CdSe Nanoplatelet Films with Controlled Orientation of their

Letter pubs.acs.org/NanoLett

CdSe Nanoplatelet Films with Controlled Orientation of their Transition Dipole Moment Yunan Gao, Mark C. Weidman, and William A. Tisdale* Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, United States S Supporting Information *

ABSTRACT: Using liquid−liquid interfacial assembly, we control the deposition of CdSe nanoplatelets into face-down or edge-up configurations. Controlled assembly, combined with back focal plane imaging, enabled unambiguous determination of the transition dipole orientation. The transition dipole moment of the emissive band-edge exciton in CdSe nanoplatelets was found to be isotropically oriented within the plane of the nanoplatelet with no measurable outof-plane component and no preference for the long- or shortaxis of the nanoplatelet. Importantly, CdSe nanoplatelet films in the face-down configuration exhibited unity dipole orientation within the plane of the film, which could improve the external efficiency of nanoplatelet LEDs, lasers, photodetectors, and photovoltaic cells beyond that which is possible with isotropic emitters. We also show that the two self-assembled configurations have different Förster energy transfer rates, as a result of different dipole orientation and internanoplatelet distance. KEYWORDS: Self-assembly, back focal plane imaging, exciton, 2D materials, energy transfer

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tional averaging.23 The self-assembly of CdSe/CdS rods has been studied by several authors17,24−26 and self-assembled stacks of CdSe nanoplatelets in solution have shown strongly polarized emission.5,27 However, a method for on-demand assembly of CdSe nanoplatelet films with controlled orientation is needed. In this work, we present a method for the controlled assembly of CdSe nanoplatelet films with entirely face-down or edge-up configurations and provide experimental determination of the exciton transition dipole orientation. In addition, we show that the assembly configuration can affect the energy transfer rate between nanoplatelets. Control of the nanoplatelet orientation was achieved by controlling surface energy of the interface during liquid−liquid interfacial assembly via introduction of soluble ligand into the subphase.17,28,29 We characterized the nanoplatelet assemblies by transmission electron microscopy (TEM) and synchrotron grazing-incidence small-angle X-ray scattering (GISAXS). Through the use of back focal plane imaging (BFP),30 we determined that bandedge exciton transition dipoles in CdSe nanoplatelets are fully oriented within the plane of the nanoplatelet and that unity inplane dipole orientation can be realized in the face-down assemblies. CdSe nanoplatelets were synthesized with oleic acid (OA) capping ligands following previously reported methods with some modifications (see Methods).9,31 In Figure 1, we show a

olloidal semiconductor nanocrystals are promising components of next-generation light emitting diodes (LEDs), lasers, solar cells, and detectors.1 In recent years, colloidal CdSe nanoplatelets have emerged as a new type of nanocrystal with distinct properties, such as narrow ensemble emission spectra, fast radiative transition rates, and anisotropic light emission, which arises from strong one-dimensional quantum confinement defined by a few-nanometer thickness and atomic uniformity.2−5 These properties make nanoplatelets particularly attractive for optoelectronics and photonics applications.6−13 In many of these applications, optical coupling efficiency can be improved by controlling the transition dipole orientation in the absorptive/emissive layer. For instance, the external quantum efficiency of LEDs can be improved by more than 50% if emissive molecules are oriented such that their transition dipole moment is parallel to the surface plane, rather than the usually random distribution of dipoles.14,15 Although such control is difficult to achieve with isotropic emitters, anisotropic nanostructures, like CdSe nanoplatelets, may offer new opportunities. Colloidal self-assembly is a powerful way to control the orientation of nanocrystals and to build macroscale functional materials from individual nanostructures.16−21 Self-assembly of anisotropic nanocrystals is particularly appealing due to the possibility for oriented assembly and shape- and orientationdependent properties.22 In addition to the technological applications, the ability to control the atomic axis of colloidal nanostructures with respect to a laboratory frame (i.e., optical, electric, or magnetic field of an experiment) enables fundamental studies not possible under conditions of orienta© 2017 American Chemical Society

