Ultrafast Cation Exchange in Supra-Quantum Dots through Nanoporous Internal Structure

Cation exchange (CE) can convert the chemical composition of nanocrystals while it preserves their size, shape, and crystal phase. Here, we report a CE reaction in a porous nanostructure of supra-quantum dots (SQDs), which are threedimensional stepwise self-assembly of quantum dots (QDs) with several tens of nm size. It shows ultrafast and complete CE reactions in the SQDs from CdSe to Cu2-xSe or Ag2Se, conserving their size and shape. The complete suppression of the 1S excitonic peak of CdSe SQD and the complete conversion of their crystal structure and chemical composition dictate the complete CE reaction in SQDs even if it has near 100 nm diameter size. The conservation of size and shape after CE reactions reveals the existence of the internal void in porous SQDs which could compensate for the expected shrinkage or expansion due to the lattice constant change before and after CE reaction. The CE reaction rate of SQDs is estimated using temporal absorbance spectra in the course of CE reaction. The CE reaction rate of SQDs showed size-independent dynamics among the SQDs with various sizes from 63 to 83 nm. Their CE reaction rate was around 0.057 s-1 which is comparable to that of 4.8 nm sized QDs. For QDs, the reaction rate was critically size-dependent, showing slower CE reaction rate as their size increases. On the other hand, SQDs showed the CE reaction rate similar to those observed by QDs of 4-5 nm in size, which is phenomenal considering the size of SQDs at least 13 times larger than the QDs. Fully accessible external cations into the porous internal structure of SQDs and their direct interaction with the internal surface of SQDs can accelerate the CE reaction. The comparable primary unit size of SQDs, similar to 4 nm, to the size of QDs explains the ultrafast and size-independent CE reaction rate of SQDs.