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Patterning of Gd2(WO4)3:Ln3+ (Ln = Eu, Tb) luminescent films by microcontact printing route
Journal of Colloid and Interface Science 365 (2012) 320–325
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Patterning of Gd2(WO4)3:Ln3+ (Ln = Eu, Tb) luminescent films by microcontact printing route Dong Wang a,b, Piaoping Yang b, Ziyong Cheng a,⇑, Wenxin Wang a, Zhiyao Hou a,b, Yunlu Dai a, Chunxia Li a, Jun Lin a,⇑ a b
State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, PR China College of Materials Science and Chemical Engineering, Harbin Engineering University, Harbin 150001, PR China
a r t i c l e
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Article history: Received 4 August 2011 Accepted 7 September 2011 Available online 12 September 2011 Keywords: Sol–gel Microcontact printing Patterning Gd2(WO4)3
a b s t r a c t Gd2(WO4)3 doped with Eu3+ or Tb3+ thin phosphor films with dot patterns have been prepared by a combinational method of sol–gel process and microcontact printing. This process utilizes a PDMS elastomeric mold as the stamp to create heterogeneous pattern on quartz substrates firstly and then combined with a Pechini-type sol–gel process to selectively deposit the luminescent phosphors on hydrophilic regions, in which a Gd2(WO4)3:Ln3+ (Ln = Eu, Tb) precursor solutions were employed as ink. X-ray diffraction (XRD), scanning electron microscopy (SEM), photoluminescence (PL) spectra, as well as low voltage cathodoluminescence (CL) spectra were carried out to characterize the obtained samples. Under ultraviolet excitation and low-voltage electron beams excitation, the Gd2(WO4)3:Eu3+ samples exhibit a strong red emission arising from Eu3+ 5D0,1,2–7F1,2 transitions, while the Gd2(WO4)3:Tb3+ samples show the green emission coming from the characteristic emission of Tb3+ corresponding to 5D4–7F6,5,4,3 transitions. The results show that the patterning of rare earth-doped phosphors through combining microcontact printing with a Pechini-type sol–gel route has potential for field emission displays (FEDs) applications. Ó 2011 Elsevier Inc. All rights reserved.
1. Introduction With the increasing development in miniaturization and microfabrication of devices, the preparation of thin films have been one of the most active research areas in recent years because of the size-/thickness-dependent properties of materials and their potential for a broad range of applications. As an important aspect, thin films that exhibit unique properties compared to bulk materials, are fundamental for electronic, coatings, displays, sensors, optical equipment, and meet the need for scaling devices to small sizes, and are therefore attractive both in theory and practice [1]. Microcontact printing (lCP), one of soft lithography methods, is an effective technique for forming patterned films with micro- and nanoscales [2]. This has attracted much interest in recent years since it provides experimental simplicity and flexibility in forming certain types of patterns. In generation, an elastomeric stamp (usually made from poly(dimethylsiloxane), PDMS) is employed as mold in the lCP process, which is fabricated by casting a prepolymer of PDMS against a original hard master mold. lCP has proven to be an feasible and facile approach that can generate patterned features on flat or nonplanar surfaces [3–6].
⇑ Corresponding authors. E-mail addresses: [email protected] (Z. Cheng), [email protected] (J. Lin). 0021-9797/$ - see front matter Ó 2011 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2011.09.008
So far the patterning of luminescent films has played an important role in high-resolution devices such as cathode-ray tubes (CRTs), electroluminescent devices (ELDs), plasma display panels (PDPs) and field emission displays (FEDs) [7]. Displays with thin film phosphors have higher contrast and resolution, superior thermal conductivity as well as a high degree of uniformity and better adhesion [8]. It is well known that metal tungstates are an important family of inorganic materials that have extensive applications in various fields [9], such as photoluminescence, fluorescent lamps, microwave applications, optical fibers, scintillator materials and X-ray intensified screen. Many research groups have undertaken investigations of the optical and luminescence properties of tungstates [10]. Nassau et al. [11] and Borchardt [12] have prepared this series of compounds and studied their structures, respectively. Until recently, Kodaira et al. [13] reported the preparation of RE2(WO4)3:Eu3+ (RE = La, Gd) powders using the Pechini sol–gel method. Nowadays, various preparation methods have been developed for the fabrication of metal tungstates materials, such as traditional solid state reaction [14], solvothermal method [15,16], low temperature solution method [17], spray pyrolysis route [18], molten salt method [19], the so-called polymeric precursor method [20,21], electrochemical method [22], microwave irradiation [23], pulsed laser deposition [24] and vapor-deposited method [25]. Compared with the above processes, the solutionbased sol–gel is a simple and economical method for making high
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quality luminescent films because it possesses a number of advantages over conventional film formation techniques, such as low temperature processing, easy coating of large surfaces, and possible formation of porous films and homogeneous multicomponent oxide films [26]. So far, some efforts have been devoted to develop various kinds of luminescent films via sol–gel method in the past years. Representative examples are Y3Al5O12:Tb [7] and Y2SiO5:Tb [8] films for cathodoluminescence, Y3Al5O12:Eu [27] films for field emission displays, Y2O3:Eu [28] and Zn2SiO4:Mn [29] films for photoluminescence. However, up to now, to the best of our knowledge, there have been no reports concerning the preparation of the patterned rare-earth ions doped Gd2(WO4)3 films via lCP method combined with the Pechini-type sol–gel method. Accordingly, here we employed these two methods to fabricate ordered Gd2 (WO4)3:Eu3+ and Gd2(WO4)3:Tb3+ dot arrays on the quartz substrates, in which the pattern printing and phosphor synthesis process can be achieved within one step and therefore reduce the cost of the production. The obtained Gd2(WO4)3:Eu3+ and Gd2(WO4)3:Tb3+ samples demonstrate the strong red and green emission under UV light or electron beam irradiation, respectively, indicating that the lCP method can be used for patterning inorganic phosphors and has potential for the applications of display devices, such as the next generation FED devices. 2. Materials and methods 2.1. Chemicals and materials Polyethylene glycol (PEG, Mw = 10,000), ethanol, n-octane, as well as nitric acid were purchased from the Beijing Chemical Company, and ammonium (meta) tungstate hydrate (H26N6O41W1218H2O) was purchased from Fluka. Citric acid monohydrate C6H8O7 H2O (P99.5%, A.R.), hydrogen peroxide (P30%, A.R.) and ethanol were all purchased from Beijing Fine Chemical Company. Gd2O3, Eu2O3 and Tb4O7 (99.99%) were purchased from Science and Technology Parent Company of Changchun Institute of Applied Chemistry. Gd(NO3)3, Eu(NO3)3 and Tb(NO3)3 were prepared by dissolving Gd2O3, Eu2O3 and Tb4O7 in dilute nitric acid HNO3, and evaporating the water in the solutions by heating. The prepolymer (Sylgard 184) and its curing agent for preparing poly(dimethylsiloxane) (PDMS) were purchased from the Dow Corning Cooperation. All the initial chemicals in this work were used without further purification.
