Emission of Cu-related complexes in ZnO:Cu nanocrystals

Photoluminescence (PL), its temperature dependence, scanning electronic microscopy (SEM) and X ray diffraction (XRD) have been applied for the comparative study of varying the emission, morphology and crystal structure of ZnO and ZnO:Cu nanocrystals (NCs) versus technological routines, as well as the dependence of ZnO:Cu NC parameters on the Cu concentration. A set of ZnO and ZnO Cu NCs was prepared by the electrochemical (anodization) method at a permanent voltage and different etching durations with follows thermal annealing at 400 °C for 2 h in ambient air. The size of ZnO NCs decreases from 300 nm×540 nm down to 200 nm×320 nm with etching duration increasing. XRD study has confirmed that thermal annealing stimulates the ZnO oxidation and crystallization with the formation of wurtzite ZnO crystal lattice. XRD method has been used for monitoring the lattice parameters and for confirming the Cu doping of ZnO Cu NCs. In ZnO Cu NCs four defect related PL bands are detected with the PL peaks at 1.95–2.00 eV (A), 2.15-2.23  eV (B), 2.43–2.50 eV (C) and 2.61–2.69 eV (D). Highest PL intensities of orange, yellow and green emissions have been obtained in ZnO Cu NCs with the Cu concentration of 2.28 at%. At Cu concentration increasing (≥2.28 at%) the PL intensities of the bands A, B, C decrease and the new PL band peaked at 2.61–2.69 eV at 10 K appears in the PL spectrum. The variation of PL intensities for all PL bands versus temperature has been studied and the corresponding activation energies of PL thermal decay have been estimated. The type of Cu-related complexes is discussed using the correlation between the PL spectrum transformation and the variation of XRD parameters in ZnO Cu NCs.     

Excimer laser accelerated hydrothermal synthesis of ZnO nanocrystals & their electrical properties

The synthesis of ZnO nanocrystals is reported using a hydrothermal chemical growth technique combined with 248 nm nanosecond excimer laser heating at fluences in the range 0-390 mJ cm⁻². The effect of laser heating in controlling the morphology of the nanocrystals is investigated using optical spectroscopy and electron microscopy characterization. Laser heating is shown to allow control of the crystal morphology from nanoparticles to nanorods as well as to modify the size distributions. The results indicate that not only does the laser accelerate the growth of nanocrystals, but can also produce crystals with a narrow size distribution possibly via photothermal size selection. An initial study of electrical conduction properties of ZnO nanocrystal thin films is also discussed.     

Effect of bi-layer ratio in ZnO/Al2O3 multilayers on microstructure and functional properties of ZnO nanocrystals embedded in Al2O3 matrix

Zinc oxide (ZnO) nanocrystals (NCs) embedded in alumina (Al₂O₃) matrix were produced via rapid thermal annealing (RTA) of pulsed laser deposited ZnO/Al₂O₃ multilayered nanostructures. The effect of the thickness ratio (R) between Al₂O₃ and ZnO in one bi-layer on the microstructure and functional properties of NCs has been investigated. Grazing incidence small angle X-ray scattering confirmed the formation of nanocrystals after RTA. Grazing incidence wide angle X-ray scattering studies revealed that ZnO NCs have a high crystalline quality with (100) as preferred orientation. Tensile strain of NCs decreases with increasing R and is correlated to the distribution of NCs. From Raman analysis, it is noticed that the phonon frequency of the E₂ mode, related to the ZnO wurtzite phase, in NCs is shifted towards that of bulk ZnO with increasing R. Photoluminescence studies revealed that the near edge peak position shifts from 382 nm to 371 nm as the ratio R changes from 1.5 to 4 and is attributed to the strain effect. The intensity of emission in the yellow–green region due to defects decreases significantly with increasing R. Current–voltage (I–V) characteristics of Al/ZnO NCs embedded in Al₂O₃/n-Si (100)/Al have shown a hysteresis behavior. The increasing width of the hysteresis with increasing R revealed that the origin of the hysteresis might be due to the existence of polar surface charges on well-separated NCs. The high-resistance and low-resistance states in I–V hysteresis curves seem to be governed by Fowler–Nordheim tunneling and Schottky emission mechanisms, respectively.     

