Nano Devices: Two Marks Important Questions and Answers

Applied Physics CSIE II

UNIT III: Nano Devices

 

IMPORTANT PART A QUESTIONS & ANSWERS

 

1. What is the difference between the band gap of a material to the nanomaterial?

In ordinary material, the band gap will be smaller. For nanomaterial, the band gap will be greater.

 

2. In quantum dot, how will be the nature of the material and band gap.

In the case of quantum dots, the smaller the particle the bigger the band gap.

 

3. What is meant by Ballistic transport?

When the mean free path of the electron is longer than the dimension of the medium through which the electron travels is called Ballistic transport.

For Ballistic transport condition L<< L_m.

 

4. Define quantum conductance.

The quantum conductance or conductance quantum 'Go' is the quantized unit of electrical conductance.

It is defined as G₀ = 2e² / h = 7.748×10^‒5 mho

 

5. Define resistance quantum.

The reciprocal of the conductance quantum is called resistance quantum (R₀).

R₀ = 1 / G₀ = h / 2e² = 12.9 kohm

 

6. Define quantum dot laser.

A quantum dot laser is a semiconductor laser that uses quantumn dots as the active medium in its light emitting region.

 

7. What are the advantages and disadvantages of quantum dot laser

Advantages:

(i) Broad spectrum with a specific wavelength of light emission can be obtained by changing dot size.

(ii) Because of very small active volume, only very less population inversion is necessary for lasing.

Disadvantages:

(i) It is very difficult to form high quality dots [Uniform size and Higher density]

(ii) Difficult to manufacture because of nanometer size.

 

8. What are the applications of quantum dot laser?

Applications

(i) QD lasers are used in medicine [optical coherence tomography].

(ii) QD lasers are used in display technologies, Spectroscopy and telecommunications. QD lasers are used in optical transmission system and optical LANS.

 

9. What is meant by bulk material?

The bulk material is a collection of atoms having property that are from individual atoms.

 

10. Define density of energy states?

Density of energy is defined as the number of available energy states per unit volume, per unit energy in a solid.

 

11. Write the equation for an electron density in a conductor at T = OK.

The electron density in a conductor at T = OK is

n_e = π/3 (8m/h²)^3/2 . E_F(0)^3/2

 

12. Whether fermi energy vary on material's size? If yes or no, justify your statement.

No, since electron density is the property of the material, the Fermi energy does not vary with materials size. Fermi energy is the same for a particle of copper as it is for a brick of copper.

 

13. What will happen to the band gap when the volume is reduced from that of a solid to a nano material?

The band gap gets bigger as the material gets smaller.

If the volume is reduced from that of a solid to that of a nano material the band gap will widen.

 

14. What is meant by Tunnelling ?

The phenomenon in which a particle, like an electron, encounters an energy barrier in an electronic structure and suddenly penetrates is known as tunnelling.

 

15. What is meant by quantum confinement?.

The effect achieved by reducing the volume of a solid so that the energy levels within it becomes discrete is called quantum confinement.

 

16. What we will observe when we decrease the size of the particle to nano size?

It we decrease the size of the particle to nano size, the decrease in confining size creates the energy levels discrete. The formation of discrete energy levels increases or widens up the band gap and finally the band gap energy also increases.

 

17. What is meant by quantum confined structure?

A quantum confined structure is one which the motion of the electron or holes are confined in one or more directions by potential barriers.

 

18. Define the term quantum well, quantum wire and quantum dot.

An electrically isolated region, like a thin film, where electrons are constrained in one dimension and exhibiting quantum behaviour is called quantum well.

An electrically isolated region, like a nano tube or nano scale wire, where electrons constrained in two dimensions and exhibiting quantum behaviour is called quantum wire.

An electrically isolated region, such as a particle or a portion of a bulk semi conductor, where electrons are constrained in all three dimensions, creating an artificial atom that exhibits quantum behaviour is called quantum dot.

