Synthesis of a NaYF₄:Yb,Er upconversion film on a silicon substrate and its tribological behavior

Abstract

In this work, amino-functionalized NaYF₄:Yb,Er upconversion nanoparticles (UCNPs) were synthesized by a hydrothermal method. Then a simple self-assembly method was carried out to prepare a NaYF₄:Yb,Er upconversion (UC) film on a silicon (Si) substrate, using γ-(2,3-epoxypropoxy)propyltrimethoxysilane (KH-560) as an intermediate coupling agent. The chemical composition and surface morphology of the UC film were characterized using X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FT-IR), water contact angle (WCA), and scanning electron microscopy (SEM) measurements. Furthermore, the tribological properties of the UC film were investigated by a UMT tester. The results indicate that the UC film has been successfully prepared on the Si substrate by the formation of chemical bonds and its lifetime is about five times longer than that of the NaYF₄:Yb,Er UCNPs on a bare Si substrate. This work provides a facile way to synthesize a NaYF₄:Yb,Er UC film with robust adhesion to the substrate, which can be applicable for other UC films.

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1. Introduction

Photons with energy less than the bandgap of the active materials cannot be absorbed by a single-junction solar cell, leading to the energy conversion efficiency of solar cells being below the Shockley–Queisser (SQ) limit.^1,2 Upconversion (UC) materials, which can absorb two or more low energy infrared photons and then convert them to a visible photon with higher energy, are considered to be one of the promising routes for overcoming the SQ limit.³ UC materials can modify the solar spectrum to make it match better with the absorption spectrum of solar cells, which is different from other methods used to improve the energy conversion efficiency of solar cells.

UC materials are always applied as a separate layer in solar cells, existing in the form of a film. Furthermore, UC films, which can combine the advantages of bulk materials and the compactness of fibers, have more advantages than bulk materials.^4,5 The use of UC films has also been proposed in the designs for multilayer optical storage disks⁶ and photovoltaic cells that capture sub-bandgap solar radiation.⁷ Yb³⁺ and Er³⁺ ion codoped NaYF₄ crystals, regarded as one of the most efficient UC materials, have become a new focus of research. Up to now, a lot of effort has been made on the synthesis and applications of NaYF₄:Yb,Er crystals or powders.^8–10 To our knowledge, the investigation of NaYF₄:Yb,Er UC films is very scarce. So it is necessary and of great significance to carry out research on NaYF₄:Yb,Er UC films.

A sol–gel method and pulsed laser deposition (PLD) are the most commonly reported methods used to synthesize UC films.^11–13 However, the sol–gel method requires post-deposition heat treatment, which may induce reactions between the phosphors and the gel, leading to the luminescence properties of the prepared film being uncontrollable. Although PLD can fabricate films with good optical properties, it is a relatively expensive and complex technique. Self-assembly has always been a facile and convenient technique to chemisorb molecules on substrate surfaces.¹⁴ It has been extensively used to prepare various kinds of films, which exhibit stable chemical and physical properties, and good covalent bonding with the substrate.^15,16 Additionally, to promote adhesion between the substrate surfaces and materials, a coupling agent, such as γ-(2,3-epoxypropoxy)propyltrimethoxysilane (KH-560), is always employed.¹⁷

As is known, the interfacial binding force is an important factor affecting the quality of thin films. Failure always happens in films which have a weak binding force to the substrate. However, due to the diversity and complexity of films, there are no standard methods which can be used to conveniently and effectively verify the binding force and evaluate the film quality. A tribological test is an effective way to investigate the wear resistance properties of materials. When a tribological test is applied to films, films which are not connected to the substrate or have a weak binding force to the substrate can be simply pushed to the sides of the contact in the course of sliding.^18,19 So the tribological results can reflect the interfacial binding force between the film and substrate to some extent.

