Solutions for Assignment-8

Transcription

1 Solutions for Assignment-8 Q1. The process of adding impurities to a pure semiconductor is called: [1] (a) Mixing (b) Doping (c) Diffusing (d) None of the above In semiconductor production, doping intentionally introduces impurities into an extremely pure intrinsic semiconductor for the purpose of modulating its electrical properties. Q2. The pentavalent impurities; i.e., antimony, arsenic, bismuth, phosphorus, etc., added to intrinsic semiconductors are called: [1] (a) Acceptor or P-type impurities (b) Donor or P-type impurities (c) Acceptor or N-type impurities (d) Donor or N-type impurities The number of electrons in pentavalent impurities i.e. antimony, arsenic, bismuth, and phosphorus are higher than the number of holes due to which they are considered as a donor or N-type impurities. Q3. The functionalization of nanomaterials is carried out through: [1] (a) Adsorption of functional groups (b) Attachment of functional groups (c) Absorption of functional groups (d) None of these Functionalization of nanomaterials includes the possibility of attaching functional groups to the nanomaterials (such as fullerene, carbon nanodots, CNTs, graphene, etc.) surface with molecular moieties possessing suitable properties that are essential for the realization of new molecular hybrid materials with novel/enhanced functions.

2 Q4. Which of the following properties can be improved through functionalization of a nanomaterial? [1] (a) Thermal stability (b) Dispersibility (c) Interfacial bonding (d) All of the above Different chemical and physical functionalizations of nanomaterials are found be effective for improving the thermal stability, electrical conductivity, processibility, dispersibility, interfacial properties and various mechanical, barrier and sensing properties. Q5. What is the effect of temperature on physical adsorption? [1] (a) Increases with increase in temperature (b) Increases with decrease in temperature (c) Decreases with increase in temperature (d) No effect Since physical adsorption is an exothermic process, it occurs more readily at lower temperatures and decreases with increase in temperature (Le-Chatelier's Principle). Q6. Which of the following bonds should be present in organosilanes? [1] (a) Si-O (b) Si-C (c) Si-Cl (d) Si-H Organosilanes consist of silane with at least one Si-C bond. Q7. Defect functionalization includes: [1] (a) Esterification (b) Fluorination (c) Hydrogenation (d) Cycloaddition

3 Defect functionalization includes amidation, esterification, thiolation, silanization and polymer grafting. Q8. Which of the following characterization techniques help to prominently identify the presence of functional groups? [1] (i) XPS; (ii) XRD; (iii) EDAX; (iv) FTIR (a) (ii) and (iv) (b) (ii) and (iii) (c) (i) and (iv) (d) (iii) and (iv) XPS and FTIR are most widely used to observe the presence of specific functional groups on the surface of a nanomaterial. Q9. Which of the following methods can be used to enable grafting on a particle through modifying the polymer end-chain after polymerization? [1] (a) Grafted from (b) Grafted onto (c) Both of these (d) None of these In grafted onto method, polymer end-chain can be modified after polymerization to enable grafting on the particle. Q10. Stöber method was originally designed for: [1] (a) Titania nanoparticles (b) Zirconia nanoparticles (c) Silica nanoparticles (d) Alumina nanoparticles Silica has been widely used since the invention of the Stöber method which was originally designed for the preparation of silica nanoparticles with well controlled spherical shape and size using alcoholic solvents, catalysts and alkoxide precursors. Q11. In an intrinsic semiconductor, the position of Fermi level lies: [2]

4 (a) At the center of forbidden energy gap (b) Near the conduction band (c) Near the valence band (d) Anywhere in the forbidden energy gap The Fermi level for n-type semiconductors is below the conduction band, for p-type semiconductors, it is above the valence band and for intrinsic (pure) semiconductors it is exactly at the center of forbidden energy gap. Q12. Choose a suitable option which provides correct explanations (Section B) for the surface modification methods (Section A): [2] Section A Section B 1. Doping (i) Gas molecules condense on the surface 2. Adsorption (ii) Attaching functional groups 3. Grafting (iii) Adding impurities or replacing host atoms 4. Functionalization (iv) Addition of polymer chains onto a surface (a) 1-(ii), 2-(iv), 3-(i), 4-(iii) (b) 1-(ii), 2-(iii), 3-(iv), 4-(i) (c) 1-(iii), 2-(i), 3-(iv), 4-(ii) (d) 1-(ii), 2-(iv), 3-(i), 4-(iii) Doping is the process of adding some impurity atoms or replacing host atoms in the semiconductor.

