This set of Nanotechnology Multiple Choice Questions & Answers (MCQs) focuses on “Synthesis of Quantum Dots”.
1. Identify a method used for the fabrication of highly ordered arrays of QDs.
a) Complementary metal-oxide semiconductor technology
b) Electrochemical assembly
c) Molecular beam expitaxy
d) Viral assembly
View Answer
Explanation: Electrochemical assembly and CVD are techniques used for the creation of highly ordered arrays of QDs. The electrochemical method involves the creation of a template by causing an ionic reaction at an electrolyte – metal interface which results in the spontaneous assembly of nanostructures along with QDs on the surface of metal. This is later used as a mask for etching these nanostructures onto a chosen substrate.
2. Which of these methods is an alternative to the high temperature dual injection synthesis?
a) Electron beam lithography
b) Colloidal synthesis
c) Electrochemical synthesis
d) Molecular seeding process
View Answer
Explanation: Molecular seeding is an alternative process to high temperature dual injection synthesis for the production of high quality quantum dots in large volumes. The process makes use of identical molecules of a molecular cluster compound as the nucleation sites for the growth of QDs. This prevents the high temperature injection. The growth of particles is maintained by the addition of precursor at intervals until the required product is obtained.
3. Match the following.
1) Focused laser beam i) Self assembled QDs from (III-V) semiconductors 2) MBE ii) Ion implantation 3) Top down process iii) Homogeneous nucleation 4) Precipitation iv) Array of dots
a) 1. iii); 2. iv); 3. i); 4. ii)
b) 1. iv); 2. iii); 3. ii); 4. i)
c) 1. iv); 2. i); 3. ii); 4. iii)
d) 1. ii); 2. iv); 3. i); 4. iii)
View Answer
Explanation: Focused ions or laser beams are used to generate arrays of 0D dots. MBE method is often used to self assemble QDs from III – V semiconductors and II – VI semiconductors using the large lattice mismatch. Top down processing methods include MBE, ion implementation, X-ray lithography etc. The precipitation process involves both nucleation and limited growth of QDs. The nucleation can be homogeneous, heterogeneous or secondary nucleation.
4. Choose the incorrect statement from the following.
a) QDs energy spectrum can be engineered by regulating its shape and size
b) MBE method of QDs synthesis is highly cost effective
c) Confinement in QDs can arise due to the electrostatic potentials
d) QDs occur spontaneously in quantum well structures
View Answer
Explanation: Quantum dots can occur spontaneously in quantum wells owing to the monolayer fluctuations in the well’s thickness. The energy spectrum of QDs can be engineered by modifying the morphology and the strength of the confinement potential. In QDs, confinement can arise from the electrostatic potential which is generated from external electrodes, doping, strain or impurities. The MBE method used for QDs production is quite expensive and lacks control over the positioning of individual dots.
5. Which among the following organism is not used in the preparation of QDs?
a) Tobacco mosaic virus
b) M13 bacteriophage virus
c) Stenotrophomonas maltophilia
d) Polyextremophile halophilic bacteria
View Answer
Explanation: Genetically engineered M13 bacteriophage viruses are used for the preparation of quantum dot biocomposite structures. Also, Stenotrophomonas maltophilia, an engineered strain of bacteria is employed for creation of inorganic quantum dots. Certain other organisms such as TMV and Fd are also known to take part in the preparation of QDs. Researchers have also found out the use of polyextremophile halophilic bacteria for preparation of salt stable fluorescent quantum dots.
6. Which of the following is a critical factor in determining the optimal conditions for growth of the QDs using the method of colloidal synthesis?
a) pH
b) Temperature
c) Moisture content
d) Pressure
View Answer
Explanation: Temperature is a critical factor in determining the optical conditions for the growth of QDs. The optimum temperature required for the process of colloidal synthesis of QDs is high enough. This facilitates rearrangement and annealing of atoms during the process. Another critical factor, the concentration of monomers, is also regulated during the crystal growth.
7. Find out the sequential order of processes involved in the formation of individual quantum dots.
a) Taping off the layer using scotch tape ➔ Lateral pattern is defined ➔ Coating of sample with resist ➔ Transfer of pattern by etching
b) Lateral pattern is defined ➔ Transfer of pattern by etching ➔ Coating of sample with resist ➔ Taping off the layer using scotch tape
c) Coating of sample with resist ➔ Lateral pattern is defined ➔ Transfer of pattern by etching ➔ Taping off the layer using scotch tape
d) Transfer of pattern by etching ➔ Coating of sample with resist ➔ Taping off the layer using scotch tape ➔ Lateral pattern is defined
View Answer
Explanation: Individual QDs are formed from 2D electron hole gases present in remotely doped quantum wells. The sample is initially coated with the resist. Electron beam lithography is used for creating a lateral pattern on the resist. It is then transferred to the electron or hole gas by etching or by lift off process. A scotch tape is often used to tap off the layer of metals.
