Unipolar assembly of zinc oxide rods manifesting polarity-driven collective luminescence

Structural and Optical Characteristics of Highly UV-Blue Luminescent ZnNiO Nanoparticles Prepared by Sol-Gel Method

A simple single pot sol-gel method is used to prepare ZnNiO nanoparticles at assorted Ni doping levels, 1, 3, 7 and 10 wt.%. Structural and optical properties of nanoparticles are studied by X-ray diffraction (XRD), UV-visible diffuse reflection spectroscopy (DRS), photoluminescence (PL) measurements, scanning electron microscopy (SEM), μ-Raman and X-ray photoelectron-spectroscopy (XPS). A single substitutional solid solution phase is detected in the wurtzite ZnNiO nanoparticles at various doping levels. XRD peak splitting and shifting is ascribed to reduced wurtzite character and presence of crystalline strain in nanoparticles at higher level of Ni doping. The Kubelka-Munk function of DRS data reveals the presence of the Burstein-Moss effect in the optical absorption of ZnNiO nanoparticles. Photoluminescence studies show intense UV-blue emission from ZnNiO nanoparticles. The UV PL also exhibits the Burstein-Moss blue shift in the ZnNiO luminescence. Raman analyses also confirms the wurtzite structure of ZnNiO nanoparticles; however, crystal structural defects and bond stiffness increase with Ni doping. The optical and structural studies presented in this work are pointing towards a multivalent Ni substitution in the nanoparticles.

Revisiting the Growth Mechanism of Hierarchical Semiconductor Nanostructures: The Role of Secondary Nucleation in Branch Formation

Although there have been advances in synthesizing hierarchical semiconductor materials,  few studies have investigated the fundamental nucleation mechanisms to explain the origins of such complex structures. Resolving these nucleation and growth pathways is technically challenging but  critical for developing predictive synthetic capabilities for the synthesis and application of new materials. In this Letter, we use state-of-the-art in situ liquid phase scanning electron microscopy (SEM) and high-resolution transmission electron microscopy in a combination with classical density functional theory (cDFT) to study the nucleation of highly branched wurtzite ZnO nanostructures via a facile, room-temperature aqueous synthesis route. Using a range of precursor concentrations, we systematically vary the hierarchical organization of these nanostructures. In situ liquid phase SEM demonstrates that all branches form through secondary nucleation and grow by classical processes. Neither random aggregation nor oriented attachment is observed. cDFT results imply that the morphological evolution with increasing [Zn²⁺] arises from an interplay between a rising thermodynamic driving force, which promotes branch number and variability of orientation, and increasing barriers to interfacial transport due to ion correlation forces that alter the anisotropic kinetics of growth. These findings provide a quantitative picture of branching that sets to rest past controversies and advances efforts to decipher growth mechanisms of hierarchical structures in real solution environments.

Native Point Defect Measurement and Manipulation in ZnO Nanostructures

This review presents recent research advances in measuring native point defects in ZnO nanostructures, establishing how these defects affect nanoscale electronic properties, and developing new techniques to manipulate these defects to control nano- and micro- wire electronic properties. From spatially-resolved cathodoluminescence spectroscopy, we now know that electrically-active native point defects are present inside, as well as at the surfaces of, ZnO and other semiconductor nanostructures. These defects within nanowires and at their metal interfaces can dominate electrical contact properties, yet they are sensitive to manipulation by chemical interactions, energy beams, as well as applied electrical fields. Non-uniform defect distributions are common among semiconductors, and their effects are magnified in semiconductor nanostructures so that their electronic effects are significant. The ability to measure native point defects directly on a nanoscale and manipulate their spatial distributions by multiple techniques presents exciting possibilities for future ZnO nanoscale electronics.

