Intraband States Research Articles

Simulation and interpretation of zinc and nitrogen dopants induced defect emissions in monoclinic gallium oxide

Zinc (Zn) and nitrogen (N) are potential acceptor dopants for inducing visible emissions and p-type conductivity in monoclinic gallium oxide (β-Ga2O3), however, the poor understanding of these dopants and their recombination process severely limited the optoelectronic applications. Here, we investigate the zinc (Zn) and nitrogen (N) dopants-induced intraband states and the broadening of defect emission bands due to vibronic coupling of highly transparent β-Ga2O3 films through chemical and optical analyses, as well as density function theory. Incorporating Zn and N in β-Ga2O3 shifts the valence band edge towards the Fermi level and introduces band tail states above the valence band maximum. The emission band from pure β-Ga2O3 becomes significantly broad with the incorporation of Zn and N resulting in two additional emission bands: green luminescence (GL) and red luminescence (RL) along with the characteristic ultraviolet luminescence (UVL) and blue luminescence (BL) of pristine β-Ga2O3. Furthermore, the defect states responsible for these UVL, BL, GL, and RL emissions and their phonon coupling strengths are estimated by simulating the spectral line shape of these emission bands using the configuration coordinate model. The simulation results indicate that the Zn and N dopants-induced intraband states are responsible for the GL, and RL bands. These intraband states are acceptors, which provide p-type conductivity in β-Ga2O3 film.

Synergizing between interband and intraband defect states in prolonging the charge carrier lifetime of InSe/SiH heterojunctions.

Experiments have demonstrated that defect states can regulate the charge carrier dynamics in heterojunctions. However, the underlying mechanism still remains under debate. Using nonadiabatic molecular dynamics, we have investigated the influence of inter and intraband defect states on charge relaxation in InSe/SiH heterojunctions. The simulations revealed that inter and intraband defect states have a weak effect on electron transfer, whereas they exert a strong influence on hole transfer and electron-hole recombination. Compared to the pristine system, the selenium vacancy creates two interband shallow electron trapping states and one intraband hole trapping state. The interband electron trapping states can capture photo-generated electrons, while the intraband hole trapping state accelerates hole transfer. The synergy between inter and intraband defect states suppresses the charge recombination by a factor of 8.3. This simulation rationalizes the influence mechanism of inter and intraband defect states on charge carrier dynamics, suggesting a valuable principle for enhancing the performance of heterojunction photocatalysts.

Reducing hot carrier cooling rate in metal halide perovskites through lead vacancies: time-domain ab initio analysis

Pb vacancies slow down hot carrier cooling dynamics in MAPbI3 perovskites by introducing intraband states that can trap holes and by decreasing the deformation of the [PbI6]4− octahedron, which leads to weakened electron–phonon coupling.

Green Light Photoelectrocatalysis with Sulfur-Doped Carbon Nitride: Using Triazole-Purpald for Enhanced Benzylamine Oxidation and Oxygen Evolution Reactions.

Materials dictate carbon neutralindustrialchemicalprocesses. Visible-light photoelectrocatalysts from abundant resources will play a key role in exploiting solar irradiation. Anionic doping via pre-organization of precursors and further co-polymerization creates tuneable semiconductors. Triazole derivative-purpald, an unexplored precursor with sulfur (S) container, combined in different initial ratios with melamine during one solid-state polycondensation with two thermal steps yields hybrid S-doped carbon nitrides (C3 N4 ). The series of S-doped/C3 N4 -based materials show enhanced optical, electronic, structural, textural, and morphological properties and exhibit higher performance in organic benzylamine photooxidation, oxygen evolution, and similar energy storage (capacitor brief investigation). 50M-50P exhibits the highest photooxidation conversion (84±3%) of benzylamine to imine at 535nm - green light for 48h, due to a discrete shoulder (≈700)nm, high sulfur content, preservation of crystal size, new intraband energy states, structural defects by layer distortion, and 10-16nm pores with arbitrary depth. This work innovates by studying the concomitant relationships between: 1) the precursor decomposition while C3 N4 is formed, 2) the insertion of Simpurities, 3) the S-doped C3 N4 property-activity relationships, and 4) combinatorial surface, bulk, structural, optical, and electronic characterization analysis. This work contributes to the development of disordered long-visible-lightphotocatalysts for solar energy conversion and storage.

