CHAPTER 1
SEMICONDUCTOR METAL OXIDES AS HUMIDITY
SENSORS
This chapter introduces the inspiration of the thesis and describes the objective of
the present investigations. It also explains the synthesis techniques of
nanostructured materials, types of humidity sensors and requirement of humidity
sensor along with experimental techniques.
1.1 Introduction
Modern technology leans heavenly on the science of measurement. The
control of industrial processes and automated systems would be very difficult without
accurate sensor devices. The widespread use of microelectronics and computers is
having a profound effect on the design of sensor systems. Nanoscience is defined as
the study of phenomena and manipulation of materials at atomic, molecular and
macromolecular scales, where properties differ significantly from those at a larger
scale; and nanotechnologies as the design, characterisation, production and
application of structures, devices and systems by controlling shape and size at the
nanometere scale. The application of nanomaterials can be historically traced back to
even before the generation of modern science and technology. Nanoscience and
nanotechnologies are widely seen as having huge potential to bring benefits to many
areas of research and application and are attracting rapidly increasing investments
from Government and from business in many parts of the world.
Nanoparticles were used as dye materials in ceramics by ancient people [1];
and colloidal gold was used in medical treatment for cure of dipsomania, arthritis etc.
As early as from 19 countries; systematic experiments conducted on nanomaterials
had also been started [2]. Material scientists and engineers now have a rapidly
evolving ability to tailor materials from the atomic scale upwards to obtain desired
properties. According to the demand of society, man discovered techniques for
1
Chapter 1
Semiconductor metal oxides as humidity sensors
producing materials that had properties superior to those of the materials occurring in
nature. The development of many advanced technologies makes the life easier. No
doubt that there is tremendous progress in the discipline of materials science and
engineering in the past few years, still there is a demand of even more sophisticated
and specialized materials. In recent years nanotechnology has become one of the most
important and exciting forefront fields in Physics, Chemistry, Engineering and
Biology. It shows great promise for providing us in the near future with many
breakthroughs that will change the direction of technological advances in a wide
range of applications. Nanoscience and nanotechnology include the areas of synthesis,
characterization, exploration, and application of nanostructured materials. The
application of nanomaterials can be historically traced back to even before the
generation of modern science and technology. Device miniaturization is also a
significant factor for the development of nanotechnology. Nanoelectronic devices
based on new nanomaterials systems and latest device structures will contribute to the
development of next generation of microelectronics. For example, single electron
transistor [3-4] and field effect transistor [5-7] based on single wall carbon nanotubes
SW(NT) are already on the way. Proto type simple logic circuits of carbon nanotubes
have already been demonstrated [8-11]. Investigations on nano systems, being widely
interdisciplinary by their nature, promote the joining and merging of the various
science and technologies such as powder technology, colloid chemistry, surface
chemistry and physics, clusters, aerosols, tribology, catalysis, simulation and
modelling, computer technique etc.
1.2 Nanomaterials
Nanomaterial is a field that provides a material science-based approach to
nanotechnology. It studies the materials with morphological features on the nanoscale
and especially those that have special properties stemming from their nanoscale
dimensions. Nanoscale is usually defined as smaller than a one tenth of a micrometer
in at least one dimension. Nanomaterials is the study of how materials behave when
their dimension are reduced to the nanoscale. The infinite possibilities that
nanotechnology has on the production of nanomaterials is going to significantly alter
the material world.
2
Chapter 1
Semiconductor metal oxides as humidity sensors
Figure 1.1: As everything nano, the field has been said real development…
The term of nanomaterials covers different types of nanosturctured materials which
posseses at least one dimension in the nanometer range. Nanomaterials have the
structural features in between of those of atoms and the bulk materials. Comparing
with bulk material, semiconducting nanomaterials can provide some special
characteristics, which have been very useful to achieve various novel devices. As
electrical, optical and chemical properties can be tuned with nanosized-particle in a
wide range. Metal oxide semiconductors have attracted much attention on the sizedependent phenomenon. In recent years, metal oxide semiconductor nanoparticles
received considerable attention as active components in a wide variety of basic
research and technological applications due to their improved electric, optical and
magnetic properties compared to their bulk counter-parts. The infinite possibilities
that nanotechnology has on the production of nanomaterials is going to significantly
alter the material world. Due to their nanometer size, nanomaterials are already
known to have many novel properties. Due to their small dimensions, nanomaterials
have extremely large surface area to volume ratio, which makes a large fraction of
atoms of the materials to be on the surface or interfacial atoms, resulting in more
surface dependent material properties. Especially when the sizes of nanomaterials are
comparable to Debye length, the entire material will be affected by the surface
properties of nanomaterials [12-13]. As with many new technologies, there seems to
3
Chapter 1
Semiconductor metal oxides as humidity sensors
be a lack of complete understanding when it comes to how nanoparticles change the
properties of materials. Over the past decade, nanomaterials have been the subject of
enormous interest [14-17]. These materials, notable for their extremely small feature
size, have the potential for wide-ranging industrial, biomedical, and electronic
applications. As a result of recent improvement in technologies to see and manipulate
these materials, the nanomaterials field has seen a huge increase in funding from
private enterprises and government, and academic researchers within the field have
formed many partnerships at individual and community levels. Nanomaterials can be
metals, ceramics, polymeric materials, or composite materials. The variety of
nanomaterials is great, and their range of properties and possible applications appear
to be enormous, from extraordinarily tiny electronic devices, including miniature
batteries, to biomedical uses, and as packaging films, superabsorbent, components of
armor, and parts of automobiles. Figure 1.2 shows the rapid development due to
nanotechnology.
Figure 1.2: Rapid development due to nanotechnology.
Nanoparticles are minute substances in the size range of 1-100 nanometers in all
directions. Typically stable substances can become highly reactive and unstable when
the particles become infinitesimal due to their extremely high surface to volume ratio.
When the size of the particles reduces to less than 100nm and all the physical and
other properties are significantly changed, then the group of particles is said as
4
Chapter 1
Semiconductor metal oxides as humidity sensors
nanomaterials. There are few techniques to synthesize nanosized particles such as
high energy ball-milling or sol-gel routes etc. Tremendous research works are going
on across the world on preparations and characterizations of nanomaterials.
The variety of nanomaterials is great, and their range of properties and
possible applications appear to be enormous, from extraordinarily tiny electronic
devices, including miniature batteries, to biomedical uses, and as packaging films,
superabsorbants, components of armor, and parts of automobiles. General Motors
claims to have the first vehicle to use the materials for exterior automotive
applications, in running boards on its mid-size vans. Editors of the “Journal Science”
profiled work that resulted in molecular-sized electronic circuits as the most important
scientific development in 2001. It is clear that researchers are merely on the threshold
of understanding and development, and that a great deal of fundamental work remains
to be done. The properties of materials can be different at the nanoscale for two main
reasons. First nanomaterials have relatively larger surface area when compared to the
same mass of material produced in a larger form. This can make materials more
chemically reactive and affect their strength or electrical properties. Second, the
quantam effects can begin to dominate the behaviour of matter at the nanoscale,
particularly at the lower end affecting the electrical, optical and magnetic behaviour
of materials. Materials can be produced that are nanoscale in one dimension, in two
dimension or in all three dimensions. Much of nanoscience and nanotechnologies are
concerned with producing new or enhanced materials.
Nanoscience and
nanotechnologies are widely seen as having huge potential to bring benefits in areas
as diverse as drug development, water decontamination, information and
communication technologies, and the production of stronger, lighter materials.
1.3 Synthesis of Nanomaterials
Synthesis methods play a very important role to control the size and surface
area of nanomaterials. There are basically two broad areas of synthesis techniques for
NSMs:
(a) Chemical Methods/ Bottom up Approach
(b) Physical Methods/ Top down Approach
1.3.1 Chemical Methods
The advantage of chemical synthesis method is its versatility in designing and
synthesizing new materials that can be refined into a final product. The chemical
5
Chapter 1
Semiconductor metal oxides as humidity sensors
process offers over other methods a good chemical homogeneity by mixing the
materials at molecular level. The major advantage of this method is to control size and
surface area of the nanomaterials. There are several methods such as:
(a) Precipitation methods:
(i) Hydrothermal or Solvothermal Synthesis
(ii) Emulsion Precipitation Method
(iii) Co-Precipitation Method
(iv) Sol-Gel Method
In all these methods, nanoparticles are found to be in the form of precipitate.
(b) Immobilization Methods
(i) Citrate Complexation Gelation Method
(ii) Penchini Method
(iii) Low Temperature Combustion Method
(a)Precipetation methods
(i) Hydrothermal or Solvothermal Synthesis
Hydrothermal technique is a promising alternative synthetic method because
of the low process temperature and very easy to control the particle size. The
hydrothermal process have several advantage over other growth process such as use
of simple equipment, catalyst free growth, low cost, large area uniform production,
environmental friendliness and less hazardous. The low reaction temperatures make
this method an attractive one for microelectronics and plastic electronics. The method
is based on the ability of water and aqueous solutions to dilute at high temperature
(500ºC) and pressure (10-80 MPa, sometimes up to 300 MPa) substances practically
insoluble under normal conditions: some oxides, silicates, sulphides. The main
parameters of hydrothermal synthesis, which define both the processes kinetics and
the properties of resulting products, are the initial pH of the medium, the duration and
temperature of synthesis, and the pressure in the system. The synthesis is carried out
in autoclaves which are sealed steel cylinders that can withstand high temperatures
and pressure for a long time. Nanopowders are normally produced by means of either
high temperature hydrolysis reactions of various compounds directly in the autoclave
or hydrothermal treatment of reaction products at room temperature; the latter case is
based on the sharp increase in the rate of crystallization of many amorphous phases in
hydrothermal conditions [18-19]. In the first case the autoclave is loaded with
6
Chapter 1
Semiconductor metal oxides as humidity sensors
aqueous solution of precursor salts, in the second case-with suspension of products
derived from solution reactions flowing under normal conditions. There is normally
no need to use special equipment and maintain a temperature gradient. Substantial
enhancement of the hydrothermal method facilitates the use of additional external
factors to control the reaction medium during the synthesis process. As of now, this
approach is implemented in the hydrothermal-microwave, hydrothermal-ultrasonic,
hydrothermal-electrochemical and hydrothermal-mechanochemical synthesis method.
There are several advantages of hydrothermal method over conventional solid state
methods. For example, compounds that have elements with an unusual oxidation state
can be synthesized such as the formation of ferromagnetic chromium (IV) oxides.
