Ferroelectric and Non-Linear Optical Nanofibers by Electrospinning: From Inorganics to Molecular Crystals

1. Introduction

Nanofibers have emerged as a promising platform for advancing optical and electrical applications due to their unique structural and functional properties. Traditional piezoelectric ceramics, such as lead zirconate titanate (PZT), zinc oxide (ZnO), and barium titanate (BaTiO₃), exhibit remarkable piezoelectric performance. However, their inherent brittleness and the need for high processing temperatures (T > 600°C) significantly constrain their practical utility [1]. These limitations have driven the search for novel ferroelectric materials that are both flexible and more easily processed. Among these alternatives, ferroelectric fiber composites fabricated by electrospinning hold significant promise. These materials not only exhibit enhanced ferroelectric, piezoelectric, pyroelectric and non-linear optical (NLO) properties, but are also flexible, biocompatible and environmentally friendly.

Ferroelectricity is a characteristic of crystals with spontaneous polarization, which can alternate between two or more equivalent orientational states under an external electric field. These materials, belonging to a subset of pyroelectric crystals, inherently exhibit piezoelectricity [2]. Importantly, many practical applications of ferroelectric materials leverage their related properties, such as pyroelectricity and piezoelectricity, rather than ferroelectricity itself. For instance, pyroelectric materials find utility in imaging and detection applications [3,4], while piezoelectric materials are widely used in electromechanical devices. Notably, the high piezoelectric coefficients of materials such as PZT have enabled advancements in microelectromechanical systems (MEMS) and energy harvesting devices, including their miniaturization [3,5,6,7,8]. Ferroelectric materials are also widely used in capacitors for microelectronics, non-volatile memories, and sensors [9,10,11,12].

A key advantage of reducing ferroelectric materials to nanoscale dimensions is the substantial decrease in voltage required to achieve coercive fields, making them highly attractive for integration into modern electronic systems. For example, nanoscale ferroelectric structures require potential differences as low as 5 V to achieve coercive fields, compared to several kV in bulk crystals or ceramics [9].

Nanostructures with reduced dimensions, in the form of fibers, wires, rods, belts, tubes, and rings, have attracted significant interest for their unique properties and potential applications in sensing, flexible electronics, and nanoscale electrical energy generation. Among these, ferroelectric nanofibers offer unique advantages, as they do not require substrates and enable new functionalities. While several fabrications methods, including chemical vapor deposition (CVD) [13,14], hydrothermal methods [15], solid state reactions [16,17] and sol-gel [18,19,20] have been used to create nanostructures, these methods often involve complex procedures, expensive equipment, or costly precursor materials.

In contrast, electrospinning is a simple, versatile and cost-effective technique capable of producing continuous, individual fibers with a variety of structures, including inclusions, core-shell and 3D architectures [21,22,23,24,25]. Electrospinning enables the fabrications of nanofibers with diameters ranging from a few micrometers to less than 100 nm. Electrospinning stands out as a straightforward and adaptable technique capable of creating nanofibers from a wide range of polymers, often incorporating organic or semi-organic nanocrystals to create hybrid composite materials. This method has also been used to align nonlinear optical (NLO) chromophores in subwavelength structures with tailored functionalities [26].

Low-dimensional nanostructures, specifically ferroelectric nanofibers, have been successfully fabricated using the versatile electrospinning technique [27,28,29,30,31,32]. These nanofibers exhibit exceptional ferroelectric properties at the nanoscale making them highly promising for applications ranging from flexible electronic devices and highly sensitive sensors to nanogenerators for power generation. Additionally, electrospinning facilitates the integrations of multiple functionalities such as combining ferroelectric and magnetic properties within a single nanostructured entity. The demand for cost-effective sensors, actuators, and electronic components, coupled with the need for increased device integration, has spurred interest in exploring nanofibers with high aspect ratios and multifunctionality. Electrospun nanofibers offer an optimal solution, combining affordability, flexibility, and versatility.

Although recent studies have reviewed ferroelectric composites [11,33,34,35,36,37,38], there remains a notable gap in the literature concerning electrospun single-phase inorganic ferroelectric nanofibers and organic ferroelectric nanofibers beyond polyvinylidene fluoride (PVDF). While the synthesis and applications of nanofibers have been extensively studied, their ferroelectric and nonlinear optical properties are often overlooked. This review aims to address this gap by focusing on the ferroelectric and nonlinear optical characteristics of electrospun nanofibers, emphasizing the unique features and potential applications of both inorganic and organic ferroelectrics.

2. Electrospinning Process and Parameters

2.1. Electrospinning

Electrospinning is a widely used technique for synthesizing nanofibers based on the stretching of a viscoelastic solution and its consequent solidification under the influence of a strong electric field. Figure 1a illustrates schematic of a typical electrospinning system used for producing fiber mats. In this method, a high DC electric field (100 – 500 kV/m) is applied to a metallic capillary containing a polymer or sol-gel precursor solution. At the tip of the capillary, an electrically charged drop forms, with electric charges evenly distributed across its surface (Figure 1b).

The interplay between the externally applied electric field, repulsive forces among surface charges, and the surface tension of the droplet induces a characteristic cone-shaped deformation known as the Taylor cone, typically exhibiting an angle of approximately 30° at its tip [22]. When the applied electric field exceeds a critical threshold, the electrostatic forces overcome the surface tension, leading to the ejection of a fine jet from the tip of the Taylor cone. The critical potential difference $V_c$ required for jet emission for the emission of the jet takes place, assuming a spherical shape of the droplet at the cone’s apex, is given by [25]:

$$V_c^2=0.36{\left({\displaystyle \frac{D}{L}}\right)}^2\left(ln\left({\displaystyle \frac{2L}{R}}\right)-{\displaystyle \frac{3}{2}}\right)\left(1.3\pi R\gamma \right) , V_C in kV$$

(1)

where R is the radius of the syringe needle, L its length, D the distance between the needle and the collector and γ the surface tension of the solution. With the numerical factors $V_C$ is expressed in kV.

Once the jet is emitted, it undergoes stretching due to the applied electric field, accompanied by instabilities arising from charge interactions. These instabilities result in the elongation and thinning of the jet, reducing its diameter as the solvent evaporates. In this way fibers are obtained that can reach, diameters down to a few tens of nanometers and lengths of several tens of meters (Figure 1c), in a controlled manner.

By tailoring the composition of the precursor solutions, it is possible to incorporate functional materials, such as ferroelectric or magnetic nanoparticles, into the fibers. This enables the creation of functionalized composite nanofibers with tunable properties for targeted applications. The high surface-to-volume ratio and aspect ratio (length-to-diameter) of the resulting fibers make electrospinning a versatile and powerful technique for fabricating micro- and nanoscale fibers with specific functionalities.

The final fiber diameter ($d_f$) is determined by the balance between surface tension and the repulsion of surface charges in the stretched jet. At the point where the jet ceases to thin further, the fiber diameter can be expressed as [25]:

$$d_f={\left(\gamma \epsilon \text{'}{\left({\displaystyle \frac{Q}{I}}\right)}^2{\displaystyle \frac{2}{\pi \left(ln\left(\chi \right)-3\right)}}\right)}^{1/3}$$

(2)

where Q is the flow rate of the fluid jet, I is the electric current, ε’ is the permittivity of the environment (air), γ is the surface tension and χ is the dimensionless wavelength of the jet’s torsional instability (proportional to the curvature radius of the bending perturbation divided by the jet radius). Consequently, the fiber diameter depends not only on the physical properties of the precursor solution (e.g., concentration, viscosity, conductivity) but also on process parameters such as the applied electric field, flow rate, and collection distance. Fiber diameter uniformity can be monitored by observing the stability of the electric current during the process. Any fluctuations in current often indicate variations in jet stability.

