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Article

Bright Green Frequency Upconversion in Catechin Based Yb /Er Codoped LaVO Nanorods Upon 980 nm Excitation 3+

3+

4

Vairapperumal Tamilmani, Astha Kumari, Vineet Kumar Rai, Balachandran Unni Nair, and Kalarical Janardhanan Sreeram J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b08510 • Publication Date (Web): 09 Jan 2017 Downloaded from http://pubs.acs.org on January 20, 2017

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The Journal of Physical Chemistry

Bright Green Frequency Upconversion In Catechin Based Yb3+/Er3+ Codoped LaVO4 Nanorods Upon 980 nm Excitation Vairapperumal Tamilmani a, Astha Kumarib, Vineet Kumar Raib, Balachandran Unni Nair a, Kalarical Janardhanan Sreeram a* a

Chemical Laboratory, CSIR-Central Leather Research Institute, Chennai-600020, Tamilnadu,

India. b

Laser and Spectroscopy Laboratory, Department of Applied Physics, Indian Institute of

Technology (Indian School of Mines), Dhanbad-826004, Jharkhand, India.

ABSTRACT. A series of Yb3+-Er3+ codoped LaVO4 phosphors using agent and phase director have been prepared by the low temperature hydrothermal synthesis technique. The sample exists in two different crystalline phases i.e. monoclinic and tetragonal, depending on the concentration of dopants. Structural, optical and thermal characterizations have been done by using the X-ray diffraction, EDX, FE-SEM, UV-Vis diffuse reflectance, FTIR, and TGA analysis. Upconversion emission (UC) based imaging and drug delivery systems have been proposed to overcome some significant drawbacks of existing systems. Though fluorides are better hosts for UC, their

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harmful character has forced researchers to look at alternatives including oxides. The frequency upconversion emission (UC) study upon excitation at 980 nm has been performed in this particular host (m-LaVO4) which is not an ideal upconverting host. The sample emits bright green color upconversion emission, along with relatively weak emissions in the blue, red and NIR regions. The nanorod shape of the developed samples has been confirmed by FE-SEM image analysis. The purity of the green color emission has been confirmed by CIE chromaticity diagram which supports its utility in the fabrication of green upconverters.

INTRODUCTION Lanthanide based phosphors, ‘also known as luminescent materials’ for lighting, telecommunications, displays, security inks and marking and probes for biosciences have occupied one-third of the total market in value terms.1-6 One of the requirements for probes in biosciences is the sensitive and specific biodetection of tumor markers for early stage cancer diagnosis and therapy. In this, the Ln3+ doped inorganic luminescent nanoparticles have attracted considerable interest.7 This is predominantly attributed to the unique luminescent properties, which includes the ability to convert near infra-red long-wavelength excitation radiation into shorter visible wavelength radiation through photon upconversion (UC) processes.8-9 Upconverting low energy photons into high energy photons is a promising technique that offers advantages such as low autofluorescence background, large anti-Stokes shifts10 (or antiRichardson shift),11 sharper emission bandwidths, high resistance to photobleaching, high penetration depth, and temporal resolution. Other advantages of lanthanides such as being able to selectively deliver drugs through photocage techniques to the target area have prompted researchers to explore possibilities of generating multifunctional lanthanide based “designer”


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nanomaterials that can have down-conversion, up-conversion, and persistent luminescence.12-13 Challenges in UC development have included difficulties in achieving efficient UC at room temperature and at low illumination intensities, low color tunability, and relatively low absorption cross sections.14 Amongst the known UC materials, the rare earth nanocrystals based UC host materials with lower phonon energy is required for obtaining highest luminescence efficiency.15 For this, the halides, due to high refractive index and high transparency have been reported as better host. But they are very sensitive to moisture and hygroscopic in nature.16 NaYF4 with Yb3+ and Er3+ dopants is known to be the most efficient NIR to visible UC material.17 In terms of stability, the oxides would be a better host, though they suffer from relatively high phonon energies.8 The use of oxides is also advantageous in comparison to fluorides, which have been reported to be often environmentally harmful.18 Amongst the oxides, the orthovanadates are potential hosts for luminescent materials. Intentional doping of non-luminescent cations for local field modifications around lanthanides has been reported to improve UC efficiency. For instance, Li+ doping in oxide hosts can produce similar results as with fluoride hosts.19 LaVO4 exists in both monoclinic (m-) and tetragonal (t-) forms, depending on reaction conditions. Though m-LaVO4 is thermodynamically more stable, the t-LaVO4 that has a structure similar to YVO4 is a promising candidate for phosphors.20 To activate upconversion emission from a t-LaVO4 host material, doping with rare earth ion such as Er3+ having low probabilities of non-radiative transitions in its excited levels has been suggested.21 Co-doping a sensitizer such as Yb3+, to improve the efficiency of energy transfer between sensitizer and activator has been suggested.20 In a recent review on upconversion efficiency, Chen et al., had detailed four key technical requirements for bioapplications of


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upconversion nanoparticles. Amongst these, monodisperse, small size, and uniform shape have been detailed as a requirement for identical optical properties, cellular uptake and biological effects.22 Through controlled synthesis, which includes hydrothermal/solvothermal methods, and tuning of experimental conditions, a variety of rare earth nanocrystals of varying shapes and having variable functional properties can be obtained.23 One of the most popular methods for synthesis of m- and t-phase LaVO4 is the use of hydrothermal synthesis in the presence of EDTA as additive.24 A replacement for Ethylenediaminetetraacetic acid (EDTA), more so in bioapplications, has been a subject matter of extensive interest. In this direction, our group has been able to successfully employ catechin, a polyphenol, as a sacrificial ligand for the phase and morphology controlled synthesis of t-LaVO4 nanoparticles.25-27 In the case of sensitizer – activator doped oxides, it has been reported that annealing of the samples improves their crystallinity and particle size distribution, leading to improved upconversion luminescence.28 In essence, based on the background information that through appropriate control on particle size, morphology, phase, through appropriate doping and treatment of samples, it is possible to obtain good upconversion luminescence. In this direction, this study attempts to develop UC emission materials from host materials like lanthanum orthovanadate, which is not an ideal upconversion host. The scheme developed and reported in this work involved the synthesis of Er3+/Yb3+ codoped lanthanum orthovanadate using catechin as morphology and phase director under hydrothermal conditions, followed by annealing. The paper reports a rare case of obtaining strong green upconversion emission in the Yb3+-Er3+ codoped LaVO4 phosphors prepared by hydrothermal synthesis technique upon excitation at 980 nm laser diode.


