Evolution Of Gap Research Articles

Band gap energy of ZnOxSe1−x over the entire composition range

A model predicting the band gap energy of ZnOxSe1−x over the entire composition range is set up. It is found that the model can provide a good description for the band gap energy of ZnOxSe1−x. It is also found that the physical mechanisms for the band gap evolution in different composition ranges are different. Although the physical mechanism for the band gap evolution in the Se-rich region differs from that in the O-rich region, the difference between the band gap reductions in the Se-rich and O-rich regions is not large. In addition, it is found that the band gap bowing in the middle composition region is smaller than that in the Se-rich and O-rich regions. The reason is that the middle composition region lies in the bandlike region while the Se-rich and the O-rich composition regions lie in the impurity-like region.

The Southeastern Anatolia Project (GAP) in Turkey: An Alternative Perspective on the Major Rationales of GAP

ABSTRACTThe Southeastern Anatolia Project (Güneydoğu Anadolu Projesi, GAP) is one of the largest development projects ever undertaken in Turkey. Launched in the 1970s, GAP initially aimed primarily at the construction of 22 dams and 19 hydroelectric power plants on the Euphrates and Tigris rivers and of extensive irrigation networks to produce hydroelectric energy and irrigate 1.8 million hectares of land in south-eastern Turkey. In later decades, the scope of GAP widened considerably as it became a more comprehensive and bold scheme of modernization and transformation. Following this expansion, GAP also aimed at entirely reorganizing the political, social, economic, and cultural landscape of south-eastern Turkey and at transforming the local population therein. Despite these developments, and despite GAP having been intensely examined in the literature, the raison d’être of the project has not yet been adequately examined from a critical perspective. Questions regarding how the state authorities rationalized the inception of GAP and which factors motivated them to undertake development activities remain under-discussed. This article seeks to analyse the major rationales of GAP together with their multifaceted implications. In this way, the article aims to present an alternative narrative on the evolution and recent status of GAP, based on empirical data.

A systematic evolution of optical band gap and local ordering in Ge1Sb2Te4 and Ge2Sb2Te5 materials revealed by in situ optical spectroscopy

A unique amalgam of fascinating potentials enabled the usage of phase change materials in rewritable optical and electrical storage technology. Furthermore, the identification of stoichiometric tuning of disorder in these materials opened up new pathways for the selection of materials for multi-bit data storage. Despite a reasonable understanding of structure and properties, an in situ study could aid in unravelling vital insights. Hence, we report on an analogy between the degree of disorder in Ge1Sb2Te4 and Ge2Sb2Te5 materials by employing novel temperature-dependent correlations of optical band gap (Eg), Tauc parameter (measure of disorder, B1/2 slope) and the change in band tail. Eg decreases from 0.8 eV (90 K) to 0.34 eV (480 K) for Ge1Sb2Te4 and from 0.82 eV (90 K) to 0.36 eV (480 K) for Ge2Sb2Te5. This trend reveals the semiconducting nature of both amorphous and crystalline phases. The change in disorder as exemplified by an increase in B1/2 of 2.7% and 14.9% during the phase transition of Ge1Sb2Te4 and Ge2Sb2Te5 respectively reveals a stronger degree of disorder in the cubic phase of Ge1Sb2Te4 as compared to the cubic phase of Ge2Sb2Te5. These findings are strongly supported by an unambiguous change in the width of the band tail. This confirms the origin of a pronounced disorder-induced localization in Ge1Sb2Te4 compared to the Ge2Sb2Te5 material.

