Activating Nitrogen for Electrochemical Ammonia Synthesis via an Electrified Transition-Metal Dichalcogenide Catalyst.

The complex interplay between local chemistry, the solvent microenvironment, and electrified interfaces frequently present in electrocatalytic reactions has motivated the development of quantum chemical methods that can accurately model these effects. Here, we predict the thermodynamics of the nitrogen reduction reaction (NRR) at sulfur vacancies in 1T'-phase MoS2 and highlight how the realistic treatment of potential within grand canonical density functional theory (GC-DFT) seamlessly captures the multiple competing effects of applied potential on a catalyst interface interacting with solvated molecules. In the canonical approach, the computational hydrogen electrode is widely used and predicts that adsorbed N2 structure properties are potential-independent. In contrast, GC-DFT calculations show that reductive potentials activate N2 toward electroreduction by controlling its back-bonding strength and lengthening the N-N triple bond while decreasing its bond order. Similar trends are observed for another classic back-bonding ligand in CO, suggesting that this mechanism may be broadly relevant to other electrochemistries involving back-bonded adsorbates. Furthermore, reductive potentials are required to make the subsequent N2 hydrogenation steps favorable but simultaneously destabilizes the N2 adsorbed structure resulting in a trade-off between the favorability of N2 adsorption and the subsequent reaction steps. We show that GC-DFT facilitates modeling all these phenomena and that together they can have important implications in predicting electrocatalyst selectivity for the NRR and potentially other reactions.

Controlling Exciton/Exciton Recombination in 2-D Perovskite Using Exciton-Polariton Coupling.

In this paper, we demonstrate that exciton/exciton annihilation in the 2D perovskite (PEA)2PbI4 (PEPI)─a major loss mechanism in solar cells and light-emitting diodes, can be controlled through coupling of excitons with cavity polaritons. We study the excited state dynamics using time-resolved transient absorption spectroscopy and show that the system can be tuned through a strong coupling regime by varying the cavity width through the PEPI layer thickness. Remarkably, strong coupling occurs even when the cavity quality factor remains poor, providing easy optical access. We demonstrate that the observed derivative-like transient absorption spectra can be modeled using a time-dependent Rabi splitting that occurs because of transient bleaching of the excitonic states. When PEPI is strongly coupled to the cavity, the exciton/exciton annihilation rate is suppressed by 1 order of magnitude. A model that relies on the partly photonic character of polaritons explains the results as a function of detuning.

Control of quantum dot emission by colloidal plasmonic pyramids in a liquid crystal.

We study the plasmon-enhanced fluorescence of a single semiconducting quantum dot near the apex of a colloidal gold pyramid spatially localized by the elastic forces of the liquid crystal host. The gold pyramid particles were manipulated within the liquid crystal medium by laser tweezers, enabling the self-assembly of a semiconducting quantum dot dispersed in the medium near the apex of the gold pyramid, allowing us to probe the plasmon-exciton interactions. We demonstrate the effect of plasmon coupling on the fluorescence lifetime and the blinking properties of the quantum dot. Our results demonstrate that topological defects around colloidal particles in liquid crystal combined with laser tweezers provide a platform for plasmon exciton interaction studies and potentially could be extended to the scale of composite materials for nanophotonic applications.

Dynamic Tuning of a Thin Film Electrocatalyst by Tensile Strain.

We report the ability to tune the catalytic activities for the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) by applying mechanical stress on a highly n-type doped rutile TiO2 films. We demonstrate through operando electrochemical experiments that the low HER activity of TiO2 can reversibly approach those of the state-of-the-art non-precious metal catalysts when the TiO2 is under tensile strain. At 3% tensile strain, the HER overpotential required to generate a current density of 1 mA/cm2 shifts anodically by 260 mV to give an onset potential of 125 mV, representing a drastic reduction in the kinetic overpotential. A similar albeit smaller cathodic shift in the OER overpotential is observed when tensile strain is applied to TiO2. Results suggest that significant improvements in HER and OER activities with tensile strain are due to an increase in concentration of surface active sites and a decrease in kinetic and thermodynamics barriers along the reaction pathway(s). Our results highlight that strain applied to TiO2 by precisely controlled and incrementally increasing (i.e. dynamic) tensile stress is an effective tool for dynamically tuning the electrocatalytic properties of HER and OER electrocatalysts relative to their activities under static conditions.

Direct Measurements of Carrier Transport in Polycrystalline Methylammonium Lead Iodide Perovskite Films with Transient Grating Spectroscopy.

