Insight into the Upconversion Luminescence of Highly Efficient

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C: Plasmonics, Optical Materials, and Hard Matter

Insight into the Upconversion Luminescence of Highly Efficient Lanthanide-Doped BiO Nanoparticles 2

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Michele Back, Enrico Trave, Pietro Riello, and Jonas J. Joos J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b00637 • Publication Date (Web): 20 Mar 2018 Downloaded from http://pubs.acs.org on March 21, 2018

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The Journal of Physical Chemistry

Insight into the Upconversion Luminescence of Highly Efficient Lanthanide-Doped Bi2O3 Nanoparticles

Michele Back,1,* Enrico Trave,1,* Pietro Riello,1 and Jonas J. Joos 2 1

Department of Molecular Sciences and Nanosystems, Università Ca’ Foscari Venezia, Via

Torino 155, 30172 Mestre-Venezia, Italy. 2

LumiLab, Department of Solid State Sciences, Ghent University, Krijgslaan 281/S1, 9000 Gent,

Belgium.

ACS Paragon Plus Environment

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ABSTRACT A series of Bi2O3 nanoparticles doped with Yb3+ and Ln3+ (Ln3+ = Er3+, Ho3+, Tm3+) ions were prepared by means of a Pechini-type sol-gel synthesis in order to develop novel approaches for the realization of high-performing upconverting nanophosphors, with controlled chromaticity output and enhanced emission efficiency. The overall upconversion mechanism originating the observed luminescence spectra is strongly influenced by the narrow bandgap of the Bi2O3 matrix (about 2.6 eV when doped at 10-12 at%), since the occurrence of optical band-to-band transitions sets such an upper energy threshold to the activation of those upconversion features characterizing the spectrum of the different Yb3+-Ln3+ systems. Moreover, as emerging from diffuse reflectance analysis performed on a series of Yb3+, Er3+ codoped samples with Yb content in the 0-20 at% range, the Bi2O3 energy gap can be properly tuned by varying the overall dopant concentration. This evidence suggests a strategy to achieve: i) chromaticity output control; ii) the realization of single-band emitters. Concerning the last point, important results were achieved for Yb3+-Er3+ and Yb3+-Tm3+ codoped samples, that behave nearly monochromatic NIR-to-red and NIR-to-NIR upconverters under 980 nm light exposure, respectively, with significant damping of those radiative components in the blue-green part of the visible spectrum. Furthermore, the emission mechanism for the investigated systems is characterized by remarkable quantum efficiency value, a fundamental parameter in view of possible application in bioimaging or anticounterfeiting fields.


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INTRODUCTION The potentialities offered by upconversion (UC) nanoparticles (NPs) doped with lanthanide ions (Ln3+) still represent a current topic of interest for many application domains, like bioimaging,1-5 photovoltaic technologies,6-8 temperature sensors,9-11 light emitting displays,12-13 lasing systems,1416

anticounterfeiting.17-19 Peculiar properties, like intense signals, sharp spectral bands, long

luminescence lifetimes, possible chromaticity control, characterize the Ln3+ radiative emissions originating from UC processes, which are basically nonlinear optical phenomenon where two- or multiphoton absorption leads to an anti-Stokes emission of higher-energy photons.20-22 Typical near-infrared (NIR) activated UC systems are based on the energy transfer between Yb3+, as the sensitizer, and a different lanthanide species, like Er3+, Ho3+ and Tm3+, as the photoluminescence (PL) activator. In this case, the large Yb3+ absorption cross-section around 980 nm can effectively promote the activator emission, usually characterized by multi-band spectral features covering from the ultraviolet to the infrared spectral region. This is due to the great abundance of meta-stable excited energy levels involved under light exposure, presented by the Ln3+ activator when in appropriate hosts. On the other hand, a possible drawback is represented by the occurrence of energy de-concentration effects that limit the efficiency of the UC process. Therefore, various approaches were attempted to realize spectral-controlled, highly efficient UC systems, by acting on the overall photophysical mechanism through antenna effects,23 energy transfer pathways,24 cross-relaxation,25,26 energy migration-mediated processes,27,28 as well as by looking for specific hosts, with improved architectures design, like core-shell assemblies or controlled incorporation of structure modifying species.29-31 Moreover, the search for chromatically-selected fluorescent probes, active in the biological optical windows falling in the red-NIR region, represents a current focus in the bioimaging technology field.


