Application of nanomaterials,assignment one by kunsa

Application of Nanomaterials [Phys8311]
Assignment ONe
Kunsa Haho
Email:kunsahaho@gmail.com
Adama Science and Techinology University
Faculity of Natural Sciences
Departement of Applied Physics
Kunsa Haho Email:kunsahaho@gmail.com Assignment one

Outline of Presentation
1 Semiconductor Nanowire FETs and SETs
2 Molecular SETs and Molecular Electronics
3 Heterostructure semiconductor lasers
4 Quantum dot lasers
5 Electrochromic Materials
6 Photochromic materials

Objective of Presentation
To present my understanding of above described concepts of
nanomaterials application areas up on reading diffrent lituretures
for a period of month.This presentation focuses on
1 physical principles
2 chemical principles
3 application areas
4 suitable materials required

Introduction
Nanomaterials are defined as the materials with at least one of its
dimension is in the range of nanometers usually 1nm -100nm.
Nanomaterials may be
Zero-dimensional (e.g., nanoparticles) −→ Quantum dots
One-dimensional (e.g., nanorods or nanotubes) −→ Quantum
wires
Two-dimensional (usually thin films) −→ Quantum wells
Nanomateirals are important in the technological applications
because of their size,they exhibit a unique physical, mechanical and
chemical properties than both atomic species and bulk materials.

Continued ...
Physical properties of nanomaterials depend on phase, color,
thickness, boiling with melting points, Morphology and
radioactivity
Mechanical properties of nanomaterials relate to electrical,
optical, magnetic, and thermal parameters
Chemical properties of nanomaterials are determined by their
chemical reactivity, oxidation, surface modification ability, and
catalytic effects.

1.Semiconductor Nanowire FETs and SETs
I.Semiconductor Nanowire FETs
Semiconductor nanowires are cylindrical semiconducting
crystals having a diameter of the order of 10- 100 nm.
semiconductor nanowires can be rationally and predictably
synthesized in single crystal forms with all key parameters
controlled, including chemical composition, diameter and
length, and doping and electronic properties.
They can be applied in the areas such as biosensers,laser
source, photodetectors,field effect transistors,single photon
source,light emitting diodes and photovoltaics.

continued...
A nanowire can constitute the channel for electrical
conduction in field-effect transistors.For example: silcon
nanowire FETS,ZnO nanowire FETs,Pbs Nanowire FETs and
GaN nanowires FETs are widely used as FET
In some litrutures semiconductor nanowire-FET configurations
can be classified as follows:
i.Planar FETs driven by a back-gate:The nanowire channel
is contacted by two lithographically defined metal leads, which
operate as source and drain contact. The wire and the metal
contacts are isolated from the substrate by a dielectric layer of
suitable composition and thickness.A conductive substrate can
be therefore used as a back gate through whose potential the
conductivity of the channel is varied.

continued...
F.g.Planar back-gate configuration and SEM image of a ZnO
back-gate transistor

continued...
ii.Planar FETs with back and top gates:
In this configuration the top gate voltage can be used to steer the
conductivity of the channel, while the back gate is used for
defining the conductivity type below the drain and source leads.In
order to realize these structures, a dielectric layer must be defined
around the nanowire,which can be obtained by controlled oxidation
(e.g. in Si) or by lithographic deposition.

continued...
F.g.Planar configuration with both back and front gates, and SEM
image of a Si NW surrounded by a SiO2 shell. The Ni leads
provide the source and drain contact, while the Al lead is the
driving gate.The substrate provides an additional back-gate

continued...
iii.Vertical FETs with surrounding gate:In this case, the
nanowire/growth substrate interface defines the source contact.
After a suitable lithographic and etching process, the lower part of
the nanowire remains recovered by a dielectric layer and a metal
contact, which acts as a gate.A metal contact, supported by a
further dielectric layer, is realized on the top of the nanowire and
acts as a drain.

continued...
F.g.Left: vertical surrounding-gate configuration, middle: scheme
of regular array vertical NW FETs based on InGaAs NWs;middle:
scheme of regular array vertical NW FETs based on InGaAs
NWs;right: SEM image of an individual Si NW FET at the
fabrication stage before the deposition of the top contact

continued...
II.Semiconductor Nanowire SETs
The single-electron tunneling (SET) transistor is a device that
has two tunnel junctions in series and a gate capacitor on
which charge can be controlled by a gate voltage source.
A semiconductor nanowire SET is a nanoscale MOSFET
where the channel is separated from the drain by thin tunnel
barriers.
The dimensions of the channel region must be small in order
to have a small intrinsic capacitance, such that the injection
of an electron in the channel can raise the potential of the
channel region by a significant (measurable) voltage. This
voltage needs to be larger than the thermal
energy,∆V > kB T
q , to observe single-electron transport

continued...
F.g.Nanowire MOSFET with constrictions between the source and
the channel and between the channel and the drain; the gate
electrode around the channel is not shown for clarity.

