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Now showing items 1 - 16 of 47

  • MoS2@VS2 Nanocomposite as a Superior Hybrid Anode Material

    Samad, Abdus   Shin, Young-Han  

    Using density functional theory, MoS2@VS2 nanocomposite is reported as a hybrid anode with upgraded electronic conductivity and Li/Na storage capacity. The chemically active monolayer VS2 can be stabilized in energy and phonon vibrations by using the monolayer MoS2 as a substrate. The stability of the chemically active monolayer VS2 is attributed to the interfacial charge accumulation between the monolayer MoS2 and VS2. The maximum specific capacity of the nanocomposite has been enhanced to 584 mAh/g both for Li and for Na storage. We attribute the high enhancement in the Li/Na storage capacity of MoS2@VS2 nanocomposite to the charge redistribution in the formation of the nanocomposite. The lithiation/sodiation open-circuit voltage range of the nanocomposite is quite feasible to be used as anode. Diffusion barriers of Li/Na ions on the surfaces of the nanocomposite are comparable to the barriers on corresponding monolayers, while at the interface the barriers are lower than that for bulk MoS2. This study utilizes different aspects of the two different materials in a hybrid anode with highly enhanced electrochemical performance.
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  • H2S adsorption process on (0001) alpha-quartz SiO2 surfaces

    Kim, Hye Jung   Jeon, Hyeongtag   Shin, Young-Han  

    We theoretically study the H2S adsorption process on (0001) alpha-quartz SiO2 surfaces, which is the preconditioning process for the atomic layer deposition growth of metal sulfide materials. The surface structures of dense and fully hydroxylated (0001) alpha-quartz SiO2 are energetically stable, but their reaction with a H2S molecule is not so active, whereas the cleaved SiO2 surface is chemically reactive to the dissociative adsorption of a H2S molecule with an adsorption energy of -3.08 eV/molecule. On the cleaved surface, we confirm that adsorbed H2S is dissociated into H and H-S fragments, and the energy barrier in this reaction process is computed as 0.042 eV. Published by AIP Publishing.
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  • Effects of Cl-Based Ligand Structures on Atomic Layer Deposited HfO2

    Park, Bo-Eun   Oh, Il-Kwon   Lee, Chang Wan   Lee, Gyeongho   Shin, Young-Han   Lansalot-Matras, Clement   Noh, Wontae   Kim, Hyungjun   Lee, Han-Bo-Ram  

    Atomic layer deposition (ALD) of HfO2 is a key technology for the application of high dielectric constant gate dielectrics ranging from conventional Si devices to novel nanodevices. The effects of the precursor on the growth characteristics and film properties of ALD HfO2 were investigated by using hafnium tetrachloride (HfCl4) and bis(ethylcyclopentadienyl)hafnium dichloride (Hf(EtCp)(2)Cl-2, Hf(C2H5C5H4)(2)Cl-2) with O-2 plasma reactant. The growth characteristics were significantly affected even by simply changing the precursor. Theoretical calculations utilizing geometrical information on the precursor and density functional theory revealed that the steric demands of the precursor ligands have a dominant effect on the different growth characteristics rather than the reaction probability of the precursor on the surface. The chemical compositional analysis results showed that the Cl residue in the HfO2 films was reduced by using Hf(EtCp)(2)Cl-2 due to the lower number of Cl atoms in each Hf precursor molecule and the relieved bridge formation Of Hf-Cl-Hf bridge on the surface compared to HfCl4. The electrical property measurement results showed significantly improved insulating properties in HfO2 using Hf(EtCp)(2)Cl-2 compared to HfCl4 due to the low concentration of Cl residue in the film. These results provide broad insights to researchers who are interested in the fabrication of high quality dielectric layers to achieve better device performance and overcome physical limitations in the nanoscale regime.
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  • Switchable polarization in an unzipped graphene oxide monolayer

