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Nanomaterials Preparation
Module2
Module 2
Classification of nanomaterials – based on dimensions (quantum dot, Q wires, Q wells)
Synthesis approach (topdown, bottom up, mention with examples which will come later)
Issues – can be covered briefly as mentioned in slides, (but can also be discussed more )
Synthesis method according to the syllabus
Nano fabrication – given in slides
Classification of nanomaterials
Nanomaterials synthesis approach
1.Top down approach: Breaking of bulk material
2.Bottom approach: Build up of material
Atommoleculecluster
Synthesis issues
• Size , shape and composition control – The monodispersity should
be very high for their uniform physical properties. ( Several
synthesis strategies were made for this control- changing the rate of
nanomaterials formation, changing the solvent, the reducing
agents, stabilizing agents etc.,)
• Stability of nanomaterials are important for their application. – Due
to higher surface energy and surface area nanomaterials tries to
always agglomerate. Several synthetic approaches were made for
stability of nanomaterials. (eg . Surface encapsulation with ligands,
dispersion in different matrixes, template approach etc.,)
• Scaling up the methods, reproducibility, building up complex
nanostructures are some of issues in prepartion of nanomaterials
Nanomaterials growth mechanism
• Nucleation
• Growth
• Nucleation without and with seeds
Homogeneous nucleation without seed :In gas phase
system – The size of the nuclei is same as the size of
gas molecules.
Heterogeneous nucleation with seed : Seeds of definite
size dispersed into liquid, gas or on solid surface
Definition: Nucleation is the process where droplets of
liquid can condense from a vapor, or bubbles of gas can
form in a boiling liquid. Nucleation can also occur in crystal
solution to grow new crystals.
Example: Dust and pollutants provide nucleation sites for
water vapor in the atmosphere to form clouds.
Seed crystals provide nucleation sites for crystal growing.
Nanomaterials growth mechanism
Nucleation
• A solution with solute exceeding the solubility or supersaturated
possesses a high Gibbs free energy; the overall energy of the system
would be reduced by segregating solute from the solution.
• The reduction of the overall Gibbs free energy of a supersaturated
solution by forming a solid phase and maintaining an equilibrium
concentration in the solution.
• This reduction of Gibbs free energy is the driving force for both nucleation
and growth.
• The change in free energy dependents on the solute concentration
Where C- conc. of solute, Co – equilibrium conc. ,  - atomic volume
• When Gv = 0 no nucleation
• When C > Co then Gv = negative, spontaneous nucleation
Nanomaterials growth mechanism
Nucleation
• For the synthesis of nanoparticles with uniform size distribution, it will be
the best if all nuclei form at the same time with the same size.
• In addition, all the nuclei will have the same subsequent growth.
Consequently, monosized nanoparticles can be obtained.
• So it is obvious that it is highly desirable to have nucleation occur in a very
short period of time.
Nanomaterials growth mechanism
Growth
• When the concentration of growth species reduces below the
minimum concentration for nucleation, nucleation stops,
whereas the growth continues
• The size distribution of nanoparticles is dependent on the
subsequent growth process of the nuclei. The growth process
of the nuclei involves multi-steps and the major steps are
(1) Generation of growth species,
(2) Diffusion of the growth species from bulk to the growth
surface
(3) Adsorption of the growth species onto the growth surface
(4) surface growth through irreversible incorporation of
growth species onto the solid surface.
Nanomaterials growth mechanism
Growth
• The Growth steps can also be further grouped into two
processes.
(1) Supplying the growth species to the growth surface is
termed as diffusion, which includes the generation,
diffusion, and adsorption of growth species onto the
growth surface
(2) whereas incorporation of growth species adsorbed on
the growth surface into solid structure is denoted as
growth.
• A diffusion-limited growth would result a different size
distribution of nanoparticles as compared with that by
growth-limited process.
Nanomaterials growth mechanism
Preparation methods
Nanomaterials preparation
Physical Methods
Ball milling
Gas condensation processing (GPC)
Laser ablation
Ion beam
Electron beam
Nanolithography
Chemical Methods
Sol-gel synthesis
Wet chemical synthesis
Precipitation method
Chemical vapour condensation
Catalytic chemical vapour deposition
Template assisted CVD
Electrochemical method
Reverse micelles
Preparation
Any Preparation technique should provide:
1. Identical size of all particles (mono sized or uniform size
distribution).
