S. K. Mandal

Electronic structure and magnetism of the diluted magnetic semiconductor Fe-doped ZnO nanoparticles

We have studied the electronic structure of Fe-doped ZnO nanoparticles, which have been reported to show ferromagnetism at room temperature, by x-ray photoemission spectroscopy, resonant photoemission spectroscopy, x-ray absorption spectroscopy, and x-ray magnetic circular dichroism (XMCD). From the experimental and cluster-model calculation results, we find that Fe atoms are predominantly in the Fe3+ ionic state with mixture of a small amount of Fe2+ and that Fe3+ ions are dominant in the surface region of the nanoparticles. It is shown that the room temperature ferromagnetism in the Fe-doped ZnO nanoparticles primarily originated from the antiferromagnetic coupling between unequal amounts of Fe3+ ions occupying two sets of nonequivalent positions in the region of the XMCD probing depth of ∼2–3 nm.

Magnetic glassy phase in Zn0.85Fe0.15O diluted magnetic semiconducting nanoparticles

The authors have investigated the ground state magnetic phase and the optical band gap of chemically synthesized Zn0.85Fe0.15O diluted magnetic semiconducting nanoparticles (∼7.5nm). The temperature dependent magnetization study shows strong irreversibility along with a cusplike anomaly, which is ascribed to a freezing to a cluster glasslike magnetic ground state. This assignment is further established by Arrott-Belov-Kouvel plots along with S-like nonsaturating magnetization versus field curves. The finite size ferromagnetic clusters which are formed due to an enhanced grain surface effect in the system undergo random dipolar intercluster interactions, giving rise to strong competitive ferromagnetic and antiferromagnetic interactions, finally leading to the freezing of those clusters.

Surface- and bulk-sensitive x-ray absorption study of the valence states of Mn and Co ions in Zn1-2xMnxCoxO nanoparticles

We have performed x-ray absorption spectroscopy (XAS) measurements on Zn1−2xMnxCoxO nanoparticles. From the XAS results, it seems that the Mn and Co ions are in a mixed-valence (2+, 3+, and 4+) state and the relative concentrations of the high-valence (3+ and 4+) Mn and Co ions are higher in the surface region than in the deep core region. We suggest that this is a distinct trend of nanoparticle diluted magnetic semiconductor (DMS) unlike the case of DMS in film and bulk forms, where the transition-metal ions are expected to be 2+.

Effect of Al concentration in grain and grain boundary region of Al-doped ZnO films: a dielectric approach

Aluminium-doped zinc oxide (ZnO : Al) films have been deposited by RF magnetron sputtering on Pyrex glass substrates. The structural, electrical and optical properties of films as a function of Al concentration have been studied. Al-doped films exhibit high optical transparency and a reduction of resistivity with increasing Al concentration. The minimum resistivity value is found to be ρ = 2.2 × 10−3 Ω cm as compared with ~104 Ω cm, for undoped ZnO. The optical band gap is observed to follow a Burstein–Moss shift with the increase in the concentration of Al doping and shows the maximum band gap of 3.53 eV. Relative variation of the grain and grain boundary scattering with doping concentration has been investigated with impedance spectroscopy.

Growth and photoluminescence characteristics of ZnO tripods

Hexagonal and preferentially oriented three-dimensional ZnO tripods have been grown on p-Si [100] substrates by a simple vapor-solid technique without using any catalysts. The tripodal-core growth route involves the formation of a hexagonal disk with [0002] larger surfaces. The tripods show irreversible shape transformation to tetrapods at higher temperature and prolonged growth time. Temperature-dependent photoluminescence characteristics of ZnO tripods have been investigated in the range from 10 to 300 K. Multiple LO phonon-assisted emissions associated with both bound and free excitons are observed for tripods, the origin of which have been explained by Permogorov’s theory for II–VI semiconductors.

