Small Exciton Research Articles

Rate of exciton scattering and exciton absorption in crystals with LO-phonon dispersion

The Ridley–Babiker relation for the LO-phonon energy is used to obtain an exact formula for the exciton scattering rate in the one-phonon approximation. The effect of LO-phonon dispersion on the frequency and temperature dependence of the exciton attenuation is studied theoretically for models of polar crystals with large and small exciton radii. It is shown that the role of LO-phonon dispersion is essentially different in processes with phonon absorption and phonon emission. For some fixed frequency interval, it is established that the effect of dispersion on exciton scattering changes with temperature. The TlBr and ZnS crystals are used as models for illustrations.

Evidence of a small exciton in theBa1−xKxBiO3superconductor and its relationship with superconductivity

The effective mass of a free-carrier plasmon is measured by momentum-resolved electron-energy-loss spectroscopy. The effective mass of the plasmon is heavy along the [100] direction and light along the [110] direction in the optimal doped regime, where T c is highest. The anisotropy of the effective mass between [100] and [110] decreases with increasing doping and decreasing of T c . In the overdoped regime with T c almost zero (normal metal state), the effective mass of the free-carrier plasmon becomes isotropic. A linear correlation between T c and the effective mass difference along the two orientations is extracted from the experimental data. The dispersion of the plasmon for the optimal doped material cannot be explained by the band structure alone, but that for the overdoped material is consistent with the band-structure calculation. This indicates that the effective mass is strongly renormalized along [100] by an additional interaction in the optimally doped material of the highest T c . Of related interest is a group of small excitons in the parent compound of Ba 1 - x K x BiO 3 (BKBO), BaBiO 3 , with assigned symmetries that are s wave (optically forbidden) at 4.2 eV, p wave at 2.1 eV (optically allowed), and d wave at 0.5 eV (optically forbidden). We propose a mixing state of free carriers and small excitons to explain the anisotropic dispersion for the optimally doped regime and a free-carrier state free of small excitons for the overdoped material (normal metal state). The linear correlation between T c and mass renormalization provides insight that the superconductivity of BKBO may be related to electronic structure, possibly to the small exciton or WZK (Wang-Zhang-Klein) exciton.

The Influence of Matrix on Quantum Size Confinement of Semiconductor Microcrystals Doped in Glass Thin Films Prepared by RF-Sputtering

Various types of semiconductor microcrystal-doped glass thin films were successfully prepared by RF-sputtering. The blue shift of the absorption edge in the visible light region, caused by the quantum confinement effect, was observed in all the thin films. The amount of the blue shift was found to depend significantly not only on the types of semiconductor dopants and the microcrystal radius but also on the type of matrix. The blue shift of semiconductor microcrystals with a small exciton Bohr radius compared with the microcrystal radius, such as that of CuCl, showed an exciton confinement effect which was theoretically predicted. On the other hand, the blue shift of microcrystals with a large Bohr radius compared with the microcrystal radius, such as that of CdSe, has been expected to follow the electron–hole independent confinement model, but it was found to deviate significantly from the theoretical curve. In order to explain this deviation, we introduced factors, such as the Coulomb interaction between electron and hole, size-dependent dielectric constants of the dopants and the dielectric constant of matrix. Consequently, it was found that the matrices have a significant influence on the electron–hole independent confinement.

Regular Arrays of (Zn,Mn)S Quantum Wires with Well-Defined Diameters in the Nanometer Range

Zn1−x Mn x S, with x varying between 0.01 and 0.30, were formed inside the ordered pore systems of different mesoporous SiO2 matrices. Because of the highly ordered structure of the hosts, regular arrays of Zn1−x Mn x S quantum wires with lateral dimensions of 3 and 5.5 nm, respectively, separated by 2-nm SiO2 barriers were obtained. The wires were characterized using photoluminescence (PL) and PL excitation (PLE) spectroscopy at liquid Helium temperatures. The PL of the wires is dominated by the 4T1 → 6A1 internal transition of the Mn2+(3d5) ions. The corresponding PLE spectra show higher internal Mn transitions as well as the band to band transition. The energies of the internal Mn transitions are typical for Mn2+ on a cation site of (II,Mn)VI semiconductors. Because of the comparable bandgaps of the SiO2 and the Zn1−x MnxS as well as the small exciton Bohr radius in (Zn,Mn)S quantum confinement effects in the wires are less than about 150 meV.

