Low temperature hydrothermal growth and optical properties of ZnO nanorods

Well‐faceted hexagonal ZnO nanorods have been synthesized by a simple hydrothermal method at relative low temperature (90°C) without any catalysts or templates. Zinc oxide (ZnO) nanorods were grown in an aqueous solution that contained Zinc chloride (ZnCl₂, Aldrich, purity 98%) and ammonia (25%). Most of the ZnO nanorods show the perfect hexagonal cross section and well‐faceted top and side surfaces. The diameter of ZnO nanorods decreased with the reaction time prolonging. The samples have been characterized by X‐ray powder diffraction (XRD) and scanning electron microscopy (SEM) measurement. XRD pattern confirmed that the as‐prepared ZnO was the single‐phase wurtzite structure formation. SEM results showed that the samples were rod textures. The surface‐related optical properties have been investigated by photoluminescence (PL) spectrum and Raman spectrum. Photoluminescence measurements showed each spectrum consists of a weak band ultraviolet (UV) band and a relatively broad visible light emission peak for the samples grown at different time. It has been found that the green emission in Raman measurement may be related to surface states. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Synthesis and optical properties of ZnO nanorods

ZnO nanorods were prepared on the silicon (100) substrates using the chemical solution deposition method (CBD) without catalyst under a low temperature (90°C). The cool water was used to dissolve the mixture of zinc nitrate hexahydrate (Zn (NO₃)₂·6H₂O) and methenamine (C₆H₁₂N₄) in order to decrease the size of ZnO nanorods. From the X‐ray diffraction (XRD) results, it can be seen that the growth orientation of the as‐prepared ZnO nanorods was (002). Scanning electron microscopy (SEM) results illustrated that the nanorods had a hexagonal wurzite structure and average diameter of about 120nm. The average diameter of nanorods prepared by the cool water process was much smaller than that by the room‐temperature (RT) water process we always used. Photoluminescence (PL) measurements were also carried out. The result showed that a blue shift in UV emission band appeared in the PL spectrum of the sample grown with cool water process, which was mainly due to the reduction of tensile strain when the diameter of the ZnO nanorods decreased. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

The effect of ZnO buffer layer on structural and optical properties of ZnO nanorods

A low‐temperature synthetic route was used to prepare oriented arrays of ZnO nanorods on ITO conducting glass substrate coated with buffer layer of ZnO seeds in an aqueous solution. The corresponding growth behavior and optical properties of ZnO nanorod arrays were studied. It was found that the nature of the buffer layer had effect on the microstructures and optical properties of the resultant ZnO nanorod arrays. X‐ray diffraction (XRD) results showed the nanorods were preferentially grown along (002) direction, but the diameter of the nanorods prepared with the buffer layer was much smaller than the without one, which can be clearly seen from the scanning electron microscopy (SEM) results. And it also found that the buffer layer was not only enhanced the density of overall coverage but also beneficial to grown the oriented arrays. Photoluminescence spectroscopy (PL) results indicated that the all the samples had the better optical behaviors. By computation, the relative PL intensity ratio of ultraviolet emission (I_UV) to deep level emission (I_DLE) of ZnO nanorods grown with the pure substrate was much higher than that of the sample with the buffer layer. The defects on the surface increased with the size reduction of nanorods caused by the buffer layer may be the main reason for it. And the small shift in the UV emission was caused by the rapid reduction in crystal size and compressive stress from Raman spectra results. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Enhancement of the emission from TeO2 nanorods by encapsulation with ZnO

TeO₂‐core/ZnO‐shell nanorods were synthesized by a two–step process comprising thermal evaporation of Te powders and atomic layer deposition of ZnO. Scanning electron microscopy images exhibit that the core‐shell nanorods are 50 ‐ 150 nm in diameter and up to a few tens of micrometers in length, respectively. Transmission electron microscopy and X‐ray diffraction analysis revealed that the cores and shells of the core‐shell nanorods were polycrystalline simple tetragonal TeO₂ and amorphous ZnO with ZnO nanocrystallites locally, respectively. Photoluminescence measurement revealed that the TeO₂ nanorods had a weak broad violet band at approximately 430 nm. The emission band was shifted to a yellowish green region (∼540 nm) by encapsulation of the nanorods with a ZnO thin film and the yellowish green emission from the TeO₂‐core/ZnO‐shell nanorods was enhanced significantly in intensity by increasing the shell layer thickness. The highest emission was obtained for 125 ALD cycles (ZnO coating layer thickness: ∼15 nm) and its intensity was much higher than that of the emission from the uncapsulated TeO₂ nanorods. The origin of the enhancement of the emission by the encapsulation is discussed in detail. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Facile synthesis of pencil‐like ZnO nanostructures at low temperature

