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Loka, Chadrasekhar,Park, Kyoung Ryeol,Lee, Kee-Sun IOP Publishing 2016 Japanese journal of applied physics Vol.55 No.1
<P>In this study, SiO2/TiO2/n-Si/Ag(Cr)/TiO2 multilayer structures have been designed and deposited by the RF and DC magnetron sputtering at room temperature. The as-deposited TiO2/glass films which are initially amorphous in nature were subjected to post annealing at 673K for anatase phase TiO2. The anatase TiO2 films showed an optical bandgap similar to 3.32 eV. The Ag(Cr)/TiO2 showed very low-emissivity (low-e) value similar to 0.081 which is evaluated by using the sheet resistance (6.51 Omega/square) of the films. All the deposited films showed high visible transmittance (similar to 81%) and high infrared reflectance (72%) which are recorded by using the UV-vis-NIR spectrophotometer. In addition, experimentally obtained optical properties were in good agreement with the simulation data. The TiO2/n-Si heterojunction concept has been employed to enhance the superhydrophilicity of the deposited multilayer stack, TiO2/n-Si/Ag(Cr)/TiO2 films exhibited best superhydrophilicity with water contact angle similar to 2 degrees. The deposited multilayer structures SiO2/TiO2/n-Si/Ag(Cr)/TiO2 and TiO2/n-Si/Ag(Cr)/TiO2 achieved significant low-e and superhydrophilicity. (C) 2016 The Japan Society of Applied Physics</P>
Enhanced transmittance of sapphire by silicon oxynitride thin films annealed at high temperatures
Loka, Chadrasekhar,Lee, Kwang,Moon, Sung Whan,Choi, YiSik,Lee, Kee-Sun Elsevier 2018 Materials letters Vol.213 No.-
<P><B>Abstract</B></P> <P>Silicon oxynitride (SiON) and SiO<SUB>2</SUB> thin films attracted considerable attention for various applications such as anti-reflection coatings and surface passivation layers. In this report, we present the enhancement of sapphire optical transmittance by over-layer deposition of the amorphous SiON by RF magnetron sputtering, and investigation on changes in microstructure and transmittance upon high-temperature annealing ∼1373 K. A thin layer of nanocrystalline SiO<SUB>2</SUB> was formed at the sapphire/film interface induced by the annealing. Remarkably, the XPS spectra revealed that the silicon-nitrogen bonds disappear in the annealed films to form a non-stoichiometric silicon oxide (SiO<SUB>x</SUB>) phase. The films showed high visible transmittance ∼92%, which is comparable to that of soda-lime glass due to a significant reduction in the refractive index 1.41 of the films after annealing at 1373 K.</P> <P><B>Highlights</B></P> <P> <UL> <LI> SiO<SUB>x</SUB> thin films were obtained by high temperature annealing of SiO<SUB>x</SUB>N<SUB>y</SUB>/sapphire film. </LI> <LI> High visible transmittance ∼92% was achieved, which is comparable to the soda-lime glass. </LI> <LI> Refractive index was greatly decreased from 1.58 to 1.41, after annealing at 1373 K. </LI> </UL> </P> <P><B>Graphical abstract</B></P> <P>[DISPLAY OMISSION]</P>
Chadrasekhar Loka,Sung Whan Moon,Yi Sik Choi,이기선 대한금속·재료학회 2018 ELECTRONIC MATERIALS LETTERS Vol.14 No.2
Transparent conducting oxides attract intense interests due to its diverse industrial applications. In this study, we report sapphiresubstrate-based TiO2/Ag/TiO2 (TAT) multilayer structure of indium-free transparent conductive multilayer coatings. The TAT thin films were deposited at room temperature on sapphire substrates and a rigorous analysis has been presentedon the electrical and optical properties of the films as a function of Ag thickness. The optical and electrical properties weremainly controlled by the Ag mid-layer thickness of the TAT tri-layer. The TAT films showed high luminous transmittance~ 84% at 550 nm along with noteworthy low electrical resistance ~ 3.65 × 10−5 Ω-cm and sheet resistance of 3.77 Ω/square,which is better are than those of amorphous ITO films and any sapphire-based dielectric/metal/dielectric multilayer stack. The carrier concentration of the films was increased with respect to Ag thickness. We obtained highest Hackke’s figure ofmerit 43.97 × 10−3 Ω−1 from the TAT multilayer thin film with a 16 nm thick Ag mid-layer.
