Enhanced Polarization Ratio of Electrospun Nanofibers with Increased

Subscriber access provided by Macquarie University

Article

Enhanced Polarization Ratio of Electrospun Nanofibers with Increased Intrachain Order by Post-Solvent Treatments Sangcheol Yoon, Siyoung Ji, Youngjun Yoo, Ji-Eun Jeong, Jeongho Kim, Han Young Woo, Byoung Choo Park, and Inchan Hwang J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.6b08277 • Publication Date (Web): 28 Nov 2016 Downloaded from http://pubs.acs.org on December 3, 2016

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

The Journal of Physical Chemistry B is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 27

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

Enhanced Polarization Ratio of Electrospun Nanofibers with Increased Intrachain Order by Post-Solvent Treatments Sangcheol Yoon,† Siyoung Ji†, Youngjun Yoo,‡ Ji-Eun Jeong§, Jeongho Kim,‡ Han Young Woo§, Byoungchoo Park*,∥, Inchan Hwang*,† †

Department of Electronic Materials Engineering, Kwangwoon University, Seoul 01897, Republic of Korea ‡

Department of Chemistry, Inha University, Incheon 22212, Republic of Korea

§

∥

Department of Chemistry, Korea University, Seoul 02841, Republic of Korea

Department of Electrophysics, Kwangwoon University, Seoul 01897, Republic of Korea

Corresponding authors: *Tel: +82-2-940-5237. E-mail address: [email protected]. *Tel: +82-2-940-8675. E-mail address: [email protected].

1 Environment ACS Paragon Plus


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 27

Abstract: Polarized emission that is beneficial to lighting and display applications can be demonstrated by aligning emissive chromophores, which can be achieved using an electrospinning technique. We investigate the photophysical properties of nanofibers based on poly[2-methoxy-5-(2-ethylhexyloxy)1,4-phenylenevinylene] (MEH-PPV): poly(ethylene oxide) (PEO) blends both with and without postsolvent treatments. Two different solvents were sequentially used with attempts to extract the insulating electrospinnable polymer and to increase the polarization ratio of the nanofiber meshes by molecular reorganization. The polarization ratio of emission from the nanofiber mehses treated by N,Ndimethylformamide (DMF) following acetonitrile solvents was found to be increased. The increase in the 0-0 emission vibronic intensity relative to the 0-1 peak and the reduction of photoluminescence bandwidth was found. In addition, the photoluminescence decays faster and the parallel component along the nanofiber axis increases after the DMF treatment, indicating the radiative recombination process becomes faster. Our results consistently show that the post-solvent treatment promotes stronger J-aggregate character with longer coherence lengths of exciton along the long axis of nanofibers, due to enhanced intrachain order.


Page 3 of 27

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1. Introduction The polarized emission source can provide beneficial properties to various applications in lighting and displays. The high polarization ratio of emission reduces glare, and thus improves a contrast ratio without polarizers when the source is used as a backlight in liquid crystal displays.1,2 The polarized emission can also be utilized in the applications optical data storage, optical communication, and stereoscopic 3D imaging systems.3-5 Efforts to realize emission with a high polarization ratio have, therefore, been made with a wide range of materials, from inorganic to organic.6-8 Conjugated polymers are a quasi one-dimensional system with intrinsic potential to emit polarized light. However, their amorphous morphology in solid state when spin-coated makes it difficult to achieve a high polarization ratio of emission from films. Several methods such as nanoimprinting, epitaxial growth, and Langmuir Blodgett deposition have been employed to create a polarized emission.9-13 Electrospinning is a simple and versatile technique that can uniaxially align chromophores by constructing nanofibers with controllable diameters ranging from tens of nanometers to micrometers.14,15 It is crucial to fabricate nanofibers as thin as possible without creating beads and disconnectivity for macromolecular alignment in the nanofibers.16-19 In addition to the elongated chromophores inside a nanofiber, nanofibers also need to be uniaxially aligned to achieve highly polarized emission from films. Various types of architectures/collectors such as very thin parallel metal wires, drums and disk collectors, and U-shaped devices have been tested to improve the degree of alignment for nanofibers.2023

Non-conjugated insulating polymers that are electrospinnable must be effectively removed to ensure

working optoelectronic devices.24-26 The elongated conjugated polymers exhibit distinctive optical properties compared with the spincoated films.27,28 In contrast to spin-coated films and solutions which exhibit the spectral features of a mixed intrachain coupling (J-aggregates) and interchain coupling (H-aggregates), except for samples particularly manipulated,28-33 conjugated polymer chains in nanofibers predominantly can have a Jaggregate character.34 Intensive research on the photophysical properties of aggregates have been conducted using both experimental and theoretical approaches. The 0-0/0-1 vibronic peak intensity ratio 3 Environment ACS Paragon Plus