In this study, we aim to provide a facile and convenient method to fabricate a NaYF₄:Yb,Er UC film. The chemical characteristics of the film and related processes have been investigated. The UC luminescence properties of the as-prepared UC film were also studied. To explore the adhesion of the film to the Si substrate, a tribological test has been carried out. The experimental results demonstrate that the NaYF₄:Yb,Er UC film has been successfully prepared and has a compact and uniform structure.

2. Experimental

2.1 Materials

Y(NO₃)₃·6H₂O (99.99%), Yb(NO₃)₃·5H₂O (99.99%), Er(NO₃)₃·5H₂O (99.99%), PEI (70 000, 50 wt%), and KH-560 were purchased from Aladdin Industrial Corporation (Shanghai). Other reagents of analytical grade were used directly without further purification. A polyethylenimine (PEI) stock solution (5 wt%) was prepared by dissolving PEI in deionized water.

2.2 Preparation

2.2.1 Synthesis of amino-functionalized NaYF₄:Yb,Er UCNPs. The amino-functionalized NaYF₄:Yb,Er UCNPs were synthesized by an improved hydrothermal method according to our previous work.²⁰ In a typical procedure, 0.78 mmol Y(NO₃)₃·6H₂O, 0.2 mmol Yb(NO₃)₃·5H₂O, 0.02 mmol Er(NO₃)₃·5H₂O and 1 mmol NaCl were firstly dissolved in 30 ml deionized water with magnetic stirring for 30 min. Then a 10 ml PEI solution was added into the above solution. After stirring for another 30 min, 6 mmol NH₄F was added. The mixture was stirred for 15 min and transferred to a 50 ml Teflon bottle held in a stainless steel autoclave, which was then sealed and maintained at 180 °C for 3 h. After the autoclave was cooled to room temperature naturally, the precipitates were collected by centrifugation and washed with deionized water several times and dried at 60 °C for 12 h.

2.2.2 Surface hydroxylation of Si substrate wafers. Silicon wafers (cut into a size of 10 mm × 10 mm) were ultrasonically cleaned with deionized water, ethanol and acetone in turn. Then the wafers were hydroxylated by immersion in piranha solution (a mixture of 7 : 3 (v/v) 98% H₂SO₄ and 30% H₂O₂) at 90 °C for 1.5 h. The wafers were rinsed with deionized water and dried under nitrogen flow after being taken out from the piranha solution.

2.2.3 Self-assembly of KH-560 film. A KH-560 solution with a volume fraction of 5% was freshly prepared by adding KH-560 into a mixture of methanol and water (volume ratio of 5 : 1) for 5 min, in which time the KH-560 molecules were considered to be hydrolyzed. The wafers in the above section were immersed in the KH-560 solution for 30 min. Then the wafers were taken out and cleaned with water 3 times, and also dried under nitrogen flow.

2.2.4 Self-assembly of NaYF₄:Yb,Er UC film. A NaYF₄:Yb,Er solution (0.05 wt%) was prepared by dissolving the amino-functionalized NaYF₄:Yb,Er UCNPs in deionized water. The KH-560 coated wafers in the last procedure were immersed in the solution and kept there for 20 h at 60 °C. Then the silicon wafers were cleaned with deionized water and dried at 80 °C for 2 h. Finally, the self-assembled NaYF₄:Yb,Er UC film was successfully obtained. For comparison, the hydroxylated Si substrate wafers from Section 2.2.2 without the KH-560 primer film had the NaYF₄:Yb,Er UCNPs assembled on them using the same method.

