Properties Of ZnO Quantum Dots Research Articles

Effect of Mn doping at different sites on the structural and electronic properties of ZnO quantum dots

We constructed ZnO quantum dots (QDs) with a diameter of 1.2[Formula: see text]nm of wurtzite structure, hydrogenated the surface and doped single Mn atoms using Zn-site substitution. We present an investigation of the structural and electronic properties of ZnO QDs of three doping sites: inner, intermediate and outer. Based on the density functional theory and the plane-wave pseudo potential method, the structural characteristics, charge distribution, conductivity and formation energy of doped ZnO QDs are analyzed. We obtained the effect of doping site depth on the properties above with the nearly optimal nanoparticle doping concentration (2.222%). The internal doping (inner and intermedium) configuration results in higher electrical conductivity and stronger bonds than the outer doping configuration. In particular, intermediate doping is highly efficient and valuable for obtaining Mn-doped ZnO nanoparticles with good structural and electronic properties. This study provides a theoretical reference for the survey of zero-dimensional dilute magnetic semiconductors.

Engineered Sericin-Tagged Layered Double Hydroxides for Combined Delivery of Pemetrexed and ZnO Quantum Dots as Biocompatible Cancer Nanotheranostics.

Despite extensive progress in the field of cancer nanotheranostics, clinical development of biocompatible theranostic nanomedicine remains a formidable challenge. Herein, we engineered biocompatible silk-sericin-tagged inorganic nanohybrids for efficient treatment and imaging of cancer cells. The developed nanocarriers are anticipated to overcome the premature release of the chemotherapeutic drug pemetrexed (PMX), enhance the colloidal stability of layered double hydroxides (LDHs), and maintain the luminescence properties of ZnO quantum dots (QDs). Materials and Methods: PMX-intercalated LDHs were modified with sericin and coupled to ZnO QDs for therapy and imaging of breast cancer cells. Results: The optimized nanomedicine demonstrated a sustained release profile of PMX, and high cytotoxicity against MDA-MB-231 cells compared to free PMX. In addition, high cellular uptake of the engineered nanocarriers into MDA-MB-231 breast cancer cells was accomplished. Conclusions: Conclusively, the LDH-sericin nanohybrids loaded with PMX and conjugated to ZnO QDs offered a promising cancer theranostic nanomedicine.

Effect of surface oxygen vacancy defects on the performance of ZnO quantum dots ultraviolet photodetector* *Project supported by the National Natural Science Foundation of China (Grant Nos. 62074148, 61875194, 11727902, 12074372, 11774341, 11974344, 61975204, and 11804335), the Youth Innovation Promotion Association of the Chinese Academy of Sciences (Grant No. 2020225), the Open Project of the State Key Laboratory of Luminescence

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 (VO) defects controlled by hydrogen peroxide (H2O2) solution treatment have been investigated. After H2O2 solution treatment, VO concentration of ZnO QDs decreased. The H2O2 solution-treated device has a higher photocurrent and a lower dark current. Meanwhile, with the increase in VO 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 VO defects. The findings in this work clarify that the surface VO defects of ZnO QDs could enhance the photoresponse speed, which is helpful for sensor designing.

Journey of ZnO quantum dots from undoped to rare-earth and transition metal-doped and their applications.

Currently, developments in the field of quantum dots (QDs) have attracted researchers worldwide. A large variety of QDs have been discovered in the few years, which have excellent optoelectronic, antibacterial, magnetic, and other properties. However, ZnO is the single known material that can exist in the quantum state and can hold all the above properties. There is a lot of work going on in this field and we will be shorthanded if we do not accommodate this treasure at one place. This manuscript will prove to be a milestone in this noble cause. Having a tremendous potential, there is a developing enthusiasm toward the application of ZnO QDs in diverse areas. Sol–gel method being the simplest is the widely-favored synthetic method. Synthesis via this method is largely affected by a number of factors such as the reaction temperature, duration of the reaction, type of solvent, pH of the solution, and the precipitating agent. Doping enhances the optical, magnetic, anti-bacterial, anti-microbial, and other properties of ZnO QDs. However, doping elements reside mostly on the surface of the QDs. The presence of doping elements inside the core is still a major challenge for doping techniques. In this review article, we have focused on pure, rare-earth, and transition metal-doped ZnO QD properties, and the various synthetic processes and applications. Quantum confinement effect is present in nearly every aspect of the QDs. The effect of quantum confinement has also been summarized in this manuscript. Furthermore, the doping of rare earth elements and transition metal, synthetic methods for different organic molecule-capped ZnO QDs, mechanisms for reactive oxygen species (ROS) generation, drug delivery system for cancer treatment, and many more application are discussed in this paper.

