Saif Ullah Awan

Ferromagnetism in Li doped ZnO nanoparticles: The role of interstitial Li

ZnO nanoparticles doped with Li (Zn1−yLiyO, y ≤ 0.1) have been investigated with emphasis on the correlation between their magnetic, electronic, and structural properties. In particular, defects such as interstitial Li and Zn atoms, substitutional Li atoms, and oxygen vacancies have been identified by X-ray photoelectron spectroscopy (XPS) and their respective roles in stabilization of the magnetic moment are discussed. X-ray diffraction (XRD) and XPS give clear evidence of Li presence at both substitutional and interstitial sites. XPS studies further show that the amount of substitutional Li defects (Lizn) and interstitial Li defects (Lii) vary non-monotonically with the Li concentration, with the Lii defects being noticeably high for the y = 0.02, 0.08, and 0.10 concentrations, in agreement with the XRD results. Magnetization studies show room temperature ferromagnetism in these nanoparticles with the moment being largest for the particles with high concentration of interstitial lithium and vice versa. Both interstitial Zn (Zni) defects and Zn-O bonds were determined from the Zn LMM Auger peaks; however, the variation of these with Li concentrations was not large. Oxygen vacancies (Vo) concentrations are estimated to be relatively constant over the entire Li concentration range. We relate the Lii and Zni defects to the formation and stabilization of Zn vacancies and thus stabilizing the p-type ferromagnetism predicted for cation (zinc) vacancy in the ZnO type oxides.ZnO nanoparticles doped with Li (Zn1−yLiyO, y ≤ 0.1) have been investigated with emphasis on the correlation between their magnetic, electronic, and structural properties. In particular, defects such as interstitial Li and Zn atoms, substitutional Li atoms, and oxygen vacancies have been identified by X-ray photoelectron spectroscopy (XPS) and their respective roles in stabilization of the magnetic moment are discussed. X-ray diffraction (XRD) and XPS give clear evidence of Li presence at both substitutional and interstitial sites. XPS studies further show that the amount of substitutional Li defects (Lizn) and interstitial Li defects (Lii) vary non-monotonically with the Li concentration, with the Lii defects being noticeably high for the y = 0.02, 0.08, and 0.10 concentrations, in agreement with the XRD results. Magnetization studies show room temperature ferromagnetism in these nanoparticles with the moment being largest for the particles with high concentration of interstitial lithium and vice versa. ...

Defects induced luminescence and tuning of bandgap energy narrowing in ZnO nanoparticles doped with Li ions

Microstructural and optical properties of Zn1−yLiyO (0.00 ≤ y ≤ 0.10) nanoparticles are investigated. Li incorporation leads to substantial changes in the structural characterization. From micro-structural analysis, no secondary phases or clustering of Li was detected. Elemental maps confirmed homogeneous distribution of Li in ZnO. Sharp UV peak due to the recombination of free exciton and defects based luminescence broad visible band was observed. The transition from the conduction band to Zinc vacancy defect level in photoluminescence spectra is found at 518 ± 2.5 nm. The yellow luminescence was observed and attributed to Li related defects in doped samples. With increasing Li doping, a decrease in energy bandgap was observed in the range 3.26 ± 0.014 to 3.17 ± 0.018 eV. The bandgap narrowing behavior is explained in terms of the band tailing effect due to structural disorder, carrier-impurities, carrier-carrier, and carrier-phonon interactions. Tuning of the bandgap energy in this class of wide bandgap...

Room temperature p-type conductivity and coexistence of ferroelectric order in ferromagnetic Li doped ZnO nanoparticles

Memory and switching devices acquired new materials which exhibit ferroelectric and ferromagnetic order simultaneously. We reported multiferroic behavior in Zn1−yLiyO(0.00≤y≤0.10) nanoparticles. The analysis of transmission electron micrographs confirmed the hexagonal morphology and wurtzite crystalline structure. We investigated p-type conductivity in doped samples and measured hole carriers in range 2.4 × 1017/cc to 7.3 × 1017/cc for different Li contents. We found that hole carriers are responsible for long range order ferromagnetic coupling in Li doped samples. Room temperature ferroelectric hysteresis loops were observed in 8% and 10% Li doped samples. We demonstrated ferroelectric coercivity (remnant polarization) 2.5 kV/cm (0.11 μC/cm2) and 2.8 kV/cm (0.15 μC/cm2) for y = 0.08 and y = 0.10 samples. We propose that the mechanism of Li induced ferroelectricity in ZnO is due to indirect dipole interaction via hole carriers. We investigated that if the sample has hole carriers ≥5.3 × 1017/cc, they can ...

