Chemical Physics Letters 644 (2016) 152–156
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Upconversion luminescence of Ho3+ /Yb3+ co-doped CaNb2 O6 thin ﬁlms Ning Li a,b , Wei Wang a,b , Pingping Duan a,b , Yinzhen Wang a,b,∗ , Xuwei Sun a,b , Juqing Di c,d , Wei Li a,b , Benli Chu a,b , Qinyu He a,b a Guangdong Provincial Key Laboratory of Quantum Engineering and Quantum Materials, School of Physics & Telecommunication Engineering, South China Normal University, Guangzhou 510006, PRC b Guangdong Engineering Technology Research Center of Efﬁcient Green Energy and Environmental Protection Materials, Guangzhou 510006, PRC c Key Laboratory of Materials for High Power Laser, Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Shanghai 201800, PRC d University of Chinese Academy of Sciences, Beijing 100049, PRC
a r t i c l e
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Article history: Received 1 September 2015 In ﬁnal form 25 November 2015 Available online 2 December 2015
a b s t r a c t Ho3+ /Yb3+ co-doped CaNb2O6 thin ﬁlms were deposited on Si(100) substrates by pulsed laser deposition and annealed at different temperature in air atmosphere. The structure and properties of the ﬁlm samples were characterized by using X-ray diffraction, atomic force microscope, Raman spectroscopy, X-ray photoelectron spectroscopy and photoluminescence spectra. It is found that the annealing temperature has a strong effect on the ﬁlm’s structure, morphology, grain size and the up-conversion luminescence properties. Upconversion luminescence intensity of Ho3+ /Yb3+ co-doped CaNb2 O6 ﬁlms is enhanced by increasing the annealing temperature. © 2015 Elsevier B.V. All rights reserved.
1. Introduction In recent years, considerable attention has been devoted to research on the rare earth ions (RE3+ ) doped upconversion (UC) luminescence materials, especially with the rapid development of semiconductor lasers and their potential applications, such as light-emitting diodes, high-density storage, display, optoelectronics, medical diagnostics, sensors and solar energy conversion [1–5]. UC is a luminescence process which can come into being higher energy photons with low energy excitation. RE3+ is very suitable to be the upconversion launch center because of the wealth of electronic level and narrow emission spectrum. Because of the special energy level structure of Ho3+ , the energy gap between its 3 K and 5 S levels with the adjacent low energy level is larger, and 8 2 thus, non-radiative relaxation is less. Also Ho3+ is easy to be directly excited by the low energy photon or the sensitization of other RE3+ because of the abundance of the Ho3+ absorption peak, such as Ho3+ and Yb3+ co-doped upconversion luminescence materials including Yb3+ /Ho3+ co-doped oxyﬂuoride glass and glass–ceramics [6–8]. It
∗ Corresponding author at: South China Normal University, Guangdong Provincial Key Laboratory of Quantum Engineering and Quantum Materials, School of Physics & Telecommunication Engineering, Guangzhou, 510006, PRC. E-mail address: [email protected]
(Y. Wang). http://dx.doi.org/10.1016/j.cplett.2015.11.041 0009-2614/© 2015 Elsevier B.V. All rights reserved.
is well known that upconversion luminescence properties strongly depend on the structure of the host materials. Columbite calcium niobate (CaNb2 O6 ), which has an orthorhombic crystal structure, has been found to have wide applications in microwave dielectrics[9,10], laser, photocatalyst, ﬁbers[13,14] and ceramics[15,16]. CaNb2 O6 can be used as a host material for phosphors because it can exhibit strong blue radiation under ultraviolet excitation of 253 nm even at room temperature . In addition it’s also reported to have been used both as a laser and a laser hostmaterial . However, there are few reports on Ho3+ /Yb3+ co-doped CaNb2 O6 thin ﬁlms. In this letter, we report the growth of Ho3+ /Yb3+ co-doped CaNb2 O6 thin ﬁlms by pulsed laser deposition (PLD). The structure and upconversion luminescence properties of the Ho3+ /Yb3+ co-doped CaNb2 O6 thin ﬁlms are investigated.
