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

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