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Abstract
Photopatternable nanoparticles can be easily dispersed into polymeric matrices and used to fabricate optoelectronic devices for display, sensing and quantum information processing applications. Here we report the first instance of a cadmium-free photopatternable quantum dot. A ligand containing dithiolane group at one end and an ene-functionalization at the other end were synthesized for this purpose. The myristic acid ligands on as synthesized red indium zinc phosphide-zinc sulfide (In(Zn)P/ZnS) quantum dots were easily replaced by the newly developed ligand by a simple sonication procedure. The functionalized quantum dots could be easily incorporated into a commercially available photoresist. The quantum dot doped photoresist was used to fabricate three-dimensional quantum dot doped hierarchical microstructures by two-photon lithography. Confocal imaging microscopy was used to verify the uniform incorporation of the nanoparticles in the hybrid microstructure.
© 2017 Optical Society of America
1. Introduction
Quantum dots (QDs) are semiconductor nanomaterials that are being studied for diverse applications such as energy harvesting, displays, quantum information processing and cellular imaging [1–4]. Functionalization of QDs plays an important role in making them stable as well as making them compatible to interactions with functional materials. Many applications involve the interplay of QDs with organics, optoelectronic polymers, bio-organic polymers or other inorganic nanomaterials [5]. Cadmium-based QDs involving combinations of elements from groups II and VI were preferred for early investigations into QDs because of their high luminescence efficiencies. However, the toxicity of cadmium-based QDs limits their use in consumer products [6]. Indium-based III-V QDs are the preferred alternative for cadmium-free QDs. Such QDs have recently found their first introduction to the consumer market in the form of Samsung quantum dot technology. But this technology for the time being remains proprietary leading to a bottle neck in the consumer electronics market for cadmium-free QDs suitable for display applications [7]. This is mainly due to the low quantum efficiencies of III-V QDs and problems with their reproducible synthesis and scaling up [8].
Photopolymerizable ligands on the surface of QDs can impart solubility and processability to QDs and facilitate their easy application. In our previous works we have also showed that the functionalization of QDs with photoreactive ligands allows the densification of QD films by light. Such densification leads to change in optical properties of the films leading to increased photoluminescence. This was found to be beneficial for electroluminescent devices that feature QDs as active layers [9]. Such functionalization also improves the uniform mixing of QDs into photoresists facilitating fabrication of quantum dot doped three-dimensional structures [9–11].
In the current work, we prepare a photopatternable cadmium-free QD. This is the first such instance to the best of our knowledge. The red In(Zn)P/ZnS alloy quantum dot was functionalized with an easy to synthesize ligand with ditholane group at one terminal and a ene (vinyl) group at the other. The functionalized QD could be easily incorporated into radical polymerizable photoresist to fabricate QD incorporated polymeric microstructures.
2. Experimental
2.1 Synthesis of ligands
The active ester of lipoic acid LP was obtained by reacting α-lipoic acid (0.27 g, 1.31 mmol) and petrafluorophenyl trifluoroacetate (0.43 ml, 2.50 mmol) in presence of triethylamine (0.36 ml, 2.53 mmol) in methylene chloride (MC) under nitrogen atmosphere for one hour. The reaction mixture was poured into MC and separated against ice water. The organic mixture was dried with anhydrous sodium carbonate, and then purified by column chromatography using MC:ethyl acetate (EA) mixture to obtain 370 mg (0.98 mmol) of LP at 75% yield. The structure of the compound was confirmed by 1H-NMR: (300 MHz, CDCl3, δ): 3.60 (m, 1H), 3.15 (m, 2H), 2.65 (t, 2H), 2.45 (m, 1H), 1.90 (m, 1H), 1.80 (m, 2H), 1.70 (m, 2H), 1.55 (q, 2H). 19F NMR (400 MHz, CDCl3, δ): −165.2 (dd), −161.2 (t), −155.4 (d).
The final vinyl-terminated ligand LN was obtained by reacting LP (990 mg, 2.65 mmol) to allylamine (0.4 ml, 5.30 mmol) in presence of the base triethylamine (0.75 ml, 5.30 mmol) in MC solvent for 6 hours at room temperature and nitrogen gas. The organic mixture was dried with anhydrous sodium carbonate, and then purified by column chromatography using a MC: EA mixture to obtain a 520 mg of brownish white compound at 80% yield. The structure of the compound was confirmed by 1H-NMR: (600 MHz, CDCl3, δ): 5.82 (m,1H), 5.55 (s, 1H), 5.15 (m, 2H), 3.85 (s, 2H), 3.55 (m, 1H), 3.20 (m, 2H), 2.45 (m, 1H), 2.15 (m, 2H), 1.90 (m, 1H), 1.75 (m, 5H), 1.45 (m, 3H).
