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Conjugated Polymer Nanoparticles Having Modified Band Gaps Assembled into Nano- and Micro-Patterned Organic Light Emitting Diodes Maura Herrera, Mohammad Abdul-Moqueet, and Mahmoud A. Mahmoud ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b02175 • Publication Date (Web): 31 Dec 2018 Downloaded from http://pubs.acs.org on January 3, 2019

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ACS Applied Nano Materials

Conjugated Polymer Nanoparticles Having Modified Band Gaps Assembled into Nano- and Micro-Patterned Organic Light Emitting Diodes Maura Herrera,‡ Mohammad Abdul-Moqueet,± Mahmoud A. Mahmoud,‡,†,±* Chemical Engineering, Department of Biomedical Engineering, ‡ Department of Chemistry † Department of Physics and Astronomy, ± the University of Texas at San Antonio, One UTSA Circle, San Antonio, TX 78249 *

E-mail: [email protected] Abstract

Nano- and micro-patterns from the Langmuir-Blodgett (LB) assembly of poly(2,5-bis(3sulfonatopropoxy)-1,4-phenylene disodium salt nanoparticles (PPP-NPs) on glass substrates exhibited a variety of structures and tunable band gaps. Tuning the band gap of optical materials used in devices based on conjugated polymers is useful for manufacturing organic light-emitting diodes and optoelectronics. The PPP-NPs are prepared by supramolecular assembly of the polymer in methanol. A highly packed, collapsed PPP-NPs monolayer assembly is obtained when the uncompressed LB film is transferred to the surface of a glass substrate. The band gap of the PPP-NPs dispersed in methanol is reduced by 0.56 eV after monolayer assembly into 2D nanostructure with an average diameter of 28±7 nm and 4.2±0.5 nm height to be 2.47 eV. This large band gap decrease is attributed to the collapse of their supramolecular assembly, the interparticle energy transfer, and change of the dielectric function from methanol to air. Due to the soft nature of the PPP-NPs, compressing the LB film resulted in a hierarchical assembly of the PPP-NPs into microdisks with a few nanometers in thickness. The single microdisk has a band gap of 1.61 eV and a broad optical spectrum composed of multiple peaks, due to random energy transfer between the PPP-NPs. When the LB film of the microdisks assembly is compressed, nanopillars of an average ~ 200 nm diameter and ~ 40 nm height are obtained, which have an intense optical signal. The band gap of the individual nanopillar is 1.6 eV. 1 ACS Paragon Plus Environment

ACS Applied Nano Materials 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

Keywords: Nano-patterns and micro-patterns, band gap, semiconducting polymer nanoparticles, Langmuir-Blodgett, OLED INTRODUCTION π-conjugated polymers (CPs)1-6 are found in many useful applications such as organic electronics,7 thin-film transistors,8 organic light emitting diodes (OLED),9 optoelectronic devices,10-12 sensors,3,13 optical filters,14 and flexible organic solar cells.5,15,16 In many of the applications of CPs, the size of their devices can be reduced to a micrometric scale.17 To better serve current and future applications of CPs, their thickness or size should be reduced to be in the nanoscale dimensions.6,18 When CPs are photoexcited, electron-hole pairs (excitons) are generated.19 These excitons may fluoresce, transfer energy to another electronic system, or get trapped losing the energy as non-radiative thermal energy.20,21 The optical and electrical properties of CPs are sensitive to changes of their assembly. Assembly of CPs into a highly packed structure leads to the rapid transfer of the excitation energy either between polymer segments of different conjugation lengths22-27 or between polymer segments and other chromophores28-31 via Förster resonance energy transfer (FRET).32-34 The energy transfer occurs in few picoseconds,23,24,27,28,33 which is much shorter than the lifetime of the excitons. Changing the conformation of the CP chains leads to the change of the optical absorption peak position and shape.35-37 Red shift in the absorption spectrum are obtained upon extending the conjugation of CP, this can be obtained if the polymers are assembled end-to-end.35,38 Conversely, the optical absorption spectrum of the CP is blue shifted when the conjugation is shortened due to the conformation of the polymer chains.38 Theoretical modeling supports the idea of the inter-chain effects in the CPs, which are important only in case of closely packed polymer systems.39 This highlights the great impact of CPs assembly on their properties.40

