12 research outputs found
Reversible Diameter Modulation of Single-Walled Carbon Nanotubes by Acetonitrile-Containing Feedstock
Changing the carbon feedstock from pure ethanol to a 5 vol % mixture of acetonitrile in ethanol during the growth of vertically aligned single-walled carbon nanotubes (SWNTs) reduces the mean diameter of the emerging SWNTs from approximately 2 to 1 nm. We show this feedstock-dependent change is reversible and repeatable, as demonstrated by multilayered vertically aligned SWNT structures. The reversibility of this process and lack of necessity for catalyst modification provides insight into the role of nitrogen in reducing the SWNT diameter
Water Encapsulation Control in Individual Single-Walled Carbon Nanotubes by Laser Irradiation
Owing
to one-dimensionality, nanoscale curvature, and high chemical
stability, single-walled carbon nanotubes (SWNTs) have unique surfaces
for gas molecules: outer surface as adsorption (exohedral) site and
inner surface that provides encapsulation (endohedral) space. Because
as-grown SWNTs have different structure (chirality and diameter) and
they are normally bundled, it is extremely difficult to investigate
the intrinsic properties of SWNTs as adsorbent. Here we demonstrate
controlling adsorption and encapsulation states of water in individual
suspended SWNTs using laser irradiation with monitoring of their behavior
by photoluminescence measurement and perform molecular dynamics simulation.
The laser heating and the pressure control make water molecules encapsulated
or ejected for SWNTs, which are individually oxidized and opened with
laser heating. The precise control of oxidization makes it possible
to observe the cluster formation of water molecules during the encapsulation
process and to confine water molecules inside SWNTs even in vacuum
Effect of Gas Pressure on the Density of Horizontally Aligned Single-Walled Carbon Nanotubes Grown on Quartz Substrates
We
investigate the influence of gas pressure on the growth of horizontally
aligned single-walled carbon nanotubes (SWCNTs) on R-cut and r-cut
crystal quartz substrates by alcohol catalytic chemical vapor deposition
(CVD). The density of horizontally aligned SWCNTs was found to depend
highly on gas pressure. A study of the SWCNT growth as a function
of CVD time revealed that the density of horizontally aligned SWCNTs
continued to increase for 10 min at reduced pressure, whereas the
density saturated rapidly at higher pressure even though catalysts
were not deactivated. We argue that variation of incubation time for
low-pressure CVD is key for independent growth of horizontally aligned
SWCNTs and hence higher density growth
On-Chip Sorting of Long Semiconducting Carbon Nanotubes for Multiple Transistors along an Identical Array
Ballistic
transport and sub-10 nm channel lengths have been achieved
in transistors containing one single-walled carbon nanotube (SWNT).
To fill the gap between single-tube transistors and high-performance
logic circuits for the replacement of silicon, large-area, high-density,
and purely semiconducting (s-) SWNT arrays are highly desired. Here
we demonstrate the fabrication of multiple transistors along a purely
semiconducting SWNT array <i>via</i> an on-chip purification
method. Water- and polymer-assisted burning from site-controlled nanogaps
is developed for the reliable full-length removal of metallic SWNTs
with the damage to s-SWNTs minimized even in high-density arrays.
All the transistors with various channel lengths show large on-state
current and excellent switching behavior in the off-state. Since our
method potentially provides pure s-SWNT arrays over a large area with
negligible damage, numerous transistors with arbitrary dimensions
could be fabricated using a conventional semiconductor process, leading
to SWNT-based logic, high-speed communication, and other next-generation
electronic devices
Highly Stable and Tunable n‑Type Graphene Field-Effect Transistors with Poly(vinyl alcohol) Films
The
intrinsic p-type behavior of graphene field-effect transistors (FETs)
under ambient conditions poses a fundamental challenge for the assembly
of complex electronic devices, such as integrated circuits. In this
work, we present a protocol for tunable n-type doping of graphene
FETs via polyÂ(vinyl alcohol) (PVA) coating. Using graphene grown by
alcohol catalytic chemical vapor deposition, functionalization of
the surface by this hydroxyl anion-rich polymer results in an evolution
of the FETs from p-type to ambipolar or n-type even under ambient
air conditions. The doping level of graphene is strongly related to
the PVA film coating parameters, such as solution concentration, hardening
temperature, and hardening time. This PVA coating proves to be a simple
and stable approach to tuning the Dirac point and doping level of
graphene, which is highly desirable and of great significance for
the future of graphene-based electronic devices
Growth Analysis of Single-Walled Carbon Nanotubes Based on Interatomic Potentials by Molecular Dynamics Simulation
Molecular
dynamics simulation was performed to understand the growth
mechanism of single-walled carbon nanotubes (SWNTs) by using the Brenner–Tersoff
potential as the interaction among carbon atoms (C–C) and the
Tersoff-type potential as the interaction between carbon and metal
(C–M) and between metal and metal atoms (M–M). The potential
