Electroluminescence from Spontaneously Generated Single-Vesicle

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Electroluminescence From Spontaneously-Generated Single Vesicle Aggregates Using Solution-Processed Small Organic Molecules Yu-Tang Tsai, Kuo-Pi Tseng, Yan-Fang Chen, Chung-Chih Wu, Gang-Lun Fan, Ken-Tsung Wong, Guillaume Wantz, Lionel Hirsch, Guillaume Raffy, Andre Del Guerzo, and Dario M. Bassani ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.5b06261 • Publication Date (Web): 05 Jan 2016 Downloaded from http://pubs.acs.org on January 9, 2016

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Electroluminescence From SpontaneouslyGenerated Single Vesicle Aggregates Using Solution-Processed Small Organic Molecules Yu-Tang Tsai,†,§ Kuo-Pi Tseng,‡ Yan-Fang Chen,†,§ Chung-Chih Wu,¶ Gang-Lun Fan,‡ KenTsung Wong*,‡ Guillaume Wantz,§ Lionel Hirsch,§ Guillaume Raffy,† Andre Del Guerzo,† and Dario M. Bassani*,† †

§

Inst. of Molecular Science, CNRS UMR 5255 and Univ. Bordeaux, F-33405 Talence, France

IMS, Univ. Bordeaux, Bordeaux INP, ENSCBP, CNRS UMR 5218, F-33400 Talence, France ‡

Department of Chemistry, National Taiwan University, Taipei 10617, Taiwan

¶

Department of Electrical Engineering, Graduate Institute of Electronics Engineering, and

Graduate Institute of Photonics and Optoelectronics, and Innovative Photonics Advanced Research Center (i-PARC), National Taiwan University, Taipei 10617, Taiwan KEYWORDS Supramolecular chemistry, soft matter, biuret, molecular electronics.

ABSTRACT Self-assembled aggregates offer great potential for tuning the morphology of organic semiconductors, thereby controlling their size and shape. This is particularly interesting

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for applications in electroluminescent (EL) devices, but there has been, to date, no reports of a functional EL device in which the size and color of the emissive domains could be controlled using self-assembly. We now report a series of molecules that spontaneously self-organizes into small EL domains of sub-micron dimensions. By tailoring the emissive chromophores in solution, spherical aggregates which have avg. size of 300 nm in diameter and emit any one color, including CIE D65 white, are spontaneously formed in solution. We show that the individual aggregates can be used in EL devices built either using small patterned electrodes or using a sandwich architecture to produce devices emitting in the blue, green, red, and white. Furthermore, sequential deposition of the three primary colors yield an RGB device in which single aggregates of each color are present in close proximity.

Miniaturizing optoelectronic components is a challenging issue in both scientific and technological investigations, and a number of different approaches have been used to create submicron sized point light-sources. Electroluminescence (EL) has been obtained from metal nanoclusters,1,

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single nano-crystals,3 or single molecules4 located between metal electrodes, and

scanning tunneling microscope (STM) probes can be used as a localized excitation source in both single molecules5,

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and quantum dot7 EL experiments. Although these provide a proof-of-

principle for electrically driven point light-sources, their usefulness in practical devices is limited due to difficulty in positioning of single objects and the fabrication of nanoscale electrodes. In contrast, top-down approaches are limited by the deposition techniques used to prepare the active layer (typically evaporation through a fine metal mask). Consequently, organic semiconductors are still highly promising (ease of fabrication, color tunability) for the construction of miniature


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light sources, and nanoscale organic light-emitting diodes8,

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have been built using e-beam

lithography or nanosphere lithography fabrication techniques. Molecular self-assembly provides a bottom-up approach towards miniaturization of electronic devices in which small molecular components spontaneously organize themselves into functional, ordered architectures during device preparation. Self-assembly is inherently compatible with low-cost high-area solutionbased processing, and is particularly appealing for electronic devices in which tailoring of specific properties (e.g. morphology, size, molecular order) is sought.10-15 Whereas a number of EL devices incorporating self-assembled architectures obtained from polymers16-20 or peptides2022

has been reported, the use of non-amphiphillic small molecules generally requires of

