Formation Of Electron-hole Pairs Research Articles

Thermoelectric properties of silicon oxide

The article describes the results obtained from the study of the thermal conductivity of granulated silicon particles covered with a layer of silicon oxide. The results of the study show that the thermal conductivity of granulated silicon increases from λ ~1,12 W/mK to λ ~2,74 W/mK at T~300÷600 K, which is consistent with the results of silicon oxide (λSiO2~1 W/mK). It was also observed that the electrical conductivity changes in the range of σ~0,0038÷0,017 (Ohm·sm)-1. The results of the study were explained based on the formation of a layer of silicon oxide on the surface of granulated silicon particles obtained by powder technology. Temperature-induced thermal voltaic effects are observed in the silicon oxide layer. This process depends on the formation of electron-hole pairs in cases with localized access, which leads to an increase in parameters λ and σ.

An efficient visible-light-driven photoconversion of nitrobenzene to aniline over PtO-decorated WO3 nanocrystals prepared by a soft template-based method

BackgroundPhotocatalysis represent a green method for oxidation and reduction reactions. This method is initiated by exciting a semiconducting material by light resulting in electron-hole (e‒‒h+) pair formation that contributes to chemical reactions. Using metal oxide photocatalysts is a green approach for selective organic transformation. Aniline (AN) is a joint compound in several chemical industries. MethodsThis work presents a simple soft template-assisted synthesis of tungsten oxide (WO3) nanocrystals followed by adding trace amounts (0.1‒0.4 wt.%) of platinum oxide (PtO) nanodots through precipitation. Significant findingsPhysicochemical description of formed heterostructures unveiled the mesoporous surface texture construction and the significance of PtO inclusion on the optical and semiconducting properties of WO3. The PtO addition to WO3 at 0.3 wt.% confirms the considerable decrease of the bandgap energy from 2.74 to 2.36 eV, revealing the excellent visible light fascination. The PtO-decorated WO3 was performed to synthesize AN from NB by photoconversion. The optimal dose of 0.3% PtO/WO3 (2.0 gL–1) revealed a complete photoconversion of NB into the beneficial AN within 40 min with magnificent reusability. The PtO loading demonstrates the enhanced e‒‒h+ separation and mobilities. This study opens the door for plenty of research toward the realization of metal oxide-based heterostructures to produce industrial chemicals.

Giant Stark Effect in Two-Dimensional Hittorf’s Phosphorene

The electric field tunable band gap and optical properties in low-dimensional materials (quantum-confined Stark effect) are very useful in applications of optoelectronics. In this paper, based on the many-body perturbation method, we investigate the evolution of the quasiparticle electronic structure, exciton, and optical properties of two-dimensional (2D) Hittorf's phosphorene under an out-of-plane electric field. Compared to other 2D monolayers, the relatively large thickness of Hittorf's phosphorene leads to a significant reduction in the quasiparticle band gap when an electric field is applied along the quantum confinement direction. The unique bilayer structure, on the other hand, guarantees a well spatial separation of photon-excited electron-hole pairs and, consequently, reduced exciton binding energy under an out-of-plane electric field. These combined effects lead to an almost fixed exciton energy and optical absorption edge at low applied electric fields. However, when the field is larger than 0.1 V/\AA{}, substantial reductions in the exciton energy and optical absorption edge are identified. For the higher-order exciton states, the involvement of more complex band-to-band electron-hole pair formation results in a nonmonotonic electric field dependence. The effective optical modulation accompanied with these giant Stark effects shows potential applications of Hittorf's phosphorene in 2D optoelectronic devices.

Optimizing molecular light absorption in the strong coupling regime for solar energy harvesting

The strong coupling of optical absorbers (e.g., molecules or semiconductors) to confined photonic modes fundamentally alters the physical properties of the coupled system via the formation of hybrid light-matter states. One potential application of strong light-matter coupling relies on exploiting it to localize light-induced charge excitation processes to small volumes of material. Applications that would benefit from this localization include thin-film photovoltaics, photodetection, photocatalysis, and others, where the overall performance depends on the ability of a material to amplify light absorption (i.e., the formation of electron-hole pairs) within specific locations in space. This contribution investigates how strong light-matter coupling affects light absorption rates in molecular absorbers coupled to photonic nanostructures. Our results show that the molecular light absorption efficiencies are highest in configurations where the strongly coupled molecules interact directly with the incoming photon flux. We also identify a nonlinear dependence in the molecular absorption as a function of concentration, unique to the strongly coupled systems. Based on these results, we propose design principles for engineering nanostructured systems that allow for high efficiencies of charge carrier localization into strongly coupled absorbers.

