Fikadu ASTU.pdf

BY
Dr.Fikadu T.
Adama, Ethiopia
May, 2019
Synthesis and Characterizations of
Nanomaterials
1
6/12/2022 Dr. Fikadu Takele

Outline:
• Introduction to nanomaterials
• Synthesis of nanomaterials and nanostructures
• Physical Chemistry of Solid Surfaces
• Synthesis of nanoparticles
• Synthesis of 1-D nanostructured materials
• Thin Film Deposition methods
• Characterization of Nanomaterials
2

1. Introduction to nanomaterials
• The principles of physics, as far as I can see, do not speak
against the possibility of maneuvering things atom by atom.”
“Put the atoms down where the chemist says, and so you make
the substance.”
3
- Richard Feynman(1959)
“There's Plenty of Room at the Bottom “
“Why cannot we write the entire 24 volumes of the
Encyclopedia Britannica on the head of a pin?”

Cont.
• Nanomaterial: refers to the matter whose length scale, in any
dimension, is approximately 1 to 100 nanometers.
• 1 nanometer (nm) = 10−9
m
• The concepts of nanotechnology are not new to nature or to
mankind. An early example of a manmade nanoprocess is stained
glass.
4

Cont.
• Materials in the nanometer scale may exhibit physical
properties distinctively different from that of bulk.
• Origins: (i) large fraction of surface atoms,
(ii) large surface energy,
(iii) spatial confinement, and
(iv) reduced imperfections.
6

Cont.
• Nanotechnology is a technology of design, fabrication and
applications of nanostructures and nanomaterials.
• Study on fundamental relationships between physical properties
and phenomena and material dimensions in the nanometer scale,
is referred to as nanoscience.
• The ability to fabricate and process nanomaterials and
nanostructures is the first corner stone in nanotechnology.
7

2. Synthesis of nanomaterials and nanostructures
• Many technologies have been explored to fabricate
nanostructures and nanomaterials.
• These technical approaches can be grouped in several ways.
8

Cont.
One way is to group them according to the growth media:
1) Vapor phase growth, including laser reaction pyrolysis for nanoparticle
synthesis and atomic layer deposition (ALD) for thin film deposition.
2) Liquid phase growth, including colloidal processing for the formation of
nanoparticles and self assembly of monolayers.
3) Solid phase formation, including phase segregation to make metallic particles
in glass matrix and two-photon induced polymerization for the fabrication of
three-dimensional photonic crystals.
4) Hybrid growth, including vapor-liquid-solid (VLS) growth of nanowires.
9

Cont.
Another way is to group the techniques according to the form of
products:
1) Nanoparticles by means of colloidal processing, flame combustion
and phase segregation.
2) Nanorods or nanowires by template-based electroplating, solution
liquid-solid growth (SLS), and spontaneous anisotropic growth.
3) Thin films by molecular beam epitaxy (MBE) and atomic layer
deposition (ALD).
4) Nanostructured bulk materials, for example, photonic bandgap
crystals by self-assembly of nanosized particles.
10

Cont.
• The most popular way of classifying the synthesis routes is based on
how the nanostructures are built, and such an approach leads to two
routes namely “bottom-up” and the “top-down” approaches.
 Top-down: size reduction from bulk materials.
 Bottom-up: material synthesis from atomic level.
• All the techniques that start with liquid and gas as the starting
material fall into “bottom-up” category.
• All the solid state routes fall into “top-down” category.
11

Cont.
Top-down routes:
• are included in the typical solid –state processing of the materials.
• This route is based with the bulk material and makes it smaller,
thus breaking up larger particles by the use of physical processes
like crushing, milling or grinding.
Bottom –Top approach:
• refers to the build-up of a material from the bottom: atom-by-
atom, molecule-by-molecule or cluster-by-cluster.
12

Top-down vs Bottom –Top approaches
13

Problem with top-down approach:
• Usually this route is not suitable for preparing uniformly
shaped materials, and it is very difficult to realize very small
particles even with high energy consumption.
• The biggest problem with top-down approach is the
imperfection of the surface structure.
• Such imperfection would have a significant impact on physical
properties and surface chemistry of nanostructures and
nanomaterials.
14

Advantages of bottom-up approach:
• To obtain nanostructures with less defects, more homogeneous
chemical composition, and better short and long range ordering.
• The bottom-up approach is driven mainly by the reduction of
Gibbs free energy, so that nanostructures and nanomaterials such
produced are in a state closer to a thermodynamic equilibrium
state.
• On the contrary, top-down approach most likely introduces
internal stress, in addition to surface defects and contaminations.
15

For the fabrication and processing of nanomaterials and
nanostructures, the following challenges must be met:
1) Overcome the huge surface energy, a result of enormous surface
area or large surface to volume ratio.
2) Ensure all nanomaterials with desired size, uniform size
distribution, morphology, crystallinity, chemical composition,
and microstructure, that altogether result in desired physical
properties.
3) Prevent nanomaterials and nanostructures from coarsening
through either Ostwald ripening or agglomeration as time
evolutes. 16

Physical Chemistry of Solid Surfaces
• Nanostructures and nanomaterials possess a large fraction of
surface atoms per unit volume.
The percentage of surface
atoms changes with the
palladium cluster diameter.
17

Cont.
• For a cube of iron of 1cm3, the percentage of surface atoms would
be only 10-5 %
• When the cube is divided into smaller cubes with an edge of l0nm,
the percentage of the surface atoms would increase to 10%.
• In a cube of iron of 1nm3, every atom would be a surface atom.
• The total surface energy increases with the overall surface area.
Table: Variation of surface energy with particle size
18

Cont.
• When the particles change from centimeter size to nanometer size,
the surface area and the surface energy increase seven orders of
magnitude.
• Due to the vast surface area, all nanostructured materials possess a
huge surface energy and, thus, are thermodynamically unstable or
metastable.
• One of the great challenges in fabrication and processing of
nanomaterials is to overcome the surface energy, and to prevent the
nanostructures or nanomaterials from growth in size, driven by the
reduction of overall surface energy.
19

Cont.
• Material or system is stable only when it is in a state with the
lowest Gibbs free energy.
• There is a strong tendency for a solid or a liquid to minimize
the total surface energy.
• There are a variety of mechanisms to reduce the overall
surface energy.
• The various mechanisms can be grouped into atomic or surface
level, individual structures and the overall system.
20

Cont.
The surface energy can be reduced through:
i. surface relaxation, the surface atoms or ions shift inwardly which
occur more readily in liquid phase than in solid surface due to
rigid structure in solids,
ii. surface restructuring through combining surface dangling bonds
into strained new chemical bonds,
iii. surface adsorption through chemical or physical adsorption of
terminal chemical species onto the surface by forming chemical
bonds or weak attraction forces such as electrostatic or van der
Waals forces, and
iv. composition segregation or impurity enrichment on the surface
through solid-state diffusion.
21

Cont.
the surface of diamond is
covered with hydrogen
and that of silicon is
covered with hydroxyl
groups through
chemisorption before
restructuring.
surface atoms shifting
either inwardly or laterally
so as to reduce the surface
energy.
illustrating the (2 X 1)
restructure of silicon {
100} surface.
22

Dimensional classification
• Nanostructured materials are those with at least one dimension
falling in nanometer scale
• Nanostructured materials are classified as Zero dimensional, one
dimensional, two dimensional, three dimensional nanostructures.
23
(a) 0D spheres and clusters; (b) 1D nanofibers, nanowires, and
nanorods; (c) 2D nanofilms, nanoplates, and networks; (d) 3D
nanomaterials.

3. Synthesis of nanoparticles
Nanoparticles have the following characteristics:
i. identical size of all particles (also called monosized or with
uniform size distribution),
ii. identical shape or morphology,
iii. identical chemical composition and crystal structure that are
desired among different particles and within individual particles,
such as core and surface composition must be the same, and
iv. individually dispersed or monodispersed, i.e. no agglomeration.
If agglomeration does occur, nanoparticles should be readily
redispersible.
24

Solidification
• Most metals are melted and then cast into semifinished or
finished shape.
Solidification of a metal can be divided into the following steps:
 Formation of a stable nucleus
 Growth of a stable nucleus.
25

Driving force: solidification
• For the reaction to proceed to the right ∆𝐺𝑣 must be negative.
• Writing the free energies of the solid and liquid as:
𝐺𝑣𝑆
= 𝐻𝑆
− 𝑇𝑆𝑆
𝐺𝑣𝐿
= 𝐻𝐿
− 𝑇𝑆𝐿
∆𝐺𝑣 = ∆𝐻 + 𝑇∆𝑆
• At equilibrium, i.e. 𝑇𝑚𝑒𝑙𝑡, then the ∆𝐺𝑉 = 0, so we can estimate
the melting entropy as:
∆𝑆 = ∆𝐻 𝑇𝑚𝑒𝑙𝑡
where ∆𝐻 is the latent heat (enthalpy) of melting.
• Ignore the difference in specific heat between solid and liquid and
we estimate the free energy difference as:
∆𝐺𝑣 = ∆𝐻 −
𝑇
𝑇𝑚𝑒𝑙𝑡
∆𝐻 =
∆𝐻 × ∆𝑇
𝑇𝑚𝑒𝑙𝑡 26
On a microscopic
scale a solid
precipitate in a
liquid matrix

