A comparative Raman study of the interaction of electron donor and acceptor molecules with graphene prepared by different methods

Interaction of tetrathiafulvalene (TTF) and tetracyanoethylene (TCNE) with few-layer graphene samples prepared by the exfoliation of graphite oxide (EG), conversion of nanodiamond (DG) and arc-evaporation of graphite in hydrogen (HG) has been investigated by Raman spectroscopy to understand the role of the graphene surface. The position and full-width at half maximum of the Raman G-band are affected on interaction with TTF and TCNE and the effect is highest with EG and least with HG. The effect of TTF and TCNE on the 2D-band is also maximum with EG. The magnitude of interaction between the donor/acceptor molecules varies in the same order as the surface areas of the graphenes. (C) 2009 Published by Elsevier B. V.

Selectivity in the Interaction of Electron Donor and Acceptor Molecules with Graphene and Single-Walled Carbon Nanotubes

Interaction of electron donor and acceptor molecules with graphene samples prepared by different methods as well as with single-walled carbon nanotubes (SWNTs) has been investigated by isothermal titration calorimetry (ITC). The ITC interaction energies of the graphene samples and SWNTs with electron acceptor molecules are higher than those with electron donor molecules. Thus, tetracyanoethylene (TCNE) shows the highest interaction energy with both graphene and SWNTs. The interaction energy with acceptor molecules varies with the electron affinity as well as with the charge-transfer transition energy for different aromatics. Metallic SWNTs interact reversibly with electron acceptor molecules, resulting in the opening of a gap.

A Raman study of the interaction of electron-donor and -acceptor molecules with chemically doped graphene

Effect of interaction of tetracyanoethylene (TCNE) and tetrathia fulvalene (TTF) with boron- and nitrogen-doped graphene has been investigated by Raman spectroscopy. The G- and 2D bands of boron- and nitrogen-doped graphenes in the Raman spectra show significantly different changes on interaction with electron-donor and -acceptor molecules. Thus, tetracyanoethylene (TCNE) and tetrathiafulvalene (TTF) have different effects on the Raman spectra of boron- and nitrogen-doped graphenes. The changes in the Raman spectra brought about by electron-donor and -acceptor molecules can be understood in general terms on the basis of molecular charge transfer. (c) 2012 Elsevier B.V. All rights reserved.

Interaction of CdSe and ZnO nanocrystals with electron-donor and -acceptor molecules

Interaction of CdSe and ZnO nanocrystals with electron-donating tetrathiafulvalene (TTF) and electron-withdrawing tetracyanoethylene (TCNE) has been investigated. Isothermal calorimetry shows CdSe nanocrystals interact more strongly with TCNE than TTF. Interaction of larger CdSe nanocrystals with TCNE causes a red-shift in the band-edge emission because of agglomeration, while the smaller CdSe nanocrystals, exhibiting stronger interaction with TCNE modify the optical gap of the nanocrystal. Luminescence of CdSe gets quenched sharply after addition of both TTF and TCNE. ZnO nanocrystals also exhibit luminescence quenching to lesser extent. Defect-emission of ZnO nanocrystals gets red or blue-shifted after interaction with TTF or TCNE respectively. (C) 2012 Elsevier B. V. All rights reserved.

Dimer formation by symbiotic donor–acceptor interaction between two molecules of a specially designed dioxomolybdenum(VI) complex containing both donor and acceptor centers – A structural, spectroscopic and DFT study

This work presents a model study for the formation of a dimeric dioxomolybdenum(VI) complex [MoO2L]2, generated by simultaneous satisfaction of acceptor and donor character existing in the corresponding monomeric Mo(VI) complex MoO2L. This mononuclear complex is specially designed to contain a coordinatively unsaturated Mo(VI) acceptor centre and a free donor group, (e.g. –NH2 group) strategically placed in the ligand skeleton [H2L = 2-hydroxyacetophenonehydrazone of 2-aminobenzoylhydrazine]. Apart from the dimer [MoO2L]2, complexes of the type MoO2L·B (where B = CH3OH, γ-picoline and imidazole) are also reported. All the complexes are characterized by elemental analysis, spectroscopic (UV–Vis, IR, 1H NMR) techniques and cyclic voltammetry. Single crystal X-ray structures of [MoO2L]2 (1), MoO2L·CH3OH (2), and MoO2L.(γ-pic) (3) have been determined and discussed. DFT calculation on these complexes corroborates experimental data and provides clue for the facile formation of this type of dimer not reported previously. The process of dimer formation may also be viewed as an interaction between two molecules of a specially designed complex acting as a monodentate ligand. This work is expected to open up a new field of design and synthesis of dimeric complexes through the process of symbiotic donor–acceptor (acid–base) interaction between two molecules of a specially designed monomer.

