Shell Thickness Dependence Research Articles

Shell effects on the dielectric properties of core-shell quantum dots.

The dielectric properties in semiconductor quantum dots are crucial for exciton formation, migration, and recombination. Different from 3D bulk materials, the dielectric response is, however, ambiguous for the small-sized 0D dots in which the effect of outer atoms on the inner atoms is usually described qualitatively. Based on the first-principles calculated electron density, the polarizability of the core-shell CdSe@ZnS wurtzite quantum dots is decomposed into the distributional contributions among which the dipole polarizability of the core is proposed to measure the shell effect on the dielectric properties of core-shell quantum dots. The shell thickness dependence on the shell effect is then studied, which is significant for the outermost shell but decays rapidly in the additional shells. Moreover, this model gives explicit physical origins of the core dipole polarizability in the core-shell QDs, which is determined by the intra-shell polarization and inter-core-shell charge transfer. Our study proposes a new approach for studying the dielectric properties of core-shell quantum dots, which is effective and extendable for other low-dimensional structures.

Hyperthermia of Magnetically Soft-Soft Core-Shell Ferrite Nanoparticles

Magnetically soft-soft MnFe2O4-Fe3O4 core-shell nanoparticles were synthesized through a seed-mediated method using the organometallic decomposition of metal acetyl acetonates. Two sets of core-shell nanoparticles (S1 and S2) of similar core sizes of 5.0 nm and different shell thicknesses (4.1 nm for S1 and 5.7 nm for S2) were obtained by changing the number of nucleating sites. Magnetic measurements were conducted on the nanoparticles at low and room temperatures to study the shell thickness and temperature dependence of the magnetic properties. Interestingly, both core-shell nanoparticles showed similar saturation magnetization, revealing the ineffective role of the shell thickness. In addition, the coercivity in both samples displayed similar temperature dependencies and magnitudes. Signatures of spin glass (SG) like behavior were observed from the field-cooled temperature-dependent magnetization measurements. It was suggested to be due to interface spin freezing. We observed a slight and non-monotonic temperature-dependent exchange bias in both samples with slightly higher values for S2. The effective magnetic anisotropy constant was calculated to be slightly larger in S2 than that in S1. The magnetothermal efficiency of the chitosan-coated nanoparticles was determined by measuring the specific absorption rate (SAR) under an alternating magnetic field (AMF) at 200-350 G field strengths and frequencies (495.25-167.30 kHz). The S2 nanoparticles displayed larger SAR values than the S1 nanoparticles at all field parameters. A maximum SAR value of 356.5 W/g was obtained for S2 at 495.25 kHz and 350 G for the 1 mg/mL nanoparticle concentration of ferrogel. We attributed this behavior to the larger interface SG regions in S2, which mediated the interaction between the core and shell and thus provided indirect exchange coupling between the core and shell phases. The SAR values of the core-shell nanoparticles roughly agreed with the predictions of the linear response theory. The concentration of the nanoparticles was found to affect heat conversion to a great extent. The in vitro treatment of the MDA-MB-231 human breast cancer cell line and HT-29 human colorectal cancer cell was conducted at selected frequencies and field strengths to evaluate the efficiency of the nanoparticles in killing cancer cells. The cellular cytotoxicity was estimated using flow cytometry and an MTT assay at 0 and 24 h after treatment with the AMF. The cells subjected to a 45 min treatment of the AMF (384.50 kHz and 350 G) showed a remarkable decrease in cell viability. The enhanced SAR values of the core-shell nanoparticles compared to the seeds with the most enhancement in S2 is an indication of the potential for tailoring nanoparticle structures and hence their magnetic properties for effective heat generation.

Shell Thickness Dependence of the Plasmon-Induced Hot-Electron Injection Process in Au@CdS Core–Shell Nanocrystals

The shell thickness dependence of the ultrafast plasmon-induced hot-electron injection process in Au@CdS core–shell nanocrystals has been studied with mid-infrared and visible femtosecond transient...

