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We demonstrate negative capacitance (NC) dispersion in ZnO by doping lesser electronegative chalcogen (Ch=S, Se, Te) elements at the oxygen (O)-site. Approximately 4.00×0.15 atomic percentage of ChOX is obtained from Rietveld refinement. Using ab initio and dielectric spectroscopy, the tailoring of charge localization around dopants and consequent charge recombination are observed to have a systematic dependence on the stabilization of NC. With the increase of dopant electronegativity difference from SOX to TeOX, the low frequency (<100Hz) dispersion of NC gradually extends to a larger frequency under lower external bias. Universal Debye response is found to govern the NC dispersion with calculated relaxation time indicating to trap mediated charge recombination. [ABSTRACT FROM AUTHOR]

Frequency-dispersive impedance analysis of CH3NH3PbI3 perovskite is carried out under the external Direct current (DC) field to investigate the interplay of dielectric polarization and delocalized carrier transport. Switching of capacitance from positive to negative values is observed in the radio frequency range (42.1–42.5 MHz) for the external bias ranging from 0–4 V. The switching frequency outlined a decreasing trend with an increase in bias. Upon fitting the experimentally obtained dispersions, a bi-relaxation mechanism is unveiled. One of its constituents arises due to the typical Maxwell–Wagner interfacial polarization between the grain cores and boundaries and acts at the lower frequencies. The other one is manifested via hopping of delocalized carriers, resulting in a high frequency degenerative pseudo inductive response. The interference of these two mechanisms is manifested into an asymmetric Breit–Wigner–Fano profile of the dielectric susceptance spectra. The results are further elaborated from a theoretical point of view involving the energy band structure, electron localization function, and Mulliken charge distribution. [ABSTRACT FROM AUTHOR]

Magnetite (Fe3O4), a representative relaxor multiferroic material, possesses fundamentally appealing multifaceted size-dependent properties. Herein, to evaluate a prototype spinel transition metal oxide (STMO), monodispersed and highly water-dispersible spherical magnetite nanoparticles (MNPs) with an enormous size range (3.7–242.8 nm) were synthesized via a facile microwave-assisted and polyol-mediated solvothermal approach at a controlled temperature and pressure using unique crystallite growth inhibitors. The excellent long-term colloidal stability of the MNPs in a polar environment and increase in their zeta potential confirmed the coordinative effect of the carboxylate groups derived from the covalent surface functionalization, which was also validated by FTIR spectroscopy, TGA and XPS analysis. The optical bandgap (Eg) between the crystal field split-off bands, which was calculated using the absorption spectra, increased gradually with a decrease in size of the MNPs within a broad UV-Vis range (1.59–4.92 eV). The red-shifting of the asymmetric Raman peaks with a smaller size and short-range electron–phonon coupling could be explained by the modified phonon confinement model (MPCM), whereas ferrimagnetic nature rejigged by superparamagnetism was verified from Mössbauer analysis. These stoichiometric, non-toxic, polar and magnetic nanocrystals are not only ideal for biomedical applications, but also suitable as electroactive porous host networks. Finally, the size-modulated MNPs were incorporated in poly(vinylidene fluoride) [PVDF]-based polytype nanogenerators as an electret filler to demonstrate their piezoelectric performance (VOC ∼115.95 V and ISC ∼1.04 μA), exhibiting substantial electromagnetic interference shielding. [ABSTRACT FROM AUTHOR]

In present era, rare earth doped nanophosphors are the key architecture in the development of white light-emitting diodes (WLEDs) and their performance has become a major topic of interest among the researchers. Still, the deficiency of orange phosphor has become a barrier for the commercialization of the WLED. In this work, single phase intense orange light emitting Na+ co-activated CaTiO3 (CTO):Sm3+ nanophosphor is synthesized via modified sol–gel method with an average particle size of 60 nm. The synthesized nanophosphor strongly exhibits bright orange emission under 408 nm excitation. Comparative photoluminescence (PL) analysis of CTO:Sm3+ and Na+ co-doped CTO:Sm3+ nanophosphors irrefutably reveals that incorporation of Na+ facilitates almost threefold increase in the luminescence intensity along with significant improvement in thermal stability. Obtained results conclusively suggest that strong orange emitting Na+ co-activated Sm3+ doped CTO nanophosphor could be a potential candidate for use in blue light chip based warm WLED and others lighting device applications. [ABSTRACT FROM AUTHOR]

Highly-luminescent nanophosphors have a decisive role in solid-state lighting (SSL) as well as in field emission display (FED) applications due to their potential use in fabrication of nanophosphor based FED and solid state display devices. Herein, the red emitting highly-luminescent Eu3+–Li+ co-doped magnesium aluminate (MgAl2O4) nanophosphors were synthesized by a customized sol–gel route with an average particle size of 18 nm, which can be easily scaled up in a large quantity. The resulting nanophosphor exhibits hypersensitive red emission, peaking at 615 nm upon 394 nm excitation. Furthermore, comparative photoluminescence (PL) studies have been carried out for Eu3+ doped and Eu3+ doped-Li+ co-doped magnesium aluminate (Li+ co-doped MgAl2O4:Eu3+) nanophosphors, which indicated that Li+ co-doping significantly improves luminescence intensity along with good crystallinity. Moreover, the charge compensation by addition of Li+ co-activator in MgAl2O4:Eu3+ lattice led to the two fold enhancement of PL intensity. The obtained results suggest that this nanophosphor could be an ultimate choice for next generation advanced luminescent nanomaterials for solid state lighting and portable FED devices. [ABSTRACT FROM AUTHOR]

