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

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molecular and organic electronics

Molecular and Organic Electronics

Thin Film Lab., University of Tehran

Reference: Nanoelectronics and Information Technology : Advanced Electronic Materials and Novel Devices

By: Rainer Waser

organic molecules hydrocarbons
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
hydrocarbons alkenes
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
hydrocarbons polyenes
Hydrocarbons: Polyenes
  • If there are more than one C=C bond  polyenes
  • Isolated, Conjugated and Cumulated
  • β-Carotene, naturally occurring polyene
hydrocarbons aromatics
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
hydrocarbons alkynes
Hydrocarbons: Alkynes
  • Alkynes: CnH2n-2 with C≡C bonds
  • sp hybridization
  • Linear structure
  • 120 pm long
electronic structure of conjugated systems
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
lcao method
LCAO Method
  • Hjk : resonance integral Sjk : overlap integral
  • Variation principle:
results for buta 1 3 diene
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
results for benzene
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
results for c 60
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
polarized molecules
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
introduction to molecular electronics
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
molecular electronics definition
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
electrodes and contacts
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
electron transport mechanisms in contacts
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
functions
Functions
  • The functions of Molecular Electronics in electronic circuits:
    • Molecular Wires, Insulators and Interconnects
    • Diodes
    • Switches and Storage Elements
    • Three-Terminal Devices
molecular wires insulators and interconnects
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

diodes
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
switches and storage elements
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
switches and storage elements1
Switches and Storage Elements
  • The potential of this switch is investigated by several molecule synthesizes
switches and storage elements2
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
three terminal devices
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
molecular electronic devices first test systems
Molecular Electronic Devices: First Test Systems
  • Scanning Probe Methods
  • Monomolecular Film Devices
  • Nanopore Concept
  • Mechanically Controlled Break Junctions
  • Electromigration Technique
scanning probe methods
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
scanning probe methods1
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
monomolecular film devices
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
cross bar arrays ram and fpga memory
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
nanopore concept
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
mechanically controlled break junctions
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
electromigration technique
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
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