Nanocrystal Volume Research Articles

Effect of the volume fraction of nanocrystals on the mechanical properties of (Fe0·9Ni0.1)86B14 amorphous/nanocrystalline alloy

The effect of the volume fraction of nanocrystals on the macroscopic mechanical properties and microscopic atomic structure of Fe-based nanocrystalline alloys, as well as the relationship between the two, is an urgent issue to be explored. Seven nanocrystalline alloys with different volume fractions of nanocrystals were formed by annealing treatment of (Fe0·9Ni0.1)86B14 amorphous precursor. Through hardness, flat plate bending, and nanoindentation experiments, it is found that when the volume fraction of nanocrystals reaches the range of 45–55 %, there is an abrupt change in the macroscopic mechanical properties. Specifically, when the volume fraction is less than this range, the properties inherit the amorphous matrix, however, larger than this range, the properties are dominated by precipitated nanocrystalline phases. Meanwhile, based on the molecular dynamics simulation method, seven Fe90Ni10 amorphous nanocrystalline models with different volume fractions of body-centered cubic nanocrystals are established, and the deformation mechanism of models and the evolution of their microstructures during the nanoindentation process are obtained intuitively. Moreover, it is further revealed that the significant changes in the Voronoi polyhedral index, which represents the microscopic atomic structure in this range of nanocrystal volume fractions, are the main reason for the abrupt changes in the mechanical properties at the macroscopic level.

Additives in Nanocrystalline Tin Dioxide: Recent Progress in the Characterization of Materials for Gas Sensor Applications.

Tin dioxide has huge potential and is widely studied and used in different fields, including as a sensitive material in semiconductor gas sensors. The specificity of the chemical activity of tin dioxide in its interaction with the gas phase is achieved via the immobilization of various modifiers on the SnO2 surface. The type of additive, its concentration, and the distribution between the surface and the volume of SnO2 crystallites have a significant effect on semiconductor gas sensor characteristics, namely sensitivity and selectivity. This review discusses the recent approaches to analyzing the composition of SnO2-based nanocomposites (the gross quantitative elemental composition, phase composition, surface composition, electronic state of additives, and mutual distribution of the components) and systematizes experimental data obtained using a set of analytical methods for studying the concentration of additives on the surface and in the volume of SnO2 nanocrystals. The benefits and drawbacks of new approaches to the high-accuracy analysis of SnO2-based nanocomposites by ICP MS and TXRF methods are discussed.

Investigation of LSPR Coupling Effects toward the Rational Design of CsxWO3–δ Based Solar NIR Filtering Coatings

AbstractThe optical range of localized surface plasmon resonance (LSPR) is extended into the infrared region, thanks to the development of highly doped semiconductor nanocrystals. Particularly, the near‐infrared (NIR) range holds a significant interest in managing solar radiation. However, practical applications necessitate the arrangement of particles, which is known to possibly impact their optical properties through LSPR coupling effects. How such coupling modifies the LSPR response in semiconductor hosts remains largely unexplored. In this study, a protocol for producing composite coatings composed of cesium‐doped tungsten bronze nanocrystals embedded in a silica matrix is presented. Achieving individual dispersion of nanocrystals is made possible through careful selection of a surface polyglycerol ligand exchange. This allows to tune the interparticle distance by adjusting the nanocrystal volume fraction in the composite. The findings demonstrate that LSPR coupling effects significantly influence the LSPR intensity of nanocrystals in the composite when the nanocrystal‐to‐nanocrystal distance matches their size. Beyond elucidating the LSPR coupling effect, this study provides insights into the potential use of Cs‐HTB nanocrystals for solar control applications. Through the optimization of morphology and film structure, remarkable selectivity is obtained in terms of maintaining good transparency in the visible range while achieving high absorption in the NIR.

Atomic scale insight into the decomposition of nanocrystalline zinc hydroxynitrate toward ZnO using Mn2+ paramagnetic probes.

