Article Cite This: J. Phys. Chem. C 2017, 121, 24837-24844
pubs.acs.org/JPCC
Origin of Shape-Dependent Fluorescence Polarization from CdSe Nanoplatelets Da-Eun Yoon,† Whi Dong Kim,† Dahin Kim,† Dongkyu Lee,† Sungjun Koh,† Wan Ki Bae,*,‡ and Doh C. Lee*,† †
Department of Chemical and Biomolecular Engineering, KAIST Institute for the Nanocentury, Korea Advanced Institute of Science and Technology (KAIST), Daejeon 34141, Korea ‡ Photoelectronic Hybrids Research Center, Korea Institute of Science and Technology (KIST), Seoul 02792, Korea S Supporting Information *
ABSTRACT: In this study, we report the shape dependence of fluorescence polarization from colloidal CdSe nanoplatelets (NPLs). Despite the symmetry of their cubic unit cell structure, CdSe nanocrystals grow into two-dimensional platelets in the presence of acetate precursors, and the resulting NPLs exhibit polarized emission. The amount of acetate salts introduced during the synthesis plays a critical role in controlling the lateral aspect ratio of CdSe NPLs. Specifically, the more the acetate hydrate presents, the more squarelike face of NPLs emerges. As a result, we achieved CdSe NPLs with varying lateral aspect ratios ranging from 1.1 to 4.5 in our experimental conditions. At the same thickness, CdSe NPLs with higher lateral aspect ratios exhibit higher fluorescence polarization. We analyzed the shape dependence by preparing films of CdSe NPLs dropcast under an electric field and measuring emission and absorption polarizations from CdSe NPL films of known orientation parameters. The emission polarization stays nearly unchanged regardless of shape anisotropy of CdSe NPLs, while the absorption polarization is affected by the lateral aspect ratio. Now that these results allude to a likelihood that absorption polarization is responsible for the shape dependence of fluorescence polarization, we design a model to assess the correlation between the geometry of NPLs and the optical transition polarization by way of the local field effect. Theoretically estimated absorption polarization also shows shape dependence similar to experimental data, which suggests that the anisotropic local field effect is a primary denominator of shape-dependent fluorescence polarization in CdSe NPLs.
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INTRODUCTION Interest in colloidal CdSe nanoplatelets (NPLs) has surged rapidly, evidenced in the series of recent findings on their unique optical properties, such as narrow photoluminescence (PL) bandwidth, giant oscillator strength transition, fast radiative recombination rate, and linearly polarized emission.1−5 By virtue of advances in the colloidal synthesis of CdSe NPLs with uniform thickness, the realization of these physical properties has become available. Earlier studies focused on the properties related to the strikingly uniform thickness, because charge carriers are strongly confined along the direction of NPL thickness.6−9 Recently, synthesis of CdSe NPLs with controlled lateral dimensions has also become available, which enables the investigation of optical properties in the context of their dependence on lateral dimensions.10−13 Now that the structural anisotropy in CdSe NPLs can be controlled, systematic investigation of the linearly polarized emission and its dependence on lateral dimensions is possible.5 Considering the isotropy in crystalline unit cell, the feasibility of changing linearly polarized emission in CdSe NPLs is particularly intriguing, as the polarized emission could enhance the efficiency of LCD device by lowering light loss when passing a polarizer filter.14,15 In general, geometric anisotropy is responsible for the degree of polarization from anisotropic nanocrystals, whose exciton fine structure and dielectric © 2017 American Chemical Society
confinement effect are strongly affected by the dimensions of the nanocrystals.16−19 For example, in the case of nanorods, the degree of polarization has a strong correlation with their aspect ratios.17,18 In the case of NPLs, because of their threedimensionally anisotropic morphology (lx ≠ ly ≠ lz), the lateral aspect ratio between two lateral edges should also be taken into account. As the thickness of CdSe NPLs dictates the emission wavelength, study of CdSe NPLs with the same thickness yet varying lateral aspect ratios would enable systematic understanding of optical polarization in the monochromatic fluorophores. While recent studies have reported on enhancing degree of polarization of ensemble CdSe NPLs via stacking into micrometer-scale aggregates4 and stretching-induced alignment,5,20 understanding on the relation between NPL morphology and polarized emission has been relatively primitive. Since fluorescence process involves absorption and emission transitions, overall fluorescence polarization is a combination of two polarizations: absorption and emission polarization. The polarizations can be explained by band-edge exciton fine structure and dielectric environment effect and largely depend Received: July 21, 2017 Revised: October 18, 2017 Published: October 18, 2017 24837
DOI: 10.1021/acs.jpcc.7b07216 J. Phys. Chem. C 2017, 121, 24837−24844
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The Journal of Physical Chemistry C
excitation light source, and a fluorescence spectrometer (Maya2000 Pro, Ocean Optics) connected with optical fiber (QP 1000-2-UV−vis, Ocean Optics) was used to measure the photoluminescence. A polarizer was placed in the emissionlight pathway, and we measured linearly polarized emission along 0° and 90° by adjusting the angle of the polarizer.
