Density Of States Profile Research Articles

Theoretical insight on the effect of middle layer specifications on electronic properties of SnS2/MX2/SnS2 Trilayer heterostructure (M = Mo, w; X = S, Se, Te)

Recently, the van der Waals (vdW) heterostructure design has gained paramount importance for realizing artificial two-dimensional (2D) material systems with tunable electronic/opto-electronic properties for different technological applications. Driven by this paradigm, in this work, a detailed theoretical analysis has been performed on the structural and electronic properties of different tri-layer van der Waals (vdW) heterostructure of SnS2/MX2/SnS2 (M = Mo, W; X = S, Se, Te) via density functional theory(DFT) based Ab-initio calculation. Moreover, for individual vdW tri-layers, two characteristics of interlayer stacking configurations corresponding to natural stacking configurations of tri-layer SnS2 and MX2 have been considered. The structural properties are evaluated from the interlayer distance, bond angle, bond length, electrostatic difference potential, electron difference density, and Mulliken charge distributions. The electronic properties of different vdW tri-layers are assessed from the interlayer charge transfer, energy band structures, and the density of states profiles, which are quantified from the energy bandgap and effective masses, which are finally compared and benchmarked with homogenous tri-layer SnS2. It has been observed that the chalcogen specification of the middle layer predominantly influences the interlayer interaction and thereby, structural/electronic properties of vdW tri-layers. The vdW tri-layers demonstrate metallic, semi-metallic, and semiconducting energy band structures (bandgap ranging from 0.138 to 0.729 eV, electron effective mass ranging from 0.286/0.286 m0 to 1.299/0.342 m0, hole effective mass ranging from 0.216/0.216 m0 to 2.825/2.927 m0) based on the middle layer MX2 specifications and stacking orientations.

Performance metrics of doped graphene nanoribbons based nanoscale interconnects: DFT and NEGF analysis

Due to the scaling of devices, the interconnects delay is the most important concern of the current technology, becomes more significant than the IC delay, and leads to overall impact on the performance of the chip. In the present work, doped Zigzag Graphene Nano ribbons (GNR) have been analyzed for the possible nanoscale interconnect applications. The investigation has been performed using the density-functional theory-based ab-initio approach, discussed in terms of structural stability of ribbons, their electronic and transport properties, and the dynamical parameters. Zigzag GNR has been decorated with itssix possible sites along the width, through P and N type impurities, to select the most favorable site for the further analysis. The stability of the systems has been analyzed in terms of formation energy, the electronic properties in terms of band structure and density of states profiles, whereas, the transport properties in terms of transmission spectrum and I–V characteristics. The magnetic inductance, kinetic inductance, electrostatic capacitance, and quantum capacitance have also been calculated as interconnect parameters. The formation energy calculations suggest that the doping at the edge sites of NRs are more favorable. Further, it has been observed that the metallicity of the doped ZGNR enhances with doping, whereas, the I–V characteristics confirms that the N doped ZGNRs have linear current with constant slope. The performance metrics of ZGNRs have also been analyzed using HSPICE, discussed in terms of delay and power delay product, shows that the N doped ZGNRs have lower delay than the other considered counterparts, defends its application as interconnects in the electronic industry.

Understanding ovonic threshold switching of GeSe chalcogenide materials using electrical methodologies for extracting density of states

As trap-related conduction is crucial in achieving ovonic threshold switching characteristics, identifying the distribution of traps in the chalcogenide matrix is essential. Herein, the density of states (DOS) near the valence band maximum level (EV) and conduction band minimum level (EC) of GeSe layers extracted using various electrical methodologies were analyzed. When electrons in the GeSe layer participate in activation under light injection, the DOS distribution at EV can be calculated. On the other hand, capacitance measurement with respect to frequencies allows the identification of the lowest energy levels of EC containing electrons that can respond to the provided AC signals. When an As dopant is introduced into the GeSe layer, the unified DOS distribution indicates a decrease in the Se element concentration owing to the over-coordinated Ge state. Therefore, increased DOS at EC is observed, which is in good agreement with the ab initio explanation. Additional simulations were performed considering plausible scenarios in which the variation in DOS profile depends on the parameters of the transport mechanism. These results reveal the ways in which threshold switching can be controlled and facilitate the understanding of electrically achieved behavior.

