Letter Cite This: Nano Lett. 2018, 18, 1600−1607
pubs.acs.org/NanoLett
Exploring Electronic Structure and Order in Polymers via SingleParticle Microresonator Spectroscopy Erik H. Horak,† Morgan T. Rea,† Kevin D. Heylman,† David Gelbwaser-Klimovsky,‡ Semion K. Saikin,‡ Blaise J. Thompson,† Daniel D. Kohler,† Kassandra A. Knapper,† Wei Wei,§ Feng Pan,† Padma Gopalan,†,§ John C. Wright,† Alán Aspuru-Guzik,‡ and Randall H. Goldsmith*,† †
Department of Chemistry, University of Wisconsin-Madison, Madison, Wisconsin 53706, United States Department of Chemistry and Chemical Biology, Harvard University, Cambridge, Massachusetts 02138, United States § Department of Materials Science and Engineering, University of Wisconsin-Madison, Madison, Wisconsin 53706, United States ‡
S Supporting Information *
ABSTRACT: PEDOT:PSS, a transparent electrically conductive polymer, finds widespread use in electronic devices. While empirical efforts have increased conductivity, a detailed understanding of the coupled electronic and morphological landscapes in PEDOT:PSS has lagged due to substantial structural heterogeneity on multiple length-scales. We use an optical microresonator-based absorption spectrometer to perform single-particle measurements, providing a bottom-up examination of electronic structure and morphology ranging from single PEDOT:PSS polymers to nascent films. Using single-particle spectroscopy with complementary theoretical calculations and ultrafast spectroscopy, we demonstrate that PEDOT:PSS displays bulk-like optical response even in single polymers. We find highly ordered PEDOT assemblies with longrange ordering mediated by the insulating PSS matrix and reveal a preferential surface orientation of PEDOT nanocrystallites absent in bulk films with implications for interfacial electronic communication. Our single-particle perspective provides a unique window into the microscopic structure and electronic properties of PEDOT:PSS. KEYWORDS: PEDOT:PSS, conjugated polymer, single-particle spectroscopy, microresonators length scales, PEDOT oligomers, 6−18-mers,14 π-stack to form small nanocrystallites from 3 to 4 oligomers for pristine films to as many as 13−14 oligomers for more conductive solvent treated films.15 The length of these nanocrystallites are limited by the destabilizing Coulombic repulsions of the charge carrier,9,15 with charge balance achieved by encapsulation with a PSS matrix. These nanocrystallites then arrange into globular conductive particles (often referred to as “gel-like particles”) in a pancake-like shape,16,17 with a phase segregated PEDOT-rich core and a PSS-rich exterior.18−20 These particles themselves are then linked via PSS-rich domains and assembled into a nanofibril geometry akin to a string of pearls, fully consistent with the one-dimensional variable range hopping conductivity found in this material.6,21 Gel-like particle15 and nanofibril21 structural motifs have also been shown to exist in solution. Upon deposition, these nanofibrils then interweave to form thin films with a PSS capping layer at the surface.19,22 Commensurately, films display highly anisotropic characteristics including (1) absorption, suggesting PEDOT oligomers lie parallel to the substrate;23 (2) X-ray scattering, suggesting π-
onjugated polymers (CPs) find extensive use in modern electronic and optoelectronic materials due to facile processability and widely tunable properties.1 Highly doped CPs are ubiquitous because of their high electrical conductivity, transparency, flexibility, and suitability for large-scale manufacturecrucial characteristics for the transparent anodes required in photovoltaic and light emitting devices, regardless of whether the active materials are small molecules, polymers,2 quantum dots,3 or perovskites.4 Poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS),5 Figure 1a, exhibits high conductivity up to6 4380 S cm−1 while remaining flexible,7 competitive with expensive, resource-limited, and inflexible indium tin oxide (ITO).8 PEDOT:PSS also finds use as a thermoelectric material,9 catalyst,10 battery stabilizer,11 ion transporter,12 and neural interface.13 In this two-component material, PEDOT is a low molecular-weight, p-doped CP responsible for high charge mobility, while PSS is a high molecular-weight, insulating polyelectrolyte responsible for maintaining charge neutrality and water solubility, Figure 1. The complex microstructure of PEDOT:PSS hinders understanding of its electronic properties, limiting improvements to conductivity and generation of new functionality. The complex microsctructure of PEDOT:PSS is highly heterogeneous on multiple length-scales. On the smallest
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© 2018 American Chemical Society
Received: October 2, 2017 Revised: January 1, 2018 Published: January 29, 2018 1600
DOI: 10.1021/acs.nanolett.7b04211 Nano Lett. 2018, 18, 1600−1607
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Figure 1. Toroidal optical microresonator-based absorption spectrometer to probe PEDOT:PSS. (a) Molecular structure of PEDOT:PSS with a bipolaron charge carrier and balancing charges. (b) Experimental apparatus of the optical microresonator-based absorption spectrometer (see Methods for details). (c) Representation of the components of PEDOT:PSS with a PSS chain (black) decorated by PEDOT oligomers (blue). This picture does not include intra- or interchain folding. (d) A photothermal map of a single PEDOT:PSS particle pumped at 1315 nm. Scale bar, 2 μm.
