Single Nanocrystals Research Articles

Water-induced construction of Cs4PbBr6/CsPbBr3 heterojunction for efficient perovskite light-emitting diode

Perovskite Cs4PbBr6/CsPbBr3 heterojunction with a strongly carrier-confined effect is emerging as a promising luminescent material for perovskite light-emitting diodes (PeLEDs). However, it is a big challenge to construct the heterojunction on a molecular scale to fully excavate this confinement effect for efficient electroluminescence efficiencies of PeLEDs. Herein, we report a water-induced construction of Cs4PbBr6/CsPbBr3 heterojunction for efficient PeLEDs via adding trace water with strong polarity. Benefiting from its strong carrier confinement effect, this heterojunction has a higher exciton binding energy (94.7 meV) and a lower nonradiative decay rate constant (0.007 ns−1) compared to that of single CsPbBr3 nanocrystals (NCs) (14.1 meV and 0.031 ns−1). Based on it, the as-fabricated PeLEDs exhibit a greatly-improved maximum current efficiency of 43.62 cd A−1 and a peak EQE of 11.31%, approximately 1.5 times that of CsPbBr3 NCs (28.43 cd A−1 and 7.78%). Evidently, this work provides an important way toward the construction of Cs4PbBr6/CsPbBr3 heterojunction and then promotes the commercial development of heterojunction-like optoelectronics.

Universal scaling laws for charge-carrier interactions with quantum confinement in lead-halide perovskites

Lead halide perovskites open great prospects for optoelectronics and a wealth of potential applications in quantum optical and spin-based technologies. Precise knowledge of the fundamental optical and spin properties of charge-carrier complexes at the origin of their luminescence is crucial in view of the development of these applications. On nearly bulk Cesium-Lead-Bromide single perovskite nanocrystals, which are the test bench materials for next-generation devices as well as theoretical modeling, we perform low temperature magneto-optical spectroscopy to reveal their entire band-edge exciton fine structure and charge-complex binding energies. We demonstrate that the ground exciton state is dark and lays several millielectronvolts below the lowest bright exciton sublevels, which settles the debate on the bright-dark exciton level ordering in these materials. More importantly, combining these results with spectroscopic measurements on various perovskite nanocrystal compounds, we show evidence for universal scaling laws relating the exciton fine structure splitting, the trion and biexciton binding energies to the band-edge exciton energy in lead-halide perovskite nanostructures, regardless of their chemical composition. These scaling laws solely based on quantum confinement effects and dimensionless energies offer a general predictive picture for the interaction energies within charge-carrier complexes photo-generated in these emerging semiconductor nanostructures.

Exciton fine structure of a single highly anisotropic CsPbBr3 nanocrystal

We measured the photoluminescence (PL) of single CsPbBr3 nanocrystals (NCs) that have a highly anisotropic shape and orthorhombic crystal phase. As the thickness of these NCs is much more smaller than the other two dimensions, they are also called nanoplatelets (NPLs). We obtain PL spectra characterized by doublets separated in energy by about 2 meV in average and showing orthogonal and linearly polarized polar lines. We identified these doublets as the two bright-exciton states of the exciton fine structure contained in the plane of the NPLs. By a comparison between theory and experiments, we were able to obtain fundamental parameters as tetragonal and orthorhombic crystal field. We measured and analysed the time-resolved PL evolution as a function of temperature of small ensemble of NPLs. We thus succeed at framing the experimental value of the bright-dark exciton splitting (5-7meV) that is slightly smaller than the theoretical value.

Single up-conversion nanocrystal as a local temperature probe of electrically heated silver nanowire.

Luminescence thermometry is a powerful technique for monitoring temperature in a sensitive, remote (through light), and minimally invasive manner. Up to now, many macroscopic and microscopic luminescence temperature probes exploiting different temperature sensing schemes have been investigated, with the majority of the studies using aggregates of nanothermometers. This work presents isolated single up-converting NaYF4:Er3+/Yb3+ nanocrystals as functional temperature indicators operating in a standard confocal microscopy configuration. More specifically, the nanocrystals were used to monitor the temperature of a single silver nanowire, whose temperature was controlled electrically via the Joule process. We demonstrate that individual nanocrystals placed near the nanowire can precisely determine the temperature distribution in its surroundings. These results, which combine nanoscopic heat generation with temperature readout using isolated nanocrystals, represent an essential step for the application of isolated single nanoprobes for luminescence thermometry at the nanoscale.

