Microwave Synthesis and Up-Conversion Properties of SHG-Active Î±

Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

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Microwave Synthesis and Up-Conversion Properties of SHG-Active α‑(La, Er)(IO3)3 Nanocrystals ́ y Riporto,‡ Yannick Mugnier,‡ Ronan Le Dantec,‡ Steṕ hanie Kodjikian,† Sylvain Regny,*,† Jeŕ em Seb́ astien Pairis,† Isabelle Gautier-Luneau,† and Geŕ aldine Dantelle*,† †

CNRS, Grenoble INP, Institut Néel, Université Grenoble Alpes, 38000 Grenoble, France SYMME, Université Savoie Mont Blanc, F-74000 Annecy, France

‡

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ABSTRACT: Pure α-La(IO3)3 and α-La0.85Er0.15(IO3)3 nanocrystals were synthesized by a microwave-assisted hydrothermal method leading to a reaction yield of 87 ± 4%. Electron microscopy and dynamic light scattering characterizations provide evidence for the formation of nanocrystals with an average size of 45 ± 10 nm for α-La(IO3)3 and 55 ± 10 nm for αLa0.85Er0.15(IO3)3. When dispersed in ethylene glycol, the nanocrystal suspensions exhibit second-harmonic generation under near-infrared excitations at 800 and 980 nm whereas additional photoluminescence by up-conversion is simultaneously observed in the case of α-La0.85Er0.15(IO3)3 nanocrystals. Quantitative assessments of the second-harmonic generation efficiency from second-harmonic scattering experiments at 1064 nm result in relatively high ⟨d⟩ coefficients measured at 8.2 ± 2.0 and 8.0 ± 2.0 pm V−1 for α-La(IO3)3 and αLa0.85Er0.15(IO3)3, respectively. The relative intensity between second-harmonic generation and photoluminescence is discussed following the excitation wavelength. markers with UV excitation (quantum dots,13 Eu3+-doped nanocrystals14 for instance), an NIR excitation allows significant minimization of tissue autofluorescence and in vivo observations with higher spatial resolution in depth. Therefore, UCNPs are emerging as a new class of fluorescent biolabels.15,16 UC efficiency in nanoparticles is linked to the crystal quality of the nanoparticles and to the lattice phonon energy of the host matrix17 because low lattice phonon energies are required to minimize nonradiative losses and thus to maximize the radiative UC emission.15,18 To this end, fluorides are extensively used as a host matrix, and among them, Yb3+/Er3+-doped NaYF4 nanoparticles have been reported to be the most efficient UC nanomaterials to produce green emission,19 and their use as biological probes is widely reported.20,21 Coupling SHG and UC processes within a single multifunctional nanoprobe could bring together the advantages of both mechanisms in terms of brightness and particle orientation dependence. Indeed, SHG and UC processes display different features that could be actually very complementary. For HNPs, the SHG signal strongly depends on the nanoparticle orientation and the intensity scales with the square of the nanocrystal volume, which makes it difficult to detect HNPs smaller than ∼20 nm.22,23 On the contrary, UC processes scale with the nanocrystal volume, as they are directly linked to the number of rare-earth doping ions,

1. INTRODUCTION Optically active nanocrystals are widely studied as they cover a large range of applications spanning from nanoprobes for bioimaging to nanoscale optoelectronics. The nonresonant nature of second-harmonic generation (SHG) and the resonant character of photoluminescence (PL) through upconversion (UC) 1 have been exploited independently according to specific requirements. For instance, in terms of increased selectivity and deep-imaging issues, an excitation wavelength in the biological transparency windows of the nearinfrared (NIR) region is of high interest when up-converting nanoparticles and nonlinear optical nanocrystals (also called harmonic nanoparticles, HNPs) are employed as exogenous optical biomarkers.2−4 Regarding HNPs, they display not only SHG signals but also higher harmonics like third- and even fourth-harmonic generation as recently demonstrated in more fundamental studies.5 Among SHG probes, organic nanocrystals show a strong absorption6,7 with a residual two-photon absorption whereas inorganic ones such as LiNbO3, KNbO3, KTP, and BaTiO3 produce more stable harmonic signals.8,9 Contrary to fluorescent markers10 and to some extent organic NPs, the absence of bleaching and blinking effects is a feature of inorganic HNPs. Regarding UC, rare-earth-doped up-converting nanoparticles (UCNPs) are commonly used to convert NIR radiation into visible light thanks to a nonlinear process where two or more low-energy photons are absorbed and converted into one higher-energy photon.11,12 Contrary to traditional fluorescent © XXXX American Chemical Society

