Order-of-Magnitude Broadband Enhanced Light Emission from

Subscriber access provided by University of Glasgow Library

Communication

Order-of-magnitude, Broadband Enhanced Light Emission from Quantum Dots Assembled in Multi-scale Phase-Separated Block Copolymers Geon Yeong Kim, Shinho Kim, Jinyoung Choi, Moohyun Kim, Hunhee Lim, Tae Won Nam, Wonseok Choi, Eugene N. Cho, Hyeuk Jin Han, ChulHee Lee, Jong Chan Kim, Hu Young Jeong, Sung-Yool Choi, Min Seok Jang, Duk Young Jeon, and Yeon Sik Jung Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.9b01941 • Publication Date (Web): 03 Sep 2019 Downloaded from pubs.acs.org on September 3, 2019

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

Order-of-magnitude, Broadband Enhanced Light Emission from Quantum Dots Assembled in Multiscale Phase-Separated Block Copolymers Geon Yeong Kim,1† Shinho Kim,2† Jinyoung Choi,1† Moohyun Kim,1 Hunhee Lim,1 Tae Won Nam,1 Wonseok Choi,1 Eugene N. Cho,1 Hyeuk Jin Han,1 ChulHee Lee,1 Jong Chan Kim,3 Hu Young Jeong,3 Sung-Yool Choi,2 Min Seok Jang,2* Duk Young Jeon,1* and Yeon Sik Jung1* 1

Department of Materials Science and Engineering, Korea Advanced Institute of Science and

Technology (KAIST), 291 Daehak-ro, Yuseong-gu, Daejeon 34141, Republic of Korea 2

School of Electrical Engineering, Korea Advanced Institute of Science and Technology (KAIST),

291 Daehak-ro, Yuseong-gu, Daejeon 34141, Republic of Korea 3

UNIST Central Research Facilities and School of Materials Science and Engineering, Ulsan

National Institute of Science and Technology (UNIST), 50 UNIST-gil, Eonyang-eup, Ulju-gun, Ulsan 44919, Republic of Korea

TABLE OF CONTENTS GRAPHIC

*

Corresponding authors, Email: [email protected] (M.S. Jang), [email protected] (D. Y.

Jeon), [email protected] (Y.S. Jung) †

These authors contributed equally to this work.

1

ACS Paragon Plus Environment

Nano Letters 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT Achieving high emission efficiency in solid-state quantum dots (QDs) is an essential requirement for high-performance QD optoelectronics. However, most QD films suffer from insufficient excitation and light extraction efficiencies along with non-radiative energy transfer between closely adjacent QDs. Herein, we suggest a highly effective strategy to enhance the photoluminescence (PL) of QD composite films through an assembly of QDs and poly(styrene-b4-vinylpyridine)) (PS-b-P4VP) block copolymer (BCP). A BCP matrix casted under controlled humidity provides multi-scale phase-separation features based on (1) sub-μm-scale spinodal decomposition between polymer-rich and water-rich phases and (2) sub-10-nm-scale microphase separation between polymer blocks. The BCP-QD composite containing bi-continuous random pores achieves significant enhancement of both light absorption and extraction efficiencies via effective random light scattering. Moreover, the microphase-separated morphology substantially reduces the Förster resonance energy transfer efficiency from 53% (pure QD film) to 22% (BCPQD composite), collectively achieving an unprecedented 21-fold enhanced PL over a broad spectral range.

KEYWORDS: Quantum dot, Block copolymer, Nanocomposite, Photoluminescence, Vaporinduced phase separation

2


Page 2 of 37

Page 3 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

For the last few decades, colloidal quantum dots (QDs) have garnered attention owing to their tunable color and narrow emission.1-7 In particular, QDs can be used as practical down-conversion materials in various types of high-performance displays and lighting such as QD liquid crystal display television (QLCD-TV)8, 9 and QD light-emitting diodes (QD-LEDs).10, 11 The efficiency of down-conversion QD films can directly affect the energy efficiency and brightness of the devices and can be improved by enhancing the combined factors of light absorption efficiency, light extraction efficiency, and internal quantum yield (QY). However, most QD films in practical devices are far less efficient than QD solution due to the inherent limitations such as insufficient light absorption, strong total internal reflection (TIR) of QD-emitted light, and non-radiative energy transfer among aggregated QDs. As a solution to these general issues that are faced with light-emitting materials, previous studies have shown that higher absorption efficiency is critical to improve the conversion of incident photons to emitted photons. For example, the incorporation of a random scattering layer can effectively increase the photoluminescence (PL) of nanowires.12 Furthermore, random scattering media exhibit excellent optical properties such as broadband,13-17 omni-directional13,

15-18

and

polarization-independent16, 17 absorption, and processing tolerance,16 which are highly desirable for conventional optoelectronics but difficult to realize by using periodic structures based on resonance phenomena. Moreover, it is desirable to reduce the efficiency of Förster resonance energy transfer (FRET) by which QDs non-radiatively transfer their energy to nearby QDs, deteriorating PL QY.19, 20 The FRET phenomenon more dominantly occurs in a film state compared to a solution state due to a substantially reduced QD-to-QD distance.19 Hence, homogeneous and controlled dispersion of QDs is critical to reduce FRET, and also to achieve uniform brightness in display and lighting

3


Nano Letters 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

devices with minimum distribution in contrast. It was suggested that increasing the average distance between emission centers can reduce the FRET, for instance, by the utilization of seeded nanorods,21 increase the thickness of the QDs shell,19, 22-23 or control the donor concentrations.24 Also importantly, the large difference in the refractive index between QDs and air causes significant TIR and thus considerable loss of emitted light. The same issue also occurs in organic light emitting diodes (OLEDs) composed of multiple layers with different refractive indices.25 As a solution, incorporating random-scattering particles or layers26, 27 was proven to be effective in achieving high outcoupling efficiency with a large viewing angle over a broad spectral range. However, despite numerous approaches reported to date, a comprehensive solution that can simultaneously resolve all of the issues mentioned above has not been developed yet. Herein, we report a marked enhancement of PL intensity from a QD composite based on a hierarchical accommodation of red-emitting QDs (CuInS2/ZnS) in a multi-scale phase-separated PS-b-P4VP BCP template. We show that the BCPs cast under a controlled humidity environment provide a randomly interconnected porous structure (via three-component spinodal decomposition among BCP/solvent/water) at a sub-micron scale – the size regime where the interaction with the visible light is maximized. Our optical calculations suggest that random scattering of light in the porous BCP-QD nanocomposite leads to 4.4- and 3.5-fold enhanced light absorption and light extraction, respectively, compared to the non-porous nanocomposite. This study also demonstrates that sub10 nm BCP microdomains can serve as hosts for QD dispersion, substantially reducing undesirable FRET among QDs. By the combined effects of the random scattering and QD dispersion in the multiscale-phase-separated BCP matrix, a 21-fold enhancement of PL intensity compared to a pure QD film is achieved, and this is further supported by an LED down-conversion device based on the BCP-QD nanocomposite.

