ( ads )
(1)
O 2 ( ads )  e  O 2 ( ads )
T < 100 °C
(2)
O2 ( ads)  e  2O( ads )
T =100- 300°C
(3)
O ( ads)  e  O 2( ads )
T > 300 °C
(4)
As shown in Fig.1.4.2 (i) the oxide ions form a surface depletion
region on the surface of the film, which creates the potential barrier.
When the sensor is exposed to a pollutant gas or vapours, the type
of oxygen formed at a certain temperature reacts with the gas, and
determines the maximum relative change in resistance by
undergoing the redox process on the surface [7-9].
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O2
Depletion region
Barrier potential
Fig 1.4.2(i) Potential barrier at grain boundary
The
important
processes
responsible
for
the
working
of
semiconductor gas/vapour sensors are

a sensitive layer that is in chemical contact with the oxide
layers,

a change in the chemistry of the sensitive layer upon
chemisorption of test gas/vapour sensor,

oxidation-reduction process.
Principally, a semiconductor sensor consists of three different parts:
a receptor, a transducer, and a conditioning module defining the
operation model [10]. Figure 1.4.2(ii) shows that the receptor part
concerns with the ability of the sensor surface to interact with the
target gas/vapour to detect the presence, and to perceive and to
transform this chemical information into a form of energy. The
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transducer part concerns with the ability of the sensor to transform
the energy into useful electrical signal [11].
Fig.1.4.2 (ii) Schematic diagram of receptor and transducer
functions of the semiconducting gas sensor [3]. L: thickness
of space charge layer, D: particle size
The receptor (the semiconductor surface) and transducer up on
contact with the reducing or oxidizing gases or vapours, alter the
surface conductivity of the material. The gas-induced changes at the
semiconductor surface are transduced by the transducer (through
the microstructure of the sensing material) into an electrical output
signal. In the case of a polycrystalline material, the grain size and
different grain intersections have a strong influence on the final
output signal [12].
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1.4.3 Choice of Semiconducting material
Semiconducting oxides are relatively wide band gap semiconductors
(Eg~3-4.5 eV). It is generally accepted that high conductivity of oxide
single crystal and polycrystalline thin films arises from the
stoichiometric deviation due to anion vacancies and/or interstitial
cations [11].
Brattain and Bardeen first reported gas sensitive effects with
Germanium since 1952 [13]. Later, in 1962, Seiyama et al [14]
demonstrated that thin films of ZnO, heated to 300°C in air, change
their conductivity in the presence of reactive gases in 1962. In the
same year, Taguchi [15] demonstrated similar properties for SnO2
with the greater stability.
A number of behavioural tendencies have been established for the
metal oxide sensors:
(i)
Relative change in resistance to gases or vapours varies for
different concentrations and temperatures. Response times
are highly dependent on temperature (they are shorter at high
temperatures), and many times responses to gases or vapours
are nonlinear as a function of concentration [16].
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The temperature has an important effect on the relative
change in resistance of semiconductor gas/vapour sensors, as
it influences the physical properties of semiconductors
(change of the free carrier concentration, Debye length), and
also because every reaction taking place at the semiconductor
surface are temperature dependent [16]. Yamazoe et al [17]
have
demonstrated
that
monitoring
the
operating
temperature, at which a semiconductor oxide sensor shows
maximum sensitivity, can be used to enhance the selectivity. It
is known that relative change in resistance of sensors,
operated in temperature range lower than 450°C, could be
controlled by either chemisorptions processes, or “redox”
processes [18-24].
(ii)
A common feature is that water vapour affects both the
conductance in air and the sensitivity to other gases. The
solution is to find materials that are less dependent on
humidity values [16].
At room temperature, the water can be adsorbed in two states
on the surface of a semiconductor: molecular water (H2O –
physi-sorption), and hydroxyl groups (OH- - chemi-sorption).
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Absorption of water vapour always produces a large increase
in the conductivity of the gas sensor [16].
(iii)
Gases/vapours with electron-accepting or electron-donating
abilities can be oxidized or reduced on a relative change in
resistance, generating ions or changing the electronic status of
oxide layers. [16].Fig.1.4.3 (i) shows the energy band diagram
for tin oxide with negatively charged adsorbed oxygen, where,
ES: potential barrier; EF: Fermi level; ED: donor level; EC: lowest
level of conduction band; EV: highest level of the valence band;
ECD: depth of donor level; ECV: energy gap between EC and EV.
The bending of band leads to the decrease of conductivity.The
sensors operate through a shift in the equilibrium of the
surface oxygen reaction by target gas or vapours [25].
Fig.1.4.3 (i) Energy diagram for SnO2 with Negatively
charged adsorbed oxygen [25]
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The electrical conduction is controlled by potential barriers
associated
conductivity
with grain
of
the
boundaries.
And,
semiconductor oxide
the
electrical
is
extremely
sensitive to the composition of the surface, which also
varies reversibly to the surface reaction involving chemisorbed
oxygen and the gas/vapours mixture component. Since the
performance of sensor with respect to sensitivity, selectivity,
and response time, are dependent on the surface reaction
between the semiconductor oxide and gas or vapour
molecules in the ambient, the particle size, microstructure,
and doping level will have a large influence on the response of
the semiconductor sensors [26].
It is known that the smaller is the activation energy of
chemisorption and the higher is the activation energy of
desorption, the more is the gas-sensing effect of adsorption
type sensors [27-29].
1.4.4 Performance parameters
The kinetics of the processes, which take place in the receptor part,
determines the sensors parameters. The most common parameters
are, sensitivity, selectivity, stability, response times, recover time,
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reliability and working temperature [16]. They are commonly given
as three “S” factors and three “R” factors, as shown in Fig.1.4.4 (i).
The sensor parameters like sensitivity, selectivity, response time and
optimal working temperature depend on several factors:

