Conduction Band Electrons Research Articles

Multiphoton induced photoluminescence during time-resolved laser-induced incandescence experiments on silver and gold nanoparticles

In conventional time-resolved laser-induced incandescence (TiRe-LII) measurements, a laser pulse heats the nanoparticles within a probe volume of aerosol, and the particle size distribution and other characteristics are inferred from the observed incandescence decay rate, which is connected to the change in sensible energy through a spectroscopic model. There is strong evidence, however, that for some aerosol systems, the incandescence signal is contaminated with other non-incandescent emission sources. Recent TiRe-LII measurements on polydisperse aerosolized silver and gold nanoparticles energized with a 1064 nm laser pulse exhibit broadband emission that is temporally aligned with the temporal profile of the laser pulse, suggesting that the signal is due to non-thermal emission. One candidate for this emission phenomenon is multiphoton-induced upconversion luminescence, in which the conduction-band electron gas is heated up to an effective lattice temperature, resulting in luminescence due to high-energy intraband transitions.

Experimental Study on the Photocatalytic Reduction of CO2 by Fe2O3/BiOIO3 Composite Photocatalyst

Photocatalytic reduction of CO2 to produce renewable hydrocarbon fuels is of great significance for solving the greenhouse effect and energy crisis. However, due to some shortcomings of the photocatalyst itself, further research is needed to realize its industrial application. In this article, we prepared Fe2O3/BiOIO3 composite photocatalyst by a simple one-step hydrothermal method, and discussed the photocatalytic activity and reaction mechanism of Fe2O3/BiOIO3 composite photocatalyst through a series of characterization and photocatalytic reduction experiments. The band gap energy of the composite photocatalyst (2.32 ev) is smaller than that of pure BiOIO3 (2.98 ev), so the composite photocatalyst can absorb visible light. Since the edge of the conduction band of BiOIO3 is more positive than the edge of the Fe2O3 conduction band, the photogenerated electrons in the Fe2O3 conduction band can be transferred to the BiOIO3 conduction band, which makes BiOIO3 more reducible and can reduce CO2 to CO. This study can provide some reference for the field of photocatalytic reduction of carbon dioxide.

Extreme Sub-Wavelength Structure Formation from Mid-IR Femtosecond Laser Interaction with Silicon.

Mid-infrared (MIR) wavelengths (2–10 m) open up a new paradigm for femtosecond laser–solid interactions. On a fundamental level, compared to the ubiquitous near-IR (NIR) or visible (VIS) laser interactions, MIR photon energies render semiconductors to behave like high bandgap materials, while driving conduction band electrons harder due to the scaling of the ponderomotive energy. From an applications perspective, many VIS/NIR opaque materials are transparent for MIR. This allows sub-surface modifications for waveguide writing while simultaneously extending interactions to higher order processes. Here, we present the formation of an extreme sub-wavelength structure formation (∼) on a single crystal silicon surface by a 3600 nm MIR femtosecond laser with a pulse duration of 200 fs. The 50–100 nm linear structures were aligned parallel to the laser polarization direction with a quasi-periodicity of 700 nm. The dependence of the structure on the native oxide, laser pulse number, and polarization were studied. The properties of the structures were studied using scanning electron microscopy (SEM), atomic force microscopy (AFM), cross-sectional transmission electron-microscopy (CS-TEM), electron diffraction (ED), and energy-dispersive X-ray spectroscopy (EDX). As traditional models for the formation of laser induced periodic surface structure do not explain this structure formation, new theoretical efforts are needed.

Conductive property of secondary minerals triggered Cr(VI) bioreduction by dissimilatory iron reducing bacteria

