Upconversion and tribological properties of β-NaYF4:Yb,Er film synthesized on silicon substrate

Applied Surface Science 371 (2016) 391–398

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Upconversion and tribological properties of ␤-NaYF4 :Yb,Er film synthesized on silicon substrate Chuanying Wang a , Xianhua Cheng a,b,∗ a b

School of Mechanical Engineering, Shanghai Jiao Tong University, Shanghai 200240, PR China State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, PR China

a r t i c l e

i n f o

Article history: Received 1 November 2015 Received in revised form 31 December 2015 Accepted 4 March 2016 Available online 7 March 2016 Keywords: ␤-NaYF4 :Yb,Er upconversion film Self-assembly Upconversion luminescence Tribological property

a b s t r a c t In this work, ␤-NaYF4 :Yb,Er upconversion (UC) film was successfully prepared on silicon (Si) substrate via self-assemble method for the first time. The chemical composition and surface morphology of the UC film were characterized by Fourier transform infrared spectroscopy (FT-IR), X-ray photoelectron spectroscopy (XPS), water contact angle (WCA), X-ray power diffraction (XRD), and scanning electron microscopy (SEM) measurements. To investigate the effects of KH-560 primer film and chemical reactions on the UC luminescence properties of ␤-NaYF4 :Yb,Er UC film, decay profiles of the 540 nm and 655 nm radiations were measured. Furthermore, tribological test was applied to qualitatively evaluate the adhesion of the UC film. The results indicate that the UC film has been successfully prepared on Si substrate by covalent chemical bonds. This work provides a facile way to synthesize ␤-NaYF4 :Yb,Er UC film with robust adhesion to the substrate, which can be applicable for other UC films. © 2016 Elsevier B.V. All rights reserved.

1. Introduction Upconversion (UC) material can absorb two or more low energy photons and generate one high energy photon [1,2]. Due to this unique optical property and its potential applications, it has attracted considerable attentions [3,4]. ␤-NaYF4 :Yb,Er UC crystal has been proved as one of the most efficient UC materials to date, many efforts have been done on the synthesis and application of ␤-NaYF4 :Yb,Er crystals [5–7]. However, the applications of bulk crystals are still restricted due to not integrating with UC devices. Films, which could combine the advantages of bulk materials and the compactness of fibers, have shown potential application prospects in UC devices. Recently, several groups have worked on the fabrication of UC films [8,9]. Moreover, transparent ␤NaYF4 :Yb,Er UC film has been applied in silicon solar cells to enhance the photovoltaic conversion efficiency [10]. Among the methods used to synthesize UC films, sol–gel method is by far the most commonly reported one [11,12]. However, the sol–gel method requires post-deposition heat treatment to reduce the concentration of quenching –OH oscillators. This may induce the reactions between phosphors and gel, giving rise to unpredictable luminescence properties of the UC film. Furthermore, rational

∗ Corresponding author at: School of Mechanical Engineering, Shanghai Jiao Tong University, Dongchuan road 800, Shanghai, PR China. E-mail addresses: [email protected], [email protected] (X. Cheng). http://dx.doi.org/10.1016/j.apsusc.2016.03.035 0169-4332/© 2016 Elsevier B.V. All rights reserved.

method used to evaluate the performance of UC film is still lacking. So it is of great importance to explore a valid method to prepare ␤-NaYF4 :Yb,Er UC film and a new way to evaluate the film quality. Self-assembly has always been a facile, simple and convenient technique to chemisorb molecules on substrate surfaces. It has been extensively used to prepare various kinds of films, which exhibit stable chemical and physical properties and strong interfacial binding force with substrates [13–15]. As is known, tribological test is an effective way to investigate the wear resistant property of materials [16,17]. In the service process, failure always happens in films which have weak binding force to substrate and easily fall off from substrate. So when tribological test is applied to films, the tribological results can evaluate the film quality and reflect the interfacial binding force between film and substrate to some extent. In this work, we aim to provide a facile, simple and convenient way to synthesize ␤-NaYF4 :Yb,Er UC film. The chemical composition of the film and reactions for the related process were investigated in details. UC luminescence properties of the as-prepared UC film were also studied. Tribological test was used to evaluate the quality of UC film and detect the interfacial binding force between film and Si substrate. The results demonstrate that the ␤-NaYF4 :Yb,Er UC film has been successfully synthesized and exhibits strong UC luminescence and robust adhesion to the substrate.

