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Nanophotonics

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Volume 6, Issue 5

Issues

Electrospinning for nano- to mesoscale photonic structures

Jack L. Skinner
  • Corresponding author
  • Mechanical Engineering, Montana Tech, Butte, MT 59701, USA
  • Montana Materials Science PhD Program, Montana Tech, Butte, MT 59701, USA
  • Montana Tech Nanotechnology Laboratory, Butte, MT 59701, USA
  • Email
  • Other articles by this author:
  • De Gruyter OnlineGoogle Scholar
/ Jessica M. Andriolo
  • Montana Tech Nanotechnology Laboratory, Butte, MT 59701, USA
  • Bioengineering, IIP PhD Program, University of Montana, Missoula, MT 59812, USA
  • Other articles by this author:
  • De Gruyter OnlineGoogle Scholar
/ John P. Murphy
  • Montana Materials Science PhD Program, Montana Tech, Butte, MT 59701, USA
  • Montana Tech Nanotechnology Laboratory, Butte, MT 59701, USA
  • Other articles by this author:
  • De Gruyter OnlineGoogle Scholar
/ Brandon M. Ross
  • Mechanical Engineering, Montana Tech, Butte, MT 59701, USA
  • Montana Tech Nanotechnology Laboratory, Butte, MT 59701, USA
  • Other articles by this author:
  • De Gruyter OnlineGoogle Scholar
Published Online: 2016-12-28 | DOI: https://doi.org/10.1515/nanoph-2016-0142

Abstract

The fabrication of photonic and electronic structures and devices has directed the manufacturing industry for the last 50 years. Currently, the majority of small-scale photonic devices are created by traditional microfabrication techniques that create features by processes such as lithography and electron or ion beam direct writing. Microfabrication techniques are often expensive and slow. In contrast, the use of electrospinning (ES) in the fabrication of micro- and nano-scale devices for the manipulation of photons and electrons provides a relatively simple and economic viable alternative. ES involves the delivery of a polymer solution to a capillary held at a high voltage relative to the fiber deposition surface. Electrostatic force developed between the collection plate and the polymer promotes fiber deposition onto the collection plate. Issues with ES fabrication exist primarily due to an instability region that exists between the capillary and collection plate and is characterized by chaotic motion of the depositing polymer fiber. Material limitations to ES also exist; not all polymers of interest are amenable to the ES process due to process dependencies on molecular weight and chain entanglement or incompatibility with other polymers and overall process compatibility. Passive and active electronic and photonic fibers fabricated through the ES have great potential for use in light generation and collection in optical and electronic structures/devices. ES produces fiber devices that can be combined with inorganic, metallic, biological, or organic materials for novel device design. Synergistic material selection and post-processing techniques are also utilized for broad-ranging applications of organic nanofibers that span from biological to electronic, photovoltaic, or photonic. As the ability to electrospin optically and/or electronically active materials in a controlled manner continues to improve, the complexity and diversity of devices fabricated from this process can be expected to grow rapidly and provide an alternative to traditional resource-intensive fabrication techniques.

Keywords: electrospinning; photonic; nanofabrication

1 Introduction

Electronic transducer systems are typically classified as either sensors or actuators. Electronic sensing components detect a physical, chemical, or optical quantity within the immediate environment, and an output signal, or actuator, which correlates to the physical, chemical, or optical quantity being detected, is generated [1]. In most cases, the electrical interface of these electronic systems is realized through integrated circuits (ICs) that provide the necessary capabilities for desired system functionality. On the micro- and nano-scale, these integrated systems are typically fabricated using two methods. The first method is referred to as a multi-chip solution that uses separate substrates for microoptoelectromechanical systems (MOEMS) and IC components. Multi-chip solutions allow the transducer and signal conditioning circuitry to be fabricated with disparate fabrication processes prior to MOEMS with ICs being combined in the final packaging step. The second method, referred to as system-on-chip (SoC), involves the use of a single silicon substrate to fabricate both MOEMS and IC components. Both multi-chip solution and SoC methods require expensive equipment and facilities that are used to produce dye (e.g. high end logic) that sells for several hundred thousand dollars per square meter. These logic and memory chips must be essentially defect-free requiring expensive metrology for processing and quality control. Silicon wafer substrates required for multi-chip and SoC methods also necessitate robotics handling for precise fabrication. A distinct need to address the issues that are inherent to complex and costly fabrication procedures is key to the progression of electronic systems technology. In addition, manufacturable technologies are critical for assembly of micro- and nano-scale photonic components that have demonstrated superior properties to their larger counterparts. As an alternative to multi-chip solution or SoC, electrospinning (ES) fabrication provides another route for MOEMS fabrication. ES requires inexpensive equipment and abundant and cheap polymeric materials and is amenable to both rigid wafer and flexible organic product lines. Such aspects make ES an attractive substitute for traditional multi-chip or SoC methods.

Organic nanofiber (ONF) systems can be readily created through the use of ES fabrication techniques. ES typically uses inexpensive polymeric materials and, in some cases, solvents, which require straightforward preparation procedures. In this way, ES allows simplistic manipulation of properties for broad-reaching applications, such as air filters, biological scaffolds, electronics, and photonic devices. This article provides an extensive review of advancements made in the area of electrospun micro- and nano-photonic devices. The following background section provides a short review of organic semiconductor systems. Subsequent information is provided on photonics integration into microelectronics, general, non-bulk (zero-, one-, two-dimensional) systems, and one-dimensional (1D) nanofiber structures. These descriptions are followed by an assessment of fabrication methods for 1D nanofiber devices and, more specifically, a focused analysis of ES used for these 1D systems as compared to traditional fabrication approaches. Immediately following the introduction and background sections, “spinnable” materials amenable to optical components will be reviewed and coupled with a discussion of the benefits and drawbacks of these materials to ES fabrication. The final sections of this article review the current state of research and up-to-date device development in the area of micro- and nano-scale photonic systems with integrated micro- and nano-scale fiber structures fabricated via ES.

2 Background

2.1 Integration of photonic components into microelectronics

Manipulation of light for use in MOEMS requires optical analogues of traditional microelectronic components. Optical analogues are referred to as photonic components and function to guide photons in a similar manner to the guiding of electrons inherent to electronics. Stand-alone photonic devices have not been actualized. As a consequence, photonic components must be integrated into pre-existing MEMS architecture. Feasibility of all-in-one photonics has been hindered by the complicated fabrication processes required, coupled with inefficient, high loss conversion of optical to electronic signals.

