Publications by authors named "Richard D Schaller"

156 Publications

Visualization of Plasmonic Couplings Using Ultrafast Electron Microscopy.

Nano Lett 2021 07 21;21(13):5842-5849. Epub 2021 Jun 21.

Center for Nanoscale Materials, Argonne National Laboratory, Lemont, Illinois 60439, United States.

Hybrids of graphene and metal plasmonic nanostructures are promising building blocks for applications in optoelectronics, surface-enhanced scattering, biosensing, and quantum information. An understanding of the coupling mechanism in these hybrid systems is of vital importance to its applications. Previous efforts in this field mainly focused on spectroscopic studies of strong coupling within the hybrids with no spatial resolution. Here we report direct imaging of the local plasmonic coupling between single Au nanocapsules and graphene step edges at the nanometer scale by photon-induced near-field electron microscopy in an ultrafast electron microscope for the first time. The proximity of a step in the graphene to the nanocapsule causes asymmetric surface charge density at the ends of the nanocapsules. Computational electromagnetic simulations confirm the experimental observations. The results reported here indicate that this hybrid system could be used to manipulate the localized electromagnetic field on the nanoscale, enabling promising future plasmonic devices.
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http://dx.doi.org/10.1021/acs.nanolett.1c01824DOI Listing
July 2021

Tunable Broad Light Emission from 3D "Hollow" Bromide Perovskites through Defect Engineering.

J Am Chem Soc 2021 May 27;143(18):7069-7080. Epub 2021 Apr 27.

Department of Chemistry, Northwestern University, Evanston, Illinois 60208, United States.

Hybrid halide perovskites consisting of corner-sharing metal halide octahedra and small cuboctahedral cages filled with counter cations have proven to be prominent candidates for many high-performance optoelectronic devices. The stability limits of their three-dimensional perovskite framework are defined by the size range of the cations present in the cages of the structure. In some cases, the stability of the perovskite-type structure can be extended even when the counterions violate the size and shape requirements, as is the case in the so-called "hollow" perovskites. In this work, we engineered a new family of 3D highly defective yet crystalline "hollow" bromide perovskites with general formula (FA)()(Pb)(Br) (FA = formamidinium (FA), = ethylenediammonium (), = 0-0.44). Pair distribution function analysis shed light on the local structural coherence, revealing a wide distribution of Pb-Pb distances in the crystal structure as a consequence of the Pb/Br-deficient nature and inclusion in the lattice. By manipulating the number of Pb/Br vacancies, we finely tune the optical properties of the pristine FAPbBr by blue shifting the band gap from 2.20 to 2.60 eV for the = 0.42 sample. A most unexpected outcome was that at > 0.33 incorporation, the material exhibits strong broad light emission (1% photoluminescence quantum yield (PLQY)) that is maintained after exposure to air for more than a year. This is the first example of strong broad light emission from a 3D hybrid halide perovskite, demonstrating that meticulous defect engineering is an excellent tool for customizing the optical properties of these semiconductors.
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http://dx.doi.org/10.1021/jacs.1c01727DOI Listing
May 2021

Dynamic lattice distortions driven by surface trapping in semiconductor nanocrystals.

Nat Commun 2021 Mar 25;12(1):1860. Epub 2021 Mar 25.

Department of Materials Science and Engineering, Stanford University, Stanford, CA, USA.

Nonradiative processes limit optoelectronic functionality of nanocrystals and curb their device performance. Nevertheless, the dynamic structural origins of nonradiative relaxations in such materials are not understood. Here, femtosecond electron diffraction measurements corroborated by atomistic simulations uncover transient lattice deformations accompanying radiationless electronic processes in colloidal semiconductor nanocrystals. Investigation of the excitation energy dependence in a core/shell system shows that hot carriers created by a photon energy considerably larger than the bandgap induce structural distortions at nanocrystal surfaces on few picosecond timescales associated with the localization of trapped holes. On the other hand, carriers created by a photon energy close to the bandgap of the core in the same system result in transient lattice heating that occurs on a much longer 200 picosecond timescale, dominated by an Auger heating mechanism. Elucidation of the structural deformations associated with the surface trapping of hot holes provides atomic-scale insights into the mechanisms deteriorating optoelectronic performance and a pathway towards minimizing these losses in nanocrystal devices.
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http://dx.doi.org/10.1038/s41467-021-22116-0DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7994579PMC
March 2021

Very Robust Spray-Synthesized CsPbI Quantum Emitters with Ultrahigh Room-Temperature Cavity-Free Brightness and Self-Healing Ability.

ACS Nano 2021 Mar 17. Epub 2021 Mar 17.

Department of Materials Science and Engineering, National Tsing Hua University, Hsinchu 30013, Taiwan.

Although colloidal lead halide perovskite quantum dots (PQDs) exhibit desirable emitter characteristics with high quantum yields and narrow bandwidths, instability has limited their applications in devices. In this paper, we describe spray-synthesized CsPbI PQD quantum emitters displaying strong photon antibunching and high brightness at room temperature and stable performance under continuous excitation with a high-intensity laser for more than 24 h. Our PQDs provided high single-photon emission rates, exceeding 9 × 10 count/s, after excluding multiexciton emissions and strong photon antibunching, as confirmed by low values of the second-order correlation function (0) (reaching 0.021 and 0.061 for the best and average PQD performance, respectively). With such high brightness and stability, we applied our PQDs as quantum random number generators, which demonstrably passed all of the National Institute of Standards and Technology's randomness tests. Intriguingly, all of the PQDs exhibited self-healing behavior and restored their PL intensities to greater than half of their initial values after excitation at extremely high intensity. Half of the PQDs even recovered almost all of their initial PL intensity. The robust properties of these spray-synthesized PQDs resulted from high crystallinity and good ligand encapsulation. Our results suggest that spray-synthesized PQDs have great potential for use in future quantum technologies (., quantum communication, quantum cryptography, and quantum computing).
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http://dx.doi.org/10.1021/acsnano.1c00733DOI Listing
March 2021

Identification of Brillouin Zones by In-Plane Lasing from Light-Cone Surface Lattice Resonances.

