Jia-Cai Nie

Strain and curvature induced evolution of electronic band structures in twisted graphene bilayer

It is well established that strain and geometry could affect the band structure of graphene monolayer dramatically. Here we study the evolution of local electronic properties of a twisted graphene bilayer induced by a strain and a high curvature, which are found to strongly affect the local band structures of the twisted graphene bilayer. The energy difference of the two low-energy van Hove singularities decreases with increasing lattice deformation and the states condensed into well-defined pseudo-Landau levels, which mimic the quantization of massive chiral fermions in a magnetic field of about 100 T, along a graphene wrinkle. The joint effect of strain and out-of-plane distortion in the graphene wrinkle also results in a valley polarization with a significant gap. These results suggest that strained graphene bilayer could be an ideal platform to realize the high-temperature zero-field quantum valley Hall effect.

Creating one-dimensional nanoscale periodic ripples in a continuous mosaic graphene monolayer.

In previous studies, it has proved difficult to realize periodic graphene ripples with wavelengths of a few nanometers. Here we show that one-dimensional (1D) periodic graphene ripples with wavelengths from 2 nm to tens of nanometers can be implemented in the intrinsic areas of a continuous mosaic (locally N-doped) graphene monolayer by simultaneously using both the thermal strain engineering and the anisotropic surface stress of the Cu substrate. Our result indicates that the constraint imposed at the boundaries between the intrinsic and the N-doped regions play a vital role in creating these 1D ripples. We also demonstrate that the observed rippling modes are beyond the descriptions of continuum mechanics due to the decoupling of graphenes bending and tensional deformations. Scanning tunneling spectroscopy measurements indicate that the nanorippling generates a periodic electronic superlattice and opens a zero-energy gap of about 130 meV in graphene. This result may pave a facile way for tailoring the structures and electronic properties of graphene.

Quantum confinement effect in ZnO thin films grown by pulsed laser deposition

High quality crystalline nanostructured ZnO thin films were fabricated by pulsed laser deposition on (0001) sapphire substrates. The films were investigated by x-ray diffraction, scanning electron microscopy, and optical absorption spectroscopy. The exciton peak energy from absorption spectra was used to determine the band gap energy Eg, which was found to increase systematically with the decrease in the mean grain size and ranged from 3.388to3.647eV. This is consistent with the quantum confinement model and, up until now, not observed in a convincing manner in the ZnO thin films.

Hierarchy of graphene wrinkles induced by thermal strain engineering

Here, we study hierarchy of graphene wrinkles induced by thermal strain engineering and demonstrate that the wrinkling hierarchy can be accounted for by the wrinklon theory. We derive an equation λ = (ky)0.5, explaining evolution of wrinkling wavelength λ with the distance to the edge y observed in our experiment by considering both bending energy and stretching energy of the graphene flakes. The prefactor k in the equation is determined to be about 55 nm. Our experimental result indicates that the classical membrane behavior of graphene persists down to about 100 nm of the wrinkling wavelength.

Superlattice Dirac points and space-dependent Fermi velocity in a corrugated graphene monolayer

Recent studies show that periodic potentials can generate superlattice Dirac points at energies

Strain-induced one-dimensional Landau level quantization in corrugated graphene

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Grain size dependence of electrical and optical properties in Nb-doped anatase TiO2

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Angle-dependent van Hove singularities and their breakdown in twisted graphene bilayers

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Landau quantization in graphene monolayer, Bernal bilayer, and Bernal trilayer on graphite surface

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d carrier induced intrinsic room temperature ferromagnetism in Nb:TiO2 film

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