Two ultrathin carbon film (10 nm, 400 mesh)–supported copper grids were treated by 25% oxygen/argon plasma for 30 s on a Fischione 1020 plasma cleaner to give hydrophilic surfaces for improved wetting of the EG solution. The fabrication of liquid cells was carried out in a nitrogen-filled glove box (oxygen content less than 10 ppm). A hexagonal superlattice was first formed on one of the treated carbon grids by drop-casting 4 μl of hexane solution (0.5 mg/ml) of PbSe nanocrystals, followed by the evaporation of hexane under 1 bar of nitrogen atmosphere in the glove box for 3 min. Then, 100 nl of the ligand removal solution (either 15 mM solution of EDA in EG or neat EG) was deposited on the other carbon grid. The two carbon grids were then sandwiched together and installed in a TEM holder (all under nitrogen atmosphere) to form a carbon grid liquid cell. The holder was then quickly removed from the glove box and inserted into the TEM for in situ characterization. The mechanism to initiate superlattice transitions is detailed in fig. S1. The Thermo Fisher Scientific (formerly FEI) ThemIS image aberration–corrected TEM at the Molecular Foundry (MF), Lawrence Berkeley National Laboratory (LBNL) and JEOL 2100 TEM at Materials Science Division, LBNL were used for in situ observations. The microscope, operating voltage, and electron beam intensity for each experiment are indicated in the corresponding movie captions.

Ex situ experiments of the superlattice phase transition were started from the hexagonal superlattices prepared by the same drop-casting method for forming in situ liquid cells. The hexagonal superlattices on a carbon grid were then immersed in 15 mM solution of EDA in EG or neat EG for 1 min to form the nanocrystal chains or the square superlattice, respectively. Ex situ HAADF-STEM characterization of PbSe nanocrystal superlattices before and after ligand displacement was performed using the probe aberration–corrected Thermo Fisher Scientific (formerly FEI) Titan 80-300 named TEAM I (MF, LBNL). High-resolution TEM imaging and EDS were performed on image aberration–corrected Thermo Fisher Scientific ThemIS (MF, LBNL).

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