Research Article Cite This: ACS Appl. Mater. Interfaces 2019, 11, 24468−24477
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Electrospun Fibers Containing Emissive Hybrid Perovskite Quantum Dots Prashant Kumar, N. Ganesh, and K. S. Narayan* Chemistry and Physics of Materials Unit, Jawaharlal Nehru Centre for Advanced Scientific Research, Bangalore - 560064, India
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S Supporting Information *
ABSTRACT: We demonstrate a single-step fabrication process of highly stable and luminescent polymer fibers embedded with quantum dots (QDs) of the organic−inorganic hybrid perovskite (OIP) (CH3NH3PbBr3) using the electrospinning process. The fiber (∼2 μm diameter) primarily consists of poly(methyl methacrylate) dispersed with clusters of OIP quantum dots. The OIP clusters are radially distributed, normal to the fiber axis. The photoluminescence quantum yield (PLQY) is high (∼80%) and comparable to that of conventional QDs. The emission maxima are tunable by varying the concentration of OIP precursor in the electrospinning solution. Submicron emission maps show an isotropic and continuous emission along the fiber, suggesting uniform distribution of QD clusters. Temperature-dependent PL response indicates features which are a function of the particle size. For small QDs, the PLQY(T) maxima are close to the ambient temperature, whereas the PLQY(T) maxima shift sizably to T < 50 K for larger QDs. Significant waveguiding of QDs emission and amplified spontaneous emission, a prerequisite for lasing, were observed in the fiber confined OIP system at room temperature. KEYWORDS: organic−inorganic hybrid perovskite, quantum dots, electrospinning, photoluminescence, quantum confinement, waveguiding, amplified spontaneous emission (ASE) moisture and oxygen.21,22 Polymer matrixes like poly(methyl methacrylate) (PMMA) and polystyrene (PS) have been used for encapsulating the OIP core,23−25 as they provide a viable orthogonal solvent processing. Additionally, by utilizing a suitable shell of appropriate dimensions and refractive index, various features like improvements in emission out-coupling, optical confinements using total internal reflection, and waveguiding of OIP emission, can be achieved. Such geometries can find application in downconverters for display and optical repeaters.26,27 Conventionally, the encapsulated OIP QDs are synthesized using a capping agent, which is a long chain alkyl group (e.g., octylamine and oleic acid),28 which attaches to the surface of QDs and provides partial protection against surface reaction. Incorporating OIP QDs into an enclosed shell, similar to the conventional core−shell semiconductor QD, requires complex chemistry with orthogonal solvents for depositing a shell layer over the active core.29 Alternative routes like coprocessing with organic polymers provide faster fabrication. A similar effect has been realized via a 1-D fiber, which is like a shell with embedded QD clusters using a facile route of electrospinning.30 The shell is formed using a clear dielectric polymer
I. INTRODUCTION In recent years, the organic−inorganic hybrid perovskites (OIPs) have been at the center of attention for their remarkable optical and electronic properties.1,2 Significant advances have been made in the development of highefficiency photovoltaics,3 light emitting diodes,4 transistors,5 lasers,6 and detectors.7 Their ability to be solution processed from low-cost precursors has led to the use of a variety of fabrication methods, giving rise to diverse morphological features ranging from single crystals to polycrystalline films and nanostructures.8,9 In general, large-scale improvements in luminescence (bright PL and EL) and stability are observed with the decrease in average grain size.4,10−14 The PL emission is highest for quantum dots (QDs) where PLQYs ∼ 80−95% are easily observed.15,16 The capped OIP QDs require an elaborate synthesis procedure, and the surface capping which remains porous results in modification of the perovskite framework and quenching of luminescence over large time scales.17,18 A desired system where the OIP core can be completely encapsulated within a nonreactive, high refractive index shell can possibly shield the OIP core more effectively. The core−shell geometry has been explored extensively for the inorganic QDs system for selectively modifying the reaction kinetics of QDs.19,20 A similar architecture where the shell is used as an encapsulation for the OIP core will have useful barrier features, which will minimize the reaction with ambient © 2019 American Chemical Society
Received: May 14, 2019 Accepted: June 17, 2019 Published: June 17, 2019 24468