4


Page 4 of 37

Page 5 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

For maximizing light-QDs interaction and minimizing the FRET effect, we designed a hierarchical structure of a BCP-QD composite film with multiple length-scale morphologies at sub-μm (Figure 1a) and sub-10 nm (Figure 1b) scales, respectively. (1) Sub-μm random pores permit the absorption and extraction enhancement by the light scattering effect. In sub-μm scale, the BCP forms a randomly porous structure with pore size ranging from a few-hundred-nm to a few-μm, which contributes to the broadband PL enhancement. (2) The sub-10 nm phase-separated structure contributes to the reduced FRET effect. In the sub-10 nm scale, the preferential segregation of functionalized QDs into the P4VP domains prohibits aggregation of QDs and enables more regular arrangement of the QDs. First, for the formation of the random network of sub-μm-scale air pores, we employed a vaporinduced phase separation (VIPS) phenomenon,28-32 which is a phase separation between polymerrich and water–rich liquid phases. VIPS occurs when a polymer solution is exposed to a nonsolvent (e.g. water) vapor, which is condensed and penetrated into the polymer solution during spin-casting due to a cooling effect by solvent evaporation. The polymer-rich phase subsequently forms a matrix with relatively faster evaporation of solvent; meanwhile, upon the slower evaporation of water, the water-rich phase is later converted into empty pores having a size range of sub-μm. Hence, the precise control of water intake during the spin-casting of the BCP-QD solution is fundamental to modulate the morphology obtained by the VIPS phenomenon.29, 31 We thus designed a tool for the control of humidity during spin-casting (Figure 1c). By varying the relative flow rate of water vapor saturated (relative humidity, RH = 100%) and completely dry air (RH = 0%), the quantitative control of RH is feasible, thus making it possible to control the porosity of the fabricated films. Key parameters affecting the porous structures will be discussed in detail in the next section.

5


Nano Letters 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

On the other hand, FRET is expected to be reduced by the uniform dispersion of QDs in the matrix. As a solution, we noticed that the microphase separation phenomenon of BCPs can provide well-defined 10-nm-scale templates for controlling the position and dispersion of QDs in the matrix. It was previously reported that co-assembly of CdSe nanorods in BCP films can generate a hierarchical superstructures,33 which suggests the possibility of controlling the positions of QDs in a desirable manner for reduction of the FRET effect. Figure 1b shows an illustration of the PSb-P4VP BCP nanostructure with embedded QDs at the nanoscale template. For the selective incorporation of QDs in the P4VP block, the QD ligand was replaced with 11-mercapto-1undecanol (MUD) with hydroxyl groups, which can bind the P4VP block via strong hydrogen bond linkages. The successful hydrogen bond formation is confirmed by Fourier transforminfrared (FT-IR) spectroscopy (Figure S1a). To investigate the optimum porous structure that can maximize the emissive properties of QDs, we compared the morphologies derived from three different polymer matrix materials. P4VP, PS, and PS-b-P4VP BCPs in dimethylformamide (DMF) solution (15 wt%) were spun-cast under an RH of 75%, resulting in homogeneous and flat morphologies, rough and non-uniform structures with spherical pores, and uniform films containing highly interconnected random pores, respectively (SEM images in Figure 2a - bottom). As mentioned above, the VIPS phenomenon is based on thermodynamic miscibility and immiscibility among the polymer, solvent (DMF), and non-solvent (water). Schematic ternary phase diagrams for the three different polymer materials are depicted (Figure 2a). The final morphology of the polymers is determined by the trajectory of compositions on the ternary phase diagrams. Considering the dynamic situation of solvent evaporation as well as water condensation and evaporation, the composition path starts from a point in the polymer-DMF two-component

6


Page 6 of 37

Page 7 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

line and subsequently crosses the three-component area with the supply of water and vaporization of DMF, gradually reaching the two-component polymer-water line after all the DMF is removed. At this stage, because of the absence of solvent, the morphology of the polymer will be maintained until all water is removed. The different miscibilities of the polymers suggests qualitatively different phase diagrams for the three polymer matrices. For example, the hydrophilic P4VP solutions would have a much wider composition range of the homogeneous phase without any morphology. In the case of hydrophobic PS, however, the phase-separated (two-phase) region will be relatively broader. The phase diagram of the amphiphilic PS-b-P4VP BCP will be between the two cases. The porous structures obtained from PS and PS-b-P4VP can be understood from the broad two-phase region due to their limited solubility in water. However, it should be emphasized that PS and PS-b-P4VP have substantially different pore morphologies, as mentioned above. As shown in the phase diagrams, the two-phase region can be divided into binodal and spinodal regions, which can be characterized by strikingly different morphologies, i.e. cellular and bi-continuous structures, respectively.30 In the ternary phase diagram, the binodal region locates at the higher polymer composition range, and the spinodal region at the lower range. The fundamental difference of the two morphologies originates from the meta-stability and instability of the ternary blends.34 For the binodal decomposition, the formation and growth of nuclei of a water phase occur to overcome an energy barrier. In this case, spherical nuclei are formed to minimize their interfacial energy. In contrast, for the spinodal decomposition, the mixture is so unstable that even a small compositional fluctuation of water concentration induces the spontaneous separation of the constituents, resulting in the highly interconnected

7


Nano Letters 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

morphology. These different decomposition mechanisms cause the contrasting intermediatemorphologies (see the schematics inserted inside blue arrows of Figure 2a). One critical factor is the change of the composition trajectory depending on the polymer. The composition path in the ternary phase diagram is affected by the rate of water condensation, which is influenced by the RH and polarity of the polymers. This means that a higher RH and a more hydrophilic polymer would facilitate water intake, and therefore the corresponding composition path will be closer to the water-solvent composition line. As presented in Figure S1b, the relative degree of hydrophilicity (P4VP > PS-b-P4VP > PS) of the polymers can be estimated from water contact angle measurement, suggesting that the composition trajectory of PS will be closest to the polymer-solvent line due to a lower water condensation rate. Thus, for the PS case, the trajectory is more likely to pass the binodal decomposition range and result in spherical pores. The highly interconnected pore morphology for the PS-b-P4VP BCP can be explained by the spinodal decomposition phenomenon, which is induced by the relatively increased water condensation rate, pushing the composition trajectory to the inside of the two-phase region. It is also noteworthy that the blending of QDs with hydrophilic ligands into the BCP system resulted in more porous structures than in the pure polymer cases (Figure S1c). We now demonstrate how the porous structure of the polymer matrix affects emission properties of the embedded QDs. The PL intensity of the BCP-QDs (QDs dispersed in BCP films with interconnected pores) film was 19 and seven times higher than those of the P4VP-QDs (composite of QDs and flat P4VP films) and PS-QD (non-uniform composite of QDs and PS with spherical pores), respectively (Figure 2b). In the case of the PS-QD film, for a more uniform dispersion of the QDs, they were capped with PS ligands. However, unlike the BCP-QD composite, the PS-QD system shows a flat surface on the entire film (Figure S1d). This may be due to the lower surface