Arrays of (Zn,Mn)S quantum wires with well-defined diameters below 10 nm

Zn 1−x Mn x S with x=1% to 30% were formed inside the ordered pore systems of different mesoporous SiO2 matrices. Due to the highly ordered host structure, regular arrays of Zn1−xMnxS quantum wires with diameters of 3.1 nm and 5.6 nm, respectively, separated by 2 nm SiO2 barriers were obtained. The wires were characterized using photoluminescence (PL), PL excitation (PLE) spectroscopy, and electron paramagnetic resonance (EPR). The T14→A16 internal transition of the Mn2+(3d5) ions dominates the PL. The corresponding PLE spectra show higher internal Mn transitions and the band-to-band transition. EPR spectra and the energies of the internal Mn transitions are typical for Mn2+ on a cation site of (II,Mn)VI semiconductors. The crystal field parameters indicate that the wires are tensilely strained. Due to the comparable band gaps of the SiO2 and the Zn1−xMnxS and the small exciton Bohr radius in (Zn,Mn)S, the confinement effects in the wires are less than 150 meV.

Capture, relaxation, and recombination in two-dimensional quantum-dot superlattices

We present a systematic study of the exciton capture, relaxation, and recombination processes in two-dimensional quantum-dot superlattices (2D QDSL's) based on time-resolved photoluminescence measurements. Due to the formation of minibands in 2D QDSL's, the capture of excitons from the miniband into some large islands is found to be a quantum capture process. The capture time increases significantly with increasing excitation density. In addition, the excitons relax rapidly within the miniband. However, the relaxation becomes slower at high excitation densities. Furthermore, the fast recombination in the miniband indicates the drastic elongation of the exciton coherence length and the significant delocalization of the wave functions. Finally, the observation of radiative recombination over a wide energy region implies the relaxation of momentum conservation or the small exciton effective mass in 2D QDSL's. All phenomena suggest that the exciton dynamics in 2D QDSL's is governed by the exciton coherence length in the miniband.

Picosecond fluorescence decay and exciton dynamics in a new far-red molecular J-aggregate system

We present and characterize a new J-aggregate-type nanostructure formed by self-assembling of 3,3′-diethylthiadicarbocyanine iodide molecules in a solution. We measure picosecond kinetics of fluorescence from the lowest exciton band, lying in the far-red wavelength range around 810–830 nm. At temperatures 4–300 K we observe a non-exponential fluorescence decay and show that this is a result of exciton–exciton annihilation in a small exciton domain regime. We also measure the temperature dependence of single-exciton radiative lifetime and give a consistent description of it in terms of a self-trapping of exciton in a discrete one-dimensional chain.

Liquid crystalline phthalocyanines in organic solar cells

Small exciton diffusion lengths, insufficient charge transport and low absorption prevent organic materials from achieving commercially useful power conversion efficiencies. The recently reported long exciton diffusion range along the columnar axes in the mesophase of disc shaped liquid crystals might represent a way to circumvent these limitations. In this paper we investigate a double-layer photovoltaic device comprising a discotic phthalocyanine derivative as electron-donor and a perylene derivative as electron-acceptor material. We fabricated one device with a pristine phthalocyanine layer and compare it with a device that was thermally treated to obtain a higher ordered mesophase.

Quantum confinement induced collapse of a large to a small radius self-trapped exciton in a permethylated polysilane chain

Low-temperature absorption and fluorescence spectra of a series of permethylated oligosilanes SinMe2n+2 in their all-anti form permits the identification of Si7Me16 as the shortest chain capable of supporting a large-radius exciton in its relaxed lowest singlet excited state, while Si6Me14 and shorter chains only carry small-radius self-trapped excitons. The following approximate one-dimensional semiconductor characteristics are derived from the data: exciton band half-width, ∼3.0 eV; lattice relaxation energy, ∼1.8 eV; and exciton–phonon coupling constant in the infinite chain, ∼0.6. ©1999 John Wiley & Sons, Inc. Int J Quant Chem 72: 361–367, 1999

Plasmon-Frenkel-exciton in a clustered solid

The standard theory of the Frenkel exciton (a small radius exciton) is applied to a fullerene 2D solid. It is the dipole collective electron excitation of a single cluster which forms the delocalized plasmon-Frenkel-exciton (PFE) in a crystal. The PFE retarded interaction is taken into account. We present transverse PFE-polariton dispersion curves along with the Coulomb problem solution for longitudinal excitation in the 2D plane.

