Article pubs.acs.org/Langmuir
Micromolding in Capillaries for Calcination-Free Fabrication of Flexible Inorganic Phosphor Films Consisting of Rare-Earth-IonDoped Nanoparticles Satoshi Watanabe,* Takeo Asanuma, Hiroshi Hyodo, Kohei Soga, and Mutsuyoshi Matsumoto Department of Materials Science and Technology, Tokyo University of Science, 6-3-1 Niijuku, Katsushika-ku, Tokyo 125-8585, Japan ABSTRACT: We discuss the micromolding in capillaries technique for the direct fabrication of calcination-free rare earth ion-doped (RE) phosphor films consisting of RE nanoparticles on plastic sheets. We synthesized two types of RE nanoparticles consisting of Y2O3 matrix doped with Er and Yb ions. Green upconversion luminescence, red upconversion luminescence, and near-infrared fluorescence appeared from the RE nanoparticles under excitation of near-infrared light. Adjusting the channel width and depth of polydimethylsiloxane molds led to control of the density of nanoparticles in the patterned RE nanoparticle films. Adjusting concentration of the RE nanoparticle dispersion and size of the RE nanoparticles allowed for the control of the density of nanoparticles in the patterned RE nanoparticle films. The density of nanoparticles in the patterned RE films on plastic sheets increased with an increase in the number of injection and drying of the RE nanoparticle dispersion. These results demonstrate that this technique enables us to directly fabricate the patterned RE phosphor films on plastic sheets, leading to the fabrication of inorganic flexible devices with small fabrication steps and material consumptions. electrophoretic deposition,36 screen printing,37 the sol−gel method,38 and the polyvinyl alcohol slurry method.39 Recently, it was reported that soft lithography using micromolding in capillaries (MIMIC) and microtransfer molding, combined with sol−gel methods allows for the direct fabrication of patterned phosphor films with low cost and without light exposure or etching processes.40−44 However, these techniques require calcination processes after the formation of the patterned precursor films, rendering the fabrication of RE phosphor films on flexible substrates difficult. We have reported calcination-free liftoff photolithography for the patterning of inorganic phosphor films consisting of RE nanoparticles on plastic sheets by using calcination-free processes.45 The emitting color of the films was controlled by adjusting the mixing ratio of the green-emitting and redemitting nanoparticles.46 However, these techniques based on photolithography have potential problems in the number of the processes and material consumption. Here, we employ MIMIC for the calcination-free fabrication of RE nanoparticle films on plastic sheets. We synthesized two types of RE nanoparticles with different diameters. The patterned RE nanoparticle films were fabricated by placing droplets of a RE nanoparticle dispersion at the open ends of polydimethylsiloxane (PDMS) molds and solid substrates. The samples were analyzed with fluorescence spectroscopy,
1. INTRODUCTION Organic solar cells,1−11 organic field effect transistors,12−19 and organic light emitting diodes20 have been extensively studied because of the small fabrication cost, abundant resources of carbon material, and flexibility. In particular, flexible devices such as displays and solar cells on plastic sheets fabricated solely with solution processes have attracted much attention for the fabrication of electronic paper and lightweight solar power modules.21,22 However, organic semiconductors have potential problems and short operation lifetime compared with inorganic semiconductors. Fabrication of inorganic ceramics on plastic sheets is required for the practical use of flexible devices under exposure to air and light. Rare-earth-ion-doped (RE) inorganic phosphor films have attracted much attention for applications in displays,23,24 threedimensional memory media,25 and photovoltaic devices26−29 because of their photonics properties and long operating lifetimes. The upconversion phenomena originates from rareearth ions doped in low-phonon inorganic materials under stepwise infrared excitations.30,31 The wavelength of upconversion luminescence can be controlled by adjusting the chemical species and the concentrations of doped rare earth ions.32 These phenomena are useful for nanofabrication in nonlinear optical fields.33 The near-infrared fluorescence of the RE phosphor films is utilized for bioimaging34 and sensing35 because it can transmit through biological samples with low biodamage. Conventional photolithography involving etching processes allows us to fabricate patterned RE phosphor films fabricated by © XXXX American Chemical Society
Received: May 13, 2013 Revised: August 7, 2013
A
dx.doi.org/10.1021/la401810x | Langmuir XXXX, XXX, XXX−XXX
Langmuir
Article
fluorescence microscopy, and field emission scanning electron microscopy (SEM). The fabrication conditions were tuned to control the density of nanoparticles in the RE nanoparticle films. MIMIC allows for direct patterning without light irradiation, etching, or liftoff, resulting in the formation of patterned RE phosphor films with little material consumption, little energy use, and few fabrication steps.