8


Page 8 of 37

Page 9 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

energy of PS, which does not allow the coexistence of the solvent-rich phase at the air-polymer interface. As the simulation results presented in Figure 4d suggest, the flat surface causes internal reflection and trapping of emitted light. Furthermore, in a humid condition, the fast influx of water into the hydrophobic PS might induce instability of PS, causing cracks and non-uniform film structures (Figure 2a and S1d). Additionally, QDs are locally aggregated in the PS matrix, as presented in Figure 4e, indicating the significant FRET effect. These combined effects may have caused less effective PL enhancement than the BCP composites. A red-shift of the PL peak in the solid-state QD compared to the solution-state QD can be used as an indicator of QD aggregation, i.e. decreased distance between the QDs. This is because QDs with a large energy bandgap transfer energy into QDs with a smaller energy bandgap in the QD aggregates.35, 36 In our PL data, the emission peak of the BCP-QD film (654 nm) shows a 10 nm red-shifted value compared to that of the solution state (644 nm), while the PS-QD and P4VP-QD respectively have the 21 nm and 18 nm red-shifted peaks compared to each solution (note: the solution peak of PS ligand-exchanged QDs is at 642 nm). This result supports a more uniform dispersion state of QDs in the BCP-QD sample via selective segregation of QDs in P4VP domains at a nanoscale dimension, implying the successful formation of hierarchical structures, as designed above (Figure 1). We also confirmed that the RH and the BCP concentrations of the cast solution play important roles in determining the final morphology of the porous BCP-QD structures. Cross-sectional SEM images of the BCP-QD films obtained with variation of the parameters are shown (Figure 3a). The samples obtained under 35% RH exhibited no porosity because of an insufficient water supply for inducing VIPS. On the contrary, with 55% and 75% RH, randomly interconnected pores were uniformly formed from 13 wt% to 15 wt% BCP solutions. However, for higher BCP

9


Nano Letters 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

concentrations (18 wt% ~), relatively spherical pores with a larger size of a few μm were formed, which is due to the inefficient mass transport of water across the surface-normal direction resulting from the dense polymer network. In order to investigate the correlation between the morphology of the BCP-QD composite and the emission property of QDs, we plotted the PL intensity depending on the RH and the BCP concentrations in the casting solution (Figure 3b). For the 35% RH condition, the PL intensity was much lower and gradually increased with the BCP concentration. In contrast, the PL intensity for the sample prepared at 75% RH was 37-fold higher than that of the 35% RH sample. Also, the PL intensity of the samples was highly dependent on BCP concentration in the solution, suggesting that the massive enhancement of PL intensity is achieved by the random pores formed via spinodal decomposition. For further confirmation, we also measured the PL intensity of the composite after removing the random pores via warm solvent-vapor annealing (WSA)37 treatment on the porous BCP-QD composite by keeping the sample under toluene vapor at a temperature of 40 °C for 2 minutes, providing mobility of the polymer chains. Figure 3c presents the gradual removal of pores with increasing WSA treatment time, which also accompanied the disappearance of surface roughness and the optical haziness (Figure S2a and S2b). As shown in Figure 3d, the removal of random pores dramatically reduced the PL intensity to approximately 5% of the original intensity, suggesting a strong correlation between random pores and PL enhancement. Note that the WSA does not damage the QDs as it entails mild annealing conditions (Figure S2c for PL spectra of pure QD films with WSA). Furthermore, a detailed analysis of the correlation between the porosity of the film and the enhancement of light absorption and extraction is presented in Supplementary Note 1 and Figure S3.

10


Page 10 of 37

Page 11 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

We compared the PL intensity of the BCP-QD and pure QD samples (Figure 4a) with a similar mass of QDs per area (Table S1 for inductively coupled plasma-mass spectrometry (ICP-MS) measurement data). Despite an almost 12% smaller amount of the elements of QDs in the BCPQD composite, its PL intensity was measured to be 21-fold higher than that of the pure QD film. Furthermore, we conducted PL analysis on a pure BCP film (Figure S4), and the negligible photoemission from the BCP was confirmed. Also, the much closer emission peak of the BCP-QD (654 nm) to that of the solution state (644 nm) indicates a uniform dispersion of QDs by the BCP (the peak of pure QD film = 667 nm). This PL result is consistent with the markedly superior brightness of the BCP-QD composite compared to the pure QD film under the same ultraviolet light source (λex = 340 nm) (Figure 4b) Absolute PL QY of the pure QD and BCP-QD films was measured as well with an integrating sphere (Table S2). A detailed discussion of the contribution of each enhancement factor (enhanced light absorption and extraction, and reduced FRET) to the absolute PL QY is in Supplementary Note 2. To understand the extraordinary PL contrast between the porous (as-spun) and non-porous (WSA 120 s) structures, we performed 3D finite difference time domain (FDTD) simulations and investigated the absorption and emission properties of these structures. In order to model the complex geometry of the porous BCP-QD structure reliably in the optical simulations, we generated quasi-random pore structures that share the important geometric features with the actual structure: the average thickness of the consisting layers (550 nm), the porosity (60%), and bicontinuous morphology (see Supplementary Note 3 and Figure S5 for details). The porous structure absorbs incident light more efficiently compared to the non-porous film. As shown in Figure 4c, the absorption enhancement factor at the excitation wavelength for the PL measurements (450 nm) is obtained as 3.5 from the measured data and 4.4 from the simulations,

11


Nano Letters 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 37

thus showing similar results. We note that the observed absorption enhancement approaches the ergodic light trapping limit,38 which can be derived from the following equation (1). 2𝑛 , which is calculated from 𝑛

4.87

(1)

1.56. This can be attributed to the sufficient randomization

of the wave vector induced by strong random scattering processes in the porous structure. The porous structure is also superior to the flat structure from the perspective of light extraction. For the non-porous flat structure, since the in-plane wave vectors of the light waves emitted from a QD are preserved upon the reflections at the interfaces, a significant portion of emitted light undergoes total internal reflection (TIR) at the top surface of the film and thus cannot outcouple to free space. For a horizontally oriented dipole (p ) in the non-porous structure, the average light extraction efficiency (avg. LEE) is calculated to be about 0.139 at the emission wavelength (654 nm) (see Figure S6 for details). For a vertically oriented dipole (p ), almost no energy can be extracted (LEE ~ 0.036), as the major portion of the light is initially emitted along a horizontal direction, as shown in the left panel of Figure 4d. In contrast, the direction of the light emitted from a QD in the porous structure is quickly randomized and loses its directional information via abundant light scattering by randomly interconnected pores and rough surface. As a result, the light extraction behavior of a horizontal and vertical dipole becomes nearly indistinguishable, as shown in the right panel of Figure 4d. More importantly, due to the wave vector randomization, the portion of light originally under the constraint of TIR acquires opportunities to outcouple to free space. Here, the efficiency of light extraction is limited by the statistical optics rather than the simple Snell’s law. Our optical simulations show that the LEE of the porous structure is about 0.36 regardless of the dipole orientation, which is enhanced roughly 3.5-fold compared to the overall

