Preparation and photoluminescence of ZnO with nanostructure by hollow-cathode discharge

Without the use of a metal catalyst in the process, ZnO with nanostructures was successfully prepared on Si (100) substrate by simple chemical vapor-deposition method. In our work, Ar was used as the plasma forming gas, O₂ was the reactive gas and metal zinc powder (99.99% purity) vaporized by cylinder hollow-cathode discharge (HCD) acted as the zinc source. The crystal structures of the as-synthesized ZnO nanostructures were characterized by X-ray diffraction (XRD); the ZnO sample growing on the wall of the crucible showed a ‘comb-like’ nanostructure, while the other one at the bottom of the crucible showed a ‘rod-like’ structure, which can be attributed to the difference of the oxygen content. The measurement on the photoluminescence (PL) performance of the ZnO nanostructures was carried out at room temperature. The results indicated that the ‘comb-shape’ ZnO nanomaterial possessed a remarkably strong ultraviolet emission peak centered at 388 nm, while ZnO nanorods, except better ultraviolet emission, also had relatively strong blue-green emission ranging from 470 to 600 nm due to the existence of oxygen vacancies. The growth mechanism of ZnO with nanostructures is also discussed in this paper.     

Ultrasound-assisted synthesis of dentritic ZnO nanostructure in ionic liquid

A facile and environment-friendly sonochemical route to fabricate well-defined dentritic ZnO nanostructures in a room-temperature ionic liquid has been reported. The structure and morphology of the synthetic branch-shaped ZnO products were characterized by X-ray diffraction and transmission electron microscopy. The photoluminescence (PL) spectrum of the synthetic dentritic ZnO nanostructures exhibits a strong ultraviolet emission at 378 nm and a weak green emission at 532 nm respectively at room temperature. A plausible formation mechanism of dentritic ZnO nanostructures was discussed in detail.     

ZnO nanostructures prepared using a vapour transport method

ZnO nanostructures were prepared on Si(100) substrates using a vapour transport technique in water vapour and oxygen gas, in the existence of Au catalyst. Synthesised in both water vapour and oxygen gas, the ZnO nanostructures presented hexagonal wurtzite structure but exhibited different growth orientations, which subsequently created diverse nanostructures. The different ZnO morphologies grown in different atmosphere are due to various growth mechanisms, which have been proposed in this article. At the end, the photoluminescence spectra of both ZnO nanostructures were measured, which revealed only a strong ultraviolet peak at about 389 nm.     

ZnO四足和多足纳米结构的制备和光致发光性能研究

采用简单的热蒸发法,在没有使用载气和催化剂的情况下成功制备出ZnO四足和多足纳米结构。采用场发射扫描电镜、X射线衍射、高分辨透射电子显微镜和荧光分光光度计研究了ZnO纳米结构的形貌、结构和光致发光性能。结果表明所合成的ZnO是由具有六方纤锌矿结构的四足和多足纳米结构组成,足部呈棒状并沿[0001]方向生长。提出了四足和多足ZnO纳米结构的生长机制。在室温下的光致发光光谱中,494nm处出现一个较强的绿色发射峰,391nm处出现一个较弱的紫外发射峰。     

ZnO nanostructured microspheres and grown structures by thermal treatment

Synthesis of flower-shaped ZnO nanostructures composed of ZnO nanosticks was achieved by the solution process using zinc acetate dihydrate, sodium hydroxide and polyethylene glycol-20000 (PEG-20000) at 180°C for 4 h. The diameter of individual nanosticks was about 100 nm. Detailed structure characterizations demonstrate that the synthesized products are wurtzite hexagonal phase, grown along the [001] direction. The infrared (IR) spectrum shows the standard peak of zinc oxide at 571 cm⁻¹. Raman scattering exhibits a sharp and strong E ₂ mode at 441 cm⁻¹ which further confirms the good crystal and wurtzite hexagonal phase of the grown nanostructures.     

Zn-catalysed growth and optical properties of modulated ZnO hierarchical nanostructures

Modulated ZnO hierarchical nanostructures have been successfully synthesised by the one-step thermal evaporation method. The ZnO hierarchical nanostructures consist of large quantities of high-dense nanowires strewn with some small balls, and these balls are connected with wires by short rods. The composition detection results show that the ball is metallic Zn, which further confirms that Zn can serve as the catalyst for vapour–liquid–solid growth of the ZnO hierarchical nanostructures. The photoluminescence spectrum of the modulated ZnO hierarchical nanostructures includes a weak ultraviolet peak centred at 380?nm and a strong green emission centred at 500?nm, which can be attributed to the exciton emission at the near-band edge and a mass of singly ionised oxygen vacancies in products, respectively.     

