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Magnetic nanoparticles have found application in medical diagnostics such as magnetic resonance imaging and therapies such as cancer treatment. In these applications, it is imperative to have a biocompatible solvent such as water at optimum pH for possible bio-ingestion. In the present work, a synthetic methodology has been developed to get a well-dispersed and homogeneous aqueous suspension of Fe3O4 nanoparticles in the size range of 8–10 nm. The surface functionalization of the particles is provided by citric acid. The particles have been characterized using transmission electron microscopy, magnetization measurements with a superconducting quantum interference device, FTIR spectroscopy (for surfactant binding sites), thermogravimetric studies (for strength of surfactant binding), and x-ray photoelectron spectroscopy and x-ray diffraction (for composition and phase information). The carboxylate functionality on the surface provides an avenue for further surface modification with fluorescent dyes, hormone linkers etc for possible cell-binding, bioimaging, tracking, and targeting.
Information about quantum dot (QD) asymmetry is derived by analyzing the polarization properties of the time-integrated four wave mixing (FWM) signal. The lowering of QD symmetry results in the splitting of bright J= +/-1 exciton states. This causes the polarization oscillation of circularly excited excitons between these two split states. In a QD ensemble with a random distribution in the exciton level splitting, this results in the decay of the difference in FWM signals observed in scattering of σ+ and σ+ polarized light on the population grating created by two σ+ pulses, the decay time reflecting the degree of QD asymmetry. We have investigated the decay time of the difference in two polarized signals for quantum dots of equal size, grown in a glass matrix under different conditions. Increasing growth temperature and decreasing growth time lead to lowering of QD symmetry. We discuss this experimental result in terms of kinetics of nanoparticle growth in glass.
Engineering of nanoparticles to detect ultraviolet light within a specified range and its feasibility to make a device has been demonstrated. It is shown that the absorption edge of a material can be shifted to significantly lower wavelengths in the UV range by using nanoparticles and that this feature can be incorporated within a device. All experimental work was focused on ZnO. Both commercially obtained ZnO nanoparticles as well as in-house synthesized ZnO nanoparticles were examined. For the in-house developed particles it was shown that varying the diameter of the ZnO nanoparticles could vary the absorption wavelength from 315 to 365 nm. Commercially available nanoparticles did not show this shift due to their relatively larger sizes (diameter ≅ 20 nm) as well as their broad size distributions.
A photocurrent effect of UV light on thin films prepared with nanoparticles has been demonstrated. Not only the optical band-gap value depends on the size of the nanoparticles but also the mobility gap of the material and, as a consequence, the onset of photocurrent.
The growth of 2D quantum dot quantum well (QDQW) nanocrystals in which a shell of CdSe is grown onto cores of ZnS and capped with a further shell of ZnS is reported. The red shift in the interband absorption and photoluminescence spectrum of the quantum dots (QDs) indicates relocalization of carriers from confinement in the ZnS core to the CdSe shell. The change in interband absorption energy utilizing the effective mass approximation with spherical symmetry was modeled, enabling an estimate of the CdSe thicknesses grown. 1.8nm and 2.5nm ZnS cores were selected as the base on which to grow the CdSe shells. Despite the 12% lattice mismatch between ZnS and CdSe, our results indicate that we have successfully grown CdSe shells approximately three monolayers thick onto 2.5nm ZnS core. Anything beyond a single monolayer of CdSe could not be grown onto the 1.8nm core, although some success was observed by incorporating a CdS graded layer in-between the ZnS core and CdSe shell. The effect of ZnS shell thickness on photoluminescence efficiency has also been studied with optimum shell thicknesses showing quantum yields as high as 52%. Growth of these nanocrystals represents a significant step in the development of strained nanocrystalline heterostructures.
Silicon/silicide/oxide nanochains, which have an alternate arrangement of silicon/copper silicide composite nanoparticles and oxide nanowires, were fabricated using silicon/oxide nanochains as templates. Photoluminescence was observed at room temperature and considered to be due to recombination of excitons in oxide. Electrostatic potential in the material was also investigated by electron holography. No visible potential bending at the silicon/silicide interface was detected.
