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Center for Organic Photonics and Electronics

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Quantitative modeling of coupling-induced resonance frequency shift in microring resonators

(Georgia Institute of Technology, 2009-12) Li, Qing ; Soltani, Mohammad ; Atabaki, Amir Hossein ; Yegnanarayanan, Siva ; Adibi, Ali

We present a detailed study on the behavior of coupling-induced resonance frequency shift (CIFS) in dielectric microring resonators. CIFS is related to the phase responses of the coupling region of the resonator coupling structure, which are examined for various geometries through rigorous numerical simulations. Based on the simulation results, a model for the phase responses of the coupling structure is presented and verified to agree with the simulation results well, in which the first-order coupled mode theory (CMT) is extended to second order, and the important contributions from the inevitable bent part of practical resonators are included. This model helps increase the understanding of the CIFS behavior and makes the calculation of CIFS for practical applications without full numerical simulations possible.
Low-voltage pentacene organic field-effect transistors with high-kappa HfO₂ gate dielectrics and high stability under bias stress

(Georgia Institute of Technology, 2009-12) Zhang, Xiaohong ; Tiwari, Shree Prakash ; Kim, Sung-Jin ; Kippelen, Bernard

Low-voltage pentacene organic field-effect transistors are demonstrated (operating voltage of −3 V) with high-κ hafnium dioxide gate dielectrics grown by atomic layer deposition at 200 °C. A high hole mobility of 0.39 cm²/V s with low threshold voltage (<−0.5 V) and low subthreshold slope of 120 mV/dec is achieved with a HfO₂ dielectric layer modified with a phosphonic acid based treatment. A high value of 94.8 nF/V s is obtained for the product of mobility and capacitance density. The devices show excellent bias stress stability with or without the phosphonic acid at the HfO₂ gate dielectric surface.
Low-voltage solution-processed n-channel organic field-effect transistors with high-k HfO₂ gate dielectrics grown by atomic layer deposition

(Georgia Institute of Technology, 2009-12) Tiwari, Shree Prakash ; Zhang, Xiaohong ; Potscavage, William J., Jr. ; Kippelen, Bernard

High performance solution-processed n-channel organic field-effect transistors based on [6,6]-phenyl C61 butyric acid methyl ester with low operating voltages (3 V) are demonstrated using a high-k hafnium dioxide gate dielectric grown by atomic layer deposition. Devices exhibit excellent n-channel performance with electron mobility values up to 0.14 cm²/V s, threshold voltages of ∼ 0.3 V, current on/off ratios >10⁵, and very low values of subthreshold slope ( ∼ 140 mV/decade).
Multiplex coherent anti-Stokes Raman scattering (MCARS) for chemically sensitive, label-free flow cytometry

(Georgia Institute of Technology, 2009-12) Camp, Charles Henry, Jr. ; Yegnanarayanan, Siva ; Eftekhar, Ali Asghar ; Sridhar, Hamsa ; Adibi, Ali

Flow cytometry is an ever-advancing high-throughput multivariate analysis tool that natively provides size and morphological information. To obtain molecular information, however, typically requires the addition of fluorophores, which are limited by spectral overlap, nonspecific binding, available conjugation chemistries, and cellular toxicity. A complementary or alternative, label-free approach to molecular information is through multiplex coherent anti-Stokes Raman scattering (MCARS), which is a coherent, nonlinear optical method that provides a wealth of molecular information by probing the Raman energies within a molecule. In this work, we demonstrate the unique capability of our MCARS flow cytometer to distinguish flowing particles and discuss system performance capabilities and possibilities.
Direct writing and characterization of poly(p-phenylene vinylene) nanostructures

(Georgia Institute of Technology, 2009-12) Wang, Debin ; Kim, Suenne ; Underwood, William D. , II ; Giordano, Anthony J. ; Henderson, Clifford L. ; Dai, Zhenting ; King, William P. ; Marder, Seth R. ; Riedo, Elisa

We report the use of thermochemical nanolithography to convert a precursor polymer film to poly(p-phenylene vinylene) with sub-100 nm spatial resolution, in ambient conditions. The local thermochemical conversion is verified by Raman spectroscopy, fluorescence imaging, and atomic force microscopy. This convenient direct writing of conjugated polymer nanostructures could be desirable for the design and fabrication of future nanoelectronic, nanophotonic, and biosensing devices.
Bond contribution model for the prediction of glass transition temperature in polyphenol molecular glass resists

(Georgia Institute of Technology, 2009-11) Lawson, Richard A. ; Yeh, Wei-Ming ; Henderson, Clifford L.

