Peter Heszler

Characterization of porous indium tin oxide thin films using effective medium theory

Effective medium theory was used to model optical properties in the 0.3 – 30 μm wavelength range for films comprised of nanoparticles of a transparent conducting oxide that are connected in a percolating network characterized by a filling factor f. The model is based on charge carrier density ne and resistivity ρ of the particles, and it enables analyses of these microscopic parameters upon posttreatment of the film. The theory was used to interpret data on spin coated layers consisting of nanoparticles of indium tin oxide (i.e., In2O3:Sn) with f close to the percolation limit. It showed that the as-deposited film contained nanoparticles with ne as large as ∼5×1020 cm−3 and ρ≈5×10−4 Ω cm. The model also provided important data on f, ne, and ρ after heat treatment of the film.

Novel amplitude and frequency demodulation algorithm for a virtual dynamic atomic force microscope

Frequency-modulated atomic force microscopy (FM-AFM; also called non-contact atomic force microscopy) is the prevailing operation mode in (sub-)atomic resolution vacuum applications. A major obstacle that prohibits a wider application range is the low frame capture rate. The speed of FM-AFM is limited by the low bandwidth of the automatic gain control (AGC) and frequency demodulation loops. In this work we describe a novel algorithm that can be used to overcome these weaknesses. We analysed the settling times of the proposed loops and that of the complete system, and we found that an approximately 70-fold improvement can be achieved over the existing real and virtual atomic force microscopes. We show that proportional-integral-differential controllers perform better in the frequency demodulation loop than conventional proportional-integral controllers. We demonstrate that the signal to noise ratio of the proposed system is 5.7 × 10(-5), which agrees with that of the conventional systems; thus, the new algorithm would improve the performance of FM-AFMs without compromising the resolution.

Advanced Agent Identification With Fluctuation-Enhanced Sensing

Conventional agent sensing methods normally use the steady state sensor values for agent classification. Many sensing elements (Hines , 1999; Ryan, 2004; Young, 2003;Qian, 2004; Qian, 2006; Carmel, 2003) are needed in order to correctly classify multiple agents in mixtures. Fluctuation-enhanced sensing (FES) looks beyond the steady-state values and extracts agent information from spectra and bispectra. As a result, it is possible to use a single sensor to perform multiple agent classification. This paper summarizes the application of some advanced algorithms that can classify and estimate concentrations of different chemical agents. Our tool involves two steps. First, spectral and bispectral features will be extracted from the sensor signals. The features contain unique agent characteristics. Second, the features are fed into a hyperspectral signal processing algorithm for agent classification and concentration estimation. The basic idea here is to use the spectral/bispectral shape information to perform agent classification. Extensive simulations have been performed by using simulated nanosensor data, as well as actual experimental data using commercial sensor (Taguchi). It was observed that our algorithms are able to accurately classify different agents, and also can estimate the concentration of the agents. Bispectra contain more information than spectra at the expense of high-computational costs. Specific nanostructured sensor model data yielded excellent performance because the agent responses are additive with this type of sensor. Moreover, for measured conventional sensor outputs, our algorithms also showed reasonable performance in terms of agent classification.

Surface energy maps of nanostructures: Atomic force microscopy and numerical simulation study

Topography and surface energy distribution of etched graphite were examined by atomic force microscopy (AFM). AFM images show atomic monolayer deep circular holes (etch pits). At certain imaging conditions, these etch pits appear surrounded by rims. Numerical simulation of AFM images reveals that the rims are formed due to an increased surface energy zone at the edges. The vertical dimension of the rim correlates with the magnitude of the local surface energy. Such a correlation between the imaging features and the surface energy profiles can be used to demarcate local chemical constituents in a composite nanomaterial.

A comparative study of size-distribution of nanoparticles generated by laser ablation of graphite and tungsten

Abstract Nanoparticles (NPs) were generated by ArF excimer laser ablation of graphite and tungsten targets in N2 ambient at atmospheric pressure. The size distribution of the particles was monitored in situ by a scanning mobility particle sizer (SMPS) system, based on differential mobility analyser. The experimental conditions made possible to record the size distributions in the 7–133-nm diameter range and results are presented for different laser fluences, repetition rates and ablated areas, respectively. Material analysis was performed by photoelectron spectroscopy (XPS), Raman spectroscopy, X ray diffraction and SEM.

