The principles of physics, as far as I can see, do not speak against the possibility of maneuvering things atom by atom.” “Put the atoms down where the chemist says, and so you make the substance.”

1. Applications of Nanomaterials (Phys 7412)
PhD Program
BY
Dr.Fikadu Takele
Adama, Ethiopia
June, 2019
6/12/2022 1

2. Topics:
1. Semiconductor Nanowire FETs and SETs
2. Molecular SETs and Molecular Electronics
3. Heterostructure semiconductor lasers
4. Quantum dot lasers
5. Electrochromic Materials
• Optical effects and quantification of colour
• Electrochromic Systems: Electrochemistry, Kinetics and Mechanism
• Construction of Electrochromic Devices
• Electrochromic Systems (Inorganic Systems/ organic Systems)
• Applications of electrochromic devices
6. Photochromic materials
• Fundamentals of Photochromic materials
• Organic Photochromic Molecules
• Multi-addressable Photochromic Materials
• Photoswitchable Supramolecular Systems
• Photochromic Bulk Materials
• Industrial Applications and Perspectives of Photochromic
materials
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3. 1. Semiconductor Nanowire FETs and SETs
• Semiconducting nanowires can serve as FETs and SETs
channels.
• Semiconducting nanowires typically have diameters in the
range 10-100nm and controlled growth can result in nanowires
that are quite and straight, with few defects.
• Both p- and n- type nanowires can be fabricated, and a variety
of devices have been demonstrated.
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4. Cont.
a) Depicts a nanowire FET, where it can
be seen that the geometry is the same
as for carbon nanotube FETs, with
nanowire replacing the tube in forming
the channel.
b) For a 17.6nm diameter GaN
nanowire, the source-drain
current versus source-drain
c) The current versus gate voltage .
Results were obtained at room temperature,
and typical FET characteristics are evident
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5. Cont.
• Semiconducting nanowire FET
can be used to form SETs at very
low temperatures.
• Dashed curves show the current-
voltage characteristics for an n-
type InP nanowire FET at room
temperature. It is evident that the
channel is ohmic.
• The solid curves were measured at
0.35K. And clear Coulomb
blockade behavior is observed.
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6. Cont.
Fig: The low-temperature source-drain conductance G , where single-electron
behavior is obiously present.
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7. 2. Molecular SETs and Molecular Electronics
• Some element
connecting two
electrodes (the source
and drain), in the
vicinity of a gate
electrode that provides
some control.
Fig: depiction of a general electronic device consisting of an element connecting
source and drain, in the presence of a control gate.
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8. Cont.
• The object inside the box labeled „element‟ may be for example,
an n- or p-type silicon channel, as in an ordinary MOSFET.
• For a nanoelectronic devices „element‟ may be a double tunnel
junction.
• The amount of current flowing is controlled by the voltage at the
gate (and, of course, the drain to source voltage).
• For digital applications, the connection should be either „on‟ or
„off‟ and for analog applications, the current ISD should vary
considerably with the gate voltage.
• In addition to the preceding choices for „element‟ there has been
considerable interest in using molecules, or chains of molecules
(including DNA strand), to connect source and drain electrodes.
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9. Cont.
• Benzene molecule
connecting source
and drain in the
vicinity of a gate
electrode having
voltage VG.
FIG: Benzene molecule with sulfur atoms connecting source and drain, in the
vicinity of gate electrode having voltage.
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10. Cont.
• For molecular devices,
electron transfer can be
described by resonant or non-
resonant tunneling.
Fig: calculated current through the device depicted in the above
fig. as a function of the gate field Eg, in V/A (Vsd=10mV).
The benzene molecule is a molecular resonant tunneling
transistor.
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11. Molecular SET
a) Molecular structure
b) Family of d.c. I-V curves recorded for several
values of the gate voltage. Left most curve is -0.4V
and other curves are in increments of -0.15V (so
that the nearly straight line is for -1V. Insert is an
AFM image of the electrodes, and the scale bar is
100nm.
6/12/2022 11

12. Cont.
• There is currently a lot of interest in developing molecular electronic
devices.
• Advantages include an implicitly bottom up approach (self assembly
based on chemistry), and extremely small device sizes.
• However, there are significant scientific and technological challenges to
overcome.
• Complications include methods of connecting molecular devices to
electrode (the metal-molecule interface often significantly impacts
devices behavior), addressing such small devices, the effect of chemical
absorption on molecules electrical behavior (this can significantly
change device characteristics), high temperature operation.
• At the current time these issues largely remain to be solved.
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13. semiconductor lasers
3 processes for interaction
between a photon and an electron:
1. Optical absorption
2. Spontaneous emission
3. Stimulated emission
• Direct recombination occur
w/out change in electron
momentum
• Indirect semiconductor change
in electron momentum for
recombination.
• For laser direct band gap.
When ℎ𝑣 < 𝐸𝑔 Semiconductor
appear as transparent.
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LASER = Light Amplification by Stimulated
Emission of Radiation