2.2. Process Parameters

The properties of the precursor solution and the corresponding processing parameters significantly influence the morphology and characteristics of the electrospun fibers. Key factors include polymer concentration, solvent properties, applied voltage, needle-to-collector distance, flow rate, and environmental conditions.

As polymer concentration or viscosity increases, the average fiber diameter tends to grow. This occurs because the higher viscosity resists elongation, slowing the jet’s thinning process. Conversely, low-viscosity solutions often result in thinner fibers or, if the concentration is too low, the formation of droplets instead of continuous fibers (a phenomenon known as electrospraying).

Solvents with low vapor pressure typically produce thinner fibers. This is because slower solvent evaporation prolongs the jet’s elongation and diameter reduction phases before solidification occurs. The choice of solvent also influences the solution’s conductivity and surface tension, both of which directly affect fiber formation. Higher solution conductivity increases the electric charge carried by the jet, enhancing the electric elongation force. As a result, the fiber diameter decreases due to increased stretching. Equation (2) illustrates this relationship, where the electric current (I) is proportional to charge density.

The applied electric field, determined by the voltage and needle-to-collector distance (Equation 1), must exceed a threshold to initiate jet emission. For larger electric fields, the fiber diameter tends to increase slightly because more fluid is ejected. However, this effect is less pronounced compared to the influence of solution viscosity.

A shorter needle-to-collector distance reduces the jet’s elongation time, resulting in thicker fibers. Conversely, increasing the distance allows more time for jet stretching and solvent evaporation, leading to thinner fibers. On the other hand, excessive distances can weaken the electric field, reducing the stability of fiber formation.

Reducing the needle radius or decreasing the flow rate results in thinner fibers. Smaller needle radii create finer initial jets, while lower flow rates reduce the amount of fluid ejected, enhancing the elongation effect.

In addition to these parameters above, the technique is also affected by the environmental conditions, such as the relative humidity and ambient temperature. Higher humidity generally leads to thinner fibers because slower drying allows extended stretching of the jet. Conversely, excessive humidity can prevent proper drying, causing the jet to break into droplets and inhibiting fiber formation. Similarly, temperature affects solvent evaporation rates, with higher temperatures facilitating faster solidification.

Electrospinning is highly adaptable, enabling the creation of nanofibers with intricate structures by varying needle and collector configurations. In coaxial electrospinning, two needles are concentrically aligned, such that one precursor solution can be delivered through an inner capillary while the other can pass through the outer capillary, creating tubular or core-shell fibers. With side-by-side needles bi- or three-phase fibers can be obtained [23,39]. Using rotating drums or parallel threads, aligned fibers can be obtained which is advantageous for applications based on fiber orientation [21,40,41]. Using more complex electrodes such as metallic wires coupled to mobile positioners, liquid collectors and assemblies [23,42,43] three-dimensional structures can be obtained.

3. Composite Ferroelectric and Multiferroic Fibers with Inorganic Inclusions

Recent studies on ferroelectric nanowires have revealed interesting size dependent properties that deviate significantly from their bulk counterparts. These nanostructures not only retain ferroelectric properties at extremely small diameters, but can also exhibit enhanced axial polarization with decreasing size [44] and significant ferroelectric and piezoelectric responses [45]. This underscores the potential for novel ferroelectric applications of functionalized nanofibers.

X-ray diffraction measurements on BaTiO₃, PbTiO₃ and PZT ferroelectric nanowires have confirmed the preservation of their polar structure. However, these nanostructures often display reduced spontaneous deformation and tetragonality compared to macroscopic crystals [46]. A key finding is that below the Curie temperature, spontaneous polarization develops parallel to the wire’s axis with the [001] direction of the tetragonal structure aligning along this axis [40,47,48]. This alignment has significant implications for the anisotropic properties of these nanowires.

Traditionally, the fabrication of ferroelectric nanowires has relied on complex and costly techniques. In this context, electrospinning emerges as a promising, cost-effective alternative for producing functionalized ferroelectric nanofibers. This technique offers advantages in terms of process control and versatility, enabling the synthesis of a wide range of ferroelectric and multiferroic nanofibers with tailored properties.

This section focuses on the synthesis of composite electrospun nanofibers based on ferroelectric and multiferroic materials. The discussion of ferroelectrics and multiferroics is linked to the deep relationship between them. In particular they share the requirement for good electrical insulation. This characteristic is crucial whether considering intrinsic multiferroics or as composite systems involving multiple materials. By exploring these relationships, we aim to elucidate the potential of electrospinning in creating multifunctional nanofibers with enhanced ferroelectric and multiferroic properties.

3.1. Ferroelectric Polymers

Ferroelectric polymers encompass a wide range of materials, including polylactic acid (PLLA), polylactic glycolic acid (PLGA), and most notably, poly(vinylidene fluoride) (PVDF) and its copolymers [41,49,50,51,52]. Among these, PVDF and its derivatives are the best-known and most widely used.

PVDF, particularly in its β phase, exhibits strong ferroelectric properties with a high electromechanical response. The electrospinning technique is especially effective for preparing PVDF nanofibers. Depending on the precursor solution PVDF concentration, nanofibers with diameters in the range 60-400 nm can be prepared [41,49,50,51,52]. Notably, electrospinning promotes the formations of the β (polar) phase in PVDF nanofibers, especially in thinner fibers [49,52], due to mechanical elongation and polarization induced by the applied electric field. In PVDF nanofibers the c-axis of crystallites tends to align along the fiber axis enhancing their piezoelectric properties.

Both natural and synthetic polyamides can display ferroelectric properties [53,54,55,56], with nylons being the most extensively researched synthetic ferroelectric polyamides [57]. The ferroelectric hysteresis characteristics identified in even-numbered nylons were initially documented in nylon-6 subjected to quenching from the melt, attributed to the autonomous rotation of dipoles within the structurally unstable nematic-type configuration [58]. The molecular mobility of dipolar entities within polyamide 6,9 was systematically investigated using a synergistic approach that combined thermo-stimulated current (TSC) and dynamic dielectric spectroscopy (DDS). Significantly, this study marked a pioneering effort in quantifying polarization and piezo/pyroelectric coefficients, thereby elucidating the distinctive ferroelectric behavior inherent in polyamide-6,9. Ferroelectric activity was detected in nylon-6,9 subsequent to polarization up to 100 MV m⁻¹ [59].

Odd-numbered nylons, including nylon-77, nylon-79, nylon-97, and nylon-99, display ferroelectric behavior due to the parallel arrangement of amide dipoles within crystalline regions. This phenomenon mirrors the characteristics observed in nylon comprised of ω-aminocarboxylic acid with an odd number of carbon atoms [60]. Nylon-11, a key odd-numbered nylon, shows promise as a functional material for electronic devices or energy storage [61,62,63]. It is crucial to control the formation of metastable crystalline phases, such as γ-piezoelectric or δ’-ferroelectric, for these applications. A recent method was described for manufacturing metastable γ-phase nylon-11 fibers using electrospinning with a customized solvent system [64].

Liquid crystal polymers represent a distinct subclass of ferroelectric polymers. These materials exhibit properties intermediate between conventional liquids and solid crystals. The occurrence of spontaneous polarization is observable in liquid crystals that have a smectic phase, which consists of rod-shaped molecules (mesogens) with a center of chirality [65].

