Fluorescence Property of ZnO Nanoparticles and the Interaction with Bromothymol Blue

We synthesized ZnO quantum dots (QDs) simply in alcoholic solution, and investigated the interaction between ZnO QDs and bromothymol blue. The structural, morphological, size and spectral properties of ZnO QDs were studied. It was found that ZnO QDs were spherical nanoparticles in the crystal structure, and the average diameter of ZnO QDs was about 4.8 nm. The excitation and emission peaks were located at 346 nm and 520 nm, respectively, which were obtained on a common fluorophotometer. The quantum yield of ZnO QDs was obtained by using quinine sulfate as a reference reagent. In addition, the fluorescence of ZnO QDs can be quenched by bromothymol blue, and the quenching mechanism was proposed in a dynamic quenching mode.     

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.     

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.     

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.     

The influence of annealing temperature on the structure,morphologies and optical properties of ZnO nanoparticles

ZnO nanoparticles (NPs) have been successfully synthesized by the simple solution method at low temperature. The effects of annealing temperature on the structure and optical properties of ZnO NPs were investigated in detail by X-ray diffraction, transmission electron microscopy (TEM), ultraviolet–visible (UV–vis) spectroscopy and photoluminescence (PL) measurements. As the annealing temperature was increased above 180 ^°C the particles morphology evolved from spherical to hexagonal shape, indicating that the average particle size increased from 11 nm to 87 nm. The UV-vis and PL spectra showed a red-shift from 3.62 to 3.33 eV when the annealing temperature was increased.     

Size-selected copper oxide nanoparticles synthesized by laser ablation

Size-tuned copper oxide nanoparticles with sizes of 9, 12, and 15 nm were fabricated by laser ablation and on-line size selection using a differential mobility analyzer at a gas pressure of 666 Pa. The dependence of the particle properties on the in situ annealing temperatures and selection sizes was investigated. The crystalline phases of the nanoparticles fabricated at temperatures below 973 K were assigned to monoclinic cupric oxide (CuO) which converted into cubic cuprous oxide (Cu₂O) when the annealing temperature was above 1,173 K. This indicates that the crystalline phases can be easily controlled by changing the annealing temperature. TEM images confirmed that well-crystallized and well-dispersed CuO and Cu₂O nanoparticles with narrow size distributions were obtained using this method. This fabrication process is useful and promising for the future investigation of the intrinsic size-dependent properties of CuO and Cu₂O.     

Photoluminescence from Zinc Oxide Quantum Dots Embedded in Silicon Dioxide Matrices

ABSTRACT

ZnO quantum dots (QDs) embedded in SiO₂ matrix are fabricated by ion implantation and annealing treatment methods. When the Zn-doping dose is (2, 3, 5, and 7) × 10¹⁶ cm^?2, the size of quantum dots is in the range of ～4–10 nm in diameter according to the XRD and HR-TEM results. Ultraviolet and green light emissions from the specimen are obtained at room temperature. With the increase of the Zn-doping dose, the PL peak in the ultraviolet region red shifts from 3.32 to 3.10 eV. This PL peak is related to the size of ZnO QDs, which is ascribed to the free exciton recombination in QDs. The green light emissions centered at 2.43 and 2.25 eV are independent of the Zn-doping dose and annealing temperature, which are attributed to the deep-level defect and the small peroxy radical (SPR) defect, respectively.     

Self-assembled SiGe quantum dots embedded in Ge matrix by Si ion implantation and subsequent annealing

We have fabricated SiGe quantum dots (QDs) by means of a two-step Si ion implantation followed by thermal rapid thermal annealing (RTA) method. SiGe QDs with the 4–6 nm diameter are formed uniformly in the near-surface region of Ge substrate. The RTA processes are performed at 800 and 900 °C for 15 s, respectively. Both experimental and theoretical analysis indicates that the higher temperature (900 °C) RTA can enhance the growth of SiGe QDs. Two photoluminescence peaks are observed near 572 and 581 nm at room temperature. The mechanism of the luminescence from SiGe QDs is discussed.     

