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  • Open Access

Study of energy transfer mechanism from ZnO nanocrystals to Eu3+ ions

Nanoscale Research Letters201611:73

https://doi.org/10.1186/s11671-016-1282-3

  • Received: 7 October 2015
  • Accepted: 26 January 2016
  • Published:

Abstract

In this work, we investigate the efficient energy transfer occurring between ZnO nanocrystals (ZnO-nc) and europium (Eu3+) ions embedded in a SiO2 matrix prepared using the sol-gel technique. We show that a strong red emission was observed at 614 nm when the ZnO-nc were excited using a continuous optical excitation at 325 nm. This emission is due to the radiative 5D0 → 7F2 de-excitation of the Eu3+ ions and has been conclusively shown to be due to the energy transfer from the excited ZnO-nc to the Eu3+ ions. The photoluminescence excitation spectra are also examined in this work to confirm the energy transfer from ZnO-nc to the Eu3+ ions. Furthermore, we study various de-excitation processes from the excited ZnO-nc and their contribution to the energy transfer to Eu3+ ions. We also report the optimum fabrication process for maximum red emission at 614 nm from the samples where we show a strong dependence on the annealing temperature and the Eu3+ concentration in the sample. The maximum red emission is observed with 12 mol% Eu3+ annealed at 450 °C. This work provides a better understanding of the energy transfer mechanism from ZnO-nc to Eu3+ ions and is important for applications in photonics, especially for light emitting devices.

Keywords

  • Zinc oxide nanocrystals
  • Energy transfer mechanism
  • Europium(III) ions
  • Photoluminescence

Background

In recent years, there has been a lot of interest in having a strong interaction between semiconductor nanocrystals and rare earth (RE) ions [1, 2]. Semiconductor nanocrystals have been used as sensitizers to excite RE ions due to their intrinsic properties such as size-dependent luminescent properties [3], large absorption cross sections [46] and broad excitation spectra [6]. In this process, semiconductor nanocrystals are excited using a light source and transfer the energy to the RE ions which can then reemit light at a different wavelength. Understanding the energy transfer mechanisms between semiconductor nanocrystals and RE ions helps in developing energy-efficient light sources such as white light sources [7], fibre amplifiers [4, 5], field emission displays [5, 8], fluorescent lamps and solid state lasers [5].

The energy transfer between several types of semiconductor nanocrystals and RE ions, such as silicon nanocrystals (Si-nc) and Er3+ ions [9], ZnO-nc and Ce3+ ions [5], ZnO-nc and Er3+ ions [10], and ZnO-nc and Tb3+ ions [11], have been studied. The energy transfer between ZnO-nc and Eu3+ ion has also been observed [6, 7, 1216], and this system has been investigated because of the usefulness of the sharp red emission from Eu3+ ions centred at 614 nm. The Eu3+ ions are usually either co-doped with ZnO-nc in a dielectric host matrix like SiO2 [6, 16] or embedded inside the ZnO-nc [7, 1215]. Co-doping of ZnO-nc and Eu3+ ions in a dielectric matrix is preferable due to the physical and chemical protection it provides to the dopants [17]. While some studies [6, 12, 13, 16] report that the energy transfer take place with the involvement of the defect states in ZnO-nc, others [14, 15] report that this energy transfer is predominantly due to the free or bound excitonic state emissions of ZnO-nc. However, a comprehensive understanding of this energy transfer mechanism and the contribution of energy transfer from the various ZnO-nc emissions has not been reported and thus needs to be investigated. In particular, this is important to develop efficient ZnO-nc and Eu3+ ion light-emitting devices and also make an important contribution to have a better understanding of the energy transfer processes from nanocrystals to RE ions, in general.

In this article, we present in detail the contribution of the various de-excitation processes of excited ZnO-nc embedded in SiO2 matrix in the energy transfer process from ZnO-nc to Eu3+ ions and we suggest a suitable mechanism for the energy transfer process. This is an extension of our earlier work [18], where the various de-excitation processes of the ZnO-nc in SiO2 were identified as being made of seven contributions. In this work, the low-cost sol-gel process was used to make the samples due to the flexibility of controlling the material composition and the structures of the thin film. Fabrication parameters of this technique like annealing temperature and Eu3+ ion concentration have been studied and optimised to achieve maximum red emission from the Eu3+ ions. We are then able to provide the best parameters in order to get the strongest energy transfer and thus get the strongest red emission at 614 nm.