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 27

in photoluminescence (PL) spectra and the radiative decay rate are a measure of intrachain order and the coherence length of excitons.35-39 As the polymer chains in nanofibers are aligned in a stretched form along the direction of a nanofiber, they are expected to have long coherence lengths as a result of enhanced intrachain order. For example, even though the spin-coated films of P3HT show a weak Haggregate character, as revealed in the enhanced 0-0 vibronic peak in absorption spectra, they present a strong characteristic of J-aggregate features when tailored by forming nanofibers.40,41 The energy transfer from the disordered and coiled to the ordered and planarized segments also is significant to understand the photophysical properties in films/solutions where aggregates and non-aggregates coexist.42,43 In this paper, we investigated the enhancement in the polarization ratio of photoluminescence by immersing the nanofiber meshes into the solvent which weakly dissolves the conjugated polymers. The blends

of

poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene]

(MEH-PPV):poly(ethylene

oxide)

(PEO) in the mixed solvents were electronspun onto the rotating drum collector. We

demonstrate that the polarization ratio of photoluminescence becomes markedly higher when immersed in solvents that can dissolve and thus wash out the electrospinnable insulating polymers, whereas the polarization ratio of absorption becomes slightly increased. We discuss the origin of the increase in the polarization ratio with the analysis of photoluminescence spectra and time-resolved photoluminescence decay, which provide information about the exciton coherence length in J-aggregates.

2. Experimental methods Materials PEO (average Mv = 600 kDa), and MEH-PPV (average Mn = 70-100 kDa) were purchased from Sigma-Aldrich and used without further purification. For spin coating and electrospinning, MEHPPV:PEO blends were dissolved in the mixture solvents of chloroform and DMF (4:1 in volume). The weight ratio of MEH-PPV:PEO was 1:3. The PEO concentration was fixed at 0.6 wt% in the solutions. The solutions were stirred at 70 °C overnight. For spin-coated films, the solutions of MEH-PPV in chlorobenzene were spin-coated at 1500 rpm for 1 min. 4 Environment ACS Paragon Plus

Page 5 of 27

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Electrospun nanofiber The electrospinning setup was structured horizontally with a stainless steel needle (Gauge no.23, inner diameter: 0.33 mm) and rotating drum collector. The entire process was carried out at room temperature and 25% - 31% relative humidity. The MEH-PPV:PEO solutions were placed in a syringe and supplied to the needle using the syringe pump (NE-1000, New Era Pump Systems Inc.) at a flow rate of 0.4 ml/h. The needle was connected to a power supply of 25 kV (FC50P2.4, Glassman High Voltage Research Inc.), and the drum collector placing 15 cm away from the tip of the needle was grounded. Nanofibers were collected on the glass substrates attached on the rotating drum collector with a diameter of 10 cm and a rotating speed of 2000 rpm. The deposition time was 20-30 min. The nanofiber meshes were immersed in acetonitrile (ACN) 6-7 times for 15 min each time to remove the PEO (ACN treatment), and then some meshes were subsequently dipped in DMF for 1 hour (DMF treatment). Characterization. The morphology of the nanofibers were examined using a scanning electron microscope (SEM, JSM-7401F, JEOL). The surface topology images of the fibers were acquired by atomic force microscopy (AFM, XE-100, Park system) with a scanning area of 10 x 10 µm2. Absorption measurements were made using a UV-visible spectrophotometer (HP8453, HP). Photoluminescence (PL) spectra were collected using a fluorescence spectrofluorometer (FP-8500, JASCO) with a xenon lamp (150 W). To avoid the influence of the intensity of the lamp, the excitation wavelength was selected at 470 nm. The PL decay times were recorded using Time-Correlated Single Photon Counting (TCSPC). The nanofiber films were excited by 100-ps laser pulses of 390 nm center wavelength and time-resolved PL from the samples was measured at 590 nm wavelength. The nominal temporal resolution of the TCSPC measurement was ~190 ps.

3. Results and Discussion Figure 1 presents the SEM images of MEH-PPV:PEO electrospun nanofibers, and it is seen that nanofibers are aligned to a particular direction. The elemental composition of a single nanofiber is difficult to determine because both MEH-PPV and PEO consist of the same atomic elements. However, 5 Environment ACS Paragon Plus


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 27

it has been reported that that the two polymers would be quite intermixed, similar to poly[(9,9dioctylfluorenyl-2,7-diyl)-co-(1,4-benzo-[2,1’-3]-thiadiazole)] (F8BT):PEO, as having more amounts of PEO in a nanofiber inhibits the formation of MEH-PPV aggregation.21,24,34 In order to extract PEO molecules from the nanofibers to have only MEH-PPV form nanofibers, the nanofiber meshes were washed with ACN, which has a differential solubility for molecules in nanofibers; it dissolves PEO and has no solubility for MEH-PPV. It has to be assured that the polymer chains still form nanofiber structures even after the solvent treatment. Figures 1(b) and 1(c) show that the meshes washed by ACN only and further washed by DMF, respectively, are still composed of nanofibers. The width of the aselectrospun nanofibers was 208 ± 49.4 nm, and the washed nanofibers shown in Figures 1(b) and 1(c) are 193 ± 49.5 and 207.7 ± 62.4 nm, respectively. Achieving almost identical widths between before and after solvent treatments might appear inconceivable if PEO is removed, since the width should be decreased. The SEM images of nanofibers shown in the lower panels of Figure 1 show that the nanofibers seem to be partially collapsed down to the substrate, while retaining their nanofiber structure.

