College of Chemistry

Nitrogen core-doping of graphene nanoribbons (GNRs) allows trigonal planar carbon atoms along the backbone of GNRs to be substituted by higher-valency nitrogen atoms. The excess valence electrons are injected into the π-orbital system of the GNR, thereby changing not only its electronic occupation but also its topological properties. We have observed this topological change by synthesizing dilute nitrogen core-doped armchair GNRs with a width of five atoms (N2-5-AGNRs). The incorporation of pairs of trigonal planar nitrogen atoms results in the emergence of topological boundary states at the interface between doped and undoped segments of the GNR. These topological boundary states are offset in energy by approximately ΔE = 300 meV relative to the topological end states at the termini of finite 5-AGNRs. Scanning tunneling microscopy (STM) and spectroscopy (STS) reveal that for finite GNRs the two types of topological states can interact through a linear combination of orbitals, resulting in a pair of asymmetric hybridized states. This behavior is captured by an effective Hamiltonian of nondegenerate diatomic molecules, where the analogous interatomic hybridization interaction strength is tuned by the distance between GNR topological modes.

Developing active, stable, and cost-effective acidic oxygen evolution reaction (OER) catalyst is a critical challenge in realizing large-scale hydrogen (H₂) production via electrochemical water splitting. Utilizing highly active and relatively inexpensive Ru is generally challenged by its long-term durability issue. Here, we explore the potential of stabilizing active Ru sites in Ru_x(Ir,Fe,Co,Ni)_1-x multicomponent alloy by investigating its phase formation behavior, OER performance, and OER-induced surface reconstruction. The alloy exhibited a multiphase structure composed of major face-centered cubic (fcc) and minor hexagonal close-packed (hcp) phases at near equimolar concentration. Machine-learned interatomic potential (MLIP) coupled with replica-exchange molecular dynamics was utilized to describe the atomic scale mixing behavior of the Ru_x(Ir,Fe,Co,Ni)_1-x catalysts and other RuIr-based alloys. The model supports our experimental findings of the well-mixed bulk fcc phase and provides an indication of the minor hcp phase formation. The optimized Ru_0.20(Ir,Fe,Co,Ni)_0.80 catalyst exhibited improved OER activity with an average overpotential of ∼237 mV measured at 10 mA cm^-2 and enhanced stability with a low activity degradation rate of ∼1.1 mV h^-1 in 24 h of operation. The acidic OER conditions induced the formation of a thin RuIr-rich oxide shell layer with a trace amount of 3d metals, where Ru was found to be relatively stabilized near the surface of the evolved nanoparticles. The machine learning-accelerated high throughput simulation protocol was further employed to screen other potential RuIr-containing quinary alloys based on expected phase stability. This work highlights the opportunity of stabilizing Ru in a multicomponent alloy matrix with improved activity and stability.

Metal halide perovskites have excellent optoelectronic properties. This study aims to determine how the optoelectronic properties of a model perovskite, cesium lead bromide (CsPbBr₃), change with length and thickness in one dimension (1D). By examining the photophysics of CsPbBr₃ quantum dots (QDs), nanowires (NWs), and nanorods (NRs), we observe the influence of confinement, exciton diffusion, and trapping on their optical properties. Our findings reveal that exciton diffusion to trap states limits the photoluminescence quantum yield (PLQY) of 1D CsPbBr₃ in the weakly confined regime (8-14 nm) and explains their long-lived exciton dynamics, while enhanced radiative rates contribute to achieving near-unity PLQY in the strongly confined regime (<7 nm). Consequently, blue-emitting, 2.4 nm-thick CsPbBr₃ NRs were 3.6X more emissive than the conventional CsPbBr₃ QDs. This study underscores how structural optimization can improve the optoelectronic performance of CsPbBr₃ and provides insight into the complex interplay of radiative and nonradiative processes in 1D ionic semiconductors.

We use path integral Monte Carlo to study the energetics of excitons in layered, hybrid organic-inorganic perovskites in order to elucidate the relative contributions of dielectric confinement and electron-phonon coupling. While the dielectric mismatch between polar perovskite layers and nonpolar ligand layers significantly increases the exciton binding energy relative to their three-dimensional bulk crystal counterparts, formation of exciton polarons attenuates this effect. The contribution from polaron formation is found to be a nonmonotonic function of the lead halide layer thickness, which is clarified by a general variational theory. Accounting for both of these effects provides a description of exciton binding energies in good agreement with experimental measurements. By studying isolated layers and stacked layered crystals of various thicknesses, with ligands of varying polarity, we provide a systematic understanding of the excitonic behavior of this class of materials and how to engineer their photophysics.

