Near-white emission observed in Dy doped AlN

Abstract

Dy doped AlN phosphors were prepared by a simple solid state route, exhibiting excellent photoluminescence. The structure, composition and morphology were investigated by means of X-ray diffraction, transmission electron microscope and energy-dispersive X-ray spectroscopy. Besides the characteristic emissions of Dy, emission from the AlN host could also be detected at room temperature. A post-thermal treatment under air atmosphere was employed to improve the photoluminescence. The blue emission and yellow emission from Dy ions contribute to the near-white emission, possessing potential applications in white LED.

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Introduction

Significant progress in solid light-emitting devices over the decades has made the development of the efficient phosphors a research hotspot.^1,2 It is known that rare-earth (RE) ions are widely used activators for phosphors utilized in light emitting diodes³ (LEDs) and flat panel displays.⁴ Different from d–d transitions of transition metal (TM) ions, f–f transitions of rare-earth⁵ (RE) ions are found to be line emissions. Besides, owing to the shielding effect of 5s² and 5p⁶ shell layer, the energies and wavelength corresponding to f–f transitions are definite, and hardly affected by the host.^6,7 Previous research has focused on trivalent europium ion, trivalent terbium ion and bivalent europium ion,^8–10 which are used as red, green and blue phosphors activator, respectively. However, trivalent dysprosium ion exerts a tremendous fascination due to the hypersensitive transition.^11,12 The characteristic emissions of Dy³⁺ ions consist of two main f–f transitions: ⁴F_9/2–⁶H_13/2 (573 nm, yellow) and ⁴F_9/2–⁶H_15/2 (482 nm, blue) transitions. ⁴F_9/2–⁶H_13/2 transition is hypersensitive transition, and the emission intensity is changed along with varying host crystals. While the emission of ⁴F_9/2–⁶H_15/2 transition is invariable with the host crystals. Accordingly, white light will be achieved by varying the host crystals, namely, adjusting the intensity of the two transitions to an appropriate ratio.¹²

Extensive investigations have focused on rare-earth (RE) ions doped oxide phosphors, such as Y₂O₃, Lu₂O₃, and Gd₂O₃.^13–15 Compared with oxide phosphors, nitride phosphors have gained widespread interest for optoelectronic and light-emitting applications.^10,16,17 AlN, GaN, InN and their alloys are frequently applied in piezoelectric material,¹⁸ deep-UV light-emitting diodes¹⁹ and field emitter materials.²⁰ However, AlN as newly developed phosphor host^21,22 has been capturing growing attention owing to its unique features, for instance, high thermal conductivity, excellent hardness and chemical stabilities, and so on. In these cases of rare-earth ions doped AlN phosphors, light emissions are mainly derived from luminescence centers in the AlN host, namely the rare-earth ions. Previous investigations generally focussed on the characteristic emissions from doped rare-earth ions, ignoring the emission from AlN host. However, it is noted that oxygen incorporated into the AlN host will give rise to light emission, and the energy of light emission is greatly affected by the amount of oxygen in the AlN crystal.²³

We successfully prepared Dy³⁺ ions doped AlN phosphor via a solid state method. The effect of the doping concentration of Dy³⁺ ions in AlN on the photoluminescence properties has been also investigated in detail. Furthermore, we discuss the luminescence properties of AlN:Dy phosphors in consideration of the emission from the AlN host. In this work, a post-thermal treatment was performed on the resulting AlN:Dy phosphors. The influence of post-thermal treatment on the luminescence of AlN:Dy phosphors was studied thoroughly.

Experimental section

Synthesis of Dy doped AlN

DyCl₃·6H₂O were prepared by dissolving Dy₂O₃ in dilute hydrochloric acid solution under magnetic stirring, followed by evaporating off excess hydrochloric acid with continuous heating. The starting materials, AlCl₃·6H₂O, DyCl₃·6H₂O and urea, were mixed thoroughly at a molar ratio of 1 − x : x : 30. Under continuous high-purity nitrogen flow, the mixed powders were first heated to 100 °C for half an hour, and subsequently heated to 900 °C for 2 hours at a heating rate of 13.3 °C min⁻¹. Light grey powders were obtained after natural cooling. Thereafter, the obtained powders underwent 2 hours heating at 650 °C under air as further post-thermal treatment and then the products were collected.