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the master. In this way, PDMS stamps with microwells array were obtained. The quartz plates were cleaned by immersing them in piranha solution (mixture of 98% H2SO4 and 30% H2O2 with v/v = 7:3) at 90 °C for 30 min (Caution! Piranha solutions are highly corrosive and potentially explosive and should be handled with great care.). Then they were rinsed with deionized water, and dried with nitrogen. The as-prepared quartz substrates exhibited hydrophilic properties because of the hydroxyl group on the substrate surface. 2.4. Fabrication of patterned Gd2(WO4)3:Ln3+ (Ln = Eu, Tb) films by microcontact printing The process of fabricating Gd2(WO4)3:Ln3+ (Ln = Eu, Tb) arrays of dots via lCP was shown in Scheme 1. This technique is based on the modification of the substrate surface and the selective film deposition on desired regions. A solution of PFOTS (1H,1H,2H,2H perfluorooctyltrichlorosilane, CF3(CF2)5CH2CH2SiCl3) in n-octane was used to ink the patterned PDMS stamp and dried under N2 stream for several minutes before transferring the SAMs to the substrate. Then, the PDMS stamp was brought into contact with the quartz substrate for 5–10 s, leading to the transferring of PFOTS molecules from the stamp to the substrate in the regions of contact. Next, the patterned substrate was used as a template for the selective deposition of the metal salts solution by spin-coating. During this spin-coating at 2000 rpm process, the precursor solution was selectively deposited on the hydrophilic regions because of the poor adhesion between the solution and the SAM. Finally, the substrate with the patterned gel was annealed at desired temperature (800 °C) with the heating rate of 2 °C min1 and held there for 3 h in air to obtain the crystalline Gd2(WO4)3:Ln3+ (Ln = Eu, Tb) films. 2.5. Characterization The X-ray diffraction (XRD) patterns of the samples were carried out on a D8 Focus diffractometer (Bruker) with use of Cu Ka radiation (k = 0.15405 nm). The patterned images were taken on a Nikon microscope (ECLIPSE Ti) with UV irradiation at 254 nm. The morphology of the crystalline samples was inspected using a field emission scanning electron microscope (SEM; S-4800, Hitachi). The photoluminescence (PL) measurements were recorded with a Hitachi F-4500 spectrophotometer equipped with a 150 W
2.2. Fabrication of Gd2(WO4)3:Ln3+ (Ln = Eu, Tb) precursor solution The doping concetrations of Eu3+ and Tb3+ were selected as 30 and 5 mol% of Gd3+ in Gd2(WO4)3 according to the previous study, respectively [26]. The stoichiometric amounts of Gd(NO3)3, Eu (NO3)3 and Tb(NO3)3 as well as H26N6O41W1218H2O were dissolved in deionized water, then the solution was mixed with citric acid as a chelating agent for the metal ions. The molar ratio of metal ions to citric acid was 1:2. A certain amount of polyethylene glycol (PEG, Mw = 10,000, 0.15 g/mL) was added to adjust the viscoelastic behavior of the solution. The solution was stirred for 2 h to obtain a homogeneous hybrid sol for further microcontact printing. 2.3. Preparation of PDMS stamps and cleaning of substrate PDMS stamp modes were fabricated by pouring the PDMS prepolymer, a 10:1 (v/v) mixture of Sylgard silicone elastomer 184, and its curing agent over a hard master prepared by photolithography. The elastomer was degassed for about 30 min at room temperature and cured at 65 °C for 4 h, then peeled gently from
Scheme 1. Schematic diagram of the experimental microcontact printing process for patterning of Gd2(WO4)3:Eu3+.