stacking both parallel and perpendicular to the substrate;24 and (3) conductivity, suggesting different charge transfer mechanisms parallel and perpendicular to the substrate.16 The base conductive unit to be examined here is the nanofibrils which exist in the aqueous dispersion and remain relatively unperturbed as they interweave to form the conductive film, where they provide the origins of observed one-dimensional conductivity. Highly doped PEDOT:PSS exhibits broad absorption in the infrared9,25 due to midgap states created during doping from charge-induced lattice relaxations. These electronic perturbations arise from injected holes producing a quinoidal distortion spread over 4−5 monomers of the CP aromatic backbone,26 collectively called a polaron. It is energetically favorable for two polarons to combine into a spin-silent bipolaron,9,27 emptying the midgap states28 and producing a single low-energy optical transition. These bipolarons can be thought of as permanent dopants (as opposed to photogenerated polarons), which undergo electronic transitions that have strong π-character. The broad, low-energy optical spectrum can imply a vanishing bandgap9,22 from band formation of the midgap states or can stem from morphological, and thus spectral, heterogeneity. Distinguishing between these limits may have implications for the mechanism of conductivity. The immense heterogeneity present in these films hampers elucidation of the internal structure and interplay of this structure with electronic properties. Single-molecule and singleparticle measurements thrive in these conditions and can directly unveil distributions of physical and electronic structures. While single-molecule fluorescence techniques have been applied to undoped semiconducting CPs,29−31 the presence of charge carriers efficiently quenches excitons,31−33 eliminating any emissive readout of doped CPs and making conductive CPs impenetrable to traditional single-molecule approaches. Absorption spectroscopy circumvents this limitation and has recently been demonstrated in undoped single CPs by Orrit and co-workers.32 Here, we employ a newly developed single-particle microresonator-based absorption
spectrometer34 to examine a doped conductive CP, PEDOT:PSS, and provide direct spectroscopic inspection of charge carrier properties. We reveal, for the first time, the electronic signatures of bulk behavior even in individual PEDOT:PSS polymers as well as new details of long-range ordering and surface orientation preference. Single-Particle Spectroscopy. To perform electronic absorption spectroscopy on individual polymers and particles of PEDOT:PSS, we use a toroidal optical microresonator as a near-field temperature sensor for photothermal imaging and spectroscopy.34−36 The Whispering-Gallery Mode (WGM) resonances of the microresonator circulate light many times around the resonator perimeter by total internal reflection, resulting in ultrahigh-quality (Q) factor resonances. These ultrahigh-Q factors offer convenient and highly sensitive readout of the local effective refractive index. Thus, changes to the effective refractive index result in WGM resonance wavelength shifts. We leverage this exquisite sensitivity to measure small temperature changes altering the refractive index via the thermo-optic coefficient. Specifically, our experiment uses two continuous-wave beams: a narrow-band wavelengthtunable laser (probe beam) tracks the resonance wavelength through evanescent coupling via a tapered optical fiber,37 while a second more broadly tunable free-space laser (pump beam) is tightly focused on the resonator surface. The pump beam excites chromophores on the resonator surface, which subsequently dissipate absorbed energy as heat, shifting the WGM resonance wavelength as detected by our probe beam, Figure 1b, enabling highly sensitive photothermal spectroscopy and microscopy.34 Control of the position, wavelength, and polarization of the free-space pump beam equips the absorption spectrometer with three avenues of sample interrogation, (1) imaging individual absorbing particles on the microresonator, (2) performing absorption spectroscopy on each individual particle, and (3) investigating the polarization-dependence of each particle’s absorption spectrum. Together, this information yields, on a per particle basis, the particle’s location, orientation, apparent 1601
