Improved Hubbard-I approximation impurity solver for quantum impurity models.

We develop a fast impurity solver which is based on the combination of Hubbard-I approximation and hybridization expansion continuous-time quantum Monte Carlo algorithm. This solver inherits the advantages of both algorithms. In order to demonstrate the power and usefulness of this solver, we use it to study the magnetic phase transitions of single-band and two-band Hubbard models in the framework of single-site dynamical mean-field theory.

Epitaxial nucleation of CVD bilayer graphene on copper.

Bilayer graphene (BLG) has emerged as a promising candidate for next-generation electronic applications, especially when it exists in the Bernal-stacked form, but its large-scale production remains a challenge. Here we present an experimental and first-principles calculation study of the epitaxial chemical vapor deposition (CVD) nucleation process for Bernal-stacked BLG growth on Cu using ethanol as a precursor. Results show that a carefully adjusted flow rate of ethanol can yield a uniform BLG film with a surface coverage of nearly 90% and a Bernal-stacking ratio of nearly 100% on ordinary flat Cu substrates, and its epitaxial nucleation of the second layer is mainly due to the active CH radicals with the presence of a monolayer-graphene-covered Cu surface.

Ultrafast growth of single-crystal graphene assisted by a continuous oxygen supply.

Graphene has a range of unique physical properties and could be of use in the development of a variety of electronic, photonic and photovoltaic devices. For most applications, large-area high-quality graphene films are required and chemical vapour deposition (CVD) synthesis of graphene on copper surfaces has been of particular interest due to its simplicity and cost effectiveness. However, the rates of growth for graphene by CVD on copper are less than 0.

Seed-Assisted Growth of Single-Crystalline Patterned Graphene Domains on Hexagonal Boron Nitride by Chemical Vapor Deposition.

Vertical heterostructures based on two-dimensional layered materials, such as stacked graphene and hexagonal boron nitride (G/h-BN), have stimulated wide interest in fundamental physics, material sciences and nanoelectronics. To date, it still remains challenging to obtain high quality G/h-BN heterostructures concurrently with controlled nucleation density and thickness uniformity. In this work, with the aid of the well-defined poly(methyl methacrylate) seeds, effective control over the nucleation densities and locations of graphene domains on the predeposited h-BN monolayers was realized, leading to the formation of patterned G/h-BN arrays or continuous films.

Evolution of domains and grain boundaries in graphene: a kinetic Monte Carlo simulation.

To understand the mystery of preferential mismatching angle of grain boundaries (GB) in multi-crystalline graphene observed experimentally, a systematic kinetic Monte Carlo simulation is designed to explore how a two-dimensional amorphous carbon system evolves into graphene domains and GBs. The details of the evolution, including the graphene domain nucleation, growth, rotation, coalescence, the corresponding GB motion, rotation and elimination, are observed. One hundred individual simulations with different initial configurations are performed and our simulation confirms that it is the Stone-Wales (SW) transformation that dominates the GB fast annealing process, and the results show that graphene domains with small angle GBs (<10°) tend to be annihilated but those with medium angles (>15°) tend to become large angle (≈30°), which is a consequence of the fact that the formation energies of GBs have two minima at 0° and 30°.

The formation mechanism of multiple vacancies and amorphous graphene under electron irradiation.

The evolution of multiple vacancies (Vns) in graphene under electron irradiation (EI) was explored systematically by long time non-equilibrium molecular dynamics simulations, with n varying from 4 to 40. The simulations showed that the Vns form haeckelites in the case with small n, while forming holes as n increases. The scale of the haeckelites, characterized by the number of pentagon-heptagon pairs, grows linearly with n.