Efficient warm-white lighting using rare-earth-element-free fluorescent materials for saving energy, environment protection and human health

Abstract

Solid-state white light emission is important for energy saving, but currently it is mainly based on environmentally unfriendly rare-earth doped phosphors or cadmium-containing quantum dots. Here, we explore an environmentally friendly approach for efficient white light emission based on ZnSe:Mn nanoparticles without rare-earth or cadmium elements. The emission is composed of a broad green-orange spectral band (525–650 nm) with the peak located at 578 nm and the color temperature is low, so it is particularly good for lighting at night to reduce risks to human health. Furthermore, the optimal absorption peak could be designed at 453 nm, which well matches the commercial blue-LED emission wavelength (445–470 nm). A quantum yield up to 84.5% could also be achieved. This rare-earth-element-free material opens up a new avenue for energy-saving, healthy, and environmentally benign lighting.

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1. Introduction

The increasing consciousness of the energy crisis makes green energy technologies very attractive for reducing power consumption. According to information from the International Energy Agency, about 20% of global electricity is consumed for illumination, so solid-state lighting is considered to be the necessary trend due to its superior energy-saving properties and long lifetime. In comparison with traditional lighting methods, the energy consumption of current LEDs is only 10–20% that of incandescent lamps and about 60% that of fluorescent lamps, so using LEDs to significantly reduce global power consumption and CO₂ emission becomes inevitable.^1,2 Therefore, the 2014 Nobel Prize in Physics was awarded to scientists who contributed to the invention of solid-state lighting.²

Current commercial white LEDs for lighting mainly rely on rare-earth doped phosphors to down-convert blue or ultraviolet (UV) light to a longer wavelength emission.^3–5 Nonetheless, rare-earth mining is a messy and polluting business that uses toxic chemicals.^6–9 Although methods like nanocrystal quantum dots (NQDs) have been proposed in recent years, efficient NQDs, such as CdSe, CdS, and Zn_xCd_1−xSe, unfortunately contain cadmium, which is still unfriendly for the environment. Progress has recently been made in the development of photoluminescent (PL) materials without rare-earth and cadmium elements. By decorating functionalized polymers on the surfaces of ZnO and Mn²⁺ doped ZnS (ZMS) nanoparticles to form core–shell structures, three physical photon-emission mechanisms for blue, green, and orange emissions can be designed in the hybrid nanocomposites,¹⁰ but the excitation is based on UV LEDs.

Because the current commercial high-efficiency blue LEDs mainly emit light at a wavelength from 445 nm to 470 nm, UV-excited nanocomposites are not suitable for such common applications. The optimal absorption band of fluorescent materials should also be in the spectral range between 445 nm and 470 nm. It is known that ZnSe is a direct bandgap semiconductor with a bandgap around 2.7–2.8 eV,^11,12 and ZnSe:Mn is a well researched phosphor currently.^13–19 However, the common synthesis ways of ZnSe-based nanoparticles for fluorescent purposes suffers the quantum confinement effect,^20–22 which might provide many advantages in other applications,^23–28 but has an adverse effect here for significantly decreasing the wavelength of absorption. The efficient excitation wavelength is mostly limited in the UV region as a result of the strong quantum confinement effect in the nanometer size.

Accordingly, here we explore an environmentally friendly avenue for efficient white light emission based on ZnSe:Mn nanoparticles with non-rare-earth, non-cadmium elements and negligible quantum confinement effect. The optimal excitation wavelength of the ZnSe:Mn nanoparticles is at 453 nm, which matches the commercial blue LED emission wavelength. In addition, by doping Mn²⁺, electrons in the ZnSe conduction band can be transferred to the Mn²⁺ energy level effectively, making ZnSe:Mn nanoparticles exhibit very high PL quantum efficiency of 84.5%. Moreover, the peak emission wavelength corresponding to the Mn²⁺ energy level is 578 nm, which produces a warm white light for human eyes.^29,30 Warm white light is better for human health than cold white light, particularly for lighting at night.^31–33 Furthermore, 578 nm is close to 555 nm, at which the human eyes have the maximum spectral sensitivity.³⁴ Therefore, our synthesized ZnSe:Mn nanoparticles (NCs) can provide energy-saving, healthy, and environmentally benign lighting.

