Thermal Healing of the Nanometer-Wide Lines of Self-Assembled Monolayer

Zhang, Zhengqing,Ahn, Yoonho,Son, Jong Yeog,Jang, Joonkyung American Chemical Society 2016 The Journal of Physical Chemistry Part C Vol.120 No.28

<P>The structural and thermal properties of the nanometer-wide lines of a self-assembled monolayer (SAM) were investigated using molecular dynamics simulations. When grown by contact printing, a linear SAM contained a significant portion (8%) of unbound molecules that are inverted among upright molecules. This paper proposes thermal annealing (300-400 K) as an efficient method to remove the unbound molecules and improve the quality of the linear SAM. The partial melting of the SAM during heating enabled the unbound molecules to flip and adsorb. With subsequent cooling, the linear SAM recovered its compact ordered structure. The molecular mechanisms and activation energies involved in the thermal annealing were elucidated.</P>

Thermal Healing of a Mixed-Thiol Monolayer at the Nanoscale

Zhang, Zhengqing,Lim, Manho,Ahn, Yoonho,Jang, Joonkyung American Chemical Society 2018 The Journal of Physical Chemistry Part C Vol.122 No.23

<P>Using molecular dynamics simulation, we study the thermal healing of a mixed-thiol monolayer grown by contact printing. By simulating the monolayers with various compositions of octadecanethiol and decanethiol molecules, we show that a mixed-thiol monolayer grown by contact printing contains a significant portion of unbound molecules which fail to bind their sulfur atoms to the underlying gold surface. A subsequent thermal annealing (300-370 K) however removes the unbound thiol molecules and gives an ordered and compact monolayer. We uncover the molecular pathways behind the removal of the unbound molecules during thermal annealing.</P> [FIG OMISSION]</BR>

Molecular Dynamics Study of the Hydrophilic-to-Hydrophobic Switching in the Wettability of a Gold Surface Corrugated with Spherical Cavities

Zhang, Zhengqing,Matin, Mohammad A.,Ha, Man Yeong,Jang, Joonkyung American Chemical Society 2016 Langmuir Vol.32 No.37

<P>This paper reports a large scale molecular dynamics (MD) simulation study of the wettability of a gold surface engraved with (hemi)spherical cavities. By increasing the depth of cavities, the contact angle (CA) of a water droplet on the surface was varied from a hydrophilic (69 degrees) to a hydrophobic value (>109 degrees). The nonmonotonic behavior of the CA vs the depth of the cavities was consistent with the Cassie-Baxter theory, as found in the experiment by Abdelsalam et al. (Abdelsalam, M. E.; Bartlett, P. N.; Kelf, T.; Baumberg, J. Wetting of Regularly Structured Gold Surfaces. Langmuir 2005, 21, 1753-1757). Depending on the depth of cavities, however, the droplet existed not only in the Cassie-Baxter state, but also in the Wenzel or an intermediate state, where the cavities were penetrated partially by the droplet.</P>

Identification and characterization of conserved and novel miRNAs in different development stages of Atrijuglans hetaohei Yang (Lepidoptera: Gelechioidea)

Zhengqing Zhang,Tianfeng Li,Guanghui Tang 한국응용곤충학회 2018 Journal of Asia-Pacific Entomology Vol.21 No.1

Atrijuglans hetaohei Yang (Lepidoptera: Gelechioidea) is a kind of walnut pest, which mainly damage Juglans regia(Juglandales: Juglandaceae) and is widely distributed in China. The information of microRNAs, e.g., the conservedand novel miRNAs of this species is still unknown. In the present study, the miRNAs in larval and adultstages of A. hetaohei were identified by high throughput sequencing with reference to the genome database of theBombyx mori. A total of 132 conserved miRNAs belonging to 64 families and seven novel miRNAs were obtained,among which 118 miRNAs were identified in both libraries, 4 and 17 miRNAs were only detected to adultspecificand larval-specific libraries, respectively. The most abundant miRNAs in A. hetaohei were miR-1a-3p, let-7, miR-2766 and miR-184. Comparative analysis revealed that 4 novel miRNAs and 47 conserved miRNAs weredifferentially expressed (P < 0.05) between two libraries. Abundance changes of miRNAs were observed fromlarva to adult: 19 miRNAs were up-regulated and 21 miRNAs were down-regulated. The metamorphosis andapoptosis associated miRNAs including miR-1a-3p, miR-133, let-7, miR-2766, miR-184 and miR-34 were discussedfor their predicted functions in insect development. The different expressions of these miRNAs wererelevant to dorso-ventral axis formation, phototransduction, insect hormone biosynthesis, circadian rhythm andmitogen-activated protein kinase (MAPK) signaling pathway. qRT-PCR analysis of ten miRNAs showed similarexpression patterns as high throughput analysis, most miRNAs were highly expressed in adult stage, while miR-8-3p, miR-305-3p and miR-2766 were highly expressed in larvae. The results preliminarily revealed the characterizationof conserved and novel miRNAs in different development stages of A. hetaohei. The different expressionsand associated functions of these miRNAs will provide a basis for understanding post-transcriptionallevel during A. hetaohei development and growth.