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Fig. 1. Proposed schematic view for the assembly of ␤-NaYF4 :Yb,Er UC film on Si substrate. Table 1 WCAs of different samples. Samples

Si/SiO2 KH-560 film ␤-NaYF4 :Yb,Er crystals ␤-NaYF4 :Yb,Er UC film

WCA/deg ∼0

55.1 ± 2.2

74.5 ± 3.1

83.2 ± 2.1

2.3. Pretreatment of Si substrate wafers Si wafers (cut in size of 10 mm × 10 mm) were ultrasonic cleaned by acetone, ethanol and water in sequence for 10 min. Then those wafers were immersed into piranha solution (a mixture of 7:3 (v/v) 98% H2 SO4 and 30% H2 O2 ) for surface hydroxylation at 90 ◦ C for 1.5 h. Then the wafers were rinsed with deionized water and dried under nitrogen flow. 2.4. Synthesis of self-assembled KH-560 film on Si substrate

Fig. 2. FTIR spectra of the amine-functionalized ␤-NaYF4 :Yb,Er crystals (a) and ␤NaYF4 :Yb,Er crystals after reacting with KH-560 (b).

2. Experimental 2.1. Materials All utilized chemicals including Polyethylenimine (PEI, 70000, 50 wt%), ␥-(2,3-epoxypropoxy) propyltrimethoxysilane (KH560), and Rare-earth nitrates (Y(NO3 )3 ·6H2 O, Yb(NO3 )3 ·5H2 O, Er(NO3 )3 ·5H2 O) of 99.99% purity were purchased from Aladdin Industrial Corporation (Shanghai). Other reagents of analytical grade were used directly without further purification. Deionized water (>18.2 M) was used throughout the experiment. PEI stock solution (5 wt%) was obtained by dissolving PEI in deionized water.

5% (vol/vol) KH-560 solution was prepared by adding KH-560 into the mixture of methanol and water (volume ratio of 5:1) and stirring for 5 min. The KH-560 was considered to dissolve and hydrolyzed in this procedure. The hydroxylated Si wafers were immersed into the above solution for 30 min. Then the wafers were taken out and cleaned by methanol for 3 times, dried by nitrogen flow. 2.5. Self-assembly of ˇ-NaYF4 :Yb,Er UC film Si substrate wafers from the last step were immersed in a solution of ␤-NaYF4 :Yb,Er crystals (0.05 wt%) for 20 h at 60 ◦ C. Then the wafers were cleaned by deionized water for several times and dried at 80 ◦ C for 2 h. For comparison, the hydroxylated Si substrate wafers from Section 2.3 without KH-560 primer film were assembled with ␤-NaYF4 :Yb,Er crystals by the same method. 2.6. Characterization

2.2. Synthesis of amine-functionalized ˇ-NaYF4 :Yb,Er crystals In a typical procedure for preparation of ␤-NaYF4 :Yb,Er crystals, 7.8 ml Y(NO3 )3 (0.1 M), 2 ml Yb(NO3 )3 (0.1 M), 2 ml Er(NO3 )3 (0.01 M), 1 mmol NaCl and 10 ml PEI solution (5 wt%) were firstly mixed together with magnetic stirring for 30 min to form a homogeneous solution. Then 12 ml NH4 F solution (1 M) was added to the mixture. Finally, the mixture was stirred for another 30 min and transferred to a 50 ml autoclave, sealed, and maintained at 180 ◦ C for 24 h. After the autoclave was cooled down naturally to room temperature, the crystals were deposited at the bottom of the vessel. The crystals were purified by centrifugation and washed with ethanol for several times, dried at 60 ◦ C for 12 h.