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Yb3+, Ln3+ codoped UCNPs with improved emission selectivity were realized, showing relevant performances in terms of continuous color output tuning over broad spectral ranges as well as single-band luminescence with suppression of other undesirable emissions. It is well known that for Yb3+-Er3+ systems the relative ratio between the Er3+ radiative features observed in the 980 nmactivated UC spectrum can be modified by acting on the Yb doping level. Up to the concentration quenching threshold, the rise of the Yb content leads to an overall increase of the PL emission intensity, but usually those components falling in the red are mainly enhanced. This is a strategy for realizing Yb3+-Er3+ based NIR-to-red upconverter.32-36 A useful route for realizing single-band Yb3+-Ln3+ codoped UC emitters is based on controlled embedding of selected ions, responsible of the activation of some mechanisms, like energy transfer or cross relaxation, which bring to a population increase for specific Ln3+ emitting states, with improvement of the related radiative transitions, at the expense of other states, involved in luminescence processes that result consequently quenched. We can mention a series of single red or NIR band-emitting fluoridebased nanophosphors, as the case of Ce3+ codoping of Yb3+-Ho3+,37-39 Dy3+ codoping of Yb3+Tm3+,26 and Mn2+ codoping of Yb3+-Er3+, Yb3+-Ho3+, Yb3+-Tm3+ systems.40,41 The aim of this work is to assess the impact of Yb3+ ions incorporation in Bi2O3 NPs doped with Ln3+ (Ln3+ = Er3+, Ho3+, Tm3+) ions, where Yb3+ acts as sensitizer for the 980 nm-activated UC PL process, as well as Bi2O3 structure modifier. In this regard, it is worth considering that Bi oxide is a narrow bandgap material and possible tunability of this parameter through the incorporation of a suitable structure modifying dopant was exploited for electronic and energy applications.42-45 Recently, we investigated the photoluminescence properties exhibited by Er3+ ions embedded in Bi2O3 NPs, observing that under 980 nm photo-excitation those Er3+ UC emissions promoted by multi-photon absorptions (i.e., third order or more) are strongly damped by competitive non-


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radiative de-excitation paths involving transitions across the host bandgap, while the ones from two-photon absorption processes are basically unaffected, resulting in predominant red-NIR luminescence features in the recorded spectra.46 But a further doping with Y3+ ions up to 40 at% promoted a progressive host bandgap widening in a range from 2.34 to 2.99 eV, resulting in an enhancement of those emissions from the higher-lying Er3+ energy levels with consequent effective fine-tuning of the chromaticity output from red to yellow-greenish. Therefore we expected the same influence on the Bi oxide bandgap by doping with Yb3+ ions, owed to the comparable ionic radius values between Yb3+ and Y3+ (0.985 and 1.019 Å, respectively)47. This was demonstrated for a series of Yb3+, Er3+ codoped NPs, where the emission behaviour under 980 nm light exposure is determined from the balance between both Yb3+-driven Er3+ sensitization and host bandgap widening effects as the Yb content increases. Furthermore, a parallel between the UC PL properties exhibited by the three different Yb3+-Ln3+ pairs allowed to highlight the impact of the reduced host bandgap on the overall photophysical mechanism at the basis of the observed upconversion processes. The main findings of this study are represented by the real control of the chromaticity output for the investigated nanophosphors, with the possibility to realize single-band emitting upconverters, and by the significant efficiency parameters estimated for the investigated optical processes, in particular UC PL quantum yield (QY) values comparable to the most efficient systems used as luminescent probes in high-contrast fluorescence imaging and multiplexed labeling.