5 Adsorption is the adhesion of atoms, ions, or molecules from a gas, liquid, or dissolved solid to a surface. This process creates a film of the adsorbate on the surface of the adsorbent. Grafting, in the context of polymer chemistry, refers to the addition of polymer chains onto a surface. Functionalization is the possibility of attaching functional groups to the nanomaterials (such as fullerene, carbon nanodots, CNTs, graphene, etc) surface with molecular moieties possessing suitable properties that are essential for the realization of new molecular hybrid materials with novel/enhanced functions. Q13. Which of the following conditions is not true for the doping of nitrogen in graphene? [2] (a) Atomic radius is almost same (b) Same crystal structure (c) Electro-positives are 2.55 for C and 3.04 for N (d) Valences are +3 for C and +3, +5 for N N-doped graphene, having following necessary conditions: Atomic radii of C and N are 70 pm and 65 pm, respectively Both have HCP crystal structure, Electro-negativities are 2.55 for C and 3.04 for N Valencies are +3 for C and +3, +5 for N Q14. Match the techniques given in Section A with their applications in Section B : [2] Section A Section B 1. Doping (i) Lenses 2. Functionalization (ii) Membranes for the separation of gases or liquids 3. Polymerization (PMMA) (iii) Molecular junctions, FETs 4. Grafting (iv) Photovoltaics, Supercapacitors (a) 1-(ii), 2-(iv), 3-(iii), 4-(i) (b) 1-(iv), 2-(i), 3-(iii), 4-(ii)

6 (c) 1-(iv), 2-(iii), 3-(i), 4-(ii) (d) 1-(ii), 2-(iii), 3-(iv), 4-(i) Doping: Electro catalysis, Li-ion battery (LIB) anodes, supercapacitors, sensing, photovoltaics, biological and biomedical applications. Functionalization: Molecular junctions, Field-effect transistors (FETs), solar thermal storage, memory devices, color sensing, biological applications. Polymerization (PMMA): Lenses, transparent aircraft enclosures, drafting equipment, outdoor signs, etc. Grafting: Membranes for the separation of gases or liquids, hydrogels, drug deliverers, thermoplastic elastomers, compatibilizers for polymer blends, polymeric emulsifiers, impact resistant plastics. Q15. Match the following techniques on the basis of their problems: [2] Techniques Problems 1. Coating (i) Agglomeration during wall formation 2. Microencapsulation (ii) Difficult to maintain the crystal structure of base material 3. Thin film (iii) Hiding functional characteristics of base material 4. Doping (iv) Damage during etching (a) 1-(ii), 2-(iv), 3-(iii), 4-(i) (b) 1-(iv), 2-(i), 3-(iii), 4-(ii) (c) 1-(iii), 2-(i), 3-(iv), 4-(ii) (d) 1-(ii), 2-(iii), 3-(iv), 4-(i) Problems during fabrication of thin film: Uniform film thickness, adhesion, cracks formation, damage thin film during etching or transfer one substrate to other, film deposition on reactor walls. Problems during microencapsulation:

7 A common problem in many microencapsulation processes is the agglomeration of capsules during wall formation. As the wall materials change from liquid to solid form they often go through a sticky stage which makes agglomeration difficult to avoid. Main problems in the coating of nanostructures: Uniformity of layer (thickness), surface roughness, agglomeration, corrosion prevention, hiding functional characteristics (optical, electrical and magnetic) of the substrate, optimal wettability, chemical composition (crystalline structure in nanometer range), standardize surface geometry. Problems during doping: It is very difficult to maintain the crystal structure of host material, during the doping process.