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Results for “electronic bandstructure”

Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 19 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2016

Multi-MHz time-of-flight electronic bandstructure imaging of graphene on Ir(111)

In the quest for detailed spectroscopic insight into the electronic structure at solid surfaces in a large momentum range, we have developed an advanced experimental approach. It combines the 3D detection scheme of a time-of-flight momentum microscope with an optimized filling pattern of the BESSY II storage ring. Here, comprehensive data sets covering the full surface Brillouin zone have been used to study faint substrate-film hybridization effects in the electronic structure of graphene on Ir(111), revealed by a pronounced linear dichroism in angular distribution. The method paves the way to 3D electronic bandmapping with unprecedented data recording efficiency.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

Electronic and thermal properties of TiFe{sub 2} compound: An ab initio study

A systematic study of electronic, and thermal properties such as the Density of states, Fermi energy, Debye temperature and specific heat coefficient, has been carried out using the results of electronic bandstructure and related characteristics of the Laves phase compound, TiFe{sub 2}. Computation of electronic bandstructure and associated properties has been carried out using the tight-binding-linear-muffin-tin-orbital (TB-LMTO) method within atomic sphere approximation (ASA). The calculated values are compared with the available results of literature.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Charge transport in organic molecular semiconductors from first principles: The bandlike hole mobility in a naphthalene crystal

Predicting charge transport in organic molecular crystals is notoriously challenging. Carrier mobility calculations in organic semiconductors are dominated by quantum chemistry methods based on charge hopping, which are laborious and only moderately accurate. We compute from first principles the electron-phonon scattering and the phonon-limited hole mobility of naphthalene crystal in the framework of ab initio band theory. Our calculations combine GW electronic bandstructures, ab initio electron-phonon scattering, and the Boltzmann transport equation. The calculated hole mobility is in very good agreement with experiment between 100–300K, and we can predict its temperature dependence with high accuracy. Here we show that scattering between intermolecular phonons and holes regulates the mobility, though intramolecular phonons possess the strongest coupling with holes. We revisit the common belief that only rigid molecular motions affect carrier dynamics in organic molecular crystals. Our paper provides a quantitative and rigorous framework to compute charge transport in organic crystals and is a first step toward reconciling band theory and carrier hopping computational methods.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Atomate: A high-level interface to generate, execute, and analyze computational materials science workflows

We introduce atomate, an open-source Python framework for computational materials science simula-tion, analysis, and design with an emphasis on automation and extensibility. Built on top of open sourcePython packages already in use by the materials community such as pymatgen, FireWorks, and custodian,atomate provides well-tested workflow templates to compute various materials properties such as elec-tronic bandstructure, elastic properties, and piezoelectric, dielectric, and ferroelectric properties. Atomatealso enables the computational characterization of materials by providing workflows that calculate X-rayabsorption (XAS), Electron energy loss (EELS) and Raman spectra. One of the major features of atomate isthat it provides both fully functional workflows as well as reusable components that enable one to com-pose complex materials science workflows that use a diverse set of computational tools. Additionally,atomate creates output databases that organize the results from individual calculations and contains abuilder framework that creates summary reports for each computed material based on multiplesimulations.

Mathew, Kiran↗
DOE OSTI2014

On the role of disorder on graphene and graphene nanoribbon-based vertical tunneling transistors

In this work, the characteristics of vertical tunneling field-effect transistors based on graphene (VTGFET) and graphene nanoribbon heterostructure (VTGNRFET) in the presence of disorder are theoretically investigated. An statistical analysis based on an atomistic tight-binding model for the electronic bandstructure along with the non-equilibrium Green's function formalism are employed. We study the dependence of the averaged density of states, transmission probability, on- and off-state conductances, on/off conductance ratio, and transfer characteristics on the substrate induced potential fluctuations and vacancies. In addition, the variabilities of the device characteristics due to the presence of disorder are evaluated. It can be inferred from the results that while introducing vacancies cause a relatively modest suppression of the transmission probability, potential fluctuations lead to the significant increase of transmission probability and conductance of the device. Moreover, the results show that the transport properties of VTGFET are more robust against disorder compared to VTGNRFET.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