Received: November 16, 2018

A

DOI: 10.1021/acs.inorgchem.8b03208 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

bar, as shown in Figure S1b. Note that the internal pressure is autogenous and cannot be independently regulated. The reaction temperature T is recorded along the synthesis through the quartz vessel via an infrared temperature sensor. After the thermal treatment, a white powder in a yellowish supernatant is filtered (round filter, type 14A, cellulose, membrane diameter 125 mm) and dried at room temperature for 1 day. The typical reaction yield is 87 ± 4%. The synthesis of Er3+-containing α-La(IO3)3 nanocrystals was conducted likewise by using stoichiometric amounts of precursors and erbium(III) chloride hexahydrate (ErCl3·6H2O, Sigma-Aldrich, 99.9%) as the Er3+ source. Different solid solutions (α-La1−xErx(IO3)3 with x = 0.05, 0.10, and 0.15) were synthesized. In this paper, as a proof of concept for the simultaneous observation of SHG and UC signals, only the properties of α-La0.85Er0.15(IO3)3 nanocrystals are discussed. 2.2. Structural and Optical Characterizations. Phase purity from X-ray powder diffraction (XRD) was assessed under ambient conditions using a Siemens D8 Advance diffractometer (Cu Kα1 radiation, 40 mA, 40 kV) in the 10−110° 2θ range with a 0.01° step size and an acquisition time of 6 s/step. Le Bail refinements achieved with the Fullprof software suite allowed derivation of the unit cell parameters and the average crystallite size.31 Dynamic light scattering (DLS) measurements were performed with a Cordouan particle size analyzer (VASCO Kin). Prior to DLS analysis, nanocrystals were dispersed at 1 mg/mL in either water or ethylene glycol (EG) and sonicated for two periods of 20 min (with a 10 min break) within an ultrasound bath at 50 °C. Zeta potential measurements were conducted on the aqueous solutions, using a Cordouan zeta potential analyzer (WALLIS). Scanning electron microscopy (FEG-SEM, Zeiss Ultra Plus, operating at 3 kV) was used to image nanocrystals from spin-coated (acceleration, 3000 rpm for 10 s; speed, 5000 rpm during 180 s) droplets of the 1 mg/mL EG suspensions deposited onto previously cleaned (acetone) Si substrates. Due to the weak stability of lanthanum iodate nanocrystals under the electron beam, transmission electron microscopy (TEM, Philips CM300, and 300 kV) experiments were carried out at liquid nitrogen temperature after drying nanocrystals previously dispersed in ethanol or EG onto carbon grids. The room temperature photoluminescence emission spectrum of α-La0.85Er0.15(IO3)3 powder under 380 nm excitation was recorded from a Safas Xenius spectrofluorometer. The UC and nonlinear optical properties were investigated from EG suspensions under different NIR excitation wavelengths (800, 980, and 1064 nm) by using a hyper-Rayleigh scattering (HRS) configuration as previously reported32 but with a tunable (700−1300 nm) vertically polarized femtosecond laser (Insight X3 Spectra-Physics, 80 MHz, 100−120 fs) as the excitation source. The incident beam was focused by a 10 cm focal length lens into a fused silica cuvette containing the dispersions. UC and second-harmonic scattering (SHS) signals were orthogonally collected by a 5 cm focal length lens and filtered by a short pass filter to remove the excitation wavelength before being focused onto the slit of a spectrometer (Andor Shamrock 193) coupled to a CCD camera (Andor iDus 401 BVF). A Dove prism was also used to vertically rotate the image focal point along the spectrometer entrance slit. To adjust the incident power, a half-wave plate associated with a polarizer cube was used to increase the mean power from 250 to 800 mW. The power-law dependences of SHS signals were then obtained from the corresponding 0.5−1.6 GW cm−2 peak intensity range at the focus of the 10 cm focusing lens. Regarding UC, as intermediate states involve long lifetimes (in the microsecond scale), the average excitation intensities were considered. Thus, the power-law dependences of UC PL signals were plotted for the average excitation intensity in the 4.5− 13 kW cm−2 range. Note that no analyzer was used in front of the detector and that prior to SHS experiments EG suspensions of αLa(IO3)3 or α-La0.85Er0.15(IO3)3 nanocrystals were sonicated for 10 min in an ultrasound bath at room temperature. SHS efficiencies of αLa(IO3)3 and α-La0.85Er0.15(IO3)3 nanocrystals were quantified for a 1064 nm excitation, using different nanocrystal concentrations and a well-characterized reference (para-nitroaniline, pNA, dissolved in methanol).32 To avoid any bias due to agglomerates, SHS

