Micro- and nanoplastics enter the environment as manufactured products in cosmetics and other industrial products, and as breakdown products of plastics of larger size (macroplastics). Recent estimates suggest that the mass of microplastics in aquatic habitats will surpass that of fish by 2050; therefore, plastic pollution is a major environmental issue faced at global scale. Marine scientists have been working in this field for over a decade, and more recently the study of microplastics has captured the attention of other scientists in the environmental field. Despite efforts focused on identifying, quantifying, and assessing the size of microplastics in the environment, standardized methods are lacking. As a result, studies cannot be compared or reproduced. Infrared (IR) microscopy has become an essential technique in the toolbox for the analysis of microplastics in environmental samples. In this work, we conducted a methodical evaluation of significant operational parameters of IR microscopy for accurately determining the size of microplastics, with the overarching goal of outlining performance parameters that will help in the standardization of microplastics analysis.

Plastic pollution is a major environmental issue faced at global scale. Micro- and nanoplastics enter the environment as manufactured products in cosmetics and other industrial products, and as breakdown products of plastics of larger size (macroplastics). According to a recent perspective paper (1), the most pessimistic estimates suggest that the mass of microplastics in aquatic habitats will surpass that of fish by 2050. Interest in the scientific community and public has exponentially increased in the past few years, resulting in a significant increase in the number of publications focused on this topic (2). Marine scientists have been working on this field for a decade, and more recently the study of microplastics has captured the attention of other scientists in the environmental field. Despite efforts focused on identifying, quantifying, and assessing the size of microplastics in the environment, standardized methods and agreement of reporting parameters are lacking (3). As a result, studies cannot be compared or reproduced. Common requests from scientists engaged in the field include the need for reference materials, proficiency tests and interlaboratory studies, and detailed descriptions of instrumental and spectral treatment parameters (2). Multiple international groups (2,3), scientific organizations (such as the Society of Environmental Toxicology and Chemistry), and standardization agencies (such as the American Society for Testing and Materials [ASTM] and the International Organization for Standardization [ISO]) are currently engaging in efforts to fulfill these requests.

 (4) provides an excellent summary of various techniques suitable of the identification of microplastics and their scope. Infrared (IR) microscopy has become an essential technique in the toolbox for the analysis of microplastics in environmental samples. IR microscopy is often used for two purposes: the identification and sizing of microplastics present in the sample. While lack of appropriate standard spectra in the libraries could cause challenges in proper identification, especially for weathered plastics, this problem can be overcome with custom-created libraries and analyzing specific regions of the spectra (unique peaks, valleys, and intensity ratios, to name a few). Therefore, the real challenge is to understand the capability of IR microscopy for microplastics analysis, mainly in terms of smallest measurable microplastic particle size.

The objective of the work presented here was to collect data to support current standardization efforts for the analysis of microplastics in environmental samples by IR microscopy. For this purpose, we conducted a methodical evaluation of significant operational parameters of IR microscopy for accurately determining the size of microplastics, including optical mode, number of scans, and background contribution from filters.

Microplastic standards were purchased from Sigma-Aldrich. They were polymer based micro particles with various diameters: 100 µm polystyrene (P/N 59336), 50 µm polymethacrylate (P/N 74161), 50 µm polystyrene (P/N 74491) and 10 µm polystyrene (P/N 72986).

Three types of filters commonly used to hold the microplastics samples for Fourier transform infrared (FT-IR) spectroscopy studies were evaluated: a stainless-steel mesh filter (Utah Biodiesel Supply, 5 μm pore size), a cellulose ester filter (Whatman, 0.6 μm pore size) and a glass fiber filter (Whatman, 934-AH grade, 1.5 μm pore size).

The measurements were carried out with a Shimadzu AIM-9000 FT-IR microscope in combination with an IRTracer-100 FT-IR spectrophotometer. Instrumental control and data acquisition were done automatically by the AIMSolution software. Data were collected in reflection mode and attenuated total reflectance (ATR) mode. When measuring in the reflection mode, background was acquired using a silver mirror. Experiments conducted in ATR mode were conducted with the Shimadzu ATR attachment with a Ge objective on the FT-IR microscope. The only post-processing steps performed were baseline correction of the data whose signal to noise ratio were calculated, and Kubelka-Munk transformation of the spectrum of the cellulose ester filter. The instrumental parameters are listed in Table I.