Cadmium sulfide quantum dots stabilized by castor oil and ricinoleic acid

Castor oil and ricinoleic acid (an isolate of castor oil) are environmentally friendly bio-based organic surfactants that have been used as capping agents to prepare nearly spherical cadmium sulfide quantum dots (QDs) at 230, 250 and 280 °C. The prepared quantum dots were characterized by Ultra violet–visible (UV–vis), Photoluminescence (PL), Transmission Electron Microscopy (TEM), High Resolution Transmission Electron Microscopy (HRTEM) and X-ray diffraction (XRD) giving an overall CdS QDs average size of 5.14±0.39 nm. The broad XRD pattern and crystal lattice fringes in the HRTEM images showed a hexagonal phase composition of the CdS QDs. The calculated/estimated average size of the prepared castor oil capped CdS QDs for various techniques were 4.64 nm (TEM), 4.65 nm (EMA), 5.35 nm (UV–vis) and 6.46 nm (XRD). For ricinoleic acid capped CdS QDs, the average sizes were 5.56 nm (TEM), 4.78 nm (EMA), 5.52 nm (UV–vis) and 8.21 nm (XRD). Optical properties of CdS QDs showed a change of band gap energy from its bulk band gap of 2.42–2.82 eV due to quantum size confinement effect for temperature range of 230–280 °C. Similarly, a blue shift was observed in the photoluminescence spectra. Scanning electron microscope (SEM) observations show that the as-synthesized CdS QDs structures are spherical in shape. Fourier transform infra-red (FTIR) studies confirms the formation of castor oil and ricinoleic acid capped CdS QDs.     

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.     

Photosensitization of multiwalled carbon nanotube scaffolds with ZnO quantum dots for photovoltaic applications

In this article, we have decorated multiwalled carbon nanotubes (MWCNTs) scaffold with ZnO quantum dots (QDs, size in the range of 2.9–4.5 nm) and investigated their prospects for photovoltaic applications. ZnO QDs, in the present study, work as photosensitizer instead of electron transporting media as used in recent conventional strategic solar cells. ZnO QDs/MWCNTs composite shows an increased visible absorbance and quenching of the broad visible emission at around ~560 nm, while only ZnO QDs exhibit a strong visible emission. An efficient electron–hole separation facilitates an increase in the short-circuit current. These results show a possibility of developing a nontoxic, ZnO QDs sensitized MWCNTs composite-based photovoltaic solar cell.     

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.     

Formation of InAs quantum dots at ultrahigh growth rates

We fabricated quantum dot (QD) structures at ultrahigh growth rates. Smaller fluctuations in QD size were observed when they were grown at a rate of 1.0 ML/s under conventional growth conditions (growth temperature of 500 °C and As₄ flux of 9×10⁻⁶ Torr). For QDs grown at high rates, growth interruption played an important role in the fabrication of QD structures; this was confirmed by carrying out reflection high-energy electron diffraction. Photoluminescence for QDs grown at high and low growth rates, with growth interruption and with low-temperature capping was observed at around 1250 nm at room temperature, indicating that high-quality QDs can be fabricated by employing high growth rates.     

CdS quantum dots in a novel glass with a very low activation energy and its variation of diffusivity with temperature

CdS quantum dots of different average sizes in the range 2 to 3.8 nm were grown by diffusion-limited growth process in indigenously made silicate glass. The absorption spectra showed a strong quantum confinement effect with a blue shift of the order of ~500 meV depending on the average size. Critical radius of quantum dots was found to be 1.8 nm. The size dispersion decreased from 15.2 to 12.5% with a 20% increase in the particle size. The activation energy for diffusion was found to be very low i.e. 193 kJ mol⁻¹ and the diffusion coefficient increased by 60% for 10 K rise in temperature. The PL emission spectra showed the presence of only deep traps around ~600 nm with a red shift of 200 nm. No shallow traps or band edge emission was observed. The PL peak position changed from 560 to 640 nm with a 35 K increase in annealing temperature.     

Liposome-coated quantum dots targeting the sentinel lymph node

Sentinel lymph node (SLN) mapping with near-infrared (NIR) quantum dot (QDs) have many advantages over traditional methods. However, as an inorganic nanomaterial, QDs have low biocompatibility and low affinity to the lymphatic system. Here, we encapsulated QDs into nanoscale liposomes and then used these liposome-coated QDs for SLN mapping. The results showed that the liposome-coated QDs exhibited core–shell characterization, and their fluorescence emission did not decrease but slightly increased after being continuously excited by a xenon lamp source (150 W) at 488 nm at 37 °C for 1 h. After storing at 4 °C for more than one and half years, the liposome-coated QDs were found to have retained their spherical structure containing a large amount of QDs. When liposome-coated QDs with average size of 55.43 nm were injected intradermally into the paw of a mouse, the SLN was strongly fluorescent within only a few seconds and visualized easily in real time. Moreover, the fluorescence of the QDs trapped in the SLN could be observed for at least 24 h. Compared with the SLN mapping of QDs absent of liposomes and liposome-coated QDs with a larger average size (100.3 and 153.6 nm), more QDs migrated into the SLN when the liposome-coated QDs with smaller average size (55.43 nm) were injected. This technique may make a great contribution to the improvement of the biocompatibility of QDs and the targeting delivery capacity of QDs into the SLN.     