2 ACS Paragon Plus Environment

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Polymer nanoparticles are prepared by two common approaches based on either directpolymerization

or

post-polymerization

fabrication.41-45

Generally,

direct

emulsion

polymerization consists of water, a monomer of low water solubility, and, if needed, a surfactant.41 The addition of a surfactant reduces the particle size that allows for better control of the size of the fabricated particles. This method produces polymer nanoparticles with a relatively small size disparity, less than 10%.41 On the other hand, post-polymerization techniques proceed through either the solvent evaporation or reprecipitation method. Solvent evaporation is carried out by dissolving the polymer in an “oil” phase of an organic solvent that is added to an aqueous solution containing a surfactant. The solvent evaporation method is applicable for polymers that are soluble in an organic solvent and water insoluble. The organic solvent evaporation proceeds by increasing the temperature, and the remaining emulsion is ultra-sonicated, which creates droplets stabilized by the surfactant.42,43 Reprecipitation of the polymer is achieved by dissolving it in an organic solvent followed by adding another solvent that the polymer is poorly soluble in. This sudden change in solubility of the polymer induces the polymer aggregation into nanoparticles.44,45 It is aimed to fabricate nano-patterned and micro-patterned OLED from poly(2,5-bis(3sulfonatopropoxy)-1,4-phenylene disodium salt-alt-1,4-phenylene) (PPP) on the surface of a glass substrate by Langmuir-Blodgett (LB) technique. PPP nanoparticles (PPP-NPs) are prepared first from supramolecular assembly of the water-soluble PPP polymer by a modified postpolymerization technique. PPP-NPs monolayer assembly is obtained under a low compression of the LB film monolayer. Upon compressing the LB film, the PPP-NPs are assembled into a 3D hierarchical structure with a diameter in micrometers and a height in nanometers. Further compression of the LB film allows for the PPP-NPs hierarchical microstructures to be squeezed



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into PPP nanopillars (PPP-NPLs) with a height and diameter in nanoscale dimensions. After compression into nanopillars, the PPP-NPs hierarchical assembled microstructure showed volume reductions of a magnitude of five. The volume reduction suggests there is packing of the polymer chains forming the PPP-NPLs. Thus, optical measurements, absorption and fluorescence, are conducted for the PPP-NPs assembly of the different structures in a nanometric scale using ultrahigh resolution spectroscopic techniques. EXPERIMENTAL Poly(2,5-bis(3-sulfonatopropoxy)-1,4-phenylene

disodium

salt-alt-1,4-phenylene),46

(PPP), and methanol were purchased from Sigma-Aldrich. The PPP-NPs were prepared as follows: Using a 30 mL glass vial, 0.1 g PPP was dissolved in to 3 mL of deionized water and was mixed with 20 mL of methanol. A white colored emulsion was obtained, which was shaken vigorously for 1 minute. The resulting emulsion was left in a tightly closed vial for 4 days. Large polymer aggregates of orange color were precipitated down, while the polymer nanoparticles dispersed in the methanol was decanted from the precipitate. In order to determine the amount of PPP assembled into nanoparticles, the precipitated orange color aggregates were dried in air and placed in an oven at 100 oC for complete dryness for 24 hours. 0.0095 g of PPP was remained in the methanol. Consequently, the concentration of PPP in methanol is 31.7 μM, as the molecular weight of PPP is > 30,000 (GPC). NanoBrook dynamic light scattering (BrookHaven, DLS) was used to determine the hydrodynamic radius of PPP-NPs. The absorption spectrum of the polymer nanoparticles solution was measured using the StellarNet dual detector super range spectrometer, UVN-SR (UV-VIS-NIR, CCD) plus NIR-512 (InGaAs PDA). The StellarNet spectrometer is sensitive in the range of 200-1700 nm. Fluorescence of the PPP-NPs in methanol was measured using the StellarNet system with an excitation source of 390 nm. Nima-KSV Langmuir-Blodgett


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trough with a surface area of 780 cm2 was used for the PPP-NPs assembly. 2 mL of PPP-NPs dispersed in methanol, equivalent to 6.33 μM, was dispersed over a water sublayer on the LB trough using a micro-syringe. The LB monolayer was left 10 minutes to allow the methanol to evaporate. The LB isotherm was measured using a platinum plate connected to the pressure sensor of the trough. The LB film of PPP-NPs was transferred to the surface of a glass substrate at surface pressures 0.04, 2, and 8 mN/m, separately, by vertical dipping technique with a lifting speed of 1 mm/second. The surface pressure was kept constant during the transfer of monolayer to the substrate. At each specific surface pressure, a transfer ratio between 0.95-1 was obtained. Due to the large surface area of the LB trough, there was negligible fluctuation of the surface pressure during the transfer of the LB film to the surface of the substrate. VTC-100 vacuum spin coater was used to make a thin film from PPP-NPs on a glass substrate, 50 μL of the PPP-NPs dispersed in methanol was dropped on a substrate spinning at a speed of 1500 rpm. To characterize the PPP-NPs, atomic force microscopy (AFM) Icon Bruker was used to image the topography of the polymer assembly. To characterize the PPP-NPs before LB assembly, a glass substrate was coated with a layer of PPP-NPs by dip coating technique. The glass substrate was dipped into the solution of PPP-NPs and moved up from the solution at a speed of 1 mm/second using the dipper of LB trough. Cytoviva hyperspectral imaging technique was used to conduct the optical measurements (absorption and fluorescence) for individual polymer nanoparticles. The hyperspectral imaging was conducted with an Olympus BX43 upright microscope frame under 100x magnification oil objectives in bright field mode. Absorption measurements were conducted using a white light source with a Specim Imspector V10E hyperspectral camera, while optical images where taken using a Qimaging CCD camera. Florescence measurements were taken using the same cameras alongside a 200W Prior Lumen 200 Metal Halide light source. The



