2D Nanostructures

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2D Nanostructures

• Thin Films

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(No Transcript)
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• Film growth methods
• Vapour-phase deposition
• Evaporation
• Molecular beam epitaxy (MBE)
• Sputtering
• Chemical vapour deposition (CVD)
• Atomic Layer deposition (ALD)
• Liquid-based growth
• Electrochemical deposition
• Chemical Bath deposition (CBD or CSD)
• Langmuir-Blodgett films
• Self-assembled monolayers (SAMs)

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• Film deposition
• Involves predominantly heterogeneous processes
• Heterogeneous chemical reactions
• Evaporation
• Adsorption desorption on growth surfaces
• Heterogeneous nucleation surface desorption on
  growth surfaces
• Most film deposition characterization processes
  are conducted under vacuum

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Fundamentals of film growth

• Thin film growth involves nucleation and growth
  on the substrate or growth surface
• Crystallinity microstructure of film is
  determined by nucleation process
• 3 Basic nucleation modes
• Island growth
• Growth species are more strongly bonded to each
  other than to the substrate
• Islands coalesce to form a continuous film
• E.g. metals on insulator substrates, alkali
  halides, graphite and mica substrates
• Layer growth
• Growth species are bound more strongly to the
  substrate than to each other
• 1st complete monolayer is formed before the
  deposition of 2nd layer occurs
• Involves in situ developed stress due to lattice
  mismatch between the deposit and the substrate
• e.g. epitaxial growth of single crystal films
• Island-layer growth
• combination of layer growth and island growth

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3 Basic nucleation modes
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Effect of growth conditions

• Nucleation models and mechanisms are applicable
  to the formation of single crystal,
  polycrystalline and amorphous deposit, and of
  inorganic, organic and hybrid deposit
• Whether the deposit is single crystalline,
  polycrystalline or amorphous depends on the
  growth conditions and the substrate

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Growth of single crystal films is most difficult
and requires

• Growth of single crystal films is most difficult
  and requires
• (i) a single crystal substrate with a close
  lattice match
• (ii) a clean surface so as to avoid possible 2o
  nucleation
• (iii) a high growth temperature so as to ensure
  sufficient mobility of the growth species
• (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

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Deposition of amorphous films

• Deposition of amorphous films typically occurs
• (i) when a low growth temperature is applied,
  there is a 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.

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Growth of polycrystalline crystalline films

• Conditions for the growth of polycrystalline
  films
• Intermediate between conditions of single crystal
  growth and amorphous film deposition
• moderate temperature to ensure a reasonable
  surface mobility of growth species
• High impinging flux of growth species

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Epitaxy

• The growth or formation of single crystal on top
  of a single crystal substrate
• Homoepitaxy
• is to grow film on the substrate, in which both
  are the same material
• No lattice mismatch between films and substrates
• Uses
• to grow better quality film
• to introduce dopants into the grown film
• Heteroepitaxy
• Films substrates are different materials
• Lattice mismatch between films and substrates
• Application of epitaxy electronic industry

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Physical vapour deposition (PVD)

• PVD is a process of transferring growth species
  from a source or target and deposit them on a
  substrate to form a film.
• The process proceeds atomically and mostly
  involves no chemical reactions.
• Methods for the removal of growth species
• Evaporation
• The growth species are removed from the source by
  thermal means
• Sputtering
• Atoms or molecules are dislodged from solid
  target through impact of gaseous ions (plasma)

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Evaporation
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Aerosol-assisted chemical vapor deposition (AACVD)
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Low pressure metal-organic chemical vapor
deposition (LP-MOCVD)
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Evaporation

• The desired vapour pressure of source material
  can be generated by simply heating the source to
  elevated temperatures
• The concentration of the growth species in the
  gas phase can be easily controlled by varying the
  source temperature and the flux of the carrier
  gas
• The resulting vapour composition often differs
  from the source composition due to pyrolysis,
  decomposition and dissociation
• It is difficult to deposit complex films. When a
  mixture of elements or compounds is used as a
  source for the growth of a complex film
• One element may evaporate faster than the another
  resulting in the depletion of the first element
• Deposition of thin films by evaporation is
  carried out in a low pressure (10-3 10-10 torr).
  It is difficult to obtain a uniform thin film
  over a large area.
• To overcome this shortfall
• Multiple sources are used instead of single point
  source
• The substrate is rotated
• Both source and substrate are loaded on the
  surface of a sphere

