Publications by authors named "Maksym V Kovalenko"

184 Publications

Exploiting the Lability of Metal Halide Perovskites for Doping Semiconductor Nanocomposites.

ACS Energy Lett 2021 Feb 21;6(2):581-587. Epub 2021 Jan 21.

Institute of Science and Technology Austria, Klosterneuburg 3400, Austria.

Cesium lead halides have intrinsically unstable crystal lattices and easily transform within perovskite and nonperovskite structures. In this work, we explore the conversion of the perovskite CsPbBr into CsPbBr in the presence of PbS at 450 °C to produce doped nanocrystal-based composites with embedded CsPbBr nanoprecipitates. We show that PbBr is extracted from CsPbBr and diffuses into the PbS lattice with a consequent increase in the concentration of free charge carriers. This new doping strategy enables the adjustment of the density of charge carriers between 10 and 10 cm, and it may serve as a general strategy for doping other nanocrystal-based semiconductors.
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http://dx.doi.org/10.1021/acsenergylett.0c02448DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7887873PMC
February 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

Monodisperse Long-Chain Sulfobetaine-Capped CsPbBr Nanocrystals and Their Superfluorescent Assemblies.

ACS Cent Sci 2021 Jan 29;7(1):135-144. Epub 2020 Dec 29.

Institute of Inorganic Chemistry, Department of Chemistry and Applied Biosciences, ETH Zürich, Vladimir Prelog Weg 1, CH-8093 Zürich, Switzerland.

Ligand-capped nanocrystals (NCs) of lead halide perovskites, foremost fully inorganic CsPbX NCs, are the latest generation of colloidal semiconductor quantum dots. They offer a set of compelling characteristics-large absorption cross section, as well as narrow, fast, and efficient photoluminescence with long exciton coherence times-rendering them attractive for applications in light-emitting devices and quantum optics. Monodisperse and shape-uniform, broadly size-tunable, scalable, and robust NC samples are paramount for unveiling their basic photophysics, as well as for putting them into use. Thus far, no synthesis method fulfilling all these requirements has been reported. For instance, long-chain zwitterionic ligands impart the most durable surface coating, but at the expense of reduced size uniformity of the as-synthesized colloid. In this work, we demonstrate that size-selective precipitation of CsPbBr NCs coated with a long-chain sulfobetaine ligand, namely, 3-(,-dimethyloctadecylammonio)-propanesulfonate, yields monodisperse and sizable fractions (>100 mg inorganic mass) with the mean NC size adjustable in the range between 3.5 and 16 nm and emission peak wavelength between 479 and 518 nm. We find that all NCs exhibit an oblate cuboidal shape with the aspect ratio of 1.2 × 1.2 × 1. We present a theoretical model (effective mass/) that accounts for the anisotropic NC shape and describes the size dependence of the first and second excitonic transition in absorption spectra and explains room-temperature exciton lifetimes. We also show that uniform zwitterion-capped NCs readily form long-range ordered superlattices upon solvent evaporation. In comparison to more conventional ligand systems (oleic acid and oleylamine), supercrystals of zwitterion-capped NCs exhibit larger domain sizes and lower mosaicity. Both kinds of supercrystals exhibit superfluorescence at cryogenic temperatures-accelerated collective emission arising from the coherent coupling of the emitting dipoles.
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http://dx.doi.org/10.1021/acscentsci.0c01153DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7845019PMC
January 2021

Hybrid 0D Antimony Halides as Air-Stable Luminophores for High-Spatial-Resolution Remote Thermography.

Adv Mater 2021 Mar 22;33(9):e2007355. Epub 2021 Jan 22.

Laboratory of Inorganic Chemistry, Department of Chemistry and Applied Bioscience, ETH Zürich, Vladimir Prelog Weg 1, Zürich, CH-8093, Switzerland.

Luminescent organic-inorganic low-dimensional ns metal halides are of rising interest as thermographic phosphors. The intrinsic nature of the excitonic self-trapping provides for reliable temperature sensing due to the existence of a temperature range, typically 50-100 K wide, in which the luminescence lifetimes (and quantum yields) are steeply temperature-dependent. This sensitivity range can be adjusted from cryogenic temperatures to above room temperature by structural engineering, thus enabling diverse thermometric and thermographic applications ranging from protein crystallography to diagnostics in microelectronics. Owing to the stable oxidation state of Sb , Sb(III)-based halides are far more attractive than all major non-heavy-metal alternatives (Sn-, Ge-, Bi-based halides). In this work, the relationship between the luminescence characteristics and crystal structure and microstructure of TPP SbBr (TPP = tetraphenylphosphonium) is established, and then its potential is showcased as environmentally stable and robust phosphor for remote thermography. The material is easily processable into thin films, which is highly beneficial for high-spatial-resolution remote thermography. In particular, a compelling combination of high spatial resolution (1 µm) and high thermometric precision (high specific sensitivities of 0.03-0.04 K ) is demonstrated by fluorescence-lifetime imaging of a heated resistive pattern on a flat substrate, covered with a solution-spun film of TPP SbBr .
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http://dx.doi.org/10.1002/adma.202007355DOI Listing
March 2021

Scalable PbS Quantum Dot Solar Cell Production by Blade Coating from Stable Inks.

ACS Appl Mater Interfaces 2021 Feb 20;13(4):5195-5207. Epub 2021 Jan 20.

Zernike Institute for Advanced Materials, Nijenborgh 4, Groningen 9747 AG, The Netherlands.

