Bottom Of Band Research Articles

Distinct Effects of Selective Sn Doping at A or B Sites in BaTiO3

The effects of selectively doping Sn into the A‐site or B‐site of BaTiO3 on its crystal structure and electronic and optical properties are systematically evaluated using first‐principles calculations, leading to several intriguing conclusions. After doping Sn at the A‐site, impurity bands originating from Sn are generated, which alter the direct bandgap to an indirect one and decrease the bandgap value, thereby reducing the electron excitation energy required. Meanwhile, (Ba0.875Sn0.125)TiO3 (A‐site) exhibits p‐type semiconductor characteristics with holes as carriers, which is beneficial for enhancing the conductivity of BaTiO3. The introduction of Sn–O interactions through A‐site Sn doping reduces the density of states of O‐2p near the valence band maximum, enhances the mobility of oxygen ions, and promotes the occurrence of redox reactions. In contrast, when Sn is doped at the B‐site, the bottom of the conduction band shifts toward higher energy levels, leading to a significant increase in the bandgap. In addition, (Ba0.875Sn0.125)TiO3 (A‐site doped) and Ba(Ti0.875Sn0.125)O3 (B‐site doped) exhibit distinct optical behaviors, thereby expanding the optical application range of BaTiO3‐based materials. This study highlights the distinct impacts of Sn doping at the A‐site or the B‐site on the modification of BaTiO3 properties, thereby paving a novel path for designing advanced materials.

Structural and electronic properties of GaN: Ab initio study within LDA and LDA+U methods

Structural and electronic properties of the GaN were simulated based on Density Functional Theory implementing Local Density Approximation methods. Hubbard U correction gives us an opportunity to find the correct energy gap for GaN in agreement with known experimental results. Choosing more accurate investigation methods leads to calculating accurate electronic band structure and in the future predicting some physical properties of related material. The bottom of the conduction band and the top of the valence band are formed mainly by p-orbitals of host Ga and N atoms. The present study shows the direct band gap character of GaN with a wurtzite structure

A first-principle study on the band structure of GePb alloys

Abstract Single crystal GePb alloys have been considered as potential direct bandgap materials for optoelectronics application. In this work, density-functional theory calculations were performed to investigate the crystalline and electronic structures of the GePb alloys. The lattice constants of the unstrained GePb alloys are found positively deviating from Vegard’s law with a bowing coefficient of 0.587 Å. GePb has a higher Poisson’s ratios than GeSn with a similar alloying concentration. With the increasing Pb concentration x in Ge1−x Pb x , a new alloying energy level brought by Pb appears at the bottom of the conduction band and continuously decreases. The new energy level is constructed to a new valley as compared to the initial Γ valley and the new energy level is acquiring its higher spectra weights with increasing Pb concentration. An indirect-to-direct bandgap transition occurs with a Pb concentration of 3.3%. The effective masses of holes and electrons in the GePb Γ valley are calculated to decrease with the increasing Pb concentration, while the effective masses of the electrons in the L valley only change slightly. The small effective masses of the electrons in the Γ valley are favorable for high-speed GePb device application.

Revisiting Band Tails and Localized States in Amorphous Semiconductors

Amorphous semiconductors, such as hydrogenated amorphous silicon (a-Si:H) and amorphous indium gallium zinc oxide (a-IGZO) have been central to the development of thin film transistors (TFTs) for large-area electronic applications such as displays. This is a result of the exceptional material uniformity that can be achieved when depositing over very large areas (~10 m2). This means that all TFTs are essentially identical over an entire large display panel. However, there is a technological price to pay – the carrier mobility is generally less in amorphous semiconductors compared to their crystalline equivalent.The origin of this reduced carrier mobility is a result of a key difference in the density of states between crystalline semiconductors and their amorphous counterparts. The long-range order of a crystalline lattice means that all carrier states in the conduction and valence bands exist over long distances. These ‘extended states’ are associated with generally high mobility carriers. However, there is no long-range order in amorphous semiconductors, but only short-range order over one to two bond lengths associated with local atomic coordination requirements (and to some degree a medium-range order over a few bond lengths). The result is that the bottom of the conduction band and top of the valence band gain ‘tail’ states which are localized in nature.Not only do these localized states frequently dominate (or at least significantly affect) conduction in amorphous semiconductors and hence the switching characteristics of TFTs, but they also have a role to play in other important processes such as hysteresis and bias stress instability. But what are these localized band tail states, how should we think about them in the context of conduction and how do they relate to extended states?These questions are revisited from the starting point that there is a perturbation in the delocalized charge concentration with position in an amorphous semiconductor compared to its crystalline counterpart, and this can be quantified as a spatially varying excess charge concentration (where a deficit is just a negative excess). The rate of spatial variation is long relative to the rate of change in the electron wavefunction, and reflects the short- and medium-range order distances. Charge conservation means that that this excess charge concentration will have a mean of zero and a Gaussian probability distribution is assumed. A model using this as a starting point has been created which reproduces band tails and which provides new insights into their physics which are discussed.

