Radial Nanowire Research Articles

Bending and reverse bending during the fabrication of novel GaAs/(In,Ga)As/GaAs core–shell nanowires monitored by in situ x-ray diffraction

We report on the fabrication of a novel design of GaAs/(In,Ga)As/GaAs radial nanowire heterostructures on a Si 111 substrate, where, for the first time, the growth of inhomogeneous shells on a lattice mismatched core results in straight nanowires instead of bent. Nanowire bending caused by axial tensile strain induced by the (In,Ga)As shell on the GaAs core is reversed by axial compressive strain caused by the GaAs outer shell on the (In,Ga)As shell. Progressive nanowire bending and reverse bending in addition to the axial strain evolution during the two processes are accessed by in situ by x-ray diffraction. The diameter of the core, thicknesses of the shells, as well as the indium concentration and distribution within the (In,Ga)As quantum well are revealed by 2D energy dispersive x-ray spectroscopy using a transmission electron microscope. Shell(s) growth on one side of the core without substrate rotation results in planar-like radial heterostructures in the form of free standing straight nanowires.

Tuning of Single Mixed (Helical) Dislocations in Core-Shell van der Waals Nanowires.

Linear defects (dislocations) not only govern the mechanical properties of crystalline solids but they can also produce distinct electronic, thermal, and topological effects. Accessing this functionality requires control over the placement and geometry of single dislocations embedded in a small host volume to maximize emerging effects. Here we identify a synthetic route for rational dislocation placement and tuning in van der Waals nanowires, where the layered crystal limits the possible defect configurations and the nanowire architecture puts single dislocations in close proximity to the entire host volume. While homogeneous layered nanowires host single screw dislocations, the synthesis of radial nanowire heterostructures (here exemplified by GeS-Ge1-xSnxS monochalcogenide core-shell nanowires) transforms the defect into a mixed (helical) dislocation whose edge/screw ratio is tunable via the core-shell lattice mismatch. The ability to design nanomaterials with control over individual mixed dislocations paves the way for identifying the functional properties of dislocations and harnessing them in technology.

Metal assisted chemical etching of silicon and solution synthesis of Cu-=SUB=-2-=/SUB=-O/Si radial nanowire array heterojunctions

Cu2O/Si radial nanowire (NWs) array heterojunctions were prepared by depositing Cu2O nanoparticles via chemical bath deposition on n-Si nanowire arrays that were fabricated by metal-assisted electroless etching. After 20 cycles of deposition, large numbers of Cu2O nanoparticles with form shells that wrap the upper segment of each Si nanowire. This method of etching offers exceptional simplicity, flexibility, environmental friendliness, and scalability for the fabrication of three-dimensional silicon nanostructures with considerable depths, because of replacement of harsh oxidants such as H2O2 and AgNO3. Keywords: Cu2O/Si NWs heterojunctions, Cu2O nanoparticles, metal-assisted electroless etching.

Triple Radial Junction Hydrogenated Amorphous Silicon Solar Cells with >2 V Open‐Circuit Voltage

When solar cells are used as the photoanode for direct water splitting, the output voltage required typically exceeds that of a single‐junction photovoltaic device. Toward this application, in this work, triple radial junction silicon nanowire (3RJ SiNW) solar cells are fabricated via a plasma‐assisted vapor‐liquid‐solid method using hydrogenated amorphous silicon (a‐Si:H) for all the absorber layers, as well as for the doped ones. A high open‐circuit voltage (VOC) of 2.05 V, short‐circuit current density (JSC) of 3.8 mA cm−2, and power conversion efficiency of 4.4% are obtained for solar cells with areas of 0.03 cm2 by optimizing the density of SiNWs grown on ZnO:Al/Ag/Corning glass substrates. For lower‐efficiency devices, however, VOC values as high as 2.2 V are consistently achieved. At these higher voltages, large variations in JSC are observed, attributed to small local variations in SiNW density. Herein, for the first time, the excellent potential of 3D radial junction solar cells for applications requiring high voltages and high surface areas, such as water splitting is demonstrated.

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).