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Figure 2. Origins of the electronic absorption spectra from homogeneous and inhomogeneous sources. (a) Distribution of the absorption crosssection (σabs) of observed PEDOT:PSS particles, each value is the average of the entire spectral window. (b) Absorption spectra of particles at specific σabs intervals (sizes). Each spectra is normalized to the mean of the curve and smoothed with a 2 nm boxcar. Each interval is offset by increasing integer values. (c) A representative set of simulated absorption spectra from electronic structure calculations with increasing amounts of homogeneous broadening. The lowest linewidth is included for illustration of the approach. Each spectra is normalized to the mean of the curve. Each set of spectra was offset for clarity. A vibrational progression was added to each calculated transition energy and each was broadened with a Lorentzian line shape (see main text and Supporting Information). (d) Three-pulse photon echo response (3PE) of PEDOT:PSS for conjugate phase matching conditions, 3PE (green) and 3PE* (orange). Measured (solid), one standard deviation confidence interval (shaded) and numerical model (dashed) are shown for each phase matching condition. The magnitude of the peak shift (Δτ), extracted from the numerical model, is shown using vertical dotted lines.
Supporting Information). Thus, we can probe individual PEDOT:PSS polymers all the way to particles of many thousands of polymers. Origins of Electronic Spectra. Bulk optical absorption of PEDOT:PSS is featureless and rises in the near IR and continues into the deep IR due to polaron and bipolaron transitions.9,25 In contrast, isolated cationic and dicationic CP oligomers show discrete spectral peaks.28,40 Surprisingly, the absorption spectra of each individual PEDOT:PSS particle is flat and independent of particle size (Figure 2b), resembling the bulk film absorption even down to a single PEDOT:PSS polymer. This behavior can be understood from two limiting cases, (1) the spectral heterogeneity from multiple bipolarons within each particle is sufficient to obscure any spectral peaks or (2) the features themselves are sufficiently homogeneously broadened that even a small degree of heterogeneity leads to the shapeless spectrum observed. The relative contribution of inhomogeneous and homogeneous broadening was investigated with electronic structure calculations and ultrafast spectroscopy. Electronic structure calculations were performed using a displacement disorder model that we previously demonstrated reproduces the experimental absorption spectrum of doped PEDOT.41 In the model, PEDOT oligomers bearing bipolarons π-stack to form 1D nanocrystallites and interact via dipolar interactions, modifying the absorption spectra.42 For large stacks, a particular realization of the disorder does not significantly affect the absorption spectra as a substantial amount of stacking arrangements have been sampled. For short stacks, the calculated spectra are very sensitive to the stacking
size, structural ordering, and electronic spectrum, painting a picture of the interplay between structural and electronic properties with unprecedented detail. The photothermal image of a PEDOT:PSS particle pumped at 1315 nm yields a near-diffraction limited point spread function (Figure 1d) with a fwhm of ∼1.3 μm. Atomic force microscopy (AFM) on particles deposited on the toroid reveal nanofibrils, confirming a consistent nanoscale morphology between the single particles and the bulk film (see Supporting Information). The observed photothermal signals were converted to absorption cross sections (σabs) with a finiteelement simulation described previously.34 The smallest observed particle has σabs of 8 × 10−15 cm2 (Figure 2a), marginally larger than the σabs of a single dye molecule.38 Values of σabs are spread over several orders of magnitude with a median value of 4.8 × 10−13 cm2. The number of bipolarons in each particle can be estimated from σabs and the per-bipolaron absorption constant (4.2 × 10 −16 cm 2 ) in a related polythiophene,39 while the number of oligomers is estimated by assuming a single bipolaron per oligomer (see Supporting Information). The range of σabs in Figure 2a corresponds to collections of approximately 20 to 420,000 oligomers. Estimation of the total σabs of a single PSS strand (Figure 1c) decorated with many PEDOT oligomers (hereafter referred to as a single PEDOT:PSS polymer) yields a value of 6.7 × 10−14 cm2 (see Supporting Information). This suggests that the lowest observed σ abs values corresponds to individual PEDOT:PSS polymers, which themselves are expected to have a range of sizes making definitive assignment difficult (see 1602