The recent development of phase transfer ligand exchange methods for PbS quantum dots (QD) has enhanced the performance of quantum dots solar cells and greatly simplified the complexity of film deposition. However, the dispersions of PbS QDs (inks) used for film fabrication often suffer from colloidal instability, which hinders large-scale solar cell production. In addition, the wasteful spin-coating method is still the main technique for the deposition of QD layer in solar cells. Here, we report a strategy for scalable solar cell fabrication from highly stable PbS QD inks. By dispersing PbS QDs capped with CHNHPbI in 2,6-difluoropyridine (DFP), we obtained inks that are colloidally stable for more than 3 months. Furthermore, we demonstrated that DFP yields stable dispersions even of large diameter PbS QDs, which are of great practical relevance owing to the extended coverage of the near-infrared region. The optimization of blade-coating deposition of DFP-based inks enabled the fabrication of PbS QD solar cells with power conversion efficiencies of up to 8.7%. It is important to underline that this performance is commensurate with the devices made by spin coating of inks with the same ligands. A good shelf life-time of these inks manifests itself in the comparatively high photovoltaic efficiency of 5.8% obtained with inks stored for more than 120 days.
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http://dx.doi.org/10.1021/acsami.0c18204DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7863069PMC
February 2021

Temperature-Dependent Charge Carrier Transfer in Colloidal Quantum Dot/Graphene Infrared Photodetectors.

ACS Appl Mater Interfaces 2021 Jan 22;13(1):848-856. Epub 2020 Dec 22.

Empa-Swiss Federal Laboratories for Materials Science and Technology, Überlandstrasse 129, CH-8600 Dübendorf, Switzerland.

Colloidal PbS quantum dot (QD)/graphene hybrid photodetectors are emerging QD technologies for affordable infrared light detectors. By interfacing the QDs with graphene, the photosignal of these detectors is amplified, leading to high responsivity values. While these detectors have been mainly operated at room temperature, low-temperature operation is required for extending their spectral sensitivity beyond a wavelength of 3 μm. Here, we unveil the temperature-dependent response of PbS QD/graphene phototransistors by performing steady-state and time-dependent measurements over a large temperature range of 80-300 K. We find that the temperature dependence of photoinduced charge carrier transfer from the QD layer to graphene is (i) not impeded by freeze-out of the (Schottky-like) potential barrier at low temperatures, (ii) tremendously sensitive to QD surface states (surface oxidation), and (iii) minimally affected by the ligand exposure time and QD layer thickness. Moreover, the specific detectivity of our detectors increases with cooling, with a maximum measured specific detectivity of at least 10 Jones at a wavelength of 1280 nm and a temperature of 80 K, which is an order of magnitude larger compared to the corresponding room temperature value. The temperature- and gate voltage-dependent characterization presented here constitutes an important step in expanding our knowledge of charge transfer at interfaces of low-dimensional materials and toward the realization of next-generation optoelectronic devices.
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http://dx.doi.org/10.1021/acsami.0c15226DOI Listing
January 2021

The dark exciton ground state promotes photon-pair emission in individual perovskite nanocrystals.

Nat Commun 2020 Nov 26;11(1):6001. Epub 2020 Nov 26.

Université de Bordeaux, LP2N, F-33405, Talence, France.

Cesium lead halide perovskites exhibit outstanding optical and electronic properties for a wide range of applications in optoelectronics and for light-emitting devices. Yet, the physics of the band-edge exciton, whose recombination is at the origin of the photoluminescence, is not elucidated. Here, we unveil the exciton fine structure of individual cesium lead iodide perovskite nanocrystals and demonstrate that it is governed by the electron-hole exchange interaction and nanocrystal shape anisotropy. The lowest-energy exciton state is a long-lived dark singlet state, which promotes the creation of biexcitons at low temperatures and thus correlated photon pairs. These bright quantum emitters in the near-infrared have a photon statistics that can readily be tuned from bunching to antibunching, using magnetic or thermal coupling between dark and bright exciton sublevels.
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http://dx.doi.org/10.1038/s41467-020-19740-7DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7691346PMC
November 2020

On the Colloidal Stability of PbS Quantum Dots Capped with Methylammonium Lead Iodide Ligands.

ACS Appl Mater Interfaces 2020 Nov 11;12(47):52959-52966. Epub 2020 Nov 11.

Zernike Institute for Advanced Materials, University of Groningen, Nijenborgh 4, Groningen 9747AG, The Netherlands.

Phase-transfer exchange of pristine organic ligands for inorganic ones is essential for the integration of colloidal quantum dots (CQDs) in optoelectronic devices. This method results in a colloidal dispersion (ink) which can be directly deposited by various solution-processable techniques to fabricate conductive films. For PbS CQDs capped with methylammonium lead iodide ligands (MAPbI), the most commonly employed solvent is butylamine, which enables only a short-term (hours) colloidal stability and thus brings concerns on the possibility of manufacturing CQD devices on a large scale in a reproducible manner. In this work, we studied the stability of alternative inks in two highly polar solvents which impart long-term colloidal stability of CQDs: propylene carbonate (PC) and 2,6-difluoropyridine (DFP). The aging and the loss of the ink's stability were monitored with optical, structural, and transport measurements. With these solvents, PbS CQDs capped with MAPbI ligands retain colloidal stability for more than 20 months, both in dilute and concentrated dispersions. After 17 months of ink storage, transistors with a maximum linear mobility for electrons of 8.5 × 10 cm/V s are fabricated; this value is 17% of the one obtained with fresh solutions. Our results show that both PC- and DFP-based PbS CQD inks offer the needed shelf life to allow for the development of a CQD device technology.
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http://dx.doi.org/10.1021/acsami.0c16646DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7705889PMC
November 2020

Unraveling the Origin of the Long Fluorescence Decay Component of Cesium Lead Halide Perovskite Nanocrystals.

ACS Nano 2020 Nov 11;14(11):14939-14946. Epub 2020 Nov 11.

IBM Research Europe-Zurich, Säumerstrasse 4, 8803 Rüschlikon, Switzerland.