Gallium arsenide, indium phosphide, and other III–V semiconductors are the chief substrate materials for photonics, while the predominant substrate in electronics is undoubtedly silicon [2]. Despite the predominant preference for these materials in their respective fields, the largely mismatched lattice constant and/or thermal expansion coefficient (TEC) makes integration challenging. Film stress created by thick structural layers also provides a limiting factor in device fabrication. Successful coalescence of photonic and microelectronic systems has been accomplished under three overarching themes. The first of these themes is hydrophilic bonding via O2 plasma-assisted or SiO2 covalent direct bonding. The second utilizes polymer to adhere silicon and III–V wafers together, and the final theme in photonic/microelectronic integration encompasses multiple bonding methods in hybrid processing techniques [3], [4].

In III–V-to-silicon bonding, high-temperature is not possible. As an alternative to typical high-temperature methods used to create strong bonding, O2 plasma surface treatment or SiO2 covalent wafer bonding (see Figure 1) has been used [2]. These methods result in robust bonding under low temperature (<400°C) [5], [6]. The first step is cleaning and priming of the SiO2 wafer surfaces. After rigorous sample cleaning, the native oxide on silicon on insulator (SOI) wafer and InP is removed in buffered hydrofluoric acid (HF) solution and 39 w/v% NH4OH, respectively, resulting in clean, hydrophobic surfaces. From there, O2 plasma-treated substrates undergo surface treatment to grow an ultra-thin layer of plasma oxide (~15 nm) [7] that creates a smooth hydrophilic surface [8]. In this way, Si-O-Si bonds of the oxide are found to be more reactive than conventional oxides formed in other cleaning processes, as well as have a higher propensity to break and form new bonds [9]. O2 ion bombardment is also used to remove hydrocarbons and water-related species on the sample surface. Cleaning in the case of direct, covalent bonding involves deposition of SiO2 on both wafer surfaces. Chemical mechanical polishing (CMP) [10] is used to reduce surface roughness to an RMS value less than 1 nm [9]. In both cases of either O2 plasma treatment or SiO2-bonded substrates, surfaces are next passivated with a high density of polar hydroxyl groups, which enables spontaneous bonding at room temperature. O2 plasma-treated samples are dipped in deionized water and dried with N2 gas or placed in vaporized NH4OH, while SiO2-treated samples are treated in diluted RCA-1 at 75°C for 10 min, then dried with dry N2 gas. In both cases, physical mating occurs at room temperature before samples are annealed at 300°C with external pressure 1 h for ultimate bond strength.

Schematic published by Liang et al. [2], comparing O2 plasma assisted and SiO2 covalent wafer bonding methods. Both methods of manufacture provide an alternative to high-temperature bonding not conducive to III–V silicon substrate processing.
Figure 1:

Schematic published by Liang et al. [2], comparing O2 plasma assisted and SiO2 covalent wafer bonding methods. Both methods of manufacture provide an alternative to high-temperature bonding not conducive to III–V silicon substrate processing.

Under the second theme of photonics and microelectronics integration, adhesive wafer bonding (see Figure 2) requires thermosetting adhesives due to the required post-bonding processing temperatures for optoelectronic components, which exceed 400°C [2]. Based on the material properties desired, different adhesives are used under various bond strength and annealing temperatures. Please refer to the extensive review on adhesives in reference [11]. Examples of adhesives used during bonding include polyimides, epoxies, spin-on-glasses, photoresists, and divinylsiloxane-benzocyclobutene (DVS-BCB).

Schematic published by Liang et al. [2], showing an overview of DVS-BCB dye-to-wafer bonding process. DVS-BCB provides high bonding strength and quality as well as a high degree of planarization and resistance to typical chemicals used in III–V processing.
Figure 2:

Schematic published by Liang et al. [2], showing an overview of DVS-BCB dye-to-wafer bonding process. DVS-BCB provides high bonding strength and quality as well as a high degree of planarization and resistance to typical chemicals used in III–V processing.

The final theme for assimilation of photonic and micro-electronics assembly requires hybridization of typical integration methods used [2], [12], [13], [14], [15]. One example of hybrid assembly is the hybrid silicon platform developed by the University of California at Santa Barbara and Intel Corporation. In this method, a hybrid structure is realized through III–V epitaxial layers transferred to an SOI waveguide through low-temperature, O2 plasma-assisted wafer bonding [2]. The thick InP is then selectively removed to create mesa structure, which enables a carrier injection scheme on the III–V region by standard photolithography and etching. Typically, the III–V mesa width is larger than the Si waveguide (1–2 μm), so that transverse confinement is determined by the SOI waveguide and not the III–V mesa. Detailed fabrication can be found in reference [16]. General structure of III–V layers consists of the following: (1) p-type InGaAs contact layer, (2) p-type InP cladding, (3) optional p-type separated confinement heterostructure (SCH) layer, (4) undoped multiple quantum well layer, (5) n-type contact layer, and (6) n-type superlattice bonding layers. Superlattice bonding layers reduce and block the TEC mismatch-induced defects [7], [17]. Generally, the fabrication process to form the hybrid Si device platform involves bonding of the III–V wafer to the patterned SOI wafer, InP substrate removal and mesa etching, and current confinement and metal contact formation. A similar refractive index among Si and III–V materials creates an optical mode in the hybrid waveguide in the Si waveguide and III–V mesa. The hybrid silicon platform provides tunable optical confinement factors in Si and III–V layers that can be optimized for best performance and highest efficiency along the length of the device.

A second hybrid method has been developed as well. Heterogeneous III–V/SOI, which was developed by Ghent University, utilizes similar device structure to that of the hybrid silicon platform with the exception that the III–V material and the Si waveguide perform independent functions [2], [18]. Thermoset polymer, DVS-BCB, is used for III–V epitaxial layer transfer [18]. Because a typical DVS-BCB layer is on the order of several hundreds of nanometers with a refractive index ~1.5 at λ=1.55 μm, a relatively thick low-index medium between III–V and Si prevents photons generated in the III–V active region from coupling into the Si waveguide. The III–V active region provides a gain for lasing, which is further benefitted by reflection at the etched laser facets. Stimulated emission leaving the edge of the laser diode requires an additional coupling structure for efficient coupling to the SOI waveguide. An optimal adiabatic inverted taper structure is employed for good coupling efficiency and fabrication tolerance, which butt-couples the bonded laser diode to a polymer waveguide. Therein, the optical mode is gradually transformed into that of the SOI waveguide by increasing the cross-sectional area of the Si waveguide. A polymer waveguide is self-aligned to the laser ridge to eliminate couple inefficiencies arising from misaligned waveguides. The Si inverted taper structure is buried underneath the polymer waveguide, and the inverted taper tip width must be sufficiently small in order for the fundamental optical waveguide mode at the tip to resemble the waveguide mode of the polymer waveguide [19].

