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A first-principles comparative study of lithium, sodium, potassium and calcium storage in two-dimensional Mg2C

Chu YZ, Yeoh KH, Chew KH

J Phys Condens Matter, 2021 Feb 17;33(7):075002.
PMID: 33152714 DOI: 10.1088/1361-648X/abc807

Two-dimensional (2D) materials have recently emerged as potential candidates for high-capacity lithium-ion batteries anode materials because of their compelling physicochemical and structural properties. In the present study, we use first-principles calculations to investigate the performance of 2D Mg2C as anode materials for Li, Na, K and Ca-ions batteries. The calculated average open-circuit voltage are 0.37, 0.50, 0.03 and 0.06 eV vs Li, Na, K, Ca. No significant structural deformations are observed on the 2D Mg2C upon the adsorption of Li, Na, K or Ca and the metallic characteristic of the 2D Mg2C is retained. The metallic behaviour of both pristine and adsorbed Mg2C ensures the desirable electric conductivity, implying the advantages of 2D Mg2C for batteries. The Na and K atoms show an extremely high diffusivity on the 2D Mg2C with a low energy barrier of 0.08 and 0.04 eV respectively, which is about an order of magnitude smaller than that of Li atom. For the Na and K atoms, the theoretical storage capacity can reach up to 1770 mAh g-1, nearly two times that of the Li atom of 885 mAh g-1. Our study suggests that the 2D Mg2C is a promising anode material which offers a fast ion diffusion and high storage capacity.
First-principles study on the atomistic corrosion processes of iron

Chew KH, Kuwahara R, Ohno K

Phys Chem Chem Phys, 2018 Jan 17;20(3):1653-1663.
PMID: 29261192 DOI: 10.1039/c7cp04022a

The corrosion of iron presents an important scientific problem and a serious economic issue. It is also one of the most important subjects in materials science because it is basically an electrochemical process and closely related to other topics such as the electrocatalysis of the oxygen reduction reaction. So far, many studies have been conducted to address the corrosion of iron, a very complicated process that occurs when iron is exposed to oxygen and water. An important question is, at which site of the iron surface the corrosion starts and how it results in the final stage of the corrosion. In the present study, as an example of superficial defects, Fe dimers sticking out of Fe(100) surfaces are considered in order to understand the iron corrosion process from first-principles using density functional theory. We found that the Fe dimers spontaneously react with O2 and H2O to form Fe2(OH)4 + 4OH-. Here, it is interesting to note that the Fe dimer plays the role of a water splitting catalyst, because the space above it is always vacant and can accept oxygen molecules many times for reacting with the surrounding water molecules. Then, if the Fe2(OH)4 molecules are detached from the surface, they react with O2 to form Fe2O(OH)4 without an activation barrier, and, in turn, the Fe2O(OH)4 and H2O molecules react to form Fe2(OH)6 complexes with an activation energy of 0.653 eV. If these complexes further dissociate into Fe(OH)3 molecules, they react with each other to form Fe2O3·2H2O with an activation energy of 0.377 eV. This work may provide useful information on possible iron corrosion processes by water in the air.
Rich antiferroelectric phase diagram of antiferroelectric-ferroelectric superlattices: internal electric field- and interface induced phase transitions

Lum CY, Lim KG, Chew KH

J Phys Condens Matter, 2020 Jun 16;32(42):425401.
PMID: 32544898 DOI: 10.1088/1361-648X/ab9d4a

We propose a thermodynamic model to the study the antiferroelectric (AFE) phase transitions in antiferroelectric-ferroelectric (AFE-FE) superlattices in which the coupling at the interface between two layers is mediated by local polarizations. Phase diagram of the AFE layer in term of the degree of interfacial effect λ and temperature T involving ferrielectric (FI) and ferroelectric (FE) phases is investigated. These two phases are stabilized by the interfacial effect and internal electric field. AFE thickness L AFE versus T phase diagram is also constructed. Intermediate regions of two-phase coexistence (IM) emerge in the λ-T and L AFE-T phase diagrams, if certain interface properties λ and layer thickness L AFE criteria are met. These IM regions are metastable states, which exist as a transition state between two phases. A tricritical point locates at the boundaries across the FI, IM and FE phases is found in the L AFE-T phase diagram. Competition among the internal electric field due to the electrostatic coupling, the FE ordering arises from the interfacial effect and the antiferroelectric ordering within the AFE layer giving rises to the rich AFE phase diagram.
Correlation between polar surface area and bioferroelectricity in DNA and RNA nucleobases

Yam SC, Zain SM, Sanghiran Lee V, Chew KH

Eur Phys J E Soft Matter, 2018 Jul 18;41(7):86.
PMID: 30014219 DOI: 10.1140/epje/i2018-11696-5