2. Identical shape or morphology.
3 Identical chemical composition and crystal structure.
4 Individually dispersed or mono dispersed i.e., no
agglomeration.
Preparation – Physical method
High-Energy ball milling (Top down approach) :
*Interest in the mineral, ceramic processing, and
powder metallurgy industry.
* Involves milling process include particle size
reduction (Fig.3).
* Restricted to relatively hard, brittle materials which
fracture and/or deform during the milling operation.
* Different purposes including; tumbler mills, attrition
mills, shaker mills, vibratory mills, planetary mills, etc.
Violent or agitation,
~50 m  nm
Schematic representation of
the principle of mechanical
milling.
*Hardened steel or tungsten carbide (WC) coated balls the basic process of
mechanical attrition (rubbing away) (Fig.3).
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Preparation – physical method
• Limitation of Ball milling: (Even though high
production rates)
1. Severe plastic deformation associated with mechanical
attrition due to generation of high temperature in the
interphase, 100 to 200º C. Thermal decomposition or
evaporation of materials
2. Difficulty in broken down to the required particle size.
3. Contamination by the milling tools (Fe) and atmosphere
(trace elements of O2, N2, in rare gases) can be a problem.
(inert condition necessary)
Preparation – Physical method
(B) Gas Condensation Processing (GPC)-Bottom-up approach:
Thermal or electric or
e- beam evaporation
(like PVD)
Metal in
crucible
Cooling
(Rotating cylinder)
Liquid N2 (-80oC)
Metal cluster
(gaseous state)
Homogenous
nucleation in gas
phase
Nanoparticles
deposits
(2-50nm)
scrapping
Collection of the
nanoparticles
General Scheme for GPC for
the nanoparticle synthesis
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•Major advantage over conventional gas
flow is the improved control of the particle
sizes.
Fig. 4 Schematic representation of typical set-up for
gas condensation synthesis of nanomaterials followed
by consolidation in a mechanical press or collection in
an appropriate solvent media.
•These methods allow for the continuous
operation of the collection device and are
better suited for larger scale synthesis of
nanopowders.
•However, these methods can only be used
in a system designed for gas flow, i.e. a
dynamic vacuum is generated by means of
both continuous pumping and gas inlet via
mass flow controller.
Limitation:1.Control of the composition of the
elements has been difficult and
reproducibility is poor.
2.Oxide impurities are often formed.
The method is extremely slow.
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Preparation
(C) Chemical Vapour Condensation (CVC) (Bottom-up approach):
•Involves pyrolysis (heat
treatment) of vapors of metal
organic precursors (starting
materials) like
Hexamethyldisilazane (CH3)3Si-NHSi-(CH3)3 to produce SiCxNyOz.
•Evaporate source in the GPC is
replaced by a hot wall reactor in
the CVC process (Fig.5).
Fig. 5 A schematic of a
typical CVC reactor
•Precursor residence time is the
key parameter to control the size
of nanoparticle here (gas flow
rate, pressure, heating
temperature can be controlled).
•Other procedure similar to GPC.
Production capabilities are much larger than in the GPC processing.
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Nanowire synthesis - CVD
Tubular furnace for synthesis of nanomaterials, nanowires by Chemical vapour deposition(CVD)
The precursor materials were converted to desired product and deposited on the substrate
The precursor material, temperature, the carrier gas are important in product formation
When catalyst is used it is known as catalytic chemical vapour deposition (CCVD)
Plasma-enhanced chemical vapor deposition (PECVD) is a
process used to deposit thin films from a gas state (vapor) to
a solid state on a substrate. Chemical reactions are involved in
the process, which occur after creation of a plasma of the
reacting gases. The plasma is generally created by RF (AC)
frequency or DC discharge between two electrodes, the space
between which is filled with the reacting gases. [edit]
Film examples & Applications
Plasma deposition is often used in semiconductor
manufacturing to deposit films conformally (covering
sidewalls) and onto wafers containing metal layers or other
temperature-sensitive structures. PECVD also yields some of
the fastest deposition rates while maintaining film quality
(such as roughness, defects/voids), as compared with sputter
deposition and thermal/electron-beam evaporation, often at
Film examples & Applications
Silicon dioxide can be deposited using a combination of
silicon precursor gasses like dichlorosilane or silane and
oxygen precursors, such as oxygen and nitrous oxide,
typically at pressures from a few millitorr to a few torr.
