Symmetry analysis and exact model for the elastic, piezoelectric, and electronic properties of inhomogeneous and strained wurtzite quantum nanostructures

International audienceA symmetry analysis and a semianalytical exact model are proposed to describe the mechanical, piezoelectric, and electronic properties of strained wurtzite quantum nanostructures with axial symmetry. An expression of the piezoelectric polarization is given as a function of inhomogeneous strains. The three-dimensional 8x8 strained kp Hamiltonian is reduced to two-dimensional using the total angular momentum representation. When the spin-orbit coupling is neglected, the Hamiltonians are reduced to 1x1 and 3x3 Hamiltonians for the states in the S-shell. For all the other shells, the fourfold degeneracy is demonstrated. Simulations are performed for InN/GaN quantum dots

On the entanglement of electrostriction and non-linear piezoelectricity in non-centrosymmetric materials

International audienceAn extended and complete thermodynamical model of third-order electro-elastic coupling is proposed with symmetry analyses and density functional theory (DFT) calculations to evaluate consistently the various linear and non-linear coefficients. It is shown that in non-centrosymmetric materials, electrostrictive and non-linear piezoelectric phenomena are strongly coupled, except for materials crystallizing in a cubic lattice associated to the 432 point group. Thorough numerical results are given for GaN and AlN compounds in the Würtzite structure. Electrostriction dominates, but non-linear elasticity and non-linear piezoelectricity must be taken into account for strain evaluation whereas non-linear piezoelectricity yields a significant correction for electric field

Two-photon transitions in triazole based quadrupolar and octupolar chromophores: a TD-DFT investigation

C. Katan present address: CNRS UMR6082 FOTON, INSA de Rennes, 20 avenue des Buttes de Coësmes, CS 70839, 35708 RENNES cedex 7, FranceInternational audienceSimultaneous absorption of two photons has gained increasing attention over recent years as it opens the way for improved and novel technological capabilities. In the search for adequate materials that combine large two-photon absorption (TPA) responses and attributes suitable for specific applications, the multibranch strategy has proved to be efficient. Such molecular engineering effort, based on the gathering of several molecular units, has benefited from various theoretical approaches. Among those, the Frenkel exciton model has been shown to often provide a valuable qualitative tool to connect the optical properties of a multibranched chromophore to those of its monomeric counterpart. In addition, recent extensions of time-dependent density functional theory (TD-DFT) based on hybrid functionals have shown excellent performance for the determination of nonlinear optical (NLO) responses of conjugated organic chromophores and various substituted branched structures. In this paper, we use these theoretical approaches to investigate the one- and two-photon properties of triazole-based chromophores. In fact, experimental data were shown to reveal quite different behaviors as compared to related quadrupolar and octupolar compounds. Our theoretical findings allow elucidating these differences and contribute to the general understanding of structure-property relations. This work opens new perspectives towards synergic TPA architectures

Modulation Response of Semiconductor Quantum-Dot Lasers

A new expression of the modulation transfer function is derived for quantum dot (QD) lasers. The analytical approach is based on a cascade relaxation model taking into account three QD energy levels such as the wetting layer (WL), the 1st excited state (ES) as well as the ground state (GS). From the analysis, we demonstrate that the carrier escape from (GS) to (ES) is responsible for a non-zero resonance frequency at low bias powers

Anisotropic and inhomogeneous Coulomb screening in the Thomas-Fermi approximation: Application to quantum dot-wetting layer system and Auger relaxation

International audienceA model for anisotropic and inhomogeneous Coulomb screening due to 2D and 3D carriers, is proposed in the Thomas–Fermi approximation. Analytical expressions for the screened interaction potentials and scattering matrix elements are obtained. This model is applied to the Auger relaxation of carriers in an InAs/InP quantum dot (QD) – wetting layer (WL) system. The influences of the QD morphology and carriers densities on screening and Auger effects are studied. 2D–2D scattering is found to be the most important process, depending especially on QD morphology. A smearing effect is associated to the wetting layer wavefunction extension along the growth axis. The screened potential is similar to a potential screened by 3D carriers

Analysis of Multivalley and Multibandgap Absorption and Enhancement of Free Carriers Related to Exciton Screening in Hybrid Perovskites