Characterization

The powder X-ray diffraction (XRD) measurements were performed on a Shimadzu 6100 diffractometer with Cu Kα radiation of 1.54059 Å at room temperature. Investigations on the microstructure of the samples were performed using transmission electron microscopy (TEM; Tecnai G2 F20), and the element composition was analyzed by energy-dispersive X-ray spectroscopy (EDX). The photoluminescence excitation and emission spectra were collected on a FLUOROMAX-4 spectrometer using xenon lamp as light source. Besides, the thermogravimetry and differential thermal analysis (TG-DTA, NETZSCH STA 449C, Germany) measurements were performed from room temperature to 1000 °C at a heating rate of 10 °C min⁻¹. The decay curves of AlN samples doped with various concentrations of Dy under excitation at 294 nm were recorded on a fluorescence spectrometer (Edinburgh FLS 920).

Results and discussion

The crystal structures were characterized by X-ray diffraction (XRD) patterns. Fig. 1a depicts the typical XRD patterns of AlN doped with various concentrations of Dy. All of the diffraction peaks are in agreement with the crystalline phase of wurtzite AlN (JCPDS file no. 25-1133). No other diffraction peaks are detected, suggesting wurtzite AlN:Dy without crystal impurities are obtained. Fig. 1b shows the local magnification of XRD patterns in the 2-theta range from 32 to 34 degrees. Clearly, the diffraction peak shifts toward larger angle with an increase in the doping concentration of Dy³⁺, which indicates the incorporation of Dy into the AlN host may lead to the local distortion of the AlN crystal lattice.

Fig. 1 (a) XRD patterns of AlN samples doped at various molar percentages of Dy; (b) enlarged figures of the strongest peak corresponding to the (100) crystal plane.

The microstructure and component of the as-prepared samples were analyzed by TEM images and EDX spectrum. The TEM images in Fig. 2 indicate that irregular AlN:0.3 mol% agglomerates prepared at 900 °C are composed of nanoparticles with diameter of 50 nm (Fig. 2b). As displayed in Fig. 2c, the spaces for the (100) and (101) lattice planes in the wurtzite AlN:Dy are determined to be 0.270 nm and 0.238 nm, respectively. Moreover, the SAED spectrum in Fig. 2d indicates polycrystalline AlN:Dy nanoparticles.

Fig. 2 (a and b) TEM images and (c) HRTEM image of AlN:0.3 mol% Dy prepared at 900 °C. (d) Corresponding SAED.

To confirm the existence of the dysprosium element in the AlN host, we characterized the X-ray energy dispersive spectrum of the AlN:0.3 mol% sample. The signal of the dysprosium element and no crystal phase with dysprosium element indicate Dy³⁺ ions have been successfully incorporated into the AlN host. Apart from the dysprosium element, aluminum, nitrogen, oxygen and carbon elements are also detected in the X-ray energy dispersive spectrum (Fig. S1†) of the AlN:0.3 mol% sample. And the atomic percentage of Al element is close to that of N element, agreeing with the theoretical proportion of AlN. Besides, oxygen is a common and inevitable impurity owing to the high solubility of oxygen in the AlN host, which accounts for the existence of O element. However, the peak of C element suggests certain carbon-containing species are retained after the high temperature reaction. And these carbon-containing species probably have influence on the PL properties, which will be discussed later.

Fig. 3 shows the photoluminescence excitation and emission spectra of AlN samples doped with various concentrations of Dy. All the AlN samples with various concentrations of Dy exhibit similar locations of excitation and emission peaks. Under 294 nm excitation, AlN:Dy samples show three sharp emission peaks at 482 nm, 573 nm and 660 nm coupled with a broad band centered at around 450 nm. Clearly, the sharp emission peaks at 482 nm, 573 nm and 660 nm correspond to the characteristic ⁴F_9/2–⁶H_15/2, ⁴F_9/2–⁶H_13/2 and ⁴F_9/2–⁶H_11/2 transitions of Dy³⁺, respectively, dominated by the ⁴F_9/2–⁶H_13/2 hypersensitive transition of Dy³⁺. Besides, the broad emission band centered at around 450 nm may be associated with the defects emission from the AlN host, because a similar broad emission band can also be observed in undoped AlN (Fig. S2†). It is a consensus that oxygen impurities in AlN host are inevitable due to the high solubility of oxygen and the strong Al–O bond. These oxygen-related defects in the AlN host are responsible for the broad emission band.

Fig. 3 Photoluminescence excitation and emission spectra of AlN samples doped with 0.05, 0.08, 0.1, 0.3 and 0.5 mol% Dy from a to e.