DOI: 10.1021/acsami.9b08409 ACS Appl. Mater. Interfaces 2019, 11, 24468−24477
Research Article
ACS Applied Materials & Interfaces into a fiber with the clusters of OIP QDs homogeneously distributed along its length.31,32 High molecular weight (∼100 kDa) and low refractive index polymers like PMMA and PS can be pulled into thin fibers of diameters ranging from 0.1 to 10 μm and of several millimeters in length, using electrospinning.33−35 Aligned and random polymer fibers are of interest for 3-D microcavity for lasers,35,36 aligned channels for field effect transistors,37,38 high response linear photodetector arrays,39−41 flexible supercapacitors,42 and many more applications. Unique 3-D geometries obtained in electrospun fibers have shown to host whispering gallery modes and allow for high quality-factor and dynamic tuning range of the propagation wavelength.43,44 In this study, we report the fabrication process to obtain polymer fibers encapsulating OIP nanoparticles formed in situ and the rich properties exhibited by this system. The PMMA solution was electrospun along with the methylammonium bromide (CH3NH3Br) and lead bromide (PbBr2) salt; the OIP QDs or nanocrystals (NCs) were formed in situ by the phase separation of OIP precursors from the PMMA matrix.45,46 Here, NC is used as a more general term for crystalline grain size ranging from 2 to 100 nm, which can be larger than the range for QDs. The fibers show high fluorescence quantum yield (∼80%) along with a continuously tunable emission band. Improvements in shielding is also reflected in the enhanced stability of the OIP NCs. Very little decrease in PLQY is observed when stored in ambient conditions (average humidity ∼ 65%) for an extended duration (∼18 months).
majority of the solvent was evaporated during the pulling of the fiber, as a consequence of an increase in surface area/ volume ratio. In addition, the fibers were annealed at elevated temperature (70 °C) for few hours. No time-dependent change in morphology was observed in the fibers, indicating almost complete removal of solvents. B. Microscopic Characterization. The dominant parameter deciding the fiber diameter appeared to be the OIP precursor’s concentration, rather than the PMMA concentration in electrospun solution. The fibers of 0.5−2 μm in width and length up to few tens of centimeters were regularly obtained for various ratios of PMMA/OIP solution. Electrospun fibers of PMMA/OIP show a light orange/yellow appearance under white light and very high luminescence in green (Figure S1c,d) when excited at 365 nm. High PL (at λ ∼ 530 nm) of fibers is comparable to that of OIP QDs; the PLQY increases with an increase in the wt % of OIP precursor. High PL intensity can be correlated with the NCs being formed within the fibers. The surface morphology and distribution of NCs within the composite fiber were investigated using noncontact atomic force microscopy (AFM). The phase image provides subsurface features by looking into the phase shift, where large positive phase shifts indicate an increase in stiffness and can be related to NCs being buried below the polymer surface. The optical and fluorescence image of a randomly picked fiber bundle is shown in Figure 1a,b, respectively. A uniform fluorescence (at λexc ∼ 480 nm) was observed for all the fibers, suggesting a homogeneous distribution of NCs within the fiber. Fiber diameter, derived from the AFM cross-sectional profile, shows a distribution from 0.5 to 2 μm (Figure 1c). The surface morphology shows a uniform profile with low surface roughness (Figure 1d). Ellipsoidal regions of a large phase shift seen in the phase image are arranged periodically along the length of the fiber, with the major axis perpendicular to the fiber axis (Figure 1e). An image highlighting these high phase shift regions is shown in Figure S3. The ellipsoidal grains (∼100 nm), visible in the phase image, were found to be the clusters of NCs when probed using transmission electron microscopy (TEM) (Figure 1f). To facilitate the TEM studies on NCs, the fiber’s PMMA matrix was dissolved in toluene, leaving NCs suspended in the solution. The NCs retain the ellipsoidal structure (which closely resembles a rice grain) in the suspension (Figure S4). We speculate that the PMMA matrix provides a confinement for the perovskite QDs in the cluster to maintain their shape. However, upon dissolving the PMMA matrix, we still continue to see the suspended clusters which suggests that the clusters are stable on their own and do not require a polymer scaffold to support and maintain their state. AFM observation shows a grain size distribution, where the length lies in the range of 50−100 nm and the width in the range of 20−50 