Determination Of Sample Thickness Research Articles

Fast determination of sample thickness through scanning moiré fringes in scanning transmission electron microscopy

Sample thickness is an important parameter in transmission electron microscopy (TEM) imaging for interpreting image contrast and understanding the relationship between properties and microstructure. In this study, we introduce a method for sample thickness determination in scanning TEM (STEM) mode based on scanning moiré fringes (SMFs). Focal-series SMF imaging is used and sample thickness can be determined in situ at a medium magnification range, with beam damage and contamination avoided to a large extent. It provides a fast and convenient approach for determining sample thickness in TEM imaging, which is particularly useful for beam-sensitive materials.

Optimized cryo-EM data-acquisition workflow by sample-thickness determination.

Sample thickness is a known key parameter in cryo-electron microscopy (cryo-EM) and can affect the amount of high-resolution information retained in the image. Yet, common data-acquisition approaches in single-particle cryo-EM do not take it into account. Here, it is demonstrated how the sample thickness can be determined before data acquisition, allowing the identification of optimal regions and the restriction of automated data collection to images with preserved high-resolution details. This quality-over-quantity approach almost entirely eliminates the time- and storage-consuming collection of suboptimal images, which are discarded after a recorded session or during early image processing due to a lack of high-resolution information. It maximizes the data-collection efficiency and lowers the electron-microscopy time required per data set. This strategy is especially useful if the speed of data collection is restricted by the microscope hardware and software, or if microscope access time, data transfer, data storage and computational power are a bottleneck.

Accuracy and precision of thickness determination from position-averaged convergent beam electron diffraction patterns using a single-parameter metric

Position-averaged convergent beam electron diffraction patterns are formed by averaging the transmission diffraction pattern while scanning an atomically-fine electron probe across a sample. Visual comparison between experimental and simulated patterns is increasingly being used for sample thickness determination. We explore automating the comparison via a simple sum square difference metric. The thickness determination is shown to be accurate (i.e. the best-guess deduced thickness generally concurs with the true thickness), though factors such as noise, mistilt and inelastic scattering reduce the precision (i.e. increase the uncertainty range). Notably, the precision tends to be higher for smaller probe-forming aperture angles.

Simultaneous determination of sample thickness, tilt, and electron mean free path using tomographic tilt images based on Beer-Lambert law.

Cryo-electron tomography (cryo-ET) is an emerging technique that can elucidate the architecture of macromolecular complexes and cellular ultrastructure in a near-native state. Some important sample parameters, such as thickness and tilt, are needed for 3-D reconstruction. However, these parameters can currently only be determined using trial 3-D reconstructions. Accurate electron mean free path plays a significant role in modeling image formation process essential for simulation of electron microscopy images and model-based iterative 3-D reconstruction methods; however, their values are voltage and sample dependent and have only been experimentally measured for a limited number of sample conditions. Here, we report a computational method, tomoThickness, based on the Beer-Lambert law, to simultaneously determine the sample thickness, tilt and electron inelastic mean free path by solving an overdetermined nonlinear least square optimization problem utilizing the strong constraints of tilt relationships. The method has been extensively tested with both stained and cryo datasets. The fitted electron mean free paths are consistent with reported experimental measurements. The accurate thickness estimation eliminates the need for a generous assignment of Z-dimension size of the tomogram. Interestingly, we have also found that nearly all samples are a few degrees tilted relative to the electron beam. Compensation of the intrinsic sample tilt can result in horizontal structure and reduced Z-dimension of tomograms. Our fast, pre-reconstruction method can thus provide important sample parameters that can help improve performance of tomographic reconstruction of a wide range of samples.

Local sample thickness determination via scanning transmission electron microscopy defocus series.

The usable aperture sizes in (scanning) transmission electron microscopy ((S)TEM) have significantly increased in the past decade due to the introduction of aberration correction. In parallel with the consequent increase of convergence angle the depth of focus has decreased severely and optical sectioning in the STEM became feasible. Here we apply STEM defocus series to derive the local sample thickness of a TEM sample. To this end experimental as well as simulated defocus series of thin Si foils were acquired. The systematic blurring of high resolution high angle annular dark field images is quantified by evaluating the standard deviation of the image intensity for each image of a defocus series. The derived dependencies exhibit a pronounced maximum at the optimum defocus and drop to a background value for higher or lower values. The full width half maximum (FWHM) of the curve is equal to the sample thickness above a minimum thickness given by the size of the used aperture and the chromatic aberration of the microscope. The thicknesses obtained from experimental defocus series applying the proposed method are in good agreement with the values derived from other established methods. The key advantages of this method compared to others are its high spatial resolution and that it does not involve any time consuming simulations.