Although secondary minerals have great potential for heavy metal removal, their impact on chromium biogeochemistry in subsurface environments associated with dissimilatory iron reducing bacteria (DIRB) remains poorly characterized. Here, we have investigated the mechanisms of biogenic secondary minerals on the rate of Cr(VI) bioreduction with shewanella oneidensis MR-1. Batch results showed that the biogenic secondary minerals, schwertmannite and jarosite, appreciably increased the Cr(VI) bioreduction rate. UV–vis diffuse reflection spectra showed that schwertmannite and jarosite are semiconductive minerals, which can be activated by MR-1, followed by transferred conduction electrons toward Cr(VI). Cyclic voltammetry and Tafel analysis suggested that the resistance of secondary minerals is a dominant factor controlling Cr(VI) bioreduction. In addition, Cr(VI) adsorption on secondary minerals through ligand exchange promoted Cr(VI) bioreduction by decreasing the electron transfer distance between MR-1 and chromate. Fe(III)/Fe(II) cycling in schwertmannite and jarosite also contributed to Cr(VI) bioreduction as reflected by X-ray photoelectron spectroscopy and Fourier transform infrared spectrometer. Complementary characterizations further verified the contributions of Fe(III)/Fe(II) cycling, Cr(VI) adsorption, and conduction band electron transfer to enhanced Cr(VI) bioreduction. This study provides new insights on the understanding of Cr(VI) bioreduction by semiconductor minerals containing sulfate in subsurface environments.

Oxygen Sensing of Pt/PEO-TiO2 in Humid Atmospheres at Moderate Temperatures

Here, we show that the presence of adsorbed water improves the oxygen-sensing properties of Pt/TiO at moderate temperatures. The studied interface is based on porous plasma electrolytic oxidized titanium (PEO-TiO) covered with platinum clusters. The electrical resistance across Pt/PEO-TiO is explained by an electronic depletion layer. Oxygen adsorbates further increase the depletion by inducing extrinsic interface states, which are occupied by TiO conduction band electrons. The high oxygen partial pressure in ambient air substantially limits the electron transport across the interface. Our DC measurements at defined levels of humidity at 30 C show that adsorbed water counteracts this shortcoming, allowing oxygen sensing at room conditions. In addition, response and recovery times from temporal oxygen exposure decrease with humidity. We attribute the effects to competing adsorption processes and reactions of water with adsorbed oxygen species and/or lattice oxygen, which involve electron re-injection to the TiO conduction band. Elevated temperatures up to 170 attenuate the effects, presumably due to the lower binding strength to the surface of molecular water compared with oxygen adsorbates.

Protonated graphitic carbon nitride/polypyrrole/reduced graphene oxide composites as efficient visible light driven photocatalysts for dye degradation and E. coli disinfection

A novel ternary PPy/P-C3N4/r-GO visible light-driven photocatalyst was fabricated via chemical polymerization of pyrrole in the presence of protonated-C3N4/reduced graphene oxide (P-C3N4/r-GO). The resulting PPy/P-C3N4/r-GO composite showed excellent photocatalytic performance under visible light for brilliant red dye (X3B) degradation and E. coli inactivation. An X3B degradation efficiency of 98% was achieved in 60 min, and an E. coli removal efficiency of 100% achieved in 90 min. Moreover, PPy/P-C3N4/r-GO displayed good recycling performance, with 90% efficiency retained over 5×3B photodegradation cycles. A plausible photocatalytic mechanism for the PPy/P-C3N4/r-GO composites was proposed, where p-n heterojunctions formed between P-C3N4 and PPy effective separate charges (photoexcited electrons accumulating in the conduction band of P-C3N4, holes accumulating in the valence band of PPy), with r-GO acting as an electron acceptor and trap. The valence band holes in P-C3N4 directly oxidize X3B, whereas conduction band electrons react with O2 to generate reactive oxygen species (superoxide and hydroxide radicals) which also degrade X3B and can inactivate E. coli. This work thus introduces an efficient metal-free ternary photocatalyst system for environmental applications.

Finite-temperature photoemission in the extended Falicov-Kimball model: a case study for Ta$_2$NiSe$_5$

Utilizing the unbiased time-dependent density-matrix renormalization group technique, we examine the photoemission spectra in the extended Falicov-Kimball model at zero and finite temperatures, particularly with regard to the excitonic insulator state most likely observed in the quasi-one-dimensional material Ta_22NiSe_55. Working with infinite boundary conditions, we are able to simulate all dynamical correlation functions directly in the thermodynamic limit. For model parameters best suited for Ta_22NiSe_55 the photoemission spectra show a weak but clearly visible two-peak structure, around the Fermi momenta k\simeq\pm k_{F}k≃±kF, which suggests that Ta_22NiSe_55 develops an excitonic insulator of BCS-like type. At higher temperatures, the leakage of the conduction-electron band beyond the Fermi energy becomes distinct, which provides a possible explanation for the bare non-interacting band structure seen in time- and angle-resolved photoemission spectroscopy experiments.

