Decrease In Nanocrystal Size Research Articles

Formation of bcc nanocrystals in Co-based amorphous alloys

The crystallization of amorphous alloys of Co-Fe-B-(Ti, Nb) system was studied by differential scanning calorimetry, X-ray diffraction, and transmission electron microscopy. It was determined that the alloying of amorphous alloys of Co-Fe-B system with elements having a bcc lattice promotes the formation of a metastable phase with this lattice during crystallization. Nanocrystal size and the fraction of the phase with a bcc lattice depend on the concentration of alloying components. When the concentration of bcc components increases, nanocrystal size decreases. When Nb (an element with a bcc lattice) is replaced by Ti (an element with an hcp lattice), the fraction of the bcc phase decreases, and multiphase crystallization (simultaneous formation of several crystalline phases) occurs. In Co56Fe16B20Ti8 alloy, the first crystallization stage consists of two steps. During the seemingly simultaneous formation of several crystalline phases, first a small quantity of the bcc phase and then Co23B6 and fcc-Cо crystals are formed sequentially. All the results obtained indicate heterogeneous nanocrystal formation with the nucleation on structurally related ordered regions (on ordered clusters which consist of alloy components with the structure related to the crystallizing phase).

Размерная зависимость упругих свойств нанокристалла аргона

AbstractThe state equation ( P ) and isothermal elastic modulus ( B ) are calculated for argon macro- and nanocrystals at T = 10 K using in the framework the RP(vac)-model of nanocrystal. The isochoric and isobaric (at P = 0) dependences of the Debye temperature (Θ), of the first (γ) and second ( q ) Grüneisen parameters, as well as the specific surface energy (σ), B and B '( P ) = (∂ B /∂ P )_ T , are studied as the functions of size and shape of the nanocrystal. As shown, the isothermally isobaric decrease in nanocrystal size is accompanied by a decrease in functions Θ, q , σ, B and B '( P ) and by an increase in the γ parameter. However, the elastic modulus rises in the case of the isothermally isochoric decrease in the nanocrystal size. When the nanocrystal deviates from its most energetically optimal shape (a cube for the RP(vac)-model), the size dependences of these functions become more noticeable.

Size Dependence of Elastic Properties of Argon Nanocrystals

The state equation (P) and isothermal elastic modulus (B) are calculated for argon macro- and nanocrystals at T = 10 K using in the framework the RP(vac)-model of nanocrystal. The isochoric and isobaric (at P = 0) dependences of the Debye temperature (Θ), of the first (γ) and second (q) Gruneisen parameters, as well as the specific surface energy (σ), B and B'(P) = (∂B/∂P)T, are studied as the functions of size and shape of the nanocrystal. As shown, the isothermally isobaric decrease in nanocrystal size is accompanied by a decrease in functions Θ, q, σ, B and B'(P) and by an increase in the γ parameter. However, the elastic modulus rises in the case of the isothermally isochoric decrease in the nanocrystal size. When the nanocrystal deviates from its most energetically optimal shape (a cube for the RP(vac)-model), the size dependences of these functions become more noticeable.

A Photoelectrochemical Device with Dynamic Interface Energetics: Understanding of Structural and Physical Specificities and Improvement of Performance and Stability

AbstractThe basic configuration of a photoelectrochemical (PEC) water splitting device contains a semiconductor junction, which separates charge carriers by developing interface energetics. Recently, porous metal oxide/semiconductor junctions have shown that flat‐band potentials (Vfb), representing interface energetics, could be dynamically changed with PEC reactions. However, it remains unclear as to what structural and physical specificities of the porous metal oxide induce the dynamic Vfb. Herein, it is demonstrated that the electrolyte permeability and nanocrystal structure of porous NiOx are crucial for the dynamic Vfb in porous NiOx integrated Si photocathodes. A comparison of the dense and porous NiOx with electrolyte impermeable and permeable features, respectively, shows that Vfb changes only in the porous NiOx. The porous NiOx also exhibits a nanocrystal structure and increased Vfb values with a decrease in nanocrystal size. As a result of the increased Vfb, the porous NiOx achieves much higher PEC performance compared to that of the dense NiOx. However, electrolyte permeability causes electrochemical decomposition of the Si component. Thus, a cointegration of the porous and dense NiOx bilayers, which ensures stable PEC operation for 10 h while achieving high potentials of 0.2 V versus reversible hydrogen electrode at a photocurrent of 10 mA cm−2, is proposed.

