Up-Conversion Emission Enhancement by Li Addition

Article pubs.acs.org/JPCC

Down-/Up-Conversion Emission Enhancement by Li Addition: Improved Crystallization or Local Structure Distortion? Daniel Avram,†,‡ Bogdan Cojocaru,§ Ion Tiseanu,† Mihaela Florea,§,∥ and Carmen Tiseanu*,† †

National Institute for Laser, Plasma and Radiation Physics, P.O. Box MG-36, RO 76900, Bucharest-Magurele, Romania Faculty of Physics, University of Bucharest, 405 Atomistilor Street, 077125 Magurele-Ilfov, Romania § Faculty of Chemistry, University of Bucharest, 4−12 Regina Elisabeta Boulevard, Bucharest, Romania ∥ National Institute of Materials Physics, 405A Atomistilor Street, 077125 Magurele-Ilfov, Romania ‡

S Supporting Information *

ABSTRACT: Local symmetry distortion by Li addition is acknowledged as an effective strategy for enhancing the luminescence of lanthanide (Ln) doped into a wide range of lattice hosts. Despite extensive literature, direct evidence that supports Li-induced modification of the local crystal-field at the Ln sites is still missing. Herein, we show that the emission enhancement by Li addition in Ln,Li−Y2O3 is due to improved crystallization and not to local structure distortion. Our approach is based on the premise that any distortion/lowering of the local symmetry would reflect into the alteration of the emission shapes and shortening of the emission decays. To this aim, we have extensively investigated the evolution with Li addition and calcination temperature of down (optical and X-ray induced) and up-conversion (UPC) emission of Ln-Y2O3 measured across the visible to near-infrared range. First, a center to center (corresponding to Ln in the C2 and S6/C3i sites of the cubic Y2O3 lattice) as well as global comparison of the emission properties of Li free and Li codoped Y2O3 are presented by use of Eu, Sm, Tb and Dy as local probes in the visible range. Next, the effect of Li on the up-conversion emission of Er- Y2O3 is analyzed in terms of UPC pathways, emission shape and intensity, decays and excitation spectra. It is concluded that Li addition does not change either the local structure around C2 or S6 Ln centers or the relative contribution of these. Moreover, it is found that the effects of Li doping on the emission properties of Ln−Y2O3 are like extending the calcination temperature of Li-free Ln− Y2O3 from 800 °C to ∼1000−1100 °C. Additionally, a relatively intense 1500 to 980 nm UPC emission is evidenced for the first time for Er−Y2O3, while a relatively intense emission around 1500 nm was measured under X-ray excitation. Taken together, our findings highlight the need for revisiting the traditional optimization strategy based on Li modification but also the promise of Er−Y2O3 nanoparticles for optical/X-ray applications in the near-infrared range. morphology,9,17,23 reduction of surface OH defects,9−14 and sensitization via oxygen vacancies induced by charge compensation.11,15,32 The nature of Li precursors as well as the preparation method were found to also play a key role on the final emission properties.19,34 Larger Ln-Y2O3 crystallites are usually obtained upon addition of Li as evidenced by X-ray diffraction (XRD) patterns9−11,14−16,21,24 and electron transmission microscopy (for example, refs 9, 17, and 23). Intuitively, crystallite size enlargement is expected to increase the contribution of core Ln at the expense of surface Ln that is exposed to efficient nonradiative emission quenching due to defects and impurities,35 thus leading to more intense emissions. On the other side, reduction of symmetry around a Ln activator is considered to arise from the size mismatch

I. INTRODUCTION Due to its facile synthesis in the nanometer regime and favorable physical properties such as a high melting point, phase stability, low phonon energy, and low thermal expansion, Y2O3 represents an excellent host material for the luminescent lanthanide ions.1−7 Li addition to Ln−Y2O3 is considered as an effective optimization strategy that reportedly boosts the emission intensity from a few times up to 2 orders of magnitude, depending on the type of Ln (co)dopants and concentrations, synthesis methods, Li concentration, and the mode emission excitation (down- or up-conversion).8−33 Despite that almost 100 publications have been published in the last two decades on Ln,Li−Y2O3, there is still no consensus being reached on the causes of Li-induced emission enhancement. Several causes for emission enhancement are being considered, such as especially the tailoring of crystal-field via lowering of the local symmetry9,13,15,16,18,21,22,24,26 or/and improved crystallization,9−11,14−16,21,32,34 but also changes of © XXXX American Chemical Society

Received: March 28, 2017 Revised: June 14, 2017

A

DOI: 10.1021/acs.jpcc.7b02897 J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry C

Li. Selected lanthanide dopants include Eu, Sm, Tb, Dy, and Er. The emission was induced by optical excitation, in both downand up-conversion modes, as well as low-energy X-ray irradiation. In cubic Y2O3, Ln dopants are assumed to be randomly distributed on the low-symmetry C2 and inversion S6/C3i higher symmetry sites of cubic Y2O3, which account for 3/4 and 1/4 of the total number of sites, respectively.1 In addition to the intensively investigated Eu,57,58 Sm, Tb, and Dy were shown recently by some of us to also exhibit strongly dissimilar emission shapes and dynamics as a function of the site they occupy,7,59 thus acting as valuable structural probes. Further, Er lanthanide is selected to probe the effect of Li on up-conversion (UPC) emission properties (i.e., UPC emission shape and intensity, UPC emission decay, and UPC excitation shape) under near-infrared excitation at ∼1500 nm. The emission properties of Li free and Ln,Li−Y2O3 are further compared to Li free Ln−Y2O3 samples that were calcined at several higher calcination temperatures (>800 °C). Taken together, our findings highlight the need for revisiting the traditional optimization strategy based on Li modification but also the promise of Er−Y2O3 nanoparticles for (bio)photonics and X-ray applications in the near-infrared range.

between ionic radii of Li and Y (0.76 to 0.9 Å, 6-fold coordination36) and oxygen vacancies following aliovalent insertion of Li, which is considered to occur either substitutionally9,12,13,15,19,24 or interstitially8 or both.10,11,14,16,18,20,22,29−31 Since lowering of the local symmetry around the Ln activator leads to the enhancement of f−f radiative transitions, local symmetry distortion by Li is a mechanism at hand that explains the emission enhancement in Y2O3 but also in a wide range of Ln-doped oxide and fluoride host materials.8,37,38 Besides the alteration of the emission shapes, distortion/lowering of the local symmetry would necessarily result in shortening of the emission decays. Yet, few reports have measured the effect of Li addition on the excited-state lifetimes of Ln in Y2O3,13,15,21,22 and these basically evidenced lengthening of the emission decays rather than their acceleration. For example, with addition of 5%Li, Liang et al.13 measured the lengthening of the4I11/2 lifetime of Er from 0.8 to 2.2 ms, while for the same intermediate state, Chen et al.21 observed a lengthening from 0.8 to 3.4 ms. Fan et al.15 reported the lengthening of the 4I13/2 level of Er from 3.14 to 13.48 ms with 5% Li addition. This leads to the reasonable assumption advanced by Cates et al.,39 that other factors, aside from or in addition to site distortion, may be responsible for the enhancement of the up-conversion emission.39 It is only recently that Wisser et al.37 demonstrated the expected correlation between the increase of quantum yield of upconversion emission and reduction of the related lifetimes, on one side, and the distortion of the local symmetry, on the other side. The system investigated was Er, Yb−NaYF4, and the local distortion was tuned by varying the cosubstitution degree of Y by a mixture of Gd and Lu ions. Use of local structure oriented techniques, such as extended X-ray absorption fine structure spectroscopy, suggested recently that the Er−O bond length and the coordination number of the Er−Er bond were altered by Li in Er−ZnO nanoparticles, leading to strong enhancement of the UPC emission intensity.40 In this particular case, Li and Ln having different valencies (1+ and 3+) from that of the host cation (2+), the substitution pathways may be quite different to that in isovalent-doped Ln oxide and fluoride materials. Thus, despite that Li addition has been established as an effective method for the enhancement of UPC emission, direct evidence that supports tailoring the local crystal field is still missing.41 Over the last three decades, several hundred studies have been devoted to up-conversion of Er single-doped or co-doped with Yb in Y2O3 by use of a continuous wave (cw) laser diode around 980 nm that matches absorptions of Er (4I15/2−4I11/2 transition) or Yb (2F7/2−2F5/2 transition). By contrast, UPC emission of Er−Y2O3 excited around 1500 nm (corresponding to the 4I15/2−4I13/2 transition) has been much less investigated, while the emission range has been limited below 900 nm, thus overlooking the NIR emission of Er at 980 nm.40−44 Er-based materials that display intense NIR-to-NIR up-conversion emission are highly attractive for (bio)photonics and photovoltaics applications.45−50 To date, only a few Er-doped materials have been shown to emit a predominant 980 nmbased emission (relative to the visible emissions located in the green and red spectral regions), under 1500 nm excitation, such as Er−CeO2 and Er−NaYF4 (nanoparticles), Er−Gd2O2S (microparticles), Er−BaTiO3 (microparticles), Er−BaY2F8 (monocrystal), and Er−LiYF4 (monocrystal).49,51−56 Herein, we assess the effect of Li addition on the luminescence properties of Ln-doped Y2O3 with specific aim to identify the origin of the emission enhancement induced by

