Highly efficient La2O3:Yb3+,Tm3+ single-band NIR-to-NIR

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Functional Inorganic Materials and Devices 2

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Highly efficient LaO:Yb ,Tm single-band NIR-to-NIR upconverting microcrystals for anti-counterfeiting applications Guojun Gao, Dmitry Busko, Reetu Joseph, Ian A. Howard, Andrey Turshatov, and Bryce Sydney Richards ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b11196 • Publication Date (Web): 24 Oct 2018 Downloaded from http://pubs.acs.org on November 2, 2018

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ACS Applied Materials & Interfaces

Highly efficient La2O3:Yb3+,Tm3+ single-band NIR-to-NIR upconverting microcrystals for anti-counterfeiting applications Guojun Gao1*, Dmitry Busko1, Reetu Joseph1, Ian A. Howard1,2, Andrey Turshatov1 and Bryce S. Richards1,2* 1

Institute of Microstructure Technology, Karlsruhe Institute of Technology, 76344 Eggenstein-

Leopoldshafen, Germany 2

Light Technology Institute, Karlsruhe Institute of Technology, Engesserstrasse 13, 76131

Karlsruhe, Germany *Corresponding

authors:

Dr.

G.

Gao:

[email protected];

Prof.

B.S.

Richards:

[email protected]

Abstract Efficient single-band NIR-to-NIR upconvrsion (UC) emission is strongly desired for many applications such as fluorescent markers, plastic recycling, and biological imaging. Herein, we report highly-efficient single-band NIR-to-NIR UC emission in La2O3:Yb3+,Tm3+ (LYT) microcrystals. Under 980 nm laser excitation, LYT exhibits NIR UC emission at ~795 nm (Tm3+: 3H

4

→ 3H6), and blue UC emission at ~476 nm; the NIR UC emission is dominant, with the

intensity ratio of the NIR to blue INIR/Iblue > 100. Remarkably, a high absolute UC quantum yield (UCQY) of 3.4 % is obtained for the single-band NIR UC emission of LYT at a relatively low excitation power density of 7.6 W/cm2. This value is much higher than the reported values of a single-band NIR UC for rare earth based UC materials in literature, such as the well-known

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benchmark UC materials of -NaYF4:Yb3+,Er3+ (~0.9%, with a excitation power density of 9 W/cm2) and Gd2O2S:Yb3+,Er3+ (~1.9%, with a excitation power density of 20 W/cm2). The high absolute UCQY of single-band NIR UC emission combined with their facile preparation hints at their potential application in anti-counterfeiting, verified by the proof-of-concept demonstration of fluorescent labeling of a transparent “IMT” pattern. Keywords: up-conversion materials, high UC quantum yield, NIR-to-NIR single band UC, Yb3+/Tm3+ UC, anti-counterfeiting

1. Introduction Up-conversion (UC) emission describes the phenomenon of converting the absorption of two or more low-energy photons to a single high-energy photon emission. Since the first successful demonstration of UC emission in 1966 by Auzel 1, trivalent lanthanide (Ln3+) activated inorganic UC materials have been intensively studied due to their potential applications in areas applications including fluorescent markers, sensors, solid-state lasers, bio-imaging, 3D volumetric displays, and solar energy harvesting.2-9 Among them, Yb3+/Er3+ activated UC materials that use Yb3+ as sensitizer have been widely recognized to be the most efficient UC materials.10-12 In these materials, Yb3+ enhances the absorption at ~980 nm and Er3+ leads to UC emission in the green and red. However, strong NIR UC emissions is of particular interest to many applications. For example, materials demonstrating efficient NIR-to-NIR UC allow the fabrication of fluorescent markers, that capitalize on the high sensitivity of silicon detectors in the NIR region.7 Furthermore, NIR-to-NIR UC emission is located within the first biological window from 650 - 1100 nm (NIR I), making them excellent candidates in the area of biological


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imaging.12-16

Upon 980 nm excitation, NIR-to-NIR UC emission is normally a two photon process. Among all Ln3+ combinations, Yb3+/Tm3+ and Yb3+/Er3+ pairs yield the most efficient NIR UC emission at ~800 nm (Tm3+: 3H4 → 3H6) and ~850 nm (Er3+: 4S3/2 → 4I13/2), respectively.17-20 For the Yb3+/Er3+ pair, the UC emission in the NIR is significantly weaker than that in the visible region. In contrast, for the Yb3+/Tm3+ pair, the NIR UC emission of Tm3+ at ~800 nm dominates the UC emission spectrum compared to the much weaker blue UC emission of Tm3+ (1G4 → 3H6) when excited at 980 nm.17,

18, 21, 22

Generally speaking, high doping levels of Tm3+ and low doping

levels Yb3+ as well as low pump power density at ~980 nm, all facilitate enhanced NIR UC emission of Tm3+.23,

24

To date, efficient UC materials have been reported in the green to red

region based on Yb3+/Er3+ activated UC materials.25 However, investigations into NIR-to-NIR UC materials are less common and their absolute UC quantum yield (UCQY) value in NIR is rarely reported (for both Yb3+/Er3+ and Yb3+/Tm3+ pairs). Furthermore, many studies on Yb3+/Er3+ and Yb3+/Tm3+ activated UC materials ignore the UC emission in NIR region, due to the aim of the development of visible UC phosphors for lighting or visual-marking applications.22,

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Additionally, real single-band NIR-to-NIR UC is extremely hard to obtain due to the accompanied strong blue UC emission for Yb3+/Tm3+ pair. In present study, we define the singleband NIR-to-NIR UC with intensity ratio of INIR/Iblue >100.

