Impact of Crystallographic Orientation Disorders on Electronic

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Letter Cite This: Nano Lett. 2018, 18, 6271−6278

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Impact of Crystallographic Orientation Disorders on Electronic Heterogeneities in Metal Halide Perovskite Thin Films Benjamin J. Foley,*,† Shelby Cuthriell,† Sina Yazdi,† Alexander Z. Chen,† Stephanie M. Guthrie,† Xiaoyu Deng,† Gaurav Giri,† Seung-Hun Lee,‡ Kai Xiao,§ Benjamin Doughty,∥ Ying-Zhong Ma,*,∥ and Joshua J. Choi*,†

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†

Department of Chemical Engineering and ‡Department of Physics, University of Virginia, Charlottesville, Virginia 22904, United States § Center for Nanophase Materials Sciences and ∥Chemical Sciences Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States S Supporting Information *

ABSTRACT: Metal halide perovskite thin films have achieved remarkable performance in optoelectronic devices but suffer from spatial heterogeneity in their electronic properties. To achieve higher device performance and reliability needed for widespread commercial deployment, spatial heterogeneity of optoelectronic properties in the perovskite thin film needs to be understood and controlled. Clear identification of the causes underlying this heterogeneity, most importantly the spatial heterogeneity in charge trapping behavior, has remained elusive. Here, a multimodal imaging approach consisting of photoluminescence, optical transmission, and atomic force microscopy is utilized to separate electronic heterogeneity from morphology variations in perovskite thin films. By comparing the degree of heterogeneity in highly oriented and randomly oriented polycrystalline perovskite thin film samples, we reveal that disorders in the crystallographic orientation of the grains play a dominant role in determining charge trapping and electronic heterogeneity. This work also demonstrates a polycrystalline thin film with uniform charge trapping behavior by minimizing crystallographic orientation disorder. These results suggest that single crystals may not be required for perovskite thin film based optoelectronic devices to reach their full potential. KEYWORDS: Lead halide perovskites, heterogeneity, crystallographic orientation, trap sites

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than those based on crystallographically disordered copper indium gallium selenide (CIGS) and CdTe.9−11 This challenge has stimulated research efforts to understand the microscopic mechanisms responsible for the spatial heterogeneity in MHP thin films. Direct assessment of electronic heterogeneity with spatial resolutions on the micronand nanometer-scales has been reported based on the measurements of photoluminescence (PL) intensity,12−14 PL lifetime and charge transfer kinetics for different grains,14−19 surface voltage, and current variations on different facets of the same crystal,20−22 and spatial dependence of femtosecond

s an emerging solution-processable class of optoelectronic materials, metal halide perovskites (MHPs) have demonstrated enormous potential for inexpensive, lightweight, and flexible solar cells with power conversion efficiencies (PCE) that are on-par with the current state-of-the-art technologies.1−4 However, the significant spatial heterogeneity in the electronic properties of the photoactive MHP thin films remains a major challenge for achieving widespread deployment of MHP solar cells.5−8 In current state-of-the-art solar cell technologies, the importance of employing electronically uniform photoactive layer materials is recognized as an essential prerequisite for achieving high PCE and reliability.7,9−11 Well-known examples include single crystal Si and epitaxial GaAs solar cells, which exhibit significantly less spatial variation in their PCEs and substantially higher performance © 2018 American Chemical Society

Received: June 14, 2018 Revised: August 21, 2018 Published: September 14, 2018 6271

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Figure 1. Grazing incidence X-ray scattering patterns of (a) the tetragonal (110) and (d) the randomly oriented MAPbI3 thin film sample. SEM images of the tetragonal (110) sample (b) and zoomed in image (c) on the region of that sample indicated by the blue square in (b), and (e) SEM image of the randomly oriented sample and (f) the corresponding zoomed in image on the region depicted by the blue square in (e). Scale bars are (b) 20 μm, (c) 500 nm, (e) 2 μm, and (f) 500 nm.