Nucleation
• The two main mechanisms by which nucleation of a solid particles
in liquid metal occurs are homogeneous and heterogeneous
nucleation.
Homogeneous Nucleation
• Homogeneous nucleation occurs when there are no special objects
inside a phase which can cause nucleation. For instance when a
pure liquid metal is slowly cooled below its equilibrium freezing
temperature to a sufficient degree numerous homogeneous nuclei
are created by slow-moving atoms bonding together in a crystalline
form.
27

Cont.
• Consider the free energy changes when some atoms in the liquid
collapse and agglomerate to form a solid of radius r.
28
• The precipitation process is seen on the atomic scale as a clustering
of adjacent atoms to form a crystalline nucleus followed by the
growth of the crystalline phase

Cont.
The energy changes involve two terms:
a) The chemical free energy change associated with the transfer of
atoms from liquid to solid state (∆𝐺𝑣);
b) The interfacial energy (𝛾) due to the creation of new interface (liquid-
solid interface).
• Assume that ∆𝐺𝑣 is the change in free energy per unit volume and
• ∆𝐺𝑇 is the total Free energy change, r is the radius of the nucleus
∆𝐺𝑇 =
4𝜋𝑟3
3
∆𝐺𝑣 + 4π𝑟2
𝛾
29

Cont.
𝑑∆𝐺𝑇
𝑑𝑟
|𝑟=𝑟∗ = 4𝜋𝑟∗2
∆𝐺𝑣 + 8𝜋𝑟0𝛾 = 0
Which implies,
𝑟∗
= −
2𝛾
∆𝐺𝑣
∆𝐺∗ 𝑟∗ =
4𝜋𝛾𝑟∗2
3∆𝐺𝑣2
𝑟∗
: critical radius;
• For r < 𝑟∗
: the growth of the droplet
∆𝐺𝑇 ↑ the embryos should shrink and
disappear!
• For r > 𝑟∗
: the growth of the droplet
∆𝐺𝑇 ↓ the nuclei could steadily grow.
• For 𝑟 = 𝑟∗ ∆𝐺 =∆𝐺∗
is the energy
barrier that a nucleation process
must overcome.
A system is in equilibrium when it is at its
most stable state when it has the lowest
value of the Gibbs free energy
𝑑∆𝐺𝑇 = 0
30
embryo
T

Cont.
31
Schematic illustrating the processes of nucleation and
subsequent growth.

Cont.
32

Heterogeneous Nucleation
• Heterogeneous nucleation is the nucleation that occurs in a liquid
on the surfaces of its container, insoluble impurities or other
structural material (catalyst) which lower the critical free energy
required to form a stable nucleus.
33

Supersaturated Solutions
• If the liquid is just at the freezing point, only a few molecules
stick, because they have comparatively high energy
• As the liquid is cooled, more molecules can form into nuclei.
• When the nucleus is big enough (because of undercooling) the
supercooled liquid suddenly changes to a solid.
• Metals often experience undercooling of 50 to 500℃
34

Cont.
• Nanoparticle Synthesis methods are:
– Colloidal Chemical Methods
– Attrition
– Pyrolysis
– RF Plasma
– Pulsed Laser Method
35

Colloidal Methods
• Colloidal chemical methods are some of the most useful, easiest,
and cheapest ways to create nanoparticles.
• Typically, a metal salt is reduced leaving nanoparticles evenly
dispersed in a liquid.
• Aggregation is prevented by electrostatic repulsion or the
introduction of a stabilizing reagent that coats the particle
surfaces.
• Particle sizes range from 1-200nm and are controlled by the initial
concentrations of the reactants and the action of the stabilizing
reagent. 36

Formation of Gold Nanoparticles
Combining hydrogen tetrachloroaurate (HAuCl4) and sodium
citrate (Na3C6H5O7) in a dilute solution.
1. Heat a solution of chloroauric acid (HAuCl4) up to reflux (boiling). HAuCl4 is a water
soluble gold salt.
2. Add trisodium citrate, which is a reducing agent.
3. Continue stirring and heating for about 10 minutes.
• During this time, the sodium citrate reduces the gold salt (Au3+) to metallic gold (Au0).
• The neutral gold atoms aggregate into seed crystals.
• The seed crystals continue to grow and eventually form gold nanoparticles.
37

Cont.
Reduction of gold ions: Au(III) + 3e- → Au(0)
38

Attrition
• Attrition is a mechanical method for creating certain types of
nanoparticles.
• Macro or micro scale particles are ground in a ball mill, a planetary
ball mill, or other size reducing mechanism.
• The resulting particles are separated by filters and recovered.
• Particle sizes range from tens to hundreds of nm (10nm-100nm).
• Broad size distribution and varied particle geometry.
• May contain defects and impurities from the milling process.
• Generally considered to be very energy intensive.
39

Ball milling
• A hollow steel cylinder
containing tungsten balls and a
solid precursor rotates about its
central axis.
• Particle size is reduced by brittle
fracturing resulting from ball-
ball and ball-wall collisions.
• Milling takes place in an inert
gas atmosphere to reduce
contamination.
40

The merits of planetary ball-mill:
1. Nanometric particle size can be achieved in a short time.
2. Synthesis of new equilibrium phases with severe localized plastic deformation of
the materials.
3. Alloying and complete solid solubility of materials can be achieved.
4. Solid state amorphization of materials can be achieved.
5. Reduction of metal oxides by hydrogen/carbon is possible.
6. Energy of the milling media can be controlled by a large number of parameters,
such as, BPMR, rpm, duration of milling, size of balls/bowls etc.
7. Mechanosynthesis of metallic component in inert atmosphere can be done very
easily.
8. The process of mechanical milling of amorphous precursors can be used to
produce bulk quantities of material with fine crystalline nanostructures.
41

The demerits of planetary ball-mill:
1. Contamination from grinding media.
2. Stickiness of material during dry grinding.
3. Excessive heating of material to be ground.
4. Combustible liquids with boiling point
42

Pyrolysis
• Pyrolysis is a popular method for creating nanoparticles, especially
oxides.
• A precursor (liquid or gas) is forced through an orifice at high
pressure and burned.
• The resulting ash is collected to recover the nanoparticles.
• Large volume of gas leads to high rate of material synthesis
43

Pyrolysis
44

RF Plasma Synthesis
• The starting material is placed in a pestle and heated under vacuum by RF
heating coils.
• A high temperature plasma is created by flowing a gas, such as He,
through the system in the vicinity of the coils.
• When the material is heated beyond its evaporation point, the vapor
nucleates on the gas atoms which diffuse up to a cooler collector rod and
form nanoparticles.
• The particles can be passivated by introducing another gas such as O2.
• In the case of Al nanoparticles the O2 forms a thin layer of AlO3 around
the outside of the particle inhibiting aggregation and agglomeration.
45

Cont.
• RF plasma synthesis is very popular method for creating
ceramic nanoparticles and powders
• Low mass yield.
46

Pulsed Laser Methods
• Pulsed Lasers have been employed in the synthesis silver
nanoparticles from silver nitrate solutions.
• A disc rotates in this solution while a laser beam is pulsed onto
the disc creating hot spots.
• Silver nitrate is reduced, forming silver nanoparticles.
• The size of the particle is controlled by the energy in the laser
and the speed of the rotating disc.
47

Cont.
Apparatus to make silver nanopartcles using pulsed laser beam that
creates hot spots on the surface of a rotating disk
48

4. Synthesis of 1-D nanostructured materials:
Nanotube, Nanowires and Nanorods
• 1-D nanostructures have been called by a variety of names including:
whiskers, fibers or fibrils, nanowires and nanorods.
• Nanotubules and nanocables are also considered one-dimensional
structures.
• Whiskers and nanorods are in general considered to be shorter than fibers
and nanowires.
• One-dimensional structures with diameters ranging from several
nanometers to several hundred microns were referred to as whiskers and
fibers.
• Nanowires and nanorods with diameters not exceeding a few hundred
nanometers
49

Cont.
The techniques can be generally grouped into four categories:
(1) Spontaneous growth:
(a) Evaporation (or dissolution)-condensation
(b) Vapor (or solution)-liquid-solid (VLS or SLS) growth
(c) Stress-induced recrystallization
(2) Template-based synthesis:
(a) Electroplating and electrophoretic deposition
(b) Colloid dispersion, melt, or solution filling
(c) Conversion with chemical reaction
(3) Electrospinning
(4) Lithography
50

Cont.
Bottom-up approaches:
1. Spontaneous growth
2. Template-based synthesis
3. Electrospinning
Top-down approaches:
4. Lithography
• Spontaneous growth commonly results in the formation single crystal
nanowires and nanorods along a preferential crystal growth direction
depending on the crystal structures and surface properties of the
nanowire materials.
• Template-based synthesis mostly produces polycrystalline or even
amorphous products
51