Fig. 1. SEM images of electrospun nanofibers (a) and (d) without solvent treatment, (b) and (e) after ACN treatment, and (c) and (f) after DMF treatment. The lower panel shows magnified images. The width of the pristine, the ACN-treated and the DMF-treated nanofibers is 208 ± 49.4, 193 ± 49.5, and 207.7 ± 62.4 nm, respectively.


Page 7 of 27

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


To clearly demonstrate that the mesh surface becomes smooth by the post-solvent treatment, the AFM images of the nanofiber meshes are shown in Figure 2. Unlike the as-spun nanofibers shown in Figure 2(a), the post-solvent treated nanofibers are partially destroyed. The average of peak-to-peak values decrease from 443.16 nm to 140.8 nm after the ACN treatment, showing that the surface of the nanofiber mesh on substrates becomes smooth. The average of peak-to-peak values after washing the ACN-treated nanofiber meshes with DMF was decreased from 140.8 nm to 115.8 nm, supporting that the nanofibers were a bit further vertically collapsed and laterally spread by DMF.

Fig. 2. AFM topography images of (a) as-electronspun, (b) ACN-treated, and (c) DMF-treated nanofibers following the ACN treatment. Scanned area is 10 × 10 µm2. (d) exhibits the average of peakto-peak values of the three different horizontal scans on three different AFM images for each meshes.

The film/mesh roughness can be briefly investigated also with the absorption spectra, which are shown in Figure 3, by looking at the absorbance at long wavelengths where MEH-PPV cannot absorb light. For MEH-PPV:PEO blends spin-coated, the film is also not so clear as normal MEH-PPV spincoated films because of PEO and DMF in solutions, causing a strong light scattering. The as-electrospun nanofiber meshes show very dissimilar absorption spectra to those of the spin-coated films, and an overall monotonic decrease with increasing wavelength with a hill peaked at ~570 nm, indicating light 7 Environment ACS Paragon Plus


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 27

scattering due to the very rough film-surface composed of nanofibers. The overall monotonic decrease feature disappears after the ACN treatment, supporting that ACN smoothed the surface of the nanofiber meshes. This demonstrates that the nanofiber is collapsed onto the substrate by ACN, most likely due to the PEO extraction because ACN dissolves PEO only. The non-zero absorbance of the nanofiber meshes at long wavelengths where MEH-PPV cannot absorb photons indicates light scattering due to the rough surface of nanofiber meshes.

Fig. 3. Absorption spectra of electrospun nanofiber meshes and MEH-PPV:PEO blends spin-coated (blue dashed).

Various kinds of conjugated polymer chains in the PEO matrix in nanofibers are known to be aligned in a stretched form along the nanofiber axis, which is difficult in the spin-coated films.21,40,41 The absorption spectra were collected with polarized light incident by placing the linear polarizer before the samples to measure the absorption polarization ratio of the nanofiber mehses (Figure 3). The absorption spectra show that the nanofiber meshes absorb more photons with a polarization parallel to the long nanofiber axis than perpendicularly polarized photons. This demonstrates that not only are the nanofibers well-aligned, but also the MEH-PPV chains in the fibers are, to some degree, uniaxially aligned along the nanofiber axis. Although the absorption polarization ratio of the as-electrospun nanofiber meshes cannot be accurately determined due to significant light scattering, that of the nanofiber films after post-solvent treatments can roughly be estimated. The ratio is shown to be 1.89 for 8 Environment ACS Paragon Plus

Page 9 of 27

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


the ACN treatment and is slightly increased to 1.98 after the further DMF treatment on the ACN-treated nanofiber meshes. The small but apparent increase in the absorption polarization ratio after the DMF treatment suggests the reorganization of the MEH-PPV chains, the details of which will be discussed later through the analysis combined with the photoluminescence data.

Fig. 4. Unpolarized photoluminescence spectra of nanofiber films. The excitation light was randomly polarized and the wavelength was 470 nm.

The spin-coated film of MEH-PPV:PEO blends absorbs photons with a peak at 515 nm wavelength. The parallel absorption spectra of the pristine, the ACN-treated and the DMF-treated nanofiber meshes peak at 560 nm, 521 nm and 524 nm, respectively, which are red-shifted compared with the spin-coated films. The ACN treatment causes a blue shift of absorption from the as-spun nanofiber meshes, but the further DMF treatment causes a red-shift from the absorption spectrum of the ACN-treated meshes, with a decrease in absorbance because DMF a bit washes out MEH-PPV. The blue/red shift is attributed to a change in conjugation lengths and/or aggregation character. If the shift of the absorption peak is caused by changes in the character of aggregates, the same effects should be seen in the photoluminescence spectra. However, the normalized spectra of the unpolarized photoluminescence emission show that the ACN-treated nanofibers emit with a peak at 592 nm, showing a slight red shift of photoluminescence compared with the as-electrospun nanofibers (589 nm), and the DMF-treated nanofibers emission peaks at 595 nm, presenting a further red shift (Figure 4). This indicates that the absorption of the pristine nanofiber meshes at long wavelengths arises from the fact that the average of conjugation lengths is 9 Environment ACS Paragon Plus


