Mn Doped ZnO Nanocrystals Research Articles

Correlation between ferromagnetism and dopant 3d metal-oxygen hybridized state lying at the bottom of conduction band in ZnO-based diluted magnetic semiconductor system

We systematically investigate the unoccupied electronic states, crystal structure, and magnetism of V- and Mn-doped ZnO nanocrystals (NCs). Post-annealing treatment at 300 °C converts diamagnetic V5+ into magnetic high-spin V3+ ions, which leads to room-temperature ferromagnetism for the V-doped NCs. In contrast, ferromagnetism does not occur for the Mn-doped NCs. Oxygen 1s x-ray absorption spectroscopy reveals that the unoccupied metal-oxygen hybridized state lies near the bottom of the conduction band for the V-doped NCs but lies far above it for the Mn-doped NCs. Therefore, the ferromagnetism in a ZnO-based diluted magnetic semiconductor system can be understood within the framework of the n-type carrier-mediated ferromagnetism model.

EGFR targeted Mn-doped ZnO fluorescent nanocrystals for cancer theranostic application

A combination of diagnosis and therapeutic treatment protocols leads to the successful management of cancer. In this context, improved diagnostic methods are needed for early cancer detection. Moreover, traditional cancer chemotherapy protocols are limited due to non-specificity and serious side effects. In the present work, we developed Mn-doped ZnO nanocrystals to manifest simultaneous drug delivery and diagnosis. Characterization results demonstrated that the synthesized nanocrystals are of nanometer range and also exhibit fluorescence property which is confirmed from fluorescence spectrophotometer and confocal microscopic study results. Cytotoxicity effects in drug resistant MCF-7 cell line showed that the nanocrystals are non-toxic up to certain concentrations, and induces toxicity at higher concentrations. However, the toxic effect of the nanocrystals was eliminated by coating a polymer on the surface of the nanocrystals. For targeted delivery, the epidermal growth factor (EGFR) antibody was surface conjugated on polymer-coated nanocrystals and the uptake of these nanocrystals was demonstrated in a drug resistant breast cancer cell line. Results depicted a higher uptake of the antibody conjugated nanocrystals compared to the unconjugated counterpart. Thus, the results suggest the potential use of EGFR conjugated Mn-doped ZnO nanocrystals in various biomedical applications, particularly in cancer diagnosis, imaging and therapy.

Microstructural, optical, magnetic and photocatalytic properties of Mn doped ZnO nanocrystals of different sizes

The present work explores the differences in the various properties of Mn doped ZnO nanocrystals when the sizes of the doped nanocrystals are significantly different. Mn doped ZnO nanocrystals of sizes ≈20 nm and ≈5 nm were synthesized by the chemical co-precipitation method. The targeted Mn doping concentrations were 2%, 6% and 10% (atomic), however, the observed doping concentrations were low (≈0.5%, ≈1% and ≈2%). The microstructural, morphological, optical and magnetic properties as well as the photocatalytic activities of the nanocrystals were investigated. The variations of the lattice strain and lattice volume with increasing Mn concentrations were found to be correlated. The un-doped and Mn doped ZnO nanocrystals were found to be ferromagnetic at room temperature. Variation of band gap with increasing Mn concentrations was observed. Significant/insignificant quenching of the photoluminescence emissions was observed for sizes ≈20 nm/≈ 5 nm after doping. The saturation magnetization was found to depend on the Mn concentrations as well as on the nanocrystal sizes. Agglomerated Mn2+ resulted in a decrease in saturation magnetization of the ≈2% Mn doped nanocrystals of sizes ≈5 nm. Correlation between the photoluminescence properties and photocatalytic activities was observed.

Microstructure, optical, dielectric and electrical characterizations of Mn doped ZnO nanocrystals synthesized by mechanical alloying