2.Molecular SETs and Molecular Electronics
Molecular electronics
Molecular electronics (it is also known as moletronics) is
defined as the field of science that investigates the electronic
and thermal transport properties of circuits in which individual
molecules (or an assembly of them) are used as basic building
blocks.
It is an interdisciplinary field that includes physics, chemistry,
materials science, and engineering.

Fig.Molecular electronics: An interdisciplinary field

continued...
Molecular electronics is one class of the nano electronics
Fig.Rough classification of nanoelectronics

Advantages of molecular technology
Size:The reduce size of small molecules (between 1 and 10
nm) could lead to a higher packing density of devices with the
subsequent advantages in cost, efficiency, and power
dissipation.
Speed: good molecular wires could reduce the transit time of
typical transistors (1014s), reducing so the time needed for an
operation.
Assembly and recognition:One can exploit specific
intermolecular interactions to form structures by nanoscale
self-assembly. Molecular recognition can be used to modify
electronic behavior, providing both switching and sensing
capabilities on the single-molecule scale

New functionalities: Special properties of molecules, like
the existence of distinct stable geometric structures or
isomers, could lead to new electronic functions that are not
possible to implement in conventional solid state devices.
Synthetic tailorability: By choice of composition and
geometry, one can extensively vary a molecules transport,
binding, optical, and structural properties.

continued...
Some Suitable molecules for molecular devices:
hydrocarbons have been extensively selected as appropriate
molecules
the most widely used molecular families for molecular devices
are oligo phenylene ethynylene (OPE), oligo phenylene
vinylene (OPV), and oligo thiophenes (OT).
OPE derivatives are conjugated molecules with a rod-like
shape , which can be used as molecular wires up to around 5
nm in length
Alkanethiols and arenethiols are extensively used in molecular
studies in which they are positioned between gold (Au)
electrodes
Another important molecule for the manufacture of molecular-
based components is deoxyribonucleic acid (DNA).

continued...
II.Molecular SETs:
When molecules are incorporated between metal electrodes,
charge transfer can occur from one side to the other. The
transfer of electrons produces current flow in the opposite
direction.
The left and right leads have a continuous band structure,
whereas the band structures are dis- continuous for the central
cluster between the leads.
Since there is a difference in the chemical potentials of the
left and right leads, electrons are transferred across the
molecular junction in order to maintain equilibrium,

continued...
f.g.Charge transfer occurs across a molecular junction to maintain
equilibrium

continued...
When the tunneling resistance is lower than the quantum
resistance, h
e2 = 26kΩ , and the energy required by an electron
to charge a junction with capacitance C, where E = e2
2C ; is
greater than the thermal energy, kBT; the single-electron
tunneling effect occurs.
The prototype benzene-1,4-dithiolate molecule was one of the
first proposals of a molecular FET.A possible setup of such
device is shown in Fig in next slide.

continued...
Fig. Three terminal setup for a molecular device made with a
benzene-1,4-dithiolate molecule

continued...
F.g.Molecular SET a)molecule structure of and
[Co(tpy−CH2)5 − SH2]2+ and [Co(tpy−SH2]2+ molecules b)Cyclic
voltammogram of [Co(tpy−SH2]2+c) I-V curve recorded for several
value of gate voltage.Left most curve is -0.4v others are an
increment of -0.15v for ( [Co(tpy−CH2)5 − SH2]2+),the upper set

3.Heterostructure semiconductor lasers
General Remarks on laser
A laser is the acronym of light amplification by stimulated
emission of radiation, is an optical quantum generator, i.e., it
is a source of coherent electromagnetic radiation.
The operation of any type of laser is based on two important
quantum phenomena: (1) the existence of states with
population inversion and (2) stimulated emission.
Laser has three basic components
Laser medium:medium where stimulated emission takes place
pump source :provides energy to the laser medium to achieve
population inversion
optical resonator:two mirror(fully and partially reflective) to
surround laser medium

Summary on semiconductor lasers:
F.g.Survey of semiconductor lasers

F.g.development of semiconductor lasers

Heterostructure semiconductor lasers
Two dissimilar semiconductors with different band gaps joined
to form what is known as a heterojunction.Heterostructures
are formed from multiple heterojunctions.
In the case of heterojunction lasers, the active layer and either
one or both of the adjacent layers are of different material.
If only one of the adjacent layers is of a different material, it is
called a simple heterojunction;
if both are different, it is called a double heterojunction.
In heterostructure semiconductor lasers, a layer of low
band gap material(and with higher refractive index) is
sandwiched between layers of a high band gap material.