Thin film and interface properties during ZnO deposition onto high-barrier hybrid/PET flexible substrates

The combination of transparent conductive oxides with high-barrier films deposited onto flexible polymeric substrates is of considerable importance in order to improve the efficiency, lifetime and stability of flexible electronic devices. In this work, ZnO thin films have been deposited onto high-barrier hybrid/PET flexible substrates by pulsed DC magnetron sputtering, at room temperature and by applying different power values on the target. The employment of in situ and real-time Vis–fUV (1.5–6.5 eV) spectroscopic ellipsometry allowed the investigation of the growth mechanisms of ZnO thin films as well as the modification procedure in the hybrid's surface. Island growth is dominant during the initial stages of deposition concerning low target power regime, whereas layer-by-layer deposition prevails at the high target power regime. The hybrid's modified layer of ～10 nm was confirmed by the transmission electron microscopy measurements which additionally revealed a columnar structure of the film with a nanocrystalline morphology. The estimated size of the nanocrystals (～15 nm and above) was compatible with atomic force microscopy (AFM) measurements. Finally, the evolution of the optical parameters (energy gap and absorption peaks) of the ZnO films during the deposition was similar.     

Magneto-optical properties of α-Fe2O3@ZnO nanocomposites prepared by the high energy ball-milling technique

Magnetic–fluorescent nanocomposites (NCs) with 10 wt% of α-Fe₂O₃ in ZnO have been prepared by the high energy ball-milling. The crystallite sizes of α-Fe₂O₃ and ZnO in the NCs are found to vary from 65 nm to 20 nm and 47 nm to 15 nm respectively as milling time is increased from 2 to 30 h. XRD analysis confirms presence of α-Fe₂O₃ and ZnO in pure form in all the NCs. UV–vis study of the NCs shows a continuous blue-shift of the absorption peak and a steady increase of band gap of ZnO with increasing milling duration that are assigned to decreasing particle size of ZnO in the NCs. Photoluminescence (PL) spectra of the NCs reveal three weak emission bands in the visible region at 421, 445 and 485 nm along with the strong near band edge emission at 391 nm. These weak emission bands are attributed to different defect – related energy levels e.g. Zn-vacancy, Zn interstitial and oxygen vacancy. Dc and ac magnetization measurements show presence of weakly interacting superparamagnetic (SPM) α-Fe₂O₃ particles in the NCs. ⁵⁷Fe-Mössbauer study confirms presence of SPM hematite in the sample milled for 30 h. Positron annihilation lifetime measurements indicate presence of cation vacancies in ZnO nanostructures confirming results of PL studies.     

Enhancement of optical switching parameter and third-order optical nonlinearities in embedded Si nanocrystals: A theoretical assessment

Third-order bound-charge electronic nonlinearities of Si nanocrystals (NCs) embedded in a wide band-gap matrix representing silica are theoretically studied using an atomistic pseudopotential approach. Nonlinear refractive index, two-photon absorption and optical switching parameter are examined from small clusters to NCs up to a size of 3 nm. Compared to bulk values, Si NCs show higher third-order optical nonlinearities and much wider two-photon absorption-free energy gap which gives rise to enhancement in the optical switching parameter.     

Facile synthesis of well-dispersed SrF2:Yb/Er upconversion nanocrystals in oleate complex systems

Well oil-dispersible SrF₂:Yb³⁺/Er³⁺ upconversion (UC) nanocrystals (NCs) were easily synthesized in the water-ethanol-oleic acid-sodium oleate complex systems. The as-prepared NCs all show size-uniformity, and their sizes, morphologies can be controlled by varying the solvent and reaction time, and rectangular SrF₂:Yb³⁺/Er³⁺ nanosheets with the sizes of 5-25 nm can be obtained. The possible mechanism on the nucleation and growth of nanocrystals occurred at the oleic acid/sodium oleate interface was also discussed. The size and morphology dependent UC luminescence behaviors have been observed in SrF₂:Yb³⁺/Er³⁺ NCs, and their UC luminescence transitions were proposed. The as-prepared UC nanocrystals are expected to fulfill the demand for biological applications.     