2. EXPERIMENTAL SECTION 2.1. Materials. Y(NO3)3, Er(NO3)3·5H2O and Yb(NO3)3·5H2O used for the preparation of RE nanoparticles were purchased from Aldrich. An aqueous solution of urea (10 mol L−1) was purchased from Kanto Chemical Corp. Acetone and chloroform used for cleaning the substrates, and urease were purchased from Wako Pure Chem. Ind., Ltd. PDMS precursor (Silpot) and its catalyst were purchased from Dow Corning Toray. 2.2. Preparation of RE Nanoparticles. RE-1 nanoparticles and RE-2 nanoparticles were synthesized using the recipe of Venkatachalam et al.31 The precursors for the fabrication of RE-1 nanoparticles were prepared by adding 4 mmol Y(NO3)3, 0.04 mmol Er(NO3)3· 5H2O, and 0.04 mmol Yb(NO3)3·5H2O to 40 mL of the 400 mmol L−1 aqueous urea solution. A nominal 10 mL of water was added to the precursors under stirring. The precursors were kept at 98 °C in a water bath for 1 h. For the synthesis of RE-2 nanoparticles, 40 mg of urease was added to the precursor before heating, and the mixture was kept at room temperature for 1 h. The precursor powders were washed centrifugally three times with distilled water, dried at 80 °C, and then calcinated at 900 °C for 30 min. The diameters of the RE-1 and RE-2 nanoparticles were estimated to be about 100 and 40 nm, respectively, using SEM. 2.3. Cleaning of Substrates. Glass plates and polyethylene terephthalate (PET) sheets used as the substrates were rinsed with acetone and then chloroform under ultrasonication for 20 min each. After being dried with nitrogen gas, the substrates were set in an ozone atmosphere generated from a PL 16-110 ultraviolet-ozone cleaner (Sen Lights Corp., Japan) for 15 min. The distance between the substrates and the light source was about 5 cm. 2.4. Fabrication of Patterned RE Nanoparticle Films by MIMIC. PDMS precursor and its catalyst at a mixing ratio of 10:1 (wt %) were slowly mixed using a glass rod in a plastic cell. The plastic cell was placed in a vacuum desiccator at a pressure less than 6.7 × 10−2 Pa for 30 min to remove air from the mixture. The mixture was poured on positive silicon masters with line patterns (width/depth = 50/2, 10/2, 5/2, 50/10, 10/10, 5/10 (μm)) and then cured at 110 °C for 30 min to polymerize the precursor films (Figure 1a). The PDMS molds were placed on glass plates or PET sheets. The RE nanoparticles in methanol were ultrasonicated for 1 h at room temperature before use to ensure that they were dispersed. Ultrasonication was carried out at an oscillating frequency of 38 kHz at an ultrasonic output power of 180 W using a US-SKS ultrasonic cleaner (SND Co. Ltd., Japan). Methanol dispersion of the RE nanoparticles was put at the open ends of the capillaries (Figure 1b). The dispersion was drawn into the capillaries due to the capillary force. After spontaneous evaporation of methanol, the PDMS molds were peeled off from the substrates (Figure 1c). 2.5. Characterization. Visible emission spectra were measured on an RF-5000 visible spectrometer (Shimadzu, Japan). Near-infrared emission spectra were obtained using a near-infrared spectrometer (AvaSpec, USA) and a semiconductor laser diode operating at 980 nm. Upconversion luminescence and near-infrared imaging was performed with an IX-71 optical microscope (Olympus, Japan) attached with a band-pass filter at 550 or 650 nm and a short-pass filter at 900 nm. SEM observations were made with an S-4200 microscope (Hitachi, Japan).
Figure 1. Patterning procedures of RE nanoparticle films on a glass plate or a plastic sheet by MIMIC: (a) Fabrication of PDMS mold, (b) injection of RE nanoparticle dispersion and (c) formation of RE nanoparticle films.