Thermal diffusivity analysis of doped poly(methyl-methacrylate) with electron donor and acceptor molecules

A computer-assisted method for analysing photoacoustic spectra has been developed in the Windows(TM) environment with the use of an easy graphical interface, the computer simulation was carried out with the aim of using the entire expression of the Rosencwaig-Gersho theory, thus permitting multiple applications. The simulation was applied to a system that mimics the electron transfer process in which the concentration of octaethylporphin donor molecules was constant whereas the concentration of duroquinone and 2,3-dichloro-5,6-dicyano-1, l-benzoquinone acceptor molecules varied. The increment of the acceptor concentration influenced the photoacoustic amplitude and phase signals. In the phase signal a significant shift to smaller values was observed, denoting a faster heat generation. The analysis of the photoacoustic signal enabled the determination of the thermal diffusivity, the result obtained through the simulation was about (7 +/- 1) x 10(-7) m(2) s(-1) indicating that changes in the photoacoustic phase signals were due to the electron transfer process rather than changes in the thermal properties of the sample.

Extended donor and acceptor molecules for organic electronics

Die Untersuchung von halbleitenden Materialien auf der Basis von organischen Molekülen stellt ein Gebiet der angewandten Forschung an der Schwelle zur industriellen Nutzung dar. Geringes Gewicht und hohe mechanische Flexibilität ermöglichen völlig neue Produkte, die mit anorganischen Halbleitern nicht zu realisieren sind. Die Herstellung von Bauteilen wie Transistoren, Solarzellen oder Leuchtdioden aus organischen Materialien ist ein komplexes Gebiet, das einer Vielzahl von unterschiedlichen Optimierungen bedarf, um eine konkurrenzfähige Leistung zu erreichen. Die synthetische organische Chemie bietet vielfältige Möglichkeiten, mit maßgeschneiderten Lösungen zum Optimierungsprozess beizutragen. Zum einen können neue aktive Materialien hergestellt werden mit besserer Leistung und leichterer Verarbeitbarkeit. Zum anderen sind Substanzen zugänglich, die z.B. bei der Ladungsträgerinjektion hilfreich sein können.rnIn dieser Arbeit wurde an beiden dieser Fronten gearbeitet. Dabei lag die Entwicklungsstrategie darin, ausgedehnte π-konjugierte Moleküle herzustellen, die entweder besonders elektronenarme Akzeptoren oder elektronenreiche Donoren darstellen. Die genaue Kontrolle der elektronischen Niveaus stellt einen wichtigen Bestandteil dar, um niedrige elektrische Kontaktbarrieren zu Metallen zu erreichen und ausreichend stabile Materialien zu erreichen.rnDer erste Fokus der Arbeiten lag in der Funktionalisierung von Coronen. Dieser PAH stellt einen guten Kompromiss bezüglich seiner Größe dar: Er ist groß genug, um Diffusion in andere Schichten von Bauteilen zu vermeiden, aber nicht zu groß, um Verarbeitung durch Vakuumsublimation zu ermöglichen. Bislang sind praktisch keine Coronen-Derivate in der Literatur beschrieben, weshalb eine neue Synthese entwickelt werden musste, die die Einführung starker Donor- und Akzeptorfunktionalitäten erlaubt. Die photochemische Cyclodehydrierung von substituierten [2.2.2]paracyclophan-trienen stellte sich als hervorragende Möglichkeit heraus, dies zu bewerkstelligen. Es wurde eine Reihe von methoxy-substitutierten Coronenen mit unterschiedlicher Symmetrie hergestellt. Mittels optischer Spektroskopie konnte gezeigt werden, dass Methoxygruppen wenig Einfluss auf die elektronischen Eigenschaften von Coronen haben. Unter Spaltung der Methylether und anschließender Oxidation allerdings sind Coronenketone zugänglich, welche bis zu drei α-Diketongruppen besitzen. Diese Moleküle sind enorm starke Akzeptoren, was durch Cyclovoltammetrie und Vergleich zu anderen Akzeptoren eindrucksvoll gezeigt werden konnte. Die Sublimation dieses Akzeptors auf die Oberfläche von Metallen zeigt einen dramatischen Einfluss auf die Austrittsarbeit dieses Metalls, was zur Herstellung eines ohmschen Kontakts zu organischen Halbleitern von außerordentlichem Nutzen ist. rnDen zweiten Teil der Arbeit bilden Benzodithiophen enthaltende Polymere, die für den Einsatz als aktive Komponente in elektronischen Bauteilen entwickelt wurden. Nach systematischer Strukturoptimierung wurde ein Polymer enthalten, welches in einem Feldeffekt-Transistor auf Standard-Silizium-Substraten Ladungsträger-Mobilitäten über 0,1 cm2/Vs erreicht mit großer Reproduzierbarkeit und ausgezeichneter Transistor-Charakteristik. Es konnte gezeigt werden, dass die durch die Monomergeometrie erzeugte Kurvung des Polymers zu einem optimalen Kompromiss aus Löslichkeit und effektiver Packung darstellt. Auf für industrielle Anwendungen besonders interessanten polymer-basierten Substraten wurde eine noch erheblich bessere Leistung gezeigt. Auf einem PET-Substrat wurden Feldeffekt-Mobilitäten von 0,5 cm2/Vs gemessen mit überzeugenden Reproduzierbarkeit und Stabilität.rnDamit konnte in der Arbeit ein bedeutender Beitrag zur Weiterentwicklung von Materialien für den Einsatz in elektronischen Bauteilen geleistet werden. Die Substanzen versprechen noch erhebliches Potenzial nach intensiver Optimierung und wurden deshalb zum Patent angemeldet.rn