surfactants or aqueous dispersions to obtain well-defined aggregates.23-29 With this in mind, we and others have designed small-molecule systems capable of selfassembling into well-defined vesicles whose shape and luminescence can be tailored across the visible spectrum by the combination of various chromophores exploiting multiple energy transfer processes demonstrated to be highly efficient in confined aggregates.30-37 Vesicles are particularly appealing for the preparation of EL devices as one may expect that they will collapse into thin disks upon deposition.38-40 This morphology is well suited for optical resonators41 and for EL materials that do not intrinsically possess high charge carrier mobility as the high aspect ratio combines a circular emission spot with a small thickness. However, the formation of artificial vesicles from surfactants or using aqueous dispersions is not readily amenable for the preparation of organic electronic devices and there have been no reports of vesicles being used as emissive centers in EL devices. We now report the generation of EL from spontaneously generated self-assembled vesicles incorporated into different device architectures based on interdigitated electrodes or sandwich-type devices. We investigate charge injection and charge


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distribution in the aggregates, EL emission profile, and discuss size limitation and device stability. Furthermore, the blending of up to three emissive chromophores allows this system to cover the entire range of the visible spectrum to produce single point emitters of any color, including CIE D65 daylight white. We also demonstrate that mixing during the deposition step is sufficiently limited to allow the fabrication of multiple-color EL devices.

Results / discussion The compounds used in this study (compound 1 – 3, Figure 1) are based on the use of a rigid, π-conjugated chromophore appended with hydrogen-bonding (H-B) biuret groups.31,

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The

photophysical properties of compound 1 – 3 in solution (THF, 1 µM) or as thin films are summarized in Table 1 and their absorption and fluorescence spectra in solution are shown in Figure S1. These compounds are not amphiphilic and, although they appear soluble in pure organic solvents such as chloroform and THF, previous experiments using small angle neutron scattering are in agreement with the presence in solution of spherical aggregates that are 200 – 500 nm in diameter possessing a thin (ca. 10 – 20 nm) wall.43 Remarkably, the self-assembly process was found to occur spontaneously in solution and is not dependent on the use of cosolvents for inducing phase-separation or de-mixing during the deposition step. The aggregates that are formed are readily visualized using SEM and TEM microscopy upon evaporation of solutions of 1 – 3 (Figure 1). Once the solvent has evaporated, the spheres collapse to discs that are 200 – 500 nm in diameter and ca. 20 – 30 nm in height (Figure S2).31, 43 All the compounds are highly emissive both in solution and in the solid.

The emission from the individual

aggregates can be further tuned by blending the compounds in solution: For example, aggregates possessing green emission are obtained by blending 2 (10%) with compound 1.


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Figure 1. Molecular structures of compounds 1 – 3 and images of the spherical aggregates formed by compound 2 upon drop-casting from dilute anhydrous THF solution (0.1 mM) onto SiO2 substrates as seen by scanning electron microscopy ((a) and (b)), by atomic force microscopy (c), and scanning transmission microscopy (d).

Table 1. Summary of photophysical properties and chromatic coordinates of compounds 1 – 3. Compound

1

Absorption

εa

Emissionb

sol / film (nm)

(104 M-1cm-1)

sol / film (nm)

360 / 360

12

340, 424 / 343, 8.8, 4.1

(0.16, 0.04) / 0.96

537 / 546

(0.16, 0.11) (0.38, 0.60) /

0.85

432

CIEc (sol) / (film)

402, 421 / 408, 430

2

ΦFb

(0.42, 0.56)


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324, 460 / 327, 8.5, 2.9

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635 / 643

(0.62, 0.38) / 0.64

467

(0.66, 0.34)

a

Extinction coefficients measured in THF (1 µM).; bFluorescence emission and quantum yields were measured on dilute (1 µM in THF, λex = 350 nm) aerated solutions, using an integration sphere coupled to a photonic multichannel analyzer (Hamamatsu C9920); cCommission Internationale de l'Eclairage chromatic coordinates of the emission from the solution and films.