12


Page 13 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

LEE of the non-porous structure (table in Figure S6). We further observed that this tendency is consistent over a broad spectral range, 500 – 700 nm (Figure S6c, S6d). Exploiting the selective conjugation property of QDs with the P4VP block, the phase separation of the two blocks enables more uniform dispersion of QDs, which is targeted for reducing FRET. Dispersion of individual QDs was observed from a top-down transmission electron microscope (TEM) images (Figure 4e). Samples of pure QD, PS-QD, P4VP-QD, and BCP-QD were prepared on copper grids with the identical casting conditions (75% RH, 0.1 wt% QD, 0.5 wt% polymers to form monolayer samples). Contrary to the aggregated QDs in the QD film, in the BCP-QD composite film, QDs are well dispersed and selectively incorporated in the cylinder P4VP domains. The assembled structure of cylindrical P4VP domains and QDs was confirmed by SEM and TEM images of mono- and multilayer BCP-QD composite films (Figure S7b and S7c). It should be noted that the original morphology of the PS-b-P4VP BCP (M.W.PS = 12 kg mol-1, M.W.P4VP = 1.7 kg mol-1) was spherical, as shown in Figure S7b (inset). The preferential segregation of functionalized QDs into the P4VP domains is supported by the morphological transition from spherical to cylindrical domains, which is indicative of volume expansion of P4VP as a result of QD incorporation.39 We also compared the degree of PL shift of the solid-state QD composite (Figure 2b) and pure QD samples (Figure 4a) in reference to the solution-state QDs, which is a good indicator of QD to QD separation as described earlier. Therefore, for a better dispersed QD composite, where the nonradiative energy transfer is less efficient, the degree of PL shift would be smaller. As Figure S8a demonstrates, the PL red-shift of BCP-QD (10 nm) was the smallest among the samples (18 – 23 nm). These combined results validate the excellent QD dispersion enabled by the BCP matrix.

13


Nano Letters 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 37

In order to compare the FRET effect, PL decay curves of the QD solution and QD and BCP-QD films were measured by time-correlated single photon counting (TCSPC) (Figure 4f). Relative to the carrier lifetime of the pure QD film, the value of the BCP-QD film largely increased and was close to that of the QD solution in which FRET hardly occurs. This indicates a substantially reduced FRET in the BCP-QD film. A detailed analysis of the PL decay components of CuInS2/ZnS40 including the calculation of average carrier lifetime and FRET efficiency was performed (Table S3). The FRET efficiency was calculated based on the following equation (2) where 𝜏

and 𝜏 denote the lifetime of the donor with and without the acceptor, respectively.41,

42

1

(2)

As the lifetime of the QD solution is considered as 𝜏 (assuming 0% FRET efficiency in a solution state due to a large enough QD-to-QD distance), the lifetime of each sample was put into 𝜏

for

the calculation. The FRET efficiency of the pure QD and porous BCP-QD films was 52.65% and 22.17%, respectively. The reduced FRET efficiency in the BCP-QD composite agrees well with the decreased PL peak shift (in reference to QD solution) compared to pure QD samples (Figure S8a), and is also consistent with the observation that, with increasing BCP concentration in the blended solution, the emission peaks become closer to that of the QD solution (Figure S8b). Overall, these calculation and characterization results clarify the individual contributions of enhanced light absorption and extraction and non-radiative energy transfer in the porous composite. The FDTD simulation provided individual enhancement factors for light absorption (4.4) and light extraction (3.5) compared to the non-porous sample. Considering that the reduced FRET effect can additionally contribute to the experimentally measured total PL enhancement factor (17.6) shown in Figure 3d, the proposed PL enhancement mechanism can be validated by 14


Page 15 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

these calculation and characterization results. Furthermore, we applied our hierarchical BCP structures to green-emitting CdSe/ZnS QDs. PL spectra of the QD solution, QD film, and BCPQD film are shown in Figure S9. The BCP - CdSe/ZnS QD composite recorded 14-fold PL enhancement compared to the pure QD sample, showing the potential generality of this PL enhancement effect by the randomly porous BCP matrix. Additionally, a comparison of peak positions, in reference to the samples with CuInS2/ZnS QDs (Figure 4a), is presented in Figure S9. CuInS2/ZnS and CdSe/ZnS QDs exhibit an identical tendency that the red-shift of PL was the smallest for the BCP-QD composite, although the degree of the PL shift is different because the thicker shell (ZnS) of the CdSe QDs reduces the FRET effect. The less effective contribution of the reduced FRET can explain the 14-fold PL enhancement in the Cd-based QDs, which is relatively smaller than the 21-fold PL enhancement of CuInS2/ZnS QDs. We present here that the BCP-QD composite can be applied as a practical down-conversion material for LEDs. The schematic in Figure 5a presents the fabrication method and diagram of the QD-LEDs. Under controlled humidity, QD and BCP-QD solutions are cast on LED chips to fabricate the QD-enhanced LEDs, where blue backlight is converted into the red color of the QDs. Operation of pure LEDs and LEDs covered with a QD film (LED/QD) and a BCP-QD film (LED/BCP-QD) is demonstrated in Figure 5b. The photographs demonstrate more reddish light emission from the BCP-QD/LED device, which is consistent with the luminance spectra presented in Figure 5c. In comparison with the sample of pure QDs, the BCP-QDs achieved seven-fold enhanced red-color luminance and consequently more efficient color conversion, as shown in the Commission Internationale de l’Éclairage (CIE) 1931 x-y color coordinates (inset of Figure 5c). The down-converted LED exhibited the CIE coordinates of (0.159, 0.036) for the QD sample and (0.249, 0.094) for the BCP-QD sample. An angle-dependent red-emitting luminance of the QD 15


Nano Letters 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

and BCP-QD samples is presented as well (Figure 5d). Compared to the QD case, a wider viewing angle is demonstrated for the BCP-QD composite (the angle from 70 to 80 °). To confirm the stability of the BCP-QD composite for the down-conversion application, we investigated the morphological and optical stability of the BCP-QD composite. With a harsh operation condition (100 ℃ for 1 day) of an LED, the randomly porous structure of the BCP-QD film was maintained well whereas the pure BCP film was transformed into a completely homogeneous film without any pores (Figure S10a). The origin of the superior thermal stability is the strong hydrogen bond linkage between the ligands of QDs and P4VP,43 resulting in highly interlinked P4VP chain structures. Figure S10b is the scheme for the arrangement of BCP-QD, where the P4VP block is interlinked via the hydrogen bonds with the ligands of QDs. Also, the outstanding optical stability of the LED/BCP-QD device, in reference to the device with the pure QD film, was confirmed (Figure S11). Compared to the pure QD case, the BCP-QD device exhibited superior optical stability under the continuous illumination of the blue LED, indicating suppressed photo-degradation of QDs. The passivation effect by BCP can inhibit the penetration of water and oxygen into the surface of QDs, suppressing the oxidation of the QD surface. This passivation effect is further supported by the maintained stability even at extreme conditions (water immersion for 5 hours) (Figure S12). The improved morphological and optical stability support a suitable application of the BCP-QD film as a down-conversion color filter. In conclusion, this study demonstrates a substantially intensified PL from selectively conjugated QDs in a multi-scale phase-separated PS-b-P4VP BCP matrix. This hierarchical BCP-QD composite can effectively and collectively enhance light absorption and extraction via (1) the highly interconnected random pore structure with sub-μm dimension formed by vapor-induced phase separation, and reduce the FRET effect via (2) the selective dispersion of QDs in the P4VP

