FILED UNDER: CHEMICAL STRUCTURE AND PROPERTIES OF MATTER | MATERIALS SCIENCE (PHYSICS) | MICRO AND NANOTECHNOLOGY | PERIODIC TABLE | RESEARCH METHODS (PHYSICS) New Layer to the Periodic Table: 2D Metals by Abe Sorber | February 2018 Thanks to their unique properties—electrical conductivity, strength and magnetism, to name a few—metals are used everywhere: from the tallest skyscrapers to the latest microelectronic devices. But there's an emerging technology in which these materials, despite their versatility, haven't yet had much success: the world of atomically thin, or twodimensional (2D), materials. The problem? Although metals are malleable on the macroscopic scale, many of them are simply reluctant to form sheets at the atomic level. Instead, their atoms prefer to cluster together into orderly, three-dimensional (3D) crystals. Janne Nevalaita/University of Jyväskylä Although metals are malleable on the macroscopic scale, many of them are simply reluctant to form sheets at the atomic level. Instead, their atoms prefer to cluster together into orderly, three-dimensional (3D) crystals. Now, two scientists from the University of Jyväskylä Nanoscience Center in Finland have perhaps laid the groundwork for making 2D metals a reality. Using computer simulations, Janne Nevalaita and Pekka Koskinen calculated a variety of 2D properties for all but the heaviest metals. ABOVE: The region of the periodic table studied by Nevalaita and Koskinen,with the chemical symbol and atomic number of each of the metals in that region. Now, two scientists from the University of Jyväskylä Nanoscience Center in Finland have perhaps laid the groundwork for making 2D metals a reality. Using computer simulations, Janne Nevalaita and Pekka Koskinen calculated a variety of 2D properties for all but the heaviest metals. Because they studied most of the metallic elements, they have in effect created a "periodic table" of 2D metals, a key starting point for new investigations published in Physical Review B. "One could say that we hit on an untouched estate, bulldozed it and created a foundation. Now others can build solid scientific structures based on that foundation," said Nevalaita in a press release. A Double-Edged Sword The most famous 2D material is graphene, which consists of carbon atoms arranged in a flat, honeycomb pattern. Graphene was first isolated in 2004, and became an instant superstar. It's stronger than steel, conducts electricity and heat better than copper, and is both stretchy and impermeable to gases. [See Building Better Sensors is Child's Play, January 2017 and Graphene Via Sticky Tape Grabs Nobel, October 2010.] Since graphene's discovery, scientists have managed to produce a variety of 2D materials. Some, like graphene, consist of a single element: for example, phosphorene (phosphorus), silicene (silicon), germanene (germanium) and borophene (boron). Others are compounds, such as hexagonal boron nitride (hBN), gallium selenide (GaSe) and molybdenum disulfide (MoS2). [See Graphene Comes of Age, January 2015.] This work has led to the production of a limited range of 2D metals, including gauzy sheets of gold (Au), tiny plates of rhodium (Rh), and, within cut-outs in a graphene sheet, minuscule patches of iron atoms (Fe). Pekka Koskinen/University of Jyväskylä The most famous 2D material is graphene, which consists of carbon atoms arranged in a flat, honeycomb pattern. Graphene was first isolated in 2004, and became an instant superstar. It's stronger than steel, conducts electricity and heat better than copper, and is both stretchy and impermeable to gases. A crucial difference between 2D metals and nonmetals is the type of bonding that occurs among their atoms. Many of the new 2D materials, including graphene, are held together by covalent bonds, in which electrons are shared between pairs of atoms. Covalent bonds are convenient for 2D structures because they assume particular directions in space, like the rigid ball-and-stick models you might see in a chemistry classroom. When bonds lie in a plane, atoms can link together in a sheet. In graphene, for instance, each carbon forms three covalent bonds, like a threepointed star. These carbons then combine to produce graphene's familiar hexagonal lattice. Metal atoms, in contrast, rely on metallic bonds, in which electrons are shared liberally among atoms. In fact, a metal is often described as an array of positively charged nuclei surrounded by a "sea" of free-flowing electrons, a description known as the Drude model. This description is too simple—scientists have needed more sophisticated theories to explain some behaviors of metals—but it embodies the principle that electrons can be "delocalized," or somehow be cut loose from an individual bond or nucleus. Mobile electrons are what give metals some of their distinctive properties, such as electrical conductivity and malleability. Similarly, in two dimensions, mobile electrons should give metals advantages over the 2D versions of nonmetals and nonmetallic compounds. Pekka Koskinen/University of Jyväskylä Metal atoms' natural tendency is to bond with neighbors in all directions. Consequently, metal atoms arrange themselves somewhat like a tidy stack of oranges at a supermarket, forming one of several tightly packed crystal structures, depending on the metal. Unfortunately, metallic bonding is a double-edged sword: it makes metals reluctant to flatten into 2D sheets. Instead, metal atoms' natural tendency is to bond with neighbors in all directions. Consequently, metal atoms arrange themselves somewhat like a tidy stack of oranges at a supermarket, forming one of several tightly packed crystal structures, depending on the metal. Therefore, predicting the mechanical properties of 2D metals was a major part of the Finnish scientists' study; it would give them a sense of which metals would be too stubborn to form sheets. "However promising the properties of the 2D metals might be, they will be irrelevant unless the metal patches can be made mechanically stable first," Koskinen said. An "Atlas" for 2D Metals Aware that structural stability is a vital consideration, Nevalaita and Koskinen set out to predict the mechanical properties of 2D metals. They studied almost every metal: from lithium (atomic number 3) to bismuth (atomic number 83), for a total of 45. For each element, they simulated three virtual sheets with different configurations: hexagonal (triangular), square and honeycomb. For each configuration they determined a suite of properties and then tabulated the results, adding a new layer of information to the familiar periodic table. Janne Nevalaita and Pekka Koskinen/Physical Review B Nevalaita and Koskinen set out to predict the mechanical properties of 2D metals. They studied almost every metal: from lithium (atomic number 3) to bismuth (atomic number 83), for a total of 45. Shown above is a periodic table of calculated results for the metals studied. "[The] periodic table usually presents the properties of elements in gas phase and in three-dimensional bulk," said Koskinen. "Our table presents the properties in two-dimensional bulk." Among the properties they calculated were cohesive energy, bond length and bulk modulus. Cohesive energy is a measure of how tightly the atoms of an element hold together; it is the amount of energy needed, per atom, to drive the atoms apart and turn the element into a gas, or "sublimate" it. A 2D metal with a high cohesive energy, for example, would be hard to sublimate. Bond length is the distance between neighboring atoms, which could vary not only among metals but also among the distinct geometric configurations of a specific metal. In general, shorter bonds are associated with stronger materials. Finally, bulk modulus is a measure of resistance to compression or stretching. The higher the bulk modulus, the less compressible and less stretchable a material is: for instance, the bulk modulus of a steel block is much higher than that of a foam sponge. Janne Nevalaita and Pekka Koskinen/Physical Review B For each element, they simulated virtual sheets with the atoms arranged in distinct geometric patterns. The lattice configurations are illustrated by the measurement of the electron density of Au (gold) in the atomic plane. A clear trend emerged when cohesive energy, bond length and bulk modulus were compared among the metals. In the middle of each of the rows of the region of the periodic table called the d-block, the metals' cohesive energies reached a maximum, their bond lengths reached a minimum, and their bulk moduli reached a maximum. In other words, the 2D metals in the middle of the d-block rows were stronger in all respects. This matches the 3D versions of the metals, which also have higher cohesive energies, shorter bond lengths and higher bulk moduli in the middle of the d-block. Janne Nevalaita and Pekka Koskinen/Physical Review B A clear