on the anisotropic morphology of particles. Therefore, for a comprehensive study on the emission and absorption polarization, the correlation between morphology of particles and optical polarization properties should be understood. Here, we demonstrate the precise control of morphology in CdSe NPLs and study how the shape anisotropy of NPLs correlates with their linearly polarized emission. For this, we controlled lateral aspect ratios of NPLs and examined the shape dependence of the linearly polarized emission. To unveil the origin of shape dependence of polarization, we analyze the emission and absorption polarization separately to decouple two factors. In addition to the experimental observation, we make theoretical estimation on the optical polarization from NPLs based on the dielectric confinement effect.
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RESULTS AND DISCUSSION We synthesized zincblende CdSe NPLs with the fixed thickness of 4.5 unit cell monolayers (5 Cd and 4 Se, denoted as 4.5 MLs hereafter) via previously reported synthetic procedures with modification (Figure S1).1,7,22 During the synthesis, we introduced cadmium acetate dihydrate that is known to trigger the lateral extension into platelets.23 The lateral extension has to do with the fact that short carboxylate molecules, such as cadmium acetate, lower the solubility of cadmium precursors in the solvent and induce the phase separation of precursors from solvent.24 Continued supply of Cd and Se adatoms results in supersaturation, where narrow facets of a zincblende CdSe crystal more readily become nucleation sites than wide surface planes, yielding anisotropic growth of CdSe into two-dimensional NPLs from isotropic crystal structures. In the anisotropic growth process, the content of cadmium acetate dihydrate influences the way lateral growth of CdSe NPLs takes place.13 We stumbled upon observation that the amount of acetate hydrate precursors determines the morphology of resulting NPLs. As shown in Figure 1, the face of NPLs becomes more squarelike with an increasing amount of the cadmium acetate dihydrate, as opposed to highly elongated rectangular shape observed in the case of low concentration of cadmium acetate dihydrate. Here, we define the lateral aspect ratio, LAR, as a ratio between the lengths of the two lateral edges, i.e., LAR = lx/ly (lx ≥ ly). From the size analysis based on transmission electron microscopy (TEM) images, the lateral aspect ratio in chosen experimental variations ranges from 1.2 to 3.8 (Figure 1e). When the lateral growth occurs at above 200 °C, water from cadmium acetate dihydrate reacts with acetate ligands and creates hydroxide anions. It is known that hydroxide anions bind to specific facets of nanocrystals and hinder the growth.25 Likewise, hydroxide anions may also be adsorbed on facets of CdSe NPLs and lead to change in morphology by altering the growth rate of the facets.13 Further increase in the LAR up to 4.5 was observed when we used other kinds of acetate salts, e.g., zinc acetate dihydrate and sodium acetate trihydrate (Figure S2). While the ranges of LAR differ in different kinds of acetate, the acetate salts have a trend in common: the higher the acetate hydrate concentration, the lower the LAR. To investigate the shape effect on their optical transition of CdSe NPLs, we obtained the fluorescence polarization of NPLs in hexane solution using the photoselection method (Figure 2).26,27 The fluorescence polarization, P, is defined as
