| 1. |
- Canestraro, Carla Daniele, et al.
(författare)
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Strong inter-conduction-band absorption in heavily fluorine doped tin oxide
- 2008
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Ingår i: Applied Surface Science. - : Elsevier BV. - 0169-4332 .- 1873-5584. ; 255:5, s. 1874-1879
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Tidskriftsartikel (refereegranskat)abstract
- The optical, electrical and structural properties of thin. film tin oxide (TO), F-doped tin oxide (FTO; n(F) approximate to 6 x 10(20) cm (3)) and highly F-doped tin oxide (hFTO; n(F) approximate to 10 x 10(20) cm (3)), grown by spray pyrolysis technique, are studied by atomic force microscopy, Hall effect, X-ray. fluorescence and transmission/reflection measurements. The resistivity (rho = 32 x 10 (4) Omega cm for intrinsic tin oxide) shows intriguing characteristics when F concentration n(F) is increased (rho = 6 x 10 (4) Omega cm for FTO but 25 x 10 (4) Omega cm for hFTO) whereas the carrier concentration is almost constant at high F concentration (n(c) approximate to 6 x 10(20) cm (3) for FTO and hFTO). Thus, F seems to act both as a donor and a compensating acceptor in hFTO. The high carrier concentration has a strong effect on the optical band-edge absorption. Whereas intrinsic TO has room-temperature band-gap energy of E-g approximate to 3.2 eV with an onset to absorption at about 3.8 eV, the highly doped FTO and hFTO samples show relatively strong absorption at 2-3 eV. Theoretical analysis based on density functional calculations of FTO reveals that this is not a defect state within the band-gap region, but instead a consequence of a hybridization of the F donor states with the host conduction band in combination with a band. filling of the lowest conduction band by the free carriers. This allows photon-assisted inter-conduction band transitions of the free electrons to energetically higher and empty conduction bands, producing the below-gap absorption peak.
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| 2. |
- Cava, Carlos E., et al.
(författare)
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Iron- and iron oxide-filled multi-walled carbon nanotubes : Electrical properties and memory devices
- 2007
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Ingår i: Chemical Physics Letters. - : Elsevier BV. - 0009-2614 .- 1873-4448. ; 444:4-6, s. 304-308
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Tidskriftsartikel (refereegranskat)abstract
- Electrical and morphological properties of neat iron- and iron oxide-filled multi-walled carbon nanotubes and its dispersion with a semiconducting polymer were investigated. The electrical properties of these carbon nanotubes changed significantly when exposed to different atmospheres. The current voltage characteristics of the films formed from the dispersions were obtained from planar devices with metallic electrodes. These devices were used as gas sensor and as memory devices where the writing, reading and erasing can be done electrically.
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| 3. |
- Aderne, Rian E., et al.
(författare)
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On the energy gap determination of organic optoelectronic materials : the case of porphyrin derivatives
- 2022
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Ingår i: Materials Advances. - : Royal Society of Chemistry. - 2633-5409. ; :3, s. 1791-1803
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Tidskriftsartikel (refereegranskat)abstract
- The correct determination of the ionization potential (IP) and electron affinity (EA) as well as the energy gap is essential to properly characterize a series of key phenomena related to the applications of organic semiconductors. For example, energy offsets play an essential role in charge separation in organic photovoltaics. Yet there has been a lot of confusion involving the real physical meaning behind those quantities. Experimentally the energy gap can be measured by direct techniques such as UV-Vis absorption, or indirect techniques such as cyclic voltammetry (CV). Another spectroscopic method is the Reflection Electron Energy Loss Spectroscopy (REELS). Regarding data correlation, there is little consensus on how the REELS' energy gap can be interpreted in light of the energies obtained from other methodologies such as CV, UV-Vis, or photoemission. In addition, even data acquired using those traditional techniques has been misinterpreted or applied to derive conclusions beyond the limits imposed by the physics of the measurement. A similar situation also happens when different theoretical approaches are used to assess the energy gap or employed to explain outcomes from experiments. By using a set of porphyrin derivatives as model molecules, we discuss some key aspects of those important issues. The peculiar properties of these porphyrins demonstrate that even straightforward measurements or calculations performed in a group of very similar molecules need a careful interpretation of the outcomes. Differences up to 660 meV (similar to 190 meV) are found comparing REELS (electrochemical) measurements with UV-Vis energy gaps, for instance. From the theoretical point of view, a reasonable agreement with electrochemical measurements of the IP, EA, and the gap of the porphyrins is only obtained when the calculations involve the full thermodynamics of the redox processes. The purpose of this work is to shed light on the differences and similarities of those aforementioned characterization methods and provide some insight that might help one to develop a critical analysis of the different experimental and theoretical methodologies.
