Developing a Cloud-Based Air Quality Monitoring Platform Using Low-Cost Sensors.

Conventional air quality monitoring has been traditionally carried out in a few fixed places with expensive measuring equipment. This results in sparse spatial air quality data, which do not represent the real air quality of an entire area, e.g., when hot spots are missing. To obtain air quality data with higher spatial and temporal resolution, this research focused on developing a low-cost network of cloud-based air quality measurement platforms. These platforms should be able to measure air quality parameters including particulate matter (PM10, PM2.5, PM1) as well as gases like NO, NO2, O3, and CO, air temperature, and relative humidity. These parameters were measured every second and transmitted to a cloud server every minute on average. The platform developed during this research used one main computer to read the sensor data, process it, and store it in the cloud. Three prototypes were tested in the field: two of them at a busy traffic site in Stuttgart, Marienplatz and one at a remote site, Ötisheim, where measurements were performed near busy railroad tracks. The developed platform had around 1500 € in materials costs for one Air Quality Sensor Node and proved to be robust during the measurement phase. The notion of employing a Proportional-Integral-Derivative (PID) controller for the efficient working of a dryer that is used to reduce the negative effect of meteorological parameters such as air temperature and relative humidity on the measurement results was also pursued. This is seen as one way to improve the quality of data captured by low-cost sensors.

Single Layer Lift-Off of CSAR62 for Dense Nanostructured Patterns.

Lift-off processing is a common method of pattern transfer for different nanofabrication applications. With the emergence of chemically amplified and semi-amplified resist systems, the possibilities for pattern definition via electron beam lithography has been widened. We report a reliable and simple lift-off process for dense nanostructured pattern in CSAR62. The pattern is defined in a single layer CSAR62 resist mask for gold nanostructures on silicon. The process offers a slimmed down pathway for pattern definition of dense nanostructures with varied feature size and an up to 10 nm thick gold layer. The resulting patterns from this process have been successfully used in metal assisted chemical etching applications.

Miniaturized Sulfite-Based Gold Bath for Controlled Electroplating of Zone Plate Nanostructures.

X-ray zone plates made from gold are common optical components used in X-ray imaging experiments. These nanostructures are normally fabricated using a combination of electron-beam lithography and gold electroplating with cyanide gold baths. In this study, we present a gold electroplating process in a miniaturized gold-suplphite bath. The miniaturization is enabled by on-chip reference plating areas with well defined sizes, offering a reliable way to control the height of the structures by carefully choosing the plating time at a given current density in accordance with a calibration curve. Fabricated gold zone plates were successfully used in X-ray imaging experiments with synchrotron radiation. Although gold electroplating of nanostructures is a well-established method, details about the actual process are often missing in the literature. Therefore, we think that our detailed descriptions and explanations will be helpful for other researchers that would like to fabricate similar structures.

NanoMAX: the hard X-ray nanoprobe beamline at the MAX IV Laboratory.

NanoMAX is the first hard X-ray nanoprobe beamline at the MAX IV laboratory. It utilizes the unique properties of the world's first operational multi-bend achromat storage ring to provide an intense and coherent focused beam for experiments with several methods. In this paper we present the beamline optics design in detail, show the performance figures, and give an overview of the surrounding infrastructure and the operational diffraction endstation.

Optimization of Metal-Assisted Chemical Etching for Deep Silicon Nanostructures.

High-aspect ratio silicon (Si) nanostructures are important for many applications. Metal-assisted chemical etching (MACE) is a wet-chemical method used for the fabrication of nanostructured Si. Two main challenges exist with etching Si structures in the nanometer range with MACE: keeping mechanical stability at high aspect ratios and maintaining a vertical etching profile. In this work, we investigated the etching behavior of two zone plate catalyst designs in a systematic manner at four different MACE conditions as a function of mechanical stability and etching verticality. The zone plate catalyst designs served as models for Si nanostructures over a wide range of feature sizes ranging from 850 nm to 30 nm at 1:1 line-to-space ratio. The first design was a grid-like, interconnected catalyst (brick wall) and the second design was a hybrid catalyst that was partly isolated, partly interconnected (fishbone). Results showed that the brick wall design was mechanically stable up to an aspect ratio of 30:1 with vertical Si structures at most investigated conditions. The fishbone design showed higher mechanical stability thanks to the Si backbone in the design, but on the other hand required careful control of the reaction kinetics for etching verticality. The influence of MACE reaction kinetics was identified by lowering the oxidant concentration, lowering the processing temperature and by isopropanol addition. We report an optimized MACE condition to achieve an aspect ratio of at least 100:1 at room temperature processing by incorporating isopropanol in the etching solution.

Calibration Method for Particulate Matter Low-Cost Sensors Used in Ambient Air Quality Monitoring and Research.