Cr2O3 +
CrO3
350ºC, 440 bar
3CrO2
H2 O
The hydrothermal method is also useful for synthesis of low temperature
phases and metastable compounds by simply using quartz ampoules. Important
advantage is that reactions do not require much time compared to conventional
methods. For instance, although a solid-state reaction can be performed in a few
weeks hydrothermal reaction can be done in a few days. Figure 1.3 shows high
pressure, high temperature autoclave used for hydrothermal synthesis of quartz
crystals.
Figure 1.3: Vertical autoclave used for hydrothermal synthesis of quartz crystals.
7
Chapter 1
Semiconductor metal oxides as humidity sensors
(ii) Emulsion Precipitation Method
This method involves the preparation of thermally stable emulsion systems
prepared by adding appropriate amounts of surfactants to a water oil system. Within
the emulsion system, there are a small number of atoms per droplet. It is necessary
that exchange of reactive species take place between droplets in order to form a stable
precipitate. From the Einstein-Smoluchowski equation, the normal rate of the particle
growth is faster than the equivalent rate of exchange between droplets. Therefore, the
nucleation and growth in emulsions are retarded in comparison to those in
homogeneous solution, avoiding the formation of large particles. Multisurfactants are
effective in forming thermally stable emulsion and controlling droplet size. Other
additives play a role as steric particle stabilizer after removal of water. Before the
particle dispersion by filtration of decantation of the organic phase, the emulsions
were prepared by mixing the oil phase (Cyclohexane or n-heptane) with tergitol
surfactants and octan-1-ol as co-surfactant. To the system stoichiometric amounts of
water were added followed by vigorous mixing until a translucent emulsion was
formed. The emulsion was added drop wise to alcohol solutions of alkoxides and
stirred for several hours. After removal of solvents in dispersion the residue was taken
up in acetone to destroy the micelles. The solid product obtained after decantation of
the organic phase was dried and transformed to nanocrystalline spinals after
calcinations.
This method provides the particular advantage of avoiding agglomeration of
the particles formed in the individual bubbles. This in turn makes possible subsequent
processing routes at unusually low temperatures [20-21]. To take full advantage of the
method for multicomponent oxides precipitation routes need to be designed so that an
intimate mixture of atoms is formed during precipitation and chemical homogeneity is
maintained during subsequent processing. This offers special challenges since
emulsion co-precipitations tend to be carried out with sample precursors that do not
affect emulsion stability but generally show a tendency to precipitate at different rates
leading to at least partial phase segregation.
(iii) Co-Precipitation Method
Solution precipitation depends on the precipitation of nanometre-sized particles
within a continuous fluid solvent. An inorganic metal salt, such as chloride, nitride
8
Chapter 1
Semiconductor metal oxides as humidity sensors
and so on, is dissolved in water. Metal cations exist in the form of metal hydrate
species, for example, Al(H2O)3+ or Fe(H2O6)3+. These hydrates are added with basic
solutions, such as NaOH. The hydrolyzed species condense and then washed, filtered,
dried and calcined in order to obtain the final product. Co-precipitation proceeds in
two stages. It begins with the entrapment of impurities during the growth of
precipitate particles upon separation and concludes with the redistribution of the
impurity between precipitate and medium. In the first stage, the impurity is trapped
either on the surface (surface co-precipitation) or inside (volume co-precipitation) the
growing particles. If the growing particles have a crystal structure, then in the case of
volume co-precipitation the impurity will become localized either at regions of the
solid phase with a perfect structure (isomorphism mixed crystal formation) or in the
vicinity of structural defects. The transfer of impurities (trace constituents) to a
precipitate concurrently with the deposition of some primary substances (macroscopic
constituent) from a solution, melts, or vapours containing several substances. Coprecipitation occurs when a solution (vapour) is supersaturated with the substance
forming the precipitate or when a melt is super cooled. Co-precipitation begins only at
the end of a latent period. The length of this period can be prolonged from
microseconds to tens of hours by altering the degree of super saturation (super
cooling), the degree of mixing, and the purity and temperature of the medium from
which the precipitate separates. Lots of investigations have been made to demonstrate
novel methods for particle synthesis or to elucidate the mechanisms of particle
formation [22]. Figure 1.4 shows schematic diagram of co-precipitation method.
9
Chapter 1
Semiconductor metal oxides as humidity sensors
Figure 1.4: Schematic diagram of co-precipitation method.
(iv) Sol-Gel Method
The sol-gel process is an wet-chemical technique widely used in the fields of
materials science and ceramic engineering. Among different synthesis methods for
preparation of oxide, sol gel method offers several advantages over other methods. As
well as this method lowers the processing temperature, better homogeneity, controlled
stoichiometry, and flexibility of forming dense monoliths, thin films, or nanoparticles.
A crucial role in the sol-gel process is played by the processes of solvent removal
from the gel. Depending on the method, the synthesis can result in various products
like xerogels, aerogels, cryogels, ambigels whose properties are different. The
common features of these products include the preservation of the nanosizes of the
structural elements and sufficiently high values of specific surface area. Figure 1.5
shows the description of sol-gel method.
10
Chapter 1
Semiconductor metal oxides as humidity sensors
Figure 1.5: Description of sol-gel method.
A sol is a dispersion of the solid particles (~ 0.1-1 μm) in a liquid where only
the Brownian motions suspend the particles. A gel is a state where both liquid and
solid are dispersed in each other, which presents a solid network containing liquid
components. Some advantages of sol-gel technique are as follows:
a) It can use to produce thin bond-coating to provide excellent adhesion between the
metallic substrate and the top coat.
b) Production of thick coating to provide corrosion protection performance is possible.
c) It can easily shape materials into complex geometries in a gel state.
d) Can produce high purity products because the organo-metallic precursor of the
desired ceramic oxides can be mixed, dissolved in a specified solvent and hydrolyzed
into a sol, and subsequently a gel, the composition can be highly controllable.
e) Can have low temperature sintering capability, usually 200-600ºC.
f) Also it provide a simple, economic and effective method to produce high quality
coatings.
Despite its advantages, sol-gel technique never arrives at its full industrial
potential due to some limitations, e.g. weak bonding, low wear-resistance, high
permeability and difficult controlling of porosity. Such methods are used primarily for
the fabrication of materials (typically metal oxides) starting from a colloidal solution
11
Chapter 1
Semiconductor metal oxides as humidity sensors
(sol) that acts as the precursor for an integrated network (or gel) of either discrete
particles or network polymers. Typical precursors are metal alkoxides and metal salts
such as chlorides, nitrates and acetates, which undergo various forms of hydrolysis
and polycondensation reactions. In this chemical procedure, the 'sol' (or solution)
gradually evolves towards the formation of a gel-like diphasic system containing both
a liquid phase and solid phase whose morphologies range from discrete particles to
continuous polymer networks [23-24]. In the case of the colloid, the volume fraction
of particles (or particle density) may be so low that a significant amount of fluid may
need to be removed initially for the gel-like properties to be recognized. The simplest
method is to allow time for sedimentation to occur, and then pour off the remaining
liquid [25-26]. Centrifugation can also be used to accelerate the process of phase
separation. Removal of the remaining liquid (solvent) phase requires a drying process,
which is typically accompanied by a significant amount of shrinkage and
densification. The rate at which the solvent can be removed is ultimately determined
by the distribution of porosity in the gel. The ultimate microstructure of the final
component will clearly be strongly influenced by changes imposed upon the structural
template during this phase of processing. Most products of sol-gel synthesis are used
as precursors in obtaining nanopowders, thin films and ceramics. The sol-gel method
has some advantages e.g. good uniformity and better controlled composition over
other ones.
(b)Immobilization Methods
(i) Citrate Complexation Gelation Method
In citrate gel methods, metal ions are stabilized by an organic network in
precursor solutions, thus fine oxide powders are obtained after a heating process [27].
They have the ability of preparing multi-component compositions with good
homogeneity and control of stochiometry. These methods utilize poly-chelates
between the C-O legends of citric acid and metal ions. The chelates undergo
polyesterification on heating with a polyfunctional alcohol. In the citrate method the
chelating process takes place during the evaporation of the precursor solution
containing metallic salts and citric acid. Further heating produces a viscous resin, and
a rigid transparent, glassy gel. Mixtures of different metal ions become immobilized
in an early stage of the formation of this rigid system [28]. This greatly reduces the
risk of segregation into different oxide compositions during subsequent calcinations.
12
Chapter 1
Semiconductor metal oxides as humidity sensors
(ii) Penchini Method
Penchini method is a method of synthesis of highly homogeneous and finely
dispersed oxide materials including formation of complexes and production of
polymer gel as an intermediate product. The method is based on an intensive blending
of positive ions in a solution, controlled transformation of the solution into a polymer
gel, removal of the polymer matrix and development of an oxide precursor with a
high degree of homogeneity. During the synthetic process, metal salts or alkoxides are
introduced into a citric acid solution with ethylene glycol [29-31]. The formation of
citric complexes is believed to balance the difference in individual behavior of ions in
solution, which results in a better distribution of ions and prevents the separation of
components at later process stages. The polycondensation of ethylene glycol and citric
acid starts above 100ºС, resulting in polymer citrate gel formation. When the heating
temperature exceeds 400ºС, oxidation and pyrolysis of the polymer matrix begin,
which lead to the formation of X-ray amorphous oxide or carbonate precursor. Further
heating of this precursor results in the formation of the required material with a high
degree of homogeneity and dispersion. Today this method is widely used in the
synthesis of dielectric, fluorescent and magnetic materials, high temperature
superconductors and catalysts as well as for the deposition of oxide films and
coatings. Main advantages include its relative simplicity, almost complete
independence of the process conditions from the chemistry of positive ions contained
in the final material and a relatively low temperature of precursor treatment, due to
which the process may occur almost completely without sintering, resulting in the
production of nanocrystal powders of refractory oxides.
