This set of Nanotechnology Multiple Choice Questions & Answers (MCQs) focuses on “Graphene”.
1. Which of the following options does not correspond to the ways in which intrinsic ripples influence electrical properties of graphene?
a) Alter band gaps
b) Impede electronic transport
c) Induce pseudo-magnetic fields
d) Create polarized carrier puddles
View Answer
Explanation: Defects present in graphene structure affect its electronic properties to a great extent. Intrinsic ripples in graphene act as defects that influence its electrical properties by changing the band gap, inducing pseudo-magnetic fields and creating polarized carrier puddles.
2. Where does the Fermi energy level lie in an ideal planar graphene structure?
a) At defects or disorders
b) Within Interatomic bonds
c) Below the conduction band
d) Junction point of valence and conduction band
View Answer
Explanation: In an ideal planar graphene structure, the conduction and the valence bands meet at the Dirac points. Fermi energy level lies at these Dirac points where the dispersion relation around the points is isotropic and linear.
3. What gives graphene a valley degeneracy of gv=2?
a) Valence Band
b) Band Gaps
c) Sets labeled as K and K’
d) Conduction Band
View Answer
Explanation: In an ideal planar graphene sample, without defects, the conduction and valence bands meet at Dirac points. These points are six locations in momentum space which are divided into two non-equivalent sets of three points each. The sets are labeled as K and K’. These sets provide graphene a valley degeneracy of gv=2.
4. Single-atom layer of graphene is not sensitive to which of the following materials?
a) Carbon nanotubes (CNTs)
b) Ferromagnetics
c) Superconductors
d) High-ĸ dielectrics
View Answer
Explanation: In single-atom layer of graphene, electron waves propagate. This makes the single-atom layer sensitive to the proximity of other materials such as high-ĸdielectrics, ferromagnetics and superconductors.
5. How do point defects like vacancies reduce electrical conductivity of graphene?
a) Alter lattice orientations
b) Inhibits the introduction of local disorders in crystalline lattice
c) Serve as scattering centers for electrons
d) Prevent doping in graphene
View Answer
Explanation: Graphene exhibits point defects such as stone wales, single vacancies, adatoms etc. In the current methods used for the preparation of graphene, point defects are inevitable. These defects serve as electron scattering centers. Consequently, the transfers of electrons get affected resulting in the decrease of the conductivity of graphene.
6. Which of the following statement is incorrect about ballistic electron transport in graphene?
a) A temperature independent mode of transport of electrons
b) It is a thermally activated mode of electron transport
c) Resistance increase suddenly at room temperature, at a certain length of 16µm
d) Ballistic electrons are similar to the ones present in cylindrical CNTs
View Answer
Explanation: Electron transport in graphene is dominated by two modes. One is the temperature independent mode while the other mode is thermally activated. Electrons travelling via ballistic mode, which is temperature independent, resemble those in cylindrical carbon nanotubes. At room temperature resistance increases instantly at a certain length of 16µm for ballistic mode.
7. Wrinkles and crumples result in varied electronic phenomena in graphene. Which among the given option is not one of them?
a) Alteration of carbon-carbon bond length
b) Carrier scattering
c) Band gap opening
d) Electron hole puddles
View Answer
Explanation: Disorders present in the form of wrinkles and crumples in graphene lattice result in several electronic phenomena. These include carrier scattering, quantum corrections, band gap opening, electron-hole puddles and suppression of weak localization.
8. Which of the following factors is not responsible for the maintenance of Quantum Hall Effect (QHE) in graphene at high temperatures?
a) Constant mobility of dirac fermions
b) High carrier concentrations
c) Large cyclotron gap
d) Electrical resistivity
View Answer
Explanation: Quantum Hall Effect in graphene survives at high temperatures due to different factors. These are mainly the large cyclotron gap, high carrier concentrations (up to 1013cm2) with only a 2D sub band filled and the unchanged mobility of dirac fermions.
9. How can graphene doped with gaseous species be returned to undoped state?
a) Extreme heating at very high temperatures in vacuum
b) Gentle heating in vacuum
c) Extreme heating at very high temperatures without vacuum
d) Gentle heating without vacuum
View Answer
Explanation: Graphene sheets are many a time doped with various gaseous species. These can be both acceptors and donors. Graphene from the doped state can be restored to its undoped and original state by gentle heating in vacuum.
10. Which of the following statement is incorrect about grain boundaries (GBs) in graphene?
a) GBs have fixed grain size that cannot be altered
b) GBs impede electronic transport
c) GBs induce scattering effect
d) GBs increase resistance of polycrystalline graphene
View Answer
Explanation: The size of grain boundaries can be altered. Studies reveal that the size of GBs has been altered during experiments, in order to check its effect on the conductivity properties of graphene.
11. Which of the following is the correct pair of edge structures for graphene nanoribbons?
a) Zig-Zag and Chiral
b) Chiral and Arm-chair
c) Zig-Zag and Armchair
d) Chiral and Bay
View Answer
Explanation: There are many edge topologies for graphene. Armchair and Zig-Zag are the two main types of edges. For graphene nanoribbons, if it is zig-zag, bandgap is zero, while if it is armchair then bandgap is non-zero.
12. Choose the incorrect option w.r.t the disorders in graphene structure
a) Crumples exhibit carrier scattering
b) Intrinsic ripples create polarized carrier puddles
c) Dislocations alter C-C bond length
d) Wrinkles close band gaps
View Answer
Explanation: Graphene structures are often characterized by defects, which influence its electronic properties. Intrinsic ripples create polarized carrier puddles, induce pseudo-magnetic fields and change band gaps. Dislocation and disclinations induce distortion in graphene lattice that alter the carbon-carbon bond length, and eventually the band structure. Wrinkles and crumples exhibit carrier scattering, band gap opening, quantum corrections etc.
13. In what way is graphene’s Quantum Hall Effect (QHE) different from the conventional quantum hall effect?
a) Plateaus occur at 5e2/h
b) Plateaus occur at 11e2/h
c) Plateaus occur at 1e2/h
d) Plateaus occur at 3e2/h
View Answer
Explanation: Graphene exhibits the quantum hall effect with respect to conductivity quantization. This effect is anomalous. It shows subtle difference from the conventional hall effect. Here plateaus occur at half integers of (4e2/h) rather than the typical (4e2/h).
14. Why is gate dependence in graphene weaker for multiple layers?
a) Sample size of multiple layered graphene is greater than single layered
b) Screening of electric field by other layers
c) Absence of electric field in graphene multi-layers
d) Presence of defects in its structure
View Answer
Explanation: Gate dependence in single layer graphene is most pronounced. For samples of graphene with multiple layers, weaker gate dependence is exhibited. This is because of the screening effect of the electric field by other layers.
15. How can the problem of graphene transistor, plagued by a low on/off ratio, be solved?
a) Carve Graphene into narrow ribbons
b) Removal of band gap
c) Dequantization of momentum of the charge carriers
d) Removal of energy range in graphene in which electron states cannot exist
View Answer
Explanation: Graphene has no band gap and hence resistivity changes are very less. Therefore, a graphene transistor by its very nature is plagued by a low on/off ratio. This problem can be sorted out to some extent by carving graphene into narrow ribbons. By doing so the momentum of the charge carriers in the transverse direction are quantized which leads to the opening of a band gap.
More MCQs on Graphene:
- Graphene MCQ (Set 2)
- Graphene MCQ (Set 3)
- Graphene MCQ (Set 4)
- Graphene MCQ (Set 5)
- Graphene MCQ (Set 6)
Sanfoundry Global Education & Learning Series – Nanotechnology.
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