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RESULTS AND DISCUSSION Crystalline character: The as prepared Yb3+-Er3+ codoped LaVO4 phosphors are brown in color, which changed to yellow on annealing. X-ray diffractograms for the samples with 4.0, 5.0, 10.0, 20.0 mol% concentration of Yb3+ ions and 2.0, 2.0, 2.0, 10.0 mol% concentration of Er3+ ions doping (One from each of the scheme with higher intensity and marked as 4Y2E, 5Y2E, 10Y2E and 20Y10E respectively) before and after annealing is presented in Figure 1 (A-D). A matching of the diffractogram with ICDD database indicates that the sample could be well indexed to JCPDS File Number 10-705226 (a= 7.4578 Å; c= 6.5417 Å, space group: I41/amd (141)), confirming the presence of pure tetragonal phase (Figure 1A). In t-LaVO4, La atom is coordinated with eight oxygen atoms to form a LaO8 dodecahedron, where La occupies D2d sites with high symmetry environment. The peak intensity of the annealed samples was higher without any alteration in crystal structure, i.e. crystallization improved without changing the crystal structure. The presence of well–resolved diffraction peaks is an indication that very highly crystalline products can be obtained at a relatively low temperature in hydrothermal reaction (180 °C).20, 29 X-ray diffractogram of 10Y2E (Figure 1B) matched with that of m-LaVO4 (JCPDS file number 50-0367), with lattice parameters a= 7.043 Å; b= 7.279 Å; c= 6.721 Å, space group (P21/n). In this structure, the La atoms are coordinated to nine oxygen atoms, thus forming a polyhedron of pentagonal interpenetrating VO43- tetrahedrons, in a C1 symmetry environment.20 The sample 5Y2E (Figure 1C) shared the same crystal structure as 10Y2E, with a few peaks matching with that of tetragonal phase. The sample 20Y10E (Figure 1D) had dominant tetragonal phase with no phase impurity.


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Figure 1. XRD profiles of 4Y2E (A), 10Y2E (B), 5Y2E (C) and 20Y10E (D). i, ii represents asprepared (Red) and annealed (Blue) Yb3+-Er3+ codoped LaVO4 UCNPs respectively. Standard Tetragonal, Monoclinic LaVO4 corresponds to JCPDS No 10-705226, 50-0367 respectively.


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In co-doped system, dopants with large ionic radii show a high tendency towards electron cloud distortion owing to increased dipole polarizability, and thus favor tetragonal LaVO4. “For same coordination numbers, the ionic radii lie in the order La3+>Er3+>Yb3+. The doping of lanthanide ions {Er3+, r=1.004 A°}30 in high concentration with a size smaller than La3+ (r= 1.160 A°)30 in LaVO4 host lattices dominate the formation of pure tetragonal-phase (with coordination number 8) LaVO4 nanoparticles. Further doping of Ln3+ ions {Yb3+, r=0.985 A°} in high concentration in LaVO4 host lattices lead to the formation of pure monoclinic phase (with coordination number 9) LaVO4 nanoparticles due to larger ionic radii of Ln3+ ions.” In addition, when the physical dimension of the particle is reduced, it leads to high surface tension which then triggers phase transformation. The combined effect of ion size disparity and morphology, tune the LaVO4 from monoclinic to tetragonal and vice-versa.30 Cell parameters decreased with increasing dopant (Yb3+, Er3+ ions) concentration, in tune with Vegard’s law (Table 2). This indicates that both Yb3+ and Er3+ ions are incorporated into the host lattice. Annealing of LaVO4: Yb3+-Er3+ nanoparticles at higher temperature do not alter the single phase characteristics of the as-prepared samples, as demonstrated by the absence of any additional diffraction lines (Supplementary information, Figure S1) in the XRD patterns of annealed samples. From the analysis of the diffractogram in Figure 1 and S1, it could be seen that the position and the intensity of the diffraction peaks were not significantly altered as doping with Yb3+ / Er3+ ions concentration varied, indicating that there is no obvious change in the crystal structure. Diffractogram of the annealed (Figure S1) and as-prepared samples (Figure S2) did not carry any signature peaks of the dopant (Yb3+ / Er3+ ions). For obtaining the optimal UC emission, it is important to obtain uniform doping. To validate the doping of the Yb3+ / Er3+ ions


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into the host lattice, Energy- dispersive X-ray spectroscopy (EDX) spectral analysis has been carried out. From the analysis (Figure S3), the presence of elements La, V, O, Yb, Er and thus the formation of desired products were confirmed. Dopants (Yb3+, Er3+) occupy La3+ sites with (Yb3+, Er3+)O9 in the case of monoclinic monazite structure and (Yb3+, Er3+)O8 in the case of tetragonal zircon structure of LaVO4.20 No difference in elemental composition has been observed in the LaVO4: Yb3+-Er3+ phosphors before and after annealing. Figure S4 and S5 ensure the homogeneous distribution of sensitizer and activators in LaVO4 host lattices.

