Core-shell Nanowires Research Articles

Simulation of a New CZTS Solar Cell Model with ZnO/CdS Core-Shell Nanowires for High Efficiency

The numerical modeling of Cu2ZnSnS4 solar cells with ZnO/CdS core-shell nanowires of optimal dimensions with and without graphene is described in detail in this study. The COMSOL Simulation was used to determine the optimal values of core diameter and shell thickness by comparing their optical performance and to evaluate the optical and electrical properties of the different models. The deposition of a nanolayer of graphene on the layer of MoS2 made it possible to obtain a maximum absorption of 97.8% against 96.5% without the deposition of graphene.The difference between generation rates and between recombination rates of electron–hole pairs of models with and without graphene is explored.The electrical parameters obtained, such as the filling factor (FF), the short-circuit current density (Jsc), the open-circuit voltage (Voc), and the efficiency (EFF) are, respectively, 81.7%, 6.2 mA/cm2, 0.63 V, and 16.6% in the presence of graphene against 79.2%, 6.1 mA/cm2, 0.6 V, and 15.07% in the absence of graphene. The suggested results will be useful for future research work in the field of CZTS-based solar cells with ZnO/CdS core-shell nanowires with broadband light absorption rates.

Formation and optical characteristics of GaN:Eu/GaN core–shell nanowires grown by organometallic vapor phase epitaxy

We report on the formation and optical characteristics of Eu-doped GaN (GaN:Eu)/GaN core–shell nanowires (NWs) grown by organometallic vapor phase epitaxy. A moderate V/III ratio and growth temperature enable the incorporation of Eu3+ ions into the GaN shells on GaN core NWs, resulting in red luminescence at room temperature whose peak intensity is more than 20 times larger than for a GaN:Eu film on planar GaN. The cathodoluminescence characterization corroborates that the observed enhanced Eu luminescence originates from the predominant formation of highly efficient Eu centers in the sidewalls of the GaN:Eu/GaN core–shell NWs.

Atomistic evidence of hydrodynamic heat transfer in nanowires

With wave-packet propagation simulations and heat flux estimation via molecular dynamics, we show that the heat flux radial distribution in silicon nanowires can be described by a mesoscopic model, the hydrodynamic heat equation. We observe Poiseuille like heat flux profile, that cannot be described by a simple kinetic model such as the Fuchs-Sondheimer model, in both pristine and core/shell nanowires. The addition of a shell does not change the shape of the radial heat flux distribution, but just modifies the maximum of the heat flux in the center of the nanowire. These results show that there is a heat flux depletion length for pristine or core shell nanowires, 1–2 nm away from the boundary of the crystalline part. The parameters of the mesoscopic model are discussed in terms of microscopic properties, including the phonon mean free path as function of frequency and the partial vibrational density of states in the different regions of the nanowire.

ZnO/Ge core–shell nanowires and Ge nanotubes fabricated by chemical vapor deposition and wet etching

One-dimensional germanium (Ge)-related nanostructures including core–shell nanowires and nanotubes with high specific surface area show enhanced performance in energy storage and electronic devices, and their structural control is important for further improving their performance and stability. In this work, we fabricated vertically formed ZnO/Ge core–shell nanowires with different shell thicknesses. The dependence of morphology, crystallinity, and internal stress of the nanowires on the shell growth time and temperature was investigated. By applying the wet-etching method to the ZnO/Ge core–shell heterojunction nanowires, we demonstrated the Ge nanotube fabrication and stress relaxation in Ge after ZnO core removal.

