Frontiers of Physics


ISSN 2095-0462 (Print)
ISSN 2095-0470 (Online)
CN 11-5994/O4
Postal Subscription Code 80-965
2019 Impact Factor: 2.502
Inorganic Two-Dimensional Nanomaterials (Eds. Changzheng Wu & Xiaojun Wu)

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Computational study on the half-metallicity in transition metal–oxide-incorporated 2D g-C3N4 nanosheets
Qian Gao (高乾), Hui-Li Wang (王会丽), Li-Fu Zhang (张丽芙), Shuang-Lin Hu (胡双林), Zhen-Peng Hu (胡振芃)
Front. Phys. .  2018, 13 (3): 138108.
Abstract   PDF (4189KB)

In this study, based on the first-principles calculations, we systematically investigated the electronic and magnetic properties of the transition metal–oxide-incorporated 2D g-C3N4 nanosheet (labeled C3N4– TM–O, TM= Sc–Mn). The results suggest that the TM–O binds to g-C3N4 nanosheets strongly for all systems. We found that the 2D C3N4–TM–O framework is ferromagnetic for TM= Sc, Ti, V, Cr, while it is antiferromagnetic for TM= Mn. All the ferromagnetic systems exhibit the half-metallic property. Furthermore, Monte Carlo simulations based on the Heisenberg model suggest that the Curie temperatures (Tc) of the C3N4–TM–O (TM= Sc, Ti, V, Cr) framework are 169 K, 68 K, 203 K, and 190 K, respectively. Based on Bader charge analysis, we found that the origin of the half-metallicity at Fermi energy can be partially attributed to the transfer of electrons from TM atoms to the g-C3N4 nanosheet. In addition, we found that not only electrons but also holes can induce half-metallicity for 2D g-C3N4 nanosheets, which may help to understand the origin of half-metallicity for graphitic carbon nitride.

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Cited: Crossref(5) WebOfScience(5)
Ultrathin nanosheets of Mn3O4: A new two-dimensional ferromagnetic material with strong magnetocrystalline anisotropy
Jun-Chi Wu, Xu Peng, Yu-Qiao Guo, Hao-Dong Zhou, Ji-Yin Zhao, Ke-Qin Ruan, Wang-Sheng Chu, Changzheng Wu
Front. Phys. .  2018, 13 (3): 138110.
Abstract   PDF (22798KB)

Two-dimensional (2D) materials with robust ferromagnetism have played a key role in realizing nextgeneration spin-electronic devices, but many challenges remain, especially the lack of intrinsic ferromagnetic behavior in almost all 2D materials. Here, we highlight ultrathin Mn3O4 nanosheets as a new 2D ferromagnetic material with strong magnetocrystalline anisotropy. Magnetic measurements along the in-plane and out-of-plane directions confirm that the out-of-plane direction is the easy axis. The 2D-confined environment and Rashba-type spin-orbit coupling are thought to be responsible for the magnetocrystalline anisotropy. The robust ferromagnetism in 2D Mn3O4 nanosheets with magnetocrystalline anisotropy not only paves a new way for realizing the intrinsic ferromagnetic behavior in 2D materials but also provides a novel candidate for building next-generation spin-electronic devices.

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Cited: Crossref(3) WebOfScience(5)
Self-folding mechanics of graphene tearing and peeling from a substrate
Ze-Zhou He, Yin-Bo Zhu, Heng-An Wu
Front. Phys. .  2018, 13 (3): 138111.
Abstract   PDF (12256KB)

Understanding the underlying mechanism in the tearing and peeling processes of graphene is crucial for the further hierarchical design of origami-like folding and kirigami-like cutting of graphene. However, the complex effects among bending moduli, adhesion, interlayer interaction, and local crystal structure during origami-like folding and kirigami-like cutting remain unclear, resulting in challenges to the practical applications of existing theoretical and experimental findings as well as to potential manipulations of graphene in metamaterials and nanodevices. Toward this end, classical molecular dynamics (MD) simulations are performed with synergetic theoretical analysis to explore the tearing and peeling of self-folded graphene from a substrate driven by external force and by thermal activation. It is found that the elastic energy localized at the small folding ridge plays a significant role in the crack trajectory. Due to the extremely small bending modulus of monolayer graphene, its taper angle when pulled by an external force follows a scaling law distinct from that in case of bilayer graphene. With the increase in the initial width of the folding ridge, the self-folded graphene, motivated by thermal fluctuations, can be self-assembled by spontaneous self-tearing and peeling from a substrate. Simultaneously, the scaling law between the taper angle and adhesive energy is independent of the motivations for thermal activation-induced self-assembly and external force tearing, providing effective insights into the underlying physics for graphene-based origami-like folding and kirigami-like cutting.