Influence of back donation effects on the structure of ZnO nanoclusters

The structure and properties of ZnO quantum dots is a very popular and rapidly growing field of research for which accurate quantum calculations are challenging to perform. Since the dependence between system size and wall time scales nonlinearly, certain compromises have to be made. A particularly important limiting factor is the size of the basis used, this is especially the case if accurate large calculations are to be carried out. In our work, we discovered that an important O(2p)->Zn(4p) back donation, which greatly influences the strength of the ZnO bond, can be reproduced only if diffuse functions are added to the basis set. We further tested the basis dependence for the magic-sized wurtzite nanophase ZnO clusters which were previously shown to be able to accurately reproduce the magnetically doped II-IV Q-dots. In this work, we outline the minimal basis sets required to properly describe ZnO bonds in a nanocluster. It was demonstrated that the rock salt nanophase is incorrectly stabilized if a basis set does not contain sufficiently diffuse functions while the correct wurtzite phase is stabilized when diffuse functions are added. This tendency, similar to that in the ZnO dimer case, was shown to stem from the incorrect lack of Zn(4p) electron density in calculations when using the diffuse-free basis set.

Concentration dependent thermo-optic properties of yellow emissive ZnO quantum dots

This paper study the thermal properties of ZnO quantum dots (QDs) synthesized by sol-gel method. The structural and morphological properties were investigated using x-ray diffraction technique (XRD) and transmission electron microscopy (TEM). Thermal properties of ZnO QDs were measured using dual beam mode matched thermal lens technique. The results reveal that the thermal diffusivity of the nanofluid depends on the concentration of ZnO QDs. There is an optimum concentration of ZnO QDs, above which thermal diffusivity increases more than that of the base fluid whereas thermal diffusivity decreases below the optimum concentration. This variation in thermal diffusivity of ZnO QDs nanofluid is explained using absorption and emission spectra.

Doping effects of ZnO quantum dots on the sensitive and selective detection of acetylene for dissolved-gas analysis applications of transformer oil

We report on the doping effect on the sensing properties of ZnO quantum dots (QDs) for the detection of acetylene. We found that In-doped ZnO (IZO) QDs exhibited a better sensing performance to 10 ppm acetylene than undoped ZnO (ZO) QDs and Al-doped ZnO (AZO) QDs. The higher sensing response of IZO QDs can be attributed to a greater number of reactive sites for detecting acetylene, which is likely to originate from the increased number of oxygen vacancies, and the larger optical band gap and surface area of IZO. This is due to a higher valence dopant and a smaller particle size. The sensing properties of IZO QDs to 10 ppm acetylene was also found to be superior to previously reported acetylene sensors that are based on semiconducting metal oxides. Furthermore, we demonstrated that 10 ppm of acetylene can be selectively detected in air within ∼100 s using a recently developed miniaturized gas chromatography (GC) integrated with the IZO QDs sensor. In addition, we found that the device can detect the major fault gases of hydrogen and acetylene separately within ∼100 s. Our study demonstrates that the device can be utilized in the GC-based on-line dissolved gas analysis to detect small amounts of acetylene gas in transformer oil.