Oxide materials exhibit excellent thermal stability in addition to their facile preparation methods, making them promising candidates for UC materials. Our recent study demonstrates that La2O3 micro-crystals are excellent UC host materials for Ln3+ luminescence centers.27 The ACS Paragon Plus Environment


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La2O3:Yb3+,Er3+ phosphors exhibited tunable UC emission colors from pure green to reddishorange by simply controlling synthesis conditions, whilst maintaining the high absolute UCQY of ~3.0%, measured at an excitation power density of 7.6 W/cm2.27 Compared to other Yb3+,Er3+based phosphors reported in the literature, the oxide based UC materials exhibit comparable UCQY values to benchmark fluoride (NaYF4:Yb3+,Er3+) and oxysulfilde (Gd2O2S:Yb3+,Er3+) hosts, both exhibiting ~3% UCQY at a excitation power density of >20 W/cm2).28,

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For the

Yb3+/Tm3+ pair, Etchart et al. reported NIR-to-NIR UC energy conversion efficiency – which is defined as the emitted power divided by absorbed power and roughly doubles the UCQY – of 1.53 ± 0.07% using Y2BaZnO5:Yb3+,Tm3+ phosphors under 977 nm excitation with a power density of 2.2 W/cm2.21, 30, 31 For the Yb3+/Er3+ pair, Pokhrel et al. reported the high NIR UCQY of 1.88 ± 0.40% for Gd2O2S:Yb3+,Er3+ with an excitation power density of 9 W/cm2.19,

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Recently, Resch-Genger and Haase’s groups reported 9 and 10% UCQY values (over the whole UC emission spectra with a power density of 20 W/cm2) for NaYF4:Yb3+,Er3+/NaYF4 core/shell nanocrystals and micro-sized NaYF4:Yb3+,Er3+ particles, respectively.33, 34 The UCQY value for a single NIR band is 0.92% (with a power density of 20 W/cm2) which much lower than for single green or red band. La2O3:Yb3+,Er3+ (LYE), Gd2O2S:Yb3+,Er3+ (GOS) and NaYF4:Yb3+,Er3+ (NYF) based phosphors are all chosen as reference materials and compared with our new La2O3:Yb3+,Tm3+ (LYT) NIR emitting phosphor in terms of intensity/brightness and absolute UCQY.

In the present study, we choose the La2O3 oxide as a host material for Yb3+/Tm3+ pair. The UC emission properties of two series of La2O3:Yb3+,Tm3+ (LYT) doped with varying amounts of Yb3+ and Tm3+ are studied in detail. The dominant NIR UC emission peak centered at ~795 nm


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can be clearly observed for our LYT materials. The optimum doping concentration of Yb3+ and Tm3+ for maximizing NIR UC emission in LYT is determined to be 6 and 0.4 mol%, respectively. LYT-4Yb.0.4Tm yields a high absolute UCQY of 3.4 % (using an excitation power density of 6.7 W/cm2) and is much higher than the values observed for the reference materials. The proof-of-concept demonstration of our “IMT” pattern demonstrates their potential application in anti-counterfeiting and fluorescent marker technologies. 2. Experimental Section Sample preparation: To determine the optimum doping concentration of Tm3+ and Yb3+ in La2O3:Yb3+,Tm3+ (LYT) and to investigate the effect of various doping concentration of Yb3+ and Tm3+ on UC emission properties of LYT, two series of LYT microcrystals were synthesized. In series I, we fix the doping concentration of Tm3+ at 0.4 mol% and vary the doping concentration of Yb3+ from 2 to 12 mol%. In series II, we fix the doping concentration of Yb3+ at 6 mol% and vary the doping concentration of Tm3+ from 0.1 to 0.9 mol%. All micro-crystalline powder samples with nominal chemical formulas of La1.992-2xO3:,xYb3+,0.4Tm3+ (LYT-xYb,0.4Tm, x = 2, 4, 6, 8, 10 and 12) and La1.88-2yO3: 6Yb3+,yTm3+ (LYT-6Yb,yTm, y = 0.2, 0.25, 0.3, 0.35, 0.4, 0.45, 0.5, 0.6 and 0.9) were synthesized by high temperature solid state reaction. Stoichiometric amounts of RE2O3 (~2 g, ChemPur, 99.9%, RE = La, Yb and Tm) were weighed, mixed, ground homogenously in an agate mortar and then sintered in alumina crucibles at 1300 - 1650 oC for 30 h in air with three intermediate grindings. The reference material of La2O3:Yb,Er (LYE) was synthesized by solid state reaction according to the method described in literature.35 The reference luminescence materials of NaYF4:Yb3+,Er3+ (NYF) and Gd2O2S:Yb3+,Er3+ (GOS) were obtained from Shanghai Huaming Gona Rare Earth New Materials Co. Ltd (China) and Maxmax (USA), respectively.