SnOxS2−x/GNS nanocomposites for reversible and high-capacity lithium-ion batteries

A SnOS/GNS nanocomposite consisting of a single phase nanocrystal (3–5 nm) improves the kinetics of the electrochemical reactions of lithium-ion batteries.

Single-atom infrared emission in doped silicon nanocrystals.

Silicon luminescence, due to silicon being abundant, non-toxic and harmless, is a topic of pivotal importance in optoelectronics and biological imaging. However, a major challenge in developing high-efficiency silicon light sources is the relatively weak allowable transitions. This study focuses on single atom-doped silicon nanocrystals (Si NCs) and theoretically investigates the emission behavior of single atoms within a tetrahedral coordination field. Doping a single atom in Si NCs can result in a ∼102 times improvement at least in the squared transition dipole moment (TDM2), and induce a spectral shift towards near- and mid-infrared wavelengths. These findings offer a strong foundation for designing Si NCs for on-chip optical communication and single photon emitters.

Local phonon imaging of AlN nanostructures with nanoscale spatial resolution.

We demonstrate local phonon analysis of single AlN nanocrystals by two complementary imaging spectroscopic techniques: tip-enhanced Raman scattering (TERS) and nano-Fourier transform infrared (nano-FTIR) spectroscopy. Strong surface optical (SO) phonon modes appear in the TERS spectra with their intensities revealing a weak polarization dependence. The local electric field enhancement stemming from the plasmon mode of the TERS tip modifies the phonon response of the sample, making the SO mode dominate over other phonon modes. The TERS imaging allows the spatial localization of the SO mode to be visualized. We were able to probe the angle anisotropy on the SO phonon modes in AlN nanocrystals with nanoscale spatial resolution. The excitation geometry and the local nanostructure surface profile determine the frequency position of SO modes in nano-FTIR spectra. An analytical calculation explains the behaviour of SO mode frequencies vs. tip position with respect to the sample.

Dynamically tunable multicolor emissions from zero-dimensional Cs3LnCl6 (Ln: europium and terbium) nanocrystals with wide color gamut.

This study demonstrates dynamically tunable multicolor emissions from a single component, zero-dimensional (0-D) cesium europium chloride (Cs3EuCl6) and cesium terbium chloride (Cs3TbCl6) nanocrystals (NCs). Highly uniform colloidal Cs3EuCl6 and Cs3TbCl6 NCs are synthesized via the heating-up method. Excitation-wavelength-dependent multicolor emissions from Cs3EuCl6 and Cs3TbCl6 NCs are observed. Under excitation of 330-400 nm, both NCs exhibit blue photoluminescence (PL). Under wavelengths shorter than 330 nm, characteristic red and green emissions are observed from Cs3EuCl6 and Cs3TbCl6, respectively, owing to the atomic emissions from the f-orbitals in trivalent europium (Eu3+) and terbium (Tb3+) ions. Cs3EuCl6 and Cs3TbCl6 NCs exhibit broadband excitation spectra and enhanced absorption properties. Particularly, Cs3EuCl6 NCs exhibit a very narrow full-width at half-maximum in both blue and red PL and no overlap between the two spectra. The photophysical properties of these NCs are further investigated to understand the multicolor PL origins by time-resolved and temperature-dependent PL measurements. Finally, the potential applications of Cs3EuCl6 and Cs3TbCl6 NCs as anti-counterfeiting inks for high-level security are demonstrated. Given their broadband excitation with enhanced absorption properties and dynamically tunable colors with a wide color gamut, Cs3EuCl6 and Cs3TbCl6 NCs have great potential as novel multicolor NC emitters for many emerging applications.

Suppression of blinking in single CsPbBr3 perovskite nanocrystals through surface ligand exchange.

Photoluminescence blinking in individual semiconducting and perovskite quantum dots reflects reduced emission quantum yield and represents an obstacle towards quantum dot applications. One of the origins of blinking is the presence of surface structural defects that can function as charge traps. To reduce the defects the surface can be modified by, e.g., covering with ligands that are more strongly bound to the surface. Here, we report exchange of ligands on the CsPbBr3 perovskite nanocrystal surface and the effect of the exchange on photoluminescence blinking. Replacement of the oleic acid and oleylamine ligands which are used in the synthesis process with quaternary amine ligands leads to substantial increase of photoluminescence quantum yield. On single particle level this is reflected by significantly improved blinking characteristics. Statistical analysis using the probability density function shows that the ligand exchange leads to longer duration of ON-times and shorter OFF-times, as well as to the presence of a higher fraction of ON-time intervals. These characteristics are not affected by sample aging within three weeks. On the contrary, storage of the samples in solution for one-to-two weeks leads to further improvement of the ON-time interval fraction statistics.

