An ongoing effort in many neuroscience laboratories is to perform chronic optical imaging and multiplexed electrophysiological recordings in awake behaving mice. We have adapted the use of conformal thin parylene C in conjunction with conductive polymer PEDOT:PSS coating on the electrode sites to realize wearable microelectrode array devices with sufficient optical transparency to allow simultaneous multiphoton imaging and optogenetic manipulation. These devices replace the current cranial glass windows providing broadband measurements of the extracellular potential as well as a see-through optical access. We have developed a mechanical solution for installation, connectorization and protection of the device and circuit board aiming for lifetime of several months while worn by a mouse. Our design allows access for high numerical aperture microscope objectives and does not obstruct the mouse face and whiskers enabling meaningful behaviors.
Electrocorticography (ECoG) is a powerful tool for direct mapping of local field potentials from the brain surface. Progress in development of high-fidelity materials such as poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) on thin conformal substrates such as parylene C enabled intimate contact with cortical surfaces and higher quality recordings from small volumes of neurons. Meanwhile, stimulation of neuronal activity is conventionally accomplished with electrical microstimulation and transcranial magnetic stimulation that can be combined with ECoG to form the basis of bidirectional neural interface. However, these stimulation mechanisms are less controlled and primitively understood on the local and cellular levels. With the advent of optogenetics, the localization and specificity of neuronal stimulation and inhibition is possible. Therefore, the development of integrated devices that can merge the sensitivity of ECoG or depth recording with optogenetic tools can lead to newer frontiers in understanding the neuronal activity.
Herein, we introduce a hybrid device comprising flexible inorganic LED arrays integrated PEDOT:PSS/parylene C microelectrode arrays for high resolution bidirectional neuronal interfaces. The flexible inorganic LEDs have been developed by the metal-organic vapor phase epitaxy of position-controlled GaN microLEDs on ZnO nanostructured templates pre-grown at precise locations on a graphene layer. By transferring it onto the microelectrode arrays, it can provides the individual electrical addressability by light stimulation patterns. We will present experimental and simulation results on the optoelectronic characteristics and light activation capability of flexible microLEDs and their evaluation in vivo.
KEYWORDS: Silicon, Epitaxy, Photovoltaics, Crystals, Scanning electron microscopy, Chemical vapor deposition, External quantum efficiency, Electrodes, Absorption, Deep reactive ion etching
Nanostructures provide novel opportunities of studying epitaxy in nano/mesoscale and on nonplanar substrates. Epitaxial
growth of silicon (Si) on the surfaces of Si nanowires along radial direction is a promising way to prepare radial p-(i)-n
junction in nanoscale for optoelectronic devices. Comprehensive studies of Si radial epitaxy in micro/nanoscale reveal
that morphological evolution and size-dependent radial shell growth rate for undoped and doped Si radial shells. Single
crystalline Si radial p-i-n junction wire arrays were utilized to fabricate photovoltaic (PV) devices. The PV devices
exhibited the photoconversion efficiency of 10%, the short-circuit current density of 39 mA/cm2, and the open-circuit
voltage of 0.52 V, respectively.
Ultrafast optical microscopy (UOM) combines a typical optical microscope and femtosecond (fs) lasers that produce
high intensity, ultrashort pulses at high repetition rates over a broad wavelength range. This enables us new types of
imaging modalities, including scanning optical pump-probe microscopy, which varies the pump and probe positions
relatively on the sample and ultrafast optical wide field microscopy, which is capable of rapidly acquiring wide field
images at different time delays, that is measurable nearly any sample in a non-contact manner with high spatial and
temporal resolution simultaneously. We directly tracked carriers in space and time throughout a NW by varying the
focused position of a strong optical “pump” pulse along the Si core-shell nanowires (NWs) axis while probing the
resulting changes in carrier density with a weaker “probe” pulse at one end of the NW. The resulting time-dependent
dynamics reveals the influence of oxide layer encapsulation on surface state passivation in core-shell NWs, as well as the
presence of strong acoustic phonon oscillations, observed here for the first time in single NWs. Time-resolved wide field
images of the photoinduced changes in transmission for a patterned semiconductor thin film and a single silicon
nanowire after optical excitation are also captured in real time using a two dimensional smart pixel array detector. Our
experiments enable us to extract several fundamental parameters in these samples, including the diffusion current,
surface recombination velocity, diffusion coefficients, and diffusion velocities, without the influence of contacts.
In recent years, transistor technology has scaled down to sub-20 nm channel length with many
performance-boosting techniques at the material and device levels in order to meet the increasing
demand for higher performance electronics. The nanowire (NW) device architecture has proven itself
as a viable candidate for the sub-20 nm generation transistors. Compared to Si NWs, the Ge/Si
core/shell NW alternative can supply larger on-current due to the increased confined hole mobility
and ohmic behavior at the Ni-alloyed drain/source contacts. It is thus important to understand
transport mechanisms in this core/shell structure, and develop pathway to realize ultra-short channel
core/shell NW field effect transistors (FETs).
In this paper, we report the growth of Ge/Si concentric NWs with precise control of Si shell
thickness. Performance of FETs fabricated from core/shell NWs exhibited a clear dependence on
NWs’ diameters, with steeper sub-threshold slopes for smaller NWs. An 18 nm diameter Ge/Si
heterostructure FET exhibited sub-threshold swing of 102 mV/decade, with a maximum
transconductance of 3.4 μS at VDS =-100 mV. Finally, transmission electron microscopy was utilized
to monitor and control the solid state reaction between Ni contacts and Ge/Si NWs, resulting in ultrascaled
channel lengths, as short as 5 nm.
This paper reports progress on the development of micromachined infrared microsensors on flexible polymer substrates. The flexible substrates were formed by spin-coating polyimide films (HD Microsystems PI-5878G) on silicon wafer carriers. Semiconducting Yttrium Barium Copper Oxide (YBCO) was used as the thermistor. The microbolometer was fabricated using a polyimide (HD Microsystems PI2737) sacrificial mesa and titanium electrode arms. The YBCO thermistor was suspended above the substrate by the electrode arms after the sacrificial layers have been removed by micromachining. The polyimide substrate was then removed from the silicon wafer carrier to complete the fabrication of the infrared microsensors on a flexible polyimide substrate. The measured thermal conductance of the microbolometers ranged from 9.07 x 10-6 W/K for a non-micromachined to 4.0 x 10-7 W/K for a micromachined sensor. As a result, the responsivity and detectivity were measured to be as high as 6.1 x 104V/W and a 1.2 x 108 cmHz1/2/W, respectively, with a 970 nA current bias. This level of performance is comparable to similar miromachined detectors fabricated on silicon substrates.
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