Perspectives on Physics: Organic Electronics and Spintronics · Perspectives on Physics: Organic...

Perspectives on Physics: Organic Electronics and

SpintronicsProf. Mats Fahlman

Surface Physics and Chemistry division, ifm

[email protected]

24/04/2009 Mats Fahlman, LiU

1 000

Stone, Wood, Copper Copper, Bronze,

Paper, GlassIron

1 800

Rubber,Steel

1 940

Steel alloys,Light alloys,

Cement,Polymers

(Nylon, etc)

20041970

Silicon!,Super Alloys

Conducting Polymers,Liquid Crystals,

Electrochromic mat.Optical fibers,

Fullerenes,Nanotubes,GMR, etc,

1 000

Stone, Wood, Copper Copper, Bronze,

Paper, GlassIron

1 800

Rubber,Steel

1 940

Steel alloys,Light alloys,

Cement,Polymers

(Nylon, etc)

20041970

Silicon!,Super Alloys

Conducting Polymers,Liquid Crystals,

Electrochromic mat.Optical fibers,

Fullerenes,Nanotubes,GMR, etc,

New Materials: a simple time line

2009


Inorganic Semiconductor Technology

• ”Lego-technology”• Atom layer by atom layer• Control the electronic and optical properties by:

• ”building blocks” (elements)• Position• Mixture• Crystalline structure


Organic Electronics

Nature:

Carbon, Oxygen, Nitrogen, Hydrogen, etc + sun

Functional and esthetically pleasingsolar cell.

The “dream”:Imitation is the sincerest form of flattery.Oscar Wilde


Nobel Prize in Chemistry 2000

”…for the discovery and developmentof conducting polymers.”

• Prof. A.J. Heeger• Prof. A.G. MacDiarmid• Prof. H. Shirakawa

1977

Polyacetylene (PA)

CC

CC

CC

CH

H

H

H

H

H

H

I2σ = 10-9 S/cm σ = 38 S/cm


Conjugated polymers

10-18 10-14 10-10 10-6 10-2 102 106S/cm

Qua

rtz

Dia

mon

d

Gla

ss

Silic

on

Ger

man

ium

Cop

per

Insulators Semi-Conductors

Metals

Polymers: Insulators and Metals


S

O O

S

O O

S+

O O

S

O O

CH2* CH *

C

NH

On

CCH3

CH3

CH2 SO3H

NNH

NH* N *x y n

**

n

SO3H

CH2* CH *

C

NH2

O

n

a)

b)

c) d) e)

* *n

C8H17 C8H17poly(dioctyl-fluorene)

S

S

S

S

n

R R

R R

Semiconductors

Insulators

PolymersMetals

Doping


Polymers can exist as:

Linear chains

Cross-linked chains Networks

Chemically identical polymers can have completely differentproperties depending on degree of cross-linking

Polymer Chains


Organic Electronics

Conducting Polymers in solution = Electronic InkThe Goal: Organic electronics printed onto paper/plastic

•Applications:- Sensors in food packaging- Large area displays-RF-ID (Barcodes, Security tags, etc)- Photovoltaics

• Cheap!• Environmentally compatible

- recycling• Existing printing techniques:

- Screen, Off-set, Ink-jet,etc


Si

Stamp ‘negative’:Polysiloxane, ’Rubber’

Pour polymerinto ‘negative’

Cure,Lift off

Stamp!

Cover stampwith InkStamp

surface

Au SubstrateAu Substrate

Ink

S S S

R R R

Magnification ofsurface area

Example : R hydrophobe =>Polymer cast from water solution goes to the empty surface areas

Soft Lithography


Organic electronics: flexibility

www.readius.com

E-book with roll out screen

E-ink display driven by polymer FET IC

http://www.readius.com/


E0

Energy of the orbital(Defined as negative)

Compare with satellite orbiting the earth

Atom nucleus

Electron-orbitals

Unlike satellites, electrons can only existin certain orbits with certain energies(quantum mechanics).

Atoms: the electron orbitals


Fermions• Electrons are fermions• Have spin 1/2• Can’t have the same quantum state (Pauli principle)• Only two e- per energy level (orbital)

E0

Bosons• Photons are bosons• Have integer spin• Can have identical quantumstates• “Unlimited” number per energylevel (orbital)

E0

ω

ω

e-

Electron spin:

Fermions - Bosons


E

E Ek

Many atoms => fine structure of many split levels with small ΔE => ‘bands’


the hydrogen atom is the only system found in our world which we can describeexactly with quantum mechanics.... for all others, approximations.