All samples were diluted first to 1 mL of Deionized water, except for the 100 μm polystyrene based microplastics sample. For reflection mode measurements, the samples were dispensed onto the filters and dried under ambient conditions. For ATR mode studies, 1–2 drops of the diluted samples or the 100 μm polystyrene-based stock sample were dispensed on a low-E microscope slide, and dried under ambient conditions.

To get a better understanding of types and distribution of the particles on the filters, infrared mapping measurements were carried out using the reflection mode on the FT-IR microscope. The information obtained from mapping is often qualitative; however, some quantitative information can also be obtained with further data analysis. In-depth measurements using more sensitive setup and parameters were carried out on areas of interest determined by mapping.

Samples containing microplastics of decreasing particle size (described in the section 

) were analyzed in reflection mode with the conditions listed in Table I. These experiments were repeated using three filter types (stainless-steel mesh filter, cellulose ester filter, and glass fiber filter) to assess the impact of the background spectra on the detection threshold of the particle sizes.

The reflection mode measurement of the samples was obtained as-is in the filter, and although this measurement mode allows for faster data acquisition with the ability to analyze multiple particles or an entire area automatically, it is susceptible to background noise from the filter element, surrounding particles, and matrix. Therefore, once the particles were identified using the reflection method, a more targeted measurement with better signal-to-noise ratio (S/N) using the ATR attachment on single particles was carried out. Samples containing microplastics of decreasing particle size were analyzed in ATR mode with the conditions listed in Table I. Because ATR is a contact-based measurement for surface measurement (no more than 2–5 µm into the surface), only particles pressed against the ATR crystal are measured, ensuring
higher accuracy.

Evaluation of S/N and Threshold of Detection Based on Sample Size

An experiment was conducted for IR spectral acquisition (reflection mode) on the 100 μm polystyrene-based sample with varying aperture sizes. The aperture sizes started from 5x5 μm, then increased to 10x10 μm. The aperture sizes were gradually increased from 10x10 μm to 100x100 μm, with an interval of 10 μm (Figure 1). This change in aperture size can be directly correlated to the difference in sample size. In addition, and more importantly, using the same particle but with different aperture sizes also ensured that any other experimental variations (inter-sample variation and instrument conditions) were eliminated. In a second experiment, the impact of the number of scans in the S/N of the measurement at each aperture size was evaluated.

Spectral Search Using the FT-IR Polymer Library

Spectral searches for all spectra were performed using polymer libraries provided by the microscope software. The Shimadzu AIM-9000 FT-IR microscope adopts a score system from 0 to 1000 to report search results. In this study, the differential derivative algorithm was used, which calculates the primary differential of each data and compares the differential factor at each point with fitting of whole curves.

Reflection Mode Mapping Measurement Results

Microplastics samples dispensed on filters were scanned automatically using the mapping function of the AIMSolution Software. Aperture sizes were adjusted according to the dimension of the microplastics particles. The ratio of the signature peak intensity over the baseline intensity could be mapped to reflect the specific microplastics abundance at the locale. Figure 2 shows the mapping area settings and the resulting chemical mapping of microplastics samples of four different sizes on a stainless-steel mesh filter.

In Figure 2, it can be observed that mapping measurement in the reflection mode can provide relatively quick qualitative analysis of the microplastics samples in a potentially large area. When measuring particles with a size of 10x10 μm, it became challenging to capture focused images of the samples because the unevenness and the structure of the filters made it almost impossible for the microplastics particles to lie within the focal plane. As a result, the FT-IR spectra acquired for the 10 μm samples tended to reflect signals contributed by a subset of particles rather than that of an exact individual particle.