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 27

large with fewer conjugation breaks rather than formation of aggregation, which is consistent with the fact that the formation of aggregation is suppressed with increasing PEO content. The small Stokes shift observed for the pristine nanofibers reflects the energy transfer between MEH-PPV chromophores might inefficiently take place, which can be explained by a small portion of short chromophores that would undergo exciton relaxation in energy. In addition, the insulating PEO molecules would act as a barrier for exciton transport from one MEH-PPV chromophore to another, reducing the energy transfer efficiency as well. The inefficient energy transfer in the presence of insulating polymers were demonstrated also from the MEH-PPV nanofibers intermixed with poly(e-caprolactone) (PLC) nanofibers.16 For the solvent-treated nanofibers, a strong blue-shifted absorption was observed compared with the pristine nanofiber meshes. This indicates that the average of conjugation lengths becomes shorter by conjugation breaks that occur possibly whilst the MEH-PPV chains are collapsed in the process of filling the void space where the PEO chains are extracted. The creation of short chromophores and the absence of PEO enable exciton transfer to longer chromophores, resulting in large Stokes shifts. The DMF treatment on nanofibers yields further interesting features in photoluminescence. The 0-0/01 vibronic peak intensity ratio is enhanced, and the bandwidth of photoluminescence is reduced. In contrast to ACN that cannot dissolve MEH-PPV, DMF can dissolve MEH-PPV with a very limited solubility. It is expected that the limited solubility might reorganize MEH-PPV chromophores and thus induce some changes in the intrachain order/interchain coupling. The absorption spectrum of the DMFtreated nanofibers is red-shifted by a few nanometers compared with that of the ACN-treated nanofibers. This red-shift can be attributed to the elongation of single chains or stronger character of J-aggregates by enhanced intrachain order. The combined analysis with the photoluminescence spectra suggests the MEH-PPV chains in the DMF-treated nanofibers have stronger J-aggregate character, as the photoluminescence also shows a slight red-shift.33 On the one hand, the different vibronic progression observed could result from a small decrease in the Huang-Rhys factor, due to a change in some vibrational displacement in the ground and/or the excited states. The enhancement in the 0-0 vibronic 10 Environment ACS Paragon Plus

Page 11 of 27

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


peak relative to the 0-1 peak reflects enhanced conjugation in the polymer chains, suggesting the reduced conformational disorder.44-46 On the other hand, it has been revealed that the 0-0/0-1 peak intensity ratio in emission is linearly proportional to the number of chromophores connected in a single chain, that is, exciton coherence length.28,35,36,47-49

Fig.

5. Photoluminescence decay dynamics from the nanofiber films: emissions (a) parallel to

nanofibers and (b) perpendicular to nanofibers.

Table 1. The fitting parameters of photoluminescence decay parallel to the nanofiber axis. The decay curves were fitted with a biexponential function, I(t) = A1·exp(-t/τ1) + A2·exp(-t/ τ2). A1

τ1 / ns

A2

τ2 / ns

As-electrospun

0.67

0.49

0.33

1.53

ACN-treated

0.77

0.44

0.23

1.21

DMF-treated

0.71

0.36

0.29

1.09



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 27

The time-resolved photoluminescence was measured to investigate the origin of the relative enhancement of the 0-0 emission peak, as the enhancement in coherence lengths results in a faster radiative decay, leading to the shorter lifetime of excitons. The time-resolved photoluminescence parallel- and perpendicular-polarized to the nanofiber axis was exhibited in Figure 5 and the fitting parameters of the parallel photoluminescence decay curves were obtained from biexponential decay fits and are summarized in Table 1. It was observed that the excitons in as-electrospun nanofibers have the longest lifetimes among the three meshes for both parallel and perpendicular emission. This can be explained by the prohibited or inefficient energy transfer down to non-radiative and/or quenching sites due to molecular insulation. A similar phenomenon has also been found from the blend of poly(diphenylene vinylene) (PDV) and PEO.50 After the removal of PEO by immersing the meshes into ACN, the photoluminescence rapidly decays. The increase in the fast decay component after solvent treatment also supports the removal of PEO. The decay dynamics of perpendicular emission are almostidentical for the ACN- and the DMF-treated nanofiber meshes, suggesting that the solvent treatment has little effects on the transition dipole moments perpendicular to the nanofibers. That is, the interchain coupling might hardly be changed by the DMF treatment. However, for the parallel emission, it was observed that photoluminescence decays faster after the DMF treatment, combined analysis with enhanced parallel emission shown in Figure 6, which we attribute to the faster radiative decay caused by the increased coherence length, resulting from higher intrachain order.

Fig. 6. Polarized photoluminescence spectra of nanofiber meshes with different post-solvent treatments.