Hybrid organic-inorganic halide perovskites have been proposed in many optoelectronic applications, but critical to their increasing functionality and utility is understanding and controlling carrier transport. Here, we use light-induced transient grating spectroscopy to probe directly carrier transport in polycrystalline methylammonium lead iodide perovskite thin films using a weakly perturbative and noncontact method. The data reveal intrinsic diffusion characteristics of the charge carriers in the material and agree well with a simulated model of charge transport in which grain boundaries act as barriers to carrier movement.

Tuning and Switching a Plasmonic Quantum Dot "Sandwich" in a Nematic Line Defect.

We study the quantum-mechanical effects arising in a single semiconductor core/shell quantum dot (QD) controllably sandwiched between two plasmonic nanorods. Control over the position and the "sandwich" confinement structure is achieved by the use of a linear-trap liquid crystal (LC) line defect and laser tweezers that "push" the sandwich together. This arrangement allows for the study of exciton-plasmon interactions in a single structure, unaltered by ensemble effects or the complexity of dielectric interfaces. We demonstrate the effect of plasmonic confinement on the photon antibunching behavior of the QD and its luminescence lifetime. The QD behaves as a single emitter when nanorods are far away from the QD but shows possible multiexciton emission and a significantly decreased lifetime when tightly confined in a plasmonic "sandwich". These findings demonstrate that LC defects, combined with laser tweezers, enable a versatile platform to study plasmonic coupling phenomena in a nanoscale laboratory, where all elements can be arranged almost at will.

Silicon Photoelectrode Thermodynamics and Hydrogen Evolution Kinetics Measured by Intensity-Modulated High-Frequency Resistivity Impedance Spectroscopy.

We present an impedance technique based on light intensity-modulated high-frequency resistivity (IMHFR) that provides a new way to elucidate both the thermodynamics and kinetics in complex semiconductor photoelectrodes. We apply IMHFR to probe electrode interfacial energetics on oxide-modified semiconductor surfaces frequently used to improve the stability and efficiency of photoelectrochemical water splitting systems. Combined with current density-voltage measurements, the technique quantifies the overpotential for proton reduction relative to its thermodynamic potential in Si photocathodes coated with three oxides (SiOx, TiO2, and Al2O3) and a Pt catalyst. In pH 7 electrolyte, the flatband potentials of TiO2- and Al2O3-coated Si electrodes are negative relative to samples with native SiOx, indicating that SiOx is a better protective layer against oxidative electrochemical corrosion than ALD-deposited crystalline TiO2 or Al2O3. Adding a Pt catalyst to SiOx/Si minimizes proton reduction overpotential losses but at the expense of a reduction in available energy characterized by a more negative flatband potential relative to catalyst-free SiOx/Si.

Experimental demonstration of photon upconversion via cooperative energy pooling.

Photon upconversion is a fundamental interaction of light and matter that has applications in fields ranging from bioimaging to microfabrication. However, all photon upconversion methods demonstrated thus far involve challenging aspects, including requirements of high excitation intensities, degradation in ambient air, requirements of exotic materials or phases, or involvement of inherent energy loss processes. Here we experimentally demonstrate a mechanism of photon upconversion in a thin film, binary mixture of organic chromophores that provides a pathway to overcoming the aforementioned disadvantages. This singlet-based process, called Cooperative Energy Pooling (CEP), utilizes a sensitizer-acceptor design in which multiple photoexcited sensitizers resonantly and simultaneously transfer their energies to a higher-energy state on a single acceptor. Data from this proof-of-concept implementation is fit by a proposed model of the CEP process. Design guidelines are presented to facilitate further research and development of more optimized CEP systems.

Do grain boundaries dominate non-radiative recombination in CH3NH3PbI3 perovskite thin films?

Here, we examine grain boundaries (GBs) with respect to non-GB regions (grain surfaces (GSs) and grain interiors (GIs)) in high-quality micrometer-sized perovskite CH3NH3PbI3 (or MAPbI3) thin films using high-resolution confocal fluorescence-lifetime imaging microscopy in conjunction with kinetic modeling of charge-transport and recombination processes. We show that, contrary to previous studies, GBs in our perovskite MAPbI3 thin films do not lead to increased recombination but that recombination in these films happens primarily in the non-GB regions (i.e., GSs or GIs). We also find that GBs in these films are not transparent to photogenerated carriers, which is likely associated with a potential barrier at GBs. Even though GBs generally display lower luminescence intensities than GSs/GIs, the lifetimes at GBs are no worse than those at GSs/GIs, further suggesting that GBs do not dominate non-radiative recombination in MAPbI3 thin films.

Large polarization-dependent exciton optical Stark effect in lead iodide perovskites.