Mn doped ZnO nanocrystals are successfully synthesized, for the first time, by mechanical alloying the stoichiometric mixture of pure ZnO and Mn powders. The structural and compositional analyses are carried out using X-ray diffraction (XRD) and high resolution transmission electron microscopy (HRTEM). Rietveld's analysis of the XRD patterns confirms the formation of single phase wurtzite Zn0.90Mn0.10O nanostructures. Different structural and microstructural parameters such as lattice parameters, crystallite size, r.m.s. strain, stacking faults etc. have been ascertained from the Rietveld analysis. UV–vis spectra are analyzed for band gap measurement and photoluminescence (PL) emission spectra have also been analyzed for estimating different structural defects and optical features of the samples. Temperature dependent dc electrical behavior is studied and the conduction mechanism has been explained by the Godet variable range hopping theory (3D G-VRH). Complex impedance analysis is used to distinguish the grain and grain boundary contribution to the system and it suggests the dominance of grain resistance in the synthesized samples. The dielectric behavior, electric modulus and ac conductivity are studied as a function of temperature (573–773 K) and frequency (100 Hz to 3 MHz). The optical and electrical characterizations predict that the synthesized materials have potential for use in optoelectronics applications.

Oriented Attachment Is a Major Control Mechanism To Form Nail-like Mn-Doped ZnO Nanocrystals.

Here, we present a controlled synthesis of Mn-doped ZnO nanoparticles (NPs) with predominantly nail-like shapes, whose formation occurs via tip-to-base-oriented attachment of initially formed nanopyramids, followed by leveling of sharp edges that lead to smooth single-crystalline "nails". This shape is prevalent in noncoordinating solvents such as octadecene and octadecane. Yet, the double bond in the former promotes oriented attachment. By contrast, Mn-doped ZnO NP synthesis in a weakly coordinating solvent, benzyl ether, results in dendritic structures because of random attachment of initial NPs. Mn-doped ZnO NPs possess a hexagonal wurtzite structure, and in the majority of cases, the NP surface is enriched with Mn, indicating a migration of Mn2+ ions to the NP surface during the NP formation. When the NP formation is carried out without the addition of octadecyl alcohol, which serves as a surfactant and a reaction initiator, large, concave pyramid dimers are formed whose attachment takes place via basal planes. UV-vis and photoluminescence spectra of these NPs confirm the utility of controlling the NP shape to tune electro-optical properties.

Mn-doped ZnO nanocrystals synthesized by sonochemical method: Structural, photoluminescence, and magnetic properties

This work reports the synthesis of Mn-doped ZnO nanostructures using ice-bath assisted sonochemical technique. The impact of Mn-doping on structural, morphological, optical, and magnetic properties of ZnO nanostructures is studied. The morphological study shows that the lower doped samples possess mixtures of nanosheets and nanorods while the increase in Mn content leads to improvement of an anisotropic growth in a preferable orientation to form well-defined edge rods at Mn content of 0.04. UV–vis absorption spectra show that the exciton peak in the UV region is blue shifted due to Mn incorporation into the ZnO lattice. Doping ZnO with Mn ions leads to a reduction in the PL intensity due to a creation of more non-radiative recombination centers. The magnetic measurements show that the Mn-doped ZnO nanostructures exhibit ferromagnetic ordering at room temperature, as well as variation of the Mn content can significantly affect the ferromagnetic behavior of the samples.

Competing effects between intrinsic and extrinsic defects in pure and Mn-doped ZnO nanocrystals

Nano-sized ZnO doped with transition metals is one of the most promising candidates in the field of diluted magnetic semiconductors unifying ferromagnetic and semiconductor properties. Promising is the exploitation of the magnetic spin of the electron and by that the application in spintronics. As the mechanism of spin coupling is still controversial, insight into the coexistence and interaction of intrinsic and extrinsic effects is vital for further technological progress. We report on the synthesis of a set of nano-sized Zn1−x Mn x O samples with a nominal concentration of x = 0.000005–0.03 and structural (XRD, TEM, and AFM), as well as electronic (PL, UV–Vis, FTIR, and EPR) investigations. In this contribution, possible interaction effects were summarized in terms of Mn doping and size. PL quenching after doping was also discussed as another aspect for the interrelations of the defects.

Structural, morphological, physical and dielectric properties of Mn doped ZnO nanocrystals synthesized by sol–gel method

Mn doped ZnO nanocrystals were synthesized by sol–gel technique and sintered at 400°C. Sintered samples characterized using X-ray diffraction (XRD), transmission electron microscopy (TEM), Fourier transform infrared (FTIR) spectroscopy and LCR-Q meter. X-ray diffraction patterns reveal that obtained samples have hexagonal (wurtzite) crystal structure. Volume of unit cell increases with Mn doping, indicating the successful incorporation of Mn ions into ZnO lattice. The morphology changed from hexagonal to spherical–hexagonal structure and size ranging from 30nm to 43nm with Mn doping. Average crystallite size of ZnO nanocrystals decreases with Mn doping. The functional groups and chemical interactions of Mn substituted ZnO samples were determined at various peaks using FTIR data and clearly confirmed that the formation of pure and Mn doped ZnO nanocrystals. Dielectric constant and dielectric loss decreases with increasing Mn concentration and frequency.