excitation light was subtracted before reaching the detectors using a long pass filter. With the use of both cameras in conjunction, it is possible to correlate the hyperspectral data with the CCD images to verify the structural features observed with its appropriate spectrum. RESULTS AND DISCUSSION Assembly of PPP Polymer into Nanostructure PPP polymer has a poly(p-phenylene) skeleton, in which each repeating unit has two phenylene groups. One of the phenylene in the repeating units has two polar sulfonate groups (see the inset of Figure 1A). Due to the good solubility of the polymer in water, the polymer chains, in principle, are expected to be unfolded in water. Figure 1A shows the absorption spectrum of PPP dissolved in water and PPP nanoparticle assembly (PPP-NPs) dispersed in methanol. Two absorption peaks at 282 and 338 nm are observed in the spectrum of PPP when dissolved in water. The absorption spectrum of the PPP-NPs dispersed in methanol showed two absorption peaks at 288 and 347 nm. On comparing the absorption spectrum of PPP in water with the PPP-NPs in methanol, the absorption peaks were slightly red-shifted and the spectral width decreased after the polymer was assembled into particles. The fluorescence spectrum of the PPP as other π-conjugated polymer is sensitive to the change of their conformation. Figure 1B is the fluorescence spectrum of PPP dissolved in water and PPP-NPs dispersed in methanol measured using a 390 nm excitation source. PPP showed a sharp fluorescence peak at 412 nm in addition to a shoulder featured at 491 nm. The fluorescence peak of the PPP was red shifted by 5 nm to 417 nm after molecular assembly into nanoparticles, while the shoulder peak of the PPPNPs was blue shifted to 487 nm. Figure S1 is the relationship between the hydrodynamic radius of the PPP-NPs dispersed in methanol and the particle relative percent. The average radius of the PPP-NPs is 70±5 nm dispersed in methanol. In order to confirm the assembly of the PPP into


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nanoparticles, a thin film from PPP-NPs dispersed in methanol was prepared by dip coating and spin coating techniques on the surface of a glass substrate. Figure 1C shows the AFM image of the PPP-NPs layer on the surface of a glass substrate prepared by dip coating, while the AFM image of PPP-NPs film spin coated on the surface of a glass substrate is shown in Figure S2. Interestingly, the polymer chains are arranged into particles in case of the dip coated and spin coated films with an average diameter and height of 25±7 nm and 3±2 nm, respectively. The reduction of the size of the polymer assembly after drying is attributed to the solvent evaporation. The volume reduction of the polymer assembly fabricated by spin coating and dip coating and the consistency of the shapes and sizes in the two films confirmed the assembly of PPP into PPP-NPs in the methanol. CPs are usually assembled either through electrostatic attraction or through π-π stacking.47 As there are no changes to the pH or the ionic strength of the PPP solution before or after adding the organic solvents, the electrostatic assembly mechanism is not applicable in this regard. The π-π stacking interaction can lead to the assembly of the polymer into different structures such as random coil, molten globule, toroid, rod, defect-coil, or defect-cylinder.36 PPP is sparingly soluble in methanol, so the PPP chains tend to assemble in a way that lowers the surface area and reduces the exposure of the sulfonate groups to the methanol solvent. This could be accomplished if the polymer self-assembles into micelles of coiled and toroidal shapes that lowers the surface area of the polymer chains and capsulate water molecules. The intramolecular assembly of PPP into PPP-NPs is responsible for the slight red shift of the absorption peaks due to π−π stacking interaction between the phenylene groups. The narrow absorption and fluorescence peaks of the PPP-NPs are indicative of a decrease in degrees of freedom, which in turn inhibit the rotational and vibrational motion of the polymer.35 The assembly of the PPP into



PPP-NPs is summarized in Figure 1D. The PPP polymer is dissolved in a small amount of water, where the polymer chains are unfolded. After adding methanol, an emulsion is formed. The PPP assembles slowly into a coiled and toroidal shapes to minimize their surface area and large polymer aggregates are precipitated. The band gap of any organic semiconductor is the energy difference between the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO). The optical band gap ‫ܧ‬௚ of PPP polymer assembled into PPP-NPs in methanol expressed in eV units can be estimated according to equation (1). Where, O௔Ǥ௘Ǥ is the absorption edge wavelength of the PPP-NPs expressed in nm unit, ݄ is plank’s constant, and ܿ is speed of light. Unlike the absorption peak ( O௠௔௫ ), which obtained at the maximum absorption, O௔Ǥ௘Ǥ is determined from the offset wavelength derived from the low energy absorption band as shown in Figure 1A. Based on this calculation the estimated band gap of PPP-NPs in methanol is 3.07 eV. ‫ܧ‬௚ ሺܸ݁ሻ ൌ ݄ ൈ