The excitation spectra, monitored with 574 nm emission of Dy, consist of a strong band centered at 294 nm and a weak peak at 350 nm. Fig. S3† displays the emission spectra of AlN:0.3 mol% excited at 294 nm and 350 nm, respectively. Upon UV excitation at 350 nm, a strong emission band centered at 450 nm is clearly observed, which is attributed to the defects emission from the AlN host. However, the peak at 574 nm corresponding to the f–f transition of Dy³⁺ is relatively weak. It is indicated that Dy³⁺ ions cannot be effectively excited or the energy transfer from the excited defect states to Dy³⁺ ions is difficult. Thus the excitation peak at 350 nm is associated with the defects in the AlN host. However, upon UV excitation at 294 nm, the characteristic emissions from Dy³⁺ ions and defects emission from the AlN host are observed. A similar excitation band centered at around 290 nm can also be observed in AlN samples doped with other RE ions, for instance, Eu²⁺ ion^3,4 and Tb³⁺ ion.²⁴ Hence, the excitation band centered at 294 nm is probably related to the transitions in the perturbed environment of the RE ions.²⁵ And the perturbed environment may arise from the oxygen defects or the local distortion of the AlN crystal lattice caused by doping RE ions. The defects state of the AlN host is excited from the ground state to the excited state by absorbing the excitation energy. And part of the energy is transferred from the corresponding excited state to the Dy³⁺ ions, leading to the emissions from Dy³⁺ ions and AlN host.

Both the excitation and emission increase along with the content of Dy³⁺ in the AlN host until this reaches around 0.3 mol% of Dy³⁺, and then decrease dramatically with further increase of the content of Dy³⁺. The decrease of PL intensity at higher doping concentration is attributed to concentration quenching, caused by energy transfer between the neighboring Dy³⁺ ions. Owing to the energy difference between ⁴F_9/2–⁶F_3/2 and ⁶H_9/2–⁶H_15/2 energy level couples is approximately equal, energy transfer between the neighboring Dy³⁺ ions results in concentration quenching, as shown in Fig. S4.† From the results of PL spectra, the optimum content of Dy³⁺ ions in the AlN host is determined to be 0.3 mol%, which suggests that the incorporation of Dy ions into AlN is difficult owing to the large difference of ionic radius between the activator (Dy³⁺, 6CN, 0.91 Å) and the host element (Al³⁺, 6CN, 0.53 Å). This is in accordance with the results involving rare earth ions doped AlN or AlON reported previously.^26,27 The critical transfer distance (R_C) between Dy³⁺ ions for energy transfer can be calculated via the following equation:²⁸

where V is the volume of the unit cell, X_C is the critical concentration of Dy³⁺ ion, N is the number of formula units per unit cell. In the case of the AlN host, the values of V and N are 41.76 ų and 0.25, respectively. And the value of R_C calculated from the equation is determined to be 47.38 Å. It is known that nonradiative energy transfer to forbidden transitions by exchange interaction occurs when R_C is about 5 Å, which is not in accordance with the case of Dy doped AlN. Thus, it is speculated that Dy is located in the octahedral interstice sites of the AlN host, instead of substitution of Dy in Al sites.²⁹

The decay curves of AlN samples doped with various concentrations of Dy are shown in Fig. S5.† The decay curves are well fitted by a single exponential function:

here I is phosphorescence intensity, A is a constant, t is time, and τ is lifetime, respectively. The variation of lifetime with the concentrations of Dy in AlN samples is shown in the inset of Fig. S5.† The lifetime decreases with increasing concentrations of Dy, which further supports the energy transfer between the neighboring Dy³⁺ ions. The quantum yields of AlN:Dy phosphors measured by integrating sphere method are listed in Table S1.† And the maximum value of quantum yield is 0.03.

In order to further improve the PL intensity, a post-thermal treatment was performed under air atmosphere, namely, all the resulting samples doped with various concentrations of Dy were reheated at 650 °C for 2 hours under air atmosphere to remove the carbon remnant. Fig. 4 reveals the effect of post-thermal treatment on the PL intensities of AlN:Dy samples. Briefly, we use the emission band centered at 450 nm and the characteristic emission at 573 nm as representatives of emissions from AlN host and Dy ions, respectively. The intensities of both AlN host emission and Dy emission are clearly increased after the post-thermal treatment, suggesting that post-thermal treatment is beneficial for the photoluminescence. However, the effect of the PL intensity enhancement is different. In the case of the AlN host emission at 450 nm, the effect of the PL intensity enhancement is minimal when the dopant concentration is 0.3 mol%, which is the optimal dopant concentration for Dy emission. As for the AlN:0.3 mol% Dy sample, most of the energy absorbed by AlN host was transferred to Dy ions, contributing to the f–f transitions of Dy, thus the energy for emission of AlN host is less. As a result, the range of the PL intensity enhancement for 450 nm emission in AlN:0.3 mol% Dy sample is minimal after the post-thermal treatment. Taking the sample doped with 0.1 mol% Dy for example to illustrate the importance of the post-thermal treatment. In the case of the AlN:0.1 mol% Dy (Fig. 5A), the PL intensities of emissions from Dy³⁺ and AlN host are increased, in comparison with the sample without post-thermal treatment. Is AlN:0.3 mol% Dy sample converted into another crystal phase during the post-thermal treatment process? We characterized the XRD pattern (Fig. 5B) of the sample after the post-thermal treatment. Clearly, no corresponding oxide or oxynitride can be detected after post-thermal treatment, indicating that the post-thermal treatment under air atmosphere did not induce another crystal phase. However, as shown in Fig. S6,† the color of the sample turns from light grey into whitish after reheated under air atmosphere. Based on above results, it is supposed that the AlN:0.1 mol% Dy sample may consist of amorphous phase impurities, such as carbon-containing species, which are related to residuals from urea decomposition. During the post-thermal treatment process, the carbon-containing species are oxidized by air and convert into corresponding oxides gases at elevated temperature.

