8.3 P Electrical Bistability

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  • 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……