Water contact angle (WCA) of the coated surface was measured by a DSA 100 contact angle meter (Kruss, Germany). Fourier transform infrared (FT-IR) spectra were recorded on a Nicolet-6700 spectrometer from Thermo Fisher. The binding energy state and element composition of films were detected by X-ray photoelectron spectroscopy (XPS, Kratos, AXIS ULTRA DLD). X-ray power diffraction (XRD) measurement was performed on a Bruker D8Advance diffractometer with Cu K␣ radiation (␭ = 0.154 nm). The 2␪ angle ranges from 10◦ to 70◦ at a scanning rate of 4◦ /min. The size and morphology of as-prepared ␤-NaYF4 :Yb,Er crystals were observed by a 120 KV biological transmission electron microscope (B-TEM; FEI, Tecnai G2 Spirit Biotwin). Morphology observation of KH-560 film was conducted by atomic force

C. Wang, X. Cheng / Applied Surface Science 371 (2016) 391–398

Fig. 3. XPS survey spectra for different films: (a) KH-560 film, (b) ␤-NaYF4 :Yb,Er UC film.

microscope (AFM, multimode nanoscope, DI, USA). 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 energy dispersive Spectrometer (EDS). The UC spectrum of the UC film was acquired on a fluorescence spectrometer (Hitachi, F-4500) using a 980 nm laser diode as the excitation source. Decay profiles of the 540 nm and 655 nm radiations were measured by square-wavemodulation of the electric current input to the 980 nm diode laser, the signals were recorded by a Tektronix TDS 5052 digital oscilloscope with a lock-in preamplifier (Stanford Research System Model SR830 DSP). The absorption spectrum of KH-560 film was studied on a UVI spectrophotometer (Thermo Electron CORP, EV300). Tribological performances were tested on a UMT-2MT tribometer (CETR, USA) under a ball-on-disk mode. 3. Results and discussion 3.1. Fabrication and characterization of ˇ-NaYF4 :Yb,Er UC film ␤-NaYF4 :Yb,Er crystals synthesized via hydrothermal method using PEI as chelating agent are amine-functionalized due to the adsorption of PEI molecule on the surface of crystals. The existence of abundant amine groups enables further surface modification of the crystals through certain chemical reactions. As reported by previous literatures [18,19], amine groups can react with epoxy groups.

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Fig. 4. Deconvoluted XPS spectra of C 1s: (a) KH-560 film, (b) ␤-NaYF4 :Yb,Er UC film.

Taking advantage of this reaction, ␤-NaYF4 :Yb,Er crystals were chemically bonded onto the KH-560 covered Si substrate (Fig. 1). To clearly demonstrate the variation of surface composition and actual occurrence of chemical reactions in the process of experiment, characterizations such as WCA measurement, FTIR and XPS analysis were carried out. WCA measurement is a simple and useful way to measure the variation of surface composition. The volume droplet was 0.6 ␮l, at least five repeat measurements were performed for each sample and the average value was taken as the resultant value. The WCAs of different samples are listed in Table 1. The WCA of Si substrate surface is about 0◦ because the surface is rich in Si-OH after pretreatment. After KH-560 film self-assembling on the Si substrate, the WCA increases to 55.1◦ . Due to the containing of epoxy groups, the surface of KH-560 film is still hydrophilic. The WCA of ␤-NaYF4 :Yb,Er UC film is 83.2◦ , indicating that the UC film is also hydrophilic. However, its WCA is slightly higher than that of ␤-NaYF4 :Yb,Er crystals self-assembled on Si substrate without KH-560 primer film. This can be ascribed to the chemical reactions between epoxy groups of KH-560 and amino groups on crystals, resulting in the decrease of hydrophilic group content on the surface of film. FTIR spectra were recorded to verify the chemical reactions between KH-560 molecules and ␤-NaYF4 :Yb,Er crystals. Fig. 2(a) shows the FTIR spectrum of the as-prepared

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C. Wang, X. Cheng / Applied Surface Science 371 (2016) 391–398 Table 2 EDS characterization of ␤-NaYF4 :Yb,Er UC film. Element

Y

Yb

Er

At.%

78.2

18.9

2.9

Table 3 Lifetimes of the 4 S3/2 and 4 F9/2 states of Er3+ ions in the as-prepared samples. Sample ␤-NaYF4 :Yb /Er crystal ␤-NaYF4 :Yb3+ /Er3+ UC film 3+

Fig. 5. Deconvoluted XPS spectra of N 1s: (a) KH-560 film, (b) ␤-NaYF4 :Yb,Er UC film.