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| 4. |
- Canestraro, Carla Daniele, et al.
(författare)
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Polarization dependence of the optical response in SnO2 and the effects from heavily F doping
- 2009
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Ingår i: Thin Solid Films. - : Elsevier BV. - 0040-6090 .- 1879-2731. ; 517:23, s. 6301-6304
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Tidskriftsartikel (refereegranskat)abstract
- The optical properties of intrinsic SnO2 (TO) and fluorine doped (FTO) are characterized in terms of the dielectric function epsilon(h omega) = epsilon(1) (h omega) + i epsilon(2)(h omega) by electronic structure calculations. The intrinsic TO shows intriguing absorption characteristics in the 3.0-8.0 eV region: (i) the low energy region of the fundamental band gap (3.2 Gamma(+)(1) (valence-band maximum to conduction-band minimum) is symmetry forbidden, and the band-edge absorption is therefore extremely weak. (ii) In the higher energy region (3.9 Gamma(+)(1), transitions (from the second uppermost valence band) is strongly polarized perpendicular to the main c axis. (iii) Transitions with polarization axis parallel to c axis are generated from Gamma(-)(2) -> Gamma(+)(1) transitions (from the third uppermost valence bands), and dominates at high energies (5.1
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| 5. |
- Pereira, Cassia Ferreira Coutinho, et al.
(författare)
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Inducing molecular orientation in solution-processed thin films of fluorene-bithiophene-based copolymer : thermal annealing vs. solvent additive
- 2024
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Ingår i: RSC Advances. - : Royal Society of Chemistry. - 2046-2069. ; 14:13, s. 9051-9061
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Tidskriftsartikel (refereegranskat)abstract
- A deep understanding of the factors influencing the morphology of thin films based on conjugated polymers is essential to boost their performance in optoelectronic devices. Herein, we investigated the electronic structure and morphology of thin films of the copolymer poly(9,9-dioctyl-fluorenyl-co-bithiophene) (F8T2) in its pristine form as well as samples processed with the solvent additive 1,8-diiodooctane (DIO) or post-processed through thermal annealing treatment. Measurements were carried out using angle-resolved S K-edge NEXAFS (near-edge X-ray absorption fine structure) in total electron yield (TEY) and fluorescence yield (FY) detection modes. Two main transitions were observed at the S 1s NEXAFS spectra: S 1s -> pi* and S 1s -> sigma* (S-C). The observed dichroism pointed to a face-on orientation of the conjugated backbone, which was significantly increased for F8T2 films processed with DIO. Resonant Auger decay spectra were obtained and analyzed using the core-hole clock (CHC) method. An enhancement in the charge transfer process was observed for thermally annealed films, especially for samples processed with DIO, corresponding to an increase in film ordering. Furthermore, the investigated films were characterized using X-ray photoelectron spectroscopy, attesting to the presence of the thiophene unit in the samples and demonstrating that some of its sulfur atoms were positively polarized in the F8T2 films. All these experimental findings were compared with molecular dynamics (MD) simulations of film evaporation with and without DIO. The use of MD, together with mathematical modeling, was able to explain the major effects found in the experiments, including the polarization of sulfur atoms. The simultaneous use of powerful spectroscopic techniques and theoretical methods shed light on key aspects linking film morphology with fabrication procedures.
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