Opto-structural and magnetic properties of fluorine doped CuO nanoparticles: An experimental study

Pristine and p-block element, Fluorine (F) doped CuO nanoparticles (NPs) were synthesized by implementing the modified sol-gel method. The structural, elemental, and phase characteristics of the nanoparticles were examined using XRD and XPS analysis. Structural analysis revealed the lowering of crystallinity and crystallite size due to F doping owing to induced lattice distortion. Reduction of phase stability of CuO giving rise to the emergence of Cu2O phase was also observed. Fused irregularly shaped particles and gradual reduction of particle size for doping were detected from the morphological analysis. Emission peak intensity in photoluminescence study sheds light on the impact of F doping on the concentration of defects like oxygen vacancy. DRS spectra attained from UV–visible spectroscopy utilized for measuring the optical band gaps suggested a decrease in optical band gap with doping which attributes to the generated intra-band defect states. Room temperature ferromagnetism (RTFM) with slight paramagnetic (PM) contribution was evident in both pure and F-doped CuO NPs. The experimental magnetization value and the BMP model were perfectly compatible, indicating that FM emerged from polaron interactions. The intriguing opto-structural and magnetic properties of F-doped CuO NPs make them a promising contender for use in solar cells, magnetic storage devices, magnetic sensors, photocatalytic, photonics, spintronics, etc. applications.

Linking optical and electronic properties to photoresponse of heterojunctions based on titania nanotubes and chromium, molybdenum, and tungsten oxides

The development of photosensitization strategies for titanium dioxide is necessary for the enhancement of its optical and electronic properties towards its application potential in solar photoelectrochemistry. In this work, significant differences in the photosensitizing capability of the 6th group transition metal oxides applied on the surface of titania nanotubes are reported. For the first time, correlations between the experimentally determined Tauc coefficients, sample photoresponse, and ab-initio simulated properties of the heterojunctions are established. Experimental results show undoubtedly that the decoration of TiO2 nanotubes with chromium oxides leads to the enhanced photoresponse, which originates from the interplay of mid-gap states and both direct and indirect nature of the transitions contributing to the optical absorption. The opposite tendency and decrease of photocurrent were found for molybdenum and tungsten oxides which exhibited forbidden nature of dominating transition. Although computations report intraband states in all interfaces, experimentally only chromium oxides contribute to the photocurrent. The uniqueness of this interface lies in the highest density of states in the vicinity of the conduction band and the low energy difference between the direct and indirect transitions of the innate chromium oxide. The obtained results demonstrate that the determination of the Tauc exponent and the nature of optical transition are more reliable experimental predictors of the photoactivity enhancement in the heterojunctions than the value of the band gap.

Red edge effect and chromoselective photocatalysis with amorphous covalent triazine-based frameworks

Chromoselective photocatalysis offers an intriguing opportunity to enable a specific reaction pathway out of a potentially possible multiplicity for a given substrate by using a sensitizer that converts the energy of incident photon into the redox potential of the corresponding magnitude. Several sensitizers possessing different discrete redox potentials (high/low) upon excitation with photons of specific wavelength (short/long) have been reported. Herein, we report design of molecular structures of two-dimensional amorphous covalent triazine-based frameworks (CTFs) possessing intraband states close to the valence band with strong red edge effect (REE). REE enables generation of a continuum of excited sites characterized by their own redox potentials, with the magnitude proportional to the wavelength of incident photons. Separation of charge carriers in such materials depends strongly on the wavelength of incident light and is the primary parameter that defines efficacy of the materials in photocatalytic bromination of electron rich aromatic compounds. In dual Ni-photocatalysis, excitation of electrons from the intraband states to the conduction band of the CTF with 625 nm photons enables selective formation of C‒N cross-coupling products from arylhalides and pyrrolidine, while an undesirable dehalogenation process is completely suppressed.

