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Khoa, Nguyen Tri,Kim, Soon Wook,Yoo, Dae-Hwang,Cho, Shinuk,Kim, Eui Jung,Hahn, Sung Hong American Chemical Society 2015 ACS APPLIED MATERIALS & INTERFACES Vol.7 No.6
<P>Heterostructures of gold-nanoparticle-decorated reduced-graphene-oxide (rGO)-wrapped ZnO hollow spheres (Au/rGO/ZnO) are synthesized using tetra-<I>n</I>-butylammonium bromide as a mediating agent. The structure of amorphous ZnO hollow spheres is found to be transformed from nanosheet- to nanoparticle-assembled hollow spheres (nPAHS) upon annealing at 500 °C. The ZnO nPAHS hybrids with Au/rGO are characterized using various techniques, including photoluminescence, steady-state absorbance, time-resolved photoluminescence, and photocatalysis. The charge-transfer time of ZnO nPAHS is found to be 87 ps, which is much shorter than that of a nanorod (128 ps), nanoparticle (150 ps), and nanowall (990 ps) due to its unique structure. The Au/rGO/ZnO hybrid shows a higher charge-transfer efficiency of 68.0% in comparison with rGO/ZnO (40.3%) and previously reported ZnO hybrids. The photocatalytic activities of the samples are evaluated by photodegrading methylene blue under black-light irradiation. The Au/rGO/ZnO exhibits excellent photocatalytic efficiency due to reduced electron–hole recombination, fast electron-transfer rate, and high charge-transfer efficiency.</P><P><B>Graphic Abstract</B> <IMG SRC='http://pubs.acs.org/appl/literatum/publisher/achs/journals/content/aamick/2015/aamick.2015.7.issue-6/acsami.5b00152/production/images/medium/am-2015-00152a_0004.gif'></P><P><A href='http://pubs.acs.org/doi/suppl/10.1021/am5b00152'>ACS Electronic Supporting Info</A></P>
Required Base Station Density for Communication-Efficient Federated Learning in Small-Cell Networks
Khoa Anh Nguyen,Quan Anh Nguyen,Jun-Pyo Hong 한국통신학회 2021 한국통신학회 학술대회논문집 Vol.2021 No.6
Federated Learning (FL) generates massive benefits of shared machine learning models without violating security and privacy requirements, making this setting relevant for many wireless applications. In this paper, we comprehensively investigate the effects of geographic node deployment on the model aggregation in federated learning on the basis of stochastic geometry-based analysis. Based on the coverage probability expression derived with stochastic geometry framework, we derive and discuss the minimum required base station density for achieving a target model aggregation rate in small-cell networks with algorithms for optimizing the target transmission rate and the base station density.
Near-Optimal Deviation-Proof Medium Access Control Designs in Wireless Networks
Khoa Tran Phan,Jaeok Park,van der Schaar, M. IEEE 2012 IEEE/ACM transactions on networking Vol.20 No.5
<P>Distributed medium access control (MAC) protocols are essential for the proliferation of low-cost, decentralized wireless local area networks (WLANs). Most MAC protocols are designed with the presumption that nodes comply with prescribed rules. However, selfish nodes have natural motives to manipulate protocols in order to improve their own performance. This often degrades the performance of other nodes as well as that of the overall system. In this paper, we propose a class of protocols that limit the performance gain from selfish manipulation while incurring only a small efficiency loss. The proposed protocols are based on the idea of a review strategy, with which nodes collect signals about the actions of other nodes over a period of time, use a statistical test to infer whether or not other nodes are following the prescribed behavior, and trigger a punishment if a deviation is inferred. We consider the cases of private and public signals and provide analytical and numerical results to demonstrate the properties of the proposed protocols.</P>
Khoa, H.X.,Tuan, N.Q.,Lee, Y.H.,Lee, B.H.,Viet, N.H.,Kim, J.S. The Korean Powder Metallurgy Institute 2013 한국분말재료학회지 (KPMI) Vol.20 No.3
$TiB_2$-reinforced iron matrix composite (Fe-$TiB_2$) powder was in-situ fabricated from titanium hydride ($TiH_2$) and iron boride (FeB) powders by the mechanical activation and a subsequent reaction. Phase formation of the composite powder was identified by X-ray diffraction (XRD). The morphology and phase composition were observed and measured by field emission-scanning electron microscopy (FE-SEM) and energy-dispersive X-ray spectroscopy (EDS), respectively. The results showed that $TiB_2$ particles formed in nanoscale were uniformly distributed in Fe matrix. $Fe_2B$ phase existed due to an incomplete reaction of Ti and FeB. Effect of milling process and synthesis temperature on the formation of composite were discussed.