allowing for the detection of smaller UCNPs as shown by Yi and Chow on a 8 nm NaYF4:Yb,Er(Tm) core,24 especially if a protective shell is added, preventing nonradiative deexcitations induced by surface traps.15 In addition, the UC signal is known to remain constant whatever the nanoparticle orientation is. Combination of SHG and UC processes within a single multifunctional nanocrystal has thus been the motivation of this work. So far, only a few studies report the elaboration of nanocrystals emitting both SHG and PL. Eu3+-doped BaTiO3 nanocrystals were produced by Rabuffetti et al., and the PL emission was attributed to the RE ion; however, the SHG properties were not specifically investigated.25 Among other nonlinear optical nanosources, core−shell nanoparticles made of KTP cores encapsulated by a LaPO4:Eu shell were developed by Mayer et al. through a two-step process. In this case, the dual emission combining the red-induced luminescence of Eu3+ and the SHG from the KTP core after shifting the excitation at 990 nm was well-detailed.26 Here, we report the preparation of Er3+-containing α-La(IO3)3 nanocrystals with the main goal of synthesizing single-phase nanocrystals with SHG and up-conversion properties from a unique excitation in the first biological transparency window (600−1000 nm). According to previous works on micron-sized lanthanum iodates, α-La(IO3)3 is known for its good SHG efficiency (comparable to that of LiIO3 at the micrometer scale 27,28) and for being a host matrix for different lanthanides.29 The choice of Er3+ ions lies on the known UC processes under excitation at 800 and 980 nm.30 In this paper, a microwave-assisted hydrothermal route is presented to prepare phase-pure α-La(IO3)3 and α-(La, Er)(IO3)3 nanocrystals. The nanocrystal morphology was investigated by electron microscopy, and the size distribution was also studied in solution after dispersion of the raw powders in water or in ethylene glycol. SHG efficiency of stable nanocrystal suspensions in ethylene glycol was quantitatively assessed from second-harmonic scattering (SHS) experiments. In the case of α-(La, Er)(IO3)3 nanocrystal suspensions, the additional UC-PL emission was investigated, and as expected, the simultaneous SHG/PL scattering could be observed from different excitation wavelengths.

2. EXPERIMENTAL DETAILS 2.1. α-La(IO3)3 and α-(La, Er)(IO3)3 Solid Solution Synthesis. After variation of the La3+:IO3− molar ratio from 1:3 to 1:20 and the synthesis parameters (temperature, pressure, duration), the typical synthesis procedure to synthesize α-La(IO3)3 was optimized as follows: precursors of lanthanum(III) chloride hexahydrate (LaCl3· 6H2O, Strem Chemicals, 99.9%) and iodic acid (HIO3, SigmaAldrich, 99.5%) were used without further purification. LaCl3·6H2O (176.7 mg, 5 × 10−4 mol) was first dissolved in 10 mL of deionized water under weak stirring, and the solution was then poured into an 80 mL quartz reactor. Iodic acid, HIO3 (263.9 mg, 1.5 × 10−3 mol), was dissolved into 10 mL of deionized water and then mixed with the lanthanum salt solution, leading to the formation of a white amorphous precipitate. After air-tight closure of the quartz vessel, a microwave (MW) heat-treatment (Anton Paar, Multiwave PRO SOLV 50 Hz) was performed with a MW power adjusted in the 0− 900 W range. The typical program for the synthesis of pure α-La(IO3) consists of a sequence of three power dwells (3 min at 800 W, 5 min at 850 W, and finally 2 min 30 s at 600 W, as illustrated in Figure S1a where the power is altered to ensure secure experimentation) during which the internal pressure P is continuously monitored and kept below the upper limit (P < 80 bar) by adjusting the MW power. The maximum temperature reaches 242 °C and the maximum pressure 79 B