Photoluminescence and electrical transport characteristics of ZnO nanorods grown by vapor-solid technique

Hexagonal shaped radial and quasialigned arrays of ZnO nanorods with diameter of about 40–60 nm have been deposited on p-Si (100) substrates by vapor-solid method using Zn as the source material. x-ray diffraction, field emission scanning electron microscopy, temperature dependent-photoluminescence, and impedance spectroscopy have been used to characterize the structural, optical and electrical transport properties of the grown nanostructures. At room temperature, a strong free excitonic emission peak at 3.311 eV with very weak defect emissions is observed. At low temperatures, near-band-edge steady-state photoluminescence spectra of ZnO nanorods are dominated by neutral-donor-bound-exciton (D0X) transitions with corresponding transverse and longitudinal optical phonon replicas. The impedance spectra as a function of bias voltage and temperature have been studied in detail. The differences in characteristics of p-Si/ZnO/Al and Al/ZnO/Al devices are discussed by using one and two RC equivalent circuits. A ...

Magnetic and optical properties of Zn1– x Fe x O ( x = 0.05 and 0.10) diluted magnetic semiconducting nanoparticles

We have investigated the magnetic and optical properties of chemically low temperature-synthesized Zn1– x Fe x O (x = 0.05 and 0.10) diluted magnetic semiconducting nanoparticles (∼7 nm). Observed magnetic behaviour of x = 0.05 samples showed that the net magnetic interaction was antiferromagnetic-like, a feature established by Curie–Weiss fit, concave Arrott–Belov–Kouvel (ABK) plots with the absence of spontaneous magnetization even at 5 K and stretched exponential-type time-dependent magnetization behaviour. Optimization of the Fe(x) dopant concentration in Zn1– x Fe x O gave the most favourable room-temperature ferromagnetism for x = 0.10, as supported by finite coercive field (∼94.4 Oe) and remanent magnetization (0.011 µB/Fe ion) from strong hysteretic magnetization vs. magnetic-field curves at room temperature. The Curie temperature of the x = 0.10 sample was estimated at ∼388 K. The existence of a room-temperature ferromagnetic phase was further established by the convex nature of the ABK plots with finite spontaneous magnetization. The observed magnetic behaviour for different x values is best explained by a magnetic polaron model.

Synthesis and temperature dependent photoluminescence properties of Mn doped Ge nanowires

Ge nanowires were grown on Au-coated Si (100) substrates using vapor-liquid-solid technique. Temperature dependent photoluminescence spectra of Mn doped nanowires showed dominant impurity emissions due to transition from an intermediate T41 to A61 ground state of Mn2+ in Ge nanowire. Magnetic hysteresis loop showed paramagnetic to ferromagnetic transition with decreasing temperature. Temperature dependent electrical resistivity indicated the formation of Mn related acceptor level in Ge at 0.159 eV from the valence band.

Magnetically tunable alternating current electrical properties of x La0.7Sr0.3MnO3–(1 − x) ErMnO3 (x = 0.1, 0.3, and 0.5) multiferroic nanocomposite

Detailed magnetically tunable ac electrical properties of x La0.7Sr0.3MnO3 (LSMO)–(1 − x) ErMnO3 (EMO) (x = 0.1, 0.3, and 0.5) multiferroic nanocomposites have been studied at 300 K in presence of varying magnetic field (Happl), applied both in parallel and perpendicular configuration with respect to the measuring electric field. AC electrical properties have exhibited significant variation with Happl for all composites, whereas for parallel configuration of Happl such effect is very feeble for x = 0.3 composite. We have attributed this anisotropic behavior to the demagnetization effect in the sample. In contrast, for x = 0.1 and 0.5 composites, no such anisotropy effect is experimentally evidenced. Impedance and real part of impedance have been found to decrease with Happl at low frequency (f) region. We attribute this observation to the depinning of the magnetic domain walls from the grain boundaries pinning centers and thereby enhancing the spin dependent transport in the composite. For x = 0.3 composi...

Enhancement of dielectric constant in transition metal doped ZnO nanocrystals

We have presented dielectric studies on Zn1−xCoxO, Zn1−xFexO, and Zn1−xFex/2Cox/2O (x = doping level) semiconducting nanoparticles (∼2–40 nm). For all those samples, dielectric constant (є) is found to exhibit a maximum with x. Enhancement of є is found to be ∼250 times for Zn0.85Co0.15O and ∼400 times for Zn0.8Fe0.1Co0.1O from that of ZnO. Presence of effective higher oxidation state of transition metals ions in ZnO due to Fe/Co doping resulting in space charge polarization possibly yields this effect. The simultaneous existence of high є, semiconducting, and ferromagnetic like behaviour at 300 K seems to propose Zn0.8Fe0.1Co0.1O, promising for technological application.