Reduced coupling of water molecules near the surface of reverse micelles

We report on vibrational dynamics of water near the surface of AOT reverse micelles studied by narrow-band excitation, mid-IR pump–probe spectroscopy. Evidence of OH-stretch frequency splitting into the symmetric and asymmetric modes is clearly observed for the interfacial H2O molecules. The polarization memory of interfacial waters is preserved over an exceptionally extended >10 ps timescale which is a factor of 100 longer than in bulk water. These observations point towards negligibly small intermolecular vibrational coupling between the water molecules as well as strongly reduced water rotational mobility within the interfacial water layer.

Investigation of the Ga-rich GaAs(001) Surface Reconstructions Stability and Interaction with Halogens

Atomic and electronic structures for a number of GaAs(001) surface geometries were studied within the density functional theory in order to re-examine the energy stability of surface reconstructions in the Ga-rich limit. It was shown that among geometries with (42) symmetry so-called ζ-model is most stable but the energetically favored Ga-rich (24) reconstructions are stabilized by dimerized Ga and As atoms. Our calculations predict the coexistence of (24) and (44) reconstructions on GaAs(001) in the Ga-rich limit. Comparative study of the halogens (F, Cl, I) adsorption on the -GaAs(001)-(42) surface were performed. The energetically preferable positions for all considered halogens are found on-top sites above dimerized and nondimerized Ga atoms. The electronic properties of the semiconductor surface and its change upon halogen adsorption are discussed. It was shown that the interaction of halogen with the Ga dimerized at-oms leads to the weakening of the chemical bonds between surface atoms that determines the initial stage of surface etching. When you are citing the document, use the following link http://essuir.sumdu.edu.ua/handle/123456789/3531

Simultaneous enhancement in open circuit voltage and short circuit current of hybrid organic-inorganic photovoltaics by inorganic interfacial modification

Here, we investigate the model poly(3-hexathiophene)/ZnO system and show that by introducing a caesium carbonate interlayer, a simultaneous increase in all photovoltaic parameters can be achieved.We kindly acknowledge Prof. Uwe Bunz and Prof. Annemarie Pucci for access to device fabrication facilities and AFM measurements, respectively. P.E.H. thanks the Excellence Initiative for Funding. A.A.B. is a Royal Society University Research Fellow.This is the author accepted manuscript. The final version is available from The Royal Society of Chemistry via http://dx.doi.org/10.1039/C5TC03206

Ultrafast Spectroscopy with Photocurrent Detection: Watching Excitonic Optoelectronic Systems at Work.

While ultrafast spectroscopy with photocurrent detection was almost unknown before 2012, in the last 3 years, a number of research groups from different fields have independently developed ultrafast electric probe approaches and reported promising pilot studies. Here, we discuss these recent advances and provide our perspective on how photocurrent detection successfully overcomes many limitations of all-optical methods, which makes it a technique of choice when device photophysics is concerned. We also highlight compelling existing problems and research questions and suggest ways for further development, outlining the potential breakthroughs to be expected in the near future using photocurrent ultrafast optical probes.A.A.B. is currently a Royal Society University Research Fellow. A.A.B. also acknowledges a VENI grant from the NWO. This project has received funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (Grant Agreement No. 639750). C.S. acknowledges funding from the Natural Science and Engineering Research Council of Canada, the Fonds de recherche du Québec–nature et technologies, the Canada Research Chair in Organic Semiconductor Materials, and the Université de Montréal Research Chair. C.S. and E.V. acknowledge collaboration with Sachetan Tuladhar, Michelle Vezie, Sheridan Few, Jenny Nelson, Hao Li, and Eric Bittner. Finally, C.S. and E.V. acknowledge essential discussions with Andy Marcus and Julia Widom for the implementation of the two-dimensional spectroscopy apparatus.This is the author accepted manuscript. The final version is available from the American Chemical Society via http://dx.doi.org/10.1021/acs.jpclett.5b0195

Influence of Fluorine and Oxygen Adsorption on the Electronic Properties of the InAs(111)A-(2*2) Surface

Atomic and electronic structures of the reconstructed InAs(111)A-(2*2) surface are studied within den-sity functional theory. The most stable adsorption positions of oxygen and fluorine on the surface are de-termined. Our calculations show that oxygen adsorption leads to appearance of electronic states in band gap and the structure of these surface states strongly depends on adsorption geometry. An increase of oxy-gen concentration and its coadsorption with fluorine leads to substantial structural changes in the surface and subsurface layers due to adsorbate penetration into semiconductor. Evolution of the electronic struc-ture upon oxygen and fluorine adsorption on the reconstructed InAs(111)A-(2*2) surface in dependence on adsorbate geometry is analyzed. It is shown that surface states induced by oxygen adsorption can be par-tially removed by fluorine, when it forms bonds with the surface indium atoms. The formation of the indi-um–fluorine or indium–oxygen bonds proceeds due to charge transfer from arsenic atoms to indium ones in the surface layers. The microscopic mechanism of fluorine influence on the interface states at InAs–oxides interface is suggested. When you are citing the document, use the following link http://essuir.sumdu.edu.ua/handle/123456789/3529

