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Core only of CdSe and core-shell quamtum dots (QDs) of CdS/ZnS, CdSe/ZnS and CdSe/ZnSe were functionalized with photosensitive monolayer to make them solution processable and photopatternable. Exchange of ligands was successfully followed using IR spectroscopic techniques. Core-shell type QDs were found to have better photoluminescence properties. Upon exposure to ultraviolet radiation these material were found to undergo polymerization forming interconnected arrays of QDs. These materials were found suitable for spin casting on organic and inorganic substrates. A highly efficient flourene-based two-photon sensitizer was mixed with QD dispersion of a urethane acrylate resin. Two-photon nanostereolithography using a mode-locked Ti:sapphire laser was applied on this resin mixture to fabricate three-dimensional (3D) microstructure. 3D microstructures fabricated were found with uniform dispersion of RGB QDs when observed through confocal microscope.
©2012 Optical Society of America
1. Introduction
Quantum dots (QDs) are tiny crystals that can trap electrons on a spatial scale as they are small enough for making the quantum effects so evident. These nanocrystals (NCs) are playing an increasingly important role in semiconductor, optical and electronic devices. Controlled fabrication of two-dimensional (2D) or three-dimensional (3D) micro- and nanoscale structures containing QDs is of great scientific importance for the development of efficient optoelectronic devices [1,2]. In recent years there have been commendable attempts to incorporate quantum dot 2D and 3D patterns through lithographic techniques [3,4]. Our group previously has reported the designing and synthesis of functionalized stable green emitting CdSe/ZnS quantum dots composed of a photopolymerizable outer corona constituting methacrylate and an inner siloxane layer, which are suitable for solution processing on both inorganic and organic substrates and their subsequent photopatterning in polymeric 3D shining microstructures [5]. In this work we are extending our previously reported method as a new way to create a solution processable RGB microstructure using photopatternable blue emitting CdS/ZnS and red emitting CdSe/ZnSe QDs along with the CdSe/ZnS green dots.
2. Experimental
2.1 Materials
Cadmium oxide (CdO, 99.99%), zinc acetate (99.9%, powder), selenium (99.9%, powder), sulfur (99.9%, powder), trioctylphosphine (TOP, 90%), oleic acid (OA, 90%), 1-octadecene (ODE, 90%), 11-mercapto-1-undecanol, 3-(trimethoxysilyl)propyl methacrylate, undecane-thiol (99.9%), trimethoxy(octyl)silane and anhydrous dimethyl sulfoxide (DMSO, 99.9 + %) were all purchased from Sigma-Aldrich and used without further purification. The urethane-acrylate resin SCR 500 was kindly provided by JSR Company, Japan.
2.2 Synthesis of CdS/ZnS, CdSe/ZnS and CdSe/ZnSe nanocrystals
The blue, green and red CdS, CdSe, CdS/ZnS, CdSe/ZnS and CdSe/ZnSe QDs respectively were synthesized by following the reported procedures [6–8]. Relative quantum yield reported for core-shell QDs are 80% for blue and green, and 60% for red dot.
2.3 Synthesis of siloxane-containing methyl methacrylate terminated QDs
Oleic acid stabilized QDs (5 mg) and 50 mg of 11-mercapto-1-undecanol were dispersed in 5 mL chloroform and 5 mL ethanol under sonication for 3 hours. Chloroform (40 mL) was added into the mixture to precipitate the 11-mercapto-1-undecanol capped NCs. The material was soluble in ethanol and DMSO. 20 mg of 11-mercapto-1-undecanol capped QDs were first dispersed in 5 mL of dry DMSO. A total of 20 mg of 11-mercapto-1-undecanol capped nanoparticles were first dispersed in 5 mL of dry DMSO, and then 100 μL of 3-(trimethoxysilyl)propyl methacrylate was added. The mixture was stirred at 50°C for 6 hours. The resulting nanoparticles were precipitated with chloroform by centrifugation. The silane containing methacrylate terminated nanoparticles were then washed with methanol and chloroform.
2.4 Microfabrication
Setup for two-photon lithography
A mode-locked Ti:sapphire laser operating at 780 nm and 80 MHz with a pulse width of less than 100 fs was used as a source for two-photon stereolithography. A high-numerical aperture lens (NA = 1.4, with immersion oil), capable of high-resolution 3D addressing of points within the photopolymerizable material, was employed in the optical system.