16


Page 16 of 37

Page 17 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

block of sub-10 nm scale domains by microphase separation of the blocks. We reveal that the bicontinuous empty pores can be obtained under systematic control of humidity and the choice of an amphiphilic BCP matrix via a three-component (BCP/solvent/water) spinodal decomposition phenomenon. Also, the selective dispersion of QDs can be induced by engineering the QD ligands to exploit the hydrogen bond linkages with the P4VP block. Our porous BCP-QD composite achieved 18- and 21-fold enhancement in PL intensity compared to the non-porous composite and pure QD film, respectively. The full-wave optical simulations demonstrate that random scattering of light in the porous composite leads to 4.4- and 3.5-fold enhanced light absorption and light extraction, respectively. The sub-μm pore structures effectively increase the optical path length of incident light approaching the ergodic limit. Through the random medium, the direction of the emitted light is quickly randomized, and the portion of light originally under the constraint of TIR acquires opportunities to be extracted. Also, the FRET efficiency of the BCP-QD film was much lower than that of the pure QD film, supporting the effectiveness of QD dispersion by the BCP microdomains. We finally demonstrated a successful application of the BCP-QD composite for down-converted LED devices, achieving seven-fold enhancement of luminance and a widened viewing angle compared to the pure QD device. We expect that further studies based on the incorporation of various light-emitting materials of high emission efficiency will realize high-performance display and lighting devices. Moreover, the fabrication strategy of the composite is versatile in allowing various flexible substrates such as polyethylene terephthalate (PET) and polyimide (PI) films (Figure S13). Also, given the lack of facile fabrication methods for well-designed multi-scale structures with an effective lightinteraction capability, our one-step assembly tool would offer a versatile and scalable methodology

17


Nano Letters 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

for various optoelectronic or photovoltaic applications where maximum device efficiency and/or minimum power consumption are pursued.

Methods Materials. Copper iodide (>99.5%), indium (III) acetate (99.99%, trace metals basis), 1-dodecanethiol (>98), 1-octadecene (90%), dodecylamine (>99%), and zinc acetate (99.99%) were purchased from Sigma Aldrich and used to synthesize CuInS2/ZnS (core/shell). 11-mercapto1-undecanol (MUD, 99%), thiol-terminated polystyrene (molecular weight (M.W.) = 12.4 kg mol1

), toluene (>99.8%), methanol (>99.8%), acetone (>99.9 %), and DMF (anhydrous, 99.8%) were

purchased from Sigma Aldrich and used to exchange the ligands. P4VP (M.W. = 12 kg mol-1), PS (M.W. = 12 kg mol-1), and PS-b-P4VP (M.W.PS = 12 kg mol-1, M.W.P4VP = 1.7 kg mol-1) were purchased from Polymer Source and used to make polymer-QD blended solutions. Synthesis and ligand exchange of CuInS2/ZnS. Copper iodide (38.1 mg), indium acetate (58.4 mg), 1-dodecanethiol (2 ml), and 1-ocatdecene (16 ml) were added in a three-neck flask and heated to 120 ℃ for 60 minutes under a vacuum. The precursor solution was heated again to 210 ℃ for 70 minutes under an argon flow (1 atm) in order to synthesize cores. For the synthesis of the ZnS shell, dodecylamine (1.0 ml) and zinc acetate (131.7 mg) were injected into the resulting solution and maintained with a temperature of 210 ℃ for 30 minutes. This step was repeated three more times for sufficient growth of the shell. To exchange the ligands of the QDs, MUD (1.634 g) was injected into the synthesized QD solution while for the homogeneous mixing of PS and QDs, the thiol-terminated PS were mixed with the QD solution by a weight ratio of 6.6 (PS ligand) to 1 (QD solution). The exchange process was taken at 210 ℃ under an argon flow (1 atm) and preceded for 30 minutes. The solution was then centrifuged for purity and dissolved into DMF. The PL QY

18


Page 18 of 37

Page 19 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

of the MUD ligand-exchanged QD solution was 42% with a reference dye of rhodamine 6G (PL QY = 95%, Sigma Aldrich). Fabrication of polymer-QD films with humidity control. In order to make polymer-QD blended solutions, P4VP, PS, and PS-b-P4VP were added to a QD solution at specific ratios, and the solutions were stirred with magnetic stirring bars at 500 r.p.m. for 3 hours. Each blended polymerQD solution was stored in a refrigerator (under 5℃) for 20 minutes to make the evaporation rate of solution slow during spin-casting, hence increasing the water penetration rate. An optical slide glass (1000 μm thickness, Marienfeld Superior) was used for the fabrication of polymer-QD films. By varying the flow of humid and dry air, the desired RH value at 25℃ was acquired, and the solution was spun-cast at 2000 r.p.m. for 200 seconds. Characterization of composite films. FT-IR spectroscopy (IFS66V/S & Hyperion 3000, Bruker Optiks) with ATR mode was used to confirm the successful conjugation of QDs with the BCP. A contact angle analyzer (Phoenix, SEO) was used to compare the relative degree of surface energy in P4V-QD, PS-QD, and BCP-QD films. Cross-sectional and top-down SEM images of composite films were observed by field emission (FE)-SEM (S4800, Hitachi) with osmium coatings. PL spectra of QD and polymer-QD composite films were measured by photoluminescence spectroscopy (MPT, PSI). AFM (XE-100, Park systems) was performed to analyze the surface morphology of the BCP-QD films. The compositions of QDs in the QD and BCP-QD samples were measured by ICP-MS (7700S, Agilent). To selectively dissolve the glass from the film, an acid solution of 70% HNO3 and 35% HCl was used. The dissolution process was carried out at 200 ℃ for 30 minutes. The diffuse transmittance and the reflectance of the BCP-QD composite samples were measured by UV-vis spectroscopy (Solid Spec–3700, Shimadzu) to obtain absorption spectra. The distribution of QDs in monolayer QD, BCP-QD, and PS-QD films was

