An aqueous method for the controlled manganese (Mn(2+)) substitution in superparamagnetic iron oxide nanoparticles for contrast enhancement in MRI.

Despite the success in the use of superparamagnetic iron oxide nanoparticles (SPION) for various scientific applications, its potential in biomedical fields has not been exploited to its full potential. In this context, an in situ substitution of Mn(2+) was performed in SPION and a series of ferrite particles, MnxFe1-xFe2O4 with a varying molar ratio of Mn(2+) : Fe(2+) where 'x' varies from 0-0.75. The ferrite particles obtained were further studied in MRI contrast applications and showed appreciable enhancement in their MRI contrast properties. Manganese substituted ferrite nanocrystals (MnIOs) were synthesized using a novel, one-step aqueous co-precipitation method based on the use of a combination of sodium hydroxide and trisodium citrate (TSC). This approach yielded the formation of highly crystalline, superparamagnetic MnIOs with good control over their size and bivalent Mn ion crystal substitution. The presence of a TSC hydrophilic layer on the surface facilitated easy dispersion of the materials in an aqueous media. Primary characterizations such as structural, chemical and magnetic properties demonstrated the successful formation of manganese substituted ferrite. More significantly, the MRI relaxivity of the MnIOs improved fourfold when compared to SPION crystals imparting high potential for use as an MRI contrast agent. Further, the cytocompatibility and blood compatibility evaluations demonstrated excellent cell morphological integrity even at high concentrations of nanoparticles supporting the non-toxic nature of nanoparticles. These results open new horizons for the design of biocompatible water dispersible ferrite nanoparticles with good relaxivity properties via a versatile and easily scalable co-precipitation route.

Resonance Raman spectroscopy of twisted interfaces in turbostratic multilayer graphene.

Turbostratic multilayer graphene presents a unique system with a large number of twisted interfaces with variable twist angles. In this work, we have systematically studied the laser excitation energy dependence of the Raman modes of turbostratic graphene. The combination of 4 different laser energies is shown to be important to reveal the twist angles ranging from 5∘to 30∘present at the same lateral position of the sample. Rotational or R-modes and D-like modes are observed, which directly arise from additional momentum transfer from the potential of corresponding superlattices. Trends in their dispersion and intensity are discussed. The resonant window for laser excitation indicates lowered positions of the van Hove singularities. Furthermore, an anomalous broadening factor of 0.17-0.265 eV is estimated for the resonance window when compared to the literature on isolated twisted bilayer graphene. Interestingly, a weak dependence of the R-modes on the laser wavelength is also observed. Finally, the dispersion of the 2D modes is also presented.

Tunable charge transport properties in non-stoichiometric SrIrO3thin films.

Delving into the intricate interplay between spin-orbit coupling and Coulomb correlations in strongly correlated oxides, particularly perovskite compounds, has unveiled a rich landscape of exotic phenomena ranging from unconventional superconductivity to the emergence of topological phases. In this study, we have employed pulsed laser deposition technique to grow SrIrO3(SIO) thin films on SrTiO3substrates, systematically varying the oxygen content during the post-deposition annealing. X-ray photoelectron spectroscopy (XPS) provided insights into the stoichiometry and spin-orbit splitting energy of Iridium within the SIO film, while high-resolution x-ray studies meticulously examined the structural integrity of the thin films. Remarkably, our findings indicate a decrease in the metallicity of SIO thin films with reduced annealing O2partial pressure. Furthermore, we carried out magneto-transport studies on the SIO thin films, the results revealed intriguing insights into spin transport as a function of oxygen content. The tunability of the electronic band structure of SIO films with varying oxygen vacancy is correlated with the density functional theory calculations. Our findings elucidate the intricate mechanisms dictating spin transport properties in SIO thin films, offering invaluable guidance for the design and optimization of spintronic devices based on complex oxide materials. Notably, the ability to tune bandwidth by varying post-annealing oxygen partial pressure in iridate-based spintronic materials holds significant promise for advancing technological applications in the spintronics domain.