ACS Nano 2021 Mar 9;15(3):5567-5573. Epub 2021 Mar 9.

Graduate Program in Applied Physics, Northwestern University, Evanston, Illinois 60208, United States.

Because of translational symmetry, electromagnetic fields confined within 2D periodic optical structures can be represented within the first Brillouin zone (BZ). In contrast, the wavevectors of scattered electromagnetic fields outside the lattice are constrained by the 3D light cone, the free-photon dispersion in the surrounding medium. Here, we report that light-cone surface lattice resonances (SLRs) from plasmonic nanoparticle lattices can be used to observe the radiated electromagnetic fields from extended BZ edges. Our coupled dipole radiation theory reveals how lattice geometry and induced surface plasmon dipole orientation affect angular distributions of the radiated fields. Using dye molecules as local dipole emitters to excite the light-cone SLR modes, we experimentally identified high-order BZ edges by directional, in-plane lasing emission. These results provide insight into nanolaser architectures that can emit at multiple wavelengths and in-plane directions simply by rotating the nanocavity lattice.
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http://dx.doi.org/10.1021/acsnano.1c00449DOI Listing
March 2021

Distance Dependence of Förster Resonance Energy Transfer Rates in 2D Perovskite Quantum Wells via Control of Organic Spacer Length.

J Am Chem Soc 2021 Mar 10;143(11):4244-4252. Epub 2021 Mar 10.

Department of Chemistry, Northwestern University, Evanston, Illinois 60208, United States.

Two-dimensional (2D) semiconductors are attractive candidates for a variety of optoelectronic applications owing to the unique electronic properties that arise from quantum confinement along a single dimension. Incorporating nonradiative mechanisms that enable directed migration of bound charge carriers, such as Förster resonance energy transfer (FRET), could boost device efficiencies provided that FRET rates outpace undesired relaxation pathways. However, predictive models for FRET between distinct 2D states are lacking, particularly with respect to the distance between a donor and acceptor. We approach FRET in systems with binary mixtures of donor and acceptor 2D perovskite quantum wells (PQWs), and we synthetically tune distances between donor and acceptor by varying alkylammonium spacer cation lengths. FRET rates are monitored using transient absorption spectroscopy and ultrafast photoluminescence, revealing rapid picosecond lifetimes that scale with spacer cation length. We theoretically model these binary mixtures of PQWs, describing the emitters as classical oscillating dipoles. We find agreement with our empirical lifetimes and then determine the effects of lateral extent and layer thickness, establishing fundamental principles for FRET in 2D materials.
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http://dx.doi.org/10.1021/jacs.0c12441DOI Listing
March 2021

Signatures of Coherent Phonon Transport in Ultralow Thermal Conductivity Two-Dimensional Ruddlesden-Popper Phase Perovskites.

ACS Nano 2021 Mar 4;15(3):4165-4172. Epub 2021 Mar 4.

Department of Mechanical Engineering, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213, United States.

An emerging class of methylammonium lead iodide (MAPbI)-based Ruddlesden-Popper (RP) phase perovskites, BAMAPbI ( = 1-7), exhibit enhanced stability to environmental conditions relative to MAPbI, yet still degrade at elevated temperatures. We experimentally determine the thermal conductivities of these layered RP phases for = 1-6, where defines the number of repeated perovskite octahedra per layer. We measure thermal conductivities of 0.37 ± 0.13/0.12, 0.17 ± 0.08/0.07, 0.21 ± 0.05/0.04, and 0.19 ± 0.04/0.03 W/m·K in thin films of = 1-4 and 0.08 ± 0.06/0.04, 0.06 ± 0.04/0.03, 0.06 ± 0.03/0.03, and 0.08 ± 0.07/0.04 W/m·K in single crystals of = 3-6. With the exception of = 1, these thermal conductivities are lower than the range of 0.34-0.50 W/m·K reported for single-crystal MAPbI. Reduced-order lattice dynamics modeling suggests that the initially decreasing trend of thermal conductivity in similarly oriented perovskites with increasing may result from the transport properties of coherent phonons, emergent from the superstructure, that do not scatter at the interfaces of organic butylammonium chains and perovskite octahedra. Reduced group velocity of coherent phonons in = 3-6, a consequence of band flattening in the phonon dispersion, is primarily responsible for their ultralow thermal conductivities. Similar effects on thermal conductivity have been experimentally demonstrated in deposited superlattices, but never in naturally defined materials such as RP phases. GIWAXS measurements reveal that higher RP phase thin films are less orientationally controlled and therefore possess apparently elevated thermal conductivities relative to single crystals of the same .
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http://dx.doi.org/10.1021/acsnano.0c03595DOI Listing
March 2021

Radiative lifetime-encoded unicolour security tags using perovskite nanocrystals.

Nat Commun 2021 02 12;12(1):981. Epub 2021 Feb 12.

Institute of Inorganic Chemistry, Department of Chemistry and Applied Bioscience, ETH Zürich, Zürich, Switzerland.