Similar reflection mapping measurements were obtained for microplastics samples held on a cellulose ester filter and a glass fiber filter. These filters were selected because they are commonly used in environmental laboratories for sample filtration. When those filters were used, signal interferences from the filters were evident (Figure 3). The cellulose ester filter showed IR peaks across the whole measured region (Figure 3e), whereas the glass fiber had IR absorption mainly in the low wavenumber region (Figure 3j). The stainless-steel mesh filter is highly reflective in the IR and can be considered to share similar features to that of the metal mirror used for background measurement; therefore, the spectra were not included in Figure 3. Figure 3 also demonstrates qualitatively how the background spectra negatively affects the detection threshold of the particle size. Overall, using the stainless-steel filter had the advantage of suppressing the signal contributions of the filters.

When measuring in the reflection mode, IR signals from the microplastics samples may be unable to be determined because of unwanted interference, like neighboring contaminants or substance attaching to the samples. It is desirable to measure in the ATR mode because it greatly assists in getting signal only from the targeted area. With a tip diameter of only 12.5 μm, the Shimadzu ATR attachment for AIM-9000 can acquire IR spectra from particles with high specificity. Figure 4 illustrates the microplastic samples and the FT-IR spectra acquired using ATR mode. However, samples were analyzed in triplicates; only one spectrum per particle size is included in the figure. The pictures on the left side are visible images captured for each of the samples analyzed. The FT-IR spectra are shown on the right side, and they illustrate the characteristic absorption bands of the microplastics. Spectral search was performed using polymer libraries provided by the microscope software; search results showed ≥75% match for the four samples analyzed, independently from the size of the particle. Qualitatively, the results demonstrate that FT-IR spectra of microplastics as small as 10 µm can be detected in ATR mode.

Evaluation of S/N and Detection Threshold

Measurements made in reflection mode are known to be more affected by the presence of interferences and background than ATR measurements; therefore, it can be assumed that the threshold of detection for determining the size of microplastics will be higher in reflection mode than ATR mode. To quantitatively determine the threshold of detection under the most challenging conditions, a series of experiments was conducted in reflection mode. FT-IR spectra were acquired on a 100 μm polystyrene particle, with variable aperture sizes gradually increasing from 5x5 μm to 100x100 μm (Figure 1); four replicate measurements at each aperture were performed. This change in aperture size can be directly correlated to the difference in sample size (See the Experimental Approach section for more details). Peak-to-peak S/N values at each aperture setting were evaluated; results are shown in Figure 4. Threshold of detection was determined based on measurements that presented a S/N > 3 (5). According to the results included in Figure 5, it can be concluded that when using the typical operational conditions of FT-IR microscopy (included in Table I), the smallest particle size that can reliably measured is 10x10 μm. From Figure 5, it was clear that the greatest increase in S/N took place when the sample (aperture) size was increased from 5x5 μm to 10x10 μm. After the sample (aperture) size was increased to 40x40 μm and higher, the rate of increase of S/N remained constant.

To further confirm the threshold of detection in terms of particle size, another study was carried out with sample sizes of 5x5 μm and 10x10 μm. In this study, the number of scans averaged to obtain a spectrum were gradually increased. The number of scans was set at 32, 64, 128, and 256 scans for the two sample sizes mentioned above. Measurements were replicated five times. Peak-to-peak S/N was evaluated at each measurement. As can be concluded from Figure 6, S/N in the 5x5 μm sample size did not show any substantial improvement with the increased number of scans. The averaged S/N values stayed close to 0.5, suggesting that only noise was observed. When the sample size was changed to 10x10 μm, S/N showed a noticeable increase as the number of scans increased. S/N should increase as more scans are averaged when a real signal is detected, this result further verifies that the signal from the 10x10 μm was real and that from the 5x5 μm sample was primarily noise; thereby, this confirms the threshold of 10x10 μm.

This work demonstrates a series of method and experimental conditions in FT-IR microscopy in both reflectance and ATR mode for identification and accurate determination of the size of microplastics. The results presented in this paper are a subset of data being prepared for standardizing the analysis of microplastics in environmental samples. We have demonstrated that, by using typical operating conditions of FT-IR microscopy, it is possible to identify and distinguish between microplastic particles as small as 10x10 μm in dimension. This threshold value held true for both the reflection measurement method (which is faster, but can experience signal interference from neighboring particles and matrix) and the ATR method, which is specific to a particle to get “cleaner” measurements but is slower because of the setup involved. In the reflection measurement itself, this threshold value was determined and verified by two separate analysis methods, further verifying the capability of the technique.

