Continuous wave near-infrared phonon-assisted upconversion in single Nd3+-doped yttria nanoparticles

Author’s Accepted Manuscript Continuous wave near-infrared phonon-assisted upconversion in single Nd+3-doped yttria nanoparticles K.C. Camargo, R.R. Pereira, L.F. dos Santos, S.R. de Oliveira, R.R. Gonçalves, L. de S. Menezes www.elsevier.com/locate/jlumin

PII: DOI: Reference:

S0022-2313(17)30790-1 http://dx.doi.org/10.1016/j.jlumin.2017.08.031 LUMIN14974

To appear in: Journal of Luminescence Received date: 7 May 2017 Revised date: 11 August 2017 Accepted date: 16 August 2017 Cite this article as: K.C. Camargo, R.R. Pereira, L.F. dos Santos, S.R. de Oliveira, R.R. Gonçalves and L. de S. Menezes, Continuous wave near-infrared phonon-assisted upconversion in single Nd+3-doped yttria nanoparticles, Journal of Luminescence, http://dx.doi.org/10.1016/j.jlumin.2017.08.031 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Continuous wave near-infrared phonon-assisted upconversion in single Nd +3 -doped yttria nanoparticles K. C. CAMARGO1, R. R. PEREIRA2, L. F. DOS SANTOS2, S. R. DE OLIVEIRA2, R. R. GONÇALVES2, L. DE S. MENEZES1, * 1

2

Departamento de Física, Universidade Federal de Pernambuco, 50670-901 Recife-PE, Brazil. Departamento de Química, Faculdade de Filosofia, Ciências e Letras de Ribeirão Preto, Universidade de São Paulo, 14040-901 Ribeirão Preto-SP, Brazil. *Corresponding author. E-mail: [email protected]

Abstract: We report on Nd3+-doped yttrium oxide polycrystalline nanoparticles synthesis, their basic structural characterization and the observation of phonon-assisted upconversion in single nanoparticles with different Nd3+ doping levels. For upconversion studies, a frequency-locked CW Ti:Sapphire laser operating @ 820 nm, resonant with a Stark level of the Nd3+ ions 4F5/2 manifold, was used to excite single nanoparticles on a home-made confocal microscope. To the best of our knowledge, luminescence ranging from 630 to 760 nm was observed for the first time from single Nd3+-doped yttria nanoparticles under such excitation conditions. Upconversion studies on different single nanoparticles revealed a good doping homogeneity and the upconversion emissions were found to be one-photon phonon-assisted processes. Keywords: Luminescent materiais; Rare-earth doped materials; Single nanoparticle spectroscopy; Upconversion; Thermometry. 1. Introduction Rare-earth (RE) doped nanoparticles (NPs) showing upconversion (UC) phenomenon have attracted attention in the recent years due to their luminescent properties [1, 2] that enable the conversion of low-energy photons into high-energy ones via multiphoton processes or in some cases via phonon-assisted transitions [3-6], allowing the signal detection at wavelengths different from the excitation one and leading to high signal-to-noise ratios [4, 6]. Among RE ions, neodymium shows strongly absorbing energy levels and emissions in the in the so-called “first biological window”, which spans over the spectral range from ~650 nm to ~950 nm, which makes this ion a good candidate for applications in biological systems [716]. In the last decade, many efforts to prepare nanoparticles using bottom up and top down strategies have been done aiming fully dispersed single NPs. In particular, the use of single NPs may provide functionalities that bulky materials or even agglomerates of NPs cannot, like to access cell organelles in order to make imaging and/or thermometry with nanometric spatial resolution. Solid state properties and applications depend on the particle’s composition as well as size and morphology, especially concerning the nanoscale.