Structural and optical characterization: The crystal structures of LYT were studied via X-ray powder diffraction patters using Cu K as radiation (1.5405 Ǻ) (Bruker D8, Cu K). The diffuse reflectance spectrum was measured with a Perkin-Elmer Lambda 950 equipped with an integrating sphere. The morphology of LYT was investigated using a scanning electron microscope (Zeiss Supra 60 VP). Commission International de I’Eclairage (CIE) 1931 chromaticity color coordinates were determined by integrating the visible PL emission spectra. UC characterization: The UC excitation spectrum was measured in a custom-built optical system whereby a tunable CW laser (SolsTiS, M2 Laser, U.K.) scanning the laser wavelength from 900 to 1000 nm with a step of 1 nm and maintaining a power density of 2 W/cm2. The UC emission spectra, decay curves and absolute UCQY were measured in a custom-built optical system using a 980 nm CW laser diode (L980P200, Thorlabs GmbH) as the excitation source. The working temperature of laser was maintained at 20.00 ± 0.01 oC (TCLDM9, Thorlabs) and the power density was kept at 0.5 W/cm2 for the collection of UC emission spectra. A CCD spectrometer (CCS200, Thorlabs) was used as a detector to record the UC emission spectra, which were subsequently corrected for its spectral response. The absolute UCQY – the ratio of emitted UC photons to absorbed photons – values were measured according to the wellestablished 2M and 3M procedures.36, 37 Only the dominant NIR UC emission band centered at ~795 nm was considered for UCQY calculations, as this emission was orders of magnitude more intense than the blue UC emission. The power density of the laser was maintained at 7.6 W/cm2 for UCQY and decay curves measurements (the radiation was additionally focused by a lens having focal distance 75 cm to increase the excitation power density). For the decay measurements, the TTL signal from the laser diode controller (ITC4001, Thorlabs) was delayed by the use of delay generator (Stanford Research Systems, DG645) in order to monitor the rise and decay times of the UC emission. The spectral separation of the PL for the lifetime ACS Paragon Plus Environment

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measurements was realized using a double monochromator (DTMS300, Bentham) and the emission at specific wavelength was detected by a photon-counting PMT (R928P, Hamamatsu), mounted in cooled housing (CoolOne, Horiba). Lifetimes were measured using TCSPC/MCS board (Timeharp 260, PicoQuant). For power dependent measurements, the power density of laser was tuned from 0.2 – 300 W/cm2 for UCQY measurements by rotating a continuously variable neutral density filter (NDC-100C-2, Thorlabs) with the aid of a computer-controlled stepper motor. Security labeling: The master pattern of the “IMT” pattern with a size of 13 mm × 6 mm was prepared on thin plastic foil and cut out using a laser cutting system (ULS VLS 3.50). The fluorescent ink was made by homogeneously mixing rubber cement (Elmer’s Products, Inc.) and LYT phosphor with weight ratio of cement: LYH phosphor = 100: 1. Subsequently, the transparent pattern of “IMT” was achieved by painting the ink through the master pattern onto a banknote. The denomination was chosen as the most counterfeited banknote within Europe.38 The CW laser (SolsTiS SolsTiS, M2 Laser, U.K.) was tuned to 980 nm and the radiation beam was expanded to create a spot with a size of ~15 mm for the material excitation. Images of the banknote with and without the 980 nm irradiation were taken via a web camera that had its NIR filter removed. A 950 nm short-pass (SP) filter was placed in front of the web camera in order to filter out the 980 nm excitation. Additional photo was done with a 700 nm SP filter to show, that normally installed IR filter on cameras is blocking the 800 nm emission of Tm3+. All measurements were performed at room temperature.

3. Results and discussion Figure 1a displays powder x-ray diffraction patterns of reference La2O3 and La1.992ACS Paragon Plus Environment


3+ 3+ 2xO3:xYb ,0.4Tm

(LYT-xYb,0.4Tm; x =4, 6, 8, 10 and 12) as a function of the doping

concentration of Yb3+. For the Yb3+ and Tm3+ free reference sample, the diffraction patterns match the standard trigonal La2O3 (P3m1, space group no. 164, ICSD no. 100204). When doping Yb3+ in La2O3 micro-crystals, two effects can be observed. Firstly, all diffraction peaks of La2O3 gradually move to higher angles with increasing Yb3+ concentration from 0 to 12 mol%. For example, the diffraction peak of (102) plane sequentially shifts from 39.57o to 39.80o, when x increases from 0 to 12. This evidences the effective substitution of smaller Yb3+ (92.5 pm in seven-fold coordination) for larger La3+ (110 pm in seven-fold coordination) sites (2d) in La2O3 lattice, which results in the shrink of the unit cell of La2O3 according to the well-known Bragg’s law. Secondly, the doping of Yb3+ in La2O3 also leads to the appearance a minor crystal phase of perovskite-type LaYbO3 (orthorhombic, Pna21, space group no. 33, ICSD no. 15093) due to the large ionic radii mismatch between Yb3+ and La3+ (~19%), and Tm3+ and La3+ (~17%). In is noteworthy that square root scale for Y-axis is applied in Figure 1a to enlarge the weak signal. Due to the low doping concentration of Tm3+ (< 1 mol%), the x-ray diffraction patterns of La1.883+,yTm3+