2.2 Synthesis of quantum dots and their functionalization
Alloy quantum dots In(Zn)P/ZnS (red) were synthesized following established procedures [12]. This method yields QDs functionalized with myristic acid (QD-MA). The myristic acid ligand was replaced by sonicating nanoparticles in presence of the ligand LN under N2 atmosphere at ambient temperature. For ligand exchange, the ligand:QD ratio was maintained at 10:1. In a typical ligand exchange reaction 320 mg of ligand was sonicated with 32 mg of QD in chloroform for one hour. After sonication, the ligand exchanged In(Zn)P/ZnS QDs were precipitated by adding dropwise into excess methanol in a centrifuge tube. After centrifugation, the supernatant was discarded and the centrifugate was redissolved in chloroform. This was again precipitated in methanol and the whole cycle of precipitation-centrifugation-dispersion was repeated 5 times.
2.3 Characterization of quantum dots and ligand
Prior to NMR the ligand LN and the precursor were purified using high pressure liquid chromatography (HPLC) using HPLC model Futecs NS-4000. The ligand was characterized by 1H-NMR spectroscopy using Varian 300 (300 MHz, Agilent Technology, USA). The 19F-NMR was carried out with 400 MHz FT-NMR (JEOL JNM-AL400, Japan). FT-IR characterization of the ligand was carried out with IRaffinity-1S (Shimadzu, Japan). The absorption and emission properties of the QDs were studied using the UV-vis spectrometer UV-3600 (Shimadzu, Japan) and PL spectrometer F-7000 fluorescence spectrophotometer (Hitachi, Japan).
2.4 Preparation of QD containing photoresist
A volume of 0.5 ml of a 12 mg/ml solution of red QDs in chloroform was added 1 g of SCR- 500 photoresist diluted with chloroform. The SCR-500 photoresist was kindly provided by the JSR company, Japan. One mg of a highly sensitive phenylenevinylene-based two-photon absorbing dye (described in section 3.3) was added to the photoresist to sensitize it [13]. The photoresist and the QDs were mixed well, the excess chloroform was evaporated away and the remaining photoresist was filtered before undergoing fabrication. The filtering step helps remove any aggregated QDs in the photoresist.
2.5 Imaging
Confocal microscopy imaging was carried out on a Zeiss LSM5 Live confocal microscope and the scanning electron microscopic images were obtained using Hitachi S4800 cold type field emission scanning electron microscope. The transmission electron microscopic (TEM) images of the QDs were obtained using JEM 3010 (JEOL Ltd., Japan).
2.6 Fabrication setup
The two-photon stereolithography setup employs a mode-locked Ti:Sapphire laser with a wavelength of 780 nm, and a repetition rate of 80 MHz and an ultrashort pulse width of 100 fs. The intensity of the laser was adjusted using a λ/2 plate; the on/off stage of the laser was controlled using a Galvano shutter. The laser was focused on the photocurable resin placed on a thin cover glass through a high numerical aperture objective lens (NA of 1.3, x100, immersion oil used). The objective lens of the fabrication system was fixed. The position of the focused laser beam inside the sample was manipulated by the movement of the piezoelectric sample stage with a resolution of 0.1 nm in the x, y and z directions. The fabrication process was monitored using a high magnification charge-coupled device (CCD) camera.
3. Results and discussion
Generally, the functionalization of QDs with patternable ligands makes them easy to incorporate into polymerized films. Doping polymeric films with QDs can be used for engineering the refractive index of polymerized films and hence their optical properties. UV irradiation of films containing polymerizable ligands has been shown to densify the films and improve their luminescent properties [9]. Polymer films and structures incorporating QDs have been studied for their properties such as photonic bandgaps, and as emissive layers in microelectronic devices [14–16]. There have been a few examples of photopatternable ligands since Kim et al. demonstrated the functionalization of QDs with thermally and photoacid cleavable t-BOK groups and fabrication of photodetector devices [17]. Lee et al. demonstrated the methacrylate-functionalization of QDs and fabrication of electroluminescent devices as well as metamaterial structures [10, 11] The vinyl-end functionalized ligands proposed in this work allow them to take part in radical initiated polymerization with acrylate based ligands as well as thiol-ene click reactions [18, 19].
3.1 Synthesis of ene-functionalized QDs
Lipoic acid and its hydrolysis product dihydrolipic acid (DHLA) are widely used as anchoring groups for functionalization of nanoparticles such as QDs and gold nanoparticles for biological and biotechnological applications [20–22]. The ligand LN was synthesized from the pentrfluorophenyl active ester of α-lipoic LP as shown in Fig. 1.