Traditional fluorescence-based tags, used for anticounterfeiting, rely on primitive pattern matching and visual identification; additional covert security features such as fluorescent lifetime or pattern masking are advantageous if fraud is to be deterred. Herein, we present an electrohydrodynamically printed unicolour multi-fluorescent-lifetime security tag system composed of lifetime-tunable lead-halide perovskite nanocrystals that can be deciphered with both existing time-correlated single-photon counting fluorescence-lifetime imaging microscopy and a novel time-of-flight prototype. We find that unicolour or matching emission wavelength materials can be prepared through cation-engineering with the partial substitution of formamidinium for ethylenediammonium to generate "hollow" formamidinium lead bromide perovskite nanocrystals; these materials can be successfully printed into fluorescence-lifetime-encoded-quick-read tags that are protected from conventional readers. Furthermore, we also demonstrate that a portable, cost-effective time-of-flight fluorescence-lifetime imaging prototype can also decipher these codes. A single comprehensive approach combining these innovations may be eventually deployed to protect both producers and consumers.
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http://dx.doi.org/10.1038/s41467-021-21214-3DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7881120PMC
February 2021

Photothermal behaviour of titanium nitride nanoparticles evaluated by transient X-ray diffraction.

Nanoscale 2021 Feb;13(4):2658-2664

Center for Nanoscale Materials, Argonne National Laboratory, Lemont, IL 60439, USA. and Department of Chemistry, Northwestern University, Evanston, IL 60208, USA and Institute for Sustainability and Energy at Northwestern, Northwestern University, Evanston, IL 60208, USA.

The photothermal properties of metal nitrides have recently received significant attention owing to diverse applications in solar energy conversion, photothermal therapies, photoreactions, and thermochromic windows. Here, the photothermal response of titanium nitride nanoparticles is examined using transient X-ray diffraction, in which optical excitation is synchronized with X-ray pulses to characterize dynamic changes in the TiN lattice. Photoinduced diffraction data is quantitatively analyzed to determine increases in the TiN lattice spacing, which are furthermore calibrated against static, temperature-dependent diffraction patterns of the same samples. Measurements of 20 nm and 50 nm diameter TiN nanoparticles reveal transient lattice heating from room temperature up to ∼175 °C for the highest pump fluences investigated here. Increasing excitation intensity drives sublinear increases in lattice temperature, due to increased heat capacity at the higher effective temperatures achieved at higher powers. Temporal dynamics show that higher excitation intensity drives not only higher lattice temperatures, but also unexpectedly slower cooling of the TiN nanoparticles, which is attributed to heating of the solvent proximal to the nanoparticle surface.
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http://dx.doi.org/10.1039/d0nr08202cDOI Listing
February 2021

Anisotropic Transient Disordering of Colloidal, Two-Dimensional CdSe Nanoplatelets upon Optical Excitation.

Nano Lett 2021 Feb 19;21(3):1288-1294. Epub 2021 Jan 19.

Nanoplatelets (NPLs)-colloidally synthesized, spatially anisotropic, two-dimensional semiconductor quantum wells-are of intense interest owing to exceptionally narrow transition line widths, coupled with solution processability and bandgap tunability. However, given large surface areas and undercoordinated bonding at facet corners and edges, excitation under sufficient intensities may induce anisotropic structural instabilities that impact desired properties. We employ time-resolved X-ray diffraction to study the crystal structure of CdSe NPLs in response to optical excitation. Photoexcitation induces greater out-of-plane than in-plane disordering in 4 and 5 monolayer (ML) NPLs, while 3 ML NPLs display the opposite behavior. Recovery dynamics suggest that out-of-plane cooling slightly outpaces in-plane cooling in 5 ML NPLs with recrystallization occurring on indistinguishable time scales. In comparison, for zero-dimensional CdSe nanocrystals, disordering is isotropic and recovery is faster. These results favor the use of NPLs in optoelectronic applications, where they are likely to exhibit superior performance over traditional, zero-dimensional nanocrystals.
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http://dx.doi.org/10.1021/acs.nanolett.0c03958DOI Listing
February 2021

Charge Transfer and Spin Dynamics in a Zinc Porphyrin Donor Covalently Linked to One or Two Naphthalenediimide Acceptors.

J Phys Chem A 2021 Jan 15;125(3):825-834. Epub 2021 Jan 15.

Department of Chemistry and Institute for Sustainability and Energy at Northwestern, Northwestern University, Evanston, Illinois 60208-3113, United States.

Quantum coherence effects on charge transfer and spin dynamics in a system having two degenerate electron acceptors are studied using a zinc 5,10,15-tri(-pentyl)-20-phenylporphyrin (ZnP) electron donor covalently linked to either one or two naphthalene-1,8:4,5-bis(dicarboximide) (NDI) electron acceptors using an anthracene (An) spacer, ZnP-An-NDI () and ZnP-An-NDI (), respectively. Following photoexcitation of and in toluene at 295 K, femtosecond transient absorption spectroscopy shows that the electron transfer (ET) rate constant for is about three times larger than that of , which can be accounted for by the statistical nature of incoherent ET as well as the electron couplings for the charge separation reactions. In contrast, the rate constant for charge recombination (CR) of is about 25% faster than that of . Using femtosecond transient infrared spectroscopy and theoretical analysis, we find that the electron on NDI in localizes onto one of the two NDIs prior to CR, thus precluding electronically coherent CR from NDI. Conversely, CR in both and is spin coherent as indicated by the observation of a resonance in the ZnP yield following CR as a function of applied magnetic field, giving spin-spin exchange interaction energies of 2 = 210 and 236 mT, respectively, where the line width of the resonance for is greater than . These data show that while CR is a spin-coherent process, incoherent hopping of the electron between the two NDIs in , consistent with the lack of delocalization noted above, results in greater spin decoherence in relative to .
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http://dx.doi.org/10.1021/acs.jpca.0c10471DOI Listing
January 2021

Singlet fission in core-linked terrylenediimide dimers.

J Chem Phys 2020 Dec;153(24):244306

Department of Chemistry and Institute for Sustainability and Energy at Northwestern, Northwestern University, Evanston, Illinois 60208-3113, USA.