Sample Thickness Determination by Scanning Transmission Electron Microscopy at Low Electron Energies

Sample thickness is a decisive parameter for any quantification of image information and composition in transmission electron microscopy. In this context, we present a method to determine the local sample thickness by scanning transmission electron microscopy at primary energies below 30 keV. The image intensity is measured with respect to the intensity of the incident electron beam and can be directly compared with Monte Carlo simulations. Screened Rutherford and Mott scattering cross-sections are evaluated with respect to fitting experimental data with simulated image intensities as a function of the atomic number of the sample material and primary electron energy. The presented method is tested for sample materials covering a wide range of atomic numbers Z, that is, fluorenyl hexa-peri-hexabenzocoronene (Z = 3.5), carbon (Z = 6), silicon (Z = 14), gallium nitride (Z = 19), and tungsten (Z = 74). Investigations were conducted for two primary energies (15 and 30 keV) and a sample thickness range between 50 and 400 nm.

Differential alternating current chip calorimeter for in situ investigation of vapor-deposited thin films

Physical vapor deposition can be used to produce thin films with interesting material properties including extraordinarily stable organic glasses. We describe an ac chip calorimeter for in situ heat capacity measurements of as-deposited nanometer thin films of organic glass formers. The calorimetric system is based on a differential ac chip calorimeter which is placed in the vacuum chamber for physical vapor deposition. The sample is directly deposited onto one calorimetric chip sensor while the other sensor is protected against deposition. The device and the temperature calibration procedure are described. The latter makes use of the phase transitions of cyclopentane and the frequency dependence of the dynamic glass transition of toluene and ethylbenzene. Sample thickness determination is based on a finite element modeling of the sensor sample arrangement. In the modeling, a layer of toluene was added to the sample sensor and its thickness was varied in an iterative way until the model fit the experimental data.

Low-energy electron scattering in carbon-based materials analyzed by scanning transmission electron microscopy and its application to sample thickness determination

High-angle annular dark-field scanning transmission electron microscopy (HAADF STEM) at low energies (≤30 keV) was used to study quantitatively electron scattering in amorphous carbon and carbon-based materials. Experimental HAADF STEM intensities from samples with well-known composition and thickness are compared with results of Monte Carlo simulations and semiempirical equations describing multiple electron scattering. A well-defined relationship is found between the maximum HAADF STEM intensity and sample thickness which is exploited (a) to derive a quantitative description for the mean quadratic scattering angle and (b) to calculate the transmitted HAADF STEM intensity as a function of the relevant materials parameters and electron energy. The formalism can be also applied to determine TEM sample thicknesses by minimizing the contrast of the sample as a function of the electron energy.

Single and Double Infrared Transitions in Rapid-Vapor-Deposited Parahydrogen Solids: Application to Sample Thickness Determination and Quantitative Infrared Absorption Spectroscopy

We present a convenient method for determining the thickness of a cryogenic parahydrogen (pH2) solid from its infrared (IR) absorption spectrum. Millimeters-thick pH2solids of exceptional optical clarity can be produced by the rapid vapor deposition method [M.E. Fajardo and S. Tam, J. Chem. Phys. 108, 4237 (1998)]. Doping of these pH2solids is readily accomplished by co-deposition of the desired impurities, making them excellent hosts for high-resolution matrix isolation spectroscopy. The intensities of the IR “double” transitions Q1(0) + S0(0) and S1(0) + S0(0) of the pH2host are insensitive to the matrix microstructure and to the presence of dopants, so these absorptions are especially well suited for thickness determinations. We calibrate the integrated absorption intensities of these two bands against the sample thicknesses determined from transmission interference fringes appearing in the same experimental spectra; we report: α˜[Q1(0) + S0(0)] = 4.84(±0.18) × 10−14cm3/s, and α˜[S1(0) S0(0)] = 0.35(±0.02) × 10−14cm3/s (95% confidence). We also discuss several other advantages of rapid-vapor-deposited pH2solids as hosts for quantitative IR absorption spectroscopy of dopant species.