Received: March 24, 2017 Revised: May 22, 2017 Published: May 23, 2017 3837

DOI: 10.1021/acs.nanolett.7b01237 Nano Lett. 2017, 17, 3837−3843

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(Figure 2, bottom), although the volume and concentration of the nanoplatelet dispersion also played a role (see Supporting Information). With 4.2 mM oleic acid in DEG and 30 μL of the nanoplatelet dispersion, the face-down configuration was achieved, whereas with 0.42 mM oleic acid in DEG and 60 μL of the same nanoplatelet dispersion the edge-up configuration was obtained, as shown in Figure 2. TEM images in Figure 2a,b show the face-down and edge-up assemblies with high fidelity up to several micrometers, and the insets show TEM images of the same films with higher magnification. Additional TEM images with other magnifications are available in the Supporting Information. Paik et al. previously studied self-assembly of gadolinium trifluoride (GdF3) nanoplatelets, having similar size to our CdSe nanoplatelets and the same nonpolar oleate capping ligands, using a liquid interface assembly method.16 They found that assembly occurred at the liquid−liquid interface, and observed that the GdF3 nanoplatelets adopted the edge-up configuration on a more polar glycol subphase (diethylene glycol) but adopted the face-down configuration on a less polar one (tetraethylene glycol). This was understood as shifting the balance of interaction strength between nanoplatelet−nanoplatelet and nanoplatelets−subphase (i.e., the interfacial energy, or surface tension), originating from van der Waals interactions between the capping ligands and the subphase molecules and strong intermolecular forces within the polar subphase.20 In our work, we tuned the interfacial energy by varying the amount of additional oleic acid added directly to the diethylene glycol (DEG) subphase. In the case of no or a small amount of oleic acid added to DEG (i.e., less than 0.42 mM), we consistently observed edge-up assembly. Furthermore, when no oleic acid was added to DEG we observed changes in nanocrystal shape and evidence of fusion of neighboring nanoplatelets (Figure S7), presumably due to detachment of surface ligands during self-assembly. However, when a small amount (0.42 mM) of oleic acid was added to the DEG subphase, these unwanted structural changes could be avoided. Evers et al. similarly observed detachment of oleic acid capping ligands from PbSe nanocrystals during self-assembly on the surface of DEG.29 They took advantage of this effect to realize ligand-free oriented nanocrystal attachment, but were also able to prevent attachment by adding excess oleic acid into the DEG subphase.29 Inspired by this work, we found that by adding a small amount of oleic acid to the DEG subphase, we could repeatedly obtain edge-up monolayer assemblies of CdSe nanoplatelets without unwanted changes to the size or shape of individual nanoplatelets (Figure 2b,d) and that increasing the oleic acid concentration yielded face-down monolayer assemblies (Figure 2a,c). With careful selection of the volume and concentration of the CdSe nanoplatelet dispersion, we were able to form large-area, phase-pure assemblies (see Supporting Information for lowermagnification/wider-area images). Self-assembled CdSe nanoplatelet monolayers were successfully transferred to a variety of substrates, including amorphous carbon and silicon nitride TEM grids, quartz glass coverslips, and SiO2/Si wafers. On solid substrates, domain sizes varied from several hundred nanometers to tens of micrometers, separated by gaps with no nanoplatelet coverage. The jagged edges of neighboring domains (Figure S6) suggest that larger domains existed on the DEG surface but were broken up during transfer to the solid substrate. Preserving uniform coverage over large areas will be needed for future device integration.

Figure 1. CdSe nanoplatelet characterization. (a) TEM image of CdSe nanoplatelets lying face-down. (b) Size histogram of nanoplatelet lateral dimensions. Gaussian fits reveal an average length of 21.9 ± 2.8 nm and width of 6.5 ± 1.2 nm. (c) Absorption (Abs) and photoluminescence (PL) spectra of nanoplatelets dispersed in hexane. On the basis of the PL peak position, the inferred nanoplatelet thickness is 1.5 nm.

transmission electron microscopy (TEM) image, size analysis, and absorption and photoluminescence (PL) spectra of the CdSe nanoplatelets. In Figure 1a, where the nanoplatelets are laying face-down on the TEM grid, the rectangular shape can be seen. TEM image analysis of more than 2000 nanoplatelets revealed an average length of 21.9 ± 2.8 nm and width of 6.5 ± 1.2 nm. The lowest-energy excitonic absorption peak is centered at 547 nm. Previous high-resolution TEM studies have shown that CdSe nanoplatelets absorbing at this energy consist of 6 layers of Cd atoms and 5 layers of Se atoms, resulting in a total thickness of 1.5 nm with effectively no variation in nanoplatelet thickness within the ensemble.3,9 To prepare monolayer CdSe nanoplatelet films, we used the liquid interface self-assembly method of Dong et al.28 Briefly, as shown in the left panel of Figure 2, a small amount of the CdSe nanoplatelet/hexane dispersion was added to the surface of a diethylene glycol (DEG) filled Teflon well. Following evaporation of the hexane solvent, a self-assembled monolayer of nanoplatelets remained on the DEG surface, which could be subsequently deposited onto a presubmerged solid substrate as the DEG was drained from the bottom of the well (see Supporting Information for details). We found that adding excess oleic acid to the DEG subphase was key to controlling the type of assembly: higher concentrations of oleic acid favored face-down assemblies (Figure 2, top), whereas lower concentrations of oleic acid favored the edge-up configuration 3838


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Figure 2. Interfacial self-assembly of CdSe nanoplatelet monolayer films. Left panel: Illustration of the equipment used for self-assembly and schematic of the face-down and edge-up configurations. (a,b) TEM images of the face-down and edge-up assemblies, respectively. (c,d) GISAXS patterns of the face-down and edge-up assemblies, respectively. DEG = diethylene glycol.

