Molecular Electronics
WHY ORGANIC MOLECULES?
• Small (nanosize)
• Tailor-made properties by synthesis of new
molecules
• large number of exact copies: control over
properties (e.g. exactly the same energy levels)
• low-cost
• complex structures built up by self-assembly
• and more…
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Classification, examples
• Bulk molecular electronics: organic compounds, e.g.
Polymer films, liquid crystal displays, organic lightemitting diode (OLED) displays, plastic transistors
• Single molecular systems:
Hybrid molecular electronics (HME): devices made of
single molecules attached to inorganic electrodes (hot
research topic)
Mono-molecular electronic devices (MME): complex
device structures built up by individual molecules (vision)
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Covalent Carbon bonds
p-orbitals
• 4 valence electrons, ground state 1s22s22p2
• hybridization: promotion to 2s2p3 ,
formation of linear combinations of the sorbital and the px- , py- , pz-orbitals, to form
-bonds
-bonds:
• Very flexible bonding, basis of all organic
molecules
Hybridization and ability to bond”sideways”
• sp3: one s-, three p-orbitals. Tetrahedral makes carbon bonds very flexible, allows
formation of complex structures
geometry (3-dim), example: diamond
Conjugated molecules: delocalized 2
• sp : one s-, two p-orbitals. Planar,
orbitals
hexagonal structure, example: graphite. One
p-orbital makes -bond
• sp: one s- and one p-orbital: linear bond.
Two p-orbitals make two -bonds
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Other bonds
• Van der Waals bonding: weak bonds, important for selfassembly
• Hydrogen bonds: ex. C - H - C , O - H - C
Contacts to electrodes:
• The molecule-electrode bonds determine charge transport
properties
• covalent bonds, strong wave function overlap => low
resistance. Example: S - Au, (thiol) used for many selfassembled monolayers
• weak van der Waals-bond => tunneling transport, high
resistance
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Single-molecule: HME and MME
Building block issues: conduction, mechanical rigidity
Polyene
Alkanes
Polythiophene
Adamantyl
Polyphenylenevinylene
Biphenyl
Polyphenyleneethynylene
Thiophenylsubstituted benzene
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Trans-acetyleneplatinum (II)
Thiophenylsubstituted benzene
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Diodes
Aviram and Ratner 1974:
Experimentally realised diodes, in
ordered molecular films
Suggested a theoretical molecular
diode
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Switches and storage elements
Switches can be classified by:
- the stimulus that triggers the switch
- the property or function that is
switched
Light-triggered switch (M. Irie)
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Catenate:
supramolecular device consisting of two interlocked rings
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Three-terminal devices
Molecules with 3 terminals? Unlikely
Instead two terminals and isolated gate
Model of potential from
buried gate
Different approach: third terminal by
”squeezing” the C60 with the tip
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Example from Chalmers: Kubatkin et al. Nature, 2003
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STM studies of single molecules
Stabilizing a single molecular wire by
embedding in alkanethiol SAM
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Monomolecular devices:
STM lithography (Reed and Tour)
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Monomolecular nanopore device: example
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Monomolecular films with crossbar arrays:
Connecting storage elements, logic structures
Optical
microscope
SEM image of a crossbar array
SEM
AFM
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Break junctions
Mechanically controlled breakage of thin
conductor, gap controlled by piezoelectric
rod
Advantages: same contact at both ends
Figure 10.5. (a) I-V
characteristics for
Benzene-1,4-dithiol
between gold electrodes.
(b) Schematic of the
molecule in contact with
gold electrodes,
represented here as three
gold atoms.
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Example of break junction experiments:
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Current transport
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Delocalized -system => Intramolecular ballistic transport
Strongly varying electronic levels depends upon molecular structure, bias
Quantized conductance G = 2e2/h for
each current mode, for ideal contacts
(rarely observed)
Discrete HOMO and LUMO levels =>
resonant tunneling => negative
dfferential resistance (NDR)
Poor overlap of -orbitals with
electrode wave functions lead to high
tunneling barrier, high resistance
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