Jaehyun Han

Highly Stretchable and Reliable, Transparent and Conductive Entangled Graphene Mesh Networks

A highly stretchable and reliable, transparent and conductive entangled graphene mesh network (EGMN) exhibits an interconnected percolation network, as usually shown in 1D nanowires, but with the electrical, mechanical, and thermal properties of 2D graphene. The unique combination of the 2D material properties and the network structure of wrinkled, waved, and crumpled graphene enables the EGMN to demonstrate excellent electrical reliability, mechanical durability, and thermal stability, even under harsh environmental and external conditions such as very high temperature, humidity, bending, and stretching. Specifically, after 100 000 cycles of bending with radius of 2 mm, the EGMN maintains its resistance similar to its initial value. The EGMN shows a steady monotonic response in resistance to strain cycles of 50 000 times with nearly constant gauge factors of 0.76, 1.67, and 2.55 at 10%, 40%, and 70% strains, respectively. Moreover, the EGMN shows very little change in resistance with the temperature increasing up to 1000 °C, by in situ thermal analysis with transmission electron microscopy and also by long-term stability testing at 70 °C and 70% relative humidity for 30 d. These results demonstrate that this novel entangled graphene mesh network can significantly broaden the application areas for various types of wearable and stretchable devices.

High-yield fabrication of suspended two-dimensional materials for atomic resolution imaging

Two-dimensional (2D) atomic crystals are very interesting materials due to their unique properties, which are significantly different than those observed in conventional three-dimensional (3D) materials. The atomic crystal structure of 2D materials is very important in determining the properties of these materials. In this comparative study, the 2D atomic structures of graphene and h-BN, synthesized using low pressure chemical vapor deposition (LPCVD), were analyzed using a spherical aberration-corrected high resolution transmission electron microscope (HRTEM) for the in situ observation of the atomic scale phenomena. In order to prepare the suspended membrane of 2D materials on a TEM grid with high yield, a two-step approach for soft removal of PMMA is employed for atomic resolution imaging. Using our new method of preparing TEM samples, we obtained fully covered and well suspended graphene and h-BN membranes on TEM mesh grids with clean surfaces and few defects. Honeycomb lattice structures of monolayer graphene and h-BN were clearly observed, and their atomic defects were analyzed as growing into circular and triangular holes, respectively, upon irradiation using an electron beam.