Organic Nanophotonics: Self-Assembled Single-Crystalline Homo

Aug 9, 2018 - Our work exhibits the high potential of the novel organic homo/heterostructures being applied in multifunctional organic integrated phot...
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Organic Nanophotonics: Self-Assembled Single-Crystalline Homo-/ Heterostructures for Optical Waveguides Zhi-Zhou Li,†,§ Yi-Chen Tao,†,§ Xue-Dong Wang,*,† and Liang-Sheng Liao*,†,‡ †

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Jiangsu Key Laboratory for Carbon-Based Functional Materials & Devices, Institute of Functional Nano & Soft Materials (FUNSOM), Soochow University, Suzhou, Jiangsu 215123, People’s Republic of China ‡ Institute of Organic Optoelectronics, Jiangsu Industrial Technology Research Institute (JITRI), Wujiang, Suzhou, Jiangsu 215211, People’s Republic of China S Supporting Information *

ABSTRACT: Homo-/heterostructures are of essential importance for the multiple input/output and the wavelength division multiplexing system in optoelectronic circuits. However, the widely used preparation method, e.g., micromanipulation, usually leads to simple and unstable physical contact. Herein, through a “bottom-up” approach, we have successfully fabricated an organic homo-/heterostructures based on the 2,2′-((1E,1′E)-1,4- phenylenebis(ethene-2,1diyl))dibenzonitrile and perylene single crystals, whose formation can be attributed to the low interplanar spacing mismatch rates, which are 2.2% for the homostructure and 4.1% for the heterostructure. More importantly, the multiple input/output channels have been demonstrated in the photonic devices based on the homo-/heterostructures, which also exhibit the novel asymmetric waveguide mode in the homostructure and the switchable transmission signals (green or yellow light) at the same output channel in the heterostructure. Our work exhibits the high potential of the novel organic homo/heterostructures being applied in multifunctional organic integrated photonics circuits at the nanoscale. KEYWORDS: homo-/heterostructure, organic semiconductor, self-assembly, single crystal, optical circuits, optical waveguides

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signals.18,19 The heterostructure that integrates different material building blocks can significantly overcome this challenge by utilizing an active/passive mixed-waveguide mode. Nevertheless, the commonly employed methods in the fabrication of homo-/heterostructures, such as micromanipulation20,21 and lithography,22 usually lead to simple physical contact, which can lead to the instability of the whole structure, a large optical loss, and limited complexity and flexibility of the structural design. Besides, from the micromanipulation and the lithography method, the laser-induced route adopted by Rajadurai Chandrasekar and his colleagues23 is also an important method for the fabrication of heterostructures. The laser was utilized to burn the microcrystals or to trigger a chemical reaction, which leads to the formation of the heterostructures. Furthermore, unique properties, such as the fluorescence change and the delay light,24 were then found. Various photonic devices, such as erasable modulators and remote sensing systems, were fabricated based on the heterostructures, which improved the high application potential of the organic heterostructures. As compared with

o date, organic optoelectronic devices based on single crystals have been realized in organic solid-state lasers (OSSLs),1−3 organic field effect transistors (OFETs),4 optical waveguides,5 and so on.6,7 However, the monotonic waveguide mode and the few output channels in a single rod or wire will restrict the further development in multifunctional optoelectronic devices.8,9 For practical applications in optoelectronic circuits, multiple input/output with diverse means of the lightwaveguide channels is the key component due to the ability to be simultaneously detected by several accessories at specific positions.10,11 Meanwhile, the unique organic homostructure gives the feasibility of more output channels and more lightwaveguide modes. Besides that, the wavelength division multiplexing (WDM) system has exhibited significant status in modern optical communication and optoelectronic integrated circuits, which requires different signals that can be generated and transmitted in the same output channel.12,13 The optical waveguide can be classified into active waveguide mode and passive waveguide mode according to the nature of the propagated optical wave.14,15 In active waveguide, the medium absorbs the source input light and propagates the emission light, while in the passive waveguide, the source input light is directly propagated in the medium.16,17 However, both the active/passive waveguides cannot generate multiple © XXXX American Chemical Society