A strong interaction of a semiconductor with a below-bandgap laser pulse causes a blue-shift of the bandgap transition energy, known as the optical Stark effect. The energy shift persists only during the pulse duration with an instantaneous response time. The optical Stark effect has practical relevance for applications, including quantum information processing and communication, and passively mode-locked femtosecond lasers. Here we demonstrate that solution-processable lead-halide perovskites exhibit a large optical Stark effect that is easily resolved at room temperature resulting from the sharp excitonic feature near the bandedge. We also demonstrate that a polarized pump pulse selectively shifts one spin state producing a spin splitting of the degenerate excitonic states. Such selective spin manipulation is an important prerequisite for spintronic applications. Our result implies that such hybrid semiconductors may have great potential for optoelectronic applications beyond photovoltaics.

Revisiting the Valence and Conduction Band Size Dependence of PbS Quantum Dot Thin Films.

We use a high signal-to-noise X-ray photoelectron spectrum of bulk PbS, GW calculations, and a model assuming parabolic bands to unravel the various X-ray and ultraviolet photoelectron spectral features of bulk PbS as well as determine how to best analyze the valence band region of PbS quantum dot (QD) films. X-ray and ultraviolet photoelectron spectroscopy (XPS and UPS) are commonly used to probe the difference between the Fermi level and valence band maximum (VBM) for crystalline and thin-film semiconductors. However, we find that when the standard XPS/UPS analysis is used for PbS, the results are often unrealistic due to the low density of states at the VBM. Instead, a parabolic band model is used to determine the VBM for the PbS QD films, which is based on the bulk PbS experimental spectrum and bulk GW calculations. Our analysis highlights the breakdown of the Brillioun zone representation of the band diagram for large band gap, highly quantum confined PbS QDs. We have also determined that in 1,2-ethanedithiol-treated PbS QD films the Fermi level position is dependent on the QD size; specifically, the smallest band gap QD films have the Fermi level near the conduction band minimum and the Fermi level moves away from the conduction band for larger band gap PbS QD films. This change in the Fermi level within the QD band gap could be due to changes in the Pb:S ratio. In addition, we use inverse photoelectron spectroscopy to measure the conduction band region, which has similar challenges in the analysis of PbS QD films due to a low density of states near the conduction band minimum.

Plasmon-Exciton Interactions Probed Using Spatial Coentrapment of Nanoparticles by Topological Singularities.

We study plasmon-exciton interaction by using topological singularities to spatially confine, selectively deliver, cotrap and optically probe colloidal semiconductor and plasmonic nanoparticles. The interaction is monitored in a single quantum system in the bulk of a liquid crystal medium where nanoparticles are manipulated and nanoconfined far from dielectric interfaces using laser tweezers and topological configurations containing singularities. When quantum dot-in-a-rod particles are spatially colocated with a plasmonic gold nanoburst particle in a topological singularity core, its fluorescence increases because blinking is significantly suppressed and the radiative decay rate increases by nearly an order of magnitude owing to the Purcell effect. We argue that the blinking suppression is the result of the radiative rate change that mitigates Auger recombination and quantum dot ionization, consequently reducing nonradiative recombination. Our work demonstrates that topological singularities are an effective platform for studying and controlling plasmon-exciton interactions.

Carrier separation and transport in perovskite solar cells studied by nanometre-scale profiling of electrical potential.

Organometal-halide perovskite solar cells have greatly improved in just a few years to a power conversion efficiency exceeding 20%. This technology shows unprecedented promise for terawatt-scale deployment of solar energy because of its low-cost, solution-based processing and earth-abundant materials. We have studied charge separation and transport in perovskite solar cells-which are the fundamental mechanisms of device operation and critical factors for power output-by determining the junction structure across the device using the nanoelectrical characterization technique of Kelvin probe force microscopy. The distribution of electrical potential across both planar and porous devices demonstrates p-n junction structure at the TiO2/perovskite interfaces and minority-carrier diffusion/drift operation of the devices, rather than the operation mechanism of either an excitonic cell or a p-i-n structure. Combining the potential profiling results with solar cell performance parameters measured on optimized and thickened devices, we find that carrier mobility is a main factor that needs to be improved for further gains in efficiency of the perovskite solar cells.

A facile solvothermal growth of single crystal mixed halide perovskite CH3NH3Pb(Br(1-x)Cl(x))3.

We demonstrate a facile synthetic approach for preparing mixed halide perovskite (CH3NH3)Pb(Br1-xClx)3 single crystals by the solvothermal growth of stoichiometric PbBr2 and [(1 - y)CH3NH3Br + yCH3NH3Cl] DMF precursor solutions. The band gap of (CH3NH3)Pb(Br1-xClx)3 single crystals increased and the unit cell dimensions decreased with an increase in Cl content x, consistent with previous theoretical predictions. Interestingly, the Cl/Br ratio in the (CH3NH3)Pb(Br1-xClx)3 single crystals is larger than that of the precursor solution, suggesting an unusual crystal growth mechanism.