Researchers have developed ferroelectric liquid crystal polymers using bases such as polysiloxanes, polyethers and polyesters, often incorporating side-chains with more than one chiral center or several side-chain structures. These ferroelectric liquid crystal polymers exhibit a wide range of physical properties, making versatile materials with ferroelectric characteristics [66].

Over the past ten years, advanced fiber mats with exceptional functionality and responsiveness have emerged, through electrospinning of polymers incorporating liquid crystals [65,67,68]. The liquid crystal component, responsible for the responsive features, is commonly encapsulated in fibers with a core/sheath structure, manufactured through coaxial electrospinning [69,70,71]. Alternatively, it can be an integral part of the polymer itself, as seen in liquid crystal elastomers [65,72,73].

3.2. Electrospinning Assisted Ferroelectric Inorganic Nanofibers

Electrospinning has emerged as a versatile technique for producing nanofibers of inorganic ferroelectric materials. The process involves mixing appropriate reagents in precursor solutions along with a transport polymer [25,74]. The morphology and diameter of the nanofibers can be controlled by adjusting the concentrations of the reagents in the precursor solutions [22,25,42,74,75].

To obtain inorganic nanofibers the prepared fiber mats must undergo a heat treatment process. This vaporizes the base polymer, promotes the coalescence and growth of the grains, and stabilizes the ferroelectric phase [42,46,76]. The heating rate must be carefully controlled to allow for slow polymer vaporization and gradual fiber formation. If the vaporization occurs too rapidly, the granular inclusions may not have sufficient time to connect, resulting in powder formation instead of fibers. Table 1 summarizes recent work on the production of inorganic ferroelectric nanofibers using this procedure, along with their respective heat treatment, purpose and potential applications.

Among the many ferroelectric materials studied, barium titanate (BaTiO₃, BTO) and lead zirconate titanate (PZT) [22,25,42,74,75,76,84,88,90,93,97,98] have received significant attention.

Barium titanate nanofibers mats have been produced with both aligned and randomly oriented nanofibers morphologies as shown in Figure 2 and Figure 3. The fibers typically have average diameters ranging from 150 to 400 nm and consist of connected BaTiO3 nanoparticles forming long filaments.

Studies on this type of inorganic ferroelectric fibers have focused mainly on their synthesis and applications. Few studies have demonstrated the existence of ferroelectricity in fibers. Different phases can coexist at room temperature (cubic, tetragonal, orthorhombic) and the reduction to the nanoscale tends to favor the stabilization of the paraelectric, non-ferroelectric cubic phase. From structural measurements and Raman spectroscopy measurements the production of nanofibers with the tetragonal ferroelectric phase was observed for nanofibers annealed in the temperature range 800-1000°C [42,46,75,97,98]. Higher annealing temperatures led to better crystallization of the BaTiO₃ nanofibers, but also to rougher nanofiber surfaces with optimized annealing temperatures in the region 900-1000°C.

The temperature dependent second harmonic generation response presented an anomaly in the vicinity of the paraelectric–ferroelectric phase transition (Figure 2d), confirming the change to the ferroelectric phase with decreasing temperature. The existence of domain structure and local switching studied by piezoelectric force microscopy (PFM) present clear evidence of the polar phase at room temperature. Using this feedback, the required conditions were determined to prepare ferroelectric fibers with high alignment and confirm the presence of the ferroelectric phase at room temperature [76].

Taking advantage of the synthesis studies, more recently, barium titanate nanofiber mats have been used as components in the development of piezoelectric nanogenerators [81]. The piezoelectric nanogenerator with BaTiO₃ electrospun nanofibers aligned vertically within the polydimethylsiloxane (PDMS) was able to produce an output power of 0.1841 µW with maximum voltage of 2.67 V and current of 261.40 nA under a low mechanical stress of 2 kPa. Additionally, the system displayed a high dielectric constant of 40.23 at 100 Hz. The harvested energy was sufficient to power a commercial LED directly or alternatively could be stored in capacitors after rectification [81].

Barium titanate electrospun nanofibers have been used as piezoelectric sensors [93,99], as seen in Figure 4. These sensors incorporate barium titanate ceramic nanofiber mats, with a polymer-like softness of 50 mN, a large Young’s modulus of 61 MPa, and an elastic strain of 0.9%, resisting fracture after deformation. The nanofiber mats generated a highly sensitive piezoelectric response, producing an output voltage of 1.05 V when subjected to a pressure of 100 kPa with a response time of 80 ms.

Beyond pure BaTiO₃ fibers, doped barium strontium titanate (Ba_0.6Sr_0.4TiO₃ or BST) [89,90] nanofibers have also been produced with diameters in the range of 100-200 nm. BST is attractive due its large dielectric constant, high tenability, low dc leakage, low loss tangent, and stable operation at high temperature. Higher calcination temperatures promoted improved BST nanofiber crystallinity [89]. The incorporation of BST fibers in a polymeric matrix has been explored for use as nanocapacitors. High energy densities and fast discharge speeds, with improved dielectric permittivity, were obtained making them potentially attractive for energy-storage applications [90].

Bismuth ferrite, BiFeO₃, nanofibers have been developed a variety of applications. Their ferroelectric properties were characterized after different annealing temperatures. Their photovoltaic performance displayed enhanced generated voltage generation compared to films [95]. The inclusion of electrospun BiFeO₃ fibres inside ferroelectric polymers, such as PVDF has been leveraged to produce flexible piezoelectric energy harvesters [94]. Additionally, the use of BiFeO₃ nanofibers in photocatalysis was explored in [96]. They were able to more efficiently degrade pollutants than in the bulk form.

Lanthanum doped bismuth titanate (Bi_3.25La_0.75Ti₃O₁₂, BLT) ferroelectric nanofibers with diameters between 100 nm and 300 nm were fabricated by electrospinning followed by annealing at 700 °C in air for 1 h [100]. Piezoeresponse force microscopy (PFM) measurements of the strain versus voltage hysteresis loops indicated ferroelectricity and an effective piezoelectric coefficient of d₃₃ = 61 pm/V was determined, about three times greater than that of a pure BLT film.

Lead zirconate titanate (PZT) ferroelectric nanofibers have been developed for flexible, wearable and self-powered energy harvesters and nanogenerators taking advantage of their significant piezoelectric coefficient [5,86,87,96]. Their synthesis conditions, in particular their annealing treatments, were studied in reference [101]. Typical fiber diameters range from 50-200 nm. Electrospun PZT piezoelectric nanogenerator fiber mats can deliver 1.63 V, with an output power of 0.03 μW [5], while oriented multilayer PZT fibers were stacked together have achieved an ultra-high output voltage of 209 V and current density of 23.5 μA/cm² [86,87].

In conclusion, electrospinning-assisted synthesis of ferroelectric inorganic nanofibers offers a promising route for developing advanced materials with tailored properties for various applications in energy harvesting, sensing, and catalysis.

3.3. Electrospinning Assisted Multiferroic Nanofibers

Beyond single-phase ferroelectrics, magnetoelectric (ME) multiferroic materials, which present simultaneous magnetic and electric ordering, have attracted significant scientific and technological interest [102]. These materials hold promise for applications in information storage, sensors, actuators, and low-power energy-efficient electronics [103,104,105,106,107]. However, in single-phase multiferroics, the microscopic mechanisms underlying ME coupling have been challenging to unravel, hindering the development of strong ME responses at useful temperatures [102].