The efficient removal of CO₂ from exhaust streams and even directly from air is necessary to forestall climate change, lending urgency to the search for new materials that can rapidly capture CO₂ at high capacity. The recent discovery that diamine-appended metal-organic frameworks can exhibit cooperative CO₂ uptake via the formation of ammonium carbamate chains begs the question of whether simple organic polyamine molecules could be designed to achieve a similar switch-like behavior with even higher separation capacities. Here, we present a solid molecular triamine, 1,3,5-tris(aminomethyl)benzene (TriH), that rapidly captures large quantities of CO₂ upon exposure to humid air to form the porous, crystalline, ammonium carbamate network solid TriH(CO₂)_1.5·xH₂O (TriHCO₂). The phase transition behavior of TriH converting to TriHCO₂ was studied through powder and single-crystal X-ray diffraction analysis, and additional spectroscopic techniques further verified the formation of ammonium carbamate species upon exposing TriH to humid air. Detailed breakthrough analyses conducted under varying temperatures, relative humidities, and flow rates reveal record CO₂ absorption capacities as high as 8.9 mmol/g. Computational analyses reveal an activation barrier associated with TriH absorbing CO₂ under dry conditions that is lowered under humid conditions through hydrogen bonding with a water molecule in the transition state associated with N-C bond formation. These results highlight the prospect of tunable molecular polyamines as a new class of candidate absorbents for high-capacity CO₂ capture.

Photosystem II (PSII) can achieve near-unity quantum efficiency of light harvesting in ideal conditions and can dissipate excess light energy as heat to prevent the formation of reactive oxygen species (ROS) under light stress. Understanding how this pigment-protein complex accomplishes these opposing goals is a topic of great interest that has so far been explored primarily through the lens of the system energetics. Despite PSIIs known flat energy landscape, a thorough consideration of the entropic effects on energy transfer in PSII is lacking. In this work, we aim to discern the free energetic design principles underlying the PSII energy transfer network. To accomplish this goal, we employ a structure-based rate matrix and compute the free energy terms in time following a specific initial excitation to discern how entropy and enthalpy drive ensemble system dynamics. We find that the interplay between the entropy and enthalpy components differ among each protein subunit, which allows each subunit to fulfill a unique role in the energy transfer network. This individuality ensures that PSII can accomplish efficient energy trapping in the reaction center (RC), effective nonphotochemical quenching (NPQ) in the periphery, and robust energy trapping in the other-monomer RC if the same-monomer RC is closed. We also show that entropy, in particular, is a dynamically tunable feature of the PSII free energy landscape accomplished through regulation of LHCII binding. These findings help rationalize natural photosynthesis and provide design principles for more efficient solar energy harvesting technologies.

Despite decades of emission control measures aimed at improving air quality, Los Angeles (LA) continues to experience severe ozone pollution during the summertime. We incorporate cooking volatile organic compound (VOC) emissions in a chemical transport model and evaluate it against observations in order to improve the model representation of the present-day ozone chemical regime in LA. Using this updated model, we investigate the impact of adopting zero-emission vehicles (ZEVs) on ozone pollution with increased confidence. We show that mitigating on-road gasoline emissions through ZEV adoption would benefit both air quality and climate by substantially reducing anthropogenic nitrogen oxides (NOx) and carbon dioxide (CO2) emissions in LA by 28 and 41% during the summertime, respectively. This would result in a moderate reduction of O3 pollution, decreasing the average number of population-weighted O3 exceedance days in August from 9 to 6 days, and would shift the majority of LA, except for the coastline, into a NOx-limited regime. Our results also show that adopting ZEVs for on-road diesel and off-road vehicles would further reduce the number of O3 exceedance days in August to an average of 1 day.

Predicting reaction kinetics in aqueous microdroplets, including aerosols and cloud droplets, is challenging due to the probability that the underlying reaction mechanism can occur both at the surface and in the interior of the droplet. Additionally, few studies directly measure the surface activities of doubly charged anions, despite their prevalence in the atmosphere. Here, deep-UV second harmonic generation spectroscopy is used to probe surface affinities of the doubly charged anions thiosulfate, sulfate, and sulfite, key species in the thiosulfate ozonation reaction mechanism. Thiosulfate has an appreciable surface affinity with a measured Gibbs free energy of adsorption of -7.3 ± 2.5 kJ mol-1 in neutral solution, while sulfate and sulfite exhibit negligible surface propensity. The Gibbs free energy is combined with data from liquid flat jet ambient pressure X-ray photoelectron spectroscopy to constrain the concentration of thiosulfate at the surface in our model. Stochastic kinetic simulations leveraging these novel measurements show that the primary reaction between thiosulfate and ozone occurs at the interface and in the bulk, with the contribution of the interface decreasing from ∼65% at pH 5 to ∼45% at pH 13. Additionally, sulfate, the major product of thiosulfate ozonation and an important species in atmospheric processes, can be produced by two different pathways at pH 5, one with a contribution from the interface of >70% and the other occurring predominantly in the bulk (>98%). The observations in this work have implications for mining wastewater remediation, atmospheric chemistry, and understanding other complex reaction mechanisms in multiphase environments. Future interfacial or microdroplet/aerosol chemistry studies should carefully consider the role of both surface and bulk chemistry.

We present an approach for augmenting Gaussian atomic orbitals with correct nuclear cusps. Like the atomic orbital basis set itself and unlike previous cusp corrections, this approach is independent of the many-body method used to prepare wave functions for quantum Monte Carlo. Once the basis set and molecular geometry are specified, the cusp-corrected atomic orbitals are uniquely specified, regardless of which density functionals, quantum chemistry methods, or subsequent variational Monte Carlo optimizations are employed. We analyze the statistical improvement offered by these cusps in a number of molecules and find them to offer similar advantages as molecular-orbital-based approaches while remaining independent of the choice of many-body method.