2. Results and discussion

2.1 Impact of alkali metal on the nanoparticles size of ZnSe:Mn

ZnSe:Mn nanoparticles were prepared by a hydrothermal method. With the doped Mn²⁺ as the activator, the nanoparticles can produce a warm white light. In order to reduce the quantum confinement effect of the nanocrystals, the synthesized nanoparticles should be large in size. It has been found that adding an alkali metal hydroxide such as KOH or NaOH can adjust and increase the size of nanoparticles.^35–37 Accordingly, we apply different amounts of KOH for the growth of our nanoparticles following a similar concept. The effect of alkali metal hydroxide on the ZnSe:Mn NCs is experimented. We also add an excess of Zn (NO₃)·6H₂O and Mn (CH₃COO)₂·4H₂O to dissociate Zn²⁺ and Mn²⁺ ions, so the solution could provide sufficient elements of Zn and Mn for the formation of a relatively large ZnSe:Mn NCs. (Detailed description is given in the Method section).

The crystal characterization of the ZnSe:Mn NCs is examined using X-ray diffraction (XRD). The result is shown in Fig. 1 (see ESI S1†). To further verify the particle size, TEM images (Fig. 2) of the nanoparticles are taken to directly observe the effect of KOH on the size of ZnSe:Mn NCs. Fig. 2(a)–(d) correspond to 0 g, 0.5 g, 2 g, and 5 g of KOH, respectively. According to the TEM images, the sizes of the nanoparticles are estimated to be 5.1 nm, 6.6 nm, 10.1 nm, and 13.1 nm, corresponding to 0 g, 0.5 g, 2 g, and 5 g of KOH, respectively. Therefore, whether XRD or TEM is used to examine the nanoparticle size, the trend is the same.

Fig. 1 XRD spectra of ZnSe:Mn NCs with 0 g, 0.5 g, 2 g and 5 g of KOH.

Fig. 2 TEM of ZnSe:Mn NCs with different amount of KOH (a) KOH 0 g, (b) KOH 0.5 g, (c) KOH 2 g, (d) KOH 5.

Next, the relationship between the KOH concentration and the optical properties of ZnSe:Mn NCs is investigated. The PLE spectra are shown in Fig. 3(a). All of the emission profiles are composed of a broad green-orange emission band (525–650 nm) with the peak located at 578 nm (see ESI S2†). The optical route is as follows. When the excitation energy, which corresponds to the blue light, matches the quantized energy levels of the ZnSe nanoparticles, electrons can transit from the valance band to the conduction band. Then the electrons at the conduction band of ZnSe nanoparticles can transfer to the energy levels of Mn²⁺ ions and transfer back to the valence band through the ₄T¹ → ₆A¹ transition, resulting in orange emission.^38,39 Because of the quantum confinement effect of the small particle size, the highest PLE intensity is under 440 nm excitation for 0 g and 0.5 g KOH. As the amount of KOH is increased to 2 g and 5 g, the best PLE intensity occurs at 450 nm excitation, which well matches the ideal bandgap of ZnSe, 2.7–2.8 eV (442–460 nm). This indicates that KOH can give rise to the absorption at the ideal bandgap corresponding to the bulk ZnSe:Mn crystals by reducing the quantum confinement effect.

Fig. 3 (a) and (b) are photoluminescence emission spectra of ZnSe:Mn NCs with different amounts of NaOH and KOH. (c) Excitation wavelength and nanoparticle size by adding different amounts of KOH and NaOH. (d) Theoretical curve and experimental results.

We further confirm the above effect by investigating the relationship between NaOH (0, 0.4, 1.6, 4 g) and optical properties of ZnSe:Mn NCs. Again, for different amounts of NaOH, the emission peak occurs at 578 nm. Also, the PLE peak shifts from below 420 nm to 447 nm as the NaOH changes from 0 g to 4 g (Fig. 3(b)), exhibiting the same trend as adding KOH. However, the PL intensity here is less than that associated with KOH. This is because the Na⁺ ion is small and thus can penetrate into the ZnSe:Mn NCs and exist therein, hence creating non-radiative recombination centers for electrons and holes and lowering the probability of radiative recombination. The results of the trends for excitation wavelength and nanoparticle size by adding different amounts of KOH and NaOH are shown in Fig. 3(c).

