A substrate for organic electronics that can do it all! (2015)

A transparent, smooth, thermally robust, conductive polyimide for flexible electronics

Advanced Functional Materials (2015)

In this work we introduce a thermally and mechanically robust, smooth transparent conductor composed of silver nanowires embedded in a colorless polyimide substrate. The polyimide is exceptionally chemically, mechanically, and thermally stable. While silver nanowire networks tend not to be thermally stable to high temperatures, the addition of a titania coating on the nanowires dramatically increases their thermal stability. This allows for the polyimide to be thermally imidized at 360 °C with the silver nanowires in place, creating a smooth (<1 nm root mean square roughness), conductive surface. These transparent conducting substrate-cum-electrodes exhibit a conductivity ratio figure of merit of 272, significantly outperforming commercially available indium-tin-oxide (ITO) coated plastics. We subject the conductive polymide to various mechanical tests and use it as a substrate for a thermally deposited, flexible, organic light-emitting diode (OLED) which shows improved device performance than a control device made on ITO coated glass.

Video above is green OLED on our conductive polyimide operating while being flexed.

Liberating the light trapped inside a white OLED (2015)

Enhanced outcoupling in organic light emitting diodes via a high-index contrast scattering layer

Published in ACS Photonics 2015

Despite high internal quantum efficiencies, planar organic light-emitting diodes (OLEDs) typically suffer from limited outcoupling efficiencies. To improve this outcoupling efficiency, we have developed a new thin (∼2 μm) light scattering layer that employs air voids (low-index scattering centers) embedded in a high-index polyimide matrix to effectively frustrate the substrate-trapped light, increasing the outcoupling efficiency. The porous polyimide scattering layers are created through the simple and scalable fabrication technique of phase inversion. The optical properties of the scattering layers have been characterized via microscopy, transmittance/haze measurements, and ellipsometry, which demonstrate the excellent scattering properties of these layers. We have integrated these films into a green OLED stack, where they show a 65% enhancement of the external quantum efficiency and a 77% enhancement of the power efficiency. Furthermore, we have integrated these layers into a white OLED and observed similar enhancements. Both the green and white OLEDs additionally demonstrate excellent color stability over wide viewing angles with the integration of this thin scattering layer.

Hybrid organic PV devices with nanowire electrodes get more efficient when lasered (2015)

Improved Efficiency of Hybrid Organic Photovoltaics by Pulsed Laser Sintering of Silver Nanowire Network Transparent Electrode

Published in ACS Applied Materials and Interfaces 2015

In this Research Article, we demonstrate pulsed laser processing of a silver nanowire network transparent conductor on top of an otherwise complete solar cell. The macroscopic pulsed laser irradiation serves to sinter nanowire-nanowire junctions on the nanoscale, leading to a much more conductive electrode. We fabricate hybrid silicon/organic heterojunction photovoltaic devices, which have ITO-free, solution processed, and laser processed transparent electrodes. Furthermore, devices which have high resistive losses show up to a 35% increase in power conversion efficiency after laser processing. We perform this study over a range of laser fluences, and a range of nanowire area coverage to investigate the sintering mechanism of nanowires inside of a device stack. The increase in device performance is modeled using a simple photovoltaic diode approach and compares favorably to the experimental data.

A see-through microphone

In this short video, we demonstrate a transparent microphone made from silver nanowire coated piezoelectric PvDF. This fun little project was a collaboration with Josh Sanz-Robinson (Advisers: Sturm,  Wagner)

The video:
(0:04)-(0:20): Vivaldi's unmistakable 'spring' is played into the microphone, one can see a small square of plastic being held in two large magnetic vices. A breadboard is visible where the piezoelectric signal is then amplified, digitized, and sent into a computer.
(0:30)-(0:42) Playback of the recorded signal, again recognizable if not maybe a little tinny. Heard laughing is Yasmin, heard talking is Josh S-R, and myself.

Using a disease model to predict user interest in social media (2014)

Epidemiological modeling of online social network dynamics

Published to the ArXiv 2014

The last decade has seen the rise of immense online social networks (OSNs) such as MySpace and Facebook. In this paper we use epidemiological models to explain user adoption and abandonment of OSNs, where adoption is analogous to infection and abandonment is analogous to recovery. We modify the traditional SIR model of disease spread by incorporating infectious recovery dynamics such that contact between a recovered and infected member of the population is required for recovery. The proposed infectious recovery SIR model (irSIR model) is validated using publicly available Google search query data for "MySpace" as a case study of an OSN that has exhibited both adoption and abandonment phases. The irSIR model is then applied to search query data for "Facebook," which is just beginning to show the onset of an abandonment phase. Extrapolating the best fit model into the future predicts a rapid decline in Facebook activity in the next few years.