2yO3:6Yb

(LYT-yTm; y = 0.1, 0.2, 0.3, 0.4, 0.5, 0.6 and 0.9) keep almost constant when

varying the doping concentration of Tm3+ from 0.1 to 0.9 mol%. Both of these effects are in line with our observations in previous works.27, 35


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Figure 1. (a) Powder x-ray diffraction patterns of reference La2O3 and La2O3:xYb,0.4Tm (x = 4, 6, 8, 10 and 12) as function of Yb3+ concentration. The reference diffraction patterns of La2O3 (pink) and LaYbO3 (orange) are tabulated in the bottom of (a). The scale in Y-axis is square root (SQR) to enlarge the weak signal. (b) SEM image of LYT-6Yb,0.4Tm with magnification of ×2000.

An scanning electron microscope image of LYT-6Yb,0.4Tm with a magnification factor of ×2000 can be seen in Figure 1b. The morphology of LYT is irregularly shaped with particle size of 2  5 μm. These particles are strongly aggregated into bigger particles of 10  20 μm due to the high sintering temperature of 1300 1600 oC.

Figure 2a illustrates the diffuse reflectance spectrum of LYT-6Yb,0.4Tm in spectral region of ACS Paragon Plus Environment


400 – 1050 nm. Five sharp absorption peaks can be observed, arising from the intraconfigurational 4fn → 4fn electronic transitions of Yb3+ and Tm3+ from the corresponding ground states to different excited states, as labeled in Figure 2a. The broad absorption band in the NIR from ~870 to 1050 nm is ascribed to Yb3+: 2F7/2 → 2F5/2. It is composed of three Stark bands, one sharp peak at ~978 nm and two shoulders at ~956 and 926 nm. The sharp absorption peaks with maxima at ~798, 694, 670 and 446 nm are attributed to the transitions of Tm3+ from the ground state of 3H6 to the labeled excited states of 3H4, 3F3, 3F2 and 1G4, respectively (Figure 2a). Additionally, the weak absorption intensity of Tm3+ in visible region (at 446 nm, Tm3+: 3H6 → 1G

4)

leads to the white nature of LYT powder under sunlight. The strong absorption intensity of

Yb3+ and Tm3+ in NIR and characteristic Stark splitting of these bands are further proof for the incorporation of Yb3+/Tm3+ on La3+ sites within the La2O3 crystal.

Figure 2. (a) Diffuse reflectance spectrum of La2O3:6Yb,0.4Tm from 450 to 1050 nm. (b) Excitation spectrum (blue line) and diffuse reflectance spectrum (black line) of La2O3:6Yb,0.4Tm from 900 to 1000 ACS Paragon Plus Environment

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nm with constant excitation power density of 2 W/cm2. (c) UC emission spectra of La2O3:6Yb,xTm (x = 0.1, 0.2, 0.3, 0.4, 0.5, 0.6 and 0.9) from 420 to 840 nm dependent on Tm3+ concentration upon excitation of a 980 nm laser diode at power density of 0.5 W/cm2. Inset of (c) presents the enlarged UC emission spectra in spectral region of 420 to 685 nm by a factor of 100. (d) UC emission spectra of La2O3:6Yb,0.4Tm in spectral region of 420 to 720 nm under irradiation of 3 W/cm2. The inset shows the image under a 980 nm laser diode at power density of 3 W/cm2. (d) UC emission intensity in NIR and blue and (e) intensity ratio of INIR/IBlue versus Tm3+ concentration. Inset of (f) illustrates CIE 1931 diagram with color coordinate (red sphere) of LYT under excitation of 980 nm. The lines in (e) and (f) are guides to the eye.

The upconversion (UC) excitation spectra are important to choose the best laser excitation wavelength, however these are rarely reported in the literature.7 In this study, it was measured by scanning the laser wavelength from 900 to 1000 nm, (1 nm step, power density of 2 W/cm2) by monitoring the UC signal of Tm3+ in NIR region, as illustrated in Figure 2b. Interestingly, the UC emission of LYT can be excited over the whole studied wavelength region of 900  1000 nm. It is composed of a sharp peak at 980 nm and two shoulders at 957 and 932 nm. It is noteworthy that the peak position and relative intensity of excitation spectrum are slightly different from those of diffuse reflectance spectrum (see Figure 2a and 2b). The absorption is only related to the transition of Yb3+ from the ground state (2F7/2) to the excited state (2F5/2). The UC emission of LYT is not only related to the absorption of Yb3+, but also the two sequential energy transfers from Yb3+ to Tm3+. Consequently, such a difference between the diffuse reflectance and emission is plausible, and this demonstrates that it is useful to measure excitation spectra for UC materials. The sharp excitation and absorption peak of Yb3+ at 980 nm matches with that of low-cost 980 nm laser diodes, which are used as excitation sources to study the UC emission properties of LYT ACS Paragon Plus Environment


as detailed below.