஼

OೌǤ೐Ǥ

PPP in H2O PPP-NPs in MeOH

0.8

0.6

0.4

0.2

0.0

Oa.e= 404 nm

Oabs.max 300

ൎ

1.0

A

350

400

450

Wavelength (nm)

500

Normalized Fluorescence Intensity

1.0

Normalized Absorbance

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

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(1)

B

PPP in Water PPP-NPs in Methanol

0.8

0.6

0.4

0.2

0.0

550

ଵଶସ଴

OೌǤ೐Ǥ ሺ௡௠ሻ

400


450

500

Wavelength (nm)

550

600

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C

D

4

Height (nm)

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


3 2 1

50

100

150

200

Length (nm)

250

300

Figure 1 A) Absorption spectrum of: PPP dissolved in water (black spectrum) and PPP-NPs dispersed in methanol, the inset is the molecular structure of PPP polymer. B) Fluorescence spectrum of PPP polymer dissolved in water and PPP-NPs dispersed in methanol. C) AFM image of PPP-NPs dip coated on the surface of a glass substrate. D) Schematic depiction of the molecular assembly of PPP into PPP-NPs. Hierarchical Assembly of Polymer Nanoparticles Langmuir-Blodgett (LB) technique has been used to assemble molecular compounds48 and colloidal nanoparticles49-51 into monolayers of different structures.52-54 To assemble the PPPNPs by LB technique, the PPP-NPs dispersed in methanol are sprayed over the water sub-layer of a LB trough. As in the case of any typical LB experiment, the PPP-NPs are organized into a monolayer on the top of the sub-layer of the LB trough after solvent evaporation. Since the PPPNPs are soft particles, the interaction of the PPP-NPs inside their LB film can be examined by studying the LB isotherm. The surface pressure of the PPP-NPs LB film was measured as a function of mechanical compression of the film. As in the case of most of LB isotherms, the LB isotherm of the PPP-NPs showed three different phases (see Figure 2A). In the first phase, the LB surface pressure did not change while the LB film was compressed, although the surface area of the LB trough was reduced from 700 to 330 cm2. The second phase was observed when the LB film was compressed to occupy an available surface area of the trough between 330 and 60



cm2. In this phase, the LB surface pressure was increased gradually while compressing the LB film. Finally, the surface pressure was increased rapidly upon reducing the trough surface area below 60 cm2 in the third phase of the LB isotherm. To evaluate whether the interaction between PPP-NPs inside the LB film is reversible, the LB trough was reopened and the LB isotherm was remeasured for the second time. Interestingly, the shape of the isotherm was changed after compressing, which suggest the irreversibility of the LB film after compression and a permanent interaction between the PPP-NPs. The LB film of PPP-NPs was transferred to the surface of a glass substrate by vertical dipping technique at LB surface pressure of 0.04, 2, and 8 mN/m. The optical image of this monolayer assembly can be seen in Figure S3. Figure 2B shows the AFM image of PPP-NPs assembly transferred to the surface of the glass substrate at a surface pressure of 0.04 mN/m, corresponding to a trough surface area of ~ 500 cm2. The PPP-NPs assembly on the surface of the glass substrate showed a granular structure with an average diameter and height of 28±7 nm 4.2±0.5 nm. Figure 2C is the AFM image of the PPP-NPs LB film transferred to the surface of the substrate at a surface pressure of 2 mN/m. Polymer microdisks (PPP-MDs) assembly on the top of a granular PPP-NPs layer are obtained at such surface pressure. The PPPMDs composed of a hierarchical assembly of PPP-NPs. Figure S4 is the optical image of the PPP-MDs on the surface of glass substrate. Figure S5A shows the statistical analysis of the diameters and heights of 100 PPP-MDs. The average height and diameter of the PPP-MDs is 6.5±3.5 nm and 1.3±0.3 μm, respectively. Unlike the assembly of hard nanoparticles in which the packing density of the nanoparticles increases inside their 2D arrays when their monolayer assembly is compressed, the soft PPP-NPs exhibited 3D assembly. The 3D assembly could be induced by the high compression of the LB film and the depletion force resulting from the displacement of the PPP-NPs. To confirm the 3D assembly of the PPP-NPs in a large scale, a