We have studied two regioisomeric terrylenediimide (TDI) dimers in which the 1-positions of two TDIs are linked via 1,3- or 1,4-phenylene spacers, mTDI and pTDI, respectively. The nature and the dynamics of the multiexciton state are tuned by altering the through-bond electronic couplings in the ground and excited states and by changing the solvent environment. Our results show that controlling the electronic coupling between the two chromophores by an appropriate choice of linker can result in independent triplet state formation, even though the initial correlated triplet pair state is confined to a dimer. Moreover, even in polar solvents, if the electronic coupling is strong, the correlated triplet pair state is observed prior to symmetry-breaking charge separation. These results point out the close relationship between the singlet, correlated triplet pair, and charge transfer states in molecular dimers.
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http://dx.doi.org/10.1063/5.0026254DOI Listing
December 2020

Photophysical implications of ring fusion, linker length, and twisting angle in a series of perylenediimide-thienoacene dimers.

Chem Sci 2020 Jul 1;11(27):7133-7143. Epub 2020 Jul 1.

Department of Chemistry , Northwestern University , 2145 Sheridan Road , Evanston , Illinois 60208 , USA . Email:

Perylenediimide (PDI) derivatives have been widely studied as electron acceptor alternatives to fullerenes in organic photovoltaics (OPVs) because of their tunable absorption in the visible range, inexpensive synthesis, and photochemical stability. A common motif for improving device efficiency involves joining multiple PDIs together through electron-rich linkers to form a twisted acceptor-donor-acceptor molecule. Molecular features such as ring fusion are further employed to modify the structure locally and in films. These synthetic efforts have greatly enhanced OPV device efficiencies, however it remains unclear how the increasingly elaborate structural modifications affect the photophysical processes integral to efficient photon-to-charge conversion. Here we carry out a systematic study of a series of PDI dimers with thienoacene linkers in which the twist angle, linker length, and degree of ring fusion are varied to investigate the effects of these structural features on the molecular excited states and exciton recombination dynamics. Spectroscopic characterization of the dimers suggest that ring fusion causes greater coupling between the donor and acceptor components and greatly enhances the lifetime of a thienoacene to PDI charge transfer state. The lifetime of this CT state also correlates well with the linker-PDI dihedral angle, with smaller dihedral angle resulting in longer lifetime. DFT and two-photon absorption TDDFT calculations were developed in-house to model the ground state and excited transitions, providing theoretical insight into the reasons for the observed photophysical properties and identifying the charge transfer state in the excited state absorption spectra. These results highlight how the longevity of the excited state species, important for the efficient conversion of excitons to free carriers in OPV devices, can be chemically tuned by controlling ring fusion and by using steric effects to control the relative orientations of the molecular fragments. The results provide a successful rationalization of the behavior of solar cells involving these acceptor molecules.
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http://dx.doi.org/10.1039/d0sc02862bDOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7654190PMC
July 2020

Transient Lattice Response upon Photoexcitation in CuInSe Nanocrystals with Organic or Inorganic Surface Passivation.

ACS Nano 2020 Oct 24;14(10):13548-13556. Epub 2020 Sep 24.

Department of Chemistry, Northwestern University, Evanston, Illinois 60208, United States.

CuInSe nanocrystals offer promise for optoelectronics including thin-film photovoltaics and printed electronics. Additive manufacturing methods such as photonic curing controllably sinter particles into quasi-continuous films and offer improved device performance. To gain understanding of nanocrystal response under such processing conditions, we investigate impacts of photoexcitation on colloidal nanocrystal lattices via time-resolved X-ray diffraction. We probe three sizes of particles and two capping ligands (oleylamine and inorganic S) to evaluate resultant crystal lattice temperature, phase stability, and thermal dissipation. Elevated fluences produce heating and loss of crystallinity, the onset of which exhibits particle size dependence. We find size-dependent recrystallization and cooling lifetimes ranging from 90 to 200 ps with additional slower cooling on the nanosecond time scale. Sulfide-capped nanocrystals show faster recrystallization and cooling compared to oleylamine-capped nanocrystals. Using these lifetimes, we find interfacial thermal conductivities from 3 to 28 MW/(m K), demonstrating that ligand identity strongly influences thermal dissipation.
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http://dx.doi.org/10.1021/acsnano.0c05553DOI Listing
October 2020

Intersubband Relaxation in CdSe Colloidal Quantum Wells.

ACS Nano 2020 Sep 31;14(9):12082-12090. Epub 2020 Aug 31.

Center for Nanoscale Materials, Argonne National Laboratory, Lemont, Illinois 60439, United States.

The dynamics of intersubband relaxation are critical to quantum well technologies such as quantum cascade lasers and quantum well infrared photodetectors. Here, intersubband relaxation in CdSe colloidal quantum wells, or nanoplatelets, is studied pump-push-probe transient spectroscopy. An initial interband pump pulse is followed by a secondary infrared push excitation, resonant with intersubband absorption, which promotes electrons from the first conduction band of the quantum well to the second conduction band. A probe pulse monitors subsequent electron cooling to the band edge of the quantum well. Using this technique, intersubband relaxation is studied as a function of critical variables such as colloidal quantum well size and thickness, surface ligand chemistry, temperature, and excitation pulse intensity. Larger quantum well sizes, judicious selection of surface ligand chemistry (.., thiolates), low temperatures, and elevated push pulse fluences slow intersubband relaxation. However, compared to resonant relaxation in colloidal quantum dots (up to hundreds of picoseconds), relaxation in colloidal quantum wells is rapid (<1 ps) under all examined conditions. These experiments indicate that rapid relaxation is driven by both LO phonon and surface scattering. The short time scale of relaxation observed in these materials may hinder intersubband technologies such as mid-infrared detectors, although such rapid relaxation may prove valuable in optical switching.
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http://dx.doi.org/10.1021/acsnano.0c05459DOI Listing
September 2020

Low-threshold laser medium utilizing semiconductor nanoshell quantum dots.