Precision printing and optical modeling of ultrathin SWCNT/C60 heterojunction solar cells.

Semiconducting single-walled carbon nanotubes (s-SWCNTs) are promising candidates as the active layer in photovoltaics (PV), particularly for niche applications where high infrared absorbance and/or semi-transparent solar cells are desirable. Most current fabrication strategies for SWCNT PV devices suffer from relatively high surface roughness and lack nanometer-scale deposition precision, both of which may hamper the reproducible production of ultrathin devices. Additionally, detailed optical models of SWCNT PV devices are lacking, due in part to a lack of well-defined optical constants for high-purity s-SWCNT thin films. Here, we present an optical model that accurately reconstructs the shape and magnitude of spectrally resolved external quantum efficiencies for ultrathin (7,5) s-SWCNT/C60 solar cells that are deposited by ultrasonic spraying. The ultrasonic spraying technique enables thickness tuning of the s-SWCNT layer with nanometer-scale precision, and consistently produces devices with low s-SWCNT film average surface roughness (Rq of <5 nm). Our optical model, based entirely on measured optical constants of each layer within the device stack, enables quantitative predictions of thickness-dependent relative photocurrent contributions of SWCNTs and C60 and enables estimates of the exciton diffusion lengths within each layer. These results establish routes towards rational performance improvements and scalable fabrication processes for ultra-thin SWCNT-based solar cells.

Energy pooling upconversion in organic molecular systems.

A combination of molecular quantum electrodynamics, perturbation theory, and ab initio calculations was used to create a computational methodology capable of estimating the rate of three-body singlet upconversion in organic molecular assemblies. The approach was applied to quantify the conditions under which such relaxation rates, known as energy pooling, become meaningful for two test systems, stilbene-fluorescein and hexabenzocoronene-oligothiophene. Both exhibit low intramolecular conversion, but intermolecular configurations exist in which pooling efficiency is at least 90% when placed in competition with more conventional relaxation pathways. For stilbene-fluorescein, the results are consistent with data generated in an earlier experimental investigation. Exercising these model systems facilitated the development of a set of design rules for the optimization of energy pooling.

Self-assembly and electrostriction of arrays and chains of hopfion particles in chiral liquid crystals.

Some of the most exotic condensed matter phases, such as twist grain boundary and blue phases in liquid crystals and Abrikosov phases in superconductors, contain arrays of topological defects in their ground state. Comprised of a triangular lattice of double-twist tubes of magnetization, the so-called 'A-phase' in chiral magnets is an example of a thermodynamically stable phase with topologically nontrivial solitonic field configurations referred to as two-dimensional skyrmions, or baby-skyrmions. Here we report that three-dimensional skyrmions in the form of double-twist tori called 'hopfions', or 'torons' when accompanied by additional self-compensating defects, self-assemble into periodic arrays and linear chains that exhibit electrostriction. In confined chiral nematic liquid crystals, this self-assembly is similar to that of liquid crystal colloids and originates from long-range elastic interactions between particle-like skyrmionic torus knots of molecular alignment field, which can be tuned from isotropic repulsive to weakly or highly anisotropic attractive by low-voltage electric fields.

Charge generation in PbS quantum dot solar cells characterized by temperature-dependent steady-state photoluminescence.

Charge-carrier generation and transport within PbS quantum dot (QD) solar cells is investigated by measuring the temperature-dependent steady-state photoluminescence (PL) concurrently during in situ current-voltage characterization. We first compare the temperature-dependent PL quenching for PbS QD films where the PbS QDs retain their original oleate ligand to that of PbS QDs treated with 1,2-ethanedithiol (EDT), producing a conductive QD layer, either on top of glass or on a ZnO nanocrystal film. We then measure and analyze the temperature-dependent PL in a completed QD-PV architecture with the structure Al/MoO3/EDT-PbS/ZnO/ITO/glass, collecting the PL and the current simultaneously. We find that at low temperatures excitons diffuse to the ZnO interface, where PL is quenched via interfacial charge transfer. At high temperatures, excitons dissociate in the bulk of the PbS QD film via phonon-assisted tunneling to nearby QDs, and that dissociation is in competition with the intrinsic radiative and nonradiative rates of the individual QDs. The activation energy for exciton dissociation in the QD-PV devices is found to be ∼40 meV, which is considerably lower than that of the electrodeless samples, and suggests unique interactions between injected and photogenerated carriers in devices.