1. Allotropes of layered phosphorus
Phosphorene, few-layer black phosphorus, can be exfoliated from its bulk counterpart. Phosphorene is a p-type semiconductor with a significant band gap. It is stable, flexible, and displays a high carrier mobility [1], suggesting its potential use in 2D electronics. Most interesting, based on ab initio density functional calculations, we found that phosphorus can form four different 2D structural phases that are almost equally stable and may be inter-connected. Connection of two planar phases resembles a sharp origami-style fold. This fold occurs naturally in this multi-phase system[2][3], whereas in single-phase systems like graphene it requires the presence of defect lines or lines of adsorbed atoms. We also find the possibility of moving this fold by structural transformation from one allotrope to another, with an unusually low activation barrier of <0.5 eV per bond. Since the electronic properties of multi-phase phosphorene may also be tuned by in-layer strain, including a semiconductor-to-metal transition, we postulate that origami-style folded phosphorene should display an unprecedented richness in its electronic behavior.
[1] H. Liu et al. ACS Nano 8, 4033 (2014)
[2] Z. Zhu and D. Tomanek, Phys. Rev. Lett. 112, 176802 (2014)
[3] J. Guan,* Z. Zhu,* and D. Tomanek, Phys. Rev. Lett. 113, 046804 (2014) (* equal contribution)
[4] J. Guan, Z. Zhu, and D. Tomanek, Phys. Rev. Lett. 113, 226801 (2014)
2. Dimaond nanowires inside carbon nanotube
Can diamond nano-wire [1] grow inside the CNTs? This open question leads me to the research topic that the possible growth mechanism of diamond nano-wire inside an armchair CNTs. I perform molecular dynamic (MD) simulations to model the reaction process between diamondoid molecules and their functional acids. I found that under certain pressure and atmosphere (like filled by H2), the molecules could dimerize inside a CNT. This indicates the trend that diamond nano-wire could form inside the CNTs.
[1] J. Zhang et al. Angew. Chem. Int. Ed. 52, 3717 (2013).
3. Electronic structure and growth mechanism of carbon foam structures
We investigated a new type of crystalline carbon foam material [1][2] beyond diamond, graphene, C60 and carbon nanotubes. Geometrically, carbon foam is assembled from bundles of (6,0) carbon nano-tubes, with the neighboring blocks sharing the same wall. Different from previous carbon materials, it is constructed by a mixture of sp2 and sp3 carbon atoms with a ration of 3:2. We found that carbon foam is rather compressible and would start folding under a certain pressure or electron doping. We also predicted that carbon foam could grow under an unequilibrium condition from the surface of transition metal like Co and Ni.
[1] Z. Zhu and D. Tomanek, Phys. Rev. Lett. 109, 135501 (2012).
[2] Z. Zhu, Z. G. Fthenakis, J. Guan and D. Tomanek, Phys. Rev. Lett.112, 026803 (2014)
4. Thermal transport of carbon nanostructures
We use nonequilibrium molecular-dynamics simulations to study the effect of structural defects on the thermal conductivity λ of graphene. Focusing on 5-7 and 5-8 defects in the graphene honeycomb lattice, we find that λ depends sensitively on whether the defects are isolated, form lines, or form extended arrangements inhaeckelites. Our results indicate that the presence of defects makes λ anisotropic and, depending on the temperature, quenches its value by one to two orders of magnitude with respect to graphene, mainly by reducing the phonon mean free path.
[1] Z. G. Fthenakis, Z. Zhu and D. Tomanek, Phys. Rev. B 83, 193405(2011).
5. Searching for largest 2D boron clusters
When the planer flake of nano-boron cluster would undergo a transition from 2D to 3D? To answer this question, I investigated the relative stability of several B19 clusters in different ionic states. The results showed that the most stable structure in anionic state is planer while in both neutral and cationic states the 3D structure is the most stable one. This means that B19 is the transition point that we are looking for.
[1] I. Boustani, Z. Zhu and D. Tomanek, Phys. Rev. B 83, 193405 (2011).