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Inorganic Chemistry experiments were conducted after a 40 min sedimentation time, and the nanocrystal mass concentrations were measured after drying of the supernatant.

account the instrumental broadening, the coherence length is estimated to be 84 ± 5 nm. The Le Bail refinement for α-La0.85Er0.15(IO3)3 leads to a smaller unit cell (a = 12.5095 Å, b = 7.0778 Å, c = 27.7639 Å, β = 101.985 Å, and V = 2404.6 Å3) which is consistent with the substitution of La3+ by Er3+ ions. Their respective ionic radii in a 9-fold coordination indeed decrease from 121.6 to 106.2 pm. The coherence length, derived from the Le Bail fitting procedure, is 57 ± 5 nm. The observed variation of coherence length between the two samples can be explained by the introduction of a doping ion into the α-La(IO3)3 structure. Doping ions indeed act as an impurity center which can promote the nucleation from a seed resulting in the so-called “heterogeneous nucleation” process. With such nucleation being thermodynamically favored, numerous nuclei are formed leading to smaller nanoparticles. A similar effect was reported in Er3+- and Yb3+-doped oxyfluoride glass-ceramics where the nature of the rare-earth ion had an impact on the final crystallite size.35,36 3.3. Morphology of α-La(IO3)3 and α-La0.85Er0.15(IO3)3 Nanocrystals and Colloidal Stability. α-La(IO3)3 and αLa0.85Er0.15(IO3)3 powders dispersed in water with a concentration of 1 mg/mL were deposited onto Si substrates and characterized by SEM (Figure S2). For each sample, most nanocrystals have a size between 20 and 50 nm, but a few larger particles are also observed. It is worth noting that the particles tend to form agglomerates, which exist in solution or could be the result of the deposition process. At this stage, no indication allows us to decipher between these two hypotheses. Regarding DLS measurements, dispersion of α-La(IO3)3 and α-La0.85Er0.15(IO3)3 nanocrystals in water results in the formation of agglomerates as indicated by the large average size at about 400 nm (Figure 2, line). It is consistent with the measured zeta potential (ζ ∼ 0 mV at pH = 7). Because colloidal suspensions of individual nanocrystals are necessary for the subsequent optical characterizations, ethylene glycol (EG), a more viscous solvent, has been chosen for the nanocrystal dispersion. The DLS size distribution by number is then centered at 50 ± 20 nm for both nanocrystal suspensions in EG (Figure 2, dashed pattern). After spin-coating droplets of both EG suspensions of αLa(IO3)3 and α-La0.85Er0.15(IO3)3 nanocrystals onto Si substrates, SEM images were recorded. They exhibit spherical nanocrystals with 95% of the measured diameters below 100 nm. Histograms by number included as insets in Figure 3 are consistent with DLS measurements. TEM images at liquid nitrogen temperature of α-La0.85Er0.15(IO3)3 only exhibit nanocrystals with a size inferior to 100 nm: most of them have a typical size in the 20−40 nm range (Figure 4a), but a few larger particles at around 60 nm can also be observed (Figure 4b). The electron diffraction pattern (Figure 4a, inset) made of numerous bright spots indicates a high degree of crystallinity. As a summary, the nanocrystal sizes and coherence lengths are gathered in Table 1. It first stands out that the nanocrystal size given by the analysis of electron microscopy images (SEM and TEM) is consistent with the hydrodynamic diameter measured by DLS in EG, knowing that DLS measurements slightly overestimate the particle size due to the presence of a surface diffuse layer. Note also that the DLS size distributions by intensity (Figure S3) point out the presence of a few larger particles (up to ∼120 nm). This is in agreement with the observation of a few large particles by electron microscopy