Magnetite (Fe 3 O 4 ), a representative relaxor multiferroic material, possesses fundamentally appealing multifaceted size-dependent properties. Herein, to evaluate a prototype spinel transition metal oxide (STMO), monodispersed and highly water-dispersible spherical magnetite nanoparticles (MNPs) with an enormous size range (3.7-242.8 nm) were synthesized via a facile microwave-assisted and polyol-mediated solvothermal approach at a controlled temperature and pressure using unique crystallite growth inhibitors. The excellent long-term colloidal stability of the MNPs in a polar environment and increase in their zeta potential confirmed the coordinative effect of the carboxylate groups derived from the covalent surface functionalization, which was also validated by FTIR spectroscopy, TGA and XPS analysis. The optical bandgap (E g ) between the crystal field split-off bands, which was calculated using the absorption spectra, increased gradually with a decrease in size of the MNPs within a broad UV-Vis range (1.59-4.92 eV). The red-shifting of the asymmetric Raman peaks with a smaller size and short-range electron-phonon coupling could be explained by the modified phonon confinement model (MPCM), whereas ferrimagnetic nature rejigged by superparamagnetism was verified from Mössbauer analysis. These stoichiometric, non-toxic, polar and magnetic nanocrystals are not only ideal for biomedical applications, but also suitable as electroactive porous host networks. Finally, the size-modulated MNPs were incorporated in poly(vinylidene fluoride) [PVDF]-based polytype nanogenerators as an electret filler to demonstrate their piezoelectric performance (V OC ∼115.95 V and I SC ∼1.04 μA), exhibiting substantial electromagnetic interference shielding.

Persistent low-frequency negative capacitance (NC) dispersion has been detected in half-metallic polycrystalline magnetite (Fe3O4) nanoparticles with varying sizes from 13 to 236 nm under the application of moderate dc bias. Using the Havriliak-Negami model, 3D Cole-Cole plots were employed to recapitulate the relaxation times (τ) of the associated oscillating dipoles, related shape parameters (α, β) and resistivity for the nanoparticles with different sizes. The universal Debye relaxation (UDR) theory requires a modification to address the shifted quasi-static NC-dispersion plane in materials showing both +ve and -ve capacitances about a transition/switching frequency (f0). A consistent blue-shift in 'f0' is observed with increasing external dc field and decreasing particle size. Based on this experimental data, a generalized dispersion scheme is proposed to fit the entire positive and negative capacitance regime, including the diverging transition point. In addition, a comprehensive model is discussed using phasor diagrams to differentiate the underlying mechanisms of the continuous transition from -ve to +ve capacitance involving localized charge recombination or time-dependent injection/displacement currents, which has been adequately explored in the scientific literature, and the newly proposed 'capacitive switching' phenomenon. An inherent non-stoichiometry due to iron vacancies [Fe3(1-δ)O4], duly validated from first principles calculations, builds up p-type nature, which consequently promotes more covalent and heavier dipoles and slows the dipolar relaxations; this is incommensurate with Maxwell-Wagner interfacial polarization (MWIP) dynamics. This combinatorial effect is likely responsible for the sluggish response of the associated dipoles and the stabilization of NC.

Graphene-based van der Waals (vdW) heterostructures can facilitate exciting charge transfer dynamics in between structural layers with the emission of excitonic quasi-particles. However, the chemical formation of such heterostructures has been elusive thus far. In this work, a simple chemical approach is described to form such van der Waals (vdW) heterostructures using few layer MoS2 sheet embedded quantum dots (QDs) and amine-functionalized graphene quantum dots (GQDs) to probe the energy transfer mechanism for tunable photoluminescence (PL). Our findings reveal an interesting non-radiative Förster-type energy transfer with the quenching of functional GQD PL intensity after GQD/MoS2 composite formation, which validates the existing charge transfer dynamics analogous to 0D and 2D systems. The non-radiative type of energy transfer characteristic from GQD into the MoS2 layer through vdW interactions has been confirmed by photoluminescence, time decay analyses and ab initio calculations with the shifting of the Fermi level in the density of states towards the conduction band in the stacked configuration. These results are encouraging for the fundamental exploration of optical properties in other chemically prepared QD/2D based heterostructures to understand the charge transfer mechanism and fingerprint luminescence quenching for future optoelectronic device and optical sensing applications.

Graphene quantum dots are known to exhibit tunable photoluminescence (PL) through manipulation of edge functionality under various synthesis conditions. Here, we report observation of excitation dependent anomalous m-n type fingerprint PL transition in synthesized amino functionalized graphene quantum dots (5-7 nm). The effect of band-to-band π*-π and interstate to band n-π induced transitions led to effective multicolor emission under changeable excitation wavelength in the functionalized system. A reasonable assertion that equi-coupling of π*-π and n-π transitions activated the heterogeneous dual mode cyan emission was made upon observation of the PL spectra. Furthermore, investigation of incremented dimensional scaling through facile synthesis of amino functionalized quantum graphene flakes (20-30 nm) revealed it had negligible effect on the modulated PL pattern. Moreover, an effort was made to trace the origin of excitation dependent tunable heterogeneous photoluminescence through the framework of energy band diagram hypothesis and first principles analysis. Ab initio results suggested formation of an interband state as a manifestation of p orbital hybridization between C-N atoms at the edge sites. Therefore comprehensive theoretical and experimental analysis revealed that newly created energy levels can exist as an interband within the energy gap in functionalized graphene quantum structures yielding excitation dependent tunable PL for optoelectronic applications.