Thermal synthesis of Ga2O3/SnO2 core–shell nanowires and their structural characterization

Ga2O3 core–SnO2 shell nanowires were synthesized by thermal evaporation of the metallic sources (Ga, Sn) in a horizontal quartz tube furnace at atmospheric pressure and in an oxidizing atmosphere. Au thin layers and 200 ppm Au colloid were applied as growth catalyst of Ga2O3 nanowires in vapor–liquid–solid method. Then the SnO2 shells were grown on the Ga2O3 nanowires cores. The configuration with highly sensitive shell and a relatively low conductivity core should enhance the efficiency of sensors based on these materials. Such narrowing of conductive area where a depletion of the charge carriers may occur, should increase the sensitivity, response and recovery time. The structural and material composition characterization of nanowires was performed by X-ray diffraction, energy-dispersive X-ray and Raman spectroscopy. The lattice parameters and strain of SnO2 shell were calculated. Using the catalyst in the form of a 200 ppm Au colloid compared to Au thin film, Ga2O3/SnO2 nanowires more similar to the core–shell structures were obtained.

Topological states in superlattices of HgTe class of materials for engineering three-dimensional flat bands

In search of materials with three-dimensional flat band dispersions, using ab initio computations we investigate how topological phases evolve as a function of hydrostatic pressure and uniaxial strain in two types of superlattices: HgTe/CdTe and HgTe/HgSe. In short-period HgTe/CdTe superlattices, our analysis unveils the presence of isoenergetic nodal lines, which could host strain-induced three-dimensional flat bands at the Fermi level without requiring doping, when fabricated, for instance, as core-shell nanowires. In contrast, HgTe/HgSe short-period superlattices are found to harbor a rich phase diagram with a plethora of topological phases. Notably, the unstrained superlattice realizes an ideal Weyl semimetal with Weyl points situated at the Fermi level. A small-gap topological insulator with multiple band inversions can be obtained by tuning the volume: under compressive uniaxial strain, the material transitions sequentially into a Dirac semimetal to a nodal-line semimetal, and finally into a topological insulator with a single band inversion.

Interface Analysis of MOCVD Grown GeTe/Sb2Te3 and Ge-Rich Ge-Sb-Te/Sb2Te3 Core-Shell Nanowires.

Controlling material thickness and element interdiffusion at the interface is crucial for many applications of core-shell nanowires. Herein, we report the thickness-controlled and conformal growth of a Sb2Te3 shell over GeTe and Ge-rich Ge-Sb-Te core nanowires synthesized via metal-organic chemical vapor deposition (MOCVD), catalyzed by the Vapor–Liquid–Solid (VLS) mechanism. The thickness of the Sb2Te3 shell could be adjusted by controlling the growth time without altering the nanowire morphology. Scanning electron microscopy (SEM) and X-ray diffraction (XRD) techniques were employed to examine the surface morphology and the structure of the nanowires. The study aims to investigate the interdiffusion, intactness, as well as the oxidation state of the core-shell nanowires. Angle-resolved X-ray photoelectron spectroscopy (XPS) was applied to investigate the surface chemistry of the nanowires. No elemental interdiffusion between the GeTe, Ge-rich Ge-Sb-Te cores, and Sb2Te3 shell of the nanowires was revealed. Chemical bonding between the core and the shell was observed.

Construction of Hierarchical CuCo2O4-Ni(OH)2 Core-Shell Nanowire Arrays for High-Performance Pseudocapacitors

The hierarchical CuCo2O4-Ni(OH)2 core-shell nanowire arrays on Ni foam were fabricated using facile and cost-effective two-step hydrothermal synthesis. The growth of CuCo2O4 nanowires was developed on Ni foam as the apposite basis of the conductive scaffold, and the ultrathin Ni(OH)2 nanowires were subsequently immobilized to form CuCo2O4-Ni(OH)2 core-shell nanowire arrays (NWAs). The prepared materials were further characterized in structural, morphological, and electrochemical properties. The obtained CuCo2O4-Ni(OH)2 pseudocapacitor electrode, incorporated by unique core-shell heterostructures nanowire arrays, exhibited great specific capacitance of 1201.67 F g-1 at 1 mA g-1, which is much higher than pristine CuCo2O4 nanowire of 638.89 F g-1 at 1 mA g-1. Simultaneously, it also has a high power density of 5.56 kW kg-1 at an energy density of 73.33 Wh kg-1 and good long-term cycling performance (~84 capacitance retention after 1000 cycles). The improved morphological and structural properties have substantiated the CuCo2O4-Ni(OH)2 core-shell nanowire arrays properties owing to higher surface active area and richer redox activity for boosting the electrochemical properties.

