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Recently, some groups have made nanofiber linear polarizers as a replacement for traditionally made scattering polarizers. Current scattering polarizers are made from thermally drawn polymer-thin films doped with birefringent dye [ ]. The thermal drawing of the poly vinyl alcohol PVA preferentially aligns the polymer chains and creates anisotropic properties within the film.
A similar effect has been shown by using the anisotropic properties of aligned ES fibers. An example of aligned polymer fibers is shown in Figure 6. Many of the polarizers made require the fiber mats to be surrounded in a matrix material to reduce the amount of scattering that occurs with uncoated fiber mats. The first use of electrospun fibers in a polarizer was shown by Katta et al.
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Electrospun PMMA fibers were aligned by using a rotating drum and had a diameter of nm with a mat thickness of 5. The collected fibers were then transferred to a glass slide with two intended functions. First, the PMMA fibers would act as a linear polarizer.
Second, the PMMA fibers would also act as a scaffold for liquid crystals to anchor and form a nematic structure. An optimized version of the liquid crystal polarizer achieved a polarizer efficiency of 0. Recent work with electrospun fiber polarizers has used PVA fibers embedded in a polymer matrix to form a thin, flexible film.
The PVA fibers are collected on a rotating drum and then transferred to a polymer solution.
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PVP dissolved in ethanol [ ] and PMMA dissolved in acetone [ ] have both been studied as the polymer solution used to create the electrospun fiber polarizer. These electrospun fiber polarizers show a significant improvement in polarizer efficiency over the current thermally drawn thin films while maintaining a simple and economical process. Improvements in this area could be accomplished through fabrication methods that do not require fibers to be physically handled during transfer.
Recently, alignment onto double-side polished silicon substrates to be used directly as a masking layer for lift off processing has been accomplished [ ]. Lack of fiber handling used in this process offers promising results for future fabrication of polarizer devices using ES. The creation of light-emitting structures for device integration via ES is gaining popularity due to its versatile and straightforward nature.
Light-emitting materials can be easily integrated into spinnable polymeric solutions prior to the ES process, and polymer fibers can be directly deposited onto planar electrodes either randomly or with a high degree of alignment. Planar device architecture can also be adapted into a coaxial cylinder, which dramatically increases the interfacial area between layers in a given volume as compared to planar geometries.
Increased interfacial area allows for a greater carrier recombination rate and a subsequent improvement in device performance.
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In both electrospun fiber mats on planar devices and in coaxial fiber geometries, there are issues in forming adequate contacts to electrodes. In coaxial fibers, a common method of forming contacts to the core material is through the use of a conductive tip of an atomic force microscope [ ], which does not match well with scalable manufacturing.
In electrospun fiber mats on planar devices, the cylindrical fibers only contact electrodes tangentially leading to inefficient charge carrier injection and extraction. Vohra et al.
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This process created more uniformly luminescing fibers and negated shunting paths by allowing a phase separation and formation of a PEO thin film layer between F8BT islands. Because annealing is a simple process amendable to maintaining high-throughput in manufacturing applications, such results are promising for future research and development of efficient charge extraction in micro- and nano-fiber devices.
Results reveal a change in the current path through device layers. Reprinted with permission from Vohra et al. ACS Nano, 5, Unlike most multi-layered OLED devices, coaxially electrospun fibers see Figure 10 can be utilized to create light-emitting electrochemical cells LECs , which only require two materials and the light-emitting layer to function efficiently, unlike most multi-layered OLED devices. Yang et al. In this research, an indium-tin oxide was evaporated in two steps to coat the coaxial fiber and form the top electrode.
Utilization of LEC technology as opposed to OLED technology for fiber devices allows the use of electrodes that can operate in atmospheric conditions [ ]. Reprinted with permission from Yang et al.
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Even simpler, single-material fibers have been utilized to create light-emitting layers in planar devices. Moran-Mirabal et al. A simple interdigitated electrode array IDE was utilized as a collection surface that was mounted on a much larger rotating grounded electrode in order to deposit fibers across the IDE array. Deposition of light-emitting fibers on IDEs in this way allows for on-chip point sources of light to be created easily with emission activation voltage determined by the pitch of the IDE.
ES has also been employed for simple production of white-light-emitting devices. Conventional white light OLEDs are multi-stacked planar devices with multiple-light-emitting layers [ ] or phosphor down-conversion layers [ ] with highly efficient white organic light-emitting diodes. Kim et al. Typically, the use of multiple dyes is restricted by donor-acceptor interactions that effectively quench the light of the donor dye molecules, which are typically blue or green emitters.
ES with multiple nozzles was used by Kim et al.
