This set of Nanotechnology Multiple Choice Questions & Answers (MCQs) focuses on “Nanowires”.
1. Which of these processes is not followed in the creation of suspended nanowires?
a) Bombardment of energetic ions towards the wire
b) Thermal oxidation of the large wire
c) Chemical etching of a larger wire
d) Indenting STM tip on the metal surface and retracing it
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Explanation: Suspended nanowires can be synthesized in a high – vacuum chamber that is held at longitudinal extremities. The procedures involved are chemical etching of a larger wire, bombardment of energetic ions towards the large wire and the indentation of the tip of a scanning tunneling microscope in the surface of a metal near its melting point followed by its retraction.
2. How does the vapour liquid solid (VLS) method proceed in producing nanowires?
a) Source is turned off to adjust size ➔ Saturation begins ➔ Source enters nanoclusters ➔ Solidification of source and outward growth from the nanocluster
b) Solidification of source and outward growth from the nanocluster ➔ Source enters nanoclusters ➔ Source is turned off to adjust size ➔ Saturation begins
c) Saturation begins ➔ Source is turned off to adjust size ➔ Solidification of source and outward growth from the nanocluster ➔ Source enters nanoclusters
d) Source enters nanoclusters ➔ Saturation begins ➔ Solidification of source and outward growth from the nanocluster ➔ Source is turned off to adjust size
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Explanation: The vapour liquid solid(VLS) process requires liquid metal catalysts, a source material from laser ablated particles or a feed gas like silane. The process proceeds with the entrance of the source into the nanoclusters, followed by its saturation. Upon achieving supersaturation, solidification of the source occurs which the grows out of the nanocluster. Switching off the source helps in adjusting the length of the nanowires generated in the procedure.
3. What is the main difference between the vapour liquid solid (VLS) and vapour solid solid (VSS) synthesis method?
a) Catalytic seed remains in solid state at high temperature annealing of substrate
b) Requirement of catalyst in VLS only
c) Production of high-quality crystalline nanowires
d) Multi-step vapour phase reaction occurring at elevated temperature
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Explanation: Vapour solid solid (VSS) growth method has quite a resemblance with the vapour liquid solid (VLS) growth method. However, in VSS method the catalytic seed remains in the solid state when the substrate is subjected to high temperature annealing. VSS method synthesizes nanowires through the thermolytic decomposition of a silicon precursor. This method mainly produces metal silicide or germanide nanowires.
4. Which of the following techniques uses ethylene glycol both as solvent and reducing agent for the production of nanowires?
a) Ion-track technology
b) Lithography
c) Polyol synthesis
d) Non-catalytic synthesis
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Explanation: Solution phase synthesis implies techniques that involve the growth of the nanowires in solution. Polyol synthesis is one such technique in which ethylene glycol is used both as a solvent and a reducing agent for the production of nanowires. This method mainly produces nanowires of gold, lead, platinum and silver.
5. What are the sequential steps occurring in the supercritical fluid-liquid-solid growth method for synthesis of nanowires?
a) Degradation of precursors ➔ Precipitation of solid crystallite ➔ Uniaxial growth of nanowires from nanocrystal seeds ➔ Dissolution of silicon and germanium in metal nanocrystals ➔ Organometallic precursors are fed into the reactor
b) Organometallic precursors are fed into the reactor ➔ Degradation of precursors ➔ Dissolution of silicon and germanium in metal nanocrystals ➔ Precipitation of solid crystallite ➔ Uniaxial growth of nanowires from nanocrystal seeds
c) Uniaxial growth of nanowires from nanocrystal seeds ➔ Dissolution of silicon and germanium in metal nanocrystals ➔ Degradation of precursors ➔ Organometallic precursors are fed into the reactor ➔ Precipitation of solid crystallite
d) Dissolution of silicon and germanium in metal nanocrystals ➔ Organometallic precursors are fed into the reactor ➔ Precipitation of solid crystallite ➔ Uniaxial growth of nanowires from nanocrystal seeds Degradation of precursors
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Explanation: Supercritical fluid liquid solid growth method is one of the solution phase techniques employed to synthesize semiconductor nanowires. In this technique metal nanocrystals are used as seeds whereas silicone or germanium organometallic substances are used as precursors. The precursors are introduced into the reactor filled with a super critical organic solvent like toluene. Thermolysis occurs leading to the degradation of the precursor and allowing their dissolution into the metal nanocrystals. A solid crystallite precipitates as more and more semiconductor solute is added from the supercritical phase. Thus, a nanowire grows uniaxially from the nanocrystal seed.
