Publications by authors named "Dmitry N Dirin"

32 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

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

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

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

Energy Transfer from Perovskite Nanocrystals to Dye Molecules Does Not Occur by FRET.

Nano Lett 2019 12 5;19(12):8896-8902. Epub 2019 Nov 5.

Institut für Experimentelle und Angewandte Physik , Universität Regensburg , Universitätsstraße 31 , 93053 Regensburg , Germany.

Single formamidinium lead bromide (FAPbBr) perovskite nanocubes, approximately 10 nm in size, have extinction cross sections orders of magnitude larger than single dye molecules and can therefore be used to photoexcite one single dye molecule within their immediate vicinity by means of excitation-energy transfer (EET). The rate of photon emission by the single dye molecule is increased by 2 orders of magnitude under excitation by EET compared to direct excitation at the same laser fluence. Because the dye cannot accommodate biexcitons, NC biexcitons are filtered out by EET, giving rise to up to an order-of-magnitude improvement in the fidelity of photon antibunching. We demonstrate here that, contrary to expectation, energy transfer from the nanocrystal to dye molecules does not depend on the spectral line widths of the donor and acceptor and is therefore not governed by Förster's theory of resonance energy transfer (FRET). Two different cyanine dye acceptors with substantially different spectral overlaps with the nanocrystal donor show a similar light-harvesting capability. Cooling the sample from room temperature to 5 K reduces the average transition line widths 25-fold but has no apparent effect on the number of molecules emitting, i.e., on the spatial density of single dye molecules being photoexcited by single nanocrystals. Narrow zero-phonon lines are identified for both donor and acceptor, with an energetic separation of over 40 times the line width, implying a complete absence of spectral overlap-even though EET is evident. Both donor and acceptor exhibit spectral fluctuations, but no correlation is apparent between the jitter, which controls spectral overlap, and the overall light harvesting. We conclude that the energy transfer process is fundamentally nonresonant, implying effective energy dissipation in the perovskite donor because of strong electron-phonon coupling of the carriers comprising the exciton. The work highlights the importance of performing cryogenic spectroscopy to reveal the underlying mechanisms of energy transfer in complex donor-acceptor systems.
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http://dx.doi.org/10.1021/acs.nanolett.9b03779DOI Listing
December 2019

Microcarrier-Assisted Inorganic Shelling of Lead Halide Perovskite Nanocrystals.

ACS Nano 2019 Oct 10;13(10):11642-11652. Epub 2019 Oct 10.

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

The conventional strategy of synthetic colloidal chemistry for bright and stable quantum dots has been the production of epitaxially matched core/shell heterostructures to mitigate the presence of deep trap states. This mindset has been shown to be incompatible with lead halide perovskite nanocrystals (LHP NCs) due to their dynamic surface and low melting point. Nevertheless, enhancements to their chemical stability are still in great demand for the deployment of LHP NCs in light-emitting devices. Rather than contend with their attributes, we propose a method in which we can utilize their dynamic, ionic lattice and uniquely defect-tolerant band structure to prepare non-epitaxial salt-shelled heterostructures that are able to stabilize these materials against their environment, while maintaining their excellent optical properties and increasing scattering to improve out-coupling efficiency. To do so, anchored LHP NCs are first synthesized through the heterogeneous nucleation of LHPs onto the surface of microcrystalline carriers, such as alkali halides. This first step stabilizes the LHP NCs against further merging, and this allows them to be coated with an additional inorganic shell through the surface-mediated reaction of amphiphilic Na and Br precursors in apolar media. These inorganically shelled NC@carrier composites offer significantly improved chemical stability toward polar organic solvents, such as γ-butyrolactone, acetonitrile, -methylpyrrolidone, and trimethylamine, demonstrate high thermal stability with photoluminescence intensity reversibly dropping by no more than 40% at temperatures up to 120 °C, and improve compatibility with various UV-curable resins. This mindset for LHP NCs creates opportunities for their successful integration into next-generation light-emitting devices.
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http://dx.doi.org/10.1021/acsnano.9b05481DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6812064PMC
October 2019

Self-Assembly of Proteinaceous Shells around Positively Charged Gold Nanomaterials Enhances Colloidal Stability in High-Ionic-Strength Buffers.