EXPERIMENTAL Materials


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Bi(NO3)3•5H2O Er(NO3)3•5H2O

(99.99%, (99.9%,

Sigma-Aldrich), Sigma-Aldrich),

Yb(NO3)3•5H2O Ho(NO3)3•5H2O

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(99.9%,

Sigma-Aldrich),

(99.9%,

Sigma-Aldrich),

Tm(NO3)3•5H2O (99.9%, Sigma-Aldrich), citric acid (99%, Carlo Erba), ethylene glycol (99.5%, Acros Organics), and HNO3 60% (Sigma-Aldrich) were used for the preparation of the samples, without further purification. Synthesis Following the synthesis route reported in Ref. 46, a Pechini-type sol-gel procedure was adopted for the preparation of a series of codoped Bi2O3 NPs. In the basic reaction, nitrate salt precursors were mixed together with citric acid and ethylene glycol. The reaction was maintained at 120°C for about 12 h and the resulting samples were calcined in a muffle oven at 750°C, in air, for 2 h. The molar proportions between the salts were chosen according to the stoichiometry of Bi2O3 and the desired dopant concentration, with the aim to maximize the optical performance of the synthesized samples. Then, following the literature and previous studies, the core of this investigation was represented by three samples containing the different Yb3+-Ln3+ pairs, which were prepared by fixing the Yb content at 10 at% and the Ln one at 2, 1, 0.2 at% for Er, Ho and Tm, respectively. Moreover, a series of Yb3+, Er3+ codoped NPs was realized by varying the Yb content in the 0-20 at% range, while Er was kept at 2 at%. In the following, the samples were labeled considering the at% content of each dopant. Characterization X-Ray Powder Diffraction (XRPD) analysis was carried out by using a Philips diffractometer with a PW 1319 goniometer with Bragg-Brentano geometry. The apparatus included a focusing graphite monochromator and a proportional counter with a pulse-height discriminator. The


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measurement was obtained by Nickel-filtered Cu Kα radiation and a step-by-step technique (steps of 0.05° in 2θ), with collection time of 30 s per step. The study of nanoparticle size and morphology, as well as EDS analysis, was realized by means of a Carl Zeiss Sigma VP Field Emission Scanning Electron Microscope (FE-SEM) equipped with a Bruker Quantax 200 microanalysis detector. The EDS spectra were collected at the same condition (20 KeV) for all the samples. Diffuse reflectance measurements were performed by collecting diffusive reflective UV-Vis (DRUV-Vis) spectra with a JASCO V-570 UV-vis spectrophotometer, equipped with an integrating sphere accessory; barium sulfate was used as reference. Considering the direct bandgap nature of the observed Bi2O3 crystalline phases, the calculation method based on the Tauc plot of the Kubelka-Munk function F(R) deriving from the acquired reflectance spectra was used to determine the optical bandgap for the investigated samples.48,49 The set-up for UC PL spectroscopy measurements included a CNI MDL-III-980 diode laser as 980 nm photon pumping source, with output power of 2W over a spot of 5×8 mm2 (power density of 5 W/cm2). The emission spectra were acquired by means of a QE65 Pro Ocean Optics spectrometer. Neutral density filters were used to attenuate the pumping radiation. Upconversion and absorption efficiencies were measured by means of an integrating sphere (152 mm diameter, coated with Spectralon), equipped with a calibrated radiometer (ILT 1700, International Light Technologies). A baffle is mounted between the sample and the detector. A 980 nm laser diode was used as excitation source and the irradiance was 1.3 W/cm².

RESULTS AND DISCUSSION


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From the morphological point of view, the synthesized NPs appear as spheroidal with size around 50 nm, as can be observed in the SEM image of Figure 1a. In addition, the EDS analysis confirmed the incorporation of the different doping species in agreement with the expected Bi:Yb:Ln doping ratio; the EDS spectrum of the Yb10Er2 sample is reported in Figure 1b. Figure S1 and S2 respectively show SEM images and EDS spectra of the Yb10Ho1 and Yb10Tm0.2 samples. Figure 1c shows the result of the XRPD measurements performed on the Yb3+, Er3+ codoped samples (YbxEr2 series). The reported series of diffraction patterns attests a structural evolution going from the tetragonal β-Bi2O3 phase (ICSD#41764) for the Ybfree sample to the cubic δ-Bi2O3 phase (ICSD#98144) as the Yb content progressively increases. This behaviour is in line with what observed in the case of Y3+, Er3+ codoped Bi2O3 NPs,46 and it is linked to the similar radius value exhibited by the two ionic species. Further details on the different Yb-Ln systems can be found in Figure S3 and S4, which respectively report the XRPD patterns of the Yb10Er2, Yb10Ho1, Yb10Tm0.2 samples, and the result of the Scherrer analysis performed on all codoped samples object of this study.


