Excitonic Structure Research Articles

Exploiting Structured Environments for Efficient Energy Transfer: The Phonon Antenna Mechanism.

A nontrivial interplay between quantum coherence and dissipative environment-driven dynamics is becoming increasingly recognized as the key for efficient energy transport in photosynthetic pigment-protein complexes, and converting these biologically inspired insights into a set of design principles that can be implemented in artificial light-harvesting systems has become an active research field. Here we identify a specific design principle, the phonon antenna, by which interpigment coherence is able to modify and optimize the way that excitations spectrally sample their local environmental fluctuations. We provide numerical simulations that suggest that the Fenna-Matthews-Olson complex of green sulfur bacteria has an excitonic structure that is close to such an optimal operating point, and place the phonon antenna concept into a broader context that leads us to conjecture that this general design principle might well be exploited in other biomolecular systems.

Multiple Exciton Generation and Recombination in Carbon Nanotubes and Nanocrystals

Semiconducting nanomaterials such as single-walled carbon nanotubes (SWCNTs) and nanocrystals (NCs) exhibit unique size-dependent quantum properties. They have therefore attracted considerable attention from the viewpoints of fundamental physics and functional device applications. SWCNTs and NCs also provide an excellent new stage for experimental studies of many-body effects of electrons and excitons on optical processes in nanomaterials. In this Account, we discuss multiple exciton generation and recombination in SWCNTs and NCs for next-generation photovoltaics. Strongly correlated ensembles of conduction-band electrons and valence-band holes in semiconductors are complex quantum systems that exhibit unique optical phenomena. In bulk crystals, the carrier recombination dynamics can be described by a simple model, which includes the nonradiative single-carrier trapping rate, the radiative two-carrier recombination rate, and the nonradiative three-carrier Auger recombination rate. The nonradiative Auger recombination rate determines the carrier recombination dynamics at high carrier density and depends on the spatial localization of carriers in two-dimensional quantum wells. The Auger recombination and multiple exciton generation rates can be advantageously manipulated by nanomaterials with designated energy structures. In addition, SWCNTs and NCs show quantized recombination dynamics of multiple excitons and carriers. In one-dimensional SWCNTs, excitons have large binding energies and are very stable at room temperature. The extremely rapid Auger recombination between excitons determines the photoluminescence (PL) intensity, the PL linewidth, and the PL lifetime. SWCNTs can undergo multiple exciton generation, while strong exciton-exciton interactions and complicated exciton structures affect the quantized Auger rate and the multiple exciton generation efficiency. Interestingly, in zero-dimensional NC quantum dots, quantized Auger recombination causes unique optical phenomena. The breakdown of the k-conversion rule and strong Coulomb interactions between carriers in NCs enhance Auger recombination rate and decrease the energy threshold for multiple exciton generation. We discuss this impact of the k-conservation rule on two-carrier radiative recombination and the three-carrier Auger recombination processes in indirect-gap semiconductor Si NCs. In NCs and SWCNTs, multiple exciton generation competes with Auger recombination, surface trapping of excitons, and cooling of hot electrons or excitons. In addition, we explore heterostructured NCs and impurity-doped NCs in the context of the optimization of charge carrier extraction from excitons in NCs.

Cryogenic Single-Nanocrystal Spectroscopy: Reading the Spectral Fingerprint of Individual CdSe Quantum Dots.

Spectroscopically resolved emission from single nanocrystals at cryogenic temperatures provides unique insight into physical processes that occur within these materials. At low temperatures, the emission spectra collapse to narrow lines, revealing a rich spectroscopic landscape and unexpected properties, completely hidden at the ensemble level. Since these techniques were first used, the technology of nanocrystal synthesis has matured significantly, and new materials with outstanding photostability have been reported. In this perspective, we show how cryogenic spectroscopy of single nanocrystals probes the fundamental excitonic structure of the band edge, revealing spectral fingerprints that are highly sensitive to a range of photophysical properties as well as nanocrystal morphology. In particular, spectral and temporal signatures of biexciton and trion emission are revealed, and their relevance to emerging technologies is discussed. Overall we show how cryogenic single nanocrystal spectroscopy can be used as a tool for understanding fundamental photophysics and guiding the synthesis of new nanocrystal materials.

