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1. Liquid crystals2. Conducting polymers3. Molecular conductors, superconductors4. Molecular electronics5. Nanomaterials
More detailed presentations on Conducting Polymers and Nanomaterials are also available on the website
Liquid crystals
Discovery:
• 1888 – Friedrich Reinitzer (Institute of Plant physiology, University of Prague)
working on cholesteryl benzoate
solid cloudy liquid clear liquid
• contacted Otto Lehmann (a German physicist)
recognized the ‘cloudy liquid’ as a new state
called it ‘liquid crystal’ (1904)
145.5oC 178.5oC
Director, n
Nematic
n
Smectic A
n
Smectic C
n n n n n
Chiral nematic
Types of liquid crystals
R CN RCN
R
CH2CH2 R
F
R
Nematic discotic
Hexagonal columnar discoticS. Chandrasekhar & coworkersBangalore
RR
R
RR
R
N
N
N
M
NN
N
N
N
OROR
ORORRO
RO
RORO
R = -(CH2)nCH3, -O(CH2)nCH3, -CH2O(CH2)nCH3 etc..
M = metal ion
Anisotropic properties
Dielectric anisoptropy, ||
Birefringence, oe nnn n refractive index e extraordinary [electric vector parallel to optic axis]o ordinary [electric vector normal to optic axis]
dielectric permittivity
Polarizability anisoptropy, ||
polarizability
P1
P2
E1
E2
"OFF" State
Light
Twisted nematic effect: Displays
Liquid crystal molecule
Plane of polarisation
P1
P2
E1
E2
X"ON" State
Light
Courtsey: http://en.wikipedia.org/wiki/File:LCD_layers.svgP1
P2E2
E1LC
Reflector
N R
CH3O N
N R
CH3O
O
N R
NC
Evolution of molecular design for LC
Chemical instability Strong colour, Negative
Colour
Conducting polymers
Natural polymers
Phenol-formaldehyde(Bakelite)
Synthetic polymers
Polyethylene Polytetrafluoroethylene(Teflon)
Polyhexamethylene adipamide(Nylon 6,6)
PolycarbonatePolyethyleneterephthalate(PET)
Discovery of conducting polymers
1862 Lethby (College of London Hospital) Oxidation of aniline in sulfuric acid
1970’s Shirakawa (Japan)
Ti(OBu)4 & Et3Al Toluene–78oC
copper-coloured film cis-polyacetylene
CH CHAcetylene gas
Ti(OBu)4 & Et3Al Hexadecane150oC
silvery filmtrans-polyacetylene
Polyacetylene (PA)
n
n
Electrical conductivity ()
cis PA 10-10 – 10-9 S cm-1 trans PA 10-5 – 10-4 S cm-1
For comparison : (copper) ~ 106 S cm-1
: (teflon) ~ 10-15 S cm-1
Doping leads to enhanced conductivity
n
n
n
+-
+ e- - e-
~ 10-5 S cm-1Semiconductor
~ 104 S cm-1
Metal
Discoverers - Nobel Prize 2000
A. Heeger, A. McDiarmid, H. Shirakawa(this photograph taken at the International Conference on
Synthetic Metals, 2000, was kindly provided by Prof. Heeger)
Polyacetylene - electronic structure
(a) (b) (c) (d) (e)
(a) ethylene(b) allyl radical(c) butadiene
-electronic energy levels and electron occupation
(d) regular trans-PA
(e) dimerised trans-PA
Examples of conducting polymers
S n Polythiophene
(PT)
n Polyparaphenylene
(PPP)
n
Polyparaphenylenevinylene (PPV)
n
O
O
N n Polypyrrole
(PPy)
O O
S n
Polyethylenedioxythiophene
(PEDOT)
Alkoxy-substitutedpolyparaphenylene
vinylene(MEH-PPV)
N
H
N N N
H
n Polyaniline
(PANI)
Electrical conductivities
10+6
10+4
10+2
100
10-2
10-4
10-6
10-8
10-10
10-12
10-14
10-16
10-18 S cm-1
CopperPlatinumBismuthGraphite
Germanium
Silicon
Polyethylene
Diamond
Quartz
ConductingPolymers
Applications of conducting polymers
Polyaniline (PANI) Transparent conducting electrodes Electromagnetic shieldCorrosion inhibitor‘Smart windows’ (electrochromism)
Polypyrrole (Ppy) Radar-invisible screen coating (microwave absorption)Sensor (active layer)
Polythiophene (PT) Field-effect transistorAnti-static coating Hole injecting electrode in OLED
Polyphenylenevinylene (PPV)Active layer in OLED
Molecular conductors, superconductors
S
S
S
S CN
CN
NC
NC
Figure 3
view perpendicular to the stack axis
TTF TCNQ
view normal to the molecular planes of TTF and TCNQ
in plane view ofTTF TCNQ
TTF-TCNQ
= 105 S cm (58 K)
Organic superconductors
S
Se
Se
Se
Se
CH3
CH3H3C
H3CTMTSF
S
S
S
S
S
SS
S
BEDT-TTF
(TMTSF)2XX = ClO4
- TC = 1.2 K (6.5 kbar) = PF6
- TC = 1.4 K
(ET)2XX = Cu(NCS)2
- TC = 11.4 K
S
S
S
S
S
S
S
S
S
S
S
S
+ +.