The simplest heterostructure laser is the
double-heterostructure laser.
A double-heterostructure semiconductor laser is consists of
different layers forming an n GaAlAs/GaAs/p GaAlAs
heterostructure grown on a highly doped GaAs substrate
(n = 2 × 1024m−3)
The active medium is undoped GaAs layer (thickness 0.2 − 1µ
m) embedded in n-doped GaAlAs and p-doped GaAlAs. Under
the action of a voltage (U ), electrons migrate from the
n-doped side and holes from the p-doped side into the the
active region (the region where free electrons and holes exist
simultaneously). This results in nonequilibrium populations of
electrons in the conduction band and of holes in the valence
band. Stimulated recombination of radiative electron- hole
pairs leads to laser radiation.

continued...
F.g.Double-heterostructure laser

The GaAs layer, with a larger refractive index than the
neighboring GaAlAs material, acts as a light guide.
Light is also reflected from the heterojunction, which helps to
confine the emitted photons to the active region.
Fg.Schematic diagram of a double-heterostructure where the active
medium (hatched area) consists of GaAs,(a), and InGaAsP, (b).

4.Quantum dot lasers
A quantum dot is a mesoscopic device that confines electrons
in a small volume in all three dimensions.Three physical
dimensions are comparable to the De Broglie wavelength.
As a result of the strong confinement imposed in all three
spatial dimensions, quantum dots are similar to atoms.
They are frequently referred to as artificial atoms,
superatoms, or quantum dot atoms.
One of the most promising applications of semiconductor
quantum dots is for diode lasers with many advantages over
bulk lasers.
A quantum dot laser is a semiconductor laser which uses a
quantum dot as an active medium.

working principle of quantum dot laser
F.g.construction of quantum dot lasers

In quantum dots, the energy levels of electron as well as of
holes are discrete.The energy levels of an electron are
En1n2n3
c =
π2~2
8me

n2
1
s1
+
n2
2
s2
+
n2
3
s3

where,n1, n2, n3 = 1, 2, 3... and si is the side length of the ith
side of the rectangular shape dot(i = 1, 2, 3)
The lowest conduction band energy level is,( for s1, s2, s3 = s),
equal to
E111
c =
3π2~2
2mes2
The next higher level is
E211
c = 1.7E111
c

continued...
The lowest energy of a radiative electron-hole pair ( for
s1, s2, s3 = s)is the gap energy of the zero-dimensional system
(as shown in the right side of fig above),
E0D
g = Eg +
3π2~2
2mr s2
where mr reduced mass
Example: GaAs quantum dot with
s1, s2, s3 = 20nm; E111
c ≈ 45meV ; E211
c ≈ 90meV ; E111
v ≈
7meV ; E211
v ≈ 14meV ; E0D
g ≈ Eg + 52meV

continued...
F.g Quantum dot laser

In the quantum dot laser (Fig.above), electrons migrate from
an n-doped region via an undoped region into the dots. Holes
migrate from a p-doped region via the undoped region into the
dots. Stimulated recombination of radiative electron-hole pairs
within the quantum dots is the source of laser radiation.
quantum dot laser can be prepared in a two ways
i.As an edge-emitting laser or
ii.As a surface-emitting laser

Unique characteristics of quantum dot lasers
The ideal quantum dot lasers would exhibit higher and
narrower gain spectrum, low threshold currents, better
stability with temperature, lower diffusion of arriers to the
device surfaces, and a narrower emission line than double
heterostructure or quantum well lasers.
Optical properties of quantum dots such as the discrete
density of states and tailored energies of optical transitions are
definitely advantageous for laser applications.
Another peculiar advantage that is inherent in
quasi-zero-dimensional structures only is that the
temperature-independent threshold current.

continued...
Comparison of quantum dot(box) lase gain with other
semiconductor laser
Fig.Plot of calculated gain coefficient vs emission wave length

continued...
Threshold current is the minimum forward-biased injection
current needed to achieve sustained laser action.
In the majority of cases the threshold current is given by
Ith(T) = Ioe
T
To
where the Io is value corresponds to T = 0C0 and T0 is the
characteristic temperature.The lower the value ofT0, the more
sensitive is the laser to changes in temperature.
T0 is larger for lower dimensionalities. T0 = ∞ for a
quantum-dot laser.
This is because in a quantum dot, the thermally induced
population of the higher states is inhibited.

continued...
Advantages of using QDL
Broad spectrum with the specific wave length light emission
can be obtained by changing dot size.
It operates even at high frequencies
It is an efficient laser
Gain is 2 to 3 times more than Quantum well lasers
Dis-dvantages of using QDL
It is very difficult to fabricate high quality quantum dots
(uniform size and density)
it is a serous problem in achieving high fill factor and high
optical confinement
it is also challenging in charge injection from contacts i.e
carrier thermalization problems.