Layered zinc hydroxynitrate (ZHN), with the chemical formula Zn5 (OH)8 (NO3)2·2H2O, exhibits a range of special properties such as anion-exchange and intercalation capacity, as well as biocompatibility, making it attractive for a large variety of applications in fields from nanotechnology to healthcare and agriculture. In this study nanocrystalline ZHN doped with 1,000ppm Mn2+ was prepared by two synthesis methods (coprecipitation and solid state reaction) using similar environment-friendly precursors. The complex morpho-structural [X-ray diffraction, scanning and transmission electron microscopy, textural analysis] and spectroscopic [Fourier transform infrared and electron paramagnetic resonance (EPR)] characterization of the two ZHN nanopowders showed similar crystalline structures with Mn2+ ions localized in the nanocrystals volume, but with differences in their morphological and textural characteristics, as well as in the doping efficiency. ZHN obtained by coprecipitation consists of larger nanoplatelets with more than two times larger specific surface area and pore volume, as well as a dopant concentration than in the ZHN sample obtained by solid state reaction. The thermal stability and the on-set of the structural phase transformation have been investigated at atomic scale with high accuracy by EPR, using Mn2+ as paramagnetic probes. The on-set of the ZHN structural phase transformation toward ZnO was observed by EPR to take place at 110°C and 130°C for the samples prepared by coprecipitation and solid state reaction, respectively, evidencing a manganese induced local decrease of the transformation temperature. Our results contribute to the selection of the most appropriate ZHN synthesis method for specific applications and in the development of new green, cost-effective synthesis routes for Mn2+ doped nano-ZnO.

Size Effect on X‐ray Scintillation Performance for Perovskite Nanocrystals Revealed by Mathematical Model

AbstractNanoscintillators (nanocrystals with X‐ray scintillation performance) lay a fundamental for their practical application in high‐resolution and low‐dose medical examination due to their excellent photoelectric properties. The optical properties and scintillation performance of nanocrystals (NCs) can be significantly affected by adjusting their size. However, the mechanism of the size effect of NCs on optical properties and X‐ray scintillation performance has not been revealed. Here, a mathematical model and experimental results are combined to demonstrate the impact of the size of NCs on their scintillation performance. The results show that the surface area and volume of NCs exhibit a positive and negative impact on the change rate of the bandgap, respectively. This model further verifies that the photoluminescence quantum yield of NCs decreases with the decrease of their size, and their light yield is remarkably affected by the size of NCs. Overall, this work provides some enlightenment for researchers to design NCs with the appropriate size for their applications in high‐performance X‐ray scintillation.

Facet Engineering for Amplified Spontaneous Emission in Metal Halide Perovskite Nanocrystals.

Auger recombination and thermalization time are detrimental in reducing the gain threshold of optically pumped semiconductor nanocrystal (NC) lasers for future on-chip nanophotonic devices. Here, we report the design strategy of facet engineering to reduce the gain threshold of amplified spontaneous emission by manyfold in NCs of the same concentration and edge length. We achieved this hallmark result by controlling the Auger recombination rates dominated by processes involving NC volume and thermalization time to the emitting states by optimizing the number of facets from 6 (cube) to 12 (rhombic dodecahedron) and 26 (rhombicuboctahedrons) in CsPbBr3 NCs. For instance, we demonstrate a 2-fold reduction in Auger recombination rates and thermalization time with increased number of facets. The gain threshold can be further reduced ∼50% by decreasing the sample temperature to 4 K. Our systematic studies offer a new method to reduce the gain threshold that ultimately forms the basis of nanolasers.

Numerical modeling of the dissolution of drug nanocrystals and its application to industrial product development.

The apparent solubility of drug nanocrystals in equilibrium was experimentally determined for a drug-stabilizer system with different particle size distributions. True supersaturation was identified for ultrafine drug nanocrystals with an almost 2-fold increase compared to the thermodynamic solubility of related coarse drug crystals, highlighting their enabling potential to enhance bioavailability. The experimental results were applied to investigate in silico the associated dissolution behavior in a closed system by numerical modeling according to the Ostwald-Freundlich and Noyes-Whitney / Nernst-Brunner equations. Calculated results were found to be in agreement with the experimental results only when the entire particle size distribution of drug nanocrystals was considered. In silico dissolution, studies were conducted to simulate the complex interplay between drug nanocrystals, dissolution conditions and resulting temporal progression during dissolution up to the equilibrium state. Calculations were performed for selected in vivo and in vitro scenarios considering different drug nanocrystal particle size distributions, drug amount, dissolution media and volume. The achieved results demonstrated the importance of ultrafine drug nanocrystals for potential bioavailability improvement and the functional applicability of the modeling approach to investigate their dissolution behavior for configurable formulation variables in product development in terms of in vivo and in vitro relevant conditions.