A new form of Ca{sub 3}P{sub 2} with a ring of Dirac nodes

We report the synthesis and crystal structure of a new high-temperature form of Ca{sub 3}P{sub 2}. The crystal structure was determined through Rietveld refinements of synchrotron powder x-ray diffraction data. This form of Ca{sub 3}P{sub 2} has a crystal structure of the hexagonal Mn{sub 5}Si{sub 3} type, with a Ca ion deficiency compared to the ideal 5:3 stoichiometry. This yields a stable, charge-balanced compound of Ca{sup 2+} and P{sup 3−}. We also report the observation of a secondary hydride phase, Ca{sub 5}P{sub 3}H, which again is a charge-balanced compound. The calculated band structure of Ca{sub 3}P{sub 2} indicates that it is a three-dimensional Dirac semimetal with a highly unusual ring of Dirac nodes at the Fermi level. The Dirac states are protected against gap opening by a mirror plane in a manner analogous to what is seen for graphene.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Predicting a quaternary tungsten oxide for sustainable photovoltaic application by density functional theory

A quaternary oxide, CuSnW{sub 2}O{sub 8} (CTTO), has been predicted by density functional theory (DFT) to be a suitable material for sustainable photovoltaic applications. CTTO possesses band gaps of 1.25 eV (indirect) and 1.37 eV (direct), which were evaluated using the hybrid functional (HSE06) as a post-DFT method. The hole mobility of CTTO was higher than that of silicon. Further, optical absorption calculations demonstrate that CTTO is a better absorber of sunlight than Cu{sub 2}ZnSnS{sub 4} and CuIn{sub x}Ga{sub 1−x}Se{sub 2} (x = 0.5). In addition, CTTO exhibits rigorous thermodynamic stability comparable to WO{sub 3}, as investigated by different thermodynamic approaches such as bonding cohesion, fragmentation tendency, and chemical potential analysis. Chemical potential analysis further revealed that CTTO can be synthesized at flexible experimental growth conditions, although the co-existence of at least one secondary phase is likely. Finally, like other Cu-based compounds, the formation of Cu vacancies is highly probable, even at Cu-rich growth condition, which could introduce p-type activity in CTTO.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

The impact of Cu on recombination in high voltage CdTe solar cells

Using photoluminescence spectroscopy, we construct a recombination model for state-of-the-art CdTe solar cells doped with Cu. Here, we observe that Cu on Cd sites form a dominant acceptor state about 150 meV from the valence band. Although it is intuitive that this state can increase hole density, we also find that this relatively shallow dopant can also limit lifetime. Consequently, CdTe solar cells doped with Cu could have a lifetime limitation inversely proportional to the hole concentration.

14 SOLAR ENERGY↗
DOE OSTI2016

Oxygen deficiency and Sn doping of amorphous Ga 2 O 3

The potential of effectively n-type doping Ga 2 O 3 considering its large band gap has made it an attractive target for integration into transistors and solar cells. As a result amorphous GaO x is now attracting interest as an electron transport layer in solar cells despite little information on its opto-electrical properties. Here we present the opto-electronic properties, including optical band gap, electron affinity, and charge carrier density, for amorphous GaO x thin films deposited by pulsed laser deposition. These properties are strongly dependent on the deposition temperature during the deposition process. The deposition temperature has no significant influence on the general structural properties but produces significant changes in the oxygen stoichiometry of the films. Here, the density of the oxygen vacancies is found to be related to the optical band gap of the GaO x layer. It is proposed that the oxygen deficiency leads to defect band below the conduction band minimum that increases the electron affinity. These properties facilitate the use of amorphous GaO x as an electron transport layer in Cu(In,Ga)Se 2 and in Cu 2 O solar cells. Further it is shown that at low deposition temperatures, extrinsic doping with Sn is effective at low Sn concentrations.

