| 1. |
- Braun, Slawomir, et al.
(författare)
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Energy-Level Alignment at Organic/Metal and Organic/Organic Interfaces
- 2009
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Ingår i: ADVANCED MATERIALS. - : Wiley. - 0935-9648 .- 1521-4095. ; 21:14-15, s. 1450-1472
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Forskningsöversikt (refereegranskat)abstract
- In this Review, we summarize recent work on modeling of organic/metal and organic/organic interfaces. Some of the models discussed have a semiempirical approach, that is, experimentally derived values are used in combination with theory, and others rely completely of calculations. The models are categorized according to the types of interfaces they apply to, and the strength of the interaction at the interface has been used as the main factor. We explain the basics of the models, their use, and give examples on how the models correlate with experimental results. We stress that given the complexity of organic/metal and organic/organic interface formation, it is crucial to know the exact way in which the interface was formed before choosing the model that is applicable, as none of the models presented covers the whole range of interface interaction strengths (weak physisorption to strong chemisorption).
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| 2. |
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| 3. |
- Braun, Slawomir, et al.
(författare)
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Energy level alignment regimes at hybrid organic–organic and inorganic–organic interfaces
- 2007
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Ingår i: Organic Electronics. - : Elsevier BV. - 1566-1199. ; 8:1, s. 14-20
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Tidskriftsartikel (refereegranskat)abstract
- Ultraviolet photoelectron spectroscopy has been used to determine the energy level alignment at interfaces of molecular hole-transporting materials and various conductive substrates. Depending on the work function of the substrate, s, a transition between two different energy level alignment regimes has been observed: namely vacuum level alignment and Fermi level pinning. The transition is associated with spontaneous positive charge transfer across the interface to the organic semiconductors above a certain material-specific threshold value of s. The charge transfer results in formation of an interfacial dipole of a magnitude that scales with s. In the vacuum level alignment regime, the hole-injection barriers scale linearly with s, while in the Fermi level pinning regime, these barriers are constant and independent of s.
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| 4. |
- Braun, Slawomir, et al.
(författare)
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Fermi level pinning at interfaces with tetrafluorotetracyanoquinodimethane (F4-TCNQ): The role of integer charge transfer states
- 2007
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Ingår i: Chemical Physics Letters. - : Elsevier BV. - 0009-2614 .- 1873-4448. ; 438:4-6, s. 259-262
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Tidskriftsartikel (refereegranskat)abstract
- The energy level alignment of vacuum deposited molecular films of tetrafluorotetracyanoquinodimethane (F4-TCNQ) on various substrates has been studied by photoelectron spectroscopy. The interfaces studied span the work function range from 3.45 to 5.8 eV. In this range, the Fermi level of the substrate is pinned in proximity to LUMO level. This indicates that a charge transfer mechanism is responsible for the observed alignment scheme. The photoelectron emission study of sub-monolayer of F4-TCNQ revealed presence of electrons in the charge transfer states at the interface. In this context the electronic structure of neutral and negatively charged F4-TCNQ has been studied theoretically and by photoelectron spectroscopy.
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| 5. |
- Braun, Slawomir, 1977-, et al.
(författare)
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Influence of the electrode work function on the energy level alignment at organic-organic interfaces
- 2007
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Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 91:20
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Tidskriftsartikel (refereegranskat)abstract
- The energy level alignment at interfaces, in stacks comprising of (4, 4′ -N, N′ -dicarbazolyl-biphenyl) (CBP), (4,4, 4″ -tris[3-methyl-phenyl(phenyl)amino]-triphenylamine) (m -MTDATA), and a conductive substrate, has been studied. We show that the alignment of energy levels depends on the equilibration of the chemical potential throughout the layer stack, while any electronic coupling between the individual layers is of lesser importance. This behavior is expected to occur for a broad class of weakly interacting interfaces and can have profound consequences for the design of organic electronic devices. © 2007 American Institute of Physics.
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| 6. |
- Braun, Slawomir, 1977-
(författare)
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Studies of Materials and Interfaces for Organic Electronics
- 2007
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Organic electronics is a rapidly evolving field with vast number of applications having high potential for commercial success. Although a great progress has been made, many organic electronic applications: organic light-emitting diodes (OLEDs), organic fieldeffect transistors (OFETs), organic solar cells, etc; still require further optimization to fulfill the requirements for successful commercialization. For many applications, available at this time organic materials do not provide satisfactory performance and stability, which hinders the possibility of a large-scale production. Therefore, the key ingredient needed for a successful improvement in performance and stability of organic electronic devices is in-depth knowledge of physical and chemical properties of molecular and polymeric materials. Since many applications encompass several thin film layers made of organics, and often also inorganic materials, the understanding of both organic-organic and hybrid interfaces is yet another important issue necessary for the successful development of organic electronics.The research presented in this thesis is based mainly on photoelectron spectroscopy, which is an experimental technique especially suited to study both surfaces and interfaces of materials. In the thesis, the properties of one of the most successful polymeric materials, poly(3,4-ethylenedioxythiophene), often abbreviated as PEDOT, have been extensively studied. The research was done in close cooperation with an industrial partner – AGFA Gevaert, Belgium. The study was focused on the exploration of the intrinsic properties of the material, such as stability, morphology and conductivity. In addition, however, a possibility of alternation of these properties was also explored. This thesis reports also about investigations of the properties of various organic-organic and hybrid interfaces. The energy level alignment at such interfaces plays important role in charge injection and performance of the thin film organic-based devices. The conditions for different energy level alignment regimes at the various interfaces have been studied. The studies on interfaces were performed in close collaboration with the R&D division of DuPont Corporation, USA. This work led to the significant advances in understanding of the interface energetics and properties of industryrelevant organic materials, as represented not only by published scientific papers, but also patent applications.