Page 13 of 27

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 6 presents the polarized emission parallel and perpendicular to the nanofibers excited with unpolarized light peaked at 470 nm. After the DMF treatment on the ACN-treated meshes, despite a small decrease in absorption, parallel emission becomes stronger while perpendicular emission is reduced, indicating that PLQY in parallel axis becomes higher and the polarization ratio of photoluminescence is increased. Figure 7 summarizes the polarization ratio of absorption and emission for the three different films. The post-solvent-treated meshes commonly show that the polarization ratio of emission is a few magnitudes higher than that of absorption. Please note that the absorption polarization ratio of as-electrospun nanofiber meshes shown in Figure 7 does not reflect a true value. The discrepancy of the polarization ratio between absorption and emission suggests that energy transfer occurs from intially excited chromophores/aggregates towards longer chromophores and/or J-aggregates with longer coherence length that are well aligned along the nanofiber axis, leading to highly polarized emission.

Fig. 7. Histogram of polarization ratio (parallel/perpendicular) of absorption and photoluminescence for different nanofiber meshes.

The photoluminescence polarization ratio is negligibly changed by the ACN treatment of nanofibers but markedly increases after the DMF treatment, from 5.26 to 8.97, while the absorption polarization ratio increases only slightly, from 1.97 to 2.12 (see histograms in Figure 7). The remarkable increase of the polarization ratio after the DMF treatment is attributed to the enhanced exciton coherence length 13 Environment ACS Paragon Plus


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 27

along the nanofiber axis due to increased intrachain order. The schematic illustration of chromophores in the nanofibers is shown in Figure 8. The stronger character of J-aggregates lead to the enhancement in the efficiency of photoluminescence parallel to the nanofiber, as a result of large transition dipole moments. Although the increased polarization ratio of photoluminescence arises from the long exciton coherence length in the J-aggregates, not all the chromophores in nanofibers would experience an enhancement in intrachain order and an increase in exciton coherence lengths. As can be seen from the polarization ratio of absorption, the increase is small compared to that of photoluminescence after the DMF treatment. We reason this that the short chromophores that are not well-aligned along the nanofiber axis do not form J-aggregates or the intrachain order is not effectively enhanced even after the DMF treatment perhaps due to the initially large degree of angles between the chromophores in single chains. By contrast, some ensembles of emissive chromophores that have long conjugation lengths are better aligned, and thus the intrachain order can effectively be enhanced by reorganization due to the DMF treatment, leading to longer exciton coherence lengths of J-aggregates that lie along the nanofiber axis.

Fig. 8. Schematic illustration of chromophores in nanofibers. The red-line represents chromophores. ET strands for energy transfer and the dashed line represents intrachain coupling.

4. Conclusions 14 Environment ACS Paragon Plus

Page 15 of 27

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


We fabricated the aligned nanofiber meshes based on blends of MEH-PPV and PEO using an electrospinning technique and achieved a high polarization ratio of photoluminescence by post-solvent treatments. The light scattering induced by the rough surface of meshes was significantly reduced after the PEO removal. The polarization ratio of photoluminescence was markedly enhanced after the DMF treatment following the ACN treatment. The changes in photoluminescence, such as the peak shift, bandwidth and the redistribution of vibronic intensities, after post-solvent treatments were observed to be too small to provide quantitative information about chain conformation and coherence lengths. Nevertheless, the qualitative analysis combined with the steady-state photoluminescence spectra and the time-resolved photoluminescence consistently demonstrate that the DMF-treated nanofibers have the increased number of emissive chromophores connected along single chains, which we attribute to higher intrachain order mediated possibly by molecular reorganization. leading to a high polarization ratio of photoluminescence .

Acknowledgements This work was supported by the Basic Science Research Program through the National Foundation of Korea (NRF), funded by the Ministry of Science, ICT & Future Planning (2014R1A1A1002217 and 2014R1A2A1A10054643). I.H acknowledges Kwangwoon University for the Research grant in 2016.

References (1) Grell, M.; Bradley, D. D. C. Polarized Luminescence from Oriented Molecular Materials. Adv. Mater. 1999, 11, 895-905. (2) Matioli, E.; Brinkley, S.; Kelchner, K. M.; Hu, Y.-L.; Nakamura, S.; DenBaars, S.; Speck, J.; Weisbuch, C. High-Brightness Polarized Light-Emitting Diodes. Light. Sci. Appl. 2012, 1, e22. (3) Wu, C. C.; Tsay, P. Y.; Cheng, H. Y.; Bai, S. J. Polarized Luminescence and Absorption of Highly Oriented, Fully Conjugated, Heterocyclic Aromatic Rigid-Rod Polymer Poly-pphenylenebenzobisoxazole. J. Appl. Phys. 2004, 95, 417-423. (4) Liedtke, A.; O'Neill, M.; Wertmöller, A.; Kitney, S. P.; Kelly, S. M. White-Light OLEDs Using Liquid Crystal Polymer Networks. Chem. Mater. 2008, 20, 3579-3586.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 27