Chembiochem 2020 01 18;21(1-2):74-79. Epub 2019 Nov 18.

Laboratory of Organic Chemistry, ETH Zürich, Vladimir-Prelog-Weg, 1-5/10, 8093, Zürich, Switzerland.

The enzyme lumazine synthase (LS) has been engineered to self-assemble into hollow-shell structures that encapsulate unnatural cargo proteins through complementary electrostatic interactions. Herein, we show that a negatively supercharged LS variant can also form organic-inorganic hybrids with gold nanomaterials. Simple mixing of LS pentamers with positively charged gold nanocrystals in aqueous buffer spontaneously affords protein-shelled gold cores. The procedure works well with differently sized and shaped gold nanocrystals, and the resulting shelled complexes exhibit dramatically enhanced colloidal stability over a wide range of pH (4.0-10.0) and at high ionic strength (up to 1 m NaCl). They are even stable over days upon dilution in buffer. Self-assembly of engineered LS shells in this way offers an easy and attractive alternative to commonly used ligand-exchange methods for stabilizing inorganic nanomaterials.
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http://dx.doi.org/10.1002/cbic.201900469DOI Listing
January 2020

High-resolution remote thermometry and thermography using luminescent low-dimensional tin-halide perovskites.

Nat Mater 2019 Aug 1;18(8):846-852. Epub 2019 Jul 1.

Laboratory of Inorganic Chemistry, Department of Chemistry and Applied Biosciences, ETH Zurich, Zurich, Switzerland.

Although metal-halide perovskites have recently revolutionized research in optoelectronics through a unique combination of performance and synthetic simplicity, their low-dimensional counterparts can further expand the field with hitherto unknown and practically useful optical functionalities. In this context, we present the strong temperature dependence of the photoluminescence lifetime of low-dimensional, perovskite-like tin-halides and apply this property to thermal imaging. The photoluminescence lifetimes are governed by the heat-assisted de-trapping of self-trapped excitons, and their values can be varied over several orders of magnitude by adjusting the temperature (up to 20 ns °C). Typically, this sensitive range spans up to 100 °C, and it is both compound-specific and shown to be compositionally and structurally tunable from -100 to 110 °C going from [C(NH)]SnBr to CsSnBr and (CNHI)SnI. Finally, through the implementation of cost-effective hardware for fluorescence lifetime imaging, based on time-of-flight technology, these thermoluminophores have been used to record thermographic videos with high spatial and thermal resolution.
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http://dx.doi.org/10.1038/s41563-019-0416-2DOI Listing
August 2019

Underestimated Effect of a Polymer Matrix on the Light Emission of Single CsPbBr Nanocrystals.

Nano Lett 2019 06 28;19(6):3648-3653. Epub 2019 May 28.

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

Lead-halide perovskite APbX (A = Cs or organic cation; X = Cl, Br, I) nanocrystals (NCs) are the subject of intense research due to their exceptional characteristics as both classical and quantum light sources. Many challenges often faced with this material class concern the long-term optical stability, a serious intrinsic issue connected with the labile and polar crystal structure of APbX compounds. When conducting spectroscopy at a single particle level, due to the highly enhanced contaminants (e.g., water molecules, oxygen) over the NC ratio, deterioration of NC optical properties occurs within tens of seconds with typically used excitation power densities (1-100 W/cm) and in ambient conditions. Here, we demonstrate that choosing a suitable polymer matrix is of paramount importance for obtaining stable spectra from a single NC and for suppressing the dynamic photoluminescence blueshift. In particular, polystyrene (PS), the most hydrophobic among four tested polymers, leads to the best optical stability, one to two orders of magnitude higher than that obtained with poly(methyl methacrylate), a common polymeric encapsulant containing polar ester groups. Molecular mechanics simulations based on a force-field approximation corroborate the hypothesis that PS affords for a denser molecular packing at the NC surface. These findings underscore the often-neglected role of the sample preparation methodologies for the assessment of the optical properties of perovskite NCs at a single-particle level and guide the further design of robust single photon sources.
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http://dx.doi.org/10.1021/acs.nanolett.9b00689DOI Listing
June 2019

Author Correction: Coherent spin dynamics of electrons and holes in CsPbBr3 perovskite crystals.

