21 research outputs found
Enhancing Reliability of Studies on Single Filament Memristive Switching via an Unconventional cAFM Approach
Memristive devices are highly promising for implementing neuromorphic functionalities in future electronic hardware, and direct insights into memristive phenomena on the nanoscale are of fundamental importance to reaching this. Conductive atomic force microscopy (cAFM) has proven to be an essential tool for probing memristive action locally on the nanoscale, but the significance of the acquired data frequently suffers from the nonlocality associated with the thermal drift of the tip in ambient conditions. Furthermore, comparative studies of different configurations of filamentary devices have proven to be difficult, because of an immanent variability of the filament properties between different devices. Herein, these problems are addressed by constraining the memristive action directly at the apex of the probe through functionalization of a cAFM tip with an archetypical memristive stack, which is comprised of Ag/Si3N4. The design of such functionalized cantilevers (entitled here as "memtips") allowed the capture of the long-term intrinsic current response, identifying temporal correlations between switching events, and observing emerging spiking dynamics directly at the nanoscale. Utilization of an identical memtip for measurements on different counter electrodes made it possible to directly compare the impact of different device configurations on the switching behavior of the same filament. Such an analytical approach in ambient conditions will pave the way towards a deeper understanding of filamentary switching phenomena on the nanoscale
Broadband laser polarization control with aligned carbon nanotubes
We introduce a simple approach to fabricate aligned carbon nanotube (ACNT)
device for broadband polarization control in fiber laser systems. The ACNT
device was fabricated by pulling from as-fabricated vertically-aligned carbon
nanotube arrays. Their anisotropic property is confirmed with optical and
scanning electron microscopy, and with polarized Raman and absorption
spectroscopy. The device was then integrated into fiber laser systems (at two
technologically important wavelengths of 1 and 1.5 um) for polarization
control. We obtained a linearly-polarized light output with the maximum
extinction ratio of ~12 dB. The output polarization direction could be fully
controlled by the ACNT alignment direction in both lasers. To the best of our
knowledge, this is the first time that ACNT device is applied to polarization
control in laser systems. Our results exhibit that the ACNT device is a simple,
low-cost, and broadband polarizer to control laser polarization dynamics, for
various photonic applications (such as material processing, polarization
diversity detection in communications), where the linear polarization control
is necessary.Comment: 5 pages, 6 figure
Extraction of some strategic elements from thorium–uranium concentrate using bioproducts of Aspergillus ficuum and Pseudomonas aeruginosa
The activity of the bioproducts from Aspergillus ficuum and Pseudomonas aeruginosa for extraction of thorium (Th4+), uranium (UO22+) and rare earth elements (REEs) from thorium–uranium concentrate was studied. P. aeruginosa produce element-specific ligand (siderophore) that is able to change pH and enhance chelation of Th4+ and UO22+. The produced siderophore at pH 5.3 has the ability to bioleach and is complexed with 68.00% of uranium and 65.00% of thorium. Also, A. ficuum produced different kinds of organic acids which leached 30.00% of uranium and 29.12% of thorium in addition to 20.00% of lanthanum, 33.00% of cerium and 2.51% of yttrium as rare earth elements at pH 3.0. Oxalic acid was efficient for Th4+, UO22+and REEs precipitation. The binocular stereo-microscope (BSM), environmental scanning electron microscope (ESEM) and X-ray diffraction (XRD) analyses confirmed the percentages of extracted metals. Exogenous polysaccharides (EPSs) seem to play an important role in bioleaching and removal of these elements. It was found that EPSs produced by A. ficuum adsorbed Th4+, UO22+ and REEs while that produced by P. aeruginosa adsorbed REEs only
Contactless Characterization of Electronic Properties of Nanomaterials Using Dielectric Force Microscopy
Characterization of electronic properties of nanomaterials usually
involves fabricating field effect transistors and deriving materials
properties from device performance measurements. The difficulty in
fabricating electrical contacts to extremely small-sized nanomaterials
as well as the intrinsic heterogeneity of nanomaterials makes it a
challenging task to measure the electronic properties of large numbers
of individual nanomaterials. Here, we utilize a scanning probe technique,
the dielectric force microscopy (DFM) to address the challenges. The
DFM technique measures the low frequency dielectric response of nanomaterials,
which is intrinsically related to their electrical conductivity. The
incorporation of a gate bias voltage in DFM measurements allows for
charge carrier density modulation, which is exploited to determine
the carrier type in nanomaterials such as semiconducting single-walled