transient absorption signal and its relaxation dynamics.12,13,23 The microscopic origin of the observed heterogeneity is currently under intense debate. Whereas most of these studies have provided evidence that the spatial heterogeneity can arise from spatially varying charge carrier trapping,13−15,17,20,24 some other studies have also identified spatially dependent photoexcited carrier diffusion characteristics as a major contribution to the heterogeneity.16,24,25 It is not yet well understood how different perovskite thin film morphology and structure give rise to these mechanisms and in which cases they are dominant or negligible. Moreover, there are conflicting conclusions in the literature on the impact of the heterogeneity on solar cell performance5,6,8 as some studies report that significant heterogeneity limits the solar cell efficiency,6,8,26 while others do not.5 These disparate conclusions reflect the complicated and intertwined nature of the morphological and electronic heterogeneities in MHP thin films. An unambiguous understanding of how morphological structures relate to electronic response requires methods that are simultaneously sensitive to both aspects, which underscores the need for multimodal imaging on the MHP thin films. Here we present a comprehensive study that employs multimodal imaging measurements on structurally distinct polycrystalline MHP thin films. Specifically, MHP thin films with random crystallographic orientation of grains were compared to those with highly uniform crystallographic orientation with respect to the substrate. A combination of confocal PL imaging and time-resolved PL (trPL) at points of interest were applied to probe the photoexcited-state properties, whereas transmission imaging and atomic force microscopy (AFM) were used to survey the morphology of the sample at the same location. This multimodal imaging approach allows the morphological and electronic hetero-

geneities to be distinguished,12 and our results identify the disorder in crystallographic orientation of grains as the predominant cause for charge trap heterogeneity. This study is the first to isolate the role of crystallographic orientation from other complicating factors such as charge diffusion in MHP thin films, and our findings resolve the seemingly conflicting reports on this intensely debated topic in the field.14,16 Also, this study adds further evidence that the surface defects are the dominant charge recombination sites in MHP thin films rather than recombination in the bulk.27−34 An important implication of this work is that a solution-processed polycrystalline perovskite thin film with a negligible spatial variation in charge trap site density reaching that of single crystals can be achieved by minimizing crystallographic orientation disorder. Results. Recently, it has been shown that surface recombination is the dominant charge carrier decay pathway in MHP thin films,27 and this result implies that the different crystal facets exposed by grains of different orientation will have varying surface recombination. On the basis of this, we hypothesized that the electronic heterogeneity observed in MHP thin films13−15,17,20,24 is dominated by disorder in the crystallographic orientation of grains and, therefore, a highly oriented thin film will exhibit homogeneous charge trapping behavior despite having grain boundaries or morphological variations. To test this hypothesis, we applied a multimodal imaging approach to image the morphological and electronic properties of individual neighboring grains. This approach for studying heterogeneity within the same sample, not absolute differences between different samples, is critically important to obtain conclusive results as preparation of MHP thin films with different crystallographic orientations requires changing the synthesis procedure, which will inevitably alter stoichiometry, 6272

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Figure 2. Normalized (a) transmission and (b) PL images of the randomly oriented sample and (c) their pixel to pixel correleation. (d) The trPL curves probed at the bright and dark regions of the PL image as indicated by the color circles in (b). Scale bars indicates 2 μm. The size of the images is 256 × 256 pixels, and the excitation fluence was 2 μJ/cm2.

tetragonal (110) orientation had a grain size of up to 40 μm, which upon close examination with SEM shows no visible smaller grains nor structures indicating multiple exposed crystallographic facets. The randomly orientated sample had approximately 1 μm sized grains (Figure 1e), consistent with previous results in the literature.38 Representative transmission and PL images acquired for the randomly oriented MAPbI3 sample are shown in Figure 2a and 2b. Optical transmission images were collected using laser light with a wavelength of 750 nm. At this wavelength, MAPbI3 has a low but finite extinction coefficient allowing for optical transmission through the film while still providing information on the overall spatial variation on optical density. Assuming that the extinction coefficient of the thin films is weakly dependent on the position, the transmission images thus provide an approximate measure of the film thickness at distinct spatial locations. The strong correlation between film thickness and transmission is shown in Supporting Information, Figure S4. The time-integrated PL image, shown in Figure 2b, was collected at the same sample region using an excitation wavelength of 500 nm and collecting emission at 750 nm using appropriate optical filters. The PL image exhibits multiple bright and dark regions, which resemble the grains in size and shape observed in the SEM image shown in Figure 1e. Visual inspection of the transmission (Figure 2a) and PL (Figure 2b) images shows lack of any apparent correlation. To quantify the degree of correlation, a two-dimensional histogram was constructed that relates the PL intensity with the transmission intensity on a pixel-to-pixel basis as shown in Figure 2c. Analysis of the correlation histogram yields a Pearson coefficient of −0.17, indicating a lack of correlation between the PL and transmission images. This means that the variations in sample thickness cannot explain the observed PL intensity variations in randomly oriented MAPbI3 thin films. Interestingly, the trPL curves measured at regions of bright and dim PL (Figure 2d) show a pronounced difference at longer times that correspond to the monomolecular recombination regime with a linear decay in a plot with a logarithm scaled PL

