Bottom Of Conduction Band Research Articles

Multiple-valley effect on modulation of thermoelectric properties of n-type ZrCuSiAs-structure oxyantimonides LnTSbO (Ln= lanthanides and T=Zn, Mn)

Thermoelectric properties of n-type LnTSbO (Ln = lanthanides and T = Zn, Mn) were firstly investigated by the first-principles method and the semi-classical Boltzmann theory. The results show that a multiple-valley structure appears around the bottom of conduction band. The valley with a high band degeneracy consists of the bands with a weak band dispersion, leading to large magnitudes of the Seebeck coefficient but low electrical conductivity. The valley with a low band degeneracy is made up of the bands with an intense band dispersion, resulting in a high electrical conductivity but small magnitudes of the Seebeck coefficient. The thermoelectric properties are dominated by the energy difference, ΔE, between the valleys. The ΔE value of LnZnSbO linearly increases with the ionic radius of Ln. The thermoelectric properties are thus effectively modulated by varying the lanthanides. As a result, LnZnSbO (Ln = Ce-Nd) with the moderate values of ΔE shows a better thermoelectric performance. The multiple-valley effect is an effective way to modulate the thermoelectric properties of n-type LnTSbO.

Modeling and analysis of I-V hysteresis behaviors caused by defects in tin perovskite thin films

We analyzed the I-V hysteresis behaviors of tin perovskite (MASnI3, MA: CH3NH3) thin films using impedance spectroscopy coupled with charge modulation spectroscopy (CMS). The capacitance-voltage (C-V) characteristics of the ITO/MASnI3/Al device showed hysteresis behaviors, in accordance with the trap filling process suggested by the I-V characteristics. The CMS measurement indicated the enlargement of the energy bandgap of the MASnI3. On the basis of these results, we proposed a model that trap states in the vicinity of the bottom of conduction band are filled, and concluded that the trap filling process occurring at Sn vacancies makes a significant contribution to the electrical properties of tin perovskite film and the hysteresis of the I-V and C-V characteristics of our device. The I-V hysteresis is suppressed with the decrease of defects such as Sn vacancies in MASnI3 films.

Cubic-like BaZrO3 nanocrystals with exposed {001}/{011} facets and tuned electronic band structure for enhanced photocatalytic hydrogen production

Facet engineering to expose specific surfaces has received rapid growth attention to promote the photocatalytic performance. In this work, we reported that BaZrO3 nanocrystals with {001}/{011} facets and corresponding higher reducing capacity could effectively improve the photocatalytic hydrogen evolution in pure water. The tuned electronic band structure arising from exposed specific {001}/{011} facets and the higher surface area are the main reasons to promote photocatalytic activity. The conduction band bottom for BaZrO3 nanocrystals with {001}/{011} facets synthesized by solvothermal method (denoted as BZO-HT) is about 0.31 eV higher than that of sample prepared by hydrothermal reaction (denoted as BZO-H). During the evaluation of photocatalytic activity in pure water, the H2 production rate for BZO-HT (27.80 μmol/g/h) is 9.4 times and six times higher than BZO-H and commercial BaZrO3 (denoted as BZO-C), respectively. This work provides a reference for other facets-related photocatalysts’ design for pure water reduction or splitting.

First-principles study of electronic properties of Cu doped Ag2S

Using first-principles calculations based on density functional theory, electronic properties of Ag2S and Cu doped Ag2S were investigated. The calculated results show that Ag2S crystal is semiconductor with a band gap of 0.92 eV which is consistent with previous PBE calculation. Three possible Cu doping configurations are investigated: Cu substitution of Ag_I, Cu substitution of Ag_II, and interstitial Cu. It is found that interstitial Cu doping reduces the band gap as the Cu concentration increases, while for Ag_I and Ag_II substitution doping the band gap is not sensitive to the impurity concentration. The band gap reduction can mainly be attributed to the downward shift of the bottom of conduction band upon Cu interstitial doping. The reduction of the band gap for Ag2S with interstitial Cu doping is consistent with experimental absorption and fluorescence spectra, where a significant red-shift after Cu doping in Ag2S crystal is observed.

Direct Z-Scheme Cs2O-Bi2O3-ZnO Heterostructures as Efficient Sunlight-Driven Photocatalysts.

