Through the use of complex core-shell structures, white light emissions are observed from a single nanoparticle doped with multiple trivalent rare-earth ions. The internal structuring of the nanoparticle to allow for a controlled degree of energy transfer a common excitation wavelength yielding the white light emission is obtained. Emissions with correlated color temperatures ranging from 2700K to 5000K were produced. The stability of the phosphors to excitation wavelength variations was examined.
© 2006 Optical Society of America
There are two general approaches to generate white light: (1) mixing three monochromatic sources (red, green, and blue) and (2) using phosphors to convert UV or blue light into a combination of red, green and blue or yellow and blue. This letter will focus on the latter technique of using a phosphor to produce white light.
At present, the most commonly used white light source for light emitting diodes (LEDs) is a GaN blue LED encapsulated with a Ce3+:YAG yellow-emitting phosphor coating. The quality of the white light is determined by the correct blue-to-yellow intensity ratio, density, particle size of phosphor and the coating quality [1, 2]. Due to the lack of a green emission, the color rendering index (CRI) is not as high as multiple phosphor systems. The color stability is also lower since the color changes with the input power .
Material alternatives of Ce3+:YAG-based phosphors include chalcogenides and silicates . In particular, single phosphors that can emit blue, green, and red are drawing attention as potential white light sources since they offer higher luminous efficiencies and lower manufacturing costs that do systems that require multiple phosphors to achieve the same blend of colors . Recently, Eu2+ and Mn2+ doped Ba3MgSiO8 phosphors integrated with a 400-nm diode to create warm white light were reported. Although, the color rendering index (CRI) was slightly higher than the commercial Ce3+:YAG, the luminous efficiency was lower . More recently, a Eu2+:Li-α-SiAlON phosphor was developed with a 460-nm chip set to create white light. The results indicated white light can be produced with different color temperature by controlling the concentration of Eu2+ in the Li-α-SiAlON, However, the luminous efficiency and the CRI values was lower then the commercial YAG:Ce3+ .
Rare earth (RE) doped nanoparticles have been synthesized that make use of Yb3+ sensitization to obtain red, green, and blue emissions from a single IR excitation wavelength [6,7]. In this paper, a method is presented for obtaining highly tailorable white light emissions from single nanoparticles when excited with a single UV wavelength. These emissions are obtained through the development of a complex core-shell particle that permit control of the energy transfer between lanthanide dopant ions . LaF3 was selected as the host materials for the active nanoparticles due to its well know properties as an optical material, which includes low vibrational energies, high thermal and chemical stability, and high solubility for optically active RE dopants. Such a UV-excited phosphor would be of interest for white LED applications as the color of the LED could be fixed during particle growth. Further the color should be stable regardless of the thickness or concentration of the phosphor conversion layer. In this way, thicker conversion layers could be used to maximize UV light conversion.
The method for producing LaF3 nanoparticles with organic ligands has been well documented in the literature. In this work, the synthesis developed by Dang, et al. , and modified by Stouwdam and van Veggel  has been used. The techniques for producing a single shell as a passive layer have been extended to produce particles composed of multiple layers of varying composition . A detailed analysis of these complex core-shell structures is in a forthcoming publication . Briefly, a solution of ammonium di-n-octadecyldithiophosphate (ADDP) and NH4F in ethanol/water was heated to 75° C. An aqueous solution with total molar Ln(NO3)3 concentration of 1.33 mmol was then added drop-wise to the stirring fluoride solution to form the core of the particles. It is important that the fluoride be slight greater than the amount prescribed by stoichiometry. This ensures complete reaction of the La and RE ions. After stirring for 10 minutes, the 1st shell was grown by the alternating addition in 10 parts of a aqueous NH4F solution and aqueous Ln(NO3)3 solution with total molar concentration of 1.33 mmol. The composition of the Ln(NO3)3 solution will be the composition of the shell. In this work, Eu(NO3)3, Tb(NO3)3, and Tm(NO3)3 were used as dopants at 20 mol % concentrations (i.e., Eu.2La.8F3, Tb.2La.8F3, and Tm.2La.8F3.). This process was repeated for each shell. The last shell was La(NO3)3 and was added so that the solution was now La3+ rich. This ensures that the nanocrystal is terminated with La atoms to which the ligands can bind. The solution was then stirred for an additional 2 hours and allowed to cool to room temperature. After cooling, the particles were cleaned by washing in ethanol and water, followed by dispersing in dichloromethane, and precipitating with the addition of ethanol. The resultant powder was dried for 2 days over P2O5 in a desiccator. The particles were dispersed in tetrahydrofuran for measurements.
