Planar Nanowires Research Articles

Fröhlich resonance splitting in hybrid GaN nanowire-Ag nanoparticle structures

Plasmonic nanoparticles (NPs) have attracted significant attention due to their unique optical properties and broad optoelectronic and photonic applications. We investigate modifications of emission in hybrid structures formed by 60 nm silver NPs and GaN planar nanowires (NWs). Bare GaN NWs exhibit photoluminescence (PL) spectra dominated by broad bands peaking at ∼3.44 eV and ∼3.33 eV, attributed to basal plane stacking faults. In hybrids, two new narrow PL lines appear at 3.36 and 3.31 eV, resulting in PL enhancement at these energies. While the 3.36 eV line in hybrid structures can be explained using the Fröhlich resonance approximation based on the electric dipole concept, the appearance of two features at 3.36 and 3.31 eV indicates the splitting of resonance lines. This phenomenon is explained in framework of theoretical model based on the interaction of the dipole with its charge image, taking into account the quadrupole moment of the silver sphere and the quadrupole field of the charge image. A good agreement is obtained between the calculated Fröhlich resonance frequencies and the experimental PL lines in hybrid structures.

Self-Competitive Growth of CsPbBr3 Planar Nanowire Array.

Semiconductor planar nanowire arrays (PNAs) are essential for achieving large-scale device integration. Direct heteroepitaxy of PNAs on a flat substrate is constrained by the mismatch in crystalline symmetry and lattice parameters between the substrate and epitaxial nanowires. This study presents a novel approach termed "self-competitive growth" for heteroepitaxy of CsPbBr3 PNAs on mica. The key to inducing the self-competitive growth of CsPbBr3 PNAs on mica involves restricting the nucleation of CsPbBr3 nanowires in a high-adsorption region, which is accomplished by overlaying graphite sheets on the mica surface. Theoretical calculations and experimental results demonstrate that CsPbBr3 nanowires oriented perpendicular to the boundary of the high-adsorption area exhibit greater competitiveness in intercepting the growth of nanowires in the other two directions, resulting in PNAs with a consistent orientation. Moreover, these PNAs exhibit low-threshold and stable amplified spontaneous emission under one-, two-, and three-photon excitation, indicating their potential for an integrated laser array.

Monte Carlo simulation of planar GaAs nanowire growth

The Monte Carlo model for the planar GaAs nanowire growth via the vapor–liquid-solid mechanism on GaAs substrate using a gallium droplet as a catalyst is realized. The effect of temperature, Ga and As2 deposition rates, substrate orientation and surface properties on the morphology of planar GaAs nanowires is considered. It is shown, that at the initial stages of nanowire growth, a 3D GaAs crystal is formed under a gallium droplet, which sets the nanowire growth direction. The range of temperatures and deposition rates of gallium and arsenic, which ensure the planar nanowire formation on the GaAs(111)A surface, is determined. The investigation of surface properties influence on the nanowire morphology showed that the arsenic diffusion influx into a gallium droplet is of critical importance for the planar GaAs nanowire growth. Ways for achieving a unidirectional growth of planar nanowires on singular and vicinal GaAs surfaces are proposed.

Guided CdTe Nanowires Integrated into Fast Near-Infrared Photodetectors.

Infrared photodetectors are essential devices for telecommunication and night vision technologies. Two frequently used materials groups for this technology are III-V and II-VI semiconductors, notably, mercury-cadmium-telluride alloys (MCT). However, growing them usually requires expensive substrates that can only be provided on small scales, and their large-scale production as crystalline nanostructures is challenging. In this paper, we present a two-stage process for creating aligned MCT nanowires (NWs). First, we report the growth of planar CdTe nanowires with controlled orientations on flat and faceted sapphire substrates via the vapor-liquid-solid (VLS) mechanism. We utilize this guided growth approach to parallelly integrate the NWs into fast near-infrared photodetectors with characteristic rise and fall times of ∼100 μs at room temperature. An epitaxial effect of the planar growth and the unique structure of the NWs, including size and composition, are suggested to explain the high performance of the devices. In the second stage, we show that cation exchange with mercury can be applied, resulting in a band gap narrowing of up to 55 meV, corresponding to an exchange of 2% Cd with Hg. This work opens new opportunities for creating small, fast, and sensitive infrared detectors with an engineered band gap operating at room temperature.