Optical phonons in core-shell semiconductor prism nanowires affected by the cross-section shape

As building blocks of modern optoelectronic devices, semiconductor core-shell nanowires are usually prisms. The Raman spectra of the nanowires can vary with the cross-section shape and size. In the framework of the dielectric continuum and Loudon's uniaxial crystal models, we investigate the influence of cross-section shape on the optical phonons in prism core-shell nanowires. It is found that while the layer components decide the modes contained in the optical phonon spectra, the shape of a cross-section mainly affects the linewidth of a spectrum. Our calculations show that the electrostatic potentials of phonons have symmetry with the cross-section, while the dispersion relationships extend to bands in prism nanowires due to the cross-section's anisotropy, in contrast with the single dispersion lines in cylindrical ones. The anisotropy of optical phonons becomes apparent in the nanowires with fewer cross-section sides. Increasing the core area can narrow the dispersion bands, approaching the single lines of cylindrical nanowires as the area gets infinitely large. Reducing the gap between the layers' components can make the anisotropy of optical phonons weaker but not disappear. The theoretical results help to interpret the variation of the Raman spectra with the cross-section shape in the nanowires and related optical and electrical properties.

Self-connected CuO–ZnO radial core–shell heterojunction nanowire arrays grown on interdigitated electrodes for visible-light photodetectors

An original photodetector system based on self-connected CuO–ZnO radial core–shell heterojunction nanowire arrays grown on metallic interdigitated electrodes, operating as visible-light photodetector was developed by combining simple preparation approaches. Metallic interdigitated electrodes were fabricated on Si/SiO2 substrates using a conventional photolithography process. Subsequently, a Cu layer was electrodeposited on top of the metallic interdigitated electrodes. The CuO nanowire arrays (core) were obtained by thermal oxidation in air of the Cu layer. Afterwards, a ZnO thin film (shell) was deposited by RF magnetron sputtering covering the surface of the CuO nanowires. The morphological, structural, compositional, optical, electrical and photoelectrical properties of the CuO nanowire arrays and CuO–ZnO core–shell nanowire arrays grown on metallic interdigitated electrodes were investigated. The performances of the devices were evaluated by assessing the figures of merit of the photodetectors based on self-connected CuO–ZnO core–shell heterojunction nanowire arrays grown on the metallic interdigitated electrodes. The radial p–n heterojunction formed between CuO and ZnO generates a type II band alignment that favors an efficient charge separation of photogenerated electron–hole pairs at the CuO–ZnO interface, suppressing their recombination and consequently enhancing the photoresponse and the photoresponsivity of the photodetectors. The electrical connections in the fabricated photodetector devices are made without any additional complex and time-consuming lithographic step through a self-connecting approach for CuO–ZnO core–shell heterojunction nanowire arrays grown directly onto the Ti/Pt metallic interdigitated electrodes. Therefore, the present study provides an accessible path for employing low dimensional complex structures in functional optoelectronic devices such as photodetectors.

In-situ generation Cu2O/CuO core-shell heterostructure based on copper oxide nanowires with enhanced visible-light photocatalytic antibacterial activity

