Bohr Diameter Research Articles

Quantum-Confined Perovskite Nanocrystals Enabled by Negative Catalyst Strategy for Efficient Light-Emitting Diodes.

The perovskite nanocrystals (PeNCs) are emerging as a promising emitter for light-emitting diodes (LEDs) due to their excellent optical and electrical properties. However, the ultrafast growth of PeNCs often results in large sizes exceeding the Bohr diameter, leading to low exciton binding energy and susceptibility to nonradiative recombination, while small-sized PeNCs exhibit a large specific surface area, contributing to an increased defect density. Herein, Zn2+ ions as a negative catalyst to realize quantum-confined FAPbBr3 PeNCs with high photoluminescence quantum yields (PL QY) over 90%. Zn2+ ions exhibit robust coordination with Br- ions is introduced, effectively retarding the participation of Br- ions in the perovskite crystallization process and thus facilitating PeNCs size control. Notably, Zn2+ ions neither incorporate into the perovskite lattice nor are absorbed on the surface of PeNCs. And the reduced growth rate also promotes sufficient octahedral coordination of PeNC that reduces defect density. The LEDs based on these optimized PeNCs exhibits an external quantum efficiency (EQE) of 21.7%, significantly surpassing that of the pristine PeNCs (15.2%). Furthermore, the device lifetime is also extended by twofold. This research presents a novel approach to achieving high-performance optoelectronic devices.

Formation of lead iodide nanoparticles using the sol–gel method and their applications

Lead iodide (PbI[Formula: see text] nanoparticles in silica have been synthesized using the sol–gel method. The aqueous method of synthesis has been used for undoped PbI2nanoparticles. These nanoparticles have been stabilized using thioglycerol acting as the capping agent. The narrow-sized PbI2nanoparticles have been obtained, which are characterized by size-dependent properties. The PbI2nanoparticles have been characterized and their properties have been investigated by using various experimental techniques like Ultraviolet–Visible (UV–Vis) Absorption Spectroscopy, X-ray Diffraction (XRD), Transmission Electron Microscopy (TEM), X-ray Photoelectron Spectroscopy (XPS), Photoluminescence (PL), Atomic Force Microscopy (AFM), etc. The lateral dimension of PbI2nanoparticles in silica varies from 9 Å to 27 Å. PbI2thin films were deposited on silicon wafers by using the dip coating method and analyzed by using various characterization techniques like Scanning Electron Microscopy (SEM), Energy Dispersive Analysis by X-rays (EDAX), Mapping, etc. The aim of this work is to synthesize PbI2nanoparticles with a size less than or comparable to the Bohr diameter of the exciton. The characterization results showed that the cluster size increases with the increasing film thickness and by the heat treatment in air atmosphere. The PbI2nanocrystals embedded in silica films have been studied in this paper. The PbI2nanoparticles and the PbI2sol–gel thin films can be used in optoelectronic device applications and chemical industrial applications. To the best of our knowledge, there are no studies on the synthesis of PbI2nanoparticles in silica using the sol–gel route and on the preparation of PbI2thin films using the dip coating method.

Size-dependent photoluminescence blinking mechanisms and volume scaling of biexciton Auger recombination in single CsPbI3 perovskite quantum dots.

Determining the correlation between the size of a single quantum dot (QD) and its photoluminescence (PL) properties is a challenging task. In the study, we determine the size of each QD by measuring its absorption cross section, which allows for accurate investigation of size-dependent PL blinking mechanisms and volume scaling of the biexciton Auger recombination at the single-particle level. A significant correlation between the blinking mechanism and QD size is observed under low excitation conditions. When the QD size is smaller than their Bohr diameter, single CsPbI3 perovskite QDs tend to exhibit BC-blinking, whereas they tend to exhibit Auger-blinking when the QD size exceeds their Bohr diameter. In addition, by extracting bright-state photons from the PL intensity trajectories, the effects of QD charging and surface defects on the biexcitons are effectively reduced. This allows for a more accurate measurement of the volume scaling of biexciton Auger recombination in weakly confined CsPbI3 perovskite QDs at the single-dot level, revealing a superlinear volume scaling (τXX,Auger ∝ σ1.96).

Strongly-confined colloidal lead-halide perovskite quantum dots: from synthesis to applications.