Photoelectrochemical investigation of charge injection efficiency for quantum dot light-emitting diode

When we applied colloidal quantum dots (QDs) for quantum dot light emitting diodes, it was well known that shell thickness played an important role in core protection, confinement of electrons and holes, and charge injection efficiency. However, although the shell thickness dependence of electroluminescence properties was reported, carrier injection efficiency has not been discussed in detail. In this paper, we investigated the effect of shell thickness on the carrier injection efficiency that was evaluated by photoelectrochemical measurements. By comparing the product of internal quantum yield of photoluminescence and the evaluated carrier injection efficiency with external quantum efficiency (EQE) for QDs with various shell thicknesses, we found that the optimal shell thickness for increasing EQE is determined by the balance between protection of QD's surface and carrier injection efficiency.

Fe3O4@HAp core–shell nanoparticles as MRI contrast agent: Synthesis, characterization and theoretical and experimental study of shell impact on magnetic properties

Magnetic core–shell Fe3O4@HAp nanoparticles (NPs) with a different HAp amount were synthesized using an original approach based on co-precipitation method combined with hydrothermal treatment (HTT) at temperatures range from 140 to 240 °C.NP morphological parameters were characterized using X-ray diffraction (XRD) and Fourier-transform infrared spectroscopy (FTIR), transmission electronic microscopy (TEM), specific surface area estimation (SSA), Mossbauer spectroscopy. As-prepared core–shell NPs are characterized by a narrow size distribution and the presence of magnetite and hydroxyapatite phase was shown. The trend of HAp amount – shell thickness dependence rely on HTT conditions which is probably due to the acceleration of crystallization process rate at higher temperatures. Core-shell NPs obtained at low temperatures possess a uniform shell with equal crystallinity and were further under study.The saturation magnetization of core–shell NPs is linearly decreasing with the increase of HAp amount from 68 emu/g (bare Fe3O4 core) to 52 emu/g (Fe3O4@20HAp). All the samples exhibited superparamagnetic behavior, the blocking temperature linear dependence on shell thickness was demonstrated.Computer simulation of blocking temperature dependence on shell thickness revealed the impact of the shell on the anisotropy constant and consequently on blocking temperature.To evaluate the efficiency of the core–shell NPs as contrast agents for magnetic resonance imaging (MRI), the samples with different HAp amount in agarose matrix were investigated. Images obtained with core–shell NPs are insignificantly less dark comparing with images obtained with bare NPs, but a higher value of T2/T1 ratio was achieved. Therefore, it can be concluded that as-prepared magnetic core–shell Fe3O4@HAp NPs can be considered as an effective contrast agent for MRI applications.

Illustrating the Shell Thickness Dependence in Alloyed Core/Shell Quantum-Dot-Based Light-Emitting Diodes by Impedance Spectroscopy

Colloidal quantum dots (QDs) are talented materials and have been extensively investigated in the field of photonics and optoelectronics due to their size-dependent optical properties. The core/she...

Temperature- and Mn2+ Concentration-Dependent Emission Properties of Mn2+-Doped ZnSe Nanocrystals.

Mn2+-doped ZnSe nanocrystals (Mn:ZnSe d-dots) with high optical quality-high dopant emission quantum yield with monoexponential dopant-emission decay dynamics-enable systematic and quantitative studies of temperature- and Mn2+ concentration-dependent optical properties of the dopant emission, especially its relationship with magnetic coupling. While temperature-dependent steady-state and transient dopant emission of d-dots with dilute Mn2+ concentrations originated from isolated Mn2+ ions, and can be quantitatively treated as a result of exciton-phonon coupling of isolated paramagnetic emission centers. Dopant emission of d-dots with high Mn2+ concentrations (up to 50% of Zn2+ ions being replaced by Mn2+ ions in the core nanocrystals) are found solely related to magnetically coupled Mn2+ emission. Magnetic coupling effects on steady-state dopant emission of d-dots with high Mn2+ concentrations are much stronger than those observed for doped bulk semiconductors, which is found to follow a strong and universe shell-thickness dependence for the epitaxial ZnSe and/or ZnS shells of the d-dots. By exciting the magnetically coupled Mn2+ ions directly, dopant-emission of d-dots with high Mn2+ concentrations exhibit monoexponential decay dynamics. In addition to this emission channel, a minor channel with slightly longer decay lifetime appears when the host nanocrystals with high Mn2+ concentrations are excited, which is barely visible at room temperature and increases its fraction by decreasing temperature.

