High Photoresponse in Conformally Grown Monolayer MoS₂ on a Rugged Substrate

Nguyen, Tri Khoa,Nguyen, Anh Duc,Le, Chinh Tam,Ullah, Farman,Tahir, Zeeshan,Koo, Kyo-in,Kim, Eunah,Kim, Dong-Wook,Jang, Joon I.,Kim, Yong Soo American Chemical Society 2018 ACS APPLIED MATERIALS & INTERFACES Vol.10 No.47

<P>Conformal growth of atomic-thick semiconductor layers on patterned substrates can boost up the performance of electronic and optoelectronic devices remarkably. However, conformal growth is a very challenging technological task, since the control of the growth processes requires utmost precision. Herein, we report on conformal growth and characterization of monolayer MoS<SUB>2</SUB> on planar, microrugged, and nanorugged SiO<SUB>2</SUB>/Si substrates via metal-organic chemical vapor deposition. The continuous and conformal nature of monolayer MoS<SUB>2</SUB> on the rugged surface was verified by high-resolution transmission electron microscopy. Strain effects were examined by photoluminescence (PL) and Raman spectroscopy. Interestingly, the photoresponsivity (∼254.5 mA/W) of as-grown MoS<SUB>2</SUB> on the nanorugged substrate was 59 times larger than that of the planar sample (4.3 mA/W) under a small applied bias of 0.1 V. This value is record high when compared with all previous MoS<SUB>2</SUB>-based photocurrent generation under low or zero bias. Such enhancement in the photoresponsivity arises from a large active area for light-matter interaction and local strain for PL quenching, wherein the latter effect is the key factor and unique in the conformally grown monolayer on the nanorugged surface.</P> [FIG OMISSION]</BR>

High-temperature-treated multiwall carbon nanotubes for hydrogen evolution reaction

Nguyen, Tri Khoa,Bannov, Alexander G.,Popov, Maxim V.,Yun, Jong-Won,Nguyen, Anh Duc,Kim, Yong Soo Elsevier 2018 International journal of hydrogen energy Vol.43 No.13

<P><B>Abstract</B></P> <P>We investigated the hydrogen evolution reaction (HER) properties of multi-wall carbon nanotubes (MWCNTs) treated at extremely high temperature (2600 °C). The heat treatment not only improves the crystallinity of the MWCNTs, but also reduces the carbon-oxygen (CO) bonding as it is replaced by the defect-carbon (<I>sp</I> <SUP>3</SUP> and CH) bonding. These modifications in the heat treated MWCNT structure lead to the increase of electrochemical charge transfer. The heat treatment of MWCNTs in the composite with Pt (MWCNT-Pt composite) further facilitates electrocatalysis. The MWCNTs-Pt composite shows strong enhancement in the HER performance with an onset of overpotential of −0.04 V vs reversible hydrogen electrode and a Tafel slope of 10.9 mV/decade. This performance is indeed better than that of Pt, which is the best working material for HER.</P> <P><B>Highlights</B></P> <P> <UL> <LI> Heat treatment effects to reorder nanotube structures by connecting the broken crystals. </LI> <LI> CO bonding density is reduced and porosity properties are increased after heat treatment. </LI> <LI> Hydrogen evolution reaction (HER) performance is enhanced after heat treatment. </LI> <LI> Charge transfer resistance in HER is strongly decreased after heat treatment. </LI> <LI> Heat-treated MWCNT-Pt composite yields the excellent HER properties. </LI> </UL> </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.

Direct C–N coupling of azoles with ethers via oxidative C–H activation under metal–organic framework catalysis

Khoa D. Nguyen,Son H. Doan,Anh N.V. Ngo,Tung T. Nguyen,Nam T.S. Phan 한국공업화학회 2016 Journal of Industrial and Engineering Chemistry Vol.44 No.-

A metal–organic framework Fe3O(BPDC)3 was synthesized, and used as a productive heterogeneouscatalyst for the direct C–N coupling of azoles with ethers via oxidative C–H activation to produce azolederivatives. The MOF-based catalyst displayed higher catalytic efficiency than many homogeneouscatalysts as well as several MOFs in the transformation. The MOF-based catalyst could be reused manytimes for the synthesis of azole derivatives by the direct C–N coupling of azoles with ethers without anoteworthy deterioration in catalytic efficiency. To the best of our knowledge, this direct C–N couplingreaction was not previously performed in the presence of heterogeneous catalysts.

