Shyuan Yang

Orientation of luminescent excitons in layered nanomaterials

In nanomaterials, optical anisotropies reveal a fundamental relationship between structural and optical properties. Directional optical properties can be exploited to enhance the performance of optoelectronic devices, optomechanical actuators and metamaterials. In layered materials, optical anisotropies may result from in-plane and out-of-plane dipoles associated with intra- and interlayer excitations, respectively. Here, we resolve the orientation of luminescent excitons and isolate photoluminescence signatures arising from distinct intra- and interlayer optical transitions. Combining analytical calculations with energy- and momentum-resolved spectroscopy, we distinguish between in-plane and out-of-plane oriented excitons in materials with weak or strong interlayer coupling-MoS₂ and 3,4,9,10-perylene tetracarboxylic dianhydride (PTCDA), respectively. We demonstrate that photoluminescence from MoS₂ mono-, bi- and trilayers originates solely from in-plane excitons, whereas PTCDA supports distinct in-plane and out-of-plane exciton species with different spectra, dipole strengths and temporal dynamics. The insights provided by this work are important for understanding fundamental excitonic properties in nanomaterials and designing optical systems that efficiently excite and collect light from exciton species with different orientations.

Clean graphene electrodes on organic thin-film devices via orthogonal fluorinated chemistry.

Graphene is a promising flexible, highly transparent, and elementally abundant electrode for organic electronics. Typical methods utilized to transfer large-area films of graphene synthesized by chemical vapor deposition on metal catalysts are not compatible with organic thin-films, limiting the integration of graphene into organic optoelectronic devices. This article describes a graphene transfer process onto chemically sensitive organic semiconductor thin-films. The process incorporates an elastomeric stamp with a fluorinated polymer release layer that can be removed, post-transfer, via a fluorinated solvent; neither fluorinated material adversely affects the organic semiconductor materials. We used Raman spectroscopy, atomic force microscopy, and scanning electron microscopy to show that chemical vapor deposition graphene can be successfully transferred without inducing defects in the graphene film. To demonstrate our transfer methods compatibility with organic semiconductors, we fabricate three classes of organic thin-film devices: graphene field effect transistors without additional cleaning processes, transparent organic light-emitting diodes, and transparent small-molecule organic photovoltaic devices. These experiments demonstrate the potential of hybrid graphene/organic devices in which graphene is deposited directly onto underlying organic thin-film structures.

Metacapacitors: Printed Thin Film, Flexible Capacitors for Power Conversion Applications

The Metacapacitors project aims to improve efficiency, functionality and form factor of offline power converters suitable for LED solid-state lighting, with a view to developing an attractive technology platform for load management and power conversion across a broad range of applications. Based on integrated switched-capacitor (SC) topologies, the project adopts an integrated approach from materials to devices to circuits. We designed capacitors based on high-κ dielectric nanocrystals, that can be prepared using high-throughput microfabrication/nanotechnology techniques, ink deposition and multilayering. The capacitor dielectric, a nanocomposite composed of (Ba, Sr)TiO3 nanocrystals in polyfurfuryl alcohol (BST/PFA, κ > 20, 100Hz-1 MHz, loss <; 0.01, 20 kHz), targets a high volumetric capacitance density and ripple current capability. The Dielectric is demonstrated to function in a finished capacitor > 1000 h at 125°C. The capacitors were board integrated with a custom hybrid-switched-capacitor-resonant dc-dc converter IC. The converter integrates a balanced SC front-end with a series resonant tank, enabling nearly lossless current regulation and tranformerless galvanic isolation. The converter IC can be stacked in the voltage domain to interface a range of inputs. The tested driver delivers about 15 W at 470 mA to a string of 12 LEDs with 90% peak efficiency.

Influence of electromigration on the maximum operating field of (Ba,Sr)TiO3/parylene-C composite capacitors

Nanoparticle/polymer composite capacitors are a promising approach for fabricating high performance capacitors at low temperatures using printing techniques. In this work, the dependence of the operating voltage of nanoparticle/polymer composite capacitors on the electromigration parameter of the electrodes is investigated. The authors have previously shown that breakdown is suppressed by selecting the polarity used in nanoparticle (Ba,Sr)TiO3/parylene-C composite film-based capacitors. In this work, the authors examine gold, silver, copper, chromium, and aluminum electrodes with comparable surface conditions. The asymmetric silver, aluminum, gold, copper, and chromium electrode devices show a 64%, 29%, 28%, 17%, 33%, improvement in the effective maximum operating field, respectively, when comparing bias polarity. The field at which filament formation is observed shows a clear dependence on the electromigration properties of the electrode material, and demonstrates that use of electromigration resistant m...

Inexpensive, Versatile, and Robust USB-Driven Sensor Platform

An ultralow cost vapor sensor platform is developed, incorporating a chemiresistor-based sensor that enables targeted sensing through surface functionalization. Humidity, ammonia, and acetone sensing are demonstrated by PEDOT-PSS, DPP-CN, and graphene surface modification, respectively. The integrated USB connector, use of a standard HMI interface, and microcontroller interface allows us installation-free data collection on any device with a USB port.

Large scale temperature monitoring system for detection of potential ebola patient

Ebola crisis has taken its human and economic toll. It is an illustrative example of how inadequate healthcare infrastructure at the community level can have global implications. We are developing a low-cost continuous temperature acquisition system with embedded functionality for monitoring at the local, national and global level. Our system aims to not only track the health of patients in a range of resource constrained environments (e.g. small villages), empowering communities to contain potential outbreaks before they become unmanageable, but also to monitor healthcare workers during the course of their contact with patients and upon return to their local communities minimizing the probability of further disease transmission. We are confident that our system can be deployed in a number of use scenarios for Ebola tracking and management to improve the efficiency of healthcare resource allocation, control the spread of disease, and ultimately save lives.