In-plasma photo-assisted etching of Si with chlorine aided by an external vacuum ultraviolet source

Photo-assisted etching of p-type Si was previously found to occur in a chlorine-containing, Faraday-shielded, inductively coupled plasma (ICP), and this was attributed to the vacuum ultraviolet (VUV) light generated by the plasma. Other causes for the very high etching rates were ruled out, including ion bombardment. In the present study, the substrate in the main Cl2/Ar ICP was subjected to extra VUV light that was generated in an independently controlled, auxiliary Ar/He ICP in tandem with the main ICP. The ICPs were separated by a tungsten mesh and a bundle of high-aspect-ratio quartz tubes in a honeycomb configuration. There was no measurable perturbation of the main plasma by the auxiliary plasma. The etching rate was found to be enhanced by 11%–51% with the additional VUV light provided by the auxiliary ICP. With absolute measurements of the auxiliary ICP photon flux at the sample surface, as described elsewhere, incredibly large etching yields of 90–240 Si atoms per photon were obtained. It is argued that etching is not a result of electron–hole pair formation but is instead ascribed to a photocatalytic chain reaction.

Excitonic-insulator instability and Peierls distortion in one-dimensional semimetals

The charge density wave instability in one-dimensional semimetals is usually explained through a Peierls-like mechanism, where the coupling of electrons and phonons induces a periodic lattice distortion along certain modes of vibration, leading to a gap opening in the electronic band structure and to a lowering of the symmetry of the lattice. In this work, we study two prototypical Peierls systems: the one-dimensional carbon chain and the monatomic hydrogen chain with accurate ab initio calculations based on quantum Monte Carlo and hybrid density functional theory. We demonstrate that in one-dimensional semimetals at $T=0$, a purely electronic instability can exist independently of a lattice distortion. It is induced by spontaneous formation of low energy electron-hole pairs resulting in the electronic band gap opening, i.e., the destabilization of the semimetallic phase is due to an excitonic mechanism.

New photosensing properties of nanostructured GdCoO3 in the ultraviolet (A)-visible-near infrared range

In this work, single-phase GdCoO3 was obtained by the solution-polymerization method, at 750 °C. Observation of its morphology by electron microscopy revealed the formation of a network of nanoparticles and abundant porosity. The bandgap energy of this material was determined by the Tauc method, giving approximately 0.54 eV, which suggested its possible application as a photosensor material. The photoconductive properties were investigated on sintered pellets made with the as-prepared powder. For their characterization, light emitting diodes (LEDs) that produce radiation from the ultraviolet-A (UV-A) to near-infrared (NIR) were used. The results revealed a reliable and reproducible photoconductive response over multiple on/off light pulses. Good sensitivity to the wavelength and optical irradiance of light was observed, confirming its possible use as a wide-spectrum photosensor. The mechanism of photoconductivity is explained by the formation of electron-hole pairs in the oxide, due to the transfer of energy from photons.

Chemiresistive NH3 detection at sub-zero temperatures by polypyrrole- loaded Sn1-xSbxO2 nanocubes.

Chemiresistive gas sensors operate mainly at high temperatures, primarily due to the need of energy for surface adsorption-desorption of analytes. As a result, the operating temperature of the chemiresistive sensors could be reduced only to room temperature. Hence, a plethora of sensing requirements at temperatures below ambient have remained outside the scope of chemiresistive materials. In this work, we have developed an antimony-doped SnO2 nanocube-supported expanded polypyrrole network that could detect low ppm ammonia gas (≤20 ppm) at sub-zero temperatures with high response (∼4), selectivity, and short response and recovery times. The low temperature chemiresistive sensing has been explained in terms of the interplay of an extended conducting network of an in situ deposited polymer, effective transport properties of majority charge carriers and a loosely bound exciton-like electron-hole pair formation and breakage mechanism.

