G.A. de Wijs

Bio: G.A. de Wijs is an academic researcher from Radboud University Nijmegen. The author has contributed to research in topics: Electronic structure & Band gap. The author has an hindex of 27, co-authored 82 publications receiving 2599 citations. Previous affiliations of G.A. de Wijs include University of Groningen & École normale supérieure de Lyon.

Modeling the polymorphism of pentacene.

Abstract: Thin films of pentacene are known to crystallize in at least four different polymorphs. All polymorphs are layered structures that are characterized by their interlayer spacing d(001). We develop a model that rationalizes the size of the interlayer spacing in terms of intralayer shifts of the pentacene molecules along their long molecular axes. It explains the wide variety of interlayer spacings, without distorting the herringbone pattern that is characteristic of many acenes. Using two simple theoretical models, we attempt to relate the intralayer shifts with the dominant, although weak, interatomic interactions (van der Waals, weak electrostatic, and covalent). For two polymorphs, a consistent picture is found. A full understanding of the other two, substrate-induced, polymorphs probably requires consideration of interlayer interactions.

Spin-polarization in half-metals (invited)

Abstract: Half-metals are defined by an electronic structure, which shows conduction by charge carriers of one spin direction exclusively. Consequently, the spin polarization of the conduction electrons should be 100%. In reality this complete spin polarization is not always observed. Since the experimental search for half-metals is tedious and the verification of the expected spin polarization is involved, electronic structure calculations have played an important role in this area. So, an important question is, how the approximations in such calculations influence the resulting spin polarization of the conduction. Another aspect is the well-known fact that bulk properties can be very different from surface and interface properties. Indeed, measurements of the spin polarization in the bulk for, e.g., NiMnSb, show results different from surface sensitive measurements. In this respect it is important to realize that the origin of half-metallic behavior is not unique. Consequently, the deviations from the bulk behavior at the surface/interface can be important. Three different categories of half-metals can be distinguished and their expected surface properties will be discussed. Finally, ways will be described to control the properties at interfaces.

Towards 100% spin-polarized charge-injection: The half-metallic NiMnSb/CdS interface

Abstract: Spin-electronics requires an electron source with a spin-polarization as high as possible. For this, half-metallic materials seem ideally suited as they exhibit 100% spin polarization. Because of its high Curie temperature and compatibility with existing semiconductor technology, NiMnSb is a most desirable half metal. However, using first-principles calculations we find that NiMnSb surfaces are not half metallic, even if they are stoichiometric and perfectly ordered. Moreover, several surface and interface sensitive experiments have reported polarizations far less than 100%. These findings are easily rationalized, as they result from the symmetry breaking at the surface. We show that it is possible to restore half metallicity at interfaces, by a proper engineering at the microscopic level. Therefore the half metal NiMnSb is, in principle, a suitable source material for 100% spin-polarized charge carriers.

The electronic structure of tantalum (oxy)nitrides TaON and Ta3N5

Abstract: Detailed results of ab initio band structure calculations for tantalum (oxy)nitrides (TaON and Ta3N5) are reported. The calculations are performed within the framework of density functional theory (DFT). We compare results obtained with the molecular dynamics pseudopotential (PP) approach of the Vienna Ab initio Simulation Program (VASP) and the Full Potential Linearized Augmented Plane Waves method (FP-LAPW) using the WIEN97 program. In agreement with neutron diffraction measurements, we show an ordering of the anions in TaON. The calculations also show that the valence band is composed mainly of the anion 2p orbitals hybridized with Ta 5d states. For TaON the top of the valence band is dominated by N 2p states. The bottom of the conduction band is mainly composed of Ta 5d states. Both TaON and Ta3N5 are semiconductors with calculated indirect band gaps of respectively 1.8 and 1.1 eV (VASP calculations) and 2.0 and 1.2 eV (WIEN97 calculations). Optical diffuse-reflectance spectra show an energy gap of 2.08 eV for Ta3N5.

Mixed Magnetism for Refrigeration and Energy Conversion

Abstract: The efficient coupling between lattice degrees of freedom and spin degrees of freedom in magnetic materials can be used for refrigeration and energy conversion. This coupling is enhanced in materials exhibiting the giant magnetocaloric effect. First principle electronic structure calculations on hexagonal MnFe(P, Si) reveal a new form of magnetism: the coexistence of strong and weak magnetism in alternate atomic layers. The weak magnetism of Fe layers (disappearance of local magnetic moments at the Curie temperature) is responsible for a strong coupling with the crystal lattice while the strong magnetism in adjacent Mn-layers ensures Curie temperatures high enough to enable operation at and above room temperature. Varying the composition on these magnetic sublattices gives a handle to tune the working temperature and to achieve a strong reduction of the undesired thermal hysteresis. In this way we design novel materials based on abundantly available elements with properties matched to the requirements of an efficient refrigeration or energy-conversion cycle.