(5) Ding, R.; Feng, J.; Zhou, W.; Zhang, X.-L.; Fang, H.-H.; Yang, T.; Wang, H.-Y.; Hotta, S.; Sun, H.-B. Intrinsic Polarization and Tunable Color of Electroluminescence from Organic Single Crystal-based Light-Emitting Devices. Sci. Rep. 2015, 5, 12445. (6) Ramos, R.; Siqueira, M. F.; Cazati, T.; Faria, R. M.; Caldas, M. J. Polarized Emission from Stretched PPV Films Viewed at the Molecular Level. Phys. Chem. Chem. Phys. 2015, 17, 2053020536. (7) Hasegawa, M.; Hirayama, Y.; Dertinger, S. Polarized Fluorescent Emission from Aligned Electrospun Nanofiber Sheets Containing Semiconductor Nanorods. Appl. Phys. Lett. 2015, 106, 051103. (8) See, G. G.; Xu, L.; Sutanto, E.; Alleyne, A. G.; Nuzzo, R. G.; Cunningham, B. T. Polarized Quantum Dot Emission in Electrohydrodynamic Jet Printed Photonic Crystals. Appl. Phys. Lett. 2015, 107, 051101. (9) Stefano, P.; Andrea, C.; Elisa, M.; Luana, P.; Roberto, C.; Dario, P. Enhancement of Light Polarization from Electrospun Polymer Fibers by Room Temperature Nanoimprint Lithography. Nanotechnology 2010, 21, 215304. (10) Laursen, B. W.; Nørgaard, K.; Reitzel, N.; Simonsen, J. B.; Nielsen, C. B.; Als-Nielsen, J.; Bjørnholm , T.; Sølling, T. I.; Nielsen, M. M.; Bunk, O., et. al. Macroscopic Alignment of Graphene Stacks by Langmuir-Blodgett Deposition of Amphiphilic Hexabenzocoronenes. Langmuir 2004, 20, 4139-4146. (11) Giancane, G.; Ruland, A.; Sgobba, V.; Manno, D.; Serra, A.; Farinola, G. M.; Omar, O. H.; Guldi, D. M.; Valli, L. Aligning Single-Walled Carbon Nanotubes By Means Of Langmuir–Blodgett Film Deposition: Optical, Morphological, and Photo-electrochemical Studies. Adv. Funct. Mater. 2010, 20, 2481-2488. (12) Schmid, S. A.; Yim, K. H.; Chang, M. H.; Zheng, Z.; Huck, W. T. S.; Friend, R. H.; Kim, J. S.; Herz, L. M. Polarization Anisotropy Dynamics for Thin Films of a Conjugated Polymer Aligned by Nanoimprinting. Phys. Rev. B 2008, 77, 115338. (13) Yanagi, H.; Morikawa, T.; Hotta, S.; Yase, K. Epitaxial Growth of Thiophene/pPhenylene Co-oligomers for Highly Polarized Light-Emitting Crystals. Adv. Mater. 2001, 13, 313-317. (14) Pagliara, S.; Camposeo, A.; Polini, A.; Cingolani, R.; Pisignano, D. Electrospun LightEmitting Nanofibers as Excitation Source in Microfluidic Devices. Lab Chip. 2009, 9, 2851-2856. (15) Moran-Mirabal, J. M.; Slinker, J. D.; DeFranco, J. A.; Verbridge, S. S.; Ilic, R.; FloresTorres, S.; Abruña, H.; Malliaras, G. G.; Craighead, H. G. Electrospun Light-Emitting Nanofibers. Nano Lett. 2007, 7, 458-463. (16) Zhong, W.; Li, F.; Chen, L.; Chen, Y.; Wei, Y. A Novel Approach to Electrospinning of Pristine and Aligned MEH-PPV Using Binary Solvents. J. Mater. Chem. 2012, 22, 5523-5530. (17) Fasano, V.; Polini, A.; Morello, G.; Moffa, M.; Camposeo, A.; Pisignano, D. Bright Light Emission and Waveguiding in Conjugated Polymer Nanofibers Electrospun from Organic Salt Added Solutions. Macromolecules 2013, 46, 5935-5942. (18) Fasano, V.; Moffa, M.; Camposeo, A.; Persano, L.; Pisignano, D. Controlled Atmosphere Electrospinning of Organic Nanofibers with Improved Light Emission and Waveguiding Properties. Macromolecules 2015, 48, 7803-7809. (19) Kuo, C.-C.; Lin, C.-H.; Chen, W.-C. Morphology and Photophysical Properties of LightEmitting Electrospun Nanofibers Prepared from Poly(fluorene) Derivative/PMMA Blends. Macromolecules 2007, 40, 6959-6966. (20) Di Camillo, D.; Fasano, V.; Ruggieri, F.; Santucci, S.; Lozzi, L.; Camposeo, A.; Pisignano, D. Near-Field Electrospinning of Light-Emitting Conjugated Polymer Nanofibers. Nanoscale 2013, 5, 11637-11642. (21) Yin, K.; Zhang, L.; Lai, C.; Zhong, L.; Smith, S.; Fong, H.; Zhu, Z. Photoluminescence Anisotropy of Uni-Axially Aligned Electrospun Conjugated Polymer Nanofibers of MEH-PPV and P3HT. J. Mater. Chem. 2011, 21, 444-448. 16 Environment ACS Paragon Plus