Spectral Tuning of Plasmonic Activity in 3D Nanostructures via High‐Precision Nano‐Printing

AbstractPlasmonic nanoparticles reveal unique optical properties and are increasingly incorporated into commercial products and technologies, ranging from photovoltaics to biological and chemical sensors. Shifting and tuning their plasmonic response according to the targeted application strongly depends on the ability to control the geometry in every detail and has not been reliably demonstrated for complex 3D nano‐architectures yet. Following that motivation, it herein presents how Focused Electron Beam Induced Deposition (FEBID), a highly flexible additive 3D direct‐write technology with spatial nano‐scale precision, is used for the controlled and tunable fabrication of plasmonically active 3D nanostructures that exhibit highly concentrated, well defined and predictable local plasmonic resonances. As model systems, planar Au nanowires and 3D nano‐tips of various geometries are prepared via FEBID and plasmonically characterized via scanning transmission electron microscopy based electron energy loss spectroscopy (STEM‐EELS) mapping measurements. The findings are complemented with corresponding plasmon simulations, revealing very good agreement with experimental findings. This way, on‐demand spectral tuning of the plasmonic response becomes accessible via upfront modeling and design of suitable 3D nanostructures, to achieve customized plasmonic responses, therefore paving the way for yet unrealized plasmonic applications in 3D space.

Recent Advances in III–V Compounds/Polymer Hybrid Solar Cells

The combination of III–V compound semiconductor materials and organic semiconductor materials to construct hybrid solar cells is a potential pathway to resolve the problems of conventional doped p–n junction solar cells, such as complexities in fabrication process and high costs. This review presents the recent progress of organic–inorganic hybrid solar cells based on polymers and III–V semiconductors, from materials to devices. The available growth process for planar/nanostructured III–V semiconductor materials, along with patterning and etching processes for nanostructured materials, are reviewed. As an emphasis of this review, advanced device structure designs are reviewed for facilitation of carrier collection and high efficiency, at planar structure level and nanowire structure level, respectively. Optimization pathways for efficiency enhancement are discussed with respect to polymer layers and surface/interface passivation, respectively. Finally, perspectives on the future development of such hybrid cells are presented.

Effect of Plasmonic Ag Nanoparticles on Emission Properties of Planar GaN Nanowires.

The combination of plasmonic nanoparticles and semiconductor substrates changes the properties of hybrid structures that can be used for various applications in optoelectronics, photonics, and sensing. Structures formed by colloidal Ag nanoparticles (NPs) with a size of 60 nm and planar GaN nanowires (NWs) have been studied by optical spectroscopy. GaN NWs have been grown using selective-area metalorganic vapor phase epitaxy. A modification of the emission spectra of hybrid structures has been observed. In the vicinity of the Ag NPs, a new emission line appears at 3.36 eV. To explain the experimental results, a model considering the Fröhlich resonance approximation is suggested. The effective medium approach is used to describe the enhancement of emission features near the GaN band gap.

Theory of diffusion-induced selective area growth of III-V nanostructures

Selective area growth of nanomembranes (NMs), nanofins, and planar nanowires (NWs) can pave the way for monolithic integration of III-V photonics with Si electronics and enable fabrication of scalable NW networks required for fundamental studies in low-temperature transport physics. Herein, we present an attempt to develop the kinetic growth theory of III-V NMs and related nanostructures in selective area epitaxy. The growth process is assumed to be driven by surface diffusion of group III adatoms. The populations of adatoms diffusing on the mask surface, NM sidewalls, and the top are described by three diffusion equations linked by six boundary conditions. The resulting growth equation provides the NM height as a function of time, slit width and pitch, and deposition conditions. The width and pitch dependences of the NM growth rate are found to be qualitatively different for different directions of the adatom diffusion fluxes (from the mask surface onto the NM or in the opposite direction). A good correlation of the model with the data on the growth kinetics of GaAs NMs by selective area molecular beam epitaxy is demonstrated.

