A population of luminous accreting black holes with hidden mergers

Major galaxy mergers are thought to play an important part in fuelling the growth of supermassive black holes. However, observational support for this hypothesis is mixed, with some studies showing a correlation between merging galaxies and luminous quasars and others showing no such association. Recent observations have shown that a black hole is likely to become heavily obscured behind merger-driven gas and dust, even in the early stages of the merger, when the galaxies are well separated (5 to 40 kiloparsecs). Merger simulations further suggest that such obscuration and black-hole accretion peaks in the final merger stage, when the two galactic nuclei are closely separated (less than 3 kiloparsecs). Resolving this final stage requires a combination of high-spatial-resolution infrared imaging and high-sensitivity hard-X-ray observations to detect highly obscured sources. However, large numbers of obscured luminous accreting supermassive black holes have been recently detected nearby (distances below 250 megaparsecs) in X-ray observations. Here we report high-resolution infrared observations of hard-X-ray-selected black holes and the discovery of obscured nuclear mergers, the parent populations of supermassive-black-hole mergers. We find that obscured luminous black holes (bolometric luminosity higher than 2x10^44 ergs per second) show a significant (P<0.001) excess of late-stage nuclear mergers (17.6 per cent) compared to a sample of inactive galaxies with matching stellar masses and star formation rates (1.1 per cent), in agreement with theoretical predictions. Using hydrodynamic simulations, we confirm that the excess of nuclear mergers is indeed strongest for gas-rich major-merger hosts of obscured luminous black holes in this final stage.Comment: To appear in the 8 November 2018 issue of Nature. This is the authors' version of the wor

Near-infrared absorbing cyanine dyes and organic-inorganic perovskites for electronic applications

Light-absorbing materials with excellent semiconducting properties are of great importance for the application in solar cells, transistors or light-emitting devices, which are emerging technologies. This thesis encompasses the synthesis and analysis of new materials for this purpose. Heptamethine cyanine dyes are strong absorbers of light in the near-infrared (NIR) energy regime with synthetically tunable absorption and redox properties, making them excellent candidates as light harvesters in organic solar cells. In particular, the absorption of sunlight invisible to the human eye allows for the fabrication of visibly transparent solar cells. In here, the exchange of counterions from cyanine iodide salts to PF6 – and $\Delta$– TRISPHAT– is presented and the effect of the counterion on the material properties of the dye is discussed. While the counterion had little to no influence on the optical and electrochemical properties in solution, a tremendous effect on the properties in the solid state was found. Thin films of the dyes with varying counterions showed differently shaped absorption bands indicating alterations in the dye aggregation behavior. Tendencies in the solid state packing of the salts with different counterions are further highlighted by X–ray crystal structures. Also the formation of bulk heterojunction blend films with [60]PCBM was strongly affected by the counterion. With the PF6 – counterion large fully phase-separated domains were obtained, whereas the $\Delta$–TRISPHAT– counterion gave a fully intermixed dye–fullerene phase. These findings will stimulate the further development of cyanine bulk heterojunction solar cells. When the heptamethine dyes were applied in semitransparent bilayer organic solar cells together with C60, a power conversion efficiency of 2.2% was achieved while maintaining a high average visible transparency of 66%. Organic–inorganic hybrid perovskites are an uprising class of compounds which have recently attracted tremendous attention due to the rapid increase in power conversion efficiency of perovskite solar cells (PSCs), reaching more than 20% in 2016. This work aimed at the enhancement of photon–to–current generation of PSCs into the NIR energy regime by utilizing heptamethine dyes as co–sensitizers. For this purpose, new heptamethine dyes with electron-donating substituents were synthesized and characterized in order to tune their redox levels to be compatible with the perovskite material. While the cyanine dyes were able to perform reasonably well as hole-transporting materials – supporting high short–circuit currents of up 15 mA cm–2, co– sensitization beyond 800 nm was not successful. Further synthetic adaptations of the dyes and improvements of the perovskite/cyanine interface might be necessary. Further investigations on organic-inorganic hybrid materials addressed the incorporation of large organic cations into low–dimensional inorganic networks. Such systems allow for the combination of desirable properties from both components, such as the structural and functional versatility of organic compounds with the stability and electrical properties of inorganic materials. Two–dimensional lead halide perovskite materials with the general formula (R-NH3)2PbX4 contain layers of PbX4 sheets alternating with layers of organic cations R-NH3 and have interesting properties such as room– temperature photo– and electroluminescence as well as high charge carrier mobilities, making them promising candidates for the application in devices such as light emitting diodes or field–effect transistors (FET). In this thesis, a method to synthesize 2D– perovskite films consisting of highly ordered crystallites is presented. In XRD spectra of the thin films very intense (00ℓ) reflexes were found from ℓ=2 up to ℓ=20, suggesting that the alternating layers of inorganic sheets and organic cations are perfectly arranged in parallel to the substrate plane. This structure is promising especially for the application in FET devices as the inorganic sheets are aligned in the direction of charge transport from source to drain electrode. Finally, an original hybrid material is presented, which incorporates NIR– heptamethine cations in an inorganic network consisting of infinite chains of face–sharing lead iodide octahedra. The structure of this hybrid was solved using X–ray crystal structure analysis and structural aspects are discussed. This “cyanine perovskite” represents the first one–dimensional lead halide perovskite incorporating a functional NIR–absorbing dye as the organic cation, which is predicted to lead to unusual optical and electrical properties through the synergistic interaction between the components

