Excitonic Structure Research Articles

Electronic excitation transfer from an organic matrix to CdS nanocrystals produced by the Langmuir–Blodgett method

The absorption, photoluminescence, and photoluminescence excitation spectra of CdS nanocrystals formed by the Langmuir–Blodgett method are explored. Features of the absorption and photoluminescence excitation spectra defined by optical transitions in the matrix and nanocrystals are identified. The efficiency of electronic excitation transfer from an organic matrix to nanocrystals is studied. It is shown that charge carriers efficiently transfer from the matrix to electron and hole size-quantization levels in nanocrystals and to acceptor defect levels in the band gap of nanocrystals. A large Stokes shift defined by fine exciton structure (bright and dark excitons) is observed. The shift is in the range 140–220 meV for nanocrystals 2.4 and 2.0 nm in radius.

Revealing Large-Scale Homogeneity and Trace Impurity Sensitivity of GaAs Nanoscale Membranes.

III-V nanostructures have the potential to revolutionize optoelectronics and energy harvesting. For this to become a reality, critical issues such as reproducibility and sensitivity to defects should be resolved. By discussing the optical properties of molecular beam epitaxy (MBE) grown GaAs nanomembranes we highlight several features that bring them closer to large scale applications. Uncapped membranes exhibit a very high optical quality, expressed by extremely narrow neutral exciton emission, allowing the resolution of the more complex excitonic structure for the first time. Capping of the membranes with an AlGaAs shell results in a strong increase of emission intensity but also in a shift and broadening of the exciton peak. This is attributed to the existence of impurities in the shell, beyond MBE-grade quality, showing the high sensitivity of these structures to the presence of impurities. Finally, emission properties are identical at the submicron and submillimeter scale, demonstrating the potential of these structures for large scale applications.

Toward an Understanding of the Excitonic Structure of the CP47 Antenna Protein Complex of Photosystem II Revealed via Circularly Polarized Luminescence.

Identification of the lowest energy pigments in the photosynthetic CP47 antenna protein complex of Photosystem II (PSII) is essential for understanding its excitonic structure, as well as excitation energy pathways in the PSII core complex. Unfortunately, there is no consensus concerning the nature of the low-energy state(s), nor chlorophyll (Chl) site energies in this important photosynthetic antenna. Although we raised concerns regarding the estimations of Chl site energies obtained from modeling studies of various types of CP47 optical spectra [Reinot, T; et al., Anal. Chem. Insights 2016, 11, 35-48] recent new assignments imposed by the shape of the circularly polarized luminescence (CPL) spectrum [Hall, J.; et al., Biochim. Biophys. Acta 2016, 1857, 1580-1593] necessitate our comments. We demonstrate that other combinations of low-energy Chls provide equally good or improved simultaneous fits of various optical spectra (absorption, emission, CPL, circular dichroism, and nonresonant hole-burned spectra), but more importantly, we expose the heterogeneous nature of the recently studied complexes and argue that the published composite nature of the CPL (contributed to by CPL685, CPL691, and CPL695) does not represent an intact CP47 protein. A positive CPL695 is extracted for the intact protein, which, when simultaneously fitted with multiple other optical spectra, provides new information on the excitonic structure of intact and destabilized CP47 complexes and their lowest energy state(s).

Colloidal quantum dots conjugated with human serum albumin – interactions and bioimaging properties

Abstract Quantum dots, due to their unique optical properties, constitute significant materials for many areas of nanotechnology and bionanotechnology. This work presents a review of researches dedicated to the interaction between quantum dots (QDs) with human serum albumin (HSA) and human cell culture as important for nanomedicine applications. The optical properties of bio-nanocomplexes formed by nanoparticles including colloidal QDs (e.g., CdTe, CdS, CdCoS) and albumin are displayed. The absorption spectra show that adding HSA to colloidal QDs leads to a gradual decrease of absorption and broadening of the exciton structure. The photoluminescence quenching results indicate that the quenching effect of QDs on HSA fluorescence depends on the size and temperature. The nature of quenching is rather static, resulting in forming QD-HSA complexes. The CdTe QD-HSA complexes show chemical stability in a PBS buffer. Furthermore, it is stable in cytoplasm and suitable for cell labeling, tracking, and other bioimaging applications.

