materials Article

Energy Transfer Efficiency from ZnO-Nanocrystals to Eu3+ Ions Embedded in SiO2 Film for Emission at 614 nm Vivek Mangalam 1 1

2

*

ID

and Kantisara Pita 1,2, *

OPTIMUS, Centre for OptoElectronics and Biophotonics, School of Electrical and Electronic Engineering, Nanyang Technological University (NTU), Block S2, 50 Nanyang Avenue, Singapore 639798, Singapore; [email protected] CINTRA, CNRS-NTU-Thales UMI 3288, Research Techno Plaza, 50 Nanyang Drive, Border X Block, Level 6, Singapore 637553, Singapore Correspondence: [email protected]; Tel.: +65-6790-6375

Received: 26 June 2017; Accepted: 8 August 2017; Published: 10 August 2017

Abstract: In this work, we study the energy transfer mechanism from ZnO nanocrystals (ZnO-nc) to Eu3+ ions by fabricating thin-film samples of ZnO-nc and Eu3+ ions embedded in a SiO2 matrix using the low-cost sol-gel technique. The time-resolved photoluminescence (TRPL) measurements from the samples were analyzed to understand the contribution of energy transfer from the various ZnO-nc emission centers to Eu3+ ions. The decay time obtained from the TRPL measurements was used to calculate the energy transfer efficiencies from the ZnO-nc emission centers, and these results were compared with the energy transfer efficiencies calculated from steady-state photoluminescence emission results. The results in this work show that high transfer efficiencies from the excitonic and Zn defect emission centers is mostly due to the energy transfer from ZnO-nc to Eu3+ ions which results in the radiative emission from the Eu3+ ions at 614 nm, while the energy transfer from the oxygen defect emissions is most probably due to the energy transfer from ZnO-nc to the new defects created due to the incorporation of the Eu3+ ions. Keywords: zinc oxide nanocrystals; energy transfer efficiency; Europium(III) ions; time-resolved photoluminescence

1. Introduction The study of the excitation of rare-earth (RE) ions through energy transfers from semiconductor nanocrystals, especially from Zinc oxide nanocrystals (ZnO-nc) has attracted a lot of attention lately. The emission from the 3.37 eV wide bandgap of ZnO-nc has been shown to efficiently excite a wide variety of RE ions ranging from Ce3+ to Yb3+ ions [1–8]. Nonetheless, the mechanism of energy transfer from ZnO-nc to RE ions is not well understood. For instance, Huang et al. [8] report that the energy transfer from ZnO-nc to RE ions, specifically Eu3+ ions, is mainly due to the defects in the ZnO-nc. However, reports by Luo et al. [5] show that the energy transfer from ZnO-nc could be due to a combination of the band edge emission from the ZnO-nc and the defects in the ZnO-nc. Furthermore, in most of these works, the study of the energy transfer from ZnO-nc is based on steady-state photoluminescence (PL) emission from the nanocrystals, while there have been fewer studies based on time-resolved photoluminescence (TRPL) measurements from the ZnO-nc [2,3]. Moreover, there have been almost no studies that aim to further the understanding of the de3cay dynamics of various ZnO-nc emission centers and their effect on the efficiency of the energy transfer mechanism from ZnO-nc to RE ions. This article is a direct continuation of our earlier work [9], in which the steady-state PL emission from ZnO-nc was studied to elucidate the contribution of seven ZnO-nc