Microscopic States and the Verwey Transition of Magnetite Nanocrystals Investigated by Nuclear Magnetic Resonance.

57Fe nuclear magnetic resonance (NMR) of magnetite nanocrystals ranging in size from 7 nm to 7 μm is measured. The line width of the NMR spectra changes drastically around 120 K, showing microscopic evidence of the Verwey transition. In the region above the transition temperature, the line width of the spectrum increases and the spin-spin relaxation time decreases as the nanocrystal size decreases. The line-width broadening indicates the significant deformation of magnetic structure and reduction of charge order compared to bulk crystals, even when the structural distortion is unobservable. The reduction of the spin-spin relaxation time is attributed to the suppressed polaron hopping conductivity in ferromagnetic metals, which is a consequence of the enhanced electron-phonon coupling in the quantum-confinement regime. Our results show that the magnetic distortion occurs in the entire nanocrystal and does not comply with the simple model of the core-shell binary structure with a sharp boundary.

Size dependence of the shape of a silicon nanocrystal during melting

Based on the experimentally established dependence of the melting temperature of a silicon nanocrystal on its size, it is shown that, as the size of a melting nanocrystal decreases, its shape also exhibits variation. Greater nanocrystals possess either rodlike or platelike shapes. As the nanocrystal size decreases, its shape transforms and approaches the shape of a cube.

Size-Dependent Melting Behavior of Colloidal In, Sn and Bi Nanocrystals

Colloidal nanocrystals are a technologically important class of nanostructures whose phase change properties have been largely unexplored. Here we report on the melting behavior of In, Sn, and Bi nanocrystals dispersed in a polymer matrix. This polymer matrix prevents the nanocrystals from coalescing with one another and enables previously unaccessed observations on the melting behavior of colloidal nanocrystals. We measure the melting temperature, melting enthalpy, and melting entropy of colloidal nanocrystals with diameters of approximately 10 to 20 nm. All of these properties decrease as nanocrystal size decreases, although the depression rate for melting temperature is comparatively slower than that of melting enthalpy and melting entropy. We also observe an elevated melting temperature during the initial melt-freeze cycle that we attribute to surface stabilization from the organic ligands on the nanocrystal surface. Broad endothermic melting valleys and very large supercoolings in our calorimetry data suggest that colloidal nanocrystals exhibit a significant amount of surface pre-melting and low heterogeneous nucleation probabilities during freezing.

On dependences of the thermoelastic properties on size and shape of an iron nanocrystal

Dependences of the elastic modulus (BT), Poisson ratio (μ), thermal expansion coefficient (αp), and specific heat (cv and cp) on the size and shape of the simple matter nanocrystal with a free surface have been studied using the model of nanocrystal in the form of a rectangular parallelepiped with a variable surface shape (RP model). Specific calculations have been carried out for bcc iron. It is shown that the elastic modulus decreases with an isomorphic (at a constant nanocrystal shape) decrease in the nanocrystal size (number N of atoms), whereas the functions μ(N), αp(N), cv(N), cp(N), and cp(N)-cv(N) increase along the isotherm. The larger the deviation of the nanocrystal shape is from the most energetically stable form (cube for the RP model), the more noticeably the change in these functions is with a decrease in nanocrystal size along the isotherm. It is shown that the size compression of the lattice parameter for a bcc iron nanocrystal increases with a decrease in the nanocrystal size, with a decrease in temperature, or with an increase in the nanocrystal shape deviation from the most energetically stable form.

On a change in the elastic properties with decreasing size of diamond, silicon, and germanium nanocrystals

The dependences of the modulus of elasticity (B) on the size (N) and the shape of the nanocrystal surface of a simple monoatomic material is studied using a model of the nanocrystal in the form of a rectangular parallelepiped with a variable surface shape. It is shown that the modulus of elasticity decreases with decreasing nanocrystal size; moreover, the dependence B(N) is less noticeable in the low-temperature range. The larger the deviation from the most stable nanocrystal shape, the more noticeable the decrease in the function B(N) as the nanocrystal size decreases along the isotherm. It is shown that the case when the function B(N) increases with decreasing nanocrystal size can occur in the low-temperature range. The dependences B(N) are calculated for diamond, Si, and Ge. It is shown that the function B(N) decreases and the Poisson coefficient increases in the case of an isomorphic decrease in the nanocrystal size. The nanocrystal is compressed at low temperatures and is stretched at high ones under the action of surface pressure. The lattice parameters of diamond, Si, and Ge nanocrystals change most noticeably at T > 1000 K.