Nat Commun 2019 Apr 15;10(1):1746. Epub 2019 Apr 15.

Experimentelle Physik 2, Technische Universität Dortmund, D-44221, Dortmund, Germany.

The original version of this Article contained an error in Fig. 2c, in which the numbers on the y-axis were given in the wrong order: '800' at the bottom through to '0' at the top. This has been corrected in both the PDF and HTML versions of the Article.
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http://dx.doi.org/10.1038/s41467-019-09821-7DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6465279PMC
April 2019

Coherent spin dynamics of electrons and holes in CsPbBr perovskite crystals.

Nat Commun 2019 02 8;10(1):673. Epub 2019 Feb 8.

Experimentelle Physik 2, Technische Universität Dortmund, D-44221, Dortmund, Germany.

The lead halide perovskites demonstrate huge potential for optoelectronic applications, high energy radiation detectors, light emitting devices and solar energy harvesting. Those materials exhibit strong spin-orbit coupling enabling efficient optical orientation of carrier spins in perovskite-based devices with performance controlled by a magnetic field. Here we show that elaborated time-resolved spectroscopy involving strong magnetic fields can be successfully used for perovskites. We perform a comprehensive study of high-quality lead halide perovskite CsPbBr crystals by measuring the exciton and charge carrier g-factors, spin relaxation times and hyperfine interaction of carrier and nuclear spins by means of coherent spin dynamics. Owing to their 'inverted' band structure, perovskites represent appealing model systems for semiconductor spintronics exploiting the valence band hole spins, while in conventional semiconductors the conduction band electrons are considered for spin functionality.
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http://dx.doi.org/10.1038/s41467-019-08625-zDOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6368575PMC
February 2019

Comparing Halide Ligands in PbS Colloidal Quantum Dots for Field-Effect Transistors and Solar Cells.

ACS Appl Nano Mater 2018 Dec 9;1(12):6882-6889. Epub 2018 Nov 9.

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

Capping colloidal quantum dots (CQDs) with atomic ligands is a powerful approach to tune their properties and improve the charge carrier transport in CQD solids. Efficient passivation of the CQD surface, which can be achieved with halide ligands, is crucial for application in optoelectronic devices. Heavier halides, i.e., I and Br, have been thoroughly studied as capping ligands in the last years, but passivation with fluoride ions has not received sufficient consideration. In this work, effective coating of PbS CQDs with fluoride ligands is demonstrated and compared to the results obtained with other halides. The electron mobility in field-effect transistors of PbS CQDs treated with different halides shows an increase with the size of the atomic ligand (from 3.9 × 10 cm/(V s) for fluoride-treated to 2.1 × 10 cm/(V s) for iodide-treated), whereas the hole mobility remains unchanged in the range between 1 × 10 cm/(V s) and 10cm/(V s). This leads to a relatively more pronounced p-type behavior of the fluoride- and chloride-treated films compared to the iodide-treated ones. Cl- and F-capped PbS CQDs solids were then implemented as p-type layer in solar cells; these devices showed similar performance to those prepared with 1,2-ethanedithiol in the same function. The relatively stronger p-type character of the fluoride- and chloride-treated PbS CQD films broadens the utility of such materials in optoelectronic devices.
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http://dx.doi.org/10.1021/acsanm.8b01696DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6317010PMC
December 2018

Electroluminescence Generation in PbS Quantum Dot Light-Emitting Field-Effect Transistors with Solid-State Gating.

ACS Nano 2018 Dec 14;12(12):12805-12813. Epub 2018 Dec 14.

Zernike Institute for Advanced Materials , University of Groningen , NL-9747AG Groningen , The Netherlands.

The application of light-emitting field-effect transistors (LEFET) is an elegant way of combining electrical switching and light emission in a single device architecture instead of two. This allows for a higher degree of miniaturization and integration in future optoelectronic applications. Here, we report on a LEFET based on lead sulfide quantum dots processed from solution. Our device shows state-of-the-art electronic behavior and emits near-infrared photons with a quantum yield exceeding 1% when cooled. We furthermore show how LEFETs can be used to simultaneously characterize the optical and electrical material properties on the same device and use this benefit to investigate the charge transport through the quantum dot film.
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http://dx.doi.org/10.1021/acsnano.8b07938DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6307172PMC
December 2018

Size-Dependent Fault-Driven Relaxation and Faceting in Zincblende CdSe Colloidal Quantum Dots.

