Vibrational, optical, and photocatalytic properties of ZnO/layered carbon nanocomposite synthesized by ball milling.

ZnO/layered carbon nanocomposites with varied sizes of ZnO nanoparticles (NPs) were synthesized by mechanical milling of mixture of ZnO NPs and carbon NPs. The NP size of ZnO was controlled with average particle sizes about 19.33, 21.87, 24.21, and 27.89 nm by varying the concentrations of carbon NPs viz 0, 2, 5, and 10 weight percent, respectively, in the mixture. Presence of carbon with ZnO in the form of composite also resulted in the enhanced shift of the band gap of ZnO due to the optical transitions in the impurity states or presence of carbon as compared to the ZnO size change alone. Additionally, the enhancement of absorbance in the visible region with an increase in carbon content was observed. Such an increase in absorbance can enhance the photocatalytic activity of ZnO NPs. Raman bands for ZnO NPs also were found to shift faster in the presence of layered carbon. The quenching of visible photoluminescence emission of ZnO NPs with an increase in concentration of carbon NPs in the composite indicated the phenomenon associated with transfer of electrons from ZnO to layered carbon helping the separation of photo-generated electrons and holes in ZnO and can lead to enhancement of the photocatalytic activity of ZnO NPs. In the photocatalytic studies, it was observed that the degradation of methylene blue (MB) dye was significantly enhanced by the increase of content of layered carbon in the nanocomposite. The sample containing 10% carbon showed the highest adsorption in dark conditions which was up to 60% of the starting strength and this was further enhanced to 88% in the presence of UV radiation. Enhanced adsorption of MB dye and the effective separation of electron-hole pairs due to charge transfer were believed to be the main causes behind such kind of improvement in the photocatalytic effects.

Collaborative spectrum sensing for cognitive visible light communications.

Cognitive visible light communication (VLC) has attracted increasing attention. By sharing underutilized VLC spectrum resources of primary users (PUs) with secondary users (SUs) opportunistically, improved spectrum utilization can be achieved without interfering with PUs. As an essential component in cognitive VLC, reliable spectrum sensing is crucial to ensure accurate cognition of PU's signal. However, due to limiting factors such as low signal-to-noise ratio (SNR) and link blocking in VLC systems, it would be difficult for a single SU to identify the status of PUs accurately and rapidly. To tackle this issue, we propose a new collaborative sensing (CS) scheme which can enhance sensing accuracy effectively by coordinating multiple SUs to participate in spectrum sensing. To evaluate the performance of the proposed CS scheme, we first develop an analytical model for the scenario of a single SU, subject to various factors such as indoor reflections and signal sampling size. Next, based on the single-SU evaluation, we further analyze the performance of the CS scheme by extending the single-SU analytical models to the multi-SU scenario. It is found that the analytical models can accurately predict the performance of the proposed CS scheme and match the results obtained by simulations. Moreover, the proposed CS scheme is effective in improving the sensing accuracy by about 40% and 10% compared with the local-sensing and the conventional CS schemes, respectively.

High performance organic thin film transistors with solution processed TTF-TCNQ charge transfer salt as electrodes.

Fabrication of high-performance organic thin film transistors (OTFTs) with solution processed organic charge transfer complex (TTF-TCNQ) film as bottom contact source-drain electrodes is reported. A novel capillary based method was used to deposit the source-drain electrodes from solution and to create the channel between the electrodes. Both p- and n-type OTFTs have been fabricated with solution deposited organic charge transfer film as contact electrodes. Comparison of the device performances between OTFTs with TTF-TCNQ as source-drain electrodes and those with Au electrodes (both top and bottom contact) indicate that better results have been obtained in organic complex film contacted OTFT. The high mobility, low threshold voltage, and efficient carrier injection in both types of OTFTs implies the potential use of the TTF-TCNQ based complex material as low-cost contact electrodes. The lower work function of the TTF-TCNQ electrode and better contact of the complex film with the organic thin film owing to the organic-organic interface results in efficient charge transfer into the semiconductor yielding high device performance. The present method having organic metal as contact materials promises great potential for the fabrication of all-organics and plastic electronics devices with high throughput and low-cost processing.

Control over multifunctionality in optoelectronic device based on organic phototransistor.

Highly stable, reproducible, photosensitive organic field-effect transistors based on an n-type organic material, copper hexadecafluorophthalocyanine, and two different polymeric gate dielectrics has been reported and their performances have been compared by evaluating the surface/interface properties. The devices produced a maximum photocurrent gain (I(light)/I(dark)) of 79 at V(G) = 7 V and showed the potentiality as multifunctional optoelectronic switching applications depending upon the external pulses. The switching time of the transistor upon irradiation of light pulse, i.e., the photoswitching time of the device, was measured to be approximately 10 ms. On the basis of optical or combination of optical and electrical pulses, the electronic/optoelectronic properties of the device can be tuned efficiently. The multifunctions achieved by the single device can ensure very promising material for high density RAM and other optoelectronic applications. Furthermore, as the device geometry in the present work is not limited to rigid substrate only, it will lead to the development of flexible organic optoelectronic switch compatible with plastic substrates.

Efficient aggregation of multiple classes of information in wireless sensor networks.

Congestion in a Wireless Sensor Network (WSN) can lead to buffer overflow, resource waste and delay or loss of critical information from the sensors. In this paper, we propose the Priority-based Coverage-aware Congestion Control (PCC) algorithm which is distributed, priority-distinct, and fair. PCC provides higher priority to packets with event information in which the sink is more interested. PCC employs a queue scheduler that can selectively drop any packet in the queue. PCC gives fair chance to all sensors to send packets to the sink, irrespective of their specific locations, and therefore enhances the coverage fidelity of the WSN. Based on a detailed simulation analysis, we show that PCC can efficiently relieve congestion and significantly improve the system performance based on multiple metrics such as event throughput and coverage fidelity. We generalize PCC to address data collection in a WSN in which the sensor nodes have multiple sensing devices and can generate multiple types of information. We propose a Pricing System that can under congestion effectively collect different types of data generated by the sensor nodes according to values that are placed on different information by the sink. Simulation analysis show that our Pricing System can achieve higher event throughput for packets with higher priority and achieve fairness among different categories. Moreover, given a fixed system capacity, our proposed Pricing System can collect more information of the type valued by the sink.