Electro-optical modulation in the blue-green spectral range using aZnSe−Zn0.75Cd0.25Sedouble superlattice at room temperature

Electroreflectance measurements performed in conjunction with absorption and photovoltage spectroscopy have allowed a determination of the modulation mechanisms that are responsible for the quantum confined Stark effect in a ${\mathrm{Z}\mathrm{n}\mathrm{S}\mathrm{e}\ensuremath{-}\mathrm{Z}\mathrm{n}}_{0.75}{\mathrm{Cd}}_{0.25}\mathrm{Se}$ double superlattice. This study indicates that the modulation of the excitonic transitions in the two superlattices (${E}_{1\mathrm{hh}}^{1},$ ${E}_{2\mathrm{hh}}^{1},$ and ${E}_{1\mathrm{hh}}^{2}$) are observed in the electroreflectance spectra. Due to the thermal effects of the photoexcited carriers the higher subband with a smaller exciton binding energy is more sensitive to the ac electric field.

Configuration of the self-trapped exciton in alkali halides

Vinti sum-rule analysis of the optical absorption of self-trapped excitons (STEs) in alkali halides reveals two STE classes: (1) Large excitons with electron orbits encompassing many ions. The STE radii correlate roughly with the dielectric constant as in the Wannier model. (2) Small excitons with orbits less than the lattice spacing. The radii are of the same size as F centers and show the same linear dependence on lattice spacing. This absorption-based classification mirrors the luminescence-based scheme of Kan'no et al. The classes correspond to on-center and off-center STEs, respectively.

Continuous photoluminescence, time resolved photoluminescence and optically detected magnetic resonance measurements of PbI 2 nanometer-sized particles, embedded in SiO 2 films

The nanometer sized particles of PbI 2 were embedded in SiO 2 films. X-ray diffraction and the TEM pictures showed the preservation of the bulk layered structure and symmetry. The PL spectrum of the nano-particles exhibited a pronounced blue shift of the exciton band due to quantum size effect. The Lead Iodide represents an exceptionally small exciton Bohr radius ( a B = 19 A ̊ ) and a special case in which m e ⪢ m h . The prepared samples contained particles with mean radii, a, in the range a B < a < 3 a B . Within this limit (with m e ⪢ m h ), the experimental results suggest that the electron is localized nearly at the center of the particle, enabling the hole to move around it. Thus, the size confinement permits the creation of an acceptor-like exciton. The PL spectrum revealed additional states, associated with stoichiometric defects either at the interior or surface sites of the nano-particles. These defects act either as donor or acceptor states. The dynamics of the various recombination processes has been investigated by measuring the time resolved PL spectra. The results show a multiexponential behavior of the various recombination emission bands, indicating the occurrence of trapping and detrapping processes. Analysis of these results suggests that the existence of surface states give rise to these complex radiative decay processes. The correlation between donor-acceptor recombination emission bands in the aforementioned samples and lattice imperfections was examined, utilizing optically detected magnetic resonance (ODMR) spectroscopy. The results identified the following imperfection sites: an acceptor site associated with an isotropic Lead vacancy defect, [ V −] pb 2+ and a donor site, associated with an anisotropic Iodine vacancy, [ V 0] Iodine .

Momentum-Dependent Charge Transfer Excitations in Sr2CuO 2Cl2 Angle-Resolved Electron Energy Loss Spectroscopy.

Electron-hole pair excitations in the insulating cuprates $Sr_2CuO_2Cl_2$ were investigated by angle-resolved electron energy loss spectroscopy. The optically allowed and optically forbidden transitions were observed to be strongly anisotropic in $Cu-O_2$ plane. The former show a large energy dispersion $\sim 1.5$eV along [110], and the latter appear at a higher energy position ($\sim 4.5$eV) only along [100], but not along [110]. We interpret these results as transitions involving excitons. A small exciton model is examined to explain both the observed features.

Coupling of Excitons in Parallel Organic and Semiconductor Quantum Wires

Abstract We study the resonance energy transfer between parallel neighboring organic and semiconductor quantum wires which is caused by the resonance dipole-dipole interaction between the wires. Due to this interaction, a new hybrid excited state arises which is a mixture of Frenkel and Wannier-Mott excitons. For the transition dipole moments oriented perpendicular to the wires, the interwire hybridization strength is nonzero even for small exciton wavevectors along the wires and decays rather slowly with increasing inter-wire spacing. This is in contrast to two-dimensional quantum wells where the dipole-dipole coupling decays fast with the increasing interwell distance and takes place only for nonzero excitonic wavevectors. The hybrid wire excitons possess both a large radius and a relatively large oscillator strength which might be especially interesting for the nonlinear optics.

