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Achieving bright photon upconversion under low irradiance is of great significance and finds many stimulating applications from photovoltaics to biophotonics. However, it remains a daunting challenge to significantly intensify upconversion luminescence in small nanoparticles with a simple structure. Herein, we report the amplification of photon upconversion through engineering interfacial density of states between the core and the shell layer in sub-10 nm colloidal rare-earth ions doped fluoride nanocrystals. Through tuning of the metal cations in the shell layer of alkaline-earth-based core/shell nanoparticles, both the interfacial phonon frequency and the density of state are evidently decreased, resulting in the luminescence intensification of up to 8224 times. The generality of this upconversion enhancement strategy has been verified through expansion of this approach to alkali-based core/shell nanoparticles. The engineering of photon density of state in such core/shell nanoparticles enables dynamic display and high-level security information storage.

Multicolor luminescent nanoparticles (NPs) show several potential emerging applications. In this work, we provide a new route that integrates the afterglow and upconversion (UC) that originate in a single activator to achieve color variations without the modulation of any other parameters. The Er3+ ions in Na3HfF7:Yb/Er NPs exhibit bright green afterglow upon X-ray irradiation and single-band red UC under 980 nm laser excitation, which are attributed to the significantly different electron population pathways. The UC intensity is stable and the afterglow decreases gradually over time, thus the output color is clearly changed from green to red naturally via illuminating the pre-X-ray-irradiated NPs with a 980 nm laser. Furthermore, the fine emission profiles of Er3+, Ho3+ and Tm3+ are achieved upon X-ray irradiation. Our results develop a new approach for time-dependent dynamic color displays and a simple route to revealing the electronic fine structures of lanthanide activators at room temperature.

Fluorescence intensity ratio-based temperature sensing with a self-referencing characteristic is highly demanded for reliable and accurate sensing. Although enormous efforts have been devoted to explore high-performance luminescent temperature probes, it remains a daunting challenge to achieve highly relative sensitivity which determines temperature resolution. Herein, we employ a novel strategy to achieve temperature probes with ultrahigh relative sensitivity through integrating both positive and negative thermal quenching effect into a hydrogel. Specifically, Er3+ ions show evidently a positive thermal quenching effect in Yb/Er:NaYF4@NaYF4 nanocrystals while Nd3+ and Tm3+ ions in a Yb2W3O12 bulk exhibit prominently a negative thermal quenching effect. With elevating temperature from 313 to 553 K, the fluorescence intensity ratio of Er (540 nm) to Nd (799 nm) and Tm (796 nm) to Er (540 nm) is significantly decreased about 1654 times and increased about 14,422 times, respectively. The maximum relative sensitivity of 15.3% K-1 at 553 K and 23.84% K-1 at 380 K are achieved. The strategy developed in this work sheds light on highly sensitive probes using lanthanide ion-doped materials.

The emission properties of lanthanide ions have been extensively investigated for their interesting physical processes and enormous applications. Conventional strategies have been used to modify luminescence properties such as temperature, pressure, and modifying components. However, the traditional methods are volatile and irreversible, which is unconducive for some optoelectronic applications. In this article, the electromechanical softness of the ferroelectric lattice is employed, which makes the strong coupling relationship between the electric field and the photonic properties of lanthanide ions. The emission intensity of the Tm3+:3H4-4H6 and 3F4-4H6 transitions was exceptionally enhanced by 2.6 and 3.2 times via ferroelectric polarization, respectively. Meanwhile, the luminescence response presents excellent reversibility and nonvolatility. This study provides a unique proposal for designing highly integrated stimuli-responsive photonic materials toward a variety of applications.

It is common that thermal quenching in luminescent materials inevitably occurs with an elevation in temperature. Herein, an inverse thermal quenching effect in lanthanide-doped upconversion nanocrystals was reported. Specifically, defects with an appropriate energy state were designed in Yb/Er (Ho, Tm) doped Na3ZrF7 nanocrystals to actualize a thermal-induced enhancement in luminescent intensity. Owing to this special temperature dependent optical property, our results reveal that these kinds of materials are very suitable for high security anti-counterfeiting applications.

Luminescent materials with negative thermal quenching effects show superior potentials in temperature sensing and anti-counterfeiting fields. Normally, smaller nanocrystals (NCs) with higher surface-to-volume ratios are used to achieve larger thermal-induced intensification of upconversion (UC) emission intensity. Herein, confining sensitizers and activators to the shell layer with two-dimensional space and introducing defect energy levels via doping low-valence ions are simultaneously applied to enhance the negative thermal quenching effect. By increasing the temperature from 293 K to 413 K, the integral UC emission intensity of 20Yb/2Er:NaGdF4 increases only by ∼2.2 times, whereas that of NaGdF4@20Ca/20Yb/2Er:NaGdF4 core–shell NCs with a similar particle size increases by ∼10.9 times.

