Anisotropic PCL nanofibers embedded with nonlinear nanocrystals as strong generators of polarized second harmonic light and piezoelectric currents

Using the electrospinning technique nanofibers consisting of organic nonlinear optical 3-nitroaniline (3NA, C6H6N2O2) nanocrystals embedded in poly-ε-caprolactone (PCL) polymer, 3NA@PCL nanofibers, were produced. Polarimetry optical second harmonic generation and X-ray diffraction studies show that 3NA push–pull molecules crystallize inside the polymer fibers with a strong preferential orientation giving rise to an alignment of the molecular dipole moments along the nanofibers longitudinal axis. This alignment strongly enhances the second order nonlinear optical response of the fibers. Intense second harmonic generation emission was observed from a single nanofiber, corresponding to an effective second order susceptibility of 80 pm V−1, four times greater than the largest second order susceptibility tensor element (21 pm V−1) associated with a macroscopic 3NA crystal. Moreover, when subjected to a modest periodically applied force of 3 N, a piezoelectric current of 70 nA generated by a 4 cm2 electrospun nanofiber mat amounted to 122 nW cm−2 of instantaneous density power, sufficient to power a LCD display. The results show that the electrospinning technique is a powerful technique to fabricate organic functional materials with oriented nanocrystals made of highly polarizable molecules, embedded in a polymer matrix.


Introduction
Low dimensional nanostructures in the form of bers, wires, rods, belts, tubes, and rings have attracted attention due to their novel properties and potential applications as sensors, exible electronic devices or nanogenerators of electrical energy. Compared to other methods of fabricating one-dimensional nanostructures, electrospinning is a simple and versatile technique capable of generating nanobers from a variety of polymers with embedded organic or semi-organic nanocrystals forming hybrid composite materials. 1À3 The technique has recently been employed to organize nonlinear optical (NLO) chromophores into subwavelength scale architectures with rationally designed functionalities. [4][5][6][7][8] Molecular materials with highly efficient nonlinear optical (NLO) properties play an important role in modern technologies with applications in light frequency conversion and integrated optics. 9 Nitroaniline derivative molecules are known for their large microscopic molecular hyperpolarizabilities and for those crystallizing into an acentric point group, a large second harmonic generation (SHG) response has been measured for most of them. The molecules have an admixture of zwitterionic D + -p-Acharacter in the ground state that reverses in the excited state. Assuming a two-level sum over-states model, this electronic structure gives origin to a strong rst hyperpolarizability due to a change in the dipole moment orientation between the ground and excited state. 10 para-Nitroaniline or 4-nitroaniline (pNA, C 6 H 6 N 2 O 2 ), which serves as a model in molecular NLO exhibits very large molecular rst order and second order polarizability. 11,12 The molecule is formed by a benzene ring with a donor NH 2 and acceptor NO 2 groups in para-positions on the benzene ring. meta-Nitroaniline or 3-nitroaniline (3NA, C 6 H 6 N 2 O 2 ), is an isomeric molecule of pNA with the acceptor NO 2 group in the position three of the benzene ring. Although their molecular dipole moments have similar magnitudes, respectively 51.46 D for pNA and 45.02 D for 3NA, 11 they crystallize in different structures. While pNA crystals have a center of symmetry, 3NA crystallizes in the polar point group mm2. 13,14 The crystal is biaxial with refractive indices n 1 ¼ 1.805, n 2 ¼ 1.715, n 3 ¼ 1.675 and the principal axis electro-optic constants are r 33 ¼ 16.7 pm V À1 , r 23 ¼ 0.1 pm V À1 , r 13 ¼ 7.4 pm V À1 at 632.8 nm, combining a low dielectric permittivity with a strong electro-optic behavior. 15,16 3NA crystals also display strong nonlinear optical effects due to its high second order susceptibility coefficients 17 Furthermore, it exhibits piezoelectric coefficients which are similar in magnitude to those of the well-known nonlinear optical and piezoelectric lithium niobate, LiNbO 3 , crystal. 18 3NA was recently reported to display ferroelectric behavior. 3,19 Good optical quality molecular organic crystals are more difficult to growth than inorganic crystals but their performance as excellent quadratic nonlinear crystals largely exceed that of inorganic ones. Growing nitroaniline derivative nanocrystals under the form of thin lms or nanobers has been recently explored with a view of developing them for applications in ultrafast optics. [20][21][22] A strong nonlinear optical response was reported from electrospun bers of 2-methyl-4-nitroaniline (MNA, C 7 H 8 N 2 O 2 ) embedded in poly-L-lactic acid (PLLA) polymer. The observed intense second harmonic response resulted from a high degree of orientation of the MNA molecules inside the nanocrystal lattice originating a net dipolar moment along the nanober longitudinal axis. 3 Moreover, it was demonstrated that by tuning the electrospinning parameters the second harmonic nonlinear response of MNA nanocrystals embedded into the nanobers increased by an order of magnitude. 6 This molecule is an engineered pNA molecule in which a methyl CH 3 group has been substituted in the 2-position of the benzene ring to achieve crystalline noncentrosymmetry. 23 Although 3NA molecules have been studied in depth both in solution and as macroscopic crystals, unlikely pNA 24-27 and MNA its properties at nanoscale have not been addressed, in particular its SHG and piezoelectric behaviour when incorporated into polymer nanobers.
In this work we report a strong polarized SHG response from a single electrospun nanober formed by well-oriented 3NA organic nanocrystals embedded in poly-3-caprolactone (PCL) polymer. An effective second order susceptibility coef-cient d 3NA@PCL eff ¼ 80 pm V À1 has been measured in a nano-ber, which is four times larger than that reported for a macroscopic crystal. This is the rst time 3NA high hyperpolarizable molecules have been integrated in functional nanobers and their crystalline nonlinear optical response studied.
Also, upon applying a periodical force of 3 N to a 3NA@PCL nanober mat, a piezoelectric current of 70 nA was generated with a maximum output power density of 122 nW cm À2 . This power is sufficient to operate a LCD display, making 3NA@PCL nanobers mats suitable for use as exible piezoelectric nanogenerators.