The crystallite size has been calculated from the highest intensity peak of the prepared phosphors with the help of XRD peak broadening estimation using the Scherrer’s equation (D = 0. 9λ/ βfCosθ), where ‘D’is the crystallite size, ‘λ’ is the X-ray wavelength and ‘θ’, ‘βf’ are Bragg’s angle and full width at half maximum (FWHM) of an estimated peak respectively.31 The Williamson-Hall (W-H) method was used to calculate the lattice strain present in the phosphors. The W-H plot is obtained from the equation βfCosθ = 4εsinθ + (0.89λ/ βfCosθ), where ε is the strain present in the prepared phosphors. The strain is calculated from the slope of a plot of βfCosθ versus 4sinθ. The crystal structure, lattice parameters, cell volume, and strain present in the developed phosphors are given in Table 2.


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Table 2. Dopant concentration, Phase composition, cell parameters and lattice strain of Yb3+-Er3+ codoped LaVO4 UCNPs. A denotes annealing at 600 °C for 4h

Sample ID

4Y2EA

Mol % of Crystal Dopants structure 3+ 3+ Yb Er 4 2 Tetragonal

4Y2E 10Y2EA

10

2

10Y2E

5Y2EA

5

2

5Y2E

20Y10EA 20Y10E *

20

10

Crystallite size* (nm) 125

Tetragonal

118

Monoclinic

117

Monoclinic

110

Monoclinic

118

Monoclinic

108

Tetragonal

97.3

Tetragonal

84.4

Lattice parameters a=7.4684 Å; c= 6.5596 Å a=7.3909 Å; c= 6.5194 Å a= 7.0412Å; b= 7.2770 Å; c= 6.7190 Å a= 7.0721 Å; b= 7.3089Å; c= 6.7484 Å a= 7.0563Å; b= 7.2927 Å; c= 6.7334 Å a= 7.0582Å; b= 7.2946 Å; c= 6.7352 Å a=7.3758 Å; c= 6.5095 Å a=7.3581 Å; c= 6.4789 Å

Cell volume ( Å3) 365.87

strain

0.00011

356.12

0.00011

332.76

0.00104

337.16

0.00023

334.91

0.00047

335.17

-0.00023

354.13

-0.00259

350.78

-0.00206

Crystallite size is determined from the powder XRD pattern according to Scherrer equation.

It can be clearly seen that the as-prepared phosphors contains nano-crystalline structures whose crystallite size after annealing had slightly increased. The lattice strain present in the phosphors was calculated from the Williamson-Hall method32 and presented in Table S1. A very small


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value of lattice strain was observed due to the mismatch of the effective ionic radii of the dopant ions (Yb3+, Er3+) and the exchange sites La3+. The positive slope value obtained in the study denotes that the tensile strain is due to the deformation along a line segment which increases in length when load is applied along that line.33 Compressive strain from the negative slope is due to the deformation along a line segment that decreases in length when a load is applied. It is quite interesting to note that the tensile strain and compressive strain is obtained for both before and after annealing and it can be concluded that the same is due to effective doping at low and high dopants concentration respectively. In case of 5Y2E, compressive strain is changed to tensile strain after annealing due to change in morphology (nanoparticles to nanorods). A change in lattice volume may lead to lattice strain change, therefore doping/dopant concentration, and shape has a distinct effect on crystal structure and lattice strain of phosphors.

Morphology: The FESEM micrographs recorded for the optimized LaVO4: Yb3+-Er3+ codoped phosphors before and after annealing is presented in Figure 2. The particles are uniform, but with irregular shapes having sizes in the range of 30-40 nm. Aggregation of nanoparticles was also observed. No significant difference in particle sizes was observed for almost all the LaVO4: Yb3+-Er3+ codoped phosphors before and after annealing of the samples as evidenced by crystallite size. This is can be attributed to random nucleation of metal-organic complexes with limited growth. From Figure 2 (a- e), it can be concluded that the morphology of the products was more or less similar, except in the case of 5Y2E (Figure 2f, g), where nanoparticles were converted into nanorods during annealing process. Crystal morphology of the UC nanophosphors was not altered with dopant concentration. Dispersibility of the nanoparticles, however, was observed to be poor for the annealed samples. From Table 2, it is understood that for 5Y2E the


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lattice parameter c changes from 6.7334 to 6.7352 Å when shape changed from nanorods to nanoparticles as the growth of nanorods orient either along [001] (c axis) or [100] (a axis) direction as observed earlier by Zhang et.al.34 The c value of the nanorods is lower than that of nanoparticles, as anisotropic growth occurs along the c axis in the nanorods. Thermodynamics illustrate that the growth along the [001] direction release more energy than that of [100] direction.33

a)

b)

c)

d)

e)

f)

g)


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Figure 2. FESEM images of 4Y2EA (a), 10Y2EA (b), 20Y10EA(c), 1Y2E (d), 1Y2EA (e), 5Y2E (f), and 5Y2EA (g) Yb3+-Er3+ codoped LaVO4 UCNPs. a, b, c, e, g and d, f represents annealed and as-prepared samples respectively.