the RE-1 and the RE-2 nanoparticles under excitation of nearinfrared light. Figure 2 panels a and b show upconversion spectra of the RE-1 nanoparticles and the RE-2 nanoparticles, respectively. Upconversion luminescence appears at 525 nm, 550 nm, and 660 nm attributed to 2 H 11/2 − 4 I 15/2 , 4 S 3/2 − 4 I 15/2 , and 4 F9/2−4I15/2 of Er3+, respectively. The green upconversion luminescence of both the RE-1 nanoparticles and the RE-2 nanoparticles is stronger than the red luminescence. Figure 2 panels c and d show near-infrared fluorescence spectra of the RE-1 nanoparticles and the RE-2 nanoparticles, respectively. Fluorescence of the RE-1 nanoparticles and the RE-2 nanoparticles appears at 1100 and 1500 nm assigned to 2 F5/2−2F7/2 of Yb3+ and 4I13/2−4I15/2 of Er3+, respectively. Fluorescence intensities of Er3+ are stronger than those of Yb3+ due to the energy transfer from excited Yb3+ to Er3+. Figure 2e shows the energy-level diagram explaining the mechanism of the upconversion luminescence and the fluorescence in this system.47,48 These results demonstrate that the RE-1 nanoparticles and the RE-2 nanoparticles serve as visible and nearinfrared emitting nanoparticles. We have reported calcination-free photolithography of RE nanoparticle films on solid substrates.45,46 Because MIMIC has been applied to the patterning of micro- and nano-particle films of polymer beads,49 we have employed MIMIC to fabricate the RE nanoparticle films on glass plates. Figure 3a shows an optical microscope image of a RE-1 nanoparticle film fabricated on a glass plate by MIMIC. It is evident that a RE-1 nanoparticle film with the line/space of 6/4 (μm) is formed. The line width of the RE-1 nanoparticle films is smaller than that of the Si molds due to the shrinkage of the
3. RESULTS AND DISCUSSION 3.1. Patterning of RE Nanoparticle Films. We study upconversion luminescence and near-infrared fluorescence of B
dx.doi.org/10.1021/la401810x | Langmuir XXXX, XXX, XXX−XXX
Langmuir
Article
Figure 2. Upconversion spectra of (a) RE-1 nanoparticles and (b) RE2 nanoparticles. Near-infrared spectra of (c) RE-1 nanoparticles and (d) RE-2 nanoparticles. (e) Energy-level diagram of Yb3+ and Er3+ systems. Full, dotted, and dashed downward arrows show radiative processes, multiphonon relaxations, and energy transfers, respectively.
Figure 3. (a) Optical microscope, (b) SEM, (c) green upconversion luminescent, (d) red upconversion luminescent, and (e) near-infrared fluorescent images of RE-1 nanoparticle films fabricated by MIMIC using a PDMS mold (width/depth = 5/2 μm) and methanol dispersion of RE nanoparticles at a concentration of 2 mg mL−1.
PDMS mold during the polymerization on the Si mold. Figure 3b shows an SEM image of the RE-1 nanoparticle film. This pattern consists of almost a single layer of RE-1 nanoparticles judging from the absence of stacked RE nanoparticles in the film. It was revealed from magnified SEM images that the diameter of the RE-1 nanoparticles was about 100 nm. These nanoparticles did not form 2D crystalline structures seen in colloid crystals. EDX measurements showed the presence of yttrium in the RE-1 nanoparticle films. Figure 3 panels c, d, and e show images of the RE-1 nanoparticle film observed at 550 nm, at 650 nm, and in the near-infrared region corresponding to upconversion luminescence, upconversion luminescence, and fluorescence, respectively. The green upconversion luminescence, red upconversion luminescence, and nearinfrared fluorescence are attributed to 4S3/2−4I15/2 transition of Er3+, 4F9/2−4I15/2 transition of Er3+, and 4I13/2−4I15/2 and 2 F5/2−2F7/2 transitions of Er3+ and Yb3+, respectively. External quantum efficiency of upconversion luminescence from the RE phosphor films (at most 10%)50 is generally smaller than that of visible fluorescence (ca. 20%)50 and near-infrared downconversion luminescence (ca. 30%).51 Upconversion luminescence from Er ions and near-infrared fluorescence from Er and Yb ions are not affected by the environments of ions such as the
Figure 4. Optical microscope images of RE nanoparticle films fabricated by using PDMS molds and methanol dispersion of RE-1 nanoparticles at a concentration of 2 mg mL−1 with variations in channel width and depth.
solvent in dispersion and the interparticle distance between adjacent RE nanoparticles in the films because the electrons in 4f orbitals that emit upconversion luminescence are protected by peripheral electrons in the 5s25p6 orbitals. Some bright spots C
dx.doi.org/10.1021/la401810x | Langmuir XXXX, XXX, XXX−XXX
Langmuir
Article
variations in channel width and depth. A decrease in line width gives rise to an increase in density of nanoparticles in the RE-1 nanoparticle film. The change in the density of nanoparticles should be brought about by a change in the amount of nanoparticle dispersion flowing in the unit cross-sectional area of the channel because the channel depth is the same. After the filling of the channel, the solvent evaporates through the outlet55 and channel walls,56 giving rise to the refilling of the channel with the nanoparticle dispersion as long as the dispersion remains at the injected side of the channel. For channels with small width, the evaporation rate through the outlet is large.55 Further, the area-per-volume ratio is large for channels with small width, resulting in a large evaporation rate per volume of solvent. These two points help explain that the density of nanoparticles in the RE nanoparticle films increases with a decrease