Nikolaos Kehagias

Tuning the Intensity of Metal‐Enhanced Fluorescence by Engineering Silver Nanoparticle Arrays

It is demonstrated that silver nanoparticle (SNP) arrays fabricated by combining nanoimprint lithography and electrochemical deposition methods can be used as substrates for metal-enhanced fluorescence, which is widely used in optics, sensitive detection, and bioimaging. The method presented here is simple and efficient at controlling the nanoparticle density and interparticle distance within one array. Furthermore, it is found that the fluorescence intensity can be tuned by engineering the feature size of the SNP arrays. This is due to the different coupling efficiency between the emission of the fluorophores and surface plasmon resonance band of the metallic nanostructures.

Reverse-contact UV nanoimprint lithography for multilayered structure fabrication

In this paper, we report results on a newly developed nanofabrication technique, namely reverse-contact UV nanoimprint lithography. This technique is a combination of nanoimprint lithography and contact printing lithography. In this process, a lift-off resist and a UV cross-linkable polymer are spin-coated successively onto a patterned UV mask-mould. These thin polymer films are then transferred from the mould to the substrate by contact at a suitable temperature and pressure. The whole assembly is then exposed to UV light. After separation of the mould and the substrate, the unexposed polymer areas are dissolved in a developer solution leaving behind the negative features of the original stamp. This method delivers resist pattern transfer without a residual layer, thereby rending unnecessary the etching steps typically needed in the imprint lithography techniques for three-dimensional patterning. Three-dimensional woodpile-like structures were successfully fabricated with this new technique.

Two-dimensional polymer photonic crystal band-edge lasers fabricated by nanoimprint lithography

We report on the fabrication and characterization of two-dimensional polymer photonic crystal band-edge lasers operating in the visible range. The components have been fabricated in a dye chromophore-loaded polymer matrix by nanoimprint lithography. High-symmetry band-edge modes are used to generate laser emission. The experimental lasing frequencies are in good agreement with those calculated using a two-dimensional plane wave algorithm. These results demonstrate the potential of nanoimprint lithography for the fabrication of two-dimensional planar photonic crystal structures in an active medium in a one-step process.

Functional patterns obtained by nanoimprinting lithography and subsequent growth of polymer brushes

In this work the growth of polymer brushes was combined with nanoimprint lithography (NIL) in order to obtain new functional nanopatterns. First, a functional thermoplastic methacrylic copolymer poly(methyl methacrylate-co-2-bromoisobutyryl-oxy-ethyl methacrylate) was synthesized. This copolymer was successfully patterned by NIL using a silicon stamp at 160 °C and 60 bar. Next, hydrophilic polymer brushes based on poly(3-sulfopropylmethacrylate) and hydrophobic polymer brushes based on a poly(fluorinated methacrylate) were grown on the imprinted surfaces. The surface properties of the patterned polymer were accordingly modified and, as a consequence, the water contact angle was modified from 80.3° to 32.5° in the case of the hydrophilic brushes and to 118.1° in the case of the hydrophobic brushes. As an application we demonstrated the use of hydrophobic polymer brushes in order to modify the surface of polymeric stamps for NIL with self-demoulding properties.

Spontaneous emission control of colloidal nanocrystals using nanoimprinted photonic crystals

The authors report on the fabrication and optical characterizations of two-dimensional photonic crystals fabricated by nanoimprint lithography in a nanocomposite polymer incorporating highly luminescent and red emitting (CdSe)ZnS core-shell colloidal nanocrystals. Photonic crystal structures enhance the light emitted from the quantum sized nanoparticles in the composite layer by slowing the propagation speed of the photons, thus increasing the coupling to the out-of-plane radiative modes. A 200% enhancement of the light collection is achieved compared to an unpatterned sample.

Photoluminescence enhancement in nanoimprinted photonic crystals and coupled surface plasmons.

A method to enhance the photoluminescence of dye chromophores-loaded by coupling the emission to surface plasmons in nanoimprinted photonic crystals is reported. A 9-fold enhancement in the spontaneous emission intensity of a rhodamine-doped polymer film is achieved on a silver layer due to surface plasmon excitation. By changing the surface plasmon frequency, this enhancement can be suppressed. When the polymer film is patterned by nanoimprint lithography with a twodimensional photonic crystal the photoluminescence intensity increases up to 27 times compared to unpatterned samples on a quartz substrate.