Pencil‐like ZnO nanostructure was synthesized by directly oxidizing granular Zn films, which was thermal deposited in a nitrogen atmosphere from Zn powder in a horizontal tube furnace. The formation of the pencil‐like structure, including a hexagonal rod and a sharp tip with diameter about 60 nm, highly depend on the thickness of the initial zinc film and the temperature of the oxidizing process. ZnO nanorods were formed in a relatively low temperature, while thicker zinc film was apt to form a dense ZnO film with tubular structures. The different structured ZnO materials showed distinguishing optical properties which indicate the intrinsic defects forming in the different growth conditions. The pencil‐like ZnO structures exhibit a relatively strong green emission attributed to the high concentrations of oxygen vacancies and its taper tip has great prospects in field‐emission devices.     

Optical and magnetic properties of (Zn,Mn)O nanostructures synthesized by CVD method

We report on microstructural, optical and magnetic properties of (Zn,Mn)O nanostructures synthesized by a chemical vapor deposition (CVD) technique. Average diameters of the as grown (Zn,Mn)O nanorods and nanowires were ∼400 nm and ∼50 nm, respectively. X‐ray diffraction (XRD) and photoluminescence (PL) spectra provided the evidence that Mn was incorporated into ZnO lattice. PL spectra of the (Zn,Mn)O nanostructures showed shift in near band edge (NBE) emission at 396 nm together with a green band (GB) emission at 510 nm and a blue band (BB) emission at 460 nm. Magnetic measurements revealed mixed magnetic phases (ferromagnetic and antiferromagnetic) in the (Zn,Mn)O nanostructures. Vapor‐solid‐solid (VSS) mechanism was thought to be responsible for the growth of the nanostructures at low temperatures. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Characterization of ZnO nanorods grown on Si substrates coated with NiCl2

ZnO nanorods were synthesized on NiCl₂‐coated Si substrates via a chemical vapor deposition (CVD) process. The as‐fabricated nanorods with diameters ranging from 150 nm to 200 nm and lengths up to several tens of micrometers grew preferentially arranged along [0001] direction, perpendicular to the (0002) plane. The clear lattice fringes in HRTEM image demonstrated the growth of good quality hexagonal single‐crystalline ZnO. Room temperature photoluminescence (PL) spectra illustrated that the ZnO nanorods exhibit strong UV emission peak and green emission peak, peak centers located at 388 nm and 506 nm. A possible growth mechanism based on the study of our X‐ray diffraction (XRD), electron microscopy and PL spectroscopy was proposed, emphasizing the effect of NiCl₂ solution (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

铟镓共掺杂对n-ZnO纳米棒/p-GaN异质结生长行为和光电性能的影响

利用低温水热法在p-GaN薄膜上生长了铟(In)和镓(Ga)共掺杂的ZnO纳米棒。X射线衍射(XRD)、X射线光电子能谱(XPS)和X射线能量色谱仪(EDS)结果表明,In和Ga已固溶到ZnO晶格中。扫描电子显微镜(SEM)结果表明, ZnO纳米棒具有良好的c轴取向性,随着In和Ga共掺杂浓度的增加,纳米棒的直径减小,密度增加。XRD结果表明,In和Ga共掺杂引起ZnO晶格常数增大,导致(002)衍射峰向低角度方向偏移。同时,ZnO的光学性质受到In和Ga共掺杂的影响。与纯ZnO相比, 共掺杂ZnO纳米棒的紫外发射峰都出现轻微红移,这是表面共振和带隙重整效应综合作用的结果。I-V特性曲线表明,随着In和Ga共掺杂浓度的增加,n-ZnO纳米棒/p-GaN异质结具有更好的导电性。     

Optimization of hydrothermal growth ZnO Nanorods for enhancement of light extraction from GaN blue LEDs

In this study, we report on the enhancement in the light extraction efficiency of GaN blue LEDs topped with ZnO nanorods. The ZnO nanorods were grown by a two-step hydrothermal synthesis with pre-coated ZnO nanoparticles under optimized condition to give the appropriate size and quality, giving an increase in the light output efficiency of 66%. This improvement is attributed to the optimal rod size and spacing with improved thermal dissipation as compared to light extraction from plain GaN surface. During the ZnO growth on the LEDs, 0.55 M of NH₃ was added and the ZnO sample was later annealed at 475 °C in N₂ ambient, to drive out interstitial oxygen atoms from the tetrahedral unstable site. As a result, a high ratio of UV to orange defect band emission was achieved. The two-step growth of ZnO nanorods on GaN LEDs was effective in generating array of ZnO nanorods which serve as reflector to enhance light extraction from LEDs.     