Photoluminescence of gamma-irradiated Beryllium oxide

This study analyses the emission and excitation spectra of Beryllium Oxide (BeO) unirradiated and irradiated with 60Co gamma radiation because of its potential use in radiation dosimetry. Deconvolution of the spectra showed that using an excitation wavelength λexc = 330 nm, four emission bands centered at 404, 430, 468 and 545 nm are obtained; These emission bands appear in both irradiated and non-irradiated BeO; however, the irradiated samples exhibited an extra emission band centered at 416 nm. The excitation spectrum was obtained by setting the emission monochromator at a wavelength λ0 = 400 nm. By means of deconvolution, it was obtained that the emission spectrum of the non-irradiated samples is constituted by two bands centered at 307 and 331 nm, while for the irradiated samples the spectrum has three components centered at 297, 334 and 317 nm. In addition, it was found that the area under the curve of all emission bands decreases after irradiation, this may be due to the formation of electron-hole pairs after irradiation, which have the effect of reducing radiative phenomena, which it could be favorable for the applications of BeO in ionizing radiation dosimetry.

Role of microwave during microwave-assisted catalytic reforming of guaiacol, syringolbio-oil as model compounds

To better understand the role of microwave and the mechanisms of thermal cracking and catalytic reforming of lignocellulosic bio-oils, this study compares the difference in the reaction characteristics between microwave-assisted and conventional heating of representative bio-oil model compounds. Syringol and guaiacol were selected as bio-oil model compounds. The effects of the heating method on the conversion of the model compound and the yield and composition of products were systematically investigated in the temperature range of 600–700 °C. The results showed that high reaction temperature and microwave-assisted heating favoured the conversion of model compounds, and the conversions of syringol were generally higher than those of guaiacol under the same conditions (temperature, heating method). The heating method was found to greatly influence the yield and composition of products. The bio-oil obtained from conventional thermal cracking of model compounds contained phenolics, aromatic hydrocarbons, aryl alkyl ethers, and oxygen-containing heterocyclic compounds. In contrast, microwave-induced polarisation of poorly stable methoxy groups promotes the formation of phenolics, the concentration of the phenolic in the guaiacol and syringol bio-oil reached 62.5–66.17 area% and 85.18–87.65 area%, respectively, in the temperature range of 600–700 °C. Catalytic reforming of model compounds showed that activated carbon catalyst drastically promoted the phenol formation, particularly under microwave-assisted heating, the phenol concentration reached 93.9–97.97 area% in bio-oil during catalytic reforming of guaiacol. The formation of electron-hole pairs on the active metal sites induced by microwave radiation possibly enhanced the demethoxylation to selectively generate phenol due to the strong electron-donating ability of the methoxyl group. Microwave-assisted catalytic reforming of the model compounds was also beneficial for H2 formation because of the direct dehydrogenation of methyl radicals and the gas-phase reactions. The H2 concentrations of 81.07–82.41 vol.% and 75.2–79.16 vol.% were obtained during microwave-assisted catalytic reforming of guaiacol and syringol, respectively.

Photocatalytic Degradation of Naphthol Green B Dye Using Coupled CdS-ZnMoO₄ in UV-A Light Irradiation.

At present, nanotechnology field development makes a major role in photocatalysis. CdS-ZnMoO₄ (36.6 wt%) coupled nanophotocatalyst is synthesized using facile hydrothermal method. The CdSZnMoO₄ catalyst shows superior photocatalytic activity in the Naphthol Green B (NGB) dye degradation in UV-A (365 nm) light irradiation. XRD analysis reveals the monoclinic structure of ZnMoO₄, cubic structure of CdS. PL shows lesser the recombination (e--h+) rate of electron-hole pair formation. UV-Vis-DRS reveals an increase in absorption in entire visible region while loading with CdS. SEM images indicate that CdS-ZnMoO₄ has nanofibrous structure. EDS reveal that Cd and S are present on the ZnMoO₄ surface. ECM indicates the division of homogeneity in elements. SAED pattern of HR-TEM images proved high crystalline properties of the catalyst. XPS reveals the different oxidation states and term-symbols of Cd, S, Zn, Mo and O in this catalyst with corresponding binding energies. CdS-ZnMoO₄ (18.5 m²g-1) has a higher surface area compared to ZnMoO₄ (10.8 m²g-1). Current densities obtained from CV reveals the higher cyclic movement of electrons (electrochemical activity) of CdS-ZnMoO₄. An effective parameters for photodegradability of NGB dye by CdS-ZnMoO₄ was analyzed.

