Optical Properties Of Nanocrystals Research Articles

Pr3+-doped YPO4 nanocrystal embedded into an optical fiber.

Optical fiber with YPO4:Pr3+ nanocrystals (NCs) is presented for the first time using the glass powder-NCs doping method. The method's advantage is separate preparation of NCs and glass to preserve luminescent and optical properties of NCs once they are incorporated into optical fiber. The YPO4:Pr3+ nanocrystals were synthesized by the co-precipitation and hydrothermal methods, optimized for size (< 100nm), shape, Pr3+ ions concentration (0.2mol%), and emission lifetime. The core glass was selected from the non-silica P2O5-containing system with refractive index (n = 1.788) close to the NCs (no = 1.657, ne = 1.838). Optical fiber was drawn by modified powder-in-tube method after pre-sintering of glass powder-YPO4:Pr3+ (wt 3%) mixture to form optical fiber preform. Luminescent properties of YPO4:Pr3+ and optical fiber showed their excellent agreement, including sharp Pr3+ emission at 600nm (1D2-3H4) and 1D2 level lifetime (τ = 156 ± 5µs) under 488nm excitation. The distribution of the YPO4:Pr3+ NCs in optical fiber were analyzed by TEM-EDS in the core region (FIB-SEM-prepared). The successful usage of glass powder-NCs doping method was discussed in the aspect of promising properties of the first YPO4:Pr3+ doped optical fiber as a new way to develop active materials for lasing applications, among others.

Effect of the bulkiness of alkyl ligands on the excited-state dynamics of ZnO nanocrystals.

Organic ligands on the surface of nanocrystals (NCs) are extremely important in influencing various physical properties, such as dispersibility, electrical properties, and optical properties. Recent studies have revealed that a slight difference in the molecular structure of aliphatic organic ligands significantly affects the dispersibility of the NCs. On the other hand, the effects of the difference in the molecular structure of ligands on the excited-state dynamics of NCs remain elusive. In this study, we synthesized a series of colloidal ZnO NCs capped with different alkyl phosphonic acids and investigated their photophysical properties using emission decay measurements and transient absorption spectroscopy. The spectral shape and lifetime of the emission originating from the surface oxygen defects of ZnO NCs are almost the same irrespective of the alkyl phosphonic ligands used, indicating that the electronic states of the surface oxygen defects are not affected by the bulkiness of the ligand. On the other hand, the emission quantum yield correlates with the rate of carrier trapping by oxygen defects, suggesting that the rate of carrier trapping reflects the number of oxygen defects. Revealing the detailed relationship between molecular structures of organic ligands and the optical properties of NCs is important for advanced photofunctional superstructures using semiconductor NCs.

Room temperature synthesis of lead-free FASnI3 perovskite nanocrystals with improved stability by SnF2 additive

Tin halide perovskites are among the candidates for replacing lead-based ones for less toxicity and comparable optical properties. However, stability remains a challenge due to the easier oxidation of Sn2+ than Pb2+. Here, for the first time, we applied the ligand-assisted reprecipitation method to synthesize CH(NH2)2SnI3 (FASnI3) orthorhombic perovskite nanocrystals with an average diameter of 7.7 nm and a photoluminescence emission at 825 ± 2 nm (1.5 eV). The influence of synthesis parameters, including precursor solvent, precipitation media, temperature, and time on optical properties of nanocrystals, was studied. By incorporating SnF2, the stability of the nanocrystals was improved, and the oxidation from FASnI3 to FA2SnI6 was significantly delayed, which was quantitively demonstrated and confirmed by observing the characteristic diffraction peaks of the perovskite phase using x-ray diffraction at various exposure time to air. The addition of SnF2 is optimized to be 6%. The FASnI3 nanocrystals stayed stable for at least 265 days under N2 storage at room temperature and relative humidity of 20%.

Mn-doped AgZnInS/ZnS nanocrystals (NCs): Effects of Zn etching on the NC optical properties