    Noor-A-Alam, Mohammad   Shin, Young-Han  

    Ferroelectricity in low-dimensional oxide materials is generally suppressed at the scale of a few nanometers, and has attracted considerable attention from both fundamental and technological aspects. Graphene is one of the thinnest materials (one atom thick). Therefore, engineering switchable polarization in non-polar pristine graphene could potentially lead to two-dimensional (2D) ferroelectric materials. In the present study, based on density functional theory, we show that an unzipped graphene oxide (UGO) monolayer can exhibit switchable polarization due to its foldable bonds between the oxygen atom and two carbon atoms underneath the oxygen. We find that a free standing UGO monolayer exhibits antiferroelectric switchable polarization. A UGO monolayer can be obtained as an intermediate product during the chemical exfoliation process of graphene. Interestingly, despite its dimensionality, our estimated polarization in a UGO monolayer is comparable to that in bulk ferroelectric materials (e.g., ferroelectric polymers). Our calculations could help realize antiferroelectric switchable polarization in 2D materials, which could find various potential applications in nanoscale devices such as sensors, actuators, and capacitors with high energy-storage density.
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  • Superionic and electronic conductivity in monolayer W2C:ab initio predictions

    Samad, Abdus   Shafique, Aamir   Kim, Hye Jung   Shin, Young-Han  

    Using density functional theory calculations, a freestanding monolayer of W2C in the 2H phase is explored to find its stability in terms of formation energy and phonon vibrations. In addition, the monolayer has a high in-plane stiffness of 278 N m(-1). Its intrinsic metallic nature, high mechanical stability, and high adsorption capability for Li/Na ions make it an appealing anode material for rechargeable Li/Na ion batteries. The anode open circuit voltages of 0.84-0.55 V for Li and 0.88-0.38 V for Na are within the voltage range of commercial anode materials. The low diffusion energy barrier for a Li (0.035 eV) or Na (0.019 eV) ion leads to superionic mobility, which causes ultrafast charge/discharge cycles. The area expansion of the fully loaded anode is negligible. Its high mechanical stiffness, superb ionic and electronic conductivity, and suitable charging voltage range are the indications of a long-life anode having a high recyclability with full recovery and fast charge/discharge processes.
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  • Computational Studies of Lead-based Relaxor Ferroelectrics

    Takenaka, Hiroyuki   Grinberg, Ilya   Shin, Young-Han   Rappe, Andrew M.  

    Relaxor ferroelectrics have been a focus of intense attention due to their fascinating physical properties. Their diffuse phase transitions have been explained by the polar nanoregion model. Nevertheless, fundamental characterization of structure and dynamics in relaxors is still a long-standing challenge. Better scientific understanding of the microscopic origins of relaxor behavior is also required to improve efficiencies of relaxor based devices. Our molecular dynamics studies in 0.75PbMg(1/3)Nb(2/3)O(3)-0.25PbTiO(3) showed good agreement with experimental data and revealed conflicts with the current polar nanoregion model. Here, we review our work and propose an alternate model for structure and dynamics in the relaxor phase.
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  • Strain engineering of phonon thermal transport properties in monolayer 2H-MoTe2

    Shafique, Aamir   Shin, Young-Han  

    The effect of strain on the phonon properties such as phonon group velocity, phonon anharmonicity, phonon lifetime, and lattice thermal conductivity of monolayer 2H-MoTe2 is studied by solving the Boltzmann transport equation based on first principles calculations. The phonon thermal transport properties of the unstrained monolayer 2H-MoTe2 are compared to those of the strained case under different biaxial tensile strains. One of the common features of two-dimensional materials is the quadratic nature near the G point of the out-of-plane phonon flexural mode that disappears by applying tensile strain. We find that the lattice thermal conductivity of the monolayer 2H-MoTe2 is very sensitive to strain, and the lattice thermal conductivity is reduced by approximately 2.5 times by applying 8% biaxial tensile strain due to the reduction in phonon group velocities and phonon lifetime. We also analyze how the contribution of each mode to lattice thermal conductivity changes with tensile strain. These results highlight that tensile strain is a key parameter in engineering phonon thermal transport properties in monolayer 2H-MoTe2.
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  • Four-States Multiferroic Memory Embodied Using Mn-Doped BaTiO3 Nanorods

    Son, Jong Yeog   Lee, Jung-Hoon   Song, Seungwoo   Shin, Young-Han   Jang, Hyun Myung  

    Multiferroics that show simultaneous ferroic responses have received a great deal of attention by virtue of their potential for enabling new device paradigms. Here, we demonstrate a high-density four-states multiferroic memory using vertically aligned Mn-doped BaTiO3 nanorods prepared by applying the dip-pen nanolithography technique. In the present nanorods array, the polarization (P) switching by an external electric field does not influence the magnetization (M) of the nanorod owing to a negligible degree of the P M cross-coupling. Similarly, the magnetic-field-induced M switching Is unaffected by the ferroelectric polarization. On the basis of these, we are able to implement a four-states nonvolatile multiferroic memory, namely, (+P,+M), (+P,-M),(-P,+M), and (-P,-M) with the reliability in the P and M switching. Thus, the present work makes an important step toward the practical realization of multistate ferrolc memories.
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  • Thermoelectric and phonon transport properties of two-dimensional IV-VI compounds