Materials 2017, 10, 930; doi:10.3390/ma10080930

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emission centers in the energy transfer process to Eu3+ ions. These seven emission centers of the ZnO-nc centered at 360, 360, 378, 396, 417, 450, 500, and 575 nm [10], werewere identified as band-edge band-edge emissions from at were identified as emissions from in centered SiO at378, 360,396, 378,417, 396,450, 417,500, 450, and 500, 575 andnm 575[10], nm [10], identified as band-edge emissions 2 , centered thethe smallest ZnO-nc, which possibly experiences some quantum confinement confinementeffect effect(QC) (QC) [11], the smallest ZnO-nc, which confinement effect (QC) [11], from smallest ZnO-nc, whichpossibly possiblyexperiences experiencessome some quantum [11], excitonic emission (EE)[12,13], [12,13],and andfive fivedifferent differentdefect defect state emissions, namely Zn defect (Zn to Zn)) excitonic emission (EE) [12,13], and five different (Zn ii to V excitonic emission (EE) state emissions, emissions,namely namelyZn Zndefect defect (Zn to i VZn [14], oxygen interstitial defect (O(O [15,16], and oxygen vacancy defects (Vooo,,, VȮ, VȮ, VӦӦ) [14,17,18], [14], oxygen interstitial defect (O ii)) i[15,16], [14,17,18], VZn ) [14], oxygen interstitial defect ) [15,16],and andoxygen oxygenvacancy vacancydefects defects (V V , VӦӦ) V ) [14,17,18], respectively. In this this work, we study the contribution of energy transfer from various ZnO-nc emission respectively. work, study contribution energy transfer from various ZnO-nc emission respectively. In In this work, wewe study thethe contribution of of energy transfer from various ZnO-nc emission 3+ 3+ 3+ ions centers to Eu ions based on TRPL measurements.We We calculate the decay time from the TRPL centers based TRPL measurements. We calculate the decay time from TRPL centers to to EuEu ions based onon TRPL measurements. calculate the decay time from thethe TRPL measurements for various ZnO-nc emission centers and then calculate the efficiency of energy measurements for various ZnO-nc emission centers and then calculate the efficiency of energy measurements for various ZnO-nc emission centers and then calculate the efficiency of energy transfer 3+ ions.to transfer from each of ofemission the ZnO-nc ZnO-nc emission centers toFurthermore, the Eu Eu3+3+ ions. ions. Furthermore, the istransfer transfer transfer from each the emission centers the the from each of the ZnO-nc centers to the Eu the Furthermore, transfer efficiency also efficiency is also also calculatedPL from steady-state PLcomparison. emission data data for comparison. The results results in this this efficiency is calculated from steady-state PL emission for comparison. The in calculated from steady-state emission data for The results in this work show high work show show high high transfer transfer efficiencies from theZn EE (at 378(at nm) and Zn to V VZn Zn centers, (at 396 396 nm) nm) emission emission work from the EE (at nm) ii to (at transfer efficiencies from theefficiencies EE (at 378 nm) and V378 396and nm)Zn emission mostly due i to Zn 3+ ions from 3+ radiative centers, mostly duefrom to the the energy transfer from ZnO-nc ZnO-nc toradiative Eu3+3+ ions ions which results results in radiative mostly due to energy transfer to Eu which to centers, the energy transfer ZnO-nc to Eu which results in emission from Euin ions at 3+ ions 3+ emission from Eu ions at 614 614 nm, nm, while the energy energy transfer from the oxygen defect emissions at 417, 417, emission from at the transfer from defect at 614 nm, while theEu energy transfer fromwhile the oxygen defect emissions atthe 417,oxygen 450, 500, and emissions 575 nm is most 450, 500, 500, and 575 energy nm is is most most probably due to to to thethe energy transfercreated from ZnO-nc ZnO-nc to the the new new defects defects 450, and nm probably due the energy transfer from to probably due to 575 the transfer from ZnO-nc new defects due to the incorporation 3+ ions, created due to the incorporation incorporation of the the Eu3+[9]. ions, as proposed proposed in our ourtransfer previous work [9]. [9]. the of Eu as in previous work of created the Eu3+ due ions, to as proposed in our previous work Understanding the energy mechanism 3+ 3+ ions 3+ Understanding the energy transfer mechanism from ZnO-nc to Eu ions will help to further Understanding the energy transfer mechanism from ZnO-nc to Eu will help to further the from ZnO-nc to Eu ions will help to further the understanding of the energy transfer process to REthe understanding of the the energy energy transferefficient processRE to RE RE ions and and in the the development of energy energy efficient understanding of process to ions in development of efficient ions and in the development of transfer energy ion-based light sources such as lasers, amplifiers, RE ion-based ion-based light sources suchbyas asthe lasers, amplifiers, phosphors, etc. that that can be be excited excited by by the the RE light sources such lasers, amplifiers, etc. can phosphors, etc. that can be excited sensitizing effect phosphors, of semiconductor nanocrystals. sensitizing effect of semiconductor nanocrystals. sensitizing effect of semiconductor nanocrystals. 2. Results and Discussion 2. Results Results and Discussion Discussion 2. and In this work, we studied the TRPL spectra of two types of thin-film samples; namely ZnO-nc in SiO2In (ZnO-nc:SiO ZnO-nc in SiOspectra with 12 mol % Eu3+ samples; ions (Eu3+ :ZnO-nc:SiO In this work, work, we we studied the TRPL TRPL spectra of two two types of thin-film thin-film samples; namely ZnO-nc in this studied the of types of namely ZnO-nc 2 ) and 2 co-doped 2 ).in 3+ 3+ 3+ The 1222 mol % Eu :ZnO-nc:SiO was used in this because 12 mol sample has22).).the SiO (ZnO-nc:SiO and ZnO-nc ZnO-nc in SiO SiO co-doped withstudy 12 mol mol % Eu Eu3+the ions (Eu3+% :ZnO-nc:SiO The SiO (ZnO-nc:SiO 22)) and in 22 co-doped with 12 % ions (Eu :ZnO-nc:SiO The 2 sample 3+ ions at 614 nm due to energy transfer from ZnO-nc, in comparison highest PL% emission from the 2Eu 12 mol mol % Eu3+3+:ZnO-nc:SiO :ZnO-nc:SiO 2 sample sample was used used in in this this study study because because the the 12 12 mol mol % % sample sample has has the the 12 Eu was 3+ 3+ 3+ with 4, 8, and 16 mol %from Eu the :ZnO-nc:SiO samples, beentransfer reported in our previous work [9]. highest PL emission emission from the Eu Eu ions ions2at at 614 nm nmwhich due to tohas energy transfer from ZnO-nc, in comparison comparison highest PL 614 due energy from ZnO-nc, in The TRPL were at each22 samples, of the peak wavelengths of the seven ZnO-nc emission with 4, 8, 8,spectra and 16 16 mol mol % %measured Eu3+3+:ZnO-nc:SiO :ZnO-nc:SiO samples, which has been been reported reported in our our previous work [9]. [9]. with 4, and Eu which has in previous work centers. The TRPL signals from the last three longest wavelengths of the seven signals, namely at 450, The TRPL TRPL spectra spectra were were measured measured at at each each of of the