Archives: Low Dimensional and Nanomaterials

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.

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!



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.

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).

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.

Direct growth of mm-size twisted bilayer graphene by plasma-enhanced chemical vapor deposition

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.

Single-step growth of graphene and graphene-based nanostructures by plasmaenhanced chemical vapor deposition

Our latest review article by Professor Nai-Chang Yeh, Chen-Chih Hsu, Jacob Bagley and
Wei-Shiuan Tseng
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