Добірка наукової літератури з теми "Single-component Nanoparticles"

By one-step mixed-solvent mediated approach, we have prepared fluorescent organic core–shell nanoparticles with an oligomer (1) derived from the Schiff base condensation reaction of 2,6-diformyl-4-methylphenol and o-phenylenediamine at room temperature. The core and shell structures are generated by the same oligomer (1) featuring the aggregation structure in core different from that in shell. The radial packing factor distribution of oligomer cluster depending on the solvent interaction in the time of nucleation is mainly responsible for the single component core–shell formation. Different morphologies of the core–shell nanospheres (CSNS) and core–shell nanohemispheres (CSNHS) were generated simply by changing the concentration of 1 in chloroform-methanol mixed solvent (1:2). We observed that fluorescent emission from those core–shell nanoparticles is intense whereas as-synthesized oligomer (1) itself is non-fluorescent in dilute solution. The enhanced emission in the core–shell form with more than 50 times increase in fluorescent quantum yield vis-à-vis 1 is a remarkable feature of the study. As UV absorption spectra of nanoparticles are blue-shifted relative to their properties in solution, the observed strong emission in the solid state makes the oligomer an outstanding exception to a well-established rule based on the molecular exciton model. The core–shell nanoparticles have been characterized by FE-SEM, TEM, XRD, nanosecond (ns) time-resolved fluorescence dynamics, UV-Vis and fluorescence spectroscopy. The longer fluorescence lifetimes (τ) of core–shell nanoparticles (3.50 ns and 3.52 ns for CSNS and CSNHS respectively) than 1 as-synthesized (1.28 ns) implies that the formation of the nanoparticles restricts the rotation and vibration of the groups in the molecules. The factor that induces fluorescent enhancement of nanoparticles is mainly ascribed to the increase of radiative rate constant (kr) and simultaneous decrease of nonradiative rate constant (knr).

The influence of various types of nanoparticle fillers with the same diameter of 20 nm were separately incorporated into a single component impregnating resin based on a polyesterimide (PEI) matrix and its subsequent changes in complex relative permittivity were studied. In this paper, nanoparticles of Al2O3 and ZnO were dispersed into PEI (with 0.5 and 1 wt.%) to prepare nanocomposite polymer. Dielectric frequency spectroscopy was used to measure the dependence of the real and imaginary parts of complex relative permittivity within the frequency range of 1 mHz to 1 MHz at a temperature range from +20 °C to +120 °C. The presence of weight concentration of nanoparticles in the PEI resin has an impact on the segmental dynamics of the polymer chain and changed the charge distribution in the given system. The changes detected in the 1H NMR spectra confirm that dispersed nanoparticles in PEI lead to the formation of loose structures, which results in higher polymer chain mobility. A shift of the local relaxation peaks, corresponding to the α-relaxation process, and higher mobility of the polymer chains in the spectra of imaginary permittivity of the investigated nanocomposites was observed.

This work reports on photothermal lens spectra of silver nanoparticles of different dimensions in the spectral region of 370–730 nm performed using an arc-lamp-based photothermal spectrophotometer. We show that the photothermal and extinction cross-section spectra of the samples are similar for nanoparticles of reduced dimensions where scattering effects are small. The results differ substantially for nanoparticles of a diameter larger than 30 nm for which scattering becomes relevant. We demonstrate that the photothermal spectrum corresponds to the absorption component of the particle’s extinction. Photothermal spectra show a clear picture of the plasmonic peaks of the nanoparticle even in the presence of high scattering. By subtracting the photothermal component from the total extinction, we extract the scattering cross-section spectra of the nanoparticles. The technique allows determination of the absorption and scattering components of the extinction providing a better understanding of the particle’s optical properties. The results agree well with the Mie approximation, which is valid for a single spherical nanoparticle. We discuss and demonstrate the application of the method to characterize particles of arbitrary shape and dimensions.