Materials and electrospinning of nanobers
Nanobers were produced by a conventional electrospinning technique described previously. 3 A 10% polymer solution formed by 0.5 g of 3NA and 0.5 g of poly-3-caprolactone (PCL, M w 80 000) on a 1 : 1 weight ratio were dissolved in 1 mL of dimethylformamide (DMF) and 4 mL of dichloromethane (DCM) solvents mixture. The chemicals were all purchased from Aldrich and used as received. The resulting clear and homogenous solution was stirred for several hours under ambient conditions prior to the electrospinning process. The precursor solution was loaded into a syringe with its needle connected to the anode of a high voltage power supply (Spellmann CZE2000). To produce the in-plane bers the spinning voltage was set at 16 kV. The distance between anode and collector was 12 cm and precursor solution ow rate of 0.15 mL h À1 was controlled by a syringe pump with attached needle of 0.5 mm diameter. The ber mat for piezoelectric measurements was collected on high purity aluminium foil which served as electrodes. For optical second harmonic generation measurements individual bers were collected using a transparent glass substrate.

Scanning electron microscopy (SEM)
The morphology, size and shape of 3NA@PCL nanobers was studied using a Nova Nano SEM 200 Scanning Electron Microscope operated at 10 KV accelerating voltage.

X-ray diffraction and Raman spectroscopy
Crystallinity and crystallographic orientation of 3NA nanocrystals inside the electrospun bers was studied by X-ray diffraction. The diffraction pattern using q-2q scans was recorded between 10 and 60 on a Philips PW-1710 X-ray diffractometer with Cu-K a radiation of wavelength 1.5406Å. The lattice planes parallel to the substrate surface were determined from the reciprocal lattice vector of modulus (2/l)sin q, with l the radiation wavelength and q the Bragg angle. Raman spectroscopy was carried out on a LabRAM HR Evolution confocal Raman spectrometer (Horiba Scientic, France) using Horiba Scientic's Labspec 6 Spectroscopy Suite Soware for instrument control, data acquisition and processing. The Raman spectra was obtained using a laser excitation with wavelength 532 nm, at 0.1% laser intensity, with 30 s acquisition time in a spectral range between 50-1750 cm À1 .