Strain‐Engineered Level Alignment in the MoTe2/WSe2 Heterobilayer

When stacked into heterostructures, layered transition metal dichalcogenides (TMDCs) exhibit peculiar electronic and optical properties that may differ substantially from those of their constituents and that largely depend on the level alignment. For example, the MoTe2/WSe2 heterobilayer exhibits a type I band lineup, which can be exploited in emitting devices but limits charge separation. In this first‐principle study based on density functional theory and many‐body perturbation theory, strain is proposed as an effective means to make MoTe2/WSe2 a type II heterostructure. By exploring several configurations where biaxial strain is applied to either or both monolayers, the top of the valence band and the bottom of the conduction band are found at different points in k‐space leading to an indirect‐to‐direct bandgap transition when the lattice constant of WSe2 is expanded by 3.2% or more. In terms of optical properties, all considered systems exhibit a first dark excitation, consistently shifting in energy with the direct electronic gap, while the absorption onset does not vary significantly with strain. Our findings suggest strain as a powerful tool for fine tuning the electronic and optical properties of TMDC heterostructures while preserving their fundamental characteristics, thus opening new avenues for designing optoelectronic applications.

What Can Chemical Bonding Tell Us about Photoinduced Phase Transition Reactions in Inorganic Semiconductors? Insight from Bismuth-Antimony Selenide.

Photoreactive self-healing semiconductors with suitable bandgaps for solar energy conversion offer an intriguing path to making resilient and low-cost photovoltaic devices through the introduction of a self-recovery path. However, only a few inorganic photovoltaic materials have such quality, and the underlying chemical properties that enable it are unknown, which poses a significant limit to our ability to study and discover new self-healing semiconductors. Recently, we have found that antimony trichalcogenide (Sb2Se3 and Sb2S3) and chalcohalides (e.g., SbSeI) can undergo a reversible photoinduced phase transition (PIPT) in which the structure is restored after photoinduced damage incurs to the materials. This group of materials offer a unique opportunity for studying PIPT and its limits. In particular, this group of materials facilitate the study of functional permutation to specific crystalline sites and to finding the limits of PIPT occurrence, which sheds light on the origin of the PIPT and self-recovery of this class of materials. Using Raman spectroscopy of thin films, and following signature vibrations of transition species, we have found that the PIPT magnitude decays upon gradual BiSb(1) substitution in a Sb2-xBixSe3 homologous series, until nearly one in five Sb ions is substituted with Bi. Then, the PIPT diminishes completely. The homologous series occurs along a transition from covalent to metavalent chemical bonding. By expanding our search, we find that a correlation between bonding type and photoreactivity does exist but conclude that it is an insufficient condition. Instead, we suggest, based on bond order and additional DFT calculations, that sufficient bonding states at the bottom of the conduction band are also required. This joint experimental and computational study pushes the limits of designing self-healing inorganic semiconductors for various applications and provides tools for further expansion.

Fe-N Co-Doped BiVO4 Photoanode with Record Photocurrent for Water Oxidation.