Figure 2c shows the UC emission spectra of LYT-6Yb,yTm (y = 0.1, 0.2, 0.3, 0.4, 0.5, 0.6 and 0.9) as function of Tm3+ concentration. Upon 980 nm excitation at a power density of 0.5 W/cm2, the NIR UC emission band with a maximum at ~795 nm and a full width at half maximum of ~13 nm (~200 cm-1) dominates the UC emission profile for all LYT samples. This emission is attributed to the typical two-photon process of Tm3+: 3H4 → 3H6. Stark split in C3v site symmetric environment leads to the asymmetry of NIR emission peak at ~795 nm.39 In addition to the strong NIR UC signal, a faint blue UC emission with a maximum at ~476 nm can be observed, this being attributed to a three-photon UC process of Tm3+: 1G4 → 3H6. Its intensity is >100 times weaker than that of the NIR band pumped with a power density of 0.5 W/cm2. This is different compared to other reports that the intensity of blue UC is only slightly weaker than that of NIR UC, such as YF3:Yb3+,Tm3+ and NaYF4:Yb3+,Tm3+.24, 40 Despite the weak intensity of blue UC compared with that of NIR UC, the bright and pure blue color can be clearly seen via the naked eye under excitation at a power density of 3 W/cm2, as shown in inset of Figure 2d. The corresponding CIE 1931 chromaticity color coordinate in blue almost remain unchanged at (0.155 ± 0.001, 0.123 ± 0.001) with varying Tm3+ and Yb3+ concentration, as shown in inset of Figure 2f. In addition, an even weaker UC emission in the red region with maxima at ~652 and 700 nm (see Figure 1d) can also be identified, attributed to a three-step UC process of 1G4 → 3F4 and two- photon UC process of 3F2,3 → 3H6, respectively. The frequently reported UC of Tm3+ in the UV region arising from the higher-order photon process (> 3) could not be detected, even when higher power densities up to 3 W/cm2 were used.40 Higher photon UC processes (>3) are strongly prohibited in LYT due to the large energy mismatch (~3600 cm-1) between Tm3+:1G4


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→1D2 (6650 cm-1) and Yb3+: 2F5/2 → 2F7/2 (~10250 cm-1) and relatively big phonon energy of La2O3 (~450 cm-1) leading to the fast multi-phonon relaxation at lower energy levels of Tm3+.

For the constant doping concentration of Yb3+, no shifts of UC peaks for any of the bands can be observed when the Tm3+ doping concentration is varied. On the contrary, the absolute intensity in NIR, red and blue as well as the intensity ratio of NIR to blue (INIR/IBlue) are highly sensitive to the Tm3+ doping concentration. The UC emission intensity of Tm3+ in NIR slightly increases with the doping concentration of Tm3+ up to 0.4 mol%, as shown in Figure 2e. LYT-6Yb,0.4Tm yields the strongest UC intensity and thus the optimum doping concentration of Tm3+ for the strongest UC emission intensity is determined to be ~0.4 mol%, which is the typical value for Tm3+.21, 30 The higher doping concentrations of Tm3+ (> 0.4 mol%) gradually quench the UC emission intensity in the NIR, attributed to efficient concentration quenching. Meanwhile, the blue UC intensity of Tm3+ decreases with increasing doping concentration of Tm3+ from 0.1 to 0.9 mol% (Figure 2c and 2e). The emission intensity ratio of INIR/IBlue increases from ~120 to 600 with increasing the doping concentration of Tm3+ (Figure 2f). This indicates the high doping concentration of Tm3+ facilitates the two photon UC process of Tm3+ in NIR owing to the enhanced cross relaxation among Tm3+ (Tm3+:1G4 + Tm3+:3F4 → Tm3+:3H4 + Tm3+:3F3), which populates NIR emission but quenches blue UC emission, which will be discussed in more detail later.

The UC emission spectra of LYT-xYb,0.4Tm (x = 2, 4, 6, 8, 10 and 12) are plotted in Figure 3a as a function of Yb3+ doping concentration under 980 nm excitation at a power density of 0.5 W/cm2. With this fixed Tm3+ concentration, the UC emission intensity of Tm3+ in NIR increases ACS Paragon Plus Environment


with increasing Yb3+ concentration up to 6 mol%. Higher Yb3+ concentration (x > 6) slightly quench the UC emission of Tm3+ indicating the pronounced concentration quenching effect of Yb3+ at high Yb3+ doping regime (6 – 12 mol%), as illustrated in Figure 3b.41-43 Consequently, the optimum doping concentration of Yb3+ and Tm3+ for the strongest UC emission intensity of LYT is determined to be 6 and 0.4 mol%, respectively. The INIR/IBlue decreases from ~300 to 150 with increasing the doping concentration of Yb3+ from 2 to 12 mol%.