Hydrophilic bonding, adhesion, and hybrid integration techniques for photonic microelectronic fabrication have generated a useful set of active and passive optical components for integration into microelectronic devices [2]. Some of the many photonic structures created include Fabry-Perot cavities [16], [19], racetrack rings [20], mode-lock lasers [21], microdisks [22], distributed feedback lasers [23], distributed Bragg reflectors [24], micro-rings [25] lasers, amplifiers [26], PIN [27], metal-semiconductor-metal junctions [28] photodetectors, electroabsorption modulators [29], Mach-Zehnder interferometers [30], micro-disk modulators [31], and high-speed switches [32]. More advanced integration circuits have also been demonstrated [28], [33]. Such examples establish the vast and pervasive nature of devices achieved when light interacting components are integrated into microelectronics structures and devices.

2.2 Organic semiconductor systems

Organic electronics such as displays, photovoltaics, and electronic circuits provide superior properties over conventional inorganic electronics due to low cost, strong mechanical properties, lightweight, and low power consumption [34]. Organic semiconductor (OSC) systems have been used to create products that cover large areas and provide modulation of mechanical properties [35]. Fabrication of OSCs allows for processing at low temperatures and on inexpensive substrates, such as plastic or paper. Implementation of these systems spans a large field of applications [35], [36], [37]. Since the discovery of electroluminescence in poly(p-phenylene vinylene) (PPV) [38], light-emitting OSCs have provided photo and electroluminescence in the UV-visible range with high emission efficiencies and large stimulated emission cross-sections [39], [40], [41], [42]. Tunable chemical synthesis methods have allowed for control over electronic band-gap structure, optical properties, and electro-chemical redox at the molecular level [43], [44], [45], [46], [47], [48], [49], [50], [51], [52] for the production of π-conjugated small molecules, oligomers, and polymers with optical and electronic properties suitable for a variety of applications of OSCs [43], [44], [45], [46], [47], [53], [54], [55], [56]. Organic light-emitting diodes (OLEDs) [38], [57], [58] and organic field-effect transistors (OFETs) [59], [60], [61], for instance, form the foundation of organic electronic circuits, and OLEDS now provide low operating voltage for efficient lighting and displays used in our everyday lives [62]. Advanced OLED systems have also utilized many advanced materials including organic molecules [63], quantum dots [64], carbon nanotubes [65], nanowires [66], and single atoms [67] or molecules [68], [69], [70], [71]. OFETs have been manufactured for a variety of applications [72], [73], [74] and exhibited improvement in performance by three to four orders of magnitude, far exceeding that of amorphous silicon-based devices [59]. Investigations of organic photoresponsive materials [75] have provided a foundation for high-performance optoelectronics for renewable energy applications. Research and development of organic semiconductor lasers (OSLs) [76] has greatly supplemented nanoimprinting capabilities as well as fabrication of tunable and compact lasers and broadband optical amplifiers. Furthermore, OSC photovoltaic devices [77], [78] provide a promising area for research. According to the National Center for Photovoltaics (NCPV) at the National Renewable Energy Laboratory (NREL) [79], polymer solar cells (PSCs) [80], [81] have demonstrated efficiencies as high as 11.5% [82]. Most recently, novel synthesis of polymer-encapsulated hybrid organic-inorganic perovskite materials with enhanced stability was accomplished and offers an exciting new material for photovoltaic cell construction [83], [84].

2.3 Organic, 1D semiconductor systems

Generally, the majority of research into organic electronic and photonic devices has focused on bulk 3D OSC systems. More recently, exploration into 1D or quasi-1D nanostructures, such as nanowires, nanorods, nanoribbons, and nanofibers for photonic and optoelectronic applications, has been studied extensively [35], [36], [37], [39], [85], [86], [87], [88], [89], [90], [91], [92], [93], [94], [95], [96], [97], [98], [99], [100], [101], [102]. The flexible properties of organic semiconductor micro- and nano-structures enable ready integration with inorganic, metallic, biological, or other organic materials for development of novel technologies.

Optoelectronic properties of organic semiconductors can characteristically be classified in terms of Frenkel-type excitons [103], [104] in which quantum confinement effects should be minimal. However, 1D nanostructures in OSCs exhibit enhanced performances compared to equivalent bulk systems [36]. Hypothesized emergent properties due to confinement of electronic, optical, magnetic, and mechanical properties due to quantum confinement of electrons or Wannier excitons in 0D, 1D, and 2D structures have driven growth of the field. The quantum confinement effect takes into account changes in atomic structure as a result of extremely small energy band structure [105], [106], [107], which ranges from 1 nm to 25 nm for typical semiconductors. In this case, the spatial size of the electronic wave function is similar to particle size, and therefore, electrons respond to this physical constraint by adjusting their energy (quantum-size effect). When critical dimensions of semiconductor devices reach or are less than the bulk semiconductor Bohr exciton radius, the overall device material becomes property size dependent. The quantum confinement effect at this scale causes increased excitonic transition energy and blue shift in the absorption and luminescence band gap energy [108], [109]. Quantum confinement will eventually result in collapse of continuous energy bands of the bulk material into discrete, atomic energy levels.

In addition to quantum confinement, 1D systems also provide an extremely high surface-area-to-volume ratio. In essence, atoms at the surface of materials have fewer adjacent counterparts as compared to atoms in the bulk of a material [110], [111]. As 1D systems reach smaller sizes, a larger fraction of atoms exists at the surface, and as this fraction increases, higher average binding energy is experienced per atom. Because the surface area-to-volume ratio scales inversely with size, there are numerous properties affected by this scaling law, among them, melting and phase transition temperature and enhanced binding strength experienced by edge and corner atoms. High surface-to-volume ratio coupled with multiple plasmon resonances in the visible or near-infrared (NIR) range is critical for surface-enhanced Raman scattering (SERS) [112] or IR spectroscopy (SEIRA) [113]. Despite the diminished cross-section that dampens plasmon resonance in nanoparticles with diameters below 15 nm [114], more complicated structures such as nanostars [115] or nanosponges [116] create large scattering cross-sections, as well as further enhanced surface-to-volume ratio.