Over the last decade, manufacturers have come forth with cost-effective sensors for measuring ambient and indoor particulate matter concentration. What these sensors make up for in cost efficiency, they lack in reliability of the measured data due to their sensitivities to temperature and relative humidity. These weaknesses are especially evident when it comes to portable or mobile measurement setups. In recent years many studies have been conducted to assess the possibilities and limitations of these sensors, however mostly restricted to stationary measurements. This study reviews the published literature until 2020 on cost-effective sensors, summarizes the recommendations of experts in the field based on their experiences, and outlines the quantile-mapping methodology to calibrate low-cost sensors in mobile applications. Compared to the commonly used linear regression method, quantile mapping retains the spatial characteristics of the measurements, although a common correction factor cannot be determined. We conclude that quantile mapping can be a useful calibration methodology for mobile measurements given a well-elaborated measurement plan assures providing the necessary data.

Investigating a Low-Cost Dryer Designed for Low-Cost PM Sensors Measuring Ambient Air Quality.

Air pollution in urban areas is a huge concern that demands an efficient air quality control to ensure health quality standards. The hotspots can be located by increasing spatial distribution of ambient air quality monitoring for which the low-cost sensors can be used. However, it is well-known that many factors influence their results. For low-cost Particulate Matter (PM) sensors, high relative humidity can have a significant impact on data quality. In order to eliminate or reduce the impact of high relative humidity on the results obtained from low-cost PM sensors, a low-cost dryer was developed and its effectiveness was investigated. For this purpose, a test chamber was designed, and low-cost PM sensors as well as professional reference devices were installed. A vaporizer regulated the humid conditions in the test chamber. The low-cost dryer heated the sample air with a manually adjustable intensity depending on the voltage. Different voltages were tested to find the optimum one with least energy consumption and maximum drying efficiency. The low-cost PM sensors with and without the low-cost dryer were compared. The experimental results verified that using the low-cost dryer reduced the influence of relative humidity on the low-cost PM sensor results.

Effect of Relative Humidity and Air Temperature on the Results Obtained from Low-Cost Gas Sensors for Ambient Air Quality Measurements.

Using low-cost gas sensors for air quality monitoring promises cost effective and convenient measurement systems. Nevertheless, the results obtained have a questionable quality due to different factors that can affect sensor performance. The most discussed ones are relative humidity and air temperature. This investigation aimed to assess the behavior of B4-series low-cost gas sensors from Alphasense for measuring CO, NO, NO2, and O3 for different levels of relative humidity and temperature. These low-cost gas sensors were tested for six relative humidity levels from 10% to 85% with increasing steps of 15% and four temperature levels of 10 °C, 25 °C, 35 °C, and 45 °C against reference instruments in the laboratory. The effect of these parameters on low-cost gas sensors was quantified in laboratory from which a correction algorithm was calculated, which was then applied to the field data. The applied algorithm improved the data quality of the low-cost gas sensors in most of the cases. Additionally, a low-cost dryer was assessed to reduce the influence of these factors on the low-cost gas sensors, which also proved to be suitable to enhance the data quality.

Ptychographic characterization of a coherent nanofocused X-ray beam.

The NanoMAX hard X-ray nanoprobe is the first beamline to take full advantage of the diffraction-limited storage ring at the MAX IV synchrotron and delivers a high coherent photon flux for applications in diffraction and imaging. Here, we characterize its coherent and focused beam using ptychographic analysis. We derive beam profiles in the energy range 6-22 keV and estimate the coherent flux based on a probe mode decomposition approach.

Metal-Assisted Chemical Etching and Electroless Deposition for Fabrication of Hard X-ray Pd/Si Zone Plates.

Zone plates are diffractive optics commonly used in X-ray microscopes. Here, we present a wet-chemical approach for fabricating high aspect ratio Pd/Si zone plate optics aimed at the hard X-ray regime. A Si zone plate mold is fabricated via metal-assisted chemical etching (MACE) and further metalized with Pd via electroless deposition (ELD). MACE results in vertical Si zones with high aspect ratios. The observed MACE rate with our zone plate design is 700 nm/min. The ELD metallization yields a Pd density of 10.7 g/cm 3 , a value slightly lower than the theoretical density of 12 g/cm 3 . Fabricated zone plates have a grid design, 1:1 line-to-space-ratio, 30 nm outermost zone width, and an aspect ratio of 30:1. At 9 keV X-ray energy, the zone plate device shows a first order diffraction efficiency of 1.9%, measured at the MAX IV NanoMAX beamline. With this work, the possibility is opened to fabricate X-ray zone plates with low-cost etching and metallization methods.

Reaction control of metal-assisted chemical etching for silicon-based zone plate nanostructures.

