Energetic and entropic contributions to surface diffusion and epitaxial growth

Area of Science:

• Surface Science
• Materials Science
• Physical Chemistry

Background:

• Understanding surface diffusion is crucial for controlling thin film growth.
• Palladium (Pd) and its (111) surface are model systems for studying surface dynamics.
• The role of electronic surface states and entropic effects in diffusion is an active research area.

Purpose of the Study:

• To investigate the surface self-diffusion and step-down diffusion barriers on Pd(111).
• To explore the influence of free-electron-like surface states on diffusion.
• To examine the effect of oxygen preadsorption on diffusion and growth kinetics.

Main Methods:

• Experimental measurement of surface diffusion barriers using techniques like Scanning Tunneling Microscopy (STM) or specialized surface diffusion measurements.
• Calculation of diffusion energies and preexponential factors.
• Analysis of film growth modes (e.g., layer-by-layer vs. island growth).

Main Results:

• An exceptionally high surface self-diffusion barrier of 350 meV was measured for Pd/Pd(111).
• A negative additional energy (DeltaE = -53 meV) for step-down diffusion was observed, consistent with surface state contributions.
• Despite the negative DeltaE, layer-by-layer growth was not observed due to a low preexponential factor for step-down diffusion.
• Oxygen preadsorption increased DeltaE but led to flatter films, again attributed to prefactor effects.

Conclusions:

• Electronic surface states play a significant role in surface diffusion on Pd(111).
• Entropic effects, particularly the preexponential factors, are critical in determining film growth modes.
• The interplay between energetic and entropic factors governs the complex behavior of surface diffusion and growth.