Band edge photoluminescence of undoped and doped TlInS2 layered crystals

Intrinsic photoluminescence (PL) of undoped and of B-, Ag- or Er-doped TlInS2 layered single crystals was investigated by confocal spectroscopy. It is found that position and intensity of PL spectral peak at 2.4eV is vastly dependent on the excitation light incidence and polarization relative to crystallographic directions. For the normal incidence to the layer plane k||c, a significant Stokes shift between the PL peak in respect to exciton absorption energy is highlighted. The shift increases to 80meV around the crystal phase transformations region of 200K. We show that presence of B and Ag impurity do not diminish the intrinsic PL emission while Er atoms incorporation enhances it substantially and modifies a fine excitonic line structure in the ferroelectric phase at low T. The obtained results imply out-of-layer plain oriented 2D excitons in TlInS2 existing at various crystal phases which are influenced by related interlayer stresses.

Excitonic structure and pumping power dependent emission blue-shift of type-II quantum dots

In this work we study theoretically and experimentally the multi-particle structure of the so-called type-II quantum dots with spatially separated electrons and holes. Our calculations based on customarily developed full configuration interaction ap- proach reveal that exciton complexes containing holes interacting with two or more electrons exhibit fairly large antibinding energies. This effect is found to be the hallmark of the type-II confinement. In addition, an approximate self-consistent solution of the multi-exciton problem allows us to explain two pronounced phenomena: the blue-shift of the emission with pumping and the large inhomogeneous spectral broadening, both of those eluding explanation so far. The results are confirmed by detailed intensity and polarization resolved photoluminescence measurements on a number of type-II samples.

Multi-excitonic (N=1,2 and 3) quantum dots in magnetic field: Analytical mapping of correlations (exchange) by multipole expansion

The understanding of the physics of exciton, bi-exciton, tri-exciton and the subsequent insight into controlling the properties of mesoscopic systems holds the key to various exotic optical, electrical and magnetic phenomena like superconductivity, Mott insulation, Quantum Hall effect etc. Many of exciton properties are similar to atomic hydrogen that attracts researchers to explore electronic structure of exciton in quantum dots, but nontriviality arises due to coulombic interactions among electrons and holes. We propose an exact integral of coulomb (exchange) correlation in terms of finitely summed Lauricella functions to examine 3-D exciton of harmonic dots confined in zero and non-zero arbitrary magnetic field. The highlight of our work is the use of exact variational solution for coloumbic interaction between the hole and the electron and evaluation of the cross terms arising out of the coupling among centre-of-mass and relative coordinates. We also have extended the size of the system to generalized N-body problem with N=3,4 for tri-exciton (e-e-h/e-h-h) ▪ and biexciton (e-h-e-h) (e-h-e-h) ▪ respectively by multipole expansions. As a probe we have examined level spacing statistics, energy spectra, specific heat (Cv), magnetization and chemical potential (μ) at low temperatures.

Quasiparticle energies, exciton level structures and optical absorption spectra of ultra-narrow ZSiCNRs

GW quasiparticle energies, exciton structures and optical absorption spectra of ultra-narrow N-ZSiCNRs.

Excitonic structure and dynamics in various photosynthetic antenna protein complexes: Holeburning and modeling studies

Excitonic structure and dynamics in various photosynthetic antenna protein complexes: Holeburning and modeling studies

Optical Studies of the Interactions CdS and CdTe Nanoparticles with a Human Serum Albumin

The interaction between CdS , CdTe nanoparticles and human serum albumin (HSA) was studied by absorption and photoluminescence spectroscopy. The performed transmission electron microscopy (TEM) analysis suggests of spherical shape of nanostructures with small size distribution. In the absorption spectra of the colloidal nanoparticles exciton band was found to be shifted to higher photon energy as compared with that for bulk crystals due to the quantum confinement effect. It was shown that addition of HSA to colloidal nanocrystals leds to a gradual decrease of absorption and broadening of exciton structure, resulting in forming nanoparticles–HSA complexes. The photoluminescence quenching results indicated that the quenching effect of nanoparticles CdTe on HSA fluorescence depend on the temperature and the nature of quenching is static. When the concentration of CdS nanoparticles in solution with HSA over 1,5 10-6 mol L-1, in addition to static quenching mechanism of photoluminescence increases of dynamic quenching mechanism.

Experimental determination of excitonic band structures of single-walled carbon nanotubes using circular dichroism spectra.

Experimental band structure analyses of single-walled carbon nanotubes have not yet been reported, to the best of our knowledge, except for a limited number of reports using scanning tunnelling spectroscopy. Here we demonstrate the experimental determination of the excitonic band structures of single-chirality single-walled carbon nanotubes using their circular dichroism spectra. In this analysis, we use gel column chromatography combining overloading selective adsorption with stepwise elution to separate 12 different single-chirality enantiomers. Our samples show higher circular dichroism intensities than the highest values reported in previous works, indicating their high enantiomeric purity. Excitonic band structure analysis is performed by assigning all observed Eii and Eij optical transitions in the circular dichroism spectra. The results reproduce the asymmetric structures of the valence and conduction bands predicted by density functional theory. Finally, we demonstrate that an extended empirical formula can estimate Eij optical transition energies for any (n,m) species.

