Temperature-dependent optical phonon shifts and splitting in cubic 10BP, natBP, and 11BP crystals

Ultra-Hard (41 GPa) Isotopic Pure 10BP Semiconductor Microwires for Flexible Photodetection and Pressure Sensing

An urgent demand for electronic and optoelectronic devices able to work in extreme environments promotes a series of research studies on semiconductor materials. Cubic boron phosphide (BP) as a semiconductor material with excellent characteristics shows great application potential. However, since the synthesis conditions required are difficult to achieve and the growth mechanism of BP is still unclear, there are few reports on the basic properties of BP and pure isotope BP, resulting in a narrow understanding of their special physical properties. Here, we successfully obtained highly pure isotopic ¹⁰BP crystals by a vapor-liquid-solid (VLS) method unconventionally designed, which successfully overcomes the thermodynamic conflict between the high melting point of the boron element and low sublimation temperature of the phosphorus element. The ¹⁰BP achieved owns an aspect ratio as high as 10⁴ and a hardness up to 41 GPa. Besides, as an indirect bandgap semiconductor, it has ultrawide red emission spectra, a p-type conductivity with extremely low resistivity, and excellent photoelectronic and piezoelectric characteristics. Furthermore, compared with other superhard semiconductors like cubic BN and diamond, ¹⁰BP has an obvious advantage of lower growth temperature (1200 °C). All these characteristics confirm the prospects owned by ¹⁰BP in its applications to the field of high-conductivity, optoelectronic, strain-sensing, and superhard semiconductors.

Super-hard "Tanghulu": cubic BP microwire covered with amorphous SiO2 balls

Superhard materials, which are widely used in metallurgy, petroleum drilling, and mechanical processing, have become the key to the development of processing and manufacturing industry. Boron phosphide is an excellent Superhard candidate material with excellent inert, high thermostability and heat conductivity. However, since synthesizing BP is a hard task, studies of its basic physical properties and applications are hindered to some extent. Here, we obtained a micron-scale “Tanghulu”, in the process of synthesizing boron phosphide single crystals using high-temperature flux method. Under a special appearance, "Tanghulu" is a superhard BP microwire covered by melted or amorphous SiO₂ and the hardness of the BP microwires is 40.16GPa. On the basis of a comprehensive material analysis, we established the formation mechanism of this Superhard “Tanghulu” as follows: during the heating process with continuous high temperature, SiO₂ molecules on the wall of quartz tube escape and diffuse freely and adhere to the boron phosphide rod-shaped single crystal, which will aggregate then under the effect of surface tension to form an isotropic spherical amorphous SiO₂ and form the “Tanghulu” finally. Our work can help to broaden the understanding of micro-scale materials.