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Figure 1. (a) FE-SEM image and (b) EDS spectrum for the Yb10Er2 sample. (c) XRPD patterns of the YbxEr2 series.

In Figure 2, the UC PL emission spectra for the three samples doped with 10 at% of Yb are showed. By referring to the energy level diagram presented in Figure 3, we can provide the assignment of the observed spectral features to the energy levels involved for any of the three Ln3+ ions. In the diagram, the main transitions taking place in the overall UC PL


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mechanism for the Yb3+-Er3+, Yb3+-Ho3+ and Yb3+-Tm3+ pairs are shown, with particular emphasis on the energy transfer mechanisms where Yb3+ ions act as donor species.

Figure 2. UC PL emission spectrum of the Yb10Er2, Yb10Ho1 and Yb10Tm0.2 samples. List of transitions labeled with cap letters: Er3+ 4S3/2 → 4I15/2 and 2H11/2 → 4I15/2 (A), 4F9/2 → 4I15/2 (B), 4I9/2 → 4I15/2 (C); Ho3+ 5F4,5S2 → 5I8 (D), 5F5 → 5I8 (E), 5F4,5S2 → 5I7 (F); Tm3+ 2F2,3 → 3H6 (G), 3H4 → 3

H6 (H). Included within the graph, digital camera images under 980 nm light exposure, collected

by using a 900 nm long-cut filter; in the case of Yb10Tm0.2 sample, the CCD camera associates the violet color to the 800 nm emission.


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Figure 3. Scheme of the energy level diagram representing UC and transition mechanisms originating the luminescence emissions observed for the Yb3+-Er3+, Yb3+-Ho3+ and Yb3+-Tm3+ systems. The brown upward arrow refers to Yb3+ ground state absorption process (GSA); solid downward arrows refer to Ln3+ radiative relaxations, with colors representing transitions in the red, green and near-IR spectral ranges; blue arrows refer to energy transfer process (ET/ETU); pink dashed arrows refer to energy back-transfer process (EBT) for the Yb3+-Er3+ system; grey wavelike arrows refer to multiphonon relaxations. The horizontal dashed orange line marks the energy threshold for Bi2O3 band-to-band transition, as estimated by the Kubelka-Munk analysis for the three codoped samples with 10 at% of Yb.

Furthermore, the diagram includes also the Bi2O3 band-to-band energy level, as estimated from the Kubelka-Munk analysis reported in Figure 4, which sets such an upper threshold for the activation of the Ln3+ transitions when operating in UC mode, as will be discussed in the following. All the spectra reported in Figure 4a are characterized by a strong absorption band peaked at 980 nm, due to the ground-to-excited state Yb3+ 2F7/2 → 2F5/2


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transition. This is a clear indication of the large cross section characterizing the Yb3+ ground-state absorption (GSA) process, which is considered as the main triggering event for the activation of the overall UC mechanism under 980 nm light exposure. From the Tauc plots in Figure 4b, it must be noticed that the estimated energy gap values scale down moving from Yb10Er2 (2.63 eV), to Yb10Ho1 (2.55 eV) and to Yb10Tm0.2 (2.53eV) samples. Therefore we suggest that this trend is linked to the progressive decrease of the whole content of lanthanide species (overall Yb and Ln at%), and that a precise control of the Bi2O3 host bandgap can be achieved through careful dopant incorporation.


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Figure 4. (a) Diffuse reflectance spectra and (b) corresponding Tauc plot of the Kubelka-Munk function F(R) for the Yb10Er2, Yb10Ho1 and Yb10Tm0.2 samples.

