Nanowire Height Research Articles

(Invited) Architectural Refinement in Core-Shell Ni/Ni(OH)2 Nanowires Array for Enhanced Supercapacitor Performance - A Comprehensive Electrochemical and Finite Element Study

We investigated the electrochemical performance and effect of different aspect ratios of core-shell Ni/Ni(OH)2 nanowires arrayed in an asymmetric supercapacitor with graphene flakes. We created thenanowires with diameters of 35 nm, 100 nm, 150 nm and different height using electrodeposition with help of Anodic Alumina Oxide template, allowing us to regulate their dimensions accurately. We identified an intriguing trend: supercapacitor performance first rises with nanowire height, but then steadily drops when a critical threshold is reached. We used Finite Element Analysis (FEA) to corroborate our experimental results and uncover the underlying electrochemical processes in order to better explain and comprehend this phenomenon. According to the FEA simulations, the nanowire length effects the distribution and accessibility of ionic species surrounding the nanowire array, which influences the charge storage and transfer processes. Figure 1

Heat transfer and visualization of flow boiling on nanowire surfaces in the microchannel

Enhancing heat transfer efficiency is crucial in heat exchange equipment. Although previous studies have focused on developing micro/nano-structured surfaces, further exploration into how surface structure can improve heat transfer efficiency (H) by altering bubble dynamics is still needed. This study aimed to innovatively analyze the impact of titanium carbide (TiC) nanowire heights—specifically 4 µm and 12 µm—on boiling heat transfer performance. Conducted under varying heat flux (Q) (50–200 W/m2) and mass flux (G) (200–300 kg/m2·s), our experiments assessed H, pressure drop (P), and local boiling curves. Using a high-speed camera, we observed complex periodic flow patterns on the 4 µm nanowire surface, including elongated bubble formation, expansion, local dryout, and subsequent fluid rewetting. Results showed that the 12 µm nanowire surface increased H by up to 19.84 % in single-phase conditions, while the 4 µm nanowire surface increased H by up to 27.9 % in two-phase conditions. These findings highlight the significant role of nanowire length and arrangement in optimizing boiling heat transfer performance. This work lays a foundation for further investigations into diverse nanowire materials and configurations.

The effect of micro-nanostructural changes on the absorption and emission characteristics of InGaAsP photocathodes

In this paper, three structures (cylinder, square column, and hexagonal prism) of InGaAsP nanowire arrays are designed based on the excellent light trapping effect of nanostructures. The effects of nanowire aperture, array period, and nanowire height on the light absorption properties are simulated and analyzed using the finite-domain time-difference (FDTD) method. The photoelectron emission capacity of the nanowire arrays was also calculated using MATLAB. The results show that the cylindrical nanowire array has phenomenon of resonance enhancement (absorption peak) in the near-infrared band of 820–1000[Formula: see text]nm, and the shift of absorption peaks can be achieved by adjusting the geometric parameters. Meanwhile, the quantum efficiency is taken to 9.98%. These simulation results provide some reference for the photocathode design of InGaAsP in the near-infrared band.

Two-Dimensional Layered Nanomaterials Steering Self-Assembly of Dodecapeptides with Three Building Blocks.

Self-assembly of peptides on layered nanomaterials such as graphite and MoS2 in the formation of long-range ordered two-dimensional nanocrystal patterns leading to its potential applications for biosensing and bioelectronics has attracted significant interest in nanoscience and nanotechnology. However, controlling the self-assembly of peptides on nanomaterials is still challenging due to the unclear role of nanomaterials in steering self-assembly. Here, we used the in-situ AFM technique to capture different changes of peptide coverage as well as lengthening and widening rates depending on peptide concentrations, show the distinct boundary dynamics of two stabilized peptide domains, and resolve the molecular resolution structural differences and specific orientation of peptide on both nanomaterials. Moreover, ex-situ results showed that the nanomaterial layers tuned the opposite changes of nanowire heights and densities and displayed the different water-resistance stabilities on both nanomaterials. This work provides a basis for understanding nanomaterials steering peptide self-assembly and using hybrid bionanomaterials as a scaffold, enabling for potential biosensing and bioelectronics applications.

