8.3 p electrical bistability

Electrical Bistability: Organic Nonvolatile Memory

Daniele Fazzi

[email protected]

Outline

6. Theoretical investigation:

3.4 Electronic structure and geometries of the charged species;

3.3 Transport properties: relevant parameters;

1. General introduction on memories: WORM, RAM, DRAM …2. Electrical Bistability: macroscopic evidence, I-V characteristics3. Architectures and Materials

4. Single Molecule memories:4.1 SPM study on Self Assembled Monolayer of BPDN molecules;4.2 SPM study on crystalline thin film: writing the single molecule;

5. Bulk memories:5.1 Organic memories with metal Nano Particles (NPs);5.2 Conjugated polymers as active materials;5.3 “Small organic molecules” with proper functional groups;

RAM: Random Access Memory

ROM: Read Only Memory

WORM: Write Once Read Many-times

DRAM: Dynamic Random Access Memory

RRAM: Resistive Random Access Memory

Flash Memory is non-volatile memory that can be electrically erased and reprogrammed. memory cards, USB flash drives (thumb

drives, handy drive, memory stick, flash stick, jump drive), general storage and transfer of data between computers and other digital products

J. Campbell Scott, Luisa D. Bozano, Adv. Mat., 19 (2007) 1452

What about inorganic memory device?

FeRAM: perovskite structure for ferroelectricmemory such as PbTixZn1-xO3 (PZT)

External electric field can polarize the material causing a distorsion of the cubic lattice (below Curie temperature)

E

Roberto Benz et al,. MSSP, 7 (2004) 349

glass substrate ITO

organic material

aluminium

300 nm

OFF state: 0ON state: 1

I

V

V switching

V switching

1

23

3’ 4

5

4. Ereasing phase: switching of the current I

5. Ereasing phase: low conductivity (σ) OFF state

1. Writing phase: low conductivity (σ) OFF state

2. Writing phase: switching of the current I

3-3’. Reading phase: high conductivity (σ) ON state

Check the state (OFF or ON) of the

memory

Electrical bistability is a reversible switching of an “active material’” between twoconducting states in response to a trigger, such as an applied voltage.

Organic memory

-4 -2 0 2 4-50-40-30-20-10

010203040

I [m

A]

V [V]

200 cycles

ION/IOFF > 2

• Vsw

• Number of cycles

• Retention time

• ION/IOFFratio

Molecules switch ON when a negative bias is applied (hole injection)

M. Caironi, et al., App. Phys. Lett., 89(2006) 243519

A case study in our laboratoriesTechnological parameters

glass substrate

ITO

organic material

aluminium

300 nm

Device Architectures

• “classical” VERTICAL architecture • “classical” COPLANAR architecture

• SPM tip + film/molecule + substrate

1) Single Molecule: 2) Bulk:

A. S. Blum et al., Nature Materials, 4 (2005) 167. Y. Yang et al., Adv. Funct. Mat., 16 (2006) 1001.

Two different approaches for organic nonvolatile memories are possible

Active Materials

Single molecule: Self Assembled Monolayer (SAM) or thin film.

Bulk materials: small molecules, polymers, host guest materials…

• medium π electrons conjugation

• functional groups with high or low electron affinity CHARGE TRAPS

According to Scott-Bozano classification, there are six type of I-V curves reported in literature, for organic memory devices and each of them is correlated to one specific bistability effect:

C. Scott and L. Bozano, Adv. Mat., 19 (2007),1452

Device strutures and materials reported in literature for RRAM (Resistive Random Access Memory)

a. homogeneous-polymer based MIM structures;

b. small-molecule-based MIM;

c. donor-acceptor complexes;

d. system within mobile ions and redox species;

e. blend of nanoparticles in organic host;

f. molecular traps doped into organic host;

A. Szymanski, D. C. Larson, M. M. Labels, Appl. Phys. Lett., 14 (1969) 88

Au/Tetracene/Al film, 450nm, vertical structure

First “small molecule” memory device

I-V curves: a general classification

Single molecule memory cells

Di Pyridyl – Di Nitro oligophenylene-ethylene dithiol

monolayer thickness = 22.3 Å

1. Homogeneus Self Assembled Monolayers of BPDN molecules

Tunneling current is mesured while the bias voltage is swept from 0 to 2 V, and back to 0 V, with the feedback turned off

The switching behaviour is a molecule based phenomenon related to the molecular electronic properties (molecular orbitals, delocalization, excited states, charge transfer states)

The I-V curves are related to a single molecule or to the intermolecular interactions effects inside the SAM?