Development of a UV-visible-NIR sensor based on LiNiO2 prepared by the coprecipitation method

The development of photonic sensors has attracted the attention of numerous research groups and companies around the world, due to their remarkable applications in diverse fields. In the case of ultraviolet sensors, wide bandgap semiconductor materials are commonly used, whereas for infrared detection, metal tellurides and sulfides are frequently employed. In this work, the photocurrent properties of LiNiO2 were investigated with the aim of developing a photonic sensor that detects radiation in the range of ultraviolet to near infrared. LiNiO2 is a well-known material that is intensively investigated for cathode material for Li-ion rechargeable batteries. In addition to its outstanding electrochemical properties, it has a narrow bandgap energy of approximately 0.6 eV, which suggests its application in photonic detectors. Single-phase LiNiO2 was prepared by a soft chemistry route (coprecipitation) at 700 °C, in air. The photodetection properties were investigated using light emitting diodes (LEDs) with various optical irradiances and exposure times. The results show uniform and reproducible response patterns over numerous on/off periods, being possible to identify variations in the optical irradiance through changes in photocurrent. The detection mechanism is based on the formation of electron-hole pairs produced during light exposure and their mobility under a bias voltage. One application of these results focuses on the development of a UV photodetector that helps prevent skin cancer and eye damage caused by sun exposure.

Magnetic, microstructural and photoactivated antibacterial features of nanostructured Co–Zn ferrites of different chemical and phase compositions

Ferrites have found extensive uses in electrical, chemical and mechanical engineering, but their potential as biomaterials remains largely unexplored. Here we report on the use of a flash method based on urea decomposition to synthesize four different compositions of cobalt and zinc ferrite, including monophasic CoFe2O4, ZnFe2O4 and Co0.5Zn0.5Fe2O4, and CoFe2O4/ZnFe2O4 nanocomposite. Zn-ferrite nanoparticles were approximately the same size as those of Co-ferrite, but were better dispersed, rougher, more crystalline and less pronouncedly faceted. The mixed, monophasic Co0.5Zn0.5Fe2O4 composition was morphologically most similar to single-phase Co-ferrite. In the biphasic mixture of CoFe2O4 and ZnFe2O4, smooth and polydisperse Co-ferrite grains were coated by finer, cuboid and monodisperse Zn-ferrite nanoparticles. The porosity and the roughness of this composition were the highest, and so was its coercivity owing to the shielding of the ferrimagnetic Co-ferrite grains by the paramagnetic Zn-ferrite nanoparticles. Expectedly, the highest saturation magnetization (66.3 emu/g) was detected in Co-ferrite and the lowest (2.8 emu/g) in Zn-ferrite. Co-ferrite also exhibited the highest magneto-crystalline anisotropy, remanence and hysteresis loop area, while the highest exchange bias and susceptibility were found in the mixed, Co0.5Zn0.5Fe2O4 composition. All four ferrite compositions were biocompatible with human fibroblasts, but they demonstrated different antibacterial activities against Gram-negative E. coli and Gram-positive S. aureus, with the biphasic mixture of CoFe2O4 and ZnFe2O4 being more effective than Co0.5Zn0.5Fe2O4, CoFe2O4 or ZnFe2O4. Further, all the compositions were more active against both bacterial species under visible light than in the dark, indicating the photocatalytic formation of electron-hole pairs and reactive oxygen species that exert a damaging effect on the bacterial cells as the prime mechanism of action.