Spontaneous Growth
• Spontaneous growth is a process driven by the reduction of
Gibbs free energy or chemical potential.
• The reduction of Gibbs free energy is commonly realized by
phase transformation or chemical reaction or the release of stress.
• For formation of nanowires or nanorods, anisotropic growth is
required, i.e. the crystal grows along a certain orientation faster
than other directions
52

Cont.
• Uniformly sized nanowires, i.e. the same diameter along the
longitudinal direction of a given nanowire, can be obtained
when crystal growth proceeds along one direction, whereas no
growth along other directions
• Morphology of final product affected by defects and
impurities.
53

Fundamentals of evaporation (dissolution)-
condensation growth
Six steps in crystal growth
54

Cont.
(1) Diffusion of growth species from the bulk (such as vapor or
liquid phase) to the growing surface, which, in general, is
considered to proceed rapid enough and, thus, not at a rate
limiting process.
(2) Adsorption and desorption of growth species onto and from the
growing surface. This process can be rate limiting, if the
supersaturation or concentration of growth species is low.
(3) Surface diffusion of adsorbed growth species. During surface
diffusion, an adsorbed species may either be incorporated into a
growth site, which contributes to crystal growth, or escape from
the surface.
55

Cont.
(4) Surface growth by irreversibly incorporating the adsorbed
growth species into the crystal structure. When a sufficient
supersaturation or a high concentration of growth species is
present, this step will be the rate-limiting process and determines
the growth rate.
(5) If by-product chemicals were generated on the surface during
the growth, by-products would desorb from the growth surface,
so that growth species can adsorb onto the surface and the
process can continue.
(6) By-product chemicals diffuse away from the surface so as to
vacate the growth sites for continuing growth. 56

Evaporation condensation
• Vapor-Solid (VS) technique.
• Simple and accessible.
• Vapor species are generated (e.g. by evaporation) and transported and
condensated onto a substrate placed in a zone with temperature lower
than that of the source material.
• Nanowires grown by this method are commonly single crystals with
few imperfections.
• The formation of nanowires is due to anisotropic growth.
• Different facets in a crystal have different growth rates.
• No control on the direction of growth of nanowire using this method.
57

Cont.
Several mechanisms are known to result in
anisotropic growth, for example:
1. Different facets in a crystal have different growth rate. For
example, in silicon with a diamond structure, the growth rate of
{111} facets is smaller than that of {110}.
2. Presence of imperfections in specific crystal directions such as
screw dislocation.
3. Preferential accumulation of or poisoning by impurities on
specific facets.
58

Dissolution condensation
• Differs from Evaporation-condensationin growth media.
• The growth species first dissolve into a solvent or a solution,
and then diffuse through the solvent or solution and deposit
onto the surface resulting in the growth of nanorods or
nanowires.
• The nanowires in this method can have a mean length of <500
nm and a mean diameter of ~60nm
59

Disadvantages of Evaporation – Condensation
• Nanowire grown by EC most likely have faceted morphology
and are generally short in length with relatively small aspect
ratios, particular when grown in liquid medium. However,
anisotropic growth induced by axial imperfections, such as
screw dislocation, microtwins and stacking faults, or by
impurity poisoning, can result in the growth of nanowires with
large aspect ratios.
60

Vapor-Liquid-Solid growth (VLS)
 A second phase material, commonly referred to as catalyst, is
introduces to direct and confine the crystal growth on a specific
orientation and within a confined area.
• The most successful method to generate single crystal nanowires in
relatively large quantities.
• Catalyst forms a liquid droplet by itself or by alloying with the
growth material, which then acts as a trap of the growth species.
• The growth species is evaporated first and then diffuses and
dissolves into a liquid droplet
• Enriched growth species in the catalyst droplets subsequently
precipitate at the substrate/liquid interface, resulting in the one-
directional growth.
61

VLS Cont.
• The diameter of each nanowire is largely determined by the size of
the catalyst droplet
Growth species in the catalyst droplets subsequently precipitates at the
growth surface resulting in the one-directional growth
62

Requirements for the VLS growth
1. The catalyst or impurity must form a liquid solution with the crystalline material
to be grown at the deposition temperature,
2. The distribution coefficient of the catalyst or impurity must be less than unity at
the deposition temperature.
3. The equilibrium vapor pressure of the catalyst or impurity over the liquid
droplet must be very small. Although the evaporation of the catalyst does not
change the composition of the saturated liquid composition, it does reduce the
total volume of the liquid droplet. Unless more catalyst is supplied, the volume
of the liquid droplet reduces. Consequently, the diameter of the nanowire will
reduce and the growth will eventually stop, when all the catalyst is evaporated.
63

Cont.
4. The catalyst or impurity must be inert chemically.
5. The interfacial energy plays a very important role. The wetting
characteristics influence the diameter of the grown nanowire. For
a given volume of liquid droplet, a small wetting angle results in a
large growth area, leading to a large diameter of nanowires.
6. For a compound nanowire growth, one of the constituents can
serve as the catalyst.
7. For controlled unidirectional growth, the solid-liquid interface
must be well defined crystallographically. One of the simplest
methods is to choose a single crystal substrate with desired crystal
orientation.
64

Stress-induced recrystallization
• Nanowires can be synthesized by stress-induced recrystallization.
• Application of pressure on solids at elevated temperatures result in the
growth of whiskers or nanowires with diameters as small as 50nm.
• Growth rate of tin whiskers increased proportionally with the applied
pressure.
• The growth proceeds from the base and not from the tip.
• The formation of metallic nanorods is likely due to the confined growth at
the surface between the metallic film and the grown nanowires, whereas
no growth is possible in other directions.
• This technique is not widely explored in the recent studies on the growth
of nanorods and nanowires.
65

Template based
• The template approach to preparing free-standing, nonoriented and
oriented nanowires and nanorods has been investigated extensively.
• The most commonly used templates are anodized alumina membrane
(AAM) and radiation track-etched polycarbonate (PC) membranes.
• Other membranes have also been used, such as nanochannel array on
glass, radiation track-etched mica, mesoporous materials, porous silicon
obtained via electrochemical etching of silicon wafer, zeolites and
carbon nanotubes.
• Biotemplates have also been explored for the growth of nanowires and
nanotubes.
66

Cont.
Alumina membranes with uniform and parallel pores:
• Are produced by the anodic oxidation of aluminium sheet in solutions of sulfuric,
oxalic, or phosphoric acids.
• The pores can be arranged in a regular hexagonal array, and densities as high as
1011pores/𝑐𝑚2 can be achieved.
• Pore size ranging from 10 nm to 100μm can be achieved .
PC membranes:
• Are made by bombarding a nonporous polycarbonate sheet, typically 6 to 20μm
in thickness, with nuclear fission fragments to create damage tracks, and then
chemically etching these tracks into pores.
• In these radiation track-etched membranes, the pores are of uniform size (as small
as 10 nm), but they are randomly distributed. Pore densities can be as high as
109 pores/𝑐𝑚2.
67

 In addition to the desired pore or channel size, morphology, size
distribution and density of pores, template materials must meet
certain requirements.
• First, the template materials must be compatible with the
processing conditions. For example, an electrical insulator is
required when a template is used in electrochemical deposition.
• Secondly, the material or solution being deposited must wet the
internal pore walls.
• Thirdly, for the synthesis of nanorods or nanowires, the
deposition should start from the bottom or from one end of the
template channel and proceed from one side to the other. 68

Cont.
• AAM and PC membranes are most commonly used for the
synthesis of nanorod or nanowire arrays.
• Each type of template has its disadvantages.
• The advantages of using PC as the template are its easy handling
and easy removal by means of pyrolysis at elevated temperatures.
• But the flexibility of PC is more prone to distortion during the
heating process, and removal of the template occurs before
complete densification of the nanorods.
• These factors result in broken and deformed nanorods.
69

Cont.
• The advantage of using AAM as the template is its rigidity and
resistance to high temperatures, which allows the nanorods to
densify completely before removal.
• This results in fairly free-standing and unidirectionally aligned
nanorod arrays with a larger surface area than for PC.
• The problem with AAM is the complete removal of the template
after nanorod growth, which is yet to be achieved when using wet
chemical etching.
 In general, electroplating and electrophoretic, deposition colloid dispersion, melt,
or solution filling and conversion with chemical reaction are template based
approach techniques.
70

Electrochemical deposition
• Also known as electrodeposition.
• Electrolysis resulting in the deposition of solid material on an
electrode. This process involves:
 oriented diffusion of charged growth species (typically
positively charged cations) through a solution when an
external electric field is applied, and
 reduction of the charged growth species at the growth or
deposition surface which also serves as an electrode.
71

Cont.
• Is only applicable to electrical conductive materials, since after the
initial deposition, the electrode is separated from the depositing
solution by the deposit and the electrical current must go through the
deposit to allow the deposition process to continue.
• Is widely used in making metallic coatings; the process is also known
as electroplating.
• When deposition is confined inside the pores of template membranes,
nanocomposites are produced.
• If the template membrane is removed, nanorods or nanowires are
prepared.
72

Cont.
• Template is attached onto the
cathode, which is subsequently
brought into contact with the
deposition solution.
• The anode is placed in the
deposition solution parallel to the
cathode.
• When an electric field is applied,
cations diffuse toward and reduce
at the cathode, resulting in the
growth of nanowires inside the
pores of template.
73