The above relation is also examined from the theoretical evaluation. The theoretical curve of the relationship between the nanoparticle size and the energy gap based on the Brus equation^40–43 is plotted in Fig. 3(d). The previous experimental sizes and absorption peak energies of ZnSe:Mn NCs (Table 1) are also marked in Fig. 3(d). It shows that our experimental results match the theoretical evaluation well. Thus the quantum confinement effect is effectively reduced with the added KOH and the photon energy of excitation is lowered.

Table 1 Relationship between particle size and energy gap

KOH (g)	0	0.5	2	5
Particle size (nm)	5.1	6.6	10.1	13.1
Excitation wavelength (nm)	435	440	447	453

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2.2 Impact and reduce of coalescence on the ZnSe:Mn nanoparticles

Owing to the small size, the nanoparticles have high surface area and tend to coalesce. Appropriate coalescence can increase the size of nanoparticles. However excessive coalescence may reduce the luminescence intensity. The reason is as follows. We can divide into two phases to discuss, excitation source (blue light) and emission light (orange light). As to excitation source, excessive coalescence prevents blue light from penetrating deeply into the ZnSe:Mn nanoparticles, so decreasing the absorption and the emission from ZnSe:Mn nanoparticles. On the other hand, the excessive coalescence makes the inside of ZnSe:Mn nanoparticles less absorb the blue light and hence can absorb the emission light, causing a strong internal consumption and scattering, as schematically shown in Fig. 3. Moreover, coalescence makes the contact area between nanoparticles increase and produce more interface defects, so the non-radiative recombination increases and reduces quantum efficient. Similar concepts have been discussed in ref. 44–46.

To solve the coalescence problem, we add an anionic surfactant and then the nanoparticles become mutually exclusive and disperse well in the solution (Fig. 4(b)).

Fig. 4 (a) Schematic diagram of coalescence (left side) and dispersion (right side); (b) schematic diagram of ZnSe:Mn NCs encapsulated by anionic surfactant; (c) effect of the growth solution with anionic surfactant; (d) effect of adding the citric acid.

Accordingly, without the coalescence, nanoparticles can well absorb and emit photons, hence enhancing the PL intensity of the nanoparticles. Fig. 4(c) shows the PL of ZnSe:Mn NCs measured at different excitation wavelengths (430 nm, 440 nm, and 450 nm) with and without the anionic surfactant. The nanoparticles with anionic surfactant give higher intensity than the nanoparticles without it, indicating that adding anionic surfactant can reduce coalescence efficiently. It is noteworthy that the anionic surfactant has relatively low toxicity compared to TOPO and HDA, which are conventionally used as a stabilizer for quantum dots and nanoparticles. TOPO and HDA are highly toxic, so their applications are limited to quantum dots.^47,48

2.3 Impact and reduce of oxidative deterioration on the ZnSe:Mn nanoparticles

Although the anionic surfactant can provide a partial function of stabilization, the large surface-to-volume ratio of nanoparticles gives a large number of dangling bonds that exist at the surface and so still tend to have oxidative deterioration.^49,50 In order to improve the oxidation resistance and stability, we add citric acid, which is a metal chelating agent and can adhere well to the exposed zinc ions to chelate the particles and prevent oxidation. Citric acid is odorless, non-toxic, and commonly used in food additives, so it is more in line with environmental health purposes.⁵¹ The PL intensity of the orange color further increases with the amount of citric acid (0, 0.05, 0.1 g), as shown in Fig. 4(d). These results demonstrate that citric acid can passivate the surface of nanoparticles, reduce the nonbonding pairs, and enhance the emission at 578 nm.

