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Center for Organic Photonics and Electronics

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Now showing 1 - 10 of 35

Photoelectron spectroscopy studies of plasma-fluorinated epitaxial graphene

(Georgia Institute of Technology, 2012-05) Sherpa, Sonam D. ; Paniagua, Sergio A. ; Levitin, Galit ; Marder, Seth R. ; Williams, M. D. ; Hess, Dennis W.

Fluorination of graphene has emerged as an attractive approach toward manipulating its physical, chemical, and electronic properties. To this end, we have demonstrated the viability of sulfur hexafluoride plasmas to fluorinate graphene as a safer alternat
Solution doping of organic semiconductors using air-stable n-dopants

(Georgia Institute of Technology, 2012-02) Qi, Yabing ; Mohapatra, Swagat K. ; Kim, Sang Bok ; Barlow, Stephen ; Marder, Seth R. ; Kahn, Antoine

Solution-based n-doping of the polymer poly{[N,N′-bis(2-octyldodecyl)-naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,5′-(2,2′-bithiophene)} [P(NDI₂OD-T₂)] and the small molecule 6,13-bis(tri(isopropyl)silylethynyl)pentacene (TIPS-pentacene) is realized with the air-stable dimers of rhodocene, [RhCp₂]₂, and ruthenium(pentamethylcyclopentdienyl)(1,3,5-triethylbenzene), [Cp*Ru(TEB)]₂. Fermi level shifts, measured by direct and inverse photoemission spectroscopy, and orders of magnitude increase in current density and film conductivity point to strong n-doping in both materials. The strong reducing power of these air-stable dopants is demonstrated through the n-doping of TIPS-pentacene, a material with low electron affinity (3.0 eV). Doping-induced reduction of the hopping transport activation energy indicates that the increase in film conductivity is due in part to the filling of deep gap states by carriers released by the dopants.
Deep in vivo two-photon imaging of blood vessels with a new dye encapsulated in pluronic nanomicelles

(Georgia Institute of Technology, 2011-03) Maurin, Mathieu ; Stéphan, Olivier ; Vial, Jean-Claude ; Marder, Seth R. ; Van der Sanden, Boudewijn

The purpose of this work was to validate the use of Pluronic fluorescent nanomicelles for in vivo two-photon imaging of both the normal and the tumor vasculature. The nanomicelles were obtained after encapsulating a hydrophobic two-photon dye: di-stryl benzene derivative, in Pluronic block copolymers. Their performance with respect to imaging depth, blood plasma staining, and diffusion across the tumor vascular endothelium was compared to a classic blood pool dye Rhodamin B dextran (70 kDa) using two-photon microscopy. Pluronic nanomicelles showed, like Rhodamin B dextran, a homogeneous blood plasma staining for at least 1 hour after intravenous injection. Their two-photon imaging depth was similar in normal mouse brain using 10 times less injected mass. In contrast with Rhodamin B dextran, no extravasation is observed in leaky tumor vessels due to their large size: 20-100 nm. In conclusion, Pluronic nanomicelles can be used as a blood pool dye, even in leaky tumor vessels. The use of Pluronic block co-polymers is a valuable approach for encapsulating twophoton fluorescent dyes that are hydrophobic and not suitable for intravenous injection.
Remote doping of a pentacene transistor: Control of charge transfer by molecular-level engineering

(Georgia Institute of Technology, 2010-09) Zhao, Wei ; Qi, Yabing ; Sajoto, Tissa ; Barlow, Stephen ; Marder, Seth R. ; Kahn, Antoine

We demonstrate that holes from a p-doped N,N′-diphenyl-N,N′-bis(1-naphthyl)-1,1′-biphenyl-4,4′-diamine (α-NPD) layer transfer to an adjacent pentacene film. The spatial separation of carriers from dopants, or remote doping, is demonstrated with a combination of photoemission spectroscopy and current-voltage measurements for a p-doped α-NPD/pentacene heterojunction. Increased conductivity of the pentacene film is observed in both nongated temperature-dependent conductivity and gated thin-film transistor measurements.
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.
Linear ripples and traveling circular ripples produced on polymers by thermal AFM probes

(Georgia Institute of Technology, 2009-06) Gnecco, Enrico ; Riedo, Elisa ; King, William P. ; Marder, Seth R. ; Szoszkiewicz, Robert