Negative spin-exchange splitting in the exciton fine structure of AlN

The exact energy position of the free exciton transition and thus the lowest band gap in bulk wurtzite AlN are still under discussion. By combined high resolution optical emission and absorption experiments on a sample with (11¯00) surface, we resolve the fine structure of the lowest energy free exciton and determine an electron-hole spin-exchange interaction constant of j=−4 meV. This results in a low energy Γ1 exciton at 6.032 eV and a high energy Γ5 component at 6.040 eV. Only the latter one is observable for (0001) oriented AlN films due to selection rules.

Excitonic processes in molecular crystalline materials

Abstract

Fine structure of the band-edge excitons and trions in CdSe/CdS core/shell nanocrystals

We present a theoretical description of excitons and positively and negatively charged trions in "giant" CdSe/CdS core-shell nanocrystals (NCs). The developed theory provides the parameters describing the fine structure of excitons in CdSe/CdS core/thick shell NCs as a function of the CdSe/CdS conduction band offset and the CdSe core radius. We have also developed a general theory describing the fine structure of positively charged trions created in semiconductor NCs with a degenerate valence band. The calculations take into account the complex structure of the CdSe valence band and inter-particle Coulomb and exchange interaction. Presented in this paper are the CdSe core size and CdSe/CdS conduction band offset dependences (i) of the positively charged trion fine structure, (ii) of the binding energy of the negatively charged trion, and (iii) of the radiative decay time for excitons and trions. The results of theoretical calculations are in qualitative agreement with available experimental data.

Can interference patterns in the reflectance spectra of GaN epilayers give important information of carrier concentration?

Low-temperature reflectance spectra of a series of Si-doped GaN epilayers with different doping concentrations grown on sapphire by metal-organic chemical vapour deposition were measured. In addition to the excitonic polariton resonance structures at the band edge, interference oscillating patterns were observed in the energy region well below the band gap. The amplitudes of these oscillation patterns show a distinct dependence on the doping concentrations of the samples. From the thin-film optical interference principle, an approach connecting the amplitude of the interference oscillations and the impurity scattering was established. Good agreement between experiment and theory is achieved.

Dielectric function and energy of the E 0 critical point of hexagonal GaN at 26 K studied by using spectroscopic ellipsometry

We report the dielectric function from 0.74 to 6.42 eV of hexagonal GaN at 26 K, along with the energies of its E0 exciton and critical point. The GaN sample is a 1.9 µm thick film deposited on a c-plane (0001) sapphire substrate by using molecular beam epitaxy. Data were obtained with a rotating-compensator spectroscopic ellipsometer. B-splines in a multilayer-structure calculation were used to extract the optical properties of the film. The 26 K E0 excitonic and critical-point structures were significantly blue-shifted and were sharpened relative to their room-temperature equivalents.

Quantum-coherent energy transfer: implications for biology and new energy technologies

The important thing in science is not so much to obtain new facts as to discover new ways of thinking about them. Sir William Henry Bragg Nobel Prize For Physics, 1915 There is a growing body of scientific evidence that light-initiated reactions in the molecular components of photosynthetic

Analysis of Excitonic and Charge Transfer Interactions from Quantum Chemical Calculations.

A procedure for a detailed analysis of excited states in systems of interacting chromophores is proposed. By considering the one-electron transition density matrix, a wealth of information is recovered that may be missed by manually analyzing the wave function. Not only are the position and spatial extent given, but insight into the intrinsic structure of the exciton is readily obtained as well. For example, the method can differentiate between excitonic and charge resonance interactions even in completely symmetric systems. Four examples are considered to highlight the utility of the approach: interactions between the nπ* states in a formaldehyde dimer, excimer formation in the naphthalene dimer, stacking interaction in an adenine dimer, and the excitonic band structure in a conjugated phenylenevinylene oligomer.

