Search

We study the optical properties of ${\mathrm{Cs}}_{3}{\mathrm{Bi}}_{2}{\mathrm{I}}_{9}$ nanoplatelets using a combination of first-principles density functional theory, $\mathit{GW}$ plus Bethe-Salpeter equation calculations, and spectroscopic experiments. We show that the material exhibits flat bands and hence high effective masses. This manifests itself in the lowest-energy transition in the absorption spectrum arising from excitons with a high binding energy of 300 meV and a Bohr radius smaller than 6 nm. Due to the indirect band gap, electrons and holes are efficiently separated in reciprocal space and recombine slowly across the band gap, leading to very weak photoluminescence. Our results resolve inconsistencies in previous studies on ${\mathrm{Cs}}_{3}{\mathrm{Bi}}_{2}{\mathrm{I}}_{9}$ and lay the groundwork for further applications of this material, reliant on charge separation.

We investigate carrier capture processes in strain-induced quantum dot structures. The quantum dots consist of a near-surface InGaAs/GaAs quantum well in which a lateral confining potential is generated by the strain from InP stressor islands grown on the sample surface. Using photoluminescence spectroscopy, we show that the rate of carrier capture into the quantum dots increases dramatically when the energetic depth of the confinement potential is reduced by enlarging the quantum well/surface separation D. While carriers in the quantum well region between the quantum dots are found to experience D-dependent nonradiative surface recombination, this process seems to be negligible for carriers in the quantum dots, presumably due to the protecting InP islands.

We investigate the exciton dynamics in composite systems of conjugated polymers and fullerene molecules by photoluminescence (PL) and femtosecond transient absorption experiments. In solid mixtures (blends) we find a strong concentration-dependent quenching of the polymer PL. This is attributed to an efficient electron transfer (ET) from the photoexcited conjugated polymer to the fullerene. The ET dynamics is directly monitored by measuring the transient stimulated emission of the conjugated polymer. The transfer rate depends linearly on the ${\mathrm{C}}_{60}$ concentration and ranges between $(66\mathrm{ps}{)}^{\ensuremath{-}1}$ and $(5\mathrm{ps}{)}^{\ensuremath{-}1}$ for concentrations from 0.5% to 5%. This dependence is in accordance with an exciton diffusion process occurring prior to the ET. The exciton diffusion length in the conjugated polymer is directly determined by measuring the PL quenching in well-defined heterostructures comprising a self-assembled fullerene monolayer and a thin spin-coated polymer layer of variable thickness. From these measurements we infer a value of 14 nm for the exciton diffusion length in ladder-type poly (p-phenylene). Our results are of direct relevance for further optimization of polymer photovoltaic devices.

We study exciton ionization induced by an axial electric field in a strongly coupled GaAs/${\mathrm{Al}}_{0.3}$${\mathrm{Ga}}_{0.7}$As superlattice at low temperatures. The field-induced ionization times of the heavy-hole 1s exciton in the miniband field regime are determined from transient four-wave-mixing experiments and theoretical model calculations. They are found to decrease with increasing field, and lie in the picosecond and subpicosecond ranges for fields on the order of a few kV/cm. They are considerably longer than those calculated for a 1s exciton in bulk GaAs. We explain this difference as a result of the nonparabolicities in the superlattice miniband dispersion. In addition to studying the field-induced ionization of the 1s resonance, we also investigate the field dependence of excitonic wave packets composed of both bound and continuum states of the exciton. It is found that the continuum components of the wave packet are more sensitive to low electric fields than the 1s component. \textcopyright{} 1996 The American Physical Society.

In low-temperature photoluminescence experiments on II-VI semiconductor quantum wells we find an energy transfer from confined quantum-well states to above-barrier states. The observed anti-Stokes barrier luminescence exhibits a characteristic intensity dependence showing that this transfer is caused by a two-step absorption process involving localized or impurity bound-exciton states in the quantum well. Time-resolved photoluminescence experiments show that the photon for the second intraband absorption step can be provided by the quantum-well luminescence, i.e., the anti-Stokes barrier luminescence is a direct consequence of photon recycling.

We report on a detailed experimental study of the homogeneous linewidth of heavy-hole excitons in semimagnetic CdTe/${\mathrm{Cd}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Mn}}_{\mathit{x}}$Te multiple quantum wells. Transient four-wave-mixing experiments are used to obtain the homogeneous linewidth by measuring the dephasing time of the excitons. We find a dephasing time of 2.2 ps corresponding to a homogeneous linewidth of 0.6 meV at low temperature and moderate exciton density. The homogeneous linewidth increases linearly with both exciton density and lattice temperature. The contributions of exciton\char21{}acoustic-phonon scattering, exciton-exciton interaction, and disorder to the homogeneous linewidth are determined.