nm. These structures cannot be interpreted as 2-D sheets as their thickness is much larger than that of sheets (3−5 nm).53 In contrast to the AFM surface features, the TEM image shows a largely shallow cluster of NCs for various grains. The regions with larger material density will appear darker due to the increase scattering of electrons. With this notion, we observe that the TEM images of clusters have a large area fraction which has lighter contrast, suggesting lower material density, which has been anticipated as hollow regions; these regions are surrounded by a continuous network of NCs. The TEM image also suggests that the individual nanocrystals are much smaller in size (5−10 nm). The electron diffraction
II. RESULTS AND DISCUSSION A. Electrospinning of PMMA/OIP Fibers. The fibers were electrospun from a single solution of PMMA/OIP precursor dissolved in dimethylformamide (DMF) (details in the Supporting Information). During electrospinning, the solution at the tip of the needle is drawn into a cone-like structure (Taylor cone), under the high field (∼1.5 × 106 V/ m).47 The surface of the cone assumes an equipotential surface, from which jets of charged particles originate. The pulling of the fiber (stretching of the solution) results from the repulsion of the like charges accumulating at the tip of the cone. By increasing the conductance (increasing ion density) of the electrospun solution, the stretching strength increases, which results in a decrease of fiber diameter,48−50 while the surface roughness decreases.51 The resulting fiber diameter is shaped by factors like electrostatic force, viscoelasticity, solvent dynamics, and air drag. Increasing the net charge density (σ) by introducing the lead and bromine ions modifies the strength of Coulombic forces; the net charge conservation equation in the solution can be written as52 It = πR2KE + 2πRνσ
(1)
where It is the constant total current in the jet, R is the jet radius, K is the solution conductance, and E is the vertical component of the electric field. In the presence of additional ions, the K and σ get modified, which results in an increased stretching force. Increased ion density of the electrospun solution also reduces the required electric field. Since the electrospinning process is driven by the repulsion of like charges, the positive and negative ions do not necessarily separate spatially, which is advantageous for the formation of OIP crystals. Remarkable improvements in fiber quality was visible even at a very small ion fraction (∼0.1 wt %). The 24469
DOI: 10.1021/acsami.9b08409 ACS Appl. Mater. Interfaces 2019, 11, 24468−24477
Research Article
ACS Applied Materials & Interfaces
to the fiber axis and can be approximated to a 2-D radial distribution in a given cross section. This pattern of growth is explained by considering the inhomogeneous phase separation during the stretching of the fiber. It is assumed that, as the stretching starts, the PMMA and OIP precursor is homogeneously mixed, which is evident from the absence of any precipitation. As the fiber stretches, the narrowing of the column results in an increase in surface area to volume ratio, which accelerates the solvent evaporation, leading to a rise in OIP precursor concentration in the solution. When the OIP precursor concentration rises above its critical concentration, it phase separates and grows into OIP NCs. Since the process is kinetically driven, many nuclei are formed which grow into a cluster of NCs. The kinetics of the growth appears to inhibit formation of large NCs. A schematic depicting the proposed mechanism for in situ formation of OIP grains during the fiber growth process is shown in Figure 1g. During the stretching of the fiber, it is pushed toward the target; the solvent continuously evaporates at the lagging edge of the fiber. The rate of stretching and solvent evaporation creates segments of fiber where the NCs nucleate and phase separate into clusters. In general, the spatial distribution of clusters does not show any periodicity, but on average, they are spatially separated by 100−200 nm. Postcrystallization, the clusters remain embedded within the polymer matrix which acts as a dense, transparent encapsulation. The size of NCs and the cluster is a function of OIP concentration in the precursor solution. Average grain size increases with an increase in OIP concentration, and at large concentrations (∼25 wt %), a complete transformation into the bulk phase is observed, which is signaled by the overlap of absorption and emission edges with that of bulk and a sharp reduction in PLQY. D. Structural Characterization. The XRD spectra of NC in the PMMA fibers show large differences from that of bulk