Jiuren Yin

Nanoscale mechanical characterization of PMMA by AFM nanoindentation: a theoretical study on the time-dependent viscoelastic recovery

Temperature-dependent indent recovery of polymethyl methacrylate is depicted by atomic force microscopy. The viscoelastic indent recovery is predicted by a numerical model based on the Boussinesq elastic theory. From the perspective of an elastic solution, viscoelastic solution for stress and displacement field is constructed through the analysis of the elasticity–viscoelasticity corresponding theory. The findings also illustrate the effect of loading condition, elastic modulus, and viscosity on the viscoelastic recovery rate.

A new two-dimensional TeSe 2 semiconductor: indirect to direct band-gap transitions

A novel two-dimensional (2D) TeSe2 structure with high stability is predicted based on the first-principles calculations. As a semiconductor, the results disclose that the monolayer TeSe2 has a wide-band gap of 2.392 eV. Interestingly, the indirect-band structure of the monolayer TeSe2 transforms into a direct-band structure under the wide biaxial strain (0.02–0.12). The lower hole effective mass than monolayer black phosphorus portends a high carrier mobility in TeSe2 sheet. The optical properties and phonon modes of the few-layered TeSe2 were characterized. The few-layer TeSe2 shows a strong optical anisotropy. Specially, the calculated results demonstrate that the multilayer TeSe2 has a wide range of absorption wavelength. Our result reveals that TeSe2 as a novel 2D crystal possesses great potential applications in nanoscale devices, such as high-speed ultrathin transistors, nanomechanics sensors, acousto-optic deflectors working in the UV-vis red region and optoelectronic devices.摘要本文基于第一性原理计算预测了一种新颖的二维稳定结构TeSe2, 结果显示单层TeSe2是一种半导体材料, 其带隙值为2.392 eV. 有趣的是单层TeSe2的间接能带在宽范围的双向负应变(0.02~0.12)作用下转变为直接能带. 比单层黑磷烯更小的有效空穴电子质量预示了TeSe2具有更高的载流子迁移速率. 此外, 对不同厚度TeSe2的声子模及光学性质也进行了计算, 结果显示不同厚度的TeSe2具有较强的光学各向异性, 尤其是多层TeSe2具有更宽的吸收波长. 这些结果表明, TeSe2作为一种新颖的二维结构在纳米器件领域具有巨大的应用潜力, 如高速超薄晶体管, 纳米力学传感器, 紫外–可见红光区声光偏振器及光电子器件等.

Strain/stress engineering on the mechanical and electronic properties of phosphorene nanosheets and nanotubes

Phosphorene is demonstrated to have a great potential in the electronics applications. In this work, the first-principle calculations are employed to predict the mechanical properties and the electronic structure of phosphorene nanosheets and nanotubes. Compared with that of nanosheets, the maximum tensile stress of nanotubes decreases from 17.66 GPa to 11.73 GPa in the zigzag direction and 7.56 GPa to 5.95 GPa in the armchair direction. The ultimate tensile strain of nanosheets is about 27% in the armchair and 25% in the zigzag directions. However, the maximum strain of the zigzag nanotubes decreases to 24% and the ultimate strain of the armchair nanotube is about 14.4%. It presents that the tensile modulus will decrease with the increasing tension, while the compression modulus increases with increasing compression. The results show that zigzag-direction stress will affect the covalent bonds largely, while the armchair-direction stress influences the lone-pair electrons more. Within the allowable strain, the band structure and effective mass of carriers are calculated. The CBM and VBM change their positions when the stress is applied. The effective mass of nanosheets and nanotubes is strongly affected by strain.

Strain engineering on transmission carriers of monolayer phosphorene

The effects of uniaxial strain on the structure, band gap and transmission carriers of monolayer phosphorene were investigated by first-principles calculations. The strain induced semiconductor-metal as well as direct-indirect transitions were studied in monolayer phosphorene. The position of CBM which belonged to indirect gap shifts along the direction of the applied strain. We have concluded the change rules of the carrier effective mass when plane strains are applied. In band structure, the sudden decrease of band gap or the new formation of CBM (VBM) causes the unexpected change in carrier effective mass. The effects of zigzag and armchair strain on the effective electron mass in phosphorene are different. The strain along zigzag direction has effects on the electrons effective mass along both zigzag and armchair direction. By contrast, armchair-direction strain seems to affect only on the free electron mass along zigzag direction. For the holes, the effective masses along zigzag direction are largely affected by plane strains while the effective mass along armchair direction exhibits independence in strain processing. The carrier density of monolayer phosphorene at 300 K is calculated about [Formula: see text] cm-2, which is greatly influenced by the temperature and strain. Strain engineering is an efficient method to improve the carrier density in phosphorene.

Two-dimensional phosphorus carbide as a promising anode material for lithium-ion batteries

Monolayer two-dimensional phosphorus carbide (γ-PC) has been intensively studied as a promising anode material for lithium-ion batteries with first-principles calculations. The results show that the monolayer γ-PC semiconductor with a direct band gap of 2.653 eV possesses excellent structural stability and deformation-resistant ability. Besides, monolayer γ-PC is also predicted to show ultrahigh conductivity. Each adsorbed Li atom binds strongly with three neighboring phosphorus atoms. The average adsorption energy of Li atoms reaches up to −1.2 eV, which is beneficial for the charging and the discharging process. Additionally, the diffusion barrier of Li can be as low as 77 meV, which is likely to greatly improve the performance of batteries. We further demonstrate that the theoretical specific capacity of monolayer γ-PC is high, up to 623.3 mA h g−1, which makes it a promising anode material for lithium-ion batteries.

Novel elastic, lattice dynamics and thermodynamic properties of metallic single-layer transition metal phosphides: 2H-M 2 P (Mo 2 P, W 2 P, Nb 2 P and Ta 2 P)

Recently, there has been a surge of interest in the research of two-dimensional (2D) phosphides due to their unique physical properties and wide applications. Transition metal phosphides 2H-M 2Ps (Mo2P, W2P, Nb2P and Ta2P) show considerable catalytic activity and energy storage potential. However, the electronic structure and mechanical properties of 2D 2H-M 2Ps are still unrevealed. Here, first-principles calculations are employed to investigate the lattice dynamics, elasticity and thermodynamic properties of 2H-M 2Ps. Results show that M 2Ps with lower stiffness exhibit remarkable lateral deformation under unidirectional loads. Due to the largest average Grüneisen parameter, single-layer Nb2P has the strongest anharmonic vibrations, resulting in the highest thermal expansion coefficient. The lattice thermal conductivities of Ta2P, W2P and Nb2P contradict classical theory, which would predict a smaller thermal conductivity due to the much heavier atom mass. Moreover, the calculations also demonstrate that the thermal conductivity of Ta2P is the highest as well as the lowest thermal expansion, owing to its weak anharmonic phonon scattering and the lowest average Grüneisen parameter. The insight provided by this study may be useful for future experimental and theoretical studies concerning 2D transition metal phosphide materials.

Effect of oxygen vacancies on Li-storage of anatase \(\hbox {TiO}_{2}\) (001) facets: a first principles study

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Molecular dynamics study on the relaxation properties of bilayered graphene with defects

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