Received: June 19, 2018 Published: August 9, 2018 A

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Figure 1. (a) Fluorescence microscope image of the as-prepared o-BCB microrods with a scale bar of 50 μm. Inset: Molecular structure of o-BCB. (b) Simulated growth morphology of the o-BCB molecules. (c) Selective area electron diffraction (SAED) pattern of one typical o-BCB organic microrod. Inset: TEM image of the corresponding o-BCB organic microrod with a scale bar of 5 μm. (d) Fluorescence microscope image of individual perylene organic microplates with a scale bar of 20 μm. Inset: Molecular structure of perylene. (e) Simulated growth morphology of the perylene molecules. (f) SAED pattern of one typical perylene microplate. Inset: Corresponding TEM image of the corresponding perylene organic microplate with a scale bar of 1 μm. (g) Schematic diagram of the self-assembly process of the organic homostructure and the heterostructure as well as the multilevel structure.

these “top-down” methods, the “bottom-up” approach can achieve more complex structures with embedded growth interfaces and high spatial and angular precision.25−27 For example, branched heterostructures composed by inorganic semiconductors including groups IV, III−V, and II−VI or a metal have been fabricated by the “bottom-up” method, which can exhibit synthetically encoded properties and function in light-emitting diode (LED) arrays, logic circuits, and biological sensors.28 Compared to the inorganic part, the organic semiconductors are more promising candidates applied for the optoelectronic integrated circuits due to the ease of roomtemperature solution processing, the simple access of molecular structure tailoring, and the capability of selfassembly for “bottom-up” fabrication.29,30 However, since the organic crystals are held together by weak van der Waals intermolecular interactions, the design and rational synthesis of organic homo-/heterostructures is still a huge challenge.31 Herein, we fabricated organic photonic devices based on novel self-assembled homo-/heterostructures with the unique properties of multiple input/output ports, asymmetric waveguide mode, and switchable transmission signals in the same output channel. The formation mechanism of the organic homo-/heterostructures based on 2,2′-((1E,1′E)-1,4phenylenebis(ethene-2,1-diyl))-dibenzonitrile (o-BCB) microrods and perylene microplates was clearly revealed as a facetselective growth principle32 with a stepwise formation process. For the organic homostructure, the highly parallel branched crystals indicate the oriented growth with the intersection angle of 58° from the trunk crystal. The formation of the

homo-/heterostructures can both be attributed to the low interplanar spacing mismatch rate, which is 2.2% for the homostructures and 4.1% for the heterostructures. Furthermore, the optical waveguide devices based on the homostructure exhibited a significant asymmetric waveguide mode that can be converted into a unique complex code for information storage. Meanwhile, the green light (λpeak = 500 nm) and yellow (λpeak = 570 nm) light signals can both be generated and transmitted in the waveguide devices based on the heterostructures simply by changing the exciting positions. Our work offers an in-depth understanding of the rational synthesis of these self-assembled organic homo-/heterostructures and the multilevel structures, which is of great significance for integrated optoelectronic circuits. In the experiment, the building blocks of the onedimensional (1D) o-BCB microrods and the 2D perylene microplates were prepared by a solution-dropping method at room temperature. In a typical process, the o-BCB molecules were well dissolved in mixed solvents of dichloromethane (DCM) and acetonitrile. Then the solution was dropped onto a quartz substrate, and the organic microstructures can be prepared after the solution evaporated. When the volume ratio of DCM to acetonitrile was set as 1:3 and the concentration is 2 mM, the o-BCB organic microrods can be obtained. As shown in Figure 1a, the length of these green-emissive o-BCB microrods ranges from 15 to 45 μm (Figure S1, Supporting Information). From the cross-profile image of the single o-BCB microrod (Figure S2), the rectangular shape has a width of ∼3 B

