Quantum wells,Quantum wires and Quantum

dots

Presented By_ To_

Sanjeeb Limbu(Reg.No:14305019) Dr.A.Subramania Sir

M.Tech.-Nanoscience & Technology Associate Professor(CNST)

Introduction

When the size or dimension of a material is continuously reduced from a large or macroscopic size, such as meter or centimetre to a very small size, the properties remain the same at first then small change begin to occur, until finally when the size drops below 100 nm. In this way formed a new structure of the material is called nanostructure. The wells, wires and dots are nothing but nanostructure of the material. The nanostructure are explain through the quantum mechanics so is called quantum wells, quantum wires and quantum dots. Its not a classical wells, wires and dots. Its also called low dimensional structure.

Classification of law dimensional materials

1.Three dimensional (3D) structure or bulk structure: No

quantisation 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. Eg.cube

2.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. Eg.nano wires,nano rod,nanotube .

Electrons confined in one direction

Quantum wells (Thin films) : Electrons

can easily move in 2 dimensions

(One dimensional quantisation)

kx ky

nz

3.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)

Electron confined in 2 dimensions

Quantum wires: Electron can freely

easily move in 1 dimension. (2 dimensional quantisation)

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)

kx

nz

ny

4.Zero-dimensional (0D) structure or quantum dot: The

extreme case of this process of size reduction in which all three dimensions reach

the low nanometre range is called a quantum dot. Eg. Nano dot

Electron confined in three dimensions

Quantum dot: electron can easily moves

in zero dimensions.

(3 dimensional quantisation)

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

ny

nz

nx

Comparison of Nanostructure:

Progressive generation of rectangular nanostructures.

Progressive generation of curvilinear nanostructures.

Density of states: The density of states of a system describes the

number of states per interval of energy of each energy level that are available

to be occupied by electrons. Density of states moving from the bulk(3D) crystal

to a quantum well(2D) ,there is a further change in the density of on moving

quantum wires(1D) and quantum dots(0D), dE

dk

dk

dN

dE

dNDoS

Structure Degree of

Confinement

Bulk Material 0D

Quantum Well 1D 1

Quantum Wire 2D

Quantum Dot 3D d(E)

dE

dN

E

E1/

Quantum wells are formed in semiconductors by having a material,

like gallium arsenide sandwiched between two layers of a material with a

wider band gap, like aluminium arsenide. (Other example: layer of indium

gallium nitride sandwiched between two layers of gallium nitride ) These

structures can be grown by molecular beam epitaxy or chemical vapor

deposition with control of the layer thickness down to monolayer's.

How to prepare a quantum well ???

Application of quantum wells:

In optical devices such as laser diodes .Formed by semiconductor materials quartz and silica(Absorb/Emit light)efficiently).

They are also used to make HEMTs (High Electron Mobility Transistors), which are used in low-noise electronics.(Heterostructure FET)

Quantum well infrared photo detectors are also based on quantum wells, and are used for infrared imaging (Absorb photons)

How to prepare a quantum wire ???

How to prepare a quantum wire ???

a) The most important application of nanowires in nanoelectronics is using them as junctions or as multi-segment nanowires or crossed nanodevices.

b) Quantum wire is used in energy storage devices

How to prepare a quantum dot ???

Application of quantum dots:

Photovoltaic devices: solar cells Biology : biosensors, imaging Light emitting diodes: LEDs Quantum computation Flat-panel displays Memory elements Photo detectors Lasers

References:

Introduction to Nanotechnology,Chgarles P.Poole,Jr

Quantum wells, Quantum wires and Quantum dots ,Paul Harrison

Solid-State Electronics 44 (2000) 2207-2212

JACS Communications

PHYSICAL REVIEW LETTERS

Quantum Wells, Quantum Wires, Quantum Dots, Quantum Limit of Conductance, Quantum Capacitance & Quantum HALL Effect,R. John Bosco Balaguru ,B. G. Jeyaprakash