J.M. Andrade, B. Ferreiro, P. López-Mahía, and S. Muniategui-Lorenzo, 

W. Cowger, A. Booth, B. Hamilton, S. Primpke, et al., 

is a Product Specialist in the Molecular Spectroscopy group at Shimadzu Scientific Instruments in Columbia, Maryland. 

is the Product Manager of the Molecular Spectroscopy group at Shimadzu Scientific instruments in Columbia, Maryland. 

 is the Environmental Market Manager at Shimadzu Scientific Instruments, in Columbia, Maryland. Direct correspondence to: 

Articles

Here we conduct an evaluation of significant operational parameters of IR microscopy for accurately determining the size of microplastics, with the overarching goal of outlining performance parameters that will help in the standardization of microplastics analysis.

Insights Toward Standardization of the Analysis of Microplastics by Infrared Microscopy

Space-based construction materials must meet a range of requirements: mechanical robustness, lightweight characteristics, and tolerance to heat (due to extreme exposure to solar radiation). There are three optical requirements for heat management: the solar reflectance must be sufficiently high, the thermal emissivity must be sufficiently high, and the surface must be diffuse. These properties ensure that the spacecraft will remain cool, and the thermal control material will perform over a wide range of exposure angles. Here, we examine a ceramic material, Z93, which is in current use on the International Space Station (!SS). Measurements of the optical properties reveal why Z93 is successful in the ISS's implementation, and also point to potential future material improvements.

odern materials are being developed to meet demanding requirements in terrestrial construction, military operations, and spacecraft. An increasingly crucial concern is heat management. This is controlled through the response to incident radiation. As an example, low-emissivity glass (low-E glass) being used in window construction reflects infrared (IR) and ultraviolet (UV) light, preventing their penetration into the structure, keeping it cooler in the summer. In the winter, this effectively reverses as the glass is used to reflect the IR (heat) into the structure, preserving the internal heat. In both cases, the impinging radiation may come from a wide range of geometries (angles).

In military applications, heat-seeking devices use IR radiation to identify and track potential targets. High-tech materials can reduce the IR signature through reflection and absorption. It is not possible to predict the exact angle at which the incoming IR-homing device will approach, meaning the material must respond over a wide range of angles of incidence as well as over an array of frequencies.

Space-based operations stretch this even further. To protect sensitive instrumentation and personnel, the surface must meet three main criteria: high solar reflectance, high thermal emissivity, and the surface must be diffuse. These ensure the vehicle remains cool and the thermal coating will operate over a wide range of exposure geometries. In a high-profile example of the impact of this, Skylab sustained damage to the micrometeoroid shield that removed much of the protection from solar radiation; this damage led to severe overheating of the interior. The orbital repair (the first of its kind) featured the addition of a solar "umbrella" shield, which restored sufficient insulation to permit operations. On the International Space Station (ISS), the lightweight ceramic material Z93 was used as an insulator, being more robust than that ultra-thin metalized mylar, such as used on the Lunar Landing Module.

We present here optical measurements on Z93 revealing the heat management characteristics of the material when exposed to intense solar radiation. 

The measurements were made using an SOC-100 Hemispherical Directional Reflectometer (HDR, Surface Optics Corporation) coupled to a Thermo Scientific Nicolet iS50FT-IR spectrometer (1,2). Figure 1 shows a schematic view of the HDR. The HDR uses a large, elliptical reflection hemisphere with the source built into the HDR at one focal point, and the sample at the other focal point. The elliptical hemisphere ensures the emitted source light is focused onto the sample over the full range of angles all at once (rather than using a directed beam, as in
integrating spheres).