Spectral analysis: Absorption spectra: A binary dopant system (Yb3+, Er3+) with different concentration of dopants was preferred for upconversion tuning. LaVO4: Yb3+-Er3+ codoped nanophosphors were collected as dried powders for spectral characterization as nanophosphors suspended in different solvents (water, ethanol) resulted in the rapid settling of the nanometer sized particles. The settling of nanoparticles in the media results in large scattering losses in the liquid medium and thus inconsistency in the spectral results was observed. UV-Vis diffuse reflectance spectroscopy was used to study the electronic transitions in LaVO4 systems. The absorption spectra of Yb3+Er3+ codoped LaVO4 nanophosphors in the 200-1200 nm region are shown in Supplementary information (Figure S6). Absorption spectra (Figure 3) is characterized by five absorption bands for the Yb3+-Er3+ codoped in LaVO4 nanophosphors, after annealing. Broad absorption bands observed in the 200- 350 nm range can be ascribed to the charge transfer from oxygen ligands to the central vanadium atoms inside the VO43- groups.35 On the basis of molecular orbital theory, the possible transitions are from the 1A2 (1T1) ground state to 1A1 (1E) and 1E (1T2) excited states of VO43- ion.36 A charge transfer band arises from the transition of 2p electrons of O2- to the empty 3d orbitals of V5+ in VO4 unit. The absorption band at ~ 974 nm is due to the absorption of Yb3+ ions corresponding to the 2F7/2→2F5/2 transition. The remaining three absorption bands are due to f-f transitions from the ground-state 4I15/2 to excited states of Er3+ ions, assigned by partial energy level diagram as shown in Figure 4a. The absorption peaks at ~ 490 nm, ~ 523 nm,


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and ~ 660 nm, are attributed to the 4I15/2→2H9/2, 4I15/2→2H11/2 and 4I15/2→4F9/2 transitions corresponding to the Er3+ ions absorption respectively. As prepared Yb3+-Er3+ codoped LaVO4 i.e. without annealing only two bands was observed at ~ 670 and ~ 1190 nm are due to 4

I15/2→4F9/2 and 3H6→3H5 transitions respectively.37 The band positions shown in Figure 3 hold

well with the reported literature.38 Lattice sites occupied by the dopants (Yb3+, Er3+) is not similar as site-to-site variations around each ion in the crystal field strength is not homogeneous, thus leading to inhomogeneous broadening in the absorption spectra. Broad absorption bands (200-350 nm) observed only for the case of 10Y2EA and 5Y2EA samples indicate that absorption intensity of the general f–f transitions of Er3+ and Yb3+ ions in the longer wavelength region are very weak in comparison with that of the VO43-groups, indicating that the excitation of Er3+ and Yb3+ ions are mainly through the VO43- groups as observed in Eu3+ doped LaVO4 phosphors.25


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Figure 3. Absorption spectra of 4Y2E (A), 10Y2E (B), 20Y10E (C), 5Y2E (D), and 1Y2E (E). i, ii represents as-prepared(Black) and annealed(Red) Yb3+-Er3+ codoped LaVO4 UCNPs respectively. Wavelength at 974 nm corresponds to 2F7/2→2F5/2 transition of Yb3+ and 490, 523, 660, 670, 1190 nm corresponds to 4I15/2→2H9/2, 4I15/2→2H11/2, 4I15/2→4F9/2, 4I15/2→4F9/2, 3H6→3H5 of Er3+respectively. The y axis drift around 400 nm and 800 nm is due to lamp change.

Upconversion spectra: The efficiency of upconversion process is determined by the nonradiative process of the materials and the multiphonon radiative decay rate that can be given by the energy gap law represented by the equation (2):39

= 1 − exp (−ℎυ⁄)

(2)

where Wn is the rate at temperature T, W0 is the rate at 0 K, n= ∆E/hυ, ∆E is the energy gap between the corresponding levels, h is the Planck’s constant and υ is the relevant phonon’s frequency.

Multiphonon nonradiative process becomes dominant with radiative relaxation when ∆E is less than or equal to five times the high-energy phonons.40 Presence of atomic groups with high vibrational frequency, such as the -OH groups, increase the nonradiative processes and thus decrease upconversion efficiency. High upconversion efficiency can be achieved by annealing process. The change in color of the powder sample from brown to light yellow during the annealing process is clearly an indication of the changes in host lattice structure. The structural


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change leads to two different color centres. Emission from the Er3+ centre evolved with annealing. When optically active ions are found close to the surface (as in this case), it leads to interactions with impurities and thus nonradiative relaxations. In this instance, an improvement in the long range ordering of Er3+ ion, better crystallinity of the product and increased crystallite size after annealing resulted in a marked upconversion emission.41

a)

b)

Figure 4. Energy level diagram for (a) direct Er3+ excitation and (b) Yb3+-Er3+ system, represents the mechanism responsible for visible emission and the color denotes the color of emitted photons.

Under a 980 nm laser excitation at room temperature, a strong green light shows up and this provides for a strong generation of upconversion fluorescence in LaVO4: Yb3+-Er3+ codoped systems. In this study, excitation power of the laser source was kept as 13.9 Wcm-2 (pump power


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=860 mW) to avoid laser induced heating. To better understand the upconversion emission of LaVO4: Yb3+-Er3+ codoped nanophosphors, the concentration of the dopant ions (Yb3+and Er3+) versus UC emission intensity was plotted for a series of samples prepared with varying dopant concentration (Supplementary information, Figure S7) and details are presented in Table-3.

Table 3. Upconversion emission intensity and G/R ratio in Yb3+-Er3+ codoped LaVO4 UCNPs.