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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

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Sputteringdc discharge system

• Ionization of inert gas e.g. Ar (by electric
  field or dc voltage)
• when inert gas ions strike the cathode (source
  target), neutral target atoms are ejected
• These atoms pass through the discharge and
  deposit on the opposite electrode (the substrate
  with growing film
• Other negatively charged species also bombard and
  interact with the surface of the substrate or
  grown film

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Deposition of insulating film

• Apply an alternate electric field to generate
  plasma between two electrodes
• Typical RF frequencies 5 30 MHz
• 13.56 MHz reserved for plasma processing by the
  Federal Communications Commission
• The target self-biases to a negative potential
  and behaves like a dc target
• Electrons are 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 capacitatively coupled to the RF
  generator
• Sputtering a mixture of elements/compounds will
  not result in a change of composition in the
  target and thus the composition of the vapour
  phase will be the same as that of the target and
  remain the same during the deposition.

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Evaporation vs Sputtering
Evaporation Sputtering
Uses low pressures (10-3 10-10 torr) Requires a relatively high pressure (100 torr)
Atoms/molecules in evaporation chamber do not collide with each other prior to arrival at the growth site Atoms/molecules in sputtering collide with each other prior to arrival at the growth site
Evaporation is desirable by thermodynamic equilibrium Sputtering is not desirable by thermodynamic equilibrium
The growth surface is not activated in evaporation The growth surface is constantly under electron bombardment and thus is highly energetic
The evaporated films consist of large grains The sputtered films consist of smaller grains with better adhesion to the substrates
Fractionation of multi-component systems is a serious challenge The composition of the target and the film can be the same
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Laser ablation

• Uses laser beams to evaporate the material
• Absorption characteristics of the material to be
  evaporated determine the laser wavelength to be
  used
• Pulsed laser beams are generally used in order to
  obtain high power density.
• Laser ablation is an effective technique for the
  deposition of complex metal oxides such as high
  Tc superconductor films.
• Advantage of Laser ablation
• The composition of the vapour phase can be
  controlled as that in the source
• Disadvantage of Laser ablation
• Complex system design
• Not always possible to find desired laser
  wavelength for evaporation
• Low energy conversion efficiency

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Electron beam evaporation

• Limited to electrically conductive source
• Advantages
• Wide range of controlled evaporation rate due to
  a high power density
• Low contamination

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Arc evaporation

• Used for evaporation of electrically conductive
  source.

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Molecular beam epitaxy (MBE)

• 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.
• Consists of realtime structural and chemical
  characterization capability
• (high energy electron diffraction (RHEED)
• X-ray photoelectric spectroscopy (XPS)
• Auger electron spectroscopy (AES)
• Can also be attached to other analytical
  instruments

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MBE Effusion cells
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MBE

• In MBE the evaporated atoms or molecules from one
  or more sources do not interact with each other
  in the vapour phase under such low pressure
• Most molecular beams are generated by heating
  solid materials placed in source cells (aka
  effusion cells or Knudssen cells)
• The atoms or molecules striking on the single
  crystal substrate results in the formation of the
  desired epitaxial film.
• The extremely clean environment, the slow growth
  rate, and independent control of the evaporation
  of individual sources enable the precise
  fabrication of nanostructures and nanomaterials
  at a single atomic layer
• Highly pure film can be obtained
• UH vacuum environment ensures absence impurity or
  contamination
• Minimal formation of crystal defects
• Slow growth rate ensures sufficient surface
  diffusion and relaxation
• Precise control of chemical composition of the
  deposit possible
• Evaporation of sources controlled individually

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Main attributes of MBE

• A low growth temperature that limits diffusion
  and hyper abrupt interfaces
• A slow growth rate that ensures a well controlled
  2D growth at a rate of 1 µm/h. A very smooth
  surface and interface is achievable through
  controlling the growth at the monoatomic layer
  level.
• A simple growth mechanism compared to other film
  growth techniques ensures better understanding
  due to the ability of individually controlled
  evaporation of sources
• A variety of in situ analysis capabilities
  provide invaluable information for the
  understanding and refining of the process

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Atomic Layer Deposition (ALD)