Enhanced proton acceleration from laser interaction with curved surface nanowire targets

A novel curved surface nanowire target is proposed to improve the cutoff energy of accelerated protons via target normal sheath acceleration. The interaction of a laser of intensity $1.37\times 10^{20}\ {\rm W}\ {\rm cm}^{-2}$ with a curved surface nanowire target is studied by two-dimensional particle-in-cell simulations. The numerical results indicate that the sheath electric field at the target rear side is significantly enhanced by this simple target design, compared with using the planar nanowire target. The transverse motion of hot electrons is effectively confined and the energy density of electrons is naturally increased. A series of simulations with various target parameters is carried out to investigate the performance of this novel target. This tailored target may provide implications for generating high-quality proton beams in experiments.

Surface steered aligned gradient inorganic lead halide perovskite CsPbBrxI3−x nanowires for use in photodetectors

Explicit control over the orientation and yield of aligned planar nanowires (NWs) offers a unique blueprint for the facile integration of sophisticated functional devices with ultra-high compactness. In this report, vapor-phase synthesis produces a massive parallel assembly of nanowires owing to the graphoepitaxial effect exhibited by the annealed M-plane sapphire surface. The nanochannels on the substrate surface serve as natural templates and define the position, growth direction, and length of NWs. Microstructure characterizations and strong band-edge photoluminescence (PL) reveal the superior quality crystal structure of CsPbBrxI3-x NWs. The NW's axially asynchronous bromide-iodide composition provides a graded bandgap with two advantages: broad gamut absorption and a dominant unidirectional built-in electric field. The electric field promotes charge carrier transportation via an energy-funneling process, as demonstrated by the outstanding performance of the CsPbBrxI3-x NW photodetector, which has a responsivity of 370 A/W and a brisk photoresponse time of about 4 ms. Bright PL observation even after 100 days of storage at room temperature (RT) and 25 % relative humidity (RH) conditions indicates a boost in monoclinic phase stability and improved environmental degradation resistance. This work presents the feasibility of massive-scale synthesis of parallel NWs and their direct integration into circuits and functional systems for smart and efficient spectrometers, image sensors, optical communication, chemical or biological detection, and other optoelectronic applications.

Lasing modes in ZnO nanowires coupled to planar metals.

Semiconductor nanowire lasers can be subject to modifications of their lasing threshold resulting from a variation of their environment. A promising choice is to use metallic substrates to gain access to low-volume Surface-Plasmon-Polariton (SPP) modes. We introduce a simple, yet quantitatively precise model that can serve to describe mode competition in nanowire lasers on metallic substrates. We show that an aluminum substrate can decrease the lasing threshold for ZnO nanowire lasers while for a silver substrate, the threshold increases compared with a dielectric substrate. Generalizing from these findings, we make predictions describing the interaction between planar metals and semiconductor nanowires, which allow to guide future improvements of highly-integrated laser sources.

P–n junctions in planar GaAs nanowires

Experimental characterization, analytical growth models, and numerical simulations are combined to define strategies for tailoring doping inside planar GaAs nanowires.

Investigation of graphene/InN nanowire based mixed dimensional barristors with widely tunable Schottky barrier for highly sensitive multimodal gas sensing applications

In this work, an electrically tunable mixed dimensional heterojunction of graphene and planar InN nanowires (NW) has been investigated as a potential candidate for highly sensitive gas sensing applications. The high surface carrier concentration of InN, which prevents it from forming good rectifying contact with graphene, has been effectively suppressed through controlled partial surface oxidation in a plasma reactor. The resulting surface modified InN NWs were found to form excellent Schottky junction with graphene, with a barrier height that is widely tunable by the gate voltage, turning it into a mixed dimensional barristor device. When utilized as a multimodal molecular sensor, utilizing both capacitive and conductive modes, it exhibited very high sensitivity, with detection limits reaching down to sub-ppb range for NO2 and low ppb range for NH3 and CO, tested in both N2 and dry air environments. Additionally, surface functionalization, using hemin/benzoic acid, of this non-conventional barristor device yielded excellent selectivity for specific analytes while maintaining the high sensitivity. The responses of the sensor were found to be minimally affected with long term exposure to atmospheric humidity, which underscores the potential of this barristor device as a highly sensitive gas sensor for practical applications.