A common signature of nearly all nanoscale emitters is fluorescence intermittency, which is a rapid switching between "on"-states exhibiting a high photon emission rate and "off"-states with a much lower rate. One consequence of fluorescence intermittency occurring on time scales longer than the exciton decay time is the so-called delayed photon emission, manifested by a long radiative decay component. Besides their dominant fast radiative decay, fully inorganic cesium lead halide perovskite quantum dots exhibit a long fluorescence decay component at cryogenic temperatures that is often attributed to the decay of the dark exciton. Here, we show that its origin is delayed photon emission by investigating temporal variations in fluorescence intensity and concomitant decay times found in single CsPbBr perovskite quantum dots. We attribute the different intensity levels of the intensity trace to a rapid switching between a high-intensity exciton state and an Auger-reduced low-intensity trion state that occurs when the excitation is sufficiently strong. Surprisingly, we observe that the exponent of this power-law-dependent delayed emission is correlated with the emission intensity, which cannot be explained with existing charge carrier trapping models. Our analysis reveals that the long decay component is mainly governed by delayed emission, which is present in both the exciton and trion state. The absence of a fine structure in trions clarifies the vanishing role of the dark exciton state for the long decay component. Our findings are essential for the development of a complete photophysical model that captures all observed features of fluorescence variations in colloidal nanocrystals.
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http://dx.doi.org/10.1021/acsnano.0c04401DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7690045PMC
November 2020

Monodisperse CoSb nanocrystals as high-performance anode material for Li-ion batteries.

Chem Commun (Camb) 2020 Nov 22;56(89):13872-13875. Epub 2020 Oct 22.

Laboratory of Inorganic Chemistry, Department of Chemistry and Applied Biosciences, ETH Zürich, Vladimir-Prelog-Weg 1, Zürich CH-8093, Switzerland.

Towards enhancement of the power density of Li-ion batteries (LIBs), antimony-based intermetallic compounds have recently attracted considerable attention as compelling anode materials owing to their high rate capability as compared to state-of-the-art graphite anodes. Here we report a facile colloidal synthesis of monodisperse CoSb nanocrystals (NCs) as a model intermetallic anode material for LIBs via the reaction between Co NCs and SbCl in oleylamine under reducing conditions. We found that ca. 20 nm CoSb NCs exhibit enhanced cycling stability as compared to larger ca. 40 nm CoSb NCs and Sb NCs with size on the order of 20 nm.
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http://dx.doi.org/10.1039/d0cc06222gDOI Listing
November 2020

Solid-State NMR and NQR Spectroscopy of Lead-Halide Perovskite Materials.

J Am Chem Soc 2020 Nov 3;142(46):19413-19437. Epub 2020 Nov 3.

Department of Chemistry and Applied Biosciences, ETH Zürich, Vladimir Prelog Weg 1-5, Zurich CH-8093, Switzerland.

Two- and three-dimensional lead-halide perovskite (LHP) materials are novel semiconductors that have generated broad interest owing to their outstanding optical and electronic properties. Characterization and understanding of their atomic structure and structure-property relationships are often nontrivial as a result of the vast structural and compositional tunability of LHPs as well as the enhanced structure dynamics as compared with oxide perovskites or more conventional semiconductors. Nuclear magnetic resonance (NMR) spectroscopy contributes to this thrust through its unique capability of sampling chemical bonding element-specifically (H, C, N, Cl, K, Br, Rb, I, Cs, and Pb nuclei) and locally and shedding light onto the connectivity, geometry, topology, and dynamics of bonding. NMR can therefore readily observe phase transitions, evaluate phase purity and compositional and structural disorder, and probe molecular dynamics and ionic motion in diverse forms of LHPs, in which they can be used practically, ranging from bulk single crystals (e.g., in gamma and X-ray detectors) to polycrystalline films (e.g., in photovoltaics, photodetectors, and light-emitting diodes) and colloidal nanocrystals (e.g., in liquid crystal displays and future quantum light sources). Herein we also outline the immense practical potential of nuclear quadrupolar resonance (NQR) spectroscopy for characterizing LHPs, owing to the strong quadrupole moments, good sensitivity, and high natural abundance of several halide nuclei (Br and I) combined with the enhanced electric field gradients around these nuclei existing in LHPs as well as the instrumental simplicity. Strong quadrupole interactions, on one side, make Br and I NMR rather impractical but turn NQR into a high-resolution probe of the local structure around halide ions.
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http://dx.doi.org/10.1021/jacs.0c07338DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7677932PMC
November 2020

Efficient Lone-Pair-Driven Luminescence: Structure-Property Relationships in Emissive 5s Metal Halides.

ACS Mater Lett 2020 Sep 4;2(9):1218-1232. Epub 2020 Aug 4.

Laboratory of Inorganic Chemistry, Department of Chemistry and Applied Biosciences, ETH Zürich, Vladimir Prelog Weg 1, CH-8093 Zürich, Switzerland.

Low-dimensional metal halides have been the focus of intense investigations in recent years following the success of hybrid lead halide perovskites as optoelectronic materials. In particular, the light emission of low-dimensional halides based on the 5s cations Sn and Sb has found utility in a variety of applications complementary to those of the three-dimensional halide perovskites because of its unusual properties such as broadband character and highly temperature-dependent lifetime. These properties derive from the exceptional chemistry of the 5s lone pair, but the terminology and explanations given for such emission vary widely, hampering efforts to build a cohesive understanding of these materials that would lead to the development of efficient optoelectronic devices. In this Perspective, we provide a structural overview of these materials with a focus on the dynamics driven by the stereoactivity of the 5s lone pair to identify the structural features that enable strong emission. We unite the different theoretical models that have been able to explain the success of these bright 5s emission centers into a cohesive framework, which is then applied to the array of compounds recently developed by our group and other researchers, demonstrating its utility and generating a holistic picture of the field from the point of view of a materials chemist. We highlight those state-of-the-art materials and applications that demonstrate the unique capabilities of these versatile emissive centers and identify promising future directions in the field of low-dimensional 5s metal halides.
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http://dx.doi.org/10.1021/acsmaterialslett.0c00211DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7491574PMC
September 2020

Supramolecular Approach for Fine-Tuning of the Bright Luminescence from Zero-Dimensional Antimony(III) Halides.