Numerous research fields, including chemistry, electronics, and medical sciences, have concentrated on the production and use of novel functional nanomaterials. Carbon, a component of all organic life forms, is essential for the creation of nanomaterials. The modern carbon-based family component known as carbonaceous quantum dots (CQD) was unintentionally discovered in 2004 while single-walled carbon nanotubes were being purified. Additionally, CQDs have exceptional qualities like outstanding photoluminescence and minimal toxic effects. Outstanding in vitro andin vivo biomedical implications of CQDs include drug/gene delivery, biosensor biotherapy, and theragnostic evolution. Also, CQDs can pass through specific body sites of endothelial inflammation (epithelium of the intestinal tract, liver, for example), tumors or penetrate capillaries due to their small size. For the same reason, nanoparticles are more suitable for intravenous administration than microparticles and also prevent particle aggregation and bypass emboli or thrombi formation. This chapter describes the most contemporary applications of CQDs in diverse biomedical fields. We hope it will provide incalculable insights to inspire discoveries on CQD and delineate a road map toward a broader range of bio applications.

A base fluid (e.g., water, ethanol, oil) in which nano-sized (1–100 nm) particles of a different material are dispersed, is known as a nanofluid. Nanofluids are attractive because the presence of the nanoparticles enhances energy transport considerably. As a result, nanofluids have higher thermal conductivity and single-phase heat transfer coefficients than their base fluids. In particular, the heat transfer coefficient increases appear to go beyond the mere thermal-conductivity effect, and cannot be predicted by traditional pure-fluid correlations such as Dittus-Boelter’s. In the nanofluid literature this behavior is generally attributed to thermal dispersion and intensified turbulence, brought about by nanoparticle motion. To test the validity of this assumption, we have considered seven slip mechanisms that can produce a relative velocity between the nanoparticles and the base fluid. These are inertia, Brownian diffusion, thermophoresis, diffusiophoresis, Magnus effect, fluid drainage and gravity. We concluded that, of these seven, only Brownian diffusion and thermophoresis are important slip mechanisms in nanofluids. Based on this finding, we developed a two-component four-equation non-homogeneous equilibrium model for mass, momentum and heat transport in nanofluids. A non-dimensional analysis of the equations suggests that energy transfer by nanoparticle dispersion is negligible, and thus cannot explain the abnormal heat transfer coefficient increases. Furthermore, a comparison of the nanoparticle and turbulent eddy scales clearly indicates that the nanoparticles move homogeneously with the fluid in the presence of turbulent eddies, so an effect on turbulence intensity is also doubtful.

In recent years, heat dissipation in micro-electronic systems has become a significant design limitation for many component manufactures. As electronic devices become smaller, the amount of heat generation per unit area increases significantly. Current heat dissipation systems have implemented forced convection with both air and fluid media. However, nanofluids may present an advantageous and ideal cooling solution. In the present study, a model has been developed to estimate the enhancement of the heat transfer when nanoparticles are added to a base fluid, in a single microchannel. The model assumes a homogeneous nanofluid mixture, with thermo-physical properties based on previous experimental and simulation based data. The effect of nanofluid concentration on the dynamics of the bubble has been simulated. The results show the change in bubble contact angles due to deposition of the nanoparticles has more effect on the wall heat transfer compared to the effect of thermo-physical properties change by using nanofluid.

Methacrylate and acrylate terminated monomers can be rapidly polymerized to polymer glasses useful in biomaterials, photolithography and rapid prototyping, optical coatings and composites. Unfortunately, polymerization shrinkage results in loss of tolerance and the development of internal stresses which can be especially critical in the case of highly crosslinked glasses. Structurally complicated oligomeric mixes of dimethacrylate monomers that exhibit a nematic liquid crystal to isotropic transition above room temperature have been synthesized in a low cost one pot synthesis to surmount the problem of polymerization shrinkage and the propensity of single component monomers to crystallize from the liquid state. Photopolymerization from the ordered liquid crystal state into a less ordered glass minimizes volumetric shrinkages to between 1–2% at greater than 90% polymerization conversion. These polymer glasses exhibited elastic bending moduli of 1.2 GPa to 1.5GPa, fracture strengths of 70–100MPa and fracture toughness of K=0.3–0.4 (MPa)1/2. In some cases the glasses exhibited ductile behavior which is unusual for highly crosslinked materials. Room temperature viscosities of 100P–2000P permit facile processing of the liquid crystal monomers with inorganic particles to make dental restorative composites.

