Designing Dual Emissions via Co-doping or Physical Mixing of

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Designing Dual Emissions via Co-doping or Physical Mixing of Individually Doped ZnO and Their Implications in Optical Thermometry Subrata Senapati, and Karuna Kar Nanda ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 27 Apr 2017 Downloaded from http://pubs.acs.org on April 29, 2017

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ACS Applied Materials & Interfaces

Designing Dual Emissions via Co-doping or Physical Mixing of Individually Doped ZnO and Their Implications in Optical Thermometry Subrata Senapati and Karuna Kar Nanda* Materials Research Centre, Indian Institute of Science, Bangalore – 560012, India. Tel. +91-080-2293 2996, Fax: +91-80-2360 7316, *E-mail: [email protected]

ABSTRACT: Here, we report on the novel design of dual emission via defect state engineering in co-doped oxide microstructures and its implication in fluorescence intensity ratio (FIR) based optical temperature sensing. Eu and Er co-dped ZnO (EuEr:ZnO) microrods prepared by hydrothermal method. The emission peaks corresponding to Eu3+ and Er3+ are observed suggesting dual emission from co-doped ZnO. Interestingly, Er3+ peak intensity decreases and that of Eu3+ increases with increase of temperature as is the case of individual doped cases and dual emission is also achieved via phyical mixing of the individual doped ZnO. The opposite trend is due to the electron transfer from the defect levels of host ZnO to Eu3+ and not to Er3+. Overall, our results pave the way in designing dual emission that can be exploited in FIR based temperature sensing. As an example, we probe temperature dependency of congo-red and polyvinyle alcohol (PVA) composite using EuEr:ZnO as optical probe for temperature sensing.

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KEYWORDS: Dual emission, Co-doping, Physical mixing, Electron transfer, Ratiometric sensing

INTRODUCTION Nanostructures with dual emission that show two well resolved intense peaks originated from two different excited sates of the materials, have attracted considerable attentions in the field of self-referenced ratio metric temperature sensing, ratio metric fluorescent pH probe and metal ion detections, single-phosphor in solid state lighting, photo-switchable marker for bio-imaging, etc.1-5 Though co-doping and wavelength mixing can yield dual emission, the common approach for it is to combine band gap engineering and doping 6-14 which is highly material specific (CdSe, ZnS/ZnSe). The general trend to achieve dual emission in selenides is either to insert transition metal ions (Cd2+, Mn2+, etc.) or to create multi core-shell structures.7,11,13,15,16 In such cases, the band gap and hence, the dual emission is very sensitive to the size/shell thickness of the nanostructures and the control over the size/thickness is crucial. On the other hand, the size-dependency of band gap is very weak for wide-band-gap oxides.17 In addition, the formation energy of oxygen vacancy in oxide materials is low creating defect levels within the energy gap that controls the emission. The color of emission can further be tuned by doping appropriate rare-earth (RE) ions (Eu, Er, Tb, Tm, etc.) which is expected to lead to interesting temperature-dependency of luminescence properties as the energy levels of different RE ions can be above or below the defect levels.18,19 Overall, the temperature dependency of co-doped oxides can be exploited for optical thermometry.


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Dual emitting materials have prominent advantages for use in self-reference fluorescence intensity ratio (FIR) based optical temperature sensing, where the intensity of emissions from two luminescence centers of a single material varies differently with temperature. FIR between two well resolved intense peaks rather than single prominent intense peak for temperature sensing application overcome various undesirable factors such as variation of probe concentrations, local environmental inhomogeneity, optoelectronic drift, etc.20-27 Dual emissions can be achieved by either co-doping various metal ions such as RE ions (Eu, Er, Tb, Pr, etc.) or transition metal ions (Mn, Cd, etc.) into different hosts, or mixing two different color emitting materials, or by controlling excitonic emissions of various materials.6,814,28,29