CdSe nanoplatelets. Spherically symmetric nanocrystals have a degenerate transition dipole oriented isotropically over the unit sphere. Anisotropic nanocrystals, however, exhibit polarized absorption and emission due to dielectric confinement effects and anisotropies in their underlying crystal structure.32−36 For instance, wurtzite CdSe nanocrystals exhibit a degenerate transition dipole oriented isotropically in the plane perpendicular to the c-axis of the unit cell,32 while elongated nanorods and nanowires exhibit linear dipoles oriented parallel to the long axis of the nanostructure.33−36 The transition dipole in CdSe nanoplatelets is predicted to be oriented in the plane of the nanoplatelet due to strong dielectric confinement effects in these atomically thin structures,3,37 which is also observed in other 2D semiconductors. 30 In agreement with these predictions, Abécassis et al. observed emission polarized parallel to the nanoplatelet plane in CdSe nanoplatelet stacks.5 However, the distribution of the transition dipole orientation in CdSe nanoplatelets has not been directly measured. To determine the transition dipole orientation in our emissive CdSe nanoplatelet films, we used back focal plane imaging (BFP). Lieb et al. have used this technique to determine the dipole orientations of single molecules,38 and Schuller et al. used it to resolve the dipole orientation in monoand a few layer MoS2.30 The technique is illustrated schematically in the left panel of Figure 3. A high numerical aperture microscope objective lens is used to resolve the angular distribution of fluorescence intensity from an emissive layer situated in the focal plane of the objective. Each point in the BFP pattern corresponds to a different angle of emission, defined by the photon momentum k and the orientation of a polarizer placed in front of the imaging array. The dipole orientation defines the emission polarization (s, p, or s−p mixed) with respect to the polarizer. Following the work of Schuller et al.,30 we model the BFP pattern as plane wave radiation originating from a thin dielectric layer (CdSe nanoplatelets) sandwiched between two semi-infinite media (air and quartz), see Figure S12. In the limit of D ≪ λ, where D

To further confirm the structure of the nanoplatelet assemblies, GISAXS was performed on assembled films on SiO2/Si substrates. As the area probed by the X-ray beam is approximately 0.5 mm2, these results further demonstrate the large-area consistency of the packing motif in these nanoplatelet films. Distinct patterns were observed for each of the two assembly types. As shown in Figure 2c, the face-down assembly showed a strong vertical GISAXS scattering peak centered at qx = 0 nm−1, qz = 1.06 nm−1. This scattering vector corresponds to a real space separation of 5.9 nm (2π/qz) in the surface normal direction, which we attribute to the distance between the inorganic nanoplatelet and the SiO2/Si substrate. This implies the formation of a self-assembled oleic acid/oleate monolayer on the SiO2 surface, so that the total separation between the inorganic CdSe nanoplatelet surface and the underlying SiO2 surface is the sum of the 1.5 nm thickness of the inorganic CdSe nanoplatelet and an additional ∼4 nm, which corresponds to the thickness of an oleic acid bilayer. Importantly, we also observed a weaker scattering peak (highlighted by the white arrow in Figure 2c) at a horizontal scattering vector of qx = 0.60 nm−1, which corresponds to a real space separation of 10.5 nm within the surface plane. This dimension agrees well with the average lateral separation between neighboring nanoplatelets in the face-down configuration observed via TEM (Figure S3). The GISAXS pattern for the edge-up assembly is shown in Figure 2d. There is a single strong lateral scattering peak at qz = 0 nm−1, qx = 1.20 nm−1, corresponding to a real space distance of 5.2 nm, which we attribute to the periodicity of face-to-face stacking of nanoplatelets in the edge-up assembly. Note that this dimension is slightly smaller than the expected 5.5 nm spacing, which may indicate some interdigitation of ligands on neighboring nanoplatelets. These results are consistent with Xray scattering studies on self-assembled GdF3 nanoplatelets.16 The ordered assemblies provided a unique opportunity to experimentally measure the transition dipole orientation in 3839


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Figure 3. Back focal plane imaging (BFP). (Left panel) A schematic of the optical system and simulated BFP images for pure in-plane and pure outof-plane dipole emission. (a) Experimental BFP image from the face-down assembly. (b) Simulated BFP image for pure in-plane dipole emission. (c) Comparison between experimental and simulated line cuts along the dashed white lines in panels a and b. (d) Experimental BFP image from the edge-up assembly. (e) Simulated BFP image for the case of a 50/50 mixture of in-plane and out-of-plane dipoles, that is, R = μa/μb = 1. (f) Comparison between experimental and simulated line cuts along the circle indicated by the dashed white lines in panels d and e.