Sample name

Mol % of Green integrated Dopants intensity Yb3+ Er3+ (G)

Red G/R ratio integrated intensity(R)

1Y2EA

1

2

26787

4188.5

6.39

2Y2EA

2

2

62799

7737

8.11

3Y2EA

3

2

82575

10112

8.16

4Y2EA

4

2

458547

39625.5

11.57

5Y2EA

5

2

299807.5

23454.5

12.78

10Y2EA

10

2

57493.5

8383.5

6.85

15Y2EA

15

2

22774

3450.5

6.60

20Y2EA

20

2

5457

1385

3.94

30Y2EA

30

2

6654

1869.5

3.55

40Y2EA

40

2

8971.5

2764.5

3.24

50Y2EA

50

2

7621.5

2446.5

3.11

Experimental conditions (i.e. Excitation source, pump power, sample concentration) remained same for all the samples. Intensity of UC emission changes as dopant concentration were varied


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(the Yb3+ concentration was varied as 1.0, 2.0, 3.0, 4.0, 5.0, 10.0, 15.0, 20.0, 30.0, 40.0, 50.0 mol %, while keeping the Er3+ concentration constant at 2.0 (Figure 5).42-43

Figure 5. Upconversion spectra of the Yb3+-Er3+ codoped LaVO4 samples. The UC emission intensity versus concentration dependent plot for different samples are shown in Figure 6 and observed that the sample 4Y2EA emits an intense and efficient green upconversion emission. The upconversion emission spectra for the optimized LaVO4: Yb3+-Er3+


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nanophosphors shown in Figure 5 exhibits an intense green band centred at ~522 nm and 552 nm due to the 2H11/2→4I15/2 and 4S3/2→4I15/2 transitions along with relatively weak emission bands in blue, red and IR regions attributed to 2H9/2→4I15/2, 4F9/2→4I15/2 and 4I9/2→4I15/2 transitions peaking at ~490 nm, 654 nm, and 802 nm respectively. The strongest upconversion emission intensity at ∼ 524 nm than other emissions i.e. non uniform intensity suggests the non-uniform population distributions. UC enhancement to the tune of ~46, ~22, and ~3 folds have been observed for the green band at 552 nm in the case of 4Y2EA, 5Y2EA, and 10Y2EA samples (one from each scheme), compared to 20Y10EA sample respectively, thus making them superior to other reported UC materials. From literature, the UC luminescence observed in this study could be attributed to lattice strain. The tensile strain in the case of 4Y2EA was the lowest. UC emission intensity decreases with increase in tensile strain, possibly because of an increase in nonradiative relaxation processes.44 Upconversion behavior of LaVO4: Yb3+-Er3+ systems can be clearly seen from the Yb3+ concentration-dependent changes reflected in Figure 6A (1-3) and 6D.


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Figure 6. (A) Effect of dopant concentration, (B)&(C) Effect of annealing, (D) Concentration dependence of 1) 4Y2EA 2) 5Y2EA 3) 10Y2EA and 4) 20Y10EA at 524 nm, 552nm, 656nm, and 802 nm respectively on the UC emission of Yb3+-Er3+ codoped LaVO4 UCNPs. i, ii represent as-prepared (Black) and annealed (Red) samples of Yb3+-Er3+ codoped LaVO4 UCNPs respectively. In order to understand the effect of catechin for efficient upconversion process, Yb3+-Er3+ codoped LaVO4 UCNPs in the presence and absence of catechin at 20.0 mol% concentration of Yb3+ ions and 2.0 mol% concentration of Er3+ ions is compared (Figure S8). Formation of the tetragonal phase, multi fold enhancement in upconversion emission could be related to the presence of catechin. From literature, this could be attributed to catechin acting as a carbon emission center.45 This justifies catechin as a phase selector, morphology director, and emission enhancer. The mechanism of UC emission observed in this study can be explained by the schematic energy level diagram of the Yb3+-Er3+ system (Figure 4b). Five different UC emission bands have been observed corresponding to the 2H9/2→ 4I15/2, 2H11/2→ 4I15/2, 4S3/2→ 4I15/2, 4F9/2→ 4I15/2 and 4I9/2→ 4I15/2 transitions of Er3+ ion peaking at ~490 nm, ~522nm, ~552 nm, ~652 nm and ~802 nm (Figure 5). Mainly the energy transfer (ET) from the sensitizer Yb3+ ion to the corresponding levels of the Er3+ ion, GSA (Ground state absorption) and ESA (Excited state absorption) processes are responsible for the enhancement and origin of the observed transitions corresponding to the Er3+ ion. The ground state atoms in the 4I15/2 level absorb energy corresponding to the 980 nm via the GSA process and get promoted to the 4I11/2 level and similarly the Yb3+ ions in the 2F7/2 level get promoted to the 2F5/2 level via the GSA process. The Er3+ ions in the 4I11/2 level again through the ESA process get promoted to the 4F7/2 level which


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further relaxes down to the two thermally coupled levels 2H11/2 and 4S3/2. The population in the 2

H11/2 and 4S3/2 emits photons at ~522 nm and ~552 nm via radiative 2H11/2→ 4I15/2, 4S3/2→ 4I15/2

transitions respectively. A part of the population in the 4I11/2 level relaxes to the 4I13/2 level, and then absorbs the excitation energy through ESA process and gets promoted to the 4F9/2 level. A radiative relaxation takes place from the 4F9/2 level to the 4I15/2, ground level emitting a photon at ~654 nm in the red region. A part of the population promoted to the 4F9/2 again absorbs energy by means of ESA process and get further promoted to the 2H9/2 level and afterwards through radiative transition relax down to the ground level, 4I15/2 and emits a photon at ~490 nm in the blue region. The ET process from the 2F5/2 level of sensitizer Yb3+ ion to the 4I11/2, 4I13/2 and 4F7/2 levels of Er3+ ion makes the population dense in the respective levels which results in more intense radiative transitions from the excited levels of Er3+ ion.46-47

Figure 7. FTIR spectrum (A) and TGA thermogram (B) of 4Y2E where a, b represent asprepared and annealed Yb3+-Er3+ codoped LaVO4 UCNPs respectively.


