MEE381 NANOTECHNOLOGY LECTURE 104.2.2014

Dr. R. PRATIBHA NALINI

UNIT - 2

Bottom up and Top down approachesBall Milling and Sol-Gel technique

UNIT 2 SCHEDULE

NANOMATERIALS SYNTHESIS

TOP DOWN APPROACH

BOTTOM UP APPROACH

•Nanomaterials are synthesized by assembling the atoms/molecules together.•Instead of taking material away to make structures, the bottom-up approach selectively adds atoms to create structures.Eg) Plasma etching, Chemical vapour deposition

•Nanomaterials are synthesized by breaking down of bulk solids into nanosizes•Top-down processing has been and will be the dominant process in semiconductor manufacturing.•Eg) Ball Milling, Sol-Gel, lithography

Basic top down approaches in nanofabrication

–Pattern transfer (lithography)–Deposition (or film growth)–Etching ( or removal of material)

Basic bottom up approaches in nanofabrication

–Chemical vapor growth: vapor-solid-liquid growth–Self assembly: colloidal chemistry

Top-down Versus Bottom-up

Etched wafer with desired pattern

Apply layer of photoresist

Expose wafer with UV light through mask and etch wafer

Start with bulk wafer

Top Down Process Bottom Up Process

Start with bulk wafer

Alter area of wafer where structure is to be created by adding polymer or seed crystals or other techniques.

Grow or assemble the structure on the area determined by the seed crystals or polymer. (self assembly)

Similar results can be obtained through bottom-up and top-down processes

Self Assembly

• The principle behind bottom-up processing.• Self assembly is the coordinated action of

independent entities to produce larger, ordered structures or achieve a desired shape.

• Found in nature.• Start on the atomic scale.

Cost of new machines and clean room environments grows exponentially with newer technologies.

With smaller geometries and conventional materials, heat dissipation is a problem.

PROBLEMS OF TOP DOWN APPROACH

Allows smaller geometries than photolithography.

Certain structures such as Carbon Nanotubes and Si nanowires are grown through a bottom-up process.

New technologies such as organic semiconductors employ bottom-up processes to pattern them.

Can make formation of films and structures much easier.Is more economical than top-down in that it does not waste material to etching.

ADVANTAGES OF BOTTOM UP APPROACH

BALL MILLING

Popular, simple, inexpensive and extremely scalable material to synthesize all classes of nanoparticles.

Can produce amorphous or nanocrystalline materials.

MECHANICAL ATTRITION MECHANISM is used to obtain nanocrystalline structures from either single phase powders or amorphous materials.

Can use either refractory balls or steel balls or plastic balls depending on the material to be synthesized.

When the balls rotate at a particular rpm, the necessary energy is transferred to the powder which in turn reduces the powder of coarse grain-sized structure to ultrafine nanorange particle.

TOP DOWN APPROACH

The energy transferred to the powder from tha balls depends on many factors such as

Rotational Speed of the ballsSize of the ballsNumber of the BallsMilling timeRatio of ball to powder massMilling medium /atmosphere

Cryogenic liquids can be used to increase the brittleness of the product

One has to take necessary steps to prevent oxidation during milling process

The selection of ball material indluences the type of material obtained. Eg) harder material balls, synthesize softer materials

Alpha-alumina and zirconia are widely used ball materials due to their high grinding resistance values.

ADVANTAGES OF BALL MILLING

Scaling can be achieved upto tonnage quantity of materials for wider applications

DISADVANTAGES OF BALL MILLING

Contamination of the milling mediaNon-metal oxides require an inert medium, and vacuum or glove box to use powder particles. So The milling process is restrictive.

SOL- GEL METHOD

WET CHEMICAL TECHNIQUE (Chemical solution deposition technique)

Produce high purity and homogeneous nanomaterials, particularly metal oxide nanoparticles

Starting material from a chemical solution leads to the formation of colloidal suspensions known as SOL

The SOL evolves towards the fomation of inorganic network containing a liquid phase called the GEL. The removal of liquid phase from the Sol yields the Gel.

The particle size and shape are controlled by the Sol/Gel transitions.

The thermal treatment (firing/calcinations) of the gel leads to further polycondensationReaction and enhances the mechanical properties of the products (i.e.) oxide nanoparticles

PRECURSORS Metal alkoxides and metal chlorides

Starting material is washed with water and dilute acid in alkaline solvent.

The material undergoes hydrolysis and polycondensation reaction which leads to the formation of colloids.

Colloid System composed of solid particles is dispersed in a solvent containing particles of size from 1nm to 1mm

The SOL is then evolved to form an inorganic network containing a liquid phase (GEL)

The Sol can be further processed to obtain the substrate in a film, either by dip coating orSpin-coating or case into a contained with desired shape or powders by calcination. The chemical reaction which takes place in the Sol-Gel metal alkoxides M(OR)2 during the hydrolysis and condensation is given by

SOL- GEL METHOD (Contd)

REFERENCE

ENGINEERING PHYSICS by V. RAJENDRAN AND THYAGARAJAN K, Tata Mc GrawHill publications

Search term “bottom up and top down approaches ball milling and sol gel” on google, freely available excerpt from Google books of the aforesaid author.

http://www.matecnetworks.org/webinars/pdf/Fabrication.pdf

Can use the above two as the study material for this topic.