150 research outputs found
Asymmetry of resonance Raman profiles in semiconducting single-walled carbon nanotubes at the first excitonic transition
Carbon nanotubes are one-dimensional nanoscale systems with strongly pronounced chirality-dependent optical properties with multiple excitonic transitions. We investigate the high-energy G mode of semiconducting single-walled nanotubes of different chiralities at first excitonic transition by applying resonant Raman spectroscopy. The G mode intensity dependence on excitation energy yielded asymmetric resonance Raman profiles similar to ones we reported for the second excitonic transition. We find the scattering efficiency to be strongest at the incoming Raman resonance. Still, the degree of asymmetry is different for the first and second transitions and the first transition profiles provide a narrower line shape due to longer exciton lifetimes. The overall scattering efficiency is up to a factor of 25 times more intense at first excitonic transition, compared to the second transition. The fifth-order perturbation theory, with implemented phonon scattering pathways between excitonic states, excellently reproduced experimental data
Principles of carbon nanotube dielectrophoresis
Dielectrophoresis (DEP) describes the motion of suspended objects when exposed to an inhomogeneous electric field. It has been successful as a method for parallel and site-selective assembling of nanotubes from a dispersion into a sophisticated device architecture. Researchers have conducted extensive works to understand the DEP of nanotubes in aqueous ionic surfactant solutions. However, only recently, DEP was applied to polymer-wrapped single-walled carbon nanotubes (SWCNTs) in organic solvents due to the availability of ultra-pure SWCNT content. In this paper, the focus is on the difference between the DEP in aqueous and organic solutions. It starts with an introduction into the DEP of carbon nanotubes (CNT-DEP) to provide a comprehensive, in-depth theoretical background before discussing in detail the experimental procedures and conditions. For academic interests, this work focuses on the CNT-DEP deposition scheme, discusses the importance of the electrical double layer, and employs finite element simulations to optimize CNT-DEP deposition condition with respect to the experimental observation. An important outcome is an understanding of why DEP in organic solvents allows for the deposition and alignment of SWCNTs in low-frequency and even static electric fields, and why the response of semiconducting SWCNTs (s-SWCNTs) is strongly enhanced in non-conducting, weakly polarizable media. Strategies to further improve CNT-DEP for s-SWCNT-relevant applications are given as well. Overall, this work should serve as a practical guideline to select the appropriate setting for effective CNT DEP
Light emission, light detection and strain sensing with nanocrystalline graphene
Graphene is of increasing interest for optoelectronic applications exploiting
light detection, light emission and light modulation. Intrinsically light
matter interaction in graphene is of a broadband type. However by integrating
graphene into optical micro cavities also narrow band light emitters and
detectors have been demonstrated. The devices benefit from the transparency,
conductivity and processability of the atomically thin material. To this end we
explore in this work the feasibility of replacing graphene by nanocrystalline
graphene, a material which can be grown on dielectric surfaces without catalyst
by graphitization of polymeric films. We have studied the formation of
nanocrystalline graphene on various substrates and under different
graphitization conditions. The samples were characterized by resistance,
optical transmission, Raman, X-ray photoelectron spectroscopy, atomic force
microscopy and electron microscopy measurements. The conducting and transparent
wafer-scale material with nanometer grain size was also patterned and
integrated into devices for studying light-matter interaction. The measurements
show that nanocrystalline graphene can be exploited as an incandescent emitter
and bolometric detector similar to crystalline graphene. Moreover the material
exhibits piezoresistive behavior which makes nanocrystalline graphene
interesting for transparent strain sensors
Separation of specific single-enantiomer single-wall carbon nanotubes in the large-diameter regime
