Core-shell Nanowire Structures Research Articles

Preparation and Assembly of Carbon Nanotube/Silica Core-Shell Nanowires to Realize High-Density and Well-Aligned Nanotube Arrays with Uniform and Tunable Pitch

This study introduces a novel method to fabricate high-density, well-aligned arrays of carbon nanotubes (CNTs) with tunable pitch, utilizing a core-shell nanowire structure of CNTs and silica. The approach involves a modified Stober process to achieve controlled deposition of silicon dioxide around individual CNTs, forming robust core-shell nanowires. By optimizing the Stober process, the shell size can be precisely controlled down to sub 10nm scale while maintaining the individualized nature of the CNTs. Densely packed and precisely aligned CNT arrays are created using Langmuir-Schaefer process. After assembly, the silica shell, acting as a physical spacer, can be easily removed by HF etching, exposing the CNTs while maintaining their alignment. The developed high-density, well-aligned CNT arrays with tunable pitch show significant promise for applications in nanoelectronics, sensors, and other emerging technologies. This methodology offers a robust and scalable approach for CNT array fabrication, opening avenues for tailoring array properties to meet specific application requirements.

Ultra-sensitive electrochemical sensor based on in situ grown ultrafine HKUST-1 nanoparticles @ graphite nanosheets and core-shell structured MoO3-polypyrrole nanowires for the detection of rutin in orange juice.

Rutin extracted from natural plants has important medical value, so developing accurate and sensitive quantitative detection methods is one of the most important tasks. In this work, HKUST-1@GN/MoO3-Ppy NWs were utilized to develop a high-performance rutin electrochemical sensor in virtue of its high conductivity and electrocatalytic activity. The morphology, crystal structure, and chemical element composition of the fabricated sensor composites were characterized by SEM, TEM, XPS, and XRD. Electrochemical techniques including EIS, CV, and DPV were used to investigatethe electrocatalytic properties of the prepared materials. The electrochemical test conditions were optimized to achieve efficient detection of rutin. The 2-electron 2-proton mechanism, consisting of several rapid and sequential phases, is postulated to occur during rutin oxidation. The results show that HKUST-1@GN/MoO3-Ppy NWs have the characteristics of large specific surface area, excellent conductivity, and outstanding electrocatalytic ability. There is a significant linear relationship between rutin concentration and the oxidation peak current of DPV. The linear range is 0.50-2000 nM, and the limit of detection is 0.27 nM (S/N = 3). In addition, the prepared electrode has been confirmed to be useful for rutin analysis in orange juice.

Morphology dependent EMI shielding performance of Ag-Ni core-shell nanowires

The proposed work reports a facile chemical reduction method for synthesizing the hierarchical silver-nickel core-shell nanowire structures with controllable morphologies by varying the Ag concentration. The growth of Ni shells around the Ag core to make thorny and smooth Ag-Ni core-shell nanowires is studied for addressing electromagnetic interference (EMI). The synthesized Ag-Ni nanowires show better EMI shielding effectiveness (SE) than Ni nanowires with much-improved absorption loss (SEA). Due to high electrical conductivity (8630 S cm−1), magnetic permeability (44.1 emu/g), and heterojunction core-shell interfaces, the bimetallic nanowires flakes exhibit exceptional average EMI SE of 90.5 dB in the X-band frequency range (8–12 GHz) with a thickness of 0.11 mm, taking specific EMI SE to 822.72 dB/mm. In addition, the EMI shielding performance of the smooth Ag-Ni nanowires showed a 45% enhancement in SEA with marginal change in reflection loss (SER) compared to the thorny ones, indicating high absorption of EM waves. The unique core-shell nanowires with high aspect ratio with tunable physical properties can be ideal candidates for futuristic applications in microwave absorption and electromagnetic interference shielding.