For the Yb10Er2 sample, the spectrum of Figure 2 is characterized by three main features: a dominant signal attributed to the Er3+ 4F9/2 → 4I15/2 transition (around 660 nm, labeled as B), falling in the red part of the visible spectrum, and two weaker emissions, respectively in the green region (around 550 nm, labeled as A), ascribed to the Er3+ 2H11/2 → 4I15/2 and 4

S3/2 → 4I15/2 transitions, and at the visible-NIR edge (around 800 nm, labeled as C),

ascribed to the Er3+ 4I9/2 → 4I15/2 transition. In the following, these three features will be referred as RED, GRN and NIR, respectively. Concerning the series of samples obtained by varying the Yb content, it was observed that the RED emission remains as the most intense for the whole explored range (0-20 at% of Yb doping). Indeed, the picture included in Figure 2 attests a bright red emission color originating under 980 nm light exposure for the Yb10Er2 sample, and all the samples belonging to the YbxEr2 series exhibit a similar chromaticity output with a dominant red component. As previously mentioned, this behaviour is usually shown for Yb3+, Er3+ codoped materials with large Yb content, and it represents a common strategy for realizing single-band red-emitting upconverters. From Figure 3, we can distinguish between two main mechanisms occurring when operating in UC mode. The first mechanism originates from a sequential two-step excitation path involving the Er3+ 4I15/2 → 4I11/2 and 4I11/2 → 4F7/2 transitions, triggered by either Er3+ absorption (GSA/ESA) or Yb3+-mediated energy transfer (ET/ETU) processes. Then, the de-excitation from the Er3+ 4F7/2 level promotes the population of the lower-lying


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energy states, from which the radiative transitions responsible for the detected spectral features take place. As emerged in the case of the Y3+, Er3+ codoped Bi2O3 NPs, the efficiency of this relaxation path, then of the resulting UC PL emission processes, is strongly influenced by possible bandgap widening effect.

Figure 5. (a) Tauc plot of the Kubelka-Munk function F(R) for the synthesized samples belonging to the YbxEr2 series, (b) trend of optical bandgap values versus Yb content (the dotted line is a guide for the eye) and (c) digital camera images of the powder samples.

In this regard, for the samples belonging to the YbxEr2 series the Tauc plot of the resulting Kubelka-Munk functions and the trend of the estimated gap values versus the Yb content are reported in Figure 5a and b, respectively. It is worth noticing that the variation


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of the Yb doping level determines a bandgap energy tunability in a range of 0.43 eV, in agreement with the behaviour evidenced for the Y3+-Er3+ system.46 As shown in Figure 5c, starting from the orange-like coloration of the Yb-free sample, the appearance of the codoped powders is characterized by a gradual whitening as the Yb content increases. This was expected in virtue of the progressive shift of the energy gap threshold towards the UV region. By considering the estimates obtained from the curves shown in Figure 5a, for the Yb10Er2 sample the host energy gap is so wide that the UC mechanism involving the Er3+ 4

F7/2 level can be considered fully activated. On the other hand, it is suggested that at low

doping level the Bi2O3 band-to-band transition could match in energy the Er3+ 4F7/2 level relaxation to the ground state, then opening the way to a non-radiative, host-mediated path competitive to the transitions towards the levels involved in the measured UC PL emissions. About the alternative UC mechanism, the two-step Er3+ excitation process includes a phonon assisted Er3+ 4I11/2 → 4I13/2 relaxation, then followed by the second upward transition to the 4F9/2 level (triggered by either Er3+ ESA or Yb3+-mediated ETU). From the 4F9/2 level, radiative de-excitation to the ground state brings to the activation of the RED emission and, after phonon assisted relaxation to the 4I9/2 level, of the NIR emission too. It must be noticed that the excited states involved in this UC mechanism fall at an energy well reduced with respect to the host bandgap. So, in principle the efficiency of the resulting radiative processes should not be affected by the occurrence of the above-described bandgap widening effect. To get deeper inside the impact of the Yb content on the UC PL properties, Figure 6 reports the variation of the Er3+ emission intensity as a function of the Yb doping level. PL spectra of the whole YbxEr2 series are shown in Figure S5. All the GRN, RED and NIR


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emissions show a general enhancement up to an optimal Yb concentration of 10-15 at%. This upward trend can be ascribed to a couple of mechanisms related to the progressive Yb content increase: (i) Er ion photosensitization, deriving from the enlarged photon absorption efficiency at 980 nm for the Yb3+ codoped material; (ii) matrix bandgap widening effect, with a consequent increase of the band-to-band transition energy that determines an efficiency enhancement for all the Er3+ radiative emissions over the analyzed spectral range, as observed in the case of the Y3+-Er3+ system.46


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Figure 6. Trend of the integrated UC PL intensity versus Yb content for the GRN, RED and NIR emissions shown by the Yb,Er codoped samples.