Mn-doped I(II)-III-VI NCs (e.g., Mn-doped AgZnInS/ZnS NCs) possessing low-energy excitation, high brightness and long fluorescence lifetimes have been desired for time-gated fluorescence biosensing/imaging. In this type of NCs, their optical properties are significantly affected by the microscopic interactions between Mn and Mn and between Mn and host NC, the compositions of NCs, and the defects in NCs. On the other hand, it is known that Zn etching to core I(II)-III-VI NCs in NC synthesis can significantly enhance the NC brightness because Zn can exchange surface atoms (e.g., Ag and In) in NCs to minimize NC surface-defects. But for Mn-doped I(II)-III-VI NCs, Zn etching could etch out not only surface-atoms of host NCs (e.g., Ag and In) but also Mn in NCs. As a result, it could significantly affect the NC compositions and the microscopic interactions between Mn and Mn as well as between Mn and host NC, and thus the optical properties of NCs (like lifetime and absorption/emission spectra). Therefore, it is needed to investigate how Zn etching would affect the optical properties of such Mn-doped NCs. In this study, a series of Mn-doped AgZnInS NCs with different Mn doping levels were prepared through nucleation doping, and then Zn etching was applied to etch these core NCs. To identify the effects of Zn etching on NC optical properties, ZnS coating (a different ZnS shelling approach by injecting Zn precursor and S precursor alternately in synthesis) was performed on the same Mn:AgZnInS NCs, and the optical properties of NCs with these two different ZnS shelling approaches were compared. Experimental results showed that under appropriate Mn doping levels in synthesis, Zn etching instead of ZnS coating can produce low-energy excitable NCs with higher QYs and longer lifetimes, which would further facilitate the use of such NCs in time-gated fluorescence measurement. To understand the reasons for the different optical properties under different ZnS shelling approaches, the material characteristics of the prepared NCs were further measured/analyzed and the possible fluorescence mechanisms were discussed.

High impact of thiol capped ZnS nanocrystals on the degradation of single and binary aqueous solutions of industrial azo dyes under sunlight

Sulfur zinc (ZnS) nanocrystals were prepared using an aqueous colloidal route with thioglycolic acid (TGA) as a stabilizer. These ZnS-TGA nanocrystals were used as a low-cost and environmental friendly alternative for the photodegradation of methylene blue (MB) and methyl orange (MO) from aqueous solutions using sunlight irradiation. Structural and optical properties of nanocrystals were determined via X-ray diffraction, Fourier transform infrared spectroscopy, Transmission electron microscopic, UV–Visible and photoluminescence spectroscopy. Results showed a blue shift of absorbance band and higher energy band gap values than bulk ZnS materials, which was in agreement with the quantum confinement effect. X-ray diffraction confirmed the formation of cubic phase of ZnS-TGA containing also a slight wurtzite phase. Calculated average size of ZnS nanocrystals by the Debye Scherrer relation was 3.5 nm. Characterization results showed the fixation of TGA on ZnS-NCs via sulphur and carboxylic functionalities on catalyst surface. Photoluminescence spectra obtained under excitation wavelength at 325 nm showed three emission bands (416, 441 and 484 nm), which were mainly related to band edge and defect-activated emissions. The photocatalytic activity of ZnS-TGA nanocrystals under sunlight irradiation to degrade tested MB and MO dyes and their mixture in aqueous solutions was evaluated. High dye degradation efficiency was achieved at a very short time (27 min) and this performance was attributed to the negative surface charge, surface area, surface defects and high dispersion of ZnS-TGA nanocrystals on the aquatic medium. Overall, this study demonstrates that these nanocrystals could be employed as low-cost nanocatalyst for the degradation of organic dyes in wastewater treatment.

Gold plasmonic enhanced luminescence of silica encapsulated semiconductor hetero-nanoplatelets.

Optical properties of nanocrystals have the potential to drive the next generation of optoelectronic devices. However, a number of technological limitations remain to be overcome. Nanocrystals' emission is strongly impacted by the chemical environment and high excitation power. In this paper, we present a preparation route to hybrid core–shell nanoplatelets encapsulated in a gold silica shield. The induced plasmon coupling offers a higher brightness at high power excitation. We also detail the highly increased fluorescence stability offered by the silica–gold-shell protection against photobleaching processes.

Engineering the synthesis procedure for PbS nanosheets with controlled optical properties in the near-infrared region

This work discloses routes of obtaining two-dimensional PbS nanostructures by chemical synthesis methods. Using the techniques of absorption and luminescence spectroscopy, their optical characteristics are revealed. Special attention is paid to the evolution of nanoplates in a colloidal solution. The processes of growth and degradation of the structure of nanoplatelets, as well as the corresponding changes in the optical properties of nanocrystals are considered.

Colloidal synthesis of lead-free all-inorganic Cs3Sb2BrxI9-x nanocrystals

Environment-friendly lead-free perovskite nanocrystals (NCs) are highly desirable in terms of optoelectronic applications. Reported herein is the synthesis of perovskite-related Cs3Sb2Br9 NCs through a simple hot-injection approach. The morphologies and optical properties of NCs were controlled and optimized by varying the reaction temperature, time, and volume of surface ligands. Meanwhile, the Cs3Sb2Br9 NCs showed bandgap emission in the blue region centered at 470 nm, and 63 nm full width at half maximum (FWHM). In addition, the Cs3Sb2Br9 NCs displayed high stability when immersed in a polar solvent like ethanol. It was also found that the bandgaps of the Cs3Sb2BrxI9-x NCs could be adjusted by regulating the composition of halogen.

