Study On The Modulation Of Rare Earth Doped Upconversion Nanoparticles Luminescence By Local Optical Field And Its Applications

Upconversion nanoparticles（UCNPs）are a specific class of optical nanomaterials doped with rare earth ions,which can convert low energy photons into high energy photons through the intermediate energy state of rare earth ions through the anti-stokes process.The upconversion luminescence（UCL）has lots of unique spectroscopic conversion abilities,which exhibit promisingapplications,including biological imaging,lasers,photovoltaics devices and photodetectors.The UCNPs under near infrared（NIR）excitation have many advantages over other nanocrystals,such as theexcellent optical stability,low biotoxicity,good biocompatibility,no photobleaching,no background noise and no blinking.Previous studies on UCNPs mainly focused on the analysis of the group spectroscopy based on the statistical average effect,however,ignored the heterogeneous information from one single UCNP.With the rapid development of nano-characterization technology,single particle spectroscopy detection has become an effective technique to solve the above problems and realize the single particle applications.The information gained by single particle spectroscopy is very important and of great value to further analysis the optical properties of single nanoparticle.However,there are still some problems like the high pumping threshold as well as low luminescent efficiency,which can restrict and impact the realization of single particle spectral detection.In this paper,we used atomic force microscopy（AFM）equipped with home-build fluorescence spectroscopy and kinetic measurements systems,combined with local optical field manipulation to carry out research,the main results are as follows:（1）We experimentally and theoretically demonstrate UCL enhancement in monolayer UCNPs using plasmon semiconductor NPs（Cs_xWO₃）and further explore its narrowband NIR photodetection application.More than three orders enhancement was obtained via the semiconductor plasmon combined with core-shell UCNPs in bothCs_xWO₃/NaYF₄/monolayer-NaYF₄:Yb³⁺,Er³⁺@NaYF₄:Yb³⁺,Tm³⁺and Cs_xWO₃/NaYF₄/monolayer-NaYF₄:Yb³⁺,Tm³⁺@NaYF₄:Yb³⁺,Er³⁺.Interestingly,the higher enhancement of core’s emissions was recorded in core-shell UCNPs by coupling with Cs_xWO₃ NPs.Finally,the high performance narrow photodetectors for 980 nm is constructed employing MAPbI₃/Cs_xWO₃/NaYF₄（15nm）/monolayer-NaYF₄:Yb³⁺,Er³⁺@NaYF₄:Yb³⁺,Tm³⁺hybrids,with narrow line width of 20 nm,high responsivity of 0.33 A/W,detectivity of 4.5×10¹⁰ Jones,and short response time of 180 ms,which is much better than the previous photodetectors.（2）In order to conquer the relatively high pumping threshold of UCNCs,we designed a novel cascade amplification strategy for UCL through cascading the superlensing effect of dielectric microlens arrays and the LSPR effect of gold nanorods,which readily leads to a UCL enhancement by more than four orders of magnitude under weak light irradiation.The optimized UCNCs composites was combined with perovskite MAPbI₃ films to prepare NIR photodetectors（PDs）based on local optical field manipulation for the first time.By accommodating multiple optical active lanthanide ions in a core-shell-shell hierarchical architecture,the PDs can detect three well-separated narrow bands in the NIR region,i.e.,808,980,and1540 nm,respectively.Due to the large UCL enhancement,the obtained PDs demonstrate extremely high responsivity of 30.73,23.15,12.20 A/W and detectivity of 5.36,3.45,1.91×10¹11 Jones for the 808,980,and 1540 nm light detection,respectively,together with short response times in the range of 80-120 ms.Moreover,we demonstrate the response to the excitation modulation frequency of a PD can be employed to discriminate the incident light wavelength.（3）We have LSPR effect combined with photonic crystal effect generated from three-dimensional opal photonic crystals（OPCs）to enhance the UCL intensities of single NaYF₄:Yb³⁺,Er³⁺and NaYF₄:Yb³⁺,Er³⁺@MnO₂ NPs.The local field modulation induced 1600 and 1400 folds UCL enhancement,respectively.The OPCs/CsxWO₃/NaYF₄:Yb³⁺,Er³⁺@MnO₂ composites single particle sensor was prepared,due to the sensitivity of NPs to dithiothreitol（DTT）.That is experimentally and theoretically demonstrated that as the UCL intensity of UCNP has been vastly increased by local field modulation,so did the DTT detection limit for single particle DTT detection varies from 1.3 to 0.65 nMol.Furthermore,we found that the single particle DTT detection has higher sensitivity,lower limit of detection and faster response time compared with the detection based on ensembles paticles.（4）Using AFM probe to manipulate the spatial position change of single UCNP relative to gold nanoroads（Au NRs）,and the influence of the above spatial position change on the UCL intensities of one UCNP was analyzed experimentally and theoretically.For single UCNP,the maximum enhancement folds based on LSPR effect of Au NR is 53 folds,and the optimal effective distance at the nanoscale is about 3 nm.Furthermore,when the two Au NRs are arranged at an angle of 180°,the central position is the area where the electric field is concentrated,and the enhancement factor of UCNP is the highest reaching 82.4 folds.The results were applied to the single particle ultra-sensitive detection of H₂S gas,which showing a good linear relationship in 0-300 ppb and the minimum detection limit is 0.8 ppb.