ACS Nano 2018 Dec 10;12(12):12558-12570. Epub 2018 Dec 10.

Istituto di Cristallografia and To.Sca.Lab , Consiglio Nazionale delle Ricerche , Via Valleggio 11 , I-22100 Como , Italy.

Surface chemistry and core defects are known to play a prominent role in governing the photophysical properties of nanocrystalline semiconductors. Nevertheless, investigating them in small nanocrystals remains a complex task. Here, by combining X-ray scattering techniques in the wide- and small-angle regions and using the Debye scattering equation (DSE) method of analysis, we unveil a high density of planar defects in oleate-terminated zincblende (ZB) CdSe colloidal quantum dots (QDs) and size-dependent faceting within a square-cuboid morphology. Atomistic models of faulted ZB nanocrystals, based on the probabilistic stacking of CdSe layers in cubic and hexagonal sequences, and data analysis point to the preferential location of faults near the center of nanocrystals. By finely modeling faulting and morphological effects on the X-ray scattering pattern, a relaxation of the Cd-Se bond distance parallel to the stacking direction, up to +3% (2.71 Å) with respect to the reference bulk value (2.63 Å), is detected, at the cubic/hexagonal transitions. The smallest nanocrystals show cubic {100} facets; {111} facets appear above 4 nm and progressively extend at larger sizes. These structural and morphological features likely vary depending on the synthesis conditions; nevertheless, since planar defects are nearly ubiquitous in CdSe QDs, the modeling approach here presented has a general validity. This work also points to the great potential of combining small- and wide-angle X-ray scattering and DSE-modeling techniques in gaining important knowledge on atomic-scale defects of semiconductor nanocrystals, underpinning the comprehension of the impact of structural defectiveness on the exciting properties of these QDs.
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http://dx.doi.org/10.1021/acsnano.8b07092DOI Listing
December 2018

Resolving the Core and the Surface of CdSe Quantum Dots and Nanoplatelets Using Dynamic Nuclear Polarization Enhanced PASS-PIETA NMR Spectroscopy.

ACS Cent Sci 2018 Sep 25;4(9):1113-1125. Epub 2018 Jun 25.

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

Understanding the surface of semiconductor nanocrystals (NCs) prepared using colloidal methods is a long-standing goal of paramount importance for all their potential optoelectronic applications, which remains unsolved largely because of the lack of site-specific physical techniques. Here, we show that multidimensional Cd dynamic nuclear polarization (DNP) enhanced NMR spectroscopy allows the resolution of signals originating from different atomic and magnetic surroundings in the NC cores and at the surfaces. This enables the determination of the structural perfection, and differentiation between the surface and core atoms in all major forms of size- and shape-engineered CdSe NCs: irregularly faceted quantum dots (QDs) and atomically flat nanoplatelets, including both dominant polymorphs (zinc-blende and wurtzite) and their epitaxial nanoheterostructures (CdSe/CdS core/shell quantum dots and CdSe/CdS core/crown nanoplatelets), as well as magic-sized CdSe clusters. Assignments of the NMR signals to specific crystal facets of oleate-terminated ZB structured CdSe NCs are proposed. Significantly, we discover far greater atomistic complexity of the surface structure and the species distribution in wurtzite as compared to zinc-blende CdSe QDs, despite an apparently identical optical quality of both QD polymorphs.
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http://dx.doi.org/10.1021/acscentsci.8b00196DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6161058PMC
September 2018

Highly Emissive Self-Trapped Excitons in Fully Inorganic Zero-Dimensional Tin Halides.

Angew Chem Int Ed Engl 2018 Aug 30;57(35):11329-11333. Epub 2018 Jul 30.

