Ali Ozhan Altun

Ultraviolet nanoimprinted polymer nanostructure for organic light emitting diode application

Light extraction efficiency of a conventional organic light emitting diode (OLED) remains limited to approximately 20% as most of the emission is trapped in the waveguide and glass modes. An etchless simple method was developed to fabricate two-dimensional nanostructures on glass substrate directly by using ultraviolet (UV) curable polymer resin and UV nanoimprint lithography in order to improve output coupling efficiency of OLEDs. The enhancement of the light extraction was predicted by the three-dimensional finite difference time domain method. OLEDs integrated on nanoimprinted substrates enhanced electroluminance intensity by up to 50% compared to the conventional device.

Metal‐Dielectric‐CNT Nanowires for Femtomolar Chemical Detection by Surface Enhanced Raman Spectroscopy

A highly sensitive substrate for surface enhanced Raman spectroscopy (SERS) is formed by arrays of gold-coated metallic carbon nanotubes having a nanoinsert of high-k dielectric (hafnia) as an energy coupling barrier. Repeated femtomolar detection of 1,2 bis-(4-pyridyl)-ethylene in solution demonstrates the critical contribution of this plasmonic energy coupling barrier to the enhanced chemical sensitivity.

Sensitive Detection of Competitive Molecular Adsorption by Surface Enhanced Raman Spectroscopy

Surface adsorption plays a critical role in a wide variety of fields from surface catalysis to molecular separation. Despite the importance, limited access to simultaneously sensitive and selective detection mechanisms has hampered the acquisition of comprehensive and versatile experimental data needed to understand the complex aspects of mixture adsorption, calling for a molecular detection method capable of obtaining the surface adsorption isotherms over a wide range of concentrations as well as distinguishing the competitive adsorption of different adsorbates. Here, we test surface-enhanced Raman spectroscopy (SERS) as an effective analysis tool of surface adsorption phenomena. Using a sensitive SERS substrate, we characterize the adsorption isotherms of chemical species of various binding energies. We obtained the isotherms for strongly binding species in a concentration range from subpicomolar to micromolar. A log-sigmoidal dependency of the SERS signals to the analyte concentration could be modeled by surface binding theories accurately using molecular dynamics simulations, thereby bringing out the potential capability of sensitive SERS for describing a single-compound adsorption process. SERS also enabled the determination of competitive adsorption isotherms from a multiple-compound solution for the first time. The successful demonstration of the sensitive and selective characterization of surface adsorption lends SERS adaptability to a cheap, facile, and effective solution for chemical analysis.

Femtomolar molecular detection with CNT based SERS substrate

We report a highly sensitive substrate for surface enhanced Raman spectroscopy (SERS) enabled by arrays of metal (gold and silver) nanowires on the template of vertically aligned (VA-) carbon nanotubes (CNTs) coated with a high-k dielectric hafnia (HfO2) layer as a potential barrier. Femtomolar detection of 1,2 bis-(4-pyridyl)-ethylene (BPE) is demonstrated with this non-resonant substrate. Comparison of SERS performance with and without the hafnia potential barrier establishes the critical contribution of this dielectric nano spacer to the large sensitivity. This behavior is attributed to the relief of electric charge leakage from metal to the CNT template in the presence of the virtual energy potential barrier. The VA-CNT substrate, when covered by dielectric barriers, can be a great template for a practical and reproducible SERS substrate.

Etch-less UV-NIL process for patterning photonic crystal structure onto OLED substrate

An etch-less ultraviolet nanoimprint lithography (UV-NIL) process is proposed for patterning a photonic crystal (PC) structure onto an organic light-emitting diode (OLED) substrate. In a conventional UV-NIL, anisotropic etching is used to remove the residual layers and to transfer the patterns onto the substrate. The proposed process does not require an etching process. In the process, a stamp with nano-scale PC patterns is pressed on the dispensed resin and UV light is then exposed to cure the resin. After tens of seconds, the stamp is separated from the patterned polymer layer on the substrate. Finally, high-refractive index material is coated onto the layer. The refractive index of the polymer should be very similar to that of glass. The enhancement of the light extraction was assessed by the three-dimensional (3D) finite difference time domain (FDTD) method. The OLED was integrated on a nanoimprinted substrate and the electro-luminance intensity was found to have increased by as much as 50% compared to a conventional device.

Stamping-based planarization of flexible substrate for low-pressure UV nanoimprint lithography.

Patterning flexible substrates in nano scale is an important and challenging issue in the fabrication of next-generation devices based on a non-silicon substrate. Step and Flash imprint lithography (S-FIL) which is a room temperature and low pressure process offers several important advantages, such as the use of a smaller and therefore cheaper stamp or the possibility of the overlay imprinting, as a transparent stamp is utilized. However, it is very difficult to perform S-FIL on a flexible substrate successfully due to the high waviness. The waviness of a flexible substrate is not a constant value in contrast to a rigid substrate. It depends on the imprint pressure applied onto the substrate. In this paper, in section two, the effect of the imprint pressure on the waviness of the surface of the flexible substrate is examined. It is proved that the waviness of the surface of the flexible substrate could not be reduced sufficiently to assure a successful imprint at low imprint pressures. In the third section, a method of patterning polymer substrates using ultra-violet nanoimprint lithography (UV-NIL) is presented. The method consists of two stages, stamping-based planarization and S-FIL. In stamping-based planarization, a planarization layer of transparent polymer is formed onto the flexible substrate. Waviness of the blank stamp (in this study, glass wafer) is transferred to the planarization layer. S-FIL is performed with the nanoimprint tool IMPRIO100 directly onto the planarization layer employing a 1 x 1 in. quartz stamp. Optical microscope and SEM images of the successfully imprinted patterns were also presented.