As visible light-driven photocatalysts in wastewater treatment, Cu2O/CuO composites have garnered considerable attention. Herein, Cu2O/CuO core–shell nanowires were fabricated directly on a Cu mesh using a simple two-step synthesis process involving a wet chemical method and rapid annealing. Unlike conventional composite nanowires, controllable core–shell nanowires exhibit high photoelectrochemical properties and overcome the problems associated with the recovery of powder-based photocatalysts. The presence and structural distribution of the Cu2O/CuO core–shell nanowires were confirmed using X-ray diffraction, X-ray photoelectron spectroscopy and transmission electron microscopy. Among the samples subjected to different rapid annealing temperatures for 180 s, the sample exposed to rapid annealing at 350 °C achieved the highest photocurrent density of −6.96 mA cm−2. In the core–shell nanowires fabricated on the samples, the ratio of Cu2O/CuO was 1:1. The photocatalytic activity of the Cu2O/CuO nanowire samples was also determined by measuring methyl blue degradation to determine their applicability in wastewater treatment. A remarkable photocatalytic degradation rate of 91.6% was achieved at a loading bias voltage of −0.5 V. The Cu2O/CuO heterojunction enhanced the photodegradation of the samples because the different bandgaps improved the dissociation of the photogenerated electron–hole pairs. Furthermore, the antibacterial activity of the Cu2O/CuO nanowires exhibited considerable resistance against Escherichia coli and photocatalytic antibacterial treatment for only 20 min under visible light killed 106 CFU/mL of E. coli. Therefore, the Cu2O/CuO controllable core–shell nanowires with a high photodegradation performance and excellent antibacterial activity under general illumination show diverse applications in water treatment.

Cyclic and tensile deformations of Gold–Silver core shell systems using newly parameterized MEAM potential

Gold–Silver (Au–Ag) core-shell nanostructures have significant applicability in stretchable and biocompatible electronics where endurance under high tensile and cyclic loading is a requirement. This work, for the first time, quantitatively investigates the role of dislocations and defect interaction governing the mechanical behavior of Au–Ag and Ag–Au Core-shell nanostructures under tensile and cyclic loading using molecular dynamics (MD) simulation. For accurate representation of the underlying physics, a novel modified embedded atomic model (MEAM) interatomic potential for pristine Au, Ag and their alloys is parameterized through two different density functional theory (DFT) schemes. Using the new potential for MD simulations, the cyclic loading properties of pristine and core-shell nanowires (NWs) in a strain range of −15%–15% for 10 cycles are conducted. The tensile behavior of pristine and core-shell NWs is also explored for temperatures between 300 K and 600 K. A comparative analysis between Core-shell structures and their pristine counterparts are carried out. Our results suggest that Ag–Au Core-shell NW exhibit superior stress-strain reversibility under cyclic loading among the structures examined. Ag–Au exhibit the highest dislocation formation and near-complete annihilation of defects consistently. Au–Ag also present improved cyclic loading properties than its pristine counterparts. For tensile loading, all four structures exhibited deterioration in strength with increasing temperature. Thermal softening is observed to be more prominent in Au–Ag core-shell NWs compared to Ag–Au. Our work lays out a foundation for exploration of mechanical properties of Au–Ag systems using the MEAM potential which will help design components for stretchable electronics and creates a pathway for further exploration of similar binary alloy systems.

Design of hybrid solar cell with GaAs1−xBix (x = 0.01) nanowire core and conformally coated P3HT/ITO shell

The optoelectronic characteristics of the vertically aligned conformally coated radial junction, core–shell nanowire (NW) hybrid solar cell (HSC) having a core active layer of GaAs0.99Bi0.01as acceptor and donor shell layer of poly(3-hexylthiophene-2,5-diyl) (P3HT), have been investigated. Since axial junction solar cells (SCs) have been experimentally showing better efficiency as compared to their radial counterpart, our objective is to improve the efficiency of radial pn junction nanowire solar cells. In order to achieve that, the geometrical parameters, including the core diameter, shell thickness comprising P3HT layer and ITO for a fixed pitch of 480 nm are optimized. A fixed thickness of 10 nm is considered for P3HT due to a diffusion length of 20 nm. 3D finite-difference time-domain (FDTD) method has been employed to examine the effect of geometrical parameter variation on carrier generation and a detailed study has been presented. The core diameter of our proposed structure is 160 nm (Rcore = 80 nm) and the thickness of the surrounding shell is considered to be 90 nm (Rshell = 90 nm) for further optoelectronic performance study. The absorption spectra, generation rate profile and electric field profiles for the wavelength (λ) range of 400–900 nm have been presented for depicting the improvement caused due to incorporation of conformal coating of P3HT on core GaAs0.99Bi0.01 NW. An extensive investigation on the variation of electrical performance parameters such as open circuit voltage (Voc), short circuit current (Jsc), fill factor (FF) and power conversion efficiency (PCE) with carrier lifetime and surface recombination velocity (SRV) is performed for a fixed core and shell doping. An efficiency of 19% is achieved for the device featuring minority carrier lifetime of 60 ns and sufficiently high SRV of 105cm/s at the contacts for lightly doped core (∼1 × 1016 cm−3) and heavily doped shell (∼1 × 1019 cm−3) and substrate (∼1 × 1019 cm−3).