Nanoscale 2020 Sep 14;12(33):17426-17436. Epub 2020 Aug 14.

The Center for Photochemical Sciences, Bowling Green State University, Bowling Green, Ohio 43403, USA.

Colloidal semiconductor nanocrystals (NCs) represent a promising class of nanomaterials for lasing applications. Currently, one of the key challenges facing the development of high-performance NC optical gain media lies in enhancing the lifetime of biexciton populations. This usually requires the employment of charge-delocalizing particle architectures, such as core/shell NCs, nanorods, and nanoplatelets. Here, we report on a two-dimensional nanoshell quantum dot (QD) morphology that enables a strong delocalization of photoinduced charges, leading to enhanced biexciton lifetimes and low lasing thresholds. A unique combination of a large exciton volume and a smoothed potential gradient across interfaces of the reported CdS/CdSe/CdS (core/shell/shell) nanoshell QDs results in strong suppression of Auger processes, which was manifested in this work though the observation of stable amplified stimulated emission (ASE) at low pump fluences. An extensive charge delocalization in nanoshell QDs was confirmed by transient absorption measurements, showing that the presence of a bulk-size core in CdS/CdSe/CdS QDs reduces exciton-exciton interactions. Overall, present findings demonstrate unique advantages of the nanoshell QD architecture as a promising optical gain medium in solid-state lighting and lasing applications.
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http://dx.doi.org/10.1039/d0nr03582cDOI Listing
September 2020

Brightly Luminescent CsPbBr Nanocrystals through Ultracentrifugation.

J Phys Chem Lett 2020 Sep 18;11(17):7133-7140. Epub 2020 Aug 18.

Materials & Nanotechnology Program, North Dakota State University, Fargo, North Dakota 58108, United States.

Using a combination of density-gradient and analytical ultracentrifugation, we studied the photophysical profile of CsPbBr nanocrystal (NC) suspensions by separating them into size-resolved fractions. Ultracentrifugation drastically alters the ligand profile of the NCs, which necessitates postprocessing to restore colloidal stability and enhance quantum yield (QY). Rejuvenated fractions show a 50% increase in QY compared to no treatment and a 30% increase with respect to the parent. Our results demonstrate how the NC environment can be manipulated to improve photophysical performance, even after there has been a measurable decline in the response. Size separation reveals blue-emitting fractions, a narrowing of photoluminescence spectra in comparison to the parent, and a crossover from single- to stretched-exponential relaxation dynamics with decreasing NC size. As a function of edge length, , our results confirm that the photoluminescence peak energy scales a , in agreement with the simplest picture of quantum confinement.
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http://dx.doi.org/10.1021/acs.jpclett.0c01936DOI Listing
September 2020

Nickel(II) Metal Complexes as Optically Addressable Qubit Candidates.

J Am Chem Soc 2020 Sep 19;142(35):14826-14830. Epub 2020 Aug 19.

Department of Chemistry, Northwestern University, Evanston, Illinois 60208, United States.

The inherent atomic level structural control of synthetic chemistry enables the creation of qubits, the base units of a quantum information science system, designed for a target application. For quantum sensing applications, enabling optical read-out of spin in tunable molecular systems, akin to defect-based systems, would be transformative. This approach would bring together molecular tunability with optical read-out technology. In theory, nickel ions in octahedral symmetry meet all the criteria for optical readout of spin. Yet, to the best of our knowledge, there are no pulse EPR studies on Ni molecules. We identified two compounds featuring highly symmetric Ni centers, thereby engendering weak zero-field splitting to enable EPR addressability: [Ni(phen)](BF) () and [Ni(pyr)](BF) () (phen = 1,10-phenanthroline; pyr = tris-2-pyridyl-methane). Crucially, these complexes feature the requisite strong field ligands to enable emission for optical addressability. We extracted axial zero-field splitting parameters of = +0.9 cm and +2.7 cm for and , respectively, enabling pulse EPR measurements. Both compounds produce emission at λ = 938-944 nm. The aggregate of these results expands the catalogue of qubit materials to Ni-based compounds and offers a future pathway for optical readout of these molecules.
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http://dx.doi.org/10.1021/jacs.0c06909DOI Listing
September 2020

Area and thickness dependence of Auger recombination in nanoplatelets.

J Chem Phys 2020 Aug;153(5):054104

Department of Chemistry, University of California, Berkeley, California 94720, USA.

The ability to control both the thickness and the lateral dimensions of colloidal nanoplatelets offers a test-bed for area and thickness dependent properties in 2D materials. An important example is Auger recombination, which is typically the dominant process by which multiexcitons decay in nanoplatelets. Herein, we uncover fundamental properties of biexciton decay in nanoplatelets by comparing the Auger recombination lifetimes based on interacting and noninteracting formalisms with measurements based on transient absorption spectroscopy. Specifically, we report that electron-hole correlations in the initial biexcitonic state must be included in order to obtain Auger recombination lifetimes in agreement with experimental measurements and that Auger recombination lifetimes depend nearly linearly on the lateral area and somewhat more strongly on the thickness of the nanoplatelet. We also connect these scalings to those of the area and thickness dependencies of single exciton radiative recombination lifetimes, exciton coherence areas, and exciton Bohr radii in these quasi-2D materials.
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http://dx.doi.org/10.1063/5.0012973DOI Listing
August 2020

Large Exciton Diffusion Coefficients in Two-Dimensional Covalent Organic Frameworks with Different Domain Sizes Revealed by Ultrafast Exciton Dynamics.

J Am Chem Soc 2020 Sep 24;142(35):14957-14965. Epub 2020 Aug 24.

