Particles In Thin Films Research Articles

Field switching of microfabricated metamagnetic FeRh MRI contrast agents

In a step towards generating switchable MRI cellular labels, we demonstrate in-situ field switching of micron scale metamagnetic Iron-Rhodium (FeRh) thin film particles. A thin-film (200 nm) FeRh sample was fabricated and patterned into an array of progressively smaller squares with sizes ranging from 500 μm down to 1 μm. The large first order phase change from antiferromagnetic to ferromagnetic state was characterized using vibrating sample magnetometry, magnetic force microscopy, and MRI. Room temperature MRI experiments sensitive to the local magnetic field surrounding the particles demonstrated the low moment state (OFF MRI contrast) at 4.7T and high moment state (ON MRI contrast) at 11.7T for the array where sizes down to 2–3 μm were observed in MRI at 50 μm resolution. The expected temperature dependent MRI contrast change was seen at 4.7T, where 10 μm particles could be observed at 150 μm resolution in the ON state. A shielded MRI insert, used to temporarily increase or decrease the magnetic field up to 0.77T amplitude, was used to reversibly switch the particle array at constant temperature and blink the particles ON and OFF at 4.7T. This work demonstrates the MRI contrast switching potential for FeRh particles with biological cell dimensions, and the use of magnetic field pulses for reversible MRI label contrast control.

Microscale Electrical Resistivity Measurements to Investigate Particle Distribution.

The functional performance of a particulate thin film depends greatly on the particle distribution that forms during drying. In situ methods for monitoring the impact of different processing parameters on the distribution of particles currently require expensive and specialized equipment. This work addresses this gap by miniaturizing a geophysical prospecting method to thin-film applications. In this method, four-electrode resistivity measurements at variable probe spacing detect changes in the vertical particle concentration profile. A heuristic colloidal drying model describes the particle distribution during drying in terms of the relative effects of Brownian diffusion, sedimentation, and evaporation. For sedimentation- and evaporation-dominated drying, the film is modeled as two stratified layers of different concentrations. Solving this model simultaneously alongside Laplace's equation for electrostatic resistance identifies the parameters necessary to distinguish between diffusion-, sedimentation-, and evaporation-dominated drying. For resistive particles in a conductive solvent, simulations predict that the normalized thickness of the top layer, δt/H0, must exceed a critical value to distinguish between different drying regimes. The heuristic model results are validated theoretically by comparison to a physics-based drying model. Model predictions are experimentally validated by fabricating a custom microlithography four-line probe device and measuring the transient resistance of systems for which the drying mechanism is known. This work offers a low-cost and in situ method to identify drying mechanisms and extract physical parameters that better characterize the processing-structure-function relationships for many coatings.

Synthesis and Characterization of Lithium-doped BaTiO3/Si(100) Thin Films with Variations of 0 %, 0.5 %, 1 % and 1.5 % as Potential Forerunners of Solar Cells

Thin films of BaTiO3/Si(100) in Lithium with doping variations of 0%, 0.5%, 1%, and 1.5% were successfully grown on a p-type Si substrate using Chemical Solution Deposition method. The films were annealed at 850 °C for 15 hours. The crystal structure was characterized using X-Ray Diffraction (XRD) with Material Analysis Using Diffraction (MAUD), Scanning Electron Microscope- Energy Dispersive X-Ray (SEM-EDX), optical absorption, and current – voltage (I-V) as potential solar cells. The results showed that the addition of Lithium doping affected the value of the lattice parameters and formed tetragonal crystals. The characterization results show that the bandgap energy value of the thin film due to lithium doping reduces the bandgap energy value because the donor atom added to a semiconductor causes the allowable energy level to be slightly below the conduction band. The presence of this new band causes the thin film bandgap energy to decrease with a five-valent tantalum dip. The morphological properties showed that the BaTiO3/Si(100) thin film particles in the deposited Lithium had a reasonably homogeneous grain. With the addition of lithium acetate as a binder into barium titanate, the grain size is getting smaller because it is suspected that the lithium-ion radius is smaller than the barium-ion radius. Measurement of I-V on the thin film shows that the output voltage value increases with more light intensity hitting the surface of the thin film. The greater the light intensity, the greater the energy of the photons, so the electrons are easier to jump. The three things above (both electrical and morphological properties) conclude that the thin films grown have the potential for solar cells.

