Electrospun Nanofibers for Optical Applications

CHAPTER

ELECTROSPUN NANOFIBERS FOR OPTICAL APPLICATIONS

19 Jianchen Hu, Ke-Qin Zhang

National Engineering Laboratory for Modern Silk, College of Textile and Clothing Engineering, Soochow University, Suzhou, Jiangsu, PR China

The optical applications of electrospun nanofibers should be separated from the applications of bulk materials or film materials, and lie in the unique optical properties of the nanofibers. These optical properties can be divided into two aspects: one is light stimulation causing a response in other properties, like conductivity; and the second is other stimulants causing optical property change, like transparency, fluorescence, different colors, and color change. From the point of view of material fabrication, some factors greatly influence the properties, including well-known factors in the electrospinning process like solution concentration, applied voltage, temperature, moisture, distance between two electrodes, jetting speed, and so on. Further treatment to the materials before or after electrospinning sometimes decides the final properties of the electrospun nanofibers. Based on this, electrospun nanofibers for special optical applications can be divided in three types: pristine polymer nanofibers; doped nanofibers; and treated nanofibers. For the first type, the optical properties depend on the pristine properties of the electrospun nanofibers. For the second type, the optical properties come mainly from the dopants, and the nanofibers play an important role in offering a one-dimensional (1D) template and confinement. As for the third type, the spun nanofibers need to be further treated by pyrolysis, water washing, or sputtering aim to remove organic components or introduce new components into the 1D structure. This chapter discusses electrospun nanofibers fabricated with these three methods, and the corresponding potential optical applications.

19.1 OPTICAL PROPERTIES OF PRISTINE ELECTROSPUN NANOFIBERS AND THE CORRESPONDING OPTICAL APPLICATIONS There is little research focusing on the optical properties or optical applications of pristine electrospun nanofibers containing only the polymer component, because most of the polymers which can be used as electrospinning materials do not have any special optical properties, such as transparency, fluorescence, controllable absorbance, and so on. Attempts have been made to rectify this lack and try to obtain electrospun nanofibers with unique optical properties. One typical example is modifying the polymer before carrying out the electrospinning process. Samuelson et al. first proposed using the electrospinning technique to fabricate nanofibrous membranes for highly responsive optical sensors based on the fluorescence quenching of the modified electrospinning materials. They chose pyrene Electrospinning: Nanofabrication and Applications. https://doi.org/10.1016/B978-0-323-51270-1.00019-4 Copyright © 2019 Elsevier Inc. All rights reserved.

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FIGURE 19.1 (A) Scheme for attaching PM to PAA. (B) Fluorescence quenching related to the concentration of Fe3þ. Reproduced with permission from Wang, X., Drew, C., Lee, S.H., Senecal, K.J., Kumar, J., Samuelson, L.A., 2002. Nano Letters 2, 1273. Copyright © 2002, American Chemical Society.

methanol (PM) as the fluorescent indicator: poly(acrylic acid) (PAA) was bonded to PM with covalent attachment (Fig. 19.1A), and the PMePAA copolymer was mixed with thermally cross-linkable polyurethane latex to form the electrospinning solution. PM was selected because it is an electronrich indicator which has a potential to react with electron-deficient quenchers, such as metal cations or nitroaromatic compounds. Fluorescence quenching when the sample was located in an atmosphere of ferric ion (Fe3þ), mercuric ion (Hg2þ), or 2,4-dinitrotoluene (DNT) proved that the optical sensor works. The sensor has two characteristics. First, compared to film with the same chemical components, the electrospun nanofiber sensor has higher sensitivity, which is believed to be attributed to the higher surface area; the nanofiber membrane has a higher surface area by one or two orders of magnitude, but the sensitivity was enhanced by two or three orders of magnitude. Secondly, when the concentration of metal cation increases, the intensity of fluorescence decreases (Fig. 19.1B); so the essence of this optical sensor is chemical sensing (Wang et al., 2002). Another visualized example for the optical properties of pristine electrospun nanofibers is the color displayed under confocal laser scanning microscopy (CLSM). Bhaskar and Lahann, (2009) designed a cojetting electrospinning system to prepare multicompartmental nanofibers by using two or more poly(lactide-co-glycolide) polymers. The fibers aggregate as bundles, but the boundaries are very clear under CLSM since each compartment has a different chemical composition. They suggested using the nanofibers and this technology in some biological applications; the color separation and overlap aspects may also be useful in the field of display (Fig. 19.2) (Bhaskar and Lahann, 2009).

