Self-assembled hierarchical nanostructured perovskites enable highly efficient LEDs via an energy cascade

Metal halide perovskites have established themselves as extraordinary optoelectronic materials, exhibiting promise for applications in large area illumination and displays. However, low luminescence, low efficiencies of the light-emitting diodes (LEDs), and complex preparation methods currently limit further progress towards applications. Here, we report on a new and unique mesoscopic film architecture featuring the self-assembly of 3D formamidinium lead bromide (FAPbBr3) nanocrystals of graded size, coupled with microplatelets of octylammonium lead bromide perovskites that enables an energy cascade, yielding very high-performance light-emitting diodes with emission in the green spectral re…

Manipulating spontaneous emission of the CsPbI3 Perovskite NCs using hybrid (HMM-Mie resonator) structures

Birefringent porous silicon membranes for optical sensing

In this work anisotropic porous silicon is investigated as a material for optical sensing. Birefringence and sensitivity of the anisotropic porous silicon membranes are thoroughly studied in the framework of Bruggeman model which is extended to incorporate the influence of environment effects, such as silicon oxidation. The membranes were also characterized optically demonstrating sensitivity as high as 1245 nm/RIU at 1500 nm. This experimental value only agrees with the theory when it takes into consideration the effect of silicon oxidation. Furthermore we demonstrate that oxidized porous silicon membranes have optical parameters with long term stability. Finally, we developed a new model …

Photoconductivity and optical properties of silicon coated by thin TiO2 film in situ doped by Au nanoparticles

Light trapping enhancement by plasmonic-active metal nanoparticles (NPs) is believed to be a promising approach to increase silicon-based solar cell efficiency. Therefore, we investigated TiO2 films in situ doped by Au NPs (TiO2:AuNPs) deposited by spin coating on a silicon substrate. Photoconductivity and optical properties of the TiO2:AuNPs/Si structures were studied in comparison with those of TiO2/Si reference samples. We found that an introduction of the 40–50 nm diameter AuNPs into the antireflective TiO2 layer deteriorates the antireflection properties and decreases the external yield of photogeneration of charge carriers. This is due to an increase of the layer reflection in the red…

Purcell-enhancement of the radiative PL decay in perylenediimides by coupling with silver nanoparticles into waveguide modes

In this work, an interesting approach to enhance the coupling between excitons and plasmons is proposed by integrating highly luminescent perylenediimides (PDIs) and Ag metal nanoparticles (MNPs) in the core of a multilayer dielectric waveguide. The combination of the weak plasmonic coupling and the high scattering of MNPs gives rise to a significant improvement of the PDI photoluminescence and Purcell factor (PF) in forward-scattering geometry. Furthermore, when the PDI-MNP system is used as the core of a multilayer waveguide, a Purcell factor enhancement larger than 10 is observed, which is explained by an increase in the exciton-plasmon coupling under the light confinement in the wavegui…

Laser ablation of a silicon target in chloroform: formation of multilayer graphite nanostructures

With the use of high-resolution transmission electron microscopy, selected area electron diffraction and x-ray photoelectron spectroscopy methods of analysis we show that the laser ablation of a Si target in chloroform (CHCl3) by nanosecond UV pulses (40 ns, 355 nm) results in the formation of about 50–80 nm core–shell nanoparticles with a polycrystalline core composed of small (5–10 nm) Si and SiC mono-crystallites, the core being coated by several layers of carbon with the structure of graphite (the shell). In addition, free carbon multilayer nanostructures (carbon nano-onions) are also found in the suspension. On the basis of a comparison with similar laser ablation experiments implement…

Short photoluminescence lifetimes in vacuum-deposited ch3nh3pbI3 perovskite thin films as a result of fast diffusion of photogenerated charge carriers