II. EXPERIMENTAL DETAILS Ln (Ln = Eu, Tb, Sm, Dy, and Er)−Y2O3 were synthesized by using the citrate complexation method.3 Eu(1%), Eu(1%) Li(5%), Tb(1%), Tb(1%) Li(5%), Sm(0.1%), Sm(0.1%) Li(5%), Dy(1%), Dy(1%) Li(5%), Er (with concentration of 1, 3, 5 and 7%), Er (1%) Li (5%), and Er(7%) Li (5%) (co)doped Y2O3 were prepared. Sm concentration was set at 0.1% due to relative intense quenching concentration induced by cross-relaxation on intermediary levels.59 To determine the effect of the Li component, which is volatile at elevated temperature, the Li, Ln−Y2O3 samples were calcined in air at 800 °C for 4 h. Li-free, Ln−Y2O3 were also calcined at 1000, 1100, and 1300 °C. Powder X-ray diffraction (XRD) patterns were recorded on a Schimadzu XRD-7000 diffractometer using Cu Kα radiation at a scanning speed of 0.10 degrees min−1 in the 15−90° 2θ range. Raman analysis was carried out with a Horiba Jobin YvonLabram HR UV−visible−NIR Raman Microscope Spectrometer at 633, 514, and 488 nm. Microbeam X-ray fluorescence (micro-XRF) spectrometry was performed on a custom-made instrument with an X-ray tube: Oxford Instruments, Apogee 5011, Mo target, focus spot ∼40 μm, max. high voltage −50 kV, max current −1 mA, Amptek X-123 complete X-ray spectrometer with a Si-PIN detector. Attenuated total reflection-Fourier transform infrared (ATR-FTIR) spectra were measured on a Spectrum Two, Perkin-Elmer spectrometer using an ATR device with a diamond crystal plate (Pike Technologies, Madison, WI). Spectra were recorded at 4 cm−1 nominal resolution and 100 scans. The photoluminescence (PL) measurements were carried out using a Fluoromax 4 spectrofluorometer (Horiba) operated in both the fluorescence and the phosphorescence mode. The repetition rate of the xenon flash lamp varied from 10 to 25 Hz, the delay after flash varied between 0.03 and 100 ms, and up to 30 flashes were accumulated per data point. Slits varied from 0.1 to 5 nm in excitation and from 0.1 to 1 nm in emission measurements, respectively. The PL decays were measured by using the “decay by delay” feature of the phosphorescence mode. The average decay lifetime was calculated as the integrated area of normalized emission decay. The PL measurements were B


Article


Figure 1. Left panel: Effects of Li addition and increase of the calcination temperature from 800 to 1100 °C on the emission spectra and decays of C2 and S6 Tb centers measured at 80 K. All the emission spectra were normalized to maximum peak intensity. The excitation and emission wavelengths ensured the best spectral separation of the distinct Tb centers.7 The schematic representations of C2 and S6 sites in cubic Y2O3 is also illustrated. Right panel: Comparison between XRD patterns of Tb−Y2O3, Tb,Li−Y2O3, and Tb−Y2O3 (1100 °C).

III. RESULTS AND DISCUSSION III.1. Effect of Li Addition on the Down-Conversion Emission of Ln Used as Luminescence Probes. To assess the effect of Li addition on the local structure of Ln−Y2O3, we select Eu (1%), Sm(0.1%), Tb(1%), and Dy (1%) as the luminescence probes.7,59,60 Selection of Li concentration at 5% is close to the value reported as optimal, above which a decline in the emission intensity of Ln activators is typically observed.9,11,14−16,18,21,22,24,29 Depending on the synthesis procedure, the type of lanthanide (co)dopants, and the way the emission was induced (down- or up-conversion excitation), smaller (2, 3%)10,12,13,19,20,30 or greater (10%)17 optimal Li concentrations were also determined. To this point, we note that in most studies, the actual Li concentration is not confirmed by quantitative analysis (i.e., inductively coupled plasma mass spectrometry),27,61,62 and thus the value may be lower than the nominal one. The XRD patterns of Ln,Li−Y2O3 were all indexed to cubic Y2O3 (JPCDS cards #01-0831), (Figure 1 and Figure S1). Insertion of Li leads to a small shift toward higher angles but also to a shape pattern in agreement with the literature.9−11,14−16,21,24 The average crystallite sizes (estimated by use of the Scherer equation) is estimated to be around 26−28 for Li-free and 44−46 nm for Li-codoped samples. Irrespective of the way that monovalent Li may be inserted into the in Y2O3 lattice (substitutionally or interstitially), the oxygen vacancies should be formed via-charge compensation. The Raman spectra (Figure S2) show that Li does not modify the shape of the phonon band spectrum characteristic of cubic Y2O3, which means that defect bands assignable to oxygen vacancies are too weak to be detected if any. Li appears to be well inserted into the lattice, as the F2g phonon mode associated with antifluorite Li2O at 520 cm−1 was not detected.62 The FTIR spectra (Figure S3) show that Li addition does not significantly modify the wide band around 3500 cm−1 associated with OH stretching of the physiosorbed water molecules. We note that, in the literature, the role of Li in reducing the surface OH groups is still unclear.9−14,16

recorded at 300 K (room temperature) and low temperature (80 K, by use of LN Temperature Measurement Dewar Accessory, Horiba Scientific). The lamp excited global measurements (global emission and excitation spectra and global emission decays) were performed at room temperature with the excitation slit set to 10−29 nm (wide bandwidth excitation) and the emission slit set from 3 to 5 nm. Siteselective, time-resolved (gated) emission spectra were recorded at room temperature using a wavelength tunable NT340 Series EKSPLA OPO (Optical Parametric Oscillator) for samples excitation at 210−2300 nm operated at 10 Hz as the excitation light source. Laser spectral width was around 5 cm−1. As the detection system, an intensified CCD (iCCD) camera (Andor Technology, iStar iCCD DH720) coupled to a spectrograph (Shamrock 303i, Andor) was used. More details of the used luminescence setup and method can be found elsewhere.3,44 For the energy dependence measurements, up to 500 accumulations were collected to obtain a better signal-tonoise ratio, and the energy of the laser pulse was modified using neutral density filters and measured with a Coherent Energy Max Laser Energy Sensor (J-10MB-HE Energy Max Sensor). The energy of the laser pulse at 1533 nm varied from 0.55 to 3.12 mJ (∼250 mW/cm2). For the emission spectra in the extended range of 300−1100 nm and 970−1700 nm, AvaSpecHS1024x58/122TEC and AvaSpec-NIR256-1.7TEC, were used, respectively. X-ray Excited Optical Luminescence (XEOL) spectra were measured by use of an X-ray tube, Oxford Instruments, Apogee 5011, Mo target, max. high voltage −50 kV, max current −1 mA. The two Avantes spectrometers were calibrated by use of an integration sphere with a 5 W halogen bulb as the calibration source, AvaSphere-50-LS-HAL12V. The emitted light was collected with a lens with focal distance of 8 mm in reflection mode on both Avantes spectrometers, and the X-ray debit dose was up to 2 Gy/s. For the digital photos registered in X-ray and UPC measurements, a low spec commercial webcam Microsoft LifeCam HD3000 not optimized for low light collection and a Canon EOS 60D were used, respectively. C


