Direct Band Gap Semiconductors with Two- and Three-Dimensional Triel-Phosphide Frameworks (Triel=Al, Ga, In).

Recently, several ternary phosphidotrielates and -tetrelates have been investigated with respect to their very good ionic conductivity, while less focus was pointed towards their electronic structures. Here, we report on a novel series of compounds, in which several members possess direct band gaps. We investigated the known compounds Li3AlP2, Li3GaP2, Li3InP2, and Na3InP2 and describe the synthesis and the crystal structure of novel Na3In2P3. For all mentioned phosphidotrielates reflectance UV-Vis measurements reveal direct band gaps in the visible light region with decreasing band gaps in the series: Li3AlP2 (2.45 eV), Li3GaP2 (2.18 eV), Li3InP2 (1.99 eV), Na3InP2 (1.37 eV), and Na3In2P3 (1.27 eV). All direct band gaps are confirmed by quantum chemical calculations. The unexpected property occurs despite different structure types. As a common feature all compounds contain EP4 tetrahedra, which share exclusively vertices for E=In and vertices as well as edges for E=Al and Ga. The structure of the novel Na3In2P3 is built up by a polyanionic framework of six-membered rings of corner-sharing InP4 tetrahedra. As a result, the newly designed semiconductors with direct band gaps are suitable for optoelectronic applications, and they can provide significant guidance for the design of new functional semiconductors.

Lithium-ion Mobility in Li6 B18 (Li3 N) and Li Vacancy Tuning in the Solid Solution Li6 B18 (Li3 N)1-x (Li2 O)x.

All-solid-state batteries are promising candidates for safe energy-storage systems due to non-flammable solid electrolytes and the possibility to use metallic lithium as an anode. Thus, there is a challenge to design new solid electrolytes and to understand the principles of ion conduction on an atomic scale. We report on a new concept for compounds with high lithium ion mobility based on a rigid open-framework boron structure. The host-guest structure Li6 B18 (Li3 N) comprises large hexagonal pores filled with ∞ 1 [ ${{}_{{\rm { \infty }}}{}^{{\rm { 1}}}{\rm { [}}}$ Li7 N] strands that represent a perfect cutout from the structure of α-Li3 N. Variable-temperature 7 Li NMR spectroscopy reveals a very high Li mobility in the template phase with a remarkably low activation energy below 19 kJ mol-1 and thus much lower than pristine Li3 N. The formation of the solid solution of Li6 B18 (Li3 N) and Li6 B18 (Li2 O) over the complete compositional range allows the tuning of lithium defects in the template structure that is not possible for pristine Li3 N and Li2 O.

Synthesis, Structure, Solid-State NMR Spectroscopy, and Electronic Structures of the Phosphidotrielates Li3 AlP2 and Li3 GaP2.

The lithium phosphidoaluminate Li9 AlP4 represents a promising new compound with a high lithium ion mobility. This triggered the search for new members in the family of lithium phosphidotrielates, and the novel compounds Li3 AlP2 and Li3 GaP2 , obtained directly from the elements via ball milling and subsequent annealing, are reported here. It was unexpectedly found through band structure calculations that Li3 AlP2 and Li3 GaP2 are direct band gap semiconductors with band gaps of 3.1 and 2.8 eV, respectively. Rietveld analyses reveal that both compounds crystallize isotypically in the orthorhombic space group Cmce (no. 64) with lattice parameters of a=11.5138(2), b=11.7634(2) and c=5.8202(1) Šfor Li3 AlP2 , and a=11.5839(2), b=11.7809(2) and c=5.8129(2) Šfor Li3 GaP2 . The crystal structures feature TrP4 (Tr=Al, Ga) corner- and edge-sharing tetrahedra, forming two-dimensional ∞ 2 T r P 2 3 - layers. The lithium atoms are located between and inside these layers. The crystal structures were confirmed by MAS-NMR spectroscopy.

Na7TaP4: A Ternary Sodium Phosphidotantalate Containing [TaP4]7- Tetrahedra.