Effect of Halogen Passivation of a Surface on Radiative and Nonradiative Transitions in Silicon Nanocrystals

Within the framework of the density functional theory, the electron structure and the rates of radiative electron–hole transitions and Auger recombination were calculated for silicon nanocrystals, up to 2.5 nm in size, on the surface of which dangling bonds were saturated with chlorine or bromine atoms. Both the radiative and Auger processes were found to be very slow compared with the case of hydrogen cover, which is evidence in favor of the effectiveness of the influence of the “surface chemistry” factor on the electronic and optical properties of nanocrystals.

Dose Dependence of Nanocrystal Formation in Helium-Implanted Silicon Layers

The possibility of nanocrystal formation in silicon layers subjected to plasma-immersion helium-ion implantation at an energy of 5 keV has been proved for the first time. The effect of the implantation dose on the microstructure of the layers has been studied by X-ray reflectometry, transmission electron microscopy and Raman scattering. It has been established that the formation of silicon nanocrystals with dimensions of 10–20 nm is accompanied by a pronounced dependence on the ion flux and occurs at a dose of 5 × 1017 cm–2 with subsequent annealing at 700–800°C. The excessive dose has been shown to cause the destruction of the upper protective sublayer and the degradation of the optical properties of nanocrystals.

Tuning the Composition of Multicomponent Semiconductor Nanocrystals: The Case of I–III–VI Materials

Among the advantages of multicomponent nanocrystals is the possibility to adjust their electronic and optical properties with composition as well as size. However, the synthesis of multicomponent nanocrystals is challenging due to the presence of several metal precursors in the reaction mixture. This review takes I–III–VI semiconductor materials as an example class of multicomponent nanocrystals to highlight the underestimated importance of composition, which can affect the electronic and optical properties of nanocrystals as much as size. We discuss synthetic strategies, which enable the composition control, and show that the ability to separately choose nanocrystal size and nanocrystal composition can be beneficial for many optoelectronic and biomedical applications.

A Modified Thermal Treatment Method for the Up-Scalable Synthesis of Size-Controlled Nanocrystalline Titania

Considering the increasing demand for titania nanoparticles with controlled quality for various applications, the present work reports the up-scalable synthesis of size-controlled titanium dioxide nanocrystals with a simple and convenient thermal treatment route. Titanium dioxide nanocrystals with tetragonal structure were synthesized directly from an aqueous solution containing titanium (IV) isopropoxide as the main reactant, polyvinyl pyrrolidone (PVP) as the capping agent, and deionized water as a solvent. With the elimination of the drying process in a thermal treatment method, an attempt was made to decrease the synthesis time. The mixture directly underwent calcination to form titanium dioxide (TiO2) nanocrystalline powder, which was confirmed by FT-IR, energy dispersive X-ray spectroscopy (EDX), and X-ray diffraction (XRD) analysis. The control over the size and optical properties of nanocrystals was achieved via variation in calcination temperatures. The obtained average sizes from XRD spectra and transmission electron microscopy (TEM) images showed exponential variation with increasing calcination temperature. The optical properties showed a decrease in the band gap energy with increasing calcination temperature due to the enlargement of the nanoparticle size. These results prove that direct calcination of reactant solution is a convenient thermal treatment route for the potential large-scale production of size-controlled Titania nanoparticles.

Optimization of Optical Properties of Nanocrystals in the Context of Functionality

Optimization of Optical Properties of Nanocrystals in the Context of Functionality

Tailoring the ligand shell for the control of cellular uptake and optical properties of nanocrystals.

In this short review, the main challenges in the use of hydrophobic nanoparticles in biomedical application are addressed. It is shown how to overcome the different issues by the use of a polymeric encapsulation system, based on an amphiphilic polyisoprene-block-poly(ethylene glycol) diblock copolymer. On the basis of this simple molecule, the development of a versatile and powerful phase transfer strategy is summarized, focusing on the main advantages like the adjustable size, the retained properties, the excellent shielding and the diverse functionalization properties of the encapsulated nanoparticles. Finally, the extraordinary properties of these encapsulated nanoparticles in terms of toxicity and specificity in a broad in vitro test is demonstrated.

