A Nanophotonic Structure Containing Living Photosynthetic Bacteria

Photosynthetic organisms rely on a series of self‐assembled nanostructures with tuned electronic energy levels in order to transport energy from where it is collected by photon absorption, to reaction centers where the energy is used to drive chemical reactions. In the photosynthetic bacteria Chlorobaculum tepidum, a member of the green sulfur bacteria family, light is absorbed by large antenna complexes called chlorosomes to create an exciton. The exciton is transferred to a protein baseplate attached to the chlorosome, before migrating through the Fenna–Matthews–Olson complex to the reaction center. Here, it is shown that by placing living Chlorobaculum tepidum bacteria within a photonic microcavity, the strong exciton–photon coupling regime between a confined cavity mode and exciton states of the chlorosome can be accessed, whereby a coherent exchange of energy between the bacteria and cavity mode results in the formation of polariton states. The polaritons have energy distinct from that of the exciton which can be tuned by modifying the energy of the optical modes of the microcavity. It is believed that this is the first demonstration of the modification of energy levels within living biological systems using a photonic structure.     

Temperature-dependent integral exciton absorption in semiconducting InP crystals

The temperature dependence of the fundamental absorption edge in free-standing “epitaxial” InP layers has been experimentally studied. The integral exciton absorption coefficient K(T) exhibits an increase at low temperatures, which is explained in terms of the exciton-polariton mechanism of light transfer in semi-conductor crystals with spatial dispersion. A critical temperature (T _c = 200 K), above which the integral absorption becomes constant, has been experimentally determined, and the corresponding critical decay parameter (Γ_c = 0.341 meV), longitudinal-transverse splitting (ħω_LT = 0.175 meV), and oscillator strength of the exciton transition (β = 0.237 × 10⁻⁴) have been calculated. The temperature dependence of the true dissipative decay has been determined.     

Fluorescence and infrared spectroscopy of electrochemically self assembled ZnO nanowires: evidence of the quantum confined Stark effect

We report room temperature fluorescence (FL) and infrared absorption (IR) spectra of spatially ordered two-dimensional arrays of vertically standing ZnO nanowires. The wires are produced by selective electrodeposition of Zn in 10-, 25- and 50-nm pores of a porous anodic alumina film, followed by chemical oxidation. Wires of different diameters show distinctly different FL emission characteristics associated with either deep level traps, or exciton recombination. The intensity of the peak caused by exciton recombination is larger than that caused by deep level traps, which is unusual in nanostructures, and attests to the high structural purity. We also see an anomalous red-shift in the FL emission spectrum which appears to be evidence of quantum confined Stark shift caused by built-in electric fields in the alumina template. The IR absorption spectra are mostly featureless and show no significant peaks indicating the absence of shallow level traps.     

Optical properties of CH3NH3PbX3 (X = halogen) and their mixed-halide crystals

Thin films of microcrystalline CH₃NH₃PbX₃ (X = halogen) as well as their mixed-halide crystals were fabricated by the spin-coating technique, and their optical properties were investigated. X-ray diffraction investigation revealed that CH₃NH₃PbBr_{3 – x} Cl _x (x = 0–3) were successfully formed on glass substrate self-assembly and oriented with the a-axis. Owing to due to their large exciton binding energy, these materials showed clear exciton absorption and free-exciton emission in the visible region at room temperature. Replacing Br with CI made it possible to control the band structure of these materials. As a result, the peak position of the exciton band shifted continuously towards blue region with increasing the CI content in the films.     

Reducing the Singlet−Triplet Energy Gap by End-Group π−π Stacking Toward High-Efficiency Organic Photovoltaics

To improve the power conversion efficiencies for organic solar cells, it is necessary to enhance light absorption and reduce energy loss simultaneously. Both the lowest singlet (S1) and triplet (T1) excited states need to energertically approach the charge-transfer state to reduce the energy loss in exciton dissociation and by triplet recombination. Meanwhile, the S1 energy needs to be decreased to broaden light absorption. Therefore, it is imperative to reduce the singlet−triplet energy gap (ΔE_ST), particularly for the narrow-bandgap materials that determine the device T1 energy. Although maximizing intramolecular push−pull effect can drastically decrease ΔE_ST, it inevitably results in weak oscillator strength and light absorption. Herein, large oscillator strength (≈3) and a moderate ΔE_ST (0.4−0.5 eV) are found for state-of-the-art A−D−A small-molecule acceptors (ITIC, IT-4F, and Y6) owing to modest push−pull effect. Importantly, end-group π−π stacking commonly in the films can substantially decrease the S1 energy by nearly 0.1 eV, but the T1 energy is hardly changed. The obtained reduction of ΔE_ST is crucial to effectively suppress triplet recombination and acquire small exciton dissociation driving force. Thus, end-group π−π stacking is an effective way to achieve both small energy loss and efficient light absorption for high-efficiency organic photovoltaics.     

