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Controlling Morphology and Aggregation in Semiconducting Polymers: The Role of Solvents on Lasing Emission in Poly[2-methoxy-5-(20 -ethylhexyloxy)-1,4-phenylene-vinylene] Minghuan Liu 1,2 , Yonggang Liu 1 , Zenghui Peng 1 , Chengliang Yang 1 , Quanquan Mu 1 , Zhaoliang Cao 1 , Ji Ma 1 and Li Xuan 1, * 1

2

*

State Key Laboratory of Applied Optics, Changchun Institute of Optics, Fine Mechanics and Physics, Chinese Academy of Sciences, Changchun 130033, China; [email protected] (M.L.); [email protected] (Y.L.); [email protected] (Z.P.); [email protected] (C.Y.); [email protected] (Q.M.); [email protected] (Z.C.); [email protected] (J.M.) University of Chinese Academy of Sciences, Beijing 100049, China Correspondence: [email protected]; Tel.: +86-0431-8617-6309

Academic Editor: Changle Chen Received: 22 May 2017; Accepted: 23 June 2017; Published: 29 June 2017

Abstract: Systematic experiments were performed to investigate solvent-dependent morphology and aggregation of the semiconducting polymer film poly[2-methoxy-5-(20 -ethyl-hexyloxy)-1,4phenylene-vinylene] (MEH-PPV), which was span-cast from nonaromatic strong polarity solvents tetrahydrofuran (THF), trichloromethane (TCM) and aromatic weak polarity solvents chlorobenzene (CB), toluene, and p-xylene. The results indicated that the conformation of the spin-cast MEH-PPV films with weak aggregation such as THF and TCM demonstrated excellent lasing emission performances because of inhibiting the fluorescence quenching induced by bi-molecule process. The Atomic Force Microscope (AFM) images confirmed the distinct morphologies of the spin-cast MEH-PPV films. The amplified spontaneous emission (ASE) was investigated in a simple asymmetric slab planar waveguide structure by methods of variable stripe length (VSL) and shifting excitation stripe (SES). The amplified spontaneous emission (ASE) experiments confirmed the distinct polymer chain conformation. The conformation, which preserved from the spin-cast process, indicated the distinct interactions between solvents and MEH-PPV polymer chains. The pure film spectra were performed to confirm the effect of distinct conformation on the material energy level. This work provides insights into the morphology and aggregation effect of the spin-cast polymer films on the performances of lasers. Keywords: semiconducting polymer; holographic polymer dispersed liquid crystal; aggregation and morphology; amplified spontaneous emission; polymer dispersed liquid crystal

1. Introduction Organic solid-state lasers (OSSLs) have attracted more attention recently [1–3]. Many materials and device configurations has been developed in this area. Semiconducting materials like semiconducting polymers are promising due to high efficiency, wide spectral coverage, and solution-based processing [4,5]. There have been no demonstrations of commercial devices under electrical injection using semiconducting polymers as active materials, largely because the presence of both injected polarons and metal electrodes quenches the luminescence and raises the threshold [1]. Fortunately, the indirect electrical pumping using inexpensive light emitting diode (LED) is feasible [6].

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Therefore, the OSSLs based on semiconducting polymers can be potentially used in sensors, optical communications, and integrated photonics [6–8]. Despite the versatility for photonic applications, some of the fundamental physical insights underlying the fabrication or optimization of practical devices based on those organic semiconducting materials remain poorly understood. The dissolution environment of the semiconducting polymers plays a vital role on the morphologies, packing conformation of the casting films [9–11] and electroluminescent performance [11,12], because the aggregation conformation in solvents will preserve the spin-cast films. The semiconducting polymer aggregation is a place where two or more chain segments come together and share their π-electron density [11]. The microstructure of solution processed thin films has been investigated by X-ray diffraction. The results demonstrate that the poly [2-methoxy-5-(20 -ethyl-hexyloxy)-1,4-phenylene-vinylene] (MEH-PPV) backbones and the planes defined by the benzene rings within the phenylene vinylene (PPV) backbones are predominantly parallel to the film plane [13]. The nanoscopic inter-chain aggregation domain formation is observed by the combination of third harmonic generation (THG) and near-field scanning optical microscopy (NSOM) [14]. Researchers Lampert et al. report the dependence of optical gain to solvents [15]. There are numerous limit reports on lasing emission of the semiconducting polymer films spin-cast from different solvents in a slab planar waveguide structure. Implement such a study will create a physical insight into the effect of solvent-dependent film morphology and aggregation on the lasing emission of spin-cast semiconducting materials. In this study, the morphology and aggregation of the spin-cast semiconducting polymer, poly[2-methoxy-5-(20 -ethyl-hexyloxy)-1,4-phenylene-vinylene] (MEH-PPV) films was systematically studied in nonaromatic strong polarity solvents tetrahydrofuran (THF), trichloromethane (TCM) and aromatic weak polarity solvents chlorobenzene (CB), toluene and p-xylene. The morphologies were investigated and imaged with an Atomic Force Microscope (AFM). The amplified spontaneous emission (ASE) was investigated in a simple asymmetric slab planar waveguide structure. In the waveguide structure, it contained a MEH-PPV layer as the core layer and a polymer-dispersed liquid crystal (PDLC) film/pre-clean glass substrate as the cladding layers. The net gain and waveguide losses of the spin-cast MEH-PPV films were also investigated and compared. The spectra of the pure films were performed. In the last section, the lasing emission of the spin-cast MEH-PPV films in a distributed feedback (DFB) configuration based on holographic polymer dispersed liquid crystal (HPDLC) were characterized to reveal the practical application. 2. Materials and Methods 2.1. Semiconducting Layer Preparation The MEH-PPV film was used as an active medium. The MEH-PPV (OLED Material Tech.) was dissolved in tetrahydrofuran (THF), trichloromethane (TCM), chlorobenzene (CB), toluene and p-xylene by weight ratio at 0.6 wt %. The chemical structures and properties of five solvents used in this study are shown in Table 1. The solutions were stirred for 48 h to ensure sufficient dissolution. A drop of MEH-PPV solution was injected onto a piece of deionized pre-clean glass substrate for spin-cast. The thickness of the MEH-PPV film was controlled by the spin speed and measured using a surface profiler (KLA Tencor P-16+). All experiments were performed in air under the same ambient circumstance. 2.2. Waveguide Structure Fabrication In order to characterize the amplified spontaneous emission (ASE) properties of MEH-PPV film spin-cast from different solvents, the polymer dispersed liquid crystal (PDLC)/MEH-PPV/glass substrate slab planar waveguide structure was fabricated. The PDLC film was fabricated on the MEH-PPV film as a cladding layer by photochemical reaction. The mixture for PDLC mainly contained acrylate monomers dipentaerythritol hydroxyl pentaacrylate (DHPA, Aldrich, 29.4 wt %, Shanghai,