Structural properties and growth mechanism of flower-like ZnO structures obtained by simple solution method

Flower-shaped zinc oxide (ZnO) structures have been synthesized in the reaction of aqueous solution of zinc nitrate and NaOH at 90 °C. To examine the morphology of ZnO nanostructures, time-dependent experiments were carried out. Detailed structural observation showed that the flower-like structures consist of triangular-shaped leaves, having sharpened tips with wider bases. Photoluminescence spectrum measured at room temperature show a sharp UV emission at 381 nm and a strong and broad green emission at 480-750 nm attributed to structural defects. A possible growth mechanism for the formation of flower-shaped ZnO structures is discussed in detail.     

Low temperature solution synthesis and characterization of ZnO nano-flowers

Synthesis of flower-shaped ZnO nanostructures composed of hexagonal ZnO nanorods was achieved by the solution process using zinc acetate dihydrate and sodium hydroxide at very low temperature of 90 °C in 30 min. The individual nanorods are of hexagonal shape with sharp tip, and base diameter of about 300-350 nm. Detailed structural characterizations demonstrate that the synthesized products are single crystalline with the wurtzite hexagonal phase, grown along the [0 0 0 1] direction. The IR spectrum shows the standard peak of zinc oxide at 523 cm⁻¹. Raman scattering exhibits a sharp and strong E₂ mode at 437 cm⁻¹ which further confirms the good crystallinity and wurtzite hexagonal phase of the grown nanostructures. The photoelectron spectroscopic measurement shows the presence of Zn, O, C, zinc acetate and Na. The binding energy ca. 1021.2 eV (Zn 2p_3/2) and 1044.3 eV (Zn 2p_1/2), are found very close to the standard bulk ZnO binding energy values. The O 1s peak is found centered at 531.4 eV with a shoulder at 529.8 eV. Room-temperature photoluminescence (PL) demonstrate a strong and dominated peak at 381 nm with a suppressed and broad green emission at 515 nm, suggests that the flower-shaped ZnO nanostructures have good optical properties with very less structural defects.     

Structure and photoluminescence of SiC/ZnO nanocomposites prepared by radio frequency alternate sputtering

SiC/ZnO nanocomposites were prepared by radio frequency alternate sputtering followed by annealing in N₂ ambient. Well-crystallized ZnO matrix was obtained after annealed at 750 °C according to X-ray diffractometer patterns. Transmission electron microscopy analyses indicated that the SiC thin layer aggregated to form SiC nanoclusters with the average size of 7.2 nm when the annealing temperature was 600 °C. When the annealing temperatures increased above 900 °C, some of the SiC nanoclusters changed into SiC nanocrystals and surfacial atoms of the SiC nanoparticles were surrounded by a layer of SiO _x (x ≤ 2) according to the Fourier transform infrared spectrums. The SiC/ZnO nanocomposites annealed at 750 °C exhibit strong photoluminescence bands ranging from 250 to 600 nm. UV light originates from the near band edge emission of ZnO and the blue emission peaked at around 465 nm (2.7 eV) may be due to the formation of emission centers caused by the defects in Si–O network, while the green-emission peak at around 550 nm (2.3 eV) may be attributed to the deep level recombination luminescence caused by the vacancies of oxygen and zinc.     

Cathodoluminescence and its mapping of flower-like ZnO, ZnO/ZnS core-shell and tube-like ZnS nanostructures

The cathodoluminescence (CL) properties including intensity and distribution of the band to band and defect emission of the flower-like ZnO, ZnO/ZnS core-shell and tube-like ZnS nanostructures have been investigated. It is indicated that the Ultraviolet (UV) emission at 380 nm of the flower-like ZnO nanostructures due to the band to band emission is weaker than their yellow emission at 600 nm induced by interstitial oxygen. Moreover, the UV emission of the ZnO nanorods unevenly distributes from the tip to the end. The UV emission on the tip is stronger than that of others due to the waveguide. On the contrary, the yellow emission at 600 nm is uniform. Furthermore, the UV emission of ZnO has been greatly enhanced and the yellow emission has been inhibited by the formation of ZnO/ZnS core-shell nanostructures in the sulfuration process due to the elimination of interstitial oxygen. However, the polycrystalline tube-like ZnS nanostructures shows the uniform and weak defect emission due to S vacancies.     

