Exotic ST12-Gemannium - Dr. Duckyong Kim
Shanghai Jan. 3, 2017 — Scientists at Geophysical Lab, Yanshan University and HPSTAR combined experimental and theoretical methods to probe the exotic properties of one special form-ST12-Ge of the element germanium. The results solve the long-standing debate on the optical and electronic properties of semiconducting Ge and suggest ST12-Ge to be a better material for infrared detection and imaging instead of single-junction solar absorbing.
Germanium as a frequently studied element in IVa element group, has the applications for next-generation computer architecture as well as implications for fundamental condensed matter physics. Germanium is used in fiber-optic systems, specialized camera and microscope lenses, circuitry, and solar cells. It is also an attractive anode material for lithium-ion batteries because it has a large theoretical charge-discharge capacity compared to graphite as well as high lithium ion diffusivity at room temperature compared to silicon.
ST12 phase for Germanium comes from its simple tetragonal structure containing 12 atoms per unit cell, resulting from slow decompression process. ST12-Ge was first observed about six decades ago, attracting both theoretical and experimental studies as a great solar absorber candidate. However, great controversies exist about its electronic structure and thermal stability due to lack of bulk pure sample.
New research published in Nature Communications (doi:10.1038/ncomms13909) from an international team Geophysical Laboratory, Yanshan Universtiy and HPSTAR, of scientists describe their work probing intrinsic semiconductor properties of bulk (~2 mm in diameter) ST12-Ge, a metastable phase of element gemanium.
“Bulk pure sample will allow for more accurate and concise electrical and optical measurements,” said Dr. Zhao, 啊lead author of the study. “However, the intrinsic properties have never been realized in ST12-Ge because the samples obtained in previous studies were either impure or too small to probe their practical performance.”
To synthesize bulk ST12-Ge, the researchers utilized a large volume high-pressure facility called multi-anvil press. To get ST12-Ge, they slowly decompressed the sample from 14 gigapascals at room temperature according to kinetic origins. ~15 mg (~2mm in diameter and length) recovered pellets were then polished for the following serials of measurements.
Bulk ST12-Ge shows both very small so-called "indirect electronic band gap", meaning that light is easily absorbed or emitted. and "direct band gap" meaning that it is much more difficult to absorb or emit light. “The smaller band gaps suggest ST12-Ge could be a good candidate for infrared detection, imaging and high-frequency and low-voltage electronic applications.” said Dr. Zhao. “Our work solves the long-standing debate regarding the band structure of ST12-Ge,”
“Future research is required to understand its structure conversion pathways, but the result of this study, experimental and theoretical, pinpoint a clear picture of the intrinsic properties of ST12-Ge, added Dr. Kim, a co-author of the study who carry out the theoretical calculation of the work and also a staff scientist of HPSTAR.
Caption: A schematic diagram of the valence and conduction bands. It is worth noting that the fundamental indirect gap is not along a high-symmetry direction.