Efficient electroluminescent materials must allow for charge injection from the electrodes and exhibit sufficiently long charge carrier lifetimes so as to render charge recombination competitive with other deactivation processes. In this aspect, supramolecular materials relying on H-bonding may be at a disadvantage because the presence of acidic protons can induce trapping of charge carriers although several reports of charge transport in such materials are known.16, 42, 44-49 To explore this point, and whether charge injection results in destabilization of the morphology, we proceeded to test charge injection in single aggregates of compound 2 using Kelvin probe force microscopy (KPFM, see Figure 2 for the basic operating principle).50 Compound 2 was selected for these studies as it possesses an energy gap that is intermediate between that of compounds 1 and 3. Using KPFM, the surface potential of an object can be mapped with a resolution of a few tens of nanometers to give information about the local work function and surface potential of the material. The morphological stability of the charged aggregates and the distribution of the accumulated charges were probed through the acquisition of time-lapsed KPFM and topology images. In a typical experiment, the aggregates were dropcast onto a SiO2 (200 nm) / Si(n+) substrate and a PtIr-coated AFM tip was used to locally inject charges by gently contacting the aggregate over a period of 3 min while applying a fixed potential (VINJ = ± 5 V with respect to the silicon wafer, tip force = 2 nN). The results for the injection of positive or negative charges into aggregates of 2 are shown in Figure 2. Prior to


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charge injection, the topography and surface potential of the aggregate (Figure 2b) shows that it possess a flat, round morphology and a homogeneously distributed surface potential which is slightly lower than that the substrate. Once injected, the charges distribute over the surface of the aggregate in a mostly uniform fashion, although some charge segregation (∆V = ± 0.15 V) across the surface can be observed. The time-lapsed KPFM images of positively charged aggregates of 2 (Figure 2c) indicate that they possess good structural stability and that the charges remain localized on the aggregate over a long period as the aggregate retains its thin disk morphology over the entire experiment. The negative charges also spread rapidly over the entire surface of the aggregate, and the surface potential decreases from –500 mV to –200 mV over 36 min and then continues to decrease slowly. It is important to note that the disk morphology obtained by the deposition of the spherical aggregate remains stable without deformation during charge injection and storage. Control experiments on clean SiO2 (200 nm) / Si(n+) substrates show that the charges spread out more uniformly and over a slightly larger area, and that the surface potential decreases less rapidly (see Figure S3). The KPFM results are particularly important in view of the molecular nature of the material, in which the aggregates are only held together by weak H-B interactions. By showing that the morphology of the aggregates does not change upon hole or electron injection (Figure 2b I – III), the KPFM experiments demonstrate the morphological stability of the aggregates towards both positive and negative charge injection where would be helpful for electron-hole pair combination, which is a crucial point for future EL applications in which the dimensions of the emitting domains are controlled by self-assembly.


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Figure 2. (a) Basic operation of a KPFM experiment: An AFM instrument equipped with a conducting tip is used to first map the surface topology (1st pass), and then to inject charges at specific locations. The surface potential is then mapped (2nd pass) by raster-scanning the sample


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away from the surface while holding the tip at a fixed potential. (b) Topology (I – III) and KPFM (IV – VI) images of single aggregates of 2 before and after hole or electron injection (VINJ = +5 V, 3 min for hole injection, –5 V, 3 min for electron injection). Scale bar is 1 µm. (c) Surface potential across the aggregate at different delays following hole or electron injection.

An important aspect to using small molecular aggregates as EL sources is their interconnection to a source of current (wiring). This requires electrical contact to be formed with the individual aggregates, which has curtailed progress to date in part due to the shifting nature of aggregate morphologies. To demonstrate the EL properties of aggregates formed by compounds 1 – 3, we proceeded to use devices composed of interdigitated Ti electrodes fabricated by E-beam lithography (Figure 3a). Titanium was chosen as the electrode material as its work function (4.33 eV) should enable it to inject both holes and electrons into the material. The gap length of about 100 nm randomly traps nanospheres that are ca. ~300 nm in diameter during deposition. The devices were mounted on an inverted microscope equipped with a sensitive EM-CCD detector to record EL signals from single aggregates. Figure 3b shows a SEM image of the interdigitated electrodes and aggregates from compound 2 dispersed onto and in-between the electrodes. Once the nanospheres are immobilized, as shown in Figure 3b, the application of a voltage across the electrodes allows charges to be injected from opposite sides of the aggregates located between the electrodes. We can expect that those aggregates possessing a combination of the lowest injection thresholds and highest charge carrier mobility will emit first as the voltage is gradually increased. Under dark conditions, the application of a DC voltage (12 V) induced emission from single points, as captured by the EM-CCD detector (Figure 3c). By overlapping the images initially obtained in reflection mode with those from EL mode, it is possible to locate


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the emissive areas as deriving from aggregates that are located between the interdigitated electrodes (Figure 3d). However, the emission was not stable over time and can shift between different locations between the electrodes.