Doxorubicin-loaded photosensitive magnetic liposomes for multi-modal cancer therapy

Multifunctional magnetic nanosystems have attracted an enormous attention of researchers for their potential applications in cancer diagnostics and therapy. The localized nanotherapies triggered by the external stimuli, like magnetic fields and visible light, are significant in clinical applications. We report a liposomal system that aims to treat cancer by magnetic hyperthermia, photodynamic therapy and chemotherapy simultaneously. The liposomes enclose clinically used photosensitizer m-THPC (Foscan) and anti-cancer drug doxorubicin, in its hydrophobic lipid bilayers, and contains magnetite nanoparticles in hydrophilic core. Three different sizes of magnetic nanoparticles (10, 22 and 30nm) and liposomes (40, 70 and 110nm) were used in this study. Magnetite single domain nanoparticles forming the magnetic core were superparamagnetic but liposomes expressed slight coercivity and hysteresis due to the clustering of nanoparticles in the core. This enhanced the heating efficiency (specific power loss) of the liposomes under an AC field (375kHz, 170Oe). Cell viability and toxicity were studied on HeLa cells using MTT assay and proteomic analysis. Confocal and fluorescence microscopy were used to study the photosensitizers profile and cells response to combined therapy. It revealed that combined therapy almost completely eliminated the cancer cells as opposed to the separate treatments. Magnetic hyperthermia and photodynamic therapies were almost equally effective whereas chemotherapy showed the least effect.

Carrier concentration dependence of ferroelectric transition in multiferroic Li doped and Li-Co co-doped ZnO nanoparticles

Dielectric measurements on both Li doped and Li-Co co-doped multiferroic nanoparticles are presented and correlated with the hole carrier concentrations measured by the Hall effect. The ferroelectric Curie temperatures lay in the range 443–512 K with the dielectric constant increasing monotonically with Li concentration. However, we find that both for these p-type systems, the Curie temperature varies non-monotonically with Li concentration, being larger for compositions with higher hole concentrations and vice versa. We find a similar trend in the variation of the magnetic moment with hole concentration. Hence, the multiferroic behavior in this system is strongly correlated with the hole concentration. The ferroelectric behavior is explained in terms of the model of electric dipoles, formed by Li off-centre impurities, interacting indirectly via the free hole carriers. The variation of the ferroelectric critical temperature with hole concentration is explained within this model in terms of the dependence...

Raman scattering and interstitial Li defects induced polarization in co-doped multiferroic Zn0.96-yCo0.04LiyO (0.00 ≤ y ≤ 0.10) nanoparticles

Structural and Raman analysis confirmed a single phase wurtzite hexagonal crystalline structure of Li co-doped ZnO nanoparticles. In the Raman backscattering spectra, ELow2and EHigh2 modes corresponded to zinc and oxygen lattice vibrations. No extra vibration modes of secondary or metallic phases of Co or Li were observed. The intensity of Raman modes decreased with increasing Li content. High resolution X-ray photoelectron spectroscopy (XPS) of Zn and Co confirmed the +2 oxidation state. The deconvolution of high resolution XPS spectra of Li showed the presence of interstitial and substitutional Li defects. The room temperature polarization lay in the range of 0.155–0.225 μC cm−2 for different compositions. We observed an interesting result in the co-doped system, that even the low Li concentration sample, e.g., Zn0.94Co0.04Li0.02O, showed a P–E hysteresis loop of polarization at ∼0.20 μC cm−2. There was higher polarization for y = 0.10, due to more interstitial Li defects, and lower polarization for y = 0.06, due to more substitutional Li defects. We suggest that polarization might have appeared and been enhanced due to the net interaction of dipoles, formed by Li off-center impurities (interstitial Li defects).

Cu co-doping effect on electronic structure and room temperature ferromagnetism of TiO2:V nanoparticles

We have synthesized co-doped Ti0.94−yV0.06CuyO2 (0 ≤ y ≤ 0.02) nanoparticles by sol–gel. Single rutile phase crystal structure has been confirmed from X-ray diffraction patterns. Decreasing trend of bandgap, resistivity and activation energy with increasing Cu co-dopant has been observed in co-doped samples. We found that the ferromagnetism of the V doped TiO2 in the rutile phase is initially strongly suppressed for very low Cu concentration but with increasing Cu content the moment increases very significantly. The non-monotonic changes in the magnetization effects are attributed to the role of Cu in introducing oxygen vacancies, i.e. excess electrons due to its lower valence compared to Ti and V. The role of oxygen vacancies in the formation of magnetic polarons may be the reason of inducing and mediation of the ferromagnetic exchange between the transition metal ions. This interpretation is supported by the resistivity and optical measurements that both reveal the existence of low lying electronic states in the Cu doped systems that lead to lowering of resistivity and the lowering of the activation energy for electronic conduction. Hence it is apparent that variable valence of the dopants and their effects on the oxygen vacancies have an important role in determining the electronic properties of magnetically doped TiO2 semiconductors.