A mirror on a moving arm automatically sweeps through the reflected radiation, sending the selected angle of reflected radiation to the FT-IR spectrometer entrance port. This reflected radiation is shown as a conical section emanating from the sample in Figure 1. The arm sweeps over the full range of polar angles and directs the light to the spectrometer. A blocker mounted in the HDR can remove the specular reflected radiation, allowing independent determination of the specular and diffuse components in the total reflection. A polarizer mounted in the HDR just in front of the entrance aperture of the spectrometer functions to account for any polarization bias in the spectrometer. Also, the sample can be heated in the sampling location, permitting emissivity measurements and the determination of the temperature dependence of the reflectivity.

The automation in the SOC-100 enables the moving arm mirror to sweep through a wide range of angles or be fixed at selected angles to probe the directional properties of the reflectivity. The light reflected from the rotating arm enters the spectrometer emission port, and then passes through the interferometer and onto the deuterated L-alanine triglycine sulfate (DLaTGS) detector.

A sample of Z93 white thermal control coating was studied. Developed for use on spacecraft, Z93 was one of the original coatings tested on the National Aeronautics and Space Administration (NASA)’s Long Duration Exposure Facility in the Thermal Control Surfaces Experiment in the 1980s (1). The coating is currently used on the surface of the ISS. It is composed of zinc oxide pigment in a potassium silicate binder. The material appears as a white ceramic. Scanning electron microscope (SEM) data reveal a microporous structure, consistent with the need in the application for a diffuse surface; this ensures reflectivity over the wide range of exposure geometries possible in space.

The reflectance of Z93 measured from 250 to 2500 nm is shown in Figure 2. This measurement is made by illuminating the sample at a particular angle and frequency and capturing the reflection over the entire hemisphere using an integrating sphere. This measurement is an example of directional hemispherical reflectance (DHR). DHR is a standard integrating sphere type of measurement, where the excitation beam is directional, while the reflectance is measured over the full hemisphere. The DHR for Z93 is low in the UV range, then quickly rises to over 90% in the visible (starting in the deep blue), and slowly tapers off in the near-infrared range (NIR). This measured reflectivity of Z93 corresponds closely to the spectral profile of extraterrestrial solar irradiance (maximum
at 500 nm).

The solar reflectance was calculated according to ASTM E903 using the spectral solar irradiance defined in ASTM E490. The solar reflectance is the percentage of solar radiation that is reflected by a material and is calculated using the equation below:

where ρ(λ) is the measured spectral reflectance and 

(λ) is the spectral solar irradiance function. Using the measurement in Figure 1, the solar reflectance was determined to be 86.5%. This means that 86.5% of the solar radiation incident on the material is reflected, or 13.5% is absorbed. This is in agreement with NASA’s results in the thermal control surfaces experiment (TCSE) and was found to be sufficiently high for operation in space (3).

The reflectance in the IR range was measured from 2 to 25 μm and is shown in Figure 3. Unlike the DHR above, the source (located at one focus of the elliptical mirror) emission is directed by the large mirror onto the sample from all angles at once, but the rotating arm collected the light reflected from the sample as an automatically selected angle; this mirror directs that reflection into the input aperture of the spectrometer. This measurement is called 

 (HDR) and is mathematically equivalent to the DHR when summed over the full range of angles. The reflectance was measured at 12 different angles ranging from 8.0° to 80.0°. It is important to have measurements across the entire hemisphere to accurately calculate emissivity. The reflectance starts high in the near-infrared spectral range up to approximately 2 to 3 μm (as shown also in Figure 2), and then drops significantly from 3 to 16 μm, and rises a small amount after 16 μm. There is an increase reflectivity with the approach to a grazing angle, which is expected.

For each reflectance spectrum above, the directional emissivity can be calculated as follows:

 is the measured reflectance at a particular angle. With sufficient directional emissivity data spanning the entire hemisphere, the total hemispherical emissivity can be calculated by integrating over the entire hemisphere:

The two calculations above were performed using temperatures ranging from 100 to 400 K, which is roughly the temperature range experienced outside the ISS. Over this range, the total emissivity remains relatively constant (Figure 4), averaging about 0.907. This means that Z93 emits 90.7% of the energy of a blackbody at the same temperature. Again, this is in agreement with NASA’s results in the TCSE and was found to be sufficiently high for operation in space (3).