Blum et al., Nature Materials, 4 (2005) 167

2. Inhomogeneus Self Assembled Monolayers

Di Pyridyl – Di Nitro oligophenylene-ethylene dithiol

C11 alkanethiol

Gold nanoparticle (2.0 nm diameter)

The I-V discontinuity corresponds to a change in the conductance state of individual molecules is

not dependent on neighbouring molecules

+

+

Activematerial

Insulator

Marker

Theoretical investigationsQuantum chemical simulations (DFT: B3PW91/6-31G**) can be used to study the MOs delocalization and the effect of different chemical groups (with different

electron affinity)

Neutral molecule (Di Nitro based molecules) J. M. Seminario et al., JPCA, 105 (2001) 791

Molecular Orbitals (MOs) simulations of neutral and charged species (at V=0)

I-V characteristic of molecule 4

Bias Voltage effects on MOs delocalization

Bias Voltage effects on energy gap

V

LUMO

HOMO

3. Crystalline thin film: writing the single molecule

4’-Cyano-2,6-Dimethyl-4-Hydroxy AzoBenzene(CDHAB)

Donor Acceptor molecule

Self organized highly ordered thin film (5 nm of thickness): STM image

All marksare in the ON state

OFF

OFF

OFF

Y. Wen et al., Adv. Mat., 18 (2006) 1983

another example…

2,2-dimethyl-α-α-α-α-tetraphenyldioxolane-4,5-dimethanol and coumarin: TADDOL-coumarin

Self AssembledMonolayer: hydrogenbonding and π-staking

Writing the single “cell” by an applied localelectric field: rapture of hydrogen bonds.

Y. Wen et al., Adv. Mat., 16 (2004) 22

1. Organic memories with metal Nano Particles (NPs) : three layer device

Al/OMO/Al

+Metal layer deposited bythermal evaporation

The metal layer must be a nanocluster one

I-V curves at various temperatures

1. The switching time is lessthen 20 ns2. The switching voltage isindipendent of the temperature

Tunneling process

Y. Yang et al., Adv. Func. Mat., 16 (2006) 1001

Conduction and switching mechanisms:

HOMO

LUMO

εF Al-n

V=0

Unbiased device:many energy wells(nanoclusters) sandwitchedbetween the organic layer

Organic layer

V≠0

The electric field polarize the Al-nlayers and organic layers

Opposite charges are induced in the Al-nlayers at the top and bottom interfaces

Lower of the interfacial gap: ON state

ΔE

2. Organic memories with Polymer/NPs: single layer device

I-V curves at different T I (ON state) vs T

Au=2.8nm

Ec = 0.1eV gained at high electric field

Charging energy in order to take place the charge transfer


ON state

3. Conjugated polymers Fluorene group: electron Donor

Oxadiazole and bipyridine group: electron Acceptor

I-V curves

Q. D. Ling et al., Polymer, 48 (2007) 5182

Electrostatic Potential Surface: positive regionnegative region

Traps for charge carrier

Mechanism of switching


4. Conjugated co-polymers with Eu complex

Eu complex (Acceptor) serve as temporary barriers totrap the charge carriers

Carbazole group: Donor

ON state

OFF StateQ. D. Ling et al., Polymer, 48 (2007) 5182

I

V

5. Conformational induced polymersNo memory effect Memory effect


Conduction and switching mechanisms:Face to face conformations

The electric field induce a face to face conformation in the PCz polymer

(PVK) No memory effect

6. Redox mechanism

HOMO

LUMO

Charge transfer complex



Our contribution

Molecules: DiPhenyl BiThiophenes (E.V. Canesi, A. Bianco, C. Bertarelli)