Efficiency of receptor part,

Efficiency of transducer part,

Properties of sensors design.
Fig. 1.4.4(i) important parameters thin film gas/vapour sensors
And efficiencies of both receptor and transducer parts in turn,
depend upon:

Electronic structure of sensing material
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
Density of surface state

Amount and quality of absorption centres

Microstructure of sensing material

Catalytic activity

Kind of detected gas or vapour.
(a)
Improvement of the performance parameter

Use of new metal oxide materials that are more sensitive to
target gases/vapours and have less effect with temperature.

A better control of the thin films microstructure using nano-
metric oxide, in order to increase the active area.

Use new catalysts/dopants/promoter.

New technologies for material deposition and a different
structure.
The sensitivity and the optimum operating temperature of sensors
are correlated to the sensitive material and the material deposition
techniques. The amount of catalysts/doping/promoter on the
sensors surface also influences sensitivity. In general, sensitivity is
enhanced either by doping, which modifies the carrier concentration
and mobility, or by micro-structural changes such as reducing the
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oxide particle size to the nano-metric scale [30]. For example, most
commonly used, SnO2, In2O3, ITO, ZnO, can be sensitised to different
gas/vapours by selecting an optimal operating temperature for the
target gases/vapours, by making micro-structural modifications or by
using dopants and catalysts. Nano-structured materials present new
opportunities for enhancing the properties and performance of
gas/vapour sensors because of their surface-to-volume ratio.
(b)
Solution to the problem of selectivity and cross sensitivity
To overcome the selectivity problem of sensors the most widely used
strategy has been to construct multisensory system, dopants, and
surface chemical modification systems [31-35]. The success of
artificial olfaction depends not only on the development of new
sensor technologies, but also on the availability of powerful pattern
recognition software.
Attempts to measure odours with electronic instruments were first
made as early as in 1950s [36], but the modern field of artificial
olfaction began in 1982 with the work of Persaud and Dodd [37],
who used a small array of gas-sensitive metal oxide devices to
classify odours. Since then, there has been a steady increase in the
number of systems using chemical sensor array. These systems are
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composed of several types of gas/vapour sensors, i.e. they use an
array
of
semiconductor
sensors
with
partially
overlapping
sensitivities, capable to obtain different response to the tested
odours [37].
The most common approach for fabricating an array of sensors is to
develop sensors with different dopants as sensing material [38]. The
literature available in the area of gas/vapours sensors on the
development of selective gas/vapours sensors system puts particular
emphasis on sensor arrays that use SnO2, In2O3, ITO, and ZnO in
association with various dopants [39]. A few examples for these
approaches are as follows:
Low concentration of NO2, CO, Ethanol, Methanol, Acetone, Benzene,
and Toluene have been detected by Cane et al. [40], with a micro
machined gas sensor array consisting of three devices working at
different temperatures. Yan et al. [41], designed a sensor array with
four specifically designed SnO2 and y-Fe2O3 sensors and used
improved back-propagation algorithm to accurately distinguish
ethanol and gasoline.
Many attempts have been made to develop electronic nose system
for applications in the fields of food, drinks, cosmetics,
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environmental monitoring, etc. [42]. Particular attention has been
paid to the cost, size and portability of these systems. Considerable
efforts have been made to study sensor arrays for the detection of
gases in a large variety of technological fields such as environmental
monitoring, food and drink analysis, medical appliances, and
industrial control systems [43-45].
Electronic nose
Gardner and Bartlett defined an electronic nose (e-nose) as [45]: “An