    Shafique, Aamir   Shin, Young-Han  

    We explore the thermoelectric and phonon transport properties of two-dimensional monochalcogenides (SnSe, SnS, GeSe, and GeS) using density functional theory combined with Boltzmann transport theory. We studied the electronic structures, Seebeck coefficients, electrical conductivities, lattice thermal conductivities, and figures of merit of these two-dimensional materials, which showed that the thermoelectric performance of monolayer of these compounds is improved in comparison compared to their bulk phases. High figures of merit (ZT) are predicted for SnSe (ZT =3D 2.63, 2.46), SnS (ZT =3D 1.75, 1.88), GeSe (ZT =3D 1.99, 1.73), and GeS (ZT =3D 1.85, 1.29) at 700 K along armchair and zigzag directions, respectively. Phonon dispersion calculations confirm the dynamical stability of these compounds. The calculated lattice thermal conductivities are low while the electrical conductivities and Seebeck coefficients are high. Thus, the properties of the monolayers show high potential toward thermoelectric applications.
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  • Adsorption and diffusion of mono,di,and trivalent ions on two-dimensional TiS2

    Samad, Abdus   Shafique, Aamir   Shin, Young-Han  

    A comparative study of the monovalent (Li, Na, and K) and multivalent (Be, Mg, Ca, and Al) metal ion adsorption and diffusion on an electronically semi-metallic two-dimensional nanosheet of 1T structured TiS2 is presented here to contribute to the search for abundant, cheap, and nontoxic ingredients for efficient rechargeable metal ion batteries. The total formation energy of the metal ion adsorption and the Bader charge analysis show that the divalent Mg and Ca ions can have a charge storage density double that of the monovalent Li, Na, and K ions, while the Be and Al ions form metallic clusters even at a low adsorption density because of their high bulk energies. The adsorption of Mg ions shows the lowest averaged open circuit voltage (0.13 V). The activation energy barriers for the diffusion of metal ions on the surface of the monolayer successively decrease from Li to K and Be to Ca. Mg and Ca, being divalent, are capable of storing a higher power density than Li while K and Na have a higher rate capability than the Li ions. Therefore, rechargeable Li ion batteries can be totally or partially replaceable by Mg ion batteries, where high power density and high cell voltage are required, while the abundant, cheap, and fast Na ions can be used for green grid applications.
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  • Piezoelectric enhancement by surface effect in hydrofluorinated graphene bilayer

    Kim, Hye Jung   Noor-A-Alam, Mohammad   Shin, Young-Han  

    We investigated the piezoelectricity of dipolar hydrofluorinated graphene (C2HF)(n) multilayers with first-principles calculations. Our results reveal that the dipole moment decreases as the number of layers increases, because electron and hole carriers are induced at the top and bottom layers due to the depolarization field. These carriers make (C2HF)(n) multilayers more stable by decreasing the depolarization field in the material. Through the calculation of the average layer piezoelectric stress constant e(31)/l in l-layer chair (C2HF)(n) multilayers, we confirmed that the piezoelectricity of the bilayer is about three times larger than that of the monolayer and bulk material. Moreover, we found that the electron and hole carriers on the top and bottom layers played a significant role in the piezoelectric enhancement of the bilayer. (C) 2015 AIP Publishing LLC.
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  • Growth Mechanisms of Carbon Nanotubes and Graphene

    Ryou, Junga   Park, Jinwoo   Shin, Young-Han   Hwang, Chanyong   Hong, Suklyun  

    To understand the initial stage in the growth of carbon nanotubes and graphene, we investigate the adsorption and diffusion of carbon atoms on the surfaces of metals such as nickel and copper. Adsorption geometries and binding energies of carbon atoms on the low-index metal surfaces and in their subsurfaces are considered. Then, the surface and subsurface diffusion barriers of a single carbon atom are calculated. On the basis of the results obtained, it is found that the carbon diffusion in nickel and copper plays an important role in the growth of these carbon-based materials.
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  • Structural and magnetic properties of Ge(0.7)Mn(0.3) thin films

    Kim, Sung-Kyu   Son, Jong Yeog   Shin, Young-Han   Jo, Moon-Ho   Park, S.   Hong, Tae Eun   Yee, K. J.  