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low-resolution AFM imaging was conducted for the PPP-NPs assembly fabricated at LB surface pressure of 2 mN/m, see Figure 3A. Finally, the PPP-NPs LB film was compressed further and transferred to the surface of a glass substrate at a surface pressure of 8 mN/m. The AFM image of the polymer assembly is shown in Figure 2D. PPP-NPLs are formed in the top of the PPP-NPs monolayer assembly. To accurately determine the average dimensions of the larger polymer nanoparticles, statistical calculations were carried out for the heights and diameters of the PPP-NPLs obtained from the low-resolution AFM image shown in Figure 3B. Figure S5B shows the statistical analysis of the diameters and heights of 125 PPP-NPLs. The average diameter and height of the PPP-NPLs is 202±39 nm and 41±10 nm, respectively. Consequently, the PPP-MDs made of hierarchical assembly of PPP-NPs are squeezed into PPP-NPLs upon increasing the LB surface pressure. The PPP-MDs suffered five times volume reduction when transformed to PPP-NPLs; this explains the high packing structure of the polymer chains inside the nanopillars. Figure S6 is the optical image of the PPP-NPLs monolayer assembly transferred to the surface of glass substrate at surface pressure 8 mN/m. The LB assembly PPP-NPs is parallel to the hard nanoparticles monolayer in the following perspective: when the LB is uncompressed, gas phase with LB surface area between 700 to 330 cm2, similar to the monolayer assembly of hard nanoparticles, the PPP-NPs assembled into a monolayer. When the LB film is compressed, LB surface area between 330 and 60 cm2, the surface pressure increased to exceed 1 mN/m and the PPP-NPs assembled into PPPNDs. The PPP-NDs assembly is maintained until the surface pressure increased to be 5 mN/m. The change of the structure of the assembly of PPP-NPs in the liquid condensed phase is analogous to the formation of nanoparticle aggregates in case of hard nanoparticle in the same



phase. When the PPP-NPs LB film is strongly compressed PPP-NPLs are formed. This obtained when surface pressure increased over 5 mN/m. Similar to the behavior of the hard nanoparticles in the solid state region, compressed LB films of PPP-NPs transferred to the substrate at any surface pressure higher than 5 mN/m have PPP-NPLs. Although, the shape of the LB isotherm of the soft polymer nanoparticles is similar to hard nanoparticles, the soft PPP-NPs inside the LB film interact in the following different ways. The unusual behavior of the PPP-NPs monolayer inside the LB film could attribute to the following reasons: 1) When the LB film of the PPP-NPs is compressed, the applied pressure is distributed homogenously on the film. This is not the case for the hard nanoparticles that are composed of two components, the capping materials flexible and hard particles, 2) Due to the porosity and good water interaction of the PPP-NPs, water causes particles to become wet at a low surface pressure. However, under compression, the water molecules gradually diffuse out of the PPP-NPs to form microdisks. Complete dehydration is obtained at a high compression and dense PPP-NPLs are formed, 3) The measured surface pressure of the assembly of the PPP-NPs is not dependent of the organization of the polymer nanoparticles only; but on the rate of water diffusion through the entire LB film. Figure 4 is a schematic depiction summarizing the LB assembly of PPP-NPs on the surface of LB trough. Unlike the LB assembly of the hard nanoparticles which is reversible, the LB assembly of the PPP-NPs is an irreversible, permanent interaction induced only by the high compression of the monolayer. Therefore, the LB isotherm showed irreversible behavior. The change of the structure of the PPP-NPs assembly on the surface of the LB is induced by the compression, not by the methanol solvent, since no large aggregate is observed in the AFM images measured for the PPPNPs layer prepared by uncompressed LB film, spin coating, and dip coating. Furthermore, the PPP-NPLs are formed only when the LB film is transferred to the surface of the substrate from


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the solid-state phase region, independent of the LB surface pressure. On the other hand, all the samples transferred to the substrate from the liquid phase region are PPP-MDs. The PPP-NPLs resulted from the compression of PPP-MDs. The separation distance between the PPP-MDs is comparable to the inter-pillar separation distance. The PPP-NPLs do not pack tightly or are in contact, even at high LB compression, since the pillars are formed through the PPP-NPs monolayer. Drying the PPP-NPLs, which has PPP-NPs in-between, does not have any influence in their shape. The LB film of PPP-NPLs with PPP-NPs distributed in a water-air interface is moved to the surface of the substrate slowly and dries upon the transfer. This confirmed that the PPP-NPLs and PPP-MDs are formed on the surface of the trough before being transferred to the substrate. Although it is more accurate to use particles/area in the LB isotherm instead of surface area, in the case of the assembly of soft nanoparticles, such as PPP-nanoparticles, using the surface area is more accurate. This is because, unlike the typical LB-assembly of hard nanoparticles, compressing the polymer nanoparticles changed them into new systems as microdisks and nanopillars of different concentrations.



A

12

B

First cycle Second cycle

ha

se 0

se en nd Co

Height Hieght(nm) (nm)

e Ph

as

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as

4

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u id

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Liq

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Solid State Phase

Surface Pressure (mN/m)

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Hieght Height (nm)

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Figure 2 A) Langmuir-Blodgett isotherm of PPP-NPs monolayer measured on the surface of water sublayer before compressing and after compressing the LB film, the three phases were observed before and after compressing (black curve), but the shape of the isotherm was changed after compressing confirming the irreversibility of the PPP-NPs LB film (red curve). AFM image of PPP-NPs LB assembly transferred to the surface of a glass substrate at surface pressure of: B) 0.04 mN/m, highly packed PPP nanostructures with granular morphology, C) 2 mN/m, microdisks are formed resulting from hierarchical assembly of the PPP-NPs, D) 8 mN/m, PPPNPLs resulted from squeezing the hierarchical assembled PPP-NPs microdisks. Topographical cross section of the polymer assembly was determined at the dashed line for B and C AFM images, while the height of the three PPP-NPLs in D was determined across the whole AFM image.