␤-NaYF4 :Yb,Er crystals, the bands at about 2845 cm−1 and 2940 cm−1 can be ascribed to the symmetrical and asymmetrical stretching vibration modes of the CH2 group, respectively [20]. The main peaks centered at 1570, 1476, 1380, and 1120 cm−1 can be attributed to the NH bending, methylene scissoring, symmetric bending of methyl, and stretching vibration of C N bond, respectively [21,22]. This result indicates the adsorption of PEI on the surface of ␤-NaYF4 :Yb,Er crystals. After reacting with KH-560, the characteristic band of amine group (1570 cm−1 ) vanishes, and the typical signal of Si O Si at 1030 cm−1 arises [23,24]. Therefore, it is rational to deduce that the amine groups of PEI can react with the epoxy groups of KH-560 molecules. To further investigate the elemental composition and elemental chemical state of the as-prepared films, XPS measurements were carried out. Fig. 3(a) shows the XPS survey spectrum of the asprepared KH-560 film. The emergence of C 1s peak illustrates that the KH-560 molecules have been assembled on Si substrate. After the assembly of ␤-NaYF4 :Yb,Er crystals, new peaks of Na 1s, F 1s, N 1s, and Y 3s appear (Fig. 3(b)), indicating the existence of ␤NaYF4 :Yb,Er crystals on KH-560 film. It is noteworthy that the Si 2s and Si 2p peaks are still visible in Fig. 3(b), this can be ascribed to the incomplete coverage of Si substrate by ␤-NaYF4 :Yb,Er crystals. Fig. 4(a) shows the C 1s XPS spectrum of KH-560 film assembled on Si substrate. Expectedly, besides the typical bonding of C C, KH560 film should have a large number of epoxy groups. Obviously, C 1s spectrum of KH-560 film in Fig. 4(a) was deconvoluted into two Gaussian peaks which can be attributed to C C (284.9 eV) and

3+

␶1 /ms (4 S3/2 )

␶2 /ms (4 F9/2 )

0.235 0.227

0.306 0.294

C O (286.4 eV), respectively [13,22]. The C O bonds were believed to stem from epoxy groups of KH-560. This result is similar with our previous work, suggesting that KH-560 film has been assembled on Si substrate [25]. Fig. 4(b) illustrates the C 1s spectrum of ␤-NaYF4 :Yb,Er UC film. It is worth noting that a new bonding peak appears at 285.4 eV, which can be assigned to C N bonds [26,27]. Obviously, the C N bond is derived from the ␤-NaYF4 :Yb,Er UC film. As ␤-NaYF4 :Yb,Er crystals have been amine-functionalized and itself contain C N bonds, basing on these results we cannot come to the conclusion that ␤-NaYF4 :Yb,Er UC film has been successfully fabricated through chemical reactions. To further investigate the chemical reactions between amine groups and epoxy groups, N 1s XPS spectra of KH-560 film and ␤-NaYF4 :Yb,Er film have been detected, as shown in Fig. 5. There are two sub-peaks appearing in different binding energies located at 398.6 (±0.3) eV and 401.2 (±0.2) eV, which are characteristic for aliphatic amine groups and protonated aliphatic amine groups, respectively [26]. Fig. 5(a) shows that the protonated species dominates for the as-prepared KH-560 film. After the assembly of ␤-NaYF4 :Yb,Er crystals, the proportion of protonated species decreases because a portion of amine groups covalently bond with epoxy groups and becomes difficult to be protonated. XPS analysis together with the results of FTIR could unambiguously support that ␤-NaYF4 :Yb,Er crystals can be chemically absorbed onto the surface of KH-560 treated Si substrate. The morphology and size of the as-prepared ␤-NaYF4 :Yb,Er crystals were investigated by TEM, as shown in Fig. 6(a). The crystals exhibit hexagonal shape with an average length of 5.5 ␮m. The phase purity and composition of the crystals were examined by XRD (Fig. 6(b)). All the diffraction peaks of the crystals can be well indexed to the standard XRD pattern of hexagonal phase NaYF4 (JCPDS No. 16-0334), implying the high crystallization of the asprepared products [28]. Fig. 7 shows the AFM morphologies of hydroxylated Si substrate and KH-560 film. As shown in Fig. 7(a) and (b), the surface of Si substrate is quite smooth (average surface roughness (Ra) of 0.333 nm) except for a few of island domains which may be caused by the impurities on Si substrate. After the assembly of KH-560 film, island structures appear on top of the surface of Si substrate (average surface roughness (Ra) of 0.424 nm), indicating the successful assembly of KH-560 film on Si substrate (Fig. 7(c) and (d)). The morphology of the as-prepared ␤-NaYF4 :Yb,Er UC film was detected by SEM, as illustrated in Fig. 8. It can be clearly seen that ␤-NaYF4 :Yb,Er crystals are distributed homogeneously throughout the substrate. The chemical composition of the UC film was further confirmed by EDS. As listed 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 reactants, indicating the successfully doping of Yb3+ and Er3+ ions into NaYF4 lattice.