Optical absorption and emission
Optical absorption measurements on a 3NA solution were carried out using a Shimadzu UV/2501PC spectrophotometer, while photoluminescence spectra were collected using a Fluo-roMax-4 spectrouorometer. For optical absorption measurements, a 1.0 Â 10 À4 M solution of 3NA was prepared in tetrahydrofuran (THF). The sample was measured in a quartz cuvette with 1 cm path length. Emission spectra from a 1.0 Â 10 À4 M solution of 3NA and from a 3NA@PCL nanober mat, were acquired using an excitation wavelength of 380 nm. The This journal is © The Royal Society of Chemistry 2020 Nanoscale Adv., 2020, 2, 1206-1213 | 1207

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Nanoscale Advances input and output slits were xed to provide a spectral resolution of 5 nm.

Optical second harmonic generation
Second harmonic generation (SHG) light efficiency of an individual 3NA@PCL ber was measured using a polarimetry setup based on a mode-locked Ti:Sapphire laser (Coherent, Mira), a 100 fs temporal pulse width and a 76 MHz repetition rate. The polarization properties of SHG were measured to assess the orientation of 3NA crystals embedded within a nanober. Two different polarization curves were taken. For the q-p conguration, the analyzer was aligned along the direction that gives rise to the maximum SHG signal while a half-wave plate placed in a stepper motor controlled rotation stage was used to vary the incident eld polarization. For the q-s conguration, a second half-wave plate behind the sample but before the analyzer was then rotated so that the detected polarization was perpendicular to that of the q-p conguration. To characterize the response of individual bers the incident beam at 800 nm was focused using an Olympus 10Â plan achromatic microscope objective to an estimated full-width at half maximum diameter of approximately 3 mm. A second 10Â microscope objective collimated the generated second harmonic light which was subsequently detected by a cooled CCD mat (Andor Newton) aer being spectrally resolved by a 0.3 monochromator (Andor Shamrock) with a 2400 lines per mm grating. The combination of the second wave plate and xed analyzer orientation allows us avoid problems such as the variation of grating efficiency with polarization, ensuring that q-p and q-s curves have the same normalization.

Piezoelectric current
The piezoelectric output voltage was measured across a 100 MU load resistance connected to a low pass lter followed by a low noise pre-amplier (Research systems SR560) before being registered by a Digital Storage Oscilloscope (Agilent Technologies DS0-X-3012A). The nanober mat was submitted to periodic mechanical forces imposed by a vibration generator (Frederiksen SF2185) with a frequency of 3 Hz imposed by a signal generator (Hewlett Packard 33120A). The forces applied were measured by a calibrated force sensing resistor (FSR402, Interlink Electronics Sensor Technology). During the electrospinning process, the electrospun bers were directly deposited on high purity aluminium foil, which served as electrodes. A mat with 4 cm 2 area and 600 mm thickness was used. All the electric contacts are very stable. During the piezoelectric measurements there are no short circuit across the ber mat, otherwise no output electric current would be measured. The sample mat is, previously to any measurements, checked for no short circuit between the electrodes.

