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Si2BN - A Graphene Analog

March 17, 2016

The metals available to ancient man were the metallic elements found in their free form in nature, or those extracted by heating of some common ores. Since gold is the least reactive metal, it occurs in nature as the metal, itself, so it was the first metal used by man. Our ancestors soon discovered how smelting of ores can release silver, mercury, copper, lead, tin, and iron.

All of these metals are relatively soft, and the major reason that iron was useful was because the accidental incorporation of carbon transformed it into steel. Eventually, it was discovered that mixing some metals in a proper proportion gave alloys with superior mechanical properties. One example of this is bronze, an alloy of copper and tin.

Bronze is stronger than copper, and it had sufficient properties to launch a new age of man called the Bronze Age. While copper has a tensile strength of 32 ksi, adding a little tin boosts this to about 45 ksi, while addition of just 1% phosphorus to produce phosphor bronze results in an alloy with a tensile strength of about 120 ksi.

Hesiod, Works and Days, lines 150-151
Hesiod, Works and Days, lines 150-151, scanned from the author's copy of ref. 1. The translation is "Their armor was of bronze, and their houses of bronze, and of bronze were their implements: there was no black iron."[1]

As metallurgy achieved a scientific basis, it was discovered that exceptional alloys could be made when several elements were combined; and, why just stop at a few, when a little pinch of one or another element has a beneficial effect. Some modern alloys are made from ten or more elements, each of which having its own particular purpose. One example is MAR-M-247,[2] a Martin Marietta nickel-based superalloy used for the hottest parts of turbine engines, such as turbine blades. This alloy, which resists oxidation and corrosion and has excellent strength at high temperature, has the following composition (mass-%):

 ElementPercentElementPercent
 Nickel59Tantalum3.0
 Tungsten10Titanium1.0
 Cobalt10Molybdenum0.7
 Chromium8.25Iron0.5
 Aluminum5.5Boron0.015

Additionally, a little hafnium or zirconium can be added to enhance ductility.

Research on graphene, the atomic layer thick allotropic form of carbon, accelerated after 2004 when a technique was discovered to cleave graphene layers from graphite and attach them to crystal wafers for measurement. This discovery was significant enough for the originators of this process, Andre Geim and Konstantin Novoselov, to earn the 2010 Nobel Prize in Physics.

Taking a cue from alloy metallurgy, scientists decided that if one type of atom works well as a two-dimensional material, perhaps similar atomic layers incorporating more than one element would have some unique properties. That's why we now have experiments involving molybdenum disulfide (MoS2), molybdenum diselenide (MoSe2), and tungsten diselenide (WSe2). I wrote about some properties of these atomic layer materials in some previous articles (Molybdenum Disulfide Circuitry, December 9, 2011, Quilting Semiconductors, September 26, 2014, and Light-Emitting Diodes of Tungsten Diselenide, March 17, 2014).

Crystal structure of molybdenum disulfideA layer of molybdenum disulfide (MoS2), a close cousin to molybdenum diselenide (MoSe2), and tungsten diselenide. Unlike graphene, it takes three atomic layers to make one layer of the compound, MoS2

(Via arXiv).

These two-dimensional materials are somewhat of a cheat. Unlike graphene, it takes three layers to get one compound layer. A group of scientists from the Institute of Electronic Structure and Laser (IESL, Heraklio, Crete, Greece), Daimler AG (Ulm, Germany), and the University of Kentucky (Lexington, Kentucky) decided that it would be interesting to see what single layer compounds of multiple types of atoms would be theoretically possible. They've found a candidate in Si2BN.[3-5]

This material is a modification of the hexagonal crystal form of boron nitride, (α-BN), which is layered like graphite (see figure). Both the boron and nitrogen atoms of boron nitride are in the plane of the single sheet. However, the material is an insulator, and it's not as useful as graphene.

A single layer of hexagonal_boron_nitride
A single layer of hexagonal boron nitride (α-BN). (Via Wikimedia Commons.)

At this point, the Si2BN material has just been predicted theoretically, it hasn't been made. The research team used an ab initio calculation of the single-atomic-layer Si2BN structure in which all of the atoms connect by with sp2 bonding; and, like graphene, there's no out-of-plane buckling.[3]

This new material has the advantage over the previously mentioned selenides and sulfides that it forms in a true single atomic layer. Also, its elements are abundant and quite inexpensive.[4] They're from the first two rows of the periodic table, and low atomic number elements are more common than those higher in the periodic table. The new material is much more stable than the graphene alternatives, and it was the only way that these three elements could bond as a single layer.[4]

Although the material has the same hexagonal structure as graphene, the atomic bonds are of different lengths.[4] While graphene has excellent mechanical properties, it isn't a semiconductor, and this precludes its easy application in electronic circuitry. The Si2BN material is metallic, but it can be easily made to be semiconducting by attaching other elements to the silicon atoms.[4] Also, the presence of silicon makes it a candidate material for hydrogen storage.[3]

Structure of di-silicon boron nitrideStructure of Si2BN. The yellow atoms are silicon, the green atoms are boron, and the blue atoms are nitrogen.

(University of Kentucky Illustration by Madhu Menon.)

The Si2BN material should be quite stable. Says Madhu Menon, physicist and coauthor of the paper describing this research,
"We used simulations to see if the bonds would break or disintegrate - it didn't happen... We heated the material up to 1,000-degree Celsius and it still didn't break."[4]

As they say, "The proof of the pudding is in the eating," and Menon is eager to have someone synthesize this material.
"We are very anxious for this to be made in the lab... The ultimate test of any theory is experimental verification, so the sooner the better!... We know that silicon-based technology is reaching its limit because we are putting more and more components together and making electronic processors more and more compact, but we know that this cannot go on indefinitely; we need smarter materials... This discovery opens a new chapter in material science by offering new opportunities for researchers to explore functional flexibility and new properties for new applications... We can expect some surprises."[4]

References:

  1. Hugh G. Evelyn-White, Trans., "Hesiod, The Homeric Hymns and Homerica," William Heinemann/The Macmillan Co. (London/New York, 1914), pp. 12-13.
  2. W. Danesi, J. Hockin, and C. Lund, "Tungsten containing alloy," US Patent No. 3,759,707, September 18, 1973.
  3. Antonis N. Andriotis, Ernst Richter, and Madhu Menon, "Prediction of a new graphenelike Si2BN solid," Phys. Rev. B, vol. 93, no. 8 (February 15, 2016), Document no. 081413, DOI:http://dx.doi.org/10.1103/PhysRevB.93.081413.
  4. UK Physicist Discovers New 2D Material that could Upstage Graphene, University of Kentucky Press Release, February 29, 2016.
  5. Dr. Madhu Menon Proposes New 2D Material, YouTube Video by the University of Kentucky, February 26, 2016. Note that the chemical symbol for boron in the periodic table of this video is wrong.

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