Carrier-mediated control over the soft mode and ferroelectricity in BaTiO3

We calculate the effects of conduction band electrons (CBEs), introduced by doping or photoexcitation, on the ferroelectricity and phonon dynamics of ${\mathrm{BaTiO}}_{3}$ (BTO). We show that CBEs destabilize ferroelectricity, which would lower the Curie temperature and coercive field, and might help to improve the speed or efficiency with which polarization domains can be switched in ferroelectric devices. We show that CBEs lower the frequencies of the ${A}_{1}$ soft and ferroelectric modes in BTO's ferroelectric phases, and raise the soft-mode frequency in its paraelectric phase. We also show that femtosecond laser pulses could be used to selectively excite a coherent ${A}_{1}$ ferroelectric-mode phonon. This would allow this much-studied excitation to be monitored by pump-probe spectroscopy as it decays into other modes. We show that many of the properties of doped and undoped BTO have simple and intuitive explanations, within an ionic picture of BTO's bonding, if it is assumed that ferroelectricity is not driven by long-range interactions, but by the attraction between Ti and O neighbors. Most of the effects of CBEs are consequences of them reducing ions' charges and increasing their polarizabilities, thereby weakening the Ti-O attraction. As the CBE density increases, so does the density of delocalized interstitial electrons, which would increase conductivity. We argue that a polar metallic phase exists if the threshold for metallic conductivity can be reached before the CBEs make polar distortions energetically unfavorable by critically weakening the Ti-O attraction.

Robust hybridization gap in the Kondo insulator YbB12 probed by femtosecond optical spectroscopy

In heavy fermions the relaxation dynamics of photoexcited carriers has been found to be governed by the low energy indirect gap, E$_{g}$, resulting from hybridization between localized moments and conduction band electrons. Here, carrier relaxation dynamics in a prototype Kondo insulator YbB${}_{12}$ is studied over large range of temperatures and over three orders of magnitude. We utilize the intrinsic non-linearity of dynamics to quantitatively determine microscopic parameters, such as electron-hole recombination rate. The extracted value reveals that hybridization is accompanied by a strong charge transfer from localized 4f-levels. The results imply the presence of a hybridization gap up to temperatures of the order of E$_{g}$/k$_{B}\approx200$ K, which is extremely robust against electronic excitation. Finally, below 20 K the data reveal changes in the low energy electronic structure, attributed to short-range antiferromagnetic correlations between the localized levels.

Metallic conduction through van der Waals interfaces in ultrathin hbox{Bi}_2hbox{Te}_3 films

While the van der Waals (vdW) interface in layered materials hinders the transport of charge carriers in the vertical direction, it serves a good horizontal conduction path. We have investigated electrical conduction of few quintuple-layer (QL) hbox {Bi}_2hbox {Te}_3 films by in situ four-point probe conductivity measurement. The impact of the vdW (Te–Te) interface appeared as a large conductivity increase with increasing thickness from 1 to 2 QL. Angle-resolved photoelectron spectroscopy and first-principles calculations reveal the confinement of bulk-like conduction band (CB) state into the vdW interface. Our analysis based on the Boltzmann equation showed that the conduction of the CB has a long mean free path compared to the surface-state conduction. This is mainly attributed to the spatial separation of the CB electrons and the donor defects located at the Bi sites.

Coexistence of Short- and Long-Range Ferromagnetic Proximity Effects in a Fe/(Cd,Mg)Te/CdTe Quantum Well Hybrid Structure.

In a Fe/(Cd,Mg)Te/CdTe quantum well hybrid structure, short-range and long-range ferromagnetic proximity effects are found to coexist. The former is observed for conduction band electrons, while the latter is observed for holes bound to shallow acceptors in the CdTe quantum well. These effects arise from the interaction of charge carriers confined in the quantum well with different ferromagnets, where electrons interact with the Fe film and holes with an interfacial ferromagnet at the Fe/(Cd,Mg)Te interface. The two proximity effects originate from fundamentally different physical mechanisms. The short-range proximity effect for electrons is determined by the overlap of their wave functions with d-electrons of the Fe film. On the contrary, the long-range effect for holes bound to acceptors is not associated with overlapping wave functions and can be mediated by elliptically polarized phonons. The coexistence of the two ferromagnetic proximity effects reveals the presence of a nontrivial spin texture within the same heterostructure.