2.3 Characterization

The binding energy state and elemental composition of the films were detected by X-ray photoelectron spectroscopy (XPS; Kratos, AXIS ULTRA DLD). Fourier transform infrared (FT-IR) spectra were recorded on a Nicolet-6700 spectrometer from Thermo Fisher. X-ray power diffraction (XRD) measurements were performed on a Bruker D8-Advance diffractometer with Cu Kα radiation (λ = 0.154 nm). The 2θ angle ranges from 10° to 70° at a scanning rate of 4° min⁻¹. The size and morphology of the as-prepared NaYF₄:Yb,Er UCNPs were observed by a 120 kV biological transmission electron microscope (B-TEM; FEI, Tecnai G2 Spirit Biotwin). The morphology and elemental composition of the as-prepared film were investigated by field emission scanning microscopy (FESEM; FEI, Sirion-200) equipped with an attachment for an energy dispersive spectrometer. The water contact angle (WCA) of the coated surface was measured by a DSA 100 contact angle meter (Kruss, Germany). At least five repeat measurements were performed for each sample and the average value was taken as the resultant value. The UC spectra of the UC film were acquired on a fluorescence spectrometer (Hitachi, F-4500) using a 980 nm laser diode as the excitation source. Tribological tests were carried out on a reciprocating ball-on-disk UMT-2MT sliding tester (CETR USA) in air at room temperature. A Si₃N₄ ball (3 mm diameter) was used as the counterface. All of the tests were conducted under the conditions of a distance of 5 mm and a frequency of 1 Hz.

3. Results and discussion

The process for the fabrication of the NaYF₄:Yb,Er UC film on a Si wafer is illustrated in Fig. 1. The self-assembled KH-560 film was firstly prepared on the hydroxylated Si wafer, the amino-functionalized NaYF₄:Yb,Er UCNPs were synthesized via a facile hydrothermal method using PEI as the chelating agent, and the NaYF₄:Yb,Er UC film was obtained by taking advantage of the reaction between the epoxy groups of the KH-560 molecules and the amino groups of the amino-functionalized NaYF₄:Yb,Er UCNPs.

Fig. 1 The schematic illustration of the assembly of the NaYF₄:Yb,Er UC film on a Si wafer.

3.1 Morphology and structure of amino-functionalized NaYF₄:Yb,Er UCNPs

The morphology and size of the NaYF₄:Yb,Er UCNPs were investigated by TEM. As shown in Fig. 2(a), the as-prepared NaYF₄:Yb,Er UCNPs are monodisperse and exhibit a spherical shape with an average size of about 25 nm, indicating that the PEI molecules can bind to the surface of the NaYF₄:Yb,Er UCNPs and effectively control the growth and prevent aggregation of the nanoparticles. Fig. 2(b) shows the XRD pattern of the as-prepared NaYF₄:Yb,Er UCNPs; we can easily find that all of the diffraction peaks can be well indexed to the standard XRD pattern of the cubic NaYF₄ phase (JCPDS no. 77-2042), indicating the good crystallization of the as-prepared samples.

Fig. 2 TEM image (a) and XRD pattern (b) of the as-prepared NaYF₄:Yb,Er UCNPs.

3.2 Fabrication and characterization of NaYF₄:Yb,Er UC film

XPS measurements were carried out to verify the surface chemical components and elemental chemical state of the as-prepared films. The C 1s XPS spectrum of the KH-560 film is shown in Fig. 3(a). It was expected that as well as the typical bonding of C–C, the KH-560 film should also have a large number of epoxy groups. As illustrated in Fig. 3(a), the C 1s XPS spectrum of the KH-560 film can be deconvoluted into two Gaussian peaks, which can be assigned to C–C (284.8 eV) and C–O (286.5 eV), respectively.^21,22 These results indicate that the KH-560 film has been successfully assembled on the Si substrate. Fig. 3(b) presents the XPS spectra of C 1s for the NaYF₄:Yb,Er UC film. When compared with Fig. 3(a), a new bonding peak at 285.6 eV emerges in Fig. 3(b). According to references, this peak is believed to result from a C–N bond.^14,23 In our experimental, only the PEI molecules capped on the NaYF₄:Yb,Er UCNPs provide nitrogen atoms, so the C–N bond is derived from the NaYF₄:Yb,Er UC film. As the PEI molecule itself contains C–N bonds, the discovery of the bonding peak at 285.6 eV cannot confirm that some C–N bonds come from the bonds between KH-560 and the NaYF₄:Yb,Er UCNPs, although the reaction between amino and epoxy groups has been proven to be feasible.^24,25 However, we can easily obtain from Fig. 3(a) and (b) that the amount of C–O groups decreases after the preparation of the NaYF₄:Yb,Er UC film on the primer KH-560 film. So it is reasonable to deduce that NaYF₄:Yb,Er UCNPs have been chemically bonded to the KH-560 film.

