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M. A. Uijttewaal

Bio: M. A. Uijttewaal is an academic researcher from Radboud University Nijmegen. The author has contributed to research in topics: Organic semiconductor & Band gap. The author has an hindex of 7, co-authored 7 publications receiving 264 citations.

Papers
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Journal ArticleDOI
TL;DR: D density functional calculations in the generalized gradient approximation are used to study a broad range of possible beta-rhombohedral structures containing interstitial atoms and partially occupied sites within a 105 atoms framework, finding the two most stable structures are practically degenerate in energy and semiconducting.
Abstract: Its low weight, high melting point, and large degree of hardness make elemental boron a technologically interesting material. The large number of allotropes, mostly containing over a hundred atoms in the unit cell, and their difficult characterization challenge both experimentalists and theoreticians. Even the ground state of this element is still under discussion. For over 30 years, scientists have attempted to determine the relative stability of α- and β-rhombohedral boron. We use density functional calculations in the generalized gradient approximation to study a broad range of possible β-rhombohedral structures containing interstitial atoms and partially occupied sites within a 105 atoms framework. The two most stable structures are practically degenerate in energy and semiconducting. One contains the experimental 320 atoms in the hexagonal unit cell, and the other contains 106 atoms in the triclinic unit cell. When populated with the experimental 320 electrons, the 106 atom structure exhibits a band ...

154 citations

Journal ArticleDOI
TL;DR: In this article, the authors present the results of a modeling study of the three-dimensional current density in single-carrier sandwich-type devices of disordered organic semiconductors.
Abstract: We present the results of a modeling study of the three-dimensional current density in single-carrier sandwich-type devices of disordered organic semiconductors. The calculations are based on a master-equation approach, assuming a Gaussian distribution of site energies without spatial correlations. The injection-barrier lowering due to the image potential is taken into account, so that the model provides a comprehensive treatment of the space-charge-limited current as well as the injection-limited current (ILC) regimes. We show that the current distribution can be highly filamentary for voltages, layer thicknesses, and disorder strengths that are realistic for organic light-emitting diodes and, that, as a result, the current density in both regimes can be significantly larger than as obtained from a one-dimensional continuum drift-diffusion device model. For devices with large injection barriers and strong disorder, in the ILC transport regime, good agreement is obtained with the average current density predicted from a model assuming injection and transport via one-dimensional filaments.

54 citations

Journal ArticleDOI
TL;DR: Despite the similarity in the crystal structures of the acene- TCNQ complexes studied here, the band structures are very different and Hole and electron transport properties are predicted to be equally good in perylene-TCNQ, in contrast to the tetracene-TCnQ, which has good transport properties for electrons only.
Abstract: The relationship between the crystal structures, band structures, and electronic properties of acene-TCNQ complexes has been investigated. We focus on the newly synthesized crystals of the charge-transfer salt tetracene-TCNQ and similar to it perylene-TCNQ, potentially interesting for realization of ambipolar transport. The band structures were calculated from first principles using density-functional theory (DFT). Despite the similarity in the crystal structures of the acene-TCNQ complexes studied here, the band structures are very different. Hole and electron transport properties are predicted to be equally good in perylene-TCNQ, in contrast to the tetracene-TCNQ, which has good transport properties for electrons only. The estimated degree of charge transfer for tetracene-TCNQ is 0.13e and for perylene-TCNQ 0.46e.

41 citations

Journal ArticleDOI
TL;DR: In this article, the anisotropy in the work function and the surface stability of a prototype half-metal was calculated, and the authors showed that the lowest work function is obtained for surfaces with the most electropositive element, whereas the stable surfaces are those containing the element with the lowest valency.
Abstract: Insight in the interplay between work function and stability is important for many areas of physics. In this paper, we calculate the anisotropy in the work function and the surface stability of $\mathrm{Cr}{\mathrm{O}}_{2}$, a prototype half-metal, and find an anisotropy of $3.8\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$. An earlier model for the relation between work function and surface stability is generalized to include the transition-metal oxides. We find that the lowest work function is obtained for surfaces with the most electropositive element, whereas the stable surfaces are those containing the element with the lowest valency. Most $\mathrm{Cr}{\mathrm{O}}_{2}$ surfaces considered remain half-metallic, thus the anisotropy in the work function can be used to realize low resistance, half-metallic interfaces.

18 citations

Journal ArticleDOI
TL;DR: In this paper, the authors used density functional calculations in the generalized gradient approximation to study a broad range of possible β-rhombohedral structures containing interstitial atoms and partially occupied sites within a 105 atoms framework.
Abstract: Its low weight, high melting point, and large degree of hardness make elemental boron a technologically interesting material. The large number of allotropes, mostly containing over a hundred atoms in the unit cell, and their difficult characterization challenge both experimentalists and theoreticians. Even the ground state of this element is still under discussion. For over 30 years, scientists have attempted to determine the relative stability of α- and β-rhombohedral boron. We use density functional calculations in the generalized gradient approximation to study a broad range of possible β-rhombohedral structures containing interstitial atoms and partially occupied sites within a 105 atoms framework. The two most stable structures are practically degenerate in energy and semiconducting. One contains the experimental 320 atoms in the hexagonal unit cell, and the other contains 106 atoms in the triclinic unit cell. When populated with the experimental 320 electrons, the 106 atom structure exhibits a band gap of 1.4 eV and an in-gap hole trap at 0.35 eV above the valence band, consistent with known experiments. The total energy of these two structures is 23 meV/B lower than the original 105 atom framework, but it is still 1 meV/B above the α phase. Adding zero point energies finally makes the β phase the ground state of elemental boron by 3 meV/B. At finite temperatures, the difference becomes even larger.