Alkali-metal-induced topological nodal line semimetal in layered XN2 (X = Cr, Mo, W)

Based on first principles calculations and the K·p effective model, we propose that alkali metal deposition on the surface of hexagonal XN2 (X = Cr, Mo, W) nanosheets induces topologically nontrivial phases in these systems. When spin orbit coupling (SOC) is disregarded, the electron-like conduction band from N-pz orbitals can be considered to cross the hole-like valence band from X-dz2 orbitals, thereby giving rise to a topological nodal line state in lithium-functionalized XN2 sheets (Li2MoN2 and Li2WN2). Such band crossing is protected by the existence of mirror reflection and time reversal symmetry. More interestingly, the bands cross exactly at the Fermi level, and the linear dispersion regions of such band crossings extend to as high as 0.9 eV above the crossing. For Li2CrN2, the results reveal the emergence of a Dirac cone at the Fermi level. Our calculations show that lattice compression decreases the thickness of a Li2CrN2 nanosheet, leading to phase transition to a nodal line semimetal. The evolution of the band gap of Li2XN2 at the Γ point indicates that the nontrivial topological character of Li2XN2 nanolayers is stable over a large strain range. When SOC is included, the band crossing point is gapped out giving rise to quantum spin Hall states in Li2CrN2 nanosheets, while for Li2MoN2, the SOC-induced gap at the crossing points is negligible.

Cs2PbI2Cl2, All-Inorganic Two-Dimensional Ruddlesden-Popper Mixed Halide Perovskite with Optoelectronic Response.

The two-dimensional Ruddlesden-Popper (RP) phases are an important class of halide perovskites with versatile optoelectronic properties. So far, only organic-inorganic hybrid RP phases involving long organic spacers were reported in this class. Here, we report an all-inorganic RP phase lead halide perovskite, Cs2PbI2Cl2 (1, I4/ mmm space group; a = 5.6385(8) Å, c = 18.879(4) Å), synthesized by a solid-state method. The compound exhibits a band gap of Eg ∼ 3.04 eV and photoconductivity. We find an anomalous band gap evolution in Cs2Pb1- xSn xI2Cl2 solid solutions. Our combined density functional theory and experimental study supports the thermodynamically stable nature of 1 as a unique ordered phase in the Cs2PbX4 (X = Cl, Br, I) system. The calculations suggest that 1 is a direct bandgap semiconductor with relatively small effective carrier mass along the in-plane direction, consistent with the experimentally observed in-plane UV-light photoresponse. We also demonstrate that 1 is promising for radiation detection capable of α-particle counting. Moreover, 1 shows markedly ambient and thermal stability.

Unrelated BCS-like pairing pseudogap and critical superconducting transition temperature in various cuprate compounds

The smooth evolution of the energy gap observed in the tunneling and angle-resolved photoemission spectra (ARPES) of high-[Formula: see text] cuprates with lowering the temperature from a pseudogap state above the critical temperature [Formula: see text] to a superconducting state below [Formula: see text], has been poorly interpreted as the evidence that the pseudogap must have the same origin as the superconducting order parameter, and therefore, must be related to [Formula: see text]. We argue that such an explanation of the tunneling gap and ARPES data is misleading. We show that the BCS-like energy gap (or pseudogap) opening in the electronic excitation spectrum of underdoped-to-overdoped cuprates at a characteristic temperature [Formula: see text] and the true superconducting order parameter appearing only at [Formula: see text] are unrelated. The superconducting phenomenon in unconventional cuprate superconductors is fundamentally different from the BCS-like pairing of fermionic quasiparticles, and the superconducting transition temperature [Formula: see text] is not determined by the BCS-like gap formation. The unusual superconducting order parameter in these high-[Formula: see text] materials appears at [Formula: see text] and coexists with the BCS-like gap (or pseudogap) below [Formula: see text].