2. Experimental The Ho3+ /Yb3+ co-doped CaNb2 O6 target was prepared by solid-state reaction methods using analytical grade CaCO3 , Nb2 O5 , Ho2 O3 and Yb2 O3 powders as the starting materials. Appropriate amounts of the starting materials for stoichiometric Ca0.88 Yb0.10 Ho0.02 Nb2 O6 were weighted and milled with zirconia balls for 24 h. After the starting powers dried and calcinated at
1073 K 973 K
873 K as-deposited JCPDS 39-1392
ﬁlm and Si substrate. The full-width at half-maximum (FWHM) of the (131) peak, the highest intensity peak among the peaks, are 1.28◦ , 0.45◦ and 0.33◦ for the CaNb2 O6 :Ho3+ /Yb3+ thin ﬁlm annealed at 973, 1073 and 1173 K, respectively. With the increase of annealing temperature, the peak intensity, especially (131) peak, becomes stronger and sharper. From the XRD data, the crystallite size can be estimated using the Scherrer’s equation.
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2θ(°) Figure 1. XRD spectra of as-deposited and annealed CaNb2 O6 :Ho3+ /Yb3+ ﬁlms.
1523 K for 10 h, the powders were pressed into disk pellets. The pellets were then sintered at 1473 K for 6 h in air. The as-prepared target showed a CaNb2 O6 crystalline phase in the XRD pattern. The Ho3+ /Yb3+ co-doped CaNb2 O6 thin ﬁlms were deposited on Si(100) substrate by PLD at room temperature, oxygen pressure of 5.2 Pa and laser pulse energy power of 300 mJ. Annealing of Ho3+ /Yb3+ co-doped CaNb2 O6 thin ﬁlms was carried out for 2 h at temperature of 873, 973, 1073 and 1173 K in air. The ﬁlms were analyzed by X-ray diffraction (XRD) to check the phases with a Rigaku D/max-IIIA X-ray diffractometer (CuK␣1 , = 1.5405 nm). The surface microstructures of Ho3+ /Yb3+ co-doped CaNb2 O6 thin ﬁlms were analyzed by atomic force microscopy (AFM) (Digital Instrument Nanoscope IIIa). Microstructure analysis of the interface between thin ﬁlms and substrates was performed by cross-sectional transmission electron microscopy (TEM, JEM-2010 FEF). The samples for TEM were made by standard methods. They were glued together face to face with epoxy resin, mechanically polished to a thickness of about 60 m, dimpled from one side to get a thickness of about 20 m at the center followed by ion milling using a Gatan precision ion polishing system. The high-resolution transmission electron microscopy (HRTEM) images were obtained with a point resolution of 0.19 nm. X-ray photoelectron spectroscopy (XPS) using an ESCALAB250 system was used to analyze the chemical valences and chemical compositions of the ﬁlms. The upconversion emission spectra were measured using the 980 nm excitation wavelength of a semiconductor laser. 3. Results and discussion Figure 1 shows the XRD patterns for as-deposited and annealed at different temperatures of CaNb2 O6 :Ho3+ /Yb3+ thin ﬁlms deposited on Si(100) substrates. There is no diffraction peak of as-deposited thin ﬁlm and annealed ﬁlm at 873 K, indicating that the CaNb2 O6 :Ho3+ /Yb3+ thin ﬁlms are amorphous. For the other samples, CaNb2 O6 :Ho3+ /Yb3+ thin ﬁlms show polycrystalline structure with the peaks including the (130) and (131) of the XRD patterns for the annealed thin ﬁlms when the annealing temperature is above 973 K. The peaks are in agreement with that of the orthorhombic columbite structure CaNb2 O6 (orthorhombic phase, space group Pbcn(60) and JCPDS no. 71–2406), the calculated lat˚ b = 5.715 Å and c = 5.184 A, ˚ this value tice constants are a = 14.834 A, is small in comparison with the bulk value of 14.926, 5.752 and ˚ The difference in lattice parameter between CaNb2 O6 ﬁlm 5.204 A. and the bulk indicates that the ﬁlm is under compression stress, the stress is mainly caused by thermal mismatch between the CaNb2 O6
0.9 ˇ cos
