The interface adhesion of CaAlSiN3_ Eu2+ phosphor/silicone used in light-emitting diode packaging_ A first principles study

The CaAlSiN3:Eu red phosphor and its silicone/phosphor composite are very promising materials used in the high color rendering white light-emitting diode (LED) packaging. However, the reliabilities of CaAlSiN3:Eu and its composite are still being challenged by phosphor hydrolysis at high humidity application condition. A fundamental understanding of the interface adhesion between silicone and CaAlSiN3:Eu is significant for the developments and applications of this material. In this work, the mechanical properties of silicone/pristine CaAlSiN3:Eu and silicone/hydrolyzed CaAlSiN3:Eu composites are experimentally measured and compared firstly, in which both the tensile strength and Young’s modulus of composite are increased after the hydrolysis reaction. Then, the first principles Density Functional Theory (DFT) calculations are used to investigate the adhesion behaviors of the silicone molecular on both the pristine and the hydrolyzed CaAlSiN3[0 1 0] at atomic level. The results show that: (1) The silicone molecular is weakly adsorbed on the pristine CaAlSiN3[0 1 0] via Van der Waals (vdW) interactions, while silicone molecular is much stronger absorbed on the hydrolyzed CaAlSiN3[0 1 0] due to the formation of hydrogen bonding at the interface; (2) The transient state calculations indicate that the sliding energy barrier of silicone on the hydrolyzed CaAlSiN3[0 1 0] is higher than that on the pristine one, as the increased adsorption energy and surface roughness. Generally, the findings in this paper can guide the phosphor selection, storage and process in LED packaging, and also assist in improving the reliability design of LED package used in high moisture condition.


Introduction
As a new generation light source, phosphor converted white lightemitting diodes (pc-WLEDs), generally constructing by a blue LED chip coated with the phosphor, are being applied in many fields, like indoor and outdoor lighting, healthcare, automotive headlamp and high-resolution displays and so on [1,2]. In a white LED package, phosphor is an integral part, working as a function of light and color conversion, and its stability directly affects the reliability of white LED [3]. Since phosphors are always sensitive to high temperature and high moisture, the thermal quenching and hydrolysis effects become critical concerns on the phosphor's reliability. Furthermore, phosphor is always mixed within silicone as a light-conversion composite. When the LED operates under harsh application environments, the silicone/phosphor interface is always suffering degradation under conditions of high temperatures [4], high blue light illumination, and high humidity [5][6][7][8][9][10][11]. Therefore, the study of interface adhesion in phosphor/silicone composite become one of fundamental researches to improve the reliability of LED package.
In recent years, CaAlSiN 3 :Eu 2+ has attracted much attention for its application in warm white or high color rendering [12][13][14]. It was firstly developed by K. Uheda et al. under a high-temperature and highpressure solid state reaction with EuN, Ca 3 N 2 , AlN and Si 3 N 4 [15] . The spectrum measurements conducted by Pan. He et al. showed that the excitation and emission spectra of CaAlSiN 3 :Eu 2+ phosphors could be effectively excited at 467 nm and exhibit a strong red emission at 668 nm, which indicates the CaAlSiN 3 :Eu 2+ phosphor as an excellent candidate for getting red emission for white light-emitting diodes [16]. According to the effective first-principles calculation, the host lattice constants [17], mechanical, electrical and optical properties [18] of CaAlSiN 3 :Eu 2+ and its derivatives were predicted. Although phosphors generally have high thermal quenching temperature and good thermal https://doi.org/10.1016/j.apsusc.2020.145251 Received 28 August 2019; Received in revised form 17 November 2019; Accepted 1 January 2020 stability, the reliability of CaAlSiN 3 :Eu 2+ red phosphor is still being challenged by the high humidity application condition. Zhu et al. [19] observed the degradation of CaAlSiN 3 :Eu 2+ in high-temperature and -pressure water stream test, which is explained as the oxidation of both the phosphor host and activator under an oxidant-gas penetration. Our research team also experimentally found the hydrolysis phenomenon of CaAlSiN 3 :Eu 2+ red phosphor, that can lower the crystallinity of CaAlSiN 3 and increases its thermal quenching effect [20].