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EXPERIMENTAL SECTION Synthesis of 4.5 Monolayer (ML) Thick CdSe Nanoplatelets (NPLs). Cadmium myristate (0.17 g (0.3 mmol)), 0.012 g (0.15 mmol) of Se, and 15 mL of 1-octadecene were loaded into a three-neck flask and degassed under vacuum at 100 °C for 1 h. Under inert atmosphere, the mixed solution was heated up to 200 °C in 7 min to yield Cd−Se complex and rapidly cooled down to room temperature by using an ice bath. Cadmium acetate dihydrate was introduced into the flask as the lateral dimension controlling agent. The reaction mixture was stirred at 100 °C for 5 min for homogeneity of reaction mixture and then heated up to 240 °C to grow CdSe NPLs. After 10 min of reaction at the elevated temperature, the flask was cooled down to room temperature to cease the reaction. Two milliliters of oleic acid was injected to enhance the colloidal stability of NPLs. Resulting CdSe NPLs were purified by precipitation (acetone) and redispersion (hexane) method and dispersed in hexane for further characterization. Measurement of Fluorescence Polarization. Spectrofluorometer (Fluorolog3 with TCSPC, Horiba Scientific) equipped with polarizers was used to measure the polarized emission. A continuous-wave xenon lamp (450 W) was used as an excitation source. Each polarizer is placed in the excitation and emission pathways, and fluorescence polarization was obtained by adjusting the alignment of polarizers (VV, VH, HH, and HV). To prevent unwanted stacking, CdSe NPLs dispersions were diluted for use in optical characterization (the optical density (O.D.) at the 1S peak position = 0.04). E-Field-Induced Alignment of CdSe NPLs. We used 50 nm thick molybdenum (Mo) electrodes on a glass substrate. The electrodes have an interdigitated pattern, with each finger 3 μm wide and separated by a gap of 3 μm. We applied an ac voltage of 100 V with a frequency of 50 kHz on electrodes to induce the alignment of NPLs. For each experiment, 7 μL of 10−6 M CdSe NPL solution was dropped on the electrode in the presence of electric field. To estimate the concentration of NPL solutions, we first calculated the extinction coefficient of CdSe NPLs for each geometry by using the equation given in the literature.21 Then we measured the optical density of NPL solutions using UV−vis spectroscopy and calculated their concentrations from the Beer−Lambert law. Measurement of Absorption and Emission Polarization. A UV−vis spectrophotometer (UV-3600, Shimadzu) was used to measure absorption spectra. We placed a polarizer in the incident-light pathway and adjusted the angle of polarizers (0° and 90°). For each sample, we obtained two absorption spectra with linearly polarized incident light of 0° and 90°. Unpolarized 385 nm UV light was used as an
P=
I − I⊥ I + I⊥
=
IVV − GIVH I , where G = HV IVV + GIVH IHH
(1)
where I∥ and I⊥ are the intensity of light with polarization, parallel and perpendicular to the excitation polarization, respectively. Each polarizer is placed in the excitation and emission pathways. The equipment setup enables facile acquisition of IVV, IVH, IHH, and IHV, where subscripts denote the configuration of excitation and emission polarization directions. For example, VH represents vertically polarized excitation beam and horizontally polarized emission. Prior to 24838
DOI: 10.1021/acs.jpcc.7b07216 J. Phys. Chem. C 2017, 121, 24837−24844
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does not affect fluorescence polarization when the solution is as dilute as O.D. at 1S peak position below 0.1 (Figure S5). Note that polarization is considered for this study instead of generally used anisotropy because of three-dimensional anisotropic geometry of NPLs. In general cases (e.g., nanorods), the fluorescence anisotropy, R = (I∥ − I⊥)/(I∥ + 2I⊥), is used for the measurement of optical anisotropy of fluorophores. The difference of I∥ and I⊥ is normalized by the total intensity, I∥ + 2I⊥, under the assumption that the emission field of fluorophore is cylindrically symmetric around the z-axis, Iz = I∥ and Ix = Iy = I⊥.26 Therefore, total intensity, IT = Ix + Iy + Iz, is turned into IT = I∥ + 2I⊥.29 On the other hand, in the case of NPLs, intensity of optical transition along the x and y axis can be easily inequivalent (Ix ≠ Iy). Since the measurement setup only allows to detect two linearly polarized components, parallel and perpendicular to the incident light, polarization is a more appropriate measure for the study about NPLs rather than anisotropy. Figure 