Liquid crystals Conducting polymers Molecular conductors, superconductors Molecular electronics Nanomaterials More detailed presentations on Conducting Polymers and Nanomaterials are also available on the website

Liquid crystals

(Institute of Plant physiology, University of Prague) 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

Types of liquid crystals Director, n Nematic n Smectic A n Smectic C n Chiral nematic

Hexagonal columnar discotic Nematic discotic Hexagonal columnar discotic S. Chandrasekhar & coworkers Bangalore

Anisotropic properties Dielectric anisoptropy,   dielectric permittivity Birefringence, n  refractive index e  extraordinary [electric vector parallel to optic axis] o  ordinary [electric vector normal to optic axis] Polarizability anisoptropy,   polarizability

Twisted nematic effect: Displays

Courtsey: http://en.wikipedia.org/wiki/File:LCD_layers.svg P1 P2 E2 E1 LC Reflector

Evolution of molecular design for LC Strong colour, Negative  Chemical instability Strong colour, Negative  Colour

Conducting polymers

Natural polymers

Synthetic polymers Polytetrafluoroethylene Polyethylene (Teflon) Phenol-formaldehyde (Bakelite) Polyethylene Polytetrafluoroethylene (Teflon) Polyhexamethylene adipamide (Nylon 6,6) Polyethyleneterephthalate (PET) Polycarbonate

Discovery of conducting polymers 1862 Lethby (College of London Hospital) Oxidation of aniline in sulfuric acid 1970’s Shirakawa (Japan) Acetylene gas Ti(OBu)4 & Et3Al Toluene –78oC Ti(OBu)4 & Et3Al Hexadecane 150oC silvery film trans-polyacetylene copper-coloured film cis-polyacetylene

Polyacetylene (PA) 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 s ~ 10-5 S cm-1 Semiconductor + e- - e- s ~ 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 -electronic energy levels and electron occupation (a) ethylene (b) allyl radical (c) butadiene (d) regular trans-PA (e) dimerised trans-PA

Examples of conducting polymers

Electrical conductivities Copper Platinum Bismuth Graphite 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 Conducting Polymers Germanium Silicon Polyethylene Diamond Quartz

Applications of conducting polymers Polyaniline (PANI) Transparent conducting electrodes Electromagnetic shield Corrosion inhibitor ‘Smart windows’ (electrochromism) Polypyrrole (Ppy) Radar-invisible screen coating (microwave absorption) Sensor (active layer) Polythiophene (PT) Field-effect transistor Anti-static coating Hole injecting electrode in OLED Polyphenylenevinylene (PPV) Active layer in OLED

Molecular conductors, superconductors

TTF-TCNQ  = 105 S cm (58 K)

Organic superconductors (TMTSF)2X X = ClO4- TC = 1.2 K (6.5 kbar) = PF6- TC = 1.4 K (ET)2X X = Cu(NCS)2- TC = 11.4 K

Oxidation of donor / Reduction of acceptor

Partial ionicity

Peierl’s instability

Organic donor molecules

Organic acceptor molecules

Molecular electronics

Molecular Rectifier

Molecular Amplifier 100 mV 20 mV Vin, Vout : input and output voltage, VP : bias voltage RP : polarisation resistance, RL : load resistance X : capacitor to isolate external circuit from bias voltage

Nanomaterials

Nanomaterials Nanoscale Concept of Molecules Size matters ! Unique effects Concept of Molecules Metal nanoparticles Parallels with molecules

Chemical Composition CuSO4.5H2O K2Cr2O7 NiCl2.6H2O

Structure Carbon Graphite Diamond Fullerene (C60)

Properties of materials depend upon : Chemical composition Structure

Size Chemical composition Structure Identical millimeter micrometer Silicon Size Chemical composition Structure Identical millimeter Silicon micrometer nanometer

8 1021 1 cm 2 cm 1 nm Surface area of 1 cube = 6 cm2 Surface area of 8 cubes = 48 cm2 2 cm Surface area = 6 x 22 = 24 cm2 1021 Total surface area = 6 x 1021 nm2 = 6 x 107 cm2 = 6000 m2 = 1.5 acre 1 nm

layer of graphite - graphene DNA 2.5 nm STEM image of a single layer of graphite - graphene Scale bar = 2 nm

Atomic Force Microscope Thickness = 2.5 nm AFM image of a monolayer of surfactants

Top-down Bottom-up

Sequential extraction of adsorbed atoms - one by one - from Germanium surface Dujardin, G., Mayne, A., Robert, O., Rose, F., Joachim, C., and Tang, H. Science 1998, 251, 1206.

AuCl3 Au Michael Faraday 1791 - 1867 P CS2 ‘finely divided metallic state’ of gold (M. Faraday, Philos. Trans. R. Soc.London, 1857, 147, 145)

Dramatic change in Colour

Plasmon Resonance Absorption

Same chemical composition but colour changes with size ! Increasing particle size Quantum dots, nanoparticles of semiconductors, of different sizes, illuminated by a single light source, emit intense fluorescence of different colours (Felice Frankel, MIT)

Fluorescence imaging in medical diagnostics Rat vasculature injected with water solution of Quantum Dots (CdSe-ZnS) Excitation at 780 nm 2-photon fluorescence at 550 nm Larson et al, Science 2003, 300, 1434 Using conventional fluorescent dyes

Carbon nanotubes - highly versatile nanomaterials The strongest and most flexible molecular material High thermal conductivity Very good electrical conductivity Can be metallic or semiconducting depending on structure Can be chemically modified

Carbon nanotube based DNA sensor CNT Carbon nanotube (CNT) array with attached DNA probe acts as an ultrasensitive sensor for detecting the hybridisation of target DNA from the sample using signals from the redox bases in the excess DNA single strands. The signal is amplified using metal ion mediators– oxidation of [Ru(bpy)3]2+ by guanine.

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