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Ordered luminescent nanohybrid thin films of Eu(BA)3Phen nanoparticle in polystyrene matrix from diblock copolymer self-assembly
Applied Surface Science 256 (2010) 2818–2825
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Ordered luminescent nanohybrid thin films of Eu(BA)3Phen nanoparticle in polystyrene matrix from diblock copolymer self-assembly Chao Wang a,b, Yaoming Zhang a,b, Xianqiang Pei a, Tingmei Wang a,*, Qihua Wang a,* a b
State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, PR China Graduate School, Chinese Academy of Sciences, Beijing 100039, PR China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 18 September 2009 Received in revised form 10 November 2009 Accepted 11 November 2009 Available online 18 November 2009
A simple route for fabricating highly ordered luminescent thin films based on hybrid material of diblock copolymer and europium complex, assisted with self-organization of polystyrene-block-poly(ethylene oxide) (PS-b-PEO) diblock copolymer upon solvent annealing, is presented. PS-b-PEO self-organized into hexagonal patterns and europium complex of Eu(BA)3Phen was selectively embedded in PS blocks after solvent annealing in benzene or benzene/water vapor. During benzene annealing, the orientation of the PEO cylindrical domains strongly depended on the Eu(BA)3Phen concentration. In contrast, when the hybrid thin films were annealed in mixture of benzene and water vapor, high degree of orientation of the PEO cylindrical domains is more easily obtained, which is independent of Eu(BA)3Phen concentration. Furthermore, preferential interaction of PEO domains with water induces a generation of nanopores in the hybrid thin film. Atomic force microscopy (AFM), scanning electron microscopy (SEM), and transmission electron microscopy (TEM) were used to characterize the long-range lateral order and phase composition of the hybrid thin films. The ordered nanohybrid thin films kept the fluorescence property of Eu(BA)3Phen and showed a strong red emission under the 254 nm light’s irradiation. The fluorescence property was confirmed by photoluminescence (PL) spectra. ß 2009 Elsevier B.V. All rights reserved.
Keywords: Block copolymer Europium complex Luminescence Nanohybrid thin film Self-assembly
1. Introduction Highly ordered nanohybrid materials in thin films have attracted much attention for their fascinating self-assembly nanostructure, as well as the potential applications in optical, optoelectronic, magnetic, and micromechanical devices. Block copolymer, one class of self-assembling materials, offer an attractive route to fabricate nanometer-scale structures, since they spontaneously form a range of periodic structures for proper compositions and under adequate conditions, owing to the microphase separation between dissimilar blocks [1–4]. Hence, much effort has been made to use self-assembled block copolymers as a tool for fabricating functional nanomaterials, such as noble metal [5–8], semiconductor [9–15], and inorganic oxide nanoparticles [16–20]. Very recent examples include the fabrication of well-defined surface patterns of functional hybrid materials in thin films based on PS-b-P4VP diblock copolymer and Au nanoparticles through metallization of the pyridine units’ in situ using a wet chemical process [21], the use of cooperative self-
* Corresponding authors. Tel.: +86 931 4968252/4968180; fax: +86 931 4968252/4968180. E-mail addresses: [email protected] (T. Wang), [email protected] (Q. Wang). 0169-4332/$ – see front matter ß 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2009.11.033
assembly of polystyrene-block-poly(2-vinylpyridine) (PS-b-P2VP) copolymer and TTIP-based sol–gel precursors to prepare 2D arrays of strings of TiO2 nanoparticles [22], and the preparation of ordered silicon nanodot arrays using linear-brush-type polystyrene-blockpolycarbosilane (PS-b-PCS) diblock copolymer thin films combined with acetone vapor annealing [23]. Since a rapid route for orienting the cylindrical microdomains of asymmetric polystyrene-blockpoly(ethylene oxide) diblock copolymer (PS-b-PEO) normal to the surface of a substrate by utilization of solvent was addressed by Russell and co-workers [24,25], examples have revealed that thin films of asymmetric (PS-b-PEO) diblock copolymer were successfully used as scaffolds for various nanostructures. One example is the fabrication of ordered inorganic oxide nanoparticles arrays which could be grown selectively on top of the PEO domains on the surface of ordered PS-b-PEO thin films by chemical vapor deposition [26,27]. Another and more extensive example involves the use of sol–gel chemistry to prepare hexagonally packed arrays of titania nanodomains with PS-b-PEO block copolymer as template [28,14,29,30]. Phosphor thin films have attracted considerable attention because of their potential application in high-resolution devices such as electroluminescent (EL) display, field emission displays (FEDs), and cathode ray tubes (CRTs) [31]. Compared with conventional powder phosphors, thin film luminescent structure used as display have higher contrast and resolution, superior
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thermal conductivity, better adhesion as well as a high degree of uniformity [31,32]. Hence, the fabrication of high-quality phosphor thin films has always been hot spot of study over the past few years [33–35]. On the other hand, the patterning technologies of phosphor screens have a great effect on the resolution of flat panel display devices [36]. Yu et al. have prepared pattered nanocrystalline phosphor films using sol–gel process combined with soft lithography [37], overcoming the high expenditure and complexity of traditional fabrication process, but the obtained patterns are in micrometer scales. Cong et al. have used micelles of PS-b-P4VP as nanoreactors to synthesize Eu(III)-block copolymer complex with the presence of 1,10-phenanthroline as cooperative ligand, and have successfully prepared hexagonally ordered luminescent thin
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films in nanometer scale [38]. However, the degree of lateral order of the thin films is low and the method is limited to the complexation between rare earth and polymer. In this work, we report the use of solvent-induced ordering to fabricate highly ordered luminescent nanohybrid thin films via spin-coating from solutions containing europium complexes of Eu(BA)3Phen and PS-b-PEO, combined with solvent annealing; the europium complex was selectively incorporated into PS domain due to its hydrophobic. Annealed in pure benzene or benzene/ water mixture vapor, the hybrid film showed highly ordered hexagonally packed PEO cylindrical microdomains embedded in PS/Eu(BA)3Phen matrix. The ordered nanohybrid thin film exhibits highly efficient red luminescence of Eu(BA)3Phen. Atomic force
Fig. 1. Height AFM images of nanohybrid thin films (a) as spun and (b) after benzene annealing and pure block copolymer (c) as spun and (b) after benzene annealing. Dark parts correspond to low height values. (e) Corresponding TEM image of (b). (f) The molecular structure of europium complexes of Eu(BA)3Phen.