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

The spatial localization of charge carriers to promote the formation of bound excitons and concomitantly enhance radiative recombination has long been a goal for luminescent semiconductors. Zero-dimensional materials structurally impose carrier localization and result in the formation of localized Frenkel excitons. Now the fully inorganic, perovskite-derived zero-dimensional Sn material Cs SnBr is presented that exhibits room-temperature broad-band photoluminescence centered at 540 nm with a quantum yield (QY) of 15±5 %. A series of analogous compositions following the general formula Cs A Sn(Br I ) (A=Rb, K; x≤1, y≤1) can be prepared. The emission of these materials ranges from 500 nm to 620 nm with the possibility to compositionally tune the Stokes shift and the self-trapped exciton emission bands.
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http://dx.doi.org/10.1002/anie.201806452DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6175341PMC
August 2018

Low-Cost Synthesis of Highly Luminescent Colloidal Lead Halide Perovskite Nanocrystals by Wet Ball Milling.

ACS Appl Nano Mater 2018 Mar 5;1(3):1300-1308. Epub 2018 Mar 5.

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

Lead halide perovskites of APbX type [A = Cs, formamidinium (FA), methylammonium; X = Br, I] in the form of ligand-capped colloidal nanocrystals (NCs) are widely studied as versatile photonic sources. FAPbBr and CsPbBr NCs have become promising as spectrally narrow green primary emitters in backlighting of liquid-crystal displays (peak at 520-530 nm, full width at half-maximum of 22-30 nm). Herein, we report that wet ball milling of bulk APbBr (A = Cs, FA) mixed with solvents and capping ligands yields green luminescent colloidal NCs with a high overall reaction yield and optoelectronic quality on par with that of NCs of the same composition obtained by hot-injection method. We emphasize the superiority of oleylammonium bromide as a capping ligand used for this procedure over the standard oleic acid and oleylamine. We also show a mechanically induced anion-exchange reaction for the formation of orange-emissive CsPb(Br/I) NCs.
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http://dx.doi.org/10.1021/acsanm.8b00038DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5999230PMC
March 2018

Surface-Engineered Cationic Nanocrystals Stable in Biological Buffers and High Ionic Strength Solutions.

Chem Mater 2017 Nov 16;29(21):9416-9428. Epub 2017 Oct 16.

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

Progress in colloidal synthesis in the last two decades has enabled high-quality semiconductor, plasmonic, and magnetic nanocrystals (NCs). As synthesized, these NCs are usually capped with long-chain apolar ligands. Postsynthetic surface functionalization is required for rendering such NCs colloidally stable in polar media such as water. However, unlike small anionic molecules and polymeric coatings, producing positively charged stable NCs, especially at high ionic strengths, has remained challenging. Here, we present a general approach to achieve aqueously stable cationic NCs using a set of small (<2.5 nm long) positively charged ligands. The applicability of this method is demonstrated for a variety of materials including semiconductor CdSe/CdS core/shell NCs, magnetic Fe@FeO, FeO, and FePt NCs, and three different classes of plasmonic Au NCs including large nanorods. The obtained cationic NCs typically have zeta potential values ranging from +30 to +60 mV and retain colloidal stability for days to months, depending on NC/ligand pair, in several biological buffers at elevated pH and in concentrated salt solutions. This allowed us to demonstrate site-specific staining of cellular structures using fluorescent cationic NCs with several different surface chemistries. Furthermore, colloidal stability of the obtained NCs in the presence of other charged species allowed the assembly of cationic and anionic counterparts driven primarily by electrostatic attraction. With this approach, we prepare highly uniform 3D and 2D binary mixtures of NCs through induced homogeneous aggregation and alternating-charge layer-by-layer deposition, respectively. Such binary mixtures may provide a new route in the engineering of nanocrystalline solids for electronics, thermoelectrics, and photovoltaics.
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http://dx.doi.org/10.1021/acs.chemmater.7b03504DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5871342PMC
November 2017

Colloidal Quantum Dot Inks for Single-Step-Fabricated Field-Effect Transistors: The Importance of Postdeposition Ligand Removal.

ACS Appl Mater Interfaces 2018 Feb 2;10(6):5626-5632. Epub 2018 Feb 2.

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

Colloidal quantum dots are a class of solution-processed semiconductors with good prospects for photovoltaic and optoelectronic applications. Removal of the surfactant, so-called ligand exchange, is a crucial step in making the solid films conductive, but performing it in solid state introduces surface defects and cracks in the films. Hence, the formation of thick, device-grade films have only been possible through layer-by-layer processing, limiting the technological interest for quantum dot solids. Solution-phase ligand exchange before the deposition allows for the direct deposition of thick, homogeneous films suitable for device applications. In this work, fabrication of field-effect transistors in a single step is reported using blade-coating, an upscalable, industrially relevant technique. Most importantly, a postdeposition washing step results in device properties comparable to the best layer-by-layer processed devices, opening the way for large-scale fabrication and further interest from the research community.
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http://dx.doi.org/10.1021/acsami.7b16882DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5814956PMC
February 2018

Stoichiometric control of the density of states in PbS colloidal quantum dot solids.