In addition, in a white LED packaging structure, phosphors are always mixed with silicone to form the high-performance composite materials [21]. However, when the phosphors composites are used in the harsh environment (e.g., high temperature and high moisture conditions), the serious lumen degradations and color shifts of a pc-WLED package were observed [22,23]. Luo et al. [7] studied the degradation mechanisms of phosphor/silicone composites used in pc-WLEDs under both high temperature and high humidity conditions. The results show that the hydrolysis of phosphors and the oxidation of silicone under a high moisture environment could accelerate the degradation of phosphor/silicone composites. Moreover, the adhesion behavior between silicone and phosphors is also a serious concern that affects the use of such material in LED packaging. Singh et al. [24] reported the lumen degradation of high-power LEDs aged under highhumidity condition and observed that the voids between silicone and phosphors can promote the adsorption of moisture on silicone, which further could result in subsequent light scattering. However, all current characterizations of phosphors and their silicone composites are studied at macro-level and the understandings on the interactions between phosphor and silicone mainly relay on the characterizations of microstructure and chemical element composition, which cannot provide a deep explanation at atomic level. The explicit physical-chemical effect of humidity condition on the adhesion behavior between silicone and phosphor is still an open question. Those insufficient investigation methods on phosphor/silicone interaction will limit the studies and applications of CaAlSiN 3 :Eu 2+ and its composite.
In this paper, the hydrolyzed CaAlSiN 3 :Eu 2+ red phosphors are obtained via a water immersion experiment, then the pristine and hydrolyzed CaAlSiN 3 :Eu 2+ red phosphors are mixed with silicone to form phosphor/silicone composites respectively. Through tensile tests, their mechanical properties are experimentally obtained and compared. After that, the adhesion properties of silicone molecular on pristine and hydrolyzed surfaces of CaAlSiN 3 [0 1 0] are investigated by using Density Functional Theory (DFT) calculations. Through comparing the adsorption energy, bonding nature, electronic structures and sliding energy barriers of silicone molecular on both pristine and hydrolyzed CaAlSiN 3 [0 1 0] surfaces, the hydrolysis effect on the interfacial and mechanical properties of CaAlSiN 3 :Eu 2+ /silicone composite are discussed.

Sample preparation and mechanical tests
A commercialized CaAlSiN 3 :Eu 2+ red phosphor is used in this study. The test sample preparation procedure for phosphor/silicone composites is shown in Fig. 1 [11,25]: Firstly, red phosphor powders were soaked in deionized water under 55°C for 1800 s and the hydrolysis reaction was occurred at the surface of phosphor powders as present in Fig. 2. Then, the phosphor powders were filtered and dried from solution. Next, the silicones KJC-1200A and KJC-1200B were mixed with a 1:1 mass ratio. The silicone mixture and the fresh and treated red phosphor powders were thoroughly mixed in a vacuum mixer with mass fractions as 5%, 10%, 15% and 20% respectively. Finally, the phosphor/silicone mixtures were poured into a polyfluortetraethylene mold and cured in a 100°C oven for 3 h. The geometric dimensions of prepared test samples with a thickness of 1 mm were designed according to the ASTM D1708 standard. The tensile test was conducted by an Electromechanical Universal Testing Machine from the MTS system (China) Co. Ltd (Model: CMT4204, accuracy: level 0.5).