Given the disparity in nanosphere size and,

presumably, electrical contacts between the nanospheres and the electrodes, it is not surprising that emission is not observed from all the aggregates simultaneously. Photoluminescence (PL) spectra measured before and after EL (Figure S4) are superimposable, which, along with the observation of repeated emission from the same locations, suggests that the aggregates are not damaged upon EL operation in agreement with the result from the KPFM experiments.

Figure 3. Schematic structure of Ti electrodes on a SiO2/Si wafer fabricated using E-beam lithography (a) and (b) SEM image of drop-cast nanospheres of compound 2 lying across the spacing between the electrodes. Optical microscopy of (c) the EL image of compound 2 nanosphere under 12 V applied voltage and (d) the overlay image of electrodes and EL from single nanosphere.

The use of a horizontal device architecture above forces charge injection to occur from opposite sides of an aggregate, resulting in charge migration over considerably long distances (≥


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100 nm) compared to typical electron / hole diffusion lengths in disordered organic materials. In contrast, a vertical (sandwich) device architecture is more similar to devices used in current OLEDs and, in this particular case, would require charge migration across only the thickness of the collapsed aggregate (ca. 20 nm, Figure S2). Ideally, the aggregates should be deposited from solution onto a transparent conducting electrode with their size defining the dimensions of the EL points. However, the use of self-assembled aggregates as the active material in EL devices must overcome the problem of direct contact (short circuit) between the top and bottom electrodes in the interstitial areas between aggregates, where no material is present. To resolve this, we employed the architecture shown in Figure 5a up-part, in which hole- and electrontransporting layers are used to fill the gaps between the aggregates and reduce short-circuit current. Device construction is simple and involves the deposition (drop-casting or spin-coating) of a dispersion of aggregates onto an ITO substrate covered with a hole-transport layer (VBFNPD)51 deposited by spin-coating and thermally cross-linked. An electron transport layer (TPBi) followed by a LiF interfacial layer and an Al electrode were then thermally evaporated on top of the aggregates. The energy level diagram for the devices constructed using compounds 1 – 3 is shown in Figure 4a. The devices were prepared either using solutions of 1, 2, or 3 (100 µM in THF) to form nano-aggregates and to give devices that produce blue, yellow, or red light, or a blend of 1, 2, and 3 (100 µM, 0.2 µM, 0.25 µM in 1, 2, and 3, respectively) to form nanoaggregates that give white light through a combination of EL and energy transfer processes. Chromatic coordinates of bulk EL from devices fabricated from aggregates of compound 1 – 3, a green EL device made from a blend of 1 and 2 (10%), and D65 white-light EL device is shown in Figure 4b. Figure 4c shows the I-V-L curves measured on devices using compound 1, 2, or 3,


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or a blend all three for generating white light, and all the devices display rectifying behavior and a turn-on voltage of 4 – 5 V that is similar for compounds 1 – 3 .

Figure 4. (a) Band energy diagram for the devices constructed using compounds 1 – 3 (HTL: VB-FNPD); (b) chromatic coordinates of bulk EL from devices fabricated from aggregates of compound 1 – 3 (blue, yellow and red triangle), a green EL device made from a blend of 1 and 2 (10%), and D65 white-light EL device (from a solution composed of 100 µM, 0.2 µM, 0.25 µM in 1, 2, and 3, respectively). ; (c) I-V-L curves measured on devices using compound 1, 2, or 3, or a blend all three for generating white light.