ΨΨΨ )(22 ,

22

22

pepp

ee

rVmmt

i rhhh +⎟

⎟⎠

⎞⎜⎜⎝

⎛∇−∇−=

∂∂

Ψ

h

)( , perV r

per ,r

the wave function describing the motion of the proton and the electron;

h "bar" = Planck's constant (h) divided by 2π;

me , mp the electron mass and the proton mass;

the Coulomb potential;

the position of electron and proton, respectively;

∇e2, ∇p

2 Laplace operators (2nd derivative in space coordinates) for the electron and proton.

Wave mechanics of the hydrogen atom


Ψ-= {2(1-S)}-1/2(A - B)

Ψ+= {2(1+S)}-1/2(A + B)

The hydrogen molecular ion (H2)


Solid of N atomsTwo atoms Six atoms

For the total number N of atoms in a solid (1023 cm–3), N energy levels split apart within a width ΔE. Leads to a band of energies for each initial atomic energy level (e.g. 3s energy band for 3s energy level).

Electrons must occupy different levels due to Pauli Exclusion

principle.

Each level contains 2(2l+1) electrons, so each band contains 2(2l+1)N electrons

From bonds to energy bands in solids


InsulatorLarge Eg

Eg

Diamond

SemiconductorMedium Eg

Eg

Si

MetalEg ~ 0

Fe

Electrons are fermions: limited numberper band!


Metal


Under constructionUnder construction

Semiconductor & Insulator


S.R. Forrest, MRS bulletin, January 2005

Photocurrent generation: Organic Solar Cells

24/04/2009 Mats Fahlman, LiUDonors Acceptors

• Soluble in samesolvents

• Phase segregatesinto networks

• Band offsets facilitate chargetransfer

Polymer solar cells: Donor and acceptormaterials


Light

R.A.J. Janssen, MRS bulletin, January 2005

Bulk heterojunction solar cells


Area of opportunityfor organic solar cells

XMarketOpportunity


Thin film transistorSemiconductor

DrainSource

GateInsulator

VGS

VDS

VDS (V)

I D(m

A)

VGS = 2 V

VGS = 3 V

VGS = 4 V

VGS = 5 V

• n-type source and drain + p-type substrate or p-type source and drain and n-type substrate• VGS creates inversion layer (attracts h+ or e- at the insulator depending on bias)• Charge is transported between source and drain in the inversion layer under VDSbias.

Field effect transistors: a short review


Thin film transistorOrganicSemiconductor

DrainSource

GateInsulator

VGS

VDS

Single crystals (vacuum deposited):Pentacene : μ ~ 1 cm2/VsSexithienyl : μ ~ 0.1 cm2/Vs

If amorphous, μ ~ 0.01 cm2/Vs

Pentacene

S

S

( )3

Sexithienyl

Organic transistors: molecular film


Thin film transistorOrganicSemiconductor

DrainSource

GateInsulator

VGS

VDS

Ordered films (solution deposited):Polyhexyltiophene : μ ~ 0.1 cm2/Vs

Disordered films (solution deposited):Polyhexyltiophene : μ ~ 10-4 cm2/VsPoly(p-penylenevinylene) : μ ~ 10-4 cm2/VsPolyfluorene: μ ~ 10-3-10-4 cm2/Vsetc

Polythiophene, PTS n

Polyfluorene, PFOn

RR

Polymer transistors


r

invD t

WLQI −= DSinvD VLWQI μ−=

DSTGSoxD V)VV(LWCI −μ=

Linear region:

LVEv DSμ=μ=

vLt r = and

)VV(CQ TGSoxinv −−=

S D

L

WQinv = Charge per unit area in inversion layertr = Transit time for charge between source and drainVT = Threshold Voltage (Qinv = 0 if VGS < VT)Cox = Gate oxide capacitance per unit area

)V1(2

)VV(LWCI DS

2TGS

oxsat,D λ+−

μ=

Saturated region:Empirically derived

Field effect transistors: a short review


Nothing new under the sun: Helmholtz Double Layer (~1850):

------------

“Metal” Electrolyte “Metal”

++ ++++ ++++ +

++++

+

+

+

+

+

+

+

+

+

+

+

+

-

-

-

-

--

--

-

-

-

-

-

-

-

++++++++++++-

-----------

Capacitor:

The surface charge is neutralized by ions at the respective interface

The surface charge potential decrease linearly from the surface to the center of the ions situated at d nm from the surface

