Conductive modes of atomic force microscopy are widely used to characterize the electronic properties of materials, and in such measurements, contact size is typically determined from current flow. Conversely, in nanodevice applications, the current flow is predicted from the estimated contact size. In both cases, it is very common to relate the contact size and current flow using well-established ballistic electron transport theory. Here we performed 19 electromechanical tests of platinum nanocontacts with in situ transmission electron microscopy to measure contact size and conductance. We also used molecular dynamics simulations of matched nanocontacts to investigate the nature of contact on the atomic scale. Together, these tests show that the ballistic transport equations under-predict the contact size by more than an order of magnitude. The measurements suggest that the low conductance of the contact cannot be explained by the scattering of electrons at defects nor by patchy contact due to surface roughness; instead, the lower-than-expected contact conductance is attributed to approximately a monolayer of insulating surface species on the platinum. Surprisingly, the low conductance persists throughout loading and even after significant sliding of the contact in vacuum. We apply tunneling theory and extract best-fit barrier parameters that describe the properties of this surface layer. The implications of this investigation are that electron transport in device-relevant platinum nanocontacts can be significantly limited by the presence and persistence of surface species, resulting in current flow that is better described by tunneling theory than ballistic electron transport, even for cleaned pure-platinum surfaces and even after loading and sliding in vacuum.
The use of sulfur as a cathode material for lithium-sulfur (Li-S) batteries has attracted significant attention due to its high theoretical specific capacity (1675 mA h g(-1)); however, practicality is hindered by a number of obstacles, including the shuttling effect of polysulfides and the low electrical conductivity of sulfur. Herein, ball milling sulfur with unzipped multiwalled carbon nanotubes (UMWNTs) was found to covalently immobilize sulfur nanoparticles to the UMWNTs and resulted in composites (designated as S@UMWNTs) with high electrical conductivity. The unzipping degree of MWNTs was first controlled to optimize the immobilization of sulfur nanoparticles to UMWNTs and the electrochemical performance of the resulting Li-S batteries. The presence of C-S covalent bonds between the UMWNTs and sulfur nanoparticles was verified using x-ray photoelectron spectroscopy and Fourier transform infrared spectroscopy, and the formation of C-S bonds was ascribed to the reactions between the mechanically-induced sulfur radicals and the functional groups of UMWNTs. As a result, when used as a cathode material for Li-S batteries, the S@UMWNTs exhibited excellent electrochemical performance, including a good long-term cycling stability and low capacity decay (e.g., ca. 0.09% per cycle over 500 charge/discharge cycles at 1 C) due to the suppression of the shuttling effect by the C-S covalent bonds.
The tunable propagation properties of MoS2 supported hybrid surface plasmon waveguides based on dielectric fiber-gap-metal substrate structures have been investigated by using the finite element method, including the effects of structural parameters, the dielectric fiber shape and carrier concentration of the MoS(2 )layer. The results reveal that as the dielectric fiber radius increases, the confinement of the hybrid mode increases, and the losses show a peak. The shape of the dielectric fiber affects the propagation properties obviously, with an optimum structural parameter (a large value of the elliptical parameter) the confinement and figure of merits increase, and the dissipation decreases simultaneously. In addition, as the carrier concentration of the MoS2 layer increases, the modulation depth of absorption reaches more than 40%, and the propagation constants manifest obvious double peaks at wavelengths of 610 nm (2.03 eV) and 660 nm (1.88 eV), coming from the excitons' absorption of the MoS2 layer. The results are very useful in helping one to understand the tunable mechanisms of hybrid mode waveguide structures and for the design of novel surface plasmonic devices in the future, e.g. absorbers, modulators, lasers, and resonators.
