Radial Heterojunction Research Articles

Spin-Resolved Magneto-Tunneling and Giant Anisotropic g-Factor in Broken Gap InAs-GaSb Core-Shell Nanowires.

We experimentally and computationally investigate the magneto-conductance across the radial heterojunction of InAs-GaSb core-shell nanowires under a magnetic field, B, up to 30 T and at temperatures in the range 4.2-200 K. The observed double-peak negative differential conductance markedly blue-shifts with increasing B. The doublet accounts for spin-polarized currents through the Zeeman split channels of the InAs (GaSb) conduction (valence) band and exhibits strong anisotropy with respect to B orientation and marked temperature dependence. Envelope function approximation and a semiclassical (WKB) approach allow to compute the magnetic quantum states of InAs and GaSb sections of the nanowire and to estimate the B-dependent tunneling current across the broken-gap interface. Disentangling different magneto-transport channels and a thermally activated valence-to-valence band transport current, we extract the g-factor from the spin-up and spin-down dI/dV branch dispersion, revealing a giant, strongly anisotropic g-factor in excess of 60 (100) for the radial (tilted) field configurations.

One-dimensional core/shell radial heterojunction with cascade type-II energy-band alignment for enhanced broadband photodetection

One-dimensional core/shell radial heterojunction with cascade type-II energy-band alignment for enhanced broadband photodetection

Silicon nanowires array capped with two shells as light-absorption antenna for self-driven broadband photodetectors

Silicon (Si) is one of the most important semiconductor materials, and Si in nanoscale has a direct bandgap; therefore, it can overcome the issues of poor light absorption for its bulk counterpart [Lu et al., Appl. Phys. Lett. 91, 263107 (2007)]. Currently, much interest is focusing on Si nanowires array (Si-NWA) for its unique characteristics, such as the enhanced light absorption and the superior electronic mobility, for photodetectors and solar cells [Ko et al., Mater. Sci. Semicond. Process. 33, 154–160 (2015) and Xie et al., ACS Nano 8, 4015–4022 (2014)]. Si-NWA or pyramid Si based photodetectors usually show higher performance than those based on the Si wafer due to the enhanced light absorption and the radial heterojunction [Coskun et al., Physica B 604, 412669 (2021) and Xiao et al., Adv. Mater. 30, 1801729 (2018)]. However, the light absorption spectrum of Si-NWA is limited within the near-infrared region, and its surface defects reduce the carriers' lifetime.

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.

MOF-on-MOF nanoarchitecturing of Fe2O3@ZnFe2O4 radial-heterospindles towards multifaceted superiorities for acetone detection

Rational integration of multifunctions is an emerging strategy to develop novel chemiresistor nano platforms with superior sensing properties. Herein, a stepwise synthesis strategy is developed to assemble two MOFs with huge mismatch into MOF-on-MOF heterojunction, which was nanoarchitectured into Fe2O3@ZnFe2O4 radial-heterospindles. The Fe2O3@ZnFe2O4 heterospindles exhibited outstanding selective acetone detection with multifaceted superiorities, such as high response, low limit of detection, fast response and recovery time, and good linearity in responses with a wide concentration window. An in-depth discussion revealed that such multifaceted sensing characters can be attributed to the distinct radial-heterostructure which proffer synergetic effects in the unimpeded gas diffusion with mesoporous sensing nanoarchitecture, favored interfacial charge transport in radial heterojunction, abundant active adsorption/reaction sites by rich oxygen vacancies, and exponential/efficient tuning of sensor resistance with barrier height. This synthetic strategy here may offer a new opportunity for fabricating dual-to-multi MOFs-based advanced nanostructures towards practical chemiresistor nanoplatforms with favorable multifaceted superiorities.

Highly integrated fiber-shaped thermoelectric generators with radially heterogeneous interlayers

The construction of high-density thermoelectric (TE) pairs on small TE devices to improve the utilization of thermal energy has garnered extensive attention. The fabrication of high-density TE pairs in fiber-shaped TE devices with curved surfaces remains challenging. Herein, we report a flexible scrolled TE module with interlayers of radial heterojunction that possesses high-density p-n junctions, accomplished by rolling a patterned carbon nanotube film (active material layer) and a cellulose acetate nanofiber membrane (insulation). This module achieves high integration of ~3.32 mm3 per TE pair and a high output voltage density of ~65.4 mV/cm2 at a temperature difference of 41 K when the length of the scrolled module is 2 cm. Furthermore, a fiber-shaped TE device is designed by interconnecting the scrolled modules. The design of p-n junctions in the axial and radial directions of the fiber-shaped device is realized, and an open-circuit voltage (Voc) density of 0.176 mV/(cm·K) is achieved. In addition, the fiber-shaped TE device can be woven with commercial yarns to construct TE fabric for harvesting the thermal energy of the human body through alternating scrolled modules. This indicates its potential application for the harvesting of wearable energy.

