Near-infrared (NIR) phosphors have received increasing attention for designing novel solid-state light sources with broadband NIR output. In this work, a novel NIR phosphor LiScP 2 O 7 :Cr 3+ (LSP:Cr 3+ ) is developed with the emissions (750−1100 nm) completely in the NIR spectral range. Under 470 nm excitation, LSP:0.06Cr 3+ shows broadband NIR emissions peaking at ∼880 nm, with a full width at half maximum (FWHM) of ∼170 nm and an internal quantum yield (IQY) of ∼38%. Moreover, photoluminescence (PL) improvements of LSP:Cr 3+ phosphors are achieved by Yb 3+ codoping, leading to the broadened FWHM (up to ∼210 nm), increased IQY (η max = ∼74%), and reduced thermal quenching. The energy transformation processes in LSP:Cr 3+ ,Yb 3+ are quantitatively analyzed on the basis of PL lifetime and QY measurements, revealing that the PL improvements by Yb 3+ codoping principally originate from the energy transfer from Cr 3+ to more efficient and thermally stable Yb 3+ emitters. Finally, NIR phosphor-converted light-emitting diodes (pc-LEDs) are fabricated by combining LSP:Cr 3+ ,Yb 3+ phosphors with blue LED chips, giving a maximum NIR output power of ∼36 mW and photoelectric efficiency of ∼12% at 100 mA drive current. The results suggest that the investigated phosphors would be promising luminescent converters for broadband NIR pc-LED applications.

By utilizing opposite luminescence temperature-dependences between lanthanide-doped microrods and nanocrystals, upconversion hybrids with color-tunable emissions are developed for more secure anticounterfeiting applications.

terfeiting nanomaterials with highlevel security features have attracted increasing interest in recent years, taking advantages of diverse luminescence characteristics of UCNCs, such as multicolor emission, [2a,3] excitation power dependence, [4] dual-mode fluorescence, [5] tunable luminescence lifetime, [6] and plasmonic coupling effects. [7] These novel UCNCs can produce color-coded or color-tunable anticounterfeiting patterns, authenticated by changing the excitation wavelength/power or utilizing the timeresolved scanning method. Although the extra anticounterfeiting features make these novel UC nanomaterials or nanocomposites very hard to duplicate, they typically require complex fabrication processes and costly reading instruments, such as ultrahigh power lasers and time-gated decoding instrumentation. Hence, the exploration of novel UC nanomaterials with high-level security, simple fabrication procedures, and convenient authentication methods is highly desired.Luminescent materials generally suffer emission loss at higher temperatures, which is well known as the thermal quenching. However, Yb-sensitized core-only UCNCs have been found to show the dramatic upconversion luminescence (UCL) increase at elevated temperatures by our group [8] and also by Zhou et al. very recently. [9] This thermally induced UCL increase phenomenon has been ascribed to temperaturedependent surface effects of UCNCs. Zhou et al. proposed a surface phonon-assisted UCL model and suggested that the heat-favorable phonons existing at the NC surface increased the energy transfer from Yb 3+ to emitting ions and induced the UCL increase at elevated temperatures. [9] Nevertheless, deep spectral analysis at various temperatures and circumstances indicated that the UCL increase should be due to the gradually attenuated quenching effect of surface-adsorbed water molecules with increasing temperature. [10] The exact mechanism behind this totally different light-heat interplay behavior at the nanoscale need to be further elucidated. [11] On the other hand, this thermally induced UCL increase phenomenon provides a novel strategy to design UC anticounterfeiting nanomaterials with strengthened security features. In the present work, based on quite different UCL temperature-dependences of active-core@inert-shell NCs (thermal quenching) and active-core@active-shell ones (thermally induced increase), This work presents a novel and highly secure anticounterfeiting strategy based on core/shell upconversion nanocrystal (UCNC) hybrids with temperature-responsive multicolor emissions. Opposite luminescent temperaturedependences are found for active-core@inert-shell (thermal quenching) and active-core@active-shell (thermally induced enhancement) UCNCs. Accordingly, their hybrids are designed to show obvious color changes with increasing temperature under 975 nm excitation. Various color-shifting pathways (from white to green, blue to green, etc.) are achieved by adjusting the core/shell NC combinations in the hybrids. Moreover, color changes of the prin...

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