To address these limitations, artificial multiferroic composites combining ferromagnetic (magnetostrictive) and ferroelectric (piezoelectric) materials have emerged as a potential alternative to single-phase multiferroics. These composites present robust magnetoelectric properties [108,109,110] with ME coupling coefficients up to ~100mVcm^-1Oe^-1 [110]. The strong ME coupling arises from the mechanical interaction between both phases, linking the electric, magnetic, and elastic fields. By carefully selecting the interface geometry and constituent materials, the ME coupling in composites can be engineered and optimized.

Various geometries have been explored for multiferroic magnetoelectric nanostructures, including self-assembled magnetostrictive nanopillars in a piezoelectric matrix, multilayers, and magnetostrictive nanograins in a piezoelectric matrix [102,111]. However, in thin-film structures, substrate clamping can significantly constrain strain-mediated interactions, limiting the achievable ME properties [112]. Interestingly, it has been predicted that substrate free structures, such as nanofibers, can exhibit ME properties several orders of magnitude higher than their thin-film counterparts [112,113].

Composite ferromagnet-ferroelectric nanofibers represent an exciting platform for such applications and can be prepared with various phase morphologies. These include core-shell structures (Table 2), Janus or side by side arrangements (Table 3), and randomly mixed composite configurations (Table 3). In core-shell nanofibers, the ferromagnetic material forms the core, surrounded by a ferroelectric shell. In Janus nanofibers, the ferromagnetic and ferroelectric phases are arranged side-by-side. In composite nanofibers, ferromagnetic and ferroelectric precursors are mixed into a single solution, creating a random distribution of the phases.

All these architectures can be fabricated using electrospinning. Core-shell nanofibers are produced using a dual syringe pump and a coaxial needle. The solution for the magnetic core is pumped through the inner part of the needle while the ferroelectric shell solution is simultaneously pumped through the outer part. Janus fibres can be fabricated with two needles are placed side-by-side and pumping the ferromagnetic and ferroelectric solutions together. For composite fibers, the single mixed solution is pumped through one needle as in conventional electrospinning.

These substrate-free, tunable nanofiber geometries provide a versatile platform to enhance magnetoelectric coupling and broaden the range of functional applications for multiferroic materials.

3.3.1. Core-Shell Multiferroic Fibers

Core-shell composite nanofibers, featuring a magnetic core surrounded by a ferroelectric shell are the most common type of multiferroic nanofibers produced by electrospinning. Various material combinations have been successfully employed to fabricate these nanofibers, including CoFe₂O₄-Pb(Zr_0.52Ti_0.48)O₃ [114], NiFe₂O₄–Pb(Zr_0.52Ti_0.48)O₃ [115,116], NdFeO₃-Pb(Zr_0.52Ti_0.48)O₃ [117], CoFe₂O₄-BiFeO₃ (see Figure 5) [118,119], NiFe₂O₄-BiFeO₃ [119], NiFe₂O₄-BaTiO₃ [120], CoFe₂O₄-Ba(Zr_0.2Ti_0.8)O₃–0.5(Ba_0.7Ca_0.3)TiO₃ [121], as summarized in Table 2.

Table 2. Multiferroic core-shell nanofibers prepared by electrospinning. The transporting polymers, high temperature procedure to vaporize the polymer and coalesce the grains to form the fibres is presented, along with the magnetoelectric coefficient, when measured. PVP - Polyvinylpyrrolidone, PMMA – Polymethyl methacrylate, PVA - Polyvinyl alcohol. MD – Magnetodielectric measurements.

Polymer	Core-Shell fibers	Heat treatment	Magnetoelectric Coefficient	Ref
PVP for CFO Mw = 1 300 000 PMMA for PZT Mw = 120 000	CoFe2O4-Pb(Zr0.52Ti0.48)O3 CFO-PZT	Nanofibers were collected on Pt/Ti/SiO2/Si substrates, dried at 120 °C for 4 h, followed by heating at 400 °C and then thermal annealing at 750 °C for 2 h in air	α31 = 29.5 V/cmOe	[114]
PVP for NFO and PZT Mw = 1 300 000	NiFe2O4–Pb(Zr0.52Ti0.48)O3 NFO-PZT	Fibers dried at 40 ℃ for 24 h and annealed in air at 650 ℃ for 1 h	Av = δHr /V Av = – 24 Oe/V Converse effect, @ 5.4 GHz [76] MD = Δε’/ε’ MD = 8 % (unassembled), -2 % (assembled in magnetic field) @ 20-22 GHz, H = 0.8 T [116]	[115,116]
PVP for NDO Mw = 1 300 000 PVA for PZT Mw = 50 000	NdFeO3- Pb(Zr0.52Ti0.48)O3 NDO-PZT	Nanofibers were kept on a hot plate to dry at 100 for 10 h, followed by annealing at 850 C for 8 h	-	[117]
PVP for CFO and BFO	CoFe2O4-BiFeO3 CFO-BFO	Nanofibers were collected to Pt/Ti/SiO2/Si substrates, dried at 120 °C for 4 h, followed by thermal annealing at 750 °C for 2 h in air	220-250 V/cm Oe -	[118]
PVP Mw = 1 300 000 For NFO and BFO	NiFe2O4-BiFeO3 NFO-BFO	The samples were dried at 80 °C for 8 h, calcined at 350 °C for 2 h, and then calcined at 700 °C for 4 h, in air.	-	[119]
PVP Mw = 1 300 000 For NFO and BTO	NiFe2O4-BaTiO3 NFO-BTO	Dried in an oven at 40 °C for 24 h, and then annealed for 1 h at 600–700 °C in air	0.4 mV/cm Oe @ 30 Hz	[120]
PVP for CFO and BCZT	CoFe2O4-Ba(Zr0.2Ti0.8)O3–0.5(Ba0.7Ca0.3)TiO3 (CFO-BCZT)	Dried at 120 °C for 90 min and calcined at temperatures between 700 °C and 1000 °C, for 1 h.	-	[121]
PVP for BNFO and PZT	Ba2Ni2Fe12O22-Pb(Zr0.52Ti0.48)O3 BNFO-PZT	Dried on a hotplate at 100 °C for 6 h and then annealed at 1100 °C for 6 h	MD = Δε’/ε’ MD = –2.4 % @ 100 Hz, H = 0.6 T	[122]
PVP for CFO and BCTSO	CoFe2O4–Ba0.95Ca0.05Ti0.89Sn0.11O3 CFO-BCTSO	Fibers dried at 80 °C under vacuum for 12 h and then annealed at 700 °C for 4 h in air	α = 0.346 V/cm Oe @ H = 1 T	[123]

Table 2. Multiferroic core-shell nanofibers prepared by electrospinning. The transporting polymers, high temperature procedure to vaporize the polymer and coalesce the grains to form the fibres is presented, along with the magnetoelectric coefficient, when measured. PVP - Polyvinylpyrrolidone, PMMA – Polymethyl methacrylate, PVA - Polyvinyl alcohol. MD – Magnetodielectric measurements.