• ALD is aka - atomic layer epitaxy (ALE) - atomic
  layer growth (ALG) - atomic layer CVD (ALCVD) -
  molecular layer epitaxy (MLE)
• differs significantly from other thin film
  deposition methods.
• The most distinctive feature of ALD is
  self-limiting growth nature, each time only one
  atomic or molecular layer can grow.
• Therefore, ALD offers the best possibility of
  controlling the film thickness and surface
  smoothness in truly nanometer or sub-nanometer
  range.
• ALD can be considered as a special modification
  of the chemical vapor deposition, or a
  combination of vapor-phase self-assembly and
  surface reaction.
• Typical ALD process
• Surface is first activated by chemical reaction.
• When precursor molecules are introduced into the
  deposition chamber, they react with the active
  surface species and form chemical bonds with the
  substrate.
• Since the precursor molecules do not react with
  each other, no more than one molecular layer
  could be deposited at this stage.
• Next, the monolayer of precursor molecules that
  chemically bonded to the substrate is activated
  again through surface reaction.
• Either the same or different precursor molecules
  are subsequently introduced to the deposition
  chamber and react with the activated monolayer
  previously deposited.
• As the steps repeat, more molecular or atomic
  layers are deposited in the way one layer at a
  time.

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The process of titania film growth by ALD.

• Substrate Hydroxylation
• Introduction of titanium precursor (titanium
  tetrachloride).
• Precursor will react with the surface hydroxyl
  groups through a surface condensation reaction
• Cl3Ti-O-Me H2O ? (HO)3Ti-O-Me HCl
• Neighboring hydrolyzed Ti precursors subsequently
  condensate to form Ti-O-Ti linkage
• (HO)3Ti-O-Me (HO)3Ti-O-Me ? Me-O-Ti(OH)2-O-Ti
  (HO)2-O-Me H2O
• By-product HCl and excess H2O removed from the
  reaction chamber.

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ZnS film growth

• Precursors ZnCl2 and H2S
• Chemisorb ZnCl2 on substrate
• Introduce H2S to react with ZnCl2 to deposit a
  monolayer of ZnS on substrate
• HCl is released as a by-product.

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Thin Films Deposited by ALD
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Requirements for ALD Precursors
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Advantages of ALD (compared to other vapor phase
deposition methods)

• (1) precise control of film thickness
• due to the nature of self-limiting process, and
  the thickness of a film can be set digitally by
  counting the number of reaction cycles.
• (2) conformal coverage.
• due to the fact that the film deposition is
  immune to variations caused by nonuniform
  distribution of vapor or temperature in the
  reaction zone.

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Applications and limitations of ALD

• ALD is an established technique for the
  production of large area electroluminescent
  displays, and is a likely future method for the
  production of very thin films needed in
  microelectronics.
• Limitations
• Many other potential applications of ALD are
  discouraged by its low deposition rate, typically
  lt0.2 nm (less than half a monolayer) per cycle.

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Self Assembly

• Self-assembly
• is a process in which a set of components or
  constituents spontaneously forms an ordered
  aggregate through their global energy
  minimization.
• is a process that ordered arrangement of
  molecules and small components such as small
  particles occur spontaneously under the influence
  of certain forces such as chemical reactions,
  electrostatic attraction, and capillary forces.
• Macromolecules (e.g. proteins, nucleic acid
  sequences, micelles, liposomes, and colloids)in
  nature adapt their final folding and
  conformation by self-assembly processes.
• Self-assembled monolayers or multiple layers of
  molecules
• In general, chemical bonds are formed between the
  assembled molecules and the substrate surface, as
  well as between molecules in the adjacent layers.
  
• The major driving force here is the reduction of
  overall chemical potential.
• A SAM is defined as a 2-D film with the thickness
  of one molecule that is attached to a solid
  surface through a covalent bond.

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Self-assembled monolayers (SAMs)

• SAMs are molecular assemblies that are formed
  spontaneously by the immersion of an appropriate
  substrate into a solution of an active surfactant
  in an organic solvent.

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SAMs3 Parts of a self-assembling surfactant
molecule

• The head group that chemisorbs on the substrate
  surface.
• very strong molecular-substrate interactions
  (e.g. covalent Si-O and S-Au bonds, ionic
  CO2-- Ag bond.
• The alkyl chain.
• The third molecular part is the terminal
  functionality
• surface functional groups in SAMs are thermally
  disordered at room temperature.
• .