Optical characteristics of structures with silicon nanowires and metal nanoparticles

To calculate the optical parameters, the finite difference method in the time domain (FDTD) was used, which can be applied to solve Maxwell’s equations. A large number of combinations of a planar structure with metal nanoparticles and a structure with nanowires and metal nanoparticles (NPs) were calculated. The height of nanowires h varied from 50 to 3000 nm, the period of the structure P was 100–600 nm, and the diameter of metal nanoparticles d was 50–400 nm. The reduction of light reflection was determined by the anti-reflection effect of the Si-NWs array itself and the direct scattering effect of metal nanoparticles. It was shown that all structures gave significantly lower reflection coefficients compared to that of a solid silicon plate.

Photoemission enhancement of InxGa1-xN nanowire array photocathode

The semiconductor material InxGa1-xN can be used in optoelectronic devices to achieve a tunable wide spectral response. Based on “Spicer's model” and “Andachi's model”, a complete theoretical model of photoemission of reflective InxGa1-xN nanowire array photocathode is established. The results show that the photocathode has a sharp cut-off characteristic, and the increase of the nanowire height is conducive to enhancing the quantum efficiency in the response band. Both oblique light and additional electric field can improve the electron collection efficiency of the anode. With an incidence angle of 0° to 40°, the collection efficiency of photocathodes with H = 800 nm is improved, with a maximum increase of 50.21%. In addition, the electron collection efficiency of nanowire photocathode with H = 400 nm, β = 20°and Eout = 0.4 V/μm can be enhanced by 121% compared to the traditional planar photocathode. This work is expected to provide theoretical guidance for researching the InGaN photocathode electron source.

Effect of substrate rotation speed on AlGaN nanowire deep ultraviolet light-emitting diodes by molecular beam epitaxy

Aluminum gallium nitride (AlGaN) nanowires by molecular beam epitaxy (MBE) have become an emerging platform for semiconductor deep ultraviolet (UV) light-emitting diodes (LEDs). Despite of the progress, much less attention has been paid to the effect of substrate rotation speed on the device performance. Herein, we investigate the effect of the substrate rotation speed on the nanowire height and diameter uniformity, as well as the electrical and optical performance of MBE-grown AlGaN nanowire deep UV LED structures with low and high substrate rotation speeds. It is found that by increasing the substrate rotation speed from 4 revolutions per minute (rpm) to 15 rpm, the statistical variation of the nanowire height and diameter is reduced significantly. Increasing the substrate rotation speed also improves the device electrical performance, with a factor of 4 reduction on the device series resistance. This improved electrical performance further transfers to the improved optical performance. The underlying mechanisms for these improvements are also discussed.

Secondary ion mass spectrometry quantification of boron distribution in an array of silicon nanowires

The development of non-planar structures such as arrays of nanowires (NWs), poses a significant challenge for dopant concentration determination. Techniques that can be readily used for 3D structures usually lack the desired sensitivity whereas secondary ion mass spectrometry (SIMS), known for its excellent detection limits, is designed to analyze flat samples. In this work, we overcome the limitation of standard SIMS approaches. NWs are covered with photoresist forming a flat surface. For high incident angle bombardment, the sputtering process becomes self-flattening, i.e. the ions collide with the sidewalls of the exposed tips of NWs at much lower angles and sputter them significantly faster. Thus, reliable information about the dopant distribution along the height of NWs can be obtained. The SIMS analysis can be performed on an array of 1000 x 1000 nanowires with a detection limit of about 5 x 1016 atoms/cm3 and a reasonable signal-to-noise ratio of about 10 dB.

Capillary spreading of ethanol-water on hierarchical nanowire surfaces with interconnected V-groove

Compared to micro/nanostructured surfaces, hierarchical structured surfaces have shown outstanding wicking capability due to the excellent combination of capillary pressure and liquid transport channel. However, the capillary wicking process of different working fluids on structured surfaces, especially hierarchical structured surfaces, was not adequately studied. In this work, a group of hierarchical nanowire surfaces with interconnected V-groove were fabricated and their capillary spreading characteristics were studied with semi-theoretical analyses and experimental tests using ethanol-water droplets with different ethanol volume fractions. The effects of surface structural parameters (nanowire diameter and nanowire height) and the ratio of surface tension to viscosity on the wicking coefficient were discussed with dimensionless wicking coefficient, overall structural contribution, Laplace pressure difference, and average volume flowrate. The wicking coefficient is proportional to the 0.5 power of the ratio of surface tension to viscosity. The hierarchical nanowire surfaces with smaller nanowire diameters possess larger Laplace pressure differences in the nanowire clusters to serve as the major driving pressure difference for capillary spreading. The surfaces with larger nanowire height transport larger volume flowrate in the V-groove.