Optical Properties and Electronic Structure of CaO

Spectra of 11 optical functions of CaO in the ranges 6.5–7.2 eV at 2 K and 0–60 eV at 77 K were determined. A total of 14 maxima and shoulders of excitons and interband transitions were found including two distinct exciton structures at 6.9 and 11.4 eV and volume and surface plasmon maxima at ~38.3 and 33.2 eV, respectively. Their principal features and general trends were established. Variable formation efficiencies of spectra of the dielectric permittivity and characteristic electron-energy losses in different spectral ranges were analyzed. The calculations used known experimental reflectance spectra and computer programs based on the Kramers–Kronig correlations and analytical formulae for the relationship between optical functions.

Effect of Spectral Density Shapes on the Excitonic Structure and Dynamics of the Fenna–Matthews–Olson Trimer from Chlorobaculum tepidum

The Fenna-Matthews-Olson (FMO) trimer (composed of identical subunits) from the green sulfur bacterium Chlorobaculum tepidum is an important protein model system to study exciton dynamics and excitation energy transfer (EET) in photosynthetic complexes. In addition, FMO is a popular model for excitonic calculations, with many theoretical parameter sets reported describing different linear and nonlinear optical spectra. Due to fast exciton relaxation within each subunit, intermonomer EET results predominantly from the lowest energy exciton states (contributed to by BChl a 3 and 4). Using experimentally determined shapes for the spectral densities, simulated optical spectra are obtained for the entire FMO trimer. Simultaneous fits of low-temperature absorption, fluorescence, and hole-burned spectra place constraints on the determined pigment site energies, providing a new Hamiltonian that should be further tested to improve modeling of 2D electronic spectroscopy data and our understanding of coherent and dissipation effects in this important protein complex.

Effects of quantum confinement on excited state properties ofSrTiO3fromab initiomany-body perturbation theory

The Ruddlesden-Popper (RP) homologous series Sr$_{n+1}$Ti$_{n}$O$_{3n+1}$ provides a useful template for the study and control of the effects of dimensionality and quantum confinement on the excited state properties of the complex oxide SrTiO$_3$. We use ab initio many-body perturbation theory within the $GW$ approximation and the Bethe-Salpeter equation approach to calculate quasiparticle energies and absorption spectrum of Sr$_{n+1}$Ti$_{n}$O$_{3n+1}$ for $n=1-5$ and $\infty$. Our computed direct and indirect optical gaps are in excellent agreement with spectroscopic measurements. The calculated optical spectra reproduce the main experimental features and reveal excitonic structure near the gap edge. We find that electron-hole interactions are important across the series, leading to significant exciton binding energies that increase for small $n$ and reach a value of 330~meV for $n=1$, a trend attributed to increased quantum confinement. We find that the lowest-energy singlet exciton of Sr$_2$TiO$_4$ ($n=1$) localizes in the 2D plane defined by the TiO$_2$ layer, and explain the origin of its localization.

Band Structure of the Rhodobacter sphaeroides Photosynthetic Reaction Center from Low-Temperature Absorption and Hole-Burned Spectra.

Persistent/transient spectral hole burning (HB) and computer simulations are used to provide new insight into the excitonic structure and excitation energy transfer of the widely studied bacterial reaction center (bRC) of Rhodobacter (Rb.) sphaeroides. We focus on site energies of its cofactors and electrochromic shifts induced in the chemically oxidized (P(+)) and charge-separated (P(+)QM(-)) states. Theoretical models lead to two alternative interpretations of the H-band. On the basis of our experimental and simulation data, we suggest that the bleach near 813-825 nm in transient HB spectra in the P(+)QM(-) state, often assigned to the upper exciton component of the special pair, is mostly due to different electrochromic shifts of the BL/M cofactors. From the exciton compositions in the charge-neutral (CN) bRC, the weak fourth excitonic band near 780 nm can be denoted PY+, that is, the upper excitonic band of the special pair, which in the CN bRC behaves as a delocalized state over PM and PL pigments that weakly mixes with accessory BChls. Thus, the shoulder in the absorption of Rb. sphaeroides near 813-815 nm does not contain the PY+ exciton band.

