Molecular and Organic Electronics

1. Molecular and Organic Electronics Thin Film Lab., University of Tehran Reference: Nanoelectronics and Information Technology : Advanced Electronic Materials and Novel Devices By: Rainer Waser

2. Organic Molecules: Hydrocarbons • Alkanes: saturated (CnH2n+2), σ bonds with sp3 hybridization • Tetrahedral arrangement • Ethane (C2H6), possible rotation around σ bond with 0.1eV energy • 154 pm C-C bond length • n>3  different isomers • Cycloalkanes ring-type structure

3. Hydrocarbons: Alkenes • Alkenes: (CnH2n), σ and π bonds with sp2 hybridization • Planar structure • 134 pm C=C bond length • Free rotation, which requires breaking bonds, is not possible • Asymmetric alkenes are slightly polar • Cis- and trans- isomers

4. Hydrocarbons: Polyenes • If there are more than one C=C bond  polyenes • Isolated, Conjugated and Cumulated • β-Carotene, naturally occurring polyene

5. Hydrocarbons: Aromatics • Aromatic Hydrocarbons (arenes): cyclic polyenes • The π-electrons are delocalized over the entire ring  undistinguishable single & double bonds • Cyclohexa-1,3,5-triene (benzene) • Polycyclic aromatic molecules: Naphtalene, Anthracene and Phenanthrene

6. Non-Hexagonal Aromatic Systems

7. Heterocyclic systems

8. Hydrocarbons: Alkynes • Alkynes: CnH2n-2 with C≡C bonds • sp hybridization • Linear structure • 120 pm long

9. Electronic Structure of π- Conjugated Systems • LCAO approx. method is used • Ψi are the AO wavefunctions of the atoms in molecules • n total number of atomic orbitals

10. LCAO Method • Hjk : resonance integral Sjk : overlap integral • Variation principle:

11. LCAO: Secular Determinant

12. Hückel Approximations

13. HMO Calculation for Ethene

14. Results for Buta-1,3-diene • Highest Occupied Molecular Orbital (HOMO) & Lowest Unoccupied Molecular Orbital (LUMO) ≡ Conduction band edge and valence band edges • HOMO-LUMO Gap (HLG) ≡ bandgap • HLG energy : energetically lowest optical absorption band

15. Energy Levels of Conjugated Hydrocarbons

16. Results for Benzene • ψ2 and ψ3 are degenerate • The total energy of π electrons for benzene is 6α+8β • The total energy of π electrons for hexa-1,3,5-triene is 6α+6.98β • The difference reveals that aromaticity yield an additional stabilization

17. Results for C60 • C60(buckminsterfullerene) molecule with 12 regular pentagons and 20 hexagons • Two bonds: • Between two hexagons : 139pm • Between hexagon & pentagon: 145pm • These lengths are between C-C and C=C lengths • σ bonds make them very stable and π bonds are delocalized • Approximately sp2 hybridization

18. Functional Groups

19. Polarized Molecules • Covalent bonding between functional groups and the rest of the molecule  polar characteristics • Electronegativity differs from an atom to another (-NO2 and –NH2) • Inductive Effect (I effect): polarization caused by functional groups which acts electrostatically along σ bonds • If functional group attract electrons: -I effect (σ-acceptor) • If functional group donates electrons: +I effect (σ-donor) • Mesomeric Effect: The functional group may attract charge density from the π-system (-M effect π-acceptor) or donates partial charge into the π-system (+M effect π-donor) from its own π- or non-bonding electrons

20. Introduction to Molecular Electronics • The ongoing feature size reduction in the Si-based technology  several physics and economic limitations  Molecular Electronics • Molecules are several order smaller than current feature sizes • They have potential to organize themselves on 2-D patterns as well as well defined supramolecular objects • Future: Ideal building blocks of high density electronic devices • The first idea that molecules can perform electronic function by Aviram and Ratner in 1974 • They theoretically suggested that a donor-spacer-acceptor acts similar to p-depletion-n junction with I-V characteristics like diodes

21. Molecular Electronics: Definition • Bulk Molecular Systems: Organic compounds (molecules, oligomers & polymers) with application in LCD, OLED and soft plastic TFTs in amorphous and polycrystalline form  The characteristics dimensions are much larger than molecule sizes • Single Molecular Systems: Individual contacts to single or small perfectly ordered array of molecules  nano-sized electronics • HME: organic molecules directly contacted by inorganic (2 or 3)electrodes • MME: All major functions of logic circuits can be integrated into molecules individually connected to each other • In MME the electrode contacts are needed only for data exchange with outside and for energy supply