Structural phase transformations in annealed cubic ZnS nanocrystals

The structural changes of cubic ZnS (cZnS) nanocrystals (NCs) doped with 0.2 at.% Mn²⁺ pulse annealed in vacuum and in air, up to 500 °C, were investigated by multifrequency electron paramagnetic resonance (EPR), X-ray diffraction (XRD), and transmission electron microscopy (TEM). The samples, prepared by a surfactant (Tween20)-assisted liquid–liquid reaction at pH = 6, consist of NCs with a tight size distribution around 3 nm and high crystallinity self-assembled into a stable mesoporous structure. The EPR spectra of the as prepared samples contain only the characteristic lines of the substitutional Mn²⁺(I) centers. No spectra from Mn²⁺ ions localized in (hydro)oxidized regions of the NCs surface were observed. The absence of such a surface layer could explain the stability of the cubic (sphalerite) structure observed by XRD and TEM in the investigated cZnS:Mn NCs annealed in vacuum up to 500 °C. The observation of the cubic-hexagonal transformation for the same NCs annealed in air supports the role of such layer in promoting this structural transformation. The narrowing of the EPR spectral lines above 200 °C with the increase in the average size of the cZnS:Mn crystallites was observed. The effect was more pronounced for the sample annealed in air. EPR also revealed the formation of minute amounts of substitutional Mn²⁺-type centers in a hexagonal ZnO structure at T ~ 300 °C, corresponding to the early stages of the thermally induced oxidation of the cZnS:Mn NCs.     

La、Ce掺杂ZnO纳米晶的发光特性

共沉淀法制备了稀土镧、铈掺杂的ZnO半导体纳米晶。X射线衍射(XRD)结果表明:掺杂的ZnO纳米晶为六方纤锌矿结构,随掺杂浓度增加ZnO粒径减小。对铈掺杂纳米ZnO,以波长380nm激发,在443nm处出现了半峰宽较窄的强的蓝光发射峰;镧掺杂ZnO纳米晶则为从418~610nm的多峰宽带发射。     

Preparation and characterization of SnS nanocrystals by a triethanolamine-assisted diethylene glycol solution synthesis

Stoichiometric, phase-controllable SnS nanocrystals (NCs) were prepared by a solution chemical synthesis using triethanolamine-assisted diethylene glycol solvent, tin(II) chloride and thioacetamide as precursors at injection temperature of 180-220 °C. The influences of triethanolamine adding amounts, injection temperature, refluxing time on crystal phase, growth morphology and optical property of the synthesized SnS NCs were investigated. The results showed that both orthorhombic (OR) and zinc-blende (ZB) phase of SnS NCs could be formed by altering triethanolamine amounts.     

Synthesis and characterisation: Zinc oxide–sulfide nanocomposites

A novel synthesis method is presented for the preparation of nanosized-semiconductor zinc oxide–sulphide (ZnO/ZnS) core–shell nanocomposites, both formed sequentially from a single-source solid precursor. ZnO nanocrystals were synthesized by a simple co-precipitation method and ZnO/ZnS core–shell nanocomposites were successfully fabricated by sulfidation of ZnO nanocrystals via a facile chemical synthesis at room temperature. The as-obtained samples were characterized by X-ray diffraction and transmission electron microscopy. The results showed that the pure ZnO nanocrystals were hexagonal wurtzite crystal structures and the ZnS nanoparticles were sphalerite structure with the size of about 10 nm grown on the surface of the ZnO nanocrystals. Optical properties measured reveal that ZnO/ZnS core–shell nanocomposites have integrated the photoluminescent effect of ZnO and ZnS. Based on the results of the experiments, a possible formation mechanism of ZnO/ZnS core–shell nanocomposites was also suggested. This treatment is suggested to improve various properties of optoelectronically valuable ZnO/ZnS nanocomposites. These nanosized semiconductor nanocomposites can form a new class of luminescent materials for various applications.     

ZnO spheres and nanorods formation: their dependence on agitation in solution synthesis

ZnO nanostructures including nanorods, dense, and partially hollow spheres were synthesized via a solution synthesis method with temperature ranging from 65 to 95 °C. Scanning electron microscopy (SEM) revealed that the diameter of the spheres is in the range of 200–500 nm. Transmission electron microscopy (TEM) showed that some of the spheres are hollow or partially hollow. Powder X-ray Diffraction (XRD) and TEM-Selected area electron diffraction (SAED) analysis showed that the spheres consist of polycrystalline nanoparticles. It was found for the first time that the agitation during the synthesis plays a critical role on morphology of the ZnO nanostructures formed in solution. The oriented attachment of nanocrystals without agitation during the synthesis could guide the nanocrystals to form an ordered nanorod structure. However, the disordered aggregation of the nanocrystals under shear force resulted in a spherical morphology. It was also found that the composition of spheres is different from that of nanorods: the spheres consist of both ZnO and Zn(OH)₂, but nanorods consist of single-crystal ZnO only. Zn(OH)₂ presented in the spheres could decompose to ZnO by calcination, resulting in the formation of hollow spheres.     

