Growth of rutile GeO₂ nanoneedles supported by in situ microscopy

Rutile GeO₂ nanoneedles were grown using a method involving eutectics of the Ge-GeO₂ system and chemical transport reactions. The synthesis was carried out at 1095–1215 K in a miniature cell, which is evacuated and sealed, enclosing all products that are generated in reactions. In this cell, one can observe the processes throughout all stages of the method in situ via video microscopy. Differences in the process development depending on the heating time and mass ratio of reagents were revealed. Depending on the latter condition, two ways for growing nanoneedles have been proposed. The needles and other reaction products were studied ex situ using SEM, dark-field and bright-field TEM, HRTEM, SAED, EDX and Raman spectroscopy. The nanoneedles are single crystals without any amorphous surface layer. They are rutile GeO₂ and belong to the tetragonal crystal system. Their longitudinal growth direction is 001. The spacings between lattice planes in the longitudinal and lateral directions are found to be 2.8 Å and 3.2 Å respectively. At higher temperatures, larger micrometre-sized crystals of rutile GeO₂ with various morphologies are formed.