(iii) Low Temperature Combustion Methods
The low temperature combustion synthesis (LCS) technique has proved to be a
novel, extremely facile, timesaving and energy-efficient route for the synthesis of
ultra fine powders [32]. This is based on gelling and subsequent combustion of an
aqueous solution containing salts of the desired metals and some organic fuels, giving
a voluminous and fluffy product with large surface area. Oxidizing metal salts such as
metal nitrates, and a combination agent (fuel) such as citrate acid, polyacrylic acid or
urea are used as starting materials. Citrate acid is more widely used, since it not only
functions as a reductant/fuel agent, but also a chelating agent. The molar ratio of fuel
to nitrates in the initial mixture imposes a great influence on calcination condition and
13
Chapter 1
Semiconductor metal oxides as humidity sensors
the subsequent characteristics of the synthesized crystallites. By controlling the
CA/NO3 ratio and calcination temperature, homogeneous crystalline spinel powders
are prepared with a nanoscale primary particle size [33].
1.3.2 Physical Methods
Several physical methods are currently in use for the synthesis and commercial
production of NSMs. Few are given as below:
1.3.2.1 Gas Condensation
Gas condensation was the first technique used to synthesize nanocrystalline
metals and alloys. In this technique, a metallic or inorganic material is vaporized
using thermal evaporation sources such as a Joule heated refractory crucibles, electron
beam evaporation devices, in an atmosphere of 1-50 m bar. In gas evaporation, a high
residual gas pressure causes the formation of ultra fine particles (100 nm) by gas
phase collision. The ultrafine particles are formed by collision of evaporated atoms
with residual gas molecules. Gas pressures greater than 3 mPa are required.
Vaporization sources may be resistive heating, high energy electron beams, low
energy electron beam and inducting heating. Clusters form in the vicinity of the
source by homogenous nucleation in the gas phase grew by incorporation by atoms in
the gas phase. It comprises of a ultra high vacuum (UHV) system fitted evaporation
source, a cluster collection device of liquid nitrogen filled cold finger scrapper
assembly and compaction device. During heating, atoms condense in the super
saturation zone close to Joule heating device. The nanoparticles are removed by
scrapper in the form of a metallic plate. Evaporation is to be done from W, Ta or Mo
refractory metal crucibles. If the metals react with crucibles, electron beam
evaporation technique is to be used. The method is extremely slow. The method
suffers from limitations such as a source-precursor incompatibility, temperature
ranges and dissimilar evaporation rates in an alloy. Alternative sources have been
developed over the years. For instance, Fe is evaporated into an inert gas atmosphere
(He). Through collision with the atoms the evaporated Fe atoms loose kinetic energy
and condense in the form of small crystallite crystals, which accumulate as a loose
powder. Figure 1.6 shows schematic diagram of the gas condensation method.
14
Chapter 1
Semiconductor metal oxides as humidity sensors
Figure 1.6
1.3.2.2 Sputtering Method
Sputtering or laser evaporation may be used instead of thermal evaporation.
Sputtering is a non-thermal process in which surface atoms are physically ejected
from the surface by momentum transfer from an energetic bombarding species of
atomic/molecular size. Typical sputtering uses a glow discharge or ion beam.
Interaction events which occur at and near the target surface during the sputtering
process in magnetron sputtering has advantage over diode and triode sputtering. In
magnetron sputtering, most of the plasma is confined to the near target region. Other
alternate energy sources which have been successfully used to produce clusters or
ultra fine particles are sputtering electron beam heating and plasma methods.
Sputtering has been used in low pressure environment to produce a variety of clusters
including Ag, Fe and Si. Figure 1.7 shows schematic diagram of sputtering method.
15
Chapter 1
Semiconductor metal oxides as humidity sensors
Figure 1.7: Diagram of sputtering method.
1.3.2.3 Chemical vapor deposition Method
Chemical vapor deposition (CVD) is a chemical process used to produce highpurity, high-performance solid materials. The process is often used in the
semiconductor industry to produce thin films. In typical CVD, the wafer (substrate) is
exposed to one or more volatile precursors, which react and decompose on the
substrate surface to produce the desired deposit. Frequently, volatile by-products are
also produced, which are removed by gas flow through the reaction chamber. CVD
involves the formation of nanomaterials from the gas phase at elevated temperatures
usually onto a solid substrate or catalyst. In this approach, vapour phase precursors
are brought into a hot-wall reactor under conditions that favour nucleation of particles
in the vapour phase rather than deposition of a film on the wall. It is called chemical
vapour synthesis or chemical vapor condensation in analogy to the chemical vapour
deposition (CVD) processes used to deposit thin solid films on surfaces. This method
has tremendous flexibility in producing a wide range of materials and can take
advantage of the huge database of precursor chemistries that have been developed for
CVD processes. The precursors can be solid, liquid or gas at ambient conditions, but
are delivered to the reactor as a vapor (from a bubbler or sublimation source, as
necessary). When a mixture of gas reactants are delivered into a reaction chamber, the
chemical reactions among the gas molecules are induced by an input of energy such
as resistant heating, laser, and plasma. Chlorides are popular reactants for the
16
Chapter 1
Semiconductor metal oxides as humidity sensors
formation of oxides because of their generally low vaporization temperature and low
cost. The typical reaction is as following:
SnCl4 (gas) + 2H2O (gas) → SnO2 (solid) + 4HCl (gas)
Another key feature of chemical vapor synthesis is that it allows formation of doped
or multi-component nanoparticles by use of multiple precursors. Schmechel et
al. prepared nanocrystalline europium doped yttria (Y2O3:Eu3+) from organometallic
yttrium and europium precursors. Senter et al. incorporated erbium into silicon
nanoparticles using disilane and an organometallic erbium compound as precursors.
Srdic et al. prepared zirconia particles doped with alumina. The powders exhibit a
narrow size distribution with an average size of about 5 nm. Advantages of CVD are
high growth rate, can deposit materials which are hard to evaporate, have good
reproducibility and can grow epitaxial films. Figure 1.8 shows the description of
CVD method.
Figure 1.8: Description of chemical vapour deposition method.
1.3.2.4 Ball Milling
Ball milling is a method of production of nano materials. This process is used
in producing metallic and ceramic nano materials. These mills are equipped with
grinding media composed of wolfram carbide or steel. A ball mill consists of a hollow
cylindrical shell rotating about its axis. The axis of the shell may be either horizontal
or at a same angle with the horizontal. The grinding media is the ball which may be
made of steel, stainless steel or rubber. The inner surface of the cylindrical shell is
usually line with an abrasion-resistant material such as manganese steel or rubber.
Ball mills rotate around a horizontal axis, partially filled with the material to be
ground plus the grinding medium. The balls rotate with high energy inside a drum
17
Chapter 1
Semiconductor metal oxides as humidity sensors
and then fall on the solid with gravity force and crush the solid into nano crystallites.
The significant advantage of this method is that it can be readily implemented
commercially. Ball milling can be used to make carbon nanotubes and boron nitride
nanotubes. It is a preferred method for preparing metal oxide nano crystals like
Cerium (CeO2) and Zinc Oxide (ZnO). Figure 1.9 shows diagram of deformations in
the material trapped between two colliding balls during ball milling.
Figure 1.9: Deformations in the material trapped between two colliding balls during
ball milling.
1.4 Film Preparation Technique
Now a day the study and application of thin film technology is entirely entered
in to almost all the branches of science and technology due to rapid development of
Nanotechnology. Tin oxide thin films have some very beneficial properties, such as
transparency for visible light, reflectivity for infrared light, and a low electrical sheet
resistance, making them suitable for a wide variety of applications such as in
transistors (Arnold et al 2003), photovoltaic cell (Cachet et al 1997), gas sensors
(Butta et al 1992), Protective and wear resistant coating on glass containers
(Nakagawa et al 1997), Infrared reflectors for glass windows (Lindner 1988) etc.
There are many methods are used to synthesize doped or un-doped tin oxide films
18
Chapter 1
Semiconductor metal oxides as humidity sensors
such as Thermal Evaporation (Comini et al 2002, Vaishnay et al 2005), Chemical
Vapor Deposition (Gorley et al 2005, Mamazza et al 2005), R.F. Magnetron Cosputtering (Jeorg et al 2006, Yoo et al 2005), Laser Pulse Evaporation (Yang and
Cheung 1982, Hui et al 2002), Spray Pyrolysis (Pirmoradi et al 2011, Lane et al 1992)
and sol-gel (Culha et al 2009). Several deposition techniques have been developed to
grow undoped and doped SnO2 films such as sputtering [34-35], sol-gel [36-37],
chemical vapor deposition (CVD) [38], sonochemical processing [39-41] and thermal
evaporation [42] spray pyrolysis, evaporation, chemical vapor deposition, sol gel
technique, magnetron sputtering pulsed laser deposition and screen printing
technique. The production of sensing layers by screen-printing technology has met the
great interest in this field. In fact, screen-printing is a simple and automated
manufacturing technique that allows the production of low cost and robust chemical
sensors with good reproducibility. Such technique allows the deposition of a
controlled amount of paste. Thick films are suitable for such sensors since the
humidity sensing properties are related to the material surface and the vapours are
always adsorbed and react with the films surface. Figure 1.10 shows the basic screen
printing process.
Figure 1.10: The basic screen printing process.
19
Chapter 1
Semiconductor metal oxides as humidity sensors
1.5 What is Sensor?
In practice, the words sensor and transducer are used interchangeably,
although the former more accurately describes the device and latter the principle
involved. A transducer is a device that transfers energy between two systems as in the
conversion of thermal into electrical energy by the Seebeck-effect thermocouple.
Figure 1.11 describes a basic sensor circuit and its relationship to the measurement
quantity.
The twelve descriptive parameters applicable to sensors are as follows:
1. Accuracy: The closeness with which a measurement approaches the true value
of a measurand, usually expressed as a percent of full-scale output.
2. Error: The deviation of a measurement from the true value of a measurand,
usually expressed as a percent of full-scale output.
3. Precision: An expression of a over some span described by the number of
significant figures available.
4. Resolution: An expression of the smallest quantity to which a quantity can be
measured.
5. Span: An expression of the extent of a measure between any two limits.
6. Range: An expression of the total extent of possible measurement values.
7. Reproducibility: It is the ability of an entire experiment or study to be
reproduced.
8. Ageing Effect: It is the effect which we see after some time on sensing material.
9. Response and Recovery Time: The time lag between initial and first change
occurring in output is taken as response and
recovery time.
10. Stability: The sensing material should be stable over long time.
20
Chapter 1
Semiconductor metal oxides as humidity sensors
Excitation
Measurand
Sensor
Transducer
principle
Interface
Circuit
Output
Signal
Figure 1.11: Sensor Circuit Elements
According to Gopel and Schierbaum [43] “sensors are devices that receive a
signal or stimulus and responds with an electrical signal”. Sensors are key elements in
the rapidly evolving fields of measurements, instrumentations and automated systems.