19


Nano Letters 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

confirmed by FE-TEM (Talos F200X, FEI). The QD film was fabricated on a holey carbon grid (Cu, 300 mesh, 50 micron), and PS-QD, P4V-QD, and BCP-QD films were fabricated on lacey carbon grids (Cu, 300 mesh) by spin-casting. To confirm the P4VP domains in monolayer BCP and BCP-QD films by SEM, a platinum incorporation was conducted for 12 hours, and then, the sample was treated with O2 plasma to etch top-coated organic layers (source power = 30 W, 40 s). The nanostructure of multilayer BCP-QD composites was confirmed by a plan-view high angle annular dark field (HAADF) scanning TEM (STEM). TCSPC (FL920, Edinburgh instruments) was used to observe the PL decay dynamics of the QD solution and QD and BCP-QD composite films at the emission wavelength of 654 nm using a 470 nm pulsed laser source. Full-wave electromagnetic simulation. Commercial finite difference time domain (Lumerical FDTD) software was used to calculate light absorption and extraction in non-porous and porous structures of BCP-QD films. Here, the frequency-dependent dielectric function of the BCP-QD composite was extracted from the measured transmission spectra of the non-porous BCP-QD films deposited on glass. The light extraction efficiency (LEE) of a single oscillating dipole was obtained as the ratio of the outcoupled light energy to the total emitted energy. The overall LEEs of the devices were calculated by averaging the LEEs of individual dipoles over their positions and orientations. Down-conversion QD-LEDs. LED chips (SMD LED 5050 B) were purchased from a commercial supplier (YNT Korea). In order to fabricate QD-LEDs by spin-casting, a LED chip was attached to a silicon wafer using a gel pad, and the product was loaded in a spin-coater for humidity control. Device characteristics of luminance and CIE coordinates were obtained by a Source Meter (Keithley 2400) and a Konica Minolta spectroradiometer (CS-2000).

20


Page 20 of 37

Page 21 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] *E-mail: [email protected] *E-mail: [email protected] ORCID Geon Yeong Kim: 0000-0002-0250-6596 Jinyoung Choi: 0000-0001-7566-183X Hyeuk Jin Han: 0000-0002-7721-9065 Hu Young Jeong: 0000-0002-5550-5298 Sung-Yool Choi: 0000-0002-0960-7146 Min Seok Jang: 0000-0002-5683-1925 Yeon Sik Jung: 0000-0002-7709-8347

21


Nano Letters 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Author Contributions G.Y.K., Y.S.J., D.Y.J., and S.-Y.C. conceived the project. G.Y.K. conducted most of the fabrication and composite-based analysis experiments. S.K. and M.S.J. performed the optical simulations and theoretical analyses. J.C., C.L., and D.Y.J. conducted the synthesis of QDs and TCSPC analysis. M.K., W.C., and E.N.C. contributed to the analysis of QD-LEDs. H.L., T.W.N., and H.J.H. contributed to microstructure analyses. J.C.K. and H.Y.J. conducted the analysis of the multilayer BCP-QD film. G.Y.K., Y.S.J., M.S.J., and S.K. wrote most of the manuscript. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This research was supported by Creative Materials Discovery Program through the National Research Foundation of Korea (NRF) funded by Ministry of Science and ICT (NRF2016M3D1A1900035 and 2016M3D1A1900038).

22


Page 22 of 37

Page 23 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

References (1)

Alivisatos, A. P. Semiconductor Clusters, Nanocrystals, and Quantum Dots. Science 1996, 271 (5251), 933–937.

(2)

Bawendi, M. G.; Carroll, P. J.; Wilson, W. L.; Brus, L. E. Luminescence Properties of CdSe Quantum Crystallites: Resonance between Interior and Surface Localized States. J. Chem. Phys. 1992, 96 (2), 946–954.

(3)

Sun, Q.; Wang, Y. A.; Li, L. S.; Wang, D.; Zhu, T.; Xu, J.; Yang, C.; Li, Y. Bright, Multicoloured Light-Emitting Diodes Based on Quantum Dots. Nat. Photonics 2007, 1 (12), 717–722.

(4)

Colvin, V. L.; Schlamp, M. C.; Alivisatos, A. P. Light-Emitting Diodes Made from Cadmium Selenide Nanocrystals and a Semiconducting Polymer. Nature 1994, 370 (6488), 354–357.

(5)

Coe, S.; Woo, W. K.; Bawendi, M.; Bulović, V. Electroluminescence from Single Monolayers of Nanocrystals in Molecular Organic Devices. Nature 2002, 420 (6917), 800– 803.

(6)

Hikmet, R. A. M.; Chin, P. T. K.; Talapin, D. V.; Weller, H. Polarized-Light-Emitting Quantum-Rod Diodes. Adv. Mater. 2005, 17 (11), 1436–1439.

(7)

Park, Y. R.; Jeong, H. Y.; Seo, Y. S.; Choi, W. K.; Hong, Y. J. Quantum-Dot Light-Emitting Diodes with Nitrogen-Doped Carbon Nanodot Hole Transport and Electronic Energy Transfer Layer. Sci. Rep. 2017, 7.

23


Nano Letters 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(8)

Chen, H.; He, J.; Wu, S.-T. Recent Advances on Quantum-Dot-Enhanced Liquid-Crystal Displays. IEEE J. Sel. Top. Quantum Electron. 2017, 23 (5), 1–11.

(9)

Samsung Display Co. A Guide to the Evolution of Quantum Dot Displays Home Page. https://pid.samsungdisplay.com/en/learning-center/white-papers.

(10) Lee, J.; Sundar, V. C.; Heine, J. R.; Bawendi, M. G.; Jensen, K. F. Full Color Emission from II ± VI Semiconductor. Adv. Mater. 2000, 12 (15), 1102–1105. (11) Kim, B.; Jun, S.; Lim, J.; Kim, Y.; Jang, H.; Jang, E. White-Light-Emitting Diodes with Quantum Dot Color Converters for Display Backlights. Adv. Mater. 2010, 22 (28), 3076– 3080. (12) Fazio, B.; Artoni, P.; Iat, M. A.; D’Andrea, C.; Lo Faro, M. J.; Del Sorbo, S.; Pirotta, S.; Gucciardi, P. G.; Musumeci, P.; Vasi, C. S.; et al. Strongly Enhanced Light Trapping in a Two-Dimensional Silicon Nanowire Random Fractal Array. Light: Sci. Appl. 2016, 5, 1–7. (13) Martins, E. R.; Li, J.; Liu, Y.; Depauw, V.; Chen, Z.; Zhou, J.; Krauss, T. F. Deterministic Quasi-Random Nanostructures for Photon Control. Nat. Commun. 2013, 4, 1–7. (14) Lee, W.-K.; Yu, S.; Engel, C. J.; Reese, T.; Rhee, D.; Chen, W.; Odom, T. W. Concurrent Design of Quasi-Random Photonic Nanostructures. Proc. Natl. Acad. Sci. 2017, 114 (33), 8734–8739. (15) Vynck, K.; Burresi, M.; Riboli, F.; Wiersma, D. S. Photon Management in TwoDimensional Disordered Media. Nat. Mater. 2012, 11 (12), 1017–1022. (16) Shigeta, H.; Kawakami, Y.; Oskooi, A.; Favuzzi, P. A.; Noda, S.; Tanaka, Y. Partially 24


