Room-temperature solution route to free-standing SiO2-capped Si nanocrystals with green luminescence

We report a new solution route for the preparation of SiO₂-capped silicon nanocrystals (Si NCs). The Si NCs terminated with SiO₂ are fully characterized by transmission electron microscopy, X-ray diffraction, UV–vis absorption, photoluminescence decay and Fourier transform infrared spectra. The photoluminescence spectra reveal that the Si NCs solution emits green luminescence at 535 and 578 nm excited at 490 nm. The origin of the green luminescence of Si NCs is studied. Our theoretical calculations reveal that the green emission is due to the surface related localized states of self-trapped excitons rather than the purely quantum-confined states, which agrees well with the experimental results. A self-trapped exciton model is proposed to take into account the stepwise localization of electron and hole at the Si–SiO₂ interface. From the localization energies the effective Bohr radii of the localized electrons and holes are estimated to be about 1.71 and 1.57 nm, respectively.     

Aqueous Phase Exfoliating Quasi‐2D CsPbBr3 Nanosheets with Ultrahigh Intrinsic Water Stability

All‐inorganic cesium lead halide perovskite nanocrystals (NCs) have emerged as attractive optoelectronic materials due to the excellent optical and electronic properties. However, their environmental stability, especially in the presence of water, is still a significant challenge for their further commercialization. Here, ultrahigh intrinsically water‐stable all‐inorganic quasi‐2D CsPbBr₃ nanosheets (NSs) via aqueous phase exfoliation method are reported. Compared to conventional perovskite NCs, these unique quasi‐2D CsPbBr₃ nanosheets present an outstanding long‐term water stability with 87% photoluminescence (PL) intensity remaining after 168 h under water conditions. Moreover, the photoluminescence quantum yields (PLQY) of quasi‐2D CsPbBr₃ NSs is up to 82.3%, and these quasi‐2D CsPbBr₃ NSs also present good photostability of keeping 85% PL intensity after 2 h under 365 nm UV light. Evidently, such quasi‐2D perovskite NSs will open up a new way to investigate the intrinsic stability of all‐inorganic perovskites and further promote the commercial development of perovskite‐based optoelectronic and photovoltaic devices.     

The inverse energy transfer between Ge nanocrystals and erbium in SiO2 and its dependence on microstructure

The electroluminescence (EL) of Er-implanted SiO₂ layers containing Ge nanocrystals (NCs) was investigated and correlated with microstructural results obtained by transmission electron microscopy, Raman spectroscopy and X-ray diffraction. In case of EL, and in contrast to the behaviour of Er-doped Si-rich SiO₂ known from the literature it appears that there is an inverse energy transfer from Er to Ge-related oxygen deficiency centres which are located at the surface of the Ge NCs or in the transition region between the NC and the SiO₂ matrix. This is indicated by the increase of the blue-violet, Ge-related EL in presence of Er, although the Ge-related photoluminescence, which was excited by UV wavelengths non-resonant to Er, decreases at the same time. The microstructural results reveal that the maximum increase of the Ge-related EL occurs when the Ge NCs are not amorphized and/or fragmented by the Er implantation but surrounded by an Er shell. Finally a qualitative model is given explaining the different behaviour of Er-implanted SiO₂ containing Ge NCs compared to the case of Si NCs by the different quality of the NC interface to the SiO₂ matrix.     

Nanosecond field-induced quenching of the luminescence from Er-doped silicon nanocrystals

Er-doped Si-rich SiO2 gate oxide layers containing silicon nanocrystals are prepared by implantation of Si+ and Er+ into SiO2 thin films. The photoluminescence from both Si nanocrystals around 700-850 nm and Er3+ ions at 1.54 microm is strongly quenched by applying electric field in the Si-rich oxide layer. The quenching time and the recovery time of the photoluminescence from Si nanocrystals are less than 50 ns under pulsed field modulation. The quenching rate of the luminescence increases with increasing the density and reducing the size of the silicon nanocrystals. Our results indicate that the fast quenching process originates from the quantum confined Stark effect and enhanced exciton ionization by carrier tunneling between the silicon nanocrystals under the high electric field.     