carbon nanotubes (SWNTs) and ZnO nanowires (ZnO NWs). This technique
avoids the need of electrical contacts and inherits the spatial mapping
capability of scanning probe microscopy, as manifested in the imaging
of intratube metallic/semiconducting junctions in SWNTs. We expect
the DFM technique to find broad applications in the characterization
of various nanoelectonic materials and nanodevices
Engineering of Molybdenum Sulfide Nanobunches on MWCNTs: Modulation of Active Sites and Electronic Conductivity via Controllable Solvothermal Deposition
Amorphous, mixed-valency, molybdenum sulfide (MoSx) with a proposed formula, [Mo(IV)4Mo(V)2(S22-)3(S2-)5](SO4)5, was grown through a one-pot, solvothermal synthesis on multi-walled carbon nanotubes (MWCNTs) in a gram-scale setup. Optimizing the loading of the active catalyst relative to the conductive support resulted in optimized catalytic performance in hydrogen evolution reaction, reaching down to one of the lowest reported overpotentials, η10 = 140 mV and η100 = 198 mV with a Tafel slope of 62 mV/dec, for the 6.5 wt % of MoSx@MWCNTs. Engineering this amorphous MoSx catalyst was made possible through control of the oxidation state of Mo to avoid the fully reduced MoS2 phases. We also demonstrate that engineering defects in the MoSx catalyst does not require sophisticated techniques (e.g., UHV deposition, ion beam sputtering, and pulsed laser ablation) but can rather be induced simply through controlling the reductive synthesis conditions
Spatially Resolved Transport Properties of Pristine and Doped Single-Walled Carbon Nanotube Networks
We use noninvasive atomic force microscopy
to probe the spatial
electrical conductivity of isolated junctions of pristine and nitric
acid treated single-walled carbon nanotube networks (SWCNT-N). By
analyzing the local IV curves of SWCNTs and bundles with various diameters,
the resistance per unit length and the contact resistance of their
junctions are estimated to be 3–16 kΩ/μm and 29–532
kΩ, respectively. We find that the contact resistance decreases
with increasing SWCNT or bundle diameter and depends on the contact
morphology, reaching a value of 29 kΩ at a diameter of 10 nm.
A nitric acid treatment moderately dopes SWCNTs and reduces their
average contact resistance by a factor of 3 while the resistance of
the nanotubes remains largely unaltered. Remarkably, the same treatment
on an SWCNT-N shows similar reduction in the sheet resistance by a
factor of 4. These results suggest that the resistance reduction mechanism
is related to the contact modulation with no major impact on conductance
of SWCNTs
LETTERS A novel hybrid carbon material
Both fullerenes and single-walled carbon nanotubes (SWNTs) exhibit many advantageous properties 1. Despite the similarities between these two forms of carbon, there have been very few attempts to physically merge them 2,3. We have discovered a novel hybrid material that combines fullerenes and SWNTs into a single structure in which the fullerenes are covalently bonded to the outer surface of the SWNTs. These fullerenefunctionalized SWNTs, which we have termed NanoBuds, were selectively synthesized in two different one-step continuous methods, during which fullerenes were formed on iron-catalyst particles together with SWNTs during CO disproportionation. The field-emission characteristics of NanoBuds suggest that they may possess advantageous properties compared with singlewalled nanotubes or fullerenes alone, or in their nonbonded configurations
Hydrogen-Driven Cage Unzipping of C<sub>60</sub> into Nano-Graphenes
Annealing of C<sub>60</sub> in hydrogen
at temperatures above the
stability limit of C–H bonds in C<sub>60</sub>H<sub><i>x</i></sub> (500–550 °C) is found to result in direct
collapse of the cage structure, evaporation of light hydrocarbons,
and formation of solid mixture composed of larger hydrocarbons and
few-layered graphene sheets. Only a minor part of this mixture is
soluble; this was analyzed using matrix-assisted laser desorption/ionization
MS, Fourier transform infrared (FTIR), and nuclear magnetic resonance
spectroscopy and found to be a rather complex mixture of hydrocarbon
molecules composed of at least tens of different compounds. The sequence
of most abundant peaks observed in MS, which corresponds to C<sub>2</sub>H<sub>2</sub> mass difference, suggests a stepwise breakup
of the fullerene cage into progressively smaller molecular fragments
edge-terminated by hydrogen. A simple model of hydrogen-driven C<sub>60</sub> unzipping is proposed to explain the observed sequence of
fragmentation products. The insoluble part of the product mixture
consists of large planar polycyclic aromatic hydrocarbons, as evidenced
by FTIR and Raman spectroscopy, and some larger sheets composed of
few-layered graphene, as observed by transmission electron microscopy.
Hydrogen annealing of C<sub>60</sub> thin films showed a thickness-dependent
results with reaction products significantly different for the thinnest
films compared to bulk powders. Hydrogen annealing of C<sub>60</sub> films with the thickness below 10 nm was found to result in formation
of nanosized islands with Raman spectra very similar to the spectra
of coronene oligomers and conductivity typical for graphene