14. Cont.
Position of Fermi level
If forward bias is applied:
• Depletion narrower
• The injected electrons and
holes will increase the
density of electrons in the
conduction band.
• The stimulated emission
rate will exceed the
absorption rate and
amplification will occur at
some value of current due
to holes in valence band.
As the current is further increased, at threshold value of the current,
the amplification will overcome the losses in the cavity and the laser
will begin to emit coherent radiation.
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15. 3. Heterostructure semiconductor lasers
• Semiconductors with different band-gaps: improved e/h
confinement.
Improved
waveguide
because the
semiconductors
have different
refractive index
A thin layer of a small
band gap
semiconductor is
sandwitched between
the two larger band
gap semiconductor
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Refractive
index
Photon
absortion

16. Double Hetero-structure (DH) laser diodes
• Improved photon confinement in the
GaAs active region due to the larger
index of refraction of GaAs (n = 3.6)
compared to the p- and n- cladding
layers (n = 3.4) light waveguide so that
light generated is confined to the
active region.
• Improved carrier confinement in the
GaAs active region due to the smaller
band gap (Eg ≈ 1.5 eV) of the GaAs
compared to the p- and n- cladding
layers (Eg ≈ 1.8 eV)
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17. 4. Quantum dot lasers
Quantum dots:
• Non-traditional semiconductor.
• Range from 2-10nm (10-50 atoms) in diameter.
• An electromagnetic radiation emitter with an easily tunable
band gap.
• Zero degrees of freedom.
6/12/2022 17

18. Cont.
• A quantum dot laser is a semiconductor laser that uses quantum
dots as an active laser medium in its light emitting region.
• Due to the tight confinement of charge carriers in quantum
dots, they exhibit an electronic structure similar to atoms.
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19. Basic characteristics of Quantum dot laser
• An ideal QDL consists of a 3D array of dots with equal size and
shape.
• Surrounded by a higher band gap materials which Confines the
injected carriers
• Embedded in an optical wavegiude consisting of lower and upper
cladding layers (n-doped and p-doped shield).
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20. QDL-Advantages
• Wavelength of light determined by the energy level not by bandgap
energy. Improved performance and increased flexibility to adjust the
wavelength
• Maximum material gain
• Low threshold current
• High output power
• Large modulation bandwidth
• Small dynamic chirp,
• Small linewidth enhancement factor
• Superior temperature stability
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21. Chromogenic systems
• Chromogenic systems change colour in response to electrical,
optical or thermal changes. These include electrochromic
materials, which change their colour or opacity on the
application of a voltage (e.g. liquid crystal displays),
thermochromic materials change in colour depending on their
temperature, and photochromic materials, which change colour
in response to light - for example, light sensitive sunglasses
that darken when exposed to bright sunlight.
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22. 5. Electrochromic Materials
• The absorption and emission spectra of certain dyes may be shifted by
hundreds of angstroms upon application of a strong electric field. This
effect is called “electrochromism”.
• 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.
• Electrochromism is the reversible and visible change in transmittance
and/or reflectance that is associated with an electrochemically induced
oxidation–reduction reaction.
• It results from the generation of different visible region electronic
absorption bands on switching between redox states.
6/12/2022 22

23. Cont.
• The color change is commonly between a transparent (“bleached”)
state and a colored state, or between two colored states.
• More than two redox states is polyelectrochromic.
• This optical change is effected by a small electric current at low dc
potentials of the order of a fraction of volts to a few volts.
• 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.
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24. Cont.
• After the resulting pulse of current has decayed and the color
change has been effected, the new redox state persists, with little
or no input of power, in the so called “memory effect”.
• The most important examples from major classes of
electrochromic materials namely transition metal oxides,
Prussian blue, phthalocyanines, viologens, fullerenes, dyes and
conducting polymers (including gels).
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25. Cont.
6/12/2022 25
Figure Generic five-layer
electrochromic device
design. Arrows indicate
movement of ions in an
applied electric field.

26. Applications of electrochromic devices
• Typically, ECD are of two types depending on the modes of device
operation, namely the transmission mode and reflectance mode.
• In the transmission mode, the conducting electrodes are transparent and
control the light intensity passing through them; this mode is used in
smart-window applications.
• In the reflectance mode, one of the transparent conducting electrodes
(TCE) is replaced with a reflective surface like aluminum, gold or
silver, which controls the reflective light intensity; this mode is useful
in rear-view mirrors of cars and EC display devices.
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27. The two modes of device operation
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Buildings (windows)
Aircraft (windows)
EC display (more reflection
mode)