Molecular features of hydration layers probed by atomic force microscopy

Zhang, Zhengqing,Ryu, Seol,Ahn, Yoonho,Jang, Joonkyung The Royal Society of Chemistry 2018 Physical chemistry chemical physics Vol.20 No.48

<P>Structurally-ordered layers of water are universally formed on a solid surface in aqueous solution or under ambient conditions. Although such hydration layers are commonly probed <I>via</I> atomic force microscopy (AFM), the current understanding on how the hydration layers manifest themselves in an AFM experiment is far from complete. By using molecular dynamics simulation, we investigate the hydration layers on a hydrophilic or hydrophobic surface probed by a nanoscale tip. We study the density and molecular orientation of water, the free energy, and the force on the tip by varying the tip-surface distance. The force-distance curve oscillates due to the transition between the mono-, bi-, and tri-layers of water confined between the tip and the surface. If both the tip and the surface are hydrophobic, water confined between the tip and the surface evaporates due to the dewetting transition, giving a hydrophobic force without oscillation. The periodicity of oscillation in the force differs from the structural periodicity of water. With a close proximity of the tip, the molecular dipoles align parallel to the surface, regardless of whether the tip and the surface are hydrophilic or hydrophobic.</P>

Molecular Dynamics Study on the Wetting Transition of a Hierarchical Groove

Zhengqing Zhang,정성욱,장준경 대한화학회 2018 Bulletin of the Korean Chemical Society Vol.39 No.3

Vortex-induced oscillations of bridges: theoretical linkages between sectional model tests and full bridge responses

Zhang, Zhitian,Ge, Yaojun,Chen, Zhengqing Techno-Press 2014 Wind and Structures, An International Journal (WAS Vol.19 No.3

Vortex-induced oscillation is a type of aeroelastic phenomenon, to which extended structures such as long-span bridges are most susceptible. The vortex-induced vibration (VIV) behaviors of a concerned bridge were investigated conventionally in virtue of wind tunnel tests on string-mounted sectional models. This necessitates the building of a linkage between the response of the sectional model and that of the prototype structure. Although many released literatures have related to this issue and provided suggestions, there is a lack of consistency among them. In this study, some theoretical models describing the vortex-induced structural motion, including the linear empirical model, the nonlinear empirical model and the modified (or generalized) nonlinear empirical model, are firstly reviewed. Then, the concept of equivalent mass density is introduced based on the principle that an equal input of energy should result in identical structural amplitudes. Based on these, the theoretical linkages between the amplitude of a section model and that corresponding to the prototype bridge are discussed with different analytical models. Theoretical derivation indicates that such connections are dependent mainly on two factors, one is the presupposed shape of deformation, and the other is the theoretical VIV model employed. The theoretical analysis in this study shows that, in comparison to the nonlinear empirical models, the linear one can result in obvious larger estimations of the full bridges' responses, especially in cases of cable-stayed bridges.