Sci Adv 2017 09 29;3(9):eaao1558. Epub 2017 Sep 29.

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

Colloidal quantum dots, and nanostructured semiconductors in general, carry the promise of overcoming the limitations of classical materials in chemical and physical properties and in processability. However, sufficient control of electronic properties, such as carrier concentration and carrier mobility, has not been achieved until now, limiting their application. In bulk semiconductors, modifications of electronic properties are obtained by alloying or doping, an approach that is not viable for structures in which the surface is dominant. The electronic properties of PbS colloidal quantum dot films are fine-tuned by adjusting their stoichiometry, using the large surface area of the nanoscale building blocks. We achieve an improvement of more than two orders of magnitude in the hole mobility, from below 10 to above 0.1 cm/V⋅s, by substituting the iodide ligands with sulfide while keeping the electron mobility stable (~1 cm/V⋅s). This approach is not possible in bulk semiconductors, and the developed method will likely contribute to the improvement of solar cell efficiencies through better carrier extraction and to the realization of complex (opto)electronic devices.
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http://dx.doi.org/10.1126/sciadv.aao1558DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5621976PMC
September 2017

An All-Solution-Based Hybrid CMOS-Like Quantum Dot/Carbon Nanotube Inverter.

Adv Mater 2017 Sep 17;29(35). Epub 2017 Jul 17.

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

The development of low-cost, flexible electronic devices is subordinated to the advancement in solution-based and low-temperature-processable semiconducting materials, such as colloidal quantum dots (QDs) and single-walled carbon nanotubes (SWCNTs). Here, excellent compatibility of QDs and SWCNTs as a complementary pair of semiconducting materials for fabrication of high-performance complementary metal-oxide-semiconductor (CMOS)-like inverters is demonstrated. The n-type field effect transistors (FETs) based on I capped PbS QDs (V = 0.2 V, on/off = 10 , S = 114 mV dec , µ = 0.22 cm V s ) and the p-type FETs with tailored parameters based on low-density random network of SWCNTs (V = -0.2 V, on/off > 10 , S = 63 mV dec , µ = 0.04 cm V s ) are integrated on the same substrate in order to obtain high-performance hybrid inverters. The inverters operate in the sub-1 V range (0.9 V) and have high gain (76 V/V), large maximum-equal-criteria noise margins (80%), and peak power consumption of 3 nW, in combination with low hysteresis (10 mV).
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http://dx.doi.org/10.1002/adma.201701764DOI Listing
September 2017

Solution-Grown CsPbBr Perovskite Single Crystals for Photon Detection.

Chem Mater 2016 Dec 21;28(23):8470-8474. Epub 2016 Nov 21.

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

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http://dx.doi.org/10.1021/acs.chemmater.6b04298DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5805401PMC
December 2016

Harnessing Defect-Tolerance at the Nanoscale: Highly Luminescent Lead Halide Perovskite Nanocrystals in Mesoporous Silica Matrixes.

Nano Lett 2016 09 25;16(9):5866-74. Epub 2016 Aug 25.

Institute of Inorganic Chemistry, Department of Chemistry and Applied Bioscience, ETH Zürich , CH-8093 Zürich, Switzerland.