Electronic properties of single Prussian Blue Analog nanocrystals determined by conductive-AFM.

We report a study of the electron transport (ET) properties at the nanoscale (conductive-AFM denoted as C-AFM hereafter) of individual Prussian Blue Analog (PBA) cubic nanocrystals (NCs) of CsCoIIIFeII, with a size between 15 and 50 nm deposited on HOPG. We demonstrate that these PBA NCs feature an almost size-independent electron injection barrier of 0.41 ± 0.02 eV and 0.27 ± 0.03 eV at the CsCoIIIFeII/HOPG and CsCoIIIFeII/C-AFM tip, respectively, and an intrinsic electron conductivity evolving from a large dispersion between ∼5 × 10-4 and 2 × 10-2 S cm-1 without a clear correlation with the nanocrystal size. The conductivity values measured on individual nanocrystals are up to fifty times higher than those reported on PBA films.

High-performance bulk Bi0.4Sb1.6Te3.0 thermoelectrics prepared from nanocrystal precursor synthesized via chemical precipitation

Bi0.4Sb1.6Te3.0 is a known p-type thermoelectric material with high thermoelectric performance near room temperature. In this study, bulk Bi0.4Sb1.6Te3.0 materials are synthesized via spark plasma sintering using single nanometer-sized nanocrystals as precursors, which are prepared in the solution phase by chemical precipitation from metallic ions. The obtained bulk material consists of submicron-to-micrometer-sized disk-shaped grains with isotropic distribution. Additionally, structural distortion such as strain and/or defects can be directly introduced into grains upon sintering, presumably due to the incomplete coalescence of the small nanocrystal precursors to form larger grains during the sintering process. The resultant bulk material exhibits high electrical conductivity and considerably low lattice thermal conductivity, due to the utilization of impurity-free nanocrystal precursors and local lattice distortions included inside grains acting as scattering nodes of phonons, respectively. The maximum dimensionless figure of merit (ZT) is achieved to be 1.42 ​at 355 ​K, and, therefore, this work provides effective methodology to fabricate high-performance chalcogenide-based thermoelectric materials.

Simultaneous Imaging of Dopants and Free Charge Carriers by Monochromated EELS.

Doping inhomogeneities in solids are not uncommon, but their microscopic observation and understanding are limited due to the lack of bulk-sensitive experimental techniques with high enough spatial and spectral resolution. Here, we demonstrate nanoscale imaging of both dopants and free charge carriers in La-doped BaSnO3 (BLSO) using high-resolution electron energy-loss spectroscopy (EELS). By analyzing high- and low-energy excitations in EELS, we reveal chemical and electronic inhomogeneities within a single BLSO nanocrystal. The inhomogeneous doping leads to distinctive localized infrared surface plasmons, including a previously unobserved plasmon mode that is highly confined between high- and low-doping regions. We further quantify the carrier density, effective mass, and dopant activation percentage by EELS and transport measurements on the bulk single crystals of BLSO. These results not only represent a practical approach for studying heterogeneities in solids and understanding structure-property relationships at the nanoscale, but also demonstrate the possibility of infrared plasmon tuning by leveraging nanoscale doping texture.