Structure-Dependent Electrical Conductance of DNA Origami Nanowires.

Exploring the structural and electrical properties of DNA origami nanowires is an important endeavor for the advancement of DNA nanotechnology and DNA nanoelectronics. Highly conductive DNA origami nanowires are a desirable target for creating low-cost self-assembled nanoelectronic devices and circuits. In this work, the structure-dependent electrical conductance of DNA origami nanowires is investigated. A silicon nitride (Si3 N4 ) on silicon semiconductor chip with gold electrodes was used for collecting electrical conductance measurements of DNA origami nanowires, which are found to be an order of magnitude less electrically resistive on Si3 N4 substrates treated with a monolayer of hexamethyldisilazane (HMDS) (∼1013 ohms) than on native Si3 N4 substrates without HMDS (∼1014 ohms). Atomic force microscopy (AFM) measurements of the height of DNA origami nanowires on mica and Si3 N4 substrates reveal that DNA origami nanowires are ∼1.6 nm taller on HMDS-treated substrates than on the untreated ones indicating that the DNA origami nanowires undergo increased structural deformation when deposited onto untreated substrates, causing a decrease in electrical conductivity. This study highlights the importance of understanding and controlling the interface conditions that affect the structure of DNA and thereby affect the electrical conductance of DNA origami nanowires.

Effect of Au film mask on antiwetting and antireflection properties of nanostructural polymer by plasma nanotexturing

Capacitively coupled radiofrequency plasma nanotexturing assisted by an Au film mask is carried out to fabricate the fluorocarbon-film decorated nanowires bundles from the semicrystalline polyethylene (PE) and amorphous PolyMethyl methacrylate (PMMA) subatrates. The spacing distance and height of the nanowires in a bundle on the PE and PMMA substrates are adjusted by the thickness of Au film mask. The complete rebound phenomenon occurs when the droplets impact with a high speed on the nanowire bundles. The nanowire bundles exhibit an improved superhydrophobicity. The narrow spacing distance between the nanowires is beneficial to the antiwetting of superhydrophobic surface. The low reflectivity of the nanowire bundles is also obtained with an incident light wavelength from 400 to 2000 nm. The antireflection property of the nanowire bundles is caused by the high height of nanowires in a bundle. The nanowire bundles on the PE and PMMA substrates prepared by the plasma nanotexturing assisted by the Au film mask have the excellent antiwetting and antireflection properties.

Retention Time Analysis in a 1T-DRAM With a Vertical Twin Gate and p+/i/n+ Silicon Nanowire

In this work, we demonstrate a one-transistor, dynamic random access memory (1T-DRAM) with a very high retention time (RT), vertical twin gates, and a p <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+</sup> /i/n <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+</sup> nanowire via well-calibrated TCAD simulations. The 4F <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> -like cell array of the proposed 1T-DRAM can be achieved by realizing twin gates vertically. This 1T-DRAM has a high read current ratio (10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sup> at 25 °C and 1-ns read duration) of state “1” to state “0,” and, even when a severe word line (WL) and bitline (BL) disturbance is considered, exhibits a RT of ~3 s at 25 °C. The long RT, considering a severe WL/BL disturbance, increases the refresh interval time. A systematic analysis shows that the gate length can be scaled down to 10 nm with an acceptable RT (~3 s) to make the fabrication easier by lowering the height of the silicon nanowire. Based on these results, we believe that our proposed 1T-DRAM will be a strong candidate for future DRAM devices.

Fabrication and Characterization of a Self-Powered n-Bi2Se3/p-Si Nanowire Bulk Heterojunction Broadband Photodetector.

In the present study, vacuum evaporation method is used to deposit Bi2Se3 film onto Si nanowires (NWs) to form bulk heterojunction for the first time. Its photodetector is self-powered, its detection wavelength ranges from 390 nm to 1700 nm and its responsivity reaches its highest value of 84.3 mA/W at 390 nm. In comparison to other Bi2Se3/Si photodetectors previously reported, its infrared detection length is the second longest and its response speed is the third fastest. Before the fabrication of the photodetector, we optimized the growth parameter of the Bi2Se3 film and the best Bi2Se3 film with atomic steps could finally be achieved. The electrical property measurement conducted by the physical property measurement system (PPMS) showed that the grown Bi2Se3 film was n-type conductive and had unique topological insulator properties, such as a metallic state, weak anti-localization (WAL) and linear magnetic resistance (LMR). Subsequently, we fabricated Si NWs by the metal-assisted chemical etching (MACE) method. The interspace between Si NWs and the height of Si NWs could be tuned by Ag deposition and chemical etching times, respectively. Finally, Si NWs fabricated with the Ag deposition time of 60 s and the etching time of 10 min was covered by the best Bi2Se3 film to be processed for the photodetector. The primary n-Bi2Se3/p-Si NWs photodetector that we fabricated can work in a self-powered mode and it has a broadband detection range and fast response speed, which indicates that it can serve as a promising silicon-based near- and mid-infrared photodetector.