Methods

The low-cost sol-gel technique was used to prepare three different types of samples, namely Eu3+ ions incorporated in SiO2 matrix (Eu3+:SiO2); ZnO nanocrystals embedded in SiO2 matrix (ZnO-nc:SiO2) and Eu3+ ions and ZnO-nc incorporated in SiO2 matrix (Eu3+ x :(ZnO-nc:SiO2) where x is the concentration of Eu3+ ions in molar fraction, and calculated using \( x=\frac{\mathrm{moles}\ \mathrm{of}\ \left({\mathrm{Eu}}^{3+}\right)}{\mathrm{moles}\ \mathrm{of}\ \left({\mathrm{Eu}}^{3+}+\mathrm{Z}\mathrm{n}+\mathrm{S}\mathrm{i}\right)} \)). Different Eu3+ x :(ZnO-nc:SiO2) samples were prepared with x ranging from 0.04 to 0.16. For ZnO-nc:SiO2 and Eu3+ x :(ZnO-nc:SiO2) samples, the Zn:Si molar ratio was maintained at 1:2. For the Eu3+:SiO2 sample, the molar ratio of Eu3+:Si was kept the same as that in the Eu3+ 0.12:(ZnO-nc:SiO2) sample. The preparation method used for the above samples is similar to that described in our previous publication [18]. In the first step of the three-step process, the precursor, the solvent and the catalyst were mixed to create the sol. Two different sols for SiO2 matrix and ZnO-nc were developed from tetraethyl orthosilicate (TEOS) and zinc acetate as precursors, respectively. To incorporate the Eu3+ ions, europium(III) nitrate was added into the TEOS sol after ageing the sol for 24 h. The two sols were then mixed together and spin coated on a (100) Si wafer substrate. These samples were soft baked and were then annealed using rapid thermal processing (RTP) at various annealing temperatures ranging from 450 to 600 °C for 1 min in an O2 environment. The formation of ZnO-nc in SiO2 using this fabrication recipe was previously studied and verified using TEM images [18]. These samples had a thickness of approximately 300 nm, which was measured using a Dektak 3 profilometer. The characterisation of the samples was done by studying room-temperature photoluminescence (PL) emission spectra and photoluminescence excitation (PLE) spectra using a spectrofluorometer (SPEX Fluorolog-3 Model FL3-11). For the PL emission spectra, the samples were excited at 325 nm using a 450-W xenon short arc lamp coupled to a monochromator and a 325 nm line filter, while the PLE spectra were obtained by measuring the 614 nm emission intensity of the samples while varying the excitation wavelength from 325 to 550 nm using the monochromator.

Results and discussion

The PL spectra of Eu3+:SiO2, ZnO-nc:SiO2 and Eu3+ 0.12:(ZnO-nc:SiO2) samples annealed at 450 °C using RTP are shown in Fig. 1. The PL emission of the SiO2 film alone (not shown) prepared using the sol-gel method showed negligible emission, which indicates that the spectra of the samples shown in Fig. 1 are not affected by the presence of the host SiO2 matrix. Firstly, we observe that the ZnO-nc:SiO2 sample shows a broadband emission from the ZnO-nc. The ZnO-nc broadband emission follows a trend similar to the one in our previous study [18]. We also observe that the Eu3+:SiO2 sample shows negligible emission at all wavelengths including at 614 nm. This is a strong evidence that the optical excitation at 325 nm does not directly excite the Eu3+ ions in the sample [19]. The Eu3+ 0.12:(ZnO-nc:SiO2) sample, however, shows a broadband emission along with two high-intensity sharp peaks at 590 and 614 nm. The 590 and 614 nm emissions are known as the 5D0 → 7F1 and 5D0 → 7F2 transitions from the Eu3+ ions [19] (see the energy level diagram in Fig. 2b). These results clearly demonstrate that the 325 nm light source excites the ZnO-nc which can then transfer the energy to the Eu3+ ions which in turn gives a strong emission in the red. In addition, we can see that the intensity of the broadband emission from the Eu3+ 0.12:(ZnO-nc:SiO2) sample between 350 and 575 nm is lower than that from the ZnO-nc:SiO2 sample, indicating that the reduction of the emission intensity is due to the energy transfer from ZnO-nc to the Eu3+ ions. The insets (a and b) in Fig. 1 show, respectively, the photograph of Eu3+:SiO2 and Eu3+ 0.12:(ZnO-nc:SiO2) samples optically excited using a mercury vapour UV lamp. The bright red emission, due to the ZnO-nc mediated excitation of the Eu3+ ions, is clearly visible from the Eu3+ 0.12:(ZnO-nc:SiO2) sample.
Fig. 1
Fig. 1

