Adsorption-induced intramolecular dipole: correlating molecular conformation and interface electronic structure

Area of Science:

• Surface Science
• Organic Electronics
• Materials Chemistry

Background:

• Understanding organic molecule-metal interfaces is crucial for organic electronic devices.
• Pentacene (PEN) and perfluoropentacene (PFP) are key organic semiconductors with differing electronic properties.
• The relationship between molecular structure, ionization energy, and charge injection barriers at interfaces requires detailed investigation.

Purpose of the Study:

• To investigate the interfacial structures and electronic properties of pentacene (PEN) and perfluoropentacene (PFP) on Cu(111).
• To elucidate the factors governing hole injection barriers at these organic-metal interfaces.
• To assess the applicability of tuning ionization energies for controlling charge injection barriers.

Main Methods:

• Utilized photoelectron spectroscopy, X-ray standing wave (XSW), and scanning tunneling microscopy.
• Performed theoretical modeling to complement experimental observations.
• Analyzed carbon bonding distances, molecular conformation, and intramolecular dipoles.

Main Results:

• Observed significantly different average carbon bonding distances for PEN (2.34 Å) and PFP (2.98 Å) on Cu(111).
• XSW data indicated an adsorption-induced nonplanar conformation for PFP, creating an intramolecular dipole (approx. 0.5 D).
• Despite differing ionization energies (PEN: 5.00 eV, PFP: 5.85 eV), comparable hole injection barriers were measured (PEN: 1.10 eV, PFP: 1.35 eV).

Conclusions:

• The nonplanar conformation and induced dipole of PFP significantly influence its interfacial behavior.
• Comparable hole injection barriers arise from a complex interplay of factors beyond just molecular ionization energy.
• The hypothesis of readily tuning charge injection barriers by adjusting molecular ionization energy at organic/metal interfaces is not universally applicable.