Probing the interaction between two single molecules: fluorescence resonance energy transfer between a single donor and a single acceptor.

We extend the sensitivity of fluorescence resonance energy transfer (FRET) to the single molecule level by measuring energy transfer between a single donor fluorophore and a single acceptor fluorophore. Near-field scanning optical microscopy (NSOM) is used to obtain simultaneous dual color images and emission spectra from donor and acceptor fluorophores linked by a short DNA molecule. Photodestruction dynamics of the donor or acceptor are used to determine the presence and efficiency of energy transfer. The classical equations used to measure energy transfer on ensembles of fluorophores are modified for single-molecule measurements. In contrast to ensemble measurements, dynamic events on a molecular scale are observable in single pair FRET measurements because they are not canceled out by random averaging. Monitoring conformational changes, such as rotations and distance changes on a nanometer scale, within single biological macromolecules, may be possible with single pair FRET.

Energy band and acceptor binding energy of GaN and AlxGa1-xN

The hole effective-mass Hamiltonian for the semiconductors of wurtzite structure is established, and the effective-mass parameters of GaN and AlxGa1-xN are given. Besides the asymmetry in the z and x, y directions, the linear term of the momentum operator in the Hamiltonian is essential in determining the valence band structure, which is different from that of the zinc-blende structure. The binding energies of acceptor states are calculated by solving strictly the effective-mass equations. The binding energies of donor and acceptor for wurtzite GaN are 20 and 131, 97 meV, respectively, which are inconsistent with the recent experimental results. It is proposed that there are two kinds of acceptors in wurtzite GaN. One kind is the general acceptor such as C, substituting N, which satisfies the effective-mass theory, and the other includes Mg, Zn, Cd etc., the binding energy of which deviates from that given by the effective-mass theory. Experimentally, wurtzite GaN was grown by the MBE method, and the PL spectra were measured. Three main peaks are assigned to the DA transitions from the two kinds of acceptor. Some of the transitions were identified as coming from the cubic phase of GaN, which appears randomly within the predominantly hexagonal material. The binding energy of acceptor in ALN is about 239, 158 meV, that in AlxGa1-xN alloys (x approximate to 0.2) is 147, 111 meV, close to that in GaN. (C) 2000 Published by Elsevier Science S.A. All rights reserved.