Illuminating Excitonic Structure in Ion-Dependent Porphyrin Aggregates with Solution Phase and Single-Particle Resonance Raman Spectroscopy

Self-assembled, excitonically coupled aggregates of 5,10,15,20-tetrakis(4-sulfonatophenyl)porphyrin (TSPP) prepared in aqueous LiCl, NaCl, KCl, CsCl, and HCl were studied with solution phase and single-particle resonance Raman (RR) spectroscopy. Solution phase excitation profiles are sharply peaked at 488.0 nm excitation for samples in HCl, LiCl, and CsCl, which show narrow J-bands in the corresponding absorption spectra, but more gently peaked for those induced by NaCl or KCl, which show broader absorption J-bands. The former three samples also exhibit larger low to high frequency mode intensity ratios with excitation near the J-band peak and smaller depolarization ratios compared to the latter two samples. Polarized spectra of individual aggregates correlate with the solution phase results, exhibiting an increase in intensities involving either incident or scattered light polarized transversely to the aggregate long axes in conditions shown previously to induce bundling of nanotubes or greater disorder. These results, along with previous absorption, resonance light scattering (RLS), and imaging data, suggest that excitonic coupling across nanotubular components in bundled aggregates leads to spectral broadening. This is attributed to increased spectral density of allowed excitonic transitions, particularly those polarized transversely to the aggregate length. Disorder leads to deviation of excitonic transition polarizations from pure axial and transverse directions, resulting in greater transverse relative to axial polarization but smaller excitonic coherence as measured by RLS intensity. These results suggest that environment-induced morphological variations can affect the energies, polarizations, and spatial structure of excitons in dye aggregates.

Carrier Self-trapping and Luminescence in Intrinsically Activated Scintillator: Cesium Hafnium Chloride (Cs2HfCl6)

There has been renewed interest in the scintillation properties of Cs2HfCl6 (CHC) primarily due to its favorable characteristics such as nonhygroscopicity, cubic structure, and fairly high light yield without any intentional doping. In this work, we report on its electronic and optical properties using first-principles calculations. As a large gap insulator (band gap ∼ 6 eV), CHC favors the formation of localized charge carriers, viz., Vk centers and electron polarons. The [HfCl6]-octahedra play a central role in trapping both types of localized carriers that leads to several low energy excitonic structures or self-trapped excitons (STEs). The observed emission spectrum of CHC is compared with our modeled STE structures and their emission energies. We find that Zr present as an unintentional impurity has high solubility in CHC and may be responsible for a secondary emission peak observed around 480 nm. Finally, we mention the electronic structure of the bromide and iodide analogues of CHC as well as the m...

Entanglement entropy of electronic excitations.

A new perspective into correlation effects in electronically excited states is provided through quantum information theory. The entanglement between the electron and hole quasiparticles is examined, and it is shown that the related entanglement entropy can be computed from the eigenvalue spectrum of the well-known natural transition orbital (NTO) decomposition. Non-vanishing entanglement is obtained whenever more than one NTO pair is involved, i.e., in the case of a multiconfigurational or collective excitation. An important implication is that in the case of entanglement it is not possible to gain a complete description of the state character from the orbitals alone, but more specific analysis methods are required to decode the mutual information between the electron and hole. Moreover, the newly introduced number of entangled states is an important property by itself giving information about excitonic structure. The utility of the formalism is illustrated in the cases of the excited states of two interacting ethylene molecules, the conjugated polymer para-phenylene vinylene, and the naphthalene molecule.

Exciton Structure and Energy Transfer in the Fenna-Matthews-Olson Complex.

The Fenna-Matthews-Olson (FMO) photosynthetic complex found in green sulfur bacteria has over the last decades been one of the favorite "model" systems for biological energy transfer. However, even after 40 years of studies, quantitative knowledge about its energy-transfer properties is limited. Here, two-dimensional electronic spectroscopy with full polarization control is used to provide an accurate description of the electronic structure and population dynamics in the complex. The sensitivity of the technique has further allowed us to spectroscopically identify the eighth bacterio-chlorophyll molecule recently discovered in the crystal structure. The time evolution of the spectral structure, covering time scales from tens of femtoseconds up to a nanosecond, reflects the energy flow in FMO and enables us to extract an unambiguous energy-transfer scheme.

Charge-tunable indium gallium nitride quantum dots

III-Nitride quantum dots have emerged as a new chip-scale system for quantum information science, which combines electrical and optical interfaces on a semiconductor chip that is compatible with non-cryogenic operating temperatures. Yet most work has been limited to optical excitations. To enable single-spin based quantum optical and quantum information research, we demonstrate here quantized charging in optically active, site-controlled III-Nitride quantum dots. Single-electron charging was confirmed by the voltage dependence of the energy, dipole moment, fine structures and polarization properties of the exciton states in the quantum dots. The fundamental energy structures of the quantum dots were identified, including neutral and charged excitons, fine structures of excitons, and A and B excitons. The results lay the ground for coherent control of single charges in III-Nitride QDs, opening a door to III-Nitride based spintronics and spin-qubit quantum information processing.