Electrospinning
• Also known as electrostatic fiber processing.
• Technique has been originally developed for generating ultrathin
polymer fibers.
• Electrospinning uses electrical forces.
• Electrospinning occurs when the electrical forces at the surface
of a polymer solution or melt overcome the surface tension and
cause an electrically charged jet to be ejected.
74

Cont.
• When the jet dries or solidifies, an electrically charged fiber
remains.
• The charged fiber can be directed or accelerated by electrical
forces and then collected in sheets or other useful geometrical
forms.
• The morphology of the fibers depends on the process
parameters, including: solution concentration, applied electric
field strength, and the feeding rate of the precursor solution.
75

Lithography
• It is also often referred to as photoengraving.
• It is the process of transferring a pattern into a reactive polymer
film, termed as resist, which will subsequently be used to replicate
that pattern into an underlying thin film or substrate.
Resists:
• Positive
• Negative
Exposure Sources:
• Light(photon)
• Electron beams
• X-ray sensitive
76

Photolithography
• Photolithographic process consists of producing a mask
carrying the requisite pattern information and subsequently
transferring that pattern, using some optical technique into a
photoactive polymer or photoresist.
There are two basic photolithographic approaches:
i. shadow printing, which can be further divided into contact
printing (or contact-mode printing) and proximity printing,
and
ii. projection printing.
• The terms printing and photolithography are used
interchangeably
77

Pattern transfer to wafer: Printing
Contact printer:
• The mask and wafer are in intimate contact
• Transfer a mask pattern into a photoresist with almost 100% accuracy and
provides the highest resolution.
• The maximum resolution is seldom achieved because of dust on substrates and
non-uniformity of the thickness of the photoresist and the substrate.
Proximity printer:
• a gap between the mask and the wafer is introduced.
• The difficulties in proximity printing include the control of a small and very
constant space between the mask and wafer, which can be achieved only with
extremely flat wafers and masks.
Projection:
• In projection printing techniques, lens elements are used to focus the mask
image onto a wafer substrate, which is separated from the mask by many
centimeters. Because of lens imperfections and diffraction considerations,
projection techniques generally have lower resolution capability than that
provided by shadow printing.
• flexible, no damage, limited resolution in single projection 78

Basic steps of the photolithographic process
• The resist material is applied as a thin coating over some base
and subsequently exposed in an image-wise fashion through a
mask, such that light strikes selected areas of the resist material.
• The exposed resist is then subjected to a development step.
• Depending on the chemical nature of the resist material: the
exposed areas may be rendered more soluble in some developing
solvent than the unexposed areas, thereby producing a positive
tone image of the mask.
• Conversely, the exposed areas may be rendered less soluble,
producing a negative tone image of the mask.
79

Cont.
• The effect of this process is to produce a three-dimensional relief image in the
resist material that is a replication of the opaque and transparent areas of the
mask.
• The areas of resist that remain following the imaging and developing processes
are used to mask the underlying substrate for subsequent etching or other image
transfer steps.
• The resist material resists the etchant and prevents it from attacking the
underlying substrate in those areas where it remains in place after development.
• Following the etching process, the resist is removed by stripping to produce a
positive or negative tone relief image in the underlying substrate.
80

Cont.
• The photolithographic
process sequences, in
which images in the
mask are transferred to
the underlying substrate
surface.
81

A complete lithographic process
Prebake
(softbake
)
Wafer with
mask film
(e.g. SiO2,
Al)
Photoresist
coating
(spin
coating)
Mask
alignment
Removal of
exposed
photoresist
Postbake
Develop-
ment
Exposure
Etching
of mask
film
Removal
of
unexposed
resist
Next process
(e.g. implantation,
deposition)
82

Electron beam lithography
• A finely focused beam of electrons can be deflected accurately
and precisely over a surface.
• When the surface is coated with a radiation sensitive polymeric
material, the electron beam can be used to write patterns of very
high resolution.
• Electrons possess both particle and wave properties.
• Their wavelength is on the order of a few tenths of angstrom,
and therefore their resolution is not limited by diffraction
considerations.
83

Cont.
• Resolution of electron beam lithography is limited by forward
scattering of the electrons in the resist layer and back scattering
from the underlying substrate.
• Electron beam lithography is the most powerful tool for the
fabrication of feathers as small as 3-5nm.
• When an electron beam enters a polymer film or any solid
material, it loses energy via elastic and inelastic collisions known
collectively as electron scattering.
• Elastic collisions result only in a change of direction of the
electrons, whereas inelastic collisions lead to energy loss.
84

Cont.
• Scattering processes lead to a broadening of the beam, i.e. the
electrons spread out.
• The magnitude of electron scattering depends on: the atomic
number, density of both the resist and substrate and the
velocity of the electrons or the accelerating voltage.
• Exposure of the resist by the forward and backscattered
electrons depends on the beam energy, film thickness and
substrate atomic number.
85

Cont.
• As the beam energy increases, the energy loss per unit path length
and scattering cross-sections decreases.
• Thus the lateral transport of the forward scattered electrons and the
energy dissipated per electron decrease while the lateral extent of
the backscattered electrons increases due to the increased electron
range.
• As the resist film thickness increases, the cumulative effect of the
small angle collisions by the forward scattered electrons increases.
86

Cont.
• Thus the area exposed by the scattered electrons at the resist-
substrate interface is larger in thick films than in thin films.
• As the substrate atomic number increases, the electron
reflection coefficient increases which in turn increases the
backscattered contribution.
• Electron beam systems can be conveniently considered in two
broad categories: those using scanned, focused electron beams
which expose the wafer in serial fashion, and those projecting
an entire pattern simultaneously onto a wafer.
87

Cont.
• Scanning beam systems can be further divided into Gausian or
round beam systems and shaped beam systems.
• All scanning beam systems have four typical subsystems:
i. electron source (gun),
ii. electron column (beam forming system),
iii. mechanical stage and
iv. control computer which is used to control the various machine
subsystems and transfer pattern data to the beam deflection
systems.
88

Cont.
• Electron sources are the same as those used in conventional
electron microscopes.
• The sources can be divided into two groups: thermionic and field
emission.
• Thermionic guns sources from a material that is heated above a
critical temperature. These sources are prepared from materials
such as tungsten, thoriated tungsten, or lanthanum hexaboride.
• Field emission sources use a high electric field surrounding a very
sharp point of tungsten. The electric field extracts electrons at the
tip of the source, forming a Gaussian spot of only a few tens of
angstroms in diameter.
89

Cont.
• It is impossible to deflect an electron beam to cover a large
area, in a typical electron beam lithography system,
mechanical stages are required to move the substrate through
the deflection field of the electron beam column.
• Stages can be operated in a stepping mode.
• Alternatively, stages can be operated in a continuous mode
where the pattern is written on the substrate while the stage is
moving.
90

X-ray lithography
The essential ingredients in X-ray lithography include:
1. A mask consisting of a pattern made with an X-ray absorbing
material on a thin X-ray transparent membrane,
2. An X-ray source of sufficient brightness in the wavelength range
of interest to expose the resist through the mask, and
3. An X-ray sensitive resist material.
There are two X-ray radiation sources:
(i) electron impact and
(ii) synchrotron sources.
• Electron impact sources produce a broad spectrum of X-rays,
centered about a characteristic line of the material, which are
generated by bombardment of a suitable target material by a high
energy electron beam.
91

Cont.
• The synchrotron or storage ring produces a broad spectrum of radiation
stemming from energy loss of electrons in motion at relativistic
energies.
• This radiation is characterized by an intense, continuous spectral
distribution from the infrared to the long wavelength X-ray region.
• It is highly collimated and confined near the orbital plane of the
circulating electrons, thereby requiring spreading in the vertical
direction of moving the mask and wafer combination with constant
speed through the fan of synchrotron radiation.
• Synchrotrons offer the advantage of high power output.
92

Cont.
• Absorption of an X-ray photon results in the formation of a
photoelectron which undergoes elastic and inelastic collisions
within the absorbing material producing secondary electrons
which are responsible for the chemical reactions in the resist
film.
• The range of the primary photoelectrons is on the order of 100-
200 nm.
• A major limitation is that of penumbral shadowing, since the X-
ray source is finite in size and separated from the mask and the
edge of the mask does not cast a sharp shadow.
93

Cont.
• Low mask contrast is another factor that degrades the pattern
resolution.
• It is very important to keep the radiation source in a small area
in order to minimize penumbral shadowing and with a
maximum intensity of X-rays to minimize exposure time.
• X-ray proximity lithography is known to provide a one to one
replica of the features patterned on the mask, and the
resolution limit of the X-ray lithography is -25 nm
94

Thin Film Deposition
• Deposition of thin films has been a subject of intensive study for
almost a century, and many methods have been developed and
improved.
• Typical steps in making thin films:
1. Emission of particles from source (heat, high voltage . . .)
2. Transport of particles to substrate
3. Condensation of particles on substrate
• Film growth methods divided into two groups: vapor-phase
deposition and liquid-based growth.
95