Article


Figure 2. Effects of Li addition and increase of the calcination temperature on global emission and excitation spectra and emission decays of Tb− Y2O3. For the emission decays, three excitation wavelengths were chosen across the Tb f−d absorptions. All the excitation and emission wavelengths are denoted in the figure.

stronger contribution is exhibited by the S6 center. This is due to the partially MD nature of 5D4−7F5 transition compared to the predominant ED nature of 5D4−7F6 one and the inversion and noninversion symmetries at S6/C2 sites.66,67 The characteristic emission decays reveal a much longer average lifetime for S6 Tb compared to C2 Tb (9.4 compared to 1.9 ms) in accordance with the forbidden nature of MD emission.7 It is observed from Figure 1 that Li addition also does not alter the emission shapes (Stark splitting pattern, peak positions, and relative intensities). In the limit of used spectral resolution (0.05 nm), the emission widths were not modified from the values of 0.2−0.25 nm. More importantly, the emission decays corresponding to either the S6 Tb or C2 Tb center in the Ln,Li−Y2O3 samples remain similar to those measured with Li free ones. It is well-known that the spontaneous emission probability or radiative transition rate is related to the line strengths of electric and magnetic dipole transitions. For the lanthanide ions, these are expressed in terms of Judd−Ofelt parameters (Ω2, Ω4, and Ω6),68,69 which are indicators of the local structure of the host in which the activator is inserted. Since the measured lifetime (τ) is connected to the radiative (τr) and nonradiative lifetimes (τnr) by the relation 1/τ = 1/τr + 1/τnr, distortions of the local symmetry are expected to reflect in the decrease of the measured lifetime (due to the increase of the probability of a radiative transition). We have also measured the emission spectra and decays of Li-free and Tb,Li−Y2O3 under lamp-based global excitation/ emission conditions (see the Experimental Details section for details on global measurements). As observed from Figure 2, Li addition induces an overall enhancement of emission of 2−2.5, which is in line with the previous reports;9,12,17,19,23 however, again Li addition does not alter to any extent the global emission shape and global decays. These were measured by shifting the excitation wavelength across 250 to 320 nm that

It is assumed that in cubic Y2O3, Ln dopants are randomly distributed on the low-symmetry C2 and inversion S6/C3i higher symmetry sites of cubic Y2O3 (Scheme S1), which account for 3/4 and 1/4 of the total number of sites, respectively.1 The absorption and emission transition probabilities of Ln are governed by site-specific selection rules: for C2 sites, both magnetic (MD) and electric dipole (ED) transitions are allowed, while for S6 sites only the magnetic dipole transitions are allowed, leading generally to emissions with order(s) of magnitude less intense than those of C2 sites.63 According to the literature, upon Li codoping of Ln−Y2O3, the small ionic radius of Li (0.76 Å compared to 0.9 Å for Y, in 6fold coordination36) enables facile insertion into Y2O3 lattice, either substitutionally9,12,13,15,19,24 or both substitutionally and interstitially10,11,14,16,18,20,22,29−31 generating strain and charge imbalance. Several scenarios have been advanced to explain the emission enhancement by Li addition: (i) the oxygen vacancies resulted from the charge-compensation can remove the inversion symmetry of S6 site, and thus the forbidden electric dipole transitions become allowed, leading to an increased number of optically active lattice sites;9,11,14−16,24 (ii) Li preferentially substitutes for C2 site, further reducing the local symmetry;64 (iii) Li addition can reduce the local symmetry at both sites;9−16,18−20,22,24,29−31 and (iv) the Ln−Ln interactions get weaker via breaking the Ln−Ln clusters in addition to reduced local symmetry.11,16,39,65 To check the above scenarios, we have pursued a comprehensive, center-to-center comparison between the emission properties of Li free and Ln,Li−Y2O3. The results are illustrated for Tb in Figure 1 and Eu, Sm, and Dy in Figures S4, S5, and S6. Shown in Figure 1 are the characteristic emissions of C2 and S6 Tb centers obtained by excitation into center related f−d absorptions at 275 and 310 nm, respectively.7 Both centers related emissions are dominated by 5D4−7F5 transition of Tb around 545 nm; however, a much D


Article


doped Er(1 and 7%)-Y2O3. The presence of the secondary phase Er2O3 was not detected. Raman spectra of Er(Li) show only phonon bands characteristic of cubic Y2O3 (not shown), while the elemental analysis by X-ray induced fluorescence confirms that the Er concentrations are close to the nominal values (Figure S7). Upon pulsed monochromatic laser excitation at 1533 nm, Er display characteristic emissions corresponding to 2H11/2, 4 S3/2−4I15/2 (green emission); 4F9/2−4I15/2 (red emission); 4 I9/2−4I15/2 and 2H11/2, 4S3/2−4I13/2 (790−850 nm) and 4 I11/2−4I15/2 (980 nm) transitions. Remarkably, for 1Er−Y2O3, almost 99.7% of the total UPC emission is concentrated in the near-infrared at 980 nm (see also the schematic representation of the involved transitions in Figure 4a). To the best of our knowledge, this is the first report on NIR (excitation at ∼1500 nm) to NIR (emission at 980 nm) UPC in Er−Y2O3. Our findings add to a few reports on an intense 980 nm emission under UPC excitation around 1500 nm measured with Er− CeO2 and Er−NaYF4 (nanoparticles), Er−Gd2O2S (microparticles), Er−BaTiO3 (microparticles), Er−BaY2F8 (monocrystal) and Er−LiYF4 (monocrystal).49,51−56 The intensity of UPC spectra increases with Er concentration up to ca. 7%, which is considered the optimized composition. It is worth mentioning that, irrespective of Er concentration and Li addition, the up-conversion emission of Er−Y2O3 is intense enough to allow the observance by the naked eye of a reddish color in ambient room light conditions. This is an important result, as efficient up-conversion of low-energy photons at 1500 nm into high-energy photons at 980 nm with Er−Y2O3 may be highly attractive for solar cells49,56 and deep penetration imaging applications.77−81 The UPC mechanism can be tentatively described as follows (a more detailed analysis will be submitted elsewhere): Upon excitation into the long-lived 4I13/2 level (lifetime of 8−10 ms40) Er can be promoted to the 4I9/2 excited state, which decays mostly nonradiatively on the 4I11/2 state. From the excited 4I11/2 level, the 4F9/2 emitting level is populated via the (4I11/2, 4I9/2) → (4F9/2, 4I13/2) energy transfer mechanism (ETU), giving rise to red UPC emission. The emission decays of the experimentally accessible emissions at 565, 660, and 800 nm were measured under both UPC and down-conversion excitation (at 488 nm, Table 1). The presence of a rise time in the 4F9/2 emission decay indicates that the most probable UPC mechanism that populates the level is ETU. The absence of a rise time of the emission decay monitored at 800 nm (4I9/2) which is supposed to feed the 4I11/2 level might suggest that ground state absorption (GSA) followed by excited state absorption (ESA) may dominate the UPC emission. However, as shown in Figure 6, the UPC decays are considerably lengthened compared to those measured upon downconversion excitation at 488 nm (Table 1) that confirm the contribution of ETU mechanism to populating the 4F9/2 state. Finally, the (4I13/2, 4I13/2)−(4I15/2, 4I9/2) ETU process followed by the 4I9/2 → 4I11/2 multiphonon relaxation likely leads to 980 nm-based emission.52,53 The gap between 4I11/2 and the next lower 4I13/2 of ∼3400 cm−1 corresponds to ca. 9 or 6 phonons (considering either the strongest or the cut-off phonon energy in Y2O3 at 380 or 600 cm−1) leading to negligible or weak multiphonon relaxation for the former level. Eventually, the energy mismatch of ∼400 cm−1 arising from the 4I13/2 −4I15/2 and 4I13/2−4I9/2 energies can be solved by a single phonon

