Man-Fai Ng

One-dimensional iron-cyclopentadienyl sandwich molecular wire with half metallic, negative differential resistance and high-spin filter efficiency properties.

We present a theoretical study on a series of novel organometallic sandwich molecular wires (SMWs), which are constructed with alternating iron atoms and cyclopentadienyl (Cp) rings, using DFT and nonequilibrium Greens function techniques. It is found that that the SMWs are stable, flexible structures having half-metallic (HM) properties with 100% negative spin polarization near the Fermi level in the ground state. Some SMWs of finite size show a nearly perfect spin filter effect (SFE) when coupled between ferromagnetic electrodes. Moreover, their I-V curves exhibit negative differential resistance (NDR), which is essential for certain electronic applications. The SMWs are the first linear molecules with HM, high SFE, and NDR and can be easily synthesized. In addition, we also analyze the underlying mechanisms via the transmission spectra and spin-dependent calculations. These findings strongly suggest that the SMWs are promising materials for application in molecular electronics.

Charge-Transfer-Based Mechanism for Half-Metallicity and Ferromagnetism in One-Dimensional Organometallic Sandwich Molecular Wires

We present a systematic theoretical study on the mechanism of half-metallicity and ferromagnetism for one-dimensional (1-D) sandwich molecular wires (SMWs) constructed with altering cyclopentadienyl (Cp) and first-row transition metal (Mt). It is unveiled for the first time that, in (MtCp) infinity, one valence electron would transfer from the Mt to the Cp ring, forming Cp (-) and Mt (+) altering structures. This electron transfer not only makes them more stable than the benzene analogues (MtBz) infinity but also leads to completely different half-metallic and ferromagnetic mechanisms. We analyze such unusual half-metallicity and ferromagnetic behaviors and explain each SMW magnetic moment quantitatively. Finally, we indicate that a Peierls transition does not occur in these 1-D SMWs.

Controlling Na diffusion by rational design of Si-based layered architectures

By means of density functional theory, we systematically investigate the insertion and diffusion of Na and Li in layered Si materials (polysilane and H-passivated silicene), in comparison with bulk Si. It is found that Na binding and mobility can be significantly facilitated in layered Si structures. In contrast to the Si bulk, where Na insertion is energetically unfavorable, Na storage can be achieved in polysilane and silicene. The energy barrier for Na diffusion is reduced from 1.06 eV in the Si bulk to 0.41 eV in polysilane. The improvements in binding energetics and in the activation energy for Na diffusion are attributed to the large surface area and available free volume for the large Na cation. Based on these results, we suggest that polysilane may be a promising anode material for Na-ion and Li-ion batteries with high charge-discharge rates.

Enhanced Li adsorption and diffusion in silicon nanosheets based on first principles calculations

First-principles density functional theory calculations are employed to investigate novel ultrathin silicon nanosheets (SiNSs) for their potential application as the anode material for Li-ion batteries. We find that Li has a higher tendency to bind on the surface of SiNS rather than penetrating through inside. The binding energies of Li show a strong dependence on the thickness of the nanosheets. The results suggest that insertion/deinsertion of Li can be controlled by using nanosheets of different thickness. More importantly, we show that there is a large increase of diffusivity in Si nanosheets as compared with the bulk case. In addition, Li diffusion shows strong dependence on the chemical functionalization of SiNSs, in which the diffusion rate is the fastest on H passivated surface as compared with the halogen passivated surfaces. Our results suggest that SiNSs are potential materials for Li-ion battery applications.

Geometry Dependent I−V Characteristics of Silicon Nanowires

The current-voltage (I-V) characteristics of small-diameter hydrogenated and pristine silicon nanowires (SiNWs) are calculated by nonequilibrium Greens function combined with density functional theory. We show that the I-V characteristics depend strongly on length, growth orientation, and surface modification of the SiNWs. In particular, a length of 3 nm is suggested for the nanowires to retrieve its intrinsic conducting properties from the influences of both the electrodes and metal/semiconductor mismatched surface contact; surface reconstruction would enhance the conductance in hydrogenated SiNW, which is explained by the extra conducting eigenchannel found in the transmission spectrum, suggesting possible surface conducting channel. Discussions with available experimental data are given.