Colloidal semiconductor nanocrystals enable the realization and exploitation of quantum phenomena in a controlled manner, and can be scaled up for commercial uses. These materials have become important for a wide range of applications, from ultrahigh definition displays, to solar cells, quantum computing, bioimaging, optical communications, and many more. Over the last decade, lead-halide perovskite nanocrystals have rapidly gained prominence as efficient semiconductors. Although the majority of studies have focused on large nanocrystals in the weak- to intermediate-confinement regime, quantum dots (QDs) in the strongly-confined regime (with sizes smaller than the Bohr diameter, which ranges from 4-12 nm for lead-halide perovskites) offer unique opportunities, including polarized light emission and color-pure, stable luminescence in the region that is unattainable by perovskites with single-halide compositions. In this tutorial review, we bring together the latest insights into this emerging and rapidly growing area, focusing on the synthesis, steady-state optical properties (including exciton fine-structure splitting), and transient kinetics (including hot carrier cooling) of strongly-confined perovskite QDs. We also discuss recent advances in their applications, including single photon emission for quantum technologies, as well as light-emitting diodes. We finish with our perspectives on future challenges and opportunities for strongly-confined QDs, particularly around improving the control over monodispersity and stability, important fundamental questions on the photophysics, and paths forward to improve the performance of perovskite QDs in light-emitting diodes.

Cu-doped SnO2 nanoparticles: size and antibacterial activity investigations.

Nowadays, the use of self-cleaning surfaces is increasing globally, especially after the COVID-2019 pandemic, and the use of nanoparticles has been shown as a plausible option for this purpose. In the present study, Cu-doped SnO2 nanocrystals were successfully synthesized (in the copper content range of 0-30 mol%) using the polymeric precursor method. The structural, morphological, vibrational, and antibacterial activity were carefully studied to unveil the effect of copper ions on the properties of the hosting matrix, aiming at maximizing the usage of Cu-doped SnO2 nanocrystals. The results show fabrication of nanoparticles near their respective exciton Bohr diameter (5.4 nm for SnO2), however, monophasic SnO2 was observed up to 15 mol%. Above this limit, a secondary CuO phase was observed, as shown by the assessed X-ray diffraction (XRD), Fourier transform infrared, and Raman spectroscopy data. Furthermore, the redshift of the primary A1g vibrational mode of SnO2 is successfully described using the phonon-confinement model, demonstrating a good relationship between the Raman correlation length (L) and the crystallite size (〈D〉), the latter determined from XRD. Regarding the antibacterial activity, assessed via the disc-diffusion testing method (DDTM) while challenging two bacterial species (S. aureus and E. coli), our results suggest a rapid diffusion of the nanoparticles out of the paper disc, with a synergistic effect credited to the Sn1-xCuxO2-CuO phases contributing to the inhibition of the bacteria growth. Moreover, the DDTM data scales with cell viability, the latter analyzed using the Hill equation, from which both lethal dose 50 (LD50) and benchmark dose (BMD) were extracted.

General Expression for the Size-Dependent Optical Properties of Quantum Dots.

While initial theories on quantum confinement in colloidal quantum dots (QDs) led to analytical band gap/size relations or sizing functions, numerical methods describe size quantization more accurately. However, because of the lack of reliable sizing functions, researchers fit experimental band gap/size data sets using models with redundant, physically meaningless parameters that break down upon extrapolation. Here, we propose a new sizing function based on a proportional correction for nonparabolic bands. Using known bulk parameters, we predict size quantization for groups IV, III-V, II-VI, and IV-VI and metal-halide perovskite semiconductors, including straightforward adaptations for negative-gap semiconductors and nonspherical QDs. Refinement with respect to experimental data is possible using the Bohr diameter as a fitting parameter, by which we show a statistically relevant difference in the band gap/size relation for wurtzite and zinc blende CdSe. The general sizing function proposed here unifies the QD size calibration and enables researchers to assess bulk semiconductor parameters and predict the size quantization in unexplored materials.

Sum frequency generation vibrational spectra of perovskite nanocrystals at the single-nanocrystal and ensemble levels

Determination of molecular structures of organic-inorganic hybrid perovskite (OIHP) nanocrystals at the single-nanocrystal and ensemble levels is essential to understanding the mechanisms responsible for their size-dependent optoelectronic properties and the nanocrystal assembling process, but its detection is still a bit challenging. In this study, we demonstrate that femtosecond sum frequency generation (SFG) vibrational spectroscopy can provide a highly sensitive tool for probing the molecular structures of nanocrystals with a size comparable to the Bohr diameter (∼10 nm) at the single-nanocrystal level. The SFG signals are monitored using the spectral features of the phenyl group in (R-MBA)PbBr3 and (R-MBA)2PbI4 nanocrystals (MBA: methyl-benzyl-ammonium). It is found that the SFG spectra exhibit a strong resonant peak at 3067±3 cm−1 (ν2 mode) and a weak shoulder peak at 3045±4 cm−1 (ν7a mode) at the ensemble level, whereas a peak of the ν2 mode and a peak at 3025±3 cm−1 (ν20b mode) at the single-nanocrystal level. The nanocrystals at the single-nanocrystal level tend to lie down on the surface, but stand up as the ensemble number and the averaged sizes increase. This finding may provide valuable information on the structural origins for size-dependent photo-physical properties and photoluminescence blinking dynamics in nanocrystals.