19.2 OPTICAL PROPERTIES OF DOPED ELECTROSPUN NANOFIBERS AND THE CORRESPONDING OPTICAL APPLICATIONS Beyond the optical properties of pristine polymer nanofibers, unique optical properties can be obtained by doping the nanofibers with photosensitive chemicals. In this state, the electrospun nanofibers act as a 1D chemical holder in which the chemicals disperse in the nanofibers, and the properties mainly originate from the doping chemicals.

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FIGURE 19.2 (A) Illustration of a cojetting system in which two or more multicompartmental microfibers can be fabricated by adding outlet streams (gray). (B) CLSM image of bicompartmental microfiber bundles. (C) Cross-sectional CLSM images of multicompartmental microfibers with clear boundaries. Scale bars in (B) and (C) represent 20 mm. Reproduced with permission from Bhaskar, S., Lahann, J., 2009. Journal of the American Chemical Society 131, 6650. Copyright © 2009, American Chemical Society.

19.2.1 PHOTOCHROMIC MOLECULE DOPED ELECTROSPUN NANOFIBERS FOR THE APPLICATION OF PHOTOSWITCHING A direct application of doped electrospun nanofibers is a photoswitch, in which photoisomerization to photochromic molecules can be introduced by light of different wavelengths, showing color variation or photocoloration to photofading that can be observed even with the naked eye in normal light. Di Benedetto et al. fabricated smart photoswitchable nanofibers by the electrospinning method, and embedded 10 ,30 -dihydro-10 ,30 ,30 -trimethyl-6-nitrospiro[2H-1-benzopyran-2,20 -(2H)indole] (6-NO2-BIPS) molecules into a poly(methyl methacrylate) (PMMA) matrix. The doped chemical has two molecular photochemical states, closed and open. The closed state can be obtained by exposing the molecule to light with a wavelength >530 nm, and the composite nanofibers appear colorless, so this state is also known as the off state. When the nanofibers are exposed to light with a wavelength <400 nm, the open form of the doped molecules emerges: the nanofibers show a pink color, and this is defined as the on state (Fig. 19.3A). The absorption reaches maximum intensity in a very short time, around 0.54 ms, so the on/off state of the photoswitch can be adjusted frequently (Fig. 19.3B). The electrospun nanofibers have another photoswitchable property, reversibly switchable wettability; 355 nm ultraviolet (UV) light makes the contact angle vary from 108 degrees to 91 degrees, whereas the film counterpart shows only a very small variation of 73 degrees to 71 degrees (Di Benedetto et al., 2008). Khatri et al. fabricated UV-response photoswitchable nanofibers with the electrospinning method and embedded 10 -30 -dihydro-8-methoxy10 ,30 ,30 -trimethyl-6-nitrospiro [2H-1-benzopyran-2,20 -(2H)-indole] (indole) and 3-dihydro-1,3, 3-trimethylspiro [2H-indole-2,30 -[3H] phenanthr [9,10-b] (1,4) oxazine] (oxazine) molecules into polyvinyl alcohol (PVA) nanofibers. The results revealed that the rate of photocoloration of PVAeindole nanofibers was five times greater than that of PVAeoxazine nanofibers, and thermal