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xenon lamp as the excitation source. The cathodoluminescence (CL) measurements were conducted in an ultrahigh-vacuum chamber (<108 Torr), where the phosphors were excited by an electron beam at a voltage range of 5 kV with a filament current 90 mA, and the spectra were recorded using an F-4500 spectrophotometer. The quantum efficiency and chromaticity coordinates of the patterned phosphor film were performed with a quantum-yield measurement system (C9920-02, Hamamatsu Photonics K.K., Japan). All the measurements were performed at room temperature. 3. Results and discussion
the square gels was observed after firing because of pyrolysis and evaporation of the organic compounds in the gel. The estimated diameter of the Gd2(WO4)3:Eu3+ dot was 6.28 ± 1.56 lm. The morphology of the dot patterns created via the lCP process remains unchanged before and after calcinations, but the size of the dots has slightly decreased with heat treatment. Meanwhile, the inset in Fig. 2b is a scanning electron microscopy (SEM) image taken in the region of the Gd2(WO4)3:Eu3+ phosphor dot, which indicates that the crystalline Gd2(WO4)3:Eu3+ patterns are composed of nanoparticles with a size between 300 and 460 nm. 3.3. Formation mechanism
3.1. Structure analysis The composition, phase purity and morphology of the asprepared samples were investigated by XRD and FE-SEM for Gd2(WO4)3:Eu3+. Fig. 1 shows the XRD patterns of the Gd2(WO4)3: Eu3+ and Gd2(WO4)3:Tb3+ patterned film annealed at 800 °C, as well as the JCPDS card (No. 23–1076) for Gd2(WO4)3 as comparison, respectively. It can be seen that there is a broad peak centered at 2h = 22° which is ascribed to amorphous SiO2 of the quartz plate. Apart from the broad peak, several weak diffraction peaks at 2h 19.0°, 28.3°, 29.7° and 47.5° are present in the XRD pattern, which are assigned to (1 1 1), (2 2 1), (0 2 3) and (2 4 2) reflections of Gd2(WO4)3 according to standard data (JCPDS Card 23–1076). The results of XRD suggest that the precursor gel on SiO2 glass has been transformed into the desired crystalline structure of Gd2(WO4)3 at 800 °C. However, besides two weak diffraction peaks, no more reflections due to Gd2(WO4)3 are present on the dot-patterned film sample. This can be assigned to the fact that the dot-patterned Gd2(WO4)3:Eu3+ thin film is discontinuous and there exists blank regions under the light spot of X-ray, which greatly decreases the diffraction intensity [30]. 3.2. Optical images Fig. 2a and d are the optical images for the patterned Gd2 (WO4)3:Eu3+ and Gd2(WO4)3:Tb3+ precursor gel films (just spincoating dried, without annealing) fabricated by the lCP technique, respectively. The average diameter of the Gd2(WO4)3:Eu3+ dots was determined to be 8.00 ± 1.28 lm. After calcination at 800 °C in air, the patterned Gd2(WO4)3:Ln3+ crystalline films were formed on the quartz plate and the corresponding optical microscope images are shown in Fig. 2b and e, respectively. A size shrinkage of
As was shown in Scheme 1, the patterning process can be described as follows: first of all, we used hydrophobic perfluorooctyltrichlorosilane (PFOTS) to rinse the patterned PDMS mold, followed by the mold contacting with the substrate. Thus, the hydrophobic PFOTS molecules were transferred to the substrate from the PDMS mold with multiple concaved microwells pattern. Then the separated square array was created on quartz plate where the square regions uncontacted with PDMS mold and still remained hydrophilic property, while the regions were enclosed by the hydrophobic PFOTS molecules. Subsequently, enough precursor solution was placed on the substrate to ensure that the whole region was covered and then spinned at 2000 rpm. During a spin coating process, the film became thinner and superfluous solution is removed. As a result, the solution could not be spread on the entire substrate. At the moment, the gravity of the film was unable to balance the intermolecular force, especially on the hydrophobic region [31]. Thus, the film was ruptured at the border where the chemical heterogeneity (hydrophobic against hydrophilic) was the highest, and subsequently dewetting occured. The volume and thickness of the film became larger in the hydrophilic region (relative to the hydrophobic region) under the action of both the chemical potential and the interfacial forces. However, the spread area was constant on this hydrophilic region, which initiated an increase in the contact angle and the curvature [32,33]. Consequently, the surface tension of the film increased with a higher curvature. Simultaneously, the H2O and EtOH were evaporated during the high speed spin-coating process, which triggered an increase in the viscosity and the intermolecular force of the film, and led to a shrinkage of the film in the hydrophilic region. Finally, the solution decayed as a dot in the center of every hydrophilic square region [34]. Fig. 2c and f are fluorescent microscope images for the patterned Gd2(WO4)3:Ln3+ (Ln = Eu, Tb) films measured under the irradiation of 254-nm UV light. A strong red and green luminescence can be seen clearly coming from the dots of the Gd2(WO4)3:Eu3+ and Gd2(WO4)3:Tb3+ pixel arrays, respectively. There was no red or green-light emission from other regions on the substrate. This indicates that there is no superfluous or undesired pattern gel left on the substrate after the pattern-transfer process. This was confirmed by energy dispersive spectroscopy (EDS) which gave the elemental analysis of different areas on the substrates. Fig. 3a shows the EDS results for the selective area within a pixel dot. The presence of gadolinium (Gd), tungsten (W), and oxygen (O) elements was observed, which can be attributed to the formation of Gd2(WO4)3:Eu3+. On the contrary, the black region outside the squares only shows gold (Au), oxygen (O) and silicon (Si) elements, which come from the quartz substrate and no characteristic peaks of gadolinium or europium elements were observed (Fig. 3b). 3.4. Optical properties
3+
3+
Fig. 1. XRD patterns of calcined Gd2(WO4)3:Eu film (a) and Gd2(WO4)3:Tb film (b) arrays fabricated by microcontact printing together with the standard data for Gd2(WO4)3 (JCPDS No. 23-1076).
Fig. 4 shows the UV–vis transmission spectrum of a patterned Gd2(WO4)3:Eu3+ film on a quartz plate. A strong absorption band
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Fig. 2. Optical images (a and d) of the patterned Gd2(WO4)3:Ln3+ (Ln = Eu, Tb) precursor gel film fabricated by lCP, (b and e) after calcination at 800 °C, and (c and f), the corresponding luminescent image under 254-nm UV light excitation. The inset in (b) is SEM image of the patterned Gd2(WO4)3:Eu3+ film, indicative of the morphology of the crystalline Gd2(WO4)3:Eu3+ particles after annealed at 800 °C.
Fig. 4. UV–vis transmission spectrum of the patterned Gd2(WO4)3:Eu3+ film on a quartz plate fabricated by the microcontact printing (lCP) process.
Fig. 3. EDS spectra of different regions from the crystalline patterned Gd2(WO4)3:Eu3+ film fabricated by microcontact printing process. (a) Spectrum obtained from the selected area inside the dots and (b) spectrum obtained from the region outside the dots.
centered at 240 nm is observed. The absorption between 400 and 800 nm (95–97% of transmittance) is very low, indicating the high transparency of the film in the visible region. Under short
wavelength ultraviolet (UV) irradiation, the as-prepared Gd2 (WO4)3:Eu3+ and Gd2(WO4)3:Tb3+ thin films exhibit red and green emission, respectively. Fig. 5 shows the excitation and emission spectra of the as-prepared films annealed at 800 °C in the UV/vis spectral range. The excitation spectrum of Gd2(WO4)3:Eu3+ (Fig. 5a, left) was obtained by monitoring the emission of Eu3+ 5D0–7F2 transition at 617 nm. It can be seen clearly that the excitation spectrum mainly consists of a broad and intense band with a maximum at 240 nm. The intensive band at 240 nm should be assigned to the WO2 4 group plus the Eu–O charge-transfer states [11,13,26]. Upon excitation at 240 nm, we can observe the strong emissions corresponding to 5 DJ–7FJ0 (J = 0, 1, 2; J0 = 0, 1, 2) transitions of Eu3+, but also a weak emission belong to the WO2 4 group from 350 to 550 nm, indicating
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Fig. 6. CIE coordinates of the patterned Gd2(WO4)3:Eu3+ (a), Gd2(WO4)3:Tb3+ (b) films fabricated by microcontact printing.