Due to emergent properties rooted in quantum confinement and high surface area-to-volume ratio, novel 1D materials and devices are continually being produced with ever-increasing applications. For example, anisotropy of organic nanowires (ONWs) facilitates the propagation of light and electricity in specific spatial directions [117]. The π–π conjugated morphology [118], [119] of ONWs [120] coupled with established charge transport according to the molecular packing orientation [121], [122] provides favorable emergent properties for electronic devices and highly efficient energy harvesting devices with extremely high aspect ratio and large surface area-to-volume ratio [123], [124], [125]. Solid, 1D organic nanorods (ONRs) with moderate aspect ratio and high surface area have provided good performance for supercapacitor electrodes [86] and greatly improved photostability for biological imaging applications [126]. Graphene plasmonics [127], [128], [129], [130] have provided gate tunability [131], [132], and the momentum mismatch between incident waves and plasmons can be overcome through the fabrication of graphene nanostructures such as nanoribbons [133], [134], [135], [136], [137], [138], [139], [140], [141].

2.4 Organic, micro- and nano-fiber systems

Of these investigations into low-dimensional nanostructure materials, ONFs have demonstrated a great potential to augment OSC systems research from fundamental science to novel device construction. For example, using nanofiber arrays, it has been demonstrated in conjugated polymers that interchain energy transfer occurs more quickly than intrachain energy migration [142], and increased stacking order of π-conjugated molecules in organic nanofibers gives them charge transport mobility and conductivity superior to that of thin films [36], [118], [143]. Supramolecular assembly of polymeric backbones also has demonstrated polarization of emitted photons [144], [145], [146], low threshold amplified spontaneous emission [147], and non-radiative energy transfer [148]. Manipulation of these fundamental properties has led to extensive innovation in the area of ONF-based devices [39], [149]. Alignment of polymer backbone structures along fiber axes forms crystal domains [150], [151], [152], which have provided a foundation for optoelectronics such as nanofiber light sources with color tunability and waveguide capabilities and nanofiber lasers [39], [146], [148], [153], [154], [155], [156], [157], [158], [159], [160], [161], [162], [163], [164], [165], [166], [167], [168], [169], [170], [171], [172], [173], [174], [175], [176], [177], [178], [179], [180], [181], [182], [183], [184], [185], [186], [187], [188], [189]. Biologically, hybrid nanofiber scaffolds have provided a cell niche conducive to improved attachment, proliferation, and cell differentiation [190], [191], [192], [193], [194], [195], [196], [197], [198], as well as targeted drug delivery carriers [198], [199], [200], [201], [202], [203], [204], [205], [206]. Filtration using ONFs [207] equipped with activated carbon has revealed adsorption of volatile organic compounds (VOCs) present in the air [208], [209], [210], and Scholten et al. reported that adsorption and desorption of VOC by electrospun nanofibrous membranes (ENMs) were faster than that of conventional activated carbon [208]. Such examples demonstrate the pervasive nature of ONF systems, which will continue to provide innovative enhancement of technologies we use every day.

2.5 Fabrication of organic, micro- and nano-fiber semiconductor systems

As the extensive applications of ONF systems experience exponential growth, research and development of the fabrication methods used for creating such devices must simultaneously experience rapid advancement for efficient and economical manufacture. One-dimensional micro- and nano-fiber devices have been prepared using ES [211], [212], [213], [214], [215], [216], [217], [218], [219], hard and soft template-assisted methods [220], [221], [222], self-assembly [223], [224], [225], scanning probe lithography (direct writing) [226], [227], [228], direct drawing [229], [230], nanoimprint lithography [131], nanofluidics [231], and physical vapor transport and deposition [232]. Review articles of these methods have been published by Kim et al. [36] and Long et al. [233].

Semiconductor chip manufacturing techniques, such as multi-chip solution and SoC, are the predominant technologies used in the semiconductor industry today [234]. Such methods require extreme precision that requires expensive metrology. According to Liddle et al. [234], cost-effective nanomanufacturing requires metrology that costs proportionally less than products produced per square meter. In the analysis by Liddle et al. areal throughput is related to feature size through Tennant’s Law [235], T=αL5, where T is areal throughput in μm2/h, L is feature length scale in nm, and α is the scaling factor that holds approximately over a wide range of nanomanufacturing technologies. Aside from ES, the smallest feature size produced has been achieved by scanning tunneling lithography (STL), which affords extremely low areal throughput. Nanoimprint lithography (NIL) has created sub 5-nm features [236] but has much higher areal throughput than STL. ES technology holds promise to surpass both of these manufacture methods and allows for production of fibers as small as 1.2 nm [237] but with potential for areal throughput that surpasses that of NIL. Coupled with the fact that an entry-level ES tool only costs approximately $20,000/unit, the small feature sizes produced and high areal throughput demonstrate the attractive capabilities of this versatile process.

Despite these benefits, ES also has drawbacks that have stunted widespread utilization outside of research and development. Aside from enhanced air filters produced at high volume, ES has demonstrated low commercial use for electronic and photonic applications. Lack of control over fiber placement during ES creates a significant decrease in predictability of electrospun device performance. Such unpredictability is not conducive to high volume production of electrospun devices. The production of polymer fiber devices with deterministic spatial control has only been accomplished through near-field ES [238]. In near-field ES, a stable fiber is deposited from spinneret to capillary within ~3 mm (prior to the chaotic region discussed in the following section), and therefore, the fiber is written nearly directly onto a surface with coordinated mechanical motion between the spinneret and the collection plate. Near-field ES, however, constricts the deposition area and does not allow easy deposition of many devices per ES run or large surface area coverage. Further complications of current ES manufacturing methods include writing fibers over complex surfaces with sharp edges and non-planar topology. Variation in electric field strength associated with non-planar surface structure adds complexity to the mechanical approach used for fiber deposition.

2.6 ES fabrication of organic, micro- and nano-fiber devices

ES was first observed in 1897 by Lord Rayleigh, with related electrospraying studied in detail in 1914 [239]. From 1934 to 1940, a series of patents were granted to Formhals for textile applications [240], [241], [242], [243], and it was the published work of Taylor in 1969 [244] that set the foundation of ES fabrication beyond the textile industry. Since its inception as a textile manufacturing process, ES manufacturing has progressed significantly. ES has produced nanofiber structures used as electron transport materials in solar cells and exhibited high photoelectric conversion efficiency due to high specific surface areas and high porosity, which creates efficient charge separation and transport. Electrospun nanofiber devices also include fuel cells, nanogenerators, and enhanced hydrogen generation via photocatalytic activity in water splitting because of large surface area-to-volume ratio and improved crystallinity [218]. The biomedical field has benefitted from ES manufacture of devices for enzyme immobilization, sensors, affinity membranes, tissue engineering scaffolds, drug delivery, and wound healing [149]. In place of catalytic synthesis, ES of polyacrylonitrile (PAN) followed by stabilization and carbonization has become a straightforward and convenient route to make continuous carbon nanofibers for use in a variety of fields from energy conversion to biomedical applications and desirable material properties (high strength, high modulus) in general [245]. ES offers opportunities of technology transfer, economic development, and ultimately, employment [246]. The promising nature indicated by the variety of applications supplemented by ES discovered thus far facilitates a productive and innovative future for ES.