Computational method
In this study, all first principles calculations were performed based on the density functional theory (DFT) as implemented in the DMol 3 package [26]. The electronic interactions were employed by using the generalized gradient approximation (GGA) with the Perdew-Burke-Ernzerhof (PBE) method [27]. Double numerical basis sets with polarization functions (DNP) were utilized. For geometry optimization calculations, a 5 × 4 × 3 Monkhorst-Pack k-point mesh for the Brillouin zone sampling was used. The convergence criteria of optimized structures are 2 × 10 −5 Ha for energy, 0.002 Ha/Å for force, and 5 × 10 −3 Å for displacement. Since the chemical elements and composition measurement by using the Energy Dispersive Spectroscopy (EDS) (see the Supplementary Information) indicate that the atomic percentage of Eu element is very small and significantly lower than the other four elements, the host CaAlSiN 3 is used in the DFT calculation to investigate the adhesion behavior between silicone and phosphor particle [25]. A unit cell of CaAlSiN 3 crystal structure was calculated with 24 atoms, placing two Al atoms and two Si atoms in the four tetrahedral sites, as shown in Fig. 3. When all the structures are fully relaxed by employing the conjugate gradient method, the lattice constants of the unit cell are a = 9.8871 Å, b = 5.7134 Å, and c = 5.1146 Å. These computed parameters are in good agreement with the previous studies [17,18].
Normally, CaAlSiN 3 surfaces with three orientations [1 0 0], [0 1 0] and [0 0 1] were reported in previous study [28]. Among these three orientations, the calculations of surface energies show that the surface, two water molecules were added to the surface structure to obtain the hydrolyzed CaAlSiN 3 [0 1 0] surface. Generally, the adhesion behaviors of a common silicone molecular on phosphor surface are very complicated, as in the local area, the adhesion modes are varied from case to case. But such local adhesion behaviors can be understood through the studies of a monomer silicone molecular on phosphor surface at different adsorption sites. Thus, we chose the monomer silicone molecular to study the adhesion property between silicone and CaAlSiN 3 , as shown in Fig. 4(b).
To investigate the adsorption of the silicone molecular on CaAlSiN 3 [0 1 0] surface, we first calculated the adsorption energy (E ad ) by using the following formulation [29], where E silicone/CaAlSiN3 is the total energy of silicone/pristine or hydrolyzed CaAlSiN 3 structure, E silicone is the energy of silicone molecular, and E CaAlSiN3 is the energy of the isolated pristine or hydrolyzed CaAlSiN 3 [0 1 0] surfaces. To link the adsorption energy to its electronic structure, we also calculated the charge density of differences for selected adsorption models via [30], where ρ silicone/CaAlSiN3 , ρ silicone and ρ CaAlSiN3 are the total charge density of the optimized silicone/CaAlSiN 3 structure, the silicone molecular and the pristine or hydrolyzed CaAlSiN 3 [0 1 0] surfaces, respectively. In addition, to investigate adhesion behavior of silicone molecular

Simulation results
Before calculating the adhesion of silicone molecular on CaAlSiN 3 , we first modeled the pristine and hydrolyzed CaAlSiN 3 [0 1 0] surfaces. On the basis of fully relaxed pristine CaAlSiN 3 [0 1 0] surface structure, the hydrolyzed surface was obtained as shown in Fig. 3(b). The modeling results show that a chemical reaction occurs at the surface of hydrolyzed CaAlSiN 3 . The H + and OH − of water molecule are bonded on the surface, forming two NeH bonds with length 1.027 Å, a SieO bond with distance 1.731 Å and an AleO bond with length 1.832 Å. Furthermore, the angle of SieNeAl at the hydrolyzed surface    Fig. 6(a), the tops of two N atoms (site-N1 for the orange ball, site-N2 for the green ball) and the tops of two Ca atoms (site-Ca1 for the black ball, site-Ca2 for the blue ball). On the hydrolyzed CaAlSiN 3 [0 1 0] surface, the tops of H atoms are considered as four adsorptions sites as depicted in Fig. 6(b), site-H1 for the orange ball, site-H2 for the green ball, site-H3 for the black ball and site-H4 for the blue ball.
