Nanocrystal Volume Research Articles

High performance hybrid near-infrared LEDs using benzenedithiol cross-linked PbS colloidal nanocrystals

We report an all solution-based processing method to produce efficient hybrid polymer–nanocrystal multilayered heterostructures for electroluminescence in the near-infrared (1050–1600nm). We employ for the first time the benzenedithiol cross-linking method to produce high-quality PbS nanocrystalline films acting both as an electron-transporting and electroluminescent layer within a near-infrared light emitting diode (LED) architecture. Due to the large volume of nanocrystals and the good carrier-transport properties within the cross-linked nanocrystalline films, this device architecture yields high emission powers and good quantum efficiencies. In contrast, using ethanedithiol cross-linking molecules results in nanocrystalline films with low photo- and electro-luminescence efficiencies.

Quantitative analysis of infrared absorption coefficient of spider silk fibers

Spider silk has a high degree of crystallinity comprising nanocrystals which are interlinked by amorphous regions containing pre-strained spidroin chains. Both, crystal volume fraction and crystal size were shown to be critical for the macroscopic mechanical properties of silk. In this study, for the first time, quantitative results were obtained from absorption IR spectra. Two main difficulties must be overcome: the cylindrical sample geometry and the absence of suitable reference spectra. We show that it is possible to get reproducible absorption coefficient spectra from dense parallel fiber grids immersed in a refractive index matching liquid. Comparison with model polypeptides yields the nanocrystal volume fraction that is close to the crystallinity values given by solid-state NMR or X-ray scattering.

Breakdown of Volume Scaling in Auger Recombination in CdSe/CdS Heteronanocrystals: The Role of the Core−Shell Interface

Spatial confinement of electronic excitations in semiconductor nanocrystals (NCs) results in a significant enhancement of nonradiative Auger recombination (AR), such that AR processes can easily dominate the decay of multiexcitons. AR is especially detrimental to lasing applications of NCs, as optical gain in these structures explicitly relies on emission from multiexciton states. In standard NCs, AR rates scale linearly with inverse NC volume. Here, we investigate multiexciton dynamics in hetero-NCs composed of CdSe cores and CdS shells of tunable thickness. We observe a dramatic decrease in the AR rates at the initial stage of shell growth, which cannot be explained by traditional volume scaling alone. Rather, fluorescence-line-narrowing studies indicate that the suppression of AR correlates with the formation of an alloy layer at the core-shell interface suggesting that this effect derives primarily from the "smoothing" of the confinement potential associated with interfacial alloying. These data highlight the importance of NC interfacial structure in the AR process and provide general guidelines for the development of new nanostructures with suppressed AR for future lasing applications.

Shape, Structural, and Energetic Effects on the Cohesive Energy and Melting Point of Nanocrystals

A nonlinear, lattice type-sensitive model, free of any adjustable parameter, has been developed to account for the shape and size dependency of the cohesive energy of free-standing nanocrystals (nanoparticles, -wires, and -films). In this model, the effects of the averaged structural and energetic properties of the surface and the volume of nanocrystals along with the first and second nearest-neighbor atomic interactions have been taken into consideration and gathered in a new parameter named as the surface-to-volume energy contribution ratio. This model has been compared to the experimental data of the cohesive energy of W and Mo nanoparticles, and the melting points of Au, Pb, Al, and Sn nanoparticles and Pb and In nanofilms. Moreover, the model has been corrected to account for the effect of substrate on the melting point of substrate-supported Sn nanodisks. It has been found that the present model has generally a good agreement with those experimental data measured by different techniques under different experimental conditions.

Fundamental aspects of activated nanocrystal luminescence and possible applications