Structure and fundamental optical absorption of CdSe films

The absorption spectra at the fundamental absorption edge of as-deposited and recrystallized CdSe films have been measured at low temperatures. The large number of crystal imperfections in as-deposited films leads to the appearance of band tails which give rise to a flat fundamental edge whose steepness is a function of the substrate temperature. After recrystallization of the films in selenium vapour the absorption edge becomes steeper and intrinsic exciton lines are observed. Below a characteristic temperature the line form of the A_{n = 1} exciton, which is of Lorentzian type, does not depend on temperature. This is related to the dominant mechanism of line broadening, which is due to the interaction with charged impurities. Above the characteristic temperature the A_n=1 exciton line can be fitted by a symmetrical Lorentzian with a linewidth that is determined by weak exciton-phonon coupling and intraband scattering.     

Direct Visualization of Exciton Transport in Defective Few-Layer WS2 by Ultrafast Microscopy

As defects usually limit the exciton diffusion in 2D transition metal dichalcogenides (TMDCs), the interaction knowledge of defects and exciton transport is crucial for achieving efficient TMDC-based devices. A direct visualization of defect-modulated exciton transport is developed in few-layer WS₂ by ultrafast transient absorption microscopy. Atomic-scale defects are introduced by argon plasma treatment and identified by aberration-corrected scanning transmission electron microscopy. Neutral excitons can be captured by defects to form bound excitons in 7.75–17.88 ps, which provide a nonradiative relaxation channel, leading to decreased exciton lifetime and diffusion coefficient. The exciton diffusion length of defective sample has a drastic reduction from 349.44 to 107.40 nm. These spatially and temporally resolved measurements reveal the interaction mechanism between defects and exciton transport dynamics in 2D TMDCs, giving a guideline for designing high-performance TMDC-based devices.     

High-Performance Fluorinated Fused-Ring Electron Acceptor with 3D Stacking and Exciton/Charge Transport

A new fluorinated electron acceptor (FINIC) based on 6,6,12,12-tetrakis(3-fluoro-4-hexylphenyl)-indacenobis(dithieno[3,2-b;2′,3′-d]thiophene) as the electron-donating central core and 5,6-difluoro-3-(1,1-dicyanomethylene)-1-indanone as the electron-deficient end groups is rationally designed and synthesized. FINIC shows similar absorption profile in dilute solution to the nonfluorinated analogue INIC. However, compared with INIC, FINIC film shows red-shifted absorption, down-shifted frontier molecular orbital energy levels, enhanced crystallinity, and more ordered molecular packing. Single-crystal structure data show that FINIC molecules pack into closer 3D “network” motif through H-bonding and π–π interaction, while INIC molecules pack into incompact “honeycomb” motif through only π–π stacking. Theoretical calculations reveal that FINIC has stronger electronic coupling and more molecular interactions than INIC. FINIC has higher electron mobilities in both horizontal and vertical directions than INIC. Moreover, FINIC and INIC support efficient 3D exciton transport. PBD-SF/FINIC blend has a larger driving force for exciton splitting, more efficient charge transfer and photoinduced charge generation. Finally, the organic solar cells based on PBD-SF/FINIC blend yield power conversion efficiency of 14.0%, far exceeding that of the PBD-SF/INIC-based devices (5.1%).     

Optical properties of lead iodide between 0.4946 and 6.185 eV

The optical and dielectric constants of PbI₂ thin films have been determined from transmittance and reflectance measurements, for photon energies between 0.4946 and 6.185 eV. The absorption coefficient, bandgap energy, and dielectric constants were determined at room temperature by the normal incidence method. The first three lines of the hydrogenic exciton series associated with the absorption edge are well resolved in reflectivity measurements. The transmittance measurements enable the evaluation of the value of bandgap energy E_g. The bandgap energy of PbI₂ at room temperature was found to be 2.55 eV. A careful analysis of the absorption coefficients indicated the crystalline character of the sample studied; a similar diagnosis was obtained from X-ray evidence. SEM analysis revealed that as the thickness of the films increases the material becomes amorphous.     