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.     

Using Zn as target to fabricate ZnO coating by thermal oxidation in air on quartz substrate

In this work, ZnO coatings were fabricated by the RF-sputtering method on quartz substrates in an inert gas ambient of Ar followed by a thermal oxidation process in air at different temperatures. The effect of thermal oxidation temperatures on the structures and photoluminescence (PL) properties of the coatings were studied. The structural characteristics of the samples were analyzed by X-ray diffraction (XRD) and atomic force microscope (AFM). The PL spectra were obtained by using a Xe laser as a light source with an excitation wavelength of 325 nm at room temperature. The force-curves were obtained by AFM. The results show that all the prepared ZnO coatings have a compact hexagonal wurtzite structure. With the increasing annealing temperature from 400 °C to 600 °C, the particle size, surface RMS roughness, photoluminescence intensity and adhesion force of the prepared ZnO coatings were increased as well.     

Morphology and photoluminescence study of electrodeposited ZnO films

ZnO films on ITO substrates and Au coated ITO substrates were fabricated by using electrodeposition technique. We carried out the experiments by adjusting the concentration of solution, potential, substrate, and temperature. The effect of temperature on the growth of the film has been examined. SEM images have shown that there are several kinds of grown competitions for the deposition of ZnO films, but three kinds of them are dominant. One is the discrete hexagonal column structure, the other is the pentagonal structure, and the third one is of well-oriented hexagonal columns with well-aligned structure. The explanation on the grown competition is discussed. ZnO hexagonal column structures with well-aligned and well-perpendicular to the surface were successfully obtained on Au/ITO substrate in aqueous solvent of electrolyte. Clearly the main columns in the film were obtained by increasing the temperature. Its photoluminescence (PL) study at low temperature exhibited the optical properties as wurtzite ZnO and indicated the existence of macrocrystalline ZnO. A better quality of ZnO columnar structures after annealing was demonstrated from PL analysis and discussion on the existence of 370 nm, 384 nm and 639 nm in the emission bands before and after annealing.     

Effect of thermal annealing in the microstructural and the optical properties of uncapped InAs quantum dots grown on GaAs buffer layers

The effect of thermal annealing on self-assembled uncapped InAs/GaAs quantum dots (QDs) has been investigated using transmission electron microscopy (TEM) and photoluminescence (PL) measurements. The TEM images showed that the lateral sizes and densities of the InAs QDs were not changed significantly up to 650 °C. When the InAs/GaAs QDs were annealed at 700 °C, while the lateral size of the InAs QDs increased, their density decreased. The InAs QDs disappeared at 800 °C. PL spectra showed that the peaks corresponding to the interband transitions of the InAs QDs shifted slightly toward the high-energy side, and the PL intensity decreased with increasing annealing temperature. These results indicate that the microstructural and the optical properties of self-assembled uncapped InAs/GaAs can be modified due to postgrowth thermal annealing.     

Microwave-Assisted Aqueous Synthesis of Highly Luminescent Carboxymethyl Chitosan-Coated CdTe/CdS Quantum Dots as Fluorescent Probe for Live Cell Imaging

This paper describes the development of a simplified and rapid method for the aqueous synthesis of quantum dots (QDs) with CdTe cores and gradient CdS external shells (CdTe/CdS QDs) aided by microwave irradiation. Several synthesis parameters, such as molar ratio of reagents, pH, reaction temperature, and reaction time, were studied in details. Under the optimized conditions, highly effective CdTe/CdS QDs could be synthesized in aqueous phase in only 15 min. In order to improve the biocompatibility of the CdTe/CdS QDs, these QDs were then interacted with carboxymethyl chitosan (CMC) so as they could be used as fluorescent probes in the aqueous phase. With the incorporation of CMC, the stability of modified QDs was found to have improved significantly (from 4 months to more than 10 months at room temperature). The photoluminescence quantum yield (PLQY) of the modified QDs could reach 75%, other parameters include a full width at half maximum of the emission (FWHM) spectrum as 40 ~ 60 nm, and an average size, estimated from electron microscopic images, as 3.5 nm. As fluorescent probes, these modified QDs were successfully used for imaging live Madin–Darby canine kidney (MDCK) cells, in which the preliminary results indicated that these modified QDs demonstrated good biocompatibility and showed promising applications for bio-labeling and imaging.