Solid solutions of CdS and ZnS: Comparing photocatalytic activity and photocurrent generation

The series of Cd1-xZnxS (x = 0–1.0) photocatalysts and Cd1-xZnxS/FTO thin film photoelectrodes were prepared. The obtained samples were studied by X-ray diffraction method (XRD), diffuse reflectance spectroscopy (DRS), scanning electron microscopy (SEM), and N2 low temperature adsorption. The photocatalysts were tested in the reaction of hydrogen production from Na2S/Na2SO3 solution under visible light irradiation. The photocurrents were measured in aqueous solution of Na2Sn and NaCl. It was shown that the target properties in the hydrogen evolution and photocurrent production are changed differently dependent on the composition of the mixed solid solutions. The highest photocatalytic hydrogen production rate was observed over Cd0.3Zn0.7S while the most effective photoelectrode was Cd0.8Zn0.2S/FTO. The Cd1-xZnxS/FTO samples were studied by the electrochemical methods in details. The factors affecting the photocatalytic activity and the photocurrent generation were found and listed for the first time. Conduction band potential (or flat band potential) and electron lifetime play a crucial role for effective photocatalytic hydrogen production over Cd1-xZnxS. Optimal photoelectrochemical characteristics were obtained in case of high values of electron lifetime. If the electron lifetimes of the tested samples have similar values, the high concentration of charge carriers is required for high photocurrent generation over Cd1-xZnxS/FTO photoelectrodes.

Photocatalytic Hydrogen Evolution Over Pt/Co-TiO2 Photocatalysts

Aims: In this study, the photocatalytic hydrogen evolution reaction from aqueous methanol was investigated upon simulated solar light using platinum loaded on cobalt doped TiO2 (Pt/Co-TiO2) composites. Background: Controversial results of cobalt-based composites create doubts about their photocatalytic activity. Thus, cobalt doped TiO2 composites were synthesized differently, and the photocatalytic activity was examined for the photocatalytic hydrogen generation. Objective: The current study aims to investigate the influence of cobalt doping and platinum loading on the photocatalytic activities of TiO2 nanoparticles for the photocatalytic H2 generation. Methods: The 0.5 wt.% Co-TiO2 and bare TiO2 photocatalysts were synthesized using two different methods, namely, reflux and hydrothermal synthesis. Additionally, the Pt deposition on the prepared Co-TiO2 and TiO2 catalysts (1 wt.% Pt) was performed using a photo-platinization method. : The as-prepared catalysts were characterized by X-ray diffraction (XRD), scanning electron microscopy/ energy dispersive X-ray analysis (SEM/EDX), transmission electron microscopy (TEM), ultraviolet- visible spectroscopy (UV-vis), X-ray photoelectron spectroscopy (XPS), electron paramagnetic resonance (EPR), and electrochemical impedance spectroscopy (EIS). Result: The XRD and EPR studies clearly indicated that the Co was incorporated into the titanium dioxide lattice. The EIS results suggested that the reduction of protons over Co-TiO2 and bare TiO2 materials was possible from a thermodynamic point of view. However, the photocatalytic results revealed that the formed amount of H2 was extremely low and close to the detection limit. The evolution of H2 from aqueous methanol (10 vol%) showed higher rates when employing 1 wt.% Pt loaded on 0.5 wt.% Co-TiO2 photocatalysts under simulated solar light irradiation. A maximum of 317 ± 44 μmol.h-1 was observed over the Pt/Co-TiO2-HT photocatalyst. Conclusion: EPR results confirmed that the cobalt ions were introduced into the TiO2 lattice by trapping the photogenerated conduction band electrons and decreasing the defects in the crystal cell. The Mott−Schottky analysis of electrochemical impedance measurements showed that all catalysts were ntype semiconductors and that cobalt doping induces impurity level within the band gap of TiO2. The experimental results of photocatalytic H2 generation from methanol-reforming demonstrated that no significant impact of Co-doping on the photocatalytic H2 formation was observed neither for bare TiO2 samples nor for the platinized materials. Based on these experimental findings, a possible mechanism for the continuous photocatalytic activity of Pt/Co-TiO2 photocatalysts under simulated solar light was proposed.