Development of dual diagnostic and therapeutic agents for cancer with heightened sensitivity, selectivity, and lower toxicity could greatly enhance the prognosis of patients suffering from this disease. Recent work by Dr. Harry Dorn of Virginia Tech has resulted in the creation of a novel nanostructure called a carbon nanotube peapod. This structure consists of multiple TNT EMF’s (Trimetallic nitride template endohedral metallofullerenes) contained within a single-walled carbon nanotube (CNT) in the form of a peapod. Using TNT EMFs containing gadolinium, this nanostructure can achieve a 40-fold improvement in magnetic resonance imaging (MRI) contrast enhancement [1]. The CNT component of the peapod can be utilized as a hyperthermia enhancer [2] and an effective platform for drug delivery [3]. This study focuses on the use of carbon nanotube peapods (CNT peapods) to absorb near infrared light and generate therapeutic heat for tumor destruction. With sufficient heating and nanoparticle targeted delivery to tumor cells, CNT peapods can potentially induce selective hyperthermia-mediated toxicity in tumors.

Nanoscale heat conduction plays a critical role in applications ranging from thermal management of nanodevices to nanostructured thermoelectric materials for solid state refrigeration and power generation. This lecture presents recent investigations in our group. The first part of the lecture demonstrates heat conduction across nanoscale interfaces formed between individual nanoscale heaters and the silicon substrate [1]. A systematic experimental study was performed of thermal transport from individual nanoscale heaters with widths ranging between 77nm-250nm to bulk silicon substrates in the temperature range of 80–300K. The effective substrate thermal conductivity was measured by joule heating thermometry. We report up to two orders of magnitude reductions in the measured effective thermal conductivity of the silicon substrate when the heater widths are smaller than the mean free path of the heat carriers in the substrate, as summarized in Fig. 1. The effective mean free path of the silicon substrate was extracted from the measurements and was found to be comparable with recent molecular dynamics simulations. A proof of concept demonstration of a novel Thermal Interface Material (TIM) is presented next. The high thermal conductivity TIM is based on a highly connected high thermal conductivity nanostructured filler network embedded in a polymer matrix where the contribution of filler-matrix interfaces to thermal resistance is minimized. It was found [2] that the thermal conductivity could be varied from ∼0.2 to 20 W/mK when the volume fraction of metallic nanoparticles was varied from 0–20%. For similar volume fractions and filler composition, microparticle based composites have two orders of magnitude lower thermal conductivities. SEM characterization and thermal transport modeling are employed to support the conclusion that morphological changes in the nano-TIM are responsible for the thermal conductivity reduction. Thermoelectric transport investigations are discussed for a novel class of highly scalable nanostructured bulk chalcogenides developed at Rensselaer Polytechnic Institute [3]. Un-optimized, single-component bulk assemblies of Bi2Te3 and Sb2Te3 single crystal nanoplates show large enhancements (25–60%) in the room temperature thermoelectric figure of merit compared with individual bulk counterparts (Table 1). Nanostructuring was found to lead to strong thermal conductivity reduction without significantly affecting the mobility of the charge carriers, as shown in Table 2. A scanning thermal microprobe technique developed for simultaneous thermal conductivity (κ) and Seebeck coefficient (α) measurements in thermoelectric films is also presented [4]. In this technique, an AC alternative current joule-heated V-shaped microwire that serves as heater, thermometer and voltage electrode, locally heats the thin film when contacted with the surface (Fig. 2). The κ is extracted from the average DC temperature rise thermal resistance of the microprobe and α from the DC Seebeck voltage measured between the probe and unheated regions of the film by modeling the heat transfer in the probe, sample and their contact area, and by calibrations with standard reference samples. Application of the technique on sulfur-doped porous Bi2Te3 and Bi2Se3 films reveals α = −105.4 and 1.96 μV/K, respectively, which are within 2% of the values obtained by independent measurements carried out using microfabricated test structures. The respective κ values are 0.36 and 0.52 W/mK, which are significantly lower than the bulk values due to film porosity, and are consistent with effective media theory. The dominance of air conduction at the probe-sample contact area determines the microscale spatial resolution of the technique and allows probing samples with rough surfaces. Non-contact mode measurement of thermal conductivity was also demonstrated and confirmed by independent characterization [5]. In non-contact mode the technique utilizes ballistic air conduction as the dominant heat transfer mechanism between the thermal probe and the sample and thus eliminates uncertainties due to solid contact and liquid meniscus conduction.