Hybrid Silver Nanocubes for Improved Plasmon-Enhanced Singlet Oxygen Production and Inactivation of Bacteria.

Plasmonic nanoparticles can strongly interact with adjacent photosensitizer molecules, resulting in a significant alteration of their singlet oxygen (1O2) production. In this work, we report the next generation of metal-enhanced 1O2 nanoplatforms exploiting the lightning rod effect, or plasmon hot spots, in anisotropic (nonspherical) metal nanoparticles. We describe the synthesis of Rose Bengal-decorated silica-coated silver nanocubes (Ag@SiO2-RB NCs) with silica shell thicknesses ranging from 5 to 50 nm based on an optimized protocol yielding highly homogeneous Ag NCs. Steady-state and time-resolved 1O2 measurements demonstrate not only the silica shell thickness dependence on the metal-enhanced 1O2 production phenomenon but also the superiority of this next generation of nanoplatforms. A maximum enhancement of 1O2 of approximately 12-fold is observed with a 10 nm silica shell, which is among the largest 1O2 production metal enhancement factors ever reported for a colloidal suspension of nanoparticles. Finally, the Ag@SiO2-RB NCs were benchmarked against the Ag@SiO2-RB nanospheres previously reported by our group, and the superior 1O2 production of Ag@SiO2-RB NCs resulted in improved antimicrobial activities in photodynamic inactivation experiments using both Gram-positive and -negative bacteria model strains.

Bandgap Engineering of Indium Phosphide-Based Core/Shell Heterostructures Through Shell Composition and Thickness.

The large bulk bandgap (1.35 eV) and Bohr radius (~10 nm) of InP semiconductor nanocrystals provides bandgap tunability over a wide spectral range, providing superior color tuning compared to that of CdSe quantum dots. In this paper, the dependence of the bandgap, photoluminescence emission, and exciton radiative lifetime of core/shell quantum dot heterostructures has been investigated using colloidal InP core nanocrystals with multiple diameters (1.5, 2.5, and 3.7 nm). The shell thickness and composition dependence of the bandgap for type-I and type-II heterostructures was observed by coating the InP core with ZnS, ZnSe, CdS, or CdSe through one to ten iterations of a successive ion layer adsorption and reaction (SILAR)-based shell deposition. The empirical results are compared to bandgap energy predictions made with effective mass modeling. Photoluminescence emission colors have been successfully tuned throughout the visible and into the near infrared (NIR) wavelength ranges for type-I and type-II heterostructures, respectively. Based on sizing data from transmission electron microscopy (TEM), it is observed that at the same particle diameter, average radiative lifetimes can differ as much as 20-fold across different shell compositions due to the relative positions of valence and conduction bands. In this direct comparison of InP/ZnS, InP/ZnSe, InP/CdS, and InP/CdSe core/shell heterostructures, we clearly delineate the impact of core size, shell composition, and shell thickness on the resulting optical properties. Specifically, Zn-based shells yield type-I structures that are color tuned through core size, while the Cd-based shells yield type-II particles that emit in the NIR regardless of the starting core size if several layers of CdS(e) have been successfully deposited. Particles with thicker CdS(e) shells exhibit longer photoluminescence lifetimes, while little shell-thickness dependence is observed for the Zn-based shells. Taken together, these InP-based heterostructures demonstrate the extent to which we are able to precisely tailor the material properties of core/shell particles using core/shell dimensions and composition as variables.

Tailorable Morphology of Core–Shell Nanofibers with Surface Wrinkles for Enhanced Gas-Sensing Properties

One-dimensional core–shell nanostructures have attracted considerable attention because of their excellent carrier transport characteristics between core and shell layers. In this work, we report the preparation of surface-wrinkled In2O3@ZnO core–shell nanofibers (IZO CSNFs) in a facile way, where the as-spun polyacrylonitrile/indium nitrate (PAN/In(NO3)3) nanofibers are used as scaffolds directly in subsequent atomic layer deposition process. The thermal decomposition of the PAN can lead to the formation of the wrinkled ZnO shells. Especially, an evolvement has been observed from solid to hollow IZO CSNFs with the increase of shell thickness, where the ZnO thick shell is robust enough and may act as another type of scaffold to tailor their morphologies. Considering the different carrier transport features in various atmospheres, the IZO-x (x is the number of ALD cycles) sensors exhibit a clear shell thickness dependence on their gas-sensing properties compared to the cases of pure In2O3 and ZnO NFs. Furt...