Controlling the kinetics of visible-light-induced photocatalytic performance of gold decorated graphitic carbon nitride nanocomposite using different proteins

In this work, the interaction of different proteins with two dimensional (2D) graphitic carbon nitride (GCN) nanosheets decorated with gold nanoparticles (Au NP) is reported along with their influence in controlling the visible-light-induced photocatalytic activity. The decoration of Au NP on 2D GCN nanosheets improves the photocatalytic activity as Au NP plays an important role in trapping and storing the conduction band electrons of GCN nanosheets. The structural, morphological and optical properties of Au-GCN (AGCN) nanocomposite have been investigated using scattering and spectroscopic techniques. Methylene blue (MB) dye was chosen as a representative system for the studying of visible-light-induced photocatalytic activity of AGCN nanocomposite in the presence of different proteins. Bovine serum albumin (BSA), trypsin and cytochrome C having different chain lengths and physicochemical properties were used to control the kinetics of the photocatalytic activity of GCN and AGCN nanocomposites. Due to the simultaneous competitive adsorption of MB and proteins, the photocatalytic activity of the nanocomposites got enhanced in the presence of BSA, while it got inhibited or slowed down in the presence of trypsin and cytochrome C, respectively. The observed photocatalytic activity could be correlated with the surface charges present on the proteins and the mechanistic understanding developed in this study can be beneficial in controlling the kinetics of the photocatalytic activity of semiconductor-based photocatalysts using proteins as activators or inhibitors.

A method for characterizing near-interface traps in SiC metal–oxide–semiconductor capacitors from conductance–temperature spectroscopy measurements

The state-of-the-art technology for gate oxides on SiC involves the introduction of nitrogen to reduce the density of interface defects. However, SiC metal–oxide–semiconductor (MOS) field-effect transistors still suffer from low channel mobility even after the nitridation treatment. Recent reports have indicated that this is due to near-interface traps (NITs) that communicate with electrons in the SiC conduction band via tunneling. In light of this evidence, it is clear that conventional interface trap analysis is not appropriate for these defects. To address this shortcoming, we introduce a new characterization method based on conductance–temperature spectroscopy. We present simple equations to facilitate the comparison of different fabrication methods based on the density and location of NITs and give some information about their origin. These techniques can also be applied to NITs in other MOS structures.

Effect of Y concentration and film thickness on microstructure and electrical properties of HfO2 based thin films

In this work, we introduced a simple solution processing method to prepare yttrium (Y) doped hafnium oxide (HfO2) based dielectric films. The films had high densities, low surface roughness, maximum permittivity of about 32, leakage current < 1.0 × 10−7 A/cm2 at 2 MV/cm, and breakdown field >5.0 MV/cm. In addition to dielectric performance, we investigated the influence of YO1.5 fraction on the electronic structure between Y doped HfO2 thin films and silicon (Si) substrates. The valence band electronic structure, energy gap and conduction band structure changed linearly with YO1.5 fraction. Given this cost-effective deposition technique and excellent dielectric performance, solution-processed Y doped HfO2 based thin films have the potential for insulator applications.

Resolving the mechanism of oxygen vacancy mediated nonradiative charge recombination in monoclinic bismuth vanadate

We report ab initio time-domain simulations of nonradiative electron-hole recombination in bismuth vanadate (BiVO4). Nonadiabatic molecular dynamics (NA-MD) simulation results demonstrate that the charge recombination in BiVO4 is greatly enhanced by a factor of ~50 after an oxygen vacancy introduced. Oxygen vacancy introduces a sub-gap state close to conduction band (CB) and capable of trapping holes, which in turn rapidly recombine with CB electrons for large transition rate. This is the first report on the excited states dynamics of BiVO4 and provide design principle for achieving high-efficiency light-harvesting devices based on BiVO4 and other transition metal oxides materials.