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Figure 3. Assessment of ordering in PEDOT:PSS. (a) Distribution of the depth of modulation (M) of observed PEDOT:PSS particles. (b) Polarization dependence of individual particles with increasing values of M. (c) Experimental density plot of the M-decay with particle size with experimentally observed values in red circles, the increasing density of observed values depicted by the blue-yellow contour plot, and the logistic function fit of the experimental data in the black line. (d) Rotational model schematic where a dipole undergoes an angular random walk, starting in blue and ending in red, controlled by random rotations by Rx, Ry, and Rz with the standard deviation of angular deviation (Δ), where smaller values of Δ create more ordered systems. (e) Rotational model density plot of the M-decay with particle size for a Δ of 2.18°. (f) Cartoon of the PSS mediated alignment of PEDOT nanocrystallites in cofacial (i) and side-by-side (ii) orientations.
broadened systems. Three-pulse photon echo (3PE) analysis can distinguish between homogeneous and inhomogeneous broadening.47 We collect the transient grating population relaxation trace and 3PE traces (Figure 2d) and find that net dephasing and population relaxation are both fast, comparable to our pulse width. Through numerical modeling (see Supporting Information), we extract a population relaxation time of 80 fs, a homogeneous dephasing time of 73 meV), and an inhomogeneous broadening factor of >43 meV. This analysis is fully consistent with the discrete widely separated energy levels of the electronic structure calculations (large inhomogeneous broadening) and the broad featureless single-polymer absorption measurements (large homogeneous broadening) The length-scale where a material transitions from molecular to bulk properties is a central theme in materials science. The optical spectra of isolated charged PEDOT oligomers28,40 is qualitatively different from the thin-film spectra. However, our observations show that at the single-polymer level this transition has already occurred with bulk-like spectra consistently observed. Further, our analysis shows that the featureless nature of the spectra derive from a combination of
arrangement, which may have substantial H- or J-aggregate character.41 Different arrangements of 100 oligomers, similar to one PEDOT:PSS polymer, generate discrete optical transitions randomly located across our spectral range (Figure 2c, bottom). The widely separated transitions suggest variability in measured optical spectra, qualitatively different than the experimental results. However, the visibility of these peaks is determined by the homogeneous linewidth of the transitions, as broader linewidths will effectively smooth out the spectrum (Figure 2c). Fourier analysis suggests a homogeneous linewidth ≥70 meV for PEDOT:PSS bipolarons (see Supporting Information). The value of this lower limit is similar to room temperature linewidths observed in single-molecule excitation spectra,43,44 where dephasing induced by environmental fluctuations are likely responsible for the majority of the linewidth, though spectral diffusion may also play a role.45 To independently assess the homogeneous linewidth, we performed ultrafast four wave mixing (4WM) spectroscopy on a drop-cast PEDOT:PSS thin film. Under Redfield Theory, the homogeneous linewidth of any transition is determined by pure dephasing and population relaxation,46 although ensemble dephasing can become relevant for very inhomogeneously 1603
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Figure 4. Surface-angle dependence of PEDOT:PSS. (a) Schematic of the relative surface normal and optical axis along the nonplanar surface of the resonator. (b) Experimentally observed polarization orientation angle (θmax) of particles on the resonator. The length of the vector is proportional to the depth of modulation (M) and the vector was chosen to point outward for clarity. (c) Relative surface angle of the collective transition dipoles of the observed particles found by comparing θmax and the radial coordinate of the resonator, along with the contributions from the three limiting orientations in (I), (II), and (III) where the parallel surface orientation, in green, originates from (I) and/or (II) and the perpendicular surface orientation, in blue, originates from (III).