2.4 Decay curves, lifetimes and EDS of ZnSe:Mn nanoparticles

Time resolved PL (TRPL) measurements are also performed to further investigate the ZnSe:Mn nanocomposites. We use a fluorescence lifetime imaging microscopy (FLIM) system to measure the decay curves and lifetimes of ZnSe:Mn nanoparticles with various processes. The emission wavelengths are monitored at 578 nm and the results are shown in Fig. 5. The sample names “NaOH” and “KOH” imply that the processes include NaOH 4 g and KOH 5 g respectively, but without dispersion and citric acid. The sample “Dispersion” implies that the process includes KOH 5 g and dispersion 5 ml. “Citric acid” implies that the process includes KOH 5 g, dispersion 5 ml and citric acid 0.1 g. The detailed values of lifetimes associated with various decay processes are presented in ESI S3.† Accordingly, we have obtained the emission lifetimes τ_Av = 0.826 ns, 3.789 ns, 4.9774 ns and 5.048 ns for the sample “NaOH”, “KOH”, “Dispersion” and “Citric Acid”, respectively. Note that the processes including dispersion and citric acid exhibit a much longer carrier lifetime. It indicates “Dispersion” and “Citric Acid” with lower defect energy levels. We use the above samples to measure the EDS spectra. The elemental concentration obtained for all the NCs is Mn = 1.5 atom%; Zn = 52.3 atom%; and Se = 46.2 atom% (ESI S4†).

Fig. 5 Lifetimes associated with various decay processes in ZnSe:Mn nanocomposites.

2.5 Dry ZnSe:Mn solution into powder

For practical applications, ZnSe:Mn in the solution has to be dried and becomes powder for later use. We divide this process into two stages, rinsing and vacuum heating. In our experiment, DI water, methanol, and isopentane are used as the rinse solution to remove 2-propanediol, respectively. First, we use DI water to rinse and heat it at 60 °C, 1 × 10⁻³ torr for 48 h. It can be clearly observed that the nanoparticles oxidize and turn scarlet, almost losing their original characteristics (Fig. 6(a) and (d)). Then we rinse them with methanol, also heating it at 60 °C and 1 × 10⁻³ torr. Compared to the process using DI water, the drying time (12 h) and oxidation are greatly reduced by the low boiling point and highly volatile properties of methanol, as seen from Fig. 6(b), indicating that oxidation is reduced by shortening the drying time. Unfortunately, the PL intensity of such powder is also low. The reason is possibly because water and methanol have free OH⁻ bonds, making the oxidative deterioration of nanoparticles occur easily.

Fig. 6 ZnSe:Mn nanoparticles obtained with the process being rinsed with (a) water, (b) methanol, (c) isopentane. PL intensity of ZnSe:Mn solution and powder with the process being rinsed with (d) water, (e) isopentane.

Based on the above experiments, the saturated carbon chain isopentane is used instead (60 °C, 1 × 10⁻³ torr) for the rinse process. The result shows that the ratio of scarlet nanoparticles increases with the drying time, but the PL intensity from such powder is much higher than from the solution type. Thus we further shorten this process time through a low pressure (24 h, 1 × 10⁻⁴ torr) to make the powder exhibit a nice yellow color (Fig. 6(c)). The corresponding PL intensity of the powder is six times higher than in the solution (Fig. 6(e)). The powder is further experimented on by placing it between a pair of cover glasses to check the PL properties. Its schematic is shown in Fig. 7(a). The material is illuminated with a commercial blue LED at the wavelength of 450 nm. Extremely bright PL with yellow-orange color can be observed, as shown from the photo in Fig. 7(b), and we use an analysis program to calculate the CIE (Ra) and CCT (K) values. The results are 50 Ra and 4879 K.

Fig. 7 (a) Schematic diagram of ZnSe:Mn NCs powder with glass covering; (b) bright PL with yellow-orange color emitted from ZnSe:Mn NCs powder excited by commercial blue LED.

Compared with other REE-free fluorescent materials for warm white lighting,^52,53 the phosphor explored in this work gives a higher quantum efficiency of 84.5%, as shown in Fig. 8 (the sample parameters of 130 °C growth temperature, 1.5 h growth time, 2 g KOH, 5 ml anionic surfactant and 0.1 g citric acid, see ESI S5 and S6†). The potential luminous efficacy of our phosphors is evaluated reaching 286 lm W⁻¹. Packaging efficiency also plays an important role in the eventual luminous efficacy. In general, packaging efficiency can reach 50–80%,⁵⁴ so the potential efficacy using our materials for blue LEDs could be as large as 229 lm W⁻¹ (see ESI S7†).