With respect to the optical thermometry, it is desirable to have dual emissions with

opposite trend in temperature dependency. Though dual emissions with opposite trend in temperature dependency have been achieved by researchers, the sensitivity and the operating temperature range is very low.6,9,10,30 In this context, it can be noted for Eu doped ZnO (Eu:ZnO) that the red emission intensity increases,18 while for Er doped ZnO (Er:ZnO), the green emission intensity decreases19 with increase in temperature. This inspired us to explore the possibility of achieving dual emissions by co-doping (Eu and Er) in ZnO and by physical mixing of individual doped ZnO and to investigate temperature-dependent luminescence keeping FIR based temperature sensing in mind. Various FIR based thermometry based on the intensity ratio between the two closely spaced thermally coupled levels (TCL) of a single RE ion have been reported that hinders proper signal discriminability due to overlapping of the peak signals and sensitivity is compromised.31,32 The advantages of employing two RE ion emissions with well separated peaks and opposite temperature dependency is to avoid overlapping and obtain desired signal discriminability, so that the limitations of sensitivity can be defeated.31,32


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Here, we report here a new approach for the novel design of dual emissions via defect state engineering and show the importance of excitation wavelength in realizing the dual emission. The co-doped ZnO shows emission peaks corresponding to Eu3+ and Er3+ only when excited with 532 nm unraveling the dual emissions. Interestingly, the intensity of Er emission decreases while that of Eu increases with temperature as is the case of individual doped cases and dual emission is achieved via phyical mixing of the individual doped ZnO. The electron transfer from the defect levels of ZnO host to Eu3+ and not to Er3+ is the key for the observed temperaturedependency. Overall, our results hold promise in designing dual emission and the temperaturedependency in various nanostructures. In this context, it is worthy to note that the observed temperature-dependency in co-doped ZnO is prerequisite for highly sensitive temperature sensing. The sensitivity is found to be comparable or superior as compared to that reported in literature and can be used in wide temperature range. As an example, temperature-dependency of congo red (CR) and poly vinyl alcohol (PVA) composite is explored using EuEr:ZnO as optical probe. Experimental detail Material Synthesis Hydrothermal synthesis of individual doped ZnO has been reported elsewhere.18,19 Rare earth ions (Eu3+ and Er3+) co-doped ZnO (EuEr:ZnO) microrods are synthesized by hydrothermal method using hydrate

zinc nitrate hexahydrate (Zn(NO3)2.6H2O), europium acetate

(Eu(CH3CO2)3·H2O),

erbium

acetate

hydrate

(Er(CH3CO2)3·H2O),

hexamethylenetetramine (HMT) (C6H12N4) as precursors. In a typical process for the synthesis of Eu3+/Er3+ co-doped ZnO, appropriate amount of Zn(NO3)2.6H2O, Eu(CH3CO2)3·H2O,


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Er(CH3CO2)3·H2O and HMT are dissolved in deionized water to obtained a molar ratio of Zn2+:(Eu3++Er3+):HMT :: 1:0.025:1 and concentration of total metal ions in the solution was kept at 0.1 M by keeping the concentration of Eu3+ to Er3+ in solution as 1:1. Then the solution was transferred to a 50 ml Teflon-lined autoclave, it was filled nearly up to 80% of its capacity. The autoclave is inserted into a muffle furnace and kept at 180 °C for 12 h. After reaction is over, autoclave is allowed to cool down naturally to room temperature. The white color samples obtained in powder form was collected from the container and washed several times with deionized water and ethanol, then dried under an incandescent bulb for overnight at 60 °C. The dried samples are taken to accomplish further characterization and analysis. Characterization Electron microscope images of the samples are obtained by field emission scanning electron microscope (FESEM, FEI Inspect F50) and transmission electron microscopy (TEM, JEM 2100F). Chemical composition was determined by energy dispersive X-ray (EDX) spectroscopy (Oxford Instrument attached with SEM, FEI) and X-ray photoelectron spectroscopy (XPS, Kratos Axis Ultra). Phase of the sample was determined by X-ray diffractometer (XRD, Philips Panalytical). Room temperature and temperature dependent Photoluminescence (PL) measurements were carried out using WITec instrument (alpha 300) with 355 and 532 nm laser as excitation sources. PL excitation (PLE) measurement is carried out using Horiba Jobin Yvon fluoromax-4 spectrofluorometer equipped with Xe lamp. Time resolved PL is carried out with Horiba Jobin Yuvon Fluorocube. Temperature dependent PL was carried out by keeping sample inside a chamber connected to programmable temperature controlling software (Linkam THMS 600). The temperature is stabilized for about 4-5 minute before acquiring the PL spectra.