One-Dimensional Organic–Inorganic Hybrid Perovskite Incorporating Near-Infrared-Absorbing Cyanine Cations

Hybrid perovskite crystals with organic and inorganic structural components are able to combine desirable properties from both classes of materials. Electronic interactions between the anionic inorganic framework and functional organic cations (such as chromophores or semiconductors) can give rise to unusual photophysical properties. Cyanine dyes are a well known class of cationic organic dyes with high extinction coefficients and tunable absorption maxima all over the visible and near-infrared spectrum. Here we present the synthesis and characterization of an original 1D hybrid perovskite composed of NIR-absorbing cyanine cations and polyanionic lead halide chains. This first demonstration of a cyanine-perovskite hybrid material is paving the way to a new class of compounds with great potential for applications in photonic devices

A transparent, solvent-free laminated top electrode for perovskite solar cells

A simple lamination process of the top electrode for perovskite solar cells is demonstrated. The laminate electrode consists of a transparent and conductive plastic/metal mesh substrate, coated with an adhesive mixture of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate), PEDOT:PSS, and sorbitol. The laminate electrode showed a high degree of transparency of 85%. Best cell performance was achieved for laminate electrodes prepared with a sorbitol concentration of ~30 wt% per milliliter PEDOT:PSS dispersion, and using a pre-annealing temperature of 120°C for 10 min before lamination. Thereby, perovskite solar cells with stabilized power conversion efficiencies of (7.6 ± 1.0)% were obtained which corresponds to 80% of the reference devices with reflective opaque gold electrodes

Conformational flexibility of palladium BINAP complexes explored by X-ray analyses and DFT studies

Several crystal structures and a theoretical DFT structure of the important catalyst (BINAP)PdCl2 (BINAP: 2,2'-bis(diphenylphosphino)-1,1'-binaphthyl) have been determined. The conformational flexibilities of the BINAP backbone and of the phenyl rings do not seem to be coupled. Two novel parameter have been introduced that define the Pi-Pi stacking between the phenyl and biaryl rings in systems similar to the BINAP ligand, as well as the delta angle that is sensitive to the important interaction of the exchangeable ligands of the palladium with the equatorial phenyl rings of the BINAP. Furthermore, the calculated bite angle is 3 degrees larger than the experimentally determined bite angles. (C) 2012 Elsevier Ltd. All rights reserved

NIR-absorbing Heptamethine dyes with tailor-made counterions for application in light to energy conversion

A method to exchange the counterion of cyanine dyes to Δ-TRISPHAT– and PF6– is presented. The influence of these counterions on the photophysical and electrochemical properties of the cyanine dye in solution is discussed, and tendencies in the solid packing are highlighted by X-ray crystal structures. The compounds were applied in semitransparent bilayer organic solar cells together with C60, and a power conversion efficiency of 2.2% was achieved while maintaining a high transparency level in the visible region of 66%

Influence of molybdenum oxide interface solvent sensitivity on charge trapping in bilayer cyanine solar cells

Bilayer organic solar cells based on trimethine cyanine donor and C60 acceptor materials have been fabricated by coating the trimethine dye from solution on molybdenum oxide (MoO3) anode buffer layer. The choice of deposition solvent drastically influences device performance, with 2,2,3,3-tetrafluoro-1-propanol (TFP) reducing the fill factor and power conversion efficiency of the device by 36 and 21%, respectively, as compared to chlorobenzene. In the case of TFP, extraction of photogenerated charge carriers by linearly increasing voltage (photo-CELIV) and capacitance-voltage analysis revealed the formation of a hole trapping zone at the molybdenum oxide interface which is also responsible for the S-shape current-voltage curve under white light irradiation. The transient charge extraction signal originating from trapped holes at the MoO3 interface could be clearly distinguished from the one relating to hole mobility in cyanine films using photo-CELIV measurements with varying delay times