Enhancing Upconversion from NaYF₄:Yb,Er@NaYF₄ Core-Shell Nanoparticles Assembled on Metallic Nanostructures.

We report a simple method for the fabrication of a three-layered plasmonic structure of silicon substrate-Au nanospheres-upconversion particles (UCNPs) that displays up to 101-fold fluorescence enhancement. Monodispersed pure hexagonal-phase NaYF4:Yb,Er core and NaYF4:Yb,Er@NaYF4 core@shell nanocrystals were prepared by a solvothermal method. Two dimensional (2D) assembled Au spheres were prepared on a Si substrate, and then, 2D arrays of UCNPs were deposited on the grown 2D monolayered Au spheres by a self-organizing process. The distance between plasmonic Au NPs and rare-earth (RE) core was finely adjusted by changing the undoped NaYF4 shell thickness. The UC emission enhancement shows a pronounced shell thickness dependence. For the non-plasmonic structured samples, a single peak in upconversion luminescence (UCL) enhancement was observed as the undoped NaYF4 shell thickness increases from 0 nm to 23.0 nm. In contrast, for the plasmonic structured samples, multi-oscillations in UCL enhancement were observed in the undoped NaYF4 shell thickness range of 0-23.0 nm, where the UCL enhancement factors of three bands (521 nm, 540 nm and 654 nm) are high up to 65, 101 and 61, respectively, at 19.6 nm-thick NaYF4 shell. The multi-oscillations in UCL enhancement in the plasmonic samples can be associated with plasmonic coupling between arrays of core-shell UCNPs with various sizes and the underlying 2D Au spheres. The related mechanisms of the UCL enhancements are discussed.

Shell-Thickness-Dependent Biexciton Lifetime in Type I and Quasi-Type II CdSe@CdS Core/Shell Quantum Dots

Suppression of Auger recombination in colloidal quantum dots (QDs) is important for their many applications, ranging from biological tagging, QD lasing, to solar energy conversion. Although it has been reported that the biexciton Auger recombination time of core/shell QDs can be significantly prolonged compared to core-only QDs, a systematic investigation of their dependence on the shell thickness is lacking. In this work, using CdSe@CdS core/shell QDs as a model system, we investigated the shell thickness dependence of biexciton lifetimes in both type I and quasi-type II QDs, prepared using large and small core sizes, respectively. We observe a strong increase of biexciton lifetime with the shell thickness and a larger saturation volume in quasi-type II CdSe@CdS QDs, compared to type I CdSe@CdS QDs. These trends can be attributed to the different thickness dependences of electron–hole wave function overlaps in these materials, which reflect their different extents of conduction band electron delocalizati...

The shell thickness and surface passivation dependence of fluorescence decay kinetics in CdSe/ZnS core-shell and CdSe core colloidal quantum dots

Femtoseconds, ultrafast, time-resolved, fluorescence upconversion spectroscopy was utilized to investigate the photo-induced emission decay dynamics of CdSe and CdSe/ZnS colloidal quantum dots whose surfaces were stabilized with octadecyl amine (ODA). The studies focused on understanding the influence of the surface stabilizing group on the fluorescence decay dynamics of CdSe and CdSe/ZnS and the effect of shell thickness on the photo-physical properties of core-shell quantum dots. The Core-shell materials showed excellent photostability, longer rise times, and lengthy decay times.