28. EC windows
• Electrochromic windows, also known as smart windows, are a
new technological arrangement for achieving energy efficiency
in buildings, with variable transmittance of light and solar
energy.
• These „„smart windows‟‟ can automatically control the amount of
light and solar energy passing through the windows which
subsequently improves indoor comfort.
• The efficiency of these windows will vary depending on their
placement, size, and local climate conditions since these factors
influence the amount of sunlight that comes in contact with these
windows.
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29. Cont.
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30. EC mirror
• Electrochromic reflecting surfaces are employed as self
darkening mirrors that regulate reflections of flashing light from
following vehicles at night so that a driver can see them without
discomfort.
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31. EC display
• Electrochromic displays can operate in either reflecting or transmitting
mode.
• They are advantageous for their low cost and low power consumption.
• There are many applications where they can be used, for example, in
goggles and motorcycle helmet visors, which can be dynamically tinted
depending on the time of day, and in paper, to create an image upon
touching it with a stylus.
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32. 6. Photochromic materials
• Photochromism is the term used for a reversible photo-induced
transformation of a molecule between two isomers whose absorption
spectra are distinguishably different.
• B having at least one absorption band appearing at longer wavelength
than those of A.
• The activating radiation generally is in the UV region (300 to 400 mm)
but could be in the visible (400 to 700 nm).
• The most prevalent photochromic systems are established to be
unimolecular reactions (A →B)
• The back reaction (B → A) can occur predominantly by a thermal
mechanism.
• The back reactions (B → A) are predominantly photochemical.
6/12/2022 32

33. Cont.
During the reversible photoisomerization, some physical properties
of photochromic compounds, may be tuned by light.
 magnetic properties,
 coordination properties,
 dipole interaction,
 refractive index,
 dielectric constant and
 geometrical structure
 absorption spectra,
 fluorescence emission,
 conjugation,
 electron conductivity,
 electrochemical properties,
6/12/2022 33

34. Cont.
• This has enlightened people to apply this kind of compounds to
perform as photochemical molecular switches.
• Molecular switches act as switching units in various optoelectronic
devices and functional materials are addressed by stimulating it
with light, electricity or chemical reagents to specifically switch
the physical properties between two states.
• Photochromic materials are very fascinating in fields such as
molecular logic gates, data recording and storage, multi-photon
devices, surface/nanoparticle devices, photo-electronic devices,
electrochemical wiring, etc.
6/12/2022 34

35. Cont.
• Thus, further creation of optoelectronic and photo-optical
devices based on photochromic molecular switches which
operate at both molecular and supramolecular levels have
recently attracted many attentions.
• Apart from these, photo-switchable compounds also have
played an important role in sensing, self-assembly and photo-
controlled biological systems.
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36. Cont.
• Among diverse photochromic
compounds, dithienylethene derivatives
have been intensively investigated for
several decades from the fundamental
and practical points of view for their
numerous potential applications as opto-
electrical devices owing to their
excellent fatigue resistance and
thermally irreversible properties, high
cyclization and cycloreversion quantum
yields, rapid response as well as
reactivity in the solid state.
Figure 1 Photochromism of
Dithienylethene (a), Spiropyran
(b) and Azobenzene (c).
6/12/2022 36

37. Applications: PC materials
i. Sunglasses: One of the most famous reversible photochromic
applications is color changing lenses for sunglasses, as found in
eyeglasses.
ii. Supramolecular chemistry: Their ability to give a light-
controlled reversible shape change.
iii. Data storage
iv. Novelty items: toys, cosmetics, clothing and industrial
applications.
v. Solar energy storage: System, for possible application to
harvest solar energy and store it for significant amounts of
time. Although storage lifetimes are attractive, for a real device it
must of course be possible to trigger the back-reaction, which
calls for further iterations in the future.
6/12/2022 37

38. Industrial Applications and
Perspectives of Photochromic
materials
• Marketed photochromic ophthalmic lenses that darkened
reversibly in sunlight owing to silver halide crystals trapped
within the matrix of the glass.
• Photochromism thus forms the basis for what has become a
global multimillion-dollar business, and a deep understanding
of the phenomenon has been fundamental to the growth of the
industries reliant on it.
• Organic photochromic materials dominate.
6/12/2022 38

39. Cont.
• However, with the fast development of polymer industrials (e.g.,
plastics), organic photochromic compounds have found their
advantages in constructing commercial photochromic materials with
greater robustness, lightness, as well as lower cost, which is
essential for commercialization.
• Therefore, 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.
• Scientific research is keen on the thermostable P‐type
photochromism, in industry, T‐type photochromism is the one that
dominates.
6/12/2022 39

40. Cont.
Figure T-type photochromes: azobenzene, spiropyran,
spirooxazine, and naphthopyran (from top to bottom).
Figure P-type photochromes:
fulgide and diarylethene (from top to
bottom).
T-type refers to those that could
undergo thermally decoloration.
P-type photochromism is thermally
irreversible, that is, all coloration
and decoloration processes are
driven only by light.
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41. Thank you!
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