The Possibility and Implications of Dynamic Nanoparticle Surfaces

Understanding the precise nature of a surface or interface is a key component toward optimizing the desired properties and function of a material. For semiconductor nanocrystals, the surface has been shown to modulate fluorescence efficiency, lifetime, and intermittency. The theoretical picture of a nanocrystal surface has included the existence of an undefined mixture of trap states that arise from incomplete passivation. However, our recent scanning transmission electron microscope movies and supporting theoretical evidence suggest that, under excitation, the surface is fluctuating, creating a dynamic population of surface and subsurface states. This possibility challenges our fundamental understanding of the surface and could have far-reaching ramifications for nanoparticle-based technologies. In this Perspective, we discuss the current theories behind the optical properties of nanocrystals in the context of fluxionality.

Optical properties of GaAs nanocrystals: influence of an electric field

A study of the electronic and optical properties of the hydrogen-terminated GaAs nanocrystals Ga₆₈As₆₈H₉₆ and Ga₉₂As₈₀H₁₀₈ is presented. In this study, their dielectric functions, refractive indices, and absorption coefficients were calculated using density functional theory (DFT). The influence of a uniform external electric field on the optical properties of the nanocrystals was also explored. The highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) for each nanocrystal were studied in the absence and the presence of the uniform external electric field. Our results indicate that the HOMO-LUMO gap decreases with increasing electric field strength. The calculated density of states revealed that the main reason for this shrinking gap is an increase in the delocalization of the gallium π-orbitals under the influence of an increasing external electric field. The permanent dipole moment and the polarizability of the nanocrystals under the induced electric field increased with increasing nanocrystal radius. The induced electric field caused a noticeable redshift in the absorption peaks. The electric field also increased the absorption intensity, particularly when the field strength was >0.25 V/Å.

Transient absorption study on the influence of several polyphenylene vinylene derivatives on the exciton lifetimes in lead(II) sulfide quantum dots

The aim of this Letter is to show the influence of several new polyphenylene vinylene (PPV) derivatives, covering surfaces of lead(II) sulfide semiconductor quantum dots, on optical properties of nanocrystals, especially on exciton lifetimes. The collected data exhibit strong dependence of the excited states lifetimes in PbS nanocrystals on the distance between the polymer chain and the quantum dot. It turns out that the presence of PPV derivatives stabilizes the created exciton, which is manifested by the increase of its lifetime. Anyway this effect weakens in PPV derivatives with longer side chains separating nanocrystal from the main conductive chain.

Facile one-step photochemical synthesis of water soluble CdTe(S) nanocrystals with high quantum yields

This paper describes a successful synthesis of water soluble CdTe(S) nanocrystals (NCs) with high quantum yields (QYs) via a mild photochemical route in one step with thioglycolic acid as the stabilizer and sulfur source. The precursor solution of mixed cadmium chloride, freshly prepared sodium hydrogen telluride and thioglycolic acid was subjected directly to UV irradiation using a 500 W high-pressure mercury lamp under the protection of N2. NCs with a small particle size (2.2 nm) and a high photoluminescence quantum yield (up to 80% at room temperature) were formed within 25 min. The NCs were characterized by UV-vis absorption and fluorescence spectroscopy, transmission electron microscopy, powder X-ray diffraction and energy-dispersive X-ray spectroscopy to evaluate their structure, size and composition. The photochemically prepared NCs were compared with CdTe NCs prepared by conventional hydrothermal synthesis and CdTe/CdS core/shell NCs obtained by post-preparative photochemical passivation. The effects of UV irradiation intensity, illumination time, temperature and concentration of Te2− on the optical properties of NCs were studied in detail.

Fluorescence Enhancement, Blinking Suppression, and Gray States of Individual Semiconductor Nanocrystals Close to Gold Nanoparticles

The optical properties of nanocrystals are drastically changed by the interaction with adjacent metal nanoparticles. By time-resolved photoluminescence spectroscopy, we investigate CdSe multishell nanocrystals coupled to self-assembled films of Au nanoparticles. The distance between emitter and metal is adjusted by coating the nanocrystals with silica shells. These NCs showed increased fluorescence intensity, a decreased fluorescence lifetime, strong blinking suppression, and fluorescence from gray states. These observations can be explained by the metal particle induced change of excitation and recombination rates.

Structure and Optical Properties of Nanocrystals and Quantum Dots with Diamond, Lonsdaleite, Sphalerite and Wurtzite Structures

Structure and Optical Properties of Nanocrystals and Quantum Dots with Diamond, Lonsdaleite, Sphalerite and Wurtzite Structures