ES fabrication requires delivery of a solvent dissolved [247], [248], [249] or solid stick [250] polymer to a spinneret (or capillary) held at a high voltage relative to a collection plate (see Figure 3). Once a polymer bead is formed just outside the tip of the spinneret (which is essentially a hollow or gauge-type needle), voltage is initiated in the collection electrode, thereby causing surface charge buildup to occur on the surface of the polymer bead. Once surface charge reaches a critical value, the polymer bead is deformed into a cone (Taylor cone). A polymer jet is then emitted from the apex of the Taylor cone to reduce the surface charge present on the polymer bead. The force required to initiate ES is described by the following formula:

Traditional electrospinning schematic. Solvent-dissolved or melted polymer is fed through a syringe to the spinneret. As polymer reaches the spinneret tip, the initiated voltage creates an electrostatic force that pulls polymer from spinneret to electrode deposition surface. Fiber path is initiated with a stable region for approximately 2–3 mm before the polymer jet becomes unstable and causes lateral perturbations prior to fiber placement.
Figure 3:

Traditional electrospinning schematic. Solvent-dissolved or melted polymer is fed through a syringe to the spinneret. As polymer reaches the spinneret tip, the initiated voltage creates an electrostatic force that pulls polymer from spinneret to electrode deposition surface. Fiber path is initiated with a stable region for approximately 2–3 mm before the polymer jet becomes unstable and causes lateral perturbations prior to fiber placement.

Fes=εrε02d2V2A(1)

where permittivity is represented by εr (relative) and ε0 (in a vacuum), A is the area of the collection plate, V is the applied voltage, and d is the separation distance between spinneret and collection surface. At the tip of the polymer cone, the exposed surface of the polymer bead is pulled by this developed electrostatic force into a micro- to nano-sized jet (see Figure 4). During solution ES, as the polymer jet is pulled toward the collection electrode, a majority of solvent used during polymer processing evaporates, and adequate electrostatic force results in fiber deposition on the collection surface (see Figure 5).

Schematic showing the evolution of an ES fiber. Polymer is forced via syringe pump or pneumatic pressure to syringe tip, forming a polymer droplet. (A) Polymer is forced via syringe pump or pneumatic pressure to syringe tip, forming a polymer droplet. (B) Electrostatic voltage initiated in the collection electrode creates an electrostatic force, which pulls the polymer jet into a Taylor cone. (C) At the tip of the Taylor cone, electrostatic force elongates the polymer further into a micro- or nano-sized jet that is pulled toward the collection surface and results in fiber deposition.
Figure 4:

Schematic showing the evolution of an ES fiber. Polymer is forced via syringe pump or pneumatic pressure to syringe tip, forming a polymer droplet. (A) Polymer is forced via syringe pump or pneumatic pressure to syringe tip, forming a polymer droplet. (B) Electrostatic voltage initiated in the collection electrode creates an electrostatic force, which pulls the polymer jet into a Taylor cone. (C) At the tip of the Taylor cone, electrostatic force elongates the polymer further into a micro- or nano-sized jet that is pulled toward the collection surface and results in fiber deposition.

SEM micrograph of nanofibers produced via solution electrospinning. Fiber diameter ranged from 800 nm to 1 μm. Polymer used during electrospinning was 8 wt% polycaprolactone in trifluoroethanol.
Figure 5:

SEM micrograph of nanofibers produced via solution electrospinning. Fiber diameter ranged from 800 nm to 1 μm. Polymer used during electrospinning was 8 wt% polycaprolactone in trifluoroethanol.

As shown in Figures 3 and 4C, the polymer jet created during solution ES is characterized by two predominant regions. In the initial, short region (microns to millimeters), the fiber is essentially straight. This region is referred to as the stable region. Following this stable region, toward collection plate, a point exists at which solvent evaporation causes lateral perturbations of the depositing fiber jet.These perturbations result in transverse fiber velocities that characterize the instability region (see Figure 3). It is this instability region that has provided so much difficulty in precise fiber control for far-field solution ES [251]. ES with a solid stick polymer is typically referred to as melt electrospinning (melt-ES). Lack of solvent in melt-ES is beneficial for two reasons. First, the lack of harsh solvent requires less precaution during polymer preparation, and second, the instability region caused by solvent evaporation is a non-issue. Melt-ES was first patented by Norton in 1936 [252], but it was not until 1981 that a three-paper series describing electrostatics and polymer melts was published by Larrondo and Manley [253], [254], [255].

During ES, charges on the metallic collection plate move instantaneously given the time scales associated with fiber deposition. Motion associated with charge in the polymer (much slower than motion of charge in metals) is dictated by ionic mobility of the polymer [256]. Resultant fiber diameter depends largely on the flow rate, applied voltage, and fluid surface tension of the melted polymer [257], [258]. Although determining specific parameters essential to a desired product made using ES may take time, these factors are relatively easily manipulated through empirical study. As stated earlier, however, instability in the fiber jet creates a lack of control over fiber placement during ES, which is problematic. The importance of deposition control during ES can best be compared to that of the electronics industry, where research and development of electronics fabrication is driven by precise spatial control of critical components [259]. Lack of control over feature placement significantly limits novel device design allowed by ES and has restricted commercial applications of ES fabrication.Efforts to control the electric field within ES must take into account the high-frequency cutoff enforced by polymer limitations. The low-frequency cut-off for dynamic field control relates to the spatial fiber deposition rate and time constants associated with the instability region.Limitations of polymers amenable to optical components are discussed in more detail following the introduction and background sections.