Modeling the effect of chirality on thermal transport in a pillared-graphene structure.

The anisotropic heat transport in graphene-CNT based materials provoked the development of three-dimensional pillared-graphene (PG) systems. In this study, we performed non-equilibrium molecular dynamics simulations to analyze PG thermal conductivity and thermal boundary conductance. For the first time, we have considered the influence of pillar chirality and the temperature effect on PG heat transport. We analyzed the influence of pillar chirality and pillar length on the in- and out-of-plane transport properties. For the temperature-dependent analysis, the chosen temperatures were in the range of 100 K to 500 K. To elucidate the mechanism underlying the heat transport, we investigated the phonon density of states (DOS) in the different regions of PG systems. The overlap factor was calculated to quantify the mismatch in the phonon DOS profiles. Across the pillar region, the overlap factor correlates directly with the thermal boundary conductance. When heat is transported in an out-of-plane direction, the zig-zag PG system performs better than the armchair PG system. The atomic arrangement at the graphene-CNT interface plays an inevitable role in limiting heat transport in PG systems. The calculated phonon energy in the zig-zag PG interface is higher than that in the armchair PG interface.

Itinerant magnetism, electronic properties and half-metallicity of Co2ZrSn and Co2HfSn Heusler alloys

Half-metallic ferromagnetic alloys are attracting considerable interest for their potential applications in spintronic devices. Co-based Heusler alloys are considered to be promising half-metallic compounds as they combine suitable magnetic, electronic and transport properties with compositional versatility and high thermal stability. In this work, Co 2 ZrSn and Co 2 HfSn Heusler alloys were studied by combining experimental and ab-initio investigations in order to accurately estimate their electronic density of states in proximity of the Fermi level and to determine their magnetic and electronic properties. Magnetization measurements, performed between 2 K and 550 K, suggest for both alloys a gradual transition from localized ferromagnetism to weak itinerant ferromagnetism with increasing temperature, well described by a simple mean field model up to the Curie temperature (454 K in Co 2 ZrSn and 432 K in Co 2 HfSn) and beyond. Ab-initio calculations were performed using two exchange-correlational functionals, PBE and PBE optimized for solids (PBEsol), in order to assess the reproducibility of theoretical results. Overall, band structures and density of states (DOS) diagrams indicated, for both compounds, the presence of a half-metallic band gap in the minority spin sub-band. The dependence of magnetic moments and band gap width on cell parameters is discussed in detail. Calculated magnetic moments and cell parameters satisfactorily match the experimental results obtained in this work and previously reported in the literature. • Itinerant weak ferromagnetic behaviour is confirmed for both alloys as they follow the Edwards-Wohlfarth model. • The mean field model indicates that the density of states profiles at the Fermi level are smooth. • Ab-initio calculation performed with PBE and PBEsol functionals show half-metallic behaviour at 0 K. • A crossover from localized magnetism to itinerant magnetism as function of temperature was found for both alloys. • The M/T crossover can be attributed to a change in spin-flip mechanism at low temperature.

Investigation of electronic and optical properties of PbxSn1-xO2 for optoelectronic applications: A TB-mBJ DFT approach