Polymer	Core-Shell fibers	Heat treatment	Magnetoelectric Coefficient	Ref
PVP for CFO Mw = 1 300 000 PMMA for PZT Mw = 120 000	CoFe2O4-Pb(Zr0.52Ti0.48)O3 CFO-PZT	Nanofibers were collected on Pt/Ti/SiO2/Si substrates, dried at 120 °C for 4 h, followed by heating at 400 °C and then thermal annealing at 750 °C for 2 h in air	α31 = 29.5 V/cmOe	[114]
PVP for NFO and PZT Mw = 1 300 000	NiFe2O4–Pb(Zr0.52Ti0.48)O3 NFO-PZT	Fibers dried at 40 ℃ for 24 h and annealed in air at 650 ℃ for 1 h	Av = δHr /V Av = – 24 Oe/V Converse effect, @ 5.4 GHz [76] MD = Δε’/ε’ MD = 8 % (unassembled), -2 % (assembled in magnetic field) @ 20-22 GHz, H = 0.8 T [116]	[115,116]
PVP for NDO Mw = 1 300 000 PVA for PZT Mw = 50 000	NdFeO3- Pb(Zr0.52Ti0.48)O3 NDO-PZT	Nanofibers were kept on a hot plate to dry at 100 for 10 h, followed by annealing at 850 C for 8 h	-	[117]
PVP for CFO and BFO	CoFe2O4-BiFeO3 CFO-BFO	Nanofibers were collected to Pt/Ti/SiO2/Si substrates, dried at 120 °C for 4 h, followed by thermal annealing at 750 °C for 2 h in air	220-250 V/cm Oe -	[118]
PVP Mw = 1 300 000 For NFO and BFO	NiFe2O4-BiFeO3 NFO-BFO	The samples were dried at 80 °C for 8 h, calcined at 350 °C for 2 h, and then calcined at 700 °C for 4 h, in air.	-	[119]
PVP Mw = 1 300 000 For NFO and BTO	NiFe2O4-BaTiO3 NFO-BTO	Dried in an oven at 40 °C for 24 h, and then annealed for 1 h at 600–700 °C in air	0.4 mV/cm Oe @ 30 Hz	[120]
PVP for CFO and BCZT	CoFe2O4-Ba(Zr0.2Ti0.8)O3–0.5(Ba0.7Ca0.3)TiO3 (CFO-BCZT)	Dried at 120 °C for 90 min and calcined at temperatures between 700 °C and 1000 °C, for 1 h.	-	[121]
PVP for BNFO and PZT	Ba2Ni2Fe12O22-Pb(Zr0.52Ti0.48)O3 BNFO-PZT	Dried on a hotplate at 100 °C for 6 h and then annealed at 1100 °C for 6 h	MD = Δε’/ε’ MD = –2.4 % @ 100 Hz, H = 0.6 T	[122]
PVP for CFO and BCTSO	CoFe2O4–Ba0.95Ca0.05Ti0.89Sn0.11O3 CFO-BCTSO	Fibers dried at 80 °C under vacuum for 12 h and then annealed at 700 °C for 4 h in air	α = 0.346 V/cm Oe @ H = 1 T	[123]

Table 3. Multiferroic composite and Janus nanofibers prepared by electrospinning. The transporting polymers, high temperature procedure to vaporize the polymer and coalesce the grains to form the fibres is presented, along with the magnetoelectric coefficient, when measured. PVP - Polyvinylpyrrolidone, MOKE – Magneto-optic Kerr effect, SHG – Second harmonic generation.

Polymer	Composite and Janus fibers	Heat treatments	Magnetoelectric Coefficient	Ref
PVP Mw = 1 300 000	Ni0.8Zn0.2Fe2O4 - Ba0.7Sr0.3TiO3 NZFO-BSTO composite fibers	Composite BSTO/NZFO fibers, with molar ratios of 95/5, 90/10, 80/20 and 70/30, were annealed at 700 °C for 2 h	MD = Δε’/ε’ MD ~18.2% @ 1 kHz, H = 6.3 kOe	[124]
PVP	CoFe2O4-BaTiO3 CFO-BTO composite fibers	Composite CFO/BTO fibers with molar ratio 50/50 were dried in an oven at 120 °C for 8 h, followed by annealing at 700 °C for 2 h	α31 = 7.8 V/cmOe	[125]
PVP Mw = 1 300 000	CoFe2O4-Pb(Zr0.52Ti0.48)O3 CFO-PZT composite fibers	Composite CFO/PZT fibers with molar ratios 0.75:1, 1:1, and 1.25:1 was dried at 120 °C for 4 h⁠, followed by heating at 400 °C and then annealed at 550 °C for 2 h in air.	-	[126,127]
PVP Mw = 1 300 000	NiFe2O4–Pb(Zr0.52Ti0.48)O3 NFO-PZT composite fibers	Composite NFO/PZT fibers with molar ratios of 0.75:1, 1:1, and 1.25:1 was dried at 120 °C for 4 h⁠, followed by heating at 400 °C and then annealed at 550 °C for 2 h in air	-	[128]
PVP Mw = 1 300 000	CoFe2O4–(Ba0.95Ca0.05)(Ti0.89Sn0.11)O3 CFO–BCTSO composite fibers	Composite BCTSn/CFO fibers with a 1:1 molar ratio was dried at 80 °C under vacuum for 12 h before being annealed at 700 °C for 4 h in an air.	-	[129]
PVP Mw = 1 300 000	CoFe2O4- BiFeO3 CFO-BFO composite fibers	Molar ratios of 1:0, 1:0.5, 1:1, 1:1.5 and 0:1. The nanofibers were dried at 60 °C for 12 h, followed by heating at 400 °C for 1.5 h and then annealed at 600 °C for 2 h in air	-	[130]
PVP Mw = 1 300 000	CoFe2O4-BaTiO3 CFO-BTO Janus fibers	The fibers were calcined at 750°C for 2 h	Changes in magnetization at the BTO ferroelectric Curie temperature [23,131,132] MOKE [133]Magnetic field-dependent polarization-resolved SHG [134]	[23,131,132,133,134,135]

Table 3. Multiferroic composite and Janus nanofibers prepared by electrospinning. The transporting polymers, high temperature procedure to vaporize the polymer and coalesce the grains to form the fibres is presented, along with the magnetoelectric coefficient, when measured. PVP - Polyvinylpyrrolidone, MOKE – Magneto-optic Kerr effect, SHG – Second harmonic generation.

Polymer	Composite and Janus fibers	Heat treatments	Magnetoelectric Coefficient	Ref
PVP Mw = 1 300 000	Ni0.8Zn0.2Fe2O4 - Ba0.7Sr0.3TiO3 NZFO-BSTO composite fibers	Composite BSTO/NZFO fibers, with molar ratios of 95/5, 90/10, 80/20 and 70/30, were annealed at 700 °C for 2 h	MD = Δε’/ε’ MD ~18.2% @ 1 kHz, H = 6.3 kOe	[124]
PVP	CoFe2O4-BaTiO3 CFO-BTO composite fibers	Composite CFO/BTO fibers with molar ratio 50/50 were dried in an oven at 120 °C for 8 h, followed by annealing at 700 °C for 2 h	α31 = 7.8 V/cmOe	[125]
PVP Mw = 1 300 000	CoFe2O4-Pb(Zr0.52Ti0.48)O3 CFO-PZT composite fibers	Composite CFO/PZT fibers with molar ratios 0.75:1, 1:1, and 1.25:1 was dried at 120 °C for 4 h⁠, followed by heating at 400 °C and then annealed at 550 °C for 2 h in air.	-	[126,127]
PVP Mw = 1 300 000	NiFe2O4–Pb(Zr0.52Ti0.48)O3 NFO-PZT composite fibers	Composite NFO/PZT fibers with molar ratios of 0.75:1, 1:1, and 1.25:1 was dried at 120 °C for 4 h⁠, followed by heating at 400 °C and then annealed at 550 °C for 2 h in air	-	[128]
PVP Mw = 1 300 000	CoFe2O4–(Ba0.95Ca0.05)(Ti0.89Sn0.11)O3 CFO–BCTSO composite fibers	Composite BCTSn/CFO fibers with a 1:1 molar ratio was dried at 80 °C under vacuum for 12 h before being annealed at 700 °C for 4 h in an air.	-	[129]
PVP Mw = 1 300 000	CoFe2O4- BiFeO3 CFO-BFO composite fibers	Molar ratios of 1:0, 1:0.5, 1:1, 1:1.5 and 0:1. The nanofibers were dried at 60 °C for 12 h, followed by heating at 400 °C for 1.5 h and then annealed at 600 °C for 2 h in air	-	[130]
PVP Mw = 1 300 000	CoFe2O4-BaTiO3 CFO-BTO Janus fibers	The fibers were calcined at 750°C for 2 h	Changes in magnetization at the BTO ferroelectric Curie temperature [23,131,132] MOKE [133]Magnetic field-dependent polarization-resolved SHG [134]	[23,131,132,133,134,135]

