chapter - iii preparation of zno...

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CHAPTER - III

PREPARATION OF ZnO NANORODS3.1 Introduction

Semiconductors with dimensions in the nanometer realm are important because

their electrical, optical and chemical properties can be tuned by changing the size of the

particles. These nanostructures have attracted much interest due to their fundamental

importance in bridging the gap between bulk matter and molecular species [1, 2]. Optical

properties of nanoparticles are of great interest for application in optoelectronics,

photovoltaics and biological sensing.

Advances in the synthesis of highly luminescent semiconductor nanocrystals

currently allow their extensive applications in different fields, ranging from

optoelectronic to bio-imaging. Depending on the applications, surface modifications of

semiconductor nanocrystals are essential. For example, a water soluble surface is

necessary for biological labels. An electron conductive layer is important for solar cells.

In recent years, one-dimensional (1D) nanostructures, such as nanorods, nanowires,

nanobelts and nanotubes have attracted much attention due to their potential applications

in a variety of novel nanodevices, such as field-effect transistors, single-electron

transistors, photodiodes, and chemical sensors. Various chemical synthetic methods have

been developed to prepare such nanostructures.

Zinc oxide is an inorganic compound with the formula ZnO. It usually appears as

a white powder, nearly insoluble in water. The powder is widely used as an additive into

numerous materials and products including plastics, ceramics, glass, cement, rubber (e.g.

car tyres), lubricants, paints, ointments, adhesives, sealants, pigments, foods (source of

Zn nutrient), batteries, ferrites, fire retardants, etc. ZnO is present in the earth crust as a

mineral zincite; however, most ZnO used commercially is produced synthetically.

In material science, ZnO is often called a II-VI semiconductor because zinc and

oxygen belong to the 2nd and 6th groups of the periodic table, respectively. This

semiconductor has several favorable properties: good transparency, high electron

mobility, wide band gap, strong room temperature luminescence, etc. Those properties

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are already used in emerging applications for transparent electrodes in liquid crystal

displays and in energy-saving or heat-protecting windows and electronic applications of

ZnO as thin-film transistor and light - emitting diodes.

Zinc oxide (ZnO) is a unique material with a direct band gap (3.37eV) and large

exciton binding energy of 60 meV [3, 4], which makes the exciton state stable even at

room temperature. ZnO has been widely used in near-UV emission, gas sensors,

transparent conductor, thin film transistors and piezoelectric applications [5–7]. Zinc is

also a very important trace element in humans [8]. The average adult body contains 3.0-

4.5 x 10−2 mmol (2-3 g) of zinc, which is found in muscle, bone, skin and plasma.

Significant amounts occur in the liver, kidney, eyes, hair etc. Zinc has been found to play

an important part in many biological systems. Therefore, ZnO is environmentally friendly

and suitable for in- vivo bio-imaging and cancer detection. Solid state white light

emitting sources based on the existing Si-based technology is one of the challenging

goals in the field of display and light emitting technology. The potential applications of

solid state white light emitting sources are immense because of their distinctive properties

like low power consumption, high efficiency, long lifetime, reduced operating costs and

free from toxic mercury. To generate white light, a blend of phosphors that emits in blue,

green and red wavelengths are required. This difficulty could be overcome by using

semiconducting nano crystalline luminescent materials, which have larger molar

absorption and exhibit broadband emission in the visible spectrum without self-

absorption. Since nanostructures possess an enormous surface-to-volume ratio, they are

suitable candidates for defect emission.

ZnO commonly exhibits luminescence in the visible spectral range due to different

intrinsic defects. This opens the possibility of use of ZnO for solid state lighting.

Controlled incorporation of defects generally involves high temperature annealing [9],

substitution [10], mechanical milling [11] etc. Due to its excellent stability and the defect

emission, ZnO has been widely used as a phosphor in vacuum fluorescent displays

(VFD). Most of the ZnO crystals have been synthesized by traditional high temperature

solid state reaction which is energy consuming and difficult to control the particle


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properties. ZnO nanoparticles [12, 13] can be prepared on a large scale at low cost by

simple solution based methods, such as chemical precipitation [14, 15], sol-gel synthesis

[16], and solvothermal/hydrothermal reaction [12, 13]. Hydrothermal technique is a

promising method because of the low process temperature and very easy to control the

particle size. The hydrothermal process have several advantages over other growth

processes 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 [17]. This method has also been successfully employed to prepare nano-scale

ZnO and other luminescent materials. The particle properties such as morphology and

size can be controlled via the hydrothermal process by adjusting the reaction temperature,

time and concentration of precursors.

3.2 Comparison of different semiconductors:

ZnO was one of the first semiconductors to be prepared in rather pure form after

silicon and germanium. It was extensively characterized as early as the 1950’s and 1960’s

due to its promising piezoelectric / acoustoelectric properties. Wide band gap

semiconductors have gained much attention during last decade because of their possible

uses as optoelectronic devices in the short wavelength and ultraviolet (UV) portion of the

electromagnetic spectrum. These semiconductors such as ZnSe, ZnS, GaN, and ZnO,

have shown similar properties with their crystal structures and band gaps. Some of the

important properties of these wide band gap semiconductors are summarized in table 3.1.

Initially, ZnSe based devices and the GaN based technologies obtained large

improvements such as blue and UV light emitting diode and injection laser. ZnSe has

produced some defect levels under high current drive. No doubt, GaN is considered to be

the best candidate for the optoelectronic devices. However, ZnO has great advantages for

light emitting diodes (LEDs) and laser diodes (LDs) over the currently used

semiconductors. ZnO as II–VI semiconductor is promising for various technological


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applications, especially for optoelectronic short wavelength light emitting devices due to

its wide and direct band gap.

The most important advantage is the high exciton binding energy (60 meV) giving

rise to efficient exitonic emission at room temperature. Since ZnO and GaN have almost

identical lattice parameters and the same hexagonal wurtzite structure, ZnO can

satisfactorily be used as lattice matched substrate in GaN based devices or vice versa.

ZnO has excellent radiation hardness among all other semiconductors. This property

facilitates the uses of ZnO based devices in space applications and high energy radiation

environments. Band gap energy can be varied from 3.3 eV to 4.5 eV with alloying

process. Hence it can be used as an active layer in the doubly confined hetero-structured

LEDs and quantum well lasers. These unique nanostructures unambiguously demonstrate

that ZnO is probably the richest family of nanostructures among all materials, both in

structure and properties [18,19].

