- List any two differences between SEM and TEM.
- SEM (Scanning Electron Microscopy): Scans the surface of a sample with electrons to produce a 3D surface image; operates in a lower vacuum.
- TEM (Transmission Electron Microscopy): Electrons pass through a thin sample to form a 2D internal structure image; requires a higher vacuum.
- Name any two Bottom-up fabrication methods of nanomaterials.
- Sol-Gel method.
- Chemical Vapor Deposition (CVD).
- What is quantum confinement effect?
Quantum confinement occurs when the size of a material is reduced to the nanoscale (comparable to the de Broglie wavelength of electrons), restricting electron movement and leading to discrete energy levels, altering optical and electrical properties.
- Find the critical angle when a light ray propagates from a medium of refractive index 1.55 to a medium of refractive index 1.475.
The critical angle \( \theta_c \) for total internal reflection is given by \( \sin \theta_c = \frac{n_2}{n_1} \), where \( n_1 = 1.55 \) and \( n_2 = 1.475 \):
\[
\sin \theta_c = \frac{1.475}{1.55} \approx 0.9516
\]
\[
\theta_c = \sin^{-1}(0.9516) \approx 72.2^\circ
\]
So, the critical angle is approximately 72.2 degrees.
- What is the meaning of CVD?
CVD stands for Chemical Vapor Deposition, a process used to deposit thin films of materials on a substrate by reacting gaseous precursors at high temperatures.
- Sol-Gel method belongs to which approach.
The Sol-Gel method belongs to the Bottom-up approach, as it involves building nanomaterials from molecular or atomic precursors through chemical reactions.
- Distinguish 1-D, 2-D, and 3-D nanomaterials.
- 1-D Nanomaterials: Confined in two dimensions (e.g., nanowires, nanotubes), length much greater than diameter.
- 2-D Nanomaterials: Confined in one dimension (e.g., graphene, thin films), large in two dimensions.
- 3-D Nanomaterials: No confinement, nanoscale in all dimensions (e.g., nanoparticles, quantum dots).
- Why do nanomaterials show different physical and chemical properties than their bulk form?
Nanomaterials have a high surface area-to-volume ratio and quantum confinement effects at the nanoscale, which alter their electronic, optical, and chemical properties compared to bulk materials.
- Mention the importance of quantum confinement at nanoscale.
Quantum confinement leads to discrete energy levels, enabling tunable optical properties (e.g., in quantum dots for displays) and enhanced electrical properties (e.g., in nanoelectronics), critical for advanced nanotechnology applications.
- What are nanomaterials?
Nanomaterials are materials with at least one dimension in the nanoscale range (1–100 nm), exhibiting unique physical, chemical, and biological properties due to their size.
- Find the condition for Bragg’s law?
a) \( n\lambda = 2d \cos 2\theta \) b) \( n\lambda = 2d \sin 2\theta \) c) \( n\lambda = 2d \cos \theta \) d) \( n\lambda = 2d \sin \theta \)
Answer d) \( n\lambda = 2d \sin \theta \)
- Write any two advantages of SEM and TEM.
- SEM: Provides detailed 3D surface imaging and is easier to prepare samples for.
- TEM: Offers high resolution (atomic level) and reveals internal structure details.
- What is X-ray diffraction? Why does it happen?
X-ray diffraction (XRD) is the scattering of X-rays by the atoms in a crystalline material, producing an interference pattern. It happens because X-rays have wavelengths comparable to atomic spacing, leading to constructive interference when Bragg’s law is satisfied.
- Does interatomic distance of a material change upon reduction of size to nanoscale?
Yes, interatomic distances can change slightly at the nanoscale due to surface effects, lattice strain, and quantum confinement, which may compress or expand the lattice compared to bulk material.
- Which are the two factors that change the properties of materials at nanoscale?
- High surface area-to-volume ratio.
- Quantum confinement effects.
- Write any four properties of nanomaterials.
- Enhanced electrical conductivity.
- Tunable optical properties (e.g., color changes in quantum dots).
- Increased chemical reactivity.
- Improved mechanical strength.
- What are the two principal factors that cause the properties of nanomaterials to differ from other materials?
- Surface Effects: High surface area-to-volume ratio increases surface energy and reactivity.
- Quantum Effects: Confinement of electrons leads to discrete energy levels, altering electronic and optical properties.
- Why are X-rays not deflected by electric and magnetic fields?
X-rays are electromagnetic waves and carry no electric charge. Since electric and magnetic fields deflect charged particles (e.g., electrons), X-rays, being neutral, pass through undeflected.