continued...
Applications of quantum dot lasers
As the wavelength is insensitive to temperature fluctuations,
quantum dot lasers are well-suited for use in optical data
communication and optical networks.
In medicine, e.g. for laser scalpels and optical coherence
tomography
They are used in optical trasmision systems and optical LANs
They used for display technologies (laser projectors, laser
television) and so on

5.Electrochromic Materials
Introduction
Oxidation and reduction of certain substance upon application
of an electrical bias can lead to distinct photo-optical and
colour changes. This phenomenon is known as
electrochromism.
Electrochromism is the reversible and visible change in
transmittance and/or reflectance that is associated with an
electrochemically induced oxidationreduction reaction.
An electrochromic material is the one that changes color in
a persistent but reversible manner by an electrochemical
reaction and the phenomenon is called electrochromism.
Electrochromic (EC) materials generally exhibit colour
changes between two coloured states or between a coloured
state and a bleached state.

continued...
The materials that change colour on passing a charge are
called electrochromes,and these can be classified into three
groups.
Type-I[Solution Electrochromes]:the colouring species
remain in solution at all times during electrochromic usage
(e.g methyl viologen).This type is used in car, anti-dazzle,
rear-view mirrors.
Type-II[Solution-Solid Electrochromes]:the reactants are in
solution but the coloured product forms a solid on the surface
of the electrode following electron transfer(e.g heptyl).This
type is used in a larger mirrors for commercial vehicles.
Type-III[Solid Electrochromes]:encompasses those where all
the materials are solids at all times,( e.g.Prussian Blue and
tungsten trioxide).This group is used in smart mirrors.

i.Optical effects and quantification of colour
Amount of colour formed: extrinsic colour
The coloured form of the electrochrome is produced by
electrochemical reaction(s) at the electrode and its reverse.
At the electrode,each redox center of the electroactive species
can accept or donate electrons from or to an external metal
connection, one center being formed per n electrons, where n
is usually one or two according to the balanced redox reaction,
Oxidisedform, O + electron(s) −→ Reducedform, R
The number of colour centres formed by the electrode
reaction, and hence the change in absorbance ∆(Abs), is in
direct proportion to the electrochemical charge passed Q,
following Faradays first law

continued...
Abs = lc
where is the extinction coefficient or molar absorptivity,c is the
concentration of the coloured species and l the spectroscopic path
length in the sample;l could be the thickness of a thin solid film of
electrochrome, or the thickness of a liquid layer containing a
dissolved chromophore.

continued...
In the case of electrochemically generated colour, ∆(Abs) is
the change in the optical absorbance is related to the change
in the concentration of chromophore generated by the
electrochemical charge passed:
∆(Abs) = l∆c
The optical absorption of an electrochrome is related to the
inserted charge per unit area Q (the charge density) by an
expression akin to the BeerLambert law , since Q is
proportional to the number of colour centres formed
∆(Abs) = log
I0
I

= ηQ

continued...
Here the proportionality factor η, the coloration efficiency, is a
quantitative measure of the electrochemically formed colour.
For an electrochromic device in transmission mode, η is
measured as the change in optical absorbance ∆(Abs) evoked
by the electrochemical charge density Q passed
η =
∆(Abs)
Q

continued...
Colour analysis of electrochromes
Currently,a new method of colour analysis used , in situ
colorimetric analysis.
This method allows the quantitative colour description of
electrochromes, as perceived by the human eye, in terms of
hue, saturation and luminance.
Colour is described by three attributes.
i.The hue,dominant wavelength or chromatic colour is the
wavelength where maximum contrast occurs in the spectral
sequence.

continued...
ii.The saturation, chroma, tone, intensity or purity:the relative
levels of white and/or black.
iii.The value, lightness or luminanceis the brightness of the
colour.
Using the three attributes of hue, saturation and luminance,
any colour can be both described and actually quantified.

ii.Electrochromic Systems: Electrochemistry, Kinetics and
Mechanism
Electrochemistry
I.Equilbrium Electrochemistry (zero-current)
An electrochemical cell comprises at least two electrodes:
each made up of two different ’charge states’ (more properly,
oxidation or redox states) of a particular chemical.
If the potential applied to the electrode in contact with both
redox states is different from this equilibrium potential then
one of two ’redox’ reactions can occur: electron
gain(reduction,as in equation below)
O + ne−
→ R
or electron loss (oxidation, the reverse of above equation)

continued...
O and R are called a redox couple and the potential of the
electrode in contact and in equilibrium with the two redox
states is the electrode potential, E0.
The electrode potential for E0,R couple is related to the ratio
of their respective concentrations by a form of the Nerest
equation.
E0,R = E0
0,R +
RT
nF
ln

[O]
[R]

where concentrations are denoted by square brackets, R is the
gas constant, F the Faraday constant, T the thermodynamic
temperature , n is the number of electrons involved in the
electron transfer reaction and E0
0,R standard electrode
potential.