depicted in Figure 3b. Collected M-values suggest widespread variation in observed ordering within each particle. Barbara and co-workers used distributions of M-values to decipher the internal structure of single semiconducting CPs with matching simulations.31 While the polydispersity of PEDOT:PSS precludes this approach, it affords an opportunity to explore how order evolves with particle size (as estimated from σabs) from the smallest polymer length-scales, a critical regime for understanding how thin-film properties derive from the properties of individual polymers. Decreasing M-values are observed for larger values of σabs (Figure 3c, red circles), demonstrating reduced oligomer alignment as particle size increases, an intuitive result as it is more difficult to align a larger number of oligomers. The shape and slope of the decay of M in Figure 3c, however, provides quantitative information about the material’s propensity for alignment. To begin, this density distribution was fit to a logistic function to phenomenologically assess the degree of ordering, eq 2,
homogeneous and inhomogeneous broadening, behavior that is consistent with the fast dephasing and large static disorder observed in the 3PE experiment on thin films. Though the thin film and single-particle experiments show very similar dephasing times, it is not obvious that this similarity should be expected. Additional mechanisms of dephasing could stem from a variety of ultrafast mechanisms present in films, including intra- or interchain energy transfer processes,48 and atomic fluctuations associated with excitations delocalized over multiple chains.49 However, because of the residual inhomogeneous broadening present in even a single PEDOT:PSS polymer, we cannot yet confirm the presence of these or other charger-transfer processes at the level of single polymers.50,51 Long-range Ordering. Electronic properties of PEDOT:PSS are intertwined with structural order, which can be interrogated via polarization dependence of the particle’s absorption, as the transition dipole of the bipolaron absorption lies parallel to the PEDOT oligomer backbone (see Supporting Information). If individual PEDOT oligomers on the PSS strand are well-aligned, the absorption will vanish when the pump beam’s polarization is perpendicular to the oligomers’ transition dipole projected in the 2D plane orthogonal to the propagation of the pump beam. If the sample is not ordered, only reduced absorption will be seen. This polarization dependence is characterized by σ(θ ) = σmax(1 − M sin 2(θ − θmax ))
M(σabs, β) =
1 1 + 10−(σabs− β)
(2)
with the lone parameter, β, the σabs at 50% falloff of oligomer alignment. Experimentally, β was determined to be 7.4 × 10−13 cm2 or ∼4000 oligomers. Comparatively, X-ray scattering suggests a nanocrystallite size of 13−14 stacked oligomers in the most crystalline postprocessed PEDOT:PSS films,15 a considerably shorter length-scale than the long-range ordering found here. The identified long-range order suggests orientations of domains are correlated, increasing electronic coupling by maximizing orbital overlap between domains, aiding charge transfer. To estimate the requisite rotational order for such a long falloff, a simple model was built to simulate M-decays, where oligomers were added with a Gaussian probability for angular
(1)
where M is the depth of modulation, σmax is the maximum σabs, and θmax is the orientation angle. M is bounded between 0 and 1 with M = 1 describing complete coalignment and high ordering within the particle and M = 0 describing uncorrelated dipole orientations and a disordered particle with various curves 1604
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Nano Letters deviation from the previous oligomer of width Δ using three orthogonal rotation matrices, Rx,y,z, Figure 3d. Larger Δ-values result in more expansive angular random walks and smaller Mvalues. Sampling this model with the empirically derived σabs distribution (see Supporting Information) recreates the experimental M-decay density distribution and β value with Δ = 2.2° (Figure 3e), suggesting an extremely strong preference to align between adjacent oligomers. This value represents an average degree of alignment, with higher alignment expected within nanocrystallites and lower alignment between nanocrystallites. The above dichotomy, small nanocrystallites implied by Xray scattering and long-range order implied by the long 50% falloff, suggests that while