The Fine-Structure Constant as a Ruler for the Band-Edge Light Absorption Strength of Bulk and Quantum-Confined Semiconductors.

Low-dimensional semiconductors have found numerous applications in optoelectronics. However, a quantitative comparison of the absorption strength of low-dimensional versus bulk semiconductors has remained elusive. Here, we report generality in the band-edge light absorptance of semiconductors, independent of their dimensions. First, we provide atomistic tight-binding calculations that show that the absorptance of semiconductor quantum wells equals mπα (m = 1 or 2 with α as the fine-structure constant), in agreement with reported experimental results. Then, we show experimentally that a monolayer (superlattice) of quantum dots has similar absorptance, suggesting an absorptance quantum of mπα per (confined) exciton diameter. Extending this idea to bulk semiconductors, we experimentally demonstrate that an absorptance quantum equal to mπα per exciton Bohr diameter explains their widely varying absorption coefficients. We thus provided compelling evidence that the absorptance quantum πα per exciton diameter rules the band-edge absorption of all direct semiconductors, regardless of their dimension.

Bulk-like ZnSe Quantum Dots Enabling Efficient Ultranarrow Blue Light-Emitting Diodes.

Blue-emitting heavy-metal free QDs simultaneously exhibiting photoluminescence quantum yield close to unity and narrow emission line widths are essential for next-generation electroluminescence displays, yet their synthesis is highly challenging. Herein, we develop the synthesis of blue-emitting QDs by growing a thin shell of ZnS on ZnSe cores with their size larger than bulk Bohr diameter. The bulk-like size of ZnSe cores enables the emission to locate in the blue region with a narrow emission width close to its intrinsic peak width. The obtained bulk-like ZnSe/ZnS core/shell QDs display high quantum yield of 95% and extremely narrow emission width of ∼9.6 nm. Moreover, the bulk-like size of ZnSe cores reduces the energy level difference between QDs and adjacent layers in LEDs and improves charge transport. The LEDs fabricated with these high-quality QDs show bright pure blue emission with an external quantum efficiency of 12.2% and a relatively long operating lifetime.

Role of secondary etching of silicon nanowires towards quantum confinement effect

The quantum confinement effect (QCE) plays a significant role in tuning the bandgap of the silicon nanowires (SiNWs) by converting from indirect to direct bandgap semiconductor. The QCE makes the silicon (Si) semiconductor suitable for efficient solar cell and optoelectronic devices. The QCE only occurs for the nanostructure of Si, whose diameter is lesser than Bohr's diameter (≈9.8 nm). Metal-assisted chemical etching (MACE) fabricates columnlike SiNWs array on the Si substrate through primary etching. Length of the fabricated NWs is in the micrometer range, and its diameter is much higher (d > 70 nm) than Bohr's diameter, which contradicts the existence of QCE in the NWs. However, secondary etching etches the sidewalls of the SiNWs forming Si nano-crystals and Si quantum dots on the NWs whose average size is lesser than Bohr's diameter. HRTEM image and mathematical analysis of the SiNWs, using Raman, XRD, and Photoluminescence (PL) characterization, calculate the average size of the nanostructures formed on the sidewalls. The analysis indicates nanostructures of size to be less than five nm, formed on the SiNWs due to secondary etching, the sole reason behind the origin of QCE.

A quantum dot model for nanoparticles in polymer nanocomposites

Nanoparticles in polymer nanocomposites can be dielectric quantum dots (DQDs). A quantum dot model is proposed for nanoparticles imbedded in polymers based on both this new concept and the multicore model proposed in 2005. QDs should be as small as de Broglie's wavelength and the exciton Bohr diameter, and accommodate a countable number of charge carriers. DQDs are a little bit larger than metal and semiconductor QDs. Some of dielectric performances such as permittivity, charge transport, space charge and treeing can be explained in terms of nonlinear multi-functional traps derived from the DQD model. It was verified that different combination of polymers and nanoparticles lead to different characteristics.