Fig. 5. PL excitation and emission spectra of the patterned Gd2(WO4)3:Ln3+ (Ln = Eu, and Tb) films.
that an efficient but not complete energy transfer from WO2 to 4 Eu3+ has occurred. The emission spectrum of Eu3+ is dominated by the hypersensitive red emission of 5D0–7F2 transition of Eu3+ at 617 nm (as indicated in Fig. 5a, right). This is because Gd2(WO4)3 belongs to space group C 62h C2=c with scheelite structure, and very similar to that of CaWO4 [13,35]. The Eu3+ ions occupy the Gd3+ without inversion center in Gd2(WO4)3 crystalline, so there is no inversion symmetry at the site of the Eu3+ ion, and then it can be concluded that the 5D0–7F2 transition dominates in the spectrum. The presence of emission lines from higher excited states of Eu3+ (5D1, 5D2) is attributed to the low vibration energy 2 of WO2 4 groups [26]. The multiphonon relaxation by WO4 is not able to bridge the gaps between the higher energy levels (5D1, 5 D2) and 5D0 level of Eu3+ completely, resulting in the weak emissions from these levels [36,37]. Fig. 5b shows the excitation and emission spectra of the as-prepared Gd2(WO4)3:5 mol% Tb3+ film. The excitation spectrum monitored with the 547 nm emission of Tb3+ (5D4–7F5) consists of a broad band with a maximum at 269 nm due to the WO2 group 4 plus the 4f8–4f75d transition of Tb3+. The presence of the excitation 3+ peak of the WO2 indi4 groups in the excitation spectrum of Tb cates that there is an energy transfer from the WO2 groups to 4 Tb3+ ions in the Gd2(WO4)3:Tb3+ luminescent pattern film. Excitation into the WO2 4 group at 269 nm yields the emissions spectrum corresponding to the f–f transitions of Tb3+, which is dominated by the green emission 5D4–7F5 transition at 547 nm (Fig. 5b). The emission spectrum still contains three other peaks centered at 491, 586 and 618 nm, originating from the characteristic 5D4–7FJ (J = 6, 4, 3) emission lines of Tb3+. It is similar with the situation of doping Eu3+, the presence of emission of Tb3+ together with
the weaker intrinsic blue emission from WO2 groups (a weak 4 broad band from 350–550 nm) suggests that an efficient but not 3+ complete energy transfer from WO2 has occurred. 4 groups to Tb Furthermore, the emission from the 5D3 level of Tb3+ is much weaker than that from the 5D4 level due to the cross relaxation effect of Tb3+ [35–39]. The chromaticity coordinates (CIE) of these patterned films are presented in Fig. 6. The coordinates of (a) x = 0.642 and y = 0.329, (b) x = 0.309 and y = 0.522 are corresponding to Gd2(WO4)3:Eu3+ and Gd2(WO4)3:Tb3+ crystal films, indicating the emissions in the red and green region, respectively. Under the excitation of a 254-nm UV light, the PL quantum efficiency for the as-obtained Gd2(WO4)3:Ln3+ (Ln = Eu and Tb) pattern samples are 23% and 6%, respectively. 3.5. Cathodoluminescence properties Under low-voltage electron beam excitation, the as-prepared Gd2(WO4)3:Ln3+ (Ln = Eu, Tb) samples also exhibit the same red and green emissions as the UV excitation, respectively. The representative CL spectra of the Gd2(WO4)3:Ln3+ samples under the excitation of electron beam (accelerating voltage 5 kV; filament current = 90 mA) are shown in Fig. 7, which have identical shapes as the PL emission spectra. However, the relative intensity of peaks in photoluminescence and cathodoluminescence spectrum varies, which may be caused by the different excitation mechanism. The electron penetration depth can be estimated using the empirical formula L [Å] = 250(A/q)(E/Z1/2)n, where n = 1.2/(1–0.29log10Z), A is the atomic or molecular weight of the material, q is the bulk density, Z is the atomic number or the number of electrons per molecule in the case of compounds, and E is the accelerating voltage (kV) [40]. Here, A = 1058.01, q = 7.49 g cm3, Z = 446, and the estimated electron-beam penetration depth in Gd2(WO4)3:Eu3+ is 2.0 nm when the accelerating voltage is 5 kV. With the increase of accelerating voltage, more plasma will be produced by the incident electrons, resulting in more Eu3+, Tb3+ ions being excited, and thus the CL intensity increases. 4. Conclusions In summary, Gd2(WO4)3:Eu3+ and Gd2(WO4)3:Tb3+ thin films have been successfully prepared by means of microcontact
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References
Fig. 7. CL spectra of the patterned Gd2(WO4)3:Ln3+ (Ln = Eu, and Tb) films.
patterning technique in conjunction with sol–gel process using the cheap and nontoxic inorganic compounds as main precursors. The rare earth ions Eu3+ and Tb3+ show their characteristic red (5D0–7F2) and green (5D4–7F5) emissions in crystalline Gd2(WO4)3 phosphor films due to an efficient energy transfer from the WO2 4 to them, respectively. These studies indicate that microcontact patterning technique is a facile and novel route for the development of patterned inorganic phosphor films that have potential in the field of light display, sensor, and optoelectronic devices.
Acknowledgments This project is financially supported by National Basic Research Program of China (Grant Nos. 2007CB935502 and 2010CB327704), National High Technology Program of China (2011AA03A407) and the National Natural Science Foundation of China (Grant Nos. NSFC 60977013, 50872131, and 20921002).