Parameters that govern the electric field that guides fiber deposition have been modeled previously, and an analytical expression for the governing electrostatic force acting on the fibers was found useful for linking field strength with deposition diameter [260]. Passive methods have also been attempted. These methods include using copper rings as lensing elements to dampen chaotic motion [261], and aperture plates that restrict and essentially “squeeze” down the electric field have also been employed to reduce resulting fiber mat spot size [262]. In addition to deposition location, alignment of polymer fibers and organic nanowires is desirable on many fronts and essential for equipment that utilizes and/or manipulates electromagnetic energy. Alignment of organic micro- or nano-fibers is beneficial for enhanced charge transport [231], [232], [263], [264], production of polarized light emission [265], [266], improved absorption and photovoltaic properties [123], [267], and enhanced crystal properties [268]. Additional benefits of parallel fiber arrangement include directional cell growth [269] and guided cell differentiation [270] as well as the achievement of high-modulus, high-strength fibers, which can be used for heat-resistant and protective clothing [271], [272]. The multitude of applications requiring alignment makes evident the need for straightforward and uninvolved processes for producing aligned fiber structures beyond that of templated strategies. Previously, alignment has been accomplished through rotating collector drums [273], [274], counter electrodes [275], and parallel auxiliary electrodes placed behind the deposition surface [276]. Variations of the latter include deposition in the gap between electrodes [277] and, moreover, deposition in the gap between collecting rings [278]. More recently, rastering during ES has created highly aligned fiber deposition [279]. In this research, fiber deposition is initiated on an electrode held at a stable, minimum voltage for ES. A second electrode placed in parallel to the stable electrode is then delivered to a sinusoidal wave input with minimum voltage below the stable electrode and maximum voltage above the stable electrode. This configuration alters preferential deposition from one electrode to the other, initiating alignment between the electrodes (see Figure 6). Further effectiveness of this system has been accomplished through increased deposition speed initiated by rotational motion of a commutator that generates a square-wave signal.

Scanning electron micrograph of polycaprolactone nanofibers produced using solution-ES. Fibers were deposited between parallel electrodes to initiate alignment and have an average diameter of 600–800 nm.
Figure 6:

Scanning electron micrograph of polycaprolactone nanofibers produced using solution-ES. Fibers were deposited between parallel electrodes to initiate alignment and have an average diameter of 600–800 nm.

The following section discusses the current state of materials research and ES assembly of organic, micro- and nano-fiber semiconductor systems, which interact with photons passively and/or actively. Up-to-date progress in construction of passive polarizers and waveguides is reviewed, followed by active photonic devices such as light-emitting fiber structures, fiber structures for photovoltaics, and optical sensing elements.

3 Materials

3.1 Polymers amenable to electrospinning

Material properties inherent to polymer(s) selected for ES can significantly impact resultant structure, size, porosity, composition, and functional capability of electrospun fibers and therefore also the resultant qualities of the bulk material fiber mat. Additionally, some polymers lack characteristics that allow them to be electrospun at all. Consequently, polymer selection is challenging in that polymers that are “spinnable” may not possess suitable material properties for a specific function and/or application.

There are two modes of failure that limit the ability of a polymer to be electrospun [280]. The first mode of failure is fiber break-up due to insufficient surface charge build-up required to overcome the cohesive surface tension of the fiber (Plateau-Rayleigh instability) [281]. Plateau-Rayleigh instability involves breakage that occurs along the fiber axis as the diameter of the polymer jet decreases [282]. Effects of Plateau-Rayleigh instability can be overcome by utilizing high-strength electric fields to subsequently increase the surface charge density on polymer jets [283], effectively balancing the cohesive and columbic forces acting on the polymer jet. However, if the field strength is too high and the surface charge density becomes too great, the polymer jet will emit microdroplets [284] in an attempt to reduce surface charge density through increased surface area. One attractive route toward mitigating Plateau-Rayleigh instabilities is through selection of appropriate polymer/solvent systems to modulate surface tension of polymer stock solutions [285].

The second mode of failure in fiber formation is caused by fiber breakage due to excessive tensile stresses that lead to brittle fracture [280]. Brittle fracture arises from the fiber behaving in a rigid manner as opposed to a viscoelastic manner, which is more favorable during ES. It is important for polymer solutions to have a high degree of elasticity in order to endure asymmetric stretching and contraction during the ES process [286]. High elasticity is typically achieved through a high degree of chain entanglement of the polymer.

Several approaches have been developed to select polymer/solvent systems amenable to ES [287], [288]. These approaches take into account the intrinsic physical properties of solvents and how they correlate to the intrinsic physical properties of polymer solutions. However, the most common approach to determining the spinnability of a polymer/solvent combination is through the empirical parametric analysis of polymer/solvent solutions.

3.2 Conjugated polymers

A conjugated polymer is a material that contains organic macromolecules with alternating π-σ bonds along the structural backbone of the molecules. Interactions between π-bonds along the backbone give rise to delocalized π-electron energy states [289]. Partial filling of energy states created through the delocalized π-electrons creates a highest occupied molecular orbital (HOMO)−lowest unoccupied molecular orbital (LUMO) energy gap in the visible to NIR range (400 nm–800 nm) [289]. Delocalized energy states are formed along the backbone of the polymer, and as a consequence, conjugated polymers exhibit anisotropic electrical and optical interactions. Anisotropy in optical and electrical interactions can be modulated through polymer chain orientation [290], [291].

3.3 Dyes

Fluorescent dyes are often used in conjunction with other optically active materials for tuning purposes [292], [293]. Absorption/emission properties of dye molecules originate from delocalized π-electrons often on large aromatic molecules. When used individually, dyes act as fluorophores and emit light upon photoexcitation. However, when dye molecules are used as dopants in active layers of OLEDs, energy transfers from the active layer occur [294], [295]. Devices that emit at multiple wavelengths can be fabricated through the use of multiple dyes in order to tune the chromacity of OLED toward white light emission [296]. In devices with a single light-emitting layer, using multiple dyes to tune for white light emission is not possible due to donor/acceptor interactions between dye molecules.

3.4 Ionic transition metal complexes

Ionic transition metal complexes (iTMCs) are composed of a transition metal, such as ruthenium or iridium, which acts as coordination centers with an array of ligands. These ligands usually consist of aromatic rings and a polyatomic counter ion for charge balance [297]. iTMCs are of particular interest due to their tunable nature, high quantum efficiencies, and simple device architecture [298]. Devices fabricated with iTMCs as the light emission material can utilize air-stable electrodes [297] and do not require electron and hole transport layers. In iTMC devices, the counter ion in the complex can diffuse through the active layer to lower the injection barrier at both the cathode and anode [298] of light-emitting devices.

4 Passive optical elements

4.1 Nanofiber waveguide

Polymer nanofibers are suitable for waveguide applications because of low attenuation, optical tunability, and favorable processability [35], [39], [299]. For a waveguide to function effectively, the waveguide core has to be surrounded by a cladding material with a comparatively lower refractive index, allowing for total internal reflection within the fiber. Often, air is the cladding material, and light will be completely confined within the waveguide due to the refractive index of most polymers being greater than 1. A majority of polymers have a refractive index of approximately 1.30–1.60 [300], making ideal waveguides.

The geometry of the waveguide and the wavelength of light will determine either single-mode or multimode propagation. When traditional optical fibers have a high number of modes, the propagation of light through the fiber can be analyzed with a geometric model and the concept of light rays [300]. As the electrospun polymer fibers decrease in size from several microns and approach sub-wavelength dimensions, the light can only behave as a wave in a single-mode propagation.