To discriminate between these two effects and, in particular, to give emphasis on the Yb3+-mediated Er3+ sensitization mechanism, we propose a parallel between the UC PL response characterizing the Yb3+-Er3+ and the Y3+-Er3+ system. Concerning the latter, we refer to the measurements performed on a series of samples already object of the work reported in Ref. 46. A typical UC PL spectrum and the variation of the resulting intensity for the GRN, RED, NIR emissions as a function of the Y content are shown in Figure S6 and S4, respectively. As expected, the behaviour in Figure S7 strongly resembles what observed in the case of the YbxEr2 series. Nevertheless, using Y3+ as codoping species, it can be stated that the increasing trend up to the optimal Y content must be solely due to the bandgap widening effect. Therefore, starting from the data reported in Figure 6 and S7, for each of the three main Er3+ emissive features the graph in Figure 7 reports the trend of the ratio between the UC PL signals originating from samples having the same Yb or Y doping content in the 0-20 at% range. Since the Yb3+ and Y3+ ions should present the same attitude as structural modifier when incorporated in a Bi2O3-based matrix, a possible influence of the progressive host bandgap widening in determining the trends in Figure 7 can be ruled out. So, the Yb/Y integrated PL ratio can be taken as a figure of merit to evaluate the real impact of the Yb3+mediated Er sensitization. By increasing the doping level, the Yb/Y ratio is enhanced up to some tens of times for the RED and the NIR emissions, reaching for the former a factor of about 50 at the optimal Yb concentration. On the other hand, the Yb/Y ratio for the GRN


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emission remains significantly lower and almost constant over the whole doping range. Therefore, it is claimed that for the Yb3+-Er3+ system the Yb3+-mediated sensitization has a fundamental role in determining the improvement of those Er3+ emissions falling in the redNIR range, while a possible contribution to the GRN intensity increase observed in Figure 6 seems rather limited. In this case, the enhancement should be mainly ascribed to the bandgap widening effect.

Figure 7. Trend of the UC PL intensity ratio versus Yb or Y content shown by the Yb-Er and YEr series. For any of the GRN, RED and NIR emissions, the ratio data were calculated using the values reported in Figure 6 and S7.

As reported in previous studies,50,51 in the framework of the overall upconversion mechanism determined by the Yb3+-Er3+ interaction, it is worth giving account of a peculiar process shown in the diagram of Figure 3. This consists in an energy back-transfer process (EBT, pink dashed lines in the diagram) from the excited Er3+ to the Yb3+ ions, whose mechanism can be summarized as Er3+ 2H11/2,4S3/2 + Yb3+ 2F7/2 → Er3+ 4I13/2 + Yb3+ 2F5/2. The following step is determined by Yb3+-to-Er3+ ETU, with consequent Er3+ upward


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transition to the 4F9/2 state from the long-living 4I13/2 one, and further feeding of those processes responsible for the radiative relaxations over the red-NIR region. Since the EBT process (and consequent ETU) must be strengthened as the Yb content increases, this effect is usually considered for explaining the red emission enhancement observed in several Yb3+-Er3+ UC PL systems. Furthermore, the occurrence of the EBT process has to cause the depletion of the Er3+ 2H11/2 and 4S3/2 states, from which the GRN emission process originates, then determining a limit to the GRN intensity enhancement owed to the larger 980 nm absorption cross section as the Yb content increases. Therefore, it is suggested that the trend observed in Figure 7 for the GRN emission is determined by the balance between the Yb3+-mediated Er sensitization and the detrimental EBT-driven depletion of the Er3+ 2

H11/2 and 4S3/2 states. To conclude the overview on the UC PL properties of the YbxEr2 series, a comment is

needed about the significant intensity reduction for all the three emissions, observed in Figure 6 at the highest Yb doping levels. This evidence presumably originates from the concentration quenching effect, which takes place since the large density of Yb3+ ions can determine the furthering of energy transfer to quenching sites, leading to the loss of excitation energy.52,53 From this results, the possibility to achieve an ideal tradeoff between undesired concentration quenching effect and enhanced absorption cross section for maximizing the optical performance of the UC system is strictly depending on a suitable choice of the concentration of both the sensitizing Yb3+ ions (that seems to be of 10-15 at% for the investigated Bi2O3-based UCNPs) and of the Ln3+ activators. Now we turn our attention to the UC PL properties characterizing the Yb-Ho and Yb-Tm systems.