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Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
Patterning of Gd2(WO4)3:Ln3+ (Ln = Eu, Tb) luminescent films by microcontact printing route Dong Wang a,b, Piaoping Yang b, Ziyong Cheng a,⇑, Wenxin Wang a, Zhiyao Hou a,b, Yunlu Dai a, Chunxia Li a, Jun Lin a,⇑ a b
State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, PR China College of Materials Science and Chemical Engineering, Harbin Engineering University, Harbin 150001, PR China
a r t i c l e
i n f o
Article history: Received 4 August 2011 Accepted 7 September 2011 Available online 12 September 2011 Keywords: Sol–gel Microcontact printing Patterning Gd2(WO4)3
a b s t r a c t Gd2(WO4)3 doped with Eu3+ or Tb3+ thin phosphor films with dot patterns have been prepared by a combinational method of sol–gel process and microcontact printing. This process utilizes a PDMS elastomeric mold as the stamp to create heterogeneous pattern on quartz substrates firstly and then combined with a Pechini-type sol–gel process to selectively deposit the luminescent phosphors on hydrophilic regions, in which a Gd2(WO4)3:Ln3+ (Ln = Eu, Tb) precursor solutions were employed as ink. X-ray diffraction (XRD), scanning electron microscopy (SEM), photoluminescence (PL) spectra, as well as low voltage cathodoluminescence (CL) spectra were carried out to characterize the obtained samples. Under ultraviolet excitation and low-voltage electron beams excitation, the Gd2(WO4)3:Eu3+ samples exhibit a strong red emission arising from Eu3+ 5D0,1,2–7F1,2 transitions, while the Gd2(WO4)3:Tb3+ samples show the green emission coming from the characteristic emission of Tb3+ corresponding to 5D4–7F6,5,4,3 transitions. The results show that the patterning of rare earth-doped phosphors through combining microcontact printing with a Pechini-type sol–gel route has potential for field emission displays (FEDs) applications. Ó 2011 Elsevier Inc. All rights reserved.
1. Introduction With the increasing development in miniaturization and microfabrication of devices, the preparation of thin films have been one of the most active research areas in recent years because of the size-/thickness-dependent properties of materials and their potential for a broad range of applications. As an important aspect, thin films that exhibit unique properties compared to bulk materials, are fundamental for electronic, coatings, displays, sensors, optical equipment, and meet the need for scaling devices to small sizes, and are therefore attractive both in theory and practice [1]. Microcontact printing (lCP), one of soft lithography methods, is an effective technique for forming patterned films with micro- and nanoscales [2]. This has attracted much interest in recent years since it provides experimental simplicity and flexibility in forming certain types of patterns. In generation, an elastomeric stamp (usually made from poly(dimethylsiloxane), PDMS) is employed as mold in the lCP process, which is fabricated by casting a prepolymer of PDMS against a original hard master mold. lCP has proven to be an feasible and facile approach that can generate patterned features on flat or nonplanar surfaces [3–6].
⇑ Corresponding authors. E-mail addresses: [email protected] (Z. Cheng), [email protected] (J. Lin). 0021-9797/$ - see front matter Ó 2011 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2011.09.008
So far the patterning of luminescent films has played an important role in high-resolution devices such as cathode-ray tubes (CRTs), electroluminescent devices (ELDs), plasma display panels (PDPs) and field emission displays (FEDs) [7]. Displays with thin film phosphors have higher contrast and resolution, superior thermal conductivity as well as a high degree of uniformity and better adhesion [8]. It is well known that metal tungstates are an important family of inorganic materials that have extensive applications in various fields [9], such as photoluminescence, fluorescent lamps, microwave applications, optical fibers, scintillator materials and X-ray intensified screen. Many research groups have undertaken investigations of the optical and luminescence properties of tungstates [10]. Nassau et al. [11] and Borchardt [12] have prepared this series of compounds and studied their structures, respectively. Until recently, Kodaira et al. [13] reported the preparation of RE2(WO4)3:Eu3+ (RE = La, Gd) powders using the Pechini sol–gel method. Nowadays, various preparation methods have been developed for the fabrication of metal tungstates materials, such as traditional solid state reaction [14], solvothermal method [15,16], low temperature solution method [17], spray pyrolysis route [18], molten salt method [19], the so-called polymeric precursor method [20,21], electrochemical method [22], microwave irradiation [23], pulsed laser deposition [24] and vapor-deposited method [25]. Compared with the above processes, the solutionbased sol–gel is a simple and economical method for making high
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quality luminescent films because it possesses a number of advantages over conventional film formation techniques, such as low temperature processing, easy coating of large surfaces, and possible formation of porous films and homogeneous multicomponent oxide films [26]. So far, some efforts have been devoted to develop various kinds of luminescent films via sol–gel method in the past years. Representative examples are Y3Al5O12:Tb [7] and Y2SiO5:Tb [8] films for cathodoluminescence, Y3Al5O12:Eu [27] films for field emission displays, Y2O3:Eu [28] and Zn2SiO4:Mn [29] films for photoluminescence. However, up to now, to the best of our knowledge, there have been no reports concerning the preparation of the patterned rare-earth ions doped Gd2(WO4)3 films via lCP method combined with the Pechini-type sol–gel method. Accordingly, here we employed these two methods to fabricate ordered Gd2 (WO4)3:Eu3+ and Gd2(WO4)3:Tb3+ dot arrays on the quartz substrates, in which the pattern printing and phosphor synthesis process can be achieved within one step and therefore reduce the cost of the production. The obtained Gd2(WO4)3:Eu3+ and Gd2(WO4)3:Tb3+ samples demonstrate the strong red and green emission under UV light or electron beam irradiation, respectively, indicating that the lCP method can be used for patterning inorganic phosphors and has potential for the applications of display devices, such as the next generation FED devices. 2. Materials and methods 2.1. Chemicals and materials Polyethylene glycol (PEG, Mw = 10,000), ethanol, n-octane, as well as nitric acid were purchased from the Beijing Chemical Company, and ammonium (meta) tungstate hydrate (H26N6O41W1218H2O) was purchased from Fluka. Citric acid monohydrate C6H8O7 H2O (P99.5%, A.R.), hydrogen peroxide (P30%, A.R.) and ethanol were all purchased from Beijing Fine Chemical Company. Gd2O3, Eu2O3 and Tb4O7 (99.99%) were purchased from Science and Technology Parent Company of Changchun Institute of Applied Chemistry. Gd(NO3)3, Eu(NO3)3 and Tb(NO3)3 were prepared by dissolving Gd2O3, Eu2O3 and Tb4O7 in dilute nitric acid HNO3, and evaporating the water in the solutions by heating. The prepolymer (Sylgard 184) and its curing agent for preparing poly(dimethylsiloxane) (PDMS) were purchased from the Dow Corning Cooperation. All the initial chemicals in this work were used without further purification.