Vortex Domain Wall Thermal Stability in Magnetic Nanodevices with In‐Plane Magnetic Anisotropy

Thermal switching of vortex domain walls (VW) in planar magnetic nanowires is a key challenge for storing data. Understanding the VW magnetization thermal switching, thermal transformation, and dynamics in magnetic nanowires is essential for the development of magnetic data storage. Thus, a micromagnetic simulation is used to investigate the effect of device temperature (T) on VW thermal switching, structural transformation, and VW dynamics in magnetic nanowires. The VW thermal nucleation is examined based on magnetic properties and magnetization in nanowires becomes more stable. Furthermore, in this study, it is found that VW thermal structural stability decreases by increasing the device temperature. The speed of VW is observed to rise exponentially by increasing the device temperature up to 600 m s−1. Thus, the VW memory is fast in writing and reading stored data at room temperature and higher. The findings reveal the potentiality of a VW simple nanomagnetic as a building block for spintronic devices such as a memory cell where the bit data are stored and VW are stable with high thermal switching stability.

Real-Time Study of Surface-Guided Nanowire Growth by In Situ Scanning Electron Microscopy.

Surface-guided growth has proven to be an efficient approach for the production of nanowire arrays with controlled orientations and their large-scale integration into electronic and optoelectronic devices. Much has been learned about the different mechanisms of guided nanowire growth by epitaxy, graphoepitaxy, and artificial epitaxy. A model describing the kinetics of surface-guided nanowire growth has been recently reported. Yet, many aspects of the surface-guided growth process remain unclear due to a lack of its observation in real time. Here we observe how surface-guided nanowires grow in real time by in situ scanning electron microscopy (SEM). Movies of ZnSe surface-guided nanowires growing on periodically faceted substrates of annealed M-plane sapphire clearly show how the nanowires elongate along the substrate nanogrooves while pushing the catalytic Au nanodroplet forward at the tip of the nanowire. The movies reveal the timing between competing processes, such as planar vs nonplanar growth, catalyst-selective vapor-liquid-solid elongation vs nonselective vapor-solid thickening, and the effect of topographic discontinuities of the substrate on the growth direction, leading to the formation of kinks and loops. Contrary to some observations for nonplanar nanowire growth, planar nanowires are shown to elongate at a constant rate and not by jumps. A decrease in precursor concentration as it is consumed after long reaction time causes the nanowires to shrink back instead of growing, thus indicating that the process is reversible and takes place near equilibrium. This real-time study of surface-guided growth, enabled by in situ SEM, enables a better understanding of the formation of nanostructures on surfaces.

Criterion for Selective Area Growth of III-V Nanowires.

A model for the nucleation of vertical or planar III-V nanowires (NWs) in selective area growth (SAG) on masked substrates with regular arrays of openings is developed. The optimal SAG zone, with NW nucleation within the openings and the absence of parasitic III-V crystallites or group III droplets on the mask, is established, taking into account the minimum chemical potential of the III-V pairs required for nucleation on different surfaces, and the surface diffusion of the group III adatoms. The SAG maps are plotted in terms of the material fluxes versus the temperature. The non-trivial behavior of the SAG window, with the opening size and pitch, is analyzed, depending on the direction of the diffusion flux of the group III adatoms into or from the openings. A good correlation of the model with the data on the SAG of vertical GaN NWs and planar GaAs and InAs NWs by molecular beam epitaxy (MBE) is demonstrated.