The enantiomer-level isolation of single-walled carbon nanotubes (SWCNTs) in high concentration and with high purity for nanotubes greater than 1.1 nm in diameter is demonstrated using a two-stage aqueous two-phase extraction (ATPE) technique. In total, five different nanotube species of âŒ1.41 nm diameter are isolated, including both metallics and semiconductors. We characterize these populations by absorbance spectroscopy, circular dichroism spectroscopy, resonance Raman spectroscopy, and photoluminescence mapping, revealing and substantiating mod-dependent optical dependencies. Using knowledge of the competitive adsorption of surfactants to the SWCNTs that controls partitioning within the ATPE separation, we describe an advanced acid addition methodology that enables the fine control of the separation of these select nanotubes. Furthermore, we show that endohedral filling is a previously unrecognized but important factor to ensure a homogeneous starting material and further enhance the separation yield, with the best results for alkane-filled SWCNTs, followed by empty SWCNTs, with the intrinsic inhomogeneity of water-filled SWCNTs causing them to be worse for separations. Lastly, we demonstrate the potential use of these nanotubes in field-effect transistors
Sensitive Detection of a Gaseous Analyte with LowâPower MetalâOrganic Framework Functionalized Carbon Nanotube Transistors
A highly sensitive and low-power sensing platform for detecting ethanol molecules by interfacing high-purity, large-diameter semiconducting carbon nanotube transistors with a metalâorganic framework layer is presented. The new devices outperform similar graphene-based metalâorganic framework devices by several orders of magnitude in terms of sensitivity and power consumption, and can detect extremely low ethanol concentrations down to sub-ppb levels while consuming only picowatts of power. The exceptional sensor performance results from the nanotube transistor\u27s high on/off ratio and its sensitivity to charges, allowing for ultra-low power consumption. The platform can also compensate for shifts in threshold voltage induced by ambient conditions, making it suitable for use in humid air. This novel concept of MOF/CNTFETs could be customized for detecting various gaseous analytes, leading to a range of ultra-sensitive and ultra-low power sensors
Vanishing Hysteresis in Carbon Nanotube Transistors Embedded in Boron Nitride/Polytetrafluoroethylene Heterolayers
Carbon nanotube fieldâeffect transistors fabricated on silicon wafers with thermal oxide often suffer from large gateâvoltage hysteresis, induced by charge trapping sites in oxides, surface hydroxyl groups, and the presence of water molecules. Surface functionalization and passivation, as well as vacuum annealing and reduced operating temperature, have shown to diminish or even eliminate hysteresis. Herein, the fabrication of nearly hysteresisâfree transistors on Si/SiO by embedding carbon nanotubes and the connecting electrodes in a hexagonal boron nitride (hâBN) bottom layer and a polytetrafluoroethylene (PTFE) top layer is demonstrated. The conditions at which catalystâfree synthesis of hâBN on SiO/Si with borazine is obtained, and the subsequent liquidâphase deposition of PTFE, are discussed. Device transfer curves are measured before and after PTFE deposition. It is found that the hysteresis is reduced after PTFE deposition, but vanishes only after a waiting period of several days. Simultaneously, the onâstate current increases with time. The results give evidence for the absence of trap states in hâBN/PTFE heterolayers and a high breakthrough field strength in those waferâscalable materials
a Raman scattering study
The longitudinal optical phonon of metallic nanotubes shifts by 23 cmâ1 to
lower energies when the nanotubes are deposited from a solution onto a
substrate. The linewidth increases by 13 cmâ1. The changes are explained in
terms of shifts in the Fermi energy that influence the Kohn anomaly in the
longitudinal optical phonon branch in metallic nanotubes. Using in situ
electrochemical Raman measurements we show that the Fermi energy is 0.16 eV
below its intrinsic value in metallic nanotubes in solution. Our results
impact the application of Raman spectroscopy to distinguish between metallic
and semiconducting tubes by examining the high-energy mode line shape
Tailoring supercurrent confinement in graphene bilayer weak links
The Josephson effect is one of the most studied macroscopic quantum phenomena
in condensed matter physics and has been an essential part of the quantum
technologies development over the last decades. It is already used in many
applications such as magnetometry, metrology, quantum computing, detectors or
electronic refrigeration. However, developing devices in which the induced
superconductivity can be monitored, both spatially and in its magnitude,
remains a serious challenge. In this work, we have used local gates to control
confinement, amplitude and density profile of the supercurrent induced in
one-dimensional nanoscale constrictions, defined in bilayer graphene-hexagonal
boron nitride van der Waals heterostructures. The combination of resistance
gate maps, out-of-equilibrium transport, magnetic interferometry measurements,
analytical and numerical modelling enables us to explore highly tunable
superconducting weak links. Our study opens the path way to design more complex
superconducting circuits based on this principle such as electronic
interferometers or transition-edge sensors
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