where = 1.5405 nm is the wavelength of the incident CuK␣ radiation, ˇ represents full-width at half-maximum of the respective peak (131) and is the Bragg diffraction angle. The average crystallite size, calculated using Scherer formula of the ﬁlms annealed at 973, 1073 and 1173 K, was about 2.89, 8.22 and 9.98 nm, respectively. With the annealing temperature at 1173 K, the FWHM of the most intense peak (131) holds a minimum value of 0.33◦ and the crystallite size possesses a maximum value of 9.98 nm. All of above show that the annealing temperature is an important factor of the crystallization process. The annealing treatment reduces the lattice strain and defects, crystallinity of the obtained ﬁlms is improved and the grain sizes become larger with elevating annealing temperatures. Figure 2 shows AFM images of the as-deposited and annealed CaNb2 O6 :Ho3+ /Yb3+ thin ﬁlms. It is found that the average grain size of the ﬁlm sample increases with the annealing temperature. The thin ﬁlm, annealed at 1173 K, has a largest grain size and best crystal quality, which is consistent with the results of XRD. The root mean square (RMS) roughness of samples as-deposited, annealed at 873, 973, 1073 and 1173 K is 4.37, 5.32, 5.78, 6.12 and 7.12 nm, respectively. And the change trends of the ﬁlms’ grain size with the annealing temperature lead that the RMS roughness value is also increases with the annealing temperature. The appropriate larger RMS roughness value is good for the ﬁlms in application, for surface roughness affects the solar absorptivity of ﬁlms. High-resolution transmission electron microscope (HRTEM) measurement was used to characterize the microstructure of the CaNb2 O6 :Ho3+ /Yb3+ thin ﬁlm annealed at 1173 K. Figure 3a shows cross-sectional TEM image of the CaNb2 O6 :Ho3+ /Yb3+ thin ﬁlms. The ﬁlm has an average thickness of about 100 nm. The ﬁlms are composed of particles, between particles have crystal boundary, which show polycrystalline ﬁlms. Polycrystalline can also be seen in the HRTEM image (Figure 2b), corresponding to the (111) crystal face, crystal plane spacing is about 0.37 nm, the direction of the two region crystal plane arrangement is not the same, showing polycrystalline structure. Additionally, electron diffraction pattern from ﬁlms is shown in Figure 2c, the diffraction pattern exhibits typical polycrystalline diffraction for the sample, the corresponding zone axis is [–101]. In order to identify the phase compositions and chemical state of elements in the ﬁlms, XPS measurement is performed for the CaNb2 O6 :Ho3+ /Yb3+ ﬁlm annealed at 1173 K shown in Figure 4. Figure 4 is the full scan spectra of the as-received and ion bombarded surfaces in the binding energy ranging from 0 to 1100 eV. The peaks of the core level from Ca, O, Nb, Yb and Ho are recorded. The carbon peak, which may be induced by the contamination during the sample preparation and handling, is distinct on the surface of the ﬁlms. The quantiﬁcation of Ca 2p, Yb 4d, Ho 4d, Nb 3d and O 1s peaks gives an average Ca/Yb/Ho/Nb/O atomic ratio of 11.12/1.11/0.22/22.22/66.67, which is nearly in agreement with the designed ratio of Ca0.88 Yb0.10 Ho0.02 Nb2 O6 . Photoluminescence is a sensitive technique for examining the ﬁlm quality, especially its optical properties. Figure 5 shows upconversion luminescence spectra of the annealed CaNb2 O6 :Ho3+ /Yb3+ thin ﬁlms under 980 nm excitation at room temperature. The UC emission is composed of the two emission peaks at 549 and 671 nm
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Figure 2. AFM image of as-deposited and annealed CaNb2 O6 :Ho3+ /Yb3+ thin ﬁlms: (a) RT, (b) 873 K, (c) 973 K, (d) 1073 K and (e) 1173 K.
OKLL Ca2s Nb3s
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100 200 300 400 500 600 700 800 900 1000 1100 1200
Binding Energy (eV) Figure 4. XPS spectra obtain from the CaNb2 O6 :Ho3+ /Yb3+ thin ﬁlm annealed at 1173 K.
F4, S2→ I8
873 K 973 K 1073 K 1173 K
Wavelength(nm) Figure 5. Upconversion luminescence spectra of the annealed CaNb2 O6 :Ho3+ /Yb3+ thin ﬁlms under 980 nm excitation at room temperature.