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Results and Discussions

(a)

(d)

(f)

(c)

(b)

(e)

(g)

(h)

(i)

Figure 1. (a) FESEM image of EuEr:ZnO microrods, (b) TEM image of a single microrod, (c) corresponding HRTEM image and (d) SAED pattern, (e) EDX spectrum of EuEr:ZnO microrods, and (f–i) EDX elemental mapping of Zn, O, Eu and Er on a single microrod surface respectively.


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The FESEM image of EuEr:ZnO samples is shown in Figure 1a. The sample consists of microrods with average diameter around 700-800 nm and average length is around few microns. TEM image of a single EuEr:ZnO microrod is shown in Figure 1b. The HRTEM image shown in Figure 1c confirms that microrods are crystalline in nature, which is in accordance with the diffraction pattern shown in Figure 1d. Figure 1e shows the EDX spectrum of EuEr:ZnO, which shows that the nanorods contain Eu and Er along with Zn and O as its main elements. The Figure 1 (f–i) show the elemental mapping of Zn, O, Eu and Er carried out on a single microrod,

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(f)

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Zn-2p 1022.1

2p3/2

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EuEr:ZnO

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Zn 2p3/2

O 1s

Eu 3d3/2 Eu 3d1/2

Intensity (a.u.)

(103)

60

(200) (112) (201)

(110)

50

2θ θ (degree)

(c)

EuEr:ZnO

Er 4d

169.8

Intensity (a.u.)

40

EuEr:ZnO

Zn 3d Zn 3p Zn 3s Eu 4d Er 4d C 1s

(101)

(102)

30

(b)

EuEr:ZnO

(002)

(100)

Intensity (a.u.)

(a)

Zn 2p1/2

confirming the high uniformity in Eu and Er doping.

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1130 1140 1150 1160 1170 1180

160

Binding energy (eV)

170

180

190

Binding energy (eV)

Figure 2. (a) XRD pattern and (b) XPS survey spectrum of EuEr:ZnO microrods. (c) High resolution Zn-2p, (d) O-1s, (e) Eu-3d, and (f) Er-4d spectra of EuEr:ZnO.


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Figure 2a shows a typical XRD pattern of EuEr:ZnO microrods. The peaks can be indexed to the crystalline wurtzite ZnO structure (JCPDS 80-0074). Figure 2b shows a typical XPS survey scan of EuEr:ZnO. The survey scan shows the peaks corresponding to Zn 2p, Zn 3p, O 1s, C 1s, Eu 3d, Eu 4d and Er 4d. The double peaks in Figure 2c at 1022.1 and 1045.2 eV are ascribed to the core level Zn 2p3/2 and Zn 2p1/2 of ZnO.33-35 The high resolution O 1s peak shown in Figure 2d is fitted into three different Gaussian peaks, representing the presence of three kinds of oxygen species. The peaks at 530.3, 532.1 and 533.7 eV are due to the crystal lattice oxygen, oxygen deficiency and adsorbed oxygen species on the surface of the EuEr:ZnO nanorod.35,36 High resolution spectrum shown in Figure 2e displays the peaks at 1135.7 and 1165.8 eV corresponding to the core level Eu 3d5/2 and Eu 3d3/2 respectively, which indicates that Eu ion is in a +3 valence state.37,38 Figure 2f corresponds to a high resolution Er-4d spectrum. The peak at 169.8 eV represents the presence of Er in trivalent state in the sample.39,40 Atomic percentage of Eu and Er in EuEr:ZnO is calculated by fitting integrated peak area of corresponding high resolution spectra and using calibrated atomic sensitivity factor. The atomic percentage of Eu and Er is found to be 1.04 and 1.32 %, respectively. Investigation of PL spectra of EuEr:ZnO are accomplished using 355 and 532 nm excitations. Figure S1 and S2 shows typical room temperature PL spectrum of EuEr:ZnO, ZnO, Eu:ZnO and Er:ZnO microrods with 355 nm excitation. It is apparent from Figure S1 and S2 that EuEr:ZnO shows UV emission due to near band edge (NBE) exitonic recombination and a broad intense defect related deep level emission (DLE) in the visible region with peak centered around 390 and 510 nm, respectively.33,36 The occurrence of visible emission is due to the radiative recombination of photo-generated holes and defect state (mainly due to the oxygen vacancy)