Effect of shell thickness on the exchange bias blocking temperature and coercivity in Co-CoO core-shell nanoparticles

The exchange bias blocking temperature distribution of naturally oxidized Co-CoO core-shell nanoparticles exhibits two distinct signatures. These are associated with the existence of two magnetic entities which are responsible for the temperature dependence of an exchange bias field. One is from the CoO grains which undergo thermally activated magnetization reversal. The other is from the disordered spins at the Co-CoO interface which exhibits spin-glass-like behavior. We investigated the oxide shell thickness dependence of the exchange bias effect. For particles with a 3 nm thick CoO shell, the predominant contribution to the temperature dependence of exchange bias is the interfacial spin-glass layer. On increasing the shell thickness to 4 nm, the contribution from the spin-glass layer decreases, while upholding the antiferromagnetic grain contribution. For samples with a 4 nm CoO shell, the exchange bias training was minimal. On the other hand, 3 nm samples exhibited both the training effect and a peak in coercivity at an intermediate set temperature Ta. This is explained using a magnetic core-shell model including disordered spins at the interface.

Interparticle interactions of FePt core and Fe3O4 shell in FePt/Fe3O4 magnetic nanoparticles

Monodisperse FePt nanoparticles were successfully synthesized using simple wet chemical method. Fe3O4 was used as a magnetic shell around each FePt nanoparticles. In FePt/Fe3O4 core/shell system, core thickness is 2 nm and shell thickness varies from zero to 2.5 nm. A theoretical model presented to calculate the shell thickness dependence of Coercivity. Presented model is compared with the results from Stoner–Wohlfarth model to interpret the shell thickness dependence of Coercivity in FePt/Fe3O4 core/shell nanoparticles. There is a difference between the results from Stoner–Wohlfarth model and experimental data when the shell thickness increases. In the presented model, the effects of interparticle exchange and random magneto crystalline anisotropy are added to the previous models of magnetization reversal for core/shell nanostructures in order to achieve a better agreement with experimental data. For magnetic shells in FePt/Fe3O4 core/shell, effective coupling between particles increases with increasing shell thickness which leads to Coercivity destruction for stronger couplings. According to the boundary conditions, in the harder regions with higher exchange stiffness, there is small variation in magnetization and so the magnetization modes become more localized. We discussed both localized and non-localized magnetization modes. For non-zero shell thickness, non-localized modes propagate in the soft phase which effects the quality of particle exchange interactions.

Strain effects on optical properties of tetrapod-shaped CdTe/CdS core–shell nanocrystals

The exciton states of strained CdTe/CdS core–shell tetrapod-shaped nanocrystals were theoretically investigated by the numerical diagonalization of a configuration interaction Hamiltonian based on the single-band effective mass approximation. We found that the inclusion of strain promotes the type-II nature by confining the electrons and holes in nonadjacent regions. This carrier separation is more efficient than that with type-II spherical nanocrystals. The strain leads to a small blue shift of the lowest exciton energy. Its magnitude is smaller than the red shift induced by increasing the shell thickness. Moreover, the larger arm diameter reduces the influence of both shell thickness and strain on the exciton energy. Considering the broken symmetry, a randomness induced carrier separation was revealed which is unique to a branched core–shell structure. The calculated shell thickness dependence of the emission energy agrees well with available experimental data.

Extinction Coefficients, Oscillator Strengths, and Radiative Lifetimes of CdSe, CdTe, and CdTe/CdSe Nanocrystals

In this paper we critically examine the literature and provide new data on fundamental optical properties of II–VI quantum dots (QDs). Specifically, we examine the integrated extinction coefficients and radiative lifetimes of different sizes of CdSe and CdTe QDs and different shell thicknesses in CdTe/CdSe core/shell QDs. We have synthesized particles having very high quantum yields and find that the measured radiative lifetimes are considerably longer and have a very different size dependence than what has been previously reported. In a simple two-level system the integrated extinction coefficients (or oscillator strengths) are related to the radiative lifetimes through the Einstein relations. The situation is more complicated in the case of II–VI QDs because of the thermal accessibility of dark states resulting from the valence band fine structure. There are significant but not equal populations in both bright and dark sublevels of the 1Se-1S3/2 exciton and in the dark 1Se-1P3/2 exciton. These Boltzmann populations depend on the QD size and shape. We find that in all three cases, quantitative or semiquantitative agreement between the measured radiative lifetimes and values calculated from the integrated extinction coefficients is obtained only if Boltzmann populations in all of the thermally accessible bright and dark states are considered. We also find that the shell thickness dependence of the radiative lifetimes of the CdTe/CdSe core/shell particles can be quantitatively understood in terms of overlap of calculated electron and hole wave functions. The results and analyses presented here clarify several discrepancies in the literature.