is the emissive layer thickness and λ is the free space wavelength, the BFP pattern is insensitive to the emissive layer thickness and analytical solutions can be obtained (see Supporting Information).30 Angle-dependent reflection and interference effects, which are s- and p-polarization dependent, define the local density of optical states (LDOS). The LDOS, together with the distribution of dipole orientations within the emissive layer, determines the image pattern observed at the back focal plane.30,38 In-plane and out-of-plane dipoles generate distinctly different BFP patterns, as shown in the simulated BFP images in the bottom left-hand corner of the figure.30,38 In this experiment, the CdSe nanoplatelet film is situated on top of a thin microscope cover glass slip (serving as the sample substrate), and the fluorescence emission measured is that which is directed into the glass substrate toward the oil immersion objective (index-matched to the glass substrate), which is located beneath the sample. In addition to the transition dipole orientation, the BFP pattern is influenced by the refractive index of the emissive layer, the glass substrate, and the medium above, which in this case is air (see Supporting Information for details of simulations).30,39 Figure 3a shows the experimental BFP pattern for the facedown assembly, and Figure 3b shows the corresponding simulated pattern assuming 100% in-plane oriented dipoles. Figure 3c compares line cuts of the experimental and simulated results along s- and p-polarized directions, defined with respect to the polarizer, showing excellent agreement (see Supporting Information for comparison to other possible dipole orientations, including partial out-of-plane components). A distinctive feature of in-plane oriented dipoles is that the p-

polarized intensity is zero at k∥ = k0, where k∥ is the in-plane photon momentum parallel to the substrate and k0 is the photon momentum in air, due to complete destructive interference of this field component. For comparison, we also measured the BFP pattern for monolayer MoS2, an atomically thin 2D semiconductor known to have an in-plane oriented transition dipole (Figure S14).30 The strong agreement between these two measurements further reinforces our conclusion that transition dipoles in CdSe nanoplatelets are oriented in the plane of the nanoplatelet. In Figure 3d, we show the experimentally measured BFP pattern for CdSe nanoplatelets in the edge-up configuration. Unlike the BFP pattern for the face-down configuration shown in Figure 3a, the edge-up BFP pattern in Figure 3d resembles neither the pure in-plane nor the pure out-of-plane simulation. However, the pattern can be simulated accurately by assuming equal contributions of in-plane and out-of-plane dipole orientations, as shown in Figure 3e. In Figure 3f ,we compare the simulated and experimentally measured angular distributions along the white dashed lines shown in Figure 3d,e. As shown in Figure 1, the nanoplatelets have a rectangular shape with lateral dimensions of 6.5 nm × 21.9 nm. In principle, this in-plane anisotropy could cause the dipole to become polarized along the long axis of the nanoplatelet as in rods and wires.33−35 However, the BFP pattern is best simulated assuming an isotropic dipole distribution within the nanoplatelet plane. Because the nanoplatelets lay on their long edge in the edge-up assembly (Figure 2b), orientation of the dipole along the long axis of the nanoplatelet would result in a BFP pattern yet again identical to the in-plane example of Figure 3a,b, which is not what is observed in Figure 3d. 3840


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Nano Letters To further quantify the orientation distribution of dipoles in the nanoplatelet plane, we simulated different combinations of long-axis (referred to here as the a-axis) and short-axis (referred to here as the b-axis) dipole contributions for the edge-up assembly, shown in Figure 3f.30 In these simulations, there are only two adjustable parameters: (1) the refractive index nN of the nanoplatelet film and (2) the ratio R = μa/μb of the dipole strength along the long (μa) and short (μb) lateral dimensions of the nanoplatelet, as illustrated in Figure 3f. The refractive index nN was calculated to be 1.67 using Bruggeman effective medium theory40 (see Supporting Information). Using this value for the refractive index, we found good agreement between experiment and simulation with R values close to 1, as shown in Figure 3d−f. Simulations with R values as small as 0.8 and as large as 1.2 already showed clear deviation from the experimental data, bounding the long axis dipole projection between 45% and 55% of the total emission. Combining these results with theory of quantum well structures,41 we feel confident concluding that CdSe nanoplatelets with the lateral dimensions studied here have a degenerate emission dipole oriented isotropically in the plane of the nanoplatelet. The difference in transition dipole orientation between the face-down and edge-up assemblies leads to different photoluminescence (PL) decay dynamics. Figure 4a compares