of channel width. Figure 4 also shows that an increase in channel depth gives rise to an increase in density of nanoparticles in the RE-1 nanoparticle films. This is caused by a large volume of the dispersion per unit area of the bottom of the channel for channels with large depth, which dominates over the effect of the small amount of the evaporated solvent described above. These results demonstrate that the adjustment of the capillary size enables us to control the density of nanoparticles in the RE-1 nanoparticle films. We performed fluorescence microscope observations of the RE-1 nanoparticle films. Figure 5 shows near-infrared fluorescent images of RE-1 nanoparticle films fabricated by using PDMS molds with variations in channel width and channel depth. The nearinfrared fluorescence is evident from all the RE-1 nanoparticle films. The upconversion luminescence was also observed from the patterned region of the RE-1 nanoparticle films. These results have demonstrated that the RE-1 nanoparticle films serve as emitting devices in the near-infrared region. The above results have shown that adjusting the channel size of the PDMS molds allows for the control of the density of nanoparticles in the RE-1 nanoparticle films. Next, we fabricated the RE nanoparticle films with variations in concentration of the RE nanoparticle dispersion and size of the RE nanoparticles. Figure 6 shows optical microscope images of the patterned RE nanoparticle films fabricated by using PDMS molds (width/ depth = 10/10 μm) and methanol dispersion of RE nanoparticles in different fabrication conditions. An increase in concentration of the RE nanoparticle dispersion gives rise to an increase in density of nanoparticles in the RE nanoparticle film. This is due to the large number of nanoparticles in the unit volume of the dispersion for a large concentration of nanoparticles. Figure 6 also shows that the density of nanoparticles in the RE nanoparticle films decreases with an increase in diameter of the nanoparticles. This may be because large particles have small diffusion coefficients.57 Solvent evaporation gives rise to the refilling of the channels with the RE nanoparticle dispersion as long as the dispersion remains at the injected sides of the channels. Because of the small diffusion coefficient of large particles, a relatively small amount of the particles flows into the channels. These results demonstrate that the density of nanoparticles in the patterned RE nanoparticle films can be controlled by adjusting the concentration of the dispersion and the diameter of the nanoparticles. Fabrication of the RE-1 nanoparticle films using the PDMS molds with a channel width of 1 μm was difficult because RE
Figure 5. Near-infrared fluorescent images of RE-1 nanoparticle films fabricated by using PDMS molds with variations in channel width and depth, and methanol dispersion of RE-1 nanoparticles at a concentration of 2 mg mL−1.
Figure 6. Optical microscope images of RE nanoparticle films fabricated by using PDMS molds (width/depth = 10/10 μm) and methanol dispersion of RE nanoparticles with variations in concentration of RE-1 nanoparticle dispersion and diameter of RE nanoparticle.
Figure 7. (a) SEM image and (b) its magnified image of a RE-2 nanoparticle film fabricated with a PDMS mold (width/depth = 50/2 μm) and methanol dispersion of RE-2 nanoparticles at 2 mg mL−1.
in upconversion luminescence and near-infrared fluorescence images are due to the formation of aggregates of RE nanoparticles, leading to relatively large numbers of RE ions per unit area of the films. These results demonstrate that the patterned RE-1 nanoparticle films can serve as visible and nearinfrared emitting devices. 3.2. Control of Fabrication Conditions of RE Nanoparticle Films. Adjusting the channel size and surface wettability allows for the control of the capillary force in the channels.49,52−54 We fabricated the RE nanoparticle films with variations in the channel width and depth of the PDMS molds. Figure 4 shows optical microscope images of the RE-1 nanoparticle films fabricated by using PDMS molds with D
dx.doi.org/10.1021/la401810x | Langmuir XXXX, XXX, XXX−XXX
Langmuir
Article
Figure 8. (a) Optical microscope, (b) green upconversion luminescence, (c) red upconversion luminescence, (d) near-infrared fluorescence, and (e) SEM images of the RE-1 nanoparticle films fabricated on PET sheets using the PDMS mold (line/depth of 50/10 μm) with variations in the number of the cycles of injection and drying of the nanoparticle dispersion.
nanoparticles should be deposited along both sides of the channels in the PDMS mold. These results have shown that we have fabricated RE-2 nanoparticle films with narrower line width than that of the channels of the PDMS molds. It is necessary to optimize experimental conditions to increase the density of nanoparticles in the RE nanoparticle films. We are trying to fabricate the patterned RE nanoparticle films at submicrometer−meter length scale by using PDMS molds with narrower channels. We have shown the formation of the RE nanoparticle films on the glass plates. This technique also allows for the fabrication of the RE nanoparticle films on PET sheets. The density of nanoparticles in the RE-1 nanoparticle films with the channel width of 50 μm is small as shown in Figures 4 and 7. To increase the density of nanoparticles, we increased the number of the cycles of the injection and drying of the RE nanoparticle dispersion at the open ends of the PDMS molds.