6. Nanowires cannot be grown in the absence of catalysts.
a) True
b) False
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Explanation: Nanowires can be grown both in the presence of or in the absence of catalysts. The absence of catalysts is rather advantageous since there is no way of catalyst contamination and we obtain pure nanowires. It also reduces the number of technological steps. Spontaneous nanowire formation by non-catalytic methods could be explained by the dislocation present in specific directions or the growth anisotropy of various crystal faces.
7. Which method of nanowire synthesis makes use of strands of nucleotides as a precursor?
a) Solution phase synthesis
b) Electrochemical deposition
c) Ion track technology
d) DNA-templated metallic nanowire synthesis
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Explanation: The DNA-templated metallic nanowire synthesis technique makes use of DNA strands that are made of nucleotides as scaffolds for metallic nanowire synthesis. It has biosensing applications, in which they allow the transduction of a DNA strand into a metallic nanowire that can be electrically detected.
8. What is the limitation of the photolithography process used for the synthesis of nanowires?
a) Limited resolution
b) Maskless method
c) Extraneous material generation
d) Expensive process
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Explanation: The resolution of photolithography is limited owing to the technique and wavelength of light used for the purpose. It is often not a suitable process for the synthesis of small nanowires. Moreover, nanowires produced using this method lacks complex electronic characteristics.
9. Why are top down methods so much in use for the fabrication of nanowires?
a) Slow production rate
b) Lack complex electronic characteristics
c) Increase the cost of production
d) Formation of ordered arrays of nanowires
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Explanation: Top down methods of fabrication of nanowires are of much interest due to the ease with which the ordered arrays of nanowires can be considered. This helps facilitate the electrical contact to the nanowires and their integration into large-scale devices. Further, these methods are quite compatible with the microelectronics industry enabling their scale up.
10. Which of these is not an advantage of the vapour liquid solid growth method employed for the synthesis of nanowires?
a) Compositional control
b) Production of superlattice structure
c) Controlled non-catalytic growth method
d) Nanometer-scale bandgap modulation
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Explanation: The vapour-liquid-solid method is advantageous for nanowire fabrication as they allow for compositional control. Moreover, dopant materials can be administered and removed easily in a repetitive manner throughout the nanowire growth, producing superlattice structures with modulated electronic properties. It has been observed that upon changing the ratio of the components in a ternary structure, nanometer-scale bandgap modulations occur and quantum-well structures are formed.
11. What is the main challenge faced by the bottom-up process in nanowire development?
a) Integration into large-scale machineries
b) Production of complex superlattice structure
c) Explicit control of nanowire composition
d) Provide template for subsequent processing
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Explanation: The main challenge faced during the development of nanowires via bottom-up processes is their integration into large scale devices. Nanowires that are grown in vertical arrays can be used in a similar fashion as top-down methods, but the bottom up processes generate nanowires that remain as disordered ‘forests’ or as solutions of nanowires.
12. Match the following.
i) Template synthesis I) Produces geometry needed for the generation of arrays of vertical nanowires ii) Electrochemical etching II) Nanowire precursors are dissolved into high boiling liquids along with catalysts iii) Selective area epitaxy III) Provides a pathway to achieve anisotropy for nanowires iv) Solution liquid solid IV) Growth in exposed areas but no deposition onto mask material
a) I-ii); II-iv); III-iii) ; IV-i)
b) I-iv); II-ii); III–i) ; IV-iii)
c) I–iii); II–i); III–iv); IV-ii)
d) I–i); II–iii); III-ii); IV-iv)
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Explanation: Templated growth method provides a pathway to achieve the anisotropy needed for nanowires. Electrochemical etching generates anodic aluminium oxide that contains a honeycomb of channels offering the geometry required for producing arrays of vertical nanowires. Selective area epitaxy (SAE) allows epitaxial growth in exposed regions but deposition onto the mask does not occur. Solution liquid solid method requires a precursor to be dissolved in high boiling liquid along with the suspended catalysts.
More MCQs on Nanowires:
Sanfoundry Global Education & Learning Series – Nanotechnology.
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