Fig. 4 PL intensities of emissions at 450 nm (A) and 573 nm (B) for AlN:Dy samples as a function of concentration of Dy before (black line) and after (red line) post-thermal treatment.

Fig. 5 Photoluminescence emission spectra (A) and XRD (B) patterns of AlN:0.1 mol% Dy sample before (a) and after (b) post-thermal treatment at 650 °C for 2 hours under the air atmosphere.

To better explain the process of the post-thermal treatment, thermogravimetry and differential thermal analysis (TG-DTA) measurements of the AlN:0.1 mol% Dy sample were conducted under air atmosphere in the range from room temperature to 1000 °C. From the TG-DTA curve (Fig. 6), the mass loss of 4.18% is observed between room temperature and 700 °C, followed by a mass increase process when temperature exceeds 700 °C. The initial mass loss with exothermic reaction occurs in the temperature range of 30–450 °C, which is attributed to the desorption of physically absorbed water and certain surface impurities. And the steeper mass loss from 450 °C to 700 °C is attributed to removal of the amorphous carbon-containing species. During this process, the amorphous carbon-containing species but not AlN itself is probably decomposed or oxidation to gases in the air atmosphere, resulting in mass loss. It is noteworthy that the amorphous carbon-containing species instead of AlN itself are oxidized, which is supported by the XRD results. A previous report³⁰ proposed that the oxidation of AlN begins at 700 °C when heated in the air. Besides, the color change after post-thermal treatment also further supports the removal of the amorphous carbon-containing species. However, the onset of mass increase coupled with endothermic reaction occurs when the temperature exceeds 700 °C, suggesting oxidation of AlN, which is in accordance with the previous report that the oxidation of AlN occurs above 700 °C in the air. Clearly, the mass will increase as AlN (40.99 g mol⁻¹) is oxidized to Al₂O₃ (101.96 g mol⁻¹). Consequently, post-thermal treatment below 700 °C under air atmosphere is a good way to enhance the PL intensity by removing the amorphous carbon-containing species left in the sample. It can be concluded from the mass loss results that the amount of amorphous carbon-containing species left in the sample is relatively low. Consequently, post-thermal treatment below 700 °C under air atmosphere is a good way to enhance the PL intensity by removing the amorphous carbon-containing species left in the sample.

Fig. 6 TG-DTA curves of the AlN:0.1 mol% Dy sample under the air atmosphere from room temperature to 1000 °C.

The CIE chromaticity diagram (Fig. 7) AlN:Dy phosphors show the emission colors for AlN:Dy phosphors in the near-white light region. The emissions from Dy and AlN host contribute to the color of AlN:Dy phosphors, leading to the near-white light. Furthermore, the emission color of AlN:Dy phosphors can be tuned by the concentrations of Dy. Accordingly, AlN:Dy phosphors is a promising material utilized in white LEDs.

Fig. 7 CIE chromaticity diagram revealing the emission colors for AlN doped with 0.05, 0.08, 0.1, 0.3 and 0.5 mol% Dy from a to e.

Conclusions

In this work, polycrystalline AlN:Dy phosphors were successfully prepared. Characteristic emissions of Dy at 483 nm, 573 nm and 660 nm coupled with a broad emission band centered at 450 were observed in the photoluminescence spectra. The broad emission is ascribed to the oxygen-related defects in the AlN host. The optimal doping concentration of Dy in the AlN host is determined to be 0.3 mol%. A post-thermal treatment to the annealed products under air atmosphere is beneficial to improve the photoluminescence properties, owing to the removal of small amounts of the amorphous carbon-containing species left in the product. The blue emission and yellow emission of AlN:Dy contribute to the near-white light, which possess potential application in white LEDs.

Acknowledgements

This work was supported by the National Natural Science Foundation of China.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra03815h

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