Two simple techniques to prepare tungsten bronze nanowires are reported in this paper. Tungsten bronze nanowires with a diameter of less than 100nm and a length of more than 10 μm have been successfully prepared by employing these techniques. A Vapor-Liquid-Solid (VLS) mechanism has been proposed to explain the growth of these tungsten bronze nanowires.
Compound semiconductor nanocrystals (quantum dots) exhibit unique size-dependent optoelectronic properties making them attractive for a variety of applications, including ultrasensitive biological detection, high-density information storage, solar energy conversion, and photocatalysis. There is presently a great need for developing scalable techniques that allow efficient synthesis, size control, and functionalization of quantum dots, without a loss of the desirable optical properties. We report experimental results on the properties and surface modification of ZnSe nanoparticles grown by a continuous vapor-phase technique utilizing an axisymmetric counterflow jet reactor. Luminescent ZnSe nanocrystals were obtained at room temperature by reacting vapors of dimethylzinc:triethylamine adduct with hydrogen selenide, diluted in a hydrogen carrier gas. The two reactants were supplied from opposite inlets of the counterflow jet configuration and initiated particle nucleation in a region near the stagnation point of the laminar flow field. Surface modification of nanoparticles by adsorption of 1-pentanethiol was used to control the rate of particle coalescence. The counterflow jet technique can be scaled up for commercial production and is compatible with other vapor-phase processing techniques used in the microelectronics industry.
This study presents the effect of gate geometry on the charging characteristics of metal nanocrystal memories. The effect is studied by varying the perimeter to area ratio, number of convex corners and concave corners of the gate of a metal-oxide-semiconductor (MOS) capacitor with embedded gold nanocrystals. It can be observed that the nanocrystal charging rate increases for a smaller perimeter to area ratio. The presence of concave and convex corners increases the nanocrystal charging rate. Based on this study it is expected that gate geometries with low perimeter to area ratio and with selected convex and concave corners would increase the nanocrystal charging rate.
CdSexTe1-x nanoparticles (with different stoichiometry ratio x) dispersed in silicon dioxide films have been grown by magnetron sputtering technique followed by thermal annealing. Effect of thermal annealing conditions on the structural, compositional, optical and electronic properties of nanoparticles has been studied using GAXRD, XPS, TEM, and spectroscopic ellipsometry techniques. A structural transformation in the nanoparticle core mediated purely by surface layer effects in the case of CdTe and a spontaneous self-organization of nanoparticles into nanorods in the case of CdSe via fractal growth has been observed. Preliminary observations from the ellipsometry measurements carried out on some of these nanoparticle films shows a blue shift of absorption edge.
The localized surface plasmon resonance (LSPR) of noble metal nanoparticles has recently been the subject of extensive studies. Previously, it has been demonstrated that Ag nanotriangles that have been synthesized using nanosphere lithography (NSL) behave as extremely sensitive and selective chemical and biological sensors. The present work reveals information regarding the long range distance dependence of the localized surface plasmon resonance (LSPR) of silver and gold nanoparticles. Multilayer adsorbates based on the interaction of HOOC(CH2)10SH and Cu2+ were assembled onto surface-confined nanoparticles. Measurement of the LSPR extinction peak shift versus number of layers and adsorbate thickness is non-linear and has a sensing range that is dependent on the composition, shape, in-plane width, and out-of-plane height of the nanoparticles. Theoretical modeling confirms and offers a mathematical interpretation of these results. These experiments indicate that the LSPR sensing capabilities of noble metal nanoparticles can be tuned to match the size of biological and chemical analytes by adjusting the aforementioned properties. The optimization of the LSPR nanosensor for a specific analyte will improve an already sensitive nanoparticle-based sensor.