Details of the confocal microscopy
Carl-Zeiss LSM5 Live confocal microscope containing external fluorescence lamps with different excitation wavelengths, namely, 405, 588, and 532 respectively for green, red, and blue photoactive materials was used.
3. Results and discussion
QDs synthesized by conventional methods when mixed into a patternable matrix, shows aggregation due to the interdigitation of stabilizing ligand alkyl terminals [9–12]. This common nanoparticles related phenomenon could be passed over by proper functionalization of QDs. Such functionalized QDs could be customized for photopatterning and also could be well integrated into photopatternable resins. We have demonstrated earlier by synthesizing photopatternable green emitting QDs which upon ultraviolet (UV) exposure are photopolymerized into an interconnected nanomatrix comprising periodically spaced particles as close-packed arrays where nanoparticles are covalently cross-linked to each other by strong rigid bonds without any aggregation [5]. These closely packed QDs in the UV processed films were observed with a PL intensity increase over an order of magnitude after photocuring. Electroluminescence (EL) devices made out of these photocured QDs containing films were observed with excellent EL efficiency. These photopatternable QD were also demonstrated as an active emitting materials, which are chemical compatible for preparing polymeric microstructures by successfully fabricating 3D shining structures from their QD dispersed urethane acrylate resins. Stretching the possibilities of this work to blue and red color QDs can achieve milestones like full color displays and 3D structures.
3.1 Synthesis
For this study involving patternable red (R), green (G), blue (B) QDs, we have synthesized a series of core-shell QDs with blue, green and red emission along with their core only structures. This was followed by their ligand exchange to make them photopatternable. Photopatternable CdSe and core-shell CdS/ZnS green QDs were synthesized following our previous report [5]. Oleic acid stabilized CdS and CdS/ZnS blue dots and oleic acid stabilized CdSe and CdSe/ZnSe red QDs were synthesized following procedures reported earlier in the literature [6–8]. They were functionalized with a methyl methacrylate (MMA) corona according to the method that used for green emitting QDs. As an initial process the oleic acid on the QDs was first replaced with mercapto undecanol, the resulting QDs were then reacted with 3-(trimethoxysilyl)propyl methacrylate to obtain the MMA-terminated QDs as given in Fig. 1 .
Fig. 1 Synthetic schemes for MMA functionalized RGB quantum dots. For (a) blue QD, CdS/ZnS, (b) green QD, CdSe/ZnS, (b) red QD, CdSe/ZnSe. Syntheses were carried out from oleic acid capped NCs for blue and green QDs while for red QDs it was from dioctylphosphine oxide (TOPO) caped NCs.
These core only and core-shell structures were examined for their PL efficiencies. Samples with comparable absorbance values in UV-vis spectra were analyzed and found with relatively very low PL for the core only structures. It is quite natural for the core only structures, as shell growth give additional advantage of better quantum confinement for the NCs [13]. Growing a shell of higher band gap over the nanocrystals confines the wave function of the electron-hole pairs more precisely resulting in a stable core-shell structure for the nanocrystals. Appearance of CdSe and CdSe/ZnSe NCs under visible light and 365 nm UV irradiation is given in Fig. 2 .Based on the result of Fig. 1, 3D structures by two-photon lithography were fabricated with core only and core-shell structures (Details of 3D structure fabrication are described in the later part of this manuscript). Through an optical microscope brightness of these structures were observed under 365 nm UV irradiation. In Fig. 3 , panels (c) and (d) shows microstructures thus fabricated based on CdSe and CdSe/ZnS QDs respectively. The core-shell structures were appeared with higher luminescence brightness as appeared in the figure. For rest of the study photopatternable core-shell type nanocrystals were used due to their better PL properties compared the core only structures.
Fig. 2 Photopatternable (a) CdSe and (b) CdSe/ZnSe NCs when observed under visible light. Photopatternable (c) CdSe and (d) CdSe/ZnSe NCs when observed under 365 nm UV light.
Fig. 3 (a) CAD master image used to prepare the 3D woodpile pattern; (b) SEM image of typical 3D pattern; (c) and (d), PL micrographs observed by 3D patterns of photocured single shell QD of CdSe and CdSe/ZnS NCs, respectively, when observed under 365 nm UV exposure.
