UNIT V – NANODEVICES AND QUANTUM COMPUTING
Introduction - Quantum confinement - Quantum structures (qualitative) - Band gap of
nanomaterial - Single Electron Transistor (SET): Tunnelling - Coulomb-blockade effect -
Carbon nanotubes: Properties and applications.
Quantum cellular automata (QCA) - Quantum system for information processing -
Characteristics and working of quantum computers - Advantages and disadvantages of
quantum computing over classical computing.
Nanotechnology is a broad term used when referring to any science or technology which
manipulates things in the atomic level, which is measured in nanometers. It is used in a number of
fields, such as biology, medicine, computers, materials, manufacturing, physics and several others.
Nanophase materials are materials with a grain size in the 1 to 100 nm range as shown in Fig5.7.
A nanometer is one-billionth of a meter - approximately 100,000 times smaller than the diameter
of a human hair. For example, the width of human hair is approximately 80,000 nanometers, and
human fingernails grow roughly one nanometer per second. A nanometer-sized particle is also
smaller than a living cell and can be seen only with the most powerful microscopes available today.
These materials have created a high interest in recent years by virtue of their unusual
Mechanical, electrical, optical and magnetic properties. Some examples are given below:
I. Nanophase ceramics are of particular interest because they are more ductile at elevated
temperatures as compared to the coarse-grained ceramics.
II. Nanostructured semiconductors are known to show various non-linear optical properties.
They are used as window layers in solar cells.
III. Nanosized metallic powders have been used for the production of gas tight materials and
porous coatings. Cold welding properties combined with the ductility make them suitable
for metal-metal bonding especially in the electronic industry.
IV. Single nanosized magnetic particles have special atomic structures with discrete electronic
states, which give rise to special properties in addition to the super paramagnetism
behaviour.
V. Nanostructured metal clusters have a special impact in catalytic applications.
Thus, nanophase materials exhibit greatly altered mechanical, optical, electrical, magnetic,
chemical and physical properties compared to their normal, large-grained counterparts with the
same chemical composition.
QUANTUM CONFINEMENT
The quantum confinement effect is observed when the size of the particle is too small to be
comparable to the wavelength of the electron.
,To understand this effect, we break the words like quantum and confinement. The word
confinement means to confine the motion of randomly moving electron to restrict its motion in
specific energy levels (discreteness). Quantum reflects the atomic dimensions of particles. So as
the size of a particle decrease till it reaches a nano scale the decrease in confining dimension makes
the energy levels discrete and this increases or widens up the band gap and ultimately the band gap
energy also increases. Due to this, there is significant change in the electronic and optical materials
of nano-dimensions as compared to the bulk materials.
QUANTUM STRUCTURES
When a bulk material is reduced in its size, atleast one of its dimensions, in the order of few
nanometers, then the structure is known as quantum structure. A quantum confined structure is
one in which the motion of the electrons or holes are confined in one or more directions by a
potential barrier.
Based on the confinement direction, a quantum confined structure will be classified into
three categories as quantum well, quantum wire and quantum dot.
Structure Quantum Confinement Number of dimensions confined
Bulk 0 3
Quantum well 1 2
Quantum wire 2 1
Quantum dot 3 0
Three-dimensional (3D) structure or bulk structure: No quantization of the
particle motion occurs i.e., the particle is free. Electron in conduction band and
holes in valence band are free to move in all three dimensions of space. E.g. cube
Two-dimensional (2D) structure or quantum well: Quantum confinement in
nanostructure-If one dimension is confined or reduced to the nanometre ranges
while other two dimensions remain large then we get a structure called quantum
well. Electrons confined in one direction. Electrons can easily move in 2 dimensions
E.g. nano wire s, nano rod, nanotube.
One-dimensional (1D) structure or quantum wire: If two dimensions are reduced
in to the nanometre range and remain large the structure to as a quantum wire. Eg.
nanoseed (Graphene). The semiconductor wires surrounded by a material with
large band gap. Surrounding material confines electron and hole in two dimensions
(carriers can only move in one dimensions) due its larger bandgap. Radius of quantum wires, nano
rods and nano tube, nano pillars (1D structures) 1-100 nm range (Typical nano scale dimension).