For the case of silicone adsorption, we modeled the adsorption in two modes: the parallel to the surface and the upright to the surface, as shown in Fig. 6(c). The calculated results for both modes on different sites of pristine and hydrolyzed CaAlSiN 3 [0 1 0] surfaces are listed in Table 1. It can be seen that the parallel adsorption configurations tend to the higher adsorption energies compared to the upright adsorption   mode. The values of charge transfer for the parallel configurations are also higher than the upright mode. We ascribe those preferences to the larger contact area of the parallel mode which promotes the interactions between silicone molecular and the surface. From Table 1, we also can find that no matter what adsorption modes, parallel or upright, the average adsorption energy of silicone molecular on the hydrolyzed surface is higher than that on the pristine surface. Now, we focus on the most thermodynamically stable adsorptions on pristine CaAlSiN 3 [0 1 0], namely the parallel model of silicone on site Ca1. As plotted in Fig. 7, the silicone molecular is adsorbed on the surface and oxygen atom moves to the position above Ca atom with distance 2.622 Å. Since the interaction with silicone molecular, the height of Ca atom is a little bit higher than the other Ca atoms. Four methyl groups of silicone are downward to the surface and the shortest distance is 2.276 Å as the length between H and N atoms. Before and after the adsorption, the angle of SieOeSi in silicone molecular changes from 136.42°to 131.0°and the length of SieO bond changes from 1.677 Å to 1.719 Å. The adhesion energy for this configuration is On the hydrolyzed CaAlSiN 3 [0 1 0] surface, the most stable adsorption site is H2 and the corresponding optimized structure is shown in Fig. 8. The silicone molecular is absorbed on the surface with SieOeSi parallel to the surface and four methyl groups downward to the surface. The adhesion energy of this configuration is −2.237 eV and the shortest distance between silicone and surface is the length between O and H atoms, 2.182 Å. Before and after the adsorption, the angle of SieOeSi slightly changes from 136.498°to 136.342°, and the length of SieO bond increases from 1.677 Å to 1.688 Å. Furthermore, according to the definition of hydrogen bond [32], we find that the silicone molecular is absorbed on the hydrolyzed CaAlSiN 3 [0 1 0] surface via hydrogen bond OeH … O, as shown in Fig. 8(d). The CDD of this configuration is plotted in Fig. 8(c), an obvious charge transfer between hydroxyl and O atom of silicone is observed, where the charges are accumulated at the side of O atom but depleted at the side of hydroxyl of surface. Generally, the total charge transfer from silicone molecular to surface is −0.028 e and hydrolyzed CaAlSiN 3 [0 1 0] surface acts as a charge donor and silicone molecular acts as an acceptor. Compare to the vdW interactions between silicone and pristine CaAlSiN 3 , the hydrogen bonding contributes a stronger intermolecular interaction. Such different bonding nature of silicone on pristine and hydrolyzed CaAlSiN 3 [0 1 0] surfaces explains the trend, shown in Table 1, that the silicone adsorbs much stronger on hydrolyzed surface.    Fig. 9(b). The energy barrier along this direction is 1.22 eV. On the hydrolyzed CaAlSiN 3 surface, when silicone slides along [1 0 0] direction, there are two transit states and one intermediate state in one period, as shown in Fig. 10(a). The energies of both transit states are 1.39 eV and 1.40 eV, respectively, so the sliding energy barrier along this direction is 1.40 eV. For the sliding along [0 0 1] direction, the silicone molecular passes a transit state and reaches the final state with energy barrier 1.31 eV, as shown in Fig. 10(b). The embedded pictures in Fig. 10 show that the minimum energy pathways for both directions are passing over the hydroxyls, not through the middle regions between hydroxyls. Compare to the sliding behaviors on pristine There are two reasons for such differences. Firstly, the hydroxyls bonded on the CaAlSiN 3 surface increase the surface roughness, which plays a role as resistance, increasing the friction between the silicone molecular and CaAlSiN 3 . Secondly, the adhesion energy of silicone on the hydrolyzed CaAlSiN 3 surface is higher than that on the pristine surface, so the higher energy is needed to break the adhesion between silicone and hydrolyzed surface. As results, both reasons cause higher energy barriers for the sliding of silicone on hydrolyzed CaAlSiN 3 [0 1 0].

