Frank Einar Kruis

Tailored nanoparticle films from monosized tin oxide nanocrystals: Particle synthesis, film formation, and size-dependent gas-sensing properties

In order to investigate the change of gas-sensitive properties of undoped tin oxide nanoparticle films depending on particle size, a thin film synthesis technique has been developed. Well-defined tin oxide nanoparticles have been prepared using a gas-phase condensation method. Pure SnO was used as starting material and was evaporated at T=820 °C. The resulting particles were sintered and crystallized in-flight at T=650 °C. Size-selected nanoparticles ranging from 10 to 35 nm were produced to form a nanoparticle film by means of electrostatic precipitation or low pressure impaction. The effect of in-flight oxidation, sintering, and crystallization on the structure, size, and size distribution of nanoparticles have been studied in detail. The samples show n-type semiconductors’ behavior like bulk SnO2. The influence of particle size on gas sensitivity and response behavior is investigated for C2H5OH at operating temperatures 200–300 °C using silicon substrates having an interdigitated contact pattern and an...

Aerodynamic Focusing of Nanoparticles: I. Guidelines for Designing Aerodynamic Lenses for Nanoparticles

Abstract This article describes the challenges in focusing nanoparticles (< 30 nm) into tightly collimated beams, and provide guidelines for designing aerodynamic lens systems for nanoparticles. The major difficulties of focusing nanoparticles arise from their low inertia and high diffusivity. Because of their low inertia, nanoparticles tend to closely follow gas streamlines; their high diffusivities lead to beam broadening and diffusional deposition. We have identified the minimum particle size that can be focused to the axis with a single lens when diffusion is neglected, assuming that the flow is continuum and subsonic. We show that lighter carrier gases are preferred for focusing small particles, and that multiple lenses operating at suboptimal Stokes numbers can be designed to focus particles smaller than was recognized previously. There exists a maximum pressure under which particles can be optimally focused, while particle diffusion and pumping requirements are minimized. Finally, we describe the procedure for designing aerodynamic lens systems for focusing nanoparticles, and present a case study of designing a single aerodynamic lens to focus 5 nm particles.

Sintering and evaporation characteristics of gas-phase synthesis of size-selected PbS nanoparticles

Abstract Gas-phase synthesis of nanoparticles is advantageous because of higher purity due to absence of liquid solvents and the possibility of reaching high temperatures, thereby extending the range of materials which can be synthesized. The synthesis of semiconducting material (PbS) via the evaporation–condensation route is here undertaken in combination with the control techniques provided by aerosol science such as size fractionation and on-line particle size measurements. Particle sizes obtained range from 3 to 50 nm with a relative standard deviation of 1.1. By means of an on-line measurement of the agglomerate diameter the evaporation and sintering characteristics of PbS nanoparticles are investigated in combination with a theoretical description of the evaporation and sintering process.

Nanoparticle charging in a twin Hewitt charger

A new unipolar charger for aerosol nanoparticles has been developed. In this twin Hewitt charger two corona discharge zones are connected by a charging zone where the nanoparticle aerosol flows. Ions move into the charging zone alternating from each corona discharging zone by means of a square-wave voltage. The operation parameters of the device have been experimentally investigated at standard conditions with the goal to optimize the extrinsic charging efficiency in N2 carrier gas. It has been found that there exists an optimal length of the charging channel for each gas flow rate through the charger which minimizes losses of charged particles and at the same time having a sufficient large niont-product. Extrinsic charging efficiencies of some 30% for particles with a diameter of 10 nm are obtained.

Band-gap tuning of PbS nanoparticles by in-flight sintering of size classified aerosols

We report about the band-gap tuning of PbS nanoparticles by in-flight sintering of size-classified aerosols. Band gaps in the range of 0.5–2.0 eV were obtained. The band gap first decreases and then increases upon increasing the sintering temperature. The decrease in the band gap is associated with the sintering of primary particles in aggregates leading to larger crystalline domains. The increase in the band gap is associated with the partial evaporation of the particles, which sets in after quasispherical and monocrystalline particles were formed by sintering. The same band gap is found for large spherical particles and aggregates of smaller primary particles, suggesting that an intimate contact between primary particles by means of sinter necks leads to a weaker quantum confinement effect than when the particles are merely touching each other.