2a displays energy-dependent fluorescence polarization of CdSe NPLs with respect to varying LARs. In all samples, the maximum value is observed near the band-edge transition, and the fluorescence polarization (P) decreases as the excitation energy increases. At high energy excitation region (λexc < 420 nm), the fluorescence polarization appears to reach a plateau. A key to understanding energy-dependent polarization is to unravel the polarization in the optical transitions of absorption and emission. Absorption polarization depends on the energy because absorption transition states are governed by the excitation energy, whereas emission polarization remains unchanged regardless of the excitation energy because the emission comes from the same band-edge transition.30 When the excitation energy exceeds the bandgap energy by far, dependence of absorption polarization on energy also becomes negligible because of very high density of electronic states, resulting in the plateau of fluorescence polarization.27,31 It is noteworthy that elongated rectangular NPLs show higher polarization values than NPLs with a lower LAR. To verify the relationship between the fluorescence polarization and the morphology of NPLs, we monitored the polarization of NPLs with exquisitely controlled LAR (Figure 2b). It turns out that the fluorescence polarization depends on the morphology; in other words, the elongation in the two-dimensional structure results in the increased polarization in optical transition upon excitation at both high energy and band edge regime. We derived equations from the relatitwon between fluorescence polarization and lateral aspect ratio of 4.5 ML CdSe NPLs with linear regression analysis:
Figure 1. (a−d) TEM images of 4.5 monolayer thick (i.e., 5 Cd and 4 Se layers) CdSe NPLs with varying lateral aspect ratios. The average lateral dimensions of CdSe NPLs (lx × ly) are displayed on TEM images. Feed ratios between cadmium acetate dihydrate (Cd(ac)2· 2H2O) and cadmium myristate (Cd(myr)2) ([Cd(ac)2·2H2O]/ [Cd(myr)2]) are (a) 0.25, (b) 0.5, (c) 0.75, and (d) 1. (e) Average lateral dimensions (lx, ly) and lateral aspect ratios (lx/ly) of samples shown in (a−d).
the measurement, absence of intrinsic polarization of equipment was verified by measuring fluorescence polarization from spherical zincblende CdSe quantum dots (QDs) that do not show optical polarization (Figure S3). To minimize the undesired interaction between nanocrystals, we diluted solution samples with the same optical density (O.D.) of 0.04 at the 1S peak position and confirmed that NPLs do not aggregate into long stacks regardless of their morphology in our measurement conditions with dynamic light scattering (DLS) measurement (Figure S4).28 We also checked that stack formation, if any,
Excitation at band edge: P = (0.021 79 × LAR) + 0.110 02
(2)
Excitation at high energy(λexc < 420 nm): P = (0.019 05 × LAR) + 0.054 27
(3)
Although the fluorescence polarization clearly shows spectrally resolved linearly polarized emission, the contribution of absorption and emission dipoles to the net polarization is not distinguished in this data.26,27 To correlate the fluorescence polarization with the morphology of NPLs in a quantitative fashion, one should devise an experimental system in which absorption and emission polarization are independently monitored on CdSe NPLs with the known orientation parameter. For that purpose, we aligned CdSe NPLs by 24839
DOI: 10.1021/acs.jpcc.7b07216 J. Phys. Chem. C 2017, 121, 24837−24844
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Figure 2. (a) Fluorescence polarization of CdSe NPLs with respect to the lateral aspect ratio. Photoluminescence excitation measurement with polarizers was carried out with NPL dispersion in hexane (O.D. at 1S peak position = 0.04). (b) Shape anisotropy dependent fluorescence polarization under band edge excitation (filled square) and high energy excitation (λmax < 420 nm, open circle). Dashed lines represent linear regression line.
