Exotic two dimensional silica – Dr. Xiao Dong
JANUARY 31, 2017
Using computer modeling, scientists from HPSTAR and Tongji University have found out three new forms of 2D-silica with exotic mechanical and electrical properties — large negative Poisson’s ratios and widest electric bandgap among all reported 2D materials . They detailed their work in the latest issue of the journal Nano Letters (DOI: 10.1021/acs.nanolett.6b03921).
Silicon dioxide is the fundamental component of glass, sand and most minerals, which is also known as one of the building units of earth crust and mantle. There are a large number of polymorphs of silica in crystalline and amorphous forms, such as quartz, and glassy SiO2. It is well known that bulk silicon dioxide is a good insulator and building materials. What about two-dimensional (2D) silicon dioxide — a layered polymorph of silicon dioxide? Will the low dimentional structural silica show novel phenomenon? These are the focus of the present work.
A team co-led by Dr. Xiao Dong, a scientist of HPSTAR, use ab initio evolutionary algorithm for crystal structure and compound prediction -- USPEX (Universal Structure Predictor: Evolutionary Xtallography) to predict intrinsic mechanical and electronic porperties of 2D crystalline freestanding silicon dioxide.
They discovered three more new 2D-silica structures in their simulations. The three new 2D-silica display comparable thermal, dynamical and mechanical properties- superhardness, wide band gap - refers to the lowest energy of light that a semiconductor can absorb and a fundamental parameter required for controlling light receiving, conversion, and transport technologies, with many typical bulk forms of silica.
What surprised the scientists most is that all the three 2D silica have large inplane negative Poisson’s ratios with the largest one being double of penta-graphene and three times of borophenes.
Usually when a material is stretched in one direction, it usually contracts in the other two directions, are said to have a positive Poisson's ratio which characterizes how a material behaves when stretched. However, some materials becoming fatter when stretched, which means they have negative Poisson’s ratio.
"A negative Poisson's ratio is theoretically possible long ago. While Two-dimensional (2D) materials- single-layer graphene, single-layer graphene ribbons, single-layer black phosphorus, cubic metals and α-Cristobalite, changed the case in their single layer structure,” said Dr. Xiao Dong.
“This surprising negative Poisson’s ratio in 2D-silica originates from the interplay of lattice symmetry and Si-O tetrahedron symmetry” explained Dr. Xiao Dong.
Furthermore, they pointed out that 2D-silica are the most insulating 2D material among reported 2D structures with the widest gap.
These exotic 2D silica with negative Poisson’s ratios and widest band gaps are expected to have great potential applications in nanomechanics and nanoelectronics.
Caption：α-2D silica with positive Posson's ratio (up left), β-2D silica, γ-2D silica, and δ-2D silica with large in-plane negative Poisson’s ratios