corresponds to overlapping range of f−d absorptions characteristic of S6 and C2 centers, as highlighted on the global excitation spectrum (also represented in Figure 2). Similar trends were measured for Eu, Sm, and Dy (Figures S4−S6). In all, the global measurements suggest that the relative occupancy of C2 and S6 sites by Ln remain essentially the same in the Li-free and Ln,Li−Y2O3. In a further set of experiments, we extended the calcination temperature of Li-free Ln−Y2O3 from 800 °C to 1000, 1100, and 1300 °C. The comparison between the emission spectra and decays of Ln−Y2O3 (800 °C), Ln,Li−Y2O3 (800 °C) and Ln-Y2O3 (1100 °C) (Figures 1−2 and Figures S4−S6) show undoubtedly that the effect of Li addition is very much like extending the calcining temperature of Li-free samples to ∼1100 °C. The comparison between selected XRD patterns also shown in Figure 1 further indicates that both Tb,Li−Y2O3, and Tb−Y2O3 calcined at 1100 °C display similar pattern structure and crystallite size (around 45 nm). In conclusion, the center to center and global comparative analysis summarized in Figures 1, 2, and S4−S6 evidence that Li addition does not change (a) either radiative or nonradiative decay components corresponding to C2 and S6 Ln centers, i.e., the local symmetry around each site and (b) the relative contribution of C2 and S6 centers. The effect of Li addition is like extending the calcination temperature of Li-free Ln−Y2O3 to ∼1100 °C. The results suggest thus that the improved crystallization rather than local symmetry distortion is the cause for the emission enhancement of Ln,Li−Y2O3. III.2. Effect of Li Addition on the Up-Conversion Emission. In the recent years, Li co-doping is extensively used as an effective way to boost the intrinsic low quantum yield of Ln up-conversion emission into wide range of oxide and fluoride hosts.8,38,70−76 Here, we investigate the effect of Li (5%) on the up-conversion emission of Er−Y2O3 excited at ∼1500 nm, corresponding to 4I15/2−4I13/2 absorption transition of Er. Similarly, to previously described Ln,Li−Y2O3, both Lifree and Er,Li−Y2O3 were calcined at 800 °C, while additional calcinations at 1000, 1100, and 1300 °C were performed for Lifree and selected Er concentration compositions. Figure 3 illustrates the XRD patterns of Er(Li)-doped Y2O3 that agree with all those of cubic Y2O3 (JPCDS cards #010831). The average crystallite sizes (estimated by use of Scherrer equation) were estimated around 26−29 nm for single Er (1, 3, 5 and 7%) doped and 44−46 nm for the Li(5%) co-

Figure 3. XRD patterns of Er(Li)−Y2O3 nanoparticles. E


Article


Figure 4. (a) Schematic representation of the excitation/emission transitions and mechanisms involved in the UPC process at 1533 nm. (b) Evolution of the UPC spectra with Er concentration and Li addition. (c) Digital images of the nanopowders (sample holder area of 14 mm × 7 mm) taken in the ambient room light conditions by use of Canon EOS 60D under exposure times of 10 s with 1600 ISO for the 1Er(Li)−Y2O3 and 1s with 400 ISO for the rest of the samples. The average power density was around 240 mW/cm2.

We pursue further a comprehensive investigation on the effects of Li addition and extending calcination temperature on the up-conversion emission intensity, shapes of the upconversion emission spectra, up-conversion emission decays, and shapes of the up-conversion excitation spectra of the optimized 7Er−Y2O3 composition. Among the 7Er−Y2O3 samples calcined at 1000, 1100, and 1300 °C, the one calcined at 1000 °C induced the closest UPC enhancement to that measured with 7Er,Li− Y2O3 (around three). In addition, per the XRD patterns also included in Figure 3, a similar average particle size of around 46 nm was measured for both 7Er−Y2O3 calcined at 1000 °C and 7Er,Li− Y2O3. Li addition only slightly modifies the shape of the UPC emission spectra of 7Er−Y2O3, that is, it slightly decreases the red emission intensity at 660 nm compared to green and NIR emissions at 565 and 980 nm, respectively (Figure S8). However, as also shown in this figure, Li-induced shape modification is quite close to that induced by extending the calcination temperature at 1000 °C. Both Li addition and calcination temperature lead to a similar number of photons required to populate the different excited states.82,83 As shown in Figure 5 slope values of 1.1−1.2 (980 nm) and 1.4−1.6 (red emission) or 1.7−1.8 (green emission) were estimated for 7Er samples series. All values fall below the

Table 1. Average Lifetimes of 7Er−Y2O3, 7Er,Li−Y2O3 and 7Er−Y2O3 (1000 °C) Measured under Down-Conversion (488 nm) and Up-Conversion Excitation (980 and 1533 nm) λem = 565 nm τava/τrb

λem = 660 nm τav/τr

7Er−Y2O3 7Er,Li−Y2O3 7Er−Y2O3 (1000 °C) 7Er−Y2O3 7Er,Li−Y2O3 7Er−Y2O3 (1000 °C) 7Er−Y2O3 7Er,Li−Y2O3

0.007 ms 0.012 ms 0.012 ms

0.03 ms 0.02 ms 0.02 ms

0.01 ms 0.01 ms 0.01 ms

0.18 ms 0.27 ms 0.26 ms

0.32 ms 0.6 ms 0.6 ms

-

0.45/0.004 ms 0.82/0.05 ms

0.18 ms 0.27 ms

7Er−Y2O3 (1000 °C)

0.83/0.035 ms

0.42/0.01 ms 0.66/0.025 ms 0.66/0.019 ms

sample λex = 488 nm

λex= 980 nmc

λex = 1533 nm

λem = 800 nm τav

0.28 ms

a

The average decay lifetime (τav) was calculated as integrated area of normalized emission decay. bThe rise time (τr) was calculated by use of single exponential fit; typical error in the average lifetime is ∼5%. c For comparison with literature data, the UPC emission decays measured under 980 nm excitation were also included (see text).

emission, likely leading to an efficient, one-phonon assisted 980 nm emission.

Figure 5. Dependence of UPC emission intensity on the laser pulse energy in log−log scale illustrated for 7Er−Y2O3, 7Er,Li−Y2O3, and 7Er−Y2O3 calcined at 1000 °C. F


Article


Figure 6. Effect of Li addition and extended calcination temperature on the emission decays of 7Er−Y2O3 measured under direct (488 nm) and UPC excitation (1533 nm).

Figure 7. Effect of Li addition and calcination temperature on the UPC excitation spectra monitoring the emissions at 980, 660, and 565 nm.

nm, but, again, the effect is essentially identical to that measured with 7Er−Y2O3 calcined at 1000 °C. Finally, we analyzed the effect of Li addition and calcination temperature on the UPC excitation spectra. Figure 7 compares the UPC excitation spectra of 7Er−Y2O3, 7Er,5Li−Y2O3 and Er−Y2O3 calcined at 1000 °C that monitor the emissions at 980, 660, and 565 nm. It is observed that neither Li addition or the increased calcination temperature alter the shapes of UPC excitation spectra, which are all dominated by narrow line around 1533 nm and accompanied by weaker absorptions on both high and low energy sides. The lines get narrower with increasing the monitored emission wavelength due to the greater number of photons responsible for the red emission (theoretically three photons) compared to those necessary for the 980 nm emission (theoretically two photons).46−48,84 In all, the shapes of UPC excitation spectra and their similarity to the absorption spectrum from ground 4I15/2 to initial excited state 4 I13/2 (spectra not included) confirm the predominance of ETU mechanism, as ground state absorption (GSA) followed by excited state absorption (ESA) would introduce additional absorption lines due to convolution of all involved intermediary states.84 We should note also that of the typical Er transitions: green (2H11/2,4S3/2−4I15/2), red (4F9/2−4I15/2), near-infrared at 800 (4I9/2−4I15/2), 980 (4I11/2−4I15/2) and 1500 nm (4I13/2−4I15/2), only the latter 4I15/2−4I13/2 absorption/emission transition has a strong magnetic dipole component.66 It is therefore expected that a significant population of Er ions located in the inversion S6 sites to be involved in the UPC excitation around 1500 nm. Unfortunately, the insufficient spectral resolution and sensitivity as well as lack of time-gated option in this near-infrared range preclude identification of Er in S6 sites. We therefore assume that the observed emissions relate mostly to Er in C2 sites. To this point, we note that, in contrast to the downconversion emission decays of Sm, Eu, Tb and Dy, the down-