8 citations


Cited by
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Journal ArticleDOI
TL;DR: Progress in the development of probes for "reactive oxygen and nitrogen" species, emphasizing the caution needed in their use is reviewed, with a focus on probes based on reduced dyes.

804 citations

Journal ArticleDOI
12 Feb 2009-Nature
TL;DR: It is found that the ionicity of the phase affects its electronic bandgap, infrared adsorption and dielectric constants, and that it arises from the different electronic properties of the B2 pairs and B12 clusters and the resultant charge transfer between them.
Abstract: Boron is an element of fascinating chemical complexity. This arises from frustration: situated between metals and insulators in the periodic table, boron has only three valence electrons that could in principle favour metallicity, yet they are sufficiently localized to give rise to an insulating state. This delicately balanced electronic structure is easily modified by pressure, temperature and impurities, making it difficult to establish boron's structure and properties. Oganov et al. have now explored the high-pressure behaviour of boron and uncovered a previously unknown ionic phase consisting of negatively charged icosahedral B12 clusters and positively charged B2 pairs. The ionicity of the new phase strongly affects many of its properties, and arises from the different electronic properties of the B12 clusters and B2 pairs and the resultant charge transfer between them. This paper has explored the high-pressure behaviour of boron and uncovered a new phase that consists of negatively charged icosahedral B12 clusters and positively charged B2 pairs. The ionicity of the new phase strongly affects many of its properties, and arises from the different electronic properties of the B12 clusters and B2 pairs and the resultant charge transfer between them. Boron is an element of fascinating chemical complexity. Controversies have shrouded this element since its discovery was announced in 1808: the new ‘element’ turned out to be a compound containing less than 60–70% of boron, and it was not until 1909 that 99% pure boron was obtained1. And although we now know of at least 16 polymorphs2, the stable phase of boron is not yet experimentally established even at ambient conditions3. Boron’s complexities arise from frustration: situated between metals and insulators in the periodic table, boron has only three valence electrons, which would favour metallicity, but they are sufficiently localized that insulating states emerge. However, this subtle balance between metallic and insulating states is easily shifted by pressure, temperature and impurities. Here we report the results of high-pressure experiments and ab initio evolutionary crystal structure predictions4,5 that explore the structural stability of boron under pressure and, strikingly, reveal a partially ionic high-pressure boron phase. This new phase is stable between 19 and 89 GPa, can be quenched to ambient conditions, and has a hitherto unknown structure (space group Pnnm, 28 atoms in the unit cell) consisting of icosahedral B12 clusters and B2 pairs in a NaCl-type arrangement. We find that the ionicity of the phase affects its electronic bandgap, infrared adsorption and dielectric constants, and that it arises from the different electronic properties of the B2 pairs and B12 clusters and the resultant charge transfer between them.

750 citations

Journal ArticleDOI
TL;DR: In this paper, the authors review the latest on geminate and nongeminate recombination in organic solar cells and discuss the key loss mechanisms in charge carrier recombination, which is one of the most promising alternative energy sources because of their ease of processing and their potential to be produced using large scale techniques such as roll-to-roll, newspaper style, coating.

538 citations

Journal ArticleDOI
TL;DR: The greatest challenges to boron chemistry are still the synthesis of monophasic products in macroscopic quantities and in the form of single crystals, the unequivocal identification and determination of crystal structures, and a thorough understanding of their electronic situation.
Abstract: Many of the fundamental questions regarding the solid-state chemistry of boron are still unsolved, more than 200 years after its discovery. Recently, theoretical work on the existence and stability of known and new modifications of the element combined with high-pressure and high-temperature experiments have revealed new aspects. A lot has also happened over the last few years in the field of reactions between boron and main group elements. Binary compounds such as B(6)O, MgB(2), LiB(1-x), Na(3)B(20), and CaB(6) have caused much excitement, but the electron-precise, colorless boride carbides Li(2)B(12)C(2), LiB(13)C(2), and MgB(12)C(2) as well as the graphite analogue BeB(2)C(2) also deserve special attention. Physical properties such as hardness, superconductivity, neutron scattering length, and thermoelectricity have also made boron-rich compounds attractive to materials research and for applications. The greatest challenges to boron chemistry, however, are still the synthesis of monophasic products in macroscopic quantities and in the form of single crystals, the unequivocal identification and determination of crystal structures, and a thorough understanding of their electronic situation. Linked polyhedra are the dominating structural elements of the boron-rich compounds of the main group elements. In many cases, their structures can be derived from those that have been assigned to modifications of the element. Again, even these require a critical revision and discussion.

494 citations

Journal ArticleDOI
TL;DR: In this paper, the growth, structure and properties of charge-transfer complexes are reviewed and underlined recent progress in their application in organic devices, as well as the challenges to understand the fundamental parameters governing their operation.
Abstract: The discovery of the organic metal TTF–TCNQ in 1973 led to an explosion of research conducted on organic charge-transfer complexes. While these materials have been studied intensely for several decades, the research was mostly aimed at the discovery of materials with high room-temperature conductivity or high-temperature superconductivity. Recently, attention has turned to technologically-relevant properties of charge-transfer complexes, such as ambipolar transport, metallicity, photoconductivity, ferroelectricity or magnetoresistance. This manuscript reviews the growth, structure and properties of charge-transfer complexes and underlines recent progress in their application in organic devices. Their prospects in future applications are discussed, as well as the challenges yet to be overcome to understand the fundamental parameters governing their operation.

341 citations