GENERAL METHODS OF PREPARATION OF NANOO)

1. GENERAL METHODS OF
PREPARATION
UNIT 2

2. Syllabus
Bottom-up Synthesis-Top-down Approach: Co-
Precipitation, Ultrasonication, Mechanical Milling,
Colloidal routes, Self-assembly, Vapour phase
deposition, MOCVD, Sputtering, Evaporation,
Molecular Beam Epitaxy, Atomic Layer Epitaxy,
MOMBE

3. INTRODUCTION
• Using the nanotechnology, we can arrange atoms
and molecules exactly as we want.
• In nano technology, manufactured products are
made from atoms.
• The properties of these products depend on how
the atoms are arranged.
• For example, if we rearrange the atoms in coal, we
get diamond. If we rearrange the atoms in sand
(and add a pinch of impurities), we get computer
chips.

4. Contd….
• Two main approaches are used for the
synthesis of nanomaterials and the
fabrications of nanostructures in
nanotechnology.
• They are:
–Bottom-up approach
–Top-down approach

5. An illustration of the top-down
approach vs. the bottom-up approach

6. BOTTOM-UP APPROACH
• Bottom-up approach refers to the construction of
nanomaterial from the bottom, i.e. atom-by-atom,
molecule-by-molecule or cluster-by-cluster.
• In bottom-up approach, the atoms or molecules are
used as the building blocks to produce
nanoparticles, nanotubes, nanorods, thin films or
layered structures.

7. Contd….

8. Contd….

9. •Synthesis of large polymer molecule is a typical example for bottom-up
approach, in which individual building blocks (i.e. monomers) are assembled
to a large molecule or polymerized into bulk material.
•Fabrication of integrated circuits (assembling different components, e.g.
resistors, capacitors, transistors etc) on a PCB
•Crystal growth, where growth species either atoms, or ions or molecules
orderly assemble into desired crystal structure on the growth surface.
•Colloidal dispersion in the synthesis of nanoparticles.
•Nanolithography and nano-manipulation.
•Production of salt and nitrate in chemical industry, the growth of single
crystals and deposition of films in electronic industry.
Examples of Bottom-Up Approach

10. Contd….
• Although such processes provide tremendous freedom
among the resultant products, the number of possible
structures to be obtained is comparatively small.
• In order to obtain the ordered structures, bottom-up
processes must be supplemented by the self-organization
of individual particles, in which the atoms or molecules
arrange themselves into a structure due to their natural
properties.
• Crystals grown for the semiconductor industry provide an
example of self assembly, as does chemical synthesis of
large molecules.

11. Contd….

12. TOP-DOWN APPROACH
• The word "top-down" means starting from large
pieces of material and producing the intended
structure by physical or mechanical or chemical
methods.
• As long as the structure are within a range of sizes
that are accessible by either mechanical tools or
photolithography processes, then top-down
processes have an unmatched flexibility in their
application.

13. Contd…..
• The principle behind the top-down approach is to
take a bulk piece of the material and then modify it
into the wanted nanostructure.
• Cutting, grinding and etching are typical fabrication
techniques, which have been developed to work on
the nano scale.
• The sizes of the nanostructures, which can be
produced with top-down techniques, are between
10 to 100 nm.

14. Contd…..

15. Contd….

16. • Examples: 1. Creating circuits on the surface of a
silicon microchip by etching.
• Attrition or milling in making nanoparticles
• NOTE: Lithography may be considered as a hybrid
approach, since the growth of thin films is bottom-
up whereas etching is top-down.
Examples of Top-Down Approach

17. Contd….

18. Pros and Cons

19. Co-Precipitation Method

20. Intro…..
• In chemistry, coprecipitation (CPT) or co-
precipitation is the carrying down by
a precipitate of substances normally soluble under
the conditions employed.
• Reactions involve the simultaneous occurrence of
nucleation, growth, coarsening and/or
agglomeration processes.
• Co-precipitation reactions exhibit the following
characteristics:
– The products are generally insoluble species formed
under conditions of high supersaturation