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Cited: Crossref(9) WebOfScience(10)
Two-dimensional aluminum monoxide nanosheets: A computational study
Shiru Lin, Yanchao Wang, Zhongfang Chen
Front. Phys. .  2018, 13 (3): 138109.
Abstract   PDF (6035KB)

By means of density functional theory (DFT) computations and particle-swarm optimization (PSO) structure searches, we herein predict five low-lying energy structures of two-dimensional (2D) aluminum monoxide (AlO) nanosheets. Their high cohesive energy, absence of imaginary phonon dispersion, and good thermal stability make them feasible targets for experimental realization. These monolayers exhibit diverse structural topologies, for instance, PmA- and Pmm-AlO possess buckled four- and sixmembered AlO rings, whereas P62-, PmB-, and P6m-AlO have pores of varied sizes. Interestingly, the most energetically preferred monolayers, PmA- and Pmm-AlO, feature wide band gaps (2.45 and 5.13 eV, respectively), which are promising for green and blue light-emitting devices (LEDs) and photodetectors.

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Cited: Crossref(3) WebOfScience(3)
Dissociation of liquid water on defective rutile TiO2 (110) surfaces using ab initio molecular dynamics simulations
Hui-Li Wang (王会丽), Zhen-Peng Hu (胡振芃), Hui Li (李晖)
Front. Phys. .  2018, 13 (3): 138107.
Abstract   PDF (26486KB)

In order to obtain a comprehensive understanding of both thermodynamics and kinetics of water dissociation on TiO2, the reactions between liquid water and perfect and defective rutile TiO2 (110) surfaces were investigated using ab initio molecular dynamics simulations. The results showed that the free-energy barrier (~4.4 kcal/mol) is too high for a spontaneous dissociation of water on the perfect rutile (110) surface at a low temperature. The most stable oxygen vacancy (Vo1) on the rutile (110) surface cannot promote the dissociation of water, while other unstable oxygen vacancies can significantly enhance the water dissociation rate. This is opposite to the general understanding that Vo1 defects are active sites for water dissociation. Furthermore, we reveal that water dissociation is an exothermic reaction, which demonstrates that the dissociated state of the adsorbed water is thermodynamically favorable for both perfect and defective rutile (110) surfaces. The dissociation adsorption of water can also increase the hydrophilicity of TiO2.

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Strategies for discovery and optimization of thermoelectric materials: Role of real objects and local fields
Hao Zhu, Chong Xiao
Front. Phys. .  2018, 13 (3): 138112.
Abstract   PDF (1232KB)

Thermoelectric materials provide a renewable and eco-friendly solution to mitigate energy shortages and to reduce environmental pollution via direct heat-to-electricity conversion. Discovery of the novel thermoelectric materials and optimization of the state-of-the-art material systems lie at the core of the thermoelectric society, the basic concept behind these being comprehension and manipulation of the physical principles and transport properties regarding thermoelectric materials. In this mini-review, certain examples for designing high-performance bulk thermoelectric materials are presented from the perspectives of both real objects and local fields. The highlights of this topic involve the Rashba effect, Peierls distortion, local magnetic field, and local stress field, which cover several aspects in the field of thermoelectric research. We conclude with an overview of future developments in thermoelectricity.

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Cited: Crossref(2) WebOfScience(2)
SiP monolayers: New 2D structures of group IV-V compounds for visible-light photohydrolytic catalysts
Zhinan Ma (马志楠), Jibin Zhuang (庄吉彬), Xu Zhang (张旭), Zhen Zhou (周震)
Front. Phys. .  2018, 13 (3): 138104.
Abstract   PDF (3718KB)

Because of graphene and phosphorene, two-dimensional (2D) layered materials of group IV and group V elements arouse great interest. However, group IV–V monolayers have not received due attention. In this work, three types of SiP monolayers were computationally designed to explore their electronic structure and optical properties. Computations confirm the stability of these monolayers, which are all indirect-bandgap semiconductors with bandgaps in the range 1.38–2.21 eV. The bandgaps straddle the redox potentials of water at pH= 0, indicating the potential of the monolayers for use as watersplitting photocatalysts. The computed optical properties demonstrate that certain monolayers of SiP 2D materials are absorbers of visible light and would serve as good candidates for optoelectronic devices.

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Interfacial properties of black phosphorus/transition metal carbide van der Waals heterostructures
Hao Yuan, Zhenyu Li
Front. Phys. .  2018, 13 (3): 138103.
Abstract   PDF (11277KB)

Owing to its outstanding electronic properties, black phosphorus (BP) is considered as a promising material for next-generation optoelectronic devices. In this work, devices based on BP/MXene (Zrn+1CnT2, T= O, F, OH, n = 1, 2) van der Waals (vdW) heterostructures are designed via first-principles calculations. Zrn+1CnT2 compositions with appropriate work functions lead to the formation of Ohmic contact with BP in the vertical direction. Low Schottky barriers are found along the lateral direction in BP/Zr2CF2, BP/Zr2CO2H2, BP/Zr3C2F2, and BP/Zr3C2O2H2 bilayers, and BP/Zr3C2O2 even exhibits Ohmic contact behavior. BP/Zr2CO2 is a semiconducting heterostructure with type-II band alignment, which facilitates the separation of electron-hole pairs. The band structure of BP/Zr2CO2 can be effectively tuned via a perpendicular electric field, and BP is predicted to undergo a transition from donor to acceptor at a 0.4 V/Å electric field. The versatile electronic properties of the BP/MXene heterostructures examined in this work highlight their promising potential for applications in electronics.

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Cited: Crossref(11) WebOfScience(11)