Dipankar Saha

In Situ Total X‐Ray Scattering Study of WO3 Nanoparticle Formation under Hydrothermal Conditions

Pair distribution function analysis of in situ total scattering data recorded during formation of WO3 nanocrystals under hydrothermal conditions reveal that a complex precursor structure exists in solution. The WO6 polyhedra of the precursor cluster undergo reorientation before forming the nanocrystal. This reorientation is the critical element in the formation of different hexagonal polymporphs of WO3.

Synthesis and structure of Bi2Ce2O7: a new compound exhibiting high solar photocatalytic activity

A facile method of solution combustion was used to synthesize a new solid solution Bi(2)Ce(2)O(7). The structure was determined from powder X-ray diffraction (PXRD) and found to crystallize in the space group Fm3m with cell parameter a = 5.46936(9) Å. The particle sizes varied from 5 to 6 nm. The degradation of cationic dye malachite green (MG) was investigated under solar radiation as the band gap of the material is 2.34 eV.

Synthesis, structure, characterization and photocatalytic activity of Bi2Zr2O7 under solar radiation

Bi2Zr2O7 was synthesized via a facile solution combustion method. Two different fuels, urea and tartaric acid were used in the synthesis, which resulted in Bi2Zr2O7 crystals with different band gaps and surface areas. The structure has been determined by Rietveld refinement followed by the difference Fourier technique. The compound crystallizes in the space group Fmm. The photocatalytic degradation of two dyes was carried out under solar radiation. Bi2Zr2O7 prepared using urea as the fuel exhibits a higher photocatalytic activity than the compound prepared using tartaric acid and comparable activity to that of commercial Evonik P-25 TiO2. It is suggested that this is due to the oxygen vacancies occurring in the two cases, the urea based compound has an occupancy of 0.216, whereas the tartaric acid based synthesis shows disorder in the oxygen position amounting to a small number of oxygen vacancies.

Atomistic modeling of the metallic-to-semiconducting phase boundaries in monolayer MoS2

Recent experimental demonstration on the coexistence of metallic and semiconducting phases in the same monolayer MoS2 crystal has attracted much attention for its use in ultra-low contact resistance-MoS2 transistors. However, the electronic structures of the metallic-to-semiconducting phase boundaries, which appear to dictate the carrier injection in such transistors, are not yet well understood. In this letter, interfacing the 2H and 1T′ polytypes appropriately, we first model the “beta” (β) and the “gamma” (γ) phase boundaries, and demonstrate good agreement with experiential results. We then apply first-principles based density functional theory to calculate the electronic structures for those optimized geometries. We further employ non equilibrium Greens function formalism to evaluate the transmission spectra and the local density of states (LDOS) in order to assess the Schottky barrier nature of the phase boundaries. Our study reveals that while the γ boundary yields p-type Schottky barrier, the β boundary leads to the distinct symmetric Schottky barrier with an atomically sharp transition region. This understanding could be useful for designing high performance transistors using phase-engineered MoS2 crystals.

Towards atomistic understanding of polymorphism in the solvothermal synthesis of ZrO

Varying atomic short-range order is correlated with the ratio of the monoclinic (m) to tetragonal (t) phase in ZrO2 nanoparticle formation by solvothermal methods. Reactions from Zr oxynitrate in supercritical methanol and Zr acetate in water (hydrothermal route) were studied in situ by X-ray total scattering. Irrespective of the Zr source and solvent, the structure of the precursor in solution consists of edge-shared tetramer chains. Upon heating, the nearest-neighbor Zr-O and Zr-Zr distances shorten initially while the medium-range connectivity is broken. Depending on the reaction conditions, the disordered intermediate transforms either rapidly into m-ZrO2, or more gradually into mixed m- and t-ZrO2 with a concurrent increase of the shortest Zr-Zr distance. In the hydrothermal case, the structural similarity of the amorphous intermediate and m-ZrO2 favors the formation of almost phase-pure m-ZrO2 nanoparticles with a size of 5 nm, considerably smaller than the often-cited critical size below which the tetragonal is assumed to be favoured. Pair distribution function analysis thus unravels ZrO2 phase formation on the atomic scale and in this way provides a major step towards understanding polymorphism of ZrO2 beyond empirical approaches.

_2

High resolution synchrotron X-ray diffraction, dielectric and Raman scattering study of a scheelite compound Li0.5Ce0.5MoO4 (LCM) revealed that it transforms to a self similar structure above 400 °C. The thermally induced isostructural phase transition (IPT), a phenomenon which has rarely been reported in the literature, is preceded by partial softening of the zone centre phonons followed by their hardening above the IPT transition temperature. The high temperature isostructural phase, which exhibits expanded lattice parameters and cell volume, nucleates and grows in the low temperature matrix over a very wide temperature range. Both the phases show nearly identical thermal expansion suggesting similarities in symmetry, unaltered coordination environments around the atoms across the transition.

nanoparticles

We present a closed-form continuous model for the electrical conductivity of a single layer graphene (SLG) sheet in the presence of short-range impurities, long-range screened impurities, and acoustic phonons. The validity of the model extends from very low doping levels (chemical potential close to the Dirac cone vertex) to very high doping levels. We demonstrate complete functional relations of the chemical potential, polarization function, and conductivity with respect to both doping level and temperature (\(T\) ), which were otherwise developed for SLG sheet only in the very low and very high doping levels. The advantage of the continuous conductivity model reported in this paper lies in its simple form which depends only on three adjustable parameters: the short-range impurity density, the long-range screened impurity density, and temperature \(T\) . The proposed theoretical model was successfully used to correlate various experiments in the midtemperature and moderate density regimes.