While lithium phosphides have been investigated intensively, very little is known about the corresponding sodium-based phosphides. Here, we report on the first ternary Na-Ta-P compound Na7TaP4, which is easily accessible via ball milling of the elements and subsequent annealing. The single crystal X-ray structure determination [monoclinic symmetry; space group P21/c; and lattice parameters a = 11.5604(4), b = 8.1530(3), c = 11.5450(5) Å, and ß = 101.602(3)°] reveals [TaP4]7- tetrahedra, which are surrounded by Na+ counterions. Na7TaP4 crystallizes in a new structure type. The structure can be described as a strongly distorted hexagonal close packing of P atoms, in which the Ta atoms are located in tetrahedral voids, and Na atoms occupy all octahedral voids and additionally 3/8 of the tetrahedral voids. The possibility to increase the ion conductivity by changing the number of charge carriers through aliovalent substitution in compounds containing [SiP4]8- and [AlP4]9- is considered. The 31P and 23Na MAS NMR as well as the Raman spectra are in accordance with the structure model, and band structure calculations predict a direct band gap of 2.9 eV.

Fast Lithium Ion Conduction in Lithium Phosphidoaluminates.

Solid electrolyte materials are crucial for the development of high-energy-density all-solid-state batteries (ASSB) using a nonflammable electrolyte. In order to retain a low lithium-ion transfer resistance, fast lithium ion conducting solid electrolytes are required. We report on the novel superionic conductor Li9 AlP4 which is easily synthesised from the elements via ball-milling and subsequent annealing at moderate temperatures and which is characterized by single-crystal and powder X-ray diffraction. This representative of the novel compound class of lithium phosphidoaluminates has, as an undoped material, a remarkable fast ionic conductivity of 3 mS cm-1 and a low activation energy of 29 kJ mol-1 as determined by impedance spectroscopy. Temperature-dependent 7 Li NMR spectroscopy supports the fast lithium motion. In addition, Li9 AlP4 combines a very high lithium content with a very low theoretical density of 1.703 g cm-3 . The distribution of the Li atoms over the diverse crystallographic positions between the [AlP4 ]9- tetrahedra is analyzed by means of DFT calculations.

Surface-Anisotropic Janus Silicon Quantum Dots via Masking on 2D Silicon Nanosheets.

Surface-anisotropic nanoparticles represent a new class of materials that shows potential in a variety of applications, including self-assembly, microelectronics, and biology. Here, the first synthesis of surface-anisotropic silicon quantum dots (SiQDs), obtained through masking on 2D silicon nanosheets, is presented. SiQDs are deposited on the 2D substrate, thereby exposing only one side of the QDs, which is functionalized through well-established hydrosilylation procedures. The UV-sensitive masking substrate is removed through UV-irradiation, which simultaneously initiates the hydrosilylation of a second substrate, thereby introducing a second functional group to the other side of the now free-standing SiQDs. This renders surface-anisotropic SiQDs that have two different functional groups on either side of the particle. This method can be used to introduce a variety of functional groups including hydrophilic and hydrophobic substrates, while the unique optoelectronic properties of the SiQDs remain unaffected. The anisotropic morphology of the QDs is confirmed through the aggregation behavior of amphiphilic Janus SiQDs at the interface of water and hexane. Additionally, anisotropic SiQDs are used to produce the first controlled (sub)monolayer of SiQDs on a gold wafer.

Supertetrahedral polyanionic network in the first lithium phosphidoindate Li3InP2 - structural similarity to Li2SiP2 and Li2GeP2 and dissimilarity to Li3AlP2 and Li3GaP2.

Phosphide-based materials have been investigated as promising candidates for solid electrolytes, among which the recently reported Li9AlP4 displays an ionic conductivity of 3 mS cm-1. While the phases Li-Al-P and Li-Ga-P have already been investigated, no ternary indium-based phosphide has been reported up to now. Here, we describe the synthesis and characterization of the first lithium phosphidoindate Li3InP2, which is easily accessible via ball milling of the elements and subsequent annealing. Li3InP2 crystallizes in the tetragonal space group I41/acd with lattice parameters of a = 12.0007(2) and c = 23.917(5) Å, featuring a supertetrahedral polyanionic framework of interconnected InP4 tetrahedra. All lithium atoms occupy tetrahedral voids with no partial occupation. Remarkably, Li3InP2 is not isotypic to the previously reported homologues Li3AlP2 and Li3GaP2, which both crystallize in the space group Cmce and feature 2D layers of connected tetrahedra but no supertetrahedral framework. DFT computations support the observed stability of Li3InP2. A detailed geometrical analysis leads to a more general insight into the structural factors governing lithium ion mobility in phosphide-based materials: in the non-ionic conducting Li3InP2 the Li ions exclusively occupy tetrahedral voids in the distorted close packing of P atoms, whereas partially filled octahedral voids are present in the moderate ionic conductors Li2SiP2 and Li2GeP2.