the peak peak wavelengths wavelengths of of the the seven seven ZnO-nc ZnO-nc emission emission The 500 and 575 nm, weresignals weak compared to three the other fourwavelengths emissions atof 360, 396signals, and 417namely nm. at centers. The TRPL signals from the the last last three longest wavelengths of the378, seven signals, namely at 450, 450, centers. The TRPL from longest the seven our575 PL nm, measurements we alsoto observed the 450, 500 at and 575 nm396 emissions 500Inand and 575 nm, were weak weak[9], compared to the other otherthat four emissions at 360, 360, 378, 396 and 417 417have nm. the 500 were compared the four emissions 378, and nm. lowest In emission These[9], weak are possibly due to larger number of phonons In our our PL PLintensity. measurements [9], weemissions also observed observed that the the 450, 450, 500 500the and 575 nm nm emissions have the the measurements we also that and 575 emissions have required compared to the emissions at emissions 360, 378, 396 417 nm, shown inof Figure 1 lowest as emission intensity. These weak weak emissions areand possibly due to the theschematically larger number number of phonons lowest emission intensity. These are possibly due to larger phonons (the energyas band diagram ZnO-nc in SiO various transitions). number ofin phonon required as compared toof the emissions at 360, 360, 378, 396 396 and 417 417 nm, shown shownThe schematically in Figure 11 required compared to the emissions at 378, and nm, schematically Figure 2 showing emissions required the 450,of 500 and 575 estimated to be around to nine times (the energy energy bandby diagram of ZnO-nc innm SiOemissions showingisvarious various transitions). Thethree number of phonon phonon (the band diagram ZnO-nc in SiO 22 showing transitions). The number of more than those required by 450, the 417 process. The probability of emissions emissions required by the the 450, 500nm andemission 575 nm nm emissions emissions is estimated estimated to be bemulti-phonon around three three to to nine times times emissions required by 500 and 575 is to around nine decreases with increasing the number of phonons required for the de-excitation process, leading to more than than those those required required by by the the 417 417 nm nm emission emission process. process. The The probability probability of of multi-phonon multi-phonon emissions emissions more a lower number excited state ZnO-ncof involved the emissions at 450,leading 500 and decreases withof increasing the number number ofelectrons phononsbeing required for the theinde-excitation de-excitation process, leading to aa decreases with increasing the phonons required for process, to 575 nm, number causing weak emissions at theseelectrons wavelengths. The energy band diagramat in450, Figure also lower number of of excited excited state ZnO-nc ZnO-nc electrons being involved involved in the the emissions emissions at 450, 500 1and and 575 lower state being in 500 575 3+ ions in the SiO matrix, illustrates the energy transfer from various ZnO-nc emission centers to Eu nm,2017, causing weak emissions emissions at at these these wavelengths. wavelengths. The The energy energy band band diagram diagram in in Figure Figure also Materials 10, 930 weak nm, causing 2 2 of119 also together withthe theenergy varioustransfer transitions to the emissions fromto and Eu3+ illustrates the energy transfer fromcorresponding various ZnO-nc ZnO-nc emission centers toZnO Eu3+3+nanocrystals ions in in the the SiO SiO matrix, illustrates from various emission centers Eu ions 22 matrix, centered at 360, 378, 417, 450, 500, and 575band nm [10], were identified band-edge emissions from ions. This figure is 396, avarious modified version ofcorresponding the diagram reported inasour earlier work [9], where together with the various transitions corresponding to the the emissions from ZnO nanocrystals nanocrystals andthe Eu3+3+ together with the transitions to emissions from ZnO and Eu thebandwidth smallest ZnO-nc, which possibly experiences some quantum confinement effect (QC) [11], the various ZnO-ncversion emission centers represented usingin the colour gradients inwhere the ions. This Thisof figure is aa modified modified version of the the bandisdiagram diagram reported in our earlier work [9], [9], where ions. figure is of band reported our earlier work excitonic emission (EE) [12,13], andZnO-nc five state emissions, Znthe defect (Zngradients i to VZnthe ) in broad vertical energy bands. Here, wedifferent note thatdefect there is a possibility ofnamely back energy transfer from the bandwidth bandwidth of the the various ZnO-nc emission centers is represented represented using the colour gradients in the of various emission centers is using colour 3+ 5 Dvacancy 3+ to(V [14], oxygen interstitial defect (O i ) [15,16], and oxygen defects o , VȮ, VӦӦ) [14,17,18], excited Eu ions to the ZnO-nc, especially from the state of Eu the V defect center in the the broad broad vertical vertical energy energy bands. bands. Here, Here, we we note note that that there there is aa possibility possibility of of back back energy energy transfer transfer from from the 0 is 55D respectively. In the this work, we study the contribution of energy transfer various ZnO-nc emission ZnO-nc with of phonons. However, we do consider ofdefect the energy the excited excited Eu3+3+cooperation ions to to the ZnO-nc, especially from the Dnot state offrom Eu3+3+the toeffect the VȮ VȮ defect centerback in the the the Eu ions the ZnO-nc, especially from the 00 state of Eu to the center in 3+ ions based on TRPL measurements. We calculate the decay time from the TRPL centers to process Eu transfer on cooperation the PL and TRPL emissions of the ZnO-nc in this study.the ZnO-nc with the cooperation of phonons. phonons. However, we do do not not consider the effect effect of of the the energy energy back back ZnO-nc with the of However, we consider measurements for various ZnO-nc emission centers andZnO-nc then calculate the efficiency of energy transfer process process on the the PL PL and TRPL TRPL emissions of the the ZnO-nc in this this study. study. transfer on and emissions of in 3+ transfer from each of the ZnO-nc emission centers to the Eu ions. Furthermore, the transfer efficiency is also calculated from steady-state PL emission data for comparison. The results in this work show high transfer efficiencies from the EE (at 378 nm) and Zni to VZn (at 396 nm) emission centers, mostly due to the energy transfer from ZnO-nc to Eu3+ ions which results in radiative emission from Eu3+ ions at 614 nm, while the energy transfer from the oxygen defect emissions at 417, 450, 500, and 575 nm is most probably due to the energy transfer from ZnO-nc to the new defects created due to the incorporation of the Eu3+ ions, as proposed in our previous work [9]. Understanding the energy transfer mechanism from ZnO-nc to Eu3+ ions will help to further the understanding of the energy transfer process to RE ions and in the development of energy efficient

