F. Della Sala

Organic single-layer white light-emitting diodes by exciplex emission from spin-coated blends of blue-emitting molecules

We report on white electroluminescence (EL) emission from a single-layer light-emitting diode based on a binary blend of organic soluble blue-emitting molecules, i.e., a diamine derivative and a substituted thiophene-1,1-dioxide. Weakly voltage-dependent white color, of coordinates (0.39, 0.40) according to the standard of the Commission Internationale de l’Eclairage, is obtained from the superposition of the blue emission from the donor and a low-energy peak due to a charge-transfer complex between the two molecules (exciplex). The EL spectrum is broader and more structured than the photoluminescence one: this could be due to the activation of exciplexes with different conformations as inferred from quantum-chemistry calculations.

Transferable tight-binding parametrization for the group-III nitrides

We present accurate tight-binding (TB) parametrizations of the quasi-particle band structures of AlN, GaN, and InN, both for the zincblende and wurtzite phases. For this purpose, an empirical sp3d5s* nearest-neighbor TB model including the spin–orbit interaction is used, ensuring the transferability of the Slater–Koster parameters to changes in the structural environment. The present approach results in excellent agreement with experimental band positions and ab initio calculations for the lowest conduction band and the valence-band splittings. As a first application, we investigate the electronic band structure of an ordered Al0.5Ga0.5N alloy in the wurtzite phase.

Meta-GGA Exchange-Correlation Functional with a Balanced Treatment of Nonlocality

We construct a meta-generalized-gradient approximation which properly balances the nonlocality contributions to the exchange and correlation at the semilocal level. This nonempirical functional shows good accuracy for a broad palette of properties (thermochemistry, structural properties) and systems (molecules, metal clusters, surfaces, and bulk solids). The accuracy for several well-known problems in electronic structure calculations, such as the bending potential of the silver trimer and the dimensional crossover of anionic gold clusters, is also demonstrated. The inclusion of empirical dispersion corrections is finally discussed and analyzed.

Frozen density embedding with hybrid functionals

The Kohn-Sham equations with constrained electron density are extended to hybrid exchange-correlation (XC) functionals. We derive the frozen density embedding generalized Kohn-Sham (FDE-GKS) scheme which allows to treat the nonlocal exact-exchange in the subsystems. For practical calculations we propose an approximated version of the FDE-GKS in which the nonadditive exchange potential is computed at a semilocal level. The proposed method is applied to compute the ground-state electronic properties of small test systems and selected DNA base pairs. The results of calculations employing the hierarchy of XC functionals BLYP/B3LYP/BHLYP and PBE/PBE0 are presented, in order to analyze the effect of nonlocal exchange contributions, and compared with reference coupled-cluster singles and doubles results. We find that the use of hybrid functionals leads to a significant improvement in the description of ground-state electronic properties of the investigated systems. The semilocal version of the FDE-GKS correctly reproduces the dipole and the electron density distribution of the exact GKS supramolecular system, with errors smaller than the ones obtained using conventional semilocal XC functionals.

Quasiparticle energies for large molecules: A tight-binding-based Green's-function approach

We present a tight-binding approach for the calculation of quasiparticle energy levels in confined systems such as molecules. The method is based on Hedins GW approximation, in which the self-energy is given as the product of the Greens function (G) and the screened Coulomb interaction (W). Key quantities in the GW formalism such as the microscopic dielectric function are expressed in a minimal basis of spherically averaged atomic orbitals. All necessary integrals are either precalculated or approximated without resorting to empirical data. The method is validated against first-principles results for benzene and anthracene, where good agreement is found for levels close to the frontier orbitals. Further, the size dependence of the quasiparticle gap is studied for conformers of the polyacenes (C{sub 4n+2}H{sub 2n+4}) up to n=30.

Two-Dimensional Scan of the Performance of Generalized Gradient Approximations with Perdew-Burke-Ernzerhof-Like Enhancement Factor.

We assess the performance of the whole class of functionals defined by the Perdew-Burke-Ernzerhof (PBE) exchange-correlation enhancement factor, by performing a two-dimensional scan of the μ and κ parameters (keeping β fixed by the recovery of the local density approximation linear response). We consider molecular (atomization energies, bond lengths, and vibrational frequencies), intermolecular (hydrogen-bond and dipole interactions), and solid-state (lattice constant and cohesive energies) properties. We find, for the energetical properties, a whole family of functionals (with μ and κ interrelated) giving very similar results and the best accuracy. Overall, we find that the original PBE and the recently proposed APBE functional [Phys. Rev. Lett.2011, 106, 186406], based on the asymptotic expansion of the semiclassical neutral atom, give the highest global accuracy, with a definite superior performance of the latter for all of the molecular properties.

Well-width dependence of the ground level emission of GaN/AlGaN quantum wells

We have performed a systematic investigation of GaN/AlGaN quantum wells grown on different buffer layers (either GaN or AlGaN) in order to clarify the role of strain, structural parameters, and built-in field in determining the well-width dependence of the ground level emission energy. We find that identical quantum wells grown on different buffer layers exhibit strong variation of the ground level energy but similar well-width dependence. The data are quantitatively explained by an analytic model based on the envelope function formalism which accounts for screening and built-in field, and by a full self-consistent tight binding model.

Excitation energies of terthiophene and its dioxide derivative: a first-principles study

Abstract The excitation spectra of terthiophene and terthiophene-S,S-dioxide are investigated by time-dependent density-functional theory (TD-DFT) calculated data are compared with recent experimental results. The second singlet–triplet excitation energy (T2) is calculated above the first singlet–singlet (S1) one. In terthiophene-S,S-dioxide the formation of a bonding interaction in the lowest unoccupied molecular orbital decreases its kinetic energy and explains the red-shift of the excitation spectrum. The inter-system crossing (ISC) rate in terthiophene-S,S-dioxide is expected to be lower than in terthiophene due to the increased T2–S1 energy gap, which is also found to not decrease with inter-ring torsion as in terthiophene.

Effects of the spontaneous polarization and piezoelectric fields on the luminescence spectra of GaN/Al0.15Ga0.85N quantum wells

Abstract We have investigated the well width dependence of the ground level emission of GaN/AlGaN quantum wells. We find that the fundamental electron–heavy-hole transition red-shifts well below the GaN bulk gap for well widths larger than 3 nm and that the luminescence intensity reduces with increasing well thickness. These effects originate from the quantum confined Stark effect caused by the strong built-in electric field induced by the spontaneous polarization charge at the GaN/AlGaN interfaces and, to a minor extent, by the piezoelectric charge, with typical strength in the MV/cm range. The experimental data are quantitatively explained by means of a self-consistent tight-binding approach which includes screening (either dielectric or by free carriers) and spontaneous polarization field.

Exchange-correlation generalized gradient approximation for gold nanostructures

We compare the performance of different exchange-correlation functionals, based on the Perdew-Burke-Ernzerhof (PBE) generalized gradient approximation, for the structural and electronic properties of gold nanostructures. In particular we consider PBEsol (constructed to correctly describe solid-state systems) and PBEint [Phys. Rev. B 82, 113104 (2010)] which was recently introduced for hybrid interfaces and preserves the correct second-order gradient expansion of exchange energy (as in PBEsol) providing as well a significant nonlocality for higher density variation (as in PBE). We find that the PBEint functional gives a well balanced description of atomization energies, structural properties, energy differences between isomers, and bulk properties. Results indicate that PBEint is expected to be the most accurate functional for medium and large size gold clusters of different shapes.