theoretical values of 2 and 3, respectively, which, according to previous studies, relate to the competition between the upconversion process and linear decay for depopulation of the intermediate excited states.82−84 We concentrate further on the effect of Li addition and calcination temperature on the UPC emission decays. Figure 6 compares the emission decays under direct and UPC excitation at 1533 nm excitation monitoring the green (565 nm), red (660 nm), and NIR (at 800 nm) emissions for the Li free, Er,Li− Y2O3, and Er−Y2O3 calcined at 1000 °C. As it can be observed in Table 1 Li induces a considerable lengthening of the UPC decays: the average lifetimes of green emission at 565 nm, red emission at 660 nm and NIR emission at 800 nm increase by 1.8, 1.6, and 1.5, respectively. As shown also in Table 1, similar lengthening effect of the UPC decays is measured with the calcined 7Er−Y2O3 at a higher temperature of 1000 °C. Lengthening effect of the excited lifetimes along with emission enhancement is contrary to what should be expected in the case of the local structure distortion: A lower local structure symmetry would enhance the radiative transition rates or decrease the excited-state lifetime decrease and concurrently enhance the quantum yield. Such a correlation has been recently demonstrated with Er, Yb−NaYF4 system37 where the local symmetry was finely tuned by varying the cosubstitution degree of Y by a mixture of Gd and Lu ions. To allow comparison with the available data published in the literature, we have also measured the UPC decays under excitation at 980 nm (Table 1, Figure S9). Chen et al.21 reported an increase of the average lifetime of green emission under UPC excitation at 980 nm by ∼1.5 times and assigned it to the lengthening of the 4I11/2 intermediate state lifetime caused by Li-induced crystal field distortions around Er ions. Similar to the literature, we observed that Li induces a lengthening of the emission decays upon UPC excitation at 980 G


Article


reflect into alteration of the emission shapes and, more significantly, shortening of the emission decays. To this aim, we have selected five types of Ln ions whose emission was induced by down-conversion optical and X-ray excitation as well as upconversion excitation. This study, although preliminary in several respects, such as the actual mechanism of Li substitution, the dependence of the emission enhancement on the excitation mode (down or up conversion) and the actual role of Li addition in reducing the surface OH groups provides unambiguous evidence that Li addition does not manipulate the local structure by changing the local symmetry around Ln sites or modify their relative occupancy. The effect of Li addition is found to be similar to extending the calcination temperature, from 800 °C to ∼1000 1100 °C, therefore assimilated to improved crystallization. Our study also exposes notable properties of Er−Y2O3 nanoparticles in the near-infrared region that render them attractive for (bio)photonics applications, including probes in the second/third biological windows. To this end, further research will explore other isostructural Y2O3 hosts for Er activator in combination with Li- and non-Li-based chemical modification strategies.

conversion emission decays of Er are prolonged (in a similar way) by Li addition and increase of calcination temperature. This may be tentatively related to nonidentification of S6 centers of Er. III.3. Effect of Li Addition on the X-ray-Induced Emission. Li addition is also used to enhance the emission of Ln in Y 2 O 3 using nonoptical excitation such as cathodoluminescence85 or X-ray excitation.86 Figure 8 shows the X-ray excited luminescence (XEOL) spectra of 1Er−Y2O3 measured in the extended 500 to 1700 nm range.

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b02897. Selected XRD patterns, Raman, FTIR and XRF spectra, additional emission spectra and decays of Eu, Sm, Dy, and Er (PDF)

Figure 8. Effect of Li addition and extended calcination temperature on the X-ray induced luminescence spectrum of 1Er−Y2O3 measured in the 500 to 1700 nm range. The digital photo was obtained by use of low spec commercial webcam (Microsoft LifeCam HD-3000 not optimized for low light collection).

■

AUTHOR INFORMATION

Corresponding Author

*Electronic mail: carmen.tiseanu@inflpr.ro.

Higher Er concentration (greater than 1%) leads to reduced intensity. Similar enhancement factor of 3 was achieved with 1Er,Li−Y2O3 and 1Er−Y2O3 calcined at 1100 °C while the spectral shapes remained unperturbed across the investigated series. In contrast to optical f−f excitation into Er absorption bands, X-ray excitation assumes several sequential processes, such as X-ray absorption, electron−hole (e−−h+) pair generation, valence and conduction band energy trapping and collection, and finally, radiative emission from Er metastable states.87,88 The relative intense XEOL is likely related to the efficient absorbance of Y host cation (the K-edge at 17.0 keV of Y falls within the diagnostic range of the Mo source) and the recognized phosphor properties of Y2O3. A notable result illustrated also in Figure 8 concerns the predominance of the NIR emission over the visible emission as around 80% of the total XEOL emission intensity of Er−Y2O3 is retained around 1500 nm but also its relative intensity. For 1Er,Li−Y2O3 and 1Er−Y2O3 calcined at 1100 °C, the overall emission is strong enough to observe by use of low spec webcam the green color associated with the much weaker green emission at 565 nm. Such X-ray excitable near-infrared emission can be advantageous for the deeper biological imaging due to reduced photon absorption, scattering and tissue autofluorescence89 reached in both excitation and emission pathways.

ORCID

Daniel Avram: 0000-0002-1893-291X Notes

The authors declare no competing financial interest.

■ ■

ACKNOWLEDGMENTS Financial support for this work was provided by ANCS LAPLAS project number 4N/2016. REFERENCES

(1) Wyckoff, R. W. G. Crystal Structures; Interscience Publishers: New York, 1964, Vol. 2. (2) Kumar Rai, V.; Pandey, A.; Dey, R. Photoluminescence Study of Y2O3:Er3+-Eu3+-Yb3+ Phosphor for Lighting and Sensing Applications. J. Appl. Phys. 2013, 113, 083104. (3) Holloway, P. H.; Trottier, T. A.; Abrams, B.; Kondoleon, C.; Jones, S. L.; Sebastian, J. S.; Thomes, W. J.; Swart, H. Advances in Field Emission Displays phosphors. J. Vac. Sci. Technol., B: Microelectron. Process. Phenom. 1999, 17 (2), 758−764. (4) Jüstel, T.; Nikol, H.; Ronda, C. New Developments in the Field of Luminescent Materials for Lighting and Displays. Angew. Chem., Int. Ed. 1998, 37 (22), 3084−3103. (5) Matsuura, D. Red, Green, and Blue Upconversion Luminescence of Trivalent-Rare-Earth Ion-Doped Y2O3 Nanocrystals. Appl. Phys. Lett. 2002, 81 (24), 4526−4528. (6) Singh, V.; Rai, V. K.; Ledoux-Rak, I.; Watanabe, S.; Rao, T. G.; Chubaci, J. F. D.; Badie, L.; Pelle, F.; Ivanova, S. NIR to Visible Upconversion, Infrared Luminescence, Thermoluminescence and Defect Centres in Y2O3: Er Phosphor. J. Phys. D: Appl. Phys. 2009, 42 (6), 065104.

