Daniel B. Straus

Headshot of me

I am a Postdoctoral Research Associate at Princeton University in the Cava lab. My research focuses on the synthesis and properties of materials for optical, electronic, and magnetic applications.

I completed my PhD in Chemistry at the University of Pennsylvania, where I focused on optical and electronic spectroscopy of 2D organic-inorganic hybrid perovskites and coupled quantum dot assemblies.

News

Our new preprint on tuning the band gap of the halide perovskite CsSrBr3 is now on ChemRxiv: 10.26434/chemrxiv-2022-bctjk-v2.

My blog post about being a parent scientist was published on the NYAS website: https://www.nyas.org/news-articles/academy-news/i-am-a-postdoc-scientist-and-i-am-a-parent/

My artwork was featured on the cover of JACS Volume 142 Issue 33

Cover of JACS Volume 142 Issue 33

Recent Publications

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. 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 is a viable method to tune the band gap of halide perovskites without introducing compensating defects that form when B-site cations of a different charge are introduced.

Schematic of 1D chains of lead bromide square pyramids highlighting asymmetric electron density

In lead(II) halide compounds including virtually all lead halide perovskites, the Pb2+ 6s lone pair results in distorted octahedra, in accordance with the pseudo-Jahn–Teller effect, rather than generating hemihedral coordination polyhedra. Here, in contrast, we report the characterization of an organic–inorganic hybrid material consisting of one-dimensional edge-sharing chains of Pb–Br square pyramids, separated by [Mn(DMF)6]2+ (DMF = dimethylformamide) octahedra. Molecular orbital analysis and density-functional theory calculations indicate that square pyramidal coordination about Pb2+ results from the occupancy of the empty ligand site by a Pb2+ lone pair that has both s and p orbital character rather than the exclusively 6s lone pair. These results demonstrate that a Pb2+ lone pair can be exploited to behave like a ligand in lead halide compounds, greatly expanding the realm of possible lead halide materials to include extended solids with nonoctahedral coordination environments.

X-ray diffraction pattern showing change in lattice parameter as C70 content increases

We describe the spontaneous chiral self-assembly of C70 with SnI4 as well as a mixture of C60 and C70 with SnI4. Macroscopic single crystals with the formula (C70)x(C60)1–x(SnI4)2 (x = 0–1) are reported. C60, which is spherical, and C70, which is ellipsoidal, form a solid solution in these crystals, and the cubic lattice parameter of the chiral phase linearly increases as x grows from 0 to 1 in accordance with Vegard’s law. Our results demonstrate that nonspherical particles and polydispersity are not an impediment to the growth of chiral crystals from high-symmetry achiral precursors, providing a route to assemble achiral particles including colloidal nanocrystals and engineered nanostructures into chiral materials without the need to use external templates or forces.

SnI4 and C60 self-assemble into a chiral material

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.

Depiction of crystal structure of CsPbI3

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.