Vortex-induced oscillations of bridges: theoretical linkages between sectional model tests and full bridge responses

Zhitian Zhang,Yaojun Ge,Zhengqing Chen 한국풍공학회 2014 Wind and Structures, An International Journal (WAS Vol.19 No.3

Vortex-induced oscillation is a type of aeroelastic phenomenon, to which extended structures such as long-span bridges are most susceptible. The vortex-induced vibration (VIV) behaviors of a concerned bridge were investigated conventionally in virtue of wind tunnel tests on string-mounted sectional models. This necessitates the building of a linkage between the response of the sectional model and that of the prototype structure. Although many released literatures have related to this issue and provided suggestions, there is a lack of consistency among them. In this study, some theoretical models describing the vortex-induced structural motion, including the linear empirical model, the nonlinear empirical model and the modified (or generalized) nonlinear empirical model, are firstly reviewed. Then, the concept of equivalent mass density is introduced based on the principle that an equal input of energy should result in identical structural amplitudes. Based on these, the theoretical linkages between the amplitude of a section model and that corresponding to the prototype bridge are discussed with different analytical models. Theoretical derivation indicates that such connections are dependent mainly on two factors, one is the presupposed shape of deformation, and the other is the theoretical VIV model employed. The theoretical analysis in this study shows that, in comparison to the nonlinear empirical models, the linear one can result in obvious larger estimations of the full bridges' responses, especially in cases of cable-stayed bridges.

Two-dimensional sheet of germanium selenide as an anode material for sodium and potassium ion batteries: First-principles simulation study

Sannyal, Arindam,Zhang, Zhengqing,Gao, Xingfa,Jang, Joonkyung Elsevier 2018 Computational materials science Vol.154 No.-

<P><B>Abstract</B></P> <P>Recently, two-dimensional layered materials have come forth as encouraging candidates for advanced electronic and optoelectronic applications. Anode materials with high energy-density and diffusion rate are fundamental features for the development of non-lithium ion batteries. Based on the density functional theory calculations, we propose a two-dimensional (2D) sheet of germanium selenide (GeSe) as a promising anode material for a sodium (Na) or potassium (K) ion battery. The phonon dispersion and formation energy verify the dynamic and thermal stability of the GeSe sheet. A substantial charge transfer from the alkali metal atoms to the GeSe sheet enhances the electrical conductivity of GeSe, favorable for an anode material. The Na or K diffusion on the GeSe sheet has a low energy barrier of 0.10 eV, giving a rapid charge/discharge rate without metal clustering. The GeSe sheet has a high theoretical capacity for both Na (707 mA h g<SUP>−1</SUP>) and K (530 mA h g<SUP>−1</SUP>) ion batteries. The GeSe sheet also gives a low and stable electrode potential comparable to that of a commercial anode material.</P> <P><B>Highlights</B></P> <P> <UL> <LI> Strong adsorption of the metal atoms prevents the metal clustering. </LI> <LI> Significant charge transfer changes the semiconducting GeSe into metallic. </LI> <LI> Alkali metal diffusion on the germanium selenide monolayer is anisotropic. </LI> <LI> The open circuit voltages are in the ranges of other commercial anode materials. </LI> <LI> The 2D germanium selenide gives very high capacity for Na-ion and K-ion batteries. </LI> </UL> </P> <P><B>Graphical abstract</B></P> <P>[DISPLAY OMISSION]</P>

Mitigation of wind-induced vibrations of bridge hangers using tuned mass dampers with eddy current damping

Huawei Niu,Zhengqing Chen,Xugang Hua,Wei Zhang 국제구조공학회 2018 Smart Structures and Systems, An International Jou Vol.22 No.6

To mitigate vibrations, tuned mass dampers (TMD) are widely used for long span bridges or high-rise buildings. Due to some durability concerns, such as fluid degradation, oil leakage, etc., the alternative solutions, such as the non-contacted eddy current damping (ECD), are proposed for mechanical devices in small scales. In the present study, a new eddy current damping TMD (ECD-TMD) is proposed and developed for large scale civil infrastructure applications. Starting from parametric study on finite element analysis of the ECD-TMD, the new design is enhanced via using the permanent magnets to eliminate the power need and a combination of a copper plate and a steel plate to improve the energy dissipation efficiency. Additional special design includes installation of two permanent magnets at the same side above the copper plate to easily adjust the gap as well as the damping. In a case study, the proposed ECD-TMD is demonstrated in the application of a steel arch bridge to mitigate the wind-induced vibrations of the flexible hangers. After a brief introduction of the configuration and the installation process for the damper, the mitigation effects are measured for the ambient vibration and forced vibration scenarios. The results show that the damping ratios increase to 3% for the weak axis after the installation of the ECD-TMDs and the maximum vibration amplitudes can be reduced by 60%.