It is widely accepted that a long photoluminescence (PL) lifetime in metal halide perovskite films is a crucial and favorable factor, as it ensures a large charge diffusion length leading to a high power conversion efficiency (PCE) in solar cells. It has been recently found that vacuumevaporated CH3NH3PbI3 (eMAPI) films show very short PL lifetimes of several nanoseconds. The corresponding solar cells, however, have high photovoltage (>1.1 V) and PCEs (up to 20%). We rationalize this apparent contradiction and show that eMAPI films are characterized by a very high diffusion coefficient D, estimated from modeling the PL kinetics to exceed 1 cm2/s. Such high D values are favorable for long di…

Role of Self-Absorption in the Photoluminescence Waveguided along CsPbBr3 Perovskite Nanocrystals Thin Films

During the last years, perovskite nanocrystals (PNCs) have been intensively studied as nanomaterials with excellent light absorption/emission properties. For example, PNCs have been successfully applied in solar cells, where the high absorption coefficient above the band gap increases the conversion efficiencies; or in optical sources, where the high quantum yield of emission at room temperature allows a low threshold of stimulated emission. In this scenario, an optical waveguide represents a suitable platform to enhance their electrooptical properties and to integrate different photonic functionalities. However, propagation of light along close packed films of PNCs is usually restricted to…

One-step formation of nanostructures on silicon surfaces using pure hydrogen-radical-initiated reactions

One-step formation of silicon nanowires, sheets, and texture surface on a silicon substrate has been achieved using hydrogen-radical etching reactions. Metallic tungsten and for comparison purposes a tungsten hot wire, were used as catalysts for the hydrogen-molecular cracking. It was shown that a variety of surface structures on silicon such as inverted pyramid texture, V-groove texture, dense silicon nanowire growth over texture, and nanosheet structure can be obtained by controlling the process conditions. The obtained results suggested that the formation of nanotungsten silicide particle is an essential prerequisite to obtain these structures. The particles work as an etching mask again…

Amplified Spontaneous Emission in Thin Films of CsPbX3 Perovskite Nanocrystals

During the last years, Metal Halide Perovskites (MHPs) have attracted special attention as an efficient conversion films for photovoltaics, or excellent gain media to construct optical sources. In spite of the fact that most of the works have been focussed on CH 3 NH 3 PbX 3 (X=Cl, Br, I) polycrystalline thin films, MHP can be also synthesized as colloidal nanocrystals. In particular, caesium lead halide perovskite CsPbX 3 nanocrystals (NCs) revealed extraordinary properties for optoelectronics. With a high quantum yield of emission (>90%) at room temperature and linewidths less than 100 meV, CsPbX 3 NCs have demonstrated favourable characteristics for active photonics. Indeed, thin films o…

Purification of Nano-Porous Silicon for Biomedical Applications

Recently, various bio-medical applications of nanoporous silicon (np-Si) have been suggested. This work investigates the biocompatibility of np-Si particles taking into account hazardous residua confined in the pores after preparation. The emphasis is on the potential application of such particles as oxygen photosensitizer for photodynamic therapy of cancer, which requires both negligible toxicity of np-Si particles in darkness and a high photo-cyto-toxic effect due to generation of singlet oxygen under illumination. Considerable amounts of water soluble toxic impurities are found to be present in the nanoporous shell of micrometer-sized np-Si particles immediately after their preparation b…

Creation and Annihilation of Nonradiative Recombination Centers in Polycrystalline Metal Halide Perovskites by Alternating Electric Field and Light

Metal halide perovskites are promising optoelectronic materials. Their electronic properties however are rather unstable which is often assigned to ion migration. Ion migration can be readily influenced by an electric field (EF). Here, the response of photoluminescence (PL) of individual MAPbX3 (MA = CH3NH3, X = I, Br) sub-micrometer-sized polycrystals to EF is studied. Alternating EF with frequency higher than 10 Hz is found to reversibly quench PL. It is proposed that an alternating EF when applied together with light increases ion migration. This leads to a shift in the equilibrium between creation and annihilation of defects toward higher concentration of nonradiative recombination cent…

Effect of alkali metal nitrate treatment on the optical properties of CsPbBr3 nanocrystal films