Fundamentals of Film Growth
• Growth of thin films, as all phase transformation, involves the
processes of nucleation and growth on the substrate or growth
surfaces.
• Three basic nucleation modes:
(1) Island or Volmer-Weber growth,
(2) Layer or Frank-van der Merwe growth, and
(3) Island-layer or Stranski-Krastonov growth.
96

Cont.
Island growth occurs when the growth species are more strongly
bonded to each other than to the substrate.
The layer growth is the opposite of the island growth, where
growth species are equally bound more strongly to the substrate
than to each other. First complete monolayer is formed, before
the deposition of second layer occurs.
The island-layer growth is anintermediate combination of layer
growth and island growth. Such a growth mode typically
involves the stress, which is developed during the formation of
the nuclei or films.
97

Cont.
Island or Volmer-Weber growth
Layer or Frank-van der Merwe growth
Island-layer or Stranski-Krastonov growth
98

Cont.
• The deposit is single crystalline, polycrystalline or amorphous,
depends on the growth conditions and the substrate.
• Deposition temperature and the impinging rate of growth species
are the two most important factors.
99

Cont.
1. Growth of single crystal films is most difficult and requires:
i. a single crystal substrate with a close lattice match,
ii. a clean substrate surface so as to avoid possible secondary
nucleation,
iii. a high growth temperature so as to ensure sufficient mobility of
the growth species and
iv. low impinging rate of growth species so as to ensure sufficient
time for surface diffusion and incorporation of growth species into
the crystal structure and for structural relaxation before the arrival
of next growth species.
100

Cont.
2. Deposition of amorphous films typically occurs:
i. when a low growth temperature is applied, there is insufficient surface
mobility of growth species and/or
ii. when the influx of growth species onto the growth surface is very high,
growth species does not have enough time to find the growth sites with the
lowest energy.
3. The conditions for the growth of polycrystalline crystalline films fall between
the conditions of single crystal growth and amorphous film deposition. In general,
the deposition temperature is moderate ensuring a reasonable surface mobility of
growth species and the impinging flux of growth species is moderately high.
101

Thin Film Deposition methods
Physical processes:
 Evaporation: Thermal, E-beam, Laser, Ion-plating.
 Sputtering: DC, RF, Magnetron, Reactive.
 Spray: Flame, Plasma.
Chemical processes:
 Chemical Vapor Deposition (CVD): Thermal, MOCVD,
PECVD.
 Plating: Electroplating, Electroless.
 Solgel
 ALE (Atomic Layer Deposition)
Molecular Beam Epitaxy
102

Physical vapor deposition (PVD)
• PVD process is a group of thin film processes in which a material is
converted into its vapor phase in a vacuum chamber and condensed onto
a substrate surface as a weak layer.
PVD processes are environmentally friendly vacuum deposition techniques
consisting of three fundamental steps:
 Vaporization of the material from a solid source assisted by high
temperature vacuum or gaseous plasma. (Synthesis of the coating vapor)
 Transportation of the vapor in vacuum or partial vacuum to the substrate
surface.
 Condensation onto the substrate to generate thin films.
• These steps are carried out inside a vacuum chamber, so evacuation of
the chamber must precede the actual PVD process.
103

Advantages and disadvantages of PVD
Disadvantage of PVD:
• The method is directional due to a large mean free path of the
vaporized source materials.
• Alloys different vapor pressures and are therefore challenging.
• Deposited film adhesion can be poor.
Advantage of PVD:
• Limited damage to sample (lower these could be some low
pressure effects such as oxygen vacancies produced in an oxide).
• Deposited metal films are very poor
104

Evaporation
• This involves vaporization of a material by way of resistive heating
• The vaporized materials is subsequently deposited on desired as a
thin film
• This process relies on the fact that when heated (particularly under
reduced pressure) there is a finite vapor pressure over all materials.
• Thermal energy is supplied by passing a large current through a
crucible/basket (a thermal tolerant material e.g. tungsten) holding
the source material
105

Cont.
• The resistivity of the crucible generates sufficient heat such that
the source material is vaporized as atom.
• The vaporized source material travels through a low pressure
space to become incident on a target sample, where it condenses
to form a film.
• The source material either sublimes (a solid to vapor), or
evaporates liquid to vapor transition via intermediate melting
from solid to liquid.
• This process is achieved in a vacuum chamber.
106

Thermal Evaporation
Difficult: High melting point materials, uniformly heating, rapidly
change of deposition rate, reactions between the source and the
heating container. 107

E-beam evaporation
• High energy focused electron
beam to heat the source material
at a small area.
• Larger deposition rate.
• Water-cooled container (cavity
or hearth): No source-container
reaction.
• Sweeping or oscillating the e-
beam to heat the source material
uniformly.
• Multiple hearth sources:
• Different source materials.
108

Sputtering
• Sputtering is to use energetic ions to knock atoms or molecules
out from a target that acts as one electrode and subsequently
deposit them on a substrate acting as another electrode.
• Although various sputtering techniques have been developed, the
fundamentals of the sputtering process are more or less the same.
Basic Techniques:
• DC (diode) sputtering
• RF (radio frequency) sputtering
• Magnetron sputtering
• Reactive sputtering
109

Sputtering Deposition Process
Sputtering:
• Ions are accelerated into target
• Some of the surface atoms are sputtered off of the target.
• These sputtered atoms “flow” across the chamber to where they
are deposited
110

DC Sputtering
• Target and substrate serve as electrodes and face each other in a
typical sputtering chamber.
• An inert gas, typically argon is introduced into the system as the
medium to initiate and maintain a discharge.
• When an electric field a dc voltage is applied to the electrodes, a
glow discharge is initiated and maintained between the
electrodes.
• Free electrons will be accelerated by the electric field and gain
sufficient energy to ionize argon atoms.
111

Cont.
• The gas density or pressure must not be too low, or else the electrons will
simply strike the anode without having gas phase collision with argon
atoms.
• However, if the gas density or pressure is too high, the electrons will not
have gained sufficient energy when they strike gas atoms to cause
ionization.
• Resulting positive ions, Ar+, in the discharge strike the cathode (the
source target) resulting in the ejection of neutral target atoms through
momentum transfer.
• The atoms pass through the discharge and deposit on the opposite
electrode (the substrate with growing film).
112

The principles of dc and RF sputtering systems
113

RF Sputtering
• For the deposition of insulating films, an alternate electric field
is applied to generate plasma between two electrodes.
• Typical RF frequencies employed range from 5 to 30MHz
However, 13.56 MHz has been reserved for plasma
processing.
• The key element in RF sputtering is that the target self-biases
to a negative potential and behaves like a dc target.
114

Cont.
• Such a self-negative target bias is a consequence of the fact that
electrons are considerably more mobile than ions and have little
difficulty in following the periodic change in the electric field.
• To prevent simultaneous sputtering on the grown film or substrate,
the sputter target must be an insulator and be capacitively coupled to
the RF generator.
• This capacitor will have a low RF impedance and will allow the
formation of a dc bias on the electrodes.
115

Cont.
• The types of plasmas encountered in thin film processing techniques and
systems are typically formed by partially ionizing a gas at a pressure well below
atmospheric.
• The plasma based film processes differ from other film deposition techniques
such as evaporation, since the plasma processes is not thermal and not
describable by equilibrium thermodynamics.
• Magnetic field has been introduced into sputtering processes to increase the
residence time of growth species in the vapor phase; such sputtering is referred
to as magnetron sputtering.
• Reactive gases have also been introduced into the deposition chamber to form
compound films, which are known as reactive sputtering.
116

Magnetron Sputter Deposition
• Magnetron sputtering is the most widely used method for vacuum
thin film deposition.
• Although the basic diode sputtering method (without magnetron or
magnetic enhanced) is still used in some application areas,
magnetron sputtering now serves over 90% of the market for sputter
deposition.
• Magnetron sputtering can be used to coat virtually anything with a
wide range of materials - any solid metal or alloy and a variety of
compounds.
117

Cont.
• A typical sputtering system consists of a vacuum chamber with
substrate holders and magnetron guns, vacuum pumps and
gauging, a gas supply system, power supplies and a computer
control system.
• A Magnetron is comprised of :
• A CATHODE = electron
source
• An ANODE = electron
collector
• A combined ELECTRIC &
MAGNETIC FIELD B X E
118

The Plasma Discharge
• Plasma is a fluid of positive ions
and electrons in a quasi-neutral
electrical state. The vessel that
contains this fluid is formed by
electric and magnetic fields.
• In many plasma coating
applications positive ions are
generated by collisions between
neutral particles and energetic
electrons. The electrons in a
plasma are highly mobile,
especially compared to the larger
ions (typically argon for
sputtering).
• Control of the highly mobile
plasma electrons is the key to all
119

Cont.
• A magnetron consists of a target with magnets arranged behind it
to make a magnetic trap for charged particles, such as argon ions,
in front of the target.
• Atoms are knocked out of the target surface by the ions - this is
sputtering. These sputtered atoms aren’t charged negatively or
positively, so they go straight out of the magnetic trap to coat the
substrate.
120