Results and discussion
Fibers morphology, optical absorption and emission The electrospun bers are uniformly shaped, yellow colored with an average diameter of 230 nm forming a mat of continuous nanobers, as indicated in Fig. 1(a) and (b) showing respectively the deposited bers and the corresponding SEM observation. There are no beads and no 3NA crystals evident on the external ber surfaces, all crystals are embedded within the ber interior. To achieve these morphological characteristics, the electrospinning experimental conditions such as solution needle feeding rate, distance from the needle to the collector and voltage applied during the electrospinning process were carefully tuned. The optical absorption and emission curves of 3NA molecules in a THF solution are shown in Fig. 2(a) and emission  1208 | Nanoscale Adv., 2020, 2, 1206-1213 This journal is © The Royal Society of Chemistry 2020 Nanoscale Advances Paper from nanocrystals embedded into electrospun bers in Fig. 2(b). The 3NA@PCL bers have a maximum emission at 550 nm similar to 3NA in solution and are transparent in the range of 450-700 nm as reported for bulk 3NA crystals. 17,18 X-ray diffraction and Raman spectroscopy Bulk crystals of 3NA crystallize in the orthorhombic Pca2 1 space group, point group mm2, with four molecules per unit cell. 13,28 The molecules are almost planar and each consists of an aromatic benzene ring with the acceptor nitro (NO 2 ) group in the para-position while the amino (NH 2 ) donor group occupies the ortho-position originating a molecular dipole moment pointing from donor to acceptor groups. The crystalline structure of 3NA can be described as all-parallel polar layers perpendicular to [100] and interconnected by weak C-H/O bonds. 29 Fig. 3(a) is the X-ray diffraction powder pattern measured on a 3NA@PCL ber mat and insets marked 3NA and PCL are those calculated for bulk 3NA crystals and PCL polymer, obtained using the program Mercury and published crystallographic information le.
The diffractograms show that for bulk 3NA the most intense Bragg reection is (211) followed by (400) and (311) both with half intensity of (211), all the others reections with much smaller intensities. For PCL there are only three intense Bragg reections with (110) an order of magnitude more intense than the other two. However, the measured X-ray pattern of an electrospun nanober mat shows that the (400) Bragg reection is the most intense and is followed by (200) reection. Furthermore, (211) reection is now roughly ve times less intense than (400). This indicates that for 3NA@PCL electrospun mats there is a strong preferential orientation as 3NA molecules crystallize inside the bers such that the crystallographic plane (400) aligns with the ber mat plane. Fig. 3(b) shows the crystal unit cell content where 3NA molecules are arranged with their molecular dipole moment (represented by arrows) adding up to a net dipole moment parallel to (400) and pointing along the polar axis. This preferential orientation is induced by the strong electric eld applied during the electrospinning process which tends to align the high molecular dipole moments within the crystal lattice, as reported before. 3 We may therefore envision a 3NA@PCL electrospun ber mat as a hybrid functional composite material formed by highly oriented 3NA nanocrystals embedded within a polymer matrix.
Polarized Raman spectroscopy indicates that some bands are absent, marked with an arrow on Fig. 3(c) and (d) in one of the spectra, consistent with the preferential nanocrystalline orientation inside the bers as concluded from the X-ray powder data collected on 3NA@PCL and above described.

Second harmonic generation
The highly oriented 3NA nanocrystalline arrangement inside the bers should lead to a very anisotropic nonlinear optical response for light traveling with its wave vector perpendicular to the ber longitudinal axis. To verify this hypothesis, the SHG efficiency of a single 3NA@PCL nanober was measured using a custom built polarimetry setup shown in Fig. 4.
The polarization properties of the generated second harmonic light (SHG) were measured to access the existence of preferred orientation for 3NA crystals embedded within the nanobers. For crystals belonging to point group mm2 and taking into account the conditions of Kleinman symmetry, 30 the second order optical polarizability components can be related to the incident fundamental electric eld amplitudes by the matrix relation, 31   This journal is © The Royal Society of Chemistry 2020 Nanoscale Adv., 2020, 2, 1206-1213 | 1209 Paper Nanoscale Advances applied along x,y,z or 1,2,3 dielectric crystal axes, respectively. For an orthorhombic lattice the dielectric axes coincide with the crystallographic axes (see Fig. 3(b) and 4). When the fundamental light is incident along the x axis, the detected signal will vary with the incident eld polarization angle q according to the following relations, The polarimetry SHG measurements were performed on a single ber and it is assumed that the light wave vector is, for most of the 3NA nanocrystals, along the dielectric x axis (see Fig. 4).
The measured polar plots, Fig. 5 were acquired by integrating the signal over 0.25 seconds with an average incident energy of 6.5 pJ. Here, the maximum of intensity occurs when the polarization of the incident and emitted light are parallel to each other and aligned with the ber longitudinal axis. For a 3NA@PCL ber, the q-p polarimetry curve of Fig. 5(a) closely approximates a single cos 4 q pattern, conrming the strong preferred crystallographic orientation of 3NA nanocrystals embedded into the PCL polymer matrix. For a bulk 3NA crystal the second order polarizability tensor element d 33 has a magnitude one order greater than d 32 , 10 therefore in q-p conguration d 33 is the main contributor to the measured intensity. For q-s conguration the measured intensity curve as shown in Fig. 5(b) is one order of magnitude smaller than q-p. Given the ratio of d 33 to d 32 one might expect that the q-s signal should be yet another order of magnitude smaller. The strong contrast in intensity between q-p and q-s curves indicates a strong degree of alignment of nanocrystals inside each ber and reinforces the conclusion taken from the analysis of X-ray diffraction data.
For comparison we present the obtained polarimetry curves measured on a (100) oriented 3NA single crystal platelet with 0.45 mm thickness under the same excitation conditions and the incident beam normally incident on the platelet. The anisotropy of polarity curves is identical to that of 3NA nanocrystals embedded into the bers, conrming the preferred orientation of the nanocrystals. Although the platelet thickness is three orders of magnitude higher than a typical nanober, remarkably the measured SHG output intensity of the platelet is lower than that of the nanober. This lower output from the bulk crystal reects the phase mismatch in the SHG generated by the platelet. There is no phase mismatch for nanocrystals inside the bers, as their size is much smaller than the coherence length of a 3NA crystal which is around 10 mm, 17 which contributes to explain the nanober enhanced SHG response, together with the high alignement of the molecular dipoles along the ber longitudinal axis.
The observed SHG enhancement displayed by electrospun bers dopped with highly hyperpolarized nitroanilines derivative molecules crystallized in a polymer matrix, has been reported before MNA nanocrystals embedded in a PLLA polymer. 3 In this work it is again demonstrated that the electrospinning technique is an efficient method for producing polymer doped nanobers with a high degree of guest molecular polar orientation within the polymer and may be used to design a class of all-organic devices.  This journal is © The Royal Society of Chemistry 2020