An unusual temperature induced isostructural phase transition in a scheelite, Li0.5Ce0.5MoO4

The effect of Bi substitution has been investigated in the palmierite type system Sr3V2O8, wherein, a new solid solution Sr3−xBi2x/3V2O8 (0 ≤ x ≤ 0.4) has been established. The structure of the composition x = 0.20 (Sr2.8Bi0.13V2O8) was solved using single-crystal X-ray diffraction (XRD) technique. The compound crystallizes in the trigonal crystal system Rm with a palmierite structural type with a = 5.6067(4) A, c = 20.0099(13) A, V = 580.64(5) A3 and Z = 3. The substituent Bi occupies a new unique 18h site resulting in a new series in palmierite class of compounds. The new solid solution has been characterized by synchrotron X-ray powder diffraction, solid-state UV-Vis diffuse-reflectance spectra, scanning electron microscopy and ionic conductivities and photocatalytic activities has been investigated. The solid solution compounds exhibited photocatalytic activity towards various classes of anionic dyes under UV radiation.

A Continuous Electrical Conductivity Model for Monolayer Graphene From Near Intrinsic to Far Extrinsic Region

Fabrication of the out-of-plane atomically sharp p–n junction by stacking two dissimilar two-dimensional materials could lead to new and exciting physical phenomena. The control and tunability of the interlayer carrier transport in these p–n junctions have a potential to exhibit new kind of electronic and optoelectronic devices. In this article, we present the fabrication, electrical, and opto-electrical characterization of vertically stacked few-layers MoTe2(p)–single-layer MoS2(n) heterojunction. Over and above the antiambipolar transfer characteristics observed similar to other hetero p–n junction, our experiments reveal a unique feature as a dip in transconductance near the maximum. We further observe that the modulation of the dip in the transconductance depends on the doping concentration of the two-dimensional flakes and also on the power density of the incident light. We also demonstrate high photo-responsivity of ~105 A/W at room temperature for a forward bias of 1.5 V. We explain these new findings based on interlayer recombination rate-dependent semi-classical transport model. We further develop first principles-based atomistic model to explore the charge carrier transport through MoTe2–MoS2 heterojunction. The similar dip is also observed in the transmission spectrum when calculated using density functional theory–non-equilibrium Green’s function formalism. Our findings may pave the way for better understanding of atomically thin interface physics and device applications.p–n junctions: unusual transfer characteristicp–n heterojunctions based on certain two-dimensional transition metal dichalcogenides display an unusual dip in the current–voltage characteristic. A team led by Anindya Das at the Indian Institute of Science in Bangalore fabricated p–n junctions made of few layers of MoTe2 and a single layer of MoS2 and observed an unexpected dip in the junction current as a function of the back gate voltage. The researchers could modulate the intensity and position of this dip feature by changing the power density of the incident light. They also performed theoretical calculations that qualitatively captured the behavior of the dip, which could be attributed to interface recombination of charge carriers. These findings further the understanding of the physics of thin-film interfaces, which can stimulate the improvement of devices.

Effect of bismuth substitution on crystal chemistry, photocatalysis and conductivity in Sr3V2O8: a new structural type in palmierite class

Abstract Reactions of hydrated zinc(II) trifluoroacetate and sodium azide with two tridentate Schiff bases HL1 (2-((E)-(2-(dimethylamino)ethylimino)methyl)-4-chlorophenol) and HL2 (2-((E)-(2-(dimethylamino)ethylimino)methyl)-4-bromophenol) under the same reaction conditions yielded two dinuclear isostructural zinc(II) complexes, [Zn(L1)(N3)]2 (1) and [Zn(L2)(N3)]2 (2), respectively. The complexes were characterized systematically by elemental analysis, UV–Vis, FT-IR, and 1H NMR spectroscopic methods. Single-crystal X-ray diffraction studies reveal that each of the dinuclear complexes consists of two crystallographically independent zinc(II) ions connected by double bridging phenoxides. All zinc(II) ions in 1 and 2 are surrounded by similar donor sets and display distorted square–pyramidal coordination geometries. The ligands and complexes reveal intraligand 1(π → π*) flourescence. The enhancement of the fluorescence intensities for the complexes compared to the ligands indicates their potential to serve as photoactive materials.