Formation and photoluminescence of Si nanocrystals in controlled multilayer structure comprising of Si-rich nitride and ultrathin silicon nitride barrier layers

The effect of ultrathin silicon nitride (Si₃N₄) barrier layers on the formation and photoluminescence (PL) of Si nanocrystals (NCs) in Si-rich nitride (SRN)/Si₃N₄ multilayer structure was investigated. The layered structures composed of alternating layers of SRN and Si₃N₄ were prepared using magnetron sputtering followed by a different high temperature annealing. The formation of uniformly sized Si NCs was confirmed by the transmission electron microscopy and X-ray diffraction measurements. In particular, the 1 nm thick Si₃N₄ barrier layers was found to be sufficient in restraining the growth of Si NCs within the SRN layers upon high annealing processes. Moreover, X-ray photoelectron spectroscopy spectra shown that films subjected to post-anneal processes were not oxidized during the annealing. X-ray reflection measurements revealed that high annealing process induced low variation in the multilayer structure where the 1 nm Si₃N₄ layers act as good diffusion barriers to inhibit inter-diffusion between SRN layers. The PL emission observed was shown to be originated from the quantum confinement of Si NCs in the SRN. Furthermore, the blue shift of PL peaks accompanied by improved PL intensity after annealing process could be attributed to the effect of improved crystallization as well as nitride passivation in the films. Such multilayer structure should be advantageous for photovoltaic applications as the ultrathin barrier layer allow better electrical conductivity while still able to confine the growth of desired Si NC size for bandgap engineering.     

Oxalic Acid Enabled Emission Enhancement and Continuous Extraction of Chloride from Cesium Lead Chloride/Bromide Perovskite Nanocrystals

All‐inorganic cesium lead halide perovskite nanocrystals (NCs) have demonstrated excellent optical properties and an encouraging potential for optoelectronic applications; however, mixed‐halide perovskites, especially CsPb(Cl/Br)₃ NCs, still show lower photoluminescence quantum yields (PL QY) than the corresponding single‐halide materials. Herein, anhydrous oxalic acid is used to post‐treat CsPb(Cl/Br)₃ NCs in order to initially remove surface defects and halide vacancies, and thus, to improve their PL QY from 11% to 89% for the emission of 451 nm. Furthermore, due to the continuous chelating reaction with the oxalate ion, chloride anions from the mixed‐halide CsPb(Cl/Br)₃ perovskite NCs could be extracted, and green emitting CsPbBr₃ NCs with PL QY of 85% at 511 nm emission are obtained. Besides being useful to improve the emission of CsPb(Cl/Br)₃ NCs, the oxalic acid treatment strategy introduced here provides a further tool to adjust the distribution of halide anions in mixed‐halide perovskites without using any halide additives.     

Alkyl Passivation and Amphiphilic Polymer Coating of Silicon Nanocrystals for Diagnostic Imaging

A method to produce biocompatible polymer‐coated silicon nanocrystals for medical imaging is shown. Silica‐embedded Si nanocrystals are formed by HSQ thermolysis. The nanocrystals are then liberated from the oxide and terminated with Si–H bonds by HF etching, followed by alkyl monolayer passivation by thermal hydrosilylation. The Si nanocrystals have an average diameter of 2.1 nm ± 0.6 nm and photoluminesce with a peak emission wavelength of 650 nm, which lies within the transmission window of 650–900 nm that is useful for biological imaging. The hydrophobic Si nanocrystals are then coated with an amphiphilic polymer for dispersion in aqueous media with the pH ranging between 7 and 10 and an ionic strength between 30 mM and 2 M , while maintaining a bright and stable photoluminescence and a hydrodynamic radius of only 20 nm. Fluorescence imaging of polymer‐coated Si nanocrystals in biological tissue is demonstrated, showing the potential for in vivo imaging.     

Growth, structure and luminescence properties of multilayer Si/β-FeSi2NCs/Si/…/Si nanoheterostructures

Multilayer structures (up to 15 layers) with β-FeSi₂ nanocrystallites (NCs) buried in silicon crystalline lattice were grown by successive repetition of reactive deposition epitaxy (RDE) or solid phase epitaxy (SPE) of thin iron film on Si(100) or Si(111) substrates and silicon molecular beam epitaxy (MBE) (100-200 nm). Cross-section high resolution transmission electron microscopy (HR TEM) images and ex situ optical and Raman spectroscopy data prove that NCs formed in silicon matrix have the structure and optical properties of β-FeSi₂. The growth conditions provide no dislocations in silicon lattice were found in the course of TEM analysis. Two types of NCs depth distribution were observed: (i) layered that corresponds to iron RDE and (ii) uniform that occurs in the case of iron SPE. The uniform NCs distribution points out the fact that during a growth process NCs moves up to the surface. In spite of small nanocrystallites size (5-50 nm) and their distribution in silicon cap layers the significant photoluminescence (PL) signal at 0.8 eV was observed for all grown samples.     