International audienceSolution-processable metal-halide perovskites recently opened a new route toward low-cost manufacture of photovoltaic cells. Converting sunlight into electrical energy depends on several factors among which a broad absorption across the solar spectrum and attractive charge transport properties are of primary importance. Hybrid perovskites meet such prerequisites, but despite foremost experimental research efforts, their understanding remains scanty. Here we show that in these materials the appropriate absorption and transport properties are afforded by the multibandgap and multivalley nature of their band structure. We also investigate the nature of the photoexcited species. Our analysis suggests exciton screening by collective orientational motion of the organic cations at room temperature, leading to almost free carriers. Molecular collective motion is also expected to couple to carrier diffusion at room temperature. In mixed halides, our interpretation indicates that doping might hinder collective molecular motions, leading to good transport properties despite alloying and local lattice strain

Electronic surface states and dielectric self-energy profiles in colloidal nanoscale platelets of CdSe

International audienceThe electronic surface states and dielectric self-energy profiles in CdSe colloidal nanoscale platelets are explored by means of an original ab initio approach. In particular, we show how the different coatings deeply modify the quantum and dielectric confinement in CdSe nanoscale platelets. Molecular coating leads to an electronic band gap free of electronic surface states as well as an optimal surface coverage. The reduced blinking in CdSe nanoscale platelets is discussed. The theoretical method here proposed allows one to go beyond the popular empirical description of abrupt dielectric interfaces by explicitly describing the nanoplatelet surface morphology and polarisability at the atomic level. This theoretical study open the way toward more precise description of the dielectric confinement effect in any hybrid system exhibiting 2D electronic properties

Carrier escape from ground state and non-zero resonance frequency at low bias powers for semiconductor quantum-dot lasers

International audienceThe three-dimensional confinement of electrons and holes in the semiconductor quantum dot (QD) structure profoundly changes its density of states compared to the bulk semiconductor or the thin-film quantum well (QW) structure. The aim of this paper is to theoretically investigate the microwave properties of InAs/InP(311B) QD lasers. A new expression of the modulation transfer function is derived for the analysis of QD laser modulation properties based on a set of four rate equations. Analytical calculations point out that carrier escape from ground state (GS) to excited state (ES) induces a non-zero resonance frequency at low bias powers. Calculations also show that the carrier escape leads to a larger damping factor offset as compared to conventional QW lasers. These results are of prime importance for a better understanding of the carrier dynamics in QD lasers as well as for further optimization of low cost sources for optical telecommunications

Comment on “Density functional theory analysis of structural and electronic properties of orthorhombic perovskite CH3NH3PbI3” by Y. Wang et al., Phys. Chem. Chem. Phys., 2014, 16, 1424–1429

International audienceYun Wang et al. used density functional theory (DFT) to investigate the orthorhombic phase of CH3NH3PbI3, which has recently shown outstanding properties for photovoltaic applications. Whereas their analysis of ground state properties may represent a valuable contribution to understanding this class of materials, effects of spin–orbit coupling (SOC) cannot be overlooked as was shown in earlier studies. Moreover, their discussion on optical properties may be misleading for non-DFT-experts, and the nice agreement between experimental and calculated band gap is fortuitous, stemming from error cancellations between SOC and many-body effects. Lastly, Bader charges suggest potential problems during crystal structure optimization

Theoretical insights into multibandgap hybrid perovskites for photovoltaic applications

Copyright 2014 Society of Photo-Optical Instrumentation Engineers. One print or electronic copy may be made for personal use only. Systematic reproduction and distribution, duplication of any material in this paper for a fee or for commercial purposes, or modification of the content of the paper are prohibited.International audienceFollowing pioneering works, the 3D hybrid lead-halide perovskites CH3NH3PbX3 (X=Cl, Br, I) have recently been shown to drastically improve the efficiency of Dye Sensitized Solar Cells (DSSC). It is predicted to open "a new era and a new avenue of research and development for low-cost solar cells ... likely to push the absolute power conversion efficiency toward that of CIGS (20%) and then toward and beyond that of crystalline silicon (25%)" (Snaith, H. J. Phys Chem. Lett. 4, 3623-3630 (2013).). Here, we investigate theoretically the crystalline phases of one of the hybrids relevant for photovoltaic applications, namely CH3NH3PbCl3. Critical electronic states and optical absorption are thoroughly investigated both in the low and high temperature phases. Our findings reveal the dramatic effect of spin orbit coupling on their multiple band gaps. Their physical properties are compared to those of conventional semiconductors, evidencing inversion of band edge states