PL spectra of Eu3+:SiO2, ZnO-nc:SiO2 and Eu3+ 0.12:(ZnO-nc:SiO2) samples. Insets a) and b) show, respectively, the photograph of Eu3+:SiO2 and Eu3+ 0.12:(ZnO-nc:SiO2) samples excited using a mercury vapour UV lamp

Fig. 2
Fig. 2

a PLE spectra of Eu3+:SiO2, ZnO-nc:SiO2 and Eu3+ 0.12:(ZnO-nc:SiO2) samples. b Schematic Dieke energy level diagram of the Eu3+ ions in SiO2

Further evidence of energy transfer from ZnO-nc to the Eu3+ ions is seen from the PLE spectra of Eu3+:SiO2, ZnO-nc:SiO2 and Eu3+ 0.12:(ZnO-nc:SiO2) samples annealed at 450 °C using RTP shown in Fig. 2a. As mentioned above in the ‘Methods’ section, the PLE spectra gives the measure of the 614 nm red emission intensity of the samples as function of varying excitation wavelengths. Firstly, we note that the PLE spectrum of the ZnO-nc:SiO2 shows a very low 614 nm emission at all excitation wavelengths. This very low 614 nm emission of the ZnO-nc:SiO2 sample is due to the slight oxygen defect emission from ZnO-nc (explained in Fig. 4), upon excitation of ZnO-nc. Figure 2a also shows the PLE spectrum of Eu3+:SiO2 sample, in which we observe the five characteristic excitation peaks of the Eu3+ ions centred at 360, 380, 392, 412 and 463 nm which are due to 7F0 → 5D4, 7F0 → 5G J(J = 2−5), 7F0 → 5L6, 7F0 → 5D3 and 7F0 → 5D2 transitions [19] of Eu3+ ions, respectively. The various excitation peaks of the Eu3+ ions in SiO2 are represented schematically in the Dieke energy level diagram [20] in Fig. 2b. The Eu3+ ions in this sample upon excitation at the five peak wavelengths directly get excited and subsequently relax to the ground state through the radiative emission at 614 nm. Interestingly, the PLE spectra of the Eu3+ 0.12:(ZnO-nc:SiO2) sample shows a strong and broad 614 nm emission profile for excitation wavelengths between 325 and 370 nm, which then reduces till 450 nm. In this broad range, the 614 nm emission of the Eu3+ 0.12:(ZnO-nc:SiO2) sample is much greater than that of both Eu3+:SiO2 and ZnO-nc:SiO2 samples. This is due to the fact that upon excitation at wavelength less than 450 nm, the ZnO-nc in the Eu3+ 0.12:(ZnO-nc:SiO2) sample were excited which then transferred the energy to the Eu3+ ions in the sample through the various excitation peaks of the Eu3+ ions at 360, 380, 392, 412 and 463 nm. The excited Eu3+ ions subsequently relaxed to the ground states giving the red emission at 614 nm. We can also observe two other striking features for the PLE of Eu3+ 0.12:(ZnO-nc:SiO2) sample. First of all, there is a bit of a plateau between 325 and 375 nm for the PLE and this is because in this range, we are exciting the ZnO-nc above its band gap where all of the ZnO-nc emission centres [18] (explained in Fig. 4) are excited, thus resulting in large energy transfer to the Eu3+ ions. Between 375 and 450 nm, as we go below the band gap of ZnO, lesser and lesser ZnO-nc emission centres are excited which results in lesser energy transfer to the Eu3+ ion. The second feature is that we see a slight bump at 392 nm for the PLE of Eu3+ 0.12:(ZnO-nc:SiO2) due to the direct strong absorption line of the Eu3+ ions noticeable in the PLE of Eu3+:SiO2, this is in addition to the contribution of ZnO emission centres at this wavelength. Thus, these results from the PLE study undoubtedly confirms the energy transfer from ZnO-nc to the Eu3+ ions which was also observed from the PL spectra of the samples.