Design and fabrication of photovoltaics based on MFS (Ag/BaTiO3/silicon p-type) structure

The experiment was carried out by growing BaTiO3 (Undoped or Li-doped) on p-type Si(100) substrates using the Chemical Solution Deposition (CSD) method and spin coating at a rotational speed of 3000 rpm for 60 s, followed by heating at 850 °C. The characterization results show that the bandgap energy value of the thin film due to lithium doping reduces the bandgap energy value. This is presumably because the donor atom added to a semiconductor causes the allowable energy level to be slightly below the conduction band. The presence of this new band causes the thin film bandgap energy to decrease with a five-valent tantalum dip. The morphological properties showed that the BaTiO3/Si(100) thin film particles in the deposited lithium had a fairly homogeneous grain. With the addition of lithium acetate as a binder into barium titanate, the grain size is getting smaller because it is suspected that the lithium-ion radius is smaller than the barium-ion radius. Measurement of I-V on the thin film shows that the output voltage value increases with more light intensity hitting the surface of the thin film. The greater the light intensity, the greater the energy of the photons, so the electrons are easier to jump. The three things above (both electrical and morphological properties) conclude that the thin films grown have the potential for photovoltaics.

Tuning the NiO Thin Film Morphology on Carbon Nanotubes by Atomic Layer Deposition for Enzyme‐Free Glucose Sensing

AbstractNanocomposites made of stacked‐cup carbon nanotubes coated with NiO (NiO/SCCNTs) via atomic layer deposition (ALD) were synthesized in order to obtain a material exhibiting enhanced and optimized electrochemical performance towards detections of glucose. The structure and morphology were characterized by transmission electron microscopy (TEM) and X‐ray diffraction (XRD). NiO deposited as nanocrystalline particles in the cubic modification, were well dispersed and directly anchored on SCCNTs forming a smooth particulate thin film, which becomes more dense with the increase of the number of ALD cycles. The NiO/SCCNTs samples with various thicknesses of the NiO coating (0.8 nm, 1.7 nm, 4.0 nm, 6.5 nm, 14.0 nm and 21.8 nm) were applied for enzyme‐free glucose sensing. Their electrochemical performance strongly depends on the thickness of the deposited NiO thin film. The best performing glucose sensors respond over a wide concentration range from 2 μM to 2.2 mM (R2=0.9979) with remarkably enhanced sensitivity (1252.3 μA cm−2 mM−1), with a limit of detection (LOD) of 0.10 μM (S/N=3) and with a fast response time (lower than 2 s). The significant performance improvement can be attributed to the conformal NiO coating, high surface to volume ratio and to the optimized thickness of the NiO thin film. The advantage of our sensors is also associated with the conductive supporting material (SCCNTs), simplicity of fabrication, high sensitivity, selectivity, stability and reproducibility for the rapid quantification of glucose.

Organic acids as electrostatic dispersing agents to prepare high quality particulate thin film

A simple electrophoretic method is developed for the fabrication of high quality electrolyte film on NiO-YSZ anode substrate for solid oxide fuel cell (SOFC) application. Aromatic carboxylic acids viz. benzoic acid and trimellitic acid have been explored as electrostatic stabilizer to prepare yttria stabilized zirconia (8YSZ) suspensions in isopropanol medium. Systematic studies on different suspension properties and deposition kinetics are carried out for each stabilizing agent to optimize the suspension chemistries and deposition conditions. Successful cathodic deposition has been achieved on polymer coated NiO-YSZ anode substrate using both benzoic acid and trimellitic acid as charging agents. Upon co-firing at 1400 °C, these films resulted in well-adhered dense YSZ coating (thickness: 7–12 μm) on the porous anode substrate. SOFC coupon cells are then fabricated by screen-printing La0.65Sr0.3MnO3 cathode layer on the top of the electrolyte. Maximum current densities as obtained from these single cells vary in the range of 1.3–1.6 A/cm2 for a cell voltage of 0.7 at 800 °C.

Can phoretic particles swim in two dimensions?