Effects of substrate temperature on microstructure, morphology and optical properties of ZnSe:Co films obtained by pulsed laser deposition

ZnSe:Co thin films were grown on sapphire (Al2O3) substrates by pulsed laser deposition at different substrate temperature. Evolutions of the microstructure, morphology and optical properties of the ZnSe:Co thin films as a function of substrate temperature were analyzed. The results indicated that the crystalline quality was promoted and the films demonstrated increasing (111) preferred orientation with increasing substrate temperature. Excellent crystalline quality and (111) preferred oriented thin film was grown at substrate temperature of 800 °C. The refractive index and dielectric constant analyzed by the transmission spectra increased with elevating substrate temperature, which may be due to the promotion of the film packing density. The thin films with wider optical band gap in range of 2.73 eV to 3.27 eV were obtained in the experiment. The band gap decreased when the substrate temperature was increased from 25 °C to 600 °C, and then increased with a further increase of substrate temperature to 800 °C. The evolution of band gap associates with the quantum confinement effect.

Evolution of the Optical Gap in the Acene Series: Undecacene.

Undecacene, the largest member of the acene family, was photogenerated in a polymer matrix under cryogenic conditions from a precursor with two α-diketone bridges. The electronic absorption of undecacene extends into the NIR region, but the spectrum is dominated by two strong absorptions in the UV/Vis range. The HOMO-LUMO transition is continuously shifted to lower energies as the number of rings of the acene increases (E vs. 1/N, N=number of rings), which is in line with the strong electron correlation calculated by DFT/MRCI.

Evolution of the Optical Gap in the Acene Series: Undecacene

AbstractUndecacene, the largest member of the acene family, was photogenerated in a polymer matrix under cryogenic conditions from a precursor with two α‐diketone bridges. The electronic absorption of undecacene extends into the NIR region, but the spectrum is dominated by two strong absorptions in the UV/Vis range. The HOMO–LUMO transition is continuously shifted to lower energies as the number of rings of the acene increases (E vs. 1/N, N=number of rings), which is in line with the strong electron correlation calculated by DFT/MRCI.

Higher Acenes by On-Surface Dehydrogenation: From Heptacene to Undecacene.

A unified approach to the synthesis of the series of higher acenes up to previously unreported undecacene has been developed through the on‐surface dehydrogenation of partially saturated precursors. These molecules could be converted into the parent acenes by both atomic manipulation with the tip of a scanning tunneling and atomic force microscope (STM/AFM) as well as by on‐surface annealing. The structure of the generated acenes has been visualized by high‐resolution non‐contact AFM imaging and the evolution of the transport gap with the increase of the number of fused benzene rings has been determined on the basis of scanning tunneling spectroscopy (STS) measurements.

Higher Acenes by On‐Surface Dehydrogenation: From Heptacene to Undecacene

AbstractA unified approach to the synthesis of the series of higher acenes up to previously unreported undecacene has been developed through the on‐surface dehydrogenation of partially saturated precursors. These molecules could be converted into the parent acenes by both atomic manipulation with the tip of a scanning tunneling and atomic force microscope (STM/AFM) as well as by on‐surface annealing. The structure of the generated acenes has been visualized by high‐resolution non‐contact AFM imaging and the evolution of the transport gap with the increase of the number of fused benzene rings has been determined on the basis of scanning tunneling spectroscopy (STS) measurements.

Effect of Pressure on Cesium Iodide Band Gap

The evolution of cesium iodide band gap as a function of pressure is studied in the range from 0 to 60 GPa. Within this range, two structural phase transitions occurred, and the band gap was affected by the compression pressure and structural rearrangement. The band gap estimation under pressure, as obtained by the density functional theory methods, successfully reproduced the experimental trend of the optical gap and electrical resistivity, namely, a general decreasing tendency, an early maximum, and a discontinuous peak around 40 GPa.