Research on nanocrystalline materials and the physics behind their properties have attracted considerable attention. A number of physical and chemical techniques have been used to synthesize different nanomaterials and nanocomposites. Optical absorption characteristics of composites containing nanosized metals or semiconductors have been investigated for potential applications in nonlinear optics and photonic crystals and also to understand the effect of particle size on the band gap of the material concerned. These materials show a large third-order nonlinear susceptibility. A polymer-matrix nanocomposite containing copper particles has been prepared by in situ chemical reduction within a polymer-metal complex solid film. The copper particle size in the order of 10 nm is controlled by the initial content of the metal ions in the complex. Their fractal pattern and the value of the fractal dimension indicate that there exists a cluster-cluster aggregation (CCA) process in the present system. Optical absorption spectra of copper-polymer nanocomposites show distinct plasma absorption bands and quantum size effect in the samples. More studies on optical properties of composites containing nanosized metals are within the Drudeframe on the basis of Mie theory, but the electrons behave in a wavelike rather than a particlelike way as the particle size decreases to below 10 nm, and the classical Drude model should be modified considering the quantum confinement effect. In this paper, the calculated blueshift of the resonance peak based on a quantum-sphere model (QSM) proposed by Huang and Lue, gives remarkable agreement with the experimental data as the size of copper particles embedded in the polymer becomes smaller.
The non-uniform size-dependent contrast of AgBr0.95I0.05 nanocrystals (NCs) ranging from 22 to 80 nm in equivalent diameter (dc) observed by cryo-energy-filtering TEM is referred to predominant excitations at the surfaces and near the edges. When the fields due to surface losses reach throughout the structure, they couple and the probability for their generation becomes periodic in the NC size. Since electronic sum rules must be satisfied, the surface excitations reduce the strength of the bulk excitations. Coupling of surface and volume losses may cause oscillations of the image intensities with the NC size. The appearance of such oscillations demonstrates a size confinement of excitations of valence electrons due to contributions to the energy-level structure from carrier confinement and surface states. The imaginary part of relative dielectric permittivity derived from electron energy-loss spectra shows an enhanced intensity of the band at 4 eV for NCs with dc = 50±4 nm as compared to those of 109±7 nm in size, while the bands at 7 eV and at 10 eV appear to be suppressed. An increase of the intensity of exciton-assisted direct interband transition at 4 eV (Γ8-, Γ6- → Γ6) correlates with the size-dependent enhancement of free exciton luminescence from AgBr NCs, when their size is less than 100 nm.
Single crystal Ge nanowires (NWs) were obtained in high yield by gas phase decomposition of germanium di-cyclopentadienylide ([Ge(C5H5)2]), at 325 °C on iron substrates. Highresolution electron microscopy (SEM/TEM) showed Ge NWs to be uniform in terms of diameter (20 nm) and length (> 25 μm). The wire growth is selective and appears to be governed by a Ge-Fe alloy epilayer formed by the reaction between Ge clusters and iron substrate, during the initial stages of the CVD process. The supersaturation of Ge-Fe solid-solution with respect to Ge content induces the spontaneous formation of single crystal germanium nuclei that act as templates for the nanowire growth. X-ray and electron diffraction revealed the NWs to be single crystals of cubic germanium with a preferred growth direction[11–2]. The proposed base-growth model on Fe substrate is supported by TEM, EDX and XPS studies.
Here we report the first study towards the integration of fullerenes and carbon nanotubes (CNT) in the gate stack of CMOS technology, which is a promising hybrid approach of top-down and bottom-up fabrication process. Prospective processes for C60 and CNT deposition over an aggressively scaled 2 nm gate oxide in the MOS capacitor structure have been monitored. CV measurements show minimal silicon contamination and interface states. Step charging at a specific voltage that corresponds to a fixed number density of C60 is used to establish the structural integrity and size-mono-dispersion of C60. The CV method can be further used to probe the charge injection into C60 and its anions to establish fundamental understanding of their molecular orbital (MO) structure.