OX

OX

S

S

tBu

tBu

tBu

tBuS

S

O

O

tBu

tBu

tBu

tBu

O

O

SS

tBu

tBu

tBu

tBu

SS

XO

OX

tBu

tButBu

tBu

Aromatic

Quinoid Aromatic Quinoid

“Linear” shape “Zeta” shape

tBu: C(CH3)3 ; X= CH3 or H

L

Z

For the same aromatic class the only difference from L to Z species is the position of the link between the phenyl group and the bithiophene unit:

different link for different electrical properties

Theoretical investigation by means of Density Functional Theory approach

The strategy adopted is:

• study of molecular structures for isolated Z and L DPBT molecules in their ground state

ground state structures, geometries of the molecular conformers, stabilization energies

• simulation of vibrational (IR and Raman) and UV-Vis absorption spectra

normal mode analysis, molecular orbitals involved in the relevant electronic transitions

• study of isolated molecules in their charged state (1+ and 2+)

reorganization energy (λ) and relative energetics

Theoretical simulations are carried out in the framework of DFT using a B3LYP hamiltonian and a double split basis set 6-31G**

Search for the key molecular parameters related to electrical bistability, charge transfer and electronic transport properties

UV-Vis absorption spectra: prediction of the electronic transitions and orbital analysis

• Red shift in the case of L molecule (observed and also predicted from ZINDO simulations);

•From orbital analysis: the L species has a conjugation path longer than the Z one.

EXPT. TEO

Z

L

transcis

HOMO

LUMO

HOMO

LUMO

Z L

HOMO

LUMO

Z LHOMO

LUMO

Electrical bistability: why charged species?

Conductance is strongly influenced by the charged state of the molecules, so different possible mechanisms for voltage-induced conductance

switching can exist. (see J.M.Seminario et al., J.P.C.A., 105 (2001) 791 )

From experimental evidence the charge injected in the organic layer is positive (hole), so the simulated charge state of the molecule is a cation (1+ or 2+).

1) DPBT(a)0 + DPBT(b)+● DPBT(a) +● + DPBT(b)0

2) DPBT(a) +● + DPBT(b) +● DPBT(a) ++ + DPBT(b)0Inter-molecular charge transfer

kET

kET

3) DPBT(a) +● + DPBT(b) ++ DPBT(a) ++ + DPBT(b)+●kET

Possible charge transfer reactions involved in the transport process:

The theoretical investigation on charge transfer is carried out in the framework of the Marcus-Hush theory for the Electron

Transfer (ET) processesRudolph A. Marcus: Nobel Prize in Chemistry 1992 "for his contributions to the theory of electron transfer reactions in chemical systems"

I

V switching

V switching

1

23

3’ 4

5

DPBT(a)0 + DPBT(b)+● DPBT(a) +● + DPBT(b)0kET

DPBT(a) +● + DPBT(b) +● DPBT(a) ++ + DPBT(b)0kET

DPBT(a) +● + DPBT(b) ++ kET DPBT(a) ++ + DPBT(b)+●

OFF State

ON State

Mobility of charge carriers

Diffusioncoefficient

Hopping rate

J. L. Bredas et al., Chem. Rev., 107 (2007) 926

Neutral State (M)

Charged State (M+●)

Relevant molecular parameters involved in the single molecule study

R. A. Marcus, Rev. Of Modern Phys., 65 (1993) 599

Transport properties: relevant parameters

classical Marcus equation

quantum mechanical corrections

P. Barbara, T. J. Meyer, M. A. Ratner, J. Phys. Chem., 100 (1996) 13148

Transition State Theory for ET Rate Constant

λ(i): reorganization energyHrp: electron tranfer integral

Reorganization energies:

2) M(a) +● + M(b) +● M(a) ++ + M(b)0kET

3) M(a) +● + M(b) ++ M(a) ++ + M(b)+●kET

1) M(a)0 + M(b)+● M(a) +● + M(b)0kET

• Higher λ for Z “TRANS”• Higher carrier mobility in the charged states for L species - ON phase

J.L. Bredas G. B. Street, Acc. Chem. Res.,18(1985) 309

λ : total reorganization energy

Vif: electron transfer integral

J.L. Bredas et al., Chem. Rev., 104 (2004) 4971

carrier mobility μhop

Electronic structure and geometries of the charged species

1) OFF state

2) ON state

3) ON state

Exotic memory……

8.3 p electrical bistability

Technology