Numerical determination of field emission performance of GaN nanowire arrays

Determining the field emission turn-on field of the GaN nanowire array can better adapt to the application of field-assisted photocathode. A numerical simulation of the field emission behavior based on the finite difference method is proposed. Considering the influence of nanowire radius, height, cathode-anode spacing and interwire spacing on the field emission of GaN nanowires. The simulation proves that field enhancement factor of a single GaN nanowire is the most excellent, while emission electrons is relatively small. When GaN nanowires have a larger aspect ratio, it will have a larger enhancement factor and a small required turn-on electric field. Due to an enhanced field shielding, the enhancement factor of the GaN nanowire array reduces as interwire spacing reduces. Cathode-anode spacing hardly influences enhancement factor of GaN nanowire array, while potential distributions are affected by cathode-anode spacing to a certain extent.

AlGaN nanowire deep ultraviolet light emitting diodes with graphene electrode

Despite graphene being an attractive transparent conductive electrode for semiconductor deep ultraviolet (UV) light emitting diodes (LEDs), there have been no experimental demonstrations of any kind of semiconductor deep UV LEDs using a graphene electrode. Moreover, although aluminum gallium nitride (AlGaN) alloys in the format of nanowires are an appealing platform for surface-emitting vertical semiconductor deep UV LEDs, in particular, at short wavelengths, there are few demonstrations of AlGaN nanowire UV LEDs with a graphene electrode. In this work, we show that transferred graphene can serve as the top electrode for AlGaN nanowire deep UV LEDs, and devices emitting down to around 240 nm are demonstrated. Compared to using metal, graphene improves both the light output power and external quantum efficiency. Nonetheless, devices with a graphene electrode show a more severe efficiency droop compared to devices with metal. Here, we attribute the heating effect associated with the large contact resistance to be the major reason for the severe efficiency droop in the devices with a graphene electrode. Detailed scanning electron microscopy and Raman scattering experiments suggest that the nanowire height nonuniformity is the main cause for the large contact resistance; this issue could be potentially alleviated by using nanowires grown by selective area epitaxy that is able to produce nanowires with uniform height. This work, therefore, not only demonstrates the shortest wavelength LEDs using a graphene electrode but also provides a viable path for surface-emitting vertical semiconductor deep UV LEDs at short wavelengths.

Comparative AFM Studies on the Morphology of Conducting Polymers/DNA Templated Nanowires

Comparative morphological studies of three conductive polymers (polyimidazole (Plm), polyindole (Pln), and polypyrrole (Ppy)) templated with deoxyribonucleic (DNA) were carried out on silicon (Si)/mica substrate with the aim of finding more insight about the extent of mixing of the polymers with the DNA and the size of the nanowires formed. Atomic force microscope (AFM) height images of bare DNA on pre-treated substrates revealed a two-dimensional network structure with an average height of approximately 1.60 ± 0.01 nm; which is close to the double stranded DNA chain height. Plm/DNA exhibits globular and agglomerate nanostructures for the concentrated polymers, while the diluted form reveals a dispersed network with a diameter of 3 - 5 nm mostly. In Pln/DNA, the AFM images show dense networks of nanowires in their concentrated form, while the diluted samples displayed individual single wires with regular and smooth morphologies. Statistical analysis of the nanowires AFM heights revealed the dominance of wires with diameters in the range of 3 - 4 nm. The most common nanowire height range was 9 - 10 nm. Comparing the three polymer AFM studies, it can be said that polypyrrole has the highest nanowires range of 9 - 10 nm, then Plm/DNA (3 - 5 nm) and the least is Pln/DNA with 3 - 4 nm range. From the nanowire sizes and morphologies (uniform, smooth and continuous), it can be concluded that both polymers have good potentials for DNA templating and application in nanoelectronic devices as they can be aligned on nanoelectrodes for passage of electricity.