Synthesis, characterization, and application of polypyrrole/TiO2 composites in photocatalytic processes: A review

Hybrid materials composed of semiconductor oxide metals and conducting polymers have been highlighted as a new class of materials, with superior properties compared to their pure constituents. Among the studied composites to photocatalytic applications, the hybrids of titanium dioxide (TiO2) and polypyrrole (PPy) are promising due to several advantages over the pure TiO2 nanoparticles. The PPy/TiO2 composite has been effectively synthetized by chemical polymerization methods as in situ polymerization, photopolymerization, electrochemical polymerization, and molecular imprinting polymerization (MIP). All the cited methods appear to be effective in reducing the band gap energy, which suggests an increase in the formation of photoexcited electron-hole pairs and, consequently, an improvement of the light absorption in the visible region (400–700 nm). In addition, the doping of PPy/TiO2 with noble metals improves the separation of charges in the semiconductor particle, inhibiting the recombination of photogenerated electron-hole pairs. All advantages are evidenced by the characterization results of SEM, TEM, HRTEM, UV-vis DRS, FTIR, XRD, PL, TGA and electrical properties. Finally, results from literature present that PPy/TiO2 composites have better photocatalytic activity than the pure TiO2, being an alternative photocatalyst promising for visible light applications. Thus, this work presents a review of the synthesis, characterization, and application of PPy/TiO2 composites in the photocatalytic processes.

Effect of spin-orbit interaction on recombination luminescence of dye in films of halogen-containing derivative poly-N-epoxypropylcarbazole

A photoluminescence kinetics of a cationic polymethine dye in composite polymer films based on halogen-containing derivatives poly-N-epoxypropylcarbazole (PEPC) has been studied. The observed luminescence kinetics is formed by the fluorescence of the dye and recombination luminescence associated with the formation and recombination of electron-hole pairs (EHP). It was shown, that measuring the external magnetic field effect on the kinetics of dye luminescence in a wide time range made it possible to determine the characteristic times of the recombination processes with the involvement of EHP formed from singlet and triplet excited electronic states of the dye molecules and main channels of electron energy transformation in composite films. An enhancement of the spin-orbit interaction in polymers with different heavy atoms leads to an increase in a value of the magnetic effect (g(B)) in the nanosecond time range and to a decrease in g(B) value in the microsecond time range.

A novel porous Fe3O4/Titanosilicate/g-C3N4 ternary nanocomposites: Synthesis, characterization and their enhanced photocatalytic activity on Rhodamine B degradation under sunlight irradiation

A magnetically separable Fe3O4/porous Titanosilicate/g-C3N4 hybrid nanocomposite (FTSCN) has been synthesized by a facile in-situ method. The synthesized nanocomposites are characterized by PXRD, FE-SEM, TEM, BET-Surface area, FT-IR, UV–vis., photoluminescence, magnetic susceptibility and photocatalytic activity. XRD, FE-SEM, TEM, FT-IR, results of FTSCN show that the formation of finely distributed Fe3O4 nanoparticles in the titanosilicate matrix and g-C3N4. BET-surface area shows that the enhancement of surface area on heterojunction. The bandwidth is calculated from diffuse reflectance ultraviolet-visible spectroscopy. The enhancement of electron-hole pair formation is observed fluorescence spectroscopy. The magnetic behaviour is also observed by magnetic susceptibility test of the nanocomposite. FTSCN nanocomposite exhibit improved photodegradation of Rhodamine B sunlight light irradiation. The ideal photocatalytic activity of FTSCN3.5 in presence of sunlight is nearly 4 to 5-fold greater than pure Fe3O4 (FO), pure titanosilicate (TS) and g-C3N4 respectively. We confirmed that the prepared FTSCN3.5 hybrid photocatalyst is very stable even after five consecutive trial runs are observed by XRD spectral data. The enhanced photocatalytic behaviour of the FTSCN3.5 hybrid nanocomposite in the presence of sunlight is because of the synergistic effect of the Fe3O4, pure titanosilicate and g-C3N4. A plausible, structure of heterojunction, physical and chemical catalytic mechanisms are also proposed.

Carbon Dots, Unconventional Preparation Strategies, and Applications Beyond Photoluminescence.

