Jason C Reed

Transparent and flexible low noise graphene electrodes for simultaneous electrophysiology and neuroimaging

Calcium imaging is a versatile experimental approach capable of resolving single neurons with single-cell spatial resolution in the brain. Electrophysiological recordings provide high temporal, but limited spatial resolution, because of the geometrical inaccessibility of the brain. An approach that integrates the advantages of both techniques could provide new insights into functions of neural circuits. Here, we report a transparent, flexible neural electrode technology based on graphene, which enables simultaneous optical imaging and electrophysiological recording. We demonstrate that hippocampal slices can be imaged through transparent graphene electrodes by both confocal and two-photon microscopy without causing any light-induced artefacts in the electrical recordings. Graphene electrodes record high-frequency bursting activity and slow synaptic potentials that are hard to resolve by multicellular calcium imaging. This transparent electrode technology may pave the way for high spatio-temporal resolution electro-optic mapping of the dynamic neuronal activity.

Graphene-enabled silver nanoantenna sensors.

Silver is the ideal material for plasmonics because of its low loss at optical frequencies but is often replaced by a more lossy metal, gold. This is because of silvers tendency to tarnish and roughen, forming Ag(2)S on its surface, dramatically diminishing optical properties and rendering it unreliable for applications. By passivating the surface of silver nanostructures with monolayer graphene, atmospheric sulfur containing compounds are unable to penetrate the graphene to degrade the surface of the silver. Preventing this sulfidation eliminates the increased material damping and scattering losses originating from the unintentional Ag(2)S layer. Because it is atomically thin, graphene does not interfere with the ability of localized surface plasmons to interact with the environment in sensing applications. Furthermore, after 30 days graphene-passivated silver (Ag-Gr) nanoantennas exhibit a 2600% higher sensitivity over that of bare Ag nanoantennas and 2 orders of magnitude improvement in peak width endurance. By employing graphene in this manner, the excellent optical properties and large spectral range of silver can be functionally utilized in a variety of nanoscale plasmonic devices and applications.

Electrogenerated Chemiluminescence from PbS Quantum Dots

We report the first observation of electrogenerated chemiluminescence (ECL) from PbS quantum dots (QDs). Different ECL intensities are observed for different ligands used to passivate the QDs, which indicates that ECL is sensitive to surface chemistry, with the potential to serve as a powerful probe of surface states and charge transfer dynamics in QDs. In particular, passivation of the QD surfaces with trioctylphosphine (TOP) increases ECL intensity by 3 orders of magnitude when compared to passivation with oleic acid alone. The observed overlap of the ECL and photoluminescence spectra suggests a significant reduction of deep surface trap states from the QDs passivated with TOP.

Voltage tuning of plasmonic absorbers by indium tin oxide

We experimentally demonstrate electrical tuning of plasmonic mid-infrared absorber resonances at 4 μm wavelength. The perfect infrared absorption is realized by an array of gold nanostrip antennas separated from a back reflector by a thin dielectric layer. An indium tin oxide active layer strongly coupled to the optical near field of the plasmonic absorber allows for spectral tunability.

Lead-salt quantum-dot ionic liquids.

The electronic energies of lead–salt quantum dots (QDs) are determined primarily by quantum confinement due to their large exciton Bohr radii. The fundamental electronic structure of the QDs (PbS and PbSe) has been worked out by Kang et al., and now research with these materials is turning towards applications. For instance, lead–salt QDs have been used as active materials in photovoltaic devices due to their size-tunable infrared (IR) absorption. They are also efficient IR emitters and could be used in biomedical imaging and in electroluminescent devices. In order for QDs to realize their full potential, their stability (e.g., photostability) and compatibility with other materials must be improved. Accordingly, much effort is devoted to surface passivation and functionalization of QDs, with increasing attention being paid to the use of ionic liquids to passivate the QD surface. Using certain ionic liquid ligands, solid materials can be transferred to a new state that exhibits liquidlike behavior at room temperature. To date, metal nanoparticles and oxide nanoparticles have been functionalized using a polymer ionic liquid. Some semiconductor nanoparticles (e.g., CdSe) functionalized using small-molecule ionic ligands have been reported. In this work, we report the first lead–salt (PbS, PbSe, and PbTe) QD ionic liquid where polymer ionic liquid ligands are used as capping ligands for QDs. The resulting amphiphilic QD ionic liquids exhibit fluidlike behavior at room temperature, even in the absence of solvents. The ionic liquid capping ligands also dramatically improve the photostability of