Three different structures of nanoparticles were produced using varying mass ratios of a Eu0.2La0.8F3 core, Tb0.2La0.8F3 shell, and Tm0.2La0.8F3 shell as shown in Fig. 1. Nanoparticle A has mass ratios of Eu:Tb:Tm of 0.50:0.75:1.00, Nanoparticle B has mass ratios of 1.00:0.75:0.75, and Nanoparticle C has mass ratios of 0.75:050:0.75.. Both sets of nanoparticles were coated with an undoped LaF3 shell as a passive protective layer.
Photoluminescence was performed with a Jobin-Yvon Fluorolog Tau 3 Fluorometer with 1 nm excitation bandpass and a 5 nm emission bandpass. The data was collected at 1 nm intervals with a 50 ms integration time. X-ray diffraction was performed with a Scintag XDS 4000 using Cu Kα radiation. Transmission electron microscopy (TEM) was performed using a Hitachi H9500 operating at an acceleration voltage of 300 kV.
From the XRD plot in Fig. 2 it is clear that crystalline LaF3 has been synthesized. The average particle size was determined from the analysis of the full-width at half maximum (FWHM) intensity of the XRD peaks to be approximately 10 nm using the Scherrer equation after the influences of instrumental broadening were removed with a crystal standard. The inset of Fig. 2 shows the e (101̄0) plane of the LaF3. It can be seen that the nanoparticles are single crystalline.
The prepared nanoparticles were characterized by photoluminescence spectroscopy. Samples were excited at 355, 356, 357, 358, 359, and 360 nm with a 1 nm slit width. The emission spectra are shown in Figs. 3(a)–3(c) for λexcitation=355, 357, 359 nm. It can be seen that the maximum excitation for the Tm3+ 450 nm peak occurs at 357 nm. The 480 and 540 nm peaks associated with the Tb3+ emissions are seen to decrease slightly with a 357 nm excitation for all samples. As with the Tb3+ peaks, the Eu3+ peaks (591, 615, 650, and 700 nm) are seen to have a minimum when excited at 357 nm and the reach a maximum intensity at an excitation of 359 nm. An excitation scan (not shown) indicates the Eu3+ has an energy level centered on 361 nm.
The emission spectra for each sample and excitation wavelength have been converted to the CIE 1931 color coordinate system as is tabulated in Table 1 and plotted in Fig. 4(a). Also included in Fig. 4(a) are the data points for Tb3+ (0.304, 0.567), Tm3+ (0.153, 0.021), and Eu3+ (0.603, 0.395). It can bee seen that Nanoparticles A are well off the Planckian locus and can be described as being too green. Nanoparticles B and C are located on the Planckian locus in the warm white light region. It is clear that there is a good deal of excitation wavelength dependent color variation.
While it is edifying to use an excitation source with a narrow FWHM, it is not necessarily a good indication of the spectra that will be obtained from a LED-excitation. For this reason, photoluminescence characterization was performed with excitation wavelengths with FWHM of ~8.5 nm which is more like the spectra that would be emitted from a UV LED. The CIE diagram in Fig. 4(b) shows the data points for excitation wavelengths of 352, 354, 357, and 360 nm. The data points have had a yellow shift, but are still located near the Planckian locus. The inset in Fig. 4(b) shows the progression of the change in color coordinates as the excitation wavelength increases with the data points shifting to the bottom right of the graph.