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Unlike devices prepared with nano-gap metal electrodes to address individual aggregates, the use of electron/hole transport layers makes it possible to observe stable and reproducible EL from single aggregates (Figure 5a). It is thus possible to use a confocal microscope to first map the location of the aggregates' EL, and then focus the objective onto individual objects for acquisition of their EL spectrum. The dispersion of colors from the individual aggregates within a single device, and across several devices is quite small as shown in Figure 5b (the emission profile and I-V-L curves of 56 additional devices are collected in the supplementary information, Figures S8 - S20). It is also possible to prepare functional devices without using a hole-transport layer, which allows us to exclude any contribution to the EL from the polymerized fluorene HTL material (see supporting information, Figures S11, S12, S13, S16, S17). The spectra obtained from the EL of individual single nanosphere of compound 1 – 3 and blend D65 white are shown in Figure 5c and the similarity with the PL in solution and in the solid is evidence that the emitting chromophore is identical in all cases. From this, we conclude that EL operation of a single aggregate does not perturb the emission of the chromophores. In the case of the white light-emitting devices, the EL emission clearly originates from the sum of the EL emission of compounds 1 – 3 as was previously demonstrated for this system using photoluminescence. The overall EL efficiency of the devices is on the order of 0.2 – 0.6 % (see Figure S5), but this value does not take into account the dark area between aggregates and should therefore be considered a low estimate of the EL efficiency of individual aggregates (the average density of aggregates reppresents 8 - 20% of the surface coverage).


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Figure 5. a) Schematic cross-section of EL devices fabricated using self-assembled nanospheres (see text for description of ETL and HTL) and corresponding wide-field microscopy images of the EL from the devices (I: 1, II: 2, III: 3, IV: D65); (b) chromatic coordinates of EL from 10 individual aggregates showing dispersion in the emission; (c) EL spectrum of an individual aggregate in devices I – III (color fill). For comparison, the bulk EL emission from the entire device is shown as a colored area.

Black solid line represents EL from a white emitting

aggregate in device IV. Its chromatic coordinates (x = 0.313, y = 0.331) correspond to those of CIE D65 white light.


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The devices obtained using the architecture shown in Figure 5a appear bright to the naked eye and are remarkably stable to ambient conditions even upon simple encapsulation with adhesive tape. Although the size of the aggregates (200 – 500 nm) is near or below the diffraction limit of the light collected (400 – 800 nm), the presence of aggregates is clearly identifiable from the EL images as they are well dispersed and highly luminescent (Figure 5a). Thanks to this setup, the intensity of EL from individual vesicles could be followed over a period of several hours. Blueemitting compounds are critical for the generation of mixed color aggregates, but generally suffer from shorter lifetimes. To test this point, we tracked the EL intensity from individual aggregates of compound 1 upon continuous operation at 8 V (Figure S6). A 30% decrease in intensity from these aggregates was observed after 6 hours of continuous EL, which is quite remarkable considering the thinness of the active layer and the absence of advanced encapsulation techniques.

We also observed reversible sporadic variations in EL intensity that

we believe may originate from exciton quenching due to the accumulation of charges in individual aggregates, although we cannot exclude alternative causes such as variations in the contact resistance to the aggregates.

Ultimately, device failure arose from delamination /

oxidation of the electrodes. This is not unexpected, given the low level of protection afforded by the adhesive tape. Nonetheless, we find that device performance remained unaffected after overnight storage under ambient conditions, and slowly decreased after several days of exposure to ambient. Fluorescence lifetime imaging of the device (λex = 380 nm) further evidences the presence of weak emission from areas between the nanospheres in which the active material was deposited as a film, which is also sometimes visible in EL. The average diameter and spectrum of the EL from the aggregates were analyzed using the confocal setup and compared to the size distribution determined from SEM. The FWHM of a


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single nanosphere EL profile is ca. 300 nm (Figure S7a), which is in good agreement with the average diameter of the spherical aggregates as determined by SEM (ca. 250 nm) in view of the diffraction-limited resolution of the optical microscope. A full statistical study of the optical size of the EL emission is given in the supplementary information (Figure S7b), where it can be seen that the average size of the EL points is near the diffraction-limit. Because of this, any EL from aggregates whose physical size is below ca. 300 nm in diameter will appear as a 300 nm spot. In this respect, the devices studied herein represent the ultimate (i.e. diffraction limited) optical resolution that can be attained. To demonstrate that the use of a supramolecular bottom-up strategy can also be used to incorporate several colors in close proximity within a single device, we successively deposited vesicles of compounds 1 – 3 on the same device. To reduce blending of the colors, the solvent was allowed to dry in-between casting each sample. The devices were then visualized using a confocal microscope equipped with filters to selectively detect emission in the blue, green and red regions of the spectrum corresponding to the EL emission from aggregates of 1, 2, and 3, respectively. The region where the three drop-cast samples met is shown in Figure 6, and it is immediately visible that aggregates of all three colors are present. It is interesting to note that in this case, the presence of the three compounds did not result in a blending of the colors, which would have generated many different colors. We do note, however, that such mixing was present in other areas of the device. Several areas where all three primary colors are present are shown separately.


