On August 6th, 2020, Professor Huaqiang Cao FRSC FIMMM from the Department of Chemistry at Tsinghua University and his collaborators Associate Professor Dan Xie from the Institute of Microelectronics at Tsinghua University, and Professor Sir Anthony K. Cheetham FRS from the Department of Materials Science and Metallurgy at the University of Cambridge (co-corresponding authors) published a research article entitled "Unzipping of black phosphorus to form zigzag phosphorene nanobelts" in Nature Communications.
In this work, an electrochemical method is used to control the concentration of oxygen molecules to prepare zigzag-phosphorene nanobelts (z-PNBs), as well as nanosheets and quantum dots, via an oxygen-driven mechanism. The mechanism of oxygen-driven directional cutting of phosphorene is revealed through theoretical calculation. Field-effect transistor (FET) devices are constructed based on the prepared z-PNBs to study the carrier transport characteristics.
Phosphorene, monolayer or few-layer black phosphorus, exhibits fascinating anisotropic properties and shows interesting semiconducting behavior. These attractive properties along a zigzag lattice direction are predicted to overstep those of an armchair lattice direction in thermal conductivity, semiconductor behavior, and mechanical strength, leading to the great potential of zigzag-phosphorene nanoribbons for a broad range of applications such as thermoelectric devices, flexible electronics, and quantum information technologies. However, due to the stability and limit of existing synthesis technology, the effective preparation of phosphorene nanobelts has become a key bottleneck in its research and application.
Inspired by the fact that black phosphorus (BP) could be oxidized and decomposed in the air, the team designed an electrochemical method to effectively adjust the ion intercalation rate and oxygen concentration around the BP by changing the current density, so as to control the dimension and size of phosphorene nanostructures, obtaining a series of phosphorene nanostructures, including nanosheets, nanobelts and quantum dots (Fig. 1). The structure characterizations show that the z-PNBs have good crystallinity and flexibility.
Fig. 1 Characterizations of z-PNBs.
Fig. 2 Mechanism of preparation of z-PNBs from bulk BP.
According to the electrochemical dissociation mechanism, the preparation process is divided into two steps, i.e., ionic intercalation and oxygen degradation (Fig. 2). In the electrochemical process, BF4- ions intercalate into the BP crystal layers along the a-axis direction of BP (i.e., [100] direction of BP , zigzag direction). At the same time, oxygen molecules are chemically adsorbed and dissociated to form dangling oxygen on the surface of BP. The hydrogen bond is also formed between dangling oxygen and H2O and the hydrolysis of P-O-P leads to the breaking of the P-P bond to form z-PNBs. The various adsorption and dissociation pathways of oxygen molecules on the phosphorene are analyzed and compared by theoretical calculation (Fig. 3). The results show that the formation of interstitial oxygen pairs is the key step to dissociate the P-P bond and finally form z-PNBs.
Fig. 3 Reaction mechanism of the oxygen-driven unzipping BP processes.
The team also designed and fabricated FET devices based on z-PNBs via a home-made Cu-grid mask method, and explored carrier the transport characteristics of z-PNBs (Fig. 4). The conversion between p-n types of devices was also realized. This work provides key materials and opens up new research methods for the application of phosphorene nanobelts in active matrix display technology, radio frequency devices and complementary metal oxide semiconductor device technology.
Fig. 4 Characterization of individual z-PNB devices.
Zhifang Liu, PhD candidate under the supervision of Professor Cao, and Yilin Sun, PhD student candidate under the supervision of Associate Professor Xie are the co-first authors. Professor Wei Li from the Center of Rare Earth and Inorganic Functional Materials, the National Institute for Advanced Materials at Nankai University and Associate Professor Jiaou Wang from the Institute of High Energy Physics at the Chinese Academy of Sciences also participated in the research. This work is supported by the National Key R & D Program of China and the National Natural Science Foundation of China.
Article link: https://www.nature.com/articles/s41467-020-17622-6