Manipulating coupling state and magnetism of Mn-doped ZnO nanocrystals by changing the coordination environment of Mn via hydrogen annealing**Project supported by the National Basic Research Program of China (Grant No. 2013CB934001) and the National Natural Science Foundation of China (Grant Nos. 51072012 and 51272015).

Mn-doped ZnO nanocrystals are synthesized by a wet chemical route and treated in H2/Ar atmosphere with different H2/Ar ratios. It is found that hydrogen annealing could change the coordination environment of Mn in ZnO lattice and manipulate the magnetic properties of Mn-doped ZnO. Mn ions initially enter into interstitial sites and a Mn3+O6 octahedral coordination is produced in the prepared Mn-doped ZnO sample, in which the nearest neighbor Mn3+ and O2 ions could form a Mn3+–O2−–Mn3+ complex. After H2 annealing, interstitial Mn ions can substitute for Zn to generate the Mn2+O4 tetrahedral coordination in the nanocrystals, in which neighboring Mn2+ ions and H atoms could form a Mn2+–O2−–Mn2+ complex and Mn–H–Mn bridge structure. The magnetic measurement of the as-prepared sample shows room temperature paramagnetic behavior due to the Mn3+–O2−–Mn3+ complex, while the annealed samples exhibit their ferromagnetism, which originates from the Mn–H–Mn bridge structure and the Mn–Mn exchange interaction in the Mn2+–O2−–Mn2+ complex.

Sol–gel synthesis and room temperature ferromagnetism in Mn doped ZnO nanocrystals

We report the synthesis of Zn1−x Mn x O nanoparticles prepared by a sol–gel processing technique for x ranging from 0 to 0.05. The structural, morphological and magnetic properties of the nanoparticles were investigated by XRD, transmission electron microscopy and superconducting quantum interference device. The structural properties showed that the obtained nanoparticles are in wurtzite phase structure with the presence of ZnMnO3 secondary phase at higher concentration. The crystallite size decreases with increasing Mn concentration and is in the range of 35–17 nm. All the samples are ferromagnetic with clear hysteresis loop at room temperature but with low values for the saturation. It was found that increasing Mn concentration is not helping the long range ferromagnetic order, which might be due to the antiferromagnetic exchanges. This ferromagnetism order is due mainly to the defects and to the substituting of Mn in the ZnO host rather than from the secondary phases of Mn.

Local Structure Investigation of Cobalt and Manganese Doped ZnO Nanocrystals and Its Correlation with Magnetic Properties

Co and Mn doped ZnO nanocrystals have been synthesized by two different routes, viz., a wet chemical method and a microwave-assisted nonaqueous method and it has been found that the samples prepared by the former method are ferromagnetic while those prepared through the later route are paramagnetic. Systematic investigation of these doped ZnO nanocrystals has been carried out by extended X-ray absorption fine structure technique to determine the changes in the local structure at the Zn and dopant sites. Co doped samples prepared by either of the techniques show almost similar behavior, with Co substituting Zn up to a 10% doping concentration, beyond which there is a signature of Co clustering. However, in the case of Mn doped samples, Mn clustering commences at lower values of doping (∼7%) for samples prepared by microwave-assisted method, while for nanocrystals prepared by the wet chemical method, Mn–K edge X-ray absorption near edge spectroscopy measurement reveals the presence of a Mn2O3 phase at lower...

Variation of the coordination environment and its effect on the white light emission properties in a Mn-doped ZnO–ZnS complex structure

Mn-doped ZnO-ZnS complex nanocrystals were fabricated through coating of dodecanethiol on Mn-doped ZnO nanocrystals. The relationship between the component of white light emission and the coordination environments of Mn-dopants were experimentally investigated. It was shown that Mn ions mainly formed Mn(3+)O6 octahedra in as prepared Mn-doped ZnO, while the Mn(3+) ions on the surface of ZnO transferred into Mn(2+) ions at the interface between ZnO and ZnS after dodecanethiol coating. The Mn(2+)S4 tetrahedron density and the orange emission intensity increased upon enhancing the dodecanethiol content. These results provide an alternative way to optimize the white emission spectrum from nanocrystals of Mn-doped ZnS-ZnO complex structures through modulation of the coordination environment of Mn ions.

