Richard Weihrich

Black Arsenic–Phosphorus: Layered Anisotropic Infrared Semiconductors with Highly Tunable Compositions and Properties

New layered anisotropic infrared semiconductors, black arsenic-phosphorus (b-AsP), with highly tunable chemical compositions and electronic and optical properties are introduced. Transport and infrared absorption studies demonstrate the semiconducting nature of b-AsP with tunable bandgaps, ranging from 0.3 to 0.15 eV. These bandgaps fall into the long-wavelength infrared regime and cannot be readily reached by other layered materials.

Reversible switching between p- and n-type conduction in the semiconductor Ag10Te4Br3

Semiconductors are key materials in modern electronics and are widely used to build, for instance, transistors in integrated circuits as well as thermoelectric materials for energy conversion, and there is a tremendous interest in the development and improvement of novel materials and technologies to increase the performance of electronic devices and thermoelectrics. Tetramorphic Ag(10)Te(4)Br(3) is a semiconductor capable of switching its electrical properties by a simple change of temperature. The combination of high silver mobility, a small non-stoichiometry range and an internal redox process in the tellurium substructure causes a thermopower drop of 1,400 microV K(-1), in addition to a thermal diffusivity in the range of organic polymers. The capability to reversibly switch semiconducting properties from ionic to electronic conduction in one single compound simply by virtue of temperature enables novel electronic devices such as semiconductor switches.

The extended stability range of phosphorus allotropes.

Phosphorus displays fascinating structural diversity and the discovery of new modifications continues to attract attention. In this work, a complete stability range of known and novel crystalline allotropes of phosphorus is described for the first time. This includes recently discovered tubular modifications and the prediction of not-yet-known crystal structures of [P12] nanorods and not-yet-isolated [P14] nanorods. Despite significant structural differences, all P allotropes consist of covalent substructures, which are held together by van der Waals interactions. Their correct reproduction by ab initio calculations is a core issue of current research. While some predictions with the established DFT functionals GGA and LDA differ significantly from experimental data in the description of the P allotropes, consistently excellent agreement with the GGA-D2 approach is used to predict the solid structures of the P nanorods.

Mineralization Routes to Polyphosphides: Cu2P20 and Cu5InP16

The element chemistry of phosphorus is one the most complex but also one of the most exciting of all chemical elements. Both the element and the binary and ternary derivatives show a great diversity in terms of reactivity, structural chemistry, and physical properties, such as polymorphism, magnetism, and superconductivity, and are applied, for example, as thermoelectrics, catalysts, or in precipitation hardening. Four allotropic modifications of phosphorus are known to date at standard conditions, namely white, violet, fibrous, and black phosphorus, besides the amorphous red phosphorus. White phosphorus comprises molecular P4 units, and black phosphorus is a layered compound, whereas violet and fibrous phosphorus are characterized by polymeric phosphorus stands of tubular [P20] 2 units connected via P2 bridges. Structural fragments of these units have been identified in amorphous red phosphorus by vibrational spectroscopy and in KP15 by structural analysis. [7] Some theoretically predicted allotropes featuring polymeric phosphorus units were successfully isolated from a copper halide matrix. P15Se and P19Se are two examples of heteroatomic polymer chains that have similar but not identical structural motifs to [P20] 2 . In the past decades elemental phosphorus was used to prepare a plethora of binary and multinary phosphides and polyphosphides. Thermodynamically and also kinetically controlled reactions were developed to derive new compounds from elemental phosphorus. Surprisingly none of the developed methods led to a binary derivative of violet or fibrous phosphorus with retention of the polyphosphide substructure. Only one ternary compound featuring a [P20] 2 unit, the direct subunit of the element structure, embedded in a copper(I) halide matrix, was reported. Recently we prepared the novel polyphosphide AgSbP14, the first pure inorganic material containing a covalent Sb P interaction, and developed a low-pressure route to black phosphorus, making this phosphorus modification commercially available and accessible for applications. Both compounds have been synthesized by a kinetically controlled reaction route using main group metal halides such as SbI3 or SnI4 as reaction promoters (mineralizers). The general reaction principle is closely related to the well-known concept of mineralization reactions described by Sch6fer. Phosphides and polyphosphides such as Zn3P2, Cu3P, LiCu2P2, and Li7Cu5P8 are considered to be promising materials for electrodes in rechargeable batteries, and a carbon–phosphorus composite was successfully tested as an electrode material. The ongoing interest in new energy storage materials on the one hand and the still not completely solved material and engineering problems with present battery systems on the other hand are stimulating the development of new synthesis routes to new materials. Polyphosphides with anisotropic subunits (2D layers) are potential candidates for intercalation reactions, as shown in case of black phosphorus. Herein we report on the CuImediated synthesis of Cu2P20 and Cu5InP16 as well as on their structures and physical properties. X-ray powder diffraction and EDX analyses for both polyphosphides substantiated the phase purity of the bulk phases and the composition of the single crystals selected for the structure determinations. The crystal structure of Cu2P20 was solved from single-crystal X-ray data at room temperature (Figure 1, top). Tubular [P20] 2 units are stacked parallel to each other and connected through tetrahedrally coordinated copper(I) ions; the Cu P bond lengths range between 2.271(3) and 2.317(3) ?. The P P bond lengths (2.154(4)–2.322(3) ?) within the tubular [P20] 2