3. RESULTS AND DISCUSSION 3.1. Lanthanum Iodates by MW-Assisted Hydrothermal Method. Lanthanum iodates are known to exhibit different polymorphs according to the synthesis methods (hydrothermal reaction, evaporation).28 La(IO3)3 materials were first synthesized in 2005 by Ok et al., following a hydrothermal method where the precursor solution was placed in a Teflon-lined autoclave and heated to 220 °C for 4 days with a large excess of iodic acid (La3+:IO3− molar ratio of 1:14).27 Taouti et al. also obtained, under hydrothermal conditions or slow evaporation of aqueous solutions, different lanthanum iodates after variation of the La3+:IO3− molar ratio between 1:4 and 1:10. Here, we developed an MW-assisted hydrothermal method, allowing a significant reduction of the reaction time and nucleation kinetics, which is favorable for the formation of nanosized crystals.33,34 The synthesis conditions (T > 240 °C, P > 70 bar, and La3+:IO3− = 1:3) were optimized for the formation of the α-La(IO3)3 phase. 3.2. Structural Studies of α-La(IO3)3 and α-La0.85Er0.15(IO3)3. The powder XRD diagrams of α-La(IO3)3 and αLa0.85Er0.15(IO3)3 nanocrystals, prepared following the power program reported in Figure S1a, are shown in Figure 1

Figure 1. Powder XRD diagrams of α-La(IO3)3 and α-La0.85Er0.15(IO3)3 nanocrystals with their corresponding Le Bail fit.

together with their Le Bail refinements in the monoclinic space group Cc. The absence of crystallized impurity can be noticed in both cases. For α-La(IO3)3, the cell parameters derived from the Le Bail refinement, a = 12.5454 Å, b = 7.0968 Å, c = 27.8304 Å, β = 102.044 Å, and V = 2423.3 Å3, are found in good agreement with the results (a = 12.526 Å, b = 7.0939 Å, c = 27.823 Å, β = 101.975 Å, and V = 2418.4 Å3) already reported by Ok et al. from single-crystal analysis.27 Taking into C


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Figure 2. Size distribution by number of (a) α-La(IO3)3 and (b) αLa0.85Er0.15(IO3)3 nanocrystals dispersed in EG (dashed) and water (line). Figure 3. SEM images of (a) α-La(IO3)3 and (b) α-La0.85Er0.15(IO3)3 nanocrystals spin-coated onto Si substrates and corresponding histograms by number. (Inset) Photographs taken 15 min after sonication of the α-La(IO3)3 and α-La0.85Er0.15(IO3)3 EG suspensions at 1 mg/mL and histograms by number for both α-La(IO3)3 and αLa0.85Er0.15(IO3)3.

(Figure 4b). Interestingly, the coherence length obtained from the Le Bail refinement is close to the nanocrystal size, thus indicating that nanoparticles are single crystals. A unique nonlinear dipole moment per nanocrystal can then be considered in the estimation of the SHG efficiency. The same assumption is also supported for α-La(IO3)3 nanocrystals even if the experimental coherence length is slightly larger than the average particle size. This is an artifact due to the presence of a few larger particles as illustrated in Figures S2 and S3. As-obtained suspensions in EG are stable for at least 40 min and can be used to quantitatively assess the SHG properties of α-La(IO3)3 and α-La0.85Er0.15(IO3)3 nanocrystals. Note that their colloidal stability has been measured over time (Figure S4) providing evidence that, below 40 min, the average nanocrystal size does not change drastically. However, as agglomeration occurs at longer aging times (above 1 h), further stabilization through surface modification would be required for bioimaging applications. 3.4. SHG Properties of α-La(IO3)3 and α-La0.85Er0.15(IO3)3. SHG properties were determined after a 40 min sedimentation time from ensemble measurements as already described elsewhere.32,37 A hyper-Rayleigh scattering (HRS) setup with different excitation wavelengths (800, 980, and 1064 nm) was used to probe SHS signals from the α-La(IO3)3