22. Electrodes and Contacts • A basic requirement for molecular electronics: the connection of the molecule to the outside world e.g. to drive current through molecules • HME: Metallic or semi-conducting electrodes • MME: (future) the replacement of metallic electrodes by molecular wires • Connection of solids and molecules: Covalent bonds and Van der Waals interaction • Covalent bonds: the best covalent link is the thiol (sulfur) group on molecule and the Au (non-oxidizing) substrate  good stability and loose enough for self-assembley • Others: Se-Au and S-Ag • Van der Waals interaction: usually between Langmuir-Blodget (LB) film and planar surface with advantage of substrate diversity

23. Electron Transport Mechanisms in Contacts • Covalent bonds: (delocalized π-electron systems on Au) short distance to metallic surface  hybridization of inner and outer extended wavefunctions • The junction acts as a waveguide for electrons • For thiol attached to the benzene: π orbitals of the benzene and the conduction band of Au overlap at the sulfur atom  relatively good contact • Van der Waals: larger distance  no wavefunction overlap (approx. independently)  Tunneling mechanism for electrons

24. Functions • The functions of Molecular Electronics in electronic circuits: • Molecular Wires, Insulators and Interconnects • Diodes • Switches and Storage Elements • Three-Terminal Devices

25. Molecular Wires, Insulators and Interconnects • In wires, the electron transport is expected to take part through the frontier orbitals of the molecule closest to the Fermi levels of the electrodes • Promising candidates: molecular wires with large delocalized π-systems (n>>1  ΔE → 0) • Wires: 1:polyene 2:poly-thiophene 3:poly-phenylene-vinylene 4:poly-phenylene-ethynelene 5: para-diacetylene-thiophenyl-substituted-benzene • Insulators: rigid molecules with non-delocalizing π-systems (non-conjugating π-systems) • The insulator should have rigidity for application as insulator in diodes • 6: alkanes (lack rigidity) 7: rigid adamantyl cage, suggested by Aviram and Ratner for diode application. 8: tetramethylsubstituted-biphenyl The delocalization of the π-system depends on the torsion angle between substituents (in 8 perpendicular π-systems  reduction in electronic communication but rigid and insulating connection) 9:trans-acetylene-platinium(II) 10: meta-diacetylene-thiophenyl-substituted-benzene

26. Diodes • The first Theoretical approach to molecular electronics by Aviram & Ratner: The π-system of donor and acceptor units are confined in two potential wells • Spacer (adamantyl cage): preserves the energy differences of the frontier orbitals, electronic transport by tunneling through the insulating rigid spacer • Positive voltage: the potential of the left lead ↑ and the right lead ↓  current flows from left LUMO 1 to HOMO2 going toward lower energies • Opposite voltage: conduction at much higher voltages • The first implementation: LB film consisting of donor-spacer-acceptor were deposited on the metallic surface followed by deposition of the top electrode • LB lacks stability due to weak Van der Waals interaction • The Alkyl chain with is vital for LB separate acceptor from the top electrode  threshold problems • Better structure: The rod-like molecule, by Reed and Tour, with a thiol function at one end was immobilized on Au surface in a Si3N4 pore  SAM film • Diode characteristics with certain threshold + NDR • The nature of this effect is not completely understood

27. Switches and Storage Elements • Some classes of molecules are stable in two different states (meta-stable or bistable) • Physical properties like conductance will defer from one state to another • Bistable molecular switches classification: • By the stimulus that triggers the switch (light, voltage or pH) • By the property or function that is switched • Light triggered switch by Irie: Two methyl-thiophene units linked with hexa-fluoro-cyclopentene • UV irradiation of 200-380 nm  closed form and 450-720 nm  open form • Advantages: excellent addressability and switching • Disadvantage: The use of light as switching trigger instead of voltage in electronic circuits

28. Switches and Storage Elements • The potential of this switch is investigated by several molecule synthesizes

29. Switches and Storage Elements • Rotaxenes and Catenanes switch as a function of applied potential between two different states • Catenanes with Two interlocked rings: • Two viologenes units • a dioxy-naphtalene unit and a tetra-thia-fulvalene (TTF) unit • It is used to build up electronic memory devices • Hysteretic rearrangement: oxidizing at +2V and reduction at -1.5V