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diagram showing showing the the energy energytransfer transferfrom fromZnO-nc ZnO-ncto toEu Eu3+3+ ions in SiO22 together Figure 1. Energy band diagram 3+ ions. various transitions transitions corresponding corresponding to to the the emissions emissions from from ZnO-nc ZnO-ncand andEu Eu3+ with the various

2.1. Transfer Efficiency from from Time-Resolved Time-ResolvedPhotoluminescence PhotoluminescenceEmission Emission 2.1. Transfer Efficiency Figure 22 shows shows the the normalized normalized TRPL TRPL spectra spectra (the (the decay decay of of the Figure the normalized normalized PL PL emission emission intensity intensity 3+ 3+ with respect 2 samples at 360, 378, 396 and 417 nm. We with respect to to time) time) of ofZnO-nc:SiO ZnO-nc:SiO2 2and andEu Eu:ZnO-nc:SiO :ZnO-nc:SiO samples at 360, 378, 396 and 417 nm. 2 3+3+ions decays faster than the sample can clearly see that the PL intensity of the sample with the Eu We can clearly see that the PL intensity of the sample with the Eu ions decays faster than the sample 3+ ions at all emission wavelengths. The decay of the PL emission in the ZnO-nc:SiO2 without Eu Eu3+ without ions at all emission wavelengths. The decay of the PL emission in the ZnO-nc:SiO2 sample is is due due to processes in in the the ZnO-nc. the sample to the the radiative radiative and and non-radiative non-radiative de-excitation de-excitation processes ZnO-nc. However, However, the 3+:ZnO-nc:SiO2 sample has additional components that contribute 3+ decay of the PL emission in the Eu decay of the PL emission in the Eu :ZnO-nc:SiO2 sample has additional components that contribute 3+ ions which results in the to the thedecay decayrate, rate, namely (i) the energy transfer from ZnO-nc Euwhich to namely (i) the energy transfer from ZnO-nc to Eu3+toions results in the radiative 3+ 3+ radiative emission from Eu ions at 614 nm, and (ii) the energy transfer from ZnO-nc the additional emission from Eu ions at 614 nm, and (ii) the energy transfer from ZnO-nc to thetoadditional new 3+ ions [9]. To study the contribution of the 3+ new defects created due to the incorporation of the Eu defects created due to the incorporation of the Eu ions [9]. To study the contribution of the energy 3+ ions, the energy transfers thetoZnO-nc toions the and Eu3+the ions and the defects created by the transfers from thefrom ZnO-nc the Eu3+ defects created by the Eu3+ ions, theEu intensities of intensities of the different TRPL spectra of the two samples are expressed using the stretched the different TRPL spectra of the two samples are expressed using the stretched exponential decay exponential decay function [19] as follows: fitting function [19]fitting as follows: t β

I (t) = Io e−( τ ) (1) (1) ( )= where I (t) is the intensity of the TRPL measurement at time t, Io is the intensity of the TRPL is the intensity of the TRPL where ( ) is the intensity of the TRPL measurement at time t, measurement at t = 0, τ is the decay time of the TRPL intensity, and β is the stretching exponential measurement at = 0, is the decay time of the TRPL intensity, and β is the stretching exponential coefficient, which is a wavelength-dependent constant. The stretched exponential decay fitting function coefficient, which is a wavelength-dependent constant. The stretched exponential decay fitting is the most widely used fitting function to represent the TRPL decay curves of ZnO-nc [19], and function is the most widely used fitting function to represent the TRPL decay curves of ZnO-nc [19], semiconductor nanocrystals in general [20–23], as they account for the complex nature of the decay and semiconductor nanocrystals in general [20–23], as they account for the complex nature of the curves which cannot be expressed using simple exponential decay functions. Using the stretch decay curves which cannot be expressed using simple exponential decay functions. Using the stretch exponential decay fitting function, we obtained very good fitting curves for the TRPL spectra of exponential decay fitting function, we obtained very good fitting curves for the TRPL spectra of the the ZnO-nc. In the stretched exponential decay equation, β ranges from 0 to 1 and is a measure of ZnO-nc. In the stretched exponential decay equation, β ranges from 0 to 1 and is a measure of interaction between identical ZnO-nc emission centers. This interaction, for instance, could be in interaction between identical ZnO-nc emission centers. This interaction, for instance, could be in the the form of the migration of excitons between ZnO-nc or the migration of carriers between identical form of the migration of excitons between ZnO-nc or the migration of carriers between identical ZnOZnO-nc defect centers [22,23]. The value of β approaches 0 when the effect of interaction between two nc defect centers [22,23]. The value of β approaches 0 when the effect of interaction between two or or more identical ZnO-nc emission centers is large, and β approaches 1 when the effect of interaction more identical ZnO-nc emission centers is large, and β approaches 1 when the effect of interaction between the identical emission centers is negligible [22,23]. For the 360-nm band edge emission (QC) between the identical emission centers is negligible [22,23]. For the 360-nm band edge emission (QC) of the ZnO-nc, the value of β is 0.65 for the sample without Eu3+ ions and 0.69 for the sample with Eu3+ of the ZnO-nc, the value of β is 0.65 for the sample without Eu3+ ions and 0.69 for the sample with ions, as shown in Table 1. However, for the 378-nm excitonic emission (EE), 396-nm Zn defect (Zni to Eu3+ ions, as shown in Table 1. However, for the 378-nm excitonic emission (EE), 396-nm Zn defect VZn ) emission, and 417-nm oxygen interstitial defect (Oi ) emission, the β values are around 0.61–0.62 (Zni to VZn) emission, and 417-nm oxygen interstitial defect (Oi) emission, the β values are around for the samples with and without Eu3+ ions. These results indicate that the 360-nm band edge emission 0.61–0.62 for the samples with and without Eu3+ ions. These results indicate that the 360-nm band edge emission (QC) experiences lesser interactions compared to the EE, Zni to VZn, and Oi emissions.

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(QC) experiences lesser interactions compared to the EE, Zni to VZn , and Oi emissions. The weaker interaction of the 360-nm band edge emission (QC) center could be due to the smaller size of the ZnO-nc, as this emission is attributed to the possible quantum confinement effect. Table 1 also show the Materials 930 4 of 9 decay time 2017, τ for10,different ZnO-nc emission centers of the ZnO-nc:SiO2 and Eu3+ :ZnO-nc:SiO2 samples. The decay time τ is primarily the lifetime of the electrons in the excited state of the ZnO-nc emission The weaker interaction of the 360-nm band edge emission (QC) center could be due to the smaller center, and its values depend on the rates of the different de-excitation processes from the ZnO-nc size of the ZnO-nc, as this emission is attributed to the possible quantum confinement effect. Table 1 emission centers. For instance, the decay time of any ZnO-nc emission center in the ZnO-nc:SiO also show the decay
time for different ZnO-nc emission centers of the ZnO-nc:SiO2 and Eu3+:ZnO- 2 ZnO − nc:SiO 2 sample τ2 samples. Theisdecay giventime by: is primarily the lifetime of the electrons in the excited state of the nc:SiO ZnO-nc emission center, and its values depend on the rates of the different de-excitation processes 1 −nc:SiOthe 2 = from the ZnO-nc emission centers. For instance, decay time of any ZnO-nc emission center in the (2) τ ZnO r R + Rnr : ) is given by: ZnO-nc:SiO2 sample (

where Rr is the rate of radiative emission and Rnr is the rate 1 of non-radiative emission from the ZnO-nc : (2) = + emission center. Similarly, the decay time of any ZnO-nc emission center in the Eu3+ :ZnO-nc:SiO2   3+