Rowland et al. observed fast picosecond Förster energy transfer in binary nanoplatelet films with the nanoplatelet donor and acceptor in a cofacial arrangement37 while Kunneman et al. found that a subportion of nanoplatelets act as quenching sites.42 Similarly, Guzelturk et al. observed that nanoplatelet stacking decreases the overall photoluminescence quantum yield and accelerates the transient PL decay.44 These works together suggest that excitons can migrate through nanoplatelet arrays via efficient Förster energy transfer and reach quenching sites before recombining radiatively. Förster energy transfer depends on the dipole orientation, the distance rDA between the donor and acceptor, and the donor and acceptor geometry, among other factors.45−47 The energy transfer rate, kET, is proportional to the dynamically averaged orientation factor ⟨κ2⟩,47 which can be calculated by integrating over the relative dipole orientation angle.48,49 In the face-down assembly, there can be three dipole alignments: parallel, head to head, and crossed with κ2 of 1, 4, and 0, respectively.47 In the edge-up assembly, there can be only parallel and crossed, as illustrated in Figure 4b. Taking the distribution probability into account, ⟨κ2⟩ = 5/4 when the nanoplatelets are laying side-by-side as in the face-down assembly and ⟨κ2⟩ = 1/2 when nanoplatelets are stacked faceto-face as in the edge-up assembly. The theoretical distance scaling of energy transfer when nanoplatelets are stacked face-to-face is kET ∝ rDA−2, where rDA = 5.2 nm is the center-to-center spacing in the edge-up assembly.46 A simple power-law distance scaling relationship does not exist for the side-by-side configuration but the interaction can be understood to drop off more quickly with increasing separation.45,46 Moreover, the center-to-center spacing in the face-down assembly, 10.5 nm, is twice as large as the separation in the edge-up assembly. These two factors contribute to substantially reduced rates of exciton diffusion in the face-down assembly, resulting in a longer exciton lifetime and slower PL decay. Dynamics in the thicker drop-cast films (Figure 3a) resemble dynamics in the edge-up assembly, because thicker films will always include some face-to-face stacking regardless of the orientation of the nanoplatelets with respect to the substrate. We have shown that by introducing soluble ligand into the DEG subphase during liquid interface assembly, films composed entirely of face-down nanoplatelets can be achieved. Moreover, we have shown that the transition dipole in these films is oriented entirely in the surface plane. Controlling the emitter dipole orientation has been an effective strategy for increasing light outcoupling efficiency in organic LEDs.14,15 However, similar strategies have not yet been applied to nanocrystal LEDs. Record-efficiency quantum dot (QD) LEDs have reached the theoretical limit for an isotropic emitter of ∼20% external quantum efficiency (EQE), with internal quantum efficiencies (IQE) of nearly 100%.50,51 Further improvements cannot be achieved without increasing the light outcoupling efficiency in these devices. Oriented nanoplatelet films, such as the ones demonstrated here, may offer a path forward. Furthermore, oriented films may increase light absorption efficiency in nanocrystal solar cells, photodetectors, and increase the performance of nanoplatelet lasers.13,52−54 Methods. Nanoplatelet Synthesis. CdSe nanoplatelets with oleic acid (OA) capping ligands were synthesized according to a previously reported method with some modifications.9,31 Specifically, selenium powder was replaced by a 0.1 M Se/1octanedecene solution (Se-ODE)55 (see Supporting Informa-

Figure 4. (a) Transient photoluminescence spectra for the face-down assembly, edge-up assembly, and drop-casted film. The inset shows decay kinetics at early times. (b) The orientation factor κ2 for different dipole alignments and center-to-center spacing in each configuration.

transient PL decay data collected from the face-down assembly, the edge-up assembly, and a drop-cast film without intentional control of the nanoplatelet orientation. The face-down assembly exhibits slower PL decay dynamics than the other two samples. We attribute this longer exciton lifetime to suppressed exciton diffusion to quenching sites in the facedown assemblies.37,42−44 3841