Page 17 of 27

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(22) Tu, D.; Pagliara, S.; Camposeo, A.; Persano, L.; Cingolani, R.; Pisignano, D. Single Light-Emitting Polymer Nanofiber Field-Effect Transistors. Nanoscale 2010, 2, 2217-2222. (23) Katta, P.; Alessandro, M.; Ramsier, R. D.; Chase, G. G. Continuous Electrospinning of Aligned Polymer Nanofibers onto a Wire Drum Collector. Nano Lett. 2004, 4, 2215-2218. (24) Vohra, V.; Giovanella, U.; Tubino, R.; Murata, H.; Botta, C. Electroluminescence from Conjugated Polymer Electrospun Nanofibers in Solution Processable Organic Light-Emitting Diodes. ACS Nano 2011, 5, 5572-5578. (25) Bedford, N. M.; Dickerson, M. B.; Drummy, L. F.; Koerner, H.; Singh, K. M.; Vasudev, M. C.; Durstock, M. F.; Naik, R. R.; Steckl, A. J. Nanofiber-Based Bulk-Heterojunction Organic Solar Cells Using Coaxial Electrospinning. Adv. Energy Mater. 2012, 2, 1136-1144. (26) Kim, T.; Yang, S. J.; Kim, S. K.; Choi, H. S.; Park, C. R. Preparation of PCDTBT Nanofibers with a Diameter of 20 nm and Their Application to Air-Processed Organic Solar Cells. Nanoscale 2014, 6, 2847-2854. (27) Köhler, A.; Hoffmann, S. T.; Bässler, H. An Order-Disorder Transition in the Conjugated Polymer MEH-PPV. J. Am. Chem. Soc. 2012, 134, 11594-11601. (28) Yamagata, H.; Hestand, N. J.; Spano, F. C.; Köhler, A.; Scharsich, C.; Hoffmann, S. T.; Bässler, H. The Red-Phase of Poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEHPPV): A Disordered HJ-Aggregate. J. Chem. Phys. 2013, 139, 114903. (29) Jakubiak, R.; Collison, C. J.; Wan, W. C.; Rothberg, L. J.; Hsieh, B. R. Aggregation Quenching of Luminescence in Electroluminescent Conjugated Polymers. J. Phys. Chem. A 1999, 103, 2394-2398. (30) Collison, C. J.; Rothberg, L. J.; Treemaneekarn, V.; Li, Y. Conformational Effects on the Photophysics of Conjugated Polymers: A Two Species Model for MEH-PPV Spectroscopy and Dynamics. Macromolecules 2001, 34, 2346-2352. (31) Nguyen, T.-Q.; Doan, V.; Schwartz, B. J. Conjugated Polymer Aggregates in Solution: Control of Interchain Interactions. J. Chem. Phys. 1999, 110, 4068-4078. (32) Kanemoto, K.; Imanaka, Y.; Akai, I.; Sugisaki, M.; Hashimoto, H.; Karasawa, T. Intrachain Photoluminescence Dynamics of MEH-PPV in the Solid State. J. Phys. Chem. B 2007, 111, 12389-12394. (33) Chakraborty, R.; Rothberg, L. J. Role of Aggregates in the Luminescence Decay Dynamics of Conjugated Polymers. J. Phys. Chem. A 2016, 120, 551-555. (34) Zhu, Z.; Zhang, L.; Smith, S.; Fong, H.; Sun, Y.; Gosztola, D. Fluorescence Studies of Electrospun MEH-PPV/PEO Nanofibers. Synth. Met. 2009, 159, 1454-1459. (35) Spano, F. C. Optical Microcavities Enhance the Exciton Coherence Length and Eliminate Vibronic Coupling in J-Aggregates. J. Chem. Phys. 2015, 142, 184707. (36) Spano, F. C.; Silva, C. H- and J-Aggregate Behavior in Polymeric Semiconductors. Annu. Rev. Phys. Chem. 2014, 65, 477-500. (37) Yamagata, H.; Spano, F. C. Strong Photophysical Similarities between Conjugated Polymers and J-aggregates. J. Phys. Chem. Lett. 2014, 5, 622-632. (38) Hestand, N. J.; Spano, F. C. The Effect of Chain Bending on the Photophysical Properties of Conjugated Polymers. J. Phys. Chem. B 2014, 118, 8352-8363. (39) Spano, F. C.; Yamagata, H. Vibronic Coupling in J-Aggregates and Beyond: A Direct Means of Determining the Exciton Coherence Length from the Photoluminescence Spectrum. J. Phys. Chem. B 2011, 115, 5133-5143. (40) Martin, T. P.; Wise, A. J.; Busby, E.; Gao, J.; Roehling, J. D.; Ford, M. J.; Larsen, D. S.; Moulé, A. J.; Grey, J. K. Packing Dependent Electronic Coupling in Single Poly(3-hexylthiophene) Hand J-Aggregate Nanofibers. J. Phys. Chem. B 2013, 117, 4478-4487. (41) Niles, E. T.; Roehling, J. D.; Yamagata, H.; Wise, A. J.; Spano, F. C.; Moulé, A. J.; Grey, J. K. J-Aggregate Behavior in Poly-3-hexylthiophene Nanofibers. J. Phys. Chem. Lett. 2012, 3, 259-263.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 27