insulating PSS layers exist between highly ordered PEDOT nanocrystallites, decreasing communication, they also mediate long-range ordering. In this picture, a diversity of PSS shell thicknesses would preclude such longrange correlations from being seen via scattering, but rotational order preservation would be seen via the modulation depth falloff in our measurement. Evidence of some order in the PSS matrix has been observed in X-ray scattering6 and absorption52 measurements. Therefore, even as these stiff PSS layers impede charge transport, their Coulombic interactions with the PEDOT nanocrystallites result in inter-nanocrystallite alignment, though we cannot distinguish between preservation of ordering along cofacial (Figure 3f, (i)) or side-by-side stacks (Figure 3f, (ii)). Thus, even as PSS layers directly exhibit a detrimental effect on conductivity, they indirectly provide a beneficial effect on conductivity by maintaining long-distance oligomer alignment. Many current efforts to enhance PEDOT:PSS conductivity focus on eliminating such PSS layers.6,8 While these efforts have successfully yielded increases of conductivity, our results suggest diminishing returns if too much PSS is removed and thus, a limit to this particular approach since the PSS also assists with alignment. Surface Ordering. The electronic structure and morphology of PEDOT PSS are both complicated by their highly anisotropic character, requiring careful examination of orientational ordering in bulk films and near interfaces. In typical single-particle and molecule optical techniques employing flat substrates, the optical axis is orthogonal to the surface plane, making it difficult to probe out of plane optical properties and providing a highly restricted view of any anisotropic character. Near-field techniques53 and defocused imaging54 can assess out-of-plane orientation of single particles but have only been demonstrated on emissive objects, while ellipsometry is inherently a bulk technique. On our toroidal microresonator spectrometer, however, particles residing on the curved toroid surface experience a surface normal that is not parallel to the optical axis, Figure 4a. Consequently, a preferred out of plane surface orientation will translate into a correlation between the orientation of the PEDOT:PSS oligomers and the radial coordinate around the resonator (Figure 4b and Supporting Information). Thus, the nonplanar shape of the toroidal resonator can be leveraged to determine the preferential surface orientation of PEDOT:PSS. Remarkably, a conspicuous bimodal distribution is observable (Figure 4c) with significant populations both perpendicular and parallel to the surface. It is instructive to consider the origins of these populations from three limiting nanocrystallite orientations (I), (II), and (III) (Figure 4c). The parallel orientation includes a combination of (I) and (II) while the perpendicular orientation is attributed to (III). While (III) is clearly visible in our
measurement, it is notably absent in the absorption profiles of bulk films from ellipsometry measurements,23 where absorption occurs predominantly in the surface plane from (I) and (II) (see Supporting Information). Our new observation implies that at the oxide surface, the first layers of PEDOT:PSS orient differently than the subsequent layers that add upon existing layers to form the bulk of the film. The conclusion stems from the different orientations observed in our single-particle experiment as compared to other experiments, where the bulk phase is the dominant contributor to ensemble-averaged measurements.23,24 This observation is an important detail for controlling morphology and potentially enhancing conductivity, as it suggests that substrate surface treatments for oxides may have little effect on bulk film ordering but significant effects on contact resistance originating from the interface of PEDOT:PSS with other layers. Interfacial molecular orientations can have a significant influence on charge-transfer characteristics.55 More study will be required to confirm if these orientations are also present at other surfaces. Further, the observation that each particle maintains a preferential direction suggests that the gel-like particles from solution are inherently asymmetric, which may impact bulk film orientation and morphology. In summary, we investigated optical and structural properties of PEDOT:PSS using a new microresonator-based absorption spectrometer. Surprisingly, direct