Accordingly, synthesis methodologies have been developed not only to succeed specific and unusual properties but also to allow interesting applications [17]. The controlled synthesis of NPs of different class of compounds, morphologies and size distribution (like dielectric, metallic, spherical, rodlike …) has led to developments in the field of materials science and their availability allowed for applications in photophysics [8], biomedicine [7-9] and nanothermometry [10-12], among other fields. Particularly interesting are NPs which can be used in biological systems as biomarkers and biothermometers [10, 13, 14]. Yttria (Y2O3) has an excellent chemical stability, relative low maximum phonon energy and high solubility for the rare-earth ions [18]. RE doped Y2O3 has been widely used in recent years for different applications, including optical displays, catalytic converters, permanent magnets, fluorescent lamps, field emission displays and cathode-ray tubes [19-21]. A huge number of synthesis methodologies of nano and microstructured Y2O3 particles have been reported, which show diverse parameters to control the particle’s structure, size and morphology [21-23]. In this work, we report on Nd3+-doped Y2O3 (Nd3+:Y2O3) polycrystalline NPs synthesis and perform structural and optical characterization studies. Besides this, by exciting single NPs with CW laser at 820 nm, we observed UC emissions in the spectral range from 630 to 760 nm, originated by the Nd3+ ions. They were determined as being result of a phonon-assisted UC process, which makes possible to apply this system in nanometric-scale thermometry involving single NPs [5, 10]. 2. Experimental Nd3+:Y2O3 particles were prepared using homogeneous precipitation method followed by thermal annealing [24, 25]. Neodymium-doped Y(OH)CO3.nH2O particles as precursor were firstly prepared via urea thermolysis by using Y(NO3)3.6H2O (99.8% purity, Sigma-Aldrich) and urea (99 – 100% purity, Cinética). An aqueous solution of Y(NO3)3 and another aqueous solution of urea were mixed at room temperature, so that in the resulting solution the final concentrations of these molecules were 0.01 and 2.00 mol.L-1 respectively. The Nd3+ was introduced as ethanolic neodymium chloride solution, which was prepared from the respective oxide (99% purity, Sigma-Aldrich,) by dissolution in aqueous hydrochloric acid solution, followed by careful drying at 80 °C with subsequent addition of anhydrous ethanol, to prepare the 0.10 mol.L-1 concentration stock solution. The Nd3+ ions concentration varied from 0.1 up to 3.0 mol% (samples Nd_01, Nd_05, Nd_10, Nd_20 and Nd_30, respectively) in relation to yttrium concentration. The final solution was placed in an isothermal bath and maintained at 80 °C for 2 hours in a closed flask. After a complete reaction, the precipitates were isolated by centrifugation, washed with distilled water and dried at 70 °C. The Nd3+:Y2O3 samples were obtained after thermal annealing of [Y(OH)CO3.nH2O] under air during 2 h at 900 °C. This temperature was reached starting from room temperature and

using a heating rate of 1°C min-1. This is important for eliminating carbonates and hydroxyls, minimizing residues in the final product. The X-ray diffraction (XRD) analysis were carried out using a Bruker D2-Phaser diffractometer with Cu-Kα radiation (λ = 1.5418 Å) in the range from 10° to 80° with 0.05° steps (0.5 s per step). The TEM measurements were conducted under a JEOL JEM-100CX II 100 kV microscope. The samples were prepared by drop-casting powders dispersion in ethanol on TEM carbon grids. The FTIR spectra were obtained on a Shimadzu IR Prestige-21 spectrometer in the 4000 - 400 cm-1 range with a 2 cm-1 resolution. KBr pellets mixed with the powder samples (KBr/powder ratio of 100:1) were employed. The diffuse reflectance spectra were recorded using a spectrophotometer (Minolta CM2600-D). When using a conventional optical microscope for e.g. investigating light emission from a single NP, fluorescence emitted by other NPs located in the perifocal regions may be detected, blurring the image formed by the light coming from the NP of interest. The confocal microscopy approach provides an improvement in both axial and lateral spatial resolutions due to the presence in the detection path of a pinhole at a position confocal to the NP investigated. This blocks light coming from NPs in perifocal regions, avoiding it to reach the photodetector. Confocal microscopy is a very useful technique, mainly due to the increased spatial resolution as compared to a normal microscope and the possibility of making slices along the optical axis, which may lead to constructing 3D images [26]. To perform single NP UC spectroscopy a homemade optical inverted microscope was employed, and the corresponding experimental setup is shown in Figure 1(a). This uses as excitation source a frequency-locked CW Ti:Sapphire laser (linewidth ~ 1 MHz) emitting at 820 nm that corresponds to the transition 4I9/2  4F5/2 , resonant to the strongest absorption peak within this excited state manifold of Nd3+ ions (as shown in Fig. 3b). A set of attenuators (neutral density filters) was used to control the excitation pump power. A lens for widefield microscopy (WL) was installed on a flip mount, so that the microscope can operate in widefield or confocal mode. A 50% beam splitter (BS) sends the excitation beam to the high-NA microscope objective (Olympus 60X Oil Objective, apochromat, coverslipcorrected, NA 1.42) which in the confocal mode focusses light on the sample, with a theoretical spot about 350 nm in diameter. In this configuration, the NPs fluorescence is collected by the same microscope objective, sent back towards the BS and the transmitted light goes through notch filter centered at 808 nm (Thorlabs NF808-34), which significantly blocks the excitation radiation. Imaging and signal recording were done in three different channels: either a simple CCD camera (Fujitsu-TCZ 984P) for coarse monitoring the sample and controlling the laser focusing on it, a spectrometer (SpectraPro SP-2500) for a recording UC spectra, or an avalanche photodiode (idQuantique, id100-50) for performing probe scanning confocal microscopy. NPs samples were prepared starting from a white powder. A diluted colloid was prepared by suspending 0.01 g of Y2O3:Nd3+ NPs in 1 mL of isopropyl alcohol. Sonication for 5 minutes was done right before spin-coating of 10 µL of the colloid

