Anisotropic Photoluminescence from Isotropic Optical Transition

Jul 9, 2018 - Department of Chemistry and James Franck Institute, University of Chicago, Chicago, Illinois 60637, United ... states are doubly degener...
0 downloads 0 Views 2MB Size
Download PDF
Letter Cite This: Nano Lett. 2018, 18, 4647−4652

pubs.acs.org/NanoLett

Anisotropic Photoluminescence from Isotropic Optical Transition Dipoles in Semiconductor Nanoplatelets Xuedan Ma,*,† Benjamin T. Diroll,† Wooje Cho,‡ Igor Fedin,‡ Richard D. Schaller,†,§ Dmitri V. Talapin,†,‡ and Gary P. Wiederrecht† †

Center for Nanoscale Materials, Argonne National Laboratory, Argonne, Illinois 60439, United States Department of Chemistry and James Franck Institute, University of Chicago, Chicago, Illinois 60637, United States § Department of Chemistry, Northwestern University, Evanston, Illinois 60208, United States Nano Lett. 2018.18:4647-4652. Downloaded from pubs.acs.org by UNIV OF SOUTH DAKOTA on 10/26/18. For personal use only.

‡

S Supporting Information *

ABSTRACT: Many important light-matter coupling and energy-transfer processes depend critically on the dimensionality and orientation of optical transition dipoles in emitters. We investigate individual quasi-two-dimensional nanoplatelets (NPLs) using higher-order laser scanning microscopy and find that absorption dipoles in NPLs are isotropic in three dimensions at the excitation wavelength. Correlated polarization studies of the NPLs reveal that their emission polarization is strongly dependent on the aspect ratio of the lateral dimensions. Our simulations reveal that this emission anisotropy can be readily explained by the electric field renormalization effect caused by the dielectric contrast between the NPLs and the surrounding medium, and we conclude that emission dipoles in NPLs are isotropic in the plane of the NPLs. Our study presents an approach for disentangling the effects of dipole degeneracy and electric field renormalization on emission anisotropy and can be adapted for studying the intrinsic optical transition dipoles of various nanostructures. KEYWORDS: CdSe/CdS core/shell nanoplatelets, optical transition dipole, higher-order Bessel-Gauss beam, emission anisotropy, electric field renormalization effect

Q

crystallographic c-axis, giving rise to a two-fold degenerate transition dipole.12−14 For most of these systems, single particle polarization spectroscopy has been utilized as an efficient tool in determining the dimensionality and orientation of the transition dipoles.13,15 Compared to organic dye molecules and spherical QDs, direct determination of transition dipoles in NPLs using polarization spectroscopy is nontrivial. This is because aside from the influence of the intrinsic transition dipole properties, renormalization effect caused by the dielectric contrast between the NPLs and the surrounding medium can cause different degrees of internal electric field attenuation for light polarized along different directions,16 leading to polarization-dependent optical transition probabilities of the NPLs. Moreover, because the transition dipoles are strongly related to the corresponding excitonic states among which the optical transitions happen, the absorption and emission transition dipoles of the NPLs could be substantially different.13 In this Letter, we address these questions by studying the absorption and emission dipoles of core/shell CdSe/CdS NPLs using single particle spectroscopy. The dimensionality and orientation of the absorption dipoles are studied by using higher-order Bessel-Gauss laser beams, which when combined with confocal microscopy are highly sensitive to surface-field

uasi-two-dimensional nanoplatelets (NPLs) possess extremely narrow spectral features due to their nearperfect monodispersity in the quantum-confined thickness dimension.1,2 Their extended plate geometry leads to large exciton coherence size and giant oscillator strength,2−4 making NPL-based optoelectronic devices promising for highly efficient dipole coupling and energy transfer processes.5,6 NPLs are also potentially interesting candidates for single photon sources in quantum information processing due to their lifetime-limited spectral line width3,7 and lateral sizedependent biexciton quantum yield.4,8 A recent demonstration9 of strong light-matter interaction between NPLs and planar microcavities at room temperature advocates the promising application of NPLs in integrated quantum photonic devices. To leverage these exceptional properties of NPLs for highperformance optoelectronic and quantum photonic applications, knowledge of their transition dipole moment is essential for efficient dipole−dipole and dipole−cavity mode coupling. Optical transition dipoles of materials can vary significantly depending on their band structures and compositions. Many organic dye molecules exhibit linear absorption and emission dipoles10,11 following the electric dipole approximation. In some larger quantum confined systems, the optical matrix elements and optical transition selection rules deviate from the simple electric dipole approximation. In the specific case of wurtzite CdSe quantum dots (QDs), the lowest-lying emissive states are doubly degenerate in the plane perpendicular to the © 2018 American Chemical Society

