Microrod Arrays Research Articles

Monolithic multiple colour emission from InGaN grown on patterned non-polar GaN

A novel overgrowth approach has been developed in order to create a multiple-facet structure consisting of only non-polar and semi-polar GaN facets without involving any c-plane facets, allowing the major drawbacks of utilising c-plane GaN for the growth of III-nitride optoelectronics to be eliminated. Such a multiple-facet structure can be achieved by means of overgrowth on non-polar GaN micro-rod arrays on r-plane sapphire. InGaN multiple quantum wells (MQWs) are then grown on the multiple-facet templates. Due to the different efficiencies of indium incorporation on non-polar and semi-polar GaN facets, multiple-colour InGaN/GaN MQWs have been obtained. Photoluminescence (PL) measurements have demonstrated that the multiple-colour emissions with a tunable intensity ratio of different wavelength emissions can be achieved simply through controlling the overgrowth conditions. Detailed cathodoluminescence measurements and excitation-power dependent PL measurements have been performed, further validating the approach of employing the multiple facet templates for the growth of multiple colour InGaN/GaN MQWs. It is worth highlighting that the approach potentially paves the way for the growth of monolithic phosphor-free white emitters in the future.

Rotation of terahertz radiation due to phonon-mediated magnetoelectric coupling in chiral selenium

The trigonal form of selenium (t-Se) has an unusual, quasi-one-dimensional, chiral crystal structure. First-principles calculations have helped us uncover a polar, optic phonon in t-Se that exhibits a diagonal magnetoelectric coupling with the electric field of the incident THz radiation, and induces a parallel magnetic field due to its inherent chirality. We show that this phonon-mediated, magnetoelectric mechanism is predicted to cause optical rotation in t-Se in the 1--3 THz range and is quite distinct from the high frequency (g80 THz) activity due to electronic excitations that has been reported previously. In the second part of the paper, we report our experimental results based on THz time-domain spectroscopy as well as direct measurements of optical rotation that confirm this prediction in an aligned, monocrystalline array of t-Se microrods. The Se microrod array not only exhibits a large birefringence ($\mathrm{\ensuremath{\Delta}}n=1.3$) but also rotates the polarization of THz radiation by \ensuremath{\sim}3\ifmmode^\circ\else\textdegree\fi{}/mm. To our knowledge, this is the only elemental solid known to rotate THz polarization, because the currently available THz rotators are based mainly on liquid crystals or metamaterials. The identification of new THz-active materials and a better understanding of the underlying physics are both clearly essential to the development of better sources, detectors and components.

Flexible NiO micro-rods/nanoporous Ni/metallic glass electrode with sandwich structure for high performance supercapacitors

Designing and preparing flexible supercapacitor electrode with high specific capacitance and good cycle stability is still a severe challenge. In this study, a sandwich-like nickel oxide/nanoporous nickel/metallic glass (NiO/np-Ni/MG) composite electrode was facilely synthesized by dealloying the Ni45Ti20Zr25Al10 MG ribbons in O2-rich HF solution. Different from the routine free-dealloying, the etching with continuous O2 flow in HF results in the in-situ growth of NiO micro-rod arrays on np-Ni surface. The shape of NiO from an initial flower-like architecture evolves to be the regular hexagonal pillars during the O2-assisted dealloying process. The possible nucleation and growth mechanism of hexagonal NiO is proposed. Moreover, the as-designed sandwich-like structure offers an excellent flexibility of the composite electrode due to the high ductile Ni-based MG interlayer. Meanwhile, the new-type NiO/np-Ni/MG electrodes demonstrate a high specific capacitance (745.3 F/cm3 at 0.5 A/cm3) in KOH solution and excellent cycling stability (92% retention after 6000 cycles at 20 mV/s). The symmetric supercapacitor (SC) devices were successfully assembled by using the dealloyed ribbons and showed a maximum volumetric energy density of about 19.82 mW h/cm3 at a power density of 0.4 W/cm3, and even kept at 8.71 mW h/cm3 when the power density increased to 1.6 W/cm3. The fully charged two SC devices connected in series could light up a red LED bulb for more than 10 min. The novel developed NiO/np-Ni/MG electrodes with good energy storage performance and excellent flexibility show highly promising application potential in wearable energy storage devices.

Wall-Mesoporous Graphitic Carbon Nitride Nanotubes for Efficient Photocatalytic Hydrogen Evolution.