Bismuth vanadate (BVO) ranks among the most promising photoanodes for photoelectrochemical (PEC) water splitting. Nonetheless, slow charge separation and transport, besides the sluggish water oxidation kinetics, are key barriers to its photoefficiency. Here, we present a co-doping strategy that significantly improves the charge separation performance of BVO photoanodes. We found that, under standard one sun illumination, the Fe-N co-doped BVO photoanode (Fe-N-BVO) by N-coordinated Fe precursor reaches a record photocurrent density of 7.01 mA cm-2 at 1.23 V vs RHE after modified a surface co-catalyst (FeNiOOH), and exhibits an outstanding stability. By contrast, much lower photocurrent density is obtained for the N-doped, Fe-doped and Fe/N-doped BVO photoanode with separated N and Fe precursors. The detailed experimental characterizations show that the high activity of the Fe-N co-doped BVO photoanode is attributed to the enhanced photo-induced bulk charge separation, as well as the accelerated surface water oxidation kinetics. XPS, EXAFS and DFT calculations clearly show that, instead of formation of deep trapping state in the individually doped BVO, the co-doping of Fe-N into BVO generates Fe-based electronic states just below the bottom of conduction band and N-derived states just above the top of valence band. Such modulations in electronic structure enable the efficient trap of the electrons and holes to enhance the separation of photo-induced carriers, but hinder the charge recombination originated from the deep trapping sites.

Fe−N Co‐Doped BiVO4 Photoanode with Record Photocurrent for Water Oxidation

AbstractBismuth vanadate (BVO) ranks among the most promising photoanodes for photoelectrochemical (PEC) water splitting. Nonetheless, slow charge separation and transport, besides the sluggish water oxidation kinetics, are key barriers to its photoefficiency. Here, we present a co‐doping strategy that significantly improves the charge separation performance of BVO photoanodes. We found that, under standard one sun illumination, the Fe−N co‐doped BVO photoanode (Fe−N−BVO) by N‐coordinated Fe precursor reaches a record photocurrent density of 7.01 mA cm−2 at 1.23 V vs RHE after modified a surface co‐catalyst (FeNiOOH), and exhibits an outstanding stability. By contrast, much lower photocurrent density is obtained for the N‐doped, Fe‐doped and Fe/N‐doped BVO photoanode with separated N and Fe precursors. The detailed experimental characterizations show that the high activity of the Fe−N co‐doped BVO photoanode is attributed to the enhanced photo‐induced bulk charge separation, as well as the accelerated surface water oxidation kinetics. XPS, EXAFS and DFT calculations clearly show that, instead of formation of deep trapping state in the individually doped BVO, the co‐doping of Fe−N into BVO generates Fe‐based electronic states just below the bottom of conduction band and N‐derived states just above the top of valence band. Such modulations in electronic structure enable the efficient trap of the electrons and holes to enhance the separation of photo‐induced carriers, but hinder the charge recombination originated from the deep trapping sites.

Three Metal Complex‐Decorated Halobismuthates: Syntheses, Structures, Semiconducting Behaviors and Theoretical Investigations

AbstractEmploying metal complex cations as the structural modifiers, three new numbers of hybrid halobismuthates family, namely [NH4][Co(phen)3]BiI6 (phen=1,10‐phenanthroline; 1), [Zn(phen)2Br][Bi(phen)Br4] ⋅ H2O (2) and [Cu(2,2′‐bipy)2Br][Cu(2,2′‐bipy)2Bi2Br9] ⋅ H2O (2,2′‐bipy=2,2′‐bipyridine; 3) have been solvothermally constructed and structurally characterized. X‐ray crystallography analyses show that all compounds characteristic of abundant weak interactions feature the discrete anionic motifs. The obtained hybrid materials possess the semiconducting nature, with the visible light responding optical band gaps of 2.13–2.71 eV. Under the alternating light irradiation, they exhibit excellent photoelectric switching properties, whose photocurrent densities (0.85, 0.24, and 0.43 μA/cm2 for compounds 1–3, respectively) compare well with some high‐performance halobismuthate competitors. Further theoretical calculations of compound 1 reveal that the photosensitive [Co(phen)3]2+ cations contribute greatly to the bottom of conduction bands (CBs) and the top of valence bands (VBs), which may be the important reason for its excellent photoelectric behavior. The analyses of Hirshfeld surface, thermal stability, and X‐ray photoelectron spectroscopy were also discussed detailedly in this paper.