This work investigates the growth of B-C-N layers by chemical vapor deposition using methylamine borane (MeAB) as the single-source precursor. MeAB has been synthesized and characterized, paying particular attention to the analysis of its thermolysis products, which are the gaseous precursors for B-C-N growth. Samples have been grown on Cu foils and transferred onto different substrates for their morphological, structural, chemical, electronic and optical characterizations. The results of these characterizations indicate a segregation of h-BN and graphene-like (Gr) domains. However, there is an important presence of B and N interactions with C at the Gr borders, and of C interacting at the h-BN-edges, respectively, in the obtained nano-layers. In particular, there is a significant presence of C-N bonds, at Gr/h-BN borders and in the form of N doping of Gr domains. The overall B: C: N contents in the layers is close to 1:3:1.5. A careful analysis of the optical bandgap determination of the obtained B-C-N layers is presented, discussed and compared with previous seminal works with samples of similar composition.
Morphology is a critical parameter for various thin film applications, influencing properties like wetting, catalytic performance and sensing efficiency. In this work, we report on the impact of oxygen partial flow on the morphology of ceramic thin films deposited by pulsed DC reactive magnetron sputtering. The influence of O-2/Ar ratio was studied on three different model systems, namely Al2O3, CuO and TiO2. The availability of oxygen during reactive sputtering is a key parameter for a versatile tailoring of thin film morphology over a broad range of nanostructures. TiO2 thin films with high photocatalytic performance (up to 95% conversion in 7 h) were prepared, exhibiting a network of nanoscopic cracks between columnar anatase structures. In contrast, amorphous thin films without such crack networks and with high resiliency to crystallization even up to 950 degrees C were obtained for Al2O3. Finally, we report on CuO thin films with well aligned crystalline nanocolumns and outstanding gas sensing performance for volatile organic compounds as well as hydrogen gas, showing gas responses up to 35% and fast response in the range of a few seconds.
Exchange bias (EB) in ferromagnet/antiferromagnet (FM/AF) core/shell nanoparticles can be used to beat the superparamagnetic limit, and these core/shell nanoparticles are commonly fabricated by the ferromagnetic cores that are naturally oxidized to form an antiferromagnetic shell. The drawbacks of this method are that the EB effect is weak and hard to be controlled due to the shell passivation effect. Thus a theoretical work is conceived where the FM/AF core/shell nanoparticles are embedded into an antiferromagnetic matrix, and an antiferromagnetic proximity effect is induced to modulate the FM/AF EB effect in a controlled way. The results show that the shell/matrix proximity may enhance the magnetic stabilization of nanoparticles to generate the core/shell EB if the matrix is a hard AF. On the other hand, a rigid core/shell EB can switch its sign through thermal training by using a softly antiferromagnetic matrix. The local magnetization behaviors and energy barrier variations during magnetizing well interpreted the simulation results. It is evidenced that the proximity effect can optimize the magnetic properties of the pinning antiferromagnetic layer at will, ranging from statically magnetic stability to dynamically magnetic relaxation. This work opens fascinating possibilities for engineering of magnetic materials with desired magnetic properties, which has led to a surge in both experimental and theoretical investigations.
Recently, there has been strong interest in flexible and wearable electronics to meet the technological demands of modern society. Environmentally-friendly and scalable electronic textiles is a key area that is still significantly underdeveloped. Here, we describe a novel strain sensor composed of aligned cellulose acetate (CA) nanofibers with belt-like morphology and a reduced graphene oxide (RGO) layer. The unique spatial alignment, microstructure and wettability of CA nanofibrous membranes facilitate their close contact with deposited GO colloids. After a portable and fast hot-press process within 700 s at 150 degrees C, the GO on CA membrane can be facilely reduced to a conductive RGO layer. Moreover, the connection among contiguous CA nanofibers and the interaction between the GO and CA substrate were both highly enhanced, resulting in superior mechanical strength with Young's modulus of 1.3 GPa and small sheet resistance lower than 10k Omega. Therefore, the conductive RGO/CA membrane was successfully utilized as a strain sensor in a broad deformation range and with versatile deformation types. Moreover, the distinctive mechanical strength under different stretch angles endowed the well-aligned RGO/CA film with intriguing sensitivity against stress direction. Such a cost-effective and environmentally-friendly method can be easily extended to the scalable production of graphene-based flexible electronic textiles.
