Conductive wires of sub-micrometer width made from platinum-carbonyl clusters have been fabricated by solution-infilling of microchannels as in microinject molding in capillaries (MIMIC). The process is driven by the liquid surface tension within the micrometric channels followed by the precipitation of the solute. Orientation of supramolecular crystalline domains is imparted by the solution confinement combined with unidirectional flow. The wires exhibit ohmic conductivity with a value of 0.2 S/cm that increases, after thermal decomposition of the platinum-carbonyl cluster precursor to Pt, to 35 S/cm.
We report detailed studies of the slow relaxation of the photoinduced excess charge carriers in organic metal-insulator-semiconductor field effect transistors consisting of poly͑3-hexylthiophene͒ as the active layer. The relaxation process cannot be physically explained by processes, which lead to a simple or a stretchedexponential decay behavior. Models based on serial relaxation dynamics due to a hierarchy of systems with increasing spatial separation of the photo-generated negative and positive charges are used to explain the results. In order to explain the observed trend, the model is further modified by introducing a gate voltage dependent coulombic distribution manifested by the trapped negative charge carriers.Field effect transistors based on organic semiconductors and conjugated polymers have spurred tremendous interest in the development of organic/plastic electronics 1,2 and optoelectronics 3,4 due to the lower cost, facile processability, and chemical tenability. Apart from the technological implications, the metal-insulator-semiconductor field effect transistor ͑MISFET͒ geometry can also serve as a model structure to study the charge transport mechanisms and transport parameters such as field effect mobility, and semiconductorinsulator interfacial density of trap states. The selective distribution of the carrier density in the FET geometry with the large concentration of the positive charges in the channel region compared to the bulk can result in interesting situation especially upon photoexcitation. Large changes in the dark drain-source current I d or I upon photoexcitation at low-light levels was recently demonstrated in FETs based on regioregular poly͑3-octylthiophene͒. 5 It was observed that in an organic FET, which is typically a p channel and activated under an enhancement mode, the incident light intensity can act as an added control parameter. 5 The electric field distribution in the device promotes the separation of the photoinduced charge carrier ͑PCC͒. The more mobile positive charges drift towards the channel whereas the negative charges find their minimum potential in the bulk and get localized in the trap sites. Upon, switching off the photoexcitation, the photoinduced current I light (t) relaxes quite slowly with an apparent persistent behavior and the sample recovers to the dark conditions over a period of several hours. This persisting effect can lead to back-gating problems in polymer FETs. The decay mechanism in the present system depends on several aspects like defects in the insulator-semiconductor interface, bulk trap densities, diffusion rates of the PCC, and V g . We examine this decay closely as a function of the gate voltage (V g ), which largely involves the neutralization of the excess negative charges in the bulk in the background of the channel current.Slow relaxation of a PCC is known to occur in poly͑phe-nylenevinylene͒ ͑PPV͒ and poly͑alkylthiophene͒ systems. [6][7][8][9] The nonequilibrium conductance following photoexcitation in these systems was interpreted in te...
3D pitch rotation of microparticles and cells assumes importance in a wide variety of applications in biology, physics, chemistry and medicine. Applications such as cell imaging and injection benefit from pitch-rotational manipulation. Generation of such motion in single beam optical tweezers has remained elusive due to complicacies of generating high enough ellipticity perpendicular to the direction of propagation. Further, trapping an extended object at two locations can only generate partial pitch motion by moving one of the foci in the axial direction. Here, we use hexagonal-shaped upconverting particles and single cells trapped close to a goldcoated glass cover slip in a sample chamber to generate complete 360 degree and continuous pitch motion even with a single optical tweezers beam. The tweezers beam passing through the gold surface is partially absorbed and generates a hot-spot to produce circulatory convective flows in the vicinity which rotates the objects. The rotation rate can be controlled by the intensity of the laser light and the thickness of the gold layer. Thus such a simple configuration can turn the particle in the pitch sense. The circulatory flows in this technique have a diameter of about 5 µm which is smaller than those reported using acousto-fluidic techniques.
Luminescence quenching in the presence of polarons is one of the major challenges in organic light emitting devices. In this work, exciton quenching in the presence of polarons is studied using phase sensitive photocurrent measurements on pentacene field effect transistors. The enhancement of conduction in the organic field effect transistors on light illumination is studied using photocurrent spectral response measurements and corresponding optical simulations. The photocurrent is shown to be governed by the polaron mobility and the exciton quenching efficiency, both of which depend on the polaron density in the channel. Two models are proposed on the exciton dynamics in the presence of gate induced polarons in the transistor channel. The first model simulates the steady-state exciton concentration profile in the presence of exciton-polaron interaction. The second one is a three-dimensional steady state exciton-polaron interaction model, which supports the findings from the first model. It is shown that the excitons quench by transferring its energy to polarons, thereby promoting the latter to high energy states in the density of states manifold. The polarons move in the higher energy states with greater microscopic mobility before thermalizing, thereby leading to an enhancement of conduction. It is observed that for the present system, where charge carrier transport is by hopping, all polarons interact with excitons. This implies that for low mobility systems, the interaction is not limited to deep trapped polarons.
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