Figure 3. SEM images of electric field induced alignment of CdSe NPLs with various lateral aspect ratios: (a) 1.2, (b) 2.5, (c) 3.2, and (d) 4.0. Orientation parameter of NPL assembly is (a) 0.53, (b) 0.46, (c) 0.55, and (d) 0.49.
and I⊥ are emission and absorption intensity parallel and perpendicular to the alignment direction, respectively (Figure 4). For the measurement of emission polarization, samples were excited by unpolarized UV light (385 nm), and emission from the samples was detected with polarizers at 0° and 90° to the direction of the electric field applied for NPL alignment (Figure 4a). It turns out that NPLs with different LARs have similar polarization values of around 0.4 regardless of their geometries (Figure 4b). In order to explain the lack of shape dependence of emission polarization, one can use a quantumwell model that takes into account electronic structures of NPLs.5,35 Based on this model, the heavy-hole (hh)-toconduction band transition has transverse electric (TE) polarization, which results in circularly polarized emission in the direction perpendicular to the plane and linearly polarized emission along the direction parallel to the quantum well layers. On the other hand, light-hole (lh)-to-conduction band transition has transverse magnetic (TM) polarization, which gives rise to linearly polarized luminescence along the thickness
dropcasting CdSe NPL dispersions (hexane) onto interdigitated electrodes in the presence of an external electric field (ac voltage of 100 V with a frequency of 50 kHz). Under the electric field, dielectric particles with a permanent or induced dipole moment experience dielectrophoretic force. This force leads NPLs to experience a torque that tends to align the major axes of NPLs in the direction parallel to the electric field during solvent evaporation.32,33 Figure 3 shows scanning electron microscopy (SEM) images of CdSe NPLs with different LAR aligned on a substrate via the aforementioned electric field assisted method. The degree of alignment is quantified by the orientation parameter, S, which is defined as S = ⟨2cos2 θ − 1⟩, with the angle θ with respect to the direction of electric field.33,34 We measured the angle of orientation, θ, of more than 1000 NPLs from randomly selected SEM images for each samples by using the ImageJ software (Figure S6). Based on this, we obtained orientation parameter, S, for each sample. For the aligned CdSe NPLs, we measured emission and absorption polarization, defined as (I∥ − I⊥)/(I∥ + I⊥), where I∥ 24840
DOI: 10.1021/acs.jpcc.7b07216 J. Phys. Chem. C 2017, 121, 24837−24844
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Figure 4. Emission and absorption polarization of aligned CdSe NPL films. (a) Photoluminescence intensity of CdSe NPLs (LAR = 4.0) parallel (0°, solid line) and perpendicular (90°, broken line) to the direction of the electric field. (b) Emission polarization of CdSe NPLs as a function of LAR. (c) Absorption spectra of CdSe NPLs (LAR = 4.0) under linearly polarized incident light with 0° (solid line) and 90° (broken line) and the energydependent absorption polarization. (d) Shape anisotropy-dependent absorption polarization of CdSe NPLs under band edge excitation (filled square) and high energy excitation (λmax < 420 nm, open circle).
