Shunhong Zhang a,b,c , Jian Zhou c , Qian Wang a,b,c,1 , Xiaoshuang Chen d,e , Yoshiyuki Kawazoe f , and Puru Jena c
a Center for Applied Physics and Technology, College of Engineering, Peking University, Beijing 100871, China;
b Collaborative Innovation Center of Inertial Fusion Sciences and Applications, Ministry of Education, Beijing 100871, China;
c Department of Physics, Virginia Commonwealth University, Richmond, VA 23284;
d National Laboratory for Infrared Physics, Shanghai Institute of Technical Physics, Chinese Academy of Sciences, Shanghai 200083, China;
e Synergetic Innovation Center of Quantum Information & Quantum Physics, University of Science and Technology of China, Hefei, Anhui 230026, China;
f Institute for Materials Research, Tohoku University, Sendai, 980-8577, Japan Edited by Ho-kwang Mao, Carnegie Institution of Washington, Washington, DC, and approved January 5, 2015 (received for review August 28, 2014)
www.pnas.org/cgi/doi/10.1073/pnas.1416591112
This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1416591112/-/DCSupplemental.
I have little knowledge on theoretical chemistry laboratories, I should try to explore that section a bit more. I am not familiar with the authors, however I enjoyed their work exposed here.
Graphite, diamond, graphene, carbon nanocones, nanochains, graphdyine, and several other predicted allotropes are composed of carbon atoms arranged in specific structures. Graphene, the 2D "miracle" material, has received a significant amount of attention since its isolation in 2004 by Novoselov and Geim, and not discovery as many reporters mistakenly state.
It has been studied since the late 1800s under the graphite oxide form. My first contact with it was the 1962 paper from Boehm, Clauss, Fischer, and Hofmann. Among the first TEM images were taken by Ruess and Vogt in 1948. From 1970s to late 1990s the studies continued, but no few-layer specimens could be easily obtained, until 2004. Note, SiC epitaxial growth of Gr has been around since the 1970s, but the quality of the obtained "monolayer graphite" (as it was initially called) was unsatisfactory for further studies; the substrate-graphite interactions were studied by Oshima and Nagashima in 1996.
There are a few predicted carbon allotropes with various designs showing more impressive properties than Gr, such as anisotropic Dirac cones (from what I understand, these allow a better control over the charge carrier mobility, in other words how fast the charges are transported through the material), inherent ferromagnetism (atom-thick magnetic sheets, probably only imagination can limit their applications), superconductivity (if found at room temperature it becomes an instant gold mine), and high catalytic reactivity (atom-thick catalytic sheets with a significant active surface/volume ratio). Impressive, but there is still work to be done until those structures become easy to obtain.
In most cases the popular carbon allotropes are arranged in hexagons, with pentagons and heptagons (sometimes even higher order polygons) being considered as structure defects, true for graphene and carbon nanotubes. However, the 5C rings are important in bucky balls and at the carbon nanotube termini. Graphene has its properties from the hexagonal structure, and in this paper the properties of a pentagonal lattice (think Cairo pentagonal tiling) are theoretically studied. An immediate observation which came to mind was the fact that there are sp2 and sp3 hybridized carbons in the structure, I was intrigued to learn how the charges migrate in it.
Interesting detail, but predictable simply from the design, is that this sheet, if stretched on one axis, expands on the one perpendicular to it as well. Imagine expanding an image from a side in a software, but maintaining its aspect ratio. The image expands on both axis. This property is described by a negative Poisson's ratio, and labels penta-graphene (pGr) as an auxetic material. This is usually seen in some polymer structures (Gore-Tex), minerals, and even paper.
From an energetic point of view, this structure is preferred to the smallest C20 fullerene, but is metastable compared to graphene. This means that its synthesis is highly plausible. This stability is in part due to the presence of sp3 bonds which release a bit of the strain. Thermal studies showed that it can handle temperature up to 1000K, meaning that this structure is positioned in a reasonably deep stability well.
Electronically, pGr is a semi-conductor with a 3.25 eV band gap [calculated through the Heyd–Scuseria–Ernzerhof (HSE06) functional]. They indicate that the electronic states near its Fermi level originate from the sp2 hybridized C atoms, and that the electron delocalization is a consequence of the sp2 pz orbitals' spatial separation from the presence of the sp3 hybridizations. I was slightly disappointed to not have seen any theoretical charge-transfer mobility calculations, then again there might be a reason for that which I am missing.
Moreover, they even considered the penta-Gr nanotubes, or "penta-tubes". Graphene nanotubes are known to have different properties (from metallic to semiconducting) depending on their zone folding [defined by the chiral vector (n, m)]. In the penta-Gr case, the authors predict "chirality-independent semiconducting carbon nanotubes". No matter the zone folding, they will more or less behave in a similar fashion. I am still trying to figure out the implications.
As soon as I finished reading the paper I considered the challenge of synthesising the material. Their proposal remains theoretical (chemical exfoliation from a T12-carbon phase). Stability domains in which both sp2 and sp3 hybridizations can form bonds in an ordered manner unfortunately surpass my knowledge base. I see this comparable to a composer who wants to harmoniously join two musical keys in one composition. Due to the metastable nature of pGr and to its symmetry, I hardly support its atomic catalytic construction route (similar to CVD for Gr) from a precursor. A step by step chemical synthesis route could be imagined. Probably a variation and succession of McMurry, Michael, and/or Suzuki reactions, allowing to make nanosheets of pGr (disclaimer: my organic chemistry is rusty). Diels-Alder could work as well. I also believe that the chemical frame of interest should be around the cyclopentadiene moiety in these cases. I hope that this material will be made a reality as soon as possible. There are reasons to believe that multilayered variations of pGr and Gr can produce intriguing semiconducting structures.
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