Department of Chemistry, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208, United States.

Large singlet exciton diffusion lengths are a hallmark of high performance in organic-based devices such as photovoltaics, chemical sensors, and photodetectors. In this study, exciton dynamics of a two-dimensional covalent organic framework, 2D COF-5, is investigated using ultrafast spectroscopic techniques. After photoexcitation, the COF-5 exciton decays via three pathways: (1) excimer formation (4 ± 2 ps), (2) excimer relaxation (160 ± 40 ps), and (3) excimer decay (>3 ns). Excitation fluence-dependent transient absorption studies suggest that COF-5 has a relatively large diffusion coefficient (0.08 cm/s). Furthermore, exciton-exciton annihilation processes are characterized as a function of COF-5 crystallite domain size in four different samples, which reveal domain-size-dependent exciton diffusion kinetics. These results reveal that exciton diffusion in COF-5 is constrained by its crystalline domain size. These insights indicate the outstanding promise of delocalized excitonic processes available in 2D COFs, which motivate their continued design and implementation into optoelectronic devices.
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http://dx.doi.org/10.1021/jacs.0c05404DOI Listing
September 2020

Using Photoexcited Core/Shell Quantum Dots To Spin Polarize Appended Radical Qubits.

J Am Chem Soc 2020 Aug 23;142(31):13590-13597. Epub 2020 Jul 23.

Department of Chemistry and Institute for Sustainability and Energy at Northwestern, Northwestern University, Evanston, Illinois 60208-3113, United States.

The synthetic tunability, flexibility, and rich spin physics of semiconductor quantum dots (QDs) make them promising candidates for quantum information science applications. However, the rapid spin relaxation observed in colloidal quantum dots limits their functionality. In the current work, we demonstrate a method to harness photoexcited spin states in QDs to produce long-lived spin polarization on an appended organic ligand molecule. We present a system composed of CdSe/CdS core/shell QDs, covalently linked to naphthalenediimide (NDI) electron-accepting molecules. The electron transfer dynamics from photoexcited QDs to the appended NDI ligands is explored as a function of both shell thickness and number of NDIs per QD. Transient EPR spectroscopy shows that the photoexcited QDs strongly spin polarize the NDI radical anion, which is interpreted in the context of both the radical pair and the triplet mechanisms of spin polarization. This work serves as an initial step toward using photoexcited QDs to strongly spin polarize organic radicals having long spin relaxation times to serve as spin qubits in quantum information science applications.
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http://dx.doi.org/10.1021/jacs.0c06073DOI Listing
August 2020

In Situ Grazing-Incidence Wide-Angle Scattering Reveals Mechanisms for Phase Distribution and Disorientation in 2D Halide Perovskite Films.

Adv Mater 2020 Aug 2;32(33):e2002812. Epub 2020 Jul 2.

Department of Chemistry, Northwestern University, 2145 Sheridan Road, Evanston, IL, 60208, USA.

2D hybrid halide perovskites with the formula (A') (A) Pb I have remarkable stability and promising efficiency in photovoltaic and optoelectronic devices, yet fundamental understanding of film formation, key to optimizing these devices, is lacking. Here, in situ grazing-incidence wide-angle X-ray scattering (GIWAXS) is used to monitor film formation during spin-coating. This elucidates the general film formation mechanism of 2D halide perovskites during one-step spin-coating. There are three stages of film formation: sol-gel, oriented 3D, and 2D. Three precursor phases form during the sol-gel stage and transform to perovskite, first giving a highly oriented 3D-like phase at the air/liquid interface followed by subsequent nucleations forming slightly less oriented 2D perovskite. Furthermore, heating before crystallization leads to fewer nucleations and faster removal of the precursors, improving orientation. This outlines the primary causes of phase distribution and perpendicular orientation in 2D perovskite films and paves the way for rationally designed film fabrication techniques.
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http://dx.doi.org/10.1002/adma.202002812DOI Listing
August 2020

Negative Pressure Engineering with Large Cage Cations in 2D Halide Perovskites Causes Lattice Softening.

J Am Chem Soc 2020 Jul 19;142(26):11486-11496. Epub 2020 Jun 19.

Department of Chemistry, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208, United States.

Organic-inorganic hybrid halide perovskites are promising semiconductors with tailorable optical and electronic properties. The choice of A-site cation to support a three-dimensional (3D) perovskite structure AMX (where M is a metal and X is a halide) is limited by the geometric Goldschmidt tolerance factor. However, this geometric constraint can be relaxed in two-dimensional (2D) perovskites, providing us an opportunity to understand how various A-site cations modulate the structural properties and thereby the optoelectronic properties. Here, we report the synthesis and structures of single-crystal (BA)(A)PbI where BA = butylammonium and A = methylammonium (MA), formamidinium (FA), dimethylammonium (DMA), or guanidinium (GA), with a series of A-site cations varying in size. Single-crystal X-ray diffraction reveals that the MA, FA, and GA structures crystallize in the same space group, while the DMA imposes the space group. We observe that as the A-site cation becomes larger, the Pb-I bond continuously elongates, expanding the volume of the perovskite cage, equivalent to exerting "negative pressure" on the perovskite structures. Optical studies and DFT calculations show that the Pb-I bond length elongation reduces the overlap of the Pb s- and I p-orbitals and increases the optical bandgap, while Pb-I-Pb tilting angles play a secondary role. Raman spectra show lattice softening with increasing size of the A-site cation. These structural changes with enlarged A cations result in significant decreases in photoluminescence intensity and lifetime, consistent with a more pronounced nonradiative decay. Transient absorption microscopy results suggest that the PL drop may derive from a higher concentration of traps or phonon-assisted nonradiative recombination. The results highlight that extending the range of Goldschmidt tolerance factors for 2D perovskites is achievable, enabling further tuning of the structure-property relationships in 2D perovskites.
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http://dx.doi.org/10.1021/jacs.0c03860DOI Listing
July 2020

Effects of Intra- and Interchain Interactions on Exciton Dynamics of PTB7 Revealed by Model Oligomers.