Surface-directed dewetting of a block copolymer for fabricating highly uniform nanostructured microdroplets and concentric nanorings.

Through a combination of nanoimprint lithography and block copolymer self-assembly, a highly regular dewetting process of a symmetric diblock copolymer occurs whereby the hierarchal formation of microdroplets and concentric nanorings emerges. The process is driven by the unique chemical properties and geometrical layout of the underlying patterned silsesquioxane micrometer-sized templates. Given the presence of nonpreferential substrate-polymer interactions, directed dewetting was utilized to produce uniform arrays of microsized droplets of microphase separated polystyrene-block-poly(methyl methylacrylate) (PS-b-PMMA), following thermal annealing at 180 °C. Microdroplets with diameters greater than 400 nm exhibited a hexagonal close-packed arrangement of nanodots on the surface with polydomain ordering. At the droplet periphery, the polydomain ordering was severely disrupted because of a higher in-plane radius of curvature. By reducing the droplet size, the in-plane radius of curvature of the microdroplet becomes significant and the PMMA cylinders adopt parallel structures in this confined geometry. Continuous scaling of the droplet results in the generation of isolated, freestanding, self-aligned, and self-supported oblique nanorings (long axis ∼250-350 nm), which form as interstitial droplets between the larger microdroplets. Optical and magnetic-based nanostructures may benefit from such hierarchal organization and self-supporting/aligned nanoring templates by combining more than one lithography technique with different resolution capabilities.

Fabrication of highly ordered sub-20 nm silicon nanopillars by block copolymer lithography combined with resist design

The control of order and orientation of the self-assembly of cylinder-forming poly(styrene-b-dimethylsiloxane) block copolymer is demonstrated. Copolymer thin films are spun-cast onto topographically patterned (well-defined rectangular cross-section channels) polyhedral-silsesquioxane-type resist templates and annealed in solvent vapor. The templates used here are fabricated by UV-curing nanoimprint lithography and the surface properties of the resist are tuned by the ligands coordinated to the resists silsesquioxane cages. Depending on the resists composition and on the surface chemistry at the base of the trench (resist or silicon), various morphologies and orientations of the polydimethylsiloxane cylinders are observed without the use of a brush layer. Some surfaces are demonstrated to be neutral for the copolymer, without any wetting layer and, under favorable conditions, highly ordered features are observed over substrate areas of about 1 cm2 (scalable to larger surfaces). Also, the possibility of using solvents widely accepted in industry for polymer spin-coating and annealing is proved. Due to the high plasma etch resistance of the polydimethylsiloxane block, self-assembled patterns can be transferred to the silicon substrate producing silicon features with aspect ratios up to 2. We demonstrate that the methodology developed here could be integrated into conventional fabrication processes and scaled to wafer production.

Enhanced light extraction in ITO-free OLEDs using double-sided printed electrodes

We show how nanoimprint lithographic techniques are particularly suited for the realization of OLED device structures. We tested them to realize nanopatterned metallic electrodes containing photonic crystals to couple the light out and plasmonic crystals showing extraordinary transmission. At similar current densities, a two-fold electroluminescence is achieved with devices having double-sided structured metallic electrodes as compared to a control OLED with an ITO anode. The use of combined nanoimprint lithography processes has the potential to expand the performance range of various organic optoelectronic devices.

Fabrication of Defect-Free Nanoimprinted Photonic Crystals for Laser Applications

The fabrication of two-dimensional photonic crystals by nanoimprint lithography is established to realize lasing oscillations at three band edge frequencies. The optically pumped lasers are based on a dye-chromophore-loaded polymer matrix on a transparent substrate. A two-dimensional plane-wave algorithm was used to design the band-edge lasers showing a good agreement with the experimental lasing frequencies. These results show that nanoimprint lithography is well-suited to the one-step fabrication of two-dimensional active-polymer photonic crystals.