nanoparticles were stuck at the open end of the PDMS molds, blocking the flow of the dispersion into the channels. Submicrometer patterns of CuSO4 crystals were fabricated by controlling the surface wettability of the molds and injecting the solution along the corners of the channels.58,59 We employed this method to fabricate the RE nanoparticle films with narrower width than that of the channels of the PDMS molds. We adjusted the surface wettability of the substrates by using the glass plates without the UV/ozone treatment, considering that the wettability of the untreated glass plates to methanol should be smaller that of the treated glass plates. Figure 7 panels a and b show SEM images of a patterned RE2 nanoparticle film fabricated with a PDMS mold (width/depth = 50/2 μm) placed on a glass plate without the UV/ozone treatment. RE-2 nanoparticles form line patterns with widths less than 10 μm. Taking into account that the width of the PDMS mold is only slightly smaller than 50 μm, RE-2 E
dx.doi.org/10.1021/la401810x | Langmuir XXXX, XXX, XXX−XXX
Langmuir
Article
Interpenetrating Network Morphology. Adv. Funct. Mater. 2005, 15, 1617−1622. (3) Li, G.; Shrotriya, V.; Hung, J.; Yao, Y.; Moriarty, T.; Emery, K.; Yang, Y. High-Efficiency Solution Processable Polymer Photovoltaic Cells by Self-Organization of Polymer Blends. Nat. Mater. 2005, 4, 864−868. (4) McNeill, C. R.; Greenham, N. C. Conjugated-Polymer Blends for Optoelectronics. Adv. Mater. 2009, 21, 3840−3850. (5) Kim, Y.; Cook, S.; Tuladhar, S. M.; Choulis, S. A.; Nelson, J.; Durrant, J. R.; Bradley, D. D. C.; Giles, M.; Mcculloch, I.; Ha, C.; Ree, M. A Strong Regioregularity Effect in Self-Organizing Conjugated Polymer Films and High-Efficiency Polythiophene:fullerene Solar Cells. Nat. Mater. 2006, 5, 197−203. (6) Peumans, P.; Uchida, S.; Forrest, S. R. Efficient Bulk Heterojunction Photovoltaic Cells Using Small-Molecular-Weight Organic Thin Films. Nature 2003, 425, 158−162. (7) Yang, X.; Loos, J.; Veenstra, S. C.; Herhees, W. J. H.; Wienk, M. M.; Kroon, J. M.; Michels, M. A. J.; Janssen, R. A. J. Nanoscale Morphology of High-Performance Polymer Solar Cells. Nano Lett. 2005, 5, 579−583. (8) Hoppe, H.; Sariciftei, N. S. Morphology of Polymer/Fullerene Bulk Heterojunction Solar Cells. J. Mater. Chem. 2006, 16, 45−61. (9) Hoppe, H.; Sariciftei, N. S. Organic Solar Cells: An Overview. J. Mater. Res. 2004, 19, 1924−1945. (10) Blom, P. W. M.; Mihailetchi, V. D.; Koster, L. J. A.; Markov, D. E. Device Physics of Polymer:Fullerene Bulk Heterojunction Solar Cells. Adv. Mater. 2007, 19, 1551−1566. (11) Watanabe, S.; Fukuchi, Y.; Fukasawa, M.; Sassa, T.; Uchiyama, M.; Yamashita, T.; Matsumoto, M.; Aoyama, T. Electron Donor and Acceptor Spatial Distribution in Structured Bulk Heterojunction Photovoltaic Devices Induced by Periodic Photopolymerization. Langmuir 2012, 28, 10305−10309. (12) Bü rgi, L.; Sirringhaus, H.; Friend, R. H. Noncontact Potentiometry of Polymer Field-Effect Transistors. Appl. Phys. Lett. 2002, 80, 2913−2915. (13) Mathijssen, S. G. J.; Cölle, M.; Mank, A. J. G.; Kemerink, M.; Bobbert, P. A.; Leeuw, D. M. Scanning Kelvin Probe Microscopy on Organic Field-Effect Transistors during Gate Bias Stress. Appl. Phys. Lett. 2007, 90, 192104. (14) Ribierre, J. C.; Watanabe, S.; Matsumoto, M.; Muto, T.; Aoyama, T. Majority Carrier Type Conversion in Solution-Processed Organic Transistors and Flexible Complementary Logic Circuits. Appl. Phys. Lett. 2010, 96, 08303. (15) Ribierre, J. C.; Fujihara, T.; Watanabe, S.; Matsumoto, M.; Muto, T.; Nakao, A.; Aoyama, T. Direct Laser Writing of Complementary Logic Gates and Lateral p-n Diodes in a SolutionProcessible Monolithic Organic Semiconductor. Adv. Mater. 2010, 22, 1722−1726. (16) Ribierre, J. C.; Watanabe, S.; Matsumoto, M.; Muto, T.; Nakao, A.; Aoyama, T. Reversible Conversion of the Majority Carrier Type in Solution-Processed Ambipolar Quinoidal Oligothiophene Thin Films. Adv. Mater. 2010, 22, 4044−4048. (17) Ribierre, J. C.; Watanabe, S.; Matsumoto, M.; Muto, T.; Hashizume, D.; Aoyama, T. Thickness Dependence of the Ambipolar Charge Transport Properties in Organic Field-Effect Transistors Based on a Quinoidal Oligothiophene Derivative. J. Phys. Chem. C 2011, 115, 20703−20709. (18) Ribierre, J. C.; Satoh, M.; Isizuka, A.; Tanaka, T.; Watanabe, S.; Matsumoto, M.; Matsumoto, S.; Uchiyama, M.; Aoyama, T. Organic Field-Effect Transistors Based on J-Aggregate Thin Films of a Bisazomethine Dye. Organ. Electron. 2012, 13, 999−1003. (19) Palermo, V.; Palma, M.; Samorì, P. Electronic Characterization of Organic Thin Films by Kelvin Probe Force Microscopy. Adv. Mater. 2006, 18, 145−164. (20) Chiesa, M.; Bu1rgi, L.; Kim, J.; Shikler, R.; Friend, R. H.; Sirringhaus, H. Correlation between Surface Photovoltage and Blend Morphology in Polyfluorene-Based Photodiodes. Nano Lett. 2005, 5, 559−563.
