High quantum yield ZnO quantum dots synthesizing via an ultrasonication microreactor method

Green emission ZnO quantum dots were synthesized by an ultrasonic microreactor. Ultrasonic radiation brought bubbles through ultrasonic cavitation. These bubbles built microreactor inside the microreactor. The photoluminescence properties of ZnO quantum dots synthesized with different flow rate, ultrasonic power and temperature were discussed. Flow rate, ultrasonic power and temperature would influence the type and quantity of defects in ZnO quantum dots. The sizes of ZnO quantum dots would be controlled by those conditions as well. Flow rate affected the reaction time. With the increasing of flow rate, the sizes of ZnO quantum dots decreased and the quantum yields first increased then decreased. Ultrasonic power changed the ultrasonic cavitation intensity, which affected the reaction energy and the separation of the solution. With the increasing of ultrasonic power, sizes of ZnO quantum dots first decreased then increased, while the quantum yields kept increasing. The effect of ultrasonic temperature on the photoluminescence properties of ZnO quantum dots was influenced by the flow rate. Different flow rate related to opposite changing trend. Moreover, the quantum yields of ZnO QDs synthesized by ultrasonic microreactor could reach 64.7%, which is higher than those synthesized only under ultrasonic radiation or only by microreactor.     

Microstructure and optical properties of ZnO nanoparticles prepared by a simple method

In this investigation, ZnO nanoparticles were prepared by a simple and rapid method. This method is based on the short time solid state milling and calcinations of zinc acetate and citric acid powders. The samples were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), Fourier transform infrared (FTIR) spectroscopy, photoluminescence and UV-vis spectroscopy. It was shown that the calcination temperature significantly affected the particle size and optical properties of the synthesized ZnO nanoparticles. Calculation based on the XRD data shows that the average sizes of ZnO particles are in agreement with those from TEM images and the size of the particles increases on increasing the calcination temperature. Also the band gap of samples decreased from 3.29 to 3.23 eV on increasing the calcination temperature from 350 to 600 °C. Photoluminescence analyses show that many defects such as interstitial zinc, zinc vacancy and oxygen vacancy are responsible for the observed optical properties.     

Synthesis of ZnO quantum dots and their agglomeration mechanisms along with emission spectra based on ageing time and temperature

The ZnO quantum dots (QDs) were synthesized with improved chemical solution method. The size of the ZnO QDs is exceedingly uniform with a diameter of approximately 4.8 nm, which are homogeneously dispersed in ethanol. The optical absorption edge shifts from 370 nm of bulk material to 359 nm of QD materials due to the quantum size effect, while the photoluminescence peak shifts from 375 nm to 387 nm with the increase of the density of ZnO QDs. The stability of ZnO QDs was studied with different dispersion degrees at 0 ℃ and at room temperature of 25 ℃. The agglomeration mechanisms and their relationship with the emission spectra were uncovered for the first time. With the ageing of ZnO QDs, the agglomeration is aggravated and the surface defects increase, which leads to the defect emission.     

A sol-gel method for preparing ZnO quantum dots with strong blue emission

ZnO quantum dots (QDs) with strong blue emission have been successfully synthesized by sol-gel method, and their crystal structures, sizes, and photoluminescence properties were characterized by X-ray diffractometer, scanning electron microscope, and ultraviolet-visible spectroscopy. It has been found that ZnO QDs had a hexagonal wurtzite crystal structure, and their average diameter was about 16.0-32.2 nm. Both the reaction time and temperature were found to have a strong influence on the average size and photoluminescence properties of ZnO QDs. Longer reaction time and higher reaction temperature resulted in larger average size for ZnO QDs. It has been shown that at reaction temperature 60 °C the emission intensity for ZnO QDs increased first with reaction time before 7 h and then decreased after 7 h. For the same reaction time 7 h, ZnO QDs synthesized at 60 °C showed the strongest emission intensity. It was found that annealing in nitrogen, vacuum, and air all resulted in an increase of the size of ZnO QDs and a reduction in their photoluminescence. The dependence of the size and properties of ZnO QDs on the reaction parameters as well as the annealing conditions has been discussed.     