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In Figure 2, the spectrum of the Yb10Ho1 sample exhibits the typical features in the visible range usually observed for Yb3+, Ho3+ codoped UC phosphors, which originate from the following transitions: Ho3+ 5F4,5S2 → 5I8 (around 550 nm, labeled as D), Ho3+ 5F5 → 5I8 (around 660 nm, labeled as E) and Ho3+ 5F4,5S2 → 5I7 (around 760 nm, labeled as F). In comparison to the Yb3+, Er3+ codoped samples, the red emissions (E and F) are less dominant in intensity with respect to the green one (D), thus the resulting eye-visible color output shown by the Yb10Ho1 sample turns to yellow-orange, as can be appreciated in the reported picture. The diagram of Figure 3 evidences that the Yb3+-mediated two-step excitation process directly promotes a Ho3+ ion from the ground to the 5F4, 5S2 excited states (pivoting on the 5

I6 level). It is worth considering that this upward transition falls in an energy range (2.15-

2.30 eV) well below the threshold value to bridge the host bandgap (2.55 eV), as well as all the radiative relaxations that determine the detected UC PL emission spectrum. Moreover, in virtue of the outcomes of the Kubelka-Munk analysis, it is suggested that this condition would be maintained also at very low Yb doping level. Therefore, in terms of band intensity ratio and emission chromaticity, we exclude that the upconversion behaviour shown by the Yb3+-Ho3+ system may be influenced by possible host bandgap widening driven by Yb content increase. Concerning the Yb10Tm0.2 sample, the spectrum of Figure 2 deviates significantly to what is usually found in literature for the largest part of Yb3+-Tm3+ UC systems. In fact, Yb3+, Tm3+ codoped phosphors usually exhibit two main features in the visible range and one in the NIR region: an intense band around 475 nm linked to the Tm3+ 1G4 → 3H6 transition, that gives the typical blue color output, and a weaker one in the red (around 650)


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linked to the Tm3+ 1G4 → 3F4 (excited-to-excited state) transition, together with an emission around 800 nm due to the Tm3+ 3H4 → 3H6 transition. In the case of the Yb10Tm0.2 sample, none of the two bands in the visible is observed, leaving the place to a faint, secondary emission around 700 nm (labeled as G) ascribed to the Tm3+ 2F2,3 → 3H6 transition, while the 800 nm band (labeled as H) appears as extremely intense. The diagram of Figure 3 gives direct account of the peculiar UC PL response exhibited by the Yb3+, Tm3+ codoped sample. Basically, the Tm3+ 1G4 level is located so high in the energy scale that the direct relaxation to the ground state is energetically larger than the Bi2O3 energy gap, as ascertained by the Kubelka-Munk analysis. In principle, this condition could determine the occurrence of host-mediated photon re-absorption process as well as energy transfer from the 1G4 level triggered by simultaneous Bi2O3 band-to-band transition, which are both detrimental for the nanophosphor UC PL efficiency. If from one hand the re-absorption process can account for the quenching of all UC PL features in the blue range, linked to 1G4 level de-excitation to the ground state, on the other hand this should not affect those radiative transitions to other lower-lying excited states, like the 1G4 → 3F4 one, leading to the emission of photons less energetic than the host bandgap. Therefore, the lack in the spectrum of Figure 2 of all the emission bands involving relaxations from the 1G4 level calls into cause the activation of competitive, non-radiative de-excitation paths determined by an energy transfer mechanism to the Bi2O3 host. The picture included in Figure 2 shows that the resulting chromatic output for the Yb10Tm0.2 sample can not be associated to a specific color. This is due to the fact that the 800 nm emission is fully dominant, and thanks to the suppression of the typical Tm3+ emissions in the visible range, the Yb3+, Tm3+ codoped Bi2O3 system can be considered as