Figure 8 shows (a) optical microscope, (b) green upconversion luminescence (c) red upconversion luminescence, (d) fluorescence, and (e) SEM images of the patterned RE-1 nanoparticle films fabricated on PET sheets with variation in the number of the cycles of the injection and drying of the nanoparticle dispersion. Optical and SEM images show that the density of nanoparticles in the RE nanoparticle films increases with an increase in the number of the cycles. Intensities of the upconversion luminescence and fluorescence also increase with an increase in the number of the cycles. These results demonstrate that the patterned RE nanoparticle films can be fabricated on the PET sheets. Adjusting the number of the cycles enables us to control the density of nanoparticles in the RE nanoparticle films. Further studies are necessary to fabricate patterned RE nanoparticle films with a large density of nanoparticles on PET sheets in one step.
4. CONCLUSION We demonstrate the direct fabrication of patterned RE nanoparticle films on glass plates and plastic sheets. Visible upconversion luminescence of Er3+, and near-infrared fluorescence of Er3+ and Yb3+ were observed from the RE nanoparticle films. The RE nanoparticle films consist of almost a single layer of the RE nanoparticles supported by SEM. Adjusting the fabrication conditions such as the concentration, the number of injection and drying cycles of the RE nanoparticle dispersion, the channel size of the PDMS molds, and the surface wettability of the substrates allows for the control of the density of nanoparticles and the width of the RE nanoparticle films. This technique enables us to directly fabricate patterned RE nanoparticle films on plastic sheets, leading to the fabrication of flexible inorganic phosphor devices with long lifetime operation. Adjusting the chemical species and concentration of the doped rare earth ions enables us to control the emission intensity of the RE nanoparticles. Direct patterning of RE nanoparticle films decreases fabrication time and material consumption. Nanopatterned RE nanoparticle films will be fabricated by combining our technique with polymer beads templates60 or with patterned water droplets on a wettabilitypatterned surface.61 To fabricate high efficient devices, deposition of visible fluorescence or near-infrared or lower conversion RE phosphor films on plastic sheets is required. These techniques allow for the future creation of flexible devices fabricated with functional nanoparticles.
■
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS This study is partly supported by JSPS KAKENHI (Grant Number 24750185) and Futaba Electronics Memorial Foundation.
■
REFERENCES
(1) Yu, G.; Gao, J.; Hummelen, J. C.; Wudl, F.; Heeger, A. J. Polymer Photovoltaic Cells: Enhanced Efficiencies via a Network of Internal Donor−Acceptor Heterojunctions. Science 1995, 270, 1789−1791. (2) Ma, W.; Yang, C.; Gong, X.; Lee, K.; Heeger, A. J. Thermally Stable, Efficient Polymer Solar Cells with Nanoscale Control of the F
dx.doi.org/10.1021/la401810x | Langmuir XXXX, XXX, XXX−XXX
Langmuir
Article