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FIGURE 19.3 (A) Off (closed) state excited by ultraviolet (UV) light (<400 nm) and on (open) state excited by visible light (530 nm) of the doped organic molecule. (B) Different colors of the photoswitchable nanofibers showing the off form (left), excited form (center), and on form (right). Reproduced with permission from Di Benedetto, F., Mele, E., Camposeo, A., Athanassiou, A., Cingolani, R., Pisignano, D., 2008. Advanced Materials 20, 314. Copyright © 2007, John Wiley and Sons.

reversibility was found to be more than twice as fast as in PVAeoxazine nanofibers. The nanofibers achieved the capability of data recording QR code multiple times (Fig. 19.4). These UV-responsive PVA nanofibers have potential as light-driven nanomaterials incorporated in sensors, sensitive displays, and optical devices such as rewritable and erasable optical storage (Khatri et al., 2015).

19.2.2 DOPING ELECTROSPUN NANOFIBERS TO FORM FLUORESCENT SENSORS FOR NITROAROMATIC EXPLOSIVE DETECTION Section 19.1 introduced the application of electrospun nanofibers for detection of the nitroaromatic explosive DNT. Wang et al. reported an interesting low-cost system based on fluorescent electrospun nanofibers for detection of buried explosives and explosive vapors. The detecting material is composed of a mat of electrospun nanofibers (thickness 1e15 mm) synthesized by polystyrene (PS) doped with pyrene (an organic molecule showing a bright cyan emission under UV excitation). For explosive optical detection the use of thin fibers is most effective, since studies on films of conjugated polymers exposed to 2,4,6-trinitrotoluene (TNT) have evidenced that fluorescence quenching shows within the

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FIGURE 19.4 Electrospun PVAeindole nanofibers used for fast recording and erasing of QR code by meaning of adjusting the color contrast with the methods of UV exposure and heat treatment. Reproduced with permission from Khatri, Z., Ali, S., Khatri, I., Mayakrishnan, G., Kim, S.H., Kim, I.S., 2015. Applied Surface Science 342, 64. Copyright © 2015, Elsevier B.V. All rights reserved.

first few tens of nm, hence film with a thickness of >25 nm shows a drop in fluorescence quenching, probably related to the limited TNT diffusion (Wang et al., 2012). The emission of the resultant nanofibers (pyrene/PS) is characterized by two main bands: the first band in the range of 370e400 nm has been ascribed to the singlet exciton emission, while the second broad band at about 470 nm has been ascribed to the excimer emission due to possible p-p assembly. When nanofibers are exposed to equilibrium vapors (about 193 ppb) of 2,4-DNT, 90% quenching of the fluorescence is obtained within 6 min, with a slight dependence on the electrospun nanofiber mat thickness. Remarkably, this material can detect other nitroaromatic compounds (like 2,6-DNT and 1,2-dinitrobenzene), as well as nonsaturated vapors of nitramines and nitrate esters. The authors also note the possibility of detecting buried 2,4-DNT (Fig. 19.5AeD) in an open environment, and a handprint polluted with explosive particulates via direct interaction (Fig. 19.5E). The overall sensing performance of this simple and economical detection material is ascribed to amplification of the quenching mechanism due to the specific conformation of the pyrene molecule and the PS (Wang et al., 2012). For the more familiar explosive TNT, a new development for detection with electrospun nanofibers is that color change can be achieved by not only optical stimulation but also chemical response. Attempting to develop portable optical devices for nitroaromatic explosive detection, Liu et al. extended the potential

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FIGURE 19.5 (A) Photos of soil with (left) and without (right) buried 2,4-DNT in Petri dishes. (B) Photo of soil with buried DNT in a flowerpot. (C) Detection of buried DNT with PS nanofibers doped with pyrene by observing emission quenching caused by leaked explosive molecules from the sample in (A). (D) Detection of buried DNT with PS nanofibers doped with pyrene as the sample in (B). (E) Florescence comparison of pyrene/PS mats without (left) or with (right) contamination of 2,4-DNT. Reproduced with permission from Wang, Y., La, A., Ding, Y., Liu, Y., Lei, Y., 2012. Advanced Functional Materials 22, 3547. Copyright © 2012, John Wiley and Sons.

