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[en] The effect of high electronic energy deposition on the structure, surface topography, optical properties, and electronic structure of cadmium sulfide (CdS) thin films have been investigated by irradiating the films with 100 MeV Ag+7 ions at different ion fluences in the range of 1012-1013 ions/cm2. The CdS films were deposited on glass substrate by thermal evaporation, and the films studied in the present work are polycrystalline with crystallites preferentially oriented along (002)-H direction. It is shown that swift heavy ion (SHI) irradiation leads to grain agglomeration and hence an increase in the grain size at low ion fluences. The observed lattice compaction was related to irradiation induced polygonization. The optical band gap energy decreased after irradiation, possibly due to the combined effect of change in the grain size and in the creation of intermediate energy levels. Enhanced nonradiative recombination via additional deep levels, introduced by SHI irradiation was noticed from photoluminescence (PL) analysis. A shift in the core levels associated with the change in Fermi level position was realized from XPS analysis. The chemistry of CdS film surface was studied which showed profound chemisorption of oxygen on the surface of CdS. (orig.)
[en] The effect of low energy argon plasma treatment on the surfaces of polyethylene terephthalate (PET) was investigated by means of contact angle measurement, X-ray photoelectron spectroscopy (XPS), Vickers’ microhardness indentation and atomic force microscopy (AFM). It was observed that the surface free energy (SFE) changes from 42.1 mJ/m2 to 85.1 mJ/m2 with the increase of plasma treatment time and the corresponding contact angle changes from 60o to 15o. The increase in SFE after plasma treatment is attributed to the functionalization of the polymer surface with hydrophilic groups. The XPS analysis shows the increase in C–O and C=O groups after plasma treatment of the polymer surface. The microhardness of the film increases with the treatment time. This may be attributed to the cross linking effect at the surface. Atomic force microscopy (AFM) reveals that average surface roughness increases from 5.8 nm to 49.7 nm as treatment time increases.
[en] There are several confronting reports of room temperature ferromagnetic (FM) ordering in bulk as well as thin films of dilutely doped or even some undoped semiconductors. We have synthesized and characterized dilute Mn-doped (2 and 4%) ZnO pellets. SQUID measurements confirm that the 2% Mn sample shows the FM ordering above the room temperature and the FM contribution coming mainly from the bulk. However, the ordering gets completely quenched for 4% Mn doping. Upon cooling down, the 2% Mn-doped sample shows further enhancement in the magnetic properties while the 4% sample did not show any FM ordering down to 5 K. The powerful X-ray photoemission spectroscopy (XPS) was employed to compare the electronic structure of these two samples. The XPS results show that the manganese shifts toward the higher valence state upon Mn doping while there is no change in the zinc and oxygen valence. The atomic concentration of divalent Mn state is found to be dominant in the ferromagnetic sample. For the non-ferromagnetic sample, a larger contribution of higher oxidation Mn states is present that is correlated to the suppressed ferromagnetism. Interestingly, the oxygen content is also found to be higher in the 4% Mn sample than that in the 2% Mn sample that has been attributed to the charge neutrality of the samples. The present study provides evidence that the magnetization originates neither from any precipitating secondary phase nor from the oxygen content but the Mn2+ state plays a significant role for the FM properties in the Mn-doped ZnO system.
[en] Through simple hydrogen annealing treatment, we observed robust inducement of room temperature ferromagnetism (RTFM) in ZnO:Co (5%) pellets. The hydrogen mediated magnetic transition is accompanied by electronic structure plus bonding modifications with no structural deviations or creation of secondary phases, as evidenced by XRD and photoemission investigations. Our findings reveal a route correlation of oxygen vacancies with the observed RTFM. In particular, we systematically investigated the time controlled re-heating consequences on hydrogenated sample. The H-induced RTFM and subsequent modifications viz. electronic structure, transport properties and bonding effect gradually retrace back upon evaporating the hydrogen.
[en] The effect of low level Co doping (5%) on polycrystalline ZnO samples has been investigated to correlate the observed changes in their magnetic state vis a vis changes in their electronic properties. Rietveld refinement of the XRD patterns confirms single phase crystallization of the samples in the wurtzite type lattice, with no evidence of any secondary phases. The as-synthesized Co-doped sample shows a paramagnetic (PM) state, however, when hydrogenated for ∼6 h, it shows a strong ferromagnetic (FM) ordering. The magnetic moment suppressed significantly when hydrogen ions were evaporated by heating and the sample turned completely paramagnetic upon long heating in air. The Co 2p X-ray photoelectron spectroscopy (XPS) results show that the substituted Co ions are in 2+ oxidation state that incorporate at Zn2+ sites and no evidence of metallic Co is observed upon hydrogenation. The XRD refinement results and the O 1s XPS results show clear evidence of oxygen depletion upon hydrogenation, followed by a complete regain upon their long heating in air. A plausible explanation for the observed room temperature ferromagnetism (RTFM) is presented in terms of oxygen vacancies, in the framework of bound magnetic polarons model. Our results evidence that the FM ordering can be switched between 'on' and 'off' by introducing (upon hydrogenation) or removing (by re-heating), respectively, the oxygen vacancies in the Co-doped ZnO matrix.