Selective area epitaxy of GaAs: the unintuitive role of feature size and pitch

Selective area epitaxy (SAE) provides the path for scalable fabrication of semiconductor nanostructures in a device-compatible configuration. In the current paradigm, SAE is understood as localized epitaxy, and is modelled by combining planar and self-assembled nanowire growth mechanisms. Here we use GaAs SAE as a model system to provide a different perspective. First, we provide evidence of the significant impact of the annealing stage in the calculation of the growth rates. Then, by elucidating the effect of geometrical constraints on the growth of the semiconductor crystal, we demonstrate the role of adatom desorption and resorption beyond the direct-impingement and diffusion-limited regime. Our theoretical model explains the effect of these constraints on the growth, and in particular why the SAE growth rate is highly sensitive to the pattern geometry. Finally, the disagreement of the model at the largest pitch points to non-negligible multiple adatom recycling between patterned features. Overall, our findings point out the importance of considering adatom diffusion, adsorption and desorption dynamics in designing the SAE pattern to create pre-determined nanoscale structures across a wafer. These results are fundamental for the SAE process to become viable in the semiconductor industry.

Combining two-photon lithography with laser ablation of sacrificial layers: A route to isolated 3D magnetic nanostructures

Three-dimensional (3D) nanostructured functional materials are important systems allowing new means for intricate control of electromagnetic properties. A key problem is realising a 3D printing methodology on the nanoscale that can yield a range of functional materials. In this article, it is shown that two-photon lithography, when combined with laser ablation of sacrificial layers, can be used to realise such a vision and produce 3D functional nanomaterials of complex geometry. Proof-of-principle is first shown by fabricating planar magnetic nanowires raised above the substrate that exhibit controlled domain wall injection and propagation. Secondly, 3D artificial spin-ice (3DASI) structures are fabricated, whose complex switching can be probed using optical magnetometry. We show that by careful analysis of the magneto-optical Kerr effect signal and by comparison with micromagnetic simulations, depth dependent switching information can be obtained from the 3DASI lattice. The work paves the way for new materials, which exploit additional physics provided by non-trivial 3D geometries.

Sub-nanometer mapping of strain-induced band structure variations in planar nanowire core-shell heterostructures

Strain relaxation mechanisms during epitaxial growth of core-shell nanostructures play a key role in determining their morphologies, crystal structure and properties. To unveil those mechanisms, we perform atomic-scale aberration-corrected scanning transmission electron microscopy studies on planar core-shell ZnSe@ZnTe nanowires on α-Al2O3 substrates. The core morphology affects the shell structure involving plane bending and the formation of low-angle polar boundaries. The origin of this phenomenon and its consequences on the electronic band structure are discussed. We further use monochromated valence electron energy-loss spectroscopy to obtain spatially resolved band-gap maps of the heterostructure with sub-nanometer spatial resolution. A decrease in band-gap energy at highly strained core-shell interfacial regions is found, along with a switch from direct to indirect band-gap. These findings represent an advance in the sub-nanometer-scale understanding of the interplay between structure and electronic properties associated with highly mismatched semiconductor heterostructures, especially with those related to the planar growth of heterostructured nanowire networks.

Enhanced Proton Acceleration from Laser Interaction with a Tailored Nanowire Target

Target normal sheath field acceleration via laser interaction with structured solid targets has been widely studied for its potential use in a wide range of applications. Here, a novel nanowire target with a corrugated front surface is proposed to improve the proton acceleration by a target normal sheath field. Two-dimensional particle-in-cell simulations demonstrated that with the existence of the corrugated surface, the cut-off energy of accelerated protons nearly doubles compared to the planar nanowire target. When interacting with the corrugated surface, the incident laser pulse is reflected multiple times, focused and reinforced in each cavity near the front surface, which leads to suppression of the reflectivity and an improvement in the absorption rate. Electrons are heated more efficiently and the sheath field at the target rear side is naturally enhanced. To further investigate the performance of this novel target, a series of simulations with various laser intensities and target sizes were also carried out. This simple target design may provide insights for experiments in the future and should arouse interest because of its wide application.