Fig. 1 (a) Synthesis of pentafluorphenyl active ester of α-lipoic acid LP, (b) synthesis of thiol-terminated ligand LN, (c) ligand exchange of myristic acid on as synthesized QDs with LN through sonication.
The red In(Zn)P/ZnS alloy quantum dots were synthesized by the procedure established by Thuy et al. [12]. The procedure yields QDs stabilized by myristic acid (QD-MA) easily dispersed in chloroform. The size of the cadmium-free core-shell QDs synthesized were found to be 7-10 nm in size as seen in the transmission electron microscopic (TEM) images in Fig. 2(a)-2(b). The composition of In(Zn)P/ZnS were analyzed by electron dispersive spectroscopy (EDS) on the particles during TEM imaging. The results can be seen in Fig. 2(c). The EDS spectrum clearly indicates the presence of phosphorus, sulfur, zinc and indium in the quantum dot. The weight percentage (Wt.%) and atomic percentage (At.%) of various elements obtained from EDS measurement of the QD are given in the table in the inset of 2(c). The myristic acid stabilizing ligands on as synthesized quantum dots can be easily replaced by sonicating them with LN ligand for an hour under N2 atmosphere. As mentioned earlier the ditholane terminal group of the ligand LN containing two sulfur atom has strong affinity for the surface of the QDs [23].
Fig. 2 Characterization of In(Zn)P/ZnS quantum dot. (a)-(b) TEM images scale bars 20 nm, and 10 nm, respectively, (c) EDS spectrum of QD: The inset shows the weight percentage (Wt.%) and atomic percentage (At.%) of different elements on the measured QD nanocrystal.
3.2 Characterization
The ligand exchange was characterized by FT-IR measurements seen in Figs. 3(a)-3(b). The stacked spectra in Fig. 3(a) compares myristic acid (top), the ligand LN (middle) and ligand functionalized QD (QD-LN) (bottom). Myristic acid data was obtained from NIST chemistry web book [24]. It is clear from Figs. 3(a)-3(b) that the amide –NH stretch at 3,290 cm−1 feature in both in LN and QD-LN. The spectrum of LN (black line) in clearly shows the vinyl -CH stretching peak at 3,080 cm−1, the presence of vinyl group in the ligand is further confirmed by the two vinyl out of plane bending vibrations at 990 cm−1 and 916 cm−1, respectively. The amide CO double bond stretch can be seen at 1640 cm−1 and the NH bending vibration at 1541 cm−1.
Fig. 3 (a) FT-IR stack spectrum comparing myristic acid (green, top) with LN (black, middle) and QD-LN (red, bottom), (b) the absorption and emission spectra of QD-MA (∆) and QD-LN (○), (c) photograph under UV irradiation of QD-MA and QD-LN.
The IR signals from ligand on QD-LN (red line, top) were observed to be rather weak even after 250 acquisition scans. The overall peaks shifted to lower energies on functionalization. The weak signals might be because of the relatively lower concentration of ligands on the surface of the QDs in QD-LN compared to an FT-IR measurement of pristine LN. The shift of peaks to lower energies might be arising from local interactions between ligands on the surface of QDs or those between ligands and QDs. This red shift can be seen clearly in the case of NH stretch, C-O double bond stretch and NH bending vibration by comparing the spectra. The vinyl peaks are not clear in the spectrum, a sharp peak occurs around 990 cm−1 but the second out of plane bending peak is not clearly seen.
The absorption and emission spectra of the QD-MA and QD-LN are given in Fig. 3(b). The absorbance and emission peaks of the QD-MA and QD-LN did not show any change on ligand exchange. The maximum emission peaks of both for QD-MA and QD-LN were at 596 nm. For the same optical density, the intensity of emission was greater for QD-MA compared to QD-LN. The decrease emission seen in Fig. 3(b) can be seen as an indicator of increased binding of the ligand with the QD surface. Previous studies on ligand exchange of QDs have established the replacement of oleic acid and the subsequent leaching of ions into solution as causes for such a decrease in emission. The bonding of the ditholane group has been proposed to be partially between the surface of the QD and the leached ions [25, 26]. Hence the decreased PL intensity in this context can be taken as an indicator of functionalization of the QD surface with LN. A photograph of dispersions of QD-MA (left) and QD-LN in chloroform under UV irradiation can be seen in Fig. 3(c).
3.3 Microfabrication
Two-photon lithography (TPL) was used for fabrication of QD incorporated three-dimensional polymeric structure. TPL is a maskless lithographic technique based on photochemical phenomena triggered by the simultaneous absorption of two photons. To fabricate quantum dot incorporated microstructure, QDs were mixed into a urethane acrylate based photoresist SCR-500, along with catalytic amounts of a highly efficient two-photon absorbing photosensitizer JS. The vinyl group on the QD would easily react with the urethane acrylate oligomers present in the photoresist during photopolymerization. The concept of three-dimensional polymeric structure incorporated with the vinyl functionalized QDs through two-photon polymerization (TPP) as well as the prominent materials in the photoresist are is summarized in Fig. 4.