grain size, surface passivation, and other parameters that influence their electronic properties. Therefore, comparison across different samples prepared through different synthesis methods is unlikely to provide conclusive information on which crystallographic orientations are intrinsically superior for device performance. In this work, we compare different grains within the same sample prepared under the same conditions. We apply this approach to different samples with varying degree of crystallographic orientation disorder separately. This allows us to obtain information on how crystallographic orientation disorder translates to electronic heterogeneity but not whether certain crystallographic orientations are intrinsically better for device performance compared to others in absolute sense. For samples, we prepared thin films of methylammonium lead iodide (MAPbI3), the most commonly studied MHP, with grains that were either randomly oriented or highly oriented with respect to the substrate. MAPbI3 films with the tetragonal (110) crystallographic orientation were prepared by modifying an MA gas treatment procedure reported in the literature.35−37 MAPbI3 samples with random crystallographic orientations were prepared using a solvent extraction method.38 Details about these sample preparation methods are provided in the Supporting Information. The morphology and crystallographic orientation of these samples were characterized using scanning electron microscopy (SEM) and grazing incidence X-ray scattering (GIXS), as shown in Figure 1. GIXS provides the two-dimensional diffraction patterns (Figure 1a,d), which can be used to quantitatively determine the degree and direction of the crystallographic orientation with respect to the substrate.39−43 Figure 1a shows a GIXS pattern from the tetragonal (110) oriented MAPbI3 thin film sample which has clear diffraction peaks corresponding to a highly preferential orientation of all grains with respect to the substrate. In contrast, a GIXS pattern from the randomly oriented MAPbI3 thin film sample (Figure 1d) shows diffraction rings, indicating a lack of any preferential orientation of the grains. The sample with highly preferential 6273

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Figure 3. Normalized (a) transmission and (b) PL images acquired for the same region of interest of the tetragonal (110) oriented MAPbI3 sample. (c) PL image acquired for a small region within (b), and (d) AFM image for the same region of interest. (e) Pixel-to-pixel correlation between transmission and PL images shown in (a,b), and (f) the trPL curves for the bright and dark regions of (c). The blue- and orange-colored circles in (c) correspond to the bright and dim regions, respectively, at which the trPL curves in (f) were taken. PL images are 300 × 300 pixels, and the excitation fluence was 5 μJ/cm2. Scale bars are (a,b,d) 20 μm and (c) 5 μm.

intensity.44−47 Such PL decay is typically attributed to trap mediated charge recombination processes.44−47 These results suggest that the observed PL intensity variation in the MAPbI3 thin film with random crystallographic orientation (Figure 2b) is in a large part due to occurrence of heterogeneous charge trapping across different grains, which is consistent with previous literature results.13,15,17,24,28 Additional data collected at different locations (Supporting Information Figures S2 and S3) is consistent with the results shown in Figure 2. The results obtained for the tetragonal (110) oriented MAPbI3 samples are dramatically different from those measured for the randomly oriented MAPbI3 thin films. First, the grains are clearly resolved in the PL images, and each grain appears to have the same behavior where the PL is brighter at the edges and dimmer in the center (Figure 3b). A PL image acquired over a smaller area denoted as a blue square in Figure 3b but with the same acquisition time did not reveal any smaller features (Figure 3c). This indicates that the observed PL features are defined by the dimensions of the grains. The difference between the centers and edges of the grains is also present in the corresponding transmission image (Figure 3a), where the edges of the grains are thinner than the center. Importantly, the pixel-to-pixel correlations between transmission and PL images are reasonably correlated with a Pearson coefficient of 0.39 (Figure 3e), indicating the PL features are largely due to the sample morphology. The trPL curves measured at chosen bright and dark regions, marked as blue and orange circles in Figure 3c, respectively, differ primarily at early times where nonlinear decay processes dominate (Figure 3f). This type of difference has been attributed to carrier redistribution within the thin film.16,24,25,44 In contrast, the slope of the trPL curves at longer times corresponding to the monomolecular recombination regime is indistinguishable, indicating that the rate of trap mediated recombination at the two locations is independent of the sample position in the tetragonal (110) oriented MAPbI3 thin films. Additional data collected at different locations (Supporting Information, Figure S1) is consistent with that