Table 3.1: Comparison of different semiconductors

Wide bandgap semi-conductor

Crystalstructure

Latticeparameter

(Å)

Eg(eVat

RT)

Melting

temp.(K)

Excitation

bindingenergy(meV)

Dielectricconstant

a b Ɛ0 Ɛ∞

ZnO Wurtzite 3.250 5.206 3.37 2248 60 8.75 3.72GaN Wurtzite 31.89 51.85 3.4 1973 21 9.5 5.15ZnSe Zinc blend 5.667 - 2.7 1790 20 7.1 5.3ZnS Wurtzite 3.824 6.261 3.7 2103 36 9.6 5.7

3.3 Fundamental properties of ZnO

3.3.1 Crystal structure

Zinc oxide crystallizes in three forms: hexagonal wurtzite, cubic zinc blend, and

the rarely observed cubic rock salt. The wurtzite structure shown in fig 3.1 is most stable

and most common at ambient conditions. The zinc blend form can be stabilized by


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growing ZnO on substrates with cubic lattice structure. In both cases, the zinc and oxide

are tetrahedral. The rock salt NaCl-type structure is only observed at relatively high

pressures - ~10 GPa. The hexagonal and cubic zinc blend ZnO lattices have no inversion

symmetry (reflection of a crystal relatively any given point does not transform it into

itself). This and other lattice symmetry properties result in piezoelectricity of the

hexagonal and zinc blend ZnO, and in pyro electricity of hexagonal ZnO. The lattice

constants are a = 3.25 Å and c = 5.2 Å; their ratio c/a ~ 1.60 is close to the ideal value for

hexagonal cell with c/a = 1.633. As in most II-VI materials, the bonding in ZnO is largely

ionic, which explains its strong piezoelectricity. Due to this ionicity, zinc and oxygen

planes bear electric charge (positive and negative, respectively). Therefore, to maintain

electrical neutrality, those planes reconstruct at atomic level in most relative materials,

but not in ZnO - its surfaces are atomically flat, stable and exhibit no reconstruction. This

anomaly of ZnO is not fully explained yet.

Fig 3.1: Schematic representation of a wurtzitic ZnO structure.

3.3.2 Basic physical properties of ZnO

Table 3.2 shows a compilation of the basic physical parameters of ZnO. Still

some uncertainty exists in these values. For example, in few reports it has been


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mentioned physical properties of only p-type ZnO and therefore the hole mobility and

effective mass are still in debates [19, 20].

Table 3.2: Basic physical properties of ZnO

Property ValueLattice parameter at 300 K

a 3.2495 Åc 5.2069 Å

c/a 1.602Density 5.606 g cm-3

Stable phase at 300 K WurtziteBond length 1.977 μm

Melting point 1975°CThermal conductivity 0.6 , 1-1.2

Static dielectric constant 8.656Refractive index 2.008, 2.029

Energy gap 3.4 eV, directExciton binding energy 60 meV

Iconicity 62%Heat capacity Cp 9.6cal/mol K

Youngs modulus E (Bulk ZnO) 111.2 ± 4.7 Gpa

3.3.3 Optical Properties

The optical properties of a semiconductor are associated with both intrinsic and

extrinsic effects. Intrinsic optical transitions take place between the electrons in the

conduction band and holes in the valence band, including exitonic effects due to the

Coulomb interaction. The main condition for exciton formation is that the group velocity

of the electron and hole is equal. Excitons are classified into free and bound excitons. In

high quality samples with low impurity concentrations, the free exciton can also exhibit

excited states, in addition to their ground-state transitions. Extrinsic properties are related

to dopants or defects, which usually create discrete electronic states in the band gap, and

therefore influence both optical absorption and emission processes. As we mentioned

above, that ZnO is a direct band semiconductor and a transparent conductive material.


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ZnO films are transparent in the wavelength range of 0.3 and 2.5 μm, and plasma edge

lies between 2 and 4 μm depending on the carrier concentration. It is well known that a

shift in the band gap edge appears with an increase in the carrier concentration. This shift

is known as Burstein-Moss shift. Optical transitions in ZnO have been studied by a

variety of experimental techniques such as optical absorption, transmission, reflection,

spectroscopic. ellipsometry, photoluminescence, cathodoluminescence, calorimetric

spectroscopy, etc. Room temperature PL spectrum of ZnO is usually composed of a near

UV-emission band (375 nm) and a green emission band (510 nm) although a yellow-

orange band (610 nm) can also be observed in some situations. The near UV-band is

closely related to the excitonic nature of the material and may be superposed with the free

exciton emission, its phonon replica, bound exciton emission, as well as bi exciton

emission. The observation of luminescence from exciton is usually difficult even at low

temperatures. This comes from a lot of factors [21]: First, the efficiency of radiative

emission is low even for direct gap semiconductors which are found to be 10-1 to 10-3. A

large part of the radiative emission comes from bound-exciton complexes and defect

centers. Secondly, exciton emission is limited by the internal reflection of the exciton and

the small escape length. As a quasi-particle, exciton moves with their group velocity

through the semiconductor. During its movement, exciton can be trapped or scattered by

impurities and phonons. When it eventually reaches the surface of the semiconductor, in

most cases, it will be reflected back into the semiconductor. Except the internal

reflection, the radiative combination yield from free-exciton is also limited by the small

escape length, which is defined as the depth from which exciton can reach the surface.

Only the free-exciton inside the escape length can have the contribution to the

luminescence. The research interest for the green band emission in ZnO can be traced

back to the early stage of last century. Due to this green emission, ZnO is considered as

an important luminescent material for the planar display and short-decay

cathodoluminescence screens. Unfortunately, the mechanisms behind this emission band

are still unclear even though the researches on this topic have been lasted for many years.

Green band emission was first attributed to an excess of zinc. Almost all the proposed


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mechanisms about the green emission are attributed to the native lattice defects except

the one that is based on the divalent Cu impurities.

3.3.4 Thermal Properties

Thermal expansion coefficient (TEC)

The change in temperature affects the lattice parameters of semiconductors. Thermal

expansion coefficients are defined as αa and αc for in and out of plane cases, respectively.

The stochiometry, presence of extended defects and free carrier concentration also affect

the thermal expansion coefficient. The X-ray powder diffraction method by Reeber was

used to measure the temperature dependence of lattice parameters of ZnO as shown in

Fig 3.2. Lattice parameters of ZnO were measured over the temperature range 4.2 - 299

K, fourth-order polynomials were fitted using the least-squares method, which gives the

minimum for the a0 parameter at 93 K. The c0 parameter has much uncertainty, did not

give any minimum value, perhaps due to its less precision and uncertainty in

measurement [22].

Fig 3.2. Wurtzite ZnO lattice parameters as a function of temperature.