Ambipolar Self-Driving Polarized Photodetection

The polarized photodetectors based on the anisotropy of low-dimensional materials have drawn extensive attention; however, the polarities of the detected photocurrents are normally independent of the light polarization, and thus, the calibration on the incident power is necessary. Here, introducing the Dember effect modulated by the photonic mechanism, we propose a core–shell nanowire structure, which can identify the photocurrent polarities under various incident light polarizations and simultaneously operate in self-driving mode. Taking the Ag@CH3NH3PbI3@ITO core–shell nanowire as an example, we perform a strict optoelectronic simulation based on finite-element method. It shows that the ambipolar signals under transverse magnetic and electric (TM and TE) incidences can indeed be achieved, with the unbiased current densities of 24.9 and −10.4 mA/cm2, respectively (light wavelength: 500 nm). We further identify that the enhanced photodetection capability comes from the Dember effect together with the specific electric field distributions controlled by the whispering-gallery mode and plasmonic resonance under TE and TM incidences. The proposed architecture is expected to realize a more accurate and calibration-free polarized photodetection with a simplified system but a higher performance, multifunction capability, and integration capability with existing electronic systems.

Modeling and Epitaxial Growth of Homogeneous Long-InGaN Nanowire Structures.

One-dimensional nanowires based on Group III-nitride materials are emerging as one of the most promising structures for applications of light-emitting diodes (LEDs), laser diodes (LDs), solar cells, and photocatalysts. However, leading to the so-called “green gap” in photonics, the fabrication of high concentration indium gallium nitride (InGaN) and long-InGaN structures remains still challenging. In this study, we performed simulations for structural modeling of uniform temperature distribution in a nanowire epitaxy, and have successfully developed high-concentration InGaN and long-InGaN nanowire heterostructures on silicon (Si) substrate using molecular beam epitaxy (MBE) system. From scanning electron microscope (SEM) and transmission electron microscope (TEM) results, it was confirmed that the various doped-InGaN nanowire structures show much higher crystal quality compared to conventional nanowire structures. By introducing a new three-step modulated growth technique, the n-/p-InGaN active regions were greatly increased and the optical properties were also dramatically improved due to reduced phase separation. In addition, a multi-band p-InGaN/GaN heterostructure was successfully fabricated with the core–shell nanowire structures, which enable the emission of light in the entire visible spectral range, and protect the InGaN surface from surface recombination. This paper offers important insight into the design and epitaxial growth of InGaN nanowire heterostructures.

Effects of HfC/PyC core-shell structure nanowires on the microstructure and mechanical properties of Hf1-xZrxC coating

To elevate the mechanical and anti-ablation properties of Hf1-xZrxC coating on C/C composites, HfC/PyC core-shell structure nanowires (HfCnw/PyC) with different PyC layer thickness were synthesized by two steps of CVD. Influences of HfCnw/PyC on the microstructure and mechanical properties of Hf1-xZrxC coating were researched. Toughening mechanism of HfCnw/PyC was also investigated. PyC layer exhibited a lamellar structure and combined well with HfCnw. After incorporating HfCnw/PyC, Hf1-xZrxC coating structure converted from columnar crystal to isometric crystal. HfCnw improved H, E, Kc and bonding strength of Hf1-xZrxC coating, which is ascribed to the nanowire pullout, debonding, bridging and crack deflection mechanism. With the PyC layer thickness increasing, H and E of the coating reduced, Kc and bonding strength of the coating increased. Because of the moderate bonding strength between HfCnw/PyC and coating matrix, lamellar structure of PyC layer and higher Kc of PyC, toughening effectiveness of the core-shell structures gradually enhanced with the PyC layer thickness increasing.