F.g.Primitive cell comprising Cu2+, Cu andZn2+, Zn half cells for
equilibrium electrochemical measurement.

continued...
In fig above the spontaneous reaction is
Cu2+
+ Zn → Cu + Zn2+
Anode(Oxidation rxn)
Zn → Zn2+
+ 2e−
and cathode(Reduction rxn)
Cu2+
+ 2e−
→ Cu

continued...
Electrochromic Operation
The electron transfer process during coloration is denoted by
the terms anodic or cathodic:Cathodically colouring materials
form colour when reduced at an electrode made negative, a
cathode, and Anodically colouring electrochromes are
coloured at an anode, or positive electrode.
Electrochromic operation involves the following two conditions
1. The number of moles of species formed at an electrode
during electrode reaction is proportional to the charge passed;
2. A given charge liberates (or deposits) masses of different
species in the ratio of their ’equivalent weights’ (relative molar
masses divided by the number of electrons involved in the
electrode reaction).

continued...
Dynamic electrochemistry
Dynamic electrochemistry is used, in which current is
passed in a controlled way, by applying a potential E to a
particular electrode that is different from the steady value.
Voltammetry is the most common of these dynamic
techniques and here it requires three
electrodes(woking,reference and counter electrodes)

continued...
f.g.Schematic representation of a three-electrode cell for
voltammetric use. Here the voltmeter V is in practice part of the
control circuitry of the potentiostat, as is implied by the dotted line

continued...
The Kinetics of Electron Transfer
The rate of electron transfer at an electrode is a function of
the gradient of electric potential applied to the electrode, and
follows the Butler-Volmer equation which, for a reduction
reaction O + ne− R is
i = nFAkf cR(exp(−αf n∅η)) − nFAkbc0(exp(−αbn∅η))
where, for brevity,∅ = F
RT ; co is the concentration of the oxidised
form and CR that of the reduced form;A is the area of the
electrode, α is a fraction termed the transfer coefficient (f and b
for forward and back reaction respectively),η is the overpotential,
(E − Eoc), where E is the potential applied to the electrode and
Eoc is the zero-current electrode potential and k is rate constants
of electron transfer.

continued
The Rate of Mass Transport
Before the electron transfer reaction can occur, of necessity
material must move from the solution bulk and approach close
to an electrode.
This movement is ’mass transport’, and proceeds via three
separate mechanisms: migration, convection and diffusion

continued...
Mass transport is formally defined as the flux ji of electroactive
species i to an electrode, as defined in the Nerest-Planck equation
ji =migration+convection+ diffusion
= µi ci
∂φ
∂x

+ ci ¯
νi − Di
∂ci
∂x

where µi is the ionic mobility of the species i;φ is the strength of
the electric field, ¯
νi is the (vectorial) velocity of solution (where
applicable), and Di and ci are respectively diffusion coefficient and
concentration of species i.

continued...
Migration is the movement of ions through solution or solid in
response to an electric field, a (positive) anode attracting any
negatively charged anions, the cathode attracting the cations
Duffussion: Diffusive behaviour obeys Ficks laws , the first
being for the flux
ji = −Di
∂ci
∂x

where∂ci
∂x is concentration gradient
Ficks second law describes the time dependence of diffusion
∂ci
∂t
= Di
∂2ci
∂x2


continued...
A useful approximate solution to Ficks second law gives
l ≈
√
Dt
where Di is the diffusion coefficient of species i, and t is the
time required for species i to move a distance l.
Another indicator of the rate of ionic movement is the ionic
mobility µ(velocity v divided by the driving field), which is
related to the diffusion coefficient D by the Nerest- Einstein
equation.
D
µ
=
kBT
ze
The Cottrell equation describes this current/time response as
i = nAF
r
D
πt

iii.Construction of Electrochromic Devices
An electrochromic device is essentially a rechargeable
battery in which the electrochromic electrode is separated by
a suitable solid or liquid electrolyte from a charge balancing
counter electrode, and the color changes occur by charging
and discharging the electrochemical cell with applied potential
of a few volts.
All electrochromic devices are electrochemical cells, so each
contains a minimum of two electrodes separated by an
ion-containing electrolyte.
Electrochromic operation of the ECD is effected by an external
power supply, either by manipulation of current or potential.