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μm. The fluorescence spectrum indicates the emission band of 490 nm (Figure S3, green line). To further reveal the self-assembly process of the o-BCB molecules, we simulated the growth morphology based on attachment energies by using the Materials Studio package (Figure 1b and Table S1). The top/bottom faces of the crystals are (001) and (00−1), while the lateral faces are (100) and (011). It is known that the triclinic crystal of o-BCB has lattice parameters of a = 3.9127(6) Å, b = 6.9657(12) Å, c = 15.611(2) Å, α = 95.798(12), β = 93.445(12), and γ = 92.199(13) (Table S2). The unit cell structure of o-BCB crystals from different views along the a/b/c axis is exhibited in Figure S4. Therefore, the clearly observed selective area electron diffraction (SAED) spots and their square symmetry suggest that the o-BCB microrods are single crystals. In Figure 1c, the triangle and circle enclosed SAED spots correspond to (100) and (020) crystal planes with d-spacing values of 3.9 and 3.5 Å, respectively. Based on the corresponding TEM image (the inset of Figure 1c), the o-BCB microrod grows along the [100] crystal direction, which is consistent with the aboveobtained rod-like growth morphology. For another building block of perylene organic microcrystals, the bright yellow edges and blackbody of the as-prepared microplates are observed (Figure 1d). The crystal structure was first reported by Donaldson.33 The fluorescence spectrum demonstrates the emission wavelength of 570 nm (orange line, Figure S3). The broad emission band can be attributed to the excimer state in the perylene microcrystals.34 The length distribution of the as-prepared organic microplates indicates an edge length of 6−10 μm (Figure S5). Impressively, the growth morphology of perylene is predicted to be plate-like, whose top/bottom faces are (100) and lateral faces are (011) and (01−1) (Figure 1e, the simulated parameters are presented in Table S3). As indicated in the SAED pattern (Figure 1f) and the corresponding transmission electron microscopy (TEM) image of one typical microplate (the inset of Figure 1f), the molecules both stack along the [010] direction (b axis) and the [001] direction (c axis), respectively (Figure S6 shows the unit cell structure of perylene crystals from different views along the a/b/c axis). Therefore, through the facile solution self-assembly approach, we can fabricate these two building blocks of 1D o-BCB microrods and 2D perylene microplates, which can be further integrated into the organic homo-/heterostructure at the micro-/nanoscale (Figure 1g). Specifically, the homostructures are composed of o-BCB microrods as both the trunk and the branch, while the heterostructures consist of o-BCB organic microrods and perylene organic microplates. Furthermore, by combining the homostructure and the heterostructure, multilevel organic microstructures can be achieved through the stepwise process. By changing the volume ratio of the DCM and the acetonitrile to 3:2, the o-BCB organic homomicrostructures can be fabricated. As shown in Figure 2a, the as-prepared organic homostructure has a comb-like shape, which consists of the trunk and branch microrods. The length of the trunk crystals can be found as ∼200 μm, and that of the branch crystals is ∼40 μm. To confirm the components of the organic homostructure, we map the fluorescence lifetimes of one typical green-emissive homo-microstructure (Figure 2b) by a confocal laser-scanning fluorescence lifetime imaging nanoscope (CL-FLIN). As we can see, the fluorescence lifetimes have a very uniform distribution in the trunk/branch

Figure 2. (a) Fluorescence microscope image of the as-prepared oBCB organic homostructure with a scale bar of 20 μm. (b) Fluorescence lifetime mapping of the homostructure with a scale bar of 20 μm. (c) Fluorescence lifetime distribution of the corresponding homostructure in (b). (d) Schematic diagram of the stepwise formation of the homostructure. Inset: SEM image of one typical homostructure with a scale bar of 20 μm. (e1−e4) Selfassembly process of the homostructure recorded at 10, 12, 14, and 16 s by a bright-field microscope. The scale bars are all 20 μm. (f) TEM image of the homostructure with a scale bar of 10 μm. Inset: Corresponding SAED pattern of the joint part in this organic homostructure. (g1−g3) SAED pattern of the corresponding trunk/ branch crystal marked as “1−3” in (f), respectively. (h) Schematic diagram of the molecular arrangement mode at the junction area of the trunk crystal and the branch crystal of the o-BCB homostructure.

microcrystals, which indicates that the branch microcrystals have the same o-BCB molecules with the same molecular packing mode as the trunk microcrystals. Besides, Figure 2c shows the fluorescence lifetime distribution of the homostructure, which demonstrates an almost perfect gauss distribution with the peak value of 9.0 ns, which is consistent with the fluorescence lifetime of an individual o-BCB microrod (Figure S7). The successful preparation of the homostructures can be attributed to the higher supersaturation degree, which was together induced by the difference in the solvent vitalization speed and the solution concentration. Since DCM has a much higher saturated vapor pressure (601.46 mmHg) than that of acetonitrile (92.34 mmHg) at room temperature (25 °C) (calculated by the Antoine equation, Lange’s Handbook of Chemistry),35 the volatile speed of DCM is much faster than that of acetonitrile. Besides that, the saturation concentration of the o-BCB in DCM (15 mM) is much higher than that in C