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application of electrospinning technology. They doped PS nanofibers with a small molecule of fluorophore tetrakis(4-methoxylphenyl)porphyrin; by introducing a porogen, the porosity of the nanofibers was much improved. The prepared nanofibers were used to detect nitroaromatic explosives and the result showed that the fluorescence quenching for different explosives was totally different, meaning good selectivity in detection. For TNT vapor, the sensitivity can be as great as a few parts per billion (Yang et al., 2011).

19.2.3 LASER DYE DOPED ELECTROSPUN NANOFIBERS FOR LASER EMISSION Li et al. studied the laser emission effect of electrospun nanofibers doped with laser dye. They doped PVA solution with rhodamine 6G (Rh6G) in different concentrations and electrospun the PVA/Rh6G composite nanofibers. As Rh6G is a fluorescent material, it was expected to harvest laser emissions when the composite nanofibers were excited by laser. Another expectation was that the generated laser emission could be propagated along the nanofibers. It was found that the concentration of the doped Rh6G and introduced pump energy influence two aspects: higher concentration corresponds to stronger emission because more Rh6G molecules increase the sensitivity; and when the concentration and/or pump energy is beyond a certain limit, the emission has a bathochromic shift of the peak which originates from the overlap of absorption and emission, the dye molecules irradiated fluorescence reabsorbed by the unexcited molecules, causing the peak to shift to long wavelength. A uniform phase with a refractive index of 1.5 (higher than air) was formed by monodispersion of Rh6G molecules in the PVA, which made it possible for light propagation in the nanofibers (Huang et al., 2017).

19.2.4 DOPANT MODIFYING THE END OF POLYMER CHAINS TO FORM FLUORESCENT AGGREGATION-INDUCED-EMISSION ACTIVE POLYMERS FOR DETECTION OF OIL ABSORPTION Normally, in doped electrospun nanofibers the dopants are physically mixed in the electrospinning solution. For example, fluorescent nanofibers are normally electrospun with polymers doped with fluorophores, like small fluorescent molecules (Camposeo et al., 2007, 2009) or fluorescent quantum dots (Lu et al., 2005; He et al., 2012). But during the process of preparing the solution, the dispersed dopants tend to aggregate, leading to nonuniform distribution of dopant in the nanofibers and nonuniformity of fluorescence. Yuan et al. developed a chemical doping process to overcome this problem, synthesizing an aggregation-induced-emission (AIE) active polymer by anchoring AIEactive initiators to the end of polymer chains through atom transfer radical polymerization (Fig. 19.6A and B). After electrospinning, a porous yellow nanofiber membrane with yellow-green fluorescence was obtained (Fig. 19.6C). Absorption of oil can switch off the fluorescence by a factor of 7.5 and desorption of oil can switch the fluorescence on again (Fig. 19.6D); this is because the absorption-induced swelling releases the spatial constraint of intramolecular rotation, and the absorbed energy is released with kinetic energy. Compared to solid nanofibers, the porous nanofibers in this study had 100 times higher specific surface area and absorbed three times as much oil, and the solid nanofibers quench only 20% when absorbing oil in a saturated state. On the other hand, physical mixing of the fluorescence molecules and the polymer when preparing nanofibers causes the nanofibers to show almost no quenching (Yuan et al., 2014).

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N

(A) N

O Br

(B)

N

MMA

O

Br n

CuBr/PMDETA initiator

O

O

N O

(C)

TPP-NI

PMMA

O

(D)

V

FIGURE 19.6 (A) Formula of AIE-active initiator. (B) Scheme of anchoring AIE-active initiators to the end of a polymer chain. (C) Illustration of the electrospinning process; the inset image shows the yellow color (dark gray in print version) of the fluorescent porous fibers. (D) Different fluorescence of PMMA porous fibers switched by oil adsorption and desorption, excited by 365 nm UV light. Reproduced with permission from Yuan, W., Gu, P.Y., Lu, C.J., Zhang, K.Q., Xu, Q.F., Lu, J.M., 2014. RSC Advances 4, 17255. Copyright © 2014, Royal Society of Chemistry.

