Kinetic Exploration of Nanoscale Polymorphs through Interface Energy Adjustment

Area of Science:

• Materials Science
• Solid-State Chemistry
• Nanotechnology

Background:

• Traditional crystal polymorphism studies rely on thermodynamics, averaging properties over time and bulk material.
• Nanocrystal phase behavior is complex and often difficult to study using conventional methods.

Purpose of the Study:

• To introduce and validate a kinetic approach for identifying crystal phases using advanced imaging techniques.
• To investigate the influence of interface energy on the stability and transitions of nanocrystal polymorphs.
• To demonstrate in situ manipulation of phase transitions through controlled size changes.

Main Methods:

• Utilizing millisecond cinematographic electron microscopy for high-speed imaging of crystal dynamics.
• Observing phase transitions in nanocrystals with diameters of a few nanometers.
• Analyzing the impact of interface energy on polymorph stability and phase transformation dynamics.

Main Results:

• Demonstrated a significant effect of interface energy on the relative stability of nanocrystal polymorphs.
• Showcased in situ control over phase transitions by adjusting nanocrystal size.
• Identified a transient, previously unknown B2 polymorph of NaI (sodium iodide) with a 1-second lifetime during sublimation.
• Successfully synthesized the B1 phase of CsCl (cesium chloride) from its liquid phase, a phase previously only observed at high temperatures (749 K).

Conclusions:

• A kinetic, time-resolved imaging approach offers new insights into crystal polymorphism at the nanoscale.
• Interface energy is a critical factor in controlling nanocrystal phase stability and enabling in situ manipulation.
• This method allows for the discovery and characterization of transient polymorphs and the synthesis of phases under novel conditions.