Thermal conductivity

Thermal conductivity (k), having a kinetic nature, is determined by vibration,

rotation and electronic degree of freedom. It is really important property of

semiconductors when these materials are used in high-power, high-temperature or


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optoelectronic devices. The electronic thermal conductivity is very small, having light

carrier concentration, which is negligible. For high pure crystals, phonon-phonon

scattering is ideally proportional toT-1 at the temperatures higher than the Debye

temperature. Point defects, such as vacancies, impurities and isotope fluctuations in ZnO

affect the thermal conductivity of ZnO material.

Fig 3.3 Thermal conductivity of fully sintered ZnO heated from room temperature to1000 °C

Fig.3.3 shows the thermal conductivity curve for a fully sintered ZnO crystal. The

thermal conductivity decreases from 37 to 4 W/m K as the temperature is increased from

room temperature to 1000°C. The dominant scattering mechanism is resistive phonon-

phonon interactions [23].

3.3.5 Electrical Properties

As a direct and wide band gap semiconductor with a large exciton binding energy

(60meV), ZnO is representing a lot of attraction for optoelectronic and electronic devices.

For example, a device made by material with a larger band gap may have a high

breakdown voltage, lower noise generation and can operate at higher temperatures with

high power operation. The performance of electron transport in semiconductor is


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different at low and high electric field. At sufficient low electric fields, the energy

distribution of electrons in ZnO is unaffected much, because the electrons can't get much

energy from the applied electrical field, as compared with their thermal energy. So the

electron mobility will be constant because the scattering rate, which determines the

electron mobility, doesn't change much. When the electrical field is increased, the energy

of the electrons from the applied electrical field is equivalent to the thermal energy of the

electron. The electron distribution function changes significantly from its equilibrium

value. These electrons become hot electrons, whose temperature is higher than the lattice

temperature. So there is no energy loss to the lattice during a short and critical time.

When the electron drift velocity is higher than its steady state value, it is possible to make

a higher frequency device.

3.3.6 Chemical Properties

ZnO occurs as white powder commonly known as zinc white or as the mineral

zincite. The mineral usually contains a certain amount of manganese and other elements

and is of yellow to red color. Crystalline zinc oxide is thermo-chromic, changing from

white to yellow when heated and in air reverting to white on cooling. This is caused by a

very small loss of oxygen at high temperatures to form the non-stoichiometric Zn1+xO,

where at 800 °C, x= 0.00007. Zinc oxide is an amphoteric oxide. It is nearly insoluble in

water and alcohol, but it is soluble in (degraded by) most acids, such as hydrochloric

acid:

ZnO + 2 HCl → ZnCl2 + H2O

Bases also degrade the solid to give soluble zincates:

ZnO + 2NaOH + H2O → Na2(Zn(OH)4)

ZnO reacts slowly with fatty acids in oils to produce the corresponding

carboxylates, such as oleate or stearate. ZnO forms cement-like products when mixed

with a strong aqueous solution of zinc chloride and these are best described as zinc

hydroxy chlorides. This cement was used in dentistry. ZnO also forms cement-like

products when reacted with phosphoric acid, and this forms the basis of zinc phosphate


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cements used in dentistry. A major component of zinc phosphate cement produced by this

reaction is hopeite, Zn3(PO4)2·4H2O. ZnO decomposes into zinc vapor and oxygen only

at around 1975 °C, reflecting its considerable stability. Heating with carbon converts the

oxide into zinc vapor:

ZnO + C → Zn + CO

Zinc oxide reacts violently with aluminum and magnesium powders, with chlorinated

rubber and linseed oil on heating causing fire and explosion hazard. It reacts with

hydrogen sulfide to give the zinc sulfide: this reaction is used commercially in removing

H2S using ZnO powder (e.g., as deodorant).

ZnO + H2S → ZnS + H2O

When ointments containing ZnO and water are melted and exposed to ultraviolet light,

hydrogen peroxide is produced.

3.4 Doping and defects in ZnO

In the recent years, much attention has been focused on wide band gap

semiconductors materials because of their excellent potential for blue light emitting

devices, short-wavelength laser diodes and detectors in UV-blue spectral region. As wide

band gap ZnO is gaining much importance for the possible application due to the

capability of ultraviolet lasing at room temperature and possibilities to engineer the band

gap. In order to attain the potential offered by ZnO, both high-quality n-and p-type ZnO

are essential. But it is very difficult to obtain the bipolar carrier doping (both n and p

types) in wide-band-gap semiconductors such as GaN and II-VI compound

semiconductors including ZnS, ZnSe, and ZnTe. Unipolar doping has not been a

surprising issue in wide-band-gap semiconductors: ZnO, GaN, ZnS, and ZnSe are easily

doped to n-type , while p-type doping is difficult. All undoped ZnO to date has been

found to be n-type, with donor concentrations typically around 1017 cm-3 for present-day

but high-quality material, but sometimes as highly doped material. The situation is

opposite for ZnTe where p-type doping is easily obtained, while n-type doping is

difficult. The main characterization techniques used to find the shallow electrical defects


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in semiconductor materials are photoluminescence and temperature dependent hall effect

measurements.

n-type doping

ZnO has wurtzite structure, where zinc is found excess in ZnO. Due to this excess

of zinc, ZnO is a non-stochiometric compound and n-type semiconductor. Undoped ZnO

shows intrinsic n-type conductivity with high electron densities of about 1021 cm-3. Zinc

interstitials Zni and the oxygen vacancy V are known to be the dominant native donor in

unintentional ZnO film. But still it is debatable issue. Photoluminescence and

temperature dependent Hall studies of electron irradiated ZnO have shown that zinc is the

most likely candidate for purely lattice-related dominant shallow donor, with an

activation energy about 30-50 meV. It has been argued that the n-type conductivity of

unintentionally doped ZnO film is only due to hydrogen (H), which is treated as a

shallow donor with activation energy of 31 meV instead of Zni. This assumption is valid

because hydrogen (H) is always present in all growth methods and can easily diffuse into

ZnO in large amounts due to it large mobility [24]. Hydrogen has been considered as a

shallow donor candidate, much research has been done on hydrogen (H) in ZnO. During

seeded chemical vapor transport (SCVT) growth of ZnO, it has been shown that

hydrogen with activation energy 39 meV acts as main donor. This donor disappears

through on annealing process. [24].