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SAED patterns of the selected areas 2 and 3 show that the branch o-BCB microrods all grow along the [001] crystal direction. What is more, the SAED pattern (the inset of Figure 2f) of the joint area of the homostructure shows the superposition of the SAED patterns of the trunk and the branch microrods. Therefore, the molecular arrangement in the branch microcrystals is the same as the trunk microcrystals and is also consistent with the individual o-BCB organic microrods. In Figure 2h, the molecular packing occurring in the trunk/ branch microcrystals are exhibited. To determine the principle of the growth angle, the interplanar spacing of the trunk/ branch crystals has been calculated. As is known to us, the interplanar spacing of the longitudinal growth direction [100] is 4.15 Å (i.e., the lattice constant a), while that of the transverse direction [010] is 6.89 Å (i.e., the lattice constant b). On considering the intersection angle, the intrinsic connection between the two interplanar spacing has been revealed, as shown in Figure 3e, which is a = 4.15 ≈ (b sin 58°)/2 = 4.06. The mismatch rate of this homostructure can be calculated by η = (a − (b sin 58°)/2)/a = 2.2%. Thus, this ultralow mismatch rate of 2.2% contributes to the growth of the branch crystals with this given intersection angle of 58°. Besides that, the molecular arrangement in the direction vertical to the substrate can be confirmed by the X-ray diffraction (XRD) patterns of the crystals. Figure S11 shows the XRD patterns of the individual o-BCB microrods and the oBCB homo-microstructures as well as the simulated patterns based on the single-crystal structure of o-BCB. By comparing with the simulated powder XRD pattern, the diffraction peaks of the o-BCB microrods can be indexed as the (002), (003), and (004) crystal faces and so on. Thus, the top face of the oBCB crystals is (001), which means that the o-BCB crystals grow along the c axis in the height direction. What is more, no extra diffraction peaks have been observed in the XRD patterns of the homostructure compared with that of the individual microrods (Figure S11), which further proved that the crystal faces of the o-BCB organic microcrystals have not been changed during the formation of the organic homostructures at the microscale. In a short conclusion, we can successfully achieve the organic homostructure at the microscale through this “bottom-up” approach. For the fabrication of the organic hetero-microstructure, we added another blocking component of perylene into the o-BCB solution system. Figure 3a shows the fluorescence microscope image of the as-prepared organic heteromicrostructure, which shows the strong green light emission from the o-BCB organic microrod and the yellow light from the perylene organic microplates (Figure S12). When the heterostructure was excited by blue light (450−480 nm), the perylene microplates and the o-BCB microrod can be distinguished more clearly (Figure S13). The XRD pattern of the heterostructures indicates the presence of the o-BCB microrods and the perylene microplates in these as-prepared hetero-microstructures (Figure S14, cyan solid line). By comparing the XRD patterns of the heterostructures with the simulated patterns of the α phase and β phase perylene crystals (Figure S14) and comparing the morphologies of these as-prepared organic microcrystals (Figure S15), the perylene crystals in the heterostructures can be confirmed as the α phase. In the TEM image of the heterostructure (Figure 3b), the SAED pattern of the perylene microplate indicates that the molecular arrangement, parallel to the o-BCB microrod, belongs to the [001]