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microscopy (AFM), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and photoluminescence (PL) spectra were used to characterize the long-range lateral order, phase composition, and fluorescence property of the hybrid thin films.
respectively, with a volume ratio of water to benzene of 0.1/0.9. The environment outside the glass vessel was maintained at 25 8C and 30% humidity. After solvent annealing for different times, the films were removed from the vessel quickly and air-dried at room temperature.
2. Experimental
2.2. Characterizations
2.1. Preparation of nanohybrid thin films
AFM studies were operated at a Nanoscope IIIa Multimode atomic force microscope (AFM, Digital Instruments) at the tapping mode. For TEM studies, mica substrates were used. These films were floated off onto a pool of deionized water and picked up with copper mesh TEM grids. TEM experiments were conducted in a JEM 2010 TEM equipped with energy dispersive spectrometer (EDS) at an accelerating voltage of 200 kV in the bright-field mode. Field emission scanning electron microscopy (FESEM) imaging was performed on a JSM-6701F operated at 5 kV. Photoluminescence (PL) measurement was performed by a Hitachi 4500 fluorescence spectrophotometer.
PS-b-PEO diblock copolymer (0.1 g) (polymer sources Inc.; Mn PEO:PS = 6400:19,000 g/mol and polydispersity of 1.04) was first dissolved in 5 mL of benzene (99.5%, Sinopharm Chemical Reagent Co. Ltd., Shanghai) and stirred for 1 h. Europium complexes of Eu(BA)3Phen was prepared according to the previous report [39]. The molecular structure of Eu(BA)3Phen is shown in Fig. 1. Eu(BA)3Phen (5 mg) was dissolved in 1 mL of benzene at 70 8C and stirred for 20 min to get clear solution. The desired amount of Eu(BA)3Phen/benzene solution was added into a PS-b-PEO solution and stirred for 12 h. The hybrid thin films were prepared on a silicon wafer by spin-coating from solutions containing Eu(BA)3Phen and PS-b-PEO at 2000 rpm for 60 s, followed by solvent annealing. The films thickness, determined by a L116E ellipsometer, was about 80.0 5 nm. Solvent annealing of the films were performed in two ways: (i) the thin films were placed in an airtight glass vessel with 70 cm3 volume along with a small vial containing pure benzene; (ii) the thin films were placed in the airtight glass vessel along with two small vials which contained benzene and water
3. Results and discussion 3.1. Solvent-annealed in pure benzene vapor Part a and b of Fig. 1 shows the AFM images of nanohybrid thin films of PS-b-PEO/Eu(BA)3Phen with a [O]/[Eu] molar ratio of 60 (oxygen in PEO to Eu of the complex) before and after benzene annealing, respectively. The results for films of the block
Fig. 2. Height AFM images of nanohybrid thin films containing different amount of Eu(BA)3Phen with changing molar ratio of oxygen in the PEO to Eu of the complex after benzene annealing for 24 h. Dark parts correspond to low height values. In the inset a Fourier transform of the AFM image shows a pattern with multiple reflections characteristic of a highly ordered hexagonal array.
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copolymer without Eu(BA)3Phen are shown in parts c and d of Fig. 1, respectively. Comparing parts a and c, it can be concluded that larger domains in part a result from the macrophase separation of Eu(BA)3Phen from PS-b-PEO. However, this morphology corresponds to a kinetically trapped structure formed during spin-coating and is far from equilibrium. After benzene annealing, the film exhibits a well-organized structure with a high degree of long-range lateral order of cylindrical microdomains (Fig. 1b). Since Eu(BA)3Phen is hydrophobic (Fig. 1f), it is expected that the macrophase-separated domains of Eu(BA)3Phen are selectively and uniformly distributed within the PS matrix during solvent annealing. From parts b and d, it is clear that the orientation of PEO cylinders transforms from parallel to perpendicular with the addition of Eu(BA)3Phen in the benzene annealed films. Similar results were observed in previous reports [40], where the addition of salt of KI gives rise to a change in the orientation of PEO cylinders from parallel to perpendicular in the benzene annealed films, by overcoming preferential interactions between PEO and the substrate because of complexation with the PEO or interaction with the substrate. However, in our study, Eu(BA)3Phen as additive is distributed in PS matrix, not PEO cylindrical microdomains. It can be concluded that the addition of Eu(BA)3Phen which is uniformly distributed in PS domains, changes the migration of PS blocks and further mitigates the PS-selectivity of the benzene vapor during solvent annealing, enabling the propagation of PS domains and PEO cylinders through the film oriented normal to film surface. The diameter of the PEO cylindrical microdomains and the repeat period of the lattice is 34.3 nm and
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20.5 nm, which is obvious different from self-assembled thin films of pure PS-b-PEO observed in Russells’ studies (43.8 nm and 23.5 nm, respectively) [25]. It could be attributed to the addition of Eu(BA)3Phen which is selectively distributed within PS matrix. In the past few years, solvent annealing, as a simple route for the control of lateral ordering and orientation of microstructures in block copolymer thin films, has been studied experimentally and theoretically [25,41–43]. By exposing the PS-b-PEO/Eu(BA)3Phen hybrid thin film to benzene vapor, the thin film is rapidly swollen. Benzene vapor, a selective solvent for PS-b-PEO, imparts mobility to the copolymer, enabling a rapid removal of defects and homogenous distribution of Eu(BA)3Phen in PS blocks. As the benzene evaporates, the concentration of benzene at the surface is lowest and a gradient in solvent concentration develops normal to the surface. Microphase separation of the PS-b-PEO/Eu(BA)3Phen occurs at the surface, the structure rapidly coarsens, and ordering propagates through the film as benzene further evaporates. Consequently, long-range lateral ordering and high degree of orientation of the hybrid thin film are achieved. The AFM images shown in Fig. 2 exhibit the dependence of orientation and lateral order of the nanohybrid thin films on Eu(BA)3Phen concentration. Prior to discussion, it should be noted that the concentration of Eu(BA)3Phen in the experiments cannot be much high, since the mixture solution of PS-b-PEO/Eu(BA)3Phen will turn turbidity when stored for a long time at a [O]/[Eu] ratio lower than 30. When the [O]/[Eu] ratio is 120, the amount of added Eu(BA)3Phen is insufficient to influence the migration and vaporselectivity of PS blocks during solvent annealing, and cylindrical
Fig. 3. Height AFM images of the nanohybrid thin films with [O]/[Eu] of 60: (a) directly after spin-coating and benzene annealing for (b) 12 h, (c) 24 h, and (d) 36 h. Dark parts correspond to low height values.
