Size-modulation of functionalized Fe3O4: nanoscopic customization to devise resolute piezoelectric nanocomposites.

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.

Negative capacitance switching in size-modulated Fe3O4 nanoparticles with spontaneous non-stoichiometry: confronting its generalized origin in non-ferroelectric materials.

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, ß) 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.

Resonant energy transfer in a van der Waals stacked MoS2 - functionalized graphene quantum dot composite with ab initio validation.

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.

Exploring the effect of hole localization on the charge-phonon dynamics of hole doped delafossite.

For weak or moderate doping, electrical measurement is not suitable for detecting changes in the charge localization inside a semiconductor. Here, to investigate the nature of charge-phonon coupling in the presence of gradually delocalized holes within a weak doping regime (~1016 cm-3), we examine the temperature dependent Raman spectra (303-817 K) of prototype hole doped delafossite [Formula: see text] (x = 0/0.03, y = 0/0.01). For both [Formula: see text] and [Formula: see text] phonons, negative lineshape asymmetry and relative thermal hardening are distinctly observed upon [Formula: see text] and [Formula: see text] doping. Using Allen formalism, charge density of states at the Fermi level per spin and molecule, and charge delocalization associated to [Formula: see text] plane, are estimated to increase appreciably upon codoping compared to the [Formula: see text]-axis. We delineate the interdependence between charge-phonon coupling constant ([Formula: see text]) and anharmonic phonon lifetime ([Formula: see text]), and deduce that excitation of delocalized holes weakly coupled with phonons of larger [Formula: see text] is the governing feature of observed Fano asymmetry ([Formula: see text]) reversal.

Local Field Enhancement-Induced Enriched Cathodoluminescence Behavior from CuI-RGO Nanophosphor Composite for Field-Emission Display Applications.

Field-emission displays (FEDs) constitute one of the major foci of the cutting edge materials research because of the increasingly escalating demand for high-resolution display panels. However, poor efficiencies of the concurrent low voltage cathodoluminescence (CL) phosphors have created a serious bottleneck in the commercialization of such devices. Herein we report a novel CuI-RGO composite nanophosphor that exhibits bright red emission under low voltage electron beam excitation. Quantitative assessment of CL spectra reveals that CuI-RGO nanocomposite phosphor leads to the 4-fold enhancement in the CL intensity as compared to the pristine CuI counterpart. Addition of RGO in the CuI matrix facilitates efficient triggering of luminescence centers that are activated by local electric field enhancement at the CuI-RGO contact points. In addition, conducting RGO also reduces the negative loading problem on the surface of the nanophosphor composite. The concept presented here opens up a novel generic route for enhancing CL intensity of the existing (nano)phosphors as well as validates the bright prospects of the CuI-RGO composite nanophosphor in this rapidly growing field.

Colossal magnetoresistance in amino-functionalized graphene quantum dots at room temperature: manifestation of weak anti-localization and doorway to spintronics.

In this work, we have demonstrated the signatures of localized surface distortions and disorders in functionalized graphene quantum dots (fGQD) and consequences in magneto-transport under weak field regime (∼1 Tesla) at room temperature. Observed positive colossal magnetoresistance (MR) and its suppression is primarily explained by weak anti-localization phenomenon where competitive valley (inter and intra) dependent scattering takes place at room temperature under low magnetic field; analogous to low mobility disordered graphene samples. Furthermore, using ab-initio analysis we show that sub-lattice sensitive spin-polarized ground state exists in the GQD as a result of pz orbital asymmetry in GQD carbon atoms with amino functional groups. This spin polarized ground state is believed to help the weak anti-localization dependent magneto transport by generating more disorder and strain in a GQD lattice under applied magnetic field and lays the premise for future graphene quantum dot based spintronic applications.

Efficient and persistent cold cathode emission from CuPc nanotubes: a joint experimental and simulation investigation.

In the current report, chemically synthesized copper phthalocyanine (CuPc) nanotubes are shown to exhibit unprecedentedly well cold cathode emission characteristics with turn-on field (3.2 V µ m(-1)) and stable emission during long intervals (200 min). Simulation of electric field distribution via finite element method around an isolated nanotube emitter in a manner parallel to the experimental setup (inter-electrode distance = 180 µm) exhibits good corroboration of theoretical premises with experimental findings. Obtained results strongly indicate CuPc nanotubes to be potential candidate as cold cathode emitter for electron emission based applications such as field emission displays and vacuum nano-electronic devices.

Amino-functionalized graphene quantum dots: origin of tunable heterogeneous photoluminescence.

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.

Charge compensation assisted enhanced photoluminescence derived from Li-codoped MgAl2O4:Eu3+ nanophosphors for solid state lighting applications.

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 Eu(3+)-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 Eu(3+) doped and Eu(3+) doped-Li(+) co-doped magnesium aluminate (Li(+) co-doped MgAl2O4:Eu(3+)) 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:Eu(3+) 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.