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B

A

Figure 3 Low magnification AFM of PPP-NPs LB assembly: A) microdisks of hierarchically assembled PPP-NPs are formed through the PPP-NPs monolayer assembly when the LB film was transferred at surface pressure of 2 mN/m, B) PPP nanopillars are formed on the top of PPPNPs monolayer assembly upon transferring the LB film at 8 mN/m.

2

1

3

Figure 4 Schematic summary of the assembly of PPP-NPs on the surface of LB trough: 1) spraying the PPP-NPs on the top of water sublayer of the LB trough, where the PPP-NPs assembled into a monolayer, 2) The PPP-NPs hierarchically assembled into microdisks when the LB film is compressed, 3) when the LB film are compressed further, the hierarchical microdisks squeezed into PPP nanopillars. Optical properties of LB assembly As in case of other CPs, the optical properties of PPP are sensitive to the change of their assembly.22-34 When the CP is closely packed, rotation of the polymer backbone becomes hindered through steric interactions, leading to changes in their optical and electrical characteristics.22,36,37,55,56 Changes in polymer conformations affect the number of diffusion energy-transfer pathways and the dynamics of the polymer excitons.22,56 The structure of the



PPP-NPs was dramatically changed when LB assembled at different magnitudes of compression, which makes it useful to study the optical properties of PPP-NPs after LB assembly. Figure 5A shows the optical properties of a single PPP-NP collected from the monolayer assembly of the polymer nanoparticles transferred to the surface of a substrate at surface pressure of 0.04 mN/m. The optical image of the PPP-NPs monolayer assembly is shown in Figure 5i. The single PPPNPs showed two absorption peaks at 415 and 443 nm. These two absorption peaks of PPP-NPs are sharp and red-shifted after transferred from the solvent to the surface of a substrate in a monolayer assembly. The sharp peak at 415 nm for the PPP-NPs in a methanol solution is related to the absorption of the π-π stacked phenylene groups, while the peak at 443 nm is representative of a transition dipole-moment along the chain of the PPP molecule, see Figure 1A.47 To confirm the optical measurements for the single PPP-NP inside the monolayer assembly, the hyperspectral optical measurements were collected for different PPP-NPs on same film. The individual PPP-NPs showed a similar absorption spectrum. Fluorescence spectrum was measured for PPP-NPs to confirm the change of their optoelectrical properties after monolayer assembly, especially after ~ 90 nm red shift in the absorption spectrum. Figure 5C is the fluorescence spectrum collected from an individual PPP-NP inside the monolayer assembly. A sharp fluorescence peak was observed at 487 nm when the individual particle was excited at 350 nm. Interestingly, the fluorescence was red shifted 70 nm after moving from methanol and monolayer assembly. Referring to the AFM images in Figure 2, the polymer nanoparticles in solution becomes elliptical nanoparticles after drying. The AFM images for the PPP-NPs film prepared by dip coating and spin coating are similar the uncompressed LB PPP-NPs monolayer assembly. However, the particle size and shape of the PPP-NPs inside the three assemblies are


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consistent and showed similar optical properties in air. This confirmed that the formation of the PPP-NPs in methanol before LB assembly. The significant red-shift of the absorption and fluorescence spectrum of the single PPPNPs after monolayer assembly on the surface of a glass substrate could be explained by the following reasons: 1) Drying the PPP-NPs improved the intramolecular π-π stacking of the phenylene groups and enhanced the PPP chains organization, 2) Drying the PPP-NPs stretched the bonds of the PPP polymer and lowered the HOMO-LUMO energy difference.57 3) The interparticle energy transfer extended the length of the conjugation, 4) tTe polymer forced planarization of the chains that extended the π conjugation,58-60 and 5) the change of the dielectric function from water to air alters the optical properties of the PPP-NPs after drying. The optical spectrum of a single PPP-MD composed of hierarchically assembled PPPNPs is shown in Figure 5A. Unlike the single PPP-NP, which demonstrated a sharp absorption spectrum, PPP-MD exhibited a broad spectrum of two peaks at 444 and 610 nm. Figure 5ii is the optical image of the PPP-MD. It is clear, that the PPP-MDs are surrounded with monolayer assembly of PPP-NPs. To confirm the effect of hierarchical assembly of the PPP-NPs on their optical properties, fluorescence measurements were conducted for an individual PPP-MD at 495 nm excitation. A fluorescence spectrum is observed at 537 nm with peak intensity lower than that of the individual PPP-NPs, the fluorescence spectrum is normalized to that of the single PPPNPs. Unlike the fluorescence spectrum of the individual PPP-NP, which is symmetrical, the fluorescence spectrum of the PPP-MD is asymmetric and has a shoulder feature. Two acceptable reasons could describe the significant red shift in the absorption and fluorescence spectra of PPPNPs after hierarchical assembly into PPP-MD: stretching the bonds of the PPP-NPs after assembly57 and the energy transfer between the PPP-NPs inside their hierarchical assembly. The