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The multiphonon relaxations (MR) i.e. rate of nonradiative channels determines the relative intensities of red and green emission in terms of surface ligand effect and is given by the equations,48 Kp= A exp-Bp and p ≈ ∆E/hω, where Kp is MR rate, A and B are host constants, p is the number of phonons, ∆E is the difference between two energy levels and hω is the maximum single energy phonon. For p≤5, nonradiative MR process precedes over radiative process. FTIR spectrum of 4Y2EA is shown in Figure 7A. A strong and weak absorption band at 815 cm-1 and 455 cm-1 is assigned to the V-O and La-O bond vibrations respectively.26,

49-51

A very broad

absorption band observed at ∼ 3700 cm-1 is characteristic of the O-H stretching vibration of adsorbed water and free O-H groups and peak around 1675 cm-1 is attributed to the bending vibration of the associated water.33 This clearly establishes that the hydration layers are bonded chemically to the LaVO4 surfaces.52-53

Water molecules bonded to the LaVO4 surfaces are either physisorbed or chemisorbed.53 Chemisorbed water molecules are bonded directly either to the surface cations (i.e. La3+, V5+) or occupy the vacant oxygen sites, whereas physisorbed water molecules are stabilized with the adjacent bridging surface OH groups.15 Two intense bands at 1552 cm-1 and 1441 cm-1 are due to the symmetrical and asymmetrical vibrations of carboxylate groups54 and arise from the high reactivity of LaVO4 surface hydration layers with the CO2 in the air form carbonate species. The presence of energetically non-equivalent surface hydration groups can be seen from the weight loss from 40-600 °C as indicated in the TGA data (Figure 7B). By annealing majority of the water molecules will be removed, followed by desorption of carbonate moiety from the nanoparticles surface.15 This hinders an easy MR between the 4I11/2 and 4I13/2 levels. Populations in the 4I13/2 level excited to the 2H11/2 level through ESA process is a direct evidence of intense


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green emission as the ESA process is more efficient. The red upconversion emission intensity is less in comparison to the green upconversion emission intensity because of the restricted relaxation channels for populating the red emitting state 4F9/2 (Figure 4b).55-56 Therefore, a prominent green emission is achieved by catechin capped Yb3+-Er3+: LaVO4 UC nanophosphors. The upconversion emission intensities of 5Y2EA and 1Y2EA (Figure 6B and 6C) increases by 37 and 50 orders of magnitude to that before annealing and this is attributed to the surface hydroxyl groups and change in surface area with shape. The shape change induces a number of surface defects in the host. Both the surface hydroxyl groups and surface defects together contribute to nonradiative relaxation rate.57 In Figure 6C, it can be seen that the population of Er3+ ions in 2H11/2 and 4S3/2 energy level at ~522 nm and ~552 nm respectively have equal intensity owing to the lower number density of the dopant ions (Yb3+ and Er3+), leading to strong green upconversion emission. Higher the surface area, higher the number of rare earth ions on the surface (or) surface defects that promotes the absorption efficiency. As annealing reduces the internal defects and increase the diffusion coefficient of atoms in the host lattice, and hence decrease the total number of crystal defects, which could serve as recombination centers for the upconverted lanthanide states is observed. It is thus clear that annealing increases the upconversion emission intensity.58 The critical distance between the activator centers can be related to upconversion luminescence intensity. According to Blasse’s equation, the critical distance (Rc) between activators for maximum emission intensity by considering optimized upconversion emission data and, can be estimated in terms of the equation (3)59 Rc= 2(3V/ 4πxcN)1/3

(3)


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where V= volume of the unit cell, xc= critical activator concentration, and N= number of available crystallographic lattice sites in the unit cell occupied by activator ions. The corresponding R (Ln3+- Ln3+) values for 4Y2EA, 10Y2EA, 5Y2EA and 20Y10EA (one sample with highest upconversion emission intensity from each of the selected scheme) samples have been calculated (Z=4, N=Z*1= 4) and Rc was determined to be 2.06 nm, 2.0 nm, 1.996 nm and 1.019 nm, respectively. This clearly indicates that critical distance between the Er3+ ions is ~2.0 nm when the doping concentration reaches 2.0 mol% of Er3+ ion till 10.0 mol% Yb3+ ion concentration and is 1.019 nm when the Er3+ ion doping concentration is 10.0 mol% and Yb3+ ion concentration reaches 20 mol%. The shorter distance between the activator ions will increase the upconversion luminescence only when the dopant ion concentration reaches a certain level.

Dexter formula has been employed to study the type of exchange interaction between the ions and the relation can be expressed as;60

=

(4)

( )

where C→ concentration, I→ intensity of upconversion, k and β are constants for each

interaction condition. (− ) is the slope of ln(I/C) versus ln(C) and θ = 6 indicates dipole-dipole, θ = 8 indicates dipole-quadrupole and θ = 10 indicates quadrupole-quadrupole interaction between the ion species.

The ln (I/C) versus ln(C) has been plotted for the Yb3+/Er3+ codoped LaVO4 nanophosphors and has been shown in Figure 8. From this plot the θ value comes out to be 6.0 in the present case. Thereby, showing the dipole-dipole interaction is operating in the Yb3+-Er3+ codoped LaVO4 nanophosphors.


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Figure 8. ln (I/C) versus ln (C) for the Yb3+-Er3+ codoped LaVO4 UCNPs.