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induced electrons.33,36 There are no apparent peaks corresponding to the Er and Eu related emissions with 355 nm excitation. Figure 3a shows a typical room temperature PL spectrum of EuEr:ZnO microrods with 532 nm excitations and the room temperature spectra of ZnO, Eu:ZnO and Er:ZnO with 532 nm excitation is presented in Figure S3 and S4, respectively for comparison. It is clear that various sharp intense peaks due to the intra-4F transitions of Eu3+ and Er3+ ions appear without any signature of the host. The whole spectrum in the Figure 3a can be divided into four regions as 530-570, 570-640, 640-670, 670-710 nm: the 530-570 and 640-670 nm regions correspond to Er3+, while 570-640 and 670-710 nm regions belong to Eu3+. Various peaks obtained for Eu3+ are at 578 nm (5D0→7F0), 588, 600 nm (5D0→7F1), 610, 617, 622 nm (5D0→7F2), and 684, 692, 700 nm (5D0→7F4).41,42 According to the selection rule, each levels can split into 2J+1(J = 0, 1, 2, etc.) sub-levels and hence, the 7F0 levels for Eu3+ has no splitting while 7F1 splits into 3 sublevels and so on.43,44 As a consequence, above mentioned main peaks along with several shoulder peaks arises for Eu3+ related emissions. Similarly, the transitions from various Er3+ exited state to ground state (4I15/2) give rises main peaks for Er3+ related emissions at 540 nm (2H11/2→4I15/2), 554, 565 nm (4S3/2→4I15/2), 661, 676 nm (4F9/2→4I15/2) along with various small peaks due to crystal field slitting.45


9


(a) (b)

5D →7F 2 0

Er3+

3

Eu3+ 2

5D →7F 0 1 5 D →7 F 4F →4I 4 9/2 15/2 0

1

Intensity (104 a.u.)

4 4 11/2 , S3/2 → I15/2

Intensity (a.u.)

2H

(K

)

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T

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Intensity (105 a.u.)

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λ (nm)

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(c)

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(d)

Er Eu

2.0 1.5 1.0 0.5 0.0

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Temperature (K)

Temperature (K) 6

(e)

(f)

5 4

I610/I565

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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3 2 1 0 100

200

300

400

500

Temperature (K)

Figure 3. (a) Room temperature PL of EuEr:ZnO rods at 532 nm excitation, (b) temperature dependant PL, (c, d) variation of Eu3+ (604-634 nm) and Er3+ (541-570 nm) integrated peak


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intensity with temperature, (e) variation of intensity ratio of Eu3+ and Er3+ peak with temperature, (f) optical photographs of EuEr:ZnO at different temperature with 532 nm excitation.

As there is only RE related various sharp emissions without any signature of host, we choose 532 nm as excitation wavelength and examine the temperature sensing ability of EuEr:ZnO. Figure 3b shows the temperature dependent PL spectra of EuEr:ZnO microrods in a temperature range of 83-493 K with an apparent dual temperature response. It is also evident that Eu3+ intensity increases and Er3+ intensity decreases with temperature in 83 to 493 K range, which is similar to Eu:ZnO and Er:ZnO (Figure S5). Interestingly, EuEr:ZnO shows prominent Er3+ 4f-transition peaks at very low temperature (83 K), while Eu3+ peaks are prominent at very high temperature (493 K). The variation of integrated peak intensities of Eu3+ (5D0→7F2 (610 nm), 604-634 nm) and Er3+ (4S3/2→4I15/2 (565 nm), 541-570 nm) transitions with temperature is shown in Figure 3c and d (as these are maximum intense peak corresponds to Eu and Er, respectively). The ratio between Eu3+ and Er3+ integrated peak intensity is plotted in Figure 3e, which clearly shows that intensity ratio increases gradually with temperature. The optical images of the emission clearly represent green Er emission at 83 K and red Eu emission at 473 K (Figure 3f). This gives an apparent visual expression of dual emitting behavior of EuEr:ZnO. Usually, the decrease in the PL intensity of the Er emission peaks46,47 is due to the temperature induced electron-phonon interaction caused by the increase in the non-radiative recombination similar to the Eu:ZnO and Er:ZnO under 355 nm excitation (Figure S6). However, the increase in the Eu3+ emission intensity is probably due to the phonon-assisted increase in the radiative recombination.18 With increase in the temperature, the phonons cause increase in the defect