Presented results of complex study of relaxation processes and interionic interaction in Y 2 SiO 5 :Pr 3+ and Lu 2 SiO 5 :Ce 3+ nanocrystals clearly show two fundamental aspects: the phonon quantum confinement gives rise to the new fluorescence dynamics of doped ions; the developed surface of nanocrystals stimulates the irregular distribution of doped ions within the nanocrystal volume and could be the reason of new atomic arrangement of nanocrystal. Fluorescence spectrum of isolated Y 2 SiO 5 :Pr 3+ nanocrystal demonstrates the intense fluorescence from the high crystal field components of split 1 D 2 manifold of Pr 3+ as the result of a suppression of phonon-assisted relaxation under the phonon quantum confinement. The direct comparison ofthe data obtained for nano-and bulk Y 2 SiO 5 :Pr 3+ crystals has revealed that the concentration threshold of luminescence quenching is strikingly low for nanocrystals. This effect is caused by uphill diffusion of doped ions and preferred Pr segregation at the nanocrystal surface layer that provides the relaxation of elastic tension arising due to the difference of ionic radii of Pr 3+ and Y 3+ . Lu 2 SiO 5 :Ce 3+ nanocrystals which average size is 5 nm do not demonstrate the effect of energy storage as the result of atomic packing changing that does not permit the existence of electronic traps.

GISAXS and GIWAXS analysis of amorphous–nanocrystalline silicon thin films

Amorphous–nanocrystalline silicon thin films were deposited by Plasma Enhanced Chemical Vapor Deposition (PECVD) on glass substrate with various silicon nano-crystal size distributions and volume fractions. The samples were examined by Grazing Incidence Small Angle X-ray Scattering (GISAXS) and Grazing Incidence Wide Angle X-ray Scattering (GIWAXS) at the Austrian SAXS beamline (Synchrotron Elettra, Trieste) using an X-ray beam energy of 8 keV. The grazing incidence angle varied from the critical angle to 0.2° above the critical angle. This allowed the examination of the samples at different depths, and the distinction of the surface scattering contribution from the particles scattering in the bulk. The sizes of the “particles” obtained from the horizontal and vertical sections of 2D GISAXS patterns were between 2 nm and 6 nm. Since GISAXS is sensitive to electron density differences (contrast) between the scattering bodies and the surrounding matrix, it is not evident whether the particles are nano-crystals or just voids embedded in amorphous matrix. However, the size of the crystals calculated from the line-shape analysis of peaks in GIWAXS spectra and the crystal size distribution obtained from High-Resolution Transmission Electron Microscopy (HRTEM) images agree well with the size of “particles” estimated from GISAXS, strongly indicating that the observed particles are silicon nano-crystals.

Magnetocaloric response of Fe75Nb10B15 powders partially amorphized by ball milling

The magnetocaloric response of mechanically alloyed Fe75Nb10B15 powders was studied for samples with different amorphous and nanocrystal volume fractions. Thermomagnetic properties scale following a Ω3 law for different milling processes, where Ω is the milling frequency. Curie temperature of the amorphous phase increases as the amorphous fraction increases due to its progressive enrichment in B. The peak magnetic entropy change, |ΔSMpk|, as well as the refrigerant capacity increase with increasing amorphous fraction. The field dependence of |ΔSMpk| can be explained by the multiphase character of the studied samples.

Strong quenching of praseodymium f–f luminescence induced by a surface of Y 2SiO 5:Pr 3+ nanocrystal

The direct comparison of the luminescence decay data obtained for nano- and bulk Y 2SiO 5:Pr 3+ crystals has revealed that the concentration threshold of luminescence quenching is strikingly low for nanocrystals. Nanocrystal inhomogeneous stress field induced by a surface stimulates the segregation of the doped Pr 3+ ions within the surface layer that provides the relaxation of elastic tension arising due to the difference of the ionic radii of Pr 3+ and Y 3+. The Pr 3+ irregular distribution in the nanocrystal volume results in the Pr 3+ local concentration increasing that facilitates the luminescence quenching.

Spectral and dynamical study of nonlinear luminescence from silicon nanocrystals excited by ultrashort pulses

We report on the spectral and dynamical properties of photoluminescence (PL) under ultrashort (femtosecond and picosecond) laser pulse excitation in Si nanocrystals prepared by Si +-ion implantation into a silica matrix. We concentrate on the luminescence properties that make it possible to distinguish several luminescence bands within a broad luminescence spectrum. In time-integrated luminescence, saturation-related changes suggest a presence of two different recombination channels connected to the nanocrystal volume states and the nanocrystal–SiO 2 interface. The saturation of the luminescence can be explained in terms of Auger recombination. We investigated in detail fast sub-nanosecond luminescence, where significant differences were observed under UV excitation (linear pump-intensity dependence, PL maximum at 450 nm) and for the excitation wavelengths in the visible part of the spectrum (superlinear pump-intensity dependence, broad luminescence band including wavelengths as short as 300 nm). We propose a rate-equations model based on the Auger and radiative/nonradiative recombination to interpret the experimental data.

