Environmentally Benign Technology for Efficient Warm-White Light Emission
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Pin-Chun Shen, Ming-Shiun Lin, and Ching-Fuh Lin*
Graduate Institute of Photonics and Optoelectronics, National Taiwan University, No.
1, Section 4, Roosevelt Road, Taipei, 10617 Taiwan (R.O.C.)
E-mail: lincf@ntu.edu.tw
Telephone: +886-2-3366 3540
Supplementary Information
Content:
1.
Characterization of ZMS nanoparticles
2.
Optical properties of ZMS nanoparticles
3.
SEM image of the PF-uncovered ZnO nanoparticles
4.
CIE chromaticity diagram and color temperature of (6%)ZMS(PF)ZnO sample pumped at different excitation wavelengths.
5.
Emission Lifetimes Calculation
6.
Experimental setup of the photoluminescence system
7.
Measuring method of the photoluminescence quantum efficiency (PLQE)
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1. Characterization of ZMS Nanoparticles
In this work, the Mn +2 -doped ZnS (ZMS) nanoparticles are prepared by hydrothermal method. Structural characterization of the ZMS nanoparticles was performed by X-ray powder diffraction (XRD), as shown in Figure S1. The three peaks located at 28.6
ﾟ , 47.5
ﾟ , and 56.3
ﾟ correspond to the (111), (220), and (311) planes, respectively. They indicate that the hydrothermally prepared ZMS nanoparticles are with the zinc blende structure 1 . The average size of ZMS nanoparticles can be calculated from Debey-Scherrer’s formula:
D=0.9λ/βcosθ (Equation S1) where D is the average particle diameter, β is the full width at half maximum
(FWHM) in radian of the diffraction peak, θ is the Bragg’s diffraction angle, and λ is the wavelength for the Kα1 component of the employed copper radiation. Thus, according to the (111) plane of XRD data, the average size of the ZMS nanoparticles is approximately 22 nm.
Figure S1 ∣ Structural characterization of ZMS nanoparticles. X-ray diffraction pattern of ZMS nanoparticles as obtained from hydrothermal synthesis.
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2. Optical Properties of ZMS Nanoparticles
Figure S2 shows the normalized photoluminescence excitation and emission spectra of ZMS nanoparticles at room temperature. The excitation spectrum monitored at 602 nm emission covers the near-UV regions from 300 nm to 400 nm.
The excitation spectrum peaks at 350 nm, which corresponds to the energy around
3.54 eV. Compared to the bulk ZnS (3.66 eV), the slight reduction of band gap for
ZMS nanoparticles may result from the decrement of electron binding energy caused by the Mn +2 impurity in ZnS lattice. The emission spectra are excitation-wavelength-independent. Excitation with light at 365, 375 and 385 nm can consistently produce a very broad band of single Mn 2+ emission. It has a peak around
602 nm and covers a wide spectral region ranging from yellow to deep red, which is associated with the Mn +2 4 T
1
6 A
1
transition 2 .
Figure S2 ∣ Optical characterization of ZMS nanoparticles. Normalized luminescence excitation and emission spectra of ZMS nanoparticles prepared by hydrothermal method.
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3. SEM image of the PF-uncovered ZnO nanoparticles
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Figure S3 ∣ SEM image of the PF-uncovered ZnO nanoparticles dispersed in toluene solvent.
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4. CIE chromaticity diagram and color temperature of (6%)ZMS(PF)ZnO sample pumped at different excitation wavelengths.
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Figure S4 ∣ CIE chromaticity diagram of (6%)ZMS(PF)ZnO sample pumped at different excitation wavelengths.
Table S1 ∣ Values of CIE coordinates and color temperature for
(6%)ZMS(PF)ZnO sample pumped at different excitation wavelengths.
Excitation Wavelength
(nm)
365
375
385
CIE Coordinates
(x, y)
(0.430, 0.404)
(0.377, 0.381)
(0.340, 0.354)
Color Temperature
(K)
3113
4114
5213
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5. Emission Lifetimes Calculation
A standard two-exponential-component model was used to study the recombination dynamics, and thus the decay curves can be described by:
I(t) = A
1
exp(-t / τ
1
) + A
2 exp(-t / τ
2
) where A
1 and A
2 are pre-exponential constants and τ
1 and τ
2 are the decay time of the two two exponential components. The emission lifetimes were then determined from the decay time of the two exponential components and the pre-exponential factors using the following relation:
τ avg
= (A
1
τ
1
+ A
2
τ
2
) / (A
1
+ A
2
)
The detailed values of lifetimes associated with various decay processes are presented in Table S2.
Table S2 ∣ Lifetimes associated with various decay processes in PF(ZnO) nanocomposites
Peak position (nm) A
1
τ
1
(ns) A
2
τ
2
(ns)
τ avg
(ns)
420
530
86.9
342.5
18.83
11.87
4743
906.8
0.04
3.25
0.379
5.935
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6. Experimental Setup of the Photoluminescence System
The experimental setup of system used for measuring the PL spectra of samples is schematically depicted in the following diagram. A 325 nm He-Cd laser, 365 nm
UV LED, 375 nm UV LED, and 385 nm UV LED were employed as excitation sources to measure the photoluminescence properties of the samples. These UV-LED excitation sources were operated at 5 V. The PL spectra of the samples (ZnO, ZnS:Mn,
PF, and ZMS(PF)ZnO nanocomposites) were measured using this photoluminescence system. The measurements were conducted at room temperature.
Figure S5 ∣ Schematic diagram of the photoluminescence system.
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7. Measuring method of the photoluminescence quantum efficiency (PLQE)
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In this work, the photon-conversion performance, namely, PLQE is defined using the following relation:
( Equation S2) where h is Planck’s constant, c is the speed of light, λ is the wavelength, and I(λ) is the intensity of PL spectrum at a wavelength of λ.
The measuring method of the PLQE, which is presented in the manuscript, is described as following:
First, we measured the PL spectrum of the reference (the excitation source, 365 nm/ 375 nm UV LED, irradiated the substrate) using an integrating sphere. The PL spectrum can be further transformed to the number of incident photons emitted from the excitation source using the relation:
( Equation S3).
Then spectrum of the incident photon number can be obtained, as depicted by
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9 yellow line in Figure S6.
Second, the PL spectrum of the sample was also measured from the same integrating sphere pumping by above-mentioned UV-LED light source (365 nm/ 375 nm). Hence we acquired a PL spectrum for the sample. It was then transformed to the spectrum of photon number using Equation S3, which can indicate the number of photons emitted as photoluminescence from the sample and the number of photons not absorbed by the sample, as depicted by green line in Figure S6.
As a result, according to these spectra of photon number, both the number of photons emitted from the sample and the number of photons absorbed by the sample can be obtained, making a comparison between the integral area of the reference and that of the sample. As shown in Figure S6, the blue area and red area represent the number of photons absorbed and the number of photons emitted, respectively.
Therefore, the PLQE can be calculated from Equation S2.
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Figure S6 ∣ Schematic diagram of absorption (blue) and emission region (red) for the sample.
References:
1.
Xu, J., Ji, W., Lin, J., Tang, S. & Du, Y. Preparation of ZnS nanoparticles by ultrasonic radiation method. Appl. Phys. A 66 , 639-641 (1998).
2.
Bhargava, R., Gallagher, D., Hong, X. & Nurmikko, A. Optical properties of manganese-doped nanocrystals of ZnS. PhRvL 72 , 416 (1994)..
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