In silico carbon molecular beam epitaxial growth of graphene on the h-BN substrate: carbon source effect on van der Waals epitaxy

Analytical Model of CVD Growth of Graphene on Cu(111) Surface

Although the CVD synthesis of graphene on Cu(111) is an industrial process of outstanding importance, its theoretical description and modeling are hampered by its multiscale nature and the large number of elementary reactions involved. In this work, we propose an analytical model of graphene nucleation and growth on Cu(111) surfaces based on the combination of kinetic nucleation theory and the DFT simulations of elementary steps. In the framework of the proposed model, the mechanism of graphene nucleation is analyzed with particular emphasis on the roles played by the two main feeding species, C and C2. Our analysis reveals unexpected patterns of graphene growth, not typical for classical nucleation theories. In addition, we show that the proposed theory allows for the reproduction of the experimentally observed characteristics of polycrystalline graphene samples in the most computationally efficient way.

Density functional tight binding study of β-Ga2O3: Electronic structure, surface energy, and native point defects

A new parameter set to model monoclinic gallium oxide, β-Ga₂O₃, with the density functional tight binding (DFTB) method is developed. Using this new parameter set, DFTB calculations of bulk electronic band structure, surface energy of low-index surfaces, and formation energy of native point vacancy defects are performed and compared with the state-of-the-art density functional theory (DFT) calculations using the advanced hybrid exchange correlation functional. DFTB calculates the bandgap energy of 4.87 eV around the Fermi energy with the conduction band approximately following the DFT study by Peelaers and Van de Walle [Phys. Status Solidi B 252, 828 (2015)]. The surface energies calculated feature the correct order of stability among low index surfaces with surface energies in semiquantitative agreement with Bermudez' report [Chem. Phys. 323, 193 (2006)]. Oxygen and gallium vacancy defect formation energies and respective transition levels calculated using DFTB with a new parameter set are in semiquantitative agreement with the previous DFT reports by Varley et al. and Zacherle et al. [Appl. Phys. Lett. 97, 142106 (2010); Phys. Rev. B 87, 235206 (2013)]. This new semiempirical parameter set for β-Ga₂O₃, validated in bulk, surface, and point properties, would be useful for large spatiotemporal quantum chemical calculations regarding β-Ga₂O₃.

Graphene growth by molecular beam epitaxy: an interplay between desorption, diffusion and intercalation of elemental C species on islands

The growth of graphene by molecular beam epitaxy from an elemental carbon precursor is a very promising technique to overcome some of the main limitations of the chemical vapour deposition approach, such as the possibility to synthesize graphene directly on a wide variety of surfaces including semiconductors and insulators. However, while the individual steps of the chemical vapour deposition growth process have been extensively studied for several surfaces, such knowledge is still missing for the case of molecular beam epitaxy, even though it is a key ingredient to optimise its performance and effectiveness. In this work, we have performed a combined experimental and theoretical study comparing the growth rate of the molecular beam epitaxy and chemical vapour deposition processes on the prototypical Ir (111) surface. In particular, by employing high-resolution fast X-ray photoelectron spectroscopy, we were able to follow the growth of both single- and multi-layer graphene in real time, and to identify the spectroscopic fingerprints of the different C layers. Our experiments, supported by density functional theory calculations, highlight the role of the interaction between different C precursor species and the growing graphene flakes on the growth rate of graphene. These results provide an overview of the main differences between chemical vapour deposition and molecular beam epitaxy growth and thus on the main parameters which can be tuned to optimise growth conditions.

An atomic carbon source for high temperature molecular beam epitaxy of graphene

We report the use of a novel atomic carbon source for the molecular beam epitaxy (MBE) of graphene layers on hBN flakes and on sapphire wafers at substrate growth temperatures of ~1400 °C. The source produces a flux of predominantly atomic carbon, which diffuses through the walls of a Joule-heated tantalum tube filled with graphite powder. We demonstrate deposition of carbon on sapphire with carbon deposition rates up to 12 nm/h. Atomic force microscopy measurements reveal the formation of hexagonal moiré patterns when graphene monolayers are grown on hBN flakes. The Raman spectra of the graphene layers grown on hBN and sapphire with the sublimation carbon source and the atomic carbon source are similar, whilst the nature of the carbon aggregates is different - graphitic with the sublimation carbon source and amorphous with the atomic carbon source. At MBE growth temperatures we observe etching of the sapphire wafer surface by the flux from the atomic carbon source, which we have not observed in the MBE growth of graphene with the sublimation carbon source.

Recent progress in van der Waals heterojunctions

Following the development of many novel two-dimensional (2D) materials, investigations of van der Waals heterojunctions (vdWHs) have attracted significant attention due to their excellent properties such as smooth heterointerface, highly gate-tunable bandgap, and ultrafast carrier transport. Benefits from the atom-scale thickness, physical and chemical properties and ease of manipulation of the heterojunctions formulated by weak vdW forces were demonstrated to indicate their outstanding potential in electronic and optoelectronic applications, including photodetection and energy harvesting, and the possibility of integrating them with the existing semiconductor technology for the next-generation electronic and sensing devices. In this review, we summarized the recent developments of vdWHs and emphasized their applications. Basically, we introduced the physical properties and some newly discovered phenomena in vdWHs. Then, we emphatically presented four classical vdWHs and some novel heterostructures formed by vdW forces. Based on their unique physical properties and structures, we highlighted the applications of vdWHs including in photodiodes, phototransistors, tunneling devices, and memory devices. Finally, we provided a conclusion on the recent advances in vdWHs and outlined our perspectives. We aim for this review to serve as a solid foundation in this field and to pave the way for future research on vdW-based materials and their heterostructures.