The n-type doping of ZnO is relatively easy as compared to p-type doping. Group

III elements Al, Ga and In as substitutional elements for Zn and group-VII elements Cl

and I as substitutional elements for O can be used as n-type dopants. Doping with Al, Ga,

and In has been attempted by many groups, resulting in high-quality, highly conductive

n- type ZnO films. Al-doped ZnO films grown by MOCVD method is highly conductive

with minimum resistivity as compared to Ga- doped ZnO films grown by chemical-vapor

deposition [18].

p-type doping

It is very difficult to obtain p-type doping in wide band gap semiconductors.

Acceptors in ZnO can also take place from both lattice defects and impurity atoms. The


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oxygen interstitial Oi and zinc vacancy Vzn are both known to be acceptors in ZnO. The

p-type doping in ZnO may be possible by substituting either group-I elements (Li, Na,

and K) for Zn sites acting as shallow acceptors and group-V elements (N, P, and As) are

found to act as deep acceptors on O sites. It was shown that group-I elements could be

better p-type dopants than group-V elements in terms of shallowness of acceptor levels.

However, group-I elements tend to occupy the interstitial sites, due to their small atomic

radii, rather than substitutional sites, and therefore, they act as donors instead of

acceptors. Moreover, significantly larger bond length for Na and K than ideal Zn–O bond

length (1.93 Å) induces lattice strain, increasingly forming native defects such as

vacancies which compensate the shallow dopants. Group V elements (N, P, As) except N,

both P and As, have a larger bonds lengths. That’s why they are likely to form antisites to

avoid the lattice strain. Unfortunately for p-conduction these elements have a tendency

towards antisite formation, i.e. they can substitute not only oxygen but also zinc atoms, in

which case they act as donors. Nitrogen (N) appears to be good candidate for a shallow

p-type dopant in ZnO with smallest ionization energy, although N is not soluble in ZnO,

and doping can be achieved by ion implantation [18, 25].

3.5 Experimental techniques

3.5.1 Introduction

A thin film is defined as a low-dimensional material created by condensing, one-

by-one, atomic/molecular/ionic species of matter. The thickness is typically less than a

few micrometers. Thin films differ from thick films. A thick film is defined as a low-

dimensional material created by thinning a three-dimensional material or assembling

large clusters/aggregates/ grains of atomic/molecular/ionic species [26]. Thin film

technology is one of the oldest arts and one of the newest sciences. Thin films have been

used for more than a half century in making electronic devices, optical coatings, hard

coatings, and decorative parts.

The first thin films were probably the deposits which Faraday [27] obtained in

1857 when he exploded metal wires in an inert atmosphere. The birth of thin films of all


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materials created by any deposition technique starts with a random nucleation process

followed by nucleation and growth stages. Nucleation and growth stages are dependent

upon various deposition conditions, such as growth temperature, growth rate, and

substrate chemistry. The nucleation stage can be modified significantly by external

agencies, such as electron or ion bombardment. Film microstructure, associated defect

structure, and film stress depend on the deposition conditions at the nucleation stage. The

basic properties of film, such as film composition, crystal phase and orientation, film

thickness, and microstructure are controlled by the deposition conditions.

Thin films exhibit unique properties that cannot be observed in bulk materials. The

properties of thin films are governed by the deposition method. Almost all thin film

deposition and processing methods are employed to characterize and measure the

properties of films. Depending upon the applications the properties of thin films can be

divided into six basic categories. They are optical, electrical, magnetic, chemical,

mechanical and thermal properties [28]. Often multiple properties are obtainable

simultaneously.

Thin films used in the semiconductor industry are deposited by a variety of

techniques. In this chapter, some of the deposition techniques usually employed to

deposit ZnO thin films and other films commonly used for photo catalytic applications

(as in this thesis) are briefly presented. As the drive to reduce production costs and the

desire to develop flexible, large-area electronics continues, the need for integrating cheap

substrates (such as glass) in the device fabrication process cannot be over emphasized.

The low melting temperature of glass places a restraint role on the device processing

temperature. In the following sections, some of the commonly used thin film deposition

processes are discussed, with particular emphasis on the dip coating process as it has

been used to deposit the ZnO semiconductor used in this work. The rest of the topics in

the chapter are devoted to the preparation techniques used to prepare the ZnO nano rods.


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3.5.2 Thin film deposition techniques

3.5.2.1 Introduction

A wide choice of preparation techniques is open to the researchers who are

interested in thin films. Thin film properties are strongly dependent on the methods of

deposition, the substrate materials, the substrate temperature, the rate of deposition and

the background pressure. The application and the properties of the given material

determine the most suitable technique for the preparation of thin films of that material.

Now-a-days there are number of deposition methods available for depositing thin films.

However, every method has its own specific limitations and involves compromises with

respect to process specifications, substrate material limitations, expected film properties

and cost. So it makes it difficult to select the best technique for any specific application.

The basic steps involved in thin film deposition techniques [28] are,

To prepare the source material.

To transport the material to the substrate.

To deposit the thin film on the substrate.

To anneal the film.

To analyze the film to evaluate the process.

The results of the analysis are then used to adjust the conditions of other steps for

film property modification. Additional process control and understanding are obtained by

monitoring the first three steps during film deposition. Analysis of the film deposition is

the final stage of the process monitoring. Monitoring is important at all steps in the thin

film process. The more monitoring that can be done during deposition, the better will be

both the control and understanding of the process. Preparation of thin films is being

advanced by numerous physical and chemical techniques.

Nano structured thin films are synthesized using a combination of approaches, for

example melting and solidification process followed by thermo dynamical treatments, or

solution/vacuum deposition. In many cases however the final product is dictated by the

kinetics of thermodynamics of systems.


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Fig 3.4 Classification of the most common deposition process

There are basically two broad areas of synthesis techniques for nano structured

thin films namely physical methods and chemical methods [42]. Classification of most

common deposition techniques are given in fig. 3.4. Each of the above methods has its

own merits and demerits.

3.5.2.2 Physical methods

Several physical methods are currently in use for the synthesis and commercial

production of nano structured materials. The PVD technique involves the physical

removal of atoms or molecules from the surface of a source material and the subsequent

deposition of a solid material onto a substrate. The physical removal of the materials is

commonly-achieved through one of the following methods:

- Evaporative deposition: The material to be deposited is heated until a high vapour

pressure is reached by an electrically resistive heating element [30]. Thermal evaporation

is achieved by electrically vapourising the solid source material in a high-vacuum


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environment using a heating element where the vapour is allowed to condense on the

substrate surface.

- Inert gas evaporation technique: The generation of atom clusters by gas phase

condensation proceeds by evaporating a pre-cursor material, either a single metal or a

compound, in a gas maintained at a low pressure, usually below 1 atm. The evaporated

atoms or molecules undergo a homogeneous condensation to form clusters via collisions

with gas atoms or molecules in the vicinity of a cold-powder collection surface. The

clusters once formed must be removed from the region of deposition to prevent further

aggregation and coalescence of the clusters [31].