Limited light absorption, inefficient electron–hole separation, and unsuitable positions of conduction band bottom and/or valence band top are three major critical issues associated with high-efficiency photocatalytic water treatment. An attempt has been carried out here to address these issues through the synthesis of direct Z-scheme Cs2O–Bi2O3–ZnO heterostructures via a facile, fast, and economic method: solution combustions synthesis. The photocatalytic performances are examined by the 4-chlorophenol degradation test under simulated sunlight irradiation. UV–vis diffuse reflectance spectroscopy analysis, electrochemical impedance test, and the observed transient photocurrent responses prove not only the significant role of Cs2O in extending light absorption to visible and near-infrared regions but also its involvement in charge carrier separation. Radical-trapping experiments verify the direct Z-scheme approach followed by the charge carriers in heterostructured Cs2O–Bi2O3–ZnO photocatalysts. The Z-scheme charge carrier pathway induced by the presence of Cs2O has emerged as the reason behind the efficient charge carrier separation and high photocatalytic activity.

Tuning the Electronic Properties of Graphene Oxide Nanoribbons Through Different Oxygen Doping Configurations

The electronic properties of armchair graphene oxide nanoribbons (AGONRs) with different doped oxygen configurations are studied based on density functional theory using first principle calculations. The electronic properties of the AGONRs are tuned by different oxygen configurations for top edges, center, bottom edges and fifth width. The AGONRs for top-edge O doping configuration are indirect band gap semiconductors with an energy gap of 1.268 eV involving hybridization among C-2p and O-2s, 2p electrons and electrical conductivity of oxygen atoms. The center and bottom edges are direct band gap semiconductors with 1.317 eV and 1.151 eV, respectively. The valence band is contributed from C-2p, O-2p and H-1s for top-edge O doping. The electronic properties of AGONRs are changed due to localization in −2.94 eV of O-2p states. The center O-doped AGONRs are n-type semiconductors with Fermi levels near the conduction band bottom. This is due to hybridization among C-2s, 2p and O-2p electrons. However, bottom-edge O-doped AGONRs are p-type semiconductors, due to the electrical conductivity of oxygen atoms. The fifth-width O-doped AGONRs are indirect band gap semiconductors with an energy gap of 0.375 eV. The projected density of states shows that the localization and hybridization between C-2 s, 2p, O-2p and H-1s electronic states are rising in the conduction band and valence band from the projected density of states. The localization is induced by O-2p electronic states at a Fermi level.

Structural, electronic and optical properties of copper-doped SrTiO3 perovskite: A DFT study

First-principles calculations based on the density function theory (DFT) are implemented to explore the effect of Cu doping on the structural, electronic and optical properties of SrTiO3 Perovskite using generalized gradient approximation (GGA) and ultrasoft pseudo-potential (USP). The effects of Cu doping on the formation energy, electronic structure and optical property are discussed here. The results of formation energies infer that Cu doping at Sr site in SrTiO3 is more favorable as compared to Ti site. Cu incorporation at Sr site affects the electronic band structure of SrTiO3 significantly by introducing new states at Gamma point consequently transforming its indirect band gap into the direct one. The relocation of density of states to lower energies, and an increased interaction between Cu atom and its neighboring atoms after Cu doping in SrTiO3 are observed. Furthermore, the partial density of states of SrTiO3 changes significantly at the bottom of conduction band and hence we conclude that the Cu doping affects the electronic band structure of SrTiO3. Cu doped SrTiO3 offers enhanced optical properties and its conversion to direct band gap makes it an appealing candidate for optoelectronic devices.

Multilayer graphene shows intrinsic resistance peaks in the carrier density dependence

Since the advent of graphene, a variety of studies have been performed to elucidate its fundamental physics, or to explore its practical applications. Gate-tunable resistance is one of the most important properties of graphene and has been studied in 1–3 layer graphene in a number of efforts to control the band gap to obtain a large on-off ratio. On the other hand, the transport property of multilayer graphene with more than three layers is less well understood. Here we show a new aspect of multilayer graphene. We found that four-layer graphene shows intrinsic peak structures in the gate voltage dependence of its resistance at zero magnetic field. Measurement of quantum oscillations in magnetic field confirmed that the peaks originate from the specific band structure of graphene and appear at the carrier density for the bottoms of conduction bands and valence bands. The intrinsic peak structures should generally be observed in AB-stacked multilayer graphene. The present results would be significant for understanding the physics of graphene and making graphene FET devices.