3.2 Characterizations
Figure 4 shows the absorption and photoluminescence spectra of different functionalized QD nanocrystals grown with different core-shell composition and with identical surface ligand conditions. The samples were observed to have relatively monodisperse particle size from the full width at half maximum (FWHM) of the PL spectra. The ligand exchange over the QD surfaces is evidenced from their IR spectra as depicted in Fig. 5 , which shows the ligand exchange over QDs with green emission. Oleic acid ligated QDs were characterized with intense signals corresponding to the antisymmetric ν(CO) stretching vibration bands of carboxylate around 1,555 cm−1 with no detectable presence of carboxylic groups (1,710 cm−1) [14]. It also displays two bands at 2922 and 2850 cm−1 corresponding to the ν(CH) vibrations of CH2 groups. For the QDs with photopatternable ligands, absence of the characteristic peak at 2500 cm−1 for S-H stretching indicates the formation of QD-S bond on the surface of the NCs. Appearance of a new band around 1070 cm−1 is observed which corresponds to the Si-O-Si vibrations.
Fig. 4 UV-vis and PL spectra of photopatternable QDs. (a) blue Cd/ZnS dots, (b) green CdSe/ZnS dots, (c) red CdSe/ZnSe dots, (d) photograph of the corresponding samples under UV light.
3.3 Microfabrication
Different lithographic techniques those have been employed in the past to achieve quantum dot embedded 3D microstructures had a pre-existing problem of detrimental aggregation effect when lithography was attempted by mixing photopatternable materials with QDs [9–12]. These effects are governed by the interactions between the surface ligands of the QDs, as well as their incompatibility with the photopatternable material in which they are dispersed.
We have successfully demonstrated that the acrylate-terminated QDs could be integrated well into acrylate and urethane acrylate resins due to their photopatternable corona and can be used for two-photon lithography (TPL) to fabricate microstructures. TPL is a fast-prototyping method that photoinduces chemical reactions in a patternable medium, allowing direct writing of microstructures. Femtosecond lasers with high repetition rates induce very specific chemical changes at the focal spot of the laser within a photoactive medium. Two-photon sensitivity is key for initiating chemical changes during microfabrication. For this reason, the patternable medium should contain a two-photon absorbing (TPA) material acting as a photosensitizer or an photoinitiator.
The spatial selectivity of the chemical process arises from the inverse dependence of two-photon absorption on the intensity of the laser beam. The two-photon dye used as the photosensitizer (spirofluorene-based TPA dye) was chosen such that the peak fluorescence of the dye does not coincide with that of the QD fluorescence during imaging the QD-impregnated structures. The fabricated structures were visualized by confocal microscopy. A Carl Zeiss LSM5 Live confocal microscope was used for visualization and the images are summarized in panels (a)-(d) in Fig. 6 . For the measurement of panel (a) visible laser line of 405 was used along with 415-480 nm filter and for panel (b) the visible laser line used was that of 488 nm and the filter was that of 500-525 nm wavelength. In the case of panel (c) it was done with laser line 532 nm in combination with a filter 550 nm-IR wavelength. Finally for the panel (d) the measurement was with all these three laser lines in operation with no filters to get white emission properties. The images discern the successful and uniform incorporation of nanocrystals within the microstructure.
Fig. 6 Confocal microscope images of 3D structure, demonstrating the uniform incorporation of RGB quantum dots throughout the structure.
The (a)-(d) panels in Fig. 6 shows the potential of these functionalized nanomaterials for their utilization in full color display devices. Charge transport layers consist of molecules or polymers having photocurable prosthetic groups such as oxetane or methacrylate can very well suited to utilize these materials for full color display device fabrication [15,16]. Attempts in this regard are in progress and the results will be reported in the due course.
4. Conclusion
Photopatternable RGB core only and core-shell type quantum dots with an inner siloxane layer and a photopatternable methacrylate corona were synthesized. The hybrid nature of the photopatternable QD makes it readily suitable for solution processing on both inorganic and organic substrates and subsequent photopatterning. Core-shell type nanocrystals were found with better PL properties. Chemical compatibility of photopatternable QD and the phenomenon of photodriven ordering in functionalized QDs was used to fabricate microstructure with uniform quantum dot dispersion. Confocal microscope images showed a uniform distribution of RGB QDs all over the microstructure.
Acknowledgments
This work was supported by the Mid-career Researcher Program (No. 2010-0000499) and the Active Polymer Center for Patterned Integration (ERC R 11-2007-050-01002-0) of the National Research Foundation of Korea. One of us, K.-S. Lee, thanks to the Asian Office of Aerospace Research and Development (AOARD), Air Force Office of Scientific Research, USA, for their support.
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