Enhancement of Luminescence Intensity in Red Emitting NaYF4:Yb

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Enhancement of Luminescence Intensity in Red Emitting NaYF4:Yb/Ho/ Mn Upconversion Nanophosphors by Variation of Reaction Parameters Kumbam Lingeshwar Reddy, Vaddepally Srinivas, Konathala Ravi Shankar, Suneel Kumar, Vipul Sharma, Ajay Kumar, Ashish Bahuguna, Kaustava Bhattacharyya, and Venkata Krishnan J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 08 May 2017 Downloaded from http://pubs.acs.org on May 9, 2017

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The Journal of Physical Chemistry

Enhancement of Luminescence Intensity in Red Emitting NaYF4:Yb/Ho/Mn Upconversion Nanophosphors by Variation of Reaction Parameters Kumbam Lingeshwar Reddy,a Vaddepally Srinivas,a Konathala Ravi Shankar,a,c Suneel Kumar,a Vipul Sharma,a Ajay Kumar,a Ashish Bahuguna,a Kaustava Bhattacharyya,b and Venkata Krishnan*a a

School of Basic Sciences and Advanced Materials Research Center, Indian Institute of Technology Mandi, Kamand, Mandi, Himachal Pradesh, 175005, India.

b

Chemistry Division, Bhabha Atomic Research Centre, Mumbai 400085, Maharashtra, India. c

Current address: School of Nano Sciences, Central University of Gujarat, Gandhinagar 382030, India Email: [email protected]

ABSTRACT In the field of biomedicine, upconversion nanoparticles have wide ranging applications from bioimaging to targeted cargo delivery, especially due to their excellent chemical and optical properties in comparison to conventional fluorophores. However, their use in biomedical applications is largely hindered due to strong absorption of short wavelength (< 600 nm) light by biological tissues/cells and feeble luminescence. Hence, it is important to develop new strategies to increase the long wavelength (red) emission efficiency. In this work, we report an effective strategy to improve the red luminescence efficiency of NaYF4:Yb/Ho/Mn upconversion nanophosphors by varying the reaction conditions. The influence of different synthesis parameters, such as solvent ratio, reaction temperature and reaction time, on the luminescence, crystal phase and morphology of the upconversion nanophosphors has been studied in detail and optimized. The improvement in the crystallinity of nanophosphors is claimed as the main origin for the increase in the red emission intensity. This work could pave way for the versatile use of these bright red emitting upconversion nanophosphors in biomedical applications.

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1. INTRODUCTION Photon upconversion materials are capable of converting higher wavelength (near infrared, NIR) light into lower wavelength (ultra-violet, UV and visible) radiation.1-3 Upconversion (UC) is due to the sequential absorption of multiple low energy photons to reach excited states, where the first excited states have longer life-times, hence the possibility of absorbing second photon to reach further higher energy levels is quite high. Relaxation from such higher energy levels leads the emission of light at lower wavelengths which typically lies between 800 nm and 350 nm.4-6 Lanthanide doped upconversion nanoparticles (UCNPs) show outstanding luminescence properties, such as sharp emission peaks, tunable emission wavelength, long Stokes shifts, extended luminescence life-time, resistance to photo bleaching, deep tissue penetration, high signal to noise ratio (SNR), etc.,7-12 which makes them a good candidate for various applications like, bioimaging,13-15 sensing,16-17 drug delivery,18 photodynamic therapy,19 etc. These extraordinary luminescent properties also make UCNPs a best alternative for conventional fluorophores.20-22 Due to unique electronic configuration and luminescent characteristics, rare earth (RE) elements doped nanoparticles are good candidates for upconversion luminescence.23-24 In most cases, a host matrix, an activator and a sensitizer are the main components that comprise the RE doped UCNPs.1 Host matrix provides essential optical properties like UC efficiency and emission profile to the UCNPs. Because of high chemical stability and low phonon energies, fluorides have been recognized as an ideal host material for UCNPs.25 The sensitizer excites with the absorption of incident light energy and transfers the same to the activator from where high energy light emits.26 Ytterbium is the most common sensitizer and the combination with erbium or holmium leads to green light emission and with thulium can produce blue light.9, 3+

27

Lithium or transition metal ions like manganese or iron co-doping

3+

along with the Yb /Er or Yb3+/Ho3+ combination leads to red emission. 14, 28-33 Emission colour can be tuned as well as intensity can be improved by varying the doping percentage of a sensitizer or activator and/or other co-dopants, which eventually lead to change of crystal phase, morphology and subsequently, the luminescence properties.34-35 Not only by changing the dopant proportions, luminescence properties can also be varied by changing the experimental conditions employed during the synthesis. Ye et al. reported the effect of reaction solution pH on the precipitated nanoparticles’ luminescence properties.36 Sarakovskis et al. and Xu et al. are examined the effect of temperature on the luminescence properties of UCNPs 37-38 and Ma et al. investigated the effect of reaction temperature as well as time on crystal phase, shape, size and upconversion luminescence (UCL) properties.39 Page 2 of 26 ACS Paragon Plus Environment