Ce3+, Tb3+) Nanocrystalline Thin Films. J. Mater. Chem. 2003, 13, 1413−1419. (42) Wang, W.; Cheng, Z.; Yang, P.; Hou, Z.; Li, C.; Li, G.; Dai, Y.; Lin, J. Patterning of YVO4:Eu3+ Luminescent Films by Soft Lithography. Adv. Funct. Mater. 2011, 21, 456−463. (43) Pang, M. L.; Lin, J.; Cheng, Z. Y.; Fu, J.; Xing, R. B.; Wang, S. B. Patterning and Luminescent Properties of Nanocrystalline Y2O3:Eu3+ Phosphor Films by Sol−Gel Soft Lithography. Mater. Sci. Eng., B 2003, 100, 124−131. (44) Pang, M. L.; Lin, J.; Fu, J.; Xing, R. B.; Luo, C. X.; Han, Y. C. Preparation, Patterning and Luminescent Properties of Nanocrystalline Gd2O3:A (A = Eu3+, Dy3+, Sm3+, Er3+) Phosphor Films via Pechini Sol−Gel Soft Lithography. Opt. Mater. 2003, 23, 547−558. (45) Watanabe, S.; Hyodo, H.; Taguchi, H.; Soga, K.; Takanashi, Y.; Matsumoto, M. Calcination- and Etching-Free Photolithography of Inorganic Phosphor Films Consisting of Rare Earth Ion Doped Nanoparticles on Plastic Sheets. Adv. Funct. Mater. 2011, 21, 4264− 4269. (46) Watanabe, S.; Hyodo, H.; Taguchi, H.; Soga, K.; Takanashi, Y.; Matsumoto, M. Calcination-Free Liftoff Photolithography of Mixed Dip-coated Films Consisting of Rare-Earth-Ion-Doped Nanoparticles on Plastic Sheets. J. Oleo Sci. 2012, 61, 565−573. (47) Gamelin, D. R.; Güdel, H. U. Upconversion Processes in Transition Metal and Rare Earth Metal Systems. Top. Curr. Chem. 2001, 214, 1−56. (48) Auzel, F. Upconversion and Anti-Stokes Processes with f and d Ions in Solids. Chem. Rev. 2004, 104, 139−174. (49) Kim, E.; Xia, Y.; Whitesides, G. M. Two- and ThreeDimensional Crystallization of Polymeric Microspheres by Micromolding in Capillaries. Adv. Mater. 1996, 8, 245−247. (50) Wright, J. C.; Fong, F. K.; Miller, M. M. Multiphonon Orbit Lattice Relaxation and Quantum Efficiency of 10μ Infrared Quantum Counter Upconversion in Rare Earth Doped Crystals. J. Appl. Phys. 1971, 42, 3806−3811. (51) Feldmann, C.; Jüstel, T.; Ronda, C. R.; Wiechert, D. U. Quantum Efficiency of Down-Conversion Phosphor LiGdF4:Eu. J. Lumin. 2001, 92, 245−254. (52) Xia, Y.; Whitesides, G. M. Soft Lithography. Angew. Chem., Int. Ed. 1998, 37, 550−575. (53) Kim, E.; Xia, Y.; Whitesides, G. M. Micromolding in Capillaries: Applications in Materials Science. J. Am. Chem. Soc. 1996, 118, 5722− 5731. (54) Kim, E.; Xia, Y.; Whitesides, G. M. Polymer Microstructures Formed by Molding in Capillaries. Nature 1995, 376, 581−584. (55) Duana, C.; Karnik, R.; Lu, M. C.; Majumdar, A. EvaporationInduced Cavitation in Nanofluidic Channels. Proc. Nat. Acad. Sci. U.S.A. 2012, 109, 3688−3693. (56) Duineveld, P. C.; Lilja, M.; Johansson, T.; Inganäs, O. Diffusion of Solvent in PDMS Elastomer for Micromolding in Capillaries. Langmuir 2002, 18, 9554−9559. (57) Li, Z. Critical Particle Size Where the Stokes-Einstein Relation Breaks Down. Phys. Rev. E 2009, 80, 061204. (58) Ransofoff, T. C.; Radke, C. J. Laminar Flow of a Wetting Liquid along the Corners of a Predominantly Gas-Occupied Noncircular Pore. J. Colloid Interface Sci. 1988, 121, 392−401. (59) Mullen, T. J.; Zhang, M.; Feng, W.; El-khouri, R. J.; Sun, L.; Yan, C.; Patten, T. E.; Liu, G. Fabrication and Characterization of RareEarth-Doped Nanostructures on Surfaces. ACS Nano 2011, 5, 6539− 6545. (60) Zhang, F.; Deng, Y.; Shi, Y.; Zhang, R.; Zhao, D. Photoluminescence Modification in Upconversion Rare-Earth Fluoride Nanocrystal Array Constructed Photonic Crystals. J. Mater. Chem. 2010, 20, 3895−3900. (61) Si, H.; Yuan, D.; Chen, J.; Chow, G.; Zhang, H. Facile Patterning of Upconversion NaYF4:Yb, Er Nanoparticles. J. Colloid Interface Sci. 2011, 353, 569−573.