We have performed computational molecular modelling to study the polarization switching and hysteresis loop behaviours of DNA and RNA nucleobases using the PM3 semi-empirical quantum mechanical approaches. All the nucleobases: adenine (A), thymine (T), guanine (G), cytosine (C), and uracil (U) were modelled. Our study indicates that all the nucleobases exhibit a zero-field polarization due to the presence of polar atoms or molecules such as amidogen and carbonyl. The shape of polarization P versus an applied electric field E hysteresis loop is square, implying typical ferroelectrics behaviour. The total energy U as a function of an applied electric field E exhibits a butterfly-like loop. The presence of zero-field polarization and ferroelectrics hysteresis loop behaviours in nucleobases may support the hypothesis of the existence of bioferroelectricity in DNA and RNA. We also found an interesting relationship between the minimum electric field required for switching [Formula: see text] and the ratio of the topological polar surface area (TPSA) to the total surface area (TSA) of a nucleobase. In particular, the [Formula: see text] of a nucleobase is inversely proportional to the TPSA/TSA ratio. This work may provide useful information for understanding the possible existence of ferroelectricity in biomaterials.
Fulltext Structured caustic vector vortex optical field: manipulating optical angular momentum flux and polarization rotation

Chen RP, Chen Z, Chew KH, Li PG, Yu Z, Ding J, et al.

Sci Rep, 2015;5:10628.
PMID: 26024434 DOI: 10.1038/srep10628

A caustic vector vortex optical field is experimentally generated and demonstrated by a caustic-based approach. The desired caustic with arbitrary acceleration trajectories, as well as the structured states of polarization (SoP) and vortex orders located in different positions in the field cross-section, is generated by imposing the corresponding spatial phase function in a vector vortex optical field. Our study reveals that different spin and orbital angular momentum flux distributions (including opposite directions) in different positions in the cross-section of a caustic vector vortex optical field can be dynamically managed during propagation by intentionally choosing the initial polarization and vortex topological charges, as a result of the modulation of the caustic phase. We find that the SoP in the field cross-section rotates during propagation due to the existence of the vortex. The unique structured feature of the caustic vector vortex optical field opens the possibility of multi-manipulation of optical angular momentum fluxes and SoP, leading to more complex manipulation of the optical field scenarios. Thus this approach further expands the functionality of an optical system.
A first-principles study of two-dimensional NbSe2H/g-ZnO van der Waals heterostructures as a water splitting photocatalyst

Yeoh KH, Chew KH, Yoon TL, Chang YHR, Ong DS

Phys Chem Chem Phys, 2021 Nov 03;23(42):24222-24232.
PMID: 34668497 DOI: 10.1039/d1cp03565g

Based on first-principles calculations, we propose a new two-dimensional (2D) van der Waals (vdW) heterostructure that can be used as a photocatalyst for water splitting. The heterostructure consists of vertically stacked 2D NbSe2H and graphene-like ZnO (g-ZnO). Depending on the stacking orders, we identified two configurations that have high binding energies with an energy band gap of >2.6 eV. These 2D systems form a type-II heterostructure which enables the separation of photoexcited electrons and holes. The presence of a strong electrostatic potential difference across the 2D NbSe2H and g-ZnO interface is expected to suppress the electron-hole recombination leading to an enhancement in the efficiency of the photocatalytic activity. Our study also shows that the 2D NbSe2H/g-ZnO vdW heterostructure has good thermodynamic properties for water splitting. Furthermore, the optical absorption of the 2D NbSe2H/g-ZnO vdW heterostructure extends into the visible light region. Our results suggest that the 2D NbSe2H/g-ZnO vdW heterostructure is a promising photocatalytic material for water splitting.
The effect of vacancy defects on the electromechanical properties of monolayer NiTe2 from first principles calculations

Yeoh KH, Chew KH, Chang YHR, Yoon TL, Ong DS

Phys Chem Chem Phys, 2022 Dec 05.
PMID: 36468660 DOI: 10.1039/d2cp05102h

The electromechanical properties of monolayer 1-T NiTe2 under charge actuation were investigated using first-principles density functional theory (DFT) calculations. Monolayer 1-T NiTe2 in its pristine form has a work area density per cycle of up to 5.38 MJ m-3 nm upon charge injection and it can generate a strain and a stress of 1.51% and 0.96 N m-1, respectively. We found that defects in the form of vacancies can be exploited to modulate the electromechanical properties of this material. The presence of Ni-vacancies can further enhance the generated stress by 22.5%. On the other hand, with Te-vacancies, it is possible to improve the work area density per cycle by at least 145% and also to enhance the induced strain from 1.51% to 2.92%. The effect of charge polarity on the contraction and expansion of monolayer 1T-NiTe2 was investigated. Due to its excellent environmental stability and good electromechanical properties, monolayer NiTe2 is considered to be a promising electrode material for electroactive polymer (EAP) based actuators.
Computational Screening of a Single-Atom Catalyst Supported by Monolayer Nb2S2C for Oxygen Reduction Reaction

Yeoh KH, Chang YHR, Chew KH, Jiang J, Yoon TL, Ong DS, et al.