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Lanthanide (Yb, Ho/Er) doped NaYF4 nanoparticles gives green emission and with the introduction of manganese as co-dopant, emission intensity increases and the emission colour changes from green to red. The influence of Mn2+ doping on crystal phase, size and luminescence has been reported. 14, 31, 33 However, the influence of the reaction parameters on the structure, morphology and luminescence of the red emitting Mn2+ doped NaYF4:Yb/Ho has not been explored yet. These studies could pave way for the development of bright red emitting upconversion nanophosphors with enhanced luminescence, suitable for several biomedical applications. Considering the importance of this requirement, we undertook the task of developing an effective strategy to improve the red emitting efficiency of NaYF4:Yb/Ho/Mn UCNPs. In this regard, we have performed detailed systematic investigations to examine the influence of different synthesis parameters, such as solvent ratio, reaction temperature and reaction time, on the structure, morphology and luminescence of NaYF4:Yb/Ho/Mn. Based on our studies, we hereby report the optimized conditions for the synthesis of bright red light emitting NaYF4:Yb/Ho/Mn nanophosphors.

2. EXPERIMENTAL SECTION 2.1. Materials Yttrium trichloride hexahydrate (YCl3.6H2O), ytterbium trichloride hexahydrate (YbCl3.6H2O), holmium trichloride hexahydrate (HoCl3.6H2O), manganese chloride tetrahydrate (MnCl2.4H2O), sodium hydroxide (NaOH), ammonium fluoride (NH4F) were purchased from Sigma Aldrich. Oleic acid (technical grade, 90%) was supplied by Alfa Aesar. All chemicals were used as received without any further purification.

2.2. Synthesis NaYF4 doped with Yb3+ (20 mol%), Ho3+ (2 mol%) and Mn2+ (40 mol%) were prepared based on the earlier reported procedure

14

by varying the reaction conditions as

mentioned below.

2.2.1. Synthesis of NaYF4:Yb/Ho/Mn by varying the reaction time In a typical synthesis procedure, 0.6 mL of 0.5 M MnCl2, 0.9 mL of 0.5 M YCl3, 1 mL of 0.2 M YbCl3, 0.1 mL of 0.2 M HoCl3 were added to a mixture of 0.3 g NaOH in 2 mL of milli-Q water, 4 mL of oleic acid (OA), 8 mL of ethanol (ET) under magnetic stirring at room temperature (RT). Then, 4 mmol of NH4F dissolved in 2 mL water was added drop Page 3 of 26 ACS Paragon Plus Environment


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wise to the above mixture. After vigorous stirring at RT for 30 minutes the whole solution mixture was transferred into a 25 mL Teflon-lined autoclave, sealed tightly and heated at 423 K for 2, 4, 6, 8 and 10 h. In each case, the whole system was allowed to cool naturally to room temperature. The final products were washed with ethanol and water mixture at least three times, by the means of centrifugation to remove any possible leftover starting materials and dried in vacuum at 333 K to obtain dried UCNP powder.

2.2.2. Synthesis of NaYF4:Yb/Ho/Mn by varying the solvent (ET/OA) ratio Similar synthesis procedure was used as above. The total volume of ET and OA was kept constant to 12 mL and the ET/OA ratio was varied between 2:10 mL, 4:8 mL, 6:6 mL, 8:4 mL, and 10:2 mL as per the experiment requirement. After mixing for 30 minutes at RT the whole solution mixture was transferred into a 25 mL Teflon-lined autoclave, sealed tightly and heated at 423 K for 10 h. The final yields were washed with ethanol and water mixture for three times and dried in vacuum at 333 K to get dried UCNP powder.

2.2.3. Synthesis of NaYF4:Yb/Ho/Mn by varying the reaction temperature Same quantities of starting materials were taken as above. Here the reaction time and solvent (ET/OA) ratio were fixed to 10 h and 12 mL (6:6 mL) respectively. But the reaction temperature was varied from 373 K, 398 K, 423 K, 448 K and 473 K for 10 h. The final products of each were washed with ethanol and water mixture for three times and dried in vacuum at 333 K to get dried UCNP powder.

2.2.4. Synthesis of NaYF4:Yb/Ho/Mn at optimized reaction conditions Same quantities of starting materials were taken as above. Based on the results obtained from the above mentioned experiments, the solvent ratio (ET/OA) was fixed to 6:6 mL. After mixing the starting materials for 30 minutes at RT the whole solution mixture was transferred into a 25 mL Teflon-lined autoclave, sealed tightly and heated at 473 K for 10 h. The final product was washed with ethanol and water mixture for three times and dried in vacuum at 333 K to get dried UCNP powder.