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China) and phthalicdiglycol diacrylate (PDDA, Eastern Acrylic Chem, 29.4 wt %, Shandong, China) and nematic liquid crystals (TEB-30A, no = 1.522, ∆n = 0.170, Silichem, 29.4 wt %, Shijiazhuang, China). Materials 2017, 10, 706 of 14 Materials 2017, 2017, 10, 706 706 of 14 14added Crosslinking monomer N-vinylpyrrolidone (NVP, Aldrich, 9.8 wt %, Shanghai, China) was 333also Materials 10, of to dilute the mixture. Rose Bengal (RB, Aldrich, 0.5 wt %, Shanghai, China) and N-phenylglycine China). Crosslinking monomer N-vinylpyrrolidone (NVP, Aldrich, 9.8 wt %, Shanghai, China) was China). Crosslinking Crosslinking monomer monomer N-vinylpyrrolidone N-vinylpyrrolidone (NVP, (NVP, Aldrich, Aldrich, 9.8 9.8 wt wt %, %, Shanghai, Shanghai, China) China) was was China). (NPG, Aldrich, 1.5 wt %, Shanghai, China) were used as photoinitiator and coinitiator, respectively. also added to dilute the mixture. Rose Bengal (RB, Aldrich, 0.5 wt %, Shanghai, China) and Nalso added to dilute the mixture. Rose Bengal (RB, Aldrich, 0.5 wt %, Shanghai, China) and Nadded to dilute the mixture. Rose Bengal (RB, Aldrich, 0.5 wt %, Shanghai, China) and NThe also mixture, which was stirred for 48 h to ensure an isotropic and homogeneous material system, phenylglycine (NPG, Aldrich, 1.5 wt %, Shanghai, China) were used as photoinitiator and coinitiator, phenylglycine (NPG, Aldrich, 1.5 wt %, Shanghai, China) were used as photoinitiator and coinitiator, phenylglycine (NPG, Aldrich, 1.5 wt %, Shanghai, China) were used as photoinitiator and coinitiator, wasrespectively. injected into an empty glass cell by capillary action in a darkroom. The empty sample The mixture, which was stirred for 48 h to ensure an isotropic and homogeneous respectively. The The mixture, mixture, which which was was stirred stirred for for 48 48 h h to to ensure ensure an an isotropic isotropic and and homogeneous homogeneous cell respectively. material system, was injected into an empty glass cell by capillary action in a darkroom. The empty wasmaterial made by two was pieces of glass substrates, one a spin-cast filmThe and the other material system, was injected into an an empty glass glass cellhad by capillary capillary actionMEH-PPV in aa darkroom. darkroom. The empty system, injected into empty cell by action in empty sample cell was made by two pieces of glass substrates, one had aµm spin-cast MEH-PPV film and the wassample a purecell glass substrate. The cell gap was controlled at 9 by spacers. The PDLC film sample cell was made by two pieces of glass substrates, one had a spin-cast MEH-PPV film and the was was made by two pieces of glass substrates, one had a spin-cast MEH-PPV film and the other was aa pure glass substrate. The cell gap was controlled at 99 μm by spacers. The PDLC film was other was pure glass substrate. The cell gap was controlled at μm by spacers. The PDLC film was photo-cured by illuminating the sample for 10 min using a 532 nm continuous frequency doubled other was a pure glass substrate. The cell gap was controlled at 9 μm by spacers. The PDLC film was photo-cured by illuminating the sample for 10 min using aa 532 nm continuous frequency doubled 3+ photo-cured by illuminating the sample for 10 min using 532 nm continuous frequency doubled Neodymµµium-doped Yttriumthe Aluminum Garnet harmonic generation [16]) laser photo-cured by illuminating sample for 10 min(Nd using:YAG, a 532 second nm continuous frequency doubled 3+ 3+:YAG, second harmonic generation [16]) Neodymμμium-doped Yttrium Aluminum Garnet (Nd 2 , asgeneration Neodymμμium-doped Yttrium Aluminum Aluminum Garnet (Nd (Nd :YAG,atsecond second harmonic generation [16]) 3+ 3+:YAG, Neodymμμium-doped Yttrium Garnet harmonic [16]) beam (New Industries Optoelectronics, Changchun, China) 10 mW/cm shown in Figure 1a. 2 as shown in Figure laser beam (New Industries Optoelectronics, Changchun, China) at 10 mW/cm laser beam beam (New (New Industries Industries Optoelectronics, Changchun, China) at 10 10 mW/cm mW/cm222,,, as as shown shown in in Figure Figure laser at An attenuator was inserted toOptoelectronics, the beam pathChangchun, to regulateChina) the beam intensity. The 2 mm initial beam 1a. An An attenuator attenuator was was inserted inserted to to the the beam beam path path to to regulate regulate the beam beam intensity. intensity. The The 22 mm mm initial initial beam beam 1a. diameter was expanded to 10 mm when it passed throughthe the expander, which contained a 20-X micro diameter was expanded to 10 mm when it passed through the expander, which contained a 20-X diameter was expanded to 10 mm when it passed through the expander, which contained a 20-X objective (Newport) and a 100 mm focal length doublet lens.lens. A pinhole (25(25 µm, Newport), which micro objective (Newport) and aa 100 mm focal length doublet A pinhole μm, Newport), micro objective (Newport) and 100 mm focal length doublet lens. A pinhole (25 μm, Newport), micro objective (Newport) and a 100 mm focal length doublet lens. A pinhole (25 μm, Newport), located at the focal of the micro objective, was used a spatial filter in thein expander. The mean which located at the focal plane of the micro objective, was used as spatial filter the expander. which located atplane the focal focal plane of the the micro objective, objective, wasasused used as aaa spatial spatial filter in the expander. expander. which located at the plane of micro was as filter in the The mean refractive index of the PDLC after photo-polymerization was 1.541 at 589 nm, which was refractive index of the PDLC after photo-polymerization was 1.541 at 589 nm, which was measured The mean mean refractive refractive index index of of the the PDLC PDLC after after photo-polymerization photo-polymerization was was 1.541 1.541 at at 589 589 nm, nm, which which was was The measured using an Abbe refractometer (2 WA, Kernco, El Paso, TX, USA). using an Abbe refractometer (2 WA, Kernco, El Paso, TX, USA). measured using using an an Abbe Abbe refractometer refractometer (2 (2 WA, WA, Kernco, Kernco, El El Paso, Paso, TX, TX, USA). USA). measured Table 1. Properties of solvents used in this work. Tetrahydrofuran (THF); trichloromethane (TCM); Table 1. Properties Properties of solventsused used in in this this work. work. Tetrahydrofuran (THF); trichloromethane (TCM);(TCM); ofof solvents work.Tetrahydrofuran Tetrahydrofuran (THF); trichloromethane Table 1. Properties Table 1. solvents used in this (THF); trichloromethane (TCM); chlorobenzene (CB). chlorobenzene (CB). chlorobenzene (CB). chlorobenzene (CB).

Materials Materials Materials Materials

Chemical Structure Chemical Structure Structure Chemical Chemical Structure O O O

THF THF THF THF TCM TCM TCM TCM

CB CB CB

Cl Cl Cl Cl

H H H H C C C C Cl Cl Cl Cl Cl Cl Cl Cl

CB

Polarity Polarity 4.2 4.2

687

687 687 687

4.4 4.4 4.4

323

323 323 323

2.7 2.7 2.7 2.7

43

43 43 43

2.4 2.4 2.4 2.4

79

79 79 79

2.5 2.5 2.5

27

27 27 27

4.2 4.2 Cl Cl Cl Cl

Volatility Volatility (mg/hour) (mg/hour)

Polarity Volatility Volatility (mg/hour) Polarity (mg/hour)

