Sonofragmentation of Ultrathin 1D Nanomaterials

A top-down method of NP synthesis that results in high-monodispersity NPs was studied. We first dispersed ultrathin Ge nanowires in dimethylformamide (DMF), and ultrasonicated the suspension with a bench-top ultrasonicator. To track fragmentation of the nanowires, we imaged the ultrasonicated sample at different time points using scanning electron microscopy (SEM) We found that the nanowires readily fragmented into <30 nm particles within 30 minutes of ultrasonication. The particle size further decreased with increasing ultrasonication time. For instance, the majority of the NPs had diameters of <10 nm with 18 h ultrasonication. As comparison, we carried out the same ultrasonication using a non-1D Ge substrate. In contrast to the nanowires, the nanopowder did not show a clear change in particle size with increasing ultrasonication time. For instance, after 18 h of ultrasonication, we observed mostly =100-300 nm particles, comparable to the size distribution of the starting material. We analyzed Ge NPs produced after 18 h of nanowire ultrasonication using transmission electron microscopy (TEM). NPs were resuspended in ethanol, filtered through a 0.2 μm filter to remove large debris and aggregates, and drop-casted and dried on a copper/carbon grid. Analysis of bright-field TEM images shows the NPs had an average size of 3.58 nm and a standard deviation of 0.74 nm, confirming generation of ultrasmall Ge NPs. Furthermore, high-resolution TEM (HRTEM) imaging of a typical Ge NP showed clear lattice fringes, indicating minimal amorphization effect during the long-term ultrasonication.