Disorder-derived, strong tunneling attenuation in bis-phosphonate monolayers.

Area of Science:

• Materials Science
• Surface Chemistry
• Organic Electronics

Background:

• Self-assembled monolayers (SAMs) are crucial for modifying surface properties.
• Understanding the structure-property relationships of phosphonic acid-based SAMs is vital for device applications.

Purpose of the Study:

• To investigate the structural and electrical properties of alkyl bisphosphonic acid (bisPA) monolayers on aluminum oxide (AlO(x)).
• To compare bisPA SAMs with alkyl monophosphonic acid (monoPA) SAMs.
• To explore the potential applications of bisPA SAMs in organic thin film devices.

Main Methods:

• Solution-based growth of bisPA and monoPA monolayers.
• Characterization using contact angle (CA), Kelvin-probe (KP), ellipsometry, infrared (IR), and X-ray photoelectron spectroscopy (XPS).
• Electrical transport measurements using current-voltage (J-V) with a hanging Hg drop top contact.

Main Results:

• BisPA monolayers are more disordered than monoPA monolayers.
• Tunneling is the dominant charge transport mechanism in both types of SAMs.
• BisPA SAMs exhibit a significantly higher tunneling decay constant (β=1.40±0.05 per carbon atom) compared to monoPA SAMs (β=0.85±0.03 per carbon atom).
• Disorder in bisPA monolayers, attributed to hydrogen bonding between distal phosphonic acid groups, enhances 'through-space' tunneling.

Conclusions:

• Alkyl bisphosphonic acids form conformationally disordered monolayers on AlO(x) surfaces.
• The observed enhanced tunneling in bisPA SAMs is linked to their molecular structure and interactions.
• BisPA SAMs show promise for gate dielectric modification in organic thin film devices due to their effective attenuation of tunnel currents.