Colloidal nanoplatelets are atomically flat, quasi-two-dimensional sheets of semiconductor that can exhibit efficient, spectrally pure fluorescence. Despite intense interest in their properties, the mechanism behind their highly anisotropic shape and precise atomic-scale thickness remains unclear, and even counterintuitive for commonly studied nanoplatelets that arise from isotropic crystal structures (such as zincblende CdSe and lead-halide perovskites). Here we show that an intrinsic instability in growth kinetics can lead to such highly anisotropic shapes. By combining experimental results on the synthesis of CdSe nanoplatelets with theory predicting enhanced growth on narrow surface facets, we develop a model that explains nanoplatelet formation as well as observed dependencies on time and temperature. Based on standard concepts of volume, surface, and edge energies, the resulting growth instability criterion can be directly applied to other crystalline materials. Thus, knowledge of this previously unknown mechanism for controlling shape at the nanoscale can lead to broader libraries of quasi-two-dimensional materials.

Ostwald ripening describes how the size distribution of colloidal particles evolves with time due to thermodynamic driving forces. Typically, small particles shrink and provide material to larger particles, which leads to size defocusing. Semiconductor nanoplatelets, thin quasi-two-dimensional (2D) particles with thicknesses of only a few atomic layers but larger lateral dimensions, offer a unique system to investigate this phenomenon. Experiments show that the distribution of nanoplatelet thicknesses does not defocus during ripening, but instead jumps sequentially from m to (m + 1) monolayers, allowing precise thickness control. We investigate how this counterintuitive process occurs in CdSe nanoplatelets. We develop a microscopic model that treats the kinetics and thermodynamics of attachment and detachment of monomers as a function of their concentration. We then simulate the growth process from nucleation through ripening. For a given thickness, we observe Ostwald ripening in the lateral direction, but none perpendicular. Thicker populations arise instead from nuclei that capture material from thinner nanoplatelets as they dissolve laterally. Optical experiments that attempt to track the thickness and lateral extent of nanoplatelets during ripening appear consistent with these conclusions. Understanding such effects can lead to better synthetic control, enabling further exploration of quasi-2D nanomaterials.

To optimize the optical properties of semiconductor nanoplatelets, simple routes to add high-quality shells are needed. We demonstrate uniform growth of CdS shells on CdSe nanoplatelets at 300 °C, overcoming limitations of previous low-temperature syntheses. We obtain core/shell nanoplatelets with spectrally narrow (20 nm) and efficient emission for shells up to 4 nm thick.

Upconversion is a photon-management process especially suited to water-splitting cells that exploit wide-bandgap photocatalysts. Currently, such catalysts cannot utilize 95% of the available solar photons. We demonstrate here that the energy-conversion yield for a standard photocatalytic water-splitting device can be enhanced under solar irradiance by using a low-power upconversion system that recovers part of the unutilized incident sub-bandgap photons. The upconverter is based on a sensitized triplet-triplet annihilation mechanism (sTTA-UC) obtained in a dye-doped elastomer and boosted by a fluorescent nanocrystal/polymer composite that allows for broadband light harvesting. The complementary and tailored optical properties of these materials enable efficient upconversion at subsolar irradiance, allowing the realization of the first prototype water-splitting cell assisted by solid-state upconversion. In our proof-of concept device the increase of the performance is 3.5%, which grows to 6.3% if concentrated sunlight (10 sun) is used. Our experiments show how the sTTA-UC materials can be successfully implemented in technologically relevant devices while matching the strict requirements of clean-energy production.

Colloidal semiconductor nanoplatelets are rectangular, quasi-two-dimensional crystallites that are only a few atomic layers thick. In the most heavily studied systems (such as CdE with E = S, Se, or Te), the highly anisotropic nanoplatelet shape forms from an isotropic cubic crystal lattice. This has been difficult to reconcile with standard nanocrystal growth models. Previously, we proposed that nanoplatelets arise due to an intrinsic kinetic instability that enhances growth on narrow crystal facets under surface-reaction-limited conditions. Here, we test this model experimentally by synthesizing small "baby" CdS nanoplatelets and performing ripening experiments. During heating, we observe transitions to thicker nanoplatelet populations that are consistent with two-dimensional Ostwald ripening. We then heat CdSe nanoplatelets in the presence of "baby" CdS nanoplatelets to form CdSe-CdS core-crown nanoplatelets. This indicates that individual nanoplatelets dissolve and transfer their material to thicker nanoplatelets, consistent with the kinetic instability model. Finally, we grow thin films of CdS nanoplatelets by combining "baby" CdS nanoplatelets with cadmium carboxylate and heating on a substrate. This shows that Ostwald ripening may also be exploited as a facile and versatile approach to nanoplatelet growth.