Exploring the Limits of Three-Dimensional Perovskites: The Case of FAPb1–xSnxBr3

Metal halide perovskites based on formamidinium (FA) show interesting properties compared to methylammonium (MA) compounds. The larger molecular size of FA compared to that of MA leads, however, to a structural instability in FAPbI3, placing FA at the border of tolerance factor for three-dimensional (3D) lead iodide perovskites. Because of the huge interest in mixed Sn/Pb perovskites, here we investigate FAPb1–xSnxBr3 perovskites in a full compositional range (0 ≤ x ≤ 1). We find a non-monotonic band gap evolution with increasing Sn content, which, through first-principles computational analyses, we relate to a distorted structure that dynamically averages to a cubic phase, as determined by X-ray diffraction. The large FA cation induces an instantaneous structure made by partly decoupled SnBr3 units, which leads to the observed band gap opening. FASnBr3 thus likely represents a limit compound of 3D perovskites. Intermediate FAPb1–xSnxBr3 compositions maintain a band gap of ∼1.8 eV up to 85% Sn content, ma...

The Southeastern Anatolia Project (GAP) revisited: The evolution of GAP over forty years

AbstractThe Southeastern Anatolia Project (Güneydoğu Anadolu Projesi, GAP) is arguably the largest regional development project ever witnessed in Turkey. Begun in the 1970s, GAP initially aimed primarily at the construction of 22 dams and 19 hydroelectric power plants on the Tigris and Euphrates rivers and of extensive irrigation networks to produce hydroelectric energy and water 1.8 million hectares of land in southeastern Turkey. Later, the scope of GAP broadened significantly as it became a more ambitious and comprehensive scheme of modernization and transformation. Following this expansion, the multidimensionality of GAP and its multifaceted implications became clearer at both the national and international level. The project grew more visible and influential not only in political and public discourses, but also in the GAP region itself. Despite these developments, however, the question of how the project’s characteristics, vocabulary, rationales, and mechanisms have changed since its inception remains underdiscussed. This article asks what GAP was in the past and what it has more recently become. It examines the gradual transformation of GAP over forty years by specifically taking into account the continuities and ruptures in development discourse, theory, and practice since the 1950s. In this way, the article aims to provide a new perspective regarding the stages through which the project has passed to reach its current form.

Composition Dependence of the Band Gap Energy of Dilute Bismuth and Nitride GaNxBiyAs1−y−z Alloy (0 ≤ x ≤ 0.05, 0 ≤ y ≤ 0.05)

The band gap evolution of dilute bismuth and nitride GaNxBiyAs1−x−y alloy is analyzed. It is found that the location of the Г conduction band minimum (CBM) of GaNxBiyAs1−x−y is determined by two factors. One is the influence of the Bi content on the Г CBM of GaAs. The other is the interaction and the hybridization between the N level and the Г CBM of GaBiyAs1−y. For the Г valence band maximum (VBM) of GaNxBiyAs1−x−y, the main factor to determine its location is the interaction and the hybridization between the Bi level and the Г VBM of GaAs. According to the physical mechanism for the band gap evolution, a model is established to depict the band gap energy of GaNxBiyAs1−x−y depending on N and Bi contents quantitatively. The results obtained using the model are consistent with the experimental data.

Sunlight-enhanced catalytic degradation over Ag–CuO nanoparticles thin films prepared by DC/RF sputtering technique

Herein, we report sunlight-activated photo-catalysis response of direct current radio frequency (DC/RF)-sputtered Ag–CuO nanoparticles thin films. We have adopted this approach for facile removal and easy recovery of thin films after use. Ag was incorporated at 2.5 and 5.4 wt% with reference to pure CuO. Structural analysis through X-ray diffraction showed successful incorporation of Ag into the CuO lattice. Surface morphological observation was carried out using a field emission scanning electron microscope, which showed a uniform deposition of the subjected thin films. The surface composition and change in the chemical state of pure and incorporated thin films were studied by X-ray photoelectron spectroscopy. In addition to structural and surface analysis, optical band gap evolution was also studied through UV–visible spectroscopy. The obtained results reflected that optical band gap decreased from 1.84 to 1.77 and 1.61 eV for 0, 2.5 and 5.4 wt% Ag, respectively. Methylene blue dye was used to investigate the photo-catalytic degradation response of deposited thin films in sunlight. Furthermore, the effect of pH and dye concentration was also taken into account. The degradation results indicate the acceleration of degradation process with the Ag incorporation into CuO lattice. Moreover, the pH also contributes positively to the degradation progression. This research study could provide a platform to understand and develop metal oxide thin films with an efficient photo-catalytic response in sunlight.