19.2.5 DOPING ELECTROSPUN NANOFIBERS WITH METAL NANOCLUSTERS FOR SELECTIVE HEAVY METAL DETECTION The heavy metals in the human body and/or in the environment can be highly alarming for humankind, and recently many efforts have been made to develop reliable and sensitive techniques for their detection. Metal nanoclusters (MNCs) are composed of several to tens of atoms, and have drawn considerable research interest due to their unique electrical, physical, and optical properties. MNCs are emerging as a very promising analytical platform for diverse sensing applications, especially selective sensing of heavy metal ions. Among the detection methods, optical active nanofibers appear to be promising carriers for MNCs in applications for real-time selective detection of heavy metals. Senthamizhan et al. developed high fluorescence and flexible gold nanoclusters (AuNCs) with a PVA nanofiber membrane (NFM) for selective mercury (Hg2þ) detection from water. The final color change

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FIGURE 19.7 Schematic diagram of the process causing selective fluorescence changes in an AuNCs/PVA NFM. Reproduced with permission from Senthamizhan, A., Celebioglu, A., Uyar, T., 2014. Journal of Materials Chemistry 2, 12717. Copyright © 2014, Royal Society of Chemistry.

linked with selective coordination of Hg2þ has demonstrated trouble-free “naked eye” colorimetric sensing (Fig. 19.7). The selectivity of the AuNCs with the PVA NFM has been shown by the response to other toxic heavy metals (lead ions Pb2þ, manganese ions Mn2þ, nickel ions Ni2þ, zinc ions Zn2þ, cadmium ions Cd2þ) in water. Furthermore, the contact mode approach has been taken into consideration for the visual fluorescent response to Hgþ, and a detection limit of 100 ppb to 1 ppm was achieved. The very promising features of high stability, flexibility, sensitivity, and selectivity have emphasized the utility of the sensor, indicating its practical applications in environmental monitoring of toxic mercury (Senthamizhan et al., 2014).

19.3 OPTICAL APPLICATIONS OF ELECTROSPUN NANOFIBERS WITH FURTHER TREATMENT Few options exist for fabrication of 1D nanostructures combining the advantages of low cost, high efficiency, and convenient assembly. Electrospinning is one method which allows fabrication of continuous fibers with diameters down to a few to hundreds of nanometers (Greiner and Wendorff, 2007). Because it is easy to dope precursors in the spinning polymer solution, and later treatment can remove the polymer component which is used as the 1D template, no other catalysts or templates are needed in this method (Wu et al., 2009). Using the dopingespinningeaftertreatment process, the doped chemicals mixed in the spinning solution can be maintained/modified in a 1D structure along with the release of gases obtained from polymer decomposition. Especially for inorganic materials with special optical properties, this method offers a new possibility for material fabrication.

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19.3.1 OPTICAL PROPERTIES OF ELECTROSPUN TiO2 NANOFIBERS TiO2 is a very popular material that can be used in many areas, and in some fields its optical properties have significant influence on the applications. In the early years research on TiO2 optical properties was limited to bulk material and nanoparticles. Especially at the nanometer scale, it was found that when the size of the nanoparticles increases from several nanometers to tens of nanometers, the absorption shifts to long wavelength. This is because the majority of the atoms congregate at the particle surface as the particle size gets smaller and smaller, and the surface atoms dominant the optical band gap; this is known as the quantum confinement effect (Anpo et al., 1987; Serpone et al., 1995). A new discovery about the optical properties of TiO2 nanofibers was made by Ramakrishna et al. (Kumar et al., 2007). They used electrospinning technology followed by sintering to prepare TiO2 nanofibers with a diameter of 60e150 nm. In the absorption spectra a red shift of around 15 nm was observed when the diameter of the fiber increased from 60 to 150 nm, but no shift was observed in the emission spectra. It is easy to understand the unchanged emission of different TiO2 nanofibers with different diameters, as the crystal grains were the same size, around 12 nm, which gives the same quantum confinement effect. Obviously, the red shift along the increase of the fiber diameter in the absorption spectra is not supposed to be explained by the size quantization effect. Ramakrishna et al. proposed that the change of optical band gap depends on the quantum confinement effect, and surface energy resulted in the surface effect of the nanofibers. The quantum confinement effect and the surface effect of the nanofibers are to be considered in other inorganic electrospun nanofibers for optical applications beyond TiO2.