Si-FeSi2/C Nanocomposite Anode Materials Produced by Two-Stage High-Energy Mechanical Milling
양윤모,Chadrasekhar Loka,김동필,주신용,문성환,최이식,박정한,이기선 대한금속·재료학회 2017 METALS AND MATERIALS International Vol.23 No.3
High capacity retention Silicon-based nanocomposite anode materials have been extensively explored for use inlithium-ion rechargeable batteries. Here we report the preparation of Si-FeSi2/C nanocomposite through scalable atwo-stage high-energy mechanical milling process, in which nano-scale Si-FeSi2 powders are besieged by thecarbon (graphite/amorphous phase) layer; and investigation of their structure, morphology and electrochemicalperformance. Raman analysis revealed that the carbon layer structure comprised of graphitic and amorphous phaserather than a single amorphous phase. Anodes fabricated with the Si-FeSi2/C showed excellent electrochemicalbehavior such as a first discharge capacity of 1082 mAh g-1and a high capacity retention until the 30thcycle. Aremarkable coulombic efficiency of 99.5% was achieved within a few cycles. Differential capacity plots of the Si-FeSi2/C anodes revealed a stable lithium reaction with Si for lithiation/delithiation. The enhanced electrochemicalproperties of the Si-FeSi2/C nanocomposite are mainly attributed to the nano-size Si and stable solid electrolyteinterface formation and highly conductive path driven by the carbon layer.
Han, Hyoung Kyu,Loka, Chadrasekhar,Yang, Yun Mo,Kim, Jae Hyuk,Moon, Sung Whan,Cho, Jong Soo,Lee, Kee-Sun Elsevier 2015 Journal of Power Sources Vol.281 No.-
<P><B>Abstract</B></P> <P>The preparation of different kinds of nanocomposite materials is a promising approach to alleviate the severe volume changes of Silicon anode materials for lithium-ion secondary batteries. In the present study, a novel nanocomposite Si<SUB>80</SUB>Fe<SUB>16</SUB>Cr<SUB>4</SUB> was synthesized by high-energy mechanical milling without noticeable contamination. The nano-indentation results revealed that the elastic recoverable energy range of the synthesized nanocomposite is 3.43 times higher than that of Si. The proposed nanocomposite milled for 8 and 10 h recorded a noteworthy reversible capacity of 841 and 812 mAh g<SUP>−1</SUP> even at 100th cycle, with excellent capacity retention. Remarkably, the nanocomposite exhibited a very low initial cycle (1st cycle) capacity loss ∼14%. The crystal separation of the less active silicide phases was determined after the extended cycling, which is advantageous for accommodating the stress produced by the volume changes of the active Si. The primary factors attributed to the excellent electrochemical performance were the size reduction of Si particles to nanometer scale, the formation of the highly elastic matrix, and separation of silicide phases after extended cycling.</P> <P><B>Highlights</B></P> <P> <UL> <LI> Nano-Si/(Fe, Cr) silicide nanocomposite formed by High-energy mechanical milling. </LI> <LI> Nanocomposite with higher elastic recoverable energy accommodates Si volume change. </LI> <LI> Remarkably, lower initial cycle capacity loss of about 14% was obtained. </LI> <LI> After extended cycling, separation of less active silicide phases was observed. </LI> </UL> </P> <P><B>Graphical abstract</B></P> <P>[DISPLAY OMISSION]</P>
Choi, Woo Jeong,Reddyprakash, M.,Loka, Chadrasekhar,Jo, Young Woong,Lee, Kee-Sun The Electrochemical Society 2019 Journal of the Electrochemical Society Vol.166 No.3
<P>In this article, we report a novel Si-Metal silicides/Carbon composite anode material for lithium-ion rechargeable batteries. The composite powder comprised of Si, FeSi<SUB>2</SUB> and CrSi<SUB>2</SUB> were synthesized by high-energy mechanical milling and then a primary carbon was formed over the Si-silicide at 900°C. The prepared composite powder was agglomerated and subsequently a thin carbon layer was coated. The X-ray diffraction results revealed that the Si and silicide crystal size decrease with respect to milling time. The critical milling time to achieve the completely nano-scale powders was 5 h. The composite powder exhibits randomly distributed carbon-coated Si-silicide. The TEM microstructure revealed homogeneous distribution of nanocomposite powder consists a very fine nanoparticles of the order of ∼30 nm. The prepared Si-silicide/C composite powders exhibited good capacity retention with an average coulombic efficiency of 99.6%. The composite powders showed good cyclability, 1076 mAh g<SUP>−1</SUP> at 50<SUP>th</SUP> cycle and 959 mAh g<SUP>−1</SUP> at 100<SUP>th</SUP> cycle. The electrode internal microstructure revealed a shell-like carbon-coated Si-silicide phases, and a complete amorphization of nanocrystalline Si during the initial cycling, while the inactive silicide phase remains unchanged. Consequently, the size reduction of Si-silicide and carbon coating over it greatly enhanced the cycling performance of the electrode.</P>