To obtain the optimum red emission intensity from the Eu3+ x :(ZnO-nc:SiO2) configuration, different samples with various annealing temperatures and different Eu3+ ion concentrations were fabricated and studied. Figure 3a presents the intensity of emission at 614 nm from the Eu3+ x :(ZnO-nc:SiO2) samples as a function of Eu3+ ion concentration (x ranging from 0 to 0.16) at various annealing temperatures from 450 to 600 °C. The corresponding PL spectra of the Eu3+ x :(ZnO-nc:SiO2) samples, with x ranging from 0 to 0.16, annealed at 450 °C is shown in Fig. 3b. Here, we observe that the red emission intensity shows a non-linear increase with increasing the Eu3+ ion concentration from 0 to 12 mol%. This is expected as increasing the Eu3+ ions decreases the distance between the ZnO-nc and the Eu3+ ions which results in enhanced energy transfer [6, 16]. Thus, greater fraction of Eu3+ ions are excited with increasing concentration resulting in enhanced red emission. A further increase in Eu3+ ion concentration to 16 mol% shows a decrease in the 614 nm emission intensity. This is attributed to Eu3+ ion concentration quenching [21], i.e. migration of energy amongst the Eu3+ ions which is non-radiatively dissipated through the quenching sites. The close proximity of the Eu3+ ions due to increasing concentration results in concentration quenching. This trend is observed in all the Eu3+ x :(ZnO-nc:SiO2) samples which were annealed at 450, 500, 550 and 600 °C. It is clearly shown here that the optimum Eu3+ ion concentration for maximum red emission is 12 mol%. This was also observed in Y2O3:Eu3+ thin film phosphors [22]. In Fig. 3a, we note that there is a small emission at 614 nm from the 0 mol% Eu3+ sample (i.e. ZnO-nc:SiO2 sample) annealed at 450 °C; this is due to the broadband nature of the ZnO-nc emission at this annealing temperature.
Fig. 3
Fig. 3

PL spectra and 614 nm red emission intensity from the Eu3+ x :(ZnO-nc:SiO2) samples. a 614 nm red emission intensity from the Eu3+ x :(ZnO-nc:SiO2) samples as a function of Eu3+ ion concentration at various annealing temperatures. b The PL spectra of the Eu3+ x :(ZnO-nc:SiO2) samples RTP annealed at 450 °C for various Eu3+ ion concentrations. c The PL spectra of the Eu3+ 0.12:(ZnO-nc:SiO2) samples annealed by RTP at various temperatures

In Fig. 3c, the corresponding PL spectra of the Eu3+ 0.12:(ZnO-nc:SiO2) samples RTP annealed at temperatures ranging from 450 to 600 °C is also shown. Here, we observe that increasing the annealing temperature leads to a reduction in the red emission intensity from the samples. This is due to the change in the nature of the emissions from the ZnO-nc embedded in the samples with increasing annealing temperature. Energy is transferred from the ZnO-nc to the Eu3+ ions due to the overlap of the broadband ZnO-nc emission spectra [18] and Eu3+ ion excitation spectra (see Fig. 2a). Within the emission range of ZnO-nc from 350 to 575 nm, we see that there is a strong overlap with Eu3+ ion absorption which is responsible for the resonant energy transfer from the ZnO-nc to the Eu3+ ions in the samples. The energy transfer from ZnO-nc results in the excitation of the Eu3+ ions from their ground state (7F0) to any of the higher states (5D2, 5D3, 5D4, 5G J(J = 2−5), 5L6) which subsequently relax back to their ground state by the radiative emissions in the red at 614 and 590 nm (see the energy level diagram in Fig. 2b). Since the broadband emission from the ZnO-nc is the largest at 450 °C annealing, the energy transfer will also be the strongest at this annealing temperature. By the same token, when the annealing temperature increases, the bandwidth of the broad emission from ZnO-nc decreases, thus resulting in decreasing the spectral overlap between the ZnO-nc broadband emission and the Eu3+ ion excitation and therefore a reduction in energy transfer from the ZnO-nc to the Eu3+ ions leading to a reduction in the red emission intensity from the Eu3+ ions. In addition, the energy transfer from ZnO-nc to the Eu3+ ions is inhibited in the samples annealed at 550 and 600 °C due to the possible formation of Zn2SiO4 at the surface of the ZnO-nc [23]. Formation of Zn2SiO4 reduces the size of ZnO-nc causing reduction of PL emission [23] from the ZnO-nc and also results in an increase in distance between ZnO-nc and Eu3+ ion which results in a reduction of energy transfer from the ZnO-nc to the Eu3+ ions. For our process, the optimum RTP annealing temperature has been found to be 450 °C.