Fluorescence dynamics and fine structure of dark excitons in semiconducting single-wall carbon nanotubes

Exact diagonalization results are reported for the bright and dark exciton structure of semiconducting single-wall carbon nanotubes in the framework of the Hubbard model combined with a small crystal approach for several values of the correlation coupling strength U/t. Our findings, in the low–intermediate correlation regime (1.5 < U/t < 2.1), show the presence of dark states above and below the first bright exciton |B〉 and can account for reported experimental values of deep triplet states below |B〉 and of a K-momentum singlet dark exciton above this state. In order to fit the temporal profile of the photoluminescence (PL) decay, a bottleneck mechanism is considered involving a few dark states, with the respective energy gaps correspondingly obtained in the above-mentioned correlation range. We find that a kinetic model with one dark state above and two below |B〉 is able to recover the observed biexponential features of the PL behaviour with a reasonable set of parameters. Within this model we attribute the long tail of the PL to a delayed luminescence process of the bright state caused by the nearby calculated dark states.

Optical characterization of band-edge property of In6S7 compound

The evaluation of band gap, exciton, and electronic structure of an energy compound is crucial and essential. We report direct optical evidence of band gap, band-edge exciton, and white-light photoelectric conversion for a solar-energy material In6S7 herein. The direct gap of In6S7 is determined to be 0.93 eV. The value is extremely suitable for absorption of full sunlight spectrum extension to near infrared region.

Site Energies of Active and Inactive Pheophytins in the Reaction Center of Photosystem II from Chlamydomonas reinhardtii

It is widely accepted that the primary electron acceptor in various Photosystem II (PSII) reaction center (RC) preparations is pheophytin a (Pheo a) within the D1 protein (Pheo(D1)), while Pheo(D2) (within the D2 protein) is photochemically inactive. The Pheo site energies, however, have remained elusive, due to inherent spectral congestion. While most researchers over the past two decades placed the Q(y)-states of Pheo(D1) and Pheo(D2) bands near 678-684 and 668-672 nm, respectively, recent modeling [Raszewski et al. Biophys. J. 2005, 88, 986 - 998; Cox et al. J. Phys. Chem. B 2009, 113, 12364 - 12374] of the electronic structure of the PSII RC reversed the assignment of the active and inactive Pheos, suggesting that the mean site energy of Pheo(D1) is near 672 nm, whereas Pheo(D2) (~677.5 nm) and Chl(D1) (~680 nm) have the lowest energies (i.e., the Pheo(D2)-dominated exciton is the lowest excited state). In contrast, chemical pigment exchange experiments on isolated RCs suggested that both pheophytins have their Q(y) absorption maxima at 676-680 nm [Germano et al. Biochemistry 2001, 40, 11472 - 11482; Germano et al. Biophys. J. 2004, 86, 1664 - 1672]. To provide more insight into the site energies of both Pheo(D1) and Pheo(D2) (including the corresponding Q(x) transitions, which are often claimed to be degenerate at 543 nm) and to attest that the above two assignments are most likely incorrect, we studied a large number of isolated RC preparations from spinach and wild-type Chlamydomonas reinhardtii (at different levels of intactness) as well as the Chlamydomonas reinhardtii mutant (D2-L209H), in which the active branch Pheo(D1) is genetically replaced with chlorophyll a (Chl a). We show that the Q(x)-/Q(y)-region site energies of Pheo(D1) and Pheo(D2) are ~545/680 nm and ~541.5/670 nm, respectively, in good agreement with our previous assignment [Jankowiak et al. J. Phys. Chem. B 2002, 106, 8803 - 8814]. The latter values should be used to model excitonic structure and excitation energy transfer dynamics of the PSII RCs.

