Short-Wave Infrared Quantum Dots with High Quantum Yield Through Mercury Cation Exchange

Colloidal semiconductor nanocrystals (NCs) are photonic materials with a combination of unique properties such as size-tunable electronic/optical energy levels, high absorption cross-section, high photoluminescence quantum yield (PLQY), long-term photochemical/photophysical stability, and versatile solution-based processability. NCs with optical transitions in the infrared are currently being developed to reduce the cost of infrared optoelectronic devices, which are currently limited to a few high-tech specialized areas, including defense and astronomy. Among the candidate NC materials, mercury chalcogenide (HgE) and mercury cadmium chalcogenide (Hg_xCd_1-xE) are leading candidates for infrared optics. In comparison with alternative infrared NCs (<i>e.g</i>., InAs and lead chalcogenides), these materials provide extremely wide spectral tunability across the near, mid, and far infrared spectra. Nevertheless, challenges remain in their advancement due to the low reduction potential of mercury ions that limit the high-temperature solvothermal synthesis and difficulties in the precise selection of mercury-to-cadmium ratios in alloys.<br/>The cation exchange (CE) reaction can create new functional NCs by replacing one type of cation with another. The product materials which often cannot be synthesized de novo, can inherit the structural integrities of the parent nanomaterials in terms of size, shape, crystal phase, and homogeneity. However, due to the diffusion-limited CE reaction mechanism, achieving fine control of the radial composition of host and guest cations has been challenging. In addition, for some material systems, the slow diffusivity of cations significantly impedes the CE reaction and requires a high reaction temperature, resulting in a loss of structural homogeneity. In this research work, we developed a new CE method that can bypass the diffusion-limited reaction pathways. In particular, we developed a CE conversion of CdSe NCs to HgSe NCs and evaluated the reaction mechanism. Through the improved reaction, Hg_xCd_1-xSe NCs exhibit a homogeneous radial distribution of cation species and enhanced overlap of electron and hole wavefunctions, as well as improved PL properties tunable across the short-wave infrared spectrum (~1000–1700 nm wavelength). After shell overcoating, these Hg_xCd_1-xSe/CdS/ZnS NCs exhibited excellent optical properties: the PLQY was above 85% and photoluminescence full-width-at-half-maximum (FWHM) was below 110 meV, properties matching those of the parental CdSe-based core/shell NC emitters.