30. Three-Terminal Devices • Very difficult implementation: 3 terminal should be made on a few nano-meters scale • Two approaches: • MME: Make a molecule with three branches, independently contacted by three leads (no implementation so far!) • HME: The third contact far away, not in contact with molecule, but able to modify the electrostatic potential inside the molecule by field effect  Field Effect Transistors • Au/Al gate electrode/Al2O3 insulating layer • C60 molecules were deposited on a metallic sub. & investigated by STM • Imaging and detecting simultaneously with squeezing the C60 • The mechanical force is the third parameter and can change the conductance by two order of magnitudes per nano-newton

31. Molecular Electronic Devices: First Test Systems • Scanning Probe Methods • Monomolecular Film Devices • Nanopore Concept • Mechanically Controlled Break Junctions • Electromigration Technique

32. Scanning Probe Methods • Scanning Probe Methods is usually used to achieve properties such as shape, size, diffusion and conductivity of individual molecules on surfaces (AFM and STM ) 1. STM as imaging and electrical measurements • Study of the conduction of rod molecules on surface: STM tip  top electrode surface bottom electrode • For this study by STM, molecules should be vertical • Van der Waals molecules prefer to lie flat on surfaces • Methods to force molecules vertical: • TripodalS-group attachments to tetrahedral molecules • Use of a carpet of upright standing insulating alkanethiols • Molecule 16, good conducting with conjugated π-system • SAM matrices of insulating dodecylmercaptan (C12H26S)  organized in domains (each domain resemble 2D crystals) followed by treating in the 16 diluted solution • STM shows that exchange takes place at domain boundaries and triple points while 16 is not observed within the domains • Conduction at slightly taller (0.7nm) molecule rods by STM tip

33. Scanning Probe Methods 2. STM as patterning tool • To circumvent the arbitrary placement of 16 in SAM matrix, the STM tip was used to pattern the SAM layer • SAM formation on gold / dilute solution of 16 and NH3 in an STM liquid cell/ applying short pulses to substrate  patterning some points/ filling of the pits with 16 • Each pit host approx. 400 molecules of 16

34. Monomolecular Film Devices • Film formation by self-assembly, vapor deposition sandwiched between two metallic leads • With a large number of molecules, the I-V properties of individual molecules are averaged out  reproducibility • Defect Problems: by deposition of the top metal layer, diffusion of metal atoms may occur through ultra thin layer short circuit • Reduction of defects by decreasing electrode area

35. Cross-bar Arrays: RAM and FPGA Memory • Fabrication of LB/Catenane cross-bar array: • Deposition of poly-Si or Pt bottom electrodes as parallel wires by e-beam lithography or nano-imprint • LB/Catenane Deposition followed by Ti layer as top contact to protect the molecule from subsequent integration steps • Top electrode deposition (Au or Al) by e-beam lithography • Ti layer is removed by etching to avoid short circuit • Voltage in range of 1-2 V for write operation to set the molecule into the high/low resistive state • Ron/Roff of 3-10 for Catenane and upto 104 for LB structure

36. Nanopore Concept • Fabrication: • Formation of a suspended Si3N4 (CVD) membrane by micromachining by KOH • A single 40nm hole is formed by e-beam lithography and RIE • RIE is adapted to form a bowl shaped pore with reduced diameters at bottom • Au top electrode by Au evaporation to fill the pore • Immersing in the 13 solution  SAM • Top Au layer deposition • Diode characteristics + NDR with Ipeak/Ivalley ratio of 1000 at 60K, exceeding the corresponding value of semiconductor tunneling diodes

37. Mechanically Controlled Break Junctions • Disadvantage of scanning probe methods • Asymmetric (shape and material) contacts • Lack of drift-stability as soon as the distance control feedback loop is switched off • High resolution lithography and shadow mask techniques allows the fabrication of metallic structures with a width of 10-20nm • To immobilize molecules between two electrodes, a notched gold wire was mechanically broken while exposing to SAM solution • The tips are then slowly moved together until the onset of conductance was achieved  Non linear curves was measured repeatedly • Advanced MCB methods using stressed Au layer for single molecule measurements  distant resolution of a tenth of Ångstrum

38. Electromigration Technique • A moderate current flow causes the electromigration of metal atoms  metal wires break up at the bottleneck  1-3nm distance • A single molecular transistor is fabricated by this technique: • Si substrate as gate electrode • SiO2 insulating layer • 2 nm spacing by electromigration