One-dimensional semiconductor nanofibers are regarded as ideal materials for electronics due to their distinctive morphology and characteristics. In this work, La-doped indium oxide (LaIno) nanofibers are fabricated as the channel layer to reduce O vacancies and the density of interface trap states; this is clearly confirmed by investigating the stability under positive bias stress and the capacitance-voltage for field-effect transistors (FETs). The In2O3 nanofiber FETs optimized by doping with 5 mol% La exhibit excellent electrical performance with a mobility of 4.95 cm(2) V-1 s(-1) and an on/off current ratio of 1.1 x 10(8). In order to further enhance the electrical performance of LaInO nanofiber FhTs, ZrAlOx, film, which has a high dielectric constant, is employed as the insulator for the LaInO nanofiber FETs. The LaInO nanofiber FhTs with ZrAlOx insulator have a high mobility of 13.5 cm(2) V-1 s(-1). These findings clearly indicate the great promise of La-doped In2O3 nanofibers in future one-dimensional nanoelectronics.
In superconducting materials a dynamical rearrangement of the vortex lattice occurs by forcing vortices at high velocities, until the system can become unstable. This phenomenon is known as vortex lattice instability, in which a sudden transition drives the superconducting system abruptly to the normal state. We present an experimental study on submicron bridges of NbN and NbTiN ultra-thin films with a thickness of few nanometers. The nanoscale effect on vortex lattice instability is investigated not only by the ultra-thin thickness in wide bridges, but also by changing the direction of the external magnetic field applied parallel and perpendicular to the c-axis epitaxial films Indeed, measurements are performed for both orientations and show the vortex lattice instability, regardless of the superconducting material. Critical currents I-c as well as instability currents I* have been compared. However, only in the parallel configuration an unusual flying birds' feature appears in the magnetic field dependence of current switching, as a consequence of the ratio I*/I-c that is approaching 1. This amazing tendency becomes relevant for practical applications involving nanostructures, since by scaling down sample thickness and rotating the external field towards the in-plane orientation, the ultra-thin film geometry can mimic the bridge narrowing down to the nanoscale.
This work studies the enhancement factor associated with a current emitted from a multi-wall carbon nanotube to an extremely small counter electrode. The experimental data show that the field enhancement factor increases by 1.15 times when the width of the counter electrode increases from 50 to 200 nm. To better understand this enhancement effect, field intensities at the emitter surface are numerically simulated. The experimental work and simulations demonstrate that the observed field enhancement results from increases in the capacitance between the emitter and counter electrode. In addition, corrugated counter electrodes are found to greatly affect both the capacitance and enhancement factor. This is because the corrugation of the anode surface raises the capacitance and thus provides a higher current. We experimentally show that an effective surface area enlargement of 1.67 times due to the corrugation provides a 1.06-fold increase of the enhancement factor. These results should assist in the future development of field emission devices based on semiconductor fabrication processes.
The recent development of the AFM-IR technique, which combines nanoscale imaging with chemical contrast through infrared spectroscopy, opened up new fields for exploration, which were out of reach for other modalities, e.g. Raman spectroscopy. Lipid droplets (LDs) are key organelles, which are associated with stress response mechanisms in cells and their size falls into that niche. LDs composition is heterogeneous and varies depending on cancer cell type and the tumor microenvironment. Prostate cancer cells show a unique lipid metabolism manifested by an increased requirement for lipid accumulation in cytosolic LDs. In the current work, AFM-IR nanoimaging was undertaken to analyze lipids in untreated and x-ray irradiated PC-3 prostate cancer cells. Cells poor in LDs showed slightly increased lipid signal in cytoplasm close to the nucleus. On the other hand, high lipid signal coming from LDs accumulation could be found in any part of the cytoplasmic region. The observed behavior was found to be independent from irradiation and its dose. According to the band assignment of the collected AFM-IR spectra, the main components of LDs were assigned to cholesteryl esters. The size of LDs present in cells poor in lipids was found to be of less than 1 mu m, whereas LDs aggregates spread out over a few microns. Analysis of AFM-IR spectra shows relative homogeneity of LDs composition in single cells and heterogeneity of LDs content within the PC-3 cell population.