Structural, electronic, and surface properties of anataseTiO2nanocrystals from first principles

The structural and electronic properties of anatase ${\text{TiO}}_{2}$ nanocrystals (NCs) are investigated through first-principles calculations. The dependence of the structural properties (e.g., NC volume variations) on the surface chemistry is discussed by considering two different surface coverages (dissociated water and hydrogens). Both prevent a pronounced reconstruction of the surface, thus ensuring a better crystalline organization of the atoms with respect to the bare NC. In particular, the results for the hydrated NC do show the largest overlap with the experimental findings. The band-gap blueshift with respect to the bulk shows up for both the bare and the hydrated NC, whereas hydrogen coverage or oxygen desorption from the bare NCs induce occupied electronic states below the conduction levels thus hindering the gap opening due to quantum confinement. These states are spatially localized in a restricted region and can be progressively annihilated by oxygen adsorption on undercoordinated surface titanium atoms. Formation energy calculations reveal that surface hydration leads to the most stable NC, in agreement with the experimental finding that the truncated bipyramidal morphology is typical of the moderate acidic environment. Oxygen desorption from the bare NC is unfavorable, thus highlighting the stabilizing role of surface oxygen stoichiometry for ${\text{TiO}}_{2}$. Available experimental data on the electronic and structural properties of ${\text{TiO}}_{2}$ NCs are summarized and compared with our results.

A phenomenological model of fluctuation electron microscopy for a nanocrystal/amorphous composite

Fluctuation electron microscopy (FEM) is a quantitative electron microscopy technique in which we use the variance V of spatial fluctuations in nanodiffraction as a function of the diffraction vector magnitude k and real-space resolution R to detect medium-range order in amorphous materials. We have developed a model for V( k, R) from a nanocrystal/amorphous composite, which is an idealized form of the medium-range order in various amorphous materials found by previous FEM measurements. The resulting expression for V( k, R) as a function of the nanocrystal size, nanocrystal volume fraction, and the sample thickness connects the FEM signal to well-defined aspects of the material's structure, emphasizes the need for samples of controlled thickness, and explains in some cases the relative height of peaks in V( k). We give an example of interpreting FEM data in terms of this model using recent experiments on amorphous Al 88Y 7Fe 5.

Composition and Size-Dependent Extinction Coefficient of Colloidal PbSe Quantum Dots

Inductively coupled plasma mass spectrometry (ICP-MS) was combined with UV–vis−NIR spectrophotometry and transmission electron microscopy to determine the nanocrystal composition and molar extinction coefficient ϵ of colloidal PbSe quantum dot (Q-PbSe) suspensions. The ICP-MS results show a nonstoichiometric Pb/Se ratio, with a systematic excess of lead for all samples studied. The observed ratio is consistent with a faceted spherical Q-PbSe model, composed of a quasi stoichiometric Q-PbSe core terminated by a Pb surface shell. At high photon energies, we find that ϵ scales with the nanocrystal volume, irrespective of the Q-PbSe size. From ϵ, we calculated a size-independent absorption coefficient. Its value is in good agreement with the theoretical value for bulk PbSe. At the band gap, ϵ is size-dependent. The resulting absorption coefficient increases quadratically with decreasing Q-PbSe size. Calculations of the oscillator strength of the first optical transition are in good agreement with theoretical ...

Comparison of Brightness of Emitting Semiconductor Nanocrystals with That of Rare-Earth Phosphor

A method was developed to compare the brightness of newly developed thin films with dispersed photoluminescent CdTe semiconductor nanocrystals (NCs) at an ultimately high concentration (ca. 0.01 M) with that of a typical commercial rare-earth phosphor (Y2O2S:Eu). We have found it is reasonable to compare their brightness under conditions of the same sample thickness, excitation intensity, and wavelength. At the weak-excitation limit, the brightness of the new films was about 30 times higher. This higher brightness is due to the higher concentration and emission efficiency of NCs, and the larger absorption cross section per unit volume of NCs. The difference in the brightness increases with the excitation intensity because rare-earth phosphors exhibit a strong saturation phenomenon due to their long emission decay time. Because these films are efficiently excited by UV–blue diodes, the power of which is steadily increasing, they will become more applicable in future.