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SAMs

• The driving force for the self-assembly includes
• electrostatic force
• hydrophobicity and hydrophilicity
• capillary force
• chemisorption.
• Types of self-assembly methods for the organic
  monolayers include
• (1) organosilicon on hydroxylated surfaces, such
  as SiO2 on Si, Al2O3 on Al, glass,
• (2) alkanethiols on gold, silver, and copper,
• (3) dialkyl sulfides on gold,
• (4) dialkyl disulfides on gold,
• (5) alcohols and amines on platinum, and
• (6) carboxylic acids on aluminum oxide and
  silver.
• Self-assembly methods grouped based on the types
  of chemical bonds formed between the head groups
  and substrates.
• (1) covalent Si-O bond between organosilicon on
  hydroxylated substrates that include metals and
  oxides,
• (2) polar covalent S-Me bond between
  alkanethiols, sulfides and noble metals such as
  gold, silver, platinum, and copper, and
• (3) ionic bond between carboxylic acids, amines,
  alcohols on metal or ionic compound substrates.

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Monolayers of Organosilicon or Alkylsilane
Derivatives

• Alkylsilanes RSiX3, R2SiX2, or R3SiX, where X
  chloride or alkoxy and R a carbon chain that
  can bear different functionalities, such as amine
  or pyridinyl.
• The formation of monolayers is simply by reacting
  alkylsilane derivatives with hydroxylated
  surfaces such as SiO2, TiO2.

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Monolayers of Organosilicon or Alkylsilane
Derivatives

• Introduction of a hydroxylated surface into a
  solution of alkyltrichlorosilane in an organic
  solvent
• After immersion, the substrate is rinsed with
  methanol, DI water and then dried.
• Organic solvent is in general required for the
  self-assembly for the alkylsilane derivatives,
  since silane groups undergo hydrolysis and
  condensation reaction when in contact with water,
  resulting in aggregation.
• For alkylsilanes with more than one chloride or
  alkoxy groups, surface polymerization is commonly
  invoked deliberately by the addition of moisture,
  so as to form silicon-oxygen-silicon bonds
  between adjacent molecules.

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Monolayers of Organosilicon or Alkylsilane
Derivatives
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Multilayers of Organosilicon or Alkylsilane
Derivatives

• The construction of an SA multilayer requires
  that the monolayer surface be modified to be a
  hydroxylated surface, so that another SA
  monolayer can be formed through surface
  condensation.
• Such hydroxylated surfaces can be prepared by a
  chemical reaction and the conversion of a
  nonpolar terminal group to a hydroxyl group.
• e.g. a reduction of a surface ester group,
• a hydrolysis of a protected surface hydroxyl
  group,
• a hydroboration-oxidation of a terminal double
  bond.
• oxygen plasma etching followed with immersion in
  DI-water
• A subsequent monolayer is added onto the
  activated or hydroxylated monolayer through the
  same self-assembly procedure and multilayers can
  be built just by repetition of this process.

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Multilayers of Organosilicon or Alkylsilane
Derivatives
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Sol-Gel Films

• Sol-gel processing
• is widely used in the synthesis of inorganic and
  organic-inorganic hybrid materials
• is capable of producing nanoparticles, nanorods,
  thin films, and monolith.
• sol-gel films are made by coating sols onto
  substrates.
• Commonly used methods for sol-gel film deposition
• spin-coating
• dip-coatings
• spray
• ultrasonically pulverized spray

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Dip-coating

• a substrate is immersed in a solution and
  withdrawn at a constant speed. As the substrate
  is withdrawn upward, a layer of solution is
  entrained, and a combination of viscous drag and
  gravitational forces determines the film
  thickness.
• Stages of the dip-coating process (next slide)
• Immersion
• Deposition drainage
• Evaporation
• Continuous
• The thickness of a dip-coated film is commonly in
  the range of 50-500 nm

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Stages of the dip-coating process
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Sol-Gel Dip-Coating
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Spin-coating

• is used routinely in microelectronics to deposit
  photoresists and specialty polymers.
• Four stages of spin coating
• delivery of solution or sol onto the substrate
  center
• spin-up
• spin-off
• evaporation (overlaps with all stages)
• After delivering the liquid to the substrate,
  centrifugal forces drive the liquid across the
  substrate (spin-up). The excess liquid leaves the
  substrate during spin off. When flow in the thin
  coating is no longer possible, evaporation takes
  over to further reduce the film thickness.
• A uniform film can be obtained when the viscosity
  of the liquid is not dependent on shear rate
  (i.e., Newtonian) and the evaporation rate is
  independent of position.