4.4

CH CH333 CH CH33

toluene toluene toluene toluene

2.4

CH CH333 CH CH33

p-xylene p-xylene p-xylene p-xylene

2.5

CH CH333 CH CH33

2.3. ASE Characterization 2.3. ASE Characterization 2.3. 2.3. ASEASE Characterization Characterization For the ASE study, the samples, which had a MEH-PPV film sandwiched between a PDLC film For the ASE study, thesamples, samples, which which had had MEH-PPV film sandwiched between PDLC film film ASE study, the samples, which film sandwiched between aa PDLC film ForFor thethe ASE study, the hadaaaMEH-PPV MEH-PPV film sandwiched between a PDLC and a glass substrate, were optically pumped by a frequency doubled passively Q-switched and aa glass glass substrate, substrate, were were optically optically pumped pumped by by aa frequency frequency doubled doubled passively passively Q-switched Q-switched and and Nd a glass substrate, were optically pumped by a frequency doubled passively Q-switched Nd3+ :YAG 3+ 3+:YAG pulsed laser (532 nm, 10 ns, 10 Hz, New Industries Optoelectronics, Changchun, China), Nd3+ :YAG pulsed pulsed laser laser (532 (532 nm, nm, 10 10 ns, ns, 10 10 Hz, Hz, New New Industries Industries Optoelectronics, Optoelectronics, Changchun, Changchun, China), China), 3+:YAG Nd pulsed laser (532 nm, 10 ns, 10 Hz, New Industries Optoelectronics, Changchun, China), as shown as shown in Figure 1b. The pumping laser beams along the sample normal were reshaped with as shown shown in in Figure Figure 1b. 1b. The The pumping pumping laser laser beams beams along along the the sample sample normal normal were were reshaped reshaped with with aaa as in Figure 1b. The pumping laser beams along the sample normal were reshaped with a cylinder cylinder lens (f 200 mm) to produce nearly rectangle pumping area on the sample. An adjustable cylinder lens lens (f (f === 200 200 mm) mm) to to produce produce aaa nearly nearly rectangle rectangle pumping pumping area area on on the the sample. sample. An An adjustable adjustable cylinder lensslit (f = 200 mm) to produce a nearly rectangle pumping area on the sample. An adjustable slit was was used to filter the central part (3 mm by 1 mm) of the pumping area to ensure uniform slit was was used used to to filter filter the the central central part part (3 (3 mm mm by by 11 mm) mm) of of the the pumping pumping area area to to ensure ensure uniform uniform slit pumping. The ASE emission from the sample edge of the waveguide was then collected using a fiberusedpumping. to filter the central part (3 mm by 1 mm) of the pumping area to ensure uniform pumping. pumping. The The ASE ASE emission emission from from the the sample sample edge edge of of the the waveguide waveguide was was then then collected collected using using aa fiberfibercoupled grating spectrometer (Sofn Instruments, Beijing, China) with aa resolution at 0.23 nm. The coupled ASE emission from the sample edge of the waveguide was then collected using a fiber-coupled coupled grating spectrometer (Sofn Instruments, Beijing, China) with resolution at 0.23 nm. grating spectrometer (Sofn Instruments, Beijing, China) with a resolution at 0.23 nm. Moreover, a non-polarized beam splitter (BS) was used in the beam path to split part of the incident Moreover, a non-polarized beam splitter (BS) was used in the beam path to split part of the incident grating spectrometer (Sofn Instruments, Beijing, China) with a resolution at 0.23 nm. Moreover, Moreover, a non-polarized beam splitter (BS) was used in the beam path to split part of the incident beams to monitor the real time pump beam energy. An optical attenuator was used to regulate the beams to to monitor monitor thesplitter real time time pump beam energy. An optical attenuator was of used toincident regulate beams the a non-polarized beam (BS) wasbeam usedenergy. in the An beam pathattenuator to split part theto to beams the real pump optical was used regulate the pump energy continuously to investigate output-emission intensity as a function of input-pumping pump energy continuously to investigate output-emission intensity as a function of input-pumping pumpthe energy continuously to investigate output-emission intensity a function of input-pumping monitor real time pump beam energy. An optical attenuator wasasused to regulate the pump energy energy fluence. energy fluence. fluence. energy continuously to investigate output-emission intensity as a function of input-pumping energy fluence.

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The net gain of the film was obtained using a variable stripe length (VSL) method [17]. One end The net gain of the was obtained variable stripe length (VSL) [17].ofOne end of the excitation stripe wasfilm in same positionusing with athe edge of the sample whilemethod the length excitation of the excitation stripe was in same position with the edge of the sample while the length of excitation stripe (l) was varied. The intensity of the ASE from the edge of the sample was measured, as shown in stripe2a. (l)Materials wasoutput varied. The intensity the ASE the edge 2017, 10, 706 of 14 Figure The ASE intensityof should be from governed by of the sample was measured, as4shown in Figure 2a. The output ASE intensity should be governed by The net gain of the film was obtainedAusing stripe length (VSL) method [17]. One end (Aλ() )IIpa variable g(λ)l p[ e edge I (λposition )I (= 1， ], sample while the length of excitation (1) g ( )l − of the excitation stripe was in same with the of  )  g ( λ ) [e  1]the (1)  ) the edge of the sample was measured, as shown stripe (l) was varied. The intensity of the ASEg (from in Figure 2a. The output ASE intensity should be governed by

where A(λ) is is a constant emissioncross crosssection, section,IpIis pumping intensity, p is thethe pumping intensity, where A(λ) a constantrelated relatedto tothe the spontaneous spontaneous emission A(the ) I p pumping g ( )l g(λ) is the net gain coefficient, and l is the length of stripe. g(λ) is the net gain coefficient, and l is the length I ( )  of the pumping [e  1]， stripe. (1) g ( )

where A(λ) is a constant related to the spontaneous emission cross section, Ip is the pumping intensity, g(λ) is the net gain coefficient, and l is the length of the pumping stripe.

(a)

(b) (a)

(b)

(c)

(d)

Figure 1. Experimental setups: (a) Schematic presentation of the experimental setup for polymer(d) for Figure 1. Experimental setups:(c) (a) Schematic presentation of the experimental setup polymer-dispersed dispersed liquid crystal (PDLC) film fabrication; (b) diagram of poly[2-methoxy-5-(2′-ethyl0 liquid crystal (PDLC) film fabrication; (b) diagram of poly[2-methoxy-5-(2 -ethyl-hexyloxy)-1,4Figure 1. Experimental setups: (a) Schematic presentation of the experimental setup for polymerhexyloxy)-1,4-phenylene-vinylene] (MEH-PPV) film amplified spontaneous emission (ASE) dispersed liquid crystal film (PDLC) film fabrication; (b) emission diagram (ASE) of poly[2-methoxy-5-(2′-ethylphenylene-vinylene] (MEH-PPV) amplified spontaneous pumping; (c) experimental pumping; (c) experimental setup of holographic polymer dispersedspontaneous liquid crystal (HPDLC) laser hexyloxy)-1,4-phenylene-vinylene] (MEH-PPV) film amplified emission (ASE) setup of holographic polymer dispersed liquid crystal (HPDLC) laser fabrication with a two-beam fabrication with a two-beam Mach-Zehnder interferometer, and; (d) HPDLC distributed feedback pumping; (c) experimental setup of holographic polymer dispersed liquid crystal (HPDLC) laser Mach-Zehnder interferometer, and; (d) HPDLC distributed feedback (DFB) laser optical performance (DFB) laser opticalwith performance characterization schematic diagram. fabrication a two-beam Mach-Zehnder interferometer, and; (d) HPDLC distributed feedback characterization schematic diagram. (DFB) laser optical performance characterization schematic diagram.

(a)

(a)

(b) (b)

2. Schematic illustration (a) variable stripe length and (b)shifting shifting excitation excitation stripe Figure 2.Figure Schematic illustration for (a)forvariable stripe length and (b) stripeexperiments. experiments. Figure 2. Schematic illustration for (a) variable stripe length and (b) shifting excitation stripe experiments.

The waveguide losses of the MEH-PPV film were characterized as shown in Figure 2b, where the

The waveguide losses of the MEH-PPV film were characterized as shown in Figure 2b, where the excitation stripe was of gradually shifted away from thecharacterized edge of the sample when keeping the excitation The waveguide theshifted MEH-PPV as shown in Figure 2b, where the excitation stripe waslosses gradually away film fromwere the edge of the sample when keeping the excitation stripe length constant [17]. The ASE intensity from the end of the excitation stripe (I0) should be constant excitation stripe was gradually shifted away from the edge of the sample when keeping the excitation stripe length constant [17].energy The ASE intensity end of the excitation (I0)edge should besample constant since the excitation of the pumpingfrom beamthe is constant. The emission stripe from the of the stripe constant [17].the ASE intensity from theand end ofabsorption/scattering the excitation (Ithe be 0 ) should sincelength thedecreased excitation energy ofThe the pumping beam constant. The from thestripe edge sample because excitation stripe wasisshifted the emission lossofwould be constant since the excitation energy of the pumping beam is constant. The emission from the edge decreased because the excitation stripe was shifted and the absorption/scattering loss would be increased with shifting. The waveguide losses follow the Beer-Lambert law of increased the sample decreased the excitation stripethe was shifted andlaw the absorption/scattering loss with shifting. because The waveguide losses follow Beer-Lambert I  I0 e x， (2) would be increased with shifting. The waveguide losses  x follow the Beer-Lambert law I  I e ， (2) where x is the shifting distance between the end0of the excitation stripe and the edge of the sample, −αx the waveguide losses, I is the ASE emission from the edge of the sample, and I0 is the (2) = Iof 0 eintensity where xαisisthe shifting distance between the Iend the ,excitation stripe and the edge of the sample, ASE emission intensity from the end of excitation stripe. α is the waveguide losses, I is the ASE emission intensity from the edge of the sample, and I0 is the where is the shifting distance between the end of the excitation stripe and the edge of the sample, ASE xemission intensity from the end of excitation stripe. α is the waveguide losses, I is the ASE emission intensity from the edge of the sample, and I0 is the ASE emission intensity from the end of excitation stripe.

