Triplet-sensitization by lead halide perovskite thin films for near-infrared-to-visible upconversion

Lead halide-based perovskite thin films have attracted great attention due to the explosive increase in perovskite solar cell efficiencies. The same optoelectronic properties that make perovskites ideal absorber materials in solar cells are also beneficial in other light-harvesting applications and make them prime candidates as triplet sensitizers in upconversion via triplet-triplet annihilation in rubrene. In this contribution, we take advantage of long carrier lifetimes and carrier diffusion lengths in perovskite thin films, their high absorption cross sections throughout the visible spectrum, as well as the strong spin-orbit coupling owing to the abundance of heavy atoms to sensitize the upconverter rubrene. Employing bulk perovskite thin films as the absorber layer and spin-mixer in inorganic/organic heterojunction upconversion devices allows us to forego the additional tunneling barrier owing from the passivating ligands required for colloidal sensitizers. Our bilayer device exhibits an upconversion efficiency in excess of 3% under 785 nm illumination

Electron-Induced Radiolysis of Astrochemically Relevant Ammonia Ices

We elucidate mechanisms of electron-induced radiolysis in cosmic (interstellar, planetary, and cometary) ice analogs of ammonia (NH3), likely the most abundant nitrogen-containing compound in the interstellar medium (ISM). Astrochemical processes were simulated under ultrahigh vacuum conditions by high-energy (1 keV) and low-energy (7 eV) electron-irradiation of nanoscale thin films of ammonia deposited on cryogenically cooled metal substrates. Irradiated films were analyzed by temperature-programmed desorption (TPD). Experiments with ammonia isotopologues provide convincing evidence for the electron-induced formation of hydrazine (N2H4) and diazene (N2H2) from condensed NH3. To understand the dynamics of ammonia radiolysis, the dependence of hydrazine and diazene yields on incident electron energy, electron flux, electron fluence, film thickness, and ice temperature were investigated. Radiolysis yield measurements versus (1) irradiation time and (2) film thickness are semiquantitatively consistent with a reaction mechanism that involves a bimolecular step for the formation of hydrazine and diazene from the dimerization of amidogen (NH2) and imine (NH) radicals, respectively. The apparent decrease in radiolysis yield of hydrazine and diazene with decreasing electron flux at constant fluence may be due to the competing desorption of these radicals at 90 K under low incident electron flux conditions. The production of hydrazine at electron energies as low as 7 eV and an ice temperature of 22 K is consistent with condensed phase radiolysis being mediated by low-energy secondary electrons produced by the interaction of high-energy radiation with matter. These results provide a basis from which we can begin to understand the mechanisms by which ammonia can form more complex species in cosmic ices

Confinement effects on multiexciton dynamics in semiconductor nanocrystals

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Chemistry, 2019Cataloged from PDF version of thesis.Includes bibliographical references (pages 125-135).Colloidal semiconductor nanocrystals are a promising platform for a number of technological developments in a wide variety of lighting applications. They also are an incredibly useful model system to interrogate fundamental carrier interactions in crystalline semiconductor lattices. This thesis investigates the properties of multiexciton states in semiconductor nanocrystals to build an understanding of what drives their emission dynamics and efficiency. A complete understanding of the processes which dominate in a wide variety of nanocrystal systems sheds light on electron-hole and exciton-exciton interactions and provides guidance on how to engineer nanocrystals for particular applications. In the first two chapters, I will build a foundation of understanding of semiconductor nanocrystal systems, and how to build up an intuitive understanding of the states in question from both fundamental modeling and chemical intuition.I also present a variety of methods which are used to interrogate the luminescent properties of these materials, with a particular focus on those utilized in this thesis. In the second chapter in particular, I focus on how photoluminescence measurements can go astray, how to identify artifacts or background signal which could bias or invalidate data, and how to eliminate these artifacts. The next chapter details the biexciton and triexciton emission dynamics and efficiency in CdSe nanocrystals. Utilizing a well-established and well-studied semiconductor systems allows nuanced interpretation of the emission dynamics, and the identification of perhaps some unexpected material properties to enhance how we imagine these highly excited states. Chapter four employs a suite of methods to begin to understand carrier-carrier interactions in cesium lead halide perovskite nanocrystals.This system provides a particularly interesting platform to investigate the effect of confinement and lattice mobility on excitonic properties. Finally, I present a few experimental directions and ideas which have not yet been explored and would provide an excellent continuation of this work.by Katherine E. Shulenberger.Ph. D.Ph.D. Massachusetts Institute of Technology, Department of Chemistr

News in Nanocrystals seminar: Self-assembly of early career researchers toward globally accessible nanoscience

In 2020, many in-person scientific events were canceled due to the COVID-19 pandemic, creating a vacuum in networking and knowledge exchange between scientists. To fill this void in scientific communication, a group of early career nanocrystal enthusiasts launched the virtual seminar series, News in Nanocrystals, in the summer of 2020. By the end of the year, the series had attracted over 850 participants from 46 countries. In this Nano Focus, we describe the process of organizing the News in Nanocrystals seminar series; discuss its growth, emphasizing what the organizers have learned in terms of diversity and accessibility; and provide an outlook for the next steps and future opportunities. This summary and analysis of experiences and learned lessons are intended to inform the broader scientific community, especially those who are looking for avenues to continue fostering discussion and scientific engagement virtually, both during the pandemic and after

