Gate tunable optical absorption and band structure of twisted bilayer graphene

We report the infrared transmission measurement on electrically gated twisted bilayer graphene. The optical absorption spectrum clearly manifests the dramatic changes such as the splitting of inter-linear-band absorption step, the shift of inter-van Hove singularity transition peak, and the emergence of very strong intra-valence (intra-conduction) band transition. These anomalous optical behaviors demonstrate consistently the non-rigid band structure modification created by the ion-gel gating through the layer-dependent Coulomb screening. We propose that this screening-driven band modification is an universal phenomenon that persists to other bilayer crystals in general, establishing the electrical gating as a versatile technique to engineer the band structures and to create new types of optical absorptions that can be exploited in electro-optical device application.Comment: 13 pages, 4 figure

Infrared study of large scale h-BN film and graphene/h-BN heterostructure

We synthesize a series of CVD h-BN films and perform critical infrared spectroscopic characterization. For high-temperature (HT, Temp = 1400 ??C) grown h-BN thin film, only E1u-mode infrared phonon is activated demonstrating highly aligned 2D h-BN planes over large area, whereas low-temperature (LT, Temp = 1000 ??C) grown film shows two phonon peaks, E1u and A2u, due to stacking of h-BN plane at tilted angle. For CVD graphene transferred on HT h-BN/SiO2/Si substrate, interband transition spectrum ??1 shifts strongly to lower energy compared with that on LT h-BN/SiO2/Si and on bare SiO2/Si substrates, revealing that the residual carrier density n in graphene is suppressed by the use of HT h-BN layer. Also, the interband transition width of ??1 defined by effective temperature is reduced from 400 K for G/SiO2/Si to 300 K for HT h-BN/SiO2/Si. The behaviors of n and effective temperature show that the HT h-BN film can decouple CVD graphene from the impurity and defect of SiO2 leading to a large scale free-standing like graphene.clos

Wafer-Scale and Wrinkle-Free Epitaxial Growth of Single-Orientated Multilayer Hexagonal Boron Nitride on Sapphire

Large-scale growth of high-quality hexagonal boron nitride has been a challenge in two-dimensional-material-based electronics. Herein, we present wafer-scale and wrinkle-free epitaxial growth of multilayer hexagonal boron nitride on a sapphire substrate by using high-temperature and low-pressure chemical vapor deposition. Microscopic and spectroscopic investigations and theoretical calculations reveal that synthesized hexagonal boron nitride has a single rotational orientation with AA' stacking order. A facile method for transferring hexagonal boron nitride onto other target substrates was developed, which provides the opportunity for using hexagonal boron nitride as a substrate in practical electronic circuits. A graphene field effect transistor fabricated on our hexagonal boron nitride sheets shows clear quantum oscillation and highly improved carrier mobility because the ultraflatness of the hexagonal boron nitride surface can reduce the substrate-induced degradation of the carrier mobility of two-dimensional materialsclos