| Mn2+-doped semiconductor nanocrystals(quantum dots)hold big application prospects in areas such as new energy conversion and optoelectronic devices due to Mn2+associated unique photoluminescent performance,for example,good emission stability,large stokes shift and μs-ms-level excited-state decay lifetime.Impressive achievements have been made on study about synthesis of Mn2+-doped nanocrystals and the relevant photophysical performance as well as understanding of their emission modulation mechanisms.However,limitations of seticonductor nanocrystals as Mn2+dopant host model gradually emerged with further exploration of Mn2+ emission modulation mechanisms.In general,nanocrystal models do not assure absolute uniformity of particle sizes and the precise regulation of concentration and location of Mn2+ dopants inside the nanocrystals is difficult.Also,defects or traps are inevitable on the surface of doped nanocrystals,of which can easily increase the complexity of the host energy levels.These limitations restrict further exploration of the emission modulation mechanism of Mn2+dopants.Metal chalcogenide supertetrahedral nanocluster is a kind of wide-gap semiconductor,which has precise structure and element composition.They also possess precise doping sites and number inside the clusters,which can be regarded as ultra-small quantum dots with precise structure.In addition,this cluster hold various intercluster assembly patterns,which can not only form crystal openframework with different dimensions,but also assemble into single crystals by discrete clusters.These characteristics provide an advantageous platform for further exploration of the photoluminescence modulation mechanisms of Mn2+ dopants.In recent years,although there are some reported studies on the luminescence properties of Mn2+based on metal chalcogenide supertetrahedral nanocluster models,they mainly focused on the changes of Mn2+emitting wavelength and the luminescence efficiency directed by the coupling interaction between Mn2+ and Mn2+.At present,many aspects of such study with clusters are highly elusive.For example,the effect of other impurities on Mn2+emission and the modulation mechanism of decay dynamics of Mn2+dopants remained less studied.In addition,previous studies on Mn2+emission behavior with cluster models were achieved with solid phase single crystal,while the studies with clusters containing Mn2+ in liquid phase are still blank.In view of this,in this thesis,we not only explored the modulation mechanism of Mn2+emission by using the solid-phase cluster model,but also studied the energy/charge transfer mechanism-relevant Mn2+photoluminescence by preparing liquid cluster dispersion samples and additionally studied Mn2+emission decay dynamics with varied intercluster models.The respective research contents are as follows:1.Study on the photoluminescence properties of Mn2+modulated by Fe2+impurities in T5-MnInS nanocluster model:Different from conventional Fe/Mn codoped chalcogenide quantum dots,the T5-MnInS nanocluster model can ensure that the Fe2+impurity doping in the core region of the cluster,where the distance between Fe2+and Mn2+is short,and there are both Mn2+-Fe2+and Mn2+-Mn2+coupling interactions which is beneficial for unraveiling the photoluminescence modulation mechanism.The photoluminescence of Mn2+dopant,including emission intensity and decay lifetime,decreased drastically due to the tiny amount introduction of Fe2+impurity.Photoluminescence excitation spectra and decay dynamics showed that excited-state Mn2+can transfer energy directly to Fe2+in addition to the host exciton.In addition,the temperature-dependent decay lifetime of the Fe2+-doped sample is found obviously different from that of the original sample.This work provides a new perspective for the photoluminescence modulation of Mn2+and will facilitate the development of more novel materials by doping a variety of impurities into a range of cluster structures.2.Photoluminescence properties of surfactant-aided dispersion of T4-MnInS nanoclusters and their energy transfer with interfacial small molecules:T4-MnInS nanoclusters in crystal lattice were dispersed in chloroform solution to form ultra-small nanoparticles with the help of surfactant DODA.T4-MnInS nanoclusters in dispersions show different emitting properties of Mn2+compared with the raw T4-MnInS crystals,such as emission redshift and decay lifetime shortening.The shortened excited state lifetime is mainly due to the consumption of Mn2+ excited state energy caused by the collision between T4 clusters and chloroform molecules.Thus,the Mn2+ luminescence lifetime can be adjusted by changing the concentration of DODA.It is found that T4MnInS can also couple with 3O2,resulting in a tiny shortened lifetime of Mn2+emission and producing 1O2 photoluminescence simultaneously.Importantly,photoluminescence test results showed that the excited Mn2+can directly transfer energy to 3O2.3.Study on direct charge transfer between Mn2+dopants in T4-MnInS clusters and interface organic amines:By selecting good T4-MnInS dispersion,we systematically studied the modulation of Mn2+emission under the coupling between T4-MnInS and small molecules(mainly organic amine molecules)at the interface.Organic amines with different structures can reduce the emission intensity of Mn2+in different degrees.Through electrochemical,steady state photoluminescence and transient absorption spectroscopy,we confirmed for the first time the direct charge transfer process between Mn2+ions and surface organic amine molecules,and identifyed the mechanism of emission quenching of Mn2+ions in semiconductor nanocrystals caused by organic amines.Based on the understanding of the proposed mechanism,we can realize the Mn2+emission modulation expectedly,including fluorescence quenching with different efficiency and fluorescence enhancement,by rationally selecting the organic amines with specific molecular structure and electronic configuration.In addition,it can also be extended to other molecules,such as H2O,to achieve the emitting enhancement of Mn2+dopants.4.Study on decay dynamics of Mn2+emission affected by T4-MnInS-based intercluster assembly patterns:With zero-dimensional T4-MnInS cluster-based crystals(D-T4)and the three-dimensional openframework crystal(L-T4)connected by T4-MnInS clusters,we systematically examined the Mn2+ photoluminescence behaviors of the two different assembly models.Compared with the D-T4 samples,the decay lifetime of Mn2+ emission in L-T4 sample with linking clusters is merely about one fifth of that in D-T4,apart from the observation of redshift emission.Structural analysis and temperature-varying emission results showed that the intercluster connection with shared S atom increases the non-radiative decay channel and caused more energy dissipation of Mn2+ excited states.Emission behavior of the self-assembled particles(A-T4)of T4-MnInS dispersion confirmed that the connection between clusters is the main cause of more energy dissipation.When T4-MnInS clusters are assembled with organic components(ammonium cations)as the intermediate,the emission lifetime of Mn2+was not shortened,although stressed by ambient forces(the emission wavelength redshifted). |
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