A schematic representation of hexagonal boratomer networks (pink) found on the hexagonal nodes and periodically in the middle of the…
A schematic representation of hexagonal boratomer networks (pink) found on the hexagonal nodes and periodically in the middle of the hexagon grown on a surface of copper atoms (brown). The researchers used a low-energy microscope (LEEM) to look at “islets” of borofene (yellow triangle in the left-hand circle) grow, change the temperature, deposition rate and other real-time growth conditions to refine the “recipe“. The islands can sit on the surface in six different orientations and can be discriminated by choosing an electron diffraction point (as it circulates in yellow) corresponding to a certain orientation (that is connected to the dashed line). Eventually, the islands grow to such an extent that they touch and meet, and the entire surface (one centimeter squared) is covered with borofene, seen in the circle to the right. The colors were added to distinguish regions with different orientations. Credit: Brookhaven National Laboratory
Borofen two-dimensional (2-D) atomic thin drill bit, a chemical element traditionally found in glass fiber insulation ̵
1; is anything but boring. Although boring is a non-metallic semiconductor in its bulk form (3-D), it becomes a metal conductor in 2-D. Borofen is extremely flexible, strong and lightweight – more than its carbon-based analog, the graph. These unique electronic and mechanical features make borofene a promising material platform for next-generation electronic devices, such as laptops, biomolecular sensors, light sensors and quantum computers.
Now, physicians from the US Department of Energy (DOE) Brookhaven National Laboratory and Yale University have synthesized borofene on large-area copper substrates (varying in size from 10 to 100 micrometer) single crystal domains (for reference, a string of human hair is about 100 microns wide). Previously, only single-colored single crystal flakes of borophene had been produced. The advance, reported on December 3 in Nature Nanotechnology represents an important step in making practical boron-based devices possible.
For electronic applications, high quality single crystal-periodic arrangements of atoms that continue throughout the crystal grid without boundaries or defects – must be distributed over large surfaces of the substrate (substrate) on which they are grown. For example, today’s microchips use single crystals of silicon and other semiconductors. Unit manufacturing also requires an understanding of how different substrates and growth conditions affect the crystal structure of the material, which determines its properties.
“We increased the size of the single-crystal domains by one million factor,” said co-author and project manager Ivan Bozovic, senior researcher and Molecular Beam Epitaxy Group Leader in Brookhaven Lab’s Condensed Materials Physics and Materials Science (CMPMS) Department and Adjunct Professor of Applied Physics at Yale University. “Large domains are required to manufacture next generation electronic devices with high electron mobility. Electrons that move easily and quickly through a crystal structure are crucial for improving device performance.”
Brookhaven Lab researcher Percy Zahl (left), Ivan Bozovic (center) and Ilya Drozdov at the Center for Functional Nanomaterials. Here they used a custom-built scanning tunnel microscope to form the surface structure of two-dimensional atomic-thin sheets of copper. Credit: Brookhaven National Laboratory
A new 2-D material
Since 2004 the discovery of the graph-single-sheet carbon atoms, which can be peeled off graphite, the core component of pencils, with Scotch tape scientists, has been in search of other 2 -D material with remarkable properties. The chemical bonds between carbon atoms that give the graph of its strength make the structure difficult to manipulate.
Theorists predicted living (besides coal on the periodic system with a smaller electron) deposited on a suitably chosen substrate could form a 2-D material similar to the graph. However, this prediction was not confirmed experimentally until three years ago, when researchers synthesized borofene for the first time. The deposited lives on silver substrates under ultra high vacuum conditions by molecular beam epitaxy (MBE), an accurately controlled atomic crystal growth technology. Soon after that, another group of scientists grew borofen on silver, but they proposed a completely different crystal structure.
“Borofen is structurally similar to the graph, with a hexagonal network of boron (instead of carbon) atoms on each of the six vertices that define the hexagon,” said Bozovic. “The boron is, however, different because it periodically has an extra boratom in the middle of the hexagon. The crystal structure tends to be theoretically stable when approximately four of each five center positions are occupied and one is available. “
According to the theory, while the number of vacancies is fixed, their arrangement is not. As long as vacant seats are distributed in a manner that maintains it stable structure (lowest energy), they can be reorganized. Because of this flexibility, the boron can have multiple configurations.
A film of the boron islands grows in real time, obtained by low energy electron microscopy. Credit: Brookhaven National Laboratory
A small step toward manufacturing of appliances
In this study, researchers first examined the time for real-time growth of borofene on silver surfaces at different temperatures. They grew the samples at Yale in an ultra high vacuum electronics microscope (LEEM) with an MBE system. During and after the growth process, they bombarded the test with a low energy electron beam and analyzed low energy electron diffraction patterns (MFD) produced as electrons reflected from the crystal surface and projected on a detector. Because the electrons have low energy, they can only reach the first atomic layers of the material. The distance between the reflected electrons (“spots” in the diffraction patterns) is related to the distance between atoms on the surface and from this information, researchers can reconstruct the crystal structure.
In this case, the patterns revealed that Borofene domains with a crystal were only tens of nanometers in size – too small for manufacturing devices and studied basic physical properties – for all growth ratios. They also determined the controversy about boron conjugation: both structures exist, but they form at different temperatures. The researchers confirmed their LEEM and LEED results through nuclear power microscopy (AFM). In AFM, a sharp tip is scanned over a surface and the measured force between the tip and the atoms on the surface is used to map the atomic arrangement.
In order to promote the formation of larger crystals, the researchers changed the substrate from silver to copper, using the same LEEM, LEED and AFM techniques. Brookhaven researchers Percy Zahl and Ilya Drozdov also depicted the surface structure at high resolution using a tailored scanning tunnel microscope (STM) with a carbon monoxide probe at the Brookhaven Center for Functional Nanomaterials (CFN) -a US Department of Energy (DOE) Office of Science User Facility. Yale Theorists Stephen Eltinge and Sohrab Ismail-Beigi performed calculations to determine the stability of the experimental structures. After identifying which structures were most stable, they simulated electron diffraction spectra and STM images and compared them with experimental data. This iterative process continued until theory and experiment were agreed.
“From theoretical insights, we expected copper to produce larger single crystals because it interacts stronger with borofene than silver,” said Bozovic. “Copper donates some electrons to stabilize borofene, but the materials do not interact too much to form a compound. Not only are the simple crystals larger, but the structures of borofene on copper differ from all those grown on silver.”  Because there are several possible breakdowns of vacancies on the surface, different crystal structures of borophene may occur. This study also showed how the boron structure can be modified by changing substrates and, in some cases, the temperature or deposition rate.
The next step is to transfer the boron sheet from the metallic copper surfaces to insulating device-compatible substrates. Thereafter, researchers will be able to accurately measure resistivity and other electrical properties that are important for device functionality. Bozovic is particularly excited to test if borofene can be manufactured superconductively. Some theorists have speculated that its unusual electronic structure can even open a path to lossless transmission of electricity at room temperature, as opposed to the ultrasound temperatures usually required for superconductivity. In the end, the goal of 2-D material research is to fine-tune the properties of these materials to suit special applications.
Boron can form a pure honeycomb graphite-like 2-D structure
Rongting Wu et al., Large-area single-crystal plates borofene on Cu (111) surfaces, Nature Nanotechnology (2018). DOI: 10,1038 / s41565-018-0317-6