Defect control and Si/Ge core–shell heterojunction formation on silicon nanowire surfaces formed using the top-down method

Control of surface defects and impurity doping are important keys to realizing devices that use semiconductor nanowires (NWs). As a structure capable of suppressing impurity scattering, p-Si/i (intrinsic)-Ge core–shell NWs with radial heterojunctions inside the NWs were formed. When forming NWs using a top-down method, the positions of the NWs can be controlled, but their surface is damaged. When heat treatment for repairing surface damage is performed, the surface roughness of the NWs closely depends on the kind of atmospheric gas. Oxidation and chemical etching prior to shell formation removes the surface damaged layer on p-SiNWs and simultaneously achieves a reduction in the diameter of the NWs. Finally, hole gas accumulation, which is important for suppressing impurity scattering, can be observed in the i-Ge layers of p-Si/i-Ge core–shell NWs.

A High‐Performance Photodetector Based on 1D Perovskite Radial Heterostructure

Abstract1D metal halide perovskites (MHPs) are known as excellent candidates for fabricating high‐performance optoelectronic devices, ranging from lasers to photodetectors (PDs). Nevertheless, their inherent limitations, such as high binding energy and limited carrier mobility, have restricted their further improvement in performance. To tackle this issue, hybrid MHP microwires (MWs) are fabricated by combining MHP MWs and an organic semiconductor via a facile self‐assembly method, forming a radial heterojunction. Meanwhile, an ultrathin insulating organic layer is introduced as an interlayer between MHP MWs and the organic semiconductor. The synthesized hybrid MWs are further applied in rigid and flexible PDs, which demonstrated a high light/dark current on‐off ratio (>2 × 105), a high responsivity (>400 A W−1), and robust bending stability. These impressive results make the hybrid MHP MWs with the unique radial heterostructure promising building blocks for high‐performance future optoelectronics.

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.

Three-dimensional a-Si/a-Ge radial heterojunction near-infrared photovoltaic detector

In this work, three-dimensional (3D) radial heterojunction photodetectors (PD) were constructed over vertical crystalline Si nanowires (SiNWs), with stacked hydrogenated amorphous germanium (a-Ge:H)/a-Si:H thin film layer as absorbers. The hetero absorber layer is designed to benefit from the type-II band alignment at the a-Ge/a-Si hetero-interface, which could help to enable an automated photo-carrier separation without exterior power supply. By inserting a carefully controlled a-Si passivation layer between the a-Ge:H layer and the p-type SiNWs, we demonstrate first a convenient fabrication of a new hetero a-Ge/a-Si structure operating as self-powered photodetectors (PD) in the near-infrared (NIR) range up to 900 nm, indicating a potential to serve as low cost, flexible and high performance radial junction sensing units for NIR imaging and PD applications.

Core-shell nanowire arrays based on ZnO and CuxO for water stable photocatalysts

Staggered gap radial heterojunctions based on ZnO-CuxO core-shell nanowires are used as water stable photocatalysts to harvest solar energy for pollutants removal. ZnO nanowires with a wurtzite crystalline structure and a band gap of approximately 3.3 eV are obtained by thermal oxidation in air. These are covered with an amorphous CuxO layer having a band gap of 1.74 eV and subsequently form core-shell heterojunctions. The electrical characterization of the ZnO pristine and ZnO-CuxO core-shell nanowires emphasizes the charge transfer phenomena at the junction and at the interface between the nanowires and water based solutions. The methylene blue degradation mechanism is discussed taking into consideration the dissolution of ZnO in water based solutions for ZnO nanowires and ZnO-CuxO core-shell nanowires with different shell thicknesses. An optimum thickness of the CuxO layer is used to obtain water stable photocatalysts, where the ZnO-CuxO radial heterojunction enhances the separation and transport of the photogenerated charge carriers when irradiating with UV-light, leading to swift pollutant degradation.

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.

Electrostatically Doped Silicon Nanowire Arrays for Multispectral Photodetectors.