Large-scale synthesis of SiC/PyC core-shell structure nanowires via chemical liquid-vapor deposition

In carbon/carbon (C/C) composites, SiC/PyC core-shell structure nanowires were successfully fabricated via chemical liquid-vapor deposition (CLVD). The influences of heat-treatment temperature on the microstructure and composition of SiC nanowires were studied, and meanwhile the growth mechanism of SiC nanowires was discussed. Additionally, the microstructure and morphology of SiC/PyC core-shell structure nanowires were also investigated. The results displayed that the low heat-treatment temperature could not meet the requirements of SiC nanowires growth, but the too high temperature made the nanowires appear agglomerate easily. Only when the heat-treatment temperature was 1800 °C, SiC nanowires possessed a uniform distribution. The diameter of SiC nanowire was about 300 nm, and there was a SiO2 layer with the thickness of about 1 nm existing on the surface of SiC nanowire. The growth behavior of SiC nanowire was governed by vapor-solid (V–S) mechanism. After the PyC deposition, SiC/PyC core-shell structure nanowires were constructed, and the nanowires were about 450 nm in diameter. These nanowires displayed a core-shell structure with three layers, which were SiC nanowire core, SiO2 interlayer and PyC shell, respectively. Meanwhile, SiC/PyC core-shell structure nanowires connected the matrices with each other, and the core-shell structure nanowires generated a stable network.

Core-shell nanowires of NiCo2O4@α-Co(OH)2 on Ni foam with enhanced performances for supercapacitors

The composites of NiCo2O4 with unique structures are extensively explored as promising electrodes. In this work, core-shell structured nanowires anchored on nickel foam are synthesized by the hydrothermal synthesis of NiCo2O4 as core and subsequent electrodeposition of α-Co(OH)2 as shell. The core-shell composites exhibit enhanced electrochemical performances ascribing to the synergistic reactions from both materials, showing higher specific capacitance than any single component. By changing the deposition time, the mass loading of α-Co(OH)2 can be easily controlled. The electrochemical performances of the hybrid electrodes are diverse with the mass loading of Co(OH)2. The optimized hybrid electrode with 3 mins electrodeposition exhibits the highest specific capacitance (1298 F g−1 at 1 A g−1) among all electrodes. The redox reaction is a main contributor to the total specific capacitance through electrochemical kinetics analysis. An asymmetric supercapacitor assembled by the optimized material as positive electrode and activated carbon as negative electrode can achieve a relatively high energy density of 39.7 Wh kg−1 at a power density of 387.5 W kg−1 (at 0.5 A g−1) in a voltage of 1.55 V.

One-Dimensional Nanostructured Oxide Chemoresistive Sensors.

Day by day, the demand for portable, low cost, and efficient chemical/gas-sensing devices is increasing due to worldwide industrial growth for various purposes such as environmental monitoring and health care. To fulfill this demand, nanostructured metal oxides can be used as active materials for chemical/gas sensors due to their high crystallinity, remarkable physical/chemical properties, ease of synthesis, and low cost. In particular, (1D) one-dimensional metal oxides nanostructures, such as nanowires, exhibit a fast response, selectivity, and stability due to their high surface-to-volume ratio, well-defined crystal orientations, controlled unidirectional electrical properties, and self-heating phenomenon. Moreover, with the availability of large-scale production methods for nanowire growth such as thermal oxidation and evaporation–condensation growth, the development of highly efficient, low cost, portable, and stable chemical sensing devices is possible. In the last two decades, tremendous advances have been achieved in 1D nanostructured gas sensors ever since the pioneering work by Comini on the development of a SnO2 nanobelt for gas sensor applications in 2002, which is one such example from which many researchers began to explore the field of 1D-nanostructure-based chemical/gas sensors. The Sensor Laboratory (University of Brescia) has made major contributions to the field of metal oxide nanowire chemical/gas-sensing devices. Over the years, different metal oxides such as SnO2, ZnO, WO3, NiO, CuO, and their heterostructures have been grown for their nanowire morphology and successfully integrated into chemoresistive gas-sensing devices. Hence in this invited feature article, Sensor Laboratory research on the synthesis of metal oxide nanowires and novel heterostructures and their characterization and gas-sensing performance during exposure to different gas analytes has been presented. Moreover, some new strategies such as branched-like nanowire heterostructures and core–shell nanowire structures adopted to enhance the performance of nanowire-based chemical sensor are presented in detail.