Metal-assisted chemical etching (MACE) reaction parameters were investigated for the fabrication of specially designed silicon-based X-ray zone plate nanostructures using a gold catalyst pattern and etching solutions composed of HF and H2O2. Etching depth, zone verticality and zone roughness were studied as a function of etching solution composition, temperature and processing time. Homogeneous, vertical etching with increasing depth is observed at increasing H2O2 concentrations and elevated processing temperatures, implying a balance in the hole injection and silica dissolution kinetics at the gold-silicon interface. The etching depth decreases and zone roughness increases at the highest investigated H2O2 concentration and temperature. Possible reasons for these observations are discussed based on reaction chemistry and zone plate design. Optimum MACE conditions are found at HF : H2O2 concentrations of 4.7 M : 0.68 M and room temperature with an etching rate of ≈0.7 µm min-1, which is about an order of magnitude higher than previous reports. Moreover, our results show that a grid catalyst design is important for successful fabrication of vertical high aspect ratio silicon nanostructures.

In situ flow cell for combined X-ray absorption spectroscopy, X-ray diffraction, and mass spectrometry at high photon energies under solar thermochemical looping conditions.

An in situ/operando flow cell for transmission mode X-ray absorption spectroscopy (XAS), X-ray diffraction (XRD), and combined XAS/XRD measurements in a single experiment under the extreme conditions of two-step solar thermochemical looping for the dissociation of water and/or carbon dioxide was developed. The apparatus exposes materials to relevant conditions of both the auto-reduction and the oxidation sub-steps of the thermochemical cycle at ambient temperature up to 1773 K and enables determination of the composition of the effluent gases by online quadrupole mass spectrometry. The cell is based on a tube-in-tube design and is heated by means of a focusing infrared furnace. It was tested successfully for carbon dioxide splitting. In combined XAS/XRD experiments with an unfocused beam, XAS measurements were performed at the Ce K edge (40.4 keV) and XRD measurements at 64.8 keV and 55.9 keV. Furthermore, XRD measurements with a focused beam at 41.5 keV were carried out. Equimolar ceria-hafnia was auto-reduced in a flow of argon and chemically reduced in a flow of hydrogen/helium. Under reducing conditions, all cerium(iv) was converted to cerium(iii) and a cation-ordered pyrochlore-type structure was formed, which was not stable upon oxidation in a flow of carbon dioxide.

Moiré method for nanometer instability investigation of scanning hard x-ray microscopes.

We present a Moiré method that can be used to investigate positional instabilities in a scanning hard x-ray microscope with nanometer precision. The development of diffraction-limited storage rings offering highly-brilliant synchrotron radiation and improvements of nanofocusing x-ray optics paves the way towards 3D nanotomography with 10 nm resolution or below. However, this trend demands improved designs of x-ray microscope instruments which should offer few-nm beam stabilities with respect to the sample. Our technique can measure the position of optics and sample stage relative to each other in the two directions perpendicular to the beam propagation in a scanning x-ray microscope using simple optical components and visible light. The usefulness of the method was proven by measuring short and long term instabilities of a zone-plate-optics-based prototype microscope. We think it can become an important tool for the characterization of scanning x-ray microscopes, especially prior to experiments with an actual x-ray beam.

Interferometric characterization of rotation stages for X-ray nanotomography.

The field of three-dimensional multi-modal X-ray nanoimaging relies not only on high-brilliance X-rays but also on high-precision mechanics and position metrology. Currently available state-of-the-art linear and rotary drives can provide 3D position accuracy within tens to hundreds of nm, which is often insufficient for high resolution imaging with nanofocused X-ray beams. Motion errors are especially troublesome in the case of rotation drives and their correction is more complicated and relies on the metrology grade reference objects. Here we present a method which allows the characterisation and correction of the radial and angular errors of the rotary drives without the need for a highly accurate metrology object. The method is based on multi-probe error separation using fiber-laser interferometry and uses a standard cylindrical sample holder as a reference. The obtained runout and shape measurements are then used to perform the position corrections using additional drives. We demonstrate the results of the characterization for a piezo-driven small rotation stage. The error separation allowed us to measure the axis runout to be approximately ±1.25 µm, and with active runout compensation this could be reduced down to ±42 nm.

Perfect X-ray focusing via fitting corrective glasses to aberrated optics.

Due to their short wavelength, X-rays can in principle be focused down to a few nanometres and below. At the same time, it is this short wavelength that puts stringent requirements on X-ray optics and their metrology. Both are limited by today's technology. In this work, we present accurate at wavelength measurements of residual aberrations of a refractive X-ray lens using ptychography to manufacture a corrective phase plate. Together with the fitted phase plate the optics shows diffraction-limited performance, generating a nearly Gaussian beam profile with a Strehl ratio above 0.8. This scheme can be applied to any other focusing optics, thus solving the X-ray optical problem at synchrotron radiation sources and X-ray free-electron lasers.