Spintronics: Fundamentals and applications

Abstract: Spintronics, or spin electronics, involves the study of active control and manipulation of spin degrees of freedom in solid-state systems. This article reviews the current status of this subject, including both recent advances and well-established results. The primary focus is on the basic physical principles underlying the generation of carrier spin polarization, spin dynamics, and spin-polarized transport in semiconductors and metals. Spin transport differs from charge transport in that spin is a nonconserved quantity in solids due to spin-orbit and hyperfine coupling. The authors discuss in detail spin decoherence mechanisms in metals and semiconductors. Various theories of spin injection and spin-polarized transport are applied to hybrid structures relevant to spin-based devices and fundamental studies of materials properties. Experimental work is reviewed with the emphasis on projected applications, in which external electric and magnetic fields and illumination by light will be used to control spin and charge dynamics to create new functionalities not feasible or ineffective with conventional electronics.

A review of electrode materials for electrochemical supercapacitors

Abstract: In this critical review, metal oxides-based materials for electrochemical supercapacitor (ES) electrodes are reviewed in detail together with a brief review of carbon materials and conducting polymers. Their advantages, disadvantages, and performance in ES electrodes are discussed through extensive analysis of the literature, and new trends in material development are also reviewed. Two important future research directions are indicated and summarized, based on results published in the literature: the development of composite and nanostructured ES materials to overcome the major challenge posed by the low energy density of ES (476 references).

Charge transport in organic semiconductors.

Abstract: 2.2. Materials 929 2.3. Factors Influencing Charge Mobility 931 2.3.1. Molecular Packing 931 2.3.2. Disorder 932 2.3.3. Temperature 933 2.3.4. Electric Field 934 2.3.5. Impurities 934 2.3.6. Pressure 934 2.3.7. Charge-Carrier Density 934 2.3.8. Size/molecular Weight 935 3. The Charge-Transport Parameters 935 3.1. Electronic Coupling 936 3.1.1. The Energy-Splitting-in-Dimer Method 936 3.1.2. The Orthogonality Issue 937 3.1.3. Impact of the Site Energy 937 3.1.4. Electronic Coupling in Oligoacene Derivatives 938

Half-metallic graphene nanoribbons

Abstract: Electrical current can be completely spin polarized in a class of materials known as half-metals, as a result of the coexistence of metallic nature for electrons with one spin orientation and insulating nature for electrons with the other. Such asymmetric electronic states for the different spins have been predicted for some ferromagnetic metals--for example, the Heusler compounds--and were first observed in a manganese perovskite. In view of the potential for use of this property in realizing spin-based electronics, substantial efforts have been made to search for half-metallic materials. However, organic materials have hardly been investigated in this context even though carbon-based nanostructures hold significant promise for future electronic devices. Here we predict half-metallicity in nanometre-scale graphene ribbons by using first-principles calculations. We show that this phenomenon is realizable if in-plane homogeneous electric fields are applied across the zigzag-shaped edges of the graphene nanoribbons, and that their magnetic properties can be controlled by the external electric fields. The results are not only of scientific interest in the interplay between electric fields and electronic spin degree of freedom in solids but may also open a new path to explore spintronics at the nanometre scale, based on graphene.

Recent Advances in Electrocatalysts for Oxygen Reduction Reaction

Abstract: The recent advances in electrocatalysis for oxygen reduction reaction (ORR) for proton exchange membrane fuel cells (PEMFCs) are thoroughly reviewed. This comprehensive Review focuses on the low- and non-platinum electrocatalysts including advanced platinum alloys, core–shell structures, palladium-based catalysts, metal oxides and chalcogenides, carbon-based non-noble metal catalysts, and metal-free catalysts. The recent development of ORR electrocatalysts with novel structures and compositions is highlighted. The understandings of the correlation between the activity and the shape, size, composition, and synthesis method are summarized. For the carbon-based materials, their performance and stability in fuel cells and comparisons with those of platinum are documented. The research directions as well as perspectives on the further development of more active and less expensive electrocatalysts are provided.