, Zero-dimensional (0D) structure or quantum dot: The extreme case of this process of size
reduction in which all three dimensions reach the low nanometer range is called quantum dot. Eg.
Nano dot. Electron confined in three dimensions Quantum dot: electron can easily
move in zero dimensions.
Electron and holes are confined in all the three dimensions of space by a
surrounding material with a larger band gap. Discrete energy levels (artificial
atoms) No quantum dots has a larger band gap like bulk semiconductor. Typical
dimensions: 1-10 nm.
SIZE DEPENDENCE OF FERMI ENERGY
In terms of distribution of energy, solids have thick energy bands, whereas atoms have thin,
discrete energy states.
π 8m 3⁄2
The electron density in a conductor at T = 0k is n= [ h2 ] EF 0 3⁄2 ------------(1)
3
h2 3n 2⁄3
Fermi energy of a conductor at T = 0K is EF 0 = [π] ------------(2)
8m
In the above equation ‘n’ is the only variable. The fermi energy depends on the number of free
N 2⁄3
electrons 'n' per unit volume 'V’. EF 0 ∝ (n)2⁄3 ∝ ( V)
Electron density is the property of the material, the fermi energy does not vary with
material's size. Femi energy is same for a particle of copper as it is for a brick of copper. Hence,
we can say that the energy states will have same range for small volume and large volume of
atoms. But for small volume of atoms,
we get larger spacing between the
energy states.
The average spacing between
the energy states is inversely
proportional to the volume of the solid.
1
∆E ∝ V
Thus, the spacing between
energy states is inversely proportional
to the volume of the solid.
BAND GAP ON NANOMATERIALS
❖ Material smaller – band gap larger
❖ Volume reduced from bulk to Nano size there is a distinct split in each sublevel.
❖ When atoms are closer sublevels split so bands get broaden.
Introduction - Quantum confinement - Quantum structures (qualitative) - Band gap of
nanomaterial - Single Electron Transistor (SET): Tunnelling - Coulomb-blockade effect -
Carbon nanotubes: Properties and applications.
Quantum cellular automata (QCA) - Quantum system for information processing -
Characteristics and working of quantum computers - Advantages and disadvantages of
quantum computing over classical computing.
Nanotechnology is a broad term used when referring to any science or technology which
manipulates things in the atomic level, which is measured in nanometers. It is used in a number of
fields, such as biology, medicine, computers, materials, manufacturing, physics and several others.
Nanophase materials are materials with a grain size in the 1 to 100 nm range as shown in Fig5.7.
A nanometer is one-billionth of a meter - approximately 100,000 times smaller than the diameter
of a human hair. For example, the width of human hair is approximately 80,000 nanometers, and
human fingernails grow roughly one nanometer per second. A nanometer-sized particle is also
smaller than a living cell and can be seen only with the most powerful microscopes available today.
These materials have created a high interest in recent years by virtue of their unusual
Mechanical, electrical, optical and magnetic properties. Some examples are given below:
I. Nanophase ceramics are of particular interest because they are more ductile at elevated
temperatures as compared to the coarse-grained ceramics.
II. Nanostructured semiconductors are known to show various non-linear optical properties.
They are used as window layers in solar cells.
III. Nanosized metallic powders have been used for the production of gas tight materials and
porous coatings. Cold welding properties combined with the ductility make them suitable
for metal-metal bonding especially in the electronic industry.
IV. Single nanosized magnetic particles have special atomic structures with discrete electronic
states, which give rise to special properties in addition to the super paramagnetism
behaviour.
V. Nanostructured metal clusters have a special impact in catalytic applications.
Thus, nanophase materials exhibit greatly altered mechanical, optical, electrical, magnetic,
chemical and physical properties compared to their normal, large-grained counterparts with the
same chemical composition.