Artificial phoretic particles swim using self-generated gradients in chemical species (self-diffusiophoresis) or charges and currents (self-electrophoresis). These particles can be used to study the physics of collective motion in active matter and might have promising applications in bioengineering. In the case of self-diffusiophoresis, the classical physical model relies on a steady solution of the diffusion equation, from which chemical gradients, phoretic flows, and ultimately the swimming velocity may be derived. Motivated by disk-shaped particles in thin films and under confinement, we examine the extension to two dimensions. Because the two-dimensional diffusion equation lacks a steady state with the correct boundary conditions, Laplace transforms must be used to study the long-time behavior of the problem and determine the swimming velocity. For fixed chemical fluxes on the particle surface, we find that the swimming velocity ultimately always decays logarithmically in time. In the case of finite Péclet numbers, we solve the full advection-diffusion equation numerically and show that this decay can be avoided by the particle moving to regions of unconsumed reactant. Finite advection thus regularizes the two-dimensional phoretic problem.

Efficient encapsulation of low dimensional particles in thin films to obtain functional coatings

The incorporation of nanoparticles into a polymeric matrix to improve the mechanical properties or add new functionalities is a very common practice in nanocomposites materials. However, the organic nature of the common matrices difficults this methodology. In this work, low dimensional particles (LDPs) are introduced in an inorganic silica matrix obtained by the sol-gel method, which provides an appropriate host to encapsulate the LDPs. The step of introducing particles with only one of their dimensions in the nanoscale, into an inorganic matrix is not defined in the literature, therefore, a study of process and efficiency of the LDPs encapsulation is carried out by means of measuring structural properties (SEM, XRD, DTA-TG, particle size) and functionalities (colorimetry and abrasion-chemical resistances) of composites in thin films.

Chemically synthesized PbS Nano particulate thin films for a rapid NO2 gas sensor

Abstract Rapid NO2 gas sensor has been developed based on PbS nanoparticulate thin films synthesized by Successive Ionic Layer Adsorption and Reaction (SILAR) method at different precursor concentrations. The structural and morphological properties were investigated by means of X-ray diffraction and field emission scanning electron microscope. NO2 gas sensing properties of PbS thin films deposited at different concentrations were tested. PbS film with 0.25 M precursor concentration showed the highest sensitivity. In order to optimize the operating temperature, the sensitivity of the sensor to 50 ppm NO2 gas was measured at different operating temperatures, from 50 to 200 °C. The gas sensitivity increased with an increase in operating temperature and achieved the maximum value at 150 °C, followed by a decrease in sensitivity with further increase of the operating temperature. The sensitivity was about 35 % for 50 ppm NO2 at 150 °C with rapid response time of 6 s. T90 and T10 recovery time was 97 s at this gas concentration.

Photovoltaic Properties of Dye-Sensitized Solar Cells Using Novel Aligned ZnO Nanorod Arrays on Sn-Doped ZnO Seeded Catalyst with Different Aspect Ratio

The Sn-doped ZnO thin films were deposited on glass and ITO by sol gel Spin Coating technique. The Structural and electrical properties of Sn-doped ZnO thin films were studied and discussed. The Sn-doped ZnO thin film particle sizes were decreased whenever the doping concentration increased. Besides that, the resistivity 7.7 x 102 Ω.cm was obtained at 2 at.% Sn-doped thin films and aligned ZnO nanorod arrays with large surface area were grown on 2 at.% Sn-doped ZnO film. Therefore, dye sensitized solar cell at 2.0 at.% Sn-doped ZnO thin films with aligned ZnO Nanorod arrays have improved in the photocurrent density and conversion efficiency.

Measuring the stopping power of α particles in compact bone for BNCT

The stopping power of α particles in thin films of decalcified sheep femur, in the range of 1.5 to 5.0 MeV incident energy, was measured by transmission of a backscattered beam from a heavy target. Additionally, the film elemental composition was determined by Rutherford Backscattering Spectrometry (RBS). These data will be used to measure boron concentration in thin films of bone using a spectrometry technique developed by the University of Pavia, since the concentration ratio between healthy tissue and tumor is of fundamental importance in Boron Neutron Capture Therapy (BNCT). The present experimental data are compared with numerical simulation results and with tabulated stopping power data of non-decalcified human bone.