Received: January 24, 2018 Revised: June 5, 2018 Published: July 9, 2018 4647

DOI: 10.1021/acs.nanolett.8b00347 Nano Lett. 2018, 18, 4647−4652

Letter

Nano Letters gradients.17 Comparing the experimentally obtained excitation patterns by raster scanning individual NPLs through the higher-order laser beams with those from vector-field simulations, we find that the absorption dipoles in NPLs are isotropic in three dimensions (3D), disregarding their lateral dimensions. Further investigation of the emission dipoles of the NPLs using correlated polarization spectroscopy reveals that emission anisotropy of the NPLs is strongly dependent on the aspect ratio of the lateral dimensions. Numerical evaluation of the influences of dipole degeneracy and renormalization effects on the emission anisotropy reveals that dielectric contrast-induced electric field renormalization is the main reason for the observed emission anisotropy and the NPLs possess two-dimensional (2D) degenerate emission dipoles. These findings can provide new insights into the design of efficient optoelectronic and hybrid photonic devices based on NPLs. Four monolayer thick (∼1.2 nm) CdSe core NPLs with various lateral sizes (∼70 to ∼440 nm2) were coated with 2 monolayer CdS conformal shells using a previously published method.18 PL spectra of the core/shell NPLs have a peak at ∼620 nm with no apparent dependence on the lateral dimensions (Supporting Information S1). For single NPL optical measurements, diluted NPL suspensions were spin coated on glass cover slides and loaded onto a home-built confocal laser scanning microscope. To study the dimensionality and orientation of the absorption dipoles, individual NPLs were raster scanned at the speed of 10 ms/pixel through higher-order Bessel-Gauss laser beams to obtain the excitation patterns. A polarization converter was used to mode convert pulsed Gaussian laser beams (1 MHz) with a wavelength of 400 nm into higher-order azimuthally polarized beams. The mean excitation power was kept below ∼5 nW, which is far below the multiexciton regime. Excitation patterns of individual emitters obtained using higher-order laser beams are characteristic of the dimensionality and orientation of their absorption dipoles. This approach has been applied to study absorption dipoles of molecules,19 SiO2 nanoparticles,20 and metallic nanoparticles.21 Figure 1b and its inset show representative doughnut-shaped excitation

patterns of NPLs by scanning them through the azimuthally polarized laser beams (see Supporting Information S2 for more examples). The NPLs experienced PL blinking during the scan, leading to a few dark pixels imposed on top of the excitation patterns. Change in the NPL lateral area has negligible influence on the excitation patterns. To determine the dimensionality and orientation of the absorption dipoles from the experimentally obtained excitation patterns, we use a vector field simulation method22,23 implemented in a homewritten Matlab program to calculate the expected excitation patterns of different absorption dipoles (see Supporting Information S3 for a detail description of the simulation method). In the simulation, we assume that the laser beam propagates along the z-axis (Figure 1a). Because azimuthally polarized laser beams do not create a longitudinal component and the total field is transverse,17 the excitation pattern of an emitter is determined by the projection of its absorption dipole in the xy-plane. This projection is dependent on the relative angle α between the transition dipole plane and xy-plane as well as the relative amplitudes of the orthogonal composition dipole components. With the increase of angle α, the excitation pattern of a 2D degenerate dipole undergoes a continuous change from the peculiar doughnut-shape to two bright off-axis lobes (Figure 1d), whereas that of a 2D anisotropic dipole persistently presents two bright off-axis lobes (Supporting Information S4). The situation for an emitter with a 3D absorption dipole is fundamentally different. For a 3D degenerate dipole, its excitation pattern remains the same doughnut-shape regardless of its orientation (Figure 1e). However, excitation patterns of an anisotropic 3D absorption dipole with unequal orthogonal dipole component amplitudes could either be doughnut-shaped or two off-axis lobes depending on its relative orientation (Figure S5). These simulation results indicate that the experimentally observed excitation patterns of the NPLs result from absorption dipoles with isotropic projections in the xy-plane. These could either be isotropic 2D in-plane dipoles (Figure 1d) or 3D dipoles with isotropic xy-plane projections (Figures 1e and S5). Because the NPLs lie mostly face-down on flat glass cover slides, determination of the exact 3D nature of their absorption dipoles based on these experimental results is not conclusive. We therefore introduce certain degrees of roughness to the glass cover slides by dry etching them with SF6 (Figure 2(b,d)). The roughness is controlled by the etch time in such a way that it causes negligible laser beam distortion but various NPL orientations. To quantitatively determine the surface roughness, we estimate the values of α from atomic force microscopy measurements by assuming that tan α = Δy/Δx. Calculation of the α-value shows that the etched substrates have an average α value of ∼30° with more than 10% of the surface having a corresponding α-value >45° and more than 23% with α > 3° (see Supporting Information S5 for details). Although the detailed orientations of each individual NPLs remain unknown, statistically the surface roughness would lead to some of the NPLs lying with certain tilt angles with respect to the substrates. In this way, if the absorption dipoles of the NPLs are only 2D degenerate, various excitation patterns and unequal intensity profiles along different directions (Figure 1d and Supporting Information Figure S4) should be observed from the NPLs dispersed on the etched substrates. However, scanning PL images of more than 150 NPLs from five samples with different lateral dimensions show the same doughnutshaped excitation pattern with similar intensity profiles along