Experimental results and explanation
It is well known that the mechanical properties of a composite are dependent on both filler's and matrix's properties and the ability to transfer stresses across the filler/matrix interface [33]. In particular, the ability to transfer stress across the interface is often discussed in terms of 'adhesion', which, in fact, is related to a complex combination of factors, such as the interfacial shear strength [34]. In the tensile test described in part 2.1, the mechanical properties of silicone/hydrolyzed CaAlSiN 3 :Eu 2+ composite were obtained and compared to the silicone/ pristine CaAlSiN 3 :Eu 2+ composite. As plotted in Fig. 11, after hydrolysis reaction, the tensile strength and Young's modulus of silicone CaAlSiN 3 :Eu 2+ composite were increased. The SEM images in Fig. 2 show that the hydrolysis reaction mainly occurred at the surface of CaAlSiN 3 :Eu 2+ and the microstructure of CaAlSiN 3 :Eu 2+ has not dramatically crashed after hydrolysis reaction. Thus, in this experiment, the main difference between hydrolyzed and pristine CaAlSiN 3 :Eu 2+ /  silicone composites came from their interfaces. Since SEM characterization shows the rougher surfaces of CaAlSiN3 CaAlSiN 3 :Eu 2+ particles after hydrolysis reaction, so we suspect that the increased friction between silicone and CaAlSiN 3 is the major contribution to the enhanced mechanical properties of silicone/CaAlSiN 3 :Eu 2+ composite. Furthermore, the DFT results in part 3.1 provide a new insight to understand the interface of silicone/CaAlSiN 3 :Eu 2+ at atom level. First, the calculated results showed that the hydrolysis reaction of CaAlSiN 3 [0 1 0] indeed is able to increase surface roughness, which further leads to the higher sliding energy barrier when silicone slides on CaAlSiN 3 [0 1 0]. It is implied that the friction on hydrolyzed surface is higher than that on pristine CaAlSiN 3 [0 1 0] surface. The DFT studies also revealed a possible bonding nature between silicone and hydrolyzed CaAlSiN 3 [0 1 0]: the hydrogen bond OeH … O. Since the strength of hydrogen bond is obviously higher than the vdW interaction, as a result, the higher adhesion energy is obtained for silicone on hydrolyzed CaAlSiN 3 [0 1 0]. These simulation results imply that the hydrolysis reaction of CaAlSiN 3 [0 1 0] can lead to a stronger interface between silicone and CaAlSiN 3 , which may improve the ability to transfer stress across the interface. Generally, the experimental and simulation results suggest that the hydrolysis reaction of CaAlSiN 3 :Eu 2+ can increase the adhesion strength between silicone and CaAlSiN 3 and enhance its composite mechanical properties.

Conclusions
In this paper, by using both experimental and theoretical methods, the effects of hydrolysis reaction of CaAlSiN 3 :Eu 2+ on the mechanical and interfacial properties of silicone/ CaAlSiN 3 :Eu 2+ composite are investigated. In experiments, the hydrolysis reaction of CaAlSiN 3 :Eu 2+ increases its surface roughness, and the tensile tests further show that both tensile strength and Young's modulus of silicone/hydrolyzed CaAlSiN 3 :Eu 2+ composite are enhanced after hydrolysis reaction. In DFT studies, we find that the adhesion of silicone molecular on the pristine CaAlSiN 3 [0 1 0] is the weak physisorption via vdW interaction while the adsorption on the hydrolyzed CaAlSiN 3 [0 1 0] surface is a more complex bonding nature: hydrogen-bonding of OeH … H. This is well corroborated with the surface electric structures where we clearly observe the corresponding redistribution of charge density on the silicone-CaAlSiN 3 interface system. In addition, using the transient state calculations, we reveal that the sliding energy barrier of silicone on hydrolyzed CaAlSiN 3 [0 1 0] is higher than that on pristine  CaAlSiN 3 [0 1 0], as the stronger adsorption energy and increased surface roughness. Generally, our experimental and simulation results consistently conclude that the hydrolysis reaction of CaAlSiN 3 :Eu 2+ will increase the adhesion of CaAlSiN 3 : Eu 2+ phosphor/silicone interface.

Author contribution
Zhen Cui contributes to the modeling and simulations. Jiajie Fan contributes to the experiments and measurements, and provides project administration and funding acquisition. Hendrik Joost van Ginkel contributes the result analysis. Xuejun Fan and Guoqi Zhang support supervision.