and α-La0.85Er0.15(IO3)3 nanocrystal suspensions in EG. Both suspensions only present a pure SHS signal (I2ω) centered at 532 nm under a 1064 nm excitation with a quadratic dependence as a function of the excitation intensity Iω (Figure 5a,b). Note that no additional PL coming from defect states was observed, unlike in KTP where Ti4+ ions are reduced into Ti3+, creating absorbing centers responsible for unwanted PL.38 In contrast, La3+ ions have a stable oxidation state and thus are unlikely to undergo any reduction process. Linear variation of the SHS signal with the nanocrystal concentration NNC is also reported in Figure 5c,d after normalization with the initial concentration N0 of the mother suspension (before the subsequent dilutions in EG). The scattered signal at 2ω can indeed be written as I2ω=G2ω NNC 2 (2) 2 2 TNC ⟨(βNC ) ⟩Iω where G2ω comprises the experimental collection efficiency and other constant factors. TNC = D

(

3ns2 2ns2

2 + nNC

) is an internal field factor calculated from DOI: 10.1021/acs.inorgchem.8b03208 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

field factor). This value is just above the ones for LiIO3 (no = 1.8559 and ne = 1.7160, λ = 1.1 μm40). ⟨β(2) NC⟩ is the volumedependent hyperpolarizability given by (2) (2) 2 ⟨βNC ⟩ = ⟨(βNC ) ⟩ = ⟨d⟩VNC with ⟨d⟩ the orientationaveraged nonlinear coefficient and VNC the nanocrystal volume derived from the DLS measurements after a 40 min sedimentation time. The external reference method with a series of paranitroaniline (pNA) solutions of maximum concentration 0.25 M in methanol was used to calibrate the optical setup and remove the experimental factor G2ω. The second-harmonic intensities resulting from HRS and SHS measurements were plotted versus the pNA and nanocrystal concentrations, respectively. From the slopes αpNA (Figure S5) and αNC (Figure 5c,d) estimated after consideration of the number density of pNA molecules and nanocrystals, the nanoparticle hyperpolarizability can be calculated from

βNC

Table 1. Average Sizes of α-La(IO3)3 and αLa0.85Er0.15(IO3)3 Nanocrystals Determined from Several Characterization Techniquesa

SEM TEM DLS in number (EG) DLS in intensity (EG) XRD

α-La(IO3)3 mean size (nm)

α-La0.85Er0.15(IO3)3 mean size (nm)

50 ± 20 30 ± 10 45 ± 20

50 ± 20 30 ± 10 55 ± 20

70 ± 20

65 ± 20

84 ± 5

57 ± 5

2 αNCsFpNA 2 αpNATNCs

βpNA

with βpNA

=

6 35

β33 and β33

= 25.9 × 10−30 esu for a vertically polarized input beam and the absence of analyzer in the detection path.41 Note that because of the observed size distributions for both nanocrystals, the mean diameter given by DLS measurements is considered for the calculation of the nonlinear ⟨d⟩ coefficient as reported in Table 2 for α-La(IO3)3 and α-La0.85Er0.15(IO3)3. The final uncertainty at ±25% for the experimental averaged coefficient measured at ∼8 pm V−1 for both materials includes all the possible contributions arising from the determination of the nanocrystal size and concentration, and from the average refractive index that was finally assumed to be in the 1.9−2.2 range. In a comparison to literature values, a ⟨d⟩ value at 15 pm V−1 was calculated by Ok et al. whereas powder SHG experiments from the Kurtz and Perry technique applied with micron-sized α-La(IO3)3 powders yielded a response similar to that of α-LiIO3.27 More recently, the SHG efficiency of αLa(IO3)3 was also qualitatively addressed28 and again found to be close to that of α-LiIO3 after application of the same grinding procedure for each material. In the case of α-LiIO3, the two nonzero nonlinear coefficients have been determined to be d31 = 7.11 pm V−1 and d33 = 7.02 pm V−1 from singlecrystal measurements.42 Taking into account the crystal class of lithium iodate (6) and space group P63,43 the orientationaveraged coefficient that would be obtained from SHS experiments can then be calculated at 8.1 pm V−1 which is completely in agreement with our experimental values for αLa(IO3)3 and α-La0.85Er0.15(IO3)3. Finally, among other HNP materials of interest, the nonresonant SH efficiency of KTP, BaTiO3, LiNbO3, and KNbO3 nanocrystals at 1064 nm has been found (with the same HRS setup9,32) to be on the same order of magnitude, thus confirming the interesting NLO properties of lanthanum iodate nanocrystals. 3.5. Simultaneous Emission of SHG and UC from αLa0.85Er0.15(IO3)3. Colloidal suspensions of α-La(IO3)3 nanocrystals only exhibit a SHS signal at 490 nm under a 980 nm excitation (Figure 6a, blue dashed line). When doped with Er3+, nanocrystal suspensions scatter several signals including SHS and the typical PL of Er3+ ions at 525, 550, and 660 nm (Figure 6a, orange solid line), as also observed in the direct PL spectrum of α-La0.85Er0.15(IO3)3 powder under 380 nm excitation (Figure S6). The quadratic character of the SHS