8. What is the simplest and cheapest method of fabrication of QDs?
a) Colloidal synthesis
b) Molecular seeding process
c) Viral assembly
d) Chemical vapour deposition
View Answer
Explanation: Quantum dots can be prepared by a number of processes from plasma synthesis to electrochemical techniques. The cheapest and the simplest method out of these is the benchtop colloidal synthesis. Large batches of QDs can be synthesized using this method. The large yield, high scalability and the convenience of benchtop conditions makes this process simple and quite promising for commercial applications.
9. What type of growth mode is followed in the synthesis of QDs via MBE?
a) Bridgman growth
b) Czochralski growth
c) Stranski – Krastanov growth
d) Verneuil growth
View Answer
Explanation: In the MBE or MOVPE method of synthesis of QDs if a material grown on a substrate leads to lattice mismatch, self-assembled QDs nucleate spontaneously under certain conditions. This leads to strain and formation of islands on top of a two dimensional wetting layer. This mode of growth is known as the Stranski – Krastanov growth.
10. Pick out the odd one from the following.
a) Indium arsenide
b) Lead selenide
c) Cadmium selenide sulphide
d) Indium phosphide
View Answer
Explanation: The colloidal synthesis method typically makes use of binary compounds such as lead sulphide, lead selenide, cadmium sulphide, indium arsenide and indium phosphite for preparing quantum dots. However, ternary compounds such as cadmium selenide sulphide can also be employed for preparing the quantum dots.
11. What is the most suitable method for large scale production of QDs?
a) Electrochemical assembly
b) Plasma synthesis
c) Electron beam lithography
d) High temperature dual injection
View Answer
Explanation: High temperature dual injection is a method employed by several companies for production of QDs. Its commercial application is because of its excellent scalability and higher yield of quantum dots (from hundreds of kilograms to tones). It can also manufacture QDs with wide range of sizes and composition. However, it is quite difficult to separate nanoparticle nucleation and growth via this method.
12. Why has plasma synthesis evolved to be one of the most popular approaches for synthesis of QDs?
a) High scalability
b) Poor dispersion of dots in organic solvents
c) High purity of QDs
d) Allow easy surface modification
View Answer
Explanation: Plasma synthesis is one of the most popular gas phase techniques for production of QDs. This is because of certain beneficial properties of this process which include – (i) Ability to regulate the morphology and composition of QDs; (ii) Doping of QDs; (iii) Surface modification of QDs by virtue of its powdered form obtained using non thermal plasma; (iv) Dispersion of QDs in various organic solvents and also in water.
13. Colloidal synthesis method involves the growth of QDs in focusing as well as defocusing regimes.
a) True
b) False
View Answer
Explanation: The growth of QDs occurs in two different regimes focusing and defocusing. Initially, at high monomer concentrations the critical size of the nanocrystals is small resulting in faster growth of smaller particles compared to large ones. This inturn leads to size distribution focusing. With the passage of time the concentration of monomer reduces leading to the increase in the critical size above average size. Thus, the size distribution defocuses.
14. Quantum dots generated using this method can be used in quantum cryptography. What is the method employed?
a) Plasma synthesis
b) Lithography
c) Colloidal synthesis
d) Molecular beam epitaxy
View Answer
Explanation: In (Ga)As quantum dots in GaAs can be grown using the molecular beam epitaxy method or metalorganic vapour phase epitaxy (MOVPE). These QDs can be used in quantum cryptography (i.e., single photon sources) and in quantum computation.
15. Genetically engineered viruses cannot recognize semiconductor surfaces.
a) True
b) False
View Answer
Explanation: Studies reveal that genetically engineered viruses can be used to recognize semiconductor surfaces through the method of selection by combinatorial phage display. This unique property of viruses can be useful for organizing inorganic QDs. Research has brought to limelight the creation of self – assembled, highly ordered, self – supporting films from a phage and Zns precursor solution.
Sanfoundry Global Education & Learning Series – Nanotechnology.
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