A comprehensive study of electronic and optical properties of PbxSn1-xO2 (x = 0.125, 0.25 and 0.375) have been performed utilizing PBE-GGA + TB-mBJ scheme employing WIEN2K package under DFT formalism. The lattice constant parameters obtained for PbxSn1-xO2 are observed to be invariant with rutile SnO2. The DoS (Density of States) profile of PbxSn1-xO2 states a dominant impact of partial states of Pb along the conduction band. A red shift of conduction band edge has been observed with increasing Pb incorporation in PbxSn1-xO2 supercells. The contribution due to partial states of O is supreme as compared to Pb along the valence band, as also encountered in SnO2. The nature of bandstructure is in parity with DoS profile. With increased incorporation of Pb atoms in PbxSn1-xO2 supercells, a significant lowering of conduction band minimum occurs thereby resulting in reduced band gap. The nature of reduction of optical band gap values is in parity with the electronic band structure. The optical parameter characteristic curves undergo redshift due to increased Pb incorporation in PbxSn1-xO2 supercells. Distinct peak values in absorption characteristic curve claims the material to have direct band gap. Higher value of static refractive index causes dispersion in the material. The real (imaginary) part of dielectric constant is consistent with the refractive index (extinction coefficient) characteristic curves. The reflection coefficient curve indicates lossy nature of the material. The reflection coefficient curve contradicts the loss coefficient characteristic curve. Hence, the consequences of this investigation claim the competency of PbxSn1-xO2 material for optoelectronic applications.

Spin polarized density functional theory\xa0calculations of the electronic structure and magnetism of the 112 type iron pnictide compound hbox {EuFeAs}_2

Using density-functional theory, we investigate the electronic, magnetic, and hyperfine-interaction properties of the 112-type iron-pnictide compound {hbox {EuFeAs}}_2, which is isostructural to the high-temperature iron-based superconductor {hbox {Ca}}_{1-x}{hbox {La}}_x{hbox {FeAs}}_2. We show that the band structure of {hbox {EuFeAs}}_2 is similar to that of the 112-type compounds’ family, with hole-like and electron-like bands at the Brillouin-zone center and corners, respectively. We demonstrate that the bands near the Fermi level originate mainly from the Fe atoms. The presence of a mixture of ionic and covalent bonding is predicted from the charge-density and atom-resolved density-of-states calculations. There is good agreement between the calculated hyperfine-interaction parameters with those obtained from the ^{57}Fe and ^{151}Eu Mössbauer measurements. The spatial distribution of atoms in {hbox {EuFeAs}}_2 leads to an in-plane 2D magnetism. Moreover, ab-initio calculations predict the compound’s magnetic moment and the magnetic moments of each constituent atom. Also, the density of states profile provides insight into the relative magnitude of these moments. Electronic structure calculations and Fermi surface topology reveal various physical and chemical properties of {hbox {EuFeAs}}_2. Valence electron density maps indicate the co-existence of a wide range of chemical bonds in this system, and based on structural properties, the transport characteristics are deduced and discussed. A thorough analysis of the atomic structure of {hbox {EuFeAs}}_2 and its role in the bond formation is presented.

Density of states analysis of electrostatic confinement in gapped graphene

We investigate the electrostatic confinement of charge carriers in a gapped graphene quantum dot in the presence of a magnetic flux. The circular quantum dot is defined by an electrostatic gate potential demarcated in a large intrinsic graphene circular ribbon which is then connected to a circular lead. Considering different regions composing our system, we explicitly determine the energy spectrum solutions in terms of Hankel functions using the scattering matrix approach. The asymptotic behavior of the Hankel functions for large arguments allowed us to calculate the density of states (DOS) and show that it has an oscillatory behavior with the appearance of some resonant peaks associated with the quantum dot bound states with definite angular momenta. It was also found that the energy gap can control the amplitude and width of these resonances and affect even their location as reflected in the density of states profile.