Cont.
121

Cont.
Advantages of Magnetron Sputtering:
• It works well with insulating targets
• High deposition rate
• Reducing electron bombardment of substrate
• Extending the operating vacuum range– ability to operate at lower
pressures
Disadvantages for Magnetron Sputtering:
• An erosion track in the target
– This leads to poor efficiency of sputtering yield versus target volume
compared to non-magnetron sputtering
• Non-uniform removal of particles from target result in non-uniform
films on substrate.
122

Comparison of evaporation and sputtering
Some major differences between evaporation and sputtering are briefly
summarized below:
1. The deposition pressure differs noticeably. Evaporation uses low
pressures typically ranging from 10−3
to 10−10
torr, whereas
sputtering requires a relatively high pressure typically of -100torr.
Atoms or molecules in evaporation chamber do not collide with each
other, whereas the atoms and molecules in sputtering do collide with
each other prior to arrival at the growth surface.
2. The evaporation is a process describable by thermodynamical
equilibrium, whereas sputtering is not.
123

Cont.
3. The growth surface is not activated in evaporation, whereas the
growth surface in sputtering is constantly under electron
bombardment and thus is highly energetic.
4. The evaporated films consist of large grains, whereas the
sputtered films consist of smaller grains with better adhesion to
the substrates.
5. Fractionation of multi-component systems is a serious challenge
in evaporation, whereas the composition of the target and the
film can be the same.
124

Molecular beam epitaxy (MBE)
• MBE can be considered as a special case of evaporation for single
crystal film growth, with highly controlled evaporation of a variety
of sources in ultrahigh-vacuum of typically 10−10
torr.
• Besides the ultrahigh vacuum system, MBE mostly consists of
realtime structural and chemical characterization capability,
including reflection high energy electron diffraction (RHEED), X-
ray photoelectric spectroscopy (XPS), Auger electron spectroscopy
(AES).
125

Cont.
• In MBE, the evaporated atoms or molecules from one or more sources do not
interact with each other in the vapor phase under such a low pressure.
• Although some gaseous sources are used in MBE, most molecular beams are
generated by heating solid materials placed in source cells, which are referred
to as effusion cells or Knudsen cells.
• A number of effusion cells are radiatically aligned with the substrates.
• The atoms or molecules striking on the single crystal substrate results in the
formation of the desired epitaxial film.
126

Cont.
• Ultrahigh vacuum environment ensures absence of impurity or
contamination, and thus a highly pure film can be readily obtained.
A number of effusion cells radiatically aligned with the substrates.
127

The main attributes of MBE
1. A low growth temperature (e.g. 550°C for GaAs) that limits diffusion and
maintains hyperabrupt interfaces, which are very important in fabricating two-
dimensional nanostructures or multilayer structures such as quantum wells.
2. A slow growth rate that ensures a well controlled two-dimensional growth at a
typical growth rate of 1𝜇𝑚/h . A very smooth surface and interface is
achievable through controlling the growth at the monoatomic layer level.
3. A simple growth mechanism compared to other film growth techniques
ensures better understanding due to the ability of individually controlled
evaporation of sources.
4. A variety of in situ analysis capabilities provide invaluable information for
the understanding and refinement of the process.
128

Chemical Vapour Deposition (CVD)
• Chemical Vapour Deposition (CVD) is a chemical process used
to produce high purity, high performance solid materials.
• Chemical Vapor Deposition (CVD) refers to the formation of a
non-volatile solid film on a substrate from the reaction of vapor
phase chemical reactants containing the right constituents.
• A reaction chamber is used for this process, into which the
reactant gases are introduced to decompose and react with the
substrate to form the film.
129

Cont.
• In a typical CVD process, the substrate is exposed to one or
more volatile precursors which react and decompose on the
substrate surface to produce the desired deposit.
• During this process, volatile by-products are also produced,
which are removed by gas flow through the reaction chamber.
130

Cont.
A typical CVD system consists of the following parts:
1) sources of and feed lines for gases;
2) mass flow controllers for metering the gases into the system;
3) a reaction chamber or reactor;
4) a system for heating up the wafer on which the film is to be
deposited; and
5) temperature sensors.
131

CVD steps
CVD steps:
• Introduce reactive gases to the chamber.
• Activate gases (decomposition) by heat or
plasma.
• Gas absorption by substrate surface .
• Reaction take place on substrate surface,
film formed.
• Transport of volatile byproducts away form
substrate.
• Exhaust waste.
CVD : deposit film through chemical reaction
and surface absorption.
132

Types of CVD
• A variety of CVD methods and CVD reactors have been developed,
depending on:
 the types of precursors used,
 the deposition conditions applied and
 the forms of energy introduced to the system to activate the
chemical reactions desired for the deposition of solid films on
substrates.
• For example, when metalorganic compounds are used as
precursors, the process is generally referred to as MOCVD
(metalorganic CVD), and when plasma is used to promote chemical
reactions, this is a plasma enhanced CVD or PECVD.
133

(Types of CVD) Cont.
APCVD (Atmospheric Pressure CVD), mass transport limited growth rate, leading
to non-uniform film thickness.
LPCVD (Low Pressure CVD):
• Low deposition rate limited by surface reaction, so uniform film thickness
(many wafers stacked vertically facing each other; in APCVD, wafers have to
be laid horizontally side by side.
• Gas pressures around 1-1000mTorr (lower P => higher diffusivity of gas to
substrate)
• Better film uniformity & step coverage and fewer defects
• Process temperature 500°C
134

Cont.
PECVD (Plasma Enhanced CVD)
• Plasma helps to break up gas molecules: high reactivity, able to
process at lower temperature and lower pressure (good for
electronics on plastics).
• Pressure higher than in sputter deposition: more collision in gas
phase, less ion bombardment on substrate
• Can run in RF plasma mode: avoid charge buildup for insulators
• Film quality is poorer than LPCVD.
• Process temperature around 100 - 400°C.
135

Cont.
MOCVD (Metal-organic CVD, also called OMVPE - organo
metallic VPE), epitaxial growth for many optoelectronic devices
with III-V compounds for solar cells, lasers, LEDs, photo-
cathodes and quantum wells.
136

CVD advantages and disadvantages
(as compared to physical vapor deposition)
Advantages:
• High growth rates possible, good reproducibility.
• Can deposit materials which are hard to evaporate.
• Can grow epitaxial films. In this case also termed as “vapor
phase epitaxy (VPE)”. For instance, MOCVD (metal-organic
CVD) is also called OMVPE (organo-metallic VPE).
• Generally better film quality, more conformal step coverage.
Disadvantages:
• High process temperatures.
• Complex processes, toxic and corrosive gasses.
• Film may not be pure (hydrogen incorporation…).
137

Wet Chemical Processes for the Synthesis of
Nanocrystalline Oxide Powders
• Sol-Gel Process
• Hydrothermal Process
• Co-Precipitation Process
• Polyol Process
• Combustion Process
138

Sol-Gel Method
• In materials science, the sol–gel process is a method for
producing solid materials from small molecules.
• Sol-gel is the multi step process, involving chemical and
physical processes associated with hydrolysis,
polymerization, gelation, condensation, drying and
densification
• The process involves conversion of monomers into a
colloidal solution (sol) that acts as the precursor for an
integrated network (or gel) of either discrete particles or
network polymers. 139

Cont.
The stages of the Sol-Gel process:
i. Hydrolysis of precursor (sol formation)
ii. Polycondensation (gelataion)
iii. Aging
iv. Drying
v. Calcination
140

Cont.
• In sol gel process initially a stable colloidal solution called sol is formed. This
process generally starts with the mixing of solid materials in water or in a suitable
solvent (usually an alcohol) at ambient or slightly elevated temperatures.
• The sol is a liquid suspension of solid particles ranging in size from 1 nm to 1
micron.
• It can be obtained by hydrolysis and partial condensation of precursors such as an
inorganic salt.
• In sol gel process, controlling the pH of starting solution is very much important
to avoid the precipitation as well as to form the homogenous gel, which can
achieved by the addition of base or acidic solutions.
141

Cont.
• The further condensation of sol particles into a three dimensional network
produces a gel material. The gel is a diphasic material in which the solids
encapsulate the solvent.
• The encapsulated liquid can be removed from a gel by either evaporative
drying or with supercritical drying /extraction.
• The resulting solid products are known as xerogel and aerogel respectively.
• When gels are dried by evaporation, the dried product is called xerogel.
• When the gels are dried by supercritical drying, the dried gel is called
aerogels. The aerogel retains high porosity and has very high pore volume.
142

General scheme of preparation by solgel method
143

Cont.
144

Cont.
Advantages of Sol-Gel Process:
• Low temperature processing and consolidation is possible.
• Smaller particle size and morphological control in powder synthesis.
• Sintering at low temperature also possible.
• Better homogeneity and phase purity compared to traditional ceramic method.
Disadvantages of Sol-Gel Process:
• Raw materials for this process is expensive (in the case of metal alkoxides)
compared to mineral based metal ion sources.
• Products would contain high carbon content when organic reagents are used in
preparative steps and this would inhibit densification during sintering.
• Since several steps are involved, close monitoring of the process is needed. 145