continued...
Thus there are two ways of coloration in ECD
a.Galvanostatic coloration uses two electrodes and
impossing a constant current.
b.potentiostatic coloration applying constant potential and
requires three electrodes
There are two modes used in the construction of an
electrochromic devices.
i.Reflected intensity mode ECD
ii.Transmitted intensity mode ECD

continued...
Fig.Schematic diagram showing the different modes of ECD
operation: (a) reflectance mode and (b) transmittance mode

continued...
For all-solid cells with reflective mode may be assembled according
to the schematic diagram below
Fig.Schematic diagram of an electrochiromic mirror operating in
reflectance mode (with the reflector as counter
electrode)(OTE:optically transparent electrode)

continued...
Transmissive cells are assembled according to the schematic
diagram
Fig.Schematic diagram of an all-solid ECD operating in
transmittance mode.

iv.Electrochromic Systems (Inorganic Systems/ organic Sys-
tems)
A wide range of both inorganic and organic chemicals exhibit
electrochromism.
Inorganic Systems
Metal oxides: includes cerium, chromium, cobalt, copper,
iridium, iron, manganese, molybdenum, nickel, nio- bium,
palladium, praseodymium, rhodium, ruthenium, tantalum,
titanium, tungsten and vanadium.
Table:selected metal oxide electrochromes

continued...
Mixed metal oxides includes,Cobalt oxide
mixtures,Molybdenum Trioxide Mixtures,Nickel oxide
mixtures,Tungsten Trioxide Mixtures,Vanadium Oxide
Mixture,Miscellaneous Metal Oxide Mixtures(e.g.cerium oxide
with either titanium dioxide or zirconium dioxide),Ternary
Oxide Mixtures and Metal Oxide-Organic Mixtures.

Organic systems
Bipyridilium Species:formed by the diquaternising of
4,4’-bipyridyl to form l,l’-disubstituted-4,4’-bipyridilium salts
Prussian Blue and Metal Hexacyano metallates
Metal Phthalocyanines
Viologens (4,4 0 -bipyridylium salts)
Polymeric Electrochromes such as polyaniline,the polypyrroles
and polythiophenes

v.Applications of electrochromic devices
The following are the some of currently confirmed areas of
application of electrochromic systems.
elements for information display
light shutters
smart windows:glazing to operate dynamically and change in
response to environmental conditions
variable-reflectance minors:Electrically switchable
automatic-dimming rear-view mirrors for cars and trucks
variable-emittance thermal radiators.

6.Photochromic materials
F.g.Classification of chromic phenomena

i.Fundamentals of Photochromic materials
Chromic materials are materials which exhibit a reversible colour
change in response to an external stimulus such as temperature
(thermochromism) and light (photochromism), chemochromism
(chemicals);electrochromism (electrical current or potential); and
ionochromism (ionic species)and so on as shown in fig above.
Photochromic is from the Greek words ”photos” and ”chroma”
meaning light and color, respectively.
Thus photochromism is the property to undergo a light-induced
reversible change of color based on a chemical reaction.

continued...
Photochromic materials are a family of compounds which
can undergo reversible photo-switches between two different
states or isomers with remarkably different properties.
The source of the colour changes is the variation in absorption
spectra of the materials across the UVvisible near- infrared
(NIR) region.
The photochromism phenomenon was first recorded on a
potassium salt of dinitromethane.
photochromic materials are very promising and potential in
several scientific research fields, ranging from chemistry,
physics and materials science to biology and nanotechnology.

continued...
In order to describe photochromism, the most common model
introduced is a simple two-way reaction between two
molecular species A and B.
A and B are separated by a potential barrier (E). If this barrier
is low, B is metastable and can revert back spontaneously to
A.Such systems are called T-type referring to the thermally
induced reaction from B to A.
On the contrary, a high barrier features a bistable system. In
this case, only photons are able to cause the reaction, and
such systems are called P-type.

continued...
F.g Photochromism: a two-way light-induced reaction between two
molecules A and B. (a) Potential energy diagram and (b) the
related schematic absorption spectra

ii.Organic Photochromic Molecules
The families of organic photochromic molecules that are
categorized by the type of chemical reaction involved during the
photochromic process,are discussed here below
Proton tranfer: e.g.salicylideneanilines (anils)
Heterolytic Cleavage:In this type of system, a covalent bond
is broken in the excited molecule,and new conformations are
created with dipolar structures e.g.
Homolytic Cleavage:In this system, with the absorption of
radiation, a bond is broken, and the dis sociation of the
molecule takes place to give two or more parts, which contain
azygous electrons e.g HABI and its derivatives

continued...
Trans-Cis Isomeration:Thermal energy can cause a change from
the trans state to the cis state, in mos cases around a central
unsaturated bond e.g. stilbene and azobenzene
Cyclization Reaction:the reaction involves six electrons
delocalized over six different atoms.This family includes fulgides,
and diarylethenes and some related families such as spirooxazine,
chromene, and fulgimides.