(21) Zhang, S.; Shen, Y.; Fang, H.; Xu, S.; Song, J.; Wang, Z. L. Growth and Replication of Ordered ZnO Nanowire Arrays on General Flexible Substrates. J. Mater. Chem. 2010, 20, 10606−10610. (22) Forrest, S. R. The Path to Ubiquitous and Low-Cost Organic Electronic Appliances on Plastic. Nature 2004, 428, 911−918. (23) Downing, E.; Hesselink, L.; Ralston, J.; Macfarlane, R. A ThreeColor, Solid-State, Three-Dimensional Display. Science 1996, 273, 1185−1189. (24) Wang, Q. H.; Bass, M. Photo-Luminescent Screens for Optically Written Displays Based on Upconversion of Near Infrared Light. Electron. Lett. 2004, 40, 987−989. (25) Ho, H. P.; Wong, W. W.; Wu, S. Y. Multilayer Optical Storage Disk Based on the Frequency Up-Conversion Effect from Rare-Earth Ions. Opt. Eng. 2003, 42, 2349−2353. (26) Richards, B. S.; Shalav, A. The Role of Polymers in the Luminescence Conversion of Sunlight for Enhanced Solar Cell Performance. Synth. Met. 2005, 154, 61−64. (27) Shalav, A.; Richards, B. S.; Trupke, T.; Kramer, K. W.; Gudel, H. U. Application of NaYF4: Er3+ Up-Converting Phosphors for Enhanced Near-Infrared Silicon Solar Cell Response. Appl. Phys. Lett. 2005, 86, 013505. (28) Shan, G. B.; Demopoulos, G. P. Near-Infrared Sunlight Harvesting in Dye-Sensitized Solar Cells via the Insertion of an Upconverter-TiO2 Nanocomposite Layer. Adv. Mater. 2010, 22, 4373−4377. (29) Strumpel, C.; McCann, M.; Beaucarne, G.; Arkhipov, V.; Slaoui, A.; Svrcek, V.; del Canizo, C.; Tobias, I. Modifying the Solar Spectrum to Enhance Silicon Solar Cell EfficiencyAn Overview of Available Materials. Sol. Energy Mater. Sol. Cells 2007, 91, 238−249. (30) Mialon, G.; Türkcan, S.; Dantelle, G.; Collins, D. P.; Hadjipanayi, M.; Taylor, R. A.; Gacoin, T.; Alexandrou, A.; Boilot, J. P. High Up-Conversion Efficiency of YVO4:Yb,Er Nanoparticles in Water down to the Single-Particle Level. J. Phys. Chem. C 2010, 114, 22449−22454. (31) Venkatachalam, N.; Saito, Y.; Soga, K. Synthesis of Er3+ Doped Y2O3 Nanophosphors. J. Am. Ceram. Soc. 2009, 92, 1006−1010. (32) Matsuura, D. Red, Green, and Blue Upconversion Luminescence of Trivalent-Rare-Earth-Ion-doped Y2O3 Nanocrystals. Appl. Phys. Lett. 2002, 81, 4526. (33) Miller, M. J.; Wei, S. H.; Parker, I.; Cahalan, M. D. Two-Photon Imaging of Lymphocyte Motility and Antigen Response in Intact Lymph Node. Science 2002, 296, 1869−1873. (34) Konishi, T.; Yamada, M.; Soga, K.; Matsuura, D.; Nagasaki, Y. PEG-Based Surface Modification on Upconversion Nanophosphors for Bio-Imaging under IR Excitation. J. Photopolym. Sci. Technol. 2006, 19, 145−150. (35) Eliseeva, S. V.; Bünzli, J. G. Lanthanide Luminescence for Functional Materials and Bio-Sciences. Chem. Soc. Rev. 2010, 39, 189− 227. (36) Eliseeva, S. V.; Bünzli, J. G. Lanthanide Luminescence for Functional Materials and Bio-Sciences. Chem. Soc. Rev. 2010, 39, 189− 227. (37) Holmes, P. J.; Loasby, R. G. In Handbook of Thick Film Technology; Electrochemical Publications; Ayr (UK), 1976; p 50. (38) Jang, J. E.; Gwak, J. H.; Jin, Y. W.; Lee, S. J.; Park, H.; Jung, J. E.; Lee, N. S. High Resolution Phosphor Screening Method for Full-Color Field Emission Display Applications. J. Vac. Sci. Technol., B 2000, 18, 1106−1110. (39) Jin, Y. W.; Jang, J. E.; Yi, W. K.; Jung, J. E.; Lee, N. S.; Kim, J. M.; Jun, D. Y.; Hong, J. P. Performance of Electrophoretic Deposited Low Voltage Phosphors for Full Color Field Emission Display Devices. J. Vac. Sci. Technol., B 1999, 17, 489−493. (40) Yu, M.; Lin, J.; Wang, Z.; Fu, J.; Wang, S.; Zhang, H. J.; Han, Y. C. Fabrication, Patterning, and Optical Properties of Nanocrystalline YVO4:A (A = Eu3+, Dy3+, Sm3+, Er3+) Phosphor Films via Sol−Gel Soft Lithography. Chem. Mater. 2002, 14, 2224−2231. (41) Yu, M.; Lin, J.; Fu, J.; Zhang, H. J.; Han, Y. C. Sol−Gel Synthesis and Photoluminescent Properties of LaPO4:A (A = Eu3+, G
dx.doi.org/10.1021/la401810x | Langmuir XXXX, XXX, XXX−XXX