The schematic of the white light particles can be seen in Figs. 1(a)–1(c). In order to create white light three different emissions are required namely red, green and blue. This was achieved by fabricating particles with a Eu3+ doped core to serve as the red emitter, surrounded by a Tb3+ doped shell to aid the energy transfer and also serve as the green emitter, a Tm3+ doped shell to serve as the blue emitter and an undoped protective LaF3 outer shell. As is shown in Fig. 5, at a wavelength of 357 nm both the Tb3+ and Tm3+ ions possess energy levels capable of excitation. On the other hand, Eu3+ does not have an energy level at 357 nm that can be efficiently pumped. As has been shown in our previous work , through the use of these core-shell structures the energy transfer between rare earth ions can be controlled. This can allow for multiple rare-earth ions to be placed into the same nanoparticle without any undesirable quenching while at the same time allowing ions to partially transfer energies such that sensitization of an acceptor ion can occur. It is well known that Tb3+ sensitizes Eu3+ [12,13] as can be seen in Figs. 1(b) and 1(c) with the Eu3+ emitting strongly when excited at wavelengths at which Tb3+ absorbs. Yet the Tb3+ emission is not fully quenched due to the core-shell structure.
After converting the spectra into the 1931 CIE color coordinate system, it can be seen that Nanoparticle A was deficient in Eu3+ and Tm3+, as indicated by its position well above the Planckian locus. By increasing the volume of the Eu3+ doped shell by 100% and decreasing the volume of the Tm3+ doped shell by 25%, the position of Nanoparticle B was shifted onto the warm white light region of the CIE diagram with a correlated color temperature (CCT) of ~3400K. In order to get closer to achromatic light, Nanoparticle C was synthesized with the volume of the Eu3+ and the Tb3+ doped shells decreased by 25 and 33%, respectively. The results are particles that emit at a CCT of ~5000K when excited at 356-357 nm. While these results are promising, the FWHM of the excitation source was ~1 nm, which is not comparable with an LED.
When the FWHM of the excitation source was increased to ~8.5 nm, as shown in Fig. 4(b), the CIE coordinates of the emission shift significantly to the yellow with CCTs of approximately 2720K and 4330K for Nanoparticles A and B with excitation of 357 nm. This can be explained by the more efficient pumping of the Tb3+ and Eu3+ ions. Tb3+ most efficiently absorbs at 350 nm and Eu3+ has an absorption at 361 nm. This can be seen in Figs. 1(a)–1(c). As the excitation wavelength is increased, the intensity of the Eu3+ emissions also increases independently of Tb3+ (480 and 540 nm) peaks. This indicates that the Eu3+ is being excited directly.
While it can be seen that the CCT of these white light emitting particles is acceptable and tailorable, the color rendering index (CRI) of the particles has not been addressed. It is expected that the combination of wavelengths λ=450, 480, 540, 590, 615, and 700 nm should give very high CRI values since it has been shown that tetrachromatic light sources (blue=450, cyan=510, green=560, red=620) can have a CRI of 95 and are accepted as suitable for most applications .
A method for developing down-conversion nano-phosphors has been presented. Through control of shell thickness, the degree of energy transfer has been controlled which allows for multiple rare earth ions that do not necessarily share a common excitation to be incorporated into a single host particle. The CCT of the emission can be controlled by the ratios of the phosphor volumes. This technique can allow for more diverse materials systems to be used together for improved phosphor engineering.
The authors gratefully acknowledge financial support from the Defense Advanced Research Projects Agency though grant # N66001-04-1-8933. The authors also wish to thank Dr. JoAn Hudson of the Clemson University Electron Microscope Laboratory for the high resolution TEM imaging.
References and links
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