ACS Mater Lett 2020 Jul 17;2(7):845-852. Epub 2020 Jun 17.

Laboratory of Inorganic Chemistry, Department of Chemistry and Applied Bioscience, ETH Zürich, Vladimir Prelog Weg 1, CH-8093 Zürich, Switzerland.

Halides of ns metal ions have recently regained broad research interest as bright narrowband and broadband emitters. Sb(III) is particularly appealing for its oxidative stability (compared to Ge and Sn) and low toxicity (compared to Pb). Square pyramidal SbX anion had thus far been the most common structural motif for realizing high luminescence efficiency, typically when cocrystallized with an organic cation. Luminescent hybrid organic-inorganic halides with octahedral coordination of Sb(III) remain understudied, whereas fully inorganic compounds show very limited structural engineerability. We show that the host-guest complexation of alkali metal cations with crown ethers fosters the formation of zero-dimensional Sb(III) halides and allows for adjusting the coordination number (5 or 6). The obtained compounds exhibit bright photoluminescence with quantum yields of up to 89% originating from self-trapped excitons, with emission energies, Stokes shifts, and luminescence lifetimes finely-adjustable by structural engineering. A combination of environmental stability and strong, intrinsic temperature-dependence of the luminescence lifetimes in the nanosecond-to-microsecond range nominate these compounds as highly potent luminophores for remote thermometry and thermography owing to their sensitivity range of 200-450 K and high specific sensitivities of 0.04 °C.
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http://dx.doi.org/10.1021/acsmaterialslett.0c00174DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7493224PMC
July 2020

Manganese(II) in Tetrahedral Halide Environment: Factors Governing Bright Green Luminescence.

Chem Mater 2019 Dec 15;31(24):10161-10169. Epub 2019 Nov 15.

Laboratory of Inorganic Chemistry, Department of Chemistry and Applied Bioscience, ETH Zürich, Vladimir Prelog Weg 1, CH-8093 Zürich, Switzerland.

Finding narrow-band light emitters for the visible spectral region remains an immense challenge. Such phosphors are in great demand for solid-state lighting and display application. In this context, green luminescence from tetrahedrally coordinated Mn(II) is an attractive research direction. While the oxide-ligand environment had been studied for decades, much less systematic efforts have been undertaken with regard to halide coordination, especially in the form of fully inorganic halide matrixes. In this study, we synthesized a series of hybrid organic-inorganic Mn(II) halides as well as a range of fully inorganic Zn halide hosts (chlorides, bromides, iodides) doped with Mn(II). In the latter, tetrahedral coordination is attained via substitutional doping owing to the tetrahedral symmetry of Zn sites. We find that the choice of the halide as well as subtle details of the crystal structure profoundly govern the photoluminescence peak positions (500-550 nm range) and emission line widths (40-60 nm) as well as radiative lifetimes (shorter for iodides) through the altered ligand-field effects and degrees of spin-orbit coupling. The photoluminescence quantum yields were as high as 70-90%. The major hurdle for the practical use of these compounds lies in their low absorption coefficients in the blue spectral regions.
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http://dx.doi.org/10.1021/acs.chemmater.9b03782DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7493303PMC
December 2019

Fast Neutron Imaging with Semiconductor Nanocrystal Scintillators.

ACS Nano 2020 Nov 18;14(11):14686-14697. Epub 2020 Sep 18.

Laboratory of Inorganic Chemistry, Department of Chemistry and Applied Biosciences, ETH Zürich, Vladimir Prelog Weg 1, Zürich, CH-8093, Switzerland.

Fast neutrons offer high penetration capabilities for both light and dense materials due to their comparatively low interaction cross sections, making them ideal for the imaging of large-scale objects such as large fossils or as-built plane turbines, for which X-rays or thermal neutrons do not provide sufficient penetration. However, inefficient fast neutron detection limits widespread application of this technique. Traditional phosphors such as ZnS:Cu embedded in plastics are utilized as scintillators in recoil proton detectors for fast neutron imaging. However, these scintillation plates exhibit significant light scattering due to the plastic-phosphor interface along with long-lived afterglow (on the order of minutes), and therefore alternative solutions are needed to increase the availability of this technique. Here, we utilize colloidal nanocrystals (NCs) in hydrogen-dense solvents for fast neutron imaging through the detection of recoil protons generated by neutron scattering, demonstrating the efficacy of nanomaterials as scintillators in this detection scheme. The light yield, spatial resolution, and neutron-vs-gamma sensitivity of several chalcogenide (CdSe and CuInS)-based and perovskite halide-based NCs are determined, with only a short-lived afterglow (below the order of seconds) observed for all of these NCs. FAPbBr NCs exhibit the brightest total light output at 19.3% of the commercial ZnS:Cu(PP) standard, while CsPbBrCl:Mn NCs offer the best spatial resolution at ∼2.6 mm. Colloidal NCs showed significantly lower gamma sensitivity than ZnS:Cu; for example, 79% of the FAPbBr light yield results from neutron-induced radioluminescence and hence the neutron-specific light yield of FAPbBr is 30.4% of that of ZnS:Cu(PP). Concentration and thickness-dependent measurements highlight the importance of increasing concentrations and reducing self-absorption, yielding design principles to optimize and foster an era of NC-based scintillators for fast neutron imaging.
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http://dx.doi.org/10.1021/acsnano.0c06381DOI Listing
November 2020

Kinetic modelling of intraband carrier relaxation in bulk and nanocrystalline lead-halide perovskites.

