Significant Enhancement of the Upconversion Emission in Highly Er3+ -Doped Nanoparticles at Cryogenic Temperatures.

Relatively low efficiency is the bottleneck for the application of lanthanide-doped upconversion nanoparticles (UCNPs). The high-level doping strategy realized in recent years has not improved the efficiency as much as expected. It is argued that cross relaxation (CR) is not detrimental to upconversion. Here we combine theoretical simulation and spectroscopy to elucidate the role of CR in upconversion process of Er3+ highly doped (HD) UCNPs. It is found that if CR is purposively suppressed, upconversion efficiency can be significantly improved. Specifically, we demonstrate experimentally that inhibition of CR by introducing cryogenic environment (40 K) enhances upconversion emission by more than two orders of magnitude. This work not only elucidates the nature of CR and its non-negligible adverse effects, but also provides a new perspective for improving upconversion efficiency. The result can be directly applied to cryogenic imaging and wide range temperature sensing.

Cross Relaxation Channel Tailored Temperature Response in Er3+ -rich Upconversion Nanophosphor.

Recently high doping of lanthanide ions (till 100 %) is realized unprecedentedly in nanostructured upconversion (UC) phosphors. However, oddly enough, this significant breakthrough did not result in a corresponding UC enhancement at ambient temperature, which hinders the otherwise very interesting applications of these materials in various fields. In this work, taking the Er3+ -rich UC nanosystem as an example, we confirm unambiguously that the phonon-assisted cross relaxation (CR) is the culprit. More importantly, combining the theoretical modeling and experiments, the precise roles of different CR channels on UC energy loss are quantitatively revealed. As a result, lowering the temperature can exponentially enhance the relevant UC luminescence by more than two orders of magnitude. Our comprehension will play an important role in promoting the UC performance and further application of high doping rare earth materials. As a proof of concept, an Er3+ -rich core/multi-shell nanophosphor is exploited which demonstrates the great potential of our finding in the field of ultra-sensitive temperature sensing.

Highly efficient upconversion emission of Er3+ in δ-Sc4Zr3O12 and broad-range temperature sensing.

Developing optical temperature sensors with a wider range, higher sensitivity and repeatability based on Er3+/Yb3+ doped upconverting phosphors has always been at the forefront of temperature measurement technologies. Here, we report the intense green upconversion luminescence in Er3+/Yb3+ doped δ-Sc4Zr3O12 for the first time and its temperature sensing performance is investigated. The structure of δ-Sc4Zr3O12 is given by Rietveld refinement of XRD data and the site occupancy of Er3+ ions has been determined. Compared with cubic Sc2O3 and ZrO2, under 972 nm excitation, the green emission from Er3+ centers in Sc4Zr3O12 is increased by 59-fold and 264-fold, respectively. By experimental analysis, this enhancement of upconversion luminescence is attributed to the low-symmetrical environment of Er3+, generation of Yb3+ clusters and high internal efficiency of Yb3+ emission in Sc4Zr3O12. In addition, the fluorescence intensity ratio of two green emission bands (2H11/2/4S3/2 → 4I15/2) is studied as a function of temperature ranging from 303 to 793 K in Sc4Zr3O12. The maximum sensitivity observed via calculation is 0.00634 K-1 at 573 K, and the sensitivity is still as high as 0.00534 K-1 at 793 K. The stability of a Sc4Zr3O12 thermometer is also examined via a recycling test. These findings suggest that δ-Sc4Zr3O12 is a promising upconversion host and could achieve high-sensitivity optical temperature sensing with a wide measuring range.

Highly Efficient Green-Emitting Phosphors Ba2Y5B5O17 with Low Thermal Quenching Due to Fast Energy Transfer from Ce3+ to Tb3.

