This document discusses organic nonvolatile memory, specifically: 1. Electrical bistability allows a material to switch between two conducting states in response to an applied voltage. This enables memory applications. 2. There are two approaches for organic nonvolatile memory - at the single molecule level using molecules or thin films, and in bulk using small molecules, polymers, or nanoparticles. 3. Density functional theory simulations can provide insight into relevant parameters like molecular orbitals, reorganization energies, and charge transfer rates that influence electrical bistability and memory performance.

1. Electrical Bistability:
Organic Nonvolatile Memory
Daniele Fazzi
daniele.fazzi@mail.polimi.it

2. Outline
1. General introduction on memories: WORM, RAM, DRAM …
2. Electrical Bistability: macroscopic evidence, I-V characteristics
3. 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;
6. Theoretical investigation:
3.3 Transport properties: relevant parameters;
3.4 Electronic structure and geometries of the charged species;

3. J. Campbell Scott, Luisa D.
Bozano, Adv. Mat., 19 (2007)
1452
RAM: Random Access Memory
ROM: Read Only Memory
WORM: Write Once Read Many-times
DRAM: Dynamic 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
RRAM: Resistive Random Access Memory

4. What about inorganic memory device?
E
FeRAM: perovskite structure for ferroelectric
memory such as PbTixZn1-xO3 (PZT)
External electric field can polarize the
material causing a distorsion of the
cubic lattice (below Curie temperature)
Roberto Benz et al,. MSSP, 7 (2004) 349

5. Organic memory
Electrical bistability is a reversible switching of an “active material’” between two
conducting states in response to a trigger, such as an applied voltage.
I V
switching 1. Writing phase: low
conductivity (σ)
V OFF state
3’
switching 4
2. Writing phase:
1
switching of the
V current I
5
2
3
3-3’. Reading phase:
Check the state high conductivity (σ)
(OFF or ON) of the ON state
memory
4. Ereasing phase:
switching of the
organic aluminium current I
material OFF state: 0
5. Ereasing phase: low
300 nm ON state: 1 conductivity (σ)
glass
ITO
substrate OFF state

6. Technological parameters
A case study in our laboratories
M. Caironi, et al.,
organic aluminium App. Phys. Lett., 89
materia (2006) 243519
l
300
glass
ITO nm
substra
te
Molecules switch ON when a negative bias
is applied (hole injection)
40
• Vsw 30
20
• Number of 10
cycles 0
I [mA]
ION/IOFF > 2
-10
• Retention -20
time -30
200 cycles
-40
• ION/IOFF -50
ratio -4 -2 0 2 4
V [V]

7. Device Architectures
• “classical” COPLANAR architecture
• “classical” VERTICAL architecture
• SPM tip + film/molecule + substrate

8. Two different approaches for organic nonvolatile memories are possible
2) Bulk:
1) Single Molecule:
A. S. Blum et al., Nature Materials, 4 (2005) 167. Y. Yang et al., Adv. Funct. Mat., 16 (2006) 1001.

9. Active Materials
Single molecule: Self
Assembled Monolayer
(SAM) or thin film.
Bulk materials:
small molecules,
polymers, host
guest materials…
• medium π electrons • functional groups with high
CHARGE TRAPS
or low electron affinity
conjugation

10. I-V curves: a general classification
Device strutures and materials
According to Scott-Bozano classification,
reported in literature for RRAM
there are six type of I-V curves reported
(Resistive Random Access Memory)
in literature, for organic memory devices
and each of them is correlated to one a. homogeneous-polymer based MIM
specific bistability effect: 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;
First “small molecule” memory device
A. Szymanski, D. C. Larson, M. M. Labels,
Appl. Phys. Lett., 14 (1969) 88
Au/Tetracene/Al film,
C. Scott and L. Bozano, Adv. Mat., 19 (2007),1452
450nm, vertical structure

11. Single molecule memory cells
1. Homogeneus Self Assembled Monolayers of BPDN molecules
monolayer
thickness = 22.3 Å
Di Pyridyl – Di Nitro oligophenylene-ethylene dithiol
Blum et al., Nature
Materials, 4 (2005) 167
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?

12. 2. Inhomogeneus Self Assembled Monolayers
Active
Di Pyridyl – Di Nitro
material
oligophenylene-ethylene
dithiol
+
Insulator
C11 alkanethiol
+
Marker
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

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

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

15. V
Bias Voltage
effects on MOs
delocalization
Bias Voltage effects on energy gap I-V characteristic of molecule 4
LUMO
HOMO

16. 3. Crystalline thin film: writing the single molecule
All marks OFF
are in the
ON state
4’-Cyano-2,6-Dimethyl-4-Hydroxy AzoBenzene
(CDHAB)
Donor Acceptor molecule
OFF
OFF
Self organized highly ordered thin film
(5 nm of thickness): STM image
Y. Wen et al., Adv. Mat., 18 (2006) 1983

17. another example… Y. Wen et al., Adv.
Mat., 16 (2004) 22
Self Assembled
Monolayer: hydrogen
bonding and π-staking
2,2-dimethyl-α-α-α-α-tetraphenyldioxolane-
4,5-dimethanol and coumarin:
TADDOL-coumarin
Writing the single “cell” by an applied local
electric field: rapture of hydrogen bonds.