Fig. 8 ZnSe:Mn with 84.5 quantum efficiency (2 g KOH, 5 ml anionic surfactant and 0.1 g citric acid).

3. Conclusions

In conclusion, we have demonstrated a novel design strategy to achieve ZnSe:Mn NCs with low quantum confinement effect with the yellow-orange emission from the 6A1 to 4T1 transition of Mn²⁺. The process of the ZnSe:Mn NCs is non-rare-earth, non-cadmium elements and matches the commercially available blue LEDs of high WPE. The corresponding emission from such nanoparticles exhibits a low color temperature of white light, which is more suitable for human health than high-color temperature light at night according to some recent studies. The PL quantum efficiency is up to 84.5%, pumped by commercial blue LEDs at the wavelength of 450 nm. The potential luminous efficacy of the warm white light is 286 lm W⁻¹ for being excited from practical blue LEDs and reduced to 229 lm W⁻¹ with the packaging loss taken into account. This novel approach opens up a new avenue for environmentally friendly, energy-saving, and healthy lighting for human life.

4. Method

4.1 Materials

Zn(NO₃)·6H₂O (Sigma-Aldrich, 99.0%), Mn(CH₃COO)₂·4H₂O (Sigma-Aldrich, 99.99%), NaBH₄ (Sigma-Aldrich, 98%), selenium powder (Alfa Aesar, 99%), 1,2-propanediol (Sigma-Aldrich, 99%), citric acid (Sigma-Aldrich, 99%), and dispersant (Information Technology Inc.). Synthesis of ZnSe:Mn Nanoparticles. ZnSe:Mn nanoparticles were prepared by a hydrothermal method. At first, 4.0 mmol zinc nitrate and 0.8 mmol manganese acetate were dissolved in 30 ml 1,2-propanediol. Then, 8.0 mmol sodium borohydride was dissolved into 10 ml de-ionized water and 4 mmol selenium powder was added into the solution. The solution was subsequently added drop-wise into the propyl glycol solution. KOH of 0, 0.5, 2 and 5 g (NaoH of 0, 0.4, 1.6 and 4 g) was dissolved into 5 ml de-ionized water respectively and added into the above mixture solution. After adding dispersant, it was continuously stirred for 10 minutes and heated at 130 °C for 1.5 h. Finally, citric acid was added into the solution and heated at 130 °C for 0.5 h. The product was filtered out, washed several times with different rinse solvents, including de-ionized water, methanol, and isopentane, and then dried under low pressure as purified ZnSe:Mn nanoparticles.

4.2 Measurements

The crystal characterization of the ZnSe:Mn nanoparticles was examined using an X-ray diffraction (XRD) meter (X’PERT) with a Cu Kα radiation source (l = 0.15406 nm) operated at 45 kV and 40 mA with a step size of 0.02u. The morphologies were imaged and analyzed using a transmission electron microscope (TEM, JEOL JEM-1200EX II). The emission and excitation properties of the ZnSe:Mn nanoparticles were measured at room temperature via using a modular fluorescence spectrophotometer (HitachiF-4500, Tokyo, Japan) with a Xenon lamp as the light source. The decay curves and lifetimes of the ZnSe:Mn nanoparticles were measured using a Fluorescence Lifetime Imaging Microscopy (FLIM) system with PicoHarp300 and TTTR Mode combined with PHR 800 router (PicoQuant). The energy-dispersive X-ray spectroscopy (EDS) of the ZnSe:Mn nanoparticles was examined using a cold-field emission scanning electron microscope (Hitachi S-4800).

Acknowledgements

We acknowledge Ministry of Science and Technology of Taiwan, National Taiwan University, and Information Technology Incorporation for supporting the research with the contract numbers: MOST 104-2119-M-002-017, MOST 103-2221-E-002-132-MY3, MOST 104-2221-E-002-139-MY3, NTU-CESRP-104R7607-1, NTU-ICRP-104R7558, MOST 104-3113-E-002-010 and MOST 104-3113-E-002-019.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra19553a

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