8.3 p electrical bistability
TRANSCRIPT
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
Conduction and switching mechanisms:
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
Q. D. Ling et al., Polymer, 48 (2007) 5182
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
Q. D. Ling et al., Polymer, 48 (2007) 5182
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
Conduction and switching mechanisms:
Q. D. Ling et al., Polymer, 48 (2007) 5182
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……