Synthesis,structural and optical characterization of Ni-doped ZnO nanoparticles

Nanocrystalline Zn_1−x Ni _x O (x = 0.00, 0.02, 0.04, 0.06, 0.08) powders were synthesized by a simple sol–gel autocombustion method using metal nitrates of zinc, nickel and glycine. Structural and optical properties of the Ni-doped ZnO samples annealed at 800 °C are characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive analysis using X-rays (EDAX), UV–visible spectroscopy and photoluminescence (PL). X-ray diffraction analysis reveals that the Ni-doped ZnO crystallizes in a hexagonal wurtzite structure and secondary phase (NiO) was observed with the sensitivity of XRD measurement with the increasing nickel concentration (x ≥ 0.04). The lattice constants of Ni-doped ZnO nanoparticles increase slightly when Ni²⁺ is doped into ZnO lattice. The optical absorption band edge of the nickel doped samples was observed above 387 nm (3.20 eV) along with well-defined absorbance peaks at around 439 (2.82 eV), 615(2.01 eV) and 655 nm (1.89 eV). PL measurements of Ni-doped samples illustrated the strong UV emission band at ~3.02 eV, weak blue emission bands at 2.82 and 2.75 eV, and a strong green emission band at 2.26 eV. The observed red shift in the band gap from UV–visible analysis and near band edge UV emission with Ni doping may be considered to be related to the incorporation of Ni ions into the Zn site of the ZnO lattice.     

Formation of ZnO nanostructures in energy-controlled hollow-type magnetron RF plasma

In this study, we investigated how zinc, sputtered from a zinc target, reacts with oxygen on the substrate to form ZnO nanostructures when the discharge parameters, such as gas flow ratio and target bias voltage, are controlled in O₂/Ar plasma. The deposits were estimated by SEM and Raman spectroscopy. Under conditions of a Zn to Ar optical emission intensity ratio of 2/1, a target voltage of − 550 V, a total pressure of 40 Pa, a substrate temperature of 150 °C, an RF power of 50 W, and a deposition time of 30 min, many vertically aligned ZnO nanorods were observed to be deposited on the substrate. The diameter of the rods was typically 50 nm. It was found that the film morphology can be controlled by the sputtering rate of zinc varied by the target bias voltage and gas flow rate.     

Morphology, structures and properties of ZnO nanobelts fabricated by Zn-powder evaporation without catalyst at lower temperature

Uniform ZnO normal nanobelts and toothed-nanobelts have been successfully synthesized respectively through pure zinc powder evaporation without catalyst at 600°C. Experimental results indicate that the key to the fabricating method is to control the gas flow rates and the partial pressures of argon, oxygen and zinc vapor. Scanning electron microscopy and high-resolution transmission electron microscopy observations show that the ZnO nanobelts have several types of single crystalline morphology. HRTEM images reveal that there are numerous screw dislocations and the growth is around the dislocations in the toothed-nanobelts. The growth of ZnO nanobelts is controlled by vapor-solid and screw dislocation mechanisms. Room temperature photoluminescence spectra of the toothed-nanobelts showed a UV emission at ∼390 nm and a broad green emission with 4 subordinate peaks at 455–495 nm.     

Induction of zinc particles on the morphology and photoluminescent property of globular Zn/ZnO core/shell nanorod heterojunction array architectures

Zn/ZnO metal/semiconductor nanostructures were successfully synthesised by a facile zinc-rich chemistry liquid-phase approach with zinc microspheres as sacrificial templates at ambient temperature. A series of globular Zn/ZnO core/shell structures and hollow microsphere architectures self-assembled by Zn/ZnO nanorod heterojunction arrays were obtained by controlling the amount of zinc particles. The structure, morphology, composition and optical properties of the products have been characterised by X-ray diffraction, scanning electron microscopy, Raman spectroscopy and photoluminescent spectroscopy. A possible growth mechanism of the Zn/ZnO nanostructures has been proposed based on the structural analysis. The growth mechanism of Zn/ZnO hollow microspheres is ascribed to Kirkendall effect. A new strong blue emission at 440 nm and a green emission around 500 nm with an enhancement over one order of magnitude compared with the pure ZnO sample have been observed. These emission bands are attributed to two kinds of mechanisms that have been discussed in detail.     

Low-temperature growth of flower-shaped UV-emitting ZnO nanostructures on steel alloy by thermal evaporation

Flower-shaped ZnO nanostructures, containing the triangular-shaped petals (sharpened tips and wider bases) have been achieved by simple thermal evaporation of high purity metallic zinc powder in the presence of oxygen at 440 degrees C on steel alloy substrate without the use of metal catalyst or additives. Detailed structural studies confirm that the obtained flower-shaped nanostructures are single crystalline and possesses a wurtzite hexagonal structure, grown along the c-axis in the [0001] direction. Raman and room temperature photoluminescence analysis substantiate a wurtzite hexagonal phase with a good crystal quality and a strong UV emission at 378 nm, respectively, indicating few or no structural defects. Additionally, a detailed possible growth mechanism has also been discussed.     

