Mahbub Hassan

Edge-enriched graphene quantum dots for enhanced photo-luminescence and supercapacitance

Graphene quantum dots (GQDs) with their edge-bound nanometer-size present distinctive properties owing to quantum confinement and edge effects. We report a facile ultrasonic approach with chemical activation using KOH to prepare activated GQDs or aGQDs enriched with both free and bound edges. Compared to GQDs, the aGQDs we synthesized had enhanced BET surface area by a factor of about six, the photoluminescence intensity by about four and half times and electro-capacitance by a factor of about two. Unlike their non-activated counterparts, the aGQDs having enhanced edge states emit enhanced intense blue luminescence and exhibit electrochemical double layer capacitance greater than that of graphene, activated or not. Apart from their use as part of electrodes in a supercapacitor, the superior luminescence of aGQDs holds potential for use in biomedical imaging and related optoelectronic applications.

Carbon functionalized TiO2 nanofibers for high efficiency photocatalysis

TiO2 nanofibers (30–50 nm diameter), fabricated by the electro-spinning process, were modified with organo-silane agents via a coupling reaction and were grafted with carbohydrate molecules. The mixture was carbonized to produce a uniform coating of amorphous carbon on the surface of the TiO2 nanofibers. The TiO2@C nanofibers were characterized by high resolution electron microscopy (HRTEM), x-ray diffraction (XRD), x-ray photoelectron (XPS), Fourier transform infrared (FTIR) and UV-vis spectroscopy. The photocatalytic property of the functional TiO2 and carbon nanocomposite was tested via the decomposition of an organic pollutant. The catalytic activity of the covalently functionalized nanocomposite was found to be significantly enhanced in comparison to unfunctionalized composite and pristine TiO2 due to the synergistic effect of nanostructured TiO2 and amorphous carbon bound via covalent bonds. The improvement in performance is due to bandgap modification in the 1D co-axial nanostructure where the anatase phase is bound by nano-carbon, providing a large surface to volume ratio within a confined space. The superior photocatalytic performance and recyclability of 1D TiO2@C nanofiber composites for water purification were established through dye degradation experiments.

High-yield aqueous phase exfoliation of graphene for facile nanocomposite synthesis via emulsion polymerization

Aqueous phase exfoliation was developed for producing high-yield graphene nanosheets from expanded graphite (EG). The process included ultrasonication with sodium dodecyl sulfate (SDS) emulsion in aqueous phase. The high throughput exfoliation process was characterized by UV-vis spectroscopy, transmission electron microscopy (TEM) and electrical impedance spectroscopy (EIS). Controlled sonication experiments revealed that optimum exfoliation corresponds to maxima in UV-vis spectra. TEM results showed that the exfoliated graphene comprised nanoflakes having ≤5 layers (~60%) and ≤10 layers for 90% of the product. The potential use of this highly dispersed graphene was demonstrated by one-pot synthesis of graphene/polymer composite via in situ emulsion polymerization with styrene. The integrated role of SDS included adsorption and exfoliation of graphite, dispersion of graphene produced and assisting with micelle formation in emulsion. The high surface area graphene nanosheets as dispersed phase in polymeric nanocomposites showed significant improvement in thermal stability and electrical conductivity.

Internet telephony: services, technical challenges, and products

The rapid proliferation of the Internet has given rise to a strong interest in carrying telephony over the Internet. Because the Internet supports data communications, a range of other services can be bundled together with Internet telephony. The Internet, however, was designed for non-real-time data communications, and hence it poses several technical challenges that must be overcome before the Internet can be successfully used for carrying telephone services. This article discusses new services we can expect from Internet telephony, the technical challenges and solutions, and the emerging products that promise to support Internet telephony.

How much of dsrc is available for non-safety use?

The Dedicated Short Range Communication (DSRC) technology is currently being standardized by the IEEE to enable a range of communication-based automotive safety applications. However, for DSRC to be cost-effective, it is important to accommodate commercial non-safety use of the spectrum as well. The co-existence of safety and non-safety is achieved through a periodic channel switching scheme whereby access to DSRC alternates between these two classes of applications. In this paper, we propose a framework that links the non-safety share of DSRC as effected by the channel switching to the performance requirements of safety applications. Using simulation experiments, we analyze the non-safety opportunity in the DSRC under varied road traffic conditions. We find that non-safety use of DSRC may have to be severely restricted during peak hours of traffic to insure that automotive safety is not compromised. Our study also provides interesting insights into how simple strategies, e.g., optimizing the message generation rate of the safety applications, can significantly increase the commercial opportunities of DSRC. Finally, we find that adaptive schemes that can dynamically adjust the switching parameters in response to observed traffic conditions may help in maximizing the commercial use of DSRC.