Phys Chem Chem Phys 2020 Aug 28;22(31):17605-17611. Epub 2020 Jul 28.

Ultrafast Optoelectronics Group, Department of Chemistry, Imperial College London, London W12 0BZ, UK.

The relaxation of high-energy "hot" carriers in semiconductors is known to involve the redistribution of energy between hot and cold carriers, as well as the transfer of energy from hot carriers to phonons. Over the past few years, these two processes have been identified in lead-halide perovskites (LHPs) using ultrafast pump-probe experiments, but their interplay is not fully understood. Here we present a practical and intuitive kinetic model that accounts for the effects of both hot and cold carriers on carrier relaxation in LHPs. We apply this model to describe the dynamics of hot carriers in bulk and nanocrystalline CsPbBr as observed by multi-pulse "pump-push-probe" spectroscopy. The model captures the slowing of the relaxation dynamics in the materials as the number of hot carriers increases, which has previously been explained by a "hot-phonon bottleneck" mechanism. The model also correctly predicts an acceleration of the relaxation kinetics as the number of cold carriers in the samples is increased. Using a series of natural approximations, we reduce our model to a simple form containing terms for the carrier-carrier and carrier-phonon interactions. The model can be instrumental for evaluating the details of carrier relaxation and carrier-phonon couplings in LHPs and other soft optoelectronic materials.
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http://dx.doi.org/10.1039/d0cp01599gDOI Listing
August 2020

Bulk and Nanocrystalline Cesium Lead-Halide Perovskites as Seen by Halide Magnetic Resonance.

ACS Cent Sci 2020 Jul 23;6(7):1138-1149. Epub 2020 Jun 23.

Department of Chemistry and Applied Biosciences, ETH Zürich, Vladimir Prelog Weg 1-5, Zürich CH-8093, Switzerland.

Lead-halide perovskites increasingly mesmerize researchers because they exhibit a high degree of structural defects and dynamics yet nonetheless offer an outstanding (opto)electronic performance on par with the best examples of structurally stable and defect-free semiconductors. This highly unusual feature necessitates the adoption of an experimental and theoretical mindset and the reexamination of techniques that may be uniquely suited to understand these materials. Surprisingly, the suite of methods for the structural characterization of these materials does not commonly include nuclear magnetic resonance (NMR) spectroscopy. The present study showcases both the utility and versatility of halide NMR and NQR (nuclear quadrupole resonance) for probing the structure and structural dynamics of CsPbX (X = Cl, Br, I), in both bulk and nanocrystalline forms. The strong quadrupole couplings, which originate from the interaction between the large quadrupole moments of, e.g., the Cl, Br, and I nuclei, and the local electric-field gradients, are highly sensitive to subtle structural variations, both static and dynamic. The quadrupole interaction can resolve structural changes with accuracies commensurate with synchrotron X-ray diffraction and scattering. It is shown that space-averaged site-disorder is greatly enhanced in the nanocrystals compared to the bulk, while the dynamics of nuclear spin relaxation indicates enhanced structural dynamics in the nanocrystals. The findings from NMR and NQR were corroborated by molecular dynamics, which point to the role of the surface in causing the radial strain distribution and disorder. These findings showcase a great synergy between solid-state NMR or NQR and molecular dynamics simulations in shedding light on the structure of soft lead-halide semiconductors.
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http://dx.doi.org/10.1021/acscentsci.0c00587DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7379391PMC
July 2020

Bright Blue and Green Luminescence of Sb(III) in Double Perovskite CsMInCl (M = Na, K) Matrices.

Chem Mater 2020 Jun 8;32(12):5118-5124. Epub 2020 Jun 8.

Laboratory of Inorganic Chemistry, Department of Chemistry and Applied Bioscience, ETH Zürich, Vladimir Prelog Weg 1, CH-8093 Zürich, Switzerland.

The vast structural and compositional space of metal halides has recently become a major research focus for designing inexpensive and versatile light sources; in particular, for applications in displays, solid-state lighting, lasing, etc. Compounds with isolated ns-metal halide centers often exhibit bright broadband emission that stems from self-trapped excitons (STEs). The Sb(III) halides are attractive STE emitters due to their low toxicity and oxidative stability; however, coupling these features with an appropriately robust, fully inorganic material containing Sb in an octahedral halide environment has proven to be a challenge. Here, we investigate Sb as a dopant in a solution-grown metal halide double perovskite (DP) matrix, namely CsMInCl:Sb (M = Na, K, = 0-100%). CsKInCl is found to crystallize in the tetragonal DP phase, unlike CsNaInCl that adopts the traditional cubic DP structure. This structural difference results in distinct emission colors, as CsNaInCl:Sb and CsKInCl:Sb compounds exhibit broadband blue and green emissions, respectively, with photoluminescence quantum yields (PLQYs) of up to 93%. Spectroscopic and computational investigations confirm that this efficient emission originates from Sb(III)-hosted STEs. These fully inorganic DP compounds demonstrate that Sb(III) can be incorporated as a bright emissive center for stable lighting applications.
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http://dx.doi.org/10.1021/acs.chemmater.0c01004DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7315817PMC
June 2020

Electrophoretic Deposition of Nanoporous Oxide Coatings from Concentrated CuO Nanoparticle Dispersions.

Langmuir 2020 Jul 6;36(28):8075-8085. Epub 2020 Jul 6.

Empa, Swiss Federal Laboratories for Materials Science and Technology, Dübendorf 8600, Switzerland.