functions for C–M and M–M bonds were established from
the results of ab initio calculations. The growth of high-quality
SWNTs was simulated at a suitable temperature and supply ratio of
carbon atoms. The potential energy of carbon atoms was strongly dependent
on the number of C–C and C–M bonds. The dependence explains
the growth process, including cap formation, its lift-off, and the
continuous SWNT growth
Diameter Modulation of Vertically Aligned Single-Walled Carbon Nanotubes
We demonstrate wide-range diameter modulation of vertically aligned single-walled carbon nanotubes (SWNTs) using a wet chemistry prepared catalyst. In order to ensure compatibility to electronic applications, the current minimum mean diameter of 2 nm for vertically aligned SWNTs is challenged. The mean diameter is decreased to about 1.4 nm by reducing Co catalyst concentrations to 1/100 or by increasing Mo catalyst concentrations by five times. We also propose a novel spectral analysis method that allows one to distinguish absorbance contributions from the upper, middle, and lower parts of a nanotube array. We use this method to quantitatively characterize the slight diameter change observed along the array height. On the basis of further investigation of the array and catalyst particles, we conclude that catalyst aggregationî—¸rather than Ostwald ripeningî—¸dominates the growth of metal particles
Temperature Distribution and Thermal Conductivity Measurements of Chirality-Assigned Single-Walled Carbon Nanotubes by Photoluminescence Imaging Spectroscopy
It is expected that
single-walled carbon nanotubes (SWCNTs) have
high thermal conductivity along the tube axis and that the thermal
conductivities depend on their structure, such as length, diameter,
chirality (<i>n</i>, <i>m</i>), and so forth.
Although many experimental measurements of the thermal conductivity
have been reported, the SWCNT structure was not characterized sufficiently.
In particular, the chirality was not assigned, and it was not confirmed
whether SWCNT was isolated or not (bundled with multiplicate SWCNTs).
Therefore, measured values widely vary (10<sup>1</sup> to 10<sup>4</sup> W/(m·K)) so far. Here, we measured the thermal conductivity
of chirality-assigned SWCNTs, which were individually suspended, by
using photoluminescence (PL) imaging spectroscopy. The temperature
distribution along the tube axis was obtained, and the temperature
dependence of the thermal conductivity was measured in a wide-temperature
range (from 350 to 1000 K). For (9, 8) SWCNTs with 10–12 μm
in length, the thermal conductivity was 1166 ± 243 W/(m·K)
at 400 K. The proposed PL imaging spectroscopy enables to measure
the thermal conductivity of SWCNTs with high precision and without
any contacts, and it is an effective method in the temperature distribution
measurements of nanomaterials
Digital Isotope Coding to Trace the Growth Process of Individual Single-Walled Carbon Nanotubes
Single-walled
carbon nanotubes (SWCNTs) are attracting increasing
attention as an ideal material for high-performance electronics through
the preparation of arrays of purely semiconducting SWCNTs. Despite
significant progress in the controlled synthesis of SWCNTs, their
growth mechanism remains unclear due to difficulties in analyzing
the time-resolved growth of individual SWCNTs under practical growth
conditions. Here we present a method for tracing the diverse growth
profiles of individual SWCNTs by embedding digitally coded isotope
labels. Raman mapping showed that, after various incubation times,
SWCNTs elongated monotonically until their abrupt termination. <i>Ex situ</i> analysis offered an opportunity to capture rare
chirality changes along the SWCNTs, which resulted in sudden acceleration/deceleration
of the growth rate. Dependence on growth parameters, such as temperature
and carbon concentration, was also traced along individual SWCNTs,
which could provide clues to chirality control. Systematic growth
studies with a variety of catalysts and conditions, which combine
the presented method with other characterization techniques, will
lead to further understanding and control of chirality, length, and
density of SWCNTs
Self-Limiting Chemical Vapor Deposition Growth of Monolayer Graphene from Ethanol
Using low-pressure chemical vapor
deposition (LPCVD), we, for the
first time, realize the self-limiting growth behavior of monolayer
graphene on commercially available electroplated copper foils from
a carbon precursor other than methane, and systematically investigate
the growth of graphene from ethanol and compare its self-limiting
behavior over copper facets with different identities. Results show
that the growth of graphene from ethanol in the LPCVD process is a
substrate-mediated process, in which the domains of graphene are determined
by the lattice axes on the copper facets. Moreover, during the evolution
of the domains, low-index copper facets of Cu(111) and Cu(100) play
a critical role in the following self-limiting process of a continuous
graphene sheet, whereas the Cu(110) and high-index facets favor nucleation
and formation of secondary layers. In addition, a high degree of similarity
exists between graphene grown from ethanol and methane, showing that,
when the carbon flux is sufficiently low, the self-limiting growth
of graphene on copper surfaces using LPCVD is independent of the precursor
structure of ethanol and methane