Figure 1. (a) Coordinate system used in this study. (b) Representative scanning PL images of the NPLs. (c) PL intensity profiles from experiment (dots) and simulation (curve). (d,e) Simulated excitation patterns of an emitter with a 2D (d) and 3D (e) degenerate absorption dipole at different tilt angles α. The 2D and 3D degenerate dipoles are simulated as two and three superimposed perpendicular linear dipoles. Scale bars: 1 μm. 4648

DOI: 10.1021/acs.nanolett.8b00347 Nano Lett. 2018, 18, 4647−4652

Letter

Nano Letters

Figure 2. (a,b) AFM images of a flat glass cover slide and a glass cover slide after being drying etched with SF6 for 3 min. (c,d) Cross-section height curves for the glass cover slides in (a,b).

different directions (Figures 1b, S3, and S4). Moreover, a reasonable agreement between the experimental intensity profiles and simulated profiles of a 3D degenerate dipole can be obtained (Figure 1c). These findings reveal that regardless of the NPL orientations, projections of their absorption dipoles in the xy-plane are always isotropic. To fulfill this condition, absorption dipoles of the NPLs at the excitation wavelength need to be 3D isotropic. This isotropic absorption dipole is most likely due to the relatively high density of electronic states at the excitation wavelength of 400 nm.24 Having determined the 3D isotropic nature of the absorption dipoles of the NPLs at the excitation wavelength, we further investigate their emission transition dipoles. To first order, the optical transition strength of an emitter is proportional to |μ·E |2, where μ is the transition dipole vector, and E is the electric field vector of the absorbed or emitted light. For a linear transition dipole oriented at a tilt angle α (the angle between the transition dipole and xy-plane) and an in-plane angle φ (the angle between the transition dipole and the light electric field) (Figure 3a), its emission intensity is proportional to its projection along the light electric field direction, that is, cos2 α cos2 φ. Rotating the polarization in the collection beam path using a linear polarizer allows the detection of the polarization degree, which is defined as P = (Imax − Imin)/(Imax + Imin), of 100% with the maximum and minimum intensities being Imax= |μ |2·|E|2·cos2 α and Imin = 0, respectively. If the emitter possesses a 2D emission dipole (Figure 3b), its emission intensity is still dependent on the tilt angle α and the in-plane angle φ, except that here φ refers to the angle between one of the transition dipole components and the light electric field. Recent ensemble studies25,26 of CdSe NPL thin films have found that their emission dipoles were oriented within the plane with no measurable out-of-plane component. Therefore, for a NPL with an in-plane 2D emission dipole lying on a flat glass cover slide, the corresponding value of α is close to 0°. Rotating a linear polarizer in the collection beam path leads to maximum and minimum emission intensities of Imax = |μ∥ |2·|E∥|2 and Imin = |μ⊥ |2·|E⊥|2 with μ∥, μ⊥ and E∥, E⊥ being the transition dipole and light electric field components along the long and short axes, respectively. By defining a dipole degeneracy factor η = |μ∥ |2/|μ⊥ |2, the polarization degree is given by