Accelerated discovery of boron-dipyrromethene sensitizer for solar cells by integrating data mining and first principle

Boron-dipyrromethene (BODIPY) is one promising class of sensitizers for dye-sensitized solar cells (DSSCs) due to unique merits of high absorption coefficient and versatile structural modification capability. However, such derivatives usually suffer from limited power conversion efficiencies (PCEs) because of narrow light absorption band and low electron injection. To aid the discovery of BODIPY sensitizers, we employ an inverse design method to design efficient sensitizers by integrating data mining and first-principle techniques. We establish robust data-mining models using genetic algorithm and multiple linear regression, where the features are filtered from 5515 descriptors and their meanings are explicitly explored for next inverse designs. Based on the features’ understanding, we design candidates NH1-6 and predict their PCEs, demonstrating remarkable enhancements (58% maximum) compared to previous works. Furthermore, their optoelectronic properties including maximum absorption wavelengths, oscillator strengths, bandgaps, transferred charges, charge transferred distances, TiO2 conduction band shifts, short-circuit currents and electron injection efficiencies simulated via first-principle calculations indicate significant increasements (93 nm, 122.41%, 23.70%, 36.36%, 471.17%, 63.64%, 28.55%, 107.86% maximum), which testifies the corresponding highly predicted PCEs and may overcome BODIPY dyes’ shortcomings. The as-designed BODIPY sensitizers can be promising candidates for DSSCs, and such method could help accelerate the discovery of other energy materials.

CuO@TiO2 and NiO@TiO2 core-shell catalysts for hydrogen production from the photocatalytic reforming of glycerol aqueous solution

Hydrogen production from the photocatalytic reforming of glycerol aqueous solution was performed on the CuO@TiO2, NiO@TiO2, NiO@CuO, and CuO@NiO core-shell nanostructured catalysts under simulated solar light irradiation. These catalysts were prepared by the combination of a modified sol-gel and a precipitation-deposition method using hydroxypropyl cellulose as structural linker and they were characterized by powder X-ray diffraction (XRD), UV-Vis diffuse reflectance spectroscopy (UV–Vis DRS), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and nitrogen physisorption isotherms techniques. The catalysts containing TiO2 as a shell and CuO as core showed much higher activity compared with those formulated with NiO@CuO, CuO@NiO, and bared CuO or NiO nanoparticles. The highest rate of hydrogen production obtained with the CuO@TiO2 catalyst was as high as 153.8 μmol·g−1h-1, which was 29.0, 24.8, 11.2 and 3.2 times greater than that obtained on CuO@NiO, NiO@CuO, TiO2 P25, and NiO@TiO2 catalyst, respectively. For the high active CuO@TiO2 catalyst, after activation of TiO2 with solar light irradiation, the conduction band electrons can be transferred to CuO core through the heterojunction in the core-shell interfaces which led to CuO gradually reduced to Cu2O, favoring the reduction of proton to release hydrogen.

Understanding of TiO2 catalysis mechanism in underwater pulsed discharge system: Charge carrier generation and interfacial charge-transfer processes

Compared with traditional photocatalysis system, TiO2 charge carrier generation and interfacial charge-transfer process may be influenced by various chemical and physical effects in underwater pulsed discharge plasma system. Here, the role of high-energy electron, ozone in TiO2 charge carrier generation and transfer process has been investigated using phenol as the probe molecule. The introduction of electron-trapping agent (KH2PO4) have an inhibiting effect on TiO2 catalytic activity, indicating high-energy electrons played a significant role in TiO2 catalytic process. EPR analysis showed that TiO2 could be activated to initiate pairs of electron-hole by high-energy electrons from plasma, and the electrons on the conduction band (CB) could be trapped on the oxygen vacancies. XPS analysis showed that the Ti3+OH species formed during discharge process due to the capture of CB electrons by Ti4+OH groups located at the TiO2 surface. The CB electrons transfer processes on TiO2 surface was strongly dependent on the redox potential of electron acceptors, which adsorbed on the TiO2 surface. The CB electrons can be transferred to dissolved O3, resulting in more OH production. Meanwhile, the CB electron also transferred to benzoquinone adsorbed on TiO2, resulting in accumulation of hydroquinone.