Molecules 2020 May 23;25(10). Epub 2020 May 23.

Department of Chemistry, Northwestern University, 2145 Sheridan Road, Evanston, IL 60208, USA.

Recent studies have shown that molecular aggregation structures in precursor solutions of organic photovoltaic (OPV) polymers have substantial influence on polymer film morphology, exciton and charge carrier transport dynamics, and hence, the resultant device performance. To distinguish photophysical impacts due to increasing π-conjugation from chain lengthening and π-π stacking from single/multi chain aggregation in solution and film, we used oligomers of a well-studied charge transfer polymer PTB7 with different lengths as models to reveal intrinsic photophysical properties of a conjugated segment in the absence of inter-segment aggregation. In comparison with previously studied photophysical properties in polymeric PTB7, we found that oligomer dynamics are dominated by a process of planarization of the conjugated backbone into a quinoidal structure that resembles the self-folded polymer and that, when its emission is isolated, this quinoidal excited state resembling the planar polymer chain exhibits substantial charge transfer character via solvent-dependent emission shifts. Furthermore, the oligomers distinctly lack the long-lived charge separated species characteristic of PTB7, suggesting that the progression from charge transfer character in isolated chains to exciton splitting in neat polymer solution is modulated by the interchain interactions enabled by self-folding.
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http://dx.doi.org/10.3390/molecules25102441DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7287679PMC
May 2020

Heat-driven acoustic phonons in lamellar nanoplatelet assemblies.

Nanoscale 2020 May 22;12(17):9661-9668. Epub 2020 Apr 22.

Center for Nanoscale Materials, Argonne National Laboratory, Lemont, IL 60439, USA.

Colloidal CdSe nanoplatelets, with the electronic structure of quantum wells, self-assemble into lamellar stacks due to large co-facial van der Waals attractions. These lamellar stacks are shown to display coherent acoustic phonons that are detected from oscillatory changes in the absorption spectrum observed in infrared pump, electronic probe measurements. Rather than direct electronic excitation of the nanocrystals using a femtosecond laser, impulsive transfer of heat from the organic ligand shell, excited at C-H stretching vibrational resonances, to the inorganic core of individual nanoplatelets occurs on a time-scale of <100 ps. This heat transfer drives in-phase longitudinal acoustic phonons of the nanoplatelet lamellae, which are accompanied by subtle deformations along the nanoplatelet short axes. The frequencies of the oscillations vary from 0.7 to 2 GHz (3-8 μeV and 0.5-1 ns oscillation period) depending on the thickness of the nanoplatelets-but not their lateral areas-and the temperature of the sample. Temperature-dependence of the acoustic phonon frequency conveys a substantial stiffening of the organic ligand bonds between nanoplatelets with reduced temperature. These results demonstrate a potential for acoustic modulation of the excitonic structure of nanocrystal assemblies in self-assembled anisotropic semiconductor systems at temperatures at or above 300 K.
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http://dx.doi.org/10.1039/d0nr00695eDOI Listing
May 2020

Water-Stable 1D Hybrid Tin(II) Iodide Emits Broad Light with 36% Photoluminescence Quantum Efficiency.

J Am Chem Soc 2020 May 29;142(19):9028-9038. Epub 2020 Apr 29.

Department of Chemistry, Northwestern University, Evanston, Illinois 60208, United States.

The optical and light emission properties of tin and lead halide perovskites are remarkable because of the robust room-temperature (RT) performance, broad wavelength tunability, high efficiency, and good quenching resistance to defects. These highly desirable attributes promise to transform current light-emitting devices, phosphors, and lasers. One disadvantage in most of these materials is the sensitivity to moisture. Here, we report a new air-stable one-dimensional (1D) hybrid lead-free halide material (DAO)SnI (DAO, 1,8-octyldiammonium) that is resistant to water for more than 15 h. The material exhibits a sharp optical absorption edge at 2.70 eV and a strong broad orange light emission centered at 634 nm, with a full width at half-maximum (fwhm) of 142 nm (0.44 eV). The emission has a long photoluminescence (PL) lifetime of 582 ns, while the intensity is constant over a very broad temperature range (145-415 K) with a photoluminescence quantum yield (PLQY) of at least 20.3% at RT. Above 415 K the material undergoes a structural phase transition from monoclinic (2/) to orthorhombic ( accompanied by a red shift in the band gap and a quench in the photoluminescence emission. Density functional theory calculations support the trend in the optical properties and the 1D electronic nature of the structure, where the calculated carrier effective masses along the inorganic chain are significantly lower than those perpendicular to the chain. Thin films of the compound readily fabricated from solutions exhibit the same optical properties, but with improved PLQY of 36%, for a 60 nm thick film, among the highest reported for lead-free low-dimensional 2D and 1D perovskites and metal halides.
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http://dx.doi.org/10.1021/jacs.0c03004DOI Listing
May 2020

Organic Cation Alloying on Intralayer A and Interlayer A' sites in 2D Hybrid Dion-Jacobson Lead Bromide Perovskites (A')(A)PbBr.

J Am Chem Soc 2020 May 27;142(18):8342-8351. Epub 2020 Apr 27.

Department of Chemistry, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208, United States.

