In-situ twistable bilayer graphene

The electrical and optical properties of twisted bilayer graphene (tBLG) depend sensitively on the twist angle. To study the angle dependent properties of the tBLG, currently it is required fabrication of a large number of samples with systematically varied twist angles. Here, we demonstrate the construction of in-situ twistable bilayer graphene, in which the twist angle of the two graphene monolayers can be in-situ tuned continuously in a large range with high precision. The controlled tuning of the twist angle is confirmed by a combination of real-space and spectroscopic characterizations, including atomic force microscopy (AFM) identification of crystal lattice orientation, scanning near-field optical microscopy (SNOM) imaging of superlattice domain walls, and resonant Raman spectroscopy of the largely enhanced G-mode. The developed in-situ twistable homostructure devices enable systematic investigation of the twist angle effects in a single device, thus could largely advance the research of twistronics.

Increased Electrical Conductivity of Carbon Nanotube Fibers by Thermal and Voltage Annealing

We report the effect of annealing, both electrical and by applied voltage, on the electrical conductivity of fibers spun from carbon nanotubes (CNTs). Commercial CNT fibers were used as part of a larger goal to better understand the factors that go into making a better electrical conductor from CNT fibers. A study of thermal annealing in a vacuum up to 800 °C was performed on smaller fiber sections along with a separate analysis of voltage annealing up to 7 VDC; both exhibited a sweet spot in the process as determined by a combination of a two-point probe measurement with a nanoprobe, resonant Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS). Scaled-up tests were then performed in order to translate these results into bulk samples inside a tube furnace, with similar results that indicate the potential for an optimized method of achieving a better conductor sample made from CNT fibers. The results also help to determine the surface effects that need to be overcome in order to achieve this.

Investigation of the Phonon Spectra in CdSe/CdS Nanoplatelets by Raman Spectroscopy

We report the phonon spectra of core/shell CdSe/CdS nanoplatelets with different shell thicknesses studied using Raman scattering. The nanoplatelets are rectangular colloidal nanocrystals, with thicknesses of core and shell layers of a few nanometers. The Raman spectra show features corresponding to the dominating longitudinal optical (LO) and surface optical (SO) phonon modes of the CdSe core in CdS shell located in the frequency regions of 200-210 and 250-290 cm-1, respectively. As the shell thickness increases, the phonon modes reveal a frequency shift and a change in intensity. The frequency shift associated with a change in the stress state in the core and shell, as well as with confinement effects is discussed. The phonon mode intensities are determined by the thickness of the shell and the proximity to resonant Raman scattering conditions.