Eur is :ZnO −nc:SiO 2 whereτR the rate of radiative emission sample is given by: and Rnr is the rate of non-radiative emission from the ZnO-

nc emission center. Similarly, the decay time of any ZnO-nc emission center in the Eu3+:ZnO-nc:SiO2 : : is given sample + :ZnO −nc:SiO 1 Eu3by: 2

=

τ

:

:

=

nr tr Rr + R 1 +R

+

+

(3)

(3)

where Rtr is the combined rate of energy transfer from ZnO-nc to Eu3+ ions which results in the where Rtr is the combined rate of energy transfer from ZnO-nc to Eu3+ ions which results in the radiative emission from the Eu3+ ions at 614 nm and energy transfer from ZnO-nc to the additional 3+ radiative emission from the Eu ions at 614 nm and energy transfer from ZnO-nc to the additional new defects created due to the incorporation of the Eu3+ ions. As expected, the decay time for the new defects created due to incorporation of the Eu3+ ions. As expected, the decay time for the 3+ 3+ the − nc:SiO2 ) is lower sample with Eu3+ 3+ions (τ Eu :ZnO than the decay time for the sample without Eu : : sample with Eu ions ( ) is lower than the decay time for the sample without Eu3+ ions ZnO − nc:SiO 2 ions (τ ) for all wavelengths, due to the additional energy transfer decay mechanisms in the : ( ) for all wavelengths, due to the additional energy transfer decay mechanisms in the Eu3+Eu :ZnO-nc:SiO 3+:ZnO-nc:SiO 2 sample. 2 sample.

Figure 2. The time-resolved photoluminescence (TRPL) spectra of the ZnO-nc:SiO2 and

Figure3+ 2. The time-resolved photoluminescence (TRPL) spectra of the ZnO-nc:SiO2 and Eu :ZnO-nc:SiO2 samples measured at (a) 360 nm; (b) 378 nm; (c) 396 nm; and (d) 417 nm, along with Eu3+ :ZnO-nc:SiO2 samples measured at (a) 360 nm; (b) 378 nm; (c) 396 nm; and (d) 417 nm, along with their respective stretched exponential fitting curves. their respective stretched exponential fitting curves.

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Table 1. The decay time (τ) and stretching exponential coefficient (β) values for the various ZnO-nc emission centers obtained time-resolved photoluminescence (TRPL)for spectra of the samples Table 1. The decay time (τ)from andthe stretching exponential coefficient (β) values the various ZnO-nc 3+ ions (Eu3+ :ZnO-nc:SiO and ZnO-nc:SiO samples, respectively). with and without Eu 2 2 emission centers obtained from the time-resolved photoluminescence (TRPL) spectra of the samples

with and without Eu3+ ions (Eu3+:ZnO-nc:SiO2 and ZnO-nc:SiO2 samples, respectively). Sample Emission Wavelength τ (ps) β Sample 2 Emission ZnO-nc:SiO 360 nmWavelength 215 ± 8(ps) 0.65 ± 0.02 (QC) 102 ± 7±8 0.69 ± ±0.02 ZnO-nc:SiO 2 2 360 nm 215 0.65 0.02 Eu3+ :ZnO-nc:SiO 3+:ZnO-nc:SiO2 378 nm 213 ± 4±7 0.62 ± ±0.01 EuZnO-nc:SiO (QC) 102 0.69 0.02 2 (EE) 84 ±213 4 ±4 0.62 ± ±0.01 Eu3+ :ZnO-nc:SiO 2 2 378 nm 0.62 0.01 ZnO-nc:SiO 396 nm 264 ±846 ± 4 0.61 ± ±0.01 3+:ZnO-nc:SiO 2 2 (EE) 0.62 0.01 EuZnO-nc:SiO (Zni to396 VZnnm ) 95 ±264 5 ±6 0.61 ± ±0.01 Eu3+ :ZnO-nc:SiO ZnO-nc:SiO 2 2 0.61 0.01 ZnO-nc:SiO 417 nm 356 ± 15 0.62 ± 0.02 2 Eu3+3+:ZnO-nc:SiO2 (Zni to VZn) 95 ± 5 0.61 ± 0.01 (Oi ) 125 ± 10 0.62 ± 0.02 Eu :ZnO-nc:SiO2 417 nm 356 ± 15 0.62 ± 0.02 ZnO-nc:SiO2 Eu3+:ZnO-nc:SiO2 (Oi) 125 ± 10 0.62 ± 0.02

From the decay time results obtained from the TRPL measurements in Table 1, the energy transfer From denoted the decay obtained fromaccording the TRPLtomeasurements in Table 1, the energy efficiency, by time ETTRPLresults , was then calculated the following equation: transfer efficiency, denoted by , was then calculated according to the following equation: 3+   2 τ Eu ::ZnO−nc:SiO Rtr : = 1 − (4) Transfer efficiency ETTRPL = r nr tr ZnO − nc:SiO (4) 2 )= R +R +R =1− τ Transfer efficiency ( : + + TRPL transfer efficiency calculated for each of the four ZnO-nc emission centers is shown in The E The T transfer efficiency calculated for each of the four ZnO-nc emission centers is shown Figure 3. 3. in Figure

Figure 3. Transfer efficiency of ZnO nanocrystals emission centers due to the incorporation of the Eu3+ Figure 3. Transfer efficiency of ZnO nanocrystals emission centers due to the incorporation of the ions, calculated from the time-resolved photoluminescence (TRPL) spectra and steady-state Eu3+ ions, calculated from the time-resolved photoluminescence (TRPL) spectra and steady-state photoluminescence (PL) emission emission data, data, along along with with their their respective respective error error bars. bars. photoluminescence (PL)