Graphitic carbon nitride (g-CN) has attracted tremendous attention as visible-light photocatalyst. However, for further improving the catalytic activity, multilevel and hierarchical nanostructuring of g-CN is highly desirable to effectively expose active sites and facilitate separation and migration of photoexciteded charge carriers for largely enhanced photocatalytic behavior. Here, we prepare wall-mesoporous graphitic carbon nitride nanotubes (g-CNNTs) by in situ annealing of urea microrod arrays preformed in virtue of a vertical gradient freeze growth (VGFG) method. Benefiting from the distinctive structural features, the hierarchical g-CNNTs exhibit a high photocatalytic H2 production rate of 8789 μmol h-1 g-1 with an excellent apparent quantum yield of 6.3 % under visible-light irradiation and long-term cycling stability. This work provides a facile and eco-friendly strategy to prepare a new type of carbon nitride-based nanostructural material for photocatalysis and environmental remediation.

UV Luminescence and Lasing in Ensembles of Zinc-Oxide Microcrystals with Copper

Microrod arrays of zinc oxide with copper have been grown by chemical vapor deposition on single-crystal sapphire substrates preliminarily covered with a nanoscale copper layer. The presence of copper (about 0.3%) in zinc-oxide microrods has been experimentally shown to increase the photoluminescence intensity of the ZnO microcrystal arrays, reduce the lasing threshold, and lead to a small (by ~2.5 nm) blue shift of the radiation intensity peak of the ZnO microcrystals. It is shown that the laser-radiation character depends on the morphology of the ZnO microrod arrays.

Imaging basal plane stacking faults and dislocations in (11-22) GaN using electron channelling contrast imaging

Taking advantage of electron diffraction based measurements, in a scanning electron microscope, can deliver non-destructive and quantitative information on extended defects in semiconductor thin films. In this work, we have studied a (11-22) semi-polar GaN thin film overgrown on regularly arrayed GaN micro-rod array templates grown by metal organic vapour phase epitaxy. We were able to optimise the diffraction conditions to image and quantify basal plane stacking faults (BSFs) and threading dislocations (TDs) using electron channelling contrast imaging (ECCI). Clusters of BSFs and TDs were observed with the same periodicity as the underlying micro-rod array template. The average BSF and TD densities were estimated to be ≈4 × 104 cm−1 and ≈5 × 108 cm−2, respectively. The contrast seen for BSFs in ECCI is similar to that observed for plan-view transmission electron microscopy images, with the only difference being the former acquiring the backscattered electrons and the latter collecting the transmitted electrons. Our present work shows the capability of ECCI for quantifying extended defects in semi-polar nitrides and represents a real step forward for optimising the growth conditions in these materials.

Piezoelectric diphenylalanine peptide for greatly improved flexible nanogenerators

We explored the piezoelectric behavior of diphenylalanine (FF) peptide using multiphysics finite element models. We compared the piezoelectric potential developed in a finite element model of an FF peptide nanowire with the potential developed in conventional piezoelectric materials. The simulation showed that the FF peptide nanowire can generate significantly higher voltage than nanowires made from piezoelectric materials such as zinc oxide, lead zirconate titanate, and barium titanate under the same force. The simulation work was expanded to create a complete model of a flexible nanogenerator based on FF peptide microrods. Based on the piezoelectric potentials calculated in the nanogenerator model, we estimated an open circuit voltage for the FF nanogenerator. To validate the model, we fabricated and demonstrated the first flexible piezoelectric nanogenerator based on vertical arrays of FF peptide microrods. We characterized the electrical behavior of the fabricated nanogenerators, and measured an open circuit voltage of up to − 0.6 V. The estimated open circuit voltage predicted by the model was in close agreement with the measured value from the fabricated nanogenerator. The results illustrate the promising potential of FF peptide as a piezoelectric material, and show the importance of finite element models for providing insight into the development of a new generation of FF peptide-enabled devices.

Synthesis of zinc oxide microrod arrays and their performance as piezo-generators

ABSTRACTZinc oxide microrods were synthesized by aqueous chemical growth using zinc nitrate and zinc acetate solutions without seed layer. Rod length of around 10 µm was obtained after 5 hr at 95°C. Solution replenishment led to a better alignment normal to the substrate; however, only zinc acetate solution resulted in bigger rods, reaching about 12 µm in length and 5 µm in diameter. X-ray diffraction revealed a pure hexagonal wurtzite structure with high crystallographic texture. Raman spectroscopy showed no evidence of lattice stress. The ZnO microrod arrays were used to fabricate piezoelectric generators, and the largest output was obtained from the generator with the higher density of rods. The use of a piezoelectric polymer in the generators led to an improved performance of at least 200% compared to reports that used a dielectric polymer.