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2.4. Laser Fabrication and Characterization

As for forwaveguide waveguide distributed feedback (DFB)application laser application and characterization, As distributed feedback (DFB) laser and characterization, a holographica holographic polymer-dispersed liquid crystal (HPDLC) [18] filmonwas on the MEH-PPV polymer-dispersed liquid crystal (HPDLC) [18] film was fabricated the fabricated MEH-PPV film as an external film as an external distributed feedback layer [19]. The prepolymerization mixture was the same distributed feedback layer [19]. The prepolymerization mixture was the same with the one usedwith for the oneThe usedinterference for PDLC. The interference optical field by twotolaser beamsthe to exposure the PDLC. optical field was created bywas twocreated laser beams exposure prepolymer prepolymer mixtureHPDLC to fabricate film, as shown 1c. A non-polarized splitter mixture to fabricate film,HPDLC as shown in Figure 1c. in A Figure non-polarized beam splitterbeam was used to 2). The curing 2 was used to split the incident laser beams by intensity with a ratio at 1:1 (3.05 mW/cm split the incident laser beams by intensity with a ratio at 1:1 (3.05 mW/cm ). The curing time was time was controlled 60 sbeams. with laser beams. the glass/MEH-PPV/HPDLC waveguide controlled at 60 s withatlaser Therefore, theTherefore, glass/MEH-PPV/HPDLC waveguide structure was structure for the waveguide HPDLC DFB laser. formed forwas theformed waveguide HPDLC DFB laser. shown in in Figure Figure 1d, 1d, the the glass/MEH-PPV/HPDLC glass/MEH-PPV/HPDLC was laser As shown was optically optically excited excited for DFB laser laser. The output application. A polarizer was used to regulate the polarization state of the pumping laser. lasing emission was collected collected with with aa fiber-coupled fiber-coupled grating grating spectrometer spectrometer with with ~32 ~32°◦ to the sample sample normal [20]. A 50 mm focal length lens was used to collect the emission light into the probe of a high normal resolution energy energy meter meter (Coherent) (Coherent) for for conversion conversionefficiency efficiencymeasurement. measurement. resolution 3. Results and Discussion 3.1. 3.1. Film Film Morphologies Morphologies The The interactions interactions between between solvents solvents and and MEH-PPV MEH-PPV chains chains vary vary intensely intensely when when MEH-PPV MEH-PPV is is dissolved in different solvents [11]. Therefore, the morphology conformation, which indicates dissolved in different solvents [11]. Therefore, the morphology conformation, which indicates the the interactions polymer chains, is preserved during the spin-cast process [21]. interactions between betweenthe thesolvents solventsand andthe the polymer chains, is preserved during the spin-cast process The thickness of the films was controlled at 80 nm. The tapping mode Atomic Force Microscope [21]. The thickness of the films was controlled at 80 nm. The tapping mode Atomic Force Microscope (AFM, Multimode 8) 8) was wasused usedtotovisually visuallycompare comparethe thefilm filmmorphologies morphologies a scan rate (AFM, BRUKER BRUKER Multimode at at a scan rate of of 5 µm/s. Figure 3 shows the film morphologies of the MEH-PPV films spin-cast from different 5 μm/s. Figure 3 shows the film morphologies of the MEH-PPV films spin-cast from different solvents. solvents. The average surface was roughness was0.716 0.673 nm, 0.716 nm, 2.02 1.82nm nm,for and 2.02 nm The average surface roughness 0.673 nm, nm, 1.31 nm, nm, 1.82 1.31 nm, and THF, TCM, for THF, TCM, CB, toluene, and p-xylene spin-cast films, respectively. The results showed that CB, toluene, and p-xylene spin-cast films, respectively. The results showed that the surface morphology the surface morphology of the MEH-PPV filmswith changed significantly with The the spin-cast solvents. of the MEH-PPV films changed significantly the spin-cast solvents. morphologies for The morphologies for nonaromatic strong polarity solvents THF and TCM were more flat and uniform nonaromatic strong polarity solvents THF and TCM were more flat and uniform than that spin-cast by than that spin-cast by aromatic CB, toluene and in p-xylene, in that Tablethe 1. aromatic weak polarity solventsweak CB, polarity toluene solvents and p-xylene, as shown Table 1.asItshown implied It implied that the conformation of MEH-PPV chains in solution preserved through the casting process conformation of MEH-PPV chains in solution preserved through the casting process and survived into and survived into the film.polarity Aromatic weak polarity such and as CB, toluenepossess and p-xylene possess the film. Aromatic weak solvents such assolvents CB, toluene p-xylene a preferential ainteraction preferential interaction withbackbone the aromatic backbone the polymer chains, a result, the polymer with the aromatic of the polymerofchains, as a result, the as polymer chains adopt a chains adopt a rigid, open conformation in solution. The nonaromatic strong polarity solvents such as rigid, open conformation in solution. The nonaromatic strong polarity solvents such as THF and TCM, THF and TCM, on possess the other hand, possess a preferential interaction polymers groups. on the other hand, a preferential interaction with the polymerswith side the groups. Thus, side the polymer Thus, the polymer chains in nonaromatic strong polarity solvents tend to coil tightly to maximize chains in nonaromatic strong polarity solvents tend to coil tightly to maximize solvent-side group solvent-side group interactions and of minimize exposure of theto aromatic backbone to the solvent. interactions and minimize exposure the aromatic backbone the solvent.

(a)

(b) Figure 3. Cont.

(c)

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(e)

(d)

3. Atomic AtomicForce ForceMicroscope Microscope (AFM) image of MEH-PPV the MEH-PPV (a) (b) THF; (b) (c) TCM; Figure 3. (AFM) image of the filmsfilms from from (a) THF; TCM; CB; (c) CB; (d) toluene (e) p-xylene. (d) toluene and (e) and p-xylene.