Preparation and Characterization of Subsurface Silver Particulate Films on Polymer Blends of Polystyrene/Poly(2-vinylpyridine)/Poly(vinylpyrollidone)/Poly(4-vinylpyridine)

Silver particulate thin films on softened polymer blends of Polystyrene (PS)/Poly(2-vinyl pyridine) (P2VP), PS/Poly(4-vinylpyridine) (P4VP), and Poly(vinylpyrollidone) (PVP)/P4VP at a rate of 0.4 nm/s held at a temperature of 457 K in vacuum of 8 × 10-6 Torr by evaporation are deposited. These silver films were characterized by their electrical behavior, optical properties and Scanning electron microscopy (SEM). Silver films deposited on softened PS, and PVP give rise to a very high room temperature resistance approaching that of the substrate resistance due to the formation of a highly agglomerated structure. On the other hand, silver films on softened P2VP and P4VP give rise to a room temperature resistance in the range of tens to a few hundred MΩ/ which is desirable for device applications. Silver films on the composites of PS/P2VP, PS/P4VP and PVP/P4VP show resistances at room temperature. The optical and plasmonic response of Ag nanoparticles onto thin layers of blends shows encapsulation of nanoparticles. The electrical properties and SEM of silver nanoparticles on the thin layers of polymer blends indicate the formation of much smaller, narrower dispersion and wide size distribution.

Debonding of particles in thin films

Thin composite films consisting of a matrix with embedded particles are currently being developed both as hard, wear resistant coatings and as functional surfaces. The effect of stiff particles in the film are studied for systems where the film is under residual tensile stresses. The particles, when they are fully bonded to the matrix, increase the stiffness of the composite film. In cases where the particles debond from the matrix material, the stiffness of the composite film decreases. The conditions under which the debonding process is stable are studied. For systems properly designed, a controlled debonding process of the particles can thus be used to reduce the stress levels in composite film lowering the risk for delamination of the composite film from the substrate as well as the risk of through cracks in the film. The work includes finite element based unit cell calculations of interface debonding between spherical particles and the film, and the release of residual stresses following this. The three dimensional unit cell calculations assume a periodic distribution of particles in the plane parallel to the substrate interface with equi-biaxial tension and periodicity with zero overall stress perpendicular to the substrate interface.

Influence of Particle Size Distribution on Micromechanical Properties of thin Nanoparticulate Coatings

In this study the production of thin nanoparticulate coatings on solid stainless-steel substrates using dip-coating was investigated. Defined particle sizes and particle size distributions of Al2O3-nanoparticles were adjusted by stirred media milling using various operating parameters. Using nanoindentation the influence of particle size and width of the particle size distribution on the mechanical properties was investigated. In particular the establishment of nanoindentation routines for particulate thin films in contrast to hard coatings is discussed. Nanoindentation appears to be an efficient method for analysing mechanical properties of said thin coatings. It will be shown, that the influence of the substrate can be neglected for small indent depth while the coating's surface roughness influences the employed routine of the nanoindentation. The effect of the median particle size and the width of the particle size distribution on the coating structure and the micromechanical coating properties will be discussed. As a result, the maximum indentation force decreases with decreasing particle size but rises again once the nanoparticles reach very small sizes. A change in the width of the particle size distribution influences the micromechanical properties and coating structure as well.

Superior Functionality by Design: Selective Ozone Sensing Realized by Rationally Constructed High‐Index ZnO Surfaces

A new technique is reported for the transformation of smooth nonpolar ZnO nanowire surfaces to zigzagged high-index polar surfaces using polycrystalline ZnO thin films deposited by atomic layer deposition (ALD). The c-axis-oriented ZnO nanowires with smooth nonpolar surfaces are fabricated using vapor deposition method and subsequently coated by ALD with a ZnO particulate thin film. The synthesized ZnO-ZnO core-shell nanostructures are annealed at 800 °C to transform the smooth ZnO nanowires to zigzagged nanowires with high-index polar surfaces. Ozone sensing response is compared for all three types of fabricated nanowire morphologies, namely nanowires with smooth surfaces, ZnO-ZnO core-shell nanowires, and zigzagged ZnO nanowires to determine the role of crystallographic surface planes on gas response. While the smooth and core-shell nanowires are largely non-responsive to varying O(3) concentrations in the experiments, zigzagged nanowires show a significantly higher sensitivity (ppb level) owing to inherent defect-rich high-index polar surfaces.

