All posts by marcus

Evidences for pressure-induced two-phase superconductivity and mixed structures of NiTe2 and NiTe in type-II Dirac semimetal NiTe2-x (x = 0.38 ± 0.09) single crystals

Bulk NiTe2 is a type-II Dirac semimetal with non-trivial Berry phases associated with the Dirac fermions. Theory suggests that monolayer NiTe2 is a two-gap superconductor, whereas experimental investigation of bulk NiTe1.98 for pressures (P) up to 71.2 GPa do not reveal any superconductivity. Here we report experimental evidences for pressure-induced two-phase superconductivity as well as mixed structures of NiTe2 and NiTe in Te-deficient NiTe2-x (x = 0.38 ± 0.09) single crystals. Hole-dominant multi-band superconductivity with the P3¯m1 hexagonal-symmetry structure of NiTe2 appears at P ≥ 0.5 GPa, whereas electron-dominant single-band superconductivity with the P2/m monoclinic-symmetry structure of NiTe emerges at 14.5 GPa < P < 18.4 GPa. The coexistence of hexagonal and monoclinic structures and two-phase superconductivity is accompanied by a zero Hall coefficient up to ∼ 40 GPa, and the second superconducting phase prevails above 40 GPa, reaching a maximum Tc = 7.8 K and persisting up to 52.8 GPa. Our findings suggest the critical role of Te-vacancies in the occurrence of superconductivity and potentially nontrivial topological properties in NiTe2-x.

Single-Step Direct Growth of Graphene on Cu Ink towards Flexible Hybrid Electronic Applications by Plasma-Enhanced Chemical Vapor Deposition

Highly customized and free-formed products in flexible hybrid electronics (FHE) require direct
pattern creation such as inkjet printing (IJP) to accelerate the product development. In this work,
we demonstrate direct growth of graphene on Cu ink deposited on polyimide (PI) by means of
plasma enhanced chemical vapor deposition (PECVD), which provides simultaneous reduction,
sintering and passivation of the Cu ink and further reduces its resistivity. We investigate the
PECVD growth conditions for optimizing the graphene quality on Cu ink, and find that the defect
characteristics of graphene are sensitive to the H2/CH4 ratio at higher total gas pressure during the
growth. The morphology of Cu ink after the PECVD process and the dependence of graphene
quality on the H2/CH4 ratio may be attributed to the difference in the corresponding electron
temperature. This study therefore paves a new pathway towards efficient growth of high-quality
graphene on Cu ink for applications to flexible electronics and Internet of Things (IoT).

Congratulations to Dr. Chien-Chang Chen!

Congratulations to Dr. Chen who successfully defended his thesis on Friday July 3rd, 2020. Kyle will be leaving for Taiwan on July 14 to take a new position at the Taiwan Semiconductor Manufacturing Company (TSMC) starting on August 3.  We all wish him the best of luck on his next journey!  Good luck and Congrats!

Direct large-area growth of graphene on silicon for potential ultra-low-friction applications and silicon-based technologies

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.

Nanoscale strain engineering of giant pseudo-magnetic fields, valley polarization, and topological channels in graphene

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.

Paper Accepted to Science Advances on Strain Engineering In Graphene

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.