Morphology‐controllable synthesis of ZnO nano‐/micro‐ structures by a solvothermal process in ethanol solution

Flower‐like ZnO nanostructures assembled by nanorods with bimodal size distribution have been synthesized by a solvothermal process in NaOH‐Et system. Various effects of the solvothermal parameters and assistant additives on the morphologies of ZnO nanostructures have been investigated. The directing effect of chloride ions have been observed in the formation of highly symmetrical 3D ZnO nanostructures. A possible mechanism has been proposed to explain the formation of ZnO nanoflowers in NaOH‐Et system. A strong near‐UV emission band centered at around 396 nm is observed in the photoluminescence spectrum of flower‐like ZnO nanostructures, indicating of their high crystal quality.     

Synthesis,structure and optical properties of zinc oxide hexagonal microprisms

Zinc oxide (ZnO) pencil‐head‐like (PHL) microprisms were synthesized by a hydrothermal route using a zinc (Zn) plate as a source and substrate. The structural analysis confirmed the formation of ZnO with hexagonal wurtzite phase on the hexagonal Zn substrate and the growth of the ZnO microparticles along the [101] direction. The room temperature photoluminescence (PL) of the ZnO microprisms showed a sharp UV emission band located at around 380 nm, which is expected to originate from the radiative recombination of free excitons. The sharp UV emission band, with a full width at half‐maximum of about 15 nm and an extremely weak visible emission, confirms the high crystal quality of the synthesized ZnO microprisms. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Characterization and optical properties of ZnS nanorods branched out from ZnS film on Zn foil

Complex nanomaterial‐film‐metal substrate architectures, which are composed of ZnS nanorods, island‐like ZnS film and Zn foil, have been formed via a simple vapor deposition route. The growth of the complex nanostructures is initiated by the preferred formation of ZnS film, and ZnS nanorods branches out from ZnS film flows a liquid‐phase epitaxial growth mode. The ZnS nanorod is switched to an angle, which may be attributed to the sudden change of vapor pressure and temperature reduction by the end of vapor deposition process. The room‐temperature photoluminescence spectrum shows that complex ZnS nanostructures have a strong blue emission band centered at about 423 nm and a weak broad green emission band centered at about 515 nm. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Sonochemical synthesis and characterization of ZnO nanorod/Ag nanoparticle composites

A simple sonochemical route for the synthesis of Ag nanoparticles on ZnO nanorods is reported. Ultrasonic irradiation of a mixture of ZnO nanorods, Ag(NH₃)₂⁺, and formaldehyde in an aqueous medium yields ZnO nanorod/Ag nanoparticle composites. The powder X‐ray diffraction of the ZnO/Ag composites shows additional diffraction peaks corresponding to the face‐center‐cubic structured Ag crystalline, apart from the signals from the ZnO nanorods. Scanning electron microscopy and transmission electron microscopy images of the ZnO/Ag composites reveal that the ZnO nanorods are coated with Ag nanoparticles with a mean size of several tens nanometer. The absorption band of ZnO/Ag composites is distinctly broadened and red‐shifted, indicating the strong interfacial interaction between ZnO nanorods and Ag nanoparticles. This sonochemical method is simple, mild and readily scaled up, affording a simple way for synthesis of other composites. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Microemulsion synthesis and characterization of aluminum doped ZnO nanorods

Aluminium doped ZnO (AZO) nanorods were synthesized by microemulsion method with different types of surfactants. Scanning electron microscopy observations show that the ZnO nanorods have diameters around about 80 nm and lengths up to several micrometers. The room temperature photoluminescence (PL) spectrum of AZO nanorods exhibited a sharp and strong ultraviolet bandgap at 383 nm and a relatively weaker emission associated with the defect level. AZO nanorods synthesized with sodium benzene sulfonate (SBS) surfactant showed lower resistivity than aluminum doped ZnO nanorods synthesized with dodecyl benzene sulfonic acid sodium salt (DBS) surfactant. Resistivity of AZO nanorods synthesized with SBS surfactant showed 2.8×10³ Ωcm. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Synthesis and photoluminescence properties of ZnO nanowire arrays

In this paper we report a chemical method named coordination reaction method to synthesize ZnO nanowire arreys. ZnO nanowires with the diameter about 80nm were successfully fabricated in the channels of the porous anodic alumina (PAA) template by the above coordination reaction method. The microstructures of ZnO/PAA assembly were characterized using scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X‐ray diffraction (XRD). The results showed that the ZnO nanowires can be uniformly assembled into the nanochannels of PAA template. The growth mechanism of ZnO nanowires and the conditions of the coordination reaction are discussed. Photoluminescence (PL) measurement shows that the ZnO/PAA assembly system has a blue emission band caused by the various defects of ZnO. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Controlled synthesis of ZnO nanostructures with different morphologies in microemulsions