Plasma-deposited silicon nitride, formed from silane and
ammonia or nitrogen, is also widely used, although it is
important to note that it is not possible to deposit a pure
nitride in this fashion. Plasma nitrides always contain a
large amount of hydrogen, which can be bonded to
silicon (Si-H) or nitrogen (Si-NH);[1] this hydrogen has an
important influence on IR and UV absorption,[2] stability,
mechanical stress, and electrical conductivity.[3]
PECVD
•
•
•
The reactor has two electrodes of different area
Use of radio frequency creates highly reactive species in low temperature plasma
Substrate can be kept low temperature ( 423-623 K)
Plasma deposition
Plasma decomposition of hydrocarbons ( acetylene)
Two electrodes with different area
Higher mobility of electrons than ions create a
sheath next to electrode with excess of ions
For good thin film deposition the plasma has to
operated at lowest possible pressure
This will increase the fraction of ions to radical
of the plasma
In pressure plasma, use of magnetic field, increase
the path length of electron and the ionisation efficiency
Preparation - Chemical Methods (Bottom-up approachs): Wet
Chemical Synthesis of nanomaterials (Sol-gel Process)
1.
Very popular & widely employed to prepare oxide materials
(SiOx).
2.
The sol-gel process: formation of a colloidal suspension (sol)
gelation of the sol to form a network in a continuous liquid
phase (gel) solid.
3.
Metal or metalloid element surrounded by various reactive
ligands (Si(OCH3)4, tetramethoxy silane, TMOS, alkoxide) is the
reactant.
4.
The starting material is processed to form a sol in contact with
water or dilute acid. Removal of the liquid from the sol yields
the gel, and the sol/gel transition controls the particle size and
shape. Calcination of the gel produces the product (eg. Oxide).
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Sol – Gel synthesis
Sol-Gel
•
•
•
•
Sol-gel processing refers to the hydrolysis and condensation of alkoxide-based
precursors such as Si(OEt)4 (tetraethyl orthosilicate, or TEOS).
The reactions involved in the sol-gel chemistry based on the hydrolysis and
condensation of metal alkoxides can be described as follows:
Classic sol-gel reaction scheme
•
•
Over all Steps:
Step 1: Formation of different stable solutions of the alkoxide (the sol).
•
Step 2: Gelation resulting from the formation of an oxide- or alcohol- bridged
network (the gel) by a polycondensation or polyesterification reaction
•
Step 3: Aging of the gel, during which the polycondensation reactions continue until
the gel transforms into a solid mass, accompanied by contraction of the gel network
and expulsion of solvent from gel pores.
•
Step 4: Drying of the gel, when water and other volatile liquids are removed from the
gel network.
– If isolated by thermal evaporation, the resulting monolith is termed a xerogel.
– If the solvent (such as water) is extracted under supercritical or near super
critical conditions, the product is an aerogel.
•
Step 5: Dehydration, during which surface- bound M-OH groups are removed, there
by stabilizing the gel against rehydration. This is normally achieved by calcining the
monolith at temperatures up to 8000C.
•
Step 6: Densification and decomposition of the gels at high temperatures (T>8000C).
The pores of the gel network are collapsed, and remaining organic species are
volatilized. The typical steps that are involved in sol-gel processing are shown in the
schematic diagram below.
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Wet chemical synthesis
• Use of chemical stabilizing agents
• Preparation of different types of nanostructures
• Stabilizing agents - eg., Citrate, Thiols, Amines,
Carboxylates, Phosphine, Phosphine oxide, Surfactants,
coordinating polymer
• Use of different reducing agents
• Coordination of stabilizing agents with the nanostructures
• Stability of the nanostructures depends on the chemical
nature of the stabilizing agents too.