Dynamic response and vibration of composite double curved shallow shells with negative Poisson's ratio in auxetic honeycombs core layer on elastic foundations subjected to blast and damping loads

Duc, Nguyen Dinh,Seung-Eock, Kim,Cong, Pham Hong,Anh, Nguyen Tuan,Khoa, Nguyen Dinh Elsevier 2017 International journal of mechanical sciences Vol.133 No.-

<P><B>Abstract</B></P> <P>The purpose of the present study is to investigate dynamic response and vibration of composite double curved shallow shells with negative Poisson's ratios in auxetic honeycombs core layer on elastic foundations subjected to blast and damping loads using analytical solution. This study considers composite double curved shallow shells with auxetic core which have three layers in which the top and bottom outer skins are isotropic aluminum materials; the central layer has honeycomb structure using the same aluminum material. Based on the first order shear deformation theory (FSDT) with the geometrical nonlinear in von Karman and using Airy stress functions method, Galerkin method and the fourth-order Runge–Kutta method, the resulting equations are solved to obtain expressions for nonlinear motion equations. The effects of geometrical parameters, material properties, elastic foundations Winkler and Pasternak, the nonlinear dynamic analysis and vibration of double curved shallow shells with negative Poisson's ratios in auxetic honeycombs core layer are studied.</P> <P><B>Highlights</B></P> <P> <UL> <LI> To investigate dynamic response and vibration of composite double curved shallow shells by using analytical solution. </LI> <LI> The composite shells have the central auxetic core layer—honeycomb structures with negative Poisson's ratio. </LI> <LI> Based on the first order shear deformation theory (FSDT). </LI> <LI> Used airy stress functions, Galerkin method and fourth-order Runge–Kutta method. </LI> <LI> The effects of geometrical parameters, material properties, elastic Winkler and Pasternak foundations, mechanical and blast loads are studied. </LI> </UL> </P>

Enzyme-mediated fabrication of an oxidized chitosan hydrogel as a tissue sealant

Phuong, Nguyen Thi,Anh Ho, Viet,Hai Nguyen, Dai,Khoa, Nguyen Cuu,Quyen, Tran Ngoc,Lee, Yunki,Park, Ki Dong Technomic Pub. Co 2015 Journal of bioactive and compatible polymers Vol.30 No.4

<P>Oxidized polysaccharide-based hydrogels have recently attracted much attention for tissue regeneration because of their biocompatibility and tissue-adhesive property. In this study, we introduce a new type of chitosan-based hydrogel as a tissue sealant, which was prepared by enzymatic mediation from periodate-oxidized chitosan–polyethylene glycol–tyramine. The oxidized chitosan backbone was expected to enhance the interconnection between the hydrogel layer and collagen in the tissues via the Schiff-base reaction. Proton nuclear magnetic resonance spectra indicated that tyramine-functionalized polyethylene glycol-nitrophenyl carbonate ester was conjugated to the oxidized chitosan. The degree of oxidation of the chitosan backbone was around 14% of the glucosamine units by proton nuclear magnetic resonance. The hydrogel was rapidly formed in situ (within a few seconds) in the presence of horseradish peroxidase and hydrogen peroxide. In vitro experiments with live/dead cell assays showed that the oxidized chitosan-based hydrogel was cytobiocompatible. The hydrogel exhibited high tissue adhesion strength on porcine skin models as well as good tissue-adhesive ability and wound healing properties on rabbit skin. These positive results could be promising for the application of oxidized chitosan-based hydrogels as a wound sealant.</P>

Bond Behavior of Steel Reinforced Bar in Geopolymer Composites

Pham Vo Anh Khoa(팜버안코아),Tan Khoa Nguyen(탄코아),Lee, Kihak(이기학) 한국콘크리트학회 2018 한국콘크리트학회 학술대회 논문집 Vol.30 No.2