This paper demonstrates a highly thermally stable and efficient green-emitting Ba2Y5B5O17:Ce3+, Tb3+ phosphor prepared by high-temperature solid-state reaction. The phosphor exhibits a blue emission band of Ce3+ and green emission lines of Tb3+ upon Ce3+ excitation in the near-UV spectral region. The effect of Ce3+ to Tb3+ energy transfer on blue to green emission color tuning and on luminescence thermal stability is studied in the samples codoped with 1% Ce3+ and various concentrations (0-40%) of Tb3+. The green emission of Tb3+ upon Ce3+ excitation at 150 °C can keep, on average, 92% of its intensity at room temperature, with the best one showing no intensity decreasing up to 210 °C for 30% Tb3+. Meanwhile, Ce3+ emission intensity only keeps 42% on average at 150 °C. The high thermal stability of the green emission is attributed to suppression of Ce3+ thermal de-excitation through fast energy transfer to Tb3+, which in the green-emitting excited states is highly thermally stable such that no lifetime shortening is observed with raising temperature to 210 °C. The predominant green emission is observed for Tb3+ concentration of at least 10% due to efficient energy transfer with the transfer efficiency approaching 100% for 40% Tb3+. The internal and external quantum yield of the sample with Tb3+ concentration of 20% can be as high as 76% and 55%, respectively. The green phosphor, thus, shows attractive performance for near-UV-based white-light-emitting diodes applications.

Enhanced ∼2 µm Emission of Tm3+ in Lu2O3 by Addition of a Trace Amount of Er3.

Er3+-induced intensity enhancement of ∼2 µm emission is observed in 2 atom % Tm3+ doped Lu2O3 under 782 nm excitation. The maximum enhancement reaches 41.9% with only 0.05 atom % Er3+. Er3+ introduces a new quantum cutting process which is proved to be a Tm3+ → Er3+ → Tm3+ forward-backward energy transfer (FBET) system. The FBET system is observed to work efficiently even at very low Er3+ concentration. Thus, energy loss due to energy migration among Tm3+ ions is suggested to be suppressed by the FBET process. The Tm3+ → Er3+ → Tm3+ FBET system may be a new route to improve the performance of Tm3+ lasers.

Improvement of Green Upconversion Monochromaticity by Doping Eu3+ in Lu2O3:Yb3+/Ho3+ Powders with Detailed Investigation of the Energy Transfer Mechanism.

The monochromaticity improvement of green upconversion (UC) in Lu2O3:Yb3+/Ho3+ powders has been successfully realized by tridoping Eu3+. The integral area ratio of green emission to red emission of Ho3+ increases 4.3 times with increasing Eu3+ doping concentration from 0 to 20 mol %. The energy transfer (ET) mechanism in the Yb3+/Ho3+/Eu3+ tridoping system has been investigated carefully by visible and near-infrared (NIR) emission spectra along with the decay curves, revealing the existence of ET from the Ho3+5F4/5S2 level tothe Eu3+5D0 level and ET from the Ho3+5I6 level to the Eu3+7F6 level. In addition, the population routes of the red-emitting Ho3+5F5 level in the Yb3+/Ho3+ codoped system under 980 nm wavelength excitation have also been explored. The ET process from the Yb3+2F5/2 level to the Ho3+5I7 level and the cross-relaxation process between two nearby Ho3+ ions in the 5F4/5S2 level and 5I7 level, respectively, have been demonstrated to be the dominant approaches for populating the Ho3+5F5 level. The multiphonon relaxation process originating from the Ho3+5F4/5S2 level is useless to populate the Ho3+5F5 level. As the energy level gap between the Ho3+5I7 level and Ho3+5I8 level matches well with that between Eu3+7F6 level and Eu3+7F0 level, the energy of the Ho3+5I7 level can be easily transferred to the Eu3+7F6 level by an approximate resonant ET process, resulting in a serious decrease in the red UC emission intensity. Since this ET process is more efficient than the ET from the Ho3+5F4/5S2 level to the Eu3+5D0 level as well as the ET from the Ho3+5I6 level to the Eu3+7F6 level, the integral area ratio of green emission to red emission of Ho3+ has been improved significantly.

Investigation of the Energy-Transfer Mechanism in Ho3+- and Yb3+-Codoped Lu2O3 Phosphor with Efficient Near-Infrared Downconversion.

A high-temperature solid-state method was used to synthesize the Ho3+- and Yb3+-codoped cubic Lu2O3 powders. The crystal structures of the as-prepared powders were characterized by X-ray diffraction. The energy-transfer (ET) phenomenon between Ho3+ ions and Yb3+ ions was verified by the steady-state spectra including visible and near-infrared (NIR) regions. Beyond that, the decay curves were also measured to certify the existence of the ET process. The downconversion phenomena appeared when the samples were excited by 446 nm wavelength corresponding to the transition of Ho3+: 5I8→5G6/5F1. On the basis of the analysis of the relationship between the initial transfer rate of Ho3+: 5F3 level and the Yb3+ doping concentration, it indicates that the ET from 5F3 state of Ho3+ ions to 2F5/2 state of Yb3+ ions is mainly through a two-step ET process, not the long-accepted cooperative ET process. In addition, a 62% ET efficiency can be achieved in Lu2O3: 1% Ho3+/30% Yb3+. Unlike the common situations in which the NIR photons are all emitted by the acceptors Yb3+, the sensitizers Ho3+ also make contributions to the NIR emission upon 446 nm wavelength excitation. Meanwhile, the 5I5→5I8 transition and 5F4/5S2→5I6 transition of Ho3+ as well as the 2F5/2→2F7/2 transition of Yb3+ match well with the optimal spectral response of crystalline silicon solar cells. The current research indicates that Lu2O3: Ho3+/Yb3+ is a promising material to improve conversion efficiency of crystalline silicon solar cell.