Figure 3. UC emission spectra of (a) La2O3:xYb,0.4Tm with different amount of Yb3+ concentration (2 – 12 mol%) upon excitation of a 980 nm laser diode with power density of 0.5 W/cm2. (b) Dependence of


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UC emission intensity in NIR versus Yb3+ concentration. (c) Absolute UC quantum yield (UCQY) of La2O3:xYb,0.4Tm as a function of Yb3+ concentration. The line in (b) is a guide to the eye.

Subsequently, we studied the absolute UCQY of LYT-xYb,0.4Tm at a power density of 7.6 W/cm2 as a function of the Yb3+ doping concentration of, as summarized in Figure 3c. Only the dominant UC emission of Tm3+ in NIR at ~795 nm is accounted for the absolute UCQY calculation. The UCQY of LYT-xYb,0.4Tm slightly increases from 3.0 % to 3.4 % with increasing Yb3+ concentration from 2 to 4 mol%. The higher Yb3+ concentration (x > 4) results in the mild decrease of UCQY from 3.4 % to 2.5 % with increasing Yb3+ concentration from 4 to 12 mol%, due to the weak concentration quenching of Yb3+. Importantly, the absolute UCQY value of LYT-4Yb,0.4Tm (3.4%) for a single NIR band is higher than the reported values for the wellknown benchmark UC materials of -NaYF4:Yb3+,Er3+ (~0.9%, with a excitation power density of 9 W/cm2)34 and Gd2O2S:Yb3+,Er3+ (~1.9%, with a excitation power density of 20 W/cm2)19.



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Figure 4. (a) Comparisons of UC emission spectra of LYT-6Yb,0.4Tm, La2O3:Yb,Er (LYE), NaYF4:Yb,Er (NYF) and La2O2S:Yb,Er (GOS) with power density of 0.5 W/cm2. (b) Comparisons of absolute UCQY of LYT-6Yb,0.4Tm, LYE, NYF and GOS with excitation power density of 7.6 W/cm2.

The UC performance of the optimized LYT-6Yb,0.4Tm phosphor in the NIR region is compared against the best reference materials in literature La2O3:Yb3+,Er3+

(LYE) and commercial

NaYF4:Yb3+,Er3+ (NYF) and Gd2O2S:Yb3+,Er3+ (GOS) by means of intensity and absolute UCQY, as summarized in Fig. 4a and 4b, respectively. LYE, NYF and GOS exhibit dominant UC emission in the red and green, and much weaker UC emission in NIR. Interestingly, LYT6Yb,0.4Tm exhibits the brightest UC intensity and highest absolute UCQY in NIR region. Upon pumping at 980 nm (power density of 0.5 W/cm2), the UC intensity of NIR emission of LYT-


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4Yb,0.4Tm is ~8, 30 and 60 times greater than that of NYF, GOS and LYE, respectively. For absolute UCQY of single-band NIR UC emission, LYT-6Yb,0.4Tm (3.2 %) is ~6, 50 and 60 times stronger than that of NYF (0.5 %), GOS (0.06 %) and LYE (0.05 %), respectively.

Figure 5. (a) UC rise (left panel) and decay (right panel) processes for blue UC emission at 476 nm for La2O3:6Yb,0.4Tm under excitation of pulsed 980 nm laser diode with duration of 20 ms and power density of 7.6 W/cm2. (b) UC rise (left panel) and decay (right panel) processes for NIR UC emission at 795 nm for La2O3:6Yb,0.4Tm under excitation of pulsed 980 nm laser diode with duration of 20 ms and power density of 7.6 W/cm2.

Figure 5a and 5b exemplarily illustrate the decay profiles of blue (476 nm) and NIR (795 nm) UC for LYT-6Yb,0.4Tm, respectively, excitation from a square-wave modulated CW 980 nm laser diode providing an excitation power density that is modulated between 7.6 W/cm2 and 0 W/cm2. The diode is modulated such that the laser is on for 10 ms to drive the sample into steady state, and then turned off for 10 ms to observe the decay from steady state. A long rise time of several ms and a short decay of hundreds of µs can be observed for decay profiles of both blue (at 476 ACS Paragon Plus Environment


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nm) and NIR (at 795 nm) UC emission. The long rise time of several ms verifies the energy transfer UC mechanisms, which will be discussed later (Fig. 6f). The rise time (to 80% of maximum intensity) of blue and NIR UC decreased from 2.10 to 1.37 ms and from 3.60 to 2.94 ms with increasing Yb3+ concentration, respectively. These long rise times are related to the lifetimes of the precursor states Yb3+:2F5/2, Tm3+:3H5, and Tm3+:3F4.