    Ge(0.7)Mn(0.3) thin films were fabricated on Al(2)O(3) (0001) and glass substrates at growth temperatures ranging from room temperature to 500 degrees C by a radio frequency magnetron sputtering. We found that the Ge(0.7)Mn(0.3) thin films showed a polycrystalline-to-amorphous transition at about 360 degrees C, and the ferromagnetic transition temperature of each thin film depends on its structure crystalline or amorphous states. Particularly, the Ge(0.7)Mn(0.3) thin films showed room temperature ferromagnetism when they were fabricated at temperatures above the crystallization temperature. (C) 2009 Elsevier B.V. All rights reserved.
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  • Density functional study of alpha-beta phase transition of polyvinylidene difluoride

    Heo, Won Joon   Kim, Won-June   Shin, Young-Han   Lee, Eok Kyun  

    We perform density functional calculations to investigate structural and dynamical properties of crystalline polyvinylidene difluoride (PVDF) associated with the transition from a to beta phase. We examine the change of the conformational energy and the corresponding structure of each phase depending on the lattice parameters of the orthorhombic crystalline structure. From this information, we construct the path that connects the point where the a phase is most stable to the point where the beta phase is most stable, and identify the sub- region in the lattice parameter space where a and beta phases have the same energy. In this sub-region, we locate the point which gives the lowest conformation energy for both a and beta phases, and examine the behaviour of the lowest energy profile and corresponding change of intermediate structures as the conformation of the PVDF chain transforms from a phase to beta phase. Finally we perform ab-initio molecular dynamics simulations and analyse the characteristic dynamics associated with transition from a to beta phase. (C) 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
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  • Influences of vacancy and doping on electronic and magnetic properties of monolayer SnS

    Ullah, Hamid   Noor-A-Alam, Mohammad   Kim, Hye Jung   Shin, Young-Han  

    Based on the first-principles calculations, we investigate the structural, electronic, and magnetic properties of defects in monolayer SnS. We study the formation and migration of vacancies at both Sn-and S-sites. In comparison to the S-site vacancy, our calculations show that creating a vacancy at the Sn-site requires lesser energy, indicating that the vacancy at the Sn-site is more likely to be formed in experiments with energetic particle irradiation. For the Sn-rich (S-rich) environment, the vacancy at the S-site (Sn-site) is more likely to be found than the vacancy at the Sn-site (S-site). Reducing the formation of vacancy clusters, the S vacancy remains at the position where it is created because of the high vacancy migration barrier. Both types of vacancies remain nonmagnetic. To induce magnetism in monolayer SnS, we also study the transition metal (TM =3D Mn, Fe, and Co) doping at the Sn-site and find a significant influence on the electronic and magnetic properties of monolayer SnS. The doping of TM changes non-magnetic monolayer SnS to magnetic one and keeps it semiconducting. Additionally, long-range ferromagnetic behavior is observed for all the doped system. Hence, doping TM atoms in monolayer SnS could be promising to realize a two-dimensional diluted magnetic semiconductor. More interestingly, all the doped TM configurations show a high spin state, which can be used in nanoscale spintronic applications such as spin-filtering devices. Published by AIP Publishing.
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  • Fast Domain Wall Switching in a Thin Ferroelectric Polymer Layer RID C-1901-2011

    Son, Jong Yeog   Shin, Yun-Sok   Shin, Young-Han   Lee, Eok Kyun   Jang, Hyun Myung  

    We investigated the canonical ferroelectric response of a thin ferroelectric polymer film using a piezoelectric force microscopy method. The thin ferroelectric poly(vinyliden fluoride-ran-trifluoroethylene) layer with a thickness of 5 nm was prepared on a (111)Pt/TiO(2)/SiO(2)/Si substrate by a Langmuir-Blodgett method. The flip speed into upward polarization in the thin ferroelectric polymer layer is faster than that into downward polarization because the adhesion strength of the fluorine atoms in the ferroelectric polymer layer with the Pt electrode is stronger than that of the hydrogen atoms. (C) 2010 The Electrochemical Society. [DOI: 10.1149/1.3507412] All rights reserved.
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