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3.2. Upconversion luminescence properties of ˇ-NaYF4 :Yb,Er UC film The typical UC emission spectrum of the as-prepared ␤NaYF4 :Yb,Er UC film excited by a 980 nm laser with a pumping power of 200 MW is shown in Fig. 9. The green emission bands at 523 nm and 540 nm correspond to the 2 H11/2 → 4 I15/2 and 4S 4 3+ ions, respectively [29]. The red 3/2 → I15/2 transitions of Er emission band at 655 nm can be assigned to the 4 F9/2 → 4 I15/2

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transition of Er3+ ions [30]. The mechanism for the UC emission of NaYF4 :Yb,Er has been well established by many literatures [31,32]. In this experiment, the UC mechanism of the film is similar to the results of our previous research [20,33]. The UC emission processes include the ground-state adsorption of Yb3+ and Er3+ ions, the excited-state adsorption of Er3+ ions, the energy-transfer UC from Yb3+ to Er3+ ions, and the cross relaxation between two nearby Er3+ ions.

Fig. 6. The TEM image (a) and XRD pattern (b) of the as-prepared ␤-NaYF4 :Yb,Er crystals.

Fig. 7. AFM 2D (a) and corresponding 3D (b) morphologies for hydroxylated Si substrate; AFM 2D (c) and corresponding 3D (d) morphologies for KH-560 film.

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Fig. 8. SEM images of ␤-NaYF4 :Yb,Er UC film at magnification of 500× (a) and 1000× (b).

Furthermore, it is noteworthy that the absorption spectrum of KH-560 film was recorded on a UVI spectrophotometer. The result shows that it has no absorption in the wavelength ranging from 400 to 1100 nm. So the re-absorption of green and red upconverted fluorescence by KH-560 film can be neglected.

3.3. Tribological properties of ˇ-NaYF4 :Yb,Er UC film

Fig. 9. The UC emission spectrum of the ␤-NaYF4 :Yb,Er UC film.

To explore the effects of the external factors (such as chemical reaction, KH-560 film et al.) on the luminescence properties of ␤-NaYF4 :Yb,Er UC film, we monitored the decay profiles of 4S 4 4F 4 3/2 → I15/2 (540 nm) and 9/2 → I15/2 (655 nm) transitions. Fig. 10 shows the normalized decay curve of the prepared samples under 980 nm laser excitation. The luminescence lifetimes of the of the 4 S3/2 and 4 F9/2 states were shown in Table 3, which were calculated by fitting each decay curve to a single exponential function. The results show that the decay time constants ␶1 and ␶2 of ␤-NaYF4 :Yb,Er crystals and the UC film are very close. As is known, the inverse of the lifetime is equal to the sum of the nonradiative transition and radiative transition [34]. So the luminescence properties of the ␤-NaYF4 :Yb,Er crystals are almost unaffected after being assembled on Si substrate.