IV. CONCLUSIONS In summary, we have evaluated the effect of Li codoping on the emission of Ln−Y2O3. Our approach is based on the premise that any distortion/lowering of the local symmetry would H


Article


(26) Fan, T.; Lü, J. Enhanced 1.5 μm and Green Upconversion Emissions in Y2O3:Er3+ Nanoparticles Codoped with Li+ Ions. Opt. Commun. 2013, 300, 5−7. (27) Liu, L.; Wang, Y.; Bai, Y.; Zhang, X.; Yang, K.; Song, Y. Effects of Alkali Metal Ions on Upconversion Photoluminescence Intensity of Er3+-Doped Y2O3 Nanocrystals. Appl. Phys. B: Lasers Opt. 2013, 110 (1), 111−115. (28) Jeong, J. H.; Bae, J. S.; Yi, S. S.; Park, J. C.; Kim, Y. S. Photoluminescence Characteristics of Li-Doped Y2O3:Eu3+ Thin-Film Phosphors on Sapphire Substrates. J. Phys.: Condens. Matter 2003, 15 (3), 567. (29) Yang, M.; Mu, L.; Mu, H.; Lü, S. Effect of Li+ Doping on the Quenching Concentration of 2H11/2/4S3/2 Level of Er3+. Int. J. Mod. Phys. B 2014, 28 (19), 1450128. (30) Meza-Rocha, A. N.; Huerta, E. F.; Caldiño, U.; Carmona-Téllez, S.; Bettinelli, M.; Speghini, A.; Pelli, S.; Righini, G. C.; Falcony, C. Dependence of the Up-Conversion Emission of Li+ Co-Doped Y2O3:Er3+ Films with Dopant Concentration. J. Lumin. 2015, 167, 352−359. (31) Yadav, R. V.; Singh, S. K.; Rai, S. B. Effect of the Li+ Ion on the Multimodal Emission of a Lanthanide Doped Phosphor. RSC Adv. 2015, 5 (33), 26321−26327. (32) Yi, S. S.; Bae, J. S.; Moon, B. K.; Jeong, J. H.; Park, J. C.; Kim, I. W. Enhanced Luminescence of Pulsed-Laser-Deposited Y2O3:Eu3+ Thin-Film Phosphors by Li Doping. Appl. Phys. Lett. 2002, 81 (18), 3344−3346. (33) Byeon, S. H.; Ko, M. G.; Park, J. C.; Kim, D. K. LowTemperature Crystallization and Highly Enhanced Photoluminescence of Gd2‑xYxO3:Eu3+ by Li Doping. Chem. Mater. 2002, 14 (2), 603−608. (34) Hong, G. Y.; Yoo, K.; Moon, S. J.; Yoo, J. S. Enhancement of Luminous Intensity of Spherical Y2O3: Eu Phosphors Using Flux during Aerosol Pyrolysis. J. Electrochem. Soc. 2003, 150 (4), H67−H71. (35) Wang, F.; Wang, J.; Liu, X. Direct Evidence of a Surface Quenching Effect on Size-Dependent Luminescence of Upconversion Nanoparticles. Angew. Chem. 2010, 122 (41), 7618−7622. (36) Shannon, R. D. Revised Effective Ionic Radii and Systematic Studies of Interatomic Distances in Halides and Chalcogenides. Acta Crystallogr., Sect. A: Cryst. Phys., Diffr., Theor. Gen. Crystallogr. 1976, 32 (5), 751−767. (37) Wisser, M. D.; Fischer, S.; Maurer, P. C.; Bronstein, N. D.; Chu, S.; Alivisatos, A. P.; Salleo, A.; Dionne, J. A. Enhancing Quantum Yield via Local Symmetry Distortion in Lanthanide-Based Upconverting Nanoparticles. ACS Photonics 2016, 3 (8), 1523−1530. (38) Han, S.; Deng, R.; Xie, X.; Liu, X. Enhancing Luminescence in Lanthanide-Doped Upconversion Nanoparticles. Angew. Chem., Int. Ed. 2014, 53 (44), 11702−11715. (39) Cates, E. L.; Wilkinson, A. P.; Kim, J. H. Delineating Mechanisms of Upconversion Enhancement by Li+ Codoping in Y2SiO5:Pr3+. J. Phys. Chem. C 2012, 116 (23), 12772−12778. (40) Bai, Y.; Wang, Y.; Yang, K.; Zhang, X.; Peng, G.; Song, Y.; Pan, Z.; Wang, C. H. The Effect of Li on the Spectrum of Er3+ in Li- and ErCodoped ZnO Nanocrystals. J. Phys. Chem. C 2008, 112 (32), 12259− 12263. (41) Chen, G.; Qiu, H.; Prasad, P. N.; Chen, X. Upconversion Nanoparticles: Design, Nanochemistry, and Applications in Theranostics. Chem. Rev. 2014, 114 (10), 5161−5214. (42) Brown, E. E.; Hömmerich, U.; Bluiett, A.; Kucera, C.; Ballato, J.; Trivedi, S. Near-Infrared and Upconversion Luminescence in Er:Y2O3 Ceramics under 1.5 μm Excitation. J. Am. Ceram. Soc. 2014, 97 (7), 2105−2110. (43) Zhang, Y.; Li, S.; Qin, F.; Zhang, Z.; Dai, Z. Ultraviolet and Visible Upconversion Luminescence in Y2O3:Er3+, Gd3+ Microcrystals by 1.55 μm Excitation. J. Opt. Soc. Am. B 2015, 32 (9), 1856−1860. (44) Wang, H.; Jiang, T.; Xing, M.; Fu, Y.; Peng, Y.; Luo, X. UpConversion Luminescence of Y2O3: Yb, Er under 1.55 μm Excitation. Ceram. Int. 2015, 41 (1), 259−263. (45) Hemmer, E.; Venkatachalam, N.; Hyodo, H.; Hattori, A.; Ebina, Y.; Kishimoto, H.; Soga, K. Upconverting and NIR Emitting Rare