Automated, low-temperature dielectric relaxation apparatus for measurement of air-sensitive, corrosive, hygroscopic, powdered samples

An automated apparatus for dielectric determinations on solid samples was designed to allow cryogenic measurements on air-sensitive, corrosive, hygroscopic, powdered samples, without determination of sample thickness, provided that it is uniform. A three-terminal design enabled measurements that were not affected by errors due to dimensional changes of the sample or the electrodes with changes in temperature. Meaningful dielectric data could be taken over the frequency range from 20 Hz to 1 MHz and the temperature range from 12 to 360 K. Tests with Teflon and with powdered NH4Cl gave results that were accurate within a few percent when compared with literature values.

Highly precise determination of optical constants and sample thickness in terahertz time-domain spectroscopy

Time-domain spectroscopy allows fast and broadband measurement of the optical constants of materials in the terahertz domain. We present a method that improves the determination of the optical constants through simultaneous determination of the sample thickness. This method could be applied to any material with moderate absorption and requires only two measurements of the temporal profile of the terahertz pulses: a reference one without the sample and one transmitted through the sample.

Interfaces and thin films as seen by bound electromagnetic waves.

This contribution summarizes the use of plasmon surface polaritons and guided optical waves for the characterization of interfaces and thin organic films. After a short introduction to the theoretical background of evanescent wave optics, examples are given that show how this interfacial "light" can be employed to monitor thin coatings at a solid/air or solid/liquid interface. Examples are given for a very sensitive thickness determination of samples ranging from self-assembled monolayers, to multilayer assemblies prepared by the Langmuir/Blodgett/Kuhn technique or by the alternate polyelectrolyte deposition. These are complemented by the demonstration of the potential of the technique to also monitor time-dependent processes in a kinetic mode. Here, we put an emphasis on the combination set-up of surface plasmon optics with electrochemical techniques, allowing for the on-line characterization of various surface functionalization strategies, e.g. for (bio-) sensor purposes.

Thin sample thickness determination by X-ray fluorescence analysis

Thin sample thickness determination by X-ray fluorescence analysis

Quantitative analysis of powdered organic matter by transmission infrared microspectroscopy using a diamond-window compression cell

An artificial coalification series of Mahakam coal (type III organic matter) was analysed by transmission FT-i.r. microspectroscopy using a diamond anvil compression cell. Micro-infrared spectra were compared with spectra recorded in global mode using the standard KBr pelletization technique. Results indicated that infrared microspectroscopy provides higher-quality spectra (baseline and signal/noise ratio) as well as more information (accurate OH and v aromatic CH integration) than the bulk approach. The interference fringes on the micro-infrared spectra were used for sample thickness determination. Direct application of the Lambert-Beer law then allowed the absorbances of major infrared bands to be used to reveal the chemical changes taking place during maturation. Aliphatic CH species were shown to decrease roughly linearly whereas aromatic CH increased exponentially with increasing pyrolysis temperature. The end member of the artificial coalification series exhibited a fourfold lower aliphatic content and fifteenfold higher aromatic content than the original sample. Although oxygenated functions (OH, CO and CO) were substantially removed during the maturation, the contrasted behaviour of individual CO bands (1710, 1735 and 1650 cm−1) strongly suggested a rearrangement among the CO functions during artificial coalification. Direct absorbances of selected infrared bands were plotted against widely used geochemical parameters (HC and OC atomic ratios and Rock-Eval hydrogen index). The good correlations obtained indicate that transmission infrared microspectroscopy using a diamond anvil compression cell is a promising technique for statistically deducing the HC, OC and HI values of kerogen.

Determination of sample thickness via scattered radiation in x‐ray fluorescence spectrometry with filtered continuum excitation

AbstractA semi‐empirical approach is described for determining the mass per unit area of a sample being analysed. The method can be used to estimate the concentration of minor and trace elements in matrices containing a substantial amount of light elements. The procedure utilizes the coherently and incoherently scattered radiation induced in the sample by the filtered continuum radiation of a rhodium x‐ray tube. The relationship between the intensity of the scattered radiation per unit mass and the average atomic number of the sample is established via calibration graphs, which can be applied for different x‐ray tube voltages and for different primary beam filters. The overall procedure was validated by the analysis of several geological standards, deposited as thin slurries of unknown thickness either on Mylar foil or on Nuclepore filters.