(622)

(440)

(400)

(b)

(211)

Intensity (arb. units)

(a)

(222)

on a glass coverslip (Menzel-Gläser #1) at 4800 rpm for 20 seconds, colloid being deposited after the spin coater reached 1000 rpm.

Nd_01

Nd_05

Nd_10

Nd_20

Nd_30

10

20

30

40

50

2 (°)

60

70

80

Fig. 1. (a) Scheme of the setup for performing UC spectroscopy. Pump: excitation source (CW Ti:Sapphire laser @ 820 nm); AS: attenuator set; WF: wide field lens; BS: beam splitter; OBJ: immersion oil objective; F: spectral filters; FM: flip mirror; L: plan convex lenses. PD: intensity reference photodiode; CCD: CCD camera; APD: Avalanche photodiode. (b) X-ray diffractograms of Y2O3 samples doped with 0.1 (Nd_01), 0.5 (Nd_05), 1.0 (Nd_10), 2.0 (Nd_20), and 3.0 (Nd_30) mol% Nd3+.

3. Results and discussions 3.1 Structural properties and optical absorption The XRD patterns of the polycrystalline NPs containing different Nd3+ concentrations are shown in Figure 1(b). The reflection peaks correspond to the pure Y2O3 body-centered cubic structure, which belongs to space group Ia3 in accordance to JCPDS card 01-074-0553, with parameter cell of a = b = c = 1.060 nm and unit cell volume of 1.1910 nm3. No changes/displacements in the peaks’ positions were observed increasing the Nd3+ concentration; moreover, no other crystalline phase was detected.

Sample

〈

〉

( 0.1 nm)

〈 〉 ( 0.1 nm)

UC emission lines (@ nm) 696

724

753

Nd_01 Nd_05

-

-

0.81

0.80

0.75

176

33

0.79

0.76

0.76

Nd_10

166

35

0.80

0.77

0.76

Nd_20

170

28

0.80

0.78

0.75

Nd_30

163

39

0.82

0.79

0.77

Table 1. Characteristics of Y2O3 samples doped with 0.1 (Nd_01), 0.5 (Nd_05), 1.0 (Nd_10), 2.0 (Nd_20), and 3.0 (Nd_30) mol% Nd3+.

In the cubic structure two symmetry sites are observed for the Y3+ ions, C2 and S6. Since the ionic radii of Y3+ (0.90 Å) and Nd3+ (0.99 Å) are close, the substitution of Y3+ by Nd3+ ions

in both C2 and S6 sites of the cubic structure is favored, without significant changes in the lattice parameters of the unit cell, even for the highest Nd3+ concentration (3.0 mol%). TEM images of the Nd3+-doped Y2O3 polycrystalline NPs and their respective histograms are presented in Figure 2. Spherical monodisperse NPs were observed exhibiting a narrow size distribution. According to Table 1, these particles show an average diameter of about 〉”), which indicates the formation of polycrystalline NPs with average 170 nm (column “〈 〉”). It is also important to stress that as the Nd3+ grain diameter about 30 nm (column “〈 content increases no significant change in their size was observed.