14 SOLAR ENERGY↗
DOE OSTI2016

Polymerization of defect states at dislocation cores in InAs

Dislocations are essentially lines of point defects which can act as recombination centers in semiconductor devices. These point defects do not behave as isolated defects. Their spatial proximity enables them to hybridize into a one-dimensional band, and the distribution of resulting defect-band states is determined by both the position of the band and its dispersion. In the case of glissile 90° partial dislocations in III-V semiconductors, the dislocation core can adopt a variety of different reconstructions. Each of these reconstructions has a different arrangement of point defects, which affects the hybridization into defect bands and their associated dispersion. Here, we illustrate these principles by performing first-principles calculations for InAs and find that some defect levels for InAs dislocations lie outside of the band gap where they cannot act as recombination centers. To provide some insight into the electronic structure of dislocations in ternary alloys, some examples relevant to InGaAs and GaAsP are included.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Normal and inverse bulk spin valve effects in single-crystal ruthenates

The current-perpendicular-to-plane magnetoresistivity (CPP-MR)/ρ c (B) is investigated in single crystal ruthenates Ca 3 (Ru 1-x Ti x ) 2 O 7 (x = 0.02). This material is naturally composed of ferromagnetic metallic bilayers (Ru,Ti)O 2 separated by nonmagnetic insulating layers of Ca 2 O 2 , resulting in tunneling magnetoresistivity. Non-monotonic ρ c (B) curves as well as the inverse spin valve effect are observed around the magnetic phase transition associating with the metal-to-insulator transition. A spin dependent tunneling model with alternate distribution of hard and soft magnetic layers [(Ru,Ti)O 2 ] is proposed to explain the exotic CPP-MR behavior. Finally, this eccentric CPP-MR behavior highlights the strong spin-charge coupling in double-layered ruthenates and provides a potential material for spintronic devices.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Emitter/absorber interface of CdTe solar cells

The performance of CdTe solar cells can be very sensitive to the emitter/absorber interface, especially for high-efficiency cells with high bulk lifetime. Performance losses from acceptor-type interface defects can be significant when interface defect states are located near mid-gap energies. Numerical simulations show that the emitter/absorber band alignment, the emitter doping and thickness, and the defect properties of the interface (i.e., defect density, defect type, and defect energy) can all play significant roles in the interface recombination. In particular, a type I heterojunction with small conduction-band offset (0.1 eV ≤ ΔE{sub C} ≤ 0.3 eV) can help maintain good cell efficiency in spite of high interface defect density, much like with Cu(In,Ga)Se{sub 2} (CIGS) cells. The basic principle is that positive ΔE{sub C}, often referred to as a “spike,” creates an absorber inversion and hence a large hole barrier adjacent to the interface. As a result, the electron-hole recombination is suppressed due to an insufficient hole supply at the interface. A large spike (ΔE{sub C} ≥ 0.4 eV), however, can impede electron transport and lead to a reduction of photocurrent and fill-factor. In contrast to the spike, a “cliff” (ΔE{sub C} < 0 eV) allows high hole concentration in the vicinity of the interface, which will assist interface recombination and result in a reduced open-circuit voltage. Another way to mitigate performance losses due to interface defects is to use a thin and highly doped emitter, which can invert the absorber and form a large hole barrier at the interface. CdS is the most common emitter material used in CdTe solar cells, but the CdS/CdTe interface is in the cliff category and is not favorable from the band-offset perspective. The ΔE{sub C} of other n-type emitter choices, such as (Mg,Zn)O, Cd(S,O), or (Cd,Mg)Te, can be tuned by varying the elemental ratio for an optimal positive value of ΔE{sub C}. These materials are predicted to yield higher voltages and would therefore be better candidates for the CdTe-cell emitter.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2018

Direct-to-indirect electronic state transition in dynamically compressed GaAs quantum wells

Dynamic compression of GaAs quantum wells was achieved to examine the direct-to-indirect transition in a reduced dimension semiconductor structure under uniaxial strain conditions. Our results show that the transformation deviates significantly from the electronic structure predictions using bulk deformation potentials. This finding is attributed to the suppression of real-space type-II transitions by quantum state interactions due to the presence of large anisotropic strains.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2018

Reliable density functional calculations for the electronic structure of thermoelectric material ZnSb