超声法制备碳量子点发光性能的影响因素分析

超声法制备碳量子点过程简单,成本低廉,不易产生二次污染,应用前景广泛。为优化超声法制备碳量子点的各工艺参数,制备了关键工艺参数不同的碳量子点样品,测试其发射与激发光谱,分析了量子点浓度,溶剂种类,辅助剂种类、浓度,超声功率、时间等参数对碳量子点发光性能影响。结果表明超声法制备的碳量子点具有激发光波长依赖性,发射峰位置随激发波长的变化而发生明显改变;碳量子点浓度增加,发光强度由于非辐射能量传递和团聚作用,先增大后减小;由于溶剂效应,碳量子点在乙醇中比在水中发光强度更强,波长更短,且浓度越大时波峰移动越明显;相比盐酸,以NaOH为辅助剂制备的碳量子点表面钝化程度高,发光强度强;增加辅助剂NaOH浓度可提高量子点表面钝化程度,增大发光强度;同等时间下增加超声功率或同等功率下适量增加超声时间,可制备更多的碳量子点样品,但超声时间过长,碳量子点容易发生团聚猝灭现象。以上影响因素分析为超声法制备碳量子点的工艺参数优化提供了理论基础,有利于碳量子点大规模低成本的生产应用。     

Effects of Annealing Temperature on Structural and Optical Properties of ZnO Thin Films

The effects of annealing temperature on the structural and optical properties of ZnO films grown on Si （100） substrates by sol-gel spin-coating are investigated. The structural and optical properties are characterized by x-ray diffraction, scanning electron microscopy and photoluminescence spectra. X-ray diffraction analysis shows the crystal quality of ZnO films becomes better after annealing at high temperature. The grain size increases with the temperature increasing. It is found that the tensile stress in the plane of ZnO films first increases and then decreases with the annealing temperature increasing, reaching the maximum value of 1.8 GPa at 700℃. PL spectra of ZnO films annealed at various temperatures consists of a near band edge emission around 380 nm and visible emissions due to the electronic defects, which are related to deep level emissions, such as oxide antisite （OZn）, interstitial oxygen （Oi）, interstitial zinc （Zni） and zinc vacancy （VZn^-）, which are generated during annealing process. The evolution of defects is analyzed by PL spectra based on the energy of the electronic transitions.     

Investigation of Cu Doping,Morphology and Annealing Effects on Structural and Optical Properties of ZnO:Dy Nanostructures

Dysprosium (Dy) doped ZnO nanosheets and nanorods were synthesized by hydrothermal method. Effects of Cu doping, morphology and annealing in Oxygen ambient on structural and optical properties of ZnO nanostructures were investigated using X–ray diffraction (XRD), scanning electron microscopy (SEM), diffuse reflectance spectra (DRS) and photoluminescence (PL) spectroscopy. This study recommends that both of intrinsic and extrinsic defects facilitate energy transfer (ET) from the ZnO host to Dy³⁺ ions and consequently have an effective role on producing intense Dy emissions at indirect excitation. The results also revealed that annealing process improved the crystal structure of ZnO nanorods due to decrease of surface; however decreased ET and Dy emissions because of diminishing in oxygen vacancy. In addition, as a result of increasing of surface area in nanorods compared to nanosheets, the oxygen vacancies and ET were enhanced. Moreover the results exhibited that electrical and optical properties of ZnO:Dy can be tuned by various amount of Dy concentrations and also Cu doping.     

巯基丙酸分子对CdSe核壳量子点和 ZnO纳米粒子薄膜之间电荷转移的影响

通过稳态光谱和时间分辨荧光光谱研究了巯基丙酸(MPA)分子对由量子点到ZnO纳米粒子薄膜的电荷转移过程的影响。研究发现,相对于CdSe纳米粒子薄膜样品,没有MPA分子参与作用的CdSe/ZnO薄膜样品和有MPA分子连接的CdSe/MPA/ZnO薄膜样品中都存在从CdSe量子点到ZnO纳米粒子薄膜的有效电荷分离过程,但是相对于CdSe/ZnO样品, CdSe/MPA/ZnO样品中电荷转移速率明显变小。这表明MPA分子本身它并不能促进CdSe到ZnO电荷分离过程,因此可以认为用金属氧化物薄膜直接吸附量子点吸收材料,将能获得高功率转换效率的量子点敏化太阳能电池。     

Effect of surface oxygen vacancy defects on the performance of ZnO quantum dots ultraviolet photodetector

The slower response speed is the main problem in the application of ZnO quantum dots (QDs) photodetector, which has been commonly attributed to the presence of excess oxygen vacancy defects and oxygen adsorption/desorption processes. However, the detailed mechanism is still not very clear. Herein, the properties of ZnO QDs and their photodetectors with different amounts of oxygen vacancy (V_O) defects controlled by hydrogen peroxide (H₂O₂) solution treatment have been investigated. After H₂O₂ solution treatment, V_O concentration of ZnO QDs decreased. The H₂O₂ solution-treated device has a higher photocurrent and a lower dark current. Meanwhile, with the increase in V_O concentration of ZnO QDs, the response speed of the device has been improved due to the increase of oxygen adsorption/desorption rate. More interestingly, the response speed of the device became less sensitive to temperature and oxygen concentration with the increase of V_O defects. The findings in this work clarify that the surface V_O defects of ZnO QDs could enhance the photoresponse speed, which is helpful for sensor designing.     

