Effect of microstructure on spontaneous polarization in amorphous solid water films

Area of Science:

• Surface Science
• Materials Science
• Physical Chemistry

Background:

• Amorphous solid water (ASW) films exhibit negative surface voltages (Vs) upon growth via vapor deposition below 110 K.
• This phenomenon is attributed to the partial alignment of water molecules during the condensation process.

Purpose of the Study:

• To investigate the origins and dependencies of surface voltage in ASW films.
• To understand the role of molecular arrangement and film structure in ASW polarization.

Main Methods:

• Kelvin probe measurements were employed to quantify surface potential.
• ASW films were grown by vapor deposition at varying temperatures and incidence angles.
• Film porosity and annealing effects on surface voltage were analyzed.

Main Results:

• Surface potential magnitude (|Vs|) increased linearly with film thickness, with a decreasing rate at higher deposition temperatures.
• |Vs| diminished with increased deposition temperature, vapor incidence angle, and irreversibly decreased upon annealing around 30 K above deposition temperature.
• Polarization was strongly correlated with film porosity, suggesting the influence of incompletely coordinated water molecules.

Conclusions:

• ASW polarization is primarily governed by dangling, unbalanced dipoles of incompletely coordinated water molecules at internal pore surfaces.
• The anisotropic film-vacuum interface plays a role in weakly aligning these dipoles.
• Observed behaviors of |Vs| align with infrared absorption data from pliable, two-coordinated surface molecules, supporting the proposed mechanism.