Phellodendron chinense Schneid: A novel yellow-emitting luminescent material for white light-emitting diodes

To facilitate the next generation of environmental material for white light emitting diodes, the discovery of natural luminesce is essential. In this study, we disclose a rare-earth free and yellow-emission phosphor, Phellodendron, which could be both excited by near ultraviolet light and blue light. The new yellow phosphor is obtained by extraction of Phellodendron chinense Schneid. The emission wavelength, full width at half maximum and CIE coordinates of extracted Phellodendron are 540 nm, 120 nm and (0.41, 0.55), respectively. The corresponding luminescent properties of Phellodendron are characterized by PL, PLE, reflection spectra, FITR and decay lifetime. Surprising thing is luminous intensity of Phellodendron phosphors excited at 380 nm was stronger than YAG:Ce phosphor by more than 139%. In addition, we firstly introduce the yellow phosphor in white LED fabrication by combining blue chip and Y3Al5O12:Ce3+ phosphor, to create warm white. For comparison, red-emission CaAlSiN3:Eu2+ phosphors are also introduced for LED package tests. The results demonstrate that Phellodendron is a potential candidate for white LED applications.

Nowadays, white light emitting diodes (W-LEDs) have provided remarkable advances in lighting technologies and displays 1 . Commercially-available W-LEDs are fabricated by combining either a blue-emitting InGaN-based LED chip covered by a yellow-emitting phosphor Y 3 Al 5 O 12 :Ce 3+ (YAG) [2][3][4][5] or n-UV LED chip with coupling red, green and blue tricolor phosphors [6][7][8][9] . A large number of luminescent materials used for LED applications are based on rare-earth ions doping technology 1, 10-12 . Given a history of ecological concerns about pollution from rare earth mines, particularly in China, there are growing social and environmental concerns about the growth of the mining and mineral processing in this sector [13][14][15] . Apart from rare-earth ions doping, nature of narrow absorption for YAG limit its application for n-UV LED application. Advent of technological advancement and economic growth have put our environment at great risk which has led researchers and scientists to develop technology that generates less negative environmental impact. We need to find some substitute for reducing the usage of rare-earth ions, preferably.
Phellodendron is a deciduous tree in the family Rutaceae that is native to east and northeast Asia 16,17 . Phellodendron bark mainly composed of berberine and palmatine [18][19][20] , which have been vastly used in Chinese traditional medicine [21][22][23][24][25] for various symptomatic treatment and pharmacological activities, such as inflammation 26,27 , anti-diarrhoea 19,25 , antitumor [28][29][30][31] , antiviral 18,32,33 , and pneumonia 34 . Aside from its importance in traditional and modern pharmaceuticals, Phellodendron bark has been one of the major sources of yellow dye in Asia for fibers and paper materials not only for its color but also for its insect repellency [35][36][37][38][39] . Berberine and palmatine have received much attention in the field of medicine, dyes and dyeing since it is one of the few cationic colorants of the natural plant dyes, and it has high alkaloids 40,41 . Furthermore, Zhang et al. found the yellow varnish on the metallic foil of a 19-20th century Tibetan altar, the presence of berberine and possible palmatine as well, by using HPLC-MS analysis 42 . Its major chromophoric substance is known to be berberine and palmatine which dyes fibers into yellow color from Phellodendron bark. To the best of our knowledge, there is no study on both the luminescence properties and LED applications of yellow material from Phellodendron bark. In this study, we firstly demonstrate that Phellodendron phosphor by extracting natural Phellodendron bark with ether and composition and luminescence of Phellodendron phosphor were investigated. In addition, in order to reduce usage of rare-earth ions, we combine commercial phosphor and natural Phellodendron phosphor by extracting.
In this study, we found a natural yellow material by extracting Phellodendron chinense Schneid. This yellow-emission phosphor is believed to replace rare earth ion-doped phosphors and reduce their usage in LED applications. Phellodendron phosphor could be pumped by near-UV light or blue light because of its nature of wide absorption band. We construct a series of LED package data by using YAG, CaAlSiN 3 :Eu 2+ and Phellodendron phosphor for comparison. The proposed approaches may be alternative combination for LED applications in the future.