21. Contd…..
– Nucleation is a key step, and a large number of small
particles will be formed
– Secondary processess, such as Ostwald Ripening and
aggregation, dramatically affect the size, morphology
and properties of the products.
– The supersaturation conditions necessary to induce
precipitation are usually the result of a chemical reaction

22. Typical Co-precipitation Methods

23. Nucleation and Growth
• Nucleation is a phenomenon of initiation of
formation of the first nanocrystal in the solution
• Nucleation is the creation of nuclei upon which
growth can occur.
• It involves the appearance of very small particles
which are capable of growing
• Nucleation plays an important role in controlling the
properties of the final product, size distribution and
nature of the phase.
• Nanoparticles need strong nucleation and slow
growth

26. Aspects of Nanoparticle Growth in
Solution

27. Tuning the Size of Nanoparticles

29. Advantages

30. Disadvantages

31. Ultrasonication (or)
Sonochemical Synthesis

32. Intro…..

33. Contd….

34. Contd….

35. Contd….

36. Contd….
• Consequently, ultrasound does not directly
interact with chemical compounds on a
molecular level.
• Sonochemistry derives from another way of
concentrating ultrasonic energy: acoustic
cavitation.

37. Contd…
• Acoustic cavitation appears in liquids at high and moderate intensities of
ultrasonic irradiation.
• A liquid expands during the expansion (negative) phase of an ultrasonic
wave.
• If the negative pressure induced by the wave in the liquid is high enough
such that the average distance between the molecules exceeds the critical
molecular distance necessary to hold the liquid intact, the liquid breaks
down and creates voids or cavities; these are cavitation bubbles.
• Once produced, these bubbles may grow until the maximum of the negative
pressure has been reached.
• In the succeeding compression cycle of the wave however, they will be
forced to contract and some of them may even disappear totally: collapsing.
• The collapse of the bubbles occurring during cavitation is more rapid than
thermal transport and generates localized hot spots, with temperature in
the order of 5000 K, pressures of 1000 atm and heating rates of more than
1010 K/s 1. These severe conditions allow the activation of reaction
mechanisms otherwise inexplicable.

38. Contd….

39. MECHANICAL MILLING

40. Intro…..
• Mechanical alloying is a simple and useful
processing technique that is now being
employed in the production of nanocrystals
and/or nanoparticles from all material classes.
• The materials are crushed mechanically in the
rotating drum by the hard balls.
• This repeated deformation can cause large
reductions in grain size to form nanoparticles.

41. Principle
•The fundamental principle of size reduction
in mechanical attrition devices lies in the
energy imparted to the sample during
impacts between the milling media.
•Useful for ceramic processing and powder
metallurgy industries

42. TYPES OF MILLING
• Different types of milling equipment are
available for mechanical alloying and
nanoparticle formation.
• They differ in their capacity, efficiency of
milling, and additional arrangements for
heat transfer and particle removal.

43. Ball Mill
• It is one of the simplest ways of making
nanoparticles of some metals and alloys in the
form of powder.
• There are many types of ball mills viz. planetary,
vibratory, rod, tumbler etc.
• A ball mill (a type of grinder) is a cylindrical
device used for grinding (or mixing) materials to
as small as few nanometers.

44. Construction
• A ball mill consists of a cylindrical capped
container that sits on two drive shafts (pulleys
and belts are used for rotary motion) or directly
connected to motor for rotation.
• Size of container, of course, depends upon the
quantity of interest.
• The container is partially filled with the material
to be ground (powder or flakes) plus the grinding
medium (hard spherical balls).
• Initial material can be of arbitrary size and
shape.

45. Contd….
• Different materials are used as grinding media,
including tungsten carbide balls, ceramic balls,
flint pebbles and stainless steel balls.
• Larger balls used for milling, produce smaller
grain size and larger defects in the particles.
• Usually 2:1 mass ratio of balls to material is
advisable.
• If the container is more than half filled, the
efficiency of milling is reduced.

