Congratulations to Chen-Hsuan Lu for passing his candidacy exam! The future in the lab awaits you! And Good Luck on your summer research trip!
Deposition of layers of graphene on silicon has the potential for a wide range of optoelectronic and mechanical applications. However, direct growth of graphene on silicon has been difficult due to the inert, oxidized silicon surfaces. Transferring graphene from metallic growth substrates to silicon is not a good solution either, because most transfer methods involve multiple steps that often lead to polymer residues or degradation of sample quality. Here we report a single-step method for large-area direct growth of continuous horizontal graphene sheets and vertical graphene nano-walls on silicon substrates by plasma-enhanced chemical vapor deposition (PECVD) without active heating. Comprehensive studies utilizing Raman spectroscopy, x-ray/ultraviolet photoelectron spectroscopy (XPS/UPS), atomic force microscopy (AFM), scanning electron microscopy (SEM) and optical transmission are carried out to characterize the quality and properties of these samples. Data gathered by the residual gas analyzer (RGA) during the growth process further provide information about the synthesis mechanism. Additionally, ultra-low friction (with a frictional coefficient ~0.015) on multilayer graphene-covered silicon surface is achieved, which is approaching the superlubricity limit (for frictional coefficients <0.01). Our growth method therefore opens up a new pathway towards scalable and direct integration of graphene into silicon technology for potential applications ranging from structural superlubricity to nanoelectronics, optoelectronics, and even the next-generation lithium-ion batteries.
We report an approach to manipulating the topological states in monolayer graphene via nanoscale strain engineering at room temperature. By placing strain-free monolayer graphene on architected nanostructures to induce global inversion symmetry breaking, we demonstrate the development of giant pseudo-magnetic fields (up to ~800 T), valley polarization, and periodic one-dimensional topological channels for protected propagation of chiral modes in strained graphene, thus paving a pathway toward scalable graphene-based valleytronics.
Our recent paper submitted to Science Advances titled “Nanoscale Strain Engineering of Giant Pseudo-Magnetic Fields, Valley Polarization and Topological Channels in Graphene”by Chen-Chih Hsu and Jiaqing Wang has been accepted. In it we explore how Nanoscale strain engineering of monolayer graphene is shown to achieve giant pseudo-magnetic fields and valley polarization.
Congratulations to Dr. Chen-Chih Hsu for successfully defending his thesis! He will be staying with us to continue his research as a staff scientist.
We are glad to welcome Chia Shuo and Akiyoshi Park as our two new graduate students to the Yeh group!
Plasma enhanced chemical vapor deposition (PECVD) techniques have been shown to be an efficient method to achieve single-step synthesis of high-quality monolayer graphene (MLG) without the need of active heating. Here we report PECVD-growth of single-crystalline hexagonal bilayer graphene (BLG) flakes and mm-size BLG films with the interlayer twist angle controlled by the growth parameters. The twist angle has been determined by three experimental approaches, including direct measurement of the relative orientation of crystalline edges between two stacked monolayers by scanning electron microscopy, analysis of the twist angle-dependent Raman spectral characteristics, and measurement of the Moiré period with scanning tunneling microscopy. In mm-sized twisted BLG (tBLG) films, the average twist angle can be controlled from 0° to approximately 20°, and the angular spread for a given growth condition can be limited to < 7°. Different work functions between MLG and BLG have been verified by the Kelvin probe force microscopy and ultraviolet photoelectron spectroscopy. Electrical measurements of back-gated field-effect-transistor devices based on small-angle tBLG samples revealed high-quality electric characteristics at 300 K and insulating temperature dependence down to 100 K. This controlled PECVD-growth of tBLG thus provides an efficient approach to investigate the effect of varying Moiré potentials on tBLG.
Monolayer transition-metal dichalcogenides (TMDCs) in the 2H-phase are semiconductors promising for opto-valleytronic and opto-spintronic applications because of their strong spin-valley coupling. Here we report detailed studies of opto-valleytronic properties of heterogeneous domains in CVD-grown monolayer WS2 single crystals.
Our latest review article by Professor Nai-Chang Yeh, Chen-Chih Hsu, Jacob Bagley and
The realization of many promising technological applications of graphene and graphene-based
nanostructures depends on the availability of reliable, scalable, high-yield and low-cost synthesis
methods. Plasma enhanced chemical vapor deposition (PECVD) has been a versatile technique
for synthesizing many carbon-based materials, because PECVD provides a rich chemical
environment, including a mixture of radicals, molecules and ions from hydrocarbon precursors,
which enables graphene growth on a variety of material surfaces at lower temperatures and faster
growth than typical thermal chemical vapor deposition. Here we review recent advances in the
PECVD techniques for synthesis of various graphene and graphene-based nanostructures.
Pdf Publication Link
The Legacy of ProfessorMildred S. Dresselhaus by Nai-Chang Yeh