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the master. In this way, PDMS stamps with microwells array were obtained. The quartz plates were cleaned by immersing them in piranha solution (mixture of 98% H2SO4 and 30% H2O2 with v/v = 7:3) at 90 °C for 30 min (Caution! Piranha solutions are highly corrosive and potentially explosive and should be handled with great care.). Then they were rinsed with deionized water, and dried with nitrogen. The as-prepared quartz substrates exhibited hydrophilic properties because of the hydroxyl group on the substrate surface. 2.4. Fabrication of patterned Gd2(WO4)3:Ln3+ (Ln = Eu, Tb) films by microcontact printing The process of fabricating Gd2(WO4)3:Ln3+ (Ln = Eu, Tb) arrays of dots via lCP was shown in Scheme 1. This technique is based on the modification of the substrate surface and the selective film deposition on desired regions. A solution of PFOTS (1H,1H,2H,2H perfluorooctyltrichlorosilane, CF3(CF2)5CH2CH2SiCl3) in n-octane was used to ink the patterned PDMS stamp and dried under N2 stream for several minutes before transferring the SAMs to the substrate. Then, the PDMS stamp was brought into contact with the quartz substrate for 5–10 s, leading to the transferring of PFOTS molecules from the stamp to the substrate in the regions of contact. Next, the patterned substrate was used as a template for the selective deposition of the metal salts solution by spin-coating. During this spin-coating at 2000 rpm process, the precursor solution was selectively deposited on the hydrophilic regions because of the poor adhesion between the solution and the SAM. Finally, the substrate with the patterned gel was annealed at desired temperature (800 °C) with the heating rate of 2 °C min1 and held there for 3 h in air to obtain the crystalline Gd2(WO4)3:Ln3+ (Ln = Eu, Tb) films. 2.5. Characterization The X-ray diffraction (XRD) patterns of the samples were carried out on a D8 Focus diffractometer (Bruker) with use of Cu Ka radiation (k = 0.15405 nm). The patterned images were taken on a Nikon microscope (ECLIPSE Ti) with UV irradiation at 254 nm. The morphology of the crystalline samples was inspected using a field emission scanning electron microscope (SEM; S-4800, Hitachi). The photoluminescence (PL) measurements were recorded with a Hitachi F-4500 spectrophotometer equipped with a 150 W
2.2. Fabrication of Gd2(WO4)3:Ln3+ (Ln = Eu, Tb) precursor solution The doping concetrations of Eu3+ and Tb3+ were selected as 30 and 5 mol% of Gd3+ in Gd2(WO4)3 according to the previous study, respectively [26]. The stoichiometric amounts of Gd(NO3)3, Eu (NO3)3 and Tb(NO3)3 as well as H26N6O41W1218H2O were dissolved in deionized water, then the solution was mixed with citric acid as a chelating agent for the metal ions. The molar ratio of metal ions to citric acid was 1:2. A certain amount of polyethylene glycol (PEG, Mw = 10,000, 0.15 g/mL) was added to adjust the viscoelastic behavior of the solution. The solution was stirred for 2 h to obtain a homogeneous hybrid sol for further microcontact printing. 2.3. Preparation of PDMS stamps and cleaning of substrate PDMS stamp modes were fabricated by pouring the PDMS prepolymer, a 10:1 (v/v) mixture of Sylgard silicone elastomer 184, and its curing agent over a hard master prepared by photolithography. The elastomer was degassed for about 30 min at room temperature and cured at 65 °C for 4 h, then peeled gently from
Scheme 1. Schematic diagram of the experimental microcontact printing process for patterning of Gd2(WO4)3:Eu3+.