Models of triplet self-trapped excitons in SiO2, HfO2, and HfSiO4

We predict by means of density functional theory simulations the structures of self-trapped triplet excitons in HfO${}_{2}$ and HfSiO${}_{4}$, and compare their properties to those of the self-trapped exciton in $\ensuremath{\alpha}$ quartz. The character of the hole and electron localization in excitons strongly depends on the geometrical structure and dielectric properties of these materials. In HfO${}_{2}$, the electron is localized predominantly on one Hf atom while the hole is localized on one or two oxygen atoms at the nearest or next-nearest-neighbor sites, depending on the crystal phase. We predict two exciton configurations in HfSiO${}_{4}$ with the excited electron localized either on a Hf or on a Si atom and the hole localized on the nearest-neighbor oxygen atoms. Excitation and luminescence energies are calculated for all triplet exciton states and compared with the available experimental data.

Excitonic signatures in the optical response of single‐wall carbon nanotubes

AbstractThe optical properties of single‐wall carbon nanotubes (SWNTs) are dominated by the excitonic character of the transitions even at room temperature. The very peculiar properties of these excitons arise from both the one‐dimensional (1D) nature of carbon nanotubes and from the electronic properties of graphene from which nanotubes are made. We first propose a brief qualitative review of the structure of the excitonic manifold and emphasize the role of dark states. We describe recent experimental investigations of this excitonic structure by means of temperature dependent PL measurements. We investigate the case of upper sub‐bands and show that high‐order optical transitions remain excitonic for large diameter nanotubes. A careful investigation of Rayleigh scattering spectra at the single nanotube level reveals clear exciton–phonon side‐bands and Lorentzian line profiles for all semi‐conducting nanotubes. In contrast, metallic nanotubes show an ambivalent behavior which is related to the reduced excitonic binding energy. magnified image Schematic of the exciton manifold in single‐wall carbon nanotubes.

Influence of hydrogen bonding on excitonic coupling and hierarchal structure of a light-harvesting porphyrin aggregate

Helical porphyrin nanotubes of tetrakis(4-sulfonatophenyl)porphyrin (TSPP) were examined in DCl/D(2)O solution using resonance Raman and resonance light scattering spectroscopy to probe the influence of hydrogen bonding on the excitonic states. Atomic force microscopy reveals similar morphology for aggregates deposited from DCl/D(2)O and from HCl/H(2)O solution. Deuteration results in subtle changes to the aggregate absorption spectrum but large changes in the relative intensities of Raman modes in the J-band excited resonance Raman spectra, revealing relatively more reorganization along lower-frequency vibrational modes in the protiated aggregate. Depolarization ratio dispersion and changes in the relative Raman intensities for excitation wavelengths spanning the J-band demonstrate interference from overlapping excitonic transitions. Distinctly different Raman excitation profiles for the protiated and deuterated aggregates reveal that isotopic substitution influences the excitonic structure of the J-band. The deuterated aggregate exhibits a nearly two-fold increase in intensity of resonance light scattering as a result of an increase in the coherence number, attributed to decreased exciton-phonon scattering. We propose that strongly coupled cyclic N-mers, roughly independent of isotopic substitution, largely decide the optical absorption spectrum, while water-mediated hydrogen bonding influences the further coherent coupling among them when they are assembled into nanotubes. The results show that, similar to natural light-harvesting complexes such as chlorosomes, hydrogen bonding can have a critical influence on exciton dynamics.

Synthesis and luminescence properties of lead-halide based organic–inorganic layered perovskite compounds (C nH 2 n+1 NH 3) 2PbI 4 ( n=4, 5, 7, 8 and 9)