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microdomains both parallel and normal to the surface coexist. With increasing Eu(BA)3Phen concentration ([O]/[Eu] = 60 and 90), highly ordered arrays of cylindrical microdomains orient normal to the surface are observed. A Fourier transform of the AFM image is shown in the inset, where a six-point pattern, with multiple higher-order reflections, is obtained, indicating the characteristic of the long-range order. Increasing the Eu(BA)3Phen concentration further ([O]/[Eu] = 30) maintains the orientation of microdomains normal to the surface; however, the lateral ordering of the cylindrical microdomains is reduced and slight macrophase separation is also observed. The annealing time dependence of the structure evolution in the nanohybrid thin films has been explored by annealing the film with [O]/[Eu] of 60 for different period of time. For the directly spincoated thin film, as Fig. 4a shows, obvious macrophase separation of Eu(BA)3Phen from PS-b-PEO is observed. After 12 h of benzene annealing, although the film shows poorly ordered morphology, macrophase-separated domains of the Eu(BA)3Phen in the film disappear (Fig. 3b), indicating that all the Eu(BA)3Phen are sequestered within the PS matrix. After annealing for 24 h, the film exhibits a well-organized structure with a high degree of long-range lateral order of cylindrical microdomains. With the annealing time increased to 36 h, no clear change of the morphology is observed. To explore the distribution of Eu(BA)3Phen in the nanohybrid films, TEM was employed to investigate the film given the good contrast that Eu(BA)3Phen can provide. Shown in Fig. 1e is the TEM image of the nanohybrid thin films removed from the substrate after benzene annealing. From the result, it can be seen that Eu(BA)3Phen nanoparticles are very fine in size, estimated to be <2 nm, and homogeneously assemble in the hexagonal-like structure of PS-b-PEO matrix. The composition of the pattered
film was checked by EDS analysis, with an detection diameter of 5 nm. In the PEO cylindrical domains, only carbon and oxygen were found, however, besides the C and O peak, europium was also observed in the PS matrix. The TEM image and EDS results confirm the selective distribution of Eu(BA)3Phen within PS matrix in the nanohybrid thin film. 3.2. Solvent-annealed in benzene/water vapor Fig. 4a shows the AFM image of the nanohybrid thin film of PS-bPEO/Eu(BA)3Phen ([O]/[Eu] = 30) after solvent annealing in benzene/water, with the volume ratio of water to benzene of 0.1/0.9. Similarly, the thin film is self-organized into hexagonally packed PEO cylindrical microdomains embedded in PS/Eu(BA)3Phen matrix, although the diameter of the nanoscopic cylindrical domains and the repeat period of the lattice have increased to 30.5 nm and 47.6 nm, respectively, due to the preferable interaction of water to PEO domains. TEM was employed to further study the nanohybrid thin film. As shown in Fig. 4b, Eu(BA)3Phen nanoparticles with diameter of 2 nm, are uniformly distributed in PS matrix, which is similar to that of the thin film after solvent annealing in pure benzene. Interestingly, some hexagonally packed nanoscopic pores are also observed in the film. The FESEM images of the film top view and cross-section show similar results to TEM, where the cylindrical nanopores, with diameter of 30 nm, span across the entire film thickness. According to previous studies [44,45], simply exposure of self-assembled BCP film to a solvent that is a good solvent for the minor component block and a nonsolvent for the major component block, the minor component is drawn to the surface of the film and surface reconstructure occurs. Consequently, a solvent-induced reconstruction of the film involving two steps is shown below to be
Fig. 4. (a) AFM images of the nanohybrid thin film ([O]/[Eu] = 30) after solvent annealing in benzene/water (0.9/0.1) for 48 h. A Fourier transform of the AFM image is shown in the inset. Dark parts correspond to low height values. (b) TEM image of the same nanohybrid thin film. (c) FESEM image of the nanohybrid thin film, and (d) corresponding cross-section view.
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responsible for the formation of nanopores. Firstly, the hybrid thin film is self-organized into hexagonally packed PEO cylindrical microdomains embedded in PS/Eu(BA)3Phen matrix in the presence of benzene vapor based on the discussion above. Secondly, due to the presence of water, a good solvent for the PEO block and a nonsolvent for the PS block, the cylindrical PEO microdomains in the ordered nanohybrid thin film is drawn to the surface of the film, leading to the generation of nanopores. The influence of Eu(BA)3Phen concentration in nanohybrid thin films on the ordering of the film has been studied. As shown in Fig. 5, for the nanohybrid thin films solvent-annealed in benzene/ water vapor mixture (water:benzene = 0.1:0.9) for 48 h, the change of Eu(BA)3Phen concentration in the nanohybrid thin films exerts little influence on the ordering, and all the films show ordered nanostructure. It appears that the presence of water is more favorable for the formation of arrays of hexagonally packed microdomains than pure benzene, without considering the influence of Eu(BA)3Phen concentration in the nanohybrid thin films. The presence of water, probably either by mitigating the PSselectivity of the vapor mixtures or by adjusting the preferential interactions between PEO domains and substrate, are more prone to induce the formation of cylindrical microdomains normal to the substrate than pure benzene vapor.