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xenon lamp as the excitation source. The cathodoluminescence (CL) measurements were conducted in an ultrahigh-vacuum chamber (<108 Torr), where the phosphors were excited by an electron beam at a voltage range of 5 kV with a filament current 90 mA, and the spectra were recorded using an F-4500 spectrophotometer. The quantum efficiency and chromaticity coordinates of the patterned phosphor film were performed with a quantum-yield measurement system (C9920-02, Hamamatsu Photonics K.K., Japan). All the measurements were performed at room temperature. 3. Results and discussion
the square gels was observed after firing because of pyrolysis and evaporation of the organic compounds in the gel. The estimated diameter of the Gd2(WO4)3:Eu3+ dot was 6.28 ± 1.56 lm. The morphology of the dot patterns created via the lCP process remains unchanged before and after calcinations, but the size of the dots has slightly decreased with heat treatment. Meanwhile, the inset in Fig. 2b is a scanning electron microscopy (SEM) image taken in the region of the Gd2(WO4)3:Eu3+ phosphor dot, which indicates that the crystalline Gd2(WO4)3:Eu3+ patterns are composed of nanoparticles with a size between 300 and 460 nm. 3.3. Formation mechanism
3.1. Structure analysis The composition, phase purity and morphology of the asprepared samples were investigated by XRD and FE-SEM for Gd2(WO4)3:Eu3+. Fig. 1 shows the XRD patterns of the Gd2(WO4)3: Eu3+ and Gd2(WO4)3:Tb3+ patterned film annealed at 800 °C, as well as the JCPDS card (No. 23–1076) for Gd2(WO4)3 as comparison, respectively. It can be seen that there is a broad peak centered at 2h = 22° which is ascribed to amorphous SiO2 of the quartz plate. Apart from the broad peak, several weak diffraction peaks at 2h 19.0°, 28.3°, 29.7° and 47.5° are present in the XRD pattern, which are assigned to (1 1 1), (2 2 1), (0 2 3) and (2 4 2) reflections of Gd2(WO4)3 according to standard data (JCPDS Card 23–1076). The results of XRD suggest that the precursor gel on SiO2 glass has been transformed into the desired crystalline structure of Gd2(WO4)3 at 800 °C. However, besides two weak diffraction peaks, no more reflections due to Gd2(WO4)3 are present on the dot-patterned film sample. This can be assigned to the fact that the dot-patterned Gd2(WO4)3:Eu3+ thin film is discontinuous and there exists blank regions under the light spot of X-ray, which greatly decreases the diffraction intensity [30]. 3.2. Optical images Fig. 2a and d are the optical images for the patterned Gd2 (WO4)3:Eu3+ and Gd2(WO4)3:Tb3+ precursor gel films (just spincoating dried, without annealing) fabricated by the lCP technique, respectively. The average diameter of the Gd2(WO4)3:Eu3+ dots was determined to be 8.00 ± 1.28 lm. After calcination at 800 °C in air, the patterned Gd2(WO4)3:Ln3+ crystalline films were formed on the quartz plate and the corresponding optical microscope images are shown in Fig. 2b and e, respectively. A size shrinkage of
As was shown in Scheme 1, the patterning process can be described as follows: first of all, we used hydrophobic perfluorooctyltrichlorosilane (PFOTS) to rinse the patterned PDMS mold, followed by the mold contacting with the substrate. Thus, the hydrophobic PFOTS molecules were transferred to the substrate from the PDMS mold with multiple concaved microwells pattern. Then the separated square array was created on quartz plate where the square regions uncontacted with PDMS mold and still remained hydrophilic property, while the regions were enclosed by the hydrophobic PFOTS molecules. Subsequently, enough precursor solution was placed on the substrate to ensure that the whole region was covered and then spinned at 2000 rpm. During a spin coating process, the film became thinner and superfluous solution is removed. As a result, the solution could not be spread on the entire substrate. At the moment, the gravity of the film was unable to balance the intermolecular force, especially on the hydrophobic region [31]. Thus, the film was ruptured at the border where the chemical heterogeneity (hydrophobic against hydrophilic) was the highest, and subsequently dewetting occured. The volume and thickness of the film became larger in the hydrophilic region (relative to the hydrophobic region) under the action of both the chemical potential and the interfacial forces. However, the spread area was constant on this hydrophilic region, which initiated an increase in the contact angle and the curvature [32,33]. Consequently, the surface tension of the film increased with a higher curvature. Simultaneously, the H2O and EtOH were evaporated during the high speed spin-coating process, which triggered an increase in the viscosity and the intermolecular force of the film, and led to a shrinkage of the film in the hydrophilic region. Finally, the solution decayed as a dot in the center of every hydrophilic square region [34]. Fig. 2c and f are fluorescent microscope images for the patterned Gd2(WO4)3:Ln3+ (Ln = Eu, Tb) films measured under the irradiation of 254-nm UV light. A strong red and green luminescence can be seen clearly coming from the dots of the Gd2(WO4)3:Eu3+ and Gd2(WO4)3:Tb3+ pixel arrays, respectively. There was no red or green-light emission from other regions on the substrate. This indicates that there is no superfluous or undesired pattern gel left on the substrate after the pattern-transfer process. This was confirmed by energy dispersive spectroscopy (EDS) which gave the elemental analysis of different areas on the substrates. Fig. 3a shows the EDS results for the selective area within a pixel dot. The presence of gadolinium (Gd), tungsten (W), and oxygen (O) elements was observed, which can be attributed to the formation of Gd2(WO4)3:Eu3+. On the contrary, the black region outside the squares only shows gold (Au), oxygen (O) and silicon (Si) elements, which come from the quartz substrate and no characteristic peaks of gadolinium or europium elements were observed (Fig. 3b). 3.4. Optical properties
3+
3+
Fig. 1. XRD patterns of calcined Gd2(WO4)3:Eu film (a) and Gd2(WO4)3:Tb film (b) arrays fabricated by microcontact printing together with the standard data for Gd2(WO4)3 (JCPDS No. 23-1076).
Fig. 4 shows the UV–vis transmission spectrum of a patterned Gd2(WO4)3:Eu3+ film on a quartz plate. A strong absorption band
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Fig. 2. Optical images (a and d) of the patterned Gd2(WO4)3:Ln3+ (Ln = Eu, Tb) precursor gel film fabricated by lCP, (b and e) after calcination at 800 °C, and (c and f), the corresponding luminescent image under 254-nm UV light excitation. The inset in (b) is SEM image of the patterned Gd2(WO4)3:Eu3+ film, indicative of the morphology of the crystalline Gd2(WO4)3:Eu3+ particles after annealed at 800 °C.
Fig. 4. UV–vis transmission spectrum of the patterned Gd2(WO4)3:Eu3+ film on a quartz plate fabricated by the microcontact printing (lCP) process.
Fig. 3. EDS spectra of different regions from the crystalline patterned Gd2(WO4)3:Eu3+ film fabricated by microcontact printing process. (a) Spectrum obtained from the selected area inside the dots and (b) spectrum obtained from the region outside the dots.
centered at 240 nm is observed. The absorption between 400 and 800 nm (95–97% of transmittance) is very low, indicating the high transparency of the film in the visible region. Under short
wavelength ultraviolet (UV) irradiation, the as-prepared Gd2 (WO4)3:Eu3+ and Gd2(WO4)3:Tb3+ thin films exhibit red and green emission, respectively. Fig. 5 shows the excitation and emission spectra of the as-prepared films annealed at 800 °C in the UV/vis spectral range. The excitation spectrum of Gd2(WO4)3:Eu3+ (Fig. 5a, left) was obtained by monitoring the emission of Eu3+ 5D0–7F2 transition at 617 nm. It can be seen clearly that the excitation spectrum mainly consists of a broad and intense band with a maximum at 240 nm. The intensive band at 240 nm should be assigned to the WO2 4 group plus the Eu–O charge-transfer states [11,13,26]. Upon excitation at 240 nm, we can observe the strong emissions corresponding to 5 DJ–7FJ0 (J = 0, 1, 2; J0 = 0, 1, 2) transitions of Eu3+, but also a weak emission belong to the WO2 4 group from 350 to 550 nm, indicating
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Fig. 6. CIE coordinates of the patterned Gd2(WO4)3:Eu3+ (a), Gd2(WO4)3:Tb3+ (b) films fabricated by microcontact printing.