Preparation and optical properties of nanocrystalline (C6H5C2H4NH3)2PbI4-doped PMMA films

Nanocrystalline (RNH3)2PbI4-doped PMMA films were successfully fabricated on glass substrates by the spin-coating technique and subsequent annealing. X-ray diffraction spectra revealed that the (RNH3)2PbI4 crystals dispersed as nanometre-sized crystals in the PMMA matrix. These films showed a strong exciton absorption band with narrow bandwidth, even at room temperature. The exciton absorption observed here could be attributed to the 6s to 6p transition of Pb2+. The stability of the (RNH3)2PbI4 crystal was improved by doping these crystals into a PMMA matrix. © 1998 Chapman & Hall     

New features of the Mott-Hubbard insulator gap of parent HTS compounds found by resonance Raman scattering and UV-excited ordinary Raman scattering

Optical absorption at the insulating gap in the parent phase of cuprate superconductors shows a broad exciton-like peak near 1.7 eV, followed by a gradual decrease in absorption persisting 1 eV above the gap. By using ultraviolet laser lines to excite Raman spectra, we have found a Raman peak 0.2 eV below the first absorption peak in insulating cuprates. The Raman peak is much narrower than the absorption peak and hasA _2g symmetry. We assign it to an exciton consisting of a hole transition from Cu to a linear combination of Cud _xy and nearest neighbor Op orbitals. We have also studied the resonance Raman profile for two-magnon Raman scattering in the same samples. We find a sharp resonance feature at about 2.7 eV, and little Raman intensity for photon energies at the 1.7 eV absorption peak. The state created at the peak must therefore be an inappropriate intermediate state for the double spin-flip Raman process.     

Plasmonic Ag@Oxide Nanoprisms for Enhanced Performance of Organic Solar Cells

Localized surface plasmon resonance (LSPR), light scattering, and lowering the series resistance of noble metal nanoparticles (NPs) provide positive effect on the performance of photovoltaic device. However, the exciton recombination on the noble metal NPs accompanying above influences will deteriorate the performance of device. In this report, surface‐modified Ag@oxide (TiO₂ or SiO₂) nanoprisms with 1–2 nm shell thickness are developed. The thin film composed of P3HT/Ag@oxides and P3HT:PCBM/Ag@oxides is investigated by absorption, photoluminescence (PL), and transient absorption spectroscopy. The results show a significant absorption, PL enhancement, and long‐lived photogenerated polaron in the P3HT/Ag@TiO₂ film, indicating the increase of photogenerated exciton population by LSPR of Ag nanoprisms. In the case of P3HT/Ag nanoprisms, partial PL quench and relatively short‐lived photogenerated polaron are observed. That indicates that the oxides layer can effectively avoid the exciton recombination. When the Ag@oxide nanoprisms are introduced into the active layer of P3HT:PCBM photovoltaic devices, about 31% of power conversion efficiency enhancement is obtained relative to the reference cell. All these results indicate that Ag@oxides can enhance the performance of the cell, at the same time the ultrathin oxide shell prevents from the exciton recombination.     

Absorption and photocurrent properties of thin ZnS films formed by pulsed-laser deposition on quartz

The absorption and photocurrent properties of thin film ZnS on quartz glass formed by pulsed-laser deposition have been studied experimentally and theoretically at room temperature. Using the Lorentzian function to describe the exciton density of states, we show that the absorption is strongly influenced by excitonic formation. The theory for the absorption, however, does not describe the PC spectra of the film since the exciton remains electrically neutral up to fields of 1 kV/cm due to the high binding energy of 36 meV. Therefore, the fundamental absorption according to density of states and Urbach rule determines the shape of the photocurrent spectrum.     

Magneto-optical studies of perylene tetracarboxylic acid diimide thin films

N,N′-Dimethyl perylene tetracarboxylic acid diimide (Me-PTCDI) thin films were prepared by vapor deposition technique. The photogeneration and recombination of singlet and triplet excitons were characterized, utilizing absorption, PL and optically detected magnetic resonance (ODMR) spectroscopy. The results suggest the creation of molecular dimers, which gives rise to the creation of split singlet exciton states and enhancement of population in the triplet exciton state.     