The Kinetics of Electron Transfer from CdS Nanorods to the MoFe Protein of Nitrogenase

Combining the remarkable catalytic properties of redox enzymes with highly tunable light absorbing properties of semiconductor nanocrystals enables the light-driven catalysis of complex, multielectron redox reactions. This Article focuses on systems that combine CdS nanorods (NRs) with the MoFe protein of nitrogenase to drive photochemical N2 reduction. We used transient absorption spectroscopy (TAS) to examine the kinetics of electron transfer (ET) from CdS NRs to the MoFe protein. For CdS NRs with dimensions similar to those previously used for photochemical N2 reduction, the rate constant for ET from CdS NRs competes with other electron relaxation processes, such that when a MoFe protein is bound to a NR, about one-half of the photoexcited electrons are delivered to the enzyme. The NR-MoFe protein binding is incomplete with more than one-half of the NRs in solution not having a MoFe protein bound to accept electrons. The quantum efficiency of ET (QEET) in these ensemble samples is similar to previously reported efficiencies of product (NH3 and H2) formation, suggesting that the enzyme utilizes the delivered electrons without major loss pathways. Our analysis suggests that QEET, and therefore the photochemical product formation, is limited at the ensemble level by the NR-MoFe protein binding and at the single-complex level by the competitiveness of ET. We characterized ET kinetics for several CdS NRs samples with varying dimensions and found that for CdS NRs with an average diameter of 4.2 nm the ET efficiency dropped to undetectable levels, defining a maximum NR diameter that should be used to photochemically drive the MoFe protein. The work described here provides insights into the design of systems with increased control of photochemical N2 reduction catalyzed by the MoFe protein of nitrogenase

Multiexciton Lifetimes Reveal Triexciton Emission Pathway in CdSe Nanocrystals

Multiexcitons in emerging semiconducting nanomaterials play a critical role in potential optoelectronic and quantum computational devices. We describe photon resolved single molecule methods to directly probe the dynamics of biexcitons and triexcitons in colloidal CdSe quantum dots. We confirm that biexcitons emit from a spin-correlated state, consistent with statistical scaling. Contrary to current understanding, we find that triexciton emission is dominated by band-edge 1S_e1S_3/2 recombination rather than the higher energy 1P_e1P_3/2 recombination

Probing Linewidths and Biexciton Quantum Yields of Single Cesium Lead Halide Nanocrystals in Solution

Cesium lead halide (CsPbX₃, X = Cl, Br, I) perovskite nanocrystals (PNCs) have recently become a promising material for optoelectronic applications due to their high emission quantum yields and facile band gap tunability via both halide composition and size. The spectroscopy of single PNCs enhances our understanding of the effect of confinement on excitations in PNCs in the absence of obfuscating ensemble averaging and can also inform synthetic efforts. However, single PNC studies have been hampered by poor PNC photostability under confocal excitation, precluding interrogation of all but the most stable PNCs, and leading to a lack of understanding of PNCs in the regime of high confinement. Here, we report the first comprehensive spectroscopic investigation of single PNC properties using solution-phase photon-correlation methods, including both highly confined and blue-emitting PNCs, previously inaccessible to single NC techniques. With minimally perturbative solution-phase photon-correlation Fourier spectroscopy (s-PCFS), we establish that the ensemble emission linewidth of PNCs of all sizes and compositions is predominantly determined by the intrinsic single PNC linewidth (homogeneous broadening). The single PNC linewidth, in turn, dramatically increases with increasing confinement, consistent with what has been found for II–VI semiconductor nanocrystals. With solution-phase photon antibunching measurements, we survey the biexciton-to-exciton quantum yield ratio (BX/X QY) in the absence of user-selection bias or photodegradation. Remarkably, the BX/X QY ratio depends both on the PNC size and halide composition, with values between ∼2% for highly confined bromide PNCs and ∼50% for intermediately confined iodide PNCs. Our results suggest a wide range of underlying Auger rates, likely due to transitory charge carrier separation in PNCs with relaxed confinement

Coherent single-photon emission from colloidal lead halide perovskite quantum dots

2017 © The Authors, some rights reserved. Chemically made colloidal semiconductor quantum dots have long been proposed as scalable and color-tunable single emitters in quantum optics, but they have typically suffered from prohibitively incoherent emission. We now demonstrate that individual colloidal lead halide perovskite quantum dots (PQDs) display highly efficient single-photon emission with optical coherence times as long as 80 picoseconds, an appreciable fraction of their 210-picosecond radiative lifetimes. These measurements suggest that PQDs should be explored as building blocks in sources of indistinguishable single photons and entangled photon pairs. Our results present a starting point for the rational design of lead halide perovskite–based quantum emitters that have fast emission, wide spectral tunability, and scalable production and that benefit from the hybrid integration with nanophotonic components that has been demonstrated for colloidal materials