Two-Dimensional Materials Continue Making Inroads
While some have just barely been synthesized and others remain in computer models, the impact of 2D materials cannot be ignored
In our last issue, we catalogued the myriad two-dimensional (2D) materials that are sometimes vying for the same applications as graphene and other times assisting it in meeting the demands of those applications. The developments that are occurring in 2D materials appear destined to make a big impact on electronics applications especially.
Silicene Is Not Suicidal
In our previous catalog of 2D materials, we highlighted recent research that indicated that silicene, which is a one-atom thick layer of silicon, had what the researchers called “suicidal tendencies.” That research out of the Netherlands was reported in January of this year. Just eight months later, an international team of researchers has shown that under the right circumstances silicene can remain stable, which should allow it to compete in the expanding arena of 2D materials.
Of course, the researchers were only capable of keeping the silicene stable in ambient air for just 24 hours, but that is long enough for researchers to continue to run test on the materials to better understand its properties and capabilities.
The circumstances that made it possible to keep the silicene stable were employing a silver substrate for the materials that was kept at a temperature of 470 Kelvin and keeping a solid silicon source at 1470 Kelvin. This arrangement allowed the researchers to place 43 monolayers of silicene on the substrate.
“Our present study shows that multilayered silicene is more conductive than single-layered silicene, and therefore opens up the possibility of using it throughout the silicon microelectronics industry,” said Paola De Padova, from Consiglio Nazionale delle Ricerche in Italy, in a press release. “In particular, we envisage the material being used as a gate in a silicene-based MOSFET (metal–oxide–semiconductor field-effect transistor), which is the most commonly used transistor in digital and analogue circuits.”
Germanene Moves From Theory to Reality
Another 2D material we catalogued in our previous newsletter was germanene, which is the 2-D version of germanium. Germanene had been theorized back in 2009, but had not been actually fabricated. However, as we reported researchers last year at Ohio State University claimed to have succeeded at growing sufficient quantities of it to measure the material’s properties in detail.
Now European researchers are claiming to be the first to successfully synthesize germanene in a process that could lead to bulk production of the elusive material. Up until this most recent research, the way proposed for producing germanene was to deposit individual germanium atoms onto a substrate under high temperatures and in an ultra-high vacuum.
While Chinese researchers have reported that they have been able to produce germanene following this process, they used platinum as the substrate material. The European researchers used gold and found that it made all the difference.
The researchers believe that they could grow germanene on thin gold films on top of a flexible substrate. This would be cheaper than the platinum substrate proposed by the Chinese researchers and could lead to large-scale production.
“The synthesis of germanene is just the very beginning of a long quest,” said Professor Guy Le Lay, from Aix-Marseille University, in a press release. “Indeed, success in the synthesis was not easy to achieve and quite demanding. A considerable amount of work is now needed to further characterize the electronic properties of the material.”
2D Materials Get Some 1D Competition
If 2D materials are so great, then a 1D material has to be even better, right? Well, yeah, if you can make it. Last year, the strongest material in the world was announced: carbyne. The problem was that it was only producible in computer models.
While producing the material is still relegated inside a computer model, researchers at Rice University are revealing through their models some new and enticing properties that make actually synthesizing the material all the more appealing.
The Rice researchers reported in the journal Nano Letters, that when the carbon chains of carbyne are stretched by as little as three percent, you widen the material’s band gap, effectively changing it from a conductor to an insulator.
The reason this occurs is a change in the way the electrons are distributed between each of the carbon atoms. There are four electrons for each carbon atom. In its relaxed state, the carbon atoms remain evenly spaced with two bonds between each of them. However, the carbon atoms are never really in a relaxed state because they are constantly moving around in a state of quantum uncertainty.
This quantum uncertainty is a kind of constant vibration that prevents carbyne from falling into a state known as Peierls distortion, in which the atomic positions of a one-dimensional material oscillate so much that the order of the material is broken.
“Peierls said one-dimensional metals are unstable and must become semiconductors or insulators,” Yakobson said in a press release. “But it’s not that simple, because there are two driving factors.”
According to Yakobson, the Peierls distortion “wants to open the gap that makes it a semiconductor” while another quantum effect called zero-point vibrational energy “wants to maintain uniformity and the metal state.”
Yakobson further explains that the zero-point vibrational energy is actually a manifestation of the quantum uncertainty.
“It’s more a blur than a vibration,” said Yakobson. “We can say carbyne represents the uncertainty principle in action, because when it’s relaxed, the bonds are constantly confused between 2-2 and 1-3, to the point where they average out and the chain remains metallic.”
While this interesting property may spur researchers to actually synthesize carbyne, it may help in working with other one-dimensional chains, such as conducting polymers, that are subject to Peierls distortions.