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On the basis of the results discussed above, simply by exposing the PS-b-PEO/Eu(BA)3Phen hybrid thin film to benzene or benzene/ water mixture vapor, highly ordered hexagonal-like structure was obtained. In a previous work of Kim et al., PS-b-PEO block copolymer has been used as a template to fabricate ordered organic–inorganic hybrid nanostructures by one-step spin-coating from solutions containing a titania precursor and the copolymer [28]. However, the obtained hybrid thin film did not show high degree of lateral ordering. In our study, solvent annealing, commonly utilized as one important approach to produce a long-range ordered thin film of a pure block copolymer, is proved to be an effective way to achieve high ordering for nanohybrid thin films. 3.3. Photoluminescence spectrum Fig. 6 shows the excitation (a) and emission (b) spectrums of the nanohybrid thin films before and after solvent annealing. As can be seen from the emission spectrum, all the samples exhibit characteristic emission peak at 592 nm and 614 nm, corresponding to (5D0–7F1, 7F2) of Eu3+, respectively. It is notable that thin films from direct spin-coating show similar emission spectrum with that of pure Eu(BA)3Phen (see Supporting Information), indicating that
Fig. 5. Height AFM images of nanohybrid thin films containing different amount of Eu(BA)3Phen with changing molar ratio of oxygen in the PEO to Eu of the complex after benzene/water annealing for 48 h. Dark parts correspond to low height values.
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Fig. 6. The excitation (a) and emission (b) spectra of nanohybrid thin films before and after solvent annealing. Both the excitation and emission slit width are selected at 5 nm.
the formation of hybrid thin films does not influence the fluorescence property of Eu(BA)3Phen. However, for the solventannealed thin films, the intensities of emitted light at 592 nm and 614 nm are relatively weak compared to those of directly spincoated thin films (Fig. 6b). To explain the above phenomenon, the excitation spectrums of the nanohybrid thin films before and after solvent annealing are recorded by monitoring the emission of Eu3+ at 614 nm. As shown in Fig. 6, in the excitation spectrum of directly spin-coated hybrid thin films, three peaks are clearly observed at 203 nm, 230 nm, and 272 nm. The former (203 nm) is due to the n ! s* transition from Phen, and the latter (230 nm and 272 nm) is due to the p ! p* transition from Phen and BA. The results indicate that a very efficient energy transfer occurs from ligand of Phen and BA to Eu3+ in the directly spin-coated hybrid thin films [46], which corresponds to the strong emission light at 592 nm and 614 nm in the emission spectrum. However, in the case of solventannealed nanohybrid thin films, besides a slight shift of the peak at 272–262 nm, a strong broadband ranging from 200 nm to 230 nm is observed in the excitation spectrum. Based on the excitation spectra of pure PS-b-PEO block copolymer (see Supporting Information), the shift of the peak at 272 nm is ascribed to the p ! p* transition from PS blocks, while the strong broadband is associated to n ! s* transition from Phen, n ! s* transition from PEO blocks, and s ! p* transition from PS blocks. The UV–vis absorption spectra of PS-b-PEO block copolymer and PS homopolymer (Mn = 20,000) thin films shows a broadband between 200 nm and 300 nm (see Supporting Information). All the results above indicate that both the ligands of Eu(BA)3Phen and PS-b-PEO block copolymer adsorb UV irradiation during emission of the ordered nanohybrid thin films. However, it should be noted that although PS blocks adsorb UV irradiation as Phen and BA do, the energy is not transferred to Eu3+ [47,48]. Given this, the weak intensity of emission light of the ordered nanohybrid thin films with solvent annealing can be explained as follows: before solvent annealing, the europium complexes of Eu(BA)3Phen within the directly spin-coated hybrid thin films congregate and macrophase separate from PS-b-PEO block copolymer (Fig. 1a), which makes no difference to the fluorescence property of Eu(BA)3Phen. As ordered nanohybrid thin films form by solvent annealing in benzene or benzene/water mixture vapor, Eu(BA)3Phen nanoparticles with diameter of 2 nm, are selectively and uniformly distributed in PS domains. The coating of PS blocks on Eu(BA)3Phen nanoparticles decrease the efficient energy transfer from ligand of Phen and BA to Eu3+ due to strong adsorption of UV irradiation of PS domains, leading to the weakening of emission intensity of Eu(BA)3Phen.