To study the effect of variation of concentration of sensitizer (Yb3+ ions) on the activator Er3+ ions, the green (G) to red (R) intensity ratio i.e.; G/R ratio for the UC emission intensity peaks at ~522 nm and ~652 nm has been calculated for the samples with Er3+ ions concentration fixed at 2.0 mol% and Yb3+ ions concentration varying as 1.0, 2.0, 3.0, 4.0, 5.0, 10.0, 15.0, 20.0, 30.0, 40.0 and 50.0 mol% i.e. for the 1Y2EA, 2Y2EA, 3Y2EA, 4Y2EA, 5Y2EA, 10Y2EA, 15Y2EA, 20Y2EA, 30Y2EA, 40Y2EA, and 50Y2EA samples (shown in Table 3). The G/R ratio first increases from 6.39 to 12.78 via 8.11, 8.16, and 11.57 up to the sample with 5.0 mol% Yb3+ ions concentration. The intensities of both green and red upconversion peaks are highest in the case of 4Y2EA sample which is the sample with optimized concentration but the ratio is more in the


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case of 5Y2EA sample due to the difference in the value of intensity of red upconversion emission peak. On increasing the concentration of Yb3+ ions beyond 5.0 mol% the ratio starts to decrease. The increase in G/R ratio up to 5.0 mol% Yb3+ ions concentration is due to the increasing intensity of the green upconversion emission peak due to efficient energy transfer from the Yb3+ ions to the Er3+ ions. This results into a more dense population in the 4F7/2 level which further results into a highly enriched 2H11/2 level via the non-radiative relaxation from the 4

F7/2 level. When the concentration of Yb3+ ions is increased beyond the 5.0 mol%, the average

distance between the Er3+ and Yb3+(Er3+) ion decreases which results into energy dissipation via the non-radiative relaxation and cross-relaxation channels.61 The cross relaxation channel 4

F7/2 (Er3+) + 2F7/2 (Yb3+)→ 4I11/2 (Er3+) + 2F5/2 (Yb3+) via the Er3+ to Yb3+ is preferential at the

same time, which results into a reduction of the 4F7/2 (Er3+) level population and hence a reduced green emission peak at ~522nm is observed.20 On the basis of the above upconversion luminescence, the emission colors can be tuned by doping with different concentration of dopants (Yb3+, Er3+). The Commission Internationale de l’Eclairage (CIE) chromaticity coordinates for LaVO4: Yb3+-Er3+ nanophosphors at pump power (860 mW) was shown in Supplementary information, Figure S9. For the calculations, GOCIE software for color coordinate calculations was used. The color coordinates remain the same for almost all the concentrations of dopants, with better chromaticity for green emitter. However, at higher concentration of Er3+, the color coordinates slowly shift towards the blue region, upon excitation by a 980 nm diode laser. Thus, prepared samples are found to be appropriate as deep-green emitting upconversion nanophosphors.


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Conclusions The Yb3+-Er3+codoped lanthanum orthovanadate phosphor samples have been synthesized successfully by using catechin as morphology and phase director under hydrothermal conditions, followed by annealing. The monoclinic and tetragonal phase of the sample has been confirmed by the X-ray diffraction analysis and the presence of all the precursor elements La, V, O, Yb, Er and thus the formation of desired products are confirmed by the EDX analysis. The improved crystallinity after annealing has been confirmed by the X-ray diffraction pattern and the developed strain throughout the prepared samples has been calculated by using the WilliamsonHall method. The nanorods shape of the developed samples has been confirmed by the FESEM images. Broad absorption bands observed in the 200- 350 nm range and the absorption peaks at ~ 490 nm, ~ 523nm, and ~ 660 nm, ~974 nm has been observed in the UV-Vis diffuse reflectance studies and have been explained properly. Bright green upconversion band, which can be seen with naked eyes, centred at ~522 nm and ~552 nm due to the 2H11/2→ 4I15/2, 4S3/2→ 4I15/2 transitions along with relatively weak UC emission bands in the blue, red and IR regions due to the 2H9/2→ 4I15/2, 4F9/2→ 4I15/2 and 4I9/2→ 4I15/2 transitions at ~490nm, ~652 nm and ~802 nm respectively have been observed upon the 980 nm laser excitation from the developed samples. About ~42 times of enhancement in the UC emission intensity has been observed for the sample with optimized dopant concentration compared to the sample with lowest upconversion emission intensity in the selected scheme. Annealing the samples at high temperature improves the crystallinity and increases the upconversion emission intensity also. Presence of different functional groups along with the vibrational frequency of vanadate ions at ~815 cm-1 have been confirmed by FTIR studies. The weight loss from 40-600 °C in the TGA analysis indicates the presence of energetically non-equivalent surface hydration groups in the phosphor samples. The


26

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critical distance between the activator Er3+ ions has been found as 2.0 nm from the Blasse’s equation for the optimum concentration of Er3+ ions at 2.0 mol%. The electric dipole-dipole exchange interaction has been found to be operating between the dopant ions by the Dexter’s relationship and calculations. The green to red ratio i.e. G/R value is found to be maximum for the sample emitting the most intense green upconversion emission i.e. for the optimized 4Y2EA sample and the values are lower for rest of the samples. An intense green upconversion emission from the prepared phosphors has been confirmed by CIE chromaticity diagram and hence the material can efficiently be used in the fabrication of green upconverters.