Structural and magnetic properties of Mn-doped ZnO nanocrystals

Mn-doped ZnO nanocrystals were successfully prepared using a novel sol–gel method followed by drying in autoclave under supercritical conditions. The estimated crystallite size is in the range of 30–50nm, in agreement with TEM analysis. Rietveld refinements confirm the formation of pure Mn-doped ZnO for lower Mn concentration. i.e. less than 5%. The lattice parameters increase with increasing Mn content according to Vegard's law due to the larger ionic radius of Mn2+ compared to that of Zn2+. Magnetic analysis reveals that increasing the doping level of Mn above 2% is not helping the long range ferromagnetic order in the sample but only enhancing the paramagnetic component. The paramagnetic susceptibility is found to increase linearly with increasing Mn concentration which suggests the formation of uncoupled magnetic moment. The estimated values for the magnetic moment per Mn atom are found to be in the range of 2–3.5µB/Mn. Ab-initio calculations also have been performed which showed that doping diamagnetic bulk ZnO with Mn induces ferromagnetic at room temperature, the total magnetic momentum increases with increasing Mn content whereas the magnetic moment of Mn is predicted to be in the range of 3–3.5μB/Mn atom which is consistent with the values obtained from magnetic measurements.

Effect of PEG on Structural and Magnetic Properties of Mn Doped ZnO Nanocrystals

Nanosized Mn doped ZnO samples were synthesized by co-precipitation method using Polyethylene glycol (PEG) as a capping agent. X- ray diffraction patterns confirm that the pure and Mn doped ZnO nanocrystals have wurtzite structure without any seconadary phases. Lattice parameters of pure and Mn doped ZnO nanocrystals increase slightly with increasing Mn concentration. The average crystalline size of pure and Mn doped ZnO nanocrystals are in the range of 14-18 nm. The X-ray density for pure and Mn doped ZnO sample is calculated using lattice parameters. It is found that almost static for Mn doped ZnO samples. In the Zn1-xMnx samples, room temperature magnetic hysteresis is observed and the saturation magnetization increases with increasing Mn content. However, these samples show room temperature ferromagnetic in nature. Result of the present investigation compared without PEG.

Elucidation of the enhanced ferromagnetic origin in Mn-doped zno nanocrystals embedded into a SiO2 matrix

The origin of the enhanced room temperature ferromagnetism in Mn-doped ZnO (ZnO:Mn) nanocrystals is investigated. ZnO:Mn nanocrystals, which were fabricated by using a laser irradiation method with a 248-nm KrF excimer laser, exhibited two-times increase in the spontaneous magnetization (∼0.4 emu/cm3 at 300 K) compared to the ZnO:Mn thin film (∼0.2 emu/cm3 at 300 K). The increased exchange integral of J 1/k B = 51.6 K in ZnO:Mn nanocrystals, in comparison with the ZnO:Mn thin film (J 1/k B = 46.9 K), is indicative of the enhanced ferromagnetic exchange interaction. This is attributed to the large number of acceptor defects in the SiO2-capped ZnO:Mn nanocrystals. Namely, the holes bound to the acceptor defects form microscopic bound-magneticpolarons with Mn ions; hence, long-range ferromagnetic coupling is enhanced. The results suggest that ferromagnetism in ZnO-based dilute magnetic semiconductors can be controlled by modulating the density of native point defects, which can be chemically and thermodynamically modified during the material synthesis or preparation.

Shape control of colloidal Mn doped ZnO nanocrystals and their visible light photocatalytic properties

For colloidal semiconductor nanocrystals (NCs), shape control and doping as two widely applied strategies are crucial for enhancing and manipulating their functional properties. Here we report a facile and green synthetic approach for high-quality colloidal Mn doped ZnO NCs with simultaneous control over composition, shape and optical properties. Specifically, the shape of doped ZnO NCs can be finely modulated from three dimensional (3D) tetrapods to 0D spherical nanoparticles in a single reaction scheme. The growth mechanism of doped ZnO NCs with interesting shape transition is explored. Furthermore, we demonstrate the tunable optical absorption features of Mn doped ZnO NCs by varying the Mn doping levels, and the enhanced photocatalytic performance of Mn doped ZnO NCs under visible light, which can be further optimized by delicately controlling their shapes and Mn doping concentrations. Our results provide an improved understanding of the growth mechanism of doped NCs during the growth process and can be potentially extended to ZnO NCs doped with other metal ions for various applications.