Building better lithium-sulfur batteries: from LiNO3 to solid oxide catalyst

Lithium nitrate (LiNO3) is known as an important electrolyte additive in lithium-sulfur (Li-S) batteries. The prevailing understanding is that LiNO3 reacts with metallic lithium anode to form a passivation layer which suppresses redox shuttles of lithium polysulfides, enabling good rechargeability of Li-S batteries. However, this view is seeing more challenges in the recent studies, and above all, the inability of inhibiting polysulfide reduction on Li anode. A closely related issue is the progressive reduction of LiNO3 on Li anode which elevates internal resistance of the cell and compromises its cycling stability. Herein, we systematically investigated the function of LiNO3 in redox-shuttle suppression, and propose the suppression as a result of catalyzed oxidation of polysulfides to sulfur by nitrate anions on or in the proximity of the electrode surface upon cell charging. This hypothesis is supported by both density functional theory calculations and the nitrate anions-suppressed self-discharge rate in Li-S cells. The catalytic mechanism is further validated by the use of ruthenium oxide (RuO2, a good oxygen evolution catalyst) on cathode, which equips the LiNO3-free cell with higher capacity and improved capacity retention over 400 cycles.

First-principles study of silicon nanowire approaching the bulk limit.

First-principles density functional theory calculations on hydrogenated silicon nanowires (SiNWs) with diameters up to 7.3 nm are carried out for comparing to experimentally relevant SiNWs and evaluating its radial doping profiles. We show that the direct band gap nature of both the small diameter (110) and (100) SiNWs fades when the diameter reaches beyond about 4 nm, where the difference of direct and indirect band gaps are close, within the experimental measurement uncertainty of ±0.1 eV, suggesting the diameter size where the gap nature transition starts. In addition, we reveal that core-surface boron (B) codoped SiNW forms more preferably at large diameter than that of the surface-surface codoped one, attributing to the lower energy configuration raised by the core B dopant at large diameter SiNW. More importantly, the diameter for such a preferential transition increases as the doping concentration decreases. Our results rationalize photoluminescent measurements and radial doping distributions of SiNWs.

Enhanced Li Adsorption and Diffusion in Single‐Walled Silicon Nanotubes: An ab Initio Study

We report a first-principles investigation of Li adsorption and diffusion in single-walled Si nanotubes (SWSiNTs) of interest to Li-ion battery anodes. We calculate Li insertion characteristics in SWSiNTs and compare them with the respective ones in carbon nanotubes (CNTs) and other silicon nanostructures. From our calculations, SWSiNTs show higher reactivity toward the adsorption of Li adatoms than CNTs and Si nanoclusters. Considering the importance of Li kinetics, we demonstrate that the interior of SWSiNTs may serve as a fast Li diffusion channel. The important advantage of SWSiNTs over their carbon analogues is a sevenfold reduction in the energy barrier for the penetration of the Li atoms into the nanotube interior through the sidewalls. This prepossesses easier Li diffusion inside the tube and subsequent utilization of the interior sites, which enhances Li storage capacity of the system. The improvements in both Li uptake and Li mobility over their analogues support the great potential of SWSiNTs as Li-ion battery anodes.

Oscillating edge states in one-dimensional MoS2 nanowires.

Reducing the dimensionality of transition metal dichalcogenides to one dimension opens it to structural and electronic modulation related to charge density wave and quantum correlation effects arising from edge states. The greater flexibility of a molecular scale nanowire allows a strain-imposing substrate to exert structural and electronic modulation on it, leading to an interplay between the curvature-induced influences and intrinsic ground-state topology. Herein, the templated growth of MoS2 nanowire arrays consisting of the smallest stoichiometric MoS2 building blocks is investigated using scanning tunnelling microscopy and non-contact atomic force microscopy. Our results show that lattice strain imposed on a nanowire causes the energy of the edge states to oscillate periodically along its length in phase with the period of the substrate topographical modulation. This periodic oscillation vanishes when individual MoS2 nanowires join to form a wider nanoribbon, revealing that the strain-induced modulation depends on in-plane rigidity, which increases with system size.

Improved binding and stability in Si/CNT hybrid nanostructures via interfacial functionalization: a first-principles study

We use first-principles calculations to investigate the geometric structure, energetics and electronic properties of silicon cluster/carbon nanotube (Si/CNT) hybrid nanostructures with potential application as Li-ion battery anodes. The effects of the main components (i.e. Si cluster, CNT support and linker) on the properties of hybrid system, such as morphology, interfacial bonding and electronic structure have been systematically evaluated. After comparing several functional groups, it has been shown that the functionalization of CNT not only increases the binding strength between Si clusters and CNT by 3 times under normal conditions, but also greatly contributes to the stability of hybrid material during lithiation. Importantly, we have shown that Li insertion leads to the weakening of Si/CNT interface, which could be one of the key reasons for the experimentally observed capacity fade in hybrid Si/CNT anodes. The structural integrity of Si/CNT nanostructure could be improved after the functionalization of CNT surface, potentially leading to the longer cycle life of the hybrid anode. Our results support recent experimental findings and reveal the importance of interface engineering in the design of hybrid nanostructures for various applications.