Prem Prabhakaran

Photopatternable quantum dots forming quasi-ordered arrays.

We have functionalized core-shell CdSe/ZnS quantum dots (QDs) with a photosensitive monolayer, rendering them solution processable and photopatternable. Upon exposure to ultraviolet radiation, films composed of this material were found to polymerize, forming interconnected arrays of QDs. The photoluminescence properties of the nanocrystal films increased with photocuring. The material was found to be suitable for spin casting and was used as the active layer in a green electroluminescent device. The electroluminescence efficiency of devices containing a photocured active layer was found to be largely enhanced when compared to devices containing nonphotocured active layers. The material also showed excellent adhesion to both organic and inorganic substrates because of the unique combination of a siloxane and a photopatternable layer as ligands. The pristine functionalized nanocrystals could easily be used for two-dimensional patterning on organic and inorganic substrates. The photopatternable quantum dots were uniformly dispersed into a photopolymerizable resin to fabricate QD embedded three-dimensional microstructures.

Quantum dots (QDs) for photonic applications

Quantum dots and their chemical adaptation for various photonic applications are presented in this review. The use of quantum dots as photoactive components in many applications requires their combination with other materials playing specific roles for separation and transport of charge carriers. Achieving good interfaces between electronically matched component materials is key to improved performance in photodetectors, photovoltaics, electroluminescence application, etc.

Photosensitive functionalized surface-modified quantum dots for polymeric structures via two-photon-initiated polymerization technique.

In this paper, the surface modification of CdSe- and CdZnS-based quantum dots (QDs) with a functional silica shell is reported. Functionalized silica shells are prepared by two routes: either by ligand exchange and a modified Stöber process or by a miniemulsion process with amphiphilic poly(oxyethylene) nonylphenylether also know as Igepal CO-520 (IG) as oligomeric amphiphile and modified silica precursors. The polymerizable groups on the functionalized silica shell allow covalent bonding to a polymer matrix and prevent demixing during polymerization and crosslinking. This allows the homogeneous incorporation of QDs in a crosslinked polymer matrix. This paper furthermore demonstrates that the resulting QDs, which are i) shielded with a proper silica shell and ii) functionalized with crosslinkable groups, can be used in two-photon-initiated polymerization processes in combination with different photoresists to obtain highly luminescent 3D structures. The resulting luminescent structures are attractive candidates for photonics and metamaterials research.

Solution processable and photopatternable blue, green and red quantum dots suitable for full color displays devices

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.

TWO-PHOTON STEREOLITHOGRAPHY

Two-photon stereolithography based on photopolymerization provides the ability to fabricate real three-dimensional (3D) microstructures beyond the resolution of focal size. In this paper, our recent research focusing on improvement of spatial resolution in two-photon stereolithography is reviewed. The influence of system and fabrication conditions in relation to the spatial resolution is discussed. For small and low aspect ratio voxels, a minimum power and minimum exposure time (MPMT) scheme is introduced. During the two-photon process, an ascending technique, wherein the truncation amount of volumetric pixels is controlled, can be applied to improve the resolution of two-dimensional patterns. 3D Microfabrication with less than 100 nm resolution can be realized by using the radical quenching effect. After the two-photon process, the resolution of fabricated patterns can be refined to 60 nm by post-processing of plasma ashing.

Energy and Charge Transfer in Nanoscale Hybrid Materials

Hybrid materials composed of colloidal semiconductor quantum dots and π-conjugated organic molecules and polymers have attracted continuous interest in recent years, because they may find applications in bio-sensing, photodetection, and photovoltaics. Fundamental processes occurring in these nanohybrids are light absorption and emission as well as energy and/or charge transfer between the components. For future applications it is mandatory to understand, control, and optimize the wide parameter space with respect to chemical assembly and the desired photophysical properties. Accordingly, different approaches to tackle this issue are described here. Simple organic dye molecules (Dye)/quantum dot (QD) conjugates are studied with stationary and time-resolved spectroscopy to address the dynamics of energy and ultra-fast charge transfer. Micellar as well as lamellar nanostructures derived from diblock copolymers are employed to fine-tune the energy transfer efficiency of QD donor/dye acceptor couples. Finally, the transport of charges through organic components coupled to the quantum dot surface is discussed with an emphasis on functional devices.

Highly biocompatible amphiphilic perylenediimide derivative for bioimaging

We report the synthesis and biological studies of a fluorescence dye with an oligoethylene glycol substituted (OEG) perylene centered dye N,N’-(2,6-diisopropylphenyl)-1-[oligo(ethylene glycol)methyl ether]-1,6,7,12-trichloroperylene-3,4:9,10-tetracarboxdiimide (PDI-OEG). The activity of the dye is juxtaposed with a precursor molecule without the OEG substitution. The OEG substitution contributes to the increased biocompatibility of PDI-OEG. Cell viability studies lead to the survival of more than 80% of the PDI-OEG cultured cells endorsing its biocompatibility. Fluorescence imaging studies were carried out using multiple cell lines. Ex-vivo studies involving nude mice were used to establish liver and lung specific organ targeting of PDI-OEG. This fluorophore is an excellent example of a stable and biocompatible red emitting small molecule for bioimaging.

Photopatternable cadmium-free quantum dots with ene-functionalization

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.

Two-photon absorption dye based on 2,5-bis(phenylacrylonitrile)thiophene with aggregration enhanced fluorescence

This paper reports the synthesis and characterization of a 2,5-bis(phenylacrylonitrile)thiophene based two-photon dye, designed to show enhancement in fluorescence quantum yield in nanoaggregated form. Strong solvatochromism has been observed and explained by the favoritism of locally excited (LE) or internal charge transfer (ICT) state depending on the solvent polarity. Aqueous dispersions of nanoparticles have been prepared and investigated regarding their optical properties which were correlated to the LE and ICT state and the molecular structure of the aggregates.

Effective direct writing of hierarchical 3D polymer micromeshes by continuous out-of-plane longitudinal scanning

AbstractNano-stereolithography, also known as two-photon direct writing is widely used for fabrication of the three-dimensional microstructure with submicron resolution. Heirarchical three-dimensional meshed microstructures are crucial for microelectromechanical systems (MEMS), photonics and biotechnological applications. Routine study for various applications using such structures requires fabrication of many sets of structures. Conventionally such meshed microstructures are constructed through layer-by-layer accumulation with discrete point-to-point laser scanning. This technique is time consuming, leading to long fabrication times placing constraints on practical applications as well as optimization of structures. In this work we propose continuous longitudinal laser scanning method as an effective means for direct writing of hierarchical three-dimensional meshed microstructures. The advantages of continuously longitudinal laser scanning method are explored for its time economy and fabrication effectiveness; a fabrication window is suggested to determine the fabrication parameter easily according to laser power and structural design.