Figure 3. (a and b) Low-magniﬁcation and high-magniﬁcation TEM images of CaNb2 O6 :Ho3+ /Yb3+ thin ﬁlm; (c) SAED pattern.
for all the ﬁlms. And the band around 671 nm is relative weak, which is attributed to the 5 F5 →5 I8 transition of Ho3+ ions. The emission band around 549 nm is very intense, which is assigned to 5 F4 , 5 S2 →5 I8 transition. It indicates that the PL spectra of the ﬁlms depend on annealing temperature signiﬁcantly. PL intensity of emission peaks increases with increase of the annealing temperature. Due to the larger grain size with increase of the annealing temperature, the density of grain boundaries is lower, which produced less scattering and dissipation of light, causing the enhancement in UC-PL intensity [19,20]. Similar increase of luminescence intensity for the ﬁlms annealed at higher temperature was reported for ZnO ﬁlms [21,22] and Bi3TiNbO9:Er3+ :Yb3+ thin ﬁlms . To understand the UC mechanisms, the pump power dependences of emissions for CaNb2 O6 :Ho3+ /Yb3+ thin ﬁlms were investigated. It is known that the UC emission intensity (I) depends on the excitation power (P) as follows. I ∝ Pn Where n is the number of pumping photons absorbed by RE ions from the ground state to the emitting excite state. The power dependence for the UC emissions in the CaNb2 O6 :Ho3+ /Yb3+ thin
N. Li et al. / Chemical Physics Letters 644 (2016) 152–156
or by excitation state absorption (ESA). Finally the excited Ho3+ ion in the 5 F4 , 5 S2 state demotes to the ground state 5 I8 with a green emission around 549 nm. There are two possible pathways for the excitation of the Ho3+ ions to the 5 F5 excited state. The one is that the Ho3+ ions relax from the 5 F4 , 5 S2 state to the 5 F5 state by the non-radiative processes; the other is that the Ho3+ ions in the 5 I6 state relaxed to the 5 I7 state, are excited to the 5 F5 state by the excited state absorption or the energy transition of the Yb3+ ions in the 2 F5/2 state. Then it can generate the red emission around 671 nm by the 5 F5 →5 I8 transition.
532nm, n=2.18 654nm, n=1.86
4. Conclusions 3.4
LogP(mW) Figure 6. Pump power dependences CaNb2 O6 :Ho3+ /Yb3+ thin ﬁlms.
This work was supported by National Nature Science Foundation of China (No. 11474104 and 51372092) and ﬁnancial support from China Scholarship Council.
Energy(10 cm )
The Ho3+ /Yb3+ co-doped CaNb2 O6 ﬁlms were successfully prepared on Si (100) substrates by pulsed laser deposition and were characterized by XRD, AFM, XPS, TEM and upconversion luminescence measurements. The results showed that the grain size and the upconversion emission of Ho3+ /Yb3+ co-doped CaNb2 O6 ﬁlms increased with the annealing temperature. Under 980 nm excitation, the UC emission is composed of the two emission band at 549 and 671 nm corresponding to the 5 F4 ,5 S2 →5 I8 and 5 F5 →5 I8 transitions of Ho3+ ions. And the luminescence intensity is higher for the sample annealed at higher temperature with the ﬁlms annealed at 873, 973, 1073 and 1173 K.
Figure 7. Simpliﬁed energy level diagram of Ho3+ and Yb3+ and UC emission processes under 980 nm excitation.
ﬁlms is shown in Figure 6. The number of phonons n is obtained from the slope of the straight line by ﬁtting a double-logarithmic plot of pump power vs. emission intensity. The values of n obtained for 549 and 671 nm bands are ∼2.18 and 1.86 for the green and red emissions, respectively. The n values suggest that upconversion in the CaNb2 O6 :Ho3+ /Yb3+ thin ﬁlms is a two-photon process. Figure 7 shows the energy level diagram of Ho3+ and Yb3+ and UC emission processes in CaNb2 O6 thin ﬁlms under 980 nm excitation. Yb3+ ions absorb the photons of 980 nm and transit from the ground state 2 F7/2 to the excited state 2 F5/2 level. The Yb3+ ions act as sensitizers, the excited Yb3+ ions in 2 F5/2 state transfer their excitation energy to Ho3+ ions. The Ho3+ ion in the ground state is excited to the 5 I6 state via ETU of neighboring Yb3+ . The excited levels 5 F4 , 5 S2 of Ho3+ ions are populated via a second ETU process
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