18. 1. Organic memories with metal Nano Particles (NPs): three layer device
Metal layer deposited by
thermal evaporation
+
The metal layer must be a
nanocluster one
Y. Yang et al., Adv. Func. Mat.,
Al/OMO/Al
16 (2006) 1001
I-V curves at various temperatures
1. The switching time is less
then 20 ns
2. The switching voltage is
indipendent of the temperature
Tunneling process

19. Conduction and switching mechanisms:
V≠0
V=0
LUMO
εF Al-n
ΔE
HOMO
Organic layer
The electric field polarize the Al-n
layers and organic layers
Opposite charges are induced in the Al-n
layers at the top and bottom interfaces
Unbiased device:many energy wells
(nanoclusters) sandwitched
between the organic layer
Lower of the interfacial gap:
ON state

20. 2. Organic memories with Polymer/NPs: single layer device
Au=2.8nm
I-V curves at different T I (ON state) vs T

21. Conduction and switching mechanisms:
ON state
Charging energy in order to take
place the charge transfer
Ec = 0.1eV gained at
high electric field

22. 3. Conjugated polymers
Fluorene group:
electron Donor
Oxadiazole and bipyridine group:
electron Acceptor
Q. D. Ling et al., Polymer, 48 (2007) 5182
I-V curves

23. Electrostatic Potential Surface:
Mechanism of switching
positive region
negative region
Traps for charge carrier Q. D. Ling et al., Polymer, 48 (2007) 5182

24. 4. Conjugated co-polymers with Eu complex
Carbazole group: Donor
Eu complex (Acceptor) serve
as temporary barriers to
trap the charge carriers
I
ON state
V
OFF State
Q. D. Ling et al., Polymer, 48 (2007) 5182

25. 5. Conformational induced polymers
No memory effect Memory effect
Q. D. Ling et al., Polymer, 48 (2007) 5182

26. Conduction and switching mechanisms:
Face to face conformations (PVK) No
memory effect
The electric field induce a face to face
conformation in the PCz polymer

27. 6. Redox mechanism
Q. D. Ling et al., Polymer, 48 (2007) 5182
Conduction and switching mechanisms:
HOMO
LUMO
Charge transfer complex

28. Our contribution

29. Molecules: DiPhenyl BiThiophenes (E.V. Canesi, A. Bianco, C. Bertarelli)
L
tBu
tBu
O
OX tBu
tBu
XO S
Z tBu
tBu
S OX
tBu
S
S
Aromatic tBu
S
S
tBu
tBu
tBu
O S tBu
OX O
tBu
S tBu
O
tBu
Aromatic Quinoid
tBu
Quinoid
“Linear” shape “Zeta” shape
tBu: C(CH3)3 ; X= CH3 or H
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

30. Theoretical investigation by means of Density Functional Theory approach
Search for the key molecular parameters related to electrical bistability,
charge transfer and electronic transport properties
The strategy adopted is:
ground state structures,
• study of molecular structures
geometries of the molecular
for isolated Z and L DPBT
conformers, stabilization
molecules in their ground state
energies
normal mode analysis,
• simulation of vibrational (IR
molecular orbitals involved in
and Raman) and UV-Vis
the relevant electronic
absorption spectra
transitions
• study of isolated molecules in reorganization energy (λ)
their charged state (1+ and 2+) 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**

31. UV-Vis absorption spectra: prediction of the electronic transitions and
orbital analysis
TEO
EXPT.
trans
Z L
cis
LUMO
Z
LUMO
L
HOMO
HOMO
• Red shift in the case of L molecule
(observed and also predicted from LUMO
ZINDO simulations); LUMO
•From orbital analysis: the L species
has a conjugation path longer than HOMO HOMO
L
the Z one. Z

32. 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+).
Possible charge transfer reactions involved in the transport process:
kET
DPBT(a) +● + DPBT(b)0
1) DPBT(a)0 + DPBT(b)+●
Inter-molecular
kET
charge transfer
2) DPBT(a) +● + DPBT(b) +● DPBT(a) ++ + DPBT(b)0
kET
3) DPBT(a) +● + DPBT(b) ++ DPBT(a) ++ + DPBT(b)+●
The theoretical investigation on charge transfer is carried out in
the framework of the Marcus-Hush theory for the Electron
Transfer (ET) processes
Rudolph A. Marcus: Nobel Prize in Chemistry 1992 quot;for his contributions to the theory of electron transfer
reactions in chemical systemsquot;

33. kET OFF State
DPBT(a) +● + DPBT(b)0
DPBT(a)0 + DPBT(b)+●
I V
switching
V 3’
switching 4
1
5
2
3
kET
DPBT(a) +● + DPBT(b) +● DPBT(a) ++ + DPBT(b)0
ON State
kET
DPBT(a) ++ + DPBT(b)+●
DPBT(a) + DPBT(b)
+● ++
Hopping rate
Mobility of
charge carriers
Diffusion
coefficient

34. Transition State Theory for ET Rate Constant
classical
Marcus
equation
quantum
mechanical
corrections
λ(i): reorganization energy
P. Barbara, T. J. Meyer, M. A. Ratner, J. Phys. Chem., 100 (1996) 13148
Hrp: electron tranfer integral
Relevant molecular parameters involved in the single
Charged State
molecule study
(M+●)
Neutral State
(M)
R. A. Marcus, Rev. Of Modern Phys., 65 (1993) 599
J. L. Bredas et al., Chem. Rev., 107 (2007) 926
Transport properties: relevant parameters

35. Reorganization energies:
J.L. Bredas et al., Chem. Rev., 104 (2004) 4971
kET
M(a) + M(b)0
+●
1) M(a)0 + M(b)+●
kET
2) M(a) +● + M(b) +● M(a) ++ + M(b)0
kET Vif: electron transfer integral
carrier
3) M(a) +● + M(b) ++ M(a) ++ + M(b)+●
mobility μhop λ : total reorganization energy
OFF state
1)
ON state
2)
J.L. Bredas G. B. Street,
Acc. Chem. Res.,18
(1985) 309
• Higher λ for Z “TRANS”
3) ON state
• Higher carrier mobility in the charged
states for L species - ON phase
Electronic structure and geometries of the charged species

36. Exotic memory……