To evaluate the performance of UC film and detect the interfacial adhesion strength between film and Si substrate, the microtribological properties of the as-prepared ␤-NaYF4 :Yb,Er UC film and ␤-NaYF4 :Yb,Er crystals assembled on bare Si substrate were investigated under reciprocating sliding motion. A Si3 N4 ball of 3 mm diameter was used as the counterface. All the tests were conducted under the conditions of a distance of 5 mm and a frequency of 1 Hz. The film fails when the coefficient of friction (COF) increases abruptly to the steady state value of about 0.8 which is the COF of bare silicon wafer sliding against ceramic ball. As shown in Fig. 11(a), the ␤-NaYF4 :Yb,Er crystals assembled on bare Si substrate keep a steady state of sliding for about 170 s before it fails. However, as a comparison, the ␤-NaYF4 :Yb,Er UC film displays good tribological property with lifetime of 850 s (Fig. 11(b)). We owe this to the different interfacial binding force between film and substrate. As is known, there are mainly two kinds of attractions leading to absorption of molecules onto solid surface, physical adsorption and chemical adsorption. The ␤-NaYF4 :Yb,Er crystals are immobilized on bare Si substrate by physical adsorption. Whereas, the ␤-NaYF4 :Yb,Er UC film is combined with Si substrate via covalent chemical bonds. Obviously, the binding force of the former is much weaker, the crystals are easily pushed to the sides in the course of sliding, resulting in the failure of the film. The tribological results demonstrate that the ␤-NaYF4 :Yb,Er UC film has been successfully synthesized on Si substrate.

Fig. 10. Decay profiles of 4 S3/2 (a) and 4 F9/2 (b) for ␤-NaYF4 :Yb,Er crystal and the UC film under 980 nm laser excitation.

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Fig. 11. Variation of COF with time for as-prepared films under an applied load of 0.5 N: (a) ␤-NaYF4 :Yb,Er crystals assembled on bare Si substrate and (b) ␤-NaYF4 :Yb,Er UC film.

4. Conclusion In this work, ␤-NaYF4 :Yb,Er UC film has been successfully assembled on KH-560 covered Si substrate via a facile self-assemble method. The chemical composition, UC luminescence and tribological properties have been investigated. The results show that ␤-NaYF4 :Yb,Er crystals are combined with Si substrate by covalent chemical bonds. Compared with ␤-NaYF4 :Yb,Er crystals, the UC luminescence properties of the as-prepared UC film are almost unchanged. The UC film displays good tribological property with lifetime of about 900s owing to the strong interfacial binding force. This work can broaden the application of ␤-NaYF4 :Yb,Er crystals and the method can be applied to synthesize other lanthanidedoped UC films. Acknowledgments We are grateful to the National Natural Science Foundation of China (Grant No. 51575341) and the help of the Instrumental Analysis Center, Shanghai Jiaotong University.

[12]

[13]

[14]

[15]

[16] [17]

[18]

[19]

[20]