Results and Discussion
The photo image of Phellodendron phosphor is shown in Fig. 1c which was extracted by using diethyl ether and collected and evaporated in rotary evaporators to obtain the diethyl ether extraction. The daylight color shown in Fig. 1c was amber yellow. In UV box, Phellodendron phosphor displayed mustard yellow and canary yellow under 254 nm and 365 nm excitation, respectively. Since the alkaloids in Phellodendron chinense Schneid were high polar, methanol-d 4 was used as a solvent to ensure that all the extract can be dissolved and avoid the inference of phenolic hydroxyl signals for NMR measurement 18 . The 1 H NMR spectrum of Phellodendron phosphor is well documented in methanol-d 4 in Fig. S2 and Table S1. The analysis of the NMR spectrum of Phellodendron phosphor revealed the extracts is Berberine 18 . FT-IR spectroscopy was used to analyze to the functional groups in Phellodendron phosphor, as shown in Fig The optical property of Phellodendron phosphor was analyzed using UV visible spectroscopy and the corresponding spectrum is shown in Fig. 2b. The broad absorption peak was from 200 nm to 500 nm. The absorption coefficient α in many amorphous semiconductors shows an exponential dependence on photon energy usually obeying the empirical relation. The band gap of Phellodendron phosphor (E g ) was determined by using Tauc's plot [43][44][45][46] .
For allowed direct transitions, n = 1/2, and for allowed indirect transitions, n = 2. From results of Fig. 2(c) and (d), Phellodendron phosphor showed both direct band gap and indirect band gap were determined to be 2.49 eV and 2.62 eV, respectively. Normalized photoluminescence excitation and photoluminescence emission spectra of Phellodendron phosphor and YAG:Ce are displayed in Fig. 2e. Phellodendron phosphor exhibits a nature of broad absorption band from 360 nm to 500 nm, indicating that it can be effectively applied in general lighting for UV, nUV and blue-pumping LED devices. The emission spectra of Phellodendron phosphor and YAG:Ce excited at 460 nm display the broad yellow-emitting peak at 543 nm and 560 nm, respectively. YAG:Ce, however, could only be excited at 460 nm according to the sharp absorption band at ~460 nm. Even though, the luminous intensity of Phellodendron phosphor is 3% of that of YAG:Ce phosphor at 460 nm excitation. Nevertheless, when Phellodendron phosphors were excited at 380 nm, the luminous intensity of Phellodendron phosphor is more than 139% than that of YAG:Ce phosphor. Phellodendron phosphor can be applied both nUV and blue chip for LED devices following to the above result. Fig. S3a shows PL/PLE spectra of commercial red-emission phosphor-CaAlSiN 3 :Eu 2+ . The emission spectra of CaAlSiN 3 :Eu 2+ phosphor is excited at 460 nm displays red-emitting peak at 620 nm.
To better explain information about the microenvironment surrounding the excited probe molecule, we recorded the time-resolved PL (TRPL) decay measurements of Phellodendron phosphor, as shown in Fig. 2f. It provides a complementary technique to study the PL mechanism of Phellodendron phosphor, using the kinetics of electron-hole recombination as a probe. The orange line in Fig. 2f displays the PL decay profile of the Phellodendron phosphor under excitation of the 260 nm laser. The PL profile can be well fitted by a sum of exponential expressions (equation 2) and the average lifetimes (τ) for bi-exponential decay of fluorescence were calculated from the decay times and pre-exponential factors, using the following equation (equation 3) as previous work 47 :  CaAlSiN 3 :Eu 2+ phosphor into phosphor paste, the color temperature of W-LEDs changed from cold color temperature to warm color temperature. Fig. S2c exhibits the CIE chromaticity diagram of enlarge of Fig. 3b with Planckian Locus line. The CIE chromaticity coordinates are shown in Table S2. Figure 3e shows the CCTs of LED (1), LED (2) and LED (3) with 15 iterations each and the summary are shown in Fig. S3c and Table S3. The range of LED (2) and LED (3) were from 3000 K to 4000 K, which are lower than that of LED (1). The average CCT of LED (1), LED (2) and LED (3) are 7623 K, 3615.7 K and 3477.6 K, respectively. It indicates that the CCT of W-LEDs could be easily controlled through the addition of Phellodendron phosphor or CaAlSiN 3 :Eu 2+ phosphor. Based on the results, the CCT of LED (2) was similar to that of LED (3). Thus, the Phellodendron phosphor could replace the commercial red phosphor CaAlSiN 3 :Eu 2+ .