Figure 3. (a,b) Projections of a linear (a) and 2D degenerate (b) transition dipole onto the sample plane. (c,d) PL timetraces of a NPL (ρ = 3.6) recorded by the detector behind the rotating linear polarizer (c) and the detector without the linear polarizer (d). (e) Ratio of the intensity curves in (c,d). (f,g) PL timetraces of a NPL (ρ = 1.5) recorded by the two detectors ((f) with the linear polarizer; (g) without the linear polarizer). (h) Ratio of the curves in (f,g). Marked areas indicate long ”dark” periods of the NPLs.

P=

(η ·|E |2 − |E⊥|2 ) (η ·|E |2 + |E⊥|2 )

(1)

Therefore, photoluminescence polarization is determined by both the dipole degeneracy factor η and the electric field intensities along different axes with P = 0 corresponding to isotropic emission. It is worth noting that because we excite the NPLs far above their emission energy with excitation intensities below saturation, in a first approximation we can treat the absorption and emission processes independently. The overall experimentally observed PL polarization P is hence the product of the absorption and emission polarizations, that is, P = Pex·Pem with Pex and Pem being the polarization degrees of the absorption and emission processes. Although we have known from the above higher-order laser studies of the NPLs that their absorption dipoles are isotropic and the excitation dipole degeneracy factor ηex = 1, the emission dipole degeneracy factor ηem remains unknown and it could be fundamentally different from ηex due to the very different excitonic states involved in the two independent processes. For quantum emitters with negligible size effects (such as small molecules) or isotropic geometries (such as spherical quantum dots), PL polarization of individual emitters has been used as a direct indication of their intrinsic transition dipole 4649

DOI: 10.1021/acs.nanolett.8b00347 Nano Lett. 2018, 18, 4647−4652

Letter

Nano Letters degeneracy because for these emitters, |E∥| = |E⊥| in eq 1. However, in the case of NPLs which have anisotropic geometries, the difference in the dielectric constants of the NPLs and their surrounding medium can lead to a renormalization of the electric field inside the NPLs with respect to the homogeneous external electric field in the surrounding medium16,27 (i.e., |E∥| ≠ |E⊥|). This renormalization effect itself can alter the probability of optical transitions along different directions and lead to a dependence of the absorption and emission on the incident light polarization. Thus, experimentally observed PL polarization of NPLs is a combined result of intrinsic optical transition dipole degeneracy and dielectric contrast-caused electric field renormalization effect. In the following, we present a general approach for disentangling these two effects and extracting intrinsic emission dipole degeneracy of a quantum emitter with an anisotropic geometry based on PL polarization spectroscopy. Because of PL blinking and PL fluctuation of the NPLs commonly attributed to charge carrier trapping at surface and/ or core/shell interface states,4,28 direct measurements of the polarization-dependent PL intensity was nontrivial. We therefore split the collected PL from the NPLs into two identical single photon avalanche diodes and place a linear polarizer in front of one of the diodes.15 This technique enables simultaneous measurements of the PL with the two detectors, and any PL intensity fluctuation/blinking that is not caused by the linear polarizer can be compensated by comparing the signals from the two detectors. Figure 3c,d shows representative PL timetraces of a NPL from a sample with an average length L of ∼15.9 nm and a width W of ∼4.4 nm detected by the two diodes. The timetrace in Figure 3c was periodically modulated by a linear polarizer at an angular speed of ∼42°/s. Despite the drastic PL fluctuation, the ratio between the two timetraces (Figure 3e) only reflects the polarization modulation and it can be fitted with a single sin2 φ function. Calculation of the polarization degree P gives a value of 23.2%. Figure 3f−h presents PL timetraces of a NPL from a sample with an average length of ∼16.2 nm and a width of ∼11.1 nm, and a polarization degree of 5.9% is obtained. We investigate polarization degrees of more than 180 NPLs from five samples with different lateral dimensions (see Supporting Information S6 for the distribution) and plot the average polarization degree values from each sample as a function of the corresponding aspect ratio (ρ = L/W) in Figure 4a. The emission polarization degree increases with the NPL lateral aspect ratio. Similar polarization behavior is observed for NPLs dispersed on the etched substrates, although the average polarization degree is slightly larger than those of the NPLs on flat substrates. Without knowing the detailed orientations of each individual NPL on the roughened substrates, we approximate their “effective” aspect ratio to be ρeff = L/(W· cos α). From the difference in the polarization degrees of the NPLs dispersed on the two different types of substrates, we can estimate that the tilt angle α of the NPLs on the etched substrates could vary between 0° and 90° with an average value of 21°, consistent with the values determined from the atomic force microscopy (AFM) measurements. However, local electromagnetic field variation caused by the roughened surface may also contribute to the discrepancy. Because of the lack of knowledge of the exact orientations of each individual NPLs, the observed discrepancy may be caused by either of the two or both effects.

