Special Collection: 无机二维纳米材料

无机二维纳米材料，通常指在一个维度上受限同时具有原子尺度厚度超薄特性的新兴纳米材料。无机二维纳米材料，拥有更丰富的元素组成、更多结构构型，带来了更多可期待的化学反应性、可调节的物性以及更有趣的能量转化和存储应用。超薄二维纳米材料，已经成为诸多基础科学问题、纳米表征以及能源应用的重要基础。近期FOP刊二维纳米材料专题（客座编辑：中国科技大学吴长征&武晓君） 总览了无机二维纳米材料的最新进展，包括计算模拟与设计、机制理解、实验合成以及能源相关应用。

Special Collection: Inorganic Two-Dimensional Nanomaterials

Eds. Changzheng Wu & Xiaojun Wu 

REVIEW

Recent progress on borophene: Growth and structures 

Longjuan Kong, Kehui Wu, Lan Chen*

Boron is the neighbor of carbon on the periodic table and exhibits unusual physical characteristics derived from electron-deficient, highly delocalized covalent bonds. As the nearest neighbor of carbon, boron is in many ways similar to carbon, such as having a short covalent radius and the flexibility to adopt sp2 hybridization. Hence, boron could be capable of forming monolayer structural analogues of graphene. Although many theoretical papers have reported finding two-dimensional allotropes of boron, there had been no experimental evidence for such atom-thin boron nanostructures until 2016. Recently, the successful synthesis of single-layer boron (referred to as borophene) on the Ag(111) substrate opens the era of boron nanostructures. In this brief review, we will discuss the progress that has been made on borophene in terms of synthetic techniques, characterizations and the atomic models. However, borophene is just in infancy; more efforts are expected to be made in future on the controlled synthesis of quality samples and tailoring its physical properties.

Strategies for discovery and optimization of thermoelectric materials: Role of real objects and local fields 

Hao Zhu, Chong Xiao*

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.

RESEARCH

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*

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.

Penta-P₂X (X=C, Si) monolayers as wide-bandgap semiconductors: A first principles prediction 

Mosayeb Naseri, Shiru Lin, Jaafar Jalilian, Jinxing Gu, Zhongfang Chen*

By means of density functional theory computations, we predicted two novel two-dimensional (2D) nanomaterials, namely P₂X (X=C, Si) monolayers with pentagonal configurations. Their structures, stabilities, intrinsic electronic, and optical properties as well as the effect of external strain to the electronic properties have been systematically examined. Our computations showed that these P₂C and P₂Si monolayers have rather high thermodynamic, kinetic, and thermal stabilities, and are indirect semiconductors with wide bandgaps (2.76 eV and 2.69 eV, respectively) which can be tuned by an external strain. These monolayers exhibit high absorptions in the UV region, but behave as almost transparent layers for visible light in the electromagnetic spectrum. Their high stabilities and exceptional electronic and optical properties suggest them as promising candidates for future applications in UV-light shielding and antireflection layers in solar cells.

Interfacial properties of black phosphorus/transition metal carbide van der Waals heterostructures 

Hao Yuan, Zhenyu Li*

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 (Zr_n+1C_nT₂, T= O, F, OH, n = 1, 2) van der Waals (vdW) heterostructures are designed via first-principles calculations. Zr_n+1C_nT₂ 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/Zr₂CO₂can 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.

SiP monolayers: New 2D structures of group IV-V compounds for visible-light photohydrolytic catalysts

Zhinan Ma, Jibin Zhuang, Xu Zhang, Zhen Zhou*

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.

Epitaxial growth of highly strained antimonene on Ag(111) 

Ya-Hui Mao, Li-Fu Zhang, Hui-Li Wang, Huan Shan, Xiao-Fang Zhai, Zhen-Peng Hu*, Ai-Di Zhao*, Bing Wang

The synthesis of antimonene, which is a promising group-V 2D material for both fundamental studies and technological applications, remains highly challenging. Thus far, it has been synthesized only by exfoliation or growth on a few substrates. In this study, we show that thin layers of antimonene can be grown on Ag(111) by molecular beam epitaxy. High-resolution scanning tunneling microscopy combined with theoretical calculations revealed that the submonolayer Sb deposited on a Ag(111) surface forms a layer of AgSb₂ surface alloy upon annealing. Further deposition of Sb on the AgSb₂surface alloy causes an epitaxial layer of Sb to form, which is identified as antimonene with a buckled honeycomb structure. More interestingly, the lattice constant of the epitaxial antimonene (5 Å) is much larger than that of freestanding antimonene, indicating a high tensile strain of more than 20%. This kind of large strain is expected to make the antimonene a highly promising candidate for roomtemperature quantum spin Hall material.

Dissociation of liquid water on defective rutile TiO₂ (110) surfaces using ab initio molecular dynamics simulations 

Hui-Li Wang, Zhen-Peng Hu, Hui Li*

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 (V_o1) 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 V_o1 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.

Computational study on the half-metallicity in transition metal–oxide-incorporated 2D g-C₃N₄nanosheets 

Qian Gao, Hui-Li Wang, Li-Fu Zhang, Shuang-Lin Hu*, Zhen-Peng Hu*

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 (T_c) 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-C₃N₄nanosheet. 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.

Two-dimensional aluminum monoxide nanosheets: A computational study 

Shiru Lin, Yanchao Wang, Zhongfang Chen*

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.

Self-folding mechanics of graphene tearing and peeling from a substrate 

Ze-Zhou He, Yin-Bo Zhu, Heng-An Wu*

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.