An empirical study of bandwidth predictability in mobile computing

While bandwidth predictability has been well studied in static environments, it remains largely unexplored in the context of mobile computing. To gain a deeper understanding of this important issue in the mobile environment, we conducted an eight-month measurement study consisting of 71 repeated trips along a 23Km route in Sydney under typical driving conditions. To account for the network diversity, we measure bandwidth from two independent cellular providers implementing the popular High-Speed Downlink Packet Access (HSDPA) technology in two different peak access rates (1.8 and 3.6Mbps). Interestingly, we observe no significant correlation between the bandwidth signals at different points in time within a given trip. This observation eventually leads to the revelation that the popular time series models, e.g. the Autoregressive and Moving Average, typically used to predict network traffic in static environments are not as effective in capturing the regularity in mobile bandwidth. Although the bandwidth signal in a given trip appears as a random white noise, we are able to detect the existence of patterns by analyzing the distribution of the bandwidth observed during the repeated trips. We quantify the bandwidth predictability reflected by these patterns using tools from information theory, entropy in particular. The entropy analysis reveals that the bandwidth uncertainty may reduce by as much as 46% when observations from past trips are accounted for. We further demonstrate that the bandwidth in mobile computing appears more predictable when location is used as a context. All these observations are consistent across multiple independent providers offering different data transfer rates using possibly different networking hardware.

Improving QoS in High-Speed Mobility Using Bandwidth Maps

It is widely evidenced that location has a significant influence on the actual bandwidth that can be expected from Wireless Wide Area Networks (WWANs), e.g., 3G. Because a fast-moving vehicle continuously changes its location, vehicular mobile computing is confronted with the possibility of significant variations in available network bandwidth. While it is difficult for providers to eliminate bandwidth disparity over a large service area, it may be possible to map network bandwidth to the road network through repeated measurements. In this paper, we report results of an extensive measurement campaign to demonstrate the viability of such bandwidth maps. We show how bandwidth maps can be interfaced with adaptive multimedia servers and the emerging vehicular communication systems that use on-board mobile routers to deliver Internet services to the passengers. Using simulation experiments driven by our measurement data, we quantify the improvement in Quality of Service (QoS) that can be achieved by taking advantage of the geographical knowledge of bandwidth provided by the bandwidth maps. We find that our approach reduces the frequency of disruptions in perceived QoS for both audio and video applications in high-speed vehicular mobility by several orders of magnitude.

Improving Cooperative Positioning for Vehicular Networks

Cooperative positioning (CP) can potentially improve the accuracy of vehicle location information, which is vital for several road safety applications. Although concepts of CP have been introduced, the efficiency of CP under real-world vehicular communication constraints is largely unknown. Our simulations reveal that the frequent exchange of large amounts of range information required by existing CP schemes not only increases the packet collision rate of the vehicular network but reduces the effectiveness of the CP as well. To address this issue, we propose simple easily deployable protocol improvements in terms of utilizing as much range information as possible, reducing range broadcasts by piggybacking, compressing the range information, tuning the broadcast frequency, and combining multiple packets using network coding. Our results demonstrate that, even under dense traffic conditions, these protocol improvements achieve a twofold reduction in packet loss rates and increase the positioning accuracy of CP by 40%.

Adaptive Position Update for Geographic Routing in Mobile Ad Hoc Networks

In geographic routing, nodes need to maintain up-to-date positions of their immediate neighbors for making effective forwarding decisions. Periodic broadcasting of beacon packets that contain the geographic location coordinates of the nodes is a popular method used by most geographic routing protocols to maintain neighbor positions. We contend and demonstrate that periodic beaconing regardless of the node mobility and traffic patterns in the network is not attractive from both update cost and routing performance points of view. We propose the Adaptive Position Update (APU) strategy for geographic routing, which dynamically adjusts the frequency of position updates based on the mobility dynamics of the nodes and the forwarding patterns in the network. APU is based on two simple principles: 1) nodes whose movements are harder to predict update their positions more frequently (and vice versa), and (ii) nodes closer to forwarding paths update their positions more frequently (and vice versa). Our theoretical analysis, which is validated by NS2 simulations of a well-known geographic routing protocol, Greedy Perimeter Stateless Routing Protocol (GPSR), shows that APU can significantly reduce the update cost and improve the routing performance in terms of packet delivery ratio and average end-to-end delay in comparison with periodic beaconing and other recently proposed updating schemes. The benefits of APU are further confirmed by undertaking evaluations in realistic network scenarios, which account for localization error, realistic radio propagation, and sparse network.

Adaptive Position Update in Geographic Routing

In geographic routing, nodes need to maintain up-to-date positions of their immediate neighbours for making effective forwarding decisions. Periodic broadcasting of beacon packets that contain the geographic location coordinates of the nodes is a popular method used by most geographic routing protocols to maintain neighbour positions. We contend that periodic beaconing regardless of network mobility and traffic pattern does not make optimal ulilisation of the wireless medium and node energy. For example, if the beacon interval is too small compared to the rate at which a node changes its current position, periodic beaconing will create many redundant position updates. Similarly, when only a few nodes in a large network are involved in data forwarding, resources spent by all other nodes in maintaining their neighbour positions are greatly wasted. To address these problems, we propose the Adaptive Position Update (APU) strategy for geographic routing. Based on mobility prediction, APU enables nodes to update their position adaptively to the node mobility and traffic pattern. We embed APU into the well known Greedy Perimeter Stateless Routing Protocol (GPSR), and compare it with original GPSR in the ns-2 simulation platform. We conducted several experiments with randomly generated network topologies and mobility patterns. The results confirm that APU significantly reduces beacon overhead without having any noticeable impact on the data throughput of the network. This result is further validated through a trace driven simulation of a practical vehicular ad-hoc network topology that exhibits realistic movement patterns of public transport buses in a metropolitan city.