Theory of MBE Growth of Nanowires on Reflecting Substrates

Selective area growth (SAG) of III-V nanowires (NWs) by molecular beam epitaxy (MBE) and related epitaxy techniques offer several advantages over growth on unpatterned substrates. Here, an analytic model for the total flux of group III atoms impinging NWs is presented, which accounts for specular re-emission from the mask surface and the shadowing effect in the absence of surface diffusion from the substrate. An expression is given for the shadowing length of NWs corresponding to the full shadowing of the mask. Axial and radial NW growths are considered in different stages, including the stage of purely axial growth, intermediate stage with radial growth, and asymptotic stage, where the NWs receive the maximum flux determined by the array pitch. The model provides good fits with the data obtained for different vapor–liquid–solid and catalyst-free III-V NWs.

Misfit Stress in Radial Core-Shell Nanowires with Diffuse Interface Boundaries

The elastic models of radial core-shell nanowires with diffuse interphase boundaries are suggested. The concept of eigenstrain is employed to consider a misfit stress distribution induced by diffusive interfaces with different range of distinctness. The eigenstrain profile described by the misfit parameter is approximated by piecewise-linear, error and arctangent functions. For these approximations the elastic stresses in core-shell nanowires are analytically derived, illustrated with plots and discussed in detail.

Kinetics of spontaneous formation of core shell structure in (In,Ga)As nanowires

A model of spontaneous formation of the core-shell structure in (In,Ga)As nanowire grown via molecular beam epitaxy without independent radial growth is proposed. Within the framework of the proposed model, the distribution of In across the axis of the nanowire was fitted. Keywords: core-shell nanowire, ternary nanowire, InGaAs nanowire, radial nanowire heterostructure.

Numerical analysis of InP based high efficiency radial junction nanowire solar cell

III-V nanowire (NW) solar cells (SCs) promise a comparable efficiency to that of their planar counterparts with additional advantage of reduced material consumption to cut the manufacturing cost. However, the disadvantage of low minority carrier lifetime due to the large surface-to-volume ratio of NWs degrades its photovoltaic performance to some extent. To overcome this issue, the structure of radial junction instead of axial junction for NWSCs has been used, but in experimental realization, defect density increases in the radial junction structure at the interface of core/shell, resulting in low performance. In this paper, we propose a high performance radial junction NWSC, consisting of p-InP NW coated by Ta 2 O 5 and ITO, and simulate it using Lumerical software. The results show that the use of ITO/Ta 2 O 5 shell over the InP NW as an electron selective contact enhances the absorption of incident photons, which in turn increases efficiency, also providing the condition that the InP required for core can be reduced without sacrificing the ideal current density, leading to a decrease in the fabrication cost. Besides, we investigate and compare the photovoltaic parameters of the structures of ITO/Ta 2 O 5 /p-InP NWSCs with circular and rectangular cross-section in terms of minority carrier life time, doping, and surface recombination velocity (SRV). It is found that circular and rectangular NWSCs with the same filling ratio possess almost the same optimized ideal photo-current such that for the carrier life time of 400 ps and the SRV of less than or equal to 10 4 cm/s at the interface of Ta 2 O 5 /InP, an efficiency as high as ~24.5% and ~25.6% can be achieved for circular and rectangular NWSCs, respectively. • Performance of cylindrical and rectangular InP NW studied using FDTD method. • Optimization of nanostructures is performed in terms of optical Jsc. • Oxide shell ITO/Ta 2 O 5 is used to enhance the absorption and J sc . • Influence of carrier lifetime, doping, and SRV on photovoltaic performance is investigated. • PCE of ~24.5% and ~25.6% is achieved with cylindrical and rectangular NW respectively.

Near-infrared detection in Si/InP core-shell radial heterojunction nanowire arrays

We have explored the photodetection properties of a core-shell nanowire (NW) heterojunction based device consisting of p-Si/n-InP with an aim to provide an alternative to Si based photodetector by integrating a III-V material, viz. InP with silicon. The core is p-Si NWs, prepared by metal assisted chemical etching of p-type Si (100) substrate at room temperature using Ag nanoparticles, whereas n-type InP shells were deposited by metal organic chemical vapor deposition. The current-voltage characteristics of the device resemble that of a p-n junction with good rectification ratio in the dark, and provides photoresponse under visible and near-infrared (NIR) spectral region at low reverse bias condition. In the NIR regime, it shows an on/off ratio of 23 with a rise and fall time of 387 ms and 424 ms, respectively, at a reverse bias voltage of 2 V. The device exhibits a peak responsivity and detectivity of 2.749 A/W and 5.52 × 1011 Jones, respectively, at 920 nm under the same reverse bias voltage. Thus p-SiNW/n-InP core-shell based device could have the potential to be developed as an optoelectronic device beyond the traditional p-n junction exploiting its radial heterojunction based structure.

