No abstract

Van der Waals layered materials with well-defined twist angles between the crystal lattices of individual layers have attracted increasing attention due to the emergence of unexpected material properties. As many properties critically depend on the exact twist angle and its spatial homogeneity, there is a need for a fast and non-invasive characterization technique of the local twist angle, to be applied preferably right after stacking. We demonstrate that confocal Raman spectroscopy can be utilized to spatially map the twist angle in stacked bilayer graphene for angles between 6.5∘ and 8∘ when using a green excitation laser. The twist angles can directly be extracted from the moiré superlattice-activated Raman scattering process of the transverse acoustic (TA) phonon mode. Furthermore, we show that the width of the TA Raman peak contains valuable information on spatial twist angle variations on length scales below the laser spot size of ∼500 nm.

We characterize InSb quantum dots induced by bottom finger gates within a nanowire that is grown via the vapor–liquid–solid process. The gates are separated from the nanowire by an exfoliated 35 nm thin hexagonal BN flake. We probe the Coulomb diamonds of the gate-induced quantum dot exhibiting a charging energy of ∼2.5 meV and orbital excitation energies up to 0.3 meV. The gate hysteresis for sweeps covering 5 Coulomb diamonds reveals an energy hysteresis of only 60 μeV between upward and downward sweeps. Charge noise is studied via long-term measurements at the slope of a Coulomb peak revealing a potential fluctuation of ∼1 μeV/Hz at 1 Hz. This makes h-BN a dielectric with the currently lowest gate hysteresis and lowest low-frequency potential fluctuations reported for low-gap III–V nanowires. The extracted values are similar to state-of-the-art quantum dots within Si/SiGe and Si/SiO2 systems.