shown in Figure 3. In order to verify that the excitation light intensity used for these measurements is within the trapmediated recombination regime, the trPL and PL intensity of the oriented sample was measured at different excitation light intensities as shown in the Supporting Information Figures S7 and S8. The existence of a correlation between the PL and transmission images for the tetragonal (110) oriented MAPbI3 sample indicates that the observed features in the PL image reflect predominantly morphological variations. In contrast, for the randomly oriented MAPbI3 samples, the lack of correlation between the PL and transmission images as well as significantly different PL decay rates in the monomolecular recombination regime suggest presence of a significant electronic heterogeneity in addition to morphological variations. The occurrence of these distinct types of heterogeneities can be visualized by constructing a map of the ratio of transmission and PL intensities. In the limit of a perfect correlation, the ratio map would be flat and featureless; whereas in the case of imperfect correlation the uncorrelated features will remain. Figure 4 shows the calculated ratio maps for both the randomly oriented MAPbI3 sample (Figure 4a) and the tetragonal (110) MAPbI3 oriented (Figure 4b) sample. The ratio map for the former exhibits significant variations with a large number of clearly visible features remaining, indicating significant contributions to the observed PL spatial heterogeneity from sources other than morphological variations. In contrast, the ratio map for the tetragonal (110) sample appears mostly flat except for the sharp features from the grain boundaries, indicating that the observed PL variations are mostly due to the difference in thickness at the edges versus the center of the grains. To further confirm these conclusions, an analysis of trPL traces was performed to quantify how the trap-mediated recombination rate varies with location. As mentioned previously, the initial nonlinear trPL decay can be attributed to carrier redistribution, whereas the linear decay at longer times to trap mediated recombination. The trPL curves can be 6274

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D is the diffusion coefficient, and n is the carrier concentration. These dynamical processes were modeled using a twodimensional cylindrical coordinate system accounting for film thickness and grain radius. We assumed the carrier flux at all grain boundaries and surfaces to be zero, and the initial carrier concentration distribution was calculated by accounting for the absorption of excitation light (Supporting Information, Figure S7) with a Gaussian beam profile (see Supporting Information on numerical modeling of carrier dynamics). As the carriers from localized photoexcitation diffuse toward the bottom of the film and radially outward, their concentration decreases and in turn results in a reduction in PL intensity. For a thicker film, during the initial short time scale where the carrier redistribution dynamics dominate, the carriers effectively diffuse into a larger volume and result in a lower carrier concentration. At longer time scales, after the carrier concentration gradient has been reduced, the influence of this diffusion process on the trPL decay kinetics becomes minimal and instead the trap mediated recombination dominates. For the tetragonal (110) oriented MAPbI3 sample, the film heights of the exact locations for the trPL measurements were taken from AFM data acquired for the same region of interest, whereas the corresponding film heights for the randomly oriented sample were estimated based on the transmission image and optical density. As shown in Figure 5, this analysis enabled us to simulate the trPL curves from the tetragonal (110) oriented sample by treating km, D, and kb as global fitting parameters. A single set of their values that best describes the trPL data acquired at both the bright and dark regions was sought during the fitting. This was possible as the difference in film thickness alone is sufficient to account for the difference in the trPL decay during the initial short time scale. However, for the randomly oriented MAPbI3 samples, no satisfactory fitting could be obtained without allowing km to vary between the trPL data from the bright and dark regions, which is consistent with our previous discussion of significantly different charge

Figure 4. Normalized transmission image divided by the normalized PL image for the randomly oriented MAPbI3 sample (a) and the tetragonal (110) oriented MAPbI3 sample (b). For the randomly oriented sample, the ratio map shows strong features. In contrast, the only remaining features in the ratio map of the tetragonal (110) oriented sample arise from sharp transmission changes at the grain boundaries. Scale bars are (a) 2 μm and (b) 20 μm.

modeled by the equation below (eq 1), which considers carrier diffusion, and first- and second-order recombination dn = D∇2 n − k mn − k bn2 dt

(1)

In eq 1, km is the rate constant for trap-mediated monomolecular recombination, which is a function of trap site density, kb is the bimolecular recombination rate constant,