Well-crystallized zinc oxide quantum dots with narrow size distribution

In this study, pulsed laser ablation, online annealing, and following size classification using a differential mobility analyzer (DMA) were employed to fabricate quantum dots (QDs) of zinc oxide (ZnO). The irregularly shaped ZnO particles were obtained at annealing temperature less than 873 K, which gradually transformed into spherical QDs with increasing the annealing temperature. Finally, ZnO QDs with narrow size distribution having spherical shapes were successfully obtained at temperatures above 1173 K under the DMA classification at a nominal size of 10 nm. TEM observation demonstrated that the ZnO QDs obtained by this process were well-crystallized single crystallites with a wurtzite structure. Further, ZnO QDs with average sizes in the range of 4.8–8.1 nm were successfully fabricated by reducing the specified sizes of DMA. These features of the fabricated ZnO QDs are favorable for investigation of intrinsic quantum size effect in ZnO.     

ZnO@GQDs核壳结构量子点的制备及性能研究

采用原位聚合法制备了以ZnO量子点为核、石墨烯量子点（GQDs）为壳的ZnO@ GQDs核壳结构量子点。通过TEM和HR-TEM对量子点进行形貌和结构的分析表征。结果表明,合成的ZnO@ GQDs核壳结构量子点为球形,粒径为～7 nm,且尺寸均匀。PL光谱研究表明,新型量子点的发射峰位于369 nm,发光峰窄、强度高;相对于ZnO的本征发射峰,GQDs的引入使得ZnO@GQDs核壳量子点的荧光发射峰出现蓝移、强度变高,从而使复合量子点的荧光具有较纯的色度和较高的强度,说明GQDs的引入具有协同优化效应。该量子点有望应用于LED显示器件。     

Influence of annealing temperature on the photoluminescence properties of ZnO quantum dots

The properties of ZnO quantum dots (QDs) synthesized by the sol-gel process are reported. The primary focus is on investigating the origin of the visible emission from ZnO QDs by the annealing process. The X-ray diffraction results show that ZnO QDs have hexagonal wurtzite structure and the QD diameter estimated from Debye-Scherrer formula is 8.9 nm, which has a good agreement with the results from transmission electron microscopy images and the theoretical calculation based on the Potential Morphing Method. The room-temperature photoluminescence spectra reveal that the ultraviolet excitation band has a red shift. Meanwhile, the main band of the visible emission shifts to the green luminescence band from the yellow luminescence one with the increase of the annealing temperature. A lot of oxygen atoms enter into Zn vacancies and form oxygen antisites with increasing temperature. That is probably the reason for the change of the visible emission band.     

Controlling the energy transfer between near-field optically coupled ZnO quantum dots

We performed time-resolved spectroscopy of ZnO quantum dots (QD), and observed exciton energy transfer and dissipation between QD via an optical near-field interaction. Two different sizes of ZnO QD with resonant energy levels were mixed to test the energy transfer and dissipation using time-resolved photoluminescence spectroscopy. The estimated energy transfer time was 144 ps. Furthermore, we demonstrated that the ratio of energy transfer between the resonant energy states could be controlled.     

Annealing effect on structure and green emission of ZnO nanopowder by decomposing precursors

ZnO nanopowder is successfully synthesized by annealing the precursors in oxygen gas using the chemical precipitation method. Structural and optical properties of thus synthesized ZnO nanopowder are characterized by scanning electron microscopy (SEM) and photoluminescence (PL). The morphology of ZnO nanopowders evolves from nanorod to cobble as annealing temperature increases from 500 to 1000~\du, while spiral structures are observed in the samples annealed at 900 and 1000~\du. The PL spectra of ZnO nanopowder consist of largely green and yellow emission bands. The green emission from ZnO nanopowder depends strongly on the annealing temperature with a peak intensity at a temperature lower than 800~℃ while the yellow emission is associated with interstitial oxygen \rm O_\i.     