Oxygen vacancy induced strong anisotropy of the thermoelectric properties of strontium barium niobate

Electronic structure and thermoelectric properties of strontium barium niobate were investigated by first principles calculations combined with the semi-classical Boltzmann theory. The band structure corresponding to lattice anisotropy shows higher band dispersion along c axis, but the conductive pathway is mainly along ab plane. As a result, the values of power factor are in the same order of magnitude along different directions. Thus, the lattice anisotropy alone cannot result in the strong anisotropic thermoelectric properties of strontium barium niobate. Oxygen vacancies not only act as charge carrier donors, but also enhance the band dispersion and change the magnetic quantum number of the electrons around the bottom of conduction band. Accordingly, the thermoelectric properties of strontium barium niobate show very strong anisotropy after oxygen reduction.

First-principles study on CH3NH3PbI3 materials for perovskite solar cells

CH3NH3PbI3 system of organic-inorganic hybrid perovskite structure solar cells absorption material was established by using first-principle calculations based on density functional theory (DFT). Orthorhombic CH3NH3PbI3 system model is optimized by different functional theories of CASTEP in Materials Studio. It revealed that the results of WC/GGA calculation method are more close to the experimental value. Electron band structure, state density are calculated in the paper and the results indicated that top orbital of the valence band is mainly contributed by Pb atom and I atomic orbital. At the same time, the conduction band bottom is mainly contributed by the orbital contribution of Pb atoms and strong σ bond is formed between terminal iodine atom and Pb atomic orbital. Moreover, the contribution of organic element density of states near the Fermi surface is very weak and the band gap of organic-inorganic hybrid CH3NH3PbI3 materials is basically determined by inorganic elements.

Energy Band Structure and Optical Properties of LiNaSO4 Crystals

The energy band structure of a LiNaSO4 has been calculated within the framework of density functional theory for space group P31c. It has been established that bandgap width Eg in the GGA approximation is 5.49 eV. The valence band top is generally formed of oxygen p-electrons, while the conduction band bottom is composed of lithium and sodium s-states. The effect of the cationic substitution Na → K → Rb → NH4 on the electron structure of LiBSO4 group crystals is considered. Based on the calculated dielectric functions, the spectral dependences of the refractive indices and extinction coefficient of a crystal were calculated and compared with experimental data.

Local coordination, electronic structure, and thermal quenching of Ce3+ in isostructural Sr2GdAlO5 and Sr3AlO4F phosphors

AbstractSr2GdAlO5:Ce and Sr3AlO4F:Ce are isostructural phosphors in which the Ce3+ 4f‐5d1 transition can be efficiently excited by a photon with energy lower than 3.1 eV. Herein, we analyze the crystal chemistry of the Ce3+ local coordination, compare the thermal quenching behavior and construct the electronic structure of Ce3+ in them. The Rietveld refinement on two occupancy models suggests that Gd3+ only occupies the 8h site in Sr2GdAlO5; this provides a hint on the preferred occupancy of dopant Ce3+ in this site. The large crystal filed splitting of Ce8h is mainly due to the fact that the 8h site is bonded to two oxygen with relatively short dSr/Gd‐O and forms a quasi‐square antiprism which experiences a large distortion. The Ce3+ 5d‐4f luminescence in Sr3AlO4F is much more stable against thermal quenching than that in Sr2GdAlO5, as evidenced by the temperature‐dependent luminescence intensity and luminescence decay studies. The energy of the O2−‐Eu3+/2+ and O2−‐Ce4+/3+ charge transfer as well as bandgap were estimated and the electronic structure of Ce3+ were constructed. A larger energy barrier ΔEdC between the Ce3+ 5d1 level and the conduction band bottom in Sr3AlO4F is seen from the Vacuum Referred Binding Energy (VRBE) diagrams which explains the higher thermal quenching temperature by thermal ionization model.

A DFT study of electronic structures and optical properties of nickel, nitrogen doped rutile TiO2

The first-principle method based on the density functional theory (DFT) was used to calculate the electronic structure and optical properties of pure, N-doped, Ni-doped and Ni/N co-doped rutile TiO2 systems. The band structures, electronic density of states and spectral absorption coefficients were calculated after geometrically optimizing the above TiO2 systems. The results show that there are impurity energy levels appear within the band gaps of Ni-, N-doped systems, which provided by N-2p orbit and Ni-3d orbit; meanwhile, more complicated impurity energy levels appear within the band gaps of N/Ni co-doped rutile TiO2. The co-doped rutile TiO2 has a better optical absorption efficiency and stronger absorption cut-off wavelength red-shift. Which can be caused by the hybridization of Ni-3d orbit and N-2p orbit provides local impurity levels, so that electrons transition occurred from the valence band maximum to the impurity levels and also from the impurity levels to the bottom of conduction band, which contributes to the electron transition and enhances the absorption performance of rutile TiO2.