Large scale synthesis of Si3N4@SiOx core-shell structure nanowires by direct nitridation

In this work, millimeter-scale Si3N4@SiOx core-shell structure nanowires were synthesized using the direct nitridation method without the use of catalysts. The silicon powders used as raw materials were pretreated at 1300 °C for 3 h under ammonia. The ammonia was then converted into a nitrogen/hydrogen gas mixture and heated at 1350 °C for another 5 h. The morphology and structure of the as-obtained nanowires were investigated in detail. The Si3N4@SiOx nanowires with a core-shell structure were composed of a crystalline α-Si3N4 core and an amorphous SiOx shell. The formation of the α-Si3N4 core nanowire is due to the reaction of the N2 gas with the byproduct SiO gas. However, the formation of the amorphous SiOx shell is related to the deposition location of the samples. For the inner part of the samples, the SiOx shell derives from the deposition of the byproduct SiO gas at 1350 °C, while the SiOx shell of the external part of the samples likely derives from the oxidation of the silicon nitride core during the cooling process.

Deformation Mechanisms and Dislocations in Nickel–Cobalt Core–Shell Nanowires Under Uniaxial Tensile Loading—A Molecular Dynamics Modeling Analysis

One-Dimensional nanostructures are of great importance due to their unique electrical, magnetic and mechanical properties. Because of the size constraints, it is impractical to obtain and understand their mechanical properties, deformation behavior and associated mechanisms experimentally, while atomistic computational modeling offers a viable approach. Several studies have focused on pure metallic and compound nanowires; however, investigations and data for core–shell nanowires is still very limited. Present work models and analyzes Nickel–Cobalt (Ni–Co) core–shell nanowire systems under uniaxial tensile loading via molecular dynamics (MD) modeling. Present studies and analysis focus on predictive mechanical properties as well as insights on deformation mechanisms and dislocations during uniaxial tensile loading. Further, pure Ni and Co nanowire systems of similar configurations are modeled, and compared to our findings of core–shell nanowire systems. Results indicate that crystal mismatch in the region of core–shell interface have a significant effect, and plays an important role in the deformation process. Evolution of dislocations during uniaxial tensile loading deformation process, following a dislocation extraction analysis are presented and discussed. Further investigations of the core–shell nanowire structures involved a systematic study on the effect of core size on the predicted mechanical properties. Results indicate that the yield strength and Young's modulus indicated a linear increase for nickel shell–cobalt core systems with a Cobalt core for increasing core sizes, while maintaining nearly similar volume, length, and outer diameter of the core–shell nanowire system. Cobalt shell–nickel core nanowire systems with nickel core however was found not to depict any such definitive characteristics.

Cu(OH)2@Cd(OH)2 core-shell nanostructure: Synthesis to supercapacitor application

The Cu(OH)2@Cd(OH)2 core-shell nanowire structure have been successfully synthesized by facile cation exchange reaction based on negative free energy of formation through topotactical transformation from their cadmium hydroxide to copper hydroxide. The encapsulation of the copper into the cadmium hydroxide nanowires was confirmed by X-ray diffraction studies, elemental analysis and Fourier transform infrared spectroscopy. Electrochemical study shows that Cu(OH)2@Cd(OH)2 core-shell nanowire structure possess high specific capacitance of 374 Fg−1 in an aqueous 1 M NaOH electrolyte. Present work demonstrates Cu(OH)2@Cd(OH)2 nanowires, synthesis and application as supercapacitive electrode.