Langmuir, 2024 Feb 08.
PMID: 38329924 DOI: 10.1021/acs.langmuir.3c03188

The search for high-performance catalysts to improve the catalytic activity for an oxygen reduction reaction (ORR) is crucial for developing a proton exchange membrane fuel cell. Using the first-principles method, we have performed computational screening on a series of transition metal (TM) atoms embedded in monolayer Nb2S2C to enhance the ORR activity. Through the scaling relationship and volcano plot, our results reveal that the introduction of a single Ni or Rh atom through substitutional doping into monolayer Nb2S2C yields promising ORR catalysts with low overpotentials of 0.52 and 0.42 V, respectively. These doped atoms remain intact on the monolayer Nb2S2C even at elevated temperatures. Importantly, the catalytic activity of the Nb2S2C doped with a TM atom can be effectively correlated with an intrinsic descriptor, which can be computed based on the number of d orbital electrons and the electronegativity of TM and O atoms.
Intramyocardial and intracoronary autologous bone marrow-derived mesenchymal stromal cell treatment in chronic severe dilated cardiomyopathy

Chin SP, Poey AC, Wong CY, Chang SK, Tan CS, Ng MT, et al.

Cytotherapy, 2011 Aug;13(7):814-21.
PMID: 21526902 DOI: 10.3109/14653249.2011.574118

BACKGROUND AIMS: Mesenchymal stromal cells (MSC) may improve cardiac function following myocardial infarction. MSC can differentiate into cardiomyocytes and endothelial cells while exerting additional paracrine effects. There is limited information regarding the efficacy of route for MSC treatment of severe dilated cardiomyopathy (DCM). The aim of this study was to demonstrate the clinical safety, feasibility and efficacy of direct intramyocardial and intracoronary administration of autologous bone marrow-derived MSC treatment for no-option patients with chronic severe refractory DCM.
METHODS: Ten symptomatic patients with DCM and refractory cardiac function, despite maximum medical therapy, were selected. Five had ischemic DCM deemed unlikely to benefit from revascularization alone and underwent bypass operations with concurrent intramyocardial MSC injection (group A). Two patients had previous revascularization and three had non-ischemic DCM and received intracoronary MSC injection (group B).
RESULTS: Group A and B patients received 0.5-1.0 × 10(6) and 2.0-3.0 × 10(6) MSC/kg body weight, respectively. All patients remained alive at 1 year. There were significant improvements from baseline to 6 and 12 months in left ventricular ejection fraction and other left ventricular parameters. Scar reduction was noted in six patients by 12 months.
CONCLUSIONS: Autologous bone marrow MSC treatment is safe and feasible for treating chronic severe refractory DCM effectively, via intracoronary or direct intramyocardial administration at prescribed doses.
Fulltext Deposition, Characterization, and Modeling of Scandium-Doped Aluminum Nitride Thin Film for Piezoelectric Devices

Zhang Q, Chen M, Liu H, Zhao X, Qin X, Wang F, et al.

Materials (Basel), 2021 Oct 27;14(21).
PMID: 34771961 DOI: 10.3390/ma14216437

In this work, we systematically studied the deposition, characterization, and crystal structure modeling of ScAlN thin film. Measurements of the piezoelectric device's relevant material properties, such as crystal structure, crystallographic orientation, and piezoelectric response, were performed to characterize the Sc0.29Al0.71N thin film grown using pulsed DC magnetron sputtering. Crystal structure modeling of the ScAlN thin film is proposed and validated, and the structure-property relations are discussed. The investigation results indicated that the sputtered thin film using seed layer technique had a good crystalline quality and a clear grain boundary. In addition, the effective piezoelectric coefficient d33 was up to 12.6 pC/N, and there was no wurtzite-to-rocksalt phase transition under high pressure. These good features demonstrated that the sputtered ScAlN is promising for application in high-coupling piezoelectric devices with high-pressure stability.
Energy Band Gap Modulation in Nd-Doped BiFeO3/SrRuO3 Heteroepitaxy for Visible Light Photoelectrochemical Activity

Tan KH, Chen YW, Van CN, Wang H, Chen JW, Lim FS, et al.

ACS Appl Mater Interfaces, 2019 Jan 09;11(1):1655-1664.
PMID: 30561192 DOI: 10.1021/acsami.8b17758

The ability of band offsets at multiferroic/metal and multiferroic/electrolyte interfaces in controlling charge transfer and thus altering the photoactivity performance has sparked significant attention in solar energy conversion applications. Here, we demonstrate that the band offsets of the two interfaces play the key role in determining charge transport direction in a downward self-polarized BFO film. Electrons tend to move to BFO/electrolyte interface for water reduction. Our experimental and first-principle calculations reveal that the presence of neodymium (Nd) dopants in BFO enhances the photoelectrochemical performance by reduction of the local electron-hole pair recombination sites and modulation of the band gap to improve the visible light absorption. This opens a promising route to the heterostructure design by modulating the band gap to promote efficient charge transfer.