It has been well‐established that light‐matter interactions, as manifested by diverse linear and nonlinear optical (NLO) processes, are mediated by real and virtual particles, such as electrons, ...phonons, and excitons. Polarons, often regarded as electrons dressed by phonons, are known to contribute to exotic behaviors of solids, from superconductivity to photocatalysis, while their role in materials’ NLO response remains largely unexplored. Here, the NLO response mediated by polarons supported by a model ionic metal oxide, TiO2, is examined. It is observed that the formation of polaronic states within the bandgap results in a dramatic enhancement of NLO absorption coefficient by over 130 times for photon energies in the sub‐bandgap regions, characterized by a 100 fs scale ultrafast response that is typical for thermalized electrons in metals. The ultrafast polaronic NLO response is then exploited for the development of all‐optical switches for ultrafast pulse generation in near‐infrared (NIR) fiber lasers and modulation of optical signal in the telecommunication band based on evanescent interaction on a planar waveguide chip. These results suggest that the polarons supported by dielectric ionic oxides can fill the gaps left by dielectric and metallic materials and serve as a novel platform for nonlinear photonic applications.
The oxygen‐deficient TiO2−x shows a large enhancement in NLO response using a model polaronic oxide, in which prominent polaronic states are located within the bandgap. The strong optical nonlinearity of the platonic TiO2−x is exploited further for the development of all‐optical switches for ultrashort pulse generation and all‐optical data processing in the near‐infrared (NIR) optical communication window.
•Cobalt ferrite nanoparticles prepared by the auto combustion method.•Dielectric relaxation was explained by impedance spectroscopy.•Interfacial polarization plays important role in cobalt ferrite ...nanoparticles.•Overlap large polaron tunneling conduction is responsible for ac conductivity.•DC conductivity is mainly due to the small polaron conduction.
The electric transport behavior of nano cobalt ferrite was studied in details within frequency window of 100Hz and 1MHz in the range of temperature of 25–200°C. No grain relaxation was observed whereas interfaces (grain boundary and electrode surface contact) became the dominant conduction regions. Both ac and dc conduction mechanism was investigated thoroughly. Overlapping of large polaron tunneling (OLPT) mechanism was found to be responsible for ac conduction process. The value obtained for mobility (10−10cm2/Vs) of charge carriers indicated the possible small polaron hopping for dc conduction process. The dc resistivity data was fitted with Mott and Davis model and the derived parameters confirmed the dc conduction of non-adiabatic nature which was due to small polaron hopping in nano cobalt ferrite.
Depiction of Mn-doped CoFe2O4 catalyst showing the generation of super oxide, oxyhydroxyl and hydroxyl free radicals under illumination of visible light.
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•Structural changes taking ...place in CoFe2O4 lattice on incorporation of Mn2+ ions.•Influence of hopping and polaron formation on the rate of degradation using Mn doped CoFe2O4 is rarely attempted.•Jumping or hopping process takes place because of valence states of two ions get interchanged at octahedral site.•Trapping/detrapping is vital for assessing semiconducting electron transport mechanism correlated with photocatalytic activity.
CoFe2O4 and MnxCo1-x Fe2O4 (x = 0.5 atom %) samples were synthesized by sol–gel combustion method. XPS spectra confirms the presence Mn and Co is in + 2 oxidation state. Binding energy values suggests Fe in + 3 oxidation state in MnCoFe2O4 catalyst. Mn0.5Co0.5Fe2O4 sample shows best fit to the Langmuir adsorption isotherm. Quantitative prediction of the carrier trapping/detrapping energy is essential for the assessing the semiconducting electron transport mechanism and formation of small-Polaron hopping correlated with photocatalytic activity. Higher photocatalytic activity is observed for Mn doped CoFe2O4 due to unique characteristic Mn2 + ion which can exist in different oxidation states. The Fe2+ ions intern get reoxidised to Fe3 + state through the multi electron reduction of oxygen. To the best of our knowledge the influence of hopping and Polaron formation in the spinel lattice for degradation of Methyl Violet by using Mn doped CoFe2O4 catalyst is rarely attempted.
We investigated the polaron-assisted nonadiabatic dynamics in protonated TiO2, as well as the polaron-H2O coupling and its effects on the relaxation of photogenerated electrons. We observed that ...different polaron hopping regimes result in varied nonadiabatic couplings and relaxations of excited electrons from the conduction band minimum to the gap states of protonated TiO2, with a weak dependence on the actual trapping site of the polaron. Surface-adsorbed H2O molecules can attract polarons toward the adsorbed Ti sites, with the coupling between H2O and the polaron being inversely proportional to their distance. Our findings suggest that the lifetime of the photogenerated charge carriers can be extended by reducing the polaron-H2O distances, with expected benefits to the efficiency of the reduced TiO2 samples for photocatalytic applications.
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Organic Electrochemical Transistors
In article number 2300308, Natalie Banerji and co‐workers present an organic electrochemical transistor (OECT) based on a conjugated polymer film. Switching on the ...transistor leads to oxidation of the polymer, forming polarons and bipolarons. The charges are compensated by ions that enter the film from the electrolyte. Using time‐resolved spectroelectrochemistry, they show that polarons prefer the ordered regions of the film, while bipolarons are generated in the amorphous regions.