(42) Consani, C.; Koch, F.; Panzer, F.; Unger, T.; Köhler, A.; Brixner, T. Relaxation Dynamics and Exciton Energy Transfer in the Low-Temperature Phase of MEH-PPV. J. Chem. Phys. 2015, 142, 212429. (43) Unger, T.; Panzer, F.; Consani, C.; Koch, F.; Brixner, T.; Bässler, H.; Köhler, A. Ultrafast Energy Transfer between Disordered and Highly Planarized Chains of Poly[2-methoxy-5-(2ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV). ACS Macro Lett. 2015, 4, 412-416. (44) Yun-Yue, L.; Chun-Wei, C.; Chang, J.; Lin, T. Y.; Liu, I. S.; Wei-Fang, S. Exciton Dissociation and Migration in Enhanced Order Conjugated Polymer/nanoparticle Hybrid Materials. Nanotechnology 2006, 17, 1260. (45) Cornil, J.; Beljonne, D.; Heller, C. M.; Campbell, I. H.; Laurich, B. K.; Smith, D. L.; Bradley, D. D. C.; Müllen, K.; Brédas, J. L. Photoluminescence Spectra of OligoParaphenyllenevinylenes: A Joint Theoretical and Experimental Characterization. Chem. Phys. Lett. 1997, 278, 139-145. (46) Chang, R.; Hsu, J. H.; Fann, W. S.; Liang, K. K.; Chang, C. H.; Hayashi, M.; Yu, J.; Lin, S. H.; Chang, E. C.; Chuang, K. R., et. al. Experimental and Theoretical Investigations of Absorption and Emission Spectra of the Light-Emitting Polymer MEH-PPV in Solution. Chem. Phys. Lett. 2000, 317, 142-152. (47) Yamagata, H.; Spano, F. C. Interplay between Intrachain and Interchain Interactions in Semiconducting Polymer Assemblies: The HJ-Aggregate Model. J. Chem. Phys. 2012, 136, 184901. (48) Spano, F. C. The Spectral Signatures of Frenkel Polarons in H- and J-Aggregates. Acc. Chem. Res. 2010, 43, 429-439. (49) Chen, G.; Sasabe, H.; Lu, W.; Wang, X.-F.; Kido, J.; Hong, Z.; Yang, Y. J-aggregation of a squaraine dye and its application in organic photovoltaic cells. J. Mater. Chem. C 2013, 1, 6547-6552. (50) Chang, M. H.; Frampton, M. J.; Anderson, H. L.; Herz, L. M. Photoexcitation Dynamics in Thin Films of Insulated Molecular Wires. Appl. Phys. Lett. 2006, 89, 232110.


Page 19 of 27

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


TOC graphic



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Fig. 1. SEM images of electrospun nanofibers (a) and (d) without solvent treatment, (b) and (e) after ACN treatment, and (c) and (f) after DMF treatment. The lower panel shows magnified images. The width of the pristine, the ACN-treated and the DMF-treated nanofibers is 208 ± 49.4, 193 ± 49.5, and 207.7 ± 62.4 nm, respectively. Figure 1 123x64mm (300 x 300 DPI)

ACS Paragon Plus Environment

Page 20 of 27

Page 21 of 27

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Fig. 2. AFM topography images of (a) as-electronspun, (b) ACN-treated, and (c) DMF-treated nanofibers following the ACN treatment. Scanned area is 10 × 10 µm2. (d) exhibits the average of peak-to-peak values of the three different horizontal scans on three different AFM images for each meshes. Figure 2 94x81mm (300 x 300 DPI)



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Fig. 3. Absorption spectra of electrospun nanofiber meshes and MEH-PPV:PEO blends spin-coated (blue dashed). Figure 3 72x54mm (300 x 300 DPI)


Page 22 of 27

Page 23 of 27

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Fig. 4. Unpolarized photoluminescence spectra of nanofiber films. The excitation light was randomly polarized and the wavelength was 470 nm. Figure 4 68x51mm (300 x 300 DPI)



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Fig. 5. Photoluminescence decay dynamics from the nanofiber films: emissions (a) parallel to nanofibers and (b) perpendicular to nanofibers. Figure 5 71x96mm (300 x 300 DPI)


Page 24 of 27

Page 25 of 27

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Fig. 6. Polarized photoluminescence spectra of nanofiber meshes with different post-solvent treatments. Figure 6 68x56mm (300 x 300 DPI)



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Fig. 7. Histogram of polarization ratio (parallel/perpendicular) of absorption and photoluminescence for different nanofiber meshes. Figure 7 66x49mm (300 x 300 DPI)


Page 26 of 27

Page 27 of 27

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Fig. 7. Histogram of polarization ratio (parallel/perpendicular) of absorption and photoluminescence for different nanofiber meshes. Figure 8 157x114mm (300 x 300 DPI)


Enhanced Polarization Ratio of Electrospun Nanofibers with Increased

Recommend Documents