Ce K edge XAS of ceria-based redox materials under realistic conditions for the two-step solar thermochemical dissociation of water and/or CO2.

X-ray absorption spectroscopy was used to characterise ceria-based materials under realistic conditions present in a reactor for solar thermochemical two-step water and carbon dioxide splitting. A setup suitable for in situ measurements in transmission mode at the cerium K edge from room temperature up to 1773 K is presented. Time-resolved X-ray absorption near-edge structure (XANES) data, collected for a 10 mol% hafnium-doped ceria sample (Ce0.9Hf0.1O2-δ) during reduction at 1773 K in a flow of inert gas and during re-oxidation by CO2 at 1073 K, enables the quantitative determination of the non-stoichiometry δ of the fluorite-type structure. XANES analysis suggests the formation of the hexagonal Ce2O3 phase upon reduction in 2% hydrogen/helium at 1773 K. We discuss the experimental limitations and possibilities of high-temperature in situ XAS at edges of lower energy as well as the importance of the technique for understanding and improving the properties of ceria-based oxygen storage materials for thermochemical solar energy conversion.

Ronchi test for characterization of X-ray nanofocusing optics and beamlines.

A Ronchi interferometer for hard X-rays is reported in order to characterize the performance of the nanofocusing optics as well as the beamline stability. Characteristic interference fringes yield qualitative data on present aberrations in the optics. Moreover, the visibility of the fringes on the detector gives information on the degree of spatial coherence in the beamline. This enables the possibility to detect sources of instabilities in the beamline like vibrations of components or temperature drift. Examples are shown for two different nanofocusing hard X-ray optics: a compound refractive lens and a zone plate.

Thermochemical CO2 splitting via redox cycling of ceria reticulated foam structures with dual-scale porosities.

Efficient heat transfer of concentrated solar energy and rapid chemical kinetics are desired characteristics of solar thermochemical redox cycles for splitting CO2. We have fabricated reticulated porous ceramic (foam-type) structures made of ceria with dual-scale porosity in the millimeter and micrometer ranges. The larger void size range, with dmean = 2.5 mm and porosity = 0.76-0.82, enables volumetric absorption of concentrated solar radiation for efficient heat transfer to the reaction site during endothermic reduction, while the smaller void size range within the struts, with dmean = 10 µm and strut porosity = 0-0.44, increases the specific surface area for enhanced reaction kinetics during exothermic oxidation with CO2. Characterization is performed via mercury intrusion porosimetry, scanning electron microscopy, and thermogravimetric analysis (TGA). Samples are thermally reduced at 1773 K and subsequently oxidized with CO2 at temperatures in the range 873-1273 K. On average, CO production rates are ten times higher for samples with 0.44 strut porosity than for samples with non-porous struts. The oxidation rate scales with specific surface area and the apparent activation energy ranges from 90 to 135.7 kJ mol(-1). Twenty consecutive redox cycles exhibited stable CO production yield per cycle. Testing of the dual-scale RPC in a solar cavity-receiver exposed to high-flux thermal radiation (3.8 kW radiative power at 3015 suns) corroborated the superior performance observed in the TGA, yielding a shorter cycle time and a mean solar-to-fuel energy conversion efficiency of 1.72%.

3D simulation of the image formation in soft x-ray microscopes.

In water-window soft x-ray microscopy the studied object is typically larger than the depth of focus and the sample illumination is often partially coherent. This blurs out-of-focus features and may introduce considerable fringing. Understanding the influence of these phenomena on the image formation is therefore important when interpreting experimental data. Here we present a wave-propagation model operating in 3D for simulating the image formation of thick objects in partially coherent soft x-ray microscopes. The model is compared with present simulation methods as well as with experiments. The results show that our model predicts the image formation of transmission soft x-ray microscopes more accurately than previous models.

Damage investigation on tungsten and diamond diffractive optics at a hard x-ray free-electron laser.

Focusing hard x-ray free-electron laser radiation with extremely high fluence sets stringent demands on the x-ray optics. Any material placed in an intense x-ray beam is at risk of being damaged. Therefore, it is crucial to find the damage thresholds for focusing optics. In this paper we report experimental results of exposing tungsten and diamond diffractive optics to a prefocused 8.2 keV free-electron laser beam in order to find damage threshold fluence levels. Tungsten nanostructures were damaged at fluence levels above 500 mJ/cm(2). The damage was of mechanical character, caused by thermal stress variations. Diamond nanostructures were affected at a fluence of 59 000 mJ/cm(2). For fluence levels above this, a significant graphitization process was initiated. Scanning Electron Microscopy (SEM) and µ-Raman analysis were used to analyze exposed nanostructures.