•Four Eu-Sr and one Nd-Sr manganite studied by X-band EPR in the range 180 – 450 K.•Eu-Sr manganites are ferromagnetic; TC decreased gradually as Eu content increased.•Linewidths of Eu-Sr manganites ...fitted to: C+(A/T)exp(-Ea/kBT); Ea = activation energy.•Linewidths of Eu-Nd manganites fitted well to the Curie-Weiss law.•Magnetization exploited to obtain values of critical exponents for Eu-Sr manganite.
Four Eu-Sr manganites, La0.7-xEuxSr0.3MnO3; x = 0.4, 0.5, 0.6, 0.7, and one Nd-Sr manganite Nd0.4La0.3Sr0.3MnO3, were studied by EPR (electron paramagnetic resonance) at 9.6 GHz at temperatures in the range 175 – 400 K. These studies were further supplemented by magnetization measurements. The positions and widths of the EPR lines of all the Eu-Sr manganite samples, containing Eu3+ ions, changed with temperature in a similar manner. The EPR data showed that these Eu-Sr manganites are ferromagnetic, whose magnetic transition temperature from the ferromagnetic to the paramagnetic state (TC) decreased gradually as the Eu content (x) increased, specifically from TC ~ 111 K (x = 0.5) to TC ~ 57 K (x = 0.7). Furthermore, in these samples the FMR (ferromagnetic resonance) lines appeared significantly above the respective TC, specifically, at 290 K (x = 0.4), 280 K (x = 0.5), 250 K (x = 0.6), together with the EPR lines. As for the sample with x = 0.7 no FMR lines were observed in the temperature range investigated here. The temperature dependence of the EPR linewidth is found to be linear in the various La0.7-xEuxSr0.3MnO3 samples, caused by the presence of conductivity due to small-polaron hopping The peak-to-peak first-derivative EPR linewidths, ΔBpp, of these Eu-doped samples fitted well above the temperature, at which the minimum of ΔBpp occurs, to the expression: ΔBpp (T) = ΔBpp,min + ATexp(-Ea/kBT), with the values of the activation energies being Ea = 0.20 eV, 0.17 eV, 0.11 eV, 0.09 eV for x = 0.4, 0.5, 0.6, 0.7, respectively. On the other hand, for the Nd-doped sample, the ΔBpp in Nd0.4La0.3Sr0.3MnO3 decreased monotonically with temperature above TC in accordance with the Curie-Weiss law as ΔBpp (T) = ΔBpp,0 + C/(T-TC). The magnetization measurements were exploited to determine TC for the various samples. The results agreed with those determined by the EPR measurements presented here. In addition, the magnetization data was analyzed to obtain the values of the critical exponents in the phase transition from the paramagnetic to the ferromagnetic state. The critical exponents, as determined for the La0.7-xEuxSr0.3MnO3 sample with x = 0.5, are β = 0.24 ± 0.02, γ = 1.10 ± 0.5, δ = 5.58.
In this work, optical properties of a tuned quantum dot/ring system with considering the electron–phonon (e-p) interaction are theoretically investigated. In particular, the second harmonic ...generation (SHG) and third harmonic generation (THG) of the system under e-p effect are determined. Here, we consider three phonon modes, longitudinal optical (LO), the surface optical (SO), and LO+SO phonons. First, the energy levels and the wave functions of the system are evaluated without e-p effect. Then, the SHG and THG are investigated with considering the LO e-p, SO e-p, and LO+SO e-p interactions. We have noticed that the e-p interaction has a great effect on the SHG and THG of our system. The effect of SO phonons on the obtained optical properties is more than the LO phonons. With considering the LO+SO-phonons, we obtain the largest influence on the optical properties.
•Electron–phonon interaction effect on optical properties of a tuned quantum dot/ring system.•The second and third harmonic generation of the system under e-p effect are determined.•Longitudinal optical, the surface optical, and LO+SO phonons are considered.•Effect of SO phonons on the obtained optical properties is more than the LO phonons.
•La0.8Pb0.2FeO3 (εr∼30,000) shows higher dielectric constant than LaFeO3 (∼14,000).•Lower A-site dopant content, the dielectric maxima shift to higher temperature.•The frequency dependence of εr and ...tanδ vs. temperature exhibit CDC like behavior.•Rg and Rgb of Pb modified LaFeO3 follow small polaron hopping conduction model.
In the present work we have illustrated the physics of the electrical characteristics of nanocrystalline La1−xPbxFeO3, (0⩽x⩽0.2) powder prepared using auto-combustion synthesis. The effect of lead doping on the dielectric, impedance and ac conductivity characteristics of lanthanum ferrite has systematically been investigated. The synthesized powders were phase pure and crystallized into centro-symmetric Pnma space group. As compared to pure LaFeO3 ceramics (dielectric constant∼14,000), the dielectric constant is grossly increased (∼30,000) in Pb doped LaFeO3. The temperature dependence of dielectric constant of 10.0 at.% Pb doped LaFeO3 exhibits dielectric maxima similar to that observed in ferroelectric ceramics with non-centrosymmetric point group. For La0.8Pb0.2FeO3 ceramics, the frequency dependence of the dielectric constant and loss tangent at various temperatures (300–450K) exhibit typical colossal dielectric constant (CDC) like behavior. From the impedance spectroscopy we have estimated the grain and grain boundary resistance and capacitance of Pb doped LaFeO3 that follow a small polaron hopping conduction model. Long range movement of the charge carriers govern the CDC behavior.