Figure 4. (a, b) Low-temperature TEM images of α-La0.85Er0.15(IO3)3 nanocrystals deposited onto carbon grid. (Inset) Typical diffraction pattern obtained at 300 kV.

method

=

a

Note that XRD gives the coherence length of the crystalline domains and not the average size.

the solvent and averaged nanocrystal refractive indices that we denote as ns and nNC, respectively. For ethylene glycol, a numerical value of ns = 1.42 was considered throughout this work.39 For the lanthanum iodates, the absence of literature values led us to consider a mean refractive index nNC at 1.9 (for the main calculations in the DLS size measurements and in the E


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Figure 5. (a, b) Power dependence of I2ω for α-La(IO3)3 and α-La0.85Er0.15(IO3)3 nanocrystals dispersed in EG showing a pure SHG signal (inset) under a 1064 nm excitation. (c, d) Intensity of the SHS signal centered at 532 nm as a function of the nanocrystal concentration NNC. Note that the x-axis scale is normalized by the initial concentration N0 of the mother suspension with N0 = 2.30 × 1012 cm−3 and N0 = 1.26 × 1012 cm−3 for the pure and doped sample, respectively, and that the initial mass concentration was 0.6 mg/mL for both suspensions.

Table 2. Effective Hyperpolarizability ⟨βNC⟩ and Orientation-Averaged Nonlinear Optical Coefficient ⟨d⟩ of α-La(IO3)3 and α-La0.85Er0.15(IO3)3 Nanocrystalsa nanomaterial

mean size [nm]

⟨βNC⟩ [10−24 esu]

⟨d⟩ [pm V−1]

α-La(IO3)3 α-La0.85Er0.15(IO3)3

45 55

0.94 ± 0.1 1.67 ± 0.3

8.2 ± 2 8.0 ± 2

populated through the nonradiative de-excitations of the S3/2 level (dashed lines in Figure 7), itself responsible for the green emission. A low GRR indicates highly probable nonradiative de-excitations between the 3000 cm−1 energy difference of the 4S3/2 to 4F9/2 levels and, to some extent, from the 4I11/2 to 4I13/2 levels also situated at ∼3000 cm−1. Consequently, the lifetime of the 4I11/2 level, an intermediate level for up-conversion under a 980 nm excitation, is shortened, leading to weak up-conversion emission. Thus, it is considered that materials with a low GRR present a weak upconversion efficiency.48 For α-La0.85Er0.15(IO3)3, the GRR is 4 at an average excitation intensity of 4.9 kW cm−2. However, as the GRR depends on the excitation intensity,49 a direct comparison between experimental values is only relevant at a similar excitation intensity. Among well-known and good upconverters, annealed YVO4:Yb,Er nanoparticles present a GRR of 6.3 at 2 kW cm−2.17 The relatively high GRR value here measured at 4 for α-La0.85Er0.15(IO3)3 is both consistent with an efficient up-conversion process and the low-energy phonon of the iodate host (150−800 cm−1 for LiIO350). Finally, α-La(IO3)3 and α-La0.85Er0.15(IO3)3 nanocrystal suspensions were also excited with a 800 nm wavelength 4

a

Note that for the point group m of the space group Cc, the orientation-averaged coefficient is given according to the individual tensor components by ⟨d2⟩ = (6/35)(d112 + d332) + (38/105)(d262 + d312 + d322 + d352) + (16/105)(d11·d26 + d11·d35 + d26·d35 + d31·d32 + d31·d33 + d32·d33) for α-La(IO3)3.