Porous Microrod Arrays Constructed by Carbon-Confined NiCo@NiCoO2 Core@Shell Nanoparticles as Efficient Electrocatalysts for Oxygen Evolution.

The study of cost-efficient and high-performance electrocatalysts for oxygen evolution reaction (OER) has attracted much attention. Here, porous microrod arrays constructed by carbon-confined NiCo@NiCoO2 core@shell nanoparticles (NiCo@NiCoO2 /C PMRAs) are fabricated by the reductive carbonization of bimetallic (Ni, Co) metal-organic framework microrod arrays (denoted as NiCo-MOF MRAs) and subsequent controlled oxidative calcination. They successfully combine the desired merits including large specific surface areas, high conductivity, and multiple electrocatalytic active sites for OER. In addition, the oxygen vacancies in NiCo@NiCoO2 /C PMRAs significantly improve the conductivity of NiCoO2 and accelerate the kinetics of OER. The above advantages obviously enhance the electrocatalytic performance of NiCo@NiCoO2 /C PMRAs. The experimental results demonstrate that the NiCo@NiCoO2 /C PMRAs as electrocatalysts exhibit high catalytic activity, low overpotential, and high stability for OER in alkaline media. The strategy reported will open up a new route for the fabrication of porous bimetallic composite electrocatalysts derived from MOFs with controllable morphology for electrochemical energy conversion devices.

Novel p-n heterojunction copper phosphide/cuprous oxide photocathode for solar hydrogen production

A Copper phosphide (Cu3P) micro-rod (MR) array, with coverage by an n-Cu2O thin layer by electrodeposition as a photocathode, has been directly fabricated on copper foil via simple electro-oxidation and phosphidation for photoelectrochemical (PEC) hydrogen production. The morphology, structure, and composition of the Cu3P/Cu2O heterostructure are systematically analyzed using a scanning electron microscope (SEM), X-ray diffraction and X-ray photoelectron spectra. The PEC measurements corroborate that the p-Cu3P/n-Cu2O heterostructural photocathode illustrates efficient charge separation and low charge transfer resistance to achieve the highest photocurrent of 430 μA cm−2 that is greater than other transition metal phosphide materials. In addition, a detailed energy diagram of the p-Cu3P/n-Cu2O heterostructure was investigated using Mott-Schottky analysis. Our study paves the way to explore phosphide-based materials in a new class for solar energy applications.

Surface modification of ZnO microrod arrays films by ion-exchange approach and their photoelectrochemical performances

ZnO microrod arrays films with the surface modification by two steps ion-exchange approach have been investigated as photoanodes in photoelectrochemical (PEC) cells. X-ray diffraction, Raman, scanning electron microscope, energy dispersive X-ray detector, UV–vis techniques and PEC measurement have been used in the pristine and surface modified ZnO microrod films. The results show that ZnS and CdS layer can be deposited on ZnO microrod surface through a two steps ion-exchange procedure. What's more, it is found that ion-exchange method is a simple approach to adjust CdS content on the samples surface via changing experimental temperature. Consequently, the PEC property of films can be improved through optimizing CdS content on the ZnO microrods surface. In this experiment, it is found that the optimized condition for preparing film is 70 °C (first step) and 100 °C (second step). These results suggest that surface tuning via ion-exchange method should represent a viable strategy to further improve the efficiency of ZnO microrods photoanodes.

A well-grown β-Ga2O3 microrod array formed from GaOOH on a Si (100) substrate and growth mechanism study

A simple two-step hydrothermal method was used to successfully fabricate a well-grown β-Ga2O3 microrod array from GaOOH on a Si (100) substrate without other heterogeneous layers.

High-performance organic circuits based on precisely aligned single-crystal arrays.

In this paper, we demonstrate high-performance organic logic circuits based on precisely controlled organic single-crystal arrays. Well-aligned microrod shaped 2,7-dioctyl[1]benzothieno[3,2-b][1]benzothiophene (C8-BTBT) single-crystal organic thin-film-transistors (OTFTs) were fabricated via solvent mediated molecular tailoring with a polymeric sacrificial layer, exhibiting saturation mobility of >2 cm2 V−1 s−1. Using this approach, precise placement of organic crystal arrays in a controlled orientation was successfully achieved, enabling the fabrication of OTFT-based inverter circuits with a gain of 1.37 (V V−1). Furthermore, it was demonstrated that, by varying the number of single-crystal microrods, the device dimension and corresponding circuit performance can be modulated. A high-performance inverter operation with various interdigitating single-crystal microrod arrays can thus be achieved.