The light scattering experiments show that the average hydrodynamic radius RH in aromatic The light scattering experiments show that the average hydrodynamic radius RH in aromatic weak polarity solvent CB is nearly double than that in nonaromatic strong polarity solvent THF. The weak polarity solvent CB is nearly double than that in nonaromatic strong polarity solvent THF. difference of average hydrodynamic radius RH confirms the distinct conformation for MEH-PPV The difference of average hydrodynamic radius RH confirms the distinct conformation for MEH-PPV polymer chains in solutions [22]. The conformations of MEH-PPV chains in solutions will preserve polymer chains in solutions [22]. The conformations of MEH-PPV chains in solutions will preserve the the MEH-PPV films when spin-cast. The microstructures of the MEH-PPV films cast from aromatic MEH-PPV films when spin-cast. The microstructures of the MEH-PPV films cast from aromatic solvents solvents CB and p-xylene and nonaromatic solvent THF are investigated by X-ray diffraction. The CB and p-xylene and nonaromatic solvent THF are investigated by X-ray diffraction. The results results indicate that the chain packings and orientations are different. When MEH-PPV films are indicate that the chain packings and orientations are different. When MEH-PPV films are span-cast span-cast from THF, the anisotropy in chain orientation is more pronounced [13]. In next section, the from THF, the anisotropy in chain orientation is more pronounced [13]. In next section, the film ASE film ASE characterization will be performed to investigate the optical performance differences of the characterization will be performed to investigate the optical performance differences of the MEH-PPV MEH-PPV films spin-cast from different solvents. films spin-cast from different solvents. 3.2. Film Film ASE ASE Characterization Characterization 3.2. The film orientation conformation of The film morphologies morphologies are are the themacroscopic macroscopicindication indicationofofthe theinner inner orientation conformation the polymer chains. Thus, the ASE experiments were performed to prove the distinct inner of the polymer chains. Thus, the ASE experiments were performed to prove the distinct inner conformation [23]. [23]. The The glass/MEH-PPV/PDLC glass/MEH-PPV/PDLC waveguide for conformation waveguidestructure structure is is aa simple simple cavity cavity geometry geometry for spectral selection even though they cannot suppress the spectral well. There is a cutoff film thickness spectral selection even though they cannot suppress the spectral well. There is a cutoff film thickness (hcutoff)) in this structure [24], below which the fundamental mode cannot propagate. This cutoff (h cutoff in this structure [24], below which the fundamental mode cannot propagate. This cutoff thickness is is given given by by thickness v n n  h  arctan u (3) u n2PDLC −， n2glass λ n n  n arctantn q 2 ASE hcuto f f = , (3) n2MEH − PPV − n2PDLC 2π n2MEH − PPV − n2PDLC where λASE is central wavelength of the guided light, nMEH-PPV is the refractive index of the MEH-PPV film, PDLC is the refractive index of the PDLC film and nglass are the refractive index of the cover glass. wherenλ ASE is central wavelength of the guided light, nMEH-PPV is the refractive index of the MEH-PPV In our case, is 633 nm, nPDLC is 1.541, andPDLC nglass isfilm 1.516. The in-plane index of MEH-PPV films film, nPDLC λ isASEthe refractive index of the and nglass are the refractive index of thevaried cover with the cast solvents. The in-plane refractive index was lower for nonaromatic solvents cast MEHglass. In our case, λASE is 633 nm, nPDLC is 1.541, and nglass is 1.516. The in-plane index of MEH-PPV PPV films than that aromatic solvents.refractive The in-plane index 1.94, 1.958, 1.969 and films varied with thecast castfrom solvents. The in-plane index was was lower for 1.948, nonaromatic solvents 1.974MEH-PPV for THF, TCM, toluene and p-xylene cast MEH-PPV Thewas refractive index cast filmsCB, than that cast from aromatic solvents.films, Therespectively. in-plane index 1.94, 1.948, was ~1.53 in the direction perpendicular to the plane of the film [25]. For the spin-cast MEH-PPV, the 1.958, 1.969 and 1.974 for THF, TCM, CB, toluene and p-xylene cast MEH-PPV films, respectively. polymer chains lie preferentially in the plane of the film [26], so we use ~1.9 in our case. The calculated The refractive index was ~1.53 in the direction perpendicular to the plane of the film [25]. For the cutoff thickness for MEH-PPV waslie23.9 nm. We chose filmofthickness at 80sonm, was spin-cast MEH-PPV, the polymerfilm chains preferentially in thethe plane the film [26], we which use ~1.9 in thick enough to support the guided light. The guided light eigenvalue equation of TE-modes in a slab our case. The calculated cutoff thickness for MEH-PPV film was 23.9 nm. We chose the film thickness waveguide is described [27]: at 80 nm, which was thick enough to support the guided light. The guided light eigenvalue equation ASE

cutoff

2

2

PDLC

glass

2

2

2

2

MEH  PPV

PDLC

MEH  PPV

PDLC

of TE-modes in a slab waveguide is described [27]:N  n N n  N ) k h  m  arctan( (n )  arctan( ) ， (4) n N n N 1 1 2 2 N 2 − n2glass 1 N 2 − n2PDLC 2 2 − N ) k h = mπ + arctan ( ) + arctan ( ) , (4) where(nN2MEH is the effective refractive index of the waveguide modes. Waveguide modeling indicated 0 − PPV n2MEH − PPV − N 2 n2MEH − PPV − N 2 that there was only one TE-mode for an 80 nm thick film and there were no TM-modes. During the photo-pumping for ASE, only the gain-narrowed peak survived and the long tails of the photoluminescence were totally compressed, when the gain over the waveguide losses. Figure 4a 2

1

2

MEH  PPV

2

2

0

2

MEH  PPV

2

1

PDLC

2

2

2

2

1

glass

2

MEH  PPV

2

2

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where N is the effective refractive index of the waveguide modes. Waveguide modeling indicated that there was only one TE-mode for an 80 nm thick film and there were no TM-modes. During the photo-pumping for ASE, only the gain-narrowed peak survived and the long tails of the photoluminescence were totally compressed, when the gain over the waveguide losses. Figure 4a shows the ASE spectra of MEH-PPV films spin-cast from THF, TCM, CB, toluene and p-xylene in the waveguide structure. The ASE spectra centered at ~633 nm and the full width at half7 maximum Materials 2017, 10, 706 of 14 (FWHM) of the spectra varied from 4.9 to 7 nm. Figure 4b shows the emission-pulse intensity as a the ASE spectra of MEH-PPV films spin-cast from THF,The TCM, CB, toluene and p-xylene in the functionshows of excitation fluence at the central peak (633 nm). output intensity increased with the waveguide structure. The ASE spectra centered at ~633 nm and the full width at half maximum increase in the excitation energy fluence. The THF-cast film possessed the minimum ASE threshold (FWHM) of the spectra varied from 4.9 to 7 nm. Figure 4b shows the emission-pulse intensity as a 2 while the p-xylene cast film possessed the maximum ASE threshold at 86.7 µJ/cm2 . at 26.7 µJ/cm function, of excitation fluence at the central peak (633 nm). The output intensity increased with the 2 , respectively. The ASEincrease threshold TCM, CB, and toluene-cast film film waspossessed 32, 52.7,the and 66.7 µJ/cm in theof excitation energy fluence. The THF-cast minimum ASE threshold 2 at 26.7 μJ/cm , whilepolarity the p-xylene cast film possessed the maximum ASE threshold 86.7performances μJ/cm2. The nonaromatic strong solvents, THF and TCM, demonstrated better at ASE 2 ASE threshold of TCM, solvents, CB, and toluene-cast filmand was p-xylene. 32, 52.7, and 66.7 μJ/cm , respectively. than theThe aromatic weak polarity CB, toluene For aromatic solvents, CB-cast The nonaromatic strong polarity solvents, THF and TCM, demonstrated better ASE performances MEH-PPV film showed better ASE performance than toluene and p-xylene. For non-aromatic solvents, than the aromatic weak polarity solvents, CB, toluene and p-xylene. For aromatic solvents, CB-cast THF cast MEH-PPV film showed better ASE performance than TCM. This indicates that the ASE MEH-PPV film showed better ASE performance than toluene and p-xylene. For non-aromatic threshold decrease solventsfilm molecule flatness both non-aromatic aromatic solvents solvents, THF with cast MEH-PPV showed better ASE for performance than TCM. Thisand indicates that the spin-castASE MEH-PPV films aswith shown in molecule Table 1.flatness The remarkable differences between the highest threshold decrease solvents for both non-aromatic and aromatic solvents spin-cast as shown in Table The remarkable differences betweenand the highest andinduces and lowest ASEMEH-PPV thresholdfilms clearly shows that 1. the solvent molecular structure polarity lowest ASE threshold clearly shows that the solvent molecular structure and polarity induces polymer conformations distinctly. The ASE emission pattern in Figure 4c,d shows a scanning electron polymer conformations distinctly. The ASE emission pattern in Figure 4c,d shows a scanning electron microscope (SEM, Hitachi S-4800) image of a PDLC film, the uniform and flat morphology confirms microscope (SEM, Hitachi S-4800) image of a PDLC film, the uniform and flat morphology confirms the excellent advantages using PDLC as as thethewaveguide claddinglayer. layer. the excellent advantages using PDLC waveguide cladding