Phen nanoparticles distributed in the PS matrix selectively. The directly spin-coated hybrid thin films with Eu(BA)3Phen macrophase separated from PS-b-PEO block copolymer, were selforganized into hexagonally packed PEO cylindrical microdomains embedded in PS/Eu(BA)3Phen matrix after solvent annealing in benzene or benzene/water. The concentration of Eu(BA)3Phen in the nanohybrid thin films have great effect on the orientation of the films during solvent annealing in benzene, and the optimal [O]/ [Eu] for the highly ordered and oriented nanohybrid thin film is 60:1. When the hybrid thin films were annealed in benzene/water mixed vapor (0.1/0.9, in volume), high ordered and well-defined lattice orientation are easily obtained, which is independent on the Eu(BA)3Phen concentration. Furthermore, PEO domains were drawn to the surface of the film due to its preferential interaction with water, leading to the formation of several nanopores in the hexagonal patterns. The ordered nanohybrid thin films kept the fluorescence property of Eu(BA)3Phen, and showed a strong red emission under the 254 nm light’s irradiation. However, solvent annealing weakened the emission intensity of the ordered nanohybrid thin films, which can be ascribed to the less efficient energy transfer from ligand to Eu3+ in Eu(BA)3Phen resulting from the coating of PS domains on Eu(BA)3Phen nanoparticles. Acknowledgments The authors would like to acknowledge the financial support of the important direction project for the knowledge innovative engineering of Chinese Academy of Sciences (Grant No. KGCX3SYW-205) and the financial support of the National 973 project of China (2007CB607601). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.apsusc.2009.11.033. References [1] [2] [3] [4] [5] [6]
4. Conclusions
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Highly ordered luminescent nanohybrid thin films were successfully prepared using self-assembly of PS-b-PEO block copolymer combined with solvent annealing, in which Eu(BA)3-
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Ordered luminescent nanohybrid thin films of Eu(BA)3Phen nanoparticle in polystyrene matrix from diblock copolymer self-assembly Chao Wang a,b, Yaoming Zhang a,b, Xianqiang Pei a, Tingmei Wang a,*, Qihua Wang a,* a b
State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, PR China Graduate School, Chinese Academy of Sciences, Beijing 100039, PR China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 18 September 2009 Received in revised form 10 November 2009 Accepted 11 November 2009 Available online 18 November 2009
A simple route for fabricating highly ordered luminescent thin films based on hybrid material of diblock copolymer and europium complex, assisted with self-organization of polystyrene-block-poly(ethylene oxide) (PS-b-PEO) diblock copolymer upon solvent annealing, is presented. PS-b-PEO self-organized into hexagonal patterns and europium complex of Eu(BA)3Phen was selectively embedded in PS blocks after solvent annealing in benzene or benzene/water vapor. During benzene annealing, the orientation of the PEO cylindrical domains strongly depended on the Eu(BA)3Phen concentration. In contrast, when the hybrid thin films were annealed in mixture of benzene and water vapor, high degree of orientation of the PEO cylindrical domains is more easily obtained, which is independent of Eu(BA)3Phen concentration. Furthermore, preferential interaction of PEO domains with water induces a generation of nanopores in the hybrid thin film. Atomic force microscopy (AFM), scanning electron microscopy (SEM), and transmission electron microscopy (TEM) were used to characterize the long-range lateral order and phase composition of the hybrid thin films. The ordered nanohybrid thin films kept the fluorescence property of Eu(BA)3Phen and showed a strong red emission under the 254 nm light’s irradiation. The fluorescence property was confirmed by photoluminescence (PL) spectra. ß 2009 Elsevier B.V. All rights reserved.
Keywords: Block copolymer Europium complex Luminescence Nanohybrid thin film Self-assembly
1. Introduction Highly ordered nanohybrid materials in thin films have attracted much attention for their fascinating self-assembly nanostructure, as well as the potential applications in optical, optoelectronic, magnetic, and micromechanical devices. Block copolymer, one class of self-assembling materials, offer an attractive route to fabricate nanometer-scale structures, since they spontaneously form a range of periodic structures for proper compositions and under adequate conditions, owing to the microphase separation between dissimilar blocks [1–4]. Hence, much effort has been made to use self-assembled block copolymers as a tool for fabricating functional nanomaterials, such as noble metal [5–8], semiconductor [9–15], and inorganic oxide nanoparticles [16–20]. Very recent examples include the fabrication of well-defined surface patterns of functional hybrid materials in thin films based on PS-b-P4VP diblock copolymer and Au nanoparticles through metallization of the pyridine units’ in situ using a wet chemical process [21], the use of cooperative self-
* Corresponding authors. Tel.: +86 931 4968252/4968180; fax: +86 931 4968252/4968180. E-mail addresses: [email protected] (T. Wang), [email protected] (Q. Wang). 0169-4332/$ – see front matter ß 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2009.11.033
assembly of polystyrene-block-poly(2-vinylpyridine) (PS-b-P2VP) copolymer and TTIP-based sol–gel precursors to prepare 2D arrays of strings of TiO2 nanoparticles [22], and the preparation of ordered silicon nanodot arrays using linear-brush-type polystyrene-blockpolycarbosilane (PS-b-PCS) diblock copolymer thin films combined with acetone vapor annealing [23]. Since a rapid route for orienting the cylindrical microdomains of asymmetric polystyrene-blockpoly(ethylene oxide) diblock copolymer (PS-b-PEO) normal to the surface of a substrate by utilization of solvent was addressed by Russell and co-workers [24,25], examples have revealed that thin films of asymmetric (PS-b-PEO) diblock copolymer were successfully used as scaffolds for various nanostructures. One example is the fabrication of ordered inorganic oxide nanoparticles arrays which could be grown selectively on top of the PEO domains on the surface of ordered PS-b-PEO thin films by chemical vapor deposition [26,27]. Another and more extensive example involves the use of sol–gel chemistry to prepare hexagonally packed arrays of titania nanodomains with PS-b-PEO block copolymer as template [28,14,29,30]. Phosphor thin films have attracted considerable attention because of their potential application in high-resolution devices such as electroluminescent (EL) display, field emission displays (FEDs), and cathode ray tubes (CRTs) [31]. Compared with conventional powder phosphors, thin film luminescent structure used as display have higher contrast and resolution, superior
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thermal conductivity, better adhesion as well as a high degree of uniformity [31,32]. Hence, the fabrication of high-quality phosphor thin films has always been hot spot of study over the past few years [33–35]. On the other hand, the patterning technologies of phosphor screens have a great effect on the resolution of flat panel display devices [36]. Yu et al. have prepared pattered nanocrystalline phosphor films using sol–gel process combined with soft lithography [37], overcoming the high expenditure and complexity of traditional fabrication process, but the obtained patterns are in micrometer scales. Cong et al. have used micelles of PS-b-P4VP as nanoreactors to synthesize Eu(III)-block copolymer complex with the presence of 1,10-phenanthroline as cooperative ligand, and have successfully prepared hexagonally ordered luminescent thin
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films in nanometer scale [38]. However, the degree of lateral order of the thin films is low and the method is limited to the complexation between rare earth and polymer. In this work, we report the use of solvent-induced ordering to fabricate highly ordered luminescent nanohybrid thin films via spin-coating from solutions containing europium complexes of Eu(BA)3Phen and PS-b-PEO, combined with solvent annealing; the europium complex was selectively incorporated into PS domain due to its hydrophobic. Annealed in pure benzene or benzene/ water mixture vapor, the hybrid film showed highly ordered hexagonally packed PEO cylindrical microdomains embedded in PS/Eu(BA)3Phen matrix. The ordered nanohybrid thin film exhibits highly efficient red luminescence of Eu(BA)3Phen. Atomic force
Fig. 1. Height AFM images of nanohybrid thin films (a) as spun and (b) after benzene annealing and pure block copolymer (c) as spun and (b) after benzene annealing. Dark parts correspond to low height values. (e) Corresponding TEM image of (b). (f) The molecular structure of europium complexes of Eu(BA)3Phen.