The electrospun polymer crystal structure has an important impact on the fiber morphology and, therefore, on its waveguide properties. More crystalline polymer fibers typically create optically smooth surfaces with minimal defects, making them ideal for waveguide applications. Fiber morphology is dependent on the ES process parameters and has an effect on the waveguide properties of a nanofiber. In a study by Enculescu et al. polyvinylpyrrolidone (PVP) fibers were doped with Rhodamine 6G (R6G) dye, and the ES parameters were varied to determine the effect of fiber morphology on the photoluminescence [301]. Samples of R6G/PVP were generated as spin-coated thin films, electrosprayed beads, electrospun beaded nanofibers, and electrospun smooth fibers. The photoluminescence was measured for all samples, and the intensity plot of photoluminescence showed a trend of decreasing intensity as well as a blue shift from the smooth to beaded fibers (see Figure 7). Beads in the fiber acted as scattering points and caused a decrease in emission intensity. The blueshift is likely an effect of the decreased critical dimension of the feature. As confinement of the light increases, there is a proportional increase in the exciton energy in the structure.

Emission intensity of thin-film and electrospun structures illuminated with a 450-W xenon lamp with monochromation for wavelength selection in excitation and emission. (Reprinted from Enculescu et al. J. Phys. Chem. Solids 75, 1365, © 2014 with permission from Elsevier).
Figure 7:

Emission intensity of thin-film and electrospun structures illuminated with a 450-W xenon lamp with monochromation for wavelength selection in excitation and emission. (Reprinted from Enculescu et al. J. Phys. Chem. Solids 75, 1365, © 2014 with permission from Elsevier).

The flat ends of polymer nanofibers have been shown to create an axial Fabry-Perot cavity [35], which can be used for spectra selection and light amplification. Studies by Camposeo et al. demonstrated laser emission from electrospun polymer nanofibers manifested via a Fabry-Perot effect in cavities along the axis of the fiber. In this research, poly(methyl methacrylate) (PMMA) fibers were doped with R6G dye. When the photoluminescence of the fibers was compared to the reference film, there was a blueshift of approximately 20.0 nm and a decrease in the full width-half-max value from 20.0 nm to 8.0 nm [153].

High-order stacking of π-conjugated molecules improves the interactions between photons and the crystal lattice and results in light propagation [35]. Defects in the crystal lattice and grain boundaries create scattering points. In ES, the molecular chains of polymer fibers are stretched, creating a highly ordered structure that allows for propagation along the π-π stacking. Much like fused silica waveguides, polymeric waveguides can be doped to add optical functionality. Liu et al. demonstrated doping in SU8-100 electrospun polymer fibers by doping with monodisperse CdSe/ZnS (core/shell) quantum dots (QDs). SU8-100 is typically used as a negative photoresist and was selected for its well-known optical transmission properties. The CdSe/ZnS QDs were incorporated to provide an in-fiber light source, as placing and coupling into electrospun subwavelength fibers is inherently difficult [156]. An external excitation source (488 nm laser) was used to excite the QDs, causing a photoluminescence propagation into the ES fiber waveguide modes. Gaio et al. studied the coupling phenomena in electrospun PMMA fibers doped with CdSeTe QD subwavelength emitters. Fibers with single QDs were excited under laser excitation (633 nm) (see Figure 8), and coupling efficiency was measured through momentum spectroscopy. In room-temperature conditions, the broadband coupling efficiency to an ONF from an individual QD was measured to be 31% of the emitted light [302].

(A) Single, free-standing ONF viewed under SEM on a TEM grid. (B) Wide-field fluorescence image of an isolated quantum dot (in the dashed box) from a fiber similar to that seen in (A). (Reprinted with from Gaio et al. ACS Nano, 10, 6125, CC-BY).
Figure 8:

(A) Single, free-standing ONF viewed under SEM on a TEM grid. (B) Wide-field fluorescence image of an isolated quantum dot (in the dashed box) from a fiber similar to that seen in (A). (Reprinted with from Gaio et al. ACS Nano, 10, 6125, CC-BY).

4.2 Polarizers

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 [303]. 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. by creating a fiber-oriented liquid crystal polarizer [304]. Electrospun PMMA fibers were aligned by using a rotating drum and had a diameter of 236 nm with a mat thickness of 5.3 μm. 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 0.92, while maintaining a transmittance of 0.48.

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. PVP dissolved in ethanol [305] and PMMA dissolved in acetone [306] 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 [279]. Lack of fiber handling used in this process offers promising results for future fabrication of polarizer devices using ES.

5 Active optical components

5.1 Electrospun light-emitting structures

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.

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 [307], 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. addressed the issue of forming efficient electrical contact to coaxial fibers through annealing poly(ethylene oxide) (PEO)/(poly[(9,9-dioctylfluorenyl-2,7-diyl)-alt-co(1,4-benzo-{2,1′3}-thiadiazole)] (F8BT) blended polymer electrospun fiber mats at 150°C for 30 min [308]. Annealing resulted in a spreading of polymer fibers and a change in electrode/F8BT interfacial area (see Figure 9). 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.

Schematic showing the effect of annealing on blended PEO/F8BT. Results reveal a change in the current path through device layers. (Reprinted with permission from Vohra et al. ACS Nano, 5, 5572. © 2011 American Chemical Society).
Figure 9:

Schematic showing the effect of annealing on blended PEO/F8BT. Results reveal a change in the current path through device layers. (Reprinted with permission from Vohra et al. ACS Nano, 5, 5572. © 2011 American Chemical Society).

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. generated a LEC made from coaxial fibers consisting of a shell composed of PEO doped with an iTMC [Ru(bpy)3]2+(PF6))2 and a liquid metal core composed of Galinstan [307]. 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 [297].

Schematic of a triaxial LEC fiber fabricated by coaxial ES and evaporation. PEO is used as a carrier polymer to allow ES of the iTMC [Ru(bpy)3]2+(PF6)−2. (Reprinted with permission from Yang et al. ACS Nano, 6, 622, © 2012 American Chemical Society).
Figure 10:

Schematic of a triaxial LEC fiber fabricated by coaxial ES and evaporation. PEO is used as a carrier polymer to allow ES of the iTMC [Ru(bpy)3]2+(PF6)2. (Reprinted with permission from Yang et al. ACS Nano, 6, 622, © 2012 American Chemical Society).

