This set of Nanotechnology Multiple Choice Questions & Answers (MCQs) focuses on “Optical Properties of Nanostructures”.
1. CdSe quantum dots can behave as photoluminescent nanoparticles.
a) True
b) False
View Answer
Explanation: Photoluminescent quantum dots of certain materials emit light owing to their fluorescent nature and their dimensions. CdSe quantum dots also exhibit this phenomenon. Their quantum dots of radius 2.9nm emit wavelengths of light of nearly 555nm. Similarly, radius of 3.4nm emit light wavelength of 580nm approximately.
2. How does plasmonics aim to improve the photonic absorption in solar cells?
a) Restrict the confinement of light inside the photoactive material
b) Obtain resonant internal light scattering on the metallic particles
c) De-excite localized plasmons using non-metallic particles
d) Weakly absorb the incident light using photovoltaic material
View Answer
Explanation: Nanoplasmonics can have a great application in the photovoltaic domain. They can be utilized to improve the photonic absorption and hence increase the efficiency of solar cells. Plasmonics aims at executing this by exciting localized plasmons using metallic nanoparticles to confine light inside the photoactive material. This can also provide beneficial resonant internal light scattering on the metallic particles.
3. Identify the optical phenomenon occurring due to mechanical bond breaking, facture and friction.
a) Photoluminescence
b) Sonoluminescence
c) Triboluminescence
d) Electroluminescence
View Answer
Explanation: Triboluminescence is an optical phenomenon taking place when materials are mechanically crushed, scratched, rubbed or pulled apart. Besides these, friction and fracture can even produce light. A very usual example is the production of light when sugar crystals are broken down or adhesive tapes are peeled. Here, the light produced is hardly visible.
4. Choose the correct statement from the given options.
a) Interferences are used in multilayer coating to enhance reflections of an optical surface
b) Use of optical coatings in optical systems is still a theoretical idea
c) There is an increase in the refractive index of interference filters in air
d) Thin film Fabry – Perot filters do not constitute photonic bandgap structures with defects
View Answer
Explanation: Multilayer coatings use interferences to reduce the reflections of an optical surface. Optical coatings are extensively used in optical systems and mainly other applications such as low-loss mirrors. There is an increase in the refractive index of interference filters when exposed to air containing moisture. This leads to the red shift of the optical properties of the interference filters. Thin film Fabry-Perot filters can be considered to be photonic bandgap structures with a defect.
5. Quantum well structures are extensively used in LEDs and laser diodes. Which of the following option does not enhance the performance of these devices?
a) Confine electrons and holes in a limited space
b) Changes in the composition of the SQ active layer
c) Guiding the output photons
d) Un-tunable emission spectrum
View Answer
Explanation: Quantum well-structures are widely used in LEDs and laser diodes to enhance the performance of the devices. This is done by confining the electrons and holes in a limited space for better recombination. Also, guiding output photons on account of the differing refractive indices of the materials can help improve the performance. A change in the composition of the SQ active layer results in colour variation from 450nm blue to 600nm yellow in the LEDs.
6. Which of the given expressions correctly represents Snell’s law?
a) n1 sin θ1 = n2 sin θ2
b) n1 sin θ2 = n2 sin θ1
c) n1 n2 = (sin θ1) (sin θ2)
d) n1 n2 = sin θ1 / sin θ2
View Answer
Explanation: Snell’s law, also known as the law of refraction or Snell-Descartes Law, states that the ratio of the sines of the angles of incidence and refraction is equivalent to the ratio of phase velocities in the two media, or equivalent to the reciprocal of the ratio of the incidences of refraction. It is given by,
⇨ sin θ2 / sin θ1 = n1 / n2
⇨ n2 sin θ2 = n1 sin θ1
where n is the refractive index and θ is the angle of incidence and refraction measured at the boundary of the two media.
7. Select a material from the given options that cannot be used to capture visible, near infrared and small wave infrared light.
a) HgCdTe
b) Radium
c) InGaAs
d) Black Silicon
View Answer
Explanation: Apart from HgCdTe and InGaAs, black silicon can be used to capture visible, near infrared and small wave infrared light. Black silicon manifests remarkable wide band antireflection properties that make it highly absorbing in visible, and in the near infrared region, below silicon bandgap.
8. Find out the incorrect statement regarding quantum wells.
a) During inter-band transition a photon is emitted
b) Used in LEDs and laser diodes
c) Bandgap engineering is possible in case of quantum wells
d) Dimensions of quantum well cannot be altered
View Answer
Explanation: In quantum well, during an interband transition, a photon is released. The dimensions of quantum well can be altered leading to the tuning of emission spectrum. This feature is called bandgap engineering. Quantum well structures find their applications in LEDs and laser diodes.
9. Pick the odd one out.
a) Fractoluminescence
b) Cathodoluminescence
c) Piezoluminescence
d) Triboluminescence
View Answer
Explanation: Cathodoluminescence is observed when electrons are bombarded on a luminescent material resulting in emission of photons that fall in visible spectrum. Triboluminescence, Fractoluminescence and Piezoluminescence are examples of mechanoluminescence that generate light when a material is acted upon by mechanical force.
10. Which of the following method is not used for the production of black silicon texture?
a) Electrochemical etching
b) Thermoform etching
c) Reactive ion etching
d) Laser-chemical etching
View Answer
Explanation: Black silicon texture can be obtained by techniques such as laser chemical etching, electrochemical etching, reactive ion etching and direct deposition of silica or polystyrene masks on silicon. Wet etching of silicon can also be done using chloroauric acid bath to make black silicon.
11. What are the factors on which the excitations of plasmons depend?
a) Wavelength of light, dielectric properties of surrounding medium
b) Specific gravity of particles, resonance of electric field
c) Angle of the reflected light, diameter of particles
d) Metal nature, incident angle of light
View Answer
Explanation: Plasmons may be excited by using metallic nanoparticles to trap light inside the photoactive substances. This excitation may depend on several factors such as metal nature, surface density of particles, their diameter, wavelength, polarization and the angle of incidence.
12. What is the expression for the intensity of light that has passed through a slab?
a) I = I0(1-IR/I0)2exp-βx
b) I = I0 exp-βx
c) I = (1-IR/I0)X
d) I = (IR/I0)+(IA/I0)
View Answer
Explanation: A beam of light upon passing through a slab loses much of its intensity due to reflection at the surface and absorption by the slab material. Rest of it gets refracted and transmitted from the other side of the slab. The intensity of this transmitted light is given by;
I = I0(1-IR/I0)2exp-βx
Sanfoundry Global Education & Learning Series – Nanotechnology.
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