Fig. 2 – Transmission Electronic Microscopy (TEM) characterization of samples. TEM images and respective size distribution histograms of Y2O3 samples doped with (A) 0.5, (B) 1.0, (C) 2.0, and (D) 3.0 mol% Nd3+.

Vibrational features can be seen in Figure 3(a), which shows the FTIR spectra of Nd3+ doped Y2O3 NPs. Very low intensity peaks about 3500 - 3400 cm-1 and 1500 - 1400 cm-1 are assigned to the vibrational modes of the hydroxyl and carbonate groups, indicating only residual content, probably on the particles’ surface. The strong peaks at 561 cm-1, 465 cm-1 and 436 cm-1 are ascribed to stretching vibration of the Y–O bond [27, 28] in agreement to the proposed cubic structure. Difuse Reflectance (%)

95

Transmittance (%)

Nd_01

Nd_05

Nd_10

Nd_20

Nd_30

(a) 4000

3500

3000

2500

2000

1500 -1

Wavenumber (cm )

1000

90 4

2

80 75

F9/2

4

85

P1/2

G11/2

4

F11/2

G9/2

4

G7/2 4

F7/2

70 65

F3/2

4 4

4

G5/2

500

4

F5/2

(b)

60 400 450 500 550 600 650 700 750 800 850 900 950

Wavelength (nm)

Fig. 3 – Optical characterization of the samples. (a) FTIR spectra of Y2O3 samples doped with 0.1 (Nd_01), 0.5 (Nd_05), 1.0 (Nd_10), 2.0 (Nd_20), and 3.0 (Nd_30) mol% Nd3+. (b) Diffuse reflectance spectrum of

Y2O3:3%Nd3+ NPs. The labels in the figure denote the manifolds related to the excited states reached via transitions from the Nd3+ ground state 4I9/2.

Since it is difficult to perform absorption experiments in single NPs as small as these under investigation, the Nd3+:Y2O3 (3.0 mol%) sample was used for performing diffuse reflectance measurements. Results are presented in Figure 3(b) and reveal manifolds with partially resolved Stark levels, characteristic of crystalline samples, related to transitions from the Nd3+ ground state 4I9/2 to excited states. This result allows us to choose a convenient wavelength for exciting the NPs 3.2 Single nanoparticle optical spectroscopy In order to enhance the transversal resolution of our microscope, increasing its ability to detect single NPs, the samples were investigated using a home-made inverted confocal microscope, as detailed in Section 2. However, the use of a confocal microscope solely does not allow us to guarantee that we are observing single NPs since our transversal resolution is of the order of 350 nm while the investigated NPs are on average 170 nm in size. To mitigate this problem and to verify the characteristics of the samples we get employing the typical preparation protocol (NPs colloid sonication and spin coating procedures), we have performed scanning electron microscopy (SEM). Results for the Nd_01 sample can be seen in Figure 4(a). In various sample areas on the coverslip we get sparse distributions of single NPs all having similar sizes of about 170 nm, laying on the coverslip at distances at least 2 µm away from each other. This tells us that with a large probability the NPs we investigate with our confocal microscope are single NPs. In addition, the SEM images show that the NPs synthesis route also provides spherical NPs following a normal size distribution with average sizes described around 170 nm, see data for samples with higher Nd3+ concentrations shown in Table 1. Figure 4(b) shows a typical single NP investigated in our experiments. With the confidence that we are looking at single NPs, we move on with making optical spectroscopy with them. In order to locate an interesting single NP for performing UC spectroscopy, we first made a 15 µm  15 µm confocal scan sending the NP luminescence to the APD, resulting in the image shown in Figure 4(c). The excitation power was 13 mW at the sample. Scan control and data acquisition were done via a LabView code. For taking this image, we integrated the APD signal for 100 ms at each pixel. Figure 4(d) shows a selected NP in a 5 µm  5 µm confocal scan. A cross section shows a spot with a ~870 nm FWHM. This value deviates from the theoretical convoluted point spread function for this system, and we attribute this to the chromatic aberration of the objective, since we optimize the signal collection at ~700 nm while the laser emits at 820 nm. Thus, the excitation beam has not a minimum spot size at the sample, being larger than that theoretically predicted.