Growth of ZnO nanostructures on Si by means of plasma immersion ion implantation and deposition

Crystalline zinc oxide (ZnO) nanostructures have been grown on Si substrates by means of Plasma Based Ion Implantation and Deposition (PIII&D) at a temperature of about 300 °C and in the presence of an argon glow discharge. In the process a crucible filled with small pieces of metallic zinc plays the role of the anode of the discharge itself, being polarized by positive DC voltage of about 400 V. Electrons produced by thermionic emission by an oxide cathode (Ba, Sr, Ca)O impact this crucible, causing its heating and vaporization of Zn. Partial ionization of Zn atoms takes place due to collisions with plasma particles. High negative voltage pulses (7 kV/40 μs/250 Hz) applied to the sample holder causes the implantation of metallic zinc into Si surface, while Zn deposition happens between pulses. After annealing at 700 °C, strong UV and various visible photoluminescence bands are observed at room temperature, as well as the presence of ZnO nanoparticles. The coated surface was characterized in detail using X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), energy dispersive spectroscopy (EDS), scanning electron microscopy (SEM), atomic force microscopy (AFM) and photoluminescence (PL) spectroscopy. XRD indicated the presence of only ZnO peaks after annealing. The composition analysis by EDS revealed distinct Zn/O stoichiometry relation depending on the conditions of the process. AFM images showed the formation of columns in the nanoscale range. Topography viewed by SEM showed the formation of structures similar to cactus with nanothorns. Depth analysis performed by XPS indicated an increase of concentration of metallic Zn with increasing depth and the exclusive presence of ZnO for outer regions. PIII&D allowed to growing nanostructures of ZnO on Si without the need of a buffer layer.     

A task specific basic ionic liquid for synthesis of flower-like ZnO by hydrothermal method

Flower-like ZnO morphology, with different shapes, have been successfully synthesized via a novel and environment-friendly hydrothermal method using zinc acetate and a task specific dicationic dibasic ionic liquid, [mmpim]₂[OH]₂, which plays an important role in fabrication of ZnO structure. The structure and morphology of the product were characterized by X-ray diffraction and scanning electron microscopy, which show different flower-like morphologies. Photoluminescence spectrum of the product exhibits a strong ultraviolet emission at 391 nm and two weak blue-green emissions at about 450 and 500 nm.     

One-step polyoxometalate-assisted solvothermal synthesis of ZnO microspheres and their photoluminescence properties

With the assistance of Keggin-type polyoxometalate (POM) H₃PW₁₂O₄₀·nH₂O (PW₁₂), ZnO microspheres have successfully been synthesized by a direct route, using zinc acetate dihydrate solid and dilute PW₁₂ absolute ethanol solution at 120 °C for 24 h. The effects of PW₁₂ concentration and reaction time were investigated. The results showed that PW₁₂ played a vital role for the formation of the ZnO microspheres during the solvothermal treatment and the original framework of PW₁₂ was not destroyed after the solvothermal treatment. Finally, the room temperature photoluminescence (PL) spectrum of the ZnO microspheres exhibited a sharp and strong ultraviolet (UV) emission at 390 nm and a quite weak visible emission at around 520 nm, respectively.     

Fabrication and growth mechanism of hexagonal zinc oxide nanorods via solution process

This article presents, the fabrication of perfectly hexagonal zinc oxide nanorods performed via solution process using zinc nitrate hexahydrate (Zn(NO₃)₂·6H₂O) and hexamethylenetetramine (HMT) at various concentrations of i.e. 1 × 10⁻³ to 10 × 10⁻² M in 50 mL distilled water and refluxed at 100 °C for 1 h. We used HMT because it acts as a template for the nucleation and growth of zinc oxide nanorods, and it also works as a surfactant for the zinc oxide structures. The X-ray diffraction patterns clearly reveal that the grown product is pure zinc oxide. The diameters and lengths of the synthesized nanorods lie in the range of 200–800 nm and 2–4 μm, respectively as observed from the field emission scanning electron microscopy (FESEM). The morphological observation was also confirmed by the transmission electron microscopy (TEM) and clearly consistent with the FESEM observations. The chemical composition was analyzed by the FTIR spectroscopy, and it shows the ZnO band at 405 cm⁻¹. On the basis of these observations, the growth mechanism of ZnO nanostructures was also proposed.     

Structural and optical properties of ZnO nanostructures grown on silicon substrate by thermal evaporation process

Two types of one-dimensional ZnO nanostructures have been synthesized on silicon substrate by the thermal evaporation of metallic zinc powder in the presence of oxygen without the use of any catalyst or additives. Detailed structural analysis revealed that the formed ZnO nanostructures are single crystalline with wurtzite hexagonal phase and grow along the [0001] direction in preference. Presence of a sharp and strong, optical phonon Raman-active E₂ (high) mode and suppressed E₁ (LO) mode in the Raman spectra, in both the cases, confirmed the good crystallinity with the wurtzite hexagonal phase for the as-grown products. A sharp and dominant near band edge emission with a suppressed green emission is observed from the as-synthesized nanostructures which affirmed the good optical properties with very less structural defects for the grown nanostructures.