Effect of annealing temperature on density of ZnO quantum dots

ZnO quantum dots (QDs) were fabricated on Si (001) substrates by pulsed laser deposition (PLD) and subsequent thermal annealing. X-ray diffraction and transmission electron microscopy analyses revealed that the ZnO QDs had polycrystalline hexagonal wurtzite structure. The size and density of ZnO QDs were investigated by atomic force microscopy. It has been found that the density decreased while the size increased with increasing annealing temperature. The analysis of size distribution of the dots shows an obvious bimodal mode according to scaling theory. The Raman spectrum shows a typical resonant multi-phonon form for the ZnO QDs. The collapse from the top of the dots was observed firstly after the samples were exposed in air for 30 days.     

Density controllable growth of ZnO quantum dots by MOCVD

High quality self-assemble ZnO quantum dots (QDs) have been successfully grown on the Si(111) substrates by metalorganic chemical vapor deposition (MOCVD). The diameter of ZnO QDs is about 10 nm in average, and the densities and the sizes of ZnO QDs can be well controlled by adjusting the growth temperature, which were evident in the SEM images. The properties and stress involved in ZnO QDs are studied by X-ray diffraction. In addition, room temperature photoluminescence spectra reveal that the ZnO QDs exhibit a band gap blue shift because of the quantum confinement effects.     

Alumina capped ZnO quantum dots multilayer grown by pulsed laser deposition

To extend the applicability of ZnO, with the bulk band gap of about 3.3 eV, into deep UV region, we have grown a multilayer of alumina capped ZnO quantum dots of mean in-plane sizes in the range of ∼1.8-3.6 nm at room temperature using alternate Pulsed Laser Deposition. Size dependent blue shift of the band gap of these dots up to ∼4.5 eV is observed in the optical absorbance spectra. The observed blue shift can be understood using the effective mass approximation in weak and strong confinement regimes.     

CdSe quantum dot sensitized solar cell based hierarchical branched ZnO nanoarrays

The hierarchical branched ZnO nanoarrays (NAs) photoanode was prepared by a two-step hydrothermal method. Vertically aligned long ZnO NWs were first synthesized using as the backbone of hierarchical branched ZnO NAs structure and high quality ZnO NAs branches were grown on the surface of backbone ZnO NAs. The structured films enhance the optical path length through the light scatting effect of branched ZnO NAs and prove the larger internal surface area in NAs film to increase quantum dots (QDs) sensitizer loadings, so the light absorption has an optimization. Compared with the cell based conventional 1D ZnO NAs, the efficiency of the new cells has a great improvement due to the increase of the short circuit current density.     

Electron dynamics in modulation p-doped InGaAs/GaAs quantum dots

We investigate the electron dynamics of p-type modulation doped and undoped InGaAs/GaAs quantum dots using up-conversion photoluminescence at low temperature and room temperature. The rise time of the p-doped sample is significantly shorter than that of the undoped at low temperature. With increasing to room temperature the undoped sample exhibits a decreased rise time whilst that of the doped sample does not change. A relaxation mechanism of electron-hole scattering is proposed in which the doped quantum dots exhibit an enhanced and temperature independent relaxation due to excess built-in holes in the valence band of the quantum dots. In contrast, the rise time of the undoped quantum dots decreases significantly at room temperature due to the large availability of holes in the ground state of the valence band. Furthermore, modulation p-doping results in a shorter lifetime due to the presence of excess defects.     

Synthesis of stable dispersion of ZnO quantum dots in aqueous medium showing visible emission from bluish green to yellow

Aqueous dispersion of 4-8 nm size stable ZnO quantum dots (QDs) exhibiting luminescence in the visible region have been synthesized by a simple solution growth technique at room temperature. Silica has been used as capping agent to control the particle size as well as to achieve uniform dispersion of QDs in aqueous medium. X-ray diffractometer (XRD) analysis reveals formation phase pure ZnO particles having wurzite (hexagonal) structure. Atomic force microscope (AFM) images show that the particles are spherical in shape, having average crystalline sizes ∼4, 5.5 and 8 nm for samples prepared at pH values of 10, 12 and 14, respectively. From the optical absorption studies, the band gap energy of QDs is found to be blue shifted as compared to bulk ZnO (3.36 eV) due to the quantum confinement effect and is consistent with the band gap calculated by using effective-mass approximation model. The photoluminescence (PL) observed in these QDs has been attributed to the presence of defect centers.