Page 24 of 37

Page 25 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

Disordered Photonic-Crystal Thin Films for Enhanced and Robust Photovoltaics. Appl. Phys. Lett. 2012, 100 (18), 181110. (17) Pan, C.-L.; Peng, C.-Y.; Chen, K.-H.; Lo, H.-C.; Huang, Y.-F.; Hsu, Y.-K.; Jen, Y.-J.; Chattopadhyay, S.; Chen, L.-C.; Lee, C.-S.; et al. Improved Broadband and QuasiOmnidirectional Anti-Reflection Properties with Biomimetic Silicon Nanostructures. Nat. Nanotechnol. 2007, 2 (12), 770–774. (18) Chen, Z. H.; Qiao, N.; Yang, Y.; Ye, H.; Liu, S.; Wang, W.; Wang, Y. Enhanced Broadband Electromagnetic Absorption in Silicon Film with Photonic Crystal Surface and Random Gold Grooves Reflector. Sci. Rep. 2015, 5, 1–8. (19) Lim, J.; Jeong, B. G.; Park, M.; Kim, J. K.; Pietryga, J. M.; Park, Y. S.; Klimov, V. I.; Lee, C.; Lee, D. C.; Bae, W. K. Influence of Shell Thickness on the Performance of LightEmitting Devices Based on CdSe/Zn 1- X Cd x S Core/Shell Heterostructured Quantum Dots. Adv. Mater. 2014, 26 (47), 8034–8040. (20) Lee, K.-H.; Park, B.; Ko, H.; Kwon, Y.; Lee, J.; Lee, C.; Yang, H.; Kang, H.-D.; Lee, J.-H. Over 40 Cd/A Efficient Green Quantum Dot Electroluminescent Device Comprising Uniquely Large-Sized Quantum Dots. ACS Nano 2014, 8 (5), 4893–4901. (21) Halivni, S.; Shemesh, S.; Waiskopf, N.; Vinetsky, Y.; Magdassi, S.; Banin, U. Inkjet Printed Fluorescent Nanorod Layers Exhibit Superior Optical Performance over Quantum Dots. Nanoscale 2015, 7 (45), 19193–19200. (22) Pal, B. N.; Ghosh, Y.; Brovelli, S.; Laocharoensuk, R.; Klimov, V. I.; Hollingsworth, J. A.; Htoon, H. “Giant” CdSe/CdS Core/Shell Nanocrystal Quantum Dots as Efficient 25


Nano Letters 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Electroluminescent Materials: Strong Influence of Shell Thickness on Light-Emitting Diode Performance. Nano Lett. 2012, 12 (1), 331–336. (23) Chern, M.; Nguyen, T. T.; Mahler, A. H.; Dennis, A. M. Shell Thickness Effects on Quantum Dot Brightness and Energy Transfer. Nanoscale 2017, 9 (42), 16446–16458. (24) Lunz, M.; Bradley, A. L.; Gerard, V. A.; Byrne, S. J.; Gun’Ko, Y. K.; Lesnyak, V.; Gaponik, N. Concentration Dependence of Förster Resonant Energy Transfer between Donor and Acceptor Nanocrystal Quantum Dot Layers: Effect of Donor-Donor Interactions. Phys. Rev. B - Condens. Matter Mater. Phys. 2011, 83 (11), 1–10. (25) Gather, M. C.; Reineke, S. Recent Advances in Light Outcoupling from White Organic Light-Emitting Diodes. J. Photon. Energy 2015, 5 (1), 053599. (26) Song, J.; Kim, K.-H.; Kim, E.; Moon, C.-K.; Kim, Y.-H.; Kim, J.-J.; Yoo, S. Lensfree OLEDs with over 50% External Quantum Efficiency via External Scattering and Horizontally Oriented Emitters. Nat. Commun. 2018, 9 (1), 3207. (27) Wang, R.; Xu, L. H.; Li, Y. Q.; Zhou, L.; Li, C.; Ou, Q. D.; Chen, J. De; Shen, S.; Tang, J. X. Broadband Light Out-Coupling Enhancement of Flexible Organic Light-Emitting Diodes Using Biomimetic Quasirandom Nanostructures. Adv. Opt. Mater. 2015, 3 (2), 203– 210. (28) Venault, A.; Chang, Y.; Wang, D.-M.; Bouyer, D. A Review on Polymeric Membranes and Hydrogels Prepared by Vapor-Induced Phase Separation Process. Polym. Rev. 2013, 53 (4), 568–626.

26


Page 26 of 37

Page 27 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

(29) Van de Witte, P.; Dijkstra, P. J. J.; van den Berg, J. W. A. W. A.; Feijen, J. Phase Separation Processes in Polymer Solutions in Relation to Membrane Formation. J. Memb. Sci. 1996, 117 (1–2), 1–31. (30) Nunes, S. P.; Inoue, T. Evidence for Spinodal Decomposition and Nucleation and Growth Mechanisms during Membrane Formation. J. Memb. Sci. 1996, 111 (1), 93–103. (31) Fashandi, H.; Karimi, M. Pore Formation in Polystyrene Fiber by Superimposing Temperature and Relative Humidity of Electrospinning Atmosphere. Polymer (Guildf). 2012, 53 (25), 5832–5849. (32) Doménech-Carbó, M. T.; Aura-Castro, E.; Domenech-Carbo, M. T. Evaluation of the Phase Inversion Process as an Application Method for Synthetic Polymers in Conservation Work. Stud. Conserv. 1999, 44 (1), 19. (33) Ploshnik, E.; Salant, A.; Banin, U.; Shenhar, R. Hierarchical Surface Patterns of Nanorods Obtained by Co-Assembly with Block Copolymers in Ultrathin Films. Adv. Mater. 2010, 22 (25), 2774–2779. (34) Alberti, S.; Gladfelter, A.; Mittag, T. Considerations and Challenges in Studying LiquidLiquid Phase Separation and Biomolecular Condensates. Cell 2019, 176 (3), 419–434. (35) Chou, K. F.; Dennis, A. M. Förster Resonance Energy Transfer between Quantum Dot Donors and Quantum Dot Acceptors. Sensors 2015, 15 (6), 13288–13325. (36) Crooker, S. A.; Hollingsworth, J. A.; Tretiak, S.; Klimov, V. I. Spectrally Resolved Dynamics of Energy Transfer in Quantum-Dot Assemblies: Towards Engineered Energy