The magneto-plasmonic properties of Ag-Co composite nano-triangle arrays are investigated. Both plasmonic and magnetic properties of the samples are found to strongly depend on the composition ratio of Ag and Co. Composite nano-triangle arrays exhibit strong plasmonic properties and low magneto-optics (MO) effect with high composition of Ag, and vice versa. The enhanced magneto-optic effect is also observed to be coincident with the localized surface plasmon resonance (LSPR) properties, i.e. the maximum Faraday effect occurs at the LSPR wavelength, which is due to locally high E-field. The composite triangle arrays with the 60% Co content show high plasmonic-MO performances characterized by magneto-optics-plasmonic correlation factor. All experimental results are confirmed by finite-difference time domain calculations.
A simple hydrogenation treatment is used to synthesize unique oxygen-deficient TiO2 with a core/shell structure (TiO2@TiO2-xHx), superior to the high H2-pressure process (under 20 bar for five days). It is demonstrated that oxygen-deficient TiO2 nanoparticle film/Si heterojunction possesses improved photoresponse performance compared to the untreated TiO2 nanoparticle film/Si heterojunction. Particularly, under 900 nm of 0.5 μW cm-2, the oxygen-deficient TiO2 nanoparticle film (TiO2@TiO2-xHx core-shell nanoparticle film)/Si heterojunction shows high responsivity (R) of 336 A W-1, prominent sensitivity (S) of 1.3 × 107 cm2 W-1, accompanied with a fast rise and decay time of 6 and 5 ms, respectively. Significantly, the detectivity (D*) of the photodetector is up to 1.17 × 1014 cm Hz1/2 W-1, which is better than that reported in metal oxide nanomaterials/Si heterojunction photodetectors, and is 4-5 orders of magnitude higher than some 2D nanosheets/Si heterojunctions of 109-1010 cm Hz1/2 W-1, indicating the excellent ability to detect weak signals. The oxygen vacancies generated in amorphous shell TiO2-xHx make the Fermi level of TiO2-x shift near the conduction band minimum and can lead to reduced dark current. The high absorption and reduced dark current of the heterojunction ensure excellent photoresponse properties of oxygen-deficient TiO2 nanoparticle film/Si heterojunction. The H-reduced oxygen-deficient amorphous shell may be an excellent candidate to enhance the photoresponse performance of metal oxide/Si heterojunction.
This paper reports on the recycling of expanded polystyrene (EPS) waste to be repurposed as EPS nanofibrous mats for air filtration applications. The EPS nanofibrous mats were prepared via electrospinning technique. The EPS solutions for producing the mats were made by dissolving the EPS waste in dimethylformamide (DMF) and d-limonene solvents. The mixing ratio of DMF and d-limonene solvents were varied to obtain EPS solutions with different surface tension and viscosity. As a result, different fiber morphology (smooth fiber, wrinkled fiber, and beaded fiber) and diameter ranging from 314 nm to 3506 nm were obtained. The synthesized EPS nanofibrous mats were characterized by scanning electron microscope, Fourier-transform infrared spectroscopy, x-ray diffraction spectroscopy, differential scanning calorimetry, mechanical strength, porosity, and water contact angle measurement apparatus. The mechanical strength measurement exhibited that the beaded fiber had the highest tensile strength and the lowest elasticity compared to wrinkled and smooth fiber. The water contact angle measurement showed that the EPS nanofibrous mats were classified as ultra-hydrophobic, which was a good criterion for air filter media. Some filtration parameters of the EPS nanofibrous mats were measured, including particle collecting efficiency, pressured drop, and quality factor. The particle collecting efficiency of each EPS nanofibrous mats was measured using monodisperse polystyrene latex (PSL) particles and PM2.5 from burning incense as the test particles. The EPS nanofibrous mats had a high collecting efficiency (up to 99.99%) and had a low pressure drop (below 70 Pa) for the face velocity of 5.4 cm s(-1). The quality factor of the EPS nanofibrous mats reached 0.10 for PSL filtration and 0.16 for PM2.5 filtration. Overall, the EPS nanofibrous mats with controlled morphology were suitable to be used as air filtration media with high mechanical strength, ultra-hydrophobic surface, and high quality factor.