The decay times of the UC emission are shorter than the rise times, as expected for these nonlinear processes. The effective lifetime, τ1/e, of NIR UC is longer than that of blue UC emission at the same Yb3+ concentration. The lifetime of NIR and blue UC emission decreases slightly from 167 to 116 µs and from 110 to 88 µs, respectively, as the Yb3+ concentration increases. This is due to the back energy transfer from Tm3+ to Yb3+ via Tm3+:1G4 + Yb3+: 2F7/2 → Tm3+:3H5 + Yb3+: 2F

5/2

and Tm3+:3H4 + Yb3+: 2F7/2 → Tm3+:3H6 + Yb3+: 2F5/2, which depopulates the UC emitting

level of Tm3+:1G4 (for blue UC) and Tm3+:3H4 (for NIR UC), respectively, at high Yb3+ concentrations.

As it is known, UC is a non-linear process and its intensity is strongly excitation power intensity dependent. Energy transfer UC, one of the most common types of UC can be approximated for small excitation power densities using Equation 1: 44, 45 IUC = µIinn

,

(1)

where, IUC is the emission intensity of UC, µ is a materials related coefficient, and n is the effective number of excitation photons required to produce the given UC emission. The involved number of photons, n, can be estimated simply via linear fitting the double logarithmic ACS Paragon Plus Environment

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relationship of UC intensity versus input power density at low excitation power density.

At low excitation power densities, the UC PLQY can be described using the following Equation 2: 28, 29

Absolute UCQY =

No. of emitted photons No. of absorbed photons

=

𝐼𝑈𝐶 αP

∝ 𝐼𝑛𝑖𝑛― 1


,

(2)


Figure 6. (a) Pump power density (0.1 – 2.5 W/cm2) dependent UC emission spectra for La2O3:6Yb,0.4Tm. (b) Double logarithmic plot of excitation power density (0.05 – 200 W/cm2) dependence of UC emission intensity in blue (476 nm, blue sold sphere), red (652 nm, red solid sphere) and NIR (798 nm, grey solid sphere) and corresponding linear fitting curves of the experimental data. (c) Dynamic UC emission intensity in NIR with different excitation power density (25, 70, 115, 160, 210 and 275 W/cm2) within 60 second. Initial (0 second) and steady state (60 second) UC spectra of La2O3:6Yb,0.4Tm upon irradiation with excitation power density of (d) 25 W/cm2 and (e) 275 W/cm2. (f) Schematically illustration of energy level diagram of Yb3+ and Er3+, and the proposed UC emission mechanism of Yb3+/Tm3+ pair in LYT under excitation at 980 nm. Solid, dash dotted and curved arrows represent absorption/emission, energy transfer and multi-photon relaxation processes, respectively. The number in circle indicates the involved photon number. ET and CR represents energy transfer and cross ACS Paragon Plus Environment

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relaxation, respectively.

where α is the absorption cross-section at excitation wavelength and P is the applied excitation power density. Thus, UCQY has gain with excitation power density when n ≥ 1 and it is important to determine the n number as a function of excitation power density. Figure 6a displays the UC emission spectra of LYT-6Yb,0.4Tm as a function of excitation power density (0.1 – 2.5 W/cm2). The decrease of incident power density leads to the nonlinear decrease of UC emission intensity of all bands. The double log-log plot of NIR, blue and red UC intensity versus pump power density (0.05 – 200 W/cm2) for LYT-6Yb,0.4Tm are illustrated in Figure 6b, solid symbols. In the low-pump power density regime (< 2 W/cm2), the NIR UC emission exhibits a quadratic increase with the pump power density (n = 2 ± 0.1) confirming its two photon energy transfer UC origin (Figure 6f). Red and blue UC emissions yield cubic behaviors with slopes of 2.8 ± 0.1 and 2.5 ± 0.1, respectively, indicating their three photon origin (Fig. 6f). In an ideal material, the slope should go to 1 at the high excitation power density limit for the upper state, when energy transfer upconversion from the lower lying intermediate states is more dominant than linear decay.46 The slope of the excitation power density dependence will be 0.5 in the case of energy transfer upconversion in heavily doped samples in the limit of high excitation power density.46-48 In our case, we have a sample which is not heavily doped, so the excitation power density dependence will reflect a behavior between the two extremes, namely for the upper states, the slopes approach unity and for the lower states the slopes approach 0.5. As opposed to this theory, the slopes at higher excitation power densities of our measured dependences for NIR, blue and red are 0.7, 0.7, and 0.6, respectively. This is caused by increased non-radiative transfer rates due to laser induced heating.