Note: Helmholtz double layer assumes rigid layers of opposite charges at the interfaces which is highly unlikely to occur in “real” situations…

Polyelectrolyte gate dielectric


Organic Electronics: Solar cells and RF-ID tags

www.konarka.com – solar cells

www.polyic.com – printed RF-ID tags, etc.

http://www.konarka.com/

http://www.polyic.com/


Diffusive reflector

Polarizing film

Glass filter

Negative electrode

Liquid Crystal film

Positive electrode

Glass filter

Polarizing film (90º angle to first film)

Cover Glass

Liquid crystal display pixel

Liquid crystal film filters light if aligned Orientation can be affected by e.g. electric currentActive matrix display: thin film transistors of amorphous siliconBacklighting provided by cold cathode fluorescent lamp


The active part is very thin (< 1 mm)

Cathode: Metal (0.1–0.5 mm)[Ca, Mg/Al]

Glass or PET (10 mm–5 mm)

Anode: Indium-Tin Oxide(0.1–0.3 mm)

transparent, e.g., in LCDs

Polymer(~ 0.1 mm)

Curtesy: R. H. Friend

Polymer Light Emitting Diode


V

Glass substrate

Polymer

Metal cathodes

Polymer LED

ITO ITO

Metal

hνh+

Conduction band

Valence band

J.H. Burroughes, et al, Nature 347 1990 p539

e-

Polymer Light Emitting Diode


300 400 500 600 700 800 900Wavelength (nm)

SS

C8H17

S

C8H17

S S n n

Nature 372 (1994) 444

EL

inte

nsity

M. Andersson, Chalmers

Color through synthesis!


Circuits made by ink-jet printing (a disruptive technology)

PLEDs: example of an active-matrix display


Organic Electronics: flat screen TVs

Sony, Samsung, etc

http://www.youtube.com/watch?v=FAk4bjdbtCI

http://www.youtube.com/watch?v=FAk4bjdbtCI


what is it?

Electronics

"1" "0"

read/write informationby charge pulses

Spintronics

"1" "0"

process information byselecting spin up/down

manipulate spin, measure a property that depends on spin

use spin directly, as in quantum computation

Spintronics


Magnetoresistance: change in resistance under an applied magnetic field

Can be positive or negative effect

Typically defined as ΔR=(R(B)-R(0))/R(0)Using “normal” mechanisms for magnetoresistance in materials, the π-conjugated molecules should have |ΔR|<10-6 at low magnetic fieldsHowever…

Magnetoresistance


Ö. Mermer,et al, PHYSICAL REVIEW B 72, 205202 2005

Organic Magnetoresistance (OMAR) effectSimple diode structureInsensitive to magnetic field (B) directionNon-magnetic electrodesNon-magnetic organic semiconductor|ΔR|~5-15% at RT!

Magnetoresistance in π-conjugated organic semiconductors (OMAR)


Some more peculiarities…

OMAR-effect changes sign depending on voltageOMAR also affects light emission: control light output by magnetic field

Ö. Mermer, et al, PHYSICAL REVIEW B 72, 205202 2005

Magnetoresistance in π-conjugated organic semiconductors (OMAR)


n: spin density at distance x from the ferromagnet/paramagnet interfacen0: spin density at distance x=0

e-e- barrier

e-Ferromagnetic half metal

Paramagnetic (semi)conductor

EF

FerromagneticHalf metal

ChangeMagnetization

”Valve” on/off!

Spin valve


“Designer magnets”, properties can be tuned by organic chemistry

Advantages:• Flexible• Low density• Potential low cost, low temperature synthesis• Variable electrical conductivity • Compatible with polymers and composites• Magneto-optic effects• …

Applications: SPIN elecTRONICS

Molecule-based magnets vs.Organic or hybrid organic/inorganic compounds• Active organic component (net spin in p-orbitals)• Passive organic component (no net spin)

Atom-based magnetsMetals or inorganic compounds withnet spin in d- or f-orbitals.