Figure 5. Simulated results (black lines) for the trPL curves measured at the bright (blue curve) and dark (orange curve) regions of the (a) tetragonal (110) oriented and (b) randomly oriented samples. (c) Simulated photoexcited carrier distributions for 150 and 300 nm films. The carriers diffuse away from the point of excitation (shown at time zero) both to the back of the thin film and radially. 6275

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on different surface terminations is the dominant origin of electronic heterogeneity in MHP thin films. It has been reported that improved passivation can reduce the heterogeneity in polycrystalline MHP thin films although not completely.49 Furthermore, the evidence currently available in the field points to the trap site heterogeneity being present in MHP thin films with significantly longer carrier lifetimes than what is observed in this work even when the films are covered with an electron transporting layer or hole transporting layer and incorporated into a solar cell.6,8 Therefore, the conclusions reached in our work are also useful for films with lower monomolecular recombination rates. Overall, the extent of this study is to identify the dominant cause of electronic heterogeneity in MHP thin films and show that crystallographic orientation plays a key role in determining the monomolecular recombination dynamics. It should be noted that our work does not identify which particular crystal facets are optimal for high device performance. Discussion. Previous studies on crystallographically disordered perovskite samples typically report evidence of charge trap site heterogeneity13,15,17,19,21 but the studies based on crystallographically oriented thin films have reached inconsistent conclusions.12,14,16,25 An enlightening case of this discrepancy is the comparison between two PL imaging studies of crystallographically oriented perovskite thin films prepared using similar methodologies but that reached opposite conclusions about the presence of charge trap site heterogeneity. Tian et al. found minimal charge trap site heterogeneity and attributed the spatial variations in PL to carrier diffusion,16 whereas de Quilettes et al. found that even after accounting for diffusion, evidence of charge trap site heterogeneity remained.24 It should be noted that both studies prepared the perovskite thin film samples with addition of lead chloride precursor, which is known to result in spatially inhomogeneous chloride distributions.14,50 Indeed, de Quilettes et al. conclusively linked the trap site heterogeneity observed in their sample to the spatial variation in chloride distribution. The degree of charge trap site heterogeneity introduced by the chloride can vary widely from laboratory to laboratory, and we speculate that the sample studied by Tian et al. was less affected. These conflicting conclusions from the literature highlight the complicating factors such as heterogeneous chloride distribution. The approach taken here in our work avoids any complications associated with addition of chlorides while applying multimodal imaging to separately reveal the heterogeneities stemming from morphological and electronic variations. Our systematic comparison of adjacent, individual grains on samples with and without preferential crystallographic orientation reveals that variations in both the charge diffusion and charge trapping can contribute to the heterogeneity in optoelectronic properties but their degree of contribution to heterogeneity varies widely depending on the perovskite thin film structure. The previous studies in the literature with seemingly conflicting conclusions can be understood in this context. The most critical insight that emerges is that the disorder in the crystallographic orientation among grains and their crystal surface termination, rather than any differences within the bulk of different grains, is the dominant source of charge trap heterogeneity in perovskite thin films. In conclusion, a major implication of our work is that electronic heterogeneity is not intrinsic to solution processed

trap mediated recombination over different randomly oriented grains. The nonlinear behavior in the short time scale decay of the trPL curves could have resulted from grain boundaries acting as barriers for carrier diffusion.25 A similar model to the one described above may be used to fit the trPL data by limiting the radius into which the carriers are allowed to diffuse. However, for the randomly oriented sample we found that inclusion of these additional variables was still insufficient to obtain satisfactory fits if both the bright and dark locations are constrained to have the same monomolecular recombination rate (see Supporting Information, Figure S10). We emphasize that in this work we place less importance on the specific mechanism responsible for the trPL decay at early times other than showing that it can be explained with a combination of carrier diffusion and film morphology. Instead, the emphasis is on our findings that the charge trap-mediated recombination is uniform across grains of MAPbI3 thin films with the same crystallographic orientation but not for randomly oriented grains. The values obtained for trap-mediated recombination rate at the different locations with high and low PL intensities (km1 and km2), the bimolecular recombination rate constant, and diffusion coefficient are summarized in Table 1. These values Table 1. Fitted Parametersa

randomly oriented tetragonal (110) oriented

km1 (μs−1)

km2 (μs−1)

D (cm2 s−1)

kb (cm3 s−1) × 1010

11 22

26 22

0.013 0.05

6.5 8.1

a

km1 and km2 denote the rate constants for trap mediated monomolecular recombination at the bright and dark regions of the PL image, respectively.

