7.4 Applications of SAMs in molecular electronics

Self-assembled monolayers (SAMs) are game-changers in molecular electronics. They form the basis for molecular junctions, wires, switches, and diodes, enabling the study of charge transport through individual molecules and creating nanoscale electronic components.

SAMs also shine in sensing applications and organic electronics. They're used to make chemical and biological sensors, protect surfaces, improve organic transistors, and enable nanoscale patterning. These applications showcase SAMs' versatility in creating functional molecular-scale devices.

Molecular Devices

Molecular Junctions and Wires

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• Molecular junctions formed by sandwiching a single molecule or a molecular monolayer between two electrodes
  • Enable the study of charge transport through individual molecules (benzene, alkanethiols)
• Molecular wires act as conductive pathways for electron transport
  • Typically consist of conjugated organic molecules with delocalized π-electron systems (polyacetylene, polyphenylene)
  • Exhibit high electrical conductivity and low resistivity compared to conventional wires

Molecular Switches and Diodes

• Molecular switches change their conductivity or other properties in response to external stimuli
  • Stimuli can be electrical, optical, magnetic, or chemical (pH, light, electric field)
  • Examples include photochromic molecules (spiropyrans, diarylethenes) and redox-active molecules (tetrathiafulvalene, ferrocene)
• Molecular diodes exhibit asymmetric current-voltage characteristics, allowing current to flow preferentially in one direction
  • Rely on the presence of electron-donating and electron-accepting groups within the molecule (rectification ratio)
  • Examples include donor-acceptor systems (phthalocyanine-perylene diimide) and asymmetric molecules (Tour wires)

Sensing Applications

Chemical and Biological Sensors

• Chemical sensors detect the presence and concentration of specific chemical species
  • SAMs can be functionalized with receptors or probe molecules that selectively bind to target analytes (metal ions, gases)
  • Binding events cause changes in the electrical, optical, or mechanical properties of the SAM (conductivity, fluorescence, mass)
• Biosensors utilize biological recognition elements (enzymes, antibodies, DNA) immobilized on SAMs to detect biological molecules
  • SAMs provide a stable and biocompatible interface for the attachment of biomolecules (gold-thiol, silane chemistry)
  • Applications include disease diagnosis, drug screening, and environmental monitoring (glucose sensors, DNA sensors)

Surface Passivation

• SAMs can passivate and protect surfaces from corrosion, oxidation, and contamination
  • Form a compact and hydrophobic barrier that prevents the penetration of unwanted species (water, oxygen, ions)
  • Commonly used in microelectronics and medical devices to improve stability and biocompatibility (silicon wafers, stainless steel implants)

Organic Electronics

Organic Field-Effect Transistors (OFETs)

• OFETs use organic semiconductors as the active layer in transistor devices
  • SAMs can modify the surface properties of the dielectric layer to improve charge carrier mobility and device performance (pentacene, rubrene)
  • SAMs can also serve as the dielectric layer itself, providing a thin and uniform insulating layer (alkylphosphonic acids on aluminum oxide)
• OFETs find applications in flexible electronics, displays, and sensors (organic light-emitting diodes, electronic paper)

Nanopatterning with SAMs

• SAMs can be used as resist layers for nanoscale patterning and lithography
  • Patterned SAMs can direct the selective deposition or etching of materials (metals, semiconductors)
  • Techniques include microcontact printing, dip-pen nanolithography, and electron-beam lithography (patterned protein arrays, nanowire arrays)
• SAM-based nanopatterning enables the fabrication of complex nanostructures and devices with high resolution and precision (sub-100 nm features)