Fig. 5. PL excitation and emission spectra of the patterned Gd2(WO4)3:Ln3+ (Ln = Eu, and Tb) films.
that an efficient but not complete energy transfer from WO2 to 4 Eu3+ has occurred. The emission spectrum of Eu3+ is dominated by the hypersensitive red emission of 5D0–7F2 transition of Eu3+ at 617 nm (as indicated in Fig. 5a, right). This is because Gd2(WO4)3 belongs to space group C 62h C2=c with scheelite structure, and very similar to that of CaWO4 [13,35]. The Eu3+ ions occupy the Gd3+ without inversion center in Gd2(WO4)3 crystalline, so there is no inversion symmetry at the site of the Eu3+ ion, and then it can be concluded that the 5D0–7F2 transition dominates in the spectrum. The presence of emission lines from higher excited states of Eu3+ (5D1, 5D2) is attributed to the low vibration energy 2 of WO2 4 groups [26]. The multiphonon relaxation by WO4 is not able to bridge the gaps between the higher energy levels (5D1, 5 D2) and 5D0 level of Eu3+ completely, resulting in the weak emissions from these levels [36,37]. Fig. 5b shows the excitation and emission spectra of the as-prepared Gd2(WO4)3:5 mol% Tb3+ film. The excitation spectrum monitored with the 547 nm emission of Tb3+ (5D4–7F5) consists of a broad band with a maximum at 269 nm due to the WO2 group 4 plus the 4f8–4f75d transition of Tb3+. The presence of the excitation 3+ peak of the WO2 indi4 groups in the excitation spectrum of Tb cates that there is an energy transfer from the WO2 groups to 4 Tb3+ ions in the Gd2(WO4)3:Tb3+ luminescent pattern film. Excitation into the WO2 4 group at 269 nm yields the emissions spectrum corresponding to the f–f transitions of Tb3+, which is dominated by the green emission 5D4–7F5 transition at 547 nm (Fig. 5b). The emission spectrum still contains three other peaks centered at 491, 586 and 618 nm, originating from the characteristic 5D4–7FJ (J = 6, 4, 3) emission lines of Tb3+. It is similar with the situation of doping Eu3+, the presence of emission of Tb3+ together with
the weaker intrinsic blue emission from WO2 groups (a weak 4 broad band from 350–550 nm) suggests that an efficient but not 3+ complete energy transfer from WO2 has occurred. 4 groups to Tb Furthermore, the emission from the 5D3 level of Tb3+ is much weaker than that from the 5D4 level due to the cross relaxation effect of Tb3+ [35–39]. The chromaticity coordinates (CIE) of these patterned films are presented in Fig. 6. The coordinates of (a) x = 0.642 and y = 0.329, (b) x = 0.309 and y = 0.522 are corresponding to Gd2(WO4)3:Eu3+ and Gd2(WO4)3:Tb3+ crystal films, indicating the emissions in the red and green region, respectively. Under the excitation of a 254-nm UV light, the PL quantum efficiency for the as-obtained Gd2(WO4)3:Ln3+ (Ln = Eu and Tb) pattern samples are 23% and 6%, respectively. 3.5. Cathodoluminescence properties Under low-voltage electron beam excitation, the as-prepared Gd2(WO4)3:Ln3+ (Ln = Eu, Tb) samples also exhibit the same red and green emissions as the UV excitation, respectively. The representative CL spectra of the Gd2(WO4)3:Ln3+ samples under the excitation of electron beam (accelerating voltage 5 kV; filament current = 90 mA) are shown in Fig. 7, which have identical shapes as the PL emission spectra. However, the relative intensity of peaks in photoluminescence and cathodoluminescence spectrum varies, which may be caused by the different excitation mechanism. The electron penetration depth can be estimated using the empirical formula L [Å] = 250(A/q)(E/Z1/2)n, where n = 1.2/(1–0.29log10Z), A is the atomic or molecular weight of the material, q is the bulk density, Z is the atomic number or the number of electrons per molecule in the case of compounds, and E is the accelerating voltage (kV) [40]. Here, A = 1058.01, q = 7.49 g cm3, Z = 446, and the estimated electron-beam penetration depth in Gd2(WO4)3:Eu3+ is 2.0 nm when the accelerating voltage is 5 kV. With the increase of accelerating voltage, more plasma will be produced by the incident electrons, resulting in more Eu3+, Tb3+ ions being excited, and thus the CL intensity increases. 4. Conclusions In summary, Gd2(WO4)3:Eu3+ and Gd2(WO4)3:Tb3+ thin films have been successfully prepared by means of microcontact
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References
Fig. 7. CL spectra of the patterned Gd2(WO4)3:Ln3+ (Ln = Eu, and Tb) films.
patterning technique in conjunction with sol–gel process using the cheap and nontoxic inorganic compounds as main precursors. The rare earth ions Eu3+ and Tb3+ show their characteristic red (5D0–7F2) and green (5D4–7F5) emissions in crystalline Gd2(WO4)3 phosphor films due to an efficient energy transfer from the WO2 4 to them, respectively. These studies indicate that microcontact patterning technique is a facile and novel route for the development of patterned inorganic phosphor films that have potential in the field of light display, sensor, and optoelectronic devices.
Acknowledgments This project is financially supported by National Basic Research Program of China (Grant Nos. 2007CB935502 and 2010CB327704), National High Technology Program of China (2011AA03A407) and the National Natural Science Foundation of China (Grant Nos. NSFC 60977013, 50872131, and 20921002).
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