Tin(II) Antimonates with Adjustable Compositions: Effects of Band‐Gaps and Nanostructures on Visible‐Light‐Driven Photocatalytic H2 Evolution

AbstractA series of tin(II)–antimonate photocatalysts with varied Sn content were prepared by altering the ion‐exchange time and reaction temperature to control their physicochemical properties, especially their band‐gaps and nanostructures. Furthermore, the effect of these catalysts on visible‐light‐driven photocatalytic H2‐evolution was also investigated. With an increase in Sn content, the narrowed band‐gaps enhanced the absorption of photons to excite the photogenerated charge carriers. A decrease in nanocrystal size approaching stoichiometric compositions impeded the recombination of the photogenerated charge carriers; the increased surface areas and pore volumes, owing to the nanostructural transformation, accelerated the redox reactions. Consequently, the photocatalytic activities gradually improved and the highest rate was observed for stoichiometric Sn2Sb2O7. As a result, the as‐prepared tin(II) antimonates—especially Sn2Sb2O7—were confirmed to be stable and efficient photocatalysts for visible‐light‐driven H2 evolution. Moreover, the activities of these photocatalysts could be improved by tuning their physicochemical properties to jointly optimize all of the processes in the photocatalytic reaction.

A study of the sensitivity of nanocrystalline indium oxide with various sizes of nanocrystals to nitrogen dioxide

The influence of NO2 adsorption on the electric conductance of the nanocrystalline indium oxide with various sizes of nanocrystals has been investigated. When the nanocrystal size decreases, the sensitivity (the ratio of the In2O3 conductance in the air and the conductance after NO2 adsorption) grows at first but then declines. An explanation for the nonmonotonous behavior of the sensitivity is offered.

Luminescence Properties Related to Defects in Y2O3:Ho3+ Nanocrystal

In this paper, Y2O3:Ho3+ nanocrystals have been prepared by a solution combustion synthesis method. The luminescence properties of defects and Ho3+ ions in yttria nanocrystal have been studied under 325 nm laser excitation. The emissions which were corresponding to 5F4-5I8, 5S2-5I8 and 5G4-5I6 trasitions of Ho3+ were measured. It was observed that the characteristic emissions of Ho3+ were gradually submerged in the broad cingulum emissions with the decrease of the nanoparticle size. The research results indicated that the broad cingulum emissions originated from the defects of nanocrystals. With the decrease of size, surface atoms which have a lot of defects increase relatively. Those surface defects can easily absorb the energy of excitation photons producing the broad spectra emissions. The electron paramagnetic resonance experiments have been performed to definitely reveal the defects information. The electron paramagnetic resonance signal has a slight shift towards the direction of weak magnetic field with the decrease of nanocrystal size, which means that the corresponding g factor becomes bigger in smaller size sample.

Interplay between Size, Composition, and Phase Transition of Nanocrystalline Cr3+-Doped BaTiO3 as a Path to Multiferroism in Perovskite-Type Oxides

Multiferroics, materials that exhibit coupling between spontaneous magnetic and electric dipole ordering, have significant potential for high-density memory storage and the design of complex multistate memory elements. In this work, we have demonstrated the solvent-controlled synthesis of Cr(3+)-doped BaTiO(3) nanocrystals and investigated the effects of size and doping concentration on their structure and phase transformation using X-ray diffraction and Raman spectroscopy. The magnetic properties of these nanocrystals were studied by magnetic susceptibility, magnetic circular dichroism (MCD), and X-ray magnetic circular dichroism (XMCD) measurements. We observed that a decrease in nanocrystal size and an increase in doping concentration favor the stabilization of the paraelectric cubic phase, although the ferroelectric tetragonal phase is partly retained even in ca. 7 nm nanocrystals having the doping concentration of ca. 5%. The chromium(III) doping was determined to be a dominant factor for destabilization of the tetragonal phase. A combination of magnetic and magneto-optical measurements revealed that nanocrystalline films prepared from as-synthesized paramagnetic Cr(3+)-doped BaTiO(3) nanocrystals exhibit robust ferromagnetic ordering (up to ca. 2 μ(B)/Cr(3+)), similarly to magnetically doped transparent conducting oxides. The observed ferromagnetism increases with decreasing constituent nanocrystal size because of an enhancement in the interfacial defect concentration with increasing surface-to-volume ratio. Element-specific XMCD spectra measured by scanning transmission X-ray microscopy (STXM) confirmed with high spatial resolution that magnetic ordering arises from Cr(3+) dopant exchange interactions. The results of this work suggest an approach to the design and preparation of multiferroic perovskite materials that retain the ferroelectric phase and exhibit long-range magnetic ordering by using doped colloidal nanocrystals with optimized composition and size as functional building blocks.