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states population due to the thermally agitated electrons from the ground states.48 As a consequence, the total population in the defect states increases and transfers to the Eu3+ excited states. This enhances the radiative recombination and hence, the intensity increases with temperature. The integral PL intensity usually decreases with temperature due to the thermal quenching as 49 Iq=Iq0/[1+Aexp(-Eq/kBT)]

(1)

where I0 is the peak intensity at temperature T=0 K, A is a constant, Eq is the activation energy in the thermal quenching process, and kB is the Boltzmann constant. Similarly, increase of PL intensity can be achived due to the thermal population and is governed by Ip=Ip0exp (-Ep/kBT)

(2)

This assumes Boltzmann function with an activation energy of Ep.50 Fitting of curves (Figure 3c) with Equation 2 and 3 for Er and Eu peak intensity, yield Eq and Ep as 68.3 and 35.6 eV, respectively. Though dual emissions with opposite trends have been achieved and explored for temperature sensing, the sensitivity is very low and the range of temperature is limited.6,9,10,30 The sensitivity of ratio-metric temperature sensing with single dopant is low as the trend of temperature-dependency of peak intensities is the same. Therefore, our co-doped samples are of paramount importance, which can easily be exploited as self-referencing optical temperature probes. It is ascertained that the sensitivity S is defined as1 S = dX/dT


(3)

12

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and the relative sensitivity SR is defined as1 S R = (dX/XdT)

(4)

where X is the intensity ratio between two emission peaks and T is the absolute temperature. As derived from Figure 3d, the sensitivity is found to be temperature-dependent (Figure S7a) and the highest sensitivity is found to be 2.8x10-2 K-1 at 453 K and the maximum relative sensitivity is found to be 1.12% K-1 at 223 K (Figure S7b), which is comparable or higher compared to previously reported relative sensitivity7,9,10,14,51 for various dual-emitting-nanostructures (Table S1). The additional advantage associated with our EuEr:ZnO is the wide range of sensing applications. Based on the results on individual Eu:ZnO18 and Er:ZnO19 samples, it is assumed that the physical mixer of Eu:ZnO and Er:ZnO may also show identical response as the EuEr:ZnO. Compared to the co-doped ZnO, the flexibility of the tuning emission intensity is very high for the physical mixer. By taking different proportion of individually doped one, the emission intensity of the both Eu and Er can be varied to achieve temperature sensing property in wide range. Overall, the sensitivity can be tuned which is not possible for co-doped case having particular content of Eu and Er. In this contest, temperature-dependent PL is carried out for the physical mixer (Eu:ZnO+Er:ZnO with 1:1 in wt% and concentration of Eu and Er both are 1.25%) with 532 nm excitation. The mixer retains its individual properties showing dual emitting behavior as is the case of EuEr:ZnO. The temperature-dependent PL for mixer is shown in Figure S8a. It shows that Er3+ emission peak is stronger at 83 K and the intensity decreases accompanied by increase in Eu3+ peak with increase of temperature and the Eu3+ emissions become prominent at 493 K. Variation of integrated intensities of Eu3+ (5D0→7F2) and Er3+