Density of States of OLED Host Materials from Thermally Stimulated Luminescence

The electronic density of states (DOS) plays a central role in controlling the charge-carrier transport in amorphous organic semiconductors, while its accurate determination is still a challenging task. We apply the low-temperature fractional thermally stimulated luminescence (TSL) technique to determine the DOS of pristine amorphous films of organic light-emitting diode (OLED) host materials. The DOS width is determined for two series of hosts, namely, (i) carbazole-biphenyl derivatives, 4,4\ensuremath{'}-bis(N-carbazolyl)-1,1\ensuremath{'}-biphenyl (CBP), 3,3\ensuremath{'}-di(9$H$-carbazol-9-yl)-1,1\ensuremath{'}-biphenyl (mCBP), and 3\ensuremath{'},5-di(9$H$-carbazol-9-yl)-[1,1\ensuremath{'}-biphenyl]-3-carbonitrile (mCBP-CN), and (ii) carbazole-phenyl (CP) derivatives, 1,3-bis(N-carbazolyl)benzene (mCP) and 9-[3-(9$H$-carbazol-9-yl)phenyl]-9$H$-carbazole-3-carbonitrile (mCP-CN). TSL originates from radiative recombination of charge carriers thermally released from the lower-energy part of the intrinsic DOS that causes charge trapping at very low temperatures. We find that the intrinsic DOS can be approximated by a Gaussian distribution, with a deep exponential tail accompanying this distribution in CBP and mCBP films. The DOS profile broadens with increasing molecular dipole moments, varying from 0 to 6 D, in a similar manner within each series, in line with the dipolar disorder model. The same molecular dipole moment, however, leads to a broader DOS of CP compared with CBP derivatives. Using computer simulations, we attribute the difference between the series to a smaller polarizability of cations in CP derivatives, leading to weaker screening of the electrostatic disorder by induction. These results demonstrate that the low-temperature TSL technique can be used as an efficient experimental tool for probing the DOS in small-molecule OLED materials.

DFT + U studies of the electronic and optical properties of ReS2 mono-layer doped with lanthanide atoms

The structural, electronic and optical properties of various elements in the lanthanide series as dopant atoms in the triclinic mono-layered rhenium disulphide (ReS2) structure was evaluated using density functional theory with the Hubbard U correction. The Hubbard U correction term was introduced to account for the presence of the electron-electron correlation in the partial filled 4f orbitals of the lanthanide dopant atoms. The calculated lanthanide ion substitutional dopant energies in the ReS2 mono-layer under both the Re-rich and S-rich conditions show that the synthesis of the lanthanide metal doped ReS2 mono-layer is feasible. The pristine ReS2 mono-layer has an electronic band gap of 1.43 eV with a non-magnetic ground state. The introduction of lanthanide atoms (Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er and Tm) with partial filled 4f orbital modifies the ReS2 mono-layer from non-magnetic to magnetic ground state. The La atom without 4f as well as the Yb and Lu atoms with completely filled 4f orbitals retains its non-magnetic ground state. The presence of the lanthanide dopant atoms in the ReS2 mono-layer modifies the electronic ground structure leading to the introduction of defect states within the energy gap and minute changes in their overall density of states profiles. The calculated optical reflectivity spectra and absorption spectra was modified with significant red-shift observed in the absorption spectra as a result of the dopant atoms. These lanthanide doped ReS2 mono-layer could find potential applications in spintronic and optoelectronic devices.

First-Principles Studies of Structural, Electronic and Magnetic Properties of the CrS, CrSe and CrTe Compounds

Using first-principles calculation within the generalized gradient approximation (GGA) and [Formula: see text] (Hubbard Coulomb onsite correction), we investigated the structural, electronic and magnetic properties of CrS, CrSe and CrTe compounds in different phases. We found that the NaCl phase is the lowest energy structure in comparison to the other investigated phases within the [Formula: see text] approach for CrS, CrSe and CrTe compounds. The CrX compounds investigated in the NaCl phase are all half-metallic ferromagnets with a net magnetic moment of 4.0[Formula: see text][Formula: see text] within the [Formula: see text] approximation. From the density of states profile and charge density distribution, the observed half-metallic ferromagnetism in the CrS, CrSe and CrTe compounds is due to the spin polarized 3[Formula: see text] orbitals of the Cr atoms. The observed stable half-metallic ferromagnetism in the NaCl phase of the CrS, CrSe and CrTe compounds should motivate further experimental studies on the possible application of these compounds as spintronic materials.