Investigation of the thermal properties of Cu–Ag core-shell nanowires using molecular dynamics simulation

Cu–Ag core-shell nanowires are a special type of nanostructured material, which are characterized by excellent oxidation resistance due to the Ag shell, together with very high conductivity and flexibility with the presence of copper nanowires (Cu NWs) in the core. This article reports a theoretical characterization of thermal properties of Cu–Ag core-shell nanowires using a non-equilibrium molecular dynamics simulation study. The important thermal properties include thermal conductivity, melting point, specific heat capacity, and coefficients of thermal expansion have been simulated against the size parameters of the nanowires. The simulation has recognized the notable phenomenon such as lower thermal conductivity as compared to the bulk copper, negative coefficients of thermal expansion and the melting point as high as 1369.8 K. The simulated results and the overall thermal behaviour of the Cu–Ag core-shell nanowires have been discussed in this article with insight into the different phenomenon occurring at the atomic scale.

Different strategies for GaN-MoS2 and GaN-WS2 core–shell nanowire growth

One-dimensional (1D) nanostructures – nanowires (NWs) – exhibit attractive properties for integration in different types of functional devices. Their properties can be enhanced even further or tuned for a specific application by combining different promising materials, such as layered van der Waals materials and conventional semiconductors, into 1D-1D core–shell heterostructures. In this work, we demonstrated growth of GaN-MoS2 and GaN-WS2 core–shell NWs via two different methods: (1) two-step process of sputter-deposition of a sacrificial transition metal oxide coating on GaN NWs followed by sulfurization; (2) pulsed laser deposition of few-layer MoS2 or WS2 on GaN NWs from the respective material targets. As-prepared nanostructures were characterized via scanning and transmission electron microscopies, X-ray diffraction, micro-Raman spectroscopy and X-ray photoelectron spectroscopy. High crystalline quality core–shell NW heterostructures with few-layer MoS2 and WS2 shells can be prepared via both routes. The experimental results were supported by theoretical electronic structure calculations, which demonstrated the potential of the synthesised core–shell NW heterostructures as photocatalysts for efficient hydrogen production from water.

Visible-light photoelectrochemical photodetector based on In-rich InGaN/Cu2O core-shell nanowire p–n junctions

n-InGaN/p-Cu2O core-shell nanowire (NW) p–n junctions enable efficient self-powered photoelectrochemical photodetectors (PEC PDs) in the visible. The photocurrent density under one-sun illumination is enhanced by 8 times compared to that of bare InGaN NW PEC PDs due to maximized photocarrier separation in the built-in electric field of the p–n junction. The responsivity reaches 173 μA/W under one-sun illumination. The response times of 30–40 ms are among the shortest achieved for PEC PDs. Together with the long-time stability and reusability, a robust, easy to fabricate, and easy to operate self-powered PEC PD is introduced.

Extended-SWIR Photodetection in All-Group IV Core/Shell Nanowires

Group IV Ge1-xSnx semiconductors hold the premise of enabling broadband silicon-integrated infrared optoelectronics due to their tunable bandgap energy and directness. Herein, we exploit these attributes along with the enhanced lattice strain relaxation in Ge/Ge0.92Sn0.08 core-shell nanowire heterostructures to implement highly responsive, room-temperature short-wave infrared nanoscale photodetectors. Atomic-level studies confirm the uniform shell composition and its higher crystallinity with respect to thin films counterparts. The demonstrated Ge/Ge0.92Sn0.08 p-type field-effect nanowire transistors exhibit superior optoelectronic properties achieving simultaneously a relatively high mobility, a high ON/OFF ratio, and a high responsivity, in addition to a broadband absorption in the short-wave infrared range. Indeed, the reduced bandgap of the Ge0.92Sn0.08 shell yields an extended cutoff wavelength of 2.1 um, with a room-temperature responsivity reaching 2.7 A/W at 1550 nm. These results highlight the potential of Ge/Ge1-xSnx core/shell nanowires as silicon-compatible building blocks for nanoscale integrated infrared photonics.