Inhomogeneous-Broadening-Induced Intense Upconversion Luminescence in Tm3+ and Yb3+ Codoped Lu2O3-ZrO2 Disordered Crystals.

Near-infrared (980 nm) to near-infrared (800 nm) and blue (490 nm) upconversion has been studied in 0.2% Tm3+ and 10% Yb3+ codoped Lu2O3-ZrO2 solid solutions as a function of the ZrO2 content in the range of 0-50%, prepared by a high-temperature solid-state reaction. The continuous enhancement of upconversion luminescence is observed with increasing ZrO2 content up to 30%. Analyses of the Yb3+ emission intensity and lifetime indicate enlarged absorption of a 980 nm excitation laser and enhanced energy transfer from Yb3+ to Tm3+ with the addition of ZrO2. The spectrally inhomogeneous broadening of the dopants in this disordered solid solution is considered to play the main role in the enhancement by providing better matches with the excitation laser line and increasing the spectral overlap for efficient energy transfer from Yb3+ to Tm3+. In addition, the inhomogeneous broadening is also validated to improve energy migration among Yb3+ ions and energy back transfer from Tm3+ to Yb3+. Hence, it is understandable that a drop in the upconversion luminescence intensity occurs as the concentration of ZrO2 is further increased from 30% to 50%. This work indicates the possibility of disordered crystals to achieve intense upconversion luminescence for biological and optoelectronic applications.

Enhancement of Eu3+ Red Upconversion in Lu2O3: Yb3+/Eu3+ Powders under the Assistance of Bridging Function Originated from Ho3+ Tridoping.

The red upconversion (UC) emission of Eu3+ ions in Lu2O3: Yb3+/Eu3+ powders was successfully enhanced by tridoping Ho3+ ions in the matrix, which is due to the bridging function of Ho3+ ions. The experiment data manifest that, in Yb3+/Eu3+/Ho3+ tridoped system, the Ho3+ ions are first populated to the green emitting level 5F4/5S2 through the energy transfer (ET) processes from the excited Yb3+ ions. Subsequently, the Ho3+ ions at 5F4/5S2 level can transfer their energy to the Eu3+ ions at the ground state, resulting in the population of Eu3+5D0 level. With the assistance of the bridging function of Ho3+ ion, this ET process is more efficient than the cooperative sensitization process between Yb3+ ion and Eu3+ ion. Compared with Lu2O3: 5 mol % Yb3+/1 mol % Eu3+, the UC intensity of Eu3+5D0→7F2 transition in Lu2O3: 5 mol % Yb3+/1 mol % Eu3+/0.5 mol % Ho3+ is increased by a factor of 8.

Low-Concentration Eu2+-Doped SrAlSi4N7: Ce3+ Yellow Phosphor for wLEDs with Improved Color-Rendering Index.

Luminescence property of low-concentration Eu2+-doped SrAlSi4N7:Ce3+ yellow phosphor is reported in this paper. Three optical centers Ce1, Ce2, and Eu2 are observed in the phosphor. Deconvolution of emission spectrum confirms the three centers to be green (530 nm), yellow (580 nm), and red (630 nm), respectively. This property promises considerable improvement of color-rendering property of a white light-emitting diode (wLED). For example, color-rendering index (CRI) of wLED fabricated by combining a blue LED chip and SrAlSi4N7:0.05Ce3+,0.01Eu2+ phosphor reaches 88. A competitive energy transfer process between Ce1-Ce2 and Ce1-Eu2 is confirmed based on Inokuti-Hirayama formula. Ratio of energy transfer rate between Ce1-Ce2 and Ce1-Eu2 (WCe1-Eu2/WCe1-Ce2) is calculated to be 2.0. This result reveals the effect of Eu2+ concentration on quantity of green and red components in SrAlSi4N7:Ce3+,Eu2+ phosphor.