Determination of Electronic Energy Levels in Type-II CdTe-Core/CdSe-Shell and CdSe-Core/CdTe-Shell Nanocrystals by Cyclic Voltammetry and Optical Spectroscopy

Two different sets of type-II core/shell semiconductor nanocrystals (NCs) are prepared and studied by optical absorption, time-resolved and time-integrated photoluminescence (PL) spectroscopy, as well as cyclic voltammetry (CV). In particular, NCs with a CdTe core and a CdSe shell of variable thickness, where the holes are localized in the core and the electrons are mainly in the shell, are compared with their inverse system of a CdSe core and a CdTe shell. All measurements are correlated to model calculations based on the effective mass approximation (EMA). The comprehensive study reveals a good congruence between optical and electrochemical measurements and theoretical modeling. In particular, we find a good coincidence between the shell-thickness dependence of the band gap as measured in PL experiments, as determined from CV data, and as calculated. Interestingly, the cyclic voltammograms, which also allow for the determination of absolute electronic energy levels, are rich in features: for various she...

Magnetic properties of core/shell nanoparticles with magnetic or nonmagnetic shells

In a model presented here, the effects of interparticle exchange and random magnetocrystalline anisotropy are added to the previous models of magnetization reversal for core/shell nanostructures in order to achieve better agreement with experimental data. The results from this model are compared with ones from the Stoner–Wohlfarth model in order to help us to interpret the shell thickness dependence of the coercivity in FePt/Fe3O4 core/shell nanoparticles and describe the difference between the results from the Stoner–Wohlfarth model and experimental data arising as the shell thickness increases. For magnetic shells in the FePt/Fe3O4 core/shell system, the effective coupling between particles increases with increasing shell thickness, which leads to coercivity destruction for stronger couplings. The nucleation mode penetrates from the soft phase into the hard phase when the exchange energy density is able to compete against the random anisotropy. According to the boundary conditions, in the harder regions with higher exchange stiffness, the magnetization variation should be small and so the magnetization modes should become more localized. We discuss localized and delocalized modes. The nonlocalized modes propagating in the soft phase for the non-zero shell thickness can have an effect on the quality of particle exchange interactions. In the case of nonmagnetic shells in the FePt/SiO2 core/shell system, the same model predicts a coercivity increase with increasing shell thickness. The increase in coercivity is due to the weakening of the particle coupling with the increasingly nonmagnetic shell. We present a model for exchange bias in these systems in order to interpret the coupling effects for the core and shell in the core/shell system. Chemical intermixing at the interface leads to ‘spin glass-like’ behavior of the core/shell interface which affects the strength of coupling of the core and shell spins. In the random anisotropy model there is not so much of an effect when just high anisotropy cores and we do not expect a major difference between the superparamagnetic limits predicted by the Stoner–Wohlfarth and exchange models. Considering a bulging mode to occur in the core will have an effect on the results due to the magnetization reversal mechanisms.

Shell thickness dependence of luminescence intensity in core/shell ZnS:Mn/ZnS nanoparticles

In the present work, ZnS-coated ZnS:Mn nanoparticles were synthesized and the ZnS shell thickness dependence of Mn emission intensity was investigated. X-ray diffraction (XRD) measurements showed that both the ZnS:Mn nanoparticles and the ZnS shells possessed a zinc blende structure. Transmission electron microscopy (TEM) images and the quantum confinement effect shown in room temperature photoluminescence excitation (PLE) spectra gave the direct evidence that ZnS shells have been successfully coated on the ZnS:Mn nanoparticles. The room temperature photoluminescence (PL) spectra revealed an increase in the Mn emission intensity followed by a steady decline as the ZnS shells thickened. The effect of ZnS shells on the luminescence properties of ZnS:Mn nanoparticles is discussed.