Colloidal lead halide perovskite nanocrystals (NCs) have recently emerged as a novel class of bright emitters with pure colors spanning the entire visible spectral range. Contrary to conventional quantum dots, such as CdSe and InP NCs, perovskite NCs feature unusual, defect-tolerant photophysics. Specifically, surface dangling bonds and intrinsic point defects such as vacancies do not form midgap states, known to trap carriers and thereby quench photoluminescence (PL). Accordingly, perovskite NCs need not be electronically surface-passivated (with, for instance, ligands and wider-gap materials) and do not noticeably suffer from photo-oxidation. Novel opportunities for their preparation therefore can be envisaged. Herein, we show that the infiltration of perovskite precursor solutions into the pores of mesoporous silica, followed by drying, leads to the template-assisted formation of perovskite NCs. The most striking outcome of this simple methodology is very bright PL with quantum efficiencies exceeding 50%. This facile strategy can be applied to a large variety of perovskite compounds, hybrid and fully inorganic, with the general formula APbX3, where A is cesium (Cs), methylammonium (MA), or formamidinium (FA), and X is Cl, Br, I or a mixture thereof. The luminescent properties of the resulting templated NCs can be tuned by both quantum size effects as well as composition. Also exhibiting intrinsic haze due to scattering within the composite, such materials may find applications as replacements for conventional phosphors in liquid-crystal television display technologies and in related luminescence down-conversion-based devices.
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http://dx.doi.org/10.1021/acs.nanolett.6b02688DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5799875PMC
September 2016

Crystal symmetry breaking and vacancies in colloidal lead chalcogenide quantum dots.

Nat Mater 2016 09 13;15(9):987-94. Epub 2016 Jun 13.

Dipartimento di Scienza e Alta Tecnologia and To.Sca.Lab, Università dell'Insubria, via Valleggio 11, I-22100 Como, Italy.

Size and shape tunability and low-cost solution processability make colloidal lead chalcogenide quantum dots (QDs) an emerging class of building blocks for innovative photovoltaic, thermoelectric and optoelectronic devices. Lead chalcogenide QDs are known to crystallize in the rock-salt structure, although with very different atomic order and stoichiometry in the core and surface regions; however, there exists no convincing prior identification of how extreme downsizing and surface-induced ligand effects influence structural distortion. Using forefront X-ray scattering techniques and density functional theory calculations, here we have identified that, at sizes below 8 nm, PbS and PbSe QDs undergo a lattice distortion with displacement of the Pb sublattice, driven by ligand-induced tensile strain. The resulting permanent electric dipoles may have implications on the oriented attachment of these QDs. Evidence is found for a Pb-deficient core and, in the as-synthesized QDs, for a rhombic dodecahedral shape with nonpolar {110} facets. On varying the nature of the surface ligands, differences in lattice strains are found.
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http://dx.doi.org/10.1038/nmat4661DOI Listing
September 2016

Hydrogen-like Wannier-Mott Excitons in Single Crystal of Methylammonium Lead Bromide Perovskite.

ACS Nano 2016 06 6;10(6):6363-71. Epub 2016 Jun 6.

Schulich Faculty of Chemistry, Russell Berrie Nanotechnology Institute, Solid State Institute , Technion, Haifa 32000, Israel.

A thorough investigation of exciton properties in bulk CH3NH3PbBr3 perovskite single crystals was carried out by recording the reflectance, steady-state and transient photoluminescence spectra of submicron volumes across the crystal. The study included an examination of the spectra profiles at various temperatures and laser excitation fluencies. The results resolved the first and second hydrogen-like Wannier-Mott exciton transitions at low temperatures, from which the ground-state exciton's binding energy of 15.33 meV and Bohr radius of ∼4.38 nm were derived. Furthermore, the photoluminescence temperature dependence suggested dominance of delayed exciton emission at elevated temperatures, originating from detrapping of carriers from shallow traps or/and from retrapping of electron-hole pairs into exciton states. The study revealed knowledge about several currently controversial issues that have an impact on functionality of perovskite materials in optoelectronic devices.
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http://dx.doi.org/10.1021/acsnano.6b02734DOI Listing
June 2016

Counterion-Mediated Ligand Exchange for PbS Colloidal Quantum Dot Superlattices.

ACS Nano 2015 Dec 4;9(12):11951-9. Epub 2015 Nov 4.

