2 Marks Question & Answers

Part - A '2' Marks Q & A

1. Define nano materials.

Nanophase materials are newly developed materials with grain particle size at the nanometre range (10-9m), i.e., in the order of 1 - 100 nm.

2. Define density of states.

It is defined as the number of available electron states per unit volume in an energy interval E and E + dE. It is denoted by Z (E).

3. Define Fermi energy.

It is defined as the highest energy level occupied by the electron at OK in metal.

4. What is a quantum confinement?

It is a process of reduction of the size of the solid such that the energy levels inside become discrete.

5. What is quantum structure?

When a bulk material is reduced in its size, atleast one of its dimension, in the order of few nanometres, then the structure is known as quantum structure.

6. Define Zener-Bloch oscillation.

It denotes the oscillation of a particle (e.g. an electron) confined in a periodic potential when a constant force is acting on it.

7. What is resonant tunneling?

The transmission probability of the double symmetric barrier is maximum and hence the tunneling current reaches peak value when energy of electron wave is equal to quantised energy state of the well.

This phenomenon is known as resonance tunneling.

8. What is single electron phenomena?

Present day, transistors require 10,000 electrons. Rather than moving many electrons through transistors, it may very well be practical and necessary to move electrons one at a time. This is known as single electron phenomena.

9. Define Coulomb-Blockade effect.

The charging effect which blocks the injection or rejection of a single charge into or from a quantum dot is called Coulomb blockade effect.

10. What is the condition for Coulomb-Blockade effect?

If two or more charges near one another, they exert coulomb forces upon each other. If two charges are the same kind, the force is repulsive. Therefore, the condition for observing coulomb blockade effects is expressed as

where C - capacitance of the quantum dot

T - temperature of the system.

WC - Charging energy and this is the energy needed to add one negatively charged electron to the dot.

11. What is single electron tunneling?

The quantization of charge can dominate and tunneling of single electrons across leaky capacitors carries the current. This is called single electron tunneling.

12. What is a Single Electron Transistor?

SET is three-terminal switching devices which can transfer electrons from source to drain one by one

13. What are the advantages of single electron transistor?

• The fast information transfer speed between cells (almost near light speed) is carried out via electrostatic interactions only.

• No wire is needed between arrays. The size of each cell can be as small as 2.5 nm. This made them very suitable for high density memory.

• This can be used for the next generation quantum computer.

14. What are the limitations of single electron transistor?

• In order to operate SET circuit at room temperature, the size of the quantum dot should be smaller than 10 nm. 

• It is very hard to fabricate by traditional optical lithography and semiconductor process.

• The methods must be developed for connecting the individual structures into logic circuits and these circuits must be arranged into larger 2D patterns.

15. What are the applications of single electron Transistor?

• A variety of digital logic functions, including AND or NOR gates, is obtained based on SET operating at room temperature.

• It is used for mass data storage.

• It is used in highly sensitive electrometer.

• SET can be used as a temperature probe, particularly in the range of very low temperatures.

• SET is a suitable measurement set-up for single electron spectroscopy.

• It is used for the fabrication of a homo-dyn receiver operating at frequencies between 10 and 300 MH.

16. What is mesoscopic structure?

The structures which have a size between the macroscopic world and the microscopic or atomic one are called mesoscopic structure.

17. What is conductance fluctuation?

Conductance (reciprocal of resistance) fluctuations in quantum physics is a phenomenon exhibited in electrical transport experiments in mesoscopic system.

18. What is quantum interference effect?

It states that much like waves in classical physics, any two (or more) quantum states can be added together ("superposed") and the result will be another valid quantum state.

19. What are the applications of quantum interference effect?

Quantum interference effect is being applied in a growing number of applications, such as the

• Superconducting Quantum Interference Device (SQUID).

• quantum cryptography

• quantum computing and quantum interference transistor.

20. What are magnetic semiconductors?

The semiconducting materials which exhibit both ferromagnetism and useful semiconductor properties are known as magnetic semiconductors.

21. Give examples for dilute magnetic semiconductor.

Oxide semiconductors:

• Zinc oxide

• Manganese-doped zinc oxide

• n-type cobalt-doped zinc oxide

Magnesium oxide:

• p-type transparent MgO films with cation vacancies

Titanium dioxide:

• Cobalt-doped titanium dioxide

• Iron-doped titanium dioxide

• Chromium-doped titanium dioxide

• Copper-doped titanium dioxide

• Nickel-doped titanium dioxide

22. What is spintronics?

The 'spin' of the electron can be used rather than its charge to create a remarkable new generation of 'spintronic' devices. These are smaller, more versatile and more robust than those currently making up silicon chips and circuit elements.

23. What is a carbon nano tube?

The carbon nanotubes are the wires of pure carbon like rolled sheets of graphite or like soda straws.

24. What are the types of carbon nano tube structure?

Three types of nanotube structures are considered by rolling a graphite sheet with different orientations about the axis.

They are

(i) Armchair structure

(ii) Zig-zag structure

(iii) Chiral structure

25. How carbon nanotubes are classified Based on the number of layers, the carbon nanotubes are classified as

(i) Single-walled (SWNTs)

(ii) Multi-walled (MWNTs).

In multi-walled nanotubes, more than one CNTs are coaxially arranged.

26. Mention any two properties of carbon nano tubes.

• Carbon nanotubes are metallic or semiconducting depending on the diameter and chirality (ie., how the tubes are rolled).

• The energy gap also varies along the tube axis and reaches a minimum value at the tube ends.

• The strength of the carbon-carbon bond is very high therefore any structure based on aligned carbon-carbon bonds will ultimately have high strength.

• One of the important properties of nanotubes is their ability to withstand extreme strain.

• Nanotubes have a high strength-to-weight ratio.

• Nanotubes have a high thermal conductivity and the value increases with decrease in diameter.

27. Write down any two applications of carbon nano tube.

• The unusual properties of carbon nano tubes have many applications such as battery electrodes, electronic devices and reinforcing fibers for stronger composites etc.,

• Carbon nanotubes can be used to make a computer switching device.

• Carbon nanotubes have many applications in battery technology. Lithium which is a charge carrier in some batteries, can be stored inside nanotube.

• Carbon nano tube can be used for storing the hydrogen which is used in the development of fuel cells.

• Nanotubes can be used to increase the tensile strength of steel.

• A plastic composite of carbon nanotubes provides light weight shielding material for electromagnetic radiation.

• Nanotubes act as catalysts for some chemical reactions.