Aligning carbon nanotubes in any way desired is very important for many fundamental and applied research projects. In this talk, I will first discuss how to grow them with controlled diameter, length, spacing, and periodicity using catalyst prepared by magnetron sputtering, electron beam (e-beam) lithography, electrochemical deposition, and nanosphere self-assembly. Then I will present our results of field emission property of both the aligned carbon nanotubes grown on flat substrates and random carbon nanotubes grown on carbon cloth. For the aligned carbon nanotubes arrays, I will present the preliminary results of using them as photonic band gap crystals and nanoantennae. As an alternative material of carbon nanotubes, ZnO nanowires have been grown in both aligned fashion on flat substrates and random fashion on carbon cloth. Using these ZnO nanowires, good field emission properties were observed. Furthermore, I will present our recent studies on the electrical breakdown and transport properties of a single suspended nanotube grown on carbon cloth by a scanning electron microscope probe incorporated into a high resolution transmission electron microscope. As part of the potential applications, I will also discuss our recent success on molecules delivery into cells using carbon nanotubes. Finally I will talk about our most recent endeavor on achieving thermoelectric figure-of-merit (ZT) higher than 2 using our unique nanocomposite approach. Plasma-enhanced chemical vapor deposition (PECVD) was discovered by my group in 1998 to be able to grow aligned carbon nanotubes [1]. Catalyst film was first deposited by magnetron sputtering. According to the thickness of the catalytic film, aligned carbon nanotubes were grown with different diameters and spacing, and different length depending the growth time. However, the two major drawbacks are 1) that the location of where the nantoube grows can not be controlled, 2) that the spacing between the nanotubes can not be varied too much. Therefore, we immediately explored to grow aligned carbon nanotubes with the location and spacing controls using e-beam lithography [2]. Unfortunately the cost is so high that the e-beam is not suited for large scale commercialization that requires only an average site density control not the exactly location, for example, electron source. It is the cost issue that made us to develop electrochemical deposition to make catalyst dots that can be separated more than 10 micormeters between dots [3]. With such arrays, we tested many samples for field emission properties and found the optimal site density [4]. However, for applications that require the location controls, for example, photonic band gap crystals, electrochemical deposition can not be satisfactory. It is this kind of application that led us to develop the nanosphere self-assembly technique in large scale [5]. For field emission, we found that ZnO nanowires are good field emitters comparable to carbon nanotubes if they are grown with the right diameter and spacing. Here I will discuss the field emission properties of ZnO nanowires as an alternative material to carbon nanotubes [6]. Us a special kind of carbon nanotubes made by PECVD, we discovered a highly efficient molecular delivery technique, named nanotube spearing, based on the penetration of Ni-particle embedded nanotubes into cell membranes by magnetic field driving. DNA plasmids encoding the enhanced green fluorescent protein (EGFP) sequence were immobilized onto the nanotubes, and subsequently speared into targeted cells. We have achieved the unprecedented high transduction efficiency in Bal17 B-lymphoma, ex vivo B cells, and primary neurons with high viability. This technique may provide a powerful tool for high efficient gene transfer in a variety of cells, especially, the hard-to-transfect cells [7]. Conventional transport studies of multiwall carbon nanotubes (MWNTs) with only the outmost wall contacted to the electrodes via side-contact shows that a MWNT is a ballistic conductor with only the outmost wall carrying current. Here we show, by using end-contact in which every wall is contacted to the electrodes, that every wall is conducting, as evidenced by a significant amount of current drop when an innermost wall is broken at high-bias. Remarkably, the breakdown of each wall was initiated in the middle of the nanotube, not at the contacts, indicating diffusive electron transport. Using end-contact, we were able to probe the conductivity wall-by-wall and found that each wall is indeed either metallic, or semiconducting, or pseudogap-like. These findings not only reveal the intrinsic physical properties of MWNTs but also provide important guidance to MWNT-based electronic devices [8]. At the end of the talk, if time permits, I will talk about our ongoing effort on improving the figure-of-merit (ZT) of thermoelectric materials using a nanocomposite strategy to mimic the structure of the superlattice of PbTe/PbSe and Bi2Te3/Sb2Te3 hoping to reduce the thermal conductivity by a factor of 2–4 while maintaining the electrical conductivity. To make a close to 100% dense nanocomposite, we started with nanoparticles synthesis, then consolidation using both the traditional hot press and the direct current fast-heat, named plasma pressure compact, to preserve the nano size of the component particles. So far, we have seen thermal conductivity decrease by a factor of 2 in the systems of Si/Ge, PbeTe/PbSe, Bi2Te3/Sb2Te3, indicating the potential of improving ZT by a factor of 2.