The recent progresses made in improving the reliability, lowering the cost of
microprocessors and interface circuits has resulted in a higher demand for sensors,
which convert physical or chemical quantities in various environments into electrical
signal. Different functions and materials have been investigated, and several devices
have been put on the market or have become part of sophisticated instrumentations
[44]. Among these materials, functional ceramics have played a major role because of
their intrinsic characteristics: they are superior in mechanical strength and chemical
resistance in most environments and in the reproducibility of the electrical properties.
They have also been widely used to satisfy diverse needs for sensing devices, and
consistent results have been obtained in the field of atmospheric sensors, i.e.,
temperature [45], humidity [46-51] and gas sensors [52]. Mainly sensor is divided into
two domains: the physical transducer and the interface layer. At the interface the
analyte interacts with a surface, producing a change in physical/chemical properties.
These changes are measured by the transducer domain and generate a related
electrical signal. The actual sensing process consists of three different parts: receptor,
transducer and operation mode [43]. The receptor is the surface of the metal oxide
where species undergo adsorption, reaction and desorption. The adsorption of species
on a solid has been divided into physisorption and chemisorption. A molecule is
considered to be chemisorbed if there is an electronic transfer between humidity and
solid and whereas there is no transfer in the case of physisorption. Ideally, the
interaction of the gaseous molecules will induce a change in the depletion layer of the
21
Chapter 1
Semiconductor metal oxides as humidity sensors
metal oxide grain. These changes are transduced into an electrical signal depending
on the microstructure of the sensitive film (the transducer). The porosity of the film,
the grain size and the different grain intersection will determine the output signal,
which takes into account the whole sensitive layer. This output single in usually
electric, although the measurement of the thermo voltage or the change in the sensor
temperature is also possible.
1.6 Requirements of a Humidity Sensor
For designing a robust humidity sensor, the sensor material should possess qualities as
under:
a) The material should have high sensitivity over a wide range of humidity and
temperature.
b) The device should also have good stability and reversibility.
c) There should not be any hysteresis in the characteristics.
d) The sensitivity should be independent of the ambient temperature.
e) Numerous materials have been utilized for humidity sensing of which the metal
oxides are physically and chemically stable and used at both room and elevated
temperatures.
f) The reliability of humidity sensors is affected by many environmental factors
such as contamination due to oil, dust and other materials in air.
g) It should have a short response and recovery times.
h) It should be less portable.
i) It should be resistive to contaminants.
j) It should have good mechanical properties and durability.
1.7 Humidity: Definition and Properties
Humidity is the amount of moisture that exists in the air. a day will be. There
are various ways, which define humidity in atmosphere. Few of them are given as
below:
1.7.1 Absolute Humidity
Absolute humidity is the ratio of mass of water vapor to unit volume of total
air and water vapor mixture can be expressed as follows:
𝑨𝑯 =
𝑴𝒘
𝑽𝒏𝒆𝒕
22
Chapter 1
Semiconductor metal oxides as humidity sensors
Where AH is absolute humidity, Mw is mass of water vapor, Vnet is total air and water
vapor mixture.
1.7.2 Relative Humidity
Relative humidity is the ratio of the partial pressure of water vapor in air-water
mixture to the saturated vapor pressure of water at prescribed temperature. RH is a
function of temperature, and thus it is a relative measurement. Relative humidity is
defined as below:
%𝑹𝑯 =
𝑷𝒗
× 𝟏𝟎𝟎
𝑷𝒔
Where Pv is partial pressure and Ps is saturated vapor pressure.
1.7.3 Specific Humidity
Specific humidity is the ratio of water vapor to dry air in a particular mass, and
is sometimes referred to as humidity ratio. Specific humidity is expressed as a ratio of
mass of water vapor Mv per unit mass of dry air Ma. This quantity is also known as
the water vapor "mixing ratio".
𝑺𝑯 =
𝑴𝒗
𝑴𝒂
Where SH is specific humidity, Mv is mass of water vapor and Ma is mass of dry air.
1.8 The Role and Applications of Humidity Sensors
Humidity plays very important role in human life. Different applications of
humidity in different fields are as follows:
a) There are many domestic applications, such as intelligent control of the living
environment in buildings, cooking control for microwave ovens and intelligent
control of laundry etc.
b) In automobile industry, humidity sensors are used in rear window defoggers
and motor assembly lines.
c) In medical field, humidity sensors are used in respiratory equipment,
sterilizers, incubators, pharmaceutical processing and biological products.
d) In agriculture, humidity sensors are used for green-house air-conditioning,
plantation protection (dew prevention), soil moisture monitoring and cereal
storage.
23
Chapter 1
Semiconductor metal oxides as humidity sensors
e) In general industry, humidity sensors are used for humidity control in
chemical gas purification, dryers, ovens, film desiccation, paper and textile
production and food processing.
f) They may be used in homes for people with illnesses affected by humidity, as
part of home heating, ventilating, and air conditioning (HVAC) systems and in
humidors or wine cellars. Humidity sensors can also be used in cars, office
and industrial HVAC systems, and in meteorology stations to report and
predict weather.
g) A person with a respiratory illness or certain allergies might use a
home humidity sensor
because
low humidity can
exacerbate
breathing
problems and cause joint pain, while high humidity encourages bacteria,
mould and fungus growth.
h) Humidity sensors can also be used in homes or museums where valuable
antiques or artwork are kept, because these items can be damaged or degraded
from constant exposure to too much moisture.
1.9 Methods for Humidity Measurement
There are various methods for measurement of humidity. Most of them using a
device called a hygrometer. The hygrometer can work in a variety of ways,
depending on the type, and is the most accurate method for determining
the humidity in the air. Some are connected to other devices called humistats, which
are connected to humidifiers and dehumidifiers, and help control the level
of humidity in the air.
The psychrometer also known as the two-bulb hygrometer which works by
using two thermometer bulbs. One bulb is dry that measures the temperature in the
air. The other bulb is covered in a substance, usually wick or muslin and then wetted
down. After being wetted, the bulb is exposed to moving air, either through a fan or
by slinging the psychrometer through the air. As the water evaporates in the moving
air, the temperature on the thermometer will drop. The amount the temperature drops
helps to tell the amount of humidity in the air.
Electric hygrometers work by measuring the electrical resistance of a
particular substance. Most substances such as lithium chloride have varying
resistances to an electrical current based on the humidity in the air. These differences
are then calculated to display the humidity.
24
Chapter 1
Semiconductor metal oxides as humidity sensors
Chemical hygrometers work by using a chemical substance and exposing it
to air. The chemical will be measured before being exposed and then again after. Any
changes in weight indicate how much humidity is in the air.
The polymeric type hygrometer consists of a capacitor with a hygroscopic
polymer dielectric. The difference in relative dielectric constants between the polymer
and the water generates a considerable variation in capacitance when water is
absorbed.
A gravimetric humidity sensor is the quartz crystal microbalance. Thin
plates of piezoelectric quartz have resonance frequencies in the MHz-range. Coated
with hygroscopic layer, the change of frequency acts as a measure for the humidity.
1.10 Resistive Humidity Sensor: Working Principle
Material conductivity varies with amount of water absorbed by it. This
principle is employed for the measurement of humidity in resistance type humidity
sensors. For dependence of conduction mechanism this type of sensor can be
subdivided into two parts:
1.10.1 Electronic Type Humidity Sensor
1.10.2 Ionic Type Humidity Sensor
1.10.1 Electronic Type Humidity Sensor
Some semiconducting oxides or composite oxides such as SnO2, ZnO, and
In2O3, etc. are wide-band gap semiconductors. H2O is adsorbed on the oxide surface
in molecular and hydroxyl forms. Water molecules are observed to increase the
conductivity of n-type semiconductor and to decrease the conductivity of p-type
semiconductor [53-54]. This effect has been attributed to the donation of electrons
from the chemically adsorbed water molecules to the ceramic surface [53]. Because
the conductivity is caused by the surface concentration of electrons, this sensing style
is usually called “electronic type”. However, the water layer formed by the physical
adsorption may be somewhat proton conductive. Therefore, at room temperatures the
conductivity of ceramic semiconducting materials is actually due to addition of both
electrons and protons (ionic), unless at high temperatures (>100ºC) moisture cannot
effectively condense on the surface. In Figure 1.12 (a), the conductivity increment is
produced by surface electron accumulation resulting from the preferential alignment
of the water dipoles [53]. Hydrogen atoms contact the surface (mostly at the oxygen
sites) and attract electrons outward. In Figure 1.12 (b) a depletion region forms
25
Chapter 1
Semiconductor metal oxides as humidity sensors
originally due to adsorbed oxygen and the released electrons may neutralize the
depletion.
Figure 1.12: Two possible mechanisms for the “donor effect” (for n-type): (a)
Electrons are attracted by the adsorbed water molecules to the semiconductor
surface and the energy bands are bended (b) Electrons are released by the
competitive
adsorption.
Since adsorbed
water molecules increase the conductivity of n-type ceramic
semiconductors, nearly humidity sensitivity is greatly dependent on adsorption and
ionization of outer molecular water as well as ionic transport on material [55-57].
With the increase of humidity, the adsorbed water makes the surface conductivity
increase due to increased charge carriers. If every phase has good humidity
sensitivity, the total sensing property will be increased; otherwise, sensing property
will be decreased. Moreover, grain boundary can increase sensing property because it
introduces many crystal defects which improve the ionization of outer molecular
water. Likewise decreased granularity has the same effect as increasing the boundary
area. On the other hand, if other ions segregate onto boundaries and inhibit adsorption
and ionization of water molecules, the sensitivity will be deteriorated.
In semiconductor, proton is the dominant carrier responsible for the electrical
conductivity. The conduction is due to the Grotthuss mechanism, through which
protons tunnel from one water molecule to the next via hydrogen bonding that
universally exists in liquid-phase water shown in Figure 1.13. This sensing
mechanism was reported about 200 years ago [58]. The mechanism of protonic
conduction inside the adsorbed water layers on the surface of the sensing materials
26
Chapter 1
Semiconductor metal oxides as humidity sensors
was discovered in study of TiO2 and α-Fe2O3 [59-60]. As shown in Figure 1.14, at the
first stage of adsorption, a water molecule is chemically adsorbed on an activated site
(a) to form an adsorption complex (b), which subsequently transfers to surface
hydroxyl groups (c). Then another water molecule comes to be absorbed through
hydrogen bonding on the two neighbouring hydroxyl groups as shown in (d).