Fig. 3 Deconvoluted XPS spectra of C 1s for the self-assembled (a) KH-560 film and (b) NaYF₄:Yb,Er UC film.

To further clarify the chemical reactions between the KH-560 molecules and the NaYF₄:Yb,Er UCNPs, FT-IR measurements were carried out. Fig. 4(a) shows the FT-IR spectrum of the as-prepared NaYF₄:Yb,Er powder, and the bands at about 2840 cm⁻¹ and 2955 cm⁻¹ are attributed to the symmetrical and asymmetrical stretching vibration modes of the CH₂ group, respectively.^26,27 The absorption bands from the internal vibration of the amide bonds appear at 1380–1628 cm⁻¹, demonstrating the presence of PEI on the surface of the NaYF₄:Yb,Er UCNPs.^27,28 After reacting with KH-560, the characteristic bands of amino groups (1570 and 1473 cm⁻¹) vanish, and the typical signals of Si–O–Si at 1035 and 920 cm⁻¹ appear.^29,30 Therefore, it is rational to deduce that the amino groups of PEI can react with the epoxy groups of the KH-560 molecules. Moreover, the C–N band at 1120 cm⁻¹ has been strengthened due to the reactions between the amino and epoxy groups.

Fig. 4 FT-IR spectra of (a) the NaYF₄:Yb,Er UCNPs and (b) the NaYF₄:Yb,Er UCNPs after reacting with KH-560 (powder).

WCA measurements are an effective way to gain an insight into the surface chemical components of the samples. As shown in Table 1, the WCA of the Si substrate is very low (∼0°) owing to the abundant surface hydroxyl groups. So this makes it possible for the KH-560 molecules to be adsorbed onto the substrate surface. The WCA results show that both the KH-560 film and the NaYF₄:Yb,Er UCNPs are hydrophilic, which can be attributed to them containing epoxy and amino functional groups, respectively. The as-prepared NaYF₄:Yb,Er UC film is also hydrophilic, with a WCA of about 86.2°. However, this value is slightly higher than the values for the KH-560 film and NaYF₄:Yb,Er UCNPs. We attribute this to the chemical reactions between the epoxy groups and amino groups, which lead to a decrease in the amount of hydrophilic groups on the surface of the film.

Table 1 WCA for different samples

Samples	Si/SiO2	KH-560 film	NaYF4:Yb,Er UCNPs	NaYF4:Yb,Er UC film
WCA/deg	∼0	53.6 ± 1.8	81.8 ± 2.1	86.2 ± 2.7

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The morphology of the as-prepared NaYF₄:Yb,Er UC film was characterized using SEM. As illustrated in Fig. 5, the NaYF₄:Yb,Er UCNPs have been successfully assembled on the substrate and the as-prepared UC film exhibits a very uniform and compact structure. The chemical composition of the UC film was further confirmed by EDS. As shown in Table 2, the results reveal that the atomic ratio of Y, Yb and Er in the final products agree well with the original molar ratios of the reactants, indicating the successful doping of Yb³⁺ and Er³⁺ ions into the NaYF₄ lattice.

Fig. 5 Low-magnification SEM image (a) and enlarged SEM image (b) of the as-prepared NaYF₄:Yb,Er UC film.