Ab initio Investigation of the Structural and Electronic Properties of Alkaline Earth Metal - TiO2 Natural Polymorphs

Titanium (IV) oxide (TiO2) has gained much attention due to its application in technologies such as optoelectronics, electronics, sensors, photocatalysts, and sustainable energy generation. However, its optical absorption falls in the ultraviolet part of the electromagnetic spectrum, resulting in a low absorption ratio of solar light. In addition, rapid electron-hole recombination limits its photocatalytic activity. To extend the application range of TiO2, the structural and chemical properties can be modified by adding various dopants to tune its electronic structure for applications within a wider range of the solar energy spectrum and ideally extend towards the visible region, which forms the dominant part of the solar energy spectrum. In this study, the structural and electronic properties of three polymorphs of TiO2 have been studied using density functional theory (DFT) as implemented in the Quantum ESPRESSO simulation package. The exchange-correlation potential has been treated with the generalised gradient approximation (GGA). Cationic substitution with non-toxic alkaline earth metal dopants Mg and Ca has been carried out with the aim of modifying the electronic structure of the polymorphs of TiO2. On 1–4% Mg and Ca cationic substitution, there is a slight expansion of the optimal unit cell volume and modulation of the band gap energy by raising the valence band maximum to higher energies. In addition, dopant inter and intra-band states are observed.

Grain Size-Engineering of PbS Colloidal Quantum Dots-Based NO2Gas Sensor

The large surface-to-volume ratio and highly reactive surfaces, combined with solution-processability make colloidal quantum dots (CQDs) promising to be the next generation of gas-sensing materials. However, the sizeengineering gas-sensing behavior of CQDs has not been further studied. Herein, we report the controllable synthesis of PbS CQDs with diameter sizes ranging from 2.7-7.0 nm through adjusting the injection temperature, precursor concentration and cooling rate. The results showed that the method of cold bath cooling exposed more {100} facets and enhanced NO2-sensing properties at room temperature. According to the first principle calculation, the {100} facets of PbS CQDs favors the competitive adsorption of NO2 with O2 molecules, which is in contrast with the {111} facets. At the same time, rapid cooling rate helps to increase the density of intra-band trap states of PbS CQDs, which may favor the interdot carrier hopping so as to transduce the charge transfer during the competitive adsorption between NO2 and O2 molecules into enhanced electrical current. This work reveals the microstructural mechanism underlying the size engineering of CQDs-based gas sensors.

Chromoselective Synthesis of Sulfonyl Chlorides and Sulfonamides with Potassium Poly(heptazine imide) Photocatalyst.

Among external stimuli used to promote a chemical reaction, photocatalysis possesses a unique one—light. Photons are traceless reagents that provide an exclusive opportunity to alter chemoselectivity of the photocatalytic reaction varying the color of incident light. This strategy may be implemented by using a sensitizer capable to activate a specific reaction pathway depending on the excitation light. Herein, we use potassium poly(heptazine imide) (K‐PHI), a type of carbon nitride, to generate selectively three different products from S‐arylthioacetates simply varying the excitation light and otherwise identical conditions. Namely, arylchlorides are produced under UV/purple, sulfonyl chlorides with blue/white, and diaryldisulfides at green to red light. A combination of the negatively charged polyanion, highly positive potential of the valence band, presence of intraband states, ability to sensitize singlet oxygen, and multi‐electron transfer is shown to enable this chromoselective conversion of thioacetates.

Engineering Individual Oxygen Vacancies: Domain-Wall Conductivity and Controllable Topological Solitons.

Nanoscale devices that utilize oxygen vacancies in two-dimensional metal-oxide structures garner much attention due to conductive, magnetic, and even superconductive functionalities they exhibit. Ferroelectric domain walls have been a prominent recent example because they serve as a hub for topological defects and hence are attractive for next-generation data technologies. However, owing to the light weight of oxygen atoms and localized effects of their vacancies, the atomic-scale electrical and mechanical influence of individual oxygen vacancies has remained elusive. Here, stable individual oxygen vacancies were engineered in situ at domain walls of seminal titanate perovskite ferroics. The atomic-scale electric-field, charge, dipole-moment, and strain distribution around these vacancies were characterized by combining advanced transmission electron microscopy and first-principle methodologies. The engineered vacancies were used to form quasi-linear quadrupole topological defects. Significant intraband states were found in the unit cell of the engineered vacancies, proposing a meaningful domain-wall conductivity for miniaturized data-storage applications. Reduction of the Ti ion as well as enhanced charging and electric-field concentration were demonstrated near the vacancy. A 3–5% tensile strain was observed at the immediate surrounding unit cells of the vacancies. Engineering individual oxygen vacancies and topological solitons thus offers a platform for predetermining both atomic-scale and global functional properties of device miniaturization in metal oxides.