M(TCNE)x molecule-based magnets


1. existence of a magnetic moment (a net spin)

2. existence of magnetic interactions between spins

3. collective behavior ordering effect of interaction > disordering effect of thermalagitation

Ferromagnet

Below Tc Above Tc TTc

Paramagnet

“Ingredients” for magnetic ordering


M(TCNE)x, x~2, M=V, Mn, Fe…

M TC (K)

V 370

Fe 95

Mn 75

Co 40

Ni 40

V(TCNE)x: semiconductor, TC above room temperature

M. Gîrtu et al 1999-2000,C. Wynn et al 1998

J.M Manriquez, G.T Yee, R.S McLean, A.J Epstein, J.S Miller, Science, v252, 1991, 1415K.I Pokhodnya, A.J Epstein, J.S Miller, Adv. Mat., 2000, v12, 410

Tetracyanoethylene(TCNE)

[TCNE]– spin density distributionSchweizer, et al, JACS 116, 7243 (1994)

M(TCNE)x molecule-based magnets


“Substrate” (m) Surface structure (<mm)

“Grain” (~μm)Cluster, tubes (~nm)

Atoms, bonds (~Å)

..

e-... . Nuclei, electrons (~<Å)

Microscope

SEM

X-ray diffractionPhotoelectron spectroscopy

Material physics: a Matreshka


ESRNMR

neutron scatteringpositron annihilation

FTIRIR

RamanHREELS

UV-Vis absorption

fluorescenceellipsometry

EELS

UPSIPES

NEXAFS EELSXES

XPSAuger

Scanning Tunneling Spectroscopy

TSC

phonons, magnons,

electronic excitation spin-excitations,core electron

excitationsnuclear

excitations

neutralspectroscopy:

ionizing spectroscopy:

transportspectroscopy:

Overview of spectroscopic techniques


X-rays, UV light

photoelectrons

kinetic energy spectrum

sample

analyzer

www.lasurface.com

Maps out occupied electronic structure:- Energy Bands- Chemical Bonds- Elements- Work function

Photoelectron Spectroscopy


Photoelectron spectroscopy

KINETIC ENERGY MEASURED

BINDING ENERGY

VAC

UU

M LEVEL

INTRAMOLECULARRELAXATION

INTERMOLECULARRELAXATION

C2

C1

hν

EB = hν - EK

e-

C1 C2

CORELEVELS

VALENCE LEVELS

UNOCCUPIEDLEVELS

V1,V2,V3

XPS


Emission part

occupiedvalencelevels

core levels

Unoccupiedlevels

ground-state

photo-excitation

Spectator-decay

Participator-decay

Excitation partX-ray absorption event

X-ray absorption (NEXAFS) probes the unoccupied states in the presence of a core hole

Element and site specific method

It’s possible to obtain magnetic information using circular polarized light (XMCD)

X-ray absorption can be used to perform resonant PES…

X-ray absorption techniques


• Selectively deposits highly conductive polymer on dielectric resin and glass without the use of electroless copper. • Formaldehyde-free, environmentally sound technology. • Fast and uniform coverage of the plated through-hole. • Costly waste treatment and microetch post-treatments are not required.

Through-hole plating in Printed Circuit Boards

http://www.baytron.com/index.php?page_id=1533&prod_service_id=&anw_id=96&operate=&suchstart=938&prodselect_5=0&suchstart=938&anwselect_14=96&suchfeld=


•Capacitors make up 90-99% of all components in most electronic devices• “Super” capacitors: Tantalum (Ta2O5) based (10-100 μF)• Traditional technology: use MnO2 as negative contact

• Problem – catches fire at high voltages!• Use Conducting polymer (PEDOT, Polyaniline) as cathode

• No fire risk• Significantly lower equivalent series resistance (ESR)

Capacitors


• Surface resistivity controllable between 10 and 106 ohm/sq• Volume conductivity ranges from 0.0001 to 5 S/cm• Can absorb up to ~90% of impinging microwave radiation • Dissipates static charge instantaneously regardless of humidityUses:EDS protection (clothes, carpets, etc)Microwave/radar absorptionResistive and microwave heating (clothes, blankets, etc)

Textile fibers can be individually coated with conducting polymers

Conductive textiles


• Conducting and semiconducting polymer films can anodically protect metals

• Can also act as on demand releasers of corrosion inhibitors

• Polyaniline typically used for steel and aluminum in construction, navy and aircrafts

Corrosion protection


Organic Electronics Research at LiU

Prof. Magnus Berggren, ITN, http://orgel.itn.liu.se/

Prof. Mats Fahlman, IFM, http://www.liu.se/ifm/surfphys/

Prof. Robert Forchheimer, ISY, http://www.bk.isy.liu.se/

Prof. Olle Inganäs, IFM, http://www2.ifm.liu.se/biorgel/

Prof. Sven Stafström, IFM, http:// cms.ifm.liu.se/theomod/compphys/

http://www.liu.se/ifm/surfphys/

http://www2.ifm.liu.se/biorgel/

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