In our previous work [18], we showed that the broad emission spectra of ZnO-nc embedded in SiO2 consists of seven Gaussian peaks centred at 360, 378, 396, 417, 450, 500 and 575 nm. The origins of these emissions have been discussed in reference [18]. The 360 and 378 nm peaks were attributed, respectively, to band edge emission from the smallest ZnO-nc which possibly experiences quantum confinement effect (labelled as QC) [24] and ZnO-nc excitonic emission (labelled as EE) [25, 26]. The 396 nm peak was attributed to the defect state electronic transition from Zn interstitial (labelled as Zni) to Zn vacancy (labelled as VZn) [17] and the remaining emission peaks were due to the electronic transition from, or to, the oxygen-related defects, namely oxygen interstitial (labelled as Oi) defect emission at 417 nm [27, 28], oxygen vacancy (labelled as VO) emission at 450 nm, singly ionised oxygen vacancy (labelled as VȮ) emission at 500 nm and doubly charged oxygen vacancy (labelled as VÖ) emission at 575 nm [17, 29, 30]. The schematic energy level diagram of the seven ZnO-nc emission centres is shown in Fig. 4c. The energies of ZnO-nc emission centres are shown as broad vertical energy bands shown with a colour gradient to indicate the broad emission bandwidth of the various ZnO-nc emission centres. In this paper, we analyse the contribution of each of these emissions or de-excitation centres in exciting the Eu3+ ions. In order to do so, the PL emission spectra of the ZnO-nc:SiO2 and Eu3+ 0.12:(ZnO-nc:SiO2) samples annealed at 450 °C were deconvoluted using the seven de-excitation centres of ZnO-nc as shown in Fig. 4a, b, respectively. The fitting parameters, such as peak wavelength and full width at half maxima of the Gaussian peaks, were kept the same as those in our previous publication [18].
Fig. 4
Fig. 4

The emission centres of ZnO-nc in (a) ZnO-nc:SiO2 and (b) Eu3+ 0.12:(ZnO-nc:SiO2) samples annealed at 450 °C. See the main text for the various ZnO-nc emission centres. (c) The schematic energy level diagram of the emission centres of ZnO-nc in SiO2

To study the contribution of energy transfer from the ZnO-nc emission centres to the Eu3+ ions the spectral overlap integral value of emission from each of the seven ZnO-nc emission centres in ZnO-nc:SiO2 sample with the Eu3+ excitation spectrum was firstly determined. The spectral overlap integral value gives a measure of the radiative energy transfer from the ZnO-nc emission centres to the Eu3+ ions. An example of the spectral overlap between Zni and Vzn defect state emission of ZnO-nc:SiO2 sample annealed at 450 °C and the Eu3+ excitation are shown in Fig. 5. The spectral overlap integral from each of the seven ZnO-nc emission centres is shown in Fig. 7 on the left axis (solid line).
Fig. 5
Fig. 5

An example of the spectral overlap. The spectral overlap of Zni to VZn defect state emission of ZnO-nc:SiO2 sample annealed at 450 °C along with the Eu3+ excitation spectrum

Furthermore, in Fig. 4a, b, we clearly observe that the emission intensities from various ZnO-nc emission centres in the Eu3+ 0.12:(ZnO-nc:SiO2) sample are lower than those in the ZnO-nc:SiO2 sample. This intensity difference is due to the energy loss from the various ZnO-nc emission centres because of the incorporation of the Eu3+ ions. Figure 6 shows the intensity differences of each of the seven ZnO-nc emission centres between ZnO-nc:SiO2 and Eu3+ 0.12:(ZnO-nc:SiO2) samples annealed at 450 °C. Thus, calculating the integral value of the difference in the emission intensities of each of the seven ZnO-nc emission centres in Fig. 6 gives a measure of energy loss from each of the ZnO-nc de-excitation centres in exciting the Eu3+ ions. The seven ZnO-nc emission centres intensity difference integral are also shown in Fig. 7 on the right axis (dotted line).
Fig. 6
Fig. 6

The intensity difference from ZnO-nc emission centres. The intensity difference between ZnO-nc:SiO2 and Eu3+ 0.12:(ZnO-nc:SiO2) samples annealed at 450 °C which is due to the incorporation of Eu3+ ions

Fig. 7
Fig. 7

A comparison of the spectral overlap integral and ZnO-nc emission intensity difference integral. The comparison for each of the seven ZnO-nc emission centres for the samples annealed at 450 °C