This paper describes the synthesis and characterization of organic–inorganic layered perovskite compounds, (C n H 2 n+1 NH 3) 2PbI 4 ( n=4, 5, 7, 8 and 9). The effect of the number of carbon atoms on luminescence properties has been examined. Thin films of microcrystalline (C n H 2 n+1 NH 3) 2PbI 4 fabricated by spin-coating are highly oriented, with the c-axis perpendicular to the substrate surface. Temperature-dependent optical absorption spectra reveal that (C n H 2 n+1 NH 3) 2PbI 4 films ( n=4, 7, 8 and 9) show the structural phase transitions. The excitonic structures of (C n H 2 n+1 NH 3) 2PbI 4 vary with the number of carbon atoms of the alkyl chain length. At low temperatures below 100 K, the lowest-energy free-exciton band of (C n H 2 n+1 NH 3) 2PbI 4 ( n=7, 8 and 9) split into three fine-structure levels. In contrast to (C n H 2 n+1 NH 3) 2PbBr 4 films, (C n H 2 n+1 NH 3) 2PbI 4 ( n=7, 8 and 9) shows no triplet exciton emission, but it shows the Stokes-shifted emission from bound excitons.

Excitons and excitonic fine structures in Si nanowires: Prediction of an electronic state crossover with diameter changes

Si nanowires have attracted considerable attention as promising candidates for electronic, thermoelectric, photonic, and photovoltaic devices, yet there appears to be only limited understanding of the underlying electronic and excitonic structures on all pertinent energy scales. Using atomistic pseudopotential calculations of single-particle as well as many-body states, we have identified remarkable properties of Si nanowires in three energy scales: (i) In the ``high-energy'' $\ensuremath{\sim}$1-eV scale, we find an unusual electronic state crossover whereby the nature of the lowest unoccupied molecular orbital (LUMO) state changes its symmetry with wire diameters for [001]-oriented wires but not for [011]-oriented wires. This change leads to orbitally allowed transitions becoming orbitally forbidden below a certain critical diameter for [001] wires. (ii) In the ``intermediate-energy'' $\ensuremath{\sim}$10${}^{\ensuremath{-}1}$-eV scale, we describe the excitonic binding, finding that in [001] wires the diameter ($D$) dependence of excitonic gap scales as $1/{D}^{1.9}$, not as $1/{D}^{1}$ as expected. The exciton binding energy increases from 52 meV at $D=7.6$ nm to 85 meV at $D=3.3$ nm and 128 meV at $D=2.2$ nm. (iii) In the ``low-energy'' $\ensuremath{\sim}$10${}^{\ensuremath{-}3}$-eV scale, we describe dark/bright excitonic states and predict how orbitally allowed transitions [in scale (i)] become spin-forbidden due to the electron-hole exchange interaction, whereas the spin-allowed states in the orbitally forbidden diameter region remain dark. The diameter dependence of the fine-structure splitting of excitonic states scales as $1/{D}^{2.3}$ in [001] wires and as $1/{D}^{2.6}$ in [011] wires. Surface-polarization effects are found to significantly enhance electron-hole Coulomb interaction, but have a small effect on the exchange fine-structure splitting. The present work provides a road map for a variety of electronic and optical effects in Si nanowires that can guide spectroscopic studies.

Effect of light scattering on the transmission spectra of organic nanocrystals

The effect of light scattering on the extinction spectra of organic nanocrystals was investigated by absorption and extinction measurement and by theoretical calculation for typical organic nanocrystals of α-perylene. In the extinction spectra, the exciton band shows a size-dependent redshift and tail toward longer wavelengths due to scattering as the crystal size is increased beyond ∼50 nm. In the absorption spectra, the exciton band blueshifts from that in the bulk crystal for crystal sizes less than ∼200 nm due to the anisotropic exciton structure in perylene. The calculation qualitatively supports these results. The effect of scattering always makes the extinction band redshift from the absorption band.

Selective-area vapor-liquid-solid growth of tunable InAsP quantum dots in nanowires

A process is described where the position, size, and cladding of an InP nanowire with an embedded InAsP quantum dot are determined by design through lithography, processing, and growth. The vapor-liquid-solid growth mode on a patterned substrate is used to grow the InP core and defines the quantum dot size to better than ±2 nm while selective-area growth is used to define the cladding thickness. The clad nanowires emit efficiently in the range λ=0.95–1.15 μm. Photoluminescence measurements are used to quantify the dependence of the excitonic energy level structure on quantum dot size for diameters 10–40 nm.