The introduction of dopants plays a key role in the physical properties of semiconductors for optoelectronic applications. However, doping is generally challenging for nanocrystals (NCs), especially for two-dimensional (2D) NCs, due to the self-annealing effect and high surface energies required for dopant addition. Here, we report an efficient doping strategy for Mn-doped 2D CsPbCl3 (i.e., Mn:CsPbCl3) nanoplatelets (NPLs) through a postsynthetic solvothermal process. While the original lightly doped 2D Mn:CsPbCl3 NPLs were obtained from growth doping, higher Mn doping efficiencies were achieved through diffusion doping under pressure-mediated solvothermal conditions, resulting in enhanced Mn photoluminescence (PL). Surprisingly, a new CsMnCl3 phase with complete dopant substitution by spinodal decomposition was observed with extended solvothermal treatment, which is confirmed by powder X-ray diffraction, X-ray absorption  fine structure, and electron paramagnetic resonance. Compared with Mn:CsPbCl3 NPLs, the pure CsMnCl3 NPLs give rise to shorter Mn PL lifetime, which is consistent with the short Mn–Mn distance within CsMnCl3 NPLs. This work provides an efficient strategy for doping inside NCs as well as new insights on the dopant concentration-dependent structural and optical properties of perovskite NCs.

The ability to dope transition-metal ions into semiconductor nanocrystals (NCs) allows for the introduction and exploitation of new extrinsic properties in the original intrinsic material. Although the synthesis of doped zero-dimensional quantum dots and one-dimensional nanorods/nanowires has been widely reported, transition-metal ion-doped two-dimensional (2D) NCs have been less explored. In this study, we developed a one-pot synthesis of Mn 2+ -doped 2D CdS (i.e., Mn:CdS) nanoplatelets (NPLs). Successful Mn doping inside the CdS NPL lattice was confirmed by electron paramagnetic resonance and X-ray diffraction measurements. Surprisingly, only CdS photoluminescence (PL), without contribution from Mn PL, was observed in the Mn:CdS NPLs, regardless of Mn doping concentration. To address the issue of poor thermal stability and improve the optical properties of the 2D Mn:CdS NPLs, we synthesized ZnS shell-passivated Mn:CdS/ZnS core/shell NPLs using a single-source shelling precursor method, which allows for ZnS surface passivation of NPLs at relatively low temperatures, while being thermally adaptable to ensure minimal NPL degradation. An extremely large exciton red shift (∼420 meV), upon ZnS shell passivation, was observed because of the increased effective thickness of the CdS core NPLs. Steady-state and time-resolved emission measurements indicate that the host−dopant energy-transfer efficiency and Mn−Mn interactions within the 2D Mn:CdS/ZnS core/shell NPLs can be fine-tuned via the dopant concentration, resulting in an intense Mn PL as well as tunable dual-band emission from the host NPLs and Mn dopants. Magnetic measurements indicate intrinsic spin states in the 2D NPLs and complex magnetic interactions at high doping concentrations, including antiferromagnetic exchange between dopants and possible dopant−surface state interaction.

In terms of producing new advances in sustainable nanomaterials, cation exchange (CE) of post-processed colloidal nanocrystals (NCs) has opened new avenues towards producing non-toxic energy materials via simple chemical techniques. The main processes governing CE can be explained by considering hard/soft acid/base theory, but the detailed mechanism of CE, however, has been debated and has been attributed to both diffusion and vacancy processes. In this work, we have performed in situ x-ray absorption spectroscopy to further understand the mechanism of the CE of copper in solution phase CdSe NCs. The x-ray data indicates clear isosbestic points, suggestive of cooperative behavior as previously observed via optical spectroscopy. Examination of the extended x-ray absorption fine structure data points to the observation of interstitial impurities during the initial stages of CE, suggesting the diffusion process is the fundamental mechanism of CE in this system.

ethanol (≥99.5%, VWR), and toluene (≥99.5%, EMD Chemicals) were used as received.Synthesis of Mn:CdS QDs. Mn(II) doped CdS QDs were synthesized through a colloidal hot-injection technique as previously described. 1 Briefly, 41.2 mg (0.130 mmol) of Cd(NO 3 ) 2 ꞏ4H 2 O, 5.82 mg (0.033 mmol) of Mn(NO 3 ) 2 ꞏH 2 O, 0.167 mL of DDT, and 10 mL of OAm were mixed in a three-neck flask. The mixture was degassed for 40 min at room temperature and another 10 min at 100 °C. The mixture was refilled with argon and kept at 110 °C for 30 min. Then, 0.667 mL of a 0.2 M solution of sulfur in OAm was swiftly injected into the flask at 160 °C. After the injection, the temperature was set at 120 °C and degassed for 10 min. The temperature was then raised to 240 °C for 5-10 min. The product was purified by adding toluene/ethanol.

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