The original Carnegie Institution for Science press release can be read here.
Hexagonal forms of silicon have been synthesized previously, but only through the deposition of thin films or as nanocrystals that coexist with disordered material. A team led by Carnegie Institution of Science’s Thomas Shiell and Timothy Strobel, and including colleagues from RMIT University (Australia) and The Australian National University, developed a new method for synthesizing a novel crystalline form of silicon, called 4H-silicon, or 4H-Si. This novel 4H-Si form has a hexagonal structure and could potentially be used to create next-generation electronic and energy devices with enhanced properties that exceed those of the “normal” cubic form of silicon used today. This newly reported synthesis pathway produces the first high-quality, bulk crystals that serve as the basis for future research activities. Their work, based in large part on research carried out at the U.S. Department of Energy’s Advanced Photon Source (APS), was published in Physical Review Letters.
Silicon plays an outsized role in human life. It is the second most abundant element in the Earth’s crust. When mixed with other elements, it is essential for many construction and infrastructure projects. And in pure elemental form, it is crucial enough to computing that the longstanding technological hub of the U.S.—California’s Silicon Valley—was nicknamed in honor of it.
Like all elements, silicon can take different crystalline forms, called allotropes, in the same way that soft graphite and super-hard diamond are both forms of carbon. The form of silicon most commonly used in electronic devices, including computers and solar panels, has the same structure as diamond. Despite its ubiquity, this form of silicon is not actually fully optimized for next-generation applications, including high-performance transistors and some photovoltaic devices. While many different silicon allotropes with enhanced physical properties are theoretically possible, only a handful exist in practice given the lack of known synthetic pathways that are currently accessible. Strobel’s lab had previously developed a revolutionary new form of silicon, called Si24, which has an open framework composed of a series of one-dimensional channels. In this new work, Shiell and Strobel’s team used Si24 as the starting point in a multi-stage synthesis pathway that resulted in highly oriented crystals in a form called 4H-silicon, named for its four repeating layers in a hexagonal structure.
“Interest in hexagonal silicon dates back to the 1960s, because of the possibility of tunable electronic properties, which could enhance performance beyond the cubic form” Strobel explained.
The 4H-Si samples were measured (Fig. 1) using synchrotron x-ray diffraction (XRD) at the HPCAT-XSD 16-ID-B x-ray beamline of the APS, an Office of Science user facility at Argonne National Laboratory. In both cases (XRD and transmission electron microscopy), the high-quality 4H-Si crystals were synthesized using high-pressure laser heating in a diamond anvil cell at HPCAT-XSD, also on the 16-ID-B beamline.
Using the advanced computing tool called PALLAS, which was previously developed by members of the team to predict structural transition pathways—like how water becomes steam when heated or ice when frozen—the group was able to understand the transition mechanism from Si24 to 4H-Si, and the structural relationship that allows the preservation of highly oriented product crystals.
“In addition to expanding our fundamental control over the synthesis of novel structures, the discovery of bulk 4H-silicon crystals opens the door to exciting future research prospects for tuning the optical and electronic properties through strain engineering and elemental substitution,” Shiell said. “We could potentially use this method to create seed crystals to grow large volumes of the 4H structure with properties that potentially exceed those of diamond silicon.”
See: Thomas B. Shiell1*, Li Zhu1, Brenton A. Cook2, Jodie E. Bradby3, Dougal G. McCulloch2, and Timothy A. Strobel1**, “Bulk Crystalline 4H-Silicon through a Metastable Allotropic Transition,” Phys. Rev. Lett. 126, 215701 (2021). DOI: 10.1103/PhysRevLett.126.215701
Author affiliations: 1Carnegie Institution for Science, 2RMIT University, 3The Australian National University
Correspondence: * tshiell@carnegiescience.edu, ** tstrobel@carnegiescience.edu
The authors thank Yue Meng and Piotr Guńka for their assistance with experimental measurements. The authors gratefully acknowledge the RMIT Microscopy and Microanalysis Facility. This work was supported by the National Science Foundation, Division of Materials Research (NSF-DMR) under Grant No. 1809756. HPCAT-XSD operations are supported by the U.S. Department of Energy National Nuclear Security Administration Office of Experimental Sciences. The Advanced Photon Source is a U.S. Department of Energy Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under Contract No. DE-AC02-06CH11357.
Copyright 2020 Carnegie Institution for Science
The U.S. Department of Energy's APS is one of the world’s most productive x-ray light source facilities. Each year, the APS provides high-brightness x-ray beams to a diverse community of more than 5,000 researchers in materials science, chemistry, condensed matter physics, the life and environmental sciences, and applied research. Researchers using the APS produce over 2,000 publications each year detailing impactful discoveries, and solve more vital biological protein structures than users of any other x-ray light source research facility. APS x-rays are ideally suited for explorations of materials and biological structures; elemental distribution; chemical, magnetic, electronic states; and a wide range of technologically important engineering systems from batteries to fuel injector sprays, all of which are the foundations of our nation’s economic, technological, and physical well-being.
Argonne National Laboratory seeks solutions to pressing national problems in science and technology. The nation's first national laboratory, Argonne conducts leading-edge basic and applied scientific research in virtually every scientific discipline. Argonne researchers work closely with researchers from hundreds of companies, universities, and federal, state and municipal agencies to help them solve their specific problems, advance America's scientific leadership and prepare the nation for a better future. With employees from more than 60 nations, Argonne is managed by UChicago Argonne, LLC, for the U.S. DOE Office of Science.
The U.S. Department of Energy's Office of Science is the single largest supporter of basic research in the physical sciences in the United States and is working to address some of the most pressing challenges of our time. For more information, visit the Office of Science website.