High Temperature Ferromagnetism and Nano-Scale Phase Separations in Diluted Magnetic Semiconductors and Oxides

In this paper the present understanding of the origin of ferromagnetic response that has been detected in a number of diluted magnetic semiconductors and diluted magnetic oxides at room temperature is outlined. It is argued that in these systems, owing to a typically small solubility of magnetic ions, crystallographic or chemical phase separation into regions with a large and a small concentration of magnetic component takes place. The ferromagnetic signatures then come from the regions with a large concentration of magnetic ions, which show non-zero spontaneous magnetization up to the blocking temperature, whose magnitude is proportional to the nanocrystal volume and magnetic anisotropy. Novel methods enabling a control of nano-assembling of magnetic nanocrystals in non-conducting matrices as well as possible functionalities of these spatially modulated magnetic systems are described. We also discuss phase separations into paramagnetic and ferromagnetic regions, which are driven by the Anderson–Mott localization and/or competing ferromagnetic and antiferromagnetic interactions. Finally, the question whether the high temperature ferromagnetism is possible in materials without magnetic ions is addressed.

Temperature dependence of coherent phonon dephasing inCsPbCl3nanocrystals

We report on the generation of coherent optical phonons in $\mathrm{Cs}\mathrm{Pb}{\mathrm{Cl}}_{3}$ nanocrystals in CsCl by femtosecond laser-pulse illumination. Four phonon modes were identified in the measured oscillations of transient transmission with frequencies matching well those of the lowest confined optical-phonon modes in $\mathrm{Cs}\mathrm{Pb}{\mathrm{Cl}}_{3}$ nanocrystals. The temperature dependence of the phonon mode softening and oscillation damping was used to identify the phonon dephasing mechanisms. It was shown that the main contribution is due to the effect of cubic anharmonicity of the lattice. The possible role of the spatial confinement of phonons in the volume of nanocrystals and the effects of the nanocrystal size distribution in the oscillation damping are also discussed.

Chemical Modification of a Porous Silicon Surface Induced by Nitrogen Dioxide Adsorption

The effect of gaseous and liquid nitrogen dioxide on the composition and electronic properties of porous silicon (PS) is investigated by means of optical spectroscopy and electron paramagnetic resonance. It is detected that the interaction process is weak and strong forms of chemisorption on the PS surface, and the process may be regarded as an actual chemical reaction between PS and NO(2). It is found that NO(2) adsorption consists in forming different surface nitrogen-containing molecular groups and dangling bonds of Si atoms (P(b)-centers) as well as in oxidizing and hydrating the PS surface. Also observed are the formation of ionic complexes of P(b)-centers with NO(2) molecules and the generation of free charge carriers (holes) in the volume of silicon nanocrystals forming PS.

Volume production of Nd2Fe14B + Fe3B nanocomposite powders and their magnetic properties in terms of exchange coupling

The present study reports the magnetic properties of the Nd 4 Fe 77.5 B 18.5 compound in term of exchange coupling between Nd 2 Fe 14 B and Fe 3 B magnetic nanocrystals in melt spun powders made by varying the quenching speed. Those magnetic properties were characterized as functions of nanocrystal size and volume fraction of each magnetic phase which was possible by employing Henkel plots (δM) and refined Mossbauer spectroscopy. The optimized magnetic properties obtained from the present volume production line were B r = 11.73 kG, i H c = 3.082 kOe, and (BH) max = 12.28 MGOe. The volume fraction of each magnetic phase for those conditions giving the grain size of 10 nm were α-Fe: 4.2%, Fe 3 B: 60.1%, and Nd 2 Fe 14 B: 35.7%. The superior magnetic properties in the Nd 2 Fe 14 B + Fe 3 B based nanocomposites were confirmed to be dependant on the volume fraction of Fe 3 B.