examination of absorption spectra in PEDOT:PSS particles from single polymers to collections of thousands showed that even the smallest particles display spectra resembling the bulk film. With complementary calculations and ultrafast spectroscopy, we showed how a combination of homogeneous and inhomogeneous broadening contributes to the single-particle spectrum. Polarization spectroscopy further revealed long-range ordering of PEDOT oligomers likely promoted by the rigid PSS matrix. Leveraging the nonplanar geometry of the toroidal microresonator identified a unique interfacial surface ordering not exhibited in the bulk. Thus, absorption spectroscopy of individual conductive polymers and polymer nanoparticles offers a new and detailed description of materials properties. Methods. Sample Preparation. Toroidal optical microresonators were fabricated with diameters of approximately 50 μm as described previously.34 These resonators were cleaned by a standard RCA clean consisting of 100 °C piranha for 15 min (1 H2O2:60 H2SO4), 75 °C SC-1 for 15 min (1 H2O2:1 NH4OH:5 H2O), 75 °C SC-2 for 15 min (1 H2O2:1 HCl:5 H2O), 60 s room temperature dilute HF dip (50 H2O:1 HF); oxygen plasma cleaning; and/or UV-ozone cleaning. This procedure produced resonators with Q-factors ranging between 106−107. The details of our Pound−Drever−Hall locking/ double-modulation scheme to provide highly sensitive photothermal measurements have been previously described.34 Pound−Drever−Hall locking error slopes were 2.8−28 mV/ fm. All resonators were photothermally imaged before PEDOT:PSS deposition to confirm that detected particles were not fabrication impurities or other contamination. PEDOT:PSS (Orgacon Dry, Sigma-Aldrich) was redispersed in milli-Q water at a concentration of 1 mg/mL. These solutions were diluted with milli-Q water to ∼2−20 ng/mL and allowed to fully solvate over ∼48 h. These solutions were then spin-coated onto the toroidal microresonators at 4000 rpm. Absorption Spectroscopy and Data Processing. The pump laser (Thorlabs TLK-L1300R) was tuned from 1280 to 1350 1605
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window. The choice of the functional affects the absolute value of the electronic excitations energy by about 100 meV. The near-field (Förster) interaction between molecular excitations in a molecular dimer, computed with the transition densities, has been fitted using the extended dipole formula.41 Then, this approximation was used for all molecules in a stack beyond the nearest neighbors, while the interactions between the nearest neighbors were computed explicitly using the transition densities. We verified that small variations in the lengths of the oligomers in the range of 8 and 12 units does not modify the qualitative structure of the spectra.
nm, limited by the tunability of the laser, while both the photothermal resonance shift and pump laser power was measured simultaneously, as described previously.34 Converting the raw resonance shift per pump power to an absorption crosssection was performed with a finite-element thermal simulation in COMSOL Multiphysics as described previously.34 The acquired absorption and polarization spectra of each particle was background subtracted by moving the pump beam to a nearby vacant portion of the toroid, ∼ 5 μm away and repeating the scan. The absorption cross-section of each particle was recorded as the average cross-section over 1280−1350 nm as these particles yielded a flat spectral response. A 2 nm boxcar smoothing was applied to all absorption spectra. Pump laser polarization was controlled with a broad-band half-wave plate (Thorlabs AHWP10M-1600) just prior to the objective to ensure proper polarization control. The background subtracted polarization traces were fit to eq 1 with both M and θmax extracted. Ultrafast Measurements. PEDOT:PSS was dropcast onto a glass microscope slide at 1 mg/mL at a tilt to ensure homogeneous film formation. The sample was heated at 100 °C for ∼15 min to evaporate water. An ultrafast oscillator (Spectra-Physics Tsunami) was used to prepare ∼35 fs seed pulses. These were amplified (SpectraPhysics Spitfire Pro XP, 1 kHz), split, and converted into 1300 nm 40 fs pulses using two separate optical parametric amplifiers (Light Conversion TOPAS-C): “OPA1” and “OPA2”. Pulses from OPA2 were split again, for a total of three excitation pulses: ω1, ω2, and ω2′. These were passed through motorized (Newport MFA-CC) retroreflectors to