OIP (Figure 2a). In contrast to the bulk spectra, the NC spectrum contains only three visible diffraction peaks, indicative of small range order, which coincides with the electron diffraction. The 2θ value for the (100) peak position increases by ∼2.03° from that of the bulk, suggesting a decrease in d spacing and hence a smaller lattice constant (∼11.6%) than that in the bulk. The shift in 2θ value of the (100) peak has a trend similar to that for submicron crystals and conventionally prepared NCs.14 Simultaneously, broadening of the NCs diffraction peak, compared to that of bulk and conventionally prepared nanocrystal, indicates a smaller domain size (diameter ∼ 1−2 nm). The 2θ and FWHM of diffraction peaks show no observable dependence on the precursor concentration (Figure S5). This suggests a smaller distribution of particle size being present at different OIP concentrations studied (0.01−5 wt %), and the broadening of the diffraction peak is dominated by the particles of smallest diameter. E. Quantum Confinement. 1. Optical Absorption and Emission. The small size of NCs leads to the quantum confinement effect, which is characterized by optical absorption and emission techniques. Absorption (α(E)) for the composite fibers with different OIP concentrations was estimated by the measurements of transmitted and scattered light intensity using an integrating sphere. Normalized α(E) for different OIP concentrations is shown along with bulk and chemically synthesized NCs in Figure 2b. The (α(E)) edge and the first excitonic absorption peak blue-shift with a
Figure 1. (a) Optical and (b) fluorescence image of OIP NCs embedded PMMA fibers, (c) AFM topography image of fibers, (d) single fiber topography and (e) phase image, (f) TEM image of OIP NCs cluster, and (g) schematic depicting proposed fiber growth method and OIP NCs formation.
pattern reveals few discrete points, suggesting low crystallinity. The prominent diffraction ring indicates a large amorphous nature of the material. C. Origin of the Unique Distribution of OIP Grain. From microscopic observation of fibers construction, it is inferred that the bulk of the fiber is composed of PMMA, while the distribution of the embedded NC clusters has a characteristic pattern. The clusters are identified in the phase image as regions of sharp phase change. The combined stiffness of the layer is higher than that of the polymer alone, which results in larger phase change. Looking at the regions of high phase contrast (Figure 1e), we can notice the ellipsoidal shape of clusters; an image with all the potential clusters is shown in Figure S3. This observation is further verified by dissolving fibers in toluene where the clusters remain undisturbed. A similar structure is observed for the clusters suspended in toluene solution, which has been dispersed on a glass slide and imaged using AFM (Figure S4). The NC clusters are ellipsoidal in shape, and the long axis is orthogonal 24470
DOI: 10.1021/acsami.9b08409 ACS Appl. Mater. Interfaces 2019, 11, 24468−24477
Research Article
ACS Applied Materials & Interfaces
signal while the scattering from the fibers remains constant. Scattering from the fibers can be removed by dissolving the fibers in a solvent orthogonal to OIP (e.g., toluene); the OIP NCs remain no longer capped, which results in increased association within the crystallites. The α(E) features of suspended NCs are closer to that of the bulk (excitonic peak at the same energy), which suggests an onset of weak aggregation within the crystallites upon dissolving the polymer matrix (Figure S6a). The composite fibers show a high PL quantum yield for all the weight ratios studied (Figure 2c). Emission per unit area is fainter at smaller OIP wt % which results from the smaller size and low density of NCs. High PL efficiency results from factors like confinement of excitons, increased radiative surface recombination, and improved light out-coupling. The blue shift in emission at lower wt % also suggests an increase in optical energy gap, which agrees well with the α(E) observation. The emission peak (Epeak) blue-shifts ∼40 nm with the decrease in OIP concentration from 5.0 to 0.16 wt %, which corresponds to an increase of ∼0.2 eV in the band gap (Figure 2d). The change in band gap at 300 K has a nonlinear dependence on OIP concentration (Figure 2e); this trend is identical for the peak width (FWHM). In general, thermal broadening of emission peak is associated with exciton− phonon coupling; an increase in FWHM of emission thus suggests a corresponding increase in exciton−phonon interaction arising from higher quantum confinement in smaller NCs. The above observations suggest a smaller crystallite size (