Interplay of piezoresponse and magnetic behavior in Bi0.9A0.1FeO2.95 (A = Ba, Ca) and Bi0.9Ba0.05Ca0.05FeO2.95 co-doped ceramics.

Extensive piezoresponse force microscopy (PFM) and magnetic force microscopy (MFM) measurements in conjunction with piezoresponse spectroscopy have been carried out on pellets of Bi0.9A0.1FeO2.95 (A = Ba, Ca) and Bi0.9Ba0.05Ca0.05FeO2.95 co-doped ceramic samples in order to characterize their ferroelectric and magnetic nature and correlate the findings with our recent far-infrared spectroscopic studies on these samples. We are able to clearly discern the switching behavior of the 71° and 109° ferroelectric domains as distinct from that of the 180° domains in both pristine and Ba-doped bismuth ferrite samples. While substitution of Ba at the Bi site in bismuth ferrite does not affect the ferroelectric and magnetic properties to a great extent, Ca-doped samples show a decrease in their d 33 values with a concomitant increase in their magnetic behavior. These results are in agreement with the findings from our far-infrared studies.

Metal-insulator transition in epitaxial Ga-doped ZnO films via controlled thickness.

Understanding and tuning of metal-insulator transition (MIT) in oxide systems is an interesting and active research topics of condensed matter physics. We report thickness dependent MIT in Ga-doped ZnO (Ga:ZnO) thin films grown by pulsed laser deposition technique. From the electrical transport measurements, we find that while the thinnest film (6 nm) exhibits a resistivity of 0.05 Ω cm, lying in the insulating regime, the thickest (51 nm) has resistivity of 6.6 × 10-4 Ω cm which shows metallic type of conduction. Our analysis reveals that the Mott's variable range hopping model governs the insulating behavior in the 6 nm film whereas the 2D weak localization (WL) phenomena is appropriate to explain the electron transport in the thicker Ga:ZnO films. Magnetoresistance study further confirms the presence of strong localization in 6 nm film while WL is observed in 20 nm and above thicker films. From the density functional calculations, it is found that due to surface reconstruction and Ga doping, strong crystalline disorder sets in very thin films to introduce localized states and thereby, restricts the donor electron mobility.

Thermal transport across wrinkles in few-layer graphene stacks.

Wrinkles significantly influence the physical properties of layered 2D materials, including graphene. In this work, we examined thermal transport across wrinkles in vertical assemblies of few-layer graphene crystallites using the Raman optothermal technique supported by finite-element analysis simulations. A high density of randomly oriented uniaxial wrinkles were frequently observed in the few-layer graphene stacks which were grown by chemical vapor deposition and transferred on Si/SiO2 substrates. The thermal conductivity of unwrinkled regions was measured to be, κ ∼ 165 W m-1 K-1. Measurements at the wrinkle sites revealed local enhancement of thermal conductivity, with κ ∼ 225 W m-1 K-1. Furthermore, the total interface conductance of wrinkled regions decreased by more than an order of magnitude compared to that of the unwrinkled regions. The physical origin of these observations is discussed based on wrinkle mediated decoupling of the stacked crystallites and partial suspension of the film. Wrinkles are ubiquitous in layered 2D materials, and our work demonstrates their strong influence on thermal transport.

g-C3N4/Ca2Fe2O5 heterostructures for enhanced photocatalytic degradation of organic effluents under sunlight.

g-C3N4/Ca2Fe2O5 heterostructures were successfully prepared by incorporating g-C3N4 into Ca2Fe2O5 (CFO). As prepared g-C3N4/CFO heterostructures were initially utilized to photodegrade organic effluent Methylene blue (MB) for optimization of photodegradation performance. 50% g-C3N4 content in CFO composition showed an enhanced photodegradation efficiency (~ 96%) over g-C3N4 (48.15%) and CFO (81.9%) due to mitigation of recombination of photogenerated charge carriers by Type-II heterojunction. The optimized composition of heterostructure was further tested for degradation of Bisphenol-A (BPA) under direct sunlight, exhibiting enhanced photodegradation efficiency of about 63.1% over g-C3N4 (17%) and CFO (45.1%). The photoelectrochemical studies at various potentials with and without light illumination showed significant improvement in photocurrent response for g-C3N4/Ca2Fe2O5 heterostructures (~ 1.9 mA) over CFO (~ 67.4 µA). These studies revealed efficient solar energy harvesting ability of g-C3N4/Ca2Fe2O5 heterostructures to be utilized for organic effluent treatment.