acetonitrile (2 mM). With the vitalization process proceeding, the relationship between the saturation concentration of the oBCB solution and the volume ratio η (η = Vacetonitrile:VDCM) indicates that the η will gradually increase, while the supersaturation degree increases. According to the Thomson−Gibbs equation Δμ = 2σv/h (Δμ is the supersaturation, σ is the specific surface energy of the crystallite, v is the volume of a single building block, and h is the distance from the center to the surface of the crystallite), the surface energy of the crystal face is proportional to the supersaturation of crystal growth units during the crystal growth.36 With the increase of the supersaturation, the chemical potential barrier should increase correspondingly. In our experiments, when the solvent volume ratio η changes from 1/4 to 1/3 and then to 3/2, the morphology of the o-BCB organic microcrystals evolves from microcubes to microrods and finally to the branched homomicrostructures (Figure S9). To further reveal the formation of the organic homostructure, the self-assembly growth process was recorded by a bright-field microscope. Figure 2d is the schematic diagram of the self-assembly process of the o-BCB homostructure, while Figure 2e−h show the bright-field images of the self-assembly process of the o-BCB organic homostructure captured at different growth times of 10, 12, 14, and 16 s, respectively. As is clearly shown in Figure 2e1−e4, the formation of the homostructure shows a stepwise growth process. As Figure 2e1 elucidates, in the first step, the o-BCB organic molecules aggregate through the intermolecular force, which induces the nucleation of the o-BCB microcrystals corresponding to the observed microstructure in Figure 2e. In step II, the o-BCB molecules start to nucleate onto the trunk microcrystal by utilizing the trunk component as a seed crystal (Figure 2e2). Then, the epitaxial growth of the branch crystals on the trunk crystal occurs in step III. As Figure 2e3 shows, during this process, both the branch crystals and the trunk crystals grow rapidly. Impressively, the nucleation process of the trunk crystals lasts 10 s, while the formation of the homostructure only lasts 6 s, which can be explained by the nucleation principles.37 The nucleation of the crystal in a homogeneous solution is much more difficult than the crystal growth process with seed crystals already existing in the solution, which contributes to the longer time. In the last step (Figure 2e4), the organic homostructure is successfully formed (the inset of Figure 2d), which shows that the branch microcrystals join well with the trunk microcrystals. The surface coverage rate on the substrate can be calculated as ∼55%. To further explore the joint level, the substrate was continually agitated during the evaporation process of the solution to damage the homostructure. As a result, the homostructure has a fracture between the branch crystal and the trunk crystal (Figure S10). It can be observed that the notch of the fractured section is deep into the trunk crystal, which demonstrates a strong interaction between the branch and the trunk crystal rather than a simple physical contact. More interestingly, in the homostructure, it can be found that the branch crystals are highly parallel with each other with the intersection angle of ∼58 degrees to the trunk crystals, which implies the oriented growth of the branch crystals. Figure 2f demonstrates the TEM image of one typical organic homostructure, and Figure 2g1−g3 show the SAED patterns of different selected areas denoted by the yellow dashed squares. As is indicated in Figure 2g1, the SAED pattern of the selected area 1 indicates that the trunk o-BCB microrod grows along the [001] crystal direction. In Figure 2g2 and g3, the two D

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direction (a axis of the o-BCB crystal, 3.91 Å) (the perylene single-crystal structure data are exhibited in Table S4). The calculated interplanar spacing mismatch rate can be as low as 4.1% = (3 × 3.91 − 11.27)/11.27, which actually promotes the epitaxial growth of the perylene microplates on the o-BCB microrods. The low interplanar spacing mismatch rate is also the key point for the formation of the heterostructures. In addition, Figure 3g shows the fluorescence lifetime mapping of one typical heterostructure. As we can see, the fluorescence lifetimes of the heterostructure can be distinctly distinguished between the perylene microplates and the o-BCB microrod. The inset of Figure 3h shows a phasor plot graph of the lifetime of the heterostructure. For every pixel in the fluorescence lifetime mapping image (Figure 3g), the CL-FLIN collects the phase delay (φ) and amplitude modulation (m) under the modulation frequency of 40 MHz. The horizontal axis g = m cos(φ) and the vertical axis s = m sin(φ). The semicircle in the graph with the center coordinates of (0.5, 0) and the radius of 0.5 is drawn by (g − 0.5)2 + s2 = 0.25, which is obtained from the monoexponential decay function τ = tan(φ)/ω = (1 − m2)0.5/mω (ω = 2π × 80 MHz). Thus, the points falling on the semicircle represent the monoexponential decay process, while the points falling inside the semicircle represent the multiexponential decay process. Figure 3h shows the fluorescence lifetime distribution of the heterostructure. By analyzing the peaks and fitting with a gauss function, the lifetime distribution peak can be separated by two peaks with the peak value of 9.5 and 10.5 ns, which can be attributed to the fluorescence lifetimes of the o-BCB and the perylene components, respectively, in the hetero-microstructures. More impressively, we can further integrate the homo-/ heterostructure into the multilevel organic microstructures. As we can see from the growth process (Figure S16a1−a4), the oBCB homostructure first formed (Figure S16a1), which then acted as the seed crystal for the growth of the perylene microplates. As clearly seen in Figure S16a2−a4, the growth process of the perylene microcrystals takes only several seconds, similar to the growth of the o-BCB branch crystals in the formation of the homostructure (Figure 2e−h). Figure S16b and c show the fluorescence microscope images of the multilevel structures under UV light (300−380 nm) and blue light (450−480 nm), respectively, where the o-BCB microrods and the perylene microplates can be clearly distinguished. In addition, the emission spectra in Figure S15e−g are collected from the selected area in Figure S16d, which further confirm the existence of the perylene and the o-BCB microcrystals. To further explore the performance of the photonic circuits made of the unique homostructure, a homemade μ-PL system with a laser light (λ = 375 nm) was utilized to excite the individual o-BCB microrod and the trunk crystal of the homostructure. As Figure S17a shows, a single o-BCB microrod has been excited by the laser at different lengths from the tip, and only two output spots can be observed at the tip of the o-BCB rod attributed to the smooth surface of the crystal. The output spectra of the tip have been collected (Figure S17b), and it can be found that the intensity decreased with an increase in the light propagation distance. To further figure out optical loss during the light propagation along the oBCB crystal, Figure S17c shows the relationship between the ratio of Iexcitation/Iout and the length from the excitation spot to the output tip. Commonly, the outcoupling intensity decreases exponentially as a function of the increase of the propagation distance, which is a typical characteristic of an active

