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Abstract There are only a few examples of nanocrystal synthesis with thallium (Tl). Here, we report the synthesis of uniform, ligand‐stabilized colloidal nanocrystals of TlBr and Tl₂AgBr₃nanocrystals with average diameter ranging between 10 and 20 nm. TlBr nanocrystals are made by hot injection of trimethylsilyl bromide (TMSBr) into solutions of oleylamine, oleic acid and octadecene with thallium (III) or thallium (I) acetate. Tl₂AgBr₃nanocrystals form when silver (I) acetate is included in the reaction. The TlBr nanocrystals have CsCl crystal structure with a direct band gap of 3.1 eV. The Tl₂AgBr₃nanocrystals have trigonal dolomite crystal structure with an indirect band gap of 3.1 eV. The TlBr nanocrystals made with thallium (III) were sufficiently uniform to assemble into face‐centered cubic (fcc) superlattices. 

Abstract A goal in the field of nanoscale optics is the fabrication of nanostructures with strong directional light scattering at visible frequencies. Here, the synthesis of Mie‐resonant core–shell particles with overlapping electric and magnetic dipole resonances in the visible spectrum is demonstrated. The core consists of silicon surrounded by a lower index silicon oxynitride (SiO_xN_y) shell of an adjustable thickness. Optical spectroscopies coupled to Mie theory calculations give the first experimental evidence that the relative position and intensity of the magnetic and electric dipole resonances are tuned by changing the core–shell architecture. Specifically, coating a high‐index particle with a low‐index shell coalesces the dipoles, while maintaining a high scattering efficiency, thus generating broadband forward scattering. This synthetic strategy opens a route toward metamaterial fabrication with unprecedented control over visible light manipulation. 

Abstract The homogeneous exciton linewidth, which captures the coherent quantum dynamics of an excitonic state, is a vital parameter in exploring light–matter interactions in 2D transition metal dichalcogenides (TMDs). An efficient control of the exciton linewidth is of great significance, and in particular of its intrinsic linewidth, which determines the minimum timescale for the coherent manipulation of excitons. However, such a control is rarely achieved in TMDs at room temperature (RT). While the intrinsic A exciton linewidth is down to 7 meV in monolayer WS₂, the reported RT linewidth is typically a few tens of meV due to inevitable homogeneous and inhomogeneous broadening effects. Here, it is shown that a 7.18 meV near‐intrinsic linewidth can be observed at RT when monolayer WS₂is coupled with a moderate‐refractive‐index hydrogenated silicon nanosphere in water. By boosting the dynamic competition between exciton and trion decay channels in WS₂through the nanosphere‐supported Mie resonances, the coherent linewidth can be tuned from 35 down to 7.18 meV. Such modulation of exciton linewidth and its associated mechanism are robust even in presence of defects, easing the sample quality requirement and providing new opportunities for TMD‐based nanophotonics and optoelectronics. 

Abstract Coupling emitters with nanoresonators is an effective strategy to control light emission at the subwavelength scale with high efficiency. Low‐loss dielectric nanoantennas hold particular promise for this purpose, owing to their strong Mie resonances. Herein, a highly miniaturized platform is explored for the control of emission based on individual subwavelength Si nanospheres (SiNSs) to modulate the directional excitation and exciton emission of 2D transition metal dichalcogenides (2D TMDs). A modified Mie theory for dipole–sphere hybrid systems is derived to instruct the optimal design for desirable modulation performance. Controllable forward‐to‐backward intensity ratios are experimentally validated in 532 nm laser excitation and 635 nm exciton emission from a monolayer WS₂. Versatile light emission control is achieved for different emitters and excitation wavelengths, benefiting from the facile size control and isotropic shape of SiNSs. Simultaneous modulation of excitation and emission via a single SiNS at visible wavelengths significantly improves the efficiency and directionality of TMD exciton emission and leads to the potential of multifunctional integrated photonics. Overall, the work opens promising opportunities for nanophotonics and polaritonic systems, enabling efficient manipulation, enhancement, and reconfigurability of light–matter interactions.