Single-ion anisotropy driven splitting of spin wave resonances in BiFeO3 at low temperature.

The spin wave resonances of BiFeO3 ceramics have been followed at low temperature through far-infrared reflectance measurements. Following the scheme of Fishman et al (2015 Phys. Rev. B 92 094422) we have been able to assign all the spin wave modes observed. A complete lifting of the degeneracies of all these modes is seen at 250 K concomitant with the increase in single-ion anisotropy. For the first time, all the spin wave modes have been observed in the infrared spectra of BiFeO3. Correlated changes in the strength and frequencies of spin wave excitations with the reported magnetic transitions at low temperature are observed. A simultaneous increase in anharmonicity of the magnetic cycloid and single-ion anisotropy with decreasing temperature results in a partial suppression of the spin wave excitations. An increase in the magnetoelectric coupling is also observed below 150 K.

Growth of sillenite Bi12FeO20 single crystals: structural, thermal, optical, photocatalytic features and first principle calculations.

Ideal sillenite type Bi12FeO20 (BFO) micron sized single crystals have been successfully grown via inexpensive hydrothermal method. The refined single crystal X-ray diffraction data reveals cubic Bi12FeO20 structure with single crystal parameters. Occurrence of rare Fe4+ state is identified via X-ray photoelectron spectroscopy (XPS) and X-ray absorption spectroscopy (XAS). The lattice parameter (a) and corresponding molar volume (Vm) of Bi12FeO20 have been measured in the temperature range of 30-700 °C by the X-ray diffraction method. The thermal expansion coefficient (α) 3.93 × 10-5 K-1 was calculated from the measured values of the parameters. Electronic structure and density of states are investigated by first principle calculations. Photoelectrochemical measurements on single crystals with bandgap of 2 eV reveal significant photo response. The photoactivity of as grown crystals were further investigated by degrading organic effluents such as Methylene blue (MB) and Congo red (CR) under natural sunlight. BFO showed photodegradation efficiency about 74.23% and 32.10% for degrading MB and CR respectively. Interesting morphology and microstructure of pointed spearhead like BFO crystals provide a new insight in designing and synthesizing multifunctional single crystals.

EXAFS and XANES investigation of (Li, Ni) codoped ZnO thin films grown by pulsed laser deposition.

Ni doped, Li doped and (Li, Ni) codoped ZnO thin films were successfully grown using a pulsed laser deposition technique. Undoped and doped ZnO thin films were investigated using extended x-ray absorption fine structure (EXAFS) and x-ray absorption near edge spectroscopy (XANES). Preliminary investigations on the Zn K-edge of the undoped and doped ZnO thin films revealed that doping has not influenced the average Zn-Zn bond length and Debye-Waller factor. This shows that both Ni and Li doping do not appreciably affect the average local environment of Zn. All the doped ZnO thin films exhibited more than 50% of substitutional Ni, with a maximum of 77% for 2% Ni and 2% Li doped ZnO thin film. The contribution of Ni metal to the EXAFS signal clearly reveals the presence of Ni clusters. The Ni-Ni distance in the Ni(0) nanoclusters, which are formed in the film, is shorter with respect to the reference Ni metal foil and the Debye-Waller factor is higher. Both facts perfectly reflect what is expected for metal nanoparticles. At the highest doping concentration (5%), the presence of Li favors the growth of a secondary NiO phase. Indeed, 2% Ni and 5% Li doped ZnO thin film shows %Nisub = 75 ± 11, %Nimet = 10 ± 8, %NiO = 15 ± 8. XANES studies further confirm that the substitutional Ni is more than 50% in all the samples. These results explain the observed magnetic properties.