Fig. 4 (a) Two-photon polymerization driven fabrication of quantum dot embedded 3D polymeric structure from the SCR-500 photoresist doped with In(Zn)P/ZnS QDs. A drop of the sample photoresist is placed on a thin cover glass substrate. The laser is focused on the sample through the cover glass using a high numerical aperture lens. (b) The main components of the TPP photoresists are urethane acrylate photoresist SCR 500, the ligand functionalized QD-LN and highly efficient two-photon photoinitaitor JS.
The addition of nanoparticles increases the threshold power of polymerization in photopolymers while also causing heating effects due to particle-laser interactions [27]. Higher loading of nanoparticles or metal/semiconductor precursors can lead to heating effects and collapse of microstructures [28]. Such effects can be controlled by regulating both the amount of nanoparticles as well as the laser power during TPL. From our previous experience with patternable semiconductor nanocrystals we have found that a nanoparticle loading of 0.2-0.6 wt.% per gram of photoresist provided stability under fabrication as well as shelf life of the prepared photoresists [9–11]. Here we have used 0.6 wt % of QD in the photoresist to avoid such effects of aggregation. To decrease the fabrication power an efficient TPA dye is used (two-photon absorption cross section 66.8 GM) [13]. The photoresist was found to have a threshold fabrication power of 17 mW.
A twisted hierarchical polymer array structure was fabricated to demonstrate the viability of uniformly QD incorporated 3D microstructure with QD-LN. The design of the 3D structure can be seen in Fig. 5(a) which features both top and side image. The SEM image of the sidewise orientation of the microstructure can be seen in Fig. 5(b)-(d). The structure is fixed well on the surface of the substrate (cover glass) on which it is fabricated. The well-formed structure also indicates the absence of heating effects associated with laser-QD interactions. Further SEM images in Fig. 5(c) show similar structures fabricated at slightly different powers 19 mW, 18 mW and 17 mW, respectively. There are not drastic changes in resolution of the structure when changing the fabrication power in this range.
Fig. 5 SEM and confocal micrographs of the 3D patterns obtained by microfabrication experiments, (a) shows the top view, and side view of the design of 3D structure, (b) SEM image of the side view of a red QD incorporated microstructure fabricated by TPL, (c) SEM image of an array of three hierarchical 3D structures fabricated by TPL. (d) SEM image of the top view of a microstructure, (e) complements the SEM image in (c) and shows the confocal microscopy image of the same three structures, (f) top view of the microstructure by confocal microscopy.
The incorporation of QD-LN into the microstructure was confirmed by imaging the structures using confocal fluorescence microscopy. The image in Fig. 4(e) corresponds to the same structures as in Fig. 4(c). It can be seen that the whole structure shows fluorescence. The intensely red aggregates that are found on the surface of the substrate as well as on the structures are due to microaggregates formed due to unreacted photoresist during developing process. The top view of the microstructure in Fig. 5(f) is an evidence of the uniformity of QD incorporation in the structures. The center portion of the structure where multiple layers overlap appears bright red while the vertices of the structure appears transparent due to the imaging of single layers constituting the microstructure.
4. Conclusions
Here we have designed, synthesized and characterized an ene-terminated ligand with a dithiolan-end group capable of co-ordinating with quantum dot surface. The ligand was successfully functionalized on red In(Zn)P/ZnS QD. The vinyl-terminated QD was easily incorporated into commercial urethane acrylate photoresist. Three dimensional hierarchical structures fabricated with the QD-doped photoresist showed a uniform distribution of QD within the sample on confocal imaging. Apart from commercial resist used in this study the cadmium-free In(Zn)P/ZnS QD with vinyl functionalization has high potential to be used in thiol-ene polymerization. Because of the wide spread application of thiol-ene polymerization these photopolymerizable cadmium-free QDs can have applications in patterning of fluorescent biologically relevant structures.
Funding
National Research Foundation of Korea (NRF) funded by the MEST (2016R1A2-B4008473); Asian Office of Aerospace and development (FA2386-11-1-4043).
Acknowledgments
This work was supported by the Mid-career Researcher Program through the National Research Foundation of Korea (NRF) funded by the MEST (2016R1A2-B4008473) and the Asian Office of Aerospace and development (FA2386-11-1-4043). One of us, K.-S. Lee thanks to the support of the Research Year Program 2016 of Hannam University.
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