Analysis of blackbody-like radiation from laser-heated gas-phase tungsten nanoparticles.

Thermal (blackbody-like) radiation that originated from laser-heated tungsten nanoparticles was measured using optical emission spectroscopy. The nanoparticles were generated via ArF excimer laser-assisted photolytic decomposition of WF6/H2/Ar gas mixtures, and the laser heating was applied parallel to the deposition. The temperature of the nanoparticles was determined, and its dependence on time, with respect to the 15-ns laser pulse (full width at half-maximum, fwhm) and laser fluence (phi), has been presented. At phi > 90 mJ/cm2, the particles reached the melting point (shortly after the laser pulse). Dominant cooling mechanisms, such as evaporation (above approximately 3000 K) and a combination of heat transfer by the ambient gas and radiative cooling (below approximately 3000 K), were observed for the nanoparticles, which were approximately 10 nm in diameter. The degree of inelasticity for the (predominantly) argon-gas collisions and the total emissivity of the particles (in the 2500-3000 K temperature region) could also be derived. The measured cooling rate and temperature data indicate that, depending on experimental parameters, evaporation and surface reactions can have a definite effect on the growth of particles.

Raman spectroscopic and atomic force microscopic study of graphite ablation at 193 and 248 nm

Experimental results are presented for the excimer laser ablation of highly oriented pyrolytic graphite at 193 and 248 nm for both single pulses and pulse trains in the fluence range of ∼1–15 J/cm2. The morphology and the depth of the ablated pits are monitored by atomic force microscopy, while the material characterization is performed by micro-Raman spectroscopy. A shift from ∼1.12 to ∼2.23 J/cm2 laser fluence is found in the single shot ablation threshold for the 248 nm laser wavelength compared to that at 193 nm. Broad D and G peaks in the Raman spectra indicate the formation of amorphous carbon layers as a result of laser irradiation with 193 and 248 nm pulses. This amorphous layer is present at lower fluences (several J/cm2) and after the very first shots. The modified layer created at 193 nm, compared to 248 nm, consists of optically denser material having more turbostratical/glassy character. The spectra do not show significant changes for fluences exceeding 6–7 J/cm2. A several hundred nanometers-high ring-like structure can be observed around the ablated pits. For laser fluences in excess of the estimated threshold at ∼6 J/cm2 (close to the aforementioned limit), the diameter of this structure increases with laser fluence. One hypothesis to explain the ring formation and the saturation of the Raman spectra supposes that the graphite melts and squirts on the laser irradiation. The ring, debris material and the amorphous layers disappear after heat treatment of the samples at 650°C, most probably by oxidative etching.

Electrical and optical properties of thin films prepared by spin coating a dispersion of nano-sized tin-doped indium oxide particles

Thin films were made by spinning a dispersion of tin-doped indium oxide particles, having an average diameter of 14 nm, onto glass substrates. As-deposited thin films displayed a resistivity ρ of 0.3 Ω m and some optical absorption. Annealing in vacuum at 200–400oC for 2 h, and subsequently in air at 500oC for 2 h, produced films with ρ ≈ 10−3 Ω m and a visible transmittance exceeding 90%. Leaving out the vacuum treatment yielded higher resistivity.

Highly Selective NO2 Gas Sensors Made of MWCNTs and WO3 Hybrid Layers

Hybrid gas sensors were fabricated by means of multiwalled carbon nanotubes (MWCNTs) covered by W O3 deposited by an advanced reactive gas deposition method. In order to increase the dispersion of ...

Size and structure of nanoparticles formed via ultraviolet photolysis of ferrocene

Iron nanoparticles enclosed in carbon shells were formed by laser-assisted chemical vapor decomposition of ferrocene (Fe(C5H5)(2)) vapor in Ar gas atmosphere. The particle size dependence on the to ...