27


Nano Letters 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Flows in Artificial Materials. Phys. Rev. Lett. 2002, 89 (18), 18–21. (37) Kim, J. M.; Kim, Y. J.; Park, W. I.; Hur, Y. H.; Jeong, J. W.; Sim, D. M.; Baek, K. M.; Lee, J. H.; Kim, M. J.; Jung, Y. S. Eliminating the Trade-off between the Throughput and Pattern Quality of Sub-15 Nm Directed Self-Assembly via Warm Solvent Annealing. Adv. Funct. Mater. 2015, 25 (2), 306–315. (38) Yablonovitch, E.; Cody, G. D. Intensity Enhancement in Textured Optical Sheets for Solar Cells. IEEE Trans. Electron Devices 1982, 29 (2), 300–305. (39) Zhang, L.; Cui, T.; Cao, X.; Zhao, C.; Chen, Q.; Wu, L.; Li, H. Inorganic-Macroion-Induced Formation of Bicontinuous Block Copolymer Nanocomposites with Enhanced Conductivity and Modulus. Angew. Chemie - Int. Ed. 2017, 56 (31), 9013–9017. (40) Wang, X.; Liang, Z.; Xu, X.; Wang, N.; Fang, J.; Wang, J.; Xu, G. A High Efficient Photoluminescence Zn-Cu-In-S/ZnS Quantum Dots with Long Lifetime. J. Alloys Compd. 2015, 640, 134–140. (41) He, T.; Chen, R.; Lim, Z. B.; Rajwar, D.; Ma, L.; Wang, Y.; Gao, Y.; Grimsdale, A. C.; Sun, H. Efficient Energy Transfer under Two-Photon Excitation in a 3D, Supramolecular, Zn(II)-Coordinated, Self-Assembled Organic Network. Adv. Opt. Mater. 2014, 2 (1), 40– 47. (42) Feng, Y.; Liu, L.; Hu, S.; Liu, Y.; Ren, Y.; Zhang, X. Förster Resonance Energy Transfer Properties of a New Type of Near-Infrared Excitation PDT Photosensitizer: CuInS2/ZnS Quantum Dots-5-Aminolevulinic Acid Conjugates. RSC Adv. 2016, 6 (60), 55568–55576.

28


Page 28 of 37

Page 29 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

(43) Rittigstein, P.; Priestley, R. D.; Broadbelt, L. J.; Torkelson, J. M. Model Polymer Nanocomposites

Provide

an

Understanding

of

Nanocomposites. Nat. Mater. 2007, 6 (4), 278–282.

29


Confinement

Effects

in

Real

Nano Letters 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

FIGURES

Figure 1. Design of a hierarchical BCP-QD composite via multi-scale phase separations. (a) An illustration of the randomly porous BCP-QD film at a sub-micrometer scale. Black arrows in the image indicate random light scattering of incident and emissive light. (b) A schematic of BCP microdomains selectively embedded with QDs at the sub-10 nm scale. Via hydrogen bonding between QDs and P4VP blocks, a fine dispersion of QDs is induced for reducing undesirable FRET.

30


Page 30 of 37

Page 31 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

Surface-ligands of QDs for the selective binding with P4VP are 11-mercapto-1-undecanol (MUD). (c) An illustration of the humidity-controlled spin-casting equipment.

31


Nano Letters 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 2. Control of random pore network via vapor-induced phase separation. (a) Schematic ternary phase diagrams for different polymer systems (P4VP, PS, and PS-b-P4VP) and resulting morphologies with cross-sectional scanning electron microscope (SEM) images (scale bars: 10 μm, 75% RH, 15 wt% polymer, and 3 wt% QD in DMF solution). Solid arrows in the phase diagrams 32


Page 32 of 37

Page 33 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

indicate expected composition paths during solvent evaporation. Schematics inserted inside blue arrows indicate the different intermediate-morphologies depending on polymer matrices. (b) Photoluminescence (PL) spectra (λex: 450 nm) of QD solution (with PS ligands and MUD ligands), P4VP-QD, PS-QD, and BCP-QD films. Dashed lines in the PL spectra indicate the position of emission peaks. The emission spectra of QD solutions are plotted with short dots.

33


Nano Letters 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 3. Correlation between porous structures and PL intensity of QDs. (a) Cross-sectional SEM images (scale bars: 5 μm) and (b) plotted PL intensity (λex: 450 nm) of BCP-QD films with different RH and BCP concentrations in DMF solution. Each average thickness of the films is presented in the cross-sectional SEM images. Error bars in the plotted PL graph represent the standard deviation of PL in each sample. (c) Cross-sectional SEM images (scale bar: 5 μm) and (d) PL spectra (λex: 450 nm) of BCP-QD films (75% RH, 15 wt% BCP, and 3 wt% QD in DMF solution) treated with warm solvent-vapor annealing (WSA).

34


Page 34 of 37

Page 35 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

Figure 4. Enhanced light absorption and extraction, and reduced FRET in BCP-QD composites. (a) PL spectra (λex: 450 nm) of QD solution, QD film, and BCP-QD film. Dashed lines in the PL spectra indicate the position of emission peaks. The emission spectra of QD solution is plotted with short dots. (b) Optical images of pure QD and BCP-QD films under ultraviolet light (λex: 340 nm) (75% RH, 15 wt% BCP, and 3 wt% QD in DMF solution). Dashed lines in the PL spectra indicate the position of emission peaks. (c) Experimentally measured (left) and simulated (right) absorption graph of the BCP-QD films. (d) Electric field distribution of dipole radiation at 35


Nano Letters 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

the center of the non-porous (left) and porous structures (right). The top and bottom panels show the emission patterns of a horizontal (x-direction) and vertical (y-direction) dipole, respectively. The scale bar is 5 μm. (e) Top-down transmission electron microscope (TEM) images of QD, PSQD, P4VP-QD, and BCP-QD films (75% RH, scale bar: 10 nm). The bottom-left insets of each image are illustrations of the QD distribution. Yellow circles indicate the positions of QDs. Squares with red dashed lines in the PS-QD film represent the locally aggregated QD patterns. White dashed lines in the BCP-QD film indicate distributed cylinder-like QD patterns. The average diameter of a QD is 2.56 nm. (f) PL decay curves of QD solution and QD and BCP-QD films (75% RH). 𝜏

represents the average carrier lifetime of each sample.

36


Page 36 of 37

Page 37 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

Figure 5. Down-conversion QD-LEDs. (a) Preparation of down-conversion QD-LEDs using porous BCP-QD composite. (b) Photographs of pure LED, LED/QD, and LED/BCP-QD devices mounted on jigs in bright and dark rooms. The dashed line in the images represents the lightemitting spots in the LED/BCP-QD device. (c) Luminance spectra of the samples. Inset is the CIE 1931 x-y color coordinates of the LED/QD and LED/BCP-QD devices, representing (0.159, 0.036) and (0.249, 0.094), respectively. (d) The angle-dependent red-emitting luminance of the LED/QD and LED/BCP-QD devices. The increased luminance at the high-angle range is due to the inherent light guiding effect of epoxy resin used as a encapsulant in the LED (LED operation voltage: 2.5 V). 37


Order-of-Magnitude Broadband Enhanced Light Emission from

Recommend Documents