2.3. Characterization The UCL measurements were performed using Agilent Technologies Cary Eclipse Fluorescence spectrophotometer equipped with a 980 nm laser diode (CW, 500 mW) as an excitation source. The samples for UCL measurements were prepared by dispersing 1 mg of Page 4 of 26 ACS Paragon Plus Environment

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NaYF4:Yb/Ho/Mn powder in 1 mL solvent (ethanol) followed by sonication for 10 min. Photographic UCL images of colloidal nanophosphors were taken by a digital camera (Sony α-6000) without adding any filter. The structural data were collected on a Rigaku Smart Lab 9 kW powder x-ray diffractometer (PXRD) with Ni-filtered Cu Kα irradiation (λ = 0.1542 nm) at 45 kV and 100 mA in 2θ ranging from 15o – 70o with a scan rate of 2o per minute with stepping size of 0.02o at room temperature. Morphology of the samples was investigated by using scanning electron microscope (SEM), FEI Nova Nano SEM-450 and transmission electron microscope (TEM), FEI Tecnai G2 20 S-twin microscope operating at 200 kV. Energy dispersive x-ray spectra (EDAX) were obtained using the same SEM and TEM instruments and elemental mapping were also performed using SEM and TEM instrument. Xray photoelectron spectroscopic (XPS) measurements were performed using SPECS instrument with a PHOBIOS 100/150 delay line detector (DLD) with 385W, 13.85 kV and 175.6 nA (sample current). We have used Al Kα (1486.6eV) dual anode as the source. The XPS was taken with pass energy of 50 eV. C-1s peak (284.5 eV) was used as an internal reference. The quantitative estimate for the elemental analysis was performed using CASA utilizing the survey scans of the specimens. The high resolution XPS data were deconvoluted to understand the different electronic states that are present in the particular element, using the CASA software. The baseline correction for each spectra was initially carried out using a Shirley background correction. The deconvolution then was carried out using a combination of Gaussian and Lorentzian curve [GL(30)- meaning Gaussian percentage is 30] in order to find out the chemical state and compound of main elements present in the passive layer.

3. RESULTS AND DISCUSSION Room temperature UCL of UCNPs synthesized under various experimental conditions was performed using 980 nm NIR laser as an excitation source. Figure 1 shows the luminescence spectra of NaYF4:Yb/Ho/Mn nanoparticles synthesized at different reaction conditions. All the samples exhibit three distinct peaks in the range of 450-670 nm. The blue emission at 485 nm was assigned to 5F3 → 5I8, the green emission at 540 nm was attributed to 5

S2 → 5I8 and the red emission at 645 nm was due to 5F5 → 5I8 transitions of Ho3+ ion.27, 40-41



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Figure 1. Room temperature UCL spectra of NaYF4:Yb/Ho/Mn NPs: (a) acquired from samples prepared using different solvent (ET/OA) volume ratios, (b) obtained from samples prepared using different time periods, (c) collected from samples prepared at different temperatures, and (d) UCL comparison of optimized sample with the other best samples synthesized by changing experimental parameters. Ethanol suspension 1 mg mL-1; excited at 980 nm. The upconversion emission intensities are drastically changed with the alteration of reaction conditions. The upconversion emission intensities in three (blue, green and red) regions were increased with the increment of ET/OA ratio from 0.2 (2:10 mL) to 1 (6:6 mL) while the reaction temperature and time were kept constant as 423 K and 10 h, respectively. Upon further increase of ET/OA ratio all the intensities weakened (shown in Figure 1a). Interestingly, the strongest luminescence was observed in the sample synthesized with the solvent (ET/OA) ratio of 6:6 mL and the luminescence intensity was fivefold higher than the intensity of the next best sample. This proves that the suitable ET/OA ratio, used for the synthesis of best luminescence UCNP, should be 1. Subsequently, we have altered the reaction time by keeping the solvent (ET/OA) ratio (8:4 mL) and reaction temperature (423 K) as constant. Here, we have adopted the synthesis procedure from our previous report.14 Hence, except the changing of one parameter, we have maintained same reaction conditions rather than implying best conditions (for example ET/OA ratio as 1), while studying the


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influence of reaction time and temperature on NaYF4:Yb/Ho/Mn nanophosphors luminescence. During the variation of reaction time, all three distinct emission peaks’ intensity was enhanced with the increase in reaction time from 2 h to 10 h. The brightest luminescence was observed in the sample heated for 10 h, which was at least two times brighter than other samples, as shown in Figure 1b. This demonstrates that the 10 h reaction time was appropriate for obtaining bright luminescent UCNPs. Then, we have kept solvent (ET/OA) ratio (8:4 mL) and reaction time (10 h) as constant and changed the reaction temperature from 373 K to 473 K. Obviously, the UCL emission intensities of the peaks were improved along with the augmentation of reaction temperature from 373 K to 473 K. UCNPs obtained after heating at 473 K shown enhanced UCL than other samples acquired on heating at different temperatures. UCL of the sample prepared at 473 K is two times greater than rest of the samples synthesized at lower temperatures, which are displayed in Figure 1c. This reveals that 473 K is the optimal reaction temperature for the preparation of high luminescent UCNP.