Growth of III-V Antimonide Heterostructure Nanowires on Silicon Substrate for Esaki Tunnel Diode

Integration of low bandgap antimonide based nanowires on Si substrate has been attracting huge attention for opto-electronic applications. In this work we demonstrated InAs/InSb and InAs/GaSb heterostructure nanowires on Si substrate by metal organic chemical vapor deposition. We grew high quality axial InSb heterostructure segment on InAs stem by self-catalyzed growth technique, which paves a way to tune the crystal structure of InSb. In case of InAs-GaSb core-shell architecture, GaSb crystal quality highly depends on InAs core. We successfully demonstrated basic electrical characteristics of InAs-GaSb core-shell nanowire which exhibits negative differential resistance at 0.8 V and peak-to-valley current ratio of 3.84.

Catalysis-Free Growth of III-V Core-Shell Nanowires on p-Si for Efficient Heterojunction Solar Cells with Optimized Window Layer

The growth of high-quality compound semiconductor materials on silicon substrates has long been studied to overcome the high price of compound semiconductor substrates. In this study, we successfully fabricated nanowire solar cells by utilizing high-quality hetero p-n junctions formed by growing n-type III-V nanowires on p-silicon substrates. The n-InAs0.75P0.25 nanowire array was grown by the Volmer–Weber mechanism, a three-dimensional island growth mode arising from a lattice mismatch between III-V and silicon. For the surface passivation of n-InAs0.75P0.25 core nanowires, a wide bandgap InP shell was formed. The nanowire solar cell was fabricated by benzocyclobutene (BCB) filling, exposure of nanowire tips by reactive-ion etching, electron-beam deposition of ITO window layer, and finally metal grid electrode process. In particular, the ITO window layer plays a key role in reducing light reflection as well as electrically connecting nanowires that are electrically separated from each other. The deposition angle was adjusted for conformal coating of ITO on the nanowire surface, and as a result, the lowest light reflectance and excellent electrical connectivity between the nanowires were confirmed at an oblique deposition angle of 40°. The solar cell based on the heterojunction between the n-InAs0.75P0.25/InP core-shell nanowire and p-Si exhibited a very high photoelectric conversion efficiency of 9.19% with a current density of 27.10 mA/cm2, an open-circuit voltage of 484 mV, and a fill factor of 70.1%.

Enhancement of Fenton processes at initial circumneutral pH for the degradation of norfloxacin with Fe@FeS core–shell nanowires

ABSTRACT The disadvantages of narrow working pH range (2.5–4.0), accumulation of iron sludge and incomplete degradation have hindered the practical application of the traditional homogeneous Fenton technique. In this research, Fe@FeS core–shell nanowires were synthesised and the innovative Fe@FeS/Fe2+/H2O2 system was adopted for norfloxacin (NOR) degradation at an initial circumneutral pH. More than 95% NOR has been removed in the Fe@FeS/Fe2+/H2O2 system within 30 min at pH 7. After investigating the concentration change of total iron, Fe2+ and H2O2 during the degradation process, NOR degradation in the Fe@FeS/Fe2+/H2O2 system might be attributed to the combined effect of homogeneous Fenton reaction and heterogeneous Fenton process. Besides that, the added Fe@FeS has accelerated Fe3+/Fe2+ redox cycle with extremely high degree. The generated reactive ●OH has been identified by electron paramagnetic resonance spectrometer results, possible degradation intermediates have also been proposed according to Gas chromatography-mass spectrometry analysis results. Moreover, Fe@FeS core–shell nanowires showed excellent reusability, it is a promising heterogeneous Fenton catalyst that is applicable for practical application.