Abstract Colloidal cesium lead bromide (CsPbBr3) perovskite nanocrystals (NCs) are fabricated as densely packed thin films using doctor blade method through a layer-by-layer assembly approach. It is found that the optical properties of the CsPbBr3 NC films are significantly improved after a ligand exchange of oleic acid (OA) and oleylamine (OAm) by sodium and potassium nitrate in ethyl acetate. Ligand-induced surface defects are passivated by the sodium and potassium cations and nitrate, which leads to the improvement in photoluminescence (PL) and life-time properties. Compared with pristine OAm-OA capped CsPbBr3 NC film, Alkali-metal nitrate salts treated films are showing higher PL intens…

Long-Lived Luminescence and Slow Carrier Diffusion in Metal Halide Perovskites as a result of Multiple Trapping and De-Trapping by Shallow Non-Quenching Traps

Production of nanometer-size GaAs nanocristals by nanosecond laser ablation in liquid.

This paper reports the formation and characterization of spherical GaAs quantum dots obtained by nanosecond pulsed laser ablation in a liquid (ethanol or methanol). The produced bare GaAs nanoparticles demonstrate rather narrow size distribution which depends on the applied laser power density (from 4.25 to 13.9 J/cm 2 in our experiments) and is as low as 2.5 nm for the highest power used. The absolute value of the average diameter also decreases significantly, from 13.7 to 8.7 nm, as the laser power increases in this interval. Due to the narrow nanoparticle size dispersion achieved at the highest laser powers two absorption band edges are clearly distinguishable at about 1.72 and 3.15 eV w…

Inhomogeneous Broadening of Photoluminescence Spectra and Kinetics of Nanometer-Thick (Phenethylammonium)2PbI4 Perovskite Thin Films: Implications for Optoelectronics

An outstanding potentiality of layered two-dimensional (2D) organic–inorganic hybrid perovskites (2DHPs) is in the development of solar cells, photodetectors, and light-emitting diodes. In 2DHPs, an exciton is localized in an atomically thin lead(II) halide inorganic layer of sub-nanometer thickness as in a quantum well sandwiched between organic layers as energetic and dielectric barriers. In previous years, versatile optical characterization of 2DHPs has been carried out mainly for thin flakes of single crystals and ultrathin (of the order of 20 nm) polycrystalline films, whereas there is a lack of optical characterization of thick (hundreds of nanometers) polycrystalline films, fundament…

Efficient Optical Amplification in a Sandwich-Type Active-Passive Polymer Waveguide Containing Perylenediimides

Polymer waveguides doped with luminescent materials serve as a suitable flexible platform for active elements (lasers and amplifiers) in on-chip optical circuits. However, at present, the best parameters (lowest thresholds) achieved with these devices are obtained with the use of the stripe excitation technique in the framework of which external illumination of an active material along the whole length of the waveguide is realized that is not convenient for the waveguide on-chip integration and requires high peak energies due to the large excitation area. In the present work, an elegant method is proposed to overcome this obstacle and provide efficient active material pumping along the whol…

UV-patternable nanocomposite containing CdSe and PbS quantum dots as miniaturized luminescent chemo-sensors

In this study, a novel multifunctional hybrid polymer-based luminescent material, particularly formulated for photolithography, was developed, fabricated and tested as a miniaturized chemosensor. This nanocomposites were formulated with either luminescent CdSe (for the visible) or PbS (for the near-IR) colloidal QDs embedded in a polyisoprene-based photoresist (PIP). We checked the sensing capability of the nanocomposite by exposing 1 cm2 CdSe nanocomposite patterns to vapours of some analyte solutions such as 2-mercaptoethanol (MET) and ethylenediamine (EDA). The transduction mechanism of the sensor is based on changes of the QD photoluminescence (PL) when molecules are adsorbed on the QD …

Simulation of surface-modified porous silicon photonic crystals for biosensing applications