Precise laser-cutting of single silver nanowires for direct measurement of SPPs propagation losses

This work presents an alternative experimental approach to directly measure propagation losses of surface plasmon polaritons (SPPs) in a single silver nanowire (AgNW). The methodology synergistically combines several techniques, including single nanowire optical shaping, accurate triggering of SPPs by upconversion nanocrystals (NCs), and precise deposition of sub-micrometer-size droplets with emitters. Indeed, a femtoliter volume of colloidal NCs deposited at one end of the nanowire and excited with a laser through an objective provides a stable source of polaritons, which propagate in the nanowire. The intensity of radiation released by the SPPs scattered out at the opposite AgNW end is a direct measure of propagation losses. This is enabled through a method of precise laser cutting of the nanowire, where the length of the nanowire can be gradually reduced by an optically controlled melting process. At the same time, both the length and diameter of the nanowire are constantly monitored using optical imaging of the complex interaction between the nanowire and polarized Gaussian beam. The optical cutting technique, implemented in this experiment, shows great potential for advanced and inexpensive shaping of the metallic nanostructures for nanophotonic applications.

Thermal barrier coatings for cellulosic substrates: A statistically designed molecular dynamics study of the coating formulation effects on thermal conductivity

A central composite design (CCD) was employed to investigate the effects of cellulose nanocrystal (CNC), cellulose nanofiber (CNF), and relative free volume on the thermal barrier properties of a pigment-based coating for cellulosic substrates, composed of calcium carbonate and poly(styrene-co-methacrylic acid) binder. Average room-temperature thermal conductivity based on three replicates was selected as response and calculated for the different coating formulations using reverse non-equilibrium molecular dynamics (RNEMD) simulation with the Müller-Plathe algorithm. The effects of CNC and relative free volume fractions on the thermal conductivity of the coating were found to be significant, while that of the CNF volume fraction was insignificant. Overall, relative free volume fraction (porosity) had much larger impact on thermal conductivity than CNC volume fraction. Moreover, a weak interaction was observed between these two significant factors. A pore size distribution analysis and average pore size calculation for the coatings (∼5.25 Å for the low and ∼6.50 Å for the high relative free volume fraction) did not reveal any significant effect of CNC on these properties at either the low or high relative free volume fraction. It is speculated that larger CaCO3-CNC interfacial phonon scattering at the low relative free volume fraction leads to the above observations.

Ultrafast intraband Auger process in self-doped colloidal quantum dots

Summary Investigating the separate dynamics of electrons and holes has been challenging, although it is critical for the fundamental understanding of semiconducting nanomaterials. n-Type self-doped colloidal quantum dots (CQDs) with excess electrons occupying the low-lying state in the conduction band (CB) have attracted a great deal of attention because of not only their potential applications to infrared optoelectronics but also their intrinsic system that offers a platform for investigating electron dynamics without elusive contributions from holes in the valence band. Here, we show an unprecedented ultrafast intraband Auger process, electron relaxation between spin-orbit coupling states, and exciton-to-ligand vibrational energy transfer process that all occur exclusively in the CB of the self-doped β-HgS CQDs. The electron dynamics obtained by femtosecond mid-infrared spectroscopy will pave the way for further understanding of the blinking phenomenon, disproportionate charging in light-emitting diodes, and hot electron dynamics in higher quantum states coupled to surface states of CQDs.

Modeling atomic layer deposition process parameters to achieve dense nanocrystal-based nanocomposites

Atomic layer deposition (ALD) is a technique capable of depositing conformal coatings in highly tortuous 3D nanostructures. One configuration that has attracted attention is nanocrystal (NC) based nanocomposite films, whereby a 3D network of randomly packed nanocrystals is infilled via ALD to yield a dense nanocomposite. In this work, we demonstrate criteria for predicting three important thermal ALD process parameters necessary to completely infill 3D NC networks: cycle number, precursor pulse time, and purge time. A description of representative pore geometry is developed using parameters of the film comprised of nanocrystals before infill, specifically NC diameter, NC volume fraction, and film thickness. This geometric description allowed for prediction of required precursor pulse times to saturate the NC film surface. A finite-difference model of water vapor transport during purging revealed that desorption kinetics can be used to predict purge times required to achieve complete infill. The model predictions show good agreement with experiments carried out by infilling films comprised of GaN NCs with ZnO by the diethylzinc/water process and films comprised of Al2O3 NCs with Al2O3 by the trimethylaluminum/water process.

Investigation on the mechanically-induced nanocrystallization in metallic glasses

Shear-induced nanocrystallization in bent ribbons of Pd40Cu30Ni10P20 metallic glass has been quantitatively investigated via synchrotron radiation. The formed nanocrystals volume fraction during deformation has been directly estimated from X-ray diffraction spectra using peaks area integration. The nanocrystallization process during deformation was found to be strongly linked with the microstructure configuration of shear bands in amorphous alloys. A constitutive model based on free volume approach has been introduced to describe the kinetic of mechanically induced nanocrystallization. The solution of the coupled constitutive equations of the model, fitted to experimental data, permits to determine the physical and mechanical parameters governing the phenomena of shear-induced crystallization in metallic glasses.