(7) Avram, D.; Cojocaru, B.; Florea, M.; Tiseanu, C. Advances in Luminescence of Lanthanide Doped Y2O3: Case of S6 Sites. Opt. Mater. Express 2016, 6 (5), 1635−1643. (8) Singh, A. K.; Singh, S. K.; Rai, S. B. Role of Li+ Ion in the Luminescence Enhancement of Lanthanide Ions: Favorable Modifications in Host Matrices. RSC Adv. 2014, 4 (51), 27039−27061. (9) Fan, T.; Zhang, Q.; Jiang, Z. The Effects of Li on the NearInfrared Luminescence Properties of Nd3+/ Li+ Codoped Y2O3 Nanocrystals. Opt. Commun. 2011, 284 (6), 1594−1597. (10) Bai, Y.; Wang, Y.; Peng, G.; Yang, K.; Zhang, X.; Song, Y. Enhance Upconversion Photoluminescence Intensity by Doping Li+ in Ho3+ and Yb3+ Codoped Y2O3 Nanocrystals. J. Alloys Compd. 2009, 478 (1−2), 676−678. (11) Li, D.; Wang, Y.; Zhang, X.; Dong, H.; Liu, L.; Shi, G.; Song, Y. Effect of Li+ Ions on Enhancement of Near-Infrared Upconversion Emission in Y2O3:Tm3+/Yb3+ Nanocrystals. J. Appl. Phys. 2012, 112 (9), 094701. (12) Pandey, A.; Rai, V. K.; Kumar, K. Influence of Li+ Codoping on Visible Emission of Y2O3:Tb3+, Yb3+ Phosphor. Spectrochim. Acta, Part A 2014, 118, 619−623. (13) Liang, H.; Zheng, Y.; Chen, G.; Wu, L.; Zhang, Z.; Cao, W. Enhancement of Upconversion Luminescence of Y2O3:Er3+ Nanocrystals by Codoping Li+−Zn2+. J. Alloys Compd. 2011, 509 (2), 409− 413. (14) Mishra, K.; Singh, S. K.; Singh, A. K.; Rai, S. B. Frequency Upconversion in Er3+ Doped Y2O3 Nanophosphor:Yb3+ Sensitization and Tailoring Effect of Li+ Ion. Mater. Res. Bull. 2013, 48 (10), 4307− 4313. (15) Fan, T.; Zhang, Q.; Jiang, Z. Enhancement of the 1.5 μm Emission in Y2O3:Er3+ Nanocrystals by Codoping with Li+ Ions. J. Opt. 2011, 13 (1), 015001. (16) Chen, G.; Liu, H.; Liang, H.; Somesfalean, G.; Zhang, Z. Upconversion Emission Enhancement in Yb3+/Er3+-Codoped Y2O3 Nanocrystals by Tridoping with Li+ Ions. J. Phys. Chem. C 2008, 112 (31), 12030−12036. (17) Sun, L.; Qian, C.; Liao, C.; Wang, X.; Yan, C. Luminescent Properties of Li+ Doped Nanosized Y2O3:Eu. Solid State Commun. 2001, 119 (6), 393−396. (18) Liang, H.; Chen, G.; Liu, H.; Zhang, Z. Ultraviolet Upconversion Luminescence Enhancement in Yb3+/Er3+-Codoped Y2O3 Nanocrystals Induced by Tridoping with Li+ Ions. J. Lumin. 2009, 129 (3), 197−202. (19) Shin, S. H.; Kang, J. H.; Jeon, D. Y.; Zang, D. S. Enhancement of Cathodoluminescence Intensities of Y2O3:Eu and Gd2O3:Eu Phosphors by Incorporation of Li Ions. J. Lumin. 2005, 114 (3−4), 275− 280. (20) Bai, Y.; Yang, K.; Wang, Y.; Zhang, X.; Song, Y. Enhancement of the Upconversion Photoluminescence Intensity in Li+ and Er3+ Codoped Y2O3 Nanocrystals. Opt. Commun. 2008, 281 (10), 2930− 2932. (21) Chen, G. Y.; Liu, H. C.; Somesfalean, G.; Sheng, Y. Q.; Liang, H. J.; Zhang, Z. G.; Sun, Q.; Wang, F. P. Enhancement of the Upconversion Radiation in Y2O3:Er3+ Nanocrystals by Codoping with Li+ Ions. Appl. Phys. Lett. 2008, 92 (11), 113114. (22) Chen, G. Y.; Liu, H. C.; Liang, H. J.; Somesfalean, G.; Zhang, Z. G. Enhanced Multiphoton Ultraviolet and Blue Upconversion Emissions in Y2O3: Er3+ Nanocrystals by Codoping with Li+ Ions. Solid State Commun. 2008, 148 (3−4), 96−100. (23) Yi, S. S.; Shim, K. S.; Yang, H. K.; Moon, B. K.; Choi, B. C.; Jeong, J. H.; Kim, J. H.; Bae, J. S. Improved Cathodoluminescent Characteristics of Y2O3:Eu3+ Thin Films by Li-Doping. Appl. Phys. A: Mater. Sci. Process. 2007, 87, 667−671. (24) Fan, T.; Zhang, Q.; Jiang, Z. Enhanced Near-Infrared Luminescence in Y2O3:Yb Nanocrystals by Codoping with Li+ Ion. Opt. Commun. 2011, 284 (1), 249−251. (25) Kumar, D.; Sharma, M.; Pandey, O. P. Effect of Co-Doping Metal Ions (Li+, Na+ and K+) on the Structural and Photoluminescent Properties of Nano-Sized Y2O3:Eu3+ Synthesized by Co-Precipitation Method. Opt. Mater. 2014, 36 (7), 1131−1138. I


Article


Gd2O3:Eu3+, a Red Phosphor Operating at Low Voltages. Appl. Phys. Lett. 2000, 77 (14), 2162−2164. (65) Huerta, E. F.; Carmona-Téllez, S.; Gallardo-Hernández, S.; Cabañas-Moreno, J. G.; Falcony, C. Up and Down Conversion Photoluminescence from Er, Yb and Li Doped Y2O3 Phosphors and Composites Films with PMMA. ECS J. Solid State Sci. Technol. 2016, 5 (7), R129−R135. (66) Dodson, C. M.; Zia, R. Magnetic Dipole and Electric Quadrupole Transitions in the Trivalent Lanthanide Series: Calculated Emission Rates and Oscillator Strengths. Phys. Rev. B: Condens. Matter Mater. Phys. 2012, 86 (12), 125102. (67) Taminiau, T. H.; Karaveli, S.; Van Hulst, N. F.; Zia, R. Quantifying the Magnetic Nature of Light Emission. Nat. Commun. 2012, 3, 979. (68) Judd, B. Optical-Absorption Intenisities of Rare-Earth Ions. Phys. Rev. 1962, 127 (3), 750−761. (69) Ofelt, G. S. Intensities of Crystal Spectra of Rare-Earth Ions. J. Chem. Phys. 1962, 37 (3), 511. (70) Zhao, C.; Kong, X.; Liu, X.; Tu, L.; Wu, F.; Zhang, Y.; Liu, K.; Zeng, Q.; Zhang, H. Li+ Ion Doping: an Approach for Improving the Crystallinity and Upconversion Emissions of NaYF4:Yb3+, Tm3+ nanoparticles. Nanoscale 2013, 5 (17), 8084−8089. (71) Nadort, A.; Zhao, J.; Goldys, E. M. Lanthanide Upconversion Luminescence at the Nanoscale: Fundamentals and Optical Properties. Nanoscale 2016, 8 (27), 13099−13130. (72) Singh, B. P.; Parchur, A. K.; Ningthoujam, R. S.; Ramakrishna, P. V.; Singh, S.; Singh, P.; Rai, S. B.; Maalej, R. Enhanced Up-Conversion and Temperature-Sensing Behaviour of Er3+ and Yb3+ Co-Doped Y2Ti2O7 by Incorporation of Li+ Ions. Phys. Chem. Chem. Phys. 2014, 16 (41), 22665−22676. (73) Guo, L.; Wang, Y.; Wang, Y.; Zhang, J.; Dong, P.; Zeng, W. Structure, Enhancement and White Luminescence of Multifunctional Lu6O5F8:20%Yb3+,1%Er3+(Tm3+) Nanoparticles via further Doping with Li+ under Different Excitation Sources. Nanoscale 2013, 5 (6), 2491−2504. (74) Cheng, Q.; Sui, J.; Cai, W. Enhanced Upconversion Emission in Yb3+ and Er3+ Codoped NaGdF4 Nanocrystals by Introducing Li+ Ions. Nanoscale 2012, 4 (3), 779−784. (75) Lin, H.; Xu, D.; Teng, D.; Yang, S.; Zhang, Y. ShapeControllable Synthesis and Enhanced Upconversion Luminescence of Li+ doped β-NaLuF4:Yb3+,Ln3+ (Ln = Tm, Ho) Microcrystals. New J. Chem. 2015, 39 (4), 2565−2572. (76) Yadav, R. S.; Yadav, R. V.; Bahadur, A.; Rai, S. B. Enhanced White Light Emission from a Tm 3+ /Yb 3+ /Ho 3+ Co-Doped Na4ZnW3O12 Nano-Crystalline Phosphor via Li+ Doping. RSC Adv. 2016, 6 (57), 51768−51776. (77) Zhan, Q.; He, S.; Qian, J.; Cheng, H.; Cai, F. Optimization of Optical Excitation of Upconversion Nanoparticles for Rapid Microscopy and Deeper Tissue Imaging with Higher Quantum Yield. Theranostics 2013, 3 (5), 306−316. (78) Liu, T. M.; Conde, J.; Lipiński, T.; Bednarkiewicz, A.; Huang, C. C. Revisiting the Classification of NIR-Absorbing Emitting Nanomaterials for in vivo Bioapplications. NPG Asia Mater. 2016, 8 (8), e295. (79) Zheng, X.; Zhu, X.; Lu, Y.; Zhao, J.; Feng, W.; Jia, G.; Wang, F.; Li, F.; Jin, D. High-Contrast Visualization of Upconversion Luminescence in Mice Using Time-Gating Approach. Anal. Chem. 2016, 88 (7), 3449−3454. (80) del Rosal, B.; Ortgies, D. H.; Fernández, N.; Sanz-Rodríguez, F.; Jaque, D.; Rodríguez, E. M. Overcoming Autofluorescence: LongLifetime Infrared Nanoparticles for Time-Gated In Vivo Imaging. Adv. Mater. 2016, 28 (46), 10188−10193. (81) Hemmer, E.; Benayas, A.; Légaré, F.; Vetrone, F. Exploiting the Biological Windows: Current Perspectives on Fluorescent Bioprobes Emitting above 1000 nm. Nanoscale Horizons 2016, 1 (3), 168−184. (82) Pollnau, M.; Gamelin, D. R.; Lüthi, S. R.; Güdel, H. U.; Hehlen, M. P. Power Dependence of Upconversion Luminescence in Lanthanide and Transition-Metal-Ion Systems. Phys. Rev. B: Condens. Matter Mater. Phys. 2000, 61 (5), 3337.