Optoelectromechanical Multimodal Biosensor with Graphene Active Region

A general, overarching theme in nanotechnology is the integration of multiple disparate fields to realize novel or expanded functionalities. Here, we present a graphene enabled, integrated optoelectromechanical device and demonstrate its utility for biomolecular sensing. We experimentally achieve an ultrawide linear dynamic sensing range of 5 orders of magnitude of protein concentration, an improvement over state-of-the-art single mode nanosensors by approximately 2-3 orders of magnitude, while retaining a subpicomolar lowest detection limit. Moreover, the ability to monitor and characterize adsorption events in the full optoelectromechanical space allows for the extraction of key intrinsic parameters of adsorbates and has the potential to extend the capabilities of nanosensors beyond the traditional binary-valued test for a single type of molecule. This could have significant implications for molecular detection applications at variable concentrations, such as early disease detection in biomedical diagnostics.

Wavelength Tunable Microdisk Cavity Light Source with a Chemically Enhanced MoS2 Emitter

In this work, we report an integrated narrowband light source based on thin MoS2 emissive material coupled to the high quality factor whispering gallery modes of a microdisk cavity with a spatial notch that enables easy out-coupling of emission while it yields high spatial coherence and a Gaussian intensity distribution. The active light emitting material consists of chemically enhanced bilayer MoS2 flakes with a thin atomic layer deposited SiO2 protective coating that yields 20-times brighter chemically enhanced photoluminescence compared to as-exfoliated monolayers on the microdisk. Quality factors ≈ 1000 are observed as well as a high degree of spatial coherence. We also experimentally achieve effective index tuning of cavity coupled emission over a full free spectral range. The thermal response of this system is also studied. This work provides new insights for nanophotonic light sources with atomically thin active media.

Plasmonically enhanced thermomechanical detection of infrared radiation.

Nanoplasmonics has been an attractive area of research due to its ability to localize and manipulate freely propagating radiation on the nanometer scale for strong light-matter interactions. Meanwhile, nanomechanics has set records in the sensing of mass, force, and displacement. In this work, we report efficient coupling between infrared radiation and nanomechanical resonators through nanoantenna enhanced thermoplasmonic effects. Using efficient conversion of electromagnetic energy to mechanical energy in this plasmo-thermomechanical platform with a nanoslot plasmonic absorber integrated directly on a nanobeam mechanical resonator, we demonstrate room-temperature detection of nanowatt level power fluctuations in infrared radiation. We expect our approach, which combines nanoplasmonics with nanomechanical resonators, to lead to optically controlled nanomechanical systems enabling unprecedented functionality in biomolecular and toxic gas sensing and on-chip mass spectroscopy.

Differential privacy for collaborative security

Fighting global security threats with only a local view is inherently difficult. Internet network operators need to fight global phenomena such as botnets, but they are hampered by the fact that operators can observe only the traffic in their local domains. We propose a collaborative approach to this problem, in which operators share aggregate information about the traffic in their respective domains through an automated query mechanism. We argue that existing work on differential privacy and type systems can be leveraged to build a programmable query mechanism that can express a wide range of queries while limiting what can be learned about individual customers. We report on our progress towards building such a mechanism, and we discuss opportunities and challenges of the collaborative security approach.

Guided wrinkling in swollen, pre-patterned photoresist thin films with a crosslinking gradient

A new thin film buckling system was developed from photosensitive SU-8 thin films doped with UV light absorbing dye, resulting in a depth-wise gradient modulus after photocrosslinking. When the film was swollen by an organic solvent, a wide range of wrinkling patterns was obtained, including lamella, peanut and semi-hexagonal patterns. Both the morphology and wavelength were found to be dependent on the original film thickness. By leveraging the thermoplasticity of SU-8, we imprinted one-dimensional (1-D) patterns on the dyed SU-8 with variable pitch and height from 1 μm to 20 μm and 15 nm to 2 μm, respectively. We then swelled the patterned films and investigated the interactions between the intrinsic buckling waves (both size and morphology) and the pre-patterns. As the pre-pattern pitch decreased, the swollen film in the patterned region evolved from isotropic wrinkles to out-of-phase, anisotropic waves, which further became in-phase when the pre-pattern pitch was smaller than the intrinsic wrinkle wavelength. For the latter, the aligned wrinkle morphology varied dramatically when the pre-pattern height decreased: from perpendicular to the pre-pattern wavevector to dual orientation with one set of wrinkles remained perpendicularly ordered and the other set of local buckling patterns aligned in parallel to the pre-pattern, and finally back to isotropic ones. Since the pre-patterns of different size and shape could be readily prepared, the combination of physical confinement together with controlled swelling in a graded thin film offers a new approach to access a wide range of controllable hierarchical patterns.