Synthesis, characterization and properties of Mn-doped ZnO nanocrystals

In this work ZnO and Mn-doped ZnO nanocrystals were synthesized by one-step aqueous solution method. X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive x-ray (EDX) analysis and photoluminescence (PL) spectroscopy have been used to characterize the samples in detail. The XRD studies revealed that the ZnO and Mn-doped ZnO had wurtzite structure (hexagonal). The composition analysis by EDX indicated the presence of small amounts of Mn. A strong and wide ultraviolet emission has been observed for the ZnO and Mn-doped ZnO nanocrystals as evidenced by the photoluminescence spectra at a wavelength of 384 and 389 nm respectively. As-synthesized ZnO and Mn-doped ZnO nanocrystals have a good crystal quality. ZnO and Mn-doped ZnO nanocrystals show diamagnetism at room temperature.

Structural, optical, electrical, and photocatalytic properties of manganese doped zinc oxide nanocrystals

Manganese-doped and undoped ZnO nanocrystals were synthesized via wet-chemical methods. The structure, physico-chemical, electrical and optical properties of the as-prepared products were characterized by using the X-ray diffraction (XRD), UV-vis diffuse reflectance spectroscopy (DRS), photoluminescence spectroscopy (PLS) and electrochemical impedance spectroscopy (EIS) techniques. The photocatalytic activity of Mn-doped ZnO nanocrystal (mixed phases) has been examined under the visible-irradiation by using photocatalytic oxidation of rhodamine B (RhB) dye as a model reaction, and compared with that of known system such as pure ZnO nanocrystal (single-phase). The results showed that Mn doped ZnO nanocrystals bleaches RhB much faster than undoped ZnO upon its exposure to the visible light. The enhancement of the photocatalytic activity was discussed as an effect due to the Mn doping in the Mn-doped ZnO semiconductors, which shifts the optical absorption edge to the visible region and alters the electron-hole pair separation conditions. These factors are responsible for the higher photocatalytic performance of Mn/ZnO composites.

Synthesis and characterization of surfactant assisted Mn2+ doped ZnO nanocrystals

We report the synthesis and characterization of Mn doped ZnO nanocrystals, both in the free standing and PVP capped particle forms. The nanocrystals size could be controlled by capping them with polyvinylpyrollidone and was estimated by X-ray diffraction and transmission electron microscopy. The chemical compositions of the products were characterized by FT-IR spectroscopy. UV–Vis absorption spectroscopy measurements reveal that the capping of ZnO leads to blue shift due to quantum confinement effect. The morphology of the particles was evaluated by Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM). Both the Thermo Gravimetric Analysis (TGA) and Differential Thermal Analysis (DTA) curves of the ZnO show no further weight loss and thermal effect at a temperature above 510°C.

The UV and blue light emission properties of Mn doped ZnO nanocrystals

We present a study of the light emission properties over wavelengths from UV to blue of Mn doped ZnO nanocrystals fabricated by means of a thermal evaporation vapor phase deposition process. The samples were grown with a Mn mole ratio in the Zn/Mn mixed source of 0% (pure ZnO sample, used as a reference), 5%, 10%, or 15% in a constant O 2/Ar gas mixture flowing at 500 °C. The pure ZnO nanocrystals exhibited a strong and predominantly UV emission peaking at 377 nm. In the photoluminescence spectra of mixed ZnO:Mn nanocrystals the major UV emission shifts from 377 to 408 nm, and a strong blue emission appears at 435 nm. The former is mainly induced by the impurity levels of Mn introduced in the band gap of the ZnO nanocrystals, while the latter is closely related to defect and Mn 2+ ions. With increasing Mn concentration the blue emission is enhanced due to the strong exchange interaction in the short range spin system and the excess impurities on the surface. The results show that the optical properties of ZnO can be tuned by the doping concentration of Mn. Mn doped ZnO nanocrystals with strong blue emission can be used in the fabrication of blue light devices.