(a)

(b)

(c)

(d)

Figure 4. The MEH-PPV Film amplified spontaneous emission (ASE) characterization: (a) ASE spectra

Figure 4. The MEH-PPV Film amplified spontaneous emission (ASE) characterization: (a) ASE spectra of MEH-PPV films in waveguide structure at an excitation fluence of 300 μJ/cm2; (b) the dependence 2 ; (b) the dependence of MEH-PPV films in waveguide structure at an excitation fluence of 300 µJ/cm of emission-pulse intensity to excitation fluence; (c) ASE emission pattern, and; (d) scanning electron of emission-pulse to of excitation (c) ASE emission pattern, and; (d) scanning electron microscope intensity (SEM) image the PDLC fluence; film. microscope (SEM) image of the PDLC film. 3.3. Optical Gain and Losses Moreand experiments 3.3. Optical Gain Losses were performed to investigate the distinct conformations of the spin-cast MEH-PPV films, which were determined by the distinct solvents polymer chains interactions. Gain

More were performed to to investigate distinctstructure conformations of5athe spin-cast andexperiments losses are intimate parameters related the ASE in athe waveguide [28]. Figure is the MEH-PPV which were determined by of the distinct solvents on polymer chainsstripe interactions. Gain and ASEfilms, intensity at the central wavelength ~633 nm dependent the excitation length with excitation fluence at 333 μJ/cm2 for gain study. The tendency of the fitting lines from Equation (1)

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losses are intimate parameters related to the ASE in a waveguide structure [28]. Figure 5a is the ASE intensity at the central wavelength of ~633 nm dependent on the excitation stripe length with excitation fluence at 333 µJ/cm2 for gain study. The tendency of the fitting lines from Equation (1) were changed from steep to gentle for different solvent-cast MEH-PPV films. The fitting net gain parameters of the Materialsspin-cast 2017, 10, 706films from THF, TCM, CB, toluene and p-xylene were 24.6, 17.5, 16.0, 8 of 14 and MEH-PPV 13, − 1 10.5 cm , as shown in Figure 5a, respectively. The nonaromatic strong polarity solvents, THF and were changed from steep to gentle for different solvent-cast MEH-PPV films. The fitting net gain TCM, possessed larger net gain than the aromatic weak polarity solvents, CB, toluene and p-xylene. parameters of the MEH-PPV spin-cast films from THF, TCM, CB, toluene and p-xylene were 24.6, For aromatic solvents, CB-cast MEH-PPV film possessed larger net gain than toluene and p-xylene, 17.5, 16.0, 13, and 10.5 cm−1, as shown in Figure 5a, respectively. The nonaromatic strong polarity whilesolvents, for nonaromatic solvents, THF larger cast MEH-PPV film a larger net solvents, gain than THF and TCM, possessed net gain than thepossessed aromatic weak polarity CB,TCM. The net-gain tends to increase with the solvent molecule flatness for both non-aromatic and aromatic toluene and p-xylene. For aromatic solvents, CB-cast MEH-PPV film possessed larger net gain than solvents spin-cast MEH-PPV films. toluene and p-xylene, while for nonaromatic solvents, THF cast MEH-PPV film possessed a larger net gain TCM. net-gainoftends increaseatwith the solvent molecule of flatness Figure 5bthan is the ASEThe intensity lightto emitted the central wavelength ~633 for nmboth as a nonfunction and aromatic solvents spin-cast MEH-PPV fluence films. at 333 µJ/cm2 . The fitting waveguide of thearomatic distance from sample edges with excitation Figure 5b is intensity of lightfrom emitted at the central of ~633 nm as awere function losses parameters ofthe theASE MEH-PPV films THF, TCM, CB,wavelength toluene and p-xylene 4.3, 4.7, 2. The fitting waveguide losses of the distance from sample edges with excitation fluence at 333 μJ/cm − 1 5.2, 6.7 and 7.2 cm , as shown in Figure 5b, respectively. Again, the nonaromatic strong polarity parameters of the MEH-PPV films from THF, TCM, CB, toluene and p-xylene were 4.3, 4.7, 5.2, 6.7 solvents, THF and TCM, possessed smaller waveguide losses than the aromatic weak polarity solvents, and 7.2 cm−1, as shown in Figure 5b, respectively. Again, the nonaromatic strong polarity solvents, CB, toluene and p-xylene. For aromatic solvents, CB cast MEH-PPV film possessed the smallest THF and TCM, possessed smaller waveguide losses than the aromatic weak polarity solvents, CB, waveguide than toluene p-xylene, while non-aromatic solvents, THFthe castsmallest MEH-PPV toluenelosses and p-xylene. For and aromatic solvents, CBfor cast MEH-PPV film possessed film possessed smallest net gain TCM.while The for losses tend to decrease solvent molecule waveguide the losses than toluene and for p-xylene, non-aromatic solvents, with THF cast MEH-PPV flatness both non-aromatic aromatic solvents spin-cast It was molecule showed that filmfor possessed the smallest and net gain for TCM. The losses tendMEH-PPV to decrease films. with solvent flatnessroughness for both non-aromatic andfor aromatic solvents spin-cast MEH-PPV films. It was showed that the surface was different MEH-PPV films spin-cast from different solvents. Therefore, the surface roughness was different for MEH-PPV films spin-cast from different solvents. Therefore, it is not surprising that the losses varied with aromatic and nonaromatic solvent-cast MEH-PPV films. it is not surprising that the the losses varied with aromatic and nonaromatic solvent-cast MEH-PPV films. The rough surface affects quality of the thin film waveguide and contributes to more scatterings The rough surface affects the quality of the thin film waveguide and contributes to more scatterings when the light is guiding in the MEH-PPV films. The intense aggregation domains formed in aromatic when the light is guiding in the MEH-PPV films. The intense aggregation domains formed in solvent-cast MEH-PPV films can also contributed to scatterings [14]. In Section 3.4, it showed that the aromatic solvent-cast MEH-PPV films can also contributed to scatterings [14]. In section 3.4, it showed ground absorption was stronger for aromatic solvent-cast MEH-PPV films at 633 nm, which leaded to that the ground absorption was stronger for aromatic solvent-cast MEH-PPV films at 633 nm, which more leaded absorption losses. to more absorption losses.

(a)

(b)

Figure 5. (a) Dependence of the film edge light intensity on the excitation stripe length with excitation

Figure 5. (a) Dependence2 of the film edge light intensity on the excitation stripe length with excitation fluence at 333 μJ/cm . The solid lines are fitting to the data using Equation (1). (b) The intensity of fluence at 333 µJ/cm2 . The solid lines are fitting to the data using Equation (1). (b) The intensity of light light emitted from the edge of the waveguide as a function of the distance between the pump stripe emitted of theatwaveguide a function of theThe distance between the pump stripe endfrom and the the edge film edge 333 μJ/cm2 as excitation fluence. solid lines are exponentially fittedend by and 2 excitation fluence. The solid lines are exponentially fitted by Equation (2). the film edge at 333 µJ/cm Equation (2).