ZnO nanostructures with different morphologies were prepared in microemulsions with ZnSO₄ and ammonia as raw materials. The effects of microemulsion types, concentration of reactants, W values, co‐surfactants, surfactants, oil phases and calcination temperatures were systematically studied. The products were characterized by X‐ray diffraction (XRD), differential scanning calorimetry and thermogravimetry (DSC‐TG), transmission electron microscopy (TEM), high‐resolution TEM (HRTEM), and photoluminescence (PL) spectrum. Results show that ZnO nanoparticles were obtained in water‐in‐oil microemulsions while ZnO nanorods are gained in bicontinuous microemulsions. Water‐in‐oil microemulsions and long carbon chains of surfactants can prevent the preferential growth of ZnO. The particle size of the products increased with the increase of W values, calcination temperatures and the concentration of reactants but decreased with the increase of the carbon chain length of surfactants, co‐surfactants and oil phases. PL spectrums show that the UV emission peak weakened and visible emission peak increased with the decrease of particle size. Meanwhile, the PL spectrums have a little red‐shifted.     

A direct suppression of green band emission of ZnO particles by adjusting the volume fraction of ethanol in reaction solutions

ZnO particles were successfully prepared by one step CTAB‐assisted hydrothermal method with different volume fraction of ethanol‐water mixture solution. The formed thorn‐ball like ZnO particles have an average size of 1 ∼ 2 μm in diameter. XRD result shows a hexagonal wurtzite structure and higher crystallinity. Room‐temperature photoluminescence shows a strong and dominated peak at ∼383 nm with a green emission at ∼510 nm. The intensity ratio between the UV and green emission increased from 1.31 to 7.53 when the volume fraction of ethanol was changed from 0% to 50%, which shows a direct suppression of structural defects just by adjusting the ethanol fraction in reaction solutions. The possible growth and luminescence mechanisms for thorn‐ball like ZnO particles are discussed.     

In-situ and ex-situ ZnO nanorod growth on ZnO homo-buffer layers

Vertically well-aligned ZnO nanorods were fabricated in-situ and ex-situ on ZnO homo-buffer layers using catalyst-free metal-organic chemical vapor deposition. Field-emission electron microscopy measurements demonstrated that the nanorods were well aligned and had a uniform diameter of 70–100 nm depending on the growth temperature, irrespective of growth conditions, in-situ and ex-situ. X-ray diffraction measurements demonstrated that the ZnO nanorods and the ZnO buffer layers had a wurtzite structure, and that the crystal quality of the nanorods grown on a smooth surface was better than that of the nanorods grown on a rough surface. Field-emission transmission electron microscopy measurements revealed the presence of a disordered layer at the interface of the nanorod and the buffer layer.     

Synthesis of ZnO nanorods by the thermal reduction process of a mixture of ZnO and Al powder

Single‐crystalline Zinc oxide (ZnO) nanorods were firstly synthesized on gold‐coated Si substrate via a simple thermal reduction method from the mixture of ZnO and Al powder. The growth process was carried out in a quartz tube at different temperature (550‐700 °C) and at different oxygen partial pressure. Their structure properties were investigated by X‐ray diffraction (XRD), scanning electron microscope (SEM), X‐ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM). The length of the as‐prepared ZnO nanorods was up to several micrometers and their diameters were about 130 nm. The X‐ray diffraction patterns, transmission electron microscopic images, and selective area electron diffraction patterns indicate that the one‐dimensional ZnO nanorods are a pure Single‐crystal and preferentially oriented in the [0001] direction. The reaction mechanism of ZnO nanorods was proposed on the basis of experimental data. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Synthesis by organic molten salt method and characterization of chalcopyrite AgInS2 nanorods

In this paper, chalcopyrite AgInS₂ nanorods were synthesized for the first time by a one‐step, ambient pressure, environment friendly organic molten salt (OMS) method at 200 °C. The as‐synthesized products were characterized by X‐ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS), respectively. The XRD results reveal that the as‐synthesized products at 120–160 °C under ambient pressure contain AgIn₅S₈ which will decrease with the increase of growth temperature. A sample containing only the chalcopyrite AgInS₂ phase is successfully obtained at 200 °C. Furthermore, the elemental compositions are found to become increasingly stoichiometric with increasing temperature. UV‐Vis and photoluminescence (PL) spectra are utilized to investigate the optical properties of AgInS₂ nanorods. By testing on UV‐Vis spectra, it is concluded that the limiting wavelength of the AgInS₂ nanorods is 661 nm and the band gap is 1.88 eV. A broad red emission band peak centered at about 1.874 eV (662 nm) is clearly observed at room temperature, and the intensity of the emission increases with excitation wavelength. In addition, the photoluminescence quantum yield (PLQY) of the nanocrystals at the excitation wavelength of 250 nm was determined to be 13.2%. A possible growth mechanism of AgInS₂ nanorods was discussed. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)