Characterization of Nanoparticles
• Nanoparticles are generally characterized by their:
 size,
 morphology and
 surface charge, using advanced microscopic techniques.
• The average particle diameter, their size distribution and charge affect the
physical stability and the in vivo distribution of the nanoparticles.
• Electron microscopy techniques are very useful in ascertaining the overall shape
of polymeric nanoparticles, which may determine their toxicity.
• The surface charge of the nanoparticles affects the physical stability and
redispersibility of the polymer dispersion as well as their in vivo performance.
146

Characterization Techniques
Two types of nanomaterial characterization:
• Spectroscopic methods
i.e. UV-VIS, DLS, etc
• Imaging methods
i.e. TEM, SEM, AFM, etc
147

0-D Nanoparticles
• Color of a nanoparticle solution is dependent on nanoparticle
size.
148

UV-Vis Absorption
• Gives quantitative measure of color.
• What wavelengths are absorbed?
• What wavelengths are transmitted?
149

Dynamic Light Scattering (DLS)
• Currently, the fastest and most popular method of determining particle
size.
• DLS is widely used to determine the size of Brownian nanoparticles in
colloidal suspensions.
• Shining monochromatic light (laser) onto a solution of spherical
particles in Brownian motion causes a Doppler shift when the light hits
the moving particle, changing the wavelength of the incoming light.
• This change is related to the size of the particle.
• It is possible to extract the size distribution and give a description of the
particle’s motion in the medium, measuring the diffusion coefficient of
the particle and using the autocorrelation function.
• The photon correlation spectroscopy (PCS) represent the most
frequently used technique for accurate estimation of the particle size and
size distribution based on DLS.
150

Dynamic Light Scattering (DLS)
DLS measures the Brownian motion of the nanoparticles and
correlates this to particle size
• DLS tell you about the size of the particles as well as the
distribution of particles within a sample (i.e. the homogeneity of
nanoparticles in a sample).
• Brownian motion of small
particles is faster than that
of large particles so the
position of small particles
lose correlation faster.
151

Brownian Motion
• A suspended particle is constantly and randomly bombarded
from all sides by molecules of the liquid. If the particle is very
small, the number of hits it takes from one side at a given time
will be stronger than the bumps from other side. This make the
particle jump. These small random jumps are what make up
Brownian motion.
152

DLS instrument
153

Imaging Methods
i. Light (Optical) Microscopy
ii. Electron Microscopy:
- TEM
- SEM
iii. Scanning Probe Microscopy
- STM
- AFM
154

Resolution Limit
• Light microscopes
– 500 X to 1500 X magnification
– Resolution of ~0.2 µm
– The lowest wavelength of visible
light is around 400 nm which
gives the optical limit of about
0.2 microns
– Limits reached by early 1930’s
• Resolution dependent on:
– wavelength of illumination ()
– Numerical Aperture (NA) of lens system
NA
d

612
.
0

155

Electron Microscopes
Wavelength of the electron dependent on:
• Electron mass (m)
• Electron charge (q)
• Potential difference to accelerate electrons (V)
• Potential difference is a way to express the momentum of electrons.
This equation is related to the de Broglie equation for the
wavelength of objects. Electrons can achieve much smaller
wavelengths than visible light
 
h
2mqV
156

• TEM is a very powerful tool for material science.
• A high energy beam of electrons is shone through a very thin sample
TEM can be used to study:
 The crystal structure and features in the structure like dislocations and grain
boundaries.
 Chemical analysis can also be performed.
 The growth of layers, their composition and defects in semiconductors.
 High resolution can be used to analyze the quality, shape, size and density of
quantum wells, wires and dots.
Transmission Electron Microscope(TEM)
157

Cont.
• Uses electrons instead of light.
• Because the wavelength of electrons is much smaller than that
of light, the optimal resolution attainable for TEM images is
many orders of magnitude better than that from a light
microscope.
• Thus, TEMs can reveal the finest details of internal structure in
some cases as small as individual atoms.
158

Imaging
• The beam of electrons from the electron gun is focused into a small,
thin, coherent beam by the use of the condenser lens.
• This beam is restricted by the condenser aperture, which excludes
high angle electrons.
• The beam then strikes the specimen and parts of it are transmitted
depending upon the thickness and electron transparency of the
specimen.
• This transmitted portion is focused by the objective lens into an
image on phosphor screen or charge coupled device (CCD) camera.
• Optional objective apertures can be used to enhance the contrast by
blocking out high-angle diffracted electrons. The image then passed
down the column through the intermediate and projector lenses, is
enlarged all the way. 159

Cont.
• The image strikes the phosphor screen and light is generated,
allowing the user to see the image.
• The darker areas of the image represent those areas of the
sample that fewer electrons are transmitted through while the
lighter areas of the image represent those areas of the sample
that more electrons were transmitted through.
160

Cont.
161

TEM
• TEM operates on different principle than SEM, yet it often
brings same type of data.
• The sample preparation for TEM is complex and time
consuming because of its requirement to be ultra thin for the
electron transmittance.
162

Specimen Preparation
• A TEM specimen must be thin enough to transmit sufficient
electrons to form an image with minimum energy loss.
• For most electronic materials, a common sequence of preparation
techniques is ultrasonic disk cutting, dimpling, and ion-milling.
• Dimpling is a preparation technique that produces a specimen with
a thinned central area and an outer rim of sufficient thickness to
permit ease of handling.
• Ion milling is traditionally the final form of specimen preparation.
In this process, charged argon ions are accelerated to the specimen
surface by the application of high voltage. The ion impingement
upon the specimen surface removes material as a result of
momentum transfer
163

Transmission Electron Microscope(TEM)
Transmission Electron
Microscope (TEM):
1. e-beam strikes sample and
is transmitted through the
sample
2. Scattering occurs
3. Un-scattered electrons pass
through sample and are
detected
164

Scanning Electron Microscope
• It is a microscope that produces an image by using an electron beam
that scans the surface of a specimen inside a vacuum chamber.
In a SEM we can study:
• Topography and morphology
• Chemistry
• Crystallography
• Orientation of grains
• In-situ experiments:
– Reactions with atmosphere
– Effects of temperature
165

The instrument in brief
166

Components of the instrument
•electron gun (filament)
• electromagnetic optics
• scan coils
• sample stage
• detectors
• vacuum system
• computer hardware and
software (not trivial!!)
167

Electron guns
• We want many electrons per time
unit per area (high current density)
and as small electron spot as
possible
• Traditional guns: thermionic
electron gun (electrons are emitted
when a solid is heated)
• W-wire, LaB6-crystal
• Modern: field emission guns (FEG)
(cold guns, a strong electric field is
used to extract electrons)
• Single crystal of W, etched to a
thin tip
168

Detectors Our traditional detectors
Backscattered electron
detector:
(Solid-State Detector)
Secondary electron
detector:
(Everhart-Thornley)
 Secondary electrons: Everhart-Thornley Detector
 Backscattered electrons: Solid State Detector
 X-rays: Energy dispersive spectrometer (EDS) 169

How the SEM works?
• The SEM uses electrons instead of light to form
an image.
• A beam of electrons is produced at the top of the
microscope by heating of a metallic filament.
• The electron beam follows a vertical path
through the column of the microscope. It makes
its way through electromagnetic lenses which
focus and direct the beam down towards the
sample.
• Once it hits the sample, other electrons ( back-
scattered or secondary ) are ejected from the
sample. Detectors collect the secondary or
backscattered electrons, and convert them to a
signal that is sent to a viewing screen similar to
the one in an ordinary television, producing an
image. 170

Signals from the sample
171

The signals come from
•Diameter of the interaction volume
is larger than the electron spot
 resolution is poorer than the size
of the electron spot
172

Secondary electrons (SE)
 Generated from the collision between the
incoming electrons and the loosely bonded
outer electrons
 Low energy electrons (~10-50 eV)
 Only SE generated close to surface escape
(topographic information is obtained)
 Number of SE is greater than the number
of incoming electrons
 We differentiate between SE1 and SE2
173

SE1
 SE2 come from a surface area that is
bigger than the spot from the incoming
electrons
 resolution is poorer than for SE1
exclusively
 The secondary electrons that are generated by the
incoming electron beam as they enter the surface
 High resolution signal with a resolution which is
only limited by the electron beam diameter
SE2
 The secondary electrons that are
generated by the backscattered
electrons that have returned to the
surface after several inelastic scattering
events
174

Backscattered electrons (BSE)
 A fraction of the incident electrons is
retarded by the electromagnetic field of
the nucleus and if the scattering angle is
greater than 180° the electron can escape
from the surface
 High energy electrons (elastic scattering)
 Fewer BSE than SE
 We differentiate between BSE1 and
BSE2
175

BSE vs SE
• SE produces higher resolution
images than BSE
• By placing the secondary
electron detector inside the lens,
mainly SE1 are detected
 Resolution of 1–2nm is possible
176

X-rays
 Photons not electrons
 Each element has a fingerprint X-ray
signal
 Poorer spatial resolution than BSE and
SE
 Relatively few X-ray signals are emitted
and the detector is inefficient
 relatively long signal collecting
times are needed
177

SEM principle
178

SEM
• Scanning electron microscopy (SEM) is giving morphological
examination with direct visualization.
• The techniques based on electron microscopy offer several
advantages in morphological and sizing analysis; however, they
provide limited information about the size distribution and true
population average.
179