In addition, we packaged LEDs with different content (0, 0.1, 2.4, 9.1, 50 wt.%) of Phellodendron phosphor shown in Fig. 3c. More Phellodendron phosphor we added, the color of emitting light is much warmer from 10000 K to 1500 K under 30 mA forward bias currents. In other words, we can easily control CCTs of LEDs through the Phellodendron phosphor addition for different applications. For luminous efficacy, the white LEDs driven of LED(1), LED(2) and LED(3) at a D.C. current of 30 mA exhibited about 61.6, 48.6 and 58.8 lm/W, respectively. By characterizing the luminesce properties of Phellodendron phosphor, we revealed that the yellow-emission peak at 543 nm and it could partially replace rare-earth elements and reduce their use in LED applications.

Conclusions
In this study, we first report luminescent properties of phosphor by extracting natural Phellodendron bark with ether. Phellodendron phosphor can be made into powder and be suspended in epoxy resin to form phosphor resin. Nuclear magnetic resonance (NMR), Photoluminescence/ photoluminescence excitation (PL/PLE), as well as CIE coordinates are characterized. The results indicated that phosphor gives an intense yellow emission peak at 540 nm under optimal excitation wavelength of 460 nm. The absorption band of phosphor was in the range of 350-525 nm, which could be excited by either n-UV chips or blue chips. The amount of phosphor contain can thus be adjusted to optimize the color intensity efficiency, CIE coordinate, correlated color temperature (CCT) and color-rendering index (CRI) of the light emitting diode (LED). Furthermore, this study helps with selection of the appropriate phosphorus content so on to optimize CIE and CCT performance.
Characterization methods. The photoluminescence (PL) and photoluminescence excitation (PLE) spectra were measured using Horiba Jobin-Yvon FluoroMax-4 spectrometer equipped with a 450 W Xenon lamp as the excitation source. All the spectra were measured with a scan rate of 150 nm min −1 . 1 H NMR spectra were measured in methanol-d 4 ( > 99.8%, Sigma-Aldrich) with a Bruker Avance 400 MHz spectrometer, using TMS as an internal standard. For each sample, 100 scans were recorded with the following parameters: 0.183 Hz/point with spectra width of 6009 Hz. The pulse width and relaxation delay are 4 s and 1.5 s, respectively. For quantitative analysis, peak area was integrated for each peak selected manually. Fourier transform infrared spectroscopy (FT-IR) spectra were collected using a Bruker Tensor 27 FT-IR spectrophotometer at a resolution of 4 cm −1 . The optical absorption spectra of the samples were carried out with an UV-VIS-NIR spectrophotometer (Hitachi U-3900, Japan) in the range of 200-800 nm. Commission International de I'Eclairage (CIE) chromaticity coordinates of the samples were measured using a Laiko DT-101 color analyzer equipped with a CCD detector (Laiko Co., Tokyo, Japan). A pulsed laser with a wavelength of 260 nm, a repetition frequency of 20 MHz, and a duration of 250 fs was used as the excitation source for time-resolved PL studies. The collected luminescence was dispersed by a 0.75 m spectrometer and detected with the photomultiplier tube. Time-resolved PL were performed using the technique of time-correlated single-photon counting (TCSPC). The steady-state and time-resolved PL was measured at room temperature. The time resolution for the time-resolved PL instrument is pico-second.