QUANTUM CONFINEMENT
The quantum confinement effect is observed when the size of the particle is too small to be
comparable to the wavelength of the electron.
,To understand this effect, we break the words like quantum and confinement. The word
confinement means to confine the motion of randomly moving electron to restrict its motion in
specific energy levels (discreteness). Quantum reflects the atomic dimensions of particles. So as
the size of a particle decrease till it reaches a nano scale the decrease in confining dimension makes
the energy levels discrete and this increases or widens up the band gap and ultimately the band gap
energy also increases. Due to this, there is significant change in the electronic and optical materials
of nano-dimensions as compared to the bulk materials.
QUANTUM STRUCTURES
When a bulk material is reduced in its size, atleast one of its dimensions, in the order of few
nanometers, then the structure is known as quantum structure. A quantum confined structure is
one in which the motion of the electrons or holes are confined in one or more directions by a
potential barrier.
Based on the confinement direction, a quantum confined structure will be classified into
three categories as quantum well, quantum wire and quantum dot.
Structure Quantum Confinement Number of dimensions confined
Bulk 0 3
Quantum well 1 2
Quantum wire 2 1
Quantum dot 3 0
Three-dimensional (3D) structure or bulk structure: No quantization of the
particle motion occurs i.e., the particle is free. Electron in conduction band and
holes in valence band are free to move in all three dimensions of space. E.g. cube
Two-dimensional (2D) structure or quantum well: Quantum confinement in
nanostructure-If one dimension is confined or reduced to the nanometre ranges
while other two dimensions remain large then we get a structure called quantum
well. Electrons confined in one direction. Electrons can easily move in 2 dimensions
E.g. nano wire s, nano rod, nanotube.
One-dimensional (1D) structure or quantum wire: If two dimensions are reduced
in to the nanometre range and remain large the structure to as a quantum wire. Eg.
nanoseed (Graphene). The semiconductor wires surrounded by a material with
large band gap. Surrounding material confines electron and hole in two dimensions
(carriers can only move in one dimensions) due its larger bandgap. Radius of quantum wires, nano
rods and nano tube, nano pillars (1D structures) 1-100 nm range (Typical nano scale dimension).
, Zero-dimensional (0D) structure or quantum dot: The extreme case of this process of size
reduction in which all three dimensions reach the low nanometer range is called quantum dot. Eg.
Nano dot. Electron confined in three dimensions Quantum dot: electron can easily
move in zero dimensions.
Electron and holes are confined in all the three dimensions of space by a
surrounding material with a larger band gap. Discrete energy levels (artificial
atoms) No quantum dots has a larger band gap like bulk semiconductor. Typical
dimensions: 1-10 nm.
SIZE DEPENDENCE OF FERMI ENERGY
In terms of distribution of energy, solids have thick energy bands, whereas atoms have thin,
discrete energy states.
π 8m 3⁄2
The electron density in a conductor at T = 0k is n= [ h2 ] EF 0 3⁄2 ------------(1)
3
h2 3n 2⁄3
Fermi energy of a conductor at T = 0K is EF 0 = [π] ------------(2)
8m
In the above equation ‘n’ is the only variable. The fermi energy depends on the number of free
N 2⁄3
electrons 'n' per unit volume 'V’. EF 0 ∝ (n)2⁄3 ∝ ( V)
Electron density is the property of the material, the fermi energy does not vary with
material's size. Femi energy is same for a particle of copper as it is for a brick of copper. Hence,
we can say that the energy states will have same range for small volume and large volume of
atoms. But for small volume of atoms,
we get larger spacing between the
energy states.
The average spacing between
the energy states is inversely
proportional to the volume of the solid.
1
∆E ∝ V
Thus, the spacing between
energy states is inversely proportional
to the volume of the solid.
BAND GAP ON NANOMATERIALS
❖ Material smaller – band gap larger
❖ Volume reduced from bulk to Nano size there is a distinct split in each sublevel.
❖ When atoms are closer sublevels split so bands get broaden.