19.3.2 ELECTROSPUN GaN, ZnO NANOFIBERS, AND APPLICATION IN UV DETECTORS As a typical example, 1D structural inorganic gallium nitride (GaN) nanowires have great potential to be used in nanometer-scale UV photodetectors due to their sensitivity to UV light (Zhong et al., 2003). In the traditional synthesis route, GaN nanowire is typically grown using a vaporeliquidesolid (VLS) mechanism with the introduction of catalysts such as Ni, Fe, Au, or In. It is difficult to control the particle size and location of these catalysts in the nanowires, so some nondeterminacy of the nanowire properties may be introduced during the synthesis process (Han et al., 1997; Chen et al., 2003; Duan and Lieber, 2000). Pan et al. electrospun a mixture solution of poly(vinyl pyrrolidone) (PVP) and gallium nitrate to form precursor composite nanofibers which were then calcined in air to remove the PVP. The formed gallium oxide nanofibers were then converted into GaN in an ammonia atmosphere at high temperature. Adoption of catalysts is avoided in this method (Wu et al., 2009). More importantly, the GaN nanowires (around 40 nm in diameter) prepared with electrospinning technology showed very good photoconductance response, even better than single crystalline GaN nanowires. When a UV light with a wavelength of 254 nm and power density of 3 mW/cm2 was turned on, the conductance was enhanced by 830 times, and when the UV light was turned off the current dropped to the initial value (Fig. 19.8A). As a comparison, GaN nanowires synthesized by the chemical vapor deposition method showed a conductance improvement of about 78 times under the same conditions (Han et al., 2004). The electrospun GaN nanofiber also showed good response speed and reversibility in a fabricated UV photo sensor. Similarly, UV nanosensers based on zinc oxide (ZnO) nanofibers were fabricated with the electrospinning technique by Zhu et al. (2009). They firstly prepared salt/polymer composite nanofibers by electrospinning a solution of zinc acetate and PVP in

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FIGURE 19.8 (A) UV detector conductance performance of electrospun GaN nanofibers. (B) UV detector conductance performance of electrospun ZnO nanofibers under different intensities of UV light. (C) Comparison of performances of UV detectors made of pristine ZnO and electrospun ZnO nanofibers, respectively. Reproduced with permission from Wu, H., Sun, Y., Lin, D.D., Zhang, R., Zhang, C., Pan, W., 2009. Advanced Materials 21, 227; Zhu, Z.T., Zhang, L.F., Howe, J.Y., Liao, Y.L., Speidel, J.T., Smith, S., Fong, H., 2009. Chemical Communications 2568. Copyright © 2008, John Wiley and Sons and 2009, Royal Society of Chemistry.

dimethyl formamide; after a pyrolysis process at 500 C to remove the polymer component, ZnO nanofibers with diameter of around 200 nm were obtained. The UV nanosensors were highly sensitive to the intensity of UV irradiation: the current increased along with the increased intensity of the UV light, especially at high bias (Fig. 19.8B). Compared to pristine ZnO nanofibers, the electrospun ZnO nanofiber device showed an improved sensitivity to UV light by a factor of 1.6 enhancement (Fig. 19.8C).