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| 7. |
- Flink, Axel, et al.
(författare)
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The location and effects of Si in (Ti1-xSix)N-y thin films
- 2009
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Ingår i: JOURNAL OF MATERIALS RESEARCH. - : Springer Science and Business Media LLC. - 0884-2914 .- 2044-5326. ; 24:8, s. 2483-2498
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Tidskriftsartikel (refereegranskat)abstract
- (Ti1-xSix)N-y (0 andlt;= x andlt;= 0.20; 0.99 andlt;= y(x) andlt;= 1.13) thin films deposited by arc evaporation have been investigated by analytical transmission electron microscopy, x-ray diffraction, x-ray photoelectron spectroscopy, and nanoindentation. Films with x andlt;= 0.09 are single-phase cubic (Ti,Si)N solid solutions with a dense columnar microstructure. Films with x andgt; 0.09 haven a featherlike microstructure consisting of cubic TiN:Si nanocrystallite bundles separated by metastable SiNz with coherent-to-semicoherent interfaces and a dislocation density of as much as 10(14) cm(-2) is present. The films exhibit retained composition and hardness between 31 and 42 GPa in annealing experiments to 1000 degrees C due to segregation of SiN, to the grain boundaries. During annealing at 1100-1200 degrees C, this tissue phase thickens and transforms to amorphous SiNz. At the same time, Si and N diffuse out of the films via the grain boundaries and TiN recrystallize.
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| 8. |
- Furlan, Andrej, 1974-, et al.
(författare)
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Synthesis of phosphorus-carbide thin films by magnetron sputtering
- 2008
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Ingår i: physica status solidi (RRL) - Rapid Research Letters. - : Wiley InterScience. - 1862-6254. ; 2:4, s. 191-193
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Tidskriftsartikel (refereegranskat)abstract
- Phosphorus-carbide, CPx (0.025≤x≤0.1) thin films have beensynthesized by magnetron sputtering from pressed graphite-phosphorustargets. The films were characterized by X-ray photoelectron spectroscopy,transmission electron microscopy and diffraction, andnanoindentation. CP0.02 exhibits C-P bonding in an amorphous structure with elements of curved grapheneplanes, yielding a material with unique short range order. These features are consistent with what has been predicted by our results of theoreticallymodeled synthetic growth of CPx. The films are mechanicallyresilient with hardness up to 24 GPa and elastic recovery upto 72%.
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| 9. |
- Jakobsson, Fredrik, et al.
(författare)
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Tuning the Energy Levels of Photochromic Diarylethene Compounds for Opto-electronic Switch Devices
- 2009
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Ingår i: JOURNAL OF PHYSICAL CHEMISTRY C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 113:42, s. 18396-18405
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Tidskriftsartikel (refereegranskat)abstract
- Diarylethene molecules are photochromics (PCs) currently investigated for use in optical write/electrical read memory applications. The impact of the photoisomerization of PCs on the device behavior is analyzed with charge transport models. These results indicate that good electrical current switching can be achieved in a device when the PCs are combined with an organic semiconductor (in multilayered structures or blends). The frontier energy levels and dipole moment of a series of diarylethene compounds have been calculated using density functional theory. A good agreement is found between the calculated electronic structure and the measured ultraviolet photoelectron spectra. Shirts in the frontier energy levels and dipole moment are generated through two different approaches for chemical modification: (i) by changing the chemical nature of the aryl rings or (ii) by adding substituents on the ethylene, bridge. The frontier energy levels can be tuned by more than 2 eV via such chemical modifications. We find that, for this family of photochromic compounds, the photoinduced current switch effect in diodes is mainly due to the modulation in the frontier energy levels rather than the changes in the amplitude of the dipole moment.
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| 10. |
- Liu, Xianjie, et al.
(författare)
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Interfacial electronic properties of pentacene tuned by a molecular monolayer of C-60
- 2009
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - College Park, MD, United States : American Physical Society. - 1098-0121 .- 1550-235X. ; 80:11, s. 115401-1-115401-7
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Tidskriftsartikel (refereegranskat)abstract
- Fine-tuning charge injection barriers between organic materials and electrodes is critical to optimize organic electronic device performance. Here we demonstrate that by modifying gold substrates with a monolayer of fullerene, significant decrease in the hole-injection barrier into pentacene films can be achieved. The insertion of the fullerene monolayer modifies the interfacial dipole and produces an interface where the pentacene molecules form a standing-up orientation with their long axis parallel to the surface normal. The latter effect lowers the vertical ionization energy of the pentacene molecules at the interface as compared to the pentacene-on-Au case, as well as improves the pi-pi overlap between the pentacene molecules that will likely enhance the transport properties in corresponding devices.
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