Effect of different pH values on growth solutions for the ZnO nanostructures

In this study, high quality zinc oxide (ZnO) nanostructures were synthesized on glass slide substrates using modified chemical bath deposition (M-CBD) method at low temperature. Through the M-CBD technique the air bubbles will be injected into aqueous growth solution. The RF magnetron sputtering method was utlized to grow ZnO seed layer on the glass substrates. The effect of different pH values of aqueous growth solution on the morphology, elemental chemical composition, crystal structural and the optical properties of ZnO nanostructures have been investigated using field emission-scanning electron microscopy (FE-SEM), Energy dispersive analysis (EDX), X-ray diffraction (XRD), and UV-Visible Spectrometer, respectively. It was observed that altering pH values from acidic to alkaline (basic) by using ammonia solution (NH₃) induced the significant change in morphology from nanorods like ZnO to nano-amber flush rose like ZnO structures. Furthermore, increased pH values had an effect on the influence intensity of the preferred orientation plane (002) and average transmittance spectrum. Whilst the absorption band edge has been shifted to a lower energy region due to the quantum size effect. It was also found that the crystal size fluctuated between 36.30 nm and 84.33 nm with a different values of pH from 6.7 to 12. The ZnO synthesized at 6.7 of pH provided the best results regarding the high aspect ratio,structural and optical properties. At this pH value, ZnO growth revealed the nanorod structure with small diameters, size and a higher energy band gap value.     

Effect of Ce and Cu co-doping on the structural,morphological, and optical properties of ZnO nanocrystals and first principle investigation of their stability and magnetic properties

Ce, Cu co-doped ZnO (Zn_1−2xCe_xCu_xO: x=0.00, 0.01, 0.02, 0.03, 0.04 and 0.05) nanocrystals were synthesized by a microwave combustion method. These nanocrystals were investigated by using X-ray diffraction (XRD), UV–visible diffuse reflectance spectroscopy (DRS), scanning electron microscopy (SEM), and vibrating sample magnetometer (VSM). The stability and magnetic properties of Ce and Cu co-doped ZnO were probed by first principle calculations. XRD results revealed that all the compositions are single crystalline. hexagonal wurtzite structure. The optical band gap of pure ZnO was found to be 3.22 eV, and it decreased from 3.15 to 3.10 eV with an increase in the concentration of Cu and Ce content. The morphologies of Ce and Cu co-doped ZnO samples confirmed the formation of nanocrystals with an average grain size ranging from 70 to 150 nm. The magnetization measurement results affirmed the antiferro and ferromagnetic state for Ce and Cu co-doped ZnO samples and this is in agreement with the first principles theoretical calculations.     

Morphology-dependent upconversion luminescence of ZnO:Er nanocrystals

Morphology impact on the upconverted luminescence of ZnO:Er³⁺ nanocrystals was studied with controllable morphology of nanorod, prickly sphere-like, column-like, branch rod, prism-like, and grain-like, prepared via the cetyltrimethylammonium bromide (CTAB)-assisted hydrothermal process. The upconversion emission of Er³⁺ with 980 nm excitation demonstrated morphology sensitivity which was related with the local environments of Er³⁺ ions in ZnO and doping efficiency. Under ultraviolet (UV) direct excitation, where exciton and defect emissions of ZnO appeared, morphology sensitivity was discussed in terms of surface-to-volume ratios.     

Synthesis of quantum dots via microreaction: structure optimization for microreactor system

Microreactor systems existed as a powerful tool for the continuous synthesis of quantum dots. However, the lack of structure optimization for the discrete units led to empirical determination of the length scale, and the properties of the formed products varied in different cases. In this article, the optimizations for the micromixer volume and capillary diameter were presented based on the synthesis of CdSe nanocrystals (NCs). Spectra investigation revealed that the application of a small convective mixer of 36 μL led to 1/3 increase of CdSe concentration in the crude solution. The enhanced mixing of the precursors in this case was also demonstrated favorable to achieve CdSe NCs with narrow PL width. Fast heating and uniform reaction condition achieved in a narrow channel favored the preparation of high quality CdSe NCs under short residence time. However, the application of wide channel did not necessarily result in CdSe NCs with poor quality. Here, we demonstrated that high-quality CdSe NCs with narrow full width at half maximum (FWHM) as 32 nm and high quantum yield (QY) 34.7% could be prepared using an 844 μm inner diameter capillary. Based on the obtained results, the scaled-up synthesis of CdSe NCs was demonstrated, and a high quantity of 0.8 g dry CdSe NCs powder (3.5 nm, σ ~ 8.2%) was obtained within 1 h.     