Figure 4. (a) Aspect ratio dependent polarization degrees of the NPL emission from experiment (red dots, NPLs on flat glass cover slides; black dots, NPLs on dry-etched substrates) and simulation (dashed curve, FDTD simulation; solid curve, analytical calculation). (b−e) Local electric field |E| distributions at the excitation wavelength for a normal incident plane wave polarized along the NPL width ((b,d)) and length ((c,e)). The dimensions of the NPLs in (b,c) and (d,e) are 30 × 7.5 nm2 and 30 × 15 nm2, respectively.

According to eq 1 the observed emission anisotropy (P ≠ 0) might be attributed to (a) anisotropic emission dipoles (μem,∥ ≠ μem,⊥) determined by the band structure of the NPLs which is related to their dimension, composition, and crystal structure;29−32 (b) renormalization effect of the optical electric field inside the NPLs caused by the differences in the dielectric constants of the NPLs and the surrounding environment.33−35 In the second case, mismatch in the dielectric constants leads to different degrees of modulation to the optically induced electric field inside the emitter with the internal electric field for light polarized along the short axis (E⊥) attenuated more than that along the long axis (E∥).34 Because the optical transition rate is proportional to the local electric field inside the NPLs, this in turn results in the probability of optical transitions for light polarized along the short axis being smaller. To determine the cause of the aspect ratio-dependent PL polarization, we evaluate the influences of the dielectric contrast and dipole degeneracy by calculating the attenuated electric field intensities inside the NPLs using two approaches. In the first approach, we simulate the electric field intensity distributions inside the NPLs using the three-dimensional finite-difference time-domain (FDTD) method. Internal electric field distributions of two NPLs with aspect ratios ρ = 4 and 2 at the excitation wavelength are plotted in (Figure 4b−e). For both NPLs, the internal electric field is attenuated strongly when the light is polarized along the width, with the attenuation being even stronger for the one with the larger aspect ratio (Figure 4b). To determine the polarization degree using eq 1, we numerically calculate the overall internal electric field intensities in NPLs for light polarized along the NPL length (|E∥|2) and width (|E⊥|2) at both the excitation and emission wavelengths. In the second approach, we calculate the internal electric field analytically. For a dielectric ellipsoid with a dielectric constant ε embedded in a medium (εm), the internal electric field of the ellipsoid (E∥,⊥) with the light polarized parallel and perpendicular to the axial direction is given by E∥,⊥ = E0·(εm/(εm + (ε − εm)·n∥,⊥)),27,36 with n∥,⊥ being the depolarization factors that are defined by the semimajor dimensions of the ellipsoid (a, c) as 4650

DOI: 10.1021/acs.nanolett.8b00347 Nano Lett. 2018, 18, 4647−4652

Letter

Nano Letters n = n⊥ =

1 − e 2 ij 1 + e yz zz − 2e) (lnjj 2e 3 k1 − e { (1 − n ) 2

(2)