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

In the past years, halide capping became one of the most promising strategies to passivate the surface of colloidal quantum dots (CQDs) in thin films to be used for electronic and optoelectronic device fabrication. This is due to the convenient processing, strong n-type characteristics, and ambient stability of the devices. Here, we investigate the effect of three counterions (ammonium, methylammonium, and tetrabutylammonium) in iodide salts used for treating CQD thin films and shed light on the mechanism of the ligand exchange. We obtain two- and three-dimensional square-packed PbS CQD superlattices with epitaxial merging of nearest neighbor CQDs as a direct outcome of the ligand-exchange reaction and show that the order in the layer can be controlled by the nature of the counterion. Furthermore, we demonstrate that the acidity of the environment plays an important role in the substitution of the carboxylates by iodide ions at the surface of lead chalcogenide quantum dots. Tetrabutylammonium iodide shows lower reactivity compared to methylammonium and ammonium iodide due to the nonacidity of the cation, which eventually leads to higher order but also poorer carrier transport due to incomplete removal of the pristine ligands in the QD thin film. Finally, we show that single-step blade-coating and immersion in a ligand exchange solution such as the one containing methylammonium iodide can be used to fabricate well performing bottom-gate/bottom-contact PbS CQD field effect transistors with record subthreshold swing.
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http://dx.doi.org/10.1021/acsnano.5b04547DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4690194PMC
December 2015

Random Lasing with Systematic Threshold Behavior in Films of CdSe/CdS Core/Thick-Shell Colloidal Quantum Dots.

ACS Nano 2015 Oct 17;9(10):9792-801. Epub 2015 Sep 17.

Materials for Electronics and Energy Technology (i-MEET), Friedrich-Alexander-Universität Erlangen-Nürnberg , Martensstraße 7, 91058 Erlangen, Germany.

While over the past years the syntheses of colloidal quantum dots (CQDs) with core/shell structures were continuously improved to obtain highly efficient emission, it has remained a challenge to use them as active materials in laser devices. Here, we report random lasing at room temperature in films of CdSe/CdS CQDs with different core/shell band alignments and extra thick shells. Even though the lasing process is based on random scattering, we find systematic dependencies of the laser thresholds on morphology and laser spot size. To minimize laser thresholds, optimizing the film-forming properties of the CQDs, proven by small-angle X-ray scattering, was found to be more important than the optical parameters of the CQDs, such as biexciton lifetime and binding energy or fluorescence decay time. Furthermore, the observed systematic behavior turned out to be highly reproducible after storing the samples in air for more than 1 year. These highly reproducible systematic dependencies suggest that random lasing experiments are a valuable tool for testing nanocrystal materials, providing a direct and simple feedback for further development of colloidal gain materials toward lasing in continuous wave operation.
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http://dx.doi.org/10.1021/acsnano.5b02739DOI Listing
October 2015

High infrared photoconductivity in films of arsenic-sulfide-encapsulated lead-sulfide nanocrystals.

ACS Nano 2014 Dec 8;8(12):12883-94. Epub 2014 Dec 8.

Institute of Semiconductor and Solid State Physics, Johannes Kepler University Linz , Altenbergerstraße 69, Linz 4040, Austria.

Highly photoconductive thin films of inorganic-capped PbS nanocrystal quantum dots (QDs) are reported. Stable colloidal dispersions of (NH4)3AsS3-capped PbS QDs were processed by a conventional dip-coating technique into a thin homogeneous film of electronically coupled PbS QDs. Upon drying at 130 °C, (NH4)3AsS3 capping ligands were converted into a thin layer of As2S3, acting as an infrared-transparent semiconducting glue. Photodetectors obtained by depositing such films onto glass substrates with interdigitate electrode structures feature extremely high light responsivity and detectivity with values of more than 200 A/W and 1.2×10(13) Jones, respectively, at infrared wavelengths up to 1400 nm. Importantly, these devices were fabricated and tested under ambient atmosphere. Using a set of time-resolved optoelectronic experiments, the important role played by the carrier trap states, presumably localized on the arsenic-sulfide surface coating, has been elucidated. Foremost, these traps enable a very high photoconductive gain of at least 200. The trap state density as a function of energy has been plotted from the frequency dependence of the photoinduced absorption (PIA), whereas the distribution of lifetimes of these traps was recovered from PIA and photoconductivity (PC) phase spectra. These trap states also have an important impact on carrier dynamics, which led us to propose a kinetic model for trap state filling that consistently describes the experimental photoconductivity transients at various intensities of excitation light. This model also provides realistic values for the photoconductive gain and thus may serve as a useful tool to describe photoconductivity in nanocrystal-based solids.
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http://dx.doi.org/10.1021/nn5067478DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4278417PMC
December 2014