Visualization of spin-polarized surface resonances in Pb-based ternary topological insulators

The formation of the topological surface state originates from bulk band inversion at the top of the valence band and the bottom of the conduction band. The transition between normal and topological insulators is known as a topological phase transition. Here we show spin-polarized electronic states of Pb-based ternary topological insulators Pb(Bi1-xSbx)2Te4 (x = 0.55, 0.70, 0.79) investigated by spin- and angle-resolved photoemission spectroscopy and first-principles calculations. We visualize a pair of spin-polarized surface resonances dispersing along the upper edge of projected bulk bands in occupied states. Interestingly, a branch of the spin-polarized surface resonances continuously connects to the topological surface state. The coexistence of the topological surface state and the spin-polarized surface resonances can be explained by considering the topological phase transition.

First‐Principle Investigation of zb‐TiGe: A Promising Narrow Bandgap Semiconductor

Using first‐principle calculations, a new compound semiconductor based on titanium (Ti) is predicted. It has been observed that Ti when alloyed with germanium (Ge) can crystallize in the zincblende symmetry, and exhibits semiconducting nature with a narrow bandgap. To test the robustness of the semiconducting nature, the structural properties and electronic band structure have been modeled using three different exchange–correlation functionals, namely, local density approximation, generalized gradient approximation, and Perdew‐Burke‐Ernzerhof for solids. With a nominal composition of 1:1, TiGe exhibits a direct bandgap of 0.58 eV at the “X” point of the Brillouin zone. From lattice dynamics, the structural stability of the compound has been established. The new semiconductor is characterized in detail for its electronic band structure, carrier effective mass, charge density distribution, and linear optical properties. Zb‐TiGe exhibits large effective mass for conduction band holes and tentatively becomes a prototype system to study heavy holes resulting in carrier condensation at the bottom of the conduction band. The linear optical properties of the TiGe are found to be suitable for photosensitive devices in the infrared region. In addition, TiGe may find application in deep‐UV and far‐UV regions because of the surface plasmon resonance at 6.4 eV.

A universal molecular oxygen-mediated photocatalysis strategy to boost visible-light induced hydrogen evolution through partial water splitting

A universal oxygen-mediated, stepwise strategy is proposed for efficiently inducing visible-light photocatalytic partial water decomposition into hydrogen over various semiconductor photocatalysts with conduction band bottoms below the single-electron oxygen reduction potential. In this scenario, molecular O2 can be transformed into reactive oxygen species, serving as both an oxidant and a homogeneous catalyst for producing hydrogen from alkaline aqueous solution containing various organic substrates. Further enhancement the performance is achieved by doping with phosphorous and oxygen, which constructs a local internal electric field and introduces sulfur vacancies, thereby facilitating the transport of photogenerated charge carriers, particularly on a representative CdS photocatalyst. The optimal hydrogen evolution performance reaches 2321.4 and 8521.4 μmol·gcatatlyst−1·h−1 in methanol and formaldehyde solution systems, respectively, with an apparent quantum efficiency exceeding 59.4 % under 450 nm visible light irradiation. Mechanistic studies demonstrate that the oxygen-mediated, sequential single-electron transfer process can occur with virtually zero activation energy.

Enhanced photocatalytic degradation of organic dyes by La3+ doped SrBi4Ti4O15

The rampant misuse of organic dyes has exacerbated environmental pollution to an alarming degree. SrBi4Ti4O15, a layered perovskite semiconductor material, demonstrates considerable potential in the degradation of organic dyes, and efforts have been made to enhance its photocatalytic efficiency. In this study, La3+ doping was utilized to synthesize SrBi4-xLaxTi4O15 (x = 0, 0.10, 0.15, 0.25, 0.40) nanosheet photocatalysts, and their photocatalytic performance and underlying mechanisms were comprehensively investigated. XRD and Raman spectroscopy revealed that La3+ doping replaced Bi3+ at the A-site, resulting in a reduction of the interlayer spacing of [TiO6] octahedra. Combined with DFT calculations, La3+ doping was found to widen the band gap of SrBi4Ti4O15 and raise the Fermi level. Additionally, La3+ doping significantly increased the ratio of the Ti 3d component at the bottom of the conduction band, favoring the migration of photogenerated electrons along the [TiO6] octahedra and accelerating the separation of photogenerated carriers. SEM and BET analyses revealed that La3+ doping not only increased the size of the nanosheets but also significantly enhanced their specific surface area, thereby exposing more active sites. Degradation experiments of Rhodamine B (Rh B) under UV light demonstrated that the photocatalytic efficiency of SrBi3.75La0.25Ti4O15 (k = 0.0514 min-1) was 2.66 times higher than that of the original SrBi4Ti4O15 (k = 0.0193 min-1). The theoretical degradation pathway of Rh B was elucidated using LC-MS analysis and the developmental toxicity of potential intermediates was analyzed. Moreover, the SrBi3.75La0.25Ti4O15 photocatalyst also exhibited excellent degradation efficacy against Methylene Blue (MB), Crystal Violet (CV), and Methyl Orange (MO). This study provides both theoretical and experimental insights into improving the photocatalytic performance of SrBi4Ti4O15, thereby enhancing its potential for industrial application.