3.3.2. Composite and Janus Multiferroic Fibers

Composite nanofibers with random mixing between both phases and Janus nanofibers have been successfully prepared by electrospinning in the systems such as Ni_0.8Zn_0.2Fe₂O₄-Ba_0.7Sr_0.3TiO₃ [124], CoFe₂O₄-BaTiO₃ [125], CoFe₂O₄-Pb(Zr_0.52Ti_0.48)O₃ [126,127], NiFe₂O₄–Pb(Zr_0.52Ti_0.48)O₃ (see Figure 6) [128], CoFe₂O₄–(Ba_0.95Ca_0.05)(Ti_0.89Sn_0.11)O₃ [129] and CoFe₂O₄- BiFeO₃ [130]. A summary of recent work is presented in Table 3.

Janus-type biphasic nanowires or nanofibers, characterized by two distinct phases in their hemi-cylindrical sections, have been successfully synthesized [23,131,132]. Unlike core-shell structures where the internal component is inaccessible, Janus-type fibers offer the advantage of exposing both phases while maintaining a substantial interfacial contact area. This unique configuration presents significant potential for enhanced strain coupling in multiferroic nanocomposites. Janus composites of CoFe₂O₄-BaTiO₃ are particularly interesting and examples are listed in Table 3.

Research on the magnetoelectric (ME) response of electrospun fibers with core-shell, composite, or Janus-type multiferroic structures remains limited. Most studies have primarily focused on processing methods and basic structural characterization, with less emphasis on comprehensive functional property analysis. Despite this, these structures have demonstrated promising multiferroic and magnetoelectric behavior, with ME coefficients ranging from 0.3-29.5 Vcm^-1Oe^-1 (see Table 2 and Table 3). This emerging field warrants further investigation.

4. Composite Fibers with Organic Ferroelectric Crystals

Beyond ferroelectric polymers, nanofibers of ferroelectric crystalline materials such as triglycine sulfate (TGS), (1,4-diazabicyclo [2.2.2]-octane) perrhenate (dabcoHReO₄) and -glycine have been developed in recent years [24,136,137]. TGS, a well-known ferroelectric material with a Curie temperature just above room temperature (TC ~ 50°C) is widely used in infrared detectors taking advantage of its high pyroelectricity.

Nanofibers formed from TGS tend to assume a core-shell structure, where the core predominantly consists of the base polymer and the TGS grains form a shell surrounding it. Piezoelectric force microscopy (PFM) and impedance spectroscopy [136,138] established the presence of ferroelectricity in these fibers with a ferroelectric transition temperature close to that of bulk TGS crystals.

Similarly, fibers were synthesized from dabcoHReO₄ (1,4-diazabicyclo [2.2.2]-octane perrhenate), a crystalline ferroelectric material known for having the highest spontaneous polarization among water-soluble organic ferroelectrics. Structural, piezoelectric (PFM) and pyroelectric characterization [24] demonstrated their ferroelectric nature and revealed promising pyroelectric and piezoelectric coefficients, making them suitable for nano energy harvesting applications. These fibers achieved a mechanical-to-electrical energy conversion efficiency of approximately 14%, Figure 7, underlining their potential for energy harvesting applications.

Electrospun composite nanofibers functionalized with a benzothiazole derivative (BZT 1), incorporating an arylthiophene π-conjugated bridge, dispersed in a poly L-lactic) acid (PLLA) matrix, were successfully synthesized [139]. These fibers were characterized for their linear and nonlinear optical properties as well as their piezoelectric and ferroelectric behavior.

Evaluation of the first molecular hyperpolarizabity β, revealed that BZT 1 is an efficient second-harmonic generator. Local piezoelectric measurements confirmed that the heterocyclic system crystallizes in a nanocrystalline acentric structure when embedded in electrospun nanofibers. Measurements of piezoelectric hysteresis loops via PFM enabled the study of local domain switching under an applied external electric field. The observed out-of-plane hysteresis loop provided direct evidence for the switching of polarization in BZT 1 nanofibers and its ferroelectric properties.

The BZT 1 compound displays a high fluorescence quantum yield of 0.58, Figure 8b,d,e present the PFM images obtained from a single BZT nanofiber deposited on a conductive substrate. The first image shows the as-electrospun nanofiber before any external polarization field was applied. In the next two images (Figure 8d–e), the out-of-plane PFM contrast is observed after applying +100 V (image d) and then -100 V DC voltage (image e) to the conductive cantilever tip for 10 s. The pronounced contrast in the PFM images arises from an induced deformation in response to the applied ‘‘read’’ ac electric field and represents the components of fiber’s polarization vector P. This contrast is roughly proportional to the longitudinal piezoelectric coefficient and is determined by the projection of the polarization vector normal to the substrate. The phase, φ, of the signal depends on the orientation of the polarization. ‘‘Bright’’ contrasts in the measured A.cos(φ) signal indicate that the polarization lies in the substrate plane, while the dark contrasts correspond to the polarization pointing downward to the surface.

The corresponding effective piezoelectric coefficient was estimated to be d₃₃ = 20 pm/V, comparable to the piezoelectric coefficient of a poled poly(vinydelenefluoride) film and dabcoHReO₄ nanofibers [24]. The switching of polarization confirms the ferroelectric nature of BZT 1, consistent with an acentric crystalline structure. Ferroelectricity is only possible in polar crystallographic point groups lacking a center of inversion [140].

Lead-free organic ferroelectric perovskite nanocrystals (MDABCO-NH₄I₃) were integrated into polymer fibers via electrospinning for mechanical energy harvesting [141], as illustrated in Figure 9. These molecular ferroelectrics, offering structural diversity and easy fabrication, exhibit piezoelectric and ferroelectric properties within the organic-inorganic hybrid materials category. The resulting flexible nanofibers act as effective piezoelectric energy harvesters, achieving an effective piezoelectric voltage coefficient, g_eff, of 3.6 VmN⁻¹ and emitting blue luminescence at 325 nm. During the ferroelectric–paraelectric phase transition, embedded MDABCO-NH₄I₃ perovskite nanocrystals display a pyroelectric coefficient comparable to state-of-the-art bulk ferroelectric materials. Moreover, these polymer embedded nanocrystals maintain stable piezoelectric output, resisting oxidation-induced degradation. This durability, combined with their flexible nature, positions these nanocrystal systems as promising candidates for energy harvesting applications.