Impact of l-Arginine and l-Histidine on the structural, optical and antibacterial properties of Mg doped ZnO nanoparticles tested against extended-spectrum beta-lactamases (ESBLs) producing Escherichia coli

Magnesium doped Zinc oxide nanoparticles (Mg:ZnO NPs) were synthesized by co-precipitation method. The synthesized Mg:ZnO NPs exhibited hexagonal wurtzite structure, which was confirmed by X-ray diffraction results. After structural confirmation of Mg doped ZnO NPs, base amino acids like l-Arginine and l-Histidine were separately incorporated with the Mg: ZnO NPs. l-Arginine added Mg:ZnO (Mg:ZnO:LA) and l-Histidine added Mg:ZnO (Mg:ZnO: LH) NPs retained the same wurtzite hexagonal structure and average crystallite sizes of Mg: ZnO:LA and Mg: ZnO:LH NPs were found to be 25 nm and 20 nm respectively. The sizes of Mg:ZnO:LH and Mg: ZnO: LA NPs decreased as compared to that of the Mg doped ZnO NPs. From the FT-IR spectra, the ZnO stretching frequencies were observed at 516, 517 and 518 cm^-1 for Mg:ZnO, Mg:ZnO: LA and Mg: ZnO:LH NPs respectively. From the FESEM images, the morphologies of ZnO:Mg and ZnO:Mg:LA NPs were spherical and the Mg: ZnO: LH NPs formed nano-flakes structure. From the EDAX study, the amount of elements incorporated in the samples was determined. The photoluminescence measurements revealed the existence of zinc vacancies, oxygen vacancies and surface defects of the samples. Antibacterial activity of the amino acid added Mg doped ZnO NPs was studied against extended-spectrum beta-lactamases (ESBLs) producing Escherichia coli (E. coli).The Minimal Inhibitory Concentration (MIC) of the LH added ZnO:Mg NPs was found to be 1000 μg/ml for which the growth of E. coli completely inhibited. l-Histidine added Mg doped ZnO NPs showed the highest antibacterial activity as compared to that of the Mg:ZnO NPs and ZnO:Mg:LA NPs.

On the interpretation of cathodoluminescence intensity maps of wide band gap nanowires

It is commonly assumed in the spatially resolved cathodoluminescence (CL) studies of wide band gap (WBG) nanowires (NWs) that the CL intensity maps of deep-level (DL) and near band edge (NBE) emission reflect the spatial distribution of defects in these structures. On this basis, crucial conclusions about the technological growth conditions of NWs are drawn. However, here we showed using three-dimensional finite element analysis that in the case of WBG NWs, which exhibit surface band bending, the CL intensity maps of DL and NBE emission do not reflect the distribution of defects but, instead, the electric field strength in NWs. In particular, we found that independently of the defect concentration distribution, the DL emission intensity is always the highest in the areas where the electric field is the strongest and the lowest where the electric field is absent, while the NBE emission intensity exhibits the opposite trends. We explained this finding by the strong influence of the electric field on the spatial distribution of radiative recombination rates. Overall, our results indicate that (i) the frequently observed spatially inhomogeneous CL intensity distribution in WBG NWs can result from the presence of electric fields but not, as widely accepted, non-uniform defect distribution and (ii) spatially resolved CL spectroscopy measurements on the WBG NWs in most cases can not provide any quantitative information about the DL defect distribution.

Metal-Coordination-Induced Fusion Creates Hollow Crystalline Molecular Superstructures

In this work, we report the formation of superstructures assembled from organic tubular crystals mediated by metal-coordination chemistry. This template-free process involves the crystallization of molecules into crystals having a rectangular and uniform morphology, which then go on to fuse together into multibranched superstructures. The initially hollow and organic crystals are obtained under solvothermal conditions in the presence of a copper salt, whereas the superstructures are subsequently formed by aging the reaction mixture at room temperature. The mild thermodynamic conditions and the favorable kinetics of this unique self-assembly process allowed us to ex-situ monitor the superstructure formation by electron microscopy, highlighting a pivotal and unusual role for copper ions in their formation and stabilization.