Investigation on properties of geopolymer mortar using preheated materials and thermogenetic admixtures

Nguyen, Khoa Tan,Le, Tuan Anh,Lee, Jaehong,Lee, Dongkyu,Lee, Kihak Elsevier 2017 Construction and Building Materials Vol.130 No.-

<P><B>Abstract</B></P> <P>Heat curing in an oven is the traditional method to obtain mechanical properties of geopolymers. This characteristic has significantly affected operations in terms of construction and energy consumption. The target of this paper is proposing alternative curing methods namely self-cured technologies for fly ash based geopolymer materials that are cured in ambient conditions without use of a heat resource. Two alternative methods are to use three different mixing processes with preheated materials and to use two thermogenetic admixtures (hot pack material and quicklime). The results show that for the mixing processes, a larger amount of provided heat energy results in a higher compressive strength of the geopolymer mortar (GM). For the use of a thermogenetic admixture, the results from compressive strength testing, Scanning Electron Microscope (SEM), energy dispersive X-ray analysis (EDX) micrographs and X-ray diffraction (XRD) analysis confirm that quicklime is more efficient than hot pack material. In the case of using quicklime, the suggested amount is about 3–5% of fly ash by mass.</P> <P><B>Highlights</B></P> <P> <UL> <LI> Proposing two new alternative curing method are using preheated materials and thermogentic admixtures. </LI> <LI> For using preheated materials, a larger amount of preheated materials results in a higher compressive strength. </LI> <LI> For using thermogentic admixtures, quicklime is more efficient than hot pack material. </LI> <LI> The suggested amount of quicklime is about 3–5% of fly ash by mass. </LI> </UL> </P>

Evaluation of the mechanical properties of sea sand-based geopolymer concrete and the corrosion of embedded steel bar

Nguyen, Khoa Tan,Le, Tuan Anh,Lee, Kihak Elsevier 2018 Construction and Building Materials Vol.169 No.-

<P><B>Abstract</B></P> <P>Portland cement concrete is a major construction and building material used all over the world. It is a composite material comprising Portland cement, coarse aggregate, fine aggregate, and water. But its increased use in construction is exhausting natural resources used in its production, making it necessary to find alternative materials. One potential method is to use sea sand as fine aggregate to produce fly ash based geopolymer concrete. In this paper, the mechanical properties of geopolymer concrete prepared with sea sand as the fine aggregate, and the corrosion of steel bar embedded in the concrete subjected to accelerated corrosion tests, were investigated. The test data revealed that for sea sand based geopolymer concrete, the compressive strength reached high values at an alkaline to fly ash ratio of 0.35–0.45. The geopolymer concrete exhibited highcompressive strength with a low aggregate to fly ash ratio. Also, there was an increase in compressive strength when the Si/Al ratio changed from 1.16 to 1.67. Furthermore, very little difference was observed between the mechanical properties of geopolymer concrete using sea sand, and river sand. Measurements of the corrosion of steel bar using a half-cell potential survey indicated that the steel in geopolymer concrete with sea sand was attacked and corroded like normal concrete. However, the potential of steel bar in geopolymer concrete was higher than in Portland cement concrete.</P> <P><B>Highlights</B></P> <P> <UL> <LI> The compressive strength of geopolymer concrete using sea sand as fine aggregate reaches the highest value at an alkaline to fly ash ratio of 0.35–0.45. </LI> <LI> For sea sand based geopolymer concrete, when the ratio of aggregate to fly ash is low, the compressive strength enhances high value. </LI> <LI> The difference in strength between specimens using river sand and sea sand is not significant. </LI> <LI> It takes more time for steel bar in geopolymer concrete using sea sand to be attacked and corroded, compared with the steel bar in normal concrete. </LI> </UL> </P>

Green Technology in Concrete Industry : Geopolymer Concrete

( Khoa Tan Nguyen ),( Tuan Anh Le ),( Namshik Ahn ) 한국건축시공학회 2011 한국건축시공학회 학술발표대회 논문집 Vol.11 No.2