0D Cs3Cu2X5 (X = I,Br, and Cl) Nanocrystals: Colloidal Syntheses and Optical Properties

0D lead‐free metal halide nanocrystals (NCs) are an emerging class of materials with intriguing optical properties. Herein, colloidal synthetic routes are presented for the production of 0D Cs₃Cu₂X₅ (X = I, Br, and Cl) NCs with orthorhombic structure and well‐defined morphologies. All these Cs₃Cu₂X₅ NCs exhibit broadband blue‐green photoluminescence (PL) emissions in the range of 445–527 nm with large Stokes shifts, which are attributed to their intrinsic self‐trapped exciton (STE) emission characteristics. The high PL quantum yield of 48.7% is obtained from Cs₃Cu₂Cl₅ NCs, while Cs₃Cu₂I₅ NCs exhibit considerable air stability over 45 days. Intriguingly, as X is changed from I to Br and Cl, Cs₃Cu₂X₅ NCs exhibit a continuous redshift of emission peaks, which is contrary to the blueshift in CsPbX₃ perovskite NCs.     

Silicon nanocrystals synthesized by electron-beam co-evaporation method and their application for nonvolatile memory

In this paper, the silicon nanocrystals (Si NCs)/SiO₂ hybrid films designed for nonvolatile memory applications are prepared by electron-beam co-evaporation of Si and SiO₂. Transmission electron microscopy images and Raman spectra verify the formation of Si NCs. Metal-oxide-semiconductor capacitor structure with Si NCs embedded in the gate oxide is fabricated to characterize the memory behaviors. High-frequency capacitance-voltage and capacitance-time measurements further demonstrate the memory effect of the structure resulting from the charging or discharging behaviors of Si NCs. It is found that the memory window can be changed by adjusting the Si/SiO₂ wt. ratio in source material. The memory devices with Si NCs/SiO₂ hybrid film as floating gate yield good retention characteristics with small charge loss.     

Highly Emissive Green Perovskite Nanocrystals in a Solid State Crystalline Matrix

Perovskite nanocrystals (NCs) have attracted attention due to their high photoluminescence quantum yield (PLQY) in solution; however, maintaining high emission efficiency in the solid state remains a challenge. This study presents a solution‐phase synthesis of efficient green‐emitting perovskite NCs (CsPbBr₃) embedded in robust and air‐stable rhombic prism hexabromide (Cs₄PbBr₆) microcrystals, reaching a PLQY of 90%. Theoretical modeling and experimental characterization suggest that lattice matching between the NCs and the matrix contribute to improved passivation, while spatial confinement enhances the radiative rate of the NCs. In addition, dispersing the NCs in a matrix prevents agglomeration, which explains their high PLQY.     

High-sensitive MIS structures with silicon nanocrystals grown via solid-state dewetting of silicon-on-insulator for solar cell and photodetector applications

This work reports an original method for the fabrication of metal–insulator–semiconductor (MIS) structures with silicon nanocrystals (Si NCs)-based active layers embedded in the insulating SiO₂ oxide, for high-performance solar cell and photodetector applications. The Si NCs are produced via the in situ solid-state dewetting of ultra-pure amorphous silicon-on-insulator (a-SOI) grown by solid source molecular beam epitaxy (SSMBE). The size and density of Si NCs are precisely tuned by varying the deposited thickness of silicon. The morphological characterization carried out by using atomic force microscopy (AFM) and scanning electron microscopy (SEM) shows that the Si NCs have homogeneous size with well-defined spherical shape and densities up to ~?10¹² /cm² (inversely proportional to the square of nominal a-Si thickness). The structural investigations by high-resolution transmission electron microscopy (HR-TEM) show that the ultra-small Si NCs (with mean diameter?~?7 nm) are monocrystalline and free of structural defects. The electrical measurements performed by current versus voltage (I–V) and photocurrent spectroscopies on the Si NCs-based MIS structures prove the efficiency of Si NCs to enhance the electrical conduction in MIS structures and to increase (×?10 times) the photocurrent (i.e., at bias voltage V = ??1 V) via the photo-generation of additional electron–hole pairs in the MIS structures. These results evidence that the Si NCs obtained by the combination of MBE growth and solid-state dewetting are perfectly suitable for the development of novel high-performance optoelectronic devices compatible with the CMOS technology.