In vitro antibacterial activity of ZnO and Nd doped ZnO nanoparticles against ESBL producing Escherichia coli and Klebsiella pneumoniae

Pure ZnO and Neodymium (Nd) doped ZnO nanoparticles (NPs) were synthesized by the co-precipitation method. The synthesized nanoparticles retained the wurtzite hexagonal structure. From FESEM studies, ZnO and Nd doped ZnO NPs showed nanorod and nanoflower like morphology respectively. The FT-IR spectra confirmed the Zn-O stretching bands at 422 and 451 cm(-1) for ZnO and Nd doped ZnO NPs respectively. From the UV-VIS spectroscopic measurement, the excitonic peaks were found around 373 nm and 380 nm for the respective samples. The photoluminescence measurements revealed that the broad emission was composed of ten different bands due to zinc vacancies, oxygen vacancies and surface defects. The antibacterial studies performed against extended spectrum β-lactamases (ESBLs) producing strains of Escherichia coli and Klebsiella pneumoniae showed that the Nd doped ZnO NPs possessed a greater antibacterial effect than the pure ZnO NPs. From confocal laser scanning microscopic (CLSM) analysis, the apoptotic nature of the cells was confirmed by the cell shrinkage, disorganization of cell wall and cell membrane and dead cell of the bacteria. SEM analysis revealed the existence of bacterial loss of viability due to an impairment of cell membrane integrity, which was highly consistent with the damage of cell walls.

Towards a highly efficient simulated sunlight driven photocatalyst: a case of heterostructured ZnO/ZnS hybrid structure

Large scale ZnO/ZnS heterostructured microflowers are fabricated through a rapid and facile strategy via microwave-assisted in situ surface sulfidation route. The as-obtained product possesses an average diameter of about 2 μm and is composed of many thin nanowires. Through a careful inspection under various growth conditions, the morphologies of the as-prepared hybrid structures could be controlled by tailoring the concentration of thioacetamide (TAA) solution during the microwave irradiation, and a possible growth mechanism was proposed. The photocatalytic experiment results for the photodegradation of eosin B under simulated sunlight irradiation revealed that the hybrid nanostructures possess significantly higher photocatalytic activity which is about triple that of the original ZnO precursors, indicating their potential applications in organically polluted water treatment. The optimal sulfidation concentration to realize the maximum photocatalytic activity in the ZnO/ZnS hybrid structures is also proposed and discussed. Meanwhile, this facile, rapid microwave-assisted strategy is scalable and can be extended to synthesize other oxide/sulfide (MOx/MSy) heterostructures.

Towards perfectly ordered novel ZnO/Si nano-heterojunction arrays

The fabrication of a highly ordered novel ZnO/Si nano-heterojuntion array is introduced. ZnO seed layer is first deposited on the Si (P<111>) surface. The nucleation sites are then defined by patterning the surface through focused ion beam (FIB) system. The ZnO nanorods are grown on the nucleation sites through hydrothermal process. The whole fabrication process is simple, facile and offers direct control of the space, length and aspect ratio of the array. It is found that ZnO/Si nanojunctions show an improved interface when subjected to heat treatment. The recrystallization of ZnO and the tensile lattice strain of Si developed during the heating process contribute the enhancement of their photoresponses to white light. The photoluminescence (PL) measurement result of nano-heterojunction arrays with different parameters is discussed.

A first-principles study of ZnO polar surface growth: adsorption of Zn(x)O(y) clusters

Adsorption of Zn(x)O(y) (x + y = 1-6) clusters on ZnO (000 ± 1) polar surfaces is studied systematically via density function theory (DFT) calculations. Different adsorption behaviors are predicted for these two surfaces. On the (0001)-Zn surface, O atoms adsorb on hollow sites at the initial stage. Then Zn atoms come in, and the stable structure becomes bulk-like for some specific clusters. On the (0001)-O surface, Zn cluster adsorption leads to stable cage structures formed by pulling substrate O out. In clusters with both Zn and O, O atoms avoid directly bonding with the surface, and no energetically favorable bulk-like structure is found. On the basis of the prediction of these surface adsorption behaviors, experimentally observed growth rate and surface roughness differences on these two polar surfaces can be understood.