Determination of surface oxygen vacancy position in SnO 2 nanocrystals by Raman spectroscopy

Raman spectra acquired from SnO 2 nanocrystals with different sizes show a size-independent Raman mode at ∼574 cm −1. The intensity increases as the nanocrystal size decreases and this tendency is contrary to that of the normal bulk Raman modes. By considering the existence of oxygen vacancies at the nanocrystal surface, we adopt the density functional theory to calculate the Raman spectra with different oxygen vacancy positions and concentrations. The results clearly demonstrate that the in-plane oxygen vacancy is responsible for the 574 cm −1 mode and the intensity enhancement is a result of the higher in-plane oxygen vacancy concentration.

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.

Negative pressures in CaWO4 nanocrystals

Tetragonal scheelite-type CaWO4 nanocrystals recently prepared by a hydrothermal method show an enhancement of its structural symmetry with the decrease in nanocrystal size. The analysis of the volume dependence of the structural parameters in CaWO4 nanocrystals with the help of ab initio total-energy calculations shows that the enhancement of the symmetry in the scheelite-type nanocrystals is a consequence of the negative pressure exerted on the nanocrystals; i.e., the nanocrystals are under tension. Besides, the behavior of the structural parameters in CaWO4 nanocrystals for sizes below 10 nm suggests an onset of a scheelite-to-zircon phase transformation in good agreement with the predictions from our ab initio calculations. CaWO4 nanocrystals exhibit a reconstructive-type mechanism for the scheelite-to-zircon phase transition that seems to follow the tetragonal path that links both structures. This result is in contrast with the mechanism recently proposed for this transition in bulk ZrSiO4 where the transition goes through an intermediate monoclinic phase.

Influence of nanocrystal size on the transport properties of Si nanocrystals

In this study, we have investigated the carrier transport mechanism across silicon nanocrystals with the Al/p-Si/Si nanocrystals/Al structure. Sizes of silicon nanocrystals were controlled at diameters of ∼6, ∼8, and ∼11 nm. It is shown that the conductivity σ of silicon nanocrystals, both as-grown and annealed, exhibits σ∝exp[−(T0/T)]1/2 behavior under low electrical fields and over a wide temperature range. The phenomenon of material constant T0 increasing with the decrease of nanocrystal size has been observed. Considering nanocrystal size effect, experimental results can be explained by the hopping-percolation model. The influence of nanocrystal size on transport properties has been discussed. Based on this model, changes in T0 after annealing treatment are attributed to an increase in effective decay length.

Effects of nanocrystal shape on the surface charge density of ionic colloidal nanoparticles

Both size and shape of nanocrystals play an important role in determination of the energy spectrum, the surface charge density, and the stability of semiconductor-based ionic colloids. When the nanocrystal size decreases, an improvement in colloidal stability is expected due to the increase of the surface charge density. A relatively small deviation from spherical-towards ellipsoidal shape may improve even further colloid stability, due to quantum confinement effects.

Absorption cross-sections and lifetimes as a function of size in Si nanocrystals embedded in SiO 2

The photoluminescence (PL) emission yield of Si nanocrystals embedded in SiO 2 depends on their size and on Si–SiO 2 interface passivation. In this work we aim at clarifying the relative importance of both contributions by studying lifetimes and absorption cross-sections as a function of size, for samples with and without passivation in forming gas. We find that while the PL lifetime increases steadily (quasi-linear dependence), the radiative lifetime increases exponentially with the nanocrystal size. Thus, as expected, radiative oscillator strengths are much smaller for large nanocrystals, but this reduction is partially compensated by a less effective quenching at interfacial non-radiative states. The absorption cross-section per nanocrystal rises as the nanocrystal size decreases, for all excitation wavelengths, implying that the variation of oscillator strength dominates over the reduction of the density of states. Passivation processes do not affect the emission mechanism and increase the emission yield while reducing the density of non-radiative recombination centers at the Si–SiO 2 interface (P b centers).