Figure 2. a) Room temperature upconversion emission spectra of NaYF4:Yb/Ho/Mn nanophosphors obtained from 6:6 mL of ET/OA after heating at 473 K for 10 h (dispersed in ethanol 1 mg mL-1). The inset shows the corresponding UCL photographic image. b) Schematic representation of the energy level diagram showing the possible upconversion mechanism of NaYF4:Yb/Ho/Mn nanophosphors. Based on the obtained results, it could be evidenced that the solvent (ET/OA) ratio of 6:6 mL, reaction temperature of 473 K for 10 h time are the best reaction conditions for the synthesis of brighter UCL NaYF4:Yb/Ho/Mn nanoparticles. This may be attributed to the enhanced synergistic interactions between the solvent (ET) and the capping agent (OA), when used in equivalent ratios. As one can see from Figure 2a, when excited at 980 nm, three UC bands with maxima at blue (485 nm), green (540 nm) and red (645 nm) regions can be observed from optimized NaYF4:Yb/Ho/Mn nanophosphors, which corresponding to 5F3 → Page 7 of 26 ACS Paragon Plus Environment


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I8, 5S2 → 5I8 and 5F5 → 5I8 transitions of Ho3+ ions. Photographic image of as prepared

UCNP colloidal solution was taken on excitation with 980 nm laser (shown in inset of Figure 2a). The mechanism for the emission of red colour is shown in Figure 2b and has been explained in detail in our previous article.14 The formulae (eq. 1 to eq. 9, in the supporting information) show the specific energy transitions of the Yb3+, Ho3+ and Mn2+ ions.42 As prepared nanophosphors UCL was measured and compared with brighter samples obtained through altering of different reaction conditions (Figure 1d). Optimized UCNPs luminescence was almost 3 times brighter than rest of all samples.

Figure 3. UCL photographic images of NaYF4:Yb/Ho/Mn nanophosphors: (a1) synthesized with different volume ratios of ET/OA at reaction temperature of 423 K for 10 h, (b1) obtained from 8:4 mL of ET/OA after heating for different reaction times at 423 K and (c1) obtained from 8:4 mL of ET/OA after heating at different reaction temperatures for 10 h. All the samples were dispersed in ethanol (1 mg mL-1) and irradiated with 980 nm laser having a power of 500 mW. Intensity of red emission as a function of (a2) solvent (ET/OA) ratio, (b2) reaction time and (c2) reaction temperature. Page 8 of 26 ACS Paragon Plus Environment

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UCL photographic images of as prepared NaYF4:Yb/Ho/Mn nanophosphors colloidal suspension are shown in Figure 3 (a1), (b1) and (c1). For precise comparison of emission intensity, all the samples concentration was maintained same as 2 mg mL-1 by dispersing in ethanol and same conditions were maintained. Figure 3a1 conveys the information about emission intensity fluctuations with solvent ratio difference during the synthesis of nanophosphors, where it can be clearly observed that the sample with ET/OA ratio of 6:6 mL is giving bright emission compared to other samples. The photographic images of luminescence intensities of the samples are in agreement with the graph plotted between the solvent ratio verses intensity (Figure 3a2). In a similar way Figure 3b1 shows the emission intensity changes with varying the reaction time of nanophosphors, where the nanophoshors obtained with 10 h heating are showing bright luminescence than other samples heated for lower time. Figure 3b1 photographic images are clearly in agreement with Figure 3b2, the graph plotted between the reaction time verses intensity. Figure 3c1 proves that the UCL emission intensity was higher in case of particles prepared at 473 K compared to rest of the samples prepared at different temperatures, which was again proved in reaction temperature graph plotted against UCL intensity (Figure 3c2).

Figure 4. Powder x-ray diffraction pattern of NaYF4:Yb/Ho/Mn nanoparticles: (a) Page 9 of 26 ACS Paragon Plus Environment