The idea that shape and structure determines functionality is one of the leiv-motifs that drives research and applications on fields such as catalysis and plasmonics. The growth and stability of metallic clusters is extensively discussed through faceting and energy minimization mechanisms, respectively. Facet truncations on the regular Mackay icosahedron (m-Ih) give rise to two sub-families exhibiting five-fold symmetry and external decahedral shape. Such successive truncations made to the regular m-Ih, led to a decahedral motif called "Decmon" (Montejano's decahedron). This structure expose facets (111) and (100), that after a total energy minimization through molecular dynamics simulations (MDs) using the embedded atom model (EAM), proved to be thermally stable. This result has been confirmed by using nano-thermodynamics. The surface energy competition between the (111) and (100) facets explains its stability at some given cluster sizes, and permits to glimpse the potential energy surface along the truncation path of nanoparticles from the decahedral (s-Dh) to icosahedral (m-Ih) structures.
Bi-containing III-V semiconductors constitute an exciting class of metastable compounds with wide-ranging potential optoelectronic and electronic applications. However, the growth of III-V-Bi alloys requires group-III-rich growth conditions, which pose severe challenges for planar growth. In this work, we exploit the naturally-Ga-rich environment present inside the metallic droplet of a self-catalyzed GaAs nanowire (NW) to synthesize metastable GaAs/GaAs1-xBix axial NW heterostructures with high Bi contents. The axial GaAs1-xBix, segments are realized with molecular beam epitaxy by first enriching only the vapor-liquid-solid (VLS) Ga droplets with Bi, followed by exposing the resulting Ga-Bi droplets to As-2 at temperatures ranging from 270 degrees C to 380 degrees C to precipitate GaAs1-xBix only under the NW droplets. Microstructural and elemental characterization reveals the presence of single crystal zincblende GaAs1-xBix axial NW segments with Bi contents up to (10 +/- 2)%. This work illustrates how the unique local growth environment present during the VLS NW growth can be exploited to synthesize heterostructures with metastable compounds.
We present a novel Cu-metal-organic framework (MOF) with two-dimensional layered topology and techniques to integrate it with flexible sensors for electrochemical detection. The unique Cu-MOF is formed by coordinating Cu2+ ions with carboxylic oxygen groups, resulting in layered structures interlayerly connected by hydrogen bonds. The resulting flexible sensors exhibit capability in detecting ascorbic acid (AA), hydrogen peroxide (H2O2) and L-Histidine (L-His) with detection limits of 2.94, 4.1 and 5.3 mu M, respectively. The linear ranges of the sensors compare favorably with other sensors based on rigid platforms that offer similar sensitivity. According to the result of cytotoxicity study, the MOFs-modified flexible sensors exhibit good biocompatibility to cells, suggesting potential use in in vivo chemical detection. The results presented here demonstrate applications of MOFs in facilitating highly stable electrochemical detection in flexible electronics, and provide fundamental knowledge about structure-dependent electrochemical properties of MOFs and changing behaviors of flexible MOFs membranes under external strain. More MOFs-based flexible sensors may be developed to explore different properties of MOFs by varying their compositions and structures for healthcare and clinic applications.