Figure 3. (a) Fluorescence microscope image of the organic heteromicrostructure consisting of the o-BCB microrod and perylene microplate. The scale bar is 20 μm. (b) TEM image of a typical organic heteromicrostructure with a scale bar of 5 μm. (c) Corresponding SAED pattern of the o-BCB microrod in the heterostructure in (b). (d) Corresponding SAED pattern of the perylene organic microplate in the heterostructure in (b). (e) FESEM image of the heterostructures. (f) Molecular arrangement of o-BCB and perylene organic molecules in the epitaxial area of the organic heterostructure. (g) Fluorescence lifetime mapping of the o-BCB organic heterostructure. The scale bar is 20 μm. (h) Fluorescence lifetime distribution of the corresponding organic heterostructure composed of the o-BCB microrod and perylene microplates. Inset: Phasor plot graph of the fluorescence lifetime.

crystal direction. Meanwhile, the molecular direction of the oBCB longitudinal direction is [100]. The SEM image of the heterostructures has been presented in Figure 3e. As we can see in the image, rectangular microcrystals and rod-like microcrystals have been obtained, which correspond to perylene and o-BCB crystals. The morphologies of perylene microcrystals and the o-BCB microcrystals did not change after the formation of the heterostructures. A compact contact can be observed at the interface of the heterostructures. What is more, the surface coverage of the heterostructures on the substrate can be calculated as ∼35% from the FESEM image. The SAED results indicate the perylene crystals and the o-BCB crystals are both single crystals with a high crystalline quality. Figure 3f shows the molecular arrangement in the joint area of the heterostructure composed of o-BCB and perylene. As we can see, the interplanar spacing of the [001]perylene direction (c axis of the perylene crystal) is 11.27 Å, which is approximately 3-fold greater than the interplanar spacing of the [100]o‑BCB E

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Figure 4. (a) Optical microscope images of the o-BCB microrod at bright field and excited by a 375 nm laser. (b) Corresponding emission spectra of the output ports a and b in (a); the inset indicates a symmetric waveguide mode. (c) Microscope images of the o-BCB homostructure at bright field and excited by a 375 nm laser. (d) Corresponding emission spectra of the output ports a′ and b′ in (c); the inset indicates an asymmetric waveguide mode. (e−i) Left: Microscope images of the o-BCB homostructure excited at different points by a 375 nm laser; right: corresponding intensity distribution of the five output channels.

intensity can be detected with the same light propagation distance (Ia:Ib ≈ 1:1, Figure 4b), which is the asymmetric waveguide in the commonly seen waveguide system. In contrast, a distinct difference can be found in the outcoupling light intensity of output a′ and output b′, although with the same light propagation distance (Ia′:Ib′ ≈ 1:4, Figure 4d). The asymmetric waveguide mode can be attributed to the different intersection angle between the light propagation direction and the branch crystals, which is an acute angle of 58° when the excited light is propagated upward (Figure 4c, A1) and an obtuse angle of 148° when the excited light is propagated downward (Figure 4c, A2). Through the optical loss coefficient κ, the light propagation distance, and the output light intensity, the light loss at the interaction region can be calculated as 4.3% for A1 and 1.1% for A2. Thus, the asymmetric waveguide mode in the homostructure can be attributed to the distinctly different optical loss at the interaction region. Besides that, different from the clean spectrum in Figure S3, Figure 4d shows the shoulder peak in the emission bands of oBCB microcrystals, which can be attributed to the selfabsorption effect. The clean emission spectrum is obtained by exciting the whole substrate. The vibration emission peaks have similar intensity and lead to a broad emission band. But the spectra in Figure 4b,c are obtained by exciting a point and collected at another output port. The propagation process of the fluorescence light in Figure 4b,c will lead to the selfabsorption effect and reduce the intensity of the shortwavelength vibration emission peak. Thus, a shoulder emission peak appeared in the emission band. To prove this, the absorption spectra of the o-BCB microcrystals have been measured, which are shown in Figure S19. As we can see, there is an overlap between the absorption spectra and the emission