trend emerged when cohesive energy, bond length and bulk modulus were compared among the metals. The density of states is illustrated by the measurement of the 2D Au (gold) configurations. So, does that mean that the strong 2D metals in the middle of the d-block would be the best candidates for 2D materials? Not necessarily. In fact, if a 2D metal's strength is very high, the metal might also be likely to fold in on itself and form a 3D structure. Instead, the 2D periodic table may point researchers to metals that have the most appropriate balance between cohesion and fragility for various purposes. "Just like an atlas, our work...provides guidelines for future research—sort of a map," said Koskinen. Electrons Are the Key Throughout its history, the periodic table has highlighted patterns among the elements. The first modern tables, culminating in the one constructed by Dmitri Mendeleev in 1869, showcased atomic weight, chemical properties, and the number of bonds that elements typically formed. Insightfully, Mendeleev inferred that gaps in his table represented undiscovered elements, and successfully predicted their properties, based on the patterns revealed in his table. Remarkably, Mendeleev's table and others, and the first successful predictions of hitherto undiscovered elements, were made without the knowledge of how elemental properties derive from subatomic particles. Today, we know that an element is distinguished by its atomic number, the number of protons it has. Furthermore, we know that an element's chemical and physical behavior is determined by the number of its outer, or valence, electrons. This is why elements can be put into columns in the periodic table; these "groups" have identical numbers of valence electrons, and therefore similar chemical properties. M. Bank An element is distinguished by its atomic number, the number of protons it has. Furthermore, an element's chemical and physical behavior is determined by the number of its outer, or valence, electrons. This is why elements can be put into columns in the periodic table; these "groups" have identical numbers of valence electrons, and therefore similar chemical properties. Perhaps not surprisingly, the trend of 2D metals growing stronger towards the middle of the d-block has to do with valence electrons. The 10 elements in each row of the d-block have electrons that fall into energy zones called "d" orbitals. There are five "d" orbitals, each of which can hold two electrons. Moving across the d-block from left to right, electrons are distributed one-by-one into these orbitals. Because electrons avoid sharing orbitals unless they have to—according to what's known as Hund's rule—the elements on the left side of the d-block have orbitals with unpaired electrons, with the fifth element from the left having the maximum of five. Continuing to the right, electrons pair up in the "d" orbitals until there are no unpaired electrons left. As it turns out, the more unpaired "d" electrons a metal has, the stronger it is. To explain this, scientists use an update of the known as the band theory of solids. In the band theory, atomic orbitals, such as the "d" orbitals, combine into giant "molecular orbitals" that cover the whole metal, whatever its shape. This ends up producing two families of this new kind of orbital: "bonding molecular orbitals" that promote bonding among atoms, and "antibonding molecular orbitals" that discourage it. Where do the d-block elements' electrons go? As you might guess, the electrons that we think of as being "unpaired" fall into bonding molecular orbitals, and those that are "paired" end up in antibonding molecular orbitals. Ultrathin, Ultra-Useful If the mechanical challenges of producing 2D metals are overcome, there will be plenty of ways to use the new materials. For example, 2D metals could be used as catalysts, substances that assist a chemical reaction without being consumed. Solid metals are already used to catalyze a variety of important reactions, such as breaking down the most harmful exhaust gases before they come out of a car's tailpipe or producing essential chemicals such as ammonia. However, some of the best present-day catalysts for these reactions—such as platinum (Pt)—are rare and very expensive. Two-dimensional metals could be used to form thinner, cheaper versions of these catalysts. Moreover, 2D versions of catalysts may be even more effective than 3D ones. According to