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a 3D framework. The A-site cations occupy the cavity within the framework. Metal halide perovskites have a relatively soft lattice and a dynamically disordered crystal structure which results in tunable charge-carrier recombination rates and other nonclassical semiconductor characteristics. [1] Metal halide perovskites have been extensively studied for solar energy conversion over the past decade due to their intriguing optoelectronic properties, including near-perfect crystalline structures, [2] tunable direct bandgaps, [3] large absorption coefficient (1.5 × 10 4 cm −1 at 550 nm), [4] high ambipolar mobility (≈20 cm 2 V −1 s −1 ), [5] long carrier diffusion lengths (100-1000 nm; ≈175 µm in MAPbI 3 single crystals) (L eff,e /L eff,h < 1), [6] small exciton binding energy (≈30 meV), [7] high defect tolerance, [8] solution processability, and low processing cost. [9] Notably, the certified power conversion efficiency (PCE) of single-junction perovskite solar cells has rapidly increased from 3.8% in 2009 to 25.2% in 2019. [10] It is also notable that charge carriers within metal halide perovskites can undergo radiative recombination to emit light, making them promising candidates as next-generation light sources for light-emitting diodes (LEDs) [11] and lasers. [12] Bulk perovskite, however, exhibits limited photoluminescence quantum yield (PLQY) due to the presence of mobile ionic defects and small exciton binding energy. [13] In contrast, perovskite nanocrystals (PNCs) possess strong quantum confinement and display improved optoelectronic properties from their bulk counterparts. [14] Notably, metal halide PNCs possess high luminescence, narrow full width at half maximum, and a composition-and size-dependent bandgap. [15] The photoluminescence (PL) of metal halide PNCs can be readily tuned from ultraviolet (UV) to near-infrared wavelengths by simply tailoring their composition or altering their size relative to their Bohr diameters.Hot-injection and ligand-assisted reprecipitation methods represent the two most-developed colloidal synthesis approaches of metal halide PNCs. [13,16] The use of organic capping ligands in synthesis enables nanoscale growth of metal halide perovskite crystals and actively passivates their surface defects, in a manner similar to that of conventional NC Metal halide perovskite nanocrystals (PNCs) have recently garnered tremendous research interest due to their unique optoelectronic properties and promising applications in photovoltaics and optoelectronics. Metal halide PNCs can be combined with polymers to create nanocomposites that carry an array of advantageous characteristics. The polymer matrix can bestow stability, stretchability, and solution-processability while the PNCs maintain their size-, shape-and composition-dependent optoelectronic properties. As such, these nanocomposites possess great promise for next-generation displays, lighting, sensing, biomedical technologies, and energy conversion. The recent advances in metal halide PNC/polymer nanocomposites are summarized here. F...

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Active antibacterial food packaging represents one of the most rapidly developing techniques in the food industry. The ability to inhibit, inactivate, kill, or maintain a low level of microbials in food products or food packages is crucial to reducing global foodborne diseases. Notably, metallic silver (Ag) and semiconducting titanium dioxide (TiO2) nanomaterials as well as their nanocomposites have been widely studied for food packaging because of their intrinsic biocidal and photocatalytic properties, respectively. However, they have inherent limitations associated with potential nanotoxicity of Ag nanoparticles and the need for ultraviolet (UV) irradiation for wide-bandgap TiO2 nanomaterials to invoke photocatalytic oxidation of microbials. In this context, upconverting nanocrystals (UCNCs) convert near infrared photons into visible and UV photons that could excite TiO2 without the use of a UV source. Herein, we designed ternary multifunctional nanocomposites composed of plasmonic Ag, photocatalytic TiO2, and upconverting NaYF4@Yb:Tm NPs as highly efficient biocidal nanomaterials under ambient light. The incorporation of UCNCs into the nanocomposites is found to be the key to enhanced antibacterial performance. Compared to Ag NP and Ag/TiO2 nanocomposite counterparts, the ternary Ag/TiO2/NaYF4@Yb:Tm nanocomposites yield superior antibacterial performance against Escherichia coli under both ambient light and solar simulator irradiation with UV photons filtered, when either applied alone or mixed with cellulose nanofibrils and then processed into films. Such multicomponent nanocomposites have great potential for use in food packaging as they reduce the amount of potentially toxic Ag used and are able to function without UV.