In this paper, we present the results of systematic test calculations for the electronic structure of thermoelectric material ZnSb using a first-principles full-potential all electron computational method. We used a linear combination of atomic orbitals (LACO) formalism, based on density functional theory (DFT). The exchange-correlation interaction potential of the many electron system was described by using a generalized gradient approximation (GGA). We compared the calculated indirect and direct band gaps as well as the effective masses of holes and electrons in ZnSb with experimental measurement results. The calculated indirect band gap of ZnSb is 0.56 eV, which agrees very well with the experimentally measured values of 0.50 eV ~ 0.61 eV. The calculated direct band gap at X point is 0.89 eV. The calculated effective masses of electrons and holes in ZnSb also agree with experimental data. The systematical test calculations as well as the comparisons of the calculated results with experimental measurements show that the obtained electronic structure of ZnSb would be reliable. We did not observe a major deficiency of the first-principles DFT calculation for the electronic structure of ZnSb, using full-potential all electron LACO method. The reported electronic structure of single crystal ZnSb from this work may provide a fundamental knowledge base for further research and applications for this important thermoelectric material.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Driving chemical interactions at graphene-germanium van der Waals interfaces via thermal annealing

Despite its extraordinary charge carrier mobility, the lack of an electronic bandgap in graphene limits its utilization in electronic devices. To overcome this issue, researchers have attempted to chemically modify the pristine graphene lattice in order to engineer its electronic bandstructure. While significant progress has been achieved, aggressive chemistries are often employed that are difficult to pattern and control. In an effort to overcome this issue, here we utilize the well-defined van der Waals interface between crystalline Ge(110) and epitaxial graphene to template covalent chemistry. In particular, by annealing atomically pristine graphene-germanium interfaces synthesized by chemical vapor deposition under ultra-high vacuum conditions, chemical bonding is driven between the germanium surface and the graphene lattice. The resulting bonds act as charge scattering centers that are identified with scanning tunneling microscopy. The generation of atomic-scale defects are independently confirmed with Raman spectroscopy, revealing significant densities within the graphene lattice. Furthermore, the resulting chemically modified graphene has the potential to impact next-generation nanoelectronic applications.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2019

Interfacial properties of penta-graphene-metal contacts

Novel properties of penta-graphene (PG) have stimulated great interest in exploring its potential for device applications. Here, we systematically study the interfacial properties of the heterojunctions constructed by stacking PG on several metal substrates (Ag, Al, Au, Cr, Cu, Pd, and Ti), which are commonly used in field-effect transistors. We consider PG as the channel material because of its semiconducting feature, while treating the metal surfaces as the electrodes. Based on first principles calculations, we show that PG preserves its pentagonal feature with some small distortions when deposited on the metal substrates but undergoes metallization due to the chemical bonding between PG and the metal surfaces. We evaluate the device potential of these PG-metal contacts by studying their tunneling barriers, orbital overlaps, and Schottky barriers. Here, we find that PG forms an n-type Schottky barrier when in contact with Al, Cu, and Ti, but forms a p-type Schottky barrier when supported on Ag, Au, Cr, and Pd. Our study sheds light on the design and fabrication of PG-based electronic devices.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2019

Atomic layer deposition of energy band tunable tin germanium oxide electron transport layer for the SnS-based solar cells with 400 mV open-circuit voltage

Tin germanium oxide, (Sn,Ge)O 2 , films were prepared using atomic layer deposition and tailored to a SnS absorber layer by incorporating various amounts of germanium into tin oxide to adjust band alignments at the interfaces of SnS/(Sn,Ge)O 2 photovoltaic devices. Carrier concentrations of (Sn,Ge)O 2 were suppressed from 10 20 to 10 18 cm -3 with germanium incorporation, with nitrogen doping further reducing carrier concentrations by another order of magnitude. Excellent tunability of both band energy levels and carrier concentrations of (Sn,Ge)O 2 allowed optimizing SnS-based solar cells. SnS/(Sn,Ge)O 2 :N devices were demonstrated, with an open-circuit voltage as high as 400 mV, due to the effective mitigation of interfacial recombination of photogenerated carriers at the SnS/(Sn,Ge)O 2 :N absorber-buffer heterojunction interface.

Sulfur compounds↗
DOE OSTI2019

First-principles study of bandgap bowing in BGaN alloys

III-nitride alloys continue to drive advances in electronic and optoelectronic devices. Recently, boron-containing nitride alloys have been explored with the goal of expanding the range of applications. Using first-principles calculations with a hybrid functional, we study the electronic structure of wurtzite BGaN alloys. Strong bandgap bowing is observed, with a concentration-dependent bowing parameter. Due to the strong bandgap bowing, the fundamental bandgap in strain-free alloys is effectively unchanged for the lowest B concentrations. A crossover from a direct to an indirect bandgap occurs for B concentrations greater than 50%.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