Scanning Probe Microscopy
• SPM is a relatively new characterization technique and has found
wide spread applications in a nanotechnology.
• Measure feedback from atomically defined tip.
• Many types of feedback (dependent on tip).
• The two major members of the SPM family are scanning
tunneling microscopy (STM) and atomic force microscopy
(AFM).
• AFM – Forces between sample and tip
• STM – Tunneling current between sample and tip
180

Scanning tunneling microscopy (STM)
• STM relies on electron tunneling, which is a phenomenon based
on quantum mechanics.
• In STM the tunneling current is given by:
𝐼 = 𝐶𝜌𝑡𝜌𝑠𝑒−𝑧𝑘
1
2
Where:
 𝑘 =
2𝑚𝑞(𝑉−𝐸)
ℎ
 z is the distance between the tip and the planar surface or
sample,
 𝜌𝑡 is the tip electronic structure, 𝜌𝑠 is the sample electronic
structure, and
 C is a constant dependent on the voltage applied between the tip
and the sample surface. 181

Cont.
• The tunneling current decays exponentially with the tip-sample
distance.
• In a typical STM, a conductive tip is positioned above the surface
of a sample.
• When the tip moves back and forth across the sample surface at
very small intervals, the height of the tip is continually adjusted to
keep the tunneling current constant.
• The tip positions are used to construct a topographic map of the
surface.
182

Cont.
• An extremely sharp tip is
mounted on to a three-
dimensional positioning
stage made of an array of
piezoelectrics.
• Such a tip would move
above the sample surface in
three dimensions accurately
controlled by the
piezoelectric arrays.
183

Cont.
• Typically the distance between the tip and the sample surface falls
between 0.2 and 0.6 nm, thus a tunneling current in the scale of 0.1-l0nA
is commonly generated. The scanning resolution is about 0.01 nm in XY
direction and 0.002 nm in Z direction, offering true atomic resolution
three-dimensional image.
STM can be operated in two modes:
• In constant current imaging a feedback mechanism is enabled that a
constant current is maintained while a constant bias is applied between
the sample and tip.
• As the tip scans over the sample, the vertical position of the tip is altered
to maintain the constant separation.
184

Cont.
• An alternating imaging mode is the constant height operation in
which constant height and bias are simultaneously maintained.
• A variation in current results as the tip scans the sample surface
because a topographic structure varies the tip-sample separation.
• The constant current mode produces a contrast directly related to
electron charge density profiles, whereas the constant height
mode permits faster scan rates.
185

How an STM Works?
• An STM works by scanning a very sharp metal wire tip over a surface.
• By bringing the tip very close to the surface, and by applying an
electrical voltage to the tip or sample, we can image the surface at an
extremely small scale – down to resolving individual atoms.
The STM is based on several principles:
• One is the quantum mechanical effect of tunneling. It is this effect that
allows us to “see” the surface.
• Another principle is the piezoelectric effect. It is this effect that allows
us to precisely scan the tip with angstrom-level control.
• Lastly, a feedback loop is required, which monitors the tunneling current
and coordinates the current and the positioning of the tip.
186

Cont.
• This is shown schematically below where the tunneling is from
tip to surface with the tip rastering with piezoelectric
positioning, with the feedback loop maintaining a current
setpoint to generate a 3D image of the electronic topography:
187

Tunneling
• Tunneling is a quantum mechanical effect.
• A tunneling current occurs when electrons move through a barrier
that they classically shouldn’t be able to move through.
• In the quantum mechanical world, electrons have wavelike
properties. When an electron moves through the barrier, it is
called tunneling.
• Quantum mechanics tells us that electrons have both wave and
particle-like properties.
• Tunneling is an effect of the wavelike nature.
188

Cont.
• When an electron (the wave) hits
a barrier, the wave doesn’t
abruptly end, but tapers off very
quickly – exponentially. For a
thick barrier, the wave doesn’t
get past.
• The scenario if the barrier is
quite thin (about a nanometer).
Part of the wave does get
through and therefore some
electrons may appear on the
other side of the barrier. 189

Tunneling in STM:
• The starting point of the electron is
either the tip or sample, depending
on the setup of the instrument.
• The barrier is the gap (air, vacuum,
liquid), and the second region is the
other side, i.e. tip or sample,
depending on the experimental
setup.
• By monitoring the current through
the gap, we have very good control
of the tip-sample distance.
data processing
and display
190

Feedback Loop
• Electronics are needed to measure the current, scan the tip, and translate
this information into a form that we can use for STM imaging.
• A feedback loop constantly monitors the tunneling current and makes
adjustments to the tip to maintain a constant tunneling current.
• These adjustments are recorded by the computer and presented as an
image in the STM software.
• Such a setup is called a constant current image.
• In addition, for very flat surfaces, the feedback loop can be turned off
and only the current is displayed.
• This is a constant height image.
191

Atomic Force Microscope (AFM)
• STM is limited to an electrically conductive surface.
• AFM was developed as a modification of STM for dielectric
materials.
• A variety of tip-sample interactions measured by an AFM,
depending on the separation.
• At short distances, the van der Waals interactions are predominant.
• Van der Waals force consists of interactions of three components:
- permanent dipoles,
- induced dipoles and
- electronic polarization.
192

Cont.
• Long-range forces act in addition to short-range forces between the
tip and sample, and become significant when the tip-sample
distance increases such that the van der Waals forces become
negligible.
• Examples of such forces include:
 electrostatic attraction or repulsion,
 current induced or static-magnetic interactions, and
 capillary forces due to the condensation of water between the
sample and tip.
193

Cont.
• The motion of a cantilever beam with an ultra small mass is
measured, and the force required to move this beam
through measurable distance 10−4
𝐴° can be as small as
10−18
N.
194

Cont.
The instrument consists of:
 a cantilever with a nanoscale
tip,
 a laser pointing at the end of a
cantilever,
 a mirror and a photodiode
collecting the reflected laser
beam, and
 a three dimensional positioning
sample stage which is made of
an array of piezoelectrics.
• The images are generated by scanning the tip across the surface.
• The AFM measures the minute upward and downward deflections of
the tip cantilever while maintaining a constant force of contact. 195

Cont.
1. Tip scans across surface
2. Laser reflects off of
cantilever to a photodetector
3. Feedback loop changes tip to
sample distance
4. Height changes recorded
• AFM provides the most accurate description of size and size
distribution and requires no mathematical treatment.
196

X-ray diffraction (XRD)
XRD is used to address all issues related to the crystal structure
of solids, including:
• lattice constants and geometry,
• identification of unknown materials,
• orientation of single crystals,
• preferred orientation of polycrystals,
• defects,
• stresses, etc.
197

Cont.
• In XRD, a collimated beam of X-rays is incident on a
specimen and is diffracted by the crystalline phases in the
specimen according to Bragg's law:
λ = 2𝑑 sin 𝜃
• The intensity of the diffracted X-rays is measured as a function
of the diffraction angle 2𝜃 and the specimen's orientation.
• This diffraction pattern is used to identify the specimen's
crystalline phases and to measure its structural properties.
198

Cont.
• XRD is nondestructive and does not require elaborate sample
preparation, wide usage of XRD method in materials
characterization.
• Diffraction peak positions are accurately measured with XRD,
which makes it the best method for characterizing homogeneous
and inhomogeneous strains.
• Homogeneous or uniform elastic strain shifts the diffraction peak
positions. From the shift in peak positions, one can calculate the
change in d-spacing, which is the result of the change of lattice
constants under a strain.
199

Cont.
• Inhomogeneous strains vary from crystallite to crystallite or
within a single crystallite and this causes a broadening of the
diffraction peaks that increase with sin𝜃.
• Peak broadening is also caused by the finite size of crystallites,
but here the broadening is independent of sin𝜃.
• When both crystallite size and inhomogeneous strain contribute
to the peak width, these can be separately determined by careful
analysis of peak shapes.
200

Cont.
• If there is no inhomogeneous strain, the crystallite size, D, can be
estimated from the peak width with the Scherrer's formula:
𝐷 =
𝐾λ
𝐵 cos 𝜃𝐵
where:
• λ is the X-ray wavelength,
• B is the full width of height maximum (FWHM) of a diffraction
peak,
• 𝜃𝐵 is the diffraction angle, and
• K is the Scherrer’s constant of the order of unity for usual crystal.
201

Cont.
• Nanoparticles often form twinned structures; therefore, Scherrer’s
formula may produce results different from the true particle sizes.
• X-ray diffraction only provides the collective information of the
particle sizes and usually requires a sizable amount of powder.
• This technique is very useful in characterizing nanoparticles.
• The film thickness of epitaxial and highly textured thin films can
also be estimated with XRD.
202

Cont.
• One of the disadvantages of XRD is the low intensity of
diffracted X-rays, particularly for low-Z materials.
• For low-Z materials, neutron or electron diffraction is more
suitable.
• Typical intensities for electron diffraction are ~108
times larger
than for XRD.
• Because of small diffraction intensities, XRD generally requires
large specimens and the information acquired is an average over
a large amount of material.
203

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