Ni(OH)2 Nanoparticles Embedded in Conductive Microrod Array: An Efficient and Durable Electrocatalyst for Alkaline Oxygen Evolution Reaction

It is extremely important to develop earth-abundant and stable oxygen evolution reaction (OER) electrocatalysts with excellent performance in alkaline media. In this work, we describe the in situ electrochemical conversion of microrod array of Ni(tetracyanoquinodimethane)2, Ni(TCNQ)2, into Ni(OH)2 nanoparticles embedded in a conductive TCNQ microrod array via anode oxidation. Such Ni(OH)2-TCNQ microarray shows OER activity needing overpotentials of 322 and 354 mV to attain current densities of 50 and 100 mA cm–2 in 1.0 M KOH, respectively. It is also extremely durable with its electrocatalytic performance being kept for at least 20 h. This work points out a stimulating approach to explore the utilization of TCNQ array as a conductive matrix for electrochemical applications.

Annealing temperature dependence of structural, optical, and thermal properties of CdSe thin films grown on porous anodic alumina

In this study, cadmium selenide (CdSe) were synthesizd by a chemical route process using l-cysteine (l-cyst) as a ligand, cadmium acetate as a Cd source and Na2SeO3 as a Se precurseur. CdSe nanocrystals were embedded in porous anodic alumina (PAA) layer by the chemical bath deposition (CBD) method. The influence of annealing temperature in air-atmosphere from 150 to 350 °C on the microstructural, optical properties, and thermal conductivity of CdSe in PAA template has been studied by scanning electron microscope (SEM), energy-dispersive X-ray spectroscopy (EDX), Raman spectroscopy, UV–Vis and photothermal deflection (PTD) techniques. The annealing temperature at 300 and 350 °C resulted in a considerable change in the surface morphology of CdSe film on the PAA layer, from a nonhomgeneous CdSe clusters into microrods array with a pyramidal ends. Raman spectroscopy measurements show an enhancement in the crystallinity of CdSe film on PAA layer and an increase of the average radius size of the nanocrystallites from 2.41 to 2.78 nm with the increase of air-annealing temperature. The optical reflectivity result of CdSe on PAA template showed that the crystal quality was significantly improved after the annealing step, bringing about an enhancement in optical absorption and a decrease in bandgap energy. The thermal conductivity Kth was studied via PTD technique, and the Kth increase was depicted with thermal treatement from 4.40 to 5.22 W M−1 K−1.

Flexible Photocatalytic Composite Film of ZnO-Microrods/Polypyrrole

Zinc oxide (ZnO)-based photoanodes with sunlight photocatalytic activity are widely used in dye-sensitized solar cells. Presently, most of such electrodes are inflexible due to the rigidness of ZnO and substrate, thus hindering their application in flexible electronics. Here, we report a flexible composite film of ZnO microrod arrays and polypyrrole (PPy) featuring significant flexibility, durability, and photocatalytic capability under visible light. In this composite film, the upper section of the ZnO microrods is coated with an approximately 400 nm thick PPy shell, and the lower section of the ZnO microrods is tightly embedded into an underlying PPy base layer, creating an integrated heterogeneous structure. The upper PPy coating shell serves as a photosensitizer for the ZnO-based photocatalysis, while the lower PPy base layer facilitates electron transport to the substrate and mechanically reinforces the ZnO microrod arrays. Under visible light, this facile structure achieves much higher photocatalytic efficiency in comparison to pure ZnO microrod arrays or PPy film, degrading methylene blue at a rate of 0.22%/min. This photocatalytic composite film may find promising applications in flexible solar cells to power stretchable and wearable electronics.

Ammonia-Assisted Wet-Chemical Synthesis of ZnO Microrod Arrays on Substrates for Microdroplet Transfer.