iii.Multi-addressable Photochromic Materials
photochromic systems with two states between open and
closed forms are highly desirable for construction of logic
elements, in which each isomer can represent 0 and 1 of a
digital binary code.
Among all the molecular devices, multi-addressable materials
for computing systems, such as logic gates, data storage
systems and field-effect transistors, aroused a wide spread
interest in scientific research.
Regarding this,multi-addressable photochromic diarylethenes
are getting much attention owing to the thermal irreversibility
and excellent fatigue resistance
Thermal stability is very critical for information storage, and
fatigue resistance allows molecular logic gates to proceed
multiple times and to be reset conventionally.

iv.Photoswitchable Supramolecular Systems
The spontaneous self-assembly of small molecules into
complex superstructures is becoming a powerful tool toward
the development of new materials and devices.
Self-assembly systems constructed by noncovalent
interactions, such as hydrogen bonding, -stacking, metal
ligand, and van der Waals interactions, have a tunable
functionality and are much more versatile than those of
polymer materials because of their dynamic nature.
Photochromic systems in which each isomer of the
photochromic compound can represent 0 or 1 of a digital code
have garnered much interest for their potential applications in
optical memory and photonic switching devices.

continued...
The recent progress on photoswitchable supramolecular
systems containing an amphiphilic system, a hostguest
system, and metal complexes, in addition to their application
as molecular devices, sensors, and bioimages
Photoreversible Amphiphilic Systems:
Amphiphilic compound contain hydrophobic and hydrophilic
moieties, possessing good solubility in organic solvents and
water and
it may self-assemble into supramolecular architectures whose
nanostructures/micro- structures can be tuned by light
irradiation.

continued...
Photoreversible Diarylethene-Based Amphiphilic System:
They have noticeably irreversible thermal photochromic
behavior, high photoisomerization quantum yields, and
outstanding fatigue resistance.
They have great potential application in high-density data
storage systems as a molecular switch with nondestructive
readout capability and as molecular devices
They are water-soluble, fluorescent switchable system and can
be used as a switchable mark for imaging living cells.

continued...
Photoreversible Azobenzene-Based Amphiphilic System:
Azobenzene is a widely used chromophore for the preparation
of light-responsive polymers because it can undergo transcis
photoisomerization in response to UV and visible light
It is the most used photochromic group in the self-assembly
process due to its large change in the structure and dipole
triggered by light.
it widely applied in hostguest systems, molecular devices, and
delivery systems
Generally, photoswitchable supramolecular systems play a vital
role in advancing supramolecular chemistry and stimuli-responsive
self-assembly research.

v.Photochromic Bulk Materials
i.Photochromic Polymers: when photochromic
chromophores are incorporated into polymer backbones or side
groups, photoirradiation brings about changes in various
properties of polymer solutions and solids. These polymers
having the chromophores are named photochromic
polymers.
ii.Single-Crystalline Photoswitches:Examples of
crystalline-state photochromic molecules are paracyclophanes ,
triarylimidazole dimer , diphenylmaleronitrile , aziridine ,
2-(2,4-dinitrobenzyl)pyridine , N-salicylideneaniline, and
triazene.
iii.Photochromic Liquid Crystals: Liquid crystals (LCs) are
fluid phases with direction dependent (i.e., anisotropic)
physical properties. They usually flow like ordinary liquids

continued...
Thermodynamically, LCs are situated between the crystalline solid
state and the liquid state.For this reason, LCs are considered as an
intermediate state or mesophase of matter also as the fourth state
of matter after solid, liquid, and gaseous states of matter.
F.g.Molecular organization in crystal, LC, and liquid states of
matter.

continued..
LCs can be classified in two categories: thermotropic LCs and
lyotropic LCs.
Thermotropic LCs can be obtained by the variations of
temperature or pressure, whereas
lyotropic LCs are fabricated by dissolving amphiphilic
materials in suitable solvents where self-assembled structures
behave as the building blocks of LC phases.
There are different classes of photochromic LCs.
Spiropyran- and Spirooxazine-Based Photochromic
Liquid Crystals:it has excellent photofatigue resistance,strong
photocoloration, and fast thermal relaxation.