Inclined and gradient Al component AlGaN nanostructures with enhanced photoelectric performance for solar-blind UV detectors

This paper designs an AlGaN nanoarray model with different Al composition distributions, and studies the effects of geometric parameters on its optical response and quantum efficiency. COMSOL Multiphysics software was used to calculate and analyze the influence of the axis inclination on the light absorption efficiency and light field distribution of the nanostructures. Compared with vertical nanostructures, tilted structure arrays have better light absorption characteristics, and the photoelectric response of gradient Al composition AlGaN photocathode is more significant at the wavelength region of 200–300 nm. Simulation studies have found that the dense nanostructured array will reabsorb part of the emitted electrons, and the use of an auxiliary external electric field can effectively improve the collection efficiency of the photocathode. Finally, when the external electric field is 2.5 V/μm, the nanowire height is 800 nm and the incident angle is 10°, the GaN-Al0.365Ga0.635N layered nanostructure array can achieve an electrical response efficiency of 30.93%.

Design and simulation of 3D perovskite solar cells based on titanium dioxide nanowires to achieve high-efficiency

The advantages of perovskite solar cells over other photovoltaic technologies in recent years lead many works to improve the efficiency of these solar cells. The short circuit current density of perovskite solar cells can be improved using 3D structures. The structure that we present is consists of a perovskite layer on a nanowire metal oxide scaffold layer. The morphology of these layers is determined by using a 3D structure algorithm. In this paper, we simulated perovskite solar cells based on the TiO2 nanowire scaffold layer, which acts as an electron transport layer and a 3D scaffold layer, by using FDTD and FE simulation methods. These methods are respectively used for optical and electrical simulations. According to our simulation results and with our proposed and optimized structure, the maximum power conversion efficiency that we have reached is 18.9%. We ignored some practical limitations to achieve this efficiency. The Period and height of nanowires for this structure are 100 nm and 400 nm respectively. All these dimensions are determined by reviewing the practical papers, so these dimensions can be applied in practical cases. Also in this study, when we apply all recombinations and practical limits, we achieved 14.7% PCE that showed 61% improvement than conventional flat structure perovskite solar cells. Both of these power conversion efficiencies that we achieved can compete against conventional silicon solar cells. Because producing these perovskite solar cells with metal oxide nanowires especially TiO2 is much more affordable and convenient than conventional silicon solar cells.

Photocurrent enhancement of Al x Ga1−x N nanowire arrays photodetector based on coupling effects of pn junction and gradient component

Ultraviolet photodetector has a variety of applications in medical diagnosis, civilian testing and military security. The enhancement of photo response has far been a hot topic regrading to the performance improvement of the devices. In this study, we proposed a self-powered photodetector based on Al x Ga1−x N nanowire arrays (NWAs) utilizing axial pn junction integrating with gradient Al component. The merit of the coupling structure is demonstrated by theoretical model and simulations. The photoelectric conversion model is built based on a continuity equation derived by its corresponding boundary conditions. The photocurrent for a single nanowire and NWAs are respectively obtained. According to the simulation results of a single nanowire, the optimal nanowire height is obtained with a photocurrent enhancement up to 330%. For NWAs, the aspect ratio of NWAs and incident angle of light synergistically determine the output photocurrent. The optimal aspect ratio for NWAs is 1:1 with an optimal incident angle of 57°. This study provides a reliable method for the design of photodetectors with micro-nano structures.

Properties of graphene deposited on GaN nanowires: influence of nanowire roughness, self-induced nanogating and defects.

We present detailed Raman studies of graphene deposited on gallium nitride nanowires with different variations in height. Our results indicate that different density and height of nanowires impact graphene properties such as roughness, strain, and carrier concentration as well as density and type of induced defects. Tracing the manifestation of those interactions is important for the application of novel heterostructures. A detailed analysis of Raman spectra of graphene deposited on different nanowire substrates shows that bigger differences in nanowires height increase graphene strain, while a higher number of nanowires in contact with graphene locally reduces the strain. Moreover, the value of graphene carrier concentration is found to be correlated with the density of nanowires in contact with graphene. The lowest concentration of defects is observed for graphene deposited on nanowires with the lowest density. The contact between graphene and densely arranged nanowires leads to a large density of vacancies. On the other hand, grain boundaries are the main type of defects in graphene on rarely distributed nanowires. Our results also show modification of graphene carrier concentration and strain by different types of defects present in graphene. Therefore, the nanowire substrate is promising not only for strain and carrier concentration engineering but also for defect engineering.