The distinct morphologies and ASEperformances performances confirm difference when The distinct morphologies and ASE confirmthe theconformation conformation difference when MEH-PPV films are spin-cast with different solvents. The conformations show intimate contact with MEH-PPV films are spin-cast with different solvents. The conformations show intimate contact with the solvent polarity and molecular structure. Aromatic weak polarity solvents such as CB, toluene, the solvent polarity and molecular structure. Aromatic weak polarity solvents such as CB, toluene, and p-xylene have a preferential interaction with the aromatic backbone of the MEH-PPV polymer and p-xylene have a preferential interaction aromatic backbone of the MEH-PPV chains, and thus the chains possess a rigidwith and the open conformation in solution. As a result, polymer it is chains, and thus the chains possess a rigid and open conformation in solution. a result, straightforward for the π-electrons on one chain to overlap with those on an adjacentAs chain when it is straightforward forthe theconformation π-electronsshows on one chainaggregation, to overlap which with those on an adjacent when spin-cast, e.g., intense corresponds to higher chain average surface roughness, ASE threshold, waveguide losses, and lower net gain properties. Nonaromatic strong polarity solvents such as THF and TCM, on the other hand, have a preferential interaction with the polymer’s side groups. Thus, the polymer chains in THF and TCM coil tightly to maximize

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spin-cast, e.g., the conformation shows intense aggregation, which corresponds to higher average surface roughness, ASE threshold, waveguide losses, and lower net gain properties. Nonaromatic strong polarity solvents such as THF and TCM, on the other hand, have a preferential interaction with the polymer’s side groups. Thus, the polymer chains in THF and TCM coil tightly to maximize solvent-side group interactions and minimize exposure of the aromatic backbone to the solvent. Thus, the conformation possesses weak π-electron interactions with adjacent polymer chains when spin-cast, e.g., weak aggregation [11,21,22], which corresponds to lower average surface roughness, ASE threshold, waveguide losses and higher net gain properties. The volatility for non-aromatic strong polarity THF and TCM occurs faster than that of aromatic weak polarity CB, toluene and p-xylene, which provides the possibility for maintaining the conformation well when spin-cast, as shown in Table 1 [29]. The major reason for the distinct ASE performances is the presence of exciton annihilation, which is described by a model expressed as [11] dN (t) N (t) β =− − √ N 2 ( t ), dt τ t

(5)

where N(t) is the time-dependent population density of emissive excitons, τ is the exciton life time, and β is the bimolecular recombination coefficient. The exciton life time is essentially identical, while the bimolecular recombination coefficient is nearly an order of magnitude larger in the aromatic weak polarity solvent-cast films than in the nonaromatic strong polarity solvent-cast films [11]. The excitons migrate to adjacent polymer chains when exciting. As a result, the exciton annihilation occurs and quenches the luminescence. The exciton annihilation is morphology-dependent, e.g., the exciton annihilation increases with aggregation. For MEH-PPV films spin-cast with nonaromatic strong polarity solvents such as THF and TCM, the morphology of the polymer chains shows weak aggregation. As a result, the exciton annihilation is suppressed in comparison with the MEH-PPV films spin-cast with aromatic weak solvents such as CB, toluene and p-xylene. Thus, it is not surprising that THF- and TCM-cast MEH-PPV films possess a lower ASE threshold and a higher net gain. 3.4. Pure Film Spectra Characterization The spectroscopic characterization was performed to further confirm the distinct conformation of the spin-cast MEH-PPV films [11,21]. The thickness of the MEH-PPV films were controlled at 80 nm. Figure 6a shows fluorescence spectra of the pure MEH-PPV films spin-cast from THF, TCM, CB, DCM, toluene and p-xylene. The reabsorption of fluorescence emision was inhibited because of the large stokes shift (~100 nm) in comparison with the absorption spectra as shown in Figure 6b [3]. The results showed that the fluorescence spectra were distinct for shapes and peak locations of the spectra. The peaks of the fluorescence spectra represented characteristic vibronic structure. The singlet S0-0 peak located at 594.2, 594.8, 597.8, 600 and 604 nm for THF, TCM, CB, toluene and p-xylene spin-cast films, respectively. The fluorescence peaks for aromatic weak polartity spin-cast MEH-PPV films were red-shifted in comparison with that spin-cast with non-aromatic strong polarity solvents [11]. The reason is the increasing of conjugation length for aromatic weak polartity spin-cast MEH-PPV films [22]. The fluorescence emission intensity decreased evidently for aromatic weak polartity spin-cast MEH-PPV films in comparison with that spin-cast with non-aromatic strong polarity solvents. Figure 6b shows absorbance spectra of the pure MEH-PPV films spin-cast from THF, TCM, CB, toluene and p-xylene. The full width at half maximum (FWHM) of all the absorbance spectra were over 100 nm. The absorbance peak located at 498, 499.8, 502.3, 505.9 and 512.8 nm for THF, TCM, CB, toluene and p-xylene spin-cast films, as shown in Figure 6b, respectively. The absorbance peaks for aromatic weak polartity spin-cast MEH-PPV films were red-shifted in comparison with that spin-cast with non-aromatic strong polarity solvents. The reason for this is the increasing of the conjugation length for aromatic weak polartity spin-cast MEH-PPV films [22]. The distinct spectral shapes and peak locations of the fluorescence and absorbance indicates the disctinct energy levels of the conformations.

The results showed that the fluorescence spectra were distinct for shapes and peak locations of the spectra. The peaks of the fluorescence spectra represented characteristic vibronic structure. The singlet S0-0 peak located at 594.2, 594.8, 597.8, 600 and 604 nm for THF, TCM, CB, toluene and p-xylene spin-cast films, respectively. The fluorescence peaks for aromatic weak polartity spin-cast MEH-PPV films were red-shifted in comparison with that spin-cast with non-aromatic strong polarity solvents Materials 2017, 10, 706 10 of 14 [11]. The reason is the increasing of conjugation length for aromatic weak polartity spin-cast MEHPPV films [22]. The fluorescence emission intensity decreased evidently for aromatic weak polartity spin-cast films inofcomparison that spin-cast with non-aromatic strong polarity We believeMEH-PPV that the difference fluorescencewith and absorbance spectra was due to distinct confromation solvents. of the spin-cast films. Materials 2017, 10, 706

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Figure 6. Pure MEH-PPV film spectra characterization: (a) fluorescence spectra and (b) absorbance spectra of the spin-cast MEH-PPV films from different solvents.

Figure 6b shows absorbance spectra of the pure MEH-PPV films spin-cast from THF, TCM, CB, toluene and p-xylene. The full width at half maximum (FWHM) of all the absorbance spectra were over 100 nm. The absorbance peak located at 498, 499.8, 502.3, 505.9 and 512.8 nm for THF, TCM, CB, toluene and p-xylene spin-cast films, as shown in Figure 6d, respectively. The absorbance peaks for aromatic weak polartity spin-cast MEH-PPV films were red-shifted in comparison with that spin-cast with non-aromatic strong polarity solvents. The reason for this is the increasing of the conjugation length for aromatic weak polartity spin-cast MEH-PPV films [22]. The distinct spectral shapes and (a) (b) peak locations of the fluorescence and absorbance indicates the disctinct energy levels of the conformations. We believe that the difference of fluorescence and absorbance spectra was due to Figure 6. Pure MEH-PPV film spectra characterization: (a) fluorescence spectra and (b) absorbance distinct confromation of the spin-cast films. spectra of the spin-cast MEH-PPV films from different solvents.

3.5. Lasing Properties 3.5. Lasing Properties For laser applicaton, a HPDLC grating was fabricated on the MEH-PPV film as the external light For laser applicaton, a HPDLC grating was fabricated on the MEH-PPV film as the external light distributed feedback geometry to form HPDLC/MEH-PPV/glass asymetric waveguide. According to distributed feedback geometry to form HPDLC/MEH-PPV/glass asymetric waveguide. According to Kogelnik’s formula of the theory of DFB lasers [30], the lasing emission wavelength in vacuum is Kogelnik’s formula of the theory of DFB lasers [30], the lasing emission wavelength in vacuum is 2neff  las 2n Λ ， (6) em ff λlas = , (6) m where neff is the effective refractive index of the lasing mode, Λ is the period of the grating, and m is where neff is theorder. effective index of the mode, Λ is the period of theperiod grating,Λ and is the diffraction Therefractive lasing wavelength canlasing be tuned by changing the grating andmthe the diffraction order. The nlasing tuned by controlled changing the grating period Λ and the effective refractive index eff. Thewavelength thickness ofcan thebe films were at 80 nm. For the diffraction effective refractive index n . The thickness of the films were controlled at 80 nm. For the diffraction eff order the 3rd, taking the value of 1.609 for effective refractive index into account, we made HPDLC order the 3rd,590 taking value 1.609 the for effective refractive index into account, wenm. made grating with nm the period toof obtain output lasing wavelength around 633.4 TheHPDLC lasing, grating with 590 nm period to obtain the output lasing wavelength around 633.4 nm. The which is coupled out via grating coupling [20,31–33], as shown in Figures 1d and 7a, shows thelasing, SEM which out period via grating coupling [20,31–33], as shownthe in Figures 1d and 7a,uniform shows the SEM image is ofcoupled the 594 nm HPDLC grating, which confirms well-defined and grating image of the 594 nm period HPDLC grating, which confirms the well-defined grating configuration. The fan-shape-like emission beams pattern is illustrated in Figureand 7b,uniform which confirms configuration. The fan-shape-like emission beams pattern is illustrated in Figure 7b, which confirms the good directionality of the HPDLC DFB laser in comparison with the ASE emission patternthe in good of the HPDLC DFB laser in comparison with the ASE emission pattern in Figure 4c. Figuredirectionality 4c.