First-Principles Study of Catalytic Activity of Cu, Ni and Pd Surface for Formaldehyde and Hypophosphite As Reducing Agents in Electroless Deposition

Electroless deposition process has been applied to various industrial fields as a fundamental technique to fabricate metal thin films. To improve the controllability of this process, investigations of the reaction mechanism and the catalytic activity of metal surfaces for each reducing agent reaction are essentially important. For systematic analyses, theoretical calculation can be employed as a powerful tool. Our previous theoretical studies suggested that the dehydrogenation step of the reducing agents could be a rate-determining step in most cases [1]. This study investigates the reaction mechanisms of reducing agents with solvent molecules on Cu, Ni, and Pd, analyzing the electronic states during the reaction to identify the origin of the catalytic activity observed in each metal surface, by using density functional theory (DFT). All calculations were performed by using Vienna Ab initio Simulation Package (VASP). The cutoff energy for the plane wave basis set was set at 400 eV. The core electrons were described by pseudopotentials constructed with the Projector Augmented Wave (PAW). The exchange-correlation energies were evaluated with the Perdew, Burke and Ernzerhof (PBE) functional. The k-point grids were taken as 3 × 3 × 1 for the geometry optimization and 15 × 15 × 1 for the density of states (DOS) calculations. Assuming that the process employs high pH solution, the molecular structures of formaldehyde and hypophosphite ions were modeled as H2CO(OH)- and H2PO2 -, respectively. The solvation structures were prepared by locating several explicit H2O molecules, which were directly interacting with the molecules of the reducing agents. Previous study of each elementary step, such as the adsorption and the dehydrogenation, indicated that H2O played an important role to determine the geometrical structure or electronic state in those steps. To obtain a reasonable result of the solid-liquid interface system, the solvation structures of each reducing agent were investigated. H2O molecules turned out to interact mainly with O atoms of formaldehyde via H atoms in the solvation structure (Fig. 1), which was almost the same as the case of hypophosphite. Due to the sterically bulky structure around O atoms, the two reducing agent molecules adsorbed on each metal surface via H atoms; H and center atoms locate on atop and bridge sites, respectively. The solvation structure prevented unreasonable dissociation of the molecules on metal surfaces, suggesting that the surrounding solvent molecules are necessary to build the practical adsorption model. The reaction energies of the dehydrogenation on each metal surface have good agreement with the catalytic activity shown in the experimental results [2]. DOS profiles demonstrate that the electronic state of the dissociated H atom of formaldehyde is sufficiently stabilized by the effective interaction with Cu surface atom, while that of hypophosphite is not. Such a detailed investigation for the electronic state of the reducing agents at each surface during the reaction is helpful to elucidate their reactivity. [1] M. Kunimoto, T. Shimada, S. Odagiri, H. Nakai, T. Homma, J. Electrochem. Soc., 158, D585-D589 (2011). [2] I. Ohno, O. Wakabayashi, S. Haruyama, J. Electrochem. Soc., 132, 2323-2330, (1985). Figure 1

First-principles study of the phase stability and elastic properties of Ti-X alloys (X = Mo, Nb, Al, Sn, Zr, Fe, Co, and O)