Figure 1.13: Brief illustration of Grotthuss mechanism.
Figure 1.14
The top water molecule condensed cannot move freely due to the restriction from the
two hydrogen bonding as shown by Figure 1.14 (d). Thus, this layer or the first
physically-adsorbed layer is immobile and there are not hydrogen bonds formed
between the water molecules in this layer. Therefore, no proton could be conducted in
this stage. As water continue to condense on the surface of semiconductor, an extra
layer on top of the first physically adsorbed layer forms. This layer is less ordered
than the first physically-adsorbed. For example, there may be only one hydrogen bond
locally. If more layers condense, the ordering from the initial surface may gradually
disappear and protons may have more freedom to move inside the condensed water
through the Grotthuss mechanism shown in Figure 1.15. In other words, from the
second physisorbed layer, water molecules become mobile and finally almost
identical to the bulk liquid water, and the Grotthuss mechanism becomes dominant.
This mechanism indicates that sensors based purely on water-phase protonic
27
Chapter 1
Semiconductor metal oxides as humidity sensors
conduction would not be quite sensitive to low humidity, at which the water vapour
could rarely form continuous mobile layers on the sensor surface. Since the
mechanism of humidity sensors is mainly associated with adsorption desorption
processes, the surface area of the sensors becomes an important factor to determine
the sensing properties. The porosity can enhance the surface area and therefore the
porous materials are considered as better sensing materials.
Figure 1.15: Mechanism of water adsorption on
electronic humidity sensor.
1.10.2 Ionic Type Humidity Sensor
In ionic-type humidity sensors, the conduction mechanism is mainly due to the
displacement of protons between the water molecules. Arai et al explains that the
conduction mechanism depends on the surface coverage of adsorbed water. Hopping
of H+ is predominant when the physisorbed HO is absent or very little. When water is
present but the coverage of the surface is not complete, H3O+ diffusion dominates.
When the H2O is abundant, the proton-transfer process dominates. Finally, at higher
RH, capillary condensation takes place in the pores according to the Kelvin equation
and the conduction mechanism becomes electrolytic.
rk =
𝟐𝑴𝜸
𝑷𝒔
𝑷
𝝆𝑹𝑻 𝑰𝒏 ( )
In the expression, rk is the Kelvin radius of the micropores, γ, ρ and M are the surface
tension (72, 75 dyn/cm at 20ºC), the density, and the molecular weight of water, P is
partial vapour pressure, Ps is the vapor pressure at saturation. The saturation
corresponds to the capillary condensation in the pores and, under a constant P and
temperature, takes place in all the pores with radii up to rk. The smaller the value of rk,
the more easily it takes place. The capillary condensation is an important factor in
ionic-type humidity sensors. For ionic sensing materials, if the humidity increases, the
conductivity decreases and the dielectric constant increases [61-62].
28
Chapter 1
Semiconductor metal oxides as humidity sensors
1.11 Characterization Techniques:
1.11.1 X-Ray Diffraction
The X-ray diffraction pattern of a pure substance is like a fingerprint of the
substance. The powder diffraction method is thus ideally suited for characterization
and identification of polycrystalline phases. The main use of powder diffraction is to
identify components in a sample by a matching procedure. Furthermore, the areas
under the peak are related to the amount of each phase present in the sample. Solid
matter can be described as:
Amorphous: The atoms are arranged in a random way similar to the disorder as find
in a liquid. Glasses are amorphous materials.
Crystalline: The atoms are arranged in a regular pattern, and there is as smallest
volume element that repeated in three dimensions which describes the crystal. X-ray
Diffraction (XRD) is a high-tech, non-destructive technique for analyzing a wide
range of materials, including fluids, metals, minerals, polymers, catalysts, plastics,
pharmaceuticals, thin-film coatings, ceramics, solar cells and semiconductors. X-ray
diffraction is a technique used to characterize the crystallographic structure, crystallite
size and preferred orientation in polycrystalline or powdered solid samples.
An electron in an alternating electromagnetic field will oscillate with the same
frequency as the field. When an X-ray beam hits an atom, the electrons around the
atom start to oscillate with the same frequency as the incoming beam. In almost all
directions we will have destructive interference, i.e. the combining waves are out of
phase and there is no resultant energy leaving the solid sample. However the atoms in
a crystal are arranged in a regular pattern, and in a very few directions we will have
constructive interference. The waves will be in phase and there will be well defined
X-ray beams leaving the sample at various directions. Hence, a diffracted beam may
be described as a beam composed of a large number of scattered rays mutually
reinforcing one another.
We have used XRD to determine the composition of our Tin oxide based
samples. XRD works by using a beam of X-rays directed at a sample to detect the
bond lengths. After the sample is loaded, the beam of X-rays moves around the
sample from 10 to 90º then using Bragg’s law the bond length can be calculated.
Bragg’s law states that when a multiple of the wavelength is divided by twice the
bond length it is equal to the sine of the angle i.e.
29
Chapter 1
Semiconductor metal oxides as humidity sensors
λ = 2d Sinθ
λ: wavelength of the X-rays
d: the spacing of the layers
Figure: 1.16
θ: the incident angle of the photons.
1.11.1.1 The Debye-Scherrer Formula
Figure 1.17 shows that the rays A, D and M make precisely the Bragg angle θB
with the reflecting planes. Ray D′, scattered by the first plane below the surface, is
one wavelength out of phase with A′, ray M′ is n wavelengths out of phase with it. At
the diffraction angle 2θB all these rays are in phase and unite to form a beam of
maximum amplitude. Ray B makes a slightly larger angle θ1 with the reflecting plane,
such that ray L' from the nth plane is (n + 1) wavelengths out of phase with B′. So the
rays scattered by the upper half of the crystal cancel exactly with those scattered by
the lower half of the crystal and θ1 is the smallest angle where complete destructive
interference occurs. This is also the case for an angle θ2 which is a bit smaller than θB
so that the path difference between the ray scattered by the first and the last plane is
(n−1) wavelengths. These are the two limiting angles where the intensity of the
diffracted beam drops to zero. This implies that the intensity is greater than zero all
the way from θ2 to θ1 as depicted in Figure 1.18.
The width of diffraction curves increases as the thickness of the crystal
decreases because the angular range (2θ1 − 2θ2) increases as n decreases. As a
measure of the peak width, the full width at half maximum (FWHM), denoted by , is
used. As an approximation  =
1
(2θ1 − 2θ2) = θ1 − θ2 is chosen, since this yields the
2
exact FWHM for a Gaussian.
The path difference equations for these two angles related to the entire thickness of
the crystal are given by:
2t sin θ1 = (n + 1) λ
2t sin θ2 = (n − 1) λ
Subtracting the above equations yields:
t(sin θ1 − sin θ2) = λ

   
   
2t cos  1 2  sin  1 2  = λ
 2 
 2 
30
Chapter 1
Semiconductor metal oxides as humidity sensors
Figure 1.17: Scattering from a finite
number of equidistant planes
Figure 1.18: FWHM for a crystal of
finite (left) and infinite (right) size
Since θ1 and θ2 are very close to θB it is reasonable to make the following
approximations:
sin θ1 + sin θ2 = 2θB
       
sin  1 2  =  1 2 
 2   2 
With these approximations we obtain
   
2t  1 2  cos θB = λ
 2 
and using the definition of the FWHM introduced above gives a crystal depth t = n.d
of
t

 cos  B
A more rigorous mathematical treatment of the problem results in the Debye-Scherrer
Formula is
D
K
 Cos
Here K is a dimensionless constant that may range from 0.89 to 1.39 depending on the
specific geometry of the scattering objects. For a perfect two-dimensional lattice,
where every point on the lattice emits a spherical wave, numerical calculations yield
31
Chapter 1
Semiconductor metal oxides as humidity sensors
the lower bound of 0.89 for K [63]. A cubic three-dimensional crystal is best
described by K = 0.94, while analytical calculations for a perfectly spherical object
yield K = 1.33. X-ray Diffraction Unit is shown in Figure 1.19.
Figure 1.19: X-ray Diffraction Unit
1.11.2 Scanning Probe Microscopy
The scanning probe microscopy (SPM) is one of the powerful modern
research techniques that allow investigating the morphology and the local properties
of the solid body surface with high spatial resolution. During last 10 years the
scanning probe microscopy has turned from an exotic technique accessible only to a
limited number of research groups, to a widespread and successfully used research
tool of surface properties. Currently, practically every research in the field of surface
physics and thin-film technologies apply the SPM techniques. The scanning probe
microscopy has formed also a basis for development of new methods in
nanotechnology, i.e. the technology of creation of structures at nanometric scales. The
scanning tunneling microscope (STM) is the first in the probe microscopes family; it
was invented in 1981 by the Swiss scientists Gerd Binnig and Heinrich Rohrer [6465]. In their works they have shown, that this is a quite simple and rather effective
way to study a surface with spatial resolution down to atomic one. Their technique
was fully acknowledged after visualization of the atomic structure of the surface of
some materials and, particularly, the reconstructed surface of silicon. In 1986, G.
Binnig and H. Rohrer were awarded the Nobel Prize in physics for invention of the
tunneling microscope. The scanning probe microscope gives researchers imaging
tools for the future as these specialized microscopes provide high image
magnification for observation of three-dimensional-shaped specimens.
32
Chapter 1
Semiconductor metal oxides as humidity sensors
Scanning probe technology at the microscopic level is found in both
academic and industrial laboratories today including physics, biology, chemistry and
are now standard analysis tools for research and development. SPM provides very
high resolution images of various sample properties. All of these microscopes work
by measuring a local property such as height, optical absorption, or magnetism with a
probe or tip placed very close to the sample.
1.11.2.1 Types
(a) Scanning Tunneling Microscopy
(b) Atomic Force Microscopy
(c) Electric Force Microscopy
(d) Magnetic Force Microscopy
(e) Near-Field Optical Microscopy
(f) Scanning Capacitance Microscopy
1.11.2.2 Basic Principle
The analysis of a surface micro relief and of its local properties is performed
by scanning probe microscopes using specially prepared tips in the form of needles.