2.2. Transfer Efficiency from Steady-State Photoluminescence Emission 2.2. Transfer Efficiency from Steady-State Photoluminescence Emission Furthermore, for comparison, the transfer efficiency of each of the seven ZnO-nc emission Furthermore, for comparison, the transfer efficiency of each of the seven ZnO-nc emission centers centers was also calculated from the steady-state PL intensity measurements of the two samples in was also calculated from the steady-state PL intensity measurements of the two samples in this study, this study, ZnO-nc:SiO 2 and Eu3+:ZnO-nc:SiO2. Savchyn et al. [24] report a similar analysis calculating ZnO-nc:SiO2 and Eu3+ :ZnO-nc:SiO2 . Savchyn et al. [24] report a similar analysis calculating the the transfer efficiency from the steady-state PL emission intensity for a silicon nanocrystals and Er3+ transfer efficiency from the steady-state PL emission intensity for a silicon nanocrystals and Er3+ ions 3+ ions system. Our results on the steady-state PL intensity for the samples without and with Eu ions system. Our results on the steady-state PL intensity for the samples without and with Eu3+ ions have have been reported in Reference [9], and we used those results to calculate the energy transfer efficiency, denoted by , in this report. The intensity of steady-state PL emission from the ZnO-

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been reported in Reference [9], and we used those results to calculate the energy transfer efficiency, denoted by ETPL , in this report. The intensity of steady-state PL emission from the ZnO-nc:SiO2 
3+ :ZnO − nc:SiO Eu ZnO − nc:SiO 3+ 2 and Eu :ZnO-nc:SiO 2 I samples can be mathematically represented 2 I by Equations (5) and (6) [24], respectively: I ZnO−nc:SiO2 ∝ N ZnO−nc:SiO2 · Rr I Eu

3+ :ZnO − nc:SiO 2

∝ N Eu

3+ :ZnO − nc:SiO 2

(5)

· Rr

(6)

where N ZnO−nc:SiO2 is the number of ZnO-nc per unit volume in the excited state of any emission 3+ center for the ZnO-nc:SiO2 sample, and N Eu :ZnO−nc:SiO2 is the number of ZnO-nc per unit volume in the excited state of any emission center for the Eu3+ :ZnO-nc:SiO2 sample. Under the steady-state condition of PL measurement, the rate equation of N ZnO−nc:SiO2 [25] can be written as follows: ∂N ZnO−nc:SiO2 = + G − N ZnO−nc:SiO2 ( Rr + Rnr ) = 0 ∂t

(7)

where G is the total number of ZnO-nc generated to the excited state of an emission center per unit volume per unit time. The value of G depends on the intensity of the steady-state PL excitation source 3+ and the absorption cross-section of the ZnO-nc. Similarly, the rate equation of N Eu :ZnO−nc:SiO2 [25] can be written as follows: ∂N Eu

3+ :ZnO − nc:SiO 2

∂t

= + G − N Eu

3+ :ZnO − nc:SiO 2


Rr + Rnr + Rtr = 0

(8)

Thus, from Equations  (7) and  (8), we can see that during the PL measurement, i.e., under a ∂N steady-state condition ∂t = 0 , N ZnO−nc:SiO2 depends only on Rr and Rnr for the ZnO-nc:SiO2 3+

sample and N Eu :ZnO−nc:SiO2 depends on Rr , Rnr , and Rtr for the Eu3+ :ZnO-nc:SiO2 sample. Here, we have assumed that the addition of Eu3+ ions into the SiO2 matrix does not affect the generation process of ZnO-nc in the excited state of any given emission center, because the intensity of the steady-state PL excitation source and the absorption cross-section of ZnO-nc are constant values in the steady-state PL measurement of the two samples. This implies that the values of G are the same for the samples without and with Eu3+ ions. Since the intensity of the PL emission is directly proportional to the number of ZnO-nc per unit volume in the excited state of an emission center, as shown in Equations (5) and (6), then using Equations (7) and (8), we have: 3+

I Eu :ZnO−nc:SiO2 = I ZnO−nc:SiO2

G · Rr Rr + Rnr + Rtr G r Rr + Rnr · R

(9)


The energy transfer efficiency from the PL steady-state measurements ETPL can then be written as follows: 3+   Rtr I Eu :ZnO−nc:SiO2 Transfer efficiency ETPL = r = 1 − (10) R + Rnr + Rtr I ZnO−nc:SiO2

The values of ETPL are shown in Figure 3, together with those of2EofTTRPL 9 , for the various ZnO-nc emission centers. The figure also shows the error bar for each of the transfer efficiency values calculated at 360, 378, 396, 417, 450,the 500,two anddifferent 575 nm methods. [10], were identified as band-edge emissions from using TRPL transfer lest ZnO-nc, which possibly experiences confinement We notice that the ETPLsome and Equantum efficiencieseffect have (QC) similar[11], values and are well within the T PL defect TRPL emission (EE) [12,13], five defect state emissions, namely (Zn i to V Zn) errorand bars ofdifferent the two measurements methods. The EZn and E transfer efficiencies are between T T gen interstitial defect (Oi)0.7 [15,16], andofoxygen vacancy defectsexcept (Vo, VȮ, [14,17,18], 0.5 and for most the ZnO-nc emissions, for VӦӦ) V defect emission at 575 nm. The transfer ely. In this work, we study the obtained contribution of energy from various ZnO-nc emission efficiencies in this work transfer are quite similar to the energy transfer efficiencies from silicon o Eu3+ ions basednanocrystals on TRPL measurements. We calculate the decay the TRPL to Er3+ ions reported by Savchyn et al.time [24],from calculated using similar methods. In our ments for variousprevious ZnO-nc work emission centers and then calculate the efficiency of energy [9], we calculated the spectral overlap integral of each of the seven ZnO-nc emissions rom each of the ZnO-nc emission centers to the Eu3+ ions. Furthermore, the transfer is also calculated from steady-state PL emission data for comparison. The results in this w high transfer efficiencies from the EE (at 378 nm) and Zni to VZn (at 396 nm) emission mostly due to the energy transfer from ZnO-nc to Eu3+ ions which results in radiative from Eu3+ ions at 614 nm, while the energy transfer from the oxygen defect emissions at 417,