(Invited) The Evolution of Inorganic Nanocrystals for Bioimaging

The first applications of luminescent nanocrystals to bioimaging were semiconductor quantum dots with optoelectronic properties that largely mirror those of organics and proteins, but with substantially increased stability and brightness that have enabled single molecule and other challenging imaging applications. Building on this success, newer nanocrystals have been engineered with optical properties unlike anything found in traditional probes, including perfect photostability,1,2 anti-Stokes emission a billion-fold more efficient than 2-photon excitation,3 and most recently, photon avalanches hosted within nanostructures.4 Avalanches are steeply nonlinear events in which outsized responses arise from a series of minute inputs. With light, photon avalanching (PA) had been observed only in bulk materials and aggregates, often at cryogenic temperatures, preventing its application to bioimaging. We recently reported the realization of PA at room temperature in sub-30 nm Tm3+-doped NaYF4 upconverting nanoparticles (UCNPs) and demonstrated their use in high-resolution imaging at wavelengths that fall within NIR spectral windows of maximal biological transparency. Avalanching nanoparticles (ANPs) can be pumped by either continuous-wave or pulsed lasers and exhibit all of the defining features of PA: clear excitation power thresholds, exceptionally long rise time at threshold, and a dominant excited-state absorption that is >10,000 times larger than ground-state absorption. Beyond the avalanching threshold, ANP emission scales with up to the 31st power of pump intensity, an extreme nonlinearity caused by the induced positive optical feedback within each nanocrystal. This enables sub-70 nm spatial resolution using only simple scanning confocal microscopy and before any computational data analysis. NaYF4 ANPs with 8-20% Tm3+content can be excited at either 1064 or 1450 nm, with avalanching emission at 800 nm. Pairing the steep nonlinearity of ANPs with existing superresolution techniques and computational methods allows for imaging with higher resolution and at ca. 100-fold lower excitation intensities than is possible with other probes. For application of ANPs to live-cell imaging, we have developed synthetic chemistry-free methods for conjugating engineered antibodies to NP-surface SpyCatcher proteins,5 which bind and spontaneously form covalent isopeptide bonds with cognate SpyTag peptides. This enables controlled and irreversible attachment of antibodies to nanoparticle surfaces, for specific targeting of cell-surface receptors in quantitative live-cell study of their distribution, trafficking, and physiology. Wu, S. et al. Non-blinking and photostable upconverted luminescence from single lanthanide-doped nanocrystals. Proc. Natl. Acad. Sci. 106, 10917–10921 (2009).Fernandez-Bravo, A. et al. Continuous-wave upconverting nanoparticle microlasers. Nat. Nanotechnol. 13, 572–577 (2018).Tian, B. et al. Low irradiance multiphoton imaging with alloyed lanthanide nanocrystals. Nat. Commun. 9, 3082 (2018).Lee, C. et al. Giant nonlinear optical responses from photon-avalanching nanoparticles. Nature 589, 230–235 (2021).Pedroso, C. C. S. et al. Immunotargeting of nanocrystals by SpyCatcher conjugation of engineered antibodies. ACS Nano 15, 18374–18384 (2021). Figure 1

High scintillation yield and fast response to alpha particles from thin perovskite films deposited by pulsed laser deposition

Although the field of solar cells is the most popular application of perovskite materials, their use in radiation detection applications is emerging. The success of perovskites as radiation detectors rests partly on the same material properties that have led to successful optoelectronics applications, meaning that other specific properties, like large stopping power, high mobility lifetime product, fast response, and large bulk resistance, play a role. In this respect, inorganic perovskites are attracting a lot of attention as scintillator materials with performances sensitive to material shape (single crystals, nanocrystals, and thin films) and growth methods. In this study, we report on the morphological, structural, and optical response of thin CsPbBr3-based perovskite films, deposited by Pulsed Laser Deposition (PLD) and post-growth annealed at 350°C in air, following excitation by different particle sources. The annealing treatment resulted in a prompt structural refinement, grain growth, and oxygen bonding to the Pb phase together with an enrichment of the surface in chemo-adsorbed oxygen probably due to Cs–O interactions, as evidenced by X-ray photoelectron spectroscopy. The film behavior under 2 MeV H+ ion beam irradiation at different fluences was analyzed together with its scintillation properties following an interaction with α particles from an Am-241 radioactive source demonstrating a very fast response for an inorganic material (∼5ns) and a photoelectron yield of about ∼47% with respect to a commercial CsI:Tl scintillator.

Exciton-Phonon and Trion-Phonon Couplings Revealed by Photoluminescence Spectroscopy of Single CsPbBr3 Perovskite Nanocrystals.

Lead halide perovskite nanocrystals (NCs) have outstanding photoluminescence (PL) properties and excellent potential for light-emitting diodes and single-photon sources. Here, we report the multiple-peak structures originating from excitons, trions, and biexcitons in low-temperature PL spectra of single CsPbBr3 NCs. We found fine-structure splitting in the PL peaks of bright excitons and biexcitons and also in the longitudinal-optical (LO)-phonon replicas of excitons. LO-phonon replicas of trions are clearly observed under strong photoexcitation, which do not show fine-structure splitting. From size-dependent analyses of these replicas, we clarified that both exciton-phonon and trion-phonon couplings become larger for smaller NCs and the coupling strengths of trions are larger than those of excitons in large NCs. These behaviors can be explained by the spatial distributions of the electron and hole wave functions in the NCs. Our findings provide essential information on electron-phonon couplings in perovskites and for the design of high-purity single-photon sources.

The Scatman: an approximate method for fast wide-angle scattering simulations.