Size‐ and Halide‐Dependent Auger Recombination in Lead Halide Perovskite Nanocrystals

AbstractLead halide perovskite nanocrystals (NCs) hold strong promise for a variety of light‐harvesting, emitting, and detecting applications, all of which, however, could be complicated by multicarrier Auger recombination. Therefore, complete documentation of the size‐ and composition‐dependent Auger recombination rates of these NCs is highly desirable, as it can guide system design in many applications. Herein we report the synthesis and Auger measurements of monodisperse APbX3 (A=Cs and FA; X=Cl, Br, and I) NCs in an extensive size range (ca. 3–9 nm). The biexciton Auger lifetime of all the NCs scales linearly with the NC volume. The scaling coefficient is virtually independent of the cation but rather depends sensitively on the anion, and is 0.035, 0.085, and 0.142 ps nm−3 for Cl, Br, and I, respectively. In all of these nanocrystals the Auger recombination is much faster than in standard CdSe and PbSe NCs (ca. 1 ps nm−3).

Size- and Halide-Dependent Auger Recombination in Lead Halide Perovskite Nanocrystals.

Lead halide perovskite nanocrystals (NCs) hold strong promise for a variety of light-harvesting, emitting, and detecting applications, all of which, however, could be complicated by multicarrier Auger recombination. Therefore, complete documentation of the size- and composition-dependent Auger recombination rates of these NCs is highly desirable, as it can guide system design in many applications. Herein we report the synthesis and Auger measurements of monodisperse APbX3 (A=Cs and FA; X=Cl, Br, and I) NCs in an extensive size range (ca. 3-9 nm). The biexciton Auger lifetime of all the NCs scales linearly with the NC volume. The scaling coefficient is virtually independent of the cation but rather depends sensitively on the anion, and is 0.035, 0.085, and 0.142 ps nm-3 for Cl, Br, and I, respectively. In all of these nanocrystals the Auger recombination is much faster than in standard CdSe and PbSe NCs (ca. 1 ps nm-3 ).

Anomalously low Rayleigh scattering by CdS/CdSe doped glasses

Light scattering (Raman, Rayleigh and Mandelshtam-Brillouin) and absorption of commercial color glasses (cut-off optical filters containing nanocrystals of ternary compounds CdSxSe1–x) were studied. It was found for the first time that the contribution of nanocrystals to the Rayleigh scattering is unprecedentedly low for some of these glasses. This conclusion is confirmed by estimation of nanocrystal size and volume fraction from low-frequency Raman scattering and absorbance spectroscopy for a chosen set of samples. Anomalously low Rayleigh scattering of nanocrystals is explained in terms of interference of light scattered by different nanocrystals (interparticle interference). The possibility of interparticle interference effects in the case of small volume fraction of nanocrystals is illustrated by computer simulation using the model of diffusion-limited phase transformation proposed earlier.

Auger Recombination Lifetime Scaling for Type I and Quasi-Type II Core/Shell Quantum Dots.

Having already achieved near-unity quantum yields, with promising properties for light-emitting diode, lasing, and charge separation applications, colloidal core/shell quantum dots have great technological potential. The shell thickness and band alignment of the shell and core materials are known to influence the efficiency of these devices. In many such applications, improving the efficiency requires a deep understanding of multiexcitonic states. Herein, we elucidate the shell thickness and band alignment dependencies of the biexciton Auger recombination lifetime for quasi-type II CdSe/CdS and type I CdSe/ZnS core/shell quantum dots. We find that the biexciton Auger recombination lifetime increases with the total nanocrystal volume for quasi-type II CdSe/CdS core/shell quantum dots and is independent of the shell thickness for type I CdSe/ZnS core/shell quantum dots. To perform these calculations and compute Auger recombination lifetimes, we developed a low-scaling approach based on the stochastic resolution of identity. The numerical approach provided a framework for studying the scaling of the biexciton Auger recombination lifetimes in terms of the shell thickness dependencies of the exciton radii, Coulomb couplings, and density of final states in quasi-type II CdSe/CdS and type I CdSe/ZnS core/shell quantum dots.

