Chemical transformations reactions, such as the cation exchange reaction, have been one of the most exciting means of studying and post-synthetically modifying nanocrystals. These reactions can create atomic arrangements that are impossible to reach in bulk materials due to kinetic limitations, and these reactions are believed to enhance diffusion beyond bulk-derived limits. In this talk I’ll first discuss cation exchange induced accelerated diffusion and then cation exchange to form a dual-interfaced heteroepitaxy.

The phenomenology of solid-state transformations and diffusion at short length scales remains poorly understood but is increasingly important for nanostructured devices that utilize “nano properties”. Using in-situ synchrotron x‑ray diffraction (XRD), we directly interrogate the structure and reaction kinetics of lead sulfide (PbS) nanocrystals transforming into cadmium sulfide (CdS) through cation exchange. The epitaxial relationship of zincblende CdS to rocksalt PbS breaks the overall symmetry of the core-shell nanocrystal without requiring the loss of unit cell symmetry, leading to anomalous peak shifts in the diffraction pattern. The magnitude of the interdiffusion coefficient, D̃, is larger by four orders of magnitude or more compared to the slowest diffusing species in our system (self-diffusion of Cd in CdS). This surprising result suggests interdiffusion is enhanced in nanocrystals. These results illustrate that the distinction between chemical diffusion in a potential gradient and diffusion at thermodynamic equilibrium has not been fully appreciated.

Additionally, through a cation exchange reaction on copper sulfide nanoparticles we have created dual interface Cu_2-xS-ZnS heterostructures, with a metastable Cu_2-xS layer. The copper sulfide phase region can be tuned to form two-dimensional (2D), single atomic layers (<1 nm). As the nanoparticles transform, we observe a solid-solid phase transformation of the copper sulfide phase from the initial low-copper phase Cu_1.8S into a higher copper phase djurleite (Cu_1.94S), but as the epitaxial strain increases a second phase transformation back to roxbyite Cu_1.8S occurs to minimize strain energy. This work demonstrates novel routes to metastable phases through strain stabilization. The copper sulfide can be etched with phosphines in oxidizing conditions. Importantly, this etching reaction is capable of removing Cu_2-xS from Cu_2-xS-ZnS epitaxial heterostructures with perfect selectivity, that is, the phosphines completely remove the Cu_2-xS without disturbing the ZnS. The etching reaction is preceded by abstraction of sulfur from the particles, destabilizing the Cu_1.81S roxbyite phase.