The size of the working part of such tips (the apex) is about ten nanometers. The usual
tip-surface distance in probe microscopes is about 0.1-10 nanometers. Various types
of interaction of the tip with the surface are exploited in different types of probe
microscopes. For example the tunnel microscope is based on the phenomenon of a
tunnelling current between a metal needle and a conducting sample; various types of
interactive force underlie the working mechanism of atomic force, magnetic force and
electric force microscopes. Activities for manipulation of atom and molecules at the
nanoscale level are dominated by the use of scanning probe microscopes in ultra-high
vacuum. Scanning Probe Microscopy (SPM) is a technique that is used to study the
properties of surfaces at the atomic level. Unlike conventional microscopy, which
uses light waves for imaging, SPM involves scanning the surface of a sample with a
very fine probe (‘tip’) and monitoring the strength of some interaction between the tip
and surface. Scanning Probe Microscopy (SPM) scans an atomically sharp probe over
a surface, typically at a distance of a few angstroms or nanometres. The interaction
between the sharp probe and surface provides 3D topographic image of surface at the
atomic scale. Compared with other instruments that open a window to a world of
molecule-sized spaces, SPMs are relatively simple, inexpensive, and easy to operate.
33
Chapter 1
Semiconductor metal oxides as humidity sensors
Especially appealing about the proximal probes is their multipurpose nature that
offers not only a view of individual atoms but also ways to pick them up, move them
around, and position them at will.
1.11.3 Scanning Electron Microscopy
Scanning electron microscopy (SEM) analyzes the surface structure of
materials. It measures and evaluates surface pitting, failure analysis, characterization
of dust, deposits, contaminants, particles, filter residues, and other applications. SEM
is a powerful and frequently used instrument, in both academia and industry, to study,
surface topography, composition, crystallography and properties on a local scale. The
spatial resolution is better than that of the optical microscope. The SEM has an
extremely large depth of focus and is therefore well suited for topographic imaging.
Besides surface topographic studies the SEM can also be used for determining the
chemical composition of a material, its fluorescent properties, the formation of
magnetic domains and so on. It is a type of electron microscope that images a sample
by scanning it with a high-energy beam of electrons in a raster scan pattern. The
electrons interact with the atoms that make up the sample producing signals that
contain information about the sample's surface topography, composition, and other
properties such as electrical conductivity. Scanning electron microscopy analyses the
surface of solid objects, producing higher resolution images than optical microscopy.
SEM produces representations of three-dimensional samples from a diverse range of
materials. Back-scatter and cathodoluminescence are used to evaluate a wide range of
samples.
1.11.3.1 Working Principle
The specimen is bombarded by a convergent electron beam, which is scanned
across the surface. This electron beam generates a number of different types of
signals, which are emitted from the area of the specimen where the electron beam is
impinging, shown in Figure 1.20. The induced signals are detected and the intensity of
one of the signals (at a time) is amplified and used to as the intensity of a pixel on the
image on the computer screen. The electron beam then moves to next position on the
sample and the detected intensity gives the intensity in the second pixel and so on.
34
Chapter 1
Semiconductor metal oxides as humidity sensors
Figure 1.20: Example of some of the different types of signals produced when
high-energy electron impinge on a material.
The working principle of the SEM is shown in Figure 1.21. For improved signal-tonoise ratio in the image, one can use a slower scan speed. This means that the electron
beam stays a longer time at one position on the sample surface before moving to the
next. This gives a higher detected signal and increased signal-to noise ratio. The types
of signals produced by an SEM include secondary electrons, back-scattered electrons
(BSE), characteristic X-rays, specimen current and transmitted electrons. Accelerated
electrons in a SEM carry significant amounts of kinetic energy, and this energy is
dissipated as a variety of signals produced by electron-sample interactions when the
incident electrons are decelerated in the solid sample. These signals include secondary
electrons (that produce SEM images), backscattered electrons (BSE), diffracted
backscattered electrons, photons, visible light (cathodoluminescence) and heat.
35
Chapter 1
Semiconductor metal oxides as humidity sensors
Figure 1.21: Schematic diagram of a SEM
Secondary electrons and backscattered electrons are commonly used for
imaging samples: secondary electrons are most valuable for showing morphology and
topography on samples and backscattered electrons are most valuable for illustrating
contrasts in composition in multiphase samples. Secondary electron detectors are
common in all SEMs, but it is rare that a single machine would have detectors for all
possible signals. The signals result from interactions of the electron beam with atoms
at or near the surface of the sample. In the most common or standard detection mode,
secondary electron imaging or SEI, the SEM can produce very high-resolution images
of a sample surface, revealing details less than 1 nm in size. Due to the very narrow
electron beam, SEM micrographs have a large depth of field yielding a characteristic
three-dimensional appearance useful for understanding the surface structure of a
sample. In the present study, the surface morphology of the samples was investigated
with a Cambridge LEO-0430 SEM (Figure 1.22).
36
Chapter 1
Semiconductor metal oxides as humidity sensors
Figure 1.22: Scanning Electron Microscopy Unit.
1.11.4 Transmission Electron Microscopy
The transmission electron microscope (TEM) operates on the same basic
principles as the light microscope but uses electrons instead of light. TEM uses high
energy electrons to penetrate through a thin (≤100 nm) sample. This offers increased
spatial resolution in imaging (down to individual atoms) as well as the possibility of
carrying out diffraction from nano-sized volumes. At a maximum potential
magnification of 1 nanometer, TEMs are the most powerful microscopes. TEMs
produce high resolution, two-dimensional images, allowing for a wide range of
educational, science and industry applications. TEM consist of the following
components: a electron source, thermionic gun, electron beam, electromagnetic
lenses, vacuum chamber, condenser, sample stage, phosphor or fluorescent screen and
computer. A Transmission Electron Microscope produces a high-resolution, black
and white image from the interaction that takes place between prepared samples and
energetic electrons in the vacuum chamber. Air needs to be pumped out of the
vacuum chamber, creating a space where electrons are able to move. The electrons
then pass through multiple electromagnetic lenses. These solenoids are tubes with coil
wrapped around them. The beam passes through the solenoids, down the column,
makes contact with the screen where the electrons are converted to light and form an
image. The image can be manipulated by adjusting the voltage of the gun to
accelerate or decrease the speed of electrons as well as changing the electromagnetic
wavelength via the solenoids. During transmission, the speed of electrons directly
correlates to electron wavelength; the faster electrons move, the shorter wavelength
and the greater the quality and detail of the image. The lighter areas of the image
represent the places where a greater number of electrons were able to pass through the
37
Chapter 1
Semiconductor metal oxides as humidity sensors
sample and the darker areas reflect the dense areas of the object. To obtain a TEM
analysis, samples need to have certain properties. They need to be sliced thin enough
for electrons to pass through, a property known as electron transparency. Samples
need to be able to withstand the vacuum chamber and often require
special preparation before viewing. Samples need to be able to withstand the vacuum
chamber and often require special preparation before viewing. Today TEMs constitute
arguably the most efficient and versatile tools for the characterization of materials
over spatial ranges from the atomic scale, through the ever-growing ‘nano’ regime
(from < 1 nm to ~ 100 nm) up to the micrometer level and beyond. In the TEM we are
usually most interested in those electrons that do not deviate far from the incidentelectron direction. This is because the TEM is constructed to gather these electrons
primarily and they also give us the information we seek about the internal structure
and chemistry of the specimen. Other forms of scattering, such as electrons which are
scattered through large angles (e.g., backscattered electrons) and electrons ejected
from the specimen (such as secondary electrons). So the electrons that hit the
specimen are often called the incident beam and those scattered by the specimen are
called scattered (or sometimes specifically, diffracted) beams. Electrons coming
through a thin specimen are separated into those that suffer no angular deviation and
those scattered though measurable angles. We call the undeviated electrons the ‘direct
beam’ (in contrast to most texts that describe this as the ‘transmitted beam’ despite the
fact that all electrons coming through the specimen have been ‘transmitted’). As the
electrons travel through the specimen they are either scattered by a variety of
processes or they may remain unaffected. The end result, however, is that a nonuniform distribution of electrons emerges from the exit surface of the specimen, as
shown schematically in Figure 1.23. Figure 1.24 shows transmission electron
microscopy unit.
38
Chapter 1
Semiconductor metal oxides as humidity sensors
Figure 1.23: (a) A uniform intensity of electrons, represented by the horizontal
lines, falls on a thin specimen. Scattering within the specimen changes both the
spatial and angular distributions of the emerging electrons. The spatial distribution
(intensity) is indicated by the wavy line. (b) The change in angular distribution is
shown by an incident beam of electrons being transformed into several forwardscattered beams.
Figure 1.24: Transmission Electron Microscopy Unit.
39
Chapter 1
Semiconductor metal oxides as humidity sensors
1.11.5 Differential Scanning Calorimetry
Differential scanning calorimetry (DSC) is a thermo analytical technique in
which the difference in the amount of heat required to increase the temperature of a
sample and reference is measured as a function of temperature. Both the sample and
reference are maintained at nearly the same temperature throughout the experiment.
Generally, the temperature program for a DSC analysis is designed such that the
sample holder temperature increases linearly as a function of time. The reference
sample should have a well-defined heat capacity over the range of temperatures to be
scanned.
The technique was developed by E. S. Watson and M. J. O'Neill in 1962 and
introduced commercially at the 1963 Pittsburgh Conference on Analytical Chemistry
and Applied Spectroscopy. The first adiabatic differential scanning calorimeter that
could be used in biochemistry was developed by P. L. Privalov and D. R.
Monaselidze in 1964. The term DSC was coined to describe this instrument which
measures energy directly and allows precise measurements of heat capacity.
The basic principle underlying this technique is that when the sample
undergoes a physical transformation such as phase transitions, more or less heat will
need to flow to it than the reference to maintain both at the same temperature.
Whether less or more heat must flow to the sample depends on whether the process is
exothermic or endothermic. For example, as a solid sample melts to a liquid it will
require more heat flowing to the sample to increase its temperature at the same rate as
the reference. This is due to the absorption of heat by the sample as it undergoes the
endothermic phase transition from solid to liquid. Likewise, as the sample undergoes
exothermic processes (such as crystallization) less heat is required to raise the sample
temperature. By observing the difference in heat flow between the sample and
reference, differential scanning calorimeters are able to measure the amount of heat
absorbed or released during such transitions. DSC may also be used to observe more
subtle phase changes, such as glass transitions. It is widely used in industrial settings
as a quality control instrument due to its applicability in evaluating sample purity and
for studying polymer curing.