Microporous core-shell Co11(HPO3)8(OH)6/Co11(PO3)8O6 nanowires for highly efficient electrocatalytic oxygen evolution reaction

Exploring high-efficient oxygen evolution reaction (OER) catalysts is critical for producing clean H2 fuel through water splitting. Co and P involved Co-Pi and cobalt phosphide OER catalysts attracted tremendous attention, while limited report on cobalt phosphite as an OER catalyst. Herein, a novel cobalt phosphite nanowires with microporous 1D channel have been succsessfully discovered with remarkable catalytic performance of a low overpotential of ˜340 mV and Tafel slope of 60 mV dec−1 at 10 mA cm−2 at pH 14. Through comprehensive structural characterization, the effective species has been clearly illustrated to be the in-situ formed heterostructural Co11(HPO3)8(OH)6/Co11(PO3)8O6 core-shell structure nanowires. This work fills the gap for exploring high efficient cobalt phosphites OER catalysts under akaline condition and paves the way for exploring novel high efficiency waster splitting catalysts.

Optimization of InAs/GaSb core-shell nanowire structure for improved TFET performance

The performance of InAs/GaSb core-shell nanowire TFET is systematically investigated for the effects of intrinsic device parameters such as channel doping, shell thickness, spacer length and source offset. Device ON-current (ION) was chosen as the key figure of merit. It is found that ION improves due to improved electrostatic control achieved by the TFET with optimum shell diameter. The maximum ION obtained for a shell thickness of 2 nm was 33.65 μA/μm and a Subthreshold Swing (SS) of 12.9 mV/decade with an ION/IOFF ratio of 1.49 × 108 for our device. Device ION can be further improved by adding an optimum spacer at the source-channel junction. It was also found that device ON-current is almost constant and does not get much affected by having a larger shell offset.

Deformation behavior of core–shell nanowire structures with coherent and semi-coherent interfaces

Abstract

Observing Electrochemical Switching Behaviors in Crossbar Core-Shell Ni/NiO Nanowires Memristor

The crossbar structure of resistive random access memory (RRAM) is the most promising technology for the development of ultra-high-density devices for future nonvolatile memory. However, only a few studies have focused on the switching phenomenon of crossbar RRAM in detail. The main purpose of this study is to understand the formation and disruption of the conductive filament occurring at the crossbar center by real-time transmission electron microscope (TEM) observation. Core-shell Ni/NiO nanowires were utilized to form a cross structure, which restricted the position of the conductive filament to the cross-center. A significant morphological change could be observed near the crossbar center, which resulted from the out-diffusion and backfill of oxygen ions. Energy dispersive spectroscopy (EDS) and electron energy loss spectroscopy (EELS) demonstrated that the movement of the oxygen ions led to the evolution of the conductive filament, followed by redox reactions. Moreover, the distinct reliability of the crossbar device was measured via ex situ experiments. In this work, the switching mechanism of the crossbar core-shell nanowire structure is beneficial to overcome the problem of nanoscale minimization. The experimental method shows high potential to fabricate high-density RRAM devices, which could be applied to 3D stacked package technology and neuromorphic computing systems. Figure 1

Controlled Formation of Radial Core-Shell Si/Metal Silicide Crystalline Heterostructures.

The highly controlled formation of "radial" silicon/NiSi core-shell nanowire heterostructures has been demonstrated for the first time. Here, we investigated the "radial" diffusion of nickel atoms into crystalline nanoscale silicon pillar 11 cores, followed by nickel silicide phase formation and the creation of a well-defined shell structure. The described approach is based on a two-step thermal process, which involves metal diffusion at low temperatures in the range of 200-400 °C, followed by a thermal curing step at a higher temperature of 400 °C. In-depth crystallographic analysis was performed by nanosectioning the resulting silicide-shelled silicon nanopillar heterostructures, giving us the ability to study in detail the newly formed silicide shells. Remarkably, it was observed that the resulting silicide shell thickness has a self-limiting behavior, and can be tightly controlled by the modulation of the initial diffusion-step temperature. In addition, electrical measurements of the core-shell structures revealed that the resulting shells can serve as an embedded conductive layer in future optoelectronic applications. This research provides a broad insight into the Ni silicide "radial" diffusion process at the nanoscale regime, and offers a simple approach to form thickness-controlled metal silicide shells in the range of 5-100 nm around semiconductor nanowire core structures, regardless the diameter of the nanowire cores. These high quality Si/NiSi core-shell nanowire structures will be applied in the near future as building blocks for the creation of utrathin highly conductive optically transparent top electrodes, over vertical nanopillars-based solar cell devices, which may subsequently lead to significant performance improvements of these devices in terms of charge collection and reduced recombination.