electronic nose is an instrument which comprises an array of
electronic chemical sensors with partial specificity and an
appropriate pattern recognition system, capable of recognizing
simple or complex odors" [45].
Fig 1.4.4 (ii) Electronic Nose system
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Fig.1.4.4 (ii) depicts the Mammalian and the Electronic nose
systems. Electronic nose is a new concept of semiconductor thin film
sensor application, which tries to mimic the human olfactory system
by using an array of electronic chemical sensors with partial
specificity and appropriate pattern-recognition electronics [46],
using artificial neural networks. Electronic noses can be applied
successfully to: -environmental monitoring; - medicine applications,
as an electronic nose can examine odors from human body and thus
can serve to recognize diseases; - Food industry, which constitutes
the largest market for electronic noses, whose applications include
quality assessment in food production and inspection of food quality
by odor. Qualitative analysis (identification of a certain gas from a
mixture of gases) and quantitative analysis (determination of each
gas concentration) can be usually achieved employing linear pattern
recognition methods such as: Principal Component Analysis (PCA)
and Discriminate Factor Analysis (DFA). Syeda Erfana Zohora et al
[47] reported detection of volatile organic compounds (VOCs) using
non-selective sensor requires an array of multiplexed sensors
followed by pattern recognition approach.
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1.5
Importance of Indium Tin Oxide ( ITO ) material
Indium tin oxide (In2O3:SnO2) is one of the most widely used transparent
conducting oxide. Its high electrical conductivity and high optical
transparency, as well as the ease with which it can be deposited as a thin
film are the attracting features which have drawn the attention of
researchers. As with all transparent conducting films, a compromise must
be made between conductivity and transparency, since increasing the
thickness and increasing the concentration of charge carriers will increase
the material's conductivity, but decrease its transparency. It has the
unusual property of remaining soft and workable at very low temperatures
[48].
1.6
Reports on ITO based gas/vapour sensors
There are various reports on the use of ITO as gas/vapour sensors. Patel et
al [49-50] have reported ITO thin film gas sensor for CO2 and CCl4 gases.
Galkidas et al [51] have reported ITO gas sensor for chlorine gas.
Sberveglieri et al [52] have reported RF Magnetron Sputtering ITO thin
films for NO2 gas sensors. Dibbren et al [53] have reported that the growth
parameter of ITO thin film strongly influences their response to gases.
Patel and Lashkari [54] have studied the effect of annealing on the
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properties of ITO film. Reactively sputtered ITO thin films have also
reported as NO and NO2 gas sensors [55]. ITO has proved to be successful
as a gas sensor and demands further exploration. V.S. Vaishnav et al have
reported the fabrication of Indium Tin Oxide thin film Gas Sensors for
Detection of Ethanol [56], and VOCs [57-58].
1.7
Aim of the Present work
The response of ITO to other gases is yet to be explored for which a
detailed study needs to be carried out and hence the present study has
been taken up on hand. A detailed investigation of the characterization of
ITO thin film, fabrication of ITO thin film sensor with promoting layers and
its application for detection of hazardous indoor pollutants like
formaldehyde, carbon tetrachloride, toluene, benzene and hydrogen
peroxide have been taken up.
Additionally, a preliminary study on the enhancement of sensitivity by the
using a special type of deposition technique, known as Chemical Vapour
Transport has been experimented and films of SnO2 with nano clusters
were deposited. A study on Flexible substrates for sensor devices is also
aimed.
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An Intelligent electronic nose, which consisted of an array of three
fabricated, selective sensors for the quantitative discrimination of benzene,
toluene and formaldehyde.
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