Reconfigurable directional selective tunneling of p-type phonons in polarized elastic wave systems

There have been many studies on the ground state of phononic crystals, but few studies on the nature of the excited state. In this paper, we introduce the asymmetric Klein tunneling method in phononic systems, which enables selective direction and control of elastic waves by inducing polarization through an external field in an artificial barrier. Using tight-binding theory, we systematically studied phononics in a super-honeycomb structure, demonstrating the anisotropic transition properties of excited state phonons through a matrix model involving the ground state’s s-type and the first excited state’s p-type wave functions. Additionally, we exploit the thermal sensitivity of epoxy to achieve localized thermal field-induced distortion of the bottom flat band, forming a tiled Dirac cone, and thereby creating a reconfigurable polarized artificial barrier in the elastic wave system. Elastic waves propagate in these systems with topological charge conservation. Combining this with the theoretical of excited-state phonons, we demonstrate asymmetric Klein tunneling with directional selectivity in the elastic wave carriers, and systematically investigate the performance of this tunneling. Furthermore, we design a four-port elastic waveguide based on the principle of asymmetric tunneling, which achieves exceptional directional selectivity of elastic wave signals by adjusting the polarization direction of barriers at the junctions. These studies not only extend the application of Klein tunneling in elastic wave systems but also open new research avenues for the study of elastic wave devices controlled by various external fields, showing direct application potential in elastic wave signal processing.

First-principles calculations of cubic boron arsenide surfaces

The properties of cubic boron arsenide (c-BAs) (100), (110), and (111) surfaces are investigated by performing first-principles calculations using the slab and Green's function surface models with different terminals. The (111) surface with As-termination is found to be the most stable structure among the studied surfaces, with its lowest surface energy (1.70–1.92 J m−2) and largest surface density (20.24 nm−2). The electronic affinity of these surfaces lie in the range 4.62–6.17 eV, which is higher than that of common semiconductor materials, such as silicon (4.05 eV) and germanium (4.13 eV), implying that the electrons at the bottom of the conduction band require more energy to escape. The surface states of the structures with As-termination in the surface band structures are generally more numerous and extended than those with B-termination. The absorption peak of the bulk c-BAs is located in the ultraviolet region, and the light absorption ranges of the surfaces are significantly extended compared with the bulk c-BAs, due to the surface states inside the bandgap.

Analysis of energy bands and density of states of CuFe(SxSeyTe1-x-y)2 by calculating

Copper based sulfur compounds have excellent optoelectronic properties which can be regulated through chemical doping, and are the candidate materials for solar cells, photocatalysis and other fields. The crystal models of CuFe(SxSeyTe1-x-y)2 (x = 0.125,0.25,0.25,0.5; y = 0.125,0.25,0.5,0.25) are established, and the energy bands and density of states of CuFe(SxSeyTe1-x-y)2 are calculated using first principles. The analysis results show that as the doping of S atoms increases, the lattice constants a and b gradually increase, while the lattice constants c and cell volume V become smaller and smaller; As the doping of Se atoms increases, the lattice constants a and b gradually increase, while the lattice constant c and cell volume V become smaller and smaller. The valence band top and conduction band bottom of CuFe(SxSeyTe1-x-y)2 (x = 0.125,0.25,0.25,0.5; y = 0.125,0.25,0.5,0.25) are at the same point G. It can be explained that CuFe(SxSeyTe1-x-y)2 is a direct bandgap semiconductor, and the bandgap widths of the selected crystal structures corresponding to the values of x and y are approximately 0.98, 0.97, 0.91 and 0.98 eV, respectively. The diagram for the density of states has peaks on both sides of the Fermi level, and other peaks in the density of states figure also correspond to the band graph. The size and trend of the peaks reflect the strength of electron delocalization.

