September 28, 2021
Observing the Etching Pathways of Faceted Semiconductor Nanocrystals with Liquid Cell Transmission Electron Microscopy
Precisely shaping the structure of semiconductor nanocrystal building blocks is crucial for realizing their full potential in applications such as solar concentrators, quantum dot displays and biological imaging. Etching and growth processes have been harnessed to fabricate delicate structures from bulk semiconductors, yet we are still exploring the transformation principles of tiny colloidal nanostructures. To provide mechanistic insights, we seek to directly record the reaction trajectories of individual nanocrystals in the native reaction environment using liquid cell transmission electron microscopy (LCTEM). The ultrathin liquid pocket containing the nanocrystals is encapsulated between two thin carbon layers, allowing the imaging electron beam to pass through without significant losses.
Past research efforts in the Alivisatos group have demonstrated the in situ observation of growth and etching pathways of metal nanocrystals. We have recently succeeded in extending this technique to capture the elusive etching trajectories of semiconductor nanocrystals with lower imaging contrast. PbSe and CdSe are prototypical groups IV-VI and II-VI semiconductors, respectively. The trajectories of isotropic PbSe nanocubes (rocksalt) selectively retain the cubic shape, which is consistent with the lower surface energy of (100) facets and a layer-by-layer etching mechanism. In contrast, the anisotropic CdSe nanorods and nanodisks (wurtzite) have facets exposing Se-rich and Cd-rich surfaces with different etching reactivity. Trajectories show that the selective etching along the less stable Se-rich facets causes the nanocrystals to change aspect ratio and even develop crater-like dents in them. When etched rapidly under high dosage of electrons, the reaction selectivity over specific facets is reduced or lost for both the PbSe and CdSe nanocrystals. In consistency with the growth mechanism learned from copious practice of nanocrystal synthesis, the microscopic trajectories observed here confirm that the etching mechanism of PbSe and CdSe nanocrystals under selective conditions can also be governed by the surface energies of crystal facets.
Past research efforts in the Alivisatos group have demonstrated the in situ observation of growth and etching pathways of metal nanocrystals. We have recently succeeded in extending this technique to capture the elusive etching trajectories of semiconductor nanocrystals with lower imaging contrast. PbSe and CdSe are prototypical groups IV-VI and II-VI semiconductors, respectively. The trajectories of isotropic PbSe nanocubes (rocksalt) selectively retain the cubic shape, which is consistent with the lower surface energy of (100) facets and a layer-by-layer etching mechanism. In contrast, the anisotropic CdSe nanorods and nanodisks (wurtzite) have facets exposing Se-rich and Cd-rich surfaces with different etching reactivity. Trajectories show that the selective etching along the less stable Se-rich facets causes the nanocrystals to change aspect ratio and even develop crater-like dents in them. When etched rapidly under high dosage of electrons, the reaction selectivity over specific facets is reduced or lost for both the PbSe and CdSe nanocrystals. In consistency with the growth mechanism learned from copious practice of nanocrystal synthesis, the microscopic trajectories observed here confirm that the etching mechanism of PbSe and CdSe nanocrystals under selective conditions can also be governed by the surface energies of crystal facets.
Chang obtained his PhD degree in chemistry from Stanford University with works focused on ultrafast nonlinear infrared spectroscopy and molecular dynamics in condensed phase. As a postdoctoral researcher in the Alivisatos group, he investigated charge transfer dynamics between quantum dots and molecular ligands under multi-excitonic conditions, in close collaboration with theory experts in the Rabani group. Besides investigating dynamics with time-resolved spectroscopy, he also enjoys learning the powerful tool of in situ electron microscopy by imaging the transformation trajectories of semiconductor nanocrystals.