constant between a particle (εCdSe ∼ 10.6) and its surrounding (εoleic acid ∼ 2.5).27,30,31 Under this condition, the electric field penetrating the particle is reduced, and it will attenuate the photon coupling to excitons, which is referred to as the local field effect.39,40 Degree of reduction in electric field penetrating an anisotropic particle varies according to the direction, and it depends on the shape of the particle.19,40 Therefore, local field effect causes the shape dependence of absorption polarization at high energy. In the case of band edge transition, electronic structure as well as local field effect needs to be considered. Should the exciton fine structure come into play and affect the shape dependence, the absorption polarization at band-edge transition would exhibit different shape dependence from the case of high-energy excitation. The fact that the shape dependence is similar in either case (Figure 4d) is a testament that electronic structure is unlikely to play a significant role in shape dependence. It leaves us a notion that shape dependence of absorption polarization is solely from local field effect. To confirm the correlation between dielectric effect and the shape dependence of absorption polarization, we quantify the local field effect along three different directions. Large difference in dielectric constant of particles and medium causes attenuation of the electric field inside of particle, Ei, compared to the external field outside of particle, Ee. For anisotropic particles, local field effect along the direction α(x, y, z) can be expressed as40,41
direction.35,36 In the case of 4.5 ML CdSe NPLs, hh−lh energy splitting is large (167 meV, ∼6.5 kBT at 298 K),1 which keeps the emission hh transition-rich. Therefore, linearly polarized emission from CdSe NPLs would result from TE polarization in the direction along the two edges of NPLs. The dominance of in-plane (x−y) transition dipole moments in emission of NPLs was revealed by the analysis of the two-dimensional kspace distribution.37 Correlating the emission polarization with LAR requires estimation of the orientation of emissive transition dipole in the NPL plane. If the NPLs have elongated-rectangular shape, transition dipole is expected to be oriented along the direction of long edge, like in the case of nanorods.16,27 However, a recent study reported that transition dipole moment of the band-edge exciton in CdSe NPLs has isotropic orientation in the plane without preference for the long or short axis of the plane.38 It means that the mere changes in LAR would not alter the polarization of emissive band-edge dipole, which coincides with our observation that emission polarization is not shape-dependent (Figure 4b). On the other hand, absorption polarization was obtained from absorption spectra for linearly polarized incident light parallel and perpendicular to the alignment direction. Similar to the fluorescence polarization (Figure 2a), absorption polarization depends on the excitation wavelength (Figure 4c). In addition, we observed that absorption polarization depends on the LAR of NPLs at both high energy and band edge excitation conditions (Figure 4d). As we mentioned above, at energies far above the band edge, electronic states form a continuum and transition dipoles can be considered to have random distribution. Nevertheless, NPLs have nonzero absorption polarization in this high-energy region because of the dielectric effect resulting from the large difference in the dielectric
E i, α =
ε 1 Ee, α with k = in 1 + nα(k − 1) εout
(4)
where depolarization factor nα can be obtained by11,42,43 24841
DOI: 10.1021/acs.jpcc.7b07216 J. Phys. Chem. C 2017, 121, 24837−24844
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The Journal of Physical Chemistry C nx =
abc 2
∫0
∞
1 (s + a 2) (s + a 2)(s + b2)(s + c 2)
exclusively proportional to the light intensity.40 Therefore, the dielectric effect model is applicable to estimate the absorption polarization. From time-averaged Poynting vector, the intensity of light is proportional to the square of the intensity of electric field (I ∝ E2),44 and it can be expressed as a function of the screening factor (I∥/I⊥ ∝ D∥/D⊥).40 Based on this, we estimated the absorption polarization of CdSe NPL film using the local field effect model and compared the results with experimental data (Figure 6). Figure 6a describes the measurement configuration of the absorption polarization on the alignment CdSe NPLs. Here, we defined the direction of electric field as 0°. Ideally, NPLs tend to be aligned parallel to the electric field; however, in reality they show distribution of alignment, whose orientation can be indexed by two angles, θ and φ (Figure 6b): θ represents the degree of alignment with respect to the electric field and φ indicates tilt angle with respect to the measurement plane. When the angle between linearly polarized light and polarizer is given by θ, the intensity of the light passing the polarizer is attenuated by cos2 θ, according to Malus’s law.45 Under the consideration of tilt angle, φ, absorption polarization of NPLs, ρ, can be expressed as
ds (5)
ny =
abc 2
∫0
∞
1 2
2
(s + b ) (s + a )(s + b2)(s + c 2)
ds (6)
nz =
abc 2
∫0
∞
1 2
2
(s + c ) (s + a )(s + b2)(s + c 2)
ds (7)
We defined lx as the length of the longest edge of NPLs, lz as the thickness of NPLs (lx > ly > lz), and a, b, and c as lx/2, ly/2, and lz/2, respectively. Here, we assumed that a2 ≫ c2 because the largest edge length of NPLs, lx, is typically over 10 nm, significantly greater than the thickness, lz (∼1.4 nm). Difference in nα along the x, y, and z directions results in the anisotropic local field effect, and this is directly related to the polarization of optical transitions. Since the depolarization factor, nα, depends on the shape of the ellipsoidal particles, electric field penetrating along each axis strongly depends on the shape of NPLs. To compare the reduction of electric field, we estimated the electric field screening factor, Dα, which is defined as E i, α 2 1 Dα = = [1 + nα(k − 1)]2 Ee, α 2
ρ= =
(8)
As shown in Figure 5, Dz (the screening factor along the thickness direction of NPLs) is significantly smaller than Dx and
I
,0◦
− I⊥,90◦
I
,0◦
+ I⊥,90◦
Dx − Dy sin 2 φ − Dz cos2 φ 2
2
Dx + Dy sin φ + Dz cos φ
(cos 2 θ − sin 2 θ) (9)
Since NPLs appear entitled to have various postures, from lying flat to vertically upright on the plane, we assumed random distribution of NPL alignment in terms of tilt angle, φ. Under this assumption, absorption polarization can be simplified by taking average in terms of tilt angle,
1 2π
2π
∫0 ρdφ.