are in general agreement with the literature.46−48 As further verification that we have captured the monomolecular rate constant correctly, we measured the trPL with a separate experiment using an ∼1 mm spot size and low fluence. The results show that the first order decay rates are indeed comparable to the values in Table 1 (see Supporting Information, Figures S5 and S6). In the case of this particular set of samples, the highly oriented sample has a faster monomolecular recombination rate than that of the randomly oriented sample. This is consistent with their relative photoluminescence quantum yields; the randomly oriented sample is approximately five times brighter than the highly oriented one at the same excitation light intensity. This mirrors the previous observation that single crystals can have inferior surface passivation compared to polycrystalline thin films depending on synthesis methods.27 Thus, our results indicate that the randomly oriented sample used in this study is better passivated due to its particular synthesis conditions compared to the highly oriented sample. Nonetheless, the salient conclusion from our results is that uniform crystallographic orientation results in uniform charge trapping behavior even with higher trap densities. This is striking because if all trap sites can be ideally all passivated, the electronic heterogeneity should disappear. The fact that our results show uniform electronic heterogeneity in the highly oriented sample despite its higher monomolecular recombination rate compared to randomly oriented sample further bolster the picture that heterogeneous charge trapping 6276

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Nano Letters MHP thin films and that films can be electronically uniform despite morphological variations and grain boundaries if the crystallographic orientation of the grains is carefully controlled. Presence of electronic heterogeneity due to different crystal surface termination is generally expected but its significance over other factors such as varying bulk crystal defects across different grain was not clear until now. This work, through a combination of novel sample preparation techniques and a multimodal imaging approach, conclusively shows that different surface termination across grains is dominant over other factors in determining electronic heterogeneity. The fact that highly oriented thin films exhibit uniform charge-trapping behavior indicates that it is possible for solution-processed polycrystalline MHP thin films to achieve electronic uniformity that approaches those of single crystals as long as the crystallographic orientation and surface termination of grains are uniform. This calls for further research efforts to obtain a deeper understanding of various relevant aspects such as the crystallization process of MHP thin films, formation of desired crystallographic orientations, the nature of charge traps on different facets, and how they can be passivated. Such insights will provide accelerated pathways to obtain the most favorable surface termination and passivation to achieve the full potential of MHPs in next generation optoelectronic technologies.

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work at ORNL was supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, Chemical Sciences, Geosciences, and Biosciences Division (Y.-Z.M. and B.D.), and the U.S. Department of Energy, Office of Science Graduate Student Research program (B.J.F.). Part of the research was performed as a user project at the Center for Nanophase Materials Sciences, which is a DOE Office of Science User Facility. This work is based upon research conducted at the Cornell High Energy Synchrotron Source (CHESS), which is supported by the National Science Foundation under award DMR-1332208.

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ABBREVIATIONS MHP, metal halide perovskite; PL, photoluminescence; trPL, time-resolved photoluminescence; AFM, atomic force microscopy; SEM, scanning electron microscope; MAPbI3, methylammonium lead iodide; MA, methylamine; GIWAXS, grazing incidence wide-angle X-ray scattering; PCE, power conversion efficiencies

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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.8b02417. Materials and methods, additional imaging and timeresolved photoluminescence, correlation between AFM and transmission images, time-resolved photoluminescence collected using conventional geometry, absorption spectroscopy, details of carrier diffusion model, analysis of impact of grain size and possible carrier diffusion across grain boundaries, intensity dependent timeresolved photoluminescence using confocal geometry (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: (B.J.F.) [email protected]. *E-mail: (Y.-Z.M.) [email protected]. *E-mail: (J.J.C.) [email protected]. ORCID

Alexander Z. Chen: 0000-0003-0625-1954 Stephanie M. Guthrie: 0000-0001-7549-9983 Gaurav Giri: 0000-0002-8504-1033 Kai Xiao: 0000-0002-0402-8276 Benjamin Doughty: 0000-0001-6429-9329 Ying-Zhong Ma: 0000-0002-8154-1006 Joshua J. Choi: 0000-0002-9013-6926 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS J.J.C. and S.-H.L. acknowledge support from the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Award Number DE-SC0016144. The 6277

DOI: 10.1021/acs.nanolett.8b02417 Nano Lett. 2018, 18, 6271−6278

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DOI: 10.1021/acs.nanolett.8b02417 Nano Lett. 2018, 18, 6271−6278