Techniques for Synthesis of Nano-materials

Akshay Tiwari and Rushabh ShahB.Tech Electrical

What are Nano-Particles

• In nanotechnology, a particle is defined as a small object that behaves as a whole unit in terms of its transport and properties.

• Particles are further classified according to size: in terms of diameter, coarse particles cover a range between 10,000 and 2,500 nanometers.

• Fine particles are sized between 2,500 and 100 nanometers.

• Ultrafine particles, or nanoparticles are sized between 100 and 1 nanometers.

Synthesis Method

• Top Down Approach:– Begins with a pattern generated on a larger scale, then reduced to nanoscale– Relatively expensive and time consuming technique– The approach use larger (macroscopic) initial structures– The structures can be externally-controlled in the processing of nanostructures

• Bottom Up Approach:– Start with atoms or molecules and build up to nanostructures– Fabrication is much less expensive– Includes the miniturization of materials components (atomic level) leading to formation of nano structures.– During Self assembly the physical forces operating at nanoscale are used to combine basic units into larger stable structure

Top-Down Approach: Lithography

Coat Protect Expose Etch Repeat

Multiple Patterned Layer of Different Materials

Photo-Lithography

Photolithography: DetailsMasking and Exposure• Align Mask to wafer before exposure• Expose resist to UV through a mask

Developing the Pattern• Resist is removed from exposed areas• Remaining resist produces masked pattern

Etching• Resist Protects selected regions during etch• Pattern is transferred to substrate material

Bottom Up Approach

1.•High precision actuators move atoms from place to place

2.•Micro tips emboss or imprint materials

3.•Electron (or ion) beams are directly moved over a surface

4.

• Chemical reactors create conditions for special growth

5.

• Biological agents sometimes used to help process

6.

• Materials are harvested for integration

Bottom Up: Molecular Assembly• Nature uses self-assembly in

infinitely subtler ways; indeed, the whole of the natural world is self assembled

• Spontaneous organization of molecules into stable, structurally well-defined aggregates (nanometer length scale).

• Molecules can be transported to surfaces through liquids to form self-assembled monolayers (SAMs).

Polythiophene wires

Self Assembly Example

Bottom Up Method: Inert-gas Condensation

• An inorganic material is vaporized inside a vacuum chamber into which an inert gas (typically argon or helium) is periodically admitted.

• Once the atoms boil off, they quickly lose their 23 energy by colliding with the inert gas.

• The vapor cools rapidly and supersaturates to form nanoparticles with sizes in the range 2–100 nm that collect on a finger cooled by liquid nitrogen.

Molecular Beam Epitaxy (MBE)A molecular beam epitaxy (MBE) machine is essentially an ultrahigh-precision, ultra clean evaporator, combined with a set of in-situ tools, such as Auger electron spectroscopy (AES) and/or reflection high energy electron diffraction (RHEED), for characterization of the deposited layers during growth.

Methods for making 1-D and 2-DNanomaterials

• The production route for 1-D rod-like nanomaterials by liquidphase methods is similar to that for the production of nanoparticles.

• CVD methods have been adapted to make 1-D nanotubes and nanowires. Catalyst nanoparticles are used to promote nucleation.

• Nanowires of other materials such as silicon (Si) or germanium (Ge) are grown by vapor-liquid-solid (VLS) methods.

Chemical Vapour Deposition

• A process involving deposition of a solid material from gaseous phase.

• Precursor gases diluted in carrier gases are delivered into the reaction chamber at ambient temperatures.

• As they pass over or come into contact with a heated substrate, they react or decompose forming a solid phase which and are deposited onto the substrate.

• Precursors for CVD processes must be volatile, but also stable enough to be able to be delivered to the reactor.

• Physical Vapour Deposition is different from cvd as it uses solid precursors whereas cvd uses gases.

Conclusions

• Two approaches used for nano-particle synthesis– Bottom-up and – Top-Down Approach

• Common Methods Used include:– Optical Lithography (Top-Down)– Inert Gas Condensation (Bottom-up)– Chemical Vapour Deposition( Bottom-up)