This set of Nanotechnology Multiple Choice Questions & Answers (MCQs) focuses on “Characteristics of Quantum Dots”.
1. How can the electronic characteristics of QDs be determined?
a) Determined by the wave function
b) Determined by the excitons
c) Determined by their shape and size
d) Determined by their photoluminescence
View Answer
Explanation: Studies suggest that electronic characteristics of QDs can be determined by their size and shape. It refers to the regulation of their emission wavelengths by tuning their size. It has been observed that smaller QDs emit shorter wavelengths as violet or blue. However bigger QDs emit longer wavelength generating colours like yellow and red.
2. Who discovered quantum dots?
a) Alexey Ekimov
b) Bill Pickering
c) Robert H. Goddard
d) Barbara McClintok
View Answer
Explanation: The QDs were discovered in a glass matrix in 1980 by the Russian physicist Alexey Ekimov. Later in 1982, these were observed in colloidal solutions by the American chemist Louis E. Brus. However, these were first theorized by Alexandar Efros.
3. Which of the following is a property observed in QDs?
a) Low quantum yield
b) High molar extinction
c) Broad emissions
d) Low photostability
View Answer
Explanation: The properties of QDs include high quantum yield, high molar extinction coefficients and high photostability. They have the ability to emit light of specific wavelengths when excited using electricity or light. Their emissions are relatively narrow and symmetrical at certain wavelengths.
4. Choose the correct statement from the following.
a) They are one dimensional entities
b) They exhibit closely spaced energy levels with reduction in size
c) They follow Mie scattering theory
d) The larger dots have a shorter lifetime with respect to fluorescence
View Answer
Explanation: QDs can be defined as point like, zero dimensional (0D) entities that have size in the order of nanometers. They exhibit various properties such as high quantum yield, melting point depression, and fluorescence and so on. The lifetime of this fluorescence is determined by the size of the dots. Larger dots have more closely shaped energy levels in which electron-hole pair can be trapped. Hence the electron – hole pair in larger dots live longer resulting in prolonged fluorescence. The optical properties of metallic QDs can be described using the Mie scattering theory.
5. Which equation can be used to calculate the confined energy levels?
a) Carothers equation
b) Einstein field equation
c) Schrödinger equation
d) Drake equation
View Answer
Explanation: The shape of quantum dots are quite complex. The calculations of the confirmed energy levels can be done using the numerical solution of the relevant Schrödinger equation. It is a linear partial differential equation that describes the wave function or state function of a quantum – mechanical system.
6. Why are quantum dots (QDs) referred to as tiny semiconductor particles?
a) Created artificially
b) Provide quantum confinement
c) Emit light of specific wavelength
d) Increase energies at which electrons and holes exist.
View Answer
Explanation: Quantum dots are referred to as tiny man-made semiconductor particles in the order of 102 to 105 nms. When semiconductor particles are reduced in size excessively, quantum effects come to play which limit the energies at which electrons and holes can exist in the particles.
7. QDs do not pose any risk to human health since they are nontoxic in nature.
a) True
b) False
View Answer
Explanation: Several research and studies based on cell cultures on quantum dots (QDs) toxicity have revealed that their potential toxicity is due to multiple factors. These include physiochemical characteristics such as size, shape, composition, surface functional groups and surface charges and their environment.
8. What is the expression for the total energy of a carrier in a cuboid – shaped dot in the infinite – depth well approximation?
a) En,m,k = (h2n2)/(8m*Lz2)+(h2m2)/(8m*Ly2)+(ћkx2)/(2m*)
b) En,k = (h2n2)/(8m*L2)+(ћ2kx2)/(2m*)
c) Em,k = (h2m2)/(8m*L2)+(ћ2kx2)/(2m*)
d) En,m,l = (h2n2)/(8m*Lz2)+(h2m2)/(8m*Ly2)+(h2l2)/(8m*Lx2)
View Answer
Explanation: The expression for the total energy for a carrier in a cuboid shaped dot in the infinite depth well approximation is given by,
En,m,l = (h2n2)/(8m*Lz2)+(h2m2)/(8m*Ly2)+(h2l2)/(8m*Lx2) (n,m,l = 1,2,3,….)
where n,m and l are quantum numbers. Lz, Ly, and Lx are the dimensions of the cuboid shaped dot and h is the Planck’s constant.
9. How can the quantum yield of quantum dots be improved?
a) Impairing the anti-oxidative system
b) Narrowing their emission spectrum
c) Altering the photostability
d) Producing them with a shell of larger band gap
View Answer
Explanation: The fluorescence quantum yield of quantum dots can be improved by creating a shell of larger bandgap semiconductor material around them. The improvement is suggested to be occurring due to the reduced access of electron and hole to non-radiative surface recombination pathways in certain cases but may also be due to Auger recombination in others.
10. Identify the phenomenon occurring as a result of recombination of excitons.
a) Fluorescence
b) Bioluminescence
c) Phosphorescence
d) Chemiluminescence
View Answer
Explanation: In semiconductors, intrinsic band gaps exist. The absorption of light leads to the excitation of electrons from valence to conduction band, leaving a hole behind. This electron and hole can bind together to form an exciton. Upon recombination of this exciton that is the return of the excited electron back to its ground state, a photon is released as light. This phenomenon is known as fluorescence.
11. How are the CdSe quantum dots toxic in nature?
a) They release free cadmium ions
b) They prevent DNA mutations
c) They inhibit formation of reactive oxygen species
d) They undergo ligand exchange
View Answer
Explanation: When exposed to UV rays these CdSe QDs release free cadmium ions that cause cellular death. Sometimes the formation of reactive oxygen species after exposure to light may damage cellular components such as proteins, lipids and DNA. There have been reports of CdSe QDs localization in cell nucleus that may induce DNA mutation while can cause severe diseases and health conditions.
More MCQs on Quantum Dots:
Sanfoundry Global Education & Learning Series – Nanotechnology.
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