signal is confirmed by its evolution as a function of the excitation intensity (Figure 6b, ■). The PL intensity also varies in a quadratic manner with the excitation power (Figure 6b, ▽ and ○), evidencing a two-photon process for SHG and UC.30,44,45 Regarding the UC efficiency, an indirect though common way to investigate it is to measure the green-to-red ratio (GRR) corresponding to the intensity ratio between the green and red emissions under a 980 nm excitation.46,47 Indeed, the 4 F9/2 level of Er3+, responsible for the red emission, is F


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Figure 6. (a) Scattering spectra of α-La(IO3)3 (dashed blue line) and α-La0.85Er0.15(IO3)3 (orange line) nanocrystal suspensions under a 980 nm excitation. Both spectra have been stacked for the sake of clarity and normalized to the amplitude of the SHS signal. (b) Evolution of the SHS intensity as a function of the peak excitation intensity and (c) PL intensities as a function of the average excitation intensity, for α-La0.85Er0.15(IO3)3. In parts b and c, the linear fits with a slope fixed to 2 are also presented.

corresponding to the electronic transition from 4I15/2 to 4I9/2. The different quadratic laws were established for all the scattered signals (Figure S7), proving the two-photon process for the SHS (400 nm) and UC (525 and 546 nm) signals. After normalization to the amplitude of the SHS signal, comparison between the scattered spectra of α-La0.85Er0.15(IO3)3 nanocrystals is reported in Figure 8 when excited at 800 nm (dashed line) and 980 nm (solid line). For iodate materials, SHG properties have been attributed to the lone pair cation, I5+,51 so that coupling between the SHG and UC processes is unlikely to occur. In addition, because of the nonresonant character of SHG at the excitation and SH frequencies for both excitation wavelengths, efficiency of the UC process can be compared and is found to be higher at 800 nm. This is likely to be due to a better absorption of Er3+ at 800 nm than at 980 nm or a more efficient UC process upon 800 nm excitation. Further experiments, involving samples

codoped with Er3+ and Yb3+, a couple which is well-known for up-conversion,52 would be necessary to discriminate between these two hypotheses.

4. CONCLUSION α-La(IO3)3 and α-La0.85Er0.15(IO3)3 nanocrystals have been successfully synthesized by a microwave-assisted hydrothermal method. This scalable approach leading to 150 mg of powder per batch in less than 30 min leads to nanocrystals with a 50 nm average size as measured by DLS and electron microscopy. SHS experiments performed for the first time on iodate nanocrystal suspensions demonstrate the attractive SH efficiency of α-La(IO3)3 and α-La0.85Er0.15(IO3)3 with an orientation-averaged ⟨d⟩ coefficient of ∼8 pm V−1 which is comparable to the state-of-the art HNP materials. Interestingly, Er-containing nanocrystals provide evidence of a simultaneous SH/PL emission under an excitation wavelength at 800 and G


Article

Inorganic Chemistry

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Microwave program details and other characterization data (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Yannick Mugnier: 0000-0002-1923-6553 Géraldine Dantelle: 0000-0001-5499-8070 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Authors from the SYMME laboratory gratefully acknowledge the financial support of the French-Switzerland Interreg V A program (project NANOFIMT), and of the 2015−2020 French Contrat Plan É tat Région (projet E-TIME).

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Figure 7. Energy levels of the UC mechanism in α-La0.85Er0.15(IO3)3 under a 980 nm excitation. Arrows correspond to radiative emissions, dashed lines to nonradiative de-excitations, and the dotted line to energy transfer between neighboring Er3+ ions.

Figure 8. Comparison of the scattered intensity from α-La0.85Er0.15(IO3)3 nanocrystal suspensions after normalization to the amplitude of the SHS signal for an excitation wavelength at 800 and 980 nm.

980 nm. These novel multifunctional biphotonic probes combine the resonant and nonresonant excitation conditions of the UC and SHG processes with an excitation in the first NIR transparency window, thus paving the way to new bioimaging applications. Among prospects, reduction of the size polydispersity and a codoping with Yb3+ and Er3+ ions are in progress to further reduce the uncertainty of the ⟨d⟩ coefficient and to increase the UC efficiency.

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