In this work realistic biosensing structures based on the integration of porous silicon photonic crystals with polymer coating technology are presented. Microcavities and rugate filters are chosen as the photonic crystal configuration. The deposition of a polymer layer on the pore walls of these structures is proposed to improve the selectivity and sensitivity of the sensing function. A complete effective refractive index model including the polymer layer, the target and external effects like silicon oxidation has been developed in order to accurately simulate the structures. It is expected that the proposed structures could be used as low cost, highly integrated and highly sensitive biolog…

Formation of Si/SiO2 Luminescent Quantum Dots From Mesoporous Silicon by Sodium Tetraborate/Citric Acid Oxidation Treatment

We propose a rapid, one-pot method to generate photoluminescent (PL) mesoporous silicon nanoparticles (PSiNPs). Typically, mesoporous silicon (meso-PSi) films, obtained by electrochemical etching of monocrystalline silicon substrates, do not display strong PL because the silicon nanocrystals (nc-Si) in the skeleton are generally too large to display quantum confinement effects. Here we describe an improved approach to form photoluminescent PSiNPs from meso-PSi by partial oxidation in aqueous sodium borate (borax) solutions. The borax solution acts to simultaneously oxidize the nc-Si surface and to partially dissolve the oxide product. This results in reduction of the size of the nc-Si core …

Facile laser-assisted synthesis of inorganic nanoparticles covered by a carbon shell with tunable luminescence

We report a one-step strategy at ambient conditions for the production of hybrid inorganic core–carbon shell nanoparticles by means of pulsed laser ablation of inorganic targets (LiNbO3, Au, and Si) in hydrocarbon liquids such as toluene and chloroform. The core of these spherical nanoparticles consists of the target material, whereas the shells are carbon structures (multilayer graphite-type carbon and amorphous carbon), which are formed due to the thermal decomposition of the organic liquid in contact with hot inorganic nanoparticles ejected from the bulk target. These carbon shells emit photoluminescence in the blue-green spectral region and the obtained luminescence, in which the lumine…

Purcell Enhancement and Wavelength Shift of Emitted Light by CsPbI3 Perovskite Nanocrystals Coupled to Hyperbolic Metamaterials

Manipulation of the exciton emission rate in nanocrystals of lead halide perovskites (LHPs) was demonstrated by means of coupling of excitons with a hyperbolic metamaterial (HMM) consisting of alternating thin metal (Ag) and dielectric (LiF) layers. Such a coupling is found to induce an increase of the exciton radiative recombination rate by more than a factor of three due to the Purcell effect when the distance between the quantum emitter and HMM is nominally as small as 10 nm, which coincides well with the results of our theoretical analysis. Besides, an effect of the coupling-induced long wavelength shift of the exciton emission spectrum is detected and modeled. These results can be of i…

Incorporation of potassium halides in the mechanosynthesis of inorganic perovskites: feasibility and limitations of ion-replacement and trap passivation

Potassium halides (KX; X = I, Br, or Cl) were incorporated as partial replacements of CsBr in the mechanosynthesis of CsPbBr3. This led to partial substitution of both monovalent ions forming mixed Cs1−xKxPbBr3−yXy perovskites. Longer photoluminescence lifetimes were also observed, possibly linked to the formation of a non-perovskite KPb2X5 passivating layer.

Novel Method of Preparation of Gold-Nanoparticle-Doped TiO2 and SiO2 Plasmonic Thin Films: Optical Characterization and Comparison with Maxwell-Garnett Modeling

SiO2 and TiO2 thin films with gold nanoparticles (NPs) are of particular interest as photovoltaic materials. A novel method for the preparation of spin-coated SiO2–Au and TiO2–Au nanocomposites is presented. This fast and inexpensive method, which includes three separate stages, is based on the in situ synthesis of both the metal-oxide matrix and the Au NPs during a baking process at relatively low temperature. It allows the formation of nanocomposite thin films with a higher concentration of Au NPs than other methods. High-resolution transmission electron microscopy studies revealed a homogeneous distribution of NPs over the film volume along with their narrow size distribution. The optica…