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Figure 6. EL image of regions from a device in which vesicles of compounds 1 – 3 in THF solution (10–4 M) were sequentially deposited by drop-casting. To avoid blending of the colors, the solvent was allowed to dry between each deposition. Areas 1 – 4 were taken from the region where the solutions met, and show the presence of the three colors in close proximity. The image was constructed using a confocal microscope equipped with filters to sequentially detect each color. See Figure S21 for the wide-field image in which regions 1 – 4 are located. The scale bar is 1µm. Conclusions In conclusion, we have shown that EL devices fabricated using a fully bottom-up strategy in which spontaneous self-assembly of vesicle-like aggregates in anhydrous conditions can be used to prepare flat, disk-like emissive areas whose size is near or below the diffraction limit of the emitted light. The EL devices prepared using a sandwich-type architecture display remarkable stability and allow the incorporation of aggregates possessing different emission characteristics in close proximity. These could be of use in the design of sub-micron light sources in which different multiple excitation wavelengths are required simultaneously, or in future highresolution displays. More importantly, the system is entirely fabricated using bottom-up selfassembly from solution, which opens up prospects of controlling the location and density of the aggregates using techniques such as spray-coating and ink-jet printing. These techniques are well adapted for up scaling to cover large areas at reasonable costs.

To the best of our

knowledge, this is the first example of the bottom-up construction of a multi-color OLED device with sub-micron elements from solution.

Methods / experimental


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Materials. Compounds 1 – 3 were prepared as previously described.31 Solutions in anhydrous THF were prepared as described below. Kelvin probe force microscopy (KPFM). The aggregates (compound 2) were deposited onto a highly-doped n-type silicon wafer (resistivity of ≈ 10-3 Ωcm), covered by a 200 nm thick SiO2 layer by drop-casting. The SiO2 surface was cleaned by a piranha solution and rinsed in deionized water. The local charge injections and KPFM experiments were performed using a Dimension Icon AFM (Bruker Corporation) under ambient atmosphere equipped with PtIrcoated tips (NANOSENSORS PPP-EFM) with a free oscillating frequency f0 ≈ 75 kHz and a spring constant k ≈ 2.8 N/m. To locally inject charges into the aggregate, the AFM tip was biased at VINJ with respect to the silicon wafer, its oscillation frequency was set to zero, and the tip was gently contacted to the surface of the aggregate with typically 2 nN contact force. A voltage was applied to the tip to inject charges, VINJ = +5 V, 3 min for hole injection, –5 V, 3 min for electron injection. Fabrication of interdigitated nano-gap electrodes. Nano-gap electrodes for devices were prepared by an electron-beam lithography system (ELS-7000EX, Maker Co. JAPAN). The positive electron-beam photoresist (PR), 495k PMMA, was first spin-coat on a fused silica substrate to form a resist layer having a thickness of 200 nm, and then pre-baked at 180 ℃ for 3 minutes. After exposure to the electron beam of 100 keV with 30 pico-ampere of current and subsequent development process, interdigitated finger patterns of the e-beam resist with 100 nm gaps were formed. The exposure was carried out for 2 µsec by a pixel map of 60000×60000 dots to give a total exposure area of 1.2×1.2 mm2. Next, a 100-nm-thick Ti layer was deposited on the interdigitated finger patterns of e-beam resist by an e-gun evaporator with a vacuum level and an