where e = 1 − a 2 /c 2 . Because the depolarization factors of a dielectric ellipsoid and rectangular plate are similar,37 we apply this method to calculate the internal electric field of the NPLs. In principle, from the simulated electric field intensities we can calculate the polarization degrees Pex and Pem at both the excitation and emission wavelengths and as a result the overall polarization degree P = Pex + Pem, provided that the corresponding dipole degeneracy factors ηex and ηem are known. Simulation results from both approaches are compared to the experimental values. Because we learn from the excitation patterns that the absorption dipole degeneracy factor ηex = 1, only the value of the emission dipole degeneracy factor ηem is adjusted for fitting. The best fitting result is achieved with ηem = 1 and the corresponding simulation results are plotted in Figure 4a (see Supporting Information S8 for other values of ηem). These findings reveal that the dielectric contrast induced renormalization effect in the NPLs is the main reason for the observed PL anisotropy. We also conclude that the 2D emission dipoles of NPLs are isotropic irrespective of their extended plate geometries. It is also worth mentioning that the origin of PL anisotropy in NPLs and QDs is fundamentally different despite the similar isotropic 2D emission dipoles in both systems. PL polarization of QDs is caused by the random orientations of the 2D emission dipoles with respect to the sample planes, whereas for NPLs it is mainly caused by the electric field renormalization effect due to their anisotropic geometries. In conclusion, we have shown that despite their 2D in-plane emission dipoles, NPLs possess 3D isotropic absorption dipoles at the excitation energy. This absorption isotropy is because of the averaging over many electronic states at the excitation energy.24 In contrast, the isotropic in-plane emission dipoles of the NPLs might be due to the heavy hole level dominant valence band top which possesses mixed in-plane px and py symmetry.26 Ultrafast thermalization of charge carriers after absorption leads to their loss of memory of the initial absorption polarization, consequently leading to decoupled absorption and emission polarizations. We also present a general approach for disentangling the effects of dipole degeneracy and renormalized electric field on PL anisotropy, which can be applied to a variety of systems for studying their intrinsic transition dipole properties. For NPLs, the aspect ratio-dependent PL polarization indicates that their lateral dimensions and the dielectric constant of the surrounding medium can be designed to achieve the most efficient dipole coupling/light extraction for specific applications. This, for instance, in combination with the size tunable LO phonon bottleneck in CdSe NPLs,7 can be applied toward highly polarized lasing applications.

■

Ensemble characterization of CdSe/CdS core/shell NPLs; experimentally obtained excitation patterns of NPLs; numerical simulation of the excitation patterns; simulated excitation patterns of absorption dipoles with different orthogonal amplitudes and orientations; distribution of α; polarization degrees of NPLs on the two different types of substrates; influence of CdS shell on the electric field distribution; emission dipole degeneracy factor dependent polarization anisotropy (PDF)

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Xuedan Ma: 0000-0002-3163-1249 Benjamin T. Diroll: 0000-0003-3488-0213 Richard D. Schaller: 0000-0001-9696-8830 Dmitri V. Talapin: 0000-0002-6414-8587 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS We thank Ralu Divan for her assistance in the dry etching process. This work was performed in part at the Center for Nanoscale Materials, a U.S. Department of Energy Office of Science User Facility, and supported by the U.S. Department of Energy, Office of Science, under Contract No. DE-AC0206CH11357. We also acknowledge support from NSF DMREF Program under awards DMR-1629601 and DMR1629383.

■

REFERENCES

(1) Son, J. S.; Wen, X.-D.; Joo, J.; Chae, J.; Baek, S.; Park, K.; Kim, J. H.; An, K.; Yu, J. H.; Kwon, S. G.; et al. Large-Scale Soft Colloidal Template Synthesis of 1.4 nm Thick CdSe Nanosheets. Angew. Chem., Int. Ed. 2009, 48, 6861−6864. (2) Ithurria, S.; Tessier, M. D.; Mahler, B.; Lobo, R. P. S. M.; Dubertret, B.; Efros, A. L. Colloidal nanoplatelets with twodimensional electronic structure. Nat. Mater. 2011, 10, 936−941. (3) Naeem, A.; Masia, F.; Christodoulou, S.; Moreels, I.; Borri, P.; Langbein, W. Giant exciton oscillator strength and radiatively limited dephasing in two-dimensional platelets. Phys. Rev. B: Condens. Matter Mater. Phys. 2015, 91, 121302. (4) Ma, X.; Diroll, B. T.; Fedin, I.; Cho, W.; Schaller, R. D.; Talapin, D. V.; Gray, S. K.; Wiederrecht, G. P.; Gosztola, D. J. Size-Dependent Biexciton Quantum Yields and Carrier Dynamics of Quasi-TwoDimensional Core/Shell Nanoplatelets. ACS Nano 2017, 11, 9119− 9127. (5) Rowland, C. E.; Fedin, I.; Zhang, H.; Gray, S. K.; Govorov, A. O.; Talapin, D. V.; Schaller, R. D. Picosecond energy transfer and multiexciton transfer outpaces Auger recombination in binary CdSe nanoplatelet solids. Nat. Mater. 2015, 14, 484−489. (6) Li, H.; Zhitomirsky, D.; Grossman, J. C. Tunable and Energetically Robust PbS Nanoplatelets for Optoelectronic Applications. Chem. Mater. 2016, 28, 1888−1896. (7) Achtstein, A. W.; Scott, R.; Kickhofel, S.; Jagsch, S. T.; Christodoulou, S.; Bertrand, G. H. V.; Prudnikau, A. V.; Antanovich, A.; Artemyev, M.; Moreels, I.; et al. p-State Luminescence in CdSe Nanoplatelets: Role of Lateral Confinement and a Longitudinal Optical Phonon Bottleneck. Phys. Rev. Lett. 2016, 116, 116802. (8) Tessier, M. D.; Spinicelli, P.; Dupont, D.; Patriarche, G.; Ithurria, S.; Dubertret, B. Efficient Exciton Concentrators Built from Colloidal