Double-Nozzle Flame Spray Pyrolysis as a Potent Technology to Engineer Noble Metal-TiO2 Nanophotocatalysts for Efficient H2 Production

Noble metal-TiO2 nanohybrids, NM0-TiO2, (NM0 = Pt0, Pd0, Au0, Ag0) have been engineered by One-Nozzle Flame Spray Pyrolysis (ON-FSP) and Double-Nozzle Flame Spray Pyrolysis (DN-FSP), by controlling the method of noble metal deposition to the TiO2 matrix. A comparative screening of the two FSP methods was realized, using the NM0-TiO2 photocatalysts for H2 production from H2O/methanol. The results show that the DN-FSP process allows engineering of more efficient NM0-TiO2 nanophotocatalysts. This is attributed to the better surface-dispersion and narrower size-distribution of the noble metal onto the TiO2 matrix. In addition, DN-FSP process promoted the formation of intraband states in NM0-TiO2, lowering the band-gap of the nanophotocatalysts. Thus, the present study demonstrates that DN-FSP process is a highly efficient technology for fine engineering of photocatalysts, which adds up to the inherent scalability of Flame Spray Pyrolysis towards industrial-scale production of nanophotocatalysts.

Sintering‐driven effects on the band gap of (Pb,La)(Ti,Ni)O3 photovoltaic ceramics

AbstractIn this work, the influence of the sintering temperature on the physical properties of (Pb0.8La0.2)(Ti0.9Ni0.1)O3 (PLT‐Ni) ceramics is reported. The experimental data revealed that the energy band gap of PLT‐Ni ceramics could be tailored from approximately 2.7 to 2.0 eV by changing the sintering temperature from 1100°C to 1250°C. It is demonstrated that the simple substitution of Ti4+ by Ni2+ cations is effective to decrease the intrinsic band gap while increasing the tetragonality factor and the spontaneous polarization. However, the additional red‐shift observed in the absorption edge of the PLT‐Ni with increasing the sintering temperature was associated with a continuous increase in the oxygen vacancies () amount. It is believed that the impact of the creation of these thermally induced is manifold. The presence of and Ni2+ ions generate the Ni2+‐ defect‐pairs that promoted both a decrease in the intrinsic band gap and an additional increase of the tetragonality factor, consequently, increasing the spontaneous polarization. The creation of Ni2+‐ defects also changed the local symmetry of Ni2+ ions from octahedral to a square pyramid, thus lifting the degeneracy of the Ni2+ 3d orbitals. With the increase in the sintering temperature, lower‐energy absorbing intraband states were also formed due to an excess of , being responsible for an add‐on shoulder in the absorption edge, extending the light absorption curve to longer wavelengths and leading to an additional absorption in “all investigated” spectrum as well.

Potassium Poly(Heptazine Imide): Transition Metal-Free Solid-State Triplet Sensitizer in Cascade Energy Transfer and [3+2]-cycloadditions.

Polymeric carbon nitride materials have been used in numerous light‐to‐energy conversion applications ranging from photocatalysis to optoelectronics. For a new application and modelling, we first refined the crystal structure of potassium poly(heptazine imide) (K‐PHI)—a benchmark carbon nitride material in photocatalysis—by means of X‐ray powder diffraction and transmission electron microscopy. Using the crystal structure of K‐PHI, periodic DFT calculations were performed to calculate the density‐of‐states (DOS) and localize intra band states (IBS). IBS were found to be responsible for the enhanced K‐PHI absorption in the near IR region, to serve as electron traps, and to be useful in energy transfer reactions. Once excited with visible light, carbon nitrides, in addition to the direct recombination, can also undergo singlet–triplet intersystem crossing. We utilized the K‐PHI centered triplet excited states to trigger a cascade of energy transfer reactions and, in turn, to sensitize, for example, singlet oxygen (1O2) as a starting point to synthesis up to 25 different N‐rich heterocycles.