Nanowires have promising applications as photodetectors with superior ability to tune absorption with morphology. Despite their high optical absorption, the quantum efficiencies of these nanowire photodetectors remain low due to difficulties in fabricating a shallow junction using traditional doping methods. As an alternative, we report nonconventional radial heterojunction photodiodes obtained by conformal coating of an indium oxide layer on silicon nanowire arrays. The indium oxide layer has a high work function which induces a strong inversion in the silicon nanowire and creates a virtual p-n junction. The resulting nanowire photodetectors show efficient carrier separation and collection, leading to an improvement of quantum efficiency up to 0.2. In addition, by controlling the nanowire radii, the spectral responses of the In2O3/Si nanowire photodetectors are tuned over several visible light wavelengths, creating a multispectral detector. Our approach is promising for the development of highly efficient wavelength-selective photodetectors.

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.

Improved Current Extraction of Cu/Si Nanowire Heterojunctions for Self-Powered Photodetecting with Insertion of MoO x Quantum Dots Film.

MoOx quantum dots were inserted between the Si nanowires (SiNWs) and Cu contacts to form the MoOx/SiNW heterojunctions via the low-temperature solution process. The common Schottky heterojunction of Cu/SiNWs is used as the referred device, and the photoelectric characteristics of Cu/MoOx/Si structures are detailedly investigated. The results indicate that the inset of MoOx between Cu and SiNWs obviously enhances photoelectric conversion efficiency from 1.58 to 3.92%, and photodetection characteristics have also improved compared to the referred device. We attribute these experimental findings to the fact that the incorporation of MoOx quantum dots into the Cu/Si heterojunction could enhance the transport of holes and inhibit the injection of electrons from Si into the top Cu electrode. In addition, it is believed that such an improved performance also comes from the improved optical absorption as well as the optimized carrier transfer and collection capability of MoOx/SiNW radial heterojunctions.

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.

Vertically arrays of Si-LaNiO3/BiFeO3/Au core-shell nano-capacitors for prominent coupled electro-optic effect

The current study delivers the first assessment of well-separated cone shaped n-type epitaxial BiFe0.9Mn0.1O3 (n-BFMO) shells integrated on LaNiO3 (LNO) buffered vertical arrays of p-type nanoporous Si nanowire cores as an absorber for high efficiency and sustainable all- oxide photovoltaic devices. The photovoltaic effect of the LNO/n-BFMO/Au radial heterojunction is investigated by registering the current-voltage curves under AM 1.5 (100 mW cm−2) illumination conditions at various ferroelectric polarization states of BFMO. The as-deposited LNO/BFMO/Au nanowire arrays fabricated by scalable Pulsed Laser deposition are completely self-polarized and produced a remarkably large open circuit photovoltage up to 0.53 V under zero poling condition. The direction of photocurrent and photovoltage is reversibly switchable in response to ferroelectric polarization switching. The short circuit photocurrent “ISC” and open circuit voltage “VOC” depict a hysteresis loop which is more or less identical to that of ferroelectric polarization and thus can be utilized as a nondestructive optical readout of the ferroelectric nonvolatile memories. By introducing special architecture for the BFO based photovoltaic device, a photoconversion efficiency ∼0.3% and External Quantum Efficiency ∼11% are achieved at zero polarization state, which can further be enhanced and modulated to ∼0.6% and 14%, respectively, at the negative poling state of BFMO.

Textured and micropillar silicon heterojunction solar cells with hot-wire deposited passivation layers

The passivating quality of hydrogenated amorphous silicon thin films is essential for achieving high open-circuit voltages in silicon heterojunction solar cells. This work reports our progress towards the use of hot wire chemical vapor deposition for fabricating these passivation layers. We have achieved 19.4% energy conversion efficiency for a solar cell with a conventional alkali-etched pyramidal texture. Next, we tested radial heterojunction devices that consist of up to 20-μm high micropillar arrays. Proper passivation of these devices is challenging, since the structures feature an enlarged surface area and vertical walls. We have made use of the ability of hot wire chemical vapor deposition to deposit conformal thin films at high deposition rate. The micropillar solar cells show only limited reduction of open-circuit voltage in comparison to a flat reference solar cell and have an improved overall performance. We created additional stacked multijunction silicon-based junctions on top of the micropillars to increase the open-circuit voltage of the solar cells for future applications in which a higher potential is required such as water splitting devices.