Low Exciton Binding Energy Research Articles

Electrically driven photon antibunching from a single molecule at room temperature

Single-photon emitters have been considered for applications in quantum information processing, quantum cryptography and metrology. For the sake of integration and to provide an electron photon interface, it is of great interest to stimulate single-photon emission by electrical excitation as demonstrated for quantum dots. Because of low exciton binding energies, it has so far not been possible to detect sub-Poissonian photon statistics of electrically driven quantum dots at room temperature. However, organic molecules possess exciton binding energies on the order of 1eV, thereby facilitating the development of an electrically driven single-photon source at room temperature in a solid-state matrix. Here we demonstrate electroluminescence of single, electrically driven molecules at room temperature. By careful choice of the molecular emitter, as well as fabrication of a specially designed organic light-emitting diode structure, we were able to achieve stable single-molecule emission and detect sub-Poissonian photon statistics.

Resonant Energy Transfer due to Exciton Coupling in Hybrid Persovskites Conjugated to GaN Semiconductors

ABSTRACTExciton-Exciton coupling in hybrid persovskites conjugated to GaN Semiconductors system has been studied. Excitons are strongly coupled in (C6H5C2H4NH3)2PbI4[bis(phenethyl-ammonium)tetraiodoplumbatel] (PEPI) is a hybrid inorganic-organic layered, with a perovskite structure quantum well (QW). The inorganic PbI4 monolayer is sandwiched between organic layers. The interaction between electron and hole forming the bound exciton is significantly stronger due to dielectric confinement. The lowest exciton binding energy is 220 meV, which is 20 times in comparison to GaAs. Gallium nitride (GaN) semiconductor has defect bound excitons with absorption band, which overlaps with the emission spectra of the PEPI system. This facilitates resonant energy transfer (RET) from the GaN defect bound exciton states to the excitons confined in the PEPI layer. We investigated the interaction by photoluminescence (PL) and found that 1s exciton in the PEPI layer strongly couples with the GaN defect level exciton, which is pronounced at lower temperature (< ∼ 100 K).

Reduction in Induced Strain in CdxZn1-xS Well Layers Using ZnS1-ySey Barrier Layers and Inhibition of Induced Strain due to Lattice Mismatch between MgzZn1-zS Cladding Layers and CdxZn1-xS/ZnS1-ySey Multiple Quantum Wells by Adjusting Mg Content

The reduction in induced-strain effect in CdxZn1-xS-based quantum wells (QWs) is important for fabrication of laser diodes (LDs). To determine the effect of the reduction in the induced strain in a CdxZn1-xS well layer which combined with a ZnS1-ySey barrier layer. By reducing the induced strain, in this system, negative conduction and valence band offsets are obtained at large Se content y, band lineups are maintained to be Type I, and slightly low exciton binding and effective band gap energies are obtained. Under this condition, the confinement efficiencies of electron and holes slightly decreases. We propose that a CdxZn1-xS/ZnS1-ySey multiple quantum well (MQW) is sandwiched between two MgzZn1-zS cladding layers. The energy difference between ZnS1-ySey and MgzZn1-zS band gaps increases with increasing Mg content z. MgzZn1-zS cladding layers are lattice-matched to CdxZn1-xS/ZnS1-ySey MQWs by adjusting z, and then the induced strain due to the MgzZn1-zS cladding layers is removed.

Self-consistent approach for calculations of exciton binding energy in quantum wells

We introduce a computationally efficient approach to calculating characteristics of excitons in quantum wells. In this approach we derive a system of self-consistent equations describing the motion of an electron–hole pair. The motion in the growth direction of the quantum well in this approach is separated from the in-plane motion, but each of them occurs in modified potentials found self-consistently. The approach is applied to shallow quantum wells, for which we obtained an analytical expression for the exciton binding energy and the ground state eigenfunction. Our numerical results yield lower exciton binding energies in comparison to standard variational calculations, while require reduced computational effort.

Theoretical study of conjugated porphyrin polymers

The optical gap of conjugated triply linked porphyrin chains is exceptionally low (∼0.5 eV). Hence, such chains are candidates for organic infrared detectors and solar cells harvesting the infrared part of the solar spectrum. However, a low exciton binding energy is required for these applications. From a theoretical analysis of excitons in long metalloporphyrin chains, we demonstrate that the binding energy is much lower than in usual conjugated polymers. Our calculated absorption spectra are in good agreement with measurements.

Comment on `Modelling the optical constants of GaAs:excitonic effects at E1,E1 + Δ1 criticalpoints'

Djurisic and Li (1999, Semiconductor Science andTechnology 14 958) have recently presented acalculation of the optical constants of GaAs in which they donot take into account excitonic effects at either E1,E1 + Δ1 or E0,E0 + Δ0 critical points (CPs).They employ band-to-band single-particle expressions withan energy-dependent pseudo-Gaussian broadening function.Their paper states that including excitonic effects at theformer CPs has `dubious physical interpretation'. It is alsoclaimed that (a) `excitonic effects are usually more pronouncedat E0,E0 + Δ0 than at the E1,E1 + Δ1 CPs',(b) `at room temperature the excitons are severelybroadened and should not contributesignificantly to the dielectric function' and (c) `excitoniceffects at E1,E1 + Δ1 critical points do not represent asignificant contribution to the dielectric function at roomtemperature for materials with low exciton binding energy'.These statements are completely incorrect, as is their fittingscheme.

The electronic structure of solid α‐sexithiophene

The properties of α‐sexithiophene (α‐T6), e.g., the pronounced one‐dimensionality of the molecule (see Figure), the layered crystal packing, and the low exciton binding energy, make it a very interesting material for electronic and optoelectronic applications. Investigations into the origin of its optical gap and conclusions drawn from them are reported. magnified image

Excitons and electron-hole plasma phase in [formula omitted] multiple quantum well laser structures grown by molecular beam epitaxy

The exciton stability in Zn 1−x Cd x Se ZnSe multiple quantum wells has been investigated by absorption and magneto-luminescence spectroscopies in undoped and modulation-doped samples. The well width and the composition dependence of the exciton binding energy has been determined experimentally and compared with the value obtained with theoretical calculations. The absorption saturation has been investigated in modulation-doped samples as a function of the sheet carrier density. The excition stability has been correlated to the actual lasing mechanism in samples with different structure and composition. Freecarrier recombination dominates the stimulated emission of Zn 1− x Cd x Se quantum wells with relatively low exciton binding energy, whereas a dominant excitonic character is found in the lasing of deep and narrow quantum wells. Stimulated emission experiments in high magnetic fields performed on various samples confirm this attribution.

Dielectric screening, exciton binding energy and the insulator-metal transition in molecular hydrogen

Using an appropriate model for dielectric screening in molecular hydrogen we calculate the static dielectric constant ϵ()) in the proximity of the insulator-metal transition. The dielectric constant ϵ(0) is constructed in terms of the energy gap, the exciton binding energy and the plasmon frequency. Considering the energy gap from the band-structure calculation of Friedli and Ashcroft [1] and making an estimate of the lower exciton binding energy in the instability region, ϵ(0) is shown to diverge at a critical density given by r s ∗ = 1.57 a.u.