The evolution of the gender test score gap through seventh grade: new insights from Australia using unconditional quantile regression and decomposition

This paper documents the patterns and examines the factors contributing to a gender gap in educational achievements in early seventh grade of schooling using a recent and nationally representative panel of Australian children. Regression results indicate that females excel at non-numeracy subjects at later grades whereas males outperform females in numeracy in all grades, whether at the mean or along the distribution of the test score. Our results also reveal a widening gender test score gap in numeracy as students advance their schooling. Regression and decomposition results also highlight the importance of controlling for pre-school cognitive skills in examining the gender test score gap.

Tensor force effect on the evolution of single-particle energies in some isotopic chains in the relativistic Hartree–Fock approximation

Within a nonlinear relativistic Hartree–Fock approximation combined with the BCS method, we study the effect of the nucleon–nucleon tensor force of the π-exchange potential on the spin- and pseudospin–orbit doublets along the Ca and Sn isotopic chains.We show how the self-consistent tensor force effect modifies the splitting of both kinds of doublets in an interdependent form, leading, quite generally, to opposite effects in the accomplishment of the spin and pseudospin symmetries (the one is restored, the other one deteriorates and vice versa). The ordering of the single-particle energy levels is crucial to this respect.Also, we observe a mutual dependence on the evolution of the shell closure gap Z = 50 and the energy band outside the core, along the Sn chain, as due to the tensor force. In fact, when the shell gap is quenched the outside energy band is enlarged, and vice versa. A reduction of the strength of the pion tensor force with respect to its experimental value from the nucleon–nucleon scattering is needed to get results closer to the experiment. Pairing correlations act to some extent in the opposite direction of the tensor term of the one-pion-exchange force.

Extension/Compression-Controlled Complete Band Gaps in 2D Chiral Square-Lattice-Like Structures

Achieving tunable band gaps in a structure by external stimuli is of great importance in acoustic applications. This paper aims to investigate the tunability of band gaps in square-lattice-like elastic periodic structures that are usually not featured with notable band gaps. Endowed with chirality, the periodic structures here are able to undergo imperfection-insensitive large deformation under extension or compression. The influences of geometric parameters on band gaps are discussed via the nonlinear finite element method. It is shown that the band gaps in such structures with curved beams can be very rich and, more importantly, can be efficiently and robustly tuned by applying appropriate mechanical loadings without inducing buckling. As expected, geometry plays a more significant role than material nonlinearity does in the evolution of band gaps. The dynamic tunability of band gaps through mechanical loading is further studied. Results show that closing, opening, and shifting of band gaps can be realized by exerting real-time global extension or compression on the structure. The proposed periodic structure with well-designed chiral symmetry can be useful in the design of particular acoustic devices.

The band gap energy of the dilute nitride alloy GaNxAsyP1−x−y (0 ≤ x ≤ 0.07, 0 ≤ y ≤ 1) depending on content

The dilute nitride alloy GaNxAsyP1−x−y is a very attractive material for solar cells. In this work, the band gap evolution for the dilute nitride alloy GaNxAsyP1−x−y is investigated. It is found that the band gap energy of the dilute nitride alloy GaNxAsyP1−x−y can be predicted by the modified band anticrossing model. It is also found that the interaction of the nitrogen level with the conduction band minimum of the host material is much stronger than the intraband coupling interaction in the conduction band minimum and the valence band maximum. In addition, it is found that the weak composition dependence of the N level is due to the N clusters. The dilute nitride alloy GaNxAsyP1−x−y should have a direct band gap no matter how large the As content is.