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Nano Letters tion (SI) for step-by-step procedures). Following the unmodified synthesis, we noticed the presence of thicker particles that were square or triangular in shape (see Figure S1 in the SI), which could not be easily removed by selective precipitation. These particles, which were not noticeable in absorption or photoluminescence spectra but were easily observed in TEM images, were particularly problematic for self-assembly because they formed defect islands in selfassembled nanoplatelet films, as shown in Figure S1b in the SI. Using the Se-ODE solution as a substitute precursor significantly avoided the occurrence of these particles, as evidenced by their absence in multiple TEM images from both drop-cast (Figure 1a and Figures S2 and S3) and self-assembled (Figure 2 and Figures S5−S11) layers. Additionally, the modified synthesis allowed more precise tuning of the nanoplatelet size, as shown in Figure S2 in the SI. TEM. TEM was performed on a FEI Tecnai Multipurpose TEM operating at 120 kV. GISAXS. GISAXS measurements were performed at the D1 beamline of the Cornell High Energy Synchrotron Source (CHESS). The X-ray beam was produced by a hardbent dipole magnet and a Mo/B4C multilayer double-bounce monochromator with the radiation having wavelength of 1.157 Å at a bandwidth of 1.5%. The GISAXS patterns were collected on a DECTRIS Pilatus3 200 K detector with a 4 s exposure time and incident X-ray beam angle of 0.3°. The sample-to-detector distance was calibrated using a silver behenate standard. Integration and processing of the patterns were performed in MATLAB. Samples were prepared using the self-assembly method described in the text and deposited on SiO2/Si wafers. Back-Focal Plane Imaging. A 405 nm diode laser (LDHDC-405M, Picoquant) operating in CW mode was focused by an oil-immersion objective (Nikon CFI Plan Apo Lambda DM 60× Oil) onto the CdSe nanoplatelet assembly, which was supported on top of a 0.13 mm thick glass cover slide. Nanoplatelet fluorescence was collected in the epi configuration by the same objective. The fluorescence passed through a dichroic mirror, the 200 mm focal length tube lens inside the microscope (Nikon Ti-U, inverted), an additional 50 mm focal length lens, polarizer, and was collected by a monochrome charge-coupled device camera (QICLICK-R-F-M-12), as schematically demonstrated in Figure 3. Transient Photoluminescence. Photoluminescence lifetime measurements were performed using time-correlated singlephoton counting (TCSPC) in an inverted optical microscope (Nikon Ti-U). The samples were excited using a 405 nm pulsed laser diode (LDHDC-405M, Picoquant, 1 MHz repetition rate, 0.5 ns pulse duration). The photoluminescence was collected in the epi configuration, passed through a dichroic mirror and long-pass filter, and focused onto a Si avalanche photodiode (Micro Photon Devices, PDM50). The detector was connected to a counting board for time correlated single photon counting (Picoquant, PicoHarp 300).

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the self-assembled CdSe nanoplatelet films, and details of the orientation factor integration calculation (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: 617.253.4975. Fax: 617.258.5766. ORCID

William A. Tisdale: 0000-0002-6615-5342 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Aaron J. Goodman and Daniel N. Congreve for monolayer MoS2 sample preparation and helpful discussions. This work was supported as part of the Center for Excitonics, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Award Number DE-SC0001088 (MIT). Y.G. was partially supported by the Office of China Postdoctoral Council under the international Postdoctoral Exchange Fellowship Program No. 20140040. GISAXS experiments were performed at the Cornell High Energy Synchrotron Source (CHESS), which is supported by the National Science Foundation and the National Institutes of Health/National Institute of General Medical Sciences under NSF award DMR1332208. Electron microscopy was performed at the MRSEC Shared Experimental Facilities at MIT, supported by the National Science Foundation under award number DMR-0819762.

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REFERENCES

(1) Kovalenko, M. V.; Manna, L.; Cabot, A.; Hens, Z.; Talapin, D. V.; Kagan, C. R.; Klimov, V. I.; Rogach, A. L.; Reiss, P.; Milliron, D. J.; Guyot-Sionnnest, P.; Konstantatos, G.; Parak, W. J.; Hyeon, T.; Korgel, B. A.; Murray, C. B.; Heiss, W. ACS Nano 2015, 9, 1012− 1057. (2) Ithurria, S.; Dubertret, B. J. Am. Chem. Soc. 2008, 130, 16504− 16505. (3) Ithurria, S.; Tessier, M. D.; Mahler, B.; Lobo, R. P. S. M.; Dubertret, B.; Efros, A. L. Nat. Mater. 2011, 10, 936−941. (4) Lhuillier, E.; Pedetti, S.; Ithurria, S.; Nadal, B.; Heuclin, H.; Dubertret, B. Acc. Chem. Res. 2015, 48, 22−30. (5) Abécassis, B.; Tessier, M. D.; Davidson, P.; Dubertret, B. Nano Lett. 2014, 14, 710−715. (6) Zhang, F.; Wang, S.; Wang, L.; Lin, Q.; Shen, H.; Cao, W.; Yang, C.; Wang, H.; Yu, L.; Du, Z.; Xue, J.; Li, L. S. Nanoscale 2016, 8, 12182−12188. (7) Fan, F.; Kanjanaboos, P.; Saravanapavanantham, M.; Beauregard, E.; Ingram, G.; Yassitepe, E.; Adachi, M. M.; Voznyy, O.; Johnston, A. K.; Walters, G.; Kim, G.-H.; Lu, Z.-H.; Sargent, E. H. Nano Lett. 2015, 15, 4611−4615. (8) Chen, Z.; Nadal, B.; Mahler, B.; Aubin, H.; Dubertret, B. Adv. Funct. Mater. 2014, 24, 295−302. (9) She, C.; Fedin, I.; Dolzhnikov, D. S.; Demortière, A.; Schaller, R. D.; Pelton, M.; Talapin, D. V. Nano Lett. 2014, 14, 2772−2777. (10) Guzelturk, B.; Kelestemur, Y.; Olutas, M.; Delikanli, S.; Demir, H. V. ACS Nano 2014, 8, 6599−6605. (11) Olutas, M.; Guzelturk, B.; Kelestemur, Y.; Yeltik, A.; Delikanli, S.; Demir, H. V. ACS Nano 2015, 9, 5041−5050. (12) Flatten, L. C.; Christodoulou, S.; Patel, R. K.; Buccheri, A.; Coles, D. M.; Reid, B. P. L.; Taylor, R. A.; Moreels, I.; Smith, J. M. Nano Lett. 2016, 16, 7137−7141.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.nanolett.7b01237. Additional experiment details, additional TEM and SEM images, BFP simulations, Bruggeman effective medium theory calculation of the effective dielectric constant of 3842


