This set of Nanotechnology Multiple Choice Questions & Answers (MCQs) focuses on “Defects in Graphene”.
1. Why does the formation of Stone Walls defect not lead to addition or removal of dangling bonds in graphene?
a) Formation energy of the defect is in the order of 10-4 eV
b) Stone Walls is a point defect
c) Creation of defect by the rotation of a single pair of atoms
d) Introduction of this defect externally is impossible
View Answer
Explanation: Stone Walls defect of graphene are points defect that are introduced in it by the rotation of a single pair of carbon atoms. This results in adjacent pairs of pentagonal and heptagonal rings. Henceforth, the formation of such defects does not lead to the addition or removal of carbon atoms or dangling bonds.
2. Which of the following is not a topological defect found in graphene?
a) Corrugations
b) Dislocations
c) Grain boundaries
d) Disclinations
View Answer
Explanation: Topological defects include dislocations, disclinations and grain boundaries. These disorders are capable of altering the lattice orientations. An intriguing feature of these defects is that they can exist in graphene without adding local disorder into crystalline lattice.
3. Choose the correct equation which can be used to derive the spatial distribution of ripples.
a) L = (4ĸ/π)(2/3TB)1/2
b) L = 4π(2ĸ/3TB)1/2
c) L = 4πĸ(2π/3TB)1/2
d) L = 4πĸ(2π2ĸ/3TB)1/2
View Answer
Explanation: The spatial distribution of ripples can be derived from the equation L = 4πĸ(2π/3TB)1/2. Here, L represents the distance between two ripples. ĸ indicates the bending rigidity. B is the two-dimensional bulk modulus and T is the absolute temperature. The equation indicates that the distance between the two ripples is inversely proportional to the square of absolute temperature and it approaches infinity at 0K.
4. Which kind of defects appears in graphene due to its fluorination or hydrogenation?
a) Adatoms
b) sp3 defects
c) Dislocations
d) Crumples
View Answer
Explanation: sp3 defects are commonly found in hydrogenated, chlorinated and fluorinated graphenes. In this defect out of plan bonding of the atoms of carbon with other elements, introduce distortions in the crystal lattice. Hence, the carbon atom converts from sp2 to sp3 hybridization. These defects usually appear in clusters rather than being isolated.
5. Which of the following property is not shown by highly ordered graphene structure?
a) High thermal conductivity
b) Opening of band gap
c) High tensile strength
d) Zero band gap
View Answer
Explanation: A highly ordered defect free and ideal graphene shows zero band gap, high thermal conductivity and high tensile strength. On introduction of defects such as wrinkles and crumples, opening of band gaps take place.
6. Why is it difficult to capture adatoms through microscopic techniques such as STM and TEM?
a) High mobility
b) Low formation energy
c) Uniform spatial distribution
d) Remain typically out of plane
View Answer
Explanation: Adatoms are considered to be out of plane carbon atoms in graphene. They have high mobility or high migration rate or high formation energy. This property makes it difficult to capture them through various microscopic techniques such as STM, TEM etc.
7. Ripples are intrinsic disorder of graphene sheets.
a) True
b) False
View Answer
Explanation: The localized π-electrons lead to asymmetric distribution of carbon-carbon bond lengths. This forces the graphene lattice to become non-planar in order to minimize free energy. Eventually, ripples are formed with heights upto 1nm. So, thermal fluctuations induced ripples are intrinsic features on graphene sheets.
8. Why does the thermal conductivity of graphene strongly depend on point defects?
a) Low sensitivity of phonon propagation in graphene
b) Scattering of phonons and their delocalized interactions
c) No effect of surface roughness on the phonon mean free path
d) Inhibition in transition of propagative mode to diffusive mode of heat transfer
View Answer
Explanation: Studies reveal that scattering of phonons by defects and their delocalized interactions result in the transition of heat transfer method from propagative mode to diffusive mode. Subsequently, the thermal conductivity of graphene strongly varies with the distribution and concentration of point defects such as stone wales and single vacancy defects.
9. What kind of deformations occurs in graphene, isotropically in two or more directions, by rapid evaporation?
a) Wrinkles
b) Dislocation
c) Ripples
d) Crumples
View Answer
Explanation: Crumples are dense deformations occurring isotropically in two or three dimensions. They are formed as a result of multi-directional forces applied on graphene during rapid evaporation. The increasing lateral compression transforms the graphene from flat, thin films to cones and finally to crumple balls.
10. These defects in graphene lattice are observed as line defects that are formed by chains of aligned dislocations. What are they?
a) Wrinkles
b) Disclinations
c) Adatoms
d) Grain boundaries
View Answer
Explanation: Grain boundaries are topological defects found in graphene. They are observed as a type of line defect formed by one dimensional chains of aligned dislocations. They are the interfaces between the domains of material with various crystallographic orientations.
11. Which of the following is not a disorder of graphene lattice?
a) Adatoms
b) Corrugations
c) Vacancies
d) Multithreading defects
View Answer
Explanation: Adatoms and vacancies are point defects found in graphene lattice. Corrugations, on the other hand, include wrinkles, crumple and ripple in graphene. A defect such as multi-threading is a software disorder and is not found in graphene lattice.
Sanfoundry Global Education & Learning Series – Nanotechnology.
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