We present subpicosecond time-resolved four-wave-mixing studies on GaAs/AlAs short-period superlattices having different miniband widths. Photon echoes are observed as a consequence of the inhomogeneous broadening of the excitonic resonance. In addition, the photon-echo signal exhibits periodic modulations as a function of the time delay between the excitation laser pulses. Free excitons and excitons bound to neutral acceptors are identified as the underlying three-level system leading to the observed photon-echo quantum beats. From the beat period we can directly infer the dissociation energy of the bound exciton. In accordance with theory, we find that the dissociation energy decreases with increasing miniband width, i.e., when the excitons become less confined. In addition, we find that the dephasing rate of bound excitons is about one order of magnitude less than that of free excitons and is mainly determined by energy relaxation.

We report on coupled electron-hole transfer, Coulomb drag, in CdSe/CdS semiconductor nanotetrapods. We demonstrate that photoexcited holes can either be transferred to the CdSe core or become trapped in one of the CdS arms. By combining time-resolved pump-probe and photoluminescence measurements we investigate how the Coulomb potential drags the electron to the hole localization site. As supported by effective-mass calculations taking into account Coulomb effects we conclude that the hole dynamics determines the fate of the electron in a coupled dynamics.

Semiconductor tetrapods are expected to exhibit isotropic optical properties due to their tetrahedral symmetry. We have investigated the optical polarization properties of individual $\mathrm{Cd}\mathrm{Se}∕\mathrm{Cd}\mathrm{S}$ tetrapods of high structural quality. In contrast to the light absorption behavior, a pronounced anisotropy is observed in the optical emission. This unexpected result can only be explained by assuming a symmetry breaking in the electronic wavefunction. Calculations reveal that slight differences between the arm diameters induce the polarized emission and a decrease in the photoluminescence intensity.

We study the temperature dependence of triplet formation and decay by considering the phosphorescence dynamics in a prototypical conjugated polymer matrix. The dynamics of triplet formation as a function of temperature are unraveled by applying an electric field during optical singlet generation and recording the phosphorescence quenching. The diffusivity of triplet excitons in conjugated polymers is shown to be independent of temperature up to $200\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. We propose a diffusion model that describes the relaxation dynamics of triplet excitons at all times through triplet-triplet annihilation and spin orbit coupling at static heavy atom sites.

Spherical CdSe nanocrystals capped by a CdS rod-like shell exhibit interesting spectral dynamics on the single particle level. Spectral boxcar averaging reveals a high degree of correlation between the emission energy, spectral linewidth, phonon coupling strength, and emission intensity of the single nanocrystal. The results can be described in terms of a spatially varying surface charge density in the vicinity of the exciton localized in the CdSe core, leading to a quantum confined Stark effect which modifies the transition energy and the radiative rate. Whereas internal charging of the particle results in a change in the nonradiative rate, surface charges primarily influence the radiative rate. Additionally, we observe characteristic spectral dynamics in frequency space, the magnitude of which depends slightly on temperature and strongly on excitation density. By distinguishing between continuous spectral jitter and discrete spectral jumping associated with a reversible particle ionization event, we can attribute the spectral dynamics to either a slowly varying surface charge density or a rapidly occurring polarization change due to a reversible expulsion of a charge carrier from the semiconductor nanostructure. Whereas the former exhibits universal Gaussian statistics, the latter is best characterized by a Lorentzian noise spectrum. The Gaussian spectral noise increases with spectral redshift of the emission and with increasing proximity of the surface charge to the localized exciton. The observation of a high degree of correlation between peak position and linewidth right up to room temperature suggests applications of the nanocrystals as extremely sensitive single charge detectors in both solid state devices and in biomolecular labeling, where highly local measurements of the dielectric environment are required. Nanoscale control of the physical shape of nanocrystals provides a versatile test bed for studying electronic noise, making the approach relevant to a wide range of conducting and emissive solid state systems.