Confinement‐induced enhancement of spin interactions in Mn‐doped II–VI semiconductor nanocrystals

AbstractThe effects of sp–d exchange interactions on the optical properties of quantum dots containing a single Mn ion are investigated within the effective mass formalism. The quantum‐mechanical eigenvalue problem of an electron‐hole pair in interaction with the Mn spin is solved analytically. In zinc‐blende semiconductors such as CdTe the fundamental absorption line splits into six components, whose relative intensities strongly depend on the Mn spin orientation with respect to the polarization of light, suggesting the possibility of optical detection of spin. In a magnetic field the splitting between the strong components of the σ− and σ+ spectra rapidly attains a saturation value close to the overall zero‐field splitting, which is inversely proportional to the nanocrystal volume. Typically, it is an order of magnitude larger than the saturation Zeeman splitting in the corresponding bulk diluted magnetic semiconductor. The numerical results for ZnSe:Mn nanocrystals show a good agreement with the published experimental data. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

The thermal, magnetic, and structural characterization of the crystallization kinetics of Fe/sub 88/Zr/sub 7/B/sub 4/Cu/sub 1/, an amorphous soft magnetic ribbon

The characterization of the crystallization kinetics of an amorphous soft magnetic alloy, Fe/sub 88/Zr/sub 7/B/sub 4/Cu/sub 1/, called NANOPERM, is presented. Vibrating sample magnetometry (VSM), X-ray diffractometry, and differential scanning calorimetry have been used to observe crystallization kinetics. The VSM observations take advantage of the Curie temperature of the amorphous phase Tc-/sub amorphous/ of the NANOPERM alloy being below its primary crystallization temperature T/sub x1/. This allows for the volume of nanocrystals transformed in the crystallization process to be inferred magnetically, as well as thermally and structurally. The Johnson-Mehl-Avrami model for isothermal crystallization kinetics is compared with the Kissinger model for nonisothermal crystallization kinetics using data gathered from the three characterization methods. Linear regression and nonlinear regression analysis of crystallization data for NANOPERM ribbon and the significance of the values that describe them, namely, the activation energy Q and the morphology index n are investigated for isothermal and constant-heating crystallization. The activation energy for NANOPERM ribbon is presented here to be in the range of Q=2.8-3.4 eV, with the crystallization kinetics proceeding by three-dimensional diffusion and immediate nucleation, where the morphology index is n=1.5. A time constant to account for initial nonisothermal conditions during isothermal heating is introduced and determined to be T=120-200 s.

Surface characteristics and dissolution behavior of plasma-sprayed hydroxyapatite coating.

One of the most important concerns with the clinical use of plasma-sprayed hydroxyapatite (HA) coatings is the resorption of the coating, and dissolution at neutral pH is one of the two major resorption mechanisms. In this study, highly crystalline pure HA powders were atmospherically plasma sprayed using various parameters. Dissolution of both HA powders and coatings was measured using a calcium ion meter. Surface characteristics, including phase, morphology, and roughness, were compared for the coatings before and after dissolution. Pulverized HA coatings exhibited significantly higher dissolution compared with the same quantity of feedstock HA powders because of the decreased crystallinity and fine crystal size of the coating. Furthermore, the dissolution decreased with the crystallinity of the coating. Dissolution of HA coatings did not show much difference with respect to the coatings in the initial stage of immersion (4 h). However, dissolution of all coatings reached saturation in a fresh physiological solution. The saturation values were much lower compared with their counterparts in the form of powders, which may imply the stability of HA coatings in long-term use. In addition to crystallinity, the particle melting status in the coatings, i.e., the volume of nanocrystals, and porosity, was found to be another important factor for the dissolution of the HA coating. X-ray diffraction patterns of HA coatings indicated the complete dissolution of impurity phases and amorphous phase after the coatings were immersed in the solution for 4 days. Coatings sprayed at lower power (27.5 kW) exhibited a pattern of crystalline HA whereas coatings sprayed at higher power (42 kW) exhibited a pattern of bone apatite. Surface morphologies showed preferential dissolution of amorphous phase in all coatings accompanied with precipitation of bone apatite observable for coatings sprayed at higher power. Surface roughness measured after the dissolution studies increased for the two coatings sprayed at lower power level but decreased for coatings sprayed at higher power level. This decrease is attributed to the better match in solubility characteristics between the fine crystals and the amorphous calcium phosphate within the coating.