kcounts/s

180 160 140 120 100 80 60 40 20 0 0.0

FWHM= 870 nm

0.5

1.0

1.5

2.0

2.5

Position (m)

3.0

3.5

(d)

(c)

Fig. 4 - Scanning Electron Microscopy (SEM) and Scanning Confocal Optical Microscopy of typical samples already on a coverslip. (a) Large scale (12.0 µm  12.0 µm) SEM picture of a typical sample, after spin-coating a dilute solution of NP on a coverslip (scale bar: 2.0 µm). One can see that the process produces sparse samples containing single NPs. (b) Small scale scanning (1.0 µm  1.0 µm), where it is possible to choose a single NP located far away from any other NP (at least by 3.2 µm, about 10 the theoretical resolution of the optical system) to perform the measurements (scale bar: 0.2 µm). (c) 15.0 µm  15.0 µm confocal scan with the APD, where one sees 3 sparsely distributed single NPs indicated by circles (scale bar: 5.0 µm). (d) A 5.0 µm  5.0 µm confocal scan around the central particle in a) (scale bar: 1.0 µm). A cross section (thin line in the middle of the figure) shows an image with ~870 nm FWHM (see inset).

Intensity (kcounts/s)

Figure 5(a) shows the UC emission spectrum of a single NP at the red-NIR regions under CW excitation at 820 nm. All 4f-4f electronic transitions of Nd3+ ions show clearly resolved Stark levels. Similar luminescence features were observed in the emission spectra acquired from different single NPs of the same sample, i.e., same nominal Nd3+ concentration. The presence of well resolved sharp peaks agrees with the distribution of lanthanide ions substituting yttrium ions at the host. As briefly mentioned before and according to previous works [29], Nd3+ ions occupy two different symmetry sites when dispersed at the Y2O3 host, substituting yttrium ions: it can be found eight yttrium at S6 symmetry sites, which contain

10 9 8 7 6 5 4 3 2 1 0

(a) 4F

9/2

4I 9/2

2H

11/2

4F 2F

11/2

4I 9/2

(b)

4I 11/2

7/2

4I 9/2

620 640 660 680 700 720 740 760 780

Wavelength (nm)

an inversion center, and twenty-four ions at C2 low symmetry sites per unit cell. Fig. 5. (a) Nd3+:Y2O3 (1.0 mol%) single NP UC spectrum for CW excitation at 820 nm excitation (power on the sample: 13 mW). The Nd3+ electronic manifolds associated to the observed emission bands have been identified and their Stark levels are resolved. (b) Phonon-assisted UC process for generating radiation at 696

nm after resonant excitation by one photon at 820 nm. The energy difference of 2,554 cm-1 for promoting the ions to the 4F9/2 state is provided by the lattice via phonon annihilation.

In order to get information about the mechanism leading to each emission, it was investigated how the fluorescence intensity of different Stark levels and electronic manifolds (Isignal) changes with the excitation density (Ipump). A set of neutral density filters is used before the microscope beam splitter for varying the excitation power and for recording the sample’s emission spectrum. In luminescent processes, in general one has , where n is the number of photons from the excitation beam needed for generating a photon from the fluorescence. In the case of downconverted, spontaneous emission, a linear response is detected with n = 1, while for UC processes, for which the luminescence emitted by the sample has a smaller wavelength than that of the excitation photons, normally one gets a nonlinear process with n > 1. When representing the dependence of the UC signal intensity as a function of the excitation density in log-log graph, the slope of the resulting straight lines is n, the number of excitation photons participating in the process. Figure 6 shows the results for the strongest emissions within each of the three most intense manifolds for the samples with 0.1, 0.5 and 1.0 mol% doping levels (a similar behavior is observed for the other two samples). The three last columns of Table 1 summarizes the n values obtained for all studied samples and considered emissions. The results show an unusual, almost linear dependence of the UC emissions on the pump power. This behavior can be understood by taking into account phonon-assisted transitions [30, 31]. In the present case, the Nd3+ ions are resonantly excited by one pump photon to the 4F5/2 manifold and the lattice provides phonons for bridging the energy gap to a higherlying emitter level, say 4F9/2 (from which the emission around 696 nm originates). Figure 5(b) illustrates this process. The sublinear dependence of all investigated emission lines is curious. As discussed before, one should expect a slope of ~1.00 for all graphs shown in Figure 6. This kind of behavior happens not only in one photon, phonon-assisted UC processes, but also in multiphoton UC processes, resonant or not [22, 30-34]. This aspect has been carefully investigated in [22], in which the authors attribute the deviation of integer values for the slopes to the influence of lower-lying states close to the emitter level. In our case, these lower-lying states can be the Stark levels within the excitation manifold or even Stark levels of the lower-lying 4F3/2 manifold, see Figure 3(b). Experiments to corroborate this model in our system will be the subject of future investigation, either by tuning the excitation laser to a transition with fewer lower lying states and/or by making temperature-dependent studies [34], exploiting some different strategies [35]. These are a particularly hard experimental task when dealing with single NPs. One should observe the behavior of the emitted fluorescence intensity (for the 3 investigated lines) as a function of Nd3+ doping level. Examination of Figure 6 shows that the UC signals get weaker as the doping level increases, which is an indicative of fluorescence quenching mechanisms. In fact, assuming a homogeneous distribution of Nd3+ ions inside the