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.nanolett.8b00347. 4651

DOI: 10.1021/acs.nanolett.8b00347 Nano Lett. 2018, 18, 4647−4652

Letter

Nano Letters Core/Crown CdSe/CdS Semiconductor Nanoplatelets. Nano Lett. 2014, 14, 207−213. (9) 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. Strong Exciton-Photon Coupling with Colloidal Nanoplatelets in an Open Microcavity. Nano Lett. 2016, 16, 7137−7141. (10) Ha, T.; Enderle, T.; Chemla, D. S.; Selvin, P. R.; Weiss, S. Single Molecule Dynamics Studied by Polarization Modulation. Phys. Rev. Lett. 1996, 77, 3979−3982. (11) Sick, B.; Hecht, B.; Novotny, L. Orientational Imaging of Single Molecules by Annular Illumination. Phys. Rev. Lett. 2000, 85, 4482− 4485. (12) Efros, A. L.; Rosen, M.; Kuno, M.; Nirmal, M.; Norris, D. J.; Bawendi, M. Band-edge exciton in quantum dots of semiconductors with a degenerate valence band: Dark and bright exciton states. Phys. Rev. B: Condens. Matter Mater. Phys. 1996, 54, 4843−4856. (13) Empedocles, S. A.; Neuhauser, R.; Bawendi, M. G. Threedimensional orientation measurements of symmetric single chromophores using polarization microscopy. Nature 1999, 399, 126−130. (14) Brokmann, X.; Coolen, L.; Dahan, M.; Hermier, J. P. Measurement of the Radiative and Nonradiative Decay Rates of Single CdSe Nanocrystals through a Controlled Modification of their Spontaneous Emission. Phys. Rev. Lett. 2004, 93, 107403. (15) Koberling, F.; Kolb, U.; Philipp, G.; Potapova, I.; Basché, T.; Mews, A. Fluorescence Anisotropy and Crystal Structure of Individual Semiconductor Nanocrystals. J. Phys. Chem. B 2003, 107, 7463−7471. (16) Rodina, A. V.; Efros, A. L. Effect of dielectric confinement on optical properties of colloidal nanostructures. J. Exp. Theor. Phys. 2016, 122, 554−566. (17) Youngworth, K. S.; Brown, T. G. Focusing of high numerical aperture cylindrical-vector beams. Opt. Express 2000, 7, 77. (18) Ithurria, S.; Talapin, D. V. Colloidal Atomic Layer Deposition (c-ALD) using Self- Limiting Reactions at Nanocrystal Surface Coupled to Phase Transfer between Polar and Nonpolar Media. J. Am. Chem. Soc. 2012, 134, 18585−18590. (19) Novotny, L.; Beversluis, M. R.; Youngworth, K. S.; Brown, T. G. Longitudinal Field Modes Probed by Single Molecules. Phys. Rev. Lett. 2001, 86, 5251. (20) Chizhik, A. M.; Chizhik, A. I.; Gutbrod, R.; Meixner, A. J.; Schmidt, T.; Sommerfeld, J.; Huisken, F. Imaging and Spectroscopy of Defect Luminescence and Electron- Phonon Coupling in Single SiO2 Nanoparticles. Nano Lett. 2009, 9, 3239−3244. (21) Failla, A. V.; Qian, H.; Qian, H.; Hartschuh, A.; Meixner, A. J. Orientational Imaging of Subwavelength Au Particles with Higher Order Laser Modes. Nano Lett. 2006, 6, 1374−1378. (22) Richards, B.; Wolf, E. Electromagnetic diffraction in optical systems II. Structure of the image field in an aplanatic system. Proc. R. Soc. London, Ser. A 1959, 253, 358−379. (23) Lieb, M. A.; Meixner, A. J. A high numerical aperture parabolic mirror as imaging device for confocal microscopy. Opt. Express 2001, 8, 458−474. (24) Richter, M. Nanoplatelets as material system between strong confinement and weak confinemen. Phys. Rev. Mater. 2017, 1, 016001. (25) Gao, Y.; Weidman, M. C.; Tisdale, W. A. CdSe Nanoplatelet Films with Controlled Orientation of their Transition Dipole Moment. Nano Lett. 2017, 17, 3837−3843. (26) Scott, R.; Heckmann, J.; Prudnikau, A. V.; Antanovich, A.; Mikhailov, A.; Owschimikow, N.; Artemyev, M.; Climente, J. I.; Woggon, U.; Grosse, N. B. Directed emission of CdSe nanoplatelets originating from strongly anisotropic 2D electronic structure. Nat. Nanotechnol. 2017, 12, 1155. (27) Cunningham, P. D.; Boercker, J. E.; Placencia, D.; Tischler, J. G. Anisotropic Absorption in PbSe Nanorods. ACS Nano 2014, 8, 581−590. (28) Tessier, M. D.; Javaux, C.; Maksimovic, I.; Loriette, V.; Dubertret, B. Spectroscopy of Single CdSe Nanoplatelets. ACS Nano 2012, 6, 6751−6758.