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Nano Letters (13) Grim, J. Q.; Christodoulou, S.; Di Stasio, F.; Krahne, R.; Cingolani, R.; Manna, L.; Moreels, I. Nat. Nanotechnol. 2014, 9, 891− 895. (14) Brütting, W.; Frischeisen, J.; Schmidt, T. D.; Scholz, B. J.; Mayr, C. Phys. Status Solidi A 2013, 210, 44−65. (15) Kim, S.-Y.; Jeong, W.-I.; Mayr, C.; Park, Y.-S.; Kim, K.-H.; Lee, J.-H.; Moon, C.-K.; Brütting, W.; Kim, J.-J. Adv. Funct. Mater. 2013, 23, 3896−3900. (16) Paik, T.; Ko, D.-K.; Gordon, T. R.; Doan-Nguyen, V.; Murray, C. B. ACS Nano 2011, 5, 8322−8330. (17) Diroll, B. T.; Greybush, N. J.; Kagan, C. R.; Murray, C. B. Chem. Mater. 2015, 27, 2998−3008. (18) Paik, T.; Diroll, B. T.; Kagan, C. R.; Murray, C. B. J. Am. Chem. Soc. 2015, 137, 6662−6669. (19) Weidman, M. C.; Smilgies, D.-M.; Tisdale, W. A. Nat. Mater. 2016, 15, 775−781. (20) Vanmaekelbergh, D. Nano Today 2011, 6, 419−437. (21) Cunningham, P. D.; Souza, J. B.; Fedin, I.; She, C.; Lee, B.; Talapin, D. V. ACS Nano 2016, 10, 5769−5781. (22) Miszta, K.; de Graaf, J.; Bertoni, G.; Dorfs, D.; Brescia, R.; Marras, S.; Ceseracciu, L.; Cingolani, R.; van Roij, R.; Dijkstra, M.; Manna, L. Nat. Mater. 2011, 10, 872−876. (23) Granados del Á guila, A.; Jha, B.; Pietra, F.; Groeneveld, E.; de Mello Donegá, C.; Maan, J. C.; Vanmaekelbergh, D.; Christianen, P. C. M. ACS Nano 2014, 8, 5921−5931. (24) Pietra, F.; Rabouw, F. T.; van Rhee, P. G.; van Rijssel, J.; Petukhov, A. V.; Erné, B. H.; Christianen, P. C. M.; de Mello Donegá, C.; Vanmaekelbergh, D. ACS Nano 2014, 8, 10486−10495. (25) Baker, J. L.; Widmer-Cooper, A.; Toney, M. F.; Geissler, P. L.; Alivisatos, A. P. Nano Lett. 2010, 10, 195−201. (26) Rizzo, A.; Nobile, C.; Mazzeo, M.; Giorgi, M. D.; Fiore, A.; Carbone, L.; Cingolani, R.; Manna, L.; Gigli, G. ACS Nano 2009, 3, 1506−1512. (27) Antanovich, A.; Prudnikau, A.; Matsukovich, A.; Achtstein, A.; Artemyev, M. J. Phys. Chem. C 2016, 120, 5764−5775. (28) Dong, A.; Chen, J.; Vora, P. M.; Kikkawa, J. M.; Murray, C. B. Nature 2010, 466, 474−477. (29) Evers, W. H.; Goris, B.; Bals, S.; Casavola, M.; de Graaf, J.; Roij, R. v.; Dijkstra, M.; Vanmaekelbergh, D. Nano Lett. 2013, 13, 2317− 2323. (30) Schuller, J. A.; Karaveli, S.; Schiros, T.; He, K.; Yang, S.; Kymissis, I.; Shan, J.; Zia, R. Nat. Nanotechnol. 2013, 8, 271−276. (31) Tessier, M. D.; Javaux, C.; Maksimovic, I.; Loriette, V.; Dubertret, B. ACS Nano 2012, 6, 6751−6758. (32) Empedocles, S. A.; Neuhauser, R.; Bawendi, M. G. Nature 1999, 399, 126−130. (33) Hu, J.; Li, L.-s.; Yang, W.; Manna, L.; Wang, L.-w.; Alivisatos, A. P. Science 2001, 292, 2060. (34) Shabaev, A.; Efros, A. L. Nano Lett. 2004, 4, 1821−1825. (35) Hadar, I.; Hitin, G. B.; Sitt, A.; Faust, A.; Banin, U. J. Phys. Chem. Lett. 2013, 4, 502−507. (36) Tice, D. B.; Weinberg, D. J.; Mathew, N.; Chang, R. P. H.; Weiss, E. A. J. Phys. Chem. C 2013, 117, 13289−13296. (37) Rowland, C. E.; Fedin, I.; Zhang, H.; Gray, S. K.; Govorov, A. O.; Talapin, D. V.; Schaller, R. D. Nat. Mater. 2015, 14, 484−489. (38) Lieb, M. A.; Zavislan, J. M.; Novotny, L. J. Opt. Soc. Am. B 2004, 21, 1210−1215. (39) Taminiau, T. H.; Karaveli, S.; van Hulst, N. F.; Zia, R. Nat. Commun. 2012, 3, 979. (40) Bruggeman, D. A. G. Ann. Phys. 1935, 24, 416−664. (41) Davies, J. H. The Physics of Low-Dimensional Semiconductors: An Introduction; Cambridge University Press: 1998. (42) Kunneman, L. T.; Schins, J. M.; Pedetti, S.; Heuclin, H.; Grozema, F. C.; Houtepen, A. J.; Dubertret, B.; Siebbeles, L. D. A. Nano Lett. 2014, 14, 7039−7045. (43) Rabouw, F. T.; van der Bok, J. C.; Spinicelli, P.; Mahler, B.; Nasilowski, M.; Pedetti, S.; Dubertret, B.; Vanmaekelbergh, D. Nano Lett. 2016, 16, 2047−2053.