1
Dx − 2 (Dy + Dz ) 1 2π (2cos2 θ − 1) ρ (θ ) = ∫0 ρdφ = 1 2π Dx + 2 (Dy + Dz ) (10)
Orientation parameter, defined as S = ⟨2cos θ − 1⟩, can be applied to obtain the absorption polarization from eq 10. If all NPLs are perfectly aligned to the direction of electric field, the orientation parameter is unity, and therefore absorption polarization is expressed by the screening factor which is a function of LAR and lx (Figure S7). As we have experimentally observed that the average orientation parameter of CdSe NPLs aligned by electric field is about 0.5 (Figure 3), our theoretical estimation of absorption polarization was made with the orientation parameter to be 0.5. As shown in Figure 6c, absorption polarization shows very similar LAR dependence between experimental and theoretical data. Therefore, it is likely that the local field effect accounts for the shapedependent absorption polarization from CdSe NPLs. 2
Figure 5. Screening factor, Dα, along x, y, and z directions as a function of LAR and lx.
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Dy (x and y axis, respectively). Also, Dx has the highest value within the entire range of LAR and lx. The large value of Dx compared to Dy and Dz indicates high probability of optical transition along elongated x-axis. In most cases, one needs to consider the exciton fine structure and selection rules for the optical transition as well as local field effect to describe the polarization properties.40 However, at energy greatly exceeding the band gap, transition dipoles have nearly isotropic distribution upon high-energy excitation, which makes intensity of absorption transition
CONCLUSION In summary, we investigated the fluorescence polarization properties of CdSe NPLs in relationship with their lateral shape anisotropy. We synthesized CdSe NPLs with controlled lateral aspect ratios ranging from 1.2 to 4.5 by varying the content of the shape anisotropy triggering agent and observed that elongated NPLs with high lateral aspect ratio exhibit higher fluorescence polarization than NPLs with low lateral aspect ratio. We conducted independent absorption and emission 24842
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Figure 6. (a) Schematic illustrating the measurement configuration of the absorption polarization on CdSe NPLs aligned by the electric field. Linearly polarized light parallel (0°) and perpendicular (90°) to the direction of electric field were used as incident light. Dotted line represents the direction of incident light propagation. (b) Angle configuration of single NPL describes its orientation. (c) Theoretical absorption polarization based on the local field effect with consideration of orientation parameter, S = 0.5, and experimentally observed absorption polarization at high energy (λexc < 420 nm) as a function of lateral aspect ratio.
polarization measurement on aligned NPLs and unveiled that the shape dependence of fluorescence polarization is largely ascribed to that of absorption polarization. We demonstrated that theoretical estimation that takes into account the local field effect agrees well with experimental results, which indeed suggests that the anisotropic local field effect is a primary denominator of shape-dependent fluorescence polarization in CdSe NPLs. As highlighted in the present study, the morphology of nanocrystals can have a strong influence on the linearly polarized emission. Particularly noteworthy are CdSe NPLs, in which optical transitions undergo polarization because of the anisotropic morphology. The present study on the correlation of shape anisotropy and optical polarization of CdSe NPLs provides an insightful avenue to enhancing polarization of a given nanomaterial.