Indium phosphide (InP) nanowires and nanotubes have been synthesized via the vapor-liquid-solid (VLS) growth mechanism. The wires as well as the tubes are crystalline and have the (bulk) zinc blende structure. Compared to the nanowires the nantubes are formed at higher temperatures. A simple model for the formation of the nanotubes is presented. The diameter of the wires and the wall thickness of the tubes can be controlled by the synthesis temperature. Photoluminescence measurements on individual wires show a strong polarization dependency. Moreover, the nanostructures exhibit a considerable blue shift with respect to bulk emission as a result of size-quantization. In addition, this blue shift indicates that the optical properties are not dominated by defect states.
Surface effects significantly influence the functionality of semiconductor nanocrystals. A theoretical understanding of these surface effects requires models capable of describing surface details at an atomic scale, passivation with molecular ligands, and few-monolayer capping shells. We present an atomistic tight-binding theory of the electronic structure and optical properties of passivated, unpassivated and core/shell nanocrystals to study these surface effects.
CuInS2 nanowires synthesized by a chemical treatment method were examined using X-ray diffraction and field emission-scanning electron microscopy (FE-SEM). The nanowires, which were identified to have a chalcopyrite structure of CuInS2, were 30–100 nm in diameter and several micrometers in length. A remarkable correlation between the color of a sample with nanowires and their FE-SEM image was disclosed. Photoluminescence spectra of the obtained nanowires were also studied. At low temperatures (∼ 10K) a broad peak centered at photon energy of 2.05 eV was observed. This energy is by 0.5 eV larger than the energy gap of the well-studied crystalline bulk samples of CuInS2. The observed rise in energy can be ascribed to quantum size effects expectedly developing in CuInS2 samples with nanosize dimensions.
During the past few decades, the density of magnetic storage has been improved considerably. To increase the storage capacity, it is necessary to reduce the size of magnetic grains. However, as the domain size decreases, their thermal stability will also decrease, which results in the loss of magnetization. To overcome the limit imposed by such superparamagnetic behavior, lots of recent research attentions have been focused on the patterned magnetic media. To maximize the storage density, it is preferable to create periodical magnetic patterns, in which single-domain magnetic dots are well separated from each other. In this experiment, we have utilized nanosphere lithography to create large-area well-ordered two dimension arrays of permalloy (Ni80Fe20) nanoparticles. Nanosphere lithography is an inexpensive, simple, parallel, and high throughput fabrication technique. We have employed monodisperse polystyrene beads with diameter of 650, 560, 440, 350, 280 nm to fabricate triangle-shaped permalloy (Ni80Fe20) nano-arrays with lateral dimension in the region of 170∼90 nm, and thickness in the region of 10∼50 nm. The magnetic behavior of these triangle-shaped nanomagnet arrays have been investigated by longitudinal magnetic optic Kerr effect (LMOKE) and magnetic force microscopy (MFM). It was found that the coercivity of the permalloy nanoparticle arrays increases with decreasing the thickness of the nanoparticle. This can be attributed to the interface effect between the arrays and the substrate.
A tight-binding hamiltonian is used to study the electronic properties of covalently-bonded, crossed (5,5) metallic nanotubes with increasing degree of disorder in the junction region. At one extreme, ideal junctions between coplanar nanotubes with a minimal number of topological defects show a good ohmic behavior. Upon increasing disorder, ohmic conduction is suppressed in favor of hopping conductivity. At the opposite extreme, strongly disordered junctions as could be obtained after electron-beam irradiation of overlayed nanotubes, display weak localization and energy quantization, indicating the formation of a quantum dot contacted to metallic nanowires by tunnel barriers.
The application of molecularly-capped gold nanoparticles (1–5 nm) in catalysis (e.g., electrocatalytic oxidation of CO and methanol) requires a thorough understanding of the surface composition and structural properties. Gold nanoparticles consisting of metallic or alloy cores and organic encapsulating shells serve as an intriguing model system. One of the challenges for the catalytic application is the ability to manipulate the core and the shell properties in controllable ways. There is a need to understand the relative core-shell composition and the ability to remove the shell component under thermal treatment conditions. In this paper, we report results of a thermogravimetric analysis of the alkanethiolate monolayer-capped gold nanoparticles. This investigation is aimed at enhancing our understanding of the relative core-shell composition and thermal profiles.