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synthesized with different volume ratios of ET/OA at reaction temperature of 423 K for 10 h, (b) obtained from 8:4 mL of ET/OA after heating for different reaction times at 423 K, (c) obtained from 8:4 mL of ET/OA after heating at different reaction temperatures for 10 h and (d) obtained from 6:6 mL of ET/OA after heating at 473 K for 10 h. The crystal phases of NaYF4:Yb/Ho/Mn nanophosphors synthesized by varying experimental parameters were systematically investigated by powder x-ray diffraction (XRD). Figure 4 displays the XRD patterns of the nanophosphors synthesized under different reaction conditions. Diffraction patterns in Figure 4a shows that with the increment of solvent volume (ET/OA) ratios from 2:10 mL to 8:4 mL results in the transformation of phase from cubic to hexagonal. NaYF4:Yb/Ho/Mn nanoparticles obtained from 2:10 mL ET/OA ratio are in cubic phase (JCPDS card No. 77-2042) and with the increasing of this ratio to 8:4 mL was lead to hexagonal phase transformation, which also correspond to the literature data (JCPDS card No. 28-1192).35 The crystal structure of cubic phase is determined with lattice parameter of a=0.547 nm, space group Fm-3m, whereas, hexagonal phased crystal structure was determined with lattice parameters of a=0.596 and c=0.353 nm, space group P63/mmc.43-44 Though the solvent (ET/OA) ratio of 10:2 produced hexagonal phased particles, but it also has some impurity peaks, for example at theta value of 23.2o, it could be the MnF3 peak (PDF card No. 00-009-0134). Figure 4b shows that the variation of reaction time also results in the phase transformation from cubic to hexagonal. At lower reaction time XRD patterns of NaYF4:Yb/Ho/Mn nanoparticles could be ascribed to a mixture of cubic and hexagonal phases of NaYF4. Upon increase in the reaction time it can be seen that the hexagonal phase increased while cubic phase decreased along with this MnF3 peak at 23.2o also vanished. Interestingly, on heating for 10 h, all the diffraction peaks were matched well with the peaks of hexagonal NaYF4. Figure 4c presents the XRD patterns of NaYF4:Yb/Ho/Mn nanophosphors obtained by heating for 10 h at different reaction temperatures with ET/OA volume ratio of 8:4 mL. Mixture of cubic and hexagonal phases of NaYF4:Yb/Ho/Mn nanoparticles were obtained on heating at lower temperatures. Upon increasing the reaction temperature hexagonal phase NaYF4 nanoparticles were predominantly obtained. An overall observation evidences that higher temperature and longer reaction time largely produces hexagonal phase NaYF4 particles. XRD pattern of UCNPs as synthesized with optimized reaction conditions (ET/OA ratio of 6:6 mL, 473 K, 10 h) is shown in Figure 4d, which indicates that hexagonal phase of NaYF4 predominantly persists, although few reflections corresponding to cubic phase NaYF4 could also be evidenced. Optical properties of UC nanocrystals can be significantly Page 10 of 26 ACS Paragon Plus Environment

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influenced by the variation of crystal structure in the host material. Usually, hexagonal phase host (or NaYF4) exhibits more UC efficiency than cubic phased one.45 This lattice phase dependent UC property can be attributed directly to the surrounded various crystal fields on trivalent lanthanide ions present in matrices of various symmetries. Compared to high symmetry hosts, low symmetry hosts characteristically employ a crystal field containing more irregular components around the dopant ions. The uneven (or irregular) components increase the electronic coupling between lanthanides 4f energy levels and higher electronic configuration and then proliferates f-f transition possibilities of the dopant ions, which eventually enhances the emission intensities in UC nanocrystals. Hence, lanthanide doped hexagonal phase NaYF4 exhibits higher luminescence efficiency than cubic crystal.1, 46-47 On further investigation to determine the presence of the constituent elements on the surface of nanophosphors, XPS studies were performed. Here, NaYF4:Yb/Ho/Mn nanophosphors obtained from 8:4 mL of ET/OA solvent volume ratio on heating at 423 K for 10 h, was taken as a representative sample and compared with NaYF4:Yb/Ho synthesized under same conditions, as the XPS studies for the NaYF4 system with different dopants of Yb/Ho/Mn and Yb/Ho would be very interesting to observe. The XPS spectra of NaYF4:Yb/Ho/Mn nanophosphors is shown in Figure 5 and the corresponding parameters are presented in Table 1, while the XPS spectra of NaYF4:Yb/Ho nanophosphors and its parameters are presented in Figure S1 and Table S1 in the supporting information section. Other characterizations of NaYF4:Yb/Ho nanoparticles were presented and explained in our earlier article.14The presence of all the constituent elements can be clearly evidenced from the survey spectrum and is also summarized in the tables. The atomic composition of Na is almost equal to the compositions of Y, Yb, Ho and Mn combined together. Fluorine atom percentage is almost 4 times higher than that of sodium, which is complementing with the expected initial atomic percentage of Na(1)Y(0.38)Yb(0.2)Ho(0.02)Mn(0.4)F(4)

nanocrystal.