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microscopy (AFM), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and photoluminescence (PL) spectra were used to characterize the long-range lateral order, phase composition, and fluorescence property of the hybrid thin films.
respectively, with a volume ratio of water to benzene of 0.1/0.9. The environment outside the glass vessel was maintained at 25 8C and 30% humidity. After solvent annealing for different times, the films were removed from the vessel quickly and air-dried at room temperature.
2. Experimental
2.2. Characterizations
2.1. Preparation of nanohybrid thin films
AFM studies were operated at a Nanoscope IIIa Multimode atomic force microscope (AFM, Digital Instruments) at the tapping mode. For TEM studies, mica substrates were used. These films were floated off onto a pool of deionized water and picked up with copper mesh TEM grids. TEM experiments were conducted in a JEM 2010 TEM equipped with energy dispersive spectrometer (EDS) at an accelerating voltage of 200 kV in the bright-field mode. Field emission scanning electron microscopy (FESEM) imaging was performed on a JSM-6701F operated at 5 kV. Photoluminescence (PL) measurement was performed by a Hitachi 4500 fluorescence spectrophotometer.
PS-b-PEO diblock copolymer (0.1 g) (polymer sources Inc.; Mn PEO:PS = 6400:19,000 g/mol and polydispersity of 1.04) was first dissolved in 5 mL of benzene (99.5%, Sinopharm Chemical Reagent Co. Ltd., Shanghai) and stirred for 1 h. Europium complexes of Eu(BA)3Phen was prepared according to the previous report [39]. The molecular structure of Eu(BA)3Phen is shown in Fig. 1. Eu(BA)3Phen (5 mg) was dissolved in 1 mL of benzene at 70 8C and stirred for 20 min to get clear solution. The desired amount of Eu(BA)3Phen/benzene solution was added into a PS-b-PEO solution and stirred for 12 h. The hybrid thin films were prepared on a silicon wafer by spin-coating from solutions containing Eu(BA)3Phen and PS-b-PEO at 2000 rpm for 60 s, followed by solvent annealing. The films thickness, determined by a L116E ellipsometer, was about 80.0 5 nm. Solvent annealing of the films were performed in two ways: (i) the thin films were placed in an airtight glass vessel with 70 cm3 volume along with a small vial containing pure benzene; (ii) the thin films were placed in the airtight glass vessel along with two small vials which contained benzene and water
3. Results and discussion 3.1. Solvent-annealed in pure benzene vapor Part a and b of Fig. 1 shows the AFM images of nanohybrid thin films of PS-b-PEO/Eu(BA)3Phen with a [O]/[Eu] molar ratio of 60 (oxygen in PEO to Eu of the complex) before and after benzene annealing, respectively. The results for films of the block
Fig. 2. Height AFM images of nanohybrid thin films containing different amount of Eu(BA)3Phen with changing molar ratio of oxygen in the PEO to Eu of the complex after benzene annealing for 24 h. Dark parts correspond to low height values. In the inset a Fourier transform of the AFM image shows a pattern with multiple reflections characteristic of a highly ordered hexagonal array.
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copolymer without Eu(BA)3Phen are shown in parts c and d of Fig. 1, respectively. Comparing parts a and c, it can be concluded that larger domains in part a result from the macrophase separation of Eu(BA)3Phen from PS-b-PEO. However, this morphology corresponds to a kinetically trapped structure formed during spin-coating and is far from equilibrium. After benzene annealing, the film exhibits a well-organized structure with a high degree of long-range lateral order of cylindrical microdomains (Fig. 1b). Since Eu(BA)3Phen is hydrophobic (Fig. 1f), it is expected that the macrophase-separated domains of Eu(BA)3Phen are selectively and uniformly distributed within the PS matrix during solvent annealing. From parts b and d, it is clear that the orientation of PEO cylinders transforms from parallel to perpendicular with the addition of Eu(BA)3Phen in the benzene annealed films. Similar results were observed in previous reports [40], where the addition of salt of KI gives rise to a change in the orientation of PEO cylinders from parallel to perpendicular in the benzene annealed films, by overcoming preferential interactions between PEO and the substrate because of complexation with the PEO or interaction with the substrate. However, in our study, Eu(BA)3Phen as additive is distributed in PS matrix, not PEO cylindrical microdomains. It can be concluded that the addition of Eu(BA)3Phen which is uniformly distributed in PS domains, changes the migration of PS blocks and further mitigates the PS-selectivity of the benzene vapor during solvent annealing, enabling the propagation of PS domains and PEO cylinders through the film oriented normal to film surface. The diameter of the PEO cylindrical microdomains and the repeat period of the lattice is 34.3 nm and
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20.5 nm, which is obvious different from self-assembled thin films of pure PS-b-PEO observed in Russells’ studies (43.8 nm and 23.5 nm, respectively) [25]. It could be attributed to the addition of Eu(BA)3Phen which is selectively distributed within PS matrix. In the past few years, solvent annealing, as a simple route for the control of lateral ordering and orientation of microstructures in block copolymer thin films, has been studied experimentally and theoretically [25,41–43]. By exposing the PS-b-PEO/Eu(BA)3Phen hybrid thin film to benzene vapor, the thin film is rapidly swollen. Benzene vapor, a selective solvent for PS-b-PEO, imparts mobility to the copolymer, enabling a rapid removal of defects and homogenous distribution of Eu(BA)3Phen in PS blocks. As the benzene evaporates, the concentration of benzene at the surface is lowest and a gradient in solvent concentration develops normal to the surface. Microphase separation of the PS-b-PEO/Eu(BA)3Phen occurs at the surface, the structure rapidly coarsens, and ordering propagates through the film as benzene further evaporates. Consequently, long-range lateral ordering and high degree of orientation of the hybrid thin film are achieved. The AFM images shown in Fig. 2 exhibit the dependence of orientation and lateral order of the nanohybrid thin films on Eu(BA)3Phen concentration. Prior to discussion, it should be noted that the concentration of Eu(BA)3Phen in the experiments cannot be much high, since the mixture solution of PS-b-PEO/Eu(BA)3Phen will turn turbidity when stored for a long time at a [O]/[Eu] ratio lower than 30. When the [O]/[Eu] ratio is 120, the amount of added Eu(BA)3Phen is insufficient to influence the migration and vaporselectivity of PS blocks during solvent annealing, and cylindrical
Fig. 3. Height AFM images of the nanohybrid thin films with [O]/[Eu] of 60: (a) directly after spin-coating and benzene annealing for (b) 12 h, (c) 24 h, and (d) 36 h. Dark parts correspond to low height values.
