UNIT-IV-Techniques to Construct Nanostructures and Characterisation of Nanoparticles

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Techniques to Construct Nanostructures and Characterisation of Nanoparticles

1. Techniques to Construct Nanostructures

Nanostructures are materials with size in the nanometer range (1–100 nm). Special techniques are used to create and study them.

A. Scanning Probe Instruments

Scanning probe instruments are used to image, measure, and manipulate materials at the nanoscale. They work by scanning a very sharp tip over the surface.

1. Scanning Tunneling Microscope (STM)

  • Uses a sharp metal tip and a conducting sample
  • When the tip comes very close to the surface, electrons “tunnel” between tip and sample
  • The tunneling current changes with surface atoms
  • Produces atomic-level images

Uses:

  • Study surface atoms of metals and semiconductors
  • Construct nanostructures atom by atom

Limitations:

  • Works only with conductive materials

2. Atomic Force Microscope (AFM)

  • Uses a very fine tip attached to a flexible cantilever
  • The tip moves over the surface and feels forces (attractive or repulsive)
  • The movement is recorded to form an image

Advantages:

  • Works with conductive and non-conductive samples
  • Can be used in air, liquid, or vacuum

Applications:

  • Surface topography
  • Nanoparticle size and shape
  • Biological samples

B. Nanoscale Lithography

Lithography means writing or patterning structures at nanoscale.

1. Photolithography

  • Uses light to transfer patterns onto a surface
  • Common in microelectronics

Limitation:

  • Resolution limited by wavelength of light

2. Electron Beam Lithography (EBL)

  • Uses a focused electron beam
  • Very high resolution (nanometer scale)

Advantages:

  • Precise patterning
  • Suitable for nanodevices

Disadvantages:

  • Slow and expensive

3. Nanoimprint Lithography

  • A mold with nanoscale patterns is pressed onto a surface
  • Simple and cost-effective

Applications:

  • Nanoelectronics
  • Sensors
  • Optical devices

2. Characterisation of Nanoparticles

Characterisation means studying size, shape, structure, surface properties, and composition of nanoparticles.

A. UV–Visible Spectroscopy

Principle:
Nanoparticles absorb or scatter UV–visible light due to electronic transitions.

Information obtained:

  • Formation of nanoparticles
  • Optical properties
  • Particle size (indirectly)

Example:

  • Metal nanoparticles show surface plasmon resonance peaks

Advantages:

  • Simple and fast
  • Non-destructive

B. Fourier Transform Infrared Spectroscopy (FTIR)

Principle:
Molecules absorb infrared radiation at specific frequencies related to chemical bonds.

Information obtained:

  • Functional groups on nanoparticle surface
  • Capping and stabilizing agents

Applications:

  • Confirm chemical composition
  • Study surface chemistry

C. Scanning Electron Microscopy (SEM)

Principle:
A focused electron beam scans the surface and produces secondary electrons.

Information obtained:

  • Surface morphology
  • Shape and size of nanoparticles

Advantages:

  • High magnification
  • Good depth of field

Limitation:

  • Lower resolution than TEM

D. Transmission Electron Microscopy (TEM)

Principle:
High-energy electrons pass through a very thin sample.

Information obtained:

  • Exact particle size
  • Shape and internal structure
  • Crystal lattice

Advantages:

  • Very high resolution
  • Atomic-level information

Limitation:

  • Sample preparation is difficult

E. Atomic Force Microscopy (AFM)

Principle:
A sharp tip scans the surface and measures forces between tip and sample.

Information obtained:

  • 2D and 3D surface images
  • Particle height and roughness

Advantages:

  • Works in liquid and air
  • No need for conductive coating

F. X-Ray Diffraction (XRD)

Principle:
X-rays are diffracted by crystal planes in nanoparticles.

Information obtained:

  • Crystal structure
  • Phase identification
  • Average crystallite size (using Scherrer equation)

Applications:

  • Confirm crystalline nature
  • Identify materials

G. Dynamic Light Scattering (DLS)

Principle:
Measures scattering of light due to Brownian motion of particles in solution.

Information obtained:

  • Hydrodynamic particle size
  • Size distribution

Advantages:

  • Quick and easy
  • Works with colloidal solutions

Limitation:

  • Less accurate for non-spherical particles



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