Photodetecting properties of single CuO\u2013ZnO core\u2013shell nanowires with p\u2013n radial heterojunction

CuO–ZnO core–shell radial heterojunction nanowire arrays were obtained by a simple route which implies two cost-effective methods: thermal oxidation in air for preparing CuO nanowire arrays, acting as a p-type core and RF magnetron sputtering for coating the surface of the CuO nanowires with a ZnO thin film, acting as a n-type shell. The morphological, structural, optical and compositional properties of the CuO–ZnO core–shell nanowire arrays were investigated. In order to analyse the electrical and photoelectrical properties of the metal oxide nanowires, single CuO and CuO–ZnO core–shell nanowires were contacted by employing electron beam lithography (EBL) and focused ion beam induced deposition (FIBID). The photoelectrical properties emphasize that the p–n radial heterojunction diodes based on single CuO–ZnO core–shell nanowires behave as photodetectors, evidencing a time-depending photoresponse under illumination at 520 nm and 405 nm wavelengths. The performance of the photodetector device was evaluated by assessing its key parameters: responsivity, external quantum efficiency and detectivity. The results highlighted that the obtained CuO–ZnO core–shell nanowires are emerging as potential building blocks for a next generation of photodetector devices.

Improvement of carrier collection in Si/a-Si:H nanowire solar cells by using hybrid ITO/silver nanowires contacts

Optoelectronic devices based on high aspect ratio nanowires bring new challenges for transparent electrodes, which can be well addressed by using hybrid structures. Here we demonstrate that a composite contact to radial junction nanowire solar cells made of a thin indium-tin oxide (ITO) layer and silver nanowires greatly improves the collection of charge carriers as compared to a single thick ITO layer by reducing the series resistance losses while improving the transparency. The optimization is performed on p-i-n solar cells comprising of dense non-vertical nanowires with a p-doped c-Si core and an ultra-thin a-Si:H absorption layer grown by plasma-enhanced chemical vapor deposition on glass substrates. The optimal hybrid contact developed in this work is demonstrated to increase the solar cell conversion efficiency from 4.3% to 6.6%.

Radial Nanowire Assemblies under Rotating Magnetic Field Enabled Efficient Charge Separation.

Developing efficient charge separation strategies is essential to achieve high-power conversion efficiency in the fields of chemistry, biology, and material science. Herein, we develop a facile strategy for fabrication of unique wafer-scale radial nanowire assemblies by exploiting shear force in rotary solution. The assembly mechanism can be well revealed by the large-scale stochastic dynamics simulation. Free electrons can be rapidly generated to produce quantitatively tunable current output when the radial nanowire assemblies rotate under the magnetic field. Moreover, the photoconductive performance of the radial semiconductor nanowire assemblies can be remarkably enhanced as the electron-hole recombination was retrained by the efficient charge separation under the rotating magnetic field. Such large-scale unique nanowire assemblies will facilitate the design of an efficient charge separation process in biosystem, sensors, and photocatalysis.

Fabrication and studies on Si/InP core-shell nanowire based solar cell using etched Si nanowire arrays

We report Si/InP core-shell nanowire radial heterojunction array based solar cell. Silicon nanowire (SiNW) arrays were fabricated by room temperature metal assisted chemical etching method on a p-type Si (100) wafer using silver nanoparticles whereas n-InP layer, as a shell, was deposited onto the SiNW arrays by atmospheric pressure metal organic chemical vapor deposition to obtain core-shell radial heterojunctions. A 100 nm transparent conductive oxide layer was deposited onto top of n-InP layer by sputtering. Transmission electron microscope images confirm the formation of Si/InP core-shell radial nanowire heterostructure. From the studies of reflectance spectroscopy, higher absorption of visible photons has been found. Current-voltage measurements on the radial core-shell nanowire heterojunction based solar cell have been taken under dark and an AM 1.5 solar radiation at room temperature. The device is found to provide a conversion efficiency of 4.39% with an open circuit voltage of 0.56 V and a short circuit current density 14.26 mA/cm2 under AM 1.5 solar radiation. The core-shell radial heterojunction solar cell on nanowire arrays shows great improvement of the performance in comparison with conventional nanowire based solar cells. Our study provides new insights into the Si/InP core-shell nanowire based heterojunction which can have potential applications in fabricating nanoscale optoelectronic devices on Si platform.

