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.

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.

Multi-peaked broad-band red phosphor Y3Si6N11:Pr3+ for white LEDs and temperature sensing.

The indispensable broad-band red phosphors for LED lighting generally show a long emission tail for wavelengths longer than 650 nm, which consumes excitation energy but contributes little luminance. Here, we report, for the first time, a broad red emission band with a steep falling edge at 652 nm, formed of widely distributed 1D2 → 3H4 emission lines of Pr3+ in Y3Si6N11 due to a large Stark splitting of the 3H4 (930 cm-1) and 1D2 (725 cm-1) levels. The red emission exhibits a 43 nm bandwidth, which is the widest in Pr3+-doped phosphors reported so far. The red Y3Si6N11:Pr3+ phosphor was applied for the fabrication of 310 nm UV chip-based white LEDs, and a high color rendering index of 96 at a low correlated color temperature of 4188 K was achieved. Furthermore, a temperature-sensing scheme was proposed based on the temperature-dependent intensity ratios of the emission lines from the thermally coupled and large Stark splitting levels of the 1D2 state. Relative sensitivities as a function of temperature were studied in the range of 93-473 K. The findings of this study indicate that Y3Si6N11:Pr3+ is an attractive broad-band red phosphor for both high color rendering white LEDs and temperature sensing applications.

Observation of a red Ce3+ center in SrLu2O4:Ce3+ phosphor and its potential application in temperature sensing.

In Ce3+ activated SrLn2O4 type phosphors (Ln = Y, Lu, Sc, etc.) only one Ce3+ center was previously reported to show a blue emission band. In this paper, we report the observation of a second Ce3+ center in SrLu2O4:Ce3+. The new center shows a red emission band peaking at 600 nm with an excitation band at 485 nm. We attributed the new center (Ce(ii)) to the substitution of the Lu3+ site and the original blue center (Ce(i)) to the substitution of the Sr2+ site. Spectroscopy studies indicate that Ce(i) centers are preferentially formed at a low doping concentration and the number ratio of Ce(i)/Ce(ii) decreases with increasing Ce3+ concentration until beyond 0.002. The fluorescence lifetimes of the two centers were measured for various doping concentrations. Energy transfer from Ce(i) to Ce(ii) was observed. It was found that the emission intensity of Ce(ii) centers reduces much faster than that of Ce(i) with increasing temperature from 83 K up to 350 K, implying their potential application in temperature sensing based on their temperature dependent intensity ratios. A relative sensing sensitivity as high as 2.28% K-1 at 283 K was achieved.

Two Ce3+ centers induced broadband emission in Y3Si6N11:Ce3+ yellow phosphor.

The Y3Si6N11:Ce3+ yellow phosphor shows a well-known ∼150 nm broad emission band, exhibiting a potential application in UV or blue based white LEDs. We report the observation of two Ce3+ emitting centers, the superposition of which forms the broad emission band. One center has a green emission band peaked at 539 nm (Ce1) with the first excitation band at 420 nm. The other has a red emission band peaked at 600 nm (Ce2) with the first excitation band at 485 nm. The two Ce3+ centers are assigned to the substitution for two Y sites in Y3Si6N11. It was found that the Ce2 emission intensity is continuously enhanced relative to that of Ce1 with an increasing Ce3+ concentration, thus leading to a redshift of the broadband. Meanwhile, a more notable fluorescence lifetime shortening of Ce1 compared to Ce2 with an increasing Ce3+ concentration was observed. These results suggest the occurrence of energy transfer from Ce1 to Ce2. The temperature-dependent luminescence intensity of Y3Si6N11:Ce3+ was also studied in the range of 93 to 473 K.

Hydrothermal Synthesis and Upconversion Properties of About 19 nm Sc2O3: Er3+, Yb3+ Nanoparticles with Detailed Investigation of the Energy Transfer Mechanism.

