Category Archives: News

Electrically Tunable and Dramatically Enhanced Valley-Polarized Emission of Monolayer WS2 at Room Temperature with Plasmonic Archimedes Spiral Nanostructures

Congratulate  Wei-Hsiang for publishing a new paper on Advanced Materials!

Abstract: Monolayer transition metal dichalcogenides (TMDs) have intrinsic valley degrees of freedom, making them appealing for exploiting valleytronic applications in information storage and processing. WS2 monolayer possesses two inequivalent valleys in the Brillouin zone, each valley coupling selectively with a circular polarization of light. The degree of valley polarization (DVP) under the excitation of circularly polarized light (CPL) is a parameter that determines the purity of valley polarized photoluminescence (PL) of monolayer WS2. Here, we report efficient tailoring of valley-polarized PL from monolayer WS2 at room temperature (RT) through surface plasmon-exciton interactions with plasmonic Archimedes spiral (PAS) nanostructures. The DVP of WS2 at RT can be enhanced from < 5% to 40% and 50% by using 2 turns (2T) and 4 turns (4T) of PAS, respectively. Further enhancement and control of excitonic valley polarization is demonstrated by electrostatically doping monolayer WS2. For CPL on WS2-2TPAS heterostructures, the 40% valley polarization is enhanced to 70% by modulating the carrier doping via a backgate, which may be attributed to the screening of momentum-dependent long-range electron-hole exchange interactions. The manifestation of electrical tunibility in the valley-polarized emission from WS2-PAS heterostructures presents a new strategy towards harnessing valley excitons for application in ultrathin valleytronic devices.

https://onlinelibrary.wiley.com/doi/10.1002/adma.202104863

Polymer-Compatible Low-Temperature Plasma-Enhanced Chemical Vapor Deposition of Graphene on Electroplated Cu for Flexible Hybrid Electronics

Congratulate Steve for publishing a new paper on ACS Applied Materials and Interfaces!

 

Abstract:

Flexible hybrid electronics and fan-out redistribution layers rely on electroplating Cu on polymers. In this work, direct low-temperature plasma-enhanced chemical vapor deposition (PECVD) of graphene on electroplated Cu over polyimide substrates is demonstrated, and the deposition of graphene is found to passivate and strengthen the electroplated Cu circuit. The effect of the H 2 /CH 4 ratio on the PECVD graphene growth is also investigated, which is shown to affect not only the quality of graphene but also
the durability of Cu. 100,000 cycles of folding with a bending radius of 2.5 mm and the corresponding resistance tests are carried out, revealing that Cu circuits covered by graphene grown with a higher H 2 /CH 4 ratio can sustain many more bending cycles. Additionally, graphene coverage is shown to suppress the formation of copper oxides in ambient environment for at least 8 weeks
after the PECVD process.

https://pubs.acs.org/doi/full/10.1021/acsami.1c11510

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.

https://authors.elsevier.com/a/1cTXE926gyzxH0

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).

https://yehgroup.caltech.edu/files/2021/02/ACS-AMI_2021_Graphene-on-Cu-Ink.pdf

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!