Enhancement of photocurrents due to the oxidation of water and organic compounds at BiZn2VO6 particulate thin film electrodes by treatment with a TiCl4 solution

Abstract Photocurrents due to water oxidation at BiZn2VO6 (Eg 2.4 eV) particulate thin film electrodes were largely enhanced by pre-treatment with an aqueous TiCl4 solution. Photocurrents for BiZn2VO6 electrodes with no TiCl4 treatment were also enhanced by the addition of organic compounds such as methanol and trimethyl amine to the aqueous electrolyte. Interestingly, such enhanced photocurrents by organic compounds were further enhanced by the TiCl4 pre-treatment. EDAX and SEM investigations showed the formation of a flock-like TiO2 overlayer on BiZn2VO6 particles after the TiCl4 treatment. The photocurrent enhancement by the TiCl4 pre-treatment is thus mainly attributed to the necking effect of the flock-like TiO2 overlayer, which facilitates the transport of photogenerated electrons within the BiZn2VO6 particulate thin film electrode.

Preparation of Micron‐Sized Spherulitic Bisphenol A Polycarbonate Particles in Thin Films

AbstractA facile technique is presented to prepare discrete µm‐sized spherulitic particles of BAPC in thin polymer films. Unlike in bulk precipitation or spray crystallization, the present technique offers a method to prepare three‐dimensional semicrystalline particles of narrow particle size distribution that can be readily isolated and collected from the glass substrate as discrete particles. We report the effects of polymer molecular weight, polymer type, and the precursor polymer film thickness on the formation of spherulitic particles and their morphologies. The three‐dimensional spherulitic particles prepared in this study have large specific surface areas, higher crystallinity and melting temperature than the bulk precipitated and crystallized polycarbonate particles.magnified image

Emission properties of impregnated cathode with nanoparticle films

The Ir nano particle thin film was grown by dcrect current magnetron sputtering at room temperature. Microstructure of thin films grown at different sputtering pressures were measured by scanning electron microscopy. The results show that the particle size of the films depends on the deposition rate in the nucleation stage of Ir films and the deposition rate can be well controlled by sputtering pressure. A new kind of cathodes are fabricated from 25% porous tungsten impregnated with 6∶1∶2-type barium calcium aluminate and coated with the nano particulate Ir thin film. The coating processes produces a film thickness of 200—500 nm and the cathodes are fired in hydrogen atmosphere for ten minutes at 1200℃ to further improve the coating microstructure. The cathodes have been studied with thermal electron microscope by which the electron-optical picture of a thermal emission cathode could be obtained. Time of flight mass spectrometer （ToFMS） has been used in a study of evaporation from impregnated cathodes. The chemical composition of evaporation of various impregnated cathodes have been measured by ToFMS. The variation of ion current of evaporants from S-type cathode and M-type cathode （coated with iridium） and n-type cathode （coated with iridium nano-particle thin film） with temperature and time have been studied and discussed. Emission current characteristics have been measured as a function of voltage and temperature.

Investigation of the effect of temperature on growth mechanism of nanocrystalline ZnS thin films

In this paper the temperature effect on the growth mechanism of ZnS thin films prepared in a chemical bath containing zinc acetate, ethylenediamine, and thioacetamide aqueous solutions has been studied in the temperature range between 25 and 75 °C. These ZnS thin films possess a nanocrystalline structure, exhibit quantum size effects due to the small crystal size and produce a blue shift in the optical spectra. This blue shift was attributed to a decrease in crystal size by using X-ray diffraction and scanning electron microscopy. The growth mechanism of the thin films is suggested to proceed by two fundamental steps: in the first step, the ZnS nanocrystallites coalesce into small grains through homogeneous nucleation in the solution phase. In the second step, eventually, these small grains or large-sized clusters diffuse and stick to the surface of the substrate to form the ZnS thin film, in a way called a cluster-by-cluster manner, resulting in particulate thin film.

Magnetization reversal and microstructure of FePt–Ag (001) particulate thin films for perpendicular magnetic recording media

FePt thin films with the addition of Ag layers have been fabricated in order to study the microstructure and magnetic properties via a molecular beam epitaxy technique on MgO (001) single-crystal substrates at 350°C. The enhancement of the coercivity is due to the film structures changed from continuous state to an isolated particulate character, not an ordering enhancement of FePt films with Ag addition. Studies of angular dependent coercivity show a tendency of a domain-wall motion shift toward rotation of reverse-domain type with the addition of Ag layers into the FePt films. The intergrain interaction was confirmed from the Kelly-Henkel plot that indicated FePt films with Ag addition can lead to the reduction of intergrain exchange coupling.