References [21] [1] F. Auzel, Upconversion and anti-stokes processes with f and d ions in solids, Chem. Rev. 104 (2004) 139–174. [2] F. Wang, X. Liu, Recent advances in the chemistry of lanthanide-doped upconversion nanocrystals, Chem. Soc. Rev. 38 (2009) 976–989. [3] G. Yi, H. Lu, S. Zhao, Y. Ge, W. Yang, D. Chen, L.H. Guo, Synthesis, characterization, and biological application of size-controlled nanocrystalline NaYF4 :Yb,Er infrared-to-visible up-conversion phosphors, Nano Lett. 4 (2004) 2191–2196. [4] D.K. Chatterjee, A.J. Rufaihah, Y. Zhang, Upconversion fluorescence imaging of cells and small animals using lanthanide doped nanocrystals, Biomaterials 29 (2008) 937–943. [5] J.H. Zeng, J. Su, Z.H. Li, R.X. Yan, Y.D. Li, Synthesis and upconversion luminescence of hexagonal-phase NaYF4 :Yb3+ ,Er3+ phosphors of controlled size and morphology, Adv. Mater. 17 (2005) 2119–2123. [6] Y. Wei, F. Lu, X. Zhang, D. Chen, Synthesis of oil-dispersible hexagonal-phase and hexagonal-shaped NaYF4 :Yb,Er nanoplates, Chem. Mater. 18 (2006) 5733–5737. [7] G. Tian, Z. Gu, L. Zhou, W. Yin, X. Liu, L. Yan, S. Jin, W. Ren, G. Xing, S. Li, Mn2+ dopant-controlled synthesis of NaYF4 :Yb/Er upconversion nanoparticles for in vivo imaging and drug delivery, Adv. Mater. 24 (2012) 1226–1231. [8] C. Lin, M.T. Berry, R. Anderson, S. Smith, P.S. May, Highly luminescent NIR-to-visible upconversion thin films and monoliths requiring no high-temperature treatment, Chem. Mater. 21 (2009) 3406–3413. [9] L. Tian, P. Wang, H. Wang, R. Liu, Hexagonal phase ␤-NaGdF4 :Yb3+ /Er3+ thin films with upconversion emission grown by electrodeposition, RSC Adv. 4 (2014) 19896–19899. [10] S. Chen, G. Zhou, F. Su, H. Zhang, L. Wang, M. Wu, M. Chen, L. Pan, S. Wang, Power conversion efficiency enhancement in silicon solar cell from solution processed transparent upconversion film, Mater. Lett. 77 (2012) 17–20. [11] S. Sivakumar, F.C.M. van Veggel, P.S. May, Near-infrared (NIR) to red and green up-conversion emission from silica sol–gel thin films made with

[22]

[23]

[24] [25] [26]

[27]

[28]

[29]

[30]