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The result was an electrospun fiber mat that was optically tunable through control of dye concentration. Figure 11 shows the resultant electrospun fiber mat, where red Rhodamine B , green Coumarin 6 , and blue Anthracene dyes were doped into a PMMA carrier polymer. In bottom right corner of far right cartoons images, the observed emission under excitation is shown. Reprinted with permission from Kim et al. ACS Appl. The ES process can result in increased crystallinity of commonly used polymers [ ] through electrostatic elongation of the polymer chains along the fiber axis.
In highly aligned polymer fibers, photoluminescence from the fibers has been polarized as well, with polarization ratios from 13 [ ] to 4 [ ]. Emission is observed to be dependent on the direction of polarization. There has been increasing interest in efficient PSCs in recent years. PSCs are of particular interest due to their ease of fabrication, which often involves solution processable techniques resulting in robust and flexible light conversion structures and devices.
The use of ES to fabricate PSC structures works well with the desired scalable manufacturing end point. ES provides fabrication capabilities for PSCs through construction of nanostructured metal oxide electron transport layers ETL [ ], [ ], [ ], [ ]. Meso-structured metallic oxides, which provide a reasonable amount of surface area for exciton dissociation, can be used for this layer [ ]. Meso-structured metallic oxides also require facile and inexpensive spin-casting or spray pyrolysis fabrication. ETLs can be further optimized toward efficient exciton dissociation through the creation nanostructured metallic oxide wire networks.
As demonstrated by Li et al. The alkoxide is hydrolyzed in the polymer nanofiber to form an amorphous oxide. Resultant electrospun fiber mats are then calcined to remove the organic polymer and form polycrystalline metallic oxide nanowires [ ]. The method outlined by Li et al. Electrospun fibers have also to be used as templates for adhesion of metal chalcogenide nanoparticles to create large quasi-nanotube structures [ ].
Cortina et al. Active layers fabricated in this manner introduce an interesting technique to fabricate heterojunctions with a large degree of optical tunability dictated by the metal chalcogenide semiconductor used to coat the electrospun scaffold. Similar to these methods, there is also interest in creating quaternary chalcogenides via ES, followed by calcination [ ].
Ozel et al. Sundarrajan et al.
A blend of P3HT and phenyl-C 61 -butyric acid methyl ester PCBM was used as a core material to create a network of nanofibers with a heterojunction-like composition. The shell material was insulating PVP, used mainly as a carrier polymer in this case, which was removed with ethanol after ES.
A method used by Sundarrajan et al. Of particular interest are optical sensors.
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Electrospun fiber mats, especially those made from fibers with porous morphologies [ ], have high surface area-to-volume ratios, making them ideal for receptor structures in sensors. Paired with the ability to easily add a degree of optical functionality via doping with a fluorescence dye, electrospun fiber mats are well suited for optical sensing applications. Fluorescence quenching triggered by an analyte takes into consideration both the loss of intensity in the signal and decay time [ ].
Electrospun optical sensors based upon fluorescence quenching are generally as sensitive as traditional thin film sensors, but the high surface area of the electrospun sensing layers gives analyte molecules better access to fluorophores, thereby improving response dynamics of the sensor [ ]. Improved response dynamics of sensors allows for sensors with faster response times. Wolf et al. Electrospun sensing layers demonstrated a response time two orders of magnitude faster than compact thin film sensing layers of the same material.
Fluorescence quenching optical sensors are attractive for explosives detection as well. Nitro-substituted groups such as 2,4-dinitrotoluene DNT and 2,4,6-trinitrotoluene TNT act as effective quenching analytes. Through the use of novel fluorophores [ ] and conjugated polymers [ ], rapid response dynamics of fluorescence quenched electrospun optical sensors can be utilized for detection of explosive compounds. Xue et al. The lower driving force for energy transfer resulted in a much stronger quenching when exposed to DNT [ ].
Time versus quenching efficiency plots for this class of sensor appeared to follow a logarithmic trend and showed that a higher overall quenching value is desirable as it produces a stronger short-term response. ES has demonstrated an efficient and simplistic alternative to traditional top-down manufacturing of micro- and nano-scale photonic devices. Fibers produced by ES provide a high surface area-to-volume ratio and novel electromagnetic properties associated with the quantum size effect.
Barriers to widespread implementation of ES include control over spatial deposition of polymer fibers and complications derived from the dynamic electric field created on and near non-planar, topographic materials. Despite these drawbacks, alignment of the polymeric backbone of nanofibers produced by ES has allowed enhanced crystallinity for passive waveguides as well as optoelectronics, photovoltaics, and nanofiber light sources with color tunability.