Influence of Interstitial Impurities (H, B, C) on Grain Boundary Cohesion in B2 Ti-based Alloys

The investigation of hydrogen, boron and carbon sorption properties at the Σ5(310) symmetrical tilt grain boundary (GB) and (310) free surface (FS) in B2 Ti-based alloys was carried out by the plane-wave pseudopotential method within density functional theory. The most preferential positions for interstitial impurities at GB were determined. It was shown that impurities sorption energies at GB depend strongly on their local environment. The analysis of electronic properties allows us to establish the microscopic na-ture of chemical bonding of all considered impurities at GB. It was shown that H decreases more signifi-cantly the surface energies than the GB energy in contrast to B and C. This results in decreasing the Grif-fith work that indicates also the decrease of the strength of grain boundary. The segregation of H at the GB makes intergranular fracture much easier because the bonding between metal atoms, which are neigh-bors of H, is weakened. The segregation behavior of hydrogen confirms it as an embrittler for B2 Ti-based alloys. At the same time boron and carbon segregation contrast to hydrogen increase the GB cohesion. When you are citing the document, use the following link http://essuir.sumdu.edu.ua/handle/123456789/3551

Improving charge separation across a hybrid oxide/polymer interface by Cs doping of the metal oxide

The process of photoinduced charge carrier separation in hybrid optoelectronics remains only partially understood, with the mechanism behind creation and dissociation of bound charge pairs (BCPs) being open questions. To investigate these processes, we employ the model hybrid ZnO/P3HT system and show that Cs doping of ZnO results in a decrease in the density of gap states at the metal oxide surface and in turn, a reduction in the yield of BCPs. This provides direct experimental evidence for a previously proposed model of BCP creation by electron trapping at the metal oxide surface states. Furthermore, Cs doping is found to substantially increase open circuit voltage in these devices without the negative effects on short circuit current that were observed in studies with other dopants. This offers new possibilities for hybrid photovoltaic devices with increased power conversion efficiencies and provides valuable insights on the charge separation processes in hybrid organic-inorganic photovoltaics.We kindly thank Prof. Annemarie Pucci and Prof. Uwe Bunz for providing access to the AFM and the device fabrication facilities, respectively. A. A. B. is a Royal Society University Research Fellow. P. E. H. and Y. V. thank the Excellence Initiative for funding.This is the author accepted manuscript. The final version is available from Wiley via http://dx.doi.org/10.1002/admi.20150061

Morphology, Temperature, and Field Dependence of Charge Separation in High-Efficiency Solar Cells Based on Alternating Polyquinoxaline Copolymer

Charge separation and recombination are key processes determining the performance of organic optoelectronic devices. Here we combine photoluminescence and photovoltaic characterisation of organic solar cell devices with ultrafast multi-pulse photocurrent spectroscopy to investigate charge generation mechanisms in the organic photovoltaic devices based on a blend of an alternating polyquinoxaline copolymer with fullerene. The combined use of these techniques enables the determination of the contributions of geminate and bimolecular processes to the solar cell performance. We observe that charge separation is not a temperature-activated process in the studied materials. At the same time, the generation of free charges shows a clear external-field and morphology dependence. This indicates that the critical step of charge separation involves the non-equilibrium state that is formed at early times after photoexcitation, when the polaronic localisation is not yet complete. This work reveals new aspects of molecular level charge dynamics in the organic light-conversion systems.We thank Maxim Pschenichnikov for useful discussions, and Ergang Wang for providing TQ1. This work was supported by the Netherlands Organization for Scientific Research (NWO) through the “Stichting voor Fundamenteel Onderzoek der Materie” (FOM). A.A.B. also acknowledges a VENI grant from NWO. A.A.B. is currently a Royal Society University Research Fellow. Photovoltaics research at Linköping was supported by the Swedish Research Council (VR), the European Commission Marie Skłodowska-Curie actions, the Swedish Energy Agency, and the Knut and Alice Wallenberg foundation (KAW).This is the author accepted manuscript. The final version is available from the American Chemical Society via http://dx.doi.org/10.1021/acs.jpcc.5b1080

Real-Time Observation of Multiexcitonic States in Ultrafast Singlet Fission Using Coherent 2D Electronic Spectroscopy

Singlet fission is the spin-allowed conversion of a spin-singlet exciton into a pair of spin-triplet excitons residing on neighbouring molecules. To rationalize this phenomenon, a multiexcitonic spin-zero triplet-pair state has been hypothesized as an intermediate in singlet fission. However, the nature of the intermediate states and the underlying mechanism of ultrafast fission have not been elucidated experimentally. Here, we study a series of pentacene derivatives using ultrafast two-dimensional electronic spectroscopy and unravel the origin of the states involved in fission. Our data reveal the crucial role of vibrational degrees of freedom coupled to electronic excitations that facilitate the mixing of multiexcitonic states with singlet excitons. The resulting manifold of vibronic states drives sub-100 fs fission with unity efficiency. Our results provide a framework for understanding singlet fission and show how the formation of vibronic manifolds with a high density of states facilitates fast and efficient electronic processes in molecular systems.This is the author accepted manuscript. The final version is available from NPG via http://dx.doi.org/10.1038/nchem.237