- Electron/ion beam evaporation: The material to be deposited is heated until a high

vapour pressure is reached by electron/ion bombardment in a "high" vacuum [32]. The

electron beam is generated by an electron gun, which uses thermionic emission of

electrons produced by heated tungsten filament. Emitted electrons are accelerated

towards an anode by a high difference of potential and a transverse magnetic field which

serves to deflect the electron beam towards a crucible.

- Sputter deposition: A glow discharge (plasma) usually localised around the "target" by a

series of magnets (as in a magnetron sputtering process) bombards the material, removing

some of the target surface atoms away as a vapour [33, 34]. This method involves the

ejection of atoms or clusters of designated materials by subjecting them to an accelerated

and highly focused beam of inert gas such as argon or helium. Sputtering process

produces films with better controlled composition, provides films with greater adhesion

and homogeneity and permits better control of film thickness.

- Pulsed laser deposition: A high power laser ablates material from the target surface into

a vapour. The atoms or molecules eroded are then transported through low pressure (to

enhance their mean free path) onto the substrate surface where condensation takes place

and the film is formed. The main disadvantage of all PVD techniques is the stringent

requirement of a vacuum system.


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3.5.2.3 Chemical methods

Chemical vapor deposition (CVD) is a chemical process used to produce high-

purity, high-performance solid materials. The process is often used in the semiconductor

industry to produce thin films. In a typical CVD process, the wafer (substrate) is exposed

to one or more volatile precursors, which react and/or 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. Micro fabrication processes

widely use CVD to deposit materials in various forms, including mono crystalline,

polycrystalline, amorphous and epitaxial.

A number of forms of CVD are in wide use and are frequently referenced in the

literature. These processes differ in the means by which chemical reactions are initiated

(e.g., activation process) and process conditions.

The classifications of CVD by operating pressure, physical charactereistics and

plasma method as follows.

1) Classified by operating pressure:

Atmospheric pressure CVD (APCVD) – CVD processes at atmospheric pressure.

Low-pressure CVD (LPCVD) – CVD processes at sub atmospheric pressures [35].

Ultrahigh vacuum CVD (UHVCVD) – CVD processes at a very low pressure.

2) Classified by physical characteristics of vapor:

Atomic layer CVD (ALCVD) – Deposits successive layers of different substances

to produce layered crystalline films.

Combustion Chemical Vapor Deposition (CCVD) –Combustion Chemical Vapor

Deposition process is an open-atmosphere, flame-based technique for depositing

high-quality thin films and nano materials.

Hot wire CVD (HWCVD) – also known as hot filament CVD (HFCVD). Uses a

hot filament to chemically decompose the source gases [36].

Metal organic chemical vapor deposition (MOCVD) – CVD processes based on

metal organic precursors.


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Hybrid Physical-Chemical Vapor Deposition (HPCVD) – Vapor deposition

processes that involve both chemical decomposition of precursor gas and

vaporization of a solid source.

3) Plasma methods:

Microwave plasma-assisted CVD (MPCVD) – CVD processes that utilize plasma

to enhance chemical reaction rates of the precursors [37].

Plasma-Enhanced CVD (PECVD) –PECVD processing allows deposition at lower

temperatures, which is critical in the manufacture of semiconductors [38].

Besides the above methods, the different wet chemical methods available for film

preparation are chemical bath deposition method, sol gel method, spin coating method

and dip coating method.

3.5.2.3.1 Chemical Bath Deposition (CBD) method

The chemical bath deposition method is one of the cheapest methods to deposit

thin films and nano materials, as it does not depend on expensive equipment and is a

scalable technique that can be employed for large area batch processing or continuous

deposition. Firstly, in 1933 Bruckman deposited PbS thin film by chemical bath

deposition (CBD) or solution grown method. The major advantage of CBD is that it

requires only solution containers and substrate mounting devices. The one drawback of

this method is the wastage of solution after every deposition. Among various deposition

techniques, chemical bath deposition yields stable, adherent, uniform and hard films with

good reproducibility by a relative simpler process. Chemical bath deposition method is

one of the suitable methods for preparing highly efficient thin films in a simpler manner.

The growth of thin films strongly depends on growth conditions, such as duration of

deposition, composition and temperature of the solution, and topographical and chemical

nature of the substrate. The chemical bath deposition involves four steps.

(1) Equilibrium between the complexing agent and water.

(2) Formation/dissociation of ionic metal-ligand complexes.

(3) Hydrolysis of the chalcogenide source and


50

(4) Formation of the solid [39]

During the step 3, the metal cations are pulled out of the solution by the desired

non-metal species provided through the hydrolysis of the chalcogenide source, to form

the solid film. The kinetics of the step 3 is highly sensitive to the solution pH and

temperature, as well as to the catalytic effects of certain solid species that may be present,

which in turn decides the rate of formation of thin film on the surface of the substrate or

bulk precipitation. During step 2, the formation of complexed metal ions allows control

over the rate of formation of solid metal hydroxides, which competes with step 4 and

which would otherwise occur immediately in the normal alkaline solutions. These steps

together determine the composition, growth rate, microstructure and surface topography

of the resulting thin films.

3.5.2.3.2 Sol-Gel Method

The sol-gel process is a wet chemical technique widely used in the fields of

material science. In this method, a set of chemical reactions irreversibly convert a

homogeneous solution of molecular reactant precursors (sol) into an infinite molecular

weight three dimensional network (gel) filling the same volume as the solution. The sol-

gel solutions are composed of a metal alkoxide and an alcohol. The metal alkoxide

undergoes a hydrolysis reaction followed by condensation polymerization. The alkoxide

species react with alcohol and produces oxide network. This oxide network extends as far

as the hydrolysis conditions permit. The oxide network (gel) so formed is a three

dimensional skeleton with interconnected pores and this can be dried and shrunk to form

a rigid solid.

In this chemical procedure, the 'sol' 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. 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. This can be accomplished in any number of ways.


51

The simplest method is to allow time for sedimentation to occur, and then pour off

the remaining liquid. 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.

Mixtures of precursors can also be used to produce binary or ternary systems each

molecular precursor has its own reaction rate that depends on the parameters such as pH

of the solvent, concentration and temperature. In a single precursor component system,

the final material can be designed to have interconnected nano scale porosity and hence a

high surface area. The percentage of porosity depends on the precursors used and the

solvent employed. One unique property of sol-gel process is the ability to go all the way

from the molecular precursor level to the product level, allowing a better control of the

whole process and the synthesis of tailor made materials for different applications.