Intrinsic Gap States in Semiconductors with Inverted Band Structure: Comparison of SnTe vs PbTe Nanocrystals

In this contribution we compare size-dependent optical properties of PbTe and SnTe nanocrystals (NCs). We demonstrate that the size dependence of the band edge absorption line and the photoluminescence (PL) of PbTe NCs can be quantitatively described by optical transitions between the lowest quantum confined states of conduction and valence bands. In contrast, the optical properties of SnTe NCs are strongly influenced by intrinsic gap states that arise from the inverted band structure that can be described by introducing a negative energy gap Eg < 0. In principle, these intrinsic gap states could be observed directly in PL and absorption of small size SnTe NCs, where the wave functions of these states are spread over the entire NC volume. In our samples, however, these transitions are not observed due to self-doping of SnTe NCs by holes created from negatively charged Sn vacancies. As a result, the absorption is blue-shifted due to filling of the gap states and confined valence band levels by holes (phenomenon known as Moss–Burstein effect), and PL is completely suppressed due to nonradiative Auger recombinations. The size-dependent Moss–Burstein effect in our NCs is quantitatively modeled by assuming a hole concentration of p ≈ 2.2 × 1020 cm–3 which corresponds to several tens of holes per NC.

Cation Effects on the Reduction of Colloidal ZnO Nanocrystals.

The effects of a variety of monatomic cations (H+, Li+, Na+, K+, Mg2+, and Ca2+) and larger cations (decamethylcobaltocenium and tetrabutylammonium) on the reduction of colloidal ZnO nanocrystals (NCs) are described. Suspensions of "TOPO"-capped ZnO NCs in toluene/THF were treated with controlled amounts of one-electron reductants (CoCp*2 or sodium benzophenone anion radical) and cations. Equilibria were quickly established and the extent of NC reduction was quantified via observation of the characteristic near-IR absorbance of conduction band electrons. Addition of excess reductant with or without added cations led to a maximum average number of electrons per ZnO NC, which was dependent on the NC volume and on the nature of the cation. Electrons are transferred to the ZnO NCs in a stoichiometric way, roughly one electron per monovalent cation and roughly two electrons per divalent cation. This shows that cations are charge-balancing the added electrons in ZnO NCs. Overall, our experiments provide insight into the thermodynamics of charge storage and relate the colloidal chemistry of ZnO with bulk oxide semiconductors. They indicate that the apparent band energies of colloidal ZnO are highly dependent on cation/electrolyte composition and concentration, as is known for bulk interfaces, and that electrons and cations are added stoichiometrically to balance charge, similar to the behavior of Li+-batteries.

Biexciton Auger Recombination Differs in Hybrid and Inorganic Halide Perovskite Quantum Dots

We use time-resolved photoluminescence measurements to determine the biexciton Auger recombination rate in both hybrid organic-inorganic and fully inorganic halide perovskite nanocrystals as a function of nanocrystal volume. We find that the volume scaling of the biexciton Auger rate in the hybrid perovskites, containing a polar organic A-site cation, is significantly shallower than in the fully inorganic Cs-based nanocrystals. As the nanocrystals become smaller, the Auger rate in the hybrid nanocrystals increases even less than expected, compared to the fully inorganic nanocrystals, which already show a shallower volume dependence than other material systems such as chalcogenide quantum dots. This finding suggests there may be differences in the strength of Coulombic interactions between the fully inorganic and hybrid perovskites, which may prove to be crucial in selecting materials to obtain the highest performing devices in the future, and hints that there could be something "special" about the hybrid materials.

Thermo-stimulated evolution of crystalline structure and dopant distribution in Cu-doped Y-stabilized ZrO2 nanopowders

This work deals with the effect of Cu doping on thermal stability of the structural properties of Y-stabilized ZrO2 nanopowders and dopants’ spatial distribution. The powders were synthesized by a co-precipitation technique, calcinated at Tc = 500–1100 °C during 2 h and studied by x-ray diffraction (XRD) and transmission electron microscopy. Calcination at Tc = 500 °C results in the formation of ZrO2 nanocrystals with tetragonal phase predominantly. The shifts of XRD peak positions of Cu-doped powders to larger angles in comparison with those of Cu-free ones testify to the Cu presence inside nanocrystals. The Tc increase results in two main processes: (i) the non-monotonic shift of XRD peak positions and (ii) the phase transformation (tetragonal to cubic and both of them to monoclinic). This observation was explained by, at first, Cu atoms incorporation into the nanocrystal volume from the surface complexes (Tc = 500–700 °C) and then their outward diffusion followed by the formation of crystalline CuO (Tc > 700 °C). Phase transformation sets in at Tc = 700 °C, when monoclinic phase appears. Its contribution rises till Tc = 1000 °C. The mechanism of monoclinic phase formation is supposed to be consisted of the out-diffusion of interstitial Cu ions due to their shift from lattice sites. This promotes an appearance of the channels for Y out-diffusion via cation vacancies and results in phase transformation. The sintering process stimulated by CuO formation is proposed to be responsible for appearance of cubic phase at 1000–1100 °C.