(29) Bhattacharyya, J.; Ghosh, S.; Gokhale, M. R.; Arora, B. M.; et al. Polarized photoluminescence and absorption in A-plane InN films. Appl. Phys. Lett. 2006, 89, 151910. (30) Spirkoska, D.; Efros, Al. L.; Lambrecht, W. R. L.; Cheiwchanchamnangij, T.; Fontcuberta i Morral, A.; Abstreiter, G. Valence band structure of polytypic zinc-blende/wurtzite GaAs nanowires probed by polarization-dependent photoluminescence. Phys. Rev. B: Condens. Matter Mater. Phys. 2012, 85, 045309. (31) Shirayama, M.; Kadowaki, H.; Miyadera, T.; Sugita, T.; Tamakoshi, M.; Kato, M.; Fujiseki, T.; Murata, D.; Hara, S.; Murakami, T. N.; et al. Optical Transitions in Hybrid Perovskite Solar Cells: Ellipsometry, Density Functional Theory, and Quantum Efficiency Analyses for CH3NH3PbI3. Phys. Rev. Appl. 2016, 5, 014012. (32) Andersen, M. L.; Stobbe, S.; Sørensen, A. S.; Lodahl, P. Strongly modified plasmon matter interaction with mesoscopic quantum emitters. Nat. Phys. 2011, 7, 215−218. (33) Wang, J.; Gudiksen, M. S.; Duan, X.; Cui, Y.; Lieber, C. M. Highly Polarized Photoluminescence and Photodetection from Single Indium Phosphide Nanowires. Science 2001, 293, 1455−1457. (34) Ruda, H. E.; Shik, A. Polarization-sensitive optical phenomena in semiconducting and metallic nanowires. Phys. Rev. B: Condens. Matter Mater. Phys. 2005, 72, 115308. (35) Ruda, H. E.; Shik, A. Polarization-sensitive optical phenomena in thick semiconducting nanowires. J. Appl. Phys. 2006, 100, 024314. (36) Kovalev, D.; Chorin, M. B.; Diener, J.; Koch, F.; Efros, A. L.; Rosen, M.; Gippius, N. A.; Tikhodeev, S. G. Porous Si anisotropy from photoluminescence polarization. Appl. Phys. Lett. 1995, 67, 1585. (37) Mejdoubi, A.; Brosseau, C. Finite-Element Simulation of the Depolarization Factor of Arbitrarily Shaped Inclusions. Phys. Rev. E 2006, 74, 031405.

4652

DOI: 10.1021/acs.nanolett.8b00347 Nano Lett. 2018, 18, 4647−4652