Electrophoretic deposition (EPD) of nanoporous oxide coatings is an interesting research avenue owing to the experimental simplicity and broad scope of applications and materials. In this study, the properties of concentrated (up to 5000 mg/L), nonaqueous CuO nanoparticle (NP) dispersions were tailored to produce micrometer-thick, nanoporous CuO films by EPD. In particular, we performed a systematic investigation of the electrophoretic mobilities and size distributions of dispersed CuO aggregates and developing agglomerates in different organic solvents for concentrations ranging from 50 to 5000 mg/L with and without surfactant addition. Time-resolved dynamic light scattering analyses showed that aggregate mobilities and agglomeration rates decrease with increasing hydrocarbon chain length of the organic solvent (from ethanol to hexanol) and thus with increasing viscosity. The highest electrophoretic mobility was obtained for CuO NP aggregates and agglomerates dispersed in ethanol as a solvent. However, the addition of ≥0.5 wt % acetylacetone as a surfactant is required to stabilize these dispersions for subsequent EPD and at the same time introduce a net attractive (electrostatic) interaction between neighboring agglomerates on the substrate to promote layer formation during the EPD step. The produced micrometer-thick nanoporous CuO coatings can serve as high surface area nanostructured materials or nanoporous scaffolds in catalysis, combustion, propellants, and nanojoining.
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http://dx.doi.org/10.1021/acs.langmuir.0c00720DOI Listing
July 2020

Silicon oxycarbide-antimony nanocomposites for high-performance Li-ion battery anodes.

Nanoscale 2020 Jul;12(25):13540-13547

Laboratory of Inorganic Chemistry, Department of Chemistry and Applied Biosciences, ETH Zürich, Vladimir-Prelog-Weg 1, CH-8093 Zürich, Switzerland. and Laboratory for Thin Films and Photovoltaics, Empa - Swiss Federal Laboratories for Materials Science and Technology, Überlandstrasse 129, CH-8600 Dübendorf, Switzerland.

Silicon oxycarbide (SiOC) has recently regained attention in the field of Li-ion batteries, owing to its effectiveness as a host matrix for nanoscale anode materials alloying with Li. The SiOC matrix, itself providing a high Li-ion storage capacity of 600 mA h g-1, assists in buffering volumetric changes upon lithiation and largely suppresses the formation of an unstable solid-electrolyte interface. Herein, we present the synthesis of homogeneously embedded Sb nanoparticles in a SiOC matrix with the size of 5-40 nm via the pyrolysis of a preceramic polymer. The latter is obtained through the Pt-catalyzed gelation reaction of Sb 2-ethylhexanoate and a poly(methylhydrosiloxane)/divinylbenzene mixture. The complete miscibility of these precursors was achieved by the functionalization of poly(methylhydrosiloxane) with apolar divinyl benzene side-chains. We show that anodes composed of SiOC/Sb exhibit a high rate capability, delivering charge storage capacity in the range of 703-549 mA h g-1 at a current density of 74.4-2232 mA g-1. The impact of Sb on the Si-O-C bonding and on free carbon content of SiOC matrix, along with its concomitant influence on Li-ion storage capacity of SiOC was assessed by Raman and 29Si and 7Li solid-state NMR spectroscopies.
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http://dx.doi.org/10.1039/d0nr02930kDOI Listing
July 2020

The Rb Bi Sb Cl Family: A Fully Inorganic Solid Solution with Room-Temperature Luminescent Members.

Angew Chem Int Ed Engl 2020 Aug 2;59(34):14490-14497. Epub 2020 Jul 2.

Laboratory of Inorganic Chemistry, ETH Zürich, 8093, Zürich, Switzerland.

Low-dimensional ns -metal halide compounds have received immense attention for applications in solid-state lighting, optical thermometry and thermography, and scintillation. However, these are based primarily on the combination of organic cations with toxic Pb or unstable Sn , and a stable inorganic luminescent material has yet to be found. Here, the zero-dimensional Rb Sb Cl phase, comprised of isolated [SbCl ] octahedra and edge-sharing [Sb Cl ] dimers, shows room-temperature photoluminescence (RT PL) centered at 560 nm with a quantum yield of 3.8±0.2 % at 296 K (99.4 % at 77 K). The temperature-dependent PL lifetime rivals that of previous low-dimensional materials with a specific temperature sensitivity above 0.06 K at RT, making it an excellent thermometric material. Utilizing both DFT and chemical substitution with Bi in the Rb Bi Sb Cl (x≤1) family, we present the edge-shared [Sb Cl ] dimer as a design principle for Sb-based luminescent materials.
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http://dx.doi.org/10.1002/anie.202003822DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7496723PMC
August 2020

Lead-Halide Scalar Couplings in Pb NMR of APbX Perovskites (A = Cs, Methylammonium, Formamidinium; X = Cl, Br, I).

Sci Rep 2020 May 19;10(1):8229. Epub 2020 May 19.

Department of Chemistry and Applied Biosciences, ETH Zürich, Vladimir-Prelog-Weg 1-5, CH-8093, Switzerland.

Understanding the structure and dynamics of newcomer optoelectronic materials - lead halide perovskites APbX [A = Cs, methylammonium (CHNH, MA), formamidinium (CH(NH), FA); X = Cl, Br, I] - has been a major research thrust. In this work, new insights could be gained by using Pb solid-state nuclear magnetic resonance (NMR) spectroscopy at variable temperatures between 100 and 300 K. The existence of scalar couplings J of ca. 400 Hz and J of ca. 2.3 kHz could be confirmed for MAPbX and CsPbX. Diverse and fast structure dynamics, including rotations of A-cations, harmonic and anharmonic vibrations of the lead-halide framework and ionic mobility, affect the resolution of the coupling pattern. Pb NMR can therefore be used to detect the structural disorder and phase transitions. Furthermore, by comparing bulk and nanocrystalline CsPbBr a greater structural disorder of the PbBr-octahedra had been confirmed in a nanoscale counterpart, not readily captured by diffraction-based techniques.
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http://dx.doi.org/10.1038/s41598-020-65071-4DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7237655PMC
May 2020

Memories in the photoluminescence intermittency of single cesium lead bromide nanocrystals.