Log fluorescence signal (arbitrary units)

NP, for a 1.0 mol% doping level, one has an average distance of ~ 3 nm between ions, which means that dipole-dipole interactions can play a significant role in fluorescence processes in such NPs. 4.2 4.0 3.8 3.6 3.4 3.2 3.0 2.8 2.6 2.4 2.2 2.0 1.8 1.6

(a) -2.0

-1.5

-1.0

-0.5

0.0

3.6 3.4 3.2 3.0 2.8 2.6 2.4 2.2 2.0 1.8 1.6 1.4 1.2 1.0

(b) -2.0

-1.5

-1.0

-0.5

0.0

3.2 3.0 2.8 2.6 2.4 2.2 2.0 1.8 1.6 1.4 1.2 1.0 0.8 0.6

(c) -2.0

-1.5

-1.0

-0.5

0.0

Log excitation density (arbitrary units)

Fig. 6- Log-log graphs of fluorescence intensity dependence on the excitation density for samples a) Nd_01; b) Nd_05; and c) Nd_10 for the emissions peaking at 696 nm (solid black squares), 724 nm (solid red circles and 753 nm (solid blue triangles).

4. Conclusions Homogeneous, spherical and well dispersed single Nd3+:Y2O3 NPs were synthesized by using a homogenous precipitation method. A narrow size distribution with a 170 nm of mean value of the polycrystalline NPs and a grain diameter round 30 nm were observed independent on the Nd3+ content. Cubic Y2O3 crystalline phase was identified and any significant changes in the lattice parameters of the unit cell were observed as the doping level, even for the highest Nd3+ concentration (3.0 mol%), which indicates that the substitution of Y3+ by Nd3+ ions in C2 and S6 symmetry sites of the cubic structure is favored. UC spectroscopy on single Nd3+:Y2O3 NPs was reported for the first time to the best of our knowledge. A sublinear (slope less but close to 1.00) dependence of the UC lines’ intensities with the excitation power was measured for all investigated emission lines, and the same behavior was observed for samples with different neodymiun concentrations. The UC mechanism was identified as one photon phonon-assisted transitions, which thus can be exploited for thermometry using single NPs. This subject is presently under investigation in our lab. Acknowledgements The authors thank Conselho Nacional de Desenvolvimento Científico e Tecnológico CNPq, Coordenação de Aperfeiçoamento de Pessoal de Nível Superior - CAPES, Fundação de Amparo à Ciência e Tecnologia do Estado de Pernambuco – FACEPE, Fundação de Amparo à Pesquisa do Estado de São Paulo - FAPESP (Brazilian agencies) and National Photonics Institute - INFo for the financial support. The authors also acknowledge Prof. José W. R.

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Graphical abstract

Nd3+-doped Y2O3 nanoparticles (~160nm) were sinthesized and morphologically-structurally characterized. Confocal microscopy and upconversion spectroscopy with single nanoparticles performed in the first biological window.