Finally, the specular and diffuse components of the total reflectance were measured at two different angles using the blocker. These measurements are shown in Figure 5. At an angle of 10°, the diffuse component is the dominant reflectance characteristic and it remains so until about 22 μm. Contrastingly, at 70°, the diffuse component drops below the specular component at about 8 μm. Generally, diffuse materials become increasingly specular in behavior at steeper angles, as seen experimentally and which can be shown with simple geometric optics arguments. Because the reflectance is quite low (emissivity is high), Z93 is an effective thermal control coating. This explains its successful use on the surface of the ISS for 10 years. It is interesting to note, Z93 transitions from diffuse to specularly reflecting at long wavelengths. By 17 μm at 70°, Z93 is about 30% reflective and mostly specular. This long-wavelength behavior could be an area for development in future thermal control coatings.

Materials involved in heat management, such as those used on space-based vehicles, require a complete understanding of reflectivity, emissivity, and direction bias in those properties. HDR provides a full range of measurement capabilities enabling improved design and fabrication of these materials. With the growth in terrestrial interest in similar performance improvements for energy conservation and military hardware protection, this type of measurement has
broad applicability.

Z93 is effective in its role as a result of combined high reflectivity and emissivity. Although the relative impact of different aspects of the optical performance varies with the angle of incidence, the overall performance remains well within the design threshold for use in extended duration space-based applications. Future material improvement may focus on the long wavelength change in performance.

Final report for the TCSE under NASA Contract NAS8-38939. D.R. Wilkes, Thermal Control Surfaces Experiment (Doc, ID 19990021250, NASA, 1999).

is a Product Manager at Thermo Fisher Scientific in Madison, Wisconsin.

are with Surface Optics Corporation in San Diego, California. Direct correspondence to: 

Measurements of the optical proper ties of a ceramic material, Z93, which is in current use on the International Space Station (ISS), reveal why this material is successful for thermal control in the ISS implementation, and also point to potential future material design improvements.

Optical Measurement of the Reflectance Behavior of Z93, the Thermal Coating on the International Space Station

Chalcogen thin films GexSb40-xSe60 with the compositionx = 12, 25, and 30 (atomic percentage) at.%, have been synthesized from elements with 99.999% purity (Ge, Sb, Se) by the conventional melt-quenching method. The quartz ampoule containing the mixture of elements with proper weight percent was evacuated down to a pressure of 10-3 Pa. The parts of synthesized glasses were served as parent material for deposition on the quartz substrate. The Swanepoel method was applied to transmission spectra given by the Lambda spectrophotometer to determine the optical properties of the thin films. The influence of the glassy composition on the optical constants of the films was studied by spectroscopic ellipsometry and correlated with UV-vis-NIR spectroscopy to determine the optical constants.

The chalcogens are the chemical elements in group 16 of the periodic table, and are also known as the oxygen family. Chalcogens consists of the elements sulfur (S), selenium (Se), tellurium (Te), and the radioactive element polonium (Po). Polonium is also a chalcogen, but is not used because of its strong radioactivity. The last chemical element in this group, Lv, is uncharacterized, and predicted to be a chalcogen as well (1). Oxygen is treated separately from the other chalcogens, because of its very different chemical behavior from sulfur, selenium, tellurium, and polonium.

Chalcogenide glasses are covalently bonded materials, and may be classified as covalent network solids. Chalcogenide materials have low band gaps, contributing to very dissimilar optical and electrical properties.

The classical chalcogenide glasses are based on As-S or Ge-S that form a strong glass within large concentration regions. Glass-forming abilities decrease with increasing molar weight of constituent elements; for example, S > Se > Te (2).

More recently, chalcogenide ternary GeSbSe glasses suitable materials for both passive and active infrared applications, such as optical fibers devices, given their thermal, mechanical, and chemical properties (3–4). Chalcogenide glass based on GeSbSe deposited on silica glass substrate has losses in optical propagation that can be achieved in photoinduced channel waveguides (5). GeSbSe system can be synthesized from a solid target using different physical vapor deposition (PVD) methods such as thermal evaporation (6–9), RF magnetron sputtering (6,10–13), or pulsed laser deposition (10,13).