The

powder

of

the

NaYF4:Yb/Ho/Mn would definitely possess adsorbed O-atom on its surface (physisorbed O2) as the sticking coefficient of O-atom is the maximum amongst the ones found in the XPS atmosphere. Therefore, from the XPS studies certain amount of physisorbed O2 is definitely expected. The excess amount of O could be ascertained to the surface -OH group, which would be inevitably present in the system. Similarly, the minor discrepancy observed in the atomic and mole percent of these elements could also be due to the fact that the XPS would only account for the elements that are present on the surface. However, it is not able to



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completely discern the bulk elemental composition which could be a different value as found from that of the surface. Table 1. Composition of different elements present on the surface of the NaYF4:Yb/Ho/Mn nanoparticles. Sample

NaYF4:Yb/Ho/Mn

Element

XPS peak (eV)

Atom %

O(1s)

529.63

41.892

F(1s)

687.70 and 685.70

36.986

Na(1s)

1074.60 and 1073.10

9.003

Y(3p)

316.85 and 304.94

5.753

Ho(4d)

161.94

4.543

Yb (4d)

349.50 and 346.10

0.525

Mn(2p)

655.60 and 643.90

1.294


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Figure 5. XPS spectra of NaYF4:Yb/Ho/Mn nanophosphors (a) survey spectrum, (b) Mn-2p, (c) Ho-4d, (d) Na-1s, (e) F-1s, (f) Yb-4p and (g) Y-3p. In Figure 5, (a) shows the survey spectrum and (b) to (g) show the XPS spectrum of the different elements present on the surface of NaYF4:Yb/Ho/Mn nanoparticles. It has been evidenced that the doping of Mn along with Yb/Ho alter the lattice structure of NaYF4 from Page 13 of 26 ACS Paragon Plus Environment


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cubic to hexagonal.14 With this in the backdrop, our XPS results would be explained to understand the alteration in the basic electronic structure as a function of different dopants and their alteration in the structural symmetry. The Figure 5b shows the XPS spectrum for that of Mn-2p, wherein the Mn 2p3/2 and Mn 2p1/2 peaks are obtained at 643.90 eV and 655.60 eV, respectively. Along with that, it has the signature to mulitplet peak splitting that is commonly observed for Mn2+.48-49 Therefore consistent with the previous literature,48-49 the presence of the Mn2+ as a dopant in the NaYF4 system could be envisaged from the XPS data. Figure 5c shows the Ho-4d peak at 161.94 eV for NaYF4:Yb/Ho/Mn nanoparticles, which is lower than the value (163.40 eV) determined for the Ho-4d peak for NaYF4:Yb/Ho nanoparticles (refer Figure S1b). The 163.4 eV peak for the Ho-4d represents the presence of Ho3+ in the nanoparticles in agreement with past literatures.50-51. However, upon Mn2+ being doped in the NaYF4 system along with Yb/Ho the binding energy of the Ho3+ gets lowered down. This would probably be due to the fact that when Mn2+ is being doped in the Y3+ sites possess lower valency as compared to that of Y3+. Therefore in order to maintain the lattice charge balance perhaps some of the Ho-4d could be missing. This will reflect in the system possessing lower binding energy with a greater FWHM, which could be seen in the Ho-4d spectra of NaYF4:Yb/Ho/Mn nanoparticles. The XPS spectrum of Na-1s of NaYF4:Yb/Ho/Mn nanoparticles shows contributions from two peaks at 1074.60 eV and 1073.10 eV (refer Figure 5d), while the corresponding spectrum of NaYF4:Yb/Ho nanoparticles shows only one peak at 1076.10 eV (refer Figure S1c). As mentioned earlier, on doping Mn along with Yb/Ho the system alters from cubic to that of hexagonal structure depending upon the concentration of Mn in the system; however the hexagonal system has two different types of Na+ ions present with different coordination sites. In one position it will be cubic mainly coordinated with the F- ions and at the other site it will be trigonal bonded with other Na+.14 Therefore the electronic density around this alkali metal in these two different coordinated Na+ present in the hexagonal unit cell of NaYF4:Yb/Ho/Mn nanoparticles will be different leading to two XPS peaks of Na+. The F-1s XPS spectrum presented in Figure 5e for NaYF4:Yb/Ho/Mn nanoparticles shows two peaks at 687.70 eV and 685.70 eV, whereas for the NaYF4:Yb/Ho nanoparticles only a single peak is obtained at 689.30 eV (see Figure S1d). The substitutional doping of Mn2+ replacing Y3+ in the NaYF4 system will lead to the formation of lattice charge imbalance. In order to maintain the lattice charge neutrality, one of the different processes that could occur would be an anionic vacancy leading to the formation of F- ion vacant sites. However, the formation of the anionic vacancy implies formation of non-stoichiometry at the Page 14 of 26 ACS Paragon Plus Environment