Absorption and emission modulation in a MoS2–GaN (0001) heterostructure by interface phonon–exciton coupling

Semiconductor heterostructures based on layered two-dimensional transition metal dichalcogenides (TMDs) interfaced to gallium nitride (GaN) are excellent material systems to realize broadband light absorbers and emitters due to their close proximity in the lattice constants. The surface properties of a polar semiconductor such as GaN are dominated by interface phonons, and thus the optical properties of the vertical heterostructure are influenced by the coupling of these carriers with phonons. The activation of different Raman modes in the heterostructure caused by the coupling between interfacial phonons and optically generated carriers in a monolayer MoS2–GaN (0001) heterostructure is observed. Different excitonic states in MoS2 are close to the interband energy state of intraband defect state of GaN. Density functional theory (DFT) calculations are performed to determine the band alignment of the interface and revealed a type-I heterostructure. The close proximity of the energy levels and the excitonic states in the semiconductors and the coupling of the electronic states with phonons result in the modification of carrier relaxation rates. Modulation of the excitonic absorption states in MoS2 is measured by transient optical pump-probe spectroscopy and the change in emission properties of both semiconductors is measured by steady-state photoluminescence (PL) emission spectroscopy. There is significant red-shift of the C excitonic band and faster dephasing of carriers in MoS2. However, optical excitation at energy higher than the bandgap of both semiconductors slows down the dephasing of carriers and energy exchange at the interface. Enhanced and blue-shifted PL emission is observed in MoS2. GaN band-edge emission is reduced in intensity at room temperature due to increased phonon-induced scattering of carriers in the GaN layer. Our results demonstrate the relevance of interface coupling between the semiconductors for the development of optical and electronic applications.

Ni doping in CuO: A highly sensitive electrode for sensing ammonia in ppm level using lanthanum gallate based electrolyte

Ni2+ doped CuO (Cu1-xNixO for 0 ≤ x ≤ 0.3) nanocrystalline powders were synthesized by combustion technique. The doped samples were found single phasic from powder XRD for up to 10 mol%, above which precipitation of NiO phase was observed. Raman study however indicated the presence of Cu2O phase in both undoped and doped samples up to 1 mol% Ni2+ doping. At and above 2 mol% doping, precipitation of the NiO phase was observed. Morphology of powders got modified drastically upon doping. Band gap showed decreasing trend with the increase of doping concentration in CuO, attributed to the formation of intra-band defect states. XPS revealed the presence of adsorbed oxygen or carbonate species on the surface. For the first time, Ni2+ doped CuO is reported as catalyst (active electrode) for ppm level NH3 sensing in amperometric mode with high sensitivity of 135.5 μA/decade at 550 °C using LaGaO3 as solid electrolyte.

Efficient photocatalytic water-splitting performance by ternary CdS/Pt-N-TiO2 and CdS/Pt-N,F-TiO2: Interplay between CdS photo corrosion and TiO2-dopping