5. Nonlinear Optical Nanofibers

Polymer-based nonlinear optical (NLO) and electro-optical (EO) materials offer an innovative alternative to conventional inorganic crystalline materials, such as lithium niobate (LiNbO₃, LBO) and potassium dihydrogen phosphate (KH₂PO₄, KDP), for device applications. Polymeric materials possess several unique features that have driven research in the field. One notable advantage is their low spectral dispersion in the refractive index over a broad range of the electromagnetic spectrum. This characteristic enables the development of traveling-wave infrared modulators operating at exceptionally high modulation frequencies, exceeding 100 GHz, while maintaining relatively long interaction lengths (up to 1 cm). Another advantage of polymeric materials is that they facilitate the fabrication of robust, low-cost optoelectronic devices through monolithic integration with semiconductor electronics.

The electrical and optical properties of organic materials can be tailored through organic synthesis, allowing for the incorporation of chromophores with significant NLO properties. This results in materials with large EO coefficients and low drive voltages. Controlling molecular morphology and dipole orientation is critical for optimizing the performance of NLO organic devices. In polycrystalline materials with randomly oriented molecules, the net effective dipole moment is negligible, limiting their utility [142].

Electrospinning provides a technique to achieve strong molecular orientation within polymeric fibers, which significantly enhances their anisotropic properties. Organic molecules embedded in electrospun fibers can exhibit high degrees of molecular order, enabling their application in nonlinear optics. [143].

An alternative method to force a macroscopic molecular orientation in polycrystalline NLO materials is the polling of polymers doped with NLO molecules by application of an strong external DC electric field. Here, the guest-host polymer is heated above its glass transition temperature and subsequently placed in a strong electric field before cooling. The molecular orientation of the NLO chromophores is then frozen during the subsequent cooling process. While effective, this method often results in significant chromophore relaxation after electric field removal, leading to decreased anisotropic properties.

In contrast, the electrospinning technique is a powerful tool for controlling the morphology and orientation of polymer chains and the embedded crystalline NLO molecules mitigating relaxation issues. It can be employed with a variety of different polymers and molecular complexes [144].

In one of the earliest examples, Isakov et al. [21] obtained strongly anisotropic optical second harmonic generation based on highly aligned nanofibers of 2-methyl-4-nitroaniline C₇H₈N₂O₂, MNA) embedded in poly(L-latic acid) (PLLA) nanofibers, fabricated by the electrospinning technique, Figure 10 a). MNA, a derivative of 4-nitroaniline or para-nitroaniline (C₇H₈N₂O₂, pNA), achieves noncentrosymmetric crystallization by substituting a hydrogen atom with a methyl group at the 2-position of the benzene ring. Crystals of MNA exhibit a very large phase matched second harmonic generation (SHG) with a figure of merit over 2 orders of magnitude greater than that of lithium niobate [142].

X-ray diffraction analyses revealed that MNA molecules within the electrospun fibers predominantly crystallized with their (010) crystallographic planes parallel to the fiber plane, Figure 10 e). Second harmonic generation polarimetry, shown in Figure 10f,g, confirmed a high degree of anisotropy in an array of MNA_PLLA electrospun nanofibers with an effective nonlinear susceptibility coefficient estimated to be 148 pm/V. The signal generated by a mat of MNA_PLLA fibers is as strong as in bulk MNA, highlighting the potential of these nanofibers for a variety of NLO applications. Furthermore, the NLO response was strongly correlated with the size of the MNA nanocrystals within the polymeric fiber host, which could be tuned by adjusting the electrospinning processing parameters. Over the range explored the nonlinear optical response of the electrospun nanofibers of MNA varied by an order of magnitude [145].

High resolution confocal laser scanning microscopy, Figure 11a,b, demonstrated that MNA nanocrystals are homogeneously distributed within the PLLA polymer fiber with particle sizes below 100 nm [145]. The mean MNA crystallite size varies with the fiber diameter: thinner fibers contain smaller crystallites. The SHG efficiency of the nanofibers varied with fiber diameter, showing reduced efficiency when the diameter approached the optical coherence length, due to phase mismatch effects, Figure 11c).

Similar advances have been made using para-nitroaniline (pNA), a prototype molecule for organic NLO materials. It has a delocalized π-π* electron system and unsaturated bridge linking a donor amino group (NH₂) and an acceptor nitro group (NO₂). Despite its large molecular hyperpolarizability [146,147,148], bulk pNA crystals belong to the centrosymmetric space group P2₁/n, preventing macroscopic second harmonic generation [149]. Embedding pNA into electrospun fibers overcame this limitation, as the process induced highly oriented mesocrystalline structures with a dominant acentric surface as shown in Figure 12 [150,151].

During the electrospinning process, pNA was incorporated into PLLA polymer fibers with a strong preferred crystallographic orientation. The X-ray diffraction pattern was dominated by the (202) Bragg reflection which was two orders of magnitude more intense than all the other observed reflections. However, all Bragg reflections could be assigned to the known centrosymmetric crystalline structure indicating that it was not a polymorph.

NLO measurements performed on a single fiber concluded that the effective mean nonlinear optical coefficient d_eff was around 42 pm/V, twice that of the highest coefficient measured on meta-nitroaniline or 3-nitroaniline (mNA or 3NA) crystal, d₃₃ = 21 pmV^-1, an established SHG crystalline compound. The results demonstrate the possibility of engineering a nanofiber array of centrosymmetric nanocrystals with a strong SHG response comparable to that of single non-centrosymmetric organic crystals. The observed effect resulted from the multiple crystal surfaces having a common orientation with highly aligned molecules forming a head-to-tail dipolar arrangement. Fluorescence lifetime microscopy, Figure 12c) indicate the presence of strong excitonic coupling between molecular units within the fibers and also in bulk pNA crystals.

Another example involves 3-nitroaniline (3NA) nanocrystals embedded in poly-ε-caprolactone (PCL), 3NA@PCL, fibers, displayed in Figure 13. The X-ray diffractogram of a 3NA@PCL nanofiber mat revealed that the (400) Bragg peak was the most intense, a notable contrast to the corresponding diffraction pattern from a bulk polycrystalline sample (Figure 13a). This indicates a strong preferential orientation within the fiber mat (Figure 13b). In the crystal unit cell, 3NA molecules are organized such that their molecular dipole moments (represented by arrows) collectively align to produce a net dipole moment parallel to (400) plane, orientated along the polar axis. In the fibers, the (400) plane aligns parallel to the surface of the fiber mat surface, so that the polar axis is directed along the longitudinal fiber axis. This preferential orientation arises due to the strong electric field applied during the electrospinning process, which aligns the high molecular dipole moments within the fibers.

The anisotropy of polarity curves measured on a nanofiber array and on a (100) oriented 3NA single crystal platelet, under identical excitation conditions, confirm the preferred orientation of the nanocrystals embedded into the fibers (Figure 13c). Notably the effective second order susceptibility measured on a single nanofiber was $d_{eff}^{3NA@PCI}=80 pm/V$, four times greater than $d_{eff}^{3NA}=21 pm/V$, the corresponding value for macroscopic 3NA crystals [26]. Consequently, the push-pull nitroaniline derivatives embedded within each fiber act as enhanced, polarized nanoemitters of second harmonic generation (SHG) light. Furthermore, the nanocrystals within the fibers are significantly smaller than the optical coherence length of a 3NA platelet crystal (approximately 10 μm), enhancing the SHG output intensity compared to bulk 3NA since phase mismatch is insignificant in the fibers.

Beyond their nonlinear optical (NLO) properties, 3NA@PCL nanofiber mats also function as electric power sources via the piezoelectric effect. Applying a periodic force induces polarization within the acentric crystalline material, enabling an external electric current to flow between the opposite faces of the mat through a load resistance (Figure 13g,h). As illustrated in Figure 13g), a periodic force of 3 N generated a voltage and current of up to 7 V and 70 nA, respectively, yielding an instantaneous power density of 122 nW/cm² [26].