Printed sub-100 nm polymer-derived ceramic structures

We proposed an unconventional fabrication technique called spin-on nanoprinting (SNAP) to generate and transfer sub-100 nm preceramic polymer patterns onto flexible and rigid substrates. The dimensions of printed nanostructures are almost the same as those of the mold, since the ceramic precursor used is a liquid. The printed patterns can be used as a replica for printing second-generation structures using other polymeric materials or they can be further converted to desirable ceramic structures, which are very attractive for high-temperature and harsh environment applications. SNAP is an inexpensive parallel process and requires no special equipment for operation.

Synthesis of Disk-Like and Flower-Like ZnO Nanostructures by Sodium Dodecyl Sulfate-Assisted non-Basic Solution Process

Disk-like and flower-like ZnO nanostructures were prepared in alcoholic solutions with SDS-free and various sodium dodecyl sulfate (SDS) concentrations by a non-basic solution method. In all cases, the obtained crystals exhibited the crystal structure of wurtzite ZnO and photoluminescence at UV and green regions. The ZnO grown in the SDS solution have a morphology of aggregated disk-like and flower-like structures. The origin of morphology different of the ZnO crystals is explained by a chemical interaction between surface of ZnO seed crystal and SDS micelles.

Polymorphous ZnO complex architectures: selective synthesis, mechanism, surface area and Zn-polar plane-codetermining antibacterial activity

Complex ZnO architectures with tunable morphologies and structures were obtained by modulating only the base type and molar ratio of base to Zn²⁺ (α) using an easy one-pot hydrothermal approach without any template or organic additive. Characterizations by X-ray diffraction, Fourier-transform infrared spectrometry, scanning electron microscopy, transmission electron microscopy, and surface area analysis were performed. The effect of the base type and base/Zn²⁺ molar ratio on the morphology and corresponding mechanism were determined. The correlations between the microstructure and properties were established. The antibacterial effect of the ZnO samples was probably due to a combination of variable factors. Better antibacterial activity is derived from more effective antibacterial surfaces, which are mainly associated with the specific surface area and Zn-polar plane. Thus, flower-like architectures with larger specific surface areas and more highly exposed (0001) Zn-polar surfaces outwards are promising structures for ZnO antibacterial agents. This work provides a guide for devising and synthesizing highly efficient antibacterial materials.

ZnS nanostructure arrays: a developing material star

Semiconductor nanostructure arrays are of great scientific and technical interest because of the strong non-linear and electro-optic effects that occur due to carrier confinement in three dimensions. The use of such nanostructure arrays with tailored geometry, array density, and length-diameter-ratio as building blocks are expected to play a crucial role in future nanoscale devices. With the unique properties of a direct wide-bandgap semiconductor, such as the presence of polar surfaces, excellent transport properties, good thermal stability, and high electronic mobility, ZnS nanostructure arrays has been a developing material star. The research on ZnS nanostructure arrays has seen remarkable progress over the last five years due to the unique properties and important potential applications of nanostructure arrays, which are summarized here. Firstly, a survey of various methods to the synthesis of ZnS nanostructure arrays will be introduced. Next recent efforts on exploiting the unique properties and applications of ZnS nanostructure arrays are discussed. Potential future directions of this research field are also highlighted.

Low-energy cathodoluminescence microscopy for the characterization of nanostructures

Spatially and spectrally resolved low-energy cathodoluminescence (CL) microscopy was applied to the characterization of nanostructures. CL has the advantage of revealing not only the presence of luminescence centers but also their spatial distribution. The use of electrons as an excitation source allows a direct comparison with other electron-beam techniques. Thus, CL is a powerful method to correlate luminescence with the sample structure and to clarify the origin of the luminescence. However, caution is needed in the quantitative analysis of CL measurements. In this review, the advantages of cathodoluminescence for qualitative analysis and disadvantages for quantitative analysis are presented on the example of nanostructures.