The ultrasmall Yb/Er: CaF 2 NCs are chosen as an example to certify the upconversion emission intensity can be greatly enhanced through combining various approaches together, including doping Na + ions to promote the growth the particle size and tailor the crystal-field environment of the emitters, control shell thickness to lower the non-radiative decay and tune Yb 3+ concentration in the shell to enhance the absorption of the excitation energy. In comparison with Na + -free 20Yb/2Er: CaF 2 NCs (~ 2 nm), the upconversion luminescence of 20Yb/2Er: CaF 2 @10Yb: CaF 2 NCs (~ 9 nm) containing ~ 0.5 mmol Na + ions in the core and 10% Yb 3+ in the shell with 5 nm thickness intensifies for about 95 times.

Lanthanide (Ln3+)-doped nanoparticles that produce strong red upconversion (UC) luminescence are desirable for biological applications. For nanoparticles, modification of their electronic properties and stabilization of their crystallographic phase are crucial for tuning their luminescence behaviors. Here, Yb3+–Er3+ doped Gd2O3 nanoparticles have been synthesized by calcining Gd2O2S in nitrogen (O2-free) and ambient air (∼21% O2), respectively. Various approaches including X-ray diffraction (XRD), transmission electron microscopy (TEM), Raman spectroscopy and thermo-gravimetric and differential thermal analyses (TG-DTA) were used to characterize the structure, morphology and phase formation mechanism of the nanoparticles. Upconversion luminescence properties were investigated by emission intensity vs. excitation power (double logarithmic relationship) and temperature dependent emission spectroscopy. The different splitting transition probabilities of the cubic and/or monoclinic phase, the probability of multiphonon relaxations and cross relaxations between Er3+ ions are responsible for the efficient red emission in the cubic Gd2O3 nanoparticles. More importantly, the lower phonon energy of the pure cubic sample has good thermal stability in the current detectable range of 20–100 °C. These results imply that the cubic Gd2O3 upconversion nanoparticles may have multiplexing functionality in bio-imaging.

After coating 20 Yb/2 Er:NaGdF4 core nanocrystals with a NaYbF4 shell, upconversion emission of the rare earth ions weakens. So far, the exact reason for this phenomenon is still unclear due to lack of the direct evidence. In this report, a core@shell@shell sandwich-like structure is designed and fabricated to investigate this phenomenon. We find that high Yb(3+) concentration in the shell has mainly two adverse impacts: it promotes not only the deleterious back energy transfer from Er(3+) in the core to Yb(3+) in the shell but also the energy transfer from Yb(3+) in the core to Yb(3+) in the shell. To obtain nanocrystals with high upconversion efficency, appropriate Yb(3+) concentration should be introduced into the shell or the transition layer.

In most of the reports on nanocrystal (NC) luminescent, the power depedence of luminescent intensity is studied using a continuous-wave laser with low power density, and the slopes of those different emission bands keep unchanged. In this paper, the up/down conversion of monodisperse Yb/Er: NaGdF4@Yb: NaGdF4@Yb: NaNdF4 NCs under the excitation of 800[Formula: see text]nm femtosecond laser was detected simultaneously by a home-built optical measurement setup. The power-dependent optical properties under the excitation of femtosecond laser are quite different from the normal situation, which is due to their high peak power and energy transfer processes: Nd[Formula: see text].

Upon introducing Ca(2+) dopants into the grain lattices by substituting Gd(3+) ions, irregular Yb/Er:NaGdF4 nanocrystals prepared through a simple solvothermal route convert into highly uniform nanorods. Meanwhile, their upconversion luminescence intensifies by about 200 times, probably due to a modification of the crystal structure of NaGdF4 and an improvement in the crystallinity of the nanophase.

A series of Er(3+)-doped (1 - x)CaF(2)-xYbF(3) (0 ≤ x ≤ 0.6) disordered solid-solution nanocrystals with various mean sizes were successfully prepared by a facile solvothermal route. Interestingly, abnormal size-dependent upconversion emissions were demonstrated in these nanocrystals for the first time. With increasing grain size, an obvious enhancement of red to green emission ratio was observed in the Er(3+) (2 mol%): 0.4CaF(2)-0.6YbF(3) nanocrystals, which is the opposite of the routine size-dependent upconversion emission behavior reported previously. Taking Eu(3+) ions as a structural probe, we investigated the influence of a disordered solid-solution structure on Ln(3+) luminescence, and proposed that Ln(3+) clusters formed in the host should play a key role to induce this unusual size-dependent upconversion emission phenomenon. As a consequence, multi-colors such as green, yellow, and red upconversion emissions can be easily realized by appropriately modifying the Yb(3+) content in the Er(3+)-doped (1 - x)CaF(2)-xYbF(3) nanocrystals. The reported results will deepen the understanding of size effects on the lanthanide upconversion in nanocrystals.