SCANNING NEAR-FIELD OPTICAL MICROSCOPY FOR MEASURING MATERIALS PROPERTIES AT THE NANOSCALE

Apertureless scanning near-field optical microscopy is a valuable tool for characterization of chemical and spectroscopic properties of the materials at the nanoscale. Description of apertureless near-field microscope is provided along with the description of a homodyne detection of the near-field signal which allows enhancing of a weak scattered radiation. Experimental evidences that homodyne detection markedly improves the signal-to-noise ratio of the detected signal are presented. A model for the dependence of the near field signal, as a function of the normal distance of the tip from the surface, is discussed. Application of a model in which the tip is represented by two spherical scatterers, one large and one small, indicates the electromagnetic field enhancement is 90 fold greater at the sharp apex of metallic probe tip.Apertureless near-field scanning infrared microscopy was employed to study samples patterned with regions of DNA and hexadecanethiol. Chemical contrast imaging was achieved by examining IR absorption in the spectral region of the phosphate stretching band of DNA molecules and harmonic demodulation of the signal scattered by the oscillating probe. IR absorption maps revealed that the IR signal was not coupled to the vertical tip motion, indicating artifact-free imaging. Monolayer-sensitive chemical imaging with a lateral spatial resolution of approximately 200 nm is demonstrated.The field enhancement in very small aperture lasers was studied using apertureless near-field microscopy. The near-field optical pattern around the aperture indicates the interference of surface plasmons with incident light. A surface plasmon point-source model has been used to determine the wavelength and the decay length of surface plasmons at the Al/silicon nitride interface. Near-field measurements also confirmed a preferred orientation of the rectangular aperture waveguide for the signal enhancement in very small aperture lasers.Optical field confinement in a ridge waveguide nanostructure designed for ultrahigh-density recording was observed using an apertureless near-field scanning optical microscope. The aperture was fabricated on a commercial edge-emitting semiconductor laser as the light source. The emission patterns are in agreement with theoretical simulation of such structures. A 90 nm x 70 nm full-width-half-maximum spot size was measured and is comparable to the ridge width of the aperture

Sensor Selection to Improve Estimates of Particulate Matter Concentration from a Low-Cost Network

Deployment of low-cost sensors in the field is increasingly popular. However, each sensor requires on-site calibration to increase the accuracy of the measurements. We established a laboratory method, the Average Slope Method, to select sensors with similar response so that a single, on-site calibration for one sensor can be used for all other sensors. The laboratory method was performed with aerosolized salt. Based on linear regression, we calculated slopes for 100 particulate matter (PM) sensors, and 50% of the PM sensors fell within ±14% of the average slope. We then compared our Average Slope Method with an Individual Slope Method and concluded that our first method balanced convenience and precision for our application. Laboratory selection was tested in the field, where we deployed 40 PM sensors inside a heavy-manufacturing site at spatially optimal locations and performed a field calibration to calculate a slope for three PM sensors with a reference instrument at one location. The average slope was applied to all PM sensors for mass concentration calculations. The calculated percent differences in the field were similar to the laboratory results. Therefore, we established a method that reduces the time and cost associated with calibration of low-cost sensors in the field

Molecular beam epitaxy of InN dots on nitrided sapphire

A series of self-assembled InN dots are grown by radio frequency (RF) plasma-assisted molecular beam epitaxy (MBE) directly on nitrided sapphire. Initial nitridation of the sapphire substrates at 900 C results in the formation of a rough AlN surface layer, which acts as a very thin buffer layer and facilitates the nucleation of the InN dots according to the Stranski-Krastanow growth mode, with a wetting layer of {approx}0.9 nm. Atomic force microscopy (AFM) reveals that well-confined InN nanoislands with the greatest height/width at half-height ratio of 0.64 can be grown at 460 C. Lower substrate temperatures result in a reduced aspect ratio due to a lower diffusion rate of the In adatoms, whereas the thermal decomposition of InN truncates the growth at T>500 C. The densities of separated dots vary between 1.0 x 10{sup 10} cm{sup -2} and 2.5 x 10{sup 10} cm{sup -2} depending on the growth time. Optical response of the InN dots under laser excitation is studied with apertureless near-field scanning optical microscopy and photoluminescence spectroscopy, although no photoluminescence is observed from these samples. In view of the desirable implementation of InN nanostructures into photonic devices, the results indicate that nitrided sapphire is a suitable substrate for growing self-assembled InN nanodots

Imaging of InGaN inhomogeneities using visible apertureless near-field scanning optical microscope

Received ( The optical properties of epitaxially grown islands of InGaN are investigated with nanometer-scale spatial resolution using visible apertureless near-field scanning optical microscopy. Scattered light from the tip-sample system is modulated by cantilever oscillations and detected at the third harmonic of the oscillation frequency to distinguish the near-field signal from unwanted scattered background light. Scattered near-field measurements indicate that the as-grown InGaN islanded film may exhibit both inhomogeneous In composition and strain-induced changes that affect the optical signal at 633 nm and 532 nm. Changes are observed in the optical contrast for large 3D InGaN islands (100's of nm) of the same height. Near-field optical mapping of small grains on a finer scale reveals InGaN composition or strain-induced irregularities in features with heights of only 2 nm, which exhibit different near-field signals at 633 nm and 532 nm incident wavelengths. Optical signal contrast from topographic features as small as 30 nm is detected

Effects of silver nanoparticles (NM-300K) on Lumbricus rubellus earthworms and particle characterization in relevant test matrices including soil.