Phase stability and elastic properties of Ti1-x-Xx alloys (X = substitutional Mo, Nb, Al, Sn, Zr, Fe, Co, and interstitial O) in body-centered cubic (bcc) (β phase) and hexagonal close-packed (hcp) (α phase) crystal structures were studied using first-principles calculations. The formation energy was used to determine the dependence of the phase stability on the atomic concentration x (0 ≤ x ≤ 0.5 for substitutional elements and 0 ≤ x < 0.02 for O) of the alloying elements and atomic configurations (ordered, disordered, and other compound structures). The disordered configurations of the atoms in the alloys were considered within the framework of the special quasi-random structures (SQS) method. The composition dependence of elastic constants, isotropic elastic moduli and density of states (DOS) were also evaluated.The predicted formation energies and elastic properties agrees well with previous experimental and theoretical results. Addition of Mo was found to stabilize the β phase significantly while destabilizing the α phase. Nb was found to be a weaker β-stabilizer. In Ti-Al and Ti-Sn systems, the compounds form preferentially at a higher concentration. Zr shows little effect on the phase stability, thus Zr is a neutral element. For Ti-O systems, the octahedral site is the most stable site for interstitial oxygen in both bcc and hcp structures. The comparison of formation energies showed that O acts as an α stabilizer. Considering the configurational entropy contribution, we concluded that temperature effect can be significant to stabilize the phases. The elastic constants calculation revealed that adding Mo, Nb, Fe, or Co increases the mechanical stability of bcc Ti, whereas bcc Ti-Al, Ti-Sn, and Ti-Zr systems are mechanically unstable for all the calculated concentrations. For their hcp counterparts, Ti-Mo and Ti-Nb have lower mechanical stability than the Ti-Al, Ti-O, and Ti-Sn systems. It was found in the electronic structures calculation that increasing addition of Nb, Mo, Fe, and Co weakens the covalent-like bonding of hcp system and strengthens the metallic bonding of bcc system. This variation of the TDOSs of these Ti alloys explains why the bcc phase becomes more stable than the hcp phase at high concentration. Due to the similarity of electronic structure, Zr does not change the DOS of the bcc and hcp phases significantly: The Fermi level in DOS profile of Ti-Zr was found to be identical to that of pure Ti system. Ti-Al and Ti-Sn alloys keep the covalent-like bonding, which explains why hcp Ti-Al and Ti-Sn are stable.

Metal-like Band Structures of Ultrathin Si {111} and {112} Surface Layers Revealed through Density Functional Theory Calculations.

Density functional theory calculations have been performed on Si (100), (110), (111), and (112) planes with tunable number of planes for evaluation of their band structures and density of states profiles. The purpose is to see whether silicon can exhibit facet-dependent properties derived from the presence of a thin surface layer having different band structures. No changes have been observed for single to multiple layers of Si (100) and (110) planes with a consistent band gap between the valence band and the conduction band. However, for 1, 2, 4, and 5 Si (111) and (112) planes, metal-like band structures were obtained with continuous density of states going from the valence band to the conduction band. For 3, 6, and more Si (111) planes, as well as 3 and 6 Si (112) planes, the same band structure as that seen for Si (100) and (110) planes has been obtained. Thus, beyond a layer thickness of five Si (111) planes at ≈1.6 nm, normal semiconductor behavior can be expected. The emergence of metal-like band structures for the Si (111) and (112) planes are related to variation in Si-Si bond length and bond distortion plus 3s and 3p orbital electron contributions in the band structure. This work predicts possession of facet-dependent electrical properties of silicon with consequences in FinFET transistor design.

Exploring the potential of boron-doped nanographene as efficient charge transport and nonlinear optical material: A first-principles study

Owing to their excellent electrochemical properties, graphenes found applications in several fields ranging from semiconductors, solar cells, field effect transistors, and nanoscale electronic devices as well as in nonlinear optical (NLO) applications. The structural features, electro-optical, charge transport and nonlinear optical properties of the boron-doped graphene (BG) compound 1 were studied using density functional theory methods The BG compound comprises a central electron deficient site of boron atoms, which can serve as electron acceptor while terminal alkoxy groups as donors leading to powerful donor-π-acceptor (D-π-A) configuration. The experimental crystal structure was successfully reproduced by optimized ground state geometry at PBE0/6-311G* level of theory for isolated molecule. The experimental lattice parameters, geometries, crystal presentation and alignment of molecules in the unit cells as well as their packing orientation of BG compound 1 was also efficiently reproduced by applying periodic boundary conditions (PBC) at PBE level. The comprehensive intramolecular charge transfer (CT) was realized from terminal rings of the HOMO to the electron deficient sites of boron atoms of the LUMO. The nature of BG compound 1 might be more towards hole transport even though its hole reorganization energy is twice than that of the electron one due to the significant higher hole transfer integral values. The superior hole transfer integrals and intrinsic mobility values of the BG compound 1 might lead remarkable hole transport contender as compared to many other organic materials. The narrow band gap, density of states profile, dielectric function, uniform conductivity functions and noteworthy electronic as well as CT properties revealed that the BG compound 1 might be proficient optoelectronic contestant having intermolecular CT as well as intramolecular CT with optimal stability. A comparison of static third-order polarizability <γ> of BG compound 1, as calculated in present investigation, was also performed with some standard NLO molecules as well as graphene nanoflakes. Moreover, longitudinal component γzzzz of parent compound has been found 12 and 4 times larger than those of previously reported open-shell poly aromatic hydrocarbons (PAH). Interestingly, by increasing the donor ability, i.e., introduction of C2H2PhNH2 groups in place of OC4H9 groups (BG compound 3) at terminal positions boosts the <γ> amplitude∼8 times than that of its parent BG compound 1.