A Novel Strategy to Synthesize Dual Blue Fluorescent‐Magnetic EuCl2 Nanocrystals via One‐Pot Method with Controlled Morphologies Using Urea

Divalent state europium nanocrystals (Eu^II NCs) have tremendous number of applications such as biological detection, solar cells, medical imaging, and so on. Here, a ready synthetic one‐pot approach to prepare dual blue fluorescent‐magnetic EuCl₂ NCs is reported. EuCl₃ is reduced by the reducing agent urea, while the oleic acid and oleyl amine are used as the capping ligand. A serial of analysis, such as X‐ray photoelectron spectroscopy, electron spin resonance, and zero‐field cooling magnetization curve/field cooling magnetization curve are performed to confirm the successful reduction of EuCl₂. It is novel that the morphology and size of produced EuCl₂ NCs could be adjusted efficiently by the reducing agent urea, including rod shaped, wire shaped, and fiber shaped. The mechanism of controlling the size and morphology of synthetic EuCl₂ NCs by the urea content is discussed in detail. What is more, it has been demonstrated that in this one‐pot method the homogeneous and complete nucleation needs to perform slowly in high temperature reaction stage, and then the uniformed crystals are gradually formed during the slow cooling process. That is an appealing strategy for tunable design and synthesis of multifunctional Eu^II NCs.     

Organic nanocrystals grown in sol–gel matrices: a new type of hybrid material for optics

We have engineered new hybrid organic–inorganic materials through a simple and generic preparation of stable organic nanocrystals grown in gel–glass matrices. This process is based on the confined nucleation and growth of dyes in the pores of dense gels. For bulk samples, narrow size distributions of particles are obtained between 10 and 20 nm in diameter. We have extended this method to the preparation of organic nanocrystals embedded in sol–gel thin films by spin-coating. For all these nanocomposite samples, we have significantly increased the stability of the dye and obtained promising linear and nonlinear optical properties. To cite this article: J. Zaccaro et al., C. R. Physique 3 (2002) 463–478.     

Electrochemical and hydrothermal deposition of ZnO on silicon: from continuous films to nanocrystals

This article presents the study of the electrochemical deposition of zinc oxide from the non-aqueous solution based on dimethyl sulfoxide and zinc chloride into the porous silicon matrix. The features of the deposition process depending on the thickness of the porous silicon layer are presented. It is shown that after deposition process the porous silicon matrix is filled with zinc oxide nanocrystals with a diameter of 10–50 nm. The electrochemically deposited zinc oxide layers on top of porous silicon are shown to have a crystalline structure. It is also shown that zinc oxide crystals formed by hydrothermal method on the surface of electrochemically deposited zinc oxide film demonstrate ultra-violet luminescence. The effect of the porous silicon layer thickness on the morphology of the zinc oxide is shown. The structures obtained demonstrated two luminescence bands peaking at the 375 and 600 nm wavelengths. Possible applications of ZnO nanostructures, porous and continuous polycrystalline ZnO films such as gas sensors, light-emitting diodes, photovoltaic devices, and nanopiezo energy generators are considered. Aspects of integration with conventional silicon technology are also discussed.     

Synthesis of ZnO nanostructures with controlled morphology and size in ionic liquids

Nanostructured ZnO has been synthesized by a hydrothermal route, using different ionic liquids (ILs) as the morphology templates. The morphology of ZnO changes from rod-like to star-like and flower-like in different ILs. A 3D nano/micro structure ZnO with unique flower-like morphology has been synthesized via the assembly of dicationic IL and [Zn(OH)₄]²⁻. The flower-like pattern was obtained in the presence of IL 1. The flower-like ZnO structure has a hexagonal prism, with a hexagonal pyramid on the tip, and diameter of ~444 nm. While the ZnO prepared in IL 2, shows uniform rod-like shape with a diameter of 91 nm, star-like morphology consisting of nanorods with diameter of ~109 nm was formed in IL 3. The XRD, SEM, and PL spectra have been employed for characterization of the synthesized ZnO nano structures.