ZnO-CuO core-shell heterojunction nanowires for optoelectronic applications

Nowadays, core-shell nanowires have attracted a great scientific interest due to their unique physical properties and advanced functionalities leading to applications in optoelectronic area [1]. Combining ZnO, an n type semiconductor with a band gap of 3.3 eV [2] and CuO, an p type semiconductor with a band gap of 1.2 eV [3], into core-shell radial heterojunction nanowires, a type II heterojunction alignment can be acquired, with a good control of the separation and recombination of the charge carriers at the interface between the two semiconductors. In this context, the ZnO-CuO core-shell heterojunction nanowire arrays were synthesized by thermal oxidation in air and radiofrequency magnetron sputtering (Figure 1). The ZnO-CuO core-shell nanowire arrays were investigated by the morphological, structural, optical, compositional and surface chemistry point of views. In order to assessed their optoelectronic properties, individual ZnO-CuO core-shell nanowires were contacted using photolithography, electron beam lithography and thin film deposition techniques. Exhibiting a rectifying behavior (Figure 2), typical for n-p diodes, these single ZnO-CuO core-shell nanowires can be used in UV photodetectors applications, such as radiation detection, air purification, advanced communications etc.

High-Efficiency Solar Cells from Extremely Low Minority Carrier Lifetime Substrates Using Radial Junction Nanowire Architecture

Currently, a significant amount of photovoltaic device cost is related to its requirement of high-quality absorber materials, especially in the case of III-V solar cells. Therefore, a technology that can transform a low-cost, low minority carrier lifetime material into an efficient solar cell can be beneficial for future applications. Here, we transform an inefficient p-type InP substrate with a minority carrier lifetime less than 100 ps into an efficient solar cell by utilizing a radial p-n junction nanowire architecture. We fabricate a p-InP/n-ZnO/AZO radial heterojunction nanowire solar cell to achieve a photovoltaic conversion efficiency of 17.1%, the best reported value for radial junction nanowire solar cells. The quantum efficiency of ∼95% (between 550 and 750 nm) and the short-circuit current density of 31.3 mA/cm2 are among the best for InP solar cells. In addition, we also perform an advanced loss analysis of the proposed solar cell to assess different loss mechanisms in the solar cell.

Breakdown of Corner States and Carrier Localization by Monolayer Fluctuations in Radial Nanowire Quantum Wells.

We report a comprehensive study of the impact of the structural properties in radial GaAs-Al0.3Ga0.7As nanowire-quantum well heterostructures on the optical recombination dynamics and electrical transport properties, emphasizing particularly the role of the commonly observed variations of the quantum well thickness at different facets. Typical thickness fluctuations of the radial quantum well observed by transmission electron microscopy lead to pronounced localization. Our optical data exhibit clear spectral shifts and a multipeak structure of the emission for such asymmetric ring structures resulting from spatially separated, yet interconnected quantum well systems. Charge carrier dynamics induced by a surface acoustic wave are resolved and prove efficient carrier exchange on native, subnanosecond time scales within the heterostructure. Experimental findings are corroborated by theoretical modeling, which unambiguously show that electrons and holes localize on facets where the quantum well is the thickest and that even minute deviations of the perfect hexagonal shape strongly perturb the commonly assumed 6-fold symmetric ground state.

Radial heterojunction based on single ZnO-CuxO core-shell nanowire for photodetector applications

ZnO-CuxO core-shell radial heterojunction nanowire arrays were fabricated by a straightforward approach which combine two simple, cost effective and large-scale preparation methods: (i) thermal oxidation in air of a zinc foil for obtaining ZnO nanowire arrays and (ii) radio frequency magnetron sputtering for covering the surface of the ZnO nanowires with a CuxO thin film. The structural, compositional, morphological and optical properties of the high aspect ratio ZnO-CuxO core-shell nanowire arrays were investigated. Individual ZnO-CuxO core-shell nanowires were contacted with Pt electrodes by means of electron beam lithography technique, diode behaviour being demonstrated. Further it was found that these n-p radial heterojunction diodes based on single ZnO-CuxO nanowires exhibit a change in the current under UV light illumination and therefore behaving as photodetectors.

