#blue phosphorus

Synthesis of pure single layer of blue phosphorus could be useful for semiconduction

NUS chemists have developed a method to synthesize monolayer blue phosphorus for potential semiconductor applications.

The ability to design and produce materials synthetically could complement the library of naturally occurring layered solids and provide opportunities to develop new materials with unique properties suited for specialized applications. The class of two-dimensional (2-D) elemental materials is particularly interesting as they represent the chemically simplest case for materials development, and serve as a model system for exploring on-surface synthesis mechanisms. One such 2-D materials is blue phosphorus, which has been calculated to possess a large band gap suitable for use in optoelectronic devices.

A research team led by Prof Chen Wei from the Department of Chemistry and Department of Physics, NUS has demonstrated experimentally that an atomically-thin layer of pure blue phosphorus can be obtained from silicon intercalation of a blue phosphorus-gold (BlueP-Au) alloy. A single layer of BlueP-Au alloy was prepared by depositing bulk black phosphorus precursors onto a heated gold (111) surface. Silicon molecules were subsequently evaporated from a heated silicon rod. These silicon atoms inserted themselves below the BlueP-Au alloy and formed a gold silicide buffer layer, breaking the molecular bonds between the phosphorus and gold atoms, thereby leading to the formation of a pure single layer of blue phosphorus on the surface.

Blue phosphorus mapped and measured for the first time

The element phosphorus can exist in various allotropes and changes its properties with each new form. So far, red, violet, white and black phosphorus have been known. While some phosphorus compounds are essential for life, white phosphorus is poisonous and inflammable and black phosphorus -- on the contrary -- particularly robust. Now, another allotrope has been identified: In 2014, a team from Michigan State University, USA, performed model calculations to predict that "blue phosphorus" should be also stable. In this form, the phosphorus atoms arrange in a honeycomb structure similar to graphene, however, not completely flat but regularly "buckled." Model calculations showed that blue phosphorus is not a narrow gap semiconductor like black phosphorus in the bulk but possesses the properties of a semiconductor with a rather large band gap of 2 electron volts. This large gap, which is seven times larger than in bulk black phosphorus, is important for optoelectronic applications.

Blue P examined at BESSY II

In 2016, blue phosphorus was successfully stabilized on a gold substrate by evaporation. Nevertheless, only now we know for certain that the resulting material is indeed blue phosphorus. To this end, a team from HZB around Evangelos Golias has probed the electronic band structure of the material at BESSY II. They were able to measure by angle-resolved photoelectron spectroscopy the distribution of electrons in its valence band, setting the lower limit for the band gap of blue phosphorus.