Black phosphorus: thickness-dependent structural stability and anisotropy
In recent years, two-dimensional nanomaterials have received widespread attention. Graphene, as the first generation of two-dimensional layered materials, has been widely used in various fields. However, the zero band gap and semi-metallic properties greatly limit its further applications. Although two-dimensional transition metal dichalcogenides (TMDs) overcome these shortcomings, their larger band gaps and lower charge transport capabilities bring new problems to their applications. In recent years, another two-dimensional nanomaterial, black phosphorus (BP), has gradually attracted people’s attention. Its band gap properties not only fill the gap between graphene and TMDs materials, but also maintain high charge mobility, large photon action and photoelectron yield. In particular, the unique wrinkled atomic arrangement of BP leads to its unique anisotropic physical and chemical properties. These characteristics of BP make it an indispensable and important material in the fields of optoelectronics, catalysis and other fields.
Thickness is an important factor affecting the properties of two-dimensional materials. Many properties of BP (band gap value, band arrangement, effective charge mass, photo-assisted oxidation properties, etc.) also depend on changes in its thickness. In fact, the essence of the thickness-dependent property changes of BP is still the regulation of its structure. The distance between adjacent BP layers and the slight changes in the arrangement of BP atoms within the layers will greatly change the electronic coupling and electron arrangement between the layers, causing corresponding electrical and optical changes. External pressure is an effective means to change the distance between BP atoms and the arrangement of atoms.
Left: The pressure-thickness structural phase diagram of BP; middle and right: the unit cell volume and compression coefficient of two types of BP with a thickness of 171 nm and 6 nm under high pressure.
Recently, Quan Zewei’s research group at Southern University of Science and Technology selected BP materials with thicknesses between 71 nm and 6 nm to study thickness-dependent structural and property changes under high pressure. Through high-pressure in-situ synchrotron radiation X-ray diffraction and Raman experiments, it was found that the phase transition sequence (A17 phase to A7 phase to SC phase) of BP with different thicknesses under high pressure is exactly the same as its bulk material, but the phase transition pressure point is significantly higher than Bulk materials, this is mainly caused by the enhanced surface energy of nanomaterials. At the same time, the high-pressure structure of BP shows obvious thickness-dependent characteristics (above). Especially between 71 nm and 13 nm, the phase transition pressure increases significantly. This shows that within this thickness range, reducing the thickness of BP will greatly promote the improvement of its structural stability. In addition, through calculation of the unit cell parameter compressibility (Kl), it is found that below 3 GPa, BP exhibits obvious thickness-dependent anisotropic structural compression, and the reduction in thickness brings about a more rigid interlayer structure.
This work is a study of the thickness-dependent structural properties of BP systems under high pressure. By summarizing the thickness-dependent structural phase diagram and anisotropic compressibility of BP materials, the thickness-pressure-structure of BP are interconnected, which provides new information and strategies for the controllable design and fabrication of BP devices. Related work was published online in Advanced Electronic Materials (DOI: 10.1002/aelm.201800712).
A review of the optical and optoelectronic properties and applications of black phosphorus
Recently, Associate Researcher Li Jia and Researcher Yu Xuefeng from the Materials Interface Research Center of the Shenzhen Institute of Advanced Technology, Chinese Academy of Sciences jointly published a paper titled “Optical and Optoelectronic Properties of Black Phosphorus and Recent Photonics” in Small Methods, a well-known publication in the field of materials science. and Optoelectronic Applications” (DOI: 10.1002/smtd.201900165), which summarizes the material properties exploration and device application research of black phosphorus in the fields of photonics and optoelectronics in recent years.
Two-dimensional black phosphorus is a new semiconductor material that has stood out and received widespread attention in recent years. With its excellent photonics and optoelectronics material properties, black phosphorus shows excellent potential in various photonic and optoelectronic device applications. First, black phosphorus has a flexible and tunable direct band gap, which provides optoelectronic devices with a wide spectrum and efficient photoelectric response spanning from the cocoline dye to the mid-infrared. Secondly, considering the two key properties of mobility and switching ratio of transistor devices, black phosphorus fills the gap between graphene and transition metal dichalcogenides, providing ideal balanced performance for transistor devices. Not only that, the in-plane anisotropy of black phosphorus has a great impact on the electrical, optical and mechanical properties of the material, thus providing an excellent platform for in-depth exploration of novel basic principles of optoelectronics and the realization of new functional applications of optoelectronic devices. Therefore, in recent years, basic research and application development on the photonics and optoelectronics properties of black phosphorus have become a hot topic for relevant scholars.
This review article first introduces the research history and material structure characteristics of black phosphorus. Based on this, the basic optical properties of two-dimensional black phosphorus include light absorption, photoluminescence, photocurrent, third harmonic, and photogenerated carrier. The research on flow dynamics and electro-optic modulation is comprehensively summarized and introduced, and the novel photonics and optoelectronics properties and applications brought by the in-plane anisotropy of two-dimensional black phosphorus are discussed. For various types of photonics related to black phosphorus, and optoelectronic device applications are given a complete overview. The authors of this article also point out the prospects and challenges of future two-dimensional black phosphorus photovoltaic applications based on their understanding of this field.