Elucidating Li-ion adsorption and diffusion behavior on the surface of Cu0.7Co2.3O4 and improvement of performance as flexible full solid-state supercapacitor

Flexible lightweight supercapacitors are now actively investigated as one of the most promising power supplies for the portable and wearable electronics. Herein, radial nanowire arrays of Cu0.7Co2.3O4 are grown on the carbon fiber for a flexible electrode by hydrothermal process and subsequent thermal annealing. The Cu0.7Co2.3O4 electrode, with large exposure of low adsorption energy sites for Li ions due to the nanowires assembled by nanoparticles, displays superior electrochemical performance in 1 M LiCl aqueous solution and demonstrates areal capacitance of 340 mF cm−2 and 685Fg-1 at scan rate of 5 mV s−1. The contribution of Faraday process and capacitive process are analyzed in detail. The density functional theory is employed to calculate the Li+ ion adsorption energy and diffusion barrier, and the results indicate that Li ion can adsorb on 001 facet of the Co2.3Cu0.7O4 easily and the Li ion diffusion barrier along 001 plane is as low as 0.16 eV. Furthermore, a flexible full solid-state supercapacitor is obtained by assembling two pieces of Co2.3Cu0.7O4 electrode face to face, which presents good performance (112.5 mF cm−2 at 5 mV s−1), stability (74% after 5000 cycles), flexibility and wide operating voltage range. Three charged solid-state supercapacitors in series can light 48 green light emitting diodes for 150 s. The Cu0.7Co2.3O4 electrode is expected to be a potential candidate for actual applications.

Complete structural and strain analysis of single GaAs/(In,Ga)As/GaAs core–shell–shell nanowires by means of in-plane and out-of-plane X-ray nanodiffraction

Typically, core–shell–shell semiconductor nanowires (NWs) made from III–V materials with low lattice mismatch grow pseudomorphically along the growth axis, i.e. the axial lattice parameters of the core and shell materials are the same. Therefore, both the structural composition and interface strain of the NWs are encoded along directions perpendicular to the growth axis. Owing to fluctuations in the supplied growth species during molecular beam epitaxy (MBE) growth, structural parameters such as local shell thickness, composition and strain may differ between NWs grown onto the same substrate. This requires structural analysis of single NWs instead of measuring NW ensembles. In this work, the complete structure of single GaAs/(In,Ga)As/GaAs core–shell–shell NW heterostructures is determined by means of X-ray nanodiffraction using synchrotron radiation. The NWs were grown by MBE on a prepatterned silicon (111) substrate with a core diameter of 50 nm and an (In,Ga)As shell thickness of 20 nm with a nominal indium concentration of 15%, capped by a 30 nm GaAs outer shell. In order to access single NWs with the X-ray nanobeam being incident parallel to the surface of the substrate, a single row of holes with a separation of 10 µm was defined by electron-beam lithography to act as nucleation centres for MBE NW growth. These well separated NWs were probed sequentially by X-ray nanodiffraction, recording three-dimensional reciprocal-space maps of Bragg reflections with scattering vectors parallel (out-of-plane) and perpendicular (in-plane) to the NW growth axis. From the out-of-plane 111 Bragg reflection, deviations from hexagonal symmetry were derived, together with the diameters of probed NWs grown under the same conditions. The radial NW composition and interface strain became accessible when measuring the two-dimensional scattering intensity distributions of the in-plane 2{\overline 2}0 and 22{\overline 4} reflections, exhibiting well pronounced thickness fringes perpendicular to the NW side planes (truncation rods, TRs). Quantitative values of thickness, composition and strain acting on the (In,Ga)As and GaAs shells were obtained via finite-element modelling of the core–shell–shell NWs and subsequent Fourier transform, simulating the TRs measured along the three different directions of the hexagonally shaped NWs simultaneously. Considering the experimental constraints of the current experiment, thicknesses and In content have been evaluated with uncertainties of ±2 nm and ±0.01, respectively. Comparing data taken from different single NWs, the shell thicknesses differ from one to another.