Carbon dots (C-dots) are generally separated into graphene quantum dots (GQDs) and carbon nanodots (CNDs) based on their respective top-down and bottom-up preparation processes. However, GQDs can be prepared by carbonization of small-molecule precursors as revealed with unconventional preparation strategies. Thus, it is their structures rather than their precursors and preparation strategy that govern whether C-dots are GQDs or CNDs. Here, the composites, structure, and electronic properties of C-dots are discussed. C-dots generally consist of a graphite-like core and amorphous oxygen-containing shell. When graphite becomes C-dots, its conduction and valence bands are separated, and the quantum confinement effect appears. Combined with the light-harvesting ability inherited from graphite, electrons in the core of C-dots are transferred from conduction to valence bands, leading to electron-hole pair formation upon light excitation. The photoexcitation activities, such as photovoltaic conversion, photocatalysis, and photodynamic therapy, are influenced by the electronic properties of the core. Different to the semiconductor properties of core, the C-dot shell is electrochemically active, leading to electrochemiluminescence (ECL). The oxygen-containing groups in shell can conjugate to functional species for use in imaging and therapy. The applications of C-dots beyond photoluminescence, including ECL, solar photovoltaics, photocatalysis, and theranostics, are reviewed.

Poly(methylmethacrylate) coating on quantum dot surfaces via photo-chemical reaction for defect passivation

An effective poly(methylmethacrylate) (PMMA) coating process via photo-chemical reaction for quantum dots (QDs) was investigated. Various concentrations of PMMA were tested and the optimal concentration was confirmed to be 0.1 mM. The quantum yield (QY%) of the 0.1 mM PMMA-coated sample showed a 27% increase in efficiency relative to the as-synthesized CdSe/ZnS QDs due to the passivation of defect states on the surface. The QY% enhanced by PMMA was further increased by light irradiation because of the strong interactions and bonding between the surface atoms and functional groups of PMMA (acrylic groups). The excitation and formation of electron-hole pairs generated by light irradiation lead to strong interactions and chemical bonding on the surface; this is applicable to other cadmium QDs. When the QDs were passivated by a photo-chemical method, the QY% increased by approximately 20% for CdSe, CdSe/ZnS, and CdSe/ZnSe QDs. In order to evaluate the long-term stability depending on the PMMA coating, the QDs were stored for 12 months. The efficiency of as-synthesized bare QDs, initially 90%, drastically decreased to 17% after 12 months, but the PMMA-coated QDs with our coating method maintained a high efficiency of over 75%.

Influence of Bi on the temperature dependent fundamental band gap parameters of GaSb1−xBix

The temperature dependent near band edge emission of LPE grown GaSbBi is studied using photoluminescence technique. A two oscillator model is used to obtain the relevant parameters that explain the temperature dependence of band gap more adequately in cryogenic region as compared to the information obtained through the conventional ‘Varshni’ model. The two oscillator model also provides some valuable information on the phonon dispersion co-efficient in GaSb and GaSbBi and its behavior vis-à-vis the bismuth content in the material. Calculations show that the relative contribution of the high energy LA phonon branch increases with increase in Bi concentration and dominate the temperature dependence of band gap. By considering the energy gap as Gibbs free energy of electron and hole pair formation, approximate analytical expressions are derived for the entropy and the enthalpy of the formation process. The entropy of formation of electron-hole pairs is found to decrease with Bi content in GaSbBi indicating an increase in ordering and a decrease of electron- phonon interaction which might explain the reduced temperature dependence of band gap of some III-V-bismides.

Review—Progress in Understanding Host-Sensitized Excitation Processes in Luminescent Materials

Numerous technologies are in use today that employ solid state luminescent materials that are excited by high-energy sources, such as X-rays, cathode rays, or vacuum ultraviolet (VUV) photons. Absorption of this energy results in the formation of electron-hole (e-h) pairs in the host material. In a doped phosphor, this energy must be transferred to the activator for luminescence to occur. The overall efficiency of host-sensitized luminescence is therefore dictated by the efficiency of energy transfer, in addition to the activator quantum efficiency. Studies of host-sensitized luminescence tend to focus on the overall efficiency, while there are fewer reports of systematic investigations aimed at quantifying the energy transfer step. Even so, important experimental and theoretical progress has been made toward improving our understanding of these processes. In this review the development of the field is discussed, with a particular focus on the study of host-to-activator energy transfer.