[en] The paper reports the application of supersonic thermal plasma expansion technique for controlled chemical synthesis of non-agglomerated, mono-dispersed nanoparticles of TiO2 and also deposition of a nanocrystalline porous coating in a single-step, continuous process. The phase transformation of the nanoparticles was investigated as a function of feed rates of the basic reactants. The possibility of the particle charging and its consequences are also discussed. The grain structure of the coating was found to retain the characteristics of the seed nanoparticles. Optical Emission Spectroscopy performed at the injection section of the reactor identified the presence of TiO and TiCl molecules, which are suspected to be intermediates in this chemical route.
[en] Effect of injection of hydrogen ions, followed by their evaporation, has been investigated in the Zn1-xFexO (x = 0.02-0.07) pellets to throw some light on electronic structure and magnetic correlations. The XRD patterns show that x ≤ 0.05 samples are single phase and the Fe ions incorporate at the Zn2+ sites, while a secondary phase ZnFe2O4 is detected for x ≥ 0.07. The 2% Fe-doped sample retains a paramagnetic ground state down to 50 K. Likewise, the 5% doped sample also shows paramagnetic state at 300 K but a weak ferromagnetic ordering stems from its cooling (Tc ∼ 160 K). Strikingly, the 5% doped sample, when annealed in hydrogen atmosphere, showed inducement of room temperature ferromagnetism. More significantly, the hydrogen-induced magnetism disappears upon evaporating the H ions by re-heating the sample. The magnetic ordering and the electronic properties exhibit a close parallelism/interplay. The X-ray photoemission spectroscopy results testify the Fe to be in mixed valent state (>2+) in paramagnetic state, however, the ferromagnetic transition stems only upon Fe3+ reducing to Fe2+, accompanied by emergence of oxygen vacancies as a parallel electronic phenomenon. Origin of H-mediated ferromagnetism is discussed in the framework of cationic vs. anionic vacancies and it is suggested that oxygen vacancies play major role in mediating the coupling.
[en] We report on the reversible manipulation of room temperature ferromagnetism in Fe (5%) doped In2O3 polycrystalline magnetic semiconductor. The X-ray diffraction and photoemission measurements confirm that the Fe ions are well incorporated into the lattice, substituting the In3+ ions. The magnetization measurements show that the host In2O3 has a diamagnetic ground state, while it shows weak ferromagnetism at 300 K upon Fe doping. The as-prepared sample was then sequentially annealed in hydrogen, air, vacuum and finally in air. The ferromagnetic signal shoots up by hydrogenation as well as vacuum annealing and bounces back upon re-annealing the samples in air. The sequence of ferromagnetism shows a close inter-relationship with the behavior of oxygen vacancies (Vo). The Fe ions tend to a transform from 3+ to 2+ state during the giant ferromagnetic induction, as revealed by photoemission spectroscopy. A careful characterization of the structure, purity, magnetic, and transport properties confirms that the ferromagnetism is due to neither impurities nor clusters but directly related to the oxygen vacancies. The ferromagnetism can be reversibly controlled by these vacancies while a parallel variation of carrier concentration, as revealed by resistance measurements, appears to be a side effect of the oxygen vacancy variation.
[en] Simultaneous self-diffusion measurements of Fe and N in amorphous Fe86N14 alloy using secondary ion mass spectroscopy (SIMS) are reported. In addition, neutron reflectivity (NR) was used to study the Fe self-diffusion in the same compound. The broadening of a tracer layer of 57Fe8615N14 sandwiched between Fe86N14 layers was observed by SIMS measurements after the annealing of films at different temperatures. A decay of the Bragg peak intensity after isothermal annealing was also observed in [Fe86N14/57Fe86N14]10 multilayers in NR. It was observed by SIMS measurements that Fe diffusion was about 2 orders of magnitude smaller than N, even though the structural relaxation times for Fe and N were almost identical. This is an important result, indicating that the relaxation time of diffusion is basically driven by the relaxation of the structure itself
[en] Influence of Co doping for In in In2O3 matrix has been investigated to study the effect on magnetic vs. electronic properties. Rietveld refinement of X-ray diffraction patterns confirmed formation of single phase cubic bixbyite structure without any parasitic phase. Photoelectron spectroscopy and refinement results further revealed that dopant Co2+ ions are well incorporated at the In3+ sites in In2O3 lattice and also ruled out formation of cluster in the doped samples. Magnetization measurements infer that pure In2O3 is diamagnetic and turns to weak ferromagnetic upon Co doping. Hydrogenation further induces a huge ferromagnetism at 300 K that vanishes upon re-heating. Experimental findings confirm the induced ferromagnetism to be intrinsic, and the magnetic moments to be associated with the point defects (oxygen vacancies Vo) or bound magnetic polarons around the dopant ions.