(a)

(b)

Figure 7. 7. (a) (a) Scanning Scanning electron electron microscope microscope (SEM) the HPDLC HPDLC film film and and (b) (b) emission emission beams beams Figure (SEM) image image of of the pattern of the waveguide HPDLC DFB laser. pattern of the waveguide HPDLC DFB laser.

Figure 8a shows the lasing spectrum from THF-cast MEH-PPV laser at excitation fluence of 83 μJ/cm2. The central wavelength of the lasing spectrum was 633.4 nm, which was consistent with the theory described by Equation (6), and the full width at half maximum (FWHM) of the lasing spectrum was 0.5 nm. The spectral compression and selection was excellent for the lasing emission spectrum of the waveguide HPDLC DFB laser in comparison with the fluorescence and ASE spectra.

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Figure 8a shows the lasing spectrum from THF-cast MEH-PPV laser at excitation fluence of 83 µJ/cm2 . The central wavelength of the lasing spectrum was 633.4 nm, which was consistent with the theory described by Equation (6), and the full width at half maximum (FWHM) of the lasing spectrum was Materials 2017, 10,0.5 706 nm. The spectral compression and selection was excellent for the lasing emission 11 of 14 spectrum of the waveguide HPDLC DFB laser in comparison with the fluorescence and ASE spectra. Figure Figure 8b 8b shows shows the the emission-pulse emission-pulse energy energy as as aa function function of ofexcitation-pulse excitation-pulse energy. energy. The The emission emission energy energy increases increases slowly slowly with with the the excitation excitation energy energy at at the the initial initial stage, stage, and and then then the the emission emission energy energy increases increases abruptly abruptly when when the the excitation excitation energy reach the lasing threshold as shown in Figure 8b. The Thelasing lasingthreshold thresholdfor for THF, THF,TCM, TCM,CB, CB,toluene tolueneand and p-xylene p-xylene spin-cast spin-cast waveguide waveguide HPDLC HPDLC DFB DFB laser laser 2 , respectively. 2, respectively. was 20 20 andand 25 µJ/cm The The slopeslope conversion efficiency of input was6.7, 6.7,11.7, 11.7,15.7, 15.7, 25 μJ/cm conversion efficiency ofpulse inputenergy pulse to output for THF,for TCM, toluene and p-xylene cast HPDLC DFB laser energy to pulse outputenergy pulse energy THF,CB, TCM, CB, toluene and p-xylene cast HPDLC DFBwas laser9.5%, was 8.2%, respectively. It indicates that the of theofspin-cast MEH-PPV 9.5%, 6.9%, 8.2%, 5.6% 6.9%,and 5.6%4.9%, and 4.9%, respectively. It indicates thatconformation the conformation the spin-cast MEHfilms make amake difference to the performance of laser devices. lasingThe threshold PPV correspondingly films correspondingly a difference to the performance of laserThe devices. lasing tends to increase with aggregation. However, the conversion decreases with aggregation. threshold tends to increase with aggregation. However, theefficiency conversion efficiency decreases with The reason is that aggregation leads to bi-molecule emission process,emission which quenches aggregation. The the reason is that the aggregation leadsnon-radiative to bi-molecule non-radiative process, the florescence [34–36]. The emission beams showed excellent as shown in Figure 8c. which quenches the florescence [34–36]. The emission beamss-polarization showed excellent s-polarization as The device lifetime which defined as thewhich numbers emission pulses when the emission shown in Figure 8c.[37–39], The device lifetime [37–39], defined as the numbers emission pulsesenergy when drops to half ofenergy the initial intensity, 72,000 pulses after h 10 Hz pumping with the 2excitation the emission drops to halfwas of about the initial intensity, was2about 72,000 pulses after h 10 Hz 2 2 fluence at with 1 mJ/cm , as shown in Figure The, as lifetime the photo-stability of the laser pumping the excitation fluence at 1 8d. mJ/cm shownconfirms in Figure 8d. The lifetime confirms the device. From our study, indication is that the the conformation aggregation with is a good photo-stability of the laserthe device. From our study, indication iswith thatweak the conformation weak candidate to be the laser active medium. aggregation is aused goodascandidate to be used as the laser active medium.

(a)

(b)

(c)

(d)

Figure 8. Lasing emission properties characterization: (a) lasing spectrum of THF-cast sample Figure 8. Lasing emission properties characterization: (a) lasing spectrum of THF-cast sample gathered gathered at an excitation fluence of 83 μJ/cm2; (b) dependence of emission-pulse energy on excitation at an excitation fluence of 83 µJ/cm2 ; (b) dependence of emission-pulse energy on excitation fluence; fluence; (c) normalized emission intensity as a function of polarizer rotation angle, (d) dependence (c) normalized emission intensity as a function of polarizer rotation angle, and;and; (d) dependence of of normalized emission intensity to pumping pulses for THF-cast laser. normalized emission intensity to pumping pulses for THF-cast laser.

4. Conclusions In conclusion, the morphology and aggregation was systematically investigated for the lasing emission of semiconducting polymer poly[2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylene-vinylene] (MEH-PPV) films with aromatic weak polarity solvents chlorobenzene (CB), toluene, p-xylene and non-aromatic strong polarity solvents tetrahydrofuran (THF) and trichloromethane (TCM). The

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4. Conclusions In conclusion, the morphology and aggregation was systematically investigated for the lasing emission of semiconducting polymer poly[2-methoxy-5-(20 -ethyl-hexyloxy)-1,4-phenylene-vinylene] (MEH-PPV) films with aromatic weak polarity solvents chlorobenzene (CB), toluene, p-xylene and non-aromatic strong polarity solvents tetrahydrofuran (THF) and trichloromethane (TCM). The results indicated that the conformation of the spin-cast MEH-PPV films with weak aggregation performed excellent lasing emission performances because of the inhibition of the fluorescence quenching induced by the bi-molecule process. The Atomic Force Microscope (AFM) images showed morphologies with different average surface roughness. The amplified spontaneous emission (ASE) experiments confirmed the distinct polymer chain conformation. The conformation, which preserved from the spin-cast process, indicated the distinct interactions between the solvents and MEH-PPV polymer chains. The distinct conformation leads to different interactions of the π-electrons on one chromophore to neighboring polymer chains. The pure film spectra were performed to confirm the effect of distinct conformation on the energy level. This study provides insight into the morphology and aggregation effect of the spin-cast polymer films on the performances of lasers. Acknowledgments: This work is supported by the National Natural Science Foundation of China (61377032 and 61378075). We also want to thank Shu Pei and Zhongxu Cui of State Key Laboratory of Applied Optics sincerely, for the helpful film thickness testing they serve. Author Contributions: Minghuan Liu conceived and designed the experiments; Minghuan Liu performed the experiments; Minghuan Liu, Quanquan Mu and Zhaoliang Cao analyzed the data; Yonggang Liu, Zenghui Peng, Chengliang Yang and Li Xuan contributed reagents/materials/analysis tools; Minghuan Liu and Ji Ma wrote the paper. All authors have approved the final article. Conflicts of Interest: The authors declare no conflict of interest.

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