broadening of the spectrum is an indication of inhomogeneous electronic interactions between the PPP-NPs inside the PPP-MDs or inside the individual PPP-NP. The optical spectra of PPP-NPLs of different diameters, resulting from squeezing the polymer microdisks, are reported in Figure 5B. Figure S6 shows the optical image of the PPPNPLs assembly, while Figure 5iii-viii are magnified optical images of the individual PPP-NPL corresponding to the absorption spectra reported in Figure 5B. PPP-NPL with a diameter of 221 nm, Figure 5iii, has a sharp absorption peak at 660 nm. This sharp peak shifted to 705 nm in case of 222 nm PPP-NPLs, Figure 5iv. The observed red shift is attributed to the height increase from 39 nm to 51 nm as the PPP-NPL although there was no significant change in the diameter from 221 to 222 nm. The optical spectra of PPP-NPL with diameter of 217, 214, and 228 nm are like that obtained in case of the 221 nm pillar. This is attributed to slight change of the heights and the small variation of the diameters of such four PPP-NPLs. The spheroid shape in Figure 5viii has two shoulders at 710 and 775 nm in addition to the main peak at 666 nm. The anisotropic shape of the spheroid, which supports two different conformations of the PPP chains, is responsible for the multiple absorption peaks. The fluorescence spectrum of an individual PPPNPL of intensity normalized to the intensity of the single PPP-NPs is shown in Figure 5C. Unlike the PPP-ND which has an asymmetric fluorescence spectrum, PPP-NPL showed symmetrical spectrum at 702 nm, but with lower intensity. The exciting optical properties of PPP-NPLs is attributed to squeezing the PPP chains inside the PPP-NPLs, which caused the following: 1) Generating a strain inside the PPP-NPLs that strongly stretched the bonds, which lowered the energy of the band gap.57 2) De-planarization of the PPP backbone could diminish the πconjugation along the polymer backbone.58 This lead to the disappearance of the absorption peak corresponding to the electric dipole-moment transition along the polymer backbone. 3)


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Enhancing the π-π stacking and improving the electronic communication between well-aligned neighboring polymer chains. See Figure S7 of the depiction scheme of four π-π stacked PPP chains.61,62 Consequently, enhancing the absorption peak attributed to electric dipole-moment transition of the stacked phenylene groups, a single absorption peak was observed at 666 nm. The band gap of PPP-NPs, PPP-NDs, and PPP-NPLs is 2.47, 1.61, and 1.60 eV, respectively. Hence, the magnitude of the band gap of PPP-NPs dispersed in methanol (3.07 eV) was reduced by 0.56, 1.46, and 1.47 eV when assembled into PPP-NPs, PPP-NDs, and PPP-NPLs, respectively. Despite, the forced organization of the polymer chains inside the PPP-NPLs lowered the band gap and narrowed the optical signal, the fluorescence efficiency is decreased.

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NPs monolayer assembly and hierarchical assembly of PPP-NPs into microdisks. B) Optical absorption spectrum of a single PPP-NPL of different diameters measured by ultrahigh resolution optical spectroscopic technique, and the corresponding optical images of the PPP-NPLs are shown in (iii-viii). Symmetrical PPP-NPLs showed a single sharp optical spectrum, while single PPP-NP and single microdisk showed two absorption peaks. C) Fluorescence spectrum of individual PPP-NP (black), microdisk (red), and PPP-NPL (blue). The fluorescence spectrum of the PPP-NPs was quenched upon hierarchical assembly or squeezing. Investigating the Conformation of Polymer after LB Assembly Raman spectroscopy measurements were conducted to determine the conformation of the PPP inside the LB assembly. Figure 6 shows the Raman spectrum of PPP-NPs, PPP-MDs, and PPP-NPLs assembly on the surface of a glass substrate. The Raman band at 1607 cm-1 is assigned as C=C vibration of π conjugation along the polymer backbone. This band becomes broader in case of the PPP-NPLs due to the squeezing of the PPP chains inside the pillars. The Raman bands, corresponding to the ring vibration, appeared at 1503 and 1311 cm-1 and are broad in case of PPP-NPs and PPP-NPLs compared to the PPP-NDs. Broadening of the Raman bands is a sign of the stress distribution upon the PPP chains. This explains the significant broadening at the 1311 cm-1 in case of PPP-NPLs, where the PPP chains are highly compressed. The sharp Raman bands of PPP-MDs could be attributed to the less stressing of the PPP chains, especially that on the surface of the microdisks. However, the PPP-NPs inside the monolayer assembly are stressed, while the PPP-NPs on the top of the PPP-MDs have freedom. The Raman band at 1194 cm-1 corresponds to the SO3- group and is very week in case of PPP-NPLs due to the polymer assembly in a way that lowered its surface charge. The Raman band assigned to O-C-O deformation appeared at 450 and 600 cm-1 in case of PPP-NPLs and PPP-NPs, respectively. The large shift and broadening of this band is resulting from stressing the PPP chains.


