1：Experimental Design and Method Selection

The synthesis of ZnSe:Mn and ZnSe nanocrystals was carried out in a microwave reactor Magnum II (ERTEC, Poland) in a Teflon vessel with a maximum applied power of 600 W and frequency of 2.45 GHz. The thermodynamic parameters of the process, such as power, reaction temperature and pressure, were controlled by Magnum V2 software.

2：Sample Selection and Data Sources

The zinc nitrate hexahydrate (Zn(NO3)2?10H2O) (≥99%, Sigma-Aldrich), sodium selenite (Na2SeO3) (99%, Sigma-Aldrich), manganese acetate tetrahydrate (Mn(CH3COO)2?4H2O) (99%, Sigma-Aldrich), ethylenediamine (NH2CH2CH2NH2) (≥98%, Carlo Erba), methyl orange (dye content 85%, Sigma-Aldrich), distilled water. All chemicals were used without further purification.

3：List of Experimental Equipment and Materials

Microwave reactor Magnum II (ERTEC, Poland), Teflon vessel, X-ray diffractometer Bruker D8 Phaser Diffractometer, SEM microscope with EDS analysis Hitachi TM 3000 Denton Vacuum, TEM measurements FEI Tecnai Osiris S/TEM, Micromeritics ASAP 2020 instrument, Nicolet 6700 FT-IR, HR2000 instrument equipped with a deuterium-halogen DH-2000-BAL light source, Spectrofluorometer Fluorolog-3 (Horiba company), Edinburgh FLS920 spectrofluorometer, UV-Vis spectrometer BLACK-Comet-SR (StellarNet Inc.), ESCAPrevac spectrometer equipped with a hemispheric XPS and AES analyzer of charged particles (VG Scienta R3000), Perkin Elmer 370 spectrometer with Lumina hollow cathode lamps (series Lumina?2), X-band Bruker ELEXSYS-580 spectrometer, photoelectric spectrometer (Instytut Fotonowy, Poland).

4：Experimental Procedures and Operational Workflow

In a typical procedure of the synthesis of ZnSe:Mn, 2.5 mmol of zinc nitrate hexahydrate, 2.5 mmol sodium selenite and 0.125 mmol of manganese (II) acetate tetrahydrate were dissolved in a solution of ethylenediamine (20 ml) and distilled water (2:1 v/v) in a Teflon vessel. Subsequently, the mixture was stirred at room temperature for 10 min. Then, the Teflon container was placed into the microwave reactor and heated according to the established temperature from 230°C to 240°C (whereas the pressure was in the range of 23 and 26 bar) for 15-25 min. The reaction mixture was then cooled to room temperature inside the reaction vessel for about 10 min. After cooling, the resulting samples were centrifuged and washed twice with distilled water. After centrifugation, nanocrystals were dried in a vacuum oven at 40°C for 18 h.

5：Data Analysis Methods

The crystallographic structure of samples was investigated using X-ray diffractometer Bruker D8 Phaser Diffractometer with CuKα radiation (λ = 1.5406 ? ), with 2θ ranging between 5-80° at the scanning rate of 0.1°. The morphology, dimensions and EDS analyses of the NCs were examined with SEM microscope with EDS analysis Hitachi TM 3000 Denton Vacuum. Transmission electron microscopy (TEM) measurements were performed on a FEI Tecnai Osiris S/TEM. The specific surface area of the ZnSe and ZnSe:Mn NCs was measured at 77 K using a Micromeritics ASAP 2020 instrument. FT-IR spectroscopy (Nicolet 6700 FT-IR) was used in order to study the presence of ligands on the surface of the NCs. Diffusion reflectance (DR) UV-Vis spectra were reordered using an HR2000 instrument equipped with a deuterium-halogen DH-2000-BAL light source at room temperature using BaSO4 as a standard. Spectrofluorometer Fluorolog-3 (Horiba company) was used to characterize emission and excitation of the ZnSe and ZnSe:Mn NCs. Luminescence lifetime measurements were performed using an Edinburgh FLS920 spectrofluorometer. UV-Vis spectra of samples in photocatalytic degradation of organic dye were recorded using UV-Vis spectrometer BLACK-Comet-SR (StellarNet Inc.). The X-ray photoelectron spectra were carried out using an ESCAPrevac spectrometer equipped with a hemispheric XPS and AES analyzer of charged particles (VG Scienta R3000). Atomic absorption spectroscopy (AAS) was also used to determine the content of manganese elements in samples. The electron paramagnetic resonance (EPR) spectra were recorded at room temperature with an X-band Bruker ELEXSYS-580 spectrometer, using the radical scavenger dimethyl pyridine N-oxide (DMPO). The photoelectrochemical characterization was performed using a photoelectric spectrometer (Instytut Fotonowy, Poland) composed of a stabilized 150W Xenon arc lamp, monochromator, and coupled with a P-IF 4.0 potentiostat.