Near Infrared Emission Of Mn²⁺-Containing Sulfide

The transition metal ion Mn²⁺activated visible luminescent materials have been extensively applied in the fields of lighting and displays,due to their unique tunable and single-band luminescence properties resulted from the 3d⁵ electron configuration of Mn²⁺.However,the emission of Mn²⁺comes from a spin and parity forbidden ⁴T₁?⁶A₁ transition,compromising the luminescence efficiency of Mn²⁺doped materials,and therefore limiting their further applications to some extent.Elevating the doping concentration has been commonly considered as a solution to enhance luminescent efficiency.While,the high-Mn²⁺-concentration not only results in luminescence quenching but also introduces additional Mn²⁺-Mn²⁺pairs species emitting near-infrared?NIR?emission.As the emerging NIR emission corresponds to additional de-excitation way for the electron at excited states,this invisible emission will inevitably decrease the visible luminescence efficiency.Despite the additional NIR emission could not serve lighting or displays,it fertilizes the tunability of Mn²⁺luminescence,offering the possibility of designing NIR emitting source for potential application at night vision,bioimaging,and telecommunication.The underlying mechanism of such NIR emissive center in Mn²⁺concentrated material,especially their behavior at different crystal structure,has not been interpreted,albeit well-known luminescent behavior of isolated Mn²⁺ion.The exploration of the origin of the Mn²⁺NIR emissive center is of significance for a profound understanding of Mn²⁺luminescent behavior and therefore designing high-efficiency Mn²⁺activated visible luminescent materials.In this dissertation,the NIR emission from Mn²⁺-activated sulfide,including different structural MnS and Mn²⁺heavily doped ZnS,have been studied.The principle and mechanism of the NIR emission are further investigated.This thesis consists of six chapters.Chapter 1 has reviewed the research progress of the visible and NIR emissions of Mn²⁺ions.The luminescence behavior of visible and NIR emissions and the crystal field effect have also been briefly introduced.In chapter 2,the experiment and measurement methods have been introduced.Chapter 3-5 have demonstrated the normal visible emission and the new emissive center,exchange-coupled Mn²⁺-Mn²⁺pair in different crystal structure,configurations,and Mn²⁺distribution by only involving single crystallographic site.An attempt has been made to provide keen insights to this NIR luminescent center including its characteristic,mechanism and tailoring behavior.The conclusion and prospect have been provided in Chapter 6.Some valuable results are obtained as follows.?1?Mn²⁺based sulfide?-MnS?rock-salt?was synthesized by using a solvothermal method.Upon 380 nm UV light excitation,simultaneous triple emissions at 710 nm,900 nm and 1380 nm were observed in this system by involving Mn²⁺at a single crystallographic site.Notably,the NIR emission centered at 1380 nm with an FWHM?full width at half maximum?bandwidth as broad as 161 nm was demonstrated at room temperature.With the analysis of crystal structure and spectroscopy,the emission bands centered at 710 nm,900 nm and 1380nm can be ascribed to the isolated Mn²⁺ion,next nearest-neighbored exchange-coupled Mn²⁺-Mn²⁺pair,and nearest-neighbored exchange-coupled Mn²⁺-Mn²⁺pair,respectively.The density functional theory?DFT?calculation of the magnetic configuration revealed that temperature-dependent luminescent spectra further demonstrated the abnormal behavior of NIR emission around antiferromagnetic transition temperature?Neel temperature?.The NIR emission beyond 1000 nm from Mn²⁺-Mn²⁺pair leads to a deeper understanding of the Mn²⁺-doped luminescent material,providing a new perspective for designing novel NIR emitting sources for various photonic applications.?2?By varying synthesis protocol,different structural MnS,g-MnS?wurtzite?,b-MnS?blende?,and?-MnS?rock-salt?were successfully synthesized.Upon varying the Mn²⁺-site coordination and/or Mn²⁺-Mn²⁺pair geometry in different structural MnS,the multiple emissions from divalent manganese are carefully investigated.The visible emission at 570 nm and NIR emission at 880 nm have been demonstrated in tetrahedralg-MnS.It is found that the visible emission and NIR emission are originated from the single Mn²⁺ion and corner-sharing exchange-coupled Mn²⁺-Mn²⁺pair,respectively.Compared study of NIR emission in tetrahedral and octahedral crystal field revealed that the emission of Mn²⁺-Mn²⁺pairs is more sensitive to the pair configuration rather than the crystal field.The direct observation of the mentioned above.The temperature-dependent emission spectra and decay of the NIR emissive center were also investigated in detail.This work provides keen insights into the luminescent behavior of Mn²⁺-Mn²⁺pair,clarifying the NIR emission mechanism.?3?With the help of nucleation-doping strategy,the MnS@ZnS nanoparticles with controlled Mn²⁺distribution was successfully obtained.Upon 320 nm excitation,dual-band emission at 585 and 720 nm were achieved,which corresponds to the single Mn²⁺ion and exchange-coupled Mn²⁺-Mn²⁺pair,respectively.By controlling the diffusion process at the MnS@ZnS interface,different radical distribution of Mn²⁺ions were obtained,therefore rationally tuning the ratio of visible and NIR emission.The relationship between Mn²⁺ions distribution and the ratio of visible and NIR emission clearly demonstrates the significant role of Mn²⁺aggregation for NIR emission.The time-resolved emission spectra and temperature-dependent spectra of the Mn²⁺-Mn²⁺pair emission in MnS@ZnS have also been investigated.The different decay time and thermal stability of visible and NIR emission find it the potential application as time-resolved imaging and temperature detector.The achievement of the NIR emission from Mn²⁺-Mn²⁺pair would facilitate the possibility of designing new NIR emission in Mn-doped nanocrystal.