The result of a DSC experiment is a curve of heat flux versus temperature or
versus time. There are two different conventions: exothermic reactions in the sample
shown with a positive or negative peak, depending on the kind of technology used in
40
Chapter 1
Semiconductor metal oxides as humidity sensors
the experiment. This curve can be used to calculate enthalpies of transitions. This is
done by integrating the peak corresponding to a given transition. It can be shown that
the enthalpy of transition can be expressed using the following equation:
ΔH = KA
Where ΔH is the enthalpy of transition, K is the calorimetric constant, and A is
the area under the curve. The calorimetric constant will vary from instrument to
instrument, and can be determined by analyzing a well-characterized sample with
known enthalpies of transition.
Differential scanning calorimetry can be used to measure a number of
characteristic properties of a sample. Using this technique it is possible to observe
fusion and crystallization events as well as glass transition temperatures Tg. DSC can
also be used to study oxidation, as well as other chemical reactions. Glass transitions
may occur as the temperature of an amorphous solid is increased. These transitions
appear as a step in the baseline of the recorded DSC signal. This is due to the sample
undergoing a change in heat capacity; no formal phase change occurs. As the
temperature increases, an amorphous solid will become less viscous. At some point
the molecules may obtain enough freedom of motion to spontaneously arrange
themselves into a crystalline form. This is known as the crystallization temperature
(Tc). This transition from amorphous solid to crystalline solid is an exothermic
process, and results in a peak in the DSC signal. As the temperature increases the
sample eventually reaches its melting temperature (Tm). The melting process results in
an endothermic peak in the DSC curve. The ability to determine transition
temperatures and enthalpies makes DSC a valuable tool in producing phase diagrams
for various chemical systems.
41
Chapter 1
Semiconductor metal oxides as humidity sensors
Figure 1.25: Differential scanning calorimeter.
1.11.6 Ultraviolet-visible spectroscopy
Ultraviolet-visible spectroscopy refers to absorption spectroscopy in the UVvisible spectral region. This means it uses light in the visible and adjacent (near-UV
and near-infrared (NIR) ranges. The absorption in the visible range directly affects the
perceived color of the chemicals involved. In this region of the electromagnetic
spectrum, molecules undergo electronic transitions [66]. This technique is
complementary to fluorescence spectroscopy, in that fluorescence deals with
transitions from the excited state to the ground state, while absorption measures
transitions from the ground state to the excited state.
The instrument used in ultraviolet-visible spectroscopy is called a UV/vis
spectrophotometer. It measures the intensity of light passing through a sample (I), and
compares it to the intensity of light before it passes through the sample (Io). The ratio
I / Io is called the transmittance, and is usually expressed as a percentage (%T). The
absorbance (A) is based on the transmittance:
𝐀 = − 𝐥𝐨𝐠 (
%𝐓
)
𝟏𝟎𝟎
42
Chapter 1
Semiconductor metal oxides as humidity sensors
Figure 1.26: Diagram of a single-beam UV/vis spectrophotometer.
Figure 1.26 shows the diagram of a single-beam UV/vis spectrophotometer.
The basic parts of a spectrophotometer are a light source, a holder for the sample, a
diffraction grating or monochromatic to separate the different wavelengths of light,
and a detector. The radiation source is often a Tungsten filament (300-2500 nm), a
deuterium arc lamp, which is continuous over the ultraviolet region (190-400 nm) or
more recently light emitting diodes (LED) and Xenon Arc Lamps for the visible
wavelengths. The detector is typically a photodiode or a CCD. Photodiodes are used
with monochromators, which filter the light so that only light of a single wavelength
reaches the detector. Diffraction gratings are used with CCDs, which collects light of
different wavelengths on different pixels.
A spectrophotometer can be either single beam or double beam. In a single
beam instrument (such as the Spectronic 20), all of the light passes through the
sample cell. Io must be measured by removing the sample. This was the earliest
design, but is still in common use in both teaching and industrial labs.
In a double-beam instrument, the light is split into two beams before it reaches
the sample. One beam is used as the reference; the other beam passes through the
sample. Some double-beam instruments have two detectors (photodiodes), and the
sample and reference beam are measured at the same time. In other instruments, the
two beams pass through a beam chopper, which blocks one beam at a time. The
detector alternates between measuring the sample beam and the reference beam.
43
Chapter 1
Semiconductor metal oxides as humidity sensors
Samples for UV/Vis spectrophotometer are most often liquids, although the
absorbance of gases and even of solids can also be measured. Samples are typically
placed in a transparent cell, known as a cuvette. Cuvettes are typically rectangular in
shape, commonly with an internal width of 1 cm. Test tubes can also be used as
cuvettes in some instruments. The type of sample container used must allow radiation
to pass over the spectral region of interest. The most widely applicable cuvettes are
made of high quality fused silica or quartz glass because these are transparent
throughout the UV, visible and near infrared regions. Glass and plastic cuvettes are
also common, although glass and most plastics absorb in the UV, which limits their
usefulness to visible wavelengths [67].
The variation in absorption coefficient as a function of photon energy for
allowed direct transitions is given by:
α =A (hν-Eg)1/2
In above equation α is the absorption coefficient, A is a constant, h is Planck’s
constant, ν is the frequency and Eg is the band gap energy. The absorption coefficient,
α, is obtained from Beer’s law
I = Io exp (-αx)
In the above equation, x is thickness of the measured sample. A plot of versus
photon energy was used to obtain the value of the direct band gap by extrapolating the
linear portion of the curves to zero absorption.
1.11.7 Fourier Transform Infrared (FTIR) Spectroscopy
FTIR spectrometers are widely used in organic synthesis, polymer science,
petrochemical engineering, pharmaceutical industry and food analysis. In addition,
since FTIR spectrometers can be hyphenated to chromatography, the mechanism of
chemical reactions and the detection of unstable substances can be investigated with
such instruments. The range of Infrared region is 12800 ~ 10 cm-1 and can be divided
into near-infrared region (12800 ~ 4000 cm-1), mid-infrared region (4000 ~ 200 cm-1)
and far-infrared region (50 ~ 1000 cm-1). The discovery of infrared light can be dated
back to the 19th century. Since then, scientists have established various ways to utilize
infrared light. Infrared absorption spectroscopy is the method which scientists use to
determine the structures of molecules with the molecules’ characteristic absorption of
infrared radiation. Infrared spectrum is molecular vibrational spectrum. When
exposed to infrared radiation, sample molecules selectively absorb radiation of
44
Chapter 1
Semiconductor metal oxides as humidity sensors
specific wavelengths which causes the change of dipole moment of sample molecules.
Consequently, the vibrational energy levels of sample molecules transfer from ground
state to excited state. The frequency of the absorption peak is determined by the
vibrational energy gap. The number of absorption peaks is related to the number of
vibrational freedom of the molecule. The intensity of absorption peaks is related to the
change of dipole moment and the possibility of the transition of energy levels.
Therefore, by analyzing the infrared spectrum, one can readily obtain abundant
structure information of a molecule. Most molecules are infrared active except for
several homonuclear diatomic molecules such as O2, N2 and Cl2 due to the zero
dipole change in the vibration and rotation of these molecules. What makes infrared
absorption spectroscopy even more useful is the fact that it is capable to analyze all
gas, liquid and solid samples. The common used region for infrared absorption
spectroscopy is 4000 ~ 400 cm-1 because the absorption radiation of most organic
compounds and inorganic ions is within this region. Infrared spectroscopy has been a
workhorse technique for materials analysis in the laboratory for over seventy years.
An infrared spectrum represents a fingerprint of a sample with absorption peaks
which correspond to the frequencies of vibrations between the bonds of the atoms
making up the material. Because each different material is a unique combination of
atoms, no two compounds produce the exact same infrared spectrum. Therefore,
infrared spectroscopy can result in a positive identification (qualitative analysis) of
every different kind of material. In addition, the size of the peaks in the spectrum is a
direct indication of the amount of material present. With modern software algorithms,
infrared is an excellent tool for quantitative analysis.
FTIR spectrometers are the third generation infrared spectrometer. FTIR
spectrometers have several prominent advantages: (1) The signal-to-noise ratio of
spectrum is significantly higher than the previous generation infrared spectrometers.
(2) The accuracy of wave number is high. The error is within the range of ± 0.01 cm-1.
(3) The scan time of all frequencies is short (approximately 1 s). (4) The resolution is
extremely high (0.1 ~ 0.005 cm-1). (5) The scan range is wide (1000 ~ 10 cm-1). (6)
The interference from stray light is reduced. Due to these advantages, FTIR
Spectrometers have replaced dispersive IR spectrometers. A block diagram of a FTIR
spectrometer is shown in Figure 1.27. A common FTIR spectrometer consists of a
source, interferometer, sample compartment, detector, amplifier, A/D convertor, and a
computer. The source generates radiation which passes the sample through the
45
Chapter 1
Semiconductor metal oxides as humidity sensors
interferometer and reaches the detector. Then the signal is amplified and converted to
digital signal by the amplifier and analog-to-digital converter, respectively.
Eventually, the signal is transferred to a computer in which Fourier transform is
carried out.
Figure 1.27: is a block diagram of an FTIR spectrometer.
A solution was developed which employed a very simple optical device called
an interferometer. The interferometer produces a unique type of signal which has all
of the infrared frequencies “encoded” into it. The signal can be measured very
quickly, usually on the order of one second or so. Thus, the time element per sample
is reduced to a matter of a few seconds rather than several minutes. Most
interferometers employ a beam splitter which takes the incoming infrared beam and
divides it into two optical beams. One beam reflects off of a flat mirror which is fixed
in place. The other beam reflects off of a flat mirror which is on a mechanism which
allows this mirror to move a very short distance (typically a few mm) away from the
beam splitter. The two beams reflect off of their respective mirrors and are
recombined when they meet back at the beam splitter. Because the path that one beam
travels is a fixed length and the other is constantly changing as its mirror moves, the
signal which exits the interferometer is the result of these two beams “interfering”
with each other. The resulting signal is called an interferogram which has the unique
property that every data point which makes up the signal has information about every
infrared frequency which comes from the source. This means that as the interferogram
is measured; all frequencies are being measured simultaneously. Thus, the use of the
46
Chapter 1
Semiconductor metal oxides as humidity sensors
interferometer results in extremely fast measurements. Because the analyst requires a
frequency spectrum in order to make identification, the measured interferogram signal
cannot be interpreted directly. A means of “decoding” the individual frequencies is
required. This can be accomplished via a well-known mathematical technique called
the Fourier transformation. This transformation is performed by the computer which
then presents the user with the desired spectral information for analysis. A Figure 1.28
shows the diagram of a Fourier-transform infrared spectrometer.