Single-shot coherent diffraction imaging (CDI) is a powerful approach to characterize the structure and dynamics of isolated nanoscale objects such as single viruses, aerosols, nanocrystals and droplets. Using X-ray wavelengths, the diffraction images in CDI experiments usually cover only small scattering angles of a few degrees. These small-angle patterns represent the magnitude of the Fourier transform of the 2D projection of the sample's electron density, which can be reconstructed efficiently but lacks any depth information. In cases where the diffracted signal can be measured up to scattering angles exceeding ∼10°, i.e. in the wide-angle regime, some 3D morphological information of the target is contained in a single-shot diffraction pattern. However, the extraction of the 3D structural information is no longer straightforward and defines the key challenge in wide-angle CDI. So far, the most convenient approach relies on iterative forward fitting of the scattering pattern using scattering simulations. Here the Scatman is presented, an approximate and fast numerical tool for the simulation and iterative fitting of wide-angle scattering images of isolated samples. Furthermore, the open-source software implementation of the Scatman algorithm, PyScatman, is published and described in detail. The Scatman approach, which has already been applied in previous work for forward-fitting-based shape retrieval, adopts the multi-slice Fourier transform method. The effects of optical properties are partially included, yielding quantitative results for small, isolated and weakly interacting samples. PyScatman is capable of computing wide-angle scattering patterns in a few milliseconds even on consumer-level computing hardware, potentially enabling new data analysis schemes for wide-angle coherent diffraction experiments.

Boosting the Self-Trapped Exciton Emission in Cs2NaYCl6 Double Perovskite Single Crystals and Nanocrystals.

Halide double perovskites have aroused substantial research interest because of their unique optical properties and intriguing flexibility for various compositional adjustments. Herein, we report the synthesis and photophysics of rare-earth element yttrium (Y)-based double perovskite single crystals (SCs) and nanocrystals (NCs). The pristine Cs2NaYCl6 bulk SCs exhibit a weak sky-blue emission with a low photoluminescence quantum yield (PLQY) of 7.68% based on the self-trapped exciton (STE), while no PL emission was observed for NCs. Excitingly, the STE emission of SCs and NCs is greatly enhanced via Sb3+ doping. The optimized Cs2NaYCl6:Sb3+ SCs and NCs exhibit high PLQYs up to 82.5% and 51.8%, respectively. Theoretical calculations and charge-carrier dynamic studies demonstrate that the giant emission enhancement after Sb3+ doping is related with the enhancement of the sensitization of the emissive STE states, the passivating of the nonradiative carrier trapping processes, and the regulation of carrier-phonon coupling.

In Situ Three-Dimensional Electron Diffraction for Probing Structural Transformations of Single Nanocrystals

Phase transitions of materials are usually accompanied by a remarkable change of properties. Tracing and identifying the structural changes at the atomic scale is the key to understanding the fundamental mechanisms behind their performances. However, the direct observation of three-dimensional structural transformations in single nanocrystals has been rarely achieved. Herein we reported the development of an in situ three-dimensional electron diffraction (3D ED) approach for single nanocrystal structural analysis by combining 3D ED with in situ transmission electron microscopy. Each data set covering a crystal tilting range of 50° was collected in around 1 min at the set temperature using an in situ heating holder. The feasibility of this protocol was proved by its successful application in investigating the metal–insulator transition (MIT) of vanadium dioxide (VO2) crystals with atomic precision. Moreover, local domains were found in the monoclinic phase before and after the annealing process, of which the orientations obey a 4-fold rotation symmetry along the a-axis.

Double-crowned 2D semiconductor nanoplatelets with bicolor power-tunable emission

Nanocrystals (NCs) are now established building blocks for optoelectronics and their use as down converters for large gamut displays has been their first mass market. NC integration relies on a combination of green and red NCs into a blend, which rises post-growth formulation issues. A careful engineering of the NCs may enable dual emissions from a single NC population which violates Kasha’s rule, which stipulates that emission should occur at the band edge. Thus, in addition to an attentive control of band alignment to obtain green and red signals, non-radiative decay paths also have to be carefully slowed down to enable emission away from the ground state. Here, we demonstrate that core/crown/crown 2D nanoplatelets (NPLs), made of CdSe/CdTe/CdSe, can combine a large volume and a type-II band alignment enabling simultaneously red and narrow green emissions. Moreover, we demonstrate that the ratio of the two emissions can be tuned by the incident power, which results in a saturation of the red emission due to non-radiative Auger recombination that affects this emission much stronger than the green one. Finally, we also show that dual-color, power tunable, emission can be obtained through an electrical excitation.

Infrared Plasmons in Single La-doped BaSnO3 Nanocrystals Revealed by Monochromated STEM-EELS

Infrared Plasmons in Single La-doped BaSnO3 Nanocrystals Revealed by Monochromated STEM-EELS