Poole–Frenkel Effect and the Opportunity of Its Application for the Prediction of Radiation Charge Accumulation in Thermal Silicon Dioxide

It is proposed that the Poole–Frenkel effect be applied to predict radiation-induced charge accumulation in thermal silicon dioxide. Various conduction mechanisms of thermal silicon dioxide are considered, the conditions of the appearance of the Poole–Frenkel effect in it are determined, and the characteristics of donor centers participating in Poole–Frenkel electrical conductivity are calculated. A donor center level at an energy of 2.34 eV below the conduction-band bottom is determined and the concentration of ionized donor centers of 1.0 × 109 cm–3 at 400 K and a field strength of 10 MV/cm is found. It is concluded that the Poole–Frenkel effect can be applied not for prediction of the absolute value of the radiation-induced charge but for comparison of the samples in terms of the ability to accumulate it.

Revealing Optical Transitions and Carrier Recombination Dynamics within the Bulk Band Structure of Bi2Se3.

Bismuth selenide (Bi2Se3) is a prototypical 3D topological insulator whose Dirac surface states have been extensively studied theoretically and experimentally. Surprisingly little, however, is known about the energetics and dynamics of electrons and holes within the bulk band structure of the semiconductor. We use mid-infrared femtosecond transient reflectance measurements on a single nanoflake to study the ultrafast thermalization and recombination dynamics of photoexcited electrons and holes within the extended bulk band structure over a wide energy range (0.3 to 1.2 eV). Theoretical modeling of the reflectivity spectral line shapes at 10 K demonstrates that the electrons and holes are photoexcited within a dense and cold electron gas with a Fermi level positioned well above the bottom of the lowest conduction band. Direct optical transitions from the first and the second spin-orbit split valence bands to the Fermi level above the lowest conduction band minimum are identified. The photoexcited carriers thermalize rapidly to the lattice temperature within a couple of picoseconds due to optical phonon emission and scattering with the cold electron gas. The minority carrier holes recombine with the dense electron gas within 150 ps at 10 K and 50 ps at 300 K. Such knowledge of interaction of electrons and holes within the bulk band structure provides a foundation for understanding how such states interact dynamically with the topologically protected Dirac surface states.

Electronic band-structure and optical constants of Pb2GeS4: Ab initio calculations and X-ray spectroscopy experiments

The electronic band-structure of the ternary sulfide Pb2GeS4 was investigated by combining experimental and theoretical methods. Binding energy (BE) values of core electrons of Pb2GeS4 are measured employing X-ray photoelectron spectroscopy (XPS) for as-synthesized and treated with Ar+-ions crystal surfaces. The XPS measurements indicate that Ar+-ion treatment does not change the BE values of the core-level electrons of atoms constituting the Pb2GeS4 single crystal as well as peculiarities of the XPS valence band (VB) spectrum. The treatment does not cause changes in the crystal surface stoichiometry. The band-structure calculations based on density functional theory (DFT) reveal total density of states and partial densities of states of Pb2GeS4 within different exchange–correlation approximations. The best fit with the experiment is derived when the DFT calculations of Pb2GeS4 employ modified Becke-Johnson potential with Hubbard-corrected functional and taking into account spin–orbit (SO) interaction. The calculations indicate that top and upper portion of the VB is composed mainly by S 3p states, its central portion is formed by Ge 4p and S 3p states, while contributions of Pb 6s states dominate at its bottom with slightly smaller contributions of Ge 4s states as well. Contributions of unoccupied Pb 6p states dominate at the conduction band (CB) bottom. Regarding the occupation of the VB by Ge 4p and S 3p states, the theoretical data are confirmed experimentally by matching the XPS VB spectrum on a common energy scale with the X-ray emission spectra representing the valence S p and Ge p states. The present calculations yield that the VB maximum is positioned at the Y point, while the CB minimum at the Г point; this fact indicates that Pb2GeS4 sulfide is an indirect-gap material. The principal optical constants are also elucidated using the ab initio DFT calculations.