Nanoscale Advances Paper
To determine the effective nonlinear susceptibility coefficient from a nanober, d 3NA@PCL eff , the SHG response of the electrospun ber was measured against an oriented beta barium borate (BBO) crystal of 1 mm thickness. This allows us to calibrate the collection efficiency of the set-up, type I phase matching occurs in BBO for an incident eld at 800 nm that propagates at 29.2 relative to the optic axis with an effective second order nonlinear coefficient of d BBO eff ¼ 2.0 pm V À1 . 32 Using an 10Â objective, the effective length over which the fundamental and second harmonic beam remains superimposed with the BBO crystal is limited by the 69 meridian walk-off angle of the extraordinarily polarized second harmonic wave to distance l BBO S of approximately 250 mm. By comparison, temporal walk-off between the fundamental and second harmonic beams is negligible under our conditions. Wang and Weiner 33 have developed a theoretical expression to estimate the efficiency of SHG by ultrashort pulses. Given the tight focus produced by the microscope objective their expression reduces to the form: Here U u and U 2u are the energies of the incident fundamental and generated second harmonic pulses while t p is the FWHM pulse duration of the fundamental beam, roughly 100 femtoseconds. At phase matching the respective refractive indices n u and n 2u are both equal to 1.660. In contrast, the submicron thickness of electrospun nanobers allows one to use the standard phase matched plane wave result 33 for SHG, which leads to a factor of L 3NA@PCL 2 /b: where b ¼ 36 mm is the confocal length of the focused incident beam and L 3NA@PCL is the thickness of an individual ber.
Taking the ratio between the two above expressions allows one to estimate the effective second order susceptibility coefficient of the electrospun bers: where we have neglected small corrections due to differences in the refractive indices. In our measurements the ratio of detected energies in q-p conguration was U 2u 3NA@PCL /U BBO 2u ¼ 0.032 while the ratio of incident fundamental energies was U 3NA@PCL u / U BBO u ¼ 10. However this latter factor should be corrected to take into account that an individual ber with a diameter of 230 nm is signicantly smaller than the estimated focused fundamental beam 1/e 2 diameter of 3.3 mm, effectively reducing the ratio of incident energies to unity.
The estimated effective second order susceptibility coefficient for the q-p conguration is approximately d 3NA@PCL eff ¼ 80 pm V À1 . This is a factor of 4 greater than the largest tensor element d 3NA

33
¼ 21 pm V À1 for bulk crystalline 3NA. 17 In conclusion, highly oriented 3NA nanocrystals inside each ber behave as strong polarized nanoemitters of SHG light. This result indicates that it is feasible to produce a highly directional SHG emitter via the electrospinning technique.