46. Process
• When the container is rotating about a horizontal axis, the
material is forced to the walls and is pressed against the
walls.
• This internal cascading effect reduces the material to a fine
powder.
• By controlling the speed of rotation of the container as well
as duration of milling, it is possible to ground the material
to fine powder (few nm to few tens of nm) whose size can
be quite uniform.
• This process may add some impurities from balls.
• The container may be filled with air or inert gas. However,
this can be an additional source of impurity, if proper
precaution to use high purity gases is not taken.

47. Contd….

48. Contd….

49. Process Parameters

50. Contd….

51. Contd…

52. Advantages

53. Disadvantages

54. SPUTTERING

55. Intro…..
• Sputtering is a process in which atoms are ejected
from a solid target material due to bombardment of
the target by energetic particles.
• Sputtering process is commonly utilized for thinfilm
deposition.
• It only happens when the kinetic energy of the
incoming particles is much higher than conventional
thermal energies (≫ 1 eV).

56. Contd…..
• In sputter deposition, some inert gas ions (e.g. argon) are incident
on a target at a high energy.
• Target material may be some alloy, ceramic or compound.
• Depending on energy of the ions and ratio of ion mass to target
atoms mass, the ion-target interaction can be a complex
phenomenon.
• The ions become neutral at the surface but due to their energy the
incident ions may get implanted, get bounced back, create
collision cascades in target atoms, displace some of the atoms in
the target creating vacancies, interstitials and other defects,
desorb some adsorbates, create photons while losing energy to
target atoms or even sputter out some target atoms/molecules,
clusters, ions and secondary electrons. Fig. 8 shows a schematic
picture of various possibilities.

57. Contd…..
• Fig. shows a schematic picture of various possibilities.

61. Sputtering Process

63. Contd…..
• Sputtering systems are often capable of depositing more
than one material simultaneously or sequentially.
• This capability is very useful in obtaining alloys and
multilayers (e.g., multilayer magnetic recording heads are
sputtered).
• The deposition rates are much higher than most CVD
techniques.
• However, due to stress accumulation and cracking, a
thickness beyond 2µm is rarely deposited with these
processes.

65. Types of Sputtering
• DC sputtering,
• Radio frequency sputtering
• Magnetron sputtering

66. DC Sputtering
• Target and substrate serve as electrodes and face each
other in a typical sputtering chamber.
• Target is held at high negative voltage [i.e. acts as cathode:
conductive material, e.g. metal] and substrate may be at
positive voltage or ground [i.e. acts as anode].
• Substrates may be simultaneously heated or cooled
depending upon the requirement.
• Once the required base pressure is attained in the vacuum
system, an inert gas (e.g. argon gas) is introduced into the
system at a pressure < 0.1 torr as the medium to initiate
and maintain a discharge.

67. Contd…
• When an electric field of several kilovolts per centimeter is
applied between the electrodes, a glow discharge is set up.
• These regions are the result of plasma, i.e. a mixture of free
electrons, ions, neutrals and photons released in various
collisions.
• The density of various particles and the length over which
they are spread and distributed depends upon the gas
pressure.

68. Contd…
• The free electrons are accelerated by the electric field, and gain
sufficient energy to ionize argon atoms.
• If the gas density or pressure is too low, then 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, then the electrons
will not gain sufficient energy when they strike gas atoms to cause
ionization.
• The positive ions, Ar+ ions, produced in the discharge strike the
cathode (the source target) resulting in the ejection of neutral target
atoms through momentum transfer.
• These ejected atoms move towards the opposite electrode (anode:
substrate) and deposit there.
• Thus, sufficiently large number of Ar+ ions are generated that can be
used to sputter the target.

70. RF Sputtering
• If the target to be sputtered is insulating, it is difficult to
use DC sputtering.
• This is because it would mean the use of exceptionally high
voltage (> 106 V) to sustain a discharge between the
electrodes.
• Such high voltage will harm the target source and produced
film.
• In DC sputtering, 100 to 3000 volts is a usual voltage.
• However if some high frequency lower voltage is applied,
the cathode and anode alternatively keep on changing their
polarity and oscillating electrons cause sufficient ionization.