Polymer waveguide couplers based on metal nanoparticle–polymer nanocomposites

In this work Au nanoparticles (AuNPs) are incorporated into poly(methyl methacrylate) (PMMA) waveguides to develop optical couplers that are compatible with planar organic polymer photonics. A method for growing AuNPs (of 10 to 100 nm in size) inside the commercially available Novolak resist is proposed with the intention of tuning the plasmon resonance and the absorption/scattering efficiencies inside the patterned structures. The refractive index of the MNP-Novolak nanocomposite (MNPs: noble metal nanoparticles) is carefully analysed both experimentally and numerically in order to find the appropriate fabrication conditions (filling factor and growth time) to optimize the scattering cross…

Ultrapure laser-synthesized Si nanoparticles with variable oxidation states for biomedical applications

We employ a method of femtosecond laser fragmentation of preliminarily prepared water-dispersed microcolloids to fabricate aqueous solutions of ultrapure bare Si-based nanoparticles (Si-NPs) and assess their potential for biomedical applications. The nanoparticles appear spherical in shape, with low size dispersion and a controllable mean size, from a few nm to several tens of nm, while a negative surface charge (−35 mV ± 0.10 according to z-potential data) provides good electrostatic stabilization of colloidal Si-NP solutions. Structural analysis shows that the Si-NPs are composed of Si nanocrystals with inclusions of silicon oxide species, covered by a SiOx (1 < x < 2) shell, while the to…

Hybrid Vapor-Solution Sequentially Deposited Mixed-Halide Perovskite Solar Cells

The recent sky-rocketing performance of perovskite solar cells has triggered a strong interest in further upgrading the fabrication techniques to meet the scalability requirements of the photovoltaic industry. The integration of vapor-deposition into the solution process in a sequential fashion can boost the uniformity and reproducibility of the perovskite solar cells. Besides, mixed-halide perovskites have exhibited outstanding crystallinity as well as higher stability compared with iodide-only perovskite. An extensive study was carried out to identify a reproducible process leading to highly crystalline perovskite films that when integrated into solar cells exhibited high power conversion…

Single-Exciton Amplified Spontaneous Emission in Thin Films of CsPbX3 (X = Br, I) Perovskite Nanocrystals

CsPbX3 perovskite nanocrystals (PNCs) have emerged as an excellent material for stimulated emission purposes, with even more prospective applications than conventional colloidal quantum dots. However, a better understanding of the physical mechanisms responsible for amplified spontaneous emission (ASE) is required to achieve more ambitious targets (lasing under continuous wave optical or electrical excitation). Here, we establish the intrinsic mechanisms underlying ASE in PNCs of three different band gaps (CsPbBr3, CsPbBr1.5I1.5, and CsPbI3). Our characterization at cryogenic temperatures does not reveal any evidence of the biexciton mechanism in the formation of ASE. Instead, the measured …

Low-dimensional non-toxic A 3 Bi 2 X 9 compounds synthesized by a dry mechanochemical route with tunable visible photoluminescence at room temperature

We have synthesized fifteen inorganic and hybrid organic-inorganic non-toxic A3Bi2X9 compounds (A = K+, Rb+, Cs+, CH3NH3+ and HC(NH2)2+; X = I−, Br−, Cl−) through dry mechanochemistry. We demonstrate that this synthetic method is very well suited to prepare compounds from poorly soluble precursors, allowing thus the preparation of so far unreported compounds. X-ray diffraction analysis demonstrates the high crystallinity of the so-formed ternary bismuth halides. Furthermore, we show that, through substitution of the A-cation and X-anion, the bandgap of these compounds can be tuned to absorb throughout the whole visible spectrum. As-prepared powders of Cs3Bi2Br9 and Cs3Bi2I9 without any pass…

Mechanisms of Spontaneous and Amplified Spontaneous Emission in CH3NH3PbI3 Perovskite Thin Films Integrated in an Optical Waveguide