The impact of silver nanoparticles (AgNP; at 0 mg Ag/kg, 1.5 mg Ag/kg, 15.4 mg Ag/kg, and 154 mg Ag/kg soil) and silver nitrate (AgNO3 ; 15.4 mg Ag/kg soil) on earthworms, Lumbricus rubellus, was assessed. A 4-wk exposure to the highest AgNP treatment reduced growth and reproduction compared with the control. Silver nitrate (AgNO3 ) exposure also impaired reproduction, but not as much as the highest AgNP treatment. Long-term exposure to the highest AgNP treatment caused complete juvenile mortality. All AgNP treatments induced tissue pathology. Population modeling demonstrated reduced population growth rates for the AgNP and AgNO3 treatments, and no population growth at the highest AgNP treatment because of juvenile mortality. Analysis of AgNP treated soil samples revealed that single AgNP and AgNP clusters were present in the soil, and that the total Ag in soil porewater remained high throughout the long-term experiment. In addition, immune cells (coelomocytes) of earthworms showed sensitivity to both AgNP and AgNO3 in vitro. Overall, the present study indicates that AgNP exposure may affect earthworm populations and that the exposure may be prolonged because of the release of a dissolved Ag fraction to soil porewater

Effect of particle size on silver nanoparticle deposition onto dielectric barrier discharge (DBD) plasma functionalized polyamide fabric

The effect on the deposition of three different size silver nanoparticles (AgNPs) onto a polyamide 6,6 (PA) fabric pre-treated using air dielectric barrier discharge (DBD) plasma was investigated. The SEM, EDS, and XPS analysis confirm that the smaller is the diameter of AgNPs, the higher the amount of adsorbed NPs on the PA. The DBD treatment on PA induces a threefold increase in Ag adsorption. The result confirms a dual effect on the wettability of the plasma treated PA substrate. AgNPs slightly enhance hydrophobicity of the PA surface and, at the same time, protect it against the plasma aging effect. The effect on the deposition of three different size silver nanoparticles (AgNPs) onto a Polyamide 6,6 (PA) fabric pre-treated using air dielectric barrier discharge (DBD) plasma was investigated. The smaller is the size, the higher the loaded AgNPs. The DBD treatment induces a threefold increase in Ag adsorption. AgNPs enhance hydrophobicity of the PA surface and reduce the plasma aging effect.Fundação para a Ciência e a Tecnologia (FCT

Cytotoxicity and ion release of alloy nanoparticles

It is well-known that nanoparticles could cause toxic effects in cells. Alloy nanoparticles with yet unknown health risk may be released from cardiovascular implants made of Nickel–Titanium or Cobalt–Chromium due to abrasion or production failure. We show the bio-response of human primary endothelial and smooth muscle cells exposed to different concentrations of metal and alloy nanoparticles. Nanoparticles having primary particle sizes in the range of 5–250 nm were generated using laser ablation in three different solutions avoiding artificial chemical additives, and giving access to formulations containing nanoparticles only stabilized by biological ligands. Endothelial cells are found to be more sensitive to nanoparticle exposure than smooth muscle cells. Cobalt and Nickel nanoparticles caused the highest cytotoxicity. In contrast, Titanium, Nickel–Iron, and Nickel–Titanium nanoparticles had almost no influence on cells below a nanoparticle concentration of 10 μM. Nanoparticles in cysteine dissolved almost completely, whereas less ions are released when nanoparticles were stabilized in water or citrate solution. Nanoparticles stabilized by cysteine caused less inhibitory effects on cells suggesting cysteine to form metal complexes with bioactive ions in media

Nanoparticle polydispersity can strongly affect in vitro dose

When nanomaterials meet the biological world, the cellular interaction of nanoparticles is routinely assessed in in vitro systems. Establishing dose–response relationships requires that the dose of nanoparticles delivered to the cell is accurate and precise. Nanoparticles as such or coated with high molecular-weight compounds are rarely uniform and the influence of heterogeneity, including polydispersity both in size and mass density, on the delivered dose is never studied before. Furthermore, a probabilistic term describing particle adherence to cells is introduced and the importance is discussed. By tracing the movement of discrete particles via modeling, it is found that the influence of heterogeneity cannot be neglected when the average particle size promotes settling over diffusion. However, the influence of polydispersity on the delivered cellular dose is less critical for particulate systems whose mean size promotes diffusion. The influence of a non-instantaneous particle association to the cell is negligible for particles whose motion is dominated by settling, but it is relevant for small particles whose motion is governed by diffusion