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• the film thickness can be controlled by adjusting
  the solution properties and the deposition
  conditions.
• In the process of creating a sol-gel coating, the
  removal of solvent or drying of the coating
  proceeds simultaneously with continues
  condensation and solidification of the gel
  network. The competing processes lead to
  capillary pressure and stresses induced by
  constrained shrinkage, which result in the
  collapse of the porous gel structure, and may
  also lead to the formation of cracks in the
  resultant films.
• The drying rate plays a very important role in
  the development of stress and formation of cracks
  particularly in the late stages and depends on
  the rate at which solvent or volatile components
  diffuse to the free surface of the coating and
  the rate at which the vapor is transported away
  in the gas.
• Stress develops during drying of a solidified
  coating due to constrained shrinkage. Solvent
  loss after solidification is a common source of
  stress in solvent-cast polymer coatings.Solvent
  content at solidification should be minimized to
  lower the stress in the coating.
• In the formation of sol-gel coating, it is very
  important to limit the condensation reaction rate
  during the removal of solvent upon drying, so
  that the volume fraction of solvent at
  solidification is kept small. To relieve
  stresses, the material can relax internally by
  molecular motion or it can deform. Internal
  relaxation slows as the material approaches an
  elastic solid and deformation is restricted by
  adherence to the substrate. Since the stress-free
  state shrinks during solidification and adherence
  to the substrate confines shrinkage in the
  coating to the thickness direction, in-plane
  tensile stresses result. Cracking is another form
  of stress relief. For sol-gel coatings, the
  formation of cracks limits the coating thickness
  commonly less than 1 micron.
• It should also been noted that sol-gel coatings
  are commonly porous and amorphous. For many
  applications, subsequent heat treatment is
  required to achieve full densification and
  convert amorphous to crystalline. Mismatch of
  thermal expansion coefficients of sol-gel
  coatings and substrates is another important
  source of stress, and a residual stress in
  sol-gel coatings can be as high as 350 MPa.
• Porosity is another important property of sol-gel
  film. Although for many applications,
  heat-treatment at elevated temperatures is
  employed to remove the porosity, the inherited
  porosity enables sol-gel film for many
  applications such as matrix of catalyst, host of
  sensing organic or biocomponents, electrode in
  solar cells. Porosity itself also renders other
  unique physical properties such as low dielectric
  constant, low thermal conductivity, etc.

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Organic-inorganic hybrids

• a new type of materials, which are not present in
  nature,
• synthesized by the sol-gel method.
• The organic component can significantly modify
  the mechanical properties of the inorganic
  component.
• The organic and inorganic components can
  interpenetrate each other on a nanometer scale.
• Depending on the interaction between organic and
  inorganic components, hybrids are divided into
  two classes
• (1) hybrids consisting of organic molecules,
  oligomers or low molecular weight polymers
  embedded in an inorganic matrix to which they are
  held by weak hydrogen bond or van der Waals force
  and
• (2) in those, the organic and inorganic
  components are bonded to each other by strong
  covalent or partially covalent chemical bonds.
• The porosity can also be controlled as well as
  the hydrophilic and hydrophobic balance. Hybrids
  with new optical or electrical properties can be
  tailored. Some hybrids can display new
  electrochemical reactions as well as special
  chemical or biochemical reactivity.

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Langmuir Blodgett Films

• LB films are monolayers and multilayers of
  amphiphilic molecules transferred from the
  liquid-gas interface (commonly water-air
  interface) onto a solid substrate
• Amphiphile
• A molecule that is insoluble in water, with one
  end that is hydrophilic (preferentially immersed
  in water) and the other that is hydrophobic
  (preferentially resides in air or in the nonpolar
  solvent)
• E.g stearic acid C17H35CO2H

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Summary

• Films with thickness lt100 nm can be deposited
  using a variety of techniques
• Methods offer varied degrees of control of
  thickness and surface smoothness
• MBE and ALD offer the most precise control of
  deposition at the single atomic level, and the
  best quality of the grown film.
• Disadvantages of MBE and ALD
• Complicated deposition instrumentation
• Slow growth rate
• SA is another method offering a single atomic
  level control
• Limitation of SA
• Limited to the fabrication of organic-inorganic
  hybrid films