Shisheng Li

Metal-Catalyst-Free Growth of Single-Walled Carbon Nanotubes

We present a metal-catalyst-free CVD process for the high-efficiency growth of single-walled carbon nanotubes (SWNTs) on surface. By applying a 30-nm-thick SiO(2) sputtering deposited Si or Si/SiO(2) wafer as substrate and CH(4) as a carbon source, dense and uniform SWNT networks with high quality can be obtained without the presence of any metal species. Moreover, a simple patterned growth approach, using a scratched Si/SiO(2) wafer as substrate, is also presented for the growth of SWNTs with good position controllability. Our finding of the growth of SWNTs via a metal-catalyst-free process will provide valuable information for understanding the growth mechanism of SWNTs in-depth, which accordingly will facilitate the controllable synthesis and applications of carbon nanotubes.

Bulk Synthesis of Large Diameter Semiconducting Single-Walled Carbon Nanotubes by Oxygen-Assisted Floating Catalyst Chemical Vapor Deposition

Semiconducting single-walled carbon nanotubes (s-SWCNTs) with a mean diameter of 1.6 nm were synthesized on a large scale by using oxygen-assisted floating catalyst chemical vapor deposition. The oxygen introduced can selectively etch metallic SWCNTs in situ, while the sulfur growth promoter functions in promoting the growth of SWCNTs with a large diameter. The electronic properties of the SWCNTs were characterized by laser Raman spectroscopy, absorption spectroscopy, and field effect transistor measurements. It was found that the content of s-SWCNTs in the samples was highly sensitive to the amount of oxygen introduced. Under optimum synthesis conditions, enriched s-SWCNTs can be obtained in milligram quantities per batch.

Surface and Interference Coenhanced Raman Scattering of Graphene

We propose a novel surface and interference coenhanced Raman scattering technique to dramatically enhance the Raman signal intensity of graphene by using a specifically designed substrate of Si capped with surface-active metal and oxide double layers (SMO). The total enhancement ratio can reach the order of 10(3) compared with the original Si substrate. Combining the visibility of graphene on the SMO substrate, we demonstrate that the tiny structure change and surface structure of graphene can be easily detected. This technique makes Raman spectroscopy a more powerful tool in the field of ultrasensitive characterization of graphene, isolated carbon nanotubes, and other film-like materials.

Growth Velocity and Direct Length- Sorted Growth of Short Single-Walled Carbon Nanotubes by a Metal-Catalyst- Free Chemical Vapor Deposition Process

We report on the observation of a very low growth velocity of single-walled carbon nanotubes (SWNTs) and consequently the direct length-sorted growth and patterned growth of SWNTs by using a metal-catalyst-free chemical vapor deposition (CVD) process proposed recently by our group, in which SiO(2) serves as catalyst. We found that the growth velocity of the SWNTs from SiO(2) catalyst is only 8.3 nm/s, which is about 300 times slower than that of the commonly used iron group catalysts (Co as a counterpart catalyst in this study). Such a slow growth velocity renders direct length-sorted growth of SWNTs, especially for short SWNTs with hundreds of nanometers in length. By simply adjusting the growth duration, SWNTs with average lengths of 149, 342, and 483 nm were selectively obtained and SWNTs as short as approximately 20 nm in length can be grown directly. Moreover, comparative studies indicate that the SiO(2) catalyst possesses a much longer catalytic active time, showing sharp contrast with the commonly used Co catalyst which quickly loses its catalytic activity. Taking advantage of the very slow growth velocity of the SiO(2) catalyst, patterned growth of SWNT networks confined in a narrow region of <5 microm was also achieved. The short SWNTs may show intriguing physics owing to their finite length effect and are attractive for various practical applications.

High temperature selective growth of single-walled carbon nanotubes with a narrow chirality distribution from a CoPt bimetallic catalyst

Chirality-controlled synthesis of single-walled carbon nanotubes (SWCNTs) is a prerequisite for their practical applications in electronic and optoelectronic devices. We report here a novel bimetallic CoPt catalyst for the selective growth of high quality SWCNTs with a narrow chirality distribution at relatively high temperatures of 800 °C and 850 °C using atmospheric pressure alcohol chemical vapor deposition. The addition of Pt into a Co catalyst forms a CoPt alloy and significantly reduces the diameters of the as-grown SWCNTs and narrows their chirality distributions.

Discovery of a new type of topological Weyl fermion semimetal state in MoxW1-xTe2.

The recent discovery of a Weyl semimetal in TaAs offers the first Weyl fermion observed in nature and dramatically broadens the classification of topological phases. However, in TaAs it has proven challenging to study the rich transport phenomena arising from emergent Weyl fermions. The series MoxW1−xTe2 are inversion-breaking, layered, tunable semimetals already under study as a promising platform for new electronics and recently proposed to host Type II, or strongly Lorentz-violating, Weyl fermions. Here we report the discovery of a Weyl semimetal in MoxW1−xTe2 at x=25%. We use pump-probe angle-resolved photoemission spectroscopy (pump-probe ARPES) to directly observe a topological Fermi arc above the Fermi level, demonstrating a Weyl semimetal. The excellent agreement with calculation suggests that MoxW1−xTe2 is a Type II Weyl semimetal. We also find that certain Weyl points are at the Fermi level, making MoxW1−xTe2 a promising platform for transport and optics experiments on Weyl semimetals.

Exciton–Plasmon Coupling and Electromagnetically Induced Transparency in Monolayer Semiconductors Hybridized with Ag Nanoparticles

Hybrid systems of excitons strongly coupled to localized surface plasmons supported by metallic nanoparticles define a new approach to control light-matter interactions. Here, we report exciton-plasmon coupling in two-dimensional (2D) semiconductors, such as MoS2 and WS2, hybridized with silver nanoparticles. Prominent photoluminescence enhancement in monolayer MoS2 was observed with localized surface plasmon resonance (LSPR) tuned to the exciton resonance. By tuning the excitation energy, the contributions from near field enhancement and radiative emission rate enhancement via Purcell effect were resolved. Strong coherent dipole-dipole coupling between excitons and LSPR in resonant condition manifests as an electromagnetically induced transparency window in the extinction spectra of the localized surface plasmon. In this strong coupling regime a new quasi-particle, known as a plexciton, is expected to exhibit distinct properties, which exist in neither of the original particles. Our results demonstrate that 2D semiconductors hybridized with plasmonic structures not only hold great promise in the applications of energy-harvesting and light-emitting devices, but also provide an attractive platform for fundamental investigations of exciton-plasmon interactions in the strong coupling regime.Exciton-plasmon coupling in hybrids of a monolayer transition metal dichalcogenide and Ag nanoparticles is investigated in the weak and strong coupling regimes. In the weak coupling regime, both absorption enhancement and the Purcell effect collectively modify the photoluminescence properties of the semiconductor. In the strong coupling regime, electromagnetically induced transparency dips are displayed, evidencing coherent energy exchange between excitons and plasmons.

Characterization of the second- and third-order nonlinear optical susceptibilities of monolayer MoS2 using multiphoton microscopy

We report second- and third-harmonic generation in monolayer MoS

Enrichment of semiconducting single-walled carbon nanotubes by carbothermic reaction for use in all-nanotube field effect transistors.

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Rapid visualization of grain boundaries in monolayer MoS2 by multiphoton microscopy

as a tool for imaging and accurately characterizing the materials nonlinear optical properties under 1560 nm excitation. Using a surface nonlinear optics treatment, we derive expressions relating experimental measurements to second- and third-order nonlinear sheet susceptibility magnitudes, obtaining values of