Effective mass approximation for two extreme semiconductors: Band gap of PbS and CuBr nanoparticles

An effective mass approximation (EMA) with finite-depth square-well potential is used to investigate the size-dependent band gap (BG) of PbS and CuBr nanoparticles embedded in different matrices. These two materials are interesting from the theoretical point of view as PbS is a low-BG material with smaller effective masses and larger dielectric constants, whereas CuBr is a wide-BG material with larger effective masses and smaller dielectric constants. Comparing the experimental BGs with our theoretical calculations, it is shown that EMA provides a better description of the experimental data, especially for CuBr, when the Coulomb interaction having the size-dependent dielectric constant is included in the calculation. Further, comparing the change in the BG of spherical nanoparticle, nanowire and thin film, it is predicted that the effective dimensionality of semiconductor nanoparticles can be increased by embedding them in another semiconducting matrix.

Monodisperse aerosol particle deposition: prospects for nanoelectronics

Abstract Nanometer-sized PbS particles are deposited in an electrostatic precipitator at atmospheric pressure on planar substrates with a rate of about 10 11 cm −2 h −1 . The low-cost aerosol apparatus consists of (i) particle generation by evaporation and subsequent coagulation, (ii) charging the particles and selection in size by a Differential Mobility Analyzer (DMA), (iii) crystallisation in an annealing step, and finally (iv) deposition of particles. Specular X-Ray diffraction and Transmission Electron Microscopy (TEM) is used to study the size and morphology of the PbS particles indicating that these particles are fully crystalline with lattice constant of bulk PbS. Laterally selective particle deposition (lithography) is aimed at by means of electrostatic control of the charged particles in the gas-phase. Best results are obtained with an electrostatic mask provided simply by a photoresist pattern.

Nanoparticles from the Gas Phase as Building Blocks for Electrical Devices

Electrical device development is driven by miniaturization and possibilities to use new chemical and physical effects. Nanotechnology offers both aspects. The structural dimensions of materials and devices are small and because of that large exchange surfaces are provided but also effects like quantum effects may occur and be used to get new or at least improved properties of nanostructured materials and devices.Nanoparticles are of special interest because of their nanodimensions in all three directions, so that nanoeffects become most prominent. They can be synthesized in solid materials, in liquids and in gases. Gas synthesis has several advantages compared to the other phases, especially the high cleanliness which can be achieved. In case of electrical devices the particles have to be deposited onto substrates in a structured way.The substrate may consist out of microelectronic devices in which the deposited nanoparticles are introduced for the basic function. In case of a transistor this would be the gate function, in case of a sensor this would be the sensing layer, where the contact with the measurement object takes place. For two kinds of particles SnO2 and PbS, synthesized in the gas phase, we demonstrate the way how to create devices with improved sensor properties.

Size-induced stability and structural transition in monodispersed indium nanoparticles

The present study reports the stability and the physical significance of the size-induced crystallographic structural transition in the gas-phase synthesized monodispersed indium nanoparticles. Transmission electron microscopy and x-ray photoelectron spectroscopy studies reveal that the formation of a thin oxide shell results in enhanced stability of indium nanoparticles. These results also show a size-induced structural transition from the bulk tetragonal to face-centered-cubic structure, which is attributed to an increase in the binding energy of core electrons of indium nanoparticles due to quantum confinement effects and the presence of a thin oxide shell.

GPU-accelerated Monte Carlo simulation of particle coagulation based on the inverse method

Simulating particle coagulation using Monte Carlo methods is in general a challenging computational task due to its numerical complexity and the computing cost. Currently, the lowest computing costs are obtained when applying a graphic processing unit (GPU) originally developed for speeding up graphic processing in the consumer market. In this article we present an implementation of accelerating a Monte Carlo method based on the Inverse scheme for simulating particle coagulation on the GPU. The abundant data parallelism embedded within the Monte Carlo method is explained as it will allow an efficient parallelization of the MC code on the GPU. Furthermore, the computation accuracy of the MC on GPU was validated with a benchmark, a CPU-based discrete-sectional method. To evaluate the performance gains by using the GPU, the computing time on the GPU against its sequential counterpart on the CPU were compared. The measured speedups show that the GPU can accelerate the execution of the MC code by a factor 10-100, depending on the chosen particle number of simulation particles. The algorithm shows a linear dependence of computing time with the number of simulation particles, which is a remarkable result in view of the n2 dependence of the coagulation.