Synthesis and Identification of Metastable Compounds: Black Arsenic—Science or Fiction?†

Back in black: All metastable and stable phases can be identified for the solid solution arsenic/phosphorus by a combination of quantum-chemical calculations and investigations of the phase formation. Reaction paths for phase formations and transitions in situ were also evaluated. The results show that orthorhombic black arsenic (o-As) is metastable in pure form and has only been previously obtained by stabilizing impurities.

Structure and electronic properties of new model dinitride systems: A density-functional study of CN2, SiN2, and GeN2

The dinitrides CN2, SiN2, and GeN2 in assumed pyrite-type structures are studied by means of density functional theory using both ultrasoft pseudopotentials and the augmented spherical wave (ASW) method. The former two materials constitute the large-x limit of the broader class of CNx and SiNx compounds, which are well known for their interesting mechanical and electronic properties. For CN2 a large bulk modulus B0 of 405 GPa was determined. While SiN2 is found to be a wide band gap compound, the calculated gaps of CN2 and GeN2 are considerably smaller. The trends in structural and electronic properties, as e.g., bond lengths, band gaps and covalency are well understood in terms of the interplay of different types of bonding.

A model study for the breaking of N2 from CNx within DFT

Abstract A hypothetical CN2 structure was investigated as a model to study the release of N2 from the octahedral hole of 3D carbon based ultra hard compounds, which is the most important drawback in the attempts to synthesize ultra hard compounds like C3N4 and C11N4. Full structure relaxations using DFT methods led to a structure at the energy minimum showing a significantly enlarged NN distance of 1.34 A compared to the molecular N2 (1.09 A). While for small volume changes a high hardness for CN2 of 405 GPa is calculated, we found that enlargements of the cell constant lead to the release of N2 that could be followed calculating the ELF and the charge transfer within the AIM theory. The whole procedure simulates an inverted “harpoon mechanism”.

Inorganic Double Helices in Semiconducting SnIP

SnIP is the first atomic-scale double helical semiconductor featuring a 1.86 eV bandgap, high structural and mechanical flexibility, and reasonable thermal stability up to 600 K. It is accessible on a gram scale and consists of a racemic mixture of right- and left-handed double helices composed by [SnI] and [P] helices. SnIP nanorods <20 nm in diameter can be accessed mechanically and chemically within minutes.

Van der Waals interactions in selected allotropes of phosphorus

Abstract Selected allotropes of phosphorus are investigated by different levels of density functional theory (DFT) calculations to evaluate the relative stability orders with a special focus on the role of van der Waals interactions. Phosphorus is an excellent reference system with a large number of allotropes. Starting from low-dimensional molecular (0D, white P) and polymer structures (1D, P nanorods) to layered (2D, black P) and tubular structures (2D and 3D, crystalline forms of red P), covalent structure motifs are interconnected by van der Waals interactions. They are a key factor for the correct energetic description of all P allotropes. A comparative study is carried out within the local density approximation (LDA) and the generalized gradient approximation (GGA), with and without implementation of a dispersion correction by Grimme (GGA-D2). Our intention is to achieve a reasonable agreement of our calculations with experimental data, the plausibility of energy values, and the treatment of long-range interactions. The effect of van der Waals interactions is exemplified for the interlayer distances of black phosphorous and its electronic structure.

First principles calculations on structure, bonding, and vibrational frequencies of SiP2

Pyrite type SiP(2) is reinvestigated by first principles calculations on various levels of functionals including local density approximation, generalized gradient approximation, Becke-Lee-Yang-Parr hybrid functional, and the Hartree-Fock method. SiP(2) is seen as a model compound with molecular [P-P] entities and [SiP(6)] octahedra. Structure and bonding are addressed by electronic structure calculations. Special attention is spent on P-P and Si-P bonds in terms of bond lengths and respective stretching modes from simulated Raman spectra. The electronic structure is analyzed in both direct and momentum space by the electron localization function and site projected density of states. The main goals of this work are to understand the nature of chemical bonding in SiP(2) and to compare and contrast the different methods of calculation.