It is still a challenging task to facilely grow microscale arrays on arbitrary substrates at low temperature conditions in solutions. Here, we have successfully formed ZnO microrod arrays on various substrates, including glass, gold coated glass, silicon wafer, and Teflon, by a single-step wet-chemical synthesis approach. We employ ammonia as the multifunctional reactant to modify the surface properties of the substrates and to regulate the pH of the reaction environment. Compared to other methods, no preloaded additives or seeds are required. The surface wettability of the ZnO microrod coated substrates can be tuned, achieving both hydrophilic and hydrophobic properties in air. We have studied both static wettability and dynamic behaviors of droplet impact or rebound on the modified substrates. We demonstrate that it is possible to achieve micromass transfer by using the hydrophobic substrate to repel water microdroplet while using the hydrophilic substrate to capture the water microdroplets utilizing their different dynamic wettability-induced responses to water droplets. We believe that the ZnO microrod array coated substrates with different static/dynamic wettability may find many potential applications, such as antiwetting, self-cleaning, inject printing, micromass transfer and capture, biomedical diagnosis, microanalysis, and so forth.

Efficient photodegradation of dye pollutants using a novel plasmonic AgCl microrods array and photo-optimized surface-enhanced Raman scattering

A novel arrayed AgCl micro-rods have in situ grown on an Ag foil successfully for the first time. The preparation process is consisted of two facile steps: (1) immersed oxidation and (2) directional ions exchange. The structure of the as-synthesized arrayed substrate has been characterized comprehensively, and the relevant growth mechanism is proposed. This highly aligned AgCl arrays show a remarkable visible-light-driven photocatalytic activity towards degrading 10−5mol/L rhodamine 6G (R6G) aqueous solution. The ultra-stable catalytic performance of the plasmonic arrays was revealed by the recycled tests in the neutral and acidic conditions. Moreover, a facile SERS substrate based on the AgCl arrays was obtained with the optimal enhancement factor (EF) of ∼3.25×107, by directly putting the substrate under a Xe lamp in 7.5min. Amazingly, the photo-optimized surface-enhanced Raman scattering (SERS) substrate still shows a stable activity for photodegrading R6G. The photocatalytic and SERS mechanism are proposed in this study.

The influence of MOVPE growth conditions on the shell of core-shell GaN microrod structures

A core-shell geometry is employed for most next-generation, three-dimensional opto-electric devices based on III–V semiconductors and grown by metal organic vapor phase epitaxy (MOVPE). Controlling the shape of the shell layers is fundamental for device optimization, however no detailed analysis of the influence of growth conditions has been published to date.We study homogeneous arrays of gallium nitride core-shell microrods with height and diameter in the micrometer range and grown in a two-step selective area MOVPE process. Changes in shell shape and homogeneity effected by deliberately altered shell growth conditions were accurately assessed by digital analysis of high-resolution scanning electron microscope images.Most notably, two temperature regimes could be established, which show a significantly different behavior with regard to material distribution. Above 900°C of wafer carrier temperature, the shell thickness along the growth axis of the rods was very homogeneous, however variations between vicinal rods increase. In contrast, below 830°C the shell thickness is higher close to the microrod tip than at the base of the rods, while the lateral homogeneity between neighboring microrods is very uniform. This temperature effect could be either amplified or attenuated by changing the remaining growth parameters such as reactor pressure, structure distance, gallium precursor, carrier gas composition and dopant materials. Possible reasons for these findings are discussed with respect to GaN decomposition as well as the surface and gas phase diffusion of growth species, leading to an improved control of the functional layers in next-generation 3D V–III devices.

Nitrogen-doped amorphous carbon-silicon core-shell structures for high-power supercapacitor electrodes

We report successful deposition of nitrogen-doped amorphous carbon films to realize high-power core-shell supercapacitor electrodes. A catalyst-free method is proposed to deposit large-area stable, highly conformal and highly conductive nitrogen-doped amorphous carbon (a-C:N) films by means of a direct-current plasma enhanced chemical vapor deposition technique (DC-PECVD). This approach exploits C2H2 and N2 gases as the sources of carbon and nitrogen constituents and can be applied to various micro and nanostructures. Although as-deposited a-C:N films have a porous surface, their porosity can be significantly improved through a modification process consisting of Ni-assisted annealing and etching steps. The electrochemical analyses demonstrated the superior performance of the modified a-C:N as a supercapacitor active material, where specific capacitance densities as high as 42 F/g and 8.5 mF/cm2 (45 F/cm3) on silicon microrod arrays were achieved. Furthermore, this supercapacitor electrode showed less than 6% degradation of capacitance over 5000 cycles of a galvanostatic charge-discharge test. It also exhibited a relatively high energy density of 2.3 × 103 Wh/m3 (8.3 × 106 J/m3) and ultra-high power density of 2.6 × 108 W/m3 which is among the highest reported values.