     

Silicon nanocrystallites produced via a chemical etching method and photoluminescence properties

Silicon nanocrystallites (NCs) were fabricated via a simple and inexpensive method. The Si powders were chemically etched in the mixture of hydrofluoric and nitric acid, followed by the ultrasonic vibration in benzene, de-ionized water, or ethanol. The as-prepared Si particles feature two different sizes of ~2 and ~10 nm, respectively. The smaller particles are Si NCs suspended in the solvent, and the larger ones are several small Si NCs wrapped in amorphous shell. As excited with line of 340–420 nm, the suspensions display violet-blue emissions, which relate to the quantum confinement effect. The photoluminescence intensity of benzene suspension is the strongest and that of ethanol is the weakest.     

High‐Brightness Blue and White LEDs based on Inorganic Perovskite Nanocrystals and their Composites

Inorganic metal halide perovskite nanocrystals (NCs) have been employed universally in light‐emitting applications during the past two years. Here, blue‐emission (≈470 nm) Cs‐based perovskite NCs are derived by directly mixing synthesized bromide and chloride nanocrystals with a weight ratio of 2:1. High‐brightness blue perovskite light‐emitting diodes (PeLEDs) are obtained by controlling the grain size of the perovskite films. Moreover, a white PeLED is demonstrated for the first time by blending orange polymer materials with the blue perovskite nanocrystals as the active layer. Exciton transfer from the blue nanocrystals to the orange polymers via Förster or Dexter energy transfer is analyzed through time resolved photoluminescence. By tuning the ratio between the perovskite nanocrystals and polymers, pure white light is achieved with the a CIE coordinate at (0.33,0.34).     

Manganese Doped Fluorescent Paramagnetic Nanocrystals for Dual‐Modal Imaging

In this work, dual‐modal (fluorescence and magnetic resonance) imaging capabilities of water‐soluble, low‐toxicity, monodisperse Mn‐doped ZnSe nanocrystals (NCs) with a size (6.5 nm) below the optimum kidney cutoff limit (10 nm) are reported. Synthesizing Mn‐doped ZnSe NCs with varying Mn²⁺ concentrations, a systematic investigation of the optical properties of these NCs by using photoluminescence (PL) and time resolved fluorescence are demonstrated. The elemental properties of these NCs using X‐ray photoelectron spectroscopy and inductive coupled plasma‐mass spectroscopy confirming Mn²⁺ doping is confined to the core of these NCs are also presented. It is observed that with increasing Mn²⁺ concentration the PL intensity first increases, reaching a maximum at Mn²⁺ concentration of 3.2 at% (achieving a PL quantum yield (QY) of 37%), after which it starts to decrease. Here, this high‐efficiency sample is demonstrated for applications in dual‐modal imaging. These NCs are further made water‐soluble by ligand exchange using 3‐mercaptopropionic acid, preserving their PL QY as high as 18%. At the same time, these NCs exhibit high relaxivity (≈2.95 mM^?1 s^?1) to obtain MR contrast at 25 °C, 3 T. Therefore, the Mn²⁺ doping in these water‐soluble Cd‐free NCs are sufficient to produce contrast for both fluorescence and magnetic resonance imaging techniques.     

Rare‐Earth‐Element‐Ytterbium‐Substituted Lead‐Free Inorganic Perovskite Nanocrystals for Optoelectronic Applications