Development of persistent phosphor of Eu2+ doped Ba2SiO4 by Er3+ codoping based on vacuum referred binding energy diagram

We have developed the Ba2SiO4: Eu2+ -Er3+ persistent phosphor. The vacuum referred binding energy (VRBE) diagram of the Ba2SiO4 host with the 4f ground states of divalent and trivalent lanthanide ions in the host band gap was constructed from the obtained spectroscopic data and the Dorenbos's procedure. From the constructed diagram, the VRBE values of the conduction band (CB) bottom and valence band (VB) top were estimated to be −0.66 and −8.27 eV, respectively. From the energy gap between the CB bottom and 4f ground states of divalent lanthanide ions, we selected the Er3+ ion as an efficient co-dopant because the trap depth was estimated to be 0.64 eV, suitable for improving the persistent luminescence duration. The Eu2+-Er3+-codoped sample showed persistent luminescence intensity of over 0.32 mcd/m2 for 7 h duration. The initial persistent luminance intensity of the Eu2+-2%Er3+-codoped sample was found to be 250 times stronger than that of the only Eu2+-doped sample.

Site Occupation of Eu2+ in Ba2- xSr xSiO4 ( x = 0-1.9) and Origin of Improved Luminescence Thermal Stability in the Intermediate Composition.

Knowledge of site occupation of activators in phosphors is of essential importance for understanding and tailoring their luminescence properties by modifying the host composition. Relative site preference of Eu2+ for the two distinct types of alkaline earth (AE) sites in Ba1.9995- xSr xEu0.0005SiO4 ( x = 0-1.9) is investigated based on photoluminescence measurements at low temperature. We found that Eu2+ prefers to be at the 9-coordinated AE2 site at x = 0, 0.5, and 1.0, while at x = 1.5 and 1.9, it also occupies the 10-coordinated AE1 site with comparable preference, which is verified by density functional theory (DFT) calculations. Moreover, by combining low-temperature measurements of the heat capacity, the host band gap, and the Eu2+ 4f7 ground level position, the improved thermal stability of Eu2+ luminescence in the intermediate composition ( x = 1.0) is interpreted as due to an enlarged energy gap between the emitting 5d level and the bottom of the host conduction band (CB), which results in a decreased nonradiative probability of thermal ionization of the 5d electron into the host CB. Radioluminescence properties of the samples under X-ray excitation are finally evaluated, suggesting a great potential scintillator application of the compound in the intermediate composition.

Entropy optimized phase transitions and improved thermoelectric performance in n-type liquid-like Ag9GaSe6 materials

Liquid-like materials have shown extremely low thermal conductivity down to the minimum level in solids and high thermoelectric (TE) figure of merit above 2.0 in a few typical Cu-based compounds. These features make liquid-like materials among the top class in TEs. However, the ultrahigh TE performance is mainly reported in p-type materials. Advanced TE technology urgently requires both excellent p- and n-type materials. In this study, we introduce entropy engineering to greatly improve the figure of merit in Ag9GaSe6-based n-type liquid-like materials. Through successfully alloying Te at Se sites, material's configurational entropy is obviously increased to optimize phase transition characters and reduce lattice thermal conductivity. In addition, the ‘electron–crystal’ behavior is well maintained to achieve good electrical properties because alloying Te at Se sites scarcely affect the bottom of conduction band. Furthermore, the origin of very low lattice thermal conductivity in the low-temperature phase is revealed and attributed to the large directional Ag vibrations. All these features make Ag9GaSe6-based compounds excellent n-type liquid-like materials. A maximum figure of merit (zT) value around 1.6 at 850 K is realized in the high entropy Ag9GaSe5.53Te0.45, which is the best value in n-type liquid-like materials reported so far.

Structural and electronic properties of low-index stoichiometric Cu2ZnSnS4 surfaces

Over the past few years, quaternary Cu2ZnSnS4 (CZTS) has attracted a great deal of attention as the most promising photovoltaic absorber layer, due to its abundance and non-toxic properties. However, the significant surface structures and properties for photo-catalytic absorption layers have not yet been studied in detail for CZTS. Hence, the surface structure and electronic properties of low-index stoichiometric CZTS surfaces are calculated based on density functional theory. The relaxation is much large for the (001), (100), (101) and (112) surfaces. Moreover, more surface states appear at the bottom of conduction band and the top of valence band. The conduction band is mainly composed of S-3p and Sn-5p orbits. The valence band top is mainly composed of S-3p and Cu-3d orbits. The band gap values of five surfaces do not vary greatly. The dangling bond density for the (112) surfaces is minimal, resulting in minimum surface energy. Finally, the equilibrium morphology of CZTS is constructed by the Wulff rule. It is found that the {101} surface is the dominant surface (72.6%). These results will help us to better understand the surface properties of absorption layer that is related to CZTS surface and provide theoretical support for future experimental studies.