19.3.3 ELECTROSPUN TRANSPARENT ELECTRODE FOR THE APPLICATION OF SOLAR CELLS AND PHOTOSENSORS For many optical devices, especially photoelectronic devices, transparent electrodes are needed. They play the important role of collecting current and letting light transmit to or release out of the function zone, such as in solar cells, light-emitting diodes, and photosensors. The currently most used transparent electrode is indium tin oxide (ITO), but the low abundance of indium makes ITO more and more expensive, and the brittleness of ITO means it is far from an ideal material for transparent electrodes, especially for flexible devices. Exploring new materials and suitable technologies is meaningful in this context. Cui et al. contributed in this field by extending electrospinning technology into metal nanofiber synthesis. Copper acetate and PVA were mixed in solution for electrospinning, and the spun precursor nanofibers were heated at 500 C in air for 2 h to remove all the polymer components and form CuO nanofibers. Following reduction in an H2 atmosphere at 300 C, Cu nanofibers were obtained (Fig. 19.9A). Compared to sputtered Cu film, the electrospun Cu nanofibers showed much higher flexibility since the high aspect ratio (above 10,000) of the nanofibers means it is easy to keep a continuous network structure to maintain conductance when introducing/releasing the strain (Fig. 19.9B). The good conductivity is mainly attributed to the fused structure, which is of benefit in lowering contact resistance (Fig. 19.9C). Considering the demands of transparency, the Cu nanofiber network is an attractive candidate since it shows uniform low absorption and reflection in the range of

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FIGURE 19.9 (A) Illustration of the preparation process for Cu nanofibers: electrospinning CuAc2/PVA composite fibers, calcinations for getting CuO nanofibers, and reduction in H2 for obtaining Cu nanofiber web. (B) Tolerance of transparent electrodes based on electrospun Cu nanofiber networks to bending degree comparing to sputtered Cu films on PDMS substrates. (C) Schematic of junctions of electrospun Cu nanofibers (below), which are different from those of solution-processed Ag nanowires (top). (D) Transmittance spectrum of electrospun Cu nanofiber webs with different sheet resistances in the range of 300e1100 nm compared to that of ITO on glass. Reproduced with permission from Wu, H., Hu, L.B., Rowell, M.W., Kong, D.S., Cha, J.J., McDonough, J.R., Zhu, J., Yang, Y., McGehee, M.D., Cui, Y., 2010. Nano Letters 10, 4242. Copyright © 2010, American Chemical Society.

300e1100 nm. As the near-infrared range offers a significant amount of solar energy, transmittance in this region is very important for solar cells (Fig. 19.9D). The electrospun Cu nanofiber electrode showed a better performance than ITO electrodes (Wu et al., 2010). To fabricate a ZnO-based photodetector, an all-electrospinning process was developed by Pan et al. Except for the ZnO component prepared by electrospinning, the transparent electrodes were created by further treatment of electrospun nanofibers. Firstly, a polyvinyl butyral (PVB) to ethanol solution was electrospun to get a freestanding PVB network on a collector plate, followed by sputtering of Pt on the collected PVB web to form a designed pattern by using a shadow mask. A thermal treatment was employed for the final forming of Pt network electrodes. The ZnO component was fabricated by the process described in Section 19.3.2, but the spun nanofibers were collected with a special collector to make the nanofibers align in uniaxial arrays. After annealing in air to remove the PVP and decompose zinc acetate into ZnO, the prepared Pt network electrodes were transferred to the aligned ZnO array. The assembled device was annealed for final formation of the photodetector. The advantages of the

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photodetector are that except for the good conductivity, the device maintained flexibility and transparency. Compared to other electrode material candidates, the Pt network electrode in this work showed similar or even better conductivity. But the good flexibility meant the device had lower conductivity recession even after hundreds of bending/releasing cycles. More importantly, the outstanding transparency compared to other electrode materials gives much lower light loss during the light detection process (Wang et al., 2017).