Lead‐(Pb‐) halide perovskite nanocrystals (NCs) are interesting nanomaterials due to their excellent optical properties, such as narrow‐band emission, high photoluminescence (PL) efficiency, and wide color gamut. However, these NCs have several critical problems, such as the high toxicity of Pb, its tendency to accumulate in the human body, and phase instability. Although Pb‐free metal (Bi, Sn, etc.) halide perovskite NCs have recently been reported as possible alternatives, they exhibit poor optical and electrical properties as well as abundant intrinsic defect sites. For the first time, the synthesis and optical characterization of cesium ytterbium triiodide (CsYbI₃) cubic perovskite NCs with highly uniform size distribution and high crystallinity using a simple hot‐injection method are reported. Strong excitation‐independent emission and high quantum yields for the prepared NCs are verified using photoluminescence measurements. Furthermore, these CsYbI₃ NCs exhibit potential for use in organic–inorganic hybrid photodetectors as a photoactive layer. The as‐prepared samples exhibit clear on–off switching behavior as well as high photoresponsivity (2.4 × 10³ A W^?1) and external quantum efficiency (EQE, 5.8 × 10⁵%) due to effective exciton dissociation and charge transport. These results suggest that CsYbI₃ NCs offer tremendous opportunities in electronic and optoelectronic applications, such as chemical sensors, light emitting diodes (LEDs), and energy conversion and storage devices.     

Characterisation of size-controlled and red luminescent Ge nanocrystals in multilayered superlattice structure

Ge nanocrystals (Ge NCs) embedded in a multilayered superlattice structure have been fabricated and investigated. The presence of Ge NCs was confirmed by Raman scattering and X-ray diffraction measurements. The average size of Ge NCs was modulated by the sputtering time of Ge-rich layer and possible mechanisms have been proposed. The blue shift of optical absorption edge was observed with the decrease of nanocrystal size. The photoluminescence showed broad bands centred at ∼ 1.77 eV and ∼ 2.01 eV for 3.9 nm and 3.0 nm nanocrystals, respectively, which are consistent with the theoretical calculation in literature. The properties of shifted optical absorption and red luminescence are tentatively explained by quantum confinement in the Ge NCs.     

A rapid and facile method for hydrothermal synthesis of CdTe nanocrystals under mild conditions

By using a novel procedure, a new kind of water-soluble CdTe nanocrystals (NCs) is synthesized in aqueous solutions at the temperature of 100 degrees C. In this procedure, tripeptide thiol glutathione was used as stabilizing agent, CdTe NCs with controllable photoluminescence wavelength from 500 nm to 680 nm were prepared within two hours. Compared with CdTe NCs prepared with thiohydracrylic acid as stabilizing agent, as-prepared NCs show much narrower photoluminescence FWHM, more symmetrical emission peak and higher photoluminescence quantum yield. The surface structure of as-prepared CdTe NCs is deduced, therefore, there may be some unreported circular structure on the surface. Experimental results show that as-prepared NCs have very good biological compatibility and they are nontoxic. And these CdTe NCs can conjugate with biological molecules for further biological luminescence study. The proposed hydrothermal synthesis procedure has the advantage of simplicity, inexpensiveness, time saving and mild operating conditions.     

Direct Bandgap Silicon: Tensile‐Strained Silicon Nanocrystals

Silicon, a semiconductor underpinning the vast majority of microelectronics, is an indirect‐gap material and consequently is an inefficient light emitter. This hampers the ongoing worldwide effort towards the integration of optoelectronics on silicon wafers. Even though silicon nanocrystals are much better light emitters, they retain the indirect‐gap nature. Here, we propose a solution to this long‐standing problem: silicon nanocrystals can be transformed into a material with fundamental direct bandgap via a concerted action of quantum confinement and tensile strain. We document this transformation by DFT calculations mapping the E( k ) band‐structure of Si nanocrystals. The experimental proofs are then given firstly by a 10 000× increase in the photon emission rate of strained silicon nanocrystals together with their altered absorbance spectra, both of which point to direct dipole‐allowed transitions, secondly by single nanocrystal spectroscopy, confirming reduced phonon energies and thus the presence of tensile strain, and lastly by photoluminescence studies under external hydrostatic pressure.     

Structured ZnO-based contacts deposited by non-reactive rf magnetron sputtering on ultra-thin SiO2/Si through a stencil mask

In this paper, we study the localized deposition of ZnO micro and nanostructures deposited by non-reactive rf-magnetron sputtering through a stencil mask on ultra-thin (10 nm) SiO₂ layers containing a single plane of silicon nanocrystals (NCs), synthetized by ultra-low energy ion implantation followed by thermal annealing. The localized ZnO-deposited areas are reproducing the exact stencil mask patterns. A resistivity of around 5 × 10^− 3 Ω cm is measured on ZnO layer. The as-deposited ZnO material is 97% transparent above the wavelength at 400 nm. ZnO nanostructures can thus be used as transparent electrodes for Si NCs embedded in the gate-oxide of MOS devices.