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with the Eu3+ excitation spectrum to obtain a measure of the energy transfer from ZnO-nc to the 3+ ,[10], werewere identified as emissions from identified asions band-edge emissions from at 614 nm. It has been found and reported in Reference [9] that Euband-edge resulting in the emission ome quantum confinement (QC) [11],[11],centers emitting at 378 and 396 nm have the highest contribution to es some quantum confinement effect the EE and effect Zni to VZn (QC) emission t statestate emissions, namely Zn defect (Zn i (Zn to VZnO-nc defect emissions, namely Zn defect i Zn to) VZn) to Eu3+ ions, while the oxygen defect emissions of ZnO-nc such as the energy transfer from en vacancy defects VȮ, VӦӦ) oxygen vacancy defects VӦӦ) [14,17,18], Oi ,(V Voo,, (V V o,, VȮ, and V [14,17,18], , which are centered at 417, 450, 500, and 575 nm, respectively, have low energy energy transfer from various ZnO-nc on of energy transfer from various ZnO-nc emission transfer contributions the Eu3+ ions. Thus, combining the results of spectral overlap integrals 2 of emission 9 to We We calculate the from decay time fromfrom the TRPL nts. calculate the decay time the[9] TRPL our previous work with the transfer efficiencies results of this work, we can deduce that and and thenas calculate the efficiency of energy dentified band-edge emissions from nters then calculate the efficiency of energy the high transfer efficiencies ETPL and ETTRPL of EE and Zni to VZn emission centers are mostly due to 3+ 3+ to the Eu Eu ions.the Furthermore, the transfer antum effect (QC) [11], nters to confinement the ions. Furthermore, the transfer energy transfer from ZnO-nc to Eu3+ ions, which results in the radiative emission from the Eu3+ ssion datanamely for comparison. The results this missions, defect i The to VZn ) in observe emission data forZn comparison. results in this that for the oxygen defect emissions of ZnO-nc such as Oi , Vo , and ions at 614(Zn nm. We also 378 and(V Zn to Zn (at emission defects o, i VȮ, VӦӦ) [14,17,18], Eancy (at nm) 378 nm) and ZnV to Vvalues Zn396 (at nm) 396 nm) emission V ,i the of the transfer efficiencies ETPL and ETTRPL are similar to those of EE and Zni to VZn 3+ 3+ to from Eu ionsemission which results ineven radiative transfer various ZnO-nc emission mO-nc ZnO-nc to Eu ions which results inthough radiative centers, the spectral overlap integral values of oxygen defect emissions are low. ansfer from the oxygen defect emissions at 417, culate the from decay time from the TRPL y transfer the oxygen defect emissions at transfer 417, This implies that the energy from these oxygen defect emission centers is mostly due to the transfer from ZnO-nc toofthe new defects hen calculate the from efficiency energy eergy energy transfer ZnO-nc to the new defects energy transfer from ZnO-nc to the additional new defects created due to the incorporation of the 3+ ions. s,Eu as proposed in our previous work [9]. [9]. in our earlier work [9]. The formation of Eu3+ ion-induced defect Furthermore, the transfer ions, as proposed inions our work Eu3+ [9],previous which was proposed 3+The ZnO-nc to Eu ions will help toimportant further the ata for comparison. results in this rom ZnO-nc to3+ Eu ions help to further the that needs to be considered when fabricating light-emitting devices centers iswill a very result ns andZn ini the energy efficient ) and toinVdevelopment Zn (at 396 use nm)of emission RE ions and the development of energy efficient which the energy transfer process from ZnO-nc to excite RE ions. This result, together with the 3+ phosphors, s, phosphors, etc.knowledge that canincan beradiative excited by the Eu ions which results ifiers, etc. that beunderstanding excited by theof ZnO-nc emission centers which contribute to the energy transfer and om the oxygen defect emissions at 417, to Eu3+ ions resulting in a strong emission at 614 nm, can help in fabricating process from ZnO-nc nsfer from ZnO-nc to theenergy-efficient new defects red light-emitting devices. future, roposed in our previous work [9]. 3.help Materials and Methods 3+ ions will ctwo to Eu to further the types of thin-film samples; namely ZnO-nc in in types of thin-film samples; namely ZnO-nc in the development of energy efficient 3+ 3+ 3+ ions 3+:ZnO-nc:SiO The (Eu samples in this study were fabricated using the low-cost sol-gel process and made into thin hwith 12 mol % Eu ions (Eu :ZnO-nc:SiO 2). The 12 mol % Eu 2). The phors, etc. that can be excited by the films of approximately 300 nm study because the 12 mol % sample has the this study because the 12 mol % sample has theby spin-coating on a Si substrate. The sol-gel process is essentially a