By varying the chemical composition of GeSbSe thin films, the network undergoes changes reflected in the structural phase transformations (14–16), to the effect of doping Sb on the electronic structure (17) and in the threshold transition in the physical parameters (18). The ternary chalcogenide glasses offer a new important application for infrared-transmitting lenses and infrared imaging sensors (3,19–21). The optical parameters determinate by spectroscopy methods are used to study thin films in optical waveguide devices, dynamic random-access memory (DRAM) materials, and electro-optical and sensor or actuator devices.

 = 12, 25, and 30 at.% were measured by a Lambda 950 spectrophotometer, and we determined the optical properties using the Swanepoel method (22). The results obtained by the ultraviolet-visible-near infrared (UV-vis-NIR) spectroscopy are correlated with spectroscopic ellipsometry measurements.

 = 12, 25 and 30 at.%, were synthesized from elements with 5N purity (Ge, Sb, Se) by conventional melt-quenching methods. The mixture of the elements with proper weight percent was placed in a quartz ampoule, and evacuated down to a pressure of 10-3 Pa. The ampoules were loaded in a rotary furnace, and heated up to 950 °C. To achieve homogeneous melting, the glassy mixture was kept at this temperature for a continuous 2 h by rotating the furnace. Then, the ampoules were pulled out, and the melts were rapidly cooled down in ice water.

The parts of synthesized glasses were served as parent material for deposition on the quartz substrate, using a resistive crucible with a 0.1 mm molybdenum sheet having the shape and size of item number 0482070 in the UMICORE catalog. Plasma cleaning with glow discharge took place at a pressure of 10-1 Pa for 12 min in the N2, and during that time, the dome with the quartz substrates was rotated at 10 rot/min. This process removed impurities on the walls from the vacuum deposition installation and on the bulk. At 4×10-3 Pa pressure, the heating resistance was coupled, by ensuring a temperature of 300 oC in the evaporation chamber while the dome rotation was kept.

The powder material was evaporated at an electric current at least 400 mA. The monitoring of the deposition control was done by the software TFCalc 3.5 for approximately 6 min, with a deposition rate of 3 nm/sec, and showed 1300 nm thickness. After evaporation, the samples were measured by a Lambda 950 spectrophotometer to determine the optical constants (J.A. Woollam Co. Inc.). Spectroscopic ellipsometry measurements were used to certify the dates.

The spectrophotometer measurements were recorded on a Lambda 950 Spectrophotometer with double beam and double monochromator atroom temperature, in the UV–vis-NIR spectral range at 266 nm/min, a scanning speed for normal incidence.

The optical system under consideration corresponds to homogeneous and uniform thin films, deposited on thick transparent substrates. Figure 1 shows the variation with respect to wavelength of the transmittance for glassy system Ge

 = 12, 25, and 30 at.%. The film has the thickness 

 the extinction coefficient. The transmission

λ(1–4) at normal incidence is given by:

 is the index of the quartz substrate and the value is 

 is thickness, φ is the phase difference between the direct and multiple reflected transmitted beams,is the absorbance and α is the absorption coefficient.

Based on Swanepoel’s method (22–23), which is based on the idea of Manifacier and associates (24) , the transmission values at the maxima and minima of the interference fringes can be obtained from equation 2 by setting the interference condition cosφ = +1 for maxima (

This method implies the calculation of the maximum 

min transmittance envelope curves by using the parabolic interpolation of the position of the experimental peak. Figure 2 exemplifies the relationship between the maximum 

min transmittance and the wavelength for Ge30Sb10Se60 films.

We mainly observe two distinct regions: a region (800 to 1800 nm) having interference fringes whose operation allows us to determine both films' refractive index and thickness, and a region (200 to 800 nm) where the absorption becomes larger, which results in a progressive damping of the interference fringes and a strong decrease of the transmittance.

The first approximate value of the real part of the refractive index 

 of the film, in the spectral region of medium and weak absorption, can be calculated using the following expression (25–27)

min are the transmission maximum and the corresponding minimum at a certain wavelength λ.

