Role of electron and hole doping in the NdNi1-xVxO3 nanostructure.

Neodymium nickelate, NdNiO3, attracts attention due to the simultaneous occurrence of several phase transitions around the same temperature. The electronic properties of NdNiO3 are extremely complex as structural distortion, electron correlation, charge ordering, and orbital overlapping play significant roles in the transitions. We report the effects of electron and hole injection via doping a single 3d metal, V, in the NdNiO3 nanostructure to understand the variations in the electronic properties without any structural distortion. A reversible resistivity modulation of more than five orders of magnitude via hole doping and complete suppression of the metal to insulator transition via electron doping is observed along with the switching of major charge carriers. The modulation of electronic properties without any structural distortion and external strain opens up new directions to consider the NdNi1-xVxO3 nanostructures applicable as emerging electronic devices.

The extent of microplastic pollution along the eastern coast of India: Focussing on marine waters, beach sand, and fish.

In this study, for the first time, we evaluated microplastic contamination in water, beach sand, and fish samples collected from the seven most famous and crowded beaches of the eastern coast of India, which cover around 1200 km. The average number of microplastics found was 80 ± 33 microplastics/m3 and 4 ± 2 microplastics/kg dry weight with a numerical abundance of polyethylene and polystyrene for water and sand samples, respectively. The polymer hazard index score, which represents the severity of the microplastics scenario in the studied locations, depicts that this coastline falls under hazard levels IV and V (most hazardous) for water and sand samples, respectively. The study revealed that approximately 30 % of the commercially important fishes collected from the locations contained microplastics with polyethylene terephthalate and polypropylene being the most abundant types. Rastrelliger kanagurta and Sardinella gibbosa were identified as the most polluted species.

Novel Study of Strain-Induced Piezoelectricity in VO2.

VO2 is well known for its dual-phase transitions, electrical and structural, at a single temperature of 340 K. The low-temperature structural phases of VO2 are different from their high-temperature counterpart in terms of structural symmetry. The strain-induced modification of the structural distortion in VO2 is studied in detail. A ferroelectric-type distortion is observed, and therefore, the piezoelectric effect in the low-temperature phases of VO2 is investigated, for the first time, by piezoresponse force microscopy. Strain is one of the factors that can modify the electronic behavior of piezoelectric materials. At the same time, the two low-temperature phases of VO2 (M1 and M2) can only be separated by the application of strain. The piezoelectric coefficient in the strained phase of VO2 was found to be 11-12 pm/V, making it eligible for piezotronic applications.

Probing phase transition in VO2 with the novel observation of low-frequency collective spin excitation.

VO2 is well known for its first order, reversible, metal-to-insulator transition (MIT) along with a simultaneous structural phase transition (SPT) from a high-temperature metallic rutile tetragonal (R) to an insulating low-temperature monoclinic (M1) phase via two other insulating metastable phases of monoclinic M2 and triclinic T. At the same time, VO2 gains tremendous attention because of the half-a-century-old controversy over its origin, whether electron-electron correlation or electron-phonon coupling trigger the phase transition. In this regard, V1-xMgxO2 samples were grown in stable phases of VO2 (M1, M2, and T) by controlled doping of Mg. We have observed a new collective mode in the low-frequency Raman spectra of all three insulating M1, M2 and T phases. We identify this mode with the breather (singlet spin excitation) mode about a spin-Pierls dimerized one dimensional spin ½ Heisenberg chain. The measured frequencies of these collective modes are phenomenologically consistent with the superexchange coupling strength between V spin ½ moments in all three phases. The significant deviation of Stokes to anti-Stokes intensity ratio of this low-frequency Raman mode from the usual thermal factor exp(hʋ/KBT) for phonons, and the orthogonal dependency of the phonon and spinon vibration in the polarized Raman study confirm its origin as spin excitations. The shift in the frequency of spin-wave and simultaneous increase in the transition temperature in the absence of any structural change confirms that SPT does not prompt MIT in VO2. On the other hand, the presence of spin-wave confirms the perturbation due to spin-Peierls dimerization leading to SPT. Thus, the observation of spin-excitations resulting from 1-D Heisenberg spin-½ chain can finally resolve the years-long debate in VO2 and can be extended to oxide-based multiferroics, which are useful for various potential device applications.

Phase-pure VO2 nanoporous structure for binder-free supercapacitor performances.

Vanadium oxides are anticipated as a high-performance energy storage electrode due to their coupled double layer and pseudo-capacitative charge storage mechanism. In the present work, we investigated the influence of different structural phases of as-grown VO2 nanoporous structure and corresponding oxidation states on the supercapacitor performance. This nanoporous structure facilitates fast ion diffusion and transport. It is shown that stoichiometric monoclinic VO2, with V oxidation state of +4, provides superior charge storage capacity with a capacitance value of 33 mF/cm2, capacitance retention of 93.7% and Coulombic efficiency of 98.2%, to those for VO2 structures with mixed oxidation states of V5+ and V4+. A comparable high energy density is also recorded for the sample with all V4+. Scanning Kelvin probe microscopy results clarify further the formation of space charge region between VO2 and carbon paper. These key findings indicate the potentiality of binder-free single phase monoclinic VO2 porous structure towards the next-generation micro-supercapacitor application.

The role of 1-D finite size Heisenberg chains in increasing the metal to insulator transition temperature in hole rich VO2.

VO2 samples are grown with different oxygen concentrations leading to different monoclinic, M1, and triclinic, T, insulating phases which undergo a first order metal to insulator transition (MIT) followed by a structural phase transition (SPT) to the rutile tetragonal phase. The metal insulator transition temperature (Tc) was found to be increased with increasing native defects. Vanadium vacancy (VV) is envisaged to create local strains in the lattice which prevents twisting of the V-V dimers promoting metastable monoclinic, M2 and T phases at intermediate temperatures. It is argued that MIT is driven by strong electronic correlation. The low temperature insulating phase can be considered as a collection of one-dimensional (1-D) half-filled bands, which undergo a Mott transition to 1-D infinitely long Heisenberg spin ½ chains leading to structural distortion due to spin-phonon coupling. The presence of VV creates localized holes (d0) in the nearest neighbor, thereby fragmenting the spin ½ chains at the nanoscale, which in turn increases the Tc value more than that of an infinitely long one. The Tc value scales inversely with the average size of the fragmented Heisenberg spin ½ chains following a critical exponent of ⅔, which is exactly the same as predicted theoretically for the Heisenberg spin ½ chain at the nanoscale undergoing SPT (spin-Peierls transition). Thus, the observation of MIT and SPT at the same time in VO2 can be explained from our phenomenological model of reduced 1-D Heisenberg spin ½ chains. The reported increase (decrease) in the Tc value of VO2 by doping with metals having valency less (more) than four can also be understood easily with our unified model, for the first time, considering finite size scaling of Heisenberg chains.