Melting and solidification study of indium and bismuth nanocrystals using reflection high-energy electron diffraction

As technology begins to utilize nanocrystals for many chemical, biological, medical, electrical, and optoelectrical applications, there is a growing need for an understanding of their fundamental properties. The study of melting and solidification of nanocrystals is of interest to fundamental understanding of the effect of reduced size and crystal shape on the solid-liquid phase transition. Melting and solidification of as-deposited and recrystallized indium and bismuth nanocrystals were studied using reflection high-energy electron diffraction (RHEED). The nanocrystals were thermally deposited on highly oriented 002-graphite substrate at different deposition temperatures. The growth dynamics of the nanocrystals was studied using in situ RHEED while the morphology and size distributions were studied using ex situ real image technique (atomic force microscopy (AFM) or scanning electron microscopy (SEM)). RHEED observation during deposition showed that 3D nanocrystals of indium are directly formed from the vapor phase within the investigated temperature range, 300 K up to 25 K below the bulk melting point of indium. On the other hand, bismuth condensed in the form of supercooled liquid droplets at temperatures above its maximum supercooling point, 125 K below the bulk melting point of bismuth. Below the maximum supercooling point, bismuth condensed in the solid phase. Post deposition real images showed that the formed nanocrystals have morphologies and size distributions that depend on the deposition temperature, heat treatment, and the amount of the deposited material. As-deposited nanocrystals are found to have different shapes and sizes, while those recrystallized from melt were formed in similar shapes but different sizes.; The change in the RHEED pattern with temperature was used to probe the melting and solidification of the nanocrystals. Melting started early before the bulk melting point and extended over a temperature range that depends on the size distribution of the nanocrystals. Nanocrystals at the lower part of the distribution melt early at lower temperatures. With the increase in temperature, more nanocrystals completely melt with the thickness of the liquid shell on the remaining crystals continuing to grow. Due to size increase after melting, recrystallized bismuth nanocrystals showed a melting range at temperatures higher than that of as-deposited. However, recrystallized indium nanocrystals showed an end melting point nearly equal to that of-the recrystallized ones except for the 1.5-ML film which showed an end melting point ∼10 K higher than that of as-deposited. (Abstract shortened by UMI.)...