We have recently shown that laser-induced heating of the sample can decrease the PLQY at higher excitation power densities. This leads to increased non-radiative rates at higher excitation power densities and a slope below 1 even for upper states.49 Likewise to test this, we conducted an investigation of the laser induced heating effect on the LYT sample. A fast UC emission intensity quenching can be observed within 1s for excitation power density > 70 W/cm2.49 The UC emission intensity in NIR of steady state (t = 60 s) decreases ~7% and 30% compared with that of initial state (t = 0 s) under irradiation of 980 nm laser at excitation power density of 25 and 275 W/cm2, respectively (see Fig. 6d and 6e, respectively). By taking in account of this heating effect, we provide the corrected excitation power density dependence curve (Fig. 6b, solid lines). Quite interestingly, the slope is not unity at high excitation power densities for the NIR emission, even after thermal correction, though it approaches unity for red and blue emissions. The reason for this is also quite self-explanatory as the state at 796 nm acts as an intermediate level for red and blue emissions which get stronger in the high excitation power density regime and hence NIR emission has a slope much less than unity.

Based on the aforementioned results, the following energy transfer upconversion mechanisms pumping at 980 nm are proposed for LYT, as schematically illustrated in Figure 6f. The successive three non-resonant energy transfer from Yb3+ to Tm3+ sequentially populates the Tm3+:3H5, Tm3+:3H4 (giving rise to NIR UC), and Tm3+:1G4 level (giving rise to blue and red UC), as described in other literature.24, 50 Due to the small energy difference between 3F3 and 3H4 levels (~1800 cm-1), the red UC at ~700 nm is an unfavorable process. The higher photon process is strongly prohibited due to the large energy mismatch (~3600 cm-1) between Tm3+:1G4 → 1D2 (6650 cm-1) and Yb3+: 2F5/2 → 2F7/2 (~10250 cm-1). The occurrence of cross-relaxation among


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Tm3+ (Tm3+:1G4 + Tm3+:3F4 → Tm3+:3H4 + Tm3+:3F3) at high doping concentration of Tm3+ strongly quenches blue UC emission leading to the increased intensity ratio of INIR/IBlue. High Yb3+ concentration facilitates the back energy transfer from Tm3+ to Yb3+ via Tm3+:1G4 + Yb3+: 2F

7/2

→ Tm3+:3H5 + Yb3+: 2F5/2 and Tm3+:3H4 + Yb3+: 2F7/2 → Tm3+:3H6 + Yb3+: 2F5/2 leading to

the decreased rise time and lifetime of blue and NIR UC emission of Tm3+.

Figure 7. The images of UC fluorescent labeling of transparent “IMT” pattern (13 mm × 6 mm) with and without irradiation of 980 nm laser with spot size of 15 mm, 950 short-pass (SP) filter in front of the camera for all the photos: (a) without 980 nm laser, (b) with 980 nm laser at a power density of 1 W/cm2 and 700 nm SP filter, (c) with 980 nm laser at a power density of 0.1 W/cm2, (d) with 980 nm laser at 1W/cm2.

The efficient single-band NIR-to-NIR UC emission of LYT makes it promising for applications in fluorescent markers, plastic recycling, biological areas, etc. A simple proof-of-concept application of fluorescent marker was demonstrated for LYT, as shown in Fig. 7a-7d. Clearly, nothing can be observed without the irradiation of 980 nm laser, as shown in Fig. 7a. Bright emission color of “IMT” pattern can be observed under the irradiation of 980 nm laser at power density of 0.1 and 1 W/cm2 (see Fig. 7c and 7d, respectively). By adding a 700 nm short-pass ACS Paragon Plus Environment


filter, no color can be observed under the irradiation of 980 nm laser that proves the detection of the NIR Tm3+ emission by a common cheap web-camera with removed IR filter (see Fig. 7b). 4. Conclusions In conclusion, we report an efficient NIR-to-NIR UC emission in La2O3:Yb3+,Tm3+ with absolute upconversion (UCQY) of 3.4%. NIR UC emission at ~795 nm (Tm3+: 3H4 → 3H6) absolutely dominates the UC emission profile for La2O3:Yb3+,Tm3+ (LYT) with intensity of NIR to blue, INIR/IBlue > 100. The optimum doping concentration of Yb3+ and Tm3+ for maximizing NIR UC emission in LYT are determined to be 6 and 0.4 mol%, respectively. Both rise time and lifetime of blue and red UC decrease with increasing Yb3+ concentration due to the back energy transfer from Tm3+ to Yb3+. The UC emission intensity and absolute UCQY value of newly developed LYT is higher than the best reference materials. The proof-of-concept of fluorescent labeling of transparent “IMT” pattern is successfully demonstrated. The high absolute UCQY combined with their easy preparation suggests their potential applications in photonic markers, plastic recycling and biological areas.

Acknowledgements The authors gratefully acknowledge financial support by Technology Transfer Project N038 between KIT and Polysecure GmbH. Furthermore, the authors would like to acknowledge the financial support provided by the Helmholtz Association: i) a Recruitment Initiative Fellowship for BSR; ii) the Science and Technology of Nanosystems (STN) program; and iii) the Helmholtz Energy Materials Foundry (HEMF) project. RJ would like to thank the German Academic Exchange Service (DAAD) for the provision of a PhD scholarship.


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Highly efficient La2O3:Yb3+,Tm3+ single-band NIR-to-NIR

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