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Doh C. Lee: 0000-0002-3489-6189 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by LG Display, Co., Ltd. and the National Research Foundation (NRF) grants funded by the Korean government (Grant NRF-2017R1A2B2011066, NRF2011-0030256, and NRF-2016M3A7B4910618). W.K.B. acknowledges financial support from the Ministry of Trade, Industry & Energy (MOTIE, 10076340) and the Korea Display Research Consortium (KDRC) program for the development of future devices technology for display industry (10051541).
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(1) Ithurria, S.; Tessier, M. D.; Mahler, B.; Lobo, R. P. S. M.; Dubertret, B.; Efros, A. Colloidal Nanoplatelets with Two-Dimensional Electronic Structure. Nat. Mater. 2011, 10, 936−941. (2) Tessier, M. D.; Javaux, C.; Maksimovic, I.; Loriette, V.; Dubertret, B. Spectroscopy of Single CdSe Nanoplatelets. ACS Nano 2012, 6, 6751−6758. (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) Abecassis, B.; Tessier, M. D.; Davidson, P.; Dubertret, B. SelfAssembly of CdSe Nanoplatelets into Giant Micrometer-Scale Needles Emitting Polarized Light. Nano Lett. 2014, 14, 710−715. (5) Cunningham, P. D.; Souza, J. B.; Fedin, I.; She, C. X.; Lee, B.; Talapin, D. V. Assessment of Anisotropic Semiconductor Nanorod and Nanoplatelet Heterostructures with Polarized Emission for Liquid Crystal Display Technology. ACS Nano 2016, 10, 5769−5781. (6) Ithurria, S.; Dubertret, B. Quasi 2D Colloidal CdSe Platelets with Thickness Controlled at the Atomic Level. J. Am. Chem. Soc. 2008, 130, 16504−16505. (7) Chen, D. D.; Gao, Y.; Chen, Y. Y.; Ren, Y.; Peng, X. G. Structure Identification of Two-Dimensional Colloidal Semiconductor Nanocrystals with Atomic Flat Basal Planes. Nano Lett. 2015, 15, 4477− 4482. (8) Biadala, L.; Liu, F.; Tessier, M. D.; Yakovlev, D. R.; Dubertret, B.; Bayer, M. Recombination Dynamics of Band Edge Excitons in QuasiTwo-Dimensional CdSe Nanoplatelets. Nano Lett. 2014, 14, 1134− 1139. (9) Lhuillier, E.; Pedetti, S.; Ithurria, S.; Heuclin, H.; Nadal, B.; Robin, A.; Patriarche, G.; Lequeux, N.; Dubertret, B. Electrolyte-Gated Field Effect Transistor to Probe the Surface Defects and Morphology in Films of Thick CdSe Colloidal Nanoplatelets. ACS Nano 2014, 8, 3813−3820.
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b07216. Experimental details of morphology control of CdSe NPLs; absorption and photoluminescence of 4.5 ML CdSe NPLs; an X-ray diffraction pattern of CdSe NPLs; TEM images of CdSe NPLs with controlled morphology by using other kinds of acetate hydrate (zinc acetate dihydrate and sodium acetate trihydrate); polarization measurement of zincblende CdSe quantum dots; dynamic light scattering distribution curves for CdSe NPLs solution; fluorescence polarization from different concentration of CdSe NPLs; angle distribution of CdSe NPLs aligned by electric field; screening factor as a function of lateral aspect ratio and lx; theoretical absorption polarization of CdSe NPLs with perfect alignment (S = 1); procedures for calculation of the depolarization factor (PDF)
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Sungjun Koh: 0000-0001-5640-4299 Wan Ki Bae: 0000-0002-3832-2449 24843
DOI: 10.1021/acs.jpcc.7b07216 J. Phys. Chem. C 2017, 121, 24837−24844
Article
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DOI: 10.1021/acs.jpcc.7b07216 J. Phys. Chem. C 2017, 121, 24837−24844