 

19. Write any two applications of quantum well, quantum wire and quantum dot.

Quantum well

1. Quantum wells are now widely used to make semiconductor layers and other important devices.

2. Quantum well infrared photodetectors are also based on quantum wells, and are used for infrared imaging.

Quantum wire

1. Quantum wires can be used for transistors.

2. A quantum wire application is nano bar codes which is used in medical field. Nano bar codes are made different quantum wires of different metals that have different reflectivity.

Quantum dots

1. Quantom dot may be used as a basic building block in making a quantum computer.

2. The quantum dot applications in various fields include blue‒laser diodes, single electron transistor, light‒emitting devices, etc

 

20. How the density of states is proportional to the energy in one dimension, two dimension and three dimension.

(i) In one dimension, density of states is proportional E^‒1/2.

(ii) In two dimension, density of states is proportional to energy E.

(iii) In three dimension, density of states is proportional to E^1/2.

 

21. Define coulomb blockade.

The resistance to electron transport caused by electrostatic coulomb forces in certain electronic structures, including quantum dots and single electron transistors is called coulomb blockade.

 

22. What is the purpose of coulomb blockade?

Coulomb blockade helps to prevent constant tunnelling to and from a quantum dot.

 

23. How coulomb blockade prevent unwanted tunnelling?

The coulomb blockade can prevent unwanted tunnelling, when energy is much higher than the thermal energy of an electron. The condition for the coulomb blockade is therefore Ec >> K_BT

 

24. What is meant by single electron transistor?

A transistor made from a quantum dot that controls the current from source to drain one electron at a time is called single electron transistor.

 

25. What are the main criteria for the single electron phenometer to occur?

For single electron phenomena to occur, we have to keep the single electron or quantum dot in isolation.

 

26. Explain the rules which used for the single electron phenomena to occur.

There are two rules:

Rule 1

The energy needed to add one electron to the dot, or charging energy, Ec, must be significantly higher than the thermal energy of an electron

i.e., Ec= e² / 2C_dot >> K_BT

Rule 2

The uncertainty of the charging energy must be less than the charging energy itself. This is accomplished if

R₁ >> h/e²

R_t = tunnelling resistance.

By following the above two rules, it becomes possible to manipulate electrons one at a time and single electron phenomena will occur.

In the single electron transistor, these two rules are used.

 

27. What is meant by Carbon nano‒tubes?

Carbon nanotubes (CNT) are molecular‒scale tubes of graphitic carbon with outstanding properties. They are among the stiffest and strongest fibres researched till date, with remarkable electronic properties and applications.

 

28. What are the types of CNT?

Types

Carbon nano‒tubes are of two types. viz

(i) (a) Single walled nano‒tubes (SWNTs)

(b) Multi walled nano‒tubes (MWNTs)

(ii) Single walled nano‒horns (SWNHs).

 

29. List out of the properties of Carbon Nano‒Tubes.

(i) CNTs have High Electrical Conductivity.

(ii) CNTs have Very High Tensile Strength.

(iii) CNTs are Highly Flexible (can be bent considerably without damage).

(iv) CNTs are Very Elastic (18% elongation to failure).

(v) CNTs have High Thermal Conductivity.

(vi) CNTs have a Low Thermal Expansion Coefficient.

(vii) CNTs are Good Electron Field Emitters.

(viii) CNTs Aspect Ratio.

 

30. Enumerate the applications of CNTs in various fields.

(i) The carbon nanotubes are very light in weight, but they are very strong, hence they are used in aerospace.

(ii) They are used in constructing nanoscale electronic devices.

(iii) Carbon nano‒tubes (CNTs) are used in battery electrodes, fuel cells, reinforcing fibers etc.

(iv) CNTs are used in the development of flat panel displays for computer monitors and televisions.

(v) Plastic Composite CNTs are used as a light weight shielding materials for protecting electromagnetic radiation.

(vi) Light weight CNTs are also used in military and communication systems, for protecting computers and electronic devices.

(vii) Semiconducting CNTs are used as switching devices.

(viii) Semiconducting CNTs are also used as chemical sensors to detect various gases.

(ix) Nano‒tubes can also serve as catalysts for some chemical reactions.

(x) The unique properties of carbon nanotubes will undoubtedly lead to many more applications in future to produce nano‒computers, plastic composites etc.