We discuss the time and temperature evolution of the nanometer-scale surface undulations (ripples) produced by a heated atomic force microscope (AFM) tip scanning across surfaces of several amorphous polymers. During linear zigzag scanning we obtain pseudolinear ripples approximately perpendicular to the fast scan direction in a range of scan rates and probe temperatures. As expected, the size of the ripples increases massively in the vicinity of the glass temperature for each polymer. We also examine a different case in which the AFM tip follows a circular path. Contrary to the ``steady{\'\' circular ripples which rotate along the scanning path during consecutive scans. The group velocity of the circular ripples is 2 orders of magnitude lower than the scan speed. We interpret the experimental data using a phenomenological model accounting for erosion and smoothing effects caused by the probing tip.
Fabrication of a blue M x N pixel organic light-emitting diode video display incorporating a thermally stable emitter

(Georgia Institute of Technology, 2009-04) Haldi, Andreas ; Kim, Jung B. ; Domercq, Benoit ; Kulkarni, Abhishek P. ; Barlow, Stephen ; Gifford, Angela P. ; Jenekhe, Samson A. ; Marder, Seth R. ; Kippelen, Bernard

A 7x11 pixel blue OLED display was fabricated using a patterned indium-tin-oxide (ITO) substrate. The fabrication process for an M x N pixel organic light-emitting diode (OLED) video display including an electrical insulating layer and a physical pixel separator layer is presented. An efficient and thermally stable blue fluorescent organic material, 6, 6'-bis((2-p-biphenyl)-4-phenylquinoline) (B2PPQ), was used in combination with an evaporated hole-transport small molecule with a high ionization potential.
Tailoring the work function of indium tin oxide electrodes in electrophosphorescent organic light-emitting diodes

(Georgia Institute of Technology, 2009) Sharma, Asha ; Hotchkiss, Peter J. ; Marder, Seth R. ; Kippelen, Bernard

We investigate the use of organic surface modifiers based on phosphonic acid anchoring groups that react with the surface of indium tin oxide (ITO) in order to modify its work function, surface energy, and the charge injection in organic multilayer electrophosphorescent devices. The phosphonic acid surface modifiers, possessing different substituting groups, are found to tune the work function of ITO in the range of 4.40–5.40 eV. These surface modifiers have been tested as an interfacial layer between the ITO anode and hole transport layers HTL that are either processed from the vapor phase or from solution. The use of this interfacial layer with a solution-processible HTL results in high quantum and luminous efficiencies of 20.6% and 68 cd/A at 100 cd/m² (17.5% and 60 cd/A at 1000 cd/m²). The enhanced performance of the devices incorporating phosphonic acid modifiers could be associated with an improved charge injection and a better compatibility with the hydrophobic nature of the organic layer. The performance of these devices is also compared to that of devices in which ITO is modified with other well-known techniques such as air plasma treatment or the use of a layer of poly (3,4-ethylenedioxythiophene)/poly (styrenesulfonate).
Effect of phosphonic acid surface modifiers on the work function of indium tin oxide and on the charge injection barrier into organic single-layer diodes

(Georgia Institute of Technology, 2009) Sharma, Asha ; Haldi, Andreas ; Hotchkiss, Peter J. ; Marder, Seth R. ; Kippelen, Bernard

We investigate the use of several phosphonic acid surface modifiers in order to increase the indium tin oxide (ITO) work function in the range of 4.90–5.40 eV. Single-layer diodes consisting of ITO/modifier/N,N′-diphenyl-N,N′-bis(1-naphthyl)-1,1′ biphenyl-4,4″ diamine (α-NPD)/Al and ITO/modifier/pentacene/Al were fabricated to see the influence of the modified ITO substrates with different work functions on the charge injection. To calculate the charge injection barrier with different surface modifiers, the experimentally measured current density-voltage (J-V) characteristics at different temperatures are fitted using an equivalent circuit model that assumes thermionic emission across the barrier between the ITO work function and the highest occupied molecular orbital of the organic material. The charge injection barrier height extracted from the model for various surface modifier-based diodes is independent of the ITO work function within the range of changes achieved through modifiers for both α-NPD and pentacene-based single-layer diodes.
Stabilization of the work function of indium tin oxide using organic surface modifiers in organic light-emitting diodes

(Georgia Institute of Technology, 2008-10) Sharma, Asha ; Kippelen, Bernard ; Hotchkiss, Peter J. ; Marder, Seth R.

We herein report on the performance and improved stability of organic light-emitting diodes (OLEDs) in which the transparent indium tin oxide (ITO) electrode is modified using organic surface modifiers based on phosphonic acid anchoring groups. In contrast to air plasma treatment, a commonly used technique to increase the work function of ITO, treatment of the ITO surface with a partially fluorinated phosphonic acid results in a comparable change in work function but with a higher stability over time. The resultant lifetime of OLEDs also increased when this phosphonic acid modified ITO was used.