Molecular and organic electronics
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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

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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


Non hexagonal aromatic systems

Non-Hexagonal Aromatic Systems


Heterocyclic systems

Heterocyclic systems


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:


Lcao secular determinant

LCAO: Secular Determinant


H ckel approximations

Hückel Approximations


Hmo calculation for ethene

HMO Calculation for Ethene


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


Energy levels of conjugated hydrocarbons

Energy Levels of Conjugated Hydrocarbons


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


Functional groups

Functional Groups


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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