71. Contd…
• In principle, 5 to 30 MHz frequency can be used and the
electrodes can be insulating.
• However, 13.56 MHz is a commonly used frequency for
deposition, as it is reserved worldwide for this purpose and
others are available for communication.
• Target itself biases to negative potential becoming cathode
when the arrangement as depicted in Fig. is used

72. Contd…
• The capacitor in the circuit will have low RF
impedance and will allow the formation of a DC bias
on the electrodes

74. Magnetron Sputtering
• Magnetron sputtering involves the creation of plasma by
the application of a large DC potential between two parallel
plates
• A static magnetic field is applied near a sputtering target
and confines the plasma to the vicinity of the target.

78. VAPOR PHASE DEPOSITION

79. Intro…..
• It generally refers to the deposition of a material on
to a substrate by the use of the gaseous phase of
the depositing material.
• These techniques have been used to produce thin
film nano materials, multilayer coatings etc.
• There are two strategies involved
– PVD – Physical Vapor Deposition
– CVD – Chemical Vapor Deposition

80. Physical Vapor Deposition (PVD)
• PVD process is a atomic deposition process in which material is
vaporized from a solid (or) liquid source in the form of atoms (or)
molecules, transported in the form of a vapor through a vacuum (or)
low pressure gaseous environment to the substrate where it
condenses.

82. Classification of PVD

83. Evaporation
• Evaporation deposition of thin films is a process where the
material to be deposited is heated to a high vapor pressure
in vacuum.
• The heating can be done either by
– Electrically resistive heating
– Electron bombardment
• The former is called thermal evaporation and the latter is
called electron beam (e-beam) evaporation.

84. Thermal Evaporation

85. Contd…

86. Contd….

87. Electron Beam (e-beam) Evaporation

88. Chemical Vapor Deposition (CVD)
• CVD process can be simply defined as the process that enables the
deposition of a solid on a heated surface from a chemical reaction in
the vapor phase.

89. Construction of CVD

90. Sequence of Gas Transport & Reaction
Processes

91. MOCVD

92. Contd….
• Metal-organic compounds (jargon:
metalorganics, metallo-organics) are a class
of chemical compounds that contain metals
and organic ligands, which confer solubility in
organic solvents or volatility.

93. Contd…
• Metalorganic vapour-phase epitaxy (MOVPE),
also known as organometallic vapour-phase
epitaxy (OMVPE) or metalorganic chemical
vapour deposition (MOCVD), is a chemical
vapour deposition method used to produce
single- or polycrystalline thin films.
• It is a process for growing crystalline layers to
create complex semiconductor multilayer
structures.
• The growth of crystals is by chemical reaction
and not physical deposition.

95. Basic principle
• n MOCVD ultrapure precursor gases are injected
into a reactor, usually with a non-reactive carrier
gas. For a III-V semiconductor, a metalorganic could
be used as the group III precursor and a hydride for
the group V precursor.
• For example, indium phosphide can be grown
with trimethylindium ((CH3)3In)
and phosphine (PH3) precursors.

96. Contd…
• As the precursors approach the semiconductor
wafer, they undergo pyrolysis and the subspecies
absorb onto the semiconductor wafer surface.
• Surface reaction of the precursor subspecies
results in the incorporation of elements into a new
epitaxial layer of the semiconductor crystal lattice.
• In the mass-transport-limited growth regime in
which MOCVD reactors typically operate, growth is
driven by super saturation of chemical species in
the vapor phase.

97. Contd….
• MOCVD can grow films containing combinations
of group III and group V, group II and group
VI, group IV.
• Required pyrolysis temperature increases with
increasing chemical bond strength of the precursor.
• The more carbon atoms are attached to the central
metal atom, the weaker the bond.
• The diffusion of atoms on the substrate surface is
affected by atomic steps on the surface.

98. Contd….
• The vapor pressure of the group III metal
organic source is an important control
parameter for MOCVD growth, since it
determines the growth rate in the mass-
transport-limited regime.

100. Comparison between Evaporation
and Sputtering