Magneto-Optical Study of Diluted Magnetic Semiconductor Nanostructures Prepared by Pulsed Laser Deposition

The CdTe/Cd_1–x Mn _x Te quantum well structures have been grown by pulsed laser deposition. X-ray diffraction analysis demonstrates high structural quality of the deposited layers and nanostructures. In the magnetoabsorption spectra the large Zeeman splitting of exciton peak which is corresponded to the heavy-hole exciton transition in the CdTe quantum well layers is revealed. The Faraday rotation spectra of the superlattices is interpreted in framework of the exciton transition for the Cd_1–x Mn _x Te layers at a higher photon energy. Influence of different factors on behavior of the Zeeman splitting and the Faraday rotation in the studied nanostructures is discussed.     

Multimagnon absorption in the optical spectrum of antiferromagnetic RbMnF3

The spectrum of optical electrodipolar absorption of antiferromagnetic RbMnF₃ in the region of the ⁶ A _1g ( ⁶ S) ⁴ E _g , ⁴ A _1g ( ⁴ G) transition has been studied experimentally in strong magnetic fields (up to300 kOe) and at the external uniaxial pressure. Employing the parameters of earlier studies of the single-particle pure exciton transition ⁶ A _1g ( ⁶ S) ⁴ E _g ( ⁴ G), the dipolar moments of the transition and the absorption maxima of its two- and three-magnon sidebands have been calculated. Also, the exciton dispersion for translationally equivalent ions and exciton-magnon interactions have been estimated. It has been shown that the whole main part (200 cm ^–1 ) of the absorption bands under consideration is likely to be formed by single-, two-, and three-magnon sidebands of this transition.     

Photovoltaic contribution of photo-generated excitons in acceptor material of organic solar cells

Contribution of exciton generation in acceptor material to the photovoltaic performance of three bulk-heterojunction organic solar cells (BHJ OSCs), PTB7:PC₇₁BM, P3HT:ICBA and P3HT:PC₆₁BM are studied. Singlet and triplet rates of absorption and dissociation and diffusion lengths are calculated and compared with those when excitons are generated in the donor of these BHJ OSCs. It is found that the rates of absorption and dissociation and diffusion lengths are comparable whether excitons are generated in donor or acceptor of these BHJ OSCs. Therefore, it is proposed that the contribution of exciton generation in acceptor may not be negligible.     

Substrate-free growth, characterization and growth mechanism of ZnO nanorod close-packed arrays

ZnO nanorod close-packed arrays are successfully fabricated in a substrate-free manner by a citric acid assisted annealing process at a low growth temperature of 400?°C. Each nanorod of ZnO nanorod close-packed arrays grows along the [0001] direction and is single crystalline with an average diameter of 50?nm, and an average length of 0.5?μm. The aspect ratio is 10. The ZnO nanorod close-packed arrays show a strong exciton absorption peak at 372?nm in UV-visible absorption spectra, exhibiting a blue-shift relative to the bulk exciton absorption (387?nm). Finally, a new growth mechanism is proposed for the substrate-free preparation of ZnO nanorod close-packed arrays by a citric acid assisted annealing process.     

Effects of Digital Doping of Mn on Giant Magneto-Optical Properties of Zn1?xMnxSe Quantum Wells

We have studied effects of the digital-doping profile of MnSe layers on the giant magneto-optical properties in Zn_1–x Mn _x Se-based quantum wells. The giant Zeeman shift energy increases monotonically with increasing spatial overlap of the exciton wavefunction with the 0.5 monatomic-thick effective Mn layers at the interfaces between the digitally doped MnSe layers and nonmagnetic ZnSe layers. Also, a field-induced enhancement factor of the excitonic photoluminescence intensity, because of the suppression of the exciton energy transfer into the d-d transition of Mn-ions, increases linearly with increasing such overlap of the exciton wavefunction with the effective Mn layer. In addition, the formation energy as large as 18.6 meV and the formation time of the magnetic polaron are determined, which are also affected by the digital-doping profile.     

Optical and photoelectric properties of barium-intercalated InSe and GaSe

Barium intercalation into InSe and GaSe layered crystals has been demonstrated by electron probe X-ray microanalysis. The transmission and photoconductivity spectra of Ba _x InSe (0 < x ≤ 1) show an additional peak and a shift of the intrinsic edge. The energy position of the main exciton maximum and the width of the lowest exciton band in Ba _x GaSe are found to be nonmonotonic functions of Ba content.