Catalyst/surfactant co-assisted colloidal synthesis and optical properties of ultrathin/ultralong ZnSe nanowires

In this work, we combine the catalytic growth and ligand control mechanisms to synthesize ultrathin/ultralong ZnSe nanowires (NWs). Ag2Se catalyst seeds initiate the NW growth and significantly cause the increase of the NW lengths up to several micrometers. The long-chain amine mixture of oleylamine and 1-dodecylamine acts as reaction medium and ligands (or surfactants) to control the sizes of Ag2Se seeds and ZnSe NWs within several nanometers (the mean diameter of NWs: ∼5.5 nm). The microstructure studies by HRTEM reveal the presence of zinc blende (ZB)/wurtzite (W) alternations and a number of stacking faults and twins within individual ZnSe ultrathin NWs. These NWs exhibit optical quantum confinement effect due to the diameters smaller than the theoretical exciton Bohr diameter of bulk ZnSe. Raman spectrum and photoelectronic performance under white light illumination are also presented.

Exciton-phonon coupling in a CsPbBr3 single nanocrystal

We have performed micro-photoluminescence measurements on a single CsPbBr3 nanocrystal (NC) with a size comparable to the Bohr diameter (7 nm). When the NC has an orthorhombic crystal symmetry, we observe an exciton fine structure composed of three peaks linearly polarized. We took advantage of the polarization properties of micro-photoluminescence to monitor in situ both the energy and linewidth of individual peaks when increasing temperature. We reveal that two regimes exist, at low and high temperature, which are dominated by acoustic or longitudinal optical phonon (Fröhlich term) couplings, respectively. The acoustic contribution does not change when the energy of the excitonic transition varies in the range of 2.46–2.62 eV, i.e., with NC sizes corresponding to this range. We find that line broadening is mainly ruled by the Fröhlich term, which is consistent with the polar nature of CsPbBr3.

Synthesis and Characterization of Ultrathin Silver Sulfide Nanoplatelets.

We report the synthesis of ultrathin silver sulfide (Ag2S) nanoplatelets (NPLs) synthesized via a one-pot method in ethylene glycol with 3-mercaptopropionic acid serving as both the sulfur precursor and the platelet ligand. The colloidally synthesized nanoplatelets are exceptionally thin, with a thickness of only 3.5 ± 0.2 Å and a 1S exciton Bohr diameter to confinement ratio of ∼12.6. The NPL growth is shown to be quantized by layer thickness using absorption and photoluminescence (PL) spectroscopy. Transmission electron microscopy, atomic force microscopy, and X-ray diffraction analyses of the NPLs show that they correspond to the (202) plane of the β-Ag2S structure. The PL quantum yield of these NPLs is ∼30%, suggesting their potential use in biomedical imaging. Optoelectronic properties were evaluated via sensitized photocurrent spectroscopy with the resulting spectra closely matching the distinctive absorption spectral shape of the Ag2S NPLs.

Highly Efficient Light-Emitting Diodes of Colloidal Metal-Halide Perovskite Nanocrystals beyond Quantum Size.

Colloidal metal-halide perovskite quantum dots (QDs) with a dimension less than the exciton Bohr diameter DB (quantum size regime) emerged as promising light emitters due to their spectrally narrow light, facile color tuning, and high photoluminescence quantum efficiency (PLQE). However, their size-sensitive emission wavelength and color purity and low electroluminescence efficiency are still challenging aspects. Here, we demonstrate highly efficient light-emitting diodes (LEDs) based on the colloidal perovskite nanocrystals (NCs) in a dimension > DB (regime beyond quantum size) by using a multifunctional buffer hole injection layer (Buf-HIL). The perovskite NCs with a dimension greater than DB show a size-irrespective high color purity and PLQE by managing the recombination of excitons occurring at surface traps and inside the NCs. The Buf-HIL composed of poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonate) (PEDOT:PSS) and perfluorinated ionomer induces uniform perovskite particle films with complete film coverage and prevents exciton quenching at the PEDOT:PSS/perovskite particle film interface. With these strategies, we achieved a very high PLQE (∼60.5%) in compact perovskite particle films without any complex post-treatments and multilayers and a high current efficiency of 15.5 cd/A in the LEDs of colloidal perovskite NCs, even in a simplified structure, which is the highest efficiency to date in green LEDs that use colloidal organic-inorganic metal-halide perovskite nanoparticles including perovskite QDs and NCs. These results can help to guide development of various light-emitting optoelectronic applications based on perovskite NCs.