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F site. Normally the non-stoichiometry at the anionic site results to the lowering of the binding energy of the anion, which is reflected in the XPS spectra of F-1s. The two different peaks for the F-1s in NaYF4:Yb/Ho/Mn nanoparticles would be due to the F- vacancy and the lattice coordinated F-. Based on the obtained results, it can be ascertained that Mn doping does not affect the Yb sites at all, as peaks corresponding to Yb-4p occurs at 349.50 eV and 346.10 eV in both NaYF4:Yb/Ho/Mn and NaYF4:Yb/Ho nanoparticles, as evidenced from Figure 5f and S1e, respectively. However, the XPS peak for the Y-3p shows a lowering in the binding energy upon Mn doping into the system. The Y-3p peaks occur at 316.85 eV and 304.94 eV in NaYF4:Yb/Ho/Mn nanoparticles (see Figure 5g), while the corresponding peaks could be evidenced at 318.28 eV and 306.35 eV in NaYF4:Yb/Ho nanoparticles (refer Figure S1f). This lowering of the binding energy due to Mn doping could also be attributed to the fact that Y3+ being present in both cubic and hexagonal lattice sites.



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Figure 6. (a) SEM image of UCNPs obtained from 6:6 mL of ET/OA after heating at 473 K for 10 h with associated (b) Na (orange), (c) Y (green), (d) F (Blue), (e) Yb (chartreuse green), (f) Mn (purple) and (g) Ho (red) elemental mappings and (h) energy dispersive x-ray (EDAX) spectrum.


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Figure 6a shows SEM image of NaYF4:Yb/Ho/Mn nanoparticles, which are obtained from optimized reaction conditions as ET/OA solvent volume ratio of 6:6 mL on heating at 473 K for 10 h. The presence of the constituent elements, sodium (Na), yttrium (Y), fluoride (F), ytterbium (Yb), manganese (Mn) and holmium (Ho) were confirmed by SEM elemental mapping of NaYF4:Yb/Ho/Mn nanoparticles and are presented in Figure 6b, c, d, e, f and g, respectively. Moreover, these elements are uniformly distributed throughout the particles. The existence of all the elements is further proved by the energy dispersive X-ray (EDAX) spectrum of the same particles (Figure 6h). SEM images of UCNPs prepared by altering the solvent volume ratio are shown in Figure S2 a) to e) in the supporting information. Due to particle agglomeration, the exact shape of the formed particles could not be vividly evidenced in the samples prepared with ET/OA ratio of 2:10 (Figure S2 e1 and e2) and 4:8 mL (Figure S2 d1 and d2). But, it can be perfectly evidenced that upon increasing ET/OA ratio, wherein the particles show clear change in morphology till the 6:6 mL (Figure S2 c1 and c2) and on more increment in the solvent ratio the particles tend to aggregate (see Figure S2 a1, a2 and b1, b2). In all the samples, all the constituent elements were present (Figure S2 a3 to e3). To further investigate the effect of reaction time on the morphology of the particles, we have changed the reaction time from 2 h to 10 h, in the synthesis. At low reaction time (2 h and 4 h), particles were not formed clearly or aggregated (Figure S3 a1, a2, b1 and b2). At high temperatures the particles were clearly separated, which can be evidenced with SEM images in Figure S3 c1, c2, d1, d2, e1 and e2. EDAX spectra of all the samples (Figure S3 a3 to e3) prove the presence of all the constituent elements in the samples. In order to understand the effect of reaction temperature of the morphology of the particles, we have synthesized NaYF4:Yb/Ho/Mn nanoparticles at different reaction temperatures from 373 K to 473 K. With the increment of reaction temperature, particles are taking hexagonal morphology (see Figure S4). At 473 K predominantly hexagonal shaped UC particles can be clearly seen in Figure S4 e1 and e2. Respective EDAX spectra (Figure S4 a3 to e3) confirm the presence of the constituent elements in all the samples.



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Figure 7. (a) TEM image, (b) HRTEM image (c) selected area electron diffraction (SAED) patterns, (d) STEM-HAADF image of NaYF4:Yb/Ho/Mn nanoparticles obtained from 6:6 mL of ET/OA after heating at 473 K for 10 h with associated (e) Na (chartreuse green), (f) Y (grey), (g) F (yellow), (h) Yb (green), (i) Ho (red), (j) Mn (purple) elemental mappings and (k) energy dispersive x-ray spectrum.


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UCNPs obtained from 6:6 mL ET/OA solvent volume ratio after heating at 473 K for 10 h were characterized by TEM. Figure 7a shows the TEM image of NaYF4:Yb/Ho/Mn nanoparticles, which clearly shows the hexagonal morphology of the particles. High resolution (HR) TEM image is shown in Figure 7b revealing the hexagonal NaYF4 structure, with a lattice distance of 0.24 nm corresponding to the d-spacing for the (200) lattice plane of the particles. The selected area electron diffraction (SAED) patterns shown in Figure 7c clearly displays the diffraction rings that match with the specific planes of hexagonal NaYF4 lattice. Figure 7d shows STEM-HAADF image of two NaYF4:Yb/Ho/Mn nanoparticles, which are obtained from optimized reaction conditions as ET/OA solvent volume ratio of 6:6 mL on heating at 473 K for 10 h. Constituent elements, sodium (Na), yttrium (Y), fluoride (F), ytterbium (Yb), holmium (Ho) and manganese (Mn) presence was again confirmed by TEM elemental mapping of NaYF4:Yb/Ho/Mn nanoparticles and are shown in Figure 7e, f, g, h, i and j respectively. Moreover it is proved that, these elements were uniformly distributed throughout the particle. The existence of all the elements is further proved by the energy dispersive X-ray (EDAX) spectrum of the same particles (Figure 7k).