The mechanical, thermodynamic, and electronic properties of cubic Au4Al crystal via first-principles calculations

The mechanical, thermodynamic, and electronic properties of cubic Au4Al intermetallic compound (IMC) crystal were explored through first-principles calculations within the generalized gradient approximation (GGA). The lattice constants and three independent elastic constants of Au4Al crystal were first calculated as a function hydrostatic pressure, and its elastic anisotropy was evaluated by the computation of the crystal direction-dependent elastic modulus and the Zener anisotropy factor. The mechanical characteristics of the crystal, such as its ductile-brittle feature and elastic anisotropy, were also obtained according to its Cauchy pressure, Zener anisotropic factor, and directional Young's modulus. In addition, the influence of hydrostatic pressure on the polycrystalline mechanical properties of Au4Al, including bulk, shear, and Young's moduli; ductility; brittleness; and Vickers hardness characteristics were also estimated. Finally, the temperature-dependent Debye temperature and heat capacity of the IMC crystal was determined using a quasi-harmonic Debye model, and its electronic band structures and density of states profiles were examined through an analysis of its electronic characteristics.The results reveal that the Au4Al crystal is not only an elastically anisotropic, low stiff and very ductile material but also a conductor. Furthermore, the bulk, shear, and Young's moduli; Zener anisotropy factor; ductility; and Vickers hardness of the crystal would increase with an increase in hydrostatic pressure. Finally, it was also found that the heat capacity of Au4Al at low temperature strictly follows the Debye T3 law.

Characterization of Carrier Transport and Trapping in Photorefractive Polymer Composites Using Photoemission Yield Spectroscopy in Air

Carrier trapping in organic photorefractive (PR) composites is affected by the bulk state (which can be varied by modifying the molecule structure and processing method used) and the presence of dipolar molecules. Such situations complicate the understanding of carrier trapping in PR composites. The density of states (DOS) provides direct information about carrier trapping in materials. Here, the first attempt to directly measure the DOS in PR composites is performed to investigate the effect of the change of the bulk state on carrier trapping. The distribution of the DOS is measured using photoemission yield spectroscopy in air. The bulk state of the PR composites is tuned based on the crystallization of dipolar nonlinear optical dyes included as a component. Thus, the bulk with a different net dipole (spatially averaged dipole) moment is prepared with the same composition as the PR materials. The edge part of the DOS profile clearly changes upon tuning the bulk state. The diminished net dipole moment achieved by the bulk state tuning causes the suppression of carrier trapping, which is well supported by the dipole trapping model. This result coincides with those obtained using photocurrent and PR grating measurements. image

Understanding defect distributions in polythiophenes via comparison of regioregular and regiorandom species