In Fig. 7, we now have the measure of the energy transfer, which is the spectral overlap integral value, and the measure of energy losses, which is ZnO-nc emission centre’s intensity difference integral value, from each of the seven ZnO-nc emission centres. Here, we observe an identical trend between the spectral overlap integral and ZnO-nc emission intensity difference integral values from the QC, EE and Zni to VZn ZnO-nc emission centres which are centred at 360, 378 and 396 nm, respectively. Amongst these, the EE and Zni to VZn have the highest spectral overlap integral and ZnO-nc emission intensity difference integral values of energy transfer. This implies that the EE and Zni to VZn ZnO-nc emission centres contribute the most to energy transfer from ZnO-nc to the Eu3+ ions. In contrast, we observe that the ZnO-nc emission intensity difference integral values from ZnO-nc emission centres like Oi, Vo and VȮ centred at 417, 450 and 500 nm, respectively, are higher than the spectral overlap integral values. We propose that this is due to defect centres induced by the incorporation of the Eu3+ ions in the Eu3+ 0.12:(ZnO-nc:SiO2) sample, providing non-radiative de-excitation paths for Oi, Vo and VȮ emissions. This means that only some portion of the Oi, Vo and VȮ ZnO-nc emissions transfer the energy to the Eu3+ ions. A large portion of the energy from the Oi, Vo and VȮ ZnO-nc emissions decays non-radiatively through the Eu3+ ion-induced defect centres. In this way, even though the spectral overlap integral values of Oi, Vo and VȮ ZnO-nc emissions are low, their ZnO-nc emission intensity difference integral values can be relatively high since these de-excitations occur via the Eu3+ ion induced defect states.

Conclusions

In conclusion, we have convincingly shown that efficient energy transfer takes place from excited ZnO-nc to Eu3+ ions embedded in a SiO2 matrix. This energy transfer gives strong red emission at 614 nm from the Eu3+ ions due to the 5D0 → 7F2 transition. This was observed using the PL emission spectra of the abovementioned samples which were optically excited using a continuous excitation at 325 nm. The energy transfer from the ZnO-nc to the Eu3+ ion was further confirmed by studying the photoluminescence excitation spectra of the samples. The dependence of the Eu3+ red emission on the annealing temperature and also on the Eu3+ concentration in the sample was studied and optimised for an annealing temperature of 450 °C and for a Eu3+ concentration of 12 mol%. We present a detailed study of the energy transfer by identifying the contribution of the seven different emission centres of ZnO-nc in exciting the Eu3+ ions. We clearly show that the EE and Zni to VZn ZnO-nc emission centres have the highest contribution to the energy transfer from ZnO-nc to the Eu3+ ions. While the Oi, Vo and VȮ ZnO-nc emission centres have low energy transfer contributions but high energy loss due to the presence of Eu3+ ions induced defect centres in the Eu3+ 0.12:(ZnO-nc:SiO2) sample. By understanding the mechanism of energy transfer from ZnO-nc to Eu3+ ions and optimising the fabrication parameters in producing the highest red emission from Eu3+ ions, we can make future proficient red luminescent solid state devices based on a low-cost technique. Extending these very important analyses to systems with other rare earth elements will also help in making efficient light sources for various applications in photonics.

Declarations

Acknowledgements

K. Pita and V. Mangalam would like to thank Academic Research Fund Tier 1 funding for the financial support of this work. C. Couteau acknowledges the France-Singapore Merlion programme and the Champagne-Ardenne region for financial support via the ‘visiting professor’ scheme.

Open AccessThis article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.

Authors’ Affiliations

(1)
OPTIMUS, Centre for OptoElectronics and Biophotonics, School of Electrical and Electronic Engineering, Nanyang Technological University (NTU), Block S2, 50 Nanyang Avenue, Singapore, 639798, Singapore
(2)
CINTRA, CNRS-NTU-Thales UMI 3288, Research Techno Plaza, 50 Nanyang Drive, Border X Block, Level 6, Singapore, 637553, Singapore
(3)
Laboratory for Nanotechnology, Instrumentation and Optics (LNIO), Charles Delaunay Institute CNRS UMR 6281, University of Technology of Troyes (UTT), 12 rue Marie Curie, 10000 Troyes, France
(4)
School of Electrical and Electronic Engineering, Nanyang Technological University (NTU), Block S2, 50 Nanyang Avenue, Singapore, 639798, Singapore

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