We investigate the nature of ultrafast exciton dissociation and carrier generation in acceptor-doped conjugated polymers. Using a combination of two-pulse femtosecond spectroscopy with photocurrent detection, we compare the exciton dissociation and geminate charge recombination dynamics in blends of two conjugated polymers, MeLPPP [methyl-substituted ladder-type poly($p$-phenylene)] and MDMO-PPV [poly(2-methoxy,5-(3,7-dimethyloctyloxy)-1,4-phenylenevinylene], with the electron accepting fullerene derivative PCBM [1-(3-methoxycarbonyl)-propyl-1-phenyl-$(6,6){\mathrm{C}}_{61}$]. This technique allows us to distinguish between free charge carriers and Coulombically bound polaron pairs. Our results highlight the importance of geminate pair recombination in photovoltaic devices, which limits the device performance. The comparison of different materials allows us to address the dependence of geminate recombination on the film morphology directly at the polymer:fullerene interface. We find that in the MeLPPP:PCBM blend exciton dissociation generates Coulombically bound geminate polaron pairs with a high probability for recombination, which explains the low photocurrent yield found in these samples. In contrast, in the highly efficient MDMO-PPV:PCBM blend the electron transfer leads to the formation of free carriers. The anisotropy dynamics of electronic transitions from neutral and charged states indicate that polarons in MDMO-PPV relax to delocalized states in ordered domains within $500\phantom{\rule{0.3em}{0ex}}\mathrm{fs}$. The results suggest that this relaxation enlarges the distance of carrier separation within the geminate pair, lowering its binding energy and favoring full dissociation. The difference in geminate pair recombination concurs with distinct dissociation dynamics. The electron transfer is preceded by exciton migration towards the PCBM sites. In MeLPPP:PCBM the exciton migration time decays smoothly with increasing PCBM concentration, indicating a trap-free exciton hopping. In MDMO-PPV:PCBM, however, the exciton migration time is found to show a threshold-like dependence on the PCBM concentration. This observation is explained by efficient interchain energy transfer in ordered MDMO-PPV domains in conjunction with exciton trapping.

Light emission at the particle plasmon frequency is observed in optically excited spherical gold nanoparticles. We find a photoluminescence efficiency of ${10}^{\ensuremath{-}6}$, which is essentially independent of particle size and four orders of magnitude higher than the efficiencies determined from metal films. Our experimental findings are explained with a process in which excited $d$-band holes recombine nonradiatively with $\mathit{sp}$ electrons, emitting particle plasmons. These plasmons subsequently radiate, giving rise to the photoluminescence observed in the experiment. We determine the quantum efficiencies involved in this process.

Fluorescence spectroscopy of conjugated polymers at the single molecule level provides unique insight into the nature of the emitting state of these organic semiconductors. We are able to verify the picture that molecular excitations form the primary photoexcitations in conjugated polymers by identifying individual chromophore units on rigid rod-like chains of a ladder-type polymer. The observation of a well-defined substructure in the vibronic progression as well as the presence of sum-frequency vibrational modes in the higher order vibrational bands demonstrate the sensitivity of the method. We find that conjugated polymers are excellent materials for single molecule experiments, exhibiting narrow transition lines accompanied only by a limited number of discrete vibrational modes offset by hundreds of ${\text{cm}}^{\ensuremath{-}1}$. We conclude that the high level of structural rigidity of the molecule as well as the presence of shielding sidegroups on the polymer chain reduces vibrational coupling both to the amorphous matrix as well as limiting the number of internal vibrational modes, in contrast to the case for small dye molecules. By studying the fluorescence from different single molecules we are able to image intramolecular and intermolecular disorder directly. We observe a distribution in energy of the electronic transitions due to the characteristic energetic disorder. The intensity of the vibronic side bands is also found to vary from molecule to molecule, which we propose to be related to conformational influence on the strength of coupling between the electronic excitation and vibrational modes. Structural relaxation and intramolecular energy transfer are studied by single molecule site-selective fluorescence. Our results suggest that even in rigid polymer molecules structural relaxation leads to a small Stokes shift of $l70\phantom{\rule{0.3em}{0ex}}{\text{cm}}^{\ensuremath{-}1}$ upon electronic excitation of a single chromophore on a polymer chain at low temperatures. The influence of vibrational and structural relaxation on intramolecular energy transfer in these multichromophoric systems is also discussed.

Strongly enhanced second-harmonic generation (SHG), which is characterized by a nearly isotropic intensity distribution, is observed for gold-glass films near the percolation threshold. The diffuselike SHG scattering, which can be thought of as nonlinear critical opalescence, is in sharp contrast with highly collimated linear reflection and transmission from these nanostructured semicontinuous metal films. Our observations, which can be explained by giant fluctuations of local nonlinear sources for SHG due to plasmon localization, verify recent predictions of percolation-enhanced nonlinear scattering.