Energy bands and acceptor binding energies of GaN

The energy bands of zinc-blende and wurtzite GaN are calculated with the empirical pseudopotential method, and the pseudopotential parameters for Ga and N atoms are-given. The calculated energy bands are in agreement with those obtained by the ab initio method. The effective-mass theory for the semiconductors of wurtzite structure is established, and the effective-mass parameters of GaN for both structures are given The binding energies of acceptor states are calculated by solving strictly the effective-mass equations. The binding energies of donor and acceptor are 24 and 142 meV for the zinc-blende structure, 20 and 131, and 97 meV for the wurtzite structure, respectively, which are consistent with recent experimental results. It is proposed that there are two kinds of acceptor in wurtzite GaN. One kind is the general acceptor such as C, which substitutes N, which satisfies the effective-mass theory. The other kind of acceptor includes Mg, Zn, Cd, etc., the binding energy of these accepters is deviated from that given by the effective mass theory. In this report, wurtzite GaN is grown by the molecular-beam epitaxy method, and the photoluminescence spectra were measured. Three main peaks are assigned to the donor-acceptor transitions from two kinds of accepters. Some of the transitions were identified as coming from the cubic phase of GaN, which appears randomly within the predominantly hexagonal material. [S0163-1829(99)15915-0].

Energy band and acceptor binding energy of GaN and AlxGa1-xN

The hole effective-mass Hamiltonian for the semiconductors of wurtzite structure is established, and the effective-mass parameters of GaN and AlxGa1-xN are given. Besides the asymmetry in the z and x, y directions, the linear term of the momentum operator in the Hamiltonian is essential in determining the valence band structure, which is different from that of the zinc-blende structure. The binding energies of acceptor states are calculated by solving strictly the effective-mass equations. The binding energies of donor and acceptor for wurtzite GaN are 20 and 131, 97 meV, respectively, which are inconsistent with the recent experimental results. It is proposed that there are two kinds of acceptors in wurtzite GaN. One kind is the general acceptor such as C, substituting N, which satisfies the effective-mass theory, and the other includes Mg, Zn, Cd etc., the binding energy of which deviates from that given by the effective-mass theory. Experimentally, wurtzite GaN was grown by the MBE method, and the PL spectra were measured. Three main peaks are assigned to the DA transitions from the two kinds of acceptor. Some of the transitions were identified as coming from the cubic phase of GaN, which appears randomly within the predominantly hexagonal material. The binding energy of acceptor in ALN is about 239, 158 meV, that in AlxGa1-xN alloys (x approximate to 0.2) is 147, 111 meV, close to that in GaN. (C) 2000 Published by Elsevier Science S.A. All rights reserved.

Estimation of electronic coupling in π -stacked donor-bridge-acceptor systems: correction of the two-state model

Comparison of donor-acceptor electronic couplings calculated within two-state and three-state models suggests that the two-state treatment can provide unreliable estimates of Vda because of neglecting the multistate effects. We show that in most cases accurate values of the electronic coupling in a π stack, where donor and acceptor are separated by a bridging unit, can be obtained as Ṽ da = (E2 - E1) μ12 Rda + (2 E3 - E1 - E2) 2 μ13 μ23 Rda2, where E1, E2, and E3 are adiabatic energies of the ground, charge-transfer, and bridge states, respectively, μij is the transition dipole moments between the states i and j, and Rda is the distance between the planes of donor and acceptor. In this expression based on the generalized Mulliken-Hush approach, the first term corresponds to the coupling derived within a two-state model, whereas the second term is the superexchange correction accounting for the bridge effect. The formula is extended to bridges consisting of several subunits. The influence of the donor-acceptor energy mismatch on the excess charge distribution, adiabatic dipole and transition moments, and electronic couplings is examined. A diagnostic is developed to determine whether the two-state approach can be applied. Based on numerical results, we showed that the superexchange correction considerably improves estimates of the donor-acceptor coupling derived within a two-state approach. In most cases when the two-state scheme fails, the formula gives reliable results which are in good agreement (within 5%) with the data of the three-state generalized Mulliken-Hush model

Energy Transfer in Nanostructured Films Containing Poly(p-phenylene vinylene) and Acceptor Species

The combination of luminescent polymers and suitable energy-accepting materials may lead to a molecular-level control of luminescence in nanostructured films. In this study, the properties of layer-by-layer (LbL) films of polyp-phenylene vinylene) (PPV) were investigated with steady-state and time-resolved fluorescence spectroscopies, where fluorescence quenching was controlled by interposing inert polyelectrolyte layers between the PPV donor and acceptor layers made with either Congo Red (CR) or nickel tetrasulfonated phthalocyanine (NiTsPc). The dynamics of the excited state of PPV was affected by the energy-accepting layers, thus confirming the presence of resonant energy transfer mechanisms. Owing to the layered structured of both energy donor and acceptor units, energy transfer varied with the distance between layers, r, according to 1/r(n) with n = 2 or 3, rather than with 1/r(6) predicted by the Forster theory for interacting point dipoles.