Observation of Resistive Switching Behavior in Crossbar Core-Shell Ni/NiO Nanowires Memristor.

The crossbar structure of resistive random access memory (RRAM) is the most promising technology for the development of ultrahigh-density devices for future nonvolatile memory. However, only a few studies have focused on the switching phenomenon of crossbar RRAM in detail. The main purpose of this study is to understand the formation and disruption of the conductive filament occurring at the crossbar center by real-time transmission electron microscope observation. Core-shell Ni/NiO nanowires are utilized to form a cross-structure, which restrict the position of the conductive filament to the crosscenter. A significant morphological change can be observed near the crossbar center, which results from the out-diffusion and backfill of oxygen ions. Energy dispersive spectroscopy and electron energy loss spectroscopy demonstrate that the movement of the oxygen ions leads to the evolution of the conductive filament, followed by redox reactions. Moreover, the distinct reliability of the crossbar device is measured via ex situ experiments. In this work, the switching mechanism of the crossbar core-shell nanowire structure is beneficial to overcome the problem of nanoscale minimization. The experimental method shows high potential to fabricate high-density RRAM devices, which can be applied to 3D stacked package technology and neuromorphic computing systems.

Toward high performance nanoscale optoelectronic devices: super solar energy harvesting in single standing core-shell nanowire.

Single nanowire solar cells show great promise for next-generation photovoltaics and for powering nanoscale devices. Here, we present a detailed study of light absorption in a single standing semiconductor-dielectric core-shell nanowire (CSNW). We find that the CSNW structure can not only concentrate the incident light into the structure, but also confine most of the concentrated light to the semiconductor core region, which boosts remarkably the light absorption cross-section of the semiconductor core. The CSNW can support multiple higher-order HE modes, as well as Fabry-Pérot (F-P) resonance, compared to the bare nanowire (BNW). Overlapping of the adjacent higher-order HE modes results in broadband light absorption enhancement in the solar radiation spectrum. Results based on detailed balance analysis demonstrate that the super light concentration of the single CSNW gives rise to higher short-circuit current and open-circuit voltage, and thus higher apparent power conversion efficiency (3644.2%), which goes far beyond that of the BNW and the Shockley-Queisser limit that restricts the performance of a planar counterparts. Our study shows that the single CSNW can be a promising platform for construction of high performance nanoscale photodetectors, nanoelectronic power sources, super miniature cells, and diverse integrated nanosystems.

Facile fabrication of gold coated nickel nanoarrays and its excellent catalytic performance towards sodium borohydride electro-oxidation

Novel Au@Ni nanoarrays electrode is facilely obtained by firstly template-assisted electro-deposition of Ni nanowire arrays (NAs), followed by galvanostatic deposition of Au catalysts onto the Ni NAs without any conductive agents and binders. The Au@Ni NAs electrode shows a rough surface and fringe with the diameter of ∼90nm, which assures a high utilization of Au catalysts and provides a large specific surface area. The elemental distribution of Ni mainly exists in the inner layer of a single Au@Ni nanowire with the diameter ∼56nm, while the elemental distribution of Au catalysts merely appears in the outer layer to form the unique core-shell nanowire structure. The Au@Ni NAs electrode reveals excellent electrochemical property and desirable stability for catalyzing NaBH4 electro-oxidation in basic solutions. The Au@Ni NAs electrode in the 2.00M NaOH and 0.24M NaBH4 solution demonstrates an oxidation current density of 2.35Amg−1 at −0.5V (vs. Ag/AgCl), which is much higher than that of the noble metal catalysts previously reported, indicating that this material may be hopefully used as anodic catalysts for applying in fuel cells.