1, which is the first approximate value of the real part of the refractive index, as calculated from equation 11, are present in Figure 3. The results are shown in Table 1, and are according to (3,27). The refractive index of the samples varies with the germanium content. We can see that the refractive index decrease when increasing the
germanium content.

The accuracy of this initial estimation of the refractive index is improved after calculating d, as will be explained below. It is necessary to take into account the basic equation for interference fringes:

 is an integer for maxima and half integer for minima.

(λ2) are the refractive indices at two adjacent maxima (or minima) at λ1 and λ2, it follows that the film thickness is given by

Equation 14 is very sensitive to errors in refractive index 

, and is not very accurate. The film thickness values obtained from equation 14 are shown in Table I. There is some dispersion in the values of 

1 as calculated from equation 11 is not accurate enough.

To determine the absorption coefficient, we used the transmittance T in the region of higher absorption (between 200 and 800 nm). The expression [2] of T in this region is reduced to

In this a region of strong absorption, the interference fringes disappear and the three curves for 

min converge to a single curve. A and B depend on the refractive index of the thin film and refractive index of the quartz substrate. The thickness d is calculated (Table I), thus we can determine the absorption coefficient α (Figure 4) by the expression

It is seen that the absorption coefficient decreases with an increase in germanium and a decrease in antimony content. To complete the calculation of the optical constants, the extinction coefficient 

 (Figure 5) is estimated from the values of the absorption coefficient α and wavelength λ, using the formula

The influence of the glassy composition on the optical constants of the films was verified by the ellipsometry. In the UV-vis-NIR spectral range, the complementary SE measurements were performed, working in the wavelength region (193–1700) nm and at an 50o angle of incidence. The transmission spectra were obtained directly by the UV-vis-NIR measurements done at zero angles of incidence (Figure 6). In the SE data analysis, the wavelength λ function of the ellipsometric angles Ψ(λ) (Figure 7a) and ∆(λ) (Figure 7b) were simulated by considering the samples as a two-layer optical system (substrate/first layer [film]/second layer [surface roughness]) and using General Oscillator mathematical models of corresponding software (data acquisition and analysis supplied by J.A. Woollam Co. Inc.). In the SE fitting procedure measurements, the surface roughness was considered as a mixture of 50% material (film) and 50% voids (air), and was modeled by applying Bruggemann’s effective medium
approximation theory (29).

From the spectroscopic ellipsometry data analysis the optical constants (refractive index 

) (Figure 8 and Figure 9) were calculated in the whole spectral range of 0.193 to 1700 nm. The results obtained by SE measurements are compared with the results calculated obtained by UV-vis-NIR spectroscopy. We can see the value for optical properties is the same, though a small increase is observed for optical properties determined by the ellipsometry

Wemple-DiDomenico (W-D) (28,30) is the model of dispersion of refractive index, and can be written as

where λ is the wavelength of the incident radiation, 

0is the average oscillator strength, and λ0 is an average oscillator wavelength. The curves of (

2 -1)-1 against (1/λ2) (Figure 10) were fitted into straight lines following Sellmeier’s dispersion formula (27–28), and the graphs were obtained to confirm formula. The values of 

According to the Wemple-DiDomenico single-oscillator model, the dispersion energy increase in the sample Ge

Determination of High Frequency Dielectric Constant

Two procedures are applied to obtain data for the refractive index 

 with the high frequency dielectric constant (29). The first procedure describes the contribution of the free carriers and the lattice vibration modes of the dispersion. The second procedure is based on the dispersion arising from the bound carriers in an empty lattice. Both models are used for the estimation of 

 for the high frequency dielectric constant ε∞.

The high frequency dielectric constant (28, 30,31) can be obtained by equation 21:

Spectroscopic ellipsometry, correlated with UV-vis-NIR spectroscopy, is used to determine the optical constants of thin films, such as in GexSb40-xSe60 chalcogenide glass.

Special Issues-08-01-2020

Insights Toward Standardization of the Analysis of Microplastics by Infrared Microscopy

Optical Measurement of the Reflectance Behavior of Z93, the Thermal Coating on the International Space Station

Characterization of GeSbSe Thin Films Synthesized by the Conventional Melt-Quenching Method