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 C₆₀ into Nano-Graphenes

Annealing of C₆₀ in hydrogen at temperatures above the stability limit of C–H bonds in C₆₀H_<i>x</i> (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 C₂H₂ 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 C₆₀ 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 C₆₀ thin films showed a thickness-dependent results with reaction products significantly different for the thinnest films compared to bulk powders. Hydrogen annealing of C₆₀ 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