19.3.4 FURTHER TREATMENT OF ELECTROSPUN NANOFIBERS FOR CONSTRUCTION OF STRUCTURAL COLOR Inspired by the structural colors from nature, attempts were made to mimic a dye-free dyeing technique. Poly(styrene-methyl methacrylate-acrylic acid) (P(St-MMA-AA)) spheres were monodispersed in a solution of PVA and water at a high concentration of 40%. The prepared composite solution was directly used for electrospinning nanofibers, forming a nonwoven fabric. The P(St-MMA-AA) spheres compactly aggregated along the nanofiber long axial, showing a hexagonal order at the surface of the nanofibers (Fig. 19.10A). But the nonwoven showed colorless state, which

FIGURE 19.10 (A) SEM image of spun PVA/P(St-MMA-AA) nanofibers with spheres arranging at the nanofiber surface in hexagonal order. (B)e(D) Optical images showing different colors of water-washed nanofibers with sphere diameters of 220, 246, and 280 nm. (E) Reflective spectra of water-washed nanofibers compare to flat photonic crystal film (inset image) with different incident angles. (F) Incident angle dependence of dark-field optical images of water-washed nanofibers (lower panel) compare to that of flat photonic crystal film (upper panel). Reproduced with permission from Yuan, W., Zhou, N., Shi, L., Zhang, K.Q., 2015. ACS Applied Materials & Interfaces 7, 14064. Copyright © 2015, American Chemical Society.

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CHAPTER 19 ELECTROSPUN NANOFIBERS FOR OPTICAL APPLICATIONS

might be because the P(St-MMA-AA) absorbs only 30% of visible light and the refractive indexes of the two polymers are very close. To form an interface with higher refractive index contrast, the researchers tried to remove the PVA and maintain the P(St-MMA-AA) spheres in the 1D structure by water washing the PVA/P(St-MMA-AA) nanofibers so that an interface between P(St-MMA-AA) spheres and air was formed. Interestingly, vivid structural color was observed, and can be tuned by using P(St-MMA-AA) spheres with different diameters. When researchers changed the P(St-MMA-AA) sphere diameter from 220 nm to 246e280 nm, the corresponding structure color changed from green (Fig. 19.10B) to red (Fig. 19.10C) to purplish red (Fig. 19.10D). For normal applications the dyed surfaces are supposed to show a stable color; in other words, the observed color should be independent of the incident angle and observing angle, which is called noniridescent color. The water-treated electrospun nanofibers showed uncontrovertible noniridescent color. The reflectance peak did not show any shift even when changing the observing angle from 0 degrees to 40 degrees (Fig. 19.10E). In comparison, the P(St-MMA-AA) spheres formed a photonic crystal film which reflected light strongly depending on the observing angle (Fig. 19.10F), which is defined as iridescent color. The significant difference originated from the microstructure of the photonic crystal film and the water-treated spun nanofibers. The arrangement of the P(St-MMA-AA) spheres in photonic crystal film is in long-range order, so the film is constructed by crystalline photonic crystals with isotropy. In contrast, the P(St-MMA-AA) spheres in water-treated nanofibers are in short-range order and long-range disorder, so the nonwoven fabric is constructed by amorphous photonic crystals. The synergistic effect of the photonic band gap related to the order arrangement and Mie scattering related to the disorder arrangement results in structural noniridescent color. This work opens a door for developing a dye-free dyeing technology using electrospinning (Yuan et al., 2015).

19.4 SUMMARY In summary, the optical applications of electrospun nanofibers depend on the nanofiber properties. The properties are strongly related to the materials and fabrication processes. From a fabrication point of view, electrospinning can be divided in three types: spinning pure polymers; spinning doped polymers; and further treatment of spun nanofibers. Doping includes both physical and chemical doping. For physical doping, it is important for the dopants to disperse uniformly in the polymer matrix with good compatibility, and avoid chemical interactions between dopants and solvents or polymers. For further treatment after spinning, treatment methods are selective based on the required properties. Normally, for thermal treatment the dopants or the pyrolysis products are the desired end result, so the decomposition temperature for the polymer should be lower than that of the dopant pyrolysis. To simplify the relationship of the fabrications and applications, the applications are related to the materials with or without treatment, before or after the electrospinning process.

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