Molecular glass resists have shown potential as replacements for polymeric resists in next generation lithography, especially extreme ultraviolet lithography. One of the main concerns about molecular resists is their glass transition temperature (Tg) which can be very low in some cases due to their small molecular size and other factors. While most of the polymeric chemically amplified resist platforms used thus far have Tg’s above 100 °C, molecular resists investigated in the literature so far have shown a wide range of measured Tg’s from near room temperature to greater than 160 °C. This potential for low Tg values and the current lack of ability to easily predict their Tg is a concern when designing new compounds because a molecular resist may be synthesized with a Tg value that is too low for the required processing conditions (e.g., allowing for dewetting of the resist, flow of the resist features, or excessive photoacid diffusion). To enable rational molecular resist design and overcome these problems, a quantitative structure-property relation model based on bond additivity that allows for the prediction of the Tg of molecular resists based on their full chemical structure has been developed in this work. The model shows a good coefficient of determination (R²) of 0.84 with experimental data, and a standard deviation of only 12 °C for 57 compounds. It works well across multiple different levels of protection, different structural moieties, different molecular sizes, and different types of protecting groups. The model was also simplified to provide a simple heuristic for predicting Tg based on only two or three structural parameters, and this easy to use simplified model provides a similar level of quantitative agreement with experimental data to the full bond additivity model.
Negative tone molecular resists using cationic polymerization: Comparison of epoxide and oxetane functional groups

(Georgia Institute of Technology, 2009-11) Lawson, Richard A. ; Noga, David E. ; Younkin, Todd R. ; Tolbert, Laren M. ; Henderson, Clifford L.

Two molecular resists with a common molecular glass core were synthesized and characterized to compare the differences between epoxide (oxirane) and oxetane functional groups for use in high resolution negative tone molecular resists. Both resists are able to obtain at least 50 nm half-pitch at a sensitivity of 75 μC/cm² under 100 keV electron-beam lithography. Due to differences in the kinetics of the cationic polymerization of epoxides as compared to oxetanes, the epoxide functionalized resist (2-Ep) was able to obtain sub-25-nm half-pitch resolution with good line edge roughness (LER) of 2.9 nm (3σ) while the oxetane resist (2-Ox) was limited to 50 nm half-pitch resolution and exhibited higher LER (3σ) of 10.0 nm. The polymerization of the oxetane functional group has slow initiation and fast propagation which leads to reduced performance in 2-Ox as compared to 2-Ep. While oxetane functionalized molecular resists can obtain reasonably good imaging performance, epoxide functional groups show more promise for use in next generation negative tone resists that have a good combination of resolution, sensitivity, and LER.
Conjugated polymer-fullerene blend with strong optical limiting in the near-infrared

(Georgia Institute of Technology, 2009-11) Chi, San-Hui ; Hales, Joel M. ; Cozzuol, Matteo ; Ochoa, Charles ; Fitzpatrick, Madison ; Perry, Joseph W.

Optical-quality, melt processable thick films of a conjugated polymer blend containing poly(2-methoxy-5-(2-ethyl-hexyloxy)-(phenylene vinylene)) (MEH-PPV), a ₆₀ derivative (PCBM) and a plasticizer (1,2-di-iso-octylphthalate) have been developed and their nonlinear absorption and optical limiting properties have been investigated. These blend materials exhibited strong optical limiting characteristics in the near infrared region (750-900 nm), with broad temporal dynamic range spanning femtosecond to nanosecond pulse widths. The dispersion of the optical limiting figure-of-merit of the MEH-PPV:PCBM:DOP blend shows a peak near the wavelength of the MEH-PPV cation, indicating an important role of one-photon and two-photon induced charge transfer in the nonlinear absorption response.
A row-action based L₁-minimization approach to robust fluorescent tomography

(Georgia Institute of Technology, 2009-10) Mohajerani, Pouyan ; Behrooz, Ali ; Eftekhar, Ali A. ; Adibi, Ali

We present a row-action method based on minimization of the L₁ norm for improving the accuracy of fluorescent tomography in reconstruction of fluorescent objects. The method is validated using a CW system and milk-based phantoms.
Nonionic photoacid generator behavior under high-energy exposure sources

(Georgia Institute of Technology, 2009-10) Lawson, Richard A. ; Noga, David E. ; Tolbert, Laren M. ; Henderson, Clifford L.

A series of nonionic photoacid generators (PAGs) are synthesized and their acid generation efficiency measured under deep ultraviolet (DUV) and electron beam exposures. The acid generation efficiency is determined with an on-wafer method that uses spectroscopic ellipsometry to measure the absorbance of an acid sensitive dye (Coumarin 6) Under DUV exposures, common ionic onium salt PAGs show excellent photoacid generation efficiency, superior to most nonionic PAGs tested in this work. In contrast, when under 100-keV high energy e-beam exposures, almost all of the nonionic PAGs show significantly better acid generation performance than the ionic onium salt PAGs tested. In particular, one nonionic PAG shows almost an order of magnitude improvement in the Dill C acid generation rate constant compared to a triarylsulfonium PAG. The high energy acid generation efficiency is found to correlate well with the electron affinity of the PAGs, suggesting that improvements in PAG design can be predicted. Nonionic PAGs merit further investigation as a means for producing higher sensitivity resists under high energy exposure sources.