• Control on the composition, size, shape of the
nanostructures
• Larger control on the reaction rates during preparation
Wet chemical synthesis
• Shape and size depends on starting materials
and reaction conditions
Wet chemical synthesis
• Preparation nanoparticles
• HAuCl4 + Stabilizing agent + NaBH4
Au nanoparticles
AgNO3 + Stabilizing agent + NaBH4
Ag nanoparticles
HAuCl4 + AgNO3 + Stabilizing agent + NaBH4
AuAg alloy NPs
Stabilizing agents – Sodium citrate, Alkanethiols, alkylammonium
salts, R- amines, R-COOH, surfactants etc
• Citrate method of Au nanoparticle preparation
• HAuCl4 + Trisodium citrate + NaBH4
Au nanoparticles
• Ratio of gold to citrate is important in size control
Wet chemical synthesis of Au NP
• 1 – AuCl4- is transferred into toluene from aqueous phase
using tetraoctylammonium salts as phase transfer agent
• 2. it is reduced in presence of thiol to give the NPs
1
2
• These NPs canbe ligand exchanged
• Alloy nanoparticles with large control in their composition
can be prepared by this method
Wet chemical synthesis
• Silver nanoparticles also can be prepared
using citrate, thiol methods
• Silver nanocubes synthesis using AgNO3 and
Ethylene glycol at 160oC
Precipitation method
Precipitation reactions involve the simultaneous
occurrence of nucleation, growth, coarsening, and/or
agglomeration processes.
Pprecipitation reactions exhibit the following characteristics:
(i) The products are generally insoluble species formed under
conditions of high supersaturation.
(ii) Nucleation is a key step, and a large number of small
particles will be formed.
(iii)Secondary processes, such as Ostwald ripening and
aggregation, dramatically affect the size, morphology, and
properties of the products.
(iv) The supersaturation conditions necessary to induce
precipitation are usually the result of a chemical reaction.
Precipitation method
•
Nanomaterials are produced by precipitation from a
solution.
•
The method involves high degree of
homogenization and low processing temperature.
•
ZnS powders were produced by reaction of aqueous
zinc salt solutions with thioacetamide (TAA).
TAA
•
Precursor zinc salts were chloride, nitric acid
solutions, or zinc salts with ligands (i.e.,
acetylacetonate, trifluorocarbonsulfonate, and
dithiocarbamate).
0.05 M Zn2+/
70oC/pH2
Eg.1: The 0.05 M cation solution was heated in a thermal bath maintained at 70° or 80 °C
in batches of 100 or 250 ml. Acid was added dropwise to bring it to a pH of 2. The
reaction was started by adding the TAA to the zinc salt solution, with the molar ratio of
TAA and zinc ions being set to an initial value of either 4 or 8.
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Template synthesis
• Template technique - Catalyst free formation
of CNT
Fabrication of nanostructures on
surfaces
Electrochemical method
•
•
•
•
Large control on the rate of electrodeposition
Substrate dependent
Product depends on the electrolytic composition
The size and shapes can be controlled by the method of
deposition
• Methods of electrochemical preparation. ( Galvanostatic,
potentiostatic, potentiodynamic, pulse method). Method
have control on the composition, size and shape.
• Nucleation and growth of nanostructured can be studied
by electrochemical method
• Different types of metal, alloy, metaloxiide nanomaterials
can be prepared by this method.
Electrodeposited nanostructures
• Au nanoclusters electrodeposited on glassy carbon electrode by
electroreduction of Au3+ solution
• Deposition was done using cyclic voltammetric method
• Nanowires of Au were deposited using polycarbonates template
Au nanocluster
Fe nanocubes
CNT with Ag NPs
Micro/Nano - Lithography
• Lithography is the technique used to transfer a
computer-generated pattern onto a substrate.
• This pattern is subsequently used to etch the
underlying tin film ( oxide, nitride).
• It is used for micro, Nano electronic fabrication
(MEMS, NEMS)
Micro/Nano lithography
Nanolithography
Nanofabrication techniques
• Scanning probe microscopy systems (STM, AFM) are
capable of controlling the movement of an atomically
sharp tip in close proximity to or in contact with a
surface with subnanometer accuracy
• Nanofabrication using microscopic probes have control
on the surface architectures and size control
• STM (scanning Tunneling), AFM( Atomic force) probes
were used to pattern or engrave the substrate surface
• Following that etching of resist surface is done to obtain
nanopatterned surfaces
Nanolithography
• The substrate is cleaned
• Substrate spin coated
with organic resist
(PMMA)
• Electron emitted from
thee biased tip is used to
pattern the resist similiar
to electron beam
lithography
• Development is done
using standard solutions
Nan- Dip Pen lithography
• AFM tip is inked with chemical of interest and brought near
to the surface
• Ink molecules flow from the pen to the surface
• Water meniscus between tip and substrate help in
diffusion and transport of molecules
• Conducting polymers, organic dyes, DNA, antibodies etc
can be fabricated
• Solid organic ink can fabricated using heated AFM
cantilever