We report on a nanoscale semiconducting optoelectronic system optimized for neuronal stimulation: the organic electrolytic photocapacitor. The devices comprise a trilayer of metal and p and n semiconductors. When illuminated in physiological solution, these metal-semiconductor devices charge up, transducing light pulses into localized displacement currents that are strong enough to stimulate cells. The devices are freestanding, requiring no wiring or external bias, and are stable in physiological conditions. We have systematically evaluated the ability of photocapacitor devices to alter the cell membrane potential of single nonexcitable cells, generate action potentials in neuronal cell cultures, and stimulate explanted light-insensitive embryonic retinas.
Structuring surface and bulk of crystalline silicon on different length scales can significantly alter its properties and possibly improve the performance of opto-electronic devices and sensors based on silicon. Different dominant feature scales are responsible for modification of some of electronic and optical properties of silicon. Several easily reproducible chemical methods for facile structuring of silicon on nano and micro-scales, based on both electroless and anodic etching of silicon in hydrofluoric acid based etchants, and chemical anisotropic etching of silicon in basic environments, are presented. We show how successive micro and nano structuring creates hierarchical silicon surfaces, which can be used to simultaneously exploit the advantages of both structuring feature length scales. Finally, we demonstrate a large increase in photocurrent obtained from a hybrid structured silicon/organic near-infrared photodetector. Improved silicon/6,6’-dibromoindigo hybrid photodiodes were prepared by nano- and micro-structuring the silicon part of the heterojunction by wet chemical etching methods. Photocurrent and spectral responsivity were improved in comparison to planar diodes by up to two orders of magnitude by optimization of the silicon structuring process. We show that the improvement in photocurrent is not due to the increase in surface area or light trapping.
Herein we report our recent efforts in employing natural materials and synthetic derivatives of natural molecules for
organic field effect transistors (OFETs) and organic photovoltaics (OPVs). We evaluated dyes from the following
chemical families: acridones, anthraquinones, carotenoids, and indigoids. These materials have proven effective in
organic field effect transistors, with mobilities in the 4 × 10-4 - 0.2 cm2/V-s range, with indigoids showing promising
ambipolar behavior. We fabricated complementary-like voltage inverters with high gains using indigoids. The
photovoltaic properties of these materials were characterized in metal-insulator-metal (MIM) diodes, as well as in donoracceptor
bilayer devices. Additionally, we have found that indigoids and Quinacridone, show long-range crystalline
order due to hydrogen binding, and have considerably higher relative permittivities (ε) compared to typical organic
semiconductors. Higher permittivities result in lower exciton binding energies and thus may lead to high photocurrents
in photovoltaic devices.
Stimuli-responsive gas permeation membranes hold substantial potential for industrial processes as well as in analytical
and screening applications. Such "smart" membrane systems, although prevalent in liquid mass-transfer manipulations,
have yet to be realized for gas applications. We report our progress in developing gas permeation membranes in which
liquid crystalline (LC) phases afford the active region of permeation. To achieve rapid and reversible switching between
LC and isotropic permeation states, we harnessed the photomechanical action of mesogenic azobenzene dyes that can
produce isothermal nematic-isotropic transitions. Both polymeric and low-molecular-weight LC materials were tested.
Three different dye-doped LC mixtures with mesogenic azo dyes were infused into commercially available track-etched
porous membranes with regular cylindrical pores (0.4 to 10.0 μm). Photoinduced isothermal phase changes in the
imbibed material produced large and fully reversible changes in the permeability of the membrane to nitrogen with 5 s of
irradiation at 2 mW/cm2. Using two measurement tools constructed in-house, the permeability of the photoswitched
membranes was determined by both variable-pressure and variable-volume methods. Both the LC and photogenerated
isotropic states demonstrate a linear permeability/pressure (ideal sorption) relationship, with up to a 16-fold difference in
their permeability coefficients. Liquid crystal compositions can be chosen such that the LC phase is more permeable than
the isotropic-or vice versa. This approach is the first system offering reversible tunable gas permeation membranes.
We have fabricated switchable gas permeation membranes in which a photoswitchable low-molecular-weight liquid
crystalline (LC) material acts as the active element. Two different LC eutectic mixtures based on cyanobiphenyls and
phenyl benzoates, respectively, were doped with mesogenic azo dyes and infused into commercially available tracketched
porous polycarbonate membranes with regular cylindrical pores (0.40 to 10.0 μm). Photo-induced isothermal
phase changes in the imbibed mesogenic material afforded large, reversible changes in the permeability of the
photoswitchable membrane to nitrogen. The membrane imbibed with the photoswitchable cyanobiphenyl LC material
demonstrated low permeability in the nematic state, while the photogenerated isotropic state demonstrated a 16×-greater
sorption coefficient. Both states obey a high linear sorption behavior in accordance with Henry's Law. The membrane
imbibed with the photoswitchable phenyl benzoate LC showed the opposite permeability behavior to the biphenylimbibed
membrane, along with nonlinear sorption behavior. Permeability switching response times for the membranes
on the order of 5 s were demonstrated using alternating UV and >420-nm radiation at an intensity of 2 mW/cm2. The
effect of thermomolecular motion on gas sorption and diffusion over the LC-isotropic phase transitions are, for the first
time, evaluated under isothermal conditions. These photoswitchable membranes are the first examples of systems that
are capable of rapid and reversible gas permeation switching. Such switchable and/or tunable membranes are in high
demand for applications in analytics, screening, and membrane reactors.
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