Organic photovoltaics (OPVs) are regarded as promising for solar-electric conversion with steadily improving power conversion efficiencies. For further progress, it is crucial to understand and mitigate defect states (traps) residing in the band-gap of OPV materials. In this work, using capacitance measurements, we analyzed two major bands in the density of states (DOS) energy spectra of defects in poly(3-hexylthiophene) (P3HT); regio-regular and regio-random species of P3HT were compared to elucidate the role of morphological disorder. To accurately interpret the obtained DOS profile, trap emission prefactors and activation energy were extracted from temperature dependent capacitance-frequency measurements, while doping, Fermi energy, built-in voltage, and energy levels of the defects were extracted from capacitance-voltage measurements. We identified that the lower energy band—misinterpreted in literature as a defect distribution—stems from free carrier response. The higher energy defect distribution band for regio-random P3HT was an order of magnitude higher than region-regular P3HT, thus stemming from morphological disorder. Impedance spectroscopy was also employed for further comparison of the two P3HT species.

Physical, mechanical, thermodynamic and electronic characterization of Cu11In9 crystal using first-principles density functional theory calculation

First-principles calculations of the physical, mechanical, thermodynamic and electronic properties of monoclinic Cu11In9 crystal are performed together with their temperature, hydrostatic pressure and direction dependences. First of all, the optimized lattice constants and energy of formation of Cu11In9 single crystal are investigated, by which its hydrostatic pressure-dependent elastic constants are characterized. Next, based on the characterized angular character of atomic bonding, Zener factor and directional Young’s modulus, the hydrostatic pressure-dependent mechanical characteristics of the single crystal, such as ductile/brittle behavior and elastic anisotropy, are examined. Moreover, the polycrystalline elastic properties of Cu11In9, including bulk, shear and Young’s modulus, and its ductile/brittle and microhardness characteristics are assessed as a function of hydrostatic pressure. After that, quasi-harmonic Debye modeling is carried out to investigate the temperature-dependent Debye temperature and heat capacity of Cu11In9 single crystal. Finally, its electronic band structures and density of states profiles are characterized through electronic characteristic analysis. An extensive comparison of the present results with the literature published data of Cu11In9, CuIn and Cu2In is performed.The present calculations indicate that Cu11In9 crystal exhibits low elastic anisotropy, low hardness, high ductility, and good electrical conductance. Moreover, its heat capacity closely follows the Debye T3-law at temperatures below the Debye temperature, and attains the Dulong–Petit limit at temperatures far above the Debye temperature. In addition, the crystal comprises higher elastic isotropy, less ductility, greater Debye temperature and better electrical conductivity than CuIn and Cu2In. Besides, the bulk, shear and Young’s modulus, and microhardness of polycrystalline Cu11In9 are all larger than those of CuIn but smaller than those of Cu2In.

First-principles density functional calculation of mechanical, thermodynamic and electronic properties of CuIn and Cu2In crystals

The study aims at assessing the mechanical, thermodynamic and electronic properties of single-crystalline and polycrystalline CuIn and Cu2In intermetallic compound (IMC) crystals using first-principles calculation based on the density functional theory within the generalized gradient approximation. The lattice constants and the five independent elastic constants of the two hexagonal single crystal structures are first calculated as a function of hydrostatic pressure, and their elastic anisotropy is examined through the computation of the crystal direction-dependent elastic modulus and the Zener anisotropy factor. Subsequently, their associated pressure-dependent polycrystalline elastic properties are also predicted, by which the ductility or brittleness of the IMC materials is characterized. Moreover, the temperature-dependent Debye temperature and heat capacity of these two IMC nanocrystals are determined using a quasi-harmonic Debye model, and their electronic band structures and density of states profiles are examined through analysis of electronic characteristics.The calculation results show that these two IMC crystals are not only an elastically anisotropic, low stiff and very ductile material but also a conductor. The elastic anisotropy, Debye temperature and heat capacity of Cu2In single crystal all surpass those of CuIn, and also, Cu2In crystal tends to be much stiffer than CuIn. Besides, the heat capacity of these two nanocrystals strictly follows with the well-known T3-law at temperature below the Debye temperature and would reach the Dulong–Petit limit at temperature above the Debye temperature.