In this paper, the physical mechanisms responsible for optical gain in ${\mathrm{CH}}_{3}{\mathrm{NH}}_{3}{\mathrm{Pb}\mathrm{I}}_{3}$ (MAPI) polycrystalline thin films are investigated experimentally and theoretically. Waveguide structures composed by a MAPI film embedded in between PMMA and silica layers are used as an efficient geometry to confine emitted light in MAPI films and minimize the energy threshold for amplified spontaneous emission (ASE). We show that photogenerated exciton density at the ASE threshold is as low as $(2.4\ensuremath{-}12)\ifmmode\times\else\texttimes\fi{}{10}^{16}\phantom{\rule{0.1em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$, which is below the Mott transition den…

Spray-driven Solid-State Halide Exchange in CsPbX3 Nanocrystal Films

Multilayers of CdSe/CdS/ZnCdS Core/Wings/Shell Nanoplatelets Integrated in a Polymer Waveguide

In this work, fabrication of multilayers of colloidal CdSe/CdS/ZnCdS core/wings/shell nanoplatelets (NPls) on solid substrates by layer-by-layer deposition technology is successfully demonstrated for the first time. Integration of multilayers of nanoplatelets into polymeric waveguide resulted in strong dichroism in absorption and polarization effects in photoluminescence with preferable absorption and emission of TE (horizontal) mode that evidences about in-plane orientation of NPls relative to the planar waveguide. The propagation of the excitation and emission light along the waveguide was thoroughly investigated both theoretically and experimentally for different number of NPl layers.

Femtosecond laser fragmentation from water-dispersed microcolloids: toward fast controllable growth of ultrapure Si-based nanomaterials for biological applications

International audience; An ultrashort laser-assisted method for fast production of concentrated aqueous solutions of ultrapure Si-based colloidal nanoparticles is reported. The method profits from the 3D geometry of femtosecond laser ablation of water-dispersed microscale colloids, prepared preliminarily by the mechanical milling of a Si wafer, in order to avoid strong concentration gradients in the ablated material and provide similar conditions of nanocluster growth within a relatively large laser caustics volume. We demonstrate the possibility for the fast synthesis of non-aggregated, low-size-dispersed, crystalline Si-based nanoparticles, whose size and surface oxidation can be controll…

Silicon Nanocrystals Produced by Nanosecond Laser Ablation in an Organic Liquid

Small (3−5 nm in diameter following HRTEM images) Si nanocrystals were produced in a two-stage process including (1) nanosecond laser ablation of a Si target in an organic liquid (chloroform) that results in formation of big composite polycrystalline particles (about 20−100 nm average diameter) and (2) ultrasonic post-treatment of Si nanoparticles in the presence of HF. The post-treatment is responsible for disintegration of the composite Si particles, release of small individual nanocrystals, and reduction of their size due to HF-induced etching of Si oxide. The downshift and broadening of the ∼520 cm−1 Raman phonon band of the small Si nanocrystals with respect to the bulk Si Raman band i…

Delayed Luminescence in Lead Halide Perovskite Nanocrystals

The mechanism responsible for the extremely long photoluminescence (PL) lifetimes observed in many lead halide perovskites is still under debate. While the presence of trap states is widely accepted, the process of electron detrapping back to the emissive state has been mostly ignored, especially from deep traps as these are typically associated with nonradiative recombination. Here, we study the photophysics of methylammonium lead bromide perovskite nanocrystals (PNCs) with a photoluminescence quantum yield close to unity. We show that the lifetime of the spontaneous radiative recombination in PNCs is as short as 2 ns, which is expected considering the direct bandgap character of perovskit…

Interpretation of the photoluminescence decay kinetics in metal halide perovskite nanocrystals and thin polycrystalline films

Abstract In this paper we present critical analysis of different points of view on interpretation of the photoluminescence (PL) decay kinetics in lead halide perovskites prepared in the form of well passivated nanocrystals (PNCs) or thin polycrystalline layers. In addition to the literature data, our own measurements are also considered. For PNCs, a strong dependence of the PL lifetimes on the type of passivating ligand was observed with a consistently high PL quantum yield. It is shown that such ligand effects, as well as a decrease in the PL lifetime with decreasing temperature, are well qualitatively explained by the phenomenological model of thermally activated delayed luminescence, in …