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microdomains both parallel and normal to the surface coexist. With increasing Eu(BA)3Phen concentration ([O]/[Eu] = 60 and 90), highly ordered arrays of cylindrical microdomains orient normal to the surface are observed. A Fourier transform of the AFM image is shown in the inset, where a six-point pattern, with multiple higher-order reflections, is obtained, indicating the characteristic of the long-range order. Increasing the Eu(BA)3Phen concentration further ([O]/[Eu] = 30) maintains the orientation of microdomains normal to the surface; however, the lateral ordering of the cylindrical microdomains is reduced and slight macrophase separation is also observed. The annealing time dependence of the structure evolution in the nanohybrid thin films has been explored by annealing the film with [O]/[Eu] of 60 for different period of time. For the directly spincoated thin film, as Fig. 4a shows, obvious macrophase separation of Eu(BA)3Phen from PS-b-PEO is observed. After 12 h of benzene annealing, although the film shows poorly ordered morphology, macrophase-separated domains of the Eu(BA)3Phen in the film disappear (Fig. 3b), indicating that all the Eu(BA)3Phen are sequestered within the PS matrix. After annealing for 24 h, the film exhibits a well-organized structure with a high degree of long-range lateral order of cylindrical microdomains. With the annealing time increased to 36 h, no clear change of the morphology is observed. To explore the distribution of Eu(BA)3Phen in the nanohybrid films, TEM was employed to investigate the film given the good contrast that Eu(BA)3Phen can provide. Shown in Fig. 1e is the TEM image of the nanohybrid thin films removed from the substrate after benzene annealing. From the result, it can be seen that Eu(BA)3Phen nanoparticles are very fine in size, estimated to be <2 nm, and homogeneously assemble in the hexagonal-like structure of PS-b-PEO matrix. The composition of the pattered
film was checked by EDS analysis, with an detection diameter of 5 nm. In the PEO cylindrical domains, only carbon and oxygen were found, however, besides the C and O peak, europium was also observed in the PS matrix. The TEM image and EDS results confirm the selective distribution of Eu(BA)3Phen within PS matrix in the nanohybrid thin film. 3.2. Solvent-annealed in benzene/water vapor Fig. 4a shows the AFM image of the nanohybrid thin film of PS-bPEO/Eu(BA)3Phen ([O]/[Eu] = 30) after solvent annealing in benzene/water, with the volume ratio of water to benzene of 0.1/0.9. Similarly, the thin film is self-organized into hexagonally packed PEO cylindrical microdomains embedded in PS/Eu(BA)3Phen matrix, although the diameter of the nanoscopic cylindrical domains and the repeat period of the lattice have increased to 30.5 nm and 47.6 nm, respectively, due to the preferable interaction of water to PEO domains. TEM was employed to further study the nanohybrid thin film. As shown in Fig. 4b, Eu(BA)3Phen nanoparticles with diameter of 2 nm, are uniformly distributed in PS matrix, which is similar to that of the thin film after solvent annealing in pure benzene. Interestingly, some hexagonally packed nanoscopic pores are also observed in the film. The FESEM images of the film top view and cross-section show similar results to TEM, where the cylindrical nanopores, with diameter of 30 nm, span across the entire film thickness. According to previous studies [44,45], simply exposure of self-assembled BCP film to a solvent that is a good solvent for the minor component block and a nonsolvent for the major component block, the minor component is drawn to the surface of the film and surface reconstructure occurs. Consequently, a solvent-induced reconstruction of the film involving two steps is shown below to be
Fig. 4. (a) AFM images of the nanohybrid thin film ([O]/[Eu] = 30) after solvent annealing in benzene/water (0.9/0.1) for 48 h. A Fourier transform of the AFM image is shown in the inset. Dark parts correspond to low height values. (b) TEM image of the same nanohybrid thin film. (c) FESEM image of the nanohybrid thin film, and (d) corresponding cross-section view.
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responsible for the formation of nanopores. Firstly, the hybrid thin film is self-organized into hexagonally packed PEO cylindrical microdomains embedded in PS/Eu(BA)3Phen matrix in the presence of benzene vapor based on the discussion above. Secondly, due to the presence of water, a good solvent for the PEO block and a nonsolvent for the PS block, the cylindrical PEO microdomains in the ordered nanohybrid thin film is drawn to the surface of the film, leading to the generation of nanopores. The influence of Eu(BA)3Phen concentration in nanohybrid thin films on the ordering of the film has been studied. As shown in Fig. 5, for the nanohybrid thin films solvent-annealed in benzene/ water vapor mixture (water:benzene = 0.1:0.9) for 48 h, the change of Eu(BA)3Phen concentration in the nanohybrid thin films exerts little influence on the ordering, and all the films show ordered nanostructure. It appears that the presence of water is more favorable for the formation of arrays of hexagonally packed microdomains than pure benzene, without considering the influence of Eu(BA)3Phen concentration in the nanohybrid thin films. The presence of water, probably either by mitigating the PSselectivity of the vapor mixtures or by adjusting the preferential interactions between PEO domains and substrate, are more prone to induce the formation of cylindrical microdomains normal to the substrate than pure benzene vapor.