Nanoscale 2020 Mar 17;12(12):6795-6802. Epub 2020 Mar 17.

Université de Bordeaux, LP2N, Talence, France.

Single cesium lead bromide (CsPbBr) nanocrystals show strong photoluminescence intermittency, with on- and off- dwelling times following power-law distributions. We investigate the correlations for successive on-times and successive off-times, and find a memory effect in the photoluminescence intermittency of such inorganic perovskite nanocrystals. This memory effect is not sensitive to the nature of the surface capping ligand and the embedding polymer. These observations suggest that photoluminescence intermittency and its memory are mainly controlled by intrinsic traps in the nanocrystals. Our findings will help optimizing light-emitting devices based on these perovskite nanocrystals.
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http://dx.doi.org/10.1039/d0nr00633eDOI Listing
March 2020

Hot Carrier Dynamics in Perovskite Nanocrystal Solids: Role of the Cold Carriers, Nanoconfinement, and the Surface.

Nano Lett 2020 Apr 12;20(4):2271-2278. Epub 2020 Mar 12.

Department of Chemistry, Imperial College London, London W12 0BZ, United Kingdom.

Carrier cooling is of widespread interest in the field of semiconductor science. It is linked to carrier-carrier and carrier-phonon coupling and has profound implications for the photovoltaic performance of materials. Recent transient optical studies have shown that a high carrier density in lead-halide perovskites (LHPs) can reduce the cooling rate through a "phonon bottleneck". However, the role of carrier-carrier interactions, and the material properties that control cooling in LHPs, is still disputed. To address these factors, we utilize ultrafast "pump-push-probe" spectroscopy on LHP nanocrystal (NC) films. We find that the addition of cold carriers to LHP NCs increases the cooling rate, competing with the phonon bottleneck. By comparing different NCs and bulk samples, we deduce that the cooling behavior is intrinsic to the LHP composition and independent of the NC size or surface. This can be contrasted with other colloidal nanomaterials, where confinement and trapping considerably influence the cooling dynamics.
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http://dx.doi.org/10.1021/acs.nanolett.9b04491DOI Listing
April 2020

Vibrational dynamics in lead halide hybrid perovskites investigated by Raman spectroscopy.

Phys Chem Chem Phys 2020 Mar;22(10):5604-5614

Institute of Solid State Physics, Technische Universitat Berlin, Hardenbergstr. 36, 10623 Berlin, Germany.

Lead halide perovskite semiconductors providing record efficiencies of solar cells have usually mixed compositions doped in A- and X-sites to enhance the phase stability. The cubic form of formamidinium (FA) lead iodide reveals excellent opto-electronic properties but transforms at room temperature (RT) into a hexagonal structure which does not effectively absorb visible light. This metastable form and the mechanism of its stabilization by Cs+ and Br- incorporation are poorly characterized and insufficiently understood. We report here the vibrational properties of cubic FAPbI3 investigated by DFT calculations on phonon frequencies and intensities, and micro-Raman spectroscopy. The effects of Cs+ and Br- partial substitution are discussed. We support our results with the study of FAPbBr3 which expands the identification of vibrational modes to the previously unpublished low frequency region (<500 cm-1). Our results show that the incorporation of Cs+ and Br- leads to the coupling of the displacement of the A-site components and weakens the bonds between FA+ and the PbX6 octahedra. We suggest that the enhancement of α-FAPbI3 stability can be a product of the release of tensile stresses in the Pb-X bond, which is reflected in a red-shift of the low frequency region of the Raman spectrum (<200 cm-1).
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http://dx.doi.org/10.1039/c9cp06568gDOI Listing
March 2020

Colloidal-ALD-Grown Core/Shell CdSe/CdS Nanoplatelets as Seen by DNP Enhanced PASS-PIETA NMR Spectroscopy.

Nano Lett 2020 May 29;20(5):3003-3018. Epub 2020 Apr 29.

Department of Chemistry and Applied Biosciences, ETH Zürich, Vladimir Prelog Weg 1-5, Zurich CH-8093, Switzerland.

Ligand exchange and CdS shell growth onto colloidal CdSe nanoplatelets (NPLs) using colloidal atomic layer deposition (c-ALD) were investigated by solid-state nuclear magnetic resonance (NMR) experiments, in particular, dynamic nuclear polarization (DNP) enhanced phase adjusted spinning sidebands-phase incremented echo-train acquisition (PASS-PIETA). The improved sensitivity and resolution of DNP enhanced PASS-PIETA permits the identification and study of the core, shell, and surface species of CdSe and CdSe/CdS core/shell NPLs heterostructures at all stages of c-ALD. The cadmium chemical shielding was found to be proportionally dependent on the number and nature of coordinating chalcogen-based ligands. DFT calculations permitted the separation of the the Cd chemical shielding into its different components, revealing that the varying strength of paramagnetic and spin-orbit shielding contributions are responsible for the chemical shielding trend of cadmium chalcogenides. Overall, this study points to the roughening and increased chemical disorder at the surface during the shell growth process, which is not readily captured by the conventional characterization tools such as electron microscopy.
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http://dx.doi.org/10.1021/acs.nanolett.9b04870DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7227022PMC
May 2020

Exclusive Electron Transport in Core@Shell PbTe@PbS Colloidal Semiconductor Nanocrystal Assemblies.

ACS Nano 2020 Mar 2;14(3):3242-3250. Epub 2020 Mar 2.

Department of Materials Science and Engineering, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro, Tokyo 152-8550, Japan.