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(4S3/2→4I15/2) transitions with temperature is shown in Figure S8b. The ratio of intensities (Eu3+/Er3+) with temperature is shown in Figure S8c for which the highest sensitivity is 3.12x10-1 K-1 at 433 K (Figure S9a) and the relative sensitivity is found to be 1.95 % K-1 at 413 K (Figure S9b). The values obtained here is higher as compared to the maximum sensitivity obtained for individual cases considering the FIR between thermalized levels which is 2.5×10-2 K-1 and 0.25×10-2 K-1 at 493 K for Er:ZnO and Eu:ZnO, respectively. Overall, not only the co-doped ZnO but the mixer can also be used as dual emitting sensors and the sensitivity can be optimized by monitoring the ratio of Er:ZnO and Eu:ZnO in the mixture. Figure S8d shows the optical images of Eu:ZnO and Er:ZnO physical mixer at different temperature with 532 nm excitation. Similar to the co-doped ZnO, the mixer also emits green color (corresponds to Er emission) at low temperature (83 K) and red color (corresponds to Eu emission) at high temperature (493 K). It is interesting to note that a yellow color emission corresponds to the mixer of green and red is observed at room temperature (293 K). It is well known that luminescence intensity of dyes is highly temperature-dependent. As a case study, we probe the temperature-dependent PL of PVA and CR composite using EuEr:ZnO as optical probes for temperature sensing. Figure 4a shows the temperature-dependent PL of the composite where the mentioned temperatures are derived from the calibration curve. Interestingly, the temperature is slightly higher/lower below/above room temperature as compared to that recorded below the glass substrate, which is probably due to the heat dissipation from the glass substrate to the surrounding environment.3 Dependency of PL intensity with calculated temperature is shown in Figure 4b. The PL intensity decreases monotonically with the increase of temperature as is the case with many systems/materials.


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45000

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


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Intensity (a.u., x10 6)

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800

λ (nm)

(b)

4 3 2 1 0

100

200

300

400

Temperature (K)

Figure 4. (a) Temperature-dependent PL spectra of congo-red and PVA composite under 532 nm excitation and (b) variation of integrated intensity with obtained temperature. Inset of (b) shows the schematic for sensor setup.

One of the possible mechanisms for the opposite trend in temperature-dependency of dual emission is the energy transfer. The efficiency of energy transfer between donor and acceptor is governed by

η = 1 - τda/τd

(5)

where τda and τd are the donor’s excited-state lifetime in the presence and absence of the acceptor, respectively. As the Er and Eu are responsible for green and red emission, Er is assumed to be the donor and Eu is the acceptor.


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(a)

5

(b)

6

7

8

9

Decay time (ns)

EuEr:ZnO-565 EuEr:ZnO-610 Er:ZnO-565 Eu:ZnO-6120

Intensity (a.u.)

Er:ZnO-565 Eu:ZnO-610 EuEr:ZnO-565 EuEr:ZnO-610

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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4

5

6

7

8

9

10

Decay time (ns)

Figure 5. PL decay curves of doped and co-doped ZnO with (a) 345 and (b) 469 nm excitation.

In order to evaluate the efficiency, time resolved PL (TRPL) studies on Eu:ZnO, Er:ZnO and EuEr:ZnO are carried out with 345 and 469 nm excitation. Figure 5 shows typical TRPL curves for ZnO, Eu:ZnO, Er:ZnO and EuEr:ZnO microrods monitoring the emission peaks at 610 and 565 nm for Eu and Er, respectively. The decay constant as evaluated from the TRPL is summarized in Table S2. It may be noted that the decay constant is dependent on excitation wavelength,52,53 and almost independent of the detection wavelength. With 345 nm excitation, the electrons get excited to the ZnO conduction band, and non-radiatively transfer to the defect states, followed by transfer to the either Eu or Er sates and then radiatively combine with holes. On the other hand, electrons directly excited to the ZnO defect states with 469 excitation, transfer to the dopant excited states. Therefore, the decay constant varies with the excitation wavelength and the overall life-time is less for 469 nm excitation as compared to the 345 nm excitation irrespective of detection wavelength. Furthermore, the time constant is nearly the same for donor with or without acceptor ruling out the possibility of energy transfer. The energy