The Sc2O3: Er3+, Yb3+ nanoparticles (NPs) with the size of about 19 nm were synthesized by a simple oleic acid-mediated hydrothermal (HT) process. X-ray diffraction (XRD), transmission electron microscopy (TEM), upconversion luminescence (UCL) spectra, and decay curves were used to characterize the resulting samples. The Sc2O3: Er3+, Yb3+ NPs made by HT method exhibit the stronger UCL, of which the red UCL are enhanced by a factor of 4, in comparison with those samples prepared by solvothermal (ST) method at the same optimized lanthanide ion concentrations. The UCL enhancement can be attributed to the reduced surface groups and longer lifetimes. Under 980 nm wavelength excitation, the decay curves of Er3+: (2H11/2, 4S3/2) → 4I15/2 and 4F9/2 → 4I15/2 emissions for Sc2O3: Er3+, Yb3+ NPs samples are both close to each other, resulting from the cross relaxation energy transfer from Er3+ to Yb3+, followed by an energy back transfer within the same Er3+-Yb3+ pair. Also, under the relatively low-power density, the slopes of the linear plots of log(I) vs. log(P) for red and green emissions are 2.5 and 2.1, implying the existence of three-photon processes. Our results indicate that Sc2O3: Er3+, Yb3+ NPs is an excellent material for achieving intense UCL with small size in the biological fields.

Efficient Blue-emitting Phosphor SrLu2O4:Ce3+ with High Thermal Stability for Near Ultraviolet (~400 nm) LED-Chip based White LEDs.

Blue-emitting phosphors for near ultraviolet (NUV) based tri-color RGB phosphor blend converted white light emitting diodes (LEDs) have been extensively investigated in the past few years. LED chip peaked near 400 nm is the most efficient among the NUV chips currently. However, most of blue phosphors show inefficient excitation around 400 nm. Herein, a novel blue phosphor SrLu2O4:Ce3+ matching well with near 400 nm chip and showing high thermal stability has been developed. The photoluminescence spectrum presents a broad emission band peaking at 460 nm with a bandwidth of nearly 90 nm. By optimizing the Ce3+ concentration, an internal quantum efficiency (IQE) as high as 76% was achieved. Furthermore, 86% of the room-temperature emission intensity is still maintained at 150 °C, indicating a good thermal stability and practicality. A series of white LEDs were fabricated based on 405 nm chips coated with a blend of the new blue phosphor with the commercial yellow and red phosphors. High color rendering indexes (≥90) were achieved while the correlated color temperature was tuneable in the range of 3094 to 8990 K. These results suggest that SrLu2O4:Ce3+ can be utilized as a blue-emitting phosphor in NUV based white LEDs.

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.

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.

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.

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.

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.

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.

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.

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.

Red emission generation through highly efficient energy transfer from Ce(3+) to Mn(2+) in CaO for warm white LEDs.

CaO:Ce(3+),Mn(2+) phosphors with various Mn(2+) concentrations were synthesized by a solid state reaction method. Efficient energy transfer from Ce(3+) to Mn(2+) was observed and it allows the emission color of CaO:Ce(3+),Mn(2+) to be continuously tuned from yellow (contributed by Ce(3+)) to red (by Mn(2+)) with an increase in Mn(2+) concentration and upon blue light excitation. The red emission becomes dominant when the Mn(2+) concentration is ≥0.014 with an energy transfer efficiency higher than 87% which can reach as high as 94% for a Mn(2+) concentration of only 0.02. A critical distance of 10.5 Å for the Ce(3+)-Mn(2+) energy transfer was determined. A faster decrease of Ce(3+) luminescence intensity in comparison with its lifetime was observed on increasing the Mn(2+) concentration. The analysis of this feature reveals that the Ce(3+) excitation energy can be completely transferred to Mn(2+) if the Ce(3+)-Mn(2+) distance is shorter than 7.6 Å. A warm white LED was fabricated through integrating an InGaN blue LED chip and a blend of two phosphors (YAG:Ce(3+) yellow phosphor and CaO:0.007Ce(3+),0.014Mn(2+) red phosphor) into a single package, which has CIE chromaticity coordinates of (x = 0.37, y = 0.35), a correlated color temperature of 3973 K and a color rendering index of 83.1. The results indicate that CaO:Ce(3+),Mn(2+) may serve as a potential red phosphor for blue LED based warm white LEDs.

