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  • Aerial Data Aggregation in IoT Networks: Hovering & Traveling Time Dilemma

    Bushnaq, Osama M.; Celik, Abdulkadir; ElSawy, Hesham; Al-Naffouri, Tareq Y.; Alouini, Mohamed-Slim (IEEE, 2018-12-13)
    The next era of information revolution will rely on aggregating big data from massive numbers of devices that are widely scattered in our environment. The majority of these devices are expected to be of low-complexity, low-cost, and limited power supply, which imposes stringent constraints on the network operation. In this regards, this paper proposes an aerial data aggregation from a finite spatial field via an unmanned aerial vehicle (UAV). Instead of fusing, relaying, and routing the data across the wireless nodes to fixed locations access points, a UAV flies over the field and collects the required data. Particularly, the field is divided into several subregions where the UAV hover over each subregion to collect samples from the underlying nodes. To this end, an optimization problem is formulated and solved to find the optimal number of subregions, the area of each subregion, the hovering locations, the hovering time at each location, and the trajectory between hovering locations such that an average number of samples are collected from the field in minimal time. The proposed the formulation is shown to be np-hard mixed integer problem, and hence, a decoupled heuristic solution is proposed. The results show that there exists an optimal number of subregions that balance the tradeoff between the hovering and traveling times such that the total time for collecting the required samples is minimized.
  • Observation of piezotronic and piezo-phototronic effects in n-InGaN nanowires/Ti grown by molecular beam epitaxy

    Tangi, Malleswararao; Min, Jungwook; Priante, Davide; Subedi, Ram Chandra; Anjum, Dalaver H.; Prabaswara, Aditya; Alfaraj, Nasir; Liang, Jian Wei; Shakfa, Mohammad Khaled; Ng, Tien Khee; Ooi, Boon S. (Elsevier, 2018-10-17)
    Group-III nitride nano-dimensional materials with noncentrosymmetric crystal structure offer an exciting area of piezotronics for energy conversion applications. We experimentally report the piezotronic and piezo-phototronic effects of n-InGaN nanowires (NWs) having an emission wavelength in the visible region (≈ 510 nm). The n-type InGaN NWs, exhibiting high structural and optical quality, were grown by plasma-assisted molecular beam epitaxy (PAMBE) on Ti/TaN/Si substrates to facilitate the direct bottom electrical contact to the NWs. Further, we use Pt/Ir conductive atomic force microscopy (c-AFM) tip as a top electrical contact to the NW. Applying compressive strain on the NWs using a c-AFM tip, the Schottky barrier height (SBH) formed at the metal-semiconductor NW interface was tuned by means of strain induced piezo-potential. Thus, we study the two-way coupling of mechanical and electrical energy results in piezotronics of n-InGaN NWs. Such measurements were further carried out under optical excitation with 405 nm laser to understand its effect on change in SBH. Thereby, we demonstrate the three-way coupling of the piezo-phototronics of n-InGaN NWs by exploiting their excellent visible optoelectronic properties. The photogenerated carriers reduce the SBH while they play a lesser role at higher tip deflection force on NWs. This revealed that at the higher strain on NW, the piezo fields screen the photoexcited carriers hence resulting in a negligible change in I-V characteristics for ≥ 6 nN tip force with and without illumination. Thus, the investigation of nanoscale piezotronic and piezo-phototronic effects of n-InGaN NWs provides an opportunity to enable piezoelectric functional devices to be used as strain-tunable, self-powered electronics and optoelectronics applications.
  • Downlink Non-Orthogonal Multiple Access (NOMA) in Poisson Networks

    Ali, Konpal S.; Haenggi, Martin; Elsawy, Hesham; Chaaban, Anas; Alouini, Mohamed-Slim (arXiv, 2018-10-15)
    A network model is considered where Poisson distributed base stations transmit to N power-domain non-orthogonal multiple access (NOMA) users (UEs) each {that employ successive interference cancellation (SIC) for decoding}. We propose three models for the clustering of NOMA UEs and consider two different ordering techniques for the NOMA UEs: mean signal power-based and instantaneous signal-to-intercell-interference-and-noise-ratio-based. For each technique, we present a signal-to-interference-and-noise ratio analysis for the coverage of the typical UE. We plot the rate region for the two-user case and show that neither ordering technique is consistently superior to the other. We propose two efficient algorithms for finding a feasible resource allocation that maximize the cell sum rate Rtot, for general N, constrained to: 1) a minimum throughput T for each UE, 2) identical throughput for all UEs. We show the existence of: 1) an optimum N that maximizes the constrained Rtot given a set of network parameters, 2) a critical SIC level necessary for NOMA to outperform orthogonal multiple access. The results highlight the importance in choosing the network parameters N, the constraints, and the ordering technique to balance the Rtot and fairness requirements. We also show that interference-aware UE clustering can significantly improve performance.
  • Layer-Dependent Rashba Band Splitting in 2D Hybrid Perovskites

    Yin, Jun; Maity, Partha; Xu, Liangjin; El-Zohry, Ahmed M.; Li, Hong; Bakr, Osman; Brédas, Jean-Luc; Mohammed, Omar F. (American Chemical Society (ACS), 2018-10-12)
    The strong spin-orbit coupling (SOC) in perovskite materials due to the presence of heavy atoms induces interesting electron-ic characteristics, such as Rashba band splitting. In spite of several recent reports on Rashba effects in 2D perovskites, the impacts of the nature of surface termination and of the number of inorganic layers on the extent of Rashba band splitting still remain to be determined. Here, using a combination of density functional theory (DFT) calculations and time-resolved laser spectroscopy, we provide a comprehensive understanding of the Rashba band splitting of the prototype 3D MAPbI3 and of 2D Ruddlesden-Popper (RP) hybrid perovskites. We demonstrate that significant structural distortions associated with different surface terminations are responsible for the observed Rashba effect in 2D perovskites. Interestingly, our theo-retical and experimental data clearly indicate that the intrinsic Rashba splitting occurs in the perovskite crystals with an even number of inorganic layers (n = 2), but not for the ones with an odd number of layers (n = 1 and n = 3). These findings not only provide a possible explanation for the elongated electron-hole recombination in perovskites but also elucidate the sig-nificant impact of the number of inorganic layers on the electronic properties of 2D perovskites.
  • Optical force decoration of 3D microstructures with plasmonic particles

    Donato, M. G.; Rajamanickam, Vijayakumar Palanisamy; Foti, A.; Gucciardi, P. G.; Liberale, Carlo; Maragò, O. M. (The Optical Society, 2018-10-12)
    Optical forces are used to push and aggregate gold nanorods onto several substrates creating surface-enhanced Raman scattering (SERS) active hot spots for Raman-based identification of proteins. By monitoring the increase of the protein SERS signal, we observe different aggregation times for different curvatures of the substrates. The slower aggregation dynamics on curved surfaces is justified by a simple geometrical model. In particular, this technique is used to decorate three-dimensional microstructures and to quickly realize hybrid micro/nanosensors for highly sensitive detection of biological material directly in a liquid environment.

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