Assemblies of colloidal semiconductor nanocrystals (NCs) in the form of thin solid films leverage the size-dependent quantum confinement properties and the wet chemical methods vital for the development of the emerging solution-processable electronics, photonics, and optoelectronics technologies. The ability to control the charge carrier transport in the colloidal NC assemblies is fundamental for altering their electronic and optical properties for the desired applications. Here we demonstrate a strategy to render the solids of narrow-bandgap NC assemblies exclusively electron-transporting by creating a type-II heterojunction shelling. Electronic transport of molecularly cross-linked PbTe@PbS core@shell NC assemblies is measured using both a conventional solid gate transistor and an electric-double-layer transistor, as well as compared with those of core-only PbTe NCs. In contrast to the ambipolar characteristics demonstrated by many narrow-bandgap NCs, the core@shell NCs exhibit exclusive n-type transport, .., drastically suppressed contribution of holes to the overall transport. The PbS shell that forms a type-II heterojunction assists the selective carrier transport by heavy doping of electrons into the PbTe-core conduction level and simultaneously strongly localizes the holes within the NC core valence level. This strongly enhanced n-type transport makes these core@shell NCs suitable for applications where ambipolar characteristics should be actively suppressed, in particular, for thermoelectric and electron-transporting layers in photovoltaic devices.
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http://dx.doi.org/10.1021/acsnano.9b08687DOI Listing
March 2020

Colloidal Antimony Sulfide Nanoparticles as a High-Performance Anode Material for Li-ion and Na-ion Batteries.

Sci Rep 2020 Feb 13;10(1):2554. Epub 2020 Feb 13.

Laboratory for Thin Films and Photovoltaics, Empa - Swiss Federal Laboratories for Materials Science and Technology, Überlandstrasse 129, CH-8600, Dübendorf, Switzerland.

To maximize the anodic charge storage capacity of Li-ion and Na-ion batteries (LIBs and SIBs, respectively), the conversion-alloying-type SbS anode has attracted considerable interest because of its merits of a high theoretical capacity of 946 mAh g and a suitable anodic lithiation/delithiation voltage window of 0.1-2 V vs. Li/Li. Recent advances in nanostructuring of the SbS anode provide an effective way of mitigating the challenges of structure conversion and volume expansion upon lithiation/sodiation that severely hinder the SbS cycling stability. In this context, we report uniformly sized colloidal SbS nanoparticles (NPs) as a model SbS anode material for LIBs and SIBs to investigate the effect of the primary particle size on the electrochemical performance of the SbS anode. We found that compared with microcrystalline SbS, smaller ca. 20-25 nm and ca. 180-200 nm SbS NPs exhibit enhanced cycling stability as anode materials in both rechargeable LIBs and SIBs. Importantly, for the ca. 20-25 nm SbS NPs, a high initial Li-ion storage capacity of 742 mAh g was achieved at a current density of 2.4 A g. At least 55% of this capacity was retained after 1200 cycles, which is among the most stable performance SbS anodes for LIBs.
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http://dx.doi.org/10.1038/s41598-020-59512-3DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7018818PMC
February 2020

A Small Cationic Organo-Copper Cluster as Thermally Robust Highly Photo- and Electroluminescent Material.

J Am Chem Soc 2020 Jan 20;142(1):373-381. Epub 2019 Dec 20.

Institut für Anorganische Chemie und Kristallographie , Universität Bremen , Leobener Straße 7 , 28359 Bremen , Germany.

Organic light-emitting diodes (OLEDs) are revolutionizing display applications. In this aspect, luminescent complexes of precious metals such as iridium, platinum, or ruthenium still playing a significant role. Emissive compounds of earth-abundant copper with equivalent performance are desired for practical, large-scale applications such as solid-state lighting and displays. Copper(I)-based emitters are well-known to suffer from weak spin-orbit coupling and a high reorganization energy upon photoexcitation. Here we report a cationic organo-copper cluster [Cu()] ( = 2,6-(PPh)CH) that features suppressed nonradiative decays, giving rise to a robust narrow-band green luminophore with a photoluminescent (PL) efficiency up to 93%. PL decay kinetics corroborated by DFT calculations reveal a complex emission mechanism involving contributions of both thermally activated delayed fluorescence and phosphorescence. This robust compound was solution-processed into a thin film in prototype OLEDs with external quantum efficiency up to 11% and a narrow emission bandwidth (65 nm fwhm).
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http://dx.doi.org/10.1021/jacs.9b10829DOI Listing
January 2020

Amplified Spontaneous Emission Threshold Reduction and Operational Stability Improvement in CsPbBr Nanocrystals Films by Hydrophobic Functionalization of the Substrate.

Sci Rep 2019 Nov 29;9(1):17964. Epub 2019 Nov 29.

Dipartimento di Matematica e Fisica "Ennio De Giorgi", Università del Salento, Via per Arnesano, 73100, Lecce, Italy.

The use of lead halide perovskites in optoelectronic and photonic devices is mainly limited by insufficient long-term stability of these materials. This issue is receiving growing attention, mainly owing to the operational stability improvement of lead halide perosvkites solar cells. On the contrary, fewer efforts are devoted to the stability improvement of light amplification and lasing. In this report we demonstrate that a simple hydrophobic functionalization of the substrates with hexamethyldisilazane (HMDS) allows to strongly improve the Amplified Spontaneous Emission (ASE) properties of drop cast CsPbBr nanocrystal (NC) thin films. In particular we observe an ASE threshold decrease down to 45% of the value without treatment, an optical gain increase of up to 1.5 times and an ASE operational stability increase of up to 14 times. These results are ascribed to a closer NC packing in the films on HMDS treated substrate, allowing an improved energy transfer towards the larger NCs within the NC ensemble, and to the reduction of the film interaction with moisture. Our results propose hydrophobic functionalization of the substrates as an easy approach to lower the ASE and lasing thresholds, while simultaneously increasing the active material stability.
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http://dx.doi.org/10.1038/s41598-019-54412-7DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6884571PMC
November 2019