Study on the Infrared and Raman spectra of Ti3AlB2, Zr3AlB2, Hf3AlB2, and Ta3AlB2 by first-principles calculations

In this paper, the crystal geometry, electronic structure, lattice vibration, Infrared and Raman spectra of ternary layered borides M3AlB2 (M = Ti, Zr, Hf, Ta) are studied by using first principles calculation method based on the density functional theory. The electronic structure of M3AlB2 indicates that they are all electrical conductors, and the d orbitals of Ti, Zr, Hf, and Ta occupy most of the bottom of the conduction band and most of the top of the valence band. Al and B have lower contributions near their Fermi level. The lightweight and stronger chemical bonds of atom B are important factors that correspond to higher levels of peak positions in the Infrared and Raman spectra. However, the vibration frequencies, phonon density of states, and peak positions of Infrared and Raman spectra are significantly lower because of heavier masses and weaker chemical bonds for M and Al atoms. And, there are 6 Infrared active modes A2u and E1u, and 7 Raman active modes, namely A1g, E2g, and E1g corresponding to different vibration frequencies in M3AlB2. Furthermore, the Infrared and Raman spectra of M3AlB2 were obtained respectively, which intuitively provided a reliable Infrared and Raman vibration position and intensity theoretical basis for the experimental study.

Energy Band Specific to Injected Li-Ion-Induced Formation of Lithium Dendrites in Garnet Solid Electrolytes.

As one of the most significant challenges in solid-state batteries, thorough investigation is necessary on the formation process of lithium dendrites in solid-state electrolytes. Here, we reveal that the growth of lithium dendrites in solid electrolytes is a physical-electrochemical reaction process caused by injected lithium ions and electron carriers, which requires a low electrochemical potential. A unique energy band specific to injected Li ions is identified at the bottom of the conduction band, which can be occupied by electron carriers from low-potential electrodes, leading to dendrite formation. In this case, it is quantitatively determined that the employed anodes with higher working voltages (>0.2 V versus Li/Li+) can effectively prevent dendrite formation. Moreover, lithium dendrite formation exclusively occurs during the charging process (i.e., lithium plating), where lithium ions meet electrons at mixed conductive grain boundaries under highly reductive potentials. The proposed model has significant scientific significance and application value.

Modification of the electronic and optical properties of YVO4:Eu induced by different charge states of the Eu ion

By employing first-principles calculations within the frame of density functional theory, we investigated the modifications of the electronic and optical properties of isolated Eu impurities within YVO4:Eu3+ compound in the presence of additional electron charges. This scenario arises when YVO4:Eu3+ is exposed to gamma radiation that ejects electrons from certain ions and makes them available for capture by others. Our findings reveal that a significant portion of the additional charge is captured by the Eu3+ ion (0.32 e), but this portion is insufficient to confirm a change in the Eu oxidation state from 3+ to 2+. Nevertheless, this capture significantly modifies the Eu electronic configuration, elevating the energy of its occupied 4f-states and bringing them closer to the conduction band bottom. This fact causes the optical absorption of Eu to be generated by 4f → 5d electron transitions, instead of 4f → 4f as in the case of the non-irradiated compound. The presented results qualitatively explain drastic changes in the Eu emission spectrum induced by gamma irradiation of the YVO4:Eu3+, perceived experimentally.

New analysis of the temperature-dependent threshold density for electron self-trapping in gaseous helium.

We present the results of a new analysis of the literature data on electron mobility μ in dense helium gas aimed at determining the existence of a threshold density for electron self-trapping in gaseous helium as a function of temperature. We have investigated the density dependence of μ and, when available, its dependence on the electric field. The experimental data are favorably rationalized by minimizing the excess free energy of the self-localized states within the optimum fluctuation model. It is shown that the formation of electron bubbles via the self-trapping phenomenon is determined by the delicate balance between the electron thermal energy, the density dependence of the electron energy at the bottom of the conduction band in the gas, and the work necessary to expand the bubble. We show that the self-trapping phenomenon is not limited to low temperatures but occurs at any temperatures for large enough densities.