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Figure 6 Raman spectrum of PPP-NPs (black), a single PPP-MD (red), and PPP-NPLs (blue), measured after 532 nm laser excitations. CONCLUSION PPP-NPs were prepared by mixing the PPP dissolved in water with methanol, which induced the polymer supramolecular assembly into particles. The PPP is well soluble in water but sparingly soluble in methanol. The optical spectra of the PPP, absorption and fluorescence, was red shifted and the peak became narrower after PPP assembly into nanoparticles. The PPPNPs were assembled into highly packed monolayer nano-patterns of rough surfaces using Langmuir-Blodgett technique. When the LB film of the PPP-NPs was compressed, due to their soft nature, they assembled hierarchically into microdisk 2D arrays of band gap of 1.61 eV. The microdisks were squeezed into PPP-NPLs, when the LB film was compressed further. Studying the LB isotherm of the PPP-NPs on the top of water sub-layer confirmed the irreversible 21 ACS Paragon Plus Environment


interaction between the nanoparticles. Due to the sensitivity of the conjugated polymer to the change of chain conformations and packing, the PPP-NPs displayed different optical properties by changing the structure of their assembly. Comparing with the absorption and florescence spectrum of PPP-NPs when dispersed in methanol, the individual PPP-NPs inside the monolayer assembly on the surface of a glass showed a red-shifted, sharp absorption and fluorescence spectrum. The individual, hierarchically assembled microdisks displayed multiple absorption peaks and an asymmetric fluorescence spectrum. This is attributed to the asymmetric energy transfer pathways between the PPP-NPs inside the 3D assembly. The inhomogeneous electronic interactions between the PPP-NPs forming the microdisks does not cause broadening of the optical spectrum, instead, the fluorescence is quenched. Single peak absorption and fluorescence spectrum was observed for PPP-NPL of different dimensions, with a band gap of 1.6 eV. Engineering the assembly to induce a tremendous reduction of the band gap of the polymer, 1.47 eV, will be useful in nano- and micro optoelectronics. The single absorption and fluorescence peaks observed for PPP-NPLs could be attributed to the disappearance of the absorption peak corresponding to the electric dipole moment transition along the polymer backbone and the enhancement of the absorption peak attributed to electric dipole moment transition of the stacked phenylene groups. Additionally, squeezing the PPP-NPs into PPP-NPLs improves the alignment of phenylene groups. PPP polymer has exciting optoelectrical properties and is thermally stable, which has potential optoelectronics applications. The enhanced optical and electrical properties of the PPP nanoparticles can be utilized to make smaller electrical devices compared to traditional PPP thin films. The advantage of having PPP nanoparticle films as opposed to traditional PPP thin films arises from the assembly of the films. By controlling the assembly, the desired properties can be


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directed to create highly localized nano-patterns of differing band gaps, which can potentially be the basis of nanoscale device fabrication. The differences in the band gaps inside of the nanopatterns can permit the creation of junctions, resulting from the energy transfer between neighboring regions. The controlled assembly introduced in this study of the nanoparticles, the microdisks, and nanopillars are instrumental in the development of organic transistors, microphotovoltaic cells, and nanometric light emitting diodes. SUPPORTING INFORMATION Figure S1 is the hydrodynamic radius of PPP-NPs dispersed in methanol. Figure S2 is an AFM image of PPP-NPs thin film prepared on the surface of a glass substrate by spin coating technique. Figure S3 and S4 optical image of PPP-NPs monolayer and PPP microdisk on the surface of glass substrate. Figure S5 is statistical analysis of the diameter and height of PPP-MDs and PPP-NPLs. Figure S6 is optical images of PPP-NPLs on a glass substrate. Figure S7 is a depiction diagram of four π-π stacked PPP chains. This material is available free of charge via the Internet at http://pubs.acs.org/. ACKNOWLEDGMENT This work was supported by the UTSA startup funding. We would like to thank Dr. Gao’s group for conducting the AFM measurements. REFERENCES (1) Bunz, U. H. F. Poly(p-phenyleneethynylene)s by Alkyne Metathesis. Acc. Chem. Res. 2001, 34, 998-1010. (2) Feng, F. D.; He, F.; An, L. L.; Wang, S.; Li, Y. H.; Zhu, D. B. Fluorescent Conjugated Polyelectrolytes for Biomacromolecule Detection. Adv. Mater. 2008, 20, 2959-2964.



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Conjugated Polymer Nanoparticles Having ... - ACS Publications

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