Laser Spectroscopy of GdPO4 . nH2O:Eu Nanomaterials.

One-dimensional GdPO4 . nH2O:Eu nanowires and nanorods of different sizes and the same structure were synthesized by hydrothermal method. Nanowire and nanorods had width and length of about 10 nm/50 nm and 80 nm/1 µm, respectively. Adjusting reaction system PH value by adding alkali metal NaOH, the size and shape of the product can be tuned. The high resolution spectra, excitation spectra, and laser selective excitation spectra at low temperature were determined. Nanorod compared with nanowire, photoluminescence was enhanced, and the excitation spectrum and laser selective excitation spectra were broadened. These results suggest that Eu3+ in GdPO4 . nH20 nanorod and nanowire were located in different local environments.

Intense Upconversion Luminescence of CaSc2 O4 :Ho(3+) /Yb(3+) from Large Absorption Cross Section and Energy-Transfer Rate of Yb(3.).

Concentration-optimized CaSc2 O4 :0.2 % Ho(3+) /10 % Yb(3+) shows stronger upconversion luminescence (UCL) than a typical concentration-optimized upconverting phosphor Y2 O3 :0.2 % Ho(3+) /10 % b(3+) upon excitation with a 980 nm laser diode pump. The (5) F4 +(5) S2 →(5) I8 green UCL around 545 nm and (5) F5 →(5) I8 red UCL around 660 nm of Ho(3+) are enhanced by factors of 2.6 and 1.6, respectively. On analyzing the emission spectra and decay curves of Yb(3+) : (2) F5/2 →(2) F7/2 and Ho(3+) : (5) I6 →(5) I8 , respectively, in the two hosts, we reveal that Yb(3+) in CaSc2 O4 exhibits a larger absorption cross section at 980 nm and subsequent larger Yb(3+) : (2) F5/2 →Ho(3+) : (5) I6 energy-transfer coefficient (8.55×10(-17) cm(3) s(-1) ) compared to that (4.63×10(-17) cm(3) s(-1) ) in Y2 O3 , indicating that CaSc2 O4 :Ho(3+) /Yb(3+) is an excellent oxide upconverting material for achieving intense UCL.

Efficient near-infrared downconversion and energy transfer mechanism of ce(3+)/yb(3+) codoped calcium scandate phosphor.

An efficient near-infrared (NIR) downconversion has been demonstrated in CaSc2O4: Ce(3+)/Yb(3+) phosphor. Doping concentration optimized CaSc2O4: 1%Ce(3+)/5%Yb(3+) shows stronger NIR emission than doping concentration also optimized typical YAG: 1%Ce(3+)/5%Yb(3+) under 470 nm excitation. The NIR emission from 900 to 1100 nm is enhanced by a factor of 2.4. In addition, the main emission peak of Yb(3+) in the CaSc2O4 around 976 nm matches better with the optimal spectral response of the c-Si solar cell. The visible and NIR spectra and the decay curves of Ce(3+): 5d → 4f emission were used to demonstrate the energy transfer from Ce(3+) ions to Yb(3+) ions. The downconversion phenomenon has been observed under the direct excitation of Ce(3+) ions. On analyzing the dependence of energy transfer rate on Yb(3+) ion concentration, we reveal that the energy transfer (ET) from Ce(3+) ions to Yb(3+) ions in CaSc2O4 occurs mainly by the single-step ET process. Considering that the luminescence efficiency of CaSc2O4: Ce(3+) is comparable to that of commercial phosphor YAG: Ce(3+), the estimated maximum energy transfer efficiency reaches 58% in the CaSc2O4: 1%Ce(3+)/15%Yb(3+) sample, indicating that CaSc2O4: Ce(3+)/Yb(3+) sample has the potential in improving the conversion efficiency of c-Si solar cells.

Blue-emitting K2Al2B2O7:Eu(2+) phosphor with high thermal stability and high color purity for near-UV-pumped white light-emitting diodes.