Assembling of a functional cyclodextrin-decorated upconversion luminescence nanoplatform for cysteine-sensing.

A novel rhodamine-oxaldehyde (RHO) functionalized ß-NaYF4:Yb(3+)/Er(3+) upconversion nanoparticle (UCNP) for specific detection of cysteine (Cys) in aqueous solution was achieved through a Förster resonance energy transfer (FRET) process. Based on self-assembling interaction, hydrophobic upconversion nanoparticles could be modified with α-cyclodextrin to make them water dispersible.

Solvothermal synthesis and upconversion properties of about 10 nm orthorhombic LuF3: Yb³âº, Er³âº rectangular nanocrystals.

The Yb(3+) and Er(3+) codoped orthorhombic LuF3 rectangular nanocrystals (NCs) with the size of about 10nm were synthesized by a facile and effective solvothermal process. X-ray diffraction (XRD), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), upconversion (UC) luminescence spectra and decay curves were used to characterize the resulting samples. Compared with YF3 and α-NaYF4 NCs, owning the similar size and the same doping levels of Yb(3+) ions and Er(3+) ions as LuF3 NCs, the green UC emission of LuF3 NCs is 18.7 times and 5.1 times stronger than that of YF3 and α-NaYF4 NCs respectively; the red UC emission of LuF3 NCs is 13.2 times and 0.6 times stronger than that of YF3 and α-NaYF4 NCs respectively. Under 980 nm wavelength excitation, the decay curves of both (4)S3/2→(4)I15/2 transition and (4)F9/2→(4)I15/2 transition exhibit a single exponential function, resulting from the fast energy migrations among Yb(3+) ions caused by the high concentration of Yb(3+) ions (20 mol%). Meanwhile, at relatively low power density, the slopes of the linear plots between log(I) and log(P) for green UC and red UC are 1.7 and 1.9 respectively, which are less than 2 due to the quenching of the thermal effect, indicating a two-photon process for them. At high power density, the slopes are decreased caused by the saturation effect. In addition, we proved the existence of the thermal effect by the pump power dependence of the intensity ratio of (2)H11/2→(4)I15/2 transition to (4)S3/2→(4)I15/2 transition.

Selectively enhanced red upconversion luminescence and phase/size manipulation via Fe(3+) doping in NaYF4:Yb,Er nanocrystals.

Red upconversion luminescence (UCL) is selectively enhanced by about 7 times via Fe(3+) codoping into a NaYF4:Yb,Er nanocrystalline lattice. The maximum red-to-green ratio (R/G) as well as the overall integrated UCL intensity features at an Fe(3+) content of 20 mol%. The size and phase of nanocrystals are simultaneously manipulated via Fe(3+) doping with various concentrations by a facile hydrothermal method. Contrary to the literature, the pure hexagonal phase appears when Fe(3+) concentrations are from 5 to 20 mol%, meanwhile, the size of NaYF4:Yb,Er nanocrystals reaches its maximum at 10 mol%. The intensified visible UCL especially the dominant red emission is mainly ascribed to the energy transfer (ET) from |(2)F7/2, (4)T1g > (Yb(3+)-Fe(3+) dimer) to (4)F9/2 (Er(3+)) states as well as the distortion of the crystalline field symmetry upon Fe(3+) codoping. Dynamic investigation of (4)S3/2 and (4)F9/2 states under the pulsed laser excitation of 980 nm along with the diffuse reflectance data further supports the proposed mechanism of UC processes. The results show the remarkable promise of Fe(3+)-codoped NaYF4:Yb,Er nanocrystals as upconverting nanoprobes with high sensitivity and penetrability in deeper tissue for multimodal biomedical imaging.