three-step process where, in the first step of chemical mixing the precursor, the solvent and the catalyst o energy transfer from ZnO-nc, in comparison due to energy transfer from ZnO-nc, in comparison were together towork create the sols. Separate sols were prepared for fabricating the SiO2 matrix ch has been reported inadded our work [9]. [9]. which has been reported in previous our previous and the ZnO-nc. The SiO sol was prepared with tetraethyl orthosilicate (TEOS) as the precursor kpeak wavelengths of the ZnO-nc emission wavelengths of seven seven ZnO-nc emission 2 dissolved in ethanol with de-ionized water, while the ZnO-nc sol was prepared with zinc acetate of the signals, namely at 450, favelengths samples; ZnO-nc inalong stthin-film wavelengths ofseven thenamely seven signals, namely at 450, 3+ ions 3+ as the emissions at 360, 378, 396 and nm. ol % Eu (Eu :ZnO-nc:SiO 2417 ). precursor The rrfour emissions atdihydrate 360, 378, 396 and 417 nm. and diethylamine as the catalyst dissolved in ethanol. In the second step, the aged for 24 h, after at the 450, 500 575 nm emissions have the because the450, 12 and mol %sols sample the d that the 500 and 575were nm has emissions have the which Europium(III) nitrate pentahydrate was added to the SiO2 matrix sol tonumber incorporate the Eu3+ ions. The two sols were then mixed to create samples with Zn:Si possibly due to ZnO-nc, the larger number of phonons y transfer from in comparison are possibly due to the larger of phonons in aschematically 1:2 schematically ratiowork and spin-coated 417 nm, shown in Figure 1 on1 a (100) Si substrate. Finally, the samples were soft baked for 5 m at been in our previous [9]. , and 396 reported and 417 nm, shown in Figure ◦ 100 ZnO-nc C and subsequently annealed at 450 ◦ C in an O2 environment for 60 s using the Rapid Thermal glengths various The number of phonon oftransitions). the transitions). seven emission wing various The number of phonon Annealing (RTP) process. The steps of fabrication are exactly same as those reported in our previous ns isof estimated be three nine times gths the seventosignals, atto450, ssions is estimated toaround be namely around three to nine times publications [9,10], and the exact ess. probability multi-phonon emissions ionsThe at 360, 378, 396of and 417 nm. process. The probability of multi-phonon emissions same samples used in our previous study of the steady-state PL 3+ emission from ZnO-nc and uired forand the de-excitation process, leading to a Eu 50, 500 emissions have the leading required for575 thenm de-excitation process, to a ions were also used for this work [9]. The formation of ZnO-nc embedded an450, SiO2and matrix involved the atin 450, 500 575 using due involved to theinlarger number of phonons eing in emissions the emissions at 500 and 575 this sol-gel recipe has been studied in detail in our group’s earlier publication [10], wherein the TEM images of the samples were analyzed to confirm the formation The energy bandband diagram Figure 7ths. nm,The shown schematically ininFigure 1 1 also energy diagram in Figure 1 also 3+ 3+ ions ZnO-nc. mission centers to of Eu ions in matrix, us The number of the phonon nc transitions). emission centers to Eu in SiO the 2SiO 2 matrix, The TRPL spectra was by exciting the sample using a femtosecond amplified-pulsed the emissions from ZnO nanocrystals and Eu3+obtained imated to be around three to nine times g to the emissions from ZnO nanocrystals and Eu3+ which waswork tuned to where 325 nm using an optical parameter amplifier and detected using a streak agram reported inlaser, our work [9], where of multi-phonon emissions dprobability diagram reported in earlier our earlier [9], camera Optronis, Kehl, Baden-Württemberg, Germany) [26]. The pulse-width and is represented using the(OptoScope colour gradients in rsthe de-excitation process, leading to aSC-10, enters is represented using the colour gradients in the repetition rate of the excitation pulse were 100 fs and 1 KHz, respectively. The PL measurements ere is the a possibility ofatback energy transfer fromfrom edthere in 450, 500energy and 575 at is emissions a possibility of back transfer 5 3+ 5 3+ were by exciting the at 325 nm using a spectrofluorometer (SPEX Fluorolog-3 Model e the D0 state ofdiagram Euof Eu to the VȮ defect in the nergy band inobtained Figure 1 center also m D0 state to the VȮ defect center in samples the 3+ ions FL3-11, NJ, USA). e do the of 2the energy back centers Euconsider ineffect theHoriba, SiO matrix, er, wenot dotoconsider not the effect ofEdison, the energy back ZnO-nc in this study. ssions from ZnO nanocrystals and Eu3+ the ZnO-nc in this study. 4. Conclusions eported in our earlier work [9], where resented using the colour gradients in measured the TRPL spectra of ZnO-nc:SiO2 and Eu3+ :ZnO-nc:SiO2 samples to In this work we possibility of back study energythe transfer from of energy transfer to the Eu3+ ions from the various emission centers of ZnO-nc. contribution te of Eu3+ to the VȮ defect center in the The TRPL spectra of the two samples were analyzed by calculating the decay time and the stretching t consider the effect of the energy back in this study.

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exponential coefficient values of the different ZnO-nc emissions centers at 360, 378, 396 and 417 nm using the stretched exponential decay function. The decay time showed that samples with Eu3+ ions experience faster decay because of the additional components, namely the energy transfer from ZnO-nc to the Eu3+ ions, which results in the radiative emission from the Eu3+ ions at 614 nm, and the energy transfer to the additional new defects created due to the incorporation of the Eu3+ ions, contributing to the decay rate. Furthermore, the transfer efficiencies of the ZnO-nc emissions calculated from the decay time are consistent with the transfer efficiencies calculated using the PL emission intensity data. These results show that the high transfer efficiencies of EE and Zni to VZn emission centers emitting at 378 and 396 nm is mostly due to the energy transfer from ZnO-nc to Eu3+ ions, resulting in an emission at 614 nm, while the energy transfer from the oxygen defect emissions at 417, 450, 500 and 575 nm is most probably due to the energy transfer from ZnO-nc to the defects created due to the incorporation of the Eu3+ ions in the Eu3+ :ZnO-nc:SiO2 samples, as proposed in our previous work. Moreover, the stretching exponential coefficient values, which are a measure of interaction between two or more identical ZnO-nc emission centers, show that the 360-nm band edge emission (QC) experiences lesser interactions compared to the EE, Zni to VZn , and Oi emissions centered at 378, 396, and 417 nm, respectively. These are important results which will help in the fabrication of future efficient light sources based on the energy transfer process from semiconductor nanocrystals. Acknowledgments: We would like to thank Sun Handong and Wang Yue from School of Physical and Mathematical Sciences, Nanyang Technological University for helping us to measure the time-resolved photoluminescence spectra of the samples used in this study. We would also like to thank Academic Research Fund Tier 1 funding for the financial support. Vivek Mangalam would like to thank Nanyang Technological University for the Research Student Scholarship provided to him. Author Contributions: Vivek Mangalam performed the experimental work, analyzed the data, and started the write-up. Kantisara Pita initiated and supervised the research work and improved the manuscript for submission and publication. Conflicts of Interest: The authors declare no conflict of interest.

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