Upconversion nanoparticles (UCNPs) are an excellent choice to construct security features against counterfeiting, owing to their unique NIR-to-VIS upconversion luminescence (UCL) characteristics. However, the application of upconversion materials is limited, due to their single and invariant emission colors. Herein, the temperature-dependent UCL properties of NaGdF:Yb/Ho (or Tm) UCNPs in the solid state have been investigated. An anomalous UCL enhancement at higher temperatures has been demonstrated for these small-sized (<10 nm) UCNPs and the underlying mechanism is discussed herein. Meanwhile, effective UCL with tunable multicolor emissions has been realized by the rational incorporation of Ho and Tm emitters into a single nanostructure. The emission colors of these Ho/Tm co-doped Na(Gd,Yb)F UCNPs can be tuned by changing the laser power or temperature, due to the different spectral sensitivities of the Tm and Ho emitters to the excitation power density and temperature. The power- and temperature-responsive color shifts of these Ho/Tm co-doped UCNPs are favorable for immediate recognition by the naked eye, but are hard to copy, offering the possibility of designing more secure anti-counterfeiting patterns.
Luminescent bulk materials generally suffer from thermal quenching, while upconversion nanocrystals (UCNCs) have recently been found to show increase of dramatic emission at elevated temperatures. A deep understanding on this quite different light–heat interaction at the nanoscale is important both scientifically and technologically. Herein, temperature-dependent upconversion luminescence (UCL) is investigated for UCNCs with various sizes, activators (Ho3+, Tm3+, Er3+), and core/shell structures. An anomalous UCL enhancement with increasing temperature is found for UCNCs with larger surface/volume ratios (SVRs). Moreover, this UCL increase shows a pronounced dependence on the SVRs, activators, emitting levels, and measuring environments. Substantial evidence confirms that the thermally induced UCL increase is primarily due to the temperature-dependent quenching effect of surface-adsorbed H2O molecules, instead of the previously proposed surface phonon-assisted mechanism. Temperature-dependent spectral investigations also show that the energy-loss process of Yb3+-sensitized UCNCs is largely due to the deactivation of Yb3+ ions caused by surface quenchers, rather than the direct quenching to activators. UCNCs with an active shell (doped with Yb3+) exhibit similar thermally induced UCL increase, due to energy migration to the surface over the Yb–Yb internet. It implies that active-core/active-shell UCNCs are susceptible to surface quenchers and would be unsuitable for applications in aqueous environments.
Small-sized upconversion nanocrystals (<10 nm) show a quite low luminescence efficiency. Even if these nanocrystals are coated by a 2 nm thick inert shell, the core/ shell nanocrystals still exhibit weak upconversion luminescence. The involved energy loss mechanism is under debate. Here, we have demonstrated that the major contribution to the low upconversion efficiency is ascribed to an overtone vibrational energy transfer from electronic transition of the Yb 3+ excited state to overtone transitions of deactivating group vibrations by dipole−dipole coupling. The maximum coupling distance reaches ∼11 nm. Moreover, we first find that an ultrathick inert shell (>11 nm) is not beneficial for upconversion luminescence due to a strong scattering effect. A novel lifetime model is proposed to precisely describe the decay times of the Yb 3+ 2 F 5/2 state and Er 3+ 4 S 3/2 state as a function of inert-shell thickness. Based on an insight into the luminescence loss, we design β-Na91%YbF 4 :9%Er@NaGdF 4 nanocrystals with a 11.2 nm inert shell to completely block surface quenching and achieve more efficient and stronger upconversion emission.
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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