Piezoelectric response
Following the remarkable SHG response, an evaluation of the piezoelectric output voltage generated by a 3NA@PCL nanober mat when deformed by an external applied force was undertaken to access their suitability for potential integration into nanoenergy harvesting processes. The piezoelectric effect results from inter-conversion between mechanical and electrical stimulus inducing a charge redistribution and separation when a mechanical force is applied to a crystalline material. For any crystallographic point group the piezoelectric and SHG crystal properties are described by the same tensor, 31 consequently the polarization P i is related to the stress tensor s j by the matrix equation: For bulk 3NA crystals the magnitude of the piezoelectric coefficients are d 33 ¼ 6.81 pCN À1 , d 32 ¼ 2.55 pCN À1 and d 15 ¼ 30.79 pCN À1 . 18 The output voltage of 3NA@PCL electrospun bers was measured by applying a periodical force perpendicularly to the ber mat and measuring the polarization along the same direction, therefore along the x axis, as indicated in Fig. 6(b). According to the orientation of 3NA crystals embedded into the electrospun bers above described and the piezoelectric tensor elements, eqn (6), the d 15 shear coefficient is the main contributor to the piezoelectric response producing a polarization P 1 ¼ d 15 s 5 , that is a force applied along x originates a polarization along the same direction due a shear stress s 5^s13 . 31 Consequently, the force applied along x(1) is transmitted across the crystal faces perpendicular to z(3) and therefore perpendicular to the molecular dipole moments, see Fig. 3 As shown in Fig. 6(a), a voltage and current up to 7 V and 70 nA, respectively, were obtained when a force of 3 N was periodically applied. Fig. 6(b) is a plot of the output voltage as a function of the applied periodical force, showing a linear increase of the response with the magnitude of the force, as expected. 3NA ber mats may be used as a power source for operating a LCD display: when pressing the electrospun ber mat the letters "nA" are turned on (see video in ESI †).
For a 3NA@PCL ber mat, an instantaneous density power of 122 nW cm À2 was achieved (calculated dividing the maximum electric power by the mat area). This power has a magnitude similar to that displayed by PVDF/PMLG [poly(vinylidene uoride)/poly(g-methyl L-glutamate)] composite bers, six times greater than reported for nanobers embedded with semiorganic ferroelectric DabcoHReO 4 (1,4-diazabicyclo[2.2.2] octane perrhenate) and one order of magnitude smaller than This journal is © The Royal Society of Chemistry 2020 Nanoscale Adv., 2020, 2, 1206-1213 | 1211

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Nanoscale Advances obtained for inorganic ceramic ferroelectric BaTiO 3 (barium titanate) and dipeptide diphenylalanine (Phe-Phe) nanobers. This is indicated in Table 1.

Conclusions
We have fabricated hybrid electrospun nanobers with nanometer thickness and lengths of several centimeters containing highly oriented nanocrystals of 3-nitroaniline, a nonlinear optical molecule with elevated hyperpolarizability, embedded in poly-3-caprolactone polymer. The estimated effective second order susceptibility measured on a single nanober can reach values as high as d 3NA@PCL eff ¼ 80 pm V À1 , a factor of four times greater than the largest tensor element d 3NA

33
¼ 21 pm V À1 associated with macroscopic 3NA crystals. Therefore 3NA nanocrystals inside each ber behave as enhanced polarized nanoemitters of SHG light.
Our results also demonstrate that by embedding 3NA molecules inside polymer bers one is able to fabricate a macroscopy mat of crystalline hybrid functional nanobers with strong piezoelectric response achieving an instantaneous power density of 122 nW cm À2 . In summary, by embedding appropriately chosen organic molecules with large individual high hyperpolarizabilities into a suitable polymer matrix, the electrospinning technique can faster crystallization with strong preferential orientation within each ber. Single nanobers can generate enhanced and strong polarized second harmonic light, whereas nanober mat efficiently convert modest applied forces into piezoelectric currents.

Conflicts of interest
The authors declare that they have no known competing nancial interests or personal relationships that could have appeared to inuence the work reported in this article.