Koskinen, scientists have discovered that the most effective regions of a 3D metal catalyst are "sharp kinks." On the edge of a kink, an atom simply has fewer neighbors than it would have if it were found on the surface of a flat slab; this makes the metal more effective as a catalyst. In 2D metals, there is only one layer of atoms, so every atom has only a few neighbors and should pack a catalytic wallop. In line with this concept, flakes of 2D rhodium (Rh) significantly outperformed current, off-the-shelf rhodium catalysts in lab tests. Shutterstock If the mechanical challenges of producing 2D metals are overcome, there will be plenty of ways to use the new ultrathin materials. Another intriguing use for 2D metals involves their unique response to light. Certain metals, when illuminated with the right frequency, develop electronic waves on the surface, resulting in heat. This phenomenon, called surface plasmon resonance, could perhaps be applied to create metallic heaters to combat cancer. Once attached to cancer cells, the metal particles could be heated with lasers, bursting the cells' walls. One problem, though, is that particles of certain metals with certain shapes will heat up only in response to particular frequencies of light. Two-dimensional metals might come to the rescue, because their particles, due to their thinness, could be made to respond to a wide range of light frequencies. Finally, metals have one feature that materials like graphene can't match: magnetism. Magnetic 2D metals could be used to make smaller and faster magnetic memory devices, such as a promising form of random-access memory (RAM) called magnetoresistive RAM (MRAM). MRAM has several ideal features: it's fast, uses little electricity, and, like a hard drive, retains information even when the power is off. Unfortunately, MRAM has not yet been miniaturized enough to compete with the types of RAM, such as DRAM and Flash, currently found in our devices. If 2D metals could shrink MRAM sufficiently, MRAM might become a useful alternative. And these are just a few applications. The list also includes quantum computing and spintronics, electronics that use electrons' spin instead of their charge. "Sky's the limit, for what the applications could be," Koskinen told ScienceNews. Discussion Questions The periodic table organizes information in a way that indicates patterns, and this can serve as an inspiration for research. Are there other materials besides 2D metals for which something like a "periodic table" might be useful? As useful as something like the periodic table is, are there drawbacks to such ways of presenting information? Journal Abstracts and Articles (Researchers' own descriptions of their work, summary or full-text, on scientific journal websites.) Nevalaita, Janne and Pekka Koskinen. "Atlas for the properties of elemental two-dimensional metals." Physical Review B (January 10, 2018) [accessed January 23, 2017]: https://journals.aps.org/prb/abstract/10.1103/PhysRevB.97.035411. Bibliography Ling, Tao, et al. "Freestanding Ultrathin Metallic Nanosheets: Materials, Synthesis, and Applications Applied Materials (July 27, 2015) [accessed February 1, 2018]: http://onlinelibrary.wiley.com/doi/10.1002/adma.201501403/abstract. Nevalaita, Janne and Pekka Koskinen. "Atlas for the properties of elemental two-dimensional metals." Physical Review B (January 10, 2018) [accessed January 23, 2017]: https://journals.aps.org/prb/abstract/10.1103/PhysRevB.97.035411. Rini, Matteo. "Synopsis: An Atlas for 2D Metals." Physics (January 10, 2018) [accessed January 22, 2018]: https://physics.aps.org/synopsis-for/10.1103/PhysRevB.97.035411. Temming, Maria. "Ultrathin 2-D metals get their own periodic table." ScienceNews (January 17, 2018) [accessed January 22, 2018]: https://www.sciencenews.org/article/ultrathin-2-d-metals-get-their-own-periodic-table?tgt=nr. University of Jyväskylä. "New research opening for atomically thin metal nanostructures." (January, 2018) [accessed January 22, 2018]: https://www.jyu.fi/en/news/archive/2018/01/tiedote-2018-01-11-15-34-16-889135. Keywords 2D metals, two-dimensional metals, periodic table for 2D metals, Janne Nevalaita and Pekka Koskinen Citation Information ( MLA ) Sorber, Abe. “New Layer to the Periodic Table: 2D Metals.” Today's Science, Infobase, Feb. 2018, https://tsof.infobaselearning.com/recordurl.aspx?wid=265745&ID=39950. Accessed 12 Apr. 2020. Copyright © 2020 Infobase. All Rights Reserved.