(44) Guzelturk, B.; Erdem, O.; Olutas, M.; Kelestemur, Y.; Demir, H. V. ACS Nano 2014, 8, 12524−12533. (45) Hernández-Martínez, P. L.; Govorov, A. O.; Demir, H. V. J. Phys. Chem. C 2013, 117, 10203−10212. (46) Liu, X.; Qiu, J. Chem. Soc. Rev. 2015, 44, 8714−8746. (47) Medintz, I.; Hildebrandt, N. FRET - Förster Resonance Energy Transfer: From Theory to Applications; Wiley-VCH Verlag GmbH & Co. KGaA, 2014. (48) Baumann, J.; Fayer, M. D. J. Chem. Phys. 1986, 85, 4087−4107. (49) Steinberg, I. Z. J. Chem. Phys. 1968, 48, 2411−2413. (50) Mashford, B. S.; Stevenson, M.; Popovic, Z.; Hamilton, C.; Zhou, Z.; Breen, C.; Steckel, J.; Bulovic, V.; Bawendi, M.; Coe-Sullivan, S.; Kazlas, P. T. Nat. Photonics 2013, 7, 407−412. (51) Dai, X.; Zhang, Z.; Jin, Y.; Niu, Y.; Cao, H.; Liang, X.; Chen, L.; Wang, J.; Peng, X. Nature 2014, 515, 96−99. (52) She, C.; Fedin, I.; Dolzhnikov, D. S.; Dahlberg, P. D.; Engel, G. S.; Schaller, R. D.; Talapin, D. V. ACS Nano 2015, 9, 9475−9485. (53) Oertel, D. C.; Bawendi, M. G.; Arango, A. C.; Bulović, V. Appl. Phys. Lett. 2005, 87, 213505. (54) Lan, X.; Voznyy, O.; García de Arquer, F. P.; Liu, M.; Xu, J.; Proppe, A. H.; Walters, G.; Fan, F.; Tan, H.; Liu, M.; Yang, Z.; Hoogland, S.; Sargent, E. H. Nano Lett. 2016, 16, 4630−4634. (55) Bullen, C.; van Embden, J.; Jasieniak, J.; Cosgriff, J. E.; Mulder, R. J.; Rizzardo, E.; Gu, M.; Raston, C. L. Chem. Mater. 2010, 22, 4135−4143.

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CdSe Nanoplatelet Films with Controlled Orientation of their

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