7 e- 7 e- 6 e-6 e-7 e-7 e-
- e-
TTF TTF+.
.
NC CN
CNNC -
NC CN
CNNC
.NC CN
CNNC
-
+ e-
TCNQ TCNQ-.
.
Oxidation of donor / Reduction of acceptor
Ea
molecule
unpaired e-
Ea = 0
Partial ionicity
/a/2a
Energy
Wave vector
Metal
0
Dimerisation
/a/2a
Energy
Wave vector
Semiconductor
0
Peierl’s instability
Organic donor molecules
S
S
S
S
S
S
S
S
S
SS
S
Se
Se
Se
Se
CH3
CH3H3C
H3C
BEDT-TTF
TMTSFTTF
N
N
R
R
R2P
N
N
R
RP+
+
Perylene
N
N
CH3H3C
H3C CH3
N
N
N
N
CH3
CH3
CH3
CH3
H3C
CH3
H3C
CH3
TMPD
TDAE
Organic acceptor molecules
NC
NC
CN
CN
TCNE
O
O
CN
CNCl
Cl
O
O
Cl
ClCl
Cl
DDQChloranil
CNNC
NC CN
N
N
CN
NC
DCNQITCNQ
Molecular electronics
N+C18H33
NC CN
CN
-
LB film of molecule XSilver electrode
Glass substrate
Electrode consisting of magnesium, silver pad &GaIn drop with gold wire
X
Figure 4
Z-type LB film
N+C18H33
NC CN
CN
-
LB film of molecule XSilver electrode
Glass substrate
Electrode consisting of magnesium, silver pad &GaIn drop with gold wire
X
Figure 4
Z-type LB film
N+C18H33
NC CN
CN
-
LB film of molecule XSilver electrode
Glass substrate
Electrode consisting of magnesium, silver pad &GaIn drop with gold wire
X
Figure 4
Z-type LB film
Molecular Rectifier
A C
B
STM piezoelectric tipmetal surface
C60 molecule
Vin
VoutA
B
CRL
RP
VP
Figure 6
X
A C
B
STM piezoelectric tipmetal surface
C60 molecule
Vin
VoutA
B
CRL
RP
VP
Figure 6
X
Vin, Vout : input and output voltage, VP : bias voltageRP : polarisation resistance, RL : load resistance
X : capacitor to isolate external circuit from bias voltage
Molecular Amplifier
20 mV 100 mV
Nanomaterials
Concept of Molecules Metal nanoparticles
Parallels with molecules
Nanoscale Size matters !
Unique effects
Nanomaterials
CuSO4.5H2O K2Cr2O7
NiCl2.6H2O
Chemical Composition
Structure
CarbonGraphite Diamond
Fullerene (C60)
Properties of materials depend upon :
Chemical composition
Structure
SizeSilicon
millimeter
micrometer
nanometer
Chemical compositionStructure Identical
Silicon
8
2 cmSurface area = 6 x 22 = 24 cm2
Surface area of 1 cube = 6 cm2 Surface area of 8 cubes = 48 cm2
1 cm
1021
1 nmTotal surface area = 6 x 1021 nm2 = 6 x 107 cm2
= 6000 m2 = 1.5 acre
STEM image of a single layer of graphite - graphene
Scale bar = 2 nm
DNA
2.5 nm
Thickness = 2.5 nm
AFM image of a monolayer of surfactants
Atomic Force Microscope
Top-down
Bottom-up
Dujardin, G., Mayne, A., Robert, O., Rose, F., Joachim, C., and Tang, H. Science 1998, 251, 1206.
Sequential extraction of adsorbed atoms -one by one - from Germanium surface
‘finely divided metallic state’ of gold (M. Faraday, Philos. Trans. R. Soc.London, 1857, 147, 145)
AuCl3 AuPCS2
1791 - 1867
Michael Faraday
Dramatic change in Colour
Plasmon Resonance Absorption
Quantum dots, nanoparticles of semiconductors, of different sizes, illuminated by a single light source,
emit intense fluorescence of different colours(Felice Frankel, MIT)
Same chemical compositionbut colour changes with size !
Increasing particle size
Rat vasculature injected with water solution of Quantum Dots (CdSe-ZnS)Excitation at 780 nm2-photon fluorescence at 550 nmLarson et al, Science 2003, 300, 1434
Fluorescence imaging in medical diagnostics
Using conventional fluorescent dyes
Computing, data storage and communication Materials Manufacturing industry Health & medicine Energy & environment Transportation & space exploration
Nanotechnology and Industry