Synthesis and evaluation of the cytotoxic and anti-proliferative properties of ZnO quantum dots against MCF-7 and MDA-MB-231 human breast cancer cells

Current trends in therapeutic research are the application of nanomaterial carriers for cancer therapy. One such molecule, ZnO, originally used in diagnosis and as a drug carrier, is gaining importance for its biological properties. Here, we report for the first time, the scope of ZnO QDs for enhanced cytotoxicity against MCF-7 and metastatic MDA-MB-231 human breast cancer cells. Unlike other ZnO nanostructures, ZnO QDs are dispersed and small sized (8–10nm) which is believed to greatly increase the cellular uptake. Furthermore, the acidic tumor microenvironment attracts ZnO QDs enhancing targeted therapy while leaving normal cells less affected. Results from MTT assay demonstrated that ZnO QDs induced cytotoxicity to MCF-7 and metastatic MDA-MB-231 breast cancer cells at very low concentrations (10 and 15μg/ml) as compared to other reported ZnO nanostructures. HEK-293 cells showed less toxicity at these concentrations. Confocal microscope images from DAPI staining and TUNEL assay demonstrated that ZnO QDs induced nuclear fragmentation and apoptosis in MCF-7 and MDA-MB-231. FACS results suggested ZnO QDs treatment induced cell cycle arrest at the G0/G1 phase in these cells. ZnO QDs drastically decreased the proliferation and migration of MCF-7 and MDA-MB-231 as seen from the results of the clonogenic and wound healing assays respectively. Furthermore, our data suggested that ZnO QDs regulated apoptosis via Bax and Bcl-2 proteins as validated by immunofluorescence and western blot. Taken together, our findings demonstrate that these ultra-small sized ZnO QDs destabilize cancer cells by using its acidic tumor microenvironment thereby inducing apoptosis and controlling the cell proliferation and migration at low dosages.

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.

Electronic and optical properties of ZnO quantum dots under hydrostatic pressure

In the present work, we studied the electronic and optical properties of ZnO quantum dots (QDs) subjected to externally applied hydrostatic pressure. Our single-particle calculations are based on the empirical pseudopotential method and the excitonic effects are considered by employing the configuration interaction approach. The optical band gap, Stokes shift, and optical emission polarization have been investigated as a function of the applied pressure. It is found that the applied pressure causes a linear increase in the optical band gap. The pressure coefficient appears to be highly size dependent, exhibiting a monotonic increase with increasing QD size. In contrast to this monotonic behavior, the applied pressure induces a nonmonotonic Stokes shift which presents a minimum value at a critical pressure. For pressures larger than this critical value, the optical emission polarization exhibits a sharp transition from in-plane to out-of-plane polarization. Finally, it is found that the critical pressure at which the crossing takes place strongly depends on the QD size, showing larger values for larger QD sizes. Beyond this crossing point, the lowest optically bright exciton state mainly originates from one Slater determinant, where both the single-particle electron and hole states have an $S$-type envelope function and the hole state originates mainly from the bulk Bloch C band.

A noncavity scheme of optical detection of high-frequency magnetic and cyclotron resonances in semiconductors and nanostructures

To perform optical detection of high-frequency magnetic resonance (ODMR) and cyclotron resonance (ODCR) with a spatial resolution, we have proposed a noncavity scheme of an ODMR-ODCR spectrometer using a quasi-optical microwave channel. The efficiency of this scheme for obtaining information on the spin properties of ZnO quantum dots and CdMnTe quantum wells and on the effective masses of carriers in crystalline silicon films with a spatial resolution within a focused laser beam is illustrated.

Linear and nonlinear intraband optical properties of ZnO quantum dots embedded in SiO2 matrix

In this work we investigate some optical properties of semiconductor ZnO spherical quantum dot embedded in an amorphous SiO2 dielectric matrix. Using the framework of effective mass approximation, we have studied intraband S-P, and P-D transitions in a singly charged spherical ZnO quantum dot. The optical properties are investigated in terms of the linear and nonlinear photoabsorption coefficient, the change in refractive index, and the third order nonlinear susceptibility and oscillator strengths. Using the parabolic confinement potential of electron in the dot these parameters are studied with the variation of the dot size, and the energy and intensity of incident radiation. The photoionization cross sections are also obtained for the different dot radii from the initial ground state of the dot. It is found that dot size, confinement potential, and incident radiation intensity affects intraband optical properties of the dot significantly.