Earth Based Nanostructures for NIR-Bioimaging. Nanoscale 2013, 5 (23), 11339−11361. (46) Goldschmidt, J. C.; Fischer, S. Upconversion for Photovoltaics − a Review of Materials, Devices and Concepts for Performance Enhancement. Adv. Opt. Mater. 2015, 3 (4), 510−535. (47) Briggs, J. A.; Atre, A. C.; Dionne, J. A. Narrow-Bandwidth Solar Upconversion: Case Studies of Existing Systems and Generalized Fundamental Limits. J. Appl. Phys. 2013, 113 (12), 124509. (48) Rüdiger, M.; Fischer, S.; Frank, J.; Ivaturi, A.; Richards, B. S.; Krämer, K. W.; Hermle, M.; Goldschmidt, J. C. Bifacial n-type Silicon Solar Cells for Upconversion Applications. Sol. Energy Mater. Sol. Cells 2014, 128, 57−68. (49) Fischer, S.; Favilla, E.; Tonelli, M.; Goldschmidt, J. C. Record Efficient Upconverter Solar Cell Devices with Optimized Bifacial Silicon Solar Cells and Monocrystalline BaY2F8:30% Er3+ Upconverter. Sol. Energy Mater. Sol. Cells 2015, 136, 127−134. (50) Zhou, B.; Shi, B.; Jin, D.; Liu, X. Controlling Upconversion Nanocrystals for Emerging Applications. Nat. Nanotechnol. 2015, 10, 924−936. (51) Florea, M.; Avram, D.; Cojocaru, B.; Tiseanu, I.; Parvulescu, V.; Tiseanu, C. Defect Induced Tunable Near Infrared Emission of Er− CeO2 by Heterovalent Co-Dopants. Phys. Chem. Chem. Phys. 2016, 18 (27), 18268−18277. (52) Fischer, S.; Fröhlich, B.; Steinkemper, H.; Krämer, K. W.; Goldschmidt, J. C. Absolute Upconversion Quantum Yield of β-NaYF4 Doped with Er3+ and External Quantum Efficiency of Upconverter Solar Cell Devices Under Broad-Band Excitation Considering Spectral Mismatch Corrections. Sol. Energy Mater. Sol. Cells 2014, 122, 197− 207. (53) Martín-Rodríguez, R.; Fischer, S.; Ivaturi, A.; Froehlich, B.; Krämer, K. W.; Goldschmidt, J. C.; Richards, B. S.; Meijerink, A. Highly Efficient IR to NIR Upconversion in Gd2O2S: Er3+ for Photovoltaic Applications. Chem. Mater. 2013, 25 (9), 1912−1921. (54) Martín-Rodríguez, R.; Rabouw, F. T.; Trevisani, M.; Bettinelli, M.; Meijerink, A. Upconversion Dynamics in Er3+-Doped Gd2O2S: Influence of Excitation Power, Er3+ Concentration, and Defects. Adv. Opt. Mater. 2015, 3 (4), 558−567. (55) Vega, M.; Fuentes, S.; Martín, I. R.; Llanos, J. Up-Conversion Photoluminescence of BaTiO3 Doped with Er3+ Under Excitation at 1500 nm. Mater. Res. Bull. 2017, 86, 95−100. (56) Favilla, E.; Cittadino, G.; Veronesi, S.; Tonelli, M.; Fischer, S.; Goldschmidt, J. C.; Cassanho, A.; Jenssen, H. P. Comparative Analysis of Upconversion Efficiencies in Fluoride Materials for Photovoltaic Application. Sol. Energy Mater. Sol. Cells 2016, 157, 415−421. (57) Tanner, P.; Wong, K. Synthesis and Spectroscopy of Lanthanide Ion-Doped Y2O3. J. Phys. Chem. B 2004, 108 (1), 136−142. (58) Buijs, M.; Meyerink, A.; Blasse, G. Energy-Transfer between Eu3+ Ions in a Lattice with 2 Different Crystallographic Sites - Y2O3Eu3+, Gd2O3-EU3+ and Eu2O3. J. Lumin. 1987, 37 (1), 9−20. (59) Lupei, A.; Tiseanu, C.; Gheorghe, C.; Voicu, F. Optical Spectroscopy of Sm3+ in C2 and C3i Sites of Y2O3 Ceramics. Appl. Phys. B: Lasers Opt. 2012, 108 (4), 909−918. (60) Avram, D.; Sanchez-Dominguez, M.; Cojocaru, B.; Florea, M.; Parvulescu, V.; Tiseanu, C. Toward a Unified Description of Luminescence−Local Structure Correlation in Ln Doped CeO2 Nanoparticles: Roles of Ln Ionic Radius, Ln Concentration, and Oxygen Vacancies. J. Phys. Chem. C 2015, 119 (28), 16303−16313. (61) George, S.; Anjana, P. S.; Deepu, V. N.; Mohanan, P.; Sebastian, M. T. Low-Temperature Sintering and Microwave Dielectric Properties of Li2MgSiO4 Ceramics. J. Am. Ceram. Soc. 2009, 92 (6), 1244− 1249. (62) Osaka, T.; Shindo, I. Infrared Reflectivity and Raman Scattering of Lithium Oxide Single Crystals. Solid State Commun. 1984, 51 (6), 421−424. (63) Tanner, P. A.; Wong, K. L. Synthesis and Spectroscopy of Lanthanide Ion-doped Y2O3. J. Phys. Chem. B 2004, 108 (1), 136−142. (64) Park, J. C.; Moon, H. K.; Kim, D. K.; Byeon, S. H.; Kim, B. C.; Suh, K. S. Morphology and Cathodoluminescence of Li-Doped J


Article

The Journal of Physical Chemistry C (83) Lüthi, S. R.; Pollnau, M.; Güdel, H. U.; Hehlen, M. P. NearInfrared to Visible Upconversion in Er3+-doped Cs3Lu2Cl9, Cs3Lu2Br9, and Cs3Y2I9 Excited at 1.54 μm. Phys. Rev. B: Condens. Matter Mater. Phys. 1999, 60 (1), 162. (84) Auzel, F.; Upconversion. and Anti-Stokes Processes with f and d Ions in Solids. Chem. Rev. 2004, 104 (1), 139−174. (85) Mao, Y.; Tran, T.; Guo, X.; Huang, J. Y.; Shih, C. K.; Wang, K. L.; Chang, J. P. Luminescence of Nanocrystalline Erbium-Doped Yttria. Adv. Funct. Mater. 2009, 19 (5), 748−754. (86) Stanton, I. N.; Belley, M. D.; Nguyen, G.; Rodrigues, A.; Li, Y.; Kirsch, D. G.; Yoshizumi, T. T.; Therien, M. J. Europium- and Lithium-Doped Yttrium Oxide Nanocrystals that Provide a Linear Emissive Response with X-ray Radiation Exposure. Nanoscale 2014, 6 (10), 5284−5288. (87) Rodnyi, P. A. Physical Processes in Inorganic Scintillators; CRC Press: New York, 1997. (88) Nikl, M. Scintillation Detectors for X-rays. Meas. Sci. Technol. 2006, 17 (4), R37. (89) Naczynski, D. J.; Sun, C.; Türkcan, S.; Jenkins, C.; Koh, A. L.; Ikeda, D.; Pratx, G.; Xing, L. X-ray-Induced Shortwave Infrared Biomedical Imaging Using Rare-Earth Nanoprobes. Nano Lett. 2015, 15 (1), 96−102.

K


Up-Conversion Emission Enhancement by Li Addition

Recommend Documents