3.5.2.3.3 Spin Coating Method

Spin coating is a fast and easy method to generate thin and homogeneous organic

films out of solutions. This method was first described by Meyerhofer et al.[40] using

several simplifications. Schematic of spin coating process is given in fig 3.5.

The four distinct stages of the spin coating process are

Deposition of the coating fluid onto the wafer or substrate

Acceleration of the substrate up to its final, desired, rotation speed

Spinning of the substrate at a constant rate; fluid viscous forces dominate the fluid

thinning behavior

Spinning of the substrate at a constant rate, solvent evaporation dominates the

coating thinning behavior


52

Fig 3.5 Scheme of Spin coating process

3.5.2.3.4 Dip Coating Method

Dip coating techniques can be described as a process where the substrate to be

coated is immersed in a liquid and then withdrawn with a well-defined withdrawal speed

under controlled temperature and atmospheric conditions. The coating thickness is

mainly defined by the withdrawal speed, by the solid content and the viscosity of the

liquid. If the withdrawal speed is chosen such that the sheer rates keep the system in the

Newtonian regime, the coating thickness can be calculated by the Landau-Levich

equation [41].

As shown by James and Strawbridge [42] for an acid catalyzed silicate sol,

thicknesses obtained experimentally fit very well to calculated ones. The interesting part

of dip coating processes is that by choosing an appropriate viscosity the coating thickness

can be varied with high precision from 20 nm up to 50 µm while maintaining high optical

quality. The schematics of a dip coating process are shown in figure 3.6.


53

Fig. 3.6: Stages of the dip coating process: dipping of the substrate into the coatingsolution, wet layer formation by withdrawing the substrate and gelation of the layer by

solvent evaporation

Fig. 3.7 Gelation process during dip coating process, obtained by evaporation of thesolvent and subsequent destabilization of the sol


54

If reactive systems are chosen for coatings, as it is the case in sol-gel type of

coatings using alkoxides or pre-hydrolyzed systems, in the so called sols, the control of

the atmosphere is indispensable. The atmosphere controls the evaporation of the solvent

and the subsequent destabilization of the sols by solvent evaporation, leads to a gelation

process and the formation of a transparent film due to the small particle size in the sols

(nm range) [43]. This is schematically shown in figure 3.7.

3.5.3 Selection of deposition techniques

Various methods were used for the preparation of ZnO thin films as discussed.

Important aspects for the selection of a deposition technique are

The suitability for given coating materials.

The precision of the layer thickness values (which may be increased with

automatic control involving in-situ growth monitoring).

The optical quality of the deposited layers.

The ability of the coatings to withstand high optical intensities.

The uniformity of layer thickness values over a larger area.

The consistency (reproducibility) and stability of obtained refractive indices.

The required substrate temperature.

The time required for the growth.

Considering the above aspects, the dip coating method is simple, inexpensive,

non-vacuum and low temperature technique for synthesizing thin films. Sol-gel dip

coating thin films are used extensively for diverse applications as protective and optical

coatings, passivation and planarization layers, sensors, high or low dielectric constant

films, inorganic membranes, electro-optic and nonlinear optical films, electro chromics,

semiconducting, anti-static coatings, superconducting films, strengthening layers and

ferroelectrics. The sol-gel technique offers a low-temperature method for synthesizing

materials that are either totally inorganic in nature or both inorganic and organic.

Dip coating method offers many advantages for the fabrication of coatings,

including excellent control of the stochiometry of precursor solutions, ease of


55

compositional modifications, customizable microstructure, ease of introducing various

functional groups or encapsulating sensing elements, repeatability of coatings, double

sided coatings, relatively low annealing temperatures, the possibility of coating

deposition on large area substrates and simple and inexpensive equipment [44]. Within

the past several years, a number of developments in precursor solutions, coating

processes and equipment have made the sol-gel technique even more widespread. It

allows optimum alignment to the desired function by individual adjustment of the coating

chemistry, highest surface homogeneity, maximum production flexibility and flexibility

in the glass substrates.

Dip coating method is more suitable to prepare materials because it permits

molecular-level mixing and processing of the raw materials and precursors at relatively

lower temperature and produces nano-structured bulk [45], powders and thin films. Sol-

gel dip coating method is a very attractive method to produce films for photo catalytic

applications for which large-area films are required at low cost. The method is often used

to produce metal oxide nano materials. So we had chosen dip coating method for the

present work.

3.5.4 Selection of substrate and cleaning process

Substrate is a passive component in the device and is required to be mechanically

stable, having a matching thermal expansion coefficient with deposited layers and remain

inert during the device fabrication [46]. Thin film coatings are extremely thin layers of

materials, which can be applied to the surface of a substrate like glass, metal, crystals, or

ceramics, to effect a change in its properties. The requirements on the substrate material

vary, depending on the application. Suitable substrates are selected for different processes

on the basis of this criterion. In the present study well-cleaned glass slides with thickness

of 1mm were chosen as substrates. The advantages of glass substrates are high

coefficients of thermal expansion, low surface roughness & flatness values, high

transmission to provide low insertion loss, excellent physical properties & chemical


56

durability, low cost and ease of availability. The hygiene of the substrate surface plays an

important role on film growth and adhesion.

Proper cleaning and preparation of substrate prior to thin film coating is essential

to the success of the film preparation. The cleaning and surface preparation conditions

must be followed immediately prior to application. Failure to adhere to these

requirements can cause adhesion loss and therefore reduce the durability and

performance level of the thin films. All substrate surfaces should be considered

contaminated and must be cleaned prior to coatings. Even freshly prepared or recently

cleaned surfaces will collect dust and dirt quickly and should be cleaned prior to film

application. It is essential to make sure all edges, corners, crevices and hard to reach areas

are cleaned well. All surfaces should be dry as well as clean.

The substrate cleaning process was started by cleaning glass surfaces with distilled

water. Then the surface was gently rubbed with acetone soaked cotton to clean the dirt

and residues. The glass substrates were then immersed into prepared chromic acid

solution. This process takes about 30 minutes to complete. Then again it was cleaned

with distilled water and wiped with acetone. Rubbing with acetone is termed as ‘solvent

clean’ [47]. Solvents can clean oils and organic residues which appear on glass surfaces.

Thereafter the cleaned substrate was put into sodium hydroxide solution for another 30

minutes. It does the process of mildly etching the surface of the glass. Again the solvent

clean process was repeated and cleaned substrates were immersed into 2- propanol in the

ultrasonic agitator for 15 minutes. In ultrasonic cleaning, dissolution of residues is

enhanced by intense local stirring action of the shock waves created in the solvent.