Even simpler, single-material fibers have been utilized to create light-emitting layers in planar devices. Moran-Mirabal et al. used iTMC [Ru(bpy)3]2+(PF6))2 doped into low weight percent (1%–3%) PEO dissolved in dry acetonitrile to form nano- and micro-fibers with electroluminescent functionality [309]. 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 [310] or phosphor down-conversion layers [311] with highly efficient white organic light-emitting diodes. Kim et al. fabricated white-light-emitting electrospun fiber mats through the use of multiple fluorescing dyes [296]. 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. to produce three types of dye-doped fibers, where spatial separation between dye molecules limited energy transfer and negated donor-acceptor interactions. 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.

Scanning electron micrographs in parallel with schematic drawings demonstrating the optical interactions and behaviors of dye-doped PMMA fibers and/or beads. 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. Mater. Interfaces, 5, 6038, © 2013 American Chemical Society).
Figure 11:

Scanning electron micrographs in parallel with schematic drawings demonstrating the optical interactions and behaviors of dye-doped PMMA fibers and/or beads. 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. Mater. Interfaces, 5, 6038, © 2013 American Chemical Society).

The ES process can result in increased crystallinity of commonly used polymers [312] through electrostatic elongation of the polymer chains along the fiber axis. When conjugated polymers are electrospun, the polymer backbone is aligned along the fiber axis, and as a consequence, delocalized π-electron energy states are aligned along the fiber axis as well. The alignment of these energy states results in anisotropic absorption/emission properties. Vohra et al. noticed the anisotropic absorption in a polymer blend of poly(styrene) (PS)/F8BT [308], which demonstrated an absorbance dependent on the polarization of light used to excite (see Figure 12). In highly aligned polymer fibers, photoluminescence from the fibers has been polarized as well, with polarization ratios from 13 [163] to 4 [166].

Fluorescent optical micrographs of the identical PS/F8BT ES fiber excited with light in the direction of the corresponding white arrow. Emission is observed to be dependent on the direction of polarization. (Reprinted with permission from Yang et al. ACS Nano, 6, 622, © 2012 American Chemical Society).
Figure 12:

Fluorescent optical micrographs of the identical PS/F8BT ES fiber excited with light in the direction of the corresponding white arrow. Emission is observed to be dependent on the direction of polarization. (Reprinted with permission from Yang et al. ACS Nano, 6, 622, © 2012 American Chemical Society).

5.2 Photovoltaic structures

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) [313], [314], [315], [316]. The interfacial area between the ETL and the active layer in PSCs is vital in exciton dissociation. Meso-structured metallic oxides, which provide a reasonable amount of surface area for exciton dissociation, can be used for this layer [317]. 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. a carrier polymer is first doped with a metal alkoxide precursor solution before being electrospun into nanometer diameter fibers. 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 [277]. The method outlined by Li et al. has been utilized to create arrays of metal oxide nanowires [313], [314], [315], [316] and has seen subsequent improvement in the efficiency of PSC devices incorporating nanostructured ETLs.

Electrospun fibers have also to be used as templates for adhesion of metal chalcogenide nanoparticles to create large quasi-nanotube structures [318]. Cortina et al. electrospun cellulose acetate (CA) nanofibers and functionalized their surface through a wet chemical precipitation of CdS nanocrystals. Coated CA fibers were then immersed in a poly(3-hexylthiophene) (P3HT) solution and drop-cast into a planar PSC device. 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 [319]. Quaternary chalcogenides such as Cu2FeSnS4 (CFTS), Cu2ZnSnS4, and Cu2ZnSnS, among others, represent an economically and ecologically friendly replacement material for chalcopyrite-based semiconductors. Ozel et al. developed a method similar to those used by Li et al. to create metal oxide nanowires that utilized an alkanethiol prior to an annealing step resulting in CFTS nanofibers.

Aside from the creation of nanostructured components of PSCs, ES can be used to directly create the active layer of PSCs. Sundarrajan et al. used coaxial ES to produce a nonwoven solar cloth, albeit with low power conversion efficiency (8.78E–8%) [320]. A blend of P3HT and phenyl-C61-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. was further optimized by Bedford et al. who used ES to instead create a suspension of P3HT/PCBM nanofibers to spin-cast onto planar PSC devices [321]. The P3HT/PCBM nanofibers created a scaffolding structure onto which a solution of P3HT/PCBM was spun-cast. PSCs created with the P3HT/PCBM nanofibers had larger short circuit current (Jsc) values, and as a result, higher efficiencies as compared to PSCs created with similar materials without nanostructured electrospun photoactive layers. Nanofibers likely improved the chain alignment of the spun-cast solution-based P3HT/PCBM leading to higher charge carrier mobility in the active layer.

6 Other applications of electrospun fibers in photonics

While a great deal of focus is placed on the OLED/PSC applications, ES finds use in a great deal of other active photonic components. Of particular interest are optical sensors. Electrospun fiber mats, especially those made from fibers with porous morphologies [322], 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 [323]. The quenching is driven by an energy transfer from the dye to the analyte molecule and/or ion [324]. 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 [323]. Improved response dynamics of sensors allows for sensors with faster response times. Wolf et al. demonstrated this with an oxygen sensor based upon an oxygen-sensitive fluorophore. 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 [325] and conjugated polymers [326], rapid response dynamics of fluorescence quenched electrospun optical sensors can be utilized for detection of explosive compounds. Xue et al. recently demonstrated an explosives detector based on a novel fluorophore that had a more favorable donor-acceptor interaction between itself and DNT compared with previous fluorophores. The lower driving force for energy transfer resulted in a much stronger quenching when exposed to DNT [325]. 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.

7 Conclusion

ES has demonstrated an efficient and simplistic alternative to traditional top-down manufacturing of micro- and nano-scale photonic devices. ES provides feature sizes that are equal to or better than NIL technology with a promise of higher areal throughput than both NIL and STL. 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. Further functionalization of polymer fibers deposited during ES has also created inexpensive fabrication of polarizers and efficient light harvesting devices. As ES fabrication moves toward controlled and enhanced deposition onto complex surfaces, the utilization for ES will experience steady growth as a fabrication technique. As ES equipment advances and becomes available to the photonics community, ES technology is well positioned to become a major player in the economical fabrication of both passive and active photonic elements.

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About the article

Received: 2016-08-09

Revised: 2016-10-29

Accepted: 2016-11-08

Published Online: 2016-12-28


Citation Information: Nanophotonics, Volume 6, Issue 5, Pages 765–787, ISSN (Online) 2192-8614, DOI: https://doi.org/10.1515/nanoph-2016-0142.

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©2017, Jack L. Skinner et al., published by De Gruyter.. This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivatives 3.0 License. BY-NC-ND 3.0

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