4. CONCLUSIONS In summary, an effective strategy was developed to improve the red emitting efficiency of NaYF4:Yb/Ho/Mn upconversion nanophosphors by altering the reaction conditions. The influence of synthesis parameters, such as solvent ratio between ethanol and oleic acid, reaction temperature and reaction time were investigated and the best suitable conditions for the preparation of highly luminescent nanophosphors were determined. As synthesized NaYF4:Yb/Ho/Mn nanophosphors were characterized for their upconversion luminescence, phase and morphology, as a function of reaction conditions. The upconversion luminescence spectra of as prepared samples demonstrated that the equal ratio (6:6 mL) of ethanol and oleic acid offers fivefold enhanced luminescence than other volume ratio combinations, reaction temperature of 473 K produced more brighter red emitting particles compared to the particles prepared at lower reaction temperatures and 10 h reaction time offered more than two fold brighter luminescence in comparison to lower reaction timings. Overall, the particles prepared using optimized reaction conditions produced three times brighter red luminescent NaYF4:Yb/Ho/Mn nanophosphors compared to rest of the reaction parameters. Based on powder XRD characterization, it can be clearly observed that the higher temperature and longer reaction time predominantly produces hexagonal phase NaYF4 particles. The presence of all the constituent elements on the surface of the nanophosphors Page 19 of 26 ACS Paragon Plus Environment


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was proved by XPS investigations. SEM images clearly show the hexagonal morphology of NaYF4:Yb/Ho/Mn nanophosphors. Elemental mapping and EDAX analysis confirm the presence of all the constituent elements. TEM, HRTEM and SAED investigations of NaYF4:Yb/Ho/Mn nanophosphors also confirm the hexagonal morphology and hexagonal phase by means of the corresponding fringes and pattern, respectively. The improvement of nanophosphors crystallinity can be claimed as the main origin for the red emission increment. This work could pave way for the versatile use of these bright red emitting upconversion nanophosphors in biomedical applications.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI:10.1021/acs.jpcc.xxxxxx. The energy transition formulas of Yb3+, Ho3+ and Mn2+ dopant ions, The elemental concentration of different elements present on the surface of the NaYF4:Yb/Ho nanoparticles (Table 1), XPS studies for (a) Na-1s, (b) F-1s, (c) Ho-4d, (d) Yb-4p, (e) Y-3p and (f) Mn-2p of NaYF4:Yb/Ho nanophosphors (Figure S1), SEM images of NaYF4:Yb/Ho/Mn nanoparticles synthesized with different volume ratios of ET/OA at reaction temperature of 423 K for 10 h and their energy dispersive x-ray spectra (EDS) (Figure S2), SEM images of NaYF4:Yb/Ho/Mn nanoparticles obtained from 8:4 mL of ET/OA after heating for different reaction times at 423 K and their EDAX spectrum (Figure S3) and SEM images of NaYF4:Yb/Ho/Mn nanoparticles obtained from 8:4 mL of ET/OA after heating at different reaction temperatures for 10 h and their EDAX spectrum (Figure S4). This information is available free of charge via the Internet at http://pubs.acs.org

AUTHOR INFORMATION Corresponding Author *Email: [email protected] ORCID Kumbam Lingeshwar Reddy: 0000-0001-6885-1328 Vaddepally Srinivas: 0000-0002-9346-8993 Konathala Ravi Shankar: 0000-0003-1837-7271 Suneel Kumar: 0000-0002-5259-1792 Vipul Sharma: 0000-0002-4460-4610 Page 20 of 26 ACS Paragon Plus Environment

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Ajay Kumar: 0000-0001-8775-4486 Ashish Bahuguna: 0000-0001-5542-1708 Venkata Krishnan: 0000-0002-4453-0914 Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS We are thankful to Advanced Materials Research Centre (AMRC), IIT Mandi for laboratory and the characterization facilities. VK acknowledges the financial support from DST, India under INSPIRE faculty grant (IFA-11CH-10). KLR, AK and VS acknowledge MHRD, India for doctoral scholarship. KRS and SK acknowledge research fellowship from UGC, India. AB acknowledges CSIR, India for senior research fellowship.

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TOC Graphic:


Enhancement of Luminescence Intensity in Red Emitting NaYF4:Yb

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