Kinetic Trapping of Charge-Transfer Molecules at Metal Interfaces.

Area of Science:

• Surface Science
• Materials Science
• Physical Chemistry

Background:

• Conjugated organic molecules commonly adsorb flat on metal surfaces.
• Recent studies indicate coverage-dependent transitions to upright-standing phases with distinct properties.
• Thermodynamic stability of upright phases is often underestimated.

Purpose of the Study:

• Investigate the energetic and kinetic factors governing organic molecule adsorption phases on metals.
• Determine the conditions under which upright-standing phases become experimentally observable.
• Clarify the role of molecular properties versus growth conditions in phase transitions.

Main Methods:

• First-principles kinetic Monte Carlo simulations to model adsorption dynamics.
• Experimental validation using infrared and X-ray photoemission spectroscopy.
• Analysis of interface energetics and growth conditions.

Main Results:

• Lower molecular density structures are kinetically favored during initial adsorption, following Ostwald's rule.
• Phase transitions to upright-standing phases are often kinetically hindered under typical surface science conditions.
• Experimental adsorption of tetracyanoethylene on Cu(111) confirms simulation findings.
• Phase transition time is primarily dictated by deposition rate, independent of molecular identity.

Conclusions:

• Upright-standing organic phases are thermodynamically stable but kinetically inaccessible under many experimental conditions.
• Kinetic barriers, rather than thermodynamic favorability, explain the rarity of observing upright phases.
• Controlling deposition rates is crucial for potentially accessing thermodynamically stable upright-standing molecular phases on metal surfaces.