Another push-pull molecule closely related to pNA is 2-amino-4-nitroaniline, (2A4NA, C₆H₇N₃O₂), which, like MNA, is an engineered molecule obtained by replacing a meta-hydrogen on the benzene ring by an amino group [152]. The NLO response of electrospun fibers mats composed by 2A4NA grains inside PLLA polymeric fibers (see Figure 14) were investigated and compared with 3NA crystals under identical conditions [153].

The average diameter of the electrospun 2A4NA@PLLA fibers was 770 nm. X-ray diffraction (XRD) patterns revealed a preferred orientation of 2A4NA and MNA nanocrystals within the polymer fibers. This orientational order was further confirmed by the anisotropy observed in SHG polarimetry curves. For both 2A4NA@PLLA and 3NA@PLLA, the q-p configuration yielded a maximum intensity nearly an order of magnitude higher than the q-s configuration. For comparison, Figure 13e) shows the polarimetry curves measured on an individual 3NA@PCL fiber under the same excitation conditions [26]. Both the 3NA@PLLA and 3NA@PCL fibers exhibited nearly identical anisotropic SHG responses, suggesting that the polymer matrix does not significantly alter the orientational distribution of the nanocrystals. There is negligible optical damage of the nanofibers during the acquisition as the curves possess symmetric maxima, and close on themselves. The nanofibers are optically robust to laser damage.

The estimated effective second-order susceptibility coefficient for 3NA@PLLA was $d_{eff}^{3NA@PLLA}=56 pm/V$, which is 1.5 times smaller than that measured for a 3NA@PCL nanofibers, $d_{eff}^{3NA@PCL}=80 pm/V$ [26]. For 2A4NA@PLLA the estimated effective second-order susceptibility coefficient was $d_{eff}^{2A4NA@PLLA}=42 pm/V$, which is among the highest values reported for organic crystals formed by simple pNA derivative push-pull molecules. The spatial distribution of second harmonic emitters within single 3NA@PLLA and 2A4NA@PLLA fibers is displayed in Figure 14c,d. The 3NA@PLLA fibers showed a relatively uniform distribution of SHG emission along the excited fiber, whereas the SHG emission from a 2A4NA@PLLA fiber is punctuated by intense spots.

Finally, in addition to charge transfer push-pull molecules, electrospun nanofibers embedded with urea, a well-known organic crystalline material, have been produced from precursor solutions with two different polymers: poly(ethylene oxide) (PEO) and poly(vinyl alcohol) (PVA). Both polymers are very soluble in water, as is urea.

Urea was one of the first organic NLO crystals available commercially. Its nonlinear optical properties been extensively studied due to its simple structure and strong second-harmonic generation (SHG) response. n its bulk crystalline form, urea exhibits an effective second-order nonlinear coefficient approximately 2.5 times greater than that of potassium dihydrogen phosphate (KDP), a widely used inorganic crystal.

With PEO, highly crystalline and orientated nanofibers containing two different polymeric host–guest inclusions were formed, Figure 15. They are the stable α complex and the metastable β complex [154,155].

The α complex has a (PEO)4–(urea)9 stoichiometry [156], and fibers were successfully prepared from a gel-like suspension of a previous inclusion compound (IC) co-crystallized complex. These highly oriented and well-aligned fibers displayed a strong molecular orientation with PEO chains confined inside urea channels. This strong molecular orientation inside the fibers was attributed to a reorientation of rigid crystalline units during the electrospinning process [157]. Remarkably, the electrospinning process stabilized the metastable β phase with a (PEO)3–(urea)2 stoichiometry resulting in an orthorhombic crystal structure as determined from the X-ray diffraction data [155].

Figure 15. (a) Wide-angle X-ray diffraction 2θ diagrams for (a) the bulk PEO−urea inclusion complex (IC) and (b) electrospun fibers. (c) Crossed-polarized optical micrograph of electrospun fibers of the PEO−urea inclusion complex [157]. Representative crossed-polarized optical micrographs of fibers prepared by electrospinning solutions with (d) 4:9 and (e) 3:2 PEO:urea molar ratios [155].

Figure 15. (a) Wide-angle X-ray diffraction 2θ diagrams for (a) the bulk PEO−urea inclusion complex (IC) and (b) electrospun fibers. (c) Crossed-polarized optical micrograph of electrospun fibers of the PEO−urea inclusion complex [157]. Representative crossed-polarized optical micrographs of fibers prepared by electrospinning solutions with (d) 4:9 and (e) 3:2 PEO:urea molar ratios [155].

Unlike the PEO-urea fibers, the PVA-urea nanofibers (1:1 polymer-to-urea mass ratio) show no evidence of inclusion complexes in their X-ray diffraction patterns. Instead, urea nanocrystals inside the PVA matrix retained the tetragonal structure of bulk urea crystals (Figure 16). However, the nanocrystalline urea within the fibers displays a strong preferential orientation with their (110) plane parallel to the fiber mat plane, Figure 16c).

The SHG response of urea was quite different in these two polymer fibers. In urea-PVA nanofibers, the urea nanocrystals generated a SHG response as intense as a pure urea powder with an average grain size of 110 µm. The SHG response of isotropic PEO-urea fibers was about two orders of magnitude less than the urea-PVA fibers, Figure 16d,e. However, no SHG signal was observed from urea-Peo fibers. These findings highlight the importance of the host polymer and the resulting crystal structure in determining the nonlinear optical properties of urea-embedded nanofibers.

Although the above results are encouraging, further research is needed to elucidate the mechanisms behind the differing SHG responses and to optimize the fabrication processes for enhanced nonlinear optical properties of organic nanocrystals embedded in polymeric electrospun fibers. Deeper insights into the process will facilitate the development of novel materials with tailored nonlinear optical characteristics

6. Conclusions

In conclusion, this review highlights the promising potential of the electrospinning technique in fabricating multifunctional ferroelectric based nanofibers from a diverse array of materials, including inorganic, semi-organic and all organic compounds. By carefully selecting an appropriate polymer matrix, these materials can be transformed into nano and micro structured hybrid functional systems. These hybrid systems encompass a range of functional materials including piezoelectric ceramics, inorganic ferroelectrics, single-phase ferroelectrics and magnetoelectric multiferroic materials, which exhibit simultaneous magnetic and electric ordering. Among the semi organic crystals, particular emphasis was placed on triglycine sulfate (TGS) due to its technological significance.

In the realm of nonlinear optics, fabrication of nanomaterials in the form of nanofibers using chromophores with high molecular hyperpolarizabilities offers an innovative alternative to conventional inorganic crystalline materials for device applications. In particular, nanofibers incorporating push-pull nitroaniline derivatives have demonstrated enhanced optical second harmonic generation, and have strong potential as nanoscale emitters of polarized light.

Additionally, crystalline hybrid nanofiber mats with pronounced piezoelectric properties have been developed, capable of efficiently converting modest applied forces into significant electrical outputs, generating appreciable piezoelectric voltages.

Looking forward, the quest for cost-effective sensors, actuators, and electronic and photonic components, alongside the increasing demand for device miniaturization and integration, positions electrospun nanofibers as a promising solution. The integration of multiple functionalities into a single nanostructure using the electrospinning technique allows researchers to addresses the key requirements of affordability, flexibility, and versatility when seeking to develop novel multifunctional device technologies.