Daniel B. Straus
I am a Postdoctoral Research Associate at Princeton University in the Cava lab. My research focuses on the synthesis and characterization of optical, electronic, and magnetic materials.
I completed my PhD in Chemistry at the University of Pennsylvania in the Kagan lab, where I focused on the synthesis and optical/electronic spectroscopy of 2D organic-inorganic hybrid perovskites and coupled quantum dot assemblies.
Now out in Chemistry of Materials: We show that CsEuBr3 and CsEuCl3 order antiferromagnetically in the bulk, in contrast to the ferromagnetism previously found for thin films and nanocrystals. This difference suggests that these materials can have their magnetic behavior tuned based on form factor.
Selected Recent Publications
Bulk CsEuBr3 and CsEuCl3 are experimentally shown to be magnetic semiconductors that order antiferromagnetically at Néel temperatures of 2.0 K and 1.0 K, respectively. Given that nanoparticles and thin films of CsEuCl3 have been reported to order ferromagnetically at a similar temperature, our observation of antiferromagnetic ordering in CsEuBr3 and CsEuCl3 expands the possible applications of halide perovskites to now include spintronic devices where both ferromagnetic and antiferromagnetic devices can be fabricated from a single material. The conclusion that CsEuCl3 can be used as a switchable magnetic material is also supported by our density-functional theory calculations.
The ability to continuously tune the band gap of a semiconductor allows its optical properties to be precisely tailored for specific applications. We demonstrate that the band gap of the halide perovskite CsPbBr3 can be continuously widened through homovalent substitution of Sr2+ for Pb2+ using solid-state synthesis, creating a material with the formula CsPb1-xSrxBr3 (0 ≤ x ≤ 1). Sr2+ and Pb2+ form a solid solution in CsPb1-xSrxBr3. Pure CsPbBr3 has a band gap of 2.29(2) eV, which increases to 2.64(3) eV for CsPb0.25Sr0.75Br3. The increase in band gap is clearly visible in the color change of the materials and is also confirmed by a shift in the photoluminescence. Density-functional theory calculations support the hypothesis that Sr incorporation widens the band gap without introducing mid-gap defect states. These results demonstrate that homovalent B-site alloying can be a viable method to tune the band gap of simple halide perovskites for absorptive and emissive applications such as color-tunable light-emitting diodes, tandem solar cells, and photodetectors.
We synthesize and characterize the magnetic and thermodynamic properties of the quasi one-dimensional organic–inorganic hybrid ANiCl3 compounds [A = N(CH3)4+, CH3NH3+, (CH3)2NH2+, C(NH2)3+, and CH(NH2)2+]. Additionally, the crystal structure of (CH3)2NH2NiCl3 is reported. These materials possess chains of face-sharing NiCl6 octahedra in a triangular array. The chains run in one direction and are separated from one another by organic cations of different sizes and geometries. In accordance with the 90° superexchange model, we find that the nature of the magnetic coupling within chains can be predicted by the value of the Ni–Cl–Ni angle. As the Ni–Cl–Ni angle decreases from 90°, the magnetic interactions become increasingly antiferromagnetic. These findings provide a foundation for predicting the magnetic properties of quasi one-dimensional organic–inorganic hybrid ANiCl3 compounds.
The design of new chiral materials usually requires stereoselective organic synthesis to create molecules with chiral centers. Less commonly, achiral molecules can self-assemble into chiral materials, despite the absence of intrinsic molecular chirality. Here, we demonstrate the assembly of high-symmetry molecules into a chiral van der Waals structure by synthesizing crystals of C60(SnI4)2 from icosahedral buckminsterfullerene (C60) and tetrahedral SnI4 molecules through spontaneous self-assembly. The SnI4 tetrahedra template the Sn atoms into a chiral cubic three-connected net of the SrSi2 type. Our results represent the remarkable emergence of a self-assembled chiral material from two of the most highly symmetric molecules, demonstrating that almost any molecular, nanocrystalline, or engineered precursor can be considered when designing chiral assemblies.
Despite the tremendous interest in halide perovskite solar cells, the structural reasons that cause the all-inorganic perovskite CsPbI3 to be unstable at room temperature remain mysterious, especially since many tolerance-factor-based approaches predict CsPbI3 should be stable as a perovskite. Here single-crystal X-ray diffraction and X-ray pair distribution function (PDF) measurements characterize bulk perovskite CsPbI3 from 100 to 295 K to elucidate its thermodynamic instability. While Cs occupies a single site from 100 to 150 K, it splits between two sites from 175 to 295 K with the second site having a lower effective coordination number, which, along with other structural parameters, suggests that Cs rattles in its coordination polyhedron. PDF measurements reveal that on the length scale of the unit cell, the PbI octahedra concurrently become greatly distorted, with one of the IPbI angles approaching 82° compared to the ideal 90°. The rattling of Cs, low number of CsI contacts, and high degree of octahedral distortion cause the instability of perovskite-phase CsPbI3. These results reveal the limitations of tolerance factors in predicting perovskite stability and provide detailed structural information that suggests methods to engineer stable CsPbI3-based solar cells.