Novel Materials and devices
The major way of achieveing targeted functions in energy and information technology is by engineering noval materials and fabricating new devices. Using accurate first principles calculations and automatic structure search methods, We work on designing novel materials and structures based on specific physics and chemistry features of the materials.
Self-assembly carbon nanotubes on diamond (100) surface
Surfaces of semiconductors are crucial for electronics, especially when the devices are reduced to the nanoscale. However, surface structures are often elusive, impeding greatly the engineering of devices. We here develop an efficient method that can automatically explore the surface structures by virtue of structure swarm intelligence. While applying the method on the "simple" diamond (100) surface, we discovered a hitherto unexpected surface reconstruction featuring self-assembly of carbon nanotubes (CNTs) arrays. The surface with self-assembled CNTs is energetically degenerate to the dimer structure, but become much favorable under a small compressive strain or at elevated temperatures. The intriguing covalent bonding between the neighboring tubes creates a unique feature of carrier kinetics ---one dimensionality of hole states versus two dimensionality of electron states, which may lead to novel design of superior electronics. Our findings highlight that the surfaces not only act as platforms of nanostructures but also play vital roles in fabricating nanomaterials and by being a functional part of the nanodevices.
- M. S. Miao, Q. M. Yan, C. G. Van de Walle, W. K. Lou, L. L. Li, K. Chang, Phys. Rev. Lett. 109, 186803 (2012).
Polarization driven topological insulator in GaN/InN/GaN quantum well
The wurtzite structure of the III-nitrides allows for the presence of spontaneous and piezoelectric polarization fields in structures grown along the  direction. The piezoelectric fields arise due to the large strain that is induced in a thin InN layer grown on GaN. We show by first-principles electronic structure calculations that this polarization can invert the bands of a GaN/InN/GaN QW when the InN thickness exceeds three monolayers (ML) along the  axis. Using an eight-band k*p model, we prove that such a system can become a TI and possess edge states in the energy gap of a Hall bar structure.
Interface-induced Topological Insulator Transition in GaAs/Ge/GaAs Quantum Wells
We demonstrate theoretically that interface engineering can drive Germanium, one of the most commonly-used semiconductors, into topological insulating phase. Utilizing giant electric fields generated by charge accumulation at GaAs/Ge/GaAs opposite semiconductor interfaces and band folding, the new design can reduce the sizable gap in Ge and induce large spin-orbit interaction, which lead to a topological insulator transition. Our work provides a new method on realizing TI in commonly-used semiconductors and suggests a promising approach to integrate it in well developed semiconductor electronic devices.