waveguide. By normalizing the outcoupled intensity (Iout) with the excitation intensity (Iex), Iout/Iex can be well fitted by a single-exponential function, Iout/Iex = y0 + A exp(−κx),37−40 where x is the propagation distance and κ is the optical loss coefficient. Figure S16c shows a good fitting by the function Iout/Iex = 0.127 + 0.739 exp(−0.011x) with the adjusted R square of 0.99, which demonstrates the optical loss coefficient of 0.011 dB·μm−1. To further clarify the unique light waveguide properties in the unique homostructure, a comparison between the waveguide mode in the single rod and the homostructure is shown in Figure 4. Figure 4a and c show microscope images of the o-BCB microrod and homostructure. The region circled by the red dashed line was excited by a laser (λ = 375 nm), while the output ports are circled by a white dashed line. As was expected, the excitation light has been successfully propagated into the branched crystals and was then output at the tips of the branch crystals. As a result, the excited light can only propagate along the x direction and only two output channels can be obtained in the o-BCB microrod, while in the homostructures, the excited light can propagate along both the x and y directions and the number of output channels was determined by the number of branch crystals. The photonic system with a single input port but multiple output ports is of essential importance for the multifunctional integrated circuits. The asymmetric waveguide means that when the light was input at the midpoint of the two output ports, the same output light signal can be obtained, while when the output signal is different, it comes to the asymmetric waveguide. Interestingly, the homostructure shows a distinct asymmetric waveguide mode, while the microrod exhibits a typical symmetric light waveguide mode. By comparing the outcoupling light spectra of the output ports a and b in Figure 4a, the same light F

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Figure 5. (a) Bright-field microscope image of the heterostructure; (b−e) microscope images of the heterostructure excited at input ports I, II, III, and IV, respectively; (f) left panel: output light spectra from channel 1 when exciting at different input ports; right panel: corresponding output intensity distribution; (g) left panel: output light spectra from channel 2 when exciting at different input ports; right panel: corresponding output intensity distribution; (h) left panel: output light spectra from channel 3 when exciting at different input ports; right panel: corresponding output intensity distribution.

Interestingly, in the waveguide devices based on the heterostructure, the green light signal (peak at 500 nm) and yellow light signal (peak at 565 nm) can be respectively generated and transmitted in the same microcrystals. Figure 5a shows the bright-field microscope image of a typical heterostructure, which was then excited by a laser (λ = 375 nm) at different positions (inputs I−IV) of the trunk crystal (Figure 5b−e). There are three output channels in the heterostructure. Channel 1 and channel 2 are both the perylene crystals, while channel 3 is the tip of the o-BCB crystal. As the input position changed, the output light from the three output channels was separately monitored. Different from Figure 4e−i, Figure 5f−h exhibit the output light spectra/ intensity change at certain output channels. Figure 5f−h can correspond to the output channels 1, 2, and 3, respectively. As the output spectra of channel 1 shows (I), when the input port I or II was excited, the green light emitted by the o-BCB crystals can be obtained, while the yellow light emitted by perylene crystals can be output when the input port III or IV is excited. Thus, the output light wavelength can be simply switched by exciting different positions of the heterostructures. The switchable output wavelength is also attributed to the unique passive/active mixed-waveguide mode. In the homostructures or the single crystals, only an active waveguide mode can be observed because the input laser light was absorbed rather than propagated. But in the heterostructures, the input laser light excites a microcrystal with the active waveguide mode, and then the fluorescence was propagated into other crystals with the passive waveguide mode, which leads to the switchable output light wavelength. Similar to the output