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On the basis of the results discussed above, simply by exposing the PS-b-PEO/Eu(BA)3Phen hybrid thin film to benzene or benzene/ water mixture vapor, highly ordered hexagonal-like structure was obtained. In a previous work of Kim et al., PS-b-PEO block copolymer has been used as a template to fabricate ordered organic–inorganic hybrid nanostructures by one-step spin-coating from solutions containing a titania precursor and the copolymer [28]. However, the obtained hybrid thin film did not show high degree of lateral ordering. In our study, solvent annealing, commonly utilized as one important approach to produce a long-range ordered thin film of a pure block copolymer, is proved to be an effective way to achieve high ordering for nanohybrid thin films. 3.3. Photoluminescence spectrum Fig. 6 shows the excitation (a) and emission (b) spectrums of the nanohybrid thin films before and after solvent annealing. As can be seen from the emission spectrum, all the samples exhibit characteristic emission peak at 592 nm and 614 nm, corresponding to (5D0–7F1, 7F2) of Eu3+, respectively. It is notable that thin films from direct spin-coating show similar emission spectrum with that of pure Eu(BA)3Phen (see Supporting Information), indicating that
Fig. 5. Height AFM images of nanohybrid thin films containing different amount of Eu(BA)3Phen with changing molar ratio of oxygen in the PEO to Eu of the complex after benzene/water annealing for 48 h. Dark parts correspond to low height values.
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Fig. 6. The excitation (a) and emission (b) spectra of nanohybrid thin films before and after solvent annealing. Both the excitation and emission slit width are selected at 5 nm.
the formation of hybrid thin films does not influence the fluorescence property of Eu(BA)3Phen. However, for the solventannealed thin films, the intensities of emitted light at 592 nm and 614 nm are relatively weak compared to those of directly spincoated thin films (Fig. 6b). To explain the above phenomenon, the excitation spectrums of the nanohybrid thin films before and after solvent annealing are recorded by monitoring the emission of Eu3+ at 614 nm. As shown in Fig. 6, in the excitation spectrum of directly spin-coated hybrid thin films, three peaks are clearly observed at 203 nm, 230 nm, and 272 nm. The former (203 nm) is due to the n ! s* transition from Phen, and the latter (230 nm and 272 nm) is due to the p ! p* transition from Phen and BA. The results indicate that a very efficient energy transfer occurs from ligand of Phen and BA to Eu3+ in the directly spin-coated hybrid thin films [46], which corresponds to the strong emission light at 592 nm and 614 nm in the emission spectrum. However, in the case of solventannealed nanohybrid thin films, besides a slight shift of the peak at 272–262 nm, a strong broadband ranging from 200 nm to 230 nm is observed in the excitation spectrum. Based on the excitation spectra of pure PS-b-PEO block copolymer (see Supporting Information), the shift of the peak at 272 nm is ascribed to the p ! p* transition from PS blocks, while the strong broadband is associated to n ! s* transition from Phen, n ! s* transition from PEO blocks, and s ! p* transition from PS blocks. The UV–vis absorption spectra of PS-b-PEO block copolymer and PS homopolymer (Mn = 20,000) thin films shows a broadband between 200 nm and 300 nm (see Supporting Information). All the results above indicate that both the ligands of Eu(BA)3Phen and PS-b-PEO block copolymer adsorb UV irradiation during emission of the ordered nanohybrid thin films. However, it should be noted that although PS blocks adsorb UV irradiation as Phen and BA do, the energy is not transferred to Eu3+ [47,48]. Given this, the weak intensity of emission light of the ordered nanohybrid thin films with solvent annealing can be explained as follows: before solvent annealing, the europium complexes of Eu(BA)3Phen within the directly spin-coated hybrid thin films congregate and macrophase separate from PS-b-PEO block copolymer (Fig. 1a), which makes no difference to the fluorescence property of Eu(BA)3Phen. As ordered nanohybrid thin films form by solvent annealing in benzene or benzene/water mixture vapor, Eu(BA)3Phen nanoparticles with diameter of 2 nm, are selectively and uniformly distributed in PS domains. The coating of PS blocks on Eu(BA)3Phen nanoparticles decrease the efficient energy transfer from ligand of Phen and BA to Eu3+ due to strong adsorption of UV irradiation of PS domains, leading to the weakening of emission intensity of Eu(BA)3Phen.
Phen nanoparticles distributed in the PS matrix selectively. The directly spin-coated hybrid thin films with Eu(BA)3Phen macrophase separated from PS-b-PEO block copolymer, were selforganized into hexagonally packed PEO cylindrical microdomains embedded in PS/Eu(BA)3Phen matrix after solvent annealing in benzene or benzene/water. The concentration of Eu(BA)3Phen in the nanohybrid thin films have great effect on the orientation of the films during solvent annealing in benzene, and the optimal [O]/ [Eu] for the highly ordered and oriented nanohybrid thin film is 60:1. When the hybrid thin films were annealed in benzene/water mixed vapor (0.1/0.9, in volume), high ordered and well-defined lattice orientation are easily obtained, which is independent on the Eu(BA)3Phen concentration. Furthermore, PEO domains were drawn to the surface of the film due to its preferential interaction with water, leading to the formation of several nanopores in the hexagonal patterns. The ordered nanohybrid thin films kept the fluorescence property of Eu(BA)3Phen, and showed a strong red emission under the 254 nm light’s irradiation. However, solvent annealing weakened the emission intensity of the ordered nanohybrid thin films, which can be ascribed to the less efficient energy transfer from ligand to Eu3+ in Eu(BA)3Phen resulting from the coating of PS domains on Eu(BA)3Phen nanoparticles. Acknowledgments The authors would like to acknowledge the financial support of the important direction project for the knowledge innovative engineering of Chinese Academy of Sciences (Grant No. KGCX3SYW-205) and the financial support of the National 973 project of China (2007CB607601). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.apsusc.2009.11.033. References [1] [2] [3] [4] [5] [6]
4. Conclusions
[7] [8] [9] [10]
Highly ordered luminescent nanohybrid thin films were successfully prepared using self-assembly of PS-b-PEO block copolymer combined with solvent annealing, in which Eu(BA)3-
[11] [12]
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