Matthew C. Beard

The combination of semiconducting properties and synthetically tunable chirality in chiral metal halide semiconductors (CMHSs) offers a compelling platform for room temperature control of electronic spin properties, leveraging effects such as chirality-induced spin selectivity (CISS). We report CISS-induced magnetoresistance (CISS-MR) exceeding 100% for spin valves operating at room temperature. The high CISS-MR is attributed to an interfacial tunneling barrier, modulated by chirality and spin, producing current dissymmetry factors ( g _c ) that surpass the limit imposed by the Julliere model, which is governed by the intrinsic spin polarization of the adjacent ferromagnetic (FM) contact. The CISS-MR exhibits a dependence on the CMHS composition, revealing structure-property relationships between CISS and structural chirality. The observed exceptionally large tunneling MR response differs from a subtle anisotropic MR arising from the proximity effect at the FM/CMHS interface in the absence of a tunneling barrier. Our study provides insights into charge-to-spin interconversion in chiral semiconductors, offering design principles to control and enhance the CISS response for functional platforms.

The interplay between carrier density regimes and the various scattering processes is fundamental for understanding charge transport efficiency in high-performance semiconductors. Combining ultrafast time-resolved terahertz spectroscopy (TRTS) and transient microwave photoconductivity (TRMC), we measure carrier mobilities over 7 orders of magnitude (1013–1020 carriers cm–3), reaching a regime where carrier–carrier scattering dominates and mobility begins to roll off. We find the Caughey–Thomas model can be effectively employed for transport modeling in 3D metal halide perovskites (MHPs), and the mobility roll-off near ∼1019 cm–3 can be tuned by MHP composition. Carrier lifetime in MHPs starts decreasing at much lower carrier densities due to interband multi-carrier recombination, while intraband carrier mobility remains unchanged as carrier–carrier scattering is screened by the presence of polarons. The validated Caughey–Thomas model provides a practical input for device simulations, prevents overestimation of diffusion length under high injection, and suggests that MHPs are excellent candidates for unipolar device applications under high carrier densities.

The directed movement of spin-bearing excitations in semiconductors can enable many potential schemes for spintronic applications. Understanding the mechanism of spin motion, as opposed to charge carrier or exciton motion, may require specialized techniques and sample conditions. Here we employ the noncontact and time-resolved technique of light-induced transient grating spectroscopy (LITG) to measure both carrier and spin motion in a perovskite semiconductor with varying crystallinity. The carrier motion aligns with expectations from past studies undertaken at low fluence, revealing an ambipolar diffusion coefficient on the order of 1 cm²/sec and diffusion length of roughly 1.5 μm for single crystals that is strongly attenuated as crystallite size decreases. The spin diffusion is measured with cross-polarized excitation beams and uncovers an intensity-dependent coefficient rising above 100 cm²/sec. The fast room-temperature spin relaxation limits the spin diffusion length, but the remarkable speedup of spin over carrier diffusion suggests a mechanism involving a combination of exchange-mediated and doping effects that enhance spin transport.

Terminating semiconductor nanocrystals with chiral organic ligands can induce chiroptical properties that combine the chiral character of the ligands with the strong and tunable optical properties of the nanocrystal. However, the synthesis of such chiral-modified nanocrystals is currently limited by solvent incompatibility between polar chiral ligands and nonpolar traditional ligands, as well as by ligand dissolution behavior that ultimately compromises nanocrystal quality and the degree to which chiroptical properties can be manipulated. In this work, we demonstrate a single-step synthesis of chiral cadmium selenide (CdSe) nanoplatelets (NPLs) that eliminates the need for postsynthetic ligand exchange in aqueous solvents, resulting in highly emissive chiral CdSe NPLs. By directly incorporating chiral aminodecanoic acid ligands during synthesis, we achieve in situ ligand binding and chirality transfer to CdSe NPLs. This approach produces CdSe NPLs with high photoluminescence quantum efficiencies of 50% and circular dichroism dissymmetry factors (g_CD) on the order of 10^–4.

Room-temperature control of spin, coupled with charge and light, remains a long-standing challenge for next-generation spin-optoelectronic applications. Although inversion symmetry breaking offers one path for spin control in semiconductors, strong spin dephasing at higher temperatures remains a persistent challenge. While hidden spin polarization (HSP) without global symmetry breaking provides another promising material design strategy, robust spin stabilization from HSP is yet to be experimentally realized. Here, we demonstrate that 2D hybrid organic-inorganic perovskites can provide spin stabilization at room temperature via hidden spin valley locking. We utilized functional nonprimary ammonium cations (NPACs) that induce giant spin splitting in 2D layers with varying spin-orbit coupling (employing Sn vs Pb) and enhanced dielectric screening, thereby restricting spin relaxation processes. Time-resolved experiments demonstrate optically generated spin-polarized carriers that persist for over four nanoseconds at room temperature without requiring an external magnetic field. Our design paradigm unlocks inversion-symmetric semiconductors with ultralong spin lifetimes, broadening the materials options for light-driven spin control.