Figure 1.28: Diagram of a Fourier-transform infrared spectrometer.
1.11.8 Atomic Force Microscopy
Atomic force microscopy (AFM) or scanning force microscopy (SFM) is a
very high-resolution type of scanning probe microscopy, with demonstrated resolution
on the order of fractions of a nanometer, more than 1000 times better than the limit.
Binnig, Quate and Gerber invented the first AFM in 1986 [90-91]. The first
commercially available AFM was introduced in 1989. The information is gathered by
"feeling" the surface with a mechanical probe. Piezoelectric elements that facilitate
tiny but accurate and precise movements on electronic command enable the very
precise scanning. Beam deflection system, using a laser and photodector to measure
the beam position as shown in Figure 1.29. The AFM consists of a cantilever with a
sharp tip (probe) at its end that is used to scan the specimen surface. The cantilever is
typically silicon or silicon nitride with a tip radius of curvature on the order of
nanometers. When the tip is brought into proximity of a sample surface, forces
between the tip and the sample lead to a deflection of the cantilever according to
47
Chapter 1
Semiconductor metal oxides as humidity sensors
Hooke's law. Depending on the situation, forces that are measured in AFM include
mechanical contact force, Vandar Waals forces, capillary forces, chemical bonding,
electrostatic forces, magnetic forces, casmir forces, salvation forces, etc. The parts of
an approach-retraction cycle of the tip area shown in Figure 1.30. The deflection is
measured using a laser spot reflected from the top surface of the cantilever into an
array of photodiodes. Other methods that are used include optical interferometry,
capacitive sensing or piezoresistive AFM cantilevers. These cantilevers are fabricated
with piezoresistive elements that act as a strain gauge. Using a Wheatstone bridge,
strain in the AFM cantilever due to deflection can be measured. If the tip was scanned
at a constant height, a risk would exist that the tip collides with the surface, causing
damage. Hence, in most cases a feedback mechanism is employed to adjust the tip-tosample distance to maintain a constant force between the tip and the sample.
Traditionally, the sample is mounted on a piezoelectric tube that can move the sample
in the z direction for maintaining a constant force, and the x and y directions for
scanning the sample. Alternatively a 'tripod' configuration of three piezo crystals may
be employed, with each responsible for scanning in the x, y and z directions. This
eliminates some of the distortion effects seen with a tube scanner. In newer designs,
the tip is mounted on a vertical piezo scanner while the sample is being scanned in X
and Y using another piezo block. The resulting map of the area s = f(x, y) represents
the topography of the sample. The AFM can be operated in number of modes,
depending on the application; such as static or contact modes a variety of dynamic or
non-contact mode.
1.11.8.1 AFM Modes of Operation
(i) Contact Mode
As the tip is raster-scanned across the surface, it is deflected as it moves over
the surface corrugation. In constant force mode, the tip is constantly adjusted to
maintain a constant deflection, and therefore constant height above the surface. This
adjustment is displayed as data. However, the ability to track the surface in this
manner is limited by the feedback circuit. Sometimes the tip is allowed to scan
without this adjustment, and one measures only the deflection. This is useful for
small, high-speed atomic resolution scans, and is known as variable-deflection mode.
Because the tip is in hard contact with the surface, the stiffness of the lever needs to
be lesser than the effective spring constant holding atoms together, which is on the
48
Chapter 1
Semiconductor metal oxides as humidity sensors
order of 1-10 N/m. Most contact mode levers have a spring constant of <1N/m. The
contact mode of AFM is shown in the Figure 1.31.
Figure 1.29: Beam deflection system, using a laser and photodector to measure the
beam position.
Figure 1.30: The parts of an approach-retraction cycle of the tip.
49
Chapter 1
Semiconductor metal oxides as humidity sensors
(ii) Non-contact Mode
The non-contact mode of AFM is shown in Figure 1.31. In this mode, the tip
of the cantilever does not contact the sample surface. The cantilever is instead
oscillated at a frequency slightly above its resonance frequency where the amplitude
of oscillation is typically a few nanometers (<10 nm). The Van Der Waals forces,
which are strongest from 1 to 10 nm above the surface, or any other long range force
which extends above the surface acts to decrease the resonance frequency of the
cantilever. This decrease in resonance frequency combined with the feedback loop
system maintains a constant oscillation amplitude or frequency by adjusting the
average tip-to-sample distance. Measuring the tip-to-sample distance at each (x, y)
data point allows the scanning software to construct a topographic image of the
sample surface. Non-contact mode AFM does not suffer from tip or sample
degradation effects that are sometimes observed after taking numerous scans with
contact AFM. This makes non-contact AFM preferable to contact AFM for measuring
soft sample. In the case of rigid samples, contact and non-contact images may look
the same. However, if a few monolayers of adsorbed fluid are lying on the surface of
a rigid sample, the images may look quite different. An AFM operating in contact
mode will penetrate the liquid layer to image the underlying surface, whereas in noncontact mode an AFM will oscillate above the adsorbed fluid layer to image both the
liquid and surface. Schemes for dynamic mode operation include frequency
modulation and the more common amplitude modulation. In frequency modulation,
changes in the oscillation frequency provide information about tip-sample
interactions. Frequency can be measured with very high sensitivity and thus the
frequency modulation mode allows for the use of very stiff cantilevers. Stiff
cantilevers provide stability very close to the surface and, as a result, this technique
was the first AFM technique to provide true atomic resolution in ultra-high vacuum
conditions.
(iii) Tapping Mode
In this mode, the cantilever is driven to oscillate up and down at near its
resonance frequency by a small piezoelectric element mounted in the AFM tip holder
similar to non-contact mode. However, the amplitude of this oscillation is greater than
10 nm, typically 100 to 200 nm. Due to the interaction of forces acting on the
cantilever when the tip comes close to the surface, Vander Waals force, dipole-dipole
50
Chapter 1
Semiconductor metal oxides as humidity sensors
interaction, electrostatic forces etc. cause the amplitude of this oscillation to decrease
as the tip gets closer to the sample.
Figure 1.31
1.11.9 Porosimetry
Porosimetry is an analytical tool used to determine various quantifiable
aspects
of
a
material's porous nature,
such
as pore
diameter,
total pore
volume, surface area, and bulk and absolute densities. The technique involves the
intrusion of a non-wetting liquid (usually mercury) at high pressure into a material
through the use of a porosimeter. The pore size can be determined based on the
external pressure needed to force the liquid into a pore against the opposing force of
the liquid's surface tension. A force balance equation known as Washburn's
equation for the above material having cylindrical pores is given as:
𝑷𝑳 − 𝑷𝑮 =
𝟒𝝈 𝒄𝒐𝒔𝜽
𝑫𝑷
Where PL = pressure of liquid, PG = pressure of gas, σ = surface tension of liquid, θ =
contact angle of intrusion liquid, DP = pore diameter.
Since the technique is usually done under vacuum, the gas pressure begins at
zero. The contact angle of mercury with most solids is between 135º and 142º, so an
51
Chapter 1
Semiconductor metal oxides as humidity sensors
average of 140º can be taken without much error. The surface tension of mercury at
20ºC under vacuum is 480 mN/m. With the various substitutions, the equation
becomes:
𝑫𝑷 =
𝟏𝟒𝟕𝟎𝒌𝑷𝒂 . 𝝁𝒎
𝑷𝑳
As pressure increases, so does the cumulative pore volume. From the cumulative pore
volume, one can find the pressure and pore diameter where 50% of the total volume
has been added to give the median pore diameter.
1.12 Object of the present investigation
There are a large number of materials and techniques used for humidity
sensing measurements depending upon the purpose of measurement and the
environment in which these measurements are made. The humidity sensors based on
electrical methods are best suited for high sensitivity and compactness of the device.
Also since the number of humidity sensitive materials is based on inorganic acetates,
halides, nitrates, sulphates, carbonates, phosphates and oxides, the choice of a suitable
material is difficult to make.
We have been interested in carrying out our investigations with material that
possess good sensitivity over the entire range of RH, i.e. from 10 to 90 %RH with
properties which are stable over time and thermal cycling after exposure to the
various chemical species likely to be present in the ambient.
Out of above mentioned humidity sensitive compounds, metal oxides were
considered to be the most promising materials for study because of their inherent
chemical and physical stability. Earlier investigations have however, shown that tin
oxide is one of the promising material among metal oxides for use in humidity and
gas sensors.
Tin dioxide (SnO2) is an n-type broad-band gap (3.6 eV) oxide semiconductor
with high chemical and mechanical stabilities [68]. SnO2, which has outstanding
optical, electrical and mechanical properties, is a versatile material and is also widely
used as the most attractive materials for gas sensor applications [69], as a catalyst
during the oxidation of organic compounds, as a key component in rechargeable Li
batteries, and as a master element in opto-electronic devices. Porous microstructure of
the materials with controlled pore size is preferred always. The sensitivity and
response time of SnO2 based sensors strongly depend on the porosity of the material.
52
Chapter 1
Semiconductor metal oxides as humidity sensors
In the present work, Tin Oxide nanomaterial and its nanocomposite have been
synthesized by using different routes [70-76] and an extensive investigation has been
carried out with its humidity sensing properties. Pellets and films of SnO2 have been
annealed at different temperatures and exposed to humidity inside a constant humidity
chamber. The variations in electrical resistance with variations in %RH have been
recorded.
1.13 Organization of the Thesis
Chapter 1 introduces the subject of study of the thesis and describes the
objective of the present investigations. It deals with the synthesis and characterization
methods of nanomaterials and their application as Humidity Sensors. The working
principles of different type of Humidity Sensors have also been described. Chapter 2
deals with synthesis of SnO2 nanomaterials through mechanochemical route, its
characterization and application as Humidity Sensor. Again in order to improve the
sensitivity of sensor, CuO has been doped in SnO2 and its humidity sensing properties
has been investigated thoroughly in Chapter 3. Chapter 4 deals with synthesis and
characterization of antimony doped tin oxide via chemical precipitation method and
their application as Humidity Sensor. Chapter 5 deals with synthesis and
characterization of zinc doped tin oxide via chemical precipitation method and their
application as Humidity Sensor. Conclusions and scope of further research work
arrived at on the basis of the studies made in the thesis are described in Chapter 6.
53