Control of Cu-doping and optical properties of ZnO quantum dots by laser ablation of composite targets

Cu doping of ZnO quantum dots (QDs) was carried out by laser ablation of Zn/Cu composite targets immersed in PVP aqueous solution. Zn/Cu core–shell particles were firstly prepared by a galvanic replacement reaction, and then they were pressed into targets with different Zn/Cu ratios for laser ablation. Under the extremely non-equilibrium conditions generated by the pulse laser used, a large amount of Cu-doped ZnO QDs were produced with ultrafine size, good dispersibility, and high stability. On the other hand, the dopant concentration was feasibly controlled from 1.8% to 4.8% by changing the atomic content of Cu in Zn/Cu composite targets. Cu-doped QDs exhibited blue emission with tunable wavelength, which was ascribed to electronic transitions from the conductor band of ZnO to the acceptor levels related to Cu dopants. As a facile and versatile technique, laser ablation is believed to be an effective way for fabricating various doped nanocrystals.

A comparative study of the structural and spectral characteristics of ZnO nanoparticles dispersed in isopropanol and polyethylene

We have developed a process for the preparation of zinc oxide nanoparticles in isopropanol and high-pressure polyethylene (HPPE) and investigated the structural and spectral properties of the ZnO-isopropanol disperse system and ZnO-HPPE nanocomposite. The results demonstrate that the luminescence properties of ZnO quantum dots can be maintained intact when the quantum dots are transferred to a chemically inert HPPE matrix. We believe that the process for the fabrication of ZnO-HPPE nanocomposites can be employed in the development of UV and visible luminescence standards.

Conventional Optics from Unconventional Electronics in ZnO Quantum Dots

We study the electronic and optical properties of ZnO quantum dots within the atomistic empirical pseudopotential framework. The highest occupied molecular orbital (HOMO) is found to be of orbital P character for structures larger than 2.6 nm in diameter. We identify the origin of this unconventional situation in the electronic character of the HOMO state, originating from an even mixture of the A- and B-bands of the Wurtzite band structure. This situation, however, does not lead to an orbitally dark exciton ground state, as one might expect. Coulomb interactions lower the bright (electron-S−hole-S) exciton below the orbitally forbidden (electron-S−hole-P) exciton to recover the conventional situation of an orbitally allowed, but spin-forbidden, exciton ground state and a Stoke’s shift originating from electron−hole exchange interactions.

Synthesis and gas sensing properties of ZnO quantum dots

ZnO nanocrystals (2.5–4.5 nm) were prepared by a wet chemical method based on alkaline-activated hydrolysis and condensation of zinc acetate solutions. Dropcasting of the nanocrystals onto alumina substrates allowed the fabrication of gas sensing devices, that were tested towards NO 2, acetone and methanol and showed promising results. At low working temperature, the ZnO quantum dots based sensors are selective to nitrogen oxide, in fact a good sensitivity is shown at 200 °C at low concentration (2 ppm), while at temperature above 350 °C, high responses are obtained for acetone and methanol. The results obtained are stimulating for further developing of nano-ZnO based sensor devices.

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.

Optical Properties of ZnO Quantum Dots in Epoxy with Controlled Dispersion

Optical properties of monodisperse colloidal ZnO quantum dots (QDs) having various degrees of dispersion in epoxy have been systematically studied. Exfoliated α-zirconium phosphate (ZrP) nanoplatel...

Fabrication and Optical Properties of ZnO Quantum Dots

Using a simple process of the deposition of ZnO thin films on SiOx/Si substrates and subsequent thermal annealing, we fabricated ZnO quantum dots embedded in silicon oxide matrix. The ZnO quantum dots were characterized using transmission electron microscopy and timeintegrated photoluminescence. The photoluminescence of the quantum dots show a blue-shift of 47 meV due to the quantum confinement effect.

Enhanced ultraviolet emission and optical properties in polyvinyl pyrrolidone surface modified ZnO quantum dots

Optical properties of ZnO quantum dots (QDs) capped with polyvinyl pyrrolidone (PVP) molecules have been investigated. It is demonstrated that surface modification by PVP can dramatically change the emission spectra of the ZnO QDs. At the optimized condition with a PVP/Zn2+ ratio of 3:5, the photoluminescence (PL) spectrum of ZnO QDs shows a strong ultraviolet (UV) emission while the low energy green emission is fully quenched. This is a result of the surface passivation of the ZnO QDs by the PVP molecules. The origin of the green emission is attributed to the surface states associated with oxygen vacancies. Temperature and excitation power dependent PL studies suggest that the UV emission is associated with localized states.