Table 2 EDS characterization of the NaYF₄:Yb,Er UC film

Element	Y	Yb	Er
At%	78.4	19.4	2.2

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3.3 UC luminescence properties of the as-prepared NaYF₄:Yb,Er UC film

Fig. 6 shows the UC emission spectrum of the as-prepared NaYF₄:Yb,Er UC film under 980 nm laser excitation (600 mW). We can easily find that the emission spectrum exhibits three distinct peaks at 520, 540 and 655 nm, which can be assigned to the ²H_11/2 → ⁴I_15/2, ⁴S_3/2 → ⁴I_15/2 and ⁴F_9/2 → ⁴I_15/2 transitions, respectively. The UC luminescence properties of the UC film are similar to the NaYF₄:Yb,Er UCNPs prepared in our previous studies,^20,31 so the UC mechanism is not discussed in this paper.

Fig. 6 UC emission spectrum of the as-prepared NaYF₄:Yb,Er UC film.

3.4 Tribological properties of the as-prepared NaYF₄:Yb,Er UC film

To evaluate the quality of the as-prepared films, tribological tests were performed on a UMT tester in reciprocal motion and the coefficient of friction (COF) for the different samples can be seen in Fig. 7. As is known, the Si substrate has poor tribological properties and a high COF (∼0.8). As illustrated in Fig. 7(a), the KH-560 film assembled on the Si substrate exhibits a low COF (∼0.26). However, its lifetime is very short, indicating the bad load-carrying capability. The as-prepared NaYF₄:Yb,Er UC film displays good tribological properties with a time of 1350 s in spite of a high COF (0.33, as seen in Fig. 7(b)). As a comparison, the tribological properties of the NaYF₄:Yb,Er UCNPs on a bare Si substrate were also investigated. As shown in Fig. 7(c), the NaYF₄:Yb,Er UCPs exhibit a COF of 0.39 and a lifetime of ∼300 s.

Fig. 7 Variation of COF with time for different samples under an applied load of 0.5 N and a constant frequency of 1 Hz: (a) KH-560 film, (b) NaYF₄:Yb,Er UC film, and (c) NaYF₄:Yb,Er UCNPs on a bare Si substrate.

By comparing the structural differences between the NaYF₄:Yb,Er UC film and the NaYF₄:Yb,Er UCNPs on the bare Si substrate, the different lifetimes of these two samples can be attributed to the different binding force strengths to the substrate. As is known, there are mainly two kinds of attractions leading to the absorption of molecules onto a solid surface, chemical adsorption and physical adsorption. According to the above analysis, the amino groups on the NaYF₄:Yb,Er UCNPs have reacted with the epoxy groups of KH-560. So the NaYF₄:Yb,Er UC film is combined with the Si substrate via covalent chemical bonds. Whereas, the NaYF₄:Yb,Er UCNPs are immobilized on the bare Si substrate mainly by physical adsorption. As chemical adsorption is much stronger than physical adsorption, the NaYF₄:Yb,Er UCNPs on the bare Si substrate can be easily pushed to the sides of the contact in the course of sliding. These results indicate the successful fabrication of the NaYF₄:Yb,Er UC film with a strong interfacial binding force to the Si substrate.

4. Conclusion

In summary, a NaYF₄:Yb,Er UC film was successfully assembled on a Si substrate via a simple and facile self-assembly method for the first time. The chemical composition and tribological properties of the UC film have been investigated. The results demonstrate that the NaYF₄:Yb,Er UCNPs are combined with the Si substrate by covalent chemical bonds. When compared with the NaYF₄:Yb,Er UCNPs assembled on a bare Si substrate, the UC film has a much longer lifetime of about 1350 s which can be ascribed to the strong interfacial binding force with the substrate. This work can broaden the applications of NaYF₄:Yb,Er crystals, especially in solar cells. Also, this method can be used to prepare other lanthanide-doped UC films.

Acknowledgements

We are grateful to the National Natural Science Foundation of China (Grant No. 51575341) and for the help of the Instrumental Analysis Center, Shanghai Jiaotong University.

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