22Z-Scheme CdS/Pt-N-TiO2 and CdS/Pt-N,FTiO2 nanocatalysts were synthesized using a sol – gel impregnation method and evaluated for their photocatalytic H2/O2 production via overall water splitting, with no extrernal electron or hole acceptors. The 0.5CdS/Pt-N-TiO2 material achieved a photocatlytic production of 639 μmol/g/h of H2 in tandem with 319 μmol/g/h of O2. The photocatalytic H2/O2 production data show that N-incorporation in the TiO2 lattice boosts overall water splitting, while F-incorporation inhibits the catalytic performance. Quantitative monitoring of the photogenerated Ti3+-surface and Ti3+-lattice electrons, as well as of the photogenerated holes (h+) by Electron Parmagnetic Resonce spectrsocopy, show that CdS/Pt-N-TiO2 achieves enhanced e−/h+ photogeneration due to intraband states generated by N-dopping, facilitating the flow of electrons via Pt to the valence band of CdS. The leaching of Cd2+ ions’ due to phootocorrosion of the CdS quantum dots, was montiored in-situ using Anodic StrippingVoltammtery (ASV). The Cd2+ leaching data reveal a severe inhibition of CdS photocorrosion ofN-dopped catalysts, CdS/Pt-N-TiO2. This reveals a dual beneficial role of N-atoms: [i] boosting the visible light photocactivity, and [ii] inhibiting CdS photocorrosoion. A consistent Z-scheme reaction mechanism is proposed for the catalytic H2/O2 production by CdS/Pt-N-TiO2 and CdS/Pt-N-F-TiO2 heterojunctions, taking into account the photoinduced e−/h+ dynamics as well as the interfacial {CdS} /{Pt-N/F-TiO2} chemistry.

Geometry-diversified Coulomb excitations in trilayer AAB stacking graphene

The lower-symmetry trilayer AAB-stacked graphene exhibits rich electronic properties and thus diverse Coulomb excitations. Three pairs of unusual valence and conduction bands create nine available interband excitations for the undoped case, in which the imaginary (real) part of the polarizability shows 1D square root asymmetric peaks and 2D shoulder structures (pairs of antisymmetric peaks and logarithm type symmetric peaks). Moreover, the low frequency acoustic plasmon, being revealed as a prominent peak in the energy loss spectrum, can survive in a narrow gap system with the large-density-of-states from the valence band. This type of plasmon mode is similar to that in a narrow gap carbon nanotube. However, the decisive mechanism governing this plasmon is the intraband conduction state excitations. Its frequency, intensity and critical momentum exhibit a non-monotonic dependence on the Fermi energy. The well-defined electron-hole excitation boundaries and the higher frequency optical plasmons are transformed by varying the Fermi energy. There remain substantial differences between the electronic properties of trilayer AAB, ABC, AAA and ABA graphene stackings.

Chemical, Structural, and Electronic Characterization of the (010) Surface of Single Crystalline Bismuth Vanadate

We have structurally, chemically and electronically characterized the most stable (010) surface of a Mo-doped BiVO4 single crystal. Low energy electron diffraction (LEED) reveals that the surface is not significantly reconstructed from a bulk termination of the crystal. Synchrotron based X-ray spectroscopies indicate no surface enhancement of any of the crystal constituents and that the Mo dopant occupies tetrahedral sites by substituting for V at the surface. Using resonant photoemission to study the valence band structure as the V L3 edge is scanned we observe an intraband gap state associated with reduced vanadium formed by the Mo doping. This state is likely associated with small polaron formation. This feature is enhanced at a photon energy that is not resonant with any of the main features in the absorption spectrum of the pristine BiVO4. This indicates that the additional electron from Mo doping likely induces further distortion of the VO4 tetrahedral units and generates a new conduction band state...

Model studies of the structure and optical properties of the TiO2(110) surface with an adsorbed Ag atom

ABSTRACTThe present studies of the atomic Ag adsorbate on the substrate TiO2(110) explore the importance of dispersion (or van der Waals) energies for determining the structure of the adsorbed Ag atom, using density functional theory (DFT) supplemented by a dispersion energy treatment, within the PBE-D3 treatment. It is also of interest to explore electronic excitation by light absorption. Electronic density of states (EDOS) are obtained without and with Ag adsorbed on the TiO2(110), to find the extent of change on the density of valence, conduction and intraband states. This is done using the hybrid HSE06 functional, which is known to provide good values for the energy band gap of the substrate. A computationally efficient PBE + BG procedure for these structures, which corrects the PBE band gap, is implemented to generate accurate EDOSs and light absorption intensities versus photon energies. This is followed by a reduced density matrix treatment of the dissipative dynamics of light absorption, generating state-to-state oscillator strengths and photoabsorbances for the pure and nanostructured TiO2(110) surfaces. Adsorption of Ag leads to a noticeable increase in light absorption at visible wavelengths, and very large increases in the UV region of the spectrum.