Novel blue-emitting K2Al2B2O7:Eu(2+) (KAB:Eu(2+)) phosphor was synthesized by solid state reaction. The crystal structural and photoluminescence (PL) properties of KAB:Eu(2+) phosphor, as well as its thermal properties of the photoluminescence, were investigated. The KAB:Eu(2+) phosphor exhibits broad excitation spectra ranging from 230 to 420 nm, and an intense asymmetric blue emission band centered at 450 nm under λex = 325 nm. Two different Eu(2+) emission centers in KAB:Eu(2+) phosphor were confirmed via their fluorescence decay lifetimes. The optimal concentration of Eu(2+) ions in K2-xEuxAl2B2O7 was determined to be x = 0.04 (2 mol %), and the corresponding concentration quenching mechanism was verified to be the electric dipole-dipole interactions. The PL intensity of the nonoptimized KAB:0.04Eu(2+) phosphor was measured to be ∼58% that of the commercial blue-emitting BaMgAl10O17:Eu(2+) phosphor, and this phosphor has high color purity with the CIE coordinate (0.147, 0.051). When heated up to 150 °C, the KAB:0.04Eu(2+) phosphor still has 82% of the initial PL intensity at room temperature, indicating its high thermal stability. These results suggest that the KAB:Eu(2+) is a promising candidate as a blue-emitting n-UV convertible phosphor for application in white light emitting diodes.

Importance of suppression of Yb(3+) de-excitation to upconversion enhancement in ß-NaYF4: Yb(3+)/Er(3+)@ß-NaYF4 sandwiched structure nanocrystals.

Nanosized Yb(3+) and Er(3+) co-doped ß-NaYF4 cores coated with multiple ß-NaYF4 shell layers were synthesized by a solvothermal process. X-ray diffraction and scanning electron microscopy were used to characterize the crystal structure and morphology of the materials. The visible and near-infrared spectra as well as the decay curves were also measured. A 40-fold intensity increase for the green upconversion and a 34-fold intensity increase for the red upconversion were observed as the cores are coated with three shell layers. The origin of the upconversion enhancement was studied on the basis of photoluminescence spectra and decay times. Our results indicate that the upconversion enhancement in the sandwiched structure mainly originates from the suppression of de-excitation of Yb(3+) ions. We also explored the population of the Er(3+4)F9/2 level. The results reveal that energy transfer from the lower intermediate Er(3+4)I13/2 level is dominant for populating the Er(3+4)F9/2 level when the nanocrystal size is relatively small; however, with increasing nanocrystal size, the contribution of the green emitting Er(3+4)S3/2 level for populating the Er(3+4)F9/2 level, which mainly comes from the cross relaxation energy transfer from Er(3+) ions to Yb(3+) ions followed by energy back transfer within the same Er(3+)-Yb(3+) pair, becomes more and more important. Moreover, this mechanism takes place only in the nearest Er(3+)-Yb(3+) pairs. This population route is in good agreement with that in nanomaterials and bulk materials.

The energy transfer mechanism in Pr(3+) and Yb(3+) codoped ß-NaLuF(4) nanocrystals.

The Pr(3+) and Yb(3+) codoped ß-NaLuF4 hexagonal nanoplates with a size of 250 nm × 110 nm were synthesized by a solvothermal process. X-Ray diffraction and scanning electron microscopy were used to characterize the crystal structure and morphology of the materials. The visible and near infrared spectra as well as the decay curves of Pr(3+):(3)P0 level were used to demonstrate the energy transfer from Pr(3+) ions to Yb(3+) ions. The downconversion phenomenon has been observed under the direct excitation of the (3)P2 level of Pr(3+). According to the analysis of the dependence of the initial transfer rate upon Yb(3+) ion concentration, it indicates that the ET from Pr(3+) ions to Yb(3+) ions is only by a two-step ET process when the Yb(3+) concentration is very low; however, with the increase of the Yb(3+) concentration, a cooperative ET process occurs and gradually increases; when the Yb(3+) ion concentration increases to 20 mol%, the ET from Pr(3+) ions to Yb(3+) ions occurs only by the cooperative ET process. When the doping concentration of Yb(3+) ions reaches 20 mol% at a fixed concentration of Pr(3+) ions (1 mol%), the theoretical quantum efficiency is 192.2%, close to the limit of 200%. The current research has great potential in improving the conversion efficiency of silicon solar cells.

Spectroscopic properties and upconversion studies in Ho(3+) /Yb(3+) Co-doped calcium scandate with spectrally pure green emission.