Through this process the glass substrates were well cleaned. Finally wet-cleaned glass

substrates were dried in a hot air oven maintained at about 70°C. Now the substrates are

ready for coating.

3.5.5 Preparation of zinc oxide seed layers

Precoating with a ZnO seed layer is crucial for the solution growth of highly

oriented ZnO nanorod arrays [48, 49]. Various techniques including radio frequency (RF)


57

sputtering, pulsed laser deposition, electron beam evaporation, thermal deposition [50],

chemical bath deposition [51], sol-gel spin coating [52]dip-coating using colloidal

solution [53]or transparent sol have been employed for the seed layer deposition. In this

work, the seed layer was prepared by the dip coating method. ZnO films can be used as

the nucleation seeds for the growth of ZnO nanorods [54].

The seed layer has an immense influence on nanorods. Control over nanorod

growth can be gained by simply varying the layer thickness, layer patterns and the

deposition techniques [55]. Among the various methods, dip coating method (automatic

dip coating system -Holmarc - HO-TH-02 shown in fig.3.8) was applied to prepare ZnO

seed layer on substrates because of its low cost and easy approach.

Fig 3.8 Automatic dip coating machine used in the present work.

All the reagents (Merck 99.99%) used in the experiment were analytically pure

and they were used without further purification. 0.1 mol of zinc acetate was dissolved

into 10ml of ethanol and 0.25ml of distilled water was introduced drop by drop through

syringe. The chemical composition was given in Table 3.3.


58

Table: 3.3 Chemical composition of ZnO seed layer solution.

Chemical Name Chemical formula Mole required Material taken

Zinc Acetate Zn(CH3 CO2) 2 0.1 0.21949 gram

Ethanol C2H5OH --- 10 ml

Deionized water H2O --- 0.25 ml

When zinc acetate is added in ethanol, the following reaction takes place initially

4Zn(Ac)2 · 2H2O → Zn4O + 7H2O+ 2HAc

Fig 3.9. Flow chart depicting the preparation of ZnO seed layer thin film.

0.1mol 0f Zinc Acetate Dihydrate+

10 ml of Ethanol+

0.25 ml of de-ionized water

Stirring for 2 hours at room temperature

Clear and homogeneous ZnO seed layersolution

Dip coating of glass substrates for 1 minin seed solution

Preheating at 70°C for 15 minutes.

Process repeated for 5 times to get desiredthickness

Annealing at 200°C for 1 hour.


59

Zn4O(Ac)6 is also called basic zinc acetate. The by-products like H2O, acetic acid

etc, can be removed by distillation. Zn4O(Ac)6 can be considered as a well designed

molecular model of ZnO. The pH of the solution was maintained at 6.5 – 7.

This solution was stirred continuously for 2 hours at room temperature for

obtaining clear homogenous solution. The stirred solution was taken in a beaker and the

well cleaned glass substrates were dipped in the solution for 1 minute at room

temperature using automatic dip coating system and preheated at 70°C and the process is

repeated for 5 times to get the thickness value of 50nm. Then this 5 layer films were

annealed in a furnace at 200°C for 1 hour to crystallize the seed particles. The flow

diagram shown in Fig. 3.9 depicts the detailed preparation procedure of ZnO nanorods.

3.5.6 Preparation of zinc oxide nanorods

Zinc oxide nanorods can be grown in different nanostructures by using low

temperature approaches. ZnO nanorods used in this research work were grown on the

surface of glass substrates. The low temperature approach used is based on the

hydrothermal method, which is substrate independent, convenient and low cost for large-

scale preparation of well-ordered ZnO nano rod arrays [56]. Hydrothermal method, as a

high performance growth technique for ZnO nanorods/nano wires, has recently received

increasing attraction due to its excellent advantages such as low cost, low temperature,

non-toxic operation and environmental cordiality. In recent studies, controllable growth

of well-aligned ZnO nanorods with high optical quality was successfully achieved on

glass substrates by hydrothermal method via adjusting the preparation parameters [57].

The growth process of ZnO nanorods can be controlled by adjusting the reaction

parameters, such as precursor concentration, growth temperature and growth time. Due to

its low cost and capability to coat large surface areas, hydrothermal method is selected

for this study.

Andres-Vergés et al [58] first reported the hydrothermal method of growing ZnO

nanostructures. However, this could not instill much interest till Vayssieres et al [59]

successfully used the method for the controlled fabrication of ZnO nanorods on glass and


60

Si substrates by the thermal decomposition of methenamine and zinc nitrate. To initiate

the growth from the substrate, a thin layer of ZnO nanoparticles was grown on the

substrate. Methenamine, also known as hexamethylenetetramine (HMT) or hexamine is a

highly water soluble, non-ionic tetradentate cyclic tertiary amine. Thermal degradation of

HMT releases hydroxyl ions which react with Zn2+ ions to form ZnO [60]. The aqueous

solutions of zinc nitrate and HMT can produce the following chemical reactions.

(CH2)6N4 + 6H2O ↔ 6HCHO + 4NH3

NH3 +H2O ↔ NH+4 + OH−

Zn(NO3)2 • 6H2O → Zn2+ + 2(NO3)- + 6H2O

2OH− + Zn2+ ↔ ZnO + H2O

The general acceptance is that, the role of HMT is to supply the hydroxyl ions to

drive the precipitation reaction [61]. The concentration of HMT plays a vital role for the

formation of ZnO nanostructure since OH- is strongly related to the reaction that

produces nanostructures. Initially, due to decomposition of zinc nitrate hexahydrate and

HMT at an elevated temperature, OH- was introduced in Zn2+ aqueous solution and their

concentrations were increased.

The separated colloidal Zn(OH)2 clusters in solution will act partly as nuclei for

the growth of ZnO nanorods. During the hydrothermal growth process, the Zn(OH)2

dissolves with increasing temperature. When the concentrations of Zn2+ and OH- reach

the critical value of the super saturation of ZnO, fine ZnO nuclei form spontaneously in

the aqueous complex solution. When the solution is supersaturated, nucleation begins.

Next the ZnO nanoparticles combine together to reduce the interfacial free energy.

This is because the molecules at the surface are energetically less stable than the

ones already well ordered and packed in the interior. Since the (0 01) face has higher

symmetry than the other faces growing along the c-axis direction, which is the typical

growth plane. The nucleation determines the surface-to-volume ratio of the ZnO nanorod.