continued...
Dithienylcyclopentenes- Based Photochromic Liquid
Crystals: are considered to be the most promising
photochromic materials for optical memory and
photoswitching applications because of their excellent fatigue
resistance and superior thermal stability of both
photoisomers,fast photocyclization, and electrical conductivity.
Azobenzene-Based Photochromic Liquid Crystals:are
widely studied and a well-known family of photochromic
compounds that undergoes transcis isomerization upon UV
irradiation

continued...
iv.Photochromic Gels:The photoresponsive gels, which can
change their properties upon light irradiation, is therefore currently
a focus of attention. Several photoresponsive gels have been pro-
posed so far with photoswitching units such as azobenzene,
spiropyran, and spirooxazine, diarylethene, salicylideneaniline,
imidazole, and stilbene

vi.Industrial Applications and Perspectives of Photochromic
materials
Organic photochromic materials have become one of the
booming fine chemical industry
There are generally two types of organic photochromic
materials: T-type and P-type.
T-type photochromism refers to those that could undergo
thermally decoloration, such as azobenzenes, spiropyrans,
spirooxazines, naphthopyrans, and so on
P-type photochromism is thermally irreversible, that is, all
coloration and decoloration processes are driven only by light.
Compounds such as dithienylethenes, fulgides, belong to
P-type photochromes.
scientific research is mostly keen on the thermostable P-type
photochromism, in industry, T-type photochromism (mostly

continued...
Commercialized T-type Photochromic Materials:
Conversion from scientific results to commercial products has
to overcome formidable technical and commercial challenges
photochromic performances and criteria to be satisfied for
commercialization are:
1 Quick coloration rate
2 Appropriate half-life of decoloration for desired applications
3 High interconversion efficiency
4 Good fatigue
5 Thermostability
6 Good solubility and malleability
Generally, polymers with flexible chains and low glass transition
temperatures (Tg ) are the best candidate in photochromic

continued...
T-type photochromic materials can be applied in the following
areascommercial T-type photochromic ophthalmic lenses have
accounted for the greatest volume usage in photochromic
industry.Photochromic surface coatings, including
photochromic inks , nail varnish , security printings [10], have
become a more popular commercial reality for applications in
esthetics, security, and authenticationsIn photochromic
plastics,Products such as drink bottles, hair clips, key fobs,
and mobile phone accessories are from photochromic PVCs
(Polyvinyl Chloride)photochromic dyes as colorants in
textilesIn a variety of fields, such as smart windows and
agricultural films, personal care and cosmetics

continued...
Commercialized P-Type Photochromic:
Major P-type photochromic materials include diarylethenes
and fulgides, with a typical cyclohexadiene ring open and close
motif.
As thermoirreversible photochromic species, P-type
photochromic materials are long time considered as potential
all-optical circuitry for next-generation computing,
switches/logic gates, memory technology, and ultra-density
data storage devices

7.References
Deborah Kane Adam Micolich Peter Roger,Nanomaterials
Science and Applications,CRC Press (2016)
Zhong Lin Wang,Handbook of Nanophase and Nanostr
Volume II:characterization
He Tian, Junji Zhang,Photochromic Materials: Preparation,
Properties and Applications-Wiley-VCH (2016)
Junji Zhang , Qi Zou , and He Tian, Photochromic Materials:
More Than Meets The Eye
Semiconductor Nanowires by Lorenzo Rigutti
Michal Vik Aravin Prince Periyasamy, Materials:Fundamentals
, Measurements ,and Applications,apple cademic press(2019)
P. M. S. Monk,R. J. Mortimer and D. R. Rosseinsky,
Electrocromism and Electrochromic decices (2007)

continued...
Peter Bamfield and Michael G. Hutchings, Chromic
Phenomena Technological Applications of Colour Chemistry
Second Edition-RSC (2010)
Dae Mann Kim Yoon-Ha Jeong,Nanowire Field Effect
Transistors: Principles and Applications springer-Verlag New
York(2014)
Zhong Lin Wang, and Nanobelts Materials, Properties and
Devices Volume 1: Metal and Semiconductor
Nanowires,Springer (2003)
Guozhen Shen Yu-Lun Chueh Nanowire Electronics,Springer
(2019)
Jean-Pierre Colinge TSMC James C. Greer,Nanowire,
Transistors Physics of Devices and Materials in One
Dimension,Cambridge University Press (2016)

continued...
Paven Thomas Mathew , Fengzhou Fang ,Advances in
Molecular Electronics: A Brief Review,ELSEVIER,Engineering
4 (2018) 760771
Hye-Mi , Jinhee Kim , Wan Soo Yun , Jong Wan Park ,
Ju-Jin Kim , Do-Jae Won , Yongku Kang ,Changjin Lee,
Molecule-based single electron transistor,ELSEVIER,Physica E
18 (2003) 243 244
George W.Hanson,Fundamentals of Nanoelectronics
michael Zwolak and Massimiliano Di Ventra, Molecular
Electronics,Virginia Polytechnic Institute and State University,
Blacksburg, Virginia 24061-0435(2003)
Karl F. Renk: Basics of Laser Physics:For Students of Science
and Engineering,Springer-Verlag Berlin Heidelberg 2012

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