La0.45Yb0.50Er0.05F3 nanoparticles, hetero-looping-enhanced energy transfer (Hetero-LEET): a new up-conversion process, J. Am. Chem. Soc. 129 (2007) 620–625. P. Jenouvrier, G. Boccardi, J. Fick, A.M. Jurdyc, M. Langlet, Up-conversion emission in rare earth-doped Y2 Ti2 O7 sol–gel thin films, J. Lumin. 113 (2005) 291–300. H.W. Tien, S.T. Hsiao, W.H. Liao, Y.H. Yu, F.C. Lin, Y.S. Wang, S.M. Li, C.C.M. Ma, Using self-assembly to prepare a graphene-silver nanowire hybrid film that is transparent and electrically conductive, Carbon 58 (2013) 198–207. M.A. Raj, S.A. John, Fabrication of electrochemically reduced graphene oxide films on glassy carbon electrode by self-assembly method and their electrocatalytic application, J. Phys. Chem. C 117 (2013) 4326–4335. P.F. Li, Y. Xu, X.H. Cheng, Chemisorption of thermal reduced graphene oxide nano-layer film on TNTZ surface and its tribological behavior, Surf. Coat. Technol. 232 (2013) 331–339. Z. Liu, D. Shu, P. Li, X. Cheng, Tribology study of lanthanum-treated graphene oxide thin film on silicon substrate, RSC Adv. 4 (2014) 15937–15944. H.J. Song, X.H. Jia, N. Li, X.F. Yang, H. Tang, Synthesis of ␣-Fe2 O3 nanorod/graphene oxide composites and their tribological properties, J. Mater. Chem. 22 (2012) 895–902. D. Shao, M. Yu, H. Sun, T. Hu, S. Sawyer, High responsivity, fast ultraviolet photodetector fabricated from ZnO nanoparticle-graphene core–shell structures, Nanoscale 5 (2013) 3664–3667. J. Ou, J. Wang, S. Liu, B. Mu, J. Ren, H. Wang, S. Yang, Tribology study of reduced graphene oxide sheets on silicon substrate synthesized via covalent assembly, Langmuir 26 (2010) 15830–15836. C. Wang, X. Cheng, Influence of Cr3+ ions doping on growth and upconversion luminescence properties of ␤-NaYF4 :Yb3+ /Er3+ microcrystals, J. Alloys Compd. 649 (2015) 196–203. M. Wang, C.C. Mi, J.L. Liu, X.L. Wu, Y.X. Zhang, W. Hou, F. Li, S.K. Xu, One-step synthesis and characterization of water-soluble NaYF4 :Yb,Er/polymer nanoparticles with efficient up-conversion fluorescence, J. Alloys Compd. 485 (2009) L24–L27. C. Zhang, R. Hao, H. Liao, Y. Hou, Synthesis of amino-functionalized graphene as metal-free catalyst and exploration of the roles of various nitrogen states in oxygen reduction reaction, Nano Energy 2 (2013) 88–97. C. Hollenstein, A. Howling, C. Courteille, D. Magni, S. Scholz, G. Kroesen, N. Simons, W. de Zeeuw, W. Schwarzenbach, Silicon oxide particle formation in RF plasmas investigated by infrared absorption spectroscopy and mass spectrometry, J. Phys. D: Appl. Phys. 31 (1998) 74–84. Z. Zhang, Catalytic effect of aluminum acetylacetonate on hydrolysis and polymerization of methyltrimethoxysilane, Langmuir 13 (1997) 473–476. C. Wang, X. Cheng, Synthesis of a NaYF4 :Yb,Er upconversion film on a silicon substrate and its tribological behavior, RSC Adv. 5 (2015) 94980–94985. G. Bai, J. Wang, Z. Yang, H. Wang, Z. Wang, S. Yang, Self-assembly of ceria/graphene oxide composite films with ultra-long antiwear lifetime under a high applied load, Carbon 84 (2015) 197–206. K.M. Kallury, R.F. Debono, U.J. Krull, M. Thompson, Covalent binding of amino carboxy, and nitro-substituted aminopropyltriethoxysilanes to oxidized silicon surfaces and their interaction with octadecanamine and octadecanoic acid studied by X-ray photoelectron spectroscopy and ellipsometry, J. Adhes. Sci. Technol. 5 (1991) 801–814. L. Yang, H. Han, Y. Zhang, J. Zhong, White emission by frequency up-conversion in Yb3+ -Ho3+ -Tm3+ triply doped hexagonal NaYF4 nanorods, J. Phys. Chem. C 113 (2009) 18995–18999. D. Yuan, M.C. Tan, R.E. Riman, G.M. Chow, Comprehensive study on the size effects of the optical properties of NaYF4 :Yb,Er nanocrystals, J. Phys. Chem. C 117 (2013) 13297–13304. Y. Li, G. Wang, K. Pan, B. Jiang, C. Tian, W. Zhou, H. Fu, NaYF4 :Er3+ /Yb3+ -graphene composites: preparation, upconversion

398

C. Wang, X. Cheng / Applied Surface Science 371 (2016) 391–398

luminescence, and application in dye-sensitized solar cells, J. Mater. Chem. 22 (2012) 20381–20386. [31] Y. Li, K. Pan, G. Wang, B. Jiang, C. Tian, W. Zhou, Y. Qu, S. Liu, L. Feng, H. Fu, Enhanced photoelectric conversion efficiency of dye-sensitized solar cells by the incorporation of dual-mode luminescent NaYF4 :Yb3+ /Er3+ , Dalton Trans. 42 (2013) 7971–7979. [32] R.B. Anderson, S.J. Smith, P.S. May, M.T. Berry, Revisiting the NIR-to-visible upconversion mechanism in ␤-NaYF4 :Yb3+ ,Er3+ , J.Phys. Chem. Lett. 5 (2013) 36–42.

[33] C. Wang, X. Cheng, Controlled hydrothermal growth and tunable luminescence properties of ␤-NaYF4 :Yb3+ /Er3+ microcrystals, J. Alloys Compd. 617 (2014) 807–815. [34] Y. Ding, X. Zhang, H. Gao, S. Xu, C. Wei, Y. Zhao, Enhancement on concentration quenching threshold and upconversion luminescence of ␤-NaYF4 :Er3+ /Yb3+ codoping with Li+ ions, J. Alloys Compd. 599 (2014) 60–64.