Gao Yang, Cao Tengfei, Cellini Filippo, Berger Claire, de Heer Walter A, Tosatti Erio, Riedo Elisa, Bongiorno Angelo
Advanced Science Research Center, City University of New York, New York, NY, USA.
School of Physics, Georgia Institute of Technology, Atlanta, GA, USA.
Nat Nanotechnol. 2018 Feb;13(2):133-138. doi: 10.1038/s41565-017-0023-9. Epub 2017 Dec 18.
Atomically thin graphene exhibits fascinating mechanical properties, although its hardness and transverse stiffness are inferior to those of diamond. So far, there has been no practical demonstration of the transformation of multilayer graphene into diamond-like ultrahard structures. Here we show that at room temperature and after nano-indentation, two-layer graphene on SiC(0001) exhibits a transverse stiffness and hardness comparable to diamond, is resistant to perforation with a diamond indenter and shows a reversible drop in electrical conductivity upon indentation. Density functional theory calculations suggest that, upon compression, the two-layer graphene film transforms into a diamond-like film, producing both elastic deformations and sp to sp chemical changes. Experiments and calculations show that this reversible phase change is not observed for a single buffer layer on SiC or graphene films thicker than three to five layers. Indeed, calculations show that whereas in two-layer graphene layer-stacking configuration controls the conformation of the diamond-like film, in a multilayer film it hinders the phase transformation.
原子级薄的石墨烯展现出迷人的机械性能，尽管其硬度和横向刚度不如金刚石。到目前为止，尚未有将多层石墨烯转变为类金刚石超硬结构的实际例证。在此我们表明，在室温下以及经过纳米压痕后，SiC(0001)上的双层石墨烯展现出与金刚石相当的横向刚度和硬度，能抵抗金刚石压头的穿孔，并且在压痕时电导率会出现可逆下降。密度泛函理论计算表明，在压缩时，双层石墨烯薄膜会转变为类金刚石薄膜，同时产生弹性形变以及从sp到sp的化学变化。实验和计算表明，对于SiC上的单个缓冲层或厚度超过三到五层的石墨烯薄膜，并未观察到这种可逆相变。实际上，计算表明，在双层石墨烯中，层堆叠构型控制着类金刚石薄膜的构象，而在多层薄膜中，它会阻碍相变。