control their relative arrival time (“delay”) at the sample: τ21 = τ2 − τ1 and τ22′ = τ2 − τ2′. The three excitation pulses were focused into the sample in a 1° right-angle isosceles triangle, as in the BOXCARS configuration.56 Each excitation beam was 67 nJ focused into a 375 μm symmetric Gaussian mode for an intensity of 61 μJ/ cm2. A new beam, emitted coherently from the sample, was isolated with apertures and passed into a monochromator (HORIBA Jobin Yvon MicroHR, 140 mm focal length) with a visible grating (500 nm blaze, 300 grooves per mm). The monochromator was set to pass all colors (0 nm, 250 μm slits) to keep the measurement impulsive. Signal was detected using an InSb photodiode (Teledyne Judson J10D-M204-R01M-3CSP28). Four wave mixing was isolated from excitation scatter using dual chopping and digital signal processing. See Supporting Information for further experimental details. Two dimensional τ21, τ22′ scans were taken for two phase matching configurations: (1) kout = k1 − k2 + k2′ (3PE) and (2) kout = k1 + k2 − k2′ (3PE*). The rephasing and nonrephasing pathways exchange their time dependence between these two configurations. Comparing both pathways, rephasing-induced peak shifts can be extracted as in 3PE.47 All data was modeled using numerical integration of the Liouville-von Neumann equation. See Supporting Information for details of the model. Electronic Structure Calculations. Electronic excitation spectra and corresponding transition densities of isolated double-charged oligomers composed of ten EDOT units were computed using time-dependent density functional theory as implemented in Turbomole 6.0.57 We used the triple-ζ valencepolarization basis sets def2-TZVP,58 and the hybrid functional B3LYP.59 Only the lowest bright electronic excitations in the oligomers were used for the spectra of oligomer stacks. This approximation is justified because the next strong excitation in the oligomer is separated from the lowest one by at least a 1 eV
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.nanolett.7b04211. (1) Single-particle spectroscopy, including (i) AFM of single PEDOT:PSS particles, (ii) relevance of deposition conditions, (iii) variability of data sets, (iv) radial deposition distribution, (v) smallest object, (vi) single PEDOT:PSS polymer assignment, (vii) homogeneous linewidth determination and limits of the finite spectral window, and (viii) contribution of spectral diffusion and charge motion. (2) Modeling crystallinity, including (i) model explanation and validation and (ii) effect of preorganization. (3) Surface orientation, including (i) relating surface orientation to toroidal radial coordinate and (ii) comparing different observed surface orderings. (4) Ultrafast measurements, including (i) thin film spectra, (ii) experimental geometry and excitation pulses properties, (iii) raw data, (iv) assignment of zero delay, (v) numerical model. (5) Electronic structure calculations (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Semion K. Saikin: 0000-0003-1924-3961 Padma Gopalan: 0000-0002-1955-640X John C. Wright: 0000-0002-6926-1837 Alán Aspuru-Guzik: 0000-0002-8277-4434 Randall H. Goldsmith: 0000-0001-9083-8592 Author Contributions
E.H.H., M.T.R., K.D.H., and F.P. acquired single-particle data with help from R.H.G.; D.G.K. and S.K.S. performed calculations with help from A.A.G.; B.J.T. and D.D.K. performed ultrafast measurements with help from J.C.W.; K.A.K. fabricated the resonators; W.W. performed atomic force measurements with help from P.G.; and E.H.H. and R.H.G. wrote the manuscript with help from all coauthors. Funding
We acknowledge support from the National Science Foundation under award DMR-1610345 (R.H.G.) and CMMI-1462771 (P.G.), the National Science Foundation through the University of Wisconsin Materials Research Science and Engineering Center (MRSEC) under award DMR-1121288 (P.G.) for use of facilities, the Center for Excitonics, an Energy Frontier Research Center funded by the U.S. Department of Energy under Award DE-SC0001088 1606
DOI: 10.1021/acs.nanolett.7b04211 Nano Lett. 2018, 18, 1600−1607
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Nano Letters
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(A.A.-G.), and the U.S. Department of Energy under Grant DEFG02−09ER46664 (J.C.W.). Notes
The authors declare no competing financial interest.
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DOI: 10.1021/acs.nanolett.7b04211 Nano Lett. 2018, 18, 1600−1607