The optical properties of a Ho(3+) /Yb(3+) co-doped CaSc2 O4 oxide material are investigated in detail. The spectral properties are described as a function of doping concentrations. The efficient Yb(3+) →Ho(3+) energy transfer is observed. The transfer efficiency approaches 50 % before concentration quenching. The concentration-optimized sample exhibits a strong green emission accompanied with a weak red emission, showing perfect green monochromaticity. The results of the spectral distribution, power dependence, and lifetime measurements are presented. The green, red, and near-infrared (NIR) emissions around 545, 660, and 759 nm are assigned to the (5) F4 +(5) S2 →(5) I8 , (5) F5 →(5) I8 , and (5) F4 +(5) S2 →(5) I7 transitions of Ho(3+) , respectively. The detailed study reveals the upconversion luminescence mechanism involved in a novel Ho(3+) /Yb(3+) co-doped CaSc2 O4 oxide material.

Tunable full-color emitting BaMg2Al6Si9O30:Eu2+, Tb3+, Mn2+ phosphors based on energy transfer.

A series of single-phase full-color emitting BaMg(2)Al(6)Si(9)O(30):Eu(2+), Tb(3+), Mn(2+) phosphors has been synthesized by solid-state reaction. Energy transfer from Eu(2+) to Tb(3+) and Eu(2+) to Mn(2+) in BaMg(2)Al(6)Si(9)O(30) host matrix is studied by luminescence spectra and energy-transfer efficiency and lifetimes. The wavelength-tunable white light can be realized by coupling the emission bands centered at 450, 542, and 610 nm ascribed to the contribution from Eu(2+) and Tb(3+) and Mn(2+), respectively. By properly tuning the relative composition of Tb(3+)/Mn(2+), chromaticity coordinates of (0.31, 0.30), high color rendering index R(a) = 90, and correlated color temperature (CCT) = 5374 K can be achieved upon excitation of UV light. Thermal quenching properties reveal that BaMg(2)Al(6)Si(9)O(30): Eu(2+), Tb(3+), Mn(2+) exhibits excellent characteristics even better than that of YAG:Ce. Our results indicate our white BaMg(2)Al(6)Si(9)O(30):Eu(2+), Tb(3+), Mn(2+) can serve as a key material for phosphor-converted light-emitting diode and fluorescent lamps.

Enriching red emission of Y3Al5O12: Ce3+ by codoping Pr3+ and Cr3+ for improving color rendering of white LEDs.

Triply doped Y3Al5O12: Ce3+, Pr3+, Cr3+ phosphors are prepared by solid state reaction. The emission spectra are enriched in the red region with the luminescence of both Pr3+ and Cr3+ through Ce3+ → Cr3+ and Ce3+ → Pr3+ → Cr3+ energy transfers. The properties of photoluminescence and fluorescence decay indicates larger macroscopic Ce3+ → Cr3+ transfer rates in the triply doped phosphors in comparison to Ce3+ and Cr3+ doubly doped one, reflecting the effect of competition between Ce3+ → Cr3+ and Ce3+ → Pr3+ transfers. White LEDs fabricated using the triply doped phosphor coated on blue LED chips show a color rendering index of 81.4 higher than that either using Ce3+ and Cr3+ doubly doped or Ce3+ singly doped phosphor.

Remarkably enhanced photoluminescence of hexagonal GdPO4.nH2O:Eu with decreasing size.

The hexagonal rhabdophane-type GdPO(4) hydrate (GdPO(4).nH(2)O) was synthesized via a simple hydrothermal process. The size and morphology of the products can be tunable by adjusting the pH of reaction systems through the addition of aqueous NaOH. The nanorods with a width of 50-100 nm and a length of about 1 microm were obtained in the absence of NaOH (pH = 2), while a significant reduction of size (width: approximately 10 nm, length: approximately 50 nm) was observed for the product synthesized in the presence of NaOH (pH = 10). Surprisingly, the small-sized product exhibits a remarkably enhanced photoluminescence quantum yield and long excited state lifetime in comparison with those of the large-sized product. This abnormal luminescence phenomenon is discussed and explained. The EDS and XPS measurements revealed the presence of Na(+) in the small-sized samples. These Na(+) cations were probably bonded to the surface O(2-) dangling bonds, which thus reduces the number of surface defects that usually serve as the nonradiative energy transfer center channels. A considerable reduction of surface defect centers results in the increase of the emission efficiency and excited state lifetime in a small-sized sample. Obviously, the controlled synthesis of rare-earth-doped nanoparticles with a small size, but with relatively strong luminescence, is significant for their applications in the areas of technologies including optoelectronics, sensing and bioimaging.