Tin Nanoparticles Exhibit Surprising Amorphous Structure and Dynamic Bonding

Tin nanoparticles (n-Sn) are expected to undergo a phase transition between semiconducting α-Sn (with a diamond structure) and metallic β-Sn (with a tetragonal structure) as a function of particle size, similar to the temperature-induced transition that occurs near room temperature. To identify their local structure and extract detailed structural parameters, researchers conducted X-ray absorption fine structure measurements on n-Sn with a diameter of 24.7 Å. The nearest-neighbor atomic distance from these measurements suggested that the local configuration of n-Sn resembles the diamond-like structure of α-Sn.

However, further experimental observations quickly revealed a more complex reality. As lead author Ikemoto notes in the paper, "However, several experimental observations have indicated that n-Sn does not adopt a perfect diamond structure like that ofα-Sn, but rather exhibits an amorphous character." This conclusion was further supported by a decrease in the coordination number, an increase in the static component of the mean-squared relative displacement of the first atomic correlation, and the disappearance of all atomic correlations beyond the nearest-neighbor.

Adding to these structural peculiarities, the bonding within n-Sn also displayed distinctive behavior. The covalent bonds in n-Sn elongate with increasing temperature, in contrast to those in α-Sn which exhibit no temperature dependence. Furthermore, the covalent bond strength of n-Sn was also found to be weaker than that of α-Sn.

These findings collectively highlight that tin nanoparticles are not merely scaled-down versions of bulk tin but possess their own unique structural and bonding dynamics. Understanding these amorphous characteristics, temperature-dependent bond changes, and weaker bond strengths is crucial for predicting and controlling the properties and potential applications of n-Sn in various fields.