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transfer from host to Eu is also rule out as there is no ZnO host related emissions observed with 532 nm excitation (Figure S3). Another possible mechanism is the electron transfer. The TRPL measurements carried out for the ZnO defect emissions in the presence and absence of dopants (Eu or Er) (Figure 6) show a change in the life-time values confirming the posibility of electron transfer from ZnO defect states to the Eu excited states. The obtained decay constant in the presence and absence of Eu and Er with excitation wavelength of 469 nm and detection wavelength of 507 nm is provided in Table 1. Based on the life-time values, it can be estimated that the electron transfer efficiency from ZnO defect states to Eu3+ excited states is 43.5% where as to Er3+ it is 4.6%, confirming the favourable electron transfer to the Eu states. Apart from this, PL excitation (PLE) study is carried out for the EuEr:ZnO by considering the emission at 610 and 565 nm for the Eu and Er peaks, respectively (Figure S10). It may be noted that no peak corresponds to the Er3+ transition is observed for 610 nm emission, whereas peaks corresponds to the host ZnO are observed at 361, 374 and 380 nm along with peaks at 394, 413, 463, 531 and 557 nm corresponding to the 7F0→5L6, and 7F0→5DJ (J = 3, 2, 1, 0) respectively for the Eu3+ transitions (Figure S10). This confirms that there is no energy transfer occurs rom the Er3+ sates to the Eu3+ states, ruling out the possibility of ion-ion energy transfer.10,20,54 The dual response behavior and electron transfer mechanism can be well explained using the Eu- and Er-ZnO band diagrams. Radiative recombination of excited state electrons and various ground state photogenerated holes gives rise to various emission bands. The direct electron transfer from conduction band to Eu3+ is most unlikely due to the radiative/non-radiative decay of ZnO excitons is much faster (>102 times)37,55 and the 532 nm excitation is insufficient to excite ZnO valence band electrons to conduction band. However, direct excitation of electrons to defect states is possible as shown in Figure 7. In this case, the electrons from ZnO transfer to dopant (4f


17


intra-states) via defect states and sharp emissions due to recombination of excited state electrons and ground state holes in the dopants are realized. As shown in Figure 7, the excited states of Eu3+ lie below, while excited states of Er3+ are at higher levels compared to the defect levels of ZnO that makes electron transfer more favorable to Eu3+ rather than Er3+. With increase in temperature, the electron population at defect levels increases that enhances the intensity of emission bands of Eu3+ as more number of electrons is available for radiative recombination (as discussed above). On the other hand, electron transfer is less favorable for Er3+ states and therefore, the intensity of emission bands decreases with temperature.

ZnO Er:ZnO Eu:ZnO

Intensity (a.u.)

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 29

4.0

4.5

Time (ns)

4

5

6

7

8

Time (ns)

Figure 6. PL decay curves of ZnO, Eu:ZnO and Er:ZnO with 469 nm excitation and 507 nm detection wavelength.


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Table 1. Decay constant evaluated from the TRPL of ZnO and Eu:ZnO.

Excitation

Detection

Average life time

wavelength (nm)

wavelength (nm)

(10-9 s)

Samples

ZnO

469

507

0.216

Eu:ZnO

469

507

0.122

Er:ZnO

469

507

0.206

Figure 7. Band diagram of EuEr:ZnO showing different energy levels and electron transfer process.


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Page 20 of 29

CONCLUSION In summary, we show that hydrothermally grown Eu and Er co-doped ZnO microrods exhibits dual emission and interestingly, Eu3+ peak intensity increases and that of Er3+ decreases with temperature. As a consequence, green emission is observed at low temperature, while red emission is realized at high temperature. The electron transfer from ZnO host to Eu3+ (not to Er3+) through defect levels is the key for the complementary temperature-dependency that pave way for the design of new dual-emitting nanostructures. The dual emission of EuEr:ZnO is explored further for temperature sensing. As an example, temperature-dependency of congo-red and PVA composite is explored using EuEr:ZnO as optical probe. Over all, the results clearly suggest that the co-doped materials can be explored as phosphors for white light emitting diodes as well. ASSOCIATED CONTENT Supporting Information Room temperature PL of EuEr:ZnO, Eu:ZnO, Er:ZnO and ZnO sample with 355 and 532 nm excitation, Temperature dependent PL of Eu:ZnO, Er:ZnO with 355 and 532 nm excitation, Absolute and relative sensitive variation with temperature for both co-doped and mixer, Tables for Decay constant and Sensitivity. The Supporting Information is available free of charge on the ACS website. AUTHOR INFORMATION Corresponding Author *[email protected]


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Table of Contents Graphic


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Designing Dual Emissions via Co-doping or Physical Mixing of

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