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a potential candidate as a single-band emitting, NIR-to-NIR UC nanophosphor operating in the first biological window, with complete suppression of the visible background noise. The above considerations fall into a general framework for the UC PL behaviour characterizing the investigated Yb3+-Ln3+ systems, which is conditioned by the fact that Bi2O3 is a narrow bandgap oxide. As stated above, band-to-band transitions across an energy gap estimated to be of around 2.3 eV for the undoped Bi2O3 host, can determine the depletion of those higher-lying Ln3+ energy levels populated through multi-step excitation mechanisms (i.e., based on three or more photon absorption under 980 nm light exposure), with consequent loss in efficiency for the related radiative relaxation processes. As expected, the outcome of the analysis of the excitation power dependence of the UC PL emissions is in line with this scenario. In fact, the slope values estimated for all the log-log plots in Figure S8 indicate that the observed emissions are basically originating from twophoton absorption processes. The optical efficiency performance of the three Yb3+, Ln3+ codoped samples embedding 10 at% of Yb was evaluated by means of the integrating sphere method.54 From this analysis, the resulting high absorption efficiency of 67±3% for all the three systems gives account of the effective capability of the material to capture the 980 nm irradiating photons. Such a large value was not unexpected on the basis of the pronounced feature originating from Yb3+ GSA absorption at 980 nm, which characterizes the reflectance spectra of Figure 4. Table 1 summarizes the quantum yield (QY) estimates obtained for the different samples, together with the data found in literature for some of the state-of-the-art, fluoride-based UCNPs containing Yb3+-Er3+ or Yb3+-Tm3+ pairs. The QY parameter is related to the


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fraction of the absorbed photons that is converted into photons emitted through UC PL mechanisms. We observe that all the systems object of this research exhibit remarkable QY values of about 2%, which are even larger than most of the phosphors reported in Table 1. Moreover, it is worth noticing that the reported measurements were obtained under moderate sample irradiance. These findings must be emphasized, because in general Ln3+ doped UCNPs hardly reach efficiency of the order of 1%, whereas values of several % can be achieved in the case of coreshell nanostructures. By considering the nature of the investigated samples, consisting in doped Bi2O3 NPs prepared by means of a quite simple synthesis procedure, the obtained results represent a promising starting point and future strategies can be developed to further enhance the UC efficiency.

Table 1. Absolute QY estimates for the Yb-Ln systems object of this research and for some stateof-the-art UC nanophosphors, measured by the integrating sphere method.


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Material

Size (nm)

QY (%)

Iex (W cm2)

Ref.

Bi2O3:Yb0.2,Er0.04 [Yb10Er2 sample] Bi2O3:Yb0.2,Ho0.02 [Yb10Ho1 sample] Bi2O3:Yb0.2,Tm0.004 [Yb10Tm0.2 sample] NaYF4:Yb0.2,Er0.02 NaYF4:Yb0.2,Er0.02 NaYF4:Yb0.2,Er0.02 NaYF4:Yb0.2,Er0.02@NaYF4 NaLuF4:Gd0.24,Yb0.2,Tm0.01 NaYF4:Yb0.25,Tm0.003@NaYF4 NaYF4:Yb0.25,Tm0.003 NaYF4:Yb0.25,Tm0.003@NaYF4 NaYF4:Yb0.8,Er0.02@CaF2 LiLuF4:Yb0.2Er0.01 LiLuF4:Yb0.2Er0.01@LiLuF4 LiLuF4:Yb0.2Er0.01@LiLuF4 LiLuF4:Yb0.2Tm0.005 LiLuF4:Yb0.2Tm0.005@LiLuF4 LiLuF4:Yb0.2Tm0.005@LiLuF4 NaGdF4:Yb0.22,Er0.025@NaYF4 NaYF4:Yb0.2,Er0.02@NaYF4

40-50 40-50 40-50 100 30 8-10 30

Insight into the Upconversion Luminescence of Highly Efficient

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