spectra in the wavelength range of 427−479 nm, which prove the appearance of the shoulder peak. The features of multiple output ports and the unique asymmetric waveguide mode can be further employed for signal recording and storage in the photonic circuits. In Figure 4e−i, we moved the input position of the laser light (λ = 375 nm), and at a certain input position, the output intensity distribution of all the output channels (1−5) was recorded (the right panel of Figure 4e−i). Based on the asymmetric waveguide properties caused by the different intersection angles, as shown in Figure 4e−i, the outcoupled intensities of the obtuse outputs are much stronger than the acute outputs. For example, there are only two major outputs in Figure 4h and only one major output in Figure 4i. According to the intersection angle, the output channels can be divided into obtuse channels and acute channels. In the right panel of Figure 4e−i, the blue column represents the strongest signal intensity, which comes from the nearest obtuse channel away from the excitation position, while cyan columns come from the other obtuse channels and the red columns come from the acute channels. When the homostructure was excited at a certain position, a certain asymmetric signal distribution as a code can be obtained, which is much more complex and more difficult to decipher than the symmetric signal distribution. The disparate output intensity distribution can be utilized to record the input position. In a practical photonic circuit, signals from more channels can be simultaneously monitored to make a more complex code. Multiple signals that can be generated and transmitted simultaneously in the same photonic circuits is the key factor of the WDM system in the photonic integrated devices. G

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channel 1, channels 2 and 3 can also output green or yellow light simply by changing the input ports (Figure 5g,h). Furthermore, radiation energy transfer between the o-BCB microcrystals and perylene microcrystals has also happened, which can be proved by the absorption spectrum and the excitation spectrum of the perylene microcrystals. As we can see in Figure S20a, there is a large overlap (430−550 nm) between the absorption spectrum of the perylene microcrystals and the emission spectrum of the o-BCB microcrystals, which leads to the reabsorption. Meanwhile, the excitation spectra of the perylene microcrystals (λem = 570 nm) indicate weak fluorescence can be emitted with the excitation wavelength of 430−550 nm. Thus, the fluorescence of o-BCB can excite the perylene microcrystals by radiation energy transfer. Figure S21 also presents the emitting mechanism of the process. However, in the heterostructure, when exciting the o-BCB microcrystals, we can only get the green output light from the perylene microcrystals (Figure 5f,g, black and red lines), and the spectrum of the output light can correspond to the o-BCB fluorescence. The absence of the perylene emission peak can be attributed to the low fluorescence efficiency of the perylene microcrystals. The fluorescence efficiency of o-BCB microcrystals is 59.9% but only 16.0% for the perylene microcrystals. Thus, the weak fluorescence from the perylene microcrystals has been covered up by the strong light signal from the o-BCB microcrystals. The output light intensity distributions of the three output channels are exhibited in the right panel of Figure 5f−h (the green columns represent the green light, while the yellow columns represent the yellow light). Similarly with Figure 4, the output intensity distribution can also be used to record the input position as a very simple information storage model. In conclusion, we have fabricated organic photonic devices based on novel self-assembled homo-/heterostructures. Through the room-temperature solution self-assembly method, organic homo-/heterostructures as well as multilevel structures were prepared, whose formation mechanism was demonstrated as stepwise facet-selective growth based on interplanar spacing matching. In the homostructures, the highly parallel branch crystals indicate the oriented growth with an intersection angle of 58°. The interplanar spacing mismatch rate is as low as 2.2% for the [100] direction and the [010] direction of the o-BCB crystals in the homostructure and 4.1% for the [001] direction of perylene crystals and the [100] direction of o-BCB crystals. Most interestingly, different from the symmetric waveguide in the single rod, the asymmetric waveguide properties can be found in the homostructures due to the different intersection angle and the disparate optical loss coefficient (4.3% for acute output and 1.1% for obtuse output). For the heterostructures, the outcoupled light color in the same output channel is switchable with the active/passive mixed-waveguide mode. Based on this asymmetric waveguide or the switchable output light color, the output intensity distributions of both the homoand heterostructures are potentially utilized as an information storage unit to record the input position. The successful preparation of organic homo-/heterostructures offers a comprehensive understanding of the formation mechanism and the preparation method, and the unique waveguide properties will potentially boost advances in integrated optoelectronic devices at the micro-/nanoscale.

Article

ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsphotonics.8b00815. Experimental details and other supporting figures/tables (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (X.-D. Wang). *E-mail: [email protected] (L.-S. Liao). ORCID

Xue-Dong Wang: 0000-0003-0935-0835 Liang-Sheng Liao: 0000-0002-2352-9666 Author Contributions §

Z.-Z. Li and Y.-C. Tao contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge financial support from the National Natural Science Foundation of China (No. 21703148), the National Key Research and Development Plan of China (Grant No. 2016YFB0400700), and the Natural Science Foundation of Jiangsu Province (BK20170330); this project is also funded by the Collaborative Innovation Center of Suzhou Nano Science and Technology (CIC-Nano), by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD), and by the “111” Project of The State Administration of Foreign Experts Affairs of China.

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