The rising star of 2D black phosphorus (phosphorene): Past, present and future prospects

(Nanowerk Spotlight) Since the ground-breaking discovery of two-dimensional (2D) black phosphorus (phosphorene), it has created global research interest and triggered ripples of excitement in the scientific community due to its intriguing optical, mechanical and electronic properties.
Researchers Prof. Neng Li (Wuhan University of Technology, China) and Dr. Wee-Jun Ong (Institute of Materials Research and Engineering, Agency for Science, Technology and Research, Singapore) have looked into the state-of-the-art development of phosphorene, including its structure, preparation routes, anisotropic properties, device applications as well as the bottlenecks encountered by the research community.
The work was published in 2D Materials ("The rising star of 2D black phosphorus beyond graphene: Synthesis, properties and electronic applications") in conjunction with the Focus Collection “Phosphorene and Related 2D Materials for Electronics and Optics”.
"By and large, layered black phosphorus displays a wrinkled structure with each P atom linked with three nearest neighbor atoms (see Figure below)," said Prof. Li. "The unique structure’s anisotropy contributes to anisotropic mechanical, thermal conduction, carrier transport and optical detection properties," he revealed.
Figure. (a) Schematic of 2D black phosphorus structures with a folded structure. (b), (c) Schematic of the monolayer blue phosphorus and black phosphorus on X-Y plane. (d) Diagram of bond angles (θ1, θ2) and bond lengths (R1, R2) of the black phosphorus structure.
"Along the Z axis, the layer-by-layer is combined through the van der Waals forces with large deformation occurred under pressure," said Pengfei Chen, a doctoral student who is the first author of the paper.
When the pressure is low, both X axis and Z axis show similar compressive properties. As indicated by a number of literature studies, the orientation of black phosphorus can be observed not only by the high resolution electron microscopy, but also by the anisotropic carriers transport. It is interesting to know that applied stress leads to a change in the conduction direction, which is effective to the mobility of electrons. Therefore, this provides an important basis for the application in mechanical devices.
"So far, phosphorene has attracted keen interests for applications in field effect transistors, optoelectronics, vapor sensors, and battery devices," elaborated Prof. Ong. "It is interesting to know that humidity sensors make great use of the instability nature of phosphorene, which even proves to display a better performance than other existing 2D materials."
The phosphorene material was relatively more sensitive to NO and NO2, which gives a strong basis to its application in nitrogen-based gas induction. As the gas molecules combine to the phosphorene surface, they play the role of donors or acceptors, leading to a change in the electronic properties.
Flexible black phosphorus field effect transistor was first obtained in 2015 when Zhu and co-workers encapsulated transistors on flexible polyimide and covered a layer of Al2O3. As such, this has increased the stability of black phosphorus in the air.
"Apart from field effect transistor, the predominant part of the optoelectronic devices is transferring incident light energy into electric signals," explained Prof. Ong. "Up to now, there are three primary mechanisms, including the photovoltaic, photo-thermoelectric, and bolometric effects," he added.
For the application in batteries, phosphorene shows high electrical conductivity. However, the volume of phosphorene significantly changes (about 300%) upon lithiation, causing the loss of electrical contact. In light of that, 2D phosphorene/graphene hybrids are developed to combine the advantages of both materials. Balancing the capacity and conductivity becomes a necessary role since the electrical contact dramatically improves with increasing amounts of graphene in the hybrid system.
"This material is unstable, and the instability mechanism is not fully understood at present," said Prof. Ong.
Thus far, scientists have isolated black phosphorus from the atmospheric environment by means of adding protective layers. This includes: (1) by employing Al2O3 or graphene as a protective layer; (2) by adopting sandwich encapsulation using boron nitride; and (3) by forming an oxidation layer. Extensive work on this area is imperative to expand the fundamental knowledge of this cutting-edge research platform.
Looking forward, there are quite a few obstacles and opportunities for the scientists to tackle. Prof. N. Li, Dr. W.-J. Ong and their teams will continue their research of this promising material from the perspective of structural design via computational and experimental studies for the application in the fields of energy, catalysis, artificial photosynthesis, electrochemistry and photochemistry.
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