Nanoscale Size Effects on Photoconductivity of Semiconducting Polymer Thin Films

Bilayer polymer photoreceptors containing a trap-free tri-p-tolylamine-doped polycarbonate (TTA:PC) layer and a conjugated polymer layer are used to study nanoscale size effects in electronic polymers. A 3-fold enhancement of photogeneration quantum efficiency is observed as the thickness of poly(2,5-pyridylenebenzobisthiazole) (PPyBT) is reduced from 100 to 10 nm. The nanometer size effects originate from the small exciton diffusion lengths and the interfacial nature of the photocarrier generation.

Nonlinear optical studies of Bloch oscillations

We have investigated the coherent dynamics of wave packets in semiconductor superlattices with an applied electric field. In particular, we have performed four-wave mixing experiments with time-resolving of the diffracted beam. The temporal behavior of the signal shows the oscillation dynamics of the Bloch wave packet. Additionally, the delayed rise of the signals clearly demonstrates the influence of many-body effects due to Coulomb coupling in the nonlinear optical response of the system. We show that the influence of the many-body effects can be tuned by changing the static applied field: for low field (small exciton binding energy), the FWM signal rises fast; for high field (large exciton binding energy), the signal rises slowly. The experimental results are well described by theory.

Photoluminescence, Reflectance, and Magnetospectroscopy of Shallow Excitons in GaN

ABSTRACTWe report several new aspects of the excitonic properties of heteroepitaxial GaN grown on sapphire or 6H-SiC. In particular, we observed the n = 2 free exciton associated with both A and B excitons (which are distinct from the n = 1 C exciton) using reflectance and 1.7 K photoluminescence. We also studied the behavior of the n = 2 A-exciton using magnetoluminescence in fields up to 12 T. The large diamagnetic shift and splitting positively confirm the identification, yielding an exciton binding energy of about 26.4 meV. Several previous identifications of the n = 2 free exciton yielding a smaller exciton binding energy are probably in error, based on our results. We have also detected the two-electron replica of the neutral donor-bound exciton for the first time in GaN and observed its splitting pattern in magnetic fields up to 12 T. This feature is 22 meV below the principal neutral donor-bound exciton peak, independently of strain shifts in the overall spectrum. It yields a precise donor binding energy of 29 meV for the shallow residual donor in material grown by metalorganic chemical vapor deposition and gas-source molecular beam epitaxy, considerably smaller than that of the residual donor reported earlier in hydride vapor phase epitaxial material (about 35.5 meV).

Hybrid excitons in parallel organic and inorganic semiconducting quantum wires.

We study excited states in parallel neighboring organic and inorganic semiconducting quantum wires. Due to the resonance dipole-dipole interaction between the wires, the hybrid exciton is a mixture of Frenkel and Wannier-Mott excitons. For the transition dipole moments oriented perpendicular to the wires, the interwire hybridization strength is nonzero even for small exciton wave vectors along the wires and decays rather slowly with increasing interwire spacing. This is in contrast to two-dimensional quantum wells where the dipole-dipole coupling decays fast with increasing interwell distance and is nonzero only for nonzero excitonic wave vectors. The hybrid wire excitons possess both a large radius and a relatively large oscillator strength, which could be especially interesting with respect to applications in nonlinear optics.

Exciton Confinement in Ultrathin Crystalline Organic Films Grown by Organic Molecular Beam Deposition

AbstractWe use the low temperature fluorescence spectra of organic multiple quantum well samples consisting of the archetype materials, 3,4,9,10 perylenetetracarboxylic dianhydride (PTCDA); 3,4,7,8 naphthalenetetracarboxylic dianhydride (NTCDA), and 3,4,9,10 perylenetetracarboxylic-bis-benzimidazole (PTCBI), to study the effects of quantum confinement on the lowest energy excited and ground electronic states of these molecular crystals. Both the Franck-Condon and the dominant ground state vibronic mode energies are observed to undergo significant shifts with decreasing PTCDA layer thicknesses (ranging from 500Å to 10Å) in PTCDA/NTCDA MQWs, while no such effects are observed for PTCBI/NTCDA MQWs. These results are interpreted in the context of confinement of spatially extended excitons in ultra thin PTCDA layers, whereas the considerably smaller radius PTCBI excitons are not affected over the range of layer thicknesses investigated. These results unambiguously rule out previous hypotheses suggesting that binding of small radius excitons to interfaces results in the blue shifts previously observed in the absorption spectra of PTCDA-based MQWs.