Hybrid layered halide perovskites have achieved impressive performance in optoelectronics. New structural types in the two-dimensional (2D) halide system such as the Dion-Jacobson phases have attracted wide research attention due to the short interlayer distance and unique layer orientation that facilitate better charge-transport and higher stability in optoelectronic devices. Here, we report the first solid solution series incorporating both A and A' cations in the 2D Dion-Jacobson family, with the general formula (A')(A)PbBr ((A' = 3-(aminomethyl)piperidinium (3AMP) and 4-(aminomethyl)piperidinium) (4AMP); A = methylammonium (MA) and formamidinium (FA)). Mixing the spacing A' cations and perovskitizer A cations generates the new (3AMP)(4AMP)(FA)(MA)PbBr perovskites. The crystallographically refined crystal structures using single-crystal X-ray diffraction data reveal that the distortion of the inorganic framework is heavily influenced by the degree of A' and A alloying. A rising fraction of 4AMP in the structure, decreases the Pb-Br-Pb angles, making the framework more distorted. On the contrary, higher FA fractions increase the Pb-Br-Pb angles. This structural evolution fine-tunes the optical properties where the larger the Pb-Br-Pb angle, the narrower the band gap. The photoluminescence emission energy mirrors this trend. Raman spectroscopy reveals a highly dynamical lattice similar to MAPbBr and consistent with the local distortion environment of the [PbBr] framework. Density functional theory (DFT) calculations of the electronic structures reveal the same trend as the experimental results where (3AMP)(FA)PbBr has the smallest band gap while (4AMP)(MA)PbBr has the largest band gap. The structural effects from solely the organic cations in the 2D system highlight the importance of understanding the high sensitivity of the optoelectronic properties on the structural tuning in this broad class of materials.
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http://dx.doi.org/10.1021/jacs.0c01625DOI Listing
May 2020

Three-Dimensional Lead Iodide Perovskitoid Hybrids with High X-ray Photoresponse.

J Am Chem Soc 2020 Apr 26;142(14):6625-6637. Epub 2020 Mar 26.

Department of Chemistry, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208, United States.

Large organic A cations cannot stabilize the 3D perovskite AMX structure because they cannot be accommodated in the cubo-octhedral cage (do not follow the Goldschmidt tolerance factor rule), and they generally template low-dimensional structures. Here we report that the large dication aminomethylpyridinium (AMPY) can template novel 3D structures which resemble conventional perovskites. They have the formula (AMPY)MI ( = 3 or 4, M = Sn or Pb) which is double of the AMX formula. However, because of the steric requirement of the Goldschmidt tolerance factor rule, it is impossible for (AMPY)MI to form proper perovskite structures. Instead, a combination of corner-sharing and edge-sharing connectivity is adopted in these compounds leading to the new 3D structures. DFT calculations reveal that the compounds are indirect band gap semiconductors with direct band gaps presenting at slightly higher energies and dispersive electronic bands. The indirect band gaps of the Sn and Pb compounds are ∼1.7 and 2.0 eV, respectively, which is slightly higher than the corresponding AMI 3D perovskites. The Raman spectra for the compounds are diffuse, with a broad rising central peak at very low frequencies around 0 cm, a feature that is characteristic of dynamical lattices, high anharmonicity, and dissipative vibrations very similar to the 3D AMX perovskites. Devices of (3AMPY)PbI crystals exhibit clear photoresponse under ambient light without applied bias, reflecting a high carrier mobility (μ) and long carrier lifetime (τ). The devices also exhibit sizable X-ray generated photocurrent with a high μτ product of ∼1.2 × 10 cm /V and an X-ray sensitivity of 207 μC·Gy·cm.
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http://dx.doi.org/10.1021/jacs.0c00101DOI Listing
April 2020

Bright Silicon Nanocrystals from a Liquid Precursor: Quasi-Direct Recombination with High Quantum Yield.

ACS Nano 2020 Apr 18;14(4):3858-3867. Epub 2020 Mar 18.

Materials and Nanotechnology Program, North Dakota State University, Fargo, North Dakota 58108, United States.

Silicon nanocrystals (SiNCs) with bright bandgap photoluminescence (PL) are of current interest for a range of potential applications, from solar windows to biomedical contrast agents. Here, we use the liquid precursor cyclohexasilane (SiH) for the plasma synthesis of colloidal SiNCs with exemplary core emission. Through size separation executed in an oxygen-shielded environment, we achieve PL quantum yields (QYs) approaching 70% while exposing intrinsic constraints on efficient core emission from smaller SiNCs. Time-resolved PL spectra of these fractions in response to femtosecond pulsed excitation reveal a zero-phonon radiative channel that anticorrelates with QY, which we model using advanced computational methods applied to a 2 nm SiNC. Our results offer additional insight into the photophysical interplay of the nanocrystal surface, quasi-direct recombination, and efficient SiNC core PL.
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http://dx.doi.org/10.1021/acsnano.9b09614DOI Listing
April 2020

Quantum Dot-Plasmon Lasing with Controlled Polarization Patterns.

ACS Nano 2020 Mar 12;14(3):3426-3433. Epub 2020 Feb 12.

Graduate Program in Applied Physics, Northwestern University, Evanston, Illinois 60208, United States.

The tailored spatial polarization of coherent light beams is important for applications ranging from microscopy to biophysics to quantum optics. Miniaturized light sources are needed for integrated, on-chip photonic devices with desired vector beams; however, this issue is unresolved because most lasers rely on bulky optical elements to achieve such polarization control. Here, we report on quantum dot-plasmon lasers with engineered polarization patterns controllable by near-field coupling of colloidal quantum dots to metal nanoparticles. Conformal coating of CdSe-CdS core-shell quantum dot films on Ag nanoparticle lattices enables the formation of hybrid waveguide-surface lattice resonance (W-SLR) modes. The sidebands of these hybrid modes at nonzero wavevectors facilitate directional lasing emission with either radial or azimuthal polarization depending on the thickness of the quantum dot film.
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http://dx.doi.org/10.1021/acsnano.9b09466DOI Listing
March 2020
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