Methods X-ray diffractogram images of the as prepared and annealed samples were collected (scan rate of 1° min-1 in the 2θ range from 10-80°) on a Rigaku Miniflex II desktop X-ray diffractometer equipped with Cu-Kα radiation (λ = 1.540562 Å). The lattice parameters and cell volume were calculated from the observed D values through a least-squares fitting method using computer program based unit cell refinement software. Morphology, and size measurements of the nanoparticles was collected using a Carl Zeiss SUPRA-55 FESEM operating at an accelerating voltage of 20 kV. Thermal analyzer TGA 1/1100 SF (METTLER TOLEDO) was used for Thermogravimetric analysis (TGA) at a heating rate of 10 °C/min under N2 atmosphere. FT-IR spectra were obtained using JASCO FT/IR 6300 spectrophotometer using the KBr pellet technique. UV-Vis diffuse reflectance spectra (UV-Vis DRS) were recorded in the absorbance mode at room temperature in the range of 400-1200 nm on Agilent Technologies (CARY -5000) double-beam spectrophotometer equipped with integrating sphere attachment using BaSO4 as the reference. The instrument was interfaced with a computer for data collection and analysis. For


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these measurements, powder samples were filled in a hole of a sample holder and the surface smoothed. The UC emission spectra of the phosphor powders were recorded from 400 to 900 nm through a Princeton triple turret grating monochromator (Acton SP-2300) attached with a photomultiplier tube (PMT) upon excitation with 980 nm continuous wave (CW) diode laser. The fluorescence was measured using the solid powder; the laser spot size was kept the same when comparing samples and the measurement spectra of all samples are recorded under the same conditions. The position of the laser relative to the samples was identical during all measurements and each sample was fixed on a metallic sample holder. Color analysis i.e. CIE (Commission Internationale de L’Eclairage) was carried out using GOCIE software.

Materials All chemicals employed in this study namely Catechin hydrate, Lanthanum (III) nitrate hexahydrate (puriss. p.a., 99.0%), Erbium (III) nitrate pentahydrate (99.9% trace metals basis), Ytterbium (III) nitrate hexahydrate (99.9% trace metals basis), sodium orthovanadate (99.98% trace metals basis), were procured from M/s. Sigma Aldrich, USA. Ethanol was purchased from Hayman speciality products. All the chemicals were used without further purifications.

UCNP Synthesis The synthesis procedure for Yb3+-Er3+ codoped LaVO4 adopted in the present study was a slightly modified version of our earlier methodology 26. Typically, 10 mL of La(NO3)3.6H2O, x mL of Yb(NO3)3.6H2O and y mL of Er(NO3)3.6H2O was added to 10 mL of catechin hydrate (5.7 mM) to form a Ln3+-Cat4- complex. After vigorous stirring for 30 minutes, Na3VO4 was added to the above solution (VO43- : La3+ = 1.05) with a pH of ∼10. Following 15 minutes of stirring, the as obtained solution was transferred into a Teflon lined bottle placed inside a


28

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stainless steel autoclave, sealed and maintained at 180 oC for 24 h. After uncontrolled cooling of the autoclave, the precipitate obtained was separated by centrifugation, washed thrice with water and ethanol in a sequential manner and then air dried at room temperature. The molar ratio of ligand (Cat4-) to La3+ ion was maintained as 1:0.05. This ratio was approximately the optimal concentration for tetragonal phase through hydrothermal synthesis route. The dried product was transferred to an alumina crucible and annealed at 600 oC for 4h. The above procedure was repeated for a range of concentrations, as per the following representation (1) and pictorially shown in Scheme 1. (1-xi-yi) La(NO3)3 + xi Yb(NO3)3 + yi Er(NO3)3

(1)

In the equation 1, For i = 1, x1 was 0.01, 0.02, 0.03, 0.04 or 0.05, y1 was 0.02; For i = 2, x2 was 0.05, 0.1, 0.15 or 0.2, y2 was 0.02; For i = 3, x3 was 0.25, y3 was 0.07; For i = 4, x4 was 0.1, 0.2, 0.3, 0.4 or 0.5, y4 was 0.02; For i = 5, x5 was 0.2 or 0.05, y5 was 0.01, 0.02, 0.03, 0.04, 0.05, 0.10 and 0.15. The composition of dopants in five schemes are presented in Table 1 a and b. Table 1a shows the details of as-prepared samples and Table 1b contains the details of annealed samples.

Scheme 1. Schematic illustration for the formation of LaVO4: Yb3+-Er3+ nanophosphors.


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Table 1. Sample name and composition of dopants in (a) As-prepared and (b) Annealed samples of Yb3+-Er3+ codoped LaVO4 UCNPs.

Sample name Mole percentage of (As-prepared) dopants

Sample name Mole percentage (Annealed) dopants Yb3+ (xi)

Er3+ (yi)

1Y2EA

1

2

2

2Y2EA

2

2

3

2

3Y2EA

3

2

4Y2E

4

2

4Y2EA

4

2

5Y2E

5

2

5Y2EA

5

2

10Y2E

10

2

10Y2EA

10

2

15Y2E

15

2

15Y2EA

15

2

20Y2E

20

2

20Y2EA

20

2

25Y7E

25

7

25Y7EA

25

7

30Y2E

30

2

30Y2EA

30

2

40Y2E

40

2

40Y2EA

40

2

50Y2E

50

2

50Y2EA

50

2

20Y1E

20

1

20Y1EA

20

1

20Y3E

20

3

20Y3EA

20

3

20Y5E

20

5

20Y5EA

20

5

20Y10E

20

10

20Y10EA

20

10

20Y15E

20

15

20Y15EA

20

15

Yb3+ (xi)

Er3+ (yi)

1Y2E

1

2

2Y2E

2

3Y2E

(a)

of

(b)


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Supporting Information. XRD profiles, Absorption, and upconversion spectra of all annealed and as-prepared Yb3+-Er3+ codoped LaVO4 UCNPs. EDAX, Elemental color mapping, and CIE color coordinates of optimized Yb3+-Er3+ codoped LaVO4 UCNPs. “This material is available free of charge via the Internet at http://pubs.acs.org.” AUTHOR INFORMATION Corresponding Author * [email protected] ACKNOWLEDGMENT VT would like to thank the Department of Science and Technology (DST), Govt. of India for the INSPIRE- Senior Research Fellowship. Authors acknowledge XIIth five year plan SURE project for their financial support. “This research work is carried out as part of Ph.D registered in University of Madras”. CSIR-CLRI Communication Number 1217.


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(18)

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