Tunable emission carbonized polymer dots (CPDs) are highly desirable for the preparation of optoelectronic devices, especially white light-emitting diodes (WLEDs). In most available studies, polychromatic CPDs are synthesized using aromatic molecules as precursors. However, few studies report the successful synthesis of polychromatic CPDs using two or more unconjugated precursors. In this work, we prepare multicolor fluorescent CPDs from a single unconjugated precursor, glucose, via a hydrothermal reaction. By controlling the particle size and degree of graphitization of the synthesized CPDs, their emission wavelength can be tuned in the range 440-625 nm (i.e., almost the entire visible region). Furthermore, the CPDs can be used to construct LEDs of varying colors, including WLEDs (CIE coordinates: 0.34, 0.36) with the correlated color temperature and color rendering index of 4997 K and 92.69, respectively. In brief, the strategy proposed in this study successfully converts unconjugated glucose into high-performance LEDs with great application potential.

Facile Synthesis of Water-Stable Multicolor Carbonized Polymer Dots from a Single Unconjugated Glucose for Engineering White Light-Emitting Diodes with a High Color Rendering Index.

The attractive features of lanthanide-doped upconversion luminescence (UCL), such as high photostability, nonphotobleaching or photoblinking, and large anti-Stokes shift, have shown great potentials in life science, information technology, and energy materials. Therefore, UCL modulation is highly demanded toward expected emission wavelength, lifetime, and relative intensity in order to satisfy stringent requirements raised from a wide variety of areas. Unfortunately, the majority of efforts have been devoted to either simple codoping of multiple activators or variation of hosts, while very little attention has been paid to the critical role that sensitizers have been playing. In fact, different sensitizers possess different excitation wavelengths and different energy transfer pathways (to different activators), which will lead to different UCL features. Thus, rational design of sensitizers shall provide extra opportunities for UCL tuning, particularly from the excitation side. In this review, we specifically focus on advances in sensitizers, including the current status, working mechanisms, design principles, as well as future challenges and endeavor directions.

Recent Development in Sensitizers for Lanthanide-Doped Upconversion Luminescence.

Co-doping mixed-valence Eu2+/3+ in a single-phase phosphor is an efficient method to realize the emission color regulation, which holds great potential for anticounterfeiting and ratiometric temperature sensing. Here, the mixed-valence Eu-doped Sr1.95+xLi1-xSi1-xAlxO4F (0 ≤ x ≤ 0.25) phosphors were designed and prepared under a reducing atmosphere. The correlation of local phase structures and luminescence properties was discussed. Replacing Si4+-Li+ ion pairs with Al3+-Sr2+ ion pairs compresses the Sr sites occupied by Eu2+, and it stabilizes Eu3+ in a reducing atmosphere and leads to the coexistence of Eu2+ and Eu3+ in single-phase Sr1.95+xLi1-xSi1-xAlxO4F:0.05Eu (0 ≤ x ≤ 0.25) phosphors. Based on the wavelength-dependent luminescence color behaviors of Sr1.95+xLi1-xSi1-xAlxO4F:0.05Eu phosphors, the fluorescent anticounterfeit papers/patterns containing Sr1.95+xLi1-xSi1-xAlxO4F:0.05Eu phosphors were the same as ordinary paper under ambient conditions. However, the hidden colors or images can be read out with green-orange luminescence under 365/300 nm light excitation. Benefiting from the diverse thermal response emission behaviors of Eu2+ (530 nm) and Eu3+ (703 nm), Sr1.95+xLi1-xSi1-xAlxO4F:0.05Eu phosphors exhibit temperature sensing performances, with the maximum absolute and relative sensitivity being 0.0294 K-1 at 573 K and 0.83% K-1 at 348 K. More importantly, Sr1.95+xLi1-xSi1-xAlxO4F:0.05Eu phosphors showed excellent stability in humid, acid, and alkali environments, which contributed to applying mixed-valence Eu2+/3+-doped Sr1.95+xLi1-xSi1-xAlxO4F to the fields of multicolor anticounterfeiting and noncontact optical thermometry.

Regulation of Local Site Structures to Stabilize Mixed-Valence Eu2+/3+ under a Reducing Atmosphere for Multicolor Photoluminescence.

Lanthanide (Ln3+)-doped near infrared (NIR)-II luminescent nanoprobes have shown great promise in many technological fields, but are currently limited by the low absorption efficiency of Ln3+ due to the forbidden 4f→4f transition. Herein, we report a novel NIR-II luminescent nanoprobe based on efficient energy transfer from Ce3+ to Er3+ and Nd3+ in sub-10 nm SrS nanocrystals (NCs), which are excitable by using a commercial blue light-emitting diode (LED). Through sensitization by the allowed 4f→5d transition of Ce3+, the NCs exhibit strong NIR-II luminescence from Er3+ and Nd3+ with quantum yields of 2.9% and 2.3%, respectively. Furthermore, by utilizing the intense NIR-II luminescence of Er3+ from the thermally coupled Stark sublevels of 4I13/2, we demonstrate the application of SrS:Ce3+/Er3+ NCs as blue-LED-excitable NIR-II luminescent nanoprobes for ratiometric thermal sensing. These findings reveal the unique advantages of SrS:Ln3+ NCs in NIR-II luminescence, which may open up a new avenue for exploring novel and versatile luminescent nanoprobes based on Ln3+-doped sulphide NCs.

/pdf/blue-led-excitable-nir-ii-luminescent-lanthanide-doped-srs-wngc8f2no5.pdf

Blue-LED-excitable NIR-II luminescent lanthanide-doped SrS nanoprobes for ratiometric thermal sensing

The authors thank the financial support of the European Research Council (ERC-AdG-2017 787510, ERC-CoG-2019 815128) and of the European Commission (EUSMI, Grant 731019). This work was performed under the Maria de Maeztu Units of Excellence Program from the Spanish State Research Agency-Grant MDM-2017-0720.

Nd3+-Doped Lanthanum Oxychloride Nanocrystals as Nanothermometers.

We report the synthesis of colloidal EuS, La2S3, and LaS2 nanocrystals between 150 and 255 °C using rare-earth iodides in oleylamine. The sulfur source dictates phase selection between La2S3 and LaS2, which are stabilized for the first time as colloidal nanocrystals. The indirect bandgap absorption of LaS2 shifts from 635 nm for nanoellipsoids to 365 nm for square-based nanoplates. Er3+ photoluminescence in La2S3:Er3+ (10%) is sensitized by the semiconducting host in the 390-450 nm range. The synthetic route yields tunable compositions of rare-earth sulfide nanocrystals. Interaction of light with these novel semiconducting nanostructures hosting rare-earth emitters should be attractive for applications that require broadband sensitization of RE emitters.

Rare-Earth Sulfide Nanocrystals from Wet Colloidal Synthesis: Tunable Compositions, Size-Dependent Light Absorption, and Sensitized Rare-Earth Luminescence.

Morgane Pellerin

Papers

Rare-Earth Sulfide Nanocrystals from Wet Colloidal Synthesis: Tunable Compositions, Size-Dependent Light Absorption, and Sensitized Rare-Earth Luminescence.

Efficient photoluminescence of isotropic rare-earth oxychloride nanocrystals from a solvothermal route