3D printer spray coater

Why pay more? Ideally for my nanoparticle (or more specific to its main function nanowire) spray coater I needed 3 axis motion, a moderately heated stage, and the ability to turn on and off the spray stream. So, I bought a vellum K8200 3D printer , which takes care of both the 3 axis motion (note the x-y belt drives are much faster than the z lead screw, but z motion usually only happens before deposition so this is not an issue) and the heated bed. The thermocouple for the bed claims that the surface can reach 75 C which is more than enough to help whisk away the organic solvent holding the nanoparticle suspension, however the nature of spray coating, which is to force evaporation makes the bed cool down rapidly- perhaps a beefier heating element would be ideal. Finally, for controlling the spray I used an unused motor controller to simply give me a 5/0 high low output from the control board, which hits a relay to a solenoid valve, holding back about 35 psi of compressed dry air. Using the solenoid valve like this does have some ramp up/down effects on the spray stream, which cause me to start and stop the spray while the nozzle is not over my sample- a small concern on the lab scale, but something which can be mitigated if needed. Total cost ~$600, compare to getting a three axis system with flow control from a company like Fisnar, and you could be adding two zeros to that price!

Integrating laser processed silver nanowires in organic electronic devices (2013)

Integrated Laser Processed Silver Nanowire Transparent Electrodes with Organic Electronic Devices

Paper presented at CLEO 2013

UV pulsed laser irradiation coupled into a metal nanowire network through plasmonic absorption leads to localized welding of junction points. This approach enables direct integration of transparent metal films with organic photovoltaic and light emitting devices.

Laser sintering silver nanowires (2012)

Direct-wright pulsed laser processed silver nanowire networks for transparent conducting electrodes:
Published in Applied Physics A in 2012

In this work, we demonstrate a direct-write refining technique in which a solution-processed nanowire network, deposited by spin coating, is exposed to monochromatic UV pulsed laser processing near a plasmonic resonance. Our results exhibit a 75 % reduction in surface resistance along with marginal improvements in optical transparency. The local nature of the laser technique enables direct-write or large area processing on a variety of substrates including flexible, and organic materials.

Detecting randomness by counting tiles (2011)

Solution to the bathroom tile problem
Published in the Princeton MAE Departmental Bulletin, 10/2011

The bathroom tile problem asks: when a tile floor (2D truncated plane) is composed of a grid of tiles in which each location is occupied by a tile of random color what is the longest span between two of the same colored tiles in either dimension along the floor. This work addresses how this longest span scales with the relative concentrations of the two tile species. Herein we provide both an analytical expression and Monte Carlo type simulation of bathroom tile floors. The punchline: the bathroom floors in the men's rooms of the engineering quad at Princeton, while lacking a repeating unit are, to a great degree of confidence, non-random. 

How to make 18 mirror segments act like one big mirror (2010)

Advanced DFS: a dispersed fringe sensing algorithm insensitive to small calibration errors

Paper presented at SPIE Space Telescopes and Instrumentation 2010

Dispersed Fringe Sensing (DFS) is an elegant method of coarse phasing segmented mirrors. DFS performance accuracy is dependent upon careful calibration of the system as well as other factors such as internal optical alignment, system wavefront errors, and detector quality. Novel improvements to the algorithm have led to substantial enhancements in DFS performance. In this paper, we present Advanced DFS, an advancement of the DFS algorithm, which allows the overall method to be less sensitive to calibration errors. This is achieved by correcting for calibration errors, which appear in the fitting equations as a signal phase term. This paper will outline a brief analytical explanation of the improvements, results of advanced DFS processed simulations and experimental advanced DFS results.

We do everything as well as we can.

“The Balinese have a saying: We have no art. We do everything as well as we can.”

-Marshall McLuhan, The medium is the massage.

Perovskite World, Confocal Microscope

Stain Glass, Optical Microscope

Shampoo, Optical Microscope

T-junction, Optical Microscope

Blanketed, Electron Microscope

Cracked, Confocal Microscope

The Street Where You Live, Confocal Microscope

Delamination, Confocal Microscope

Birds Home, Confocal Microscope