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evaporation rate of approximately 1*10-6 Torr and 0.3 Å/s, respectively. Finally, the interdigitated Ti electrodes (with 100-nm gaps) were formed through the lift-off process. Fabrication and measurements of layered nano-aggregate EL device. The electroluminescent devices having the device structure of Figure 5a were fabricated using a commercially available ITO coverslip as an anode and LiF/Al as a cathode. The structure of the device included a layer of indium tin oxide (ITO) with a sheet resistance of 30-60 Ω.square This substrate underwent a wet cleaning procedure of successive 30 min ultrasonic bath treatment in trichloroethylene, ethanol, and deionized water at room temperature. The substrates subsequently underwent a UVozone cleaning treatment and a layer of VB-FNPD was spin coated from a 1 wt % chlorobenzene dispersion at 1000 rpm for 30s. The hole transport layer was thermally polymerized by heating to 180 °C under nitrogen for 1 h. Then, the emitting layer (EML) was deposited by spin-coating (see supporting information for details) on top of the VB-FNPD layer. Solutions of compounds 1, 2, and 3 were prepared by adding the appropriate volume of THF (dried over Na/ benzophenone and distilled prior to use) to a glass vial containing a solid sample of the compound to produce a 10-4 M solution. Dissolution was achieved by gentle shaking of the vial. The solutions were drop-cast onto the substrates using a syringe to deposit approximately 50 µL of solution, and the solvent was allowed to evaporate in air. For white-light emitting aggregates, a solution of 1, 2, and 3 in THF was used (100 µM, 0.2 µM, 0.25 µM in 1, 2, and 3, respectively). After the nanoaggregates were drop-cast, the electron transport layer, TPBi, was thermally evaporated under vacuum (ca. 10-6 mbar) at a low deposition rate of 0.3 Å s-1. The deposited thickness (100 nm) was monitored using a thickness monitor inside the vacuum chamber close to the substrate holder. Thicknesses were checked with a Tencor AS-IQ profilometer. Bilayer cathodes of lithium fluoride LiF (1 nm thick) and aluminium Al (120 nm thick) were then evaporated


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through a shadow mask. All devices had an active area of 10 mm2. Samples were then stored and characterized under inert atmosphere in a nitrogen glovebox (O2 and H2O < 1 ppm). Currentvoltage-luminance (I-V-L) curves were measured using a Keithley 2400 Source-measurement unit (SMU) coupled to a photodiode calibrated with a Minolta CS-100 luminance meter or Hamamatsu C9920-12, external quantum efficiency measurement system coupled to an integration sphere. Optical image measurements. The optical images and EL properties of the devices prepared using nano-aggregates were measured by confocal fluorescence microscopy (CFM). The devices were protected with adhesive tape placed on the cathode contact prior to removing them from the glove box and were found to be stable to ambient for 2 – 3 days. The CFM used was a Picoquant Microtime 200 based on an inverted Olympus IX71 microscope equipped with a 100 × objective (UPLSAPO, N.A. = 1.4), a PicoHarp 300 multichannel single photon counter, and two MPD SPADs. A Ti–Sapphire laser chain composed of a Coherent Mira and a frequency doubler (pulses of 4–6 ps at 385 nm) was used for PL measurements. The EL devices were placed flat on the sample tray and the set-up was used to collect the EL by transmission through the dichroic optics before being focused on a 50 µm pinhole. After the pinhole, light can be diverted into an Andor SR300i spectrometer equipped with a Newton EM-CCD for spectroscopy measurements or to time-resolved avalanche photodiodes (MPD).

ASSOCIATED CONTENT Supporting Information. Spectral information for compounds 1 – 3, additional details for KPFM measurements, EL device characterization for 56 devices prepared using vesicles formed


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from compounds 1 – 3 are provided. Supporting Information is available online from the journal homepage (www.acs.org).

AUTHOR INFORMATION Corresponding Author *Dario M. Bassani, E-mail: [email protected]; Fax: +335 4000 6994; Ken-Tsung Wong, E-mail: [email protected]; Fax: +886-2-3366-1667 Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ACKNOWLEDGMENT The authors dedicate this article to Prof. Tien-Yau Luh on the occasion of his 70th birthday. We are grateful for the help of Mr. Po Han Wang in the graphical representation in the TOC and for financial support from LabEx AMADEus (ANR-10-LABX-0042-AMADEUS through grant ANR-10-IDEX-0003-02) and the Agence Nationale de la Recherche - Ministry of Science and Technology (MOST) Taiwan joint funding (grants ANR-13-IS07-0001 EVOLVE and 103-

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Graphical Abstract


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Electroluminescence from Spontaneously Generated Single-Vesicle

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