Then incorporation of growth units into crystal lattice of the nanorods by dehydration

reaction takes place. It is concluded that the growth habit is determined by


61

thermodynamic factor and by concentration of OH- as the kinetic factor in aqueous

solution growth.

During the formation process of ZnO, mono Zn–OH is formed during the

hydrolysis reaction and Zn–OH aggregation results in the formation of crystalline nuclei

and the primary particle size depends on the aggregation degree. Finally the particles in

the film are oxidized and turned into oxide form during calcinations and annealing at

higher temperature promotes the formation of Zn–O–Zn bonds. The pre-heat treatment at

70°C between each five times of dip coating induces generation of more nuclei. This

facilitates the subsequent crystal growth process, accompanied by the diffusion of Zinc

species towards the nucleated grains resulting in grain growth and formation of nano

crystalline ZnO nano rod.

ZnO nanorods have been prepared onto well cleaned glass substrates by

hydrothermal method. In the present work zinc nitrate (Merck 99.9%) has been used as

the zinc precursor and the matrix sol was prepared by mixing it with

hexamethylenetetramine (HMT) (Aldrich 99.9%) and de-ionized water at room

temperature. De-ionized water is used as a solvent. Hydrothermal treatments were carried

out by suspending the ZnO seed coated glass substrates in upside down position in a glass

beaker filled with aqueous solution 0.02 mol of zinc nitrate and 0.1 mol of

hexamethylenetetramine (HMT). The final composition of the solution in molar ratio was

1:5 (zinc nitrate: HMT). The same treatment was carried out for 1:10 and 1:15 molar

concentrations also. The chemical compositions were given in Tables 3.4 to 3.6.

Table:3.4 Chemical composition of ZnO growth layer solution with1:5 concentration.

Chemical Name Chemical formula Mole required Material takenZinc Nitrate Zn(NO3)2 0.02 mol 0.118988 grams

Hexamethylenetetramine

(CH2)6 N4 0.1 mol 0.28038 grams

Deionized water H2O --- 20 ml


62

Table:3.5 Chemical composition of ZnO growth layer solution with 1:10 concentration.



(CH2)6 N4 0.2 mol 0.56076 grams

Deionized water H2O --- 20 mlTable:3.6 Chemical composition of ZnO growth layer solution with 1:15 concentration.



(CH2)6 N4 0.3 mol 0.84114 grams

Deionized water H2O --- 20 ml

Fig 3.10. Flow chart depicting the preparation of ZnO nanorods.

0.02mol 0f Zinc Nitrate+

0.1 mol of Hexamethylenetetramine+

De-ionized water


ZnO Growth layer solution

Hydrothermal process at 90°C for 4 hoursin hot air oven

Rinsing with de-ionized water to removeimpurities

Annealing at 300°C, 400°C and 500°C for1 hour.


63

The final mixture solution was stirred through magnetic stirrer for about 2 hours.

During the growth process the solution was heated with a hot air oven and maintained at

90°C for 4 hours.

At the end of the growth period, the substrates were taken from the solution and

immediately rinsed with de ionized water to remove the residuals from the surface and

dried in air at room temperature. The prepared films were annealed at 300°C, 400°C and

500°C for 1 hour respectively. The flow diagram shown in Fig. 3.10 depicts the detailed

preparation procedure of ZnO nanorods.

3.5.7 Preparation of Al, Sr and Li doped ZnO nanorods

Group IV semiconductors such as silicon, germanium, and silicon carbide, the

most common dopants are acceptors from Group III or donors from Group V elements.

Optical and electrical properties of Al-doped ZnO thin films, prepared using sol-gel

technique were reported [63]. Both ZnO and ZnO: Al films were preferentially oriented

along c-axis and showing an optical transmittance of ~90 %. Electrical resistivity was

decreasing with increase in the film thickness. In an interesting paper, influence of

different dopant elements on the structural and electrical properties of spray pyrolysed

ZnO thin films was investigated [64]. It was seen that, as the dopant concentration

increased, the film growth became non-oriented with poor crystallinity. SEM and TEM

micrographs showed that amount of dopant influenced the microstructure of the film to a

great extent.

Holmelund et al reported properties of pure and St doped ZnO films [PLD

deposited] [65]. Pushparajah et al studied the physical properties of Li doped ZnO films,

prepared using spray pyrolysis technique [62]. On Li-doping, the sample became

amorphous and resistivity also increased.

In the present work aluminium, strontium and lithium doped ZnO nanorods thin

films have been prepared on to well cleaned glass substrates by hydrothermal method.

Zinc nitrate (Merck 99.9%) was used as the zinc precursor, aluminium nitrate, strontium

chloride and lithium chloride (Merck 99.9%) were used as dopants and the matrix sol was


64

prepared by mixing hexamethylenetetramine (HMT) (Aldrich 99.9%) with de-ionized

water at room temperature. De-ionized water was used as a solvent.

Table:3.7 Chemical composition of ZnO growth layer solution with various dopants.



(CH2)6 N4 0.2 mol 0.56076 grams

Deionized water H2O --- 20 mlAluminium Nitrate Al(NO3)3 0.02 mol 0.150052 gramsStrontium Chloride Sr Cl2 0.02 mol 0.106648 gramsLithium Chloride Li Cl 0.02 mol 0.118988 grams

Fig 3.11. Flow chart depicting the preparation of ZnO nanorods with dopants.

0.02mol 0f Zinc Nitrate+

0.02 mol of dopant+

0.2 mol of Hexamethylenetetramine+

De-ionized water


ZnO Growth layer solution with dopant

Hydrothermal process at 90°C for 4 hoursin hot air oven

Rinsed with de-ionized water to removeimpurities

Annealed at 500°C for 1 hour.


65

Hydrothermal treatments were carried out by suspending the ZnO seed coated

glass substrates in upside down position in a glass beaker filled with aqueous solution

0.02 mol of zinc nitrate, 0.02 mol of dopant and 0.2 mol of hexamethylenetetramine

(HMT). The final composition of the solution in molar ratio was zinc nitrate: dopant:

HMT = 1:1:10. The chemical compositions were given in table 3.7.

The final mixture solution was stirred through magnetic stirrer for about 2 hours.

During the process the solution was heated with a hot air oven and maintained at 90°C

for 4 hours. At the end of the growth period, the substrates were taken from the solution

and immediately rinsed with de ionized water to remove the residuals from the surface

and dried in air at room temperature and then annealed in air. The prepared films were

annealed at 500°C for 1 hour. The flow diagram shown in fig. 3.11 depicts the detailed

preparation procedure of Al, St and Li doped ZnO nanorods.


66

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