Time-Resolved Photoluminescence Properties of ZnO Thin Films Evaporated by Atomic Layer Deposition

We have studied time-resolved photoluminescence (PL) properties of ZnO thin films which were grown by layer-by-layer manner using atomic layer deposition (ALD) method. The deposited film thickness was estimated by X-ray reflectance spectroscopy, which proved that the ALD system deposited -1.57 Å/cycle in thickness. As-grown films showed typical band-edge and surface defect induced emissions. The evaluated PL lifetimes showed interesting trend as the film thickness approaching to exciton Bohr diameter. Moreover, PL lifetime imaging technique clearly showed the characteristic lifetime modulation with changing film thickness in both band-edge and surface defect emissions.

Characterization of CdSe-nanocrystals used in semiconductors for aerospace applications: Production and optical properties

Semiconductor nanocrystals (NC’s) are the materials with dimensions less than 10nm. When the dimensions of nanocrystals are reduced the bulk bohr diameter, the photo generated electron-hole pair becomes confined and nanocrystal exhibits size dependent upon optical properties. This work is focused on the studying of CdSe semiconductor nanocrystals. These nanocrystals are considered as one of the most widely studies semiconductors because of their size – tunable optical properties from the visible spectrum. CdSe-nanocrystals are produced and obtained throughout the experimental setup initiated at Nano-NRIAG Unit (NNU), which has been constructed and assembled at NRIAG institute. This unit has a specific characterization for preparing chemical compositions, which may be used for solar cell fabrications and space science technology. The materials prepared included cadmium oxide and selinid have sizes ranging between 2.27nm and 3.75nm. CdSe-nanocrystals are synthesized in “TOP/TOPO (tri–octyl phosphine/tri–octyl phosphine oxide). Diagnostic tools, include UV analysis, TEM microscope, and X-ray diffraction, which are considered for the analytical studies of the obtained materials. The results show that, in this size regime, the generated particles have unique optical properties, which is achieved from the UV analysis. Also, the TEM image analysis shows the size and shape of the produced particles. These studies are carried out to optimize the photoluminescent efficiency of these nanoparticles. Moreover, the data revealed that, the grain size of nanocrystals is dependent upon the growth time in turn, it leads to a change in the energy gap. Some applications of this class of materials are outlined.

Structural, FTIR and photoluminescence properties of ZnS:Cu thin films by chemical bath deposition method

ZnS:Cu thin films were deposited on glass substrate using chemical bath by neutral pH solution synthesis route. Cu is varied between 0 and 0.1M%. The XRD measurement shows hexagonal structure with the average crystalline size between 1.67 and 2.47nm (below Bohr diameter). Energy dispersive X-ray spectrum reveals the presence of Cu in the ZnS lattice. The increase of lattice parameters, the reduction of particle size and a small shift in XRD peaks by Cu-doping reveals the substitution of Cu2+ ions into the ZnS lattice. The observed red shift of green emission from 506 to 519nm in photoluminescence spectra confirms the presence of Cu2+ ions that are embedded in the ZnS matrix.

Influence of a Thickness and Percentage of Ge in Optical Properties of Thin Films of Si1-xGex Grown by LPCVD

Optical properties of Si1-xGex thin films obtained by LPCVD, with different thickness and percentages of Ge are investigated in this work. Raman spectroscopy was employed for the determination of Ge percentage (x) and strain (ε) present on the films, using equations that involve the frequencies of Si-Si, Si-Ge and Ge-Ge Raman vibrational modes. From these calculations, an increase in the percent of Ge is observed with the rise of time of deposition. The refractive index of each film was determined by Ellisometry, where an abrupt increase of this parameter is detected for films of lower thickness. This result could be due to the quantum confinement effects as a consequence of the thickness of these films, which is comparable with the excitonic Bohr diameter of Ge. The best result was obtained to the film Si0.63Ge0.37 grown at 30s with a refractive index n= 3.141 for λ= 632.9 nm, thickness of 62.7 nm and a density of nanoparticle about 1.0x1010 cm-2 with an average size of ~ 47 nm.

Band-gap tuning at the strong quantum confinement regime in magnetic semiconductor EuS thin films

Ultraviolet-visible absorption spectra of nanoscaled EuS thin films reveal a blue shift of the energy between the top-valence and bottom-conduction bands. This